Oxidation of Ascorbate by Ni(III) Complexes with Tetraaza-macrocyclic Ligands in Neutral Aqueous Solutions. A Pulse-Radiolysis Study

Israel Zilbermann; Avraham Meshulam; Haim Cohen; Dan Meyerstein

doi:10.1080/10610270108027487

Oxidation of Ascorbate by Ni(III) Complexes with Tetraaza-macrocyclic Ligands in Neutral Aqueous Solutions. A Pulse-Radiolysis Study

Israel Zilbermann, Avraham Meshulam, Haim Cohen, Dan Meyerstein

Research output: Contribution to journal › Article › peer-review

6 Scopus citations

Abstract

The mechanisms and kinetics of oxidation of ascorbate, AH^-, by Ni(III)Lⁱ_aq and by LⁱNi(III) (HPO₄)₂^- complexes (L¹ = meso-(5,12)-7,7,14,14-hex-amethyl-1,4,8,11-tetraazacyclotetradecane; L² = 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane) in neutral aqueous solutions have been investigated. The oxidation of ascorbate by the LⁱNi(III) (HPO₄)₂^- and Ni(III)L¹_aq proceeds via two consecutive reactions well separated in time. The products of the first reaction are the A^.- radical anion and the corresponding Ni(II) complex. The oxidations by the LⁱNi(III)(HPO₄)₂^- complexes proceed via the outer sphere mechanism, whereas the detailed mechanism of reaction of Ni(III)L¹_aq cannot be determined. The rate of reaction decreases with the increase in the concentration of phosphate, thus indicating that LⁱNi(III)(HPO₄)(H₂O)⁺ and LⁱNi(III)OH²⁺ are stronger oxidizing agents than LⁱNi(III)(HPO₄)₂^-. The oxidation of ascorbate by Ni(III)L²_aq proceeds via three consecutive reactions which are well separated in time. Thus the results clearly point out that this process occurs via the inner sphere mechanism. The first transient observed is tentatively identified as L²(H₂O)Ni(II)(A^.-)²⁺, i.e., an unexpected complex of the ascorbate anion radical. Also in this process the last transient observed is the A^.- anion radical. The stabilization of the ascorbyl radical in a transient complex might be of biological significance.

Original language	English
Pages (from-to)	325-332
Number of pages	8
Journal	Supramolecular Chemistry
Volume	13
Issue number	2
DOIs	https://doi.org/10.1080/10610270108027487
State	Published - 2001
Externally published	Yes

Bibliographical note

Funding Information:
We express our sincere gratitude for Professor Dr. Joachim Sieler (Leipzig University, Germany) for his active interest in our work. The present study was in part supported by a visiting Stipendium from the Sächsische Staatsministerium für Wissenschaft und Kunst (KVD), which is gratefully acknowledged.

Keywords

Ascorbate
Ascorbyl radial
Mechanism
Nickel(III)
Oxidation

Access to Document

10.1080/10610270108027487

Cite this

@article{fea4276e694d43d6ae290bb378fedf6a,

title = "Oxidation of Ascorbate by Ni(III) Complexes with Tetraaza-macrocyclic Ligands in Neutral Aqueous Solutions. A Pulse-Radiolysis Study",

abstract = "The mechanisms and kinetics of oxidation of ascorbate, AH-, by Ni(III)Liaq and by LiNi(III) (HPO4)2- complexes (L1 = meso-(5,12)-7,7,14,14-hex-amethyl-1,4,8,11-tetraazacyclotetradecane; L2 = 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane) in neutral aqueous solutions have been investigated. The oxidation of ascorbate by the LiNi(III) (HPO4)2- and Ni(III)L1aq proceeds via two consecutive reactions well separated in time. The products of the first reaction are the A.- radical anion and the corresponding Ni(II) complex. The oxidations by the LiNi(III)(HPO4)2- complexes proceed via the outer sphere mechanism, whereas the detailed mechanism of reaction of Ni(III)L1aq cannot be determined. The rate of reaction decreases with the increase in the concentration of phosphate, thus indicating that LiNi(III)(HPO4)(H2O)+ and LiNi(III)OH2+ are stronger oxidizing agents than LiNi(III)(HPO4)2-. The oxidation of ascorbate by Ni(III)L2aq proceeds via three consecutive reactions which are well separated in time. Thus the results clearly point out that this process occurs via the inner sphere mechanism. The first transient observed is tentatively identified as L2(H2O)Ni(II)(A.-)2+, i.e., an unexpected complex of the ascorbate anion radical. Also in this process the last transient observed is the A.- anion radical. The stabilization of the ascorbyl radical in a transient complex might be of biological significance.",

keywords = "Ascorbate, Ascorbyl radial, Mechanism, Nickel(III), Oxidation",

author = "Israel Zilbermann and Avraham Meshulam and Haim Cohen and Dan Meyerstein",

note = "Funding Information: We express our sincere gratitude for Professor Dr. Joachim Sieler (Leipzig University, Germany) for his active interest in our work. The present study was in part supported by a visiting Stipendium from the S{\"a}chsische Staatsministerium f{\"u}r Wissenschaft und Kunst (KVD), which is gratefully acknowledged.",

year = "2001",

doi = "10.1080/10610270108027487",

language = "אנגלית",

volume = "13",

pages = "325--332",

journal = "Supramolecular Chemistry",

issn = "1061-0278",

publisher = "Taylor and Francis Ltd.",

number = "2",

}

TY - JOUR

T1 - Oxidation of Ascorbate by Ni(III) Complexes with Tetraaza-macrocyclic Ligands in Neutral Aqueous Solutions. A Pulse-Radiolysis Study

AU - Zilbermann, Israel

AU - Meshulam, Avraham

AU - Cohen, Haim

AU - Meyerstein, Dan

N1 - Funding Information: We express our sincere gratitude for Professor Dr. Joachim Sieler (Leipzig University, Germany) for his active interest in our work. The present study was in part supported by a visiting Stipendium from the Sächsische Staatsministerium für Wissenschaft und Kunst (KVD), which is gratefully acknowledged.

PY - 2001

Y1 - 2001

N2 - The mechanisms and kinetics of oxidation of ascorbate, AH-, by Ni(III)Liaq and by LiNi(III) (HPO4)2- complexes (L1 = meso-(5,12)-7,7,14,14-hex-amethyl-1,4,8,11-tetraazacyclotetradecane; L2 = 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane) in neutral aqueous solutions have been investigated. The oxidation of ascorbate by the LiNi(III) (HPO4)2- and Ni(III)L1aq proceeds via two consecutive reactions well separated in time. The products of the first reaction are the A.- radical anion and the corresponding Ni(II) complex. The oxidations by the LiNi(III)(HPO4)2- complexes proceed via the outer sphere mechanism, whereas the detailed mechanism of reaction of Ni(III)L1aq cannot be determined. The rate of reaction decreases with the increase in the concentration of phosphate, thus indicating that LiNi(III)(HPO4)(H2O)+ and LiNi(III)OH2+ are stronger oxidizing agents than LiNi(III)(HPO4)2-. The oxidation of ascorbate by Ni(III)L2aq proceeds via three consecutive reactions which are well separated in time. Thus the results clearly point out that this process occurs via the inner sphere mechanism. The first transient observed is tentatively identified as L2(H2O)Ni(II)(A.-)2+, i.e., an unexpected complex of the ascorbate anion radical. Also in this process the last transient observed is the A.- anion radical. The stabilization of the ascorbyl radical in a transient complex might be of biological significance.

AB - The mechanisms and kinetics of oxidation of ascorbate, AH-, by Ni(III)Liaq and by LiNi(III) (HPO4)2- complexes (L1 = meso-(5,12)-7,7,14,14-hex-amethyl-1,4,8,11-tetraazacyclotetradecane; L2 = 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane) in neutral aqueous solutions have been investigated. The oxidation of ascorbate by the LiNi(III) (HPO4)2- and Ni(III)L1aq proceeds via two consecutive reactions well separated in time. The products of the first reaction are the A.- radical anion and the corresponding Ni(II) complex. The oxidations by the LiNi(III)(HPO4)2- complexes proceed via the outer sphere mechanism, whereas the detailed mechanism of reaction of Ni(III)L1aq cannot be determined. The rate of reaction decreases with the increase in the concentration of phosphate, thus indicating that LiNi(III)(HPO4)(H2O)+ and LiNi(III)OH2+ are stronger oxidizing agents than LiNi(III)(HPO4)2-. The oxidation of ascorbate by Ni(III)L2aq proceeds via three consecutive reactions which are well separated in time. Thus the results clearly point out that this process occurs via the inner sphere mechanism. The first transient observed is tentatively identified as L2(H2O)Ni(II)(A.-)2+, i.e., an unexpected complex of the ascorbate anion radical. Also in this process the last transient observed is the A.- anion radical. The stabilization of the ascorbyl radical in a transient complex might be of biological significance.

KW - Ascorbate

KW - Ascorbyl radial

KW - Mechanism

KW - Nickel(III)

KW - Oxidation

UR - http://www.scopus.com/inward/record.url?scp=0035567174&partnerID=8YFLogxK

U2 - 10.1080/10610270108027487

DO - 10.1080/10610270108027487

M3 - ???researchoutput.researchoutputtypes.contributiontojournal.article???

AN - SCOPUS:0035567174

SN - 1061-0278

VL - 13

SP - 325

EP - 332

JO - Supramolecular Chemistry

JF - Supramolecular Chemistry

IS - 2

ER -

Oxidation of Ascorbate by Ni(III) Complexes with Tetraaza-macrocyclic Ligands in Neutral Aqueous Solutions. A Pulse-Radiolysis Study

Abstract

Bibliographical note

Keywords

Access to Document

Other files and links

Fingerprint

Cite this