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

Frustrated Lewis pairs (FLPs) based on nitrogen heterocycles (pyridine, pyrazole, and imidazole) with a silane or germane group in the α-position of a nitrogen atom have been considered as potential molecules to sequestrate carbon dioxide. Three stationary points have been characterized in the reaction profile: a pre-reactive complex, an adduct minimum, and the transition state connecting them. The effect of external (solvent) or internal (hydroxyl group) electric fields in the reaction profile has been considered. In both cases, it is possible to improve the kinetics and thermodynamics of the complexation of CO2 by the FLP and favor the formation of adducts.


INTRODUCTION
Carbon dioxide, CO 2 , is a fascinating molecule. It is small, stable, and available in abundance; however, this apparently inoffensive compound has often its name written in capital letters next to the global warming phenomenon. Because of its greenhouse effect, 1−3 the capture of carbon dioxide by accessible and cheap compounds is an active research area. There are a plethora of methods and patterns enabling the capture of CO 2 4−7 based on two major concepts: absorption and adsorption. 4 In the adsorption process, the idea is to find compounds that, once they are organized into a surface, can create interactions with the carbon dioxide to capture it. The main goal here is to find a compound that is able to change the electronic distribution of CO 2 to make this very stable compound a little more reactive. In previous works, it has been shown that carbon dioxide is able to form complexes with phosphines, 8−11 sulfur dioxide, 12 pyridine derivatives, 13−15 imidazole, 16−18 and other heterocycles. 19,20 It has also been proved that CO 2 can form adducts with carbenes 21−27 and frustrated Lewis pairs (FLPs), 28−33 some of them including silicon and germanium as Lewis acid centers. 34−36 In this article, we explore the use of FLP based on Nheterocycles with a silane or germane group in the α-position of a nitrogen atom as potential molecules to form adducts with CO 2 (Scheme 1). The nitrogen atom could act as a Lewis base (LB) and the silane or germane group as a Lewis acid (LA). 37−39 The intramolecular disposition of the LA and LB adds rigidity to these systems, which should minimize the entropic effects. Derivatives of the studied molecules have been synthesized, and in some cases, their X-ray structure has been reported. 40−45 Three stationary points have been characterized along the reaction coordinate between the FLPs and CO 2 : two minima, the pre-reactive complex and the adduct, and a TS connecting both minima. We explore the effect on the kinetics and thermodynamics of the reaction by means of an electric field generated by a solvent (external) and a hydroxyl group in the heterocyclic ring.

COMPUTATIONAL DETAILS
All of the structures presented in this work were optimized at the MP2 computational level 46 with the jul-cc-pVTZ basis set. 47 This basis set corresponds to the aug-cc-pVTZ 48 for all atoms except hydrogen where cc-pVTZ is used. The energy minima and TS structures (zero and one imaginary frequency, respectively) were confirmed by frequency computations. The Gaussian-16 scientific software 49 was used in these calcu-lations, and the coordinates of the stationary points are gathered in Table S1 of the Supporting Information (SI).
The solvent effect was simulated using the PCM model 50 with the dielectric constant for acetonitrile (ϵ = 35.69). This solvent was selected based on previous studies that show a small complexation with the FLP before reacting with CO 2 , which could prevent the adduct formation between FLP and CO 2 . 51 The electron density of the systems was analyzed within the quantum theory of atoms in molecules (QTAIM) 52,53 and AIMAll software. 54 Based on this method, the electron density critical points are located and, using the signature of the second derivative (Laplacian), these critical points can be classified as nuclear attractors, bond, ring, and cage critical points. The characteristics of the bond critical points (BCP) provide important information about the contact between the two atoms involved.
We used the NBO method, 55 with the NBO-7 version 56 of the program connected with the Gaussian-16 program, to evaluate the stabilization due to the charge transfer between occupied and empty orbitals specially in intermolecular interactions. These calculations were carried out with the M06-2X functional 57 with the geometries obtained at the MP2 level to account for electronic correlation.

RESULTS AND DISCUSSION
We study the formation of adducts between CO 2 and 10 FLP molecules based on N-heterocycles with silane and germane groups displayed in Figure 1. This section is divided into four parts. The first part analyzes the electronic properties of the isolated FLP and CO 2 molecules. In the second part, the three stationary points of the reaction of the FLP + CO 2 in a vacuum are considered. In the third section, we discuss the solvent effect, and the last section is focused on the effect of including a hydroxyl group near the GeH 2 F moiety.  Table 1.
The negative values of the extreme MEPs associated with nitrogen in the FLPs range between −108 and −153 kJ mol −1 , being larger in imidazole than in pyrazole and pyridine derivatives; the two latter show similar values. The σ-hole associated to the Si−F/Ge−F bond presents positive values of the extreme MEPs between 40 and 134 kJ mol −1 . The largest and smallest values in the two series (Si and Ge derivatives)  correspond to the Imi_2 and Py compounds, respectively. The replacement of Si by Ge in FLPs increases the extreme MEP values of the σ-hole, by 35 kJ mol −1 on average, while the absolute value of the minimum associated with the lone pair in nitrogen decreases only by 6 kJ mol −1 . Thus, we expect that the Ge derivatives should form stronger complexes with CO 2 as compared to the complexes of the corresponding Si derivatives.
3.2. FLP + CO 2 Reaction in Gas Phase. Three stationary points were characterized between the FLP and CO 2 . Initially, an energy minimum complex was obtained between the two systems that can evolve through a TS to an adduct (see Figure  3 for two examples). Only in the case of the Pz_3_Ge + CO 2 system, the adduct was not located and all of the attempts to locate it evolved spontaneously toward the complex. The following nomenclature will be used in this article to differentiate the three stationary points: FLP:CO 2 for the complex, FLP/CO 2 for the TS, and FLP−CO 2 for the adduct, that is :, /, and −.
The N···C and O···Si/Ge interatomic distances in the three stationary points are gathered in Table 2 and the relative energies with respect to the isolated monomers along the reaction coordinate are represented in Figure 4 and listed in Table S2 of the SI. The profile of the free energy evolution ( Figure S1) is similar to Figure 4 but with more positive values due to the entropic effects. A graphical representation of the evolution of four interatomic distances [N−C, O−Si/Ge, C− O(1), and C−O(2)] along the reaction coordinate is included in the SI ( Figure S2).
3.2.1. FLP:CO 2 Complexes. The FLP:CO 2 complexes present two tetrel bond interactions, 60−63 with both molecules acting simultaneously as a tetrel donor and an acceptor, which could favor a cooperative effect. The two intermolecular distances that characterize the interactions range between 2.77 and 2.68 Å for N···C and between 2.80 and 3.29 Å for the O··· Si/Ge ones. The shortest distances in both series are found in the Imi_2 complexes. Both tetrel bond distances in the Ge series are shorter than in the Si one, in agreement with the MESP results of the isolated FLP molecules discussed previously.
The binding energies of the FLP:CO 2 complexes range between −30 and −21 kJ mol −1 , which is similar to other complexes involving CO 2 . 10,19,24,25 In agreement with the MESP values and the intermolecular distances, the strongest complexes in both series correspond to the Imi_2 complexes. The complexes in the Ge series are on average 3.5 kJ mol −1 stronger than the corresponding ones in the Si series. These results are in agreement with previous reports that have shown that Ge is a better tetrel bond donor than Si. 62 3.2.2. FLP−CO 2 Adducts. The second minimum found in the reaction coordinate corresponds to the FLP−CO 2 adducts. As indicated previously, in the case of Pz_3_Ge−CO 2 , all  Figure 3. Stationary points of the PySi + CO 2 and Pz_1_Ge + CO 2 systems. The numbering used to identify the two oxygens of CO 2 has been indicated.
The Journal of Physical Chemistry A pubs.acs.org/JPCA Article attempts to locate an adduct spontaneously evolve toward the complex previously discussed. In these minima, the N−C distances range between 1.50 and 1.62 Å and the O(1)−Si/Ge distances between 1.93 and 2.40 Å. The Si/Ge atoms are penta-coordinated with a bipyramidal arrangement. The shortest distances are found in the Pz_1 adducts in both series while the longest, in the Si series, correspond to the Pz_3_Si−CO 2 adduct, not present in the Ge series as mentioned above. As opposed to the trends observed in the complexes, shorter distances are found in the adducts of Si compounds as compared to the Ge series for both N−C and O−Si/Ge parameters. The relative energies of the adducts range between +23 and −28 kJ mol −1 . They can be divided into three groups: • Less-stable adducts than the corresponding isolated reactants: Pz_3_Si, Imi_4_Si, and Imi_4_Ge.
• More-stable adducts than reactants and complexes: Pz_1_Si and Pz_1_Ge. With respect to the energies of the TS, they can be divided into two groups: • Positive relative energies (less stable than the isolated reactants) are found in four cases: imi_4_Si, Pz_1_Si and Pz_3_Si, and imi_4_Ge. • Negative relative energies (more stable than the isolated reactants) are found in five cases: Imi_2_Si/Ge, Imi_2_Ge, Pz_1_Ge, Py_Si, and Py_Ge.
The activation barriers (energy difference between the adduct and the TS) range between 17 and 47 kJ mol −1 .
3.2.4. Overall Analysis in Gas Phase. The geometrical and energetic values of the stationary points along the reaction coordinate have been used to evaluate the two parameters γ and β, defined in eqs 1 and 2, respectively, proposed by where A and C indicate adducts and complexes, respectively. The values obtained for the two parameters have been gathered in Table S3.  (Table S3). The value of β will be close to 1 when the geometries of adducts and TS are very similar; otherwise, the values of β are close to −1 when the geometries of the TS and complexes are similar. In the present case, they range between 0.29 and 0.81, which indicates that all of the TS geometries are more similar to the adducts than the complexes as indicated by the positive values of β. In this set of parameters, the largest values of γ are associated to those of β, showing a second order polynomial relationship between the two parameters (R 2 = 0.97).
As regards to the electron density properties, the values of the electron density properties at the bond critical points of the two tetrel contacts in the stationary points along the reaction coordinate (Table S4) were characterized using the properties at the N−C and Si/Ge−O BCPs (Table S5) The NBO analysis shows that two interactions between occupied and empty orbitals are the most important to explain the attractive forces between the two molecules along the reaction coordinate, the N(lp) → BD* CO and O(lp) → BD* Si/Ge−F. They have been represented in Figure 5 for the Pz_1_Si:CO 2 complex. In agreement with the QTAIM results, the NBO charge-transfer stabilization energies (Table S6) increases when going from the complexes to the adducts through the TS, with C−N contacts considered as bonded in all adducts and the Pz_3_Si and Imi_4_Ge TS structures.
3.3. Solvent Effects. The study of the reaction in gas phase shows very different dipole moments in complexes (between 0.27 and 4.31 D) and adducts (between 4.12 and 8.86 D). Thus, a priori, the presence of solvents could stabilize The Journal of Physical Chemistry A pubs.acs.org/JPCA Article in a larger degree the adducts as compared to the complexes, thus changing the energy profiles in the reaction. Consequently, the effect of acetonitrile was considered for all cases using the PCM model. The solvent effect destabilizes the complexes by 9 kJ mol −1 on average when compared to the analogous results in gas phase ( Figure 6 and Table S7). In contrast, the adducts in the PCM model (acetonitrile) are more stable by 23 kJ mol −1 on average than without solvent. In addition, the inclusion of the solvent model allows to locate the adduct of Pz_3Ge, which could not be located in the gas phase.
Another interesting effect of the solvent is that now, in the PCM model (acetonitrile), all of the adducts are more stable than the complexes and the isolated monomers, except for Pz_3_Si and Pz_3_Ge, where the adducts present relative energy of +4.9 and +3.6 kJ mol −1 , respectively.
The overall effect of the solvent on the energy and geometry of the stationary points are reflected in the calculated γ and β parameters that are smaller (more negative in the case of γ) in the PCM (acetonitrile) to the ones in the gas phase (Table  S8). In the PCM model, the values of the γ parameter are negative since all of the reactions are exothermic, and the β ones are positive but small, an indication that the TS geometries are intermediate between the two minima. The only exception is the reaction involving Pz_3_Ge that show positive values of γ (+0.41) and a value of β of 0.64.
The ρ BCP values (Table S9) obtained for the intermolecular interactions using the PCM (acetonitrile) model are slightly smaller than those found in the complexes and TS in gas phase, in agreement with the longer interatomic distances found when the system is solvated than in vacuo. In contrast, the ρ BCP values in the adducts are slightly larger since in this case, the bond distances are sorter when solvated than in vacuo.
To get more insight into the effect of the solvent in the energy profile, the effect of eight solvents with a large variety of dielectric constants was considered in the Imi_2_Ge + CO 2 reaction. We considered the following solvents: argon (ϵ =  Figure S4). The larger the dielectric constant, the more exothermic the reaction is (in Figure 7, we display the energy profile for the five selected solvents). In addition, a reduction of the activation energy is observed following the postulate of Laidler and Landskrener that relates this parameter with (1 − 1/ϵ). 71 In fact, a linear correlation between E a and (1 − 1/ϵ) is obtained with a R 2 value of 0.99.

Inclusion of Hydroxyl
Group. An alternative method to induce a field effect in the region where the reaction  The Journal of Physical Chemistry A pubs.acs.org/JPCA Article between the FLP molecule and CO 2 occurs is to modify the chemical composition of the reactants. Thus, we decided to add a hydroxyl group near the GeH 2 F group of the FLP in three molecules (Figure 8). The Cartesian coordinates of the stationary points along the reaction coordinate characterized in this section are gathered in Table S10.
The most stable configuration of the FLP-OH molecules shows a hydrogen bond between the hydroxyl group and the fluorine atom of the GeH 2 F group. The intramolecular interaction affects the MEP, increasing the σ-hole associated to the Ge−F bond (between 22 and 45 kJ mol −1 ) (Table 3).
Thus, the stabilization in the complexes increases between 1 and 4 kJ mol −1 when compared to the analogous systems without the hydroxyl group (see Figure 9 and Table S11). Even larger effects are observed in the stabilization of the TS (between 6 and 12 kJ mol −1 ) and the adducts (between 17 and 35 kJ mol −1 ). Thus, with the inclusion of the OH group in the molecules, the adducts of the Pz_1_Ge_OH and Py_Ge_OH systems are more stable than the complexes, as opposed to the corresponding cases with no hydroxyl group.
In analogy with the energy results, the intermolecular N···C and Ge···O distances are shorter in the three stationary points (Table S12) when the hydroxyl groups are added to the FLP molecules except for the N···C distance in the TS that are about 0.1 Å longer.

CONCLUSIONS
A theoretical study of the reaction of FLP based on nitrogen heterocycles with silane/germane groups in α to one nitrogen with CO 2 has been carried out. The results obtained here support the following conclusions: • The molecular electrostatic potential of the isolated FLPs and CO 2 shows the complementarity required for the formation of the pre-reactive complexes and further the adducts. • In the gas phase, the two minima (pre-reactive complexes FLP:CO 2 and adducts FLP−CO 2 ) are found for all of the FLPs except for the Pz_3_Ge + CO 2 system where only the pre-reactive complex was located. The pre-reactive complexes, FLP:CO 2 , are more stable than the corresponding adducts FLP−CO 2 except for the Pz_1_Si and Pz_1_Ge cases. • The inclusion of the solvent effect changes the stability found in gas phase due to the large dipole moment found on the adducts that show larger solvation energies than the complexes. Relationships between the reaction energy and the barrier with the inverse of the dielectric constant have been obtained. • The inclusion of a hydroxyl group in the heterocyclic ring of the FLPs has shown to be an alternative method to change the energy profile favoring the adducts.
■ ASSOCIATED CONTENT

* sı Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpca.1c04787. Geometries and energies of the optimized systems; relative energy of the stationary points; γ and β parameters; electron density properties at the intermolecular bond critical points; classification of the tetrel bond contacts; NBO charge-transfer stabilization energies; electronic energy vs free energy profile; evolution of selected distances along the reaction coordinate; ρ BCP vs interactomic distance; and adduct-complex energy difference vs the inverse of the dielectric constant (PDF)    Figure 9. Potential energy surface of the reactions between FLP-OH + CO 2 . For comparative purposes, the values of the corresponding systems without hydroxyl group has been added in parenthesis.
The Journal of Physical Chemistry A pubs.acs.org/JPCA Article