Electrochemical oxidation of Mo(CO)4(LL) and Mo(CO)3(LL)(CH3CN): Generation, infrared characterization, and reactivity of [Mo(CO)4(LL)]+ and [Mo(CO)3(LL)(CH3CN)]+ (LL = 2,2′-bipyridine, 1,10-phenanthroline and related ligands)

Mo(CO)4(LL) complexes, where LL = polypyridyl ligands such as 2,2′-bipyridine and 1,10-phenanthroline, undergo quasi-reversible, one-electron oxidations in methylene chloride yielding the corresponding radical cations, [Mo(CO)4(LL)]+. These electrogenerated species undergo rapid ligand substitution in the presence of acetonitrile, yielding [Mo(CO)3(LL)(CH3CN)]+; rate constants for these substitutions were measured using chronocoulometry and were found to be influenced by the steric and electronic properties of the polypyridyl ligands. [Mo(CO)3(LL)(CH3CN)]+ radical cations, which could also be generated by reversible oxidation of Mo(CO)3(LL)(CH3CN) in acetonitrile, can be irreversibly oxidized yielding [Mo(CO)3(LL)(CH3CN)2]2+ after coordination by an additional acetonitrile. Infrared spectroelectrochemical experiments indicate the radical cations undergo ligand-induced net disproportionations that follow first-order kinetics in acetonitrile, ultimately yielding the corresponding Mo(CO)4(LL) and [Mo(CO)2(LL)(CH3CN)3]2+ species. Rate constants for the net disproportionation of [Mo(CO)3(LL)(CH3CN)]+ and the carbonyl substitution reaction of [Mo(CO)3(LL)(CH3CN)2]2+ were measured. Thin-layer bulk oxidation studies also provided infrared characterization data of [Mo(CO)4(ncp)]+ (ncp = neocuproine), [Mo(CO)3(LL)(CH3CN)]+, [Mo(CO)3(LL)(CH3CN)2]2+ and [Mo(CO)2(LL)(CH3CN)3]2+ complexes.

Graphical abstractThe reaction pathways followed by electrogenerated [Mo(CO)4(LL)]+ and [Mo(CO)3(LL)(CH3CN)]+ were elucidated using electrochemical and infrared spectroelectrochemical techniques. Trends in rate constants are related to the steric and electronic properties of the polypyridyl ligands. Several novel molybdenum(I) radical species, including [Mo(CO)4(ncp)]+, and molybdenum(II) oxidation products, [Mo(CO)3(LL)(CH3CN)2]2+ and [Mo(CO)2(LL)(CH3CN)3]2+, were spectroscopically characterized.Figure optionsDownload full-size imageDownload as PowerPoint slide