Slow proton transfer to coordinated carboxylates: studies on [Ni(O2CR){PhP(CH2CH2PPh2)2}]+ (R = Et or Ph)

Abstract

The reactions between [Ni(O2CR)(triphos)]+ (R = Et or Ph, triphos = PhP(CH2CH2PPh2)2) and mixtures of lutH+ and lut (lut = 2,6-dimethylpyridine) have been studied in MeCN at 25.0 °C using stopped-flow spectrophotometry. The kinetics and spectroscopic changes indicate an equilibrium reaction, presumably involving protonation of an oxygen site (the only sites on the complex containing lone pairs of electrons). Proton transfer is slow and comparison of the kinetic data shows that the rates are insensitive to the R substituent. Using the kinetic data, the pKas of [Ni(HO2CR)(triphos)]2+ (pKa = 14.5) have been calculated showing that when coordinated to the {Ni(triphos)}2+ site, RCO2H is about 8 pKa units more acidic than the free acid. Comparison of the kinetic results on the reactions of [Ni(O2CR)(triphos)]+ with mixtures of lutH+ and lut and those of the analogous [Ni(S2CR)(triphos)]+ show that protonation at oxygen is at least 7.6 × 103 times faster than to sulfur, and the coordinated carboxylic acid is ca. 8 pKa units less acidic than the corresponding coordinated carboxydithioic acid.