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 II (η 1 -O 2 •- ), is dominated by hydrogen atom transfer, the majority of four-coordinate Cu II (η 1 -O 2 •- ) 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 2 - Me BPA ( 1 ), 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 3 -TMPA ( 2 ). Kinetic isotope effect (KIE) measurements and correlation of second-order rate constants ( k 2 's) versus oxidation potentials ( E ox ) for a range of phenols indicates that the complex [Cu II (η 1 -O 2 •- )( 1 )] + reacts with phenols via a similar hydrogen atom transfer (HAT) mechanism to [Cu II (η 1 -O 2 •- )( 2 )] + . However, [Cu II (η 1 -O 2 •- )( 1 )] + performs HAT much more quickly, with its k 2 for reaction with 2,6-di- tert -butyl-4-methoxyphenol (MeO-ArOH) being >100 times greater. Furthermore, [Cu II (η 1 -O 2 •- )( 1 )] + can oxidize C-H bond substrates possessing stronger bonds than [Cu II (η 1 -O 2 •- )( 2 )] + is able to, and it reacts with N -methyl-9,10-dihydroacridine ( Me AcrH 2 ) approximately 200 times faster. The much greater facility for substrate oxidation displayed by [Cu II (η 1 -O 2 •- )( 1 )] + is attributed to it possessing higher inherent electrophilicity than [Cu II (η 1 -O 2 •- )( 2 )] + , 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.