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Synthesis, Structure, and Magnetic Properties of Closely Related Mono- and Di-Cu(II) Schiff Base Complexes: Formation of Water Clusters and Hydrogen-Bonded Networks

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posted on 06.01.2010, 00:00 by Snehadrinarayan Khatua, Jina Kang, Jung Oh Huh, Chang Seop Hong, David G. Churchill
The mononuclear complex [Cu(rslysH)(OAc)]·4.5H2O (1) [rslys = 6-amino-2-[(2-hydroxybenzylidene)amino]hexanoate] has been prepared via an easy “one-pot” synthetic method in which the corresponding d/l-lysine-derived Schiff base ligand is generated in situ. Further, the dinuclear acetate-bridged Cu(II) complex, [Cu2(rslys)2(μ-OAc)]·(OH)·6H2O (2) was obtained as a minor product along with the mononuclear compound from a “one pot” crystallization. These complexes have been characterized by UV/vis, IR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and single-crystal X-ray diffraction. The molecular structures of 2 confirm its dimeric nature with dicopper(II) phenolate and acetate-bridge environments. Variable-temperature magnetic analysis revealed a weak antiferromagnetic coupling interaction for the dicopper species mediated by the phenolate and bridging acetate groups. Large numbers of solvent water molecules are present in the lattice of the mononuclear compound 1. The carboxylate oxygen of the Schiff base ligand and terminal -NH2 groups engage in intermolecular hydrogen bonding with solvent water molecules, giving one- to three-dimensional (1- to 3-D) networks. The lattice water molecules form cyclic octameric water clusters and 1-D polymeric water cluster chains, which also assist the formation of 3-D hydrogen-bonded networks. Also in compound 2, 3-D hydrogen bonded networks are observed, but in contrast to the cyclic octanuclear water clusters and 1-D cluster chain for 1, infinite P- and M-helical water cluster chains and two-dimensional (2-D) sheets are present. A hexadecanuclear water cluster may be regarded as a formal building units for the 2-D sheet. The racemic d-/l-lysine used herein as a source of the Schiff base ligand led to racemic hydrogen-bonded networks containing enantiomeric complexes of d- and l-lysine Schiff base ligands in the packing structures for compounds 1 and 2.