How to Make a Major Shift in a Redox Potential:  Ligand Control of the Oxidation State of Dimolybdenum Units

A compound reported earlier (<i>Polyhedron</i> <b>1989</b>, <i>8</i>, 2339) as (Bu<i><sup>n</sup></i><sup></sup><sub>4</sub>N)<sub>2</sub><i>H</i><sub>2</sub>{Mo<sub>2</sub>[Mo(CO)<sub>4</sub>(PhPO<sub>2</sub>)<sub>2</sub>]<sub>2</sub>} has been reexamined. We find that the hydrogen atoms in this formula are not present. Therefore, the complex must be considered as having a central triply bonded Mo<sub>2</sub><sup>6+</sup> unit, instead of a quadruply bonded Mo<sub>2</sub><sup>4+</sup> unit. Our conclusion is based on a variety of experimental evidence, including X-ray crystal structures of four crystal forms, as well as the neutron crystal structure of one. This explains the relatively long Mo−Mo bond lengths found in the range 2.1874(7)−2.2225(7) Å and the absence of a δ → δ* transition in the visible spectrum. From electrochemistry we also find that the diphosphonate ligand has such an exceptional ability to stabilize higher oxidation states that even common solvents such as CH<sub>2</sub>Cl<sub>2</sub> and C<sub>2</sub>H<sub>5</sub>OH readily oxidize the Mo<sub>2</sub><sup>4+</sup> unit that is introduced from the Mo<sub>2</sub>(O<sub>2</sub>CCH<sub>3</sub>)<sub>4</sub> or [Mo<sub>2</sub>(O<sub>2</sub>CCH<sub>3</sub>)<sub>2</sub>(NCCH<sub>3</sub>)<sub>6</sub>](BF<sub>4</sub>)<sub>2</sub> employed in the preparation. The only chemically reversible wave at <i>E</i><sub>1/2</sub> = −1.54 V vs Ag/AgCl corresponds to the reduction process Mo<sub>2</sub><sup>6+</sup> → Mo<sub>2</sub><sup>5+</sup>.