A pH-metric, UV, NMR, and X-ray crystallographic study on arsenous acid reacting with dithioerythritol

András Kolozsi, A. Lakatos, G. Galbács, Anders Østergaard Madsen, Erik Larsen, B. Gyurcsik

Research output: Contribution to journalArticle

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Abstract

The aqueous solutions of arsenous acid with the meso and racemic forms of 1,4-dithiol-butane-2,3-diol, namely, dithioerythritol (dte) and dithiothreitol (dtt), respectively, were titrated pH-metrically in different molar ratios. The pKa values determined for As(OH)3, and dtt were in good accordance with the literature data, and we determined for the first time the pKa value of dte. The deprotonation steps of both M (As(OH) 3 considered as a central metal ion) and H2L components dte and dtt (considered as ligands) appeared at a higher pH in the titration curves of the ternary systems (M, H2L, H+) than in the individual component. This unusual observation is explained by the condensation reactions between the reagents taking place in the pH <8 range. In the solutions of CAS(III) > 5·10-3 M, the precipitate formed upon mixing the arsenous acid and H2L solutions in neutral medium, and the formation of the precipitate shifted toward acidic pH on the increase of the total concentrations. This indicated that pH-metry can follow the reactions only in an indirect way. Useful, but not satisfactory, information can be obtained by means of this method alone. Combined with NMR and UV spectroscopic measurements it is revealed that depending on the As(III)/H2L molar ratio, different complexes form in the solutions. In the species with 1:2 composition, one of the ligands is strongly bound to the arsenic(III) probably via its two thiolate, while the second one is attached only weakly. The crystal structure of an As(III)-dte crystal of 1:1 composition, grown from ethanolic solution, shows that As(III) binds the ligand through its three p-orbitals in a manner similar to that expected in aqueous solution. While the uptake of the second ligand cannot be detected by pH-metry, the decomposition of thioether bonds above pH ∼10 is confirmed by the change in UV spectra at ∼265 nm to be a base-consuming process. In such alkaline solutions, most probably, rearrangement of the bonding scheme occurs, resulting in ligands being bound to the arsenic(III) through the oxygen donor atoms.

Original languageEnglish
Pages (from-to)3832-3840
Number of pages9
JournalInorganic Chemistry
Volume47
Issue number9
DOIs
Publication statusPublished - May 5 2008

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Dithioerythritol
Nuclear magnetic resonance
X rays
nuclear magnetic resonance
acids
Dithiothreitol
Ligands
ligands
x rays
Arsenic
Precipitates
arsenic
precipitates
Deprotonation
Condensation reactions
aqueous solutions
Sulfides
Ternary systems
Chemical analysis
Titration

ASJC Scopus subject areas

  • Inorganic Chemistry

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A pH-metric, UV, NMR, and X-ray crystallographic study on arsenous acid reacting with dithioerythritol. / Kolozsi, András; Lakatos, A.; Galbács, G.; Madsen, Anders Østergaard; Larsen, Erik; Gyurcsik, B.

In: Inorganic Chemistry, Vol. 47, No. 9, 05.05.2008, p. 3832-3840.

Research output: Contribution to journalArticle

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T1 - A pH-metric, UV, NMR, and X-ray crystallographic study on arsenous acid reacting with dithioerythritol

AU - Kolozsi, András

AU - Lakatos, A.

AU - Galbács, G.

AU - Madsen, Anders Østergaard

AU - Larsen, Erik

AU - Gyurcsik, B.

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N2 - The aqueous solutions of arsenous acid with the meso and racemic forms of 1,4-dithiol-butane-2,3-diol, namely, dithioerythritol (dte) and dithiothreitol (dtt), respectively, were titrated pH-metrically in different molar ratios. The pKa values determined for As(OH)3, and dtt were in good accordance with the literature data, and we determined for the first time the pKa value of dte. The deprotonation steps of both M (As(OH) 3 considered as a central metal ion) and H2L components dte and dtt (considered as ligands) appeared at a higher pH in the titration curves of the ternary systems (M, H2L, H+) than in the individual component. This unusual observation is explained by the condensation reactions between the reagents taking place in the pH <8 range. In the solutions of CAS(III) > 5·10-3 M, the precipitate formed upon mixing the arsenous acid and H2L solutions in neutral medium, and the formation of the precipitate shifted toward acidic pH on the increase of the total concentrations. This indicated that pH-metry can follow the reactions only in an indirect way. Useful, but not satisfactory, information can be obtained by means of this method alone. Combined with NMR and UV spectroscopic measurements it is revealed that depending on the As(III)/H2L molar ratio, different complexes form in the solutions. In the species with 1:2 composition, one of the ligands is strongly bound to the arsenic(III) probably via its two thiolate, while the second one is attached only weakly. The crystal structure of an As(III)-dte crystal of 1:1 composition, grown from ethanolic solution, shows that As(III) binds the ligand through its three p-orbitals in a manner similar to that expected in aqueous solution. While the uptake of the second ligand cannot be detected by pH-metry, the decomposition of thioether bonds above pH ∼10 is confirmed by the change in UV spectra at ∼265 nm to be a base-consuming process. In such alkaline solutions, most probably, rearrangement of the bonding scheme occurs, resulting in ligands being bound to the arsenic(III) through the oxygen donor atoms.

AB - The aqueous solutions of arsenous acid with the meso and racemic forms of 1,4-dithiol-butane-2,3-diol, namely, dithioerythritol (dte) and dithiothreitol (dtt), respectively, were titrated pH-metrically in different molar ratios. The pKa values determined for As(OH)3, and dtt were in good accordance with the literature data, and we determined for the first time the pKa value of dte. The deprotonation steps of both M (As(OH) 3 considered as a central metal ion) and H2L components dte and dtt (considered as ligands) appeared at a higher pH in the titration curves of the ternary systems (M, H2L, H+) than in the individual component. This unusual observation is explained by the condensation reactions between the reagents taking place in the pH <8 range. In the solutions of CAS(III) > 5·10-3 M, the precipitate formed upon mixing the arsenous acid and H2L solutions in neutral medium, and the formation of the precipitate shifted toward acidic pH on the increase of the total concentrations. This indicated that pH-metry can follow the reactions only in an indirect way. Useful, but not satisfactory, information can be obtained by means of this method alone. Combined with NMR and UV spectroscopic measurements it is revealed that depending on the As(III)/H2L molar ratio, different complexes form in the solutions. In the species with 1:2 composition, one of the ligands is strongly bound to the arsenic(III) probably via its two thiolate, while the second one is attached only weakly. The crystal structure of an As(III)-dte crystal of 1:1 composition, grown from ethanolic solution, shows that As(III) binds the ligand through its three p-orbitals in a manner similar to that expected in aqueous solution. While the uptake of the second ligand cannot be detected by pH-metry, the decomposition of thioether bonds above pH ∼10 is confirmed by the change in UV spectra at ∼265 nm to be a base-consuming process. In such alkaline solutions, most probably, rearrangement of the bonding scheme occurs, resulting in ligands being bound to the arsenic(III) through the oxygen donor atoms.

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