Kinetic, DFT and TD-DFT studies on the mechanism of stabilization of pyramidal H3PO3 at the [Mo3M′S4(H2O)10]4+clusters (M′ = Pd, Ni)

Abstract

The kinetics of reaction between the [Mo₃M′S₄(H₂O)₁₀]⁴⁺ clusters (M′ = Pd, Ni) and H₃PO₃ has been studied in 4.0 M Hpts/Lipts (pts⁻ = p-toluenesulfonate). For both complexes there is an initial kinetic step with small absorbance changes that corresponds to substitution of the water coordinated to Pd by a molecule of tetrahedral H₃PO₃. For the Pd complex, tautomerization of H₃PO₃ occurs in a slower kinetic step with much larger absorbance changes; it leads to formation of [Mo₃Pd(pyr-H₃PO₃)S₄(H₂O)₉]⁴⁺ in which H₃PO₃ adopts a pyramidal structure, but the process is not as favored as for H₃PO₂. The kinetics of this second step is independent of the concentration of H₃PO₃ but dependent on the concentration of Hpts on the supporting electrolyte. For the Ni complex, the second step is severely hindered and its kinetics could not be studied. DFT calculations indicate that tautomerization of H₃PO₃ is expected to be less favoured than that of H₃PO₂, both processes being less favored at the Ni cluster than at its Pd analogue. With regard to the tautomerization mechanism, the calculations indicate that the mechanism previously proposed for H₃PO₂ can be the same for H₃PO₃, although the initial H-shift can also occur through a protonation–deprotonation sequence with participation of external protons instead of a second molecule of the phosphorus acid. TD-DFT studies have been also carried out to understand the similarity between the spectra of the starting complex and the reaction intermediate formed in the first kinetic step as well as the large spectral changes associated to the tautomerization process. Although it contains contributions from several transitions, the intense band observed for clusters containing coordinated pyr-H₃PO₃ involves essentially ligand-to-metal charge-transfer (LMCT) transitions.