Influence iron-iron distance on the thermal decomposition of ammonium perchlorate. New catalysts for the highly efficient combustion of solid rocket propellant

Article abstract of DOI:10.1016/j.jorganchem.2019.121020

Five homobimetallic compounds derived from indacene and p-phenylene ligands with different Fe–Fe distances have been synthesized. The catalytic effect on the thermal decomposition of Ammonium Perchlorate (AP) of each complex is analyzed, establishing comp

Full text of DOI:10.1016/j.jorganchem.2019.121020

Journal Pre-proof
Influence iron-iron distance on the thermal decomposition of ammonium perchlorate.
New catalysts for the highly efficient combustion of solid rocket propellant
Juan Luis Arroyo, Paula Povea, Rodrigo Faúndez, María Belén Camarada,
Christopher Cerda-Cavieres, Gabriel Abarca, Juan Manuel Manriquez, Cesar
Morales-Verdejo
PII:
S0022-328X(19)30463-2
DOI:
https://doi.org/10.1016/j.jorganchem.2019.121020
JOM 121020
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 10 September 2019
Revised Date: 5 November 2019
Accepted Date: 6 November 2019
Please cite this article as: J.L. Arroyo, P. Povea, R. Faúndez, Marí.Belé. Camarada, C. Cerda-
Cavieres, G. Abarca, J.M. Manriquez, C. Morales-Verdejo, Influence iron-iron distance on the
thermal decomposition of ammonium perchlorate. New catalysts for the highly efficient combustion
of solid rocket propellant, Journal of Organometallic Chemistry (2019), doi: https://doi.org/10.1016/
j.jorganchem.2019.121020.
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Influence iron-iron distance on the thermal decomposition of ammonium
perchlorate. New catalysts for the highly efficient combustion of solid rocket
propellant
Juan Luis Arroyo,1 Paula Povea,1 Rodrigo Faúndez,2 María Belén Camarada,3
Christopher Cerda-Cavieres,3 Gabriel Abarca,3 Juan Manuel Manriquez,2 Cesar
Morales-Verdejo 3,*
1
Laboratorio de Materiales Energéticos, Instituto de Investigaciones y Control del
10 Ejército de Chile (IDIC), Av. Pedro Montt 2136, Santiago, Chile.
11 2 Departamento de Química Inorgánica, Facultad de Química, Pontificia Universidad
12 Católica de Chile, Vicuña Mackenna 4860, Macul, Santiago, Chile.
3
13
Centro Nanotecnología Aplicada (CNAP), Facultad de Ciencias, Universidad Mayor,
14 Camino la Pirámide 5750, Huechuraba, Santiago, Chile.
15
16 * Corresponding author: cesar.morales@umayor.cl (Cesar Morales-Verdejo)
17
18 Keywords: bimetallic compounds, ammonium perchlorate, burning rate catalyst,
19 propellant
20
21 Abstract
22 Five homobimetallic compounds derived from indacene and p-phenylene ligands with
23 different Fe-Fe distances have been synthesized. The catalytic effect on the thermal
24 decomposition of Ammonium Perchlorate (AP) of each complex is analyzed,
25 establishing comparisons with catocene (Cat) and ferrocene (Fc). Cyclic voltammetry
26 showed quasi-reversible redox potential with higher electron-transfer ability than
27 ferrocene and catocene. An anti-migration study of these compounds is carried out,
28 compared with those of Cat and Fc. Compound 5, bearing a higher molecular weight,
29 displayed a better anti-migration performance than catocene and ferrocene. In
30 addition, the synthesized compounds have shown a shift on the peak temperature to

31 lower values as well as an increase in the released heat values during thermal
32 decomposition of AP.
33
34 1. Introduction
35 Rockets constitute a very important system in all kinds of missiles, used to deliver
36 warheads to targets as well as launching satellites into orbit. There are highly effective
37 burn-rate catalysts for AP containing composite solid propellant and are excellent
38 candidates for application in rocket engines with high thrust and acceleration power
39 in comparison with Fe2O3 [1]. Ferrocene derivatives have been reported to impart
40 high rates to HTPB-based composite propellants. Within these series, the bi-ferrocene
41 complexes have better performances than the monometallic analogues, thereby
42 improving the performance of the composite solid propellant [2–7].
43 Dual-core Fc derivatives as well as dendrimer substances with iron atoms near a
44 ferrocene moiety have suggested as a key step the combustion of these ferrocenyl
45 derivatives initially generates Fe2O3 nanoparticles[4,7–9]. These nanoparticles have a
46 larger surface area than simple hematite particles and display excellent catalytic
47 activity on the thermal decomposition of AP [8]. Besides, as mentioned in the findings
48 of X. Liu and coworkers, a decreased tendency of the catalytic effect on the thermal
49 decomposition of AP becomes evident as the non-conjugated alkylene spacers grow in
50 size in the ionic binuclear ferrocene compounds [2].
51 Even though ferrocene derivatives BR catalysts have extraordinary effects in the
52 enhancement of the burning rates of composite solid propellant, some drawbacks
53 must be thoroughly addressed, such as an undesired sublimation during curing and
54 processing, as well as migrating to the surface on the propellants after extended
55 periods of time in storage [3,10–13]. These above-mentioned deficiencies can produce
56 long-term undesirable results such as storage issues [14–16], lifespan reduction,
57 changes on the initial burning parameters [3] as well as damage to the propellant
58 system [17]. To study the previously mentioned issues even further, we have
59 synthesized and tested bimetallic compounds derived from ferrocene. Our research
60 group has designed the synthetic route to obtain a complete series of binuclear
61 organometallic complexes derived from different bridging ligands such as s-indacene,

62 pentalene, p-phenylene and naphthalene [6,18–24]. In order to understand the
63 electronic properties allowing the intermetallic communication in the mixed-valence
64 organometallic derivatives, some of these complexes have previously been reported
65 [10, 12], but were not studied as burning rate catalysts. These compounds have very
66 high molecular weights, an advantageous factor in overcoming migration and
67 sublimation issues.
68
The current work describes the catalytic activity of the neutral homobimetallic
69 complexes derived from s- and as-Indacene and p-Phenylene in comparison with
70 ferrocene (Fc) and 2,2-bis(ethylferrocenyl)propane (Catocene: Cat). Since the semi-
71 rigid bridged ligands, p-Phenylene, p-Biphenylene, and p-Terphenylene and rigid
72 ligands s- and as-indacene have high molecular weights, their heavier respective
73 complexes contribute by lowering migration and sublimation losses. Additionally,
74 complexes of the respective bridging ligands have different iron-iron metal distance
75 values as well as different redox potential values. We therefore attempt to study the
76 role of the redox behavior as well as the Fe-Fe distance on the thermal decomposition
77 of AP.
78
79 2. Experimental Section
80 2.1. General Information of the catalysts
81 All manipulations were carried out under a pure nitrogen atmosphere by using a
82 vacuum atmosphere dry box equipped with a Model HE 493 Dri-Train purifier or with
83 the use of a vacuum line by using standard Schlenk tube techniques.
84 Reagent grade solvents were distilled under an atmosphere of nitrogen from sodium
85 benzophenone for toluene, hexane and thf (previously distilled from AlLiH4) and from
86 P2O5 for acetonitrile and dichloromethane.
87 The synthesis of the following compounds has been reported previously: Fe(acac)2;
88 acac: acetylacetonate [25]; Cp*Fe(acac); Cp*: pentamethylcyclopentadienyl [25]; s-Ic’:
89 4,8-dimethyl-2,6-diethyl-symmetric-indacene [26]; as-Ic’: 2,7-diethyl-asymmetric-
90 indacene [27]; p-Ph’: p-Bis(2,3,4,5-tetramethylcyclopentadienyl)benzene [28]; p-
91 BiPh’: p-Bis(2,3,4,5-tetramethylcyclopentadienyl)biphenylene [28]
[Cp*Fe-s-Ic’-
92 FeCp*] (1) [5]; [Cp*Fe-p-Ph’-FeCp*] (3) and [Cp*Fe-p-BiPh’-FeCp*] (4) [28]

93 4,4’’−Dibromo-1,1’:4’,1’’-terphenyl,
2,3,4,5-tetramethylcyclopent-2-en-1-one,
94 suspension of sodium pentamethylcyclopentadienyl (NaC5Me5) in THF 0.5 M and
95 ferrocene (purchased from Aldrich).
96 Elemental analyses (C and H) were made with a Fisons EA 1108 microanalyzer. 1H and
97 13C NMR spectra were recorded on Bruker AC-400, Bruker AC-200P, and Bruker AC 80
98 Spectrometers. Chemical shifts were reported in ppm relative to residual solvents and
99 were assigned using 2D NMR tools. All peaks reported were singlets, unless otherwise
100 specified.
101
102 2.2 Synthesis Organic and Organometallic Compounds
103 2.2.1 p-Bis(2,3,4,5-tetramethylcyclopentadienyl)terphenylene). p-TerPh’.
104 To 2.5 g (6.4 mmol) of 4,4’’−Dibromo-1,1’:4’,1’’-terphenyl dissolved in 50 mL of
105 anhydrous diethyl ether was added dropwise 3.0 mL (6.1 mmol) of 2.0 M n-
106 butyllithium in hexanes. After the solution was stirred for 20 min, a solution of 2,3,4,5-
107 tetramethylcyclopent-2-en-1-one (0.86 mL, 6.1 mmol) in 5.0 mL of diethyl ether was
108 added. The mixture was then refluxed for 1 +h. The reaction was cooled to room
109 temperature, and 3.0 mL (6.1 mmol) of 2.0 M n-butyllithium was added. The mixture
110 was stirred for 1 h. A solution of 0.86 mL (6.1 mmol) of 2,3,4,5-tetramethylcyclopent-
111 2-en-1-one dissolved in 5.0 mL of ether was then added dropwise.
112 Stirring was continued for 1 h. Then, the ethereal solution was quenched with
113 aqueous saturated ammonium chloride, the layers were separated, and the aqueous
114 phase was extracted with 30 mL of ether. The solution was concentrated to 30 mL,
115 and 0.5 g of p-toluenesulfonic acid dihydrate was added. After 2 h of stirring a
116 precipitate formed. The solid was filtered, washed with water and then with
117 methanol, and finally dried. The crude product (0.8 g) was recrystallized from 50 mL
118 of hot chloroform to afford 0.65 g (21.0% yield) of yellow crystals.
119 Anal. Calcd for C36H38: C, 91.86; H, 8.14. Found: C, 91.83; H, 8.17.
120 1H NMR (400 MHz, C6D6) δ 1.04 (d, 6H), 1.79 (S, 6H), 1.87 (s, 6H), 2.02 (S, 6H), 3.14 (q,
121 2H), 7.52 (s, 4H), 7.57 (d, 4H), 7.65 (d, 4H).

122 13C NMR (101 MHz, C6D6) δ 142.27, 140.77, 140.72, 140.33, 140.03, 139.80, 137.67,
123 137.32, 136.24, 134.92, 131.66, 128.80, 128.57, 126.86, 49.87, 14.59, 12.45, 11.51,
124 10.68.
125
126 2.2.2 [Cp*Fe-as-Ic’-FeCp*] (2)
127 A suspension of NaC5Me5 in THF 0.5 M (3.8 mL, 1.9 mmol) was added slowly to a THF
128 solution (40 mL) of Fe(acac)2, (0.48 g, 1.9 mmol) cooled to -78 ºC. The reaction
129 mixture was warmed to room temperature. After 1 h the deep red solution was cooled
130 to -78 ºC and a suspension of the dilithium salt of ligand 2,7-diethyl-as-indacene
131 (prepared from 2,7-diethyl-as-indacene (0.20 g, 0.95 mmol) and 2.0 M n-butyllithium
132 (0.95 mL, 1.90 mmol) in 20 mL of THF) was added. The mixture was warmed to room
133 temperature and stirred for 1 h. Then, the solvent was removed, and the product was
134 dissolved with toluene, filtered to remove the insoluble Na(acac) and Li(acac), and
135 washed three times. After removal of the solvent via vacuum, dark-red powder
136 unstable to air was obtained. Yield: 35.7 % (0.20 g).
137 Anal. Calcd for C36H46Fe2: C, 73.23; H, 7.85. Found: C, 73.28; H, 7.90.
138 1H NMR (400 MHz, C6D6) δ 1.15 (t, 6H), 1.76 (s, 30H), 2.26 (q, 4H), 3.21 (dd, 4H), 6.51
139 (s, 2H).
140 13C NMR (101 MHz, C6D6) δ 140.94, 132.49, 121.80, 77.74, 59.06, 29.03, 18.57, 9.94.
141
142 2.2.3 [Cp*Fe-p-TerPh’-FeCp*] (5)
143 A suspension of NaC5Me5 in THF 0.5 M (2.56 mL, 1.28 mmol) was added slowly to a
144 THF solution (40 mL) of Fe(acac)2, (0.32 g, 1.28 mmol) cooled to -78 ºC. The reaction
145 mixture was warmed to room temperature. After 1 h the deep red solution was cooled
146 to -78 ºC and a suspension of the dilithium salt of ligand p-TerPh’ (prepared from p-
147 TerPh’ (0.30 g, 0.64 mmol) and 2.0 M n-butyllithium (0.64 mL, 1.28 mmol) in 30 mL of
148 THF) was added. The mixture was warmed to room temperature and stirred for 2 h.
149 Then, the solvent was removed, and the product was dissolved with toluene, filtered
150 to remove the insoluble Na(acac) and Li(acac), and washed three times. After removal
151 of the solvent via vacuum, dark-red powder unstable to air was obtained. Yield: 18.4
152 % (0.10 g).

153 Anal. Calcd for C57H70Fe2: C, 78.98; H, 8.14. Found: C, 78.86; H, 8.22.
154 1H NMR (400 MHz, CDCl3) δ 1.53 (s, 30H), 1.62 (s, 24H), 7.37 (s, 4H), 7.41 (d, 4H), 7.49
155 (d, 4H).
156 13C NMR (101 MHz, C6D6) δ 142.43, 140.88, 140.48, 140.20, 137.84, 137.47, 136.40,
157 135.08, 128.73, 127.91, 127.03, 125.31, 59.87, 29.83, 19.38, 14.75, 12.61, 12.53, 11.67,
158 10.76.
159
160 2.3 Thermal analysis
161 DSC analysis was performed on a DSC 822e METTLER TOLEDO instrument
162 respectively at a heating rate of 5 ºC·min-1 under nitrogen in the range of 80–500 ºC.
163 To investigate the catalytic performance of the compounds derived from indacenes
164 and p-phenylenes on the thermal decomposition of AP, specific amounts of the
165 complexes and AP were mixed and ground in a certain weight ratio for DSC analysis.
166
167 2.4 Electrochemical measurements
168 A Voltalab (PGZ100) potentiostat electrochemical workstation was used to measure
169 the electrochemical properties of the samples at ambient temperature (20 ºC). A
170 conventional three-compartment cell was employed throughout the work. A platinum
171 disk electrode (2 mm diameter) was used as a working electrode. The counter
172 electrode was a large area coiled Pt wire, separated from the electrolytic solution by a
173 sintered glass frit. Before each experiment, the working electrode was polished to a
174 mirror finish with an aqueous alumina slurry (particle size 0.3 and 0.05 micron) on
175 micro-cloth pads, rinsed thoroughly with water and dried. All potentials are referred
176 to a Ag/AgCl (KCl,1 M) electrode. Each working solution was purged with high purity
177 argon for 15 min prior to each experiment, and a lower flow was maintained over the
178 solution during the measurements. Tetrabutylammonium tetrafluoroborate
179 (Bu4N•BF4) was dried at 110 ºC and kept into a dryer until used as supporting
180 electrolyte. Dichloromethane was anhydrated with phosphorus pentoxide under N2
181 atmosphere reflux. Cyclic voltammetry measurements were carried out in anhydrous
182 dichloromethane with a concentration of 0.01 M Bu4N•BF4, 5 mM analyte, at a scan
183 rate of 100 mV•s−1.

184
185 2.5 Computational details
186 Geometries were fully optimized at the density functional theory (DFT) level, as
187 implemented in Gaussian16 software [29]. The functional and basis set were selected
188 according to a previous study on ferrocenium/ferrocene system, reported by Flores-
189 Leonar and coworkers [30], where the dual corrected ωB97X-D functional [31]
190 proved to be consistent throughout the analysis of geometry, in combination with a
191 basis set SDD [32] and cc-pVTZ [33], for Fe and Cp respectively. The molecular
192 structures were fully optimized in gaseous phase and vibrational frequencies
193 calculations were performed at the same level of theory as the geometry
194 optimizations to confirm that the stationary points were minima at the potential
195 energy surface. A tight SCF convergence criteria (10−8 a.u.) was used in all
196 calculations.
197
198
199 3. Results and Discussion
200 Preparation of the complexes (2) and (5) proceeds straightforwardly by previously
201 reported route [28,34]. Details on the thorough characterization by means NMR
202 spectroscopy is in the supporting information file. The reaction of the dilithium ligand
203 salts with (Cp*)Fe(acac) affords the title compounds, which is easily separated from
204 Na(acac) and Li(acac) byproducts. The unstable air black-red powder compounds
205 were obtained, analytically pure, in approximately 20 % yield.
206 Since ferrocene-type complexes are known to be effective catalysts on the thermal
207 decomposition of AP, the activity of the binuclear compounds (1), (2), (3), (4) and (5)
208 as burning rate catalysts in comparison with ferrocene (Fc) and catocene (Cat) was
209 carried out. Figure 1 shows the catalysts used in this work.

210
Fe
Fe
Fe
Fe
Fe
Fe
Fe
(2)
(1)
(Cat)
(Fc)
Fe
Fe
Fe
Fe
Fe
Fe
(5)
(3)
(4)
211
212
Figure 1: Ferrocene (Fc), catocene (Cat), bimetallic (1, 2, 3, 4 and 5) catalysts
213
214
compared in this work.
215 3.1 Redox properties
216 The redox properties of ferrocene, catocene, and all the previously mentioned
217 bimetallic compounds were analyzed by means of cyclic voltammetry.
218 Cyclic voltammetry profiles were recorded between -1.5 and 1.5 V at a sweep rate of
219 100 mV·s-1. The first and second oxidation potential for each compound were
220 obtained from the first cycle in anhydrous dichloromethane, as listed in Table 1.
221 Neutral compounds (1), (2), and (3) displayed two oxidation peaks associated to the
222 oxidation of each ferrocene center in the molecule. The first and second oxidation
223 potentials are correlating to the metal pointed in comparison with the mono- and
224 binuclear species, as already reported [19,28,35,36]. Compounds (4) and (5) only
225 presented one oxidation peak corresponds to the simultaneous transfer of two
226 electrons per dimer, as observed by E. Bunel and coworkers [28] (Fig. 2).
227
228
229

230
231
Figure 2: Cyclic Voltammograms of complexes (2), (4) and (5) in acetonitrile/0.1 M n-
232
233
234
Bu4NBF4. Scan rate 100 mV/s

235 Table 1: Oxidation potentials for ferrocene, catocene and neutral complexes derived
236 from indacenes and
237
p-Phenylenes (mV vs Ag/AgCl)
Complexes
Eox1(0/1+)
Eox2(1+/2+)
∆Eox
-----
Ferrocene (Fc)
Catocene (Cat)
-----
+2 +3
500 Fe
/Fe
152
973
640
626
-
+2 +2 +2 +3
346 Fe
+2 +3 +3 +3
498 Fe
483 Fe
770 Fe
459 Fe
/Fe /Fe /Fe
/Fe /Fe /Fe
[Cp*Fe-s-Ic’-FeCp*][5] (1)
+2 +2 +2 +3
+2 +3 +3 +3
/Fe /Fe /Fe
-490 Fe
/Fe /Fe /Fe
[Cp*Fe-as-Ic’-FeCp*] (2)
+2 +2 +2 +3
/Fe /Fe /Fe
+2 +3 +3 +3
/Fe /Fe /Fe
130 Fe
[Cp*Fe-
[Cp*Fe-
[Cp*Fe-
p-Ph’-FeCp*][6] (3)
p-BiPh’-FeCp*] (4)
p-TerPh’-FeCp*] (5)
+2 +2 +2 +3
+2 +3 +3 +3
/Fe /Fe /Fe
-167 Fe
/Fe /Fe /Fe
-
-
+2 +2 +2 +3
/Fe /Fe /Fe
91 Fe
-
+2 +2 +2 +3
276 Fe
/Fe /Fe /Fe
238
239
240
Note: Eox1 (0/1+) and Eox2 (1+/2+) are the first and second oxidation potentials (mV), ∆Eox is the subtraction between
the first and second oxidation potentials in the complex Eox1 (0/1+) and Eox2 (1+/2+). (*) According to reference [2,37]
the potential oxidation was reported with Ag/AgCl as reference electrode.
241
242 The first oxidation of compounds (1) and (3) appears at more cathodic potentials
243 compared to compounds (2), (4) and (5). This evidences that (1) and (3) require a
244 lower amount of energy than (2), (3) and (5), to form the respective cation.
245 In all cases, the first oxidation potential exhibited lower values than ferrocene and
246 catocene.
247 The thermodynamic stability of the mixed-valence species related to the electronic
248 interactions is associated with the term ΔEox [38]. As previously reported[5], the
249 difference of potential between the two oxidation peaks is ΔEox = 973 mV for
250 compound (1), giving a high comproportionation constant (Kc = 2.81x1016) [39],
251 suggesting a high stability for the intervalence system (1+), in comparison with the
252 compounds (2) (Kc = 6.6x1010) and (3) (Kc = 3.8x1010), corroborating the above
253 described. In the case of the catocene (Cat), as we described [5,6] the difference of

254 potential between the two oxidation peaks is small (ΔEox = 151 mV) and low
255 comproportionation constant (Kc = 3.57x102), which is indicative that the metals are
256 noninteracting either because the ligand does not provide an electronic coupling
257 pathway.
258 The ΔEox value of zero for the compounds (4) and (5), indicates a reduced
259 delocalization due to the increased distance between the iron centers.
260 Additionally, the redox properties may help us in better understanding the electron-
261 transfer mechanism of the compounds and its effect on the catalytic performance on
262 the thermal decomposition of AP.
263
264 3.2. Computational characterization
265 Given the fact that both moieties may lie on the same side of the bridging ligand plane
266
(syn) or on different sides (anti), two different molecular configurations are possible.
267 The geometry optimization of anti-configuration of the neutral bimetallic complexes
268 has been performed considering that the metal atoms coordinate the C5 rings, as
269 usually observed experimentally [36,40,41]. As expected and for sterically related
270 effects [42], the anti-configuration was always found to be slightly more stable than
271 the syn isomer; therefore, we have focused our theoretical calculations on the anti
-
272 configuration (Figure 3).
273 All complexes in the neutral state were optimized in a vacuum with the ωB97X-D
274 functional in combination with a basis set SDD[43] and cc-pVTZ[44] for Fe and Cp,
275 respectively. Frequency analysis in gas phase was performed on each optimized
276 structure to verify the existence of a minimum in the potential energy surface.
277 Performing single point calculations on the gas phase optimized structures. Figure 2
278 shows the optimized structures in gas phase, in order to determine the distance
279 between the metallic iron centers.
280
281

5.68 Å
(Cat)
282
5.73 Å
(1)
283
5.08 Å
(2)
284
285
286

8.74 Å
(3)
287
13.11 Å
(4)
288
289
17.27 Å
(5)
290
291
292
293
Figure 3: Optimized structures at ωB97X-D/SDD/cc-pVTZ level in gas phase.
294
295
296
Table 2 (vide infra) shows that the compound (2) has the shortest distance between
the metal centers iron (5.08 Å), even less than catocene (5.68 Å) and its analog
compound s-indacene derivate (1) (5.73 Å). Regarding the compound (3) (8.74 Å) has

297 a greater distance than compounds (1), (2) and (Cat); and as expected a shorter
298 distance to the compounds (4) and (5), 13.11 Å and 17.27 Å, respectively.
299 Furthermore, these iron-iron distances would help us in better understanding the reaction
300 mechanism of the compounds and of the catalytic performance on the thermal
301 decomposition of AP.
302
303 3.3 Anti-migration studies
304 Solid propellant samples were prepared in order to explore anti-migration behavior of
305 the bimetallic compounds. Detail of the samples preparation is shown in Table S1
306 supplementary information. The samples were first filled in glass tubes and put for
307 the curing process at 65 ºC for seven days. Then blank simulative solid propellant was
308 filled in the end of the tubes. Anti-migration studies were carried out in an oven at 65
309 ºC for 21 days.
310 Figure 5 shows the results of anti-migration studies. Ferrocene showed the maximum
311 migration with a value of 4.0 cm in 21 days. Samples of catocene and bimetallic
312 compounds showed anti-migration behavior comparable with ferrocene (Figure 4,
313 Figure S2.1 and S2.3). On long-term storage, ferrocene inevitably reached the ends of
314 the tube (Supplementary Material Figure S2.2), while indacenes and p-phenylenes
315 derivatives moved to a lesser extent. The migration distances of these samples are
316 plotted in Figure 5. It can be seen that compound (5) has the best anti-migration
317 property among the samples, due to its high molecular weight (866 g/mol). In general,
318 the neutral compounds present anti-migration behavior mostly because of its higher
319 molecular weight (590 – 866 g/mol) than ferrocene (186 g/mol) and catocene (484
320 g/mol). Higher molecular weight leads to stronger van der Waals forces. However,
321 this force is not strong enough to prevent migration on long-time storage for
322 compounds (1), (2), (3), and (4).

323
324
Figure 4: Anti-migration studies photos of compound (5) on days a) 7, b) 14 and c) 21
325
at 65 ºC.
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1
2
3
4
5
Catocene Ferrocene
COMPOUNDS
7 d
14 d 21 d
326
327
Figure 5: Evaluation of the migration tendency of (1), (2), (3), (4) and (5) along
migration distance and for different aging time (the bars in each parallel row from left
to right represent a compound was aged for 7, 14 and 21 days, respectively at 65 °C).
328
329
330
331
332
333

334
335
336
337
338
339
340
341
342
3.4 Catalytic Effects on thermal decomposition of ammonium perchlorate
The catalytic effects of the bimetallic compounds on the thermal decomposition of AP
were investigated by DSC measurements with a heating rate of 5 ºC·min-1 in N2
atmosphere in the range of 140–450 ºC. Figure 6 (Fig S3.1-S3.5 supporting
information) shows the DSC curves of both AP and the mixture of AP with the title
compound. In general, the catalytic activity of a BR catalyst in solid propellant can be
assessed by studying its effect on the thermal degradation of AP by DSC instrument.
The weight percentage of ferrocene-based BR catalysts used in AP were 2, 4 and 6
wt%.
343
344
Figure 6: DSC curves of AP with different percentage (wt%) of compound (4).
345
346
347
348
349
The fact that the endothermic process of AP peaked at 249 ºC is due to the crystal
transformation from orthorhombic to cubic phase, displaying almost no shift (max. ca.
4 ºC) and exhibited a similar shape with 2–6 wt% of (1), (2), (3), (4) and (5) as
additives. Both exothermic stages are evidenced firstly by the low-temperature

350 decomposition (LTD) (peak temperature at 320 ºC), and secondly by the high-
351 temperature decomposition (HTD) (peak temperature at 418 ºC) of AP. Each of these
352 phases is respectively attributed to a partial decomposition of AP to form some
353 intermediate product, to a later and eventual complete decomposition to volatile
354 products. The exothermic processes are significantly affected by the addition of the
355 catalysts derived from indacenes and p-phenylenes, where the wide decomposition
356 temperature range of AP was considerably narrowed and the released heat of AP
357 increased. DSC curves of pure AP and AP with 2 wt% of the bimetallic catalysts are
358 almost the same (Figure 6 and S3.1-S3.5 supplementary material).
359 The exothermic process suggests that the high temperature decomposition (HTD) of
360 AP with the bimetallic compounds occurs predominantly via the thermal
361 decomposition. This implies that the additives have a greater catalytic effect on the
362 HTD of AP than on the earlier stage. However, when wt% of bimetallic complexes was
363 further increased, from 2% to 6%, the catalytic effect did not improve, with the
364 exception of compound (1), obtaining the best catalytic effects at 4 w%.
365 Therefore, 4 wt% was chosen as the optimum amount of each compound for the
366 evaluation of their catalytic effects. After adding 4 wt% of the compounds (1), (2), (3),
367 (4) and (5), the highest thermal decomposition temperatures are 321, 320, 319, 330
368 and 340 ºC respectively, decreased by 97, 98, 99, 88 and 78 ºC compared with pure
369 AP, which indicates the additives have an evident catalytic effect on the thermal
370 decomposition of AP.
371 It is noted that all the DSC curves displayed a similar shape with one peak, except that
372 of compound (4) (Figure 6), suggesting that an identical decomposition mechanism of
373 the mixture systems should be expected.
374 When comparing the monometallic compound ferrocene with all the bimetallic
375 complexes the heat released is greater. In fact, the heat released of compound (4)
376 (2122 J·g-1) is almost similar to the catocene (2472 J·g-1) (see Table 2).
377
378
379
380

381 3.5 Possible burning rate catalytic mechanism of ferrocene-based compounds
382 on the thermal decomposition of AP
383 Four catalytic mechanisms have been proposed: electron transfer mechanism, proton
384 transfer mechanism, forming transition materials mechanism and acid-base
385 interaction mechanism. [45–47]
386 In order to clarify the possible type of reaction mechanism and considering that
387 combustion is a drastic oxidation-reduction process, our research group has
388 suggested a relationship between the oxidation potential and the catalytic effect on
389 the AP thermal decomposition, as summarized in Table 2. In each case, compounds Fc,
390 Cat and the bimetallic substances, behave whether by lowering the HTD temperature
391 value and/or a higher heat release.
392 As it is accepted by other authors[48], the low temperature decomposition (LTD) is
393 governed by the electron transfer process. This decomposition is the most important
394 factor governing the decomposition of NH4ClO4, followed by exothermic peak (HTD)
395 associated with the further decomposition of HClO4, which gets converted into small
396 molecules, such as H2O, HCl, N2 and O2. In this context, bimetallic compounds play a
397 vital role in accelerating the electron transfer process, initiating the decomposition
398 process. Therefore, there is a clear relationship between oxidation potential and
399 catalytic effect. Regarding Fc, Cat and bimetallic compounds, it is possible to
400 appreciate a general tendency associated to oxidation potentials versus shifting of the
401 HTD peak of AP to lower temperatures. All these compounds present low oxidation
402 potential in comparison with AP, which could be the key for the candidates as BR
403 catalysts. These results suggest that the addition of bimetallic catalyst leads probably
404 to the decomposition of AP through electron transfer mechanism. However, in
405 relation to the bimetallic compounds (1) and (2), it is not possible to see a clear
406 tendency with the oxidation potential versus the shift of the HTD peak of AP to the
407 left, because, probably, another important factor is affecting the catalytic effect (vide
408 infra). But if we look at compounds (3), (4) and (5), it is possible to see a tendency
409 with oxidation potential versus the shift of the HTD peak of AP. Furthermore, it is not
410 possible to clarify a tendency with the heat released for each compound, indeed the
411 heat released by Cat is still higher than the others catalysts reported here.

412 Table 2: Summary table, relationship between the oxidation potential and the catalytic
413 effect (4 wt%) on AP thermal decomposition of the ferrocene derivatives
Compounds
Eox1 vs
Ag/AgCl (mV)
HTD of AP
(ºC)
Heat released
(J·g-1)
Distance
(Å)
NH4ClO4 (AP)
825
500
-490
130
-167
91
418
360
321
320
319
330
340
346
943
-
(Fc)
(1)
1313
1594
1870
1560
2122
1212
2472
-
5.73
5.08
8.74
13.11
17.27
5.68
(2)
(3)
(4)
(5)
276
346
(Cat)
414
415 It can be also noted that the final decomposition temperatures of AP with complex (3)
416 as additive is lower than the other complexes, implying that the effects of the
417 bimetallic compound on the thermal degradation of AP are greater than catocene.
418
419 Regarding transition materials mechanism, it has been widely accepted that
420 ferrocenyl-bimetallic derivatives tend to form Fe2O3 particles [4,7-9]. The systems
421 having a higher content of iron per molecule, greater number of electron donor
422 groups and the Fe atoms closer to one another, could better promote the formation of
423 nanoscale hematite particles [9]. If we compare the iron-iron distances in the family of
424 the rigid bridging ligands, s- and as-indacene, we could see a tendency in their Fe-Fe
425 distance and the catalytic effect on the thermal decomposition of AP, (2) ≥ (1). In the
426 same way, the complexes derived from semi rigid bridging ligands, p-phenylene, p-
427 Biphenylene and p-Terphenylene display a marked catalytic tendency of the effect on
428 the thermal degradation of AP, (3) > (4) > (5).
429 This could confirm the mechanism proposed about Ferrocenyl-bimetallic derivatives
430 during the burn-up phase, responsible for generating high surface area Fe2O3
431 nanoparticles, more effective in comparison with common powdered Fe2O3. Zain-ul et

432
433
al. and Gao et al. [8,49] gave a more visual description of the burning process. This
promotion stems from the unique structure of the bimetallic complexes (Figure 7).
434
435
436
437
438
439
440
441
442
443
444
Figure 7: Possible formation mechanism of nanoscale Fe2O3 from the bimetallic
molecule.
4. Conclusion
All compounds showed much slower migration rates than that of catocene, mostly due
to their molecular weights. In addition, the bimetallic complexes showed
quasireversible redox potential values displaying lower energy charge-transfer in a
similar situation to that of ferrocene and catocene, which may come in handy to
further study how redox processes may affect the decomposition of AP.

445 The catalytic results by means of DSC techniques confirmed that the bimetallic
446 complexes derived from rigid bridging ligands, s- and as-indacenes; and semi rigid
447 bridging ligands, p-phenylene, p-Biphenylene and p-Terphenylene, have high catalytic
448 activity for the thermal decomposition of ammonium perchlorate. Catalytically,
449 compound (3) is the most active despite the fact its released heat is somewhat lower.
450 From the results obtained, we can state that for the fine-tuning of catalyst design on
451 the thermal decomposition of AP, it is indeed necessary to consider Fe-Fe distance
452 factors and the oxidation potential values, perhaps opening fields on further research
453 for newer molecules based on intrametallic distance values. Despite the effect that BR
454 catalysts cannot be uniquely analyzed upon the influence on the thermal
455 decomposition of AP, the results confirm the bimetallic compounds could have
456 promising applications as burning rate catalysts in composite solid propellants.
457
458 Acknowledgements
459 We gratefully acknowledge the financial support from FONDECYT grants 1161297,
460 1180023 and Laboratory of Energy Materials from Institute of Research and Control
461 (IDIC) of the Chilean Army. Powered@NLHPC: This research was partially supported
462 by the supercomputing infrastructure of the NLHPC (ECM-02)
463
464
465 Supplementary material
466 NMR characterization, Migration studies and DSC curves to different wt% for all
467 compounds related to this article can be found at supplementary data.
468
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HIGHLIGHTS
Influence iron-iron distance on the thermal decomposition of ammonium
perchlorate. New catalysts for the highly efficient combustion of solid rocket
propellant
Juan Luis Arroyo,1 Paula Povea,1 Rodrigo Faúndez,2 María Belén Camarada,3
Christopher Cerda-Cavieres,3 Gabriel Abarca,3 Juan Manuel Manriquez,2 Cesar
Morales-Verdejo 3,*
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•
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Complexes show catalytic activity on thermal decomposition of AP.
These compounds show a shift on the peak temperature of AP to left.
These compounds increase released heats during thermal decomposition of
AP.
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•
Catalytic activities are examined by DSC techniques.
Theoretical calculations are carried out to gain further understanding.

Declaration of interests
☐ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: