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.
Highlights
Given the imminent consumption of fossil fuels, alternative energy sources such as the hydrogen economy should receive the baton
Back in the 80s, Kubas et al reported that transition metals can form σ -complexes binding dihydrogen molecules in a η2 fashion
Geometries have been fully optimized at the Density Functional Theory (DFT) level in gas phase using the M06 density functional [63] [64] as implemented in Gaussian 09 [65]
Summary
Given the imminent consumption of fossil fuels, alternative energy sources such as the hydrogen economy should receive the baton. Morris has proposed an empirical, simple equation, based on ligand acidity constants, to estimate the pKaTHF or pKaDCM values of hydride and dihydrogen species [37]. Geometries have been fully optimized at the DFT level in gas phase using the M06 density functional [63] (with an ultrafine grid) [64] as implemented in Gaussian 09 [65] This functional correctly accounts for dispersion interactions and is recommended for transition metal chemistry [66,67] and water clusters [68]. Acid constants can be estimated by computing the standard Gibbs energy in solution of the corresponding deprotonation process. Detailed ࢞G values can be found in the Supporting Information
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
