New Tricks by Old Anions: Hydrogen Bonded Hexacyanoferrous Anionic Networks

Hexacyanoferrates are well-known to form metal organic frameworks by coordination to metal atoms or acting as hydrogen bond acceptors. In this paper we report a new type of hexacyanoferrate self-assembly, based on direct hydrogen bonding of partially protonated hexacyanoferrate anions. By preparing a series of 15 hexacyanoferrates with various organic bases, we have found that protonated hexacyanoferrates (present in 10 structures) readily form chains (two structures), two-dimensional (four structures), or three-dimensional networks (four structures), whereby the dimensionality of the network generally increases with the protonation degree of the hexacyanoferrates. The exact mode of the self-assembly, including the network type, depends on fine interplay of the pK(a) value of the base, its steric properties, and the stoichiometry of the formed solid.