Second sphere control of spin state: Differential tuning of axial ligand bonds in ferric porphyrin complexes by hydrogen bonding

摘要: Abstract An iron porphyrin with a pre-organized hydrogen bonding (H-Bonding) distal architecture is utilized to avoid the inherent loss of entropy associated H-Bonding from solvent (water) and mimic behavior metallo-enzyme active sites attributed interactions site 2nd sphere residues. Resonance Raman (rR) data on these complexes indicate that an axial ligand like hydroxide can result in both stronger or weaker Fe(III)–OH bond relative complexes. The 6-coordinate (6C) bearing water derived ligands, trans imidazole thiolate stabilize low spin (LS) ground state (GS) when complex without stabilizes high (HS) state. DFT calculations reproduce trend experimental provide mechanism how indeed lead metal bonds donates H-Bond accepts H-Bond. computational results explain weak (due H-Bonding) stabilization synthetic mimics enzymes containing sites. bound reduced ferrous only strengthen Fe(II)–OH2 thus exclusion hydrophilic residues O2 binding/activating heme proteins necessary inhibition binding by water. These help demonstrate predominant role played subtle changes its orientation determining geometric electronic structure based nature.