An unusual exchange between alkylidyne alkyl and bis(alkylidene) tungsten complexes promoted by phosphine coordination: kinetic, thermodynamic, and theoretical studies

Laurel A Morton; Xin-Hao Zhang; Ruitao Wang; Zhenyang Lin; Yun-Dong Wu; Zi-Ling Xue

doi:10.1021/ja0467263

An unusual exchange between alkylidyne alkyl and bis(alkylidene) tungsten complexes promoted by phosphine coordination: kinetic, thermodynamic, and theoretical studies

J Am Chem Soc. 2004 Aug 25;126(33):10208-9. doi: 10.1021/ja0467263.

Authors

Laurel A Morton¹, Xin-Hao Zhang, Ruitao Wang, Zhenyang Lin, Yun-Dong Wu, Zi-Ling Xue

Affiliation

¹ Department of Chemistry, The University of Tennessee, Knoxville, TN 37996, USA.

PMID: 15315409
DOI: 10.1021/ja0467263

Abstract

d0 Tungsten alkylidyne alkyl complex (Me3SiCH2)3W(CSiMe3)(PMe3) (4a) was found to undergo a rare, PMe3-promoted exchange with its bis(alkylidene) tautomer (Me3SiCH2)2W(=CHSiMe3)2(PMe3) (4b). Thermodynamic studies of the exchange showed that 4b is favored and gave Keq and the enthalpy and entropy of the equilibrium: DeltaH degrees = -1.8(0.5) kcal/mol and DeltaS degrees = -1.5(1.7) eu. Kinetic studies of the alpha-H migration between 4a and 4b by variable-temperature NMR gave rate constants k1 and k-1 for the reversible reactions and activation enthalpies and entropies: DeltaH1 = 16.2(1.2) kcal/mol and DeltaS1 = -22.3(4.0) eu for the forward reaction (4a --> 4b); DeltaH2 = 18.0(1.3) kcal/mol and DeltaS2 = -20.9(4.3) eu for the reverse reaction (4b --> 4a). Ab initio calculations at the B3LYP level revealed that PMe3 binds with the bis(alkylidene) tautomer relatively more strongly than with the alkylidyne tautomer and thus stabilizes the bis(alkylidene) tautomer.