Group 3 and Group 13 Metal Hydride Compounds

Group 3 and Group 13 Metal Hydride Compounds DISSERTATION zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) im Fach Chemie der Fakultät für Biologie, Chemie und Geowissenschaften der Universität Bayreuth vorgelegt von Dipl. Chem. Tobias Bauer geboren in Regensburg Bayreuth, 2013 Group 3 and Group 13 Metal Hydride Compounds DISSERTATION zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) im Fach Chemie der Fakultät für Biologie, Chemie und Geowissenschaften der Universität Bayreuth vorgelegt von Dipl. Chem. Tobias Bauer geboren in Regensburg Bayreuth, 2013 The following work has been carried out in the period September 2009 to July 2013 at the Lehrstuhl für Anorganische Chemie II of the Universität Bayreuth under the supervision of Prof. Dr. Rhett Kempe. This thesis fulfills the requirements for the doctoral degree of the Falkultät für Biologie, Chemie und Geowissenschaften at the Universität Bayreuth. Thesis submitted: 03/07/2013 Thesis accepted: 10/07/2013 Scientific Colloquium: 31/10/2013 Current dean of faculty: Prof. Dr. Rhett Kempe Examination Committee: First referee: Prof. Dr. R. Kempe Second referee: Prof. Dr. R. Schobert Third referee: Prof. Dr. A. Fery Chairman: Prof. Dr. A. Greiner „Das Außerordentliche geschieht nicht auf glattem, gewöhnlichem Wege.“ Johann Wolfgang von Goethe Alphabetical list of abbreviations alane aluminum hydride Ap aminopyridinate, aminopyridinato ligand ApH aminopyridine °C degree celsius Cp cyclopentadienyl ligand ELI-D electron localizability indicator Et ethyl Gu guanidinate, guanidinato ligand GuH guanidine NMR nuclear magnetic resonance spectroscopy thf/THF tetrahydrofuran PEt3 triethylphosphine Ph phenyl py pyridinyl PyAp deprotonated N-(2,6-diisopropylphenyl)-6-(pyrrolidin-1-yl)pyridin-2-amine QTAIM quantum theory of atoms in molecules XRD single crystal X-ray structure analysis Table of Contents 1 Summary/Zusammenfassung ............................................................. 1 1.1 Summary .................................................................................................... 1 1.2 Zusammenfassung .................................................................................... 4 2 Introduction ......................................................................................... 7 3 Overview of Thesis Results .............................................................. 13 3.1 Synopsis ................................................................................................... 13 3.2 Individual Contribution to Joint Publications ....................................... 19 4 The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations ..................................................................................... 21 4.1 Introduction .............................................................................................. 22 4.2 Results and Discussion .......................................................................... 23 4.3 Conclusions ............................................................................................. 32 4.4 Experimental Section .............................................................................. 32 4.5 Acknowledgments ................................................................................... 36 4.6 References ............................................................................................... 36 4.7 Supporting Information ........................................................................... 40 4.8 References ............................................................................................... 45 5 Synthesis and Structure of Aminopyridinato and Guanidinato Ligand Stabilized Al-H Complexes ........................................................ 46 5.1 Introduction .............................................................................................. 46 5.2 Results and Discussion .......................................................................... 47 5.3 Conclusions ............................................................................................. 53 5.4 Experimental Section .............................................................................. 54 5.5 References ............................................................................................... 56 6 Synthesis and Structure of a Trinuclear Yttrium Polyhydride Cluster Stabilized by a Bulky Guanidinato Ligand ............................... 58 6.1 Introduction .............................................................................................. 58 6.2 Results and Discussion .......................................................................... 59 6.3 Conclusions ............................................................................................. 65 6.4 Acknowledgments ................................................................................... 66 6.5 Experimental Section .............................................................................. 66 6.6 References ............................................................................................... 67 7 Ternary Rare-Earth Transition-Metal Polyhydride Cluster Compounds ............................................................................................. 69 7.1 Introduction .............................................................................................. 69 7.2 Results and Discussion .......................................................................... 70 7.3 Conclusions ............................................................................................. 75 7.4 Acknowledgments ................................................................................... 75 7.5 References ............................................................................................... 76 7.6 Supporting Information ........................................................................... 78 7.7 General ..................................................................................................... 78 7.8 Details of the X-ray crystal structure analyses ..................................... 79 7.9 Synthesis and characterization of the cluster compounds ................. 80 7.10 References ............................................................................................... 81 8 List of Publications ........................................................................... 82 9 Acknowledgements / Danksagung ................................................... 85 9.1 Acknowledgments ................................................................................... 85 9.2 Danksagung ............................................................................................. 87 10 Declaration / Erklärung: .................................................................... 89 2. Introduction 7 2 Introduction Metal-carbon and metal-hydrogen bonds are at the very heart of coordination chemistry. Molecular metal hydrides, in general, are a fascinating class of compounds regarding their structure, reactivity and applications. They are key intermediates in a plethora of selective stoichiometric transformations and/or catalytic cycles. The first well-defined transition metal hydrides, (CO)4FeH2 and (CO)4CoH, were prepared by Hieber and co-workers in 1931 and 1932, respectively.[1,2] These quite unstable compounds remained as laboratory curiosities for over 20 years. The next milestone dates back to the year 1955 with the discovery of (C5H5)2ReH by Birmingham and Wilkinson[3] and (C5H5)(CO)3MH (M = Cr, Mo, W) by Fischer and coworkers.[4] Two years later, Chatt, Duncanson, and Shaw prepared the exceptionally stable hydride compound trans-(PEt3)2ClPtH.[5] Since then, rapid development in this field took place and by the year 1965 over 200 derivatives were reported in some 300 publications.[6] In 2001 the Nobel Prize in chemistry was awarded jointly to Knowles, Nyori and Sharpless for asymmetric catalysis (Knowles and Nyori for contributions on asymmetric hydrogenation). This can be seen as one magic moment of metal hydride chemistry. Research interest in the main group metal hydrides was documented alike.[7] The s-block metal hydrides are salt-like and the p-block metal hydrides form covalently bonded molecules comparable to the ones formed by the dand f-block metals. Especially aluminum hydrides, first prepared by Stecher and Wiberg in 1942,[8] received much attention due to promising applications as reducing agents in organic synthesis[9] and for the reduction of metal complexes[10]. Furthermore, alanes are used in hydroamination reactions[11] and as precursors for metal organic chemical vapor deposition processes.[12] A more convenient preparation method was reported by Finholt, Bond, and Schlesinger in 1947.[13] Pioniering work on amine complexes of alane dates back to the early 1950s and the early 1960s.[14,15] Since then, much effort has been devoted to extend the field of alane chemistry. Transition metal σ-alane complexes[16] were prepared due to promising applications. Recent studies on alanes focus on applications as hydrogen storage materials.[17,18] Moreover, a guanidinato ligand stabilized adduct of dialane (Al2H4) was reported to feature a direct Al-Al bond.[19] Another prominent and rich field of metal hydrides is found to be the hydrides of rare earth metals (group 3 metals and lanthanoid metals [Ce-Lu]). These compounds often aggregate and build up polyhydride clusters. Rare earth (poly)hydride compounds possess a fascinating variety of unique structural motifs and chemical properties. The early work on lanthanoid hydrides is reviewed by Bos and Gayer and covers the period from 1891 to 1966.[20] 2. Introduction 8 Since then, it took over a decade until the first example of a molecular lanthanoid hydride was reported. Schumann and co-workers prepared [(C5H5)2LuH(thf)] (thf = tetrahydrofuran) by hydrogenolysis of the corresponding alkyl or aryl precursor in 1981.[21] From there on, research interest increased and an oddless number of cyclopentadienyl stabilized hydride and alkyl complexes of the rare earths became known.[22] Various applications and reactivities like hydrogenation reactions,[23] hydroboration reactions,[24] hydroamination reactions,[25] hydrosilylation reactions,[26] hydrophosphination reactions[27] and polymerization processes[28] of alkenes by cyclopentadienyl-type rare earth metal hydrides and alkyls are published. Hence, permanent interest in this type of compounds arose. Recently, a shift from cyclopentadienyl ligand sandwichand half-sandwich (poly)hydride complexes towards alternatively supported hydride compounds has taken place.[29] Mainly, because of their promising new applications and reactivities. Nonetheless, rare earth metal hydride compounds supported by ligands other than Cp and its derivatives still lack in number. The most used and important Cp alternatives are amido[30] (Scheme 1, right) and alkoxy ligands (Scheme 1, center). They have proven to be suitable for the stabilization of electron poor transition, main group and rare earth metal ions in different oxidation states. Scheme 1. Commonly used ligand types for the stabilization of metal hydrides (R, R’ = aryl, alkyl or silyl, M = Main group, transition or rare earth metal). The aminopyridinato ligand, a subclass of the amido ligand family, derived from deprotonated 2-aminopyridines, has been prominently used in the renaissance of amido metal chemistry.[31] Two different binding modes are known (Scheme 2) and many substitution patterns to fine tune the steric bulk of the ligand are possible. Starting from 2,6-dibromopyridine, firstly a substituted phenyl group is introduced via Kumada coupling and secondly, a derivative of aniline is introduced via Buchwald-Hartwig aryl amination. Scheme 2. Binding modes of aminopyridinato ligands (R, R' = aryl, alkyl or silyl, M = Main group, transition or rare earth metal, M' = transition metal). 2. Introduction 9 [Ru(PhNpy)2)PPh3)2] was the first example of a strained η2-coordinated aminopyridinato ligand stabilized complex, described by Cotton and co-workers in 1984.[32] In 1991 Gambarotta and co-workers reported on the first vanadium compound stabilized by an aminopyridinato ligand.[33] Kempe et al. prepared the first corresponding group 3 metal complex in 1997.[34] Another subclass of the amido ligands are guanidinato ligands derived from deprotonated guanidines, which are comparable to the aminopyridinato ligands regarding their binding mode (Scheme 3). Their substitution pattern is more variable than for the aminopyridinato ligands, due to substitution on the nitrogen atoms. Recently, this ligand class was comprehensively reviewed by Jones.[35] Synthesis of guanidinato ligands is achieved via a direct approach starting from substituted carbodiimides, which are reacted, with lithiated secondary amine derivatives. The resulting lithium complexes of the ligands can be used in salt metathesis reactions towards metal halides or can be hydrolyzed to afford the protonated ligands. These protonated ligands can be used in alkane or amine elimination routes. Lappert and co-workers published the first transition metal guanidinato ligand stabilized complex in 1970.[36] Scheme 3. Binding modes of guanidinato ligands (R, R',R'', R''' = aryl, alkyl or silyl, M = Main group, transition or rare earth metal, M' = transition metal). Firstly, this work was focused on synthesis and characterization of guanidinato ligand stabilized aluminum dimethyl complexes. These complexes were examined regarding the substituents R'' and R'''. Dependency of the steric bulk towards the (M)N-C-N(M) angle was observed (Scheme 3, left, M = Al). Secondly, synthesis and structure of Ap and Gu ligand stabilized Al-H complexes was discussed. The reaction of a sterically bulky guanidine with lithium alanate was examined. A rare example of a σ-alane lithium complex was observed. Thirdly, a guanidinato ligand stabilized yttrium dialkyl complex was synthesized and characterized. Its ability towards hydrogenolysis using H2 was investigated. The resulting trinuclear yttrium polyhydride cluster compound possesses highly dynamic behavior of the hydrides and the guanidinato ligands, as observed by variable temperature 1H NMR spectroscopy. 2. Introduction 10 Fourthly, the first examples of ternary rare earth-transition metal polyhydride cluster compounds were shown. Cluster formation proceeded through C–H bond activation of the Cp ligands that stabilize the transition metal-containing educt. Quantum chemical calculations of the electronic structure showed ionic W–H∙∙∙Lu interactions and a covalent, polar Lu–Re bond. [1] W. Hieber, F. Leutert, Naturwissenschaften 1931,19, 360–361. [2] W. Hieber, F. Mühlbauer, E. A. Ehmann, Berichte der deutschen chemischen Gesellschaft (A and B Series) 1963, 65, 1090–1101. [3] G. Wilkinson, J. M. Birmingham, J. Am. Chem. Soc. 1955, 77, 3421–3422. [4] E. O. Fischer, W. Hafner, H. O. Stahl, Z. Anorg. Allg. Chem. 1955, 282, 47–62. [5] J. Chatt, L. A. Duncanson, B. L. Shaw, Proc. Chem. Soc. 1957, 343. [6] For selected review articles on molecular transition metal hydrides, please see: a) J. Chatt, Science 1968, 160, 723–729; b) H. D. Kaesz, R. B. Saillant, Chem. Rev. 1972, 72, 231–281; c) A. J. Hoskin, D. W. Stephan, Coord. Chem. Rev. 2002, 233–234, 107– 129; d) G. S. McGrady, G. Guilera, Chem. Soc. Rev. 2003, 32, 383–392. [7] For selected review articles on main group metal hydrides, please see: a) S. Aldridge, A. J. Downs, Chem. Rev. 2001, 101, 3305–3365, b) S. K. Mandal, H. W. Roesky, Acc. Chem. Res. 2012, 45, 298–307. [8] O. Stecher, E. Wiberg, Berichte der deutschen chemischen Gesellschaft (A and B Series) 1942, 75, 2003–2012. [9] J. Málek, M. Cerný, Synthesis 1972, 217–234. [10] B. M. Bulychev, Polyhedron 1990, 9, 387–408. [11] H. Haubenstock, E. L. Eliel, J. Am. Chem. Soc. 1962, 84, 2363–2368. [12] J. A. Jegier, W. L. Gladfelter, Coord. Chem. Rev. 2000, 206–207, 631–650. [13] A. E. Finholt, A. C. Bond, H. I. Schlesinger, J. Am. Chem. Soc. 1947, 69, 1199–1203. [14] E. Wiberg, H. Graf, R. Usón, Z. Anorg. Allg. Chem. 1953, 272, 221–232. [15] J. K. Ruff, M. F. Hawthorne, J. Am. Chem. Soc. 1960, 82, 2141–2144. [16] I. M. Riddlestone, S. Edmonds, P. A. Kaufman, J. Urbano, J. I. Bates, M. J. Kelly, A. L. Thompson, R. Taylor, S. Aldridge, J. Am. Chem. Soc. 2012, 134, 2551−2554. [17] a) L. Schlapbach, A. Züttel, Nature 2001, 414, 353-358; b) E. David, J. Mater. Proc. Technol. 2005, 162, 169–177; c) U. Eberle, M. Felderhoff, F. Schüth, Angew. Chem. 2009, 121, 6732–6757; Angew. Chem. Int. Ed. 2009, 48, 6608–6630, d) S. F. Matar, Prog. Solid State Chem. 2010, 38, 1-37; e) S. F. Matar, Prog. Solid State Chem. 2012, 40, 31–40. [18] For recent examples see: a) A. Züttel, Mater. Today 2003, 24–33; b) M. Latroche, J. Phys. Chem. Solids 2004, 65, 517–522; c) W. Grochala, P. P. Edwards, Chem. Rev. 2. Introduction 11 2004, 104, 1283–1315; d) S. Harder, J. Spielmann, J. Intemann, H. Bandmann, Angew. Chem. 2011, 123, 4242–4246; Angew. Chem. Int. Ed. 2011, 50, 4156–4160; e) P. Jochmann, J. P. Davin, T. P. Spaniol, L. Maron, J. Okuda, Angew. Chem. 2012, 124, 4528–4531; Angew. Chem. Int. Ed. 2012, 51, 4452–4455. [19] S. J. Bonhady, D. Collis, G. Frenking, N. Holzmann, C. Jones, A. Stasch, Nat. Chem. 2010, 2, 865–869. [20] W. G. Bos, K. H. Gayer, J. Nuc. Mat. 1966, 18, 1–30. [21] H. Schumann, W. Genthe, J. Organomet. Chem. 1981, 213, C7–C9. [22] For selected review articles on cyclopentadienyl lanthanoid hydrides, please see: a) H. Schumann, J. A. Meese-Marktscheffel, L. Esser, Chem. Rev. 1995, 95, 865–986; b) M. Ephritikhine, Chem. Rev. 1997, 97, 2193–2242. [23] a) W. J. Evans, I. Bloom, W. E. Hunter, J. L. Atwood, J. Am. Chem. Soc. 1983, 105, 1401–1403; b) G. Jeske, H. Lauke, H. Mauermann, H. Schumann, T. J. Marks, J. Am. Chem. Soc. 1985, 107, 8091–8103; c) D. Stern, M. Sabat, T.J. Marks, J. Am. Chem. Soc. 1990, 112, 9558–9575; d) V. P. Conticello, L. Brard, M. A. Giardello, Y. Tsuji, M. Sabat, C. L. Stern, T. J. Marks, J. Am. Chem. Soc. 1992, 114, 2761–2762. [24] a) K. N. Harrison, T. J. Marks, J. Am. Chem. Soc. 1992, 114, 9220–9221; b) E. A. Bijpost, R. Duchateau, J. H. Teuben, J. Mol. Catal. 1995, 95,121–128; [25] a) P. W. Roesky, T. E. Müller, Angew. Chem. 2003, 115, 2812–2814; Angew. Chem. Int. Ed. 2003, 42, 2708–2710; b) S. Hong, T. J. Marks, Acc. Chem. Res. 2004, 37, 673–686; c) K. C. Hultzsch, Adv. Synth. Catal. 2005, 347, 367–391. [26] G. A. Molander, J. A. C. Romero, Chem. Rev. 2002, 102, 2161–2186. [27] a) M. R. Douglass, T. J. Marks, J. Am. Chem. Soc. 2000, 122, 1824–1825; b) A. Kawaoka, T. J. Marks, J. Am. Chem. Soc. 2004, 126, 12764–12765; c) A. Kawaoka, T. J. Marks, J. Am. Chem. Soc. 2005, 127, 6311–6324. [28] a) Z. Hou, Y. Wakatsuki, Coord. Chem. Rev. 2002, 231, 1–22; b) H. Yasuda, J. Organomet. Chem. 2002, 647, 128–138; c) Y. Nakayama, H. Yasuda, J. Organomet. Chem. 2004, 689, 4489–4498. [29] For selected review articles on non-cyclopentadienyl lanthanoid hydrides and their applications, please see: a) M. Konkol, J. Okuda, Coord. Chem. Rev. 2008, 252, 1577– 1591; b) A. A. Trifonov, Coord. Chem. Rev. 2010, 254, 1327–1347. [30] M. F. Lappert, P. P. Power, A. R. Sanger, R. C. Srivastava, Metal and Metalloid Amides, Ellis Norwood Ltd., Chichester, 1980. [31] R. Kempe, Angew. Chem. 2000, 112, 478–504; Angew. Chem. Int. Ed. 2000, 39, 468– 493. [32] A. R. Chakravarty, F. A. Cotton, E. S. Shamshoum, Inorg. Chim. Acta 1984, 86, 5–11. 2. Introduction 12 [33] J. J. H. Edema, S. Gambarotta, A. Meetsma, A. L. Spek, N. Veldman, Inorg. Chem. 1991,30, 2062–2066. [34] R. Kempe, A. Spannenberg, Z. Kristalogr. NCS 1997, 212, 487–489. [35] C. Jones, Coord. Chem. Rev. 2010, 254, 1273–1289. [36] G. Chandra, A. D. Jenkins, M. F. Lappert, R. C. Srivastava, J. Chem. Soc. 1970, 2550– 2558. 3. Overview of Thesis Results 13 3 Overview of Thesis Results This thesis comprises four publications, which are presented in chapters 4 to 7. The individual contributions to joint publications are pointed out in chapter 3.2. In the following, the central theme of the thesis is summarized. 3.1 Synopsis The main task of this thesis was to increase the small number of structurally fully characterized group 3 and group 13 metal hydride compounds. Furthermore, group 3 and group 13 metal alkyl compounds were synthesized (and characterized). These alkyl compounds were synthesized as precursors and their ability to afford hydride compounds was studied. Supporting ligands for all complexes presented herein were restricted to aminopyridinato, guanidinato and phenolato ligands. These types of ligands are used to a very slight extent in group 3 and group 13 metal hydride chemistry. Chapter 4 deals with new aluminum alkyl compounds stabilized by guanidinato ligands. Guanidinato ligand stabilized aluminum dialkyls were synthesized and structurally characterized. Structural data of these compounds based on single crystal X-Ray structure analysis led to a concept of shortening metal-metal bonds. Due to this ligand based concept the best suited ligand yielding a stable Cr-Cr compound featuring the shortest metal-metal bond observed to date was found. In diguanidinato dichromium complexes the length of the quintuple bond can be influenced by the substituent at the central carbon atom of the used ligand. To find the guanidinato ligand forming the shortest Cr-Cr quintuple bonded complex, the dependency of the relevant N-C-N angle in the guanidinato ligand from the introduced substituent was investigated. Fine tuning of the ligands steric bulk was essential. Guanidinato ligand stabilized aluminum dialkyls were expected to be well suited for such a ligand fine-tuning. The tetrahedral coordination avoided inter-ligand repulsion and the smooth synthesis via alkane elimination from aluminum trialkyls allowed for an easy access. Four different guanidinato ligand stabilized aluminium dialkyls were synthesized. These compounds were isolated in good yields (> 80 %). 3. Overview of Thesis Results 14 Figure 3.1. Crystal structure of [(MPipGu)AlMe2] and [{(MPipGu)Cr}2]. Structural data of the corresponding aluminum dialkyls showed promising (Al)N-C-N(Al) angles for the guanidinato ligands bearing a 2,6-dimethylpiperidine and a diisopropylamine backbone, respectively. The found (Al)N-C-N(Al) angles were 107.68(12)° (2,6dimethylpiperidine) and 107.39(15)° (diisopropylamine). These two potential ligands were examined towards ultra short metal-metal distances. Figure 3.2.N-C-N angles for all structurally investigated Al complexes. The guanidinato ligand carrying the 2,6-dimethylpiperidine backbone was found to be the optimal ligand. The reduction of its chromium(II) chloride ate-complex yielded a quintuply bonded bimetallic complex with a Cr-Cr-distance of 1.7056 (12) Å. Moreover, these guanidinato ligand stabilized aluminum dialkyls were thought of as precursors to aluminum hydride 3. Overview of Thesis Results 15 compounds. Transformation of the alkyl compounds using H2 and phenylsilane did not afford the corresponding hydride compounds. So, a direct approach to molecular alanes stabilized by N-ligands was carried out. Chapter 5 deals with the synthesis and structure of rare aminopyridinato and guanidinato ligand stabilized aluminum hydride compounds. Only a small number of structurally fully characterized amidinato, aminopyridinato and guanidinato ligand stabilized alanes are known until now. Starting from AlH3, the direct approach to afford N-ligand stabilized aluminum hydride compounds was studied. The reaction of the aminopyridine N-(2,6diisopropylphenyl)-6-(pyrrolidin-1-yl)pyridin-2-amine (PyApH) and the guanidine N,N'-bis(2,6diisopropylphenyl)piperidine-1-carboximidamide (PipGuH) with freshly prepared AlH3 was investigated. For both N-ligands the formation of a dimeric, double hydrogen bridged aluminum dihydride complex was observed. In these isostructural dimeric complexes, the aluminum centers are five-coordinated by two N atoms (from the N-ligands), two µ2-bridging hydrides and a terminal hydride. The aminopyridinato ligand stabilized compound is unstable and intramolecular ligand redistribution reaction leading to monomeric [(PyAp)2AlH] was observed even at room temperature. The formation proceeded (most likely) via AlH3 formation and its decomposition to Al and H2. The guanidinato ligand stabilized complex was found to be more stable and no ligand transfer was observed up to 50°C. Furthermore, the reaction of (2R,6S,Z)- N,N'-bis(2,6-diisopropylphenyl)-2,6-dimethylpiperidine-1-carboximidamide (MPipGuH) with LiAlH4 was examined. A rare example of a σ-alane lithium complex, namely [(MPipGu)(H)2Al(µH)Li(thf)3], was synthesized in 81 % yield. Figure 3.3. Crystal structure of the σ-alane lithium complex [(MPipGu)(H)2Al(µ-H)Li(thf)3]. In this compound, the aluminum center was five-coordinated. The guanidinato ligand was bound in a N,N’-dihapto-chelating mode. Two terminal hydrides and one bridging hydride to a THF stabilized lithium atom accomplished the coordination sphere around the aluminum atom. This complex could be a suitable precursor to synthesize other (example given) σ-alane 3. Overview of Thesis Results 16 transition metal or σ-alane lanthanoid complexes. Based on the knowledge, that the guanidinato ligand stabilized aluminum dialykls were not able to undergo hydrogenolysis to afford the corresponding hydride compounds, examinations of group 3 metals were carried out. Chapter 6 deals with the synthesis and structure of a trinuclear yttrium polyhydride compound stabilized by a guanidinato ligand. The synthesis and structural determination of the first rare earth“(LnH2)3“ polyhydride stabilized by a guanidinato ligand was achieved. An yttrium alkyl complex was thought of as a promising starting material and its behavior to hydrogenolysis using H2 was examined. The reaction of equimolar amounts of yttrium trialkyl complex ([YR3(thf)2]) (R = CH2Si(CH3)3, thf= tetrahydrofuran) with the guanidine N,N'-bis(2,6-diisopropylphenyl)piperidine1-carboximidamide (PipGuH) gave the resulting guanidinato ligand stabilized yttrium dialkyl complex ([PipGu)YR2(thf)]. This complex features a N,N’-dihapto-guanidinato ligand, two alkyl moieties and one THF molecule coordinated to the Y atom. In contrast to the aluminum dialkyl compounds, hydrogenolysis of this precursor with H2 (2 bar) yielded clean formation of the corresponding guanidinato ligand stabilized trinuclear yttrium hexahydride cluster compound [{(PipGu)YH2}3(thf)2]. Figure 3.4. Crystal structure of the cluster core unit of [{(PipGu)YH2}3(thf)2] (guanidinato ligands only shown as NCN moieties for clarity). The isolated yield was 96 %. Single crystal X-Ray structure analysis revealed a triangle defined by the three yttrium atoms. Each yttrium atom carried a guanidinato ligand in the same N,N’-dihapto-chelating mode like in the precursor dialkyl complex, as was revealed by XRD analysis. Moreover, coordinated THF molecules to two of the three yttrium atoms were found. Highly dynamic behaviour of the hydrido and the guanidinato ligands was observed by variable temperature 1H NMR spectroscopy. Lanthanoid polyhydride cluster possess various, interesting structural motifs and, despite cyclopentadienyl ligand based compounds, are small in number. Moreover, a broad variety of applications are known. This type of compound was thought of as 4. The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations 23 Scheme 1. The role of the stabilizing ligand on the metal-metal distance in complexes having a (formal) quintuple bond. The substituents on top (red) alter the N-C-N angle (blue) and compress the metal-metal multiple bond. We report here the results of a systematic search for the shortest metal-metal (quintuple) bond. The finally obtained distance is 1.7056(12) Å. Furthermore, we point out at a few limitations of the above introduced ligand based metal-metal bond shortening concept, the main one being the formation of a different coordination isomer, an unsupported Cr(I) dimer with a significantly lower bond order. Quintuple bonding has gained a lot of attention meanwhile. The di-metallic (chromium or molybdenum) platform is well suited to activate small molecules.[15] 4.2 Results and Discussion The hypothesis we developed from the state of the art in making ultra-short chromium-chromium quintuple bonds basically means the Cr-Cr distance is determined by the substituent R linked to the central carbon atom of the guanidinato (or amidinato) ligand (Scheme 2). Scheme 2. Synthesis of the Al-complexes 5-8. 4. The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations 24 In order to find the guanidinate ligand forming the shortest Cr-Cr quintuple bond the dependence of the relevant N-C-N angle in the guanidinate ligand from the introduced substituent R (Scheme 2) was investigated. Aluminum dialkyls were expected to be well suited for such a study. The tetrahedral coordination avoids interference with the remaining ligands and the smooth synthesis via alkane elimination from commercially available trialkyls allows for an easy access. The aluminum guanidinates 5-8 (Scheme 2, Figure 1) were synthesized and characterized via Xray crystal structure analysis. We observed that increasing the steric demand on the back bone from pipiridine to diisopropylamine decreases the NCN bond angle from 109.8(3) in 5 to 107.39(15) in 8 (Figure 1).[16] Thus, the ligands 3 and 4 should give Cr complexes with even shorter metal-metal bond distances than 2. The Cr-Cr complex stabilized by 2 is featuring the shortest metal-metal bond [1.7293 (12) Å] observed in a stable molecule yet.[13b] Figure 1.Molecular structure of 7 with the hydrogen atoms omitted for clarity and the crucial N-C-N angle for all structurally investigated Al complexes (R = 2,6-diisopropylphenyl).Selected bond lengths [Å] and angles [°]: Al1N2 1.9245(13), Al1-N1 1.9318(13), Al1-C1 1.958(2), Al1-C2 1.9609(19); N2-C5-N1 107.68(12), N2-Al1-N1 69.09(5), N2-Al1-C1 113.78(7), N1-Al1-C1 120.33(8), N2-Al1-C2 118.70(7), N1-Al1-C2 113.44(7), C1-Al1-C2 114.32(9). The reactions of the lithium guanidinates, Li[(2,6-dimethylpipiridine)C(NAr)2] and Li[(diisopropylamine)C(NAr)2][17] made from 3 and 4, with CrCl2 in THF afforded, after removal of the solvent and subsequent extraction with ether, the corresponding Cr(II) atecomplexes 9 and 10, respectively as blue crystalline materials in good yields (Scheme 4). 4. The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations 25 Scheme 4. Synthesis of 9 and 10. The 1H NMR spectra showed only broad signals due to the presence of paramagnetic Cr(II) ions. Both complexes were structurally investigated by X-ray crystal structure analysis. The observed structural motif has been recently reported for diketiminate ligands.[18] The molecular structure of 9 is shown in Figure 2. Its magnetic moments (μB) was determined to be 4.54. From the initially selected guanidines 3 and 4, ligand precursor 4 carries the bulkiest substituent and for the corresponding Al complex 8 the smallest N-C-N angle was observed (Figure 1). Figure 2. Molecular structure of 9 [ORTEP representation (on the 50 % probability level) for all non carbon atoms); Hydrogen atoms have been omitted for clarity. Selected bond lengths [Å] and angles [°]: C1-N1 1.348(2), C1-N2 1.352(3), C1-N3 1.375(3), Li1-O1 1.936(4), Li1-O2 1.941(4), Li1-Cl1 2.347(4), Li1-Cl2 2.361(4), Li1-Cr1 3.209(4), N1-Cr1 2.0527(16), N2-Cr1 2.0455(16), Cr1-Cl2 2.3492(6), Cr1-Cl1 2.3691(6); N1-C1-N2 109.73(16), N1-C1-N3 126.79(17), N2-C1-N3 123.48(17), N2-Cr1-N1 65.19(6), N2-Cr1-Cl2 99.46(5), N1-Cr1-Cl2 164.62(5), N1-Cr1-Cl1 101.51(5), Cl2-Cr1-Cl1 93.86(2). 4. The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations 26 Thus, we became interested to use 10, which is stabilized by deprotonated 4. The reduction of 10 with KC8 and work up in hexane led to a monomeric Cr0 complex (compound 11, Scheme 5), in which the central Cr atom is sandwiched between two arene units of two guanidinate ligands (Figure 3). Scheme 5. Synthesis of the Cr complex 11. Not only the η6-coordination of the arene unit is limited to the bridging Cr, but the same arene unit also coordinates one K in the same fashion. Furthermore, the guanidinate ligands in 11 are acting as an amide coordinating the K atom through N1. Figure 3. Molecular structure of 11; Hydrogen atoms have been omitted for clarity. Selected bond lengths [Å] and angles [°]:C5-N2 1.310(5), C5-N1 1.359(5), C5-N3 1.424(6), N1-K1 2.688(4), Cr1–Ar centroid 1.667, K1–Ar centroid 2.798, O1-K1 2.682(4), O2-K1 2.748(4); N2-C5-N1 122.9(4), N2-C5-N3 117.3(4), N1-C5-N3 119.8(4), C5-N1-K1 129.6(3), O1-K1-N1 119.59(13), O1-K1-O2 82.70(13), N1-K1-O2 139.72(13). 4. The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations 27 The K atoms are further coordinated by two thf molecules. The central structural motif resembles the classic bis(benzene)chromium structure.[19] Since the reduction of 10 leads to an over-reduced product, we repeated the reaction more than five times also with varied amount of potassium graphite. The results were similar. We obtained 11 and leftover starting material (10) with lower amounts of the reducing agent. From these studies, we concluded that the steric bulk of the substituent in the backbone of 5 is already too large to stabilize a complex having a quintuple bond and continued with attempts based on 4 (or the dichloride 9). Reduction of 9 with KC8 in THF resulted in a sudden color change from royal blue to orange red (Scheme 6). After work up, 12 was isolated as purple needles at room temperature. The crystal structure of 12 reveals a compound where the two guanidinate ligands do not act as bridging ligands. They coordinate to each Cr atom in a chelating fashion giving rise to an unsupported Cr-Cr-bond. The Cr-Cr bond axis is collinear to the C2 axis of NCN moiety of the guanidinate ligand. A Cr-Cr bond length of 2.652(2) Å is observed for 12. Scheme 6. Synthesis of 12 and 13. The molecular structure of 12 is shown in Figure 4. The conjugated NCN moiety shows very similar C-N distances [C1-N2 1.360(7), C1-N3 1.363(7), C1-N1 1.374(7) Ǻ]. 4. The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations 28 Figure 4. Molecular structure of 12. Hydrogen atoms have been omitted for clarity. Selected bond lengths [Å] and angles [°]: C1-N1 1.360(7), C1-N3 1.363(7), C1-N2 1.374(7), N1-Cr1 2.036(5), N2-Cr1 2.045(5), Cr1-Cr1 2.652(2); N1-C1-N3 125.5(6), N1-C1-N2 108.9(5), N3-C1-N2 125.5(6), N1-Cr1-N2 66.06(18), N1-Cr1-C1 32.84(18). Unsupported chromium-chromium bonds are rare. Pioneering work in this regard was reported by the Gambarotta group.[20] They synthesized N-ligand stabilized Cr(II) with a rather weak bond between the two metal atoms. Dimers of Cr(I) such as those observed herein are difficult to obtain, because a variety of “side reactions” have to be avoided. Complexes of Cr(I) become mononuclear if the stabilizing ligand is too bulky.[11,21] Arene sandwich complexes can be formed if aromatic solvents are used.[22] The presence of dinitrogen can lead to N2 complexes.[23] Furthermore, bridging of the aryl substituents of the N-ligand has to be avoided.[24] X-ray crystal structure analysis, magnetic data and electronic structural calculations (vide infra), IR data and reaction with CCl4 (no formation of CHCl3)[25] indicate that no bridging hydrides are present in 12. Interestingly, the second and third crop of crystallization during the synthesis of 12 did not afford needles but orange red plates. The Xray structural analysis revealed a bridged homobimetallic compound (13) with an exceptionally short metal-metal distance of 1.7056 (12) Å (Figure 5). A second crystal gave rise to a structure with a Cr-Cr distance of 1.7061(9) Å. It is the shortest Cr-Cr distance as well as the shortest metal-metal bond reported for a stable compound yet. A distance approaching 1.70 Å is interesting in a few regards. For instance, the chromium-chromium bond length of transient Cr2 molecule which can be generated by laser-evaporation of the metal and via flash photolysis of Cr(CO)6 is in the same distance range.[26,27] This compound has a formal sextuple bond. Furthermore, a similar distance as for the metal-metal bond in 13 was found recently for the longest alkane C-C bond[28] [1.704 (4) Å]. This essentially means a metal-metal bond and a C-C bond of an alkane can be of similar length. The Cr-N 4. The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations 29 bond lengths [1.992(4), 1.993(4), 2.008(4) and 2.011(4) Å] are comparable to the ones in the already known quintuply bonded chromium complexes but shorter than Cr-N bond distances [2.036(5) and 2.045(5) Å] observed for 12.[7,9] The C-N bond distances [1.390(6) Ǻ] for the non-coordinating nitrogen are slightly longer than the C-N bond distances of chromium coordinated nitrogen atoms [1.346(6) and 1.336(6) Å]. Figure 5. Molecular structure of 13. Hydrogen atoms and one hexane molecule have been omitted for clarity. Selected bond lengths [Å] and angles [°]: C1-N2 1.345(6), C1-N1 1.363(6), C1-N3 1.390(6), C13-N5 1.336(6), C13-N4 1.346(6), C13-N6 1.390(6), N1-Cr2 1.992(4), N2-Cr1 1.993(4), N4-Cr2 2.008(4), N5-Cr1 2.011(4), Cr1Cr2 1.7056(12); N2-C1-N1 112.1(4), N2-C1-N3 124.1(4), N1-C1-N3 123.8(4), Cr2-Cr1-N2 98.27(12), N2-Cr1-N5 164.29(17), Cr1-Cr2-N1 97.26(12), N1-Cr2-N4 165.17(17). Interestingly, parallel to our investigation the Jones group synthesized and characterized an iron(I) high-spin complex based on 3 with a very short Fe-Fe bond [2.1270(7) Å] that displays significant multiple-bond character.[29] The room temperature magnetic moment of 12 is µB 4.66 which is higher than the theoretically expected value for two S = 2/2 chromium centers (theoretical value of µB = 4.00), but lower than the theoretical value for two S = 3/2 chromium centers (theoretical value of µB =5.48). This is in good agreement with a Cr-Cr bond with an effective bond order of 1.25 (vide infra), where two of the five electrons are involved in metal-metal bond formation. Upon cooling a continuous decrease of the magnetic moment down to µB 0.58 was observed. This behavior is best explained with antiferromagnetic interactions between the remaining unpaired electrons of the two chromium centers. The experimental data were fit assuming two antiferromagnetically coupled S = 3/2 centers with H = -JS1S2. The obtained coupling constant J = -62 (1) cm-1 (g = 2, TIP = 0.0013(1) cm3·mol-1) is indicative of strong 4. The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations 30 antiferromagnetic interactions between the two chromium centers. The room temperature magnetic moment of 13 is µB 2.27 which is indicative of an S = 0 ground state of the dinuclear chromium complex with a paramagnetic impurity (Chromium (I) with S = 5/2). This value does not change significantly upon cooling. The experimental data of 13 were fit assuming an S = 0 ground state and a temperature independent paramagnetism TIP due to Zeeman perturbation. The best fit for compound 13 was found with a paramagnetic impurity PI = 5.0 % per Cr (S = 5/2) and TIP = 787·10-6 cm3·mol-1. Impurities in this %-range are not unusual for the very reactive quintuple bonds.[8,15a] In addition, herein, impurities of 12 may play a role. Figure 6.Active orbitals for structure 12 and their occupation numbers in the ground state. Finally, multiconfigurational quantum chemical calculations using the CASSCF/CASPT2 method[30] were performed to examine the electronic structure of these two coexisting Cr2-guanidinate compounds and in particular, the unique bonding of the unsupported Cr2 unit in 12 and compared it to the bonding in 13. Various dichromium systems, analogues to 13 are known to feature a quintuple metal-metal bond, despite the different ligands or oxidation state of the Cr atom. The metal-metal bonding is quantified in terms of effective bond order (EBO), defined as (ηb - ηa)/(ηb + ηa), where ηb is the occupation number for the bonding natural orbital and ηa is the occupation number for the corresponding antibonding natural orbital. The ground state of 12 has a highly multiconfigurational singlet nature, which is practically degenerate (< 2 kcal/mol) with the triplet and quintet states. Inspection of the natural orbital occupation numbers (Figure 6) indicates that all the 3d orbitals, except one /* pair, are singly occupied, which gives a minor contribution to the Cr- 4. The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations 31 Cr bond: the EBO value for the  bond is 0.11, and the corresponding  and  values are 0.16 and 0.04, respectively. One electron from each Cr atom is involved in Cr-N interaction with the ligands (those orbitals are not included in the complete active space). Interestingly, the strongest bond in the Cr2 unit is the  bond formed from the interaction of the 4s orbitals, with an EBO of 0.94. This results in a total bond order of 1.25 and an electronic configuration (Cr-Cr)4s2(Cr-Cr)3d1(Cr-Cr)3d*1(Cr-Cr)3d1(Cr-Cr)3d*1(Cr-Cr)3d2(Cr-Cr)3d*2. Thus, the long 2.65 Å Cr-Cr bond in this unsupported Cr(I) dimer bears a single 4s–4s interaction with the 3d shells antiferromagnetically coupled into a net singlet state. Table 1. Effective bond order for 12 and 13:, ,  contributions and total EBO values. Cr2guanidinate compound[13b] is given for comparison. EBO 12 13 Cr2-guanidinate[31e]  1.05 0.84 0.83  0.16 1.66 1.62  0.04 1.43 1.35 Total bond order 1.25 3.93 3.80 In contrast to 12, the short Cr-Cr bond in 13 is a formal quintuple bond with the Cr 3d orbitals forming the metal-metal multiple bond, whereas the pair of Cr 4s orbitals is directly involved in the Cr-N interaction with the ligands. The N atoms interact with the same weight with the Cr-Cr core as indicated by the shape of the Cr-N molecular orbitals (see Figure 7). The total EBO value of 3.93 (see Table 1) is slightly larger than the value of 3.80[31e] computed for the Cr2-guanidinate system[13b] which holds the previous record for the shortest Cr-Cr bond. Inspection of Table 1 indicates that the shortening (ca. 0.02 Å) of the metalmetal bond is accompanied by a slight increase of the strength of one of the  bonds. Analogously to other dichromium species,[31e,32] the closed-shell configuration (Cr-Cr)3d2(Cr-Cr)3d4(Cr-Cr)3d4(Cr-N)4s2 dominates the multideterminantal wave function with a total weight of 70 %. 4. The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations 32 Figure 7.Active orbitals for structure 13 and their occupation numbers in the ground state. The 1H NMR of 13 shows well resolved single signal set indicative of a diamagnetic compound. Complex 12 is rather stable in solution and does not show any decomposition or conversion to 13 as monitored by NMR spectroscopy using a C4D8O solution. 4.3 Conclusions In conclusion, we report on a rational approach to the complex having the shortest metal-metal bond. The key to isolate it, was a sterically tailor made guanidinate ligand. The metal-metal distance observed is of the same length as the longest C-C bond in stable alkanes. The ligand-based quintuple bond shortening concept has a few limitations. Most importantly, the formation of coordination isomers in which inter-ligand repulsion is minimized. It is assumable that additional shortening is difficult to accomplish since we reached the end of the stability gap. 4.4 Experimental Section General: All manipulations were performed with rigorous exclusion of oxygen and moisture in Schlenk-type glassware on a dual manifold Schlenk line or in N2 filled glove box (mBraun 120-G) with a high-capacity recirculator (<0.1ppm O2). Solvents were dried by distillation from sodium wire / benzophenone. Commercial CrCl2 (Alfa Aesor) was used as received. Compounds 2, 3, 4 and 8 were prepared according to published literature.[16,17a,33] Deuterated solvents were obtained from Cambridge Isotope Laboratories and were degassed, dried and distilled prior to use. NMR spectra were recorded on Varian 300 MHz and Varian 400 MHz at 4. The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations 39 2009, 471, 1–10; e) G. La Macchia, G. Li Manni, T. K. Todorova, M. Brynda, F. Aquilante, B. O. Roos, L. Gagliardi, Inorg. Chem. 2010, 49, 5216–5222. [32] G. Li Manni, A. Dzubak, A. Mulla, D. W. Brogden, J. F. Berry, L. Gagliardi, Chem. Eur. J. 2012, 18, 1737–1749. [33] S. Ge, A. Meetsma, B. Hessen, Organometallics 2008, 27, 3131–3135. [34] A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano, C. Giacovazzo, A. Guagliardi, A. G. G. Moliterni, G. Polidori and R. Spagna, J. Appl. Cryst. 1999, 32, 115–119. [35] SHELX97 Programs for Crystal Structure Analysis (Release 97-2). G. M. Sheldrick, Institut für Anorganische Chemie der Universität, Tammanstrasse 4, D-3400 Göttingen, Germany, 1998. [36] L. J. Farrugia, J. Appl. Cryst. 1999, 32, 837–838. 4. The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations 40 4.7 Supporting Information X-ray crystallographic data including tables, details of the magnetic and computational study Figure S1. Molecular structure of 5 [ORTEP representation (on the 50 % probability level) for all non carbon atoms]; Hydrogen atoms have been omitted for clarity. Figure S2. Molecular structure of 6 [ORTEP representation (on the 50 % probability level) for all non carbon atoms]; Hydrogen atoms have been omitted for clarity. Figure S3. Molecular structure of 10 [ORTEP representation (on the 50 % probability level) for all non carbon atoms]; Hydrogen atoms have been omitted for clarity. 4. The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations 41 Table S1. Crystal data for Al compounds 5, 6 and 7. compound 5 6 7 Empirical formula C32H50AlN3 C29H46AlN3 C34H54AlN3 Formula weight 503.73 463.67 531.78 crystal system Hexagonal Hexagonal Monoclinic space group P3(2)21 P3(2)21 P2(1)/c a [Å] 14.9090(7) 14.4200(6) 9.4030(5) b [Å] 14.9090(7) 14.4200(6) 31.9350(17) c [Å] 12.2510(6) 12.2130(5) 11.2800(6) α [deg]  [deg] 106.059(4) γ [deg] V, [Å3] 2358.30(19) 2199.29(16) 3255.0(3) crystal size, [mm3] 0.41 x 0.36 x 0.35 0.34 × 0.33 × 0.29 0.67 x 0.55 x 0.48 calcd, [g cm-3] 1.064 1.050 1.085 µ, [mm-1] (Mo K) 0.087 0.089 0.088 T, [K] 133(2) 133(2) 133(2)  range, [deg] 1.5825.69 1.6325.60 1.2825.80 no. of reflections unique 2991 2769 6171 no. of reflections obs. [I > 2 ( I )] 2394 2580 5293 no. of parameters 187 157 343 wR2 (all data ) 0.1083 0.0785 0.1226 R value [I>2 (I)] 0.0463 0.0352 0.0477 4. The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations 42 Table S2. Crystal data for Cr compounds 9, 10, 11, 12 and 13. compound 9 10 11 12 13 Empirical formula C40H64Cl2CrLiN3O2 C39H64Cl2CrLiN3O2 C78H128CrK2N6O4 C64H96Cr2N6 C70H108Cr2N6 Formula weight 748.78 736.77 1344.06 526.73 1137.62 crystal system Triclinic Monoclinic Monoclinic Triclinic Triclinic space group P-1 C2/c P2(1)/n P-1 P-1 a [Å] 11.2860(6) 16.0290(5) 10.6720(6) 10.8060(8) 12.6160(5) b [Å] 13.7030(6) 16.4480(5) 18.8650(10) 10.9290(9) 15.1330(6) c [Å] 14.689(8) 16.8070(7) 20.0340(10) 15.8220(12) 19.4170(7) α [deg] 102.318(4) 90.514(6) 112.679(3)  [deg] 94.489(4) 108.140(3) 104.381(4) 70.900(6) 95.143(3) γ [deg] 107.100(4) 69.395(6) 98.334(3) V, [Å3] 2096.95(18) 4210.8(3) 3907.0(4) 1601.6(2) 3341.0(2) crystal size, [mm3] 0.36 x 0.32 x 0.24 0.23 x 0.21 x 0.18 0.42 x 0.27 x 0.07 0.31 x 0.29 x 0.15 0.21 × 0.20 × 0.12 calcd, [g cm-3] 1.186 1.162 1.142 1.092 1.131 µ, [mm-1] (Mo K) 0.435 0.432 0.302 0.379 0.368 T, [K] 133(2) 133(2) 133(2) 193(2) 133(2)  range, [deg] 1.44-25.67 1.82-25.67 1.51-25.67 2.49-25.66 1.15-25.69 no. of reflections unique 7899 3961 7380 6018 12613 no. of reflections obs. [I > 2 ( I )] 6333 2919 3630 1959 4306 no. of parameters 442 219 424 335 694 wR2 (all data ) 0.1120 0.1025 0.1781 0.1685 0.1419 R value [I>2 (I)] 0.0418 0.0463 0.0794 0.0718 0.0611 Susceptibility measurements: Magnetic susceptibility measurements were carried out with a Quantum Design MPMS-XL SQUID magnetometer in the range from 2 to 300 K at 2 T (12) and 0.5 T (13). The powdered sample 12 was placed in a gelatin capsule, fixed in a nonmagnetic sample holder and measured in the RSO mode. The powdered sample 13 was placed in a quartz glass holder, fixed in a non-magnetic sample holder and measured in the DC mode. The magnetic data were corrected for the diamagnetic contribution of the sample holder and the quartz glass or gelatin capsule, respectively. The molar susceptibility data were corrected using the Pascal constant. The experimental data of 13 were fit assuming an S = 0 ground state and a temperature independent paramagnetism TIP due to Zeeman perturbation. The best fit for compound 13 was found with a paramagnetic impurity PI = 5.0 % per Cr (S = 5/2) and TIP = 787·10-6 cm3·mol-1. 4. The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations 43 Figure S4. Temperature dependence of the MT product of 12. The solid line represents the fit with H = -JS1S2 (S = 3/2), J = -62 (1) cm-1 and TIP = 0.0013(1) cm3·mol-1 assuming g = 2. Figure S5. A) Temperature dependence of the molar susceptibility of 13 (open circles). The solid line reproduces the best fit with the parameters PI = 5.0 % per Cr (S = 5/2) and TIP = 787·10-6 cm3·mol-1. B) Temperature dependence of the MT product of 13. 050 100 150 200 250 300 0 1 2 3 molT[cm3Kmol-1] T [K] 050 100 150 200 250 300 0.0 0.5 1.0 MT[cm3Kmol-1] T [K] a) b) 050 100 150 200 250 300 0,00 0,01 0,02 0,03 0,04 0,05 M[cm3mol-1] T [K] 4. The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations 44 Figure S6. The temperature dependence of the molar magnetic susceptibility of 11. The open points are the observed susceptibility, the black line reproduces the best fit with the parameters PI = 0.55 % (per Cr, S = 3/2 and Weiss constant θ = –1 K) and TIP = 26∙10-6 cm3∙mol-1. Computational details: Quantum chemical calculations were performed using the multiconfigurational Complete Active Space SCF (CASSCF)[1] method, followed by second-order perturbation theory (CASPT2).[2] Relativistic all electron ANO-RCC basis sets with triple-zeta quality (ANO-RCC-VTZP) were used on chromium and nitrogen[3] and minimal basis sets (ANO-RCC-MB) on carbon and hydrogen.[4] Scalar relativistic effects were included using the Douglas-Kroll-Hess Hamiltonian.[5] The computational costs arising from the two-electron integrals were drastically reduced by employing the Cholesky decomposition (CD) technique[6] combined with the Local Exchange (LK) screening.[9] In the CASSCF treatment, the complete active space contains ten electrons in twelve active orbitals (10/12). This space comprises all 3d and 4s orbitals forming the Cr-Cr bond, namely one 4s, one 3d, two 3d and two 3d bonding and the corresponding antibonding orbitals. In the subsequent CASPT2 calculations, orbitals up to and including the 2p for Cr and 1s for C and N were kept frozen. The Frozen Natural Orbital approach with 70% of the virtual orbitals taken into account was applied to CASPT2 (FNO-CASPT2) for saving disk requirements and reducing computational costs.[10] Ci symmetry was imposed. The Cr-Cr bonding is quantified in terms of effective bond order (EBO), defined as (ηb - ηa)/(ηb+ηa), where ηb is the occupation number for the bonding natural orbital andηa is the occupation number for the corresponding antibonding natural orbital. The CASSCF/CASPT2 approach has proven to be very successful in the studies of metal-metal bonded compounds.[11,14] All calculations were performed with the MOLCAS 7.4 package.[15] 4. The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations 45 4.8 References [1] B.O. Roos, P. R. Taylor, P. E. M. Siegbahn, Chem. Phys. 1980, 48, 157-173. [2] K. Andersson, P.-A. Malmqvist, B.O. Roos, J. Chem. Phys. 1992, 96, 1218-1226. [3] B. O. Roos, R. Lindh, P. A. Malmqvist, V. Veryazov, P. O. Widmark, J. Phys. Chem. A 2005, 108, 2851-2858. [4] P. O. Widmark, P. A. Malmqvist, B. O.Roos, Theor. Chim. Acta 1990, 77, 291-306. [5] B. A. Hess, Phys. Rev. A 1986, 33, 3742-3748. [6] F. Aquilante, P.A. Malmqvist, T. B. Pedersen, A. Gosh and B. O. Roos, J. Chem. Theory Comp. 2008, 4, 694-702. [7] F. Aquilante, T. B. Pedersen, R. Lindh, B. O. Roos, A. S. de Meras, H. Koch, J. Chem. Phys. 2008, 129, 024113. [8] F. Aquilante, L. Gagliardi, T. B. Pedersen, R.Lindt, J. Chem. Phys. 2009, 130, 154107. [9] F. Aquilante, T. B. Pedersen, R. Lindh, J. Chem. Phys. 2007, 126, 194106. [10] F. Aquilante, T. K. Todorova, L. Gagliardi, T. B. Pedersen, B. O. Roos, J. Chem. Phys. 2009, 131, 034113. [11] M. Brynda, L. Gagliardi, B. O. Roos, Chem. Phys. Lett. 2009, 471, 1-10. [12] G. La Macchia, L. Gagliardi, P. P. Power, M. Brynda, J. Am. Chem. Soc. 2008, 130, 5104-5114. [13] L. Gagliardi, B. O. Roos, Inorg. Chem. 2003, 42, 1599-1603. [14] B. O. Roos, A. Borin, L. Gagliardi, Angew. Chem. Int. Ed. 2007, 46, 1469-1472. [15] G. Karlström, R. Lindh, P. A. Malmqvist, B.O. Roos, U. Ryde, V. Veryazov, P. O. Widmark, M. Cossi, B. Schimmelpfennig, P. Neogrady, L. Seijo, Comput. Mater. Sci. 2003, 28, 222-239. 5. Synthesis and Structure of Aminopyridinato and Guanidinato Ligand Stabilized Al-H Complexes 46 5 Synthesis and Structure of Aminopyridinato and Guanidinato Ligand Stabilized Al-H Complexes Tobias Bauer,[a] Winfried P. Kretschmer,[a] Muhammad Hafeez[a,b], and Rhett Kempe*[a] [a] T. Bauer, Dr. W. P. Kretschmer, Dr. M. Hafeez, Prof. Dr. R. Kempe, Lehrstuhl Anorganische Chemie II, Universität Bayreuth, Universitätsstrasse 30, NW I, 95440 Bayreuth (Germany), Fax: (+49) 921552157, E-mail: [email protected] [b] Dr. M. Hafeez, Department of Chemistry, University of Azad Jammu and Kashmir, Muzaffarabad 13100, Azad Kashmir,(Pakistan). To be submitted. Keywords: Alanes•σ-alanes • N-ligands Abstract: The reaction of the aminopyridine N-(2,6-diisopropylphenyl)-6-(pyrrolidin-1-yl)pyridin2-amine (PyApH) and the guanidine N,N'-bis(2,6-diisopropylphenyl)piperidine-1carboximidamide (PipGuH) with (freshly prepared) AlH3 was investigated. For both N-ligands the formation of a dimeric, double hydrogen bridged aluminum dihydride complex is observed. The aminopyridinate is unstable and an intramolecular ligand redistribution reaction leading to monomeric [(PyAp)2AlH] is observed. The formation proceeds (most likely) via AlH3 formation and its decomposition to Al and H2. The guanidinate was found to be more stable and no ligand transfer was observed up to 50°C. Furthermore, the reaction of (2R,6S,Z)-N,N'-bis(2,6diisopropylphenyl)-2,6-dimethylpiperidine-1-carboximidamide (MPipGuH) with LiAlH4 was examined. The σ-alane lithium complex [(MPipGu)(H)2Al(µ-H)Li(thf)3] was formed in 81 % yield. It could be a suitable educt to synthesize other (for instance) σ-alane transition metal complexes. 5.1 Introduction Aluminum is the third most abundant element and the most abundant metal in the earth`s crust. It`s hydride, alane, is prominently used in organic synthesis as reducing agent.[1] Moreover, it can be used to reduce metal complexes.[2] Furthermore, alanes have interesting applications in hydroalumination reactions[3], as precursors in metal organic chemical vapour 5. Synthesis and Structure of Aminopyridinato and Guanidinato Ligand Stabilized Al-H Complexes 47 deposition,[4] and as hydrogen storage materials.[5,6] In addition, transition metal σ-alane complexes are of great interest.[7] Novel alanes could extent or improve these potential applications. Herein we report on synthesis and structure of aminopyridinato and guanidinato ligand stabilized Al-H complexes. All complexes presented herein were characterized via single crystal X-ray structure analysis (XRD). Until now, only a small number of structurally fully characterized amidinato and guanidinato ligand stabilized alanes are known. Recently, a guanidinato ligand stabilized adduct of dialane (Al2H4) with an aluminum-aluminum bond was reported.[8] 5.2 Results and Discussion Aminopyridinato ligands (Ap, Scheme 1, left)[9,10,11] are bidentate, monoanionic ligands related to guanidinato ligands (Gu, Scheme 1, right)[12]. Both ligand classes are able to stabilize a broad variety of metal ions.[12,13] N NR R' N RN N R' R'' R''' Scheme 1. Aminopyridinato ligands and the related guanidinato ligands (R, R’, R’’, R’’’ = aryl, alkyl or silyl substituents). As one can see, both ligand families can be fine-tuned with regard to their electron donating abilities and the steric bulk via the substituents (R and R’ in the Ap system, R, R’, R’’ and R’’’ in the Gu system). Firstly, we started with studies on Ap ligand stabilized alanes. We did chose N-(2,6-diisopropylphenyl)-6-(pyrrolidin-1-yl)pyridin-2-amine (PyApH, 1a) due to its electron donating ability.[14] Treatment of one equivalent of the aminopyridine 1a in toluene with one equivalent of in situ prepared AlH3 in a 1:2 mixture of ether/toluene lead to the formation of the dimeric aluminium hydride species [(PyApAlH2)2] 2a in 54% yield (Scheme 3). Crystals suitable for XRD analysis could be grown by storage of a concentrated toluene solution of 2a at -40°C. The molecular structure of compound 2a is shown in Figure 1.Experimental details of the XRD studies can be found in Table 1. NMR studies of compound 2a revealed, as expected, a single set of proton resonances for the equivalent Ap ligands and a very broad singlet at  = 5.00 ppm for the four aluminium hydrides. This indicates fluxional, dynamic behaviour in solution. 5. Synthesis and Structure of Aminopyridinato and Guanidinato Ligand Stabilized Al-H Complexes 48 Scheme 3. Synthesis of the dimeric compound 2a. Two doublets at  = 5.07 and 5.19 ppm belonging to the aromatic protons in the pyridine ring (3 and 5 position) are quite up field for aromatic protons. This can be explained by the increased electron donating ability of the PyAp ligand due to the pyrolidinyl moiety. Another confirmation for the increased electron donating ability of PyAp is found in the crystal structure of 2a. The sum of all angles around N3 is 359.8°, indicating a nearly perfect planar sp2 hybridized N atom. The torsion angle by which N3 is shifted out of the plane is only 2.7°. Moreover, the distance between N3 and C5 is 1.3475(10) Å, lying between a N-Csp2 double bond (1.28 Å in average) and a N-Csp2 single bond (1.48 Å in average).[15] The amido N-Al distance of 1.8742(6) Å and the pyridine N atom to Al distance of 2.0358(7) Å indicate that the anionic charge of the Ap ligand is localized at the amido N atom. The aluminum centers are five-coordinated by two N atoms (from the Ap ligand), a terminal and two bridging hydrides. Figure 1. Molecular structure of compound 2a with 50% thermal ellipsoids. Carbon atoms are displayed as spheres, Hydrogen atoms, except of the hydrides, are omitted for clarity. Selected bond lengths [Å] and bond angles [°]:Al1– H1 1.479(9), Al1–H2 1.624(9), C5–N3 1.3475(10), C5–N2 1.3498(10), N1–Al1 1.8742(6), N2–Al1 2.0358(7), N1–C1– N2 107.53(6). 5. Synthesis and Structure of Aminopyridinato and Guanidinato Ligand Stabilized Al-H Complexes 55 6.8 Hz, CH(CH3)2), 2.63 (m, 4H, CH2), 3.02 (m, 4H, CH2), 3.59 (sept., 2H, CH(CH3)2), 3.65 (sept., 2H, CH(CH3)2), 5.21 (d, 2H, JHH = 7.7 Hz, m-C5N1H3), 5.27 (d, 2H, JHH = 7.7 Hz, mC5N1H3), 6.94 (t, JHH = 8.0 Hz, 2H, pC5N1H3), 7.10-7.19 (m, 6H, arom. CH). 13C NMR (100 MHz, C6D6, 298 K): δ = 22.09, 24.65, 25.04, 25.17, 27.22, 28.41, 28.94, 45.13, 46.94, 91.85, 93.91, 123.27, 124.30, 125.66, 127.70, 140.27, 140.87, 146.67, 147.53, 154.86, 166.71. Synthesis of [{(PipGu)Al(H)(µ-H)}2] (2b): To a freshly prepared solution of AlH3 in ether (15 mL, 2 mmol) was added a solution of PipGuH (895.4 mg, 2 mmol) in ether (10 mL). The reaction mixture was stirred at 300 rpm over night. Concentration of the mixture to approximately 5 mL, followed by heating until the saturated solution started to boil, gave colorless crystals suitable for X-ray structure analysis upon storage at ambient temperature. Yield: 0.828 g (87 %). C60H92Al2N6 (951.38): Calcd. C 75.75, H 9.75, N 8.83; found. C 75.30, H 9.61, N 8.78; 1H NMR (300 MHz, C6D6, 298 K): δ = 0.83 (m, br., 12H CH2), 1.29 (d, 24H, CH(CH3)2), 1.34 (d, 24H, CH(CH3)2), 2.73 (m, 8H, N(CH2)2), 3.68 (sept, 8H, CH(CH3)2), 4.83 (s, br, 4H Al-H), 7.07 – 7.11 (m, 12H, m-C5H3, p-C5H3). 13C NMR (100 MHz, C6D6, 298 K): δ = 23.23, 23.49, 24.78, 26.12, 28.47, 47.67, 105.19, 123.88, 139.12, 144.53, 163.67. Synthesis of [(MPipGu)(H)2Al(µ-H)Li(thf)3](4c): LiAlH4 (151.8 mg, 4 mmol) was dissolved in ether (10 mL) at 0° C and MPipGuH (1.902 g, 4 mmol) in ether (10 mL) was added slowly. The reaction mixture was allowed to warm to ambient temperature and was stirred at 300 rpm overnight. The solvent was removed under reduced pressure and the crude residue was extracted with THF. Concentration of the clear solution gave colorless crystals after storage at 10° C overnight. Yield: 2.359 g (81 %). C44H75AlLiN3O3 (728.01): Calcd. C 72.59, H 10.38, N 5.77; found. C 72.92, H 10.82, N 6.55; 1H NMR (300 MHz, C6D6, 298 K): δ = 0.77 (d, 6H, JHH = 7.0 Hz, N{CH(CH3)}2),0.86-1.23 (m, 6H, CH2), 1.32 (m, br, 12H, THF), 1.37 (d, 12H, JHH = 6.8 Hz, CH(CH3)2), 1.41 (d, 12H, JHH = 6.8 Hz, CH(CH3)2), 3.43 (m, 12H, THF),3.6-4.3 (m, br, 3H. Al-H), 3.81 (sept., 4H, CH(CH3)2), CH(CH3)2), 3.93 (sept., 2H, NCH(CH3)), 7.02-7.19 (m, 6H, arom. CH). 13C NMR (100 MHz, C6D6, 298 K): δ = 13.65, 21.37, 23.31, 25.56, 26.94, 28.46, 29.78, 48.43, 123.72, 145.08, 163.40. 5. Synthesis and Structure of Aminopyridinato and Guanidinato Ligand Stabilized Al-H Complexes 56 5.5 References [1] J. Málek and M. Cerný, Synthesis 1972, 217–234. [2] B. M. Bulychev, Polyhedron 1990, 9, 387–408. [3] H. Haubenstock, E. L. Eliel, J. Am. Chem. Soc. 1962, 84, 2363–2368. [4] J. A. Jegier, W. L. Gladfelter, Coord. Chem. Rev. 2000, 206–207, 631–650. [5] a) L. Schlapbach, A. Züttel, Nature 2001, 414, 353-358; b) E. David, J. Mater. Proc. Technol. 2005, 162, 169–177; c) U. Eberle, M. Felderhoff, F. Schüth, Angew. Chem. 2009, 121, 6732–6757; Angew. Chem. Int. Ed. 2009, 48, 6608–6630, d) S. F. Matar, Prog. Solid State Chem. 2010, 38, 1-37; e) S. F. Matar, Prog. Solid State Chem. 2012, 40, 31–40. [6] For recent examples see: a) A. Züttel, Mater. Today 2003, 24–33; b) M. Latroche, J. Phys. Chem. Solids 2004, 65, 517–522; c) W. Grochala, P. P. Edwards, Chem. Rev. 2004, 104, 1283–1315; d) L. Hou, Renew. Sustain. Energy Rev. 2005, 9, 395–408; e) L. Zhou, Y. Zhou, Y. Sun, Int. J. Hydrogen Energy, 2006, 31, 259–264; f) U. Eberle, G. Arnold, R. V. Helmholt, J. Power Sour. 2006, 154, 456–460; g) B. Sakintuna, F. LamariDarkrim, M. Hirscher, Int. J. Hydrogen Energy 2007, 32, 1121–1140; h) B. Bogdanović, U. Eberle, M. Felderhoff, F. Schüth, Scripta Mat. 2007, 56, 813–816; i) S. Harder, J. Spielmann, J. Intemann, H. Bandmann, Angew. Chem. 2011, 123, 4242–4246; Angew. Chem. Int. Ed. 2011, 50, 4156–4160; h) P. Jochmann, J. P. Davin, T. P. Spaniol, L. Maron, J. Okuda, Angew. Chem. 2012, 124, 4528–4531; Angew. Chem. Int. Ed. 2012, 51, 4452–4455; j) J. Intemann, J. Spielmann, P. Sirsch, S. Harder, Chem. Eur. J. 2013, DOI: 10.1002/chem.201300684 [7] I. M. Riddlestone, S. Edmonds, P. A. Kaufman, J. Urbano, J. I. Bates, M. J. Kelly, A. L. Thompson, R. Taylor, S. Aldridge, J. Am. Chem. Soc. 2012, 134, 2551−2554. [8] S. J. Bonhady, D. Collis, G. Frenking, N. Holzmann, C. Jones, A. Stasch, Nat. Chem. 2010, 2, 865–869. [9] For review articles on aminopyridinato ligands, see: a) R. Kempe, H. Noss, T. Irrgang, J. Organomet. Chem. 2002, 647, 12–20; b) R. Kempe, Eur. J. Inorg. Chem. 2003, 791– 803. [10] For discussions on ligand binding modes, see: S. Deeken, G. Motz, R. Kempe, Z. Anorg. Allg. Chem. 2007, 633, 320–325. [11] For details on the synthesis of aminopyridines via Pd-catalyzed aryl amination route, see: a) S. Wagaw, S. L. Buchwald, J. Org. Chem. 1996, 61, 7240–7241; b) T. Schareina, G. Hillebrand, H. Fuhrmann, R. Kempe, Eur. J. Inorg. Chem. 2001, 2421–2426. 5. Synthesis and Structure of Aminopyridinato and Guanidinato Ligand Stabilized Al-H Complexes 57 [12] For review articles on guanidinato ligands and their general applicability, see: a) F. T. Edelmann, Adv. Organomet. Chem. 2008, 57, 183-352; b) C. Jones, Coord. Chem. Rev. 2010, 254, 1273–1289. [13] For examples of the general applicability of Ap ligands, see: a) G. Glatz, G. Motz, R. Kempe, Z. Anorg. Allg. Chem. 2008, 634, 2897–2902. G. Glatz, S. Demeshko, G. Motz, R. Kempe, Eur. J. Inorg. Chem. 2009, 1385–1392. [14] a) M. Hafeez, W. P. Kretschmer, R. Kempe, Eur. J. Inorg. Chem. 2011, 5512–5522; b) M. Hafeez, W. P. Kretschmer, R. Kempe, Z. Anorg. Allg. Chem. 2012, 638, 324–330. [15] F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen, J. Chem. Soc., Perkin Trans. 2 1987, S1–S19. [16] M. L. Cole, C. Jones, P. C. Junk, M. Kloth, A. Stasch, Chem. Eur. J. 2005, 11, 44824491. [17] H.-J. Himmel, Inorg. Chem. 2007, 46, 6585-6593. [18] F. M. Brower, N. E. Matzek, P. F. Reigler, H. W. Rinn, C. B. Roberts, D. L. Schmidt, J. A. Snover, K. Terada, J. Am. Chem. Soc. 1976, 98, 2450–2453. [19] a) W. P. Kretschmer, B. Hessen, A. Noor, N. M. Scott, R. Kempe, J. Organomet. Chem. 2007, 692, 4569–4579; b) C. Döring, R. Kempe, Eur. J. Inorg. Chem. 2009, 412–418. [20] R. Duchateau, A. Meetsma, J. H. Teuben, Chem. Commun. 1996, 223–224. [21] S. K. T. Pillai, W. P. Kretschmer, M. Trebbin, S. Förster, R. Kempe, Chem. Eur. J. 2012, 18, 13974–13978. [22] a) A. Heine, D. Stalke, Angew. Chem. 1992, 104, 941–942; Angew. Chem. Int. Ed. 1992, 31, 854–855; b) M. G. Gardiner, C. l. Raston, Coord. Chem. Rev. 1997, 166, 1–34; c) H. Nöth, A. Schlegel, J. Knizek, I. Krossing, W. Ponikwar, T. Seifert, Chem. Eur. J. 1998, 4, 2191–2203. [23] A. Noor, T. Bauer, T. K. Todorova, B. Weber, L. Gagliardi, R. Kempe, Chem. Eur. J. 2013, 19, 9825–9832.. [24] G. Jin, C. Jones, P. C. Junk, K.-A. Lippert, R. P. Rose, A. Stasch, New J. Chem. 2009, 33, 64–75. [25] A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano, C. Giacovazzo, A. Guagliardi, A. G. G. Moliterni, G. Polidori, R. Spagna, J. Appl. Cryst. 1999, 32, 115–119. [26] G. M. Sheldrick, Acta Cryst. 2008, A64, 112–122. [27] L. J. Farrugia, J. Appl. Cryst. 1999, 32, 837–838. 6. Synthesis and Structure of a Trinuclear Yttrium Polyhydride Cluster Stabilized by a Bulky Guanidinato Ligand 58 6 Synthesis and Structure of a Trinuclear Yttrium Polyhydride Cluster Stabilized by a Bulky Guanidinato Ligand Tobias Bauer[a], and Rhett Kempe*[a] [a] T. Bauer, Prof. Dr. R. Kempe, Lehrstuhl Anorganische Chemie II, Universität Bayreuth, Universitätsstrasse 30, NW I, 95440 Bayreuth (Germany), Fax: (+49) 921552157, E-Mail: [email protected] To be submitted. Keywords: Polyhydride Cluster Compounds • Rare Earths • X-Ray Diffraction • Hydride Ligands • N-Ligands 6.1 Introduction Polyhydride complexes of the rare earth (RE) metals (group 3 metals and lanthanoid metals [Ce-Lu]) have fascinated chemists due to their reactivity and structural motifs. Furthermore, RE (poly)hydrides are among the most reactive compounds known.[1] Recently, the interest in RE hydrides has shifted from monohydride L2LnH complexes[2] to dihydride LLnH2 complexes. The dihydride complexes tend to aggregate and vary in structure ranging from hexanuclear,[1e,3a-c] pentanuclear,[3c] tetranuclear,[3b-j,3o] trinuclear[3b,3k-o] to dinuclear complexes.[3r] The nuclearity mainly dependents from the steric bulk of the ancillary ligand used. An increase in the steric demand of the ancillary ligand seems to lead to a decrease in nuclearity. Structurally fully characterized (trustable determination of the positions of the hydrogen atoms) lanthanide polyhydride complexes still lack in number. Until now, mostly sterically demanding cyclopentadienyl derivatives,[3d-h,3j] scorpionato [tris(pyrazolyl)hydroborate] ligands,[3a,b] tetraazacycloamido[3l] and aminopyridinato ligands[3m,q] have been used to stabilize the “(LnH2)x” unit. Herein, we report on synthesis and structure of the first RE (LnH2)3 polyhydride stabilized by a guanidinato ligand. 6. Synthesis and Structure of a Trinuclear Yttrium Polyhydride Cluster Stabilized by a Bulky Guanidinato Ligand 59 6.2 Results and Discussion All complexes synthesized were characterized by NMR spectroscopy, elemental analysis and single crystal X-ray structure analysis (XRD). First, we synthesized the guanidinate yttrium dialkyl complex 1 (Scheme 1). The equimolar reaction of thf stabilized yttrium trialkyl (Y(CH2SiMe3)3thf2) with the guanidine (Z)-N,N'-bis(2,6-diisopropylphenyl)piperidine-1carboximidamide (PipGuH) in n-hexane afforded clean formation of the guanidinato ligand stabilized yttrium dialkyl complex [PipGuY(CH2SiMe3)2thf] 1 in 80 % yield (Scheme 1). NMR investigation of compound 1 showed, as one would expect for a mononuclear complex, a single set of proton resonances for the guanidinato ligand, one signal set for the two alkyl moieties and one signal set for the coordinated thf. The YCH2 resonance (D6-benzene, rt) for 1 is found at δ = -0.26 ppm with a coupling constant JYH = 3.0 Hz. This is in good comparison to related NMR studies (D6-benzene, rt) on guanidinato ligand stabilized yttrium dialkyl [(ArNC(NMe2)NAr)Y(CH2SiMe3)2thf][4] (δ = -0.31, JYH = 2.9 Hz), amidinato ligand stabilized yttrium dialkyl [PhC-(NAr)2]Y(CH2SiMe3)2thf][5] (δ = -0.11, JYH = 3 Hz) and aminopyridinato ligand stabilized yttrium dialkyl [(Ar)6-{(2,4,6-triisopropylphenyl) pyridine-2-yl)amido}Y(CH2SiMe3)2thf][6] (δ = -0.42 JYH = 3.0 Hz) (Ar = 2,6-diisopropylphenyl) complexes. Scheme 1. Synthesis of 1. Single crystals suitable for XRD analysis were grown from a saturated hexane/toluene (1:1 ratio) solution by slowly cooling to -40° C. The molecular structure of compound 1 is depicted in Figure 1. Compound 1 crystallizes in the monoclinic spacegroup P2(1)/n and features a N,N’-dihapto-guanidinato ligand, two alkyl moieties and one thf molecule coordinated to the Y atom. The O atom of the thf molecule occupies a position roughly in the plane defined by the central yttrium atom and the two nitrogen atoms of the guanidinato ligand. To minimize steric repulsion with the ligand 2,6-diisopropylphenyl groups, the two alkyl moieties occupy positions above and below this plane. The bond lengths from the central yttrium atom to the ligand nitrogen atoms are 2.349(2) and 2.335(2) Å. In comparison to the related complex reported by Hessen and co-workers [(ArNC(NMe2)NAr)Y(CH2SiMe3)2thf][4] 6. Synthesis and Structure of a Trinuclear Yttrium Polyhydride Cluster Stabilized by a Bulky Guanidinato Ligand 60 (Ar = 2,6-diisopropylphenyl) the found C–Y bond distances of 2.378(3) and 2.391(3) Å in compound 1 are slightly longer than the ones in the Hessen compound (2.374(4) and 2.384(4) Å). Figure 1. Molecular structure of compound 1. Ellipsoids are drawn on the 50 % probability level. Carbon atoms are shown as spheres and hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]:C2–Y1 2.378(3), C3–Y1 2.391(3), C1–N1 1.346(4), C1–N2 1.350(4), C1–N3 1.366(4), N1–Y1 2.349(2), N2–Y1 2.335(2), O1–Y1 2.382(2), N1 C1 N2 111.9(3). Hydrogenolysis of compound 1 at 0°C (2 bar H2 pressure) afforded clean formation of the trinuclear polyhydride complex [{(PipGu)YH2}3thf2] (2, Scheme 2) as indicated by the NMR studies and XRD analysis. Scheme 2. Synthesis of 2. 6. Synthesis and Structure of a Trinuclear Yttrium Polyhydride Cluster Stabilized by a Bulky Guanidinato Ligand 61 1H NMR investigations of compound 2, in contrast to the studies of compound 1, gave rather broad resonance peaks of the three guanidinato ligands, the two coordinated thf molecules and a sharp quartet for the six hydrides (JYH = 18.5 Hz). In solution, we face a complex dynamic behavior. The three guanidinato ligands are not equivalent which can be explained by the fact, that two of the three yttrium centers have a coordinated thf molecule and therefore the rotation in two of the three ligands is hindered at room temperature. All six hydrides give rise to a single quartet at δ = 6.26 ppm due to coupling with the three yttrium atoms. This shows that the hydrides, in solution, are all equivalent and very fluxional in the time scale of NMR spectroscopy and are not distinguishable like in the solid-state structure. Reports on other trinuclear yttrium hexahydride complexes are in agreement with our finding.[3b,p,l] To get a deeper insight into the dynamic behavior of compound 2 we carried out variable temperature 1H NMR studies (Figure 2). Upon heating to 100°C the three guanidinato ligands give rise to a single set of proton resonances as can be seen in Figure 2 B). This confirmed that the broadening of the ligand signals are due to hindrance in rotation at room temperature and/or an equilibrium in coordination and decoordination of thf. The quartet at δ = 6.26 ppm shows peak broadening down to -35°C. At this temperature, the hydrides start to become inequivalent and their fast skipping is hindered as seen in Figure 2 C). Unfortunately, the limiting spectra where the µ3and µ2-hydrides become distinguishable could not be obtained. 6. Synthesis and Structure of a Trinuclear Yttrium Polyhydride Cluster Stabilized by a Bulky Guanidinato Ligand 62 Figure 2. A) 1H NMR spectrum of 2 (D8-toluene, rt, 7.6 to 0.4 ppm). B) 1H high temperature NMR spectra of 2 (D8toluene, 100°C, 7.6 to 0.4 ppm). C) 1H variable low temperature NMR spectra of 2 (D8-toluene, -65 to 10°C, 6.50 to 5.95 ppm). A molecular structure of compound 2 is depicted in Figure 3 and a more detailed view of the core structure with peripheral ligands reduced is shown in Figure 4. Selected interatomic distances and bond lengths are summarized in Table 1. Details on the XRD analysis of compound 1 and 2 are given in Table 2. 6. Synthesis and Structure of a Trinuclear Yttrium Polyhydride Cluster Stabilized by a Bulky Guanidinato Ligand 63 Figure 3. Molecular structure of compound 2. Ellipsoids are drawn on the 50 % probability level. Carbon atoms are shown as spheres and non hydride hydrogen atoms are omitted for clarity. Figure 4. Detailed view of the cluster core with peripheral ligands drawn as NCN moieties and thf molecules as O atoms. 6. Synthesis and Structure of a Trinuclear Yttrium Polyhydride Cluster Stabilized by a Bulky Guanidinato Ligand 64 Table 1. Selected interatomic distances and bond lengths [Å] of compound 2. N–Y (average value) 2.387 O–Y (average value) 2.362 Y(1,2,3)–H5 2.22(4), 2.16(4), 2.12(4) Y(1,2,3)–H6 2.11(4), 2.11(4), 2.24(4) Y(1,2)–H1 2.20(4), 2.17(4) Y(2,3)–H2 2.09(4), 2.17(4) Y(1,3)–H3 2.14(4), 1.93(3) Y(1,3)–H4 2.01(4), 2.04(4) Y1–Y2 3.5457(6) Y1–Y3 3.1697(5) Y2–Y3 3.4293(6) Compound 2 crystalizes in the monoclinic spacegroup P2(1)/c with a n-hexane molecule per asymmetric unit. Each Y atom in complex 2 bears a guanidinato ligand in the same N,N’- dihapto mode like in the precursor complex 1 but with longer Y-N distances due to higher coordination number of the yttrium atoms. The three yttrium atoms define a triangle. Four of the six hydride ligands bridge one of the three Y---Y edges in a µ2 mode, meaning one of the edges is bridged by two µ2-hydrides while the other two edges are bridged by only one µ2-hydride. The last two hydrides are capping the sides of the Y3 plane in a µ3 fashion. The Y---Y edge bridged by two hydrides is significantly shorter than the two other edges (3.1697(5) Å in comparison to 3.4293(6) and 3.5457(6) Å, respectively). This Y---Y distance is the second shortest ever reported, the shortest being 3.1648(7) Å reported by Hou and co-workers.[3p] Other (YH2)3 cluster compounds have Y-H and Y---Y distances in the range from 2.06(4) to 2.37(4) Å and 3.1648(7) to 3.6841(2) Å.[3b,p,l] 7. Ternary Rare-Earth Transition-Metal Polyhydride Cluster Compounds 71 on report of Slater.[15] 1H NMR studies at room temperature showed singlets in the hydride region with a 2:1 ratio for the six hydride ligands in compound 1 at  = –13.69 (4H, JWH = 83.3 Hz) and –13.27 ppm (2H, JWH = 104.1 Hz). These signals show an upfield shift in comparison to the signal at –12.26 ppm for the educt [Cp2WH2]. The observed JWH coupling constant of 83.3 Hz is greater than that for [Cp2WH2] of 73.0 Hz. The same effect was reported for the comparable compounds [Cp*2Y(µ-η1:η5-C5H4)(µ-H)2WCp] (JWH = 78.0 Hz, Cp* = pentamethylcyclopentadienyl)[8a] and [(Et3P)2(H)Ir(µ-η1:η5-C5H4)H2WCp] with JWH coupling constants of 92.4 Hz and 95.2 Hz.[16] The second JWH coupling constant of 104.1 Hz is even greater and might indicate that the hydride ligands on the bridging tungstenocene moiety are bound in a µ3-fashion by W3, Lu1 and Lu2. The C–H activated Cp ligands showed signals that one would expect for a mirror symmetric C5H4 moiety with two singlets per activated Cp ligand giving three sets of signals in a 2:1:1 ratio. Figure 1. ORTEP drawing of compound 1 with 50% thermal ellipsoids. Hydrogen atoms have been omitted for clarity. Selected bond lengths [Å]: Lu1–O1 2.107(11), Lu2–O2 2.081(10), Lu1–W2 3.1211(10), Lu2–W1 3.1155(9), Lu1–C1 2.485(14), Lu2–C1 2.558(14), Lu1–C28 2.554(14), Lu2–C28 2.493(15), Lu1–C63 2.366(17), Lu2–C23 2.315(17), Cpcentroid–W1 1.965, C5H4centroid–W1 1.927, Cpcentroid–W2 1.954, C5H4centroid–W2 1.935, C5H4centroid–W3 1.932 (average value). The selective cluster formation via C–H bond activation indicative by the good isolated yield of 1 inspired us to investigate the formation of ternary polyhydride clusters. We chose [Lu(OAr)(Cp2Re)R(thf)] as a promising and rather reactive educt (Scheme 2).[13c] Reacting equimolar amounts of [Lu(OAr)(Cp2Re)R(thf)] with [Cp2WH2] in benzene at room temperature lead to the formation of the trimetallic polyhydride cluster compound 2a in 48% yield (Scheme 2). To the best of our knowledge, compound 2a is the first example of a RE-metal polyhydride cluster featuring three different metals as was revealed by XRD and NMR studies (Figure 2). 7. Ternary Rare-Earth Transition-Metal Polyhydride Cluster Compounds 72 Compound 2a is well soluble in aromatic solvents and shows no solubility in aliphatic hydrocarbons. Scheme 2: Synthesis of 2a and 2b. One of the lutetium centers in 2a is five coordinate by one phenolato ligand, three C–H activated Cp rings, and a rhenium atom. The other lutetium center has the coordination number ten containing a phenolato ligand, three C–H activated Cp ligands, and two tungsten atoms each bridged by two µ2-hydrides. Figure 2.ORTEP drawing of 2a with 50% thermal ellipsoids. Hydrogen atoms have been omitted for clarity. Selected bond lengths [Å]: Lu1–O1 2.109(5), Lu2–O2 2.082(5), Lu1–W1 3.1760(5), Lu1–W2 3.2033(5), Lu2–Re1 2.7986(5), Lu1–C10 2.434(8), Lu1–C15 2.608(8), Lu1–C30 2.543(9), Lu2–C10 2.464(7), Lu2–C15 2.440(8), Lu2–C30 2.430(8), Cpcentroid–W1 1.889, C5H4centroid–W1 1.867, Cpcentroid–W2 1.984, C5H4centroid–W2 1.918, Cpcentroid–Re1 1.869, C5H4centroid–Re1 1.857. The Lu–Re bond distance is 2.7986(5) Å, which is way shorter than the sum of the covalent radii of rhenium and lutetium (3.38 Å) based on Alvarez and co-workers data,[14] shorter 7. Ternary Rare-Earth Transition-Metal Polyhydride Cluster Compounds 73 than the sum of the atomic radii in crystals (3.1 Å) based on Slater,[15] and even shorter than the bond lengths in [Lu(Cp2Re)3] (2.8773(8), 2.8899(7) and 2.8913(8) Å)[13c] and in [Lu(OAr)(ReCp2)R(thf)] (2.8498(6) Å).[13e] The Lu–W bond lengths are 3.1760(5) and 3.2033(5) Å. As such, they are longer than in 1, but still shorter than the sum of the covalent radii[14] and the sum of the atomic radii.[15] The hydride signals in the 1H NMR spectrum of compound 2a are shifted upfield again to  = –13.61 ppm and show a JWH coupling constant of 82.5 Hz which is in good agreement with the finding for compound 1 (JWH = 83.3 Hz). There are no further hydride signals present indicating a non hydride-bridged Lu–Re bond. Each of the phenolato ligands shows one set of signals at room temperature. Finally, we became interested in synthesizing the molybdenum analogue of 2a. The equimolar reaction of [Cp2MoH2] with [Lu(OAr)(Cp2Re)R(thf)] in benzene gave the isostructural heteromultimetallic polyhydride cluster 2b in 52% yield (Figure 3 and Scheme 2). The Lu–Mo bond distances of 3.2025(12) and 3.1613(9) Å are shorter than the sum of the covalent radii of 3.41 Å based on report of Alvarez and co-workers,[14] and in good agreement with the sum of the atomic radii of 3.2 Å based on report of Slater.[15] Again, an upfield shift of the singlet at  = – 10.25 ppm accounting for four hydride protons can be seen in the 1H NMR spectrum, recorded at room temperature, in comparison to the singlet at  = –8.80 ppm for the hydrides in [Cp2MoH2]. Figure 3.ORTEP drawing of 2b with 50% thermal ellipsoids. Hydrogen atoms have been omitted for clarity. Selected bond lengths [Å]: Lu1–O1 2.111(7), Lu2–O2 2.074(7), Lu1–Mo1 3.2025(12), Lu1–Mo2 3.1613(9), Lu2–Re1 2.8058(6), Lu1–C5 2.412(10), Lu1–C11 2.608(13), Lu1–C28 2.591(12), Lu2–C5 2.484(10), Lu2–C11 2.403(12), Lu2– C28 2.426(11), Cpcentroid–Mo1 1.982, C5H4centroid–Mo1 1.917, Cpcentroid–Mo2 1.965, C5H4centroid–Mo2 1.888, Cpcentroid– Re1 1.879, C5H4centroid–Re1 1.852. Quantum chemical calculations were performed to locate the missing hydride-atomic positions in compound 2a and to analyze their role in W–Lu bonding compared to Re–Lu 7. Ternary Rare-Earth Transition-Metal Polyhydride Cluster Compounds 74 bonding in the cluster compounds 2. For that purpose, full structure optimization has been invoked on a slightly simplified model structure 2a’ (substituting H for the tert-butyl groups) for compound 2a. For the scalar-relativistic (ZORA approach)[17] calculations with the ADF[18] program system the DFT/BP86[19] method and internal Slater-type triple-zeta basis sets with two sets of polarization functions (“TZ2P”, with frozen small core) have been employed. After 100 optimization cycles the –OPhen groups were fixed, and the remaining atom positions were further optimized. At gradients of less than 0.3 mHartreeÅ–1, the procedure was finished. The final structure, especially the inner metal part, is very similar to the one from experimental structure determination. Figure 4. Optimized structure of model 2a’ with ELI-D/QTAIM basin intersections. Hydridic H atoms are displayed as blue spheres, intersection of corresponding ELI-D basin yields a region (deep blue) contained in the QTAIM H atom, a region (red) contained in the W QTAIM atom, and a region (light blue) contained in Lu1 atom; intersection of the ELI-D Lu-Re bond basins yields a region (red) belonging to the Re atom, and a region (light blue) belonging to the Lu2 atom; all basins are cropped at density values below 0.0001 e/Bohr–3. It displays distances d(Lu1–W) of 3.13 and 3.14 Å (vs. 3.18 and 3.20 Å from experimental structure determination), d(Lu2–Re1) = 2.81 Å (vs. 2.80 Å from experiment), and d(W–H) between 1.72 and 1.73 Å with angles H-W-H of 87.3° and 87.6°. Under the same computational conditions a separate structure optimization of the [Cp2WH2] molecule yields d(W–H) = 1.71 Å and angle H-W-H = 78.6°, which shows that the distances W–H only marginally increase upon coordination to Lu, whereas the angle H-W-H notably widens by 9°. Concerning the electronic structure, a HOMO-LUMO gap of 1.5 eV is found, in which the HOMO, HOMO-1 and HOMO-2 can be classified as nominal Re(5d) orbitals. As has been previously done,[9c, 13c,d,e] position-space bonding analysis (program DGrid[20]) by using the electron density (QTAIM method)[21] and the electron localizability indicator (ELI-D)[22] has been employed. The negative values Qeff(H) = –0.35 ±0.01 for the QTAIM effective charges of the 7. Ternary Rare-Earth Transition-Metal Polyhydride Cluster Compounds 75 four W-H-Lu1 bridging H atoms was consistent with their hydridic character. These values were found to be very similar to the ones Qeff (H) = –0.30 in the isolated [Cp2WH2] molecule. Applying the method of ELI-D/QTAIM basin intersection[23] yields that W–H bonding is polar-covalent with 70% of the ELI-D basin integrated charge density of each hydridic H atom belongs to the corresponding QTAIM H atom and 26% to the QTAIM W atom (Figure 4). Only a tiny amount of 3% is contained in the QTAIM Lu1 atom. Taking into account the positive effective charge Qeff (Lu1) = +1.9, this finding is to be interpreted as a very ionic type of bonding interaction H–Lu1, which is consistent with the virtually unchanged distances d(W–H) compared to isolated [Cp2WH2]. Concerning direct W–Lu1 bonding a corresponding ELI-D maximum was not displayed; however owing to the bridging H atoms, this would not be expected to occur. As a signature of W–Lu1 bonding, inside the quadrilateral W-H’-Lu1-H’’, a region with negative values of the Laplacian of ELI-D,[22c] which extends perpendicular to the W–Lu1 interconnection line, can be found. Such a region does not occur in the isolated molecule [Cp2WH2], but is found also for the Lu2–Re bonding situation, in which ELI-D attractors additionally signify the covalent bonding interaction. In complete analogy to previous cases of polar-covalent rare earthtransition metal bonding,[9c, 13,c,d,e] unsupported Lu2–Re bonding is indicated by corresponding ELI-D maxima with 1.30 electrons in two Lu2–Re bonding ELI-D basins (merged into one superbasin). The ELI-D/QTAIM intersection procedure showed that 79% of the basin population is contained in the Re, and 16% in the Lu QTAIM atom, which is similar to previous cases. 7.3 Conclusions In conclusion, transition metal dihydride complexes of the formula [Cp2MH2] (M = Mo, W) react with rare earth metal bisand monoalkyl complexes and undergo multiple C–H bond activation steps, leading to binary and ternary RE-metal polyhydride cluster compounds. The ternary clusters display polar ReLu bonds and W–H∙∙∙Lu interactions, where the hydride atoms are polar-covalently coordinated to W, and the interaction with Lu is very ionic. Additionally, a covalent direct interaction W–Lu is indicated by ELI-D analysis. Concerning the broader strategy of building higher aggregated ternary RE–TM polyhydride clusters [Cp2MH2] represents a promising transition metal building block. In the future work, we are interested in studying the reactivity of the cluster compounds introduced herein. 7.4 Acknowledgments Financial support by the Deutsche Forschungsgemeinschaft (DFG, KE 756/ 21-1, WA 956/3-1) is gratefully acknowledged. 7. Ternary Rare-Earth Transition-Metal Polyhydride Cluster Compounds 76 Supporting Information available: Supporting information for this article contains detailed information on the synthesis and characterization of the compounds described herein, as well as crystallographic details of the structures determined by XRD. It is available on the WWW under http://dx.doi.org/10.1002/chem.201301290. 7.5 References [1] For some examples, see: a) R. M. Bullock, M. H. Voges, J. Am. Chem. Soc. 2000, 122, 12594–12595; b) R. Noyori, Angew. Chem. 2002,114, 2108–2123; Angew. Chem. Int. Ed. 2002, 41, 2008–2022; c) C. Deutsch, N. Krause, B. H. Lipshutz, Chem. Rev. 2008, 108, 2916–2927; d). H. Nakazawa, M. Itazaki, Top. Organomet. Chem. 2011, 33, 27–81. [2] a) L. Schlapbach, A. Züttel, Nature 2001, 414, 353–358; b) E. David, J. Mater. Proc. Technol. 2005, 162, 169–177; c) B. Sakintuna, F. Lamari-Darkrim, M. Hirscher, Int. J. Hydrogen Energy 2007, 32, 1121–1140; d) S. F. Matar, Prog. Solid State Chem. 2010, 38, 1–37; e) S. F. Matar, Prog. Solid State Chem. 2012, 40, 31–40. [3] NiMH materials are used in batteries of hybrid cars, for example in the Toyota Prius [4] T. Shima, Y. Luo, T. Stewart, R. Bau, G. J. McIntyre, S. A. Mason, Z. Hou, Nat. Chem. 2011, 3, 814–820. [5] W. J. Evans, J. H. Meadows, T. P. Hanusa, J. Am. Chem. Soc. 1984, 106, 4454–4460. [6] The localization of H atoms is a fundamental problem in polyhydride complexes of heavy atoms: G. G. Hlatky, R. H. Crabtree, Coord. Chem. Rev. 1985, 65, 1–48. [7] a) M. L. H. Green, A. K. Hughes, D. M. Michaelidou, P. Mountford, J. Chem. Soc., Chem. Commun. 1993, 591–593; b) D. M. Michaelidou, M. L. H. Green, A. K. Hughes, P. Mountford, A. N. Chernega, Polyhedron 1995, 14, 2663–2675. [8] a) N. R. Radu, P. K. Gantzel, T. D. Tilley, J. Chem. Soc., Chem. Commun. 1994, 11751176; b) Y. Takenaka, Z. Hou, Organometallics 2009, 28, 5196–5203; c) T. Shima, Z. Hou, Chem. Eur. J., 2013, 19, 3458–3466 [9] a) D. Alvarez, Jr., K. G. Caulton, W. J. Evans, J. W. Ziller, J. Am. Chem. Soc. 1990, 112, 5674–5676; b) D. Alvarez, Jr., K. G. Caulton, W. J. Evans, J. W. Ziller, Inorg. Chem. 1992, 31, 5500–5508; c) M. V. Butovskii, O. L. Tok, F. R. Wagner, R. Kempe, Angew. Chem. 2008, 120, 6569–6572; Angew. Chem. Int. Ed. 2008, 47, 6469–6472; d) T. Shima, Z. Hou, Chem. Lett. 2008, 37, 298–299; e) T. Shima, Z. Hou, Organometallics 2009, 28, 2244–2252. [10] A. P. Sobaczynski, T. Bauer, R. Kempe, Organometallics, 2013, 32, 1363–1369. [11] P. Cui, T. P. Spaniol, J. Okuda, Organometallics, 2013, 32, 1176–1182. 7. Ternary Rare-Earth Transition-Metal Polyhydride Cluster Compounds 77 [12] For recent reviews see: a) D. Patel, S. T. Liddle, Rev. Inorg. Chem. 2012, 32, 1–22; b) B. Oelkers, M. V. Butovskii, R. Kempe, Chem. Eur. J. 2012, 18, 13566–13579. [13] a) I. P. Beletskaya, A. Z. Voskoboynikov, E. B. Chuklanova, N. I. Kirillova, A. K. Shestakova, I. N. Parshina, A. I. Gusev, G. K.-I. Magomedov, J. Am. Chem. Soc. 1993, 115, 3156–3166; b) P. L. Arnold, J. McMaster, S. T. Liddle, Chem. Commun. 2009, 818– 820; c) M. V. Butovskii, C. Döring, V. Bezugly, F. R. Wagner, Y. Grin, R. Kempe, Nat. Chem. 2010, 2, 741–744; d) C. Döring, A.-M. Dietel, M. V. Butovskii, V. Bezugly, F. R. Wagner, R. Kempe, Chem. Eur. J. 2010, 16, 10679-10683; e) M. V. Butovskii, O. L. Tok, V. Bezugly, F. R. Wagner, R. Kempe, Angew. Chem. 2011, 123, 7873–7840; Angew. Chem. Int. Ed. 2011, 50, 7695-7698; f) M. P. Blake, N. Kaltsoyannis, P. Mountford, J. Am. Chem. Soc. 2011, 133, 15358–15361. [14] B. Cordero, V. Gómez, A. E. Platero-Prats, M. Revés, J. Echeverría, E. Cremades, F. Barragán, S. Alvarez, Dalton Trans. 2008, 2832–2838. [15] J. C. Slater, J. Chem. Phys. 1964, 41, 3199–3204. [16] P. S. Pregosin, A. Togni, L. M. Venanzi, Angew. Chem. 1981, 93, 684; Angew. Chem. Int. Ed. 1981, 20, 668–669. [17] E. van Lenthe, A. E. Ehlers and E. J. Baerends, J. Chem. Phys. 1999, 110, 8943–8953. [18] a) G. te Velde, F. M. Bickelhaupt, S. J. A. van Gisbergen, C. Fonseca Guerra, E. J. Baerends, J. G. Snijders and T. Ziegler, J. Comput. Chem. 2001, 22, 931–967; b) C. Fonseca Guerra, J.G. Snijders, G. te Velde, E. J. Baerends, Theor. Chem. Acc. 1998, 99, 391–403; c) ADF2012.01, SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Netherlands, http://www.scm.com [19] a) A. D. Becke, Phys. Rev. A 1988, 38, 3098-3100; b) J. P. Perdew, Phys. Rev. B 1986, 33, 8820-8824. [20] M. Kohout, program DGrid, version 4.6, Radebeul, Germany, 2012. [21] R. F. W. Bader, Atoms in Molecules: A Quantum Theory, Oxford University Press, Oxford, 1994. [22] a) M. Kohout, Faraday Discuss. 2007, 135, 43–54; b) F. R. Wagner, V. Bezugly, M. Kohout, Yu. Grin, Chem. Eur. J. 2007, 13, 5724–5741; c) F. R. Wagner, M. Kohout, Yu. Grin, J. Phys. Chem. A 2008, 112, 9814–9828. [23] a) S. Raub, G. Jansen, Theor. Chem. Acc. 2001, 106, 223–232; b) G. Jansen, M. Schubart, B. Findeis, L. H. Gade, I. J. Scowen, M. McPartlin, J. Am. Chem. Soc. 1998, 120, 7239–7251. 7. Ternary Rare-Earth Transition-Metal Polyhydride Cluster Compounds 78 7.6 Supporting Information 7.7 General All manipulations were performed with rigorous exclusion of oxygen and moisture in Schlenktype glassware on a dual manifold Schlenk line or in a N2 filled glove box (mBraun 120-G) with a high-capacity recirculator (<0.1ppm O2). Solvents were dried by distillation from sodium wire/benzophenone. Commercial [Cp2WH2] (ABCR) was used as received. [Lu(OAr)R2(thf)2],[1] [Lu(OAr)(ReCp2)R(thf)],[1] [Cp2MoH2][2] and [Cp2ReH][3] were prepared according to published procedures. Deuterated solvents were obtained from Cambridge Isotope Laboratories and were degassed, dried and distilled prior to use. NMR spectra were recorded on Varian Unity 300 MHz and Varian Unity 400 MHz instruments at ambient temperature. The chemical shifts are reported in ppm relative to the internal TMS or residual solvent signals. Elemental analyses (CHN) were determined using a Vario EL III instrument. X-ray crystal structure analyses were performed by using a STOE-IPDS II equipped with an Oxford Cryostream low-temperature unit. Structure solution and refinement was accomplished using SIR97,[4] SHELXL97[5] and WinGX.[6] Crystallographic details are summarized in Table 1. CCDC-932376 (for 1), -932377 (for 2a), and -932378 (for 2b) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. 7. Ternary Rare-Earth Transition-Metal Polyhydride Cluster Compounds 79 7.8 Details of the X-ray crystal structure analyses Table 1: Details of the X-ray crystal structure analyses. Compound 1 2a 2b Crystal system triclinic monoclinic monoclinic space group P-1 P21/c P21/c a [Å] 14.5190(7) 19.4320(5) 19.2850(5) b [Å] 14.5450(7) 18.5290(4) 18.1160(5) c [Å] 16.5350(8) 19.4310(5) 19.4870(5) α [˚] 86.651(4) 90.00 90.00  [˚] 68.746(4) 119.476(2) 117.975(2) γ [˚] 76.211(4) 90.00 90.00 V [Å3] 3159.0(3) 6090.7(3) 6012.6(3) Z 2 4 4 ρ (calcd.) [g cm-3] 1.950 2.026 1.859 µ [mm-1] 8.580 8.999 5.709 T [K] 133 133 133 2θ range [˚] 2.64-50.39 2.41-50.06 2.39-53.91 Reflections unique 10601 10239 12771 Refl. obsv. [I > 2σ(I)] 5588 7554 7715 Parameters 709 706 640 R1, wR2 [I > 2σ(I)] 0.0506, 0.1134 0.0336, 0.0703 0.0555, 0.1348 R1, wR2 (all data) 0.0935, 0.1217 0.0526, 0.0741 0.0929, 0.1474 7. Ternary Rare-Earth Transition-Metal Polyhydride Cluster Compounds 80 7.9 Synthesis and characterization of the cluster compounds Synthesis of [C58H74Lu2O2W3] (1): [Cp2WH2](71 mg, 225 µmol) was dissolved in benzene (1.5 mL) and added to a solution of [Lu(OAr)R2(thf)2] (105 mg, 150 µmol) in benzene (1.5 mL). The reaction mixture was kept at room temperature for 24 hours without stirring to form a yellow crystalline precipitate. Yield: 0.096 g, 75%. Yellow prism like crystals suitable for X-Ray structure analysis where grown at the layer interface by freezing and layering both educt benzene solutions and letting them thaw slowly. C58H74Lu2O2W2 (1704.66): Calcd. C 40.87, H 4.38; found C 41.25, H 4.35; 1H NMR (400 MHz, [D6]benzene): δ = -13.69 (s, 4H, JWH = 83.3 Hz, W-H), -13.27 (s, 2H, JWH = 104.1 Hz, W-H), 1.66 (s, 36H, C(CH3)3), 4.13 (s, 4H, C5H4), 4.24 (s, 10H, C5H5), 4.36 (s, 4H, C5H4), 4.53 (s, 2H, C5H4), 4.64 (s, 2H, C5H4), 5.16 (s, 2H, C5H4), 5.54, (s, 2H, C5H4), 6.85 (t, 2H, JHH = 7.7 Hz, p-C14H21O), 7.38 (d, 4H, JHH = 7.7 Hz, m-C14H21O) ppm.Due to the poor solubility of compound 1 meaningful 13C NMR experiments were not possible. Synthesis of [C58H73Lu2O2ReW2] (2a): To [Cp2WH2] (31.6 mg, 100 µmol) in benzene (1 mL) was added [Lu(OAr)(Cp2Re)R(thf)] (85.6 mg, 100 µmol) in benzene (2 mL). The reaction mixture turned from light yellow to orange within one hour. Concentration in vacuum yielded yellow block-like crystals at 10° C. Yield: 0.408 g, 48%.C58H73Lu2O2ReW2 (1706.02): Calcd. C 40.83, H 4.31; found C 40.29, H 4.16; 1H NMR (400 MHz, [D6]benzene): δ = -13.61 (s, 4H, JWH = 82.5 Hz, W-H), 1.53 (s, 18H, C(CH3)3), 1.65 (s, 18H, C(CH3)3), 4.20 (s, 5H, (C5H5)Re), 4.24 (s, 10H, (C5H5)W), 4.40 (s, 2H, C5H4), 4.55 (s, 2H, C5H4), 4.83 (s, 2H, C5H4), 5.21 (s, 2H, C5H4), 6.83 (t, 1H, JHH = 8,0 Hz, pC14H21O), 6.85 (t, 1H, JHH = 8.2 Hz, p-C14H21O), 7.29 (d, 2H, JHH = 7.7 Hz, m-C14H21O), 7.38 (d, 2H, JHH = 7.7 Hz, m-C14H21O);13C NMR (100 MHz, [D6]benzene): δ = 32.28 (C(CH3)3), 35.48 (C(CH3)3), 64.55 ((C5H5)Re), 71.62 ((C5H5)W), 73.73 (C5H4), 78.10 (C5H4), 117.67 (p-C14H21O), 125.63 (m-C14H21O), 137.90 (o-C14H21O), 163.16 (i-C14H21O) ppm. Synthesis of [C58H73Lu2Mo2O2Re] (2b): To [Lu(OAr)(Cp2Re)R(thf)] (0.428 g, 500 µmol) in benzene (10 mL) was added [Cp2MoH2] (0.114 g, 500 µmol) in benzene (5 mL). The yellow solution turned dark orange to brown within one hour. Concentration in vacuum gave yellow crystals with a block shaped habit at 10° C. Yield: 0.198 g, 52%. C58H73Lu2Mo2O2Re (1530.22): Calcd. C 45.52, H 4.81; found C 45.02, H 4.32;1H NMR (400 MHz, [D6]benzene): δ = -10.25 (s, 4H, Mo-H), 1.52 (s, 18H, C(CH3)3), 1.65 (s, 18H, C(CH3)3), 4.20 (s, 5H, (C5H5)Re), 4.24(s, 2H,C5H4), 4.36 (s, 10H, (C5H5)Mo), 4.53 (s, 9. Acknowledgements / Danksagung 87 9.2 Danksagung Mein aufrichtiger Dank gilt meinem akademischen Lehrer Prof. Dr. Rhett Kempe für die Möglichkeit, dieses sehr interessante Thema in seinem Arbeitskreis zu bearbeiten, die unermüdliche Diskussionsbereitschaft und das stete Interesse am Fortgang der Arbeit, so wie die gewährte wissenschaftliche Freiheit bei der Bearbeitung des Themas. Weiterhin danke ich Ihm für die exzellenten Arbeitsbedingungen und die Einführung in die Röntgeneinkristallstrukturanalyse. Dr. Winfried P. Kretschmer danke ich für stete Diskussionsbereitschaft, Motivation, Unterstützung und für das Korrekturlesen meiner Publikationen. Dr. Christian Döring danke ich herzlich für die Einführung in die Lanthanoidchemie und für die gemeinsam verbrachte Zeit im Labor und außerhalb des Labors. Dr. Awal Noor möchte ich aufrichtig danken für die Motivation, Unterstützung und stete Hilfsbereitschaft in vielerlei Hinsicht. Dr. Torsten Irrgang danke ich vielmals für das Korrekturlesen der Zusammenfassung und der Einleitung. Ein besonderes Dankeschön gilt den von mir betreuten Praktikanten Thomas Wittmann, Sabrina Sachau, Andreas Mark, Michael Vogel und Franziska Speckner für ihre Hilfe und Arbeit. Herzlich bedanken möchte ich mich bei Walter Kremnitz, Heidi Maisel, Simone Ott, Marlis Schilling, Sandra Keller und Anna-Maria Dietel für die Hilfestellungen und Unterstützung bei Verwaltungsangelegenheiten und Zuarbeiten im Laboralltag. Bei meinen Laborkollegen Dr. Winfried Kretschmer, Christian Hübner, Isabelle Hass und AnnaMaria-Dietel möchte ich mich für die gute Arbeitsatmosphäre und Hilfsbereitschaft bedanken. 9. Acknowledgements / Danksagung 88 Meinen Arbeitskollegen und dem gesamten Arbeitskreis Kempe, Dr. Christine Denner, AnnaMaria-Dietel, Julia Ewert, Daniel Forberg, Martin Friedrich, Isabelle Haas, Muhammad Hafeez, Justus Hermannsdörfer, Toni Hille, Christian Hübner, Dr. Torsten Irrgang, Dr. Winfried P. Kretschmer, Sonja Lippert, Georg Lochner, Stefan Michlik, Dr. Awal Noor, Johannes Obenauf, Dr. Benjamin Oelkers, Simone Ott, Saravana Pillai, Dr. Sadaf Qayyum, Susanne Ruch, Sabrina Sachau, Stefan Schwarz, Adam Sobaczynski, Emmanuel Sobgwi Tamne, Theresa Winkler und Muhammad Zaheer danke ich ganz besonders für die gute Arbeitsatmosphäre und die schöne Zeit auch abseits des Laboralltags. Des Weiteren bin ich ihnen sehr dankbar für die Hilfe und Unterstützung, die ich in vielerlei Hinsicht erfahren habe. Meiner liebenden Familie, meinen Geschwistern Deborah Bauer, Rebekka Bauer, Manuel Bauer, Dorothea Bauer, Daniel Bauer und Tamara Baciu danke ich aus tiefstem Herzen für all die Aufmunterung während der Zeit meiner Promotion. Bei meinen Eltern, besonders bei meinem Vater Ulrich Bauer, möchte ich mich für die Hilfe und Unterstützung in jeglicher Hinsicht aufrichtig bedanken. Ich bin sehr glücklich und stolz Teil dieser tollen Familie zu sein. Zuletzt möchte ich mich bei Brina für all ihre Liebe, ihr Verständnis, die Unterstützung, aufmunternden Worte und ihren unerschütterlichen Glauben und niemals endendes Vertrauen in mich aufrichtig und aus tiefstem Herzen bedanken. 10. Declaration / Erklärung 89 10 Declaration / Erklärung: I hereby declare that I have written this work by myself and that no other sources than those mentioned in this work have been used. This work has so far neither been submitted to the Faculty of Biology, Chemistry and Earth Sciences at the University of Bayreuth nor to any other scientific institution for the purpose of a doctoral thesis. I never finally failed a similar doctoral examination at any other university. Hiermit versichere ich an Eides statt, dass ich die vorliegende Arbeit selbstständig und nur unter Verwendung der angegebenen Hilfsmittel und Quellen angefertigt habe. Diese Arbeit wurde bisher weder an der Fakultät für Biologie, Chemie und Geowissenschaften der Universität Bayreuth noch einer anderen wissenschaftlichen Einrichtung zum Zwecke der Promotion eingereicht. Ich habe keine gleichartige Doktorprüfung an einer anderen universitären Hochschule endgültig nicht bestanden. Tobias Bauer