SYNTHESIS, STRUCTURE, AND REDOX PROPERTIES OF THE MOLYBDENUM(VI) COMPLEXES [MO(ETA(5)-C5PH4R)O2X] (R = 2,5-DIMETHOXYPHENYL, X = BR OR ALKOXIDE)

The first high-oxidation-state, molybdenum complexes with a pentaarylc yclopentadienyl ligand have been prepared. Oxidation of [Mo(C5Ph4R)(CO )(3)Br] (R = 2,5 dimethoxyphenyl) with dioxygen in boiling toluene pro vides a high yield route to the 16-electron, molybdenum(VI) complex [M o(C5Ph4R)O2Br]. Reactions of this complex with aliphatic alcohols (R ' ' OH), but not with phenols, afforded the alkoxide complexes [Mo(C5Ph4 R)O-2(OR '')]. The complexes are unstable to hydrolysis; the cyclopent adiene C5Ph4RH is isolated from reactions with water. Crystal structur e analyses of the cyclopentadiene C5Ph4RH and of [Mo(C5Ph4R)O-2(OMe)] were made. The orientation of the dimethoxyphenyl substituent, with th e o-methoxy group directed either towards or away from the metal centr e, leads to proximal and distal rotamers Mr each complex. The proximal rotamer of [Mo(C5Ph4R)O-2(OMe)] crystallises. The NMR spectra of the complexes show peaks for the proximal and,distal rotamers. A variable- temperature H-1 NMR study of [Mo(C5Ph4R)O2Br] allowed calculation of t he rotational barrier for the dimethoxyphenyl substituent: Delta G(dou ble dagger) = 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 c a. -1.55 V.