Synthesis of two nickel (II) complexes bearing pyrrolide-imine ligand and their catalytic effects on thermal decomposition of ammonium perchlorate

Accepted Manuscript Synthesis of two nickel (II) complexes bearing pyrrolide-imine ligand and their catalytic effects on thermal decomposition of ammonium perchlorate Ji-Bin Zhuo, Zai-He Ma, Cai-Xia Lin, Li-Li Xie, Sha Bai, Yao-Feng Yuan PII: DOI: Reference:

S0022-2860(14)01241-1 http://dx.doi.org/10.1016/j.molstruc.2014.12.038 MOLSTR 21188

To appear in:

Journal of Molecular Structure

Received Date: Revised Date: Accepted Date:

15 October 2014 10 December 2014 10 December 2014

Please cite this article as: J-B. Zhuo, Z-H. Ma, C-X. Lin, L-L. Xie, S. Bai, Y-F. Yuan, Synthesis of two nickel (II) complexes bearing pyrrolide-imine ligand and their catalytic effects on thermal decomposition of ammonium perchlorate, Journal of Molecular Structure (2014), doi: http://dx.doi.org/10.1016/j.molstruc.2014.12.038

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Synthesis of two nickel (II) complexes bearing pyrrolide-imine ligand and their catalytic effects on thermal decomposition of ammonium perchlorate ∗

Ji-Bin Zhuoa, Zai-He Maa, b, Cai-Xia Lin a, Li-Li Xie a, Sha Bai c, Yao-Feng Yuan a a

Department of Chemistry, State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou 350116, People’s Republic of China.

b

Qingdao Double-Peach Specialty Chemicals (Group) Co., ltd. Qingdao 266031, People’s Republic of China.

c

Beth Israel Deaconess Medical Center (BIDMC), Harvard Medical School, Boston, MA 02215, USA.

Abstract: Two pyrrolide-imine chelating Ni(II) complexes {[2-(2-CH3O-C6H4-NCH)C4H3N]2Ni (2a) and [(Fc-NCH)]C4H3N]2Ni (2b, Fc = ferrocenyl)} were prepared via treating corresponding Schiff base with 0.5 equiv. NiCl2·6H2O in moderate yields. The crystal structures of 2a and 2b were determined by single-crystal X-ray diffraction. Atom Ni(II) of 2a was coordinated by two pyrrolide-imine ligands in trans position to display a twisted octahedral coordination geometry. Ni(II) of 2b had a distorted square-planar geometry, bonded with two ferrocenyl pyrrole-imine ligands, each ferrocene and pyrrole of ligands adopting a trans conformation. The UV-vis spectroscopy and electrochemical measurements were investigated. The catalytic efficiency of the complexes on the thermal decomposition of ammonium perchlorate (AP) was studied by differential scanning calorimetry (DSC) and thermogravimetry (TG). Compared with the thermal decomposition of pure AP, the decomposition temperatures were decreased by 27 ˚C, 77 ˚C, 88 ˚C and 172 ˚C, respectively when 1a, 1b, 2a and 2b were added in AP. The results indicated that the Ni(II) complex 2b bearing ferrocene-based pyrrolide-imine N,N-chelate ligand displayed an excellent catalytic efficiency on the thermal decomposition of AP. Keywords: Ni(II) complex; Pyrrolide-imine; Crystal structure; UV-Vis spectroscopy; Electrochemical measurement; Thermal decomposition of ammonium perchlorate

∗

Corresponding authors’ address: Department of Chemistry, Fuzhou University, Fuzhou 350116, China. Tel./fax: +86 591 22866161. E-mail address: [email protected] (Y.F. Yuan).

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1. Introduction The great interest in ferrocenyl-substituted heterocyclic compounds is associated not only with the unusual properties that the ferrocene system imparts to the heterocyclic fragment but also with the redox activity of the ferrocene system [1-4]. In recent years, the investigations on the synthesis of ferrocene-based heterocyclic compounds have been reported frequently due to their potential applications as electrochemical sensors [5-12], fluorescent probe [13, 14], metal complexes [15] and heterogeneous catalysts [16, 17]. Therefore, the integration of one or more ferrocene units into a heterocyclic compound has been recognized as an attractive way to entitle a molecule with new functions. Researchers have been focusing on the study of heterocyclic derivatives connecting ferrocenyl unit, for their advantages on controllability in terms of burning-rate catalysts [18-23]. Compared with the traditional combustion catalysts, ferrocene-based heterocyclic compounds possess many advantages, such as low viscosity, flammability, high molecular weight and thermal stability [18, 24]. Many examples of the successful application of ferrocenyl derivative as high-burning-rate catalyst have been reported in these few years [12, 22, 25, 26]. Pyrrolide-imine compounds have been widely studied in the field of applied chemistry, especially served as catalysts with high performance [27-30], which are crucial ligands from the aspect of coordination chemistry [29, 30]. We envision that introducing the pyrrolide-imine to ferrocene-based heterocyclic compounds could derive new materials with unique electrochemical properties and functional molecules of high catalytic activity [31-33]. In addition, ammonium perchlorate (AP) is the common oxidizer in composite propellants, and the thermal decomposition characteristics of AP directly influence the behavior of composite solid propellant combustion [34, 35]. It has been reported that the activity of catalytic decomposition of AP would enhance with an addition of burning-rate catalysts comprising nickel, because the nickel at the molecular level on the propellant surface may contribute to the catalytic effect when the compounds decompose [36, 37]. However, there are very few reports on nickel complexes as potentially high-burning-rate catalysts. Therefore, we reported a convenient and efficient preparation method of pyrrolide-imine chelate nickel complexes 2a and 2b. Molecular structures and cyclic voltammetry (CV) studies of redox properties of 2b were investigated. In order to confirm potentially burning-rate catalytic activity of 2a and 2b, the catalytic properties on thermal decomposition of AP were studied. Pyrrolide-imine chelate nickel complex 2b containing ferrocenyl unit was found to an excellent catalyst for thermal 2

decomposition of AP.

2. Experimental 2.1. General All chemicals were commercially available. All reactions were performed under an atmosphere of dry nitrogen. Melting points (m.p.) were determined with a 4X micro melting point apparatus. Infrared spectra (IR) were recorded with a Spectrum-2000 FT-IR spectrum in KBr pellets in the range of 4000-400 cm-1. Nuclear magnetic resonance spectra (1H and 13C NMR) were measured in CDCl3, with tetramethylsilane (TMS) as the internal standard, with a Bruker AV600 spectrometer at ambient temperature. Electrospray ionization mass spectrometry (ESI-MS) was performed with an X7 ICP-MS instrument. 1-aminoferrocene was synthesized according to the previously reported protocol [38]. (Insert Scheme 1 near here) Single crystals of 1b, 2a and 2b were grown by slow diffusion of petroleum ether into the solution of pure product in dichloromethane over several days, respectively. X-ray structural measurements was carried out on a Rigaku RAXISIV CCD diffract meter with a graphite-monochromator Mo-Kα radiation (λ = 0.071073 nm) at 293(2) K. All diffraction data were collected by scanning in a certain mode and refined in Lp factor. All data were corrected by semi-empirical method using SADABS program, the SAINT program was used for integration of the diffraction profiles [39]. The structure was solved by the direct methods using SHELXS program of the SHELXL-97 [40]. The position coordinates and each anisotropic thermal parameter of nonhydrogen atoms were refined by full-matrix least-squares on F2 through confirmation. All hydrogen atoms were generated geometrically assigned appropriated isotropic thermal parameters and included in the final calculations. The crystallographic data, collection parameters and refinement parameters were summarized in Table 1, and the atomic numbering scheme adopted and selected bond distances (Å) and angles (°) were listed in Table S1~S3. (Insert Table 1 here) The DFT calculations were carried out with the Gaussian 03 suite of programs [41]. Theoretical calculations were performed at the DFT level using the B3LYP. Vibrational frequency calculations were measured at the same level of theory to verify the nature of the stationary points [42].

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The UV-visible spectroscopy was tested by Perkin Elmer lambda 900 UV/VIS/NIR in acetonitrile (c = 1.0×10-5 M). With this method, quartz material was used as cuvette, whose volume was 1.0 × 1.0 × 4.0 cm3. The extinction coefficient was calculated according to Lambert-Beer's law [43]. The electrochemical characterization was tested by a CHI620C electrochemical workstation via CV in CH3CN at room temperature. In this method, (n-Bu4N)PF6 (c = 0.10 M) was used as the supporting electrolyte. A three-electrode cell was used, consisting of a glassy carbon working electrode, a platinum wire auxiliary electrode and Ag/AgCl (3.0 mol/L KCl) reference electrode. The scan rate of CV reached 0.10 V-1. The surface of glassy carbon electrode was polished with 0.05 nm α-Al2O3 slurry on emery paper and rinsed with doubly distilled water. The system was degassed by purging nitrogen for more than 5 minutes before each electrochemical measurement. Differential scanning calorimetry (DSC) and thermogravimetry (TG) analysis on pure AP and AP with an addition of 3 wt % of 1-2a or 1-2b (totally 5 mg for each mixture) were dried under 0.1 MPa at 20 °C, then recorded with a Simultaneous DSC-TG Thermal Analyzer SDT Q600 instrument. The experiments were conducted under nitrogen atmosphere (20 mL N2/min) at heating rate of 10 ˚C/min from 50 to 500 ˚C.

2.2. Synthesis of Ligands 2.2.1 Synthesis of ligand 1a 1a was synthesized by modifying the reported procedure [44]. Catalytic amount of p-toluene sulfonic acid (20 mg, 0.10 mmol) was added to a mixture of pyrrole-2-carbaldehye (380 mg, 4.0 mmol) and 1-amino-2-methoxybenzene (490 mg, 4.0 mmol) in methanol (10 mL) under stirring condition in nitrogen atmosphere. The resulting mixture was heated to reflux for 20 h, and then cooled to room temperature. After removal of volatiles under reduced pressure, the residue was mixed with THF and filtered through a pad of celite. The resulting solution was concentrated to give a brown solid, which was further purified by re-dissolved in THF/ether (1: 4). The solution was kept at -18 ˚C for overnight, and brown crystalline solid was collected by filtration. The solid was washed with ether and further dried under vacuum to obtain 1a. Yield 270 mg (68 %). m.p. 235-236 ˚C . 1H NMR (400 MHz, CDCl3) δ 10.07 (s, 1H, NH), 8.30 (s, 1H, CH=N), 7.20 (dt, J = 7.6, 1.6 Hz, 1H, Ar-H), 7.05 (d, J = 1.6 Hz, 1H, Ar-H), 7.01 (d, J = 7.6 Hz, 1H, Ar-H), 6.98 (d, J = 7.6 Hz, 1H, Ar-H), 6.88 (d, J = 1.2 Hz, 1H, Py-H), 6.70 (dd, J = 1.2, 3.6 Hz, 1H, Py-H), 6.30 (d, J = 3.6 Hz, 1H, Py-H), 3.88 (s, 3H, CH3). 13C NMR (100 MHz, CDCl3) δ 152.52, 150.71, 141.82, 130.95, 126.63, 123.32, 121.36, 120.74, 116.69, 111.42, 4

110.47, 55.84. MS (ESI) m/z: 200.5. Anal. Calcd for C12H12N2O: C 71.98, H 6.04, N 13.99. Found: C 71.96, H 6.30, N 13.83. 2.2.2 Synthesis of ligand 1b Substituting amino-ferrocene for 1-amino-2-methoxybenzene, ligand 1b was yielded as yellow crystalline solid. Yield 800 mg (72%). m.p. 135-136 ˚C (CH2Cl2/n-hexane). 1H NMR (400 MHz, CDCl3) δ 10.58 (s, 1H, NH), 8.45 (s, 1H, CH=N), 7.02 (d, J = 1.2 Hz, 1H, Py-H), 6.64 (dd, J = 1.2, 3.6 Hz, 1H, Py-H), 6.34 (d, J = 3.6 Hz, 1H, Py-H), 4.50 (s, 2H, Cp-H), 4.21 (s, 2H, Cp-H), 4.18 (s, 5H, Cp-H). 13C NMR (100 MHz, CDCl3) δ 148.33, 130.54, 123.16, 114.85, 109.59, 106.73, 69.53, 67.61, 62.36. IR (KBr, v/cm-1): 3375, 3099, 1603, 1542, 1466, 1413, 1299, 1242, 1223, 1114, 1104, 1028, 999, 928, 882, 837, 813, 739, 685, 650, 672, 592, 531, 492, 469. MS (ESI) m/z: 279.7. Anal. Calcd for C15H14FeN2: C 64.78, H 5.07, N 10.07. Found: C 64.70, H 5.38, N 9.89.

2.3 Synthesis of complexes 2.3.1 Synthesis of [2-(2-CH3O-C6H4-NCH)C4H3N]2Ni (2a) A solution of 1a (200 mg, 1.0 mmol), NaOH (40 mg, 1.0 mmol) and NiCl2·6H2O (120 mg, 0.50 mmol) in methanol (20 mL) was stirred and heated at reflux for 4 h under nitrogen atmosphere. All volatiles were removed under reduced pressure and brown solid was obtained. The solid was dissolved in CH2Cl2/n-hexane (1: 4) and kept at -18 ˚C for overnight. Brown crystalline solid was collected after filtration. Yield 190 mg (82%). m.p. 218-220 ˚C (CH2Cl2/n-hexane). 1H NMR (400 MHz, CDCl3) δ 8.58 (s, 2H, CH=N), 7.77 (dd, J = 1.6, 8.0 Hz, 2H, Ar-H), 7.57 (d, J = 1.6 Hz, 2H, Ar-H), 7.28 (d, J = 1.2 Hz, 2H, Py-H), 7.19 (d, J = 8.0 Hz, 4H, Ar-H), 7.01 (dd, J = 1.2, 3.6 Hz, 2H, Py-H), 6.46 (dt, J = 3.6 Hz, 2H, Py-H), 3.94 (s, 6H, CH3). 13C NMR (100 MHz, CDCl3) δ 152.53, 151.16, 141.83, 130.96, 126.15, 123.45, 121.25, 120.64, 116.70, 111.58, 110.18, 55.85. IR (KBr, v/cm-1): 3436, 3060, 2920, 1625, 1599, 1560, 1494, 1461, 1445, 1382, 1287, 1251, 1236, 1189, 1175, 1114, 1095, 1029, 978, 947, 892, 799, 745, 680, 608, 560. MS (ESI) m/z: 457.0. Anal. Calcd for C24H22N4NiO2: C 63.06, H 4.85, N 12.26. Found: C 63.27, H 4.93, N 12.35. 2.3.2 Synthesis of [(Fc-NCH)]C4H3N] 2Ni (2b) The synthetic procedure of 2b was the same as that for 2a except that 1b was used instead of 1a. Dark brown solid was collected. Yield 200 mg (65%). m.p. 278-280 ˚C (CH2Cl2/n-hexane). 1H NMR (400 MHz, CDCl3) δ 8.28 (s, 2H, CH=N), 7.25 (d, J = 1.2 Hz, 2H, Py-H), 6.49 (dd, J = 1.2, 3.6 Hz, 2H, Py-H), 6.03 (d, J = 3.6 Hz, 2H, Py-H), 4.46 (s, 4H, Cp-H), 4.18 (s, 4H, Cp-H), 4.17 (s, 10H, Cp-H). 13C 5

NMR (100 MHz, CDCl3) δ 148.23, 131.33, 122.32, 114.75, 110.30, 105.80, 69.43, 66.74, 62.26. IR (KBr, v/cm-1): 3441, 2925, 1602, 1412, 1323, 1247, 1134, 1103, 1091, 1051, 1030, 1000, 955, 928, 882, 822, 810, 740, 676, 661, 608, 534, 490. MS (ESI) m/z: 612.3. Anal. Calcd for C30H26Fe2N4Ni: C 58.79, H 4.28, N 9.14. Found: C 58.59, H 4.63, N 8.84.

3. Results and Discussion 3.1 Synthesis and Characterization Following the synthetic procedure, complexes 2a and 2b were obtained as brown red solid by reaction of NiCl2·6H2O with 2.0 equiv. of corresponding ligand 1 in CH3OH at reflux under nitrogen atmosphere (Scheme 1). Both of the complexes were stable in air. In the 1H NMR spectra of the complexes, the characteristic resonances for the acidic protons of complexes disappeared compared with those of ligands, confirming the formation of coordinate bonds between ligands and nickel atom. There were some proton signals slightly shifted in the 1H NMR spectra of the complexes compared with those of relative ligands (Figures S1-S4). However, there was not much difference between complexes and corresponding ligands in the 13C NMR spectra (Figures S5-S8).

3.2 Analysis of structure 3.2.1 Molecular Structure of 2a As shown in Figure 1a, complex 2a crystallizes in the orthorhombic system. The structure of 2a is solved in P212121 space group. The nickel (II) atom is hexacoordinated, the coordination geometry is distorted from octahedral as ascertained by the observed τ value of 0.27 (τ = (β–α)/60 where α and β are the largest coordination angles, β = N2–Ni–N4 169.39° and α = N1–Ni–O1 152.93°) [45]. The two pyrrolide-imine moieties as chelating ligands bonded with the nickel (II) center through nitrogen atom and oxygen atom in a trans manner. The nickel–nitrogen and nickel–oxygen lengths for the octahedron (Ni–N1 bond: 2.023 Å, Ni–N2 bond: 2.010 Å, Ni–N1 bond: 2.030 Å, Ni–N4 bond: 2.014 Å, Ni–O1 bond: 2.292 Å, Ni–O2 bond: 2.406 Å) are comparable to those found in similar types of nickel (II) octahedral complexes [46-48]. However, the dihedral angles of benzene ring and pyrrole ring in two ligands are different, with angles of 6.83° and 19.36°, respectively. The N=C bond lengths of the two ligands are not equal (N2=C5 bond: 1.291 Å, N4=C17 bond: 1.315 Å). All above indicate the two ligands in complex are not crystallographically identical. The adjacent molecules are stacked through

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the C–H···π contacts (Table 2). Through the weak intermolecular interactions, the adjacent molecules are held together and display a layered structure (Figure 1b). (Insert Figure 1a-b here) 3.2.3 Molecular Structure of 1-2b The ORTEP views of 1-2b (Figure 2a and Figure 2b) indicate that both of 1b and 2b belong to the orthorhombic system with space group Pbca. Nickel(II) of 2b has a distorted square-planar structure with two ferrocenyl pyrrole-imine ligands. Two bidentate ligands are coordinated with the nickel (II) center. The coordination sites in the complex are the pyrrolide-imine group (Ni1–N1 bond: 2.039 Å, Ni–N2 bond: 1.949Å, Ni–N3 bond: 1.939 Å, Ni–N4 bond: 2.017 Å). Bond distances and angles of the coordination polyhedron are analogous to those found in other complexes of Ni(II) involving N–donor atoms [47]. The molecular structure of ferrocene and pyrrole in the ligand of 2b adopts a trans conformation, which is the same as that of 1b. All the cyclopentadiene (Cp) rings in ferrocenyl unit of 1b and 2b are eclipsed (1b: pseudo-torsion angles of C1-Cg-Cg-C10: 4.56°, 2b: pseudo-torsion angles of C1-Cg-Cg-C10: 8.39º and C21-Cg-Cg-C26: 0.59º) [49]. The Cp ring (C1-C5) of substituted ferrocenyl unit and the pyrrole ring in 1b are almost in the same plane (dihedral angle is 4.53°), which indicate the pyrrole rings are almost coplanar with the Cp rings of ferrocenyl unit, forming a large conjugated system. However, the highly distorted square-planar geometry leads to result that the pyrrole ring and corresponding Cp ring are no longer coplanar in 2b (dihedral angles are 42.42° and 48.31°, respectively). The two ligands in 2b are also not crystallographically identical. Each mono-nuclear in unit cell shows different intramolecular C–H···π contacts (Figure 2c), and geometric features of the C–H···π interactions are also given in Table 2. (Insert Figure 2a-c here) (Insert Table 2 here)

3.3 Analysis of UV–vis absorption spectroscopy of compounds The UV–vis absorption spectra of theses compounds (c= 10-5 M) in CH3CN solutions showed the absorption bands with λmax at 352 nm of 1a and 323 nm of 1b, which could be attributed to the absorption peaks of pyrrolide-imine (Figure 3). The absorption bands of 2a and 2b appeared at 415 nm and 364 nm, respectively. In comparison with 1a and 1b, the absorption peaks in the UV spectra of 2a 7

and 2b were wider and shifted towards long wavelength, which were due to the ligand-to-metal charge transfer (LMCT) under the electron withdrawing effect of metal. In addition, in complex 2 the outer electron distribution of atom Ni(II) was 3d8, the non-full shell structure, causing d-d* transition, which exhibited as a small peak close to 500 nm in their UV spectra [50]. (Insert Figure 3 here)

3.4. Electrochemical behavior of 1b and 2b The effect of scan rate on the CV of 2b was investigated (Figure 4a) and the two ferrocenyl fragments in 2b were electrochemically equivalent after the formation of complex. Both the anodic and cathodic peak currents were linear to the square roots of scan rate (v1/2) at different scan rates, indicating a diffusion-controlled process in the electrochemical behavior of 2b [51]. CVs of ferrocene, 1b and 2b at scan rate of 0.1 V·s-1 were measured in acetonitrile at room temperature (Figure 4b) and the results were summarized in Table 3. The oxidation and reduction current peaks (∆E) of 1b and 2b were typically separated by 0.066 and 0.068 V, respectively, close to that of simple ferrocene (0.070 V). The values of ∆E implied ferrocenyl unit had good reversibility, which was further supported by values of ipa/ipc (nearly equal to 1). The potential values (Ep) for 1b and 2b slightly shifted to negative potential compared with ferrocene. This was because the imino group in 1b and 2b could increase the electron density of ferrocene, making ferrocenyl unit easier to oxidize. [52] (Insert Figure 4a, 4b here) (Insert Table 3 here) DFT computational studies were performed to verify the electron transfer mechanism of 2b. The schematic representation of the molecular frontier orbitals of 2b was exhibited in Fig. 5b, the HOMO-2 (-5.509 eV) and HOMO-3 (-5.520 eV) of 2b were dominated by each ferrocene unit, respectively. The results conformed the two ligands in 2a were not identical, which was associated with crystal description above. However, the energy gap between two orbitals was quite small (Δ =0.011 eV). In the electrochemical behavior, the separation of two ferrocene units of 2b was difficult in the solution of CH3CN containing (n-Bu4N)PF6 was applied as a supporting electrolyte [53], only a reversible one-electron redox was seen. (Insert Figure 5 near here)

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3.5. Catalytic effect on the thermal decomposition of AP The thermal decomposition of AP was affected by the catalytic burning rate, which was analyzed by DSC curves (Figure 6a) and TG curves (Figure 6b), respectively. From Figure 6a we noticed that the DSC curves for thermal decomposition of pure AP showed two stages. In the first stage, the endothermic peak appeared at 246 ˚C due to its transition from orthorhombic form to cubic form. In the second stage, the main exothermic peak appeared at relatively higher temperature of 462 ˚C, indicating complete decomposition of the intermediate products [23, 54, 55]. (Insert Figure 6a, 6b near here) The DSC curves for decomposition of AP in addition of 1a, 1b, 2a or 2b showed noticeable differences in the decomposition patterns of AP. In the first stage, the endothermic peaks at about 243-245˚C in all samples exhibited a similar shape, indicating that additives had little effect on the crystallographic transition temperature of AP. In the second stage, dramatic changes in the exothermic peaks of AP decomposition were observed. After adding 1a, 1b, 2a or 2b, the exothermic peaks became sharper and appeared at a lower temperature (435 ˚C, 385 ˚C, 374 ˚C and 290 ˚C, respectively), that was, and the additives lowered the thermal decomposition temperature of AP by 27 ˚C, 77 ˚C, 88 ˚C and 172 ˚C, respectively. 2b (ΔT = 172 ˚C) had better catalytic efficiency on AP than 1b (ΔT = 77 ˚C) and 2a (ΔT = 88 ˚C), which indicated both ferrocene-based moiety and coordinated metal nickel had a promoting effect on thermal decomposition of AP. While the mass fraction of additives would little affect their catalytic efficiency on AP (Figure S9). The results and argument confirmed that ferrocene-based nickel complex 2b was a kind of potentially high-burning-rate catalyst. To gain more insight into the catalytic efficiency of these compounds, the relationship between decomposition temperature (Tp) and heating rate (β) were listed in the Table S4 to describe the Kissinger correlation, and then the apparent activation energy (Ea) and pre-exponential factor (A) were obtained by the Kissinger correlation [56, 57]. For AP, Ea was calculated to be 104 kJ·mol-1. In the presence of additives, Ea was changed to be 100, 92.2, 91.7 and 79.8 kJ·mol-1 with additives 1a, 1b, 2a and 2b, respectively. Compensation parameter (Sp) was chosen to describe the reaction ability of the system, the smaller value of Sp was, the better catalytic efficiency of additives on thermal decomposition temperatures of AP would be [56, 58]. Analysis of the data in Table 4, value of Sp after 2b (3 wt%) added to the AP was significantly less than that of other additives (1a, 1b and 2a), which was the direct evidence for the high catalytic activity of complex 2b. However, different equivalent 9

additives (2a and 2b) did not much influence the value of Sp (Table S5), consistent with the conclusion of the DSC curves. (Insert Table 4 near here) Furthermore, the weight loss of AP was quantitatively determined from the TG curves of pure AP and AP with additives (Figure 6b). The TG curve of pure AP exhibited that the weight loss required the only one step, and the final thermal decomposition temperature was about 461 ˚C. The final thermal decomposition temperatures of AP with 1a, 1b, 2a or 2b were at about 429 ˚C, 384 ˚C, 374 ˚C and 290 ˚C, respectively and it also indicated that 2b had the best catalytic efficiency on AP. The above results further supported the discussions in the DSC curves.

4. Conclusion In this paper, two pyrrolide-imine Ni(II) complexes 2a and 2b were synthesized and characterized by 1H NMR,

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C NMR, IR and MS. The crystal structures of the complexes were

determined by single-crystal X-ray diffraction. The Ni(II) of 2a and 2b was coordinated by two corresponding ligands 1 in trans position, respectively. Their spatial structures were stacked through hydrogen bonds in the solid state. The absorption peaks of complexes were red-shift in the UV-vis spectra compared with the corresponding ligands. Furthermore, electrochemical investigations showed two ferrocenyl units in 2b were electrochemical equivalent, and the ferrocenyl units of 1b and 2b implied good reversibility, easier to oxidize compared with simple ferrocene. The DSC and TG measurements confirmed the significant catalytic effects of 1b, 2a and 2b on the reducing of the decomposition temperature of AP, and 2b was of highest catalytic effect (by 172 ˚C). We expected that the ferrocene-based nickel complex derivatives would have great value in high-burning-rate catalyst for composite solid propellants.

5. Acknowledgments We are grateful for the financial support from the National Natural Science Foundation of China (No. 21172036 and 21372043) and the Research Fund for the Doctoral Program of Higher Education of China (No. 20113514110002)

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13

2

o

2

2 o

2

2 o

Scheme 1. The synthesis of complexes 2a and 2b

14

Figure 1a. ORTEP view of 2a (50% probability level), all hydrogen atoms were omitted for clarity.

Figure 1b. Cell packing diagram of 2a (viewed down the b axis), the 2D supramolecular layers formed by the C–H···π contacts.

15

Figure 2a. ORTEP view of 1b (50% probability level), all hydrogen atoms were omitted for clarity.

Figure 2b. ORTEP view of 2b (50% probability level), all hydrogen atoms were omitted for clarity.

Figure 2c. Cell packing diagram of 2b (viewed down the b axis), the 2D supramolecular layers by the C–H···π contacts in 2b. 16

Ligand 1a Ligand 1b Complex 2a Complex 2b

Absorbance / 10-5 M cm-1

0.6

352

0.5

323

0.4

365

0.3

415

0.2 *

0.1

d-d transition

0.0 300

400

500

600

700

Wavelength / nm

Figure 3. UV spectra for compounds 1a-b and 2a-b.

Current Intensity /A

b

4.0x10

-5

3.0x10

-5

2.0x10

-5

1.0x10

-5

Ferrocene Ligand 1b Complex 2b

0.0 -1.0x10

-5

-2.0x10

-5

-3.0x10

-5

0.0

0.2

0.4

0.6

0.8

1.0

Pontential /V

Figure 4. (a) CV spectra and current peak currents of 2b obtained at different scan rates in CH3CN. (b) CV spectra of ferrocene, 1b and 2b at scan rate of 0.10 V·s-1.

18

HOMO (-5.133 eV)

HOMO-1 (-5.263 eV)

HOMO-2 (-5.509 eV)

HOMO-3 (-5.520 eV)

Figure 5. The energy diagram of the first four HOMOs of 2b.

246 °C AP 243 ° C AP + 3% 1a 244 °C AP + 3% 1b 245 °C AP + 3% 2a 244 ° C AP + 3% 2b

b 462 °C

Weight/%

Heat Flow / (mW/mg)

a

385 °C 435° C

AP AP + 3% 1a AP + 3% 1b AP + 3% 2a AP + 3% 2b 461 °C 429 ° C 384 °C 374 °C

374 °C

290 °C

290 °C 200

250

300

350

400

Temperature/°C

450

500

200

250

300

350

400

450

500

Temperature/°C

Figure 6. DSC (a) and TG (b) curves of AP, AP+3% 1a, AP+3% 1b, AP+3% 2a and AP+3% 2b.

19

Table 1 Crystallographic data and structure refinement parameters Complex

1b

2a

2b

Empirical formula

C15H14FeN2

C24H22N4NiO2

C30H26NiFe2N4

Formula weight

278.13

457.15

612.97

Temperature (K)

293(2) K

293(2) K

293(2) K

Wavelength (Å)

0.71073

0.71073

0.71073

Crystal system

Orthorhombic

Orthorhombic

Orthorhombic

Space group

P212121

Pbca

Pbca

A(Å)

7.5006(15)

8.3738(17)

19.294(4)

b(Å)

10.659(2)

15.751(3)

12.567(3)

c(Å)

15.782(3)

33.188(7)

21.365(4)

α(°)

90

90

90

β(°)

90

90

90

γ(°)

90

90

90

1261.8(4)

4377.3(2)

5180.2(2)

3

V (Å ) Z

4

8

8

Dcalc (g•cm-3)

1.464

1.387

1.584

F(000)

800

2750

3864

Crystal size (mm)

0.50 × 0.40 × 0.20

0.40 × 0.40 × 0.20

0.20 × 0.18 × 0.15

θ range (°)

3.21 to 27.49

3.31 to 27.52

3.24 to 27.53

-9 ≤ h ≤ 9;

-10 ≤ h ≤ 10;

-24 ≤ h ≤25;

-13 ≤ k ≤ 11;

-20 ≤ k ≤ 20;

-16 ≤ k ≤ 15;

-20 ≤ l ≤ 20

-30 ≤ l ≤ 43

24 ≤ l ≤ 27

10564

34459

41310

2887

4683

5948

[R(int) = 0.0469]

[R(int) = 0.0867]

[R(int) = 0.0757]

0.995

0.997

0.998

Full-matrix

Full-matrix

Full-matrix

least-squares on F2

least-squares on F2

least-squares on F2

R1 = 0.0570

R1 = 0.0792

R1 = 0.0922

Index ranges Reflections collected Independent reflections Data Completeness Refinement method Final R indices [I>2σ (I)] R indices (all data) 2

Goodness-of-fit on F

wR2 = 0.1274

wR2 = 0.2041

wR2 = 0.2562

R1 = 0.0623

R1= 0.0885

R1= 0.0983

wR2 = 0.1307

wR2 = 0.2377

wR2 = 0.2590

1.137

1.992

1.722

20

Table 2 Distances (Å) and angles (π) of C–H···π interactions for 2a and 2b

2a 2b

H···π(Å) 2.70

C–H···π(o) 178

C–Cg(Å) 3.66

C15–H15···Cgb

2.73

135

3.48

C16–H16···Cgc

2.71

137

3.48

C15–H12c···Cg

a

a

Cg : centre of gravity of ring [C18-C19-C20-C21-C22-C23]. Cgb: centre of gravity of ring [C21-C22-C23-C24-C25]. Cgc: centre of gravity of ring [C1-C2-C3-C4-C5].

Table 3 Cyclic voltammograms data of ferrocene, 1b and 2b a ferrocene

1b

2b

b

0.510

0.476

0.468

c

0.440

0.408

0.402

0.070

0.068

0.066

ipa/ipce

1.03

0.97

0.98

Ep /Vf

0.475

0.442

0.435

Epa(free)/V Epc(free)/V ∆E/Vd

a

All potential data were determined in CH3CN containing 0.1 mol·L-1 (n-Bu4N)PF6 as the supporting electrolyte. A three-electrode cell was used, consisting of a glassy carbon working electrode, a platinum wire auxiliary electrode and Ag/AgCl (3.0 mol/L KCl) reference electrode. b

Epa: anodic peak potential. c Epc: cathodic peak potential. d ∆E = Epa−Epc.

e

ipa and ipc represent the anodic and cathodic peak currents.

f

Ep : redox peak potential, Ep ＝(Epa＋Epc)/2.

Table 4 Thermal decomposition parameters of AP and AP with additivesa. Tp/˚Cb

Ea/(kJ·mol-1)c

lg[A d/s-1]

Spe

AP

462

104

4.98

20.9

AP+1a

435

100

5.01

20.0

AP+1b

385

92.2

4.94

18.7

AP+2a

374

91.7

5.03

18.2

AP+2b

290

79.8

5.07

15.7

a

the additives are 3wt %, heating rate is 10˚C/ min, errors are estimated to be < 10%.

b

Tp: decomposition temperature. c Ea: apparent activation energy. d A: pre-exponential factor.

e

Sp: compensation parameter, Sp = Ea/lgA.

21

Graphical abstract

Research Highlights: 

Two novel pyrrolide-imine chelate complexes 2a and 2b are synthesized in moderate yields.



Molecular structure of 1b, 2a and 2b are characterized by single-crystal X-ray diffraction analysis.



The structure-property relationship of 1-2a and 1-2b are investigated based on UV-Vis spectroscopy and electrochemical measurements.



2b has an excellent catalytic effect on the decomposition of ammonium perchlorate.

22