SYNTHESIS AND REACTIVITY OF ETA-6-ARENE DERIVATIVES OF NIOBIUM(II), NIOBIUM(I), AND NIOBIUM(O)

The primary reduction products in the Al/AlX3/NbX5/arene system, namely Nb(eta-6-arene)(AlX4)2, X = Cl, arene = benzene, hexamethylbenzene; X = Br, arene = hexamethylbenzene, have been isolated. Tetrahydrofuran at room temperature reacts with Nb(eta-6-hexamethylbenzene)(AlBr4)2, to give the aluminium-free complex Nb2(eta-6-hexamethylbenzene)2Br4, whose crystal structure was determined by X-ray diffraction methods. In the presence of tetrahydrofuran or dimethoxyethane, the eta-6-mesitylene-tetrahaloaluminato complexes are further reduced by aluminium to bis-mesityleneniobium(0), Nb(mes)2, which undergoes: a) disproportionation with CO to [Nb(mes)2(CO)]+[Nb(CO)6]-, b) oxidation by V(CO)6 or (4,4'-dimethyl-2,2'-dipyridyl)(BPh4)2 under carbon monoxide to [Nb(mes)2(CO)]X, X = [V(CO)6]- or BPh4-. The structure of [Nb(mes)2(CO)] BPh4 has been determined by X-ray diffraction methods. In the presence of CO, Nb(mes)2 is reduced by CoCp*2 [Cp* = eta-5-C5(CH3)5] to [Nb(CO)6]-, which was isolated as its CoCp*2+ derivative; it efficiently deoxygenates CO2 to CO in the presence of carbon monoxide acceptors such as the [Nb(mes)2(THF)]+ cation.