Slow magnetic relaxation in mononuclear octahedral manganese(III) complexes with dibenzoylmethanide ligands

Article abstract of DOI:10.1002/ejic.201402964

The structural characterization and magnetic studies of three mononuclear Mn^III complexes based on the dibenzoylmethanido (dbm^-) ligand, Mn(dbm)₃ (1) and [Mn(dbm)₂(L)₂](ClO₄) (L = dimethyl

Full text of DOI:10.1002/ejic.201402964

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DOI:10.1002/ejic.201402964
Slow Magnetic Relaxation in Mononuclear Octahedral
Manganese(III) Complexes with Dibenzoylmethanide
Ligands
Lei Chen,[a] Jing Wang,[a] Yuan-Zhong Liu,[a] You Song,[a]
Xue-Tai Chen,*[a] Yi-Quan Zhang,*[b] and Zi-Ling Xue[c]
Keywords: Manganese / N,O ligands / Single-molecule magnets / Magnetic properties
The structural characterization and magnetic studies of three
mononuclear MnIII complexes based on the dibenzoylmeth-
anido (dbm–) ligand, Mn(dbm)3 (1) and [Mn(dbm)2(L)2](ClO4)
(L = dimethyl sulfoxide, 2; L = pyridine, 3), are reported. The
MnIII ions are in an axially elongated octahedral coordination
obtained by fitting magnetic-susceptibility and magnetiza-
tion data. Ab initio calculations also show the axial magnetic
anisotropy in the molecules of 1–3. With an applied dc field,
the ac susceptibility measurements reveal slow magnetic re-
laxation in 1–3. The possible relationship between the struc-
geometry. The axial zero-field-splitting parameters D, rang- tures and the magnetic properties in these complexes are dis-
ing from –3.42 to –4.52 cm–1 for the three complexes, were
cussed.
ions with large magnetic anisotropies. Since Ishikawa et
al.[5] reported the first example of a mononuclear lantha-
nide-based SMM, so-called single-ion magnets (SIMs), the
slow magnetic relaxation behavior has been observed in
many mononuclear complexes containing lanthanide or ac-
tinide ions.[6]
Introduction
Superparamagnetic discrete molecules exhibit slow mag-
netic relaxation and other quantum properties, and they are
known as single molecule magnets (SMMs).[1] The slow
magnetic relaxation of SMMs arises from the reversal of
spin overcoming an energy barrier U, which is based on the
ground-state spin S and the uniaxial anisotropy D with U
= |D|S2 for integer and U = |D|(S2 – 1/4) for half-integer
spins. Therefore, achieving a large spin value (S), increasing
the uniaxial anisotropy (D), and the combination of these
two approaches have been employed to increase the spin-
reversal barrier and to improve the magnetic properties of
SMMs. Initially, the study of SMMs had been focused on
transition-metal clusters, especially on polynuclear Mn clus-
ters,[2] because it is easier to design and synthesize such
clusters with a strong magnetic coupling between the metal
centers than to control the anisotropy of a molecule. The
family of SMMs has since been extended to polynuclear
3d–4f[3] and lanthanide clusters[4] by introducing lanthanide
More recently, mononuclear complexes based on transi-
tion-metal ions have exhibited SMM behavior as well.[7–11]
Mononuclear MnIII complexes are of particular interest,
because the largest family of polynuclear SMMs contains
5
the MnIII ion. The d4 MnIII ion has an Eg ground state in
an octahedral environment, such that the first-order orbital
contribution is quenched. However, because of the Jahn–
Teller effect in octahedral, high-spin MnIII complexes, the
5
5
5Eg ground state is split into A1g and B1g, which corre-
sponds to an axial elongation with the zero-field-splitting
(ZFS) parameter D Ͻ 0 in most cases or to an axial com-
pression resulting in D Ͼ 0 in rare cases.[12] Therefore, the
highly elongated configuration attracted considerable atten-
tion in the search for new MnIII SIMs with negative D val-
ues. So far, three samples of MnIII-based SIMs have been
reported. Ishikawa et al.[11a] presented an MnIII–salen-type
complex with a diamagnetic [CoIII(CN)6]3– unit, [Mn-
{5-TMAM(R)-salmen}(H2O)–Co(CN)6]·7H2O·MeCN {5-
TMAM(R)-salmen = (R)-N,NЈ-(1-methylethylene)bis[(5-tri-
methylammoniomethyl)salicylideneiminate]}, which shows
magnetic relaxation properties without frequency-depend-
ent maxima in the frequency range 1–1500 Hz. Sanakis et
al.[11b] observed field-induced slow magnetic relaxation in
the mononuclear octahedral manganese(III) complex
[Mn{(OPPh2)2N}3] with a frequency dependence of the
out-of-phase ac susceptibility signals at the low temperature
[a] State Key Laboratory of Coordination Chemistry, Nanjing
National Laboratory of Microstructures, School of Chemistry
and Chemical Engineering, Nanjing University,
Nanjing 210093, China
E-mail: xtchen@netra.nju.edu.cn
http://chem.nju.edu.cn/english/facultylr.asp?fln=CHEN,Xuetai
[b] Jiangsu Key Laboratory for NSLSCS, School of Physical
Science and Technology, Nanjing Normal University,
Nanjing 210023, China
E-mail: zhangyiquan@njnu.edu.cn
http://physics.njnu.edu.cn/szdw/2014-10/111956_147489.html
[c] Department of Chemistry, University of Tennessee
Knoxville, Tennessee 37996, USA
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402964.
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of 2.0 K, but no frequency-dependent maxima were found
in the frequency range 1–10000 Hz. Vallejo et al.[11c]
have observed out-of-phase maxima at frequencies in the
range 1–10000 Hz in the six-coordinate MnIII complex
(Ph4P)[Mn(opbaCl2)(py)2] [H4opbaCl2
dichloro-ortho-phenylenebis(oxamic acid), Ph4P+ = tetra-
phenylphosphonium cation]. These three known MnIII
=
N,NЈ-3,4-
-
based SIMs exhibit an axial elongation because of the
Jahn–Teller effect. Their axial ZFS parameters D were de-
termined to be –3.3, –3.4, and –3.42 cm–1 by EPR (electron
paramagnetic resonance) spectroscopy, and the effective en-
ergy barriers are 9.3, 7.8, and 12.6 cm–1, respectively.[11]
More examples of six-coordinate MnIII SIMs would reveal
the correlation between structures and dynamic magnetic
properties. Herein, we report the dynamic magnetic proper-
ties of three mononuclear MnIII complexes with the chelat-
ing dibenzoylmethanido (dbm–) ligand, [Mn(dbm)3] (1),
[Mn(dbm)2(L)2](ClO4) (L = DMSO = dimethyl sulfoxide, 2;
L = py = pyridine, 3). These complexes show large, negative
D values and field-induced slow magnetic relaxation. Com-
plex 3 exhibits maxima of the out-of-phase ac susceptibility
signals with energy barriers of Ueff = 18.5 cm–1, which is
a value higher than those in the three reported Mn-based
SIMs.[11]
Results and Discussion
Structures of 1–3 are presented in Figure 1. Crystallo-
graphic data, data collection and refinement parameters are
listed in Table S1, and the corresponding bond lengths and
angles around the MnIII ions are given in Table 1. In 1, the
MnIII ion is coordinated by three dbm– ligands, and the
six oxygen atoms form a Jahn–Teller-distorted environment
with an axial elongation along the O1–Mn–O4 direction
(Figure 1, top). A similar coordination sphere was reported
for [Mn{(OPPh2)2N}3][11b,13] and [Mn(trop)3].[14] For 2, the
MnIII ion is located in a six-coordinate octahedral environ-
ment with a static Jahn–Teller distortion, in which the four
O donor atoms of two chelating dbm– ligands define the
equatorial plane, whereas the remaining O atoms from two
DMSO molecules reside in the axial positions (Figure 1,
middle). The axial Mn–O bonds [2.169(3) and 2.177(3) Å]
are longer than the equatorial Mn–O bonds [from
1.9061(18) to 1.9104(18) Å; average of 1.908 Å]. Moreover,
the equatorial O–Mn–O bond angles are in the range
87.88(11)–91.96(8)°, which is close to the angle for an ideal
octahedron (90°) (Table 1). The sulfur atom and one methyl
group of each DMSO ligand are disordered at two posi-
tions. Two phenyl rings from the same dbm– ligand are
nearly coplanar with a small twist angle of 7.84°. The struc-
tural features of 3 are similar to those of 2. As in 2, the
coordinated atoms of the equatorial plane are provided by
exactly two dbm– ligands, but the axially coordinated N
atoms originate from two coordinated pyridine ligands
(Figure 1, bottom). For one of the dbm– ligands, the di-
hedral angle between two phenyl rings is 51.60°, but the
angle in the other dbm– ligand is 3.07°.
Figure 1. ORTEP drawings of 1 and the cations of 2 and 3. H
atoms are omitted for clarity.
As depicted in Table 1, the axially elongated Mn–O or
Mn–N bonds for 1–3 are longer than the equatorial Mn–O
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Table 1. Selected bond lengths [Å] and angles [°] for 1–3.
the values of χMT start to decrease and finally reach 2.21,
2.09, and 1.95 cm3 Kmol–1 at 1.8 K for 1, 2, and 3, respec-
tively. Because of the absence of close Mn···Mn contacts in
the crystal packing, the downturn below 50 K is most likely
a result of the magnetic anisotropy of the MnIII ions with
S = 2 rather than of intermolecular interactions. Field-de-
pendent magnetization measurements were performed be-
tween 1.8 and 5 K at applied magnetic fields in the range
1–7 T, as shown in Figures 3 and S4–S6. With increasing
magnetic field, the magnetization for 1, 2, and 3 increases
and reaches values of 3.28, 3.36, and 3.06 Nβ, respectively,
at 7 T. However, no saturation was achieved. The lack of
saturation at 7 T and the non-superposition of M vs. H/T
1
2
3
Equatorial Mn–O
positions
1.902(3)
1.927(3)
1.929(3)
1.941(3)
87.90(14)
88.74(14)
90.61(15)
92.74(14)
2.116(3)
2.141(4)
1.9061(18)
1.9061(18)
1.9104(18)
1.9104(18)
87.88(11)
88.20(11)
91.96(8)
91.96(8)
2.169(3)
2.177(3)
1.9025(16)
1.9117(16)
1.9130(16)
1.9195(15)
87.35(7)
89.12(7)
91.59(7)
92.37(7)
2.286(2)
O–Mn–O
Axial
Mn–O/Mn–N
positions
2.300(2)
O–Mn–O/N–Mn–N 175.61(13)
177.96
178.77(8)
bonds, and the order of the axial bond lengths in the three
complexes is 3 Ͼ 2 Ͼ 1. The axial bond angles are almost
perfectly linear. The sums (Σ) of the absolute values of devi-
ation from 90° of the twelve cis angles for 1, 2, and 3 are
23.59°, 13.65°, and 27.14°, respectively, which indicates the
small distortion from the ideal octahedron for the three
complexes.[15] To further estimate the degree of distortion,
the continuous-shape-measurement (CSM) analysis was
performed by using the SHAPE 2.1 program. The values
obtained relative to the octahedron are 0.327, 0.418, and
0.931 for 1, 2, and 3, respectively.[15a,15b] Evidently, the devi-
ation of 3 is the most significant. The closest intermolecular
Mn–Mn distances for 1, 2, and 3 are 9.83(4), 7.69(9),
and 8.28(5) Å, respectively, which indicates no close inter-
molecular exchange pathways.
The polycrystalline sample of 1–3 used for the magnetic
measurements was characterized by powder X-ray diffrac-
tion analyses. The diffraction patterns are consistent with
those calculated from the single-crystal X-ray diffraction
data (Figures S1–S3, Supporting Information). Direct-cur-
rent magnetic susceptibility measurements were carried out
in the temperature range 1.8–300 K at 1 kOe (for 1 and 2)
and 2 kOe (for 3), as shown in Figure 2. At room tempera-
ture, the χMT values for 1, 2, and 3 are 3.37, 3.13, and
2.96 cm3 Kmol–1, respectively, which is in agreement with
an S = 2 spin center with g values of 2.12, 2.04, and 1.99,
respectively. Upon lowering the temperature, the values of
χMT remain relatively constant. Below approximately 50 K,
Figure 3. Variable-temperature, variable-field dc magnetization
data collected on pure polycrystalline samples of 1 (a), 2 (b), and
3 (c). Fields of 1–7 T were used from 1.8 to 5 K. Solid lines indicate
the best fits obtained with the PHI program.[16]
Figure 2. Variable-temperature dc susceptibility data of polycrys-
talline samples of 1–3. Solid lines indicate the best fits obtained
with the PHI program.[16]
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curves further point to the presence of ZFS of the S = 2
ground state.
To evaluate the axial (D) and rhombic (E) ZFS param-
eters, the magnetic susceptibility and the magnetization
data were simultaneously fit by using the PHI program[16]
and the anisotropic spin Hamiltonian given in Equation (1).
2
2
2
ˆ
ˆ
ˆ
ˆ
ˆ
H = D[Sz – S(S + 1)/3] + E(Sx – Sy ) + μB(gxSxBx + gySyBy +
ˆ
gzSzBz)
(1)
Here, D and E represent the axial and rhombic ZFS pa-
rameters, and gx, gy, and gz are the anisotropic g factors.
The best fits afforded the final parameters for 1–3 given
in Table 2. It is clear that good agreement was achieved be-
tween the experimental and the simulated data for χMT vs.
T (Figure 2) and M vs. H curves at various temperatures
(Figure 3). It should be noted that 1 and 3 have been
studied by Gatteschi et al.[17] and Aromi et al.[18] by using
high-frequency and -field (HF) EPR spectroscopy, respec-
tively. Evidently, the D and E values obtained here are con-
sistent with the values determined by HFEPR (D =
–4.35 cm–1, E = 0.28 cm–1 for 1; D = –4.5 cm–1, E =
–0.4 cm–1 for 3).
Table 2. Results of the fittings of the dc magnetic data carried out
with the PHI program[16] for 1–3.
gz
gx,y
D [cm–1]
E [cm–1]
Residual
1
2
3
2.03
1.92
1.97
2.16
2.11
2.01
–4.52
–3.42
–4.46
0.71
0.74
0.93
0.00013
0.00041
0.000047
To reveal the SMM characteristics of 1–3, frequency-de-
pendent alternating-current (ac) magnetic susceptibility was
measured in the temperature range 1.8–10 K. No out-of-
phase ac susceptibility (χMЈЈ) signal was observed under an
applied dc field of zero for 1–3 (Figures S8–S10) because of
the quantum tunneling of the magnetization (QTM)
through the spin-reversal barrier. Hence, to reduce the
QTM effect, a dc field of 1500 or 2000 Oe was employed in
the ac measurements for 1–3. As shown in Figure 4, with
an applied dc field, only a frequency dependence of the χMЈЈ
signal was observed for 1 and 2. However, obvious peaks
of the χMЈЈ signal appear for 3 at the frequencies in the
range 250–1488 Hz with an applied dc field of 1500 Oe. The
temperature and frequency dependence of the ac suscep-
Figure 4. Temperature dependence of the out-of-phase ac magnetic
susceptibility from 1.8 to 3.5 K under applied dc fields, measured
on pure polycrystalline samples of 1 (a), 2 (b), and 3 (c). The solid
lines are given as guide to the eye.
tibilities (Figures 4 and S11–S13) implies that all complexes on transition metal ions.[7–11] For 1, the energy barrier (4|D|
exhibit SMM characteristics.
= 18.2 cm–1) calculated from HFEPR data is considerably
In the cases of 1 and 2, because of the lack of observed larger than the effective energy barrier (11.5 cm–1), and this
peaks of the χMЈЈ signal, the ac susceptibility data were may be explained by the existence of non-negligible QTM.
treated by a model that assumes only single slow relaxation However, in 2, the value of the energy barrier (4|D| =
of magnetization by using the relationship ln(χMЈЈ/χMЈ) = 13.7 cm–1) calculated from the dc magnetization measure-
ln(ωτo) + Ueff/kBT, where ω (= 2πν) is the frequency of the ments is smaller than the effective energy barrier 16.8 cm–1.
ac field, τo is the pre-exponential factor, and Ueff is the ef- The latter value may have been overestimated and should
fective energy barrier.[19] The average values of Ueff are be less than 13.7 cm–1. This might reflect the limitation of
11.5 cm–1 (τo ≈ 2.3ϫ10–7 s) and 16.8 cm–1 (τo ≈ 9.4ϫ10–9 s) the model used to evaluate the effective energy barriers,
for 1 and 2, respectively (Figures 5 and 6). The τo values which assumes only single slow relaxation of magnetization.
obtained here are comparable to those found in SIMs based It should be noted that the presence of disorder in the
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molecular structure of 2 likely results in multiple relaxation
processes,[20] which would render the model not appropri-
ate.
Figure 7. Relaxation time of the magnetization: ln(τ) vs. T–1 plot
at an applied field of 1.5 kOe for 3. The solid lines represent
Arrhenius fits. The data were derived from the maxima of χMЈЈ vs.
T at different frequencies.
Figure 5. ln(χMЈЈ/χMЈ) vs. T–1 plot and the best linear fits under the
2.0 kOe applied field for 1.
The magnetic parameters of 1–3 and the three known
MnIII SIMs[11] are listed in Table 3. The effective energy
barrier of 3 (18.5 cm–1) is significantly higher than those of
the other MnIII SIMs. The energy barriers Ueff for the field-
induced slow magnetic relaxation vary with the dc field
used.[21] Therefore, the Ueff values are only compared if the
same dc fields are used in the ac magnetic susceptibility
measurements. The Ueff values of 1–3 and of the three re-
ported MnIII SIMs were obtained under different dc fields.
However, careful examination of Table 3 could reveal some
trends of the effective energy barriers. The effective barrier
Ueff for these MnIII SIMs seems to be influenced by the
structural types and the ZFS parameters D. The six com-
plexes could be divided into two types. One type, 1 and
[Mn{(OPPh2)2N}3], has three chelating ligands. The other
type, the remaining four complexes, contains chelating li-
gands and two monodentate ligands in the axial positions.
In the two complexes of the first type, the larger Ueff value
corresponds to the larger D value. A Ueff value of 11.5 cm–1
Figure 6. ln(χMЈЈ/χMЈ) vs. T–1 plots and the best linear fits under
the 1.5 kOe applied field for 2.
For 3, as depicted in Figure 4c, frequency-dependent for 1 with a D value of –4.55 cm–1 is larger than 7.8 cm–1
maxima of the out-of-phase ac magnetic susceptibility were for [Mn{(OPPh2)2N}3] with a D value of –3.4 cm–1. Both
observed under a 1.5 kOe applied field. Cole–Cole plots Ueff values were obtained with the similar dc fields (2000
were established and fitted by using the generalized Debye vs. 2250 Oe). The same trend is observed for complexes 2
functions (Figure S14). The fitting parameters are summa- and 3 of the second type. The Ueff value of 3 (18.5 cm–1)
rized in Table S2 in the Supporting Information. The pa- obtained with a dc field of 1500 Oe is larger than that of 2
rameters α (α indicates the deviation from the pure Debye (Ͻ13.7 cm–1) under the same conditions. It is noteworthy
model) in the range 0.04–0.13 imply a narrow width of re- that the magnetic parameters obtained so far are limited,
laxation times. The Arrhenius plot, as depicted in Figure 7, and more experimental work is needed to reach a more reli-
was constructed by using the peaks of the out-of-phase χMЈЈ able conclusion.
signals from the temperature-dependent data at different
To further confirm our conclusions about the magnetic
frequencies (Figure 4c). A fit to the linear relationship af- anisotropy obtained from the above magnetic analyses,
fords the effective spin-reversal barrier of Ueff = 18.5 cm–1 Orca 3.02 calculations[22] were performed with the general-
(τo = 9.2ϫ10–8). Only the thermally activated process was ized-gradient-approximation (GGA) PBE (Perdew–Burke–
observed, which indicates that the QTM mediated by the Ernzerhof)[23] functional. This is a good choice, as con-
hyperfine, dipolar interactions and the transverse anisot- firmed by Neese et al.[24] As expected, other functionals
ropy (E) is suppressed at the applied field of 1.5 kOe. It based on GGA yield very similar results and are therefore
should be noted that the energy barrier in 3 is comparable not reported. The spin–orbit coupling (SOC) operator used
to the value estimated from HFEPR data (4|D|
=
was the efficient implementation of the multicenter spin–
18.0 cm–1), which further implies that the applied field of orbit mean-field (SOMF) concept developed by Hess et
1.5 kOe fully suppresses the QTM effect.
al.[25] The spin–spin contributions (SSC) to the D values
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Table 3. Magnetic parameters for 1–3 and for the reported MnIII SIMs.
Type
Complex
D [cm–1]
E [cm–1]
Ueff [cm–1]
τo [s]
Hdc [Oe][a]
Ref.
1
[Mn(dbm)3] (1)
–4.55[b]
–3.4[b]
–3.42[c]
–4.5[b]
–3.3[b]
0.28[b]
11.5
7.8
2.3ϫ10–7
8.2ϫ10–8
9.4ϫ10–9
9.2ϫ10–8
8.0ϫ10–8
2.9ϫ10–7
1.24ϫ10–7
2000
2250
1500
1500
0
4500
1000
this work
[12b]
[Mn{(OPPh2)2N}3]
2
[Mn(dbm)2(DMSO)2] (2)
[Mn(dbm)2(pyridine)2] (3)
[Mn{5-TMAM(R)-salmen}(H2O)Co(CN)6]
0.74[c]
–0.40[b]
0[b]
16.8[d]
18.5
9.3
this work
this work
[12a]
11.5
12.6
[11c]
(Ph4P)[Mn(opbaCl2)(py)2]
–3.42[b]
–0.15[b]
[a] The applied fields used for ac magnetic susceptibility measurements. [b] Obtained by HFEPR spectroscopy. [c] Obtained by fitting the
dc magnetic data. [d] This value was overestimated and should be less than 13.7 cm–1.
were also included, although they are very small for our MnIII in 3 are responsible for its larger D value compared
complexes.[24] The coupled-perturbed (CP) method pro- to 2.
posed by Neese,[24] which uses revised prefactors for the
spin-flip terms and solves a set of coupled perturbed equa-
tions for the SOC perturbation, was used. CASSCF (com- Conclusions
plete active space self-consistent field) calculations with
We have described the magnetic properties of three mo-
four electrons in the five Mn 3d-based orbitals [CAS(4,5)],
nonuclear MnIII complexes with octahedral coordination
spectroscopy-oriented CI (SORCI),[26] and difference-dedi-
environments. The ZFS parameters D for 1–3 were deter-
cated configuration interaction (DDCI3)[27] on top of the
mined by fitting the magnetic susceptibility and the magne-
CAS(4,5) reference states were carried out. In the calcula-
tization data. The large negative D values are consistent
tions, the orbitals were determined for the average of 5 S =
with the Jahn–Teller axis elongation of the octahedral ge-
2 and 45 S = 1 roots. All calculations were performed with
ometry. Complexes 1–3 show frequency dependence of their
triple-ζ and one polarization function TZVP[28] basis set for
ac susceptibility under applied dc fields. Maxima of the out-
all atoms. Tight convergence criteria were used to ensure
of-phase ac susceptibility signals are observed in 3. The ef-
that the results are well converged with respect to technical
fective spin-reversal barrier of Ueff = 18.5 cm–1 for 3 is
parameters. In the above calculations of D and E employing
higher than those of the other MnIII SIMs. Possible reasons
SORCI and DDCI3, the SOC and SSC contributions were
for the dynamic properties of 1–3 were discussed. A more
both included. The calculated values of the ZFS parameters
reliable relationship between the structure and the magnetic
for 1–3 are shown in Table 4, and they agree with the values
properties requires additional studies of other MnIII SIMs.
obtained from analysis of the dc magnetic data. Further-
more, the magnetic anisotropic axis, calculated by using
DDCI3, locates the axial Mn–L bond (Figure S7), and indi-
cates that the ligand field exhibits higher axial character in
Experimental Section
the axial direction.
General: Unless otherwise stated, all chemicals were purchased
from commercial sources and used without further purification.
nBu4NMnO4 was prepared as previously reported.[29] [Mn(dbm)3]
(1), [Mn(dbm)2(L)2](ClO4) (L = DMSO, 2; L = py, 3) were pre-
pared according to a modified method of Aromi et al., which was
originally used to prepare 3.[18] Elemental analyses were performed
with an Elementar Vario ELIII elemental analyzer. The powder
XRD patterns were measured at room temperature with a Bruker
D8 Advance X-ray diffractometer.
Table 4. Calculated and experimental D, E [cm–1], and g values (x,
y, z) for the ground state of complexes 1–3 obtained by using PBE,
SORCI, and DDCI3.
PBE
D
SORCI
D
DDCI3
D
g[a]
E
E
E
1
2
3
–2.45 0.08 –3.35 0.06 –4.07 0.05 1.995, 1.992, 1.965
–2.03 0.09 –3.03 0.06 –3.64 0.01 1.994, 1.994, 1.969
–2.60 0.13 –3.66 0.01 –3.95 0.01 1.995, 1.994, 1.969
Synthesis of [Mn(dbm)3] (1): To a solution of Mn(ClO4)2·6H2O
(0.225 g, 0.62 mmol) in acetonitrile (10 mL) was slowly added a
purple solution of nBu4NMnO4 (0.055 g, 0.15 mmol) in acetonitrile
(2 mL). Solid Hdbm (0.42 g, 1.86 mmol) was added, and the mix-
ture was stirred for 10 min and then filtered. The filtrate was left
in a beaker until crystallization started and afforded dark crystals.
The yield was 56% with respect to Mn. C45H33MnO6 (724.69):
calcd. C 74.58, H 4.59; found C 74.29, H 4.74.
[a] Obtained by DDCI3.
From Table 4, the D values of 1–3 calculated by using
DDCI3 are closest to the experimental values. The struc-
tures of 2 and 3 are very similar, but their D values are
somewhat different. We wondered whether the pyridine li-
gands in 3 lead to the larger D value compared to the
DMSO ligands in 2. To investigate this, we used two pyr-
idine ligands to substitute two DMSO molecules in 2 and
to obtain a model molecule 2a. The distances Mn–N in 2a
are the same as those in 3. The D value of 2a calculated by
using DDCI3 is –3.94 cm–1, which is almost the same as
Synthesis of [Mn(dbm)2(DMSO)2](ClO4) (2): To a yellowish solu-
tion of Mn(ClO4)2·6H2O (0.225 g, 0.62 mmol) and DMSO (1.0 mL,
11.6 mmol) in acetonitrile (10 mL) was slowly added a purple solu-
tion of freshly prepared nBu4NMnO4 (0.055 g, 0.15 mmol) in
acetonitrile (2 mL). Solid Hdbm (0.35 g, 1.55 mmol) was added im-
mediately to the resulting dark brown solution, and the mixture
that of 3. Thus, we believe that the pyridine ligands around was stirred for a few hours and then filtered. The filtrate was kept
Eur. J. Inorg. Chem. 0000, 0–0
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FULL PAPER
2006; c) R. Winppeny, Molecular Cluster Magnets, World Sci-
entific, Singapore, 2012.
in a beaker until crystallization occurred. Red-brown crystals were
isolated with a yield of 20% with respect to Mn. C34H32ClMnO10S2
(755.13): calcd. C 54.08, H 4.27; found C 53.85, H 4.59.
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X-ray Structure Determination: Single-crystal X-ray diffraction
data for 1–3 were collected with a Bruker APEX DUO dif-
fractometer with a CCD area detector by using graphite-mono-
chromated Mo-Kα radiation (λ = 0.71073 Å) at room tempera-
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data and determining lattice parameters. Data were integrated with
the SAINT program. Absorption corrections were applied by using
SADABS.[31] The structures of complexes 1–3 were solved with the
program SHELXS-97 and subsequently completed by using the
full-matrix least-squares technique with the SHELXL 97 pro-
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and S, O, N, and C atoms were subsequently identified by using
difference Fourier maps. All non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen atoms on organic
ligands were set in calculated positions and generated by the riding
model. The structures of complexes 1[17,33] and 3[18] were previously
reported, but we recollected the data on our own samples to con-
firm their structures. CCDC-1017697 (for 2), -1018575 (for 3), and
1018576 (for 1) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
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Magnetic Measurements: The magnetic properties were recorded at
fields up to 7 T between 1.8 and 300 K with a Quantum Design
SQUID VSM magnetometer (for 1 and 2) at 1 kOe and with a
Quantum Design MPMS-XL17 SQUID instrument (for 3) at
2 kOe. The temperature- and frequency-dependent ac susceptibility
data were collected by using an oscillating ac field of 2.0 Oe and
ac frequencies ranging from 1 to 1000 Hz for 1 and 2, whereas an
oscillating ac field of 5.0 Oe and ac frequencies ranging from 1 to
1500 Hz were used for 3. The magnetic susceptibility data were
corrected for the sample holder as well as for diamagnetism of the
constituent atoms (estimated by using Pascal constants).
Supporting Information (see footnote on the first page of this arti-
cle): Parameters of crystal data collection and refinement, experi-
mental and calculated powder XRD patterns, magnetization mea-
surements, frequency- and temperature-dependent ac susceptibility
data, and Cole–Cole plots.
Acknowledgments
We are grateful for the financial support from the National Basic
Research Program of China (No. 2013CB922102 to X.-T. C. and
Y. S.) and from the Priority Academic Program for the Develop-
ment of Jiangsu Higher Education Institutions (to Y.-Q. Z.). The
donors of the American Chemical Society Petroleum Research
Fund are also acknowledged for partial support of this research
(to Z.-L. X.).
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Received: October 9, 2014
Published Online: ᭿
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Date: 08-12-14 13:18:34
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FULL PAPER
Slow Magnetic Relaxation
L. Chen, J. Wang, Y.-Z. Liu, Y. Song,
X.-T. Chen,* Y.-Q. Zhang,*
Z.-L. Xue .......................................... 1–9
Three mononuclear MnIII complexes based
on the dibenzoylmethanido ligand and hav-
ing negative zero-field-splitting parameters
D exhibit field-induced slow relaxation be-
havior.
Slow Magnetic Relaxation in Mononuclear
Octahedral Manganese(III) Complexes
with Dibenzoylmethanide Ligands
Keywords: Manganese / N,O ligands / Sin-
gle-molecule magnets / Magnetic properties
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim