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

Comparison of the substrate reactivity of four- versus five-coordinate end-on superoxocopper(II) complexes is complicated by the relative scarcity of the former and the difference in donor atoms that have been used to support these two geometries. Herein, we report a four-coordinate end-on superoxocopper(II) complex supported by a sterically encumbered bis(2-pyridylmethyl)amine ligand, dpb2-MeBPA (1), and comparison of 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). These ligands differ by the presence/absence of a single pyridine donor. Kinetic isotope effect (KIE) measurements and correlation of second order rate constants (k2’s) versus oxidation potentials (Eox) for a range of phenols indicates that the complex [Cu(𝜂1-O2•−)(1)]+ reacts with phenols via a similar hydrogen atom transfer (HAT) mechanism as [Cu(𝜂1-O2•−)(2)]+. However, [Cu(𝜂1-O2•−)(1)]+ performs HAT much more quickly, with its k2 for reaction with 2,4-di-tert-butyl-4-methoxyphenol (MeO-ArOH) being > 100 times greater. Furthermore, [Cu(𝜂1-O2•−)(1)]+ can oxidize C-H bond substrates possessing stronger bonds than [Cu(𝜂1-O2•−)(2)]+, and it reacts with N-methyl-9,10-dihydroacridine (MeAcrH) approximately 200 times faster. The much greater facility for substrate oxidation displayed by [Cu(𝜂1-O2•−)(1)]+ is attributed to its more open coordination sphere, which allows easier substrate approach than [Cu(𝜂1-O2•−)(2)]+. These observations are of relevance to enzymes in which four-coordinate end-on superoxocopper(II) intermediates, rather than five-coordinate congeners, are routinely invoked as the active oxidants responsible for substrate oxidation.