Dynamic Motion of Organic Ligands in Polar Layered Cobalt Phosphonates

Article abstract of DOI:10.1002/chem.201801301

By introducing the polar methoxy group into phenyl- or benzyl-phosphonate ligands, four cobalt phosphonates with layered structures are obtained, namely, [Co(4-mopp)(H₂O)] (1), [Co(4-mobp)(H₂O)] (2), [Co(3-mopp)(H₂O)] (3),

Full text of DOI:10.1002/chem.201801301

A Journal of
Accepted Article
Title: Dynamic Motion of Organic Ligands in Polar Layered Cobalt
Phosphonates
Authors: Zhong-Sheng Cai, Norihisa Hoshino, Song-Song Bao, Jiage
Jia, Tomoyuki Akutagawa, and Li-Min Zheng
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201801301
Link to VoR: http://dx.doi.org/10.1002/chem.201801301
Supported by

10.1002/chem.201801301
Chemistry - A European Journal
FULL PAPER
Dynamic Motion of Organic Ligands in Polar Layered Cobalt
Phosphonates
Zhong-Sheng Cai,[a] Norihisa Hoshino,[b] Song-Song Bao,[a] Jiage Jia,[a] Tomoyuki Akutagawa,*[b] and Li-Min
Zheng*[a]
Abstract: By introducing polar methoxy group into phenyl- or benzyl-phosphonate ligands, four cobalt phosphonates with layered structures
are obtained, namely, Co(4-mopp)(H2O) (1), Co(4-mobp)(H2O) (2), Co(3-mopp)(H2O) (3) and Co(3-mobp)(H2O) (4), where 4- or 3-moppH2 is
(4- or 3-methoxyphenyl)phosphonic acid and 4- or 3-mobpH2 is (4- or 3-methoxybenzyl)phosphonic acid. Compounds 1, 2 and 4 crystallize in
polar space groups Pmn21 or Pna21, while compound 3 crystallizes in a centrosymmetric space group P21/n. The layer topologies in the four
structures are similar and can be viewed as perovskite-type, where the edge-sharing [Co4O4] rhombuses are capped by the PO3C groups. The
phenyl and Me-O groups in compounds 1-3 are heavily disordered, while that in 4 is ordered. Structural comparison based on the data at 296
K and 123 K reveals distinct dynamic motion of the organic groups in compounds 1 and 2. The fluctuation of polar Me-O groups in these two
compounds is confirmed by the dielectric relaxation measurements. In contrast, the fluctuation of polar groups in compounds 3 and 4 is not
evident. Interestingly, the dehydrated samples of 3 and 4 (3-de and 4-de) exhibit one-step and two-step phase transitions associated with the
motion of polar organic groups, as proved by the DSC and dielectric measurements. Magnetic properties of compounds 1-4 are investigated,
and strong antiferromagnetic interactions are found to mediate between the magnetic centers through -O(P) and O-P-O bridges.
organic ligands.[20-26] However, the rotation frequency of the
ligands can be affected or controlled by the structures such as the
shape and volume of the pores, temperature and other external
Introduction
stimulus. In some cases, temperature dependent order-disorder
transition occurs for the polar rotation components leading to
interesting dielectric or ferroelectric properties.[27-30] Further, when
Studies on the dynamic motions in crystalline solids have been a
challenging but very interesting task for the development of
molecular machines and devices.[1-3] While most of the work have
been focused on the discrete molecules, for example molecular
gyroscopes,[4-8] much less has been explored for materials with
extended network structures.[1,9,10]
Metal-organic frameworks which show precise structures that
can be designed and constructed through judicious choice of
metal ions and ligands offer an unprecedented structural and
chemical diversity. A number of new architectures with various
pore sizes, shapes and chemical functions have been obtained,
which show promising potential applications in gas storage and
separation,[11-14] catalysis,[15,16] proton conduction[17,18] and
magnetism[19] etc. MOFs can also be good candidates for
studying the molecular motor dynamics because of their porosity,
thermal stability and flexibility, which allow the free rotation of
a
paramagnetic metal ion is introduced to the system,
multifunctional materials can be obtained.[31-33] Nevertheless,
dynamic motion on non-porous layered MOF materials has rarely
been documented.[34-36]
Scheme 1. The chemical structures of the ligands.
[a]
Dr. Z.-S. Cai, Prof. Dr. S.-S. Bao, Jiage Jia, Prof. Dr. L.-M. Zheng
State Key Laboratory of Coordination Chemistry, School of
Chemistry and Chemical Engineering, Collaborative Innovation
Center of Advanced Microstructures
Metal phosphonates are an important class of organic-
inorganic hybrid materials showing versatile structures and
intriguing functions.[37,38] Most of them were reported to exhibit
layered or pillared layered structures in which the inorganic layers
of M-PO3 with different topologies are separated by the organic
components. The layered inorganic backbone can provide rigid
frames to serve as the stator, and the organic part can undergo
rotation between the layers. Thus layered or pillared layered metal
phosphonates can provide good candidates for constructing
amphidynamic crystals. Surprisingly, however, dynamic motions
of organic ligands in metal phosphonate systems have never
been described thus far, partly because of their dense and
Nanjing University
Nanjing 210023, P. R. China
E-mail: lmzheng@nju.edu.cn
http://hysz.nju.edu.cn/lmzheng/
Dr. Norihisa Hoshino, Prof. Dr. Tomoyuki Akutagawa
Institute of Multidisciplinary Research for Advanced Materials
(IMRAM), Tohoku University,
Sendai 980-8577, Japan
E-mail: akuta@tagen.tohoku.ac.jp
[b]
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/chem.201801301
Chemistry - A European Journal
FULL PAPER
Table 1. Crystallographic data for compounds 1-4 at 296 K.
Compound
Formula
M
1
2
3
4
C7H9CoO5P
263.04
C8H11CoO5P
277.07
C7H9CoO5P
263.04
C8H11CoO5P
277.07
crystal system
space group
a / Å
orthorhombic
Pmn21 (No. 31)
5.6146(8)
17.656(3)
4.8382(7)
90
orthorhombic
Pmn21 (No. 31)
5.7121(6)
17.8425(19)
4.8150(5)
90
monoclinic
P21/n (No. 14)
4.8553(4)
32.281(2)
5.6773(4)
91.788(1)
889.39(11), 4
1.964
orthorhombic
Pna21 (No. 33)
35.022(3)
4.8180(5)
5.7163(5)
90
b / Å
c / Å
 / 
V / Å3, Z
Dc / g cm-3
479.62(13), 2
1.820
490.74(9), 2
1.875
964.55(15), 4
1.908
F(000)
266
282
532
564
GoF on F2
R1, wR2[a] [I>2
(all data)
1.000
1.001
1.007
0.970

(I)]
0.0289, 0.0661
0.0315, 0.0672
0.54, -0.60
1828984
0.0466, 0.0866
0.0640, 0.0956
0.118, -0.875
1828986
0.0339, 0.0658
0.0559, 0.0721
0.446, -0.405
1529373
0.0487, 0.1196
0.0632, 0.1313
0.883, -0.576
1828990
(

)
,
min / e Å-3
max
CCDC number
aR1 = Σ||Fo| - |Fc||/Σ|Fo|, wR2 = [Σw(Fo2 - Fc2)2/Σw(Fo2)2]1/2
Table 2. The Co-O and Co···Co distances (Å) and O-Co-O and Co-O-Co angles () in structures 1-4 at 296 K.
Compound
Co-O
O-Co-O
Co···Coa
3.729(1)
∠Co-O-Co
1
2
3
4
2.059(1) - 2.229(1)
2.056(5) - 2.226(5)
2.055(2) - 2.278(2)
2.049(5) - 2.267(5)
66.8(1) - 175.7(2)
66.5(2) - 174.8(3))
66.3(1) - 174.1(1)
66.7(2) - 173.3(2)
120.6(1)
122.7(1)
3.759(1)
3.701(1), 3.815(1)
3.760(1)
119.6(1), 122.0(1)
121.1(1), 123.9(1)
a The Co···Co distance over -O bridge
nonporous nature and the presence of inter-molecular
interactions which block the free motion of the organic moieties
within the structure. To promote the rotation frequency of the
organic groups in metal aromatic phosphonates, the introduction
of a proper substituent into the aromatic rings can be a useful
strategy to weaken the inter-molecular interactions such as -
interactions. In particular, when the substituent is polar, dielectric
or even ferroelectric properties associated with order-disorder
phase transitions could be observed. Following this train of
thought, we designed and synthesized four aromatic
phosphonate ligands containing para- or meta-substituted polar
Me-O group, namely, (4- or 3-methoxyphenyl)phosphonic acid [4-
or 3-moppH2] and (4- or 3-methoxybenzyl)phosphonic acid [4- or
3-mobpH2] (Scheme 1). The latter two are more flexible due to the
presence of an additional methyl group.
In this paper, we report four cobalt phosphonates based on
these phosphonate ligands, formulated as Co(4-mopp)(H2O) (1),
Co(4-mobp)(H2O) (2), Co(3-mopp)(H2O) (3) and Co(3-
mobp)(H2O) (4). Single crystal structural determinations reveal
that compounds 1, 2 and 4 are polar. While compound 3 is non-
polar, the structure of which has already been described in a
recent work.[39] All four compounds display similar layered
structures with the layer topology of perovskite-type. The organic
groups in compounds 1-3 are heavily disordered at both 296 K
and 123 K, but those in 4 are well ordered even at 296 K. The
anisotropic parameter analysis reveals distinct fluctuation of the
Ph-OMe groups in the para-substituted compounds 1 and 2, also
supported by dielectric measurements. More interestingly, the
dehydrated samples 3-de and 4-de show temperature dependent
dielectric anomalies related to phase transitions. Finally, the
magnetic properties of 1-4 are studied, and dominant
antiferromagnetic interactions are found in all compounds.
Results and Discussion
Description of structures 1-4
Single crystal structures of compounds 1-4 were measured at 296
K. Although structure 3 at room temperature was already reported,
the crystallographic data and selected structural parameters of all
four compounds are given for a comparison (Tables 1 and 2). It is
clear that three of the four structures (1, 2, 4) are polar, while
compound 3 is centrosymmetric.
Compound 1 crystallizes in the polar space group Pmn21 (No.
31). Its asymmetric unit consists of one CoII, one 4-mopp2- and
one coordinated water molecule (Figure S3). The Co1 atom has
a distorted octahedral geometry surrounded by six oxygen atoms,
five from phosphonate ligands (O1A, O1B, O1C, O1D and O2)
and one from water molecule (O1W). The Co-O bond lengths fall
in the range of 2.059(1) - 2.229(1) Å. The 4-mopp2- serves as a
penta-dentate ligand, two of its three phosphonate oxygen atoms
act as -O bridges to connect the CoII ions. An inorganic layer
structure is thus constructed which contains edge-sharing [Co4O4]
This article is protected by copyright. All rights reserved.

10.1002/chem.201801301
Chemistry - A European Journal
FULL PAPER
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
(k)
(l)
Figure 1. Structures of compounds 1 (a, b, c), 2 (d, e, f), 3 (g, h, i) and 4 (j, k, l). Ellipsoid representation of the ligands showing the motion of the disordered atoms
and phenyl groups (a, d, g, j), single inorganic layers in the ac or bc plane (b, e, h, k) and packing diagrams viewed along the c-axis (c, f) or a-axis (i) or b-axis (l).
rhombus (Figure 1). Within the rhombus, the Co···Co distance
over -O is 3.729(1) Å and the Co-O-Co angle is 120.6(1). The
layer is not flat, but slightly undulated along the b-axis. The PO3C
groups are capped on the rhombus with the organic tails filling in
the interlayer space (Figure 1c). The layer topology can be
approximately described as a perovskite-type, similar to those of
M(PhPO3)(H2O) (M = Mg, Zn, Mn, Ca, Cd).[40, 41] Notably, the
neighboring phosphonate ligands are apart from each other by
4.838(1) Å such that - interactions are not present in the
structure. The phenyl and Me-O groups are disordered over two
sites (Figure 1a). Van der Waals interactions are dominant
between the layers.
This article is protected by copyright. All rights reserved.

10.1002/chem.201801301
Chemistry - A European Journal
FULL PAPER
The structure of compound 2 is similar to that of 1 except that
the benzyl group replaces the phenyl group in 1 (Figure 1d). It
also crystallizes in the polar space group Pmn21, and shows a
layer structure containing edge-sharing [Co4O4] rhombus.
Compared with those in 1, the Co···Co distance over the -O
bridge [3.759(1) Å] and the Co-O-Co angle [122.7(1)] are slightly
larger, while the adjacent P···P distance [4.815(1) Å] is slightly
shortened. More significant difference is that the Ph-OMe group
as a whole are disordered over two sites in 2, unlike compound 1
in which the phenyl and Me-O groups are disordered individually
(Figure 1).
U33 > U22. Hence the fluctuation of the Me-O group is speculated
to occur in the ac plane (Figure 1d). At 123 K, the U11 values of
the disordered atoms become much smaller, resulting in
parameters U11 ≈ U22 ≈ U33. The nearly isotropic behavior
suggests the blocking of the motion at low temperature.
In the case of compound 3, the tensor values of the disordered
atoms at 296 K follow the sequence of U11 ≈ U33 > U22 (Table
S11). However, the difference between the U11 (or U33) and U22
values is much smaller than those in 1 and 2, indicating a slight
fluctuation of the atoms in the ac plane. At 123 K, the values of
the tensors decrease (U11 < U22 ≈ U33), revealing the blocking
of fluctuation.
When meta-substituted Me-O groups are introduced into the
pheny or benzyl phosphonate ligands, compounds 3 and 4 are
obtained. Although both show analogous layer structures,
compound 4 crystallizes in a polar space group Pna21 (No. 33),
while 3 crystallizes in a centrosymmetric space group P21/n. In
these two cases, the [Co4O4] rhombus becomes slightly distorted.
For 3, the Co···Co distances are 3.701(1) and 3.815(1) Å and the
Co-O-Co angles are 119.6(1) and 122.0(1). For 4, the edge
distance is the same [Co···Co: 3.760(1) Å] but the Co-O-Co
angles are 121.1(1) and 123.9(1). The dangling Ph-OMe groups
are disordered over three different sites in 3, but completely
ordered in 4 (Figure 1).
Dielectric properties
The comparison of anisotropic parameters for structures 1-3 at
296 and 123 K reveals the distinct fluctuation of the phenyl and
Me-O groups in the ac plane in compounds 1 and 2. The free
rotation of the organic groups can be monitored using techniques
2
such as solid state H NMR,[21-26] neutron inelastic scattering,[43]
Terahertz vibration,[44] and dielectric measurements.[27-30] The
latter concerns with the reorientational motions of the dipolar
groups.[45] For compounds 1-4, although para-substituted phenyl
groups are apolar, the Me-O and meta-substituted phenyl groups
are polar. Therefore, dielectric relaxation properties are
investigated to probe the influence of the substitutional position of
the grafted Me-O groups on the ligand dynamics.
Comparison of anisotropic parameters of 1-3 at 296
K and 123 K
Considering that the organic groups of the phosphonate ligands
in compounds 1-3 are heavily disordered, we measured the three
structures at 123 K to check whether an order-disorder phase
transition occurs at lower temperature (Tables S5-S8). The
results demonstrate that all three structures remain almost
unchanged upon cooling to 123 K with only tiny modification of
their structural parameters. Nevertheless, a careful analysis of the
structures reveals that the shapes of the atomic ellipsoids of the
disordered atoms exhibit visible changes on cooling, indicating a
temperature dependent dynamic motion of these atoms.
Table S9 shows the anisotropic parameters of the disordered
atoms in 1, e.g. the U11, U22 and U33 tensors which correspond
to fluctuations along the a-, b- and c-axis, respectively.[42] At room
temperature, the tensor values for the C1 atom are 0.030, 0.022
and 0.022 Å2, indicating an isotropic behavior. For C4 to C8 atoms
of the phenyl group, the U11 values are 3-4 times U22 or U33
suggesting a strong anisotropic fluctuation along the a-axis. For
C2 and C3, the values are U11 ≈ U33 > U22. Similar situation is
found for the polar Me-O group. The tensor values follow the
sequence of U11 >> U33 > U22 for the C11, C12 and O3 atoms.
These results suggest that the phenyl and Me-O groups in
compound 1 can fluctuate in the ac plane at room temperature
(Figure 1a).
When the temperature is lowed to 123 K, the U11 tensor values
for the atoms C4 to C8, C11, C12 and O3 decrease significantly,
associated with the increase of the corresponding U22 values.
Obviously, the dynamic motions of these atoms are hampered at
low temperature.
For compound 2, the tensor values of the disordered atoms of
the benzyl group (C1 to C7) at 296 K follow the sequence of U11 >>
U22 ≈ U33 (Table S10), which is consistent with the pendulum-
like movements of the benzyl groups in 2. For the atoms of the
Me-O group (C8 and O3), the tensor values follow the order U11 >>
(a)
(b)
(c)
(d)
Figure 2. The ” vs. T plots at various frequencies for compounds 1 (a), 2 (b), 3
(c) and 4 (d).
The dielectric measurements were performed on compounds
1-4 using pressed plate samples in the frequency range of 31.6 -
1000 kHz and temperature range of 170 - 250 K. Figures 2a and
2b show the temperature dependent imaginary signals (”) of the
dielectric constant for compounds 1 and 2. At a fixed frequency
(f), the curves show maximum at a certain temperature (ca. 200 –
220 K) in both cases. The maximum is related to the dipolar
This article is protected by copyright. All rights reserved.

10.1002/chem.201801301
Chemistry - A European Journal
FULL PAPER
relaxation process in the samples, i.e., the rotation of the polar
Me-O groups. When the frequency of the applied electric field is
(a)
(b)
increased, the maximum of the peak shifts to
a higher
temperature. This kind of frequency dependent behavior indicates
a thermally activated single relaxation process.[24] Thus the
relaxation rate (1/), characterizing the dielectric relaxation
process, should follow the Arrhenius law (eq. 1),
Ea
1

(1)
0 exp(
)
kBT
where kB corresponds to the Boltzmann’s constant, T is the
temperature, Ea is the activation energy for the relaxation process,
and 0 is the pre-exponential factor. The ” vs. T plots will reach
a maximum when  = 1, where  is the angular frequency (=
2f). Thus, the frequency of the applied field can be used to
estimate the relaxation rate at the temperature where ” shows
the maximum. The obtained Ea are 20.6 and 21.5 kcal/mol, and
0 are 3.8×1026 and 1.4×1028 s-1 for compounds 1 and 2,
respectively (Figure S7). The energy barriers are larger than
those reported in the literature for the flip or rotational motion of
the aromatic rings in different MOFs (4.0 – 15.1 kcal/mol),[20-26]
which can be explained by the fact that the layer structures of 1
and 2 are more dense than those of MOFs. The latter often
contains micro-pores to impart substantial free volume around the
rotator ligands.
Notably, the Ea values for the two compounds are quite close
to each other, but the 0 are significantly different by two orders
of magnitude. The larger 0 in compound 2 suggests a faster
dipolar relaxation process related to Me-O groups, which is
attributed to their structural difference. In 2, the (4-methoxybenzyl)
phosphonate ligand has an additional –CH2- group comparing
with the (4-methoxyphenyl)phosphonate ligand in 1, which
increases the flexibility and hence the rotation of the organic
groups.
(c)
(d)
Figure 3. Temperature- and frequency-dependent dielectric constants together
with the DSC diagrams in the heating process for compounds 3-de (a) and 4-
de (b). Temperature- and frequency-dependent dielectric constants for
compounds 3 (c) and 4 (d) are also presented as a comparison.
the fluctuation of the organic moieties.
Thermal analyses were performed under nitrogen flow in the
temperature range 30 – 500 C (Figure S8). The weight losses
between 128 and 174 C are 6.7% for 1, 6.6% for 2, 7.0% for 3
and 6.2% for 4, corresponding to the release of one water
molecule (calcd. 6.8% for 1 and 3, 6.5% for 2 and 4). Above 174
C, a wide plateau is observed until the compounds are
decomposed at ca. 370 C for 1 and ca. 410 C for 2-4.
Remarkably, the simultaneously measured heat-flow curves
(Figure S9) reveal an endothermic peak at 248 C for 3, and two
endothermic peaks at 203 and 245 C for 4. On the contrary, no
peak appears in the range 200 - 350 C for 1 and 2. It is evident
that phase transitions occur in the dehydrated samples of 3 and 4
(abbreviated as 3-de and 4-de) above 200 C, but not in those of
1 and 2 (abbreviated as 1-de and 2-de).
To further confirm the phase transition, DSC measurements
were conducted for the four dehydrated samples obtained by
heating the samples at 180 C for one hour. No peak is observed
for compounds 1-de and 2-de (Figure S10) and their PXRD
patterns remaining the same between 180 and 300 C (Figure
S11), thus excluding any phase transitions in this temperature
range for the two samples.
For compound 3-de, the heating and cooling processes give a
distinct endothermic peak at 251 C and an exothermic peak at
229 C, respectively (Figures 3 and S12). Clearly a phase
transition occurs which is reversible upon heating and cooling.
The phase transition can be monitored by the variable
temperature PXRD measurements. As shown in Figure S13, the
PXRD pattern of 3-de (after heating 3 to 180 C) is quite similar
to that of the pristine compound 3, indicating that the original
layered structure is maintained after dehydration. However, when
the temperature is increased to 220 C, additional diffraction
For compound 3, the ” value increases only slightly with
increasing temperature (Figure 2c), in agreement with the
centrosymmetric nature of the structure. Although compound 4 is
polar, the ” signals are much weaker than those observed in 1
and 2. An unobtrusive maximum appears at ca. 200 K when the
frequency reaches 1000 kHz (Figure 2d). The frequency
dependence of the ” signals is not evident below 250 K, attributed
to the ordering arrangement of the organic groups in 4.
Phase transitions in the dehydrated samples of
compounds 3 and 4
Both the structural and dielectric measurements reveal that the
dynamic motion of the Me-O groups can more easily occur in the
para-substituted aromatic metal phosphonates of 1 and 2 than in
the meta-substituted ones of 3 and 4. This could be due to the
larger steric hindrance in the meta-substituted compounds,
providing less free volume proximate to the rotator. It is noticed
that all of compounds 1-4 contain a coordination water molecule
pointing out to the interlayer space. It forms hydrogen bonds with
the phosphonate oxygen atoms within the inorganic layer, and
also with the –CH- moieties from the phenyl or benzyl groups with
the shortest C(H)···O distance being 3.550 Å for 1, 7.287 Å for 2,
3.347 Å for 3, and 3.635 Å for 4. The removal of the coordination
water molecule could weaken the interactions between the
organic groups and the layered inorganic backbone, and facilitate
This article is protected by copyright. All rights reserved.

10.1002/chem.201801301
Chemistry - A European Journal
FULL PAPER
peaks appear at 11.3 and 16.9, the intensity of which increases
with increasing temperature. In contrast, the peak intensities at
11.1 and 16.6 diminish and finally disappear at 280, forming a
new phase of the material. Unfortunately, we are not able to solve
the crystal structure of this new phase because its single
crystallinity cannot be maintained after dehydration and phase
transition processes.
45.5
K
for compounds 1-4, respectively, indicating the
propagation of moderate to strong AF interactions between the
cobalt(II) centers.[46] In order to estimate the strength of AF
interactions, one simple phenomenological eq. 2 can be used,[52]
(2)
T  Aexp(E1 /kT)Bexp(E2 /kT)
where A+B equals the Curie constant (C), and E1 and E2
correspond to “activation energy” related to the spin-orbital
coupling and to the AF interaction, respectively. As shown in
Figure 4, all the experimental data of the present work are well
fitted by this model. The obtained parameters are summarized in
Table 3. The E1/k values are consistent with those give in the
literature for both the effects of spin-orbital coupling and site
distortion (E1/k of the order of 100 K).[53] For E2/k, compounds 1
and 3 have similar values of 7.2 and 7.4 K, smaller than that for
compounds 2 and 4 (8.6 and 8.1 K). This result suggests the
strength of the AF coupling in the four compounds having the
sequence of J1 ≈ J3 < J2 ≈ J4, consistent with the sequence for
AF ordering temperatures (6.5, 10, 6 and 9 K based on the peak
of M vs. T plots) (Figure S17).
The magnetization (M) curves were measured at 1.9 K for the
four compounds (Figure S19). In all cases, the magnetizations per
Co increase almost linearly with increasing magnetic field and
reach values of 0.74, 0.74, 0.90 and 0.80 N at 70 kOe for 1-4,
respectively. These values are much smaller than the saturation
value of ca. 2.3 N for CoII (Seff = 1/2, g = 4.6). This difference is
in line with the strong AF interaction as well as anisotropic effect
in the compounds.[54]
The DSC curve for compound 4-de is more complicated (Figure
S14). Two successive endothermic peaks are observed at 202
and 245 C in the heating process, suggesting the presence of
two-step phase transitions. This is confirmed by the variable
temperature PXRD measurements (Figure S15). At 180 C, the
diffraction peak at 5.30o splits into two peaks at ca. 5.08o and
5.38o, indicating the coexistence of two phases. The intensity of
the former increases with increasing temperature. This process
should attribute to the first phase transition in compound 4-de.
When the temperature reaches 240 C, the diffraction peak at
5.08 further splits into two peaks, corresponding to the second
phase transition. Surprisingly, in the cooling process of the DSC
measurement three exothermic peaks around 238, 195 and 138
C are observed, suggesting three successive phase transitions
upon cooling. The curve measured in the temperature range 100
- 220 oC shows a pair of peaks at 202 C and 138 C, indicating
that the transition at 138 oC corresponds to the first transition at
202 C in the heating mode.
In order to get insight into the origin of these phase transitions,
dielectric relaxation spectroscopy was measured. The real
component of the dielectric constant (’) would show drastic
changes when the phase transition arises from the motion and /
or ordering of the polar groups. Indeed, both 3-de and 4-de show
frequency-independent anomalies (Figures 3 and S16). The
maxima are at ca. 263 C for 3-de, and 201 and 251C for 4-de,
in agreement with the DSC and PXRD measurements upon
heating. The results demonstrate that the motion of the polar
phenyl and Me-O groups should be responsible to the phase
transitions in compounds 3-de and 4-de.
Magnetic properties
The cobalt compounds are magnetically attractive due to the
presence of strong spin-orbit coupling of the CoII ion.[46] A number
of cobalt phosphonates have been reported to show interesting
magnetic behavior such as ferrimagnetism, weak ferromagnetism
and metamagnetism.[47,48] Polar cobalt compounds can offer
unique opportunities for exploring new multifunctional materials
such as multiferroic materials combining polarity and
magnetism.[49-51] Bearing this in mind, we also studied the
magnetic properties of compounds 1-4. Figures 4 and S17 show
the temperature dependent magnetic susceptibilities measured
under a dc field of 1 kOe and a temperature range of 1.9 - 300 K.
At room temperature, the mT products for compounds 1 - 4 are
3.08, 3.20, 3.38 and 3.31 cm3 mol-1 K, respectively. These values
are much larger than the spin-only value of 1.88 cm3 mol-1 K for
isolated CoII ions (S = 3/2 and g = 2.0), attributed to the orbital
contribution of the octahedral cobalt(II) ion. Upon cooling, the mT
values decrease continuously and approach to 0.10, 0.09, 0.13
and 0.10 cm3 mol-1 K at 1.9 K for the four compounds, arising from
antiferromagnetic (AF) interactions and / or spin-orbital couplings
in the materials. The Weiss constants () based on the Curie-
Weiss fitting of the data above 80 K are -84.1, -46.7, -59.1 and -
Figure 4. The mT vs. T plots of the four compounds, solid lines are the best fit
based on eq. 2.
Table 3. The obtained parameters based on eq. 2
A
[cm3 mol-1 K]
2.46
B
[cm3 mol-1 K]
1.28
-E1/k
[K]
-E1/k
[K]
Compound
1
2
3
4
-89.4
-65.4
-78.2
-64.2
-7.2
-8.6
-7.4
-8.1
2.20
1.50
2.42
1.56
2.30
1.52
Proper magneto-structural relationship should be built to
explain the change of AF ordering temperature from compound to
compound. Generally, the magnetic coupling mediated through
O-P-O bridge is weak. In this case, the -O(P) bridges should play
a key role for the mediation of magnetic interactions. Within the
layer, the shortest Co···Co distances of the compounds are in the
range of 3.70 - 3.82 Å (Table 2). The similar distances in
This article is protected by copyright. All rights reserved.

10.1002/chem.201801301
Chemistry - A European Journal
FULL PAPER
compounds 2 - 4 are not in accordance with their difference in AF
ordering temperature. While the average Co-O-Co angles of
compounds 1 and 3 are similar and 2o smaller than the similar
values of 2 and 4. The strength of AF coupling can be dependent
on the value of Co-O-Co angles, the larger values in compounds
2 and 4 will result in stronger AF interactions or higher AF ordering
temperatures and vice versa.[55]
Synthesis of the phosphonate ligands. The ligands were
synthesized through traditional Arbuzov reaction with or without the
catalyst of NiCl2. For (4- or 3-methoxyphenyl)phosphonic acid, 11 mmol
triethyl phosphite was drop-wise added to a mixture of 10 mmol bromide
and 1 mmol NiCl2 in 100 mL three-necked bottle under the flow of N2 at
150 oC during a period of ca. 0.5 hour. The mixture was further heated for
5 hours, and then cooled to room temperature. To the brown mixture was
added 50 mL 1 mol/L HCl and the mixture was extracted by ethyl acetate
(50 mL × 3). The organic layers were combined and dried by Na2SO4.
Afterwards, the solvent was removed by distillation, and 20 mL
concentrated HCl was added to the resulted oil, and the mixture was
refluxed for 24 hours. Pale yellow solids were obtained after removing the
solvent, which were washed by CH3CN and dried in air resulting in a white
solid. Yield: 63.2 % and 66.8 % for 4-moppH2 and 3-moppH2, respectively.
Elemental anal. calcd. for C7H9O4P: C, 44.69; H, 4.82 %. Found: C, 43.95;
H, 4.87 % (for 4-moppH2); C, 43.99; H, 4.89 % (for 3-moppH2).
Conclusions
We report for the first time the dynamic motions of the organic
groups in layered metal phosphonate systems. The structures of
four cobalt phosphonate compounds Co(4-mopp)(H2O) (1), Co(4-
mobp)(H2O) (2), Co(3-mopp)(H2O) (3) and Co(3-mobp)(H2O) (4),
where 4- or 3-moppH2 is (4- or 3-methoxyphenyl)phosphonic acid
and 4- or 3-mobpH2 is (4- or 3-methoxybenzyl)phosphonic acid,
have been studied at 296 and 123 K. All bear similar layer
topologies containing edge-sharing [Co4O4] rhombuses.
Structural comparison of the anisotropic parameters obtained at
296 and 123 K indicates the fluctuation of the phenyl and Me-O
groups in 1 and 2. The motion of polar Me-O groups in the two
compounds is further confirmed by dielectric measurements.
Considering that compounds 1 and 2 differ from 3 and 4 by the
substitution positions of the Me-O group (para- or meta-), the
dynamic motion of the organic groups in metal aromatic
phosphonates can be tuned by the positional isomerism of the
ligand which cause different steric hindrance. Remarkably, the
dehydrated compounds 3-de and 4-de show one- or two-step
phase transitions above 200 oC, evidenced by both DSC and
dielectric measurements. The phase transitions could be
associated with the order-disorder or positional changes of the
polar phenyl and methoxy groups. The work provides successful
examples for the exploration of dynamic ligand motions in
nonporous layered metal phosphonates.
The (4- or 3-methoxybenzyl)phosphonic acid was synthesized based
on similar route without NiCl2 catalyst. Yield: 84.6 % and 89.0 % for 4-
mobpH2 and 3-mobpH2, respectively. Elemental anal. calcd. for C8H11O4P:
C, 47.53; H, 5.48 %. Found: C, 47.20; H, 5.08 % (for 4-mobpH2); C, 47.13;
H, 5.67 % (for 3-mobpH2).
Synthesis of Co(4-mopp)(H2O) (1). CoCl2·6H2O (1.0 mmol, 234 mg)
and 4-moppH2 (0.5 mmol, 92 mg) were added to 4 mL distilled water and
the pH value was adjusted to 3.8 - 4.0 with 0.5 M NaOH. Then the mixture
was transferred to a 24 mL Teflon-lined autoclave and kept at 100 oC for
two days. After cooling to room temperature, purple-red polycrystals were
obtained and washed by distilled water. Yield: 16.5 mg (13 %, based on 4-
moppH2). The PXRD measurement confirmed the purity of the phase.
Elemental anal. calcd. for C7H9CoO5P: C, 31.96; H, 3.45. Found: C, 32.57;
H, 3.79 %. IR (cm-1): 3463(m), 3007(w), 1599(s), 1572(m), 1502(m),
1460(m), 1290(m), 1254(s), 1182(m), 1141(s), 1097(s), 1028(m), 964(s),
926(m), 696(m).
Synthesis of Co(4-mobp)(H2O) (2). The procedure was similar to the
synthesis of compound 1 except that 4-mobpH2 was used instead of 4-
moppH2. Yield: 23.5 mg (17 %, based on 4-mobpH2). Elemental anal. calcd.
for C8H11CoO5P: C, 34.68; H, 4.00. Found: C, 35.39; H, 4.30 %. IR (cm-1):
3466(m), 2952(w), 1612(m), 1511(s), 1418(w), 1301(m), 1244(s), 1175(w),
1153(w), 1089(s), 1033(m), 961(s), 841(m), 820(w), 780(w), 704(w),
673(w).
Experimental Section
Synthesis of Co(3-mobp)(H2O) (4). CoCl2·6H2O (1.0 mmol, 234 mg)
and 3-mobpH2 (0.5 mmol, 92 mg) were added to 4 mL distilled water and
the pH value was adjusted to 3.8 - 4.0 with 0.5 M NaOH. Then the mixture
was transferred to a 24 mL Teflon-lined autoclave and kept at 100 oC for
one day. After cooling to room temperature, purple-red polycrystals were
obtained and washed by distilled water. Yield: 19.7 mg (14 %, based on 3-
mobpH2). Elemental anal. calcd. for C8H11CoO5P: C, 34.68; H, 4.00. Found:
C, 34.91; H, 4.00 %. IR (cm-1): 3422(m), 2979(w), 1594(s), 1492(m),
1457(m), 1432(w), 1319(w), 1278(w), 1247(s), 1162(m), 1086(s), 1036(m),
961(s), 861(m), 796(m), 770(w), 737(w), 688(m).
Materials and physical measurements: The reagents were obtained
from commercial sources and used without further purification. Compound
Co(3-mopp)(H2O) (3) was synthesized according to the previously
reported method.[39] Elemental analyses for C, H, N were performed on an
Elementar Vario MICRO elemental analyzer. The infrared spectra were
recorded on a Bruker Tensor 27 spectrometer with pressed KBr pellets in
the 400 - 4000 cm-1 region. Thermogravimetric analyses were performed
on a Mettler Toledo TGA/DSC 1 STARe instrument in the range of 30-500
oC under a nitrogen flow at a heating rate of 10 oC min-1. Powder X-ray
diffraction patterns (PXRD) were recorded on a Bruker ADVANCE D8
instrument using Cu-K radiation. The magnetic susceptibility data were
obtained using polycrystalline samples by a Quantum Design MPMS
SQUID VSM magnetometer. DSC curves were measured on a Mettler
Toledo DSC823e instrument under a nitrogen flow at a heating rate of 10
oC min-1. The temperature-dependent dielectric constants were measured
using the two-probe AC impedance method at frequencies from 1 kHz to
1 MHz (Hewlett-Packard, HP4194A) using the temperature controller of a
Linkam LTS-E350 system. The electrical contacts were prepared using
gold paste (Tokuriki 8560) to attach the 25-m  gold wires to the 3-mm 
compressed pellet.
Single-crystal structure determination. Single crystals of dimensions
0.5 × 0.05 × 0.05 mm3 for 1, 0.2 × 0.05 × 0.05 mm3 for 2, 0.05 × 0.04 ×
0.03 mm3 for 3 and 0.04 × 0.04 × 0.05 mm3 for 4 were selected for indexing
and intensity data collection on a Bruker APEX II CCD diffractometer using
graphite-monochromatized Mo K radiation ( = 0.71073 Å) at 296 K and
123 K. A hemisphere of data were collected using a narrow-frame method
with scan widths of 0.30° in  and an exposure time of 40 s/frame. The
data were integrated using the Siemens SAINT program,[56] with the
intensities corrected for Lorentz factor, polarization, air absorption, and
absorption due to variation in the path length through the detector face
plate. Absorption corrections were applied. The structures were solved by
This article is protected by copyright. All rights reserved.

10.1002/chem.201801301
Chemistry - A European Journal
FULL PAPER
direct methods and refined on F2 by full-matrix least-squares using
SHELXTL.22.[57] All the non-hydrogen atoms were located from the Fourier
maps, and were refined anisotropically. The disordered atoms in the
compounds were refined anisotropically. The crystallographic data of the
compounds are listed in Table 1 and S5. The selected bond lengths and
angles are given in Tables S1-S4 and S6-S8.
[26] C. S. Vogelsberg, F. J. Uribe-Romo, A. S. Lipton, S. Yang, K. N. Houk,
S. Brown, M. A. Garcia-Garibay, Proc. Natl. Acad. Sci. USA 2017, 114,
13613–13618.
[27] W. Zhang, H.-Y. Ye, R. Graf, H. W. Spiess, Y.-F. Yao, R.-Q. Zhu, R.-G.
Xiong, J. Am. Chem. Soc. 2013, 135, 5230−5233.
[28] W.-Y. Zhang, Y.-Y. Tang, P.-F. Li, P.-P. Shi, W.-Q. Liao, D.-W. Fu, H.-Y.
Ye, Y. Zhang, R.-G. Xiong, J. Am. Chem. Soc. 2017, 139, 10897−10902.
[29] P. Jain, N. S. Dalal, B. H. Toby, H. W. Kroto, A. K. Cheetham, J. Am.
Chem. Soc. 2008, 130, 10450 – 10451.
[30] G.-C. Xu, X.-M. Ma, L. Zhang, Z.-M. Wang, S. Gao, J. Am. Chem. Soc.
2010, 132, 9588 - 9890.
Acknowledgements
[31] G.-C. Xu, W. Zhang, X.-M. Ma, Y.-H. Chen, L. Zhang, H.-L. Cai, Z.-M.
Wang, R.-G. Xiong, S. Gao, J. Am. Chem. Soc. 2011, 133, 14948 –
14951.
Financial support by the National Natural Science Foundation of China
(21731003) and the National Key R&D Program of China
(2017YFA0303203) is acknowledged.
[32] P. Jain, V. Ramachandran, R. J. Clark, H. D. Zhou, B. H. Toby, N. S.
Dalal, H. W. Kroto, A. K. Cheetham, J. Am. Chem. Soc. 2009, 131, 13625
– 13627.
Keywords: Molecular motion • layer • cobalt phosphonate •
dielectric • magnetic • polar
[33] Y. Tian, S. Shen, J. Cong, L. Yan, S. Wang, Y. Sun, J. Am. Chem. Soc.
2016, 138, 782 - 785.
[34] B. Kundys, A. Lappas, M. Viret, V. Kapustianyk, V. Rudyk, S. Semak, C.
Simon, I. Bakaimi, Phys. Rev. B, 2010, 81, 224434.
[1]
[2]
C. S. Vogelsberg, M. A. Garcia-Garibay, Chem. Soc. Rev. 2012, 41,
1892–1910.
[35] A. O. Polyakov, A. H. Arkenbout, J. Baas, G. R. Blake, A. Meetsma, A.
Caretta, P. H. M. Van Loosdrecht, T. T. M. Palstra, Chem. Mater. 2012,
24, 133 – 139.
P. H. Colberg, S. Y. Reigh, B. Robertson, R. Kapral, Acc. Chem. Res.
2014, 47, 3504 - 3511;
[36] W.-Q. Liao, Y. Zhang, C.-L. Hu, J.-G. Mao, H.-Y. Ye, P.-F. Li, S. D. Huang,
R.-G. Xiong, Nat. Commun. 2015, 6, 7338.
[3]
[4]
[5]
M. G. L. van den Heuvel, C. Dekker, Science 2007, 317, 333 - 336.
C. J. Bruns, J. F. Stoddart, Acc. Chem. Res. 2014, 47, 2186 - 2199.
T. Akutagawa, H. Koshinaka, D. Sato, S. Takeda, S. Noro, H. Takahashi,
R. Kumai, Y. Tokura, T. Nakamura, Nat. Mater. 2009, 8, 342 – 347.
H. Kitagawa, Y. Kobori, M. Yamanaka, K. Yoza, K. Kobayashi, Proc. Natl.
Acad. Sci. USA 2009, 106, 10444 - 10448;
[37] A. Clearfield, K. Demadis, Metal Phosphonate Chemistry: From
Synthesis to Applications, 2012, Royal Society of Chemistry.
[38] S.-S. Bao, G. K. H. Shimizu, L.-M. Zheng, Coord. Chem. Rev. 2018, DOI:
10.1016/j.ccr.2017.11.029
[6]
[7]
[8]
[9]
[39] Z.-S. Cai, Y. Shi, S.-S. Bao, Y. Shen, X.-H. Xia, L.-M. Zheng, ACS Catal.
2018, 8, 3895−3902.
Q.-C. Zhang, F.-T. Wu, H.-M. Hao, H. Xu, H.-X. Zhao, L.-S. Long, R.-B.
Huang, L.-S. Zheng, Angew. Chem. Int. Ed. 2013, 52, 12602 - 12605.
W. Li, C.-T. He, Y. Zeng, C.-M. Ji, Z.-Y. Du, W.-X. Zhang, X.-M. Chen, J.
Am. Chem. Soc. 2017, 139, 8086−8089.
[40] G. Cao, H. Lee, V. M. Lynch, T. E. Mallouk, Inorg. Chem. 1988, 27, 2781-
2785.
[41] G. Cao, V. M. Lynch, L. N. Yacullo, Chem. Mater. 1993, 5, 1000-1006.
[42] M. Tadokoro, Y. Ohhata, Y. Shimazaki, S. Ishimaru, T. Yamada, Y.
Nagao, T. Sugaya, K. Isoda, Y. Suzuki, H. Kitagawa, H. Matsui, Chem.
Eur. J. 2014, 20, 13698 - 13709.
S. Bracco, M. Beretta, A Cattaneo, A Comotti, A Falqui, K. Zhao, C.
Rogers, P. Sozzani, Angew. Chem. Int. Ed. 2015, 54, 4773 - 4777.
[10] A. Comotti, S. Bracco, A. Yamamoto, M. Beretta, T. Hirukawa, N. Tohnai,
M. Miyata, P. Sozzani, J. Am. Chem. Soc. 2014, 136, 618−621.
[11] J.-R. Li, R. J. Kuppler, H.-C. Zhou, Chem. Soc. Rev. 2009, 38, 1477 –
1450.
[43] D. I. Kolokolov, H. Jobic, A. Stepanov, M. Plazanet, M. Zbiri, J. Ollivier,
V. Guillerm, T. Devic, C. Serre, G. Férey, Eur. Phys. J. Special Topics
2010, 189, 263.
[12] H. Furukawa, U. Müller, O. M. Yaghi, Angew. Chem. Int. Ed. 2015, 54,
3417 – 3430.
[44] M. R. Ryder, B. Van de Voorde, B. Civalleri, T. D. Bennett, S.
Mukhopadhyay, G. Cinque, F. Fernandez-Alonso, D. De Vos, S. Rudić,
J.-C. Tan, Phys. Rev. Lett. 2017, 118, 255502.
[13] Z. Fang, B. Bueken, D. E. De Vos, R. A. Fischer, Angew. Chem. Int. Ed.
2015, 54, 7234 – 7254.
[45] D. W. Fu, W. Zhang, H. L. Cai, Y. Zhang, J. Z. Ge, R. G. Xiong, S. D.
Huang, J. Am. Chem. Soc. 2011, 133, 12780 - 12786;
[46] M. Kurmoo, Chem. Soc. Rev. 2009, 38, 1353 - 1379.
[47] S.-S. Bao, L.-M. Zheng, Coord. Chem. Rev. 2016, 319, 63–85.
[48] L.-R. Guo, S.-S. Bao, B. Liu, D. Zeng, J. Zhao, J. Du, L.-M. Zheng, Chem.
Eur. J. 2012, 18, 9534 - 9542.
[14] M. L. Foo, R. Matsuda, S. Kitagawa, Chem. Mater. 2014, 26, 310 - 322.
[15] T. Zhang, W. Lin, Chem. Soc. Rev. 2014, 43, 5982 – 5993.
[16] J. Li, Q.-L. Zhu, Q. Xu, Chem. Commun. 2014, 50, 5899 - 5901.
[17] M. Yoon, K. Suh, S. Natarajan, K. Kim, Angew. Chem. Int. Ed. 2013, 52,
2688 – 2700.
[18] P. Ramaswamy, N. E. Wong, G. K. H. Shimizu, Chem. Soc. Rev. 2014,
43, 5913 - 5932.
[49] G. Nénert, U. Adem, E. M. Bauer, C. Bellitto, G. Righini and T. T. M.
Palstra, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 78, 054443
[50] J. Huang, S.-S. Bao, L.-S. Ling, H. Zhu, Y.-Z. Li, L. Pi, L.-M. Zheng, Chem.
Eur. J. 2012, 18, 10839–10842.
[19] G. M. Espallargas, E. Coronado, Chem. Soc. Rev. 2018, 47, 533-557.
[20] A. Schneemann, V. Bon, I. Schwedler, I. Senkovska, S. Kaskel, R. A.
Fischer, Chem. Soc. Rev. 2014, 43, 6062 - 6096.
[51] X.-J. Yang, S.-S. Bao, M. Ren, N. Hoshino, T. Akutagawa, L.-M. Zheng,
Chem. Commun. 2014, 50, 3979-3981.
[21] S. Horike, R. Matsuda, D. Tanaka, S. Matsubara, M. Mizuno, K. Endo, S.
Kitagawa, Angew. Chem. Int. Ed. 2006, 45, 7226 – 7230.
[22] D. I. Kolokolov, H. Jobic, A. G. Stepanov, V. Guillerm, T. Devic, C. Serre,
G. Férey, Angew. Chem. Int. Ed. 2010, 49, 4791 - 4794.
[23] S. L. Gould, D. Tranchemontagne, O. M. Yaghi, M. A. Garcia-Garibay, J.
Am. Chem. Soc. 2008, 130, 3246 – 3247.
[52] J.-M. Rueff, N. Masciocchi, P. Rabu, A. Sironi, A. Skoulios, Chem. Eur.
J. 2002, 8, 1813 - 1820.
[53] R. L. Carlin, Magnetochemistry, Springer, Berlin, Heidelberg, 1986.
[54] G. E. Kostakis, S. P. Perlepes, V. A. Blatov, D. M. Proserpio, A. K. Powell,
Coord. Chem. Rev. 2012, 256, 1246 - 1278.
[24] S. Devautour-Vinot, G. Maurin, C. Serre, P. Horcajada, D. P. da Cunha,
V. Guillerm, E. de S. Costa, F. Taulelle, C. Martineau, Chem. Mater. 2012,
24, 2168−2177.
[55] J. B. Goodenough, Magnetism and the Chemical Bond, John Wiley &
Sons, New York, London, 1963.
[56] SAINT, Program for Data Extraction and Reduction, Siemens Analytical
X-ray Instruments, Madison, WI, 1994 - 1996.
[25] K. Zhu, V. N. Vukotic, C. A. O’Keefe, R. W. Schurko, S. J. Loeb, J. Am.
Chem. Soc. 2014, 136, 7403−7409.
[57] SHELXTL (version 5.0), Reference Manual, Siemens Industrial
Automation, Analytical Instruments, Madison, WI, 1995.
This article is protected by copyright. All rights reserved.

10.1002/chem.201801301
Chemistry - A European Journal
FULL PAPER
FULL PAPER
Layered metal phosphonates Co(4-
mopp)(H2O) (1), Co(4-mobp)(H2O) (2),
Co(3-mopp)(H2O) (3) and Co(3-
mobp)(H2O) (4), where 4- or 3-
moppH2 is (4- or 3-
Zhong-Sheng Cai,a Norihisa Hoshino,b
Song-Song Bao,a Jiage Jia,a Tomoyuki
Akutagawa,*b and Li-Min Zheng*a
Page No. – Page No.
methoxyphenyl)phosphonic acid and
4- or 3-mobpH2 is (4- or 3-
Dynamic Motion of Organic Ligands
in Polar Layered Cobalt
Phosphonates
methoxybenzyl)phosphonic acid, are
reported. The phenyl and Me-O
groups rotate in 1 and 2 but not in 3
and 4. The dehydrated compounds 3-
de and 4-de show one- or two-step
phase transitions above 200 oC.
This article is protected by copyright. All rights reserved.