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 W2(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 [W2(dpa)3Cl2][BPh4] (3). The [W2(dpa)3Cl2]+ cation is better prepared by oxidation of 1 with SO2Cl2. Compound 1 was characterized by X-ray crystallography and cyclic-voltammetry, and is compared with its earlier reported molybdenum analogue, Mo2(dpa)4 (2). One-electron oxidation products of 1 and 2, [W2(dpa)4][BPh4] (1BPh4) and [Mo2(dpa)4][BPh4] (2BPh4), respectively, have also been synthesized. The crystallographically determined metal−metal distances of 2.23 Å and 2.14 Å in 1BPh4 and 2BPh4, respectively, are in agreement with metal−metal bond orders of 3.5. Unlike most previously reported Mo25+ and W25+ compounds, the primary coordination spheres around the M2-units in 1/1BPh4 and 2/2BPh4 remain unchanged upon one-electron oxidation, because the tridentate dpa ligand hinders axial coordination of exogenous ligands.