Synthesis, characterization and oxygen atom transfer reactivity of a pair of Mo(iv)O- and Mo(vi)O2-enedithiolate complexes – a look at both ends of the catalytic transformation

Abstract

Two new molybdenum complexes (Bu₄N)₂[Mo^IVO(ntdt)₂] (1) and (Ph₄P)₂[Mo^VIO₂(ntdt)₂] (2) (ntdt = 2-naphthyl-1,4-dithiolate) were synthesized using asymmetric dithiolene precursors and were characterized as structural models for the active site of arsenite oxidase, a molybdopterin bearing enzyme. The ligand was obtained readily by a two-step synthesis starting from 2-bromo-2′-acetonapthone. Complexes 1 and 2 were obtained by reaction of the resulting 4-naphthyl-1,3-dithiol-2-one with metal precursors trans-[MoO₂(CN)₄]⁴⁻ and cis-[MoO₂(NCS)₄]²⁻ respectively. Notably and to the best of our knowledge, this work constitutes the first utilization of the latter in dithiolene chemistry. 1 and 2 were characterized by NMR and IR spectroscopy, by cyclic voltammetry, mass spectrometry, elemental analysis and in case of 1 by single-crystal X-ray diffraction. The molecular structure of compound 1 exhibits the less common cis isomeric form (i.e. the naphthyl groups of the 2-naphthyl-1,4-dithiolate ligands are located on the same side of the MoS₄ square base). Structural, spectroscopic and electrochemical data are discussed in context. The catalytic oxo-transfer properties of 1 and 2 were investigated by oxo-transfer reactions from DMSO to PPh₃ with varied catalyst : PPh₃ ratios. Interestingly, the oxygen atom transfer reaction from DMSO to PPh₃ starting from compound 2 was found to be more efficient under the given conditions than when the reduced catalyst 1 was employed as initial species. The two catalytic systems are discussed and compared in terms of their reactivity.