How acid can become a dihydrogen complex in water? A DFT study

To accurate know the acidity of a dihydrogen molecule coordinated to a transition metal ion in water medium is an issue of interest in many areas, from electrochemistry to enzymes and catalysis. However, experimental determination of this magnitude is challenging, and very few values have been reported. In this article we describe a computational protocol, based on DFT calculations and employing a discrete continuum solvent representation, to estimate pK(a)(water) of transition metal dihydrogen complexes. In this approach the number of solvent molecules explicitly included in the calculations is determined by the convergence with the solvation Gibbs energy of the proton in the solvent. The approach has been initially validated with experimental data in tetrahydrofuran (THF) solvent. Using (THF)(3) clusters a mean absolute deviation from experiments of only 1.4 pKa unit is achieved. In water the convergence is reached with (H2O)(10) clusters. Using them in a discrete-continuum model, the pK(a)(water) of twelve dihydrogen complexes experimentally characterized in water have been computed. pKa water values span a wide range, from 23 to -4, illustrating how coordination to a transition metal modifies the dihydrogen acidity. Decomposition of the Delta G of the acid-base equilibrium in two contributions, one intrinsic to the complex and another one accounting for solvent effects enables a deeper analysis of the dihydrogen acidities. (C) 2021 The Author(s). Published by Elsevier B.V.

Automated summary

This article presents a computational protocol to estimate the pK(a)(water) of transition metal dihydrogen complexes in water medium. Experimental validation and computational results show that coordination to a transition metal ion affects the acidity of dihydrogen. Decomposition of the Delta G of the acid-base equilibrium enables a deeper analysis of dihydrogen acidities.