title: Second sphere control of spin state: Differential tuning of axial ligand bonds in ferric porphyrin complexes by hydrogen bonding.
abstract: An iron porphyrin with a pre-organized hydrogen bonding (H-Bonding) distal architecture is utilized to avoid the inherent loss of entropy associated with H-Bonding from solvent (water) and mimic the behavior of metallo-enzyme active sites attributed to H-Bonding interactions of active site with the 2nd sphere residues. Resonance Raman (rR) data on these iron porphyrin complexes indicate that H-Bonding to an axial ligand like hydroxide can result in both stronger or weaker Fe(III)-OH bond relative to iron porphyrin complexes. The 6-coordinate (6C) complexes bearing water derived axial ligands, trans to imidazole or thiolate axial ligand with H-Bonding stabilize a low spin (LS) ground state (GS) when a complex without H-Bonding stabilizes a high spin (HS) ground state. DFT calculations reproduce the trend in the experimental data and provide a mechanism of how H-Bonding can indeed lead to stronger metal ligand bonds when the axial ligand donates an H-Bond and lead to weaker metal ligand bonds when the axial ligand accepts an H-Bond. The experimental and computational results explain how a weak Fe(III)-OH bond (due to H-Bonding) can lead to the stabilization of low spin ground state in synthetic mimics and in enzymes containing iron porphyrin active sites. H-Bonding to a water ligand bound to a reduced ferrous active site can only strengthen the Fe(II)-OH2 bond and thus exclusion of water and hydrophilic residues from distal sites of O2 binding/activating heme proteins is necessary to avoid inhibition of O2 binding by water. These results help demonstrate the predominant role played by H-Bonding and subtle changes in its orientation in determining the geometric and electronic structure of iron porphyrin based active sites in nature.
authors.affiliation: Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata, West Bengal 700032, India.
authors.forename: Kaustuv
authors.initials: K
authors.lastname: Mittra
authors.affiliation: Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata, West Bengal 700032, India.
authors.forename: Kushal
authors.initials: K
authors.lastname: Sengupta
authors.affiliation: Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata, West Bengal 700032, India.
authors.forename: Asmita
authors.initials: A
authors.lastname: Singha
authors.affiliation: Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata, West Bengal 700032, India.
authors.forename: Sabyasachi
authors.initials: S
authors.lastname: Bandyopadhyay
authors.affiliation: Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata, West Bengal 700032, India.
authors.forename: Sudipta
authors.initials: S
authors.lastname: Chatterjee
authors.affiliation: Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata, West Bengal 700032, India.
authors.forename: Atanu
authors.initials: A
authors.lastname: Rana
authors.affiliation: Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata, West Bengal 700032, India.
authors.forename: Subhra
authors.initials: S
authors.lastname: Samanta
authors.affiliation: Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata, West Bengal 700032, India. Electronic address: icad@iacs.res.in.
authors.forename: Abhishek
authors.initials: A
authors.lastname: Dey
mesh_terms: D005290:Ferric Compounds / Q000737:chemistry*; D006860:Hydrogen Bonding; D011166:Porphyrins / Q000737:chemistry*; D013059:Spectrum Analysis, Raman
chemical_list: D005290:Ferric Compounds; D011166:Porphyrins
keywords: Hydrogen bonding; Iron hydroxide vibration; Resonance Raman; Spin state