Proton NMR Study of the Deprotonation of Axial Imidazole Ligands in Low-Spin Ferric Porphyrin Complexes

The proton NMR spectra of a series of low-spin ferric bis(imidazolate) and mixed-ligand cyanide-imidazolate complexes of natural porphyrin derivatives have been recorded, assigned, and compared to those of the analogous complexes of neutral imidazole. Deprotonation of an axial imidazole leads to hyperfine shift changes for porphyrin substituents that reflect primarily a decrease in ligand → metal π charge transfer. The axial ligand hyperfine shifts are separated into their dipolar and contact contributions by using the magnetic anisotropy data derived from low-temperature ESR spectra of both the imidazole and imidazolate complexes. The imidazole 2′-H peak is shown to shift characteristically upfield upon deprotonating a coordinated imidazole, suggesting that this resonance may serve as a probe of the extent of imidazolate character of the proximal histidine in cytochrome b₅ and heme peroxidases.