Investigation of the structure and phase transitions of the polymeric inorganic–organic hybrids: [M(Im)₄V₂O₆]_∞; M = Mn, Co, Ni, Im = imidazole

The polymeric isomorphous hybrid inorganic–organic vanadium oxide compounds [M(Im)₄V₂O₆]_∞, M = Mn, Co, Ni, Im = imidazole, were investigated between 100 (2) and 295 (2) K by single-crystal X-ray diffraction. The compounds contain two-dimensional polymeric sheets packed perpendicularly to c* and undergo a reversible order–disorder phase transition. The room-temperature P4₂/n disordered phase (Z = 8) is reversibly transformed to the I4₁/a ordered phase (Z = 32) below 281 K for the Mn and 175 K for the Co compounds, requiring a change of the hydrogen-bond connectivity for two of the eight imidazoles per asymmetric unit of the I4₁/a structure. The ordered phase of the Ni compound was found to have the space group P2/n (Z = 8) at 100 K. Models for the phase-transition mechanisms are considered.