Molybdenum(VI) oxosulfato complexes in MoO 3-K 2S 2O 7-K 2SO 4 molten mixtures: Stoichiometry, vibrational properties, and molecular structures

Angelos G. Kalampounias, George Tsilomelekis, Rolf W. Berg, Soghomon Boghosian

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Abstract

The structural and vibrational properties of molybdenum(VI) oxosulfato complexes formed in MoO 3-K 2S 2O 7 and MoO 3-K 2S 2O 7-K 2SO 4 molten mixtures under an O 2 atmosphere and static equilibrium conditions were studied by Raman spectroscopy at temperatures of 400-640 °C. The corresponding composition effects were explored in the XMoO 3 0 = 0-0.5 range. MoO 3 undergoes a dissolution reaction in molten K 2S 2O 7, 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 MoO 3 + nS 2O 7 2- →C 2n- was inferred by exploiting the Raman band intensities, and it was found that n = 1. Therefore, depending on the MoO 3 content, monomeric MoO 2(SO 4) 2 2- and/or associated [MoO 2(SO 4) 2] m 2m- complexes are formed in the binary MoO 3-K 2S 2O 7 molten system, and pertinent structural models are proposed in full consistency with the Raman data. A 6-fold coordination around Mo is inferred. Adjacent MoO 2 2+ cores are linked by bidentate bridging sulfates. With increasing temperature at concentrated melts (i.e., high XMoO 30), the observed spectral changes can be explained by partial dissociation of [MoO 2(SO 4) 2] m 2m- by detachment of S 2O 7 2- and formation of a Mo-O-Mo bridge. Addition of K 2SO 4 in MoO 3-K 2S 2O 7 results in a "follow-up" reaction and formation of MoO 2(SO 4) 3 4- and/or associated [MoO 2(SO 4) 3] m 4m- complexes in the ternary MoO 3-K 2S 2O 7-K 2SO 4 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 V s and V as Mo(=O) 2 modes of MoO 2(SO 4) 2 2- and [MoO 2(SO 4) 2] m 2m- and (2) at 935 (polarized) and 895 (depolarized) cm -1 for the respective modes of MoO 2(SO 4) 3 4- and [MoO 2(SO 4) 3] m 4m-. The results were tested and found to be in accordance with ab initio quantum chemical calculations carried out on [MoO 2(SO 4) 3] 4- and [{MoO 2} 2(SO 4) 4(μ-SO 4) 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.

Original languageEnglish (US)
Pages (from-to)8861-8872
Number of pages12
JournalJournal of Physical Chemistry A
Volume116
Issue number35
DOIs
StatePublished - Sep 6 2012
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry

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