Dihydrohexacyanoferrates of N-heterocyclic cations

Abstract Dihydrohexacyanoferrates (II and III) of aromatic N-heterocyclic cations X+ (such as N-methylquinoxalinium, 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 X 2 + H 2 [ 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⋯N, N–H⋯O and O–H⋯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 H2Bpy2+ and HPypy2+ cations and the [FeII(CN)4(CNH)2]2− anion.