2 DIFFERENT (OXALATO)(BIPYRIDINE)COPPER(II) COMPLEXES IN ONE SINGLE-CRYSTAL - CRYSTAL-STRUCTURES AND MAGNETIC-PROPERTIES OF [CU2(BIPY)2(H2O)2(C2O4)]X2.[CU(BIPY)(C2O4)](X = NO3-, BF4- OR CLO4-)

Three complexes of formula [Cu2(bipy)2(H2O)2(C2O4)]X2.[Cu(bipy)(C2O4)] (bipy = 2,2'-bipyridine; C2O42- = oxalate, X = NO3- 1, BF4- 2 or ClO4- 3 have been synthesised and their crystal structures determined by single-crystal X-ray diffraction methods at room temperature. They are all isostructural and crystallize in the monoclinic system, space group C2/c, Z = 4, with a = 21.739(2), b = 10.458(1), c = 16.023(2) angstrom, beta = 95.69(1)-degrees for 1, a = 22.740(5), b = 10.508(1), c = 16.129(2) angstrom, beta = 92.32(1)-degrees for 2, and a = 22.819(3), b = 10.583(2), c = 16.389(2) angstrom, beta = 91.52(2)-degrees for 3. The crystal structures of 1 and 2 were determined from Patterson and Fourier analysis and refined by full-matrix least-squares techniques, whereas that of 3 was solved by direct methods. Final values of the reliability factors R (R') were 0.029 (0.051 ) for 1, 0.033 (0.046) for 2 and 0.057 (0.059) for 3 with 3290, 3526 and 1972 observed reflections respectively. Their structures consist of cationic centrosymmetric dinuclear [Cu2(bipy)2(H2O)2(C2O4)]2+ units, neutral axiosymmetric mononuclear [Cu(bipy)(C2O4)] entities and either NO3-, BF4- or ClO4- as counter ion. Each copper atom of the dinuclear species is in a square-pyramidal environment with two oxalate oxygen and two bipyridine nitrogen atoms as a base and a water molecule at the apical position. The copper atom of the mononuclear complex is in a slightly tetrahedrally distorted square comprised of two bipyridine nitrogen and two oxalate oxygen atoms. In both complexes one or two more distant atoms of the counter ion completes a (5 + 1) or a (4 + 2) copper co-ordination, respectively. The mono- and di-nuclear entities form an alternating chain via weak interactions through counter ions and copper atoms. Variable-temperature (20-300 K) magnetic susceptibility measurements revealed a strong antiferromagnetic interaction within the dinuclear unit, the singlet-triplet energy gap being -386, -378 and -376 cm-1 for 1, 2 and 3, respectively. The chi(M)T versus T curve for all three complexes exhibits a plateau at T <80 K which corresponds to the Curie law expected for the mononuclear complex. The magnitude of the exchange coupling in this series has been analysed in the framework of a simple orbital model.