Porphyrins spin forms

The large number of electronic configuration of iron porphyrins with oxidative states of 2+ or 3+, high and low spin forms, and charge transfer states with different axial ligands offer the possibility of a number of non-radiative decay pathways ( ). In... 

Trimethylphosphine and mixed phenyl, methyl phosphines readily bind to both Fe and Fe porphyrins to form low-spin complexes. The (/ -OCH3)4TPPFe (PMc3)2 and OEPFe (PMe3)2 complexes have been characterized stracturally, as well as by Fe NMR (5pe = 7670 and 7873 ppm, respectively) and Mdssbauer (for the OEP... 

Characteristic differences in skeletal mode wavenumbers in IR spectra were seen for high- and low-spin forms of [Fe(pybzim)3](C104)3.H20, where pybzim = 2-(2 -pyridyl)benzimidazole.287 The resonance Raman spectra of cross-trans-linked iron(II) basket-handle porphyrins gave vsFe-Him2 200 209 cm, vsFe py2 182-205 cm-1 (assignments were helped by deuteriation experiments).288... 

The replacement of an alkyl ligand by an aryl ligand on Fe(Por)(R) for a given porphyrin macrocycle increases the fraction of the complex present in a high spin form. This is shown in Table 4 and explains why the only high spin states are seen for Fe(Por)(R) with perfluoro ligands. 

Self-exchange rates of the three tetrakis-(4-pyridyl)porphine complexes [Fe(P)(imidazole)a] +/ +, [Fe(P)(OH)(OH2)] +/ +, and [Fe(P)(OHa)]6+/ + have been estimated as > 10 , > 10 , and 1.2 x 10 respectively. Of the iron(iii) complexes in this series, the first is low-spin, the second is believed to be an equilibrium mixture of high-spin and low-spin forms, and the third is high-spin. These results support a previous suggestion that electron transfer to or from a low-spin iron porphyrin system has a negligible Franck-Condon barrier. ... 

With more bulky porphyrins like TMP, a stable low-spin monomer Rh(TMP) can be isolated (g = 2.65, g = 1.915), which forms a paramagnetic CO adduct. 

The electronic absorption spectra of the products of one-electron electrochemical reduction of the iron(III) phenyl porphyrin complexes have characteristics of both iron(II) porphyrin and iron(III) porphyrin radical anion species, and an electronic structure involving both re.sonance forms Fe"(Por)Ph] and tFe "(Por—)Ph has been propo.sed. Chemical reduction of Fe(TPP)R to the iron(II) anion Fe(TPP)R) (R = Et or /7-Pr) was achieved using Li BHEt3 or K(BH(i-Bu)3 as the reductant in benzene/THF solution at room temperature in the dark. The resonances of the -propyl group in the F NMR spectrum of Fe(TPP)(rt-Pr) appear in the upfield positions (—0.5 to —6.0 ppm) expected for a diamagnetic porphyrin complex. This contrasts with the paramagnetic, 5 = 2 spin state observed... 

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