A phenol–imidazole pro-ligand that can exist as a phenoxyl radical, alone and when complexed to copper(ii) and zinc(ii)

Abstract

A new N,O-bidentate, phenol–imidazole pro-ligand 2′-(4′,6′-di-tert-butylhydroxyphenyl)-4,5-diphenyl imidazole (LH) has been designed, synthesised, and characterised. LH possesses no readily oxidisable position (other than the phenol) and involves o- and p-substituents on the phenol ring that prevent radical coupling reactions. LH undergoes a reversible one-electron oxidation to generate the corresponding [LH]˙⁺ radical cation that possesses phenoxyl radical character. The unusual reversibility of the [LH]/[LH]˙⁺ redox couple is attributed, at least in part, to a stabilisation of [LH]˙⁺ by intramolecular O–H⋯N hydrogen bonding. The compounds [CuL₂] (1) and [ZnL₂] (2) have been synthesised and characterised structurally, spectroscopically, and electrochemically. The crystal structures of 1·4DMF, 1·3MeOH, and 2·2.5MeCN·0.3CH₂Cl₂ have been determined and each shown to possess an N₂O₂-coordination sphere, the geometry of which varies with the nature of the metal and the nature of the co-crystallised solvent. 1 and 2 each undergo two, reversible, ligand-based, one-electron oxidations, to form, firstly, [M(L)(L˙)]⁺ and secondly [M(L˙)₂]²⁺. The [M(L)(L˙)]⁺ (M = Cu, Zn) cations have been generated by both electrochemical and chemical oxidation and their [ML₂][BF₄] salts isolated as air-stable, dark green, crystalline solids. The UV/vis, EPR, and magnetic characteristics of these compounds are consistent with each cation involving an M^II (M = Cu or Zn) centre bound to a phenoxide (L⁻) and a phenoxyl radical (L˙). The structural information obtained by a determination of the crystal structures of [CuL₂][BF₄]·2CH₂Cl₂ and [ZnL₂][BF₄]·2CH₂Cl₂·0.75pentane fully supports this interpretation. For each of these salts, there is a clear indication that the coordinated phenoxyl radical is involved in intramolecular π–π stacking interactions that parallel those in galactose oxidase.