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

Abstract 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 pKawater 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 pKawater of twelve dihydrogen complexes experimentally characterized in water have been computed. pKawater values span a wide range, from 23 to -4, illustrating how coordination to a transition metal modifies the dihydrogen acidity. Decomposition of the ∆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.