Frustrated Lewis Pair Research Articles

Comparative Investigation on the Catalytic Potential of Borylated Triazine‐Based FLPs for CO2 Sequestration

AbstractTo address the increasing levels of carbon dioxide (CO2) in the atmosphere, various heterocyclic N/B‐based intramolecular frustrated Lewis pairs (IFLPs) have been experimentally and theoretically investigated. Reports have shown them to be a potential catalyst for CO2 sequestration, however, a comparative rationale of these IFLPs has not been reported. Thus, in the present work, we have proposed an uninvestigated IFLP based on i. e., 2‐boranyl‐1,3,5‐triazine (BT) and compared with previously reported borylated imidazole (Im) and pyrimidine (Prm) for CO2 abduction. The density functional theory (DFT) base calculations have been carried out to probe the reaction mechanism and thorough natural bond orbital analysis has been carried out to get deeper insights into the catalytic behavior of the proposed IFLPs. Further, the effect of substituting the hydrogens at the ring and the B atom of the considered IFLP, by −CH3, −Cl and −OH, on the catalytic behavior has been studied. The results show that the borylated triazines can be a potential catalyst for CO2 sequestration. Also, the catalytic behavior of the presented IFLPs can be modulated by appropriated substitutions.

Frustrated Lewis Pairs on Zr Single Atoms Supported N‐Doped TiO2‐x Catalysts for Electrochemical Nitrate Reduction To Ammonia

AbstractThe electrochemical reduction of nitrates (NO3RR) for ammonia synthesis at room temperature holds immense potential. One key challenge is the adsorption and activation of NO3−, along with the provision of sufficient active hydrogen to accelerate the hydrogenation process. Here, the study prepares N‐doped TiO2‐x supported by Zr single atoms (Zr‐TiON) with rich oxygen vacancies (Ov), in which unsaturated Zr (Lewis acidic, LA) sites together with oxygen atoms around Ov (Lewis base, LB) form frustrated Lewis acid‐base pairs (FLPs). At −60 mA cm−2, NH3 Faradaic efficiency reaches 94.8%, corresponding to the production rate of 663.15 µmol h−1 mgcat−1. The yield rate is up to 26.16 mmol h−1mgcat−1 at −1 A cm−2 in flowing electrolyzer. Theoretical calculations and in situ spectroscopy analysis reveal that the interaction between LA and LB sites in FLPs plays a crucial role in facilitating adsorption and activation of electron‐rich NO3− and electron‐deficient *H. The presence of enhanced FLPs significantly reduces the energy barrier for H2O dissociation, lowering it to 0.20 eV, which facilitates subsequent hydrogenation reactions. The abundance of *H accelerates hydrogenation process, thereby enhancing the activity of NO3RR. This FLP design offers a promising approach for paving the way for the development of highly efficient NO3RR catalysts.

Alkane Elimination Preparation of Heterobimetallic MoAl Tetranuclear and Binuclear Complexes Promoting THF Ring Opening

We report a straightforward alkane elimination strategy to prepare well-defined heterobimetallic Al/Mo species. Notably, the reaction of the monohydride complex of molybdenum, Cp*MoH(CO)3, with triisobutyl aluminum affords a new heterobimetallic [MoAl]2 tetranuclear compound, [Cp*Mo(CO)(µ-CO)2Al(iBu)2]2, (1), featuring a 12-membered C4O4Mo2Al2 ring in which isocarbonyls bridge the Mo and Al centers. The addition of pyridine to this complex successfully results in the dissociation of the dimer into a new discrete binuclear complex, [Cp*Mo(CO)2(µ-CO)Al(Py)(iBu)2], (2). Switching the nature of the Lewis base from pyridine to tetrahydrofuran does not lead to the THF analogue of adduct 2, but rather to a complex reaction where one of the identified products corresponds to a tetranuclear species, [Cp*Mo(CO)3(μ-CH2CH2CH2CH2O)Al(iBu)2]2, (3), featuring two bridging alkoxybutyl fragments originating from the C-O ring opening of THF. Compound 3 adds to the unusual occurrences of THF ring opening by heterobimetallic complexes, which is evocative of masked metal-only frustrated Lewis pair behavior and highlights the high reactivity of these Al/Mo assemblies.

Molecular Dihydrogen Activation by (C5Me5)M/N (M=Rh, Ir) Transition Metal Frustrated Lewis Pairs: Reversible Proton Migration to, and Proton Abstraction from, the C5Me5 Ligand.

The masked transition-metal frustrated Lewis pairs [Cp*M(κ3N,N',N''-L)][SbF6] (Cp*=η5-C5Me5; M=Ir, 1, Rh, 2; HL=pyridinyl-amidine ligand) reversibly activate H2 under mild conditions rendering the hydrido derivatives [Cp*MH(κ2N,N'-HL)][SbF6] observed as a mixture of the E and Z isomers at the amidine C=N bond (M=Ir, 3Z, 3E; M=Rh, 4Z, 4E). DFT calculations indicate that the formation of the E isomers follows a Grotthuss type mechanism in the presence of water. A mixture of Rh(I) isomers of formula [(Cp*H)Rh(κ2N,N'-HL)][SbF6] (5 a-d) is obtained by reductive elimination of Cp*H from 4. The formation of 5 a-d was elucidated by means of DFT calculations. Finally, when 2 reacts with D2, the Cp* and Cp*H ligands of the resulting rhodium complexes 4 and 5, respectively, are deuterated as a result of a reversible hydrogen abstraction from the Cp* ligand and D2 activation at rhodium.

Isomorphic Insertion of Ce(III)/Ce(IV) Centers into Layered Double Hydroxide as a Heterogeneous Multifunctional Catalyst for Efficient Meerwein-Ponndorf-Verley Reduction.

The development of highly active acid-base catalysts for transfer hydrogenations of biomass derived carbonyl compounds is a pressing challenge. Solid frustrated Lewis pairs (FLP) catalysis is possibly a solution, but the development of this concept is still at a very early stage. Herein, stable, phase-pure, crystalline hydrotalcite-like compounds were synthesized by incorporating cerium cations into layered double hydroxide (MgAlCe-LDH). Besides the insertion of well-isolated cerium centers surrounded by hydroxyl groups, the formation of hydroxyl vacancies near the aluminum centers, which were formed by the insertion of cerium centers into the layered double hydroxides (LDH) lattice, was also identified. Depending on the initial cerium concentration, LDHs with different Ce(III)/Ce(IV) ratios were produced, which had Lewis acidic and basic characters, respectively. However, the acid-base character of these LDHs was related to the actual Ce(III)/Ce(IV) molar ratios, resulting in significant differences in their catalytic performance. The as-prepared structures enabled varying degrees of transfer hydrogenation (Meerwein-Ponndorf-Verley MPV reduction) of biomass-derived carbonyl compounds to the corresponding alcohols without the collapse of the original lamellar structure of the LDH. The catalytic markers through the test reactions were changed as a function of the amount of Ce(III) centers, indicating the active role of Ce(III)-OH units. However, the cooperative interplay between the active sites of Ce(III)-containing specimens and the hydroxyl vacancies was necessary to maximize catalytic efficiency, pointing out that Ce-containing LDH is a potentially commercial solid FLP catalysts. Furthermore, the crucial role of the surface hydroxyl groups in the MPV reactions and the negative impact of the interlamellar water molecules on the catalytic activity of MgAlCe-LDH were demonstrated. These solid FLP-like catalysts exhibited excellent catalytic performance (cyclohexanol yield of 45%; furfuryl alcohol yield of 51%), which is competitive to the benchmark Sn- and Zr-containing zeolite catalysts, under mild reaction conditions, especially at low temperature (T = 65 °C).

MOFs-derived (Fe3O4/Fe2O3)OH@C-X in-situ form frustrated Lewis acid-base pairs for visible-light-driven photocatalytic CO2 reduction

Frustrated Lewis pairs (FLPs) have demonstrated their capacity to capture and react with CO2, presenting a novel method for CO2 reduction. Despite this, their practical application remains limited due to a complex preparation process, poor stability, and regeneration challenges. We address these issues in this study by pyrolyzing a metal-organic framework (MOF) to form in-situ FLPs on an iron oxide surface. The resulting (Fe3O4/Fe2O3)OH@C-400 composite exhibits a significantly improved CO2 reduction rate of 173.3 μmol g−1 h−1, which is approximately 13.4 times higher than that of Fe2O3 and 10.9 times higher than that of Fe3O4. Both theoretical and experimental findings confirm that the creation of heterostructured hollow spheres and in-situ FLPs enhances visible light absorption, improves the separation and transfer of photogenerated carriers, and increases the photocatalytic efficiency for CO2 reduction. The H/D kinetic isotope effect suggests that water plays a crucial role in the limiting step of CO2 photoreduction. In-situ FTIR results corroborate the formation of COOH* as a critical intermediate in photocatalytic CO2 reduction over (Fe3O4/Fe2O3)OH@C-X. This research provides a novel insight into catalyst material design and mechanism research for photocatalytic CO2 reduction.

Excited state dynamics of P-H···H-B and P-H···H-Si hydride-hydride interactions and potential dehydrogenation reactions in frustrated lewis pair

Dihydrogen interactions plays a crucial role in frustrated Lewis pairs (FLPs) strategy and promotes dehydrogenation reactions due to its low directionality and saturability. This work focuses on the excited-state dynamics of the P-H···H-B and P-H···H-Si intramolecular hydride-hydride interactions in 4-bis(pentafluorophenyl)-borane-5-dimesitylphosphino-9,9-dimethylxanthine (FPB-MP-MX) and 5-dimethylsilyl-9,9-dimethylxanthene-4-yl-diphenylphosphonium cation (MSi-PP-MX), respectively. The interaction between B-H, C-H and π electron of benzene ring are also considered. Upon photoexcitation to the first singlet (S1) state, the P-H···H-B of FPB-MP-MX and the P-H···H-Si of MSi-PP-MX are both strengthened, while B-H···π and C−H···π interaction are all weakened. These changes are induced by the charge transfer from the 9,9-dimethyl-xanthene moiety to the electron-deficient phosphorus cation and mesitylene group, which are demonstrated by the molecule orbital, electron–hole, and charge analysis. In addition, the potential energy curve of FPB-MP-MX scanned the P-H···H-B shows low energy barriers of 20.68 kcal mol−1 in the S0 state, implying the potential dehydrogenation reaction with lower energy of hydrogen product. In contrast to FPB-MP-MX, the potential energy curve of MSi-PP-MX scanned the P-H···H-Si has large energy barrier of 42.66 kcal mol−1 in the S0 state and 32.01 kcal mol−1 in the S1 state, which demonstrates the difficulty of dehydrogenation reaction. This works presents reasonable interpretation on the dynamics changes of dihydrogen bonding and dehydrogenation reaction of frustrated Lewis pairs upon photoexcitation.

Stereocontrol via Propeller Chirality in FLP-Catalyzed Asymmetric Hydrogenation.

Utilization of chiral frustrated Lewis pairs as catalysts in enantioselective hydrogenation of unsaturated molecules represents a promising approach in asymmetric synthesis. In our effort to improve our current understanding of the factors governing the stereoselectivity in these catalytic processes, herein we examined the mechanism of direct hydrogenation of aromatic enamines catalyzed by a binaphthyl-based chiral amino-borane. Our computational analysis reveals that only one particular conformer of the key borohydride reaction intermediate can be regarded as a reactive form of this species. This borohydride conformer has a well-defined chiral propeller shape, which induces facial selectivity in the hydride transfer to pro-chiral iminium intermediates. The propeller chirality of the reactive borohydride conformer is generated by the axially chiral binaphthyl scaffold of the amino-borane catalyst through stabilizing π-π stacking interactions. This new computational insight can be readily used to interpret the high degree of stereoinduction observed for these reactions. We expect that the concept of chirality relay could be further exploited in catalyst design endeavors.

Optimizing low-temperature CO2 methanation through frustrated Lewis pairs on Ni/CeO2 catalysts

The promising realm of carbon dioxide (CO2) conversion offers a compelling avenue to advance environmental sustainability. This study introduces a novel approach by utilizing frustrated Lewis pairs (FLPs) for efficient low-temperature CO2 methanation. Through a tailored ammonium bicarbonate (AB) precipitation method, we engineered Ni/CeO2 catalysts abundant in FLPs. Various morphologies, including aggregated flakes, broom-like structures, and irregular granules, were achieved by modulating the AB-to-cerium molar ratio. The distinctive broom-like Ni/(0.06AB)CeO2 catalyst, comprised of nanorods with abundant FLPs, exhibited outstanding low-temperature CO2 methanation performance: 67.5 % CO2 conversion, 99.6 % CH4 selectivity, and 0.27 s−1 CO2 turnover frequency (TOFCO2) at 240 °C, with excellent stability in a 100-h test at 300 °C. Furthermore, leveraging in-situ DRIFTS and in-situ Raman techniques, and DFT study, we unraveled CO2 hydrogenation pathways and found a dual interplay of CO and formate pathways in the low-temperature methanation over Ni/(0.06AB)CeO2 catalyst.

Capture of CO2 by Melamine Derivatives: A DFT Study Combining the Relative Energy Gradient Method with an Interaction Energy Partitioning Scheme.

A theoretical study of the interaction between melamine and CO2 was carried out using density functional theory (DFT) with the B3LYP-D3(BJ)/aug-cc-pVTZ level of theory. The presence of anions interacting with melamine transforms the weakly bonded tetrel complexes into adducts. Thus, melamine acts as an FLP (frustrated Lewis pair) with acid groups (NHs as hydrogen bond donors) and a base group (N of the triazine ring). The application of the relative energy gradient formalism (REG) along the reaction coordinate has demonstrated that the ability of the melamine-anion systems to capture CO2 is linked to its capacity to polarize the CO2 molecule. These results have been confirmed by placing the melamine:CO2 complex in a uniform electric field with different strengths.

Visible-Light Photocatalytic H2O2 Production Boosted by Frustrated Lewis Pairs in Defected Polymeric Carbon Nitride Nanosheets

Visible-Light Photocatalytic H₂O₂ Production Boosted by Frustrated Lewis Pairs in Defected Polymeric Carbon Nitride Nanosheets

Restructuring Surface Lewis Pairs of FAU Zeolite through N Doping for Boosting the Toluene Side-Chain Alkylation Performance.

Toluene side-chain alkylation with methanol for the styrene monomer formation remains a great challenge. An optimal synergy between acidic and basic sites on zeolites is required for an efficient catalysis process. It is important to modulate the surface Lewis acid-base pairs precisely. Herein, we report a strategy to restructure the surface Lewis acid-base pairs in cesium-modified X zeolite (CsX) by N doping. In the process of toluene side-chain alkylation, the CsX-BN-600 catalyst, where N species is doped into the framework of the X zeolite, exhibits 2.7 times the styrene formation rate and a much better selectivity of 85.7% in comparison to the parent CsX of 70.1% selectivity to styrene at the same reaction conditions. The introduction of N species into zeolites acts as a new Lewis base site and optimizes the Lewis sites due to its ability of electron donation. Meanwhile, the frustrated Lewis pair (FLP) between the deprotonated framework nitrogen in X zeolite and positively polarized C species in the side-chain alkylation reaction is created. Furthermore, the N doping contributes to the generation of the active intermediates of HCOO* and H3CO*. These reasons favor the superiority of the catalyst through N doping.

Biomimetic Frustrated Lewis Pair Catalysts for Hydrogenation of CO to Methanol at Low Temperatures.

The industrial production of methanol through CO hydrogenation using the Cu/ZnO/Al2O3 catalyst requires harsh conditions, and the development of new catalysts with low operating temperatures is highly desirable. In this study, organic biomimetic FLP catalysts with good tolerance to CO poison are theoretically designed. The base-free catalytic reaction contains the 1,1-addition of CO into a formic acid intermediate and the hydrogenation of the formic acid intermediate into methanol. Low-energy spans (25.6, 22.1, and 20.6 kcal/mol) are achieved, indicating that CO can be hydrogenated into methanol at low temperatures. The new extended aromatization-dearomatization effect involving multiple rings is proposed to effectively facilitate the rate-determining CO 1,1-addition step, and a new CO activation model is proposed for organic catalysts.

(Pyridin-2-ylmethyl)triel Derivatives as Masked Frustrated Lewis Pairs. Interactions and CO2 -Sequestration.

The isolated (pyridin-2-ylmethyl)triel derivatives (triel=B, Al and Ga) show an intramolecular N⋅⋅⋅Tr triel bond as shown by compounds found in the Cambridge Structural Database and DFT calculations. The possibility to use them as masked frustrated Lewis pairs (mFLP) has been explored theoretically concerning their reaction with CO2 . The adduct formation proceeds in two steps. In the first one, the (pyridin-2-ylmethyl)triel derivatives break the intramolecular N⋅⋅⋅Tr bond assisted by CO2 and in the second step the adduct is formed with Tr-O and N-C covalent bonds. The corresponding energy minima and transition states (TS) of the reaction have been characterized and analyzed. The distortion/interaction model analysis of the stationary points indicates that the whole process can be divided in two parts: reorganization of the mFLP in the first steps of the reaction while the reaction with CO2 (associated to the distortion of this molecule) is more important in the formation of the final adduct. In all cases studied, the final products are more stable than the starting molecules that combine with reasonable TS energies indicating that these reactions can occur.

CO2 reduction using aluminum hydride: Generation of in-situ frustrated Lewis pairs and small molecule activation therein.

CO2 reduction is appealing for the long-term production of high-value fuels and chemicals. Herein, using density functional theory (DFT) based calculations, we study the CO2 reduction pathway to formic acid using aluminum hydride and phosphine derivatives. Our primary focus is on aluminum hydride derivatives, aimed at improving the efficiency of the CO2 reduction process. Substituents with σ-donating properties at the aluminum center are discovered to lower the activation barriers. We demonstrate how di-tert-butylphosphine oxide (LB-O)/di-tert-butylphosphine sulfide (LB-S)/di-tert-butylphosphanimine (LB-N) work together with aluminum hydride to facilitate CO2 reduction process and generate in-situ frustrated Lewis pairs (FLPs), such as FLP-O, FLP-S, and FLP-N. The activation strain model (ASM) analysis reveals the significance of strain energy in determining activation barriers. EDA-NOCV and PIO analyses elucidate the orbital interactions at the corresponding transition states. Furthermore, the study delves into the activation of various small molecules, such as dihydrogen, acetylene, ethylene, carbon dioxide, nitrous oxide, and acetonitrile, using those in-situ generated FLPs. The study highlights the low activation barriers and emphasizes the potential for small molecule activation in this context.

CeO2-Based Frustrated Lewis Pairs via Defective Engineering: Formation Theory, Site Characterization, and Small Molecule Activation.

Activation of small molecules is considered to be a central concern in the theoretical investigation of environment- and energy-related catalytic conversions. Sub-nanostructured frustrated Lewis pairs (FLPs) have been an emerging research hotspot in recent years due to their advantages in small molecule activation. Although the progress of catalytic applications of FLPs is increasingly reported, the fundamental theories related to the structural formation, site regulation, and catalytic mechanism of FLPs have not yet been fully developed. Given this, it is attempted to demonstrate the underlying theory of FLPs formation, corresponding regulation methods, and its activation mechanism on small molecules using CeO2 as the representative metal oxide. Specifically, this paper presents three fundamental principles for constructing FLPs on CeO2 surfaces, and feasible engineering methods for the regulation of FLPs sites are presented. Furthermore, cases where typical small molecules (e.g., hydrogen, carbon dioxide, methane oxygen, etc.) are activated over FLPs are analyzed. Meanwhile, corresponding future challenges for the development of FLPs-centered theory are presented. The insights presented in this paper may contribute to the theories of FLPs, which can potentially provide inspiration for the development of broader environment- and energy-related catalysis involving small molecule activation.

Heterolytic H2 Activation in Heterogeneous Hydrogenation/Hydroprocessing Catalysis

AbstractIn heterogeneous catalysis, heterolytic H2 activation for (selective) hydrogenation and hydroprocessing reactions involves the dissociation of adsorbed H2 molecules into proton (Hδ+) and hydride (Hδ−) on the catalyst surface. This approach offers several advantages, including high selectivity for polar bond (s), a low energy barrier for H2 dissociation, a high capacity for reaction‐favorable H2 adsorption, and reduced catalyst poisoning. This requires the construction of frustrated Lewis pairs on the catalyst surface, satisfying specific criteria, such as having an abundant quantity of Lewis pairs with steric hindrance and maintaining a certain distance of 3–5 Å between the pairs. This review highlights intrinsic catalyst properties for heterolytic H2 activation based on state‐of‐the‐art reports. The main components necessary for this activation include supports with strong basic sites and/or oxygen vacancies, and/or metals of single atom. For this purpose, designed catalytic materials aim to strengthen the Lewis acidity and basicity, improve the polarization of Lewis pairs, enrich oxygen vacancies, maximize the interfacial area between metal species and Lewis base, and enhance metal–support interaction. Therefore, heterogeneous catalysts retaining such heterolytic H2 activation characteristics will be significantly effective in various hydrogenation and hydroprocessing reactions.

Design and Synthesis of Heterogeneous Frustrated Lewis Pairs for Hydrogenation: From Molecular Immobilization to Defects Engineering

AbstractThe concept of “Frustrated Lewis pairs” (FLPs), which emerged from the discovery that H2 can be reversibly activated by combinations of sterically encumbered Lewis acids and bases. Since then, the field of FLP chemistry has expanded enormously. One of the most remarkable achievements is the development of FLP catalysts for hydrogenation, which are environmentally friendly and have potential industrial application prospects. In recent years, this unique catalysis concept has pushed the study of FLP chemistry to a new direction: heterogeneous FLP catalysts. This review outlines the recent research progress of new strategies to design and synthesize heterogeneous FLPs for hydrogenation reaction, and prospects the development of novel heterogeneous FLP catalysts.

Review of Theoretical and Computational Studies of Bulk and Single Atom Catalysts for H2 S Catalytic Conversion.

Catalytic conversion of hydrogen sulfide (H2 S) plays a vital role in environmental protection and safety production. In this review, recent theoretical advances for catalytic conversion of H2 S are systemically summarized. Firstly, different mechanisms of catalytic conversion of H2 S are elucidated. Secondly, theoretical studies of catalytic conversion of H2 S on surfaces of metals, metal compounds, and single-atom catalysts (SACs) are systematically reviewed. In the meantime, various strategies which have been adopted to improve the catalytic performance of catalysts in the catalytic conversion of H2 S are also reviewed, mainly including facet morphology control, doped heteroatoms, metal deposition, and defective engineering. Finally, new directions of catalytic conversion of H2 S are proposed and potential strategies to further promote conversion of H2 S are also suggested: including SACs, double atom catalysts (DACs), single cluster catalysts (SCCs), frustrated Lewis pairs (FLPs), etc. The present comprehensive review can provide an insight for the future development of new catalysts for the catalytic conversion of H2 S.

Reactivity of Frustrated Lewis Pair: Carbocation versus Radical Intermediates.

Recent reports of radical formation within frustrated Lewis pairs (FLPs) suggested that single-electron transfer (SET) could play an important role in their chemistry especially for C-C coupling. In sharp contrast, our extensive dispersion-corrected DFT calculations show that although reactive benzhydryl radical along with phosphine radical cation species can be kinetically generated from bulky phosphines and benzhydryl cation, direct P-C hetero-coupling may lead to bulky phosphonium cation as reactive carbocation transfer reagents to styrene substrates, which is kinetically much more favorable than the recently proposed radical C-C coupling between benzhydryl radical and styrene. Similarly, meta-stable radical cation Mes3 P+ ⋅ salt is also kinetically accessible via SET reactions of Mes3 P and B(C6 F5 )3 with 0.5 equivalent of p-O2 C6 Cl4 .