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

A new N,O-bidentate, phenol–imidazole pro-ligand 2ʹ-(4ʹ,6ʹ-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 MII(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 (PI)–(PI) stacking interactions that parallel those in galactose oxidase.