Synthesis, structure and reactivity of Pd and Ir complexes based on new lutidine-derived NHC/phosphine mixed pincer ligands

This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. Accepted Manuscript Dalton Transactions www.rsc.org/dalton Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1 Please do not adjust margins Please do not adjust margins Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/ Synthesis, structure and reactivity of Pd and Ir complexes based on new lutidine-derived NHC/phosphine mixed pincer ligands Práxedes Sánchez, a Martín Hernández-Juárez, a Eleuterio Álvarez, a Margarita Paneque, a, * Nuria Rendón a and Andrés Suárez a, * Coordination studies of new lutidine-derived hybrid NHC/phosphine ligands (CNP) to Pd and Ir have been performed. Treatment of the square-planar [Pd(CNP)Cl](AgCl 2 ) complex 2a with KHMDS produces the selective deprotonation at the CH 2 P arm of the pincer to yield the pyridine-dearomatised complex 3a. A series of cationic [Ir(CNP)(cod)] + complexes 4 has been prepared by reaction of the imidazolium salts 1 with Ir(acac)(cod). These derivatives exhibit in the solid state, and in solution, a distorted trigonal bipyramidal structure in which the CNP ligands adopt an unusual C (axial) -N (equatorial) -P (equatorial) coordination mode. Reactions of complexes 4 with CO and H 2 yield the carbonyl species 5a(Cl) and 6a(Cl), and the dihydrido derivatives 7, respectively. Furthermore, upon reaction of complex 4b(Br) with base, selective deprotonation at the methylene CH 2 P arms is observed. The, thus formed, deprotonated Ir complex 8b reacts with H 2 in a ligand-assisted process leading to the trihydrido complex 9b, which can also be obtained by reaction of 7b(Cl) with H 2 in the presence of KO t Bu. Finally, the catalytic activity of Ir-CNP complexes in the hydrogenation of ketones has been briefly assessed. Introduction Metal complexes based on lutidine-derived PNP pincer ligands have gained considerable attention due to their applications in organometallic chemistry and catalysis (Fig. 1a). 1 In these derivatives metal-ligand cooperativity, triggered by deprotonation of the methylene arms of the ligand accompanied by dearomatisation of the pyridine ring, has led to unique reactivity in the activation of a diversity of X-H (X = H, C, O, N) bonds. While significantly less studied, analogous complexes based on CNC pincers (C stands for a N-heterocyclic carbene, NHC) have also been described (Fig. 1b). 2-4 As shown with Ru-CNC complexes, 3,4 these derivatives can also be deprotonated at the methylene CH 2 N bridges, and participate in metal-ligand cooperation processes. Furthermore, the larger Py-CH 2 -NHC linkage, which forms 6-membered metallacycles upon coordination, confers a greater flexibility on the ligand in comparison to 5-membered rings formed in PNP pincers. This flexibility should permit stabilizing metal complexes in a variety of coordination geometries, a relevant issue in catalysis where the intermediates in the catalytic cycle may need to adopt different structural arrangements. For example, while PNP ligands have only exhibited meridional coordination modes, facial coordination of CNC ligands in Ru complexes has been observed. 3 More remarkable, as demonstrated by the Pidko´s group, is that the increased flexibility of the CNC pincer results in enhanced reactivities towards H 2 and CO 2 in comparison to Ru-PNP systems. 4b N R2P PR2 N N N N N R R N N PR2 N R M-CNC M-PNP M-CNP (this work) [M] [M] [M] N N PR2 N R M-CNP [M] Danopoulos, Braunstein et al. (2015) a) b) c) d) Fig. 1 General structure of metal complexes with a) lutidinederived PNP ligands, b) lutidine-derived CNC ligands, c) deprotonated picoline-derived CNP ligands, and d) lutidinederived CNP ligands (this work). In addition, non-symmetric pincer ligands, i.e. having two inequivalent flanking donor groups, allow for a larger electronic and steric diversity derived from the potential Page 1 of 12 Dalton Transactions Dalton Transactions Accepted Manuscript ARTICLE Journal Name 2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins tuning of two different side donors. 5 With respect to lutidinederived pincer complexes, some examples of PNP’ 6 and CNC’ 4a derivatives have been reported. Unsymmetrical PNX and CNX pincer complexes have also been described, although these derivatives are usually of the type PNN 1 and CNN 7 where hemilabile coordination of the N-donor flanking group has been proposed. In marked contrast, complexes based on CNP ligands having a pyridine central moiety and in which the two side functionalities are two significantly different strong σ - donors, such as a phosphine and a NHC, have not been investigated. In fact, a limited number of hybrid tridentate ligands possessing both phosphine and NHC donors have been reported, and these have either a ligand backbone based on a 1,3-disubstituted phenyl ring, 8a or a different arrangement of the donor moieties, where the NHC group is the central unit of the pincer ligand. 8b-f Also, recently Danopoulos, Braunstein et al. have prepared Co and Cr complexes based on deprotonated NHC/phosphine mixed pincer ligands having a central picoline motif (Fig. 1c) . 9 Based on these precedents, we aimed to develop a new class of ligands having NHC and phosphine side donors and a lutidine central fragment (CNP, Fig. 1d). A fundamental difference of these ligands with the previous picoline-based pincer derivatives reported by Danopoulos, Braunstein et al. resides in the presence of a methylene linker between the pyridine and the NHC functionalities, which could also be susceptible to deprotonation and should enhance pincer flexibility. In this contribution, we report on the synthesis of the precursors of these ligands as well as their coordination to Pd and Ir complexes. In particular, the ability of CNP ligands to adapt to different coordination geometries and participate in ligand-assisted processes has been assessed. Scheme 1 Syntheses of CNP ligands precursors. Results and discussion Syntheses of imidazolium salts 1 Syntheses of imidazolium salts 1a(Cl) , 1b(Cl) and 1b(Br) were effected as shown in Scheme 1. Derivative 1a(Cl) was prepared by reaction of the corresponding 2-chloromethyl-6- imidazolylmethyl-pirydine with diphenyl phosphine in the presence of KO t Bu. Alternatively, in the case of salts 1b(Cl) and 1b(Br) , higher product yields were obtained when diphenyl phosphine-borane adduct was used for the introduction of the P-donor fragment, followed by phosphine deprotection by simple treatment with refluxing MeOH. The CNP ligands precursors were obtained with moderate to good yields (55- 85%) as white to brown solids. Synthesis and deprotonation of Pd-CNP complex 2a For an evaluation of the coordination capabilities of these new CNP ligands, we initially studied the formation of Pd derivatives. Thus, salt 1a(Cl) was reacted with Ag 2 O in CH 2 Cl 2 , followed by addition of PdCl 2 (cod) to yield complex 2a (Scheme 2). 10 The spectroscopic data support the formation of a complex in which the CNP ligand is coordinated to the metal centre as a pincer. For example, its 1 H NMR spectrum shows distinctly two signals for the bridging methylenes. The CH 2 P protons appear at 4.32 ppm (d, 2 J HP = 11.3 Hz), whereas the NCH 2 hydrogens produce a singlet at 6.05 ppm. The 13 C{ 1 H} NMR spectrum exhibits a doublet at 166.4 ppm with a large 2 J CP (183 Hz, carbene carbon, C 2 , of the NHC moiety), indicating the trans disposition of the NHC and phosphine moieties. Scheme 2 Synthesis and reactivity of Pd complex 2a . A single crystal X-ray diffraction study of 2a confirmed the proposed structure (Fig. 2). Thus, complex 2a in the solid state is comprised of a Pd atom in an square-planar coordination geometry, with the carbene and phosphine fragments of the pincer disposed trans to each other (C 2 (NHC)-Pd-P = 168.65 o ), and the chloride ligand trans to the pyridine (N(Py)-Pd-Cl = 175.03 o ). The NHC-Pd-Py chelate ring has a boat conformation as determined by the torsion angle C(14)-N(3)-Pd(1)-C(1) of 32.4 o , whereas the 5-membered ring involving the phosphine donor has an envelope conformation with a C(18)-N(3)-Pd(1)- P(1) torsion angle of 28.4 o . Since it can be expected that CNP ligands can be deprotonated in either the CH 2 P or CH 2 -NHC arms, the acid/base responsiveness of 2a was tested by adding KHMDS to a suspension of the complex in THF. 11 In the 1 H NMR spectrum of the resulting product 3a , a significant up-field shift for the signals of the pyridine protons (5.55-6.46 ppm) in accord with the dearomatisation of this moiety is observed. Meanwhile, the =CHP fragment produces a singlet signal at 3.41 ppm (integrating to 1H) in the 1 H NMR spectrum and a doublet at 63.3 ppm ( 1 J CP = 66 Hz) in the 13 C{ 1 H} NMR experiment, evidencing the selective deprotonation of the CNP ligand at the CH 2 P arm. N N X N Ar X i) N N PPh2 N Ar X i) for 1a(Cl): Ph2PH + KOtBu in MeCN/THF for 1b(Cl) and 1b(Br): Ph2P(BH3)H + KOtBu in MeCN/THF; then MeOH, reflux 1a(Cl); Ar = Mes, X = Cl 1b(Cl); Ar = 3,5-Xyl, X = Cl 1b(Br); Ar = 3,5-Xyl, X = Br Page 2 of 12Dalton Transactions Dalton Transactions Accepted Manuscript Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3 Please do not adjust margins Please do not adjust margins Fig. 2 ORTEP drawing at 30% ellipsoid probability of the cationic component of complex 2a . Hydrogen atoms and solvent molecules have been omitted for clarity. Selected bond lengths [Å] and angles [ o ]: Pd(1)-C(1) 2.038(6), Pd(1)-N(3) 2.071(4), Pd(1)-P(1) 2.2623(15), Pd(1)-Cl(1) 2.2758(14), C(1)- Pd(1)-P(1) 168.65(16), N(3)-Pd(1)-Cl(1) 175.03(14), P(1)-Pd(1)- Cl(1) 93.97(5), P(1)-Pd(1)-N(3) 81.34(14), C(1)-Pd(1)-Cl(1) 95.93(16), C(1)-Pd(1)-N(3) 88.6(2). Synthesis and structural features of Ir-CNP complexes Reaction of imidazolium salts 1 with Ir(acac)(cod) provided cationic olefin complexes 4 , isolated as yellow to orange solids in moderate to good yields (30-80%) (Scheme 3). These derivatives are stable in the solid state to the atmospheric agents, and have been fully characterised by NMR. For example, in the 1 H NMR spectrum of 4b(Br) , the CH 2 P protons are diastereotopic and appear as doublet of doublets at δ 3.36 ( 2 J HP = 2.1 Hz) and 4.17 ppm ( 2 J HH = 15.5 Hz and 2 J HP = 11.6 Hz). Similarly, protons for the CH 2 N bridge produce two doublets at δ 5.65 and 6.91 ( 2 J HH = 14.1 Hz). The 13 C{ 1 H} NMR spectrum shows a doublet at 164.8 ppm for the C 2 NHC carbon with a very small 2 J CP coupling constant of 8 Hz. These data suggest a cis coordination of the phosphine and NHC donors, also confirmed in the solid state by a single crystal X-ray diffraction study of 4b(BAr F ) (Fig. 3), obtained by anion exchange in complex 4b(Br) with NaBAr F . The structure of complex 4b(BAr F ) is best described as adopting a distorted trigonal bipyramidal geometry despite the acute P-Ir-N(Py) bond angle of 75.98(8) o . The CNP ligand exhibits a facial coordination, with the NHC donor in the apical position (P-Ir-C(NHC) angle of 95.61(10) o ). In addition, the sixmembered chelate ring involving the NHC and pyridine donors adopts a boat–like conformation as defined by the dihedral angle C(13)-N(3)-Ir(1)-C(1) of −50.6 o , while the chelate ring containing the phosphine fragment exhibits an envelope conformation with a C(17)-N(3)-Ir(1)-P(1) angle of 29.5 o . This facial coordination mode is unprecedented in M-PNP complexes and may be ascribed to the larger flexibility of the six-membered Py-M-NHC chelate ring, as previously observed in Ru-CNC complexes. 3 In addition, the C (axial) -N (equatorial) - P (equatorial) coordination mode of the pincer differs significantly from previously reported pentacoordinated d 8 pincer complexes, 12 for which an eq-ax-eq distribution is usually observed. 13 Finally, as observed with other pentacoordinated diolefin Ir complexes, 13 the distance from the Ir atom to the centroid of the C=C bonds is slightly longer for the alkene coordinated trans to the NHC than for the olefin placed in the meridional position ( ∆ d(Ir-centroid C=C) = 0.14 Å). Fig. 3 ORTEP drawing at 30% ellipsoid probability of the cationic component of complex 4b(BAr F ) . Hydrogen atoms and solvent molecules have been omitted for clarity. Selected bond lengths [Å] and angles [ o ]: Ir(1)-C(1) 2.029(4), Ir(1)-N(3) 2.253(3), Ir(1)-P(1) 2.3412(9), Ir(1)-C(31) 2.263(4), Ir(1)-C(32) 2.217(4), Ir(1)-C(35) 2.140(3), Ir(1)-C(36) 2.101(4), C(1)-Ir(1)- P(1) 95.61(10), N(3)-Ir(1)-P(1) 75.98(8), N(3)-Ir(1)-C(1) 81.07(13). As exemplified with complex 4a(Cl) , a dynamic behaviour in solution for complexes 4 has been evidenced by NMR. VT- 1 H NMR spectra of 4a(Cl) registered in the temperature range between 50 and − 80 o C show sharp signals for the resonances attributable to the CNP ligand. In contrast, broad signals at 2.98 and 3.49 ppm, integrating for two protons each, are observed for the olefinic protons at 25 o C. 1 H- 1 H COSY and 1 H- 1 H NOESY experiments indicate that each signal is produced by protons of different olefinic moieties; i.e. H b1 ,H b2 and H a1 ,H a2 produce signals at 2.98 and 3.49 ppm, respectively (Fig. 4a). In addition, these signals broaden upon lowering the temperature and eventually split at temperatures below − 25 o C into two sets of two signals each, appearing at δ 2.23 (H b2 ) and 3.39 (H b1 ), and 2.86 (H a2 ) and 3.90 (H a1 ), respectively. An approximate value of ∆ G ‡ = 10.9 kcal mol -1 at the coalescence temperature (244 K) can be estimated for the fluxional process. This dynamic behaviour can be ascribed to alkene site exchange allowed by the decoordination of the C=C fragment trans to the NHC moiety to produce the distorted tetrahedral intermediate A , followed by re-coordination of the free olefin moiety to the opposite side without a net change of the fac coordination mode of the CNP ligand (Fig. 4a). In addition, the 1 H, 1 H-exchange spectroscopy (EXSY) experiment of 4a(Cl) registered at 50 o C demonstrates the existence of additional dynamic processes with higher energy barriers. Thus, intense exchange cross-peaks between the signals for the olefinic protons appearing at 2.98 and 3.49 ppm are observed, which can be explained by the formal rotation of Page 3 of 12 Dalton Transactions Dalton Transactions Accepted Manuscript ARTICLE Journal Name 4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins the diolefin ligand allowed by the decoordination of one of the Ir-alkene bonds (Fig. 4a). Furthermore, the observation of strong correlation peaks between the resonances of the o-, m- and pprotons of one of the PPh groups with the aromatic protons of the other phenyl group, as well as between the signals of the methylene protons in each of the CH 2 P and CH 2 N arms, indicates that the CNP pincer in complexes 4 also undergoes structural changes, which could be assigned to a slow interconversion between the two enantiomeric forms of the complex. Previously, mirror-image isomer exchange involving a pseudo-Berry rotation has been observed for pentacoordinated pincer Ir complexes containing diolefin ligands. 13c However, this process should not be possible in complexes 4 due to the C (axial) -N (equatorial) -P (equatorial) coordination mode of the pincer. Since, as discussed above, olefin decoordination seems facile, the observed fluxional process could likely involve the intermediacy of the square-planar structure B (Fig. 4b). Scheme 3 Synthesis and reactivity of Ir-CNP complexes 4 . Fig. 4 Proposed dynamic processes in solution operating in the cationic part of complexes 4 (positive charges have been supressed for clarity; PR 2 = PPh 2 , Ar = mesityl or 3,5-xylyl). Page 4 of 12Dalton Transactions Dalton Transactions Accepted Manuscript Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5 Please do not adjust margins Please do not adjust margins To evaluate the donating properties of the CNP ligands, we prepared the carbonyl derivative 5a(Cl) by bubbling CO through a CH 2 Cl 2 solution of complex 4a(Cl) (Scheme 3). Signals of the bridging CH 2 P and CH 2 N protons in the 1 H NMR spectrum support a planar coordination of the CNP pincer. Thus, the CH 2 P protons produce a doublet signal at 4.18 ppm ( 2 J HP = 10.0 Hz) while the CH 2 N hydrogens appear as a singlet at 6.11 ppm. In the IR spectrum, the carbonyl ligand absorbs at 1985 cm -1 , which is a higher frequency than that corresponding to the ( t Bu-PNP)-Ir analogue (1964 cm -1 ), 14 suggesting a lower electron density at the metal centre in the Ir-CNP system. The CO ligand is detected in the 13 C{ 1 H} NMR spectrum by the appearance of a doublet signal at 177.2 ppm ( 2 J CP = 10 Hz), while the C 2 NHC carbon appears at 178.1 ppm ( 2 J CP = 99 Hz). Interestingly, complex 5a(Cl) reversibly coordinates a new CO molecule yielding complex 6a(Cl) , as inferred from the absorption bands corresponding to the CO ligands, which appear in the IR spectrum at 1946 and 2021 cm -1 . 15 In the 1 H NMR spectrum of 6a(Cl) , the presence of a singlet signal at 6.09 ppm (2H) for the CH 2 N arm and a doublet at 4.29 ppm (2H, 2 J HP = 10.9 Hz) attributable to the CH 2 P moiety suggests the existence of a symmetry plane containing the CNP-Ir coordination plane and points out to a meridional coordination of the pincer ligand, 13b at variance with the coordination geometry in the also pentacoordinated compounds 4 . As determined by VT- 1 H NMR spectroscopy (see SI), carbonyl complexes 5a(Cl) and 6a(Cl) exhibit a dynamic behaviour in solution, which equilibrates the two otherwise diastereotopic hydrogens of both methylene bridges. In square-planar Pd 16 and octahedral Ru complexes incorporating CNC ligands, 3b similar dynamic processes have been ascribed to a slow interconversion between the two twisted conformations adopted by both C 2 (NHC)-N(Py)-M chelate rings of the pincer ligand. Similarly, the observed fluxionality in derivatives 5a(Cl) and 6a(Cl) can be attributed to the fast atropoisomerism between the two limiting enantiomeric forms shown in Fig. 5. Fig. 5 Interconversion between the limiting conformations of 5a(Cl) and 6a(Cl) . Complexes 4a(Cl) and 4b(Cl) react with H 2 producing the dihydrido derivatives 7 and cyclooctene (Scheme 3). At room temperature, the 1 H NMR spectrum of complex 7a(Cl) shows two doublets of doublets at –20.19 ( 2 J HP = 13.8 Hz, 2 J HH = 7.0 Hz) and –23.30 ppm ( 2 J HP = 18.9 Hz) due to the hydrido ligands placed trans to the pyridine and trans to the chloride, respectively. In the 13 C{ 1 H} NMR spectrum, the C 2 NHC appears at 172.9 ppm as a doublet signal ( 2 J CP = 119 Hz). Exposure of a sample of 7a(Cl) in CD 2 Cl 2 to deuterium gas (2 bar) or addition of CD 3 OD causes fast H/D exchange of the hydrido ligands. Furthermore, the structural features of complex 7a(Cl) have been studied in the solid state by single crystal X-ray diffraction (Fig. 6). This derivative has an octahedral geometry with the two hydrido ligands occupying mutually cis positions and the CNP ligand adopting a meridional coordination, as defined by the C(1)-Ir(1)-P(1) angle value of 166.5(3) o . The chelate ring incorporating the NHC fragment has a boat-like conformation as shown by the C(14)-N(3)-Ir(1)-C(1) torsion angle of –26.5(9) o , whereas an envelope conformation for the N(Py)-Ir-P ring is observed with a C(18)-N(3)-Ir(1)-P(1) dihedral angle of –14.9(8) o . Fig. 6 ORTEP drawing at 30% ellipsoid probability of complex 7a(Cl) . Hydrogen atoms, except hydrido ligands, and solvent molecules have been omitted for clarity. Selected bond lengths [Å] and angles [ o ]: Ir(1)-C(1) 2.037(10), Ir(1)-N(3) 2.192(9), Ir(1)- P(1) 2.262(3), Ir(1)-H(1)Ir 1.600, Ir(1)-H(2)Ir 1.599, Ir(1)-Cl(1) 2.509(3), C(1)-Ir(1)-P(1) 166.5(3), N(3)-Ir(1)-H(1)Ir 173.9, P(1)- Ir(1)-H(1)Ir 92.7, P(1)-Ir(1)-N(3) 82.2(3), C(1)-Ir(1)-H(1)Ir 95.4, C(1)-Ir(1)-N(3) 89.0(4). Deprotonation and ligand-assisted H 2 activation We have also explored the deprotonation of the Ir-CNP complexes 4 . 17 Treatment of 4b(Br) with KO t Bu produces the selective deprotonation of the CH 2 P arm (Scheme 4). The resulting complex 8b is characterised in the 1 H NMR spectrum by the presence of significantly high-field shifted signals for the pyridine protons (5.6-6.4 ppm), evidencing the dearomatisation of the pyridine ring. The =CHP proton appears as a singlet at 3.86 ppm, while the CH 2 -NHC hydrogens generate two doublets at 4.93 and 5.29 ppm ( 2 J HH = 13.6 Hz). In the 13 C{ 1 H} NMR spectrum, the resonance caused by the C 2 NHC carbon appears as an overlapped doublet at 170.6 ppm. Although the J CP value cannot be unambiguously calculated, a Page 5 of 12 Dalton Transactions Dalton Transactions Accepted Manuscript ARTICLE Journal Name 6 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins value of 2 to 20 Hz is estimated, suggesting a cis coordination of the phosphine and NHC fragments. This coordination mode is further supported by the existence of strong NOE contacts between the protons of the xylyl and PPh 2 groups. Deprotonated complex 8b reacts with H 2 at 0 o C to produce the trihydrido derivative 9b in a ligand-assisted process leading to the re-aromatisation of the pyridine fragment. Complex 9b is only stable under an atmosphere of H 2 and can be also obtained by reaction of 7b(Cl) with KO t Bu followed by exposure to H 2 . The trihydrido complex 9b shows in the 1 H NMR spectrum a doublet of doublets at –9.98 ppm (2H, 2 J HP = 18.2 Hz, 2 J HH = 4.8 Hz) due to the apical hydrido ligands and a doublet of triplets at –19.64 ppm (1H, 2 J HP = 14.4 Hz) produced by the hydride trans to the pyridine N. The C 2 of the NHC fragment appears in the 13 C{ 1 H} NMR spectrum as a doublet at 176.9 ppm with a large 2 J CP (121 Hz), in agreement with a trans disposition of the NHC and phosphine donors of the CNP ligand. Scheme 4 Selective deprotonation of complex 4b(Br) , and formation of complex 9b . Hydrogenation of ketones catalysed by Ir-CNP complexes In order to assess the catalytic potential of these Ir-CNP complexes, their performance in the hydrogenation of ketones was studied (Table 1). 18 In the presence of KO t Bu, complex 4a(Cl) smoothly catalysed the hydrogenation of acetophenone under 4 bar of H 2 at 30 o C in 2-methyltetrahydrofuran, using a S/C/B ratio of 100/1/15 (entry 1). By using the same pressure, catalyst loading could be decreased to a S/C ratio of 250 after heating to 60 o C (entry 2). Also, at this temperature, a lower H 2 pressure (1 bar) could be employed (entry 3). Under the latter conditions, complex 4b(Cl) was found to be slightly less active, whereas a significantly lower catalytic activity was obtained with the carbonyl derivative 5a(Cl) (entries 4 and 5). Finally, the hydrogenation of a series of ketones was performed with complex 4a(Cl) . High conversions were obtained in the case of acetophenone derivatives substituted with p-methoxi, pchloro and o-bromo substituents (entries 6-8). Alternatively, the presence of fluoro substituents seems somewhat detrimental since 2-fluoroacetophenone was reduced with a slightly lower yield (entry 9). Also, the hydrogenation of a cyclic ketone, α -tetralone, proceeded with a high conversion (entry 10). Conclusions In summary, Pd and Ir complexes based on novel lutidinederived CNP pincer ligands have been synthesised. The flexibility of the chelating Py-CH 2 -NHC fragment of the ligands allows for both facial and meridional coordination modes in five- and six-coordinated Ir-CNP complexes. Furthermore, selective deprotonation of the CH 2 P arm in Ir-CNP complexes promotes ligand-assisted H-H activation, leading to active species in ketone hydrogenation. Further studies involving the application of metal complexes based on CNP ligands in X-H (X = H, C, heteroatom) bond activation and as catalysts in the (de)hydrogenation of polar substrates are currently in progress in our laboratory. Table 1 Hydrogenation of ketones catalysed by Ir-CNP complexes a Entry Ketone Ir-CNP T ( o C) Conv. (%) 1 b Acetophenone 4a(Cl) 30 95 2 b,c 60 93 3 60 >99 4 4b(Cl) 60 98 5 5a(Cl) 60 43 6 4´-Methoxiacetophenone 4a(Cl) 80 96 7 4´-Chloroacetophenone >99 8 2´-Bromoacetophenone 99 9 2´-Fluoroacetophenone 82 10 α -Tetralone 89 a Reaction conditions, unless otherwise noted: 1 bar of H 2 , 2- methyltetrahydrofuran, S/C/B = 100/1/15, base: KO t Bu, 16 h. [S] = 0.13 M. Conversion was determined by 1 H NMR spectroscopy. b 4 bar of H 2 . c S/C/B = 250/1/15. Experimental General procedures All reactions and manipulations were performed under nitrogen or argon, either in a Braun Labmaster 100 glovebox or using standard Schlenk-type techniques. All solvents were distilled under nitrogen with the following desiccants: sodium-benzophenone-ketyl for diethyl ether (Et 2 O) and tetrahydrofuran (THF); sodium for hexane, pentane and toluene; CaH 2 for dichloromethane (CH 2 Cl 2 ) and acetonitrile (CH 3 CN); and NaOMe for methanol (MeOH). 1-(3,5- dimethylphenyl)-1H-imidazole and 1-(2,4,6-trimethylphenyl)-1H- imidazole were prepared as previously described. 19 Ir(acac)(cod) 20 and NaBAr F21 were synthesized according to literature procedures. All other reagents were purchased from commercial suppliers and Page 6 of 12Dalton Transactions Dalton Transactions Accepted Manuscript Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 7 Please do not adjust margins Please do not adjust margins used as received. NMR spectra were obtained on Bruker DPX-300, DRX-400, AVANCEIII/ASCEND 400R or DRX-500 spectrometers. 31 P{ 1 H} NMR shifts were referenced to external 85% H 3 PO 4 , while 13 C{ 1 H} and 1 H shifts were referenced to the residual signals of deuterated solvents. All data are reported in ppm downfield from Me 4 Si. All NMR measurements were carried out at 25 o C, unless otherwise stated. NMR signal assignations were confirmed by 2D NMR spectroscopy ( 1 H- 1 H COSY, 1 H- 1 H NOESY, 1 H- 13 C HSQC and 1 H- 13 C HMBC). HRMS data were obtained on a JEOL JMS-SX 102A mass spectrometer at the Instrumental Services of Universidad de Sevilla (CITIUS). ESI-MS experiments were carried out in a Bruker 6000 apparatus by the Mass Spectrometry Service of the Instituto de Investigaciones Químicas. Elemental analyses were run by the Analytical Service of the Instituto de Investigaciones Químicas in a Leco TrueSpec CHN elemental analyzer. IR spectra were acquired on a Bruker Tensor 27 instrument. Synthesis of imidazolium salts 1 Imidazolium salts 1 were synthesised in two steps from the corresponding 2,6-bis(halomethyl)pyridines and imidazoles as shown below. [2-(Diphenylphosphinyl)methyl-6-(3-mesitylimidazolium-1- yl)methyl]pyridine chloride, 1a(Cl) A solution of 2,6-bis(chloromethyl)pyridine (4.00 g, 22.7 mmol) and 1-mesityl-1H-imidazole (2.12 g, 11.4 mmol) in toluene (80 mL) was refluxed for 7 days. The precipitate was filtered, washed with cold THF (2 x 20 mL) and pentane (3 x 20 mL), and dried under vacuum to yield [2-chloromethyl-6-(3-mesitylimidazolium-1- yl)methyl]pyridine chloride as a brown solid (2.91 g, 70%). 1 H NMR (400 MHz, CDCl 3 ): δ 10.56 (s, 1H, H arom Imid), 8.13 (s, 1H, H arom Imid), 8.06 (d, 3 J HH = 7.7 Hz, 1H, H arom Py), 7.84 (dd, 3 J HH = 7.7 Hz, 3 J HH = 7.7 Hz, 1H, H arom Py), 7.48 (d, 3 J HH = 7.7 Hz, 1H, H arom Py), 7.10 (s, 1H, H arom Imid), 6.99 (s, 2H, 2 H arom Mes), 6.24 (s, 2H, CH 2 N), 4.63 (s, 2H, CH 2 Cl), 2.33 (s, 3H, CH 3 ), 2.04 (s, 6H, 2 CH 3 ). 13 C{ 1 H} NMR (125 MHz, CDCl 3 ): δ 156.0 (C q arom), 151.9 (C q arom), 141.5 (C q arom), 140.8 (CH arom), 138.9 (CH arom), 134.3 (2 C q arom), 130.8 (C q arom), 130.0 (2 CH arom), 125.1 (CH arom), 124.1 (CH arom), 124.0 (CH arom), 122.8 (CH arom), 52.3 (CH 2 N), 45.1 (CH 2 Cl), 21.2 (CH 3 ), 17.7 (2 CH 3 ). HRMS (ESI): m/z 326.1411 [(M−Cl) + ] (exact mass calculated for C 19 H 21 ClN 3 : 326.1424). In subsequent step, to a solution of PPh 2 H (1.08 g, 5.80 mmol) in THF (10 mL) was added a solution of KO t Bu (0.650 g, 5.80 mmol) in THF (10 mL). The resulting mixture was stirred for 5 min, and added to a solution of [2-chloromethyl-6-(3-mesitylimidazolium-1- yl)methyl]pyridine chloride (2.00 g, 5.52 mmol) in MeCN (40 mL). The suspension was stirred overnight, and MeOH (15 mL) was added to quench the reaction. Solvent was evaporated, and the residue was extracted with CH 2 Cl 2 (2 x 15 mL). The solid obtained after removal of the solvent was washed with diethyl ether (3 x 20 mL) and pentane (3 x 20 mL). Imidazolium salt 1a(Cl) was isolated as a light brown solid (2.40 g, 85%). 1 H NMR (500 MHz, CDCl 3 ): δ 10.30 (s, 1H, H arom Imid), 7.85 (s, 1H, H arom Imid), 7.60 (d, 3 J HH = 7.5 Hz, 1H, H arom Py), 7.54 (dd, 3 J HH = 7.6 Hz, 3 J HH = 7.6 Hz, 1H, H arom Py), 7.36 (m, 4H, 4 H arom PPh), 7.28 (m, 6H, 6 H arom PPh), 7.12 (s, 1H, H arom Imid), 7.03 (d, 3 J HH = 7.7 Hz, 1H, H arom Py), 7.01 (s, 2H, 2 H arom Mes), 5.94 (s, 2H, CH 2 N), 3.59 (s, 2H, CH 2 P), 2.33 (s, 3H, CH 3 ), 2.01 (s, 6H, 2 CH 3 ). 31 P{ 1 H} NMR (202 MHz, CD 2 Cl 2 ): δ −11.7. 13 C{ 1 H} NMR (126 MHz, CD 2 Cl 2 ): δ 158.8 (d, J CP = 8 Hz, C q arom), 152.7 (C q arom), 141.4 (C q arom), 138.7 (d, J CP = 3 Hz, CH arom), 138.5 (d, J CP = 15 Hz, 2 C q arom), 137.9 (CH arom), 134.7 (2 C q arom), 133.0 (d, J CP = 19 Hz, 4 CH arom), 131.2 (C q arom), 129.9 (2 CH arom), 129.0 (2 CH arom), 128.7 (d, J CP = 7 Hz, 4 CH arom), 124.0 (d, J CP = 5 Hz, CH arom), 123.9 (CH arom), 122.7 (CH arom), 121.3 (d, J CP = 2 Hz, CH arom), 53.7 (CH 2 N), 38.3 (d, J CP = 17 Hz, CH 2 P), 21.1 (CH 3 ), 17.7 (2 CH 3 ). HRMS (ESI): m/z 476.2243 [(M−Cl) + ] (exact mass calculated for C 31 H 31 N 3 P: 476.2256). [2-(Diphenylphosphinyl)methyl-6-(3-(3,5-xylyl)imidazolium-1- yl)methyl]pyridine chloride, 1b(Cl) A solution of 2,6-bis(chloromethyl)pyridine (3.96 g, 22.5 mmol) and 1-xylyl-1H-imidazole (1.93 g, 11.2 mmol) in THF (20 mL) was heated to 45 °C for 7 days. The solution was reduced to half the initial volume by solvent evaporation, and Et 2 O (10 mL) was added to precipitate the product. The solid was filtered, washed with Et 2 O (2 x 10 mL) and pentane (3 x 5 mL) and dried under vacuum. [2- Chloromethyl-6-(3-(3,5-xylyl)imidazolium-1-yl)methyl]pyridine chloride was isolated as a white solid (2.21 g, 57%). 1 H NMR (400 MHz, CDCl 3 ): δ 11.09 (s, 1H, H arom Imid), 8.19 (d, 3 J HH = 7.2 Hz, 1H, H arom Py), 8.09 (s, 1H, H arom Imid), 7.89 (dd, 3 J HH = 8.0 Hz, 3 J HH = 7.2 Hz, 1H, H arom Py), 7.54 (m, 2H, H arom Py + H arom Imid), 7.23 (s, 2H, 2 H arom Xyl), 7.11 (s, 1H, H arom Xyl), 6.10 (s, 2H, CH 2 N), 4.69 (s, 2H, CH 2 Cl), 2.36 (s, 6H, 2 CH 3 ). 13 C{ 1 H} NMR (101 MHz, CDCl 3 ): δ 156.1 (C q arom), 151.7 (C q arom), 140.9 (CH arom), 140.5 (2 C q arom), 136.4 (CH arom), 134.3 (CH arom), 132.1 (CH arom), 125.2 (CH arom), 124.1 (C q arom), 123.8 (CH arom), 120.31 (CH arom), 119.58 (2 CH arom), 52.7 (CH 2 N), 45.4 (CH 2 Cl), 21.4 (2 CH 3 ). HRMS (ESI): m/z 312.1256 [(M−Cl) + ] (exact mass calculated for C 18 H 19 ClN 3 : 312.1268). In a subsequent step, to a solution of Ph 2 P(BH 3 )H (0.288 g, 1.44 mmol) in THF (10 mL) was added a solution of KO t Bu (0.161 g, 1.44 mmol) in THF (5 mL). The mixture was stirred for 10 min, and added to a suspension of [2-chloromethyl-6-(3-(3,5-xylyl)imidazolium-1- yl)methyl]pyridine chloride (0.500 g, 1.44 mmol) in MeCN (10 mL). The resulting suspension was stirred overnight, and MeOH (10 mL) was added to quench the reaction. The solvent was evaporated under vacuum, and the solid was extracted with CH 2 Cl 2 (3 x 10 mL). Solvent removal followed by washings with Et 2 O (2 x 10 mL) yields a light orange solid which should correspond to the borane adduct of 1b(Cl). This solid was dissolved in MeOH (10 mL), and the solution was transferred to a Fisher-Porter vessel and heated to 75 o C for 24 h. Volatiles were removed under vacuum, and MeOH (10 mL) was newly added and the previous procedure repeated. The resulting solid was washed with toluene (2 x 5 mL) and Et 2 O (3 x 5 mL) to give an off-white solid (0.444 g, 62%). 1 H NMR (400 MHz, CD 2 Cl 2 ): δ 11.28 (s, 1H, H arom Imid), 7.74 (d, 3 J HH = 7.6 Hz, 1H, H arom Py), 7.64 (dd, 3 J HH = 7.7 Hz, 3 J HH = 7.7 Hz, 1H, H arom Py), 7.55 (d, 3 J HH = 1.1 Hz, 1H, H arom Imid), 7.44 (m, 4H, 4 H arom), 7.36 (m, 7H, 7 H Page 7 of 12 Dalton Transactions Dalton Transactions Accepted Manuscript ARTICLE Journal Name 8 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins arom), 7.29 (s, 2H, 2 H arom Xyl), 7.21 (s, 1H, H arom Xyl), 7.12 (d, 3 J HH = 7.8 Hz, 1H, H arom Py), 5.91 (s, 2H, CH 2 N), 3.69 (s, 2H, CH 2 P), 2.45 (s, 6H, 2 CH 3 ). 31 P{ 1 H} NMR (121 MHz, CD 2 Cl 2 ): δ −11.8. 13 C{ 1 H} NMR (101 MHz, CD 2 Cl 2 ): δ 159.0 (d, J CP = 8 Hz, C q arom), 152.6 (C q arom), 141.0 (2 C q arom), 138.5 (d, J CP = 15 Hz, 2 C q arom), 138.0 (CH arom), 136.6 (CH arom), 134.9 (C q arom), 133.0 (d, J CP = 19 Hz, 4 CH arom), 131.9 (CH arom), 129.1 (2 CH arom), 128.7 (d, J CP = 7 Hz, 4 CH arom), 124.2 (d, J CP = 5 Hz, CH arom), 123.7 (CH arom), 121.8 (CH arom), 120.2 (CH arom), 119.7 (2 CH arom), 54.0 (CH 2 N), 38.3 (d, J CP = 16 Hz, CH 2 P), 21.3 (2 CH 3 ). HRMS (ESI): m/z 462.2082 [(M−Cl) + ] (exact mass calculated for C 30 H 29 N 3 P: 462.2094). [2-(Diphenylphosphinyl)methyl-6-(3-(3,5-xylyl)imidazolium-1- yl)methyl]pyridine bromide, 1b(Br) A solution of 1-(3,5-xylyl)-1H-imidazole (1.00 g, 5.81 mmol) in THF (20 mL) was added to a solution of 2,6-bis(bromomethyl)pyridine (3.08 g, 11.6 mmol) in THF (20 mL). The solution was stirred for 7 days at room temperature. The resulting precipitate was filtered, washed with cold THF (2 x 10 mL) and hexane (2 x 10 mL), and dried to give [2-bromomethyl-6-(3-(3,5-xylyl)imidazolium-1- yl)methyl]pyridine bromide as a light brown solid (2.00 g, 79%). 1 H NMR (500 MHz, CD 2 Cl 2 ): δ 10.86 (s, 1H, H arom Imid), 7.98 (s, 1H, H arom Imid), 7.80 (d, 3 J HH = 7.6 Hz, 1H, H arom Py), 7.75 (s, 1H, H arom Imid), 7.72 (dd, 3 J HH = 7.7 Hz, 3 J HH = 7.6 Hz, 1H, H arom Py), 7.42 (d, 3 J HH = 7.6 Hz, 1H, H arom Py), 7.34 (s, 2H, 2 H arom Xyl), 7.12 (s, 1H, H arom Xyl), 5.98 (s, 2H, CH 2 N), 4.51 (s, 2H, CH 2 Br), 2.36 (s, 6H, 2 CH 3 ). 13 C{ 1 H} NMR (125 MHz, CD 2 Cl 2 ): δ 157.4 (C q arom), 152.9 (C q arom), 141.0 (2 C q arom), 138.9 (CH arom), 136.1 (CH arom), 134.7 (C q arom), 132.0 (CH arom), 124.0 (CH arom), 123.9 (CH arom), 123.6 (CH arom), 120.7 (CH arom), 119.7 (2 CH arom), 53.9 (CH 2 N), 34.0 (CH 2 Br), 21.2 (2 CH 3 ). HRMS (ESI): m/z 356.0751 [(M−Br) + ] (exact mass calculated for C 18 H 19 BrN 3 : 356.0757). In a subsequent step, to a solution of Ph 2 P(BH 3 )H (0.483 g, 2.42 mmol) in THF (10 mL) was added a solution of KO t Bu (0.271 g, 2.42 mmol) in THF (10 mL). The resulting mixture was stirred for 10 min, and added to a solution of [2-bromomethyl-6-(3-(3,5- xylyl)imidazolium-1-yl)methyl]pyridine bromide (1.01 g, 2.39 mmol) in MeCN (20 mL). The suspension was stirred overnight, and MeOH (15 mL) was added to quench the reaction. The solvent was evaporated under vacuum, and the resulting solid was extracted with CH 2 Cl 2 (3 x 10 mL). Solvent removal followed by washings with Et 2 O (2 x 10 mL) yields a light orange solid which should correspond to the borane adduct of 1b(Br). This solid was dissolved in MeOH (10 mL), and the solution was transferred to a Fisher-Porter vessel and heated to 75 o C for 24 h. Volatiles were removed under vacuum, and MeOH was newly added and the previous procedure was repeated. The resulting solid was washed with toluene (10 mL) and Et 2 O (10 mL) to give an off-white solid (0.693 g, 55% yield). 1 H NMR (500 MHz, CD 2 Cl 2 ): δ 10.77 (s, 1H, H arom Imid), 7.65 (d, 3 J HH = 7.6 Hz, 1H, H arom Py), 7.61 (t, 3 J HH = 1.2 Hz, 1H, H arom Imid), 7.56 (m, 2H, H arom Py + H arom Imid), 7.38 (m, 4H, 4 H arom PPh), 7.32 (s, 2H, 2 H arom Xyl), 7.29 (m, 6H, 6 H arom PPh), 7.13 (s, 1H, H arom Xyl), 7.07 (d, 3 J HH = 7.6 Hz, 1H, H arom Py), 5.84 (s, 2H, CH 2 N), 3.62 (s, 2H, CH 2 P), 2.37 (s, 6H, 2 CH 3 ). 31 P{ 1 H} NMR (202 MHz, CD 2 Cl 2 ): δ −11.7. 13 C{ 1 H} NMR (125 MHz, CD 2 Cl 2 ): δ 159.0 (d, J CP = 8 Hz, C q arom), 152.3 (C q arom), 141.0 (2 C q arom), 138.4 (d, J CP = 15 Hz, 2 C q arom), 137.9 (CH arom), 135.9 (CH arom), 134.8 (C q arom), 132.9 (d, J CP = 19 Hz, 4 CH arom), 131.9 (CH arom), 129.1 (2 CH arom), 128.7 (d, J CP = 7 Hz, 4 CH arom), 124.2 (d, J CP = 5 Hz, CH arom), 123.9 (CH arom), 121.6 (CH arom), 120.3 (CH arom), 119.7 (2 CH arom), 54.0 (CH 2 N), 38.3 (d, J CP = 16 Hz, CH 2 P), 21.3 (2 CH 3 ). HRMS (ESI): m/z 462.2089 [(M−Br) + ] (exact mass calculated for C 30 H 29 N 3 P: 462.2099). Synthesis of Pd-CNP complexes 2a and 3a Complex 2a. A solution of 1a(Cl) (0.100 g, 0.20 mmol) in CH 2 Cl 2 (7 mL) was added Ag 2 O (0.049 g, 0.21 mmol). The suspension was stirred for 24 h, and filtered. To the resulting solution was added PdCl 2 (cod) (0.057 g, 0.20 mmol). After stirring for 4 h, solvent was removed under vacuum, and the residue was washed with Et 2 O (2 × 5 mL), extracted with CH 2 Cl 2 (2 × 5 mL), and crystallized from a CH 2 Cl 2 /toluene solvent mixture. Pale yellow solid (0.096 g, 60%). Anal. calcd (%) for C 31 H 30 AgCl 3 N 3 PPd: C 46.8; H 3.8; N 5.3; found: C 47.2; H 3.7; N 4.8. 1 H NMR (500 MHz, CD 2 Cl 2 ): δ 8.30 (d, 3 J HH = 7.6 Hz, 1H, H arom Py), 8.21 (s, 1H, H arom NHC), 8.05 (dd, 3 J HH = 7.5 Hz, 3 J HH = 7.5 Hz, 1H, H arom Py), 7.92 (d, 3 J HH = 7.6 Hz, 1H, H arom Py), 7.73 (dd, 3 J HP = 12.1 Hz, 3 J HH = 8.1 Hz, 4H, 4 H arom PPh), 7.58 (t, 3 J HH = 7.5 Hz, 2H, 2 H arom PPh), 7.50 (m, 4H, 4 H arom PPh 2 ), 7.03 (s, 2H, 2 H arom Mes), 6.95 (s, 1H, H arom NHC), 6.05 (s, 2H, CH 2 N), 4.32 (d, 2 J HP = 11.3 Hz, 2H, CH 2 P), 2.39 (s, 3H, CH 3 ), 2.17 (s, 6H, 2 CH 3 ). 31 P{ 1 H} NMR (121 MHz, CD 2 Cl 2 ): δ 34.8. 13 C{ 1 H} NMR (126 MHz, CD 2 Cl 2 ): δ 166.4 (d, J CP = 183 Hz, C-2 NHC), 161.9 (d, J CP = 7 Hz, C q arom), 155.1 (C q arom), 142.1 (CH arom), 139.6 (C q arom), 135.8 (C q arom), 135.5 (2 C q arom), 133.3 (d, J CP = 11 Hz, 4 CH arom), 132.7 (2 CH arom), 129.7 (d, J CP = 12 Hz, 4 CH arom), 129.0 (2 CH arom), 126.6 (d, J CP = 46 Hz, 2 C q arom), 126.4 (CH arom), 125.4 (d, J CP = 10 Hz, CH arom), 124.0 (d, J CP = 5 Hz, CH arom), 123.4 (d, J CP = 5 Hz, CH arom), 55.2 (CH 2 N), 41.7 (d, J CP = 28 Hz, CH 2 P), 21.3 (CH 3 ), 18.6 (2 CH 3 ). MS (ESI, CH 2 Cl 2 ): m/z (%): 616 (100) [(M−AgCl 2 ) + ]. Complex 3a. To a suspension of 2a (0.025 g, 0.04 mmol) in THF (2 mL) was added KHMDS (0.007 g, 0.04 mmol). The mixture was stirred for 30 min, and solvent was evaporated under reduced pressure. The residue was washed with Et 2 O (2 x 2 mL) and extracted with THF (2 x 2 mL). Solvent removal under vacuum provides complex 3a as an orange solid (0.020 g, 85%). An analytical pure sample of 3a could not be obtained due to significant decomposition of the complex during purification. 1 H NMR (400 MHz, THF-d 8 ): δ 7.71 (m, 4H, 4 H arom PPh 2 ), 7.44 (s, 1H, H arom NHC), 7.22 (m, 6H, 6 H arom PPh 2 ), 6.98 (s, 1H, H arom NHC), 6.87 (s, 2H, 2 H arom Mes), 6.46 (ddd, 3 J HH = 8.9 Hz, 3 J HH = 6.4 Hz, 5 J HP = 2.8 Hz, 1H, H c ), 6.30 (d, 3 J HH = 8.7 Hz, 1H, H b ), 5.55 (d, 3 J HH = 6.3 Hz,1H, H d ), 4.87 (s, 2H, CH 2 N), 3.41 (s, 1H, H a ), 2.30 (s, 3H, CH 3 ), 2.08 (s, 6H, 2 CH 3 ). 31 P{ 1 H} NMR (162 MHz, THF-d 8 ): δ 25.3. Page 8 of 12Dalton Transactions Dalton Transactions Accepted Manuscript