Burning Rate Performance Study of Ammonium Perchlorate Catalyzed by Heteroleptic Copper(I) Complexes with Pyrazino[2,3-f][1,10]phenanthroline-Based Ligands

Article abstract of DOI:10.1002/ejic.202001092

This contribution describes the catalytic effect of heteroleptic Cu(I) complexes with pyrazino[2,3-f][1,10]phenanthroline-based ligands (C1-7) on the thermal decomposition of ammonium perchlorate (AP). The complexes C2 and C4-7 were synthesized and characterized by NMR, HRMS and, in the case of C7, by X-ray diffraction. The burning rate performance of C1-7 on thermal decomposition of AP was studied by differential scanning calorimetry technique. The effect of the counter ion type, the number of metal centers and ligand substitution was evaluated. These AP+complex mixtures exerted a shift to lower ignition temperatures and an increase in the released heats during thermal decomposition compared with pure AP. The compound C4 showed the higher catalytic effect among the complexes series, and C5 exerted the highest energy release, in contrast to the other evaluated complexes and to the reported compounds CuO, [Cu₂(en)₂(HBTI)₂]₂ and Catocene.

Full text of DOI:10.1002/ejic.202001092

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doi.org/10.1002/ejic.202001092
Burning Rate Performance Study of Ammonium
Perchlorate Catalyzed by Heteroleptic Copper(I) Complexes
with Pyrazino[2,3-f][1,10]phenanthroline-Based Ligands
Manuel A. Escobar,[a] Cesar Morales-Verdejo,*[b] Juan Luis Arroyo,[c] Paulina Dreyse,[d]
Iván González,[e] Iván Brito,[f] Desmond MacLeod-Carey,[g] David Moreno da Costa,[a] and
Alan R. Cabrera*[a]
This contribution describes the catalytic effect of heteroleptic
Cu(I) complexes with pyrazino[2,3-f][1,10]phenanthroline-based
ligands (C1-7) on the thermal decomposition of ammonium
perchlorate (AP). The complexes C2 and C4-7 were synthesized
and characterized by NMR, HRMS and, in the case of C7, by X-
ray diffraction. The burning rate performance of C1-7 on
thermal decomposition of AP was studied by differential
scanning calorimetry technique. The effect of the counter ion
type, the number of metal centers and ligand substitution was
evaluated. These AP+complex mixtures exerted a shift to lower
ignition temperatures and an increase in the released heats
during thermal decomposition compared with pure AP. The
compound C4 showed the higher catalytic effect among the
complexes series, and C5 exerted the highest energy release, in
contrast to the other evaluated complexes and to the reported
compounds CuO, [Cu2(en)2(HBTI)2]2 and Catocene.
1. Introduction
Currently, research on energetic materials is directed towards
the synthesis of simple molecules with high density, high heat
resistance and low sensitivity.[1] Ammonium perchlorate (AP) is
a common oxidizer in composite solid propellants. The thermal
[a] Dr. M. A. Escobar, Dr. D. Moreno da Costa, Dr. A. R. Cabrera
Departamento de Química Inorgánica,
Facultad de Química y de Farmacia,
Pontificia Universidad Católica de Chile,
Vicuña Mackenna 4860, Macul, Santiago, Chile
E-mail: arcabrer@uc.cl
decomposition of AP has
a close relationship with the
combustion process of the propellants.[2] Nowadays, in the field
of the solid-propellant rocket motor, the lower combustion
temperature and higher specific volume of combustion of AP
could improve the specific impulse of the rocket motor and
decrease its internal erosion.[2a] Therefore, the combustion effect
of a burning rate (BR) catalysts on the composite propellants’
combustion behavior is usually assessed by its impact on AP’s
thermal degradation.[3] Many effective combustion catalysts,
such as ferrocene derivatives,[4] metal oxides,[5] nanoparticles[6]
and transition-metal coordination complexes[1b,7] have been
reported as BR catalysts.
[b] Dr. C. Morales-Verdejo
Universidad Bernardo OHiggins,
Facultad de Ciencias de la Salud,
Centro Integrativo de Biología y Química Aplicada (CIBQA),
General Gana 1702, Santiago, Chile
E-mail: cesar.morales@ubo.cl
[c] J. L. Arroyo
Laboratorio de Materiales Energéticos,
Instituto de Investigaciones y Control del Ejército de Chile, IDIC,
Av. Pedro Montt 2136, Santiago, Chile
[d] Dr. P. Dreyse
Departamento de Química,
Universidad Técnica Federico Santa María,
Av. España 1680, Valparaíso, Chile
[e] Dr. I. González
CuO powder has been used as combustion catalyst, were its
particles, with smaller sizes and better dispersion, exhibit a
better catalytic effect compared with bulky grains or coarsely
dispersed catalysts.[6f] On the other hand, when copper
coordination complexes are used they produce CuO nano-
particles after the burnt up. These nanoparticles with high
surface areas are much more effective in comparison with
common metal oxide powder.[6f] Gao and co-workers gave a
more visual description of the burning process.[1b] Under
heating, the monometallic copper compound begins to decom-
pose under the attack of the oxygen atom formed from the
AP’s decomposition. Then the organic ligands are oxidized to
generate carbon dioxide, molecular nitrogen, and other gas-
eous products of the decomposition of the ligand, according to
its structure. Meanwhile, the copper atoms and the oxygen
atoms react quickly to form nanoscale CuO fine particles. These
Laboratorio de Química Aplicada,
Instituto de Investigación y Postgrado,
Facultad de Ciencias de la Salud,
Universidad Central de Chile,
Lord Cochrane 418, Santiago, Chile
[f] Dr. I. Brito
Departamento de Química,
Facultad de Ciencias Básicas,
Universidad de Antofagasta,
Av. Angamos 601, Antofagasta, Chile
[g] Dr. D. MacLeod-Carey
Universidad Autónoma de Chile,
Facultad de Ingeniería,
Instituto de Ciencias Químicas Aplicadas,
Inorganic Chemistry and Molecular Materials Center,
El Llano Subercaseaux 2801, San Miguel,
Santiago, Chile
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202001092
Part of the “Inorganic Chemistry in Latin America” Special Collection.
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particles possess a larger specific surface area than the standard
purpose, regardless of its well know applications in several
subjects.[15]
Previous studies of Cu(N,N) complexes for thermal decom-
position of AP has been reported. In 2011 Yang et al. reported a
bistetrazole dinitrate Cu(II) complex [Cu(Mtta)2(NO3)2] as addi-
tive for the AP burning.[1b] The nitrogen content (%N) is round
CuO powder. These nanosized CuO particles, coming from
thermal decomposition of copper complexes, are excellent BR
catalyst and can accelerate AP’s thermal decomposition, as
mentioned above.[1b,6f,7]
Considerable attention has been paid in the study of
complexes with ligands that contain high-energy NÀ N and CÀ N
bonds, because the promising applications as burning catalyst
of solid-propellants.[8] In contrast to conventional energetic
materials, this class of substances does not gain its energy from
the oxidation of a carbon backbone or a fuel but rather from
their high heats of formation. For pyrotechnics, these high-
energy density materials serve as potential propellants, coloring
agents, and fuels – eventually in combination with less-toxic
metal ions such as Cu(II) instead of Ba(II). In addition, nitrogen-
rich materials combine several advantages: (i) only or mostly
gaseous products (smokeless combustion), (ii) high heats of
formation, (iii) high propulsive power, (iv) high specific impulse,
and (v) high flame temperatures.[8b]
Among the most studied polypyridine ligands are the
phenanthrolines (phen) and their derivatives.[9] These have a
rigid conjugated structure caused by its fused rings core, with
at least two N atoms being juxtaposed. This spatial position of
N atoms generates that phen can rapidly coordinate to metal
ions, producing highly stable compounds. This chelate effect is
a critical feature for obtaining stable complexes and, for that
reasons, our research focus on the structure of pyrazino[2,3-
f][1,10] phenanthroline (ppl) ligands. This contains a phen
scaffold fused to pyrazine moiety. These structures have more
nitrogen content than phen, and in some cases, can offer twice
as many pairs of nitrogen atoms for bimetallic coordination.
Also, ppl ligands provides new substitution sites for further
structure improvement, hydrogen bridge sites and π-π inter-
molecular interactions.[10] Despite the wide variety of applica-
°
66%. The high-temperature decomposition round to 318.0 C
and the heat released is 3950 J×gÀ 1. By other hand, Liu et al.
synthesized a bimetallic complex employing diethylamine and
5,5’-(1H-imidazole-4,5-diyl)bis(1H-tetrazole) as ligands. The au-
thors found that the high-temperature decomposition was
À 1
°
336.1 C, with an energy released round to 2526.0 J×g (see
Figure 1).[7]
In order to provide more in-depth knowledge in the
development of copper complexes as additives for burning of
ammonium perchlorate-based propellants, we report the syn-
thesis and characterization of heteroleptic Cu(I) complexes (C1-
7), with different ppl-based ligands and a diphosphine ligand
(bis[2-(diphenylphosphino)-phenyl]ether, POP). This strongly
hindered phosphine ligand improves the mixed-ligands com-
plexes’ stability by reducing the flattening effect over the Cu(I)
metal, preventing another molecule’s bonding in the fifth-
coordination site.[16] These heteroleptic Cu(I) complexes are
easily synthesized and characterized by spectroscopic and
spectrometric media, in contrast to Cu(II) complexes and copper
oxide nanoparticles. In a more technical advantage, these
compounds’ ionic nature and their high molecular weights
gather the ideal characteristics to contribute to zero migration
and losses by sublimation during curing and processing the
composite solid propellant. The catalytic performance toward
the AP’s thermal decomposition was explored by differential
scanning calorimetry for all compounds here reported.
tions founded for phen- and ppl-based ligands, e.g., bioinor- 2. Results and Discussion
ganic chemistry,[11] optoelectronics,[12] catalysis,[13] etc., there is
no literature about its energetic property so far. On the other
hand, several metal centers have been tested as efficient
catalysts for the decomposition of solid propellants.[14] However,
well-defined molecular Cu(I) complexes, composed of phen-
and ppl-based ligands, have never been studied for this
2.1. Synthesis of heteroleptic Cu(I) complexes
Heteroleptic Cu(I) complexes (C1-7) of the type [Cu(N,N)(POP)]+
were synthesized according to the literature methods, through
strict stoichiometric control of the reaction. C1 and C3
complexes have been previously reported.[17] In this work, six
Figure 1. (a) Previous works reported in Cu(II) complexes for AP burning. (b) Our work displayed in this article.
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N,N ligands derived from ppl (L1-6), and commercially available
For C7, crystals suitable for XRD analysis were obtained by
controlled evaporation of a CH2Cl2 solution of the complex into
toluene. Its molecular structure is shown in Figure 2 and
selected distances, angles, and torsions are summarized in
Table S1, Table S2, and Table S3, respectively. Both metal
centers exhibited equal bond distances and angles between
each other, which agrees with the symmetry and planarity of
the N,N ligand. This equivalent environment surrounding the
copper centers can be seen in all angles and torsions involved,
e.g., the angles P1-Cu1-P2 and P1i-Cu1i-P2i have the same value
POP phosphine were used. The N,N ligands corresponds to: L1:
ppl;[18] L2: dppz;[19] L3: deppl;[20] L4: n2ppl;[21] L5: sppl;[22] L6: ppz.[23]
The complexes C1-7 were obtained in a two-step reaction: (1)
the addition of one molar equivalent of the N,N ligand to a
solution of [Cu(CH3CN)4]X (X: BF4 or PF6) in acetonitrile / CH2Cl2
(1:1), followed by (2) the addition of one molar equivalent of
the POP ligand (see Scheme 1). The five new heteroleptic Cu(I)
complexes (C2 and C4-7) were obtained with yields between
80–90%. All synthesized complexes showed high air and
thermal stability in the solid state. Furthermore, these were
stable in CH2Cl2 solution for several days. Once Cu(I) complexes
were successfully synthesized and isolated, characterization
through NMR, HRMS, and X-ray diffraction (XRD) for C7 was
achieved.
i
i
i
i
°
(115.95(8) ), or the torsions N1-C5-C6-N2 and N2-C6-C5-N1
°
have the value of À 2.3(9) . Besides, C7 exhibit distorted
tetrahedral geometry in both metal centers, with angles
°
°
ranging from 80.7(2) (N1-Cu1-N2) to 114.9(2) (P2-Cu1-N2). In
addition, L4 shown elevated planarity expected for this type of
N,N ligand, despite the coordination of two metal centers, with
torsions of 176.7(6) (C4-C5-C6-N2) and 177.4(6) (N1-C5-C4-C4).
The spectroscopic and spectrometric characterization of C2
and C4-7 is consistent with the molecular structures shown in
Scheme 1. In the case of C2 and C4-6, the NMR and HRMS data
are consistent with mononuclear heteroleptic Cu(I) complexes.
i
°
°
For a more detailed XRD characterization, see the ESI.
1
The H NMR of these complexes exhibited a 1:1 integral ratio
between the N,N, and the POP ligands protons. The most
significant signals to appreciate this are the protons in para
positions to the N-phenanthroline and to the P-oxyphenyl
moieties. These signals are in approximately 9.55 ppm and
2.2. Redox properties
The redox properties of all compounds were studied by cyclic
voltammetry in dichloromethane solution. This, to evaluate
their oxidation and reduction abilities, according to the nature
of the N,N ligand, number of metallic centers and type of
counter anion. The assignment of the redox processes was
carried out by comparison with electrochemical data reported
for similar Cu(I) complexes.[16c,25] The measured redox potentials
of copper compounds are collected in Table 1, and voltammet-
ric profiles are shown in Figure 3. It should be pointed out that
cyclic voltammetry measurements exhibited quasi-reversible
processes and low defined redox waves (especially in oxida-
tions). Possibly due to adsorption phenomena and/or exchange
1
6.77 ppm, respectively. On the other hand, in the H NMR and
13C NMR of C5,
a pseudo-diasterotopic behavior in the
phenanthroline part of L4 is observed, exerted by the
asymmetric pyrazole segment in the ligand. The 31P{1H} NMR for
complexes C2 and C4-6 showed typical chemical shift related
with a bidentate POPÀ Cu bond formation, in À 10.4 ppm for the
four complexes.[24] Regarding the molecular structure of C7, it
was confirmed the binuclear nature of the heteroleptic complex
1
by NMR, HRMS, and XRD. In this case, the H NMR exhibited a
2:1 integral ratio between the POP and the N,N ligand, as was
observed in the same signals as for C2 and C4-6, which appears
in 9.69 ppm and 6.66 ppm. For a detailed 1D and 2D NMR
characterization, as well as HRMS, see the ESI.
reactions, obtaining
a dynamic equilibrium between the
heteroleptic and the homoleptic complexes, similar to the
previously described for analogous Cu(I) complexes.[16c] In fact,
Scheme 1. Synthetic route of heteroleptic Cu(I) complexes studied (C1-7).
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Figure 2. ORTEP plot of compound C7, with partial numeration scheme. Hydrogen atoms were removed for clarity. Thermal ellipsoids were drawn with 30%
of probability (Symmetry code i: +x, 3/2-y, +z).
Table 1. Values of the redox processes for complexes C1-7.[a,b]
Compound
Eox(Cu2+/+) [V]
Ered(L/LÀ ) [V]
C1
C2
C3
C4
C5
C6
C7
1.32
1.32
1.32
1.32
1.32
1.31
1.47
À 1.41
À 1.17
À 1.19
À 1.17
À 1.12
À 0.91
À 0.81
[a] L is referred to N,N ligand. [b] Dichloromethane/TBAPF6 0.1 M, vs. Ag/
AgCl.
after continuous scans, some peaks associated with adsorbed
species increased their currents, changing the original voltam-
metric profiles. Therefore, the redox potentials for all complexes
were determined from the first scan.
The oxidation potentials of the Cu(I) mono- and bimetallic
compounds are less positive compared to the oxidation
potential of metal in CuO, since the redox process from Cu(II) to
Cu(III) involve very high energy (~2.5 V).[26] This evidence is
important to ensure the in situ formation of CuO nanoparticles,
from the Cu(I) complexes, during the burning process. The
voltammetric profiles exhibit quasi-reversible behavior at pos-
itive potentials, attributed to Cu(I)/Cu(II) oxidation in the copper
complexes, with contributions of POP ligand.[16c,25c] For the
oxidation processes of C1-6, the oxidation potentials are around
1.32 V, due the similar structure between them. The only
differentiation factor corresponds to the N,N ligand, and no
strong contribution on the value is observed. Only C7 is more
positive, were the ligand (ppz) is a bridge that links two Cu(I)
metal centers, resulting in the oxidation process at 1.47 V.
The reduction processes, attributable to N,N ligands, can be
observed towards the cathodic sweep. These follow a trend
from lower to higher negative potential as C7<C6<C5<C4-
C2<C3<C1, where the less negative value is assigned to C7
Figure 3. Cyclic voltammetry profiles recorded in dry dichloromethane
solution at 0.1 VsÀ 1 for C1-7.
(À 0.81 V), the closest to 0 potential of the series. Similar
behavior has been reported on bimetallic Re(I) complexes with
ppz-based ligands.[12d] Regarding the counterion effect on C1-2
complexes, a less negative reduction potential is observed
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À
À
°
using PF6 (C2: À 1.17 V) instead of BF4 (C1: À 1.41 V). This
temperature at 293.7 C) and the high-temperature decomposi-
°
difference can be explained by the diffusion of each counterion
tion (HTD) stage (peak temperature at 429.3 C) of the AP are
À
À
species towards the electrodes. As PF6 is bigger than BF4 , this
should diffuse slowly and therefore, reduces at a more negative
potential.
significantly affected by the addition of all compounds, except
for compound C2. The latter exerted a small increase in the
°
thermal decomposition temperature (413.6 C), suggesting that
the counterion [PF6]À deactivates the catalytic effect, producing
a retarding effect, compared with its analogue C1. After adding
C1, C3-7 the highest thermal decomposition temperatures are
2.3. Catalytic Effects on thermal decomposition of ammonium
perchlorate and possible burning rate catalytic mechanism
°
367.0, 370.4, 359.4, 370.8, 386.9, and 368.0 C, respectively,
decreased by 47.4, 44.0, 55.0, 43.4, 17.5, and 46.4 C compared
°
The copper compounds were explored as promoters to the
thermal decomposition of ammonium perchlorate (AP), the key
component of solid composite propellants. The catalytic effects
of the copper complexes on the thermal decomposition of AP
were investigated by DSC measurements with a heating rate of
5 C/min in N2 atmosphere in the range of 140–450 C. Figure 4
shows the DSC curves of both AP and the mixture of AP with
the title compound. In this work, 5 mass% of catalyst was used.
It was observed that the phase transition endothermic process
with pure AP. These results indicate that copper complexes
have a positive catalytic effect on AP’s thermal decomposition
(Table 2), where C4 exhibited the lowest decomposition tem-
perature.
Furthermore, the exothermic peaks become sharper, in-
dicating the decomposition process of AP becomes more
quickly. It is noted that the DSC curves for compounds C3-5
and C7 only one exothermic peak became sharper, suggesting
that the decomposition process of AP occurred more quickly.
Regarding complexes C1 and C6 displayed a similar shape with
two peaks in each DSC curve, implying that an identical
decomposition mechanism of the mixture systems should be
expected, except that of pure AP and C2.
In their DSC curves, it was noticed that the exothermic heats
were in the range of 2053–2820 J/g. In general, the contribution
of the Cu(I) complexes to the catalytic burning of AP
corresponds to a growth of the heat released in order of more
than 2 times. The mixtures AP+C1 and AP+C2 contribute with
2.74 and 2.18 times over than AP alone, respectively (entries 2
and 3, Table 2). The difference on the results from AP+C1 and
AP+C2 is attributed to the counterion in each complex, where
°
°
°
of AP, peaked at 243 C, has almost no shift and exhibit a similar
shape with 5 wt% for all the catalysts here reported. This
indicates that the catalysts exert little effect on the AP’s
crystallographic transition temperature, as usually observed.
Both the low-temperature decomposition (LTD) stage (peak
À
PF6 could affect to the process by inhibiting or retarding the
burning process. Between AP+C3, AP+C4 and AP+C6
(entries 4, 5 and 7, respectively), having the same nitrogen
content, the heat released with AP+C6 is 4.9% higher than
that of AP+C3, and AP+C6 is 4.2% higher than AP+C4. When
AP+C1 and AP+C7 are compared in their role as catalysts of
the burning process, a difference of 5.1% of heat released is
determined in favor of AP+C1, confirming that a monometallic
copper complex is more efficient than a binuclear complex for
Figure 4. DSC curves of AP and mixture of AP with 5 mass% of C1-7.
Table 2. Comparison of the catalytic effects of copper complexes with classical catalysts on the thermal decomposition of AP.[a]
Heat released ratio[b]
°
Entry
Compound
Wt [%]
HTD of AP [ C]
Heat released
[%] N
[J gÀ 1
]
1
2
3
4
5
6
7
8
NH4ClO4 (AP)
AP+C1
AP+C2
AP+C3
AP+C4
AP+C5
AP+C6
414.4
367.0
413.6
370.4
359.4
370.8
386.9
368.0
345.6
318.0
336.1
346.0
384.0
943.4
3.4
6.7
6.7
6.3
6.3
8.7
6.3
4.2
–
1
5
5
5
5
5
5
5
3
2581.5
2053.0
2441.8
2459.3
2820.9
2568.1
2448.8
1321.4
3950.0
2526.0
2476.0
NR
2.74
2.18
2.59
2.61
2.99
2.72
2.60
1.40
4.19
2.68
2.63
–
AP+C7
9
AP+CuO[6f]
AP+[Cu(Mtta)2(NO3)2][1b]
10
11
12
13
33
10
5
66.0
7.8
–
[7]
AP+[Cu2(en)2(HBTI)2]2
AP+Catocene[4d]
AP+Cu2O[27]
6
–
[a] NR: Not reported. [b] Ratio between the heat released by the mixture and the heat released by the neat AP.
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the burning process. The AP+C5 (entry 6, Table 2) showed the
[2,3-f][4,7]phenanthroline ligands showed high catalytic activity
for the thermal decomposition of ammonium perchlorate. The
compound C4 exhibited a higher catalytic effect than the other
complexes, observed as the lowest thermal decomposition
temperature in the study. In terms of the energy released, the
best mixture was AP+C5 in contrast to the other mixtures of
AP+Complex (almost 3 times higher than neat AP), attributable
to the nitrogen-content of C5, the highest in the Cx series.
Besides, C5 showed highest energy release than the reported
compounds CuO, [Cu2(en)2(HBTI)2]2 and Catocene. On the other
hand, the efficiency of compound C2 was diminished presum-
higher heat released in comparison with the other copper
complexes, exhibiting 8.4% more heat release in comparison
with AP+C1. This is due to the higher nitrogen content in C5,
where the 1-methyl-1H-pyrazole scaffold plays a key role. This is
consistent with the literature findings. Yang et al. described the
employment of 4,5-bis(1H-tetrazol-5-yl)-1H-imidazole, a nitro-
gen rich compound, as ligand for metal-organic catalysts for
thermal decomposition of AP.[7] The authors sustain the
selection of this compound as high energy species, because its
capacity to increase the energy of composite propellants as
consequence of the nitrogen content of the ligand.[7–8]
À
ably for the retardant effect due to counterion PF6 . Regarding
The analysis of AP+C5 against the reported catalyst, AP+
[Cu(Mtta)2(NO3)2], is 28.6% in favor of AP+[Cu(Mtta)2(NO3)2].
However, the nitrogen content in the latter is 7.6 times higher
than AP+C5. A second comparison can be established between
AP+[Cu2(en)2(HTBTl)2]2 against AP+C5, where heat released is
in the order of 10.5% in favor of AP+C5. Finally, by a quick
inspection, AP+C5 has higher heat released against AP+CuO
and AP+Catocene. Concerning AP+Cu2O (entry 13, Table 2),
Ramdani et al. reported a study of Cu2O microparticles with and
without additives. The proportions of the additives and AP in
the mixture were respectively chosen as 6% and 94%,
the effect of the bimetallic complex C7, we confirmed that a
monometallic nature of the complex its more efficient than a
bimetallic one. Though the effect of BR catalysts cannot only be
focused on the influence on the thermal decomposition of AP,
the results confirm that these copper derivatives could have
potential applications as burning rate catalysts in solid
composite propellants, obtaining long periods of storage time,
increase lifespan without presenting changes on the initial
burning parameters, due to their ionic character.
obtaining a catalytic effect on AP’s thermal decomposition, Experimental Section
°
shifting the HTD at 384 C, comparable with some of our
catalysts.[27]
General Information
Finally, the mixtures AP+Ligands are an inefficient catalyst
All reagents were purchased from commercial sources, unless
otherwise specified and used as received. Ligands and complexes
C1 and C3 were prepared as described in literature.[17–23] NMR
spectra were recorded on NMR Bruker AV 400. Chemical shifts are
given in parts per million relative to TMS [1H and 13C, δ(SiMe4)=0]
or an external standard [δ(CFCl3)=0 for 19F NMR]. Most NMR
assignments were supported by additional 2D experiments. HRMS-
ESI-MS experiments were carried out using a Thermo Scientific
Exactive Plus Orbitrap Spectrometer. Cyclic voltammograms were
recorded using a PalmSens 3 Potentiostat, in a three-electrode cell
configuration with a platinum disc working electrode of 0.02 cm2
geometric area, a saturated Ag/AgCl reference electrode and a
platinum wire counter electrode. All electrochemical measurements
were carried out in anhydrous dichloromethane solutions of Cu(I)
complexes (1 mM) with tetrabutylammonium hexafluorophosphate
(TBAPF6) (0.1 M) as the supporting electrolyte at a scan rate of
0.1 VsÀ 1. Before each measure all solutions were degassed with N2.
For X-ray crystal structure analysis, data sets were collected with a
STOE IPDS II two-circle-diffractometer using Mo Kα radiation (λ=
0.71073 Å). The intensities were corrected for absorption by an
empirical correction with X-Area.[28] The structures were solved by
direct methods (SHELXS)[29] and refined by full-matrix least-squares
calculations on F2 (SHELXL-97). Anisotropic displacement parame-
ters were refined for all non-hydrogen atoms.
for the burning of AP, compared to their respective mixtures of
AP+Complexes, mainly because the ligands do not produce an
active enough catalytic species to ensure a well-performed
burning process. Summarizing, C5 is close to the described
catalytic efficiencies. Even more, in some cases, this complex
displayed higher energy at lower concentrations of the catalyst,
and a lower percentage of nitrogen per molecule, improving
the thermal decomposition of AP significantly.
3. Conclusion
Five new heteroleptic Cu(I) complexes (C2 and C4-7) were
synthesized and isolated in good yields (80-90%). These
complexes were highly air- and thermally stable in the solid
state and in CH2Cl2 solution for several days. Besides, C2 and
C4-7 were characterized through NMR, HRMS, and X-ray
diffraction (for C7). For C2 and C4-6, this is consistent with
mononuclear heteroleptic Cu(I) complexes. For C7, the binu-
clear nature of the heteroleptic complex was confirmed by
NMR, HRMS, and XRD.
All compounds showed a quasi-reversible redox process to
the positive potential associated with Cu(I)/Cu(II) oxidation. The
oxidation potentials of all evaluated complexes were less
positive than the oxidation potential of metal in CuO, showing
higher oxidation ability for AP’s catalytic process. For C7, the
oxidation potential was more positive due to the bridge nature
of the ligand ppz.
Thermal analysis
DSC and TG analysis were performed on an 822e Mettler Toledo
and TGA/SDTA 851e Mettler Toledo instruments respectively at a
°
°
heating rate of 5 C/min under nitrogen in the range of 80–500 C.
To investigate the catalytic performance of the phenanthroline
derivative copper(I) complexes for the thermal decomposition of
AP, specific amounts of the complexes and AP were mixed and
ground in a certain weight ratio for DSC analysis.
The catalytic evaluation results, by mean DSC technique,
confirmed that the copper complexes derived from pyrazino
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298 K): δ/ppm=À 10.4 (s). 11B NMR (160 MHz, CDCl3, 298 K): δ/
Synthesis of the compounds
ppm=À 0.69 (s). HRMS (ESI): m/z [M]+ for C52H36CuN4OP2S: calc:
889.1381; found: 889.1444.
General synthetic procedure of complexes
Complex C7. Isolated as a red crystalline material in 90% yield
(224.2 mg, 0.14 mmol). 1H NMR (400 MHz, CDCl3, 298 K): δ/ppm=
9.76 (d, J=8.4 Hz, 2H), 9.66 (s, 2H), 8.51 (d, J=4.7 Hz, 2H), 7.96 (dd,
J=8.4, 4.8 Hz, 2H), 7.39–7.28 (m, 24H), 7.08-6.97 (m, 12H), 6.88 (t,
J=7.5 Hz, 8H), 6.73 (m, 4H), 6.64 (m, 8H). 13C{1H} NMR (100 MHz,
CDCl3, 298 K): δ/ppm=158.5 (t, JC-P =5.8 Hz), 150.8, 148.4, 141.1,
For complexes C2 and C4-6, a solution of the corresponding ppl
derivative ligand (1 eq) in CH2Cl2 was added dropwise to a solution
of [Cu(CH3CN)4]X (X=À BF4 or À PF6) (1 eq) in a mixture of acetonitrile
/ CH2Cl2 (1:1). The reaction mixture was stirred for 30 minutes at
room temperature. Then, a CH2Cl2 solution of (Oxydi-2,1-phenylene)
bis(diphenylphosphine) (POP) (1 eq) was added and stirred for 90
minutes at room temperature. For bimetallic complex C7, a solution
of [Cu(CH3CN)4]PF6 (2 eq), in a mixture of acetonitrile / CH2Cl2 (1:1),
139.9, 135.9, 134.6, 133.9 (t, JC-P =8.4 Hz), 132.4, 132.1 (t, JC-P
7.8 Hz), 131.0, 129.9 (dt, JC-P =155.6, 17.6 Hz), 129.8, 129.7 (t, JC-P
5.0 Hz), 128.7 (t, JC-P =4.6 Hz), 127.8, 127.2, 125.4, 123.5 (t, JC-P
=
=
=
was added dropwise to
a
solution of pyrazino[2,3-f][4,7]
15.6 Hz), 120.6. 19F NMR (400 MHz, CDCl3, 298 K): δ/ppm=À 152.7
(s). 31P{1H} NMR (160 MHz, CDCl3, 298 K): δ/ppm=À 10.0 (s). 11B NMR
(160 MHz, CDCl3, 298 K): δ/ppm=À 0.78 (s). HRMS (ESI): m/z [M]+2
for C86H64Cu2N4O2P4: calc: 1434.2572; found: 1434.2527, m/z [MBF4]+
for C86H64BCu2F4N4O2P4: calc: 1523.2588; found: 1523.2534.
phenanthroline (ppz) ligand (1 eq) in CH2Cl2. The reaction mixture
was stirred for 30 minutes at room temperature. Then, a POP (2 eq)
solution in CH2Cl2 was added and stirred for 90 minutes at room
temperature. For all complexes, the volatiles were removed in
vacuum and the crude product was purified by crystallization using
°
CH2Cl2 / Hexane mixture at À 20 C. For additional experimental
Deposition Number 2042067 (for C7) contains the supplementary
crystallographic data for this paper. These data are provided free of
charge by the joint Cambridge Crystallographic Data Centre and
Fachinformationszentrum Karlsruhe Access Structures service
www.ccdc.cam.ac.uk/structures.
details, 2D NMR and assignment data see the ESI.
Complex C2. Isolated as a yellow powder in 85% yield (151.6 mg,
0.15 mmol). H NMR (400 MHz, CDCl3, 298 K): δ/ppm=9.47 (d, J=
8.3 Hz, 2H), 9.04 (s, 2H), 8.83 (d, J=4.8 Hz, 2H), 7.81 (dd, J=8.3,
4.8 Hz, 2H), 7.40–6.85 (m, 26H), 6.72 (m, 2H). 13C{1H} NMR (100 MHz,
CDCl3, 298 K): δ/ppm=158.4 (d, JC-P =6.1 Hz), 151.6, 145.6, 144.3 (t,
1
J
C-P =2.1 Hz), 139.7, 134.6, 134.4, 133.2, 133.1, 133.0, 132.2, 130.7,
Acknowledgements
130.5, 130.3, 129.2, 129.0 (t, JC-P =4.8 Hz), 128.4, 126.4, 125.4, 124.0
(d, JC-P =14.9 HZ), 120.6. 19F NMR (400 MHz, CDCl3, 298 K): δ/ppm=
À 73.5 (JF-P: 712 Hz). 31P{1H} NMR (160 MHz, CDCl3, 298 K): δ/ppm=
We gratefully acknowledge the financial support from FONDECYT
grant 1161297, 1180673, and 1201173. FONDEQUIP program EQM
160042, EQM 120021, EQM 130021, and EQM 180024. Office of
Naval Research (ONR) grant N62909-18-1-2180, USM PM_I_2020_
31 project., and Laboratory Energy Materials from Institute of
Research and Control (IDIC) of the Chilean Army.
À 10.4 (s), À 144.3(JP-F
:
712 Hz). HRMS (ESI): m/z [M]+ for
C50H36CuN4OP2: calc: 833.1660; found: 833.1606.
Complex C4. Isolated as a yellow powder in 89% yield (75.9 mg,
0.15 mmol). H NMR (400 MHz, CDCl3, 298 K): δ/ppm=9.58 (d, J=
1
8.1 Hz, 2H), 9.02 (d, J=4.7 Hz, 2H), 7.98 (dd, J=8.1, 4.7 Hz, 2H),
7.49–6.98 (m, 26H), 6.78 (m, 2H), 4.61 (c, J=7.1 Hz, 4H), 1.52 (t, J=
7.1 Hz, 6H). 13C{1H} NMR (100 MHz, CDCl3, 298 K): δ/ppm=164.3,
158.5, 152.8, 145.1, 145.0, 139.4, 135.1, 134.5, 133.1 (t, JC-P =8.2 Hz),
132.3, 130.4, 130.3, 129.2, 129.0 (t, JC-P =4.8 Hz), 127.2, 126.9, 125.4,
123.8 (t, JC-P =15.2 Hz), 120.6, 63.4, 14.2. 19F NMR (400 MHz, CDCl3,
298 K): δ/ppm=À 154.0 (s). 31P{1H} NMR (160 MHz, CDCl3, 298 K): δ/
ppm=À 10.4 (s). 11B NMR (128 MHz, CDCl3, 298 K): δ/ ppm=-0.63
(s). HRMS (ESI): m/z [M]+ for C56H44CuN4O5P2: calc: 977.2083; found:
977.2020.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: Ammonium perchlorate · Burning rate · Copper ·
Heterogeneous catalysis · N ligands
Complex C5. Isolated as a yellow powder in 86% yield (105.9 mg,
0.11 mmol). H NMR (400 MHz, CDCl3, 298 K): δ/ppm=9.52 (d, J=
1
8.2 Hz, 1H), 9.50 (d, J=8.2 Hz, 1H), 8.85 (s, 1H), 8.81 (d, J=4.8 Hz,
1H), 8.78 (d, J=4.8 Hz, 1H), 7.81 (dd, J=8.1, 4.8 Hz, 1H), 7.70 (dd,
J=8.1, 4.9 Hz, 1H), 7.34–6.99 (m, 26H), 6.82 (m, 2H), 4.56 (s, 3H). 13C
{1H} NMR (100 MHz, CDCl3, 298 K): δ/ppm=158.6 (t, JC-P =6.0 Hz),
151.1, 150.9, 150.4, 145.3, 144.3, 140.1, 138.6, 134.9, 134.5, 134.5,
133.2, 133.1 (t, JC-P =8.1 Hz), 132.2, 130.6 (d, JC-P =9.3 Hz), 130.3,
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Manuscript received: December 3, 2020
Revised manuscript received: March 18, 2021
Accepted manuscript online: March 22, 2021
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