Experimental Insights into the Structure and Reactivity of Frustrated Lewis Pairs

Abstract

Frustrated Lewis pairs (FLPs), namely the combination of a Lewis acid (LA) and a Lewis base (LB) that cannot fulfil their natural propensity to form stable Lewis adducts, have been recently exploited to activate small molecules, such as H2 and CO2, and have drawn much interest, due to their applicability in metal‐free catalytic reactions. This review focuses mainly on the experimental studies of both the structure and reactivity of FLPs providing a starting body of evidence to elucidate the mechanism of their action. Important insights coming from NMR spectroscopy, X‐ray diffraction and other experimental techniques are highlighted and discussed. In particular, evidence about the tendency of LA and LB to interact in the absence of a substrate are commented on for both inter‐ and intramolecular FLPs, underlining the role played by the structure of LA and LB in determining the level of interaction. Next, the effect of the structure of FLPs on the kinetics and thermodynamics of substrate activation is illustrated, with a particular focus on catalytic hydrogenation reactions.

Disclosing the multi-faceted world of weakly interacting inorganic systems by means of NMR spectroscopy

“…25 Experimental validation of such a mechanistic scheme is hampered by difficulty in obtaining clear indications about the formation and structure of LA⋯LB in solution. 26 Deep insights into the level of interaction between frustrated phosphines and boranes have been obtained recently by exploiting NOE and diffusion NMR techniques. 27 Fig.…”

Calorimetric Study of the Activation of Hydrogen by Tris(pentafluorophenyl)borane and Trimesitylphosphine

“…Lewis acid–base pairs that are sterically or otherwise prevented from forming a normal dative adduct, “frustrated Lewis pairs” (FLPs), − have received tremendous attention in the past decade due to their notable ability to activate small molecules. These unexpected findings have found applications in catalysis, − organic synthesis, − and CO 2 capture − and have introduced a new paradigm for conceptualizing chemical reactivity.…”

Convenient and accurate insight into solution-phase equilibria from FlowNMR titrations

“…To confirm the formation of a monomeric 1 : 1 adduct we applied diffusion ordered spectroscopy (DOSY) to quantify the hydrodynamic radius of the product formed. 79 Capitalising on the abundant signal intensity and large chemical shift dispersion, 19 F DOSY spectra were acquired periodically throughout the titration (Fig. S35 †).…”

“…The degree of this frustration has been found to lead to interesting reactivities and subsequently widespread use in small molecule activation and catalysis. 4,60,[79][80][81][82][83][84][85] An important metric for the reactivities and understanding of association in FLPs is their respective binding constant (hence their degree of frustration) in solution. 61,86 Sterically hindered pyridines in particular and their association with BCF have been studied to determine their binding strength.…”