COPPER(II) COMPLEXES OF 2 NEW OXAMIDATE BIS-TETRADENTATE SCHIFF-BASE LIGANDS

Two novel symmetrical bis-tetradentate Schiff base ligands, H2L1 and H 4L2, have been prepared from the condensation of N,N'-bis(3-aminopropy l) oxamide with pyridine-2-carbaldehyde and pyrrol-2-carbaldehyde, res pectively. The ligands react with copper(II) nitrate in basic medium t o yield the dinuclear complexes [Cu2L1](NO3)(2) and [Cu2L2]. [Cu2L1](N O3)(2) reacts with sodium azide to yield the neutral dinuclear complex [Cu2L1(N-3)(2)], which was structurally characterized by X-ray diffra ction. The complex crystallizes in the monoclinic system, space group P2(1)/c with cell constants: a = 7.916(2) Angstrom, c = 20.110(4) Angs trom; beta = 95.20(3)degrees, V = 1255.4(4) Angstrom(3), Z = 2. The st ructure consists of neutral quasi-planar centrosymmetric dinuclear cop per(II) molecules, where two crystallographically equivalent copper(II ) ions are bridged by an oxamidate group in li ans conformation, resul ting in a copper-copper distance of 5.271 Angstrom. Each copper(II) io n occupies the N3O inner Site of the doubly-deprotonated L-1 ligand wi th bond distances of ca 2.0 Angstrom, whereas a nitrogen atom from the azide group completes the coordination polyhedron with a longer bond distance of 2.340(4) Angstrom. All the dinuclear complexes exhibited s trong antiferromagnetic exchange interaction. The magnetic data were a nalyzed by using the Bleaney-Bowers equation (H = -JS(1)S(2)) leading to singlet-triplet energy values (J) of 425.5 cm(-1) for [Cu2L1](NO3)( 2), 427.9 cm(-1) for [Cu2L2] and 424.1 cm(-1) for [Cu-2(L-1)(N-3)(2)]. These J values are a consequence of the ability of the oxamidate-brid ge to propagate the antiferromagnetic exchange, interaction between me tal ions which allow a large overlap of the d(x2-y2) magnetic orbitals centered on each copper(II) ion. (C) 1998 Elsevier Science Ltd. All r ights reserved.