Synthesis, structure, and redox properties of the molybdenum(VI) complexes [Mo(δ5-C5Ph4R)O2X] (R = 2,5-dimethoxyphenyl; X = Br or alkoxide)

Abstract

The first high-oxidation-state, molybdenum complexes with a pentaarylcyclopentadienyl ligand have been prepared. Oxidation of [Mo(C₅Ph₄R)(CO)₃Br] (R = 2,5-dimethoxyphenyl) with dioxygen in boiling toluene provides a high yield route to the 16-electron, molybdenum(VI) complex [Mo(C₅Ph₄R)O₂Br]. Reactions of this complex with aliphatic alcohols (R″OH), but not with phenols, afforded the alkoxide complexes [Mo(C₅Ph₄R)O₂(OR″)]. The complexes are unstable to hydrolysis; the cyclopentadiene C₅Ph₄RH is isolated from reactions with water. Crystal structure analyses of the cyclopentadiene C₅Ph₄RH and of [Mo(C₅Ph₄R)O₂(OMe)] were made. The orientation of the dimethoxyphenyl substituent, with the o-methoxy group directed either towards or away from the metal centre, leads to proximal and distal rotamers for each complex. The proximal rotamer of [Mo(C₅Ph₄R)O₂(OMe)] crystallises. The NMR spectra of the complexes show peaks for the proximal and distal rotamers. A variable-temperature ¹H NMR study of [Mo(C₅Ph₄R)O₂Br] allowed calculation of the rotational barrier for the dimethoxyphenyl substituent: ΔG^‡ = 68.6 ± 0.8 kJ mol^–1. A Mo^VI–Mo^V couple is observed for [Mo(C₅Ph₄R)O₂Br] at –0.55 V (vs. the ferrocene–ferrocenium couple), and the corresponding couples for the alkoxide complexes occur at ca. –1.55 V.