Understanding Disorder and Crystallographic Phase Changes in Metalloporphyrins

A temperature-dependent crystallographic phase change has been observed in [Co(TPP)(NO)]. Crystal structures at multiple temperatures reveal that the system changes from a system with eightfold disorder at 250 K to one that is more ordered below 195 K. Interestingly, at 190 K, it was possible to trap the structure in its transition state, resulting in three distinct structures of [Co(TPP)(NO)]. The phase change is reversible and has given insight into the nature of nitrosyl disorder in the porphyrin system as a dynamic disorder. The reproducibility of the phase change has been studied through both DSC measurements and crystallographic unit cell indexing on several crystals. In an attempt to understand the reason for the phase change, [Fe(TPP)Cl] and [Mn(TPP)(NO)(MeOH)] were synthesized and studied crystallographically at multiple temperatures.