The Key Role of Hydrogen Bonding in the Nuclearity of Three Copper(II) Complexes with Hydrazone-derived Ligands and Nitrogen Donor Heterocycles

Three new Cu(II) complexes of formula [Cu(L-1)(pyz)(CH3OH)]ClO4 (1), [Cu(L-1)(4,4'-bpy)(ClO4)]center dot 0.5H(2)O (2) and [{Cu(L-2)(ClO4)}(2)(mu-4,4'-bpy)] (3) have been synthesised by using pyrazine (pyz) and 4,4'-bipyridine (4,4'-bpy) and tridentate O,N,O-donor hydrazone ligands, (LH)-H-1 and (LH)-H-2, obtained by the condensation of 1,1,1-trifluoro-2,4-pentanedione with salicyloylhydrazide and benzhydrazide, respectively. The ligands and their complexes have been characterized by elemental analyses, FT-IR, and UV-Vis spectroscopies. Single crystal X-ray structure analysis evidences the metal ion in a slightly deformed square pyramidal geometry in all the complexes. However complexes 1 and 2 are mononuclear with pyz and 4,4'-bpy, respectively, showing an unusual monodentate behavior, while complex 3 is dinuclear with 4,4'-bpy adopting the typical bridging coordination mode. Self assembly of the complex units by hydrogen bonding interactions produces one-dimensional arrangement in each crystal packing. The magnetic characterization of complex 3 indicates a weak antiferromagnetic exchange interaction between the Cu(II) ions (J = -0.96 cm (1)) mediated through the long 4,4'-bpy bridge. Electrochemical behavior of the complexes is also discussed.