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

Preparation and characterization of novel alkyl alkylidyne (Me 3 SiCH 2 )3W(≡CSiMe 3 )(PMe3) (la) and bis-alkylidene (Me3SiCH2)2W(=CHSiMe3) 2 (PMe3) (1b) and studies of the exchange between alkylidyne la and bis-alkylidene 1b are reported. An adduct between PMe 3 and alkyl alkylidyne (Me 3 SiCH 2 ) 3 W≡ CSiMe 3 (2a), (Me3SiCH2)3W(≡CSiMe 3 )(PMe3) (la), 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 la ⇄ 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: ΔH° = -1.8(0.5) kcal/mol and ΔS° = -1.5(1.7) eu. The α-H exchange between la 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 (la → 1b) and ΔH ‡ = 18.0(1.3) kcal/mol and ΔS ‡ = -21(4) eu for the reverse reaction (1b → 1a). An adduct between (Me 3 SiCH 2 ) 3 W≡CSiMe 3 and PMe 2 Ph, (Me3SiCH2) 3 W(≡CSiMe 3 )(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 [K eq ' = 4.65(0.11) at 303 K] than the la ⇄ 1b equilibrium. The forward 3a → 3b conversion in the PMe 2 Ph complexes is slower than the la → 1b conversion at 303 K, whereas the reverse 3b → 3a conversion is slightly faster than the 1b → la conversion.