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-tert-butyl-2-phenoxo)­amido)­rhenium­(V), (ONOCat)­ReO­(PPh3), reacts with molecular oxygen to give triphenylphosphine
oxide and the dimeric rhenium­(VII) complex fac,anti-(ONOCat)­Re­(O)­(μ-O)2Re­(O)­(ONOCat). The ONO ligand adopts an unusual fac 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 N-heterocyclic carbene ligand IMes to fac,anti-(ONOCat)­Re­(O)­(μ-O)2Re­(O)­(ONOCat) cleaves the dimer into monomeric C1-symmetric fac-(ONOCat)­ReO2(IMes). The monorhenium­(VII) complex is deoxygenated
by PMe2Ph to give the rhenium­(V) compound (ONOCat)­ReO­(IMes), which can be independently prepared by ligand substitution
of (ONOCat)­ReO­(PPh3). The degree of stereochemical
rigidity exhibited by the dioxo compound, as established by dynamic
NMR spectroscopy, excludes the intermediacy of mer-(ONOQ)­ReVO2(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.