Unprecedented copper(II) mediated in situ formation of gem-diol binuclear complexes: a combined experimental and computational study

The two alkoxo bridged complexes [Cu-2(L-1)(2)(HL2)(2)(H2O)](NO3)(2)center dot 2H(2)O 1 and [Cu-2(L-1)(2)(HL2)(2)](NO3)(2)center dot H2O2 have been synthesized by metal assisted hydrolysis of N'-[(E)-phenyl(pyridin-2-yl) methylidene] furan-2-carbohydrazide and N'-[(E)-phenyl(pyridin-2-yl) methylidene] acetohydrazide and characterized by various physicochemical techniques, where L-1 is 2-benzoylpyridine and HL2 is phenyl(pyridin-2-yl) methanediol. The molecular structures of the complexes have been determined by single crystal X-ray diffraction analyses. The distances between the two metal centers, viz., Cu(1)center dot center dot center dot Cu(2) are 3.027 angstrom for 1 and 3.023 angstrom for 2. The molecular structures of both complexes consist of gem-diols. Low temperature magnetic susceptibility measurements reveal antiferromagnetic interactions with J values of -12.90 cm(-1) for 1 and -12.97 cm(-1) for 2. X-band ESR spectra showed typical S = 1 signals for both complexes. The zero-field splitting parameter (D) values estimated from the spectra of complexes 1 and 2 are 0.0030 and 0.011 cm(-1), respectively in the polycrystalline state. Electrochemical studies of binuclear complexes evidence two irreversible one electron transfer reduction waves (E-pc1 = 0.142 to -0.126, E-pc2 = -0.480 to -0.188 V). The electronic spectra of the complexes have been explained by TD-DFT calculations. Both complexes 1 and 2 were tested as catalysts for the oxidation of the model substrate 3,5-di-tertbutylcatechol (3,5-dtbc) to 3,5-di-tert-butylquinone (3,5-dtbq) and can be considered as functional models for catechol oxidase. These complexes also catalyze the dismutation of superoxide at biological pH; complex 2 is more SOD active than 1.