The 1H NMR spectra of iron(III) quinoxalinotetraphenylporphyrin ((QTPP)FeIIIX n ), iron(III) (methylquinoxalino)tetraphenylporphyrin ((MQTPP)FeIIIX n ), and iron(III) pyrazinotetraphenylporphyrin ((PTPP)FeIIIX n ) have been studied to elucidate the impact of an aromatic extension of a single pyrrole ring on the electronic structure of the corresponding high- and low-spin iron(III) porphyrins. The 1H NMR spectra of the complexes with the following axial ligands have been reported:  chloride, iodide, cyanide, pyridine-d 5 (py-d 5), 4-aminopyridine (4-NH2py), and imidazole (ImH). Modification of the tetraphenylporphyrin by addition of the quinoxaline (pyrazine) fragment results in stabilization of the rare low-spin iron(III) (d xz d yz )4(d xy )1 electronic ground state in the presence of axial cyanide or pyridine ligands. The more common (d xy )2(d xz d yz )3 electronic ground state has been established for [(QTPP)FeIII(4-NH2py)2]+ and [(QTPP)FeIII(ImH)2]+ species. To account for the substituent contribution, the Hückel linear combination of atomic orbitals (LCAO) method has been used to determine the molecular orbitals involved in the spin density delocalization. The deviation from Curie law observed for [(QTPP)FeIII(CN)2]- suggests Boltzmann equilibrium {(d xz )2(d yz )2(d xy )2(Ψ- 1)1 ↔ (d xz )2(d yz )2(d xy )1(Ψ- 1)2} ⇌ (d xz )1(d yz )2(d xy )2(Ψ- 1)2 ⇌ (d xz )2(d yz )1(d xy )2(Ψ- 1)2 where Ψ- 1 is related to the a2u orbital of a regular porphyrin. For the first time in the group of low-spin iron(III) tetraarylporphyrins, the sign reversal of the isotropic shift was directly observed for pyrrole-proton resonances. The structure of (QTPP)FeIIICl was determined by X-ray crystallography. (QTPP)FeIIICl crystallizes in the monoclinic space group P21/c with a = 18.016(5) Å, b = 11.399(3) Å, c = 21.996(5) Å, β = 112.22(5)°, and Z = 4. The refinement of 548 parameters and 2696 reflections yields R 1 = 0.0654, R w2 = 0.1717. The (QTPP)FeIIICl presents features of the high-spin five-coordinate iron(III) tetraphenylporphyrin. The quinoxalinotetraphenylporphyrin macrocycle assumes a saddle-shape geometry.

Treatment of iron(II) porphyrins, (py)2FeII(OEP), (py)2FeII(EtioP), (py)2FeII(DeuteroP), (py)2FeII(MesoP), and (py)2FeII(ProtoP), where OEP are the dianions of octaethylporphyrin, etio-1 porphyrin, deuteroporphyrin-IX dimethyl ester, mesoporphyrin-IX dimethyl ester, and protoporphyrin-IX dimethyl ester, with hydrogen peroxide in pyridine-d 5 at −30 °C in the strict absence of dioxygen is shown to result in clean oxygenation of the heme and the formation of oxophlorin complexes, (py)2Fe(OEPO), (py)2Fe(EtioPO), (py)2Fe(DeuteroPO), (py)2Fe(MesoPO), and (py)2Fe(ProtoPO). Reactions have been monitored by 1H NMR spectroscopy. The product oxophlorin complexes are stable as long as the samples are protected from exposure to dioxygen. The hyperfine shift patterns and the relative intensities of the individual resonances have been analyzed in terms of a model in which the location of the meso oxygen substituent dominates the pattern of spin density distribution. The resulting 1H NMR spectra obtained from oxidation of the unsymmetrically substituted hemes, (py)2FeII(DeuteroP), (py)2FeII(MesoP), and (py)2FeII(ProtoP), with hydrogen peroxide have been analyzed. These spectra show that the four isomeric oxophlorin products are formed in a nonrandom fashion. Solvent effects can produce a significant alteration in the regiospecificity of heme oxygenation.