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

Abstract

SynopsisLinear 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.AbstractAbstract ImageKinetics 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)