2D molecular square grid of Cobalt(II) with tridentate phenylglycinate and mandelate: Structure and magnetism

Article abstract of DOI:10.1016/j.jpcs.2003.08.013

The hydrothermal synthesis, single crystal X-ray structures and magnetic properties of two layered cobalt-carboxylate complexes, _∞ ²[Co^II(O₂C^*CH(OH)C ₆H₅)₂] (1) and

Full text of DOI:10.1016/j.jpcs.2003.08.013

Journal of Physics and Chemistry of Solids 65 (2004) 55–60
www.elsevier.com/locate/jpcs
2D molecular square grid of Cobalt(II) with tridentate phenylglycinate
and mandelate: structure and magnetism
Hitoshi Kumagaia, Yoshimi Okab, Katsuya Inouea, Mohamedally Kurmooc,
*
aApplied Molecular Science, Institute for Molecular Science (IMS), Nishigounaka 38, Myoudaiji, Okazaki 444-8585, Japan
bJapan Department of Structural Molecular Science, The Graduate University for Advanced Studies, Nishigounaka 38, Myoudaiji,
Okazaki 444-8585, Japan
c
´
Institut de Physique et Chimie des Materiaux de Strasbourg, 23 Rue du Loess, BP 43, 67034, Strasbourg, Cedex2, France
Received 29 June 2003; revised 7 August 2003; accepted 11 August 2003
Abstract
The hydrothermal synthesis, single crystal X-ray structures and magnetic properties of two layered cobalt-carboxylate complexes,
21[CoII(O2CpCH(OH)C6H5)2] (1) and 12 [CoII(O2CpCH(NH2)C6H5)2] (2), where O2CCH(OH)C6H5 is mandalate and O2CCH(NH2)C6H5 is
phenylglycinate, are described. Pale pink crystals of 1 and 2 were obtained by the reaction of cobalt nitrate and the enantiomer-pure acids at
120 8C. In each case, the structure consists of stacks of quasi square-grid polymeric sheets consisting of carboxylato- bridges, M–O–C–O–
M, and the presence of both D- and L-enantiomers of the ligands segregated on each face of the layer. The ligands exhibit both chelating and
bridging functions with the carboxylate group adopting an anti–anti mode. The magnetic properties are characteristic of weakly interacting
paramagnets where the moments are elevated by an important orbital contribution via spin-orbit coupling.
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
through-space interaction and (b) the magnetic properties of
multi-layer systems with different types of exchange
mechanism. Examples of magnetic triangular lattice are
those of hydroxides [6] or silicates [7]; those with hexagonal
symmetry are oxalates [8,9] and metal-P2S6 [10] and for
square-grid there are halide and oxide perovskites [11],
thiocyanate [12], cyanides [13], dicyanamides [14] and
carboxylates [15]. Here, we present two examples of layered
compounds having M–O–C–O–M bridges where the
bridge is made by the carboxylate groups of mandelic acid
(a-hydroxyphenylacetic acid) or phenylglycine (a-amino-
phenylacetic acid). The two acids were expected to also
provide the additional property of optical activity.
The design of polymeric coordination networks has
advanced considerably due principally to the potential
applications for sorption, selective catalysis, ion-exchange
and also for their magnetic properties [1–3]. Amongst these
systems, 2D layered compounds have a very special place
[4,5]. Layered compounds have been known for a very long
time and many of them have been exploited for their
intercalation chemistry. Several structural topologies within
the layers have been described including triangular-,
hexagonal- and square-grid networks. Our ongoing interest
in this area has focused on the development of novel layered
magnetic compounds [6] for they belong to a special class
amongst magnetic materials and are used as models for the
understanding of (a) the presence of long range magnetic
ordering, theoretically not possible for 2D-Heisenberg
system but made possible by the presence of a weak dipolar
A search through Chemical Abstracts for coordination
complexes of cobalt with mandelic acid gave eight hits
where the main interests were the characterisation of the
molecular structure by infrared, circular dichroism, for-
mation constants and thermal properties [16]. No charac-
terisation of any crystal phase was reported. Similar search
for phenylglycine of cobalt gave one hit reporting its
complexation property in solution [17].
*
Corresponding author. Tel.: þ33-38810-7247; fax: þ33-38810-7133.
E-mail addresses: kurmoo@ipcms.u-strasbg.fr (M. Kurmoo); e1254@
mosk.tytlabs.co.jp (Hitoshi Kumagai).
0022-3697/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2003.08.013

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H. Kumagai et al. / Journal of Physics and Chemistry of Solids 65 (2004) 55–60
2. Experimental
have been applied. Details of crystallographic data are
collected in Table 1. The crystal data have been deposited
at CCDC, Cambridge, UK and given the reference numbers
CCDC 213444 and 213445.
All chemicals of purity higher than 98% were purchased
from Tokyo Kasei and Aldrich and were used without
further purification.
2.1. Preparation of [Co(O2CCH(OH)C6H5)2] (1)
and [Co(O2CCH(NH2)C6H5)2] (2)
3. Results and discussions
As the interest in the area of coordination polymers is
becoming more apparent by the number of interesting low
dimensional and framework materials that are being
synthesised [1], the research area is expanding in various
directions and in particular, that where the ligands contain
carboxylate groups. One is to functionalise the aromatic
rings or alkyl chains to which the carboxylate group is
attached with nitrogen donor groups [19], in particular, to
provide further coordination capabilities for specific
functions. Our present interest was to use ligands with a
coordinating group on the a-carbon of acetic acid and also
to choose those that have optical activity. Thus, in this work
reactions were performed with Co(II) and two chiral mono-
carboxylic acids, mandelic acid and phenylglycine
(Scheme 1). The ligand contains either an –OH group or
an –NH2 group, respectively, on the a-carbon of acetic
acid. In both cases, we choose the enantiomer pure starting
materials for the reactions. However, the single crystal
structure analyses revealed the formation of compounds
containing racemic mixture of the two enantiomers, which
presumably produced during the hydrothermal reactions.
The lack of any circular dichroism in the UV–vis spectra of
1 and 2 dispersed in KBr disks confirms the presence of both
enantiomers to produce non-chiral compounds. This is not
so surprising since it is well known that racemisation of the
ligands is favoured in the presence of bases [20].
The synthesis of the two compounds was performed in
exactly the same way in 120 ml capacity autoclave. In each
case, Co(NO3)2·6H2O (1.0 g, 0.0035 mol) was dissolved in
distilled water (ca. 20 ml) and a solution of the acid,
enantiomer-pure L-mandelic acid (HO2CCH(NH2)C6H5)
(0.52 g, 0.0035 mol) or D-phenylglycine (HO2CCH(NH2)-
C6H5) (0.52 g, 0.0035 mol), and NaOH (0.14 g, 0.0035 mol)
in distilled water (ca. 20 ml) was added. The mixture was
placed in the Teflon liner of the autoclave, sealed and heated
to 120 8C for 1 day. It was then allowed to cool to room
temperature. In both cases pale pink plates were obtained
which were harvested, washed with water and acetone and
allowed to dry in air.
2.2. X-ray crystallography and structure solution
The single crystals were selected and glued on the tip of
glass fibers with epoxy resin. Their diffraction data were
collected on a Bruker SMART APEX CCD area detector
employing v-scan mode at room temperature. The diffract-
ometer is equipped with a graphite monochromated Mo Ka
˚
(0.7107 A) radiation. The data were corrected for Lorentz and
polarisation effects. The structure was solved by direct
methods and expanded using Fourier techniques [18]. The
non-hydrogen atoms were refined anisotropically. The final
cycle of the full-matrix least-squares refinement was based on
the number of observed reflections and n variable parameters.
They converged (large parameter shift was s times its e.s.d.)
3.1. Structure of [Co(O2CCH(OH)C6H5)2] (1)
and [Co(O2CCH(NH2)C6H5)2] (2)
P
P
with agreement factors of R ¼ llFol 2 lFcll= lFol, Rw ¼
P
P
2
2
½ wðlFol 2 lFclÞ = wlFol Š1=2: No extinction corrections
Table 1
The asymmetric unit contains one ligand and one half of
cobalt atom in each case. A drawing of the asymmetric unit
and the atomic numbering scheme is shown in Fig. 1. A
summary of the crystal data of the two compounds is given
in Table 1. The selected bond distances and angles of 1 and
Crystallographic data for [Co(O2CCH(OH)C6H5)2] (1) and [Co(O2-
CCH(NH2)C6H5)2] (2)
Complex
1
2
Empirical formula
Formula weight
Crystal system
Space group
˚
a (A)
˚
b (A)
O6C16H12Co
359.20
O24C16H16CoN2
359.25
Monoclinic
P21=c (#14)
15.512(3)
4.8857(9)
9.365(2)
99.979(3)
699.0(2)
2
Monoclinic
P21=c (#14)
15.478(2)
4.9948(6)
9.457(1)
99.809(2)
720.4(2)
2
˚
c (A)
b (8)
3
˚
V (A )
Z
Dc (g cm23
)
1.706
1.656
m (Mo Ka) (cm21
R; Rw
)
20.61
16.36
0.091, 0.144
0.050
0.097, 0.119
0.040
R1
Scheme 1.

H. Kumagai et al. / Journal of Physics and Chemistry of Solids 65 (2004) 55–60
57
Table 3
Selected bond distances (A) and angles (8) of [Co(O2CCH(NH2)C6H5)2] (2)
˚
Atom
Atom
Distance
Co(1)
Co(1)
Co(1)
O(1)
O(2)
N(1)
2.047(2)
2.247(2)
2.114(3)
Atom
O(1)
O(1)
O(1)
O(1)
O(1)
O(2)
O(2)
O(2)
N(1)
Atom
Co(1)
Co(1)
Co(1)
Co(1)
Co(1)
Co(1)
Co(1)
Co(1)
Co(1)
Atom
O(10)
O(20)
N(10)
O(2)
N(1)
O(20)
N(1)
N(10)
N(10)
Angle
180.0
94.99(8)
100.40(9)
85.01(8)
79.60(9)
180.0
89.85(8)
90.15(8)
180.0
1 and 2 are severely distorted by compression along the
b-axis and elongation along the c-axis. Consequently,
the two short axes become non-equivalent in 1 and 2. The
shortest distances of 1 and 2 are very similar to those found
in Co(H2O)2Ln. However, the area defined by the four
Fig. 1. Atomic numbering scheme for Co(phenylglycinate)2 (2); the same
scheme is adopted for Co(mandelate)2 (1) by replacing N(1) by O(3).
2
2
˚
˚
nearest cobalt neighbours is 45.75 A for 1, 47.23 A for 2
2
and lies between 46.92 and 47.73 A for Co(H2O)2Ln. This
˚
2 are given in Tables 2 and 3, respectively. The common
key feature of the structures is the stacking of square-grid
layers of cobalt-oxygen separated by the phenyl rings of
the ligands (Figs. 2 and 3). The layers are parallel to the bc
plane separated by the ligands along the a-axis, giving
suggests that the grids are highly flexible but retain their
planarity.
The cobalt ions exhibit octahedral geometry comprising
four oxygen atoms of carboxylate groups and two oxygen
atoms of –OH groups for 1 or two nitrogen atoms of –NH2
groups for 2. These O (OH) or N (NH2) atoms coordinate
in trans-positions. In contrast to the coordination of
Co(H2O)2Ln the additional coordinating site of the ligands
occupy the sites of the water molecules. Consequently, the
presence of the –OH or –NH2 group results in the
chemical formula CoL2 (L ¼ mandelate or phenylglyci-
nate) without the need of coordinated water. The –OH and
–NH2 groups coordinate to Co(II) centers to give O–C–
C–OH (–NH2) five-membered chelate ring. The Co–O
and Co–N are both normal. However, the bonding of the
cobalt to the –OH or –NH2 groups exerts severe distortion
of the octahedra. The bond angles lie in the range 78.40(7)
to 101.60(7)8 and 79.60(9) to 100.40(9)8 for 1 and 2,
respectively. Although, these distortions are present in
the Co(H2O)2Ln compounds, they are more pronounced for
1 and 2. The structural difference between the two
Co(H2O)2Ln and CoL2 is principally due to the small
chelate angles of the ligands. The third coordination by the
ligands through the additional group on the a-carbon exerts
severe distortion on the carboxylate groups and conse-
quently, it results in the compression of the squares
compared to Co(H2O)2Ln.
˚
interlayer distances of ca. 15.5 A. The interlayer separation
is very close to that found in Co(H2O)2(trans-cinnamate)2
[21]. Each layer contains octahedral Co(II) ions connected
by four equatorially coordinated O–C–O bridges of the
carboxylate groups. Within the layer, the three distinct
˚
Co–Co distances are 4.89, 5.28 and 9.37 A 1 and 4.99,
˚
5.35 and 9.46 A for 2. Square arrangement of Co(II)
ions was previously found in Co(H2O)2Ln (n ¼ 1 : benzene
1,4-dicarboxylate or terephthalate and 4-carboxycinna-
mate, n ¼ 2 : trans-cinnamate) type compounds and
Co(formate)2(urea)2 [22,23]. In contrast to the latter,
Table 2
˚
Selected bond distances (A) and angles (8) of [Co(O2CCH(OH)C6H5)2] (1)
Atom
Atom
Distance
Co(1)
Co(1)
Co(1)
O(1)
O(2)
O(3)
2.048(2)
2.160(2)
2.114(2)
Atom
O(1)
O(1)
O(1)
O(1)
O(2)
O(1)
O(2)
O(20)
O(3)
Atom
Co(1)
Co(1)
Co(1)
Co(1)
Co(1)
Co(1)
Co(1)
Co(1)
Co(1)
Atom
O(10)
O(20)
O(30)
O(2)
O(20)
O(3)
O(3)
O(3)
O(30)
Angle
180.0
97.42(7)
101.60(7)
82.58(7)
180.0
We anticipated that the use of optically pure ligands may
result in optically active coordination polymers as has been
seen in some cases [24]. In fact, the racemisation of the
ligand in the presence of a base, which is needed for the
reactions, resulted in compounds having a racemic mixture
78.40(7)
90.08(7)
89.92(7)
180.0

58
H. Kumagai et al. / Journal of Physics and Chemistry of Solids 65 (2004) 55–60
Fig. 2. View of the layer structures of (a) Co(mandelate)2 (1) and (b) Co(phenylglycinate)2 (2) showing the three coordination bonds of each ligand. The
benzene rings are omitted for clarity.
of the two enantiomers (Fig. 4). Interestingly, within
the space group of the crystals the two enantiomers are
forcibly segregated on each side of the layer, respectively.
The mid-infrared spectra of the compounds measured by
transmission in a KBr pellet display bands at 3255 cm21
(OH), 1573 cm21 (antisymmetric carboxylate stretching) and
1408 cm21 (symmetric carboxylate stretching) for
Co(mandelate)2; 3358 and 3247 cm21 (NH), 1566 cm21
(antisymmetric carboxylate stretching) and 1395 cm21
(symmetric carboxylatestretching) forCo(phenylglycinate)2.
The large difference between the antisymmetric and
symmetric stretching modes of the carboxylate is in
agreement with that expected for an anti–anti coordination
mode [25].
distortion of the octahedral may have resulted in a lower
spin-orbit coupling [26]. However, a small but finite
exchange interaction between neighbouring cobalt atoms
cannot be excluded.
The magnetic properties of the compounds were
measured on polycrystalline samples by use of a SQUID
(Quantum Design) magnetometer in the temperature range
2–300 K and field up to 5 T. The results are shown in
Figs. 5 and 6. The absolute values and the temperature
dependence of the moments, represented as the product of
susceptibility and temperature, are typical for the presence
of an orbital contribution for Co(II) [26]. The data above
100 K were fitted to the Curie–Weiss law with Curie
constant 3.3 ^ 0.2 cm3 K mol21 and Weiss temperature of
211.1 ^ 0.1 K for 1 and 28.5 ^ 0.3 K for 2. These
values are in good agreement with those expected for
almost non-interacting Co(II). The Weiss constants
are, however, less than that estimated (ca. 220 K) if
a spin-orbit contribution is included for a perfect
octahedral complex of cobalt(II), suggesting that the severe
Fig. 3. View of the structure of Co(phenylglycinate)2 showing the stacking
of the layers along the a-axis. Note that there is no interleaving of the
phenyl rings.

H. Kumagai et al. / Journal of Physics and Chemistry of Solids 65 (2004) 55–60
59
Fig. 4. View of the layer structure of Co(phenylglycinate)2 highlighting the segregation of the enantiomers D and L on the opposite sides of the layer. The carboxy
carbon atom is drawn in large grey ball, nitrogen and hydrogen in small light grey balls. The hardedness is shown by the tetrahedral coordination of the a carbon.
4. Conclusion
they are segregated on the two faces of the layer by
symmetry. The coordination of –OH or –NH2 groups yield
CoL2 type compounds in which the same square-grid
structure was found as in other Co(H2O)2Ln. The non-
interacting moment on the cobalt atoms suggests that the O–
C–O bridges are less effective for magnetic exchange [6,22].
In summary, two new layered compounds have been
identified in the hydrothermal reaction of Co(II) and
mandelic acid or phenylglycine in alkaline media. Racemi-
sation of the ligands occurred during the hydrothermal
reaction results in the coordination of both enantiomers and
Fig. 5. Temperature dependence of the xT product and 1=x for
Co(mandelate)2 (circles) and Co(phenylglycinate)2 (crosses).
Fig. 6. Isothermal magnetisation of Co(mandelate)2 (circles) and
Co(phenylglycinate)2 at 2 K. Lines are guides to the eyes.

60
H. Kumagai et al. / Journal of Physics and Chemistry of Solids 65 (2004) 55–60
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This work was funded by a Grant-in-Aid for scientific
research from the Ministry of Education, Science, Sports,
and Culture (Japan) and the CNRS (France). H.K. thanks the
JSPS for a Young Scientist Fellowship for his stay in
Strasbourg.
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