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

Abstract

The kinetics of the reaction of Mo(VI)(S2C6H4)3 with organic phosphines to produce the anionic Mo(V) complex, Mo(V)(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 Mo(VI)(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, Mo(VI)(S2C6H4)3(HO)-. This is followed by the concerted abstraction of 2e-, H+ by the Mo(VI) center to give Mo(IV)(S2C6H4)3(2-), H+, and the corresponding phosphine oxide. However, this Mo(IV) complex product is oxidized rapidly to Mo(V)(S2C6H4)3- via comproportionation with unreacted Mo(VI)(S2C6H4)3. The Mo(V) complex thus formed can be oxidized to the starting Mo(VI) complex upon admission of O2. Consequently, Mo(VI)(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(VI) complex reduction increases as does the phosphine basicity: (p-CH3C6H4)3P > (C6H5)3P > (p-ClC6H4)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 Mo(VI)(S2C6H4)3 as electron acceptor.

The different kinetic and mechanistic behaviors of molybdenum and tungsten in the reduction of tris(benzene‐1,2‐dithiolato)Mo(VI) and W(VI) complexes by ascorbic acid in aqueous media

“…Therefore, the metal center at Mo and W tris(dithiolenes) is described as a V state rather than a VI oxidation state, preventing coordination sphere expansion. On the other hand, the cleavage of the W-S bond is entirely consistent with the OAT reactivity exhibited by these complexes toward oxo acceptors as organic phosphines [48] or sulfite in aqueous media [47].…”

“…However, very recently, we have shown that the reduction of these complexes by using some chemical reductants in the presence of water cannot be explained by simple outer-sphere electron transfer reactions. For instance, [Mo(S 2 C 6 H 4 ) 3 ] oxidizes sulfite to sulfate [47] and organic phosphines to their corresponding oxides [48], through a mechanism involving probably the initial coordination of water to the molybdenum center, followed by the oxygen atom transfer (OAT) to the substrate to give its oxidized form.…”

Lucigenin-Tris(2-carboxyethyl)phosphine Chemiluminescence for Selective and Sensitive Detection of TCEP, Superoxide Dismutase, Mercury(II), and Dopamine

“…The nucleophilic attack of H 2 O or OH – toward the phosphorus atom of R 3 P •+ generates phosphoranyl radical species (R 3 P • –OH), which further undergo several radical chain processes (including O 2 •– and H 2 O 2 ) and subsequently gives the R 3 PO as the final stable product (eq 2 in Scheme B) . Previous reports of unusual reducing properties of R 3 P by utilizing H 2 O or OH – as a source of oxygen rather than O 2 further supports this mechanism. Luc •+ then react with R 3 P • –OH radicals to generate the excited N -methylacridone (emitter) and R 3 PO.…”

“…•− and H 2 O 2 ) and subsequently gives the R 3 P O as the final stable product (eq 2 in Scheme 1B). 52 Previous reports of unusual reducing properties of R 3 P by utilizing H 2 O or OH − as a source of oxygen rather than O 2 54 further supports this mechanism. Luc •+ then react with R 3 P • −OH radicals to generate the excited N-methylacridone (emitter) and R 3 PO.…”

DFT Study on the Interaction of Tris(benzene-1,2-dithiolato)molybdenum Complex with Water. A Hydrolysis Mechanism Involving a Feasible Seven-Coordinate Aquomolybdenum Intermediate

“…Perhaps the most amazing aspect of tris-dithiolato chemistry is the reactivity toward water which is not expected to be relevant due to the stability of the Mo–S bond and the electronic structure of tris-dithiolato complexes. However, oxygen exchange between water and phosphines or sulfite mediated by [Mo­(C 6 H 4 S 2 ) 3 ] has been reported. , The mechanism is supposed to involve initial coordination of water to the molybdenum–dithiolato species . The oxo-transfer activity of neutral tris-dithiolato complexes is difficult to understand based only on the d 1 , or more probably d 2 , electronic structure of metal center, which would make the postulated initial formation of a heptacoordinated aquo tris-dithiolato intermediate an unlikely event .…”