DFT at its best: metal- versus ligand-centered reduction in nickel hydroporphyrins

DFT calculations, using the PW91, OLYP, and B3LYP functionals, have provided some of the first estimates of the relative energies of the Ni(I) and Ni(II) ligand anion radical states of hydroporphyrin complexes. Although the three functionals chosen sometimes yield discordant results, the results of this study are essentially functional-independent. For isobacteriochlorin derivatives, our calculations predict that the Ni(I) state may be favored by 0.5 eV or more, relative to the ligand anion radical state. For other hydroporphyrins, however, electrochemical studies indicate a much finer balance, which may be tipped one way or the other, depending on the substituents. DFT calculations nicely capture these rather subtle substituent effects. In particular, our results support and extend Bruckner and co-workers' finding (from electrochemical studies) that the most rigid, strongly ruffled nickel chlorins prefer to reduce on the macrocycle, whereas their more flexible congeners undergo metal-centered reduction.