Dihydrohexacyanoferrates of N-heterocyclic cations

Dihydrohexacyanoferrates (II and III) of aromatic N-heterocyclic cations X+ (such as N-methyiquinoxalinium, pyridinium, dipyridinium) and X2+ (such as pyridylpyridinium, dipyridinium) are synthesized and characterized. For the first time, the crystal structures of acidic dihydrohexacyanoferrates are described. The formation of the X2+H2[Fe(CN)(6)] and X2+H2[Fe(CN)(6)] species which contain the [Fe(CN)(6)](4-) and [Fe(CN)(4)(CNH)(2)](2-) anions from acidic solutions occurs after the formation of the H[Fe(CN)(6)](3-) species as can be established from the outer-sphere charge transfer (OSCT) bands in the absorption spectra. The crystal structures of these species contain extensive network of intermolecular N-H center dot center dot center dot N, N-H center dot center dot center dot O and O-H center dot center dot center dot N hydrogen bonds which link the hexacyanoferrate anions with solvent water (if present) and N-heterocyclic cations if the later can participate in the H-bond formation. In the crystals of dihydrohexacyanoferrates, the H-bond networks can be two-dimensional (species 1) and three-dimensional (species 2-7). The lack of acidic protons for the H-bond network formation can be compensated by solvent water molecules. The H-bond network plays an important role in stabilization of such strongly-acidic species such as the H(2)Bpy(2+) and HPypy(2+) cations and the [Fe-11(CN)(4)(CNH)(2)](2-) anion. (C) 2007 Elsevier B.V. All rights reserved.