Five Nitrogen Oxidation States from Nitro to Amine: Stabilization and Reactivity of a Metastable Arylhydroxylamine Complex

Redox
noninnocent ligands enhance the reactivity of the metal they
complex, a strategy used by metalloenzymes and in catalysis. Herein,
we report a series of copper complexes with the same ligand framework,
but with a pendant nitrogen group that spans five different redox
states between nitro and amine. Of particular interest is the synthesis
of a unprecedented copper­(I)-arylhydroxylamine complex. While hydroxylamines
typically disproportionate or decompose in the presence of transition
metal ions, the reactivity of this metastable species is arrested
by the presence of an intramolecular hydrogen bond. Two-electron oxidation
yields a copper­(II)-(arylnitrosyl radical) complex that can dissociate
to a copper­(I) species with uncoordinated arylnitroso. This combination
of ligand redox noninnocence and hemilability provides opportunities
in catalysis for two-electron chemistry via a one-electron copper­(I/II)
shuttle, as exemplified with an aerobic alcohol oxidation.

氧化还原非无辜配体（redox noninnocent ligands）可增强其所配位金属的反应活性，这一策略已被金属酶所采用，并应用于催化领域。本文报道了一系列具有相同配体骨架的铜配合物，其带有一个可在硝基与氨基之间呈现五种不同氧化还原态的悬挂氮基团。尤为值得关注的是，我们合成了一种前所未有的铜(I)-芳基羟胺配合物。通常情况下，羟胺类化合物在过渡金属离子存在时易发生歧化或分解反应，但该亚稳物种的反应性因分子内氢键的存在而受到抑制。两电子氧化过程可生成铜(II)-芳基亚硝酰自由基配合物，该配合物可解离为带有未配位芳基亚硝酰的铜(I)物种。配体氧化还原非无辜性与半配位性的结合，为通过一价/二价铜(I/II)穿梭机制实现两电子化学的催化反应提供了可能，这一点在需氧醇氧化反应中得到了验证。