Seven-Coordinate Group 6 Metal Hydrides Obtained by H2 Activation at B(C6F5)3 Adducts of N2 Complexes: Frustrated Lewis Pair-Type Reactivity of The B-N Linkage

The adducts 2M,R of general formula trans-[(L)M{R2P(CH2)2PR2}2{N2B(C6F5)3}] (L=ø or N2, M=Mo or W, R=Et or Ph), formed from Lewis acid-base pairing of B(C6F5)3 to a dinitrogen ligand of zero-valent group 6 bis(phosphine) complexes trans-[M{R2P(CH2)2PR2}2(N2)2] are shown to react with dihydrogen to afford hepta-coordinated bis(hydride) complexes [M(H)2{R2P(CH2)2PR2}{N2B(C6F5)3}] 3M,R which feature the rare ability to activate both dinitrogen and dihydrogen at a single metal center, except in the case where M=Mo and R=Ph for which fast precipitation of insoluble [Mo(H)4(dppe)2] (dppe=1,2-bis(diphenylphosphino)ethane) occurs. The frustrated Lewis pair (FLP)-related reactivity of the B-N linkage in compounds 3W,R was explored and led to distal N functionalization without involvement of the hydride ligands. It is shown in one example that the resulting bis(hydride) diazenido compounds may also be obtained through a sequence involving first FLP-type N-functionalization followed by oxidative addition of H2. Those oily compounds were found to have limited stability in solution or in their isolated states. Finally, treatment of 3W,Et with the Lewis base N,N-dimethylaminopyridine (DMAP) affords the simple but unknown bis(hydride)-dinitrogen species [W(H)2(depe)2(N2)] 11Et (depe=1,2-bis(diethylphosphino)ethane) which direct, selective formation from trans-[W(N2)2(depe)2] is not possible.

Experimental and computational aspects of molecular frustrated Lewis pairs for CO2 hydrogenation: en route for heterogeneous systems?

Catalysis plays a crucial role in advancing sustainability. The unique reactivity of frustrated Lewis pairs (FLPs) is driving an ever-growing interest in the transition metal-free transformation of small molecules like CO2 into valuable products. In this area, there is a recent growing incentive to heterogenize molecular FLPs into porous solids, merging the benefits of homogeneous and heterogeneous catalysis - high activity, selectivity, and recyclability. Despite the progress, challenges remain in preventing deactivation, poisoning, and simplifying catalyst-product separation. This review explores the expanding field of FLPs in catalysis, covering existing molecular FLPs for CO2 hydrogenation and recent efforts to design heterogeneous porous systems from both experimental and theoretical perspectives. Section 2 discusses experimental examples of CO2 hydrogenation by molecular FLPs, starting with stoichiometric reactions and advancing to catalytic ones. It then examines attempts to immobilize FLPs in porous matrices, including siliceous solids, metal-organic frameworks (MOFs), covalent organic frameworks, and disordered polymers, highlighting current limitations and challenges. Section 3 then reviews computational studies on the mechanistic details of CO2 hydrogenation, focusing on H2 splitting and hydride/proton transfer steps, summarizing efforts to establish structure-activity relationships. It also covers the computational aspects on grafting FLPs inside MOFs. Finally, Section 4 summarizes the main design principles established so far, while addressing the complexities of translating computational approaches into the experimental realm, particularly in heterogeneous systems. This section underscores the need to strengthen the dialogue between theoretical and experimental approaches in this field.

Synthesis and Photochemical Uncaging of Alkene-Protected, Polymer-Bound Vicinal Frustrated Lewis Pairs

Polymeric materials bearing Frustrated Lewis Pair (FLP) functionality are promising candidates for use as heterogeneous catalysts and adaptive materials, but synthetic access to FLP-functional polymers remains limited due to the incompatibility of FLPs with standard polymerization chemistries. Herein, we describe a synthetic approach that "cages" highly reactive vicinal phosphine-borane FLPs as covalent alkene adducts, which are stable to Ni-mediated vinyl addition polymerization. We discovered that the caged FLP adducts can be photochemically activated to liberate vicinal FLPs, enabling spatiotemporally controlled release of FLPs from polymeric precursors.

Structural Flexibility is a Decisive Factor in FLP Dihydrogen Cleavage with Tetrahedral Lewis Acids: A Silane Case Study

Dihydrogen activation is the paradigmatic reaction of frustrated Lewis pairs (FLPs). While trigonal-planar Lewis acids have been well established in this transformation, tetrahedral Lewis acids are surprisingly limited. Indeed, several cases were computed as thermodynamically and kinetically feasible but exhibit puzzling discrepancies with experimental results. In the present study, a computational investigation of the factors influencing dihydrogen activation are considered by large ensemble sampling of encounter complexes, deformation energies and the activation strain model for a silicon/nitrogen FLP and compared with a boron/phosphorous FLP. The analysis adds the previously missing dimension of Lewis acids' structural flexibility as a factor that influences preexponential terms beyond pure transition state energies. It sheds light on the origin of "overfrustration" (defined herein), indicates structural constraint in Lewis acids as a linchpin for activation of weak donor substrates, and allows drawing a more refined mechanistic picture of this emblematic reactivity.

Aluminum in Frustrated Lewis Pair Chemistry

This review article describes the development of the use of aluminum compounds in the chemistry of frustrated Lewis pairs (FLPs) over the last 14 years. It also discusses the synthesis, reactivity and catalytic applications of intermolecular, intramolecular and so-called hidden FLPs with phosphorus, nitrogen and carbon Lewis bases. The intrinsically higher acidity of aluminum compounds compared to their boron analogs opens up different reaction pathways. The results are presented in a more or less chronological order. It is shown that Al FLPs react with a variety of polar and non-polar substrates and form both stable adducts and reversibly activate bonds. Consequently, some catalytic applications of the title compounds were presented such as dimerization of alkynes, hydrogenation of tert-butyl ethylene and imines, C-F bond activation, reduction of CO2, dehydrogenation of amine borane and transfer of ammonia. In addition, various Al FLPs were used as initiators in polymerization reactions.

Yb/Si frustrated Lewis pairs with a labile naphthalenyl bridge

The first examples of RE/Si FLPs (RE: rare-earth metal, FLPs: frustrated Lewis pairs), namely Yb/Si FLPs were synthesized, where Yb⋯Si distances are in the range of 3.55 to 3.72 Å. These FLPs react with triphenylphosphine sulfide and aryl isocyanide to produce novel silylyne group transfer products through dissociation of naphthalene.

Exploring the Impact of Elements on the Reactivity of a Straightforward Procedure for Generating Vinyl-Carbazole Derivatives via a Frustrated Lewis Pair Mechanism

The effect of chemical element on the reactivity for carbazolation reaction of phenylacetylene utilizing G13(C6F5)3 (Lewis acid) and G15-carbazole (Lewis base) was theoretically investigated using density functional theory (M06-2X-D3/def2-TZVP), where G13 represents Group 13 elements and G15 represents Group 15 elements. Through activation strain model (ASM) analysis, it is apparent that the reactivity of the entire carbazolation reaction is chiefly governed by the structural strain energy of the alkyne fragment. In other words, if G13(C6F5)3 or G15-carbazole features an atomic radius that is either too small (e.g., B atom) or too large (e.g., Tl or Bi atom), it results in inadequate orbital overlap between the reactants due to the impact of steric effects. This, in turn, results in an elevation of the activation energy for such reactions, thereby impeding the alkyne from undergoing the carbazole catalytic reaction. In light of the above analyses, our theoretical findings suggest that, except for Tl(C6F5)3, the other four Lewis acid catalysts (B(C6F5)3, Al(C6F5)3, Ga((C6F5)3, and In((C6F5)3) demonstrate effectiveness in catalyzing the carbazolation reaction of alkyne alongside with N-carbazole. Additionally, it is anticipated that, among the five categories of G15-carbazole molecules studied, only N-carbazole can participate in the carbazolation reaction with alkyne catalyzed by B(C6F5)3, considering both kinetic and thermodynamic factors at room temperature. Our theoretical investigations, as outlined in this study, indicate that the carbazolation reaction of the alkyne, catalyzed by G13(C6F5)3 and G15-carbazole, follows Hammond's postulate. To put it more plainly, when the transition state of the chemical reaction occurs earlier, it results in a decrease in activation energy.

Coordination of Al(C6F5)3 vs. B(C6F5)3 on group 6 end-on dinitrogen complexes: chemical and structural divergences

The coordination of the Lewis superacid tris(pentafluorophenyl)alane (AlCF) to phosphine-supported, group 6 bis(dinitrogen) complexes [ML2(N2)2] is explored, with M = Cr, Mo or W and L = dppe (1,2-bis(diphenylphosphino)ethane), depe (1,2-bis(diethylphosphino)ethane), dmpe (1,2-bis(dimethylphosphino)ethane) or 2 × PMe2Ph. Akin to tris(pentafluorophenyl)borane (BCF), AlCF can form 1 : 1 adducts by coordination to one distal nitrogen of general formula trans-[ML2(N2){(μ-η1:η1-N2)Al(C6F5)3}]. The boron and aluminium adducts are structurally similar, showing a comparable level of N2 push-pull activation. A notable exception is a bent (BCF adducts) vs. linear (AlCF adducts) M-N-N-LA motif (LA = Lewis acid), explained computationally as the result of steric repulsion. A striking difference arose when the formation of two-fold adducts was conducted. While in the case of BCF the 2 : 1 Lewis pairs could be observed in equilibrium with the 1 : 1 adduct and free borane but resisted isolation, AlCF forms robust 2 : 1 adducts trans-[ML2{(μ-η1:η1-N2)Al(C6F5)3}2] that isomerise into a more stable cis configuration. These compounds could be isolated and structurally characterized, and represent the first examples of trinuclear heterometallic complexes formed by Lewis acid-base interaction exhibiting p and d elements. Calculations also demonstrate that from the bare complex to the two-fold aluminium adduct, substantial decrease of the HOMO-LUMO gap is observed, and, unlike the trans adducts (1 : 1 and 1 : 2) for which the HOMO was computed to be a pure d orbital, the one of the cis-trinuclear compounds mixes a d orbital with a π* one of each N2 ligands. This may translate into a more favourable electrophilic attack on the N2 ligands instead of the metal centre, while a stabilized N2-centered LUMO should ease electron transfer, suggesting Lewis acids could be co-activators for electro-catalysed N2 reduction. Experimental UV-vis spectra for the tungsten family of compounds were compared with TD-DFT calculations (CAM-B3LYP/def2-TZVP), allowing to assign the low extinction bands found in the visible spectrum to unusual low-lying MLCT involving N2-centered orbitals. As significant red-shifts are observed upon LA coordination, this could have important implications for the development of visible light-driven nitrogen fixation.

Use of Microwave Dielectric Spectroscopy for the In Actu Assessment of Frustrated Lewis Pair Encounter Complexes

Frustrated Lewis pairs (FLPs) offer an important and promising paradigm for main group catalysis. Reported here is the use of microwave dielectric spectroscopy for the in actu detection of FLP encounter complexes. This technique focuses on the room-temperature measurement of the loss component of microwave permittivity (ε2) over the bandwidth from 0.5 to 6.8 GHz. The microwave loss measured for a Lewis pair in a toluene host solution is compared with the losses of the individual components when measured separately, and the difference in loss Δε2 is used to characterize the electrostatic interaction between the pair. The Δε2 value shows a direct correlation with an ability for the FLP encounter complex to split hydrogen gas and abstract hydrogen from γ-terpinene and has led to the identification of a novel FLP encounter complex, tris-pentafluorophenyl borane-eucalyptol pairing.

Oxidation-dependent Lewis acidity in chalcogen adducts of Sb/P frustrated Lewis pairs

The reactions of the frustrated Lewis pair (F5C2)2SbCH2P(tBu)2 with oxygen, sulphur, selenium and tellurium led to the mono-oxidation products (F5C2)2SbCH2P(E)(tBu)2 (E = O, S, Se, Te). Further oxidation of these chalcogen adducts with tetrachloro-ortho-benzoquinone (o-chloranil) gave (F5C2)2Sb(CH2)(μ-E)P(tBu)2·CatCl (CatCl = o-O2C6Cl4) with a central four-membered ring heterocycle for E = O, S, and Se. For E = Te the elimination of elemental tellurium led to an oxidation product with two equivalents of o-chloranil, (F5C2)2SbCH2P(tBu)2·2CatCl, which is also accessible by reaction of (F5C2)2SbCH2P(tBu)2 with o-chloranil. The synthesised compounds were characterised by NMR spectroscopy and X-ray structure analyses, and the structural properties were analysed in the light of the altered Lewis acidity due to the oxidation of the antimony atoms.

A catalytic collaboration: pairing transition metals and Lewis acids for applications in organic synthesis

The use of metal catalysts to accelerate an organic transformation has proven indispensable for access to structural motifs having applications across medicinal, polymer, materials chemistry, and more. Most catalytic approaches have cast transition metals in the "leading role"; these players mediate important reactions such as C-C cross coupling and the hydrogenation of unsaturated bonds. These catalysts may require collaboration, featuring Lewis acidic or basic additives to promote a desired reaction outcome. Lewis acids can serve to accelerate reactions by way of substrate stabilization and/or activation, and as such, are valuable in optimizing catalytic transformations. A burgeoning area of chemical research which unifies these concepts has thus sought to develop transition metal complexes having ambiphilic (containing a Lewis basic and acidic unit) ligands. This approach takes advantage of metal-ligand cooperativity to increase the efficiency of a given chemical transformation, leveraging intramolecular interactions between a transition metal and an adjacent secondary ligand site. While this has shown significant potential to facilitate challenging and important transformations, there remains unexplored depth for creativity and future advancement. This Frontier highlights inter- and intramolecular combinations of transition metals and Lewis acids that together, provide a collaborative platform for chemical synthesis.

Ab initio investigation on the mechanism of SO2 activation by P/B intermolecular frustrated Lewis pairs

CONTEXT

In silico study investigates the activation of sulfur dioxide by newly designed frustrated Lewis pairs, i.e., [P(tBu)3…B(C2NBSHF2)3], where the Lewis acid part is a super Lewis acid. The activation process involves the making of P-S and B-O bonds, leading to the formation of an FLP-SO2 adduct. The calculated results demonstrate that the activation of SO2 by the FLP is almost barrierless and exothermic. Exploration of the impact of the solvent environment on the feasibility and energetics of the reaction has been investigated. The exothermicity is increasing in nonpolar solvents.

METHODS

This study focuses on understanding the electronic activity of SO2 activation by FLP with the help of the Minnesota 06 functional, M06-2X (global hybrid functional with 54% HF exchange) along with Pople's basis set, 6-311G (d, p). Principal interacting orbital and extended transition state-natural orbitals for chemical valence studies, giving impactful insight into the favorable orbital interaction and electron transfer in this reaction. Furthermore, useful CDFT descriptors such as reaction force constant and reaction electronic flux profiles along the intrinsic reaction coordinate give insights into the synchronicity and total electronic activity of the reaction.

Lewis Acid-Driven Inverse Hydride Shuttle Catalysis

Inverse hydride shuttle catalysis provides a multicomponent platform for the highly efficient synthesis of alkaloid frameworks with exquisite diastereoselectivity. However, a number of limitations hinder this method, primarily the strict requirement for highly electron-deficient acceptors. Herein, we present a general Lewis acid-driven approach to address this constraint, and have developed two broad strategies enabling the modular synthesis of complex azabicycles that were entirely unattainable using the previous method. The enhanced synthetic flexibility facilitates a streamlined asymmetric cyclization, leading to a concise total synthesis of the alkaloid (-)-tashiromine.

Homogeneous and Heterogeneous Frustrated Lewis Pairs for the Activation and Transformation of CO2

Due to the serious ecological problems caused by the high CO2 content in the atmosphere, reducing atmospheric CO2 has attracted widespread attention from academia and governments. Among the many ways to mitigate CO2 concentration, the capture and comprehensive utilization of CO2 through chemical methods have obvious advantages, whose key is to develop suitable adsorbents and catalysts. Frustrated Lewis pairs (FLPs) are known to bind CO2 through the interaction between unquenched Lewis acid sites/Lewis base sites with the O/C of CO2, simultaneously achieving CO2 capture and activation, which render FLP better potential for CO2 utilization. However, how to construct efficient FLP targeted for CO2 utilization and the mechanism of CO2 activation have not been systematically reported. This review firstly provides a comprehensive summary of the recent advances in the field of CO2 capture, activation, and transformation with the help of FLP, including the construction of homogeneous and heterogeneous FLPs, their interaction with CO2, reaction activity, and mechanism study. We also illustrated the challenges and opportunities faced in this field to shed light on the prospective research.

Engineering Frustrated Lewis Pair Active Sites in Porous Organic Scaffolds for Catalytic CO2 Hydrogenation

Frustrated Lewis pairs (FLPs), featuring reactive combinations of Lewis acids and Lewis bases, have been utilized for myriad metal-free homogeneous catalytic processes. Immobilizing the active Lewis sites to a solid support, especially to porous scaffolds, has shown great potential to ameliorate FLP catalysis by circumventing some of its inherent drawbacks, such as poor product separation and catalyst recyclability. Nevertheless, designing immobilized Lewis pair active sites (LPASs) is challenging due to the requirement of placing the donor and acceptor centers in appropriate geometric arrangements while maintaining the necessary chemical environment to perform catalysis, and clear design rules have not yet been established. In this work, we formulate simple guidelines to build highly active LPASs for direct catalytic hydrogenation of CO2 through a large-scale screening of a diverse library of 25,000 immobilized FLPs. The library is built by introducing boron-containing acidic sites in the vicinity of the existing basic nitrogen sites of the organic linkers of metal-organic frameworks collected in a "top-down" fashion from the CoRE MOF 2019 database. The chemical and geometrical appropriateness of these LPASs for CO2 hydrogenation is determined by evaluating a series of simple descriptors representing the intrinsic strength (acidity and basicity) of the components and their spatial arrangement in the active sites. Analysis of the leading candidates enables the formulation of pragmatic and experimentally relevant design principles which constitute the starting point for further exploration of FLP-based catalysts for the reduction of CO2.

Understanding the CO capture reaction through electronic structure analysis of four-membered-ring group-13/N- and B/group-15-based Lewis acid-base pairs

Incomplete combustion yields a significant byproduct, known for its high toxicity to humans: gas phase carbon monoxide (CO). This study utilized several advanced theoretical methods to examine the factors contributing to the activation energy involved in CO capture by a frustrated Lewis pair (FLP) and to forecast the potential success of the CO capture reaction. The current theoretical findings indicate that among the four-membered-ring Group-13/N-FLP and B/Group-15-FLP molecules, only the B/N-based FLP-type molecule effectively captures CO, considering both thermodynamics and kinetics. According to the results obtained through energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV), it can be concluded that the donor-acceptor (singlet-singlet) model, rather than the electron-sharing (triplet-triplet) model, effectively characterizes the electronic structures in the CO trapping reaction involving four-membered-ring G13/G15-FLPs. Theoretical findings, derived from EDA-NOCV and frontier molecular orbital theory, demonstrate that the CO capture reaction by G13/G15-FLP involves two distinct bonding interactions. The first interaction is characterized by FLP-to-CO forward bonding, with the lone pair of G15 (G13/G15-FLP) donating to the empty p-π* orbital of carbon (CO), which predominates. The second interaction involves CO-to-FLP backward bonding, where the empty σ* orbital of G13 (G13/G15-FLP) accepts the lone pair of carbon (CO), albeit to a lesser extent. In summary, our theoretical findings indicate that the G13-C and G15-C bonds in the G15/G15-TS species with a four-membered ring can be classified as two dative single bonds. The importance of the interaction between Lewis bases and CO surpasses that of the interaction between Lewis acids and CO. Theoretical evidences in this study demonstrate a linear connection between the G13-G15 bond length within the four-membered-ring G13/G15-FLP and the activation barrier linked to CO capture. The activation strain model analysis in this study suggests that the activation energy required for bond formation primarily depends on the geometric deformation energy of G13/G15-FLP in capturing CO. Our DFT investigation shows that Hammond's postulate is obeyed by the CO catching reaction of the four-membered-ring G13/N-FLP, meaning that an earlier transition state is associated with a lower activation barrier, but not with the CO catching reaction of the four-membered-ring B/G15-FLP.

Frustrated Lewis pair-mediated hydro-dehalogenation: crucial role of non-covalent interactions

CONTEXT

Organic halides stand as invaluable reagents with diverse applications in synthetic chemistry and various industrial processes. Despite their utility, concerns arise due to their inherent toxicity. Addressing these apprehensions, hydro-dehalogenation has emerged as a promising strategy involving the replacement of halogen atoms with hydrogen atoms to transform toxic organic halides into hydrocarbons. This study delves into the computational exploration of hydro-dehalogenation reactions of benzyl halide, mediated by frustrated Lewis pairs (FLPs), using density functional theory (DFT). The reactions entail the formation of FLP1 or FLP2 in the presence of TMP or lutidine with B(C6F5)3, respectively. This is followed by heterolytic cleavage of dihydrogen and subsequent reaction with benzyl halides. Non-covalent interaction analysis underscores the significance of π-π stacking and CH-π interactions in stabilizing transition states. Additionally, the activation strain model (ASM) dissects activation energies, revealing the substantial impact of strain energy on reaction barriers. Energy decomposition analysis (EDA) offers insights into the contributions of electrostatic, orbital, and dispersion energies to the overall attractive interaction energy. The investigation extends to hydro-dehalogenation reactions of ethyl halides, uncovering distinct mechanisms and activation barriers. This comprehensive analysis illuminates the intricacies of hydro-dehalogenation reactions, providing valuable insights into their mechanisms and paving the way for future studies in this field.

METHODS

Geometry optimizations were carried out at the M06-2X/def2-SVP level of theory, which was performed using the Gaussian 16 program. Solvent-corrected single-point energies were also calculated using the polarizable continuum model (PCM) at the PCM(chloroform)-M06-2X/def2-TZVP//M06-2X/def2-SVP level of theory. The Gibbs free energy correction was determined from computations performed at the M06-2X/def2-SVP level of theory. Principal interacting orbital (PIO) analysis was conducted using the NBO 6.0 software. The nature of bonding in the respective transition state (TS) structures was analyzed using atoms-in-molecules (AIM) analyses. Additionally, the presence of non-covalent interactions (NCI) was exemplified using Multiwfn software.

On the mechanism of sp2 C-H borylation using ortho-N-substituted pyridinium cations

ortho-N-Substituted pyridinium cations with the weakly coordinating anion [B(C6F5)4]- have been studied and crucial structural features in the sp2 C-H borylation catalysis of 3-methylthiophene have been identified. The electron-deficiency of the aromatic core of the cation is essential for activity together with accessible protons. The spectroscopic yield of the borylation of 3-methylthiophene with catecholborane (CatBH) was optimized up to 86% and the method was further applied to other substrates such as N-alkylbenzenes. A mechanistic DFT study revealed the rate-limiting step in the catalysis to be the liberation of molecular H2 (ΔG‡ = 27.5 kcal mol-1), whereas the overall reaction was found to be exergonic by 5.1 kcal mol-1.

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.

Theoretical Investigation on Dialumenes toward Dihydrogen Activation: Mechanism and Ligand Effect

Herein, we report a computation study based on the density functional theory calculations to understand the mechanism and ligand effect of the base-stabilized dialumenes toward dihydrogen activation. Among all of the examined modes of dihydrogen activation using the base-stabilized dialumene, we found that the concerted 1,2-hydrogenation of the Al═Al double bond is kinetically more preferable. The concerted 1,2-hydrogenation of the Al═Al double bond adopts an electron-transfer model with certain asynchrony. That is, the initial electron donation from the H-H σ bonding orbital to the empty 3p orbital of the Al1 center is followed by the backdonation from the lone pair electron of the Al2 center to the H-H σ antibonding orbital. Combined with the energy decomposition analysis on the transition states of the concerted 1,2-hydrogenation of the Al═Al double bond and the topographic steric mapping analysis on the free dialumenes, we ascribe the higher reactivity of the aryl-substituted dialumene over the silyl-substituted analogue in dihydrogen activation to the stronger electron-withdrawing effect of the aryl group, which not only increases the flexibility of the Al═Al double bond but also enhances the Lewis acidity of the Al═Al core. Consequently, the aryl-substituted dialumene fragment suffers less geometric deformation, and the orbital interactions between the dialumene and dihydrogen moieties are more attractive during the 1,2-hydrogenation process. Moreover, our calculations also predict that the Al═Al double bond has a good tolerance with the stronger electron-withdrawing group (-CF3) and the weaker σ-donating N-heterocyclic carbene (NHC) analogue (e.g., triazol carbene and NHSi). The reactivity of the dialumene in dihydrogen activation can be further improved by introducing these groups as the supporting ligand and the stabilizing base on the Al═Al core, respectively.

Asymmetric catalysis by chiral FLPs: A computational mini-review

Steric hindrance in Lewis acid (LA) and Lewis base (LB) obstruct the Lewis acid-base adduct formation, and the pair was termed as frustrated Lewis pair (FLP). In the past 16 years, the field of enantioselective catalysis by chiral FLPs has been slowly growing. It was shown that chiral LAs are significant as they are involved in the hydrogen transfer (HT) step to the imine, resulting in enantioselectivity. After H2 activation, the borohydride can exist in a number of plausible conformations and their stability is governed by the presence of noncovalent interaction through C-H····π and π····π interactions. However, LBs are not ideal for asymmetric induction as they compete with the imine substrate as a counter LB. Further, the proton transfer from chiral LB to the imine does not induce any chirality as chirality develops in the HT step. However, intramolecular FLPs with chiral scaffold are very efficient as they possess an optimum distance between LA and LB, which facilitates the H2 activation but precludes the adduct formation of the small molecules substrate with the LA component. This mini-review summarizes computational investigation involving chiral LA and LB, and discusses intramolecular FLPs in the enantioselective catalysis.

Significantly enhanced catalytic performance of Pd nanocatalyst on AlOOH featuring abundant solid surface frustrated Lewis pair for improved hydrogen activation

The catalytic performance of a catalyst is significantly influenced by its ability to activate hydrogen. Constructing frustrated Lewis pairs (FLPs) with the capacity for hydrogen dissociation on non-reducible supports remains a formidable challenge. Herein, we employed a straightforward method to synthesize a layered AlOOH featuring abundant OH defects suitable for constructing solid surface frustrated Lewis pair (ssFLP). The results indicated that the AlOOH-80 (synthesized at 80 °C) possessed an appropriate crystalline structure conducive to generating numerous OH defects, which facilitated the formation of ssFLP. This was further evidenced by the minimal water adsorption in the AlOOH-80, inversely correlated with the quantity of defects in the catalyst. As expected, the Pd loaded onto AlOOH (Pd/AlOOH-80) exhibited excellent catalytic activity in hydrogenation reactions, attributed to abundant defects available for constructing ssFLP. Remarkably, the Pd/AlOOH-80 catalyst, with larger-sized Pd nanoparticles, displayed notably superior activity compared to commercial Pd/Al2O3 and Pd/C, both featuring smaller-sized Pd nanoparticles. Evidently, under the influence of ssFLP, the size effect of Pd nanoparticles did not dominate, highlighting the pivotal role of ssFLP in enhancing catalytic performance. This catalyst also exhibited exceptionally high stability, indicating its potential for industrial applications.

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.

Unraveling Reactivity Pathways: Dihydrogen Activation and Hydrogenation of Multiple Bonds by Pyramidalized Boron-Based Frustrated Lewis Pairs

The activation of H2 by pyramidalized boron-based frustrated Lewis Pairs (FLPs) (B/E-FLP systems where "E" refers to N, P, As, Sb, and Bi) have been explored using density functional theory (DFT) based computational study. The activation pathway for the entire process is accurately characterized through the utilization of the activation strain model (ASM) of reactivity, shedding light on the underlying physical factors governing the process. The study also explores the hydrogenation process of multiple bonds with the help of B/N-FLP. The research findings demonstrate that the liberation of activated dihydrogen occurs in a synchronized, albeit noticeably asynchronous, fashion. The transformation is extensively elucidated using the activation strain model and the energy decomposition analysis. This approach suggests a co-operative double hydrogen-transfer mechanism, where the B-H hydride triggers a nucleophilic attack on the carbon atom of the multiple bonds, succeeded by the migration of the protic N-H.

Triple para-substitution reactions of B(C6F5)3 and [(C6F5)3PF]+ with P(SiMe3)3

para-Substitution reactions on C6F5 rings of Lewis acids have been exploited to achieve triply substituted derivatives. The reaction of B(C6F5)3 with P(SiMe3)3 ultimately affords the Lewis acid B(C6F4P(SiMe3)2)31. This species binds Lewis bases affording the adducts LB(C6F4P(SiMe3)2)3 (L = MeCN 2, OPEt33, PMe34, PBu35) and reacts with LiMe to give the salt [Li][MeB(C6F4P(SiMe3)2)3]·3THF 6. It also reacts with H2O to give (L)B(C6F4PH2)3 (L = H2O 7, MeCN 8). In an analogous fashion, [(C6F5)3PF][B(C6F5)4] was converted to [FP(C6F4P(SiMe3)2)3] [B(C6F5)4] 9 and subsequently to [(MeO)P(C6F4PH2)3][B(C6F5)4] 10.

"Homoleptic" Tetracoordinate Boron Compounds

"Homoleptic" tetracoordinate boron compounds, in which the central boron atom links to four identical atoms, are a special and important family of boron compounds. During the past decades, they have been extensively employed in inorganic, organic, macromolecular, and materials chemistry. Many of them exhibit a diverse range of outstanding properties, and therefore, the synthesis and application of those compounds have emerged as a hot research topic in modern boron chemistry. This review summarizes and discusses the "homoleptic" tetracoordinate boron compounds, which are organized according to the kinds of atoms coordinated to the central boron.

Hexacoordinated tin complexes catalyse imine hydrogenation with H2

Frustrated Lewis pair (FLP) hydrogenation catalysts predominantly use alkyl- and aryl-substituted Lewis acids (LA) that offer a limited number of combinations of substituents, limiting our ability to tune their properties and, ultimately, their reactivity. Nevertheless, main-group complexes have numerous ligands available for such purposes, which could enable us to broaden the range of FLP catalysis. Supporting this hypothesis, we demonstrate here that hexacoordinated tin complexes with Schiff base ligands catalyse imine hydrogenation via activation of H2(g). As shown by hydrogen-deuterium scrambling, [Sn(tBu2Salen)(OTf)2] activated H2(g) at 25 °C and 10 bar of H2. After tuning the ligands, we found that [Sn(Salen)Cl2] was the most efficient imine hydrogenation catalyst despite having the lowest activity in H2(g) activation. Moreover, various imines were hydrogenated in yields up to 98% thereby opening up opportunities for developing novel FLP hydrogenation catalysts based on hexacoordinated LA of main-group elements.

Mechanistic Insight Into the Reactivity of Frustrated Lewis Pairs: Liquid-State NMR Studies

Frustrated Lewis pairs (FLPs) have been widely investigated as promising catalysts due to their metal-free feature and ability to activate small molecules. Over the last few years, the structure, dynamics and interactions between the Lewis centers and their effects on the reactivity with different substrates have been studied. Nuclear magnetic resonance (NMR) is a powerful tool in studying the reaction intermediates, kinetics and mechanism of frustrated Lewis pairs (FLPs). Various NMR experiments have been applied to precisely determine the association or cooperativity of FLPs and one or two-dimensional spectra were obtained. Herein, insights coming from NMR spectroscopy for FLPs are presented, the structure and reactivity of FLPs in solution are described, and their effects on the kinetics and mechanism of different substrates are also illustrated in this review.

Small molecules (CO2 , iPrNCO, and iPrNCNiPr) activation by the metallomimetics (μ-Hydrido) diborane anion: A DFT investigation on mechanism and chemoselectivity controlling

Main-group metallomimetics provide a new way to replace transition metal complexes to activate inert small molecules under mild conditions. In this work, the activation mechanisms of CO2 , iPrNCO, and iPrNCNiPr by (μ-Hydrido) diborane anion ([1H]- ) have been investigated by density functional theory (DFT) calculations. Two different activation sites, BB versus BH bond of [1H]- , are investigated and compared. The results show that these inert molecules can be activated by [1H]- through cycloadditions under mild conditions. The reactions with iPrNCO and iPrNCNiPr are dynamic and thermodynamic controlling, the obtained products are related not only to the energy barrier but also to the stability of the products. Moreover, the competition for BB/BH bond site activation is directly related to the steric effect of small molecules. CO2 , which is without steric hindrance, can only be activated by the BB bond, whereas iPrNCNiPr can only be activated by the BH bond due to the large steric effect. The medium iPrNCO can be activated not only by the BB bond but also by the BH bond. Our study provides theoretical explanations for the reaction activity and chemoselectivity controlling of the title reaction, and displays the potential applications for compounds containing boron-boron bonds and inert small molecule activation.

Asymmetric Transfer Hydrogenation of 2-Substituted Quinoxalines with Regenerable Dihydrophenanthridine

The asymmetric hydrogenation of quinoxalines represents one of the most efficient approaches for the synthesis of optically active tetrahyroquinoxalines. In this paper, we demonstrate a metal-free asymmetric transfer hydrogenation of 2-substituted quinoxalines with regenerable dihydrophenanthridine under H2 using a combination of chiral phosphoric acid and achiral borane as catalysts. A wide range of optically active 2-substituted tetrahydroquinoxalines were produced in high yields with ≤98% ee.

Plant-based calcium silicate from rice husk ash: A green adsorbent for free fatty acid recovery from waste frying oil

Driven by the urgent need for a solution to tackle the surge of rice husk (RH) and waste frying oil (WFO) waste accumulation at a global scale, this report highlights the use of calcium silicates (CS) extracted from acid-pre-treated rice husk ash (RHA) for free fatty acid (FFA) removal from WFO as conventional RHA shows limited FFA adsorption performance. A novel alkaline earth silicate extraction method from acid-pre-treated RHA was outlined. The structural and behavioural attributes of the synthesised CS were identified through BET, SEM-EDS, and XRD analyses and compared to those of RHA. Notable morphology and structural modification were determined, including reducing specific surface areas, mitigating from amorphous to crystalline structure with regular geometric forms, and detecting Si-O-Ca functional groups exclusive to CS adsorbents. A comparison study showed superior lauric acid (LA) adsorption performance by CS absorbents over acid-pre-treated RHA, with a significant increase from 0.0831 ± 0.0004 mmol LA/g to 2.5808 ± 0.0011 mmol LA/g after 60 min. Recognised as the best-performing CS adsorbent, CS-1.0 was used for further investigations on the effect of dosage, LA concentration, and temperature for efficient LA adsorption, with up to 100% LA removal and 5.6712 ± 0.0016 mmol LA/g adsorption capacity. The adsorption isotherm and kinetic studies showed LA adsorption onto CS-1.0 followed Freundlich isotherm with KF = 0.0598 mmol(1-1/n) L(1/n) g-1 & Qe,cal = 3.1696 mmol g-1 and intraparticle diffusion model with kid = 0.1250 mmol g-1 min0.5 & Ci = 0.9625 mmol g-1, indicating rapid initial adsorption and involvement of carboxylate end of LA and the calcium ions on the CS-1.0 in the rate-limiting step. The high equilibrium adsorption capacity and LA adsorption rate indicated that the proposed CS-1.0 adsorbent has excellent potential to recover FFA from WFO effectively.

Frustrated Lewis pairs on pentacoordinated Al3+-enriched Al2O3 promote heterolytic hydrogen activation and hydrogenation

As an emerging class of metal-free catalysts, frustrated Lewis pairs (FLPs) catalysts have been greatly constructed and applied in many fields. Homogeneous FLPs have witnessed significant development, while limited heterogeneous FLPs catalysts are available. Herein, we report that heterogeneous FLPs on pentacoordinated Al3+-enriched Al2O3 readily promote the heterolytic activation of H2 and thus hydrogenation catalysis. The defect-rich Al2O3 was prepared by simple calcination of a carboxylate-containing Al precursor. Combinatorial studies confirmed the presence of rich FLPs on the surface of the defective Al2O3. In contrast to conventional alumina (γ-Al2O3), the FLP-containing Al2O3 can activate H2 in the absence of any transition metal species. More importantly, H2 was activated by surface FLPs in a heterolytic pathway, leading to the hydrogenation of styrene in a stepwise process. This work paves the way for the exploration of more underlying heterogeneous FLPs catalysts and further understanding of accurate active sites and catalytic mechanisms of heterogeneous FLPs at the molecular level.

A review of frustrated Lewis pair enabled monoselective C-F bond activation

Frustrated Lewis pair (FLP) bond activation chemistry has greatly developed over the last two decades since the seminal report of metal-free reversible hydrogen activation. Recently, FLP systems have been utilized to allow monoselective C-F bond activation (at equivalent sites) in polyfluoroalkanes. The problem of 'over-defluorination' in the functionalization of polyfluoroalkanes (where multiple fluoro-positions are uncontrollably functionalized) has been a long-standing chemical problem in fluorocarbon chemistry for over 80 years. FLP mediated monoselective C-F bond activation is complementary to other solutions developed to address 'over-defluorination' and offers several advantages and unique opportunities. This perspective highlights some of these advantages and opportunities and places the development of FLP mediated C-F bond activation into the context of the wider effort to overcome 'over-defluorination'.

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 .

Cooperative small molecule activation by apolar and weakly polar bonds through the lens of a suitable computational protocol

Small molecule activation processes are central in chemical research and cooperativity is a valuable tool for the fine-tuning of the efficiency of these reactions. In this contribution, we discuss recent and remarkable examples in which activation processes are mediated by bimetallic compounds featuring apolar or weakly polar metal-metal bonds. Relevant experimental breakthroughs are thoroughly analyzed from a computational perspective. We highlight how the rational and non-trivial application of selected computational approaches not only allows rationalization of the observed reactivities but also inferring of general principles applicable to activation processes, such as the breakdown of the structure-reactivity relationship in carbon dioxide activation in a cooperative framework. We finally provide a simple yet unbiased computational protocol to study these reactions, which can support experimental advances aimed at expanding the range of applications of apolar and weakly polar bonds as catalysts for small molecule activation.

Metallomimetic C-F Activation Catalysis by Simple Phosphines

Delivering metallomimetic reactivity from simple p-block compounds is highly desirable in the search to replace expensive, scarce precious metals by cheap and abundant elements in catalysis. This contribution demonstrates that metallomimetic catalysis, involving facile redox cycling between the P(III) and P(V) oxidation states, is possible using only simple, cheap, and readily available trialkylphosphines without the need to enforce unusual geometries at phosphorus or use external oxidizing/reducing agents. Hydrodefluorination and aminodefluorination of a range of fluoroarenes was realized with good to very good yields under mild conditions. Experimental and computational mechanistic studies show that the phosphines undergo oxidative addition of the fluoroaromatic substrate via a Meisenheimer-like transition state to form a fluorophosphorane. This undergoes a pseudotransmetalation step with a silane, via initial fluoride transfer from P to Si, to give experimentally observed phosphonium ions. Hydride transfer from a hydridosilicate counterion then leads to a hydridophosphorane, which undergoes reductive elimination of the product to reform the phosphine catalyst. This behavior is analogous to many classical transition-metal-catalyzed reactions and so is a rare example of both functional and mechanistically metallomimetic behavior in catalysis by a main-group element system. Crucially, the reagents used are cheap, readily available commercially, and easy to handle, making these reactions a realistic prospect in a wide range of academic and industrial settings.

The Underlying Catalytic Role of Oxygen Vacancies in Fatty Acid Methyl Esters Ketonization over TiOx Catalysts

Recently, interest in converting bio-derived fatty acid methyl esters (FAMEs) into added-value products has significantly increased. The selectivity of ketonization reaction in the conversion of the FAMEs has significantly hampered the efficiency of this process. Herein, this work reports the preparation of catalysts with different levels of oxygen vacancies while the crystal phase remained unchanged. The catalyst with the highest level of oxygen vacancy exhibited the maximum selectivity. The density functional theory (DFT) simulation showed an increase in interatomic distances leading to the formation of frustrated Lewis pairs (FLPs) upon the creation of oxygen vacancies. The surface measurements, type and density of acid sites of the catalysts, showed that the Lewis acid sites enhanced the selectivity for ketone production; while Bronsted acid sites increased the formation of by-products. Moreover, the ketone formation rate was directly proportional to acid density. The findings of this research provide a different approach for catalyst design, based on defects engineering and their effect on the surface activity, which could be used for enhancing the catalytic performance of novel metal oxides.

Quo Vadis CO2 Activation: Catalytic Reduction of CO2 to Methanol Using Aluminum and Gallium/Carbon-based Ambiphiles

We report on so-called "hidden FLPs" (FLP: frustrated Lewis pair) consisting of a phosphorus ylide featuring a group 13 fragment in the ortho position of a phenyl ring scaffold to form five-membered ring structures. Although the formation of the Lewis acid/base adducts was observed in the solid state, most of the title compounds readily react with carbon dioxide to provide stable insertion products. Strikingly, 0.3-3.0 mol% of the reported aluminum and gallium/carbon-based ambiphiles catalyze the reduction of CO2 to methanol with satisfactory high selectivity and yields using pinacol borane as stoichiometric reduction equivalent. Comprehensive computational studies provided valuable mechanistic insights and shed more light on activity differences.

Metal-Free Heterogeneous Asymmetric Hydrogenation of Olefins Promoted by Chiral Frustrated Lewis Pair Framework

The development of metal-free and recyclable catalysts for significant yet challenging transformations of naturally abundant feedstocks has long been sought after. In this work, we contribute a general strategy of combining the rationally designed crystalline covalent organic framework (COF) with a newly developed chiral frustrated Lewis pair (CFLP) to afford chiral frustrated Lewis pair framework (CFLPF), which can efficiently promote the asymmetric olefin hydrogenation in a heterogeneous manner, outperforming the homogeneous CFLP counterpart. Notably, the metal-free CFLPF exhibits superior activity/enantioselectivity in addition to excellent stability/recyclability. A series of in situ spectroscopic studies, kinetic isotope effect measurements, and density-functional theory computational calculations were also performed to gain an insightful understanding of the superior asymmetric hydrogenation catalysis performances of CFLPF. Our work not only increases the versatility of catalysts for asymmetric catalysis but also broadens the reactivity of porous organic materials with the addition of frustrated Lewis pair (FLP) chemistry, thereby suggesting a new approach for practical and substantial transformations through the advancement of novel catalysts from both concept and design perspectives.

Mechanistic Study of B(C6F5)3-Catalyzed Transfer Hydrogenation of Aldehydes/Ketones with PhSiH3 and Stoichiometric Water

A DFT study was performed on the mechanisms of B(C6F5)3-catalyzed transfer hydrogenation of aldehydes/ketones, using PhSiH3 and stoichiometric water. Path B2 includes a stepwise Piers SN2-Si process, H- transfer, and hydrolysis desilylation of siloxane, in which the hydrolysis desilylation step is rate-determining. Path C1 is first determined, involving a B(C6F5)3-catalyzed concerted addition step of 2H2O to carbonyl generating R1R2C(OH)2, a subsequent SN2-Si dehydroxylation step of R1R2C(OH)2 giving R1R2C=OH+ and (C6F5)3B-H-, and final H- transfer producing the respective alcohol R1R2CHOH. A B(C6F5)3-catalyzed H2 generation process (Path H0) is determined. Path B2 is the only mechanism for the stepwise method. Using a one-time one-pot feeding method, alkyl/aryl aldehydes, dialkyl ketones, and alkyl aryl ketones (1a-g) can be reduced into alcohols chemoselectively and effectively at room temperature. More than 1 equiv of water over substrates is necessary. Herein, Path C1 is the dominant transfer hydrogenation pathway, and the H2 generation is efficiently inhibited, by the competitive advantage of Path C1 and initial dominant existence of the complexes IM0 and IM1-x. The diaryl ketones (1h,1i) cannot be efficiently reduced into the respective alcohols using the one-time feeding one-pot method. The barriers of C-TS1-h/i are obviously higher than those of C-TS1-a-g, attributed to the electron-donating and space effects of the two aryls on carbonyl C. The possible Paths B2 and C1 of transfer hydrogenation have no competitive advantage with Path H0. The DFT results are consistent with the experiments.

Catalyst-Free Transfer Hydrogenation from Amine-Borane Small Oligomers

Amine-borane dimers and oligomers with varied steric and electronic profiles were prepared via capping agent-controlled AA/BB polycondensations. They were used for transfer hydrogenations to aldehydes, ketones, imines as well as electron-poor alkene/alkyne moieties. The amine-borane Lewis-paired oligomers and the congested bis(amine-borane)s provided the highest yields. This was likely helped by facilitated dissociation (oligomers) or H-bond assistance. In the case of the oligomers, the second equivalent of H2 present was also engaged in the reaction. Solid-state NMR characterization provides evidence that the boron-containing materials obtained after transfer dehydrogenation are highly similar to those obtained from thermal dehydrogenation. The oligomers bridge the gap between simple amine-borane molecular reductants and the poly-amine-boranes and provide a full picture of the reactivity changes at the different scales.

A crystalline aluminium-carbon-based ambiphile capable of activation and catalytic transfer of ammonia in non-aqueous media

Despite recent achievements in the field of frustrated Lewis pairs (FLPs) for small molecule activations, the reversible activation and catalytic transformations of N-H-activated ammonia remain a challenge. Here we report on a rare combination of an aluminium Lewis acid and a carbon Lewis base. A so-called hidden FLP consisting of a phosphorus ylide featuring an aluminium fragment in the ortho position of a phenyl ring scaffold is introduced. Although the formation of the Lewis acid/base adduct is observed in the solid state, which at first glance leads to formally quenched FLP reactivity, we show that the title compound readily reacts with non-aqueous ammonia thermoneutrally and splits the N-H bond reversibly at ambient temperature. In addition, NH3 transfer reactions mediated by a main-group catalyst are presented. This proof-of-principle study is expected to initiate further activities in utilizing N-H-activated ammonia as a readily available, atom-economical nitrogen source.

Halide-Coupled Double Electron Transfer with Electron-Rich Diboranes

The ditriflato-diborane B2 (μ-hpp)2 (OTf)2 (hpp=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate) acts as a stable surrogate of the elusive dication [B2 (hpp)2 ]2+ , being both electrophilic (vacant boron p orbitals) and nucleophilic (filled B-B bond orbital). This combination of seemingly contrasting behaviors could be used to develop a metallomimetic diborane chemistry, with Lewis σ-basic and π-acidic substrates being bound and reduced at the diborane. Here, we report on a novel reaction type within this general theme, in which double electron transfer from the diboron unit to the boron-bound organic substrate is coupled with halide transfer in the other direction. Novel diborylated dienamines are synthesized in this way. The scope of this unprecedented reaction motif and the reaction pathways are elucidated.

An orbitally adapted push-pull template for N2 activation and reduction to diazene-diide

A Lewis superacidic bis(borane) C6F4{B(C6F5)2}2 was reacted with tungsten N2-complexes [W(N2)2(R2PCH2CH2PR2)2] (R = Ph or Et), affording zwitterionic boryldiazenido W(ii) complexes trans-[W(L)(R2PCH2CH2PR2)2(N2{B(C6F5)2(C6F4B(C6F5)3})] (L = ø, N2 or THF). These compounds feature only one N-B linkage of the covalent type, as a result of intramolecular boron-to-boron C6F5 transfer. Complex trans-[W(THF)(Et2PCH2CH2PEt2)2(N2{B(C6F5)2C6F4B(C6F5)3})] (5) was shown to split H2, leading to a seven-coordinate complex [W(H)2(Et2PCH2CH2PEt2)2(N2{B(C6F5)2}2C6F4)] (7). Interestingly, hydride storage at the metal triggers backward C6F5 transfer. This reverts the bis(boron) moiety to its bis(borane) state, now doubly binding the distal N, with structural parameters and DFT computations pointing to dative N→B bonding. By comparison with an N2 complex [W(H)2(Et2PCH2CH2PEt2)2(N2{B(C6F5)3}] (10) differing only in the Lewis acid (LA), namely B(C6F5)3, coordinated to the distal N, we demonstrate that two-fold LA coordination imparts strong N2 activation up to the diazene-diide (N22-) state. To the best of our knowledge, this is the first example of a neutral LA coordination that induces reduction of N2.

Stereoselective Hydrosilylation of 1,2-Diketones Catalyzed by Chiral Frustrated Lewis Pairs

The asymmetric reduction of 1,2-diketones for the synthesis of optically active 1,2-diols, especially 1,2-anti-diols, remains a formidable challenge. In this paper, we describe the first highly stereoselective hydrosilylation of unsymmetrical vicinal diketones with PhSiH3 by using a chiral frustrated Lewis pair (FLP) catalyst, giving a variety of 1,2-diaryl-1,2-anti-diols in high yields with excellent d.r. values and up to 97 % ee. The chiral FLP catalyst exhibits the ability to control regio-, diastereo- and enantioselectivites concurrently.

Homolytic C-H Bond Activation by Phosphine-Quinone-Based Radical Ion Pairs

Herein, we present the formation of transient radical ion pairs (RIPs) by single-electron transfer (SET) in phosphine-quinone systems and explore their potential for the activation of C-H bonds. PMes3 (Mes=2,4,6-Me3 C6 H2 ) reacts with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) with formation of the P-O bonded zwitterionic adduct Mes3 P-DDQ (1), while the reaction with the sterically more crowded PTip3 (Tip=2,4,6-iPr3 C6 H2 ) afforded C-H bond activation product Tip2 P(H)(2-[CMe2 (DDQ)]-4,6-iPr2 -C6 H2 ) (2). UV/Vis and EPR spectroscopic studies showed that the latter reaction proceeds via initial SET, forming RIP [PTip3 ]⋅+ [DDQ]⋅- , and subsequent homolytic C-H bond activation, which was supported by DFT calculations. The isolation of analogous products, Tip2 P(H)(2-[CMe2 {TCQ-B(C6 F5 )3 }]-4,6-iPr2 -C6 H2 ) (4, TCQ=tetrachloro-1,4-benzoquinone) and Tip2 P(H)(2-[CMe2 {oQtBu -B(C6 F5 )3 }]-4,6-iPr2 -C6 H2 ) (8, oQtBu =3,5-di-tert-butyl-1,2-benzoquinone), from reactions of PTip3 with Lewis-acid activated quinones, TCQ-B(C6 F5 )3 and oQtBu -B(C6 F5 )3 , respectively, further supports the proposed radical mechanism. As such, this study presents key mechanistic insights into the homolytic C-H bond activation by the synergistic action of radical ion pairs.

Frustrated Lewis Pair Chelation in the p-Block

Frustrated Lewis pairs (FLPs) have been the subject of considerable study since the field's inception. While much of the research into FLPs has centered around small molecule activation for diverse stoichiometric and catalytic transformations, intramolecular FLPs also show promise as chelating ligands. The cooperative action of Lewis basic and acidic moieties enables intramolecular FLPs to stabilize low oxidation state centers and (consequently) reactive molecular fragments through a donor-acceptor approach, making them an attractive ligand class in main group element chemistry. This review outlines the state of FLP chelation to date throughout the p-block, encompassing primarily groups 13-16.

Asymmetric catalysis with FLPs

Chiral catalysts play a crucial role in the realm of asymmetric catalysis. Since their breakthrough discovery in 2006, chiral frustrated Lewis pairs (FLPs) have risen as a novel catalyst category for a broad range of metal-free asymmetric reactions. This review provides an overview of the remarkable progress made in this field over the past 15 years. The design and synthesis of chiral FLPs and their applications in hydrogenation, hydrosilylation, transfer hydrogenation, and various other reactions are summarized and highlighted.

Donor-Acceptor Activation of Carbon Dioxide

The activation and functionalization of carbon dioxide entails great interest related to its abundance, low toxicity and associated environmental problems. However, the inertness of CO2 has posed a challenge towards its efficient conversion to added-value products. In this review we discuss one of the strategies that have been widely used to capture and activate carbon dioxide, namely the use of donor-acceptor interactions by partnering a Lewis acidic and a Lewis basic fragment. This type of CO2 activation resembles that found in metalloenzymes, whose outstanding performance in catalytically transforming carbon dioxide encourages further bioinspired research. We have divided this review into three general sections based on the nature of the active sites: metal-free examples (mainly formed by frustrated Lewis pairs), main group-transition metal combinations, and transition metal heterobimetallic complexes. Overall, we discuss one hundred compounds that cooperatively activate carbon dioxide by donor-acceptor interactions, revealing a wide range of structural motifs.

Aluminum(III) Cations [(NHC) ⋅ AlMes2 ]+ : Synthesis, Characterization, and Application in FLP-Chemistry

The three-coordinate aluminum cations ligated by N-heterocyclic carbenes (NHCs) [(NHC) ⋅ AlMes2 ]+ [B(C6 F5 )4 ]- (NHC=IMeMe 4, IiPrMe 5, IiPr 6, Mes=2,4,6-trimethylphenyl) were prepared via hydride abstraction of the alanes (NHC) ⋅ AlHMes2 (NHC=IMeMe 1, IiPrMe 2, IiPr 3) using [Ph3 C]+ [B(C6 F5 )4 ]- in toluene as hydride acceptor. If this reaction was performed in diethyl ether, the corresponding four-coordinate aluminum etherate cations [(NHC) ⋅ AlMes2 (OEt2 )]+ [B(C6 F5 )4 ]- 7-9 (NHC=IMeMe 7, IiPrMe 8, IiPr 9) were isolated. According to a theoretical and experimental assessment of the Lewis-acidity of the [(IMeMe ) ⋅ AlMes2 ]+ cation is the acidity larger than that of B(C6 F5 )3 and of similar magnitude as reported for Al(C6 F5 )3 . The reaction of [(IMeMe ) ⋅ AlMes2 ]+ [B(C6 F5 )4 ]- 4 with the sterically less demanding, basic phosphine PMe3 afforded a mixed NHC/phosphine stabilized cation [(IMeMe ) ⋅ AlMes2 (PMe3 )]+ [B(C6 F5 )4 ]- 10. Equimolar mixtures of 4 and the sterically more demanding PCy3 gave a frustrated Lewis-pair (FLP), i.e., [(IMeMe ) ⋅ AlMes2 ]+ [B(C6 F5 )4 ]- /PCy3 FLP-11, which reacts with small molecules such as CO2 , ethene, and 2-butyne.