Nonclassical redox reactions in organometallic complexes involve oxidation or reduction of surrounding ancillary ligands whose frontier orbitals are accessible as an electron source or sink in support of metal-based bond formation or scission. Initial studies of pyridine-N-oxide deoxygenation by the oxobis(3,5-di-tert-butylcatecholato)molybdenum(VI) fragment suggested a mechanism involving rate-limiting N-O bond cleavage simultaneous with formation of a ligand-oxidized dioxomolybdenum(VI) complex. In order to discourage ligand dissociation upon oxidation, a tripodal tris(aminophenol) ligand, MeClampH6, was prepared and its κ7 Mo(VI) and κ6 Ti(IV) complexes characterized. The 18-electron (MeClamp)Mo possesses significant ligand-metal π donation and is unreactive with inner-sphere oxygen-atom donors. The strength of π donation can be estimated from optical spectroscopy as about 40 kcal/mol per π bond. The 16-electron (MeClamp)Ti is diamagnetic and adopts an unsymmetrical structure that allows mixing of a ligand nonbonding orbital with a ligand-metal bonding orbital. Thermal population of the triplet state is noticeable at room temperature with a singlet-triplet gap of 3.43 ± 0.19 kcal/mol determined by NMR spectroscopy. The complex displays the ability to catalytically oxidize benzylic and aliphatic alcohols to aldehydes or ketones under aerobic conditions, possibly involving ligand-centered dehydrogenation by this coordinatively saturated complex.

Structure, bonding, and reactivity of d0 dioxolene and iminoxolene molybdenum and titanium complexes

251st National Meeting (2016)

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