Less Frustration, More Activity—Theoretical Insights into Frustrated Lewis Pairs for Hydrogenation Catalysis

Abstract

The field of frustrated Lewis pair (FLP) chemistry has seen rapid development in only a few years. FLPs have performed most spectacularly in hydrogenation catalysis: a wide variety of FLP‐based systems can catalyze the hydrogenation of a range of different substrates, including imines, enamines, ketones, alkynes, and alkenes. However, FLP‐based hydrogenation catalysts are yet to match the efficiency of their transition‐metal counterparts. The current investigation reveals an important aspect of FLPs that can be exploited to improve their efficiency, that is, the more sterically hindered the FLP catalyst is, the lower is its turnover frequency. Full quantum chemical calculations with DFT for a family of different, experimentally known hydrogenation FLP catalysts shows that superior FLP catalysts can be designed by reducing the frustration (by reducing the steric demand and acid/base strength) in the FLP. However, as lowering the steric demand without reduction in the frustration can result in unwanted side reactions, the design of the most efficient FLP catalysts depends on tuning the system so that both the steric demand and the frustration are reduced appropriately.

Metal-Free Borylation of Heteroarenes Using Ambiphilic Aminoboranes: On the Importance of Sterics in Frustrated Lewis Pair C–H Bond Activation

“…20b Recently, a computational study was reported proposing that smaller FLPs could yield more active catalysts for the hydrogenation reaction, notably using analogues 2 and 4. 25 Our experimental results support such hypothesis for the C-H bond activation process, but also illustrate the care that needs to be taken when doing computational design. Indeed, the authors proposed 3-centers-2-electrons resting states (Table 3C) for 2 and 4 rather than the experimentally characterized B-H bridged dimers (Table 3D).…”

“…However, it is important to mention that faster reactivity might ensue when only one of the Lewis pair components is required for a reaction to take place, such as in the interaction of a B–H with a protic reagent . Recently, a computational study was reported proposing that smaller FLPs could yield more active catalysts for the hydrogenation reaction, notably using analogues 2 and 4 . Our experimental results support such a hypothesis for the C–H bond activation process, but also illustrate the care that needs to be taken when doing computational design.…”

Evidence for Genuine Bimetallic Frustrated Lewis Pair Activation of Dihydrogen with Gold(I)/Platinum(0) Systems

“…turnover frequency). 22 With this in mind and in the light of the FLP-like reactivity exhibited by heterodinuclear complexes with an explicit M-M bond, 17,23 we decided to investigate the effects derived from finely tuning the balance between metal-metal bond formation 24 and complete metallic frustration in our gold(I)/platinum(0) system. To switch between the two extreme scenarios we have targeted herein three Lewis acidic gold complexes stabilized by terphenyl phosphines, PR2Ar' (R = alkyl; Ar' = C6H3-2,6-Ar2), whose steric parameters were recently investigated by our group (Figure 1).…”

Surprisingly Flexible Oxonium/Borohydride Ion Pair Configurations

“…In addition to phenomena of hydrogen bonding and ion pairing, what makes the complex [ether-H (+) -ether]­[(EWG) 3 B–H (−) ] particularly interesting is that it can be the product of the heterolytic H 2 -cleavage in ether solutions of borane Lewis acids (Scheme ). , Traditionally, transition-metal complexes have played a central role in activation of H 2 and enabling hydrogenation reactions. − Relatively recently, it had been demonstrated that the key step of H 2 -activation can be achieved using frustrated Lewis pairs (FLPs) with main group elements. − At present, there is a continuing surge of interest in the metal-free activation of H 2 by various types of FLPs and the so-called FLP-catalysis of the hydrogenations of a variety of substrates including those with the unsaturated NC and OC bonds. ,,,,− Since Stephan’s seminal work, many reports describe such FLP reactions in the nondonor hydrocarbon or chlorinated/brominated solvents (toluene, dichloromethane, bromobenzene, etc.) and moderately strong donor-solvents (ethers) as well; in the interest of space we are able to mention only a small part of relevant literature.…”