Enhancement of London Dispersion in Frustrated Lewis Pairs: Towards a Crystalline Encounter Complex

Abstract

The encounter complex, i.e. the pre-organized assembly consisting of a Lewis acid and a Lewis base, is a fundamental concept in frustrated Lewis pair (FLP) chemistry. However, this donor-acceptor complex is challenging to study due to its transient nature. Here, we present a combined theoretical and experimental investigation on the potential isolation of an encounter complex enabled by enhancement of London dispersion forces between a sterically encumbered Lewis acid and base pair. Guided by computational analyses, the FLP originating from the bulky triarylamine N(3,5-tBu2C6H3)3 and the novel triarylborane B(3,5-tBu2C6H3)3 was investigated, leading to the isolation of a 1:1 co-crystal of both FLP components.

Dispersion Energy‐Stabilized Boron and Phosphorus Lewis Pairs

“…The standard approach for modeling the association of FLPs involves starting from gas phase geometries at 0 K and comparing the corresponding association energies or individual contributions thereof, such as the dispersion energy. [8] Computational studies, especially ones that include free energies of association, are rare for FLPs, [6a,14] and thermostatistical corrections and solvation contributions are usually calculated using a protocol that starts from the gas phase geometries at 0 K. This was also our starting point, but, whereas geometries were described at the PBEh-3c [15] level after a CREST [16] conformer search with GFN2-xTB [17] that provided BÀ P bond lengths very similar to those in the crystal structure (see Supporting Information Table S37), but the calculated ΔG°2 98 with these geometries were clearly too exergonic compared to the measured values (Table S32). Furthermore, the experimentally observed trend of stronger association with increasing ligand size, due to enhanced dispersion interactions, was not reproduced either.…”

“…Only recently, Slootweg et al introduced tBu-groups into intermolecular FLPs based on triarylboranes and triarylamines in order to achieve efficient electron transfer but thermodynamic parameters were not studied in detail by experimental nor computational methods. [8] However, only joined computational and experimental studies can provide the necessary interface to improve both the quantum mechanical methods and the desired molecular properties. In this light, high-level quantum chemistry in combination with reliable experimental data is of utmost importance for advancing the theoretical and molecular understanding.…”

Infrared Spectroscopy Evidence of Weak Interactions in Frustrated Lewis Pairs Formed by Tris(pentafluorophenyl)borane

“…Moreover, F. Holtrop et al . recently investigated the formation of two amine‐borane FLPs encounter complexes, where they showed the potential for isolating the encounter complex in solid state by increasing the London dispersion forces between the Lewis acid and the Lewis base [20] …”

“…[19] Moreover, F. Holtrop et al recently investigated the formation of two amine-borane FLPs encounter complexes, where they showed the potential for isolating the encounter complex in solid state by increasing the London dispersion forces between the Lewis acid and the Lewis base. [20] Aiming at the encounter complexes characterization, molecular spectroscopies, especially, Raman spectroscopy can be used to probe molecular vibrations, what would bring relevant parameters concerning structural modifications in FLPs. However, as we showed in a recent paper, conventional Raman spectroscopy does not give any relevant spectral information about the FLP formation, while by using resonance Raman spectroscopy, we found evidence of a charge transfer complex formed due the Mes 3 P/B(C 6 F 5 ) 3 encounter complex.…”

Molecular Wind-Up Meter for the Quantification of London Dispersion Interactions

“…Examples of conformational balances are plentiful, such as Trögers-base derived systems for alkyl–alkyl and alkyl–aryl interactions, bicyclic N -arylimide for aryl–aryl interactions, − dibenzobicyclononane (BCN), , imidazoline-2-thione, cyclooctatetraene, and diarylthioureas . Dissociative systems include H-bound heterocyclic compounds, , mono- or diatomic complexes, − and heterodissociative systems based on [10]-cycloparaphenylene and C 60 , which balance stabilizing and repulsive interactions. , Comparing different balances is challenging, though, due to the distance dependence of the underlying Lennard-Jones potentials as well as entropic and solvent effects.…”