Oxygen Atom Transfer Reactions with Molybdenum Cofactor Model Complexes That Contain a Tetradentate OSSO-Type Bis(phenolato) Ligand

The catalytic activity of molybdenum complexes in oxidation state +IV, +V, and +VI stabilized by a tetradentate dithiaalkanediyl-2,2′-bis­(phenolato) (OSSO)-type ligand was tested in the oxygen atom transfer (OAT) reaction from DMSO to PPh3. The molybdenum­(IV) complexes [(OSSO)­MoCl2] (1a, b) were prepared from [MoCl4(NCMe)2] and the corresponding bis­(phenol) (OSSO)­H2. The molybdenum­(V) complexes [(OSSO)­MoOCl] (2a, b) were obtained by salt metathesis either from MoOCl4 or from MoOCl3 with the corresponding bis­(phenolate) salts [(OSSO)­M2] (M = Li, K). The reaction with MoOCl4 proceeds via the intermediate [(OSSO)­MoOCl2] (A), which disproportionates into 2 and insoluble oligomers. Single crystal X-ray diffraction of 1a and 2b revealed octahedral cis-α-coordination of the metal center. EPR spectra of 2a and 2b show the characteristic signal of a molybdenum d1-spin system which is not coupled to the ligand. OAT reactions in the presence of 1–2 or of the bis­(oxo) complex [(OSSO)­MoO2] (3) revealed that the molybdenum­(IV) complex 1b exerts the high catalytic activity with complete conversion of PPh3 to OPPh3 within 30 min at 60 °C. The high catalytic activity of 1 compared to that of 2–3 is explained by the formation of the unstable intermediate A, which decomposes to 2 in the absence of PPh3