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
PY - 2001/1/1
Y1 - 2001/1/1
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 - Article
AN - SCOPUS:0035567174
SN - 1061-0278
VL - 13
SP - 325
EP - 332
JO - Supramolecular Chemistry
JF - Supramolecular Chemistry
IS - 2
ER -