Restraining the motion of a ligand for modulating the structural phase transition in two isomorphic polar coordination polymers

Article abstract of DOI:10.1039/c4dt00675e

A structural phase transition induced by ligand motion was found in a new polar coordination polymer: [Cu(NCS)₂(4-APy)₂]_n (4-APy = 4-aminopyridine). Restraining such motion in an isomorphic compound [Cu(NCS)₂(4-MeAPy)₂]_n (4-MeAPy = 4-methylaminopyridine) results in distinct phase transition behaviour. These findings provide a new clue for modulating phase transition behaviour in known materials. the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00675e

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Restraining the motion of a ligand for modulating
the structural phase transition in two isomorphic
polar coordination polymers†
Cite this: Dalton Trans., 2014, 43,
9008
Received 6th March 2014,
Accepted 21st March 2014
a
a
a
a
a,b
Bao-Ying Wang, Wei-Jian Xu, Wei Xue, Rui-Biao Lin, Zi-Yi Du,
a
a
a
DOI: 10.1039/c4dt00675e
Dong-Dong Zhou, Wei-Xiong Zhang* and Xiao-Ming Chen
www.rsc.org/dalton
A structural phase transition induced by ligand motion was found molecular level so as to design and synthesize new, desired
7
in a new polar coordination polymer: [Cu(NCS)
2
(4-APy)
2 n
] (4-APy = phase transition materials. To date, even though various types
4
-aminopyridine). Restraining such motion in an isomorphic com- of structural phase transitions have been documented in CPs,
8
2 2 n
pound [Cu(NCS) (4-MeAPy) ] (4-MeAPy = 4-methylaminopyri- such as order–disorder of guest, and the displacement
dine) results in distinct phase transition behaviour. These findings of some organic moieties, their microscopic mechanistic
provide a new clue for modulating phase transition behaviour in details are still unclear. In this scenario, studying
9
known materials.
molecular motion in the lattice of those CPs will increase our
knowledge on phase transitions, as well as their utility for new
devices.
During our research on molecular motion in specific con-
fined environments in CPs, a structural phase transition
induced by ligand motion was found in a polar chain complex:
Over the past decades, solid structural phase transitions have
attracted considerable interest not only because of their funda-
mental significance as a ubiquitous critical phenomenon, but
10
1
also for their potential technological applications in the develop-
ment of switching dielectrics, sensors, smart materials and
[
Cu(NCS)
ing this motion of the ligand in an isomorphic compound
Cu(NCS) (4-MeAPy) ] (2) (4-MeAPy = 4-methylaminopyridine),
2 2 n
(4-APy) ] (1) (4-APy = 4-aminopyridine). By restrain-
2
tiny machines. On a microscopic level, the concerted mole-
cular motion, which accompanies large deformations and/or
distortions of the lattice, usually plays a significant role in trig-
gering such structural phase transitions. Therefore, the poten-
tial to rationally control molecular motion is crucial for the
design and construction of a material with a desired structural
phase transition.
motion of protons causes distinct ferroelectric phase tran-
sition behaviour. Controlling the 180° flip-flop molecular
random motion of a rotator gives rise to a phase transition
from disordered phase at high temperature to ordered phase
at low temperature. However, up to now, it remains a major
[
2
2 n
distinct phase transition behaviour was obtained.
Green block crystals of 1 were obtained by evaporation of a
methanol solution containing stoichiometric Cu(SCN)2 and
3
4-APy ligands. In situ single-crystal X-ray analyses revealed that
3
b,4
For example, modulating the dynamic
1
crystallizes in the orthorhombic space group Cmc2 at 353 K
1
(
the high temperature phase, 1α) and in the space group Pbn2
11
1
5
at 103 K (the low temperature phase, 1β). In 1α, as shown in
Fig. 1a, the Cu1 atom is located at a crystallographic mirror
plane, and is approximately in square-pyramidal coordination
geometry. Four nitrogen atoms from two 4-APy ligands and
4
6
challenge to manifest molecular motion in solids.
−
two NCS groups form the basal plane [Cu–N 1.947(3)–2.027(3)
Currently, coordination polymers (CPs) have received con-
siderable attention because of their diversity of structures and
Å], forming a Cu(NCS) (4-APy) unit. A sulfur atom (S1B) from
2
2
the neighbouring Cu(NCS)
2
2
(4-APy) unit occupies the apical
1
functions. Their designable/tuneable characteristics enable
position of the Cu1 atom [Cu1–S1B 2.879(2) Å], and thus, these
Cu(II) ions are linked into one-dimensional square-wavy chains
CPs to be a promising platform for controlling motion at the
with a [–Cu–NCS–] repeating unit along the c axis by bridging
−
μ
1,3-NCS groups (Fig. 1b). Moreover, the Cu1 atom also weakly
a
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory
interacts with the sulfur atom from the adjacent bridging
NCS groups [Cu⋯S1C 3.531(2) Å]. It seems that the S1 atom
might coordinate to two Cu atoms with equal Cu⋯S dis-
tances. However, the 4-APy ligands array along the chain in a
typical herringbone fashion (Fig. S5†), restricting the distance
of Photoelectronic Materials and Technologies, School of Chemistry & Chemical
Engineering, Sun Yat-Sen University, Guangzhou, 510275, P. R. China.
E-mail: zhangwx6@mail.sysu.edu.cn
−
2
+
b
College of Chemistry & Chemical Engineering, Gannan Normal University, Ganzhou
3
†
41000, P. R. China
Electronic supplementary information (ESI) available: Experimental details and
2
+
between the S1-atom-interacting Cu
ions [Cu1B⋯Cu1C
characterization. CCDC 980360–980363. For ESI and crystallographic data in CIF
or other electronic formats see DOI: 10.1039/c4dt00675e
6.397(1) Å]. Thus the shorter and longer Cu⋯S distances are
9008 | Dalton Trans., 2014, 43, 9008–9011
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general positions, due to the loss of mirror symmetry at low
temperature (Fig. S7†). The local coordination environment of
the Cu1 atom was found to be slightly changed, and the 1D
2 n
polar polymeric nature of the [Cu(NCS) ] chain was retained
in 1β, though the intrachain Cu1–S1B bond length was
reduced to 2.834(1) Å and the weak-coordinated Cu1⋯S1C dis-
tance was reduced to 3.486(1) Å (Fig. 1e). Moreover, the
observed Flack parameter of 0.02(1) in 1β indicates that the
polarity did not change during the phase transition.
Thermogravimetric (TG) analyses reveal that 1 is stable up
to 393 K. Moreover, in the differential scanning calorimeter
(
3
DSC) plots, an endothermic/exothermic peak appeared at
41.8/338.7 K in the heating/cooling run (Fig. S6 and S8†), con-
firming the reversibility of the phase transition for 1. The
lambda-shaped peaks and minor heat hysteresis (ca. 3 K)
imply the phase transition might be a continuous one. Given
that the entropy change ΔS = R ln N (R is the gas constant, and
N is the ratio of possible configurations in the disordered
system), ΔS is 0.36 J mol−
1
K
−1
and N is close to 1.0, further
Fig. 1 The coordination environments of the Cu(II) ion in 1α (a) and 1β
(
d). The one-dimensional chain of 1α (b) and 1β (e). The packing struc-
supporting the second-order characteristic for this structural
phase transition. In addition, the phase transition was further
confirmed for 1 by variable-temperature powder X-ray diffrac-
tion analysis (Fig. S9†).
ture of 1α (c) and 1β (f) viewed along with the c axis. Hydrogen bonds
are represented by red dotted lines. Symmetry codes: (A) 1 − x, y, z; (B)
1
− x, 2 − y, 0.5 + z; (C) 1 − x, 2 − y, −0.5 + z.
During the structural transition from 1α to 1β, a notable
displacement of the NCS− groups from the mirror plane
2 n
alternately present, eventually resulting in a polar [Cu(NCS) ]
−
chain. Besides that, terminal NCS groups were found to be passing through the [Cu(NCS) ] chains was found. In 1β, the
2
n
−
2
-fold disordered about the mirror plane. Due to the inter- bridging NCS groups markedly deviated from the least-squares
chain hydrogen-bonding interactions between the terminal plane of the N1, Cu1, N2 and S1A atoms [0.805(1) Å for S1 and
−
−
NCS groups and the amino groups of the two adjacent chains 0.343(2) Å for C1, respectively]. Moreover, the terminal NCS
3.426(6) Å for N4#–H⋯S2, Fig. 2a and Table S3†], each chain groups become ordered in 1β with deviations from this plane
[
interacts with four neighbouring chains with the same [0.185(1) Å for S2 and 0.053(2) Å for C2, respectively]. As a
polarity, resulting in a polar three-dimensional (3D) supra- result, two different interchain hydrogen-bonding interactions
molecular network (Fig. 1c). The observed Flack parameter of are found [3.545(3) Å for N5#–H⋯S2 and 3.490(4) Å for N6$–
0
.03(2) indicates the pure polarity of the entire tested single H⋯S2, respectively, see Fig. 1f]. This fact implies that the
motion of the NCS− groups, especially the hydrogen-bonding
crystal.
Compared to that of 1α, as depicted in Fig. 1d, the asym- involved terminal NCS− groups, may be the essential reason
metric unit in 1β doubled and all the atoms appeared at the for the phase transition.
Fig. 2 From the perspective view of the 3D packing along the b axis, the terminal NCS− groups were found to be 2-fold disordered in 1α (a) but
ordered and restrained by methyl groups from the adjacent chains in 2 (b) at 353 K. The dash lines stand for the hydrogen-bonding interactions. For
clarity, the space-filling spheres of terminal NCS− groups and amino/methylamino groups are set at 30% transparent, and the carbon and hydrogen
atoms of the pyridyl groups are shown in pale green for the farther chains and gray for the closer chains. Symmetry codes: for 1α, (#) −0.5 + x, 1.5 −
y, −0.5 + z; ($); 1.5 − x, 1.5 − y, −0.5 + z; for 2, (#) 0.5 + x, 1.5 − y, 0.5 + z; ($) 0.5 − x, 1.5 − y, 0.5 + z.
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In order to gain further insight into the phase transition such as polarity are retained even though the phase transition
details of 1, we modified the hydrogen donor by introducing a behaviour is tuned.
methylamino group, and obtained its isomorphic compound
In conclusion, a structural phase transition induced by
−
2
, supposing that motion of the terminal NCS groups may be ligand motion has been demonstrated in a 3D hydrogen-
suppressed by the methyl group, and thus, result in distinct bonded polar supramolecular network consisting of one-
phase transition behaviour. Indeed, for 2, the single-crystal dimensional Cu(II) chains. Moreover, restraining the motional
diffractions at 103 and 353 K revealed that it crystallizes in the groups via steric hindrance resulted in distinct phase tran-
space group Cmc2 , the same as that of 1α. Similar to 1α, at sition behaviour in its isomorphic compound. This fact
1
3
53 K, polar [Cu(NCS)
2
]
n
chains with alternate weak- demonstrates that a slight modulation of the specific confined
coordinated Cu⋯S interactions [3.884(2) Å] and Cu–S co- environment around the motional units is a moderate way to
ordinated bonds [2.774(2) Å] are found in 2 (Fig. S10†). The control the motion of ligands and to tune the structural phase
interchain hydrogen-bond interactions and the resulting 3D transition yet keep the crystal packing unchanged. The present
supramolecular network of 2 are almost the same as those of strategy provides a new clue for the design and synthesis of
−
1
α, except that the terminal NCS groups in 2 are found to be new phase transition materials or tuning phase transition
orderly located on the mirror plane at 353 K. As shown in behaviours for known materials.
−
Fig. 2b, each terminal NCS group is attracted by two N–H
groups via hydrogen-bonding interactions, and is repulsed by
two methyl groups from the opposite side via steric exclusion.
−
As a result, the motion of each terminal NCS group is
Acknowledgements
restrained along the a and c directions, thus the structure of 2
was found to be almost unchanged when the temperature was This work was supported by the NSFC (21290173, 21121061
lowered to 103 K. The absence of a phase transition in the and 21301198), the 973 Project (2012CB821706), and NSF of
temperature region of 200–373 K was further confirmed by Guangdong (S2012030006240). W.-X. Z. is grateful for “100
DSC measurements for 2 (Fig. S11†). Though a phase tran- Talents Program of SYSU” initial funding.
sition of 2 might occur at a lower temperature, which was not
detected in our investigated temperature range (T > 103 K), the
phase transition temperature of 2 must be at least 237 K lower
than that of 1. This fact clearly indicates that the microscopic
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9010 | Dalton Trans., 2014, 43, 9008–9011
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