A tungsten silyl alkylidyne complex and its bis(alkylidene) tautomer. Their interconversion and an unusual silyl migration in their reaction with dioxygen

Bis(alkylidene) complex W(=CH-t-Bu)(2)(CH2-t-Bu)(Si-t-BuPh2) (1a) has been found to be in equilibrium with its alkyl alkylidyne tautomer W( C-t-Bu)(CH2-t-Bu)(2)(Si-t-BuPh2) (1b). Bis(alkylidene) complexes are believed to be intermediates in a-H transfer in alkylidyne complexes W( CSiMe3)(CH2-t-Bu)(3) and W( C-13-t-Bu)(CH2-t-Bu)(3). The current study represents a rare observation of an exchange between a bis(alkylidene) and an alkyl alkylidyne complex. Thermodynamics and kinetics of the exchange 1a reversible arrow 1b have been studied. The equilibrium constant K-eq for the exchange 1a reversible arrow 1b is 3.34(0.05) at 287(1) K, and the thermodynamic parameters of the equilibrium measured are Delta H degrees = -3.8(0.8) kJ/mol, Delta S degrees = -3(3) J/mol(.)K, and Delta G degrees(287K) = -3(2) kJ/mol. The rate constants k(1) and k(-1) were obtained from 2D exchange spectroscopy between 267 and 297 K and are 0.34(0.03) and 0.10(0.01) s(-1), respectively, at 287 K. For the conversion 1a -> 1b, activation enthalpy and entropy are Delta H-1(double dagger) = 75(5) kJ/mol, Delta S-1(double dagger), = 8(25) J/mol(.)K; for the conversion 1b -> 1a, Delta H--1(double dagger) = 78(5) kJ/mol, Delta S--1(double dagger) = 8(25) J/mol(.)K. In the reaction of 1 with O-2, the silyl ligand in 1b was found to undergo an unprecedented migration to the alkylidyne ligand to give the alkylidene oxo complex W(=O)[=C(t-Bu)(Si-t-BuPh2)](CH2-t-Bu)(2) (2). The structure of 2 has been determined. Density functional theory calculations have been conducted for a series of model 1,2-migration reactions. The results suggest that the formation of 2 might be initiated by a 1,2-silyl migration to generate a triplet W-alkylidene intermediate, which is then trapped by O-2.