Axial ligation in water-soluble copper porphyrinates: contrasts between EPR and UV–vis

Copper porphyrinates are generally known to display a less diverse reactivity compared to their iron counterparts. Here, we examine a water-soluble copper porphyrinate for its ability to engage in reactions involving axial ligation to the copper or possible redox cycling. Although UV–vis spectra indicate an expected lack of reactivity, electron paramagnetic resonance spectra (EPR) reveal an unexpected wealth of changes in electronic structures at the copper, induced by potential ligands such as imidazole or nitrite, but also by seemingly unexpected candidates for ligands, such as 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS) and guaiacol, as well as by dithionite.