2,2'-BIPYRIMIDINE (BIPYM)-BRIDGED DINUCLEAR COMPLEXES .2. SYNTHESIS, CRYSTAL-STRUCTURE AND MAGNETIC-PROPERTIES OF [FE2(H2O)8(BIPYM)][SO4]2.2H2O AND [FE2(H2O)6(BIPYM)(SO4)2]

Two new dinuclear iron(II) complexes of formulae [Fe2(H2O)8(bipym)][SO4]2.2H2O 1 and [Fe2(H2O)6-(bipym)(SO4)2] 2 (bipym = 2.2'-bipyrimidine) have been synthesised and their crystal structures determined by single-crystal X-ray diffraction. Crystals of 1 and 2 are monoclinic, space group P2(1)/c with a = 8.138(1), b = 11.661(2), c = 11.886(2) angstrom, beta = 91.85(l)-degrees and Z = 2 for l and space group P2(1)/n with a = 6.275(2), b = 13.550(4), c = 10.937(2) angstrom, beta = 96.43(2)-degrees and Z = 2 for 2. The structure of 1 consists of centrosymmetric dinuclear cations [Fe2(H2O)8(bipym)]4+, unco-ordinated sulfate anions and water of crystallization whereas that of 2 is made up of neutral dinuclear [Fe2(H2O)6(bipym) (SO4)2] units. The coordination geometry around each iron atom is that of a highly distorted octahedron: the Fe(II)-N distances are longer (average value 2.22 angstrom) than the Fe(II)-O ones (average values 2.11 and 2.09 in 1 and 2, respectively). In both complexes, the bipyrimidine group joins two adjacent iron atoms acting in a bis(chelating) fashion. The C-C bond between the pyrimidine rings of bipym is perpendicular to the Fe ... Fe vector giving two five-membered chelate rings, the bite angle of bipym being 74.9(l )-degrees in 1 and 74.1 (1 )-degrees in 2. The intramolecular metal-metal separation is 5.836(l ) and 5.909(l ) angstrom in 1 and 2, respectively. Magnetic susceptibility versus temperature data for both compounds were fitted to the Heisenberg-Dirac-Van-Vleck S1 = S2 = 2 spin exchange model with J = -3.4 cm-1, g = 2.28 and theta = -0.7 cm-1 for 1 and J = -3.1 cm-1, g = 2.23 and theta = 1.3 cm-1 for 2. The efficiency of bipym to transmit electronic effects between iron(II) ions is compared to that of related oxygen-donor potentially bis(chelating) ligands.