Smooth transition between SMM and SCM-type slow relaxing dynamics for a 1-D assemblage of {Dy(nitronyl nitroxide)2} units

Article abstract of DOI:10.1039/b918554b

A model example for size effects on the dynamic susceptibility behavior is provided by the chain compound [{Dy(hfac)₃NitPhIm₂} Dy(hfac)₃] (NitPhIm = 2-[4-(1-imidazole)phenyl]nitronyl nitroxide radical). The Arrhenius plot reveals two relaxation regimes attributed to SMM (Δ = 17.1 K and τ₀ = 17.5 × 10^-6 s) and SCM (Δ = 82.7 K and τ₀ = 8.8 × 10^-8 s) behaviors. The ferromagnetic exchange among the spin carriers has been established for the corresponding Gd derivative.

Full text of DOI:10.1039/b918554b

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Smooth transition between SMM and SCM-type slow relaxing dynamics
for a 1-D assemblage of {Dy(nitronyl nitroxide)2} unitsw
Ruina Liu,a Licun Li,*a Xiaoling Wang,a Peipei Yang,a Chao Wang,a Daizheng Liaoa and
Jean-Pascal Sutter*bc
Received (in Cambridge, UK) 8th September 2009, Accepted 16th February 2010
First published as an Advance Article on the web 12th March 2010
DOI: 10.1039/b918554b
A
model example for size effects on the dynamic
operative between the Ln ion and the radical ligands, and
between the {Ln(Nit)2} units and the further Ln ions bridging
the units. The single-crystal X-ray structures and magnetic
behaviors for the two 1-D compounds are described.
susceptibility behavior is provided by the chain compound
[{Dy(hfac)3NitPhIm2}Dy(hfac)3] (NitPhIm = 2-[4-(1-imidazole)-
phenyl]nitronyl nitroxide radical). The Arrhenius plot reveals
two relaxation regimes attributed to SMM (D = 17.1 K and
s0 = 17.5 ꢀ 10ꢁ6 s) and SCM (D = 82.7 K and s0 = 8.8 ꢀ 10ꢁ8 s)
behaviors. The ferromagnetic exchange among the spin carriers
has been established for the corresponding Gd derivative.
Reaction of 2-[4-(1-imidazole)phenyl]-4,4,5,5-tetramethyl-
imidazoline-1-oxyl-3-oxide (NitPhIm) radical with Ln(hfac)3
(hfac = hexafluoroacetyl acetonate) yielded deep blue plate-
like crystals of [{Ln(hfac)3}2(NITPhIm)2] 1 (Gd) and 2 (Dy).
Single-crystal X-ray diffraction analyses reveal that
complexes 1 and 2 are isomorphous (see ESIw). Therefore,
only the structure of complex 1 will be briefly described. As
shown in Fig. 1, there are two independent Gd atoms in the
asymmetric unit. Their coordination spheres comprise two
oxygen atoms from two nitroxide groups for Gd(1) and two
nitrogen atoms from two imidazole rings for Gd(2), and are
completed to eight by the oxygen atoms of three didentate hfac
ligands. The bond lengths are comparable to those of the
reported Gd complexes with nitronyl nitroxides.13–15 Each
NitPhIm acts as bridging ligand and is coordinated to two
different Gd atoms through the oxygen atom of the nitronyl
nitroxide group and the nitrogen atom of the imidazole ring to
develop an one-dimensional chain having a ‘head-to-head’
motif. The Gd(1)ꢂ ꢂ ꢂGd(2) distance is 10.032 A and the shortest
interchain Gdꢂ ꢂ ꢂGd separation is 10.199 A.
Low dimensional molecule-based magnetic systems attract a
renewed interest since it has been shown that they may exhibit
slow dynamics for the relaxation of their magnetization.1,2
Such materials known as single molecule magnets (SMMs)
and single chain magnets (SCMs) not only permitted the
observation of fascinating quantum phenomena3 but they
are also investigated as potential candidates for future high-
density data storage materials.4 The 1-D systems are especially
promising because of their virtual infinite spin ground state
and intrinsic structural anisotropy.5,6 Among the various
chemical routes investigated to obtain and improve the
characteristics of those compounds, a strategy involving
metal ions with organic radicals as ligands has proven very
successful.7,8 For instance with anisotropic Ln ions a family of
SCMs with different relaxation features could be obtained.9
For this series but in general for SCMs, the one-dimensional
systems are not perfect and the actual material comprises spin
chains of various lengths. As a result the signature for finite
size effects is often detected by a crossover between two
regimes in the dynamics of the relaxation of the magnetization
in the lower temperature domain.10–12 Here we report on a
compound formed by {Dy(Nit)2} units (Nit stands for nitronyl
nitroxide radical) assembled in a chain by the means of an
additional Ln for which the AC magnetic susceptibility
revealed two limit regimes in accordance with a SMM and a
SCM, respectively. The transition between the two regimes
is smooth with AC susceptibility features characteristic for
a size distribution of the spin system. The homologous Gd
compound shows that ferromagnetic interactions are
The variable-temperature magnetic susceptibilities for 1 and 2
were measured from 300 to 2.0 K in an applied field of 1 kOe.
The powder from crushed crystals of 2 was dispersed in grease
to avoid orientation in the field. The wMT vs. T plots are shown
in Fig. 2. At 300 K, the wMT value for 1 is 16.64 cm3 molꢁ1 K,
which is consistent with the expected value of 16.51 cm3 molꢁ1 K
for the isolated spins two SGd1 = SGd2 = 7/2 and two
Srad1 = Srad2 = 1/2. With lowering the temperature, the
wMT value increases more and more rapidly to reach
23.92 cm3 molꢁ1 K at 2.0 K. Such a behavior is characteristic
a Department of Chemistry, Nankai University, Tianjin 300071, China.
E-mail: li_licun@hotmail.com
b CNRS, LCC (Laboratoire de Chimie de Coordination), 205,
route de Narbonne, F-31077 Toulouse, France
c
´
Universite de Toulouse, UPS, INPT, LCC, F-31077 Toulouse,
France. E-mail: sutter@lcc-toulouse.fr
w Electronic supplementary information (ESI) available: Preparation,
crystal data, and magnetic properties for 1 and 2 (pdf document).
CCDC 728975 (1) and 728977 (2). For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/b918554b
Fig. 1 Molecular structure of [{Gd(hfac)3}2(NITPhIm)2] 1. Fluorine
atoms are omitted for clarity.
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Below 4 K a drastic modification of the curvature can be
noticed, suggesting an upturn for lower T. The field dependence
of the magnetization recorded at 2 K (Fig. S9, ESIw) exhibits
fast increase of the magnetization for low fields followed be a
steady increase to become 12 mB at 50 kOe without reaching
saturation.
The temperature dependence of the ac magnetic susceptibility
for 2 at different frequencies (Hac = 3 Oe) has been investi-
gated in the absence and with an applied static field. In the
absence of field, a signal for the imaginary component of the
susceptibility, wM0 0, is found for temperatures below 15 K and
the signal is frequency dependent (Fig. S10, ESIw). However,
the curve diverges from the characteristic Gaussian-shape;
signal is weak and steadily increases for lower temperatures,
no maximum is observed down to 2.0 K. Such a behavior
suggests occurrence of relaxation mechanisms such as
quantum tunneling of the magnetization.22 It can be seen from
Fig. 3 (left) that the dynamic AC response is notably changed
wh0e0n recorded with a static field of 3 kOe. The signal for
wM deviates from zero below 16 K and well-shape peak-like
Fig. 2 wMT vs. T plot for 1 (circles) and 2 (squares). The solid line
represents the calculated behavior for 1 (see text).
of a system with ferromagnetic interactions. Moreover, this
latter value is larger than the 20.25 cm3 molꢁ1 K expected for a
S = 9/2 (the {Gd(Nit)2}) and a Gd ion with no interaction,
thus ferromagnetic interactions also take place between these
spin units along the chain. Based on above structural analysis,
three main exchange pathways should be operative: (i) the
magnetic interaction between Gd(1) ion and the directly
coordinated nitroxide group (J1); (ii) the magnetic coupling
between the two coordinated nitroxide groups through Gd(1)
(J2); (iii) the magnetic coupling between Gd(2) and nitroxide
group through phenyl and imidazole rings. The latter is
anticipated to be weak.16 Hence, the magnetic behavior of 1
can be treated as that of one Rad–Gd–Rad magnetic unit plus
an uncoupled Gd(III) ion, there weak exchange interaction
being considered within the mean field approximation (zJ0).17
The magnetic data were analyzed by a theoretical expression
curves are obtained. Both the in-phase and out-of-phase
00
susceptibilities are strongly frequency-dependent. For wM
the low and high frequency signals are rather sharp but for
,
intermediate frequencies the curves become broader. The
00
occurrence of two distinct peaks for wM is evident for
0
frequencies between 200 and 600 Hz. For wM two maxima
centered at ca. 3 and 13 K are clearly visible. The Arrhenius
plot obtained from these data is given in Fig. 3 (right).
Obviously two relaxation regimes take place with a smooth
transition between them. Analysis of the linear parts of the
curve on the basis of the Arrhenius law for a thermally
activated mechanism, t = t0 exp(D/kBT) where t is the
relaxation time and D the effective energy barrier for reversal
of the magnetization, yielded for the low-temperature
(see ESIw) deduced from the spin Hamiltonian
ꢁJ1(SRad1SGd1 + SGd1SRad2) ꢁ J2Srad1Srad2. Best fit to the
experimental data yielded g = 2.0, J1 = 3.53 cmꢁ1, J2
H =
=
ꢁ8.93 cmꢁ1, zJ0 = 0.017 cmꢁ1. These results indicate that the
Gd–Nit interactions are ferromagnetic whereas interaction
between radicals through the Gd atom is antiferromagnetic,
as already observed in such compounds.13,14 Not unexpectedly,
the ferromagnetic interaction between the nitroxide and the
Gd atom through phenyl and imidazole rings is quite weak due
to poor spin delocalization.16,18 The M vs. H curve at 2.0 K is
shown in Fig. S6 (ESIw). A magnetization of 15.90 mB is
reached at 5 T, in agreement with 16 mB expected for the
ferromagnetic arrangement of the spins. For the lower fields
the magnetization for 1 is above the magnetization calculated
with the Brillouin function for non-coupled S = 9/2 and
S = 7/2 spin centers (g = 2.0, T = 2 K); this is a further
confirmation of the ferromagnetic interactions taking place
between the LnRad2 units and the bridging Ln ion. AC
magnetic susceptibility measurements show that no magnetic
ordering occurs above 2 K for 1 (Fig. S8).
For complex 2, the wMT value at room temperature is
28.5 cm3 molꢁ1 K, close to expected value of 29.1 cm3 molꢁ1
K
Fig.
3 (Left) Temperature dependence of the out-of-phase ac
for two uncorrelated Dy(III) (a 6H15/2 ion) and two uncorrelated
S = 1/2 spins. The wMT value is almost constant until about
130 K where it decreases to reach 23.20 cm3 molꢁ1 K at 2.0 K.
This decrease can be ascribed to the crystal field effect for the
Dy ion leading to a deviation from a Curie behavior.19–21
susceptibilities measured in H = 3 kOe dc field for 2. (Right)
Arrhenius plot (circles), the lines show the linear fit to the data, in
blue low T/frequency domain, in red high T/frequency domain. TB
00
corresponds to the temperature of the maximum of wM for a given
frequency.
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(frequency) domain D = 17.1 K and t0 = 17.5 ꢀ 10ꢁ6 s,
We are grateful for financial support from the National
and for the high T (frequency) part D = 82.7 K and t0
8.8 ꢀ 10ꢁ8 s.
=
Natural Science Foundation of China (Nos. 50672037,
20971072, 90922032), the NSF of Tianjin (No.
09JCYBJC05600), and the French Research Agency (Agence
Nationale de la Recherche, reference ANR-09-BLAN-0054-01).
The AC data for compound 2 can be seen as a snapshot
revealing the variety of spin-arrangements existing for this
well-defined chemical system in the considered temperature
domain. The signal in the low-temperature domain (2–4 K) is
attributed to a SMM, typically the {DyNit2} units, whereas
signature for a SCM is found above 10 K. The hypothesis
that these unusual AC features result from two independent
anisotropic centers,23 i.e. the {DyNit2} unit and the bridging
Dy ion, can be discarded because of the ferromagnetic inter-
actions existing between the Ln centers as shown for 1.
Moreover, the two energy barriers (D) obtained for 2 well
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00
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2568 | Chem. Commun., 2010, 46, 2566–2568