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.

A multi-FLP approach for CO2 capture: investigating nitrogen, boron, phosphorus and aluminium doped nanographenes and the influence of a sodium cation

The reactivity of B3N3-doped hexa-cata-hexabenzocoronene (B3N3-NG), Al3N3-NG, B3P3-NG and Al3P3-NG, models of doped nanographenes (NGs), towards carbon dioxide was studied with density functional theory (DFT) calculations at the M06-2X/6-311++G(3df,3pd)//M06-2X/6-31+G* level of theory. The NG systems exhibit a poly-cyclic poly-frustrated Lewis pair (FLP) nature, featuring multiple Lewis acid/Lewis base pairs on their surface enabling the capture of several CO2 molecules. The capture of CO2 by these systems was investigated within two scenarios: (A) sequential capture of up to three CO2 molecules and (B) capture of CO2 molecules in the presence of a sodium cation. The resulting adducts were analyzed in terms of the activation barriers and relative stabilities. The presence of aluminium atoms changes the asynchrony of the reaction favoring the aluminium-oxygen bond and influences the regioselectivity of the multi-capture. A cooperative effect is predicted due to π-electron delocalization, with the sodium cation stabilizing the stationary points and favoring the addition of CO2 to the NGs.

PIO and IBO analysis to unravel the hidden details of the CO2 sequestration mechanism of aromatically tempered N/B-based IFLPs

Enhancing the catalytic reactivity of Frustrated Lewis Pairs (FLPs) in various activities such as CO2 activation and sequestration has recently gained interest among researchers around the globe. A recent investigation showed the use of aromaticity as a tool to modulate the catalytic behaviour of FLPs, establishing a whole new dimension in this area. In this work, aromatically tempered N/B-based intramolecular frustrated Lewis Pairs (IFLPs) are proposed for CO2 sequestration. Density functional theory (DFT)-based calculations were carried out to probe the reaction mechanism. The detailed mechanistic investigation was carried out using intrinsic reaction coordinate (IRC), principal interacting orbital (PIO), intrinsic bond orbital (IBO) and natural bonding orbital (NBO) analyses. The results show that aromatic gain in the system at the basic sites lowers the activation barrier, whereas the antiaromatic gain results in increased activation energy. The sequestration mechanism was found to be an asynchronous concerted mechanism, and polar solvents result in higher asynchronicity. This work, for the first time, reports asynchronicity in the catalytic behavior of aromatically tempered IFLPs, which can be crucial to designing better IFLPs for CO2 sequestration.

Unprecedented Activation of CO2 by α-Amino Boronic Acids

Efficient and environmentally benign transformation of carbon dioxide (CO2) into valuable chemicals is mainly obstructed by the lack of suitable catalysts. To date, various catalysts have already been investigated for the conversion of CO2 molecules, but still finding metal-free, simple, and environment-friendly catalysts is a topic of utmost interest among researchers. Thus, in this regard, the present work projects α-amino boronic acids (AABs) as a metal-free and simple catalyst for CO2 activation. The density functional theory (DFT)-based calculations have been carried out to explore the catalytic potential of AABs. The detailed electronic structure analysis of the considered AABs unveils the catalytic similarities with frustrated Lewis pairs (FLPs) in a gas phase. Interestingly, a peculiar catalytic action of AABs has been observed in the presence of solvents. The contrasting catalytic behavior of AABs in solvents has been extensively investigated by employing principal interacting orbital (PIO), intrinsic bond orbital (IBO), and natural bond orbital (NBO) analyses along the reaction paths. The results of the orbital studies provide concrete ground for the observed reaction mechanism. Further, the energetic analysis of the reaction of CO2 with AABs reveals that <5 kcal/mol energy is required for activation in a solvent phase, and the formed adducts are readily active. These observations show that AABs can be considered as an efficient catalyst for CO2 activation.

Reactivity of a model of B3P3-doped nanographene with up to three CO2 molecules

The reactivity of a B₃P₃-doped hexa-cata-hexabenzocoronene, as a model of nanographene (B₃P₃-NG), towards carbon dioxide was studied at the DFT M06-2X/6-311++G(3df,3pd)//M06-2X/6-31+G* level of theory. This compound can be classified as a poly-cyclic poly-Frustrated Lewis Pair (FLP) system, as it presents more than one Lewis Acid/Lewis Base pair on its surface, making the capture of several carbon dioxide molecules possible. Two scenarios were considered to fully characterize the capture of CO₂ by this multi-FLP system: (i) fixation of three CO₂ molecules sequentially one by one; and (ii) simultaneous contact of three CO₂ molecules with the B₃P₃-NG surface. The resulting adducts were analyzed as function of activation barriers and the relative stability of the CO₂ capture. A cooperativity effect due to the π-delocalization of the hexa-cata-hexabenzocoronene is observed. The fixation of a CO₂ molecule modifies the electronic properties. It enhances the capture of additional CO₂ molecules by changing the acidy and basicity of the rest of the boron and phosphorus atoms in the B₃P₃-NG system.

Stereoselective Activation of Small Molecules by a Stable Chiral Silene

The reaction of the dilithium salt of the enantiopure (S)-BINOL (1,1'-bi-2-naphthol) with two equivalents of the amidinate-stabilized chlorosilylene [LPh SiCl] (LPh =PhC(NtBu)2 ) led to the formation of the first example of a chiral cyclic silene species comprising an (S)-BINOL ligand. The reactivity of the Si=C bond was investigated by reaction with elemental sulfur, CO2 and HCl. The reaction with S8 led to a Si=C bond cleavage and concomitantly to a ring-opened product with imine and silanethione functional groups. The reaction with CO2 resulted in the cleavage of the CO2 molecule into a carbonyl group and an isolated O atom, while a new stereocenter is formed in a highly selective manner. According to DFT calculations, the [2+2] cycloaddition product is the key intermediate. Further reactivity studies of the chiral cyclic silene with HCl resulted in a stereoselective addition to the Si=C bond, while the fully selective formation of two stereocenters was achieved. The quantitative stereoselective addition of CO2 and HCl to a Si=C bond is unprecedented.

Transition-Metal-Free Heterocyclic Carbon-Boron Bond Formation

Heteroaryl boronic acids and esters are extremely important and valuable intermediates because of their wide application in the synthesis of marketed drugs and bioactive compounds. Over the last couple of decades, the construction of highly important heteroaryl carbon-boron bonds has created huge attention. The transition-metal-free protocols are more green, less sensitive to air and moisture, and also economically advantageous over the transition-metal-based protocols. The transition-metal-free C-H borylation of heteroarenes and C-X (X=halogen) borylation of heteroaryl halides represents an excellent approach for their synthesis. Also, various cyclization and alkyne activation protocols have been recently established for their synthesis. The goal of this review article is to summarize the existing literature and the current state of the art for transition-metal-free synthesis of heteroaryl boronic acid and esters.

Boron based intramolecular heterocyclic frustrated Lewis pairs as organocatalysts for CO2 adsorption and activation

The massive increase in the amount of carbon dioxide (CO₂ ) in the atmosphere has led to serious environmental problems. One of the best ways to tackle this problem is the CO₂ capture and its utilization as a C1 carbon source for the production of industrially valuable chemicals. But the thermodynamic stability of the CO₂ molecule poses a great challenge in its transformation. Since the last two decades, various metal-based and organic catalysts have been developed for the adsorption and activation of CO₂ . Among all the catalysts the Frustrated Lewis pairs (FLPs) have been shown great potential in CO₂ capture and conversion. Thus, in the present work, Intramolecular Frustrated Lewis pairs (IFLP) based on N-Heterocycles with boron group functionalization at the α-position to N has been theoretically investigated for CO₂ activation. Thorough orbital analysis has been carried out to investigate the reactivity of the proposed catalytic systems. The result shows that the considered IFLPs are capable of activating CO₂ with minimum energy requirements. The CO₂ activation energy range between 8 and 14 kcal/mol. The non-polar solvent was found to be the suitable medium for the reaction. Also, the reversibility of the adducts formed with the IFLPs can be controlled by appropriate substitution at B atom in the IFLPs.

Exploring the Reactivity of B-Connected Carboranylphosphines in Frustrated Lewis Pair Chemistry: A New Frame for a Classic System

The primary phosphines MesPH2 and tBuPH2 react with 9-iodo-m-carborane yielding B9-connected secondary carboranylphosphines 1,7-H2 C2 B10 H9 -9-PHR (R=2,4,6-Me3 C6 H2 (Mes; 1 a), tBu (1 b)). Addition of tris(pentafluorophenyl)borane (BCF) to 1 a, b resulted in the zwitterionic compounds 1,7-H2 C2 B10 H9 -9-PHR(p-C6 F4 )BF(C6 F5 )2 (2 a, b) through nucleophilic para substitution of a C6 F5 ring followed by fluoride transfer to boron. Further reaction with Me2 SiHCl prompted a H-F exchange yielding the zwitterionic compounds 1,7-H2 C2 B10 H9 -9-PHR(p-C6 F4 )BH(C6 F5 )2 (3 a, b). The reaction of 2 a, b with one equivalent of R'MgBr (R'=Me, Ph) gave the extremely water-sensitive frustrated Lewis pairs 1,7-H2 C2 B10 H9 -9-PR(p-C6 F4 )B(C6 F5 )2 (4 a, b). Hydrolysis of the B-C6 F4 bond in 4 a, b gave the first tertiary B-carboranyl phosphines with three distinct substituents, 1,7-H2 C2 B10 H9 -9-PR(p-C6 F4 H) (5 a, b). Deprotonation of the zwitterionic compounds 2 a, b and 3 a, b formed anionic phosphines [1,7-H2 C2 B10 H9 -9-PR(p-C6 F4 )BX(C6 F5 )2 ]- [DMSOH]+ (R=Mes, X=F (6 a), R=tBu, X=F (6 b); R=Mes, X=H (7 a), R=tBu, X=H (7 b)). Reaction of 2 a, b with an excess of Grignard reagents resulted in the addition of R' at the boron atom yielding the anions [1,7-H2 C2 B10 H9 -9-PR(p-C6 F4 )BR'(C6 F5 )2 ]- (R=Mes, R'=Me (8 a), R=tBu, R'=Me (8 b); R=Mes, R'=Ph (9 a), R=tBu, R'=Ph (9 b)) with [MgBr(Et2 O)n ]+ as counterion. The ability of the zwitterionic compounds 3 a, b to hydrogenate imines as well as the Brønsted acidity of 3 a were investigated.

Proton to hydride umpolung at a phosphonium center via electron relay: a new strategy for main-group based water reduction

Generation of dihydrogen from water splitting, also known as water reduction, is a key process to access a sustainable hydrogen economy for energy production and usage. The key step is the selective reduction of a protic hydrogen to an accessible and reactive hydride, which has proven difficult at a p-block element. Although frustrated Lewis pair (FLP) chemistry is well known for water activation by heterolytic H-OH bond cleavage, to the best of our knowledge, there has been only one case showing water reduction by metal-free FLP systems to date, in which silylene (SiII) was used as the Lewis base. This work reports the molecular design and synthesis of an ortho-phenylene linked bisborane-functionalized phosphine, which reacts with water stoichiometrically to generate H2 and phosphine oxide quantitatively under ambient conditions. Computational investigations revealed an unprecedented multi-centered electron relay mechanism offered by the molecular framework, shuttling a pair of electrons from hydroxide (OH-) in water to the separated proton through a borane-phosphonium-borane path. This simple molecular design and its water reduction mechanism opens new avenues for this main-group chemistry in their growing roles in chemical transformations.

Diverse Uses of the Reaction of Frustrated Lewis Pair (FLP) with Hydrogen

The articulation of the notion of "frustrated Lewis pairs" (FLPs) emerged from the discovery that H₂ can be reversibly activated by combinations of sterically encumbered main group Lewis acids and bases. This has prompted numerous studies focused on various perturbations of the Lewis acid/base combinations and the applications to organic reductions. This Perspective focuses on the new directions and developments that are emerging from this FLP chemistry involving hydrogen. Three areas are discussed including new applications and approaches to FLP reductions, the reductions of small molecules, and the advances in heterogeneous FLP systems. These foci serve to illustrate that despite having its roots in main group chemistry, this simple concept of FLPs is being applied across the discipline.

The Reaction of CO2 with a Borylnitrene: Formation of an 3-Oxaziridinone

The reaction of a borylnitrene with carbon dioxide is studied under cryogenic matrix isolation conditions. Photogenerated CatBN (Cat=catecholato) reacts with CO2 under formation of the cycloaddition product CatBNCO2 , a 3-oxaziridinone derivative, after photoexcitation (>550 nm). The product shows Fermi resonances between the CO stretching and ring deformation modes that cause unusual 13 C and 18 O isotopic shifts. A computational analysis of the 3-oxaziridinone shows this cyclic carbamate to be less strained than an α-lactone or an α-lactame.

Small Molecule Activation with Intramolecular "Inverse" Frustrated Lewis Pairs

The intramolecular "inverse" frustrated Lewis pairs (FLPs) of general formula 1-BR₂ -2-[(Me₂ N)₂ C=N]-C₆ H₄ (3-6) [BR₂ =BMes₂ (3), BC₁₂ H₈ , (4), BBN (5), BBNO (6)] were synthesized and structurally characterized by multinuclear NMR spectroscopy and X-ray analysis. These novel types of pre-organized FLPs, featuring strongly basic guanidino units rigidly linked to weakly Lewis acidic boryl moieties via an ortho-phenylene linker, are capable of activating H-H, C-H, N-H, O-H, Si-H, B-H and C=O bonds. 4 and 5 deprotonated terminal alkynes and acetylene to form the zwitterionic borates 1-(RC≡C-BR₂ )-2-[(Me₂ N)₂ C=NH]-C₆ H₄ (R=Ph, H) and reacted with ammonia, BnNH₂ and pyrrolidine, to generate the FLP adducts 1-(R₂ HN→BR₂ )-2-[(Me₂ N)₂ C=NH]-C₆ H₄ , where the N-H functionality is activated by intramolecular H-bond interactions. In addition, 5 was found to rapidly add across the double bond of H₂ CO, PhCHO and PhNCO to form cyclic zwitterionic guanidinium borates in excellent yields. Likewise, 5 is capable of cleaving H₂ , HBPin and PhSiH₃ to form various amino boranes. Collectively, the results demonstrate that these new types of intramolecular FLPs featuring weakly Lewis acidic boryl and strongly basic guanidino moieties are as potent as conventional intramolecular FLPs with strongly Lewis acidic units in activating small molecules.

Hydroalkylation of Aryl Alkenes with Organohalides Catalyzed by Molybdenum Oxido Based Lewis Pairs

Three molybdenum(VI) dioxido complexes [MoO2(L)2] bearing Schiff base ligands were reacted with B(C6F5)3 to afford the corresponding adducts [MoO{OB(C6F5)3}(L)2], which were fully characterized. They exhibit Frustrated Lewis-Pairs reactivity when reacting with silanes. Especially, the [MoO{OB(C6F5)3}(L)2] complex with L=2,4-dimethyl-6-((phenylimino)methyl)phenol proved to be active as catalyst for the hydroalkylation of aryl alkenes with organohalides and for the Atom-Transfer Radical Addition (ATRA) of organohalides to aliphatic alkenes. A series of gem-dichloride and gem-dibromide compounds with potential for further derivatization were synthesized from simple alkenes and organohalides, like chloroform or bromoform, using low catalyst loading.

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.

CO2 Capture and in situ Catalytic Transformation

The escalating rate of fossil fuel combustion contributes to excessive CO₂ emission and the resulting global climate change has drawn considerable attention. Therefore, tremendous efforts have been devoted to mitigate the CO₂ accumulation in the atmosphere. Carbon capture and storage (CCS) strategy has been regarded as one of the promising options for controlling CO₂ build-up. However, desorption and compression of CO₂ need extra energy input. To circumvent this energy issue, carbon capture and utilization (CCU) strategy has been proposed whereby CO₂ can be captured and in situ activated simultaneously to participate in the subsequent conversion under mild conditions, offering valuable compounds. As an alternative to CCS, the CCU has attracted much concern. Although various absorbents have been developed for the CCU strategy, the direct, in situ chemical conversion of the captured CO₂ into valuable chemicals remains in its infancies compared with the gaseous CO₂ conversion. This review summarizes the recent progress on CO₂ capture and in situ catalytic transformation. The contents are introduced according to the absorbent types, in which different reaction type is involved and the transformation mechanism of the captured CO₂ and the role of the absorbent in the conversion are especially elucidated. We hope this review can shed light on the transformation of the captured CO₂ and arouse broad concern on the CCU strategy.

Interaction of Metal Oxido Compounds with B(C6 F5 )3

Lewis acid-base pair chemistry has been placed on a new level with the discovery that adduct formation between an electron donor (Lewis base) and acceptor (Lewis acid) can be inhibited by the introduction of steric demand, thus preserving the reactivity of both Lewis centers, resulting in highly unusual chemistry. Some of these highly versatile frustrated Lewis pairs (FLP) are capable of splitting a variety of small molecules, such as dihydrogen, in a heterolytic and even catalytic manner. This is in sharp contrast to classical reactions where the inert substrate must be activated by a metal-based catalyst. Very recently, research has emerged combining the two concepts, namely the formation of FLPs in which a metal compound represents the Lewis base, allowing for novel chemistry by using the heterolytic splitting power of both together with the redox reactivity of the metal. Such reactivity is not restricted to the metal center itself being a Lewis acid or base, also ancillary ligands can be used as part of the Lewis pair, still with the benefit of the redox-active metal center nearby. This Minireview is designed to highlight the novel reactions arising from the combination of metal oxido transition-metal or rare-earth-metal compounds with the Lewis acid B(C₆ F₅ )₃ . It covers a wide area of chemistry including small molecule activation, hydrogenation and hydrosilylation catalysis, and olefin metathesis, substantiating the broad influence of the novel concept. Future goals of this young and exciting area are briefly discussed.

Preparation of the Borane (Fmes)BH2 and its Utilization in the FLP Reduction of Carbon Monoxide and Carbon Dioxide

Treatment of 1,3,5-tris(trifluoromethyl)benzene with n-BuLi, followed by H₃ B⋅SMe₂ and subsequent hydride removal gave the (Fmes)BH₂ reagent, which was isolated as a SMe₂ stabilized monomer or a ligand free (μ-H)₂ -bridged dimer. Reaction with Mes₂ P(vinyl) gave the respective ethylene-bridged P/B(Fmes)H FLP. It reduced carbon monoxide to the formyl stage and carbon dioxide to the formaldehyde oxidation state. Most new compounds were characterized by X-ray diffraction.

Heterolytic Si-H Bond Cleavage at a Molybdenum-Oxido-Based Lewis Pair

The reaction of a molybdenum(VI) oxido imido complex with the strong Lewis acid B(C6 F5 )3 gave access to the Lewis adduct [Mo{OB(C6 F5 )3 }(NtBu)L2 ] featuring reversible B-O bonding in solution. The resulting frustrated Lewis pair (FLP)-like reactivity is reflected by the compound's ability to heterolytically cleave Si-H bonds, leading to a clean formation of the novel cationic MoVI species 3 a (R=Et) and 3 b (R=Ph) of the general formula [Mo(OSiR3 )(NtBu)L2 ][HB(C6 F5 )3 ]. These compounds possess properties highly unusual for molybdenum d0 species such as an intensive, charge-transfer-based color as well as a reversible redox couple at very low potentials, both dependent on the silane used. Single-crystal X-ray diffraction analyses of 2 and 4 b, a derivative of 3 b featuring the [FB(C6 F5 )3 ]- anion, picture the stepwise elongation of the Mo=O bond, leading to a large increase in the electrophilicity of the metal center. The reaction of 3 a and 3 b with benzaldehyde allowed for the regeneration of compound 2 by hydrosilylation of the benzaldehyde. NMR spectroscopy suggested an unusual mechanism for the transformation, involving a substrate insertion in the B-H bond of the borohydride anion.

Hydrogen cleavage by solid-phase frustrated Lewis pairs

We report the direct synthesis of a solid-phase frustrated Lewis pair (s-FLP) by combining a silica-supported Lewis acid ([triple bond, length as m-dash]SiOB(C6F5)2, s-BCF) with a Lewis base (tri-tert-butylphosphine, (t)Bu3P) to give [[triple bond, length as m-dash]SiOB(C6F5)2][(t)Bu3P]. Reaction of this s-FLP with H2 under mild conditions led to heterolytic H-H bond cleavage and the formation of [[triple bond, length as m-dash]SiOB(H)(C6F5)2][(t)Bu3PH].

Theoretical studies on bridged frustrated Lewis pair (FLP) mediated H2 activation and CO2 hydrogenation

DFT calculations show that H₂ and CO₂ activation by bridged phosphane/borane frustrated Lewis pairs (FLPs) experiences a one-step concerted mechanism with small reaction barriers.

(Dicyclohexyl(2-(dimesitylboryl)phenyl)phosphine: en route to stable frustrated Lewis pairs-hydrogen adducts in water

The new ansa-phosphinoborane (dicyclohexyl(2-(dimesitylboryl)phenyl)phosphine was synthesized via an one-pot protocol in 67% yield. The compound has been characterized by 1H, 13C, 11B and 31P NMR, and its solid-state structure determined by a single crystal X-ray diffraction analysis. The ansa-phosphinoborane does not react with molecular hydrogen or water at room or elevated temperature. According to performed DFT studies, heterolytic splitting of water or hydrogen by the phosphinoborane are both endergonic but close in thermodynamics. In polar solvents, such as in methanol or acetonitrile, addition of hydrogen is energetically more favorable than of water.

Novel B(Ar')2(Ar'') hetero-tri(aryl)boranes: a systematic study of Lewis acidity

A series of homo- and hetero-tri(aryl)boranes incorporating pentafluorophenyl, 3,5-bis(trifluoromethyl)phenyl, and pentachlorophenyl groups, four of which are novel species, have been studied as the acidic component of frustrated Lewis pairs for the heterolytic cleavage of H2. Under mild conditions eight of these will cleave H2; the rate of cleavage depending on both the electrophilicity of the borane and the steric bulk around the boron atom. Electrochemical studies allow comparisons of the electrophilicity with spectroscopic measurements of Lewis acidity for different series of boranes. Discrepancies in the correlation between these two types of measurements, combined with structural characterisation of each borane, reveal that the twist of the aryl rings with respect to the boron-centred trigonal plane is significant from both a steric and electronic perspective, and is an important consideration in the design of tri(aryl)boranes as Lewis acids.

A combined "electrochemical-frustrated lewis pair" approach to hydrogen activation: surface catalytic effects at platinum electrodes

Herein, we extend our "combined electrochemical-frustrated Lewis pair" approach to include Pt electrode surfaces for the first time. We found that the voltammetric response of an electrochemical-frustrated Lewis pair (FLP) system involving the B(C6 F5 )3 /[HB(C6 F5 )3 ](-) redox couple exhibits a strong surface electrocatalytic effect at Pt electrodes. Using a combination of kinetic competition studies in the presence of a H atom scavenger, 6-bromohexene, and by changing the steric bulk of the Lewis acid borane catalyst from B(C6 F5 )3 to B(C6 Cl5 )3 , the mechanism of electrochemical-FLP reactions on Pt surfaces was shown to be dominated by hydrogen-atom transfer (HAT) between Pt, [PtH] adatoms and transient [HB(C6 F5 )3 ](⋅) electrooxidation intermediates. These findings provide further insight into this new area of combining electrochemical and FLP reactions, and proffers additional avenues for exploration beyond energy generation, such as in electrosynthesis.

Metal-free hydrogenation catalyzed by an air-stable borane: use of solvent as a frustrated Lewis base

In recent years 'frustrated Lewis pairs' (FLPs) have been shown to be effective metal-free catalysts for the hydrogenation of many unsaturated substrates. Even so, limited functional-group tolerance restricts the range of solvents in which FLP-mediated reactions can be performed, with all FLP-mediated hydrogenations reported to date carried out in non-donor hydrocarbon or chlorinated solvents. Herein we report that the bulky Lewis acids B(C6Cl5)x(C6F5)(3-x) (x=0-3) are capable of heterolytic H2 activation in the strong-donor solvent THF, in the absence of any additional Lewis base. This allows metal-free catalytic hydrogenations to be performed in donor solvent media under mild conditions; these systems are particularly effective for the hydrogenation of weakly basic substrates, including the first examples of metal-free catalytic hydrogenation of furan heterocycles. The air-stability of the most effective borane, B(C6Cl5)(C6F5)2, makes this a practically simple reaction method.

Carbene-9-BBN ring expansions as a route to intramolecular frustrated Lewis pairs for CO2 reduction

Reactions of phosphine-derived carbenes with 9-borabicyclo[3.3.1]nonane (9-BBN) result in ring-expansion reactions to generate novel intramolecular frustrated Lewis pairs (FLPs). These FLPs effect the catalytic reduction of CO2 in the presence of boranes to give BOB and methoxy-borate species.

Borane-protected cyanides as surrogates of H-bonded cyanides in [FeFe]-hydrogenase active site models

Triarylborane Lewis acids bind [Fe2(pdt)(CO)4(CN)2](2-) [1](2-) (pdt(2-) = 1,3-propanedithiolate) and [Fe2(adt)(CO)4(CN)2](2-) [3](2-) (adt(2-) = 1,3-azadithiolate, HN(CH2S(-))2) to give the 2:1 adducts [Fe2(xdt)(CO)4(CNBAr3)2](2-). Attempts to prepare the 1:1 adducts [1(BAr3)](2-) (Ar = Ph, C6F5) were unsuccessful, but related 1:1 adducts were obtained using the bulky borane B(C6F4-o-C6F5)3 (BAr(F)*3). By virtue of the N-protection by the borane, salts of [Fe2(pdt)(CO)4(CNBAr3)2](2-) sustain protonation to give hydrides that are stable (in contrast to [H1](-)). The hydrides [H1(BAr3)2](-) are 2.5-5 pKa units more acidic than the parent [H1](-). The adducts [1(BAr3)2](2-) oxidize quasi-reversibly around -0.3 V versus Fc(0/+) in contrast to ca. -0.8 V observed for the [1](2-/-) couple. A simplified synthesis of [1](2-), [3](2-), and [Fe2(pdt)(CO)5(CN)](-) ([2](-)) was developed, entailing reaction of the diiron hexacarbonyl complexes with KCN in MeCN.

Phosphine catalyzed reduction of CO2 with boranes

Phosphine is shown to catalyse the reaction of CO2 with 9-BBN to give mixtures of HCO2(B(C8H14)) 3, H2C(OB(C8H14))24 and MeOB(C8H14) 5; at 0.02 mol% of tBu3P 5 is obtained in 98% yield at 60 °C with TON of almost 5500 and a TOF of 170. Under stoichiometric conditions the species (R3PCH2O)(HC(O)O)B(C8H14) (R = tBu 1,4-MeC6H42) were isolated and characterized.

An electrochemical study of frustrated Lewis pairs: a metal-free route to hydrogen oxidation

Frustrated Lewis pairs have found many applications in the heterolytic activation of H2 and subsequent hydrogenation of small molecules through delivery of the resulting proton and hydride equivalents. Herein, we describe how H2 can be preactivated using classical frustrated Lewis pair chemistry and combined with in situ nonaqueous electrochemical oxidation of the resulting borohydride. Our approach allows hydrogen to be cleanly converted into two protons and two electrons in situ, and reduces the potential (the required energetic driving force) for nonaqueous H2 oxidation by 610 mV (117.7 kJ mol(-1)). This significant energy reduction opens routes to the development of nonaqueous hydrogen energy technology.

Formation, structural characterization, and reactions of a unique cyclotrimeric vicinal Lewis pair containing (C6F5)2P-Lewis base and (C6F5)BH-Lewis acid components

A unique vicinal Lewis pair cyclotrimer has been synthesized and characterized with respect to its structure and reactivity.