Computationally Designed 1,2,4-Triazolylidene-Derived N-Heterocyclic Olefins for CO2 Capture, Activation, and Storage.

In this article, triazolylidene-derived N-heterocyclic olefins (trNHOs) are designed using computational quantum tools, and their potential to promote CO2 sequestration is tested and discussed in detail. The low barrier heights related to the trNHO-mediated process indicate that the tailored compounds are very promising for fast CO2 sequestration. The systematic analysis of the presence of distinct substitutes at different N positions of the trNHO ring allows us to rationalize their effect on the carboxylation process and reveal the best N-substituted trNHO systems for CO2 sequestration and improved trNHO carboxylates for faster CO2 capture/release.

Introduction

Carbon dioxide (CO2) sequestration, activation, and catalytic transformation have attracted much attention in the last years mainly because of its increasing concentration in the atmosphere and its potential as a C1 source.[1−5] However, the successful chemical incorporation of CO2 usually requires extreme reaction conditions because of its kinetic and thermodynamic stability.[1,2]

There are a variety of structurally different chemical systems that are considered as catalytic mediators of CO2 transformations.[4−6] In 2013, Lu and collaborators described[7] the synthesis and characterization of a variety of N-heterocyclic olefins (NHOs), which are able to sequestrate CO2 yielding NHO carboxylates (NHO–CO2) (Scheme ). The synthesized NHO–CO2 compounds were also efficient organocatalysts in further transformations. Since then, both NHO and NHO–CO2 species have been explored to promote different reactions.[7−17]

Scheme 1

CO2 Capture by NHO Yielding NHO–CO2 Adducts

Substitutions at N- and C-position of the imidazolium ring influence the carboxylation rate of NHO by CO2 and the NHO–CO2 stability.[7,18] Therefore, the catalytic activities of NHO and NHO–CO2 compounds can be modulated by modifications in the NHO ring structure.

To date, all of the explorations involving NHOs and CO2 capture, however, have relied on NHOs derived from the imidazolylidene structure, which prompted us to investigate the carboxylation process and the related properties of NHOs and NHO–CO2 containing the 1,2,4-triazolylidene (tr) ring. Figure shows the general structure of the NHOs derived from the imidazolylidene structure (generic NHO) and those derived from the triazolylidene class, which is considered in the present work (trNHO). N-heterocyclic carbenes (NHCs) from the triazolylidene family, for example, show remarkable activity in a varied range of catalytic processes, and their catalytic reactivity and selectivity are mainly governed by the presence of different N-aryl substituents.[19−22]

Figure 1

Overview of the present study: generic structure of an imidazolylidene-derived NHO (NHO), a triazolylidene-derived NHO (trNHO), reference NHO, reference trNHO, and the explored substituents (R).

With all of this in mind, the present work uses computational quantum chemistry tools to analyze the structure and stability of free trNHOs and their carboxylation reaction with CO2. This chemical process provides a way to capture CO2 by yielding trNHO–CO2; hence, these carboxylates are key species in capturing/releasing CO2. Moreover, as either NHOs or their carboxylates play a role in a growing range of CO2 transformations, the exploration of this trNHO class contributes to the development of new and suitable organocatalysts to be used in CO2 incorporation transformations.

Methods and Models

Computational Methods

Geometry optimizations and frequency calculations of all chemical species were performed using the MP2 method[23,24] along with the 6-31+G(d,p) basis set.[25,26] The solvation effects of CH2Cl2 (dichloromethane, DCM), an usual solvent in the NHO-organocatalyzed CO2 reactions, were included with the SMD (IEF-cPCM) method.[27−29] This level of theory is referred to as MP2(DCM)/6-31+G(d,p). The MP2(DCM)/6-31+G(d,p) approach was tested for optimizations of a NHO–CO2 adduct in a chemical system chosen among those characterized by Lu and collaborators.[7] This level of theory showed to be efficient and reliable to determine its structure. Details and comparisons of the calculated and experimental structures are given in Table S1. The nature of all structures, as minima or transition states (TSs), was confirmed by calculating and diagonalizing the matrix of the second derivatives of the energy (Hessian); TSs are identified by only one imaginary frequency. Minimum energy path[30] (MEP) calculations were employed to connect reactants to products using the GS2 algorithm[31] with a range of step sizes from 0.01 to 0.50 (amu)1/2 a0. All of these calculations were performed using the GAMESS-US[32,33] program package.

The relative energies were improved by single-point energy (SP) calculations using the domain-based local pair natural orbital-coupled cluster method with single and double excitations and perturbative triple corrections (DLPNO-CCSD(T)).[34,35] For the DLPNO-CCSD(T) calculations, the cc-pVTZ basis set and the correlation fitting auxiliary basis set cc-pVTZ/C were used.[36−39]

As thermal corrections based on the harmonic approximation are prone to error,[40] the DLPNO-CCSD(T) relative energies include only zero-point vibrational energy (ZPE) obtained from the MP2(DCM)/6-31+G(d,p) calculations. Hence, unless otherwise specified, all relative energies discussed in the text are obtained from EcorrectedDLPNO-CCSD(T), according to eq . For selected trNHO-mediated processes, values of ΔGRXN and ΔG‡ are provided in Table S2. The corrected DLPNO-CCSD(T) energies, EcorrectedDLPNO-CCSD(T), were calculated bywhere EMP2(DCM)/6-31+G(d,p) and ESPMP2/6-31+G(d,p) are electronic energies in DCM and gas phase, respectively. Their difference corresponds to a solvation correction to the ESPDLPNO-CCSD(T) electronic energy. All DLPNO-CCSD(T) calculations were carried out using ORCA.[41]

We applied the DF-RKS/PBE/def2-TZVP[42−44] level of theory, with the univ-JKFIT[42] fitting basis set, to perform the intrinsic bond orbital (IBO) analysis, as implemented in IboView program,[45] which allows us to follow the electron flow along the reaction path.[46] Additionally, a quantitative interpretation of bonding can be done by assigning the electrons in a doubly occupied IBO to individual atoms.[47]

To investigate the nature and magnitude of the interactions between the trNHO and CO2 moieties in the TSs, the Su–Li energy decomposition analysis (Su–Li EDA)[48] was performed at MP2/6-31+G(d,p) level of theory, and the interaction terms are corrected for the basis set superposition error (BSSE). All Su–Li EDA calculations were performed with GAMESS-US. The total interaction energy, ΔEint, is decomposed into five energy terms: electrostatic (ΔEes), exchange (ΔEex), repulsion (ΔErep), polarization (ΔEpol), and dispersion ΔEdispTo further explore and visualize the noncovalent interactions, the NCI (noncovalent interactions) method was applied.[49,50] This approach localizes the noncovalent interactions between the atoms and classifies them according to their strength and type. All NCI computations were carried out with the NCI-Plot package[51] using electronic densities from the MP2/6-31+G(d,p) calculations.

Chemical Model Systems

Inspired by the work of Lupton[52] and Smith[20] on the study of N-substituents in NHC-mediated processes, we selected 11 distinct substituents to investigate. They are shown in Figure and labeled as 1a–1k. The NHO and trNHO with both N2-methyl and N4-methyl units are the reference systems (Figure ).

First, the effects of the 1a–1k substituents at the N2 and N4 positions of trNHO were considered separately: the 1a–1k groups were considered at the N2 atom or at the N4, that is, while varying the groups at N2, N4 remained attached to the methyl group as it is in the reference system and vice versa. In sequence, the combined effect of some substituents was further explored.

Results and Discussion

This section is organized as follows: first, a detailed comparison of the CO2 sequestration performed by the reference systems, NHO and trNHO (Figure ), is presented; next, the effects of N-substituents on the carboxylation process within the trNHO class is discussed; finally, the combined effect of selected substituents is described and the optimal trNHO systems are presented.

CO2 Capture Mechanism by the Reference NHO and trNHO Systems

The energy profile for the carboxylation process between each reference system and CO2 is shown in Figure . Along this work, the labels containing subscripts vdw, TS, and add refer to van der Waals complexes, TSs, and adducts, respectively.

Figure 2

Relative energy profiles of the NHO + CO2 and trNHO + CO2 carboxylation processes obtained using the EcorrectedDLPNO-CCSD(T) energies and NCI analysis using its color scale: green for weak interactions, red for strong nonbonded overlaps, and blue for strong attractive interactions; all energies are given in kcal mol–1 with respect to the separated reactants.

In addition, M06-2X/6-31+G(d,p)[53] calculations were used to assess the nucleophilicity of the free trNHO and NHO systems using the nucleophilicity index (N) defined by N = EHOMO(Nu) (eV) – EHOMO(TCE) (eV), with Nu = NHO, trNHO, and tetracyanoethylene (TCE) as reference, as described by Domingo and co-authors.[54] The density functionals (DFs), M06-2X, B3LYP-D3,[55,56] and wB97X-D[57−60] were applied for some of our trNHO carboxylation processes, and their overall trend is in line with the current DLPNO-CCSD(T) discussions. More details are given in Table S3. The Mulliken charge analysis shows that the charge of the CNHOα and CtrNHOα are practically the same (−0.609 and −0.611, respectively). According to the nucleophilicity index, these free species are strong nucleophiles (N ≥ 3.9 eV)[61] with NNHO = 5.3 eV and NtrNHO = 4.7 eV.

Both reactions start with the formation of a van der Waals complex, NHO and trNHO, both at −1.0 kcal mol–1 in the explored potential energy surface. The NHO and trNHO systems are predicted to have a singlet electronic ground state; their triplet electronic states are 91.9 and 91.7 kcal mol–1 (MP2(DCM)/6-31+G(d,p) level of theory) higher in energy, respectively.

We note that this CO2 transformation process should be faster using the NHO system. However, the activation energy barrier, ΔE‡, of the trNHO-mediated process is only 2.3 kcal mol–1 higher. Therefore, the new trNHO system is a suitable chemical system for CO2 sequestration as good as their counterpart NHOs. Both carboxylation reactions are exoergic.

The NHO–CO2 adduct, with a Cα–CCO bond length of 1.57 Å, is more stable than trNHOadd by 6.0 kcal mol–1, in which the just formed C–C bond is slightly longer, 1.58 Å. Lu and co-workers[7] pointed out that, in general, NHO–CO2 species with longer Cα–CCO bond distances are decarboxylated more easily, and the length of the Cα–CCO bond is correlated to the catalytic activity. Moreover, they observed that the length of the CNHCα–CCO bond is significantly shorter, in the range of 1.52–1.53 Å,[62−64] than that of the CNHOα–CCO bond (1.55–1.60 Å), and that the reactivity of NHO–CO2 systems toward nucleophile-promoted reactions was superior. More details about their optimized geometric parameters are seen in Supporting Information.

The CO2 sequestration by trNHO and NHO was carried out using the NCI (Figure ) and IBO (Figure ) approaches. With the color scale adopted by the NCI analysis, we can visualize and identify the type of interaction present: green indicates weak interactions such as dispersion forces; red is used for strong nonbonded overlaps, and blue stands for strong attractive interactions. Both the NHO and trNHO systems exhibit extended green regions, indicating how important are noncovalent interactions to drive the reaction. In the NHO and trNHO species, the blue area indicates a strong interaction, that is, the formation of the C–C bond. For the adducts, the green area is reduced, and weak forces are present between the oxygen atoms of CO2 and respective rings.

Figure 3

Six active IBOs for the NHO- and trNHO-mediated processes; electron pairs are IBOs of the same color. The active IBO that changed the most along the reaction path, numbered as 1, has its fraction of electrons assigned to individual atoms shown in parentheses.

Following the active IBOs along the reaction path, a detailed mechanism in terms of curly arrows can be derived. More details about the theory is given elsewhere,[45−47] and here we retain ourselves to the interpretation of results. Six active IBOs were seen to change their nature during the reaction, which are displayed in Figure . The IBO that changes the most along the reaction path is identified as 1 in Figure . For this IBO, we show in parentheses the fraction of electrons assigned to the individual atoms. Following IBO 1 in both types of mediated process, we see that the π bond between C3 and C6 turns into a new σ bond, which is mainly composed of C6 and CCO atoms; the C3 has a small contribution of 0.056 (0.063) electrons for this new C–C bond in NHO–CO2 (trNHO–CO2) systems. It is interesting to note how the C6–CCO bond formation in the TSs is translated by the NCI and the IBO approaches: NCI indicating a stronger interaction between C6 and CCO in blue and IBO indicating the bond formation by changes in the fraction of electrons seen between these atoms.

Effect of N-Substituents on CO2 Capture

Next, changes in the activation energy barriers (ΔE‡) and reaction energies (ΔERXN) resulting from the presence of different groups at N2 and N4 positions of the trNHO ring were analyzed. A similar analysis for the NHO system was not performed, as it has been done in detail by Dong and collaborators.[18] The goal of the current step is to rationalize, for the first time, how we can modulate the energetics of CO2 capture in trNHO-mediated processes by varying the N-substituents of the triazolium ring.

To do the comparison, we use as reference NHO and trNHO with methyl groups at both N2 and N4 positions. First, the effect of substituents on trNHO was considered separately at N2 and N4 positions, that is, by varying the groups at N2 from 1a to 1k with a methyl group at N4 and then varying the groups at N4 with a methyl group at N2. The results of this first step are summarized in Table . For completeness, the values of ΔE‡ and ΔERXN for the reference systems are also given in Table . The combined effect of changing the groups attached at N2 and N4 was explored further with substituents that reduced the barrier the most.

Table 1

Activation Energy Barrier (ΔE‡) and Reaction Energy (ΔERXN) in kcal mol–1 for the Carboxylation Process Involving the Reference NHO, the Reference trNHO, and trNHO with Different Substituents (R Ranges from 1a to 1k) at N2 or N4a

N2 substituents			N4 substituents
	ΔE‡	ΔERXN	ΔE‡	ΔERXN
NHO	3.2	–12.5	3.2	–12.5
trNHO	5.5	–6.5	5.5	–6.5
1a	5.8	–5.7	5.2	–6.6
1b	5.9	–5.3	5.4	–5.9
1c	7.2	–4.0	5.6	–6.7
1d	7.5	–3.8	6.1	–6.1
1e	4.5	–8.0	4.3	–8.0
1f	7.9	–1.4	7.9	–2.8
1g	7.5	–2.0	7.5	–3.1
1h	6.9	–4.0	5.9	–5.9
1i	5.9	–5.6	4.2	–7.4
1j	6.0	–5.4	4.6	–10.9
1k	6.5	–5.1	5.9	–7.0

The relative energies were obtained using the EcorrectedDLPNO-CCSD(T) values.

According to our calculations, all of the explored CO2 fixation processes are exoergic. Substituents 1f and 1g lead to less stable carboxylates, regardless of the position (N2 or N4), with decarboxylation barriers in the range of 9.3–10.7 kcal mol–1. The most exoergic process (ΔERXN) involves the group 1j attached at the N4 position. Substitutions at the N4 atom are more effective in stabilizing the adducts than substitutions made at N2. Therefore, improved carboxylates for CO2 storage might be better tailored by changing the groups at N4.

Groups 1f and 1g are responsible for the largest activation barriers (ΔE‡ > 7.0 kcal mol–1) for substitutions carried out at the N4 atom. Among the N2 set, groups 1c, 1d, 1f, and 1g follow this same trend. According to the ΔE‡ values in Table , the fastest trNHO-mediated process should be observed when the −CH3 group is replaced by the −C(CH3)3 (1e) group for functionalizations at N2; at N4, this is accomplished using the methoxyaryl group 1i. In addition, the replacement of the methyl group at N4 by 1a, 1b, 1e, 1i, or 1j also results in activation barriers with a lower energy than the reference value (5.5 kcal mol–1).

Next, we are interested in the nature and magnitude of the interaction energies between the trNHO and CO2 moieties in all of the TSs related to the carboxylation process mediated by a particular N-substituted trNHO species. To do that, the Su–Li EDA analysis was performed. The main goal is to identify the relative contributions of the individual energy terms to the stabilization/destabilization of TSs and, hence, the terms that most affect the rate of reaction. Moreover, such decomposition analysis might be extrapolated to also shed some light on the relative stability of van der Waals complexes and adducts.

In addition, distortion energy (ΔEdist), the energy necessary to distort the fragment to that in the TS geometry, is also listed. Distortion energies for the CO2 and the many N-substituted trNHO fragments in the TS geometry are ΔEdist(CO and ΔEdist(trNHO), respectively. The addition of reactant distortion energies and the ΔEint term provides a value that correlates better with ΔE‡.[65] The results are summarized in Table . Along the discussion that follows, the chemical systems are named using labels that contain the identification of the group (1a–1k) and the position where it is attached (N2 or N4).

Table 2

Su–Li EDA Analysis and Distortion Energies (ΔEdist(CO and ΔEdist(trNHO)) of Reacting Fragments in TSsa

	ΔEes	ΔEex	ΔErep	ΔEpol	ΔEdisp	ΔEint	ΔEdist(CO2)	ΔEdist(trNHO)	ΔE‡
NHOTS	–28.8	–45.9	86.2	–13.4	–3.2	–5.1	6.4	1.3	3.2
trNHOTS	–32.5	–52.5	99.8	–17.5	–2.6	–5.3	8.2	1.9	5.5
trNHOTS-1a-N2	–34.5	–56.9	108.2	–19.7	–3.4	–6.2	9.1	1.7	5.8
trNHOTS-1b-N2	–34.7	–57.3	109.1	–19.8	–3.4	–6.1	9.0	1.9	5.9
trNHOTS-1c-N2	–35.0	–57.3	109.3	–20.5	–3.3	–6.9	9.7	2.4	7.2
trNHOTS-1d-N2	–35.1	–57.8	110.3	–21.0	–3.4	–7.0	10.0	2.8	7.5
trNHOTS-1e-N2	–30.9	–49.8	94.2	–16.3	–3.1	–5.9	7.8	1.4	4.5
trNHOTS-1f-N2	–38.6	–64.1	123.3	–25.0	–2.5	–6.9	11.4	2.8	7.9
trNHOTS-1g-N2	–38.1	–62.7	120.4	–24.2	–2.4	–7.0	11.1	2.7	7.5
trNHOTS-1h-N2	–35.5	–58.8	112.4	–21.4	–3.4	–6.7	10.1	2.7	6.9
trNHOTS-1i-N2	–35.3	–58.2	110.3	–20.2	–3.1	–6.4	9.1	2.1	5.9
trNHOTS-1j-N2	–35.6	–59.0	112.1	–20.6	–3.1	–6.2	9.2	2.1	6.0
trNHOTS-1k-N2	–34.9	–57.1	108.7	–20.5	–3.4	–7.2	9.7	2.2	6.5
trNHOTS-1a-N4	–33.1	–55.3	104.8	–18.7	–3.7	–6.0	8.7	2.2	5.2
trNHOTS-1b-N4	–33.0	–55.4	105.0	–18.7	–3.7	–5.8	8.6	2.3	5.4
trNHOTS-1c-N4	–34.5	–56.1	106.6	–19.6	–3.3	–6.9	9.1	2.4	5.6
trNHOTS-1d-N4	–34.7	–56.5	107.5	–19.8	–3.4	–6.8	9.2	2.4	6.1
trNHOTS-1e-N4	–31.5	–51.6	97.4	–16.8	–3.5	–5.9	7.9	1.8	4.3
trNHOTS-1f-N4	–36.6	–61.4	117.7	–22.7	–2.9	–6.0	10.3	2.6	7.9
trNHOTS-1g-N4	–36.3	–60.9	116.7	–22.4	–3.0	–5.9	10.2	2.4	7.5
trNHOTS-1h-N4	–34.8	–56.9	108.4	–20.1	–3.4	–6.8	9.4	2.0	5.9
trNHOTS-1i-N4	–36.3	–59.5	112.8	–21.4	–3.6	–7.9	9.7	2.5	4.2
trNHOTS-1j-N4	–36.1	–59.1	112.1	–21.3	–3.5	–7.9	9.7	2.7	4.6
trNHOTS-1k-N4	–34.2	–55.7	105.8	–19.4	–3.4	–6.8	9.0	2.0	5.9

For completeness, the activation energy barrier (ΔE‡) is also included. The listed values are in kcal mol–1.

The trNHOTS-1e-N2 species, which is the TS associated to the fastest carboxylation reaction (ΔE‡ = 4.5 kcal mol–1) among the N2 systems set, shows the lowest reactant distortion energy (ΔEdist = ΔEdist(CO + ΔEdist(trNHO) = 9.2 kcal mol–1) and repulsion interaction energy (ΔErep = 94.2 kcal mol–1), while the slowest process has the largest ΔErep (123.3 kcal mol–1); it is also true that this latter species also exhibits the highest ΔEdist. Indeed, all of the fluoroaryl substituents at N2 have the largest distortion energies and ΔErep.

Within the N4 systems set, the structure with the largest ΔEdist and ΔErep is trNHO-1f-N4; it is related to the carboxylation reaction with the slowest rate. Among the TSs with ΔE‡ < 5.0 kcal mol–1, a unique trend is not seen, which suggests that there is more than one way to stabilize the TS. The trNHO-1e-N4 appears to take advantage of a small ΔEdist to provide a TS, which is only 4.3 kcal mol–1 higher in energy than its reactants, whereas the other two need larger stabilizing energy terms, mainly the polarization and exchange energies, to counterbalance their higher ΔEdist.

Improving the trNHO Systems

In the present subsection, the effects of the simultaneous replacement of both −CH3 groups at N2 and N4 positions are investigated. Assuming that an additive effect of the N-substituents groups would be observed, those that reduced the activation barrier the most, ΔE‡ < 5.0 kcal mol–1, were selected. For all investigated structures, group 1e was fixed at N2 as it was the only group within this set that provided the desired barrier lowering. Among the N4 systems set, groups 1e, 1i, and 1j were chosen to be explored (see Table ). Therefore, our current model systems always have 1e attached to N2; however, they differ as the groups bonded to the N4 atom, which is 1e, 1i, or 1j, labeled as trNHO-1e-N2-1e-N4, trNHO-1e-N2-1i-N4, and trNHO-1j-N2-1e-N4, respectively, and their optimized structures are given in Figure . The idea behind this protocol is to tailor better N-substituted trNHO systems for faster CO2 sequestration and/or provide more stable carboxylates, which are responsible for CO2 storage.

Figure 4

Relative energy profiles for the carboxylation process of (A) trNHO-1e-N2-1j-N4, (B) trNHO-1e-N2-1i-N4, and (C) trNHO-1e-N2-1e-N4. For completeness, the energetic of the reaction between the reference system, trNHO, and CO2 is also indicated in (D). The energies are obtained using the EcorrectedDLPNO-CCSD(T) values and are given in kcal mol–1 with respect to the separated reactants.

Figure also presents the MEPs for the mediated reactions; for completeness, the energetic of the reaction between the reference system, trNHO, and CO2 is also indicated. All of the new redesigned trNHO systems show faster carboxylation than the reference system. The major improvements are seen with respect to the stabilization of the carboxylates systems. All carboxylates are now more stable than the trNHO–CO2 reference. The trNHOs trNHO-1e-N2-1i-N4 and trNHO-1e-N2-1j-N4 yield carboxylates with energies of −12.0 and −11.7 kcal mol–1, respectively, with respect to their reactants. These values are even better than that described for the trNHO-1j-N4 system (−10.9 kcal mol–1, Table ) and closer to the NHO–CO2 value.

To follow the C–C bond formation between CO2 and trNHO systems, the IBO related to it is shown in Figure . The van der Waals complexes, TSs, and adducts associated to the carboxylation process of trNHO-1e-N2-1e-N4, trNHO-1e-N2-1j-N4, and trNHO-1e-N2-1i-N4 are displayed.

Figure 5

IBO related to C–C bond formation between CO2 and trNHO systems; the fractions of electrons assigned to individual atoms are shown in parentheses. The carbon atoms are labeled as indicated in Figure .

In all of the van der Waals complexes, the electrons of the π bond between C3 and C6 are mainly localized at C6. The carbon atoms are labeled as indicated in Figure . In the trNHOvdW-1e-N2-1e-N4 system, the fraction of electrons in C6 is higher than that in the trNHOvdW-1e-N2-1i-N4 and trNHOvdW-1e-N2-1j-N4 species. This former van der Waals complex is the most stable one; hence, such polarization appears to contribute with such stabilization, even though its reacting fragments are separated by 3.42 Å. More details about their structures are given in the Supporting Information. A similar trend is observed along the TSs of these systems. Their C6–CCO bond lengths were found to be 2.39, 2.29, and 2.29 Å for trNHOTS-1e-N2-1e-N4, trNHOTS-1e-N2-1i-N4, and trNHOTS-1e-N2-1i-N4, respectively. When the reaction is complete, a new C–C bond is formed. There are contributions of C3, C6, and CCO to this new σ bond. The three adducts have the same C6–CCO bond length (1.57 Å), which is comparable to our reference trNHO–CO2 (1.58 Å) and NHO–CO2 (1.57 Å) values. Hence, the adduct stability and decarboxylation energy are not exclusively dependent on the this bond distance.

In Figure , the differences in the components of interaction energy and distortion energy of the TSs related to trNHOTS-1e-N2-1e-N4, trNHOTS-1e-N2-1i-N4, and trNHOTS-1e-N2-1j-N4 with respect to the reference system trNHOTS are presented.

Figure 6

Differences in the components of the interaction energy and distortion energy for TSs trNHOTS-1e-N2-1e-N4, trNHOTS-1e-N2-1i-N4, and trNHOTS-1e-N2-1j-N4 with respect to the reference system trNHOTS. X = es, ex, rep, pol, disp, int, and dist.

It is clear that the repulsion term, which arises from the partially overlapping electron densities of the interacting reacting fragments, has an impact on the energy of the TSs: the ΔErep of trNHOTS-1e-N2-1e-N4, the lowest-energy TS, is smaller than that of trNHOTS; the opposite behavior is seen in the other two TSs. Large distortions lead to larger barriers; for trNHOTS-1e-N2-1e-N4, the distortion energy is kept below the reference system as well as its energy.

Overall, a lowering of the TS energy can be mainly attributed to a combination of favorable exchange and orbital polarization interactions and a low distortion energy and repulsion interaction between the reacting systems. The polarization interaction and exchange terms, in particular, can be related to the IBO shown in Figure . ΔEpol refers to an orbital relaxation energy arising from the bond formation, polarization, and charge-transfer processes, whereas ΔEex represents the stabilizing interaction due to the exchange of electrons between the monomers. During the formation of a new σ C–C bond, the orbitals change significantly; trNHOTS-1e-N2-1i-N4 and trNHOTS-1e-N2-1j-N4 have larger fractions of electrons being transferred among C3, C6, and CCO and hence larger ΔEpol and ΔEex terms compared to those of trNHOTS. However, the superior values of polarization, exchange interactions, and other stabilizing terms in trNHOTS-1e-N2-1i-N4 and trNHOTS-1e-N2-1j-N4 are not enough to better compensate the CO2 distortion and ΔErep. Therefore, a trade-off between ΔEpol + ΔEex and ΔEdist + ΔErep is important in designing trNHO systems for a faster CO2 capture.

Conclusions

In the present study, new trNHOs systems were designed using computational chemistry. Their ability to capture CO2 is comparable to that of its NHO counterpart. The reference trNHO–CO2 is less stable than the reference NHO–CO2, and its decarboxylation energy barrier is 3.7 kcal mol–1 lower than that of NHO–CO2; therefore, the trNHO adduct releases CO2 more easily.

The presence of different substituents at different N positions of the trNHO ring can favor either a faster carboxylation or a more stable adduct. Sole substitutions at the N4 atom are more effective in stabilizing the adducts than the respective one made at the N2 position; therefore, N4-functionalized systems should be better for CO2 storage; among the discussed systems, trNHO-containing methoxyaryl can be a good choice. On the other hand, if free trNHO catalysts are the choice and CO2 has to be released, those containing fluoroaryl groups might be a better system.

To result in even faster carboxylation/decarboxylation processes and more stable adducts, a combined substitution at the N2 and N4 positions of the trNHO ring can be made. An additive effect, which might be expected, of the substitution of both N2 and N4 was not observed. The lowering of the energy barrier can be accomplished by a combination of favorable exchange and orbital polarization interactions and a low distortion energy and repulsion interaction between the reacting systems. More stable carboxylates, better systems for CO2 storage, can be tailored by the functionalization using methoxyaryl groups at N4 and the −C(CH3)3 substituent at N2.