P. Hambright

Reduction potentials of seventy-five free base porphyrin molecules: Reactivity correlations and the prediction of potentials

Abstract Phase sensitive fundamental harmonic ac voltammetric techniques have been used to measure the reduction potentials of seventy-five metal-free meso and β-pyrrole substituted porphyrins in DMF solution. Correlations are explored between the reduction potentials and porphyrin and metalloporphyrin reactivity. Such porphyrin reduction potentials can be reliably predicted using assigned substitutent partial potential values.

The kinetics of zinc(II) incorporation into fifty-five free base porphyrins in DMF: structure-reactivity correlations

Abstract The kinetics of zinc(II) incorporation into fifty-five free base porphyrins of varying charges and structures were investigated in dimethylformamide at 25 °C. The rate law is the same as found by Tanaka and co-workers ( Bull Chem. Soc. Jpn., 57 (1984) 204) for tetraphenylporphyrin. One interpretation is that Zn(II) forms a complex with the free base porphyrin H 2 ue5f8P and in a k 1 pathway, the coordinated Zn(II) enters the porphyrin, while in a k 2 route, a second zinc is needed to produce the final product. While little correlation is found between the rate constants and overall porphyrin peripheral charge, k 2 varies over a 26 000 fold range for uncharged species. The replacement of hydrogens by two bulky substituents in the ortho -phenyl positions of tetraphenyl type porphyrins leads to a 200–500 fold decrease in k 2 , and such ortho groups might inhibit porphyrin ring deformation. Phenyl rings substituted in the para position by groups such as −NH 2 ,−OCH 3 and −OH produce more reactive porphyrins that if such substituents are present in the meso -phenyl positions, indicating electron donation to the porphyrin ring by resonance effects. This implies coplanarity between at least one phenyl ring and the porphyrin plane in a distorted metal-porphyrin activated complex. Mercury(II) markedly catalyzes the Zn(II)/ H 2 ue5f8P reactions through the very rapid formation of Hg(II)P, which is 10 4 times more reactive with Zn(II) than H 2 ue5f8P itself. The predeformed octabromo-tetramesitylporphyrin metallates ∼4 × 10 3 times faster than the tetramesitylporphyrin.

Kinetics of the reactions of divalent copper, zinc, cobalt, and nickel with a deformed water soluble centrally monoprotic porphyrin

Abstract The kinetics of incorporation of Cu2+, Zn2+, Co2+, and Ni2+ into the free base H2-BrP(4)4+ and centrally monoprotic H-BrP(4)3+ forms of the deformed and water soluble beta-octabromo-tetrakis(N-methyl-4-pyridyl)porphyrin were studied at 25xa0°C, I=0.10 (NaNO3/HNO3). Below pH 7, the reactions were first-order in porphyrin and metal, and the rate constants increased with an increase in pH. The monocation H3-BrP(4)5+ did not incorporate metal ions, and H-BrP(4)3+ reacted several hundred times faster than H2-BrP(4)4+. The incorporation rate constants for the pre-deformed H2-BrP(4)4+ were ∼103 times larger as compared to the more planar tetrakis(N-methyl-4-pyridyl)porphyrin. For both porphyrins, the reactivity order was Cu2+>Zn2+>Co2+>Ni2+. While Cu2+ reacted ∼104 times more rapidly than did Ni2+, the ratios of the metalation rate constants of H-BrP(4)3+ to the water exchange rate constants of the metal ions varied by no more than a factor of 4 across the series.

Metal-porphyrin interactions—VI The reactivities of several ferric porphyrin monomers with cyanide compared with ligand reactions of iron and cobalt porphyrins reconstituted with proteins[1]

Abstract A study of the hydrolysis and kinetics and equilibrium behavior of cyanide addition to the monomeric iron(III) complexes of meso, proto and deuteroporphyrin-IX in 2 per cent sodium lauryl sulfate-0·1 M tetramethyl ammonium bromide, 25° is reported. The reactivity parameters are compared to reactions of the same Co(II) and Fe(II) porphyrin types reconstituted to myoglobins and hemoglobins.

Interactions of cyanide with a water soluble ruthenium porphyrin

Abstract The reactions of cyanide with the mono-carbonyl complex of ruthenium(II)-uroporphyrin-I (COue5f8 Ru(II)ue5f8P) were studied in aqueous solution at 25 °C, ionic strength 0.1 (NaCl). The equilibrium constant for mono-cyano (CO,CN)ue5f8Ru(II)ue5f8P formation was 1.1 ± 0.1 x 10 4 M −1 , which was independent of pH between 6 and 12. The kinetics of the incorporation of the first cyanide were first order in porphyrin and cyanide concentration, with a specific rate constant of 2.3 ± 0.2 M −1 s −1 at pH 11. Under an inert atmosphere, the di-cyano (NC) 2 ue5f8Ru(II)ue5f8P forms from the (CO,CN) adduct, with a rate law first order in porphyrin and independent of cyanide concentration. This is ascribed to a rate limiting dissociation of CO, with a specific rate constant of 4.6 ± 0.3 X 10 −5 s −1 . Upon adding di-oxygen in the presence of excess cyanide at pH 11, both the (CO, CN) and di-cyano-Ru(II) complexes were oxidized to the di-cyano-Ru(III) porphyrin, which could be reduced with dithionite into the di-cyano-Ru(II) adduct. In acid solution, both cyanides were lost from the di-cyano-Ru(II) porphyrin, and addition of carbon monoxide at this stage reformed the starting material, COue5f8Ru(II)ue5f8P. The applications of ruthenium porphyrins as in vivo cyanide scavenging drugs is discussed.

Mössbauer, magnetic susceptibility, radiolytic and photochemical studies of europium and lutetium porphyrins

Abstract The chemistry of the lanthanide complexes of europium and lutetium with several porphyrin molecules were probed in both the solid state and in solution. Mossbauer and variable temperature magnetic susceptibility studies indicated that the trivalent state of europium was formed under the synthetic conditions. Electron addition to both europium(III) and lutetium(III) porphyrins by photochemical and radiolytic techniques resulted in reduction of the porphyrin ring rather than the coordinated trivalent metal ion, and the resulting π-radical anion disproportionated into the initial metalloporphyrin and the ring reduced metalloporphyrin di-anion. The reduction potentials in solution for mono- and di-anion formation of the lutetium porphyrin were measured by electrochemical procedures. While the lutetium(III) porphyrin is stable for years at pH 7, the metal is rapidly removed from the metalloporphyrin by protons in the di-anion state. The photophysics of the lutetium(III) porphyrin in solution are dominated by the triplet state, and the reactivity of this triplet with dioxygen, NADH and persulfate is reported.

Tetrabenzoporphyrins: Metal Incorporation and Exchange Kinetics, Ligational Equilibria and Pulse Radiolysis Studies

The solution chemistry of tetrabenzoporphyrin (TBP) and several of its metal derivatives were compared with that of the corresponding octaethylporphyrins. Kinetic studies were done on zinc and cadmium ion incorporation and the exchange reactions of cadmium porphyrins with zinc. Formation constants of Cd(II), Cu(II), VO(IV), Ni(II), Zn(II) and Mg(II) porphyrins with pyridine were determined for both compounds. The spectra of the radical cations of Ni(II), Mg(II) and H2-TBP were obtained, as well as that of the radical anion of Mg(II)-TBP. Aside from the new Ni(I)-TBP, no long-lived intermediates, as noted with other porphyrins, were observed upon reduction of the tetrabenzoporphyrins. The low basicity and rigidity of the tetrabenzoporphyrin nucleus may explain many of its relative reactivity properties.

Kinetics of the demetallation of water soluble lanthanide porphyrins by ethylenediaminetetraacetic acid

Abstract The kinetics of the demetallation of six water soluble trivalent lanthanide tetrakis (iV-methyl-4-pyridyl)-porphyrins (Lu, Yb, Dy, Gd, Sm and Nd-TMPyP) by EDTA (ethylenediaminetetraacetic acid) were studied at 25°C between pH 5 and 10. The reactions were first order in porphyrin, and the kinetics indicated complexation between the HY3- form of EDTA and the monohydroxylanthanide porphyrin. The rate determining step was either the dissociation of this complex into products (k1), or, if the complex was a dead-end intermediate, the reaction between the uncomplexed reactants, k2. The rate laws for either pathway were the same, and thus the mechanisms kinetically indistinguishable. For all of the lanthanide porphyrins, an equation of the form log (k2) = 44.9 (Ro)—39.4 described the results, with Ro the ionic radii of the lanthanide ions in Angstroms. A linear relationship was also found between log (k2) for EDTA and log (kH+) for the acid catalyzed solvolysis reactions of the complexes.

Anti-Cyanide Drugs: Kinetics of the Removal of Copper(II) and Nickel(II) from N-Methyl-tetra(4-sulfonatophenyl)porphyrins by Cyanide. LD50s of Common Metalloporphyrins and Metallophthalocyanines

The purpose of this project was to develop prophylactic water-soluble anti-cyanide drugs. The 24 h mean LD50s in mice of 13 common metalloporphyrins and metallophthalocyanines are reported. At doses of 1/4, 1/16 and 1/64 of the LD50 administered 15 or 60 min prior to a 2LD50 challenge of sodium cyanide, none of these compounds protected mice against cyanide. While Co(H2O)6Cl2 and Ni(H2O)6Cl2 were found to be effective, they are too toxic to be useful. A stable metalloporphyrin that could scavenge cyanide by donating metal ions to produce non-toxic polycyano species in the presence of cyanide would be desirable. We found that nickel(II) and copper(II) are removed from their N-methyl-tetra(4-sulfonato)porphyrins by cyanide and hydrogen cyanide. These reactions are not observed with the parent tetra(4-sulfonatophenyl)porphyrins. Equilibrium constants for cyanide addition to both types of metalloporphyrins and the rate laws for metal removal by cyanide and HCN are reported. The medical implications of these findings are discussed.

Redox chemistry of water-soluble vanadyl porphyrins

Abstract In aqueous solution, pulse radiolytic studies have shown that vanadyl porphyrins undergo redox reactions only at the porphyrin ring. The resultant porphyrin radical anions and cations are unstable with respect to disproportionation. Steady state reduction, both radiolytic and photochemical, of the vanadyl porphyrins results in formation of phlorins, porphodimethenes and chlorins depending upon pH and the nature of the porphyrin periphery groups. There is no evidence to show formation of products in which the central vanadyl ion has been reduced.