Metalloporphyrin axial ligation is an important process in catalysis and the enzymatic chemistry of proteins and is metal center dependent. Direct structural dynamics measurements on different metalloporphyrins using X-ray transient absorption spectroscopy (XTA) have brought new insight into this extensively studied process. This review uses two representative open shell metalloporphyrins, nickel tetramesitylporphyrin (NiTMP) and iron protoporphyrin (FePP), to demonstrate the capability of XTA in resolving both electronic and nuclear structures of these porphyrins in axial ligation processes. A surprisingly broad 3d orbital energy level distribution has been related to differences in the conformational distribution of NiTMP in the ground and excited state, which suggests a unified mechanism for axial ligation in open shell metalloporphyrins driven by transient vacancies in the 3dz2 orbital, which is aligned with the axial ligation coordinate. XTA studies also show that dynamic and structural differences in the CO dissociation process are influenced by the metal site environments, demonstrated by the comparison of FePP in solution and heme in myoglobin. These results imply a porphyrin conformational control of ligation and therefore insight into metalloporphyrin catalyst design involving control the axial ligation/deligation processes as well as deduction of enzymatic regulation of ligand binding.
- Excited state molecular structures
- Photodissociation of the ligand
- X-ray transient absorption
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry
- Materials Chemistry