Dispersion-Oriented Soft Interaction in a Frustrated Lewis Pair and the Entropic Encouragement Effect in its Formation

The origin of the stability of a frustrated Lewis pair (FLP) tBu(3)P:B-(C6F5)(3) is investigated computationally to demonstrate the importance of the dispersion interaction. To this end, the interaction between alkyl-substituted phosphines (Me3P and tBu(3)P) and hexafluorobenzene (C6F6) is first investigated. Driven by the lone-pair to pi-orbital interaction, the binding energy is found to be even larger than usual pi-pi interaction energies between small aromatic compounds. This character, which is inherited to fluorophenyl-substituted B(C6F5)(3) in the FLP, induces large flexibility in the FLP over the molecular surface of B(C6F5)(3). This soft interaction, in turn, causes an entropic stabilization of the FLP formation in comparison with classical Lewis pairs based on close and tight P-B dative bonds. It also suggests a diverse nature of the FLP when it is involved in chemical reactions. Even with the cooperative participation of the perfluorophenyl groups, a detailed inspection of the FLP interaction potential energy surface indicates that the boron atom is still the major interaction site for the pair formation. This non-negligible direct P-B interaction, which is related also to the soft nature of the borane frontier orbital, is further supported by substantial spatial overlap between the frontier orbitals on the phosphine/borane fragments and their interaction energy estimations.