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

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 MoVI–MoV 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.