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(η¹-O₂^•-), is dominated by hydrogen atom transfer, the majority of four-coordinate Cu^II(η¹-O₂^•-) 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, dpb₂-^MeBPA (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, dpb₃-TMPA (2). Kinetic isotope effect (KIE) measurements and correlation of second-order rate constants (k₂'s) versus oxidation potentials (E_ox) for a range of phenols indicates that the complex [Cu^II(η¹-O₂^•-)(1)]⁺ reacts with phenols via a similar hydrogen atom transfer (HAT) mechanism to [Cu^II(η¹-O₂^•-)(2)]⁺. However, [Cu^II(η¹-O₂^•-)(1)]⁺ performs HAT much more quickly, with its k₂ for reaction with 2,6-di-tert-butyl-4-methoxyphenol (MeO-ArOH) being >100 times greater. Furthermore, [Cu^II(η¹-O₂^•-)(1)]⁺ can oxidize C-H bond substrates possessing stronger bonds than [Cu^II(η¹-O₂^•-)(2)]⁺ is able to, and it reacts with N-methyl-9,10-dihydroacridine (^MeAcrH₂) approximately 200 times faster. The much greater facility for substrate oxidation displayed by [Cu^II(η¹-O₂^•-)(1)]⁺ is attributed to it possessing higher inherent electrophilicity than [Cu^II(η¹-O₂^•-)(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.