Activation of Small Molecules and Hydrogenation of CO2 Catalyzed by Frustrated Lewis Pairs

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

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

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.

(Pyridin-2-ylmethyl)triel Derivatives as Masked Frustrated Lewis Pairs. Interactions and CO2 -Sequestration

The isolated (pyridin-2-ylmethyl)triel derivatives (triel=B, Al and Ga) show an intramolecular N⋅⋅⋅Tr triel bond as shown by compounds found in the Cambridge Structural Database and DFT calculations. The possibility to use them as masked frustrated Lewis pairs (mFLP) has been explored theoretically concerning their reaction with CO2 . The adduct formation proceeds in two steps. In the first one, the (pyridin-2-ylmethyl)triel derivatives break the intramolecular N⋅⋅⋅Tr bond assisted by CO2 and in the second step the adduct is formed with Tr-O and N-C covalent bonds. The corresponding energy minima and transition states (TS) of the reaction have been characterized and analyzed. The distortion/interaction model analysis of the stationary points indicates that the whole process can be divided in two parts: reorganization of the mFLP in the first steps of the reaction while the reaction with CO2 (associated to the distortion of this molecule) is more important in the formation of the final adduct. In all cases studied, the final products are more stable than the starting molecules that combine with reasonable TS energies indicating that these reactions can occur.

Systematic Assessment of the Catalytic Reactivity of Frustrated Lewis Pairs in C-H Bond Activation

Unreactive C-H bond activation is a new horizon for frustrated Lewis pair (FLP) chemistry. This study provides a systematic assessment of the catalytic reactivity of recently reported intra-molecular FLPs on the activation of typical inert C-H bonds, including 1-methylpyrrole, methane, benzyl, propylene, and benzene, in terms of density functional theory (DFT) calculations. The reactivity of FLPs is evaluated according to the calculated reaction thermodynamic and energy barriers of C-H bond activation processes in the framework of concerted C-H activation mechanisms. As for 1-methylpyrrole, 14 types of N-B-based and 15 types of P-B-based FLPs are proposed to be active. Although none of the evaluated FLPs are able to catalyze the C-H activation of methane, benzyl, or propylene, four types of N-B-based FLPs are suggested to be capable of catalyzing the activation of benzene. Moreover, the influence of the strength of Lewis acid (LA) and Lewis base (LB), and the differences between the influences of LA and LB on the catalytic reactivity of FLPs, are also discussed briefly. This systematic assessment of the catalytic activity of FLPs should provide valuable guidelines to aid the development of efficient FLP-based metal-free catalysts for C-H bond activation.

Ru(II) Complexes with Protic- and Anionic-Naked-NHC Ligands for Cooperative Activation of Small Molecules

A set of ruthenium(II)-protic-N-heterocyclic carbene complexes, [Ru(NNCH )(PPh3 )2 (X)]Cl (1, X=Cl and 2, X=H) and their deprotonated forms [Ru(NNC)(PPh3 )2 (X)] (1', X=Cl and 2', X=H), in which NNC is a new unsymmetrical pincer ligand, are reported. The four complexes are interconvertible by simple acid-base chemistry. The combined theoretical and spectroscopic investigations indicate charge segregation in anionic-NHC complexes (1' and 2') and can be described from a Lewis pair perspective. The chemical reactivity of deprotonated complex 1' shows cooperative small molecule activation. Complex 1' activates H-H bond of hydrogen, C(sp3 )-I bond of iodomethane, and C(sp)-H bond of phenylacetylene. The activation of CO2 using anionic NHC complex 1' at moderate temperature and ambient pressure and subsequent conversion to formate is also described. All the new compounds have been characterized using ESI-MS, 1 H, 13 C, and 31 P NMR spectroscopy. Molecular structures of 1, 2, and 2' have also been determined with single-crystal X-ray diffraction. The cooperative small molecule activation perspective broadens the scope of potential applications of anionic-NHC complexes in small molecule activation, including the conversion of carbon dioxide to formate, a much sought after reaction in the renewable energy and sustainable development domains.

A theoretical study of the reaction of borata derivatives of benzene, anthracene and pentacene with CO2

A theoretical study of the reaction between several borataacenes (1-methylboratabenzene, 9-methyl-9-borataanthracene and cis and trans diboratapentacene) and CO2 has been carried out at the M06-2X computational level. The influence of a counterion (potassium cation), the cation complexation by 18-crown-6-ether and solvent effects have been explored. The computational results predict anti/syn selectivity as found experimentally in the cis- and trans-diboratapentacene reaction with CO2 (Baker et al., J. Am. Chem. Soc., 2023, 145, 2028).

Insights into Single-Electron-Transfer Processes in Frustrated Lewis Pair Chemistry and Related Donor-Acceptor Systems in Main Group Chemistry

The activation and utilization of substrates mediated by Frustrated Lewis Pairs (FLPs) was initially believed to occur solely via a two-electron, cooperative mechanism. More recently, the occurrence of a single-electron transfer (SET) from the Lewis base to the Lewis acid was observed, indicating that mechanisms that proceed via one-electron-transfer processes are also feasible. As such, SET in FLP systems leads to the formation of radical ion pairs, which have recently been more frequently observed. In this review, we aim to discuss the seminal findings regarding the recently established insights into the SET processes in FLP chemistry as well as highlight examples of this radical formation process. In addition, applications of reported main group radicals will also be reviewed and discussed in the context of the understanding of SET processes in FLP systems.

Frustrated Lewis Pairs: Bonding, Reactivity, and Applications

The outstanding capability of Frustrated Lewis Pair (FLP) catalysts to activate small molecules has gained significant attention in recent times. Reactivity of FLP is further extended toward the hydrogenation of various unsaturated species. Over the past decade, this unique catalysis concept has been successfully expanded to heterogeneous catalysis as well. The present review article gives a brief survey on several studies on this field. A thorough discussion on quantum chemical studies concerning the activation of H₂ is provided. The role of aromaticity and boron-ligand cooperation on the reactivity of FLP is discussed in the Review. How FLP can activate other small molecules by cooperative action of its Lewis centers is also discussed. Further, the discussion is shifted to the hydrogenation of various unsaturated species and the mechanism regarding this process. It also discusses the latest theoretical advancements in the application of FLP in heterogeneous catalysis across various domains, such as two-dimensional materials, functionalized surfaces, and metal oxides. A deeper understanding of the catalytic process may assist in devising new heterogeneous FLP catalysts through experimental design.

Screening Carbon-Boron Frustrated Lewis Pairs for Small-Molecule Activation including N2 , O2 , CO, CO2 , CS2 , H2 O and CH4 : A Computational Study

Dinitrogen (N₂ ) activation is particularly challenging under ambient conditions because of its large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap (10.8 eV) and high bond dissociation energy (945 kJ mol^-1 ) of the N≡N triple bond, attracting considerable attention from both experimental and theoretical chemists. However, most effort has focused on metallic systems. In contrast, nitrogen activation by frustrated Lewis pairs (FLPs) has been initiated recently via theoretical calculations. Here we perform density functional theory (DFT) calculations to screen a series of experimentally viable FLPs for small-molecule activation including N₂ , O₂ , CO, CO₂ , CS₂ , H₂ O and CH₄ . In addition, aromaticity is found to play an important role in most of these small-molecule activation. The particularly thermodynamic stabilities of the activation products and low reaction barriers could be a step forward for the development of FLP towards small-molecule activation including N₂ , inviting experimental chemists' verification.

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.

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.

Use of 5,10-Disubstituted Dibenzoazaborines and Dibenzophosphaborines as Cyclic Supports of Frustrated Lewis Pairs for the Capture of CO2

The reactivity of 5,10-disubstituted dibenzoazaborines and dibenzophosphaborines towards carbon dioxide was studied at the DFT, M06-2X/def2-TZVP, computational level. The profile of this reaction comprises of three stationary points: the pre-reactive complex and adduct minima and the transition state(TS) linking both minima. Initial results show that dibenzoazaborines derivatives are less suitable to form adducts with CO₂ than dibenzophosphaborine systems. The influence of the basicity on the P atom and the acidity on the B center of the dibenzophosphaborine in the reaction with CO₂ was also explored. Thus, an equation was developed relating the properties (acidity, basicity and boron hybridization) of the isolated dibenzophosphaborine derivatives with the adduct energy. We found that modulation of the boron acidity allows to obtain more stable adducts than the pre-reactive complexes and isolated monomers.

Frustrated Lewis Pairs in Heterogeneous Catalysis: Theoretical Insights

Frustrated Lewis pair (FLP) catalysts have attracted much recent interest because of their exceptional ability to activate small molecules in homogeneous catalysis. In the past ten years, this unique catalysis concept has been extended to heterogeneous catalysis, with much success. Herein, we review the recent theoretical advances in understanding FLP-based heterogeneous catalysis in several applications, including metal oxides, functionalized surfaces, and two-dimensional materials. A better understanding of the details of the catalytic mechanism can help in the experimental design of novel heterogeneous FLP catalysts.