Hydrothermal synthesis, structure and magnetism of square-grid cobalt(II)-carboxylate layered compounds with and without pillars

Article abstract of DOI:10.1039/b202995b

The hydrothermal synthesis, single crystal X-ray structures and magnetic properties of two layered cobalt(II)-carboxylate complexes, _∞²[Co^II(H₂O)₂ (O₂CCHCHC₆H₅)₂] (1) and _∞²[Co^II(H₂O)₂ (O₂CCHCHC₆H₄CO₂)_2/2] (2), are described. Pale red crystals of Co(H₂O)₂L₂, L = trans-cinnamate (C₉H₇O₂^-) (1) or L₂ = 4-carboxycinnamate (C₁₀H₆O₄^2-) (2), were obtained at 120°C. The structures consist of square-grid 2D-coordination polymeric sheets, ... -OCO-Co(H₂O)₂-OCO-Co(H₂O)₂- ..., separated by C₆H₅-CH=CH- for (1) or pillared by -C₆H₄-CH=CH- for (2). The magnetism was studied as a function of temperature and magnetic field. In both cases the magnetic moment decreases on lowering the temperature due to spin-orbit coupling and no interaction between cobalt ions. The data can alternatively be fitted to an unrealistic quadratic layer model for S = 3/2 without taking into account the effect of spin-orbit.

Full text of DOI:10.1039/b202995b

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Hydrothermal synthesis, structure and magnetism of square-grid
cobalt(II)-carboxylate layered compounds with and without pillars
Hitoshi Kumagai,a,b Yoshimi Oka,b,c Katsuya Inoueb and Mohamedally Kurmoo*a
a Institute de Physique et Chimie des Matériaux de Strasbourg, 23 Rue du Loess 67037
Strasbourg Cedex, France. E-mail: kurmoo@ipcms.u-strasbg.fr
b Applied Molecular Science, Institute for Molecular Science (IMS), Nishigounaka 38,
Myoudaiji, Okazaki 444-8585, Japan
c Department of Structural Molecular Science, The Graduate University for Advanced Studies,
Nishigounaka 38, Myoudaiji, Okazaki 444-8585, Japan
Received 26th March 2002, Accepted 3rd July 2002
First published as an Advance Article on the web 14th August 2002
The hydrothermal synthesis, single crystal X-ray structures and magnetic properties of two layered cobalt()-
2
2
carboxylate complexes, ∞[CoII(H2O)2(O2CCHCHC6H5)2] (1) and ∞[CoII(H2O)2(O2CCHCHC6H4CO2)2/2] (2), are
described. Pale red crystals of Co(H2O)2L2, L = trans-cinnamate (C9H7O2Ϫ) (1) or L2 = 4-carboxycinnamate
(C10H6O42Ϫ) (2), were obtained at 120 ЊC. The structures consist of square-grid 2D-coordination polymeric sheets,
ؒ ؒ ؒ –OCO–Co(H O) –OCO–Co(H O) – ؒ ؒ ؒ , separated by C H –CH᎐CH– for (1) or pillared by –C H –CH᎐CH–
᎐
᎐
2
2
2
2
6
5
6
4
for (2). The magnetism was studied as a function of temperature and magnetic field. In both cases the magnetic
moment decreases on lowering the temperature due to spin–orbit coupling and no interaction between cobalt ions.
The data can alternatively be fitted to an unrealistic quadratic layer model for S = 3/2 without taking into account
the effect of spin–orbit.
bi-dentate of syn–syn, syn–anti, and anti–anti types, and tri-
Introduction
dentate), (b) the stable coordination geometries (tetrahedral,
pyramidal and octahedral) of Co() and (c) the presence of
and the number of coordinated water or hydroxide.13,14 The
isolation of single phases is a problem under certain conditions
and polymorphism is an additional problem in some cases.
Controlled hydrothermal synthesis is becoming one of the most
reliable techniques for producing single phases of high crystal-
linity.15 For example, 1D, 2D and 3D compounds have been
isolated and structurally characterised.16–18 Interestingly, some
of the compounds are found to have cavities and channels with
incorporated solvents, which may be removed without altering
the framework.19
Among the layered phases, two families containing dicarb-
oxylates have been isolated and fully characterised structurally,
viz. (a) Co5(OH)8L2ؒnH2O which are ferrimagnets with Curie
temperatures of 60 K20 and (b) Co2(OH)2L2 which are meta-
magnets exhibiting large coercive fields at low temperatures.6,21
In both cases, the metals are arranged in a triangular lattice
and the principal magnetic exchange is via the µ3-bridging
hydroxide. The ferrimagnetic compounds were also obtained
with several mono-carboxylates but they were poorly crystal-
lised for crystal structure determination.20 In contrast, no
layered metamagnet has been isolated so far with a mono-
carboxylate. A third family of layered compounds were also
obtained with both mono- and di-carboxylates and coordinated
water molecules instead of hydroxide.6 The resulting layers have
different structural connectivity and magnetic topology which
result in no long range magnetic ordering. The two examples
presented here (see Scheme 1) belong to the latter family.
Layered solids are an increasingly important class of materials
of interest in the areas of sorption, separation and catalysis, as
exemplified by layered double hydroxides, silicates and metal-
dihalides, -dichalcogenides and -phosphates.1 Transition metal
coordination polymers are a newly added family to this list.
Several families have been synthesised and characterised and
their chemical and physical properties evaluated. The layers
may comprise of a M–O network where the oxygen is part of
a phosphate, oxalate, sulfate, silicate or hydroxide,2 or a M–S
network as in MPS3, or MS2,3 or a M–X network as in MX2 and
A2MX4.4 Only few of the layered M–O networks are known
where the oxygen is from a carboxylate group.5–10 Here, we
report two such examples, where the layer consists of a square-
grid net of octahedral coordinated cobalt atoms connected
by O–C–O bridges. In one case the layers are pillared by
the organic backbone of the di-carboxylate and in the other
they are separated by the mono-carboxylate. Similar nets have
been observed for Co(H2O)2terephthalate,6 Co(formamide)2-
7
(formate)2 and Mn(H2O)2acetylene-dicarboxylate,8 where the
metal has octahedral coordination and for two compounds
with the same layer topology but with tetrahedral coordinated
cobalt and saturated alkyl di-carboxylate.9,10 Among these
compounds, the Co-formate is the only one to exhibit long
range magnetic ordering with a spontaneous magnetization due
to non-collinearity of the sub-lattice magnetization. Other
known ligands forming this kind of square-grid layers are
thiocyanate and dicyanamide with manganese.11,12
Metal carboxylate complexes have been the subjects of study
for many years where the interest has focused on both structure
and magnetism.13 The reactions of a carboxylic or di-carboxylic
acid with a cobalt ion are known to result in several complexes
depending on the concentration, the pH, the temperature
and the synthetic method employed. The wide range of stable
compounds is due to several parameters, viz. (a) the range of
the coordination modes of the carboxylate ion (mono-dentate,
Experimental
Preparation of complexes
All chemicals (Tokyo Kasei Co. and Aldrich) were used as
received without further purification.
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J. Chem. Soc., Dalton Trans., 2002, 3442–3446
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b202995b

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Table 1 Summary of crystal data
Compound
1
2
Formula
Mw
C18H18CoO6
389.27
C10H10CoO6
285.12
Crystal size/mm
Crystal system
Space group
0.1 × 0.1 × 0.05
Monoclinic
C2/c
0.15 × 0.1 × 0.1
Monoclinic
P21/c
a/Å
b/Å
c/Å
36.564(5)
6.4037(9)
7.327(1)
92.101(4)
1714.5(4)
4
1.032
1957
1394 [I > 2.00σ(I )]
0.043 (0.127)
11.3236(4)
6.6084(4)
7.2011(6)
101.948(5)
527.2(1)
β/Њ
Scheme
1
The molecular structures of trans-cinnamate and 4-
carboxycinnamate.
V/Å3
Z
2
µ(Mo-Kα)/mmϪ1
No. reflections measured
No. observations
R (Rw)
1.642
1317
1061 [I > 3.00σ(I )]
0.051 (0.071)
Co(H2O)2(trans-cinnamate)2 1. Co(NO3)2ؒ6H2O (0.5 g, 0.0017
mol) was dissolved in distilled water (ca. 20 ml) and a solution
of trans-cinnamic acid, C6H5CHCHCO2H, (0.51 g, 0.0034 mol)
and NaOH (0.13 g, 0.0033 mol) in distilled water (ca. 20 ml)
was added. The mixture was placed in the Teflon liner of an
autoclave, sealed and heated to 120 ЊC for 1 day. It was then
allowed to cool to room temperature in a water bath. Pale red
crystals were obtained which were washed with water and
acetone and dried in air. Anal. calc. (%): CoC18O6H18: C, 55.54;
H, 4.66. Found: C, 54.67; H, 4.62. Infrared/cmϪ1: 680m, 720 m,
780m, 880m, 980m, 1070w, 1250w, 1420s, 1450m, 1540s, 1580m,
1640s, 3020w, 3060w, 3080w, 3240mbr, 3340mbr, 3460mbr.
CCDC reference numbers 182521 and 182522.
See http://www.rsc.org/suppdata/dt/b2/b202995b/ for crystal-
lographic data in CIF or other electronic format.
Physical measurements
Infrared spectra were recorded by transmission through KBr
pressed pellets or thin films deposited on KBr plates by use of a
MATTSON or a JASCO FTIR interferrometer. The magnetic
properties of the complexes were studied by Quantum Design
MPMS-XL SQUID magnetometers in the temperature range
2–300 K and fields up to 5 Tesla. A Princeton Applied Research
Vibrating Sample magnetometer was used to measure the iso-
thermal magnetization in field up to 2 Tesla. X-Ray powder
diffraction data were collected on a Siemens D-500 diffracto-
meter equipped with Co-Kα (λ = 1.789Å) radiation at room
temperature.
Co(H2O)2(4-carboxycinnamate) 2. Co(NO3)2ؒ6H2O (1.0 g,
0.0035 mol) was dissolved in distilled water (ca. 20 ml) and a
solution of 4-carboxycinnamic acid, HO2CC6H4CHCHCO2H,
(0.38 g, 0.002 mol) and NaOH (0.16 g, 0.004 mol) in distilled
water (ca. 20 ml) was added. The mixture was placed in the
Teflon liner of an autoclave that was then sealed and heated
to 120 ЊC for 3 days. The bomb was placed in a water bath
and allowed to cool to room temperature. Pink powder of
Co2(OH)2carboxycinnamate and red crystals of Co(H2O)2carb-
oxycinnamate were obtained. The crystals were first separated
from the pink powder by decantation and then manually under
a microscope. The crystals were finally washed with water and
acetone and allowed to dry in air. Anal. calc. (%) for
CoO6C10H10: C, 42.13; H, 3.54. Found: C, 42.17; H, 3.50.
Infrared/cmϪ1: 485w, 492w, 560w, 704mbr, 800m, 862w, 890w,
986m, 1015w, 1112w, 1138w, 1182w, 1212w, 1255w, 1292w,
1368vs, 1410s, 1536vs, 1585vs, 1636s, 3316m, 3430msh.
Results and discussion
Preparation of the complexes
The choice of 4-carboxycinnamic acid (eight carbon atom
bridge) was made so as to have a connector of length inter-
mediate between terephthalic acid (six carbon atom bridge)
and 4,4Ј-diphenyldicarboxylate (ten carbon atom bridge) for
the study of the structural and magnetic properties of the
metamagnets, Co2(OH)2L2.6,21 Trans-cinnamic acid was chosen
to remove the connection between the layers. We first observe
that the blue–green ferrimagnets, Co5(OH)8L2ؒnH2O, are
formed easily with the two acids. While we were able to syn-
thesise the pink metamagnet Co2(OH)2(4-carboxycinnamate),
we could not detect this phase with trans-cinnamic acid. In
addition, good quality crystals of the di-aquo phase, Co-
(H2O)2L2, have been obtained with both acids.
Crystallography
For each compound, single crystals were glued onto the tip of a
glass fibre. Intensity data were collected at room temperature on
a Bruker SMART APEX CCD area detector (for 1) and a
Kappa-CCD Nonius diffractometer (for 2) using graphite
monochromated Mo-Kα (λ = 0.71073 Å) radiation (ω-scan
mode). The data were corrected for Lorentz and polarization
effects. The structure was solved by direct methods and
expanded using Fourier techniques. The hydrogen atoms of the
acid in 1 were located from difference Fourier maps and the
hydrogen atoms of water were placed at ideal positions. The
non-hydrogen atoms were refined anisotropically. The final
cycle of full-matrix least-squares refinement for 1 was based on
1957 observed reflections [I > 2.00σ(I )] and 115 variable
parameters; it converged to unweighted and weighted agreement
factors of R = Σ||Fo| Ϫ |Fc||/Σ|Fo| of 0.043 and Rw = [Σw(|Fo| Ϫ
|Fc|)2/Σw|Fo|2]1/2 of 0.127. The final cycle of full-matrix least-
squares refinement for 2 was based on 1061 observed reflections
[I > 3.00σ(I )] and 88 variable parameters; it converged to R =
0.051 and Rw = 0.071. No extinction corrections have been
applied. Toward the end of the refinement of the structure of
2 the carbon atoms C(3) and C(4) appeared to be disordered
and were finally refined as 50% occupancy at the two sites. A
summary of the crystal data is collected in Table 1.
Crystal structures
The crystallographic data appear in Table 1 and a selection of
the relevant bond lengths and angles are given in Table 2. The
atom numbering schemes are shown in Fig. 1.
The structure of the two compounds consist of layers of
a square-grid cobalt–oxygen net separated by the organic
moieties. Each layer contains trans-Co(H2O)2 connected by four
O–C–O bridges of the carboxylate groups (Fig. 2). The square
arrangement is similar to that found in [Co(H2O)2BDC] (BDC
= benzene 1,4-dicarboxylate or terephthalate). [Co(H2O)2BDC]
crystallises in the monoclinic system C2/c, a = 18.274(3), b =
6.548(9), c = 7.296(1) Å, β = 98.6(3)Њ, V = 862.5 Å3 and Z = 4.6
Within the layer the shortest Co–Co distance in 1 is 4.9 Å and
the second nearest is 6.4 Å. For 2 the shortest Co–Co distance
is 4.9 Å and second nearest is 6.6 Å. These distances are
comparable to those found in the terephthalate derivative that
J. Chem. Soc., Dalton Trans., 2002, 3442–3446
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Table 2 Selected bond distances (Å) and angles (Њ) for 1 and 2
1
Co1–O1
Co1–O2
2.096(2)
2.066(2)
Co1–O3
2.091(2)
O1–Co–O2
O1–Co–O3
O2–Co–O3
94.66(9)
92.28(9)
94.80(9)
O1–Co–O2Ј
O1–Co–O3Ј
O2–Co–O3Ј
85.34(9)
87.72(9)
85.20(9)
2
Co–O1
Co–O2
2.107(3)
2.083(3)
Co–O3
2.094(3)
O1–Co–O2
O1–Co–O3
O2–Co–O3
91.5(1)
87.3(1)
88.5(1)
O1–Co–O2Ј
O1–Co–O3Ј
O2–Co–O3Ј
88.5(1)
92.7(1)
91.5(1)
Fig. 2 View of a layer along the stacking axis (a*-axis) showing the
alternating orientation of the octahedra.
Fig. 1 Coordination around the cobalt atoms and the adopted atom
numbering for 1 (a), thermal ellipsoids at 50% probability, and 2 (b).
Fig. 3 View of the arrangement of the organic backbone in the
galleries in 1 (a) and 2 (b). Due to the disorder of C(3) and C(4) the
cinnamic backbone in 2 appears as naphthalene (see text).
also contains Co in octahedral coordination; however, they
are longer than those found in the anhydrous pimelate9 and
glutarate10 derivatives that contain cobalt in tetrahedral
coordination. This is consistent with the shortening of the
Co–O bond lengths from octahedral (2.1 Å) to tetrahedral
(1.9 Å) coordination.
The square-grid of cobalt is stabilised by covalent Co–O
bonds and O–C–O bridging units. The Co–O bond lengths vary
from 2.066 to 2.107 Å for the two compounds. These may be
compared to the pimelate and glutarate derivatives that vary
from 1.95 to 1.99 Å. It is to be noted that the structure is stable
with or without the pillars. In the present compounds two
bonded water molecules in the trans-positions complete the
octahedron geometry. The Co–OH2 bond lengths are 2.09 Å for
1 and 2.08 Å for 2. The octahedra are considerably distorted
with O–Co–O angles in the range 85.20(9)–94.80(9)Њ. To satisfy
the constraint imposed by the rigid OCO bridges upon the
layers, the octahedra are tilted from one another. The dihedral
angles are 123.1Њ for 1, 123.5Њ for 2, and 125.1Њ for Co-
(H2O)2BDC.6
One of the important structural differences between the two
compounds is that the layers in 1 are separated by the cinna-
mate ion (Figs. 3a and 4a) and the layers in 2 are connected by
the carboxycinnamate ion (Figs. 3b and 4b) in a similar fashion
as that in the terephthalate derivative.6 To our knowledge the
two examples are rare cases, which have been characterised
structurally, where the same inorganic layer is separated by a
carboxylate spacer in one case and connected in the other. The
interlayer distance is 18.3 Å in 1, 11.3 Å in 2, and 9.1 Å in the
terephthalate derivative. The increase from 9.1 Å for a bridging
unit of six carbon atoms (terephthalate) to 11.3 Å for one with
eight carbon atoms (carboxycinnamate) is as expected. This is
comparable to those observed for the Co2(OH)2L2 series, 9.9 Å
for L2 = terephthalate and 12.1 Å for L2 = carboxycinnamate.21
However, the large difference in interlayer distance between 1
and 2 is due to (a) the presence of twice the number of acid
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Fig. 5 Temperature dependence of the product of susceptibility and
temperature (circles) and the inverse susceptibility for 2 (diamonds).
Fig. 4 View of the zig–zag arrangement of the aromatic rings within
the galleries for 1 (a) and 2 (b).
units in 1 and (b) the non-interdigitation of the cinnamate ion
in 1 due to crowding and consequently to the repulsion of the
benzene rings. It is interesting to note that in neither of them
is solvent water molecule included in the lattice, although the
synthesis is performed in water. This may be due to the lack of
available polar groups for hydrogen bond formation. The extent
of void space in 1, as suggested by the lack of residual electron
density in the difference Fourier maps, compared to 2 can be
seen in the calculated densities, 1.51 g cmϪ3 for 1 and 1.81 g
cmϪ3 for 2. It is also to be noted that these densities are still
lower than that of the terephthalate derivative (2.00 g cmϪ3). It
would be interesting to check if the layers in 1 can be further
separated by intercalation of neutral compounds.
Fig. 6 Isothermal magnetization of 2 at 2 K; line is a guide to the eye.
Magnetic properties
The magnetic properties of compounds 1 and 2 were studied
as a function of temperature (2–300 K) in a fixed magnetic
field and as a function of field up to 5 Tesla at different fixed
temperatures; the data for 2 are shown in Figs. 5 and 6. The
moment decreases in both cases as the temperature of the
sample is lowered. The data above 150 K can be fitted to a
Curie–Weiss function with Curie constants, 2.78 cm3 K molϪ1
for 1 and 2.95 cm3 K molϪ1 for 2 and Weiss constants Ϫ24.05 K
for 1 and Ϫ18.6 K for 2. The effective magnetic moment (µeff)
per cobalt ion [4.54 µB (1) and 4.7 µB (2)] is consistent with those
expected for octahedral cobalt(). The values are typical for
cobalt() ion in an octahedral environment with an enhanced
moment due to orbital contribution and a lowering of the
moment at low temperatures due to the effect of spin–orbit
coupling.22,23 A slight discontinuity is observed in 1 around
50 K due to unavoidable contamination from small particles
of the ferrimagnetic compound, Co5(OH)8(cinnamate)2ؒnH2O
which is not even seen under a microscope!
around 15 K.9,10 In both cases, the high temperature behaviour
is described as paramagnetic with Weiss constants of Ϫ23
2
K. On the one hand, a small rise in the susceptibility below 10
K for the pimelate compound and the appearance of a weak
hysteresis at 4.2 K were interpreted as being due to a small
canting.9 On the other hand, the rise at low temperatures in Co-
glutarate was associated with the presence of a paramagnetic
impurity.10 In contrast, the cobalt-formate compound exhibits a
field dependent magnetization below 9 K and a clear hysteresis
loop that were associated with canting.7
The temperature dependence of the magnetic susceptibility
of Co-glutarate in the range 15–300 K has been modelled by
use of Lines’ model for a quadratic layer antiferromagnet of
S = 3/2.10,24 This model was also used by Rettig et al. for
Co-formate. Although reasonable agreement was obtained, the
latter authors argued against such a model on the grounds that
it ignores the effect of spin–orbit coupling. Having clean sets of
data for 2 and Co(H2O)2BDC we attempted to fit the data with
the Lines’ model.7 It gives visually good fits but the values of
J and g vary enormously depending on the range of tempera-
ture over which the fit was performed. For example the g-value
varies from a modestly acceptable 1.9 to an unrealistic 12. The
instability of the fits is due to the lack of any maximum in the
The magnetic properties of the square-grid layered com-
pounds of cobalt referred to above have all been studied as a
function of temperature and magnetic field.7,9,10 They were
found to be quite different. For example, the two compounds
with tetrahedral coordinated cobalt centres exhibit maxima
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temperature dependence of the susceptibility results in several
possible solutions.25 The use of the function given by Mabbs
and Machin22 that take into account the effect of spin–orbit
coupling does not quite represent the observed data. Whether
this is due to severe distortion of the cobalt coordination from
exact octahedral for which the function was worked out is not
clear.
The isothermal magnetization at 2 K (Fig. 6) exhibits a con-
tinuous increase to a saturation value of 2.2 0.2 µB per cobalt
ion. No hysteresis was observed. The curve is consistent with
that expected for a Brillouin function. Its saturation value is
within the range expected for free cobalt ion with an effective
S = 1/2 and anisotropic g-values. It further confirms that the
cobalt ions do not interact.
Conclusion
Three distinct layered phases have been identified in the
hydrothermal reaction of Co() and terephthalate or carboxy-
cinnamate in alkaline media; a blue–green ferrimagnet [Co5-
(OH)8L2ؒnH2O], a light pink metamagnet [Co2(OH)2L2], and a
pale red paramagnet [Co(H2O)2L2]. Interestingly, the meta-
magnetic phase is not stabilised by mono-carboxylate. The
two examples presented here are rare cases for which we have
demonstrated structurally that the inorganic layers can be
either separated by spacers or pillared by connectors. The bent
O–C–O bridges are found to be poor exchange pathways for
long range magnetic ordering to occur compared to the linear
N–C–N bridges found in dicyanamide complexes.26–29
Acknowledgements
18 (a) O. M. Yaghi, H. Li, C. Davis, D. Richardson and T. L. Groy, Acc.
Chem. Res., 1998, 31, 474; (b) M. Eddouadi, D. A. Moler, H. Li,
B. Chen, T. M. Reineke, M. O’Keeffe and O. M. Yaghi, Acc. Chem.
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This work was funded by the CNRS-France and a Grant-in-
Aid for scientific research from the Ministry of Education,
Science, Sports, and Culture. H. K. thanks the JSPS (Japan)
for a Young Scientist Fellowship for his stay in Strasbourg.
M. K. thanks the Royal Society of Chemistry (UK) for an author
travel grant. We also thank Drs A. Decian and N. Kyritsakas
from the X-ray facilities in Strasbourg.
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