Abstract

The redox conversions of hydroxylamine (NH2OH) are biologically relevant because this molecule serves as a nitric oxide donor in mammals under certain conditions. The analogous arylhydroxylamine compounds (ArNHOH, where Ar is an aryl group) display similar chemistry, with the advantage of fewer side-reactions. Thus, there has recently been increased interest into investigations of the reactivity of arylhydroxylamines with metal complexes as mimics of the biologically relevant reactions of NH2OH with copper or iron. Oxidation of arylhydroxylamines yields nitrosoarenes (ArNO), which are structurally and electronically versatile redox-active molecules. Their corresponding metal complexes are stable mimics of O2 and HNO complexes transiently found in nature and catalytic cycles. This stability allows for a systematic survey of the structure and redox state of ArNO metal complexes. The metal-mediated transformations between ArNHOH, ArNO and other relevant nitrogen-based groups, however, are not well understood. Herein, in two different reports, we address the reactivity of ArNHOH and ArNO groups with copper complexes.
In the first study, a series of complexes is prepared by self-assembly of copper(I) precursors and arylnitroso species. The nature of the copper(I) supporting ligand (bi-, tri- or tetradentate, as well as secondary vs. tertiary amine donors) and the electronic nature of the arylnitroso species (electron-donating or withdrawing substituents) are varied. The stoichiometry of the reaction, the topology, and the electronic properties of the adducts are characterized by UV-Vis spectroscopy, single crystal X-ray diffraction, and DFT methods. The more electron-rich ligands and the more electron-poor arylnitroso species lead to an inner-sphere electron transfer and formation of copper(II)-(arylnitrosyl radical) complexes bearing a linkage topology that depends on the denticity of the supporting ligand. These results provide a canvas by which to predict the products of similar self-assembled redox reactions.
In the second study, the synthesis of an unprecedented copper(I)-arylhydroxylamine complex is reported. The reactivity of the arylhydroxylamine is arrested by the presence of an intramolecular hydrogen bond that stabilizes the weakly bound hydroxylamine. Upon two-electron oxidation, a copper(II)-(arylnitrosyl radical) complex is formed, which crystallizes as a copper(I) species with an uncoordinated arylnitroso function. This validates that such ligands may enable ligand redox chemistry and hemilability in copper complexes. Such tethered arylnitroso complexes present new opportunities for two-electron chemistry in earth-abundant metal catalysis.
In a nutshell, this thesis highlights unique electron-transfer events and redox conversions about the N-O bond in copper complexes. The main finding is the characterization of an intermediate redox state in-between ArNHOH and ArNO, namely the arylnitrosyl radical anion, ArNO-, which is stable only when coordinated.