Chemistry of Intermolecular Frustrated Lewis Pairs in Motion: Emerging Perspectives and Prospects

Abstract

This feature article describes the chemistry in motion of frustrated Lewis pairs (FLPs). With state‐of‐the‐art ab initio molecular dynamics (AIMD) simulations supplemented by minimum energy path (MEP) and potential energy surface (PES) calculations, we examine the binding of CO2 and the heterolytic cleavage of H2 by a Lewis base (LB), tBu3P, and a Lewis acid (LA), B(C6F5)3. We strive to uncover and understand mechanistic implications of the physical laws that govern the behavior of a LB and a LA when they react with a third species (e.g., CO2 or H2) at finite temperature. The approximation that we necessarily must make at present is to forgo the quantization of the movement of atoms in favor of the Born‐Oppenheimer molecular dynamics (BOMD), which unfold according to the classical (Newton’s) laws of motion. However, strict quantum chemical theory is used to compute all of the forces that govern the dynamics of the macromolecular FLP system. Using physical reasoning and innovative computer simulations, we show that multi‐scale motion is the predominant mechanistic aspect in reactions of the tBu3P/B(C6F5)3 FLP, as well as, conceivably, those of other similar intermolecular FLPs. Insight achieved thus far leads to a novel activity model for intermolecular FLPs and specific predictions, which could be useful for future experimental and theoretical studies of FLP and other chemistries.

A Prediction of Proton‐Catalyzed Hydrogenation of Ketones in Lewis Basic Solvent through Facile Splitting of Hydrogen Molecules

“…However, the field is still dominated by metal‐based chemistry. At present, nonmetal activation (splitting) of H 2 is limited to just a few families of chemical systems, namely, singlet carbenes, combinations of the phospine/borane species and their derivatives, which constitute a family of the so‐called “frustrated” Lewis pairs (FLP), and a stable diradical digermyne …”

“…The hydride-type attacko nc arbon is reminiscent of the splitting of H 2 by singlet carbenes except that, in this case, aL ewis base from the surrounding environment (solvent) is necessaryf or polarizationo f H 2 and acceptance of the protonr esulting from the heterolytic splitting of H 2 .Because of steadily growingi nteresti nh ydrogenation across chemicald isciplines and extensive industrial use of hydrogenation reactions, it is perhaps next to impossible to overstate the general importance of H 2 activation and its role in the hydrogenationo ft he unsaturated carbon-hetero atom and carboncarbon bonds. [1][2][3][4][5] However,t he field is stilld ominated by metal-based chemistry.A tp resent,n onmetal activation (splitting) of H 2 is limited to just af ew familieso fc hemicals ystems, namely,s inglet carbenes, [6] combinations of the phospine/ borane species and their derivatives, which constitute af amily of the so-called "frustrated" Lewisp airs (FLP), [7][8][9][10][11][12][13][14] and as table diradical digermyne. [15] One particularly significant development in the area of nonmetalc atalysis is as eries of reports of novel hydrogenations of carbonylc ompounds,t hat is, solutions of aL ewis acid in moderately Lewis basic solvent(dioxane, THF,etc.)…”

Carbonyl Activation by Borane Lewis Acid Complexation: Transition States of H2 Splitting at the Activated Carbonyl Carbon Atom in a Lewis Basic Solvent and the Proton‐Transfer Dynamics of the Boroalkoxide Intermediate

“…According to the literature, it is not quite certain what the rate‐limiting step is in the thus far computed reaction profiles involving Brønsted acid activation/catalysis . For a comparison of the TS calculations describing competing reaction pathways of the FLP activation of H 2 , significant uncertainties can be expected to arise from the non‐static (fluid) nature of 1) interactions of solvent molecules with the ionic intermediate A2 and 2) interactions of the solvated ketone with the ionic intermediate B2 . The latter is particularly important because intermediates alike B2 are regarded as plausible source of the activated proton and hydride for hydrogenation (reduction) of the solvated substrate.…”

Theory‐Based Extension of the Catalyst Scope in the Base‐Catalyzed Hydrogenation of Ketones: RCOOH‐Catalyzed Hydrogenation of Carbonyl Compounds with H2 Involving a Proton Shuttle

“…In the reported TSs of this reaction, H 2 is polarized by LA and LB into H (δ−) −H (δ+) state with the hydride‐like H (δ−) and the proton‐like H (δ+) atoms . Typically in the reaction given in Equation (1), LAs are boranes with electron‐withdrawing substituents, that is, B(C 6 F 5 ) 3− n (C 6 Cl 5 ) n with n =0–3, in combination with phosphine LB species, as imines/amines, or carbonyl compounds and ethers with nitrogen or oxygen atoms as sufficiently potent LB centers .…”