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

Abstract

Reaction of [Ru(PPh 3 ) 3 HCl] with LiCH 2 TMS, MgMe 2 , and ZnMe 2 proceeds with chloride abstraction and alkane elimination to form the biscyclometalated derivatives [Ru(PPh 3 )(C 6 H 4 PPh 2 ) 2 H][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 LiCH 2 TMS yields [Ru(PPh 3 )(C 6 H 4 PPh 2 ) 2 H][Li(12crown-4) 2 ] (2). These four complexes demonstrate increasing interaction between M′ and the hydride ligand in the [Ru(PPh 3 )(C 6 H 4 PPh 2 ) 2 H] − 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-(PPh 3 ) 2 (C 6 H 4 PPh 2 )(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(PPh 3 ) 2 (C 6 H 4 PPh 2 )(ZnMe) 2 ][BAr F 4 ] ( 6) can be trapped and structurally characterized. 4 reacts with H 2 at −40 °C to form [Ru(PPh 3 ) 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.

Metal‐only Lewis Pairs of Rhodium with s, p and d‐Block Metals

“…The occupancies of the donor NBOs are 1.75 and 1.84 electrons for the Rh LP and the Al LV,a valence sp 3 hybrid, respectively.T he RhÀMi nteractions in the adducts with the two tetrylenes, 1•SnCl 2 and 1•GeCl 2 ,w hich were assigned the highest covalent character according to the AIM analysis, are described very differently by the NBO analysis:w hereas the donor-acceptor description is used for the former,o ne bonding NBO was localized between Rh and Ge in the latter (Figure 7). Close inspectiono ft he NLMO associated with the donor NBO of the d (Rh) !p (Sn) interaction in 1•SnCl 2 (DE ij = 55.2 kcal mol À1 )r eveals that it has the highestm ixingo f acceptorm etal orbitals of all analogous NLMOs in this study, with 81.9 %R ha nd 15 %S nc omposition, [86] whereas the NLMO associated with the s (Rh-Ge) NBO of 1•GeCl 2 has an even higher mixing of Ge orbitals, although it is heavily weighted towards the Rh atom:7 1% Rh (sd 2 )a nd 23 %G e( p), with about 2% mixingf rom each Pa tom. This can be compared with the s (Rh-H) NBO of 2,w hich has about 55 %R hc haracter and 45 %H character.T he bonding in this case is pure covalent from the localized orbital perspective.N evertheless, the s (Rh-P) !p (Sn) interaction is also dominant in 1•SnCl 2 , [31b] with DE ij = 62.8 kcal mol À1 .T he involvement of the RhÀPb onds in the RhÀGe interaction of 1•GeCl 2 is described in terms of donor-acceptor interactions: s (Rh-Ge) !s* (Rh-P) and s* (Rh-Ge) !…”

“…The Rh−M interactions in the adducts with the two tetrylenes, 1⋅SnCl 2 and 1⋅GeCl 2 , which were assigned the highest covalent character according to the AIM analysis, are described very differently by the NBO analysis: whereas the donor–acceptor description is used for the former, one bonding NBO was localized between Rh and Ge in the latter (Figure 7). Close inspection of the NLMO associated with the donor NBO of the d (Rh) → p (Sn) interaction in 1⋅SnCl 2 (Δ E ij =55.2 kcal mol −1 ) reveals that it has the highest mixing of acceptor metal orbitals of all analogous NLMOs in this study, with 81.9 % Rh and 15 % Sn composition, [86] whereas the NLMO associated with the σ (Rh‐Ge) NBO of 1⋅GeCl 2 has an even higher mixing of Ge orbitals, although it is heavily weighted towards the Rh atom: 71 % Rh (s d 2 ) and 23 % Ge ( p ), with about 2 % mixing from each P atom. This can be compared with the σ (Rh‐H) NBO of 2 , which has about 55 % Rh character and 45 % H character.…”

Bonding and Reactivity of a Pair of Neutral and Cationic Heterobimetallic RuZn2 Complexes

“…In this particular case, both Ru and Zn centers are coordinatively unsaturated, and this “dual unsaturation” allows them to act cooperatively in the stoichiometric activation of H 2 to give IV . 12 We have interpreted Ru–Zn bonding within complex III and other related Ru–Zn complexes 13 , 14 in terms of a donor–acceptor interaction between a Ru(0) metal center and Z-type Zn-based acceptor ligands.…”

Palladium‐Catalysed C−H Bond Zincation of Arenes: Scope, Mechanism, and the Role of Heterometallic Intermediates

“…We show for the first time that heterometallic Pd—Zn compounds can engage in a series of steps required for catalytic turnover (oxidative addition, ligand exchange, reductive coupling). While there is pioneering work demonstrating some of these steps in isolation, [20, 21] this is the first time they have been integrated into an efficient catalytic cycle.…”

“…We have conducted adetailed analysis of the mechanism of C À Hzincation, which shows that heterometallic complexes involving both Pd and Zn play av ital role in the catalytic cycle.W es how for the first time that heterometallic Pd-Zn compounds can engage in as eries of steps required for catalytic turnover (oxidative addition, ligand exchange, reductive coupling). While there is pioneering work demonstrating some of these steps in isolation, [20,21] this is the first time they have been integrated into an efficient catalytic cycle.…”