Reversible high-temperature phase transition of a manganese(II) formate framework with imidazolium cations
A new metal–formate framework, poly[1H-imidazol-3-ium [tri-μ2-formato-manganese(II)]], {(C3H5N2)[Mn(HCOO)3]}n, was synthesized and its structural phase transition was studied by thermal analysis and variable-temperature X-ray diffraction analysis. The transition temperature is around 435 K. The high-temperature phase is tetragonal and the low-temperature phase is monoclinic, with a β angle close to 90°. The relationship of the unit cells between the two phases can be described as:aHT= 0.5aLT+ 0.5bLT;bHT= −0.5aLT+ 0.5bLT;cHT = 0.5cLT. In the high-temperature phase, both the framework and the guest 1H-imidazol-3-ium (HIm) cations are disordered; the HIm cations are located about 2mmsites and were modelled as fourfold disordered. The Mn and a formate C atom are located on fourfold rotary inversion axes, while another formate C atom is on a mirror plane. The low-temperature structure is ordered and consists of two crystallographically independent HIm cations and two crystallographically independent Mn2+ions. The phase transition is attributable to the order–disorder transition of the HIm cations.