Assessing Tetrel-Based Neutral Frustrated Lewis Pairs for Catalytic Hydrogenation

Abstract

Frustrated Lewis pair (FLP) chemistry has gained significant attention in the past decade as the main group hydrogenation catalysts. The majority of the reported FLP catalysts utilize either boron-containing Lewis acids (LAs) or alternatively isolobal and iso­(valence)­electronic tetrylium analogues for hydrogenation. Recently, a new family of intramolecular FLPs R3E-CH2-PR′2 (E = Si, Ge, Sn) featuring neutral sp3 tetrel atoms as LA centers were reported for activation of a range of small molecules under ambient conditions. They have been shown to perform heterolytic cleavage of the hydrogen molecule, a key step in hydrogenation catalysis. In this work, we computationally examine their ability to perform catalytic hydrogenation using CO2, carbonyls, and imines as substrates. Our mechanistic analyses reveal that the transfer of the activated hydrogens from FLP to the unsaturated bond follows a concerted yet asynchronous pathway. The sequence of proton/hydride transfer is largely influenced by the donor/acceptor centers of the FLP as well as the corresponding unsaturated functional group. We find that by varying the donor centers in these FLPs, the dihydrogen release barrier can be tuned, and nitrogen-bearing FLPs have a lower dihydrogen release barrier than those with phosphorus. The reactivity trends are rationalized within the framework of the activation strain model. Finally, two major side reactions pertinent to the performance of the hydrogenation cycle, such as dimer formation of the catalyst and decomposition through reductive elimination, are also investigated in detail.

In Silico Investigation of the Mechanism of Disulfide Bond Dissociation by New Frustrated Lewis Pairs

“…Natural Bonding Orbital (NBO) Study. For a better understanding about the electron transfer (ET) process, 29,30,36 Natural bonding orbital (NBO) analysis is also conducted. The analysis revealed two significant interactions among P( t Bu) 3 , [(CH 3 ) 2 CH] 2 S 2 , and B(C 2 NBSHF 2 ) 3 .…”

“…FLPs employ sterically demanding groups to prevent the formation of stable Lewis acid–base adducts, thereby maintaining synergistic Lewis acidic and basic effects. This concept has been applied to achieve metal-free hydrogenation and activation of various small molecules, such as H 2 , THF, CO 2 , N 2 O, alkenes, and terminal alkynes. − The experimental studies of FLPs have received significant theoretical attention. ,− The ability of FLPs to cleave S–S bonds was demonstrated by Stephan and co-workers in 2009. They found that the S–S bond of R–S–S–R (R = Ph or i Pr) could be readily cleaved by FLPs, such as ( t Bu) 3 P/B­(C 6 F 5 ) 3 , under ambient conditions (25 °C, 1 atm), yielding thiophosphonium thioborate zwitterions.…”

Advances in CO2 activation by frustrated Lewis pairs: from stoichiometric to catalytic reactions

“…136 Similarly, Pati and co-workers computationally explored the ability of the FLP system (F 5 C 2 ) 3 E–CH 2 –D( t Bu) 2 where E = Si, Ge, Sn and D = P, N to act as hydrogenation catalysts using CO 2 as a substrate. 137 For the FLPs where D = N, simultaneous proton and hydride migration take place, whereas for D = P FLPs, proton transfer is followed by hydride transfer. NBO analysis shows that LP(O) → σ* (D–H) and σ(E–H) → π* (CO) dominate along the energy profile.…”

Recent Advances in Group 14 and 15 Lewis Acids for Frustrated Lewis Pair Chemistry

“…Results also suggest switching the donor site from P to N significantly affects the H 2 activation profiles as the latter FLPs follow a synchronous mechanism for H 2 activation, resulting in lower activation barriers. Inspired by this, the ability of these geminal FLPs to perform as hydrogenation catalysts, particularly towards C=X (X=O, N) are also computationally investigated [58] …”

“…Inspired by this, the ability of these geminal FLPs to perform as hydrogenation catalysts, particularly towards C=X (X=O, N) are also computationally investigated. [58]…”