The Metal or the Ligand? The Preferred Locus for Redox Changes in Oxygen Atom Transfer Reactions of Rhenium Amidodiphenoxides

The rhenium­(V) oxo complex oxo­(triphenylphosphine) (bis­(3,5-di-<i>tert</i>-butyl-2-phenoxo)­amido)­rhenium­(V), (ONO^Cat)­ReO­(PPh₃), reacts with molecular oxygen to give triphenylphosphine oxide and the dimeric rhenium­(VII) complex <i>fac</i>,<i>anti</i>-(ONO^Cat)­Re­(O)­(μ-O)₂Re­(O)­(ONO^Cat). The ONO ligand adopts an unusual <i>fac</i> geometry, presumably to maximize π donation to rhenium; strong π donation is substantiated by the intraligand bond distances (metrical oxidation state = −2.24(9)). Addition of the <i>N</i>-heterocyclic carbene ligand IMes to <i>fac</i>,<i>anti</i>-(ONO^Cat)­Re­(O)­(μ-O)₂Re­(O)­(ONO^Cat) cleaves the dimer into monomeric <i>C</i>₁-symmetric <i>fac</i>-(ONO^Cat)­ReO₂(IMes). The monorhenium­(VII) complex is deoxygenated by PMe₂Ph to give the rhenium­(V) compound (ONO^Cat)­ReO­(IMes), which can be independently prepared by ligand substitution of (ONO^Cat)­ReO­(PPh₃). The degree of stereochemical rigidity exhibited by the dioxo compound, as established by dynamic NMR spectroscopy, excludes the intermediacy of <i>mer</i>-(ONO^Q)­Re^VO₂(IMes) in this oxygen atom transfer reaction. Thus, oxygen atom transfer takes place preferentially by direct reduction of the oxorhenium­(VII) moiety (classical oxygen atom transfer) rather than through initial internal electron transfer and ligand-centered reduction of an oxorhenium­(V)-iminoquinone