Slow magnetic relaxation in mononuclear tetrahedral cobalt(II) complexes with 2-(1H-imidazol-2-yl)phenol based ligands

Article abstract of DOI:10.1016/j.crci.2012.07.005

Three mononuclear cobalt(II) complexes based on the 2-(4,5-diphenyl-1H- imidazol-2-yl)phenol ligand and its nitro and methoxy derivatives have been synthesized and investigated. For two of these complexes, which were derived from the parent ligand and its nitro derivative, the molecular structures have been determined by X-ray crystallography. A distorted tetrahedral coordination environment is observed for the cobalt(II) centers with two bidentate N,O ligands. The magnetic properties have been studied by susceptibility and magnetization measurements revealing the presence of high-spin cobalt(II) ions with a considerable zero-field splitting in the order of D = -35 to -41 cm ^-1 for the three complexes. With an applied dc field the ac susceptibility data reveal a slow magnetic relaxation which is characteristic for single-molecular magnet (SMM) behavior.

Full text of DOI:10.1016/j.crci.2012.07.005

C. R. Chimie 15 (2012) 929–936
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Full paper/Memoire
Slow magnetic relaxation in mononuclear tetrahedral cobalt(II)
complexes with 2-(1H-imidazol-2-yl)phenol based ligands
Axel Buchholz a, Abiodun O. Eseola a,b, Winfried Plass a,
*
a Lehrstuhl fu¨r Anorganische Chemie II, Institut fu¨r Anorganische und Analytische Chemie, Friedrich-Schiller-Universita¨t Jena, Humboldtstr. 8, 07743 Jena, Germany
b Department of Chemical Sciences, Redeemer’s University, Redemption City, Ogun State, Nigeria
A R T I C L E I N F O
A B S T R A C T
Article history:
Three mononuclear cobalt(II) complexes based on the 2-(4,5-diphenyl-1H-imidazol-2-
yl)phenol ligand and its nitro and methoxy derivatives have been synthesized and
investigated. For two of these complexes, which were derived from the parent ligand and
its nitro derivative, the molecular structures have been determined by X-ray
crystallography. A distorted tetrahedral coordination environment is observed for the
cobalt(II) centers with two bidentate N,O ligands. The magnetic properties have been
studied by susceptibility and magnetization measurements revealing the presence of
high-spin cobalt(II) ions with a considerable zero-field splitting in the order of D = –35 to –
41 cmꢀ1 for the three complexes. With an applied dc field the ac susceptibility data reveal a
slow magnetic relaxation which is characteristic for single-molecular magnet (SMM)
behavior.
Received 18 March 2012
Accepted after revision 17 July 2012
Available online 23 August 2012
Keywords:
Cobalt(II)
Magnetic properties
Single-molecule magnets
N,O ligands
Zero-field splitting
´
ß 2012 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
the resulting complexes is concerned, it is unfortunately
not strictly leading to ferromagnetic coupling between the
There is contemporary growing interest in the chemis-
try of paramagnetic compounds due to their potential
applications as magnetic materials [1–4]. Since the
discovery of the [Mn12O12(OAc)16(H2O)4] {Mn12} as the
first single-molecular magnet (SMM) [5,6], the quest had
been opened for polynuclear metal complexes with ever
larger spin ground states [7]. This is based on the fact, that
the slow relaxation of the magnetic moment is a crucial
condition and is related to the anisotropy barrier U of the
system, which depends on the total spin S and the easy-
axis anisotropy parameter D with U = jDjS2 for integer and
U = jDjS2–1/4 for half-integer spins. In this context rational
design strategies have been developed leading to parallel
spin alignment with the spin-polarization mechanism as a
commonly employed recipe [8]. Although this approach
allows the implementation of symmetry conditions that
are advantageous as the magnetic relaxation behavior of
metal centers [9–13].
However, in recent years the focus in the search for new
SMMs has been shifted from optimizing the total spin S
towardsthe creation of systems withlarger jDj, i.e. increasing
the local zero-field splitting (ZFS) parameters in the system,
as it has been recognized that the anisotropy barrier U is
virtually independent of S [14,15]. This has already led to
significant advances for polynuclear systems [16–19].
Recently, it has been shown that SMM behavior can also
be observed for mononuclear complexes. Beside lanthanide
based systems [20–24] also 3d metal ions came into focus, i.e.
iron(II) [25–27] and cobalt(II) complexes [28,29].
In general the magnetic relaxation of SMMs is not solely
governed by the thermal relaxation process determined by
the anisotropy barrier U, as additional quantum tunneling
of magnetization (QTM) plays an important role which
leads to relaxation even at temperatures well below those
corresponding to the thermally accessible barrier [4]. It is
therefore of major interest to control the QTM process in
SMMs for potential future applications in fields like
molecular spintronics, information storage, and quantum
* Corresponding author.
E-mail address: sekr.plass@uni-jena.de (W. Plass).
1631-0748/$ – see front matter ß 2012 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2012.07.005

930
A. Buchholz et al. / C. R. Chimie 15 (2012) 929–936
computing [30–34]. Besides dipolar and hyperfine inter-
actions the transverse anisotropy (E) is responsible for
resonant QTM at zero field. The latter can be avoided by
utilizing half-integer spin systems, as the mixing of ꢁ MS
levels by E is forbidden according to Kramers’ theorem [35].
We therefore started to explore high-spin cobalt(II)
complexes possessing an S = 3/2 ground state in order to
find systems with appropriate magnetic properties. Such
systems with tetrahedral coordination geometry generally
show a considerable ZFS [36–39] which strongly depends
on the distortion from the ideal Td symmetry [40]. Studies
on 2-(1H-imidazol-2-yl)phenol based ligands and their
zinc(II) and nickel(II) complexes have shown that these
ligands support neutral complexes with tetrahedral
coordination geometry [41–43]. Herein, we describe the
synthesis and magnetic characterization of a series of new
mononuclear cobalt(II) complexes based on 2-(1H-imida-
zol-2-yl)phenol ligands.
0,65 mmol); yield 181 mg (0,23 mmol, 96%). Anal. calc.
for C42H28CoN6O6 (2); M.wt. (771.66): C, 65.37; H, 3.66; N,
10.89. Found: C, 64.81; H, 3.81; N, 10.95. IR: 3246(w),
3055(w), 1603, 1563, 1484(s), 1302(s), 1272(s), 1128(s),
887, 832, 767, 752, 739, 694(s), 659, 625, 586, 506,
460 cmꢀ1
.
2.2.3. [Co(L3)2] (3)
Cobalt(II) acetate tetrahydrate (47 mg, 0.19 mmol), 2-
methoxy-6-(4,5-diphenyl-1H-imidazol-2-yl)phenol
(182 mg, 0.53 mmol); yield 132 mg (0,18 mmol, 95%). Anal.
calc. for C46H40CoN4O5 (3ꢃEtOH); M.wt. (787.78): C, 70.13;
H, 5.12; N, 7.11. Found: C, 69.81; H, 5.18; N, 7.26. IR:
3065(w), 2962(w), 2830(w), 1604(w), 1474(s), 1445, 1391,
1322, 1235(s), 1214(s), 1196, 1179, 1074(s), 857, 836,
766(s), 740, 728, 695(s), 637, 582, 510, 455 cmꢀ1
.
2.3. Magnetic measurements
2. Experimental
The magnetic properties were measured on a Quantum
Design MPMS-5 SQUID Magnetometer in the temperature
range from 300 to 2 K at static fields up to 5 T. The
polycrystalline samples were mounted in gel capsula. The
data were corrected for the diamagnetism of the sample
holder and capsula as well as for the diamagnetic
contribution of the ligand. Measurements of the ac
susceptibility were performed with an alternating field
of 3 Oe at constants dc fields of 0, 400, and 1000 Oe in the
frequency range of 10 to 1500 Hz and at temperatures from
2 to 15 K. The experimental data were fitted for each
temperature to Eq. (1). This allows one to extract the
2.1. Materials and instrumentation
Abbreviations used throughout the text:
ꢂ
ꢂ
ꢂ
L1 = 2-(4,5-diphenyl-1H-imidazol-2-yl)phenol;
L2 = 2-(4,5-diphenyl-1H-imidazol-2-yl)-4-nitrophenol;
L3 = 2-(4,5-diphenyl-1H-imidazol-2-yl)-6-methoxyphenol.
All starting materials were obtained commercially as
reagent grades and used without further purification. The 2-
(1H-imidazol-2-yl)phenol based ligands were prepared
according to recently published procedures [42]. The
synthesis of complex [Co(L1)2] (1) was initially described
by MacDermott [44]. IR spectra were recorded on a Bruker
Equinox spectrometer with a diamond ATR unit in the range
of 4000 to 370 cmꢀ1. Elemental analyses were performed on
a Leco CHNS-932 or El Vario III elemental analyzers.
adiabatic and isothermal susceptibility (xs and
magnetization relaxation time ( c) and the distribution
width ( ) using OriginPro 8.5:
x0), the
t
a
x0
ꢀ
xs
x
ðvÞ ¼ xs
þ
(1)
1ꢀ
a
1 þ ðivtc
Þ
2.4. Crystal structure analyses
2.2. Synthesis of the complexes
Single crystals for complexes 1 and 2 were obtained by
recrystallization from MeOH and DMF, respectively.
Suitable crystals were selected while covered with mother
liquor under a polarizing microscope and fixed on fine
glass fibers. The crystallographic data was collected on a
Nonius KappaCCD diffractometer using graphite-mono-
The cobalt(II) salt and the ligand were dissolved in 10 mL
ethanol and stirred at reflux for 30 minutes. The reaction
mixture was cooled to room temperature and the product
isolated by filtration. The orange-pink polycrystalline sam-
ples were washed with a few milliliters of ethanol and dried
overnight in a desiccator over silica at room temperature.
chromated Mo-K radiation (l = 71.073 pm). A summary
a
2.2.1. [Co(L1)2] (1)
of crystallographic and structure refinement data is given
in Table 1. Data were corrected for Lorentz and polarization
effects, but not for absorption effects. The structures were
solved by direct methods with shelxs-97 and refined by
full-matrix least-squares techniques against Fo2 using
shelxl-97 [45]. Anisotropic thermal parameters were used
for all non-hydrogen atoms. Hydrogen atoms were
calculated and treated as riding atoms with fixed thermal
parameters, except for the hydrogen atoms at the imidazol
moieties in compound 1.3MeOH. These hydrogen atoms
were found during structure solution and refined iso-
tropically. For two of the DMF solvent molecules in
2.2.5DMF a disorder over two positions was found with an
Cobalt acetate(II) tetrahydrate (174 mg, 0.72 mmol), 2-
(4,5-diphenyl-1H-imidazol-2-yl)phenol (451 mg, 1.44 mmol);
yield 550 mg (0,71 mmol, 99%). Anal. calc. for C46H42
CoN4O4 (1ꢃ2EtOH); M.wt. (773.76): C, 71.41; H, 5.47; N,
7.24. Found: C, 71.39; H, 5.57; N, 7.28. IR: 3060(w),
2970(w), 1605, 1477(m), 1452(m), 1307(m), 1245, 1144,
1045, 855, 765(s), 752(m), 694(s), 609, 506, 464,
434 cmꢀ1
.
2.2.2. [Co(L2)2] (2)
Cobalt(II) acetate tetrahydrate (58 mg, 0.24 mmol), 2-
(4,5-diphenyl-1H-imidazol-2-yl)-4-nitrophenol (234 mg,

A. Buchholz et al. / C. R. Chimie 15 (2012) 929–936
931
Table 1
Summary of the crystallographic and structure-refinement data for
compounds [Co(L1)2].3MeOH (1.3MeOH) and [Co(L2)2].2.5DMF
(2.2.5DMF).
Compound
1.3MeOH
2.2.5DMF
Empirical formula
Formula weight
Crystal size (mm)
Crystal system
Space group
a (pm)
C45H42CoN4O5
777.76
C49.5H45.5CoN8.5O8.5
954.37
0.5 ꢄ 0.5 ꢄ 0.5
monoclinic
P21/c
0.8 ꢄ 0.5 ꢄ 0.5
monoclinic
C2/c
1963.62(4)
2630.85(6)
1561.92(3)
109.4760(10)
7.6072(3)
8
3618.24(7)
1532.45(4)
16.2967(4)
100.091(2)
8.8964(4)
8
b (pm)
c (pm)
b
(8)
V (nm3)
Z
Dcalcd (g cmꢀ3
)
1.358
1.425
m
(mmꢀ1
)
0.504
0.454
u
range of data collection (8) 2.08, 27.49
2.36, 27.48
26227
Fig. 1. Molecular structure of one of the two symmetry-independent
complex molecules [Co(L1)2] (1) in crystals of 1.3MeOH. Thermal
ellipsoids are drawn at the 50% probability level. Hydrogen atoms are
omitted for clarity.
Measured reflections
Unique reflections (Rint
Goodness-of-fit on F2
41808
)
17305 (0.0731) 10160 (0.0534)
1.149
1.091
R1(I>2
s
(I))a
0.0788
0.1495
0.0623
0.1348
2
b
wR2 (all data, F0
)
P
P
a
R1
=
jjFoj–jFcjj/ jFoj
P
P
b
wR2 = [ w(Fo2–Fc2)2/ w(Fo2)2]1/2
obtained for complexes 1 and 2 by recrystallization from
methanol and DMF solution, respectively. However, the
investigations of the magnetic properties were performed
on the dried, polycrystalline samples obtained from the
reaction in ethanol solution.
occupation factor of 0.5, whereas for the additional DMF
molecule a partial occupation of 0.5 without disorder was
observed. CCDC 869901 and CCDC 869902 contain the
supplementary crystallographic data for the structures
1.3MeOH and 2.2.5DMF, respectively. These data can be
obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest.cif, on quoting the appropriate depository number.
3.2. Structural data
Complex 1 crystallizes in the monoclinic space group
P21/c with two crystallographically independent complex
molecules in the asymmetric unit. The molecular structure
of one of the symmetry-independent molecules is depicted
in Fig. 1 and selected bond lengths and angles are given in
Table 2. In the crystal structure three additional co-
crystallized methanol molecules per complex unit are
present.
3. Results and discussion
3.1. Synthesis
Complex
2 is found to crystalize together with
The 2-(1H-imidazol-2-yl)phenol based ligands 2-(4,5-
diphenyl-1H-imidazol-2-yl)phenol (L1), 2-(4,5-diphenyl-
1H-imidazol-2-yl)-4-nitrophenol (L2), and 2-(4,5-diphe-
nyl-1H-imidazol-2-yl)-6-methoxyphenol (L3) depicted in
Scheme 1 are employed in the synthesis of complexes.
The reaction of cobalt(II) acetate tetrahydrate in
ethanol solution with the ligands L1, L2, and L3 results in
the formation of the corresponding neutral cobalt(II)
additional co-crystallized DMF molecules in the mono-
clinic space group C2/c. The asymmetric unit contains only
one complex molecule and the DMF molecules disordered
over several crystallographic positions. The molecular
structure of the complex molecule 2 is depicted in Fig. 2
and the corresponding bond lengths and angles around the
cobalt(II) ion are listed in Table 3.
In both complexes the cobalt(II) ions are found in a
distorted tetrahedral geometry. The coordination sphere is
complexes with
a 1:2 stoichiometry. The pale pink
cobalt(II) complexes [Co(L1)2] (1), [Co(L2)2] (2) and
[Co(L3)2] (3) were isolated as polycrystalline materials
and characterized by elemental analysis and IR spectros-
copy. In order to establish the molecular structures, single
crystals suitable for X-ray crystallography have been
Table 2
Bond lengths (pm) and angles (8) at the cobalt(II) ions in complex [Co(L1)2]
(1) for both symmetry-independent molecules in the crystals of 1.3MeOH.
Con1–On1
193.3(3)
192.0(3)
Con1–On2
192.1(3)
192.4(3)
X
Con1–Nn1
197.3(3)
197.2(3)
HL1: X = Y = H
H
N
Con1–Nn3
197.6(3)
196.7(3)
On1–Con1–On2
On1–Con1–Nn1
On1–Con1–Nn3
On2–Con1–Nn1
On2–Con1–Nn3
Nn1–Con1–Nn3
105.56(12)
93.86(12)
125.13(13)
132.17(13)
94.54(12)
109.31(13)
106.13(12)
94.51(13)
128.90(13)
123.74(13)
94.28(13)
112.24(14)
HL2: X = NO2, Y = H
N
HL3: X = H, Y = OMe
HO
Y
Scheme 1.

932
A. Buchholz et al. / C. R. Chimie 15 (2012) 929–936
Table 3
Bond lengths (pm) and angles (8) at the cobalt(II) ion in complex [Co(L2)2]
(2).
Co1–O1
191.3(2)
Co1–O4
191.4(2)
Co1–N1
197.6(2)
Co1–N4
197.1(2)
O1–Co1–O4
O1–Co1–N1
O1–Co1–N4
O4–Co1–N1
O4–Co1–N4
N1–Co1–N4
114.26(9)
94.39(9)
128.37(10)
124.62(10)
94.56(9)
103.32(10)
established by the phenolate oxygen atoms and the
imidazol nitrogen atoms of two ligand fragments. The
aromatic phenol and imidazole parts of each ligand are
nearly coplanar (see Figs. 1 and 2). The coordination
geometry of the cobalt centers of complexes 1 and 2 are
very similar in both structures and, moreover, comparable
to other examples with substituted 2-(1H-Imidazol-2-
yl)phenol based ligands [46,47]. This particularly holds for
the two symmetry-independent molecules of 1 found in
the crystal structure, which correspond to the enantio-
meric pair given by the chirality at the cobalt center.
However, the crystal packing enforces deviations in the
Fig. 3. Overlay of the two symmetry-independent complex molecules
[Co(L1)2] (1) in crystals of 1.3MeOH. The structure of molecule 1 was
inverted, the positions of the atoms Con1, On1, Nn1, On2 and Nn3 (n =1,2)
were used to fit the overlay.
tetrahedral and square planar by the corresponding shape
measures, S(Td) and S(D4h). These values give a measure for
the difference from ideal geometry, where zero indicates
ideal geometry. Data is given in Table 4. For all three
complex molecules of 1 and 2 these values indicate a
distorted tetrahedral coordination geometry with the
distortion about 1/4 of the full way between tetrahedral
and square planar.
outer ligand sphere namely the aromatic rings (due to
p-p
and van der Waals interactions), as illustrated by an overly
of the two independent molecules in Fig. 3. The average
deviation of the positions of the atoms Con1, On1, Nn1, On2
and Nn3 (n = 1,2) used to fit the overlay of the two
symmetry-independent molecules of 1 is only 11 pm.
The angles between the two mean planes formed by the
two ligand systems at the pseudotetrahedral cobalt centers
of complexes 1 (72.68 and 75.88) and 2 (74.98) considerably
deviate from the ideal 908 angle expected for a tetrahedral
coordination geometry. An alternative approach for the
related symmetry characterization of coordination poly-
hedra around metal ions is the so-called continuous shape
measure [48,49], which gives a quantitative measure for
The packing in the crystal structure of 1.3MeOH is
primarily determined by hydrogen bonding interactions
involving the phenolate oxygen (On1 and On2) and the
protonated imidazole nitrogen atoms (Nn1 and Nn3) of the
ligands and the co-crystallized methanol molecules. This
leads to chains for both of the crystallographically
independent molecules along the [001] direction with
distances between the hydrogen-bonded heavy atoms in
the range of 268–273 and 285–300 pm for OꢃꢃꢃO and NꢃꢃꢃO,
respectively. Within the chains also
p-p interactions
the distortion of
a
given coordination sphere from
between the aromatic rings of the ligands are observed.
Finally, the aggregation of these chains is solely established
via van der Waals interactions. This leads to Co–Co
distances within the chains of about 790 pm and to
interchain Co–Co distances larger than 946 pm.
In contrast to 1.3MeOH, the crystal structure of
2.2.5DMF exhibits hydrogen bonds only between the
protonated nitrogen atoms N2 and N5 of the imidazole
fragments of the coordinated ligands and the carbonyl
groups of a co-crystallized DMF molecule, the latter
showing a two-fold disorder. The observed N ꢃ ꢃ ꢃ O dis-
tances between the hydrogen bonded heavy atoms lie
Table 4
CSM parameters for the coordination geometries of the cobalt(II) ions in 1
and 2 (for notation see text).
1 (n = 1)
1 (n = 2)
2
S(Td)
3.20
19.20
28%
2.70
20.90
26%
2.74
20.59
26%
S(D4h
)
a
F
(Td ! D4h
)
Fig. 2. Molecular structure of the complex molecule [Co(L2)2] (2) in
crystals of 2.2.5DMF. Thermal ellipsoids are drawn at the 50% probability
level. Hydrogen atoms are omitted for clarity.
a
F(Td ! D4h
) gives the angular fraction along the path from a
tetrahedron to a square, for definition see Ref. [50].

A. Buchholz et al. / C. R. Chimie 15 (2012) 929–936
933
within the rang of 262 and 290 pm. The overall aggregation
of the molecules in the crystal structure is solely given by
van der Waals interactions, leading to intermolecular Co–
Co distances larger than 849 pm.
experimental room temperature
x
T values of 2.75 emu
K molꢀ1 for 1, 2.67 emu K molꢀ1 for 2, and 2.55 emu K
molꢀ1 for 3 are significantly larger than the anticipated
spin-only value for S = 3/2 of 1.87 emu K molꢀ1. However,
these values are well within the range typically observed
for highly anisotropic cobalt(II) ions with significant spin-
orbit coupling [36–39]. Upon lowering the temperature
3.3. Magnetic data
The molar magnetic susceptibilities of dried polycrys-
talline samples of complexes 1, 2, and 3 were measured in
the temperature range from 300 to 2 K and applied static
magnetic fields in the range of 1 to 5 kOe. Within this range
no field dependence was observed. The variable tempera-
the
xT values only slightly decrease. Below about 100 K the
xT values slowly show a significant decrease to values of
about 1.7 to 2.0 emu K molꢀ1 at 2 K. The observed
temperature profiles are characteristic for Curie-type
behavior of noninteracting mononuclear cobalt(II) ions,
where the decrease at low temperature is attributed to the
anisotropy of the cobalt(II) ions rather than intermolecular
antiferromagnetic interactions, which is consistent with
the large Co–Co distances observed in the crystal packing
ture
xT plots for 1–3 are depicted in Fig. 4. The
a
of 1 and 2. The experimental xT data were analyzed using
the spin Hamiltonian given in Eq. (2), which includes axial
ZFS and Zeeman interactions:
ꢀ
ꢁ
1
m
BBS þ D S2z ꢀ SðS þ 1Þ
(2)
ˆ
H ¼ g
3
The best fit including a correction term xTip for
uncompensated temperature-independent magnetism
results in g = 2.39, jDj=17 cmꢀ1 and
x
Tip = 2.9 ꢄ 10ꢀ4 emu
molꢀ1 for complex 1, g = 2.30, jDj = 22 cmꢀ1 and
x
Tip = 6.0 ꢄ 10ꢀ4 emu molꢀ1 for complex 2, and g = 2.29,
jDj =18 cmꢀ1 and
x
Tip = 2.9 ꢄ 10ꢀ4 emu molꢀ1 for complex
3. This leads to a good description of the experimental data
in the temperature range from 300 to 15 K. Nevertheless,
for temperatures below 15 K the predicted values are
slightly smaller than those from experiment, which might
be attributed to intermolecular interactions. However, the
inclusion of an appropriate term in the Hamiltonian did not
lead to a significant improvement of the fit. For this reason
and to avoid overparametrization, effects from intermo-
lecular interactions were not considered here. It should be
noted, that it is difficult to determine the axial ZFS
parameter D from magnetic susceptibility data derived
from measurements on polycrystalline samples [51].
Besides the absolute value, this particularly holds for the
sign of D which cannot unambiguously be determined on
b
c
this basis, as the variations of
xT are in general not very
sensitive in this respect. However, a more sensitive probe
in determining D values is the magnetization at interme-
diate fields and appropriate low temperatures, which is
based on the deviation from the anticipated Brillouin
behavior.
Variable-temperature magnetization measurements
for complexes 1–3 were performed at fields ranging from
0 to 5 T and temperatures between 2 and 5 K (Figs. S1 and
S2). The M vs. H data show an increase of the magnetization
which becomes almost linear for higher fields without
saturation to reach values of 2.13 mB for 1, 2.02 mB for 2,
and 2.14 mB for 3 (at 2 K and 5 T). The observed lack of
saturation together with the spreading of the curves in the
M vs. H/T plots (Fig. S2) indicates the presence of a
considerable ZFS in all three complexes. The fit on the M vs.
H/T data was performed utilizing a full-matrix diagonal-
ization approach [52] for the Hamiltonian given in Eq. (2)
Fig. 4. Plots for xT vs. T (black squares) and the best fit according to Eq. (2)
(red lines) for complex 1 (top), 2 (middle), and 3 (bottom); applied
magnetic field 2 kOe (for parameters see text).

934
A. Buchholz et al. / C. R. Chimie 15 (2012) 929–936
leading to D values of –41 cmꢀ1 for 1 and –35 cmꢀ1 for 2
and 3. For this fit the experimental data measured at 2 K
was excluded, due to the obvious presence of additional
low temperature effects like e.g. intermolecular interac-
tions. This is clearly substantiated by comparing the chi-
squared test parameters which are considerably smaller by
almost one order of magnitude upon excluding the 2 K data
set. Optimization for a positive sign of D in all cases does
not converge to a reliable fit, indicating the right choice of
sign. Not unexpectedly, the obtained absolute values
deviate from the once derived from the susceptibility
data, as the weighting in the magnetization based fit is
clearly more emphasized on the relevant low temperature
and high field data.
a
The SMM behavior of complexes 1–3 was investigated
by frequency-dependent ac susceptibility measurements
b
in the temperature range from 2 to 10 K. The in-phase (x0
and out-of-phase (x00) component of the ac susceptibility
x0 – ix00) were measured using an alternating field of
3 Oe. Without an applied dc field no imaginary signal (x00
)
(x =
)
could be observed for any of the compounds. For complex 2
even with an applied dc field only a marginal frequency
dependence was observed for the in-phase component x0
at temperatures below 5 K (Fig. S3). Moreover, as depicted
in Fig. 5 only a very small out-of-phase signal x00 was
observed with no peak characteristic down to 2 K. For
complex 1, however, the data depicted in Fig. 6 shows a
clear out-of-phase signal below 8 K.
The maximum for x00 vs. temperature shifts from 3.5 K
for 10 Hz to 6.5 K for 1143 Hz. Although a similar behavior
is found for complex 3, clear defined maxima in tempera-
ture dependence of the out-of-phase signal x00 are only
observed for higher frequencies, as depicted in Fig. 7.
The signals clearly deviate from the expected Lorentzian
shape, with a considerable remaining intensity of x00 at the
low temperature side which indicates an additional relaxa-
tion process to be operative. At higher dc field (1000 Oe) the
signal intensity of x00 increases and the maxima become
more pronounced. It seems that the additional relaxation
process can be suppressed by higher dc fields.
Fig. 6. Top: Temperature dependence of the out-of-phase ac
susceptibility for complex [Co(L1)2] (1) at different frequencies; lines
are guides for the eye. Bottom: Cole–Cole plot for
1 at different
temperatures with an applied dc field of 400 Oe; solid lines represent
the best fit (for parameters see text).
the adiabatic (xs for
xo for
! 1) as well as the magnetization relaxation
time c(T) and a which describes its width of distribution
[53–55]. In the case of a paramagnetic compound obeying
the Curie law the isothermal susceptibility, o, represents
v
! 0) and isothermal susceptibility
(
v
t
Fitting the data of complexes
1 and 3 for each
temperature to Eq. (1) leads to parameter sets including
x
the corresponding dc susceptibility. The obtained param-
eter sets can best be represented by a Cole–Cole plot as
depicted in Figs.
6 and 7 for complexes 1 and 3,
respectively. The simulations with the derived parameter
sets are in excellent agreement with the experimental data
particularly for higher temperatures at both applied dc
fields (400 and 1000 Oe). However, in the case of low
temperatures the measured data at high frequencies,
representing data point at the very low end of the
respective x0 range, deviate from the simulated semicircle,
again indicating a second relaxation process. Especially in
the case of complex 1 this leads to a considerable
uncertainty for the low temperature parameter sets at 2
and 2.5 K, as the number of available data points is
insufficient for reliable fits. Therefore these data were not
included in the corresponding fits and are not represented
in the Cole–Cole plots depicted in the Fig. 6.
Fig. 5. Temperature dependence of the out-of-phase ac susceptibility for
complex [Co(L2)2] (2) at different frequencies with an applied field of
400 Oe; lines are guides for the eye.
For the parameter
a, representing the width of
distribution of the magnetization relaxation times, a value

A. Buchholz et al. / C. R. Chimie 15 (2012) 929–936
935
a
b
Fig. 8. Arrhenius plots of relaxation times as ln(tc
complexes 1 and 3; lines represent linear fits to the high-temperature
)
vs. 1/T for the
data of measurements at dc fields of 400 and 1000 Oe (see text for details).
regime. This is independent from the applied dc field
leading to an almost perfect overlay of the data obtained
for both fields (400 and 1000 Oe). In contrast, for complex 3
a much more pronounced temperature dependence is
observed for tc which, moreover, is different for the two
applied dc fields. However, at higher temperatures these
values approach each other, indicating a field-independent
high temperature process. Whereas the low temperature
data indicates a field-dependent relaxation process as
expected form the x00 behavior (vide supra). Unfortunately
their is a considerable uncertainty in the determination of
Fig. 7. Top: Temperature dependence of the out-of-phase ac
susceptibility for complex [Co(L3)2] (3) at different frequencies; lines
the tc values from Cole–Cole plots for data points at higher
temperatures, as in these cases only the minor part of the
corresponding semicircles are observed. Therefore only
data up to a range of 6 to 7.5 K depending on the applied dc
field were used in the fit to Eq. (3). For complex 1 this leads
are guides for the eye. Bottom: Cole–Cole plot for
3 at different
temperatures with an applied dc field of 400 Oe; solid lines represent
the best fit (for parameters see text).
to Ueff = 49 K and
t
0 = 7.5 ꢄ 10ꢀ8 s at an applied dc field of
close to 1 is expected for classical spin-glass systems (a = 1
means an infinitely wide distribution), whereas it will tend
to 0 for a single relaxation time. In the case of 1 a decreases
from 0.41 at 3.0 K to 0.014 at 6.5 K with an applied dc field
of 400 Oe. With the higher dc field of 1000 Oe these values
are even smaller, ranging from 0.19 at 3.5 K to 0.008 at
7.5 K.
In case of a classical thermally activated relaxation
process [56], which takes place by excitation from a ground
state Ms = ꢁ 3/2 level to the higher energy Ms = ꢁ 1/2 levels
via the absorption of phonons from the lattice and de-
excitation to the final state in which phonons are emitted, the
anisotropic energy barrier Ueff can be determined from the
Arrhenius law given in Eq. (3). For a graph of ln(tc) vs. 1/T this
should result in a linear dependence for the high temperature
regime of the relaxation from which Ueff and the attempt time
to can be extracted.
400 Oe and for data points within the range of 5.0 to 6.0 K.
Whereas for the higher dc field of 1000 Oe data from
temperature range of 6.5 to 7.5 K could be used leading to
Ueff = 89 K and
t
0 = 1.0 ꢄ 10ꢀ10 s. In the case of complex 3
the fits lead to parameter sets of Ueff = 42 K and
t
0 = 1.4 ꢄ 10ꢀ7 s for an applied dc field of 400 Oe (5.0 to
6.0 K) and Ueff = 63 K and
t
0 = 2.6 ꢄ 10ꢀ9 s for 1000 Oe (5.5
to 6.5 K).
From these energy barriers (U = jDj(S2–1/4)) D values
can be deduced for complexes 1 and 3 which range from 17
to 31 cmꢀ1 and 15 to 22 cmꢀ1, respectively. Even the upper
limits of these estimated values, which correspond to the
more reliable data sets measured at applied dc fields of
1000 Oe, are considerably smaller than those determined
from the dc magnetization measurements. This however is
not unexpected, as similar observations have been
reported for tetrahedral cobalt(II) complexes in the
literature [28,29], and may by explained by the presence
of non-negligible QTM in these complexes.
tc
¼
t
0expðUeff =kTÞ
(3)
The corresponding Arrhenius plots for complexes 1 and
4. Conclusion
3 are depicted in Fig. 8.
For complex 1 a clear temperature dependence for
observed with the high temperature data approaching the
Arrhenius behavior expected for the thermally activated
t
c is
We have synthesized a series of air-stable neutral
mononuclear cobalt(II) complexes [Co(L)2] utilizing a
sterically demanding bidentate ligand system which is

936
A. Buchholz et al. / C. R. Chimie 15 (2012) 929–936
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base on the 2-(1H-imidazol-2-yl)phenol core fragment.
This leads to a significantly distorted tetrahedral coordi-
nation geometry at the cobalt(II) ions. As a consequence a
considerable spin-orbit coupling is observed for these
complexes. The reported complexes appear to be the first
examples of mononuclear tetrahedral cobalt(II) complexes
with coordinated N,O ligands to exhibit slow magnetic
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We thank the Deutsche Forschungsgemeinschaft (DFG)
(PL 155/11-1 and PL 155/12-1) for generous funding of this
project.
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Appendix A. Supplementary data
CCDC 869901 and CCDC 869902 contain the supplemen-
tary crystallographic data for the structures 1ꢃ3 MeOH and
2ꢃ2.5 DMF, respectively. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request.cif, on quoting the appro-
priate depository number. Supplementary data associated
with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.crci.2012.07.005.
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