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

The reactivity of zinc verdoheme, [Zn(II)(OEOP)](O(2)CCH(3)) 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, dimethylamide), 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-H(2)O-[Zn(II)(OEB-10,19-(CN)(2))](2) and mu-H(2)O-[(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(II) 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.

Alkyl exchange reactions of organocobalt porphyrins. A bimolecular homolytic substitution reaction.

Reaction of organocobalt(III) porphyrins with a cobalt(II) complex of a distinguishable porphyrin or tetrapyrrole resulted in the reversible exchange of the organic axial ligand. The exchange reaction was facile in such solvents as benzene, toluene, dichloromethane, chloroform, and pyridine; was unaffected by total exclusion of light; was faster than would be expected for a homolytic process given known Co-C bond dissociation energies; and was of broad scope with respect to the organic ligand. Methyl, benzyl, primary alkyl, secondary alkyl, and acyl groups exchanged, but phenyl groups did not. The position of the exchange equilibrium was independent of the direction of approach and was nonstatistical. The relaxation to equilibrium appeared to be consistent with that of a second-order process. The rate of the reaction varied with the identity of the R group in the order Bzl > or = Me > Et approximately n-Pr > i-Pr > i-Bu > acetyl approximately neopentyl approximately 2-adamantyl. However, the total variation in reaction rates was remarkably small. Attempts to find evidence of free-radical intermediates by trapping with TEMPO or CO or by alkyl group interchange with an excess of an alkyl halide of a distinct alkyl group were unsuccessful over a time scale comparable to multiple half-lives of the exchange reaction. In addition, no rearrangement products were detected in exchange reactions of the 5-hexenyl group. Use of cobalt porphyrin reactants that were sterically encumbered on both faces with groups large enough to prevent formation of a bridged, Co-C-Co structure resulted in a 5 or more order of magnitude decrease in the rate of methyl exchange, if not its outright cessation, when run with total exclusion of light. The decrease in the rate of the thermal exchange process revealed the existence a slow photochemical exchange process that was driven by room lights. All evidence was consistent with a bimolecular S(H)2 mechanism for the thermal exchange mechanism.

Cis-influence of hydroporphyrin macrocycles on axial ligation equilibria and alkyl exchange reactions of alkylcobalt(III) porphyrin complexes.

Stability constants are reported for the coordination of pyridine and substituted pyridines to the alkylcobalt(III) complexes of octaethylporphyrin (OEP), t-octaethylchlorin (OEC), and ttt-octaethylisobacteriochlorin (OEiBC) in toluene solution. The stability constants correlate with the base strength of the nitrogenous ligand. A cis-influence of the macrocycle saturation level on the stability constants is observed. Stability constants for coordination of a given pyridine ligand to an alkylcobalt(III) complex are roughly 10 times smaller than the stability constants for the corresponding cobalt(II) complex. Analysis of a thermodynamic cycle demonstrates that this leads to decreased stability of the complex with respect to Co-C bond homolysis upon ligand coordination, a "base-on" effect. Alkyl exchange occurs between cobalt complexes of different tetrapyrroles. Equilibrium data establish that the exchange is nonstatistical and that the Co-C bond is stabilized by increasing the saturation of the tetrapyrrole macrocycle.