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Kim HW, Rhee YM: Dispersion-oriented soft interaction in a frustrated Lewis pair and the entropic encouragement effect in its formation. Chemistry. 2009 Dec 14;15(48):13348-55.
The origin of the stability of a frustrated Lewis pair (FLP) tBu (3) P:B (C (6) F (5))(3) is investigated computationally to demonstrate the importance of the dispersion interaction. To this end, the interaction between alkyl-substituted phosphines (Me (3) P and tBu (3) P) and hexafluorobenzene (C (6) F (6)) 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 (C (6) F (5))(3) in the FLP, induces large flexibility in the FLP over the molecular surface of B (C (6) F (5))(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. |
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