A structurally perfect S = 1/2 metal-organic hybrid kagome antiferromagnet

Article abstract of DOI:10.1021/ja709991u

We report the synthesis and magnetic characterization of Cu(1,3-bdc) (1,3-bdc = 1,3-benzenedicarboxylate), a novel structurally perfect S = 1/2 metal-organic hybrid kagome antiferromagnet exhibiting properties consonant with spin frustration. In the kagome structure, each monodentate carboxylate ligand bridges two copper ions in the kagome plane, and the three-atom bridge allows for the shortest known metal-metal distance in any metal-organic hybrid kagome reported to date. The compound shows antiferromagnetic mean nearest-neighbor superexchange, as evidenced by ν_CW = -33 K. The compound orders ferromagnetically at 2 K and has a frustration parameter, f = 16.5, indicative of a spin-frustrated compound. Copyright

Full text of DOI:10.1021/ja709991u

Published on Web 02/15/2008
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A Structurally Perfect S ) /2 Metal-Organic Hybrid Kagome´ Antiferromagnet
Emily A. Nytko,† Joel S. Helton,‡ Peter Mu¨ller,† and Daniel G. Nocera*,†
Departments of Chemistry and Physics, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139-4307
Received November 5, 2007; E-mail: nocera@mit.edu
Geometric spin frustration occurs when the structural arrange-
ment of spins precludes the simultaneous satisfaction of all nearest-
neighbor interactions.1,2 The corner-sharing equilateral triangles of
a kagome´ lattice present an excellent construct on which to study
spin frustration; spin liquid physics can result when the kagome´
1
lattice features S )
/
spins on lattice vertices.3-5 However,
2
compounds bearing the kagome´ structural motif are rare,6-10 and
exceptionally so when an organic moiety is a structural constituent
of the kagome´ triangles.11-14 Previous studies of spin frustration
on metal-organic hybrid kagome´ compounds have been compli-
cated by long exchange pathways,15,16 triangulated17,18 or interpen-
etrating structures,19 and spin centers composed of dimers displaying
intradimer exchange interactions.20-23 There are some structurally
distorted metal-organic hybrid kagome´s with isolated metal ions
(Co-Zn-Cd) on the vertices11 and fewer structurally perfect
Figure 1. Crystal structure of 1. Left: Interlayer coordination environment
projected parallel to the crystallographic c-axis. Right: The organic-
inorganic hybrid kagome´ lattice, projected perpendicular to the c-axis. Phenyl
ring atoms and guest water molecule have been omitted for clarity. Selected
interatomic distances and angles: Cu(1)-O(1), 1.9265(9); Cu(1)-O(2),
1.9428(9); Cu(1A)‚‚‚O(2B), 2.8396(9); Cu(1)-C(1A), 1.2685(15); O(2A)-
C(1), 1.2689(14) Å; O(1)-Cu(1)-O(2), 92.14(4)°; O(2)-Cu(1)-O(1A),
87.86(4)°; Cu(1)-O(1)-C(1A), 115.67(8)°; Cu(1)-O(2A)-C(1),
132.42(8)°.
1
systems (In, V, Zn);12-14 none are composed of S ) /2 magnetic
ions. We report herein the synthesis and magnetic characterization
of the first structurally perfect metal-organic hybrid kagome´ with
1
a spin of S ) /2 on lattice vertices.
We recently employed a hydrothermal approach to prepare the
1
first structurally perfect S ) /2 kagome´ all-inorganic antiferro-
magnet, herbertsmithite ZnCu3(OH)6Cl2.9 This remarkable material
shows no long-range ordering to 50 mK,24 despite having strongly
antiferromagnetic nearest-neighbor superexchange, as evidenced by
by elemental analysis (see SI, Figure S7). Solubility appears to play
an important role in the formation of 1, as related compounds Cu-
(H2O)(1,3-bdc)‚H2O and Cu24(1,3-bdc)24(DMF)14(H2O)10‚(H2O)10-
(DMF)6(C2H5OH)6 can be obtained from reactions with cupric
acetate and cupric nitrate, respectively.27,28
ΘCW ) -314 K. We have subsequently sought to generalize the S
1
)
/ kagome´ lattice by discovering new systems that can be
2
prepared in crystalline form.
The single-crystal X-ray structure of 1 is shown in Figure 1.
Details of the structure solution and refinement are provided in the
SI. The compound crystallizes in the hexagonal space group
P63/m. The highly symmetric structure has only one-half of a
crystallographically independent Cu(II) ion, which resides in a
nearly square planar coordination environment with four mono-
dentate carboxylate ligands. Each carboxylate group bridges two
copper ions to form a kagome´ lattice in the ab plane composed of
{Cu3(OCO)3} triangles. The in-plane Cu-Cu distance is 4.5541-
(2) Å (4.5528(3) Å for 2, Cu(1,3-bdc)‚0.11 H2O, the water-
containing species), and the distance between the kagome´ planes
(d002) is 7.9716(3) Å (7.9775(6) Å for 2). The layers stack in AA
fashion. 1,3-bdc ligands serve to link the layers together into a three-
dimensional metal-organic framework. The structure has hexagonal
channels, with a solvent-accessible void of 45 Å3 and a cross-
channel Cu-Cu distance of 9.1081(2) Å (9.1056(3) Å for 2). The
guest water molecule residing in the center of this channel in the
kagome´ plane can be removed by heating the crystals to 240 °C
without significant decomposition (see SI, Figure S2).
Initial treatment of Cu(OH)2 with isophthalic acid (1,3-bdcH2)
under hydrothermal conditions led to the formation of a novel
metal-organic hybrid kagome´, Cu(1,3-bdc) (1), according to the
condensation reaction below:
Cu(OH)2 + 1,3-bdcH2 f Cu(1,3-bdc) + 2H2O (1)
The product mixture also contains 1,3-bdcH2 and a copper-
containing ligand oxidation byproduct. (Ligand oxidation of 1,3-
bdcH2 in Cu(II)-containing solutions has been previously re-
ported.25,26) To increase the solubility of 1,3-bdcH2 and to buffer
the reaction solution, imidazole was added to subsequent reactions.
Additionally, Cu(OH)F was included as a mineralizing agent to
promote crystal formation. Large (up to 2 mm on an edge and 2.2
mg), dark blue hexagonal plates of 1 were separated from a product
mixture containing starting materials and a copper-containing ligand
oxidation byproduct, C32H24Cu6O26 (3) (see Supporting Information,
SI). 1 may contain an incidental guest water molecule upon
structural analysis, so crystals of 1 were heated prior to magnetic
measurements in order to remove the incidental solvent molecule
(see SI). The purity of 1 was confirmed by similarities between
simulated and experimental powder X-ray diffraction (pXRD) and
Fitting the high-temperature inverse susceptibility data (150-
350 K) of a ground powder of 1 yields ΘCW ) -33 K, which
suggests that the mean nearest-neighbor superexchange interaction
is antiferromagnetic (see SI, Figure S3). The zero-field-cooled and
field-cooled magnetic susceptibility of 1 versus temperature are
shown in Figure 2a. A plot of øT vs T is provided in the SI, Figure
† Department of Chemistry.
‡ Department of Physics.
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10.1021/ja709991u CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
metal-organic hybrid kagome´ to date. The ability to obtain crystals
of moderate size opens the way for magnetoanisotropy measure-
ments and, with still larger crystal sizes, inelastic neutron scattering
measurements. Owing to the S ) 1/2 spin on a kagome´ lattice, the
discovery of inorganic-organic hybrid materials such as 1 provides
a venue for the exploration of quantum disordered ground states in
two dimensions.
Acknowledgment. This work made use of the Shared Experi-
mental Facilities supported by the MRSEC Program of the National
Science Foundation under award number DMR 02-13282. We thank
NSF for providing E.A.N. with a predoctoral fellowship, Mr. T.
McClure and Dr. S. Chu for experimental assistance, and Drs. B.
Bartlett, D. Villagra´n, and D. Grohol and Profs. M. Shores and Y.
S. Lee for helpful discussions.
Supporting Information Available: Complete synthetic protocol
and characterization of CuC8H4O4; crystallographic tables and X-ray
crystallographic information, in CIF format, for 1-3. This material is
available free of charge via the Internet at http://pubs.acs.org.
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