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Helder M. Marques - One of the best experts on this subject based on the ideXlab platform.
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Probing the nature of the Co(III) ion in Corrins: The reactions of aquacyano-5-seco-cobyrinic acid heptamethyl ester with anionic ligands
Inorganica Chimica Acta, 2019Co-Authors: Monika Nowakowska, Susan M. Chemaly, Amanda L. Rousseau, Patrick Govender, Pradeep R. Varadwaj, Arpita Varadwaj, Koichi Yamashita, Helder M. MarquesAbstract:Abstract The substitution of H2O in aquacyano-5,6-dioxo-5,6-seco-heptamethyl-cob(III)yrinate, (aquacyano-5-seco-cobester, [AC-5-seco-Cbs]+), in which the C5–C6 bond of the Corrin ring of this vitamin B12 derivative is cleaved, by a variety of anionic ligands is reported. The pKa for ionization of coordinated water decreases from 9.8 ± 0.3 in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, [ACCbs]+) to 7.28 at 25 °C (ΔH = –88 ± 17 kJ mol−1 and ΔS = –434 ± 56 J K−1 mol−1) in [AC-5-seco-Cbs]+. The pKa, confirmed by determining the pH-dependence of coordination of SO32– by [AC-5-seco-Cbs]+, shows Co(III) in this complex behaves much more like Co(III) in simple hexacoordinate complexes than in intact cobalt Corrins. A comparison of log K values for coordination of CN–, SO32–, NO2–, N3– and S2O32– by [ACCbs]+ and [AC-5-seco-Cbs]+ demonstrates that cleavage of the Corrin ring significantly decreases the affinity of Co(III) for the softer ligands CN–, SO32– and, more marginally, NO2–. However, [AC-5-seco-Cbs]+ has a higher affinity for N3– and S2O32– than [ACCbs]+. These trends correlate with the position of the ligands in the spectrochemical series (N3–
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Probing the Nature of the Co(III) Ion in Corrins: Comparison of Reactions of Aquacyanocobyrinic Acid Heptamethyl Ester and Aquacyano-Stable Yellow Cobyrinic Acid Hexamethyl Ester with Neutral N‑Donor Ligands
2013Co-Authors: Susan M. Chemaly, Monika Nowakowska, Louise Kendall, Dale Pon, Christopher B. Perry, Helder M. MarquesAbstract:Equilibrium constants (log K) for substitution of coordinated H2O in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, ACCbs) and aquacyano-stable yellow cobyrinic acid hexamethyl ester (aquacyano-stable yellow cobester, ACSYCbs), in which oxidation of the C5 carbon of the Corrin interrupts the normal delocalized system of Corrins, by neutral N-donor ligands (ammonia, ethanolamine, 2-methoxyethylamine, N-methylimidazole, and 4-methylpyridine) have been determined spectrophotometrically as a function of temperature. Log K values increase with the basicity of the ligand, but a strong compensation effect between ΔH and ΔS values causes a leveling effect. The aliphatic amines with a harder donor atom produce ΔH values that are more negative in their reactions with ACSYCbs than with ACCbs, while the softer, aromatic N donors produce more negative ΔH values with ACCbs than with ACSYCbs. Molecular modeling (DFT, M06L/SVP, and a quantum theory of atoms in molecules analysis of the electron density) shows that complexes of the aliphatic amines with SYCbs produce shorter and stronger Co–N bonds with less ionic character than the Co–N bonds of these ligands with the cobester. Conversely, the Co–N bond to the aromatic N donors is shorter, stronger, and somewhat less ionic in the complexes of the cobester than in those of the SYCbs. Therefore, the distinction between the harder Co(III) in ACSYCbs and softer Co(III) in ACCbs, reported previously for anionic ligands, is maintained for neutral N-donor ligands
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Probing the nature of the Co(III) ion in Corrins: comparison of reactions of aquacyanocobyrinic acid heptamethyl ester and aquacyano-stable yellow cobyrinic acid hexamethyl ester with neutral N-donor ligands.
Inorganic chemistry, 2012Co-Authors: Susan M. Chemaly, Monika Nowakowska, Louise Kendall, Dale Pon, Christopher B. Perry, Helder M. MarquesAbstract:Equilibrium constants (log K) for substitution of coordinated H2O in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, ACCbs) and aquacyano-stable yellow cobyrinic acid hexamethyl ester (aquacyano-stable yellow cobester, ACSYCbs), in which oxidation of the C5 carbon of the Corrin interrupts the normal delocalized system of Corrins, by neutral N-donor ligands (ammonia, ethanolamine, 2-methoxyethylamine, N-methylimidazole, and 4-methylpyridine) have been determined spectrophotometrically as a function of temperature. Log K values increase with the basicity of the ligand, but a strong compensation effect between ΔH and ΔS values causes a leveling effect. The aliphatic amines with a harder donor atom produce ΔH values that are more negative in their reactions with ACSYCbs than with ACCbs, while the softer, aromatic N donors produce more negative ΔH values with ACCbs than with ACSYCbs. Molecular modeling (DFT, M06L/SVP, and a quantum theory of atoms in molecules analysis of the electron density) sh...
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The cis influence of the Corrin in vitamin B12 models
Chemical Physics Letters, 2012Co-Authors: Patrick Govender, Christopher B. Perry, Isabelle Navizet, Helder M. MarquesAbstract:Abstract Density Functional Theory (DFT) and Quantum Theory of Atoms in Molecules (QTAIM) calculations on [NH 3 [Co(III)(C10-X-Corrin)]–CH 3 ] + , where the C10–H of Corrin is replaced by electron-donating or –withdrawing groups, X, explore the cis influence in these vitamin B 12 models. As the electron donating ability of X increases, a normal trans influence is observed: the Co–NH 3 bond weakens while the Co–CH 3 bond strengthens. Surprisingly, though, the Co–CH 3 bond dissociation energy decreases monotonically as the Co–C bond strengthens. This is found to be a consequence of the extent of shortening of the Co–NH 3 during the homolysis reaction.
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Probing the nature of the Co(III) ion in Corrins: a comparison of the thermodynamics and kinetics of the ligand substitution reactions of aquacyanocobyrinic acid heptamethyl ester and stable yellow aquacyanocobyrinic acid heptamethyl ester.
Inorganic chemistry, 2011Co-Authors: Susan M. Chemaly, Melissa Florczak, Heinrich Dirr, Helder M. MarquesAbstract:Equilibrium constants (log K) for the substitution of coordinated H(2)O in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, ACCbs) and stable yellow aquacyanocobyrinic acid heptamethyl ester (stable yellow aquacyanocobester, ACSYCbs), in which oxidation of the C5 carbon of the Corrin interrupts the normal delocalized system of Corrins, by ligands with soft (CN(-), SO(3)(2-), and S(2)O(3)(2-)) and hard (NO(2)(-) and N(3)(-)) donors have been determined. The ligands with a harder donor atom (N in N(3)(-) and NO(2)(-)) produce ΔH values that are more negative in their reactions with ACSYCbs than with ACCbs. If the donor atom is softer (C in CN(-) and S in SO(3)(2-)), then ΔH is less positive, or more negative, for reactions with ACCbs than with ACSYCbs. The softer metal in ACCbs has a preference for softer ligands and the harder metal in ACSYCbs for the harder ligands. A kinetics study in which CN(-) substitutes H(2)O on Co(III) shows that ACCbs is more labile than ACSYCbs; the second-order rate constant k(II) is between 4.6 (at 5 °C) and 2.6 (at 35 °C) times larger. ΔH(‡) for the reaction of CN(-) with ACCbs is smaller by some 12 kJ mol(-1) than that for the reaction with ACSYCbs, consistent with an earlier transition state in which bonding between the softer metal of ACCbs and the ligand is greater than that of ACSYCbs with its harder metal. This difference in ΔH(‡) makes ACCbs over 100 times more labile, although the effect is masked by a ΔS(‡) value that is over 30 J K(-1) mol(-1) more negative. There is a significant increase in the inertness of Co(III) upon a decrease in the extent of conjugation of the Corrin ligand. Modifying the electronic structure of the equatorial ligand in the cobalt Corrins can modify the thermodynamics and kinetics of its reactions with exogenous ligands.
Susan M. Chemaly - One of the best experts on this subject based on the ideXlab platform.
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Probing the nature of the Co(III) ion in Corrins: The reactions of aquacyano-5-seco-cobyrinic acid heptamethyl ester with anionic ligands
Inorganica Chimica Acta, 2019Co-Authors: Monika Nowakowska, Susan M. Chemaly, Amanda L. Rousseau, Patrick Govender, Pradeep R. Varadwaj, Arpita Varadwaj, Koichi Yamashita, Helder M. MarquesAbstract:Abstract The substitution of H2O in aquacyano-5,6-dioxo-5,6-seco-heptamethyl-cob(III)yrinate, (aquacyano-5-seco-cobester, [AC-5-seco-Cbs]+), in which the C5–C6 bond of the Corrin ring of this vitamin B12 derivative is cleaved, by a variety of anionic ligands is reported. The pKa for ionization of coordinated water decreases from 9.8 ± 0.3 in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, [ACCbs]+) to 7.28 at 25 °C (ΔH = –88 ± 17 kJ mol−1 and ΔS = –434 ± 56 J K−1 mol−1) in [AC-5-seco-Cbs]+. The pKa, confirmed by determining the pH-dependence of coordination of SO32– by [AC-5-seco-Cbs]+, shows Co(III) in this complex behaves much more like Co(III) in simple hexacoordinate complexes than in intact cobalt Corrins. A comparison of log K values for coordination of CN–, SO32–, NO2–, N3– and S2O32– by [ACCbs]+ and [AC-5-seco-Cbs]+ demonstrates that cleavage of the Corrin ring significantly decreases the affinity of Co(III) for the softer ligands CN–, SO32– and, more marginally, NO2–. However, [AC-5-seco-Cbs]+ has a higher affinity for N3– and S2O32– than [ACCbs]+. These trends correlate with the position of the ligands in the spectrochemical series (N3–
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Probing the Nature of the Co(III) Ion in Corrins: Comparison of Reactions of Aquacyanocobyrinic Acid Heptamethyl Ester and Aquacyano-Stable Yellow Cobyrinic Acid Hexamethyl Ester with Neutral N‑Donor Ligands
2013Co-Authors: Susan M. Chemaly, Monika Nowakowska, Louise Kendall, Dale Pon, Christopher B. Perry, Helder M. MarquesAbstract:Equilibrium constants (log K) for substitution of coordinated H2O in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, ACCbs) and aquacyano-stable yellow cobyrinic acid hexamethyl ester (aquacyano-stable yellow cobester, ACSYCbs), in which oxidation of the C5 carbon of the Corrin interrupts the normal delocalized system of Corrins, by neutral N-donor ligands (ammonia, ethanolamine, 2-methoxyethylamine, N-methylimidazole, and 4-methylpyridine) have been determined spectrophotometrically as a function of temperature. Log K values increase with the basicity of the ligand, but a strong compensation effect between ΔH and ΔS values causes a leveling effect. The aliphatic amines with a harder donor atom produce ΔH values that are more negative in their reactions with ACSYCbs than with ACCbs, while the softer, aromatic N donors produce more negative ΔH values with ACCbs than with ACSYCbs. Molecular modeling (DFT, M06L/SVP, and a quantum theory of atoms in molecules analysis of the electron density) shows that complexes of the aliphatic amines with SYCbs produce shorter and stronger Co–N bonds with less ionic character than the Co–N bonds of these ligands with the cobester. Conversely, the Co–N bond to the aromatic N donors is shorter, stronger, and somewhat less ionic in the complexes of the cobester than in those of the SYCbs. Therefore, the distinction between the harder Co(III) in ACSYCbs and softer Co(III) in ACCbs, reported previously for anionic ligands, is maintained for neutral N-donor ligands
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Probing the nature of the Co(III) ion in Corrins: comparison of reactions of aquacyanocobyrinic acid heptamethyl ester and aquacyano-stable yellow cobyrinic acid hexamethyl ester with neutral N-donor ligands.
Inorganic chemistry, 2012Co-Authors: Susan M. Chemaly, Monika Nowakowska, Louise Kendall, Dale Pon, Christopher B. Perry, Helder M. MarquesAbstract:Equilibrium constants (log K) for substitution of coordinated H2O in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, ACCbs) and aquacyano-stable yellow cobyrinic acid hexamethyl ester (aquacyano-stable yellow cobester, ACSYCbs), in which oxidation of the C5 carbon of the Corrin interrupts the normal delocalized system of Corrins, by neutral N-donor ligands (ammonia, ethanolamine, 2-methoxyethylamine, N-methylimidazole, and 4-methylpyridine) have been determined spectrophotometrically as a function of temperature. Log K values increase with the basicity of the ligand, but a strong compensation effect between ΔH and ΔS values causes a leveling effect. The aliphatic amines with a harder donor atom produce ΔH values that are more negative in their reactions with ACSYCbs than with ACCbs, while the softer, aromatic N donors produce more negative ΔH values with ACCbs than with ACSYCbs. Molecular modeling (DFT, M06L/SVP, and a quantum theory of atoms in molecules analysis of the electron density) sh...
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Probing the nature of the Co(III) ion in Corrins: a comparison of the thermodynamics and kinetics of the ligand substitution reactions of aquacyanocobyrinic acid heptamethyl ester and stable yellow aquacyanocobyrinic acid heptamethyl ester.
Inorganic chemistry, 2011Co-Authors: Susan M. Chemaly, Melissa Florczak, Heinrich Dirr, Helder M. MarquesAbstract:Equilibrium constants (log K) for the substitution of coordinated H(2)O in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, ACCbs) and stable yellow aquacyanocobyrinic acid heptamethyl ester (stable yellow aquacyanocobester, ACSYCbs), in which oxidation of the C5 carbon of the Corrin interrupts the normal delocalized system of Corrins, by ligands with soft (CN(-), SO(3)(2-), and S(2)O(3)(2-)) and hard (NO(2)(-) and N(3)(-)) donors have been determined. The ligands with a harder donor atom (N in N(3)(-) and NO(2)(-)) produce ΔH values that are more negative in their reactions with ACSYCbs than with ACCbs. If the donor atom is softer (C in CN(-) and S in SO(3)(2-)), then ΔH is less positive, or more negative, for reactions with ACCbs than with ACSYCbs. The softer metal in ACCbs has a preference for softer ligands and the harder metal in ACSYCbs for the harder ligands. A kinetics study in which CN(-) substitutes H(2)O on Co(III) shows that ACCbs is more labile than ACSYCbs; the second-order rate constant k(II) is between 4.6 (at 5 °C) and 2.6 (at 35 °C) times larger. ΔH(‡) for the reaction of CN(-) with ACCbs is smaller by some 12 kJ mol(-1) than that for the reaction with ACSYCbs, consistent with an earlier transition state in which bonding between the softer metal of ACCbs and the ligand is greater than that of ACSYCbs with its harder metal. This difference in ΔH(‡) makes ACCbs over 100 times more labile, although the effect is masked by a ΔS(‡) value that is over 30 J K(-1) mol(-1) more negative. There is a significant increase in the inertness of Co(III) upon a decrease in the extent of conjugation of the Corrin ligand. Modifying the electronic structure of the equatorial ligand in the cobalt Corrins can modify the thermodynamics and kinetics of its reactions with exogenous ligands.
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Probing the nature of the Co(III) ion in Corrins: the structural and electronic properties of dicyano- and aquacyanocobyrinic acid heptamethyl ester and a stable yellow dicyano- and aquacyanocobyrinic acid heptamethyl ester.
Inorganic chemistry, 2011Co-Authors: Susan M. Chemaly, Kenneth L Brown, Manuel A. Fernandes, Orde Q. Munro, Craig D. Grimmer, Helder M. MarquesAbstract:A stable yellow derivative of cobyrinic acid heptamethyl ester, (5R,6R)-Coα,Coβ-dicyano-5,6-dihydro-5-hydroxy-heptamethylcob(III)yrinate-c,6-lactone (DCSYCbs), was prepared from dicyanocobyrinic acid heptamethyl ester (DCCbs). The C5 carbon is oxidized and the c side chain cyclized to form a lactone at C6; the 13 atom, 14 π-e(-) delocalized system of Corrins is interrupted, giving a triazamethine system with four conjugated double bonds between N22 and N24 and an isolated double bond between N21 and C4. Stable yellow aquacyanocobyrinic acid heptamethyl ester (ACSYCbs) was prepared by driving off HCN with N(2) in a methanol/acetic acid solution. The electronic spectra of DCCbs and DCSYCbs appear similar except that the bands in DCSYCbs are shifted to shorter wavelengths and the γ-band is much less intense. The experimental spectra were adequately modeled using TD-DFT at the PBE1PBE/6-311G(d,p) level of theory. DCSYCbs crystallizes in the space group P2(1)2(1)2(1) (R(1) = 6.08%) with Z = 4, including one methanol solvent molecule and one water molecule per cobester. The addition of a hydroxyl group at C5 causes loss of the double bond between C5 and C6 and elongation of the C5-C6 bond. From a combination of two-dimensional (1)H TOCSY and ROESY NMR spectra and (1)H/(13)C HSQC and HMBC data, the complete (1)H and (13)C NMR assignments of DCSYCbs were possible, except for two of the ester methyl groups and the (13)C resonances of the two axial cyanide ligands. The latter were assigned using relative chemical shifts calculated by GIAO-DFT methods. The (59)Co resonance of DCCbs was observed at 4074 ppm while that of DCSYCbs is shifted downfield to 4298 ppm. Comparison with available (59)Co data of analogous systems suggests that the more π-conjugated Corrin of DCCbs interacts more strongly with the metal than the less extensively conjugated macrocycle of DCSYCbs. As the strength of the interaction between Co(III) and an equatorial macrocycle increases, ν(CN) of axially coordinated CN(-) shifts to lower frequency; in DCSYCbs and DCCbs ν(CN) occurs at 2138 and 2123 cm(-1), respectively. Hence the Corrin ligand in DCCbs interacts more strongly with the metal than the stable yellow Corrin ligand, with its diminished conjugation. The UV-vis spectral data and DFT-calculated MOs are consistent with greater overlap between the Corrin and the metal orbitals in DCCbs relative to DCSYCbs, which gives the metal in the former a softer, more covalent character.
Kenneth L Brown - One of the best experts on this subject based on the ideXlab platform.
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Probing the nature of the Co(III) ion in Corrins: the structural and electronic properties of dicyano- and aquacyanocobyrinic acid heptamethyl ester and a stable yellow dicyano- and aquacyanocobyrinic acid heptamethyl ester.
Inorganic chemistry, 2011Co-Authors: Susan M. Chemaly, Kenneth L Brown, Manuel A. Fernandes, Orde Q. Munro, Craig D. Grimmer, Helder M. MarquesAbstract:A stable yellow derivative of cobyrinic acid heptamethyl ester, (5R,6R)-Coα,Coβ-dicyano-5,6-dihydro-5-hydroxy-heptamethylcob(III)yrinate-c,6-lactone (DCSYCbs), was prepared from dicyanocobyrinic acid heptamethyl ester (DCCbs). The C5 carbon is oxidized and the c side chain cyclized to form a lactone at C6; the 13 atom, 14 π-e(-) delocalized system of Corrins is interrupted, giving a triazamethine system with four conjugated double bonds between N22 and N24 and an isolated double bond between N21 and C4. Stable yellow aquacyanocobyrinic acid heptamethyl ester (ACSYCbs) was prepared by driving off HCN with N(2) in a methanol/acetic acid solution. The electronic spectra of DCCbs and DCSYCbs appear similar except that the bands in DCSYCbs are shifted to shorter wavelengths and the γ-band is much less intense. The experimental spectra were adequately modeled using TD-DFT at the PBE1PBE/6-311G(d,p) level of theory. DCSYCbs crystallizes in the space group P2(1)2(1)2(1) (R(1) = 6.08%) with Z = 4, including one methanol solvent molecule and one water molecule per cobester. The addition of a hydroxyl group at C5 causes loss of the double bond between C5 and C6 and elongation of the C5-C6 bond. From a combination of two-dimensional (1)H TOCSY and ROESY NMR spectra and (1)H/(13)C HSQC and HMBC data, the complete (1)H and (13)C NMR assignments of DCSYCbs were possible, except for two of the ester methyl groups and the (13)C resonances of the two axial cyanide ligands. The latter were assigned using relative chemical shifts calculated by GIAO-DFT methods. The (59)Co resonance of DCCbs was observed at 4074 ppm while that of DCSYCbs is shifted downfield to 4298 ppm. Comparison with available (59)Co data of analogous systems suggests that the more π-conjugated Corrin of DCCbs interacts more strongly with the metal than the less extensively conjugated macrocycle of DCSYCbs. As the strength of the interaction between Co(III) and an equatorial macrocycle increases, ν(CN) of axially coordinated CN(-) shifts to lower frequency; in DCSYCbs and DCCbs ν(CN) occurs at 2138 and 2123 cm(-1), respectively. Hence the Corrin ligand in DCCbs interacts more strongly with the metal than the stable yellow Corrin ligand, with its diminished conjugation. The UV-vis spectral data and DFT-calculated MOs are consistent with greater overlap between the Corrin and the metal orbitals in DCCbs relative to DCSYCbs, which gives the metal in the former a softer, more covalent character.
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side chain entropy and the activation of organocobalamins for carbon cobalt bond homolysis synthesis characterization and thermolysis of the neopentyl derivative of a unique cobalamin analog lacking a c side chain
Inorganic Chemistry, 1997Co-Authors: Kenneth L Brown, Shifa Cheng, Jeffrey D Zubkowski, Edward J ValenteAbstract:Hydrodeamination of the c-amino derivative, 5, of cyanocobalamin (CNCbl) with hydroxylamine-O-sulfonic acid in aqueous base leads to an extensively rearranged product instead of the c side chain truncated derivative, 1, expected from simple deamination. The rearranged product (CNCbl-8-butanamide) crystallizes in the orthorhombic system, space group P212121 with unit cell dimensions a = 16.041(11), b = 21.94(2), and c = 25.43(2) A. It is devoid of substituents at Corrin ring C(7) but quarternized at C(8) with an “upwardly” pseudoaxial methyl group and a d side chain expanded by one methylene group to a butanamide. The Corrin ring of this rearranged derivative is significantly flatter (Corrin ring fold angle 9.9°) than CNCbl itself (fold angle 18.0°). Conversion of CNCbl-8-butanamide to its neopentyl derivative (NpCbl-8-butanamide), a NpCbl analog which lacks a c acetamide side chain, permits a quantitative assessment of the influence of thermal motions of the c side chain on the entropy of activation for c...
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Heteronuclear NMR studies of cobalt Corrinoids—17. characterization of neopentylcobinamide and neopentyl-13-epicobinamide by 1H and 13C NMR spectroscopy: Inferred Corrin ring conformations from chemical shift differentials
Polyhedron, 1995Co-Authors: Kenneth L Brown, Daniel R. EvansAbstract:Abstract While the 1H and 13C NMR spectra of neopentylcobalamin (NpCbl) and its epimer at Corrin ring C(13), neopentyl-13-epiCbl (Np-13-epiCbl), are extremely broad, apparently due to chemical exchange between the base-on and base-off species, the cobinamide derivatives, neopentyl cobinamide (NpCbi+) and neopentyl-13-epicobinamide (Np-13-epiCbi+), have sharp, well-resolved NMR spectra. The 1H and 13C NMR spectra of NpCbi+ and Np-13-epiCbi+ have now been completely assigned by modern two-dimensional NMR methodologies. Comparison of the 13C spectra of these two complexes shows that significant chemical shift differences occur at a variety of Corrin ring and peripheral carbon atoms and are not localized near the site of epimerization. Similarly, comparison of the 13C NMR spectra of NpCbi+ and 5′-deoxyadenosylcobinamide (AdoCbi+) shows differences at many Corrin ring and peripheral carbons. A first attempt at discerning differences in Corrin ring conformation from such differences in 13C chemical shift has been made by comparing the X-ray crystal structures and 13C NMR spectra of 5′-deoxyadenosylcobalamin (coenzyme B12, AdoCbl) and cyanocobalamin (vitamin B12, CNCbl). After elimination of carbon atoms whose chemical shifts are likely to be significantly affected by differences in the inductive effect of the Ado and CN ligands, and after consideration of differential anisotropic shielding effects in the two complexes due to the presence or absence of the Ado ligand, the difference in magnetic anisotropy of the central cobalt atom, the change in position of the axial nucleotide and differences in the magnetic anisotropy of the Corrin ligand, 15 peripheral carbon atoms [C(2), C(18), C(20), C(25), C(26), C(30), C(36), C(37), C(41), C(46), C(47), C(48), C(54), C(55), C(60)] emerge as candidate “reporter” carbons whose 13C chemical shifts may be useful in deducing conformational differences in cobalt Corrinoids. Application of this method to adeninylpropylcobalamin (AdePrCbl), for which the X-ray crystal structure and absolute NMR assignments are known, correctly predicts the gross conformational differences between the Corrin ring of AdePrCbl and that of CNCbl. Use of these reporter carbon chemical shifts suggests that in NpCbi+, the fold angle, defined as the angle between the “northern” and “southern” planes of the Corrin ring, is reduced relative to AdoCbi+. Comparison of the chemical shifts of the reporter carbon atoms in NpCbi+ and Np-13-epiCbi+ suggests that the fold angle in the former is larger than that for the latter.
Ming-chang Lin - One of the best experts on this subject based on the ideXlab platform.
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On the Reduction of O₂ on Cathode Surfaces of Co–Corrin and Co–Porphyrin: A Computational and Experimental Study on Their Relative Efficiencies in H₂O/H₂O₂ Formation
The Journal of Physical Chemistry, 2020Co-Authors: Wen-fei Huang, Sun-tang Chang, Hsin-chih Huang, Chen-hao Wang, Li-chyong Chen, Kuei-hsien Chen, Ming-chang LinAbstract:The mechanisms for O₂ reduction and H₂O/H₂O₂ formation on Co–Corrin and Co–porphyrin cathode surfaces of the proton exchange membrane fuel cell (PEMFC) systems have been studied by hybrid Hartree–Fock/density functional theory (B3LYP) calculations with the LANL2DZ basis set. The calculations show that the reduced Co–Corrin with a single negative charge (Co–Corrin–) is more reactive than the neutral Co–Corrin and the doubly charged Co–Corrin²–. Both O₂ and O adsorptions are most stable on Co–Corrin–, rather than Co–Corrin or Co–Corrin²–. The potential energy profiles show that the decomposition of O₂ on both Co–Corrin and Co–Corrin– can take place energetically favorably without thermal activation. The formation of H₂O and H₂O₂ are predicted to occur by two separate reaction paths: the HO path and the HOO path. The HO path with H₂O as the predominant product on the reduced Co–Corrin– surface, the energetically favored surface, under operational cathodic conditions, which is consistent with recent experimental findings, wherein the PEMFCs with pyrolyzed vitamin B12 containing Co–Corrin as catalysts loaded at the cathode, can deliver up to 14.5 A cm–³ at 0.8 V with IR compensation. A similar calculation performed for a Co–porphyrin system shows a significantly less efficient O₂ reduction, consistent with the experiment results of the PEMFC power output studies.
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On the Reduction of O-2 on Cathode Surfaces of Co-Corrin and Co-Porphyrin: A Computational and Experimental Study on Their Relative Efficiencies in H2O/H2O2 Formation
The Journal of Physical Chemistry C, 2020Co-Authors: Wen-fei Huang, Sun-tang Chang, Hsin-chih Huang, Chen-hao Wang, Li-chyong Chen, Kuei-hsien Chen, Ming-chang LinAbstract:The mechanisms for O2 reduction and H2O/H2O2 formation on Co–Corrin and Co–porphyrin cathode surfaces of the proton exchange membrane fuel cell (PEMFC) systems have been studied by hybrid Hartree–F...
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on the reduction of o on cathode surfaces of co Corrin and co porphyrin a computational and experimental study on their relative efficiencies in h o h o formation
The Journal of Physical Chemistry, 2020Co-Authors: Wen-fei Huang, Sun-tang Chang, Hsin-chih Huang, Chen-hao Wang, Li-chyong Chen, Kuei-hsien Chen, Ming-chang LinAbstract:The mechanisms for O₂ reduction and H₂O/H₂O₂ formation on Co–Corrin and Co–porphyrin cathode surfaces of the proton exchange membrane fuel cell (PEMFC) systems have been studied by hybrid Hartree–Fock/density functional theory (B3LYP) calculations with the LANL2DZ basis set. The calculations show that the reduced Co–Corrin with a single negative charge (Co–Corrin–) is more reactive than the neutral Co–Corrin and the doubly charged Co–Corrin²–. Both O₂ and O adsorptions are most stable on Co–Corrin–, rather than Co–Corrin or Co–Corrin²–. The potential energy profiles show that the decomposition of O₂ on both Co–Corrin and Co–Corrin– can take place energetically favorably without thermal activation. The formation of H₂O and H₂O₂ are predicted to occur by two separate reaction paths: the HO path and the HOO path. The HO path with H₂O as the predominant product on the reduced Co–Corrin– surface, the energetically favored surface, under operational cathodic conditions, which is consistent with recent experimental findings, wherein the PEMFCs with pyrolyzed vitamin B12 containing Co–Corrin as catalysts loaded at the cathode, can deliver up to 14.5 A cm–³ at 0.8 V with IR compensation. A similar calculation performed for a Co–porphyrin system shows a significantly less efficient O₂ reduction, consistent with the experiment results of the PEMFC power output studies.
Monika Nowakowska - One of the best experts on this subject based on the ideXlab platform.
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Probing the nature of the Co(III) ion in Corrins: The reactions of aquacyano-5-seco-cobyrinic acid heptamethyl ester with anionic ligands
Inorganica Chimica Acta, 2019Co-Authors: Monika Nowakowska, Susan M. Chemaly, Amanda L. Rousseau, Patrick Govender, Pradeep R. Varadwaj, Arpita Varadwaj, Koichi Yamashita, Helder M. MarquesAbstract:Abstract The substitution of H2O in aquacyano-5,6-dioxo-5,6-seco-heptamethyl-cob(III)yrinate, (aquacyano-5-seco-cobester, [AC-5-seco-Cbs]+), in which the C5–C6 bond of the Corrin ring of this vitamin B12 derivative is cleaved, by a variety of anionic ligands is reported. The pKa for ionization of coordinated water decreases from 9.8 ± 0.3 in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, [ACCbs]+) to 7.28 at 25 °C (ΔH = –88 ± 17 kJ mol−1 and ΔS = –434 ± 56 J K−1 mol−1) in [AC-5-seco-Cbs]+. The pKa, confirmed by determining the pH-dependence of coordination of SO32– by [AC-5-seco-Cbs]+, shows Co(III) in this complex behaves much more like Co(III) in simple hexacoordinate complexes than in intact cobalt Corrins. A comparison of log K values for coordination of CN–, SO32–, NO2–, N3– and S2O32– by [ACCbs]+ and [AC-5-seco-Cbs]+ demonstrates that cleavage of the Corrin ring significantly decreases the affinity of Co(III) for the softer ligands CN–, SO32– and, more marginally, NO2–. However, [AC-5-seco-Cbs]+ has a higher affinity for N3– and S2O32– than [ACCbs]+. These trends correlate with the position of the ligands in the spectrochemical series (N3–
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Probing the Nature of the Co(III) Ion in Corrins: Comparison of Reactions of Aquacyanocobyrinic Acid Heptamethyl Ester and Aquacyano-Stable Yellow Cobyrinic Acid Hexamethyl Ester with Neutral N‑Donor Ligands
2013Co-Authors: Susan M. Chemaly, Monika Nowakowska, Louise Kendall, Dale Pon, Christopher B. Perry, Helder M. MarquesAbstract:Equilibrium constants (log K) for substitution of coordinated H2O in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, ACCbs) and aquacyano-stable yellow cobyrinic acid hexamethyl ester (aquacyano-stable yellow cobester, ACSYCbs), in which oxidation of the C5 carbon of the Corrin interrupts the normal delocalized system of Corrins, by neutral N-donor ligands (ammonia, ethanolamine, 2-methoxyethylamine, N-methylimidazole, and 4-methylpyridine) have been determined spectrophotometrically as a function of temperature. Log K values increase with the basicity of the ligand, but a strong compensation effect between ΔH and ΔS values causes a leveling effect. The aliphatic amines with a harder donor atom produce ΔH values that are more negative in their reactions with ACSYCbs than with ACCbs, while the softer, aromatic N donors produce more negative ΔH values with ACCbs than with ACSYCbs. Molecular modeling (DFT, M06L/SVP, and a quantum theory of atoms in molecules analysis of the electron density) shows that complexes of the aliphatic amines with SYCbs produce shorter and stronger Co–N bonds with less ionic character than the Co–N bonds of these ligands with the cobester. Conversely, the Co–N bond to the aromatic N donors is shorter, stronger, and somewhat less ionic in the complexes of the cobester than in those of the SYCbs. Therefore, the distinction between the harder Co(III) in ACSYCbs and softer Co(III) in ACCbs, reported previously for anionic ligands, is maintained for neutral N-donor ligands
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Probing the nature of the Co(III) ion in Corrins: comparison of reactions of aquacyanocobyrinic acid heptamethyl ester and aquacyano-stable yellow cobyrinic acid hexamethyl ester with neutral N-donor ligands.
Inorganic chemistry, 2012Co-Authors: Susan M. Chemaly, Monika Nowakowska, Louise Kendall, Dale Pon, Christopher B. Perry, Helder M. MarquesAbstract:Equilibrium constants (log K) for substitution of coordinated H2O in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, ACCbs) and aquacyano-stable yellow cobyrinic acid hexamethyl ester (aquacyano-stable yellow cobester, ACSYCbs), in which oxidation of the C5 carbon of the Corrin interrupts the normal delocalized system of Corrins, by neutral N-donor ligands (ammonia, ethanolamine, 2-methoxyethylamine, N-methylimidazole, and 4-methylpyridine) have been determined spectrophotometrically as a function of temperature. Log K values increase with the basicity of the ligand, but a strong compensation effect between ΔH and ΔS values causes a leveling effect. The aliphatic amines with a harder donor atom produce ΔH values that are more negative in their reactions with ACSYCbs than with ACCbs, while the softer, aromatic N donors produce more negative ΔH values with ACCbs than with ACSYCbs. Molecular modeling (DFT, M06L/SVP, and a quantum theory of atoms in molecules analysis of the electron density) sh...
