Bis- and tris-(oxalato)ferrate(III) complexes as precursors of polynuclear compounds

The preparation and crystal structure of two oxalato-bridged Na-I - Fe-III compounds, [Na3Fe(ox)(3)(H2O)(4)] . H2O (1) and [Fe-II(phen)(3)][NaFe(ox)(3)(H2O)(3)] . 4H(2)O (5), two mononuclear Fe-III complexes AsPh4[Fe(bipy)(ox)(2)] . H2O ( 3) and AsPh4[Fe(phen)(ox)(2)] . H2O (4) and an oxalato-bridged Fe-III compound [AsPh4](4)[ Fe-2(ox)(5)] . 5H(2)O (2) (ox = oxalate dianion, bipy = 2,2'- bipyridine, phen = 1,10-phenantroline and AsPh4+ = tetraphenylarsonium cation) are reported here. The structure of 1 consists of infinite anionic [NaFe(ox)(3)](2-) layers linked trough centrosymmetric [Na-4(H2O)(8)](4+) tetranuclear units yielding a three-dimensional motif. Crystallization water molecules ensure the cohesion of the crystal lattice in 1. The iron environment is distorted octahedral being surrounded by six oxygen atoms of three oxalate groups, with an average iron to oxalate-oxygen bond distance of 2.012(3) Angstrom. There are four crystallographically independent sodium atoms, all of which have a highly distorted octahedral environment. Six oxalate-oxygen atoms are bound to Na(1) and Na( 2), whereas the other two sodium atoms are surrounded by four water oxygen and two oxalate oxygen atoms [ Na( 3)] and three water oxygen and three oxalate oxygen atoms [ Na( 4)]. The structure of compound 2 consists of centrosymmetric dinuclear [Fe-2(ox)(5)](4-) anions, AsPh4+ cations and crystallization water molecules. The anionic unit contains two Fe(III) atoms, four bidentate and one bridging oxalato ligands. Each iron atom has a distorted octahedral environment. The structures of 3 and 4 consist of discrete [Fe(AA)( ox)(2)](-) [AA = bipy (3) and phen (4)] anions, tetraphenylarsonium cations and uncoordinated water molecules which are linked by hydrogen bonds. Each iron atom is six-coordinated with two nitrogen atoms from the bidentate AA ligand and four oxalate-oxygen atoms building a distorted octahedral environment. The structure of compound 5 consists of anionic [Fe(ox)(3)Na(H2O)(3)](2-) chains, tris-chelated low-spin iron(II) [Fe(phen)(3)](2+) units and lattice water molecules linked to the chains through hydrogen bonds in which the water molecules and oxygen atoms of the oxalate groups are involved. The chains contain Fe3+ and Na+ cations alternatively bridged by oxalate groups, the tris-chelated [Fe(ox)(3)](3-) unit acting as a bidentate ligand through one oxalato group toward a central sodium atom. Magnetic susceptibilty measurements in the temperature range 2.0 - 290 K for 2 that reveal the occurrence of antiferromagnetic coupling ( J = 26.5 cm(-1), the Hamiltonian being defined as H = - 2JS(1)S(2)) between the high-spin iron(III) ions across the oxalato bridge.