Tungsten alkyl alkylidyne and bis-alkylidene complexes. Preparation and kinetic and thermodynamic studies of their unusual exchanges

Preparation and characterization of novel alkyl alkylidyne (Me3SiCH2)(3)W(equivalent to CSiMe3)(PMe3) (1a) and bis-alkylidene (Me3SiCH2)(2)W(=CHSiMe3)(2)(PMe3) (1b) and studies of the exchange between alkylidyne la and bis-alkylidene 1b are reported. An adduct between PMe3 and alkyl alkylidyne (Me3SiCH2)(3)W CSiMe3 (2a), (Me3SiCH2)(3)W( CSiMe3)(PMe3) (1a), was found to undergo a rare reversible transformation to its bis-alkylidene tautomer (Me3SiCH2)(2)W(=CHSiMe3)(2)(PMe3) (1b). The X-ray crystal structure of 1b has been determined. The bis-alkylidene tautomer 1b is favored in the 1a reversible arrow 1b equilibrium with K-eq 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: Delta H degrees = -1.8(0.5) kcal/mol and Delta S degrees= -1.5(1.7) eu. The (alpha-H exchange between 1a and 1b follows first-order reversible kinetics. The activation parameters are Delta H-double dagger = 16.2(1.2) kcal/mol and Delta S: = -22(4) eu for the forward reaction (1a -> 1b) and Delta H-double dagger = 18.0(1.3) kcal/mol and Delta S-double dagger -21(4) eu for the reverse reaction (1b -> 1a). An adduct between (Me3SiCH2)(3)W CSiMe3 and PMe2Ph, (Me3SiCH2)(3)W( CSiMC3)(PMe2Ph) (3a), was found to undergo a similar reversible transformation to its bis-alkylidene tautomer (Me3SiCH2)(2)W(=CHSiMe3)(2)(PMe7Ph) (3b). The 3a reversible arrow 3b equilibrium is shifted more to the alkyl alkylidyne adduct 3a [K-eq' = 4.65(0.11) at 303 K] than the 1a reversible arrow 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 lb - la conversion.