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

Abstract

The bulky 2,6-di-tert-butyl-4-nitrophenolate ligand forms complexes with [Tp^tBuCu^II]⁺ and [Tp^tBuZn^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 κ²-nitronate structure, while the Zn analogue has a four-coordinate κ¹-nitronate ligand. 4-Nitrophenol, without the 2,6-di-tert-butyl substituents, instead binds to [Tp^tBuCu^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^tBuM^II]⁺ unit. Tp^tBuCu^II(κ²-O₂N^tBu₂C₆H₂O) reacts with the hydroxylamine TEMPO–H (2,2,6,6-tetramethylpiperidin-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.