Sterically directed nitronate complexes of 2,6-di-tert-butyl-4-nitrophenoxide with Cu(II) and Zn(II) and their H-atom transfer reactivity

The bulky 2,6-di-tert-butyl-4-nitrophenolate ligand forms complexes with [Tp(tBu)Cu(II)](+) and [Tp(tBu)Zn(II)](+) binding via the nitro group in an unusual nitronato-quinone resonance form (Tp(tBu) = hydro-tris(3-tert-butyl-pyrazol-1-yl)borate). The Cu complex in the solid state has a five-coordinate kappa(2)-nitronate structure, while the Zn analogue has a four-coordinate kappa(1)-nitronate ligand. 4-Nitrophenol, without the 2,6-di-tert-butyl substituents, instead binds to [Tp(tBu)Cu(II)](+) through the phenolate oxygen. This difference in binding is very likely due to the steric difficulty in binding a 2,6-di-tert-butyl-phenolate ligand to the [Tp(tBu)M(II)](+) unit. Tp(tBu)Cu(II)(kappa(2)-(O2NBu2C6H2O)-Bu-t) reacts with the hydroxylamine TEMPO-H (2,2,6,6-tetramethyl-piperidin-1-ol) by abstracting a hydrogen atom. This system thus shows an unusual sterically enforced transition metal-ligand binding motif and a copper-phenolate interaction that differs from what is typically observed in biological and chemical catalysis.