c6dt00979d

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Cite this: Dalton Trans., 2016, 45,
9279
Received 12th March 2016,
Accepted 29th April 2016
DOI: 10.1039/c6dt00979d
www.rsc.org/dalton
Two field-induced slow magnetic relaxation
processes in a mononuclear Co(II) complex with a
distorted octahedral geometry†
Jing Li,
a
Yuan Han,
a
Fan Cao,
a
Rong-Min Wei,
a
Yi-Quan Zhang*
b
and You Song*
a
A distorted octahedral Co
II
complex is reported with homoscorpionate ligands. This complex comprised a
field-induced single-molecule magnet, showing two slow relaxation processes under a low dc field (<800
Oe) and only one process under a high dc field (≥800 Oe), which was an unusually discovery for 3d metal
ions. On the basis of the ac magnetic data, we show for the first time that one of the slow relaxation pro-
cesses in the low dc field originates from intermolecular dipolar interactions. Interestingly, the Raman
process is predominant in the spin reversal relaxation process. The origin of the behaviours of the
complex was elucidated by ab initio calculations.
Introduction
The scorpionate ligand,
1
a tridentate ligand binding to a metal
ion in a fac manner, is considered to be a fertile field of remark-
able scope. It has been applied in many research fields, such as
catalysis,
1b
biochemistry
1c
and molecular magnetism.
1d
Four
generations of scorpionate ligands have been discovered over
the past 50 years. Hydridotris(pyrazole) borate (Tp), tetra(pyra-
zole) borate (pzTp) and hydridotris(3,5-dimethylpyrazole) borate
(Tp*) are the first-generation ligands that have been used most
extensively due to the ease of their synthesis from readily avail-
able and low-cost starting materials.
Single-molecule magnets (SMMs) and single-chain magnets
(SCMs)
2
have attracted a lot of research attention in chemistry,
physics and material science. From a scientific point of view,
they can be used to investigate phenomena such as the
quantum tunnelling of magnetisation (QTM),
3
quantum phase
interference
4
and magneto-chiral effects.
2c
Furthermore, they
have the potential to be applied in high density magnetic
storage, quantum computation and spintronic devices.
3c,5
In
particular, owing to the slow magnetic relaxation and magnetic
hysteresis resulting from a purely molecular origin, they have
attracted intense interest over the last two decades. For a
superior SMM, a high blocking temperature (T
B
) and large
energy barrier (U) to magnetisation reversal are necessary. The
energy barrier can be described as U=|D|S
2
(or U=|D|(S
2
−1/4)
for half integer S),
6
and thus, is a result of the combined effect
from the ground-state spin Sand the negative zero field split-
ting (ZFS) parameter D. However, it has been shown that the
energy barrier Uaffected by the spin Sis limited because the
ZFS parameter Dis proportional to S
−2
.
7
Overall, the enhance-
ment in magnetic anisotropy Dis regarded as a more meaning-
ful strategy to increase the blocking temperature T
B
and energy
barrier U. Based on the above considerations, lots of low-
dimensional magnets with enhanced U
eff
and high T
B
have
been reported.
8–10
Co
II
ion with a d
7
electron configuration
usually has a strong unquenched orbital momentum under
most different coordination geometries.
11–17
Additionally, Co
II
-
based complexes have a high accessibility for their design and
synthesis under mild reaction conditions.
18
Consequently, Co
II
has become a “star”ion to build molecule-based magnets,
especially for SMMs and SCMs. In this theme, herein, we
report a Co
II
complex [Co(Tp*)
2
](1)
19
in a distorted octahedral
coordination environment, which showed SMM behaviour.
Interestingly, two relaxation processes of magnetisation were
observed in complex 1, with the origins of the two processes
an open puzzle in the field of mononuclear SMMs. In the
present work, through analysing the slow relaxation behaviour
under different external dc fields and in a magnetic dilution
system, two processes could be clearly assigned as an inter-
molecular dipolar interaction and a spin reversal. Herein, we
report the single-crystal structure and the magnetic properties
of complex 1in detail.
†Electronic supplementary information (ESI) available: Auxiliary magnetic data
and the methods for ab initio calculation. CCDC 1477206. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c6dt00979d
a
State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of
Advanced Mirostructures, School of Chemistry and Chemical Engineering, Nanjing
University, Nanjing 210093, China. E-mail: yousong@nju.edu.cn
b
Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology,
Nanjing Normal University, Nanjing 210023, China.
E-mail: zhangyiquan@njnu.edu.cn
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Experimental
All the reagents and solvents were purchased from commercial
sources and used as received. Potassium hydridotris(3,5-
dimethylpyrazole)borate (KTp*) was prepared by reported
procedures.
20
Cobalt bis-hydridotris(3,5-dimethylpyrazole)borate [Co(Tp*)
2
](1)
KTp* (0.2 mmol, 58 mg) and Co(ClO
4
)
2
·6H
2
O (0.1 mmol,
36.5 mg) were added to 1 mL MeOH. After the metallic salt
was dissolved, 10 mL CH
2
Cl
2
and 1 mL i-PrOH were added.
20 min later, the yellow solution was filtered and left in a
beaker until crystallisation occurred. Yellow crystals were iso-
lated with a yield of 93%. Elemental analysis calcd (%) for
C
30
H
44
B
2
CoN
12
: C (55.15), N (25.72), H (6.79); found: C (54.92),
N (25.51), H (6.46). IR data ATR (cm
−1
): 3412.92s, 3122.66s,
2506.76w, 1617.77w, 1541.92s, 1444.44s, 1401.65s, 1201.28s,
1063.80(s), 1042.29s, 845.87m, 807.29m, 766.62m, 697.78w,
646.95m, 465.97m. The diluted sample, i.e. complex 1′,wassyn-
thesised in the same way, but starting with Co(ClO
4
)
2
·6H
2
O:
Zn(ClO
4
)
2
·6H
2
O = 1 : 9. The crystal structures were determined by
the single-crystal diffraction as isostructural with 1.Thedilution
ratios were confirmed by ICP-AES analyses as 10 ± 0.4%.
Physical measurements and magnetic measurements
The IR spectra measurements were carried out with a Nexus
870 FT-IR spectrometer using KBr pellets in the range of
400–4000 cm
−1
. Elemental analyses of C, H and N were
recorded on a PerkinElmer 240C elemental analyser. ICP-AES
analyses were recorded on an Optima 5300 DV. The alternating
current magnetic data were measured on a MPMS Squid VSM
magnetometer with an ac field of 2 Oe and frequencies varying
over the range of 1 to 999 Hz. The direct current magnetic data
were measured at a temperature between 1.8 K and 300 K, and
the magnetisation isothermal measurements were made in
sections between 0 and 7 T on a MPMS-XL7 SQUID magneto-
meter. Experimental susceptibilities were corrected for the dia-
magnetism as estimated from Pascal’s tables
21
and, for the
sample holder, by a previous calibration.
Ab initio calculations for complex 1
To obtain the d-orbital energies, Orca 3.0.3 calculations
22a
were performed with the popular B3LYP hybrid functional pro-
posed by Becke
22b,c
and Lee et al.
22d
Triple-ζwith one polaris-
ation function TZVP
22e,f
basis set was used for all the atoms.
Scalar relativistic treatment (ZORA) was used in all calcu-
lations. A large integration grid (grid = 6) was applied to Co
II
for the ZORA calculations. Tight convergence criteria were
selected to ensure that the results were well converged with
respect to the technical parameters.
Considering the effects of the dynamical electronic a corre-
lation based on the complete active space self-consistent field
(CASSCF) using the MOLCAS 7.8 program package
23
and the
complete active space second-order perturbation theory
(CASPT2) was performed on the complete structure of complex
1. For the first CASSCF calculation, the basis sets for all atoms
were atomic natural orbitals from the MOLCAS ANO-RCC
library: ANO-RCC-VTZP for the magnetic centre ion Co
II
, VTZ
for close N and VDZ for distant atoms. The calculations
employed the second-order Douglas–Kroll–Hess Hamiltonian
approach, where scalar relativistic contractions are taken into
account in the basis set. Afterwards, the effect of the dynami-
cal electronic correlation was applied using CASPT2. Then, the
spin–orbit coupling was handled separately in the restricted
active space state interaction (RASSI-SO) procedure. The active
electrons in 10 active spaces considering the 3d double-shell
effect included all seven 3d electrons, and the number of
mixed spin-free states was 50 (all those from the 10 quadru-
plets and all those from the 40 doublets).
X-ray crystallography
The crystallographic data of complexes 1and 1′were collected
on a Bruker Smart CCD area-detector diffractometer with
Mo-Kαradiation (λ= 0.71073 Å) by using the ωscan mode at
296 K. The diffraction data were treated using SAINT,
24a
and
all absorption corrections were applied by using SADABS.
24b
All the non-hydrogen atoms were located by Patterson’s
method
24c
using the SHELXS program of the SHELXTL
package and by subsequent difference Fourier syntheses. The
hydrogen bonded to carbon were determined theoretically and
refined with isotropic thermal parameters riding on their
parents. All non-hydrogen atoms were refined by full-matrix
least-squares on F
2
. All the calculations were performed by the
SHELXTL-97 program.
24d
Results and discussion
Structural descriptions
Complex 1crystallises in the triclinic space group P1
ˉand is
composed of a neutral molecule with a simple molecular struc-
ture, as shown in Fig. 1a, which is formed by two tridentate
homoscorpionate ligands: bis-hydridotris(3,5-dimethyl-
pyrazole) and a single chelating Co
II
ion. The central Co
II
ion
is coordinated to six nitrogen atoms, giving rise to a distorted
octahedron. The Co–N bond lengths are 2.133 Å, 2.146 Å and
2.151 Å, respectively. For a regular octahedral coordination
Fig. 1 (a) Structure of complex 1. The hydrogen atoms have been
omitted for clarity. Orientations of the local main magnetic axes of the
ground doublets on the magnetic centre ion (green arrow). (b) 3d
energy levels of complex 1.
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geometry, there are three C
4
and four C
3
symmetry axes, with
coordinate bond angles of only 90 and 180°. However, for
complex 1, the angles (Table S1†) indicate that the octahedral
configuration is distorted and elongated along the lining of
B⋯Co⋯B
i
(symmetry code i: −x,−y,−z), one of the four C
3
axis, which destroys all the C
4
symmetry and the other three C
3
symmetry axes (Fig. 1a).
The angle of B⋯Co⋯B
i
is retained at 180° and the different
Co–N bond lengths indicate that the lining of B⋯Co⋯B
i
defines the quasi-S
6
axis of the D
3d
point group. Thus, the
molecular geometry of complex 1can be regarded as a special
distorted octahedron with a S
6
symmetry axis. Based on this
configuration, the 3d energy levels of complex 1were obtained
(Fig. 1b). There was no special interaction (hydrogen bond or
π⋯πinteraction) except for van der Waals’forces in the crystal
lattice. All the anisotropic axes of Co
II
were parallel to each
other and in the same orientation along the lining of
B⋯Co⋯B
i
. The shortest distance between the paramagnetic
Co
II
ions of neighbouring units was 8.825 Å while the next-
nearest neighbours were separated by 10.826 Å (Fig. S1b†).
Magnetic data
The EPR spectrum of complex 1, involving a single peak at g
∥
=
8.47 and g
⊥
= 1.18, was reported by Tierney
19a
at the X-band
(9 GHz) at low temperatures, indicating that complex 1
belongs to the easy-axis system. More recently, Gao’s
group
14b,25
reported a series of trigonal prismatic Co
II
com-
plexes exhibiting slow magnetic relaxation with high energy
barriers at zero external dc field. Complex 1has a very similar
splitting (Fig. 1b) to the d-orbitals in Gao’s report.
25
It is of
particular interest to investigate its single-molecule magnetic
properties, especially, under a completely different coordi-
nation geometry.
25
Temperature-dependent direct current (dc) magnetic sus-
ceptibility data were measured for a polycrystalline sample of
complex 1from 1.8 to 300 K in an applied field of 2 kOe
(Fig. 2). The room-temperature χ
M
Tvalue was 3.01 cm
3
Kmol
−1
,
which is larger than 1.87 cm
3
K mol
−1
(mononuclear high spin
Co
II
,S= 3/2, g= 2.0), owing to the significant orbital contri-
bution to the net magnetic moment.
11,18,25,26
The χ
M
Tvalues
remained constant down to 110 K, and then slightly decreased
to 2.32 cm
3
K mol
−1
at 1.8 K. The decrease observed below
110 K was most likely due to the intrinsic magnetic anisotropy
of the Co
II
ions rather than from antiferromagnetic inter-
actions between the metal ions. The field-dependent magneti-
sations were collected at applied magnetic fields of 0–7T,
ranging from 1.8 to 7 K (Fig. 2 inset). The magnetisation
increases rapidly below 1.5 T at 1.8 K. In higher fields, the
magnetisation curve follows a linear slope to reach 2.34Nμ
B
for
an isolated Co
II
ion (gS =2×3/2=3Nμ
B
per Co
II
ion) without
saturation. The lack of saturation and the non-superposition
on the curves of the Mvs.H/Tplot (Fig. S2†) suggest the pres-
ence of strong magnetic anisotropy.
14a,17,26d
In order to probe the low-temperature relaxation dynamics
of complex 1, temperature and frequency dependence of the
alternating current (ac) magnetic susceptibility measurements
were performed on the polycrystalline samples. Unfortunately,
there was no signal for the out-of-phase ac susceptibility (χ″
M
)
observed under a zero applied dc field, while a small external
dc field could induce strong frequency-dependent ac suscepti-
bilities (Fig. S3†). The absence of a slow relaxation behaviour
under zero applied fields could be a result of a strong QTM.
27
When the dc field was applied, two slow magnetic relaxations
were observed in the χ″
M
vs.νplot below 600 Oe, while only
one process was higher than 800 Oe. Hence, 400 Oe and 1 kOe
were chosen to perform additional ac measurements in the
temperature range of 1.8 to 6.2 K (Fig. 3 and S4†).
Under 400 Oe of a dc field, two slow magnetic relaxations
could be clearly observed in the frequency-dependent ac sus-
ceptibilities in the low temperature regime (1.8–3 K) (Fig. 3a
Fig. 2 Temperature dependence of χ
M
Tunder a 2 kOe applied dc field
at 1.8–300 K for a polycrystalline sample of complex 1by MPMS-XL7
SQUID. The solid red line represents the calculated magnetic suscepti-
bilities with the intermolecular interaction zJ’of 1set to −0.05 cm
−1
.
Inset: Experimental Mvs.Hplots at different temperatures.
Fig. 3 Frequency dependence of the out-of-phase (χ’’
M
) AC suscepti-
bilities and variable temperature Cole–Cole plots under 400 Oe (a, b)
and 1 kOe (c, d) dc field (1–999 Hz, by MPMS Squid VSM) at indicated
temperatures for complex 1.
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and b). The Cole–Cole plots of χ″
M
vs.χ′
M
show the superposi-
tion of two semicircles, and were fitted by the CCFIT program
28
to a linear combination of two modified Debye functions with
the αparameters always less than 0.46, thus spanning a large
range (Table S2†). These phenomena clearly indicate that there
exist two slow relaxations of magnetisation in complex 1and
that they both have an influence on each other with much
quenched QTM, leading to a large and inconstant αvalue. This
suggests that the two slow relaxations result from the inter-
molecular dipolar interaction and the spin reversal.
With a higher external dc field of 1 kOe, only one slow mag-
netic relaxation was observed in the whole temperature regime
(Fig. 3c and d) and the low-temperature relaxation dis-
appeared. The Cole–Cole plots were also fitted by the general-
ised Debye function with the αparameters in a narrow range
of 0–0.21 (Table S3†). Elimination of the low-temperature relax-
ation process can be ascribed to the fact that the inter-
molecular interaction and QTM are stronger in low
temperature than in high one under the weak thermal effect.
2a
A large number of reported mononuclear Co
II
-based
SMMs
13,15–17
show a field-induced magnetic relaxation behav-
iour because of the quenched QTM.
In order to further confirm the conclusion that the second-
ary slow relaxation process in the low temperature regime
under the 400 Oe dc field originates from intermolecular
dipolar interactions, ac magnetic susceptibility measurements
were carried out for complex 1′with a 10-fold magnetic site
dilution of complex 1by Zn
II
ion. Interestingly, the diluted
complex showed a frequency-dependent magnetic behaviour
in the absence of an external dc field (Fig. S5†). This behaviour
is common for all lanthanide ions, but is rare for 3d transition
metal ions. Owing to magnetic dilution, the dipolar inter-
actions are not effective in promoting the tunnelling, which
even disappears. The Cole–Cole plots (Fig. S6†) were fitted by
the generalised Debye function, with αparameters near to zero
(Table S4†), describing an almost pure single relaxation
process. Moreover, frequency-dependent ac magnetic suscepti-
bilities with a 400 Oe external dc field showed obvious differ-
ences (Fig. S7, S8 and S10†). Compared with Fig. 3b, the
secondary relaxation (Fig. S10†) in the low temperature regime
was quite weakened, proving that this relaxation process
results from intermolecular dipolar interactions. Besides, com-
pared with Fig. 3c, the high temperature process did not
change in the doping system (Fig. S7, S9 and S11†), indicating
that it resulted from a spin reversal.
A plot of ln(τ)vs.T
−1
based on the variable-frequency sus-
ceptibility data were fitted with Arrhenius’law τ=τ
0
exp(ΔE/
k
B
T), giving the thermal energy barrier for the relaxation of
magnetisation as ΔE= 36.8 K with τ
0
= 1.47 × 10
−7
s (400 Oe)
and ΔE= 30.5 K with τ
0
= 4.85 × 10
−7
s (1 kOe) (Fig. 4). The
relaxation time under a higher applied dc field is longer,
which is in agreement with the previous report.
29
Given the
coexistence of multiple relaxations, for the relaxation data, it
would be more reasonable to model with QTM, Raman and
Orbach relaxation processes.
26b
A fitting with eqn (1) (where
τ
QTM
is the quantum tunnelling of the magnetisation relax-
ation time, Cis the coefficient of Raman process, U
eff
is the
energy barrier to magnetisation reversal and k
B
is the Boltz-
mann constant) accords well with the data over the whole
temperature regime, with parameters of τ
QTM−1
= 4.31 s
−1
(10.16 s
−1
), C= 0.00307 s
−1
K
−8.04
(0.017 s
−1
K
−7.3
), n= 8.04
(7.3), τ
0
= 1.44 × 10
−7
s (7.13 × 10
−7
s) and U
eff
= 38.59 K
(34.72 K) under a 400 Oe (and 1 kOe) external dc field. The
unusual parameters (τ
QTM−1
, especially) obtained from the
entire temperature range fitting might be due to the slow relax-
ation process pathways changing under a different dc field.
τ1¼τQTM1þCT nþτ01expðUeff =kBTÞð1Þ
CASPT2 calculations showed that we could not use the zero-
filed splitting parameters Dand Eto depict its magnetic aniso-
tropy for complex 1, since the splitting of the two lowest
Kramers doublets (KDs) was much larger than the energy sep-
aration between the two lowest spin-free states in complex 1
(see Tables S7 and S8†). From Table S8,†the energy separation
between the lowest two KDs of 1is 217.6 cm
−1
, which is much
larger than its experimental energy barrier (c. 38 K). Based on
the observed gvalue of the lowest KDs, the ground and the
first excited states of 1are not pure Ising states and are mixed
by several m
J
states, which may cause a large QTM between
these states.
The following conclusions can be drawn from the above cal-
culated results. Eqn (1) (a combination of all the processes)
may not be a good choice to fit the energy barrier. The Orbach
relaxation process and QTM (with an added external dc field
and the cobalt ion as a Kramer ion) can both be ignored, and
the direct and Raman processes are then the main com-
ponents in the relaxation process. Therefore, the data was
fitted with eqn (2) (where Ais the coefficient of a direct
process, His the external dc field, Tis the temperature and C
is the coefficient of the Raman process) in the whole tempera-
ture regime. The best fits (Fig. 5), irrespective of the initial
values, were obtained at A= 2.10 × 10
−6
S
−2
Oe
−2
(1.57 × 10
−6
S
−2
Oe
−2
), C= 0.051 s
−1
K
−7.04
(0.023 s
−1
K
−7.38
) and n= 7.04
(7.38) under a 400 Oe (and 1 kOe) external dc field, which are
all in accordance with expected values for Kramer ion SMMs
where n≥4.
26b
The small value of Aimplies a very small con-
tribution of the single phonon direct relaxation mechanism.
Fig. 4 Arrhenius plots of ln(τ)vs. the inverse temperature T
−1
, as calcu-
lated from data at a dc field of 400 Oe (a) and 1 kOe (b). Blue lines show
the fit of the data to the Arrhenius expression τ=τ
0
exp(U
eff
/kT ); red
lines show the fit to the data using eqn (1).
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The doped system was also fitted by eqn (2) (Fig. S12†), with
parameters of A= 0 (0), C= 0.051 s
−1
K
−6.98
(0.026 s
−1
K
−7.29
)
and n= 6.98 (7.29) under a 400 Oe (and 1 kOe) external dc
field. From the fitting results, the Raman and direct processes
describe the slow relaxation behaviour very well, while the
Raman process is predominant in the whole temperature
regime. By comparing the calculated energy with the experi-
mental results, eqn (2) can be improved for this Co
II
system.
τ1¼AH 2TþCT nð2Þ
Conclusions
We report a field-induced SMM with two slow relaxation pro-
cesses under a low dc field (<800 Oe) and only one slow relax-
ation process under a high dc field (≥800 Oe), which is rare and
distinct from the previously reported complexes.
30
The secondary
relaxation in the low temperature regime was strongly dependent
on the intermolecular dipolar interaction and the other process
istheresultofthespinreversal.Overall,acomprehensive
picture of the relaxation pathway can be drawn under different
applied external dc fields, which expands the magnetic relax-
ation rule in lanthanides- and actinides-based SMMs to 3d
single-ion magnets. These results are important to unify the
understanding of the slow relaxation dynamics within SMMs.
Acknowledgements
This work was supported by Major State Basic Research Devel-
opment Program (2013CB922102), National Natural Science
Foundation of China (21571097 and 21171089), Natural
Science Foundation of Jiangsu Province of China (BK20130054
and BK20151542), Specialized Research Fund for the Doctoral
Program of Higher Education and a Project Funded by the Pri-
ority Academic Program Development of Jiangsu Higher Edu-
cation Institutions.
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