Tungsten Alkyl Alkylidyne and Bis-alkylidene Complexes. Preparation and Kinetic and Thermodynamic Studies of Their Unusual Exchanges

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Abstract

Preparation and characterization of novel alkyl alkylidyne (Me3SiCH2)3W(⋮CSiMe3)(PMe3) (1a) and bis-alkylidene (Me3SiCH2)2W(CHSiMe3)2(PMe3) (1b) and studies of the exchange between alkylidyne 1a and bis-alkylidene 1b are reported. An adduct between PMe3 and alkyl alkylidyne (Me3SiCH2)3W⋮CSiMe3 (2a), (Me3SiCH2)3W(⋮CSiMe3)(PMe3) (1a), was found to undergo a rare reversible transformation to its bis-alkylidene tautomer (Me3SiCH2)2W(CHSiMe3)2(PMe3) (1b). The X-ray crystal structure of 1b has been determined. The bis-alkylidene tautomer 1b is favored in the 1a ⇌ 1b equilibrium with Keq ranging from 12.3(0.2) at 278(1) K to 9.37(0.12) at 303(1) K, giving the thermodynamic parameters for the equilibrium: ΔH° = −1.8(0.5) kcal/mol and ΔS° = −1.5(1.7) eu. The α-H exchange between 1a and 1b follows first-order reversible kinetics. The activation parameters are ΔH⧧ = 16.2(1.2) kcal/mol and ΔS⧧ = −22(4) eu for the forward reaction (1a → 1b) and ΔH⧧ = 18.0(1.3) kcal/mol and ΔS⧧ = −21(4) eu for the reverse reaction (1b → 1a). An adduct between (Me3SiCH2)3W⋮CSiMe3 and PMe2Ph, (Me3SiCH2)3W(⋮CSiMe3)(PMe2Ph) (3a), was found to undergo a similar reversible transformation to its bis-alkylidene tautomer (Me3SiCH2)2W(CHSiMe3)2(PMe2Ph) (3b). The 3a ⇌ 3b equilibrium is shifted more to the alkyl alkylidyne adduct 3a [Keq‘ = 4.65(0.11) at 303 K] than the 1a ⇌ 1b equilibrium. The forward 3a → 3b conversion in the PMe2Ph complexes is slower than the 1a → 1b conversion at 303 K, whereas the reverse 3b → 3a conversion is slightly faster than the 1b → 1a conversion.

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