- Email: [email protected]
External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs Fazl-i-Sattar a,1, Arsalan Ahmed b,1, Habib Ullah c, Zakir Ullah c,d, Muhammad Tariq a, Khurshid Ayub b,* a
National Center of Excellence in Physical Chemistry, University of Peshawar, Peshawar, KPK, 25120, Pakistan Department of Chemistry, COMSATS University, Abbottabad Campus, Abbottabad, KPK, 22060, Pakistan c Institute of Chemistry, University of Peshawar, Peshawar, KPK, Pakistan d Department of Chemistry, Korea Advanced Institute of Sciences and Technology (KAIST), Daejeon, 34141, Republic of Korea b
highlights Photoswitchable hydrogen
graphical abstract catalysis
recombination
is
for re-
ported for the first time. The catalyst is rationally designed by studying proton and hydride affinities. Dithienylethene
photochrome
offer promising application for photoswitchable catalysis. The hydrogen recombination in closed form is much faster than that of the open isomer.
article info
abstract
Article history:
Photoswitchable catalysis involves changes in properties of a catalyst based on difference
Received 4 July 2019
in electronic and steric factors. These changes can selectively turn “ON” and “OFF” the
Received in revised form
catalytic activity by an external stimulus, light. Herein, we report dithienylethene based
20 September 2019
photoswitchable frustrated Lewis pairs for facile hydrogen recombination. The diary-
Accepted 7 October 2019
lethene moiety is not serving as template to alter the catalytic activity rather it is the core
Available online xxx
part of the catalyst. The rational design principle involves study of proton and hydride affinities of Lewis acid and base functionalities installed on the diarylethene photoswitch
Keywords:
pair. Proton and hydride affinities differ significantly between open and closed isomers
Dithienylethene
that help in designing an active molecule for hydrogen recombination. The proton and
Frustrated lewis pairs
hydride affinities are then utilized to formulate best relative positions of Lewis acid and base on a single molecule in order to liberate one H2 molecule per photochrome. Energy
* Corresponding author. E-mail address: [email protected] (K. Ayub). 1 Both authors contributed equally. https://doi.org/10.1016/j.ijhydene.2019.10.051 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
2
international journal of hydrogen energy xxx (xxxx) xxx
Switchable catalysis
barriers for hydrogen recombination are calculated for open and closed isomers. The
Hydrogen recombination
closed isomer releases the hydrogen with a low activation barrier of 1.9 kcal mol1 whereas the open isomer requires relatively high barrier of 4.7 kcal mol1. The pronounced differences in energy barrier illustrate the potential of photoswitchable hydrogen recombination with diarylethene photochrome. The chemically stored hydrogen in frustrated Lewis pairs can be liberated in a controlled fashion through external stimulus for catalytic hydrogenation reactions. The present study will provide new dimensions to the scientific community for exploration of other systems with even better selectivities. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Photoswitchable catalysis involves alteration in the intrinsic catalytic properties of an active species (catalytically) through a reversible photochemical transformation [1]. The catalyst may be active or inactive in its initial state however, photoinduced transformation may either alter the rate of a catalyzed reaction (A / B; k1 s k2) or may initiate a different reaction (C /D). Photoswitchable catalysts generally involves attachment of a photochromic species to a catalyst [2,3]. The photochromic moiety exists in two isomeric forms which are interconvertible through irradiation with photons of different wavelengths [4]. Therefore, photoswitchable catalysts offer different reactivities and selectivities via interconversion between two states through an external stimulus (light). The modulation of the activity is independent of the diffusion limits that are generally associated with other chemical processes. The requirements for a photoswitchable catalyst are essentially at least the same as required by a photochromic system; the interconversion between both states (forward and backward) occurs with high efficiency by using lights of different wavelengths without formation of any photostationary states [1]. Moreover, the catalyst should be fatigue resistant under photoirradiation conditions [5]. However, there are certain other requirements which must be met by an efficient photocatalyst; the photoisomerization should cause significant alteration in steric or electronic properties of the catalyst which should ultimately change the catalytic activity or selectivity [6]. The photochromic moiety should be capable of absorbing light at a wavelength which should not excite other components of the catalyst [1]. A number of unique approaches to meet these requirements have been reported in the literature. A few well known photochromic moieties used in photoswitchable catalysis are stilbenes [7], azobenzenes [8], spiropyrans [9] and diarylethenes [6,10]. Azobenzenes and stilbenes which involve E / Z isomerization have been used to change the sterics by photochemical isomerization which significantly alters the catalytic activity [7,8]. Diarylethenes, which involve electrocyclic ring closing reaction, have been utilized for switching of steric and electronic properties of catalysts [5]. Photoinduced changes in the charge distribution have been used to alter the photocatalytic activities of spiropyran based photoswitchable catalysts [9].
The very first example of photoswitchable homogeneous catalysis is reported by Ueno, Takahashi, and Osa in 1981 where UV induced photoisomerization of azobenzene was coupled with a b-cyclodextrin catalyzed ester hydrolysis reaction (Fig. 1) [11]. The E-azobenzene on b-cyclodextrin in 1 blocked one end of the b-cyclodextrin, which prevented binding of the ester substrate in the b-cyclodextrin cavity, (hydrolysis inhibition). Upon UV irradiation (l ¼ 365 nm), the catalyst isomerized to the Z form 2, in which ester can be accommodated in the cyclodextran moiety which resulted in a 5-fold increase in the rate of hydrolysis of p-nitrophenylacetate. The literature reveals a number of other reports where photoswitches have been applied to homogeneous and heterogeneous catalysis [12e15]. The reactivity of dithienylethene photoswitchable catalyst can be altered by changes in sterics or electronics (vide supra). Branda and co-workers were successful in changing the steric properties of a copper(1)cyclopropanation catalyst through photocyclization of a dithienylethene photochrome [16]. A chiral bis-(oxazoline) dithienylethene ligand was developed which would chelate with a Cu(I) in its open form (6) to facilitate enantioselective cyclopropanation of styrene (Fig. 2). However, upon irradiation, the bis(oxazoline) dithienylethene ring was closed to form 7 where oxazoline moiety was not able to chelate the copper atom. Very poor enantioselectivity was observed with the closed isomer. The enantioselective activity was regenerated when the ligand was irradiated with visible light (l ¼ 434 nm). Besides sterics, electronic properties of dithienylehtenes are also modulated/altered by photoisomerization that can also affect the activity of a catalyst. An excellent example of photoswitchable catalytic activity via electronic modulation was reported by Bielawski and coworkers [17]. They prepared DTE-annulated NHC precatalyst 8 which underwent expected electrocyclic ring closing on exposure to UV light under neutral and basic conditions. The reverse reaction (electrocyclic ring opening) was achieved upon exposure to visible light (Fig. 3). The open form 8 was quite efficient in catalyzing the transesterification and amidation reaction. The open form 9, obtained through UV irradiation of 8, was not effective in catalyzing the transesterification and amidation reactions. The rate of amidation reaction could be toggled easily between slow and fast reactions by exposure to UV and visible light, respectively. Spectroscopic analysis revealed that the resting state of the active catalyst was an imidazolinium
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
international journal of hydrogen energy xxx (xxxx) xxx
3
Fig. 1 e Capping b-cyclodextrin with an azobenzene moiety facilitated photoswitchable catalytic ester hydrolysis [11]. Reprinted with permission from Ref. [1].
Fig. 2 e Chelation of a Cu(I) Atom in the chiral pocket of 6 facilitated stereoselective cyclopropanations; UV irradiation to form 7 disrupted the Cu chelation and decreased the stereoselectivity [16]. Reprinted with permission from Ref. [1].
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
4
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 3 e Ring-open isomer 8 of a DTE-annulated N-heterocyclic carbenes facilitated transesterifications, amidations, and ring opening polymerizations in ambient light, whereas the corresponding reactions are inhibited upon photoinduced formation of ring-closed isomer 9 [17]. Reprinted with permission from Ref. [1].
species 11 which converted into an inactive NHC alcohol 12 upon UV irradiation and photocyclization. Subsequent irradiation with visible light reversed the photocyclization and converted the NHC-alcohol adduct back to the active imidazolium species, which re-engaged the catalytic cycle. We became interested in exploring the photoswitchable catalysis based on dithienylethene photochrome for transition metal free hydrogen activation through frustrated Lewis pairs (FLP) concept. The concept of transition metal free hydrogen activation through FLP dates back to 2006 when Stephan and coworkers reported (C6H2Me3)2PH(C6F4)BH(C6F5)2 (Me ¼ methyl), which cleanly loses hydrogen above 100 C [18]. Remarkably, the dehydrogenated product (C6H2Me3)2P(C6F4) B(C6F5)2 is stable and reacts under 1 atm of H2 at 25 C to reform the starting complex (Fig. 4). They understand that this novelty in the reaction is due to the steric hindrance of both Lewis acid and Lewis base.
The focus in the literature is on metal free hydrogen activation through frustrated Lewis pairs. However, the reverse reaction is also quite important because it can generate molecular hydrogen which can be used for various catalytic processes. Moreover, if H2 release can be achieved then these systems may provide a way for chemical storage of hydrogen from which hydrogen can be regenerated as and when desired. Hydrogen storage [19e27] and controlled release of hydrogen [28,29] is an active area of research in the recent times for efficient utilization of hydrogen as fuel [24,25,30,31] and in catalysis. The motivation of the current work is to design photoswitchable frustrated Lewis pairs for controlled activation and release of hydrogen (reversible) through external stimulus. In our approach, dithienylethene moiety is functionalized with appropriate Lewis acidic and basic groups therefore, the photochrome itself behaves as a frustrated Lewis pair. The dithienylethene moiety is chosen because the
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 4 e Reversible hydrogen activation through frustrated Lewis pairs [18].
photoisomerization not only changes the sterics but also the electronic structure is altered. Both thienyl rings are communicating electronically in the closed form (closed circuit) whereas upon isomerization, the electronic communication is lost in the open form. In our approach, first of all, changes in the Lewis acidity and basicity upon photoisomerization are studied. The results are then analyzed to designing a photochromic FLP for hydrogen recombination.
Computational methodology All calculations are performed within hybrid GGA B3LYP [32] exchange correlation functional with 6-31G (d) basis set [33] using GAUSSIAN 09 [34]. Frustrated Lewis pairs for hydrogen recombination are relatively large systems therefore, choice of Gaussian program package is quite useful because Gaussian provides very effect analytic gradient for density functional theory. The equilibrium geometries of all systems are therefore efficiently localized [35]. The B3LYP method, which consists of three parameter hybrid functional of Becke [32] in conjunction with the correlation functional of Lee, Yang and Parr [36], provides a nice balance between cost and accuracy [37e39]. It is known to perform very well for the prediction of proton and hydride affinities of various systems [40e43]. Moreover, B3LYP has well established reliability for studying reaction mechanism [44e47] besides its accuracy for geometric and electronic [48e52,57] and opto-electronic [53,54] properties. Transition states for hydrogen activation are located through quadratic synchronous transition method. The optimized structures are confirmed through frequency analysis as true minima (lack of any imaginary frequency) or transition states (one imaginary frequency). Moreover, imaginary frequencies of transition states were also evaluated to confirm that their associated eigenvector corresponds to the motion along the reaction coordinates. The reported energies are zero-point corrected energies. The positions 5, 4 and 2 are the only available position on the dithienylethene skeleton where Lewis acid or Lewis base groups can be installed. These positions are suitably substituted with Lewis acid and basic groups for hydride and proton affinities, respectively [55]. In order to decrease the computational cost, simplified version of these Lewis acids and bases are used because Lewis acidity and basicity of these groups are not influenced by sterics. The bulk in FLP chemistry are used for blocking the reaction between Lewis acid and Lewis base [56]. Our purpose here is to explore change in electronic properties upon switching and its effect on hydrogen activation therefore, a simplified model is quite reliable. Hydride affinity (HA) (thermochemical property) can be defined as the enthalpy of reaction in Equation (Eq.) 1a or
hydride affinities of cations can also be determined as the enthalpy of reaction for Eq. (1b). AH / A þ H
(1a)
AH / Aþ þH
(1b)
Eq. (1a) shows that hydride affinities are readily obtained from enthalpies of formation of AH and A, as shown in mathematical expression of eq. (2). HAðAÞ ¼ DHf ðAÞ þ DHf ðH Þ DHf ðAH Þ
(2)
HAs can be calculated using eq. (2) having already known heats of formation. HA ¼ EHydride complex e (Ehydride þ EMolecule)
(3)
Similarly, proton affinity (PA) can be defined as the enthalpy of a reaction in eq. (4a) or proton affinity of anion can also be determined as enthalpy of reaction for eq. (4b). A þ Hþ / AHþ
(4a)
A þ Hþ / AH
(4b)
Proton affinity can also be defined as the negative of reaction enthalpy (DH) at 298 K and for the reaction (eq. (4b)) can be calculated from the following equation. PA ¼ DH ¼ ðDE þ RTÞ
(5)
The proton and hydride affinities are exothermic but negative sign for exothermic reactions is not shown by convention.
Results and discussion Hydride affinity First BH2 group is introduced at 5, 4 and 2 positions (see Fig. 5 for numbering scheme) of the open dithienylethene (DTEo) to get an estimate of hydride affinities for these positions. Due the symmetry of the molecule, these are the only chemically distinct position on the dithienylethene skeleton. For the open isomer, the lowest hydride affinity is calculated when BH2 group is present at position 2 (117.4 kcal mol1). The hydride affinity is increased to 119.3 kcal mol1 for 5-BH2 DTEo 21. For DTEo, the highest hydride affinity is calculated for 4-BH2 system 23 (121.0 kcal mol1). Charges on the DTEo are analyzed to rationalize the trends in hydride affinities. Charge analysis reveals that the carbon
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 5 e Numbering scheme of open and closed diarylethene for discussion.
bearing BH2 group in 4-BH2 DTEo 23 has positive charge of 0.307. Moreover, the boron of BH2 is also positively charged 0.146. This means that the BH2 group in this system is not stabilized by the adjacent carbon (electron deficient) which increases the susceptibility of boron for electron rich species. Therefore, the hydride affinity is higher in this case. For 5- and 2-BH2 dithienylethenes (21 and 25), the charge on boron is relatively high; 0.216 and 0.239 for 21 and 25, respectively but at the same time, the carbon bearing the BH2 also gets negatively charged, 0.693 and 1.284. The boron in these structures is stabilized by the adjacent negative charge which renders the BH2 less susceptible for hydride affinity. The effect is more pronounced for 4-BH2 therefore, 4-BH2 DTEo has the lowest hydride affinity. In a similar fashion, the hydride affinities of closed isomers of dithienylethene (DTEc) photoswitches are determined at the same positions. The hydride affinities of DTEc photoswitch with BH2 group at positions 5, 4 and 2 are 127.5, 121.8 and 122.3 kcal mol1, respectively (Table 1). The DTEc system is substantially different than the open form mainly due to extended conjugation between both thiophene rings (Fig. 6). Moreover, the substituent on position 2 is perpendicular to the plane of the ring therefore, it is not affected by conjugation/ electronic effects. Since DTEc has complete delocalization of electrons therefore, resonance structures play important role in deciding the hydride affinities. Among 5 and 4 positions, position 4 has high charge density which results in lowering of hydride affinity of 4-BH2 DTEc.
Proton affinity The proton affinity (PA), of an anion or of a neutral atom or molecule is a measure of its gas-phase basicity. Theoretically, PAs of open and closed DTE photoswitch are determined by the same method as are followed for HAs of open and closed DTE photoswitch. The PAs for DTEc at positions 5, 4 and 2 are 220.3, 228.5 and 231.1 kcal mol1, respectively (Table 1). The proton affinities are expected to follow a trend opposite to that of hydride affinities because of the similar reasons described above. The observed trends of proton affinities are similar to the expected ones. For 2-NH2 dithienylethene, the nitrogen atom as well as the neighboring carbons are negatively charged which increases the susceptibility of the NH2 for proton abstraction (highest proton affinity is calculated). For closed isomer, the highest proton affinity is also calculated for 2-NH2 DTEc which is mainly attributed to perpendicular orientation of NH2 group with respect to the plane of the ring. The amino group is therefore not affected by electronic effect and high proton affinity is calculated. The proton affinities of NH2 at other positions (5- and 4-) are lower than that of NH2 at position 2. This trend is expected because proton affinities follow a trend opposite to the hydride affinities. The hydride affinities are increased and proton affinities are decreased upon cyclization. Delocalization of lone pair of S atom along with p bond in DTEc occurs in the entire molecule which results in a negative charge at position
Table 1 e Hydride and proton affinities of open and closed dithyenylethene photoswitch. Position 5-BH2 4-BH2 2-BH2
HA (DTEo) 21 23 25
119.3 121.0 117.4
22 24 26
HA (DTEc)
Position
127.5 121.8 122.3
5-BH2 4-BH2 2-BH2
PA (DTEo) 27 29 31
220.3 228.5 231.1
PA (DTEc) 28 30 32
220.2 226.8 241.8
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
international journal of hydrogen energy xxx (xxxx) xxx
7
Fig. 6 e Delocalization in closed dithienylethene photoswitch. 5 and 5’. When a NH2 group is attached at 5 and 5’ positions, the proton affinities are influenced by these negative charges.
Proton and hydride affinities of complex DTEo and DTEc photoswitch Next, HAs are studied for DTEo and DTEc photoswitch containing both groups (NH2 and BH2) placed on the same molecule at suitable positions. The above illustrated individual hydride and proton affinities are useful to get an idea about the hydride and proton affinities for different positions. Since hydrogen splits heterolytically on NH2 and BH2 groups therefore, molecules having one substituent on a photochrome (either NH2 or BH2) would require two molecules of photochromes for the activation of one H2 molecule. In order to dissociate one H2 molecule per photochrome, two functional groups are installed on a single photochrome. Moreover, the purpose here is to evaluate whether the presenece of a lewis acid affects the strength of the base and vice versa. Position 2 is not studied further because the substituents at position 2 lie perpendicular to the plane of the dithienylethene scaffold and does not communicate electronically with other substituents present on the photochrome. Moreover, this position is highly susceptible for oxidation which results in the elimination of the substituents. We have considered only two positions (5 and 4) which results in the generation of three structures for proton and hydride affinities (Fig. 7). The hydride affinities are reduced under the influence of NH2. For example, the hydride affinity of 5-NH2-4-BH2 DTEo is 118.6 kcal mol1 (Table 2) compared to 121.0 kcal mol1 for 4BH2 DTEo. Similarly, the hydride affinities of 5-NH2-50 -BH2 or 4-NH2-5-BH2 DTEo are 118.2 and 115.4 kcal mol1, respectively compared to 119.3 kcal mol1 for 5 BH2 DTEo. The decrease in hydride affinity is mainly due to electron donating nature of NH2 group which gives its electron density to dithienylethene which makes the BH2 group less acidic and hence hydride affinity decreases. The hydride affinity of DTEc also decreases but with a pronounced effect. For example, the hydride affinity of 5-NH2-4-BH2 DTEc 34 is 119.1 compared to 121.8 kcal mol1 for 4-BH2 DTEc 24. The more pronounced effect in the closed isomer is due to better communication between donor and acceptor. The hydride affinities of the closed isomers are greater than those of the open isomers, very similar to the mono substituted BH2. The proton affinities of the DTEo are also reduced under the influence of BH2. The BH2 group takes the electron density from NH2 which leaves the NH2 group less electron
rich. The decrease in the basicity of NH2 group results in drop in proton affinity. The proton affinity of 5-NH2-4-BH2 DTEo 33 is 215.2 kcal mol1 compared to 220.3 for 5-NH2 DTEo 27. The proton affinities for other position are also decreased. The maximum decrease in proton affinity is observed for 4-NH2-5-BH2 DTEo 37 where the proton affinity is 222.1 kcal mol1 compared to 228.5 for 4-NH2 DTEo 29. The decrease in proton affinity is observed more intensively in the closed isomer. After establishing the hydride and proton affinities of group in 33-38, the next step is to choose a system for hydrogen activation. The dithienylethene photoswitch is a positive photochrome where colorless open form is stable which closes to form a thermally less stable closed form. The closed form is less stable, therefore, to generate a good FLP catalyst, it is necessary to that H2 addition on a FLP catalysis does not disturb this natural stability trend rather it should add up. The above results reveal that 5,4 and 5,50 combination do not disturb this natural difference to any appreciable extent. Since 5,4 and 5,50 combinations are favorable therefore, we have taken 5,5’ combination for the activation of H2 molecule.
Optimized geometry of open dithienylethene (DTEo) þH2 First, the reaction of H2 molecule with the open form of a judiciously chosen FLP is studied. For the reactants, a van der Waals type complex is observed where two molecules of FLP interact with H2 molecule in such a way that one hydrogen atom (of H2 molecule) interacts with the nitrogen atom of one FLP whereas the other hydrogen atom interacts with boron atom of the other FLP molecule. The HeH bond length in the reactant (van der Waals complex vdW-O) is 0.74 A whereas N1eH1 and B1eH2 bond distances are 2.71 A and 4.71 A, respectively. Details of other geometric parameters are given in Table 3. Transition state (TS-O) for hydrogen activation is located at a barrier of 30.9 kcal mol1 from the vdW-O (Fig. 8). The energy of activation is relatively low and should be accessible at slightly elevated temperature. The geometric parameters are also analyzed to account for changes during the reaction. N1eH1 bond distance in reactant (vdW-O) is 2.71 A, which is reduced to 1.23 A in TS-O. Similarly, B1eH2 bond distance is also decreased from 4.77 A in the starting vdW-O complex to 1.36 A in the TS-O. Shortening of N1eH1 and B1eH2 bond is accompanied by elongation of HeH bond. The bond distance is increased from 0.74 A in vdW-O to 1.07 A in the TS-O. All these changes are supportive of the
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
8
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 7 e Bis functionalized Dithienylethenes for hydrogen activation.
Table 2 e Hydride Affinities of open and closed dithienylethene (DTE) photoswitch. Molecule DTEo
DTEc
Substituents 5-NH2-4-BH2 5-NH2-50 -BH2 4-NH2-5-BH2 5-NH2-4-BH2 5-NH2-50 -BH2 4-NH2-5-BH2
33 35 37 34 36 38
HA
PA
118.6 118.2 115.4 119.1 123.5 122.9
215.2 217.1 222.1 213.9 212.9 223.3
PA of mono 27 27 29 28 28 30
220.3 220.3 228.5 220.2 220.2 226.8
HA of mono 23 21 21 24 22 22
121.0 119.3 119.3 121.8 127.5 127.5
Table 3 e Important geometrical parameters of open dithienylethene(DTEo) þ H2. Parameters Bond Distance A
Bond Angle
Dihedral Angle F
Eact ER
Atoms
Reactant
N1eH1 H1eH2 B1eH2 N1eH C5eN1 N1eB1 C5’-B1 B1eF C5eN1eH1 N1eH1eH2 B1eH2eH1 C5’-B1-H2 HeN1eH FeB1eF C5eN1eH1eH2 C5’-B1-H2-H1 B1eH2eH1eN1 Hydrogen Activation (kcal mol1) 30.9 26.2
2.71 0.74 4.71 1.01 1.39 3.62 1.52 1.33 127.4 170.8 56.8 117.3 108.8 116.0 176.0 179.5 5.7
Transition State
Product
1.23 1.07 1.36 1.02 1.43 2.78 1.57 1.40 119.1 166.7 102.5 106.1 106.6 110.2 166.2 179.3 5.9 Hydrogen Recombination (kcal 4.7 26.2
1.07 2.99 1.21 1.04 1.45 3.11 1.59 1.51 109.0 121.8 52.0 117.0 34.8 109.9 139.6 158.9 91.0 mol1)
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
international journal of hydrogen energy xxx (xxxx) xxx
9
Fig. 8 e Potential energy profile diagram of hydrogen recombination reaction in closed and open dithienylethene photochromes. All energies are in kcal mol¡1 relative to open van der Waals complex vdW-O at 0 kcal mol¡1 and the bond distances are in Angstrom. progression of the reaction. Animation of the imaginary frequency also supports the breakage of HeH bond on FLP. Close analysis of the geometric parameters of the TS-O reveals that the transition state is late in nature as expected
from Hammonds’ postulate for an endothermic reaction. The energy of the reaction is 26.2 kcal mol1. The hydrogen molecule is completely broken in the product (charge separated complex CS-O) with the concomitant formation of NeH
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
10
international journal of hydrogen energy xxx (xxxx) xxx
Table 4 e Important geometrical parameters of closed dithienylethene (DTEc)þH2. Parameters Bond Distance A
Bond Angle
Dihedral Angle F
Eact ER
Atoms
Reactant
N1eH1 H1eH2 B1eH2 N1eH C5eN1 N1eB1 C5’-B1 B1eF C5eN1eH1 N1eH1eH2 B1eH2eH1 C5’-B1-H2 HeN1eH FeB1eF C5eN1eH1eH2 C5’-B1-H2-H1 B1eH2eH1eN1 Hydrogen Activation (kcal mol1) 35.6 33.7
and BeH bonds. The HeH distance in the product is 2.99 A whereas the N1eH1 and B1eH2 bond lengths are 1.07 and 1.21 A, respectively. Besides the above-mentioned bonds, certain bond lengths in the vicinity of the reaction site are also changed during the course of the reaction. For example, C5’-B1 bond distance also increases gradually from 1.52 A in vdW-O to 1.59 A in CS-O. The increase in the C5’-B1 bond length reflects decrease in the interaction of boron with dithienylethene core due to the attachment of hydride with boron moiety. Besides bond lengths, certain bond angles also change as the reaction makes progress. The simulated bond angle of C5eN1eH1 in the vdW-O complex is 127.4 which decreases to 119.1 in the TS-O and finally to 109.0 in the product. A similar type of trend is observed for N1eH1eH2 where the bond angle decreases from 170.8 in reactant to 121.8 in the product. On the other end, the bond angle change in B1eH2eH1 during the reaction is a bit different. Bond angle first increases from 56.8 to 102.5 as the reaction proceeds from reactant to the transition state, and then decreases to 52.0 in product. Thermodynamics of the reaction mentioned above reveal that the reaction is a thermodynamically unfavorable one despite low activation barriers for hydrogen activation. The reaction in the reverse direction i.e., hydrogen generation or hydrogen recombination is quite facile kinetically and thermodynamically. The activation barrier for generation of hydrogen is merely 4.7 kcal mol1 where the reaction for hydrogen recombination is exothermic by 26.2 kcal mol1 (Fig. 8). Therefore, these dithienylethene based frustrated Lewis pairs can be used for facile generation of H2 molecule.
Optimized geometry of closed dithienylethene (DTEc) photoswitch þ H2 After establishing that the DTEo based frustrated Lewis pairs can be used for facile recombination of H2 molecule, it is worth studying how hydrogen recombination progresses with the closed isomer. Surprisingly the activation barrier for hydrogen
4.74 0.74 3.55 1.01 1.36 3.98 1.52 1.33 124.7 43.5 162.3 114.9 115.4 115.6 63.0 89.9 120.1
Transition State
Product
1.20 1.10 1.35 1.02 1.44 3.01 1.58 1.43 111.6 167.3 114.8 105.0 110.8 111.4 87.7 133.8 17.4 Hydrogen Recombination (kcal 1.9 33.7
1.10 2.24 1.24 1.02 1.45 3.16 1.59 1.38 109.3 98.4 78.6 112.3 108.4 106.7 110.7 122.0 176.4 mol1)
recombination is reduced to merely 1.9 kcal mol1 starting from charge separated complex (CSeC). The N1eH1 and B1eH2 bond lengths in CS-C are 1.10 and 1.24 A, respectively. The lower activation barrier for hydrogen recombination in the closed isomer is due to close resemblance of the TS-C with the CS-C, as compared to that of the open isomer. For example, the N1eH1 bond distance in charge separated complex CS-C is 1.10 A compared to 1.20 A in the TS-C (compared 1.07 A with 1.23 A for the corresponding open dithienylethene complex and transition state, respectively). Similarly, the B1eH2 bond distances in the CS-C and TS-C for DTEc are 1.24 A and 1.35 A, respectively (compare 1.21 A and 1.36 A for the corresponding bond distances in DTEo CS-O and TS-O). The H1eH2 distance in the CS-C is 2.24 A which decreases to 1.10 A in the TS-C. The A. H1eH2 distance in the hydrogen recombined vdW-C is 0.74 A, N1eH1 and B1eH2 distances in the vdW-C are 4.74 and 3.55 respectively. The geometric parameters of the vdW-C differ significantly from that of the open isomer (see Tables 3 and 4). Quite similar to the open DTE isomer, the hydrogen recombination in the closed isomer is also a thermodynamically favorable process where the reaction is exothermic by 33.7 kcal mol1.
Conclusions Photoswitchable catalysis involves changes in properties of a catalyst based on difference in electronic and steric factors. Herein, we report diarylethene based photoswitchable frustrated Lewis pairs for reversible hydrogen activation and recombination. The diarylethene moiety is not a template to alter the catalytic activity rather it is a core part of the catalyst. The rational design principle involves study of proton and hydride affinities of Lewis acids and base functionalities installed on the diarylethene photoswitch pair. Proton and hydride affinities differ significantly between open and closed isomers that help in designing an active molecule for
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
international journal of hydrogen energy xxx (xxxx) xxx
hydrogen recombination. Hydride affinities of the open isomer are lower than the hydride affinities of the closed isomers whereas the proton affinities follow an opposite trend. The proton and hydride affinities are then utilized to formulate best relative positions of Lewis acid and base on a single molecule in order to generate one H2 molecule for photochrome. Energy barriers are calculated for hydrogen splitting and hydrogen recombination for open and closed isomers. The open isomer splits the hydrogen with an activation barrier of 30.9 kcal mol1 whereas the closed isomer splits the hydrogen with relatively high activation barrier of 35.6 kcal mol1. Quite contrary to hydrogen splitting, the hydrogen recombination reaction is quite facile with dithienylethene photochrome. The activation barrier for the open isomer is 4.7 kcal mol1 whereas the activation barrier in the closed isomer is merely 1.9 kcal mol1. The pronounced differences in energy barriers between closed and open isomer combined with low activation barrier for hydrogen recombination in photochromic dithienylethene illustrate the potential of photoswitchable hydrogen recombination with diarylethene photochrome. The chemically stored hydrogen in frustrated Lewis pairs can be liberated in a controlled fashion through external stimulus for catalytic hydrogenation reactions. The present study will provide new dimensions to the scientific community for exploration of other systems with even better selectivities.
Acknowledgement The authors acknowledge financial support from Higher Education Commission of Pakistan via Grant # 2981.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.051.
references
[1] Neilson BM, Bielawski CW. Illuminating photoswitchable catalysis. ACS Catal 2013;3:1874e85. https://doi.org/10.1021/ cs4003673. [2] Du¨rr H, Bouas-Laurent H. Photochromism. Elsevier; 2003. https://doi.org/10.1016/B978-0-444-51322-9.X5000-3. [3] Irie M. Photochromism: memories and SwitchesIntroduction. Chem Rev 2000;100:1683e4. https:// doi.org/10.1021/cr980068l. [4] Canary JW. Molecular switches edited by ben L. Feringa,. Adv Synth Catal 2002;344:563. https://doi.org/10.1002/16154169(200207)344:5<563::AID-ADSC563>3.0.CO;2-M. [5] Irie M. Diarylethenes for memories and switches. Chem Rev 2000;100:1685e716. https://doi.org/10.1021/cr980069d. [6] Matsuda K, Irie M. Diarylethene as a photoswitching unit. J Photochem Photobiol C Photochem Rev 2004;5:169e82. https://doi.org/10.1016/j.jphotochemrev.2004.07.003. [7] Papper V, Likhtenshtein GI. Substituted stilbenes: a new view on well-known systems. J Photochem Photobiol A Chem 2001;140:39e52. https://doi.org/10.1016/S1010-6030(00)00428-7.
11
[8] Bandara HMD, Burdette SC. Photoisomerization in different classes of azobenzene. Chem Soc Rev 2012;41:1809e25. https://doi.org/10.1039/C1CS15179G. [9] Berkovic G, Krongauz V, Weiss V. Spiropyrans and spirooxazines for memories and switches. Chem Rev 2000;100:1741e54. https://doi.org/10.1021/cr9800715. [10] Tian H, Wang S. Photochromic bisthienylethene as multifunction switches. Chem Commun 2007:781e92. https:// doi.org/10.1039/B610004J. [11] Ueno A, Takahashi K, Osa T. Photocontrol of catalytic activity of capped cyclodextrin. J Chem Soc Chem Commun 1981:94. https://doi.org/10.1039/c39810000094. [12] Fruk L, Rajendran V, Spengler M, Niemeyer CM. Lightinduced triggering of peroxidase activity using quantum dots. Chembiochem 2007;8:2195e8. https://doi.org/10.1002/ cbic.200700594. [13] Niazov T, Shlyahovsky B, Willner I. Photoswitchable electrocatalysis and catalyzed chemiluminescence using photoisomerizable monolayer-functionalized surfaces and Pt nanoparticles. J Am Chem Soc 2007;129:6374e5. https:// doi.org/10.1021/ja0707052. [14] Tian H, Yang S. Recent progresses on diarylethene based photochromic switches. Chem Soc Rev 2004;33:85. https:// doi.org/10.1039/b302356g. [15] Cacciapaglia R, Di Stefano S, Mandolini L. The bis-barium complex of a butterfly crown ether as a phototunable supramolecular catalyst. J Am Chem Soc 2003;125:2224e7. https://doi.org/10.1021/ja029331x. [16] Sud D, Norsten TB, Branda NR. Photoswitching of stereoselectivity in catalysis using a copper dithienylethene complex. Angew Chem 2005;117:2055e7. https://doi.org/ 10.1002/ange.200462538. [17] Neilson BM, Bielawski CW. Photoswitchable organocatalysis: using light to modulate the catalytic activities of Nheterocyclic carbenes. J Am Chem Soc 2012;134:12693e9. https://doi.org/10.1021/ja304067k. [18] Welch GC, Juan RRS, Masuda JD, Stephan DW. Reversible, metal-free hydrogen activation. Science 2006;314:1124e6. https://doi.org/10.1126/science.1134230. 80-. [19] Rad AS, Ayub K. How can nickel decoration affect H 2 adsorption on B 12 P 12 nano-heterostructures? J Mol Liq 2018;255:168e75. https://doi.org/10.1016/j.molliq.2018.01.149. [20] Nisa RU, Ayub K. Mechanism of Zn(OTf) 2 catalyzed hydroaminationehydrogenation of alkynes with amines: insight from theory. New J Chem 2017;41:5082e90. https:// doi.org/10.1039/C7NJ00312A. [21] Rad AS, Ayub K. Coordination of nickel atoms with Al12X12(X ¼ N, P) nanocages enhances H2adsorption: a surface study by DFT. Vacuum 2016;133. https://doi.org/10.1016/ j.vacuum.2016.08.017. [22] Ayub K. Transportation of hydrogen atom and molecule through X12Y12nano-cages. Int J Hydrogen Energy 2017;42. https://doi.org/10.1016/j.ijhydene.2017.02.202. [23] Rad AS, Ayub K. Enhancement in hydrogen molecule adsorption on B12N12nano-cluster by decoration of nickel. Int J Hydrogen Energy 2016;41. https://doi.org/10.1016/ j.ijhydene.2016.08.158. [24] Jain IP. Hydrogen the fuel for 21st century. Int J Hydrogen Energy 2009;34:7368e78. https://doi.org/10.1016/ j.ijhydene.2009.05.093. [25] Elam C. Realizing the hydrogen future: the International Energy Agency’s efforts to advance hydrogen energy technologies. Int J Hydrogen Energy 2003;28:601e7. https:// doi.org/10.1016/S0360-3199(02)00147-7. [26] Kaczmarek Ł, Warga T, Zawadzki P, Makowicz M, Bucholc B, Kula P. The influence of the hydrogenation degree on selected properties of graphene as a material for reversible
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
12
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
international journal of hydrogen energy xxx (xxxx) xxx
H2 storage. Int J Hydrogen Energy 2019;44:23149e59. https:// doi.org/10.1016/j.ijhydene.2019.06.007. Yanxing Z, Maoqiong G, Yuan Z, Xueqiang D, Jun S. Thermodynamics analysis of hydrogen storage based on compressed gaseous hydrogen, liquid hydrogen and cryocompressed hydrogen. Int J Hydrogen Energy 2019;44:16833e40. https://doi.org/10.1016/ j.ijhydene.2019.04.207. Liu J, Lan L, Li R, Liu X, Wu C. Agglomerated AgePd catalyst with performance for hydrogen generation from formic acid at room temperature. Int J Hydrogen Energy 2016;41:951e8. https://doi.org/10.1016/j.ijhydene.2015.10.144. Loges B, Boddien A, Junge H, Beller M. Controlled generation of hydrogen from formic acid amine adducts at room temperature and application in H 2/O 2 fuel cells. Angew Chem Int Ed 2008;47:3962e5. https://doi.org/10.1002/ anie.200705972. Mori D, Hirose K. Recent challenges of hydrogen storage technologies for fuel cell vehicles. Int J Hydrogen Energy 2009;34:4569e74. https://doi.org/10.1016/ j.ijhydene.2008.07.115. MOMIRLAN M, VEZIROGLU T. The properties of hydrogen as fuel tomorrow in sustainable energy system for a cleaner planet. Int J Hydrogen Energy 2005;30:795e802. https:// doi.org/10.1016/j.ijhydene.2004.10.011. Becke AD. Density-functional thermochemistry. III. The role of exact exchange. J Chem Phys 1993;98:5648e52. https:// doi.org/10.1063/1.464913. Hehre WJ, Ditchfield R, Pople JA. Selfdconsistent molecular orbital methods. XII. Further extensions of Gaussiandtype basis sets for use in molecular orbital studies of organic molecules. J Chem Phys 1972;56:2257e61. https://doi.org/ 10.1063/1.1677527. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Scalmani G, Barone V, Petersson GA, Nakatsuji H, Li X, Caricato M, Marenich AV, Bloino J, Janesko BG, Gomperts R, Mennu B. Gaussian 16, revision C.01. 2016 [No Title, (n.d.]. dos Santos RB, Rivelino R, Mota F de B, Gueorguiev GK. Effects of N doping on the electronic properties of a small carbon atomic chain with distinct sp2 terminations: a firstprinciples study. Phys Rev B 2011;84. https://doi.org/10.1103/ PhysRevB.84.075417. 075417. Lee C, Yang W, Parr RG. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys Rev B 1988;37:785e9. https://doi.org/10.1103/ PhysRevB.37.785. Arshad M, Bibi A, Mahmood T, Asiri A, Ayub K. Synthesis, crystal structures and spectroscopic properties of triazinebased hydrazone derivatives; a comparative experimentaltheoretical study. Molecules 2015;20:5851e74. https:// doi.org/10.3390/molecules20045851. Hung TQ, Thang NN, Hoang DH, Dang TT, Ayub K, Villinger A, Lochbrunner S, Flechsig G-U, Langer P. Synthesis and Properties of 5,7-Dihydropyrido[3,2- b :5,6- b0 ]diindoles. Eur J Org Chem 2015;2015:1007e19. https://doi.org/10.1002/ ejoc.201403035. Rahman MH, Thakur JS, Rimai L, Perooly S, Naik R, Zhang L, Auner GW, Newaz G. Dual-mode operation of a Pd/AlN/SiC device for hydrogen sensing. Sens Actuators B Chem 2008;129:35e9. https://doi.org/10.1016/j.snb.2007.07.064. ZB. Hydride affinities of some Vianello R, Peran N, Maksic substituted alkynes: prediction by DFT calculations and rationalization by triadic formula. J Phys Chem A 2006;110:12870e81. https://doi.org/10.1021/jp064463o. Zhu X-Q, Wang C-H. Hydride affinity scale of various substituted arylcarbeniums in acetonitrile. J Phys Chem A 2010;114:13244e56. https://doi.org/10.1021/jp109149x.
[42] Lee H-M, Lee G-Y. DFT studies on the proton affinities of oxazole. J Korean Chem Soc 2007;51:7e13. https://doi.org/ 10.5012/jkcs.2007.51.1.007. [43] Rivera A, Moyano D, Maldonado M, Rı´os-Motta J, Reyes A. FTIR and DFT studies of the proton affinity of small aminal cages. Spectrochim Acta Part A Mol Biomol Spectrosc 2009;74:588e90. https://doi.org/10.1016/j.saa.2009.07.009. [44] Rodrı´guez-Otero J. Study of the electrocyclization of ( Z )-Hexa-1,3,5-triene and its heterosubstituted analogues based on ab initio and DFT calculations. J Org Chem 1999;64:6842e8. https://doi.org/10.1021/jo990881w. [45] Ess DH, Houk KN. Activation energies of pericyclic reactions: performance of DFT, MP2, and CBS-QB3 methods for the prediction of activation barriers and reaction energetics of 1,3-dipolar cycloadditions, and revised activation enthalpies for a standard set of hydrocarbon. J Phys Chem A 2005;109:9542e53. https://doi.org/10.1021/jp052504v. [46] Cheong PH-Y, Morganelli P, Luzung MR, Houk KN, Toste FD. Gold-catalyzed cycloisomerization of 1,5-allenynes via dual activation of an ene reaction. J Am Chem Soc 2008;130:4517e26. https://doi.org/10.1021/ja711058f. [47] Shi F-Q, Li X, Xia Y, Zhang L, Yu Z-X. DFT study of the mechanisms of in water Au(I)-Catalyzed tandem [3,3]Rearrangement/Nazarov reaction/[1,2]-hydrogen shift of enynyl acetates: a proton-transport catalysis strategy in the water-catalyzed [1,2]-Hydrogen shift. J Am Chem Soc 2007;129:15503e12. https://doi.org/10.1021/ja071070þ. [48] Ullah H, Shah A-U-HA, Bilal S, Ayub K. Doping and dedoping processes of polypyrrole: DFT study with hybrid functionals. J Phys Chem C 2014;118. https://doi.org/10.1021/jp505626d. [49] Bibi S, Ullah H, Ahmad SM, Ali Shah A-U-H, Bilal S, Tahir AA, Ayub K. Molecular and electronic structure elucidation of polypyrrole gas sensors. J Phys Chem C 2015;119. https:// doi.org/10.1021/acs.jpcc.5b03242. [50] Ullah H, Shah A-U-HA, Ayub K, Bilal S. Density functional theory study of poly(o -phenylenediamine) oligomers. J Phys Chem C 2013;117. https://doi.org/10.1021/jp311526u. [51] Ullah H, Shah A-U-HA, Bilal S, Ayub K. DFT study of polyaniline NH3, CO2, and CO gas sensors: comparison with recent experimental data. J Phys Chem C 2013;117. https:// doi.org/10.1021/jp407132c. [52] Ayub K. Are phosphide nano-cages better than nitride nanocages? A kinetic, thermodynamic and non-linear optical properties study of alkali metal encapsulated X 12 Y 12 nanocages. J Mater Chem C 2016;4:10919e34. https://doi.org/ 10.1039/C6TC04456E. [53] Irfan M, Iqbal J, Sadaf S, Eliasson B, Rana UA, Ud-din Khan S, Ayub K. Design of donor-acceptor-donor (D-A-D) type small molecule donor materials with efficient photovoltaic parameters. Int J Quantum Chem 2017;117. https://doi.org/ 10.1002/qua.25363. e25363. [54] Ans M, Iqbal J, Ahmad Z, Muhammad S, Hussain R, Eliasson B, Ayub K. Designing three-dimensional (3D) nonfullerene small molecule acceptors with efficient photovoltaic parameters. ChemistrySelect 2018;3:12797e804. https://doi.org/10.1002/slct.201802732. [55] Stephan DW. “Frustrated Lewis pairs”: a concept for new reactivity and catalysis. Org Biomol Chem 2008;6:1535. https://doi.org/10.1039/b802575b. [56] Stephan DW, Erker G. Frustrierte Lewis-Paare: metallfreie Wasserstoffaktivierung und mehr. Angew Chem 2010;122:50e81. https://doi.org/10.1002/ange.200903708. [57] Broitman E, Gueorguiev G, Furlan A, Son N, Gellman A, Stafstrom S, Hultman L. Water adsorption on fullerene-like carbon nitride overcoats. Thin Solid Films 2008;517:1106e10.
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs Fazl-i-Sattar a,1, Arsalan Ahmed b,1, Habib Ullah c, Zakir Ullah c,d, Muhammad Tariq a, Khurshid Ayub b,* a
National Center of Excellence in Physical Chemistry, University of Peshawar, Peshawar, KPK, 25120, Pakistan Department of Chemistry, COMSATS University, Abbottabad Campus, Abbottabad, KPK, 22060, Pakistan c Institute of Chemistry, University of Peshawar, Peshawar, KPK, Pakistan d Department of Chemistry, Korea Advanced Institute of Sciences and Technology (KAIST), Daejeon, 34141, Republic of Korea b
highlights Photoswitchable hydrogen
graphical abstract catalysis
recombination
is
for re-
ported for the first time. The catalyst is rationally designed by studying proton and hydride affinities. Dithienylethene
photochrome
offer promising application for photoswitchable catalysis. The hydrogen recombination in closed form is much faster than that of the open isomer.
article info
abstract
Article history:
Photoswitchable catalysis involves changes in properties of a catalyst based on difference
Received 4 July 2019
in electronic and steric factors. These changes can selectively turn “ON” and “OFF” the
Received in revised form
catalytic activity by an external stimulus, light. Herein, we report dithienylethene based
20 September 2019
photoswitchable frustrated Lewis pairs for facile hydrogen recombination. The diary-
Accepted 7 October 2019
lethene moiety is not serving as template to alter the catalytic activity rather it is the core
Available online xxx
part of the catalyst. The rational design principle involves study of proton and hydride affinities of Lewis acid and base functionalities installed on the diarylethene photoswitch
Keywords:
pair. Proton and hydride affinities differ significantly between open and closed isomers
Dithienylethene
that help in designing an active molecule for hydrogen recombination. The proton and
Frustrated lewis pairs
hydride affinities are then utilized to formulate best relative positions of Lewis acid and base on a single molecule in order to liberate one H2 molecule per photochrome. Energy
* Corresponding author. E-mail address: [email protected] (K. Ayub). 1 Both authors contributed equally. https://doi.org/10.1016/j.ijhydene.2019.10.051 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
2
international journal of hydrogen energy xxx (xxxx) xxx
Switchable catalysis
barriers for hydrogen recombination are calculated for open and closed isomers. The
Hydrogen recombination
closed isomer releases the hydrogen with a low activation barrier of 1.9 kcal mol1 whereas the open isomer requires relatively high barrier of 4.7 kcal mol1. The pronounced differences in energy barrier illustrate the potential of photoswitchable hydrogen recombination with diarylethene photochrome. The chemically stored hydrogen in frustrated Lewis pairs can be liberated in a controlled fashion through external stimulus for catalytic hydrogenation reactions. The present study will provide new dimensions to the scientific community for exploration of other systems with even better selectivities. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Photoswitchable catalysis involves alteration in the intrinsic catalytic properties of an active species (catalytically) through a reversible photochemical transformation [1]. The catalyst may be active or inactive in its initial state however, photoinduced transformation may either alter the rate of a catalyzed reaction (A / B; k1 s k2) or may initiate a different reaction (C /D). Photoswitchable catalysts generally involves attachment of a photochromic species to a catalyst [2,3]. The photochromic moiety exists in two isomeric forms which are interconvertible through irradiation with photons of different wavelengths [4]. Therefore, photoswitchable catalysts offer different reactivities and selectivities via interconversion between two states through an external stimulus (light). The modulation of the activity is independent of the diffusion limits that are generally associated with other chemical processes. The requirements for a photoswitchable catalyst are essentially at least the same as required by a photochromic system; the interconversion between both states (forward and backward) occurs with high efficiency by using lights of different wavelengths without formation of any photostationary states [1]. Moreover, the catalyst should be fatigue resistant under photoirradiation conditions [5]. However, there are certain other requirements which must be met by an efficient photocatalyst; the photoisomerization should cause significant alteration in steric or electronic properties of the catalyst which should ultimately change the catalytic activity or selectivity [6]. The photochromic moiety should be capable of absorbing light at a wavelength which should not excite other components of the catalyst [1]. A number of unique approaches to meet these requirements have been reported in the literature. A few well known photochromic moieties used in photoswitchable catalysis are stilbenes [7], azobenzenes [8], spiropyrans [9] and diarylethenes [6,10]. Azobenzenes and stilbenes which involve E / Z isomerization have been used to change the sterics by photochemical isomerization which significantly alters the catalytic activity [7,8]. Diarylethenes, which involve electrocyclic ring closing reaction, have been utilized for switching of steric and electronic properties of catalysts [5]. Photoinduced changes in the charge distribution have been used to alter the photocatalytic activities of spiropyran based photoswitchable catalysts [9].
The very first example of photoswitchable homogeneous catalysis is reported by Ueno, Takahashi, and Osa in 1981 where UV induced photoisomerization of azobenzene was coupled with a b-cyclodextrin catalyzed ester hydrolysis reaction (Fig. 1) [11]. The E-azobenzene on b-cyclodextrin in 1 blocked one end of the b-cyclodextrin, which prevented binding of the ester substrate in the b-cyclodextrin cavity, (hydrolysis inhibition). Upon UV irradiation (l ¼ 365 nm), the catalyst isomerized to the Z form 2, in which ester can be accommodated in the cyclodextran moiety which resulted in a 5-fold increase in the rate of hydrolysis of p-nitrophenylacetate. The literature reveals a number of other reports where photoswitches have been applied to homogeneous and heterogeneous catalysis [12e15]. The reactivity of dithienylethene photoswitchable catalyst can be altered by changes in sterics or electronics (vide supra). Branda and co-workers were successful in changing the steric properties of a copper(1)cyclopropanation catalyst through photocyclization of a dithienylethene photochrome [16]. A chiral bis-(oxazoline) dithienylethene ligand was developed which would chelate with a Cu(I) in its open form (6) to facilitate enantioselective cyclopropanation of styrene (Fig. 2). However, upon irradiation, the bis(oxazoline) dithienylethene ring was closed to form 7 where oxazoline moiety was not able to chelate the copper atom. Very poor enantioselectivity was observed with the closed isomer. The enantioselective activity was regenerated when the ligand was irradiated with visible light (l ¼ 434 nm). Besides sterics, electronic properties of dithienylehtenes are also modulated/altered by photoisomerization that can also affect the activity of a catalyst. An excellent example of photoswitchable catalytic activity via electronic modulation was reported by Bielawski and coworkers [17]. They prepared DTE-annulated NHC precatalyst 8 which underwent expected electrocyclic ring closing on exposure to UV light under neutral and basic conditions. The reverse reaction (electrocyclic ring opening) was achieved upon exposure to visible light (Fig. 3). The open form 8 was quite efficient in catalyzing the transesterification and amidation reaction. The open form 9, obtained through UV irradiation of 8, was not effective in catalyzing the transesterification and amidation reactions. The rate of amidation reaction could be toggled easily between slow and fast reactions by exposure to UV and visible light, respectively. Spectroscopic analysis revealed that the resting state of the active catalyst was an imidazolinium
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
international journal of hydrogen energy xxx (xxxx) xxx
3
Fig. 1 e Capping b-cyclodextrin with an azobenzene moiety facilitated photoswitchable catalytic ester hydrolysis [11]. Reprinted with permission from Ref. [1].
Fig. 2 e Chelation of a Cu(I) Atom in the chiral pocket of 6 facilitated stereoselective cyclopropanations; UV irradiation to form 7 disrupted the Cu chelation and decreased the stereoselectivity [16]. Reprinted with permission from Ref. [1].
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
4
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 3 e Ring-open isomer 8 of a DTE-annulated N-heterocyclic carbenes facilitated transesterifications, amidations, and ring opening polymerizations in ambient light, whereas the corresponding reactions are inhibited upon photoinduced formation of ring-closed isomer 9 [17]. Reprinted with permission from Ref. [1].
species 11 which converted into an inactive NHC alcohol 12 upon UV irradiation and photocyclization. Subsequent irradiation with visible light reversed the photocyclization and converted the NHC-alcohol adduct back to the active imidazolium species, which re-engaged the catalytic cycle. We became interested in exploring the photoswitchable catalysis based on dithienylethene photochrome for transition metal free hydrogen activation through frustrated Lewis pairs (FLP) concept. The concept of transition metal free hydrogen activation through FLP dates back to 2006 when Stephan and coworkers reported (C6H2Me3)2PH(C6F4)BH(C6F5)2 (Me ¼ methyl), which cleanly loses hydrogen above 100 C [18]. Remarkably, the dehydrogenated product (C6H2Me3)2P(C6F4) B(C6F5)2 is stable and reacts under 1 atm of H2 at 25 C to reform the starting complex (Fig. 4). They understand that this novelty in the reaction is due to the steric hindrance of both Lewis acid and Lewis base.
The focus in the literature is on metal free hydrogen activation through frustrated Lewis pairs. However, the reverse reaction is also quite important because it can generate molecular hydrogen which can be used for various catalytic processes. Moreover, if H2 release can be achieved then these systems may provide a way for chemical storage of hydrogen from which hydrogen can be regenerated as and when desired. Hydrogen storage [19e27] and controlled release of hydrogen [28,29] is an active area of research in the recent times for efficient utilization of hydrogen as fuel [24,25,30,31] and in catalysis. The motivation of the current work is to design photoswitchable frustrated Lewis pairs for controlled activation and release of hydrogen (reversible) through external stimulus. In our approach, dithienylethene moiety is functionalized with appropriate Lewis acidic and basic groups therefore, the photochrome itself behaves as a frustrated Lewis pair. The dithienylethene moiety is chosen because the
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 4 e Reversible hydrogen activation through frustrated Lewis pairs [18].
photoisomerization not only changes the sterics but also the electronic structure is altered. Both thienyl rings are communicating electronically in the closed form (closed circuit) whereas upon isomerization, the electronic communication is lost in the open form. In our approach, first of all, changes in the Lewis acidity and basicity upon photoisomerization are studied. The results are then analyzed to designing a photochromic FLP for hydrogen recombination.
Computational methodology All calculations are performed within hybrid GGA B3LYP [32] exchange correlation functional with 6-31G (d) basis set [33] using GAUSSIAN 09 [34]. Frustrated Lewis pairs for hydrogen recombination are relatively large systems therefore, choice of Gaussian program package is quite useful because Gaussian provides very effect analytic gradient for density functional theory. The equilibrium geometries of all systems are therefore efficiently localized [35]. The B3LYP method, which consists of three parameter hybrid functional of Becke [32] in conjunction with the correlation functional of Lee, Yang and Parr [36], provides a nice balance between cost and accuracy [37e39]. It is known to perform very well for the prediction of proton and hydride affinities of various systems [40e43]. Moreover, B3LYP has well established reliability for studying reaction mechanism [44e47] besides its accuracy for geometric and electronic [48e52,57] and opto-electronic [53,54] properties. Transition states for hydrogen activation are located through quadratic synchronous transition method. The optimized structures are confirmed through frequency analysis as true minima (lack of any imaginary frequency) or transition states (one imaginary frequency). Moreover, imaginary frequencies of transition states were also evaluated to confirm that their associated eigenvector corresponds to the motion along the reaction coordinates. The reported energies are zero-point corrected energies. The positions 5, 4 and 2 are the only available position on the dithienylethene skeleton where Lewis acid or Lewis base groups can be installed. These positions are suitably substituted with Lewis acid and basic groups for hydride and proton affinities, respectively [55]. In order to decrease the computational cost, simplified version of these Lewis acids and bases are used because Lewis acidity and basicity of these groups are not influenced by sterics. The bulk in FLP chemistry are used for blocking the reaction between Lewis acid and Lewis base [56]. Our purpose here is to explore change in electronic properties upon switching and its effect on hydrogen activation therefore, a simplified model is quite reliable. Hydride affinity (HA) (thermochemical property) can be defined as the enthalpy of reaction in Equation (Eq.) 1a or
hydride affinities of cations can also be determined as the enthalpy of reaction for Eq. (1b). AH / A þ H
(1a)
AH / Aþ þH
(1b)
Eq. (1a) shows that hydride affinities are readily obtained from enthalpies of formation of AH and A, as shown in mathematical expression of eq. (2). HAðAÞ ¼ DHf ðAÞ þ DHf ðH Þ DHf ðAH Þ
(2)
HAs can be calculated using eq. (2) having already known heats of formation. HA ¼ EHydride complex e (Ehydride þ EMolecule)
(3)
Similarly, proton affinity (PA) can be defined as the enthalpy of a reaction in eq. (4a) or proton affinity of anion can also be determined as enthalpy of reaction for eq. (4b). A þ Hþ / AHþ
(4a)
A þ Hþ / AH
(4b)
Proton affinity can also be defined as the negative of reaction enthalpy (DH) at 298 K and for the reaction (eq. (4b)) can be calculated from the following equation. PA ¼ DH ¼ ðDE þ RTÞ
(5)
The proton and hydride affinities are exothermic but negative sign for exothermic reactions is not shown by convention.
Results and discussion Hydride affinity First BH2 group is introduced at 5, 4 and 2 positions (see Fig. 5 for numbering scheme) of the open dithienylethene (DTEo) to get an estimate of hydride affinities for these positions. Due the symmetry of the molecule, these are the only chemically distinct position on the dithienylethene skeleton. For the open isomer, the lowest hydride affinity is calculated when BH2 group is present at position 2 (117.4 kcal mol1). The hydride affinity is increased to 119.3 kcal mol1 for 5-BH2 DTEo 21. For DTEo, the highest hydride affinity is calculated for 4-BH2 system 23 (121.0 kcal mol1). Charges on the DTEo are analyzed to rationalize the trends in hydride affinities. Charge analysis reveals that the carbon
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
6
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 5 e Numbering scheme of open and closed diarylethene for discussion.
bearing BH2 group in 4-BH2 DTEo 23 has positive charge of 0.307. Moreover, the boron of BH2 is also positively charged 0.146. This means that the BH2 group in this system is not stabilized by the adjacent carbon (electron deficient) which increases the susceptibility of boron for electron rich species. Therefore, the hydride affinity is higher in this case. For 5- and 2-BH2 dithienylethenes (21 and 25), the charge on boron is relatively high; 0.216 and 0.239 for 21 and 25, respectively but at the same time, the carbon bearing the BH2 also gets negatively charged, 0.693 and 1.284. The boron in these structures is stabilized by the adjacent negative charge which renders the BH2 less susceptible for hydride affinity. The effect is more pronounced for 4-BH2 therefore, 4-BH2 DTEo has the lowest hydride affinity. In a similar fashion, the hydride affinities of closed isomers of dithienylethene (DTEc) photoswitches are determined at the same positions. The hydride affinities of DTEc photoswitch with BH2 group at positions 5, 4 and 2 are 127.5, 121.8 and 122.3 kcal mol1, respectively (Table 1). The DTEc system is substantially different than the open form mainly due to extended conjugation between both thiophene rings (Fig. 6). Moreover, the substituent on position 2 is perpendicular to the plane of the ring therefore, it is not affected by conjugation/ electronic effects. Since DTEc has complete delocalization of electrons therefore, resonance structures play important role in deciding the hydride affinities. Among 5 and 4 positions, position 4 has high charge density which results in lowering of hydride affinity of 4-BH2 DTEc.
Proton affinity The proton affinity (PA), of an anion or of a neutral atom or molecule is a measure of its gas-phase basicity. Theoretically, PAs of open and closed DTE photoswitch are determined by the same method as are followed for HAs of open and closed DTE photoswitch. The PAs for DTEc at positions 5, 4 and 2 are 220.3, 228.5 and 231.1 kcal mol1, respectively (Table 1). The proton affinities are expected to follow a trend opposite to that of hydride affinities because of the similar reasons described above. The observed trends of proton affinities are similar to the expected ones. For 2-NH2 dithienylethene, the nitrogen atom as well as the neighboring carbons are negatively charged which increases the susceptibility of the NH2 for proton abstraction (highest proton affinity is calculated). For closed isomer, the highest proton affinity is also calculated for 2-NH2 DTEc which is mainly attributed to perpendicular orientation of NH2 group with respect to the plane of the ring. The amino group is therefore not affected by electronic effect and high proton affinity is calculated. The proton affinities of NH2 at other positions (5- and 4-) are lower than that of NH2 at position 2. This trend is expected because proton affinities follow a trend opposite to the hydride affinities. The hydride affinities are increased and proton affinities are decreased upon cyclization. Delocalization of lone pair of S atom along with p bond in DTEc occurs in the entire molecule which results in a negative charge at position
Table 1 e Hydride and proton affinities of open and closed dithyenylethene photoswitch. Position 5-BH2 4-BH2 2-BH2
HA (DTEo) 21 23 25
119.3 121.0 117.4
22 24 26
HA (DTEc)
Position
127.5 121.8 122.3
5-BH2 4-BH2 2-BH2
PA (DTEo) 27 29 31
220.3 228.5 231.1
PA (DTEc) 28 30 32
220.2 226.8 241.8
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
international journal of hydrogen energy xxx (xxxx) xxx
7
Fig. 6 e Delocalization in closed dithienylethene photoswitch. 5 and 5’. When a NH2 group is attached at 5 and 5’ positions, the proton affinities are influenced by these negative charges.
Proton and hydride affinities of complex DTEo and DTEc photoswitch Next, HAs are studied for DTEo and DTEc photoswitch containing both groups (NH2 and BH2) placed on the same molecule at suitable positions. The above illustrated individual hydride and proton affinities are useful to get an idea about the hydride and proton affinities for different positions. Since hydrogen splits heterolytically on NH2 and BH2 groups therefore, molecules having one substituent on a photochrome (either NH2 or BH2) would require two molecules of photochromes for the activation of one H2 molecule. In order to dissociate one H2 molecule per photochrome, two functional groups are installed on a single photochrome. Moreover, the purpose here is to evaluate whether the presenece of a lewis acid affects the strength of the base and vice versa. Position 2 is not studied further because the substituents at position 2 lie perpendicular to the plane of the dithienylethene scaffold and does not communicate electronically with other substituents present on the photochrome. Moreover, this position is highly susceptible for oxidation which results in the elimination of the substituents. We have considered only two positions (5 and 4) which results in the generation of three structures for proton and hydride affinities (Fig. 7). The hydride affinities are reduced under the influence of NH2. For example, the hydride affinity of 5-NH2-4-BH2 DTEo is 118.6 kcal mol1 (Table 2) compared to 121.0 kcal mol1 for 4BH2 DTEo. Similarly, the hydride affinities of 5-NH2-50 -BH2 or 4-NH2-5-BH2 DTEo are 118.2 and 115.4 kcal mol1, respectively compared to 119.3 kcal mol1 for 5 BH2 DTEo. The decrease in hydride affinity is mainly due to electron donating nature of NH2 group which gives its electron density to dithienylethene which makes the BH2 group less acidic and hence hydride affinity decreases. The hydride affinity of DTEc also decreases but with a pronounced effect. For example, the hydride affinity of 5-NH2-4-BH2 DTEc 34 is 119.1 compared to 121.8 kcal mol1 for 4-BH2 DTEc 24. The more pronounced effect in the closed isomer is due to better communication between donor and acceptor. The hydride affinities of the closed isomers are greater than those of the open isomers, very similar to the mono substituted BH2. The proton affinities of the DTEo are also reduced under the influence of BH2. The BH2 group takes the electron density from NH2 which leaves the NH2 group less electron
rich. The decrease in the basicity of NH2 group results in drop in proton affinity. The proton affinity of 5-NH2-4-BH2 DTEo 33 is 215.2 kcal mol1 compared to 220.3 for 5-NH2 DTEo 27. The proton affinities for other position are also decreased. The maximum decrease in proton affinity is observed for 4-NH2-5-BH2 DTEo 37 where the proton affinity is 222.1 kcal mol1 compared to 228.5 for 4-NH2 DTEo 29. The decrease in proton affinity is observed more intensively in the closed isomer. After establishing the hydride and proton affinities of group in 33-38, the next step is to choose a system for hydrogen activation. The dithienylethene photoswitch is a positive photochrome where colorless open form is stable which closes to form a thermally less stable closed form. The closed form is less stable, therefore, to generate a good FLP catalyst, it is necessary to that H2 addition on a FLP catalysis does not disturb this natural stability trend rather it should add up. The above results reveal that 5,4 and 5,50 combination do not disturb this natural difference to any appreciable extent. Since 5,4 and 5,50 combinations are favorable therefore, we have taken 5,5’ combination for the activation of H2 molecule.
Optimized geometry of open dithienylethene (DTEo) þH2 First, the reaction of H2 molecule with the open form of a judiciously chosen FLP is studied. For the reactants, a van der Waals type complex is observed where two molecules of FLP interact with H2 molecule in such a way that one hydrogen atom (of H2 molecule) interacts with the nitrogen atom of one FLP whereas the other hydrogen atom interacts with boron atom of the other FLP molecule. The HeH bond length in the reactant (van der Waals complex vdW-O) is 0.74 A whereas N1eH1 and B1eH2 bond distances are 2.71 A and 4.71 A, respectively. Details of other geometric parameters are given in Table 3. Transition state (TS-O) for hydrogen activation is located at a barrier of 30.9 kcal mol1 from the vdW-O (Fig. 8). The energy of activation is relatively low and should be accessible at slightly elevated temperature. The geometric parameters are also analyzed to account for changes during the reaction. N1eH1 bond distance in reactant (vdW-O) is 2.71 A, which is reduced to 1.23 A in TS-O. Similarly, B1eH2 bond distance is also decreased from 4.77 A in the starting vdW-O complex to 1.36 A in the TS-O. Shortening of N1eH1 and B1eH2 bond is accompanied by elongation of HeH bond. The bond distance is increased from 0.74 A in vdW-O to 1.07 A in the TS-O. All these changes are supportive of the
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
8
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 7 e Bis functionalized Dithienylethenes for hydrogen activation.
Table 2 e Hydride Affinities of open and closed dithienylethene (DTE) photoswitch. Molecule DTEo
DTEc
Substituents 5-NH2-4-BH2 5-NH2-50 -BH2 4-NH2-5-BH2 5-NH2-4-BH2 5-NH2-50 -BH2 4-NH2-5-BH2
33 35 37 34 36 38
HA
PA
118.6 118.2 115.4 119.1 123.5 122.9
215.2 217.1 222.1 213.9 212.9 223.3
PA of mono 27 27 29 28 28 30
220.3 220.3 228.5 220.2 220.2 226.8
HA of mono 23 21 21 24 22 22
121.0 119.3 119.3 121.8 127.5 127.5
Table 3 e Important geometrical parameters of open dithienylethene(DTEo) þ H2. Parameters Bond Distance A
Bond Angle
Dihedral Angle F
Eact ER
Atoms
Reactant
N1eH1 H1eH2 B1eH2 N1eH C5eN1 N1eB1 C5’-B1 B1eF C5eN1eH1 N1eH1eH2 B1eH2eH1 C5’-B1-H2 HeN1eH FeB1eF C5eN1eH1eH2 C5’-B1-H2-H1 B1eH2eH1eN1 Hydrogen Activation (kcal mol1) 30.9 26.2
2.71 0.74 4.71 1.01 1.39 3.62 1.52 1.33 127.4 170.8 56.8 117.3 108.8 116.0 176.0 179.5 5.7
Transition State
Product
1.23 1.07 1.36 1.02 1.43 2.78 1.57 1.40 119.1 166.7 102.5 106.1 106.6 110.2 166.2 179.3 5.9 Hydrogen Recombination (kcal 4.7 26.2
1.07 2.99 1.21 1.04 1.45 3.11 1.59 1.51 109.0 121.8 52.0 117.0 34.8 109.9 139.6 158.9 91.0 mol1)
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
international journal of hydrogen energy xxx (xxxx) xxx
9
Fig. 8 e Potential energy profile diagram of hydrogen recombination reaction in closed and open dithienylethene photochromes. All energies are in kcal mol¡1 relative to open van der Waals complex vdW-O at 0 kcal mol¡1 and the bond distances are in Angstrom. progression of the reaction. Animation of the imaginary frequency also supports the breakage of HeH bond on FLP. Close analysis of the geometric parameters of the TS-O reveals that the transition state is late in nature as expected
from Hammonds’ postulate for an endothermic reaction. The energy of the reaction is 26.2 kcal mol1. The hydrogen molecule is completely broken in the product (charge separated complex CS-O) with the concomitant formation of NeH
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
10
international journal of hydrogen energy xxx (xxxx) xxx
Table 4 e Important geometrical parameters of closed dithienylethene (DTEc)þH2. Parameters Bond Distance A
Bond Angle
Dihedral Angle F
Eact ER
Atoms
Reactant
N1eH1 H1eH2 B1eH2 N1eH C5eN1 N1eB1 C5’-B1 B1eF C5eN1eH1 N1eH1eH2 B1eH2eH1 C5’-B1-H2 HeN1eH FeB1eF C5eN1eH1eH2 C5’-B1-H2-H1 B1eH2eH1eN1 Hydrogen Activation (kcal mol1) 35.6 33.7
and BeH bonds. The HeH distance in the product is 2.99 A whereas the N1eH1 and B1eH2 bond lengths are 1.07 and 1.21 A, respectively. Besides the above-mentioned bonds, certain bond lengths in the vicinity of the reaction site are also changed during the course of the reaction. For example, C5’-B1 bond distance also increases gradually from 1.52 A in vdW-O to 1.59 A in CS-O. The increase in the C5’-B1 bond length reflects decrease in the interaction of boron with dithienylethene core due to the attachment of hydride with boron moiety. Besides bond lengths, certain bond angles also change as the reaction makes progress. The simulated bond angle of C5eN1eH1 in the vdW-O complex is 127.4 which decreases to 119.1 in the TS-O and finally to 109.0 in the product. A similar type of trend is observed for N1eH1eH2 where the bond angle decreases from 170.8 in reactant to 121.8 in the product. On the other end, the bond angle change in B1eH2eH1 during the reaction is a bit different. Bond angle first increases from 56.8 to 102.5 as the reaction proceeds from reactant to the transition state, and then decreases to 52.0 in product. Thermodynamics of the reaction mentioned above reveal that the reaction is a thermodynamically unfavorable one despite low activation barriers for hydrogen activation. The reaction in the reverse direction i.e., hydrogen generation or hydrogen recombination is quite facile kinetically and thermodynamically. The activation barrier for generation of hydrogen is merely 4.7 kcal mol1 where the reaction for hydrogen recombination is exothermic by 26.2 kcal mol1 (Fig. 8). Therefore, these dithienylethene based frustrated Lewis pairs can be used for facile generation of H2 molecule.
Optimized geometry of closed dithienylethene (DTEc) photoswitch þ H2 After establishing that the DTEo based frustrated Lewis pairs can be used for facile recombination of H2 molecule, it is worth studying how hydrogen recombination progresses with the closed isomer. Surprisingly the activation barrier for hydrogen
4.74 0.74 3.55 1.01 1.36 3.98 1.52 1.33 124.7 43.5 162.3 114.9 115.4 115.6 63.0 89.9 120.1
Transition State
Product
1.20 1.10 1.35 1.02 1.44 3.01 1.58 1.43 111.6 167.3 114.8 105.0 110.8 111.4 87.7 133.8 17.4 Hydrogen Recombination (kcal 1.9 33.7
1.10 2.24 1.24 1.02 1.45 3.16 1.59 1.38 109.3 98.4 78.6 112.3 108.4 106.7 110.7 122.0 176.4 mol1)
recombination is reduced to merely 1.9 kcal mol1 starting from charge separated complex (CSeC). The N1eH1 and B1eH2 bond lengths in CS-C are 1.10 and 1.24 A, respectively. The lower activation barrier for hydrogen recombination in the closed isomer is due to close resemblance of the TS-C with the CS-C, as compared to that of the open isomer. For example, the N1eH1 bond distance in charge separated complex CS-C is 1.10 A compared to 1.20 A in the TS-C (compared 1.07 A with 1.23 A for the corresponding open dithienylethene complex and transition state, respectively). Similarly, the B1eH2 bond distances in the CS-C and TS-C for DTEc are 1.24 A and 1.35 A, respectively (compare 1.21 A and 1.36 A for the corresponding bond distances in DTEo CS-O and TS-O). The H1eH2 distance in the CS-C is 2.24 A which decreases to 1.10 A in the TS-C. The A. H1eH2 distance in the hydrogen recombined vdW-C is 0.74 A, N1eH1 and B1eH2 distances in the vdW-C are 4.74 and 3.55 respectively. The geometric parameters of the vdW-C differ significantly from that of the open isomer (see Tables 3 and 4). Quite similar to the open DTE isomer, the hydrogen recombination in the closed isomer is also a thermodynamically favorable process where the reaction is exothermic by 33.7 kcal mol1.
Conclusions Photoswitchable catalysis involves changes in properties of a catalyst based on difference in electronic and steric factors. Herein, we report diarylethene based photoswitchable frustrated Lewis pairs for reversible hydrogen activation and recombination. The diarylethene moiety is not a template to alter the catalytic activity rather it is a core part of the catalyst. The rational design principle involves study of proton and hydride affinities of Lewis acids and base functionalities installed on the diarylethene photoswitch pair. Proton and hydride affinities differ significantly between open and closed isomers that help in designing an active molecule for
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
international journal of hydrogen energy xxx (xxxx) xxx
hydrogen recombination. Hydride affinities of the open isomer are lower than the hydride affinities of the closed isomers whereas the proton affinities follow an opposite trend. The proton and hydride affinities are then utilized to formulate best relative positions of Lewis acid and base on a single molecule in order to generate one H2 molecule for photochrome. Energy barriers are calculated for hydrogen splitting and hydrogen recombination for open and closed isomers. The open isomer splits the hydrogen with an activation barrier of 30.9 kcal mol1 whereas the closed isomer splits the hydrogen with relatively high activation barrier of 35.6 kcal mol1. Quite contrary to hydrogen splitting, the hydrogen recombination reaction is quite facile with dithienylethene photochrome. The activation barrier for the open isomer is 4.7 kcal mol1 whereas the activation barrier in the closed isomer is merely 1.9 kcal mol1. The pronounced differences in energy barriers between closed and open isomer combined with low activation barrier for hydrogen recombination in photochromic dithienylethene illustrate the potential of photoswitchable hydrogen recombination with diarylethene photochrome. The chemically stored hydrogen in frustrated Lewis pairs can be liberated in a controlled fashion through external stimulus for catalytic hydrogenation reactions. The present study will provide new dimensions to the scientific community for exploration of other systems with even better selectivities.
Acknowledgement The authors acknowledge financial support from Higher Education Commission of Pakistan via Grant # 2981.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.051.
references
[1] Neilson BM, Bielawski CW. Illuminating photoswitchable catalysis. ACS Catal 2013;3:1874e85. https://doi.org/10.1021/ cs4003673. [2] Du¨rr H, Bouas-Laurent H. Photochromism. Elsevier; 2003. https://doi.org/10.1016/B978-0-444-51322-9.X5000-3. [3] Irie M. Photochromism: memories and SwitchesIntroduction. Chem Rev 2000;100:1683e4. https:// doi.org/10.1021/cr980068l. [4] Canary JW. Molecular switches edited by ben L. Feringa,. Adv Synth Catal 2002;344:563. https://doi.org/10.1002/16154169(200207)344:5<563::AID-ADSC563>3.0.CO;2-M. [5] Irie M. Diarylethenes for memories and switches. Chem Rev 2000;100:1685e716. https://doi.org/10.1021/cr980069d. [6] Matsuda K, Irie M. Diarylethene as a photoswitching unit. J Photochem Photobiol C Photochem Rev 2004;5:169e82. https://doi.org/10.1016/j.jphotochemrev.2004.07.003. [7] Papper V, Likhtenshtein GI. Substituted stilbenes: a new view on well-known systems. J Photochem Photobiol A Chem 2001;140:39e52. https://doi.org/10.1016/S1010-6030(00)00428-7.
11
[8] Bandara HMD, Burdette SC. Photoisomerization in different classes of azobenzene. Chem Soc Rev 2012;41:1809e25. https://doi.org/10.1039/C1CS15179G. [9] Berkovic G, Krongauz V, Weiss V. Spiropyrans and spirooxazines for memories and switches. Chem Rev 2000;100:1741e54. https://doi.org/10.1021/cr9800715. [10] Tian H, Wang S. Photochromic bisthienylethene as multifunction switches. Chem Commun 2007:781e92. https:// doi.org/10.1039/B610004J. [11] Ueno A, Takahashi K, Osa T. Photocontrol of catalytic activity of capped cyclodextrin. J Chem Soc Chem Commun 1981:94. https://doi.org/10.1039/c39810000094. [12] Fruk L, Rajendran V, Spengler M, Niemeyer CM. Lightinduced triggering of peroxidase activity using quantum dots. Chembiochem 2007;8:2195e8. https://doi.org/10.1002/ cbic.200700594. [13] Niazov T, Shlyahovsky B, Willner I. Photoswitchable electrocatalysis and catalyzed chemiluminescence using photoisomerizable monolayer-functionalized surfaces and Pt nanoparticles. J Am Chem Soc 2007;129:6374e5. https:// doi.org/10.1021/ja0707052. [14] Tian H, Yang S. Recent progresses on diarylethene based photochromic switches. Chem Soc Rev 2004;33:85. https:// doi.org/10.1039/b302356g. [15] Cacciapaglia R, Di Stefano S, Mandolini L. The bis-barium complex of a butterfly crown ether as a phototunable supramolecular catalyst. J Am Chem Soc 2003;125:2224e7. https://doi.org/10.1021/ja029331x. [16] Sud D, Norsten TB, Branda NR. Photoswitching of stereoselectivity in catalysis using a copper dithienylethene complex. Angew Chem 2005;117:2055e7. https://doi.org/ 10.1002/ange.200462538. [17] Neilson BM, Bielawski CW. Photoswitchable organocatalysis: using light to modulate the catalytic activities of Nheterocyclic carbenes. J Am Chem Soc 2012;134:12693e9. https://doi.org/10.1021/ja304067k. [18] Welch GC, Juan RRS, Masuda JD, Stephan DW. Reversible, metal-free hydrogen activation. Science 2006;314:1124e6. https://doi.org/10.1126/science.1134230. 80-. [19] Rad AS, Ayub K. How can nickel decoration affect H 2 adsorption on B 12 P 12 nano-heterostructures? J Mol Liq 2018;255:168e75. https://doi.org/10.1016/j.molliq.2018.01.149. [20] Nisa RU, Ayub K. Mechanism of Zn(OTf) 2 catalyzed hydroaminationehydrogenation of alkynes with amines: insight from theory. New J Chem 2017;41:5082e90. https:// doi.org/10.1039/C7NJ00312A. [21] Rad AS, Ayub K. Coordination of nickel atoms with Al12X12(X ¼ N, P) nanocages enhances H2adsorption: a surface study by DFT. Vacuum 2016;133. https://doi.org/10.1016/ j.vacuum.2016.08.017. [22] Ayub K. Transportation of hydrogen atom and molecule through X12Y12nano-cages. Int J Hydrogen Energy 2017;42. https://doi.org/10.1016/j.ijhydene.2017.02.202. [23] Rad AS, Ayub K. Enhancement in hydrogen molecule adsorption on B12N12nano-cluster by decoration of nickel. Int J Hydrogen Energy 2016;41. https://doi.org/10.1016/ j.ijhydene.2016.08.158. [24] Jain IP. Hydrogen the fuel for 21st century. Int J Hydrogen Energy 2009;34:7368e78. https://doi.org/10.1016/ j.ijhydene.2009.05.093. [25] Elam C. Realizing the hydrogen future: the International Energy Agency’s efforts to advance hydrogen energy technologies. Int J Hydrogen Energy 2003;28:601e7. https:// doi.org/10.1016/S0360-3199(02)00147-7. [26] Kaczmarek Ł, Warga T, Zawadzki P, Makowicz M, Bucholc B, Kula P. The influence of the hydrogenation degree on selected properties of graphene as a material for reversible
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051
12
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
international journal of hydrogen energy xxx (xxxx) xxx
H2 storage. Int J Hydrogen Energy 2019;44:23149e59. https:// doi.org/10.1016/j.ijhydene.2019.06.007. Yanxing Z, Maoqiong G, Yuan Z, Xueqiang D, Jun S. Thermodynamics analysis of hydrogen storage based on compressed gaseous hydrogen, liquid hydrogen and cryocompressed hydrogen. Int J Hydrogen Energy 2019;44:16833e40. https://doi.org/10.1016/ j.ijhydene.2019.04.207. Liu J, Lan L, Li R, Liu X, Wu C. Agglomerated AgePd catalyst with performance for hydrogen generation from formic acid at room temperature. Int J Hydrogen Energy 2016;41:951e8. https://doi.org/10.1016/j.ijhydene.2015.10.144. Loges B, Boddien A, Junge H, Beller M. Controlled generation of hydrogen from formic acid amine adducts at room temperature and application in H 2/O 2 fuel cells. Angew Chem Int Ed 2008;47:3962e5. https://doi.org/10.1002/ anie.200705972. Mori D, Hirose K. Recent challenges of hydrogen storage technologies for fuel cell vehicles. Int J Hydrogen Energy 2009;34:4569e74. https://doi.org/10.1016/ j.ijhydene.2008.07.115. MOMIRLAN M, VEZIROGLU T. The properties of hydrogen as fuel tomorrow in sustainable energy system for a cleaner planet. Int J Hydrogen Energy 2005;30:795e802. https:// doi.org/10.1016/j.ijhydene.2004.10.011. Becke AD. Density-functional thermochemistry. III. The role of exact exchange. J Chem Phys 1993;98:5648e52. https:// doi.org/10.1063/1.464913. Hehre WJ, Ditchfield R, Pople JA. Selfdconsistent molecular orbital methods. XII. Further extensions of Gaussiandtype basis sets for use in molecular orbital studies of organic molecules. J Chem Phys 1972;56:2257e61. https://doi.org/ 10.1063/1.1677527. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Scalmani G, Barone V, Petersson GA, Nakatsuji H, Li X, Caricato M, Marenich AV, Bloino J, Janesko BG, Gomperts R, Mennu B. Gaussian 16, revision C.01. 2016 [No Title, (n.d.]. dos Santos RB, Rivelino R, Mota F de B, Gueorguiev GK. Effects of N doping on the electronic properties of a small carbon atomic chain with distinct sp2 terminations: a firstprinciples study. Phys Rev B 2011;84. https://doi.org/10.1103/ PhysRevB.84.075417. 075417. Lee C, Yang W, Parr RG. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys Rev B 1988;37:785e9. https://doi.org/10.1103/ PhysRevB.37.785. Arshad M, Bibi A, Mahmood T, Asiri A, Ayub K. Synthesis, crystal structures and spectroscopic properties of triazinebased hydrazone derivatives; a comparative experimentaltheoretical study. Molecules 2015;20:5851e74. https:// doi.org/10.3390/molecules20045851. Hung TQ, Thang NN, Hoang DH, Dang TT, Ayub K, Villinger A, Lochbrunner S, Flechsig G-U, Langer P. Synthesis and Properties of 5,7-Dihydropyrido[3,2- b :5,6- b0 ]diindoles. Eur J Org Chem 2015;2015:1007e19. https://doi.org/10.1002/ ejoc.201403035. Rahman MH, Thakur JS, Rimai L, Perooly S, Naik R, Zhang L, Auner GW, Newaz G. Dual-mode operation of a Pd/AlN/SiC device for hydrogen sensing. Sens Actuators B Chem 2008;129:35e9. https://doi.org/10.1016/j.snb.2007.07.064. ZB. Hydride affinities of some Vianello R, Peran N, Maksic substituted alkynes: prediction by DFT calculations and rationalization by triadic formula. J Phys Chem A 2006;110:12870e81. https://doi.org/10.1021/jp064463o. Zhu X-Q, Wang C-H. Hydride affinity scale of various substituted arylcarbeniums in acetonitrile. J Phys Chem A 2010;114:13244e56. https://doi.org/10.1021/jp109149x.
[42] Lee H-M, Lee G-Y. DFT studies on the proton affinities of oxazole. J Korean Chem Soc 2007;51:7e13. https://doi.org/ 10.5012/jkcs.2007.51.1.007. [43] Rivera A, Moyano D, Maldonado M, Rı´os-Motta J, Reyes A. FTIR and DFT studies of the proton affinity of small aminal cages. Spectrochim Acta Part A Mol Biomol Spectrosc 2009;74:588e90. https://doi.org/10.1016/j.saa.2009.07.009. [44] Rodrı´guez-Otero J. Study of the electrocyclization of ( Z )-Hexa-1,3,5-triene and its heterosubstituted analogues based on ab initio and DFT calculations. J Org Chem 1999;64:6842e8. https://doi.org/10.1021/jo990881w. [45] Ess DH, Houk KN. Activation energies of pericyclic reactions: performance of DFT, MP2, and CBS-QB3 methods for the prediction of activation barriers and reaction energetics of 1,3-dipolar cycloadditions, and revised activation enthalpies for a standard set of hydrocarbon. J Phys Chem A 2005;109:9542e53. https://doi.org/10.1021/jp052504v. [46] Cheong PH-Y, Morganelli P, Luzung MR, Houk KN, Toste FD. Gold-catalyzed cycloisomerization of 1,5-allenynes via dual activation of an ene reaction. J Am Chem Soc 2008;130:4517e26. https://doi.org/10.1021/ja711058f. [47] Shi F-Q, Li X, Xia Y, Zhang L, Yu Z-X. DFT study of the mechanisms of in water Au(I)-Catalyzed tandem [3,3]Rearrangement/Nazarov reaction/[1,2]-hydrogen shift of enynyl acetates: a proton-transport catalysis strategy in the water-catalyzed [1,2]-Hydrogen shift. J Am Chem Soc 2007;129:15503e12. https://doi.org/10.1021/ja071070þ. [48] Ullah H, Shah A-U-HA, Bilal S, Ayub K. Doping and dedoping processes of polypyrrole: DFT study with hybrid functionals. J Phys Chem C 2014;118. https://doi.org/10.1021/jp505626d. [49] Bibi S, Ullah H, Ahmad SM, Ali Shah A-U-H, Bilal S, Tahir AA, Ayub K. Molecular and electronic structure elucidation of polypyrrole gas sensors. J Phys Chem C 2015;119. https:// doi.org/10.1021/acs.jpcc.5b03242. [50] Ullah H, Shah A-U-HA, Ayub K, Bilal S. Density functional theory study of poly(o -phenylenediamine) oligomers. J Phys Chem C 2013;117. https://doi.org/10.1021/jp311526u. [51] Ullah H, Shah A-U-HA, Bilal S, Ayub K. DFT study of polyaniline NH3, CO2, and CO gas sensors: comparison with recent experimental data. J Phys Chem C 2013;117. https:// doi.org/10.1021/jp407132c. [52] Ayub K. Are phosphide nano-cages better than nitride nanocages? A kinetic, thermodynamic and non-linear optical properties study of alkali metal encapsulated X 12 Y 12 nanocages. J Mater Chem C 2016;4:10919e34. https://doi.org/ 10.1039/C6TC04456E. [53] Irfan M, Iqbal J, Sadaf S, Eliasson B, Rana UA, Ud-din Khan S, Ayub K. Design of donor-acceptor-donor (D-A-D) type small molecule donor materials with efficient photovoltaic parameters. Int J Quantum Chem 2017;117. https://doi.org/ 10.1002/qua.25363. e25363. [54] Ans M, Iqbal J, Ahmad Z, Muhammad S, Hussain R, Eliasson B, Ayub K. Designing three-dimensional (3D) nonfullerene small molecule acceptors with efficient photovoltaic parameters. ChemistrySelect 2018;3:12797e804. https://doi.org/10.1002/slct.201802732. [55] Stephan DW. “Frustrated Lewis pairs”: a concept for new reactivity and catalysis. Org Biomol Chem 2008;6:1535. https://doi.org/10.1039/b802575b. [56] Stephan DW, Erker G. Frustrierte Lewis-Paare: metallfreie Wasserstoffaktivierung und mehr. Angew Chem 2010;122:50e81. https://doi.org/10.1002/ange.200903708. [57] Broitman E, Gueorguiev G, Furlan A, Son N, Gellman A, Stafstrom S, Hultman L. Water adsorption on fullerene-like carbon nitride overcoats. Thin Solid Films 2008;517:1106e10.
Please cite this article as: Fazl-i-Sattar et al., External stimulus controlled recombination of hydrogen in photochromic dithienylethene frustrated lewis pairs, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.051