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Article Cite This: Cryst. Growth Des. 2019, 19, 3660−3669

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Crystal Solvates of Energetic 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12hexaazaisowurtzitane Molecule with [bmim]-Based Ionic Liquids Ivan V. Fedyanin,*,† Konstantin A. Lyssenko,†,‡ Leonid L. Fershtat,§ Nikita V. Muravyev,∥ and Nina N. Makhova§

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†

A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova St. 28, 119991 Moscow, Russian Federation ‡ Department of Physical Chemistry, Faculty of Chemistry, Lomonosov Moscow State University, 1-3 Leninskiye Gory, Moscow 119991, Russia § N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prosp. 47, 119991 Moscow, Russian Federation ∥ N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, Kosygin Str., 4, 119991 Moscow, Russian Federation S Supporting Information *

ABSTRACT: Crystallization of different biological targets and pharmaceuticals from ionic liquids (ILs) could result in previously unknown polymorph modifications. Crystallization of high-energy materials (HEMs) from ILs has not been studied so far. This paper is devoted to the first attempts to crystallize a highly energetic compound2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (HNIW or CL-20) from different ILs. However, instead of the possible polymorphs these investigations unexpectedly resulted in multicomponent crystal solvates containing an HNIW molecule and anions and cations of two ILs1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]). Their structure was determined by single-crystal X-ray diffraction analysis. The HNIW molecule in both structures adopts different but the most common conformations, similar to the β- and γ-polymorphs of the pure HNIW. Crystal packing was analyzed in terms of extended Hirshfeld surface analysis that allowed distinguishing the contacts between different solvate components. In HNIW· [bmim][PF6] intermolecular bonding interactions were also analyzed by topological analysis of theoretical (B3LYP-D3/POBTZVP) electron density distribution. It was found that, despite small difference in the ionic liquid, the arrangement of crystal components in two solvates differs significantly. In HNIW·[bmim][PF6] with a chainlike packing pattern of the HNIW molecules the HNIW···HNIW interactions play a much lesser role than in HNIW·[bmim][BF4] with layerlike arrangement of the energetic molecules. The presence of the anion and cation of the ionic liquid significantly decreases the melting point and density of the materials, relative to ε-HNIW. Importantly, the sensitivity to impact (18 J) measured for HNIW·[bmim][BF4] is also significantly lower as compared to ε-HNIW (4 J).

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INTRODUCTION Ionic liquids (ILs)low-melting salts (Tm < 373 K), which commonly consist of a voluminous organic cation and an organic or inorganic anionover the last years attract a significant attention as the substitutes of traditional organic solvents. ILs have considerable advantages over traditional organic solvents, in particular, a negligible vapor pressure, an ability to remain in liquid state over a wide temperature range, high thermal stability, significant ionic conductivity, structural tunability, powerful solvating properties, and recyclability. The unique physical and solvent properties of ILs allow their use not only for the “green” organic synthesis but also for crystallization processes.1−7 The first investigations of crystallization processes from ILs were performed for the preparation of polymorph modifications of different drugs and proteins.8,9 These investigations have shown that crystallization from ILs could result in such polymorphs that were not formed upon crystallization from common organic solvents.10 These specific © 2019 American Chemical Society

properties of ILs could be very useful for the preparation of both new and known polymorph modifications of highenergetic materials (HEMs). Although some ILs with specific anions/cations can be considered as energetic materials themselves,11 to the best of our knowledge, a crystallization of conventional HEMs from ILs has not been studied so far. One of the important attributes of ILs is the ability for finetuning their physical and chemical properties by selection of the appropriate cations and anions.12 Most common ILs contain heterocyclic cations (e.g., imidazolium, pyrrolidinium, triazolium, etc.) and inorganic anions (e.g., BF4, PF6, HSO4, N(CN)2, etc.) including energetic (e.g., NO3, N3, Pic, etc.)13 and borohydride-based anions (e.g., BH4, BH3CN).14 In this work, eight commercially available ILs with different Received: December 10, 2018 Revised: May 23, 2019 Published: June 4, 2019 3660

DOI: 10.1021/acs.cgd.8b01835 Cryst. Growth Des. 2019, 19, 3660−3669

Crystal Growth & Design

Article

well-suited, since it is suggested that both salt and neutral coformers are solid at normal conditions, and only for those ILs that have melting points above room temperature can such a term be adopted. But the multicomponent specific crystal structures described in this paper, prepared from solid HNIW and liquid at room-temperature ILs, should be called crystal solvates. These structures could even be called “pseudopolymorphs”,22 since they differ only in anions (BF4− vs PF6−) that are similar in nature, but we are among those researchers who prefer not to use this arguable term.23

heterocyclic cations and inorganic anions were selected (Table 1). The first four ILs (entries 1−4) contain the same cation (1Table 1. Studied Ionic Liquids abbreviated name 1. [bmim][BF4] 2. [bmim][PF6] 3. [bmim] [HSO4] 4. [bmim] [N(CN)2] 5. [emim][OTf] 6. [bmpyrr] [OTf] 7. [emim] [N(CN)2] 8. [bmpyrr] [N(CN)2]

full name 1-butyl-3-methylimidazolium tetrafluoroborate 1-butyl-3-methylimidazolium hexafluorophosphate 1-ethyl-3-methylimidazolium hydrogen sulfate 1-butyl-3-methylimidazolium dicyanimide 1-ethyl-3-methylimidazolium trifluoromethanesulfonate 1-butyl-3-methylpyrrolidinium trifluoromethanesulfonate 1-ethyl-3-methylimidazolium dicyanamide 1-butyl-3-methylpyrrolidinium dicyanamide

crystallization result 1:1 solvate 1:1 solvate

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ε-form ε-form

EXPERIMENTAL SECTION

Crystallization. All the ionic liquids used in crystallization experiments were supplied from Merck KGaA and used without further purification. In general, HNIW (100 mg) was mixed with a corresponding IL (2 mL), and then acetonitrile (10−40 mL depending on the IL) was added dropwise under magnetic stirring until complete dissolution of initial compound. The obtained solution was left to stand at 20 °C followed by slow evaporation of acetonitrile until the formation of crystals (7−30 d), which were studied by powder and single-crystal X-ray diffraction analysis. Crystallization results are provided below in Table 1. X-ray Diffraction Study. X-ray diffraction experiments were performed on a Bruker APEX DUO diffractometer equipped with APEX II CCD detector using graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å). Frames were integrated using the Bruker SAINT24 software package using a narrow-frame algorithm, and a semiempirical absorption correction was applied with the SADABS25 program using intensity data of the equivalent reflections. The structures were solved using dual-space methods with SHELXT26 and refined in anisotropic approximation with SHELXL27 program. Hydrogen atoms were placed in calculated positions and refined in a riding model with Uiso = 1.5Ueq and Uiso = 1.2Ueq of the connected methyl and all other carbon atoms. Crystal data and experimental and refinement details are provided in Table 2. Periodic DFT Calculations. The density functional theory (DFT) calculations of crystal structures of the HNIW·[bmim][PF6] solvate, as well as of the α-polymorph of [bmim][PF6] and the β-, γ-, and εpolymorphs of HNIW, were performed with CRYSTAL17 software suite.28 The B3LYP29 functional with the third version of dispersion

“α-form” 1:2 solvate ε-form ε-form

butyl-3-methylimidazolium) and four different anions (BF4, PF6, HSO4, and N(CN)2). Other ILs considered contain in pairs two different cations with the same anions (entries 5 and 6 −OTf and also 7 and 8 −N(CN)2). It is important to emphasize that the used ILs contained the overlapping sets of anions and cations, which could help to establish some trends in crystallization behavior of HEMs. It is worth noting that, although HNIW is partly soluble in ILs at high temperatures, to avoid heating the energetic material the actual crystallization described in this paper was performed from ternary mixtures of the HNIW, IL, and acetonitrile. Unfortunately, to this moment all attempts to obtain new polymorphs of HNIW using ILs failed. In general, the most thermodynamically stable and widespread ε-polymorph of HNIW was crystallized. To our surprise, the crystallization of HNIW from three investigated ILs (viz., 1, 2, and 6) leads to multicomponent structures containing HNIW molecule, as well as the anion and the cation of the IL. Unfortunately, the structure containing HNIW and 1-butyl-3-methylpyrrolidinium trifluoromethanesulfonate ([bmpyrr][OTf]) turned out to be highly disordered, and we were not able to find a suitable model to fulfill the requirements for publication of a singlecrystal structure. The model for disordered cation, cell parameters, and powder diffraction pattern for this solvate are given in Supporting Information (Figures S3, S7, and S8). But two other ionic liquids, both containing 1-butyl-3methylimidazolium (hereafter bmim) cation and fluorinated anions (PF6− and BF4−) formed stable multicomponent structures that are ordered. According to Cambridge Structural Database (CSD),15 the only two examples of an ionic structure containing the HNIW moiety known so far are recently obtained ionic cocrystal with 1-amino-3-methyl-1,2,3-triazolium nitrate (1-AMTN) 16 and a cocrystal with 1,4dimethylpiperazin-1-ium formate, published only in CSD.17 Despite their intrinsic ionic nature, the phenomenon of cocrystallization is known for ILs for many years.12,18,19 Strictly speaking, the term “co-crystal” is not appropriate for a multicomponent crystal containing the IL moiety, if we use the most strict or common definitions adopted so far.20,21 Even the special term “salt co-crystal”21 used for multi component structures containing both ionic and neutral components is not

Table 2. Crystallographic Data for IL Solvates HNIW· [bmim][PF6] and HNIW·[bmim][BF4]

CCDC deposit no. total formula formula weight temperature (K) crystal system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z/Z′ dcalc (g cm−3) 2θmax (deg) reflns collected/unique reflns with [I > 2σ(I)] R1(F2) wR2 GOF Δρmin/Δρmax 3661

HNIW·[bmim][PF6]

HNIW·[bmim][BF4]

1844075 C14H21F6N14O12P 722.42 120 monoclinic P21/n 13.7333(15) 10.6913(12) 17.8276(19) 90.340(3) 2617.5(5) 4/1 1.833 60 34 216/7625 5216 0.0582 0.1573 1.054 −0.359/0.906

1844076 C14H21BF4N14O12 664.26 120 monoclinic Pn 7.570 00(10) 26.6121(5) 12.6961(2) 95.2520(10) 2546.94(7) 8/2 1.732 70 100 082/22 288 18 776 0.0404 0.0969 1.009 −0.448/0.644



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Figure 1. Conformation of the HNIW molecule in the IL solvates, projection to the average plane of a six-membered ring. Anisotropic displacement parameters are drawn at 50% probability level. correction and Becke-Johnson dumping,30 B3LYP-D3(BJ), was used in combination with POB-TZVP basis set.31 Atomic coordinates were optimized using experimental unit cell parameters and symmetry. Shrinking factor 4 4 4 was used for Monkhorst−Pack grid, yielding 27 k points in irreducible Brillouin zone for α-[bmim][PF6] and βHNIW and 30 k points for other structures. For calculation of the lattice energy of HNIW polymorphs, the wave functions of the isolated molecules in crystal geometry were computed. To account for the correction of basis set superposition error (BSSE), the algorithm implemented in CRYSTAL17 was used to apply the counterpoise approach: the energy of isolated molecule in crystal geometry was calculated with additional basis functions placed around it in atomic positions within the radius of 6 Å (MOLEBSSE keyword). For calculation of binding energy of an uncharged part of the HNIW· [bmim][PF6] solvate the electrons and nuclear charges were removed for the IL part of the crystal leaving the basis functions (GHOSTS keyword). Topological analysis of the theoretical ρ(r) distribution within the Atoms in Molecules (AIM) theory,32 and integration of atomic properties were performed with TOPOND17 program integrated in the CRYSTAL17 package.

by orientation of nitro groups relative to molecular core (see, for instance, Figure S13 in Supporting Information). The conformation in HNIW·[bmim][PF6] structure resembles the one found in β-HNIW polymorphic form, and both molecules in HNIW·[bmim][BF4] adopt the conformation found in γ-HNIW. The availability of different multicomponent structures of HNIW makes it possible to analyze the relative abundance of different molecular conformations in solid state. According to CSD,15 42 unique structures contain an HNIW molecule, of them 38 being cocrystals or solvates; in two structures (CSD AZACIP and ZEBJOH) nitro groups of HNIW are apparently disordered, and the molecule cannot be assigned to the specific conformation. In the remaining data set of 40 structures six structures contain two and one structure contains four symmetry-independent molecules in the unit cell. It is reasonable to consider each of the symmetry-independent molecules as a representative of the molecular conformation. Therefore, 49 conformations were compared in total. The full list of structures is provided in Supporting Information, Table TS1. It turns out that the diversity of conformations of the HNIW molecule in this data set is covered by four conformations found in its polymorphs. The only exception is one of the symmetry-independent molecules in the 1:1 cocrystal with tris[1,2,5]oxadiazolo[3,4-b:3′,4′-d:3″,4″-f]azepin-7-amine34 (CSD TETTAQ), incorporating one of the nitro groups connected to N atom of the six-membered ring in an axial position. In all other structures the nitro groups connected to N1 and N2 atoms of the six-membered ring (Figure 1) are closer to equatorial position, and the main difference is in the position of NO2 groups connected to other four N atoms. In the majority of structures HNIW molecules adopt either β- or γ-conformation, and the frequency of their occurrence is approximately equal (19 and 21 cases). The conformation found in the most thermodynamically stable33,35 ε-polymorph is observed only in five cases, and conformation found in the high-pressure ζ-polymorph is found in only two cocrystals. These observations are in line with relative stability of HNIW conformers in gas phase (β < γ < ε < ζ), calculated at different theory levels.36,37 Thus, we can propose that the thermodynamic stability of ε-HNIW is a tradeoff between the molecular conformation and intermolecular interactions. Note that there is no apparent relation between the coformer and the conformation of HNIW molecule in the structures considered,

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RESULTS AND DISCUSSION Both multicomponent crystals discussed below were obtained in 1:1 composition of HNIW and IL components. The structure HNIW·[bmim][BF4] crystallizes in polar space group Pn with two symmetry-independent sets of the components (Z′ = 2). The latter crystal is pseudo-centrosymmetric; the HNIW molecule and the anion but not the cations are related by symmetry elements of P21/n space group, and the cations are fully disordered in this case. The absence of 21 axis in the structure is supported by intensity statistics for systematic absences: although the intensity of such reflections is ∼3 times lower than for other nonexistent elements, the I/σ(I) ratio is almost the same as for all reflections. The conformation of HNIW molecules in two IL solvates is different (Figure 1). It is reasonable to compare these conformations with those found in polymorphs and other cocrystals of HNIW. As of now, the crystal structures of all four polymorphic forms of pure HNIW reported by Russell et al.33 have been determined by X-ray diffraction (the fifth αpolymorph turned out to be a non-stoichiometric hydrate). Polymorphs β, γ, and ε are stable at ambient conditions, while ζ is a high-pressure polymorph. In all these polymorphs the HNIW molecule has its own conformation that differs mainly 3662



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Figure 2. Conformations of bmim+ cations in the IL solvates of HNIW. Anisotropic displacement parameters are drawn at 50% probability level. The values of torsional angles for HNIW·[bmim][PF6] and HNIW·[bmim][BF4] (two independent cations A and B), in order: C1A-N2A-C5AC6A 95.5(4), 121.5(2) and 93.7(3)° (intentionally taken positive); N2A-C5A-C6A-C7A 177.4(3), −179.37(17), 176.3(2); C5A-C6A-C7A-C8A −178.5(3), −177.76(19), 161.4(2)°.

Figure 3. Fragments of crystal packing in HNIW·[bmim][PF6] crystal, (a) view along b axis, (b) view along [−1 2 1] direction. The HNIW molecules, bmim+ cations, and PF6− anons are drawn in green, blue, and red colors.

Intermolecular Interactions. Visual analysis of crystal packing patterns demonstrates clear difference between the two solvates. In HNIW·[bmim][PF6] the bmim+ cations form geometric layers parallel to the ac diagonal (Figure 3a); these layers are interleaved by alternating “chains” of HNIW molecules along [−1 2 1] crystallographic direction, which are separated by the anions (Figure 3b). The cation layers can also be distinguished in HNIW·[bmim][BF4] (Figure 4), but in this solvate HNIW molecules also form layerlike pattern, and anions are incorporated between the two layers. It is interesting that, in the latter structure, symmetry-independent cations interact directly with each other, while each HNIW layer is built only by one of the independent molecules. Geometry analysis of intermolecular interactions has revealed the presence of very short intermolecular contacts in both crystals. Aside from multiple C−H···O, C−H···F, N··· N, and O···N, they include rather rare F···N contacts (2.643 Å in HNIW·[bmim][BF4]); many of these interatomic distances are significantly shorter than the sum of van der Waals radii of the corresponding atoms. Although short contacts usually

since different conformations are observed in cocrystals with chemically and geometrically similar molecules. Moreover, in some structures symmetry-independent HNIW molecules adopt different conformation. Finally, it is not surprising that the conformations of the HNIW molecules in the IL solvates described in this paper fall into two most-abundant cases. The butyl chain of bmim+ (Figure 2) in both structures adopts trans/trans (TT) conformation of n-butyl,38 which is found to be the most common conformation in crystal structures containing this cation39 and is also found in βpolymorph of the pure [bmim][PF6] ionic liquid. However, the geometry of cation is different in the orientation of this nbutyl chain relative to the plane of the heterocycle that is defined by C1−N2−C5−C6 torsion angle. In HNIW·[bmim][PF6] and one of the cations of HNIW·[bmim][BF4] this angle is close to optimal value of 90°,38 but it is equal to 121.5(2)° in second independent molecule of the latter structure; this difference in conformation of the cation explains the pseudosymmetry described above. 3663



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definition, the HS analysis is performed separately for each of the crystallographically independent species of the crystal structure. In polymorphs and/or structures with several symmetryindependent molecules the ratios of contacts of different types is a distinct characteristic of the molecule and are well-suited for visualization and direct comparison of the structures.43 In multicomponent crystals these ratios can also be calculated for each of the coformers, but in such a case they contain only aggregate information about all interactions of the molecule with its local environment, and these interactions strongly depend on the nature of coformers. For instance, HS analysis was already performed for some cocrystals and solvates of HNIW.44 It is not surprising that in the cocrystal with chemically similar coformer HMX45 (CSD ZEBJOH) the distribution of the types of intermolecular contacts resembles the one in pure HNIW polymorphs, in cocrystal with planar nitrogen-rich and hydrogen-free molecule BTF46 (CSD PEHSUS) the HNIW molecule participates in more O···N interactions, while in structure with coformers that contain multiple phenyl44 or alkyl47 substituents C−H···O contacts dominate in crystal packing. As expected, the types of intermolecular contacts of the HNIW molecule in the IL solvates (Figure 5) are very different compared to pure HNIW polymorphs and structures analyzed in ref 44. In addition to contacts with fluorine atoms, the contribution of C−H···O is significant, apparently due to interactions with alkyl chains of the bmim+ cations. It was interesting to analyze, to what extent the intermolecular contacts between HNIW molecules are retained in multicomponent structures. To separate contribution from different pairs of components to the surface areas, we performed an extended analysis of HS for each coformer with an in-house program using the output of TONTO.48 The results are summarized in Figure 5 and Tables 3 and 4 (see Supporting Information for technical details and extended data). Note that data on HS analysis in Table 4 is not symmetric, and it should be read as “the component A (in a column) has common area with component B (in a row)”; the comparison of the data in rows is meaningless, since they were calculated for different HS.

Figure 4. Fragment of crystal packing in HNIW·[bmim][BF4] crystal, view along a axis. The symmetry-independent HNIW molecules are drawn with light and dark green, bmim+ cations with dark and light blue, and BF4− anions with light and dark red colors.

stabilize the crystal lattice, in some cases they can be treated as “forced”. Therefore, we used more sophisticated approaches to analyze intermolecular interactions described below. The Hirshfeld surface (HS) analysis is a popular method for quantifying and visualizing intermolecular interactions. In the same manner as atomic weight function was introduced by Hirshfeld for his definition of atom in molecule,40 a molecular weight function can be calculated to define the contribution of the pro-molecule of interest into the pro-crystal density:41 w(r ) =

∑A ∈ molecule ρA (r ) ∑A ∈ crystal ρA (r )

=

ρpromolecule (r ) ρprocrystal (r )

The molecular HS is then defined as a surface with w(r) = 0.5, and a molecule in crystal is a region of space enclosed by this surface, where w(r) ≥ 0.5. To attribute a region of the HS to a specific interatomic contact, the distances to the nearest atoms inside (di) and outside (de) the surface are used.42 By

Figure 5. Relative contribution of the contacts of different type to HS area of the HNIW molecule. Top two lines: full data for IL solvates; middle three lines: polymorphs of pure HNIW; bottom two lines: only HNIW···HNIW interactions in the IL solvates. Values for two crystallographically independent molecules in HNIW·[bmim][BF4] are averaged. 3664



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that surfaces of the anions have zero intersection areas in both IL solvates, and therefore they do not interact directly with each other; this is just a quantitative manifestation of their space separation. On the other hand, the HS areas that correspond to cation···cation contacts are notable, especially for HNIW·[bmim][BF4]. The detailed analysis shows the absence of direct contacts between the heterocyclic fragments of cations, and most of bmim+...bmim+ contacts are between the atoms of alkyl fragments (58% of in [bmim][PF6] and 61% in [bmim][BF4] solvates) and between alkyl fragments and heterocycles. In both cases anions interact mostly with HNIW molecule and not with cations; the same is true for cations in HNIW·[bmim][PF6], but in HNIW·[bmim][BF4] for cation the distribution of bmim+···HNIW and bmim+ ···bmim + contacts is almost equal. The contacts of the HNIW molecule clearly demonstrate the differences between two structures. In HNIW·[bmim][PF6] only 22.6% of HS area correspond to HNIW···HNIW interactions, while in HNIW·[bmim][BF4] this value is as great as 45.9%. These two values are a manifestation of different packing type of HNIW molecules, chains and layers in former and latter cases. It is interesting that not only the total area of HNIW··· HNIW but also types of interactions between the molecules change upon solvate formation. In [bmim][PF6] solvate the ratio of O···H contacts in chains is even higher than in pure HNIW polymorphs, whereas in the structure containing [bmim][BF4] O···N contacts are characterized by even higher HS area, due to presence of T-shaped N···O contact in the latter solvate. In general, the ratios of different types of HNIW···HNIW interactions are not similar to those in the polymorphs. It would be interesting to compare the results of a quasigeometrical HS anaysis with more rigorous approach based on a strict definition of the bonding interaction. In such approach, R. F. Bader’s Atoms in Molecules theory, an interaction is defined as bonding by the presence of (3,−1) or “bond critical point” (BCP) of ρ(r) function.32 Unfortunately, because of the nature of the anion that exhibits thermal motion of the fluorine atoms, it was not possible to perform a high-resolution X-ray diffraction experiment, and topological analysis for HNIW· [bmim][PF6] was performed on theoretical (B3LYP-D3/POBTZVP) periodic ρ(r) function; the atomic positions were optimized to eliminate the forces on atoms. Because of pseudosymmetry and doubled number of atoms in the independent part of the unit cell, geometry optimization and consequent analysis was not feasible for HNIW·[bmim][BF4]. All bonding intermolecular interactions were found as BCPs, and the whole list is provided in Supporting Information, Table TS8. Note that, due to geometry optimizations, these contacts are slightly different from those that would be observed in experimental structure. The energy of individual intermolecular interactions (EEML) was estimated with the Espinosa−Molins−Lecomte (EML) correlation49 as EEML = −0.5a03v(r), where a0 is the Bohr radius, and v(r) is the potential energy density in a BCP. The theoretical justification of this empirical correlation was recently proposed by us,50 and it is successfully applied to a wide range of inter- and intramolecular contacts.51 However, this correlation cannot be reliable generally52 and seems to overestimate the energy of very short interactions, where the repulsive component dominates over exchange-correlation stabilization.53 Indeed, the lattice energy calculated by EML equation for the ε-HNIW

Table 3. Hirshfeld Surface Areas of the Coformers in the IL Solvates and Volumes Enclosed by These Surfacesa HNIW·[bmim][PF6] component 2

HS area, Å HS volume, Å3 Bader volume, Å3 component HS area, Å

2

HS volume, Å3

PF6−

bmim+

301.7 99.7 342.9 81.2 355.5 91.7 HNIW·[bmim][BF4]

221.7 217.2 206.3

HNIW

BF4−

bmim+

303.4 308.5 348.2 351.8

80.4 80.9 57.5 57.7

221.2 221.8 214.5 217.1

HNIW

a

For HNIW·[bmim][BF4] data are provided for two symmetryindependent molecules. The Bader molecular volume is calculated from B3LYP-D3/POB-TZVP optimized structure.

Table 4. Relative Contribution of Contacts between the Components of the IL Solvates to the Crystal Packing (in Columns)a HNIW·[bmim][PF6] component HNIW PF6− bmim+ HNIW PF6− bmim+

HNIW PF6− bmim+

component HNIW BF4− bmim+

HNIW

PF6−

HS area with, % 22.6 74.9 25.6 0.0 51.8 25.1 AIM, contacts with, %, (number) 14.5 73.9 (8) (17) 30.9 0.0 (17) (0) 54.5 26.1 (30) (6) EEML, contacts with, %, (energy, kcal/mol) 16.1 82.3 (10.9) (24.9) 36.9 0.0 (24.9) (0.0) 47 17.7 (31.7) (5.4) HNIW·[bmim][BF4] HNIW

BF4−

HS, contacts with, % 45.9 64.9 17.2 0.0 37.0 35.2

bmim+ 66.3 10.3 23.4 76.9 (30) 15.4 (6) 7.7 (3) 82.5 (31.7) 14.0 (5.4) 3.5 (1.3) bmim+ 44.2 11.7 44.1

a For HNIW·[bmim][PF6] the values are based on HS analysis as well as on topological analysis of DFT-based ρ(r). For HNIW·[bmim][BF4] only HS analysis is reported; the values are averaged for symmetry-independent components of the same type.

The ratios of intercomponent contacts can be influenced by the size and the shape of the coformers and therefore their accessible surface areas. However, in two IL solvates the HS areas of HNIW and cations are very similar, and for anions the difference is notable but still does not exceed 25% (see Table 3). Table 4 demonstrates the relative areas of HS of the coformers corresponding to contacts to the same and other two components. First of all, on the one hand, it seems obvious 3665



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this value is comparable with an area of HS of HNIW molecule that corresponds to HNIW···HNIW interactions (22.6%, see Table 4). Therefore, the interaction energy between the molecules of the same type is to some extent proportional to the common area of their HSs, at least in the absence of strong specific interactions. Although this observation requires additional justification, a surface area (together with other properties, such as electrostatic potential) is used in empirical schemes proposed for sublimation enthalpy calculation.55 Note that integration of theoretical ρ(r) over atomic basins of HNIW·[bmim][PF6] lead to virtually complete charge transfer from the cation (+1.03 e) to the anion (−0.96 e), while charge on the HNIW moiety (−0.07 e) is insignificant. The integrated molecular volumes are comparable with those obtained by HS analysis (Table 3), and they are slightly higher for the HNIW and the anion and lower for the cation. Unfortunately, all X-ray diffraction experiments for β-HNIW possessing the same molecular conformation were performed at room temperature, and calculated molecular volume for this polymorph (ca. 366 Å3) is higher than for molecule in [bmim][PF6] solvate, but considering the ca. 2% difference of the unit cell volume between the room temperature and 100 K,56 the HNIW molecule in this solvate occupies almost the same volume as in pure β-HNIW. Physical Properties. To determine how the presence of the IL component modifies the physical property of the material, thermal behavior was studied on the crystalline samples. The resulting curves are shown in Figure 6. All curves

containing very short O···O interactions is significantly higher than that for two other polymorphs (see Supporting Information, Table TS6). It is not surprising that all shortest interactions in HNIW· [bmim][PF6] with interatomic distances less than the sum of van der Waals radii by more than 0.2 Å were found as BCPs in topological analysis of ρ(r), and their energies were estimated to be in a range of 1.5−2.5 kcal/mol (due to empirical nature of EEML the precision of energy values is chosen arbitrary). Higher interatomic distances did not always correspond to bonding interactions. In total, 65 symmetry-independent bonding contacts were found; the longest was C−H···O (3.286 Å), with EEML = 0.3 kcal/mol. As can be seen from Table 4, the relative number of bonding interactions between the coformers in HNIW·[bmim][PF6] in general correlate with results of the HS analysis; the same result was demonstrated recently by us for totally different system, polymorphs of 8-hydroxyquinoline.54 Notable differences are observed only for cation···cation interactions, since only three interactions are found between the H atoms of the cations in the topological analysis of ρ(r). Moreover, even the relative energy of interactions between the components estimated by EEML (Table 4) resembles the ratios of HS intersection areas and interaction numbers. In contrast to 8hydroxyquinoline discussed in ref 54, in the IL solvate there are no strong interactions such as H-bonds, and the distribution of interaction energies is more or less uniform for all contact types. Because of the ionic nature, it is hardly possible to calculate the lattice energy for ILs and their solvates. Moreover, the lattice energy, as commonly defined for molecular solids, does not have a measurable reference quantity in the case of ILs. The measurement of sublimation enthalpy ΔHsub that is directly related to the strength of intermolecular interactions in molecular solids is hardly possible for ILs due to their extremely low volatility. Instead, the bonding lattice energy (Elatt,EML) was calculated for the optimized α-[bmim][PF6] structure as sum of EEML of all intermolecular contacts, and it is equal to 33.4 kcal/mol. The same quantities calculated for β- and γ-polymorphs of HNIW are equal to 38.3 and 37.8 kcal/mol and are comparable with values calculated by UNI force field (38.6 and 34.2 kcal/mol) and values obtained directly from B3LYPD3/POB-TZVP periodic calculations (43.9 and 46.6 kcal/ mol). The Elatt,EML for the HNIW·[bmim][PF6] solvate was calculated to be 74.2 kcal/mol, that is by 2.5 kcal/mol greater than the sum of Elatt,EML of the components. The estimation of the formation energy calculated directly from B3LYP-D3/ POB-TZVP results as ΔE = E(HNIW·[bmim][PF6]) − E(βHNIW) − E(α-[bmim][PF6]) and is equal to −6.4 kcal/mol. Both estimations should not be considered as undoubtedly proper values, since Elatt,EML is empirical, and ΔE is subject to BSSE error that cannot be simply corrected for ionic structures. However, these numbers are comparable and indicate that EEML estimations are in general reliable in our case and can be used to compare the relative contributions of specific interaction types to the lattice energy. It is also possible to separate the contribution from uncharged part, for example, HNIW···HNIW interactions to the lattice energy. This energy calculated at B3LYP-D3/POBTZVP level is equal to 8.3 kcal/mol, which comprises ca. 18% of the lattice energy of β-HNIW polymorph (43.9 kcal/mol, see Supporting Information for details). It is interesting that

Figure 6. Differential scanning calorimetry (DSC) and thermogravimetric (TG) mass loss signals for obtained solvates and neat HNIW under linear heating with 5 K min−1 rate. (inset) The magnified DSC data for melting of solvates and phase transition of HNIW.

reveal the melting phenomena at temperatures within 105− 151 °C range, while the neat HNIW reveals no melting prior to decomposition with the only endothermic effect to be the phase transition of ε- to γ-polymorph (164 °C). After they melted, the solvates reveal the decomposition with onset temperatures near 190 °C, that is, lower than for pure HNIW at 234 °C. Further heating reveals the endothermic process above 250 °C, apparently corresponding to the ionic liquid evaporation and pyrolysis of the residue after CL-20 decomposition. Optical image of the sample after experiment shows the evidence of liquefaction with gas formation and decomposition in liquid phase. The magnitude of the mass loss within the stages for HNIW·[emim][OTf] is higher than that for HNIW·[bmim][PF6] and HNIW·[mim][BF4] in agreement with their crystallization composition (1:2, 1:1, and 1:1, respectively) Cooling of the melt from temperatures before decomposition leads to the same crystal solvate phase that was 3666



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proved by powder XRD experiments (see Supporting Information). The important physical property relevant to energetic materials is sensitivity to destructive stimuli, such as impact or friction. From the detailed analysis of crystal structures presented below, it is expected that the sensitivity of the crystal solvates should be significantly lower as compared to pure HNIW, because the fraction of the intermolecular interactions between the energetic parts is decreased. The density of the crystal solvates is aslo significantly lower: dcalc at room temperature is equal to 2.044 g cm−1 for ε-HNIW (CSD PUBMUU02) and 1.673, 1.754, and 1.543 g cm−1 for [bmim][BF4], [bmim][PF4], and [bmpyrr][OTf] solvates. The impact sensitivitiy was determined experimentally using a STANAG protocol and BAM-type apparatus for HNIW· [bmim][BF4], the solvate with the higher fraction of HNIW··· HNIW contacts. The sensitivity turned out to be 18 ± 3 J, which is significantly lower than the value for the pure εHNIW (4.1 ± 1.5 J).

CCDC 1844075−1844076 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Corresponding Author

*Phone: +7 499 1359214. E-mail: [email protected]. ORCID

Ivan V. Fedyanin: 0000-0002-4953-3874 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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Notes

The authors declare no competing financial interest.

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CONCLUSIONS In summary, previously unknown HEMs crystallization from ILs has been performed. The crystal structures obtained in this paper are the first examples of crystal solvates constructed from the energetic HNIW molecule and liquid at room temperature ILs[bmim][PF6] and [bmim][BF4]. Despite the similarity of the ILs, both structures have very different crystal packing as well as conformations of the crystal components. The conformations of the HNIW molecule belong to two most abundant cases, also found in β- and γ-polymorphs of the pure HNIW, for [bmim][PF6] and [bmim][BF4] crystal solvates, respectively. An extended quantitative analysis of the Hirshfeld surface analysis clearly demonstrates the common features and the difference between two crystal solvates. In the case of HNIW·[bmim][PF6] with chainlike arrangement of energetic molecules their interactions with cations play the most significant role, while in HNIW·[bmim][BF4] with layerlike packing the HNIW···HNIW interactions are dominant. The anions interact mostly with HNIW molecules in both cases, but direct cation···anion interactions are scarce. Topological analysis of theoretical (B3LYP-D3/POB-TZVP) electron density distribution for the HNIW·[bmim][PF6] solvate supports the results of HS analysis in terms of the ratios and relative energies of interactions between the components. The bonding lattice energy estimated for the latter crystal solvate from ab initio calculations shows the energetic stabilization upon solvate formation. The presence of the cations and anions of the ionic liquid decreases the density of the materials, as well as their melting temperatures and sensitivity to impact. This work will be helpful for understanding cocrystallization, polymorphism, and other phenomena of crystal engineering and supramolecular chemistry.

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AUTHOR INFORMATION

ACKNOWLEDGMENTS This study was financially supported by Russian Foundation for Basic Research (Project No. 16-29-01042). REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b01835. Overlays between the HNIW molecules in the IL solvates and polymorphs, crystallographic parameters for heavily disordered structure, powder diffraction patterns and IR spectra of crystal solvates, CSD search, Hirshfeld surface analysis, periodic DFT calculations (PDF) 3667



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