Self-assembled cobalt(II) Schiff base complex: Synthesis, structure, and magnetic properties

Article abstract of DOI:10.1007/s00706-011-0521-7

A mononuclear complex [CoL₂Cl₂]·3.5H ₂O (L = 2-[(2,2-diphenylethylimino)methyl]pyridine-1-oxide) has been synthesized and characterized by X-ray structure analysis. The crystal structure confirms the formation of an interesting porous framework with channel diameters of about 8 A through weak C-H···π and C-H···Cl interactions. The magnetic properties of this complex have also been studied, and the susceptibility and magnetization data were analyzed in terms of the spin Hamiltonian formalism. They confirm substantial zero-field splitting, D/hc = 75 cm^-1. Springer-Verlag 2011.

Full text of DOI:10.1007/s00706-011-0521-7

Monatsh Chem (2011) 142:789–795
DOI 10.1007/s00706-011-0521-7
ORIGINAL PAPER
Self-assembled cobalt(II) Schiff base complex: synthesis,
structure, and magnetic properties
•
Cyril Rajnak Jan Titis Roman Boca
•
•
´
Jan Moncol’ Zdenka Padelkova
´
ˇ
ˇ
•
´
ˇ
ˇ
´
Received: 8 February 2011 / Accepted: 2 May 2011 / Published online: 31 May 2011
Ó Springer-Verlag 2011
Abstract A mononuclear complex [CoL2Cl2]ꢀ3.5H2O
(L = 2-[(2,2-diphenylethylimino)methyl]pyridine-1-
oxide) has been synthesized and characterized by X-ray
structure analysis. The crystal structure confirms the for-
mation of an interesting porous framework with channel
Introduction
Interest in new research into the coordination chemistry of
Co(II) compounds is already substantial but is still
increasing, due to their unique magnetic properties [1].
This uniqueness makes the Co(II) ion suitable for the
preparation of new metal clusters with single-molecule
magnet (SMM) behavior that show fascinating physical
properties (such as slow magnetic relaxation and quantum
tunneling) [2]. The requirements for SMMs are well
known: (1) large-spin ground state; (2) large magnetic
anisotropy. However, it is also very useful if such com-
pounds possess interesting molecular or crystal structures.
Much desired are, for example, molecules that are suitable
for the self-assembled construction of well-ordered multi-
dimensional structures [3].
˚
diameters of about 8 A through weak C–Hꢀꢀꢀp and
C–HꢀꢀꢀCl interactions. The magnetic properties of this
complex have also been studied, and the susceptibility and
magnetization data were analyzed in terms of the spin
Hamiltonian formalism. They confirm substantial zero-
field splitting, D/hc = 75 cm-1
.
Keywords Transition metal compounds ꢀ
X-ray structure determination ꢀ Magnetic properties ꢀ
UV/Vis spectroscopy
Transition metal complexes show magnetic anisotropy
as a consequence of the zero-field splitting (ZFS) that splits
the states belonging to the same S-multiplet [4]. The zero-
field splitting can be viewed at several levels of approxi-
mation. At the bottom level is the concept of the spin
Hamiltonian. The spin Hamiltonian recognizes only the
spin operators acting on the basis set of spin functions. The
spin operators are preceded by magnetic parameters (D, E,
gz, gx, gy) that measure the extent of the spin–spin and
spin–field interaction; these parameters are considered the
molecular constants characteristic for the particular system.
They can be retrieved by fitting the experimental data. The
magnetic parameters originate in the electronic structure
of the complex and are modulated by various structural
factors of the coordination polyhedra.
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-011-0521-7) contains supplementary
material, which is available to authorized users.
´
ˇ
C. Rajnak ꢀ J. Titis (&)
Department of Chemistry, University of SS. Cyril and
Methodius, 917 01 Trnava, Slovakia
e-mail: jan.titis@ucm.sk
ˇ
R. Boca ꢀ J. Moncol’
Institute of Inorganic Chemistry, Technology and Materials,
Slovak University of Technology, 812 37 Bratislava, Slovakia
The primary aim of this work was to discern correlations
between the magnetic and structural properties of metal
complexes. Unlike the traditional magnetostructural J cor-
relations [5], we proposed a new type of magnetostructural
ˇ
´
Z. Padelkova
Department of General and Inorganic Chemistry,
Faculty of Chemical Technology, University
of Pardubice, Pardubice, Czech Republic
123

´
C. Rajnak et al.
790
D correlation. The latter interrelates the tetragonal distor-
tion of an octahedral pattern, Dstr, with the axial zero-field
splitting parameter D which results from the analysis of
magnetic data. The D correlations were originally reported
for a series of mononuclear nickel(II) complexes [6–8].
Attempts to relate structural and magnetic anisotropy in
mononuclear Co(II) complexes appeared recently [9–11].
The Co(II) ion differs from the Ni(II) cases in that the
ground electron term (4T1g) in the octahedral geometry is
orbitally degenerate. On tetragonal compression it splits
4
4
into the terms A2g (ground) and Eg (excited), while on
tetragonal elongation the opposite is true (4Eg is the ground
term) [9]. Several compounds with homogeneous and het-
erogeneous donor sets that are suitable for searching for
magnetostructural correlations have been gradually pre-
pared. In order to enrich this series of Co(II) complexes, we
focused on bidentate Schiff base ligands possessing N and
O donor atoms. Such ligands usually occupy four coordi-
nation sites in the equatorial plane, whereas the two axial
positions are filled by other available donors.
Fig. 1 ORTEP drawing of molecular complex 1 (thermal ellipsoids
are drawn at the 50% probability level) [symmetry codes: (i) x, y, z;
(ii) –x, y ? 1/2, -z ? 1/2; (iii) –x, -y, -z; (iv) x, -y -1/2, z – 1/2]
Table 1 Crystallographic data for 1
Formula
C40H36Cl2CoN4O2ꢀ3.5H2O
789.631
Results and discussion
Formula weight (g mol-1
Crystallographic system
Space group
)
Monoclinic
P21/c
The title complex 1 has a composition which corresponds
to the formula [CoL2Cl2]ꢀ3.5H2O, where L is the Schiff
base bidentate ligand based upon the pyridine-N-oxide unit.
This ligand was prepared by a Schiff condensation between
2,2-diphenylethylamine and 1-oxypyridine-2-carbaldehyde
at a ratio of 1:1 (Scheme 1; L = 2-[(2,2-diphenyleth-
ylimino)methyl]pyridine-1-oxide). The complex was
prepared by combining CoCl2ꢀ6H2O with purified ligand in
a methanol/water solution. Pink crystals obtained within
3 days were collected by filtration. The composition and
structure of 1 (Fig. 1) were determined by single-crystal
X-ray analysis. Important crystallographic data are col-
lected in Table 1.
˚
a (A)
10.596(4)
10.535(6)
19.667(11)
90.000(4)
113.614(4)
90.000(4)
2,011.57(18)
2
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
Volume (A )
Z
Dcalc (Mg m-3
l (mm-1
)
1.213
)
0.596
F(000)
762
Compound 1 crystallizes in a monoclinic system with
the space group P21/c. The inversion center of this complex
molecule is located at the Co(II) ion, which is positioned in
a distorted octahedral environment consisting of nitrogen
atoms of the azomethine group, two oxygen atoms of the
N-oxide system (equatorial plain), and two chloride ligands
Temperature (K)
150(2)
Reflections collected
Data/restrains/parameters
Goodness-of-fit on F2
4,059
2,817/0/223
1.033
Final R indices [I [ 2r(I)]
Absorption correction
R = 0.0449, Rw = 0.0920
Empirical (w-scan)
(axial sites). The geometry of the {CoN2O2Cl2} chromo-
phore is characterized by the bond lengths Co1–Cl1
O
N
O
˚
˚
˚
(2.492 A), Co1–N2 (2.081 A), Co1–O1 (2.033 A), and the
bond angles Cl1–Co1–N2 (90.98°), O1–Co1–N2 (84.82°),
Cl1–Co1–O1 (89.45°). Further important structural data are
collected in Table 2. Note that the arrangements of the
atoms Co, N2, C6, C5, N1, and O1 constitute a chelate ring
with significant deformations. In particular, the bite angle
+
N
O
N
- H2O
H2N
L
Scheme 1
123

Self-assembled cobalt(II) Schiff base complex
791
structure, should be noted in particular. Within the unit
cell, four monomeric units are connected via the C–Hꢀꢀꢀp
interactions to the ring (Fig. 2a). Specifically, the geo-
Table 2 Selected bond lengths and bond angles for 1
˚
Bond lengths (A)
Co1–Cl1
2.492(7)
2.033(18)
2.081(2)
1.270(3)
O1–N1
N1–C1
N1–C5
N2–C7
1.328(3)
1.346(4)
1.365(3)
1.461(3)
˚
metric data for these contacts are H7BꢀꢀꢀC13 2.703 A,
Co1–O1
˚
˚
H6ꢀꢀꢀC11 3.087 A, H4ꢀꢀꢀC19 2.851 A, and H4ꢀꢀꢀC18
Co1–N2
˚
2.858 A. This interesting ring arrangement originates in the
N2–C6
significant flexibility of the ligand L and the symmetric
nature of the complex. It is then found that the packing in
the bc plane results in a 2D network containing cavities
Bond angles (°)
Cl1–Co1–O1
Cl1–Co1–N2
Cl1–Co1–Cl1i
Cl1–Co1–O1i
Cl1–Co1–N2i
O1–Co1–N2
O1–Co1–O1i
O1–Co1–N2i
89.45(5)
90.98(6)
180.00
N2–Co1–N2i
Co1–O1–N1
Co1–N2–C6
Co1–N2–C7
N2–C7–C8
N2–C6–C5
C9–C8–C15
O1–N1–C1
180.00
119.56(14)
124.10(17)
117.35(15)
109.32(18)
125.20(2)
112.97(19)
116.70(2)
˚
with a diameter of about 8 A (Fig. 2b). The complex
molecules are also connected along the crystallographic
90.55(5)
89.02(6)
84.82(7)
180.00
a axis, through the C–HꢀꢀꢀCl interactions (H3ꢀꢀꢀCl1 2.732
˚
A; see Fig. 2c), and ultimately form channels filled with
solvent molecules (Fig. 2d).
The electronic spectrum of the studied complex shows
two highly visible principal d–d bands in the regions
14,000–16,000 and 17,000–22,000 cm-1, which are split
due to the symmetry lowering of the ligand field (Fig. 3).
Although the first visible d–d transition appears
95.18(7)
is rather small (O1–Co1–N2 84.82°). It is evident that the
effort of L to achieve an ideal bite angle (90°) subsequently
deforms the O1–N1–C1 angle, the value of which is then
\120° (see Table 2). Moreover, due to the flexibility of the
ligand L and the electronic predispositions of sp2 oxygen of
the N-oxide, the pyridine units are not situated in the plane
of the {N2O2} donors: the dihedral angle N2–Co1–O1–N1
is 48.23°.
For a more detailed stereochemical analysis, we utilized
the parameter Dstr = Rax – Req as a measure of the
tetragonal distortion of the chromophore [6]. For com-
plexes containing qualitatively different M–L bonds, the
correction to the heterogeneity of the donor set reads
4
4
4
at *10,000 cm-1 [D1 = T2g(4Eg, B2g) / T1g(4A2g),
Oh(D4h) approximation], its intensity is weak [13]. Over
25,000 cm-1, intense charge-transfer (CT) transitions are
observed.
An angular overlap model (AOM) was adopted for more
precise interpretation of the spectral data. To this end, a
simple molecular orbital calculation based on extended
Hu¨ckel theory (EHT) was performed to figure out the one-
electron orbital energies for 1 using its crystallographic
coordinates [14]. It has already been shown that the EHT
MO energies are useful for estimating the AOM parameters
er and ep [15]. In the D4h symmetry approximation,
the relations are: E(z2) = er(xy) ? 2er(z), E(x2 - y2) =
3er(xy), E(xz, yz) = 2ep(xy) ? 2ep(z), and E(xy) =
4ep(xy), where E is the energy shift relative to the unper-
turbed d-orbitals. Five d-character EHT MOs of 1 are
depicted in Fig. 4.
ꢀ
ꢀ
ꢀ
Dstr ¼ ðdi ꢁ diÞz ꢁ ½ðdi ꢁ diÞx þ ðdi ꢁ diÞyꢂ=2;
ð1Þ
ꢀ
where di is the mean distance for a given bond (in this case
ꢀ
i = N, O, Cl). Values of di have been taken from
compounds containing [Co(NH3)6]2?, [Co(H2O)6]2?, and
[CoCl6]4- complexes, giving rise to dðCo ꢁ NÞ = 2.185 A,
ꢀ
˚
The energy of the d-orbitals in the free Co(II) ion is
-13.18 eV, and thus the AOM parameters are: er(z) =
4,700, er(xy) = 2,500, ep(z) = 263, and ep(xy) = 222
cm-1. The nephelauxetic ratio was set to b = B/B0 = 0.80
(the free ion Racah parameter is B0 = 980 cm-1). Finally,
the calculation was performed using the AOMX program
[16]. The main components of the calculated energy
ꢀ
˚
ꢀ
˚
dðCo ꢁ OÞ = 2.085 A, and dðCo ꢁ ClÞ = 2.475 A [12].
Under these assumptions, the analysis for the complex 1
yields the bond-length shifts D(Co–N) = -10.40 pm,
D(Co–O) = -5.10 pm, and D(Co–Cl) = ?1.70 pm, so
the formal directions of the individual M–L vectors
are |Co–N|ax, |Co–Cl|eq, and |Co–O|eq, yielding
Dstr = -8.70 pm. This means that the effective distortion
of the complex refers to a compressed tetragonal bipyramid.
An analogous procedure was performed to quantify the
in-plane distortion
4
4
spectrum (ideal D4h) are: A2g (ground term), Eg (644),
4Eg (8,529), 4B2g (9,970), 2Eg (14,021), 2A2g (14,555), 4A2g
4
4
(16,543), B1g (16,582), and Eg (19,922 cm-1). Note that
the distinctive bands in the 14,000–15,000 cm-1 region
were assigned to spin-forbidden transitions to 2Eg and 2A2g
terms. We assume that these bands can borrow their
intensities from the close-lying spin-allowed transitions.
Above 20,000 cm-1, all d–d transitions are spin-forbidden.
According to Fig. 3, the reconstruction of the energy
transitions is fairly good, so the effective tetragonal
ꢀ
ꢀ
Estr ¼ ½ðdi ꢁ diÞx ꢁ ðdi ꢁ diÞyꢂ=2
ð2Þ
so that the rhombic structural parameter for 1 is ?3.40 pm.
The most important intermolecular contacts in the
crystal of 1 are presented in Fig. 2. The weak C–Hꢀꢀꢀp and
C–HꢀꢀꢀCl interactions, which lead to the formation of a 3D
123

´
C. Rajnak et al.
792
Fig. 2 Crystal packing for 1.
a System of C–Hꢀꢀꢀp contacts in
the unit cell; b packing in the bc
plane; c C–HꢀꢀꢀCl interactions
along the crystallographic
a axis; d channel formation
along the crystallographic a axis
(a perspective view)
4Eg
Fig. 3 Electronic d–d spectrum
of 1 in the visible region (left)
and the corresponding term
diagram (right). Vertical bars
are the calculated AOM
2.5
2.0
1.5
1.0
0.5
0.0
0.6
0.4
0.2
4T1g
CI
4P
4A2g
Δ3
15B
2G
CT
4B+3C
energies of the D4h terms
4A2g
4B1g
Δ2
9
10 11 12 13 14 15 16
10Dq
4F
4B2g
4T2g
2A2g
4Eg
4Eg
4Eg
2Eg
Δ1
4B1g
8Dq
4B2g
4A2g
4Eg
4T1g
Δ
ax > 0
4A2g
D4h
8
10
12
14
16
18
20
22
24
26
28
R3
Wavenumber /103 cm−1
Oh
compression is satisfactorily confirmed. The full output of
the AOMX program can be found in the Electronic sup-
plementary material.
taken at T0 = 2.0 and 4.6 K. The magnetic data for com-
plex 1 are presented in Fig. 5. These show paramagnetic
behavior typical of mononuclear Co(II) complexes with
large zero-field splitting (ZFS). The spin-only value of the
effective magnetic moment for the S = 3/2 spin system is
leff = 3.87lB (g = 2.0). The observed value at T = 293 K
is leff = 4.35lB, showing that g [ 2 holds true. On cool-
ing, the effective magnetic moment stays almost constant
The magnetic data were taken with a SQUID apparatus
(MPMS-XL-7, Quantum Design) in two modes: (1) tem-
perature dependence of the magnetic susceptibility
(B0 = 0.1 T), which is transformed to the effective mag-
netic moment; (2) isothermal magnetization until B = 7 T,
123

Self-assembled cobalt(II) Schiff base complex
793
Fig. 4 d-Character MOs and
their one-electron energies
generated by the EHT method
Fig. 5 Magnetic data for 1.
Temperature dependence of the
effective magnetic moment
(molar susceptibility) and field
dependence of the molar
magnetization at two
temperatures. Empty circles
experimental data, lines fitted
5
4
3
2
1
0
3
2
1
0
B = 0.1 T
T = 2.0 K
T = 4.6 K
10
5
μ
μ
μ
0
0
20
40
0
50
100
150
200
250
300
0
1
2
3
4
5
6
7
T/K
B/T
S
ꢁ2
2
2
2
2
ꢁ1
until T = 70 K, which reflects the Curie law. Below this
temperature, the effective magnetic moment gradually
decreases until a nonzero low-temperature limit. This
behavior is a fingerprint of considerable zero-field splitting.
From a theoretical point of view, the magnetism of
Co(II) coordination compounds is rather complex: (1) the
spin-Hamiltonian formalism is fulfilled only for tetrago-
^
^
^
^
~
~
ꢀ
^
H ¼ ꢀh ½DðSz ꢁ S =3Þ þ EðSx ꢁ SyÞꢂ þ ꢀh lBðB ꢀ gꢀ ꢀ SÞ
ð3Þ
was used as an appropriate model. Here, D is the axial ZFS
parameter, E is the rhombic ZFS parameter, and g is the
gyromagnetic ratio tensor. Other variables and symbols
have their usual meanings.
In the case of high-spin d7 systems, a theoretical
analysis based upon the crystal-field approach predicts
that gz = 2.0 holds true [17]. A molecular-field (MF)
correction improved the reproduction of the low-temper-
ature susceptibility data. This correction was applied in
the form
4
nally compressed complexes where the A2g state refers to
the ground one; (2) on approaching the octahedral limit, 12
energy levels become thermally populated (as they arise
4
4
from the splitting 4A2g ? Eg / T1g), so a more complex
theoretical treatment is necessary [17]. For our purpose,
two independent sets of magnetic data (v vs. T at B = 0.1 T
and M vs. B at T = 2.0 and 4.6 K) were subjected to
simultaneous theoretical analysis. Since the X-ray structure
and the electronic spectra analysis confirmed the tetrago-
nally compressed structure, a spin-Hamiltonian of the form
Â
À
Á
Ã
vcor ¼ v= 1 ꢁ zj=NAl0l2B ꢀ v þ a;
ð4Þ
where the zj parameter accounts for the nearest neighbors.
The last quantity a compensates for uncertainties in
123

´
C. Rajnak et al.
794
determining the underlying diamagnetism, and it accounts
for the temperature-independent paramagnetism.
(12 Union Road, Cambridge CB2 1EZ, UK; fax: (?44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk).
An advanced fitting procedure gave the following set of
magnetic parameters: gx = 2.356, gy = 2.515, D/hc = 75.
10 cm-1, E/hc = 4.84 cm-1, zj/hc = -0.011 cm-1, and
a = -9.01 9 10-9 m3 mol-1 (R = 0.0067).
2-[(2,2-Diphenylethylimino)methyl]pyridine-1-oxide)
(C20H18N2O)
An amount of 0.66 g 1-oxypyridine-2-carbaldehyde
(4.68 mmol) dissolved in 20 cm3 methanol were added to
0.92 g of 2,2-diphenylethylamine (4.68 mmol) dissolved in
20 cm3 methanol. The mixture was refluxed for 30 min,
cooled, and filtered. Recrystallization from ethanol affor-
ded 0.90 g (60%) as yellow crystals. M.p.: 114 °C; 1H
NMR (300 MHz, DMSO-d6): d = 4.38 (C9, 2H, d), 4.59
(C10, 1H, t), 7.27 (CHAr, 1H, dddd), 7.39 (C5, 1H, ddd),
7.51 (C2, 1H, ddd), 7.72 (C3, 1H, ddd), 8.16 (C7, 1H, s)
ppm; 13C NMR (75 MHz, DMSO-d6): d = 42.9 (C10),
62.3 (C9), 125.2 (CHAr), 135.8 (C3), 143.1 (C11, C18),
It is reiterated that in the solid state the magnetic
anisotropy for six-coordinated Co(II) complexes adopts
large values. In the case of 1, the anisotropy values are:
D = 75.1 cm-1 versus Dstr = -8.7 pm and E = 4.8 cm-1
versus Estr = 3.4 pm. For instance, the results for other
chromophores are: {CoO6}, [Co(H2O)6](6-OHnic)2 ?
D = 126 cm-1 versus Dstr = ?7.23 pm [10]; {CoN2-
O2O02}, [Co(ac)2(H2O)2(MeIm)2] ? D = 95 cm-1 versus
Dstr = -11.9 pm [9]. As can be seen, the ZFS energy gap
is much larger for an elongated bipyramid (Dstr [ 0). This
is probably due to the presence of the orbital angular
momentum in the effective ground term (4Eg). According
to our previous findings, these experimental relations are in
good agreement with the crystal-field modeling, and could
fit into the overall correlation [9].
ꢀ
149.9 (C5), 163.7 (C7) ppm; IR (KBr): m = 754 (Ar–H,
wag), 1,291 (N–O, stretch), 1,636 (C=N, stretch) cm-1
.
Bis[2-[(2,2-diphenylethylimino)methyl]pyridine-1-
oxide)]cobalt(II)dichloride hydrate
(1, C40H36Cl2CoN4O2ꢀ3.5H2O)
An amount of 0.12 g CoClꢀ6H2O (0.50 mmol) dissolved in
10 cm3 methanol ? water were added to 0.30 g 2-[(2,2-
diphenylethylimino)methyl]pyridine-1-oxide) (1.0 mmol)
dissolved in 30 cm3 methanol. The mixture was refluxed
for 2 h, cooled, and filtered. Dark pink block-shaped
crystals were grown by evaporating the solution within
3 days. The yield was 0.17 g (57%).
Experimental
NMR spectra were recorded at 300 MHz for 1H and 75 MHz
for 13C with a Varian Gemini 200. IR spectra were measured
on a Magna FTIR 750 spectrometer (Nicolet) using KBr
pellets in the 7,000–400 cm-1 region. Electronic spectra
were measured in Nujol mull on a Specord 200 (Analytical
Jena) in the range 50,000–9,000 cm-1. Magnetic suscepti-
bility and magnetization measurements were done using a
SQUID magnetometer (Quantum Design) from 2 K at
B = 0.1 T. The magnetization data were taken at T = 2.0
and 4.6 K. Raw susceptibility data were corrected for
underlying diamagnetism using the set of Pascal constants.
The effective magnetic moment was calculated in the usual
manner: leff/lB = 798(v0T)1/2 when SI units are employed.
The crystallographic data collection was carried on a
Bruker–Nonius Kappa CCD diffractometer equipped with
Acknowledgments Slovak Grant Agencies (VEGA 1/1005/09,
1/0052/11 and APVV-VVCE-0004-07) and the Ministry of Education
of the Czech Republic (Project VZ0021627501) are acknowledged for
their financial support of this work.
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´
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