Magnetic, thermal, and neutron diffraction studies of a coordination polymer: bis(glycolato)cobalt(ii)

Article abstract of DOI:10.1039/C8DT04358B

The two-dimensional quadratic lattice magnet, bis(glycolato)cobalt(ii) ([Co(HOCH₂CO₂)₂]), showed antiferromagnetic ordering at 15.0 K and an abrupt increase in magnetisation at H = 22?600 Oe and 2 K, thereby acting as a metamagnet. Heat capacity measurements revealed that the associated entropy change ΔS around the transition temperature was evaluated to be 6.20 J K^?1 mol^?1 and that the Co(ii) ion had the total angular momentum of J = 1/2 at low temperatures. Neutron diffraction studies suggested that the magnetic moment vectors of the Co(ii) ions had an amplitude of 3.59μ_B and were not aligned in a fully antiparallel fashion to those of their neighbours, which caused canting between the magnetic moment vectors in the sheet. The canting angle was determined to be 7.1°. Canting induced net magnetisation in the sheet, but this magnetisation was cancelled between sheets. The magnetisations in the sheets were oriented parallel to the magnetic field at the critical magnetic field.

Full text of DOI:10.1039/C8DT04358B

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Magnetic, thermal, and neutron diffraction studies
of a coordination polymer: bis(glycolato)cobalt(II)†
Cite this: Dalton Trans., 2019, 48,
333
Tomohiro Nakane,a Shota Yoneyama,b Takeshi Kodama,b Koichi Kikuchi,b
Akiko Nakao,c Takashi Ohhara,d Ryuji Higashinaka,e Tatsuma D. Matsuda,e Yuji Aokie
f
and Wataru Fujita
*
The two-dimensional quadratic lattice magnet, bis(glycolato)cobalt(II) ([Co(HOCH2CO2)2]), showed anti-
ferromagnetic ordering at 15.0 K and an abrupt increase in magnetisation at H = 22 600 Oe and 2 K,
thereby acting as a metamagnet. Heat capacity measurements revealed that the associated entropy
change ΔS around the transition temperature was evaluated to be 6.20 J K−1 mol−1 and that the Co(II) ion
had the total angular momentum of J = 1/2 at low temperatures. Neutron diffraction studies suggested
that the magnetic moment vectors of the Co(II) ions had an amplitude of 3.59μB and were not aligned in a
fully antiparallel fashion to those of their neighbours, which caused canting between the magnetic
moment vectors in the sheet. The canting angle was determined to be 7.1°. Canting induced net magneti-
sation in the sheet, but this magnetisation was cancelled between sheets. The magnetisations in the
sheets were oriented parallel to the magnetic field at the critical magnetic field.
Received 1st November 2018,
Accepted 28th November 2018
DOI: 10.1039/c8dt04358b
rsc.li/dalton
shift in the structural transition temperature upon cooling,
while no shift in the ferromagnetic transition temperature was
Introduction
Coordination polymers exhibit unique magnetic properties observed.3 The origin of the structural phase transition was
such as high-Tc ferri/ferromagnetism, photomagnetism, attributed to the positions of hydrogen atoms in hydrogen
single-chain quantum magnetism, magnetic bistabilities, bonds.
and guest-induced magnetic changes.1 Recently, we have
In this work, we focused on a coordination polymer, bis(gly-
reported that a coordination polymer, bis(glycolato)copper(II) colato)cobalt(II) ([Co(HOCH2CO2)2], 1), whose crystal structure
[Cu(HOCH2CO2)2], exhibited a structural phase transition had previously been reported by Medina et al.;4 this material is
between its high-temperature and low-temperature phases; isostructural with the copper derivative [Cu(HOCH2CO2)2].
this phase transition was associated with a large thermal hys- Fig. 1 shows a schematic representation of the crystal structure
teresis of approximately 220 K upon cooling and 280 K upon of 1. This material has a two-dimensional lattice network com-
heating, with ferromagnetic ordering at 1.1 K.2 Deuterium sub- prising Co(II) ions coordinated equatorially to two glycolate
stitution in the hydroxy groups induced a large 220 K to 280 K anions in a bidentate fashion through hydroxy and carboxylate
oxygen atoms, and axially to two glycolate carbonyl oxygen
atoms, to form coordination sheets (Fig. 1(a)). The planes of
the five-membered chelate rings are roughly perpendicular
with respect to each other. There are O–H⋯O hydrogen bonds
aDepartment of Information and Basic Science, Graduate School of Natural Science,
Nagoya City University, 1 Yamanohatam Mizuho-cho, Mizuho-ku, Nagoya 467-8501,
Japan
bDepartment of Chemistry, Tokyo Metropolitan University, 1-1 Minami-osawa,
Hachioji 192-0397, Japan
between hydroxy hydrogen atoms and the outer carbonyl
oxygen atoms of the carboxylates in the intersheets, as shown
in Fig. 1(b). Magnetic interactions between Co(II) ions are
expected to occur via the carboxylates, so that 1 shows a
unique magnetic behaviour. However, there have been no mag-
netic studies or structural analyses at various temperatures
such as those established for the Cu derivative. Hence, we
investigated the magnetic and thermal properties of 1, as well
as its magnetic anomaly at 15.0 K and the anomalous field
dependence of magnetisation at 2 K. We also elucidated the
origin of the magnetic anomaly by means of neutron diffrac-
tion studies.
cComprehensive Research Organization for Science and Society, Tokai,
Ibaraki 319-1106, Japan
dJ-PARC Center, Japan Atomic Energy Agency, Tokai, Naka, Ibaraki 319-1195, Japan
eDepartment of Physics, Tokyo Metropolitan University, 1-1 Minami-osawa,
Hachioji 192-0397, Japan
fGeneral Education, Faculty of Science and Technology, Seikei University,
3-3-1 Kichijoji-kita machi, Musashino 180-8633, Japan.
E-mail: fujitaw@st.seikei.ac.jp; Fax: +81-422-37-3781; Tel: +81-422-37-3792
†Electronic supplementary information (ESI) available: Photograph of crystals
and neutron diffraction measurements. CCDC 1876541–1876543. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8dt04358b
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grease used to hold the sample on the mount. The molar unit
of the heat capacity Cp was chosen as the quantity per mole of
Co(HOCH2COO)2. The specific heat capacity Cmag was
obtained by subtracting the lattice contribution from the total
heat capacity Cp. The lattice heat capacity Clattice was deter-
mined from the Cp data in the 30–50 K temperature range
using the following equation:
Cp ¼ aT 3 þ bT 5 þ cT 7 þ dT 9 þ eT ꢀ2
;
ð1Þ
where the last term stands for the high-temperature limit of
the specific heat capacity,6 and the remaining terms corres-
pond to the lattice heat capacity Clattice. The coefficients
obtained following fitting were: a = 0.001047, b = −6.5835 ×
10−7, c = 2.1773 × 10−10, d = −2.8193 × 10−14, and e = 1129.1.
The magnetic entropy was obtained by integrating (Cp
−
Clattice)/T in the 0–50 K temperature region.
Neutron diffraction experiments were carried out on a
SENJU (BL 18) MLF, J-PARC single-crystal TOF neutron diffract-
ometer.7 The J-PARC accelerator power was 200 kW. The data
were collected at wavelengths of 0.4–4.0 Å (1st frame) at 298 K,
25 K (>TN), and 5 K (<TN). The crystal structure was first ana-
lysed at 300 K to verify the quality of a single crystal. The
crystal structures were then re-determined at 25 K and 5 K in
order to examine the structural phase transitions in the
5–300 K temperature range, and to locate magnetic peaks
below the magnetic-transition temperature. Experiments at 5 K
were also carried out at wavelengths of 4.0–8.0 Å (2nd frame)
in order to collect magnetic peaks in the low Q-range.
Diffraction data were processed using the STARGazer data-
reduction program to provide HKL-F data.8 Crystal and mag-
netic-structure analyses were performed using the JANA2006
programs.9 Crystallographic data have been deposited at
the Cambridge Crystallographic Data Centre (CCDC
1876541–1876543 for compound 1 at 5 K, 20 K and 298 K,
respectively).
Fig. 1 Schematic representation of the coordination polymer bis(glyco-
lato)cobalt(II) ([Co(HOCH2CO2)2], 1). (a) The structure of the coordination
sheet. The sheets are parallel to the (1 0 −1) plane. The broken lines indi-
cate the axial Co–O bonds. (b) Atomic alignment of the hydrogen bond
formed between intersheets.
Experimental
Compound 1 was prepared following a reported procedure.4
Accordingly, 100 mL of an aqueous solution containing 2.00 g
Results and discussion
Magnetic properties
(8.03 mmol) of cobalt acetate tetrahydrate and 1.23
g
(16.1 mmol) of glycolic acid was maintained at 90 °C for 13 d.
At the end of this period, dark reddish block crystals were Fig. 2 shows the results of magnetic measurements carried out
obtained. Single crystals were grown to a maximum size of on 1. Fig. 2(a) displays the paramagnetic susceptibility (χp) of 1
5.0 mm × 2.0 mm × 2.0 mm (Fig. S1†). Anal. Calcd for as a function of temperature, measured in a magnetic field of
C4H6O6Co3: C, 22.98%; H, 2.89%; found: C, 23.02%; H, 2.89%. 50 Oe. The χp value increased gradually with decreasing tem-
Magnetic studies were performed on a polycrystalline perature, reaching a sharp maximum at 15.2 K, after which it
sample, which was prepared by crushing single crystals of 1, at decreased rapidly below this temperature, which is suggestive
temperatures down to 2 K using a Quantum Design Co. Ltd of a magnetic phase transition. Fig. 2(b) shows the χpT vs. T
MPMS SQUID magnetometer. The raw experimental data were plot of 1; the χpT value was 3.006 emu K mol−1 at 300 K. A
corrected for the diamagnetic susceptibility of 1 (−81.22 × gradual decrease in the χpT value was observed upon cooling.
10−6 emu mol−1) as determined from Pascal’s constants.5 The Data in the 150–300 K range were fitted to the Curie–Weiss
molar unit of paramagnetic susceptibility χp was chosen as the law,5 with C = 3.476 emu K mol−1 and θ = −44.3 K, where C
quantity per mole of Co(HOCH2COO)2.
and θ are the Curie and Weiss constants, respectively. The
Heat-capacity measurements were carried out for a block effective magnetic moment was determined to be 5.27μB from
crystal on a Quantum Design physical property measurement the Curie constant, which is consistent with that expected for
system (PPMS) equipped with a relaxation-type calorimeter. a high spin state in an octahedral Co(II) ion, as proposed by
Data were corrected for the sample mount and the Apiezon Mabbs and Machin,10 who also proposed that an isolated octa-
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2 K. On the other hand, Co(HCOO)2·2H2O is also a coordi-
nation polymer in which Co(II) ions are bridged by carboxyl-
ates, although the bridging structure is different.12 In this
material, antiferromagnetic interactions exist between the
Co(II) ions, and an antiferromagnetic phase transition is
observed at 5.4 K, which suggests that the strengths of the
superexchange interactions via the carboxylate groups
influence the bridging structure.
Fig. 2(c) shows the dependence of magnetisation on the
field at 2 and 25 K. At 25 K, which is above the magnetic tran-
sition temperature, magnetisation increases linearly with the
increasing magnetic field. At 2 K, below the magnetic tran-
sition temperature, magnetisation also increases linearly in
the lower magnetic-field region. A rapid increase in magnetisa-
tion, of ∼2300 erg Oe−1 mol−1, was observed at around a mag-
netic field of 22 600 Oe. Once again, magnetisation increased
linearly at fields above 25 000 Oe. The magnetisation of 1
reached ∼5000 erg Oe−1 mol−1 at 70 000 Oe and was not satu-
rated. This behaviour is similar to those observed for
metamagnets.5
Heat capacity
Fig. 3 shows the temperature dependence of the heat capacity
(Cp) of 1 in the 2–50 K temperature range. The heat capacity
exhibits an anomaly at 15.0 K, which supports the existence of
magnetic ordering at this temperature. The total magnetic
entropy was evaluated to be 6.20 J K−1 mol−1 by integrating
Cp/T with respect to T. This value is similar to the expected ΔS
value for the magnetic ordering of 1 mol with J = 1/2 (R ln(2J + 1) =
5.76 J K−1 mol−1).13 This result indicates that an octahedral
Co(II) ion has a total angular momentum quantum number of
J = 1/2 in the ground state. This is consistent with the litera-
ture11 and with the magnetic behavior shown in this work.
Neutron diffraction
Fig. 2 Magnetic properties of 1. (a) Temperature dependence of the
paramagnetic susceptibility χp for non-oriented microcrystals under
50 Oe. (b) The χpT vs. T plot for non-oriented microcrystals. The solid
curves indicate the theoretical best fits of the Curie–Weiss law. (c) Field
dependence of the magnetization for non-oriented microcrystals of 1 at
2 K and 25 K.
We investigated 1 by neutron diffraction at 5 K, 20 K, and
298 K, and determined its crystallographic and magnetic struc-
tures. The structural parameters summarised in Table 1 are
based on neutron diffraction data and are consistent with the
results from the X-ray diffraction studies.4 There are no signifi-
hedral Co(II) ion has a Weiss constant θ of −20 K, which
results in a gradual transformation of the total angular
momentum quantum number (J = 3/2) at high temperature to
J = 1/2 at low temperature.10 The θ value of −44.3 K for 1
suggests spin–orbit coupling in each Co(II) ion; furthermore, it
indicates that antiferromagnetic interactions arise between
Co(II) ions via carboxylate groups that link the molecules in the
complex.
Kurmoo et al. reported the magnetic properties of bis(man-
delato)cobalt(II), where mandelate is a phenyl-substituted
α-hydroxycarboxylate.11 Although this material also forms a
two-dimensional quadratic coordination sheet similar to that
observed for 1, magnetic interactions between Co(II) ions were
weak and no magnetic phase transition was observed down to Fig. 3 Temperature dependence of the heat capacity Cp for 1.
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Table 1 Crystallographic parameters for 1
T/K
298 K
20 K
5 K
Crystal system
Space group
Monoclinic
P21/n
a/Å
b/Å
c/Å
β/Å
V/Å3
Z
5.180(3)
7.860(5)
8.659(5)
105.129(8)
340.3(4)
2
5.170(3)
7.697(5)
8.701(5)
104.492(8)
335.2(4)
2
5.143(3)
7.656(5)
8.653(5)
104.429(8)
330.0(3)
2
R1
wR2
0.0783
0.1070
0.0680
0.1116
0.0692
0.1235
cant structural differences among the Co–O coordination bond
lengths, O–H bond lengths, or O–H⋯O (hydrogen bond)
interatomic distances over the investigated temperature range
(Tables S1 and S2†). We previously reported that the structural
phase transition of bis(glycolato)copper(II) [Cu(HOCH2CO2)2]
is triggered by the thermal motion of hydrogen atoms in
hydrogen bonds and the elongations of axial Cu–O bonds.3 In
this study, compound 1 showed no structural phase transition
in the 2–300 K temperature range; the Co(II) ions do not
undergo large Jahn–Teller distortions and possess elongated
axial bonds. This suggests that hydrogen bonds and elongated
coordination bonds, which are affected by the positive charges
of protons, play important roles, as observed during the struc-
tural phase transition of the copper derivative.
Fig. 4 Magnetic structure for 1 at 2 K. The blue, red, brown and white
spheres are cobalt, oxygen, carbon and hydrogen atoms. The green
arrows are magnetic moment vectors. The unit cell is of the crystal
structure.
At 5 K, magnetic reflections with a propagation vector q =
(0.5 0 0.5) were observed, as shown in Fig. S2.† The unit cell
volume of the magnetic structure is twice as large as that of
the crystal structure, which is characteristic of antiferro-
magnetic ordering. The magnetic structure is described by the
P21/c magnetic space group with a (−a + b) × b × (−a − b)
supercell. Magnetic structure analyses were performed using
555 magnetic reflections (334 (I > 3σ)); this refinement gave
reliable factors, namely R = 11.76% and wR = 21.63%. Fig. 4
shows the magnetic structure of 1 at 5 K. The green arrows
correspond to the magnetic moment vectors of Co(II) ions;
their magnitudes of 3.59(7)μB are consistent with those of
Co(II) ions in octahedral environments.11 The magnetic moments
are almost parallel to the coordination sheet and are not
aligned in an exactly antiparallel fashion with their neigh-
bours, which causes canting toward the b axis in the sheet.
The canting angle to the neighbouring vector was determined
to be 7.1°. Although the canting of magnetic moments is
found in many Co(II) compounds,14 their magnetic structures
have rarely been determined by neutron diffraction using
single crystals.
Fig. 5 Schematic presentation of (a) rearrangement of magnetic
moment vectors on Co(II) ions in the coordination sheet. Blue spheres
are cobalt ions. Green arrows are the magnetic moment vectors. Red
arrows indicate the direction of the magnetization in the coordination
sheet. Direction of magnetization in the coordination sheets for 1 at 2 K
(b) below Hc and (c) above HC.
We next interpreted the anomalous field dependence of
this material below the transition temperature. Fig. 5 shows a
schematic representation of the magnetic structure and mag-
netisation of the coordination sheets in 1. Because of the anti-
ferromagnetic ordering in the material at 15.0 K, the magnetic
moment vectors of the Co(II) ions are aligned in an antiparallel ling.5 The molecules in the complex are oriented in two ways
fashion. A divalent cobalt ion in an octahedral environment (Fig. 1(a)), and each complex molecule has its own axis of easy
has a highly anisotropic g-tensor arising from spin–orbit coup- magnetisation. Based on the orientation of the molecules in
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the complex and their magnetic moment vectors, canting KAKENHI Grant Number 16K05942. X-ray structure analyses
occurs due to a competition between antiparallel alignments and magnetic experiments were conducted at the Institute of
that cause antiferromagnetic interactions, and the anisotropies Molecular Science, supported by the Nanotechnology Platform
of the Co(II) ions. As a result, a small net magnetisation occurs Program (Molecule and Material Synthesis) of the Ministry of
in the coordination sheet toward the b axis, as shown in Education, Culture, Sports, Science and Technology (MEXT),
Fig. 5(a). Hence, each coordination sheet is spontaneously Japan. Neutron diffraction experiments were performed using
magnetised toward the b axis, and magnetisation cancels out SENJU under the trial use program (2016A0016) of J-PARC and
in the intersheets (Fig. 5(b)). It is likely that weak antiferro- CROSS.
magnetic interactions occur between the sheets via the hydro-
gen bonds.15 The magnetisation in each sheet of this material
is oriented toward the magnetic field below the critical field of
22 600 Oe (Fig. 5(c)). As shown in Fig. 2(c), a rapid increase of
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which is consistent with the observed rapid increase; the
assumption shown in Fig. 5 is supported by this value. The
magnetisation increases with the increasing magnetic field
above the critical magnetic field because the canting angle
increases.
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Conclusions
We carried out magnetic, thermal, and neutron diffraction
studies on the coordination polymer, bis(glycolato)cobalt(II)
([Co(HOCH2CO2)2]). This material showed antiferromagnetic
ordering at 15.0 K and a rapid increase in magnetisation at
H = 22 600 Oe at 2 K. This magnetic behaviour was similar to
that of a traditional metamagnetic material. Neutron diffrac-
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Acknowledgements
This study was partially supported by a Grant from the Faculty
of Science and Technology, Seikei University, and by JSPS
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