Benzimidazole-triazole ligands with pendent triazole functionality: unexpected formation and effects on copper-catalyzed aerobic alcohol oxidation

A series of benzimidazole triazole ligands (NN') having a pendent triazole arm with different triazole substituents including CH2Ph (3a), cyclo-C6H11 (3b), and CH2SiMe3 (3c) were obtained in moderate yields from Cu-catalyzed oxidative C-N cyclization of the respective amine-triazole compounds N, N'-bis((1R- 1,2,3-triazol-4-yl) methyl) benzene-1,2-diamine (2a-2c). Treatment of CuCl2 with one equiv. of the benzimidazole-triazole ligands afforded the corresponding Cu-II complexes with the empirical formula of Cu(NN') Cl-2 (4a-4c). Crystal structures of 4b and 4c reveal mononuclear and dinuclear Cu-II complexes, respectively. Despite the differences in triazole substituents and their solid state structures, ESR spectra indicate the same molecular structures in CH3CN solution whereas CV data suggest similar redox potentials for 4a-4c. Catalytic activities for aerobic oxidation of benzyl alcohol to benzaldehyde follow this trend: 4c > 4a > 4b. In addition, the catalytic system 4c/TEMPO/Cu-0/NMI (TEMPO = 2,2,6,6-tetramethyl- piperidinyloxyl, NMI = N-methylimidazole) exhibited high activities for oxidation of activated alcohols (i.e., benzyl alcohol derivatives and allylic alcohol) in CH3CN at room temperature.