Molybdenum(VI) Oxosulfato Complexes in MoO₃–K₂S₂O₇–K₂SO₄ Molten Mixtures: Stoichiometry, Vibrational Properties, and Molecular Structures

Abstract

The structural and vibrational properties of molybdenum­(VI) oxosulfato complexes formed in MoO₃–K₂S₂O₇ and MoO₃–K₂S₂O₇–K₂SO₄ molten mixtures under an O₂ atmosphere and static equilibrium conditions were studied by Raman spectroscopy at temperatures of 400–640 °C. The corresponding composition effects were explored in the <i>X</i>_MoO₃⁰ = 0–0.5 range. MoO₃ undergoes a dissolution reaction in molten K₂S₂O₇, and the Raman spectra point to the formation of molybdenum­(VI) oxosulfato complexes. The MoO stretching region of the Raman spectrum provides sound evidence for the occurrence of a dioxo Mo­(O)₂ configuration as a core. The stoichiometry of the dissolution reaction MoO₃ + <i>n</i>S₂O₇^2– → C^2<i>n</i>– was inferred by exploiting the Raman band intensities, and it was found that <i>n</i> = 1. Therefore, depending on the MoO₃ content, monomeric MoO₂(SO₄)₂^2– and/or associated [MoO₂(SO₄)₂]_<i>m</i>^2<i>m</i>– complexes are formed in the binary MoO₃–K₂S₂O₇ molten system, and pertinent structural models are proposed in full consistency with the Raman data. A 6-fold coordination around Mo is inferred. Adjacent MoO₂²⁺ cores are linked by bidentate bridging sulfates. With increasing temperature at concentrated melts (i.e., high <i>X</i>_MoO₃⁰), the observed spectral changes can be explained by partial dissociation of [MoO₂(SO₄)₂]_<i>m</i>^2<i>m</i>– by detachment of S₂O₇^2– and formation of a MoOMo bridge. Addition of K₂SO₄ in MoO₃–K₂S₂O₇ results in a “follow-up” reaction and formation of MoO₂(SO₄)₃^4– and/or associated [MoO₂(SO₄)₃]_<i>m</i>^4<i>m</i>– complexes in the ternary MoO₃–K₂S₂O₇–K₂SO₄ molten system. The 6-fold Mo coordination comprises two oxide ligands and four O atoms linking to coordinated sulfate groups in various environments of reduced symmetry. The most characteristic Raman bands for the molybdenum­(VI) oxosulfato complexes pertain to the Mo­(O)₂ stretching modes: (1) at 957 (polarized) and 918 (depolarized) cm^–1 for the ν_s and ν_as Mo­(O)₂ modes of MoO₂(SO₄)₂^2– and [MoO₂(SO₄)₂]_<i>m</i>^2<i>m</i>– and (2) at 935 (polarized) and 895 (depolarized) cm^–1 for the respective modes of MoO₂(SO₄)₃^4– and [MoO₂(SO₄)₃]_<i>m</i>^4<i>m</i>–. The results were tested and found to be in accordance with ab initio quantum chemical calculations carried out on [MoO₂(SO₄)₃]^4– and [{MoO₂}₂(SO₄)₄(μ-SO₄)₂]^8– ions, in assumed isolated gaseous free states, at the DFT/B3LYP (HF) level and with the 3-21G basis set. The calculations included determination of vibrational infrared and Raman spectra, by use of force constants in the Gaussian 03W program