SYNTHESIS, CRYSTAL-STRUCTURE AND MAGNETIC-PROPERTIES OF THE ALTERNATING CHAIN [VO(H2O)5][CU2(BIPYM)2(H2O)(SO4)3]CENTER-DOT-7H2O (BIPYM = 2,2'-BIPYRIMIDINE)

The new compound [VO(H2O)5][Cu2(bipym)2(H2O)(SO4)3].7H2O 1 (bipym = 2, 2'-bipyrimidine) has been synthesised and its crystal structure determ ined by single-crystal X-ray diffraction [monoclinic, space group P2(1 )/c, a = 9.944(3), b = 1 3.928(6), c = 26.275(6) angstrom, beta = 99.5 9(2)degrees and Z = 4]. Its structure consists of anionic alternating zigzag chains of bipym-bridged copper(II) ions with aqua and sulfato l igands. The electroneutrality is achieved by pentaaquaoxovanadium(IV) cations. The coordination geometry around the copper(II) ions can be d escribed as distorted CuN4O2 octahedra: that around Cu(1) comprises th ree nitrogens from bipym and an oxygen from sulfate in the basal posit ions and a nitrogen from bipym and an oxygen of another sulfate group in the axial sites; three nitrogens from bipym and an oxygen from sulf ate occupy the equatorial positions around Cu(2), whereas a nitrogen f rom bipym and an oxygen from water fill the axial sites. The bipyrimid ine group joins two adjacent copper atoms acting in a bis(chelating) f ashion. The vanadium atom in the pentaaquavanadyl cation is approximat ely at the centre of an octahedron defined by five oxygens from water molecules and an oxo group. The V(IV)-O (oxo) bond distance [1.577(5) angstrom] is shorter than that of V(IV)-O(water) (average 2.025 angstr om). The intrachain copper-copper separations are 5.505(2) and 5.675(2 ) angstrom. Variable-temperature (4.2-300 K) magnetic susceptibility d ata were fitted by the alternating-chain spin-exchange model [H = -J S IGMA (S2i.S2i-1 + alpha S2i.S2i + 1), alpha = alternation parameter] a nd to the usual Bleaney-Bowers expression for a copper(II) pair with t he inclusion in both cases of the Curie law for an isolated S = 1/2 sp ecies in order to account for the presence of the vanadyl cation. The values of the exchange coupling are discussed in the light of the stru ctural data and compared to those reported for other bipym-bridged cop per(II) complexes.