Comparison of the Chemical Properties of Iron and Cobalt Porphyrins and Corrins

Density functional calculations have been used to compare various geometric, electronic and functional properties of iron and cobalt porphyrin (Por) and corrin (Cor) species. The investigation is sed on octahedral M-II/III complexes (where M is the metal) with 0 axial imidazole ligands (as a model of b and c type cytochromes) or with one imidazole and one methyl ligand (as a model of methylcobolamin). However, we have also studied some five-coordinate M-II complexes with an imidazole ligand and four:,coordinate M-I/II complexes without any axial ligands as models of 1 other intermediates in the reaction cycle of coenzyme B-12. The central cavity of the corrin ring is smaller than that of porphine. We show that the cavity of corrin is close to ideal for low-spin Co-III, Co-II, and Co-I with the axial ligands encountered in biology, whereas the cavity in porphine is better suited for intermediate-spin states, Therefore, the low-spin state of Co is strongly favoured in complexes with corrins, whereas there is a small energy difference between the various spin states in iron porphyrin species. There are no clear differences for the reduction potentials of the octahedral complexes, but [Co(i)Cor] is more easily formed (by at least 40 kJ mole(-1)) than [Fe(i)Por]. Cobalt and corrin form a strong Co-C bond that is more stable against hydrolysis than iron and porphine. Finally, Fe-II/III gives a much lower reorganization energy than Co-II, this is owing to the occupied d(z) orbital in Co-II. Altogether, these results give some clues about how nature has chosen the tretrapyrrole rings and their central metal ion.