Oxidation Chemistry of Axially Protected Mo2 and W2 Quadruply Bonded Compounds

Reported is a facile, high-yielding one-pot synthesis of the quadruply bonded ditungsten (II,II) compound W(2)(dpa)(4) (1) (dpa = 2,2'-dipyridylamide), which was obtained from W(CO)(6) at high temperature in naphthalene. A similar reaction in 1,2-dichlorobenzene furnished a ditungsten (III, III) species as the major product that was crystallized as [W(2)(dpa)(3)Cl(2)][BPh(4)] (3). The [W(2)(dpa)(3)Cl(2)](+) cation is better prepared by oxidation of 1 with SO(2)Cl(2). Compound 1 was characterized by X-ray crystallography and cyclic-voltammetry, and is compared with its earlier reported molybdenum analogue, Mo(2)(dpa)(4) (2). One-electron oxidation products of 1 and 2, [W(2)(dpa)(4)][BPh(4)] (1BPh(4)) and [Mo(2)(dpa)(4)][BPh(4)] (2BPh(4)), respectively, have also been synthesized. The crystallographically determined metal-metal distances of 2.23 A and 2.14 A in 1BPh(4) and 2BPh(4), respectively, are in agreement with metal-metal bond orders of 3.5. Unlike most previously reported Mo(2)(5+) and W(2)(5+) compounds, the primary coordination spheres around the M(2)-units in 1/1BPh(4) and 2/2BPh(4) remain unchanged upon one-electron oxidation, because the tridentate dpa ligand hinders axial coordination of exogenous ligands.