Tetragonal to triclinic--a phase change for [Fe(TPP)(NO)].

The temperature dependence of the crystalline phase of (nitrosyl)(tetraphenylporphinato)iron(II), [Fe(TPP)(NO)], has been explored over the temperature range of 33-293 K. The crystalline complex is found in the tetragonal crystal system at higher temperatures and in the triclinic crystal system at lower temperatures. In the tetragonal system, the axial ligand is strongly disordered, with the molecule having crystallographically required 4/m symmetry, leading to eight distinct positions of the single nitrosyl oxygen atom. The phase transition to the triclinic crystal system leads to a partial ordering with the molecule now having inversion symmetry and disorder of the axial nitrosyl ligand over only two positions. The increase in ordering allows subtle molecular geometry features to be observed; in particular, an off-axis tilt of the Fe-N(NO) bond from the heme normal is apparent. The transition of the reversible phase change begins at about 250 K. This transition has been confirmed by both X-ray diffraction studies and a differential scanning calorimetry study.