Enabling sustainable bifunctional photocatalytic synthesis by boosted defective carbon nitride with frustrated Lewis pairs

ConspectusThe discovery of reversiblehydrogenation using metal-free phosphoboratespecies in 2006 marked the official advent of frustrated Lewis pair(FLP) chemistry. This breakthrough revolutionized homogeneous catalysisapproaches and paved the way for innovative catalytic strategies.The unique reactivity of FLPs is attributed to the Lewis base (LB)and Lewis acid (LA) sites either in spatial separation or in equilibrium,which actively react with molecules. Since 2010, heterogeneous FLPcatalysts have gained increasing attention for their ability to enhancecatalytic performance through tailored surface designs and improvedrecyclability, making them promising for industrial applications.Over the past 5 years, our group has focused on investigating andstrategically modifying various types of solid catalysts with FLPsthat are unique from classic solid FLPs. We have explored systematiccharacterization techniques to unravel the underlying mechanisms betweenthe active sites and reactants. Additionally, we have demonstratedthe critical role of catalysts’ intrinsic electronic and geometricproperties in promoting FLP formation and stimulating synergisticeffects. The characterization of FLP catalysts has been greatly enhancedby the use of advanced techniques such as synchrotron X-ray diffraction,neutron powder diffraction, X-ray photoelectron spectroscopy, extendedX-ray absorption fine structure, elemental mapping in scanning transmissionelectron microscopy, electron paramagnetic resonance spectroscopy,diffuse-reflectance infrared Fourier transform spectroscopy, and solid-statenuclear magnetic resonance spectroscopy. These techniques have provideddeeper insights into the structural and electronic properties of FLPsystems for the future design of catalysts.Understanding electrondistribution in the overlapping orbitalsof LA and LB pairs is essential for inducing FLPs in operando in heterogeneouscatalysts through target electron reallocation by external stimuli.For instance, in silicoaluminophosphate-type zeolites with weak orbitaloverlap, the adsorption of polar gas molecules leads to heterolyticcleavage of the Alδ+–Oδ− bond, creating unquenched LA–LB pairs. In a Ru-doped metal–organicframework, the Ru–N bond can be polarized through metal–ligandcharge transfer under light, forming Ru+–N– pairs. This activation of FLP sites from the framework representsa groundbreaking innovation that expands the catalytic potential ofexisting materials. For catalysts already employing FLP chemistryto dynamically generate products from substrates, a complete mechanisticinterpretation requires a thorough examination of the surface electronicproperties and the surrounding environment. The hydrogen spilloverability on the Ru-doped FLP surfaces improves conversion efficiencyby suppressing hydrogen poisoning at metal sites. In situ H2–H2O conditions enable the production of organicchemicals with excellent activity and selectivity by creating newbifunctional sites via FLP chemistry. By highlighting the novel FLPsystems featuring FLP induction and synergistic effects and the selectionof advanced characterization techniques to elucidate reaction mechanisms,we hope that this Account will offer innovative strategies for designingand characterizing FLP chemistry in heterogeneous catalysts to theresearch community.

Advancing CO2 hydrogenation to formic Acid: DFT insights into Frustrated Lewis Pair−Functionalized UiO−67 catalysts

Frustrated Lewis pair (FLP) chemistry, a concept that has emerged in the last decade, presents unique systems capable of metal-free hydrogenation by using simple combinations of cooperative main group element components. This finding is particularly interesting given that hydrogenation is a chemical reaction that is performed on a huge scale worldwide with current technologies relying on metal-based catalysts. Over the last 10 years, the range of FLP catalysts has been probed and expanded. At the same time, the concept has also been exploited to employ simple combinations of main group reagents to activate a range of other small molecules. In some cases, these …

The past decade has seen the subject of transition metal-free catalytic hydrogenation develop incredibly rapidly, transforming from a largely hypothetical possibility to a well-established field that can be applied to the reduction of a diverse variety of functional groups under mild conditions. This remarkable change is principally attributable to the development of so-called 'frustrated Lewis pairs': unquenched combinations of bulky Lewis acids and bases whose dual reactivity can be exploited for the facile activation of otherwise inert chemical bonds. While a number of comprehensive reviews into frustrated Lewis pair chemistry have been published in recent years, this tutorial review aims to provide a focused guide to the development of efficient FLP hydrogenation catalysts, through identification and consideration of the key factors that govern their effectiveness. Following discussion of these factors, their importance will be illustrated using a case study from our own research, namely the development of FLP protocols for successful hydrogenation of aldehydes and ketones, and for related moisture-tolerant hydrogenation.

Exploring the metal-free catalytic reduction of CO2 to methanol with saturated adamantane scaffolds of phosphine-borane frustrated Lewis pair: A DFT study

The mechanism of dihydrogen activation has been theoretically investigated by means of DFT calculation. An experimentally synthesized bridged P/B frustrated Lewis pair (FLP) and two designed FLPs are used for this purpose. The model FLPs 2 and 3 are more efficient than FLP 1 for H2 activation as revealed by the thermochemical and kinetic data. A significant amount of electron density is transferred from H2 molecule to the FLPs at the transition states (TSs) during the process of H2 activation, and this is greater at the corresponding TSs of FLPs 2 and 3 than that of FLP 1. The NICS(0) and NICS(1zz) of the boron heterocycle at the FLPs 2 and 3, and at the corresponding TSs and the product geometries of H2 activation demonstrate that the anti-aromatic character of the rings in the FLPs is remarkably reduced at the TSs and finally at the products and that is most likely responsible for enhanced activity of FLPs 2 and 3 by decreasing the activation barrier.

Frustrated Lewis pairs (FLPs) with a unique “push–pull” effect can effectively activate many types of molecules to obtain unanticipated catalytic activity. Herein, FLPs are introduced into polymeric carbon nitride (CN), and their functions in the photocatalytic synthesis of H2O2 are studied. The FLPs in B-doped CN (BCN) are constituted by electron-deficient boron as Lewis acid sites and nitrogen neighbored with cyano groups as Lewis base sites. The formation of FLPs can improve the light absorption ability and the separation of photogenerated carriers. The FLPs afford strong adsorption of O2, but cannot produce H2O2 directly because the strong activation of oxygen bonds leads to oxygen bond scission during reduction. The FLPs enhance H2O2 production through the effective activation of ethanol (ETOH) by the “push–pull” effect of FLPs. The reduction of O2 to H2O2 is found through •O2– and 1O2 species. The photocatalytic H2O2 production rate on BCN can reach 51,008 μM g–1 h–1, which is over 12 times that of pristine CN (4113 μM g–1 h–1). This study not only provides an effective approach for enhancing photocatalytic H2O2 production but also deepens the understanding of the role of FLPs in molecule activation.

While the frustrated Lewis pair (FLP) concept has been successfully extended to heterogeneous catalysis, the underlying factors governing the catalytic performances of FLPs and designing strategies remain elusive. Herein, a theoretical study is performed to design metal-free heterogeneous FLPs with tunable activity for hydrogen dissociation. The designed FLPs constructed by functionalizing the N-doped zigzag graphene edge with eight functional groups −BX2 (X = F, Cl, Br, H, CH3, CF3, CN, NO2) can readily heterolytically dissociate H2 (H2 → Hδ− + Hδ+) with reaction barriers varying from 0.15 to 0.70 eV, showing their comparable activity to homogeneous FLPs. More importantly, FLP acidities of designed FLPs are linearly correlated with the reaction energies of H2 dissociation, suggesting the significant role of FLP acidity in determining their catalytic activity. Further calculations show that the reaction barriers of hydrogenation of CO2 also linearly correlate with the reaction energies of H2 dissociation and accordingly are governed by FLP acidity. Overall, this study provides a route for designing metal-free heterogeneous FLPs on graphene materials and discloses a close relationship between the composition (N···BX2), the electronic structure (FLP acidity), and the functionality (catalytic ability) of the FLPs.

Frustrated Lewis pairs (FLPs) are the combination of Lewis acid and base motifs where steric hindrance prevents strong adduct formation. Accordingly, the ability of FLPs in small molecule activation and their capability in hydrogen cleavage led to their use in the hydrogenation of a wide range of unsaturated substrates. Here, we investigated theoretically the ability of three intramolecular phosphorus/boron FLPs as bifunctional catalysts in the metal-free hydrogenation of dimethylacetylene to cis-alkene. The mechanism of this hydrogenation reaction, based on the boron acceptor [including –OR substituents (B(OR)2), where R is an aliphatic or aromatic branch] and phosphorus donor, has been explored. Based on the results obtained, it was confirmed that the H2 splitting reaction and the formation of the phosphonium–borohydride motifs for these FLPs are endothermic. It has been shown that these FLPs have a moderate ability in H–H bond splitting. Also, the capability of the boron atom in FLPs on the hydrogenation reaction was investigated. The reduction steps of the mechanism showed an exothermic nature. This result revealed that the presence of the boron as a Lewis acid, with a very limited Lewis acidity, improves the catalytic hydrogenation reaction significantly. Finally, it was confirmed that the proposed FLPs in the cis-hydrogenation of alkynes will be effective.

Frustrated Lewis Pairs (FLP) are an important advance in metal-free catalysis due to their ability to activate a variety of small molecules. Many studies have focused on a very limited sample of Lewis acids and bases. Herein, we disclose an automated exploration algorithm using density functional methods, artificial neural networks (ANNs), and a molecule builder that incentivizes the exploration of favorable FLP space for the activation of methane via two mechanisms: deprotonation and hydride abstraction. The exploration algorithm creates FLPs with different Lewis acids (LA), Lewis bases (LB), and their substituents (LA/LB), which proved successful in quickly converging in the favorable chemical space, suggesting chemically sound structures, and generating thousands of potential candidates for methane activating FLPs. By modeling thousands of reactions, an FLP database of methane activation was created, allowing one to data mine properties, e.g., adduct bond length, highest occupied molecular orbital-lowest-unoccupied molecular orbital (HOMO-LUMO) gap, global electrophilicity index, favored Lewis acids/bases/substituents, and substituent steric volume. These properties not only successfully narrow the FLP chemical space but also provide meaningful insight into the chemical nature of competent methane activators. The machine learning discovery strategy disclosed here is general enough to be applicable to many chemical optimization tasks. This study also investigates the efficacy of a Machine-Learned Force Field (MLFF) in predicting the formation energies of Frustrated Lewis Pairs (FLPs). Our model, exhibiting a test error of ±10 kcal/mol, highlighted impressive computational efficiency by enabling the calculation of all possible FLP permutations within our chemical space. The MLFF demonstrated proficiency in predicting energies, providing a significant acceleration compared to quantum mechanics methods. However, challenges emerged in accurately capturing forces, necessitating recourse to classical force fields for reliable structure relaxation. The present study sheds light on the MLFF's potential as a tool for rapid energy predictions, emphasizing the need for further refinement to enhance its accuracy, particularly in force predictions, to expand its utility in chemical simulations.

The chemistry of frustrated Lewis pair (FLP) is widely explored in the activation of small molecules, the hydrogenation of CO2, and unsaturated organic species. A survey of several experimental works on the activation of small molecules by FLPs and the related mechanistic insights into their reactivity from electronic structure theory calculation are provided in the present review, along with the catalytic hydrogenation of CO2. The mechanistic insight into H2 activation is thoroughly discussed, which may provide a guideline to design more efficient FLP for H2 activation. FLPs can activate other small molecules like, CO, NO, CO2, SO2, N2O, alkenes, alkynes, etc. by cooperative action of the Lewis centers of FLPs, as revealed by several computational analyses. The activation barrier of H2 and other small molecules by the FLP can be decreased by utilizing the aromaticity criterion in the FLP as demonstrated by the nucleus independent chemical shift (NICS) analysis. The term boron-ligand cooperation (BLC), which is analogous to the metal-ligand cooperation (MLC), is invoked to describe a distinct class of reactivity of some specific FLPs towards H2 activation.

Review: 109 refs.

The emergence of frustrated Lewis pairs (FLPs) has created a whole new dimension in the development of metal free catalysts for CO2 sequestration. Efforts have been made to enhance the catalytic activity of the FLPs. The aromatic modulation of the catalytic sites has been successfully demonstrated to enhance the activity towards CO2. Although various aromatically modified geminal FLPs have been investigated for CO2 capture, the catalytic space of these FLPs has not been fully resolved yet. Thus, to fulfil the knowledge gap in the understanding of the catalytic behaviour and to extend the concept of aromatically enhanced FLPs, in the present study all the possible combinations of aromatic and antiaromatic modulations of the acidic and basic sites have been proposed and examined using density functional theory based orbital analysis. Further to verify the results obtained from the orbital analysis and to fully explore the catalytic space of the proposed systems, free energy landscapes have been examined using metadynamics simulations. The detailed intrinsic bond orbital (IBO) and principal interacting orbital (PIO) analyses capture crucial details of the reactions. Furthermore, evolution of anisotropy of induced current density (AICD) along the reaction justifies the effect of aromatic/antiaromatic modulation on the catalytic sites. The results show that highly asynchronous mechanisms have been found due to the aromatic/antiaromatic modulations. The simultaneous favourable aromatic/antiaromatic modification on the acidic and basic sites may greatly reduce the CO2 activation barrier. The enhancement of the acidic character of the B atom in the intramolecular FLPs (IFLPs) leads to a thermodynamically more feasible reaction with stable CO2 adducts. The acidic site has been found to play a major role in controlling the kinetics and thermodynamics of the reaction. This study provides valuable insights into the catalytic realm of the aromatically modified FLPs, which can be utilized to design more efficient and specific next-generation FLPs.

The past decade has seen the emergence of a new concept in main-group chemistry: ‘frustrated Lewis pairs’ (FLPs) are combinations of a Lewis acid (LA) and base (LB) that are prevented from forming the classically-expected adduct. By displaying simultaneously acidic and basic behaviour, these systems have been shown to be capable of activating a wide range of chemical bonds, in a manner highly reminiscent of transition-metal (TM) compounds. Chief among these reactions is the activation of H2, which can then be transferred from the FLP to an appropriate substrate. FLPs have thus provided an entirely new class of homogeneous hydrogenation catalysts, which do not require the use of TMs. Nevertheless, prior to the work described herein, such catalysis had not been successfully applied to the hydrogenation of organic carbonyl functional groups, and had been found to be extremely sensitive to the presence of moisture. This thesis describes work that has successfully overcome these limitations.

Solid frustrated Lewis pair (FLP) shows remarkable advantages in the activation of small molecules such as CO2, owing to the strong orbital interactions between FLP sites and reactant molecules. However, most of the currently constructed FLP sites are randomly distributed and easily reunited on the surface of catalysts, resulting in a low utilization rate of FLP sites. Herein, atomic tungsten-based FLP (N···WSA FLP) sites are constructed for photocatalytic CO2 conversion through introducing W single-atoms into polymeric carbon nitride. In the atomically dispersed N···WSA FLP, the electron-deficient W single-atom acts as the Lewis acid (LA), and the adjacent electron-rich N atom acts as the Lewis base. Through the combination of various characterizations, including pyridine-IR, in situ diffuse reflectance infrared Fourier transform spectroscopy, CO2-temperature programmed desorption, and theoretical calculations, the positive effects of N···WSA FLP on photocatalytic CO2 reduction are well revealed. The N···WSA FLP can effectively adsorb CO2 to form an unusual W-O-C-N structure with significant d-p orbital interactions, which leads to an interesting "push-push" electron transfer effect. The π back-donation from W 5d to the antibonding orbital (2π) of CO2 realizes reverse electron transfer from the W single-atom to the O site, while the electrons are transferred from the electron-rich N site to the electropositive C site via Lewis acid-base interactions, therefore effectively breaking the C═O bond to activate CO2 molecules and boost CO2-to-CO performance. This work provides a brand new route for the research on high-efficiency activation of small molecules based on single-atom-based FLP catalysts.