Ionic Crystal Containing Protons (H+) as Counter Cations: Preparation and Crystal Structure of a Salt of 4,4′-Bipiperidine-1,1′-diium and Hexacyanidoferrate(II)

A molecular salt with a bulky cation and anion, (H2bipip)(H)2[Fe(CN)6] (1) (H2bipip2+: 4,4’-bipiperidine-1,1’-diium), has been prepared. The salt comprises protons (H+) as counter cations. Single-crystal X-ray structure analysis revealed that the proton is trapped between two cyanido ligands of [Fe(CN)6]4– anions, forming a very short N⋯H+⋯N hydrogen bond.