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.

Mono(Lewis Base)-Stabilized Gallium Iodide: An Unexplored Class of Promising Ligands

Quantum-chemical (DFT) calculations on hitherto unknown base(carbene)-stabilized gallium monoiodides (LB→GaI) suggest that these systems feature one lone pair of electrons and a formally vacant p-orbital - both centered at the central gallium atom - and exhibit metallomimetic behavior. The calculated reaction free energies as well as bond dissociation energies suggest that these LB→GaI systems are capable of forming stable donor-acceptor complexes with group 13 trichlorides. Examination of the ligand exchange reactions with iron and nickel complexes indicates their potential use as ligands in transition metal chemistry. In addition, it is found that the title compounds are also able to activate various enthalpically robust bonds. Further, a detailed mechanistic investigation of these small molecule activation processes reveals the non-innocent behavior of the carbene (base) moiety attached to the GaI fragment, thereby indicating the cooperative nature of these bond activation processes. The energy decomposition analysis (EDA) and activation strain model (ASM) of reactivity were also employed to quantitatively understand and rationalize the different activation processes.

Insights into the Reactivity of the Ring-Opening Reaction of Tetrahydrofuran by Intramolecular Group-13/P- and Al/Group-15-Based Frustrated Lewis Pairs

A theoretical study concerning key factors affecting activation energies for ring-opening reactions of tetrahydrofuran (THF) by G13/P-based (G13 = B, Al, Ga, In, and Tl) and Al/G15-based (G15 = N, P, As, Sb, and Bi) frustrated Lewis pairs (FLPs) featuring the dimethylxanthene scaffold was performed using density functional theory. Our theoretical findings indicate that only dimethylxanthene backbone Al/P-Rea (Rea = reactant) FLP-type molecules can be energetically favorable to undergo the ring-opening reaction with THF. Our theoretical evidence reveals that the shorter the separating distance between Lewis acidic (LA) and Lewis basic (LB) centers of the dimethylxanthene backbone FLP-type molecules, the greater the orbital overlaps between the FLP and THF and the lower the activation barrier for such a ring-opening reaction. Energy decomposition analysis (EDA) evidence suggests that the bonding interaction for such a ring-opening reaction is predominated by the donor-acceptor interaction (singlet-singlet interaction) compared to the electron-sharing interaction (triplet-triplet interaction). In addition, the natural orbitals for chemical valence (NOCV) evidence demonstrate that the bonding situations of such ring-opening reactions can be best described as FLP-to-THF forward bonding (the lone pair (G15) → the empty σ*(C-O)) and THF-to-FLP back bonding (the empty σ*(G13) ← filled p-π(O)). The EDA-NOCV observations show that the former plays a predominant role and the latter plays a minor role in such bonding conditions. The activation strain model reveals that the deformation energy of THF is the key factor in determining the activation energy of their ring-opening reactions. Comparing the geometrical structures of the transition states with their corresponding reactants, a linear relationship between them can be rationally explained by the Hammond postulate combined with the respective activation barriers calculated in this work.

Bonding Situation of σ-E-H Complexes in Transition Metal and Main Group Compounds

The ambiguous bonding situation of σ-E-H (E=Si, B) complexes in transition metal compounds has been rationalized by means of Density Functional Theory calculations. To this end, the combination of the Energy Decomposition Analysis (EDA) method and its Natural Orbital for Chemical Valance (NOCV) extension has been applied to representative complexes described in the literature where the possible η¹ versus η² coordination mode is not unambiguously defined. Our quantitative analyses, which complement previous data based on the application of the Quantum Theory of Atoms in Molecules (QTAIM) approach, indicate that there exists a continuum between genuine η¹ and η² modes depending mainly on the strength of the backdonation. Finally, we also applied this EDA-NOCV approach to related main-group species where the backdonation is minimal.

Divergent CO2 Activation by Tuning the Lewis Acid in Iron-Based Bimetallic Systems

Bimetallic motifs mediate the selective activation and functionalization of CO₂ in metalloenzymes and some recent synthetic systems. In this work, we build on the nascent concept of bimetallic frustrated Lewis pairs (FLPs) to investigate the activation and reduction of CO₂ . Using the Fe⁰ fragment [(depe)₂ Fe] (depe=1,2-bis(diethylphosphino)ethane) as base, we modify the nature of the partner Lewis acid to accomplish a divergent and highly chemoselective reactivity towards CO₂ . [Au(PMe₂ Ar)]⁺ irreversibly dissociates CO₂ , Zn(C₆ F₅ )₂ and B(C₆ F₅ )₃ yield different CO₂ adducts stabilized by push-pull interactions, while Al(C₆ F₅ )₃ leads to a rare heterobimetallic C-O bond cleavage, and thus to contrasting reduced products after exposure to dihydrogen. Computational investigations provide a rationale for the divergent reactivity, while Energy Decomposition Analysis-Natural Orbital for Chemical Valence (EDA-NOCV) method substantiates the heterobimetallic bonding situation.

Understanding the reactivity of frustrated Lewis pairs with the help of the activation strain model-energy decomposition analysis method

This Feature article presents recent representative applications of the combination of the Activation Strain Model of reactivity and the Energy Decomposition Analysis methods to understand the reactivity of Frustrated Lewis Pairs (FLPs). This approach has been helpful to not only gain a deeper quantitative insight into the factors controlling the cooperative action between the Lewis acid/base partners but also to rationally design highly active systems for different bond activation reactions. Issues such as the influence of the nature of the FLP antagonists or the substituents directly attached to them on the reactivity are covered herein, which are crucial for the future development of this fascinating family of compounds.

Carbon Dioxide Reduction by an Al–O–P Frustrated Lewis Pair

The reaction of tBu₂P(O)H with Bis₂AlH (Bis = CH(SiMe₃)₂) afforded the adduct tBu₂P(H)–O–Al(H)Bis₂ (3). It slowly releases H₂ to form the first oxygen-bridged geminal Al/P frustrated Lewis pair tBu₂P–O–AlBis₂. It is capable of reversibly binding molecular hydrogen to afford 3, shown by NMR and H/D scrambling experiments, and forms a 1,2-adduct with CO₂. Importantly, the H₂ adduct 3 reduces CO₂ in a stoichiometric reaction leading to the formic acid adduct tBu₂P(H)–O–Al(CO₂H)Bis₂. The formation of the different species was explored by density functional theory calculations which provide support for the experimental results. All products were characterized by NMR spectroscopy as well as X-ray diffraction experiments and elemental analyses.

Addition vs. reduction: the geminal FLP Bis₂Al–O–PtBu₂ can reversibly bind molecular hydrogen, it reacts with CO₂ to give an adduct, and its hydrogen adduct reduces CO₂ to an adduct of formic acid.

Significant Insight into the Origin of Reaction Barriers Determining Dihydrogen Activation by G13-P-P (G13 = Group 13 Element) and G15-P-Ga (G15 = Group 15 Element) Frustrated Lewis Pairs

The heterolytic cleavage of H₂ by multiply bonded phosphorus-bridged G13-P-P-Rea (G13 = B, Al, Ga, In, and Tl) and G15-P-Ga-Rea (G15 = N, P, As, Sb, and Bi) frustrated Lewis pairs (FLPs) has been theoretically investigated using density functional theory calculations. For the above nine FLP-type molecules, our theoretical findings suggest that only Al-P-P-Rea, Ga-P-P-Rea, and In-P-P-Rea can undergo the energetically feasible H₂ activation reaction from kinetic and thermodynamic viewpoints. Our study based on the activation strain model (ASM) reveals that gaining a better orbital overlap between G13-P-P-Rea and G15-P-Ga-Rea molecules and H₂ affected the reaction barriers through the atomic radius of G13 and G15. According to our energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) results, the bonding of these H₂ activation reactions involving G13-P-P-Rea and G15-P-Ga-Rea is dominated by the donor-acceptor interaction (singlet-singlet interaction) rather than the electron-sharing interaction (triplet-triplet interaction). Moreover, our EDA-NOCV evidence reveals that the best description for the above bonding situations is the lone pair(G15) → σ*(H₂) interaction rather than the empty p-π-orbital(G13) ← σ(H₂) interaction. In particular, the findings in this work based on theoretically calculated geometries and the corresponding relative free energies of the stationary points combined with the results from the above sophisticated methods nicely agree with the famous Hammond postulate.

Investigating Tetrel-Based Neutral Frustrated Lewis Pairs for Hydrogen Activation

Tetrel Lewis acids are a prospective alternative to commonly employed neutral boranes in frustrated Lewis pair (FLP) chemistry. While cationic tetrylium Lewis acids, being isolobal and iso(valence)electronic, are a natural replacement to boranes, neutral tetrel Lewis acids allude as less trivial options due to the absence of a formally empty p orbital on the acceptor atom. Recently, a series of intramolecular geminal FLPs (C₂F₅)₃E-CH₂-P(tBu)₂ (E = Si, Ge, Sn) featuring neutral tetrel atoms as acceptor sites has been reported for activation of small molecules including H₂. In this work, through density functional theory computations, we elucidate the general mechanistic picture of H₂ activation by this family of FLPs. Our findings reveal that the acceptor atom derives the required Lewis acidity utilizing the antibonding orbitals of its adjacent bonds with the individual contributions depending on the identity of the acceptor and the donor atoms. By varying the identity of the Lewis acid and Lewis base sites and attached substituents, we unravel their interplay on the energetics of the H₂ activation. We find that switching the donor site from P to N significantly affects the synchronous nature of the bond breaking/formations along the reaction pathway, and as a result, N-bearing FLPs have a more favorable H₂ activation profile than those with P. Our results are quantitatively discussed in detail within the framework of the activation-strain model of reactivity along with the energy-decomposition analysis method. Finally, the reductive elimination decomposition route pertinent to the plausible extension of the H₂ activation to catalytic hydrogenation by these FLPs is also examined.

Origin of asynchronicity in Diels-Alder reactions

Asynchronicity in Diels-Alder reactions plays a crucial role in determining the height of the reaction barrier. Currently, the origin of asynchronicity is ascribed to the stronger orbital interaction between the diene and the terminal carbon of an asymmetric dienophile, which shortens the corresponding newly formed C-C bond and hence induces asynchronicity in the reaction. Here, we show, using the activation strain model and Kohn-Sham molecular orbital theory at ZORA-BP86/TZ2P, that this rationale behind asynchronicity is incorrect. We, in fact, found that following a more asynchronous reaction mode costs favorable HOMO-LUMO orbital overlap and, therefore, weakens (not strengthens) these orbital interactions. Instead, it is the Pauli repulsion that induces asynchronicity in Diels-Alder reactions. An asynchronous reaction pathway also lowers repulsive occupied-occupied orbital overlap which, therefore, reduces the unfavorable Pauli repulsion. As soon as this mechanism of reducing Pauli repulsion dominates, the reaction begins to deviate from synchronicity and adopts an asynchronous mode. The eventual degree of asynchronicity, as observed in the transition state of a Diels-Alder reaction, is ultimately achieved when the gain in stability, as a response to the reduced Pauli repulsion, balances with the loss of favorable orbital interactions.

Understanding the reactivity of geminal P/B and P/Al frustrated Lewis pairs in CO2 addition and H2 activation

Understanding CO₂ addition and H₂ activation by geminal P/Z (Z = B, Al) frustrated Lewis pairs (FLPs) is crucial for the development of transition metal-free catalysts for hydrogenation of unsaturated compounds and CO₂ conversion. The distortion-interaction energy decomposition and natural bond orbital were used to gain insights into the reactivity of CO₂ addition and H₂ activation. Remarkably, the orbital interactions in the reactions of CO₂ and H₂ with geminal P/B FLPs are stronger than those in the reactions with more reactive geminal P/Al FLPs, which contain a stronger Lewis acid. The fact that geminal P/B FLPs react with a higher energy barrier than geminal P/Al FLPs is actually due to the unfavorable core repulsion contribution to the interaction energy and the higher distortion energy found for the geminal P/B FLPs. In addition, we revealed that although the interaction energy is higher for H₂ activation, the distortion energy is significantly lower for CO₂ addition. Thus, geminal P/Z FLPs are more reactive toward CO₂ than toward H₂. Overall, the reactivity and thermodynamic stability for the reactions of geminal P/Z FLPs with CO₂ and H₂ are not affected by the type of bridging carbon (sp³-C or sp²-C) but mainly influenced by the substituents on P and Z (B or Al). An insight into the thermodynamic stability of the H₂ activation product is gained by quantifying the hydride affinity, the proton affinity, and the stability of the zwitterionic product relative to the prototypical Lewis acid-base pair, P^tBu₃/B(C₆F₅)₃.

Factors Controlling the Aluminum(I)-meta-Selective C-H Activation in Arenes

The so far poorly understood factors controlling the complete meta-selectivity observed in the C-H activation reactions of alkylarenes promoted by aluminyl anions have been explored in detail by means of Density Functional Theory calculations. To this end, a combination of state-of-the-art computational methods, namely the activation strain model of reactivity and energy decomposition analysis, has been applied to quantitatively unveil the origin of the selectivity of the transformation as well as the influence of the associated potassium cation. It is found that the selectivity takes place during the initial nucleophilic addition step where the key LP(Al)→π*(C=C) molecular orbital interaction is more stabilizing for the meta-pathway, which results in a stronger interaction between the reactants along the entire transformation.

Rational Development of a Metal-Free Bifunctional System for the C-H Activation of Methane: A Density Functional Theory Investigation

The activation or heterolytic splitting of methane, a challenging substrate usually restricted to transition metals, has so far proven elusive in experimental frustrated Lewis pair (FLP) chemistry. In this article, we demonstrate, using density functional theory (DFT), that 1-aza-9-boratriptycene is a conceptually simple intramolecular FLP for the activation of methane. Systematic comparison with other FLP systems allows to gain insight into their reactivity with methane. The thermodynamics and kinetics of methane activation are interpreted by referring to the analysis of the natural charges and by employing the distortion-interaction/activation strain (DIAS) model. These showed that the nature of the Lewis base influences the selectivity over the reaction pathway, with N Lewis bases favoring the deprotonation mechanism and P bases the hydride abstraction one. The lower barrier of activation for 1-aza-9-boratriptycene and the higher products stability are due to a better interaction energy than its counterparts, itself due to electrostatic interactions with the methane moiety, favorable orbital overlaps allowed by the side-attack, and space proximity between the B and N atoms.

Chemical reactivity from an activation strain perspective

Chemical reactions are ubiquitous in the universe, they are at the core of life, and they are essential for industrial processes. The drive for a deep understanding of how something occurs, in this case, the mechanism of a chemical reaction and the factors controlling its reactivity, is intrinsically valuable and an innate quality of humans. The level of insight and degree of understanding afforded by computational chemistry cannot be understated. The activation strain model is one of the most powerful tools in our arsenal to obtain unparalleled insight into reactivity. The relative energy of interacting reactants is evaluated along a reaction energy profile and related to the rigidity of the reactants' molecular structure and the strength of the stabilizing interactions between the deformed reactants: ΔE(ζ) = ΔEstrain(ζ) + ΔEint(ζ). Owing to the connectedness between the activation strain model and Kohn-Sham molecular orbital theory, one is able to obtain a causal relationship between both the sterics and electronics of the reactants and their mutual reactivity. Only when this is accomplished one can eclipse the phenomenological explanations that are commonplace in the literature and textbooks and begin to rationally tune and optimize chemical transformations. We showcase how the activation strain model is the ideal tool to elucidate fundamental organic reactions, the activation of small molecules by metallylenes, and the cycloaddition reactivity of cyclic diene- and dipolarophiles.

Mechanistic insights into the insertion and addition reactions of group 13 analogues of the six-membered N-heterocyclic carbenes: interplay of electrophilicity, basicity, and aromaticity governing the reactivity

Three fundamental concepts (aromaticity/basicity/electrophilicity), being heavily used in modern chemistry, have been applied in this work to study the chemical reactivity of six-membered-ring group 13 N-heterocyclic carbenes (G13-6-Rea; G13 = group 13 elements) using density functional theory (BP86-D3(BJ)/def2-TZVP). G13-6-Rea is isolobal to benzene. Two model reactions have been used in the present study: the insertion reaction of G13-6-Rea with methane and the [1 + 2] cycloaddition reaction of G13-6-Rea with ethene. Our theoretical analysis reveals that the chemical reactivity of B-6-Rea, Al-6-Rea, and Ga-6-Rea is governed by their HOMO (the sp²-σ lone pair orbital on the G13 element), and thus they can be considered nucleophiles. Conversely, the chemical behavior of In-6-Rea and Tl-6-Rea is determined by their LUMO (the vacant p-π orbital on the G13 element), and thus they can be considered electrophiles. On the basis of the VBSCD (valence bond state correlation diagram) model and ASM (activation strain model), this theoretical evidence demonstrates that the origin of activation barriers for the above model reactions is due to the atomic radius of the pivotal group 13 element in the six-membered-ring of G13-6-Rea. Accordingly, our theoretical conclusions suggest that the lower the atomic number and the smaller the atomic radius of the G13 atom, the higher the aromaticity of the six-membered-ring of G13-6-Rea and the smaller the singlet–triplet energy splitting ΔE_st of this N-heterocyclic carbene analogue, which will facilitate its chemical reactions. The theoretical findings originating from this study allow many predictions in experiments to be made.

On the basis of sophisticated theoretical models, the theoretical study demonstrated that the atomic radius of a group 13 element in a six-membered-ring NHC analogue plays an important role in determining its chemical reactivity.

A Zwitterionic Phosphonium Stannate(II) via Hydrogen Splitting by a Sn/P Frustrated Lewis-Pair and Reductive Elimination

The reactivity of the frustrated Lewis pair (FLP) (F5 C2 )3 SnCH2 P(tBu)2 (1) was investigated with respect to the activation of elemental hydrogen. The reaction of 1 at elevated hydrogen pressure afforded the intramolecular phosphonium stannate(II) (F5 C2 )2 SnCH2 PH(tBu)2 (3). It was characterized by means of multinuclear NMR spectroscopy and single crystal X-ray diffraction. NMR experiments with the two isotopologues H2 and D2 showed it to be formed via an H2 adduct (F5 C2 )3 HSnCH2 PH(tBu)2 (2) and the subsequent formal reductive elimination of pentafluoroethane; this is supported by DFT calculations. Parahydrogen-induced polarization experiments revealed the formation of a second product of the reaction of 1 with H2 , [HP(tBu)2 Me][Sn(C2 F5 )3 ] (4), in 1 H NMR spectra, whereas 2 was not detected due to its transient nature.

Rationalizing the AlI -Promoted Oxidative Addition of C-C Versus C-H Bonds in Arenes

The factors controlling the oxidative addition of C-C and C-H bonds in arenes mediated by Al^I have been computationally explored by means of Density Functional Theory calculations. To this end, we compared the processes involving benzene, naphthalene and anthracene which are promoted by a recently prepared anionic Al^I -carbenoid. It is found that this species exhibits a strong tendency to oxidatively activate C-H bonds over C-C bonds, with the notable exception of benzene, where the C-C bond activation is feasible but only under kinetic control reaction conditions. State-of-the-art computational methods based on the combination of the Activation Strain Model of reactivity and the Energy Decomposition Analysis have been used to rationalize the competition between both bond activation reactions as well as to quantitatively analyze in detail the ultimate factors controlling these transformations.

Understanding the role of frustrated Lewis pairs as ligands in transition metal-catalyzed reactions

The role of frustrated Lewis pairs (FLPs) as ligands in gold(i) catalyzed-reactions has been computationally investigated by using state-of-the-art density functional theory calculations. To this end, the nature of (P,B)-FLP-transition metal interactions in different gold(i)-complexes has been first explored in detail with the help of the energy decomposition analysis method, which allowed us to accurately quantify the so far poorly understood AuB interactions present in these species. The impact of such interactions on the catalytic activity of gold(i)-complexes has been then evaluated by performing the Au(i)-catalyzed hydroarylation reaction of phenylacetylene with mesitylene. With the help of the activation strain model of reactivity, the factors governing the higher activity of Au(i)-complexes having a FLP as a ligand as compared to that of the parent PPh₃ system have also been quantitatively identified.

Understanding chemical reactivity using the activation strain model

Understanding chemical reactivity through the use of state-of-the-art computational techniques enables chemists to both predict reactivity and rationally design novel reactions. This protocol aims to provide chemists with the tools to implement a powerful and robust method for analyzing and understanding any chemical reaction using PyFrag 2019. The approach is based on the so-called activation strain model (ASM) of reactivity, which relates the relative energy of a molecular system to the sum of the energies required to distort the reactants into the geometries required to react plus the strength of their mutual interactions. Other available methods analyze only a stationary point on the potential energy surface, but our methodology analyzes the change in energy along a reaction coordinate. The use of this methodology has been proven to be critical to the understanding of reactions, spanning the realms of the inorganic and organic, as well as the supramolecular and biochemical, fields. This protocol provides step-by-step instructions-starting from the optimization of the stationary points and extending through calculation of the potential energy surface and analysis of the trend-decisive energy terms-that can serve as a guide for carrying out the analysis of any given reaction of interest within hours to days, depending on the size of the molecular system.

Reduction of Benzonitriles via Osmium-Azavinylidene Intermediates Bearing Nucleophilic and Electrophilic Centers

The reduction of the N≡C bond of benzonitriles promoted by OsH₆(PⁱPr₃)₂ (1) has been studied. Complex 1 releases a H₂ molecule and coordinates 2,6-dimethylbenzonitrile to afford the tetrahydride OsH₄{κ¹- N-(N≡CC₆H₃Me₂)}(PⁱPr₃)₂ (2), which is thermally stable toward the insertion of the nitrile into one of the Os-H bonds. In contrast to 2,6-dimethylbenzonitrile, benzonitrile and 2-methylbenzonitrile undergo insertion, via Os(η²-N≡CR) intermediates, to give the azavinylidene derivatives OsH₃(═N═CC₆H₄R)(PⁱPr₃)₂ [R = H (3) or Me (4)]. The analysis by means of computational tools (EDA-NOCV) of the bonding situation in these compounds suggests that the donor-acceptor nature of the osmium azavinylidene bond dominates over the mixed electron-sharing/donor-acceptor and pure electron-sharing bonding modes. The N atom is strongly nucleophilic, whereas one of the hydrides is electrophilic. In spite of the different nature of these centers, the migration of the latter to the N atom is kinetically prevented. However, the use of water as a proton shuttle allows hydride migration, as a consequence of a significant decrease in the activation barrier. The resulting phenylaldimine intermediates evolve by means of orthometalation to give OsH₃{κ²- N, C-(NH═CHC₆H₃R)}(PⁱPr₃)₂ [R = H (5) or Me (6)]. The presence of electrophilic and nucleophilic centers in 3 confers upon it the ability to activate σ-bonds, including H₂ and pinacolborane (HBpin). The reaction with the latter gives OsH₃{κ²- N, C-[N(Bpin)═CHC₆H₄]}(PⁱPr₃)₂ (7).

Aromaticity can enhance the reactivity of P-donor/borole frustrated Lewis pairs

Herein we introduce a novel concept in FLP chemistry: aromaticity as the key factor enhancing the reactivity of FLPs.