Ring-opening and meso substitution from the reaction of cyanide ion with zinc verdohemes

The reactivity of zinc verdoheme, [Zn-II(OEOP)](O2CCH3) where OEOP is the monoanion of octaethyl-5-oxaporphyrin, with cyanide ion has been shown to be a complex process that involves not only the expected ring-opening of the macrocycle, as occurs with other nucleophiles (methoxide, methanethiolate, di-methylamide), but also substitution at one or two of the meso positions. The ring-opened products have been subjected to crystallographic study. The structures of mu -H2O-{Zn-II(OEB-10,19-(CN)(2))}(2) and mu -H2O-{(Zn-II(OEB-10,15,19-(CN)(3))}(2) both consist of two helical tetrapyrrole subunits that are coordinated to a zinc ion through four Zn-N bonds. The two zinc ions are coordinated to a bridging water molecule that is also hydrogen bonded to a lactam oxygen atom at one end of each tetrapyrrole subunit. Thus the chiral sense of one helical Zn-II(OEB-10, 19-(CN)(2)) portion is transmitted to the other Zn-II(OEB-10,19-(CN)(2)) unit and the resulting binuclear unit is chiral. In contrast Co-II(OEB-15,19-(CN)(2)), which was obtained by the insertion of Co(H) into the free ligand, is monomeric with a four-coordinate cobalt ion. A series of DFT geometry optimization calculations were performed on zinc complexes of 5-oxaporphyrins (verdoheme), verdins (bilindione), 4-cyano-5-oxaporphyrins, and 19-cyanoverdins in an effort to gain insights to the features of these complexes and the reactions that lead to meso-cyano-substituted cyanoverdins.