Porphyrins high spin complexes

The complex Mn(acac)3 has two forms, one of which shows a substantial tetragonal elongation (two Mn—O = 2.12 A four Mn—O = 1.93 A) as do other high-spin complexes such as porphyrin adducts the other form shows a moderate tetragonal compression (two Mn—O = 1.95 A four Mn—O = 2.00 A). 

Thus, Brill and Sandberg pointed out that whenever imidazole is coordinated to the ferriprotoporphyrin (H, 79, 82) the difference spectra of low-spin vs. high-spin complexes are characterized by an absorption band below 250 nm (A 236-23s approximately 6-12 X 10 M cm" ). Such a diagnostic band, attributable to charge transfer transitions from L5 to porphyrin orbitals, was noted in the difference spectrum of ferricata-lase cyanide (low spin) vs. catalase (high spin) 74), thereby favoring L5 = His. 

Six-coordinate complexes of several Fe porphyrins, having one pyridine and one isothiocyanate ligand, have been reported. Interestingly, the pyridine/N-bound thiocyanate anion combination appears to lie close to the 5=1/2 — 5/2 spin crossover point for the TPPFe complex and, depending on the crystal lattice, exists as either a low-spin or a thermal equilibrium between a low-spin and a high-spin complex, while the OEPFe complex is high-spin at all temperatures. ... 

Five typical RR spectra of Fe(OEP) derivatives are displayed in Fig. 7, where the spectra are arranged in the decreasing order of the core-size. [Fe(OEP)(Me2SO)2] (Me2SO dimethylsulfoxide) and Fe(OEP)Br stand for the six- and five-coordinate ferric high-spin complexes, respectively. Fe(OEP)(Im)2 and [Fe(OEP)(Im)2] (Im imidazole) are typical low-spin complexes of the ferrous and ferric porphyrins, respectively. 

The basic requirements for synthetic heme models are, therefore, a five-coordinate iron(II) porphyrin in a high-spin state, with a proximal nitrogen base and a vacant site for dioxygen. Futhermore, the formation of chemochromes by axial coordinaton of two N-bases to the iron(II) pro-phyrin should be prevented.Several approaches were put forward. It is found that a sterically hindered axial base, such as C2-dimethylimidazole, favors formation of a five-coordinate high-spin complex. Furthermore, covalent attachment of the axial ligand or steric hindrance of one of the two faces of the heme can also be used to prevent axial... 

The EPR-silent [Fe(Papy3)(NO)] complex is very stable, allowing for the preparation of crystaUine solids. The NO-release represented by reversing reaction (10) is not available thermally, but it becomes possible upon illumination (21). In a similar way, the compoimd [Fe(bpb)(N0)(N02)] complex was synthesized upon bubbhng NO into an oxygen-free acetonitrile solution of the low-spin [Fe (bpb)(py)2] ion (a nonheme planar analog of the iron(III)-porphyrinate) (57). Notice that the high-spin complex [Fe (bpb)(Cl)2] did not afford any isolable nitrosyl, a fact that has been employed to propose that the spin state of the iron center of the precursor complex dictates its affinity toward NO. 

Alkyliron(lll) porphyrin complexes are air. sensitive, and when exposed to oxygen under ambient conditions the products are the very stable iron(IIl) /t-oxo dimers, [Fe(Por)]20. A more careful investigation revealed that the reaction of the alkyl complexes with oxygen proceeds via insertion of O2 into the Fe—C bond. " When a solution of Fe(Por)R (R = Me, Et, i-Pr) is exposed to O2 at —70 C, the characteristic H NMR spectrum of the low spin iron alkyl complex disappears and is replaced by a new, high spin species. The same species can be generated from the reaction of an alkyl hydroperoxide with Fe(Por)OH, and is formulated as... 

The first test case was the ferrous high-spin state (Fe, S = 2) in the picket-fence porphyrin acetate complex [Fe(CH3COO)(TPpivP)] [13, 23], which is a model for the prosthetic group termed P460 of the multiheme enzyme hydroxyl-amine oxidoreductase from the bacterium Nitrosomonas europeae. Both the picket-fence porphyrin and the protein P460 exhibit an extraordinarily large quadrupole splitting, as observed by conventional Mossbauer studies [56]. 

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