Characterization of imidazolate-bridged dinuclear and mononuclear hydroperoxo complexes

Dinucleating ligands having two metal-binding sites bridged by an imidazolate moiety, Hbdpi, HMe(2)bdpi, and HMe(4)bdpi (Hbdpi = 4,5-bis(di(2-pyridylmethyl)aminomethyl)imidazole, HMe(2)bdpi = 4,5-bis((6-methyl-2-pyridylmethyl)(2-pyridylmethyl)aminomethyl)imidazole HMe(4)bdpi = 4,5-bis(di(6-methyl-2-pyridylmethyl)aminomethyl)imidazole), have been designed and synthesized as model ligands for copper-zinc superoxide dismutase (Cu,Zn-SOD). The corresponding mononucleating ligands, MeIm(Py)(2), MeIm(Me)(1), and MeIm(Me)(2) (MeIm(Py)(2) = (1-methyl-4-imidazolylmethyl)bis(2-pyridylmethyl)amine, MeIm(Me)(1) = (1-methyl-4-imidazolylmethyl)(6-methyl-2-pyridylmethyl) (2-pyridylmethyl)amine, MeIm(Me)(2) = (1-methyl-4-imidazolyl-methyl)bis(6-methyl-2-pyridylmethyl)amine), have also been synthesized for comparison. The imidazolate-bridged Cu(II)-Cu(II) homodinuclear complexes represented as [Cu-2(bdpi)(CH3CN)(2)](ClO4)(3). CH3CN . 3H2O (1), [Cu-2(Me(2)bdpi)-(CH3CN)(2)](ClO4)(3) (2), [Cu-2(Me(4)bdpi)(H2O)(2)](ClO4)(3). 4H(2)O (3), a Cu(II)-Zn(II) heterodinuclear complex of the type of [CuZn(bdpi)(CH3CN)(2)](ClO4)(3). 2CH(3)CN (4), Cu(II) mononuclear complexes of [Cu(MeIm(Py)(2))(CH3CN)](ClO4)(2). CH3CN (5), [Cu(MeIm(Me)(1))(CH3CN)](ClO4)(2) (6), and [Cu(MeIm(Me)(2))(CH3CN)] (ClO4)(2) (7) have been synthesized and the structures of complexes 5-7 determined by X-ray crystallography. The complexes 1-7 have a pentacoordinate structure at each metal ion with the imidazolate or 1-methylimidazole nitrogen, two pyridine nitrogens, the tertiary amine nitrogen, and a solvent (CH3CN or H2O) which can be readily replaced by a substrate. The reactions between complexes 1-7 and hydrogen peroxide (H2O2) in the presence of a base at -80 degreesC yield green solutions which exhibit intense bands at 360-380 nm, consistent with the generation of hydroperoxo Cu-(II) species in all cases. The resonance Raman spectra of all hydroperoxo intermediates at -80 degreesC exhibit a strong resonance-enhanced Raman band at 834-851 cm(-1), which shifts to 788-803 cm(-1) (Deltav= 46 cm(-1)) when O-18-labeled H2O2 was used, which are assigned to the O-O stretching frequency of a hydroperoxo ion. The resonance Raman spectra of hydroperoxo adducts of complexes 2 and 6 show two Raman bands at 848 (802) and 834 (788), 851 (805), and 835 (789) cm(-1) tin the case of (H2O2)-O-18, Deltav = 46 cm(-1)), respectively. The ESR spectra of all hydroperoxo complexes are quite dose to those of the parent Cu(II) complexes except 6. The spectrum of 6 exhibits a mixture signal of trigonal-bipyramid and square-pyramid which is consistent with the results of resonance Raman spectrum.