SYNTHESIS, CRYSTAL-STRUCTURE AND MAGNETIC-PROPERTIES OF [FE-2(BPYM)(C5O5)(2)(H2O)(4)]CENTER-DOT-2H(2)O AND 2 POLYMORPHS OF [FE-2(BPYM)(C4O4)(2)(H2O)(6)]CENTER-DOT-2H(2)O (BPYM = 2,2'-BIPYRIMIDINE)

Two dinuclear iron(II) complexes of formulae [Fe-2(bpym) (C5O5)(2)(H2O )(4)]. 2H(2)O (1) and [Fe-2(bpym) (C4O4)(2)(H2O)(6)]. 2H(2)O (2) (bpym = 2,2-bipyrimidine, C5O52- = dianion of croconic acid (4,5-dihydroxyc yclopent-4-ene-1,2,3-trione) and C4O42- = dianion of squaric acid (3,4 -dihydroxycyclobut-3-ene-1,2-dione)) were prepared and their crystal s tructures (at 103 K) determined by X-ray diffraction methods. The stru cture of 1 consists of neutral centrosymmetric [Fe-2(bpym) (C5O5)(2)(H 2O)(4)] units and water molecules of crystallization which are linked by an extensive network of hydrogen bonds. The coordination geometry a round each iron atom is that of a compressed octahedron with bpym nitr ogen atoms and croconate oxygen atoms in the equatorial positions, and water molecules in the axial positions. The croconate and bpym ligand s adopt bidentate and bisbidentate coordination modes respectively. Co mpound 2 exhibits two different polymorphic forms, denoted 2i and 2ii. The two molecular structures are made up of neutral centrosymmetric [ Fe-2(bpym) (C4O4)(2)(H2O)(6)] units and water of hydration, the main d ifference between the two forms being in the packing and hydrogen bond pattern. Each iron atom has a highly distorted octahedral geometry wi th bpym nitrogen atoms, one squarate oxygen and a water molecule in eq uatorial positions, and water molecules in axial positions. Squarate a nd bpym ligands adopt monodentate and bisbidentate coordination modes respectively. The intradimer metal-metal separations are 5.829(2) (1), 5.869(1) (2i) and 5.941(3) Angstrom (2ii). The magnetic behaviour of 1 and 2i in the temperature range 290-2 K is characteristic of an intr adimer antiferromagnetic coupling, the susceptibility curves exhibitin g maxima at 11.5 (1) and 18.5 K (2i). A magnetostructural comparison w ith other bpym-bridged iron(II) complexes is carried out. (C) 1998 Els evier Science S.A. All rights reserved.