Ferrocene-based sulfonyl dihydropyrazole derivatives: Synthesis, structure, electrochemistry and effect on thermal decomposition of NH4ClO 4

Article abstract of DOI:10.1016/j.molstruc.2014.03.013

Three ferrocene-based sulfonyl-substituted dihydropyrazoles 3a-c have been synthesized, from the corresponding α,β-unsaturated diketones, and fully characterized. The crystal structures of 3a-c have been confirmed by X-ray crystallography, and electrochemistry behaviors of 3a-c have been examined by cyclic voltammetry (CV). Representatively, the mechanism of the electron transfer in redox process of 3a has been verified by density functional theory (DFT) calculation. It has been found that the activity of catalytic decomposition of ammonium perchlorate (AP) is significantly lowered (by 62.9-104.7 °C) with an addition of 3a-c. We expect that the ferrocene-based sulfonyl dihydropyrazole derivatives would have a great value in burning rate catalyst as composite solid propellants.

Full text of DOI:10.1016/j.molstruc.2014.03.013

Journal of Molecular Structure 1067 (2014) 112–119
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Ferrocene-based sulfonyl dihydropyrazole derivatives: Synthesis,
structure, electrochemistry and effect on thermal decomposition
of NH4ClO4
Ji-Bin Zhuo a, Heng-Dong Li a, Cai-Xia Lin a, Li-Li Xie a,b, Sha Bai c, Yao-Feng Yuan a,b,
⇑
a Department of Chemistry, Fuzhou University, Fuzhou 350108, China
b State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou 350002, China
c Beth Israel Deaconess Medical Center (BIDMC), Harvard Medical School, Boston, MA 02215, USA
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
Three ferrocene-based sulfonyl-
substituted dihydropyrazoles 3a–c
are synthesized in moderate yields.
Molecular structure of 3a has been
characterized by single-crystal X-ray
diffraction analysis for a first time.
The electrochemical behaviors of 3a–
c are discussed; the electrochemical
inequivalence of 3a was confirmed by
DFT.
ꢀ
Dinuclear ferrocenyl pyrazole 3a
shows a greater catalytic effect on the
decomposition temperature of AP
than 3b–c.
a r t i c l e i n f o
a b s t r a c t
Article history:
Three ferrocene-based sulfonyl-substituted dihydropyrazoles 3a–c have been synthesized, from the
Received 30 December 2013
Received in revised form 7 March 2014
Accepted 10 March 2014
corresponding
a,b-unsaturated diketones, and fully characterized. The crystal structures of 3a–c have
been confirmed by X-ray crystallography, and electrochemistry behaviors of 3a–c have been examined
by cyclic voltammetry (CV). Representatively, the mechanism of the electron transfer in redox process
of 3a has been verified by density functional theory (DFT) calculation. It has been found that the activity
of catalytic decomposition of ammonium perchlorate (AP) is significantly lowered (by 62.9–104.7 °C)
with an addition of 3a–c. We expect that the ferrocene-based sulfonyl dihydropyrazole derivatives would
have a great value in burning rate catalyst as composite solid propellants.
Available online 25 March 2014
Keywords:
Dihydropyrazole
Ferrocene
Ó 2014 Elsevier B.V. All rights reserved.
Sulfonyl
Structure
Electrochemistry
Ammonium perchlorate
Introduction
the pyrazole ring to deliver the corresponding derivatives with
alternative functions used in electrochemical sensors [8–10],
Pyrazole and its derivatives are widely used in the field of med-
icine and pesticide [1–7]. Different substituents are introduced to
molecular recognitions [11–13] and decomposition catalysts
[14,15]. Specifically, ferrocene has unique aromatic electron-rich
potential and electrochemical reversibility [16,17]. Its derivatives
have many advantages, including good mobility, flammability,
high molecular weight and thermal stability, compared to the
traditional combustion catalysts [18,19]. Currently, researchers
⇑
Corresponding author at: Department of Chemistry, Fuzhou University, Fuzhou
350108, China. Tel./fax: +86 591 22866161.
E-mail address: yaofeng_yuan@fzu.edu.cn (Y.-F. Yuan).
http://dx.doi.org/10.1016/j.molstruc.2014.03.013
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

J.-B. Zhuo et al. / Journal of Molecular Structure 1067 (2014) 112–119
113
have been focusing on the study of pyrazole heterocyclic deriva-
tives with a connection of ferrocenyl functional unit, for their
advantages in controllability in terms of burning catalysts [20–
22]. The research on ferrocene catalytic activity for AP decomposi-
tion, is to explore the basis of such catalysts catalyzing composite
solid propellant combustion [23,24], However, the ferrocene
combustion catalysts have problems of diffusion and evaporation
during processing, significantly limiting the service life of propel-
lants [25]. We envision that by introducing a polar group to the
molecular structure, the van der Waals’ force between a ferrocene
derivative and the propellant molecule could be increased, leading
to less volatile products [26–29]. The combination of sulfonyl
groups and ferrocene pyrazoline rings could derive new materials
with unique electrochemical properties and functional molecules
of high catalytic activity.
We report three ferrocene-based sulfonyl dihydropyrazole
derivatives 3a. See for example, molecular structures and the CV
studies of redox properties of these compounds have been investi-
gated. Representatively, elctrochemical inequivalence of the two
ferrocenyl units of 3a is confirmed by DFT calculation. The catalytic
properties of 3a–c on thermal decomposition of AP have been stud-
ied to determine the catalytic effect on burning rate of the sulfonyl
ferrocene dihydropyrazoles.
(k = 0.071073 nm) at 293(2) K. All diffraction data were collected
by scanning in a certain mode and refined in Lp factor. The position
coordinates and each anisotropic thermal parameter of nonhydro-
gen atoms were refined by full-matrix least-squares on F2 through
confirmation. All structures were solved by direct methods and
refined with SHELX-97 program suite [30]. All hydrogen atoms
were generated geometrically assigned appropriated isotropic
thermal parameters and included in the final calculations.
The DFT calculations were carried out with the Gaussian 03 suite
of programs [31]. 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 sta-
tionary points [32]. Crystallographic data and structure refinement
parameters were summarized in Table 1.
The electrochemical characterization was tested by a CHI620C
electrochemical workstation via CV in acetonitrile. In this method,
(n-Bu4N)PF6 (c = 0.10 mol/L) was used as the supporting electrolyte.
A three-electrode cell was used, consisting of a glassy carbon work-
ing electrode, a platinum wire auxiliary electrode and Ag/AgCl
(3.0 mol/L KCl) reference electrode. The scan rate of CV reached
100 m Vꢁ1. The surface of glassy carbon electrode was polished with
0.3 and 0.05 nm a-Al2O3 slurry on emery paper and rinsed with dou-
bly distilled water. The system was degassed by purging nitrogen
for more than 5 min before each electrochemical measurement.
Differential scanning calorimetry (DSC) and thermogravimetry
(TG) analysis on pure AP and AP with an addition of 5 wt% of 3a–
c (totally 5 mg for each mixture) were dried under 0.1 MPa at
Experimental
General
20 °C, then recorded with
a Simultaneous DSC-TG Thermal
Analyzer SDT Q600 instrument. The experiments were conducted
under a nitrogen atmosphere (20 mL N2/min) at heating rate of
10 °C/min from 50 to 500 °C.
All chemicals were commercially available. Acetonitrile was
distilled with CaH2 and used immediately. CH2Cl2 is dichlorometh-
ane. PE is petroleum ether. All reactions were performed under an
atmosphere of dry nitrogen. Melting points (m.p.) were deter-
mined with a 4X micro melting point apparatus. Infrared spectra
(IR) was recorded with a Spectrum-2000 FT-IR spectrum in KBr
pellets in the range 4000–400 cmꢁ1. Nuclear magnetic resonance
spectra (1H and 13C NMR) was measured in CDCl3, with tetrameth-
ylsilane (TMS) as the internal standard, with a Bruker AV600 spec-
trometer at ambient temperature. Electrospray ionization mass
spectrometry (ESI-MS) was performed with an X7 ICP-MS instru-
ment. The synthesis of 1-phenylsulfonyl-3-ferrocenyl-4,5-dihy-
dro-1H-pyrazole derivatives 3a–c (Scheme 1) were carried out by
condensing the corresponding ferrocenyl pyrazoline derivatives
2a–c benzenesulfonyl chloride [16,17,27]. The unstable pyrazoline
derivatives 2a–c were generated in situ from cycloaddition of the
Synthesis of 1-phenylsulfonyl-3,5-ferrocenyl-4,5-dihydropyrazole
derivatives (3a)
To a stirred solution of
a,b-unsaturated diketone 1a (1.0 g,
2.4 mmol) in ethanol (10 mL) was charged with hydrazine hydrate
(4.0 mL, 80 mmol) in one portion at 20 °C. The resulting mixture
was heated at reflux for 30 min, and then concentrated to yield
2a as a red viscous liquid. The unstable intermediate 2a was imme-
diately dissolved in CH2Cl2 (10 mL). To the above solution was
added a solution of benzenesulfonyl chloride (0.60 mL, 5.0 mmol)
in CH2Cl2 (10 mL) dropwise over 10 min at 20 °C. The resulting
mixture was further stirred at 40 °C for 30 min, and then cooled
to room temperature. The reaction mixture was charged with CH2-
Cl2 (20 mL), washed with water (50 mL ꢂ 3), then partitioned be-
tween H2O and CH2Cl2. The organic extract was dried (MgSO4)
and concentrated to give a red viscous liquid, which was further
purified by silica gel chromatography to yield 3a (0.61 g, 45%) as
a,b-unsaturated diketones with hydrazine hydrate.
Salmon pink single crystals of 3a–c were grown by slow
diffusion of petroleum ether into the solution of pure product in
acetonitrile over several days, respectively. The spatial structures
of 3a–c were determined with X-ray crystallography. X-ray struc-
tural measurements were carried out with a Rigaku RAXISIV CCD
a yellow powder. M.p. 254–255 °C (CH2Cl2/PE). IR (KBr), v/cmꢁ1
:
diffractometer with a graphite-monochromator Mo K
a
radiation
3436, 3083, 2929, 1671, 1585, 1445, 1352, 1171, 1105, 822, 724,
Fe
O
C
Fe
Fe
R
R
R
C
H
C
H
i
ii
N
O
N
S
N
HN
O
1a-c
2a-c
a: R = Fc; b: R = p-bromopheny; c: R = Ph;
3a-c
Scheme 1. Reagents and conditions: (i) N2H4ꢃH2O, 80 °C; (ii) benzenesulfonyl chloride, CH2Cl2, 40 °C.

114
J.-B. Zhuo et al. / Journal of Molecular Structure 1067 (2014) 112–119
Table 1
Crystallographic data and structure refinement parameters of 3a–c.
Compound
3a
3b
3c
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C
29H26Fe2N2O2S
C25H21BrFeN2O2S
549.26
293(2)
0.71073
Monoclinic
P2(1)/c
13.087(3)
9.844(2)
17.752(4)
90.00
99.42(3)
90.00
C25H22FeN2O2S
470.37
293(2)
578.29
293(2)
0.71073
Monoclinic
P2(1)/c
19.586(4)
10.936(2)
11.649(2)
90.00
92.71(3)
90.00
0.71073
Orthorhombic
P2(1)2(1)2(1)
5.9466(12)
11.653(2)
31.274(6)
90.00
b (Å)
c (Å)
a
(°)
b (°)
(°)
90.00
90.00
c
V (Å)
2492.4(9)
4
1.541
1.278
1472
2256.2(8)
4
1.617
2.558
1112.0
2167.2(8)
4
1.442
0.817
Z
Dcalc (g cmꢁ3
)
Absorption coefficient (mmꢁ1
F (000)
)
1280
Crystal size (mm)
Index ranges
0.25 ꢂ 0.25 ꢂ 0.20
ꢁ22 6 h 6 25
ꢁ13 6 k 6 14
ꢁ14 6 l 6 15
20,546
0.30 ꢂ 0.25 ꢂ 0.20
ꢁ15 6 h 6 13
ꢁ11 6 k 6 11
ꢁ21 6 l 6 21
15,470
0.50 ꢂ 0.30 ꢂ 0.20
ꢁ7 6 h 6 7
ꢁ15 6 k 6 15
ꢁ38 6 l 6 40
18,264
Reflections collected
Independent reflections
h range for data collection
Absorption correction
Refinement method
5163 [R(int) = 0.0495]
3.24–27.55°
Multi-scan
Full-matrix least-squares on F2
1.274
5709/0/325
R1 = 0.0706
wR2 = 0.2127
R1 = 0.0847
wR2 = 0.2495
3462 [R(int) = 0.0660]
3.14–25.00°
Multi-scan
Full-matrix least-squares on F2
1.296
3972/0/289
R1 = 0.0803
wR2 = 0.2321
R1 = 0.1031
wR2 = 0.2700
4625 [R(int) = 0.0516]
3.14–27.51°
Multi-scan
Full-matrix least-squares on F2
1.170
4947/0/280
R1 = 0.0509
wR2 = 0.1421
R1 = 0.0573
wR2 = 0.1611
Goodness-of-fit on F2
Data/restraints/parameters
Final R indices [I > 2
r (I)]
R indices (all data)
689. 1H NMR (400 MHz, CDCl3): d 7.93 (d, J = 6.8 Hz, 2H, Ph-H), 7.54
(d, J = 6.8 Hz, 3H, Ph-H,), 4.92 (s, 1H, Pz-H) (Pz: parazole), 4.76 (d,
1H, Cp-H) (Cp: cyclopentadiene), 4.57 (d, 1H, Cp-H), 4,42 (d, 2H,
Cp-H), 4.22 (m, 9H, Cp-H), 4.00 (s, 5H, Cp-H), 3.29 (m, 2H, Pz-H).
matography. M.p. 203–204 °C (CH2Cl2/PE). IR (KBr), v/cmꢁ1: 3062,
2926, 1596, 1485, 1446, 1351, 1171, 828, 763, 699. 1H NMR
(400 MHz, CDCl3): d 7.98 (d, J = 6.8 Hz, 2H, Ph-H), 7.59 (m,
J = 6.8 Hz, 3H, Ph-H), 7.46 (m, J = 7.2 Hz, 2H, Ph-H), 7.41 (t,
J = 7.2 Hz, 2H, Ph-H), 7.35 (t, J = 7.2 Hz, 1H, Ph-H), 4.88 (m,
J = 11.2, 9.2 Hz, 1H, Pz-H), 4.70 (s, 1H, Cp-H), 4.53 (s, 1H, Cp-H),
4.40 (d, 2H, Cp-H), 3.98 (s, 5H, Cp-H), 3.38 (d, J = 17.2, 11.2 Hz,
1H, Pz-H), 3.01 (d, J = 17.2, 11.2 Hz, 1H, Pz-H). 13C NMR
(100 MHz, CDCl3): d 159.25 (Pz-C), 141.26 (Ph-C), 135.83 (Ph-C),
133.27 (Ph-C), 128.87 (Ph-C), 128.80 (Ph-C), 128.01 (Ph-C),
126.62 (Ph-C), 74.41 (Cp-C),70.81 (Cp-C), 70.59 (Cp-C), 69.59
(Cp-C), 68.40 (Cp-C), 67.61 (Cp-C), 64.55 (Pz-C), 45.27 (Pz–C). MS
(ESI) m/z: [M + H] 471.00, [M+] calcd. 470.36. Anal. calcd. for
13C NMR (100 MHz, CDCl3):
d 160.21 (Pz-C), 137.01 (Ph-C),
133.06 (Ph-C), 128.65 (Ph-C), 128.61 (Ph-C), 74.30 (Cp-C), 70.79
(Cp-C), 70.38 (Cp-C), 69.53 (Cp-C), 69.15 (Cp-C), 68.64 (Cp-C),
67.53 (Cp-C), 59.67 (Pz-C), 43.02 (Pz-C). MS (ESI) m/z: [M + H]
578.92, [M+] calcd. 578.28. Anal. calcd. for C29H26Fe2N2O2S: C
60.30, H 4.61, N 5.09; found: C 60.23, H 4.53, N 4.84.
Synthesis of 1-phenylsulfonyl-3-ferrocenyl-5-(4-bromophenyl)-4,5-
dihydropyrazole (3b)
The complex was prepared by manner analogous of that of 3a. A
yellow powder 3b (0.48 g, 39%) was obtained after silica gel chro-
matography. M.p. 265–266 °C (CH2Cl2/PE). IR (KBr), v/cmꢁ1: 3442,
3088, 2917, 1599, 1487, 1447, 1352, 1167, 1107, 822, 719, 689. 1H
NMR (400 MHz, CDCl3): d 7.96 (m, J = 6.8 Hz, 2H, Ph-H), 7.60 (m,
J = 6.8 Hz, 3H, Ph-H), 7.53 (d, J = 8.0 Hz, 2H, Ph-H), 7.34 (d,
J = 8.0 Hz, 2H, Ph-H), 4.83 (m, J = 11.2, 9.2 Hz, 1H, Pz-H), 4.70 (s,
1H, Cp-H), 4.51 (s, 1H, Cp-H), 4.40 (d, 2H, Cp-H), 3.97 (s, 5H, Cp-
H), 3.39 (d, J = 17.2, 11.2 Hz, 1H, Pz-H), 2.96 (d, J = 17.2, 9.2 Hz,
1H, Pz-H). 13C NMR (100 MHz, CDCl3): d 159.15 (Pz–C), 140.89
(Ph-C), 135.59 (Ph-C), 133.41 (Ph-C), 132.01 (Ph-C), 128.84 (Ph-
C), 128.80 (Ph-C), 128.37 (Ph-C), 121.98 (Ph-C), 69.75 (Cp-C),
67.19 (Cp-C), 64.00 (Pz-C), 45.10 (Pz-C). MS (ESI) m/z: [M + H]
549.00, [M+] calcd. 549.26. Anal. calcd. for C25H21BrFeN2O2S: C
54.74, H 4.03, N 4.99; found: C 54.67, H 3.85, N 5.10.
Synthesis of 1-phenylsulfonyl-3-ferrocenyl-5-benzoyl-4,5-dihydro-
1H-pyrazole (3c)
The complex was prepared by manner analogous of that of 3a. A
yellow powder 3c (0.64 g, 48%) was obtained after silica gel chro-
Fig. 1a. ORTEP view of 3a (50% probability level), I and II refer to the two ferrocenyl
units.

J.-B. Zhuo et al. / Journal of Molecular Structure 1067 (2014) 112–119
115
Table 2
H-bonding geometry (Å, o) for 3a–b.
Type
DAH
(Å)
Hꢃ ꢃ ꢃA
Dꢃ ꢃ ꢃA
DAHꢃ ꢃ ꢃA
(°)
(Å)
(Å)
3a C15AH15ꢃ ꢃ ꢃO1 Intra. 0.95
C13AH13ꢃ ꢃ ꢃO1 Inter. 1.00
C19AH19ꢃ ꢃ ꢃO1 Inter. 0.96
2.48
2.38
2.51
2.59
2.58
2.52
2.85(8)
3.22(3)
3.31(3)
3.43(9)
3.50(1)
3.38(2)
104
141
142
151
155
152
3b C5AH5ꢃ ꢃ ꢃO1
Inter. 0.93
3c C13AH13ꢃ ꢃ ꢃO1 Inter. 0.97
C25AH25ꢃ ꢃ ꢃO1 Inter. 0.92
Table 3
Distances (Å) and angles (
p
) of CAHꢃ ꢃ ꢃ
p
contacts for 3a–b.
Hꢃ ꢃ ꢃ (Å)
p
CAHꢃ ꢃ ꢃp (°)
Fig. 1b. Hydrogen bonds between adjacent molecules of 3a.
3a
C8AH8ꢃ ꢃ ꢃCga
2.74
2.85
2.65
2.98
2.85
2.87
2.96
2.77
141
143
150
139
98
C1AH1ꢃ ꢃ ꢃCgb
C23AH23 ꢃ ꢃ ꢃCgc
C10AH10ꢃ ꢃ ꢃCgd
C25AH25ꢃ ꢃ ꢃCge
C16AH16ꢃ ꢃ ꢃCgf
C4AH4ꢃ ꢃ ꢃCgg
C25H22FeN2O2S: C 63.72, H 4.82, N, 5.95; found: C 63.84, H 4.71, N
5.96.
3b
3c
127
167
97
Results and discussion
C21AH21ꢃ ꢃ ꢃCgh
a
b
c
Crystal structure of 3a–c
Cg: (Cp ring of 3a C20AC24).
Cg: (Cp ring of 3a C25AC29).
Cg (Ph ring of 3a).
Cg: (Ph ring of 3b C20A25).
Cg (Pz ring of 3b).
Cg: (Cp ring of 3c C1AC5).
Cg: (Ph ring of 3c C20AC25).
Cg: (Pz ring of 3c).
Fig. 1a shows the molecule structure of 3a, indicates that 3a
crystallizes in monoclinic crystal system, space group P21/c, Se-
lected bond distances and angles are included in Tables S1 and
S2. The structure consists of a benzenesulfonyl substituted pyra-
zole ring and two ferrocenes, attached to C11 and C13. The five
atoms of the pyrazole ring are almost in the same plane. Their bond
lengths tend to conjugate (C11AC12 bond: 1.489 Å; N1AN2 bond:
1.433 Å; C11@N2 bond: 1.306 Å). The Cp ring (C6AC10) of ferroce-
nyl unit I is almost parallel to the pyrazole ring (dihedral angle
4.56°), indicating that there is a conjugation among them. The
bond length of C6AC11 (1.456 Å) is greater than a regular C@C
double bond but shorter than a CAC single bond [17]. The two
Cp rings of ferrocenyl unit I are eclipsed (pseudo-torsion angles
of C1-Cg-Cg-C6: 4.21°) [33], while the two Cp rings of ferrocenyl
unit II are halfway between eclipsed and staggered (pseudo-tor-
sion angles of C20-Cg-Cg-C25: 19.60°). Coplanarity of the substi-
tuted Cp ring (C6AC10) of ferrocenyl unit I and pyrazole ring in
3a has an impact on intramolecular forces. The crystal packing of
d
e
f
g
h
3a (Fig. 1b) shows that molecules are linked into a one-dimen-
sional chain along the c axis by intermolecular hydrogen bonding
interactions and CAHꢃ ꢃ ꢃ
p contacts (Tables 2 and 3). Interactions
of molecules lead to a stable spatial structure by parallel stacking
(Fig. 1c).
Different from 3a, 3b or 3c contains only one ferrocenyl unit,
however, three compounds are similar on the spatial structure
(Figs. 2 and 3). The plane of the pyrazole ring of 3b has a larger
degree of distortion than in 3c. However, the dihedral angles
between the Cp ring (C6AC10) of ferrocenyl unit and pyrazole ring
are small (12.48° for 3b; 4.13° for 3c). Compared with our previous
Fig. 1c. Cell packing diagram of 3a, viewed down the c axis.

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J.-B. Zhuo et al. / Journal of Molecular Structure 1067 (2014) 112–119
Fig. 2. ORTEP view of 3b (50% probability level).
Fig. 4a. Cyclic voltammetry of ferrocene and 3a–c at a scan rate of 0.1 V/s.
Table 4
Cyclic voltammograms datas of ferrocene and 3a–c.
Compound
Epa (V)
Epc (V)
DE (V)
Ep (V)
DEp (V)
Ferrocene
3a, Fc unit I
3a, Fc unit II
3b
0.496
0.706
0.562
0.690
0.707
0.421
0.623
0.481
0.610
0.631
0.458
0.664
0.522
0.652
0.669
0.075
0.083
0.081
0.080
0.076
0.142
3c
Epa: anodic oxidation potential; Epc: cathodic reduction potential; Ep: redox peak
potential;
Ep = (Epa + Epc)/2;
D
E = Epa ꢁ Epc
;
D
Ep(3a) = Ep(3aFc(I)) ꢁ Ep(3aFc(II)).
The monoferrocenyl pyrazole 3b and 3c show only one ferroce-
nyl oxidation in their CV spectra. The oxidation potential of
ferrocenyl group of the p-bromophenyl substituted pyrazole 3b
(0.690 V) is lower than that of the phenyl-substituted pyrazole ring
3c (0.707 V), indicating a lower degree of electronic interaction
between the ferrocenyl unit and pyrazole core than 3c. It was in
congruent with that the pyrazole ring and Cp ring (C6AC10) of
3c had a higher degree of conjugated system than that of 3b.
Fig. 3. ORTEP view of 3c (50% probability level).
work [34], bond lengths and angles of pyrazole ring and benzene-
sulfonyl group in 3b and 3c are similar to those observed in 3a
(Tables S1 and S2). In the crystal lattices of both 3b and 3c
molecules are associated though hydrogen bonds to form a spiral
one-dimensional structure (Fig. S2) and held together by CAHꢃ ꢃ ꢃ
p
contacts and stacked along the a axis (Fig. S3). The related data of
hydrogen bonds and CAHꢃ ꢃ ꢃ
p contacts are also given in Tables 2
and 3.
Electrochemical behavior of 3a–c
In the electrochemical analysis, the redox properties of 3a–c
(c = 10ꢁ3 mol/L) are studied by cyclic voltammetry (CV) and differ-
ential pulse voltammetry (DPV). The electrochemical process of
ferrocenyl moieties could be assigned as electrochemically quasi-
reversible. Two redox peak potentials (Ep) of 3a (Fig. 4a and
Table 4), for electrochemically reversible process are observed at
0.522 V and 0.664 V. The Ep of 3b and 3c are 0.652 V and 0.669 V,
respectively. All of them are higher than that of ferrocene
(0.458 V). Simultaneously, the potential difference of anodic oxida-
tion and cathodic reduction (
II were observed at 0.083 V and 0.081 V, the
D
E) of 3a for two ferrocenyl unit I and
E of 3b and 3c were
D
0.080 V and 0.076 V, respectively, and all of them are identical with
ferrocene (0.075 V), which can confirm the electrochemical revers-
ible oxidation process of ferrocenyl units of 3a–c.
Fig. 4b. Cyclic voltammetry of 3a at scan rates of 0.02, 0.04, 0.06, 0.08 and 0.1 V/s.

J.-B. Zhuo et al. / Journal of Molecular Structure 1067 (2014) 112–119
117
Fc
I
Fc
I
Fc
Fc
II
Fc
II
Fc
II
- e , oxidation
N
N
N
I
N
N
N
e , reduction
+
O S O
Ph
O S O
Ph
O S O
Ph
Scheme 2. Electron transfer mechanism of 3a.
Analysis of the CV of 3a, two ferrocenyl units, monoferrocenyl pyr-
azoles 3b and 3c, varying from 0.02 to 0.10 V/s, indicates a diffu-
sion-controlled process (Fig. 4b). Both the anodic and cathodic
DFT study on electron transfer
In order to propose the electron transfer mechanisms and elct-
rochemical inequivalence of two ferrocenyl units in 3a, the molec-
ular frontier orbitals of 3a (Fig. 5) was exhibited by means of
simplified theoretical modeling. Quantization calculation shows
that ferrocenyl Cp rings are rich in negative charges and the iron
atom is positive charged. In addition, a large number of positive
charges are also enriched in S and C11 (Table S3). The ferrocenyl
unit II, with HOMO (ꢁ5.495 eV) and HOMO-1 (ꢁ5.501 eV), tend
to lose an electron faster than the ferrocenyl unit I, with HOMO-
2 (ꢁ5.850 eV) and HOMO-3 (ꢁ5.948 eV). Thus, the ferrocenyl unit
II is more inclined to serve as an electron donor than the ferrocenyl
unit I. This result indicates that ferrocenyl unit II oxidizes first, in
peak currents are linear with the square root of scan rates (
3a undergoes two successive reversible one-electron-oxidation
processes. The potential difference of two redox peaks ( Ep) of
v
1/2).
D
ferrocenyl unit I and II is 0.142 V. The potential of the second wave
(Ep ¼ 0:664 V) of 3a and oxidation peak potential of 3b (Ep ¼
0:610 V), 3c (Ep ¼ 0:631 V) occur at nearly the same location
(Fig. 4a), the phenomenon manifesting that the redox couple is as-
signed to ferrocenyl unit II. The electrochemical behavior process
of 3a is viewed in Scheme 2. The first oxidation step occurs on ferr-
ocenyl unit II at Epa of 0.562 V, then the second redox wave of unit I
emerges at Epa = 0.706 V.
HOMO (-5.495 eV)
HOMO-1 (-5.501 eV)
HOMO-2 (-5.850 eV)
HOMO-3 (-5.948 eV)
Fig. 5. The structure of 3a optimized by theoretical calculations and energy diagram of the first four HOMOs of 3a.

118
J.-B. Zhuo et al. / Journal of Molecular Structure 1067 (2014) 112–119
a large conjugated system. Adjacent molecules are connected by
the hydrogen bonding to present a stable spatial structure. The
cyclic voltammetry shows that the two quasi-reversible oxidation
waves of 3 are diffusion-controlled. Specifically with 3a, ferrocenyl
unit II is more electron rich to serve as an electron donor than ferr-
ocenyl unit I, further confirmed by DFT calculation. The results
from DTA and TG analysis indicate that 3a–c is of high catalytic ef-
fect on lowering the decomposition temperature of AP (by 62.9–
104.7 °C) with 5 wt% addition of each. Experimental results show
that the sulfonyl substituent added to the dihydropyrazole ring
could lower the thermal decomposition temperature of AP.
Meanwhile, dinuclear ferrocenyl substituted further reduces the
migration and evaporation due to its steric hindrance. We expect
that the novel dinuclear ferrocenyl pyrazole derivatives would be
more widely used in high burning rates solid propellants.
Acknowledgements
We are grateful for the financial support from the National
Natural Science Foundation of China (No. 21172036) and the
Research Fund for the Doctoral Program of Higher Education of
China (No. 20113514110002).
Fig. 6. DSC curves of AP, AP + 5% 3a, AP + 5% 3b and AP + 5% 3c.
consistency with the electrochemical analysis that the first redox
wave is associated with the oxidation of ferrocenyl unit II (Fig. 4a).
Appendix A. Supplementary material
Catalytic effect on the decomposition of ammonium perchlorate (AP)
Hydrogen bonds between adjacent molecules of 3b and 3c were
given in Fig. S1. Cell packing diagram of 3b and 3c were given in
Fig. S2. TG curves of AP, AP+5% 3a, AP+5% 3b and AP+5% 3c were
given in Fig. S3. Selected bond lengths for 3a–c were given in
Table S1. Selected angle between two bonds for 3a–c were given
in Table S2. Atomic natural charge of molecules 3a–c were given
in Table S2. Crystallographic data for the structures reported in this
paper has been deposited with the Cambridge Crystallographic
Data Centre as supplementary publications No. CCDC 800549
(3a), CCDC 985889 (3b) and CCDC 985981 (3c). Copies of the data
can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336 033; e-mail: de-
posit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk). Supplemen-
tary data associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.molstruc.2014.03.013.
The decomposition of AP, affected by the catalytic burning rate,
is analyzed using DSC curves (Fig. 6) and TG curves (Fig. S3). The
DSC curves of AP shows that the additives 3a–c (245.3 °C,
246.8 °C and 246.5 °C, respectively) have little effect on the crystal-
lographic transition temperature of AP (246.3 °C) from orthorhom-
bic form to cubic form [35,36]. After the addition of 3a–c, the
exothermic peaks become sharper, indicating the decomposition
process of AP becomes more quickly. The result suggests that
3a–c have a promoting effect on thermal decomposition of AP.
The original exothermic peak of pure AP appears at 459.6 °C, and
shift to a lower temperature (354.9 °C, 392.1 °C, 396.7 °C) when
3a–c are mixed, respectively. The TG curves indicates that temper-
atures of initial thermal decompositions of four samples (pure AP
and AP with 3a–c additives) are between 280 and 285 °C, while
that of complete decomposition temperatures are 459.6 °C,
354.9 °C, 396.3 °C, 403.9 °C, respectively. Specifically, 3a shows a
greater catalytic effect on the decomposition temperature of AP
than 3a and 3b. This indicates that the catalytic effect on AP of
dinuclear ferrocenyl pyrazole is more effective than mononuclear
ferrocenyl pyrazole, which is due to the pyrazole substituent on
the Cp ring of dinuclear ferrocenyl moiety, the bulky substituent
can hinder the migration in AP. Combined with our earlier work,
dinuclear ferrocene-based acyl dihydropyrazole only lower the
thermal decomposition temperature by 74.3 °C [27], the sulfonyl
substituent added to the dinuclear ferrocene-based dihydropyraz-
ole ring could lower the thermal decomposition temperature
(104.7 °C) than acyl substituent, which indicating that the sulfonyl
substituent has significance in overcoming migration and evapora-
tion of catalyst. The results and arguments confirm dinuclear
ferrocenyl dpyrazole 3a has a promoting effect on thermal decom-
position of AP.
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