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

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journal contribution
posted on 17.03.2020, 19:15 by Fedor M. Miloserdov, Nasir A. Rajabi, John P. Lowe, Mary F. Mahon, Stuart A. Macgregor, Michael K. Whittlesey
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.