Aqueous Chemistry of the Metallocene [Cp₂MoCl₂]BF₄: Evidence of Autocatalytic Molybdenum(V) Reduction in Water

The aqueous chemistry of the air-stable Mo­(V) metallocene [Cp2MoCl2]­BF4 (1) yields an unexpected autocatalytic reduction when water is added to an acetonitrile solution of 1. While 1 yields the expected stable Cp–Mo ligation and rapid chloride hydrolysis in water, a Mo­(V) → Mo­(IV) reduction to the metallocene Cp2MoCl2 (2) was evident. Under acidic conditions (pH ∼2) or trace amounts of water this reduction was slow enough to be monitored spectroscopically, and it is shown to be autocatalytic in aqueous 2. No reaction occurs when 1 and 2 are in the dichloride form in acetonitrile (i.e., no water). It is hypothesized that the added water serves two roles. First it initially reduces a small population of 1 to 2, and then as the aquated Mo­(IV) metallocene, it catalyzes the reduction of the remaining Mo­(V) in water. This is the first aqueous investigation of the Mo­(V) metallocene, and it shows a novel and unprecedented autocatalytic reduction that is mediated by water.