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

Article abstract of DOI:10.1021/om049031j

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-BuPh ₂) (1b). Bis(alkylidene) complexes are believed to be intermediates in α-H transfer in alkylidyne complexes W(=CSiMe₃)(CH ₂-t-Bu)₃ and W(=¹³C-t-Bu)(CH ₂-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 ? 1b have been studied. The equilibrium constant K_eq, for the exchange 1a ? 1b is 3.34(0.05) at 287(1) K, and the thermodynamic parameters of the equilibrium measured are ?H° = -3.8(0.8) kJ/mol, ?S° = -3(3) J/mol-K, and ?G°_287K = -3(2) kJ/mol. The rate constants k₁ 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 ?H? ₁ = 75(5) kJ/mol, ?S?₁ = 8(25) J/mol-K; for the conversion 1b - 1a, ?H?_-1 = 78(5) kJ/ mol, ?S? _-1 = 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.