| Abstrakt: |
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 LHradical cation that possesses phenoxyl radical character. The unusual reversibility of the LHLHredox couple is attributed, at least in part, to a stabilisation of LHby intramolecular O–HN hydrogen bonding. The compounds CuL21 and ZnL22 have been synthesised and characterised structurally, spectroscopically, and electrochemically. The crystal structures of 1·4DMF, 1·3MeOH, and 2·2.5MeCN·0.3CH2Cl2have 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. 1and 2each undergo two, reversible, ligand-based, one-electron oxidations, to form, firstly, MLLand secondly ML22. The MLLM Cu, Zn cations have been generated by both electrochemical and chemical oxidation and their ML2BF4 salts isolated as air-stable, dark green, crystalline solids. The UVvis, EPR, and magnetic characteristics of these compounds are consistent with each cation involving an MIIM 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 CuL2BF4·2CH2Cl2and ZnL2BF4·2CH2Cl2·0.75pentane 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. |
