Combined kinetic and DFT studies on the stabilization of the pyramidal form of H3PO2 at the heterometal site of [Mo3M′S4(H2O)10]4+clusters (M′ = Pd, Ni)

Abstract

Kinetic and DFT studies have been carried out on the reaction of the [Mo₃M′S₄(H₂O)₁₀]⁴⁺clusters (M′ = Pd, Ni) with H₃PO₂ to form the [Mo₃M′(pyr-H₃PO₂)S₄(H₂O)₉]⁴⁺ complexes, in which the rare pyramidal form of H₃PO₂ is stabilized by coordination to the M′ site of the clusters. The reaction proceeds with biphasic kinetics, both steps showing a first order dependence with respect to H₃PO₂. These results are interpreted in terms of a mechanism that involves an initial substitution step in which one tetrahedral H₃PO₂ molecule coordinates to M′ through the oxygen atom of the PO bond, followed by a second step that consists in tautomerization of coordinated H₃PO₂ assisted by a second H₃PO₂ molecule. DFT studies have been carried out to obtain information on the details of both kinetic steps, the major finding being that the role of the additional H₃PO₂ molecule in the second step consists in catalysing a hydrogen shift from phosphorus to oxygen in O-coordinated H₃PO₂, which is made possible by its capability of accepting a proton from P–H to form H₄PO₂⁺ and then transfer it to the oxygen. DFT studies have been also carried out on the reaction at the Mo centres to understand the reasons that make these metal centres ineffective for promoting tautomerization.