Unusual Oxidation of Phosphines Employing Water as the Oxygen Atom Source and Tris(benzene-1 ,2-dithiolate)molybdenum(Vl) as the Oxidant. A Functional Molybdenum Hydroxylase Analogue System.

The kinetics of the reaction of MoVI(S2C6H4)3 with organic phosphines to produce the anionic Mo(V) complex, MoV(S2C6H4)3, and phosphine oxide have been investigated. Reaction rates, monitored by UV-vis stopped-flow spectrophotometry, were studied in THF/H2O media as a function of the concentration of phosphine, molybdenum complex, pH, and water concentration. The reaction exhibits pH-dependent phosphine saturation kinetics and is first-order in complex concentration. The water concentration strongly enhances the reaction rate, which is consistent with the formation of MoVI(S2C6H4)3(H2O) adduct as a crucial intermediate. The observed pH dependence of the reaction rate would arise from the distribution between acid and basic forms of this adduct. Apparently, the electrophilic attack by the phosphine at the oxygen requires the coordinated water to be in the unprotonated hydroxide form, MoVI(S2C6H4)3(HO)-. This is followed by the concerted abstraction of 2e-, H+ by the Mo(Vl) center to give MoVI(S2C6H4)32-, H+, and the corresponding phosphine oxide. However, this Mo(IV) complex product is oxidized rapidly to MoV(S2C6H4)3 via comproportionation with unreacted MoVI(S2C6H4)3. The Mo(V) complex thus formed can be oxidized to the starting Mo(Vl) complex upon admission of O2. Consequently, MoVI(S2C6H4)3 is a catalyst for the autoxidation of phosphines in the presence of water. Additionally, there was a detectable variation in the reactivity for a series of tertiary phosphines. The rate of Mo(Vl) complex reduction increases as does the phosphine basicity: (p-CH3C6H4)3P > (C6H5)3P > (p-CIC6H4)3P. Oxygen isotope tracing confirms that water rather than dioxygen is the source of the oxygen atom which is transferred to the phosphine. Such reactivity parallels oxidase activity of xanthine enzyme with phosphine as oxygen atom acceptor and MoVI(S2C6H4)3 as electron acceptor. [ABSTRACT FROM AUTHOR]