Synthesis, structural aspects and magnetic properties of an unusual 2D thiocyanato-bridged cobalt(II)-Schiff base network

Article abstract of DOI:10.1016/j.ica.2010.07.069

A new two-dimensional (2D) thiocyanato-bridged cobalt(II) network formulated as [LCo₂(NCS)₂]_n (1), has been synthesized with the Schiff base ligand N,N′-bis(3- methoxysalicylidenimino)-1,3-diaminopropane (H₂L) a

Full text of DOI:10.1016/j.ica.2010.07.069

Inorganica Chimica Acta 363 (2010) 3981–3986
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, structural aspects and magnetic properties of an unusual 2D
thiocyanato-bridged cobalt(II)–Schiff base network
a
b
c
d
a,*
Santarupa Thakurta , Ray J. Butcher , Carlos J. Gómez-García , Eugenio Garribba , Samiran Mitra
a
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
Department of Chemistry, Howard University, 2400 Sixth Street, NW Washington, DC 20059, USA
Instituto de Ciencia Molecular (ICMol), Parque Científico, University of València, 46980 Paterna, Spain
Department of Chemistry and Center for Biotechnology Development and Biodiversity Research, University of Sassari, Via Vienna 2, I-07100 Sassari, Italy
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
2 2 n
A new two-dimensional (2D) thiocyanato-bridged cobalt(II) network formulated as [LCo (NCS) ] (1), has
Received 5 January 2010
Received in revised form 18 July 2010
Accepted 24 July 2010
0
been synthesized with the Schiff base ligand N,N -bis(3-methoxysalicylidenimino)-1,3-diaminopropane
L) and thiocyanate anions. This novel layered compound has been completely characterized by
(H
2
elemental analysis, FT-IR, UV–Vis spectroscopy and the structure has been established by single crystal
X-ray diffraction studies. The structure of 1 consists of a doubly phenoxo-bridged dimer comprising
two different cobalt(II) centers with different coordination geometries (octahedral and tetrahedral).
The 2D network is accomplished by bridging thiocyanate ligands, connecting the dimeric motifs in an
end-to-end fashion. Variable-temperature magnetic susceptibility and EPR measurements reveal
predominant antiferromagnetic exchange interaction in the complex.
Available online 30 July 2010
Keywords:
Cobalt(II)
Schiff base
Thiocyanato-bridged 2D polymer
Mixed Oh-Td geometries
Antiferromagnetic coupling
Ó 2010 Elsevier B.V. All rights reserved.
1
. Introduction
The rational design and construction of novel polymeric coordi-
both ferro and antiferromagnetic interactions between the para-
magnetic metal ions [13–17], depending on the bridging modes
and the localized structural details. However, in contrast to the
vast chemistry of azido-bridged complexes, only limited number
of structurally as well as magnetically characterized thiocyanato-
bridged compounds of divalent first row transition metals are re-
nation complexes have attracted considerable attention in recent
years not only due to their fascinating structural networks but also
for their interesting properties and applications in fields as cataly-
sis, ion-exchange, non-linear optical and magnetic materials. Thus,
a number of covalent polymers with wide variety of topologies of
one-, two-, or three-dimensional frameworks including chain,
ladder, sheet, diamondoid, rectangular-grid, honeycomb, etc., have
been established [1–6]. Several factors, such as the nature of
the metal ions, the geometry of the organic ligands and the suitable
reaction conditions, should be taken into consideration for the
preparation of desired metal–organic frameworks (MOF) having
extended networks. In this context, selection of appropriate short
bridging ligand often turns out to be the key step for synthesis of
such polymeric assemblies. Due to the versatile coordination
modes of the ambidentate thiocyanate ligand [7,8], this linear
triatomic pseudohalide has become a common bridging block to
prepare these multi-dimensional complexes. As a bridging ligand
the thiocyanate anion can usually link a pair of metal centers in
II
ported in literature and most of the fully studied complexes are Ni
II
II
and Cu dimers, 1D chains and 2D layers [18–21]. However, for Co
compounds, the existing results are quite scarce. Thus, there are
only four fully structurally and magnetically characterized –SCN–
II
Co bridged systems (all of them present double –SCN– bridges
II
and octahedrally coordinated Co ions): (i) the alternating chain
3 n
compound [Co(l1,3-SCN)(l-MPyO)(SCN)(CH OH)] , where DFT
calculations and experimental measurements show that the Co–
SCN–Co double bridge gives rise to a very weak antiferromagnetic
coupling [22], (ii) the 2D compound [Co(
where a very weak ferromagnetic coupling is predicted for the
double –SCN– bridge [23], (iii) the 1D compound [Co(SCN)
(bim)], that presents a weak ferromagnetic coupling through a
double –SCN– bridge [24] and (iv) the 2D compound [Co(SCN)
2 n
l1,3-SCN) (l1,6-dmpzdo)]
2
-
2
-
(bte)], that presents a metamagnetic behavior with competing
weak ferro (through double –SCN– bridges) and antiferromagnetic
coupling (through the bte ligand) [24].
either an end-on (
l1,1-NCS,
l1,1-SCN) [9,10] or an end-to-end
(l
1,3-NCS) [11,12] configuration. Also, thiocyanate can mediate
An effective synthetic strategy to construct controllable
polymeric frameworks is self-assembly [25] of different building
components, like metal–ion templates, organic blockers and
*
Corresponding author. Tel.: +91 033 2414 6666x2779; fax: +91 033 2414 6414.
E-mail address: smitra_2002@yahoo.com (S. Mitra).
0
020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.07.069

3
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S. Thakurta et al. / Inorganica Chimica Acta 363 (2010) 3981–3986
bridging units. This assembly process can be successfully applied in
trile solvent mixture (3:1 v/v) was added drop-wise. The resulting
deep brown solution was stirred for 15 minutes followed by the
addition of sodium thiocyanate (0.162 g, 2 mmol) in a minimum
volume of methanol. The reaction mixture was stirred at room
temperature for 30 min and then it was left undisturbed. After
4 days, olive-brown prismatic single crystals suitable for X-ray
0
0
the presence of rigid N,N -bidentate spacers and pyrazine, 4,4 -
bipyridine, imidazole, etc., have been frequently employed to pro-
duce numerous multi-dimensional arrays and networks [26–30].
On the other hand, an assembly process containing flexible multid-
entate ligands is usually very complicated, especially in the case of
Schiff base ligands which can adopt different kinds of chelating and
bridging modes depending on the reaction conditions. Owing to
their conformational flexibility such ligands are capable to induce
a wide variety of structural topologies among which robust 2D
and 3D covalent networks of transition metal complexes are very
rare. Thus, the development of specific supramolecular 2D and
diffraction appeared. Yield: 72%. Anal. Calc. for C21
2 4 4 2
H20Co N O S :
C, 43.91; H, 3.51; N, 9.75. Found: C, 43.88; H, 3.54; N, 9.79%.
2.3. Physical measurements
The Fourier transform infrared spectra of the ligand and the
complex were recorded on a Perkin–Elmer Spectrum RX I FT-IR
3
D architectures based on Schiff bases still remains as a major
challenge for chemists.
spectrophotometer with KBr pressed pellets in the range 4000–
The present work stems from our interest to prepare coordina-
tion polymers of cobalt(II) in combination with N,O-donor Schiff
bases and thiocyanate anions. Our research group has already re-
ported some thiocyanato-bridged multinuclear metal complexes
using different tri and tetradentate Schiff base ligands [31–33].
Here, we have utilized the potentially hexadentate compartmental
ꢀ1
2
00 cm . The electronic spectra were recorded on a Perkin–Elmer
Lambda 40 (UV–Vis) spectrometer using HPLC grade acetonitrile as
solvent in the range 800–200 nm. Elemental analyses were carried
out on a Perkin–Elmer 2400 II Elemental Analyser. Electron para-
magnetic resonance (EPR) spectra of the complex were recorded
on polycrystalline sample as a function of temperature with an
X-band Bruker EMX spectrometer. The magnetic susceptibility
measurements were carried out in the temperature range 2–
0
Schiff base N,N -bis(3-methoxysalicylidenimino)-1,3-diaminopro-
pane (H
1) where doubly phenoxo-bridged dimeric building blocks are
connected by 1,3-thiocyanate bridges to generate a layered 2D
2 2 2 n
L) to synthesize a novel infinite 2D polymer [LCo (NCS) ]
(
3
00 K with an applied magnetic field of 0.1 T on polycrystalline
l
sample of complex 1 (mass = 44.64 mg) with a Quantum Design
MPMS-XL-5 SQUID magnetometer. The isothermal magnetization
was performed on the same sample at 2 K with magnetic fields
up to 5 T. The susceptibility data were corrected for the sample
holders previously measured using the same conditions and for
the diamagnetic contributions of the salt as deduced by using Pas-
network. Here we report the synthesis, spectroscopic, structural
and magnetic characterization of complex 1 which is, to the best
II
of our knowledge, the first Co Schiff base polymeric complex com-
prising a dinuclear repeating unit having both octahedral and tet-
II
rahedral Co centers. Furthermore, the title compound is also the
II
first fully structurally and magnetically characterized Co complex
ꢀ6
ꢀ1
cal’s constant tables (
.4. Crystallographic data collection and structure refinements
The intensity data collection of 1 was performed with an Oxford
v
dia = ꢀ310.4 ꢂ 10 emu mol ).
presenting a single
. Experimental
.1. Materials
All the chemicals and solvents employed for the synthesis were
l1,3-thiocyanate bridge.
2
2
2
Diffraction Gemini R-Ruby diffractometer using multi scan tech-
nique at 296 K. Data collection and unit cell refinement were per-
formed using CRYSALISPRO software [34]. The structure was solved by
direct methods with SHELXS and refined by full-matrix least squares
of analytical grade. 1,3-Diaminopropane and 3-methoxy-2-
hydroxybenzaldehyde were purchased from Aldrich Co., USA. So-
dium thiocyanate (Aldrich, USA) was used as received. Cobalt(II)
perchlorate hexahydrate was prepared by the treatment of cobalt
carbonate (E. Merck, India) with 80% perchloric acid (E. Merck, In-
dia) followed by slow evaporation. It was then filtered through a
2
methods based on F with SHELXL-97 [35]. The non-hydrogen atoms
were refined with anisotropic factors. All hydrogen atoms were
positioned geometrically and treated as riding on the bound atom.
All calculations and graphical works for the complex have been
done using PLATON [36] and WINGX [37] programs. Selected crystallo-
graphic data and some important structure refinement parameters
of complex 1 are summarized in Table 1.
2
fine glass-frit and preserved in a CaCl desiccator.
Caution! Perchlorate salts in presence of organic materials are
potentially explosive. They should be prepared in small amounts
and handled with extreme care.
3
. Results and discussion
2.2. Syntheses
A number of discrete 3d–4f hetero-metallic compounds have al-
ready been synthesized from the di-compartmental Schiff base H
and its close derivatives [38–41] where the internal N cavity of
the Schiff base is adequate for complexation of the 3d ions and the
2 2
external O cavity is well suited for the 4f ions. But N + O O
homo-metallic 3d–3d complexes are very rare as the ionic radius of
the 3d metal usually fits in only one of the two cavities. In case of
2
L
2
2
.2.1. Synthesis of the Schiff base ligand
2 2
O
0
.2.1.1.
N,N -bis(3-methoxysalicylidenimino)-1,3-diaminopropane
(H
2
L). The ligand H
hydroxybenzaldehyde (20 mmol, 3.04 g) and 1,3-diaminoprapane
10 mmol, 0.833 ml) in 50 ml of methanol at 65 °C for 1 h. Upon
2
L was prepared by refluxing 3-methoxy-2-
2
O
2
2
O
2
(
cooling the solution, a deep-yellow crystalline solid was precipi-
tated which was collected as the ligand for the synthesis of com-
2
complex 1, the multidentate H L ligand serves as a dinucleating
one, bridging two cobalt(II) ions through two phenolic oxygen
atoms to accommodate the cobalt(II) ions in two different com-
partments. In this regard, complex 1 represents a unique example
to study the magnetic properties of homo-metallic transition metal
compounds in such ligand environment. Furthermore, here we
have strategically increased the nuclearity of the complex by utiliz-
ing thiocyanate as suitable bridging co-ligand which allows con-
trolled aggregation of the dimeric units affording the robust two-
dimensional polymeric network.
plex 1. Yield: 90%. Anal. Calc. for C19
N, 8.18. Found: C, 66.71; H, 6.42; N, 8.08%. FT-IR (KBr, cm ):
(C@N) 1640. UV–Vis (k, nm): 258, 398.
22 2 4
H N O : C, 66.65; H, 6.48;
ꢀ1
m
2
2
ClO
.2.2. Synthesis of the complex
.2.2.1. [LCo (NCS) (1). To a 20 ml methanolic solution of Co(-
ꢁ6H O (0.732 g, 2 mmol), a solution of the Schiff base ligand
L (0.342 g, 1 mmol) dissolved in 15 ml of a methanol–acetoni-
2
2 n
]
4
)
2
2
H
2

S. Thakurta et al. / Inorganica Chimica Acta 363 (2010) 3981–3986
3983
Table 1
Summary of crystal data and refinement details for 1.
Empirical formula
Formula weight
Crystal dimension (mm)
Crystal system
Space group
a (Å)
C
21
H
20Co
2 4 4 2
N O S
574.39
0.23 ꢂ 0.25 ꢂ 0.45
orthorhombic
P b c n
11.0904(3)
11.5669(3)
18.3876(5)
90
b (Å)
c (Å)
b (°)
3
V (Å )
2358.79(11)
4
Z
T (K)
D
296(2)
1.617
1.619
calc (g cmꢀ3)
ꢀ
1
l
(mm
)
F(0 0 0)
1168
h (°)
Total data
4.64–32.55
17 571
Unique data
3982
Observed data [I > 2
r
(I)]
1507
0.0457
0.1413
1.01
a
R
b
R
w
2
Goodness-of-fit (GOF) on F , S
R
D
D
int
0.0650
0.44
ꢀ0.36
ꢀ
3
q
maximum (e Å
)
q
minimum (e Åꢀ3)
a
R =
R
(|F
R
o
ꢀ F
c
|)/
R
|F
o
|
b
2
2
½
R
w
= { [w(|F
o
ꢀ F
c o
|) ]/R[w|F | ]}
3.1. Crystal structure description
Fig. 1b. Assembly of dinuclear units through end-to-end thiocyanate bridges.
(
Symmetry code: i x + 1/2, y + 1/2, ꢀz + 1/2, ꢀx + 1/2, y + 1/2, z).
ii
The structure of complex 1 consists of an infinite 2D layer
formed by the polymeric [LCo (NCS) system where the building
block is the dimeric [LCo (NCS) ] moiety incorporating the Schiff
2
2 n
]
2
2
Table 2
2ꢀ
base L and two thiocyanate anions as shown in Fig. 1a. The
Selected bond lengths and bond angles for 1.
dimeric motifs are connected to each other by out-of-plane, end-
Bond lengths (Å)
to-end (
network of complex 1 (Fig. 1b). All the selected bond distances
and angles for complex 1 are listed in Table 2. The [LCo (NCS)
unit has two crystallographically different metal centers Co(1)
and Co(2), and one half of the unit is symmetry related to the other
l
1,3-) thiocyanate bridges to generate the extended
Co(1)–O(1)* 2.006(2)
Co(1)–O(1) 2.006(2)
Co(1)–N 2.029(3)
Co(2)–O(1) 1.995(2)
Co(2)–O(1)* 1.995(2)
Co(2)–N(1A)* 1.970(3)
Co(2)–N(1A) 1.970(3)
S(1A)–C(1A) 1.660(9)
N(1A)–C(1A) 1.143(4)
2
2
]
Co(1)–N* 2.029(3)
Co(1)–S(1A)i 2.727(7)
ii
Co(1)–S(1A) 2.727(7)
Bond angles (°)
O(1)*–Co(1)–O(1) 78.51(12)
O(1)–Co(1)–N 91.47(10)
N–Co(1)–N* 98.56(17)
N(1A)*–Co(2)–N(1A) 112.4(2)
N(1A)*–Co(2)–O(1) 117.90(12)
N(1A)–Co(2)–O(1) 113.00(11)
O(1)–Co(2)–O(1)* 79.01(12)
C(1A)–N(1A)–Co(2) 169.2(3)
Co(1)–O(1)–Co(2) 101.24(9)
i
O(1)*–Co(1)–S(1A) 89.4(3)
i
O(1)–Co(1)–S(1A) 79.41(19)
i
ii
S(1A) –Co(1)–S(1A) 165.6(3)
O(1)–Co(1)–N* 169.95(10)
i
Symmetry transformations used to generate equivalent atoms: * ꢀx + 1, y, ꢀz + 1/2
ii
0
x + 1/2, y + 1/2, ꢀz + 1/2 ꢀx + 1/2, y + 1/2, z x ꢀ 1/2, y ꢀ 1/2, ꢀz + 1/2.
half because of the presence of a crystallographic twofold axis
passing through the Co(1)ꢁ ꢁ ꢁCo(2) vector (symmetry code: *
ꢀ
x + 1, y, ꢀz + 1/2). The compartmental Schiff base H
2
L ligand
cavity, while both
ðimineÞ ðphenolicÞ
binds Co(1) center in the inner N
O
2
2
deprotonated phenolic oxygens [O(1) and its symmetry related
counterpart O(1)*] further bridge to Co(2) center forming a
II
II
bis(
l
2
2
-phenoxo)-bridged Co –Co motif. Interestingly, the bridging
segment is fully planar; the Co(1)–O(1)–Co(2) bridge angle
2
Co O
and the Co(1)ꢁ ꢁ ꢁCo(2) distance within the dinuclear core are
°
1
01.24(9) and 3.092 Å, respectively. The two thiocyanate groups
which are bonded to the Co(2) center through the N atoms
[
N(1A) and its symmetry related counterpart N(1A)*] act as inter-
0
molecular bridges and are bonded to the Co(1) centers of the adja-
cent molecules (symmetry code:
through the S atoms [S(1A) and its symmetry related counterpart
S(1A)*] to provide an infinite 2D network extended by Co-NCS-Co
0
x ꢀ 1/2, y ꢀ 1/2, ꢀz + 1/2)
Fig. 1a. ORTEP representation of the dimeric unit complex 1. Thermal ellipsoids
have been drawn at 40% probability level. (Symmetry code: * ꢀx + 1, y, ꢀz + 1/2).

3
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S. Thakurta et al. / Inorganica Chimica Acta 363 (2010) 3981–3986
linkages (Fig. 2) with Coꢁ ꢁ ꢁCo separation across the chain of
.164 Å. The most noticeable feature of this complex is the varia-
6
tion in the coordination environments of the two metal centers
within the dimeric repeat unit. Thus, the geometry of Co(1) can
be best described as distorted octahedral (Fig. 1b) with the equato-
rial plane being formed by two imine nitrogen atoms and two
bridging phenoxo oxygen atoms of the deprotonated ligand. The
axial sites of the octahedron are occupied by two sulfur atoms
i
ii
[
S(1A) and S(1A) ] of two
l(N,S)-bridging NCS groups, belonging
i
to neighbouring molecules (symmetry code:
x + 1/2, y + 1/2,
ꢀx + 1/2, y + 1/2, z). The axial Co–S bond lengths
2.727(7) Å] are considerably longer compared to the bond dis-
ii
ꢀ
z + 1/2,
[
tances in the equatorial plane [2.006(2)–2.029(3) Å], even after
taking into account the larger covalent radius of sulfur. The Co–
Ophenoxo and Co–Nimine bond lengths are consistent with the corre-
sponding values observed in other Schiff base cobalt(II) complexes
[
[
42]. The ranges of cisoid [78.51(12)–99.90(2)°] and transoid
165.6(3)–169.95(10)°] angles reflect significant distortion of the
CoN
2 2 2
O S chromophore from ideal octahedral geometry. The
(pheno-
geometry of the Co(2) center residing in the outer O
2
lic)
(methoxy)
O
2
compartment can be described as a distorted tetrahe-
Fig. 3. A perspective view of highly corrugated 2D layer along crystallographic b
dral environment comprising the two bridging phenoxo oxygen
atoms of L and the two nitrogen atoms from the two coordinated
axis.
2ꢀ
thiocyanate ions (Fig. 1b). The values of the Co–Omethoxy distances
CoII based on a Schiff base ligand. Recently, a hetero-metallic
(
2.533 Å) indicate that they are large enough to discard the eventu-
II
II
Cu –Gd thiocyanato-bridged coordination polymer derived from
the H L ligand was reported [46] where only one of the thiocyanate
ligands participates in bridging, resulting in an alternating zigzag
ality of Co–O bonds, so the methoxy groups remain non-coordi-
nated. The Co–N(thiocyanate) bond distances [1.970(3) Å] are in
agreement with those observed in other structurally characterized
2
II
II
2ꢀ
1D chain structure. In contrast to this Cu –Gd complex, both thio-
cyanate ligands are involved in bridging in case of complex 1,
therefore the structure is a very original square 2D network.
[
Co(NCS)
4
]
complexes [43]. The N(1A)–Co(2)–N(1A)* angle with
a value of 112.4(2)° approximate the ideal tetrahedral angle,
whereas the O(1)–Co(2)–O(1)* angle [79.01(12)°] is much smaller
in order to fulfill the geometrical constraint of the ligand L2 . Sim-
ilar ranges of bond angles have previously been reported for some
tetrahedral complexes having bidentate chelating ligands and ter-
ꢀ
3.2. Infrared spectra
ꢀ
ꢀ
minal co-ligands such as Cl , NCS , etc. [44,45]. Although the O
coordination sphere around Co(2) atom is quite distorted, the angle
between the CoN and the CoO planes (85.84°) is close to the an-
2
N
2
The solid state Fourier transform infrared spectrum of complex
ꢀ
1
1 shows a strong absorption band at 1603 cm
that can be
assigned to the imine stretching frequency of the coordinated
ligand, whereas for the free ligand molecule the same band is ob-
2
2
gle of 90° in a perfect tetrahedron. The NCS groups are almost lin-
ear with N(1A)–C(1A)–S(1A) angle of 177.2(6)° while the Co–N–C
linkages are bent with a Co(2)–N(1A)–C(1A) angle of 169.2(3)°,
deviating from 180° expected for the sp hybrid orbital of the nitro-
gen atom. Overall, the octahedral and tetrahedral cobalt(II) centers
of the adjacent units are connected sequentially through end-to-
end thiocyanate bridges generating a square 2D network (Fig. 2).
Moreover, Fig. 3 illustrates a highly corrugated view of the layered
polymer along crystallographic b axis with the thiocyanate bridges
propagating like waves. To the best of our knowledge, complex 1 is
the first mixed phenoxo- and thiocyanato-bridged 2D polymer of
ꢀ1
served at 1640 cm . The shift of this band towards lower frequen-
cies on complexation with the metal suggests coordination to the
metal ion through the imine nitrogen atom [47,48]. The phenolic
ꢀ1
C–O stretching is observed at 1272 cm for 1. The ligand coordi-
nation to the metal center is further evidenced by prominent bands
ꢀ1
appearing at nearly 455 and 375 cm which can be attributed to
ꢀ1
m
(Co–N), and m(Co–O), respectively. The bands at 2930 cm can
be correlated to the alkyl C–H bond stretching of the alkoxy groups.
The end-to-end bridging mode of the thiocyanate group is con-
ꢀ1
firmed by the splitting of the absorption band at ca. 2117 cm cor-
responding to the C@N asymmetric stretching that appears now at
m
ꢀ
1
2
140 and 2088 cm indicating, respectively, the S- and N-coordi-
nation modes of the thiocyanate ligand [49].
3.3. Electronic spectra
Complex 1 exhibits intense band at 350–385 nm which can be
attributed to the charge transfer transition from the coordinated
unsaturated ligands to the metal ion (LMCT) [50]. Again, the strong
bands at about 245 and 380 nm can be assigned to the intraligand
p
? p* and n ? p* transitions, respectively. The visible region of
the spectrum is dominated by tetrahedral electronic transitions,
since d–d bands in tetrahedral crystal fields are typically more in-
tense (by about 10ꢂ) than those in octahedral environments given
the p–d mixing due to the absence of an inversion center in the for-
mer. Thus the absorption bands occurring in the 536–646 nm re-
4
4
II
gion correspond to the transition
ion [51,52].
2 1
A ? T (P) of tetrahedral Co
Fig. 2. A perspective view of the square-grid 2D network along crystallographic c
axis.

S. Thakurta et al. / Inorganica Chimica Acta 363 (2010) 3981–3986
3985
3.4. Magnetic studies
0.25
The thermal variation of the molar magnetic susceptibility per
0
0
.20
.15
II
Co dimer times the temperature (
v
m
T) for complex 1 shows a va-
lue of ca. 0.33 emu K mol at room temperature (Fig. 4). Upon
cooling down the sample, the T product shows a continuous de-
crease to reach a value of ca. 0.23 emu K mol at 20 K. Below this
temperature T shows a much more pronounced decrease to
ꢀ
1
v
m
ꢀ1
v
m
0.10
ꢀ1
reach a value of ca. 0.08 emu K mol at 2 K. This behavior suggests
that complex 1 presents a strong antiferromagnetic coupling that
cancels, almost completely, the magnetic moment even at high
temperatures. An additional proof of the presence of a strong anti-
ferromagnetic coupling in this complex comes from the isothermal
0
0
.05
.00
0
.0
0.5
1.0
1.5
2.0
2.5
magnetization at 2 K that shows a very low saturation value of ca.
-
1
II
H/T (T.K
)
0
.22
6].
l
B
(Fig. 5), well below the expected value for a Co dimer [53–
5
Fig. 5. Isothermal magnetization at 2 K of complex 1.
As mentioned above, the strong antiferromagnetic coupling is
clearly confirmed with the low temperature value
0.16 emu K mol per Co ion), which is significantly lower than
m
v T
ꢀ1
II
of six for the other CoII ion [60,61] (complex 1 is, thus, the third
example of this type and the first one magnetically characterized).
The remaining 12 complexes present coordination indices of 4 for
(
II
the values usually observed in tetrahedral and octahedral Co com-
plexes, where the observed T values are in the ranges 2.4–2.9
v
m
ꢀ1
II
II
and 2.7–3.4 emu K mol , respectively, per Co ion [57–59]. Since
both Co centers. Therefore, it is not surprising that all the mag-
ꢀ1
the
v
m
T value at room temperature (0.32 emu K mol per dimer)
neto-structural correlations established for double alkoxo- or
II
represents only ca. 5% of the expected value for two non interact-
phenoxo-bridged Co complexes assume a tetrahedral or distorted
II
ꢀ1
II
ing Co ions (ca. 6 emu K mol ), this small magnetic contribution
may have two possible origins: (i) a small amount of monomeric
tetrahedral geometry for the Co centers [62], but so far none has
been carried out for mixed four and six coordination indices. Fur-
thermore, these studies are inadequate and uncertain because of
the scarcity of structurally and magnetically characterized com-
pounds of this type, precluding any application to complex 1
[62]. Nevertheless, we can conclude that complex 1 presents a
II
Co impurity and (ii) a contribution due to the incomplete cancel-
lation of the magnetic moments of both Co ions since they present
II
different coordination environments and, therefore, different g val-
ues. The thermal behavior of this residual magnetic moment shows
II
maximum at ca. 3 K in the
v
m
versus T plot (inset in Fig. 4), indicat-
strong antiferromagnetic coupling between the two Co ions
ing the presence of a weak antiferromagnetic interaction that sup-
through the double phenoxo bridge owing to the presence of a high
degree of overlap of the magnetic orbitals and that the low residual
magnetic moment observed in this compound can be reasonably
attributed to the incomplete cancellation of the magnetic moments
given the different coordination environments (and, therefore,
ports the second possible origin for this magnetic signal. Note that
II
a hypothetical magnetic signal arising from isolated Co monomers
II
(
due to Co vacancies in a small fraction of the dimers) should
rather behave in a paramagnetic way. Furthermore, the long Co–
SCN–Co bridges observed in complex 1 are expected to give rise
to a weak magnetic coupling, in agreement with the observed
behavior in the very few magnetically characterized thiocyanate-
II
magnetic moments) for both Co ions.
3.5. EPR spectra
II
bridged Co complexes [22–24].
In order to explain the presence of such a strong antiferromag-
netic coupling in complex 1, we have performed a careful search in
the CCDC database. This search shows that there are only 21 struc-
EPR spectra on the polycrystalline powder of 1 were recorded at
liquid helium (10 K), liquid nitrogen (100 K) and room temperature
298 K). In all the cases, no signal was observed, demonstrating the
antiferromagnetic coupling between the cobalt ions, observed for
(
II
turally characterized Co complexes with a double alkoxo (or
II
phenoxo) bridge in which at least one of the Co ions presents a
II
other di-(l-phenoxo) Co complexes [63].
II
coordination index of four. Out of these 21 complexes, 14 are Co
As an example, the RT spectrum of 1 is shown in Fig. S1 (Supple-
mentary material). The EPR signal is due only to the S = 0 spin state,
suggesting that the antiferromagnetic coupling is so strong that the
excited states are not populated even at rather high temperatures.
This confirms the results of the variable-temperature magnetic
susceptibility measurements that indicate a strong intradimeric
antiferromagnetic interaction already at room temperature. The
region with an asterisk is attributable to a small amount of a
monomeric impurity, detected also by the molar susceptibility
measurements (see Fig. 4).
dimers and among these 14, only two exhibit a coordination index
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0.050
0
0
0
0
0
0
.045
.040
.035
.030
.025
.020
In a mixture of non-coordinating solvents, like CH
2 2
Cl /toluene
5
0:50 v/v or CHCl /toluene 60:40 v/v, EPR spectra of 1 are very
3
similar to those obtained on the polycrystalline powder; this
means that the solid state structure is retained in these experimen-
tal conditions.
0.015
0
2
4
6
8
10 12 14
T (K)
0
50
100
150
T (K)
200
250
300
4
. Conclusion
In summary, we have reported herein a novel coordination
polymer of cobalt(II) ion prepared from a versatile Schiff base li-
Fig. 4. Thermal variation of the
temperature region of the molar susceptibility (
v
m
T product for complex 1. Inset shows the low
vs. T).
v
m

3
986
S. Thakurta et al. / Inorganica Chimica Acta 363 (2010) 3981–3986
gand and thiocyanate bridging ligands. The primary building block
of complex 1 is a very rare dimeric unit containing octahedral and
tetrahedral cobalt(II) centers bridged by two phenolic oxygen
atoms of the deprotonated ligand L2 . These dimeric motifs are
connected to each other through end-to-end bridging thiocyanate
groups resulting in the formation of an original square 2D infinite
network. Complex 1 represents the first magnetically and structur-
ally characterized complex presenting tetrahedral and octahedral
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5
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Acknowledgements
Santarupa Thakurta gratefully acknowledges the Council of
Scientific and Industrial Research, New Delhi, India for the award
of Senior Research Fellowship to her (CSIR Sanction No.09/
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MANet network of excellence), the Spanish Ministerio de Educa-
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2
Appendix A. Supplementary material
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CCDC 754994 contains the supplementary crystallographic data
for complex 1. These data can be obtained free of charge from The
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data_request/cif. Supplementary data associated with this article
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