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Linear Free Energy Relationships for Complex Formation Reactions between Carboxylic Acids and Palladium(II). Equilibrium and High-Pressure Kinetics Study

Shi, Tiesheng LU and Elding, Lars Ivar LU (1997) In Inorganic Chemistry 36(4). p.528-536
Abstract (Swedish)

Synopsis
Linear free energy relationships based on equilibrium and kinetics measurements, including variable-temperature and high-pressure data, for complex formation reactions and aquations of square-planar palladium(II) complexes are derived and discussed. In particular, data for an extensive series of carboxylic acids of widely different basicities and steric properties, acting as nucleophiles toward Pd(II) have been determined.

Abstract
Abstract Image
Kinetics for complex formation between Pd(H2O)42+ and formic, butyric, dl-lactic, 2-methyllactic, methoxyacetic, malonic, succinic, oxydiacetic, l-malic, and citric acids has been studied in an aqueous acidic medium by use of variable-temperature and -pressure... (More)

Synopsis
Linear free energy relationships based on equilibrium and kinetics measurements, including variable-temperature and high-pressure data, for complex formation reactions and aquations of square-planar palladium(II) complexes are derived and discussed. In particular, data for an extensive series of carboxylic acids of widely different basicities and steric properties, acting as nucleophiles toward Pd(II) have been determined.

Abstract
Abstract Image
Kinetics for complex formation between Pd(H2O)42+ and formic, butyric, dl-lactic, 2-methyllactic, methoxyacetic, malonic, succinic, oxydiacetic, l-malic, and citric acids has been studied in an aqueous acidic medium by use of variable-temperature and -pressure stopped-flow spectrophotometry. Kinetics traces for reactions between the metal ion and formic, butyric, lactic, 2-methyllactic, methoxyacetic, oxydiacetic, and citric acids can be described by single exponentials, which are assigned to the formation of monodentate complexes:  Pd(H2O)42+ + RCOOH ⇌ Pd(H2O)3OOCR+ + H3O+ (k1, k-1). Equilibrium constants K1 for lactic, 2-methyllactic, methoxyacetic, oxydiacetic, and citric acid reactions calculated from spectrophotometric equilibrium measurements and from kinetics (K1 = k1/k-1) are in good agreement. There is a linear correlation between the stability constants β1 of the carboxylato complexes and the first dissociation constants Ka1 of the carboxylic acids as expressed by log β1 = (0.48 ± 0.03)pKa1 + (2.1 ± 0.1). The formation rate constants k1 are insensitive to the basicity and steric properties of the carboxylic acids at 25 °C, due to an excellent isokinetic relationship between ΔH1⧧ and ΔS1⧧ with an isokinetic temperature of 292 K, suggesting also that all of the carboxylic acids react via the same mechanism. Rate constants k-1 are correlated with pKa1 of the entering carboxylic acids according to log k-1 = (0.47 ± 0.06)pKa1 − (0.7 ± 0.2), indicating that the weaker the carboxylic acid, the larger k-1. These facts together with the observation that a weak carboxylic acid is prone to form a strong complex as shown by the correlation between log β1 and pKa1 are interpreted in terms of a proton-assisted reaction mechanism which is further supported by the ionic strength dependence of the rate constant k-1, consistent with a direct attack of an oxonium ion on Pd(H2O)3OOCR+ for the reverse reaction. High-pressure stopped-flow measurements at 25 °C give activation volumes ΔV1⧧ = −4.9 ± 0.2 cm3 mol-1, ΔV-1⧧ = −2.7 ± 0.5 cm3 mol-1 for malonic acid, and ΔV1⧧ = −3.5 ± 0.2 cm3 mol-1, ΔV-1⧧ = −1.9 ± 0.4 cm3 mol-1 for citric acid, respectively. Existing data so far for Pd(H2O)42+ complex formation reactions obey a linear correlation between ΔV1⧧ and partial molar reaction volumes ΔV1° according to ΔV1⧧ = (0.92 ± 0.04)ΔV1° − (2.2 ± 0.2) cm3 mol-1, the slope of 0.92 indicating that bond-making between palladium and the entering ligands largely dominates the formation of the transition state. There are no linear correlations between ΔV1⧧ and partial molar volumes VL of the entering ligands, as has been claimed in previous literature for related reactions, or between ΔV1⧧ and ΔS1⧧. (Less)
Abstract
Synopsis
Linear free energy relationships based on equilibrium and kinetics measurements, including variable-temperature and high-pressure data, for complex formation reactions and aquations of square-planar palladium(II) complexes are derived and discussed. In particular, data for an extensive series of carboxylic acids of widely different basicities and steric properties, acting as nucleophiles toward Pd(II) have been determined.
Abstract
Abstract Image
Kinetics for complex formation between Pd(H2O)42+ and formic, butyric, dl-lactic, 2-methyllactic, methoxyacetic, malonic, succinic, oxydiacetic, l-malic, and citric acids has been studied in an aqueous acidic medium by use of variable-temperature and -pressure stopped-flow... (More)
Synopsis
Linear free energy relationships based on equilibrium and kinetics measurements, including variable-temperature and high-pressure data, for complex formation reactions and aquations of square-planar palladium(II) complexes are derived and discussed. In particular, data for an extensive series of carboxylic acids of widely different basicities and steric properties, acting as nucleophiles toward Pd(II) have been determined.
Abstract
Abstract Image
Kinetics for complex formation between Pd(H2O)42+ and formic, butyric, dl-lactic, 2-methyllactic, methoxyacetic, malonic, succinic, oxydiacetic, l-malic, and citric acids has been studied in an aqueous acidic medium by use of variable-temperature and -pressure stopped-flow spectrophotometry. Kinetics traces for reactions between the metal ion and formic, butyric, lactic, 2-methyllactic, methoxyacetic, oxydiacetic, and citric acids can be described by single exponentials, which are assigned to the formation of monodentate complexes:  Pd(H2O)42+ + RCOOH ⇌ Pd(H2O)3OOCR+ + H3O+ (k1, k-1). Equilibrium constants K1 for lactic, 2-methyllactic, methoxyacetic, oxydiacetic, and citric acid reactions calculated from spectrophotometric equilibrium measurements and from kinetics (K1 = k1/k-1) are in good agreement. There is a linear correlation between the stability constants β1 of the carboxylato complexes and the first dissociation constants Ka1 of the carboxylic acids as expressed by log β1 = (0.48 ± 0.03)pKa1 + (2.1 ± 0.1). The formation rate constants k1 are insensitive to the basicity and steric properties of the carboxylic acids at 25 °C, due to an excellent isokinetic relationship between ΔH1⧧ and ΔS1⧧ with an isokinetic temperature of 292 K, suggesting also that all of the carboxylic acids react via the same mechanism. Rate constants k-1 are correlated with pKa1 of the entering carboxylic acids according to log k-1 = (0.47 ± 0.06)pKa1 − (0.7 ± 0.2), indicating that the weaker the carboxylic acid, the larger k-1. These facts together with the observation that a weak carboxylic acid is prone to form a strong complex as shown by the correlation between log β1 and pKa1 are interpreted in terms of a proton-assisted reaction mechanism which is further supported by the ionic strength dependence of the rate constant k-1, consistent with a direct attack of an oxonium ion on Pd(H2O)3OOCR+ for the reverse reaction. High-pressure stopped-flow measurements at 25 °C give activation volumes ΔV1⧧ = −4.9 ± 0.2 cm3 mol-1, ΔV-1⧧ = −2.7 ± 0.5 cm3 mol-1 for malonic acid, and ΔV1⧧ = −3.5 ± 0.2 cm3 mol-1, ΔV-1⧧ = −1.9 ± 0.4 cm3 mol-1 for citric acid, respectively. Existing data so far for Pd(H2O)42+ complex formation reactions obey a linear correlation between ΔV1⧧ and partial molar reaction volumes ΔV1° according to ΔV1⧧ = (0.92 ± 0.04)ΔV1° − (2.2 ± 0.2) cm3 mol-1, the slope of 0.92 indicating that bond-making between palladium and the entering ligands largely dominates the formation of the transition state. There are no linear correlations between ΔV1⧧ and partial molar volumes VL of the entering ligands, as has been claimed in previous literature for related reactions, or between ΔV1⧧ and ΔS1⧧. (Less)
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organization
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Contribution to journal
publication status
published
subject
keywords
Palladium(II), Carboxylic acids, Complex formation, linear free energy relationship, LFER, High-pressure kinetics, Fast reactions , Stopped flow, Stability constants, Isokinetic relation, Activation parameters, Palladium(II), Carboxylic acids, Complex formation, Linear Free Energy Relations, LFER, Fast kinetics, High-pressure kinetics, Reaction Mechanism, Activation Parameters
in
Inorganic Chemistry
volume
36
issue
4
pages
9 pages
publisher
The American Chemical Society
external identifiers
  • scopus:0001002411
ISSN
1520-510X
DOI
10.1021/ic9613523
language
English
LU publication?
yes
id
b8fa23e6-1545-4969-9819-4a6e2734dda3
date added to LUP
2017-01-04 12:24:48
date last changed
2017-05-03 18:10:38
@article{b8fa23e6-1545-4969-9819-4a6e2734dda3,
  abstract     = {Synopsis<br/>Linear free energy relationships based on equilibrium and kinetics measurements, including variable-temperature and high-pressure data, for complex formation reactions and aquations of square-planar palladium(II) complexes are derived and discussed. In particular, data for an extensive series of carboxylic acids of widely different basicities and steric properties, acting as nucleophiles toward Pd(II) have been determined.<br/>Abstract<br/>Abstract Image<br/>Kinetics for complex formation between Pd(H2O)42+ and formic, butyric, dl-lactic, 2-methyllactic, methoxyacetic, malonic, succinic, oxydiacetic, l-malic, and citric acids has been studied in an aqueous acidic medium by use of variable-temperature and -pressure stopped-flow spectrophotometry. Kinetics traces for reactions between the metal ion and formic, butyric, lactic, 2-methyllactic, methoxyacetic, oxydiacetic, and citric acids can be described by single exponentials, which are assigned to the formation of monodentate complexes:  Pd(H2O)42+ + RCOOH ⇌ Pd(H2O)3OOCR+ + H3O+ (k1, k-1). Equilibrium constants K1 for lactic, 2-methyllactic, methoxyacetic, oxydiacetic, and citric acid reactions calculated from spectrophotometric equilibrium measurements and from kinetics (K1 = k1/k-1) are in good agreement. There is a linear correlation between the stability constants β1 of the carboxylato complexes and the first dissociation constants Ka1 of the carboxylic acids as expressed by log β1 = (0.48 ± 0.03)pKa1 + (2.1 ± 0.1). The formation rate constants k1 are insensitive to the basicity and steric properties of the carboxylic acids at 25 °C, due to an excellent isokinetic relationship between ΔH1⧧ and ΔS1⧧ with an isokinetic temperature of 292 K, suggesting also that all of the carboxylic acids react via the same mechanism. Rate constants k-1 are correlated with pKa1 of the entering carboxylic acids according to log k-1 = (0.47 ± 0.06)pKa1 − (0.7 ± 0.2), indicating that the weaker the carboxylic acid, the larger k-1. These facts together with the observation that a weak carboxylic acid is prone to form a strong complex as shown by the correlation between log β1 and pKa1 are interpreted in terms of a proton-assisted reaction mechanism which is further supported by the ionic strength dependence of the rate constant k-1, consistent with a direct attack of an oxonium ion on Pd(H2O)3OOCR+ for the reverse reaction. High-pressure stopped-flow measurements at 25 °C give activation volumes ΔV1⧧ = −4.9 ± 0.2 cm3 mol-1, ΔV-1⧧ = −2.7 ± 0.5 cm3 mol-1 for malonic acid, and ΔV1⧧ = −3.5 ± 0.2 cm3 mol-1, ΔV-1⧧ = −1.9 ± 0.4 cm3 mol-1 for citric acid, respectively. Existing data so far for Pd(H2O)42+ complex formation reactions obey a linear correlation between ΔV1⧧ and partial molar reaction volumes ΔV1° according to ΔV1⧧ = (0.92 ± 0.04)ΔV1° − (2.2 ± 0.2) cm3 mol-1, the slope of 0.92 indicating that bond-making between palladium and the entering ligands largely dominates the formation of the transition state. There are no linear correlations between ΔV1⧧ and partial molar volumes VL of the entering ligands, as has been claimed in previous literature for related reactions, or between ΔV1⧧ and ΔS1⧧.},
  author       = {Shi, Tiesheng and Elding, Lars Ivar},
  issn         = {1520-510X},
  keyword      = {Palladium(II),Carboxylic acids,Complex formation,linear free energy relationship,LFER,High-pressure kinetics,Fast reactions ,Stopped flow,Stability constants,Isokinetic relation,Activation parameters,Palladium(II),Carboxylic acids,Complex formation,Linear Free Energy Relations,LFER,Fast kinetics,High-pressure kinetics,Reaction Mechanism,Activation Parameters},
  language     = {eng},
  month        = {02},
  number       = {4},
  pages        = {528--536},
  publisher    = {The American Chemical Society},
  series       = {Inorganic Chemistry},
  title        = {Linear Free Energy Relationships for Complex Formation Reactions between Carboxylic Acids and Palladium(II). Equilibrium and High-Pressure Kinetics Study},
  url          = {http://dx.doi.org/10.1021/ic9613523},
  volume       = {36},
  year         = {1997},
}