Quantum chemistry of FLPs and their activation of small molecules: methodological aspects

We discuss methodological aspects of the computation of structures and energies of three common FLPs which are able to activate dihydrogen under ambient conditions. The effect of London dispersion corrections by the DFT-D3 scheme and solvent as well as rovibrational corrections to yield free reaction enthalpies in solutions are described. Common density functionals of semi-local, hybrid, and double-hybrid type as well as (SCS-)MP2 wave function based methods with very large AO basis sets are investigated. It is found that reliable structures (in comparison to X-ray data) are already obtained using relatively cheap DFT methods like TPSS-D3/TZ. The variations between different density functionals for electronic reaction energies are small to moderate (1-2 kcal/mol which is about 10% of the H₂ was addition energy). Dispersion corrections are found to be essential for accurate thermochemistry. Computed free H₂ reaction enthalpies in the gas phase are close to zero while values computed in common solvents with the COSMO-RS continuum solvation model are strongly exergonic (about -10 kcal/mol in CH₂Cl₂). This new finding emphasizes the important role of the solvent for FLP chemistry involving zwitterionic species. According to our results the future for reliable quantum chemistry of FLP processes is bright.