Sulfate-Bridged Dimeric Copper(II) Complexes with Three-Dimensional Network: Synthesis, Structure and DFT Studies

The 1:1 reaction of the diacetyl monoxime-2-pyridyl hydrazone HL1 (1) and the pyridine-2-carboxaldehyde-2-imidazoline hydrazone hydrobromide HL2 (2) with copper(II) sulfate pentahydrate in methanol affords a series of new sulfate-bridged dimeric copper(II) complexes, [Cu-2(II)(HL1)(2)(mu-SO4)(2)]center dot 4H(2)O (1a) and [Cu-2(II)(HL2)(2)(mu-SO4)Br-2]center dot 2H(2)O (2a), respectively. The room temperature magnetic moments of the complexes 1a and 2a are 1.82 mu(beta) and 1.84 mu(beta), respectively. The bands appearing in the UV region (200-340 nm) are characteristic of the ligands HL1 (1) and HL2 (2). In complexes 1a and 2a, these ligand centred bands are accompanied by multiple bands extending into the visible region (370-480 nm), The association constant (K-assi UV/Vis) was found to be (1.120 +/- 0.002) x 10(5) for 1a and (1.196 +/- 0.002) x 10(4) for 2a at 298 K determined using UV/Vis spectroscopy. On excitation at 285 nut, ligand I emits strongly in the 364 rim region due to an intraligand (1)(pi-pi*) transition. Upon complexation with copper(II), the emission peak is slightly blue shifted (354 nm, FIFo 0.89) with a little quenching in the emission intensity as expected for divalent. copper, Complexes la and 2a exhibit two consecutive oxidation couples for two copper(II) centres in deionised distilled water tinder a nitrogen atmosphere. DFT and TDDFT calculations were performed and the results are highly consistent with the spectroscopic behaviour Of the complexes. The molecular structures of dinuclear la (Cu center dot center dot center dot Cu 4.555 angstrom) and 2a (Cu center dot center dot center dot Cu 6.106 angstrom) have been determined by single-crystal X-ray diffraction studies. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)