Aqueous Chemistry of the Metallocene [Cp<sub>2</sub>MoCl<sub>2</sub>]BF<sub>4</sub>: Evidence of Autocatalytic Molybdenum(V)
Reduction in Water
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Abstract
The
aqueous chemistry of the air-stable Mo(V) metallocene [Cp<sub>2</sub>MoCl<sub>2</sub>]BF<sub>4</sub> (<b>1</b>) yields an
unexpected autocatalytic reduction when water is added to an acetonitrile
solution of <b>1</b>. While <b>1</b> yields the expected
stable Cp–Mo ligation and rapid chloride hydrolysis in water,
a Mo(V) → Mo(IV) reduction to the metallocene Cp<sub>2</sub>MoCl<sub>2</sub> (<b>2</b>) 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 <b>2</b>. No reaction occurs when <b>1</b> and <b>2</b> 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 <b>1</b> to <b>2</b>, 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