A Four-Coordinate\nEnd-On Superoxocopper(II) Complex:\nProbing the Link between Coordination Number and Reactivity

Although the reactivity\nof five-coordinate end-on superoxocopper(II)\ncomplexes, Cu<sup>II</sup>(η<sup>1</sup>-O<sub>2</sub><sup>•–</sup>), is dominated by hydrogen atom transfer, the majority of four-coordinate\nCu<sup>II</sup>(η<sup>1</sup>-O<sub>2</sub><sup>•–</sup>) complexes published thus far display nucleophilic reactivity. To\ninvestigate the origin of this difference, we have developed a four-coordinate\nend-on superoxocopper(II) complex supported by a sterically encumbered\nbis(2-pyridylmethyl)amine ligand, dpb<sub>2</sub>-<sup>Me</sup>BPA\n(<b>1</b>), and compared its substrate reactivity with that\nof a five-coordinate end-on superoxocopper(II) complex ligated by\na similarly substituted tris(2-pyridylmethyl)amine, dpb<sub>3</sub>-TMPA (<b>2</b>). Kinetic isotope effect (KIE) measurements\nand correlation of second-order rate constants (<i>k</i><sub>2</sub>’s) versus oxidation potentials (<i>E</i><sub>ox</sub>) for a range of phenols indicates that the complex\n[Cu<sup>II</sup>(η<sup>1</sup>-O<sub>2</sub><sup>•–</sup>)(<b>1</b>)]<sup>+</sup> reacts with phenols via a similar\nhydrogen atom transfer (HAT) mechanism to [Cu<sup>II</sup>(η<sup>1</sup>-O<sub>2</sub><sup>•–</sup>)(<b>2</b>)]<sup>+</sup>. However, [Cu<sup>II</sup>(η<sup>1</sup>-O<sub>2</sub><sup>•–</sup>)(<b>1</b>)]<sup>+</sup> performs\nHAT much more quickly, with its <i>k</i><sub>2</sub> for\nreaction with 2,6-di-<i>tert</i>-butyl-4-methoxyphenol (MeO-ArOH)\nbeing >100 times greater. Furthermore, [Cu<sup>II</sup>(η<sup>1</sup>-O<sub>2</sub><sup>•–</sup>)(<b>1</b>)]<sup>+</sup> can oxidize C–H bond substrates possessing stronger\nbonds than [Cu<sup>II</sup>(η<sup>1</sup>-O<sub>2</sub><sup>•–</sup>)(<b>2</b>)]<sup>+</sup> is able to,\nand it reacts with <i>N</i>-methyl-9,10-dihydroacridine\n(<sup>Me</sup>AcrH<sub>2</sub>) approximately 200 times faster. The\nmuch greater facility for substrate oxidation displayed by [Cu<sup>II</sup>(η<sup>1</sup>-O<sub>2</sub><sup>•–</sup>)(<b>1</b>)]<sup>+</sup> is attributed to it possessing higher\ninherent electrophilicity than [Cu<sup>II</sup>(η<sup>1</sup>-O<sub>2</sub><sup>•–</sup>)(<b>2</b>)]<sup>+</sup>, which is a direct consequence of its lower coordination number.\nThese observations are of relevance to enzymes in which four-coordinate\nend-on superoxocopper(II) intermediates, rather than their five-coordinate\ncongeners, are routinely invoked as the active oxidants responsible\nfor substrate oxidation.