Reduction of [ML(alkyne)2(η-C5R‘5)]+ (M = Mo or W, L = MeCN or CO, R‘ = H or Me, C5R‘5 = C5HPh4): Characterization of Radical Intermediates in the Reductive Coupling of Coordinated Alkynes

Abstract

The complexes [M(NCMe)(RC2R)2(η-C5R‘5)]+ (M = Mo or W, R = Me or Ph; R‘ = H or Me, C5R‘5 = C5HPh4) undergo one-electron reduction to [M(NCMe)(RC2R)2(η-C5R‘5)], which rapidly lose acetonitrile to give the 17-electron complexes [M(RC2R)2(η-C5R‘5)]. The stability of [M(RC2R)2(η-C5R‘5)] depends on R and R‘; for R = Ph, the radical is sufficiently stable so that its reduction to the 18-electron anion [M(RC2R)2(η-C5R‘5)]- is detected in the cyclic voltammogram. Chemical reduction of [Mo(NCMe)(PhC2Ph)2(η-C5HPh4)]+ with [Co(η-C5H5)2] gives the air-sensitive solid [Mo(PhC2Ph)2(η-C5HPh4)], characterized as an alkyne-based radical by ESR spectroscopy, which gives [Mo(NCMe)(PhC2Ph)2(η-C5HPh4)]+ when treated with [Fe(η-C5H5)2]+ in acetonitrile. The carbonyl cations [M(CO)(RC2R)2(η-C5R‘5)]+ (M = Mo, W) undergo two sequential one-electron reductions, the first of which is reversible and gives the 19-electron species [M(CO)(RC2R)2(η-C5R‘5)]. The 19-electron radical [Mo(CO)(PhC2Ph)2(η-C5Me5)] has been characterized in solution by IR and ESR spectroscopy.