Polymorphs of phenazine hexacyanoferrate(II) hydrate: supramolecular isomerism in a 2D hydrogen-bonded network

The crystal structures of two polymorphs of a phenazine hexacyanoferrate(II) salt/cocrystal, with the formula (Hphen)₃[H₂Fe(CN)₆][H₃Fe(CN)₆]·2(phen)·2H₂O, are reported. The polymorphs are comprised of (Hphen)₂[H₂Fe(CN)₆] trimers and (Hphen)[(phen)₂(H₂O)₂][H₃Fe(CN)₆] hexamers connected into two-dimensional (2D) hydrogen-bonded networks through strong hydrogen bonds between the [H₂Fe(CN)₆]^2- and [H₃Fe(CN)₆]^- anions. The layers are further connected by hydrogen bonds, as well as through π-π stacking of phenazine moieties. Aside from the identical 2D hydrogen-bonded networks, the two polymorphs share phenazine stacks comprising both protonated and neutral phenazine molecules. On the other hand, the polymorphs differ in the conformation, placement and orientation of the hydrogen-bonded trimers and hexamers within the hydrogen-bonded networks, which leads to different packing of the hydrogen-bonded layers, as well as to different hydrogen bonding between the layers. Thus, aside from an exceptional number of symmetry-independent units (nine in total), these two polymorphs show how robust structural motifs, such as charge-assisted hydrogen bonding or π-stacking, allow for different arrangements of the supramolecular units, resulting in polymorphism.