Magnetic resonance investigations of iron tetrapyrrolic macrocycles

Abstract

¹H NMR and EPR techniques were used to investigate the electron spin distribution and electronic ground state in several iron tetrapyrrolic macrocycles. The first macrocycle studied is corrole, including [(Me₈C)FeCl] (Me₈C = 2,3,7,8,12,13,17,18-octamethylcorrole) and [(7,13-Me₂Et₆C)FeCl] (7,13-Me₂Et₆C = 7,13-dimethyl-2,3,8,12,17,18-hexaethylcorrole) and four meso-substituted corrolates--[(TPCorr)FeCl], [(4-NO2TPCorr)FeCl], [(4-MeOTPCorr)FeCl] and [(TPCorr)FeClO₄] (TPCorr = 5,10,15-triphenylcorrole). These chloroiron corrolates were all found to be S = 3/2 intermediate-spin iron(III) π cation radical complexes, with the corrole radical strongly antiferromagnetically coupled to the spins of the iron, leading to an overall spin of 1 and large negative π spin densities on the meso positions. Upon addition of imidazole ligands, [(Me₈C)FeCl] and [(7,13-Me₂Et₆C)FeCl2] change to bis imidazole low-spin iron(III) π cation radical species at low temperature. There is little or no ferromagnetic coupling between the radical and the iron center, resulting in large position pi spin densities on the meso positions. The binding of cyanide to [(7,13-Me₂Et₆C)FeCl] causes autoreduction of the complex. An excess of cyanide in the solution can reduce the bis-cyanide complex, a low-spin iron(III) π cation radical which is produced first upon addition of cyanide, to the mono-cyanide complex, which is a normal low-spin iron(III) five-coordinate complex. The redox reaction occurs on the corrole ring instead of at the iron center. Proton relaxation times (T₁ and T₂) of a pyrrole-CH₃ peak from the heme domain of the chicken liver sulfite oxidase were measured by NMR methods. From the relaxation times, it is found that the sulfite oxidase enzyme tumbles as the whole protein rather than the larger Mo domain and the smaller heme domain tumbling somewhat independently. The last macrocycles investigated are chlorins and mono-oxochlorin. Both high-spin tetraphenylchlorinatoiron(III) chloride (TPCFeCl) and octaethylchlorinatoiron(III) chloride (OECFeCl) and their low-spin complexes with different imidazole and pyridine ligands were studied by NMR and EPR. The full peak assignments were made for all high-spin and low-spin species from COSY, NOESY, NOE difference and saturation transfer experiments. The NMR results show that, like TPPFe(III) and TMPFe(III) complexes, the low-spin TPCFe(III) complexes change their ground state from (dxy)²(dxzdyz)³ to (dxzdyz)⁴(dxy)¹ with decrease in the donor strength of the axial ligands, while OECFe(III) complexes keep their ground state unchanged (always (dxy)²(dxydyz)³) with different axial ligands in the temperature range of NMR experiments (+30°C to -90°C)). However, EPR data show that both TPCFe(III) and OECFe(III) complexes have the trend of change to (dxzdyz)⁴(dxy)¹ ground state with weak donor ligands (such 4-cyanopyridine). The electronic structure of [OECFe(t-BuNC)₂]⁺ is the (dxzdyz)⁴(dxy)¹ ground state with a low-lying (dxy)²(d xzdyz)³ excited state. The chlorin ring of [OECFe(tBuNC)₂]⁺ is probably ruffled, as in [OEPFe(t-BuNC)₂]⁺. The NMR spectrum of [OECFe(t-BuNC)₂]⁺ is characterized by the large downfield shift of the pyrrolene protons, indicating the involvement of the A-1 orbital in the spin distribution mechanism. [mono-oxo-OECFe(Im-d₄)₂]Cl (mono-oxo-OEC = 2-oxo-3,3',7,8,12,13,17,18-octaethyl-chlorin) is a low-spin Fe(III) complex with (dxy)²(dxzdyz)³ ground state. The pattern of the chemical shifts of the pyrrole-CH2 and meso protons is similar to that of [OECFe(Im-d₄)₂]Cl, except that more peaks were observed due to its lower symmetry. Finally, DFT calculation on high-spin iron (III) chlorin was carried out to predict the Fermi contact shifts and spin distribution mechanism.