An unsymmetrical coordination environment leading to two slow relaxation modes in a Dy2 single-molecule magnet

Article abstract of DOI:10.1039/c1cc14223b

A Dy₂ single-molecule magnet was isolated using a mixed ligand strategy in which the Dy^III ions adopt distinct coordination environments. This leads to two unique relaxation modes due to a single-ion type relaxation mechanism. Energy

Full text of DOI:10.1039/c1cc14223b

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 10993–10995
www.rsc.org/chemcomm
COMMUNICATION
An unsymmetrical coordination environment leading to two slow
relaxation modes in a Dy2 single-molecule magnetw
Po-Heng Lin,a Wen-Bin Sun,*b Mang-Fei Yu,b Guang-Ming Li,b Peng-Fei Yan*b and
Muralee Murugesu*a
Received 13th July 2011, Accepted 24th August 2011
DOI: 10.1039/c1cc14223b
A Dy2 single-molecule magnet was isolated using a mixed ligand
strategy in which the DyIII ions adopt distinct coordination
environments. This leads to two unique relaxation modes due
to a single-ion type relaxation mechanism. Energy barriers were
obtained from individually fitting the overlapping peaks.
unsymmetrical coordination environments leading to two slow
relaxation modes.
Our synthetic strategy relies on the use of polydentate
salen-type ligands (Fig. S1, ESIw) in order to encapsulate
lanthanide ions.1b,e,5 Furthermore, the phenol group provides
an efficient superexchange pathway between metal centres. In
addition, to promote encapsulation/stabilization of oxophilic
lanthanide ions, an ancillary b-diketonate ligand was employed
as a co-ligand. Therefore, DyIII(acac)3ꢀH2O was used as the metal
precursor in our reaction mixtures. A slurry of salen-type H2L1
(Fig. S1, ESIw) with the metal precursor in CH2Cl2 followed
by Et2O diffusion leads to the formation of yellow crystals of
[Dy2(L1)2(acac)2(H2O)]ꢀ2CH2Cl2 (1) in 61% yield in 7 days.
Complex 1 crystallizes in the monoclinic P21/n space group.
The structure is composed of two DyIII ions located in the
N2O2 coordination pockets of two L12ꢁ and two acacꢁ ligands
(Fig. 1). A unique structural feature of this complex is the
coordination environment forming the coordination poly-
hedra; the Dy1 atom is octacoordinate whereas the Dy2 ion
is heptacoordinate.
Single-Molecule Magnets (SMMs) continue to fascinate scientific
communities thanks to their unique intrinsic magnetic properties.1
More specifically, the discovery of single-ion lanthanide complexes
exhibiting slow relaxation of the magnetisation boosted the
research in isolating high-energy barrier SMMs.2 Lanthanide
SMMs continue to flourish in this regard and new high
relaxation barriers are being reached.1a,3 Recently, Evans, Long
and co-workers were able to promote strong communication
between DyIII ions via a dinitrogen radical which yielded
hysteresis loops opening at record temperatures.1a Although
this result is an outstanding breakthrough in this field, promoting
interactions between lanthanide ions continues to be a difficult
challenge owing to poor orbital overlap between the 4f core
orbitals of lanthanide ions. In fact, the observed slow relaxation
in polynuclear lanthanide SMMs are generally due to single-ion
relaxation mechanisms.3 Therefore, understanding the relaxation
mechanism in polynuclear lanthanide SMMs is key. We have
recently reported a centrosymmetric dinuclear Dy2 complex
where a doping method was employed in order to elucidate the
influence of the neighbouring ion in a single-ion relaxation
mechanism.4 However, coordinatively unsymmetrical dinuclear
SMMs are rare and we believe they represent an alternative
way to study single-ion relaxation mechanisms in multinuclear
systems with several relaxation pathways. Herein, we report
a unique study of a dinuclear lanthanide complex with
The Dy1 ion adopts a distorted square antiprism geometry
whereas a distorted capped trigonal prism is found for the Dy2
ion. In both cases the observed distortions arise from idealised
geometry due to ligand bite angles. In the case of Dy1, one
slightly bent tetradentate L12ꢁ ligand (N1, N2, O1, O2) forms
one of the distorted square antiprism basal planes while two
acacꢁ ligands form the other. Similarly, the Dy2 ion lies
almost in the central distorted square plane formed by O1,
O2, O3, O4 atoms, two nitrogen atoms (N3, N4) above and
a Department of Chemistry and Centre for Catalysis Research and
Innovation, University of Ottawa, 10 Marie-Curie, Ottawa, ON,
Canada K1N 6N5. E-mail: m.murugesu@uottawa.ca;
Tel: +1 613 562 5800-2733
b Key Laboratory of Functional Inorganic Material Chemistry,
Heilongjiang University, Ministry of Education, No. 74, Xuefu Road,
Nangang District, Harbin 150080, P.R. China.
E-mail: wenbinsun@yahoo.cn, yanpf@vip.sina.com;
Tel: +86-451-86608042
w Electronic supplementary information (ESI) available: Detailed
synthetic procedure, additional magnetic measurements. CCDC
830604, 837060. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1cc14223b
Fig. 1 Left: Molecular structure of [Dy2(L1)2(acac)2(H2O)]. Right:
Polyhedral representation of the dinuclear core emphasising the distorted
square antiprism (blue) and capped trigonal prism (orange). Colour code:
Dy (orange), O (red), N (blue). H atoms were omitted for clarity.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 10993–10995 10993

View Article Online
one coordinated water molecule (O9) below the plane leading
to the distorted capped trigonal prism geometry. An intra-
molecular Dy1ꢀ ꢀ ꢀDy2 distance of 3.84 A and the average
Dy–O–Dy angle of 110.81 are similar to those recently reported
for Dy2 complexes.1b,e Careful inspection of the packing
arrangement reveals the closest intermolecular Dy2ꢀ ꢀ ꢀDy2
distance of 5.90 A. Both polyhedra share one edge formed
by phenoxide oxygen atoms (O2, O1). The latter bridging
oxygens provide a weak magnetic superexchange pathway
between the lanthanide ions.
the magnetisation indicate the presence of significant magnetic
anisotropy and/or low-lying excited states. It is further confirmed
by the M vs. H/T plot where the isofield lines do not super-
impose on a single master-curve as expected for an isotropic
system with a well-defined ground state. It is worth mentioning
that the M vs. H data do not exhibit hysteresis loops above
2.5 K with sweep rates used in the SQUID magnetometer.
However, ac susceptibility measurements were carried out in
order to probe potential slow relaxation of the magnetisation
indicative of SMM behaviour.
In order to probe the magnetic behaviour of 1, variable
temperature direct and alternating current (dc and ac) suscepti-
bility measurements were carried out on a freshly filtered poly-
crystalline sample. At room temperature (Fig. 2), the observed
wT value of 28.26 cm3 K molꢁ1 is in excellent agreement with
the theoretical value of 28.34 cm3 K molꢁ1 for two non-
interacting DyIII ions (6H15/2, S = 5/2, L = 5, g = 4/3, wT =
14.17 cm3 K molꢁ1). Upon decreasing the temperature, the wT
product remains constant until 100 K and then decreases slightly
to reach 25.93 cm3 K molꢁ1 at 12 K. The latter decrease is
most likely due to depopulation of the Stark sublevels and/or
significant magnetic anisotropy present in DyIII systems. Finally,
below 12 K a sharp increase can be observed and the wT
reaches a maximum value of 26.72 cm3 K molꢁ1 at 2.5 K. Such
an increase can be attributed to weak intramolecular ferro-
magnetic interactions between the DyIII ions. Such observation
of ferromagnetic interactions is rare in lanthanide systems as
4f ions couple weakly due to the core orbital nature of lanthanide
ions.1e Furthermore, strong magnetic anisotropy as well as
thermal population of low-lying excited states generally tend
to mask these weak interactions.
The temperature dependence of the ac susceptibility, measured
under a zero dc field, reveals frequency dependant in-phase
(w0) and out-of-phase (w0 0) signals (Fig. 3 and Fig. S2, ESIw).
This clearly indicates slow relaxation of the magnetisation
arising from SMM behaviour. Under zero field measurements
(Fig. 3, top) the presence of a full broad peak can be observed
with a maximum located around 6 K at 1500 Hz. Furthermore, a
broad shoulder between 8 and 12 K can be seen as well as a tail
of the peak below 5 K. The former shoulder indicates most
Recently, we reported two similar phenoxide bridged
dinuclear core Dy2 complexes exhibiting similar behaviour in
which both DyIII ions are arranged centrosymmetrically.1e
Thus the coordination environments are identical for both
metal centres in the reported system. Metal–metal distances
are comparable (3.80 A vs. 3.84 A) whereas the Dy–O–Dy
angles are slightly smaller than in 1 (107.71 vs. 110.81). The
field dependence of the magnetisation below 8 K reveals a
relatively rapid increase of the magnetisation at low fields and
then a very slow linear increase without clear saturation (Fig. 2,
inset). The high field linear variation and non-saturation of
Fig. 3 Frequency dependence of the out-of-phase (w0 0) ac susceptibility
from 2.5 K to 15 K under an applied zero (top) and 1000 Oe (middle)
static field at indicated frequencies. Fitting of the relaxation time using
Arrhenius law (middle inset). Fitting of the peak position to determine
the individual relaxation time (bottom).
Fig. 2 Temperature dependence of the wT product at 1000 Oe for 1.
Inset: M vs. H/T plot at temperatures below 8 K.
c
10994 Chem. Commun., 2011, 47, 10993–10995
This journal is The Royal Society of Chemistry 2011

View Article Online
likely the presence of a secondary relaxation process involved
whereas the latter tail is indicative of quantum tunnelling of
the magnetisation often seen in lanthanide SMMs.1e,2 In order
to minimize the occurring quantum tunnelling, ac susceptibility
measurements were carried out under a static dc field (Fig. 3,
middle, and Fig. S3, ESIw).
The Cole–Cole plots from the zero field measurements indicate
a near semicircle as expected for one relaxation mode (Fig. 4,
top). However, for the in-field ac measurements clear distortion
of the semi-circles indicates the presence of two relaxation modes
(Fig. 4, bottom). It is clearly evident that such in-field measure-
ments are ideal for reducing QTM and highlighting multiple
relaxation modes in polynuclear lanthanide SMMs.
The thermal dependence of the imaginary susceptibility
under an optimum field of 1000 Oe reveals an enhancement
of the peak feature as the quantum tunnelling is reduced
(Fig. 3, middle). At higher frequencies both peaks remain
overlapping but are clearly observable. Two peaks clearly
indicate the presence of two relaxation processes, which can
be attributed to single-ion relaxation of Dy1 and Dy2 ions.
This interesting feature is due to the presence of two coordi-
natively distinct DyIII ions where the subtle differences are due
to distinct ligand fields.6 This presumably leads to different
energies for first excited Karmers doublets in each Dy sites.
Extracting energy barriers from overlapping relaxation processes
is tricky and to our knowledge has never been reported.
We were able to employ deconvolution analysis techniques
utilised in NMR studies to fit such overlapping peaks to
estimate the relaxation barriers (Fig. 3, bottom). The analysis
was performed on individual relaxation data using a multi-peak
Gaussian fit with Origin software. Fitting of the individual peaks
is given in Fig. S4 (ESIw). Once the peak maxima positions are
calculated it is possible to assess the values of the anisotropic
barriers by considering a thermally activated model (Arrhenius
law, t = t0exp(Ueff/kT)). The effective energy barriers obtained
are Ueff = 36 K (t0 = 4.2 ꢂ 10ꢁ7 s) and Ueff = 80 K (t0 =
8.3 ꢂ 10ꢁ8 s) for the low and high temperature domains,
respectively (Fig. 3, middle inset). Such values are comparable
to other reported Dy2 SMMs.1b,e,2 These relaxation barriers
primarily originate from single-ion anisotropy.
In conclusion, we were able to synthesize an unsymmetrical
Dy2 complex with distinct coordination environments inducing
two individual relaxation processes. Both relaxations are pre-
dominantly single-ion in origin with relaxation barriers due to
different ligand fields and coordination geometries. At this
stage it is not possible to clearly identify which relaxation peak
corresponds to which lanthanide ion. We are currently exploring
the possibility of introducing a different neutral ligand such as
pyridine to replace the water molecule (O9) to vary the ligand
field around the seven coordinate Dy2 ion and thus potentially
promote a different relaxation process. This may be the way to
elucidate accurately the influence of the ligand field around the
DyIII ions.
This work was financially supported by the NSFC of China
(21102039, 21072049 and 50903028) and Educational Committee
of Heilongjiang Province (10td03 and 11551336), NSERC-DG,
CFI, ORF and ERA.
Notes and references
1 (a) J. D. Rinehart, M. Fang, W. J. Evans and J. R. Long, Nat. Chem.,
2011, 3, 538; (b) J. Long, F. Habib, P.-H. Lin, I. Korobkov,
G. Enright, L. Ungur, W. Wernsdorfer, L. F. Chibotaru and
M. Murugesu, J. Am. Chem. Soc., 2011, 133, 5319; (c) M. Mannini,
F. Pineider, C. Danieli, F. Totti, L. Sorace, P. Sainctavit, M.-A. Arrio,
E. Otero, L. Joly, J. C. Cezar, A. Cornia and R. Sessoli, Nature, 2010,
468, 417; (d) J. K. Tang, I. Hewitt, N. T. Madhu, G. Chastanet,
W. Wernsdorfer, C. E. Anson, C. Benelli, R. Sessoli and A. K. Powell,
Angew. Chem., Int. Ed., 2006, 45, 1729; (e) P.-H. Lin, T. J. Burchell,
R. Clerac and M. Murugesu, Angew. Chem., Int. Ed., 2008, 47, 8848;
(f) C. J. Milios, A. Vinslava, W. Wernsdorfer, S. Moggach, S. Parsons,
S. P. Perlepes, G. Christou and E. K. Brechin, J. Am. Chem. Soc.,
2007, 129, 2754.
2 (a) D.-P. Li, X.-P. Zhang, T.-W. Wang, B.-B. Ma, C.-H. Li, Y.-Z. Li
and X.-Z. You, Chem. Commun., 2011, 47, 6867; (b) S.-D. Jiang,
B.-W. Wang, H.-L. Sun, Z.-M. Wang and S. Gao, J. Am. Chem.
Soc., 2011, 133, 4730; (c) D.-P. Li, T.-W. Wang, C.-H. Li, D.-S. Liu,
Y.-Z. Li and X.-Z. You, Chem. Commun., 2010, 46, 2929;
(d) A. Yamashita, A. Watanabe, S. Akine, T. Nabeshima,
M. Nakano, T. Yamamura and T. Kajiwara, Angew. Chem., Int.
Ed., 2011, 50, 4016; (e) H. L. C. Feltham, Y. Lan, F. Klower,
¨
L. Ungur, L. F. Chibotaru, A. K. Powell and S. Brooker, Chem.–Eur.
J., 2011, 17, 4362; (f) P.-E. Car, M. Perfetti, M. Mannini,
A. Favre, A. Caneschi and R. Sessoli, Chem. Commun., 2011,
47, 3751; (g) M. A. AlDamen, J. M. Clemente-Juan, E. Coronado,
C. Mart-Gastaldo and A. Gaita-Ari, J. Am. Chem. Soc., 2008,
130, 8874; (h) N. Ishikawa, M. Sugita, T. Ishikawa, S. Koshihara
and Y. Kaizu, J. Am. Chem. Soc., 2003, 125, 8694.
3 (a) P.-H. Lin, T. J. Burchell, L. Ungur, L. F. Chibotaru,
W. Wernsdorfer and M. Murugesu, Angew. Chem., Int. Ed., 2009,
48, 9489; (b) R. J. Blagg, C. A. Muryn, E. J. L. McInnes, F. Tuna
and R. E. P. Winpenny, Angew. Chem., Int. Ed., 2011, 50, 6530.
4 F. Habib, P.-H. Lin, J. Long, I. Korobkov, W. Wernsdorfer and
M. Murugesu, J. Am. Chem. Soc., 2011, 133, 8830.
5 (a) X.-P. Yang, R. A. Jones, M. M. Oye, A. L. Holmes and
W.-K. Wong, Cryst. Growth Des., 2006, 6, 2122; (b) X.-P. Yang
and R. A. Jones, J. Am. Chem. Soc., 2005, 127, 7686; (c) X.-P. Yang,
R. A. Jones and W.-K. Wong, Dalton Trans., 2008, 1676;
(d) P.-F. Yan, S. Chen, P. Chen, J.-W. Zhang and G.-M. Li,
CrystEngComm, 2011, 13, 36.
Fig. 4 Cole–Cole plots for zero field (top) and 1000 Oe (bottom)
static field.
6 L. Sorace, C. Benelli and D. Gatteschi, Chem. Soc. Rev., 2011, 40, 3092.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 10993–10995 10995