Examination of oxygen atom transfer reactivity of heteroscorpionate dioxo-Mo(VI) complexes: Geometric isomers, solvent effect, intermediates, and catalytic oxidation

Inspired by Nature-Functional Analogues of Molybdenum and Tungsten-Dependent Oxidoreductases

Throughout the previous ten years many scientists took inspiration from natural molybdenum and tungsten-dependent oxidoreductases to build functional active site analogues. These studies not only led to an ever more detailed mechanistic understanding of the biological template, but also paved the way to atypical selectivity and activity, such as catalytic hydrogen evolution. This review is aimed at representing the last decade’s progress in the research of and with molybdenum and tungsten functional model compounds. The portrayed systems, organized according to their ability to facilitate typical and artificial enzyme reactions, comprise complexes with non-innocent dithiolene ligands, resembling molybdopterin, as well as entirely non-natural nitrogen, oxygen, and/or sulfur bearing chelating donor ligands. All model compounds receive individual attention, highlighting the specific novelty that each provides for our understanding of the enzymatic mechanisms, such as oxygen atom transfer and proton-coupled electron transfer, or that each presents for exploiting new and useful catalytic capability. Overall, a shift in the application of these model compounds towards uncommon reactions is noted, the latter are comprehensively discussed.

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

Four dioxidomolybdenum(VI) complexes of the general structure [MoO₂L₂] employing the S,N-bidentate ligands pyrimidine-2-thiolate (PymS, 1), pyridine-2-thiolate (PyS, 2), 4-methylpyridine-2-thiolate (4-MePyS, 3) and 6-methylpyridine-2-thiolate (6-MePyS, 4) were synthesized and characterized by spectroscopic means and single-crystal X-ray diffraction analysis (2-4). Complexes 1-4 were reacted with PPh₃ and PMe₃, respectively, to investigate their oxygen atom transfer (OAT) reactivity and catalytic applicability. Reduction with PPh₃ leads to symmetric molybdenum(V) dimers of the general structure [Mo₂O₃L₄] (6-9). Kinetic studies showed that the OAT from [MoO₂L₂] to PPh₃ is 5 times faster for the PymS system than for the PyS and 4-MePyS systems. The reaction of complexes 1-3 with PMe₃ gives stable molybdenum(IV) complexes of the structure [MoOL₂(PMe₃)₂] (10-12), while reduction of [MoO₂(6-MePyS)₂] (4) yields [MoO(6-MePyS)₂(PMe₃)] (13) with only one PMe₃ coordinated to the metal center. The activity of complexes 1-4 in catalytic OAT reactions involving Me₂SO and Ph₂SO as oxygen donors and PPh₃ 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 [MoO₂(PymS)₂] (1) and [MoO₂(6-MePyS)₂] (4) are similarly efficient catalysts, while complexes 2 and 3 are only moderately active. In the catalytic oxidation of PMe₃ with Me₂SO, complex 4 is the only efficient catalyst. Complexes 1-4 were also found to catalytically reduce NO₃^- with PPh₃, although their reactivity is inhibited by further reduced species such as NO, as exemplified by the formation of the nitrosyl complex [Mo(NO)(PymS)₃] (14), which was identified by single-crystal X-ray diffraction analysis. Computed ΔG^⧧ values for the very first step of the OAT were found to be lower for complexes 1 and 4 than for 2 and 3, explaining the difference in catalytic reactivity between the two pairs and revealing the requirement for an electron-deficient ligand system.

Molybdenum complexes with a μ-O{MoO2}2 core: their synthesis, crystal structure and application as catalysts for the oxidation of bicyclic alcohols using N-based additives

Binuclear molybdenum(vi) complexes with a μ-O{MoO₂}₂ core as catalysts for the oxidation of bicyclic alcohols (fenchyl alcohol and isoborneol), using 30% H₂O₂ as the oxidant in the presence of NEt₃ as the additive, are reported.