A Four-Coordinate End-On Superoxocopper(II) Complex: Probing the Link between Coordination Number and Reactivity
Suman Debnath, Shoba Laxmi, Olivia McCubbin Stepanic, Sebastian Y. Quek, Maurice van Gastel, Serena DeBeer, Tobias Krämer, Jason England
Abstract
High Resolution Image Download MS PowerPoint Slide 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.