Iron Salts with the Tetracyanidoborate Anion: [FeIII(H2O)6][B(CN)4]3, Coordination Polymer [FeII(H2O)2{κ2N[B(CN)4]}2], and [FeII(DMF)6][B(CN)4]2

By Nitschke, Christian & Köckerling, Martin
Published in Inorganic Chemistry NULL 2011

Abstract

The reaction of Fe(OH)3 with tetracyanidoboronic acid, H[B(CN)4]·xH2O, in water leads to the first examples of tetracyanidoborates with a triply charged metal cation, [FeIII(H2O)6][B(CN)4]3 (1). Using elemental iron powder as starting material, [FeII(H2O)2 κ2Î?B(CN)4] 2] (2) is obtained. Anhydrous iron(II) tetracyanidoborate, which is synthesized by heating of 2, is soluble in dry dimethylformamide. After evaporation of the DMF solvent, single crystals of the third title compound, [FeII(DMF)6][B(CN)4]2 (3), are obtained. Compound 3 is the first metal tetracyanidoborate soluble in nonpolar solvents. The title compounds have been characterized by single-crystal X-ray diffraction (1 rhombohedral, R3Ì…c (no. 167), a = 14.9017(7) Ã…, c = 20.486(1) Ã…, Z = 6; 2 tetragonal, I4Ì…2d (no. 122), a = 12.3662(3) Ã…, c = 9.2066(4) Ã…, Z = 4; 3 triclinic, P1Ì… (no. 2), a = 8.6255(3) Ã…, b = 11.0544(4) Ã…, c = 12.2377(5) Ã…, Z = 1). The metal ions in all three compounds are octahedrally coordinated. Whereas 1 and 3 are built up from isolated complex ions, 2 comprises a coordination polymer, in which the FeII ion is coordinated by two oxygen atoms of two water molecules in a trans orientation and four nitrogen donor atoms of the [B(CN)4]- groups, which bridge between neighboring iron ions. The iron(III) ion in 3 is in a perfect octahedral environment, which is formed by the O atoms of 6 molecules of water. The single-crystal X-ray structures, vibrational spectra, thermal properties, solubilities, and electrochemical characteristics are reported and compared with those of other known tetracyanidoborates.

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