Electron-Transfer-Induced Interconversion of Alkyne and Vinylidene Chromium Complexes: A Quantitative Study

Neil G. Connelly*, Maria Cristina Lagunas, Bernhard Metz, William E. Geiger, Michael J. Shaw, Anne L. Rieger, Philip H. Rieger

*Corresponding author for this work

Research output: Contribution to journalArticle

74 Citations (Scopus)

Abstract

UV irradiation of [Cr(CO)3(η-C6Me6)] with Me3SiC=CSiMe3 in THF gives the orange vinylidene complex [Cr(CO)2{η-C=C(SiMe3)2}(η-C6Me6)] (V) which undergoes oxidative isomerization to the alkyne cation [Cr(CO)2-(η-Me3SiC=CSiMe3)(η-C6Me6)]+ (A+), isolable as the orange [PF6]- salt. The temperature dependence (from 228 to 295 K) of the rate of isomerization of [Cr(CO)2{η-C=C(SiMe3)2}(η-C6Me6)]+ (V+) to A+, determined by cyclic voltammetry, gives ΔS+ = -6 ± 13 J mol-1 K-1 and ΔH+ = 61 ± 3 kJ mol-1. The vinylidene cation (V+) has been generated at low temperature, by reacting V with [FeCp2]+, and characterized by ESR spectroscopy. Paramagnetic cation A+ undergoes reductive isomerization to V; the rate of isomerization A to V, determined by cyclic voltammetry (from 279 to 293 K) and spectroelectrochemistry (at 242 K), gives AS* = 84 ± 4 J mol-1 K-1 and ΔH+ = 97 ± 1 kJ mol-1. At temperatures above 233 K, the isomerization of A to V is catalyzed by V+, via the cross reaction V+ + A ⇄ V + A+. The thermodynamic and kinetic parameters are in accord with a mechanism for the isomerization of A to V in which the alkyne ligand slips to η1-coordination prior to the transition state. Accordingly, the isomerization of V+ to A+ resembles the 1,2-sigmatropic shift of a SiMe3 group in a purely organic system.

Original languageEnglish
Pages (from-to)12202-12208
Number of pages7
JournalJournal of the American Chemical Society
Volume117
Issue number49
DOIs
Publication statusPublished - 01 Jan 1995
Externally publishedYes

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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