A Four-Coordinate End-On Superoxocopper(II) Complex: Probing the Link between Coordination Number and Reactivity.

Although the reactivity of five-coordinate end-on superoxocopper(II) complexes, Cu(η-O), is dominated by hydrogen atom transfer, the majority of four-coordinate Cu(η-O) complexes published thus far display nucleophilic reactivity. To investigate the origin of this difference, we have developed a four-coordinate end-on superoxocopper(II) complex supported by a sterically encumbered bis(2-pyridylmethyl)amine ligand, dpb-BPA (), and compared its substrate reactivity with that of a five-coordinate end-on superoxocopper(II) complex ligated by a similarly substituted tris(2-pyridylmethyl)amine, dpb-TMPA (). Kinetic isotope effect (KIE) measurements and correlation of second-order rate constants ('s) versus oxidation potentials () for a range of phenols indicates that the complex [Cu(η-O)()] reacts with phenols via a similar hydrogen atom transfer (HAT) mechanism to [Cu(η-O)()]. However, [Cu(η-O)()] performs HAT much more quickly, with its for reaction with 2,6-di--butyl-4-methoxyphenol (MeO-ArOH) being >100 times greater. Furthermore, [Cu(η-O)()] can oxidize C-H bond substrates possessing stronger bonds than [Cu(η-O)()] is able to, and it reacts with -methyl-9,10-dihydroacridine (AcrH) approximately 200 times faster. The much greater facility for substrate oxidation displayed by [Cu(η-O)()] is attributed to it possessing higher inherent electrophilicity than [Cu(η-O)()], which is a direct consequence of its lower coordination number. These observations are of relevance to enzymes in which four-coordinate end-on superoxocopper(II) intermediates, rather than their five-coordinate congeners, are routinely invoked as the active oxidants responsible for substrate oxidation.