A reversible phase transition and dielectric anomaly in a spherical molecule [(3,2,1-dabco)2PbBr6]

Article abstract of DOI:10.1016/j.inoche.2020.107932

Organic-inorganic hybrid materials were attracted most interests due to their potential applications in phase transition and photoelectric fields. Within this context, a spherical molecule 1, 5-diazabicyclo[3,2,1]octane (3,2,1-dabco) was used to react with PbBr₂ in concentrated HBr acid and led to a discrete organic inorganic hybrid compound [(3,2,1-dabco)₂PbBr₆] (1). Thermodynamic DSC analyses and dielectric measurement of 1 showed an apparently reversible phase transition at about 427 K. The variable temperature X-ray single crystal diffraction demonstrated compound 1 underwent an order-disorder transition from orthorhombic to cubic systems at room and high temperatures.

Full text of DOI:10.1016/j.inoche.2020.107932

Journal Pre-proofs
A reversible phase transition and dielectric anomaly in a spherical molecule
[
(3,2,1-dabco) PbBr ]
2
6
Xiuli You, Xingcong Wang, Zhenhong Wei
PII:
S1387-7003(20)30299-9
DOI:
https://doi.org/10.1016/j.inoche.2020.107932
INOCHE 107932
Reference:
To appear in:
Inorganic Chemistry Communications
Received Date:
Revised Date:
Accepted Date:
10 March 2020
3 April 2020
8 April 2020
Please cite this article as: X. You, X. Wang, Z. Wei, A reversible phase transition and dielectric anomaly in a
spherical molecule [(3,2,1-dabco) PbBr ], Inorganic Chemistry Communications (2020), doi: https://doi.org/
2
6
1
0.1016/j.inoche.2020.107932
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A reversible phase transition and dielectric anomaly in a spherical molecule [(3,2,1-dabco) PbBr ]
2
6
1
1
, 2
Xiuli You , Xingcong Wang , Zhenhong Wei*
1
Jiangxi key laboratory of organic chemistry, Jiangxi science and technology normal university,
Nanchang 330013, China
2
College of chemistry, Nanchang university, Nanchang city, 330031, China
Corresponding Email: weizh@ncu.edu.cn
Abstract
Organic-inorganic hybrid materials were attracted most interests due to their potential applications in
phase transition and photoelectric fields. Within this context, a spherical molecule 1, 5-
diazabicyclo[3,2,1]octane (3,2,1-dabco) was used to react with PbBr in concentrated HBr acid and
2
led to a discrete organic inorganic hybrid compound [(3,2,1-dabco) PbBr ] (1). Thermodynamic DSC
2
6
analyses and dielectric measurement of 1 showed an apparently reversible phase transition at about
27 K. The variable temperature X-ray single crystal diffraction demonstrated compound 1 underwent
4
an order-disorder transition from orthorhombic to cubic systems at room and high temperatures.
Keywords: phase transition, dielectric anomaly, organic-inorganic hybrid, spherical molecule, crystal
structure
Introduction
Phase transition materials can absorb or release a large amount of latent heat, and it will become the
1

best green environmental protection carrier for energy saving and environmental protection[1-5]. On
the basis of composition, phase transition materials can be divided into inorganic and organic phase
transition materials, in which inorganic materials can provide good optoelectronic properties and high
thermal stability[6, 7], organic materials have an advantage on mechanical tunability and structural
multiplicity[8, 9]. In order to integrate the comprehensive functions of organic and inorganic phase
transition materials into one molecule, the organic-inorganic hybrid materials are attracted more
attentions because they are kinds of excellent composite materials with superior optical, electrical,
thermal, magnetic, and mechanical properties[10-16].
As it to know, the most common general formula of an organic-inorganic compound is ABX , where
3
A is an organic molecule, B is a metal, and X is a halogen[17-19]. The metal B and halogen X form
octahedra [BX ] through coordination, thus 3D to 1D inorganic moieties are constructed by sharing
6
edges or corners of octahedra[20-22]. The organic molecule A as the role of templating agents locate
in cavities surrounded by BX octahedra or between the inorganic layers, have considerable freedom
6
of motion, so they tend to exhibit kinetic turbulence at relatively high temperatures and become ordered
as temperatures decrease, thus leading to structure reversible phase transition. This order-disorder
phase transitions have been found in a substantial number of organic-inorganic hybrid materials[23-
2
7].
According to the numbers of inorganic dimension, the organic-inorganic hybrid compounds can be
divided into 3D, 2D 1D and 0D four categories[28-31]. As to 3D type, the number is scare, and the
famous 3D organic inorganic hybrid perovskite structure is methylammonium lead iodide (MAPbI3),
which possesses an appropriate bandgap energy and large absorption coefficient, and its photoelectric
conversion efficiency (PCE) has raised from 3.9% to 23% in just few years[32]. On the other hand,
2

the 2D layered and 1D chain organic-inorganic hybrid compound structures are intensively reported
and their processability and the adjustable structures have potential application in phase transition
materials[33-37]. However, organic-inorganic hybrid materials with lower dimensions are rare and
need to be further discovered and studied.
In recent years, with our in-depth research on organic-inorganic hybrid materials, a new discrete hybrid
compound [(3,2,1-dabco) PbBr ] with reversible phase transition was prepared by reacting a spherical
2
6
molecule 1,5-diazabicyclo[3,2,1]octane (3,2,1-dabco) with PbBr in concentrated HBr aqueous
2
solution. Herein, we report the synthesis, characterization and phase transition of compound 1.
Results and discussion
Preparation and phase transition
1
,5-diazabicyclo[3,2,1]octane (3,2,1-dabco) was a isomer of 1,4-diazabicyclo[2,2,2]octane (2,2,2-
dabco), both are spherical molecules. Reaction of [3,2,1-dabco] with PbBr gave a high phase
2
transition temperature compound [(3,2,1-dabco) PbBr ] (1) with relatively high yield. Compound 1
2
6
can be kept stability in air for a month. The differential scanning calorimetry (DSC) curve of 1 was
shown in Figure 1. During the heating and cooling runs, compound 1 exhibited an endothermic peak
at 427 K, an exothermic peak at 414 K, indicating compound 1 experienced a reversible phase
transition. Notably, the observed thermal hysteresis 13 K in the heating and cooling processes
represented a first-order phase transition[38,39]. The corresponding entropy changes (ΔS) was
-1
-1
calculated to be approximately 12.5 J mol K for the phase transition at T according to the records
c
of DSC curve. Based on the Boltzmann equation, ΔS = RlnN, (where R is the gas constant and N is
the ratio of the numbers of respective geometrically distinguishable orientations), the N is calculated
3

to be 4.5, which suggests an ordered−disordered phase transition of the spherical [3,2,1-debaco]
component in 1. Such high transition temperatures have also been found in other organic-inorganic
hybrid compound phase transition materials[40, 41].
Figure 1 The DSC curve of 1 in heating / cooling runs.
As we all know, the phase transition will be accompanied by abnormal physical properties near the
phase transition point of the structure[42, 43]. The dielectric constant of 1 was measured as a function
of temperature at 1 MHz frequency. As shown in Figure 2, the real part ε′ and imaginary part ε′′ (ε =
ε′ − iε″, where ε″ is imaginary part of ε) of dielectric constants of compacted pellets of polycrystalline
sample 1 both show a step-like change during heating and cooling with the anomalous peak at T =
c
4
27 K. The maximum peak in real part is more than 2 times larger than that of the peak valley. It is
more effective to prove that this phase transition was corresponded to the order and disorder of [3,2,1-
2
+
dabco] cations, which is consistent with the DSC analysis.
4

Figure 2. (a) Real part and (b) imaginary part of dielectric constants of 1 at 1MHz frequency in a
heating / cooling run.
Variable temperature X-ray single structure
In order to accurately analyze the origin of high phase transition, variable-temperature single X-ray
diffraction analysis of 1 was performed at 298 K and 403 K respectively. The phase above T was
c
defined as high temperature phase (HTP) and the phase below T as the low temperature phase (LTP).
c
At LTP, compound 1 crystallized in the orthorhombic system, adopted centrosymmetric Pnnm space
group (point group mmm) with a = 9.4410(5) Å, b = 13.3307(7) Å, c = 9.3549(4) Å, and V = 1177.36(10)
3
2+
4−
Å . The asymmetric unit contains half of the [3,2,1-dabco] cation and one-fourth of [PbBr ] anion
6
2
+
as shown in Figure 3. The [3,2,1-dabco] cations are located on the 2-fold rotational axis parallel to
4
−
the crystallographic b-axis, and the [PbBr ] anions are situated on the mirror plane perpendicular to
6
the b-axis and a 2-fold rotational axis passes through the axis of octahedron [PbBr ]. As shown in
6
4
−
Table 1s, in the discrete [PbBr ] anion, the octahedral geometry is slightly distorted with the Pb−Br
6
bond lengths varying from 3.0141(12) Å to 3.0157(8) Å, and the bond angles in the cis position are at
the ranges of 83.37(3)- 96.63(3)°, in the trans position are 180°, which are compared to the
2
+
analogue[44]. Interestingly, the protonated spherical [3,2,1-dabco] cation exhibits ordered state, and
5

4
−
connected with the discrete [PbBr ] anion through N−H···Br hydrogen bonds. The protonated H
6
atoms are linked to the two terminal Br atoms to give an infinite linear organic-inorganic hybrid chain
along c direction.
Figure 3 The structure of compound 1 at LTP.
At HTP, compound 1 crystallized in the cubic system, Fm3푚 space group with a = b = c = 13.4704(5)
3
4−
Å, V = 2444.2(3) Å . In the structure, the [PbBr ] octahedron exhibited regular configuration with
6
six Pb−Br bond lengths at 3.012(2) Å and the bond angles Br-Pb-Br in cis and trans positions were at
9
0° and 180° to satisfy the high symmetry sites. The central Pb2+ ion was located on the
crystallographic mirror plane, the terminal I ions are on the fourfold axis. On the contrary, the [3,2,1-
2
+
dabco] cation was located on the special high symmetry site of m3m point group, consequently
required a total disorder. Because of the inconsistency between crystallographic symmetry and
2
+
molecular symmetry, the [3,2,1-dabco] cation was modeled with a spherical structure discarding its
chemical sense, Figure 4. This suggests that the cation with dynamic disorder is nearly freely rotating
at the HTP, which was also observed in those with spherical geometries, such as [3-oxo-
6

quinuclidinium]ClO [45] and [MeHdabco]RbI [46]. Thus, the order-disorder transition of organic
4
3
cation induces the phase transitions in compound 1.
Figure 4. The structure of compound 1 at HTP
Experimental
Synthesis: 1,5-diazabicyclo[3.2.1]octane was synthesized according to the reported literature by
reactions of homopiperazine with formaldehyde aqueous solution in toluene/H O systems at 130 ℃
2
for six hours[47]. The obtained 1,5-diazabicyclo[3.2.1]octane (0.112 g, 1 mmol) was reacted with
PbBr (0.3610 g, 1.0 mmol) in 47% hydrobromic acid solution (10.0 mL) on the heating of 60 ℃ for
2
half hour, leading to the formation of white block crystals 1, which were isolated by filtration and dried
under vacuum. Yield: 0.52 g, 65%. Anal. Calcd for C H N PbBr : C 15.75, H 3.08, N 6.12%. Found:
1
2
28
4
6
C 15.49, H 3.13, N 5.93%.
Measurements. The methods of DSC, dielectric experiments were described in detail previously.
Intensity data of structure 1 were collected at 298 K and 400 K on a Rigaku synergy diffractometer
using Mo Kα (λ = 0.71075 Å) radiation. The structures were solved by the direct methods and refined
2
by full-matrix least-squares refinement base on F with the SHELXL program[48]. Selected
7

crystallographic data and structure refinement are listed in Table 2s.
Conclusion
In summary, a new discrete organic-inorganic hybrid compound [(3.2.1-dabco) PbBr ] (1) exhibited a
2
6
reversible first-order phase transition which was proved by DSC and dielectric constant. The variable
temperature single crystal structure analysis revealed the origin of the reversible phase transition was
2
+
attributed to order and disorder of spherical geometry of [3.2.1-dabco] cation at LTP and HTP.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (21661021, 21865015).
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1
2

Author contributions
Zhenhong Wei designed research;
Xiuli You and Xingcong Wang performed research and solved the crystal structure.
1
3

Declaration of Interest Statement
We declare that we have no financial and personal relationships with other
people or organizations that can inappropriately influence our work, there is no
professional or other personal interest of any nature or kind in any product, service
and/or company that could be construed as influencing the position presented in, or the
review of the manuscript entitled.
Zhenhong Wei (on the behalf of all the authors)
Nanchang University
1
4

Graphic abstract
A discrete organic-inorganic hybrid compound [(3.2.1-dabco) PbBr ] exhibited a reversible phase
2
6
2
+
transition which was originated from the order to disorder of spherical [3.2.1-dabco] cation at RTP
and HTP.
1
5

Highlights
1
. A spherical molecule 1, 5-diazabicyclo[3,2,1]octane was chose to react with PbBr to give an
2
organic-inorganic compound.
2
. The obtained hybrid compound showed a reversible phase transition.
3
. The variable temperature X-ray single crystal diffraction revealed the phase transition was derived
from the order and disorder of spherical molecule at room temperature and high temperature.
1
6