Phosphonothioate hydrolysis through selective P-S bond scission by molybdenum metallocenes

We present an overview of the phosphonothioate hydrolytic chemistry promoted by molybdenum organometallics as well as new results on solvent effects. The metallocene bis(η ⁵-cyclopentadienyl) molybdenum(IV) dichloride (Cp₂MoCl₂ Cp=η⁵-C ₅H₅) and the methylated analog, (CH₃Cp) ₂MoCl₂ hydrolyzes the compound O,S-diethyl phenylphosphonothioate (DEPP) whose core functional group mimics the neurotoxin VX. This is one of the few examples of phosphonothioate degradation yielding exclusively the desired P-S bond scission under mild aqueous conditions (pH 7, 25°C). The solvent composition affects the monomer-dimer equilibrium of aquated molybdocenes which in turn dictates the rate of DEPP hydrolysis. Kinetic and equilibrium results are presented to show that the monomer is the active species. Moreover, photoelectron spectroscopy data indicate that while the methylated (CH₃Cp)₂MoCl₂ has a higher monomer concentration compared to Cp₂MoCl₂, the former has a diminished electrophilic molybdenum center that is key in phosphonothioate hydrolysis.