Sterically Directed Nitronate Complexes of 26-Di-tert-Butyl-4-Nitrophenoxide with Cu(II) and Zn(II) and Their H-Atom Transfer Reactivity

摘要

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]⁺ throught the phenolate oxygen. This difference in binding is very likely due to the steric difficulty of 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.