Synthesis, crystal structure, and catalytic studies on dinuclear copper(II) mesocates

Imine based bis-bidentate ligands H-2-m-xysal, (L1H2); H-2-m-xysal-Cl, (L2H2); H-2-m-xysal-Br, (L3H2); H-2-m-xysal-OCH3, (L4H2); H-2-m-xysal-(t-Bu)(2), (L5H2) were synthesized and characterized. These substituted 1,3-bis(hydroxylbenzyl)-diaminoxylene dianion ligands upon treating with copper(II) acetate in 2: 2 equivalent of L:M ratio, resulted in a series of binuclear [Cu-2(m-xysal)(2)] neutral complexes 1-5. The crystal structures determined for the complexes 1 and 2 indicate a dinuclear association. The CH center dot center dot center dot pi interaction observed between the metal-chelate ring and the hydrogens associated with m-xylene spacer moiety being first in this series of complexes, is demonstrated to stabilize the helical conformation through intramolecular self assembly process. The position of the resonance on the EPR spectra and the absence of Delta Ms = +/-1 feature for the complexes 2, 3, and 5 obtained for room temperature solid state samples revealed that the metal centers though exist in the dinuclear unit, they are separated from each other and possess a non-interacting monomer-type metal-metal association. The Cu(II) centers in all these complexes possessing an intermediate geometry between tetrahedral and square planar, an appropriate catalytic study converting 4-nitrobenzaldehye to corresponding nitroaldol was carried out using complex 5. (C) 2011 Elsevier B.V. All rights reserved.