Dicopper moieties stabilized by Fréchet-type dendrons: Syntheses and structural characterizations

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Five paddlewheel-shaped dicopper molecules, 1, 2a, 2b, 3 and 4, were synthesized from the reaction of copper acetate and different generations of Fréchet-type dendrons (HG1-HG4). All compounds showed diagnostic bands in their FTIR spectra for the coordinated carboxylate groups with separations of COasym and COsym lower than 200 cm-1 suggesting the formation of dimers. A broad absorbance band centered at 700, 695, 695 and 693 nm was observed for compounds 1-4 respectively, which is ascribed to the d-d charge-transfer in Cu(II) complexes. A LMCT absorption was observed near 304, 290, 298 and 294 nm for compounds 1-4 respectively. Magnetic susceptibility measurements for 1, 2a and 3 at 1.53, 1.53 and 1.7 respectively in the solid state at 293 K are also suggestive of a dimeric arrangement. The structures of HG2, 1, 2a, and 2b were characterized unambiguously with single crystal X-ray diffraction. Three molecules constituted the asymmetric unit of HG2 in the monoclinic space group P21/n and resulted in dimers formed via intermolecular carboxylic acid-type hydrogen bonds. Compounds 1, 2a, and 2b contain centrosymmetric paddlewheel-shaped dinuclear copper molecules bridged by four carboxylate groups in syn, syn arrangements. The copper centers in compounds 1, 2a, and 2b are all coordinated by four carboxylate O atoms in a square pyramidal geometry and the ligands on the apical positions are disordered and occupied by either water, THF, pyridine or methanol molecules. In 1, all the hydroxyl O atoms are involved in hydrogen bonding, which results in the formation of a 3D network. π-π stacking interactions involving phenyl rings were observed in the crystal structures of HG2, 2a and 2b. © 2014 Elsevier Ltd. All rights reserved.

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