Oxygen Atom Transfer Reactivity of Molybdenum(VI) Complexes Employing Pyrimidine- and Pyridine-2-thiolate Ligands.

Four dioxidomolybdenum(VI) complexes of the general structure [MoOL] employing the S,N-bidentate ligands pyrimidine-2-thiolate (PymS, ), pyridine-2-thiolate (PyS, ), 4-methylpyridine-2-thiolate (4-MePyS, ) and 6-methylpyridine-2-thiolate (6-MePyS, ) were synthesized and characterized by spectroscopic means and single-crystal X-ray diffraction analysis (-). Complexes - were reacted with PPh and PMe, respectively, to investigate their oxygen atom transfer (OAT) reactivity and catalytic applicability. Reduction with PPh leads to symmetric molybdenum(V) dimers of the general structure [MoOL] (-). Kinetic studies showed that the OAT from [MoOL] to PPh is 5 times faster for the PymS system than for the PyS and 4-MePyS systems. The reaction of complexes - with PMe gives stable molybdenum(IV) complexes of the structure [MoOL(PMe)] (-), while reduction of [MoO(6-MePyS)] () yields [MoO(6-MePyS)(PMe)] () with only one PMe coordinated to the metal center. The activity of complexes - in catalytic OAT reactions involving MeSO and PhSO as oxygen donors and PPh as an oxygen acceptor has been investigated to assess the influence of the varied ligand frameworks on the OAT reaction rates. It was found that [MoO(PymS)] () and [MoO(6-MePyS)] () are similarly efficient catalysts, while complexes and are only moderately active. In the catalytic oxidation of PMe with MeSO, complex is the only efficient catalyst. Complexes - were also found to catalytically reduce NO with PPh, although their reactivity is inhibited by further reduced species such as NO, as exemplified by the formation of the nitrosyl complex [Mo(NO)(PymS)] (), which was identified by single-crystal X-ray diffraction analysis. Computed Δ values for the very first step of the OAT were found to be lower for complexes and than for and , explaining the difference in catalytic reactivity between the two pairs and revealing the requirement for an electron-deficient ligand system.