Zn-Promoted C–H Reductive Elimination and H2 Activation via a Dual Unsaturated Heterobimetallic Ru–Zn Intermediate

Reaction of [Ru(PPh3)3HCl] with LiCH2TMS, MgMe2, and ZnMe2 proceeds with chloride abstraction and alkane elimination to form the bis-cyclometalated derivatives [Ru(PPh3)(C6H4PPh2)2H][M′] where [M′] = [Li(THF)2]+ (1), [MgMe(THF)2]+ (3), and [ZnMe]+ (4), respectively. In the presence of 12-crown-4, the reaction with LiCH2TMS yields [Ru(PPh3)(C6H4PPh2)2H][Li(12-crown-4)2] (2). These four complexes demonstrate increasing interaction between M′ and the hydride ligand in the [Ru(PPh3)(C6H4PPh2)2H]− anion following the trend 2 (no interaction) < 1 < 3 < 4 both in the solid-state and solution. Zn species 4 is present as three isomers in solution including square-pyramidal [Ru(PPh3)2(C6H4PPh2)(ZnMe)] (5), that is formed via C–H reductive elimination and features unsaturated Ru and Zn centers and an axial Z-type [ZnMe]+ ligand. A [ZnMe]+ adduct of 5, [Ru(PPh3)2(C6H4PPh2)(ZnMe)2][BArF4] (6) can be trapped and structurally characterized. 4 reacts with H2 at −40 °C to form [Ru(PPh3)3(H)3(ZnMe)], 8-Zn, and contrasts the analogous reactions of 1, 2, and 3 that all require heating to 60 °C. This marked difference in reactivity reflects the ability of Zn to promote a rate-limiting C–H reductive elimination step, and calculations attribute this to a significant stabilization of 5 via Ru → Zn donation. 4 therefore acts as a latent source of 5 and this operational “dual unsaturation” highlights the ability of Zn to promote reductive elimination in these heterobimetallic systems. Calculations also highlight the ability of the heterobimetallic systems to stabilize developing protic character of the transferring hydrogen in the rate-limiting C–H reductive elimination transition states.


[Ru(PPh 3 )(C 6 H 4 PPh 2 ) 2 H][Li(THF) 2 ] (1).
To an agitated suspension of  (t, 2 J PP = 20 Hz, C 6 H 4 PPh 2 (cis to RuH)), -27.9 (t, 2 J PP = 20 Hz, C 6 H 4 PPh 2 (trans to RuH)). 13 Table S4 and Figures S9-S15. The reaction was monitored further at 273 K and finally at 298 K. These data are summarized in Table S5 and To visualize more clearly the color change over the course of the reaction, a repeat run was carried out in a J. Young's resealable ampule. The ampule was charged with a magnetic stir bar and a THF (3 mL) solution of 3 (56 mg, 0.054 mmol) under 1 atm of H 2 at 263 K (ice / NaCl). The reaction mixture changed from red to colorless upon stirring, but upon stirring being halted, became red again. This process was reproducible was over several minutes (see accompanying ESI video file). Ultimately, complete conversion of 4 to a mixture of fac-8-Zn S23 and mer-8-Zn was confirmed by analysis of an aliquot of the solution by 1 H and 31 P NMR spectroscopy.   [d] fac-8-Zn was observed in less than 1 mol % quantity.

S-4 Crystallographic Details
Data for 1, 3 and 4THF were collected using an Agilent SuperNova instrument (using Cu-K radiation) while those for 2, 4THF/4ClTHF, 4 and 6 were obtained using an Agilent Xcalibur diffractometer and a Mo-K source. All experiments were conducted at 150 K, with the exception of that for compound 3 (vide infra). Using Olex2, 7 all structures were solved with the olex2.solve 8 structure solution program and subsequently refined using the SHELXL program. 9 While refinements were largely unremarkable, there are some points which merit note as follows.
The hydride ligand in 1 was located and refined without restraints. There is a little smearing of the electron density in the region of the THF ligands. However, efforts to model same were abandoned, on the basis that a stable disorder model could not be achieved without the inclusion of extensive restraints.
The asymmetric unit in 2 contains one cation, one anion and three molecules of benzene.
The hydride ligand in the former was located and refined subject to being a distance of 1.6 Å from Ru1. The cation was (surprisingly) ordered. There is evidence for some disorder in the guest benzene based on C83, but this was not modeled. The highest, residual, electron-density maximum is located at a chemically insignificant distance from the transition metal.
C60-C62 were modeled for 60:40 disorder in the structure of 3. Distance restraints were used in the disordered region. H1 was located and refined freely. Data were collected at 200 K, as the crystal was seen to crack and degrade at 150 K -possibly due to a phase transition.
In 4THF, the asymmetric unit comprises one molecule of the ruthenium-zinc complex and two regions of solvent. The hydride ligand (H1) was located and refined freely. Each of the two solvent regions contain one molecule of THF, with the moieties based on O2 and O3 being disordered in 70:30 and 65:35 ratios, respectively. Distance and ADP restraints were included in S39 disordered regions to assist convergence. The assignment of the oxygen atoms in the solvent entities is somewhat tentative due to the smearing of the electron density in these regions.
The asymmetric unit in 4THF/4ClTHF contains one molecule of a ruthenium-zinc complex and two regions of solvent. The methyl ligand attached to Zn1, in the former, was seen to be disordered in a 65:35 ratio with a chloride ligand which means that the gross crystal contains two distinct compounds. The hydride ligand (H1) was located and refined freely. Each of the two solvent regions contain one molecule of THF, with both solvent molecules being disordered in a 65:35 ratio. Distance and ADP restraints were included in disordered regions to assist convergence. The assignment of the oxygen atoms in the solvent entities is somewhat tentative due to the smearing of the electron density in these regions.
The hydride ligand (H1) in the structure of 4 was located and refined without restraints.
One phenyl ring (attached to P3) was modeled to take account of 55:45 disorder. The component parts therein were treated as rigid hexagons in the final least-squares and some soft ADP restraints were also included for partial occupancy carbon atoms. The asymmetric unit in 6 comprises one cation and one anion. The fluorine atoms attached to C82 were modeled to take account of 60:40 disorder in the final least-squares, while refinement of those bonded to C86        Figure S32. 13    THF + THFd 8 Figure S37. 13

Computational Details
DFT calculations were run with Gaussian 09 (Revision D.01). 10 Ru, Mg, Zn and P centers were described with the Stuttgart RECPs and associated basis sets 11 and 6-31G** basis sets were used for all other atoms. 12 A set of d-orbital polarization functions was also added to P (d =0.387) 13 and together this combination is termed BS1. Optimizations employed the BP86 functional 14 Table S8). The effect of dispersion was also considered with Grimme's D3 parameter set with Becke-Johnson damping 24 Figure S49.