Cocrystallization of energetic Mn(II) complex with nitrogen-rich ligand SCZ and oxygen-rich ligand TNR

Abstract Based on the advantages of energetic complexes and cocrystallization, a novel energetic complex cocrystal [Mn(SCZ)3](TNR) (H2O) · [Mn(SCZ)2(H2O)(TNR)](H2O) (SCZ: semicarbazide, TNR: 2,4,6-trinitroresorcinol) was synthesized through a one-step reaction. This cocrystal contains equal units of [Mn(SCZ)2(H2O)(TNR)](H2O) and [Mn(SCZ)3]TNR(H2O). The molecules of the two units arrange mutually crosswise in the cocrystal and the benzene rings of TNR can form one-dimensional self-assemblies through π-π stacking. The thermal decomposition of the cocrystal is complicated with one endothermic process and three exothermic processes in the DSC curve. The complex [Mn(SCZ)2(H2O)(TNR)] · 3(H2O) was synthesized and the temperature of the major exothermic peak of the cocrystal is higher than observed for this complex. Graphical Abstract


Introduction
Energetic materials that can store and release substantial amounts of chemical energy are extensively used in military and civilian areas [1]. The development of energetic materials still focuses on exploring new energetic compounds of excellent performance [2]. The most studied topic in the preparation of energetic compounds is the facile synthesis of new energetic complexes [3]. However, as the structures and synthetic procedures of new ligands becoming more and more complicated [4], the synthetic difficulties and high costs of novel ligands may limit their practical application. Cocrystallization is an effective method to alter and improve the properties of energetic materials [5]. Synthesis and investigation of energetic cocrystals have drawn wide attention and usually only organic energetic compounds can be used for cocrystallization [6], while reports of coordination energetic complex cocrystals are rare [7]. A possible reason lies in the requirements mismatch to generate complexes and cocrystals. The unique coordination modes of the ligands and central metals usually cannot satisfy the strict requirements on the spatial structure and interaction of components to form cocrystals simultaneously. Therefore, research on the preparation and properties of complex cocrystals is very challenging.
We draw on the concept of molecular design of organic energetic cocrystal materials that energetic cocrystals can be realized based on p-p stacking and C-H … nitro interactions [8] to try to get complex cocrystals and study their properties. We selected semicarbazide (SCZ) as the ligand and 2,4,6-trinitroresorcinol (TNR) as the counterion. SCZ is economically feasible and stable under ambient conditions of temperature and pressure [9]. As a derivative of hydrazine, SCZ can ensure considerable nitrogen content. Foremost it can form chelating energetic materials (CEMs) [10]. The TNR molecule contains one benzene ring, two hydroxyl groups and three nitro groups and is also a good ligand for energetic complexes. It can provide multiple oxygen atoms from nitro and hydroxyl groups that are beneficial to the oxygen balance of the materials [11,12].
In O) were analyzed. The non-isothermal kinetic parameters, the critical temperature of the thermal explosion and the entropy of activation (DS 6 ¼ ), enthalpy of activation (DH 6 ¼ ) and free energy of activation (DG 6 ¼ ) of the cocrystal were first studied. This work is helpful to develop and make up for the deficiency of preparation technology of cocrystal complexes.

Materials and physical techniques
All reagents (analytic grade) were purchased from reputational vendors and used without purification. Elemental analyses were performed with an automatic trace element analyzer Flash EA 1112 full. DSC measurement was performed with a Pyris-1 differential scanning calorimeter. The sample was powdered and sealed in aluminum pans with a linear heating rate of 10 K min À1 from 350 to 850 K. Thermogravimetric (TG) measurements were performed with a Pyris-1 thermogravimetric analyzer (TG-DTG) of TA instruments with the linear heating rate of 10 K min À1 from 350 to 850 K.

Synthesis
The entire system of the synthesis experiment should be set up behind an explosionproof shield and the product should be dried in an explosion-proof dryer.

Synthesis of [Mn(SCZ) 3 ](TNR)(H 2 O) Á [Mn(SCZ) 2 (H 2 O)(TNR)](H 2 O)
An aqueous solution containing manganese carbonate (10 mmol, 1.15 g, Sinopharm Chemical Reagent Co., Ltd., AR) was charged into a glass reactor in a thermo-water bath. The solution was stirred with a mechanical agitator and heated to 385 K. TNR (10 mmol, 2.43 g, commercial product and recrystallized in distilled water before its use in the synthesis) with a stoichiometric ratio of 1:1 was added to the above solution. Stirring was continued until the solution became totally clear. Semicarbazide hydrochloride (30 mmol, 3.35 g, Sinopharm Chemical Reagent Co., Ltd., AR) was dissolved in distilled water (30 mL). The pH was adjusted to 6-7 with solid Na 2 CO 3 (14.6 mmol, 1.55 g, Sinopharm Chemical Reagent Co., Ltd., AR). After it was added to the above clear solution, continuous stirring was maintained at 385 K for 15 min. The solution was cooled to room temperature naturally. The precipitate was collected by filtration. It was washed with distilled water and ethanol, respectively. Yield: 57% (2.92 g). Single crystals suitable for X-ray measurement were obtained by evaporation of the mother liquor at room temperature over 14 days. Elemental analysis calcd for Mn 2 C 17 N 21 O 24 H 33 (molar mass 1025.52 g mol À1 ) (%): C, 19.91; H, 3.22; N, 28.70 (17) 11.226 (2) 12.428 (2) 6.7846 (14) c (Å) 15.043 (2) 25.580 (5)

Results and discussion
In the cocrystal, molecule A and B mutually crosswise arranged which makes good use of the space. The nitro groups and the electron-poor p-system of the benzene rings in the adjacent units interact with each other and make p-p stacking systems. Benzene rings of TNR overlap with each other in a parallel face-to-face way through the p-p stacking (with face distance of 3.6-4.2 Å), which together with the p-p stacking, electrostatic interaction, vdW, and hydrogen bonding contribute to the formation of the cocrystal [8], as shown in Figure 3. (Selected hydrogen bonds are listed in Table  S3 in the Supporting Information.) The has a lower density and smaller spatial hindrance than the cocrystal, and Mn 2þ tends to coordinate to form octahedron configurations.

Thermal decomposition and non-isothermal kinetics analysis
Differential scanning calorimeter (DSC) was used to investigate the thermal behavior of the cocrystal. The DSC curves of the cocrystal and [Mn(SCZ) 2 (H 2 O)(TNR)] Á 3(H 2 O) with a linear heating rate of 10 K min À1 are shown in Figures 5(a) and 6(a), respectively. Thermal decomposition of the cocrystal is complicated with one endothermic process and three exothermic processes in the DSC curve. The endothermic process was observed between 382.15 and 412.80 K with a peak temperature of 400.45 K, indicating loss of water in the cocrystal. The peak temperature of the three exothermic stages occur at 480.75, 551.65 and 656.45 K, respectively. The area of the third exothermic peak is large which indicates that the reaction of this stage produces a lot of heat. The multiple ring structure and the chelating effect lead to multi-step thermal decomposition reaction. The DSC curve of [Mn(SCZ) 2    Kissinger's method [19] and Ozawa-Doyle's method [20] combined with the first exothermic peak temperatures of DSC curves measured at four different heating rates of 5, 10, 15 and 20 K min À1 were applied to study the kinetics parameters of the thermal decomposition reaction. The values of the first exothermic peak temperatures are the average of three single measurements. Kissinger and Ozawa-Doyle equations are shown in Equations (1) and (2). The apparent activation energy E, pre-exponential factor A, linear coefficient R c and standard deviations S of the cocrystal were determined and are shown in Table 3.
Kissinger equation Ozawa-Doyle equation where T p is the peak temperature (K), R is the gas constant (8.314 J K À1 mol À1 ), b is the linear heating rate (K min À1 ) and G(a) is the reaction mechanism function. Table 4. Thermal parameters of the cocrystal. The Arrhenius equation of the decomposition of the cocrystal is shown in the following equation: ln k ¼ 50:36À215:9 Â 10 3 =RT (3) The values of the peak temperature corresponding to b!0 are obtained according to Equation (4) [21]. The value is shown in Table 4, where a, b and c are coefficients.
The corresponding critical temperature of thermal explosion (T b ) can be obtained by Equation (5) [22] shown in Table 4. The T b value of the cocrystal is higher than the traditional diazodinitrophenol (DDNP) (458.15 K) [23] which indicates that the cocrystal is more thermally stable.
The entropy of activation (DS 6 ¼ ), enthalpy of activation (DH 6 ¼ ) and free energy of activation (DG 6 ¼ ) of the decomposition reaction were obtained by Equations (6)-(8) [21] shown in Table 4. The positive value of DG 6 ¼ indicates that the exothermic decomposition processes of the cocrystal must proceed under the heating condition.
DG 6 ¼ ¼ DH 6 ¼ ÀTDS 6 ¼ where k B is the Boltzmann constant (1.381 Â 10 À23 J K À1 ) and h is the Planck constant (6.626 Â 10 À34 J s). O) was also synthesized. It has similar thermal performance to the cocrystal of one endothermic process and three exothermic processes. The temperature of the major exothermic peak of the cocrystal is higher than [Mn(SCZ) 2 (H 2 O)(TNR)] Á 3(H 2 O). The values of T b and DG 6 ¼ indicate that the cocrystal is more thermally stable than DDNP and the exothermic decomposition processes of the cocrystal must proceed under the heating condition. The mutually crosswise arrangement of the two-unit molecules and overlap of benzene rings of TNR result in good thermal stabilities of the energetic complex cocrystal.