Intramolecular Frustrated Lewis Pair Research Articles

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.

Complexes of a Frustrated Lewis Pair-Supported P(-1) Ligand.

Several complexes of the intramolecular frustrated Lewis pair (FLP)-supported P(-1) ligand [iPr2P(C6H4)BCy2{P}]- are presented (Cy=cyclohexyl). Chief among these is the first example of a monomeric zinc bis(phosphido) complex, which was synthesized as a potential precursor for the solution-phase deposition of Zn3P2. While this goal was ultimately unsuccessful, the Zn(II) complex acts as a convenient springboard to other metal phosphide species via transmetallation: affording a tellurium bis(phosphido) complex and a formal adduct of the phosphorus subhalide PPCl2. Trapping experiments show that the PPCl2 adduct can also be prepared directly through the in situ reduction of PCl3 in the presence of an intramolecular FLP ligand. Lastly, we report a formal η2-phosphaborene complex of cobalt(-1) which is isoelectronic to olefin complexes, and explore its bonding via density functional theory (DFT) computations.

What Determines the Lewis Acidity of a Bismuthane? Towards Bi-Based FLPs.

Aiming at intramolecular frustrated Lewis pairs (FLPs) based on soft Lewis acidic bismuth centers, a phosphine function was combined with a dichloridobismuthane unit on a phenylene backbone utilizing a scrambling approach. The reaction between two equivalents of BiCl3 and (o-(Ph2P)C6H4)3Bi yielded (o-(Ph2P)C6H4)BiCl2(THF), the structure of which indicated Bi…P interactions and thus a pronounced Lewis acidity at the bismuth center that was confirmed by the Gutmann-Beckett method. However, the system turned out to be insufficient to be utilized for FLP reactivity. Hence, the chloride ligands were exchanged by iodide and C2F5 substituents, respectively. Despite a lower electronegativity the iodide compound exhibits a shorter Bi…P contact, while the C2F5 substituents led to a further decrease of the Lewis acidity, despite their high group electronegativity. DFT calculations rationalized this by a quenching of the Lewis acidity inherent to the σ*(Bi-C) orbital by negative hyperconjugation from occupied p-orbitals at the F atoms. Furthermore, it turned out that the strength of the covalent Bi-X σ-bond is a more important factor than the charge at Bi in determining the energetic accessibility and thus Lewis acidity of the antibonding σ*(Bi-C) orbital.

A Novel Trifunctional Catalyst Enables Radical–Radical Coupling via Intramolecular Frustrated Lewis Pairs

A Novel Trifunctional Catalyst Enables Radical–Radical Coupling via Intramolecular Frustrated Lewis Pairs

Novel Trifunctional Intramolecular Frustrated Lewis Pair Derived From Aminoboronic Acid for Converting CO2 Into Valuable Chemicals

ABSTRACTThe conversion of CO2 into valuable chemicals remains a significant challenge for achieving environmental sustainability, primarily due to the stability of the CO2 molecule. This necessitates the development of efficient and ecofriendly catalysts. In recent years, frustrated Lewis pairs (FLPs) have shown promise for CO2 utilization. In this study, we introduce α‐aminodiboronic acid (DBA), a novel trifunctional aminoboronic acid, as an intramolecular FLP for converting CO2 into cyclic carbonate and formic acid. Using density functional theory (DFT) calculations, we explored the reaction mechanism and investigated DBA's electronic structure through molecular electrostatic potential surface (MESP) and natural bond orbital (NBO) analyses. Our results reveal that one −B (OH)2 group induces an unusual state of frustration in the molecule due to charge transfer from the nitrogen atom's lone pair to the π* orbitals, enhancing catalytic performance. The additional −B (OH)2 group serves as an anchoring site for reactive species. The epoxide activation energy is reduced by approximately 27 kcal/mol compared to the uncatalyzed reaction, and the reduction of CO2 occurs with a requirement of 26 kcal/mol. The additional −B (OH)2 plays a crucial role in the catalytic mechanism and minimizes the energies of various structures observed in the reaction path. The reaction energetics align with structural analysis observations, marking this study as the first report on single‐molecule trifunctional FLPs for transforming CO2 into valuable chemicals.

P-Diarylboryl Halothiophenols as Multifunctional Catalysts via Photoactive Intramolecular Frustrated Lewis Pairs.

p-Diarylboryl halothiophenols are developed and unequivocally characterized. Their photophysical properties and catalytic performance are unveiled by experimental and theoretical investigations. This novel class of triarylboranes behaves as a Brønsted acid to generate the corresponding borylthiophenolate that can absorb visible light to undergo intramolecular charge transfer to form a radical pair consisting of a boron radical anion and thiyl radical, which acts as a single-electron reductant while engaging in hydrogen atom transfer to regenerate the parent borylthiophenol. The synthetic relevance of this mode of action is demonstrated by the establishment of unique catalysis that integrates three different yet tunable functions in a single catalytic cycle, thereby allowing borylthiophenols to solely promote the assembly of sterically congested 1,2-diols and 1,2-aminoalcohol derivatives via radical-radical cross-coupling.

Comparative Investigation on the Catalytic Potential of Borylated Triazine‐Based FLPs for CO2 Sequestration

AbstractTo address the increasing levels of carbon dioxide (CO2) in the atmosphere, various heterocyclic N/B‐based intramolecular frustrated Lewis pairs (IFLPs) have been experimentally and theoretically investigated. Reports have shown them to be a potential catalyst for CO2 sequestration, however, a comparative rationale of these IFLPs has not been reported. Thus, in the present work, we have proposed an uninvestigated IFLP based on i. e., 2‐boranyl‐1,3,5‐triazine (BT) and compared with previously reported borylated imidazole (Im) and pyrimidine (Prm) for CO2 abduction. The density functional theory (DFT) base calculations have been carried out to probe the reaction mechanism and thorough natural bond orbital analysis has been carried out to get deeper insights into the catalytic behavior of the proposed IFLPs. Further, the effect of substituting the hydrogens at the ring and the B atom of the considered IFLP, by −CH3, −Cl and −OH, on the catalytic behavior has been studied. The results show that the borylated triazines can be a potential catalyst for CO2 sequestration. Also, the catalytic behavior of the presented IFLPs can be modulated by appropriated substitutions.

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.

Exploring the electronic and steric effects on the dimerization of intramolecular frustrated Lewis pairs: a comparison between aminoboranes and aminoalanes.

The dimerization of intramolecular aminoborane and aminoalane frustrated Lewis pairs was investigated using density functional theory. We systematically varied the substituents to gradually increase their bulkiness, including H, CH3, t-Bu, Ph, and Mes groups. Starting from the most stable conformer of the monomers, a frustrated Lewis pair or classic Lewis adduct, we studied the dimerization process for all systems, revealing significant variations in the Gibbs free energy. Dimerization was favored in four aminoboranes and six aminoalanes, depending on the specific combinations of substituents. Applying an energy decomposition analysis, we found that the preparation energy of the monomers and the non-orbital interactions between them are the primary contributors to the observed energetic differences, showing a clear linear relationship. Additionally, we analyzed the electronic effects by increasing the acidity of the Lewis acid, observing a shift toward endergonic and exergonic directions in aminoboranes and aminoalanes, respectively. This shift was attributed to the stabilization of a classic Lewis adduct. This study underscores three crucial factors influencing dimer formation: (i) substituent size, (ii) stabilization of the classic Lewis adduct conformation, and (iii) covalent radii of the Lewis centers. Understanding these factors is essential for designing FLPs and preventing unwanted dimerization that could affect their catalytic performance in H2 activation processes.

Insight into acrolein activation by P/B intramolecular frustrated Lewis pairs

AbstractThe study investigates the reactivity of a cyclic five‐membered intramolecular P/B frustrated Lewis pair towards acrolein through a cycloaddition reaction. Intrinsic reaction coordinate (IRC) calculations suggest the single‐step mechanism. It has been observed that the cycloaddition reaction occurs through a concerted mechanism in both the presence and absence of the catalyst. Analysis of reaction force and reaction electronic flux provides valuable information about the total work required and electronic activity along the IRC. Additionally, natural bonding orbital (NBO) analyses enrich the understanding of the mechanism in terms of the electron transfer process during the chemical reaction.

Frustrated Lewis Pair Adduct of Atomic P(−1) as a Source of Phosphinidenes (PR), Diphosphorus (P2), and Indium Phosphide

AbstractWe report phosphinidenes (PR) stabilized by an intramolecular frustrated Lewis pair (FLP) chelate. These adducts include the parent phosphinidene, PH, which is accessed via thermolysis of coordinated HPCO. The reported FLP‐PH species acts as a springboard to other phosphorus‐containing compounds, such as FLP‐adducts of diphosphorus (P2) and InP3. Our new adducts participate in thermal‐ or light‐induced phosphinidene elimination (of both PH and PR, R=organic group), transfer P2 units to an organic substrate, and yield the useful semiconductor InP at only 110 °C from solution.

Frustrated Lewis Pair Adduct of Atomic P(-1) as a Source of Phosphinidenes (PR), Diphosphorus (P2 ), and Indium Phosphide.

We report phosphinidenes (PR) stabilized by an intramolecular frustrated Lewis pair (FLP) chelate. These adducts include the parent phosphinidene, PH, which is accessed via thermolysis of coordinated HPCO. The reported FLP-PH species acts as a springboard to other phosphorus-containing compounds, such as FLP-adducts of diphosphorus (P2 ) and InP3 . Our new adducts participate in thermal- or light-induced phosphinidene elimination (of both PH and PR, R=organic group), transfer P2 units to an organic substrate, and yield the useful semiconductor InP at only 110 °C from solution.

Understanding the enhanced reactivity of strained intramolecular Frustrated Lewis Pairs

AbstractThe poorly understood factors controlling the enhanced reactivity of strained intramolecular frustrated Lewis pairs (FLPs) having a rigid biphenylene tether have been quantitatively explored in detail by means of computational methods. With the help of the activation strain model of reactivity and the energy decomposition analysis methods, the challenging allene activation reaction has been selected and compared to the analogous process mediated by a related intramolecular FLP having a more flexible tether, which is significantly less reactive. In addition, the influence of the nature of the Lewis acid atom on the reactivity of the strained FLP has been considered as well showing that the reactivity steadily decreases when going down in group 13.

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.

Metal-free catalytic conversion of CO2into methanol: local electrophilicity as a tunable property in the design and performance of aniline-derived aminoborane-based FLPs

A deeper computational mechanistic study of an environmentally friendly metal-free CO2reduction process towards MeOH is presented, using a previously tested intramolecular frustrated Lewis pair (2-[bis(R)boryl]-N,N-dimethylaniline) as catalyst and H2as reducing agent.

New insights into H2 activation by intramolecular frustrated Lewis pairs based on aminoboranes: the local electrophilicity index of boron as a suitable indicator to tune the reversibility of the process.

A large set of intramolecular aminoborane-based FLPs was studied employing density functional theory in the H2 activation process to analyze how the acidity and basicity of boron and nitrogen atoms, respectively, affect the reversibility of the process. Three different linkers were employed, keeping the C-C nature in the connection between both Lewis centers: -CH2-CH2-, -CH[double bond, length as m-dash]CH-, and -C6H4-. The results show that significant differences in the Gibbs free energy of the process are found by considering all the combinations of substituents. Of the 75 systems studied, only 9 showed the ability to carry out the process reversibly (ΔGH2 in the range of -3.5 to 2.0 kcal mol-1), where combinations of alkyl/aryl or aryl/alkyl in boron/nitrogen generate systems capable of reaching reversibility. If the alkyl/alkyl or aryl/aryl combination is employed, highly exergonic (non-reversible H2 activation) and endergonic (unfeasible H2 activation) reactions are found, respectively. No appreciable differences in the linker were found, allowing us to continue the analysis with the most entropically favorable linker, the -C6H4- linker. From this, 25 different FLP systems of type 2-[bis(X)boryl]-(Y)aniline (X: H, CF3, C6F5, PFtB, FMes and Y: H, CH3, t-but, Ph, Mes) can be formed. By analyzing the electronic properties of each system, we have found that the condensed-to-boron electrophilicity index ωB+ is inversely related to the ΔGH2. Interestingly, two relationships were found; the first is for alkyl groups (Y: CH3 and t-but) and the second for aryl groups (Y: H, Ph, and Mes), which is intimately related to the proton affinity of each aniline. In addition, it is quite interesting when the frustration degree, given by B⋯N distance dB-N, is brought together with ωB+, since the quotient has unit energy/length corresponding to unit force; concomitantly, a measure of the FLP strength in H-H bond activation can be defined. With this finding, a rational design of this kind of FLP can be performed by analyzing the acidity of boron through condensed-to-boron electrophilicity and knowing the nature of the substituent of nitrogen according to whether the Y is alkyl or aryl, optimizing the H2 reversible activation in a rational way, which is crucial to improve the catalytic performance.

In Silico Partial N2 to NH3 Conversion with a Light Atom Molecule.

N2 can be stepwise converted in silico into one molecule NH3 and a secondary amide with a bond activator molecule consisting only of light main group elements. The proposed N2 -activating pincer-related compound carries a silyl ion (Si(+) ) center as well as three Lewis acidic (-BF2 ) and three Lewis basic (-PMe2 ) sites, providing an efficient binding pocket for gaseous N2 within the framework of intramolecular frustrated Lewis pairs (FLP). In addition, it exhibits supportive secondary P-B and F⋅⋅⋅B contacts, which stabilize the structure. In the PSi(+) -N-N-BP environment the N≡N triple bond is extended from 1.09 Å to remarkable 1.43 Å, resembling a N-N single bond. The strongly activated N-N-fragment is prone to subsequent hydride addition and protonation steps, resulting in the energy efficient transfer of two hydrogen equivalents. The next hydride added causes the release of one molecule NH3 , but leaves the ligand system as poisoned R3 Si(+) -NH2 -PMe2 or R3 Si(+) -NH3 dead-end states behind. The study indicates that approximately tetrahedral constrained SiBP2 -pockets are capable to activate N2 , whereas the acid-rich SiB3 - and SiB2 P-pocktes, as well as the base-rich SiP3 -pockets fail, hinting towards the high relevance of the acid-base proportion and relative orientation. The electronic structure of the N2 -activated state is compared to the corresponding state of a recently published peri-substituted bond activator molecule featuring a PSi(+) -N-N-Si(+) P site (S. Mebs, J. Beckmann, Physical Chemistry Chemical Physics 2022, 24, 20953-20967).

Recyclable cyclic bio-based acrylic polymer via pairwise monomer enchainment by a trifunctional Lewis pair.

The existing catalyst/initiator systems and methodologies used for the synthesis of polymers can access only a few cyclic polymers composed entirely of a single monomer type, and the synthesis of such authentic cyclic polar vinyl polymers (acrylics) devoid of any foreign motifs remains a challenge. Here we report that a tethered B-P-B trifunctional, intramolecular frustrated Lewis pair catalyst enables the synthesis of an authentic cyclic acrylic polymer, cyclic poly(γ-methyl-α-methylene-γ-butyrolactone) (c-PMMBL), from the bio-based monomer MMBL. Detailed studies have revealed an initiation and propagation mechanism through pairwise monomer enchainment enabled by the cooperative and synergistic initiator/catalyst sites of the trifunctional catalyst. We propose that macrocyclic intermediates and transition states comprising two catalyst molecules are involved in the catalyst-regulated ring expansion and eventual cyclization, forming authentic c-PMMBL rings and concurrently regenerating the catalyst. The cyclic topology of the c-PMMBL polymers imparts an ~50 °C higher onset decomposition temperature and a much narrower degradation window compared with their linear counterparts of similar molecular weight and dispersity, while maintaining high chemical recyclability.

Novel Insight into the Molecular Frustration of IFLPs Based on Boron-Functionalized Pyrimidines for CO2 Sequestration.

The emergence of frustrated Lewis pairs (FLPs) as organocatalysts for CO2 sequestration has opened a new dimension in this area. To date, various inter- and intramolecular frustrated Lewis pairs have been experimentally and theoretically explored for CO2 activation, still there is plenty of room for new insights into FLPs. Thus, in the present study intramolecular frustrated Lewis pairs (IFLPs) based on boron-functionalized pyrimidines have been proposed for CO2 activation and computationally investigated to gain new insights into the molecular frustration. The extensive natural bond orbital (NBO) analysis unveils an interesting relationship between the orbital charge transfer and the activity. The result shows that the greater the charge transfer between the acidic and basic sites, the higher will be the frustration in the molecule. Also, the presence of atoms bearing a lone pair attached to the acidic site relieves the frustration by charge transfer. Based on the orbital charge transfer, the predicted activity order for the proposed IFLPs is supported by the energetics for the reaction of CO2 with the IFLPs. Further, the activation strain analysis (ASA) provides a different viewpoint about the reactivities of the IFLPs and highlights the importance of the geometrical structure of the catalyst. Furthermore, the ab initio molecular dynamics (AIMD) uplights the reversibility of the formed products.

Ambiphilic Behavior of Ge(II)-Pseudohalides in Inter- and Intramolecular Frustrated Lewis Pair Alkyne Addition Reactions.

The reaction of the germylene chloride (NacNac)GeCl (1, NacNac = CH{(CMe)(2,6-iPr2C6H3N)}2), phenylacetylene, and B(C6F5)3 gives the intermolecular frustrated Lewis pair (FLP) addition product 2. In this case, the Ge(II) center acts as a base. In contrast, the analogous reaction of germylene thiocyanate 3 reacts independently with B(C6F5)3 to give the germylene cation salt [(NacNac)Ge][SCNB(C6F5)3] 4. Subsequent in the presence of alkynes, the Ge(II) cation and γ-C of 4 act as a Lewis acidic and basic center, respectively, to affect the addition of alkynes, affording products [(NacNac)Ge(RCCR')][SCNB(C6F5)3] 5 and 6. Compound 4 also reacts with Me3SiCN to give the cyanide-bridged Ge/B species 7, which also reacts with phenylacetylene to give CN abstraction and intramolecular addition yielding the salt [(NacNac)Ge(PhCCH)][NCB(C6F5)3] 8. Despite the similarity of 1 and 3, DFT calculations show that the highest occupied molecular orbital (HOMO) of 1 is mainly located at the more sterically hindered germylene center, while the HOMO of 3 is located on the less sterically hindered NCS group, prompting markedly different FLP addition products.