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Kinetics and mechanism for reduction of halo- and haloam(m)ine platinum(IV) complexes by L-ascorbate

Lemma, Kelemu LU ; House, Donald A.; Retta, Negussie and Elding, Lars Ivar LU (2002) In Inorganica Chimica Acta 331. p.98-108
Abstract
Reduction of the model platinum(IV) complexes cis-[PtCl4(NH3)(2)] (1), trans-[PtCl4(NH3)(2)] (2). trans-[PtCl2(en)(2)](2+) (3), trans-[PtBr2(NH3)(4)](2+) (4), [PtCl6](2-) (5), and [PtBr6](2-) (6) with L-ascorbic acid (H(2)Asc) in 1.0 M aqueous medium at 25 degreesC in the region 1.75 less than or equal to pH less than or equal to 7.20 has been investigated using stopped-flow spectrophotometry. The redox reactions follow the rate law: -d[Pt(IV]/dt = k[H(2)Asc](tot)[Pt(IV)] where k is a pH-dependent second-order rate constant and [H(2)Asc](tot), the total concentration of ascorbic acid. The pH-dependence of k is attributed to parallel reduction of Pt(IV) by the protolytic species HAsc(-) and Asc(2-). Analysis of the kinetics data reveals... (More)
Reduction of the model platinum(IV) complexes cis-[PtCl4(NH3)(2)] (1), trans-[PtCl4(NH3)(2)] (2). trans-[PtCl2(en)(2)](2+) (3), trans-[PtBr2(NH3)(4)](2+) (4), [PtCl6](2-) (5), and [PtBr6](2-) (6) with L-ascorbic acid (H(2)Asc) in 1.0 M aqueous medium at 25 degreesC in the region 1.75 less than or equal to pH less than or equal to 7.20 has been investigated using stopped-flow spectrophotometry. The redox reactions follow the rate law: -d[Pt(IV]/dt = k[H(2)Asc](tot)[Pt(IV)] where k is a pH-dependent second-order rate constant and [H(2)Asc](tot), the total concentration of ascorbic acid. The pH-dependence of k is attributed to parallel reduction of Pt(IV) by the protolytic species HAsc(-) and Asc(2-). Analysis of the kinetics data reveals that the ascorbate anion Asc(2-) is up to seven orders of magnitude more reactive than HAsc(-) while H(2)Asc is unreactive. Electron transfer from HAsc(-)/Asc(2-) to the Pt(IV)) compounds is suggested to take place by a mechanism involving a reductive attack on any one of the mutually trans-halide ligands by Asc(2-) and/or HAsc(-) forming a halide-bridged activated complex, The rapid reduction of these complexes supports the assumption that ascorbate Asc(2-) might be an important reductant at physiological conditions for anticancer active Pt(IV) pro-drugs capable of undergoing reductive trans elimination. The parameters DeltaH(not equal) and DeltaS(not equal) for reduction of Pt(IV) with Asc(2) hake been determined from the study of the temperature dependence of k. (C) 2002 Elsevier Science B.V. All rights reserved. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
L-ascorbic acid, Reduction, Kinetics , Platinum(IV) complexes, Reaction mechanism
in
Inorganica Chimica Acta
volume
331
pages
98 - 108
publisher
Elsevier
external identifiers
  • wos:000175179700015
  • scopus:0037187532
ISSN
0020-1693
DOI
10.1016/S0020-1693(01)00762-9
language
English
LU publication?
yes
id
cab81e3b-b2a4-4b25-8532-a6ac9b28fc44 (old id 339593)
date added to LUP
2007-10-31 11:10:56
date last changed
2017-09-10 04:27:08
@article{cab81e3b-b2a4-4b25-8532-a6ac9b28fc44,
  abstract     = {Reduction of the model platinum(IV) complexes cis-[PtCl4(NH3)(2)] (1), trans-[PtCl4(NH3)(2)] (2). trans-[PtCl2(en)(2)](2+) (3), trans-[PtBr2(NH3)(4)](2+) (4), [PtCl6](2-) (5), and [PtBr6](2-) (6) with L-ascorbic acid (H(2)Asc) in 1.0 M aqueous medium at 25 degreesC in the region 1.75 less than or equal to pH less than or equal to 7.20 has been investigated using stopped-flow spectrophotometry. The redox reactions follow the rate law: -d[Pt(IV]/dt = k[H(2)Asc](tot)[Pt(IV)] where k is a pH-dependent second-order rate constant and [H(2)Asc](tot), the total concentration of ascorbic acid. The pH-dependence of k is attributed to parallel reduction of Pt(IV) by the protolytic species HAsc(-) and Asc(2-). Analysis of the kinetics data reveals that the ascorbate anion Asc(2-) is up to seven orders of magnitude more reactive than HAsc(-) while H(2)Asc is unreactive. Electron transfer from HAsc(-)/Asc(2-) to the Pt(IV)) compounds is suggested to take place by a mechanism involving a reductive attack on any one of the mutually trans-halide ligands by Asc(2-) and/or HAsc(-) forming a halide-bridged activated complex, The rapid reduction of these complexes supports the assumption that ascorbate Asc(2-) might be an important reductant at physiological conditions for anticancer active Pt(IV) pro-drugs capable of undergoing reductive trans elimination. The parameters DeltaH(not equal) and DeltaS(not equal) for reduction of Pt(IV) with Asc(2) hake been determined from the study of the temperature dependence of k. (C) 2002 Elsevier Science B.V. All rights reserved.},
  author       = {Lemma, Kelemu and House, Donald A. and Retta, Negussie and Elding, Lars Ivar},
  issn         = {0020-1693},
  keyword      = {L-ascorbic acid,Reduction,Kinetics ,Platinum(IV) complexes,Reaction mechanism},
  language     = {eng},
  month        = {03},
  pages        = {98--108},
  publisher    = {Elsevier},
  series       = {Inorganica Chimica Acta},
  title        = {Kinetics and mechanism for reduction of halo- and haloam(m)ine platinum(IV) complexes by L-ascorbate},
  url          = {http://dx.doi.org/10.1016/S0020-1693(01)00762-9},
  volume       = {331},
  year         = {2002},
}