Equilibrium and kinetic studies on ligand substitution reactions of chloromethyl(aquo)cobaloxime with aromatic and aliphatic N-donor ligands

Reddy, D. Sudarshan; Satyanarayana, S.

doi:10.1007/bf02704257

J Chem Sci

2003

DOI: 10.1007/bf02704257

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Equilibrium and kinetic studies on ligand substitution reactions of chloromethyl(aquo)cobaloxime with aromatic and aliphatic N-donor ligands

D. Sudarshan Reddy

S. Satyanarayana

Abstract: Equilibria and kinetics of the reactions of chloromethyl(aquo)cobaloxime with histamine, histidine, glycine and ethyl glycine ester were studied as a function of pH at 25°C, 1⋅0 M ionic strength (KCl) by spectrophotometric techniques. Comparison of equilibrium constants and rate constants tells that the order is K Hisdn > K Hiamn > K Gly > K EtGlyest. The rate of substitution of H 2 O varies with the pKa of the incoming ligand and nucleophilic participation of the ligand in the transition state. The rate const… Show more

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Cited by 3 publications

References 26 publications

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Atrans influence study in propyl (aquo)cobaloxime by imidazoles and amino acids

Madhuri

Satyanarayana

2005

J Chem Sci

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Substitution reactions of propyl cobaloxime with imidazole, substituted imidazoles, histidine, histamine, glycine and ethyl glycine ester are carried out as a function of pH. Trends in the formation constants are explained based on the steric hindrance, extent of π-bonding and σ-donor capacity of the incoming ligand. Molecular mechanics is used to theoretically determine the bond length and bond strain values by MM2 parametrization and these are correlated with the experimental data.

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Atrans influence study in propyl (aquo)cobaloxime by imidazoles and amino acids

Madhuri

Satyanarayana

2005

J Chem Sci

Self Cite

View full text Add to dashboard Cite

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The inorganic chemistry of the cobalt corrinoids – an update

Marques

2023

Journal of Inorganic Biochemistry

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Probing the nature of the Co(iii) ion in cobalamins: a comparison of the reaction of aquacobalamin (vitamin B12a) and aqua-10-chlorocobalamin with some anionic and N-donor ligands

Knapton

Marques

2005

Dalton Trans.

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To probe the cis effect of the corrin macrocycle in vitamin B12 derivatives, equilibrium constants for the substitution of coordinated H2O in aquacobalamin (vitamin B12a, H2OCbl+) and in aqua-10-chlorocobalamin, H2O-10-ClCbl+, (in which Cl has replaced the C10 H) by an exogenous ligand, L (L = an anion, NO2-, SCN-, N3-, OCN-, S2O3(2-), NCSe- or a neutral N-donor, CH3NH2, pyridine, imidazole) have been determined. The cis influence reported in the electronic spectra of the cobalamins is observed in the spectra of L-10-ClCbls as well. Anionic ligands bind more strongly to H2O-10-ClCbl+ than to H2OCbl+ with log K values between 0.10 and 0.63 (average 0.26) larger; the converse is true for the neutral N-donor ligands, where log K is between 0.17 and 0.3 (average 0.25) smaller. Semi-empirical molecular orbital (SEMO) calculations using the ZINDO/1 model on the hydroxo complexes show that charge density is delocalised from the axial donor atom to the metal and Cl. This explains why coordinated OH- is a poorer base in HO-10-ClCbl than in HOCbl and the pK(a) of H2O-10-ClCbl+ is lower than that of H2OCbl+. It further explains why, because of the ability of the metal in concert with the C10 Cl to accept charge density from the ligand, an anionic ligand will bind more strongly to Co(III) in H2O-10-ClCbl+ than in H2OCbl+. The kinetics of the replacement of coordinated H2O by two probe ligand, pyridine and azide, were determined. The rate constants for substitution of H2O in H2O-10-ClCbl+ by pyridine show saturation, whilst those for substitution by N3- do not; this is inconsistent with a purely dissociative mechanism and the reactions proceed through an interchange mechanism. The values of the activation parameters are more positive for the reaction between these ligands and H2OCbl+, than for their reaction with H2O-10-ClCbl+. This is interpreted to mean that the transition state in the reaction of H2O-10-ClCbl+ occurs earlier along the reaction coordinate. In the temperature range studied, H2O-10-ClCbl+ reacts more slowly with py and N3- than does H2OCbl+. SEMO calculations indicate that as the Co-O bond to the departing H(2)O molecule is stretched, the charge density on Co in H2OCbl+ is always lower than on Co in H2O-10-ClCbl+. This suggests that the former is a better electrophile towards the incoming ligand, and offers an explanation for the kinetics observations.

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Equilibrium and kinetic studies on ligand substitution reactions of chloromethyl(aquo)cobaloxime with aromatic and aliphatic N-donor ligands

Cited by 3 publications

References 26 publications

Atrans influence study in propyl (aquo)cobaloxime by imidazoles and amino acids

Atrans influence study in propyl (aquo)cobaloxime by imidazoles and amino acids

The inorganic chemistry of the cobalt corrinoids – an update

Probing the nature of the Co(iii) ion in cobalamins: a comparison of the reaction of aquacobalamin (vitamin B12a) and aqua-10-chlorocobalamin with some anionic and N-donor ligands

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