Single molecule magnet behavior observed in a 1-D dysprosium chain with quasi-D5h symmetry

Article abstract of DOI:10.1039/c5dt04147c

Two one-dimensional (1-D) chain complexes with pentagonal bipyramidal Dy^III centers have been synthesized and magnetically characterized. Field-induced single molecule magnet behavior has been revealed in both compounds, which is still rarely reported in a lanthanide compound with a pentagonal bipyramidal coordination geometry. Their crystal field parameters and orientations of the magnetic easy axes were obtained from the simulation of the magnetic data and the electrostatic model calculation.

Full text of DOI:10.1039/c5dt04147c

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Single molecule magnet behavior observed in a
1
-D dysprosium chain with quasi-D symmetry†
5
h
Cite this: DOI: 10.1039/c5dt04147c
Received 22nd October 2015,
Accepted 11th November 2015
a,b
a
a
b
b
Xing-Cai Huang, Ming Zhang, Dayu Wu,* Dong Shao, Xin-Hua Zhao,
a
b
Wei Huang and Xin-Yi Wang*
DOI: 10.1039/c5dt04147c
www.rsc.org/dalton
1
3−
7
Two one-dimensional (1-D) chain complexes with pentagonal Er) and [Ln(SiW11
O
39
)
2
]
(Ln = Dy, Ho, Er, Yb), lanthanide
III
8
−
9
bipyramidal Dy centers have been synthesized and magnetically β-diketone, and [Ln(DOTA)(H
2
O)] (Ln = Dy, Er, Yb) systems.
characterized. Field-induced single molecule magnet behavior has Organometallic double-decker lanthanide SMMs, (Cp*)Er-
2
−
−
been revealed in both compounds, which is still rarely reported in (COT), Ln(COT) (Ln = Er, Dy), and [Ln(COT″)
a lanthanide compound with a pentagonal bipyramidal coordi- Ce), have high rank rotation axes (C , S , C axis). Recently,
nation geometry. Their crystal field parameters and orientations of strong magnetic anisotropy and SMM behaviour have been
the magnetic easy axes were obtained from the simulation of the reported in Ln[N(SiMe (Ln = Er, Dy) and their related com-
2
] (Ln = Er, Dy,
10
∞
8
8
3 2 3
) ]
1
1
magnetic data and the electrostatic model calculation.
pounds with a C3v symmetry.
In this regard, lanthanide complexes with pentagonal bi-
Since the first discovery of the single-molecule magnet (SMM)
III
pyramidal geometry (D5h), especially for the Kramers Dy
+
−
behaviour of the sandwich complex [N(C
4
H
9
)
4
] [LnPc
2
] (Ln =
6
III 4
(
H15±2) and Er ( I15±2) ions, are rarely reported to behave as
1
Tb, Dy) in 2003, mononuclear SMMs, commonly termed as
single-ion magnets (SIMs), have been intensively studied
because of their potential applications in molecular spintro-
12
13
14
15
16
SMMs, such as Dy, Er, Dy2, Zn Dy and Fe Dy systems.
2
2
In these systems, the pentagonal geometry of a five-fold axis
played a predominant role in the suppression of the quantum
2
3
nics and quantum-computing devices. These mononuclear
systems provide excellent models for understanding the
−1
tunnelling and resulted in the high energy barrier of 305 cm
(
439 K) for the ZnDyZn compound compared to similar com-
4
magnetic anisotropy, i.e., especially single-ion anisotropy. In
15
pounds of octahedral geometry. Also, an energy barrier of
lanthanide mononuclear SMMs, single-ion magnetic aniso-
tropy depends on the competition between electrostatic
−1
3
19 cm (459 K) is observed for the FeDyFe compound, which
16
is the highest anisotropy barrier for 3d–4f SMMs. Previously,
5
crystal-field interaction and spin–orbit coupling. The large
we adopted the pentadentate ligand to successfully design the
unquenched orbital moment of lanthanides gives rise to their
large magnetic anisotropy, making lanthanides good candi-
dates for the construction of SMMs. To suppress the quantum
tunnelling of magnetization frequently observed in the lantha-
nide SMMs, a ligand field of axial symmetries, such as D ,
17
3
d SMMs with quasi-D5h geometry. As these ligands can
occupy the five equatorial coordination sites, a pentagonal
bipyramidal geometry can be achieved if the axial positions
are occupied by only two ligands. As an excellent pentadentate
4
d
ligand, H
2
valdien (N1, N3-bis(3-methoxysalicylidene)diethyl-
D
5h, D∞h, S
8
4
(I ), D6d and so on, is usually preferred. For
enetriamine) was widely used; and a series of SMMs based on
example, the following SMMs contain lanthanides of approxi-
18
19
this ligand were obtained, including the Dy ,
Dy ,
12
2
±
±0 6
9−
mate D4d local symmetry: [Pc′ Ln] , [Ln(W O ) ] (Ln = Ho,
20
21
2
5 18 2
Co
2
Dy
2
,
4 2
and Ni Ln (Ln = Tb, Dy and Ho) compounds. But
III
in these complexes, Dy centres mostly adopted an eight-
coordinate ligand environment. To lower the coordination
number, bulky anions with large steric effects are available for
constructing seven-coordinate complexes to obtain the penta-
gonal bipyramidal geometry (D5h).
a
Jiangsu 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
b
State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of
Based on the above consideration, we successfully syn-
III
Advanced Microstructures, School of Chemistry and Chemical Engineering, Nanjing
University, Nanjing, 210093, China. E-mail: wangxy66@nju.edu.cn
thesized a series of one-dimensional Ln coordination poly-
mers (experimental details in the ESI†) by combining the
†
Electronic supplementary information (ESI) available: Experimental details,
X-ray crystallography data, XRD, additional magnetic measurement data. CCDC
415417, 1415418, 1432155–1432157 and 1432220. For ESI and crystallographic
pentadentate ligand valdien and a bulky anion diphenyl phos-
−
phate ([(PhO) PO ] ) (Scheme 1). Herein, we report the synth-
1
2
2
data in CIF or other electronic format see DOI: 10.1039±c5dt04147c
eses, crystal structures and magnetic properties of two 1D Dy
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environment. The three nitrogen atoms and two oxygen atoms
from the valdien ligands occupy the equatorial position of the
III
Dy center, while the axial positions are occupied by one
−
oxygen atom from the [(PhO)
2
PO
2
)] ligand and one chloride
−
anion in 1 and two oxygen atoms from two [(PhO) PO )]
2
2
ligands in 2 (Fig. 1c and d). As the pentagonal bipyramidal
geometry is distorted, the characteristic bond angles of the
]− as coordination geometry deviate from the ideal values for an
ideal D5h symmetry (Table S3, ESI†). The axial bond angles
2 2 2
Scheme 1 The ligand H valdien and the bulky anion [(PhO) PO
the coligand.
(
O5–Dy1–Cl1 and O5–Dy1–O6) are 162.87(7)° and 163.10(1)°
2
2
for 1 and 2, respectively. Using the program SHAPE 2.1, the
III
compounds [DyNa(valdien)((PhO) PO )Cl] (1) and [Dy(valdien)- continuous shape measures (CShM’s) of the Dy centres rela-
(PhO) PO )] (2) in detail. As expected, the Dy center adopts tive to the ideal pentagonal bipyramid were calculated to be
2 2 n
2
2
n
III
(
pentagonal bipyramidal geometry. Field-induced slow mag- 0.775 and 0.496 for 1 and 2, respectively (Table S4, ESI†). Sep-
III
netic relaxation, characteristic of SMM behavior, was observed arated by the anions and the bulky ligands, the Dy centres
in both compounds.
are isolated from each other: the shortest intra- and interchain
III
III
Yellow block single crystals of compounds 1 and 2 were pre- Dy ⋯Dy distances are 8.3733(6) and 8.9941(5) Å for 1, and
pared by liquid diffusion of isopropyl ether into the mixture 6.0538(8) 11.0268(5) Å for 2, respectively (Fig. S2, ESI†), which
III
III
solution containing
PhO) PO H±NaOH or (PhO)
ing isostructural complexes 3–6 in the ESI†). Single-crystal
H
2
valdien, Et
PO H±Et N (see the details includ- chain lanthanide SMMs.
Direct-current (dc) magnetic susceptibilities of both 1 and 2
3
N, DyCl
3
·6H
2
O
and fall in the range of the Dy ⋯Dy distances in the reported 1D
2
3
(
2
2
2
2
3
X-ray diffraction analyses revealed that compound 1 crystallizes were determined on polycrystalline samples in the temperature
in the orthorhombic space group Pbca and compound 2 in the range of 2.0–300 K under 1000 Oe (Fig. 2a and b). The χ T
M
3
−1
monoclinic space group P2 ±c (Table S1, ESI†). Although both values at 300 K for 1 and 2 are 13.87 and 14.53 cm mol K,
1
3
compounds are one-dimensional coordination polymers, the respectively, which are close to the expected values (14.17 cm
chains are different depending on the deprotonated bases mol K) for an isolated Dy ion (S = 5±2, L = 5, J = 15±2, H15±2,
−
1
III
6
used in the reaction: the chains in 1 are formed by the [DyNa- g = 4±3). Upon cooling, the χ T values for 1 and 2 gradually
M
−
−
3
−1
(
2 2
valdien)((PhO) PO )] units bridged by the Cl anions, while decrease to 8.67 and 8.92 cm mol K at 2.0 K. This behaviour
the chains in 2 are formed by the [Dy(valdien)] units bridged is mainly due to the thermal depopulation of the Dy Stark
by the O–P–O group of diphenyl phosphate (Fig. 1a and b). sublevels. The magnetization of both 1 and 2 at 2 K increases
+
III
Every valdien ligand in 1 uses both its coordination pockets up to 5.26 and 5.56μ at 70 kOe (Fig. 2a and b, inset), respecti-
B
III
I
(
N O and O pockets) to embrace both the Dy and Na ions, vely, which is clearly lower than the expected saturation value
respectively; while for 2, the O pocket of the valdien ligand is for one uncorrelated Dy centre (10μ ), indicating the pres-
empty. Interestingly, both Dy centres in compounds 1 and 2 ence of magnetic anisotropy and±or low-lying excited states.
3 2 4
III
4
III
B
III
are seven-coordinate with a distorted pentagonal bipyramidal
To understand the magnetic anisotropy of the Dy ion for
both 1 and 2, their magnetic susceptibility and magnetization
data (Fig. 2a and b) were analysed simultaneously with the
crystal field theory using the PHI program. The Crystal-Field
Hamiltonian is expressed in the equation as follows:
24
N
k
X X X
q
ki
q
ki
k
ˆ
ˆ
HCF ¼
σ B θkO
ð1Þ
i
i¼1 k¼2;4;6 q¼ꢀk
Fig. 1 The 1D chains of 1 (a) and 2 (b), and the coordination polyhedron
around Dy centres with pentagonal bipyramidal geometry (D5h) for
Fig. 2
M
χ T vs. T plots for 1 (a) and 2 (b) at 1000 Oe from 2–300 K and
III
M vs. H plots for 1 and 2 at 2 K (inset), the red solid lines are fitted and
1
(c) and 2 (d).
simulated by the PHI program.
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k
i
q
k
where σ
are the orbital reduction parameters, B
are the shapes in the high frequency region and irregular curves in
crystal field parameters (CFPs) in Steven’s notation, θ are the the low frequency region, respectively (Fig. S7, ESI†). According
k
q
operator equivalent factors, and Oˆ are the operator equi- to the generalized Debye model for the high frequency
ki
2
7
valents. Considering the slightly distorted pentagonal bipyrami- region, the Cole–Cole plots at 1.8–4.0 K of 1 and at 1.8–5.0 K
dal geometry of D symmetry, only the parameters (B , B and of 2 (Fig. S7, ESI†) can be fitted well, with α values in the range
) are adopted to avoid overparametrization.
0
0
5
h
2
4
0
6
15,25
0
2
B
B
The CFPs (B
,
of 0.10–0.22 for 1 and 0.07–0.14 for 2 (Tables S7 and S8, ESI†),
and g ) were respectively, indicating a relatively narrow distribution of the
0
0
6
4
and B
) and the anisotropy g-factors (g
x
, g
y
z
employed to fit the magnetic susceptibility and magnetization relaxation processes. To estimate the anisotropy energy barrier
data. As can be seen in Fig. 2, both the susceptibility and the (Ueff), the high temperature data (Fig. 4) were fitted according
−
1
−1
magnetization data in the whole temperature range and up to to the Arrhenius law τ = τ
0
B
exp(−Ueff±k T). The effective
−
1
the highest magnetic field can be fitted well with one set of energy barrier U was estimated to be 14.23 cm (20.48 K)
eff
−
1
0
parameters (Table S5, ESI†). With the obtained fitting para- and 13.70 cm (19.70 K) for 1 and 2, respectively, with τ =
meters (CFPs and g-factors), the magnetic data of 1 and 2 are 2.56 × 10− s for 1 and τ
7
= 6.12 × 10 s for 2. It is noteworthy
−7
0
simulated to obtain the energy-levels, the eigenstates and that the plots of ln(τ) versus 1±T exhibit multiple relaxation pro-
g-tensors. From the simulation of the ground state multiplet cesses under 1000 Oe field. In general, n = 9 for Kramers ions,
6
H
15±2 (energy-levels are listed in Table S6, ESI†), the energy smaller n values in the range of 1–6 can be considered as
2
8
gap between the ground state Kramers doublets (m = ± 13±2) reasonable for an optical acoustic Raman process. Consider-
J
= ± 11±2) are 41.63 cm− for 1 ing the spin-lattice relaxation of both Raman and Orbach pro-
1
and the first excited states (m
J
and 45.65 cm− for 2, respectively. The local g-tensors on the cesses at low temperatures, we could fit all the magnetic data
1
29
−
1
n
−1
ground doublets of the Dy sites show large g values, g = 15.46 with eqn (1) τ = CT + τ
for 1 and g
the easy axial magnetic anisotropy of both compounds.
exp(−U ±k T), giving C = 100.9
z
z
0
eff
B
−
1
−3.5
, n = 3.5, Ueff = 49.94 cm−1, τ
−8
z
= 17.44 for 2 (ideally g
x
= g
y
= 0, g
z
= 20), indicating
s
K
0
= 1.50 × 10 s for 1 and
2
3c,26
−1
−2.0, n = 2.0, Ueff = 53.25 cm , τ
−1
−8
C = 505.0 s
K
0
= 3.03 × 10
s
To probe the slow magnetic relaxation, the temperature and for 2 (Fig. 4), which are close to the obtained energy gap from
frequency-dependent ac susceptibilities were determined in
the temperature range of 1.8–10 K for 1 and 2. Under a zero dc
field, no out-of-phase ac susceptibility (χ″) signal was observed
above 1.8 K for both compounds, probably due to the fast
QTM (Fig. S3 and S4, ESI†). However, to suppress the QTM
effect, the 1000 Oe dc field was applied and both the in-phase
(
χ′) and out-of-phase (χ″) for 1 and 2 showed obvious tempera-
ture and frequency dependence (Fig. 3, S5 and S6, ESI†), indi-
cating that the slow magnetic relaxation originated from the
III
single-ion magnetic behaviour of the Dy ion, as observed for
III
23
other 1D or 2D Dy compounds with SMM behavior.
The relaxation time (τ) can be obtained from the frequency-
Fig. 4 The plots of ln(τ) versus 1/T at the 1000 Oe dc field for 1 (a) and
dependent ac susceptibility data (χ′ and χ″), and the corres- 2 (b). The blue and red solid lines represent the fitting by the Arrhenius
ponding Cole–Cole plots exhibit asymmetrically semi-circular law for the high temperature region and for all the data, respectively.
Fig. 3 Frequency dependence of the in-phase (χ’) and out-of-phase (χ’’) ac susceptibility data under the 1000 Oe dc field range from 1.8 to 4.0 K
for 1 (a) and the range from 1.8 to 5.0 K for 2 (b). The solid lines are a guide for the eye.
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2
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8
9
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III
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III
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31
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III
2
Dy coordination polymers display single molecule magnet
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We acknowledge the financial support by the Priority Aca-
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cation Institutions. This experimental work is financially funded
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