An Undecanuclear Ferrimagnetic Cu9Dy2 Single Molecule Magnet Achieved through Ligand Fine-Tuning

Article abstract of DOI:10.1021/acs.inorgchem.6b00490

We describe the concept of increasing the nuclearity of a previously reported high-spin Cu₅Gd₂ core using a "fine-tuning" ligand approach. Thus, two Cu₉Ln₂ coordination clusters, with Ln = Dy (1) and Gd (2), wer

Full text of DOI:10.1021/acs.inorgchem.6b00490

Communication
pubs.acs.org/IC
An Undecanuclear Ferrimagnetic Cu9Dy2 Single Molecule Magnet
Achieved through Ligand Fine-Tuning
Irina A. Kuhne,† George E. Kostakis,‡,§ Christopher E. Anson,† and Annie K. Powell*,†,#
̈
†Institut fur Anorganische Chemie, Karlsruhe Institute of Technology (KIT), Engesserstrasse 15, D-76131 Karlsruhe, Germany
̈
‡Department of Chemistry, School of Life Sciences, University of Sussex, Arundel Building 305, BN1 9QJ Sussex, U.K.
§Science and Educational Center of Physics of Noneqiliubrium Open Systems, Samara State Aerospace University (National Research
University), Moskovskoye Shosse 34, Samara 443086, Russia
#Institute of Nanotechnology, KIT, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
S
* Supporting Information
rarely can the SMM behavior be probed solely by ac
measurments.7
ABSTRACT: We describe the concept of increasing the
nuclearity of a previously reported high-spin Cu5Gd2 core
using a “fine-tuning” ligand approach. Thus, two Cu9Ln2
coordination clusters, with Ln = Dy (1) and Gd (2), were
synthesized with the Gd compound having a ground spin
state of 17/2 and the Dy analogue showing single-molecule-
magnet behavior in zero field.
We show here how a ligand engineering approach applied to a
system with a favorable core spin topology leads to a CuII/DyIII
SMM. We took our previously reported Cu5Gd2 CC, for which
we were unable to isolate the DyIII analogue, as a starting point.
This has a high S = 17/2 ground spin state and is synthesized using
the Schiff-base ligand H4L1,15 based on o-vanillin and tris-
(hydroxymethyl)aminoethane (tris). Given that one or even two
hydroxyl groups of the tripodal alcohol unit may not be involved
in coordination but can form inter- and intramolecular hydrogen
bonds to solvent molecules, neighboring clusters, or internal OH
groups,16−18 we decided to replace one of the CH2OH alcohol
groups with a CH3 methyl unit to give the ligand H3L2. This
results in the cancellation of hydrogen bonding involving this
arm, allowing for a nuclearity expansion. Indeed, this approach
has been used in CoII,III CCs but led not only to cluster expansion
but also to a change in the core motif.17,18 In the compounds we
report here, this approach has been successfully applied to
expanding Cu5Ln2 by the addition of two Cu2 units to give a
Cu9Ln2 topology with the same core motif as that in Cu5Gd2 for
both Ln = GdIII and DyIII, with the latter showing SMM behavior.
n the burgeoning area of 3d/4f coordination clusters (CCs)
Ifor those showing single-molecule-magnet (SMM) behavior,
it is becoming increasingly clear that the correct blend of 3d and
4f ions and the resulting topology of the clusters are important
factors governing this behavior.1 For 3d/4f SMMs, the magnetic
behavior can show two slow relaxation regimes corresponding to
the 4f single ion and to the exchange-coupled 3d/4f processes, as
evidenced within the frequency window of routine alternating-
current (ac) susceptibility studies. Here, in contrast to 4f single-
ion systems,2,3 these can obey an Arrhenius law, with two linear
dependences corresponding to the two relaxation pathways, as
was shown for a CoII DyIII2 system.4 Generally, such systems can
2
C o m p o u n d
[ C u D y ( μ - O H ) ( μ -
be treated within the coupled 3d/4f regime as giant spin systems.
As such, they show relatively low effective energy barriers to spin
inversion (Ueff) compared with the very high barriers often
predicted for 4f single-ion systems.5 However, this is
compensated for by a suppression of quantum tunneling effects,
which are particularly complicated and prevalent for 4f single
ions. Thus, for high-spin 3d/4f systems, it can be possible to
observe and analyze the relaxation data within the usual
frequency window of ac susceptibility measurements in a zero
applied direct-current (dc) field for both the single-ion and
exchange-coupled cases.4
9 2 3 4 3
Br)2(L2)2(HL2)4(Br)2(NO3)2(MeOH)4]·6MeOH (1·
6MeOH) was synthesized by mixing a ligand solution of H3L2
(0.25 mmol) and triethylamine (1.0 mmol) with a solution of
CuBr2 (0.25 mmol) and Dy(NO3)3·6H2O (0.25 mmol) in 10.0
mL of methanol. 1·6MeOH crystallizes in the triclinic space
group P1 with Z = 2 (Figure 1).
̅
The central CuII ion, Cu(1), of this undecanuclear CC sits on
an inversion center. The basic core (Figure S1) is essentially the
same as that of the previously reported Cu5Gd2 cluster.15 The
{Cu5Dy2} unit is bridged by four μ3-hydroxides, O(1), O(1′),
O(2), and O(2′), each of which bridges between two CuII and
one DyIII ion. Cu(1) provides the common vertex of two Cu
triangles, which are linked together by one μ3-bridging bromide
ligand, Br(1) or Br(1′), albeit with rather long bond distances of
between 2.8604(13) and 2.9627(13) Å.
CCs combining highly anisotropic 4f ions such as DyIII with
the quantum spin CuII d9 ion6 have received less attention than
other combinations. Among the relatively few examples of such
SMMs,7−9 the ones with high nuclearities above ten7,10−14 are
even rarer. These have the advantage that the molecules possess
high spins and are well separated within the crystal structure.
Although these systems can show slow relaxation of magnet-
ization, as evidenced by frequency-dependent ac susceptibilities,
the maxima of the characteristic out-of-phase signals are often
shifted beyond the available frequency window.11,12 Thus, only
Cu(1) exhibits a 4 + 2 distorted octahedral coordination
sphere with bromides on the elongated Jahn−Teller (JT) axis.
Received: March 2, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b00490
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
expected for isotropic systems. The saturation value of 17.05 μB
at 7 T and 2 K is much lower than the expected 23.0 μB for two
GdIII (S = 7/2) and nine CuII (S = 1/2) ions, which are uncoupled
or completely ferromagnetically coupled, suggesting that there is
a degree of antiferromagnetic coupling. If we assume that there is
a similar spin orientation within the Cu9Gd2 core (2; Figures 3
and S5), and therefore in Cu9Dy2, as in the previously reported
Cu5Gd2,15 we expect ferromagnetic Cu−Dy interactions through
double oxo bridges,24 while the Cu−Cu interactions through μ3-
Br ions are strongly antiferromagnetic.25 The additional two
copper dinuclear units on both sides of the core [Cu(4) and
Cu(5) and symmetry equivalents] are antiferromagnetically
coupled, as suggested from their oxo-bridged angles of 98.6(3)°
for Cu(4)−O(13)−Cu(5) and 96.5(2)° for Cu(4)−O(6)−
Cu(5).26,27 Within a giant spin model, this leads to a predicted
value of 17.0 μB for the magnetization and therefore a spin
ground state of S = 17/2, in excellent agreement with the
measured one (17.05 μB). For compound 1, the magnetization
measurements show no saturation up to 7 T for compound 1
(Figure S4). For the higher temperature isotherms, the values
rapidly increase at low temperatures before following a more
gradual linear increase without saturation.
Figure 1. Molecular structure of Cu9Dy2 (organic H atoms omitted for
clarity; μ-bridging bonds highlighted in orange, benzene rings
highlighted according to coordination mode; see Figure S1).
The Cu(4) ions also have a JT distorted octahedral coordination
environment but with one bromide, Br(2), and one methanol,
O(8), in the axial positions. Cu(2) and Cu(3) have square-
pyramidal coordination. Cu(5) has a square-planar ligand
environment (Figure S2). The DyIII ions are nine-coordinate.
The six Schiff-base ligands show three different coordination
modes (Scheme S1). Four of these are doubly deprotonated with
the loss of H atoms on the phenol and one diol O atom and
bridge two or three metal ions, while the remaining two ligands
are completely deprotonated and bridge four metal ions. This has
been shown for Cu/Dy complexes with Schiff-base ligands based
on o-vanillin providing two pockets for metal-ion coordina-
tion.19−23 One pocket is formed by the phenoxo O atom and the
imine N atom, which in the current compound is always
occupied by a CuII ion, similar to previously reported Cu/Ln
complexes.11,19 The other pocket, defined by the phenoxy and
methoxy O atoms, can incorporate a CuII or DyIII ion or remain
vacant. One of the alcohol arms is always coordinated to one CuII
ion, assisting with chelation of this metal ion in the first pocket as
well as having the possibility of bridging to a second CuII ion,
with or without the help of the second diol arm. The Cu−O and
Cu−N bond lengths are between 1.891(6) and 2.086(5) Å, while
the bond lengths on the JT axis are extended with 2.65 Å for
Cu(4)−O(8).
The ac susceptibility measurements performed on compound
1 display frequency-dependent peaks in the out-of-phase signals
(Figures 2 and S6) even in the absence of an applied external
Figure 2. Frequency dependence of χM″ for Cu9Dy2 (1) in a zero
applied dc field in the range between 1.8 and 3.5 K (left) and Cole−Cole
plot between 1.8 and 2.6 K. The solid lines are fits of the experimental
data (right).
The static (dc) magnetic properties for both compounds with
the formulas 1·6MeOH·4H2O and 2·7H2O (formulas consistent
with C, H, and N analyses on fresh samples) were studied
between 1.9 and 300 K under an applied field of 1000 Oe (Figure
S3). The χMT value of 34.3 cm3·K/mol of 1·6MeOH·4H2O at
room temperature is in good agreement with what is expected for
nine noninteracting CuII (S = 1/2, g = 2, C = 0.375 cm3·K/mol)
and two DyIII (S = 5/2, 6H15/2, g = 4/3, C = 14.17 cm3·K/mol) ions
(31.7 cm3·K/mol). The χMT value of 18.9 cm3·K/mol for the
isostructural GdIII-containing compound 2·7H2O is in good
agreement (S = 7/2, 8S7/2, g = 2, C = 7.88 cm3·K/mol) with the
expected value of 19.09 cm3·K/mol. Upon lowering the
temperature, the χT product of 1 decreases until it reaches the
minimum value of 30.1 cm3·K/mol at 26 K. For compound 2·
7H2O, the χT product stays almost constant upon lowering of
the temperature to 40 K. Upon further cooling, the χT values for
both compounds continuously increase to reach a value of 52.2
cm3·K/mol at 1.9 K for 1 and 32.7 cm3·K/mol for 2. This is in line
with a ferrimagnetic spin topology among the metal centers.
For compound 2, the magnetization shows a clear saturation
above 3 T and the reduced magnetization (Figure S4) shows
superposition of the three isotherms onto one master curve, as
field. This points to a lack of quantum tunneling in this frequency
range, contrary to what is usually the case for Cu/Ln clusters.28,29
The signals are temperature-dependent between 1.8 and 2.6 K
over the entire available frequency range. Above 2.8 K, the peaks
are shifted to higher frequencies and the maxima are beyond the
available frequency window. The data between 1.8 and 2.6 K
were used to extract the relaxation time as a function of the
temperature. The ln(τ) versus 1/T points fit a linear curve,
suggesting that an Orbach process is operative over the whole
studied temperature and frequency range. A fit to the Arrhenius
equation (Figure S7) gives Ueff = 16.1 K and τ0 = 3.6 × 10−7 s (R
= 0.99). Cole-Cole plots of χM′ versus χM″ between 1.8 and 2.6 K
(Figure 2) have semicircular profiles, indicative of a single
relaxation process. The plot was fitted with CC-Fit,28 which uses
a generalized Debye model.30,31 The extracted parameters (see
the Supporting Information) are similar to those reported for the
Cu8Dy9 complex.9
A topological analysis of the isostructural compounds 1 and 2,
taking the long Cu−Br bond distances into account, in line with
the magnetic exchange pathways and spin topology, was
performed using TOPOS software32 (originally developed to
describe metal-organic frameworks) and adopting the NDk-m33
B
DOI: 10.1021/acs.inorgchem.6b00490
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
symbolism for application to CCs. This shows that the clusters
can be represented as 1,2,3,4,6M11-1 (Figure 3). This appears
REFERENCES
(1) Kostakis, G. E.; Powell, A. K. Chem. - Eur. J. 2010, 16, 7983.
(2) Rechkemmer, Y.; Fischer, J. E.; Marx, R.; Dorfel, M.; Neugebauer,
P.; Horvath, S.; Gysler, M.; Brock-Nannestad, T.; Frey, W.; Reid, M. F.;
Van Slageren, J. J. Am. Chem. Soc. 2015, 137, 13114.
■
̈
(3) Pedersen, K. S.; Dreiser, J.; Weihe, H.; Sibille, R.; Johannesen, H.
V.; Sørensen, M. A.; Nielsen, B. E.; Sigrist, M.; Mutka, H.; Rols, S.;
Bendix, J.; Piligkos, S. Inorg. Chem. 2015, 54, 7600.
(4) Mondal, K. C.; Sundt, A.; Lan, Y.; Kostakis, G. E.; Waldmann, O.;
Ungur, L.; Chibotaru, L. F.; Anson, C. E.; Powell, A. K. Angew. Chem.,
Int. Ed. 2012, 51, 7550.
(5) Rinehart, J. D.; Long, J. R. Chem. Sci. 2011, 2, 2078.
(6) Schnack, J. Condensed Matter 2015, 1−8.
́
(7) Liu, J.-L.; Lin, W.-Q.; Chen, Y.-C.; Gomez-Coca, S.; Aravena, D.;
Ruiz, E.; Leng, J.-D.; Tong, M.-L. Chem. - Eur. J. 2013, 19, 17567.
(8) Costes, J.-P.; Auchel, M.; Dahan, F.; Peyrou, V.; Shova, S.;
Wernsdorfer, W. Inorg. Chem. 2006, 45, 1924.
(9) Aronica, C.; Chastanet, G.; Pilet, G.; Le Guennic, B.; Robert, V.;
Wernsdorfer, W.; Luneau, D. Inorg. Chem. 2007, 46, 6108.
(10) Dermitzaki, D.; Raptopoulou, C. P.; Psycharis, V.; Escuer, A.;
Perlepes, S. P.; Stamatatos, T. C. Inorg. Chem. 2015, 54, 7555.
(11) Iasco, O.; Novitchi, G.; Jeanneau, E.; Luneau, D. Inorg. Chem.
2013, 52, 8723.
(12) Leng, J.-D.; Liu, J.-L.; Tong, M.-L. Chem. Commun. 2012, 48,
5286.
(13) Iasco, O.; Novitchi, G.; Jeanneau, E.; Wernsdorfer, W.; Luneau, D.
Inorg. Chem. 2011, 50, 7373.
Figure 3. Topological analysis of complex 2 (right) compared with the
previously reported Cu5Gd2 complex13 (left) (GdIII ions, turquoise; CuII
ions, dark blue) and spin orientation of the metal centers in Cu9Gd2 (2).
The associated Schiff-base ligands are shown below.
to be the first example of this topology ever reported in 3d, 4f, or
3d/4f polynuclear CC chemistry. The fact that this core motif is a
decorated version of that in the [Cu5Gd2]15 complex (3,6M7-2)
is easily verified by graphical screening. When the molecular
formula, the synthetic recipe, and the organic ligands used to
build these entities are scrutinized, it is clear that the presence of
the CH3 group in complex 1 in place of the OH group in the
Cu5Gd2 complex15 favors the addition of two Cu2 units, resulting
in the formation of the undecanuclear species as a result of the
deletion of hydrogen bonding with adjacent or solvent
molecules. Other modifications to the ligand offer further
possibilities for structural tuning.
(14) Baskar, V.; Gopal, K.; Helliwell, M.; Tuna, F.; Wernsdorfer, W.;
Winpenny, R. E. P. Dalton Trans. 2010, 39, 4747.
(15) Wu, G.; Hewitt, I. J.; Mameri, S.; Lan, Y.; Cler
Qiu, S.; Powell, A. K. Inorg. Chem. 2007, 46, 7229.
́
ac, R.; Anson, C. E.;
(16) Zhou, Q.; Yang, F.; Liu, D.; Peng, Y.; Li, G.; Shi, Z.; Feng, S.
Dalton Trans. 2013, 42, 1039.
(17) Jia, Z.-Q.; Sun, X.-J.; Hu, L.-L.; Tao, J.; Huang, R.-B.; Zheng, L.-S.
Dalton Trans. 2009, 6364.
We have successfully used a ligand modification approach to
increase the nuclearity of the high-spin Cu5Gd2 to give a Cu9Ln2
system (Ln = Gd, Dy) while maintaining the same core topology
and spin structure, as revealed from the study of Cu9Gd2. The
DyIII analogue shows SMM behavior even in the absence of an
applied field, as revealed by the ac susceptibility measurements.
(18) Zhou, H.; Chen, Y.-Y.; Yuan, A.-H.; Shen, X.-P. Inorg. Chem.
Commun. 2008, 11, 363.
(19) Kuhne, I. A.; Magnani, N.; Mereacre, V.; Wernsdorfer, W.; Anson,
̈
C. E.; Powell, A. K. Chem. Commun. 2014, 50, 1882.
(20) Kajiwara, T.; Nakano, M.; Takahashi, K.; Takaishi, S.; Yamashita,
M. Chem. - Eur. J. 2011, 17, 196.
(21) Xue, S.; Guo, Y. N.; Zhao, L.; Zhang, H.; Tang, J. Inorg. Chem.
2014, 53, 8165.
(22) Zhang, P.; Zhang, L.; Lin, S.; Tang, J. Inorg. Chem. 2013, 52, 6595.
(23) Sieklucka, B.; Podgajny, R.; Pinkowicz, D.; Nowicka, B.;
Korzeniak, T.; Bałanda, M.; Wasiutynski, T.; Pełka, R.; Makarewicz,
́
M.; Czapla, M.; Rams, M.; Gaweł, B.; Łasocha, W. CrystEngComm 2009,
11, 2032.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b00490.
(24) Bencini, A.; Benelli, C.; Caneschi, A.; Carlin, R. L.; Dei, A.;
Gatteschi, D. J. Am. Chem. Soc. 1985, 107, 8128.
(25) Laborda, S.; Clerac, R.; Anson, C. E.; Powell, A. K. Inorg. Chem.
2004, 43, 5931.
Synthesis, magnetic data, and topological analysis (PDF)
Crystallographic details in CIF format (CIF)
Crystallographic details in CIF format (CIF)
(26) Merz, L.; Haase, W. J. Chem. Soc., Dalton Trans. 1980, 74, 875.
(27) Crawford, V. H.; Richardson, H. W.; Wasson, J. R.; Hodgson, J.
D.; Hatfield, W. E. Inorg. Chem. 1976, 15, 2107.
AUTHOR INFORMATION
Corresponding Author
*E-mail: annie.powell@kit.edu. Tel: +49 721 608 42135.
Author Contributions
■
(28) Maity, M.; Majee, M. C.; Kundu, S.; Samanta, S. K.; Sanudo, E. C.;
̃
Ghosh, S.; Chaudhury, M. Inorg. Chem. 2015, 54, 9715.
(29) Wen, H.-R.; Bao, J.; Liu, S.-J.; Liu, C.-M.; Zhang, C.-W.; Tang, Y.-
Z. Dalton Trans. 2015, 44, 11191.
All authors have given approval to the final version of the
manuscript.
Notes
(30) Chilton, N. F. CC-Fit, http://www.nfchilton.com/cc-fit.html.
(31) Guo, Y.-N.; Xu, G.-F.; Guo, Y.; Tang, J. Dalton Trans. 2011, 40,
9953.
(32) Blatov, V. A.; Shevchenko, A. P.; Proserpio, D. M. Cryst. Growth
Des. 2014, 14, 3576.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(33) Kostakis, G. E.; Blatov, V. A.; Proserpio, D. M. Dalton Trans. 2012,
41, 4634.
DFG CFN and the Helmholtz Gemeinschaft POF STN provided
funding. We thank Abhishake Mondal and Valeriu Mereacre for
collecting the magnetic data. I.A.K. thanks Amy-Jane Hutchings
for her great help during this project.
C
DOI: 10.1021/acs.inorgchem.6b00490
Inorg. Chem. XXXX, XXX, XXX−XXX