On the thermodynamic stability of the intermolecular association between Lewis acids and Lewis bases: a DFT study

The intermolecular association of twelve combinations of six different Lewis acids and Lewis bases (i.e., R(3)A-BR3' where A = B and Al; B = N and P; R = H, F, and C6F5; R' = H, CH3, and C(CH3)(3)) was theoretically described by means of DFT calculations using the dispersion-corrected omega B97x-D and B97D functionals in conjunction with the 6-311++G(2d,2p) basis set including toluene as solvent through the PCM-SMD implicit solvent scheme. All the studied Lewis pairs appeared to be stable on the basis of computed BSSE-corrected interaction energies; however, the free energies of formation computed in solution (Delta G(solv)) indicate that three Lewis acid-base combinations can be considered frustrated Lewis pairs (FLPs). Besides, the four features that characterize FLPs are: (1) large distances between the acid and base centers, (2) negligible changes in the geometry of the acid, (3) weak interaction energies, and (4) non-covalent dispersion energy contributing to almost the entire interaction energy. In the present work, we introduce two ad hoc indexes intended to quantify separately the electronic and steric factors, which have a direct effect in the intermolecular association of Lewis acids and Lewis bases and can be used to distinguished FLPs from classical Lewis adducts. Based on the aforementioned ad hoc indexes, the existence of a new kind of complexes that are "intermediate" between classical complexes and FLPs is proposed.