Iron porphyrin complex with hydrogen bonding (H-Bonding) distal site stabilize a low spin (LS) ground state while a complex with a hydrophobic distal site and bearing the same axial ligand as the former stabilizes a high spin (HS) ground state. DFT calculations indicate that while H-Bonding to an anionic ligand weakens its bonding with a metal, H-Bonding to a neutral ligand like H2O significantly strengthens its bonding to a metal. The stronger FeIII-OHx bond in the complex bearing a H-Bonding distal site, which bears H-Bonding distal site, leads to stabilization of a LS ground state. The reorganization energies (?) of these thiolate and imidazole bound iron porphyrin sites for both high and low spin states are experimentally determined. The data indicate that the ? of a low LS thiolate bound iron porphyrin complex (PSRFe-OH2) is comparable to a LS imidazole bound complex with a trans hydroxide ligand (PImFe-OH) but greater than those reported for histidine bound LS heme sites (e.g. heme a). DFT calculations reproduce these data and indicate that the dominant anisotropic p covalency of thiolate ligand in the PSRFe-OH2 complex and the PCET involved in the reduction of the PImFe-OH complex is responsible for the higher l in these systems. The computed ? depends on both nature of axial ligand and spin state and presence of hydrogen bonding to the axial ligands. These results are cogitated considering the nature of the redox active molecular orbital (RAMO) involved and large electronic relaxation present in the thiolate bound active sites.  The control of ? by axial ligand and spin state can tune the reactivities of these biological active sites.

Bio-inspired electrodes modelling the active site of cytochrome P450: Electronic structure ? ? ? reactivity

248th National Meeting (2014)

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