Metallo-N-Heterocycles - a New Family of Hydrogen Storage Material

Energy Storage Materials 26 (2020) 198–202

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Energy Storage Materials

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Metallo-N-Heterocycles - A new family of hydrogen storage material

Khai Chen Tan a,b,1, Yang Yu a,c,1, Ruting Chen d,1, Teng He a,*, Zijun Jing a,c, Qijun Pei a,c, Jintao Wang a,c, Yong Shen Chua b,**, Anan Wu d,***, Wei Zhou e, Hui Wu e,****, Ping Chen a a Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China b School of Chemical Sciences, Universiti Sains Malaysia, 11800, Minden, Penang, Malaysia c University of Chinese Academy of Sciences, Beijing, 100049, China d Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China e NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD, 20899-6102, USA

ARTICLE INFO ABSTRACT

Storing hydrogen efficiently in condensed materials is a key technical challenge. Tremendous efforts have been Dedicated to the Dalian Institute of Chemical given to inorganic hydrides containing B–H, Al–H and/or N–H bonds, while organic compounds with a great Physics, Chinese Academy of Sciences on the variety and rich chemistry in manipulating C–H and unsaturated bonds, however, are undervalued mainly occasion of its 70th anniversary. because of their unfavourable thermodynamics and selectivity in dehydrogenation. Here, we developed a new family of hydrogen storage material spanning across the domain of inorganic and organic hydrogenous com- Keywords: Metallo-N-Heterocycle pounds, namely metallo-N-heterocycles, utilizing the electron donating nature of alkali or alkaline earth metals to Thermodynamic alteration tune the electron densities of N-heterocyclic molecules to be suitable for hydrogen storage in terms of thermo- Reversible hydrogen storage dynamic properties. Theoretical calculations reveal that the enthalpies of dehydrogenation (ΔHd) of these metallo-N-heterocycles are dependent on the electronegativity of the metals. In line with our calculation results, sodium and lithium analogues of pyrrolides, imidazolides and carbazolides of distinct structures were synthesized and characterized for the first time, where the cation-π interaction was identified. More importantly, a reversible hydrogen absorption and desorption can be achieved over lithium carbazolide which has a hydrogen capacity as 1 high as 6.5 wt% and a suitable enthalpy of dehydrogenation of 34.2 kJ mol -H2 for on-board hydrogen storage.

1. Introduction meet the practical requirements for hydrogen storage in terms of thermodynamic/kinetic properties, reversibility, hydrogen capacity, safety Finding a practical hydrogen storage material possesses a daunting and cost etc., a breakthrough in the development of novel material sys- challenge in the implementation of hydrogen economy [1–3]. Storing tems is highly needed. The idea we discuss in the present work is the hydrogen chemically in condensed materials provides a safe and efﬁcient hybridization of inorganic and organic hydrides [11–14], in particular way to solve this problem [4,5]. Tremendous research efforts have been N-heterocycles, to form a new family of hydrogen storage material made on the development of inorganic hydrides such as alanates [6], namely metalorganic hydrides. Actually, in chemistry, a hydride is a amide-hydride composites [7], borohydrides [8], etc. in the past two compound having negatively-charged hydrogen anion [15]. However, decades. Comparatively less attention was given to organic hydrides [9, nowadays, hydrogenous compounds having hydrogen bonded to metals 10], undervaluing the potential of the large family of organic com- or non-metal in ionic, metallic or covalent manner are collectively pounds. This is probably due to the difﬁculties in manipulating the referred as hydrides [1,16]. Therefore, we use the term metalorganic thermodynamic properties and the selectivity in the dehydrogenation of hydrides here. We demonstrated that three N-heterocycles (pyrrole, organic hydrides. Given that no materials developed to date could fully imidazole, carbazole) can be transformed into novel metallated

* Corresponding author. ** Corresponding author. *** Corresponding author. **** Corresponding author. E-mail addresses: [email protected] (T. He), [email protected] (Y.S. Chua), [email protected] (A. Wu), [email protected] (H. Wu). 1 These authors contributed equally. https://doi.org/10.1016/j.ensm.2019.12.035 Received 28 October 2019; Received in revised form 1 December 2019; Accepted 21 December 2019 Available online 24 December 2019 2405-8297/© 2019 Elsevier B.V. All rights reserved. K.C. Tan et al. Energy Storage Materials 26 (2020) 198–202 derivatives via facile reaction with hydrides of alkali or alkaline earth dehydrogenation enthalpy (ΔHd) of tetrahydropyrrole (H-PY), tetrahy- metals (Li, Na, K, Mg, and Ca). Two new structures were resolved. The droimidazole (H-IMD) and dodecahydrocarbazole (H-CZ) and found that hydrogen storage thermodynamic properties of the newly developed the ΔHd of H-PY is in agreement with the value from experiment [10,18]. materials depend strongly on electronegativity of the metal as indicated Since we used an improved method to calculate ΔHd, the results are by our DFT calculations. Our experimental results further proved that slightly different from the calculated values from other methods [10,19]. lithium carbazolide, as a representing metalorganic hydride, exhibits We then performed the gas-phase calculations on the attractive properties in terms of hydrogen capacity (6.5 wt%) and metallo-N-heterocycles and reveal significant decreases of ΔHd of the 1 reversibility (enthalpy of dehydrogenation, ΔHd ¼ 34.2 kJ mol -H2). metallated systems compared to those of the pristine systems (Fig. 1b and Tables S1–S5). Noticeably the ΔHd decreases with Pauling electronega- 2. Results and discussion tivity, Z, of the metals (Fig. 1b and Tables S2–S5). We further quantified the amount of charge transfer from metal to aromatic ring, and consid- 2.1. Thermodynamic calculations ered it as a descriptor of the metallation effect (Fig. 1c and Tables S2–S5). Nearly linear relationships between ΔHd and the charge transferred were The common feature of N-heterocycles (pyrrole, imidazole and obtained, agreeing well with the electronegativity effect of metals and carbazole) investigated hereinafter is that they have N–H bond where the manifesting that the ΔHd could be manipulated via adjusting metals. δþ electropositive H may be replaced by alkali and alkaline earth metal Although both H-lean and H-rich metallo-N-heterocycles can be stabi- cations (M ¼ Li, Na, K, Mg, Ca) to form metallo-N-heterocycles. It was lized, the conjugation effect is significantly greater than hyper- reported that the reactions between alkali or alkaline earth metal hy- conjugation effect of the same type, i.e., the electrons transferred from δþ drides and organics with active H (such as N–HorO–H) are thermo- metal to aromatic ring in H-lean compounds are more easily delocalized dynamically favourable and experimentally feasible [11–13]. than that of H-rich compounds, resulting in more stabilized H-lean Consequently, the metallations investigated in the present study could be counterparts as well as substantial decrease in ΔHd upon metallation. feasible as shown in the reactions 1 to 3 in the experimental section. We This could be the reason to the linear relationship between ΔHd and then focus on the effect of metallation on the heat flow of hydrogen charge transfer from metal to aromatic ring (Fig. 1c and Tables S2–S5). It absorption and desorption over the tetrahydropyrrolide/pyrrolide is also shown that ΔHd of M-H-PY and M-H-IMD decrease more signifi- (M-H-PY/M PY for short), tetrahydroimidazolide/imidazolide cantly than that of M-H-CZ, which could be ascribed to the fact that the (M-H-IMD/M-IMD for short), and dodecahydrocarbazolide/carbazolide donated electrons from metal in M-CZ can be delocalized in 3 rings of the (M-H-CZ/M-CZ for short) pairs as shown in Fig. 1a using X1s method carbazole, averaging the metallation effect. Theoretical investigations by [17] which combines the DFT method with a neural network correction Jessop et al. disclosed nicely that electron donating substituents such as for a more accurate prediction of heat of formation of large molecules –OH, –NH2 etc. can decrease the ΔHd of cyclohexane from ca.68to65 1 than normal DFT method. and 62 kJ mol -H2, respectively, where the extent of variation is less To verify the accuracy of X1s method in determining the thermody- than 10% [10,18]. The change in ΔHd is also insignificant when a methyl 1 namic properties of N-heterocycles, we firstly calculated the group is substituted on imidazole (39.7 kJ mol -H2) to form

Fig. 1. (a) Proposed reactions for reversible hydrogen storage using metallo-N-heterocycles. (b) ΔHd of different metallo-N-heterocycles versus electronegativities of H and metals. (c) ΔHd versus the charge transfer from metal to aromatic rings. (d) The length of C–Hatα site of M-H-CZ versus electronegativities of H and metals. *For the ease of illustration, M in (a) is mono-valent.

199 K.C. Tan et al. Energy Storage Materials 26 (2020) 198–202

1 1-methylimidazole (38.9 kJ mol -H2). Comparatively, metallation ex- products were characterized using liquid state nuclear magnetic reso- erts a much stronger effect owing to a stronger electron donating ability nance (NMR). However, no proper solvent was found for imidazolides of of alkali or alkaline earth metal (see Fig. 1b and Tables S2–S5). For sodium and lithium, other metallated products clearly show the disap- instance, hydrogen desorption from lithium imidazolide (13.8 kJ pearance of the protons of N–H group of pyrrole and carbazole at 10.7 1 1 mol -H2) is even exothermic! Another interesting finding is that the ppm and 11.3 ppm, respectively. The H resonances of pyrrolides and lengths of C–H bond at the α-site of H-rich CZ increase upon metallation carbazolides are shifted upfield, indicating more shielding effect upon as shown in Fig. 1d and Table S6, manifesting the C–H bond can be metallation (Fig. 2b). In addition, strong 7Li and 23Na signals were activated to certain extent and likely to dissociate firstly upon dehydro- detected in the pyrrolide and carbazolide samples (Figs. S1 and S2) 13 genation. Considering the hydrogen storage capacity and ΔHd of these further evidence the substitution of H by Li or Na. C NMR spectra also metalorganic hydrides predicted in Fig. 1, we synthesized lithium and reveal the impact of metallation: the C atom closes to the N atom has a sodium analogues of pyrrolide, imidazolide and carbazolide to verify downfield shift, while the C atom further to N atom has a upfield shift these conceptual materials. (Fig. 2c), which is consistent with our simulation on 13C NMR signals of sodium carbazolide and pyrrolide (Figs. S3 and S4). 2.2. Syntheses and crystal structures of metalorganic hydrides All the metallated samples present distinct XRD patterns from the parent pyrrole, imidazole and carbazole (Fig. 2a). The structures of some fi As shown in Table S2, lithium and sodium analogues of metallo-N- of the aromatic metallo-N-heterocycles are dif cult to be resolved due to heterocycles are more appealing in terms of hydrogen storage capacity, their poor crystallinity or interference of impurity phases. Fortunately, enthalpy of dehydrogenation and ease of synthesis. Therefore, we the structures of sodium analogues of pyrrolide and imidazolide were fi attempted to synthesize lithium and sodium analogues of metallo-N- successfully identi ed by direct space methods. DFT calculations were heterocycles using LiH and NaH as the metal precursors to react with then performed to optimize the atomic positions (see supporting infor- pyrrole, imidazole and carbazole, respectively, in a molar ratio of 1:1 mation, Tables S7 and S8) and the fully relaxed structures are used in the bond length discussion below. Rietveld refinements with rigid body according to equations 1–3 in experimental part. One equivalent of H2 was released in the mechanical milling process of LiH/NaH with pyrrole constraints on organic anions based on the DFT optimized structure and imidazole, suggesting a complete conversion to metallo-pyrrolides models were conducted using the synchrotron and laboratory XRD data and imidazolides, respectively. New phases were detected by X-ray (Figs. S5 and S6). Sodium pyrrolide and imidazolide possess monoclinic (Cc (No. 9)), and orthorhombic (Pna21 (No. 33)) structures, respectively diffraction (XRD, Fig. 2a). However, the reactions between hydrides and þ (Tables S7–S11). As shown in Fig. 2d and e, in the two structures, Na carbazole can only take place at elevated temperatures (~250 C) giving δ rise to metallo-carbazolides (see XRD patterns in Fig. 2a). The metallated forms ionic bonds with the N of the aromatic N-heterocycles via

Fig. 2. (a) XRD patterns of sodium and lithium analogues of pyrrolides, imidazolides and carbazolide. (b) 1H NMR spectra of synthesized metallo-pyrrolides and -carbazolides as compared to their molecular counterparts in d-DMSO. (c) 13C NMR spectra of synthesized metallo-pyrrolides and -carbazolides as compared to their þ molecular counterparts in d-DMSO. Crystal structures (left) and local coordination of Na cation (right) of (d) sodium pyrrolide and (e) sodium imidazolide. Blue sphere: nitrogen, green sphere: sodium, black sphere: carbon and pink sphere: hydrogen.

200 K.C. Tan et al. Energy Storage Materials 26 (2020) 198–202

þ electrostatic attraction. In sodium imidazolide, each Na cation binds to Fig. 3c, an exothermic feature is observed, from which a ΔHofca. 34.2 1 the N of four neighboring imidazolide anions, forming a distorted kJ mol -H2 for hydrogenation can be derived (see supporting infor- tetrahedral coordination with Na–N bond distances ranging from 2.4196 mation) that is in excellent agreement with the DFT prediction (33.7 kJ 1 to 2.4455 Å to 2.5031–2.5174 Å. The imidazolide anions are closely mol -H2 for dehydrogenation, Fig. 1). linked by such Na–N interactions assembling a strongly bonded three- Catalytic dehydrogenation is always a challenging task in organic dimensional network. At the same time, a weak cation-π interaction be- hydrogen carriers [20–24]. In the dehydrogenation of Li–H-CZ, Ru par- tween sodium and the adjacent aromatic ring was observed. While in the ticles, however, is not effective. We, therefore, tried commercial 5 wt% þ cases of sodium pyrrolide, besides Na–N bonding, Na cation also in- Pd/C and 5 wt% Rh/Al2O3 to improve the kinetics of hydrogen desorp- teracts with the face of an adjacent aromatic ring via cation-π interaction. tion process. Fortunately, both catalysts were capable of selectively The pyrrolide anions are chained together through both Na–N bondings converting Li–H-CZ back to Li-CZ at 200 C, recorded a conversion yield (2.3927–2.3979 Å and 2.4776–2.6814 Å) and the cation-π interaction of 72% and 44%, respectively. (Fig. 3d and Table S12). Because the þ with the Na on top of π face of the pyrrolide rings, creating a 3-D mixture of the reactant and catalyst are in slurry form, we suppose higher arrangement (Fig. 2d). The determined crystal structures provide conversion can be achieved upon sufficient contact between the Li–H-CZ insightful details about cation-anion arrangement configurations and and the catalysts. It is noteworthy that neat dodecahydrocarbazole shows interactions, which will conversely help hone the DFT calculations on no conversion for dehydrogenation under the same conditions, which their thermodynamic parameters. may be due to the unfavourable dehydrogenation thermodynamics (Table S12). Development of efficient catalysts is highly demanded for both 2.3. Hydrogen absorption and desorption dehydrogenation and hydrogenation of these metallo-N-heterocycles. Although no report was found on the catalyst development for these As shown in Fig. 1c, the dehydrogenation thermodynamics can be newly developed materials, the concept and knowledge on the cyclo- manipulated by varying metals. For instance, the ΔHd of H-CZ/CZ pair alkanes and N-heterocycles will definitely shine light on the fabrication can be significantly reduced upon metallation to a suitable range of of suitable catalysts for metallo-N-heterocycles. Furthermore, it is sci- 1 26.4–40.0 kJ mol -H2. Of all the studied pairs, the Li–H-CZ/Li-CZ pair entific interests to explore other forms of energy, such as light, to drive 1 with ΔHd of 33.7 kJ mol -H2 and the theoretical hydrogen storage ca- the hydrogenation and dehydrogenation under the help of catalyst, pacity of 6.5 wt% appears to be attractive. Therefore, Li–H-CZ/Li-CZ pair which are hot topics in the hydrogenation/dehydrogenation of organics was investigated for the demonstration of the reversible hydrogen stor- recently [25,26]. age. Because the hydrogenation (including the activation of H–H and – C–C bonds) would need to overcome certain kinetic barrier, we intro- 3. Conclusion duced fresh-made Pt, Pd, Rh and Ru particles to catalyse the hydrogen absorption over lithium carbazolide (Figs. S7 and S8), and found that Ru As demonstrated in this study, alkali or alkaline earth metals can be outperforms others, i.e., a complete hydrogenation can be achieved even utilized as strong electron donating substituents for N-heterocycles, under 100 C and 70 bar of H2 as evidenced by the amount of hydrogen creating a new family of hydrogen storage materials with tunable ther- absorbed (Fig. 3a) and the NMR observation of Li–H-CZ as the only modynamic properties. Three types of metallo-N-heterocycles were product (Fig. 3b and Fig. S9). studied in the present study. The Li-CZ pair serves as an example Because of the high selectivity towards Li–H-CZ, we employed C80 to demonstrating the capability of the metalorganic hydride for hydrogen measure the heat of hydrogenation of Li-CZ to Li–H-CZ. As shown in

Fig. 3. (a) Conversion of lithium carbazolide catalyzed by Ru particles. (b) 1H NMR spectra of the neat and hydrogenated lithium carbazolide in d-DMSO. (c) Measurement on the heat of hydrogenation of lithium carbazolideusing C80. (d) 1H NMR spectra of the neatlithium carbazolide and the dehydrogenated lithium dodecahydrocarbazolides catalyzed by Pd/C and Rh/Al2O3 at 200 C.

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