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 pK_as of [Ni(HO₂CR)(triphos)]²⁺ (pK_a = 14.5) have been calculated showing that when coordinated to the {Ni(triphos)}²⁺ site, RCO₂H is about 8 pK_a 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 pK_a units less acidic than the corresponding coordinated carboxydithioic acid.

Graphical abstract

Keywords:

• Nickel
• Protonation
• Kinetics
• Mechanism
• Carboxylate

Acknowledgements

A.A. thanks the Iraqi Ministry of Higher Education and Scientific Research for a studentship.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

Dedicated to Professor Dr Rudi van Eldik on the occasion of his 70th birthday.

¹ Calculated pK_a (in water) using Advanced Chemistry Development (ACD/Labs) Software V11.02 (© 1994-2014 ACD/Labs), then correcting to the pK_a in MeCN using equations in reference 12.