Oxo and Nitrido Bridged Hemin Dimers

Iron porphyrins, the prosthetic group of the heme proteins are of course of great interest. Tetraphenylporphine, (TPPH2) (Fig. 2) is not a naturally occurring porphyrin. But because of its ease of preparation and close similarity to other porphyrins, tetraphenylporphinatoiron (TFPFe) derivatives have been extensively studied. Hemes are Fe(II) compounds such as TPPFe(II), and hemins are rive coordinate Fe(III) derivatives such as TFPFe(III) X (X = halide, pseudohalide, carboxylate, alkoxide, etc.)16). Six coordinate Fe(III) prophyrins are known as hemichromes, such as [TPPFe(III) L2] X (L = pyridine, piperidine, imidazole, etc.)16). Hemichromes are usually low spin, S = 1/2, systems 0 eff 1-9 B.M.) and hemins are of the high spin, 

About ten years ago, on the basis of kinetic studies, an oxo bridge between two Fe(III) porphyrins was proposed as an intermediate in the oxidation of an Fe(II) heme by 0219) and on the basis of analytical data as a possible product of that reaction20). In 1969 solid [(porphyrin) Fe(III)-0-Fe(III) (porphyrin)] dimers such as((TPPFe(III)20), 

Infrared spectra of oxo bridged Fe(III) complexes show a strong Fe-O-Fe asymmetric stretch at about 850 cm-110) and the hematins are no exception24 26). For example (TPPFe)20 presents bands at 870 (vs) and 855 (m) on-1 21). Most hematins only have one infrared band in the asymmetric Fe-O-Fe region and the reason for the splitting of the band for the TPP derivative is not known. 

The expected observation of a symmetric Fe-O-Fe stretch at about 250 cm-1 for (TPPFe)20 has not been realized, since no bands appear below 350 cm-1 in the resonance Raman spectra. That has been interpreted as due to the much greater enhancement of ligand-related modes with excitation in the visible region (514.5 nm)27). 

The accurate solid state determination of the magnetic susceptibility of(TPPFe)20 has not been reported and NMR studies in solution30) produced values of — J = 

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