MAGNETOSTRUCTURAL EFFECTS OF THE JAHN-TELLER DISTORTIONS ON 2,2'-BIPYRIMIDINE (BPM-)BRIDGED DINUCLEAR COPPER(II) COMPLEXES - CRYSTAL-STRUCTURES AND MAGNETIC-PROPERTIES OF [CU2(BPM)(H2O)(4)(SO4)(2)]CENTER-DOT-3H(2)O AND [CU-2(BPM)(H2O)(8)](SO4)(2)CENTER-DOT-2H(2)O

Two new copper(II) compounds of formula [Cu-2(bpm)(H2O)(4)(SO4)(2)]. 3H(2)O (1) and [Cu-2(bpm)(H2O)(8)](SO4)(2) . 2H(2)O (2) (bpm = 2,2'-bipyrimidine;) have been synthesized and structurally characterized by X-ray diffraction. Compound 1 crystallizes in the monoclinic system, space group P2(1)/n, with a = 7.651(1) Angstrom, b = 25.119(3) Angstrom, c = 11.050(1) Angstrom, beta = 108.80(1)degrees, Z = 4 and V = 2010.4(4) Angstrom(3), whereas compound 2 is orthorhombic, space group Pbcn, with 13.401(2) Angstrom, b = 12.116(2) Angstrom c = 14.178(4) Angstrom, Z = 4, and V = 2302.0(6) Angstrom(3). The structure of 1 consists of a 2D-array of copper(II) ions bridged by bis-bidentate bpm groups and bis-unidentate sulfate. Each copper atom in 1 is in an elongated octahedral CuO6 surrounded by two water molecules and two nitrogen atoms from bpm building the basal plane and two sulfate oxygens in the axial positions. The structure of 2 is made up of noncentrosymmetric dinuclear [Cu-2(bpm)(H2O)(8)](4+) cationic units, uncoordinated sulfate anions, and water of crystallization. Both copper(II) ions in 2 have distorted octahedral CuO6 surroundings, one being compressed and the other elongated. Compounds 1 and 2 exhibit antiferromagnetic coupling, which is relatively strong in the former (J(singlet-triplet energy gap) = -159 cm(-1)) and weak in the latter (J = -24 cm(-1)). The influence of the distortions (elongation or compression) of the copper environment on the exchange coupling in 1 and 2 is analyzed by extended Huckel calculations in the framework of a simple orbital model.