ja0c01062_si_001.pdf (4.4 MB)
Zn-Promoted C–H Reductive Elimination and H2 Activation via a Dual Unsaturated Heterobimetallic Ru–Zn Intermediate
journal contribution
posted on 2020-03-17, 19:15 authored by Fedor M. Miloserdov, Nasir A. Rajabi, John P. Lowe, Mary F. Mahon, Stuart A. Macgregor, Michael K. WhittleseyReaction
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.
