Structural variation and magnetic properties of tetrakis(μ2-carboxylate)-bridged dicopper(II) complexes of betaines with different axial ligands

Three tetrakis(mu(2)-carboxylate)-bridged dicopper(II) complexes of betaines, [Cu-2(pybet)(4)(H2O)(2)](NO3)(2)(ClO4)(2). 2H(2)O (pybet = pyridinioacetate, C5H5N+CH2CO2-, pybet) (1), [Cu-2(pybet)(4)(H2O)Cl](ClO4)(3). 2H(2)O (2) and [Cu-2(ppbet)(4)Cl2](ClO4)(2) (ppbet = pyridiniopropionate, C5H5N+CH2CH2CO2-) (3), have been synthesized and characterized by X-ray crystallography. These complexes all comprise [Cu-2(mu(2)-carboxylate)(2)] structures (Cu ... Cu = 2.677(1)-2.760(1) Angstrom) with each copper(II) atom being coordinated in a square;pyramidal CuO4L environment, in which the axial ligands (L) are two aqua molecules (Cu-O = 2.093(2) Angstrom), one aqua molecule and one chloride ion (Cu-O = 2.130(8), Cu-Cl = 2.358(4) Angstrom) and, two chloride ions (Cu-Cl = 2.454(2) Angstrom) in complexes 1, 2 and 3, respectively. The intramolecular Cu Cu distance may correlate to the mutual repulsion between the axial and basal metal-ligand bonds. Temperature-dependent magnetic susceptibility data of the powder samples of the complexes have been fitted, yielding the intramolecular coupling constant 2J values of -340, -321 and -301 cm(-1) for 1, 2 and 3, respectively. The magnitudes of 2J may be inversely correlated to the displacement of the metal atom from the basal plane. (C) 1998 Elsevier Science Ltd. All rights reserved.