Copper porphyrins are generally known to show a less diverse reactivity as compared to their iron counterparts, both redox-wise and in terms of axial ligation. Reported here are density functional theory (DFT) results on models of copper (II)-porphyrins (models of [5,10,15,20-tetrakis(N-methylpyridyl-4)porhinato]copper(II)tetratosylate) with a set of axial ligands – nitrite (both the nitrogen-bound isomer and the oxygen-bound isomer, i.e., nitro and nitrito), imidazole, two forms of phenol (neutral and anionic), and water - related to an unexpected range of electronic structures detectable in electron paramagnetic resonance (EPR) spectra of a water-soluble copper porphyrin with water, nitrite, imidazole, dithionite, 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS) and guaiacol. Computed spin densities and atomic charges reveal various degrees of influence of the axial ligands on the Cu-porphyrin electronic structure, which may be related to the notably different changes induced by each ligand (imidazole, nitrite, guaiacol and ABTS) on the EPR superhyperfine splitting, but with an unexpectedly strong dependence on choice of the computational methodology. Thus, at the B3LYP/6-31G** level the copper spin densities are predicted to range from 0.69 to 0.73 depending on the axial ligand, this contribution being located on the dx2-y2 orbital in a range from 65% to 100%. BP86/DN** results tend to favor a distinctly larger spread of these values.

Roxana-Viruca Ţolan, Alexandru Lupan and Radu Silaghi-Dumitrescu