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.

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.

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.

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.

A Facile Preparation of N-Heterocyclic Olefins: Ring-Opening Polymerization of ß-Butyrolactone and Frustrated Lewis Pair Reactivity.

A direct synthesis of N-heterocyclic olefins (NHOs) and their mesoionic congeners (mNHOs) from N-heterocyclic carbenes and N-aziridinylimines is reported. The reaction provided diverse functionalized (m)NHOs and π-extended analogues. The prepared NHOs initiated the ring-opening polymerization of ß-butyrolactone, and insertion of aldehyde and nitrile into an NHO-B(C6 F5 )3 adduct was demonstrated.

Fluorination/Dearomatization of C6F5 Groups: An FLP Route to an Electrophilic Borane and Non-Coordinating Anions.

B(C6F5)3 and the corresponding anion [B(C6F5)4]- are ubiquitous in main group and transition metal chemistry. Known derivatives are generally limited to the incorporation of electron donating substituents. Herein we describe electrophilic fluorination and dearomatization of such species using XeF2 in the presence of BF3 or Lewis acidic cations. In this fashion the anions [HB(C6F5)3]-, [B(C6F5)4]- and [(C6F5)3BC≡NB(C6F5)3]-, are converted to [FB(C6F7)3]-, [B(C6F7)4]-, and [(C6F7)3BC≡NB(C6F7)3]-, respectively. Similarly, the borane adducts (L)B(C6F7)3 (L=MeCN, OPEt3) are produced. These rare examples of electrophilic attack of electron deficient rings proceed as [XeF][BF4] acts as a frustrated Lewis pair effecting fluorination and dearomatization of C6F5 rings.

Complexes of a Frustrated Lewis Pair-Supported P(-1) Ligand.

Several complexes of the intramolecular frustrated Lewis pair (FLP)-supported P(-1) ligand [iPr2P(C6H4)BCy2{P}]- are presented (Cy = cyclohexyl). Chief among these is the first example of a monomeric zinc bis(phosphido) complex, which was synthesized as a potential precursor for the solution-phase deposition of Zn3P2. While this goal was ultimately unsuccessful, the Zn(II) complex acts as a convenient springboard to other metal phosphide species via transmetallation: namely, a tellurium bis(phosphido) and a formal adduct of the phosphorus subhalide PPCl2. Trapping experiments show that the PPCl2 adduct can also be prepared directly through the in situ reduction of PCl3 in the presence of an intramolecular FLP ligand. Lastly, we report a formal η2-phosphaborene complex of cobalt(-1) which is isoelectronic to olefin complexes, and explore its bonding via density functional theory (DFT) computations.

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.

Unveiling the Obscure Potential of o-carborane Based IFLPs for CO2 Sequestration.

Sequestering carbon dioxide (CO2) from the atmosphere is necessary to achieve a sustainable environment. The emergence of frustrated Lewis pairs (FLPs) has opened a new dimension in the development of organocatalysts for CO2 sequestration. To date, various FLPs have been developed for CO2 sequestration, yet the quest for robust FLPs continues. Based on the intriguing electronic effects of the carborane polyhedral, o-carboranes can be projected as a versatile bridging unit for intramolecular FLPs (IFLPs). In the present work, o-carborane based IFLPs AlP, BP, AlN and BN have been proposed for the activation of CO2 molecule. The density functional theory (DFT) based calculations and thorough orbital analysis have been carried out to extensively study the electronic structure of the o-carborane unit. The proposed IFLPs were systematically compared with their corresponding phenyl bridged analogues to assess the effect of o-carborane bridging unit on the reactivity of the acidic and basic sites. The results show that the o-carborane supported IFLPs are more reactive towards CO2 than the phenyl bridged IFLPs. Also, placing the basic site on the B atom at the 4th position of the o-carborane bridge rather than the C atom at the 2nd position results in more reactive IFLPs.

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.

Recent Advances in Asymmetric Catalysis Using p-Block Elements.

The development of new methods for enantioselective reactions that generate stereogenic centres within molecules are a cornerstone of organic synthesis. Typically, metal catalysts bearing chiral ligands as well as chiral organocatalysts have been employed for the enantioselective synthesis of organic compounds. In this review, we highlight the recent advances in main group catalysis for enantioselective reactions using the p-block elements (boron, aluminium, phosphorus, bismuth) as a complementary and sustainable approach to generate chiral molecules. Several of these catalysts benefit in terms of high abundance, low toxicity, high selectivity, and excellent reactivity. This minireview summarises the utilisation of chiral p-block element catalysts for asymmetric reactions to generate value-added compounds.

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.

Finding Natural, Dense, and Stable Frustrated Lewis Pairs on Wurtzite Crystal Surfaces for Small-Molecule Activation.

The surface frustrated Lewis pairs (SFLPs) open up new opportunities for substituting noble metals in the activation and conversion of stable molecules. However, the applications of SFLPs on a larger scale are impeded by the complex construction process, low surface density, and sensitivity to the reaction environment. Herein, wurtzite-structured crystals such as GaN, ZnO, and AlP are found for developing natural, dense, and stable SFLPs. It is revealed that the SFLPs can naturally exist on the (100) and (110) surfaces of wurtzite-structured crystals. All the surface cations and anions serve as the Lewis acid and Lewis base in SFLPs, respectively, contributing to the surface density of SFLPs as high as 7.26×1014 cm-2. Ab initio molecular dynamics simulations indicate that the SFLPs can keep stable under high temperatures and the reaction atmospheres of CO and H2O. Moreover, outstanding performance for activating the given small molecules is achieved on these natural SFLPs, which originates from the optimal orbital overlap between SFLPs and small molecules. Overall, these findings not only provide a simple method to obtain dense and stable SFLPs but also unfold the nature of SFLPs toward the facile activation of small molecules.

Construction of Frustrated Lewis Pairs in Poly(heptazine Imide) Nanosheets via Hydrogen Bonds for Boosting CO2 Photoreduction.

The creation of frustrated Lewis pairs on catalyst surface is an effective strategy for tuning CO2 activation. The critical step in the formation of frustrated Lewis pairs is the spatial effect of proximal Lewis acid-Lewis base pairs. Here, we demonstrate a facile surface functionalization methodology that enables hydrogen bonding between N and H atoms to mediate the construction of frustrated Lewis pairs in poly(heptazine imide), thereby increasing the propensity to activate CO2 molecules. Experimental and theoretical results show that the construction of active hydrogen bonding regions can facilitate the bending of CO2 molecules. Furthermore, the delocalization of electron clouds induced by the hydrogen bonding-mediated frustrated Lewis pairs can promote the heterolytic cleavage and photocatalytic conversion of CO2. This work highlights the potential of utilizing hydrogen bonding-mediated strategy in heterogeneously photocatalytic activation of CO2 over polymer materials.

Transition Metal-Free Catalytic C-H Zincation and Alumination.

C-H metalation is the most efficient method to prepare aryl-zinc and -aluminium complexes that are ubiquitous nucleophiles. Virtually all C-H metalation routes to form Al/Zn organometallics require stoichiometric, strong Brønsted bases with no base-catalyzed reactions reported. Herein we present a catalytic in amine/ammonium salt (Et3N/[(Et3N)H]+) C-H metalation process to form aryl-zinc and aryl-aluminium complexes. Key to this approach is coupling an endergonic C-H metalation step with a sufficiently exergonic dehydrocoupling step between the ammonium salt by-product of C-H metalation ([(Et3N)H]+) and a Zn-H or Al-Me containing complex. This step, forming H2/MeH, makes the overall cycle exergonic while generating more of the reactive metal electrophile. Mechanistic studies supported by DFT calculations revealed metal-specific dehydrocoupling pathways, with the divergent reactivity due to the different metal valency (which impacts the accessibility of amine-free cationic metal complexes) and steric environment. Notably, dehydrocoupling in the zinc system proceeds through a ligand-mediated pathway involving protonation of the ß-diketiminate Cγ position. Given this process is applicable to two disparate metals (Zn and Al), other main group metals and ligand sets are expected to be amenable to this transition metal-free, catalytic C-H metalation.

Reactivity and Steric Parameters from 2D to 3D Bulky Pyridines: Increasing Steric Demand at Nitrogen with Chiral Azatriptycenes.

Sterically hindered pyridines embedded in a three-dimensional triptycene framework have been synthesized, and their resolution by chiral HPLC enabled access to unprecedented enantiopure pyridines exceeding the known steric limits. The design principles for new axially chiral pyridine derivatives are then described. To rationalize their associations with Lewis acids and transition metals, a comprehensive determination of the steric and electronic parameters for this new class of pyridines was performed. This led to the general parameterization of the steric parameters (percent buried volume %VBur, Tolman cone angle θ, and He8_steric descriptor) for a large set of two- and three-dimensional pyridine derivatives. These parameters are shown to describe quantitatively their interactions with carbon- and boron-centered Lewis acids and were used to predict the ΔG° of association with the prototypical B(C6F5)3 Lewis acid widely used in frustrated Lewis pair catalysis. This first parameterization of pyridine sterics is a fundamental basis for the future development of predictive reactivity models and for guiding new applications of bulky and chiral pyridines in organocatalysis, frustrated Lewis pairs, and transition-metal catalysis.

Metal-Free Catalytic Hydrogenolysis of Chlorosilanes into Hydrosilanes with "Inverse" Frustrated Lewis Pairs.

The challenging metal-free catalytic hydrogenolysis of silyl chlorides to hydrosilanes is unlocked by using an inverse frustrated Lewis pair (FLP), combining a mild Lewis acid (Cy2 BCl) and a strong phosphazene base (BTPP) in mild conditions (10 bar of H2 , r. t.). In the presence of a stoichiometric amount of the base, the hydrosilanes R3 SiH (R=Me, Et, Ph) are generated in moderate to high yields (up to 95 %) from their chlorinated counterparts. A selective formation of the valuable difunctional monohydride Me2 SiHCl is also obtained from Me2 SiCl2 . A mechanism is proposed based on stoichiometric experiments and DFT calculations; it highlights the critical role of borohydride species generated by the heterolytic splitting of H2 .

Differences in the Reactivity of Geminal Si-O-P and Al-O-P Frustrated Lewis Pairs.

The new oxygen-bridged geminal Si/P Frustrated Lewis Pair (FLP) tBu2 P-O-Si(C2 F5 )3 (2) is able to reversibly bind carbon dioxide at ambient temperature. We compared its reactivity towards benzil, but-3-en-2-one, nitriles and phenylacetylene to that of the Al/P FLP tBu2 P-O-AlBis2 (Bis=-CH(SiMe3 )2 ) (1). When reacted with benzil, both, 1 and 2, form the 1,2-addition product, but in the Si/P FLP 2, the second carbonyl function additionally binds to the silicon atom. With but-3-en-2-one 2 forms the 1,2-addition product, while 1 binds in 1,4-position. The reaction with acetonitrile yielded an unexpected etheneimine adduct for both systems, while only 1 reacted with tert-butylnitrile. With benzonitrile and acrylonitrile, 2 showed reversible addition to the C≡N bond and 1 forms a stable adduct with benzonitrile. Solely 1 shows reactivity towards phenylacetylene affording a mixture of the CH deprotonation adduct tBu2 P(H)-O-AlBis2 (CCPh) and the FLP -C≡C 1,2-addition adduct under ring formation. All compounds were characterized by multinuclear NMR spectroscopy, XRD and elemental analysis.

Assessing Combinations of B(C6 F5 )3 and N2 -Derived Molybdenum Nitrido Complexes for Heterolytic Bond Activation.

Two different dinitrogen-derived molybdenum nitrido complexes varying by their geometry, ligand spheres and oxidations states were shown to engage their N ligand in dative bonding with the strong Lewis acid B(C6 F5 )3 . The stable adducts were assessed for frustrated Lewis pair-type heterolytic E-H bond splitting of hydrosilanes (E=Si) and HB(C6 F5 )2 . Whereas Si-H bond activation was achieved, HB(C6 F5 )2 was shown to substitute B(C6 F5 )3 in a quantitative or equilibrated fashion, depending on the nature of the nitrido complex. No B-H bond splitting was observed. Thermodynamics of these reactions, computed by DFT, are in agreement with the experimental outcomes.

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.