The Association of Two "Frustrated" Lewis Pairs by State-of-the-Art Quantum Chemical Methods

State-of-the-art quantum chemical methods have been applied to describe the association of two frustrated Lewis pairs (FLPs), B(C6F5)(3)/PR3 (1: R= 2,4,6-Me3C6H2; 2: R=CMe3), with different steric demands of the base component. Interaction energies are calculated at the dispersion-corrected DFT, MP2 (second-order Moller-Plesset), and DLPNO-CCSD(T) (domain-based local pair natural orbital-coupled cluster, including single, double and perturbative triple excitations) levels of theory, combined with extended triple-or quadruple-zeta AO (atom-centered orbital) basis sets. Thermostatistical contributions to the free binding energy are calculated from harmonic frequencies at the efficient HF-3c (minimal basis Hartree-Fock with three corrections) level, while solvation effects in benzene are accounted for by the COSMO-RS (conductor-like screening model for realistic solvents) continuum model. Comparison with the recently measured experimental value for the free association energy of the FLP 1 reveals agreement between theory and experiment within the estimated error bars. The computed gas phase interaction energies for both FLPs are similar (about -13 kcal mol(-1)), with only small variations (about -3 kcal mol(-1)) for various quantum chemical methods, when London dispersion interactions are accounted for properly. The association of the more "frustrated" FLP 1 is mainly driven by nondirectional dispersion forces, resulting in non-preferential orientations, which is in agreement with experimental results. On the other hand, in FLP 2 with the "smaller" base, the boron and phosphorous atoms face each other in the favored complex structure, indicating a weak P-B donor-acceptor interaction. This conformation of 2 seems to be more suitable for small molecule (e.g., H-2) activations.