Heterobimetallic ruthenium – zinc complexes with bulky N-heterocyclic carbenes : syntheses , structures and reactivity †

The ruthenium–zinc heterobimetallic complexes, [Ru(IPr)2(CO)ZnMe][BAr F 4] (7), [Ru(IBiox6)2(CO)(THF) ZnMe][BAr4] (12) and [Ru(IMes)’(PPh3)(CO)ZnMe] (15), have been prepared by reaction of ZnMe2 with the ruthenium N-heterocyclic carbene complexes [Ru(IPr)2(CO)H][BAr F 4] (1), [Ru(IBiox6)2(CO)(THF)H][BAr F 4] (11) and [Ru(IMes)(PPh3)(CO)HCl] respectively. 7 shows clean reactivity towards H2, yielding [Ru(IPr)2(CO) (η-H2)(H)2ZnMe][BAr4] (8), which undergoes loss of the coordinated dihydrogen ligand upon application of vacuum to form [Ru(IPr)2(CO)(H)2ZnMe][BAr F 4] (9). In contrast, addition of H2 to 12 gave only a mixture of products. The tetramethyl IBiox complex [Ru(IBioxMe4)2(CO)(THF)H][BAr F 4] (14) failed to give any isolable Ru–Zn containing species upon reaction with ZnMe2. The cyclometallated NHC complex [Ru(IMes)’ (PPh3)(CO)ZnMe] (15) added H2 across the Ru–Zn bond both in solution and in the solid-state to afford [Ru(IMes)’(PPh3)(CO)(H)2ZnMe] (17), with retention of the cyclometallation.


Introduction
Heterobimetallic complexes featuring a transition metal (TM) in partnership with a Lewis acidic (LA), typically main group element, have been the focus of considerable interest 1 because of their potential to bring about the cooperative activation of E-H (E = H, N, Si etc.) bonds. 2 The most commonly found heterobimetallic complexes feature a late transition metal (groups 8-10) and an element from group 13 ( particularly B and Al) and, in many cases, are readily prepared by salt elimination reaction of a TM anion with a halide of the LA. 3 While this approach is very flexible in that there are many possible TM and LA fragments that can be combined in this way, one (if not both) of the partners is typically left coordinatively saturated, reducing the subsequent reactivity for bond activation processes.An alternative approach which has been employed, although less frequently, is an alkane elimination pathway via the reaction of a TM hydride precursor with a LA hydrocarbyl reagent. 4This synthetic approach does come with potential issues (e.g. the use of highly pyrophoric group 13 trialkyls, cost of Ga/InMe 3 etc.), but does allow access to heterobimetallic complexes with unsaturation at both centres, thereby opening up an opportunity to probe true TM-LA cooperativity.

Reactivity of 1 towards ZnR 2 reagents
The methyl zinc analogue of 4, [Ru(IPr) 2 (CO)ZnMe][BAr F 4 ] (7), was prepared by subjecting a C 6 H 5 F solution of 1 to a slight excess of a toluene solution of ZnMe 2 .7 was isolated as a dark red solid in good yield (73%) and exhibited diagnostic low frequency 1 H (δ -0.86) and 13 C (δ -0.7) NMR resonances for the Zn-Me group, along with high frequency 13 C signals (δ 200.6 and 188.0) arising from the presence of the carbenic and carbonyl carbons respectively.The structure of 7 was confirmed by X-ray crystallography (Fig. 1), which revealed a Ru-Zn distance of 2.3997(8) Å, comparable to that in 4 (2.4069(7)Å).There was no reaction between 1 and ZnPh 2 (even upon heating to 70 °C) presumably due to the unfavourable combination of bulky substituents on the NHC and Zn.
Upon exposure of a fluorobenzene solution of 7 to 1 atm H 2 , an instantaneous change in colour from red-orange to colourless was observed, resulting from the formation of the dihydrogen dihydride complex, [Ru(IPr)   The Ru-Zn complexes 8 and 9 were characterised crystallographically (Fig. 2).As anticipated, a comparison of these two complexes to their ZnEt analogues 5 and 6 (Scheme 3) shows the same patterns i.e. elongation of the Ru⋯Zn distance relative to 4 and 7, less asymmetry of the Ru-H-Zn distances for H trans to CO and greater association with Ru for H trans to either an agostic interaction (6 and 9) or a dihydrogen ligand (5 and 8).

Synthesis and reactivity of Ru(IBiox) complexes
Given the success of IPr in allowing access to isolable [Ru(NHC) 2 (CO)H] + and [Ru(NHC) 2 (CO)ZnR] + species, we turned our attention to the IBiox class of NHCs introduced by Glorius, 12 on the basis that they are also known to be sterically demanding and flexibly restricted.Moreover, in spite of their use for the preparation of low-coordinate Rh and Ir complexes, 13 we were aware of just a single example of a Ru-IBiox complex at the outset of our work. 14mploying previous methodology, 15  4 ] (14, Scheme 5) respectively.The X-ray structures of neither 11 nor 14 (Fig. 3) showed any agostic interactions to the substituents on the IBiox ligands (e.g.shortest C-H⋯Ru in 11 is 3.182 Å). 13a,c In order to relieve the electron-deficiency of the Ru(II) centres, a THF ligand resides in the metal coordination sphere of each complex trans to CO. 17 The X-ray crystal structure of 10 and 13, along with those of the cations in 11 and 14, are shown in Fig. 3.A listing of metrical data for the compounds is given in Table 1.As expected, all four compounds exhibited square pyramidal geometries with the hydride ligand in an apical position.Analysis of the NHC tilting angle Θ NHC (Ru-C NHC -centroid NHC ) 18 revealed angles of >170°in all cases, showing that, despite the coordinative unsaturation at ruthenium, the IBiox ligands remain free of any structural distortions akin to those seen in some [M(IBiox) 3 ] + (M = Rh, Ir) species.13c Efforts to generate new Ru-Zn containing complexes through reaction of 11 and 14 with ZnMe 2 was successful only in the case of the former, 19 which generated [Ru(IBiox6) 2 (CO) (THF)ZnMe][BAr F 4 ] (12, Fig. 4 and Scheme 4).A comparison between the structures of 4 and 12 yield some superficial similarities and some interesting differences.Both structures contain two trans NHC ligands and coordination bonds in the equatorial plane of which two are common, namely, one to a zinc centre and one to a CO ligand.In 4, the remaining site is occupied by a bifurcated agostic interaction, while in 12, there is coordination of a THF molecule.In gross terms, the structures of both cations overlay reasonably well, but the biggest significant difference between them lies in the relative orientations of the NHC ligands.In 4, the angle between the mean planes based on the 5-membered NHC rings is relatively staggered at 102°, while the comparable angle in 12 (32°) reflects a more eclipsed carbene conformation.The arising steric ramifications are that the C NHC -Ru-C NHC angle of 177.32(19)Å in 4 is noticeably more linear than the 170.96 (10)°angle observed in 12.It is possible that the significantly shorter Ru-Zn distance of 2.3819(4) Å in 12 (cf.2.4069(7) Å in 4) may reflect the less encumbered access of the zinc ligand, towards the ruthenium centre, via the opposite face of the cation to the NHC ligand fold.
Upon exposure to either 1 or 5 atm H 2 , NMR spectra of fluorobenzene solutions of 12 exhibited signals for free IBiox6 as well as the salt [IBiox6•H][BAr F 4 ].Any products of initial reaction with H 2 therefore appear to be of only limited stability.

Ru-Zn bond formation from a neutral Ru-H precursor
The premise behind the initial synthesis of 1 was that addition of ZnR 2 to an electrophilic ruthenium hydride complex would both resonances correlated to a 13 C NMR signal at δ 32. 20he similarity of chemical shifts and J values for both species (e.g. each exhibited a high frequency resonance for the carbenic carbon with a 2 J CP value of >80 Hz, indicative of a trans IMes-Ru-PPh 3 geometry) suggested that they were most likely conformers.There was a slight solvent dependence on the solution ratio of 15a : 15b (88 : 12 and 82 : 18 in C 6 D 6 and THF-d 8 respectively).The two species were shown to be in exchange in THF-d 8 by EXSY, although NOESY measurements failed to divulge any information as to the spatial difference between 15a and 15b.We were unable to establish any difference   The X-ray crystal structures of 15 and 17 (Fig. 5) show clearly the transformation of 5-coordinate 15 to six-coordinate 17 upon reaction with H 2 .As anticipated (vide supra), elongation of the Ru-Zn distance from 2.3677(3) Å to 2.4828(3) Å takes place upon H 2 addition.Both bridging hydrogens were located and refined without restraints.As in 6 and 9, the hydride trans to CO was more evenly shared between Ru and Zn than that, which in the case of 17, lies trans to the methylene group of the activated IMes ligand.As a result of cyclometallation, neither 15 nor 17 showed a strictly linear C IMes′ -Ru-P geometry (172.32(6) and 169.10(6)°respectively). 15 exhibited a particularly noticeable distortion of the angle at the cyclometallated methylene carbon (Ru(1)-C(3)-C(4) = 83.39(12)°). 21eliminary studies to investigate the mechanism of formation of 17 revealed that exposure of 17 to D 2 (1 atm) resulted in slow (1 day, room temperature) deuterium incorporation into both Ru-H-Zn positions, but no H/D exchange at RuCH 2 .This excludes exchange taking place via a reversible reductive elimination pathway involving both RuH and RuCH 2 . 22The viability of an alternative pathway through phosphine dissociation was probed by reaction of 17 with 5 equiv.P(p-tolyl) 3 .Slow PPh 3 /P (p-tolyl) 3 was indeed observed, but the relevance of this to the H/D exchange was complicated by the appearance of other low frequency proton signals arising from the decomposition of 17 that can be seen in solution over 1-2 days.

Conclusions
We have reported that ZnMe 2 reacts with both cationic and neutral ruthenium hydride precursors containing bulky N-heterocyclic carbene ligands to afford new heterobimetallic complexes containing Ru-Zn bonds.The IPr complex [Ru(IPr) 2 (CO)ZnMe][BAr F 4 ] (7) proved to be similar in terms of both structure and reactivity towards H 2 to the previously reported ZnEt derivative 4. Use of the bulky IBiox carbene ligands met with varying levels of success; the cyclohexyl substituted derivative IBiox6 gave [Ru(IBiox6) 2 (CO)(THF) ZnMe][BAr F 4 ] (12), whereas the analogous tetramethyl IBioxMe 4 derivative could not be isolated.Of particular interest was the formation of the neutral complex [Ru(IMes)′(PPh 3 )(CO) ZnMe] (15), which added H 2 across the Ru-Zn bond whilst retaining the cyclometallated NHC ligand.As noted above, this behaviour contrasts with the reversal of cyclometallation that is brought about upon exposure of [Ru(IMes)′(PPh 3 ) 2 (CO)H] to H 2 .20a This, together with the fact that 4 reacts with HBcat to bring about dehydrocoupling (and generation of 5) 6b in contrast to 1 which reacts with HBcat with loss of H 2 and formation of the stable boryl complex [Ru(IPr) 2 (CO)Bcat][BAr F 4 ], 5 provides evidence for very different reactivity between Ru-Zn and Ru-H containing species.Indeed, efforts to probe the reactivity of 15 towards a broader range of E-H bonds, as well as prepare derivatives of the complex containing other metallated ligands, are in progress.

X-ray crystallography
Data for 7, 9, 11 and 12 were collected using an Agilent Xcalibur diffractometer while those for 8, 10, 13, 14, 15 and 17 were obtained using an Agilent SuperNova instrument (Table 2).All experiments were conducted at 150 K, solved using charge-flipping algorithm implemented in Olex2 24 and refined using SHELXL. 25In structures where disorder was observed in a [BAr F 4 ] anion, C-F, F⋯F, C⋯F and ADP restraints were applied, on merit.Otherwise, refinements were largely straightforward.Hence, only points of merit will be detailed hereafter.The asymmetric unit in 7 comprises one cation and one anion.The hydrogens attached to C55 in the former were located and refined subject to having similar C-H bond distances and to being equidistant from each other.F7, F8 and F9 were each disordered over 2 sites in the anion.H1, H2 and H3 in the cationic portion of compound 8 were readily located and, after some effort, an assignment was also made for H4.The associated U iso values were refined freely, and that for H4 is somewhat higher than one might expect.However, this may well reflect some movement in the ligated dihydrogen, wherein the constituent atoms were refined subject to being equidistant from Ru1 and at a distance of 0.75 Å from each other (the refined H-H distance, on this basis, is 0.75(1) Å).The bridging hydrides were refined without restraints.Residual electron density maxima in this structure are in the region of the anion CF 3 groups, five of which merited disorder modelling.In particular, F7-F12 were each refined over 2 positions in a 50 : 50 disorder ratio while F1-F3 exhibited 70 : 30 disorder.Moreover, the entire CF 3 moieties based on C71 and C80 were refined to take account of 70 : 30 and 55 : 45 disorder levels, respectively.
In 9, the asymmetric unit contains one cation, one anion and one molecule of CH 2 Cl 2 .H1 and H2 in the cation were located a refined without restraints.The hydrogens attached to C26 were similarly located and refined subject to being located at a distance of 0.98 Å from C26. Fluoride disorder was modelled for two of the [BAr F 4 ] CF 3 moieties.In particular, F1-F3 were disordered over two sites in a 75 : 25 ratio while F13-15 were disordered over three sites in a 50 : 40 : 10 ratio.The solvent molecule exhibited 55 : 45 disorder and C-Cl distances were restrained to being similar in both moieties.ADP restraints were included for fractional occupancy atoms.The asymmetric unit in 10 equates to half of one molecule of the complex, and half of a benzene molecule.The chloride and carbonyl ligands within the metal complex are disordered with each other in a 50 : 50 ratio.The hydride ligand (which is likely to be disordered over 2 sites) could not be reliably located and, hence, was omitted from the refinement.One cation, one anion and two independent fluorobenzene halves constitute the asymmetric unit in 11.The solvent moieties are proximate to crystallographic inversion centres which serve, in each case, to generate the remaining molecule portions.The halides in these solvent moieties are necessarily disordered and, hence, exhibit half site-occupancies.75 : 25 disorder was also modelled for C22 in the cation, with chemically similar distances Some disorder modelling was necessary in both that cation and the anion present in the asymmetric unit of compound 12.In the cation, this pertained to 55 : 45 disorder confined to atoms C37 and C38 in the THF ligand.Chemically equivalent distances involving the partial occupancy atoms were restrained to being similar in the final least-squares and some ADP restraints were also included for same.Four of the CF 3 groups in the anion were seen to exhibit disorder.In particular, the fluorine atoms attached to C47, C56, C63 and C72 were each modelled over 2 sites, in ratios of 55 : 45, 60 : 40, 55 : 45 and 75 : 25, respectively.The asymmetric unit in 13 comprises half of a molecule, with the central ruthenium located at a crystallographic inversion centre.This necessarily means that the chloride and carbonyl ligands are disordered in a 50 : 50 ratio.An exemplary diffraction pattern was observed for crystal of compound 14 where the asymmetric unit was seen to contain one cation and one anion.There was no evident twinning but, yet, the structural motif itself is riddled with disorder.While this was successfully modelled, it has inevitably resulted in the addition of a large number of restraints to the model, as both carbene ligands in the cation were seen to be disordered in a 50 : 50 ratio.The carbene carbons are, in each ligand, common to both components.In addition, C26 in the THF ligand was also seen to exhibit disorder, which optimally refined to a 60 : 40 ratio.Distance similarity restraints and ADP restraints were added to the model for the cation, on merit, in the final refinement cycles.In the BAr F 4 anion, three of the rings were seen to be disordered in an 80 : 20 ratio.The moiety based on C36 did not exhibit disorder to a level that could be credibly modelled, although the CF 3 group based on C42 was treated for 70 : 30 disorder.
In 15, both H3a and H3b were located and subsequently refined subject to each being a distance of 0.98 Å from C3.In a similar vein, the hydrogens attached to C3 were also readily located in 17, and each refined subject to being situated at distance of 0.95 Å from the parent atom.Finally, the bridging hydride ligands were also located in this compound and refined without restraints.

Conflicts of interest
There are no conflicts of interest to declare.
crystallographically as measurements of a number of different single crystals only ever afforded the same structure as shown in Fig. 5. Upon addition of 1 atm H 2 to a deep red-orange C 6 D 6 solution of 15, an instant colour change to very pale ensued from formation of [Ru(IMes)′(PPh 3 )(CO)(H) 2 ZnMe] (17).The 1 H NMR spectrum showed the presence of two doublet hydride resonances at δ −6.77 ( 2 J HP = 14.9 Hz) and δ −9.19 ( 2 J HP = 5.0 Hz), alongside a higher frequency doublet at δ 3.22 and doublet of doublets at δ 1.83, consistent with addition of H 2 across the Ru-Zn bond rather than reversal of the IMes cyclometallation.This irreversibility contrasts with what we observed previously in the case of the related cyclometallated hydride derivative [Ru(IMes)′(PPh 3 ) 2 (CO)H], which reacted with H 2 to form [Ru(IMes)(PPh 3 ) 2 (CO)H 2 ]. 20a Equally surprisingly, monitoring of the reaction with H 2 in the solid-state by IR spectroscopy showed complete depletion of ν CO for 15 at 1860 cm −1 and appearance of a new carbonyl absorption band at 1941 cm −1 for 17 upon stirring a ground up microcrystalline sample of the former under 1 atm H 2 for 2 days at room temperature.

Table 2
Crystal data and structure refinement details for compounds 7-15 and 17 The hydride was also located and is disordered in a 50 : 50 ratio.The associated metal-hydride distances were refined subject to a 1.6 Å, Ru-H, distance restraint.Anion disorder was limited to the halides in five of the CF 3 functionalities.Specifically, the fluorines attached to C46, C54, C55, C63 and C70 exhibited disorder ratios of 65 : 35, 55 : 45, 50 : 50, 75 : 25 and 75 : 25, respectively.