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Cite this: Chem. Sci., 2022, 13, 11422
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Coordination polymer-forming liquid Cu(2isopropylimidazolate)†
Teerat Watcharatpong, ‡a Taweesak Pila, ‡a Thana Maihom, bTomohiro Ogawa, c Takuya Kurihara, c Koji Ohara, d Tadashi Inoue,
Hiroyasu Tabe, c Yong-Sheng Wei, c Kanokwan Kongpatpanich *aand Satoshi Horike *acfThestructure

of

the

melt

state

of

one-dimensional(1D)

coordinationepolymercrystalCu(isopropylimidazolate) (melting temperature Tm ¼ 143  C) was characterized by DSC, variabletemperature PXRD, solid-state NMR (SSNMR), viscoelastic measurements, XAS, and DFT-AIMD
Received 8th June 2022
Accepted 9th September 2022

calculations. These analyses suggested “coordination polymer-forming liquid” formation with preserved

DOI: 10.1039/d2sc03223f

adjacent chains are the keys to the rarely observed polymer-forming liquid. The melt structure isreminiscent of the common 1D organic polymer melts such as entanglement or random coil structures.

rsc.li/chemical-science

coordination bonds above Tm. Variable chain conﬁgurations and moderate cohesive interaction in

Introduction
In the molten state of representative one-dimensional (1D)
organic polymers such as polyethylene, polypropylene, and soon, they take random coil structures and form entanglement
networks if their molar mass is high enough.1 Isotropic andexible structures are possible due to the strong covalent bonds
with sp3 carbons and resulting high internal freedom of thepolymer chains. Understanding the structure and dynamics oforganic polymer melts has been essential for materials engineering such as bers by melt spinning, and mechanically
tough lms by oriented crystallization of melts.2
Recently, crystal melting and glass formation of coordination polymers (CPs) or metal–organic frameworks (MOFs) havegained much attention.3–12 The CP/MOF glasses are regarded asa

Department of Materials Science and Engineering, School of Molecular Science andEngineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210,
Thailand. E-mail: horike@icems.kyoto-u.ac.jpbDepartment of Chemistry, Faculty of Liberal Arts and Science, Kasetsart University,
Kamphaengsaen Campus, Nakhon Pathom 73410, ThailandcInstitute for Integrated Cell-Material Sciences-VISTEC Research Center, Institute forAdvanced Study, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501,
Japan

a new family of glass types,13 with unique non-crystalline
properties including gas permeability, conductivity, phaseswitching, emission, and encapsulation.11,14–19 Although there
are many studies about the structure of these glasses, thestructural investigation of the melt state is yet limited. In thecase of three-dimensional (3D) frameworks, including[Zn(imidazolate)1.75(benzimidazolate)0.25](ZIF-62),20
thebridging ligands facilitate the partial breaking of coordination
bonds upon melting, and thus promote a lower fusion ofenthalpy DHfus.9,21,22 For example, the reduction in the averagecoordination number of 3D MOF (ZIF-4, [Zn(imidazolate)2])
upon melting is 10–18% by MD simulations.21,23 The strength ofcoordination bonds is smaller than that of covalent bonds, andthe network exibility of 3D CP/MOFs is smaller than that of sp3
carbon-based organic polymers in general.
1D CPs are structurally more related to the common 1D
organic polymers, but there is no report that the melt state iscomposed of polymeric structures, because of the same reasons
as for 3D frameworks. It is a challenge to nd CPs whose melt
state is composed of polymeric structures without bondbreaking. The polymer-forming melt behavior is relevant toorganic polymers, and it would open various ways of materials
fabrication of CP/MOF bers and lms, and engineering suchas injection molding.

dDiﬀraction and Scattering Division, Japan Synchrotron Radiation Research Institute(JASRI), Sayo 679-5198, Hyogo, Japane

Department of Macromolecular Science, Graduate School of Science, Osaka University
Toyonaka, Osaka 657-0043, Japan

Results and discussionf

To nd the coordination polymer-forming melt, we employed
1D CPs having a variety of polymer congurations with strong
coordination bonds. We selected a 1D CP Cu(ipim) (1, Hipim ¼
2-isopropylimidazole).24 The previous paper reported the crystal
structure and diﬀerential scanning calorimetry (DSC) of 1. The 1

Department of Synthetic Chemistry and Biological Chemistry, Graduate School ofEngineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan† Electronic supplementary information (ESI) available: Synthesis, TGA, PXRD,
PDF analysis, MD simulation, time-temperature superposition master curves,and EXAFS tting parameters. See https://doi.org/10.1039/d2sc03223f‡ These authors contributed equally to this manuscript.

11422 | Chem. Sci., 2022, 13, 11422–11426© 2022 The Author(s). Published by the Royal Society of ChemistryView Article Online

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shows an endothermic peak at 143  C in the DSC upscan suggesting the crystal melting. The unique feature of the crystal
structure of 1 is that the unit cell contains 6.5 crystallographically independent Cu(ipim) units to form wavy 1D chains(Fig. 1A).
Each chain is packed in parallel to one another through vander Waals interactions. One of the imidazolate rings showsa two-site disorder. The crystal structure has eight diﬀerent
angles of N–Cu–N, ranging from 172.2 to 180.0 (Fig. S10A).†
The dihedral angle between adjacent imidazolato rings also
varies signicantly. These structural features suggest that the
1D chains in 1 form a variety of conformations to enlarge thefusion of entropy DSfus, although strong metal–azolate bonds
would preserve the polymeric structure. According to the literature, the crystalline powder of 1 was synthesized andconrmed by powder X-ray diﬀraction (PXRD).24 The SEM imageof 1 (Fig. 1B) showed a brous morphology reected by theanisotropic 1D chain assembly. Thermogravimetric analysis(TGA) of 1 (Fig. S1†) showed no weight loss up to 340  C indicating that the composition of 1 does not change up to thistemperature. DSC of 1 (Fig. 1C) with in situ sample imaging
showed a sharp endothermic peak onset at 143  C, indicating
the crystal melting of 1. Hereaer, we call the melt state of 1 as
1-melt. DHfus calculated by DSC is 6.2 kJ mol1 and the entropyof fusion DSfus is 14.9 J mol1 K1. They are comparable to thoseof ZIF-62 (DHfus ¼ 3.5 kJ mol1, DSfus ¼ 5.0 J mol1 K1) whichis composed of strong Zn2+–N bonds.12,25 In the cooling processof 1-melt in DSC, a sharp exothermic peak was observed at
112  C (DHcrys ¼ 5.0 kJ mol1, DScrys ¼ 13.8 J mol1 K1). The
peak corresponds to the crystallization of 1-melt to 1, as we
observed an identical PXRD pattern for the cooled sample at
25  C.
The facile crystallization of 1-melt to 1 by gentle cooling isunusual among the other reported CP/MOFs.12 Most of the melt
states of the reported CP/MOFs transform to the glassy state by

Chemical Science
cooling because of high viscosity, and one example has clearlyshown the crystallization from the melt so far.9 We conducted
a rapid quenching of the 1-melt by soaking in liquid N2, but it
showed crystallization as conrmed by PXRD. Four heating/cooling cycles of DSC are shown in Fig. 1C, which show thesame traces for all the cycles. PXRD of 1 at 25  C aer four cycles
of DSC measurements was identical to that of 1 (Fig. S2).† Thus,
the crystal melting and recrystallization behavior are reversible.
Variable temperature (VT)-PXRD was performed to investigate the structural transformation from 1 to 1-melt (l ¼ 0.99998
Å, Fig. 1D). PXRD patterns of 1 in the range of 25 to 120  C were
unchanged. In contrast, we observed a mixed pattern of crystalline and amorphous states at 150  C which is just above themelting temperature Tm (143  C) by DSC. This indicates thecoexistence of amorphous (melt) and crystalline states.
Although we should take diﬀerent temperature ramping rates of
DSC and PXRD into account, the mixed phase at near Tm would
be intrinsic for 1 and it reminds us of the imperfect crystallinityof common organic polymers. PXRD patterns above 175  Csuggest that the sample totally transformed to 1-melt. VT 1H
magic-angle spinning solid-state NMR was performed to study
the structure and dynamics of 1 and 1-melt (Fig. 1E). Proton
peaks at 1.4, 3.2, and 6.6 ppm correspond to the –CH3, alkyl CH
and N–CH, and NH groups of the ipim ligand, respectively. Eachpeak is broadened at 30  C, and spinning side-bands areobserved in the broad frequency region, suggesting the low
mobility of the structure. The spectra were unchanged at 120  C
indicating that the structure and dynamics of 1 are intact as
supported by VT-PXRD. The spectrum of 1-melt at 157  Cshowed sharp peaks in addition to the broad peaks. The sharp
peaks indicate the enhanced mobility of the ligands due to the
crystal melting. The broad residual peaks are ascribed to microcrystals remaining above Tm, which is also consistent with VTPXRD. The sharp peaks have spinning side-bands, suggesting
the presence of weak anisotropic nuclear spin interactions (i.e.

(A) Crystal structure. Cu: orange; C: gray; N: dark blue and (B) SEM image of 1. Isopropyl groups are omitted. (C) DSC proﬁles on the ﬁrst
cycle (sky blue) and the second to fourth cycles (red) of 1. 10 K min1 under N2 ﬂow. (D) VT-PXRD of 1. Wavenumber was l ¼ 0.99998 Å. (E) 1H
magic-angle spinning solid-state NMR of 1 at temperatures of 30, 60, 90, 120, and 157  C. Asterisk denotes spinning sidebands.
Fig. 1© 2022 The Author(s). Published by the Royal Society of ChemistryChem. Sci., 2022, 13, 11422–11426 | 11423

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1

H dipolar interaction and/or chemical shi anisotropy). Thismeans that the dynamics of the ligands in 1-melt are anisotropic even under the melt state.
X-ray scattering measurements were performed to obtainpair distribution function (PDF) data of 1 and 1-melt at various
temperatures (Fig. 2A). A simulated PDF prole from a single
crystal X-ray structure of 1 (Fig. S4A and S5†) was used to assign
the peaks. The dominant components of three prominent peaks
at 6.10, 11.6, and 17.1 Å observed at 30  C are the nearest
neighbor, second-neighbor, and third-neighbor correlations of
two intrachain Cu+ ions respectively (①  ③). We also foundthe correlations of N–N overlap in the peaks ① and ② though
the intensities are lower than those of Cu–Cu. This is because N
atoms are the rst neighboring atoms and coordinate with Cu+
ions in a linear fashion. These three peaks ①  ③ in G(r) aremeasured at 175 and 195  C which are above Tm are shown. The
peak intensities and position of the nearest intrachain Cu–Cu
neighbor (①) at 175 and 195  C are unchanged as those at 30  C.
This suggests that the Cu-ipim-Cu bridges remain above Tm.
The intensities of the two peaks of second/third neighbors ②
and ③ at 175 and 195  C are reduced compared with the case at
30  C, suggesting lower structural periodicity due to thermal
activation.
To distinguish the contributions of both intrachain andinterchain Cu–Cu to the PDF results, we generated a PDF proleby creating a modied crystal structure of 1 leaving only theFig. 2 (A) Partial crystal structure of 1 showing the intrachain Cu–Cu
correlations at approximately ① 6.10, ② 11.6, ③ 17.1 Å and radial
distribution function of 1 at 30  C and 1-melt at 175, 195 and 215  C. (B)Distribution plots of Cu–Cu distances of 1(model) (blue) and 1melt(model) (red) generated from MD simulation. S.D. is the standard
deviation, and N is the number of the population.

11424 | Chem. Sci., 2022, 13, 11422–11426

Edge Article
atoms forming a single chain to exclude the interchain correlations (Fig. S4B).† The 7.45 and 13.5 Å peaks corresponding toCu–Cu are not observed. This suggests that these peaks are
attributed to interchain Cu–Cu. In Fig. 2A, the peak at 7.45 Å is
suﬃciently intense at 175  C, while the 13.5 Å peak is signicantly attenuated. At 215  C, both peaks almost disappear.
Considering that peaks ① and ② retain some intensity at
215  C, it is suggested that the correlations in interchain Cu–Cu
tend to be smaller than those in intrachain Cu–Cu upon heating. We also observed the shi of these two peaks ② and ③ tobe shorter upon heating, and this is because of the higher
degree of conformation. The presence of these correlation
peaks above Tm indicates that the 1-melt is composed of coordination polymers having suﬃcient molecular weight.
Theoretical calculations were conducted to understand thestructural transformation upon melting. DFT was applied tostructural optimization of the crystal structure of 1, and then ab
initio molecular dynamics (AIMD) simulations were executed at
25 and 175  C named as 1(model) and 1-melt(model), respectively (Fig. S6–S8).† We found that the intramolecular reduced
pair distribution function G(r) produced from 1-melt(model) issimilar to that of 1 (Fig. S9†) by observing peaks at 6.1, 11.6, and17.1 Å in Fig. 2A. This indicates that the 1-melt(model)
successfully describes the structure in 1-melt. A distribution of
N–Cu–N bond angles of 1 (model) and 1-melt(model) (Fig. S13†)
shows that the larger standard deviation in 1-melt(model)
indicates the greater structural conformation of the 1-D chain
upon melting. Fig. 2B shows the distribution of Cu–Cu
distances in 1(model) and 1-melt(model). 1-melt(model) has
a broader distribution and it also supports the higher disorderof the melt state with a 1D chain conguration.
We measured variable temperature X-ray absorption spectroscopy (XAS) at 25, 175, 300, 90, and 25  C to quantify thecoordination numbers (CNs) of Cu+ using 1(model) and 1melt(model). The results are summarized in Table S1.† The CNs
are estimated to be 1.97 at 175  C and 1.79 at 300  C. Thissuggests that the CN is almost unchanged above Tm and this
supports the discussion about the retention of the 1D chain
structures above Tm without bond breaking. The lower CN at
300  C increases upon cooling and it is 1.98 at 25  C. This
indicates that the coordination bonds of N–Cu reform below Tmbecause of less thermal activation.
Viscoelastic properties of 1 and 1-melt were studied further
to support the discussion of the structure of 1-melt. The
samples were loaded into a rheometer at 250  C and cooled
stepwise. The strain sweep and frequency sweep test results
under the isothermal conditions are shown in Fig. 3A and B,
respectively. In the melting state, the real part of the complex
modulus, G0 , is approximately 106 Pa, and did not depend on
the frequency (Fig. 3A). This indicates that the molten state hasa high viscosity and requires a certain external force to deform.
This property suggests the elasticity and the existence of the
network structure. The storage modulus of the compoundshould be much lower in case the structure does not remain as
a 1D network. In addition, careful observation revealed that themoduli at diﬀerent temperatures could superpose with eachother with a horizontal shi and make a composite curve© 2022 The Author(s). Published by the Royal Society of ChemistryView Article Online

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Chemical Sciencevariable temperature PXRD, SSNMR, XAS, viscoelastic
measurements, and DFT-AIMD calculations suggested that the“polymer-forming liquid” is the most probable structure above
Tm. It is essentially diﬀerent from 3D MOF cases which requirebond-breaking upon melting. Variable chain congurations
attributed to the linear coordination geometry and moderatecohesive interaction in adjacent chains are the keys to thepolymer-forming liquid. The melt structure is reminiscent of
the common 1D organic polymer melts, and the utilization of
the polymer melt would expand the materials engineering
including oriented, mechanically tough lms or bers.

Author contributions
T. W. and T. P.: data curation, investigation, validation, writing– original dra. T. M. and T. O. and T. K. and K. O. and T. I. and
H. T. and Y. -S. W.: formal analysis, investigation, resources,
validation, writing – review & editing. K. K. and S. H.: conceptualization, funding acquisition, project administration,
supervision, writing – review & editing. All authors criticallyreviewed and revised the manuscript dra and approved thenal version for submission.

Conﬂicts of interest(A) Frequency sweep and (B) strain sweep viscoelastic
measurements of 1 from 235 to 115  C under N2 ﬂow. (C) Fibers of 1
prepared by a melt-quench process by use of hot press and collectedby spinning around the glass tube.
Fig. 3(master curve, Fig. S14),† which is called the time–temperature
superposition principle (method of reduced variables). Thesefeatures are characteristic of ordinary exible organic polymericsystems such as polystyrene. On the other hand, the linear
viscoelastic regime was less than 1% and decreased slightly withdecreasing temperature (Fig. 3B). The stronger strain dependence of modulus than that in exible polymers suggests thatthe network in the molten state is an entangled network of stiﬀ
chains. Another possibility is that the system contains anaggregate of solid particles.26 These observations lead to the
proposed model that there are both microcrystalline and melt
phases in this temperature region, which coincides with PXRD
and SSNMR results. The rheological characterization also suggested that the structure of 1-melt is composed of long linear
molecules. We prepared ber-shaped samples from hot-pressed
1-melt by use of a rheometer machine. The brous sample wascollected by pulling up 1-melt (Fig. 3C), and we can collect thesebers by use of a glass tube. The ber sample showed crystalline
PXRD the same as the simulated pattern of the crystal structureof 1 (Fig. S15†).

There are no conicts to declare.

Acknowledgements
The work was supported by the Japan Society for the Promotionof Science (JSPS) for a Grant-in-Aid for Scientic Research (B)(JP18H02032),
Challenging
Research(Exploratory)(JP19K22200), Transformative Research Areas (A) “Supraceramics” (JP22H05147) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan and theMinistry of Higher Education, Science, Research and Innovation (RGNS 63-251) and the National Research Council ofThailand (No. N42A650196). We acknowledge Prof. Kazuki
Nakanishi for valuable discussion, Dr Tsuyoshi Watanabe for
XAS measurements at SPring-8, and Ms. Vetiga Somjit and Mr
Kotaro Ohara for the processing of XAS data. The synchrotron
radiation experiments were performed using BL02B2 and
BL14B2 beamlines at SPring-8 with the approval of JASRI(Proposal Nos. 2019B1093 and 2022A1813). The computational
resources were provided by the ASEAN HPC Fugaku Trial 2022
by the A*STAR Computational Resource Centre. This work used
computational resources of the supercomputer Fugaku
provided by RIKEN through the HPCI System Research Project(Project ID: hp220211).

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Conclusions
The work elucidated that the melt state of 1D CP Cu(ipim)(Hipim ¼ 2-isopropylimidazole) is composed of polymeric
networks without signicant coordination bond breaking. DSC,

© 2022 The Author(s). Published by the Royal Society of Chemistry

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