Synthetic-method-dependent magnetic relaxation in a cobalt(II) phosphonate chain compound

Article abstract of DOI:10.1021/ic5021737

A polar cobalt(II) phosphonate Co(bamdpH₂)(H₂O) (1) [bamdpH₄ = (benzylazanediyl)bis(methylene)diphosphonic acid] is reported. It shows a linear chain structure. The neighboring chains are connected by moderately strong hydrogen bonds forming a supramolecular layer. The interlayer spaces are filled with the organic groups of the phosphonate ligands. Compound 1 displays the coexistence of single-chain magnet behavior and canted antiferromagnetism below 2.8 K. Moreover, the magnetic dynamics is strongly dependent on the synthetic methods, a phenomenon that has not been documented before.

Full text of DOI:10.1021/ic5021737

Article
pubs.acs.org/IC
Synthetic-Method-Dependent Magnetic Relaxation in a Cobalt(II)
Phosphonate Chain Compound
Zhong-Sheng Cai,† Min Ren,† Song-Song Bao,† Norihisa Hoshino,‡ Tomoyuki Akutagawa,‡
and Li-Min Zheng*,†
†State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing
210093, China
‡Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Sendai 980-8577, Japan
S
* Supporting Information
ABSTRACT: A polar cobalt(II) phosphonate Co(bamdpH2)(H2O) (1) [bamdpH4 = (benzylazanediyl)bis(methylene)-
diphosphonic acid] is reported. It shows a linear chain structure. The neighboring chains are connected by moderately strong
hydrogen bonds forming a supramolecular layer. The interlayer spaces are filled with the organic groups of the phosphonate
ligands. Compound 1 displays the coexistence of single-chain magnet behavior and canted antiferromagnetism below 2.8 K.
Moreover, the magnetic dynamics is strongly dependent on the synthetic methods, a phenomenon that has not been
documented before.
To elaborate on these unusual forms of magnetism, we
report a case of a new cobalt phosphonate, namely, Co-
(bamdpH2)(H2O) (1), where bamdpH4 represents
(benzylazanediyl)bis(methylene)diphosphonic acid
[C6H5CH2N(CH2PO3H2)2]. This compound crystallizes in a
polar space group, where considerable frequency dependence of
the ac susceptibilities is observed within a canted antiferro-
magnetic ground state below 2.8 K. More interestingly, the
relaxation property of the compound is dependent on the
samples obtained under different synthetic conditions. A
comparative study on the sample-dependent magnetic behavior
of a closely related SCM compound Co(p-Me-C6H5CH2N-
(CH2PO3H)2)(H2O) (2)5 is also presented.
INTRODUCTION
■
Owing to the potential applications in information storage and
molecular spintronics there are increasing interests in the
development of magnets of molecular and nanosizes.1 As such
single-molecule (SMM)2 and single-chain magnets (SCM)3 are
highly regarded as materials for the future as they display
magnetic hysteresis below their blocking temperature (TB) and
most importantly exhibit slow magnetization relaxation on fairly
long time scales. In the chemistry of SCMs one chooses
moment carriers with significant uniaxial anisotropy, for
4,5
6
7
8
9
10
I
example, CoII, MnIII, FeII, LnIII, ReIV, and UO2 , to
provide an Ising system with a sizable energy barrier for
moment reversal. It is also important that the moments are
strongly coupled within the chain (J) with negligible interchain
interaction (J′).3 Thus far a number of SCMs have been
reported satisfying the above conditions.11 However, con-
troversial examples concerning the coexistence of slow
relaxation pertaining to SCM and long-range effects such as
canted antiferromagnetism12 and metamagnetism11d,13 have
also been observed. Moreover, the relaxation mechanism of
SCMs is not fully understood. Although most reported SCMs
show one relaxation process, dual-relaxation processes are
found in a few cases.13d,14
EXPERIMENTAL SECTION
■
Materials and Measurements. The reagents and solvents
employed were obtained from commercial sources and used without
further purification. ((Benzylazanediyl)bis(methylene))diphosphonic
acid (bamdpH4) was prepared according to the literature.15 Elemental
analyses for C, H, and N were performed on an Elementar Vario
Received: September 6, 2014
© XXXX American Chemical Society
A
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MICRO elemental analyzer. Infrared spectra were recorded on a
Bruker TENSOR 27 IR spectrometer with pressed KBr pellets in the
400−4000 cm−1 region. Thermogravimetric analyses were performed
on a METTLER TOLEDO TGA/DSC 1 STARe instrument in the
range of 30−800 °C under nitrogen flow at a heating rate of 10 °C
min−1. Powder X-ray diffraction patterns were recorded on a Bruker
D8 ADVANCE XRD instrument using Cu Kα radiation. Magnetic
susceptibility data were obtained on polycrystalline samples using a
Quantum Design MPMS-XL7 SQUID magnetometer.
Synthesis of 1-bulk-a. Co(NO3)2·6H2O (58.3 mg, 0.2 mmol)
and bamdpH4 (57.9 mg, 0.2 mmol) were added to 8 mL of distilled
water, and the pH value was adjusted to 3.60 with 0.5 M NaOH. Then
the mixture was transferred to a 25 mL Teflon-lined autoclave and
kept at 140 °C for 3 days. After cooling to room temperature, red
crystals were obtained. Crystals were washed with distilled water and
dried in air. Yield: 20.0 mg (27% based on Co). Anal. Calcd for
C9H15CoNO7P2: C, 29.21; H, 4.09; N, 3.78. Found: C, 29.31; H, 4.37;
N, 3.74. IR (KBr, cm−1): 3577(s), 3337(s), 3244(m), 3023(w),
2967(w), 2896(w), 2336(w), 1649(m), 1239(m), 1204(m), 1171(s),
1135(s), 1029(s), 955(s), 915(s), 861(w), 833(w), 787(w), 744(m),
703(m), 631(m), 578(m), 559(m), 458(m), 426(w).
Table 1. Crystallographic Data for 1
empirical formula
fw
C9H15CoNO7P2
370.09
cryst syst
space group
a (Å)
monoclinic
Cc
8.5361(3)
26.5298(11)
6.2175(2)
113.3120(10)
1293.08(56), 4
1.901
b (Å)
c (Å)
β (deg)
V (Å3), Z
Dc (g cm−3
)
F (000)
756
a
b
R1, wR2 [I > 2σ(I)]
0.0191, 0.0428
0.0202, 0.0431
0.997
a
b
R1, wR2 (all data)
goodness-of-fit
Flack parameter
0.02(1)
(Δρ)max, (Δρ)min (e Å−3
)
0.339, −0.335
905563
CCDC number
a
b
2
2
R1 = Σ||Fo| − |Fc||/Σ|Fo|. wR2 = [Σw(Fo − Fc2)2/Σw(Fo )2]1/2
.
Synthesis of 1-bulk-b. Co(CH3COO)2·4H2O (49.9 mg, 0.2
mmol) and bamdpH4 (58.2 mg, 0.2 mmol) were added to 6 mL of
distilled water, and the pH value was adjusted to 3.54 with 0.5 M
HNO3. Then the mixture was transferred to a 25 mL Teflon-lined
autoclave and kept at 140 °C for 3 days. After cooling to room
temperature, red crystals were obtained, washed with distilled water,
and dried in air. Yield: 23.2 mg (31% based on Co). Anal. Calcd for
C9H15CoNO7P2: C, 29.21; H, 4.09; N, 3.78. Found: C, 29.21; H, 4.17;
N, 3.86.
Synthesis of 1-bulk-c. Co(CH3COO)2·4H2O (49.1 mg, 0.2
mmol) and bamdpH4 (94.7 mg, 0.32 mmol) were added to 6 mL of
distilled water, and the pH value was about 3.48 without adding any
other reagent. Then the mixture was transferred to a 25 mL Teflon-
lined autoclave and kept at 140 °C for 3 days. After cooling to room
temperature, red crystals were obtained, washed with distilled water,
and dried in air. Yield: 24.5 mg (33% based on Co). Anal. Calcd for
C9H15CoNO7P2: C, 29.21; H, 4.09; N, 3.78. Found: C, 29.36; H, 4.50;
N, 3.77.
Synthesis of 1-bulk-d. Co(NO3)2·6H2O (60.9 mg, 0.2 mmol)
and bamdpH4 (58.4 mg, 0.2 mmol) were added to 6 mL of distilled
water, and the pH value was adjusted to 3.49 using 33.7 mg of 4,4′-
bipyridine. Then the mixture was transferred to a 25 mL Teflon-lined
autoclave and kept at 140 °C for 3 days. After cooling to room
temperature, red crystals were obtained, washed with distilled water,
and dried in air. Yield: 20.0 mg (27% based on Co).
Synthesis of 1-bulk-e. Co(NO3)2·6H2O (60.7 mg, 0.2 mmol)
and bamdpH4 (59.5 mg, 0.2 mmol) were added to 6 mL of distilled
water, and the pH value was adjusted to 3.15 using 27.1 mg of 4,4′-
bipyridine. Then the mixture was transferred to a 25 mL Teflon-lined
autoclave and kept at 140 °C for 3 days. After cooling to room
temperature, red crystals were obtained, washed with distilled water,
and dried in air. Yield: 11.8 mg (16% based on Co).
X-ray Crystallographic Analyses. Single crystals were selected
for indexing and intensity data collection on a Bruker SMART APEX
CCD diffractometer using graphite-monochromatized Mo Kα
radiation (λ = 0.71073 Å) at room temperature. Data were integrated
using the Siemens SAINT program16 with the intensities corrected for
Lorentz factor, polarization, air absorption, and absorption due to
variation in the path length through the detector face plate. Absorption
corrections were applied. Structures were solved by direct methods
and refined on F2 by full-matrix least-squares using SHELXTL.17 All
non-hydrogen atoms were located from the Fourier maps and refined
anisotropically. All H atoms were refined isotropically with the
isotropic vibration parameters related to the non-hydrogen atoms to
which they are bonded. Crystallographic data of compound 1 are listed
in Table 1, and selected bond lengths and angles are given in Table 2.
Table 2. Selected Bond Lengths (Angstroms) and Angles
(degrees) for 1
a
Co1−O5B
Co1−O1W
Co1−O1
2.044(2)
2.111(2)
2.120(2)
91.5(1)
Co1−O4A
Co1−O4
Co1−N1
2.122(2)
2.161(2)
2.257(2)
O5B−Co1−O1W
O5B−Co1−O1
O1W−Co1−O1
O5B−Co1−O4A
O1W−Co1−O4A
O1−Co1−O4A
O1−Co1−N1
O4A−Co1−N1
O4−Co1−N1
P1−O1−Co1
C1−N1−Co1
C2−N1−Co1
O5B−Co1−O4
O1W−Co1−O4
O1−Co1−O4
O4A−Co1-O4
O5B−Co1−N1
O1W−Co1−N1
P2−O4−Co1C
P2−O4−Co1
P2−O5−Co1D
C3−N1−Co1
Co1A−O4A−Co1
165.0(1)
93.8(1)
82.9(1)
91.7(1)
174.5(1)
84.7(1)
108.8(1)
82.8(1)
88.4(1)
92.7(1)
100.0(1)
116.3(1)
114.0(1)
152.4(1)
109.4(1)
122.2(1)
80.0(1)
165.1(1)
84.4(1)
114.0(1)
105.8(1)
107.6(1)
a
Symmetry codes: (A) x, −y + 1, z + 0.5; (B) x, y, z + 1; (C) x, −y +
1, z − 0.5; (D) x, y, z − 1.
RESULTS AND DISCUSSION
■
Crystal Structure of 1. Although samples 1-bulk-a−e were
synthesized under different experimental conditions using
different starting materials and/or pH adjusters, general
characterization such as powder XRD, IR, TG, and elemental
analyses reveals that they are identical in both composition and
structure (Figures S1−S3, Supporting Information). Thus, a
single crystal was picked up from 1-bulk-a. Single-crystal
structural analysis reveals that 1 crystallizes in a polar space
group Cc. The asymmetric unit consists of one CoII, one
bamdpH22−, and one coordinated water molecule. The Co1
atom has a distorted octahedral environment surrounded by
one nitrogen atom, four phosphonate oxygen atoms, and one
water molecule [Co1−O 2.044(2)−2.161(2) Å, Co1−N
2.257(2) Å] (Figure 1a). The equivalent Co1 atoms are
bridged by μ3-O(P) and O−P−O units forming a zigzag chain
running along the c axis (Figure 1b). The Co···Co distance over
the μ3-O(P) is 3.750(1) Å. Strong hydrogen-bond interactions
are found between the chains [O6···O2i 2.544(2) Å; O6−
H6A−O2i 172.1(1)°; symmetry code (i) x + 1, y, z], resulting
in a supramolecular layer in the ac plane (Figure S4, Supporting
B
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Figure 1. (a) Building unit of compound 1 with atom-labeling scheme
(40% probability). (b) Chain structure of 1. (c) Structure of 1 packed
along the c axis. All H atoms are omitted for clarity. CoO5N and PO3C
polyhedra are shown in blue and gray, respectively.
Information). The layer is polar due to the alternative parallel
arrangement of the positively charged {CoO5N} and negatively
charged {PO3C} polyhedra along the a axis. The shortest Co···
Co distance between the chains within the layer is 8.120(1) Å.
The phenylmethyl groups reside on opposite sides of the layer.
The layers are stacked along the b axis, resulting in a polar
three-dimensional supramolecular structure (Figure 1c). The
π−π interactions are negligible between the neighboring phenyl
rings as the dihedral angle is 23.9(1)° and center-to-center
distance is 4.208(1) Å. The shortest Co···Co distance between
the layers is 11.920(1) Å.
Magnetic Properties. The magnetic behaviors of samples
1-bulk-a−c are studied in detail. Figure 2a shows the dc
magnetic susceptibilities (H = 2 kOe) plotted as χMT vs T
which are almost the same for the three samples. χMT at 300 K
is 3.14/3.11/3.13 cm3 K mol−1 per Co for 1-bulk-a/1-bulk-b/
1-bulk-c, much larger than the spin-only value of 1.88 cm3 K
mol−1 expected for spin S = 3/2 with g = 2.0 due to the orbital
contribution from the octahedral CoII ion. The susceptibility
data above 100 K follow the Curie−Weiss law with the Curie
constant (C) and Weiss constant (θ) of 3.43/3.18/3.44 cm3 K
mol−1 and −23.7/−28.5/−27.6 K for 1-bulk-a/1-bulk-b/1-
bulk-c, respectively (Figure S5, Supporting Information). The
Weiss temperature θ, slightly more negative than that expected
for the presence of spin−orbit coupling, indicates antiferro-
magnetic (AF) interaction between nearest neighbors.18 Upon
cooling, χMT decreases continuously and approaches a
minimum of 0.68/0.72/0.73 cm3 K mol−1 at 8 K followed by
a sharp increase to a value of 1.67/1.90/1.85 cm3 K mol−1 at 3.5
K, below which χMT decreases again. Since the chain could be
considered as an isotropic Heisenberg chain with S = 3/2 in the
high-temperature region, Fisher’s chain model19 scaled to S =
3/2 is applied to simulate the susceptibility data above 100 K
(Figure S6, Supporting Information). The best fits result in
intrachain coupling constants J = −8.99/−10.07/−9.94 K and g
= 2.71/2.70/2.70 for 1-bulk-a/1-bulk-b/1-bulk-c, respectively.
The g values are close to those obtained from Curie constants
(g = 2.71/2.60/2.71).
Figure 2. (a) χMT vs T plots of the three samples. (Inset) M vs H
plots. (b) Temperature dependence of the dc and ac susceptibilities for
sample 1-bulk-a. Solid lines are a guide for the eye.
bifurcation below 2.8 K, indicating the onset of a blocking of
the moments (Figures 2b and S7, Supporting Information).
The sharpness of the blocking and its independence on the
applied field suggests the occurrence of long-range magnetic
ordering (LRO). The isothermal magnetization displays no
hysteresis loop down to 1.8 K (Figure S8, Supporting
Information). The initial magnetization is almost linear in
field until it saturates at ca. 0.2 Nβ and then increases gradually
to 0.70
0.01 Nβ at 70 kOe (Figure 2a, inset). The latter
values are much lower than that for a fully aligned moment of
2.3 Nβ. The behavior can be regarded as a canted
antiferromagnet with a canting angle estimated from
sin−1(0.2/2.3) of 5°. 1-bulk-a, 1-bulk-b, and 1-bulk-c behave
in a similar manner.
A remarkable difference is observed in the ac susceptibility
data. Figure 3 and Figures S9 and S10, Supporting Information,
give the temperature-dependent ac susceptibility measured at
The zero-field-cooled (ZFC) and field-cooled (FC, Hdc = 0.8
or 10 Oe) magnetic susceptibilities for 1-bulk-a show a
Figure 3. χ′ and χ″ versus T plots with eye-guided lines of 1-bulk-a
measured at zero dc field (Hac = 1.4 Oe).
C
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zero dc field (Hac = 1.4 Oe) and at frequencies of 1−1500 Hz.
In all cases, both the in-phase (χ′) and the out-of-phase (χ″)
signals show frequency dependence below 2.8 K, indicating a
slow relaxation of magnetization. If we compare the χ′′ signals
at 1 Hz, however, it is clear that the peak appearing at 2.0 K in
1-bulk-a vanishes in 1-bulk-b and 1-bulk-c (Figure S11,
Supporting Information), suggesting that the relaxation in 1-
́
bulk-a is much slower. A least-squares fit using the Arrhenius
relation τ(T) = τ0 exp(Δeff/kBT) based on peaks in the χ′′ vs T
plots results in energy barriers (Δeff/kB) and pre-exponential
factors (τ0) of 37.0/27.9/30.4 K and 1.52 × 10−9/1.38 × 10−8/
9.22 × 10−10 s, respectively, for the three samples. The τ0 values
fall in the range found for SCMs. The parameter f = (ΔTp/Tp)/
Δlog f is estimated as 0.10/0.14/0.16 for 1-bulk-a/1-bulk-b/1-
bulk-c, precluding the spin-glass behaviors.20
The sharp blocking of the moments at 2.8 K associated with
the onset of the imaginary ac susceptibility for all frequencies
studied indicates a long-range magnetic ordering (LRO) in
favor of SCM behavior. Oka et al. has recently shown that LRO
can be established for a diamond chain isolated at 17 Å without
any chemical connection between the chains.21 They argue for
the first time that the magnetic domain structure within the
chains of such a network will dominate the dynamics.21
Consequently, the dynamics are similar to that for SCMs, as the
extracted parameters above demonstrate, due to the very
anisotropic domain shape. The results are consistent with those
found for the mineral K2CoII (OH)2(SO4)3(H2O)2, where a
3
Figure 4. (a) χ′′ vs frequency, and (b) ln(τ/s) vs T−1 plots for samples
1-bulk-a, 1-bulk-b, and 1-bulk-c. Solid lines are a guide for the eye in a
and best fitting in b.
strong frequency dependence of the ac susceptibilities was
found within the LRO state determined by neutron
diffraction,22 which was associated with elongated domain.
The difference in dynamics could be clearly visualized in the
χ′, χ′′ vs frequency curves, measured between 1.8 and 2.6 K
under zero dc field (Hac = 1.4 Oe). χ′′ increases at the expense
of χ′ (Figures S12, Supporting Information, and 4a). The peaks
of χ′′ signals at the same temperature shift to higher
frequencies from 1-bulk-a to 1-bulk-c, revealing the gradual
decrease of relaxation time. The relaxation time can be
extracted by fitting the Cole−Cole plots23 using the generalized
Debye model. A least-squares fit based on the Arrhenius
relationship results in parameters Δeff/kB = 34.1/30.3/29.8 K
and τ0 = 1.78 × 10−9/ 4.83 × 10−9/1.42 × 10−9 s for 1-bulk-a,
1-bulk-b, and 1-bulk-c, respectively (Figure 4b). The
parameters are slightly different from those obtained from the
temperature-dependent ac susceptibility data. However, the
frequency dependence fit could be more reliable because the
whole curve is used for fitting instead of peak maximum only.
The distribution coefficient α values are in the range of 0.29−
0.41/0.28−0.40/0.23−0.36 for the three samples with decreas-
ing temperature (Figure S13, Tables S1−S3, Supporting
Information), indicating a relatively broad distribution of the
relaxation time. The large distribution of the relaxation time is
also demonstrated by the fact that χ′′ shows signals below 2.8
K, where the ZFC-FC curves also show bifurcation.
only difference is the Flack parameter. For 1-bulk-a, the Flack
parameter is close to zero for one crystal, while it is 0.49−0.76
for the other four crystals. For 1-bulk-b, the Flack parameter is
close to zero for three crystals but 0.79−0.80 for the other two
crystals. If structures with Flack parameters of 0 and 1 can be
viewed as a pair of polar isomers, both 1-bulk-a and 1-bulk-b
contain a mixture of two isomers. Therefore, observation of a
single relaxation process in each sample cannot be explained by
the presence of two polar isomers.
It is noted that samples 1-bulk-a, 1-bulk-b, and 1-bulk-c
were synthesized under similar experimental conditions except
−
for the inorganic salts. The NO3 anion is used in preparing
sample 1-bulk-a, while CH3COO− is employed in preparing
sample 1-bulk-c. For 1-bulk-b, both NO3− and CH3COO− are
involved in the reaction mixture. As demonstrated by Sessoli
and co-workers, the crystalline defects and chemical mod-
ifications can also be present in the undoped pure compound
on the order of a few per thousand spins.11a,24,25 Although the
anions are not involved in the structures of the final products,
they could disturb the crystallization process of compound 1,
generating different amounts of defects within the crystalline
samples. Unfortunately, direct observation or analysis of the
absolute amounts of the defects is difficult, especially for
molecular systems.25 However, defects do play a key role in the
magnetization relaxation of SCMs by dividing the chains into
short segments.11a,24
Observation of different magnetic dynamics in samples 1-
bulk-a, 1-bulk-b, and 1-bulk-c is unprecedented because they
are structurally and chemically the same. Considering that
compound 1 crystallizes in a polar space group Cc, we wonder
whether the direction of the polar axis could have any impact
on the magnetic behavior. Thus, five single crystals were each
randomly selected from 1-bulk-a and 1-bulk-b, respectively,
and subjected to structural determination. The results
demonstrate that all show the same structure with almost the
same cell parameters (Table S4, Supporting Information). The
It is well known that excitations nucleating close to a defect
site cost one-half the energy of those inside the chain in the
SCMs. At high temperature, the magnetic correlation length
(ξ) is shorter than the chain length (⟨L⟩) and the spin flip
occurs with Δ = 4J. At low temperature, ξ is much longer than
⟨L⟩; hence, Δ = 2J. The crossover temperature from the
D
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infinite-size to finite-size regime depends on J and ⟨L⟩ and
becomes higher for shorter chains. As expected from the
Glauber model in a 1D anisotropic system, a linear fit in the
ln(χ′T) vs T−1 plot in the temperature range 2.9−3.5 K gives
Δξ = 19.9/20.2/22.2 K for 1-bulk-a/1-bulk-b/1-bulk-c,
respectively, on the basis of the expression χT = Ceff exp(Δξ/
kBT), where Ceff is the effective Curie constant and Δξ is the
energy required to create a domain wall along the chain (Figure
S14, Supporting Information). Therefore, the exchange
coupling constant within the chain can be estimated as |J| =
10.0/10.1/11.1 K for the three samples, which are close to
those obtained from Fisher’s chain model (J = −8.99/−10.07/
−9.94 K). The energy barriers are expected to be ca. 40 and 20
K in the infinite-size and finite-size regions, respectively. The
energy barriers of the three samples obtained based on the
Arrhenius law (29.8−34.1 K) are between them, indicating the
coexistence of chains in the two regimes.
The frequency-dependent ac susceptibility data of the three
samples are measured at 1.8−2.2 K (Figures S20, Supporting
Information, and 5a). Slow magnetization relaxation is observed
The effective chain lengths could be related to the defects in
the three samples. As demonstrated by Bogani et al.,24 the
higher concentration of defects will contribute to higher
crossover temperature, i.e., higher deviation temperature from
the linear dependence of ln(χ′T) vs T−1 curve. The deviation
temperatures are 2.86/2.89/2.93 K for 1-bulk-a/1-bulk-b/1-
bulk-c (Figure S14, Supporting Information), suggesting the
increase of defects in the sequence of 1-bulk-a < 1-bulk-b < 1-
bulk-c. In addition, Figure 4b shows that the relaxation time
decreases in the sequence 1-bulk-a > 1-bulk-b > 1-bulk-c,
indicating that the effective chain length decreases in the same
sequence.24a Thus, a higher concentration of defects contrib-
utes to a shorter effective chain length in the three samples.
To check the effect of the other templates such as organic
molecules, samples 1-bulk-d and 1-bulk-e were obtained
following a similar synthetic procedure to sample 1-bulk-a
except that the pH was adjusted by 4,4′-bipyridine to 3.49 and
3.15, respectively. The SCM behavior is again observed for 1-
bulk-d and 1-bulk-e with energy barriers of 39.6/40.5 K and τ0
values of 1.30 × 10−10/2.67 × 10−10 s, respectively (Figures
S15−S17, Supporting Information). The barriers are close to
that expected for the SCM in the infinite region. The results
suggest that the length of chain segments in compound 1 may
be adjusted by the presence of suitable counteranions or
organic templates. The experiments are reproducible, although
the values of Δeff and τ0 can be slightly different.
With the above observations, we wonder whether it is
possible to observe two relaxation processes in a SCM system
simply by changing the synthetic condition. Hence, we examine
in a similar eye the properties of the structurally similar
compound Co(p-Me-C6H5CH2N(CH2PO3H)2)(H2O) (2) in
which the phosphonate ligand has an additional methyl group.5
This compound crystallizes in a centrosymmetric (P21/c)
instead of a polar space group and has been described to be a
SCM.5 For a comparison, three samples of 2 were hydro-
thermally synthesized at 140 °C using different cobalt salts and
pH adjusters, e.g., Co(NO3 )2 /p-Me-C6 H5CH2 N-
(CH2PO3H2)2/NaOH for 2-bulk-a, Co(CH3COO)2/p-Me-
C6H5CH2N(CH2PO3H2)2/HNO3 for 2-bulk-b, and Co-
(CH3COO)2/p-Me-C6H5CH2N(CH2PO3H2)2 for 2-bulk-c.
Although the synthetic conditions are different from that
reported in the literature where tetramethylammonium chloride
was involved and the reaction was carried out at 180 °C for 4
days,5b samples 2-bulk-a, 2-bulk-b, and 2-bulk-c are the same
as compound 2, as proved by powder XRD and IR
measurements (Figures S18 and S19, Supporting Information).
Figure 5. (a) χ″ vs frequency plots. (b)Logarithmic magnetization
relaxation time (τ) versus T−1 plots with Arrhenius fitting for samples
2-bulk-a, 2-bulk-b, and 2-bulk-c.
in all cases as expected. For samples 2-bulk-b/2-bulk-c, only
one relaxation process appears with energy barriers of 33.4/
39.3 K and pre-exponential factors of 2.87 × 10−10/7.01 ×
10−12 s. Interestingly, 2-bulk-a shows a double-relaxation
process with energy barriers of 38.2 and 22.4 K and τ0 of
2.66 × 10−10 and 2.93 × 10−9 s, respectively (Figures S21,
Supporting Information, and 5b). Noting that the energy
barrier of the high-frequency relaxation is almost one-half of
that of low-frequency relaxation in sample 2-bulk-a,13d,14 we
propose that the presence of two relaxation processes could be
attributed to the coexistence of both the infinite- and the finite-
size regimes in this sample.
CONCLUSIONS
■
We prepared five samples of a chain compound Co(bamdpH2)-
(H2O) (1) under slightly different synthetic conditions using
different cobalt salts or organic templates. Magnetic studies
reveal that in all cases the sharp imaginary ac susceptibility can
be observed at one fixed temperature for all frequencies,
suggesting the occurrence of LRO. In addition, dynamics are
present and similar to SCMs due to similar domain shapes. The
magnetization relaxation is significantly affected by the
synthetic methods and templates used, which has not been
documented before in the literature. Further, observation of
double-relaxation processes in sample 2-bulk-a suggests that
the origin of the dual relaxation in SCM systems could be due
to the presence of both the infinite- and the finite-size regimes.
E
dx.doi.org/10.1021/ic5021737 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(6) (a) Cler
́
ac, R.; Miyasaka, H.; Yamashita, M.; Coulon, C. J. Am.
This work highlights the importance of defects in the
understanding of magnetic dynamics of structures having
chains of magnetic moment carriers and also provides a new
route to control the chain length and hence the energy barrier
of SCMs.
Chem. Soc. 2002, 124, 12837−12844. (b) Bai, Y.-L.; Tao, J.;
Wernsdorfer, W.; Sato, O.; Huang, R.-B.; Zheng, L.-S. J. Am. Chem.
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Commun. 2014, 50, 2120−2122.
ASSOCIATED CONTENT
* Supporting Information
■
S
Structural data, IR, PXRD, TG, and additional magnetic data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(7) (a) Wang, S.; Zuo, J.-L.; Gao, S.; Song, Y.; Zhou, H.-C.; Zhou, Y.-
Z.; You, X.-Z. J. Am. Chem. Soc. 2004, 126, 8900−8901. (b) Kajiwara,
T.; Nakano, M.; Kaneko, Y.; Takaishi, S.; Ito, T.; Yamashita, M.;
Igashira-Kamiyama, A.; Nojiri, H.; Ono, Y.; Kojima, N. J. Am. Chem.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: lmzheng@nju.edu.cn.
■
́
Brefuel, N.; Duhayon, C.; Paulsen, C.; Barra, A.-L.; Ramasesha, S.;
Sutter, J.-P. J. Am. Chem. Soc. 2010, 132, 6047−6056. (d) Liu, T.;
Zheng, H.; Kang, S.; Shiota, Y.; Hayami, S.; Mito, M.; Sato, O.;
Yoshizawa, K.; Kanegawa, S.; Duan, C.-Y. Nat. Commun. 2013, 4,
2826−2832.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(8) (a) Bogani, L.; Sangregorio, C.; Sessoli, R.; Gatteschi, D. Angew.
Chem., Int. Ed. 2005, 44, 5817−5821. (b) Bernot, K.; Bogani, L.;
Caneschi, A.; Gatteschi, D.; Sessoli, R. J. Am. Chem. Soc. 2006, 128,
7947−7956.
Financial support by the National Basic Research Program of
China (2010CB923402, 2013CB922102), the NSF of Jiangsu
Province of China (No. BK2009009), and the NSF of China
(No. 21371094) is acknowledged. We appreciate Prof. M.
Kurmoo for kind proof-reading and comments and Prof. E.
Coronado for valuable discussions. We also thank Prof. Li Pi for
kind help in magnetic measurements.
́
(9) (a) Harris, T. D.; Bennett, M. V.; Clerac, R.; Long, J. R. J. Am.
Chem. Soc. 2010, 132, 3980−3988. (b) Feng, X.; Liu, J.; Harris, T. D.;
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(10) Mougel, V.; Chatelain, L.; Hermle, J.; Caciuffo, R.; Colineau, E.;
Tuna, F.; Magnani, N.; de Geyer, A.; Pec
Chem., Int. Ed. 2014, 53, 819−823.
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