A mixed-valence metallogrid [CoIII2CoII2] with an unusual electronic structure and single-ion-magnet characterization

Article abstract of DOI:10.1039/c7dt00564d

The reaction of the multisite coordination ligand (H₂L) with Co(Ac)₂·4H₂O in the absence of any base affords a homometallic tetranuclear mixed-valence complex, [Co₄(L)₄(CH₃CO₂)

Full text of DOI:10.1039/c7dt00564d

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
rsc.li/dalton

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Mixed-Valence Metallogrid [CoIII CoII ] with Unusual Electronic
2
2
Structure and Single-Ion-Magnet Characterization
Received 00th January 20xx,
Accepted 00th January 20xx
Wei Huang,a Feifei Pan,a Zhenxing Wang,b Yan Bai,c Xuejun Feng,*a Jiande Gu,a Zhong-Wen Ouyangb and Dayu Wu*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
The reaction of the multisite coordination ligand (H2L) with Co(Ac)2 · 4H2O in the absence of any base affords a
homometallic tetranuclear mixed-valence complex, [Co4(L)4(CH3CO2)2(CH3OH)2]·Et2O (1)
. The mixed-valence
metallogrid [Co4(L)4(CH3CO2)2 (CH3OH)2]·Et2O (1) has been theoretically and experimentally analyzed to assign the valence
and spin state in the form of trans-[CoIIIls-CoIIhs-CoIIIls-CoIIhs]. HF-EPR reveals the presence of axial anisotropy (D = -34.4 cm-1)
with a significant transverse component (E = 9.5 cm-1) for the local high spin cobalt centers. Slow magnetic relaxation
effects were observed in the presence of a dc field, demonstrating field-induced single ion magnet behavior, which is
associated with the unusual electronic structure of Co(II) within the metallogrid.
the ligand donor characteristics can then provide a means of tuning
the magnetic anisotropy despite of positive or negative sign.8
Previously, we reported the trinuclear linear CoIII-CoII-CoIII mixed-
valence single-molecule magnets.9 Magnetic analyses revealed that
central Co(II) was magnetically isolated by two terminal
diamagnetic Co(III) subunits, in which the energy scale of anisotropy
may stem from the transverse zero-field splitting. Here, we
employed the multi-site coordination donor to self-assemble the
Introduction
The transition-metal mononuclear complexes rather than
polynuclear aggregates have shown promise as single-molecule-
magnet (SMM) candidates.1 For transition-metal clusters, the height
of the energy barrier and therefore the SMM behavior depends on
the large-spin multiplicity of the ground state (ST) and the Ising-type
magnetic anisotropy of the molecule (D < 0).2 However, slow
magnetic relaxation for mononuclear first-row transition metal
complexes was frequently prohibited because fast quantum
tunneling magnetization (QTM) may prevent the magnetization
relaxation through a thermally activated mechanism, and in most
cases it is effective to exert a small magnetic field to suppress
tunnelling and induce the slow magnetization relaxation (so-called
field-induced single-ion magnet).3,4 It is believed that the low
coordination environment of first-row transition metal complexes
forms a relatively weak ligand field, resulting in the d orbital energy
splitting with a small separation between the electronic ground
state and the excited states, thus maximizing the spin–orbit
coupling to enhance the magnetic anisotropy.5-7 Indeed, variation of
tetranuclear
[Co4(L)4(CH3CO2)2(CH3OH)2]·Et2O (1), in the form of CoIII-CoII-CoIII-CoII
valence alignment, in which is doubly deprotonated
mixed-valence
cobalt
complex,
L
a
multidentate ligand as depicted in Scheme 1. Complex 1 contains
two magnetically-isolated weakly coordinated octahedral Co(II) ions
with axial anisotropy and large transverse component. Slow
relaxation of the magnetization that is characteristic of Single-
Molecule Magnet (SMM) behavior was observed.
Experimental Section
Reagents and General Procedures
All of the reagents employed were commercially available and were
used without further purification. Perdeuterated dimethyl sulfoxide
(DMSO-d6) was purchased from Alfa Aesar Co. Ltd. Elemental
analyses (C, H, and N) were conducted with a PerkinElmer 2400
analyzer. Micro-IR spectroscopy studies were performed on a
Nicolet Magna-IR 750 spectrophotometer in the 4000−400 cm−1
aJiangsu Key Laboratory of Advanced Catalytic Materials and Technology,
Collaborative Innovation Center of Advanced Catalysis & Green Manufacturing,
School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu
213164, China; E-mail: wudy@cczu.edu.cn; fengxuejun@cczu.edu.cn
bWuhan National High Magnetic Field Center, Huazhong University of Science and
Technology, 430074 Wuhan, China
1
region (w, weak; b, broad; m, medium; s, strong) by a KBr disk. H
c Key Laboratory of Polyoxometalate Chemistry of Henan Province, School of
Chemistry and Chemical Engineering, Henan University, Kaifeng, 475004, People’s
Republic of China
†Electronic Supplementary Informaꢀon (ESI) available: Additional magnetic,
structural, and spectroscopic characterization data. This material is available free of
charge via the Internet at http://pubs.acs.org. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/x0xx00000x
NMR spectra were obtained from a solution in deuterated DMSO
using a Bruker-400 spectrometer (s, singlet; d, doublet; t, triplet; m,
multiplet; dd, double doublet).
Synthetic route of ligand H2L is shown in Scheme 1. 2-oxo-
quinoline-3-carbaldehyde was prepared according to the literature
method.10 A mixture ethanolic solution (80mL) of benzoyl hydrazine
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(10mmol, 1.36g) and 2-oxo-quinoline-3-carbaldehyde (10mmol, variable-field (0–5.0 T) magnetization measurements at low
1.73 g) was refluxed for 2 h under nitrogen atmosphere; after it was temperatures in the range of 2.0–10.0 K were carried out with a
cooled to room temperature, a pale yellow solid was obtained by Quantum Design SQUID magnetometer. Alternating current (ac)
filtration under reduced pressure. The crude product was washed susceptibility measurements were performed under an oscillating
with cold ethanol and dried in vacuo. Yield: 86%. 1H NMR (in DMSO- field of 3 Oe and ac frequencies in the range from 1 to 1500 Hz. The
d6): δ 8.73(s, 1H), 8.49(s, 1H), 7.96(t, 2H), 7.87(t, 1H), 7.61(m, 1H), magnetic data were corrected for the sample holder and the
7.55(m, 4H), 7.46(t, 1H), 7.36(d, 1H), 7.23(t, 1H). Elemental analysis intrinsic diamagnetic contributions from Pascal constants.17 Dc and
for C17H13O2N3, Calculated: H, 4.50; C, 70.09; N, 14.42. Found: H, ac magnetic measurements were carried out by restraining the
4.75; C, 69.38; N, 14.67%.
sample in eicosane in order to prevent any torquing due to its
Preparation of compound 1. A solution of Co(OAc)2·4H2O magnetic anisotropy.
(0.10mmol, 0.025g) in methanol (10ml) was added to a methanol HF-EPR
solution of ligand H2L(0.1mmol, 0.029g) in the absence of any base, HF-EPR measurements were performed on a locally developed
the mixture solution was stirred for another 15 min. The suspension spectrometer at the Wuhan National High Magnetic Field Center,
was then filtered, and the filtrate was diffused with diethyl ether. using a pulsed magnetic field of up to 30 T.18,19
Dark-red block-shaped single crystals suitable for X-ray diffraction Theoretical Investigation
analysis were obtained after several days. Yield: 68%. IR(KBr Gaussian 09 was used for all the computations and the Minnesota
pellet,cm-1)：3404(s), 1613(s), 1590(w), 1562(w), 1508(m), 1494(m),
density functional M06-L and M06-2x and the valence double zeta
basis set, augmented with d-type polarization functions as well as
the diffuse functions for heavy elements and p-type polarization
functions for H, namely 6-31+G(d,p) were employed.
1438(w), 1381(w), 1185(m), 781(w), 754(w), 702(w), 668(w). Anal.
Calcd for C78H68Co4N12O15: H, 4.16; C, 56.81; N, 10.19. Found: H,
4.43; C, 57.05; N, 10.65.
In the model complex, the doubely deprotonated multidentate
ligand (L in the complex) was simplified by eliminating the phenyl
groups at the terminal of the ligand.(Fig. S2, ESI).
Scheme 1. The synthesis route of ligand H2L.
O
O
N
H
POCl3
DMF
H
Mulliken spin density values of main atoms, the total energy (in
Hartree) and the cartesian coordinates for the optimised structure
of the complex are provided in the supporting materials.(ESI, Table
S2-3) The optimized geometry of the modeled complex is depicted
in supporting materials, Fig. S3, in which the N—Co(II) atomic
distances amount 2.06 to 2.09 A and the O—Co(II) distances varies
from 1.98 to 2.23 A. These values are close to those observed in the
crystal measurements. The model adopted in the DFT study
reasonably represents the complex 1. MO analysis reveals that the
dz2 and dx2-y2 of Co(III) are unoccupied in the complex (Fig. S4).
H
HAc
O
N
O
N
Cl
H
2-oxo-quinoline-3-carbaldehyde
O
NH2
N
O
H
N
H
H
N
O
N
H
O
N
O
MeOH
H2L
H
Single-Crystal Structure Determination
Table 1. Summary of crystallographic data for 1.
The diffraction intensity data of complex 1 at 120(2) K were
collected on a Bruker APEX-II CCD with graphite-monochromated
Mo Kα radiation (λ = 0.71073 Å). Data collection, data reduction,
and cell refinement were performed by using the Bruker Instrument
Service v4.2.2 and SAINT V8.34A software.11,12 Structures were
solved by direct methods using the SHELXS program, and
refinement was performed using SHELXL based on F2 through full-
matrix least-squares routine.13 Absorption corrections were applied
upon using multiscan program SADABS.14 Hydrogen atoms of
organic ligands were generated geometrically by the riding mode,
and all the non-hydrogen atoms were refined anisotropically
through full-matrix least-squares technique on F2 with the SHELXTL
program package.15,16A summary of the crystallographic data and
refinement parameters is shown in Table 1. Selected bond lengths
and bond angles for 1 are listed in Table S1.
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C78H68Co4N12O15
1649.16
Triclinic
P-1
14.9882(11)
15.0573(12)
17.6800(13)
74.232(2)
82.418(2)
66.125(2)
3510.2(5)
2
b (Å)
c (Å)
α(°)
β(°)
γ(°)
V (Å3)
Z
F (000)
1696
Goodness-of-fit on F2
R1, wR2 (I>2σ(I)a
R1, wR2 (all data)a
Residuals (e Å–3)
1.077
0.0614 0.1805
0.0898 0.2443
1.041, -1.067
Magnetic Measurement
aR1 = Σ||Fo| − |Fc||/Σ|Fo|; wR2 = [Σ[w(Fo − Fc )2]/Σ[w(Fo2)2]]1/2 bGOF =
.
[Σ[w(Fo2 − Fc2)2]/(Nobs − Nparams)]1/2, based on the data I > 2σ(I).
2
2
Variable-temperature (2.0–300 K) direct current (dc) magnetic
susceptibility measurements under an applied field of 1.0 kG, and
2 | J. Name., 2012, 00, 1-3
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localized on the d orbitals (dxy, dx2-y2, and dz2) for each high-spin
Co(II) ion.(Fig.1(d)) It should be noted that the spin-spin interaction
between Co(II) ions can be ignored. The DFT calculations of the
open-shell singlet state of this model complex results in the same
total energy as compared to that based on the septet state
calculations.
Results and Discussion
X-ray Structure Analysis
The crystal structure of complex 1 contains neutral tetranuclear
[Co4(L)4(CH3CO2)2(CH3OH)2] unit in addition to an ether molecule as
shown in Fig. 1.20 Within the individual ligand, the C−N bond
distances of 1.336(3), 1.365(7) and 1.317(7) Å for C7−N1, C16−N3
and C17−N3 bonds, respecꢀvely, are the intermediate between the
normal single bond and double one, suggesting that the protons on
all the carbohydrozone and imine groups get lost and L acts as a
dianionic ligand in the complex (denoted as L). Two corner cobalt
atoms Co1 and Co3, with the separation of 5.995 Å, are
octahedrally coordinated to two chelating ligands and form CoN2O4
coordination sphere with the comparable Co-O and Co−N bond
distance varying from 1.856(4) to 1.917(4) Å, the bite angle in the
two ligands is very close to 90o, and the full set of angles about Co1
range between 83.2o and 93.3o, while another pair of corner cobalt
atoms, Co(2) or Co(4), are separated by 6.125 Å and each one is
located around the heavily distorted octahedral sphere by two
monodentate N atoms from two different ligands, one methanol
molecule and one acetate anion with M-L distance of ca. 2.0 Å. In
addition, two weakly coordination sites exist around Co4 with
Co4…O2 separation being 2.601 Å and Co4…O8 (2.578 Å). The
octahedral coordination about Co4 is severely distorted with the
bite angle O2-Co-N3 and O8-Co4-N12 of ca. 55o. The similar
coordination sphere is found on its diagonal Co2, and the weak
coordination is evidenced by Co2…O4 contact of 2.355 Å and
Co2…O5 of 2.544 Å. As expected, the low-spin Co3+ ions allow the
Fig. 1. (a) Ligand structure and its deprotonation formation upon metal
coordination. (b) Crystal structure of the complex 1 with the selected atom
scheme and the solvent molecule omitted for clarity. (c) The total spin
density of the model complex. (d) The singly-occupied molecular orbits
(SMOs) localized on Co(2). Similar SMOs on Co(4) are not displayed here for
clarity.
ligand donor atoms to be significantly shorter (∼0.2 Å) than the
analogous distances involving the high-spin Co2+ ions. The bond
distances around the Co(2) and Co(4) center are obviously longer
than those of Co(1) and Co(3); two Co−N bond distances vary
between 2.028(5) and 2.044(4) Å and Co-O distances range from
1.981(4) to 2.065(4) Å with the exceptionally long Co…O weak
coordination, all of which are indicative of high-spin Co2+ (S = 3/2)
for Co(2)/Co(4) and low-spin Co3+ (S = 0) for Co(1)/Co(3),
respectively. The oxidation state of four cobalt ions was assigned on
the basis of these bond length considerations, charge balance, and
BVS calculations. BVS values of 4.49, 2.27, 4.48 and 2.28 for the
Co(1), Co(2), Co(3) and Co(4) centers, respectively, support the
valence assignment on the basis of bond lengths.21 The analysis of
X-ray diffraction data can give rise to a mixed-valence metallogrid
complex of the CoIII−CoII−CoIII−CoII form. The UV-vis-NIR spectra was
further checked to show no inter-valence charge transfer band
(IVCT).(Fig. S5, ESI) Hence, the compound 1 is a valance-localized
(type-I) mixed valence system without electronic interactions
between Co(II) and Co(III).
Magnetic Study
Temperature- and field-dependent magnetic data are depicted in
Fig.2, where χM is the molar magnetic susceptibility per CoIII2CoII
2
unit. The χMT value of 5.15 emu K mol-1 at room temperature
corresponds to the value expected for two S = 3/2 ions with g =2.34.
These values substantially exceed the spin-only value for two high-
spin cobalt(II) ions (S = 3 / 2 , 1.875 cm3mol−1 K with g = 2). This is
indicative of an unquenched orbital contribution of the Co2+ ion in a
distorted-octahedral geometry. When the temperature is further
lowered, the χMT decreases pronounced to reach the values of 3.52
cm3 mol−1 K at 2 K. This behavior is due to the local anisotropy of
the Co2+ ion promoted by the spin-orbit coupling (SOC) rather than
intramolecular interactions between paramagnetic Co(II) ions
through the diamagnetic CoIII linkage. The data could be analyzed
through a Hamiltonian for mononuclear model that takes into
account spin−orbit coupling, axial distortion of the octahedral
geometry, and Zeeman interactions:27
ꢋ
3
2
3
ꢀ ꢁ ꢂ ꢃ ꢄꢅꢆꢇ ꢈ Δꢉꢆꢊ ꢂ ꢃ ꢌ ꢈ ꢍꢎꢂ ꢃ ꢄꢆ ꢈ ꢏꢐSꢑH ꢒ1ꢓ
2
3
2
where κ is the orbital reduction factor, λ is the spin−orbit
To further confirm the X-ray diffraction results on assigning the
valence and spin state, the density functional theory (DFT)
calculations were used to optimize the geometries and electronic
structure using the Gaussian 09 code.22-25 The total spin density of
the model system as depicted in Fig. 1(c) clearly indicates that the
spin density of the system is largely limited on the two high-spin
cobalt ions.26 The successive molecular orbital (MO) analysis
demonstrates that three singly occupied MOs (SMO) are well-
4
coupling parameter, and Δ is the axial orbital splitting of the T1g
term. The factor −3/2 comes from the fact that the real angular
4
momentum for the T1g ground state in an ideal Oh geometry is
4
equal to the angular momentum of the P free ion term multiplied
by − 3/2. The best-fit parameters were as following: κ = 0.79, λ =
−57, Δ = 351 cm−1 , R = 6 × 10−5. The fitting parameters are in good
accordance with previously reported values for distorted-
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octahedral Co(II) complexes and indicate the distortion from the was examined as shown in Fig. 3(a) (Fig. S8, SI). Debye model was
ideal octahedral configuration.28
used to extract relaxation times (τ) at different temperatures. The
To further determine the magnitude and sign of the anisotropy results were employed in constructing the Arrhenius plots [τ = τ0
parameter, we determined the field dependence of the exp(Ueff/kBT)] shown as inset of Fig. 3b. Assuming a thermally
magnetization of complex 1 at fields ranging from 0.05 to 5 T activated mechanism, a fit to the linear data would provide the
between 2 and 10 K (inset, Fig. 2). The M vs H/T plots for 1 are not activation energy (Ueff = 17.7 cm−1) and the pre-exponential factor
superimposed on a single master curve, clearly indicating the (τ0 = 5.5 × 10−8 s). Obviously, Ueff is much smaller than the zero-field
presence of a significant magnetic anisotropy. The magnetization of energy gap of magnitude 2D as predicted by the static magnetic
the sample at 5 T and 2 K is M = 4 µB, which is far below the measurements. This means that the derived energy barrier cannot
expected saturation value for two S = 3/2 and g = 2.34 (Msat = 7.0 be correct. Similar observations have recently been reported for
µB) and is another indication of large ZFS. Indeed, the low- other mononuclear cobalt complex.8f
temperature experimental magnetization data were then analyzed
(a)
1.0
0.8
0.6
0.4
0.2
0.0
2 K
2.25 K
2.5 K
2.75 K
3 K
3.25 K
3.5 K
3.75 K
4 K
using ANISOFIT version 2.0, which takes into account the axial (D)
and transverse (E) magnetic anisotropies in the spin Hamiltonian29:
2
Ĥ = DŜz2 + E(Ŝx2 − Ŝy ) + gμBŜH
(2)
where S is the spin ground state, g is the average g factor, μB is
the Bohr magneton, and H is the magnetic field. The best-fit values
of the parameters were as follows: D = -34.8(5) cm−1, E = 6.7(6)
cm−1, and g = 2.5(2) with f = 0.0010 (solid lines in Fig. 2, inset).
Optimization by setting the initial D to a positive value does not
converge to a reliable fit, indicating the correct choice of the sign.
Because D is negative, the Ms = ± 3/2 Kramers doublet is below the
Ms = ± 1/2 Kramers doublet.
4.25 K
1
10
100
1000
4
z2
ν
/ Hz
d
5.0
4.5
4.0
3.5
3.0
(b)
-5.5
-6.0
-6.5
-7.0
-7.5
-8.0
-8.5
-9.0
-9.5
Obs
Calc
3
2
1
0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
experimental data
Orbach+Raman
Orbach
d
d
x2-y2
xy
β
Ν
0.05T
0.1T
0.5T
1T
3T
5T
-6.0
χ
χ
χ
χ
-6.5
-7.0
-7.5
-8.0
-8.5
-9.0
-9.5
dxz
Fitting
τ
dyz
0.0
0.5
1.0
1.5
2.0
2.5
/
τ
H / T (T / K)
0
50
100
150
200
250
300
Temperature (K)
Fig. 2 (Left) Temperature dependence of χMT for complex 1 under the
magnetic field of 0.25 T. Solid lines represent the calculated result with the
Hamiltonian of eq 1. Inset is low-temperature magnetization data for 1
collected in the temperature range of 2−10 K under various applied dc fields
of 0.05−5 T. Solid red lines correspond to best fits obtained with ANISOFIT
2.0.19 (Right) Removal of the degeneracy of the d orbitals on Co2 and Co4
and their relative energy level diagrams of d orbitals.
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
lnT/K
0.25
0.30
0.35
0.40
T-1 / K-1
0.45
0.50
Fig. 3 (a) Frequency dependence of out-of-phase (χM") ac susceptibilities for
1 under 1000 Oe dc field. The solid lines are the best fitting to the
generalized Debye model. (b) Natural logarithm of the relaxation time lnτ vs
T-1. The curved red line is the fit to sum of Raman and Orbach processes with
Ueff = 68.8 cm-1 (fixed). Inset is power law for 1 between 2.0 K and 3.5 K.
The large and negative zero-field splitting D parameter for
compound 1 indicate the existence of an important axial anisotropy
and therefore the possibility of slow magnetic relaxation. Under a
zero dc field, no maximum in the out-of-phase ac susceptibility
signal (χM”) of both the frequency and temperature dependent
plots was observed for 1 (Fig. S7, ESI†), which is due to very fast
quantum tunnelling of magnetization (QTM). Although no peaks of
χM” are observed under zero dc magnetic field, non-zero χM” signals
appeared under an applied field of 1.0 kG (this field was chosen
because it induces slower relaxation (Fig. S7 and Table S4, ESI),
The Cole−Cole plots for compound 1 at different temperatures
under a 1.0 kG dc field also display signatures of slow magnetic
relaxation (Fig. 4). The Cole−Cole diagrams in the temperature
range 2−4.25 K for 1 exhibit semicircular shapes. The fit of the χM”
vs χM′ data at each temperature using the generalized Debye mode
(see equation S1 in the Supporting Information) yielded the values
of χT(isothermal susceptibility), χS (adiabatic susceptibility), and α
(this parameter determines the width of the distribution of
relaxation times, so that α = 1 corresponds to an infinitely wide
distribution of relaxation times, whereas α = 0 represents a
indicating
a slowly relaxing magnetic moment. To obtain
quantitative information regarding the spin relaxation barrier, the
frequency dependence of χM″ and χM′ at the different temperatures
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12
relaxation with a single time constant). Fitting the data gives the
calculated values of α within the relatively wide range of 0.002-0.2
at the different temperatures (Table S5, Supporting Information),
which is indicative of multiple relaxation processes and the
presence of a non-negligible remaining QTM relaxation.
gx = gy = 2.58(5) gz = 2.30(5)
D =
34.4(6)cm−1
E = 9.5(5) cm−1
z
y
10
8
−
6
1.0
4
2 K
x
0.8
2.25 K
2.5 K
2.75 K
2
0.6
3 K
3.25 K
3.5 K
3.75 K
0
0
50
100
150
200
250
300
0.4
4 K
4.25 K
Frequency (GHz)
0.2
Sim, D > 0
Expt.
0.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
χM' / cm3mol-1
Fig. 4. Cole−Cole plot of χM″ vs χM′ over the temperature range of 2.0−4.25 K
under a 1000 Oe applied dc field for compound 1. The solid line represents a
least-squares fitting of the data using the generalized Debye mode.
Sim, D < 0
To resolve this discrepancy, we recorded high-field, high-frequency
electron paramagnetic resonance (HF-EPR) spectra at different
frequencies and up to an applied magnetic field of 10 T of complex
1 to confirm the magnetization results, and specifically the negative
sign of D parameters (Fig. 5). The low-temperature powder data are
typical for an anisotropic system described by the zero-field
0
2
4
6
8
10
Magetic Field (T)
Fig. 5 (a) Frequency dependence of the high-frequency EPR peak positions
deduced from studies of a powder sample of 1 at 4.2 K. (b) The best
simulation and experimental spectra of 120 GHz in derivative mode at 4.2 K.
2
Hamiltonian H = DŜz2 + E(Ŝx2 − Ŝy ) + gμBŜH. Here, three components
The observation of field-induced slow magnetization relaxation in 1
is difficult to understand. Another mononuclear CoII complex with D
< 0 has been reported recently,8f the quantum relaxation processes
are hindering the observation of the full barrier. However, in the
case of 1 this explanation does not seem to be pertinent, since
quantum tunneling relaxation only contributes in the zero field.
Instead, Raman relaxation prevails at most temperatures, and the
exponential Orbach pathway becomes important at the highest
temperatures only. Nevertheless, the Raman process still influences
greatly the overall relaxation properties, so the observed barrier of
magnetization reversal (Ueff = 17.7 cm−1) is far lower than the
barrier of Orbach process only (U = 68.8 cm−1). With the fixed zero-
field energy, we revisit the analysis of the ac susceptibility data by
considering the additional relaxation process. We have fitted the
data in Fig. 3b as a sum of the Orbach and Raman processes with
the equation(1) τ−1 =CTn + τ0−1 exp(−Ueff/kBT), keeping the effective
energy barrier fixed at Ueff = 68.8 cm-1. The best fit is obtained for C
are observed for complex 1, corresponding to transitions between
two Kramers doublets (ms = ±1/2 and ±3/2) according to the
selection rule Δms = ±1, with one low field component [effective
Landé constant gx,eff = 6.66(7) for 1 separated from two high-field
components [gy,eff = 2.62(8) and gz,eff = 1.65(8). This pattern of g
values is characteristic of an orbitally nondegenerate ground state
with a negative D value. To precisely determine the axial D and
transverse E terms, spectral simulations were performed under the
Hamiltonian mentioned above when considering spin ground state
anisotropy. Assuming uniaxial anisotropy D is close to that
evaluated from M versus H/T analysis, a good simulation of the
spectra can be achieved as shown in Fig. 5b by setting the
anisotropic g tensor.30 The parameters are as follows: gx = gy =
2.58(5), gz = 2.30(5), D = -34.4(6) cm−1, and E = 9.5(5) cm-1. This is a
clear evidence that the D value is in agreement with the result of
magnetization data because an alternative simulation with the
same absolute positive D value yields an even larger discrepancy.31
We have also tried to neglect the E parameter from the spin
Hamiltonian, however, the simulation results are very poor, and no
better results are obtained by allowing for an isotropic g tensor.
-1
= 2.6±0.3, n=6.7±0.3, and τ0 = 1.8 × 1015 s-1 for 1. We found that
the relaxation times obey a power law (τ~T-n; n=6.9±0.1) in the 2.0-
3.5 K temperature range (Fig. 3b, inset), which is indicative that
compound 1 involves a dominant Raman processes.8h
Conclusions
In conclusion, we reported the syntheses, structures, and magnetic
properties of a new CoIII2CoII tetranuclear mixed-valence complex
2
with general formula [Co4(L)4(CH3CO2)2 (CH3OH)2]·Et2O (1). The
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electronic structures of four cobalt ions were experimentally and
theoretically analyzed. DFT calculations provide a direct assignment
of the spin distribution. Although both magnetism and HF-EPR
revealed compound 1 has significant uniaxial anisotropy D value,
they do not exhibit slow relaxation of the magnetization at zero
magnetic field. This somewhat surprising behavior is due to the fast
quantum tunneling of the magnetization (QTM), the dominant
relaxation pathway at zero field. However, under a small magnetic
field, the QTM relaxation pathway is suppressed, the relaxation of
the magnetization is slowed down, and compound 1 show slow
magnetism relaxation behavior. Through control of redox property
of transition metal centers, complex 1 is also a promising building
block for larger strongly exchange coupled clusters to suppress
quantum tunneling.
(d) R. Herchel, L. Vahovska, I. Potocnak and Z.
Travnicek, Inorg. Chem., 2014, 53, 5896-5898. (e) T.
Jurca, A. Farghal, P. H. Lin, I. Korobkov, M. Murugesu
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Rechkemmer, F. D. Breitgoff, M. vander Meer, M.
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Neese, B. Sarkar and J. van Slageren, Nature Commu.,
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ACKNOWLEDGMENT
(9) D. Wu, X. Zhang, P. Huang, W. Huang, M. Ruan and Z.
W. Ouyang, Inorg. Chem., 2013, 52, 10976-10982.
(10) M. K. Singh, A. Chandra, B.Singh and R.M. Singh,
Tetrahedron Lett., 2007, 48, 5987-5990.
We are thankful for financial support by the Priority Academic
Program Development of Jiangsu Higher Education Institutions. This
experimental work was financially supported by the National
Natural Science Foundation of China (Grants 21471023
&
21671027), Jiangsu Provincial QingLan Project, and Jiangsu Province
Key Laboratory of Fine Petrochemical Engineering (KF1302).
(11) Bruker SAINT, v8.34A; Bruker AXS Inc: Madison, WI,
2013.
(12) Bruker APEX2, v2014.5−0; Bruker AXS Inc: Madison,
WI, 2007.
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Magnetically isolated weakly coordinated cobat(II) ions with unusual electronic structure in
mixed-valence metallogrid show field-induced slow magnetization relaxation.
1