Symmetry-Based Magnetic Anisotropy in the Trigonal Bipyramidal Cluster
Cai-Feng Wang, Jing-Lin Zuo,* Bart M. Bartlett, You Song, Jeffrey R. Long,* and Xiao-Zeng You
Coordination Chemistry Institute and the State Key Laboratory of Coordination Chemistry, Nanjing UniVersity,
Nanjing 210093, China, and Department of Chemistry, UniVersity of California, Berkeley, California 94270-1460
Received March 15, 2006; E-mail: email@example.com; firstname.lastname@example.org
For more than a decade, it has been recognized that molecules
possessing a high-spin ground state, S, and a negative uniaxial
anisotropy, D, can exhibit slow magnetic relaxation at low
temperatures.1Such “single-molecule magnets” have received
considerable attention, owing in part to the possibility that they
might, one day, find applications in high-density information
storage, quantum computing, or spin-based molecular electronics.2
Since any of these potential applications would benefit from the
use of molecules with higher blocking temperatures than currently
known,3the development of synthetic methods for producing high-
spin, high-anisotropy molecules has become a major research focus.
While much of this research has been directed toward transition
metal-oxo clusters,1,3,4some success has also been achieved with
use of cyanide as a bridging ligand.5Here, organic blocking ligands
can be utilized to direct the assembly of specific structures, wherein
variation of the metal ions can provide adjustability to both spin
state and magnetic anisotropy. Recently, we showed that the
incorporation of low-spin iron(III) centers into the face-centered
cubic cluster [Tp8(H2O)6Cu6Fe8(CN)24]4+led to single-molecule
magnet behavior, despite the apparent Oh symmetry of its core
structure.6We now demonstrate that employing a tridentate blocking
ligand on the CuIIcenters in this system generates a trigonal
bipyramidal cluster, [Tp2(Me3tacn)3Cu3Fe2(CN)6]4+, in which the
reduced symmetry affords a significantly increased anisotropy
Synthesis of the new cluster proceeds via reaction of [(Me3tacn)-
Cu(H2O)2](ClO4)2 (Me3tacn ) N,N′,N′′-trimethyl-1,4,7-triazacy-
clononane)7with (Bu4N)[TpFe(CN)3] (Tp-) hydrotris(pyrazolyl)-
borate)8in a mixture of ethanol and acetonitrile. Diffusion of diethyl
ether vapor into the resulting solution affords dark brown block-
shaped crystals of [Tp2(Me3tacn)3Cu3Fe2(CN)6](ClO4)4‚2H2O (1)
in 82% yield.9
The crystal structure of 1 features the pentanuclear [Tp2(Me3-
tacn)3Cu3Fe2(CN)6]4+cluster depicted in Figure 1. Here, two
opposing [TpFe(CN)3]-complexes are connected via a triangle of
three [(Me3tacn)Cu]2+units to give a trigonal bipyramidal metal-
cyanide core with approximate D3hsymmetry. We note that this
core geometry has been obtained previously with other metals and
different capping ligands.5b,eThe cyanide bridges connecting metal
centers in 1 are quite close to linearity, with Fe-CtN and Cu-
NtC angles falling in the range 168.1(2)-178.6(3)°. The coordina-
tion at each CuIIcenter is square pyramidal, with the cyanide
nitrogen atoms lying in the basal plane. The Fe-C bond distances
range from 1.911(3) to 1.938(3) Å, in good agreement with those
observed previously for structures containing [TpFe(CN)3]-.6,8
Importantly, the local three-fold symmetry axes associated with the
coordination environments of the two low-spin FeIIIcenters are
roughly collinear, such that one might expect the anisotropy
stemming from orbital angular momentum to be maximized.10
Within the crystal structure, the closest contacts between metal
centers in neighboring clusters are 8.43 Å, with no intervening
hydrogen-bonding pathways. Thus, we can anticipate that the
magnetic properties observed above 1.8 K will be mainly those of
the isolated cluster units.
dc magnetic susceptibility measurements were performed on a
polycrystalline sample of 1 at 2 kOe in the temperature range 1.8-
300 K (see Figure 2). At room temperature, the compound exhibits
?MT ) 2.14 emu‚K/mol, which is higher than the value of 1.875
emu‚K/mol expected for a spin-only contribution from two low-
spin FeIII(S )1/2) and three CuII(S )1/2) ions in the absence of
any exchange coupling. As the temperature is lowered, ?MT rises
to a maximum of 5.35 emu‚K/mol at 6.0 K.
Figure 1. Structure of the trigonal bipyramidal cluster [Tp2(Me3tacn)3Cu3-
Fe2(CN)6]4+in 1. Orange, green, magenta, gray, and blue spheres represent
Fe, Cu, B, C, and N atoms, respectively; H atoms are omitted for clarity.
Selected mean interatomic distances (Å) and angles (deg): Fe-C 1.926(9),
Fe-N 1.96(1), Cu-NCN1.98(1), Cu-Ntacn2.10(7), CtN 1.133(9), Fe‚‚‚
Cu 5.02(3), Cu‚‚‚Cu 6.7(2), Fe-CtN 177(1), Cu-NtC 172(3), C-Fe-C
88(2), C-Fe-N 92(2), N-Fe-N 89.0(8), NCN-Cu-NCN88(2), NCN-Cu-
Figure 2. dc magnetic susceptibility of 1 recorded under a 2 kOe measuring
field. Inset: Reduced magnetization data for 1 at low temperatures. Solid
lines represent fits to the data (see text for details).
Published on Web 05/12/2006
7162 9 J. AM. CHEM. SOC. 2006, 128, 7162-7163
10.1021/ja061788+ CCC: $33.50 © 2006 American Chemical Society
This behavior typifies ferromagnetic coupling between the orthogo- Download full-text
nal spin orbitals of octahedral FeIII(t2g) and square pyramidal CuII
(b1g) centers, resulting in an S )5/2ground state. Below 6 K, ?MT
decreases, suggesting the presence of significant zero-field splitting
in the ground state. Note that the observation that ?MT is
independent of field strength for applied fields below 2 kOe (see
Figure S1) precludes the presence of significant intermolecular
interactions. Fitting the 2 kOe ?MT data using the isotropic spin
Hamiltonian H ˆ ) -2J(S ˆFe(1)+ S ˆFe(2))‚(S ˆCu(1)+ S ˆCu(2)+ S ˆCu(3)), which
includes only nearest-neighbor exchange, gave g ) 2.245(4), J )
8.5(1) cm-1, and TIP ) -1.0 × 10-3emu/mol (R ) 1.2 × 10-3).
The large Lande ´ splitting can be ascribed to a combination of the
orbital contributions from the low-spin FeIIIcenters11and individual
factors of >2 associated with the CuIIcenters. The magnitude of
the exchange coupling is somewhat lower than the 15 cm-1
estimated for [Tp8(H2O)6Cu6Fe8(CN)24]4+,6but slightly greater than
the 5 or 6 cm-1observed for other molecules with FeIII-CN-CuII
The magnetization data collected for 1 at temperatures between
1.8 and 5 K are also consistent with an S )5/2ground state (see
inset in Figure 2). Here, the non-superposition of the isofield lines
confirms the presence of significant zero-field splitting. Fitting the
data using ANISOFIT 2.013afforded zero-field splitting parameters
of D ) -5.7 cm-1and E ) 1.4 × 10-3cm-1, with g ) 2.53.
Importantly, the axial anisotropy is much greater than that associated
with the S ) 7 ground state of the face-centered cubic cluster
[Tp8(H2O)6Cu6Fe8(CN)24]4+, for which D ) -0.16 cm-1.6This
difference likely arises due to the lower core symmetry of the
molecule, which does not lead to near-cancellation of individual
ion anisotropy terms. Based on the observed values of S and D,
[Tp2(Me3tacn)3Cu3Fe2(CN)6]4+should be a single-molecule magnet
with a spin reversal barrier of U ) (S2-1/4)|D| ) 34 cm-1.
As shown in Figure 3, the low-temperature ac susceptibility data
obtained for 1 are indeed indicative of single-molecule magnet
behavior. The ?M′′ values for a given frequency attain a maximum
that shifts to lower temperature upon decreasing the frequency. The
?M′′ peak positions were determined using fits to Lorentzian lines,
and the plot of ln τ versus 1/T (see inset) follows the Arrhenius
expression ln τ ) Ueff/kBT + ln τ0. Least-squares fitting gave τ0)
4.8 × 10-8s and an effective spin-reversal barrier of Ueff) 16
cm-1. The reduction by roughly 50% in the observed barrier height
may be attributable to the usual thermally activated tunneling
The foregoing results demonstrate the enormous impact that
cluster symmetry can have on magnetic anisotropy, with a ligand-
induced switch from a cubic to a linear arrangement of FeIIIcenters
leading to significant enhancement of the anisotropy barrier. Of
particular import is the observation that the resulting barrier is
substantial despite the ground-state spin being just S )5/2. Future
efforts will therefore focus on assembly of linear vertex-sharing
trigonal bipyramid oligomers of the type [Tp2(Me3tacn)3nCu3nFen+1-
(CN)6n](3n+1)+(n ) 2, 3, 4, ...; S )(4n+1)/2) via three-component
reactions incorporating [Fe(CN)6]3-.
Acknowledgment. This research was funded by the National
Natural Science Foundation of China (20531040 and 90501002)
and the U.S. National Science Foundation (ECS-0210426). We also
thank the Program for New Century Excellent Talents in University
of China (NCET-04-0469) and the University of California
President’s Postdoctoral Fellowship Program for support.
Supporting Information Available:
additional magnetic characterization data (PDF); an X-ray crystal-
lographic file (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
Experimental details and
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Figure 3. Out-of-phase component of the ac magnetic susceptibility data
for 1, recorded with switching frequencies of 1 (O), 10 (0), 100 (4), 499
(]), 997 (+), and 1488 (×) Hz. Inset: Arrhenius law fit of peak maximum
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C O M M U N I C A T I O N S
J. AM. CHEM. SOC. 9 VOL. 128, NO. 22, 2006 7163