Synthesis, crystal structures, and catalytic activity of dicopper(II) complexes with dinucleating tetraimidazole ligands [Cu-2(RO)(mbipl)](ClO4)(2) (R=Me, Et, 2-Pr; Hmbipl equals 1,5-bis[bis[(1-methyl-4-imidazolyl)methyl]amino]-3-pentanol) and [Cu-2(MeO)(pbipl)](ClO4)(2) (Hpbipl equals 1,5-Bis[bis[(1-isopropyl-4-imidazolyl)methyl]amino]-3-pentanol)

Synthesis, crystal structures, and catalytic activity are reported for dinuclear copper(II) complexes of new dinucleating tetraimidazole ligands, 1,5-bis[bis[(1-methyl-4-imidazolyl)methyl]amino]-3-pentanol (Hmbipl) and 1,5-bis[bis[(1-isopropyl-4-imidazolyl)methyl]amino]-3-pentanol (Hpbipl). These ligands have been prepared by a reductive coupling of 1,5-diamino-3-pentanol with 4 equiv of 1-alkyl-4-formylimidazole (alkyl = methyl and isopropyl) by using sodium cyanoborohydride in quantitative yields. The ligand Hmbipl forms [Cu-2(MeO)(mbipl)](ClO4)(2) (1) in MeOH. The methoxide bridge of 1 is easily exchanged with EtOH and 2-PrOH during recrystallization from such solvents, yielding ethoxide-and 2-propoxide-bridged complexes [Cu-2(RO)(mbipl)](ClO4)(2) (R = Et, 2; R = 2-Pr, 3). The ligand Hpbipl forms [Cu-2(MeO)(pbipl)](ClO4)(2) (4) in MeOH. Compounds 1-4 have been characterized by elemental analyses, FAB mass, electronic absorption, and IR spectra, magnetic susceptibilities, and cyclic voltammograms. Crystal structures of 2 . 2MeCN and 3 . CH2Cl2 have been determined by X-ray analyses, showing that the coordination geometry about each copper ion in 2 and 3 is distorted trigonal bipyramidal. Compounds 1-4 catalyze the quantitative oxidation of 2,4-di-tert-butylphenol (DBP) to 3,3',5,5'-tetra-tert-butyl-2,2'-dihydroxybiphenyl with H2O2. A hydroperoxodicopper(II) complex 5 which is generated from 4 as an active intermediate is detected by UV-vis spectroscopy. Compound 4 shows the highest activity among the present complexes because of its high reaction rate and high durability. Kinetic studies show that the reaction is first order with respect to both DBP and the catalyst. The second-order rate constant, k(2), is 5.3 M-1 s(-1). It seems that the bulky N-isopropyl groups enhance the catalytic activity of 4.