Synthesis crystal structure and magnetic properties of [Fe2(bpym)(C5O5)2(H2O)4]·2H2O and two polymorphs of [Fe2(bpym)(C4O4)2(H2O)6]·2H2O (bpym = 2,2′-bipyrimidine)

Two dinuclear iron(II) complexes of formulae [Fe2(bpym)(C5O5)2(H2O)4]·2H2O (1) and [Fe2(bpym)(C4O4)2(H2O)6]·2H2O (2) (bpym = 2,2-bipyrimidine, C5O52 = dianion of croconic acid (4,5-dihydroxycylopent-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 structures (at 103 K) determined by X-ray diffraction methods. The structure of 1 consists of neutral centrosymmetric [Fe2(bpym)(C5O5)2(H2O)4] units and water molecules of crystallization which are linked by an extensive network of hydrogen bonds. The coordination geometry around each iron atom is that of a compressed octahedron with bpym nitrogen atoms and croconate oxygen atoms in the equatorial position and water molecules in the axial positions. The croconate and bpym ligands adopt bidentate and bisbidentate coordination modes respectively. Compound 2 exhibits two different polymorphic forms, denoted 2i and 2ii. The two molecular structures are made up of neutral centrosymmetric [Fe2(bpym)(C4O4)2(H2O)6] units and water of hydration, the main difference between the two forms being in the packing and hydrogen bond pattern. Each iron atom has a highly distorted octahedral geometry with bpym nitrogen atoms, one squarate oxygen and a water molecule in equatorial positions, and water molecules in axial positions. Squarate and 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) Å (2ii). The magnetic behaviourof 1 and 2i in the temperature range 290-2 K is characteristic of an intradimer antiferromagnetic coupling, the susceptibility curves exhibiting maxima at 11.5 (1) and 18.5 K (2i). A magnetostructural comparison with other bpym-bridged iron(II) complexes is carried out.