The effect of axial ligands and porphyrin substituents on the electronic ground state of low-spin iron(III) porphyrinates

Abstract

To aid in the understanding of the electronic states of low-spin, bis-pyridine and -imidazole model complexes of the cytochromes b variable temperature proton NMR spectroscopy and EPR spectroscopy have been employed on a series of complexes of tetramesitylporphinatoiron(III), tetra(2,6-dibromophenyl)porphinatoiron(III) perchlorate, tetra(2,6-dichlorophenyl)porphinatoiron(III) perchlorate, tetra(2,6-difluorophenyl)porphin-atoiron(III) perchlorate, and tetra(2,6-dimethoxyphenyl)-porphinatoiron(III) perchlorate where the axial ligands were 4-dimethylaminopyridine, 3,4-dimethylpyridine, 3,5-dimethylpyridine, 4-methylpyridine, 3-methylpyridine, methylpyridine, 3-chloropyridine, 3-cyanopyridine, 4-cyanopyridine, N-methylimidazole, and 2-methylimidazole. Both paramagnetic iron(III) and diamagnetic cobalt(III) porphyrinates were synthesized. The 25°C proton NMR spectra of the cobalt(III) complexes were used to estimate the diamagnetic shifts of the corresponding iron(III) complexes. The proton NMR spectra of the iron(III) complexes were recorded from -90°C to +40°C. Small isotropic shifts and anti-Curie behavior in the pyrrole-proton resonances of the lower basicity pyridine complexes of tetramesitylporphinatoiron(III)+ indicate a (dxz,dyz)⁴(dxy)¹ ground state, and axial ∼4K EPR spectra of the lower basicity complexes and large gmax or rhombic EPR spectra of the higher basicity pyridine complexes of all of the iron(III) porphyrinates studied indicate a shift in electronic ground state from (dxz,dyz)⁴(dxy)¹ for the lower basicity pyridine complexes to (dxy)²(dxz,dyz)³ for the higher basicity pyridine complexes in all cases. The dipolar and contact contributions to the isotropic shift were determined based on estimates of the ground-excited state crossover point and geometric factors obtained from crystal structure data. Variable temperature NOESY and COSY spectra were successful in distinguishing meta from para resonances in the NMR spectra of the tetra-(2,6-dichlorophenyl)porphinatoiron(III) perchlorate bis-2-methylimidazole complex. X-ray crystallographic studies indicated parallel orientation of the axial ligands in tetra-(2,6-difluorophenyl)porphinatoiron(III) perchlorate bis-3,5-dimethylpyridine.