A Cationic Unsaturated Platinum(II) Complex that Promotes the Tautomerization of Acetylene to Vinylidene

COMMUNICATION A Cationic Unsaturated Platinum(II) Complex that Promotes the Tautomerization of Acetylene to Vinylidene Laura Ortega-Moreno, Riccardo Peloso, Joaquín López-Serrano, Javier Iglesias-Sigüenza, Celia Maya and Ernesto Carmona*[a] Dedication ((optional)) Abstract: Complex [PtMe2(PMe2ArDipp2)] (1) that contains a tethered terphenyl phosphine (ArDipp2 = 2,6-(2,6-iPr2C6H3)2C6H3) reacts with [H(Et2O)2]BArF (BArF = B[3,5-(CF3)2C6H3]4) to give the solvent complex [PtMe(S)(PMe2ArDipp2)]+ (2·S). Although the solvent molecule was easily displaced by a Lewis base (e.g. CO and C2H4) to afford the corresponding adducts, treatment of 2·S with C2H2 yielded instead the allyl complex [Pt(η3-C3H5)(PMe2ArDipp2)]+ (6) via the alkyne intermediate [PtMe(η2-C2H2)(PMe2ArDipp2)]+ (5). Deuteration experiments with C2D2 and kinetic and theoretical investigations demonstrated that the conversion of 5 into 6 involved a Pt(II)-promoted HCCH to :C=CH2 tautomerization in preference over acetylene migratory insertion into the Pt-Me bond. Cationic organometallic complexes are valuable reagents that find numerous applications in catalysis.[1-3] Low-coordinate cationic methyl derivatives of Pt(II) are of particular importance, for they combine high metal electrophilicity with the accessible coordination sites and the intrinsic reactivity of the metal-carbon σ-bond.[4-6] -Acidic Pt(II) and Au(I) systems catalyze many electrophilic activations of alkenes and alkynes.[7] Carbene complexes, including metal-vinylidenes,[8-9] are often proposed as active intermediates, but in most instances there is no direct evidence for their involvement.[7a] Vinylidene complexes of transition metals have been widely investigated, the most convenient method to induce the alkyne-to-vinylidene rearrangement being the use of electron-rich metal fragments.[9a] By contrast, there is a scarcity of information about Pt and Au analogues,[10-12] which might be associated with the -acid nature of compounds of these metals.[7] Phosphines are one of the most important families of auxiliary ligands.[13] Following earlier work,[14] we describe herein the synthesis and structural characterization of the cationic, unsaturated complex [PtMe(S)(κ2-P,C-PMe2ArDipp2)]+ (2·S), that features bidentate coordination of the bulky phosphine, PMe2ArDipp2 (Dipp = 2,6-iPrC6H3; ArDipp2 = 2,6-(2,6- iPrC6H3)2C6H3).[15,16] Although complex 2·S is not truly a lowcoordinate species,[17] it exhibits a weakly-bonded molecule of solvent (S = H2O, Et2O, THF) and a labile Pt···Carene interaction. Therefore it could be viewed as a source of the two-ligand Pt fragment [PtMe(PMe2ArDipp2)]+, for which rich electrophilic reactivity can be foreseen. While no C—C bond formation was observed for C2H4 under ambient conditions, C2H2 yielded initially an unprecedented[18,19] [PtMe(η2-C2H2)(PMe2ArDipp2)]+ complex (5), that experienced facile C—C coupling to afford the -allyl complex [Pt(η3-C3H5)(PMe2ArDipp2)]+ (6). The use of C2D2 provided unequivocal proof for a CH≡CH to :C=CH2 tautomerism induced by the Pt(II) center, also supported by computational studies. Protonation (CD2Cl2, 80 ºC) of complex 1 that features κ2- P,C phosphine coordination[20] (see the Supporting Information) with [H(Et2O)2]BArF (BArF = B[3,5-(CF3)2C6H3]4) did not permit observation of a transient -CH4 complex.[2c] Liberation of CH4 occurred instead with formation of complexes [PtMe(S)(PMe2ArDipp2)]+ (2·S) (Scheme 1) featuring rather large 1JPPt and 3JPtH (Pt—Me) couplings of about 4800 and 77 Hz, respectively. These complexes exhibited dynamic behavior at 25 ºC,[21] although below 10 ºC the molecular structure became rigid on the NMR time scale. The Dipp rings give rise below 25 ºC to two septets (2.38 and 2.28 ppm in C2D2Cl4) and four doublets (between 1.52 and 1.06 ppm) for the iPr substituents. At higher temperatures (Figure S12) the two rings become equivalent (above 110 ºC; ΔGǂ = 19 ± 2 kcal·mol1 at the coalescence temperature of 100 ºC). Scheme 1. Generation of complexes 2·S. Single crystals of 2·S could only be obtained after prolonged cooling at 20 ºC of CH2Cl2/Et2O solutions of the complex. Adventitious H2O originated crystals of the aquo complex [PtMe(H2O)(PMe2ArDipp2)]BArF 2·H2O in preference to those of 2·Et2O or 2·CH2Cl2. The H2O ligand is trans to P (Figure 1) and forms a Pt—O bond with a normal[22] length of 2.143(4) Å. Other metric parameters have the expected values, though the Pt···Carene interaction (Pt1-C22 in Figure 1) of 2.348(5) Å is significantly longer than normal olefinic Pt(II)—C [a] Dr. L. Ortega-Moreno, Dr. R. Peloso, Dr. J. López-Serrano, Dr. C. Maya, Prof. Ernesto Carmona Instituto de Investigaciones Químicas (IIQ), Departamento de Química Inorgánica and Centro de Innovación en Química Avanzada (ORFEO-CINQA). Universidad de Sevilla and Consejo Superior de Investigaciones Científicas (CSIC). Avenida Américo Vespucio 49, 41092 Sevilla (Spain) E-mail: gu[email protected] [b] Dr. J. Iglesias-Sigüenza, Departamento de Química Orgánica, Universidad de Sevilla and Centro de Innovación en Química Avanzada (ORFEO-CINQA), C/Prof. García González 1, 41012 Sevilla, Spain. Supporting information for this article is given via a link at the end of the document. COMMUNICATION bonds (ca. 2.11-2.16 Å).[23-25] The X-ray data therefore support the notion that cations 2·S could behave as a source of the electrophilic [PtMe(PMe2ArDipp2)]+ fragment. Figure 1. ORTEP view of the cation of complex 2·H2O; most hydrogen atoms are excluded for clarity and thermal ellipsoids are set at 40% level probability. Selected bond distances (Å) and angles (º): Pt1-O1 2.143(4), Pt1-C1 2.034(6), Pt1-P1 2.170(1), Pt1-C22 2.348(5), O1-Pt1-C1 83.2(2), C1-Pt1-P1 89.6(2), P1-Pt1-C22 83.9(1), C22-Pt1-O1 103.1(2). We sought empirical evidence for the electron density at the Pt(II) center of 2·S by carrying out its reaction with CO. Treatment of 2·S with CO (2 bar, 20 ºC) gave the carbonyl complex [PtMe(CO)(PMe2ArDipp2)]+, (3), with a κ2-P,C tethered phosphine in a structure akin to 2·S. Its carbonyl IR stretching frequency at 2127 cm1 is only somewhat smaller than the 2143 cm1 value of free CO. For comparison, cationic bis(diimine) complexes [PtMe(CO)(N^N)]+ [25] and recently reported [Pt(PCP)(CO)]+ derivatives[26] have 𝜈(CO) values of 2103-2116 cm1 and 2111-2145 cm1, respectively. Complex 2·S offered also the possibility of accommodating Pt—alkene or Pt—alkyne functions cis to the Pt—Me bond, thus amenable to experience insertion reactions.[24,27] These compounds are related to the catalyst resting state involved in the Pd- and Ni-catalyzed olefin polymerization[1a] and in the Ptcatalyzed dimerization of ethylene.[22,28-29] Addition of C2H4 to CH2Cl2 solutions of complex 2·S led to the adduct [PtMe(η2-C2H4)(PMe2ArDipp2)]+ (4), exhibiting once again chelating κ2-P,C phosphine ligation. The Pt—C2H4 linkage undergoes fast C2H4 rotation at 20 ºC, with a barrier Gǂ = 11 ± 1 kcal·mol1 at the coalescence temperature of 25 ºC (Figure S13). It also experiences associative exchange with excess free C2H4 (EXSY NMR between 0 and 25 ºC, Gǂ = 16 ± 2 kcal·mol1). At room temperature, it gives rise to a broad 13C{1H} NMR resonance at 102.2 ppm, thereby having an olefin coordination shift to lower frequencies, , of nearly 21 ppm. For comparison, in dicationic [Pt(PNP)(C2H4)]2+,  = 45 ppm.[7c,30] This observation is indicative of a significantly decreased contribution of the -back-donation in complex 4,[31] in agreement with the high -acidity of the Pt(II) center in 2·S. Complex 4 is nonetheless stable at 20 ºC towards C2H4 migratory insertion into the Pt—Me bond, although under excess of C2H4 (2 bar) becomes a catalyst precursor for C2H4 dimerization[22,28,29] above 55 ºC (see the Supporting Information). Related diimine Pt catalysts required heating for 2 days at 100 ºC.[28] When complex 2·S was treated with C2H2 at room temperature (2 bar, CH2Cl2 solution) a clean reaction yielded the η3-allyl complex [Pt(η3-C3H5)(PMe2ArDipp2)]+ (6), through the intermediacy of the -C2H2 compound [PtMe(η2- C2H2)(PMe2ArDipp2)]+ (5) (Scheme 2). Monitoring of the reaction by variable temperature 1H and 31P{1H} NMR spectroscopy revealed that C2H2 coordination takes place even at 80 ºC, forming adduct 5 that features a 31P{1H} resonance at 11.5 ppm, with a 1JPPt of 4282 Hz. This species contains Pt—Me ( 0.60, 3JHP = 3 Hz, 2JHPt = 76 Hz) and Pt—η2-C2H2 linkages, the latter giving rise to a 1H NMR doublet centered at 3.08 ppm (3JHP = 4 Hz, 2JHPt = 50 Hz) and to a 13C doublet resonance centered at 77.7 ppm, with 1JCH, 2JCH, and 2JCP couplings of 263, 44, and 12 Hz, respectively. Scheme 2. Formation of the -allyl complex 6 from 2·S and C2H2 via 2- acetylene adduct 5. A second intermediate formed above 60 ºC and survived for a short time at room temperature before it converted into the η3-allyl complex 6 that was the only observable reaction endproduct. Its NMR characterization was therefore attempted at 20 ºC. It contains an intact Pt—Me unit, as hinted by a 1H NMR resonance at  0.43 ppm (2JHPt = 74 Hz) and a corresponding 13C{1H} signal at 6.6 ppm (2JCP = 6 Hz). In addition, a 13C resonance was recorded at 221.2 ppm. Since in a recently reported cationic Pt(II) alkylidene a 13C signal at 209 ppm was assigned to the alkylidene carbon nucleus,[14a] it is tempting to ascribe the observed 221.2 ppm resonance to the Pt-bonded carbon of the Pt=C=CH2 unit. The corresponding CH2 resonance could not be assigned with confidence and therefore we defer a definite structural proposal for this intermediate until additional data become available. Noteworthy, for a recently reported Au(I) vinylidene, the key resonances of the - and -carbon of the vinylidene ligand could not be detected despite extended acquisition. 13C labeling was needed to identify two broad multiplets at 206 and 112 ppm for this vinylidene fragment.[10] Unequivocal evidence for the participation of a Pt(II)- vinylidene in the formation of complex 6 was provided by isotopic labelling studies using C2D2. The newly formed allyl moiety of 6-d2 contained two D atoms bound to a terminal allyl carbon atom. Furthermore, the use of C2D2 yielded a kinetic isotope effect kH/kD of 2.7 ± 0.3 suggesting that acetylene C—H bond cleavage was involved in the rate-determining step. Computational studies at the DFT M06/6-31g(d,p)/SDD level were developed and two mechanistic routes were explored, namely, migratory insertion of the coordinated C2H2 molecule of 5 into the Pt—Me bond and a 1,2-hydrogen shift within the 2- bound alkyne. ΔG≠,DCM values of 26.1 kcal·mol-1 and 19.4 kcal·mol-1 were respectively found (Figure 2).[32] The latter path led in its first elemental step to the vinylidene ligand of intermediate A, with energy only 6.5 kcal·mol-1 above 5. Fast CH3 migration to the Pt=C bond (ΔG≠,DCM = 5.9 kcal·mol-1 from A) yields a second intermediate B that originates 6 after a facile, Pt-assisted H migration from CH3 to the central Pt-bonded COMMUNICATION carbon atom. The overall energy return ΔG0,DCM is of 40.9 kcal·mol-1. The calculations support therefore that the vinylidene route to the 3-allyl complex 6 is preferred over the classical migratory insertion reactivity. Figure 2. Free energy profile for the transformation of 5 to 6 via acetylene to vinylidene tautomerization. The small energy difference computed for the tautomeric structures 5 and A (6.5 kcal·mol-1) is rather intriguing. To gain further information, the donor-acceptor properties of the C2H2 and vinylidene ligands of 5 and A were analyzed by means of the Energy Decomposition Analysis (EDA; Table 1)[33] and Natural Orbital for Chemical Valence (NOCV) schemes.[34] For the sake of completeness, the Pt—C2H4 linkage of complex 4 was also investigated computationally. The results of this analysis confirm that the C=CH2 ligand has the largest fragment interaction energy (ΔEint: -72.2 vs. -32.5 (η2-C2H4) and -31.6 (η2- C2H2) kcal∙mol-1). The Pt(II)-vinylidene interaction has a strong electrostatic component (see Table 1 and the Supporting Information for further details)[7a,35] and it is largely responsible for the accessibility of the vinylidene isomer under mild conditions (Δ(ΔEint) between 2-HC≡CH and :C=CH2 coordination amounts 40.6 kcal∙mol-1). Table 1. EDA analysis of the [Pt]-η2-C2H4, [Pt]-η2-C2H2 and [Pt]=C=CH2 interactions in 4, 5 and B optimized at the BP86/TZP level imposing Cs symmetry. [Pt]- η2-C2H4 [Pt]- η2-C2H2 [Pt]=C=CH2 ΔEint[a] -32.5 -31.6 -72.2 ΔEPauli 148.0 153.2 267.3 ΔEelstat[b] -106.7 (59.1%) -110.2 (59.6%) -218.6 (64.4%) ΔEorb[b] -73.8 (40.9%) -74.6 (40.4%) -120.9 (35.6%) ΔE(A')[c] -47.61 (64.6%) -45.8 (61.3%) -103.3 (85.4%) ΔE(A")[c] -26.14 (34.4%) -28.9 (38.7%) -17.7 (14.6%) [a] kcal∙mol-1; [b] Percentage of electrostatic/covalent contribution to the attractive interactions; [c] Percentage contribution to the orbital interactions. In conclusion, this work proves that the electrophilic Pt(II) center of the cationic methyl-acetylene complex 5, readily accessible from 2·S and C2H2, induces a C—C bond forming reaction through an isomeric methyl-vinylidene structure in preference to classical migratory insertion reactivity. Acknowledgements Financial support (FEDER support, Subprogramas Ramón y Cajal and Juan de la Cierva, grant BES-2011-048035) from Spanish Ministry of Science (Projects CTQ2013-42501-P, CTQ2014-51912-REDC and CTQ-2014-52769-C3-3-R) and the Junta de Andalucía (Grant FQM-119 and project P09-FQM-4832 and FQM-2126) is gratefully acknowledged. The use of the facilities of the Galicia Supercomputing center (CESGA) and the SGAI-CSIC (Cluster Trueno) are gratefully acknowledged. Keywords: phosphane ligands • -acid complex • vinylidenes • platinum • DFT [1] a) S. D. Ittel, L. K. Johnson, M. Brookhart, Chem. Rev. 2000, 100, 1169; b) D. S. McGuinness, Chem. Rev. 2011, 111, 2321; c) J. R. Khusnutdinova, D. Milstein, Angew. Chem. Int. Ed. 2015, 54, 12236. [2] a) M. Joost, A. Amgoune, D. Bourissou, Angew. Chem. Int. Ed. 2015, 54, 15022; b) J. J. Verendel, O. Pàmies, M. Diéguez, P. G. Andersson, Chem. Rev. 2014, 114, 2130; c) W. H. Bernskoetter, C. K. Schauer, K. I. Goldberg, M. 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[35] a) M. S. Nechaev, V. M. Rayon, G. Frenking, J. Phys. Chem. A., 2004, 108, 3134; b) C. Massera, G. Frenking, Organometallics 2003, 22, 2758; c) S. F. Vyboishchikov, G. Frenking, Chem. Eur. J., 1998, 4, 1428; d) G. Frenking, M. Solà, S. F. Vyboishchikov, J. Organomet. Chem., 2005, 690, 6178. COMMUNICATION Entry for the Table of Contents (Please choose one layout) Layout 1: COMMUNICATION The cationic complex [PtMe(S)(PMe2ArDipp2)]+ reacted with acetylene to yield the allyl complex [Pt(η3-C3H5)( PMe2ArDipp2)]+, formally the product of a classical migratory insertion reaction. Nevertheless, deuteration experiments with C2D2 and kinetic and theoretical investigations demonstrated that the transformation occurred via a Pt(II)- promoted HC≡CH to :C=CH2 tautomerization. L. Ortega-Moreno, R. Peloso, J. López- Serrano, Javier Iglesias-Sigüenza, C. Maya and E. Carmona* Page No. – Page No. A Cationic Unsaturated Platinum(II) Complex that Promotes the Tautomerization of Acetylene to Vinylidene