A Four-Coordinate End-On Superoxocopper(II) Complex: Probing the Link between Coordination Number and Reactivity.
J Am Chem Soc
; 146(34): 23704-23716, 2024 Aug 28.
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| ID: mdl-39192778
ABSTRACT
Although the reactivity of five-coordinate end-on superoxocopper(II) complexes, CuII(η1-O2â¢-), is dominated by hydrogen atom transfer, the majority of four-coordinate CuII(η1-O2â¢-) 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 (Eox) for a range of phenols indicates that the complex [CuII(η1-O2â¢-)(1)]+ reacts with phenols via a similar hydrogen atom transfer (HAT) mechanism to [CuII(η1-O2â¢-)(2)]+. However, [CuII(η1-O2â¢-)(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, [CuII(η1-O2â¢-)(1)]+ can oxidize C-H bond substrates possessing stronger bonds than [CuII(η1-O2â¢-)(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 [CuII(η1-O2â¢-)(1)]+ is attributed to it possessing higher inherent electrophilicity than [CuII(η1-O2â¢-)(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.
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J Am Chem Soc
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2024
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Article