Using the bidentate ligand L = 1-methyl-2-(alkylthiomethyl)-1H-benzimidazole with mixed imidazole-N and thioether-S functions, the authors were able to observe valence tautomer (redox isomer) equil. (L)CuII(Q-II) .dblharw. (L)Cu1(Q-1) by EPR spectroscopy (Q = alkyl-substituted 3-tert-butyl-o-benzoquinones). These systems are the 1st to show such behavior outside amine oxidase enzymes, where the intramol. electron transfer is essential for the activation of O2 by copper(I). In contrast, coligands contg. exclusively thioether functions yield only copper(I)/o-semiquinone radical complexes, while the use of 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3TACN) gave a structurally and spectroelectrochem. characterized copper(II)/catecholate complex (Me3TACN)Cu(Q3) (Q3 = 3,5-di-tert-butylcatecholate) in which the square-pyramidally coordinated metal forms one weak and two strong bonds to the three nitrogen donor atoms. Most remarkably, rather small modifications of the quinone shift the valence tautomer equil.: the use of slightly more electron-rich methoxy-rather than alkyl-substituted 3-tert-butyl-o-benzoquinones and L resulted in semiquinone/copper(I) formation exclusively. on SciFinder(R)
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