Sequestration of Carbon Dioxide with Frustrated Lewis Pairs Based on N-Heterocycles with Silane/Germane Groups

Puppeteering the reactivity of frustrated Lewis pairs toward CO2via coordination dichotomy in bridging units

o-Carborane can be regarded as one of the most versatile bridging units for intramolecular frustrated Lewis pairs (IFLPs) due to the coordination dichotomy of the cage. The acidity and basicity of the active centres can be modulated to control the thermodynamic, kinetic, and mechanistic pathways of the reaction with CO2 simply by positioning the active centres at different coordination sites on the carborane unit.

Unveiling the Obscure Potential of O-Carborane Based IFLPs for CO2 Sequestration

Sequestering carbon dioxide (CO2) from the atmosphere is necessary to achieve a sustainable environment. However, the thermodynamic stability of the CO2 molecule poses a significant challenge to its capture, necessitating catalysts that can overcome this stability. The emergence of frustrated Lewis pairs (FLPs) has opened a new dimension in the development of organocatalysts for CO2 capture and utilization. To date, various FLPs have been developed for CO2 sequestration, yet the quest for robust FLPs continues. Based on the intriguing electronic effects of the carborane polyhedral, o-carboranes can be projected as a versatile bridging unit for intramolecular FLPs (IFLPs). In the present work, o-carborane based IFLPs i. e., 1-Al(CH3)2-2-P(CH3)2-1,2-C2B10H10, 1-B(CH3)2-2-P(CH3)2-1,2-C2B10H10, 1-Al(CH3)2-2-N(CH3)2-1,2-C2B10H10, 1-P(CH3)2-2-B(CH3)2-1,2-C2B10H10 abbreviated as AlP, BP, AlN and BN have been proposed for the activation of CO2 molecule. The density functional theory (DFT) based calculations and thorough orbital analysis have been carried out to extensively study the electronic structure of the o-carborane unit. The proposed IFLPs were systematically compared with their corresponding phenyl bridged analogues to assess the effect of o-carborane bridging unit on the reactivity of the acidic and basic sites. The results show that the o-carborane supported IFLPs are more reactive towards CO2 than the phenyl bridged IFLPs. Also, placing the basic site on the B atom at the 4th position of the o-carborane bridge rather than the C atom at the 2nd position results in more reactive IFLPs.

Orbital and free energy landscape expedition towards the unexplored catalytic realm of aromatically modified FLPs for CO2 sequestration

The emergence of frustrated Lewis pairs (FLPs) has created a whole new dimension in the development of metal free catalysts for CO2 sequestration. Efforts have been made to enhance the catalytic activity of the FLPs. The aromatic modulation of the catalytic sites has been successfully demonstrated to enhance the activity towards CO2. Although various aromatically modified geminal FLPs have been investigated for CO2 capture, the catalytic space of these FLPs has not been fully resolved yet. Thus, to fulfil the knowledge gap in the understanding of the catalytic behaviour and to extend the concept of aromatically enhanced FLPs, in the present study all the possible combinations of aromatic and antiaromatic modulations of the acidic and basic sites have been proposed and examined using density functional theory based orbital analysis. Further to verify the results obtained from the orbital analysis and to fully explore the catalytic space of the proposed systems, free energy landscapes have been examined using metadynamics simulations. The detailed intrinsic bond orbital (IBO) and principal interacting orbital (PIO) analyses capture crucial details of the reactions. Furthermore, evolution of anisotropy of induced current density (AICD) along the reaction justifies the effect of aromatic/antiaromatic modulation on the catalytic sites. The results show that highly asynchronous mechanisms have been found due to the aromatic/antiaromatic modulations. The simultaneous favourable aromatic/antiaromatic modification on the acidic and basic sites may greatly reduce the CO2 activation barrier. The enhancement of the acidic character of the B atom in the intramolecular FLPs (IFLPs) leads to a thermodynamically more feasible reaction with stable CO2 adducts. The acidic site has been found to play a major role in controlling the kinetics and thermodynamics of the reaction. This study provides valuable insights into the catalytic realm of the aromatically modified FLPs, which can be utilized to design more efficient and specific next-generation FLPs.

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.

Understanding the coupling of non-metallic heteroatoms to CO2 from a Conceptual DFT perspective

CONTEXT

A Conceptual DFT (CDFT) study has been carry out to analyse the coupling reactions of the simplest amine (CH3NH2), alcohol (CH3OH), and thiol (CH3SH) compounds with CO2 to form the corresponding adducts CH3NHCO2H, CH3OCO2H, and CH3SCO2H. The reaction mechanism takes place in a single step comprising two chemical events: nucleophilic attack of the non-metallic heteroatoms to CO2 followed by hydrogen atom transfer (HAT). According to our calculations, the participation of an additional nucleophilic molecule as HAT assistant entails important decreases in activation electronic energies. In such cases, the formation of a six-membered ring in the transition state (TS) reduces the angular stress with respect to the non-assisted paths, characterised by four-membered ring TSs. Through the analysis of the energy and reaction force profiles along the intrinsic reaction coordinate (IRC), the ratio of structural reorganisation and electronic rearrangement for both activation and relaxation energies has been computed. In addition, the analysis of the electronic chemical potential and reaction electronic flux profiles confirms that the highest electronic activity as well as their changes take place in the TS region. Finally, the distortion/interaction model using an energy decomposition scheme based on the electron density along the reaction coordinate has been carried out and the relative energy gradient (REG) method has been applied to identify the most important components associated to the barriers.

METHODS

The theoretical calculation were performed with Gaussian-16 scientific program. The B3LYP-D3(BJ)/aug-cc-pVDZ level was used for optimization of the minima and TSs. IRC calculations has also been carried out connecting the TS with the associated minima. Conceptual-DFT (CDFT) calculations have been carried out with the Eyringpy program and in-house code. The distortion/interaction model along the reaction coordinate have used the decomposition scheme of Mandado et al. and the analysis of the importance of each components have been done with the relative energy gradient (REG) method.

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.

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.

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).

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.

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.

A theoretical study of the valence tautomerism of 1H-pyrazolium-4-olates (X = O) and related compounds (X = S, Se, NH): relative stabilities, protonation effects, and tautomerization barriers

The valence isomerism of a series of heterocyclic mesomeric betaines (HMBs) belonging to class 5, called pseudo-semi-conjugated HMBs, has been studied theoretically both the neutral and the protonated species. These HMBs are 1H-pyrazol-2-ium-4-olates and related compounds where the oxygen atom has been replaced by S, Se atoms, and an NH group. The main conclusion of the present work is that the ring/open valence tautomerism is possible both for neutral and protonated although it has never been observed experimentally.