Preparation of Tungsten Alkyl Alkylidene Alkylidyne Complexes and Kinetic Studies of Their Formation

摘要

An equilibrium mixture of alkyl alkylidyne W(CH_2SiMe_3)_3(≡ CSiMe_3)(PMe_3) (1a) and its bis-(alkylidene) tautomer W(CH_2SiMe_3)_2(= CHSiMe_3)_2(PMe_3) (1b) has been found to undergo an α-hydrogen abstraction reaction in the presence of PMe_3 to form alkyl alkylidene alkylidyne W(CH_2SiMe_3)(= CHSiMe_3)-(≡ CSiMe_3)(PMe_3)2. In the presence of PMe_3, the formation of 2 follows first-order kinetics, and the observed rate constant was found to be independent of the concentration of PMe_3. The activation parameters for the formation of 2 are ΔH = 28.3(1.7) kcal/mol and ΔS = 3(5) eu. In the presence of PMe_2Ph, an equilibrium mixture of W(CH_2SiMe_3)_3(≡ CSiMe_3)(PMe_2Ph) (3a) and its bis(alkylidene) tautomer W(CH_2SiMe_3)_2 (= CHSiMe_3)_2(PMe_2Ph) (3b) was similarly converted to W(CH_2SiMe_3)(= CHSiMe_3)(≡ CSiMe_3)(PMe_2Ph)_2. The observed rate of this reaction was also independent of the concentration of PMe_2Ph. These observations suggest a pathway in which the tautomeric mixtures 1a,b and 3a,b undergo rate-determining, α-hydrogen abstraction, followed by phosphine coordination, resulting in the formation of the alkyl alkylidene alkylidyne complexes 2 and 4.