Reactions of monomeric cobalt-oxygen complexes.  I.  Thermodynamics of reaction of molecular oxygen with five- and six-coordinate amine complexes of a cobalt porphyrin

Walker, F. Ann

doi:10.1021/ja00785a026

Cited by 132 publications

(70 citation statements)

References 2 publications

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“…The cobalt porphyrins ( Earlier measurements of P1/2 of simple cobalt porphyrins such as cobalt protoporphyrin IX dimethyl ester-(NMeIm) (29), cobalt(p-methoxyphenyl)porphyrin-(NMeIm) (9,30), and cobalt tetratolylporphyrins with appended bases (31) showed very low affinities, 300 times worse than our models or the…”

Section: Methodscontrasting

confidence: 59%

Model compounds for the T state of hemoglobin.

Collman

Brauman

Doxsee

et al. 1978

Proc. Natl. Acad. Sci. U.S.A.

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02 binding to a series of ferrous and cobaltous "picket fence" porphyrins-is reported. N-Methylimidazole and covalently attached imidazoles give 02 binding to ferrous porphyrins with AH' =--16.2 kcal/mol (-67.7 kJ/mol) and ASO = -40 eu (standard state, 1 atmosphere OaJ. Similar studies with cobaltous porphyrins yield AH' = -12.8 kcal/mol (-53.5 kJ/mol) and AS' = -39 eu. These values match well those of myoglobin and isolated subunits of hemoglobin and their cobalt reconstituted analogues. 1,2-Dimethylimidazole has been successfully used to mimic the presumed restraint of T state hemoglobin. In direct analogy to the decreased cooperativity shown by cobalt-substitute hemoglobin, model cobalt porphyrins show a smaller decrease in 02 affinity than the analogous iron porphyrins when the axial base is hindered. Thermodynamic data are presented. The molecular mechanism of cooperativity in hemoglobin is discussed. The mechanism of cooperativity in hemoglobin (Hb) is of continuing interest (1-3). Model porphyrins capable of reversible oxygen binding have contributed to a fuller understanding of myoglobin (Mb) and other monomeric hemoproteins (4-6). Very little work, however, has appeared on models for the low affinity, T state, of Hb. The effect of steric restraint built into the porphyrin (7,8) or the axial base (9-13) has not been fully explored. We wish to report a full study of 02 binding to both iron and cobalt "picket fence" porphyrins with hindered and unhindered imidazoles. In addition, we make here a preliminary report on the synthesis and characterization of "picket fence" porphyrins with covalently attached axial bases. MATERIALS AND METHODSmeso-Tetra(a,a,a,a-o-pivalamidophenyl)porphyrin (H2T-PivPP) was prepared as described (4). Cobalt was inserted by use of anhydrous CoC12 in a solution of tetrahydrofuran with a trace of 2,6-lutidine at 500 under N2. Further purification consisted of chromatographic separation on Woelm neutral alumina. Details are presented elsewhere (14). meso-were prepared by the reaction of 4-(N-imidazolyl)butyl chloride and 5-(N-imidazolyl)valeryl chloride, respectively, with meso-tri(aaao-pivalamidophenyl)-(3-o-aminophenylporphyrin. Extreme care must be taken to prevent exposure of the porphyrins to 02 and light due to singlet molecular oxygen production catalyzed by the metal-free porphyrin (15) and trapping by the attached imidazole (16). Chemical structures of these porphyrins are shown in Fig. 1. The meso-tri(a,a,a-o-pivalamidophenyl)-fl-o-aminophenylporphyrin was prepared by the reaction of the readily available meso-tetra(aa,a,a-o-aminophenyl)-porphyrin (4) with limited amounts of pivaloyl chloride [(CHF)3COC1] and isolated by column chromatography. Iron was easily inserted under inert atmosphere by use of anhydrous FeBr2 in 1:1 benzene/tetrahydrofuran with a trace of 2,6-lutidine, followed by chromatographic filtration through a short plug of alumina. All intermediates and porphyrins described above have been well characterized by elemental analysis, UV/visible, nuclear ...

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Section: Methodscontrasting

confidence: 59%

Model compounds for the T state of hemoglobin.

Collman

Brauman

Doxsee

et al. 1978

Proc. Natl. Acad. Sci. U.S.A.

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“…When cobalt(II) is oxidized or reduced to cobalt(III) or cobalt (I) respectively, the overall catalytic activity will decrease. The mechanism shown in Scheme 2.7 was first presented by Karmilova et 76 and cobaloxime derivatives. 77,78 The dioxygen affinities, however, have an inverse dependence on temperature and for porphyrins in the most favourable case only one percent will form an oxygen adduct at room temperature.…”

Section: Catalyst Deactivationmentioning

confidence: 88%

“…77,78 The dioxygen affinities, however, have an inverse dependence on temperature and for porphyrins in the most favourable case only one percent will form an oxygen adduct at room temperature. 76 Therefore, oxidation by oxygen will probably occur via trapping of a polymeric radical by oxygen followed by combination of the resulting peroxo-radical and the cobalt complex. 1,79 On the other hand cobalt hydrides of cobaloximes behave as weak acids (pK a ~ 10) 30 and can be deprotonated to form a cobalt(I) species.…”

Section: Catalyst Deactivationmentioning

confidence: 99%

Shining a light on catalytic chain transfer

et al. 2002

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The catalytic chain transfer polymerization of styrene is only truly effective when the reaction mixture is exposed to (UV‐)light. The apparent chain transfer constant depends inversely on radical concentration and can be increased up to 8000. These results can be explained by combining aspects of both catalytic chain transfer and the formation of cobalt‐carbon bonds. For the catalytic chain transfer polymerization of n‐butyl acrylate a chain transfer constant of 650 was found. The resulting transfer coefficient has the same order of magnitude as the one for n‐butyl methacrylate. This means that the absence of an α‐methyl group hardly influences the transfer step itself. Furthermore, the effect of possible impurities on the catalytic chain transfer polymerization of methyl methacrylate is investigated.

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“…Soc., in press) and 1,2,.. .39 A that are observed in these centrosymmetric six-coordinate species display the full effect of the odd electron in the 3d22 orbital of the cobalt atom. Equilibrium measurements of the complexing reactions in toluene solution show that most of the enthalpy change favoring the formation of the six-coordinate species is, in fact, associated with the initial formation of the five-coordinate complex (19); the decrease in enthalpy accompanying the complexing of the second axial. ligand is apparently <2 kcal/mol.…”

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confidence: 99%