Effects of Ionic Liquids on the Thermodynamics of Hydrogen Activation by Frustrated Lewis Pairs: A Density Functional Theory Study**

Abstract

Nowadays, hydrogen activation by frustrated Lewis pairs (FLPs) and their applications are one of the emerging research topics in the field of catalysis. Previous studies have shown that the thermodynamics of this reaction is determined by electronic structures of FLPs and solvents. Herein, we investigated systems consisting of typical FLPs and ionic liquids (ILs), which are well known by their large number of types and excellent solvent effects. The density functional theory (DFT) calculations were performed to study the thermodynamics for H2 activation by both inter‐ and intra‐molecular FLPs, as well as the individual components. The results show that the computed overall Gibbs free energies in ILs are more negative than that computed in toluene. Through the thermodynamics partitioning, we find that ILs favor the H−H cleavage elemental step over the elemental steps of proton attachment, hydride attachment and zwitterionic stabilization. Moreover, the results show that these effects are strongly dependent on the type of FLPs, where intra‐molecular FLPs are more affected compared to the inter‐molecular FLPs.

Hydrogen Activation by Frustrated and Not So Frustrated Lewis Pairs Based on Pyramidal Lewis Acid 9-Boratriptycene: A Computational Study

“…Thus, the activation energies for the reaction in the gas phase satisfactory reproduce values in the solvents. Significant additional energetic stabilization of the final ionic pair [HP t Bu 3 ] + [HB­(C 6 H 4 ) 3 CH] − in solution is similar to the values for other hydrogen splitting products reported in the literature. , …”

“…Significant additional energetic stabilization of the final ionic pair [HP t Bu 3 ] + [HB(C 6 H 4 ) 3 CH] − in solution is similar to the values for other hydrogen splitting products reported in the literature. 57,58 Comparison of the Studied Systems. Table 6 summarizes the most important characteristics of the studied systems with respect to potential catalytic activity: the activation barriers and the reaction energies of heterolytic hydrogen splitting.…”

“…A highly polar ion pair is stabilized in solution depending on the polarity of the solvent. The solvent contributions to the Gibbs energy of the reaction of H 2 activation by different intermolecular boron-phosphorus FLPs are from −4 to −32 kJ·mol –1 in toluene and ca. −42 kJ·mol –1 in dichloromethane …”

“…It should be taken into account that the reaction of H 2 activation by FLP is strongly affected by the environment. 34 Solvent influence for FLP chemistry was theoretically investigated by explicit 56 as well as implicit 57 , 58 solvation methods. A highly polar ion pair is stabilized in solution depending on the polarity of the solvent.…”

“…Significant additional energetic stabilization of the final ionic pair [HP t Bu 3 ] + [HB(C 6 H 4 ) 3 CH] − in solution is similar to the values for other hydrogen splitting products reported in the literature. 57 , 58 …”