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

Abstract

The two alkoxo bridged complexes [Cu₂(L¹)₂(HL²)₂(H₂O)](NO₃)₂·2H₂O 1 and [Cu₂(L¹)₂(HL²)₂](NO₃)₂·H₂O 2 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 L¹ 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)⋯Cu(2) are 3.027 Å for 1 and 3.023 Å 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-tert-butylcatechol (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.