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Exploring the reactivity of carbene supported diboraanthracene towards dihydrogen activation
Polyhedron 170 (2019) 666–673
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Polyhedron journal homepage: www.elsevier.com/locate/poly
Exploring the reactivity of carbene supported diboraanthracene towards dihydrogen activation Chayanika Kashyap, Shahnaz S. Rohman, Sabnam S. Ullah, Ankur K. Guha ⇑ Advanced Computational Chemistry Centre, Department of Chemistry, Cotton University, Panbazar, Assam 781001, India
a r t i c l e
i n f o
Article history: Received 28 April 2019 Accepted 19 June 2019 Available online 2 July 2019 Keywords: NHC Diboraanthracene H2 activation Quantum chemical calculation Reversibility
a b s t r a c t Quantum chemical calculations have been carried out on some N-heterocyclic carbene (NHC) stabilized boraanthracenes to investigate their possibility to act as H2 activators. Different coordination modes such as normal, abnormal and remote NHC are considered. Moreover, a 1,3,2,5-diazadiborinine molecule, which is experimentally known to activate H2 has also been considered for comparison. All the studied systems have a lower HOMO–LUMO gap than this molecule, an important factor for rendering higher reactivity. Quasiclassical trajectory for the reaction between H2 and these molecules indicates a single dynamically concerted step. Electronic structure analysis reveals synergism between donation and back donation in the activation process. The effect of substituents has also been studied which reveals that electron withdrawing substituents increase the activation barrier while electron donating substituents decrease it. The position of the substituents is also very crucial so far as the reactivity is concerned. With remote NHC stabilized boraanthracenes, it may be possible to achieve a low kinetic barrier and thermodynamic reversibility for activation of H2. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Since the first observation of H2 splitting by Sabatier [1], most small molecule activation have been dominated by transition metal complexes [2]. However, the last decade has witnessed a tremendous advancement in metal free activation of small molecules [3]. Among them are the group 14 dimetallynes or dimetallenes [4], stable singlet carbenes [5], and frustrated Lewis pairs (FLPs) [6]. Recently, we have theoretically designed a handful of FLP based on carbene-borane system and elucidate their capability to act as reversible catalyst for dihydrogen activation [7]. Boron containing heteroarenes have shown promise towards activation of enthalpically strong bonds [8–13]. Among them, the dianionic 9,10-biboraanthracene (I, Scheme 1) [8]. N-heterocyclic carbene (NHC) stabilized neutral 9,10-diboraanthracene (II) [9] 1,3,2,5diazadiborinine (III/IV) [10,11] and 1,4,2,5-diazadiborinine (V) [12]. However, among these boron-heteroarenes, only the dianionic species I and the neutral species IV were shown to activate enthalpically strong H2 bonds [8,12] while the others are capable of activating different enthalpically strong bonds. Hydrogen being the most abundant element on earth, H2 is extensively used in
⇑ Corresponding author. E-mail address: [email protected] (A.K. Guha). https://doi.org/10.1016/j.poly.2019.06.043 0277-5387/Ó 2019 Elsevier Ltd. All rights reserved.
many biological [14] and synthetic [15] processes. Therefore, activation of H2 is extremely important just like other enthalpically strong bonded small molecules. The limitation of H2 activation by neutral boron-heteroarenes is described due to the intrinsic synthetic challenges to construct such a neutral and highly reactive aromatic platform [12]. Kinjo et al. [12] envisioned that extension of a 6p-system over the central ring may engineer a small HOMO– LUMO gap in the neutral derivatives imparting higher reactivity. They could successfully design a neutral boron-heteroarene IV which possessed a small HOMO–LUMO gap and could successfully activate H2 molecule [12]. The HOMO–LUMO gap of these molecules is a very important parameter for H2 activation. During the activation of H2, the HOMO of these molecules will donate to the HAH r* antibonding orbital while the LUMO will accept electron from the HAH r bonding orbital. Thus, a smaller HOMO–LUMO gap is very crucial to achieve higher reactivity. We also envision that 9,10-diboraanthracene (II) stabilized by a NHC may get similar delocalization over the central ring due to the extended conjugation and may result in a small HOMO–LUMO gap which may render higher reactivity. We, therefore, investigated the reactivity of NHC stabilized 9,10-diboraanthracene (II) towards H2 activation. In addition, herein, we have also investigated the reactivity of hitherto unknown 9,10-diboraanthracene (II) stabilized by abnormal [16] and remote carbenes [17]. Scheme 2 provides the schematic representation of the considered molecules in this study. Electron
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Scheme 1. Boron-heteroarenes capable of activating small molecules.
Scheme 2. Schematic representation of 9,10-diboraanthracenes (1–3) supported by normal (1), abnormal (2) and remote carbenes (3) as well as Kinjo’s 1,3,2,5diazadiborinine (4) molecule considered in this study.
their respective Hessian (matrix of analytically determined second order derivative of energy) led to all real valued frequencies while the transition states were characterized as stationary points with one imaginary frequency. The transition states were further verified by intrinsic reaction coordinate (IRC) [19] analysis at the same level of theory. All energies were zero point corrected. Natural bond orbital (NBO, version 3.1) [20] analyses were carried out to understand the electronic feature of the catalytic process. Nucleus independent chemical shift (NICS) [21] calculation was carried out by using the GIAO approach in order to measure the extent of aromaticity. NICS calculations were performed by placing a ghost atom (symbol Bq) 1 Å above the geometric mean of the central ring containing the boron atoms.
donating (OMe) and electron withdrawing (NO2) substituents have also been considered. The 1,3,2,5-diazadiborinine molecule (4) reported by Kinjo et al. [12] has also been considered for comparison. We have adopted a simplified nomenclature throughout the text. For example, 1H means 9,10-diboraanthracene with L = 1 and R = H. 2. Computational details All the structures were fully optimized without any symmetry constraint using M06-2X/6-31+G* level of theory [18a] as this functional has been reported to produce better result in predicting the general trends in the conformer relative energies and identifying the global minimum conformer [18b]. Moreover, this functional is prescribed as one of the best for main group thermochemistry [18a]. Harmonic frequency calculations were performed at the same level of theory to understand the nature of the stationary states in the potential energy surface. All reactants, intermediates and products were verified as local minima by confirming that
Fig. 2. Frontier orbitals of 1H and 4 (Hydrogen atoms are omitted for clarity): (a) HOMO of 1H (b) LUMO of 1H (c) HOMO of 4 (d) LUMO of 4 and (e) HOMO–LUMO gap (eV) of all the molecules.
Fig. 1. Optimized geometries of the molecules with R = H for 1–3. Bond lengths are in Å.
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Table 1 Activation barrier and formation energies (kcal/mol) for H2 activation. Values within parenthesis refers to the activation energies with substituents attached to the carbon atom nearest to the boron centre (see Scheme 3 below). Reactions
Ea
1H + H2 1NO2 + H2 1Meo + H2 2H + H2 2NO2 + H2 2MeO + H2 3H + H2 3NO2 + H2 3MeO + H2 4 + H2
5.52 18.69 (42.4) 13.17 (43.9) 9.12 21.19 (43.2) 16.29 (37.2) 6.68 9.14 (60.0) 5.22 (50.6) 15.61
DG 39.37 36.11 40.55 49.15 45.53 50.75 20.20 22.02 20.17 33.86
( 21.2) ( 24.5) ( 23.6) ( 21.9) ( 14.3) ( 16.3)
Integration of the trajectories of H2 addition has been performed using BOMD option. Charge decomposition analyses (CDA) [22] have been performed using the Multwfn program [23]. All calculations were performed using the Gaussian 16 suite of programs [24].
3. Results and discussion The optimized geometries of all the molecules with R = H are shown in Fig. 1. The central diboron ring in all the molecules is perfectly planar. The B-Cc (Cc means carbenic carbon) distance is shortest in 3H which might be due to the stronger r donating ability of remote carbenes [25]. The carbenes attached to B atoms
Fig. 3. Optimized geometries of the transition states of 1H, 2H, 3H and 4 and the products. Bond lengths are in Å. Expect the reactant H2 molecule, all other hydrogen atoms are omitted for clarity.
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in 1–3 are tilted from the plane of the central diboron ring. The phenyl groups attached to the B atoms in 4 adopt a perfectly perpendicular arrangement with respect to the central diboron ring. Since, the HOMO–LUMO gap of these molecules is a very important property with regards to their reactivity towards H2 molecules [12], we have calculated the HOMO–LUMO gap of all these molecules (Fig. 2). The HOMO and LUMO in DFT theory are nothing but Kohn-Sham orbitals. These Kohn-Sham orbitals and their associated eigenvalues are very suitable for qualitative chemical applications. The HOMO of these molecules will interact with the r* orbital of HAH bond while the LUMO will interact with the r orbital of HAH molecule. Thus, lower HOMO–LUMO gap is expected to increase the reactivity of these species towards H2 activation [12]. The HOMO of 1H and 4 are similar in nature in that they feature a p bonding interaction between B and C atoms in 1H and BAN AC p interactions in 4. The LUMO is a p* orbital delocalized over almost over the entire molecule. Interestingly, the presence of carbene donors (as in 1–3) significantly lowers the HOMO–LUMO gap (values are given in Table S1, supporting information) as compared to 4 which is expected to have a smaller HOMO–LUMO gap due to the presence of extended p-conjugation [12]. Thus, lower HOMO– LUMO gaps in 1–3 compared to 4 may lead to their higher reactivity which in turn, may lead to their feasibility to activate dihydrogen molecules. It is to be noted that remote carbene containing diboraanthracene 3 has the smallest HOMO–LUMO gap and is expected to be the most effective in activating dihydrogen. However, the substituents OMe and NO2 have very little effect on the HOMO–LUMO gaps of these molecules. We then turned our attention to investigate the possibility of dihydrogen activation by 1–3. Table 1 contains the activation barrier (Ea) and Gibbs free energies (DG) of the product formation. We would like to point out here that we were unable to optimize the pre-reactive complex as during optimization, the pre-reactive complexes either end up in transition state like structures with first or higher order saddle points or they end up in product geometries. We also tried them from an IRC pathway. They lead to the same result and hence, we have not discussed the formation of any pre-reactive complex during this activation pathway. It is evident from Table 1 that all these NHC-supported diboraanthracenes need to overcome small to moderate activation barriers to achieve H2 activation. The formation energies are all significantly negative suggesting spontaneity of all these reactions. We have also included 1,3,2,5-diazadiborinine (4) reported by Kinjo et al. [12] for comparison. It is to be noted that the parent diboraanthracenes 1H, 2H and 3H have lower activation barriers than NO2 and MeO substituted ones. The electron withdrawing nature of NO2 reduces the electron density of the central ring and thereby increases the activation barrier while the electron donating nature of MeO decreases the activation barrier. Interestingly all the parent molecules 1H, 2H and 3H have lower activation barriers than the substituted ones as well as 4 which was shown to activate H2 experimentally [12]. Fig. 3 contains the optimized geometries of the transition states of 1H, 2H and 3H and 4. The two boron atoms in TS1H, TS2H and TS3H are distorted upward from the plane of the other carbon atoms of the central ring while for TS4, the two boron atoms are in the same plane as the other carbon atoms of the central ring. This could possibly the reason behind the longer B-H distance in TS4. The computed natural charges on both the H atoms in TSs and in products are almost equal suggesting homolytic cleavage of the HAH bond. Further, to investigate the effect of the substituents R at the carbon atom closer to the boron atom, we have considered the molecules shown in Scheme 3. The position of the R group closer to B atom is expected to affect the reactivity of these molecules towards H2 activation. The activation energies as well as the Gibbs free energies of formation are collected in Table 1. It is evident from
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Scheme 3. Schematic representation of (1–3) having substituents R at the carbon atom closer to the boron atom.
Table 1 that introduction of R closer to B atom dramatically increases the activation barrier. Moreover, the Gibbs free energy changes are also less exergonic suggesting that the position of R is very important so far as the activity of these molecules is concerned. We therefore, did not considered these molecules shown in Scheme 3 for further analysis. The trajectories of H2 addition to 1H, 2H, 3H and 4 were explored by employing quasiclassical trajectories (QCTs) using (U)M062X/6-31+G* level with Born-Oppenheimer molecular dynamics (BOMD) option in Gaussian 16. Fig. 4 shows the trajectories of H2 addition at different intervals of time (in fs). It is evident from Fig. 4 that the formation of the first B-H bond takes about 26, 27, 29 and 32 fs for 1H, 2H, 3H and 4 respectively. This suggests that the formation of the first B-H bond takes almost equal time for all these molecules. However, the time gap between the formation of the first and second B-H bonds are 6, 8, 9 and 13 fs. All these time gaps are very short suggesting that the addition of H2 to these molecules occurs in a single, dynamically concerted step. Further to understand the electronic feature behind this activation process, we have carried out natural bond orbital analysis of the transition state structures TS1H of 1H. Fig. 5 shows the frontier TS orbitals of TS1H. uTS 1 is the B-H r bonding orbital while u2 is the HAH r bonding orbital. The occupancies of these orbitals are significant which suggest that formation of the B-H bond has started in TS1H. It should also be noted that the HAH r antibonding (uTS 3 ) orbital in TS1H has occupancy of 0.12 e which suggests that breaking of the HAH bond has started in TS1H. Thus, NBO analysis reveals that both B-H bond making and HAH bond breaking have started in the transition states. Hence, the synergism between the donation from HAH r bonding orbital and backdonation to HAH r antibonding orbital is the dominating electronic feature for the activation process. Further to quantify the charge donation and back-donation, we have carried out CDA of the transition states. As the reactants and the H2 molecule are closed shell species, the application of CDA to quantify charge donation/backdonation will be appropriate. Table 2 contains the numerical data. Since the residue terms are nearly zero, the interaction can be described by familiar Dewar-Chatt-Duncanson donor–acceptor model [26]. The CDA results clearly quantify that there is significant donation from the HAH r bond to the empty orbitals on B in the ring as well as significant back donation to the HAH r* antibonding orbital supporting that bond breaking and making process have already started in the transition states. The repulsive terms are all negative indicating reduced closed-shell repulsion [22]. Further, we have performed Bickelhaupt’s activation strain model [27] of chemical reactivity to have a better understanding
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Fig. 4. Plot of typical QCTs for the H2 addition to the 1H, 2H, 3H and 4 respectively in different intervals of time (fs).
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Fig. 5. Frontier occupied molecular orbitals of TS1H with their occupancies (in e).
Table 2 Charge decomposition analyses (CDA) of the transition states calculated at M06-2X/631+G* level of theory. Here donation means donation from the HAH r bonding orbital to empty orbitals of B while the back-donation means back-donation to HAH r* antibonding orbital from the boron ring. Entry
Donation (d)
Back-donation (b)
TS1H TS1NO2 TS1MeO TS2H TS2NO2 TS2MeO TS3H TS3NO2 TS3MeO TS4
0.092 0.068 0.072 0.105 0.099 0.097 0.112 0.109 0.114 0.082
0.062 0.052 0.066 0.073 0.071 0.067 0.081 0.078 0.068 0.054
Repulsion (r) 0.331 0.312 0.322 0.348 0.339 0.333 0.314 0.331 0.317 0.319
Residue (D) 0.023 0.021 0.023 0.032 0.031 0.011 0.022 0.026 0.023 0.028
of the variation of the activation energy with the degree of deformation that the reactant undergoes during the activation process. In this model, the activation energy (DEà) of the transition state is decomposed into strain energy DEàstrain and interaction energy DEàint between the reactants. The calculated values of DEàstrain and DEàint are collected in Table 3. We observed a very good correlation (Fig. 6) between the activation energy and strain energy (R2 = 0.96) which implies that an increase in strain increases the reaction barrier. This is also in tune with Hammond’s postulate [28]; the lower the DEàstrain value, the closer the TS is to the reactant, and hence, the lower the activation barrier. Recently, we [7] and previously others [29] also noticed the same correlation. We then turned our attention to investigate the effect of the HOMO–LUMO gap on activation barriers as previously suggested by Kinjo et al. [12] that a smaller HOMO–LUMO gap increases the reactivity of the system, i.e., decreases the activation barrier. Barring a few discrepancies, a good correlation is obtained (Fig. 7) between the HOMO–LUMO gap and activation energies which is in agreement with the previous suggestion by Kinjo et al. [12]. Thus engineering a small HOMO–LUMO gap for these kind of systems is expected to increase their reactivity by lowering the activation barriers. All the NHC supported boraanthracenes
have a lower HOMO–LUMO gap and hence are expected to show a better H2 activating ability. Reversibility of these systems is an important aspect to consider. Since the storage of hydrogen is a difficult phenomenon, and hence, a reversible system may become an efficient one for the purpose. Can the reversibility of H2 addition be possible? A closer look at Table 1 suggests that a lower kinetic barrier and lesser exergonicity were found for 3. The activation barrier for 3 lies between 5 and 9 kcal/mol while the exergonicities of the reactions lies between 20 and 22 kcal/mol. What may cause these systems to have lower kinetic barrier as well as lesser exergonicity of the reaction? Recently, we have shown that reduction in aromatic character may lead to better reversibility in terms of less exergonicity of product formation of frustrated Lewis pairs [7].
Fig. 6. Correlation plot of strain energies (kcal/mol) of the considered molecules versus the activation barriers (kcal/mol).
Table 3 M06-2X/6-31+G* single point calculations of the strain energy (DEàstrain, in kcal/mol) and interaction energy (DEàint, in kcal/mol) of the transition states. Molecule H
1 1NO2 1Meo 2H 2NO2 2MeO 3H 3NO2 3MeO 4
DE à 4.3 12.9 8.2 7.6 14.3 13.1 5.1 7.4 4.33 13.43
DEàint 3.2 9.5 7.2 5.6 11.2 7.6 4.5 2.2 3.9 8.8
DEàstrain 7.1 15.3 12.1 10.2 16.3 15.4 7.7 10.3 6.2 14.2
Fig. 7. A plot of HOMO–LUMO gap (eV) and activation energy (kcal/mol) of the studied systems.
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reveals that bond making and bond breaking have started in the transition states which might be a manifestation of synergism between donation from HAH r bonding orbital and back donation to HAH r* antibonding orbital. The strain energy (DEàstrain) increases with an increase in the reaction barrier (DEà) which is also in tune with previous studies [7,29] and Hammond’s postulate [28]. The reduction in aromatization effect may help in gaining reversibility towards H2 activation by 3. Recently, Wang et al. have described doubly reduced 9,10-dihydro-9,10-diboraanthrancenes as a mimic of transition metal because they not only activate H2 but also subsequently use both hydrogen atoms for further synthesis [31]. We therefore, feel that our study will provide impetus to the experimentalists to have a closer look at these boraanthracenes for possible H2 activation and explore their reactivity further. Acknowledgments Fig. 8. Correlation between DG of product formation against the NICS-1ZZ values of the transition states.
Our recent analysis is in accordance with the previous results of Wang et al. [29a] Previously, Devarajan et al. [30] have shown that for the reversible activation of H2 by a main group divalent atom, the higher the activation barrier lower the possibility of achieving reversibility. We are, therefore, interested in investigating whether aromatization has any effect on exergonicity of the presently studied systems or not. If the aromatization effect is reduced, the hydride product is destabilized and hence, a lower exergonicity may be obtained. For this, we have performed nucleus independent chemical shift (NICS) [21] analysis by placing a ghost atom (symbol Bq) 1 Å above the geometric mean of the central ring (designated as NICS-1ZZ) of transition states. Fig. 8 depicts the variation of NICS-1ZZ values (numerical values are given in Table S2, supporting information) in the transition states against the DG of product formation. A nice correlation has been obtained (R2 = 0.82) between the chosen parameters. Lower aromaticity (less negative NICS-1ZZ values) of the transition states results in less exergonicity of the product formation. For example, the NICS-1ZZ values of the transition states of 3 are less negative (less aromatic) and consequently their product formation is less exergonic (Table 1). Hence, lower aromaticity in the transition states may lead to better reversibility of the system. Thus, among the studied systems, remote NHC stabilized boraanthracenes 3 have statistically better reversibility as they possess a low kinetic barrier as well as lower exergonicity of the reaction. 4. Conclusions Quantum chemical calculations have been carried out on Nheterocyclic carbene (NHC) stabilized boraanthracenes to investigate their possibility towards H2 activation. Experimentally known normal NHC (1) [9] and hitherto unknown abnormal (2) and remote (3) NHC stabilized boraanthracenes have been considered in this study. Moreover, the experimentally well characterized 1,3,2,5-diazadiborinine (4) molecule has also been considered for comparison. The engineering of a small HOMO–LUMO gap has been suggested to impose enough reactivity to 4 towards H2 activation [12]. All these NHC stabilized boraanthracenes (1–3) have a lower HOMO–LUMO gap compared to 4 which suggest that 1–3 may have potential towards H2 activation. In fact, a very good correlation is obtained between the HOMO–LUMO gap and the activation energies. The position of the substituents R is very crucial for their reactivity towards H2 activation. Quasiclassical trajectory for the reaction between H2 and these molecules indicates a single dynamically concerted step. The electronic structure analysis
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Exploring the reactivity of carbene supported diboraanthracene towards dihydrogen activation Chayanika Kashyap, Shahnaz S. Rohman, Sabnam S. Ullah, Ankur K. Guha ⇑ Advanced Computational Chemistry Centre, Department of Chemistry, Cotton University, Panbazar, Assam 781001, India
a r t i c l e
i n f o
Article history: Received 28 April 2019 Accepted 19 June 2019 Available online 2 July 2019 Keywords: NHC Diboraanthracene H2 activation Quantum chemical calculation Reversibility
a b s t r a c t Quantum chemical calculations have been carried out on some N-heterocyclic carbene (NHC) stabilized boraanthracenes to investigate their possibility to act as H2 activators. Different coordination modes such as normal, abnormal and remote NHC are considered. Moreover, a 1,3,2,5-diazadiborinine molecule, which is experimentally known to activate H2 has also been considered for comparison. All the studied systems have a lower HOMO–LUMO gap than this molecule, an important factor for rendering higher reactivity. Quasiclassical trajectory for the reaction between H2 and these molecules indicates a single dynamically concerted step. Electronic structure analysis reveals synergism between donation and back donation in the activation process. The effect of substituents has also been studied which reveals that electron withdrawing substituents increase the activation barrier while electron donating substituents decrease it. The position of the substituents is also very crucial so far as the reactivity is concerned. With remote NHC stabilized boraanthracenes, it may be possible to achieve a low kinetic barrier and thermodynamic reversibility for activation of H2. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Since the first observation of H2 splitting by Sabatier [1], most small molecule activation have been dominated by transition metal complexes [2]. However, the last decade has witnessed a tremendous advancement in metal free activation of small molecules [3]. Among them are the group 14 dimetallynes or dimetallenes [4], stable singlet carbenes [5], and frustrated Lewis pairs (FLPs) [6]. Recently, we have theoretically designed a handful of FLP based on carbene-borane system and elucidate their capability to act as reversible catalyst for dihydrogen activation [7]. Boron containing heteroarenes have shown promise towards activation of enthalpically strong bonds [8–13]. Among them, the dianionic 9,10-biboraanthracene (I, Scheme 1) [8]. N-heterocyclic carbene (NHC) stabilized neutral 9,10-diboraanthracene (II) [9] 1,3,2,5diazadiborinine (III/IV) [10,11] and 1,4,2,5-diazadiborinine (V) [12]. However, among these boron-heteroarenes, only the dianionic species I and the neutral species IV were shown to activate enthalpically strong H2 bonds [8,12] while the others are capable of activating different enthalpically strong bonds. Hydrogen being the most abundant element on earth, H2 is extensively used in
⇑ Corresponding author. E-mail address: [email protected] (A.K. Guha). https://doi.org/10.1016/j.poly.2019.06.043 0277-5387/Ó 2019 Elsevier Ltd. All rights reserved.
many biological [14] and synthetic [15] processes. Therefore, activation of H2 is extremely important just like other enthalpically strong bonded small molecules. The limitation of H2 activation by neutral boron-heteroarenes is described due to the intrinsic synthetic challenges to construct such a neutral and highly reactive aromatic platform [12]. Kinjo et al. [12] envisioned that extension of a 6p-system over the central ring may engineer a small HOMO– LUMO gap in the neutral derivatives imparting higher reactivity. They could successfully design a neutral boron-heteroarene IV which possessed a small HOMO–LUMO gap and could successfully activate H2 molecule [12]. The HOMO–LUMO gap of these molecules is a very important parameter for H2 activation. During the activation of H2, the HOMO of these molecules will donate to the HAH r* antibonding orbital while the LUMO will accept electron from the HAH r bonding orbital. Thus, a smaller HOMO–LUMO gap is very crucial to achieve higher reactivity. We also envision that 9,10-diboraanthracene (II) stabilized by a NHC may get similar delocalization over the central ring due to the extended conjugation and may result in a small HOMO–LUMO gap which may render higher reactivity. We, therefore, investigated the reactivity of NHC stabilized 9,10-diboraanthracene (II) towards H2 activation. In addition, herein, we have also investigated the reactivity of hitherto unknown 9,10-diboraanthracene (II) stabilized by abnormal [16] and remote carbenes [17]. Scheme 2 provides the schematic representation of the considered molecules in this study. Electron
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Scheme 1. Boron-heteroarenes capable of activating small molecules.
Scheme 2. Schematic representation of 9,10-diboraanthracenes (1–3) supported by normal (1), abnormal (2) and remote carbenes (3) as well as Kinjo’s 1,3,2,5diazadiborinine (4) molecule considered in this study.
their respective Hessian (matrix of analytically determined second order derivative of energy) led to all real valued frequencies while the transition states were characterized as stationary points with one imaginary frequency. The transition states were further verified by intrinsic reaction coordinate (IRC) [19] analysis at the same level of theory. All energies were zero point corrected. Natural bond orbital (NBO, version 3.1) [20] analyses were carried out to understand the electronic feature of the catalytic process. Nucleus independent chemical shift (NICS) [21] calculation was carried out by using the GIAO approach in order to measure the extent of aromaticity. NICS calculations were performed by placing a ghost atom (symbol Bq) 1 Å above the geometric mean of the central ring containing the boron atoms.
donating (OMe) and electron withdrawing (NO2) substituents have also been considered. The 1,3,2,5-diazadiborinine molecule (4) reported by Kinjo et al. [12] has also been considered for comparison. We have adopted a simplified nomenclature throughout the text. For example, 1H means 9,10-diboraanthracene with L = 1 and R = H. 2. Computational details All the structures were fully optimized without any symmetry constraint using M06-2X/6-31+G* level of theory [18a] as this functional has been reported to produce better result in predicting the general trends in the conformer relative energies and identifying the global minimum conformer [18b]. Moreover, this functional is prescribed as one of the best for main group thermochemistry [18a]. Harmonic frequency calculations were performed at the same level of theory to understand the nature of the stationary states in the potential energy surface. All reactants, intermediates and products were verified as local minima by confirming that
Fig. 2. Frontier orbitals of 1H and 4 (Hydrogen atoms are omitted for clarity): (a) HOMO of 1H (b) LUMO of 1H (c) HOMO of 4 (d) LUMO of 4 and (e) HOMO–LUMO gap (eV) of all the molecules.
Fig. 1. Optimized geometries of the molecules with R = H for 1–3. Bond lengths are in Å.
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Table 1 Activation barrier and formation energies (kcal/mol) for H2 activation. Values within parenthesis refers to the activation energies with substituents attached to the carbon atom nearest to the boron centre (see Scheme 3 below). Reactions
Ea
1H + H2 1NO2 + H2 1Meo + H2 2H + H2 2NO2 + H2 2MeO + H2 3H + H2 3NO2 + H2 3MeO + H2 4 + H2
5.52 18.69 (42.4) 13.17 (43.9) 9.12 21.19 (43.2) 16.29 (37.2) 6.68 9.14 (60.0) 5.22 (50.6) 15.61
DG 39.37 36.11 40.55 49.15 45.53 50.75 20.20 22.02 20.17 33.86
( 21.2) ( 24.5) ( 23.6) ( 21.9) ( 14.3) ( 16.3)
Integration of the trajectories of H2 addition has been performed using BOMD option. Charge decomposition analyses (CDA) [22] have been performed using the Multwfn program [23]. All calculations were performed using the Gaussian 16 suite of programs [24].
3. Results and discussion The optimized geometries of all the molecules with R = H are shown in Fig. 1. The central diboron ring in all the molecules is perfectly planar. The B-Cc (Cc means carbenic carbon) distance is shortest in 3H which might be due to the stronger r donating ability of remote carbenes [25]. The carbenes attached to B atoms
Fig. 3. Optimized geometries of the transition states of 1H, 2H, 3H and 4 and the products. Bond lengths are in Å. Expect the reactant H2 molecule, all other hydrogen atoms are omitted for clarity.
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in 1–3 are tilted from the plane of the central diboron ring. The phenyl groups attached to the B atoms in 4 adopt a perfectly perpendicular arrangement with respect to the central diboron ring. Since, the HOMO–LUMO gap of these molecules is a very important property with regards to their reactivity towards H2 molecules [12], we have calculated the HOMO–LUMO gap of all these molecules (Fig. 2). The HOMO and LUMO in DFT theory are nothing but Kohn-Sham orbitals. These Kohn-Sham orbitals and their associated eigenvalues are very suitable for qualitative chemical applications. The HOMO of these molecules will interact with the r* orbital of HAH bond while the LUMO will interact with the r orbital of HAH molecule. Thus, lower HOMO–LUMO gap is expected to increase the reactivity of these species towards H2 activation [12]. The HOMO of 1H and 4 are similar in nature in that they feature a p bonding interaction between B and C atoms in 1H and BAN AC p interactions in 4. The LUMO is a p* orbital delocalized over almost over the entire molecule. Interestingly, the presence of carbene donors (as in 1–3) significantly lowers the HOMO–LUMO gap (values are given in Table S1, supporting information) as compared to 4 which is expected to have a smaller HOMO–LUMO gap due to the presence of extended p-conjugation [12]. Thus, lower HOMO– LUMO gaps in 1–3 compared to 4 may lead to their higher reactivity which in turn, may lead to their feasibility to activate dihydrogen molecules. It is to be noted that remote carbene containing diboraanthracene 3 has the smallest HOMO–LUMO gap and is expected to be the most effective in activating dihydrogen. However, the substituents OMe and NO2 have very little effect on the HOMO–LUMO gaps of these molecules. We then turned our attention to investigate the possibility of dihydrogen activation by 1–3. Table 1 contains the activation barrier (Ea) and Gibbs free energies (DG) of the product formation. We would like to point out here that we were unable to optimize the pre-reactive complex as during optimization, the pre-reactive complexes either end up in transition state like structures with first or higher order saddle points or they end up in product geometries. We also tried them from an IRC pathway. They lead to the same result and hence, we have not discussed the formation of any pre-reactive complex during this activation pathway. It is evident from Table 1 that all these NHC-supported diboraanthracenes need to overcome small to moderate activation barriers to achieve H2 activation. The formation energies are all significantly negative suggesting spontaneity of all these reactions. We have also included 1,3,2,5-diazadiborinine (4) reported by Kinjo et al. [12] for comparison. It is to be noted that the parent diboraanthracenes 1H, 2H and 3H have lower activation barriers than NO2 and MeO substituted ones. The electron withdrawing nature of NO2 reduces the electron density of the central ring and thereby increases the activation barrier while the electron donating nature of MeO decreases the activation barrier. Interestingly all the parent molecules 1H, 2H and 3H have lower activation barriers than the substituted ones as well as 4 which was shown to activate H2 experimentally [12]. Fig. 3 contains the optimized geometries of the transition states of 1H, 2H and 3H and 4. The two boron atoms in TS1H, TS2H and TS3H are distorted upward from the plane of the other carbon atoms of the central ring while for TS4, the two boron atoms are in the same plane as the other carbon atoms of the central ring. This could possibly the reason behind the longer B-H distance in TS4. The computed natural charges on both the H atoms in TSs and in products are almost equal suggesting homolytic cleavage of the HAH bond. Further, to investigate the effect of the substituents R at the carbon atom closer to the boron atom, we have considered the molecules shown in Scheme 3. The position of the R group closer to B atom is expected to affect the reactivity of these molecules towards H2 activation. The activation energies as well as the Gibbs free energies of formation are collected in Table 1. It is evident from
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Scheme 3. Schematic representation of (1–3) having substituents R at the carbon atom closer to the boron atom.
Table 1 that introduction of R closer to B atom dramatically increases the activation barrier. Moreover, the Gibbs free energy changes are also less exergonic suggesting that the position of R is very important so far as the activity of these molecules is concerned. We therefore, did not considered these molecules shown in Scheme 3 for further analysis. The trajectories of H2 addition to 1H, 2H, 3H and 4 were explored by employing quasiclassical trajectories (QCTs) using (U)M062X/6-31+G* level with Born-Oppenheimer molecular dynamics (BOMD) option in Gaussian 16. Fig. 4 shows the trajectories of H2 addition at different intervals of time (in fs). It is evident from Fig. 4 that the formation of the first B-H bond takes about 26, 27, 29 and 32 fs for 1H, 2H, 3H and 4 respectively. This suggests that the formation of the first B-H bond takes almost equal time for all these molecules. However, the time gap between the formation of the first and second B-H bonds are 6, 8, 9 and 13 fs. All these time gaps are very short suggesting that the addition of H2 to these molecules occurs in a single, dynamically concerted step. Further to understand the electronic feature behind this activation process, we have carried out natural bond orbital analysis of the transition state structures TS1H of 1H. Fig. 5 shows the frontier TS orbitals of TS1H. uTS 1 is the B-H r bonding orbital while u2 is the HAH r bonding orbital. The occupancies of these orbitals are significant which suggest that formation of the B-H bond has started in TS1H. It should also be noted that the HAH r antibonding (uTS 3 ) orbital in TS1H has occupancy of 0.12 e which suggests that breaking of the HAH bond has started in TS1H. Thus, NBO analysis reveals that both B-H bond making and HAH bond breaking have started in the transition states. Hence, the synergism between the donation from HAH r bonding orbital and backdonation to HAH r antibonding orbital is the dominating electronic feature for the activation process. Further to quantify the charge donation and back-donation, we have carried out CDA of the transition states. As the reactants and the H2 molecule are closed shell species, the application of CDA to quantify charge donation/backdonation will be appropriate. Table 2 contains the numerical data. Since the residue terms are nearly zero, the interaction can be described by familiar Dewar-Chatt-Duncanson donor–acceptor model [26]. The CDA results clearly quantify that there is significant donation from the HAH r bond to the empty orbitals on B in the ring as well as significant back donation to the HAH r* antibonding orbital supporting that bond breaking and making process have already started in the transition states. The repulsive terms are all negative indicating reduced closed-shell repulsion [22]. Further, we have performed Bickelhaupt’s activation strain model [27] of chemical reactivity to have a better understanding
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Fig. 4. Plot of typical QCTs for the H2 addition to the 1H, 2H, 3H and 4 respectively in different intervals of time (fs).
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Fig. 5. Frontier occupied molecular orbitals of TS1H with their occupancies (in e).
Table 2 Charge decomposition analyses (CDA) of the transition states calculated at M06-2X/631+G* level of theory. Here donation means donation from the HAH r bonding orbital to empty orbitals of B while the back-donation means back-donation to HAH r* antibonding orbital from the boron ring. Entry
Donation (d)
Back-donation (b)
TS1H TS1NO2 TS1MeO TS2H TS2NO2 TS2MeO TS3H TS3NO2 TS3MeO TS4
0.092 0.068 0.072 0.105 0.099 0.097 0.112 0.109 0.114 0.082
0.062 0.052 0.066 0.073 0.071 0.067 0.081 0.078 0.068 0.054
Repulsion (r) 0.331 0.312 0.322 0.348 0.339 0.333 0.314 0.331 0.317 0.319
Residue (D) 0.023 0.021 0.023 0.032 0.031 0.011 0.022 0.026 0.023 0.028
of the variation of the activation energy with the degree of deformation that the reactant undergoes during the activation process. In this model, the activation energy (DEà) of the transition state is decomposed into strain energy DEàstrain and interaction energy DEàint between the reactants. The calculated values of DEàstrain and DEàint are collected in Table 3. We observed a very good correlation (Fig. 6) between the activation energy and strain energy (R2 = 0.96) which implies that an increase in strain increases the reaction barrier. This is also in tune with Hammond’s postulate [28]; the lower the DEàstrain value, the closer the TS is to the reactant, and hence, the lower the activation barrier. Recently, we [7] and previously others [29] also noticed the same correlation. We then turned our attention to investigate the effect of the HOMO–LUMO gap on activation barriers as previously suggested by Kinjo et al. [12] that a smaller HOMO–LUMO gap increases the reactivity of the system, i.e., decreases the activation barrier. Barring a few discrepancies, a good correlation is obtained (Fig. 7) between the HOMO–LUMO gap and activation energies which is in agreement with the previous suggestion by Kinjo et al. [12]. Thus engineering a small HOMO–LUMO gap for these kind of systems is expected to increase their reactivity by lowering the activation barriers. All the NHC supported boraanthracenes
have a lower HOMO–LUMO gap and hence are expected to show a better H2 activating ability. Reversibility of these systems is an important aspect to consider. Since the storage of hydrogen is a difficult phenomenon, and hence, a reversible system may become an efficient one for the purpose. Can the reversibility of H2 addition be possible? A closer look at Table 1 suggests that a lower kinetic barrier and lesser exergonicity were found for 3. The activation barrier for 3 lies between 5 and 9 kcal/mol while the exergonicities of the reactions lies between 20 and 22 kcal/mol. What may cause these systems to have lower kinetic barrier as well as lesser exergonicity of the reaction? Recently, we have shown that reduction in aromatic character may lead to better reversibility in terms of less exergonicity of product formation of frustrated Lewis pairs [7].
Fig. 6. Correlation plot of strain energies (kcal/mol) of the considered molecules versus the activation barriers (kcal/mol).
Table 3 M06-2X/6-31+G* single point calculations of the strain energy (DEàstrain, in kcal/mol) and interaction energy (DEàint, in kcal/mol) of the transition states. Molecule H
1 1NO2 1Meo 2H 2NO2 2MeO 3H 3NO2 3MeO 4
DE à 4.3 12.9 8.2 7.6 14.3 13.1 5.1 7.4 4.33 13.43
DEàint 3.2 9.5 7.2 5.6 11.2 7.6 4.5 2.2 3.9 8.8
DEàstrain 7.1 15.3 12.1 10.2 16.3 15.4 7.7 10.3 6.2 14.2
Fig. 7. A plot of HOMO–LUMO gap (eV) and activation energy (kcal/mol) of the studied systems.
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reveals that bond making and bond breaking have started in the transition states which might be a manifestation of synergism between donation from HAH r bonding orbital and back donation to HAH r* antibonding orbital. The strain energy (DEàstrain) increases with an increase in the reaction barrier (DEà) which is also in tune with previous studies [7,29] and Hammond’s postulate [28]. The reduction in aromatization effect may help in gaining reversibility towards H2 activation by 3. Recently, Wang et al. have described doubly reduced 9,10-dihydro-9,10-diboraanthrancenes as a mimic of transition metal because they not only activate H2 but also subsequently use both hydrogen atoms for further synthesis [31]. We therefore, feel that our study will provide impetus to the experimentalists to have a closer look at these boraanthracenes for possible H2 activation and explore their reactivity further. Acknowledgments Fig. 8. Correlation between DG of product formation against the NICS-1ZZ values of the transition states.
Our recent analysis is in accordance with the previous results of Wang et al. [29a] Previously, Devarajan et al. [30] have shown that for the reversible activation of H2 by a main group divalent atom, the higher the activation barrier lower the possibility of achieving reversibility. We are, therefore, interested in investigating whether aromatization has any effect on exergonicity of the presently studied systems or not. If the aromatization effect is reduced, the hydride product is destabilized and hence, a lower exergonicity may be obtained. For this, we have performed nucleus independent chemical shift (NICS) [21] analysis by placing a ghost atom (symbol Bq) 1 Å above the geometric mean of the central ring (designated as NICS-1ZZ) of transition states. Fig. 8 depicts the variation of NICS-1ZZ values (numerical values are given in Table S2, supporting information) in the transition states against the DG of product formation. A nice correlation has been obtained (R2 = 0.82) between the chosen parameters. Lower aromaticity (less negative NICS-1ZZ values) of the transition states results in less exergonicity of the product formation. For example, the NICS-1ZZ values of the transition states of 3 are less negative (less aromatic) and consequently their product formation is less exergonic (Table 1). Hence, lower aromaticity in the transition states may lead to better reversibility of the system. Thus, among the studied systems, remote NHC stabilized boraanthracenes 3 have statistically better reversibility as they possess a low kinetic barrier as well as lower exergonicity of the reaction. 4. Conclusions Quantum chemical calculations have been carried out on Nheterocyclic carbene (NHC) stabilized boraanthracenes to investigate their possibility towards H2 activation. Experimentally known normal NHC (1) [9] and hitherto unknown abnormal (2) and remote (3) NHC stabilized boraanthracenes have been considered in this study. Moreover, the experimentally well characterized 1,3,2,5-diazadiborinine (4) molecule has also been considered for comparison. The engineering of a small HOMO–LUMO gap has been suggested to impose enough reactivity to 4 towards H2 activation [12]. All these NHC stabilized boraanthracenes (1–3) have a lower HOMO–LUMO gap compared to 4 which suggest that 1–3 may have potential towards H2 activation. In fact, a very good correlation is obtained between the HOMO–LUMO gap and the activation energies. The position of the substituents R is very crucial for their reactivity towards H2 activation. Quasiclassical trajectory for the reaction between H2 and these molecules indicates a single dynamically concerted step. The electronic structure analysis
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