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

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 (H G1 –H G4 ). All compounds showed diagnostic bands in their FTIR spectra for the coordinated carboxylate groups with separations of CO asym and CO sym 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 H G2, 1, 2a, and 2b were characterized unambiguously with single crystal X-ray diffraction. Three molecules constituted the asymmetric unit of H G2 in the monoclinic space group P 2 1 / 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 H G2, 2a and 2b.