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, CuII(η1-O2•–), is dominated by hydrogen atom transfer, the majority of four-coordinate
CuII(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 (Eox) for a range of phenols indicates that the complex
[CuII(η1-O2•–)(1)]+ reacts with phenols via a similar
hydrogen atom transfer (HAT) mechanism to [CuII(η1-O2•–)(2)]+. However, [CuII(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, [CuII(η1-O2•–)(1)]+ can oxidize C–H bond substrates possessing stronger
bonds than [CuII(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 [CuII(η1-O2•–)(1)]+ is attributed to it possessing higher
inherent electrophilicity than [CuII(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.

尽管五配位端基超氧合铜(II)配合物（five-coordinate end-on superoxocopper(II) complexes）CuII(η¹-O₂•–)的反应活性以氢原子转移（Hydrogen Atom Transfer, HAT）为主，但迄今为止已报道的多数四配位CuII(η¹-O₂•–)配合物却表现出亲核反应活性。为探究这一差异的起源，我们开发了一种由空间位阻较大的双(2-吡啶甲基)胺配体dpb2-MeBPA(1)支撑的四配位端基超氧合铜(II)配合物，并将其底物反应活性与由相似取代基修饰的三(2-吡啶甲基)胺配体配位的五配位端基超氧合铜(II)配合物dpb3-TMPA(2)进行了对比。动力学同位素效应（Kinetic Isotope Effect, KIE）测量，以及一系列酚类底物的二级速率常数(k₂)与氧化电位(Eox)的相关性分析表明，配合物[CuII(η¹-O₂•–)(1)]+与酚类的反应遵循与[CuII(η¹-O₂•–)(2)]+类似的氢原子转移机制。然而，[CuII(η¹-O₂•–)(1)]+的氢原子转移反应速率要快得多：其与2,6-二叔丁基-4-甲氧基苯酚(MeO-ArOH)反应的二级速率常数高出两个数量级以上。此外，[CuII(η¹-O₂•–)(1)]+能够氧化键能更高的C-H键底物，且与N-甲基-9,10-二氢吖啶(MeAcrH₂)的反应速率快出约200倍。[CuII(η¹-O₂•–)(1)]+更强的底物氧化能力，归因于其相较于[CuII(η¹-O₂•–)(2)]+具有更高的本征亲电性，而这一差异直接源于前者更低的配位数。上述发现与相关酶体系具有重要关联——在这些酶中，四配位端基超氧合铜(II)中间体而非其五配位同系物，通常被认定为介导底物氧化的活性氧化剂。