Heptamolybdate: a highly active sulfide oxygenation catalyst

Abstract

The sulfide oxygenation activities of both heptamolybdate ([Mo₇O₂₄]⁶⁻, [1]⁶⁻) and its peroxo adduct [Mo₇O₂₂(O₂)₂]⁶⁻ ([2]⁶⁻) were examined in this contribution. [Mo₇O₂₂(O₂)₂]⁶⁻ was prepared in a yield of 65% from (NH₄)₆[Mo₇O₂₄] (1a) upon treatment of 10 equiv. of H₂O₂ and structurally identified through single crystal X-ray diffraction study. (nBu₄N)₆[Mo₇O₂₂(O₂)₂] (2b) is an efficient catalyst for the sequential oxygenation of methyl phenyl sulfide (MPS) by H₂O₂ to the corresponding sulfoxide and subsequently sulfone with a 100% utility of H₂O₂. Surprisingly, (nBu₄N)₆[Mo₇O₂₄] (1b) is a significantly faster catalyst than 2b for MPS oxygenation under identical conditions. The pseudo-first order k_cat constants from initial rate kinetics are 54 M⁻¹ s⁻¹ and 19 M⁻¹ s⁻¹ for 1b and 2b, respectively. Electrospray ionization mass spectrometry (ESI-MS) investigation of 1b under the catalytic reaction conditions revealed that [Mo₂O₁₁]²⁻ is likely the main active species in sulfide oxygenation by H₂O₂.