Probing the Charge Transfer in a Frustrated Lewis Pair by Resonance Raman Spectroscopy and DFT Calculations

Abstract

A classical Lewis adduct derives from a covalent bond between a Lewis acid and a base. When the adduct formation is precluded by means of steric hindrance the association of the respective acid‐base molecular system is defined as a frustrated Lewis pair (FLP). In this work, the archetypal FLP Mes3P/B(C6F5)3 was characterized for the first time by resonance Raman spectroscopy, and the results were supported by density functional theory (DFT) calculations. The charge transfer nature of the lowest energy electronic transition, from phosphine to borane, was confirmed by the selective enhancement of the Raman bands associated to the FLP chromophore at resonance condition. Herein, we demonstrate the use of resonance Raman spectroscopy as a distinguished technique to probe the weak interaction involved in FLP chemistry.

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

“…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. Vibrational bands of modes associated with both acid and base were enhanced by the resonance Raman effect, confirming the charge transfer from the phosphine to borane [21] …”

“…Vibrational bands of modes associated with both acid and base were enhanced by the resonance Raman effect, confirming the charge transfer from the phosphine to borane. [21] Infrared spectroscopy (IR) provides information about vibrational modes which dynamics result in a dipole moment fluctuation. In this way, considering the Lewis acid and base structure, it is clearly notable that the B(C 6 F 5 ) 3 (named as BCF) Lewis acid has many polar CÀ F bonds attached to phenyl groups that can be probed by IR, allowing to access relevant information about the chemical environment on the Lewis acid.…”

Evidence for the encounter complex in frustrated Lewis pair chemistry

“…83,84 This chargetransfer process was confirmed by EPR spectroscopy and transient absorption spectroscopy, and has enabled the encounter complex to be directly studied using UV-Vis spectroscopy and resonance Raman spectroscopy. 88 The studies discussed in this article have provided evidence for the encounter complex in FLP chemistry. However, there are still questions to be answered to expand knowledge and application in this area of chemistry:…”

“…83,84 This charge-transfer process was confirmed by EPR spectroscopy and transient absorption spectroscopy, and has enabled the encounter complex to be directly studied using UV-Vis spectroscopy and resonance Raman spectroscopy. 88…”

“…The charge transfer in PMes 3 /B(C 6 F 5 ) 3 was further studied by resonance Raman spectroscopy by Ando and co-workers. 88 Resonance Raman spectroscopy can provide information on vibrational modes that are associated with a particular electronic transition. The authors showed that the Raman spectrum of the FLP was the same as the superposition of the spectra of the individual components, consistent with the previous spectroscopic evidence that there is very limited interaction between the acid and base in solution.…”