Reactivity of a Frustrated Lewis Pair and Small-Molecule Activation by an Isolable Arduengo CarbeneB{3,5-(CF3)2C6H3}3Complex

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.

Single electron reduction of NHC-CO2-borane compounds

The carbon dioxide radical anion [CO2˙-] is a highly reactive species of fundamental and synthetic interest. However, the direct one-electron reduction of CO2 to generate [CO2˙-] occurs at very negative reduction potentials, which is often a limiting factor for applications. Here, we show that NHC-CO2-BR3 species - generated from the Frustrated Lewis Pair (FLP)-type activation of CO2 by N-heterocyclic carbenes (NHCs) and boranes (BR3) - undergo single electron reduction at a less negative potential than free CO2. A net gain of more than one volt was notably measured with a CAAC-CO2-B(C6F5)3 adduct, which was chemically reduced to afford [CAAC-CO2-B(C6F5)3˙-]. This room temperature stable radical anion was characterized by EPR spectroscopy and by single-crystal X-ray diffraction analysis. Of particular interest, DFT calculations showed that, thanks to the electron withdrawing properties of the Lewis acid, significant unpaired spin density is localised on the carbon atom of the CO2 moiety. Finally, these species were shown to exhibit analogous reactivity to the carbon dioxide radical anion [CO2˙-] toward DMPO. This work demonstrates the advantage provided by FLP systems in the generation and stabilization of [CO2˙-]-like species.

Activation of SF5CF3 by the N-Heterocyclic Carbene SIMes

The greenhouse gas SF5CF3 was photochemically activated with SIMes (1,3-Bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene) to give 1,3-dimesityl-2,2-difluoroimidazolidine (SIMesF2), and 1,3-dimesitylimidazolidine-2-sulfide, as well as the trifluoromethylated carbene derivative 1,3-dimesityl-2-fluoro-2-trifluoromethylimidazolidine. CF3 radicals, as well as SF4, serve presumably as intermediates of the conversions. In addition, the photochemical activation of SF5CF3 was performed in the presence of triphenylphosphine. The formation of triphenyldifluorophosphorane and triphenylphosphine sulfide was observed.

Activation of H-H, HO-H, C(sp2)-H, C(sp3)-H, and RO-H Bonds by Transition-Metal Frustrated Lewis Pairs Based on M/N (M = Rh, Ir) Couples

Reaction of the dimers [(Cp*MCl)₂(μ-Cl)₂] (Cp* = η⁵-C₅Me₅) with Ph₂PCH₂CH₂NC(NH(p-Tolyl))₂ (H₂L) in the presence of NaSbF₆ affords the chlorido complexes [Cp*MCl(κ²N,P-H₂L)][SbF₆] (M = Rh, 1; Ir, 2). Upon treatment with aqueous NaOH, solutions of 1 and 2 yield the corresponding complexes [Cp*M(κ³N,N′,P-HL)][SbF₆] (M = Rh, 3; Ir, 4) in which the ligand HL presents a fac κ³N,N′,P coordination mode. Treatment of THF solutions of complexes 3 and 4 with hydrogen gas, at room temperature, results in the formation of the metal hydrido-complexes [Cp*MH(κ²N,P-H₂L)][SbF₆] (M = Rh, 5; Ir, 6) in which the N(p-Tolyl) group has been protonated. Complexes 3 and 4 react with deuterated water in a reversible fashion resulting in the gradual deuteration of the Cp* group. Heating at 383 K THF/H₂O solutions of the complexes 3 and 4 affords the orthometalated complexes [Cp*M(κ³C,N,P-H₂L_-H)][SbF₆] [M = Rh, 7; Ir, 8, H₂L_-H = Ph₂PCH₂CH₂NC(NH(p-Tolyl))(NH(4-C₆H₃Me))], respectively. At 333 K, complexes 3 and 4 react in THF with methanol, primary alcohols, or 2-propanol giving the metal-hydrido complexes 5 and 6, respectively. The reaction involves the acceptorless dehydrogenation of the alcohols at a relatively low temperature, without the assistance of an external base. The new complexes have been characterized by the usual analytical and spectroscopic methods including the X-ray diffraction determination of the crystal structures of complexes 1–5, 7, and 8. Notably, the chlorido complexes 1 and 2 crystallize both as enantiopure conglomerates and as racemates. Reaction mechanisms are proposed based on stoichiometric reactions, nuclear magnetic resonance studies, and X-ray crystallography as well as density functional theory calculations.

In solution, masked transition-metal frustrated Lewis pairs (TMFLPs) give rise to the corresponding TMFLP species which activate dihydrogen, water, and alcohols following FLP reaction pathways. When D₂O or alcohols with deuterated OH groups were employed, H/D exchange at the Cp* ligand (involving C(sp³)−H activation) was observed. C(sp²)−H bond activation involving orthometalation of the p-Tolyl ring was also observed.

Geometrical influence on the non-biomimetic heterolytic splitting of H2 by bio-inspired [FeFe]-hydrogenase complexes: a rare example of inverted frustrated Lewis pair based reactivity

Despite the high levels of interest in the synthesis of bio-inspired [FeFe]-hydrogenase complexes, H2 oxidation, which is one specific aspect of hydrogenase enzymatic activity, is not observed for most reported complexes. To attempt H-H bond cleavage, two disubstituted diiron dithiolate complexes in the form of [Fe2(μ-pdt)L2(CO)4] (L: PMe3, dmpe) have been used to play the non-biomimetic role of a Lewis base, with frustrated Lewis pairs (FLPs) formed in the presence of B(C6F5)3 Lewis acid. These unprecedented FLPs, based on the bimetallic Lewis base partner, allow the heterolytic splitting of the H2 molecule, forming a protonated diiron cation and hydrido-borate anion. The substitution, symmetrical or asymmetrical, of two phosphine ligands at the diiron dithiolate core induces a strong difference in the H2 bond cleavage abilities, with the FLP based on the first complex being more efficient than the second. DFT investigations examined the different mechanistic pathways involving each accessible isomer and rationalized the experimental findings. One of the main DFT results highlights that the iron site acting as a Lewis base for the asymmetrical complex is the {Fe(CO)3} subunit, which is less electron-rich than the {FeL(CO)2} site of the symmetrical complex, diminishing the reactivity towards H2. Calculations relating to the different mechanistic pathways revealed the presence of a terminal hydride intermediate at the apical site of a rotated {Fe(CO)3} site, which is experimentally observed, and a semi-bridging hydride intermediate from H2 activation at the Fe-Fe site; these are responsible for a favourable back-reaction, reducing the conversion yield observed in the case of the asymmetrical complex. The use of two equivalents of Lewis acid allows for more complete and faster H2 bond cleavage due to the encapsulation of the hydrido-borate species by a second borane, favouring the reactivity of each FLP, in agreement with DFT calculations.

A masked form of an O-borylated Breslow intermediate for the diastereoselective FLP-type activation of aldehydes

Breslow intermediates are very often elusive species whose application in Frustrated Lewis Pair chemistry is unprecedented. We describe herein the use of a masked form of an O-Borylated Breslow (OBB) intermediate that performs FLP-type activation of the carbonyl function of five different benzaldehyde derivatives with complete diastereoselectivity. The resulting compounds are characterised in solution by NMR spectroscopy (compounds 4 - 8 ) and in solid state by X-Ray diffraction analysis (compounds 4 - 6 ). A combined kinetic and theoretical investigation reveals the associative nature of the rate determining step and suggests that the OBB intermediate part is never released during the whole process.

A boron-transfer mechanism mediating the thermally induced revival of frustrated carbene-borane pairs from their shelf-stable adducts

Chemists have designed strategies that trigger the conformational isomerization of molecules in response to external stimuli, which can be further applied to regulate the complexation between Lewis acids and bases. We have recently developed a system in which frustrated carbene-borane pairs are revived from shelf-stable but external-stimuli-responsive carbene-borane adducts comprised of N-phosphine-oxide-substituted imidazolylidenes (PoxIms) and triarylboranes. Herein, we report the detailed mechanism on this revival process. A thermally induced borane-transfer process from the carbene carbon atom to the N-phosphinoyl oxygen atom initiates the transformation of the carbene-borane adduct. Subsequent conformational isomerization via the rotation of the N-phosphinoyl group in PoxIm moieties eventually leads to the revival of frustrated carbene-borane pairs that can cleave H₂. We believe that this work illustrates an essential role of dynamic conformational isomerization in the regulation of the reactivity of external-stimuli-responsive Lewis acid-base adducts that contain multifunctional substituents.

Progressing the Frustrated Lewis Pair Abilities of N-Heterocyclic Carbene/GaR3 Combinations for Catalytic Hydroboration of Aldehydes and Ketones

Exploiting the steric incompatibility of the tris(alkyl)gallium GaR₃ (R = CH₂SiMe₃) and the bulky N-heterocyclic carbene (NHC) 1,3-bis(tert-butyl)imidazol-2-ylidene (ItBu), here we report the B-H bond activation of pinacolborane (HBPin), which has led to the isolation and structural authentication of a novel ion pair, [{ItBu-BPin}⁺{GaR₃(μ-H)GaR₃}^-] (2). Contrastingly, neither ItBu or GaR₃ was able to react with HBPin under the conditions of this study. Combining an NHC-stabilized borenium cation, [{ItBu-BPin}⁺], with an anionic dinuclear gallate, [{GaR₃(μ-H)GaR₃}^-], 2 proved to be unstable in solution at room temperature, evolving to the abnormal NHC-Ga complex [BPinC{{N(tBu)]₂CHCGa(R)₃}] (3). Interestingly, the structural isomer of 2, with the borenium cation residing at the C4 position of the carbene, [{aItBu-BPin}⁺{GaR₃(μ-H)GaR₃}^-] (4), was obtained when the abnormal NHC complex [aItBu·GaR₃] (1) was heated to 70 °C with HBPin, demonstrating that, under these forced conditions, it is possible to induce thermal frustration of the Lewis base/Lewis acid components of 1, enabling the activation of HBPin. Building on these stoichiometric studies, the frustrated Lewis pair (FLP) reactivity observed for the GaR₃/ItBu combination with HBPin could then be upgraded to catalytic regimes, allowing the efficient hydroboration of a range of aldehydes and ketones under mild reaction conditions. Mechanistic insights into the possible reaction pathway involved in this process have been gained by combining kinetic investigations with a comparative study of the catalytic capabilities of several gallium and borenium species related to 2. Disclosing a new cooperative partnership, reactions are proposed to occur via the formation of a highly reactive monomeric hydride gallate, [{ItBu-BPin}⁺{GaR₃(H)}^-] (I). Each anionic and cationic component of I plays a key role for success of the hydroboration, with the nucleophilic monomeric gallate anion favoring the transfer of its hydride to the C═O bond of the organic substate, which in turn is activated by coordination to the borenium cation.

Si(II) Cation-Promoted Formation of an Abnormal NHC-Bound Silylene and a cAAC-Silanyl Radical Ion

The reaction of amidinatosilylene LSi(:)Cl [L = PhC(NtBu)₂] with N-heterocyclic carbene I_Ar [:C{N(Ar)CH}₂, where Ar = 2,6-iPr₂C₆H₃] and NaOTf in tetrahydrofuran (THF) facilely afforded a silicon(II) cation [LSi(:)-_aI_Ar]⁺OTf^- (1⁺OTf^-), where I_Ar isomerizes to abnormal N-heterocyclic carbene _aI_Ar, coordinating to the silicon(II) center. Its Ge homologue, [LGe(:)-_aI_Ar]⁺OTf^- (2⁺OTf^-), was also accessed via the same protocol. For the formation of 1⁺, we propose that an in situ-generated Si(II) cation [LSi(:)]⁺ under the treatment of LSi(:)Cl with NaOTf may isomerize I_Ar in THF. In contrast, the replacement of I_Ar with cyclic alkyl(amino) carbene (cAAC) furnished a cAAC-silanyl radical ion [LSi(H)-cAAC]^•+(LiOTf₂)^- [3^•+(LiOTf₂)^-], which may undergo an abstraction of the H radical from THF. All of the products were characterized by nuclear magnetic resonance spectroscopy, electron paramagnetic resonance, and X-ray crystallography, and their bonding scenarios were investigated by density functional theory calculations. These studies provide new perspective on carbene-silicon chemistry.

Halogen Complexes of Anionic N-Heterocyclic Carbenes

The lithium complexes [(WCA-NHC)Li(toluene)] of anionic N-heterocyclic carbenes with a weakly coordinating anionic borate moiety (WCA-NHC) reacted with iodine, bromine, or CCl4 to afford the zwitterionic 2-halogenoimidazolium borates (WCA-NHC)X (X=I, Br, Cl; WCA=B(C6 F5 )3 , B{3,5-C6 H3 (CF3 )2 }3 ; NHC=IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, or NHC=IMes=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene). The iodine derivative (WCA-IDipp)I (WCA=B(C6 F5 )3 ) formed several complexes of the type (WCA-IDipp)I⋅L (L=C6 H5 Cl, C6 H5 Me, CH3 CN, THF, ONMe3 ), revealing its ability to act as an efficient halogen bond donor, which was also exploited for the preparation of hypervalent bis(carbene)iodine(I) complexes of the type [(WCA-IDipp)I(NHC)] and [PPh4 ][(WCA-IDipp)I(WCA-NHC)] (NHC=IDipp, IMes). The corresponding bromine complex [PPh4 ][(WCA-IDipp)2 Br] was isolated as a rare example of a hypervalent (10-Br-2) system. DFT calculations reveal that London dispersion contributes significantly to the stability of the bis(carbene)halogen(I) complexes, and the bonding was further analyzed by quantum theory of atoms in molecules (QTAIM) analysis.

Incorporation of H2O and CO2 into a BN-embedded 3aH-3a1H-acephenanthrylene derivative

A fused tetracyclic BN-species 1 featuring nucleophilic nitrogen and electrophilic boron centers behaves as a reactive N/B frustrated Lewis pair (FLP) for small molecule activation. Specifically, the O-H and C[double bond, length as m-dash]O bonds have been cleaved by 1 with the formation of fused borinic acid 2, borenium species 3, anionic boranuidacarboxylic acid 4 and oxadiazaborolidinone 5, respectively. Quantum-mechanical calculations are conducted to comprehensively understand the activation processes of small molecules by 1.

Noble Gas Binding Ability of an Au(I) Cation Stabilized by a Frustrated Lewis Pair: A DFT Study

The noble gas (Ng) binding ability of a monocationic [(FLP)Au]⁺ species has been investigated by a computational study. Here, the monocationic [(FLP)Au]⁺ species is formed by coordination of Au(I) cation with the phosphorous (Lewis base) and the boron (Lewis acid) centers of a frustrated Lewis pair (FLP). The bonds involving Au and P, and Au and B atoms in [(FLP)Au]⁺ are partially covalent in nature as revealed by Wiberg bond index (WBI) values, electron density analysis and energy decomposition analysis (EDA). The zero point energy corrected bond dissociation energy (D₀), enthalpy and free energy changes are computed for the dissociation of Au-Ng bonds to assess the Ng binding ability of [(FLP)Au]⁺ species. The D₀ ranges from 6.0 to 13.3 kcal/mol, which increases from Ar to Rn. Moreover, the dissociation of Au-Ng bonds is endothermic as well as endergonic for Ng = Kr-Rn, whereas the same for Ng = Ar is endothermic but exergonic at room temperature. The partial covalent character of the bonds between Au and Ng atoms is demonstrated by their WBI values and electron density analysis. The Ng atoms get slight positive charges of 0.11–0.23 |e|, which indicates some amount of charge transfer takes place from it. EDA demonstrates that electrostatic and orbital interactions have equal contributions to stabilize the Ng-Au bonds in the [(FLP)AuNg]⁺ complex.

Halogenated triarylboranes: synthesis, properties and applications in catalysis

Halogenated triarylboranes (BAr₃) have been known for decades, however it has only been since the surge of interest in main group catalysis that their application as strong Lewis acid catalysts has been recognised. This review aims to look past the popular tris(pentafluorophenyl)borane [B(C₆F₅)₃] to the other halogenated triarylboranes, to give a greater breadth of understanding as to how tuning the Lewis acidity of BAr₃ by modifications of the aryl rings can lead to improved reactivity. In this review, a discussion on Lewis acidity determination of boranes is given, the synthesis of these boranes is discussed, and examples of how they are being used for catalysis and frustrated Lewis pair (FLP) chemistry are explained.

Normal to abnormal ItBu·AlH3 isomerization in solution and in the solid state

The first example of solid state normal to abnormal NHC complex isomerization is reported.

A molecular electron density theory study of the insertion of CO into frustrated Lewis pair boron-amidines: a [4 + 1] cycloaddition reaction

The insertion of CO into hydrogenated boron-amidine 1 yielding five-membered diazaborolone (5DAB) 3 has been studied within the molecular electron density theory (MEDT) at the DFT ωB97X-D/6-311G(d,p) level. This is a domino process comprised of two consecutive reactions: (i) the dehydrogenation of 1 yielding the frustrated Lewis pair (FLP) boron-amidine 4, which quickly equilibrates with four-membered diazaborolone (4DAB) 2; and (ii) the addition of CO into FLP 4, yielding the final 5DAB 3. Analysis of the Gibbs free energies indicates that the extrusion of H2 demands a high ΔG≠ of 28.6 kcal·mol-1, being endergonic by 6.7 kcal·mol-1. The subsequent addition of CO into FLP 4 presents a low ΔG≠ of 15.0 kcal·mol-1; formation of 5DAB 3 being exergonic by -5.7 kcal·mol-1 from hydrogenated boron-amidine 1. An analysis of the bonding changes along the insertion of CO in a smaller FLP model indicates that this reaction can be considered a [4 + 1] cycloaddition reaction taking place via a five-membered pseudocyclic transition state associated with a two-stage one-step mechanism. Analysis of the conceptual DFT reactivity indices suggests that the initial attack of CO on FLP 4 is an acid/base process in which the carbenoid carbonyl character allows CO to participate as a Lewis base, rather than a nucleophilic/electrophilic interaction. The results arising from the analysis of the Parr functions, however, coincide with this behaviour.

Dihydrogen activation by intermolecular rare-earth aryloxide/N-heterocyclic carbene Lewis pairs

Cooperative H₂ activation can be readily achieved by combination of homoleptic rare-earth aryloxides with N-heterocyclic carbenes under mild conditions.

Sulfur, mercury, and boron adducts of sydnone imine derived anionic N-heterocyclic carbenes

The sydnone imines (5-benzoylimino)-3-(2-methoxyphenyl)-sydnone imine and molsidomine were deprotonated at C4 to give sydnone imine anions which can be represented as anionic N-heterocyclic carbenes, respectively. Trapping reactions with sulfur gave unstable sydnone imine sulfides which were stabilized by the formation of methyl thioethers, methyl sulfoxides, gold complexes [(PPh₃)Au–S-sydnone imine] and a bis(ligand) mercury(ii) complex. The latter possesses a tetrahedral coordination of the mercury central atom to the sulfur atoms with the N6 nitrogen atoms coordinating as neutral ligands. Water converted the molsidomine anion into ethyl(2-morpholino-2-thioxoacetyl)carbamate. Mercury(ii)chloride and triphenylborane were employed to trap the sydnone imine carbenes as mercury complexes as well as BPh₃ adducts.

The sydnone imines (5-benzoylimino)-3-(2-methoxyphenyl)-sydnone imine and molsidomine were deprotonated at C4 to give sydnone imine anions which can be represented as anionic N-heterocyclic carbenes, respectively.

Selective Three-Component Coupling for CO2 Chemical Fixation to Boron Guanidinato Compounds

A selective three-component coupling was employed to fix carbon dioxide to boron guanidinato compounds. The one-pot reaction of carbon dioxide, carbodiimides, and borylamines (ArNH)BC₈H₁₄ afforded the corresponding 1,2-adducts {R(H)N}C{N(Ar)}(NR)(CO₂)BC₈H₁₄. Alternatively, the reaction with p-MeOC₆H₄NC or 2,6-Me₂C₆H₃NC gave the corresponding isocyanide 1,1-adducts { i-PrHN}C{N(p-Me-C₆H₄)}(N i-Pr){CNAr}BC₈H₁₄. The molecular structures of products (2,6- i-Pr₂C₆H₃NH)BC₈H₁₄ 7, { i-Pr(H)N}C{N(p-MeC₆H₄)}(N i-Pr)(CO₂)BC₈H₁₄ 9, {Cy(H)N}C{N( p-MeC₆H₄)}(Cy)(CO₂)BC₈H₁₄ 13, and { i-PrHN}C{N( p-MeC₆H₄)}(N i-Pr){CNR″}BC₈H₁₄ (R″ = p-MeOC₆H₄, 2,6-Me₂C₆H₃) 14 and 15 were established by X-ray diffraction. Density functional theory calculations at the M05-2X level of theory revealed that CO₂ fixation and formation of the corresponding adduct is exothermic and proceeds via a nonchelate boron guanidinato intermediate.

Solid state frustrated Lewis pair chemistry

In solution the PCy3/B(C6F5)3 pair is rapidly deactivated by nucleophilic aromatic substitution. In the solid state deactivation is effectively suppressed and the active frustrated phosphane/borane Lewis pair splits dihydrogen or adds to sulfur dioxide. A variety of phosphane/B(C6F5)3 pairs have been used to carry out active FLP reactions in the solid state. The reactions were analyzed by DFT calculations and by solid state NMR spectroscopy. The solid state dihydrogen splitting reaction was also carried out under near to ambient conditions with suspensions of the non-quenched phosphane/borane mixtures in the fluorous liquid perfluoromethylcyclohexane.

A twist on Hellwinkel’s salt, [P(2,2′-biphenyl)2]+[P(2,2′-biphenyl)3]–

Treating PCl5 with C12H8Li2, generated from either C12H10, C12H8Br2, or C12H8I2, affords three products in different ratios depending on the source of the lithiated biphenyl. Hellwinkel’s salt [P(C12H8)2][P(C12H8)3] ([1][2]) and another product [P(C12H8)(C24H16)][P(C12H8)3] ([1′][2]) were obtained by reacting PCl5 with 2,2′-dilithiobiphenyl [Route A: 2.5 equiv.; obtained from biphenyl, n-BuLi, and TMEDA; Route B: 3.0 equiv.; obtained from 2,2′-diiodobiphenyl and n-BuLi; Route C: 4.0 equiv.; obtained from 2,2′-dibromobiphenyl and n-BuLi]. The synthesis, isolation, and characterization of the chiral spiro-compound [1′][2] and the characterization of the pentavalent phosphorane [P(C12H8)2(C12H9)] (3) are reported. The complex [1′][2] was characterized by 31P{1H} NMR spectroscopy, X-ray crystallography, and mass spectrometry. The pentavalent compound (3) was characterized by 31P{1H} NMR spectroscopy and X-ray crystallography.

An unexpected Staudinger reaction at an N-heterocyclic carbene-carbon center

The previously unreported carbene-phosphine adduct (IPr)PCl2N3 [IPr = (HCNDipp)2C:; Dipp = 2,6-iPr2C6H3] was synthesized and used as a synthon toward the elusive dichlorophosphazene monomer unit, [Cl2P=N]. (IPr)PCl2N3 was found to undergo halide and azide abstraction when combined with various electrophiles and its thermolysis yielded the unexpected Staudinger reaction product (IPr=N)PCl2.

Designing effective 'frustrated Lewis pair' hydrogenation catalysts

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.

A strategy for developing metal-free hydrogenation catalysts: a DFT proof-of-principle study

Using DFT computations, a metal-free strategy has been formulated to activate hydrogen reversibly and to construct hydrogenation catalysts, calling for experimental realizations.

Metal-Free Borylation of Heteroarenes Using Ambiphilic Aminoboranes: On the Importance of Sterics in Frustrated Lewis Pair C-H Bond Activation

Two novel frustrated Lewis pair (FLP) aminoboranes, (1-Pip-2-BH₂-C₆H₄)₂ (2; Pip = piperidyl) and (1-NEt₂-2-BH₂-C₆H₄)₂ (3; NEt₂ = diethylamino), were synthesized, and their structural features were elucidated both in solution and in the solid state. The reactivity of these species for the borylation of heteroarenes was investigated and compared to previously reported (1-TMP-2-BH₂-C₆H₄)₂ (1; TMP = tetramethylpiperidyl) and (1-NMe₂-2-BH₂-C₆H₄)₂ (4; NMe₂ = dimethylamino). It was shown that 2 and 3 are more active catalysts for the borylation of heteroarenes than the bulkier analogue 1. Kinetic studies and density functional theory calculations were performed with 1 and 2 to ascertain the influence of the amino group of this FLP-catalyzed transformation. The C-H activation step was found to be more facile with smaller amines at the expense of a more difficult dissociation of the dimeric species. The bench-stable fluoroborate salts of all catalysts (1F-4F) have been synthesized and tested for the borylation reaction. The new precatalysts 2F and 3F are showing higher reaction rates and yields for multigram-scale syntheses.

Design and reactions of a carbon Lewis base/boron Lewis acid frustrated Lewis pair

The conjugated dienamine 4 selectively adds Piers' borane [HB(C₆F₅)₂] to give the enamine/borane system 5, which features a boratirane structure by internal enamine carbon Lewis base to boron Lewis acid interaction. Compound 5 behaves as a C/B frustrated Lewis pair and undergoes typical addition reactions to benzaldehyde, several nitriles and to sulfur dioxide.This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

Trans-Metal-Trapping Meets Frustrated-Lewis-Pair Chemistry: Ga(CH2SiMe3)3-Induced C-H Functionalizations

Merging two topical themes in main-group chemistry, namely, cooperative bimetallics and frustrated-Lewis-pair (FLP) activity, this Forum Article focuses on the cooperativity-induced outcomes observed when the tris(alkyl)gallium compound GaR₃ (R = CH₂SiMe₃) is paired with the lithium amide LiTMP (TMP = 2,2,6,6-tetramethylpiperidide) or the sterically hindered N-heterocyclic carbene (NHC) 1,3-bis(tert-butyl)imidazol-2-ylidene (I^tBu). When some previously published work are drawn together with new results, unique tandem reactivities are presented that are driven by the steric mismatch between the individual reagents of these multicomponent reagents. Thus, the LiTMP/GaR₃ combination, which on its own fails to form a cocomplex, functions as a highly regioselective base (LiTMP)/trap (GaR₃) partnership for the metalation of N-heterocycles such as diazines, 1,3-benzoazoles, and 2-picolines in a trans-metal-trapping (TMT) process that stabilizes the emerging sensitive carbanions. Taking advantage of related steric incompatibility, a novel monometallic FLP system pairing GaR₃ with I^tBu has been developed for the activation of carbonyl compounds (via C═O insertion) and other molecules with acidic hydrogen atoms such as phenol and phenylacetylene. Shedding new light on how these non-cocomplexing partnerships operate and showcasing the potential of gallium reagents to engage in metalation reactions or FLP activations, areas where the use of this group 13 metal is scant, this Forum Article aims to stimulate more interest and activity toward the advancement of organogallium chemistry.

CO/CO and NO/NO coupling at a hidden frustrated Lewis pair template

N-Allyltetramethylpiperidine is readily isomerized to the corresponding enamine by treatment with catalytic amounts of B(C6F5)3. It adds HB(C6F5)2 at the nucleophilic enamine carbon atom to form a C/B Lewis adduct. This reacts with two molar equivalents of carbon monoxide by selective head to tail coupling to give a five-membered C2O2B heterocycle. In contrast the enamine/HB(C6F5)2 Lewis pair reacts with two molar equiv. of nitric oxide by head to head coupling. This reaction probably proceeds via equilibrium with the corresponding vicinal N/B Lewis pair. Most products were characterized by X-ray diffraction.