Heme oxygenase (HO) is the only enzyme in mammals known to catalyse the physiological degradation of unwanted heme into biliverdin, Fe ion and CO. The process involves introduction of the hydroxyl group at one of itsmeso-positions as the first fundamental step of the heme cleavage process. It was also found thatmeso-amino heme undergoes similar ring-cleavage process while reacting with dioxygen in presence of pyridine as an axial ligand. The present paper briefly reviews the reactions of modelmeso-hydroxylated heme and its analogues with dioxygen, and their relevance in the heme degradation process.

Synthesis, structure and properties of new five-coordinate phenolate complexes of di-iron(III) bisporphyrin are reported here, in which phenol binds in $\eta^1$-fashion as an axial ligand. The solid and solution EPR at 120K and ¹H NMR spectral pattern in solution provide unequivocal evidence for the high spin (S = 5/2) nature of the complex. Mulliken spin density calculation using DFT demonstrates the positive spin densities at the meso carbons and negative spin densities at the methylene carbons and, as a result, the meso and methylene protons are shifted in the upfield and down field regions, respectively in the ¹H NMR spectra of the molecule. Also, the ortho- and para-protons of the phenolate ligands are observed to be shifted in the upfield region while meta-protons are shifted downfield. The alternating shift pattern, which is the opposite sign of the chemical shifts for meta-versus ortho- and para-protons, was also explained due to negative and positive spin densities, respectively on the carbons and indicative of 𝜋 spin delocalization on the phenolate ligand. Thus, the calculated spin density maps accounted for the essential ¹H NMR spectroscopic features that are observed here for the phenolate complexes of di-iron(III) bisporphyrin. The temperature dependence of the signals follows the Curie law which is indicative of single spin state throughout the temperature range of −40 to +40°C. The single crystal X-ray structure of the corresponding chloro derivative, trans 1,2-bis(chloroiron(III) octaethyl porphyrinyl)ethene, has also been reported here which authenticates the high-spin nature of the complex.

Highly flexible Zn(II)1,2-bis(meso-octaethylporphyrin)ethane (1) has been used as host in which two porphyrin rings are found to be face-to-face in non-coordinating solvents. Upon addition of one relatively smaller 4,4′-dipyridine (L¹) and one extended N,N′-bispyridine-4-yl-methylene ethylenediamine (L²) guest ligands, the syn conformation of 1 is switched to the anti complexes 1·(L¹)₂ and 1·L², respectively. Single crystal X-ray structures of all the complexes are reported in which a stable one-dimensional coordination polymer is produced only in 1·L² that is, to the best of our knowledge, the first structural report of 1D-coordination polymer with porphyrin dimer. Solution structures of the complexes along with binding studies in solution between 1 and L have also been investigated. The morphology of the polymeric complex 1·L² on silicon wafer surface was examined by Atomic Force Microscopy (AFM) in which the crystalline islands of well defined facets of size ranging from 200-550 nm perimeter and a height of 20-40 nm have been observed.

A series of complex with a general formula of M(Fc₂Ph₂P) [Fc₂Ph₂P = 5, 10-bisferrocenyl-15,20-bisphenylporphyrin (2−); M = Fe(III)Cl Fe(III)(ClO₄) and Cu(II)] have been synthesized and characterized. The single crystal X-ray structure of Cu^II(Fc₂Ph₂P) has been reported in which two ferrocene moieties are in anti form with respect to each other. The ferrocenyl groups of Cu^II(Fc₂Ph₂P) are more easily oxidized via a single two-electron quasi-reversible process compared to the free base ligand in which two 1e-oxidative response separated by 0.23 V are observed. Electrochemical study of Fe^III(Fc₂Ph₂P)Cl revealed ferrocenebased two-electron quasi-reversible oxidation at 0.72 V indicating no observable coupling of the ferrocene moieties. The higher oxidation state of Fe(III) reduces the electron releasing tendency of the porphyrin ring and thus make the ferrocene oxidation difficult. The porphyrin, however, lack substituents at the 𝛽-pyrrolic positions, and the ferrocenyl moieties are therefore free to rotate. The observed electrochemical analyses thus demonstrate that the oxidation of the ferrocene subunit is strongly affected by porphyrin ring as well as the central metal through extended 𝜋-conjugation.

The multiheme cytochrome c involves extensive interaction among the heme centers to enable them to perform a wide variety of enzymatic activities. In an attempt to exploit such heme–heme interactions in the synthetic diheme, an ethane-bridged diiron(III) porphyrin dimer has been utilized that can switch easilybetween syn and anti conformations due to highly flexible nature of the bridge. Upon protonation using 5% aqueous Brønsted acid, the dichloromethane solution of diiron(III)-µ-oxo porphyrin dimer immediately changes its color from green to red leading to the formation of a series of µ-hydroxo complexes. However, long exposure of the Brønsted acid converts the µ-hydroxo complex to five-coordinate diiron(III) porphyrin dimer in which the counter anion switches its position to act as an axial ligand. In the present study, the investigation has been extended further using Brønsted acids containing strongly coordinating anions such as HN₃, HNCS and l-(+)-Lactic acid (HLA) in which the anions eventually coordinate to iron centers directly as axial ligands. A comprehensive account of the anion-mediated spin state change in the ethane-bridged diiron(III) porphyrin dimer has also been presented here.