Ln2(C2O4)(O2CCH2OH)4 (Ln ) Gd(III), Tb(III)): UnprecedentedInterpenetrating 4,6-Connected Net Built on Unusual Rectangularand Distorted Hexagonal Lanthanide Nodes

Feng Luo,† Gary J. Long,‡ Yun-xia Che,† and Ji-min Zheng*,†

Department of Chemistry, Nankai UniVersity, Tianjin, China, and Department of Chemistry, MissouriUniVersity of Science and Technology, UniVersity of Missouri-Rolla, Rolla, Missouri 65409-0010

ReceiVed June 17, 2008; ReVised Manuscript ReceiVed July 19, 2008

ABSTRACT: Hydrothermal synthesis was used to prepare Ln2(C2O4)(O2CCH2OH)4 (Ln ) Gd(III)/1 Tb(III)/2) from HCO2CH2OH andC2O4

2-, the latter being formed in situ from HCO2CH2OH. The new compounds crystallize with the same unique 2-fold interpenetrating4,6-connected net described by (32 ·62 ·72)(34 ·42 ·64 ·75) topology, a topology that is based on a combination of highly unusual rectangularand distorted hexagonal lanthanide nodes. Gd2(C2O4)(O2CCH2OH)4, 1, exhibits fully paramagnetic behavior with µeff ) 7.938(5) µB perGd(III) ion; in the solid state, Tb2(C2O4)(O2CCH2OH)4, 2, exhibits very strong luminescence.

The “node-and-spacer” synthetic approach proposed by Wellsand O’Keeffe for controlling the topology of complex bondingnetworks is now widely used in crystal engineering because it isan effective synthetic approach to new networks that may serve as“templates” for new functional materials.1,2 The simplest metal-organic coordination networks consist of unitary nets with one typeof node, a node whose topology is determined by its localgeometry.3 Based on the consideration of the geometry of thesenodes, Zaworotko has used various vertex-linked polygons and/orpolyhedra, viz. the VLPP method, to delineate their well-definedmetal-organic frameworks.4

As indicated by Zaworotko, this approach is most useful forbinary nets containing pairs of polygonal or polyhedral molecularbuilding blocks; even the simplest combinations of these buildingblocks can yield a broad structural diversity as is evidenced by(3,4)-connected nets.5 These nets include alternate connections (a)between trigonal and square nodes that will generate two differentnets, namely, the Pt3O4 and twisted boracite nets, (b) betweentrigonal and tetrahedral nodes that will generate two additional nets,namely, the boracite and C3N4 nets, and (c) between trigonal andsquare nodes with, in part, directly connected square nodes, namely,the “so-called” tfz net.3,5 Zaworotko and his co-workers have alsodefined several ternary nets, more specifically both the USF-3,4,5net which contains trigonal, square, and tetrahedral nodes, and anunusual 3,24-connected net.6 Recently, by using the above VLPPapproach, we have described both another ternary net that is alsocomprised of trigonal, square, and tetrahedral nodes, and two 3,8-connected nets that are based upon the trigonal and bisdisphenoidnodes.7 Herein, the VLPP approach is employed to synthesize anunprecedented 4,6-connected net, see Figure 1, that is based onthe highly unusual combination of rectangular and distortedhexagonal lanthanide nodes.

Polymers 1 and 2 were synthesized by the hydrothermal self-assembly of Gd2O3, or Tb3O4, and HCO2CH2OH, see Scheme 1(Supporting Information). In this synthesis, CuSO4 is required andserves as a possible catalyst for the in situ formation of C2O4

2-

from HCO2CH2OH; other copper(II) salts, such as Cu(NO3)2, CuCl2,and Cu(ClO4)2, may also serve as the copper source. Suchhydrothermal preparation is not very sensitive to reaction temper-ature, and 1-2 can be obtained between 160 and 200 °C. Onceformed, single crystals of 1-2 are air-stable and insoluble incommon solvents such as water, methanol, ethanol, acetonitrile,and DMF.

Single crystal X-ray diffraction results indicate that 1-2 areisomorphous and crystallize with the tetragonal P4(2)/nbc spacegroup; herein, only the structure of 1 is discussed in detail. In 1,there are two unique Gd(III) ions, both of which have a bicappedtrigonal-prismatic coordination environment of eight oxygens. Forthe Gd1 site this environment consists of eight CO2CH2OH-

oxygens from both carboxyl and hydroxyl groups, whereas for theGd2 site it consists of four CO2CH2OH- carboxylate oxygens andfour C2O4

2- oxygens. The Gd-O bond lengths vary from 2.2819(17)to 2.4818(15) Å, distances that are comparable with those observedin related Gd(III)-containing compounds.8 The CO2CH2OH- andC2O4

2- ligands have µ2:η2:η1 and µ2:η2:η2 coordination modes,respectively (see Scheme 1).

If the C2O42- ligands are not considered, then the combination

of Gd1, Gd2, and the CO2CH2OH- ligands would yield a PtS matrixin which Gd1 and Gd2 act as rectangular and tetrahedral nodes,respectively, nodes that are alternately linked. Then, as shown inFigure 1 and Figure S2, by including the C2O4

2- ligands, the Gd2tetrahedral nodes in the PtS net are linked together by the linearC2O4

2- spacers, producing a 4,6-connected net with (32 · 62 · 72)(34 ·42 ·64 ·75) topology.9 Concurrently, the tetrahedral nodes expandto be 6-connected nodes, an expansion that is best understood inthe following fashion. First, the Gd2 node connects to two additionalGd2 nodes and four Gd1 nodes; the arrangement of the Gd2 nodewith one Gd2 node and two Gd1 nodes yields a rectangular planerather than a tetrahedron and, thus, the Gd2 node is not anoctahedral, trigonal prismatic, or metaprismatic node, even though6-connected nodes usually exhibit an octahedral geometry.3 Second,the two planes formed by the above-mentioned arrangement arenot coplanar, but rather have a dihedral angle of 90° and, thus, aregular hexagonal geometry is excluded. Because of the above-mentioned 90° dihedral angle, we consider the Gd1 node to be adistorted hexagonal node. Finally, the two identical 4,6-connectednets interpenetrate to produce a dense packed structure (Figure 2).

A number of transition-metal based 4-connected square planarand/or tetrahedral nodes have been reported.10 In contrast, becausetrivalent rare-earths are seldom considered to be suitable as four-connected nodes, only a few rare-earth centered 4-connected nodeshave been reported to date.11 A close inspection of these examplesindicates that they are all based on a tetrahedral rare-earth centerednode; to our knowledge there is no report of a square planar orrectangular node. Furthermore, it seems that rare-earth basedframeworks with hexagonal or distorted hexagonal nodes have neverbeen reported.

As has been indicated by O’Keeffe, the arrangement of regulartetrahedral or regular octahedral coordination in a 4,6-connectedstructure is impossible; the best and most often encountered

* To whom correspondence should be addressed. E-mail: jmzheng@nankai.edu.cn.

† Nankai University.‡ University of Missouri-Rolla.

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10.1021/cg800640u CCC: $40.75 2008 American Chemical SocietyPublished on Web 08/28/2008

compromises are the Al2O3 net, and the expanded Fe2(SO4)3

structure.3,5a Other particularly interesting 4,6-connected nets arethe augmented Al2O3 and Co3[Re6Se8(CN)6]2 nets with square planar4-connected nodes.3,5a The vertex-linked approach indicates thatthere are 32 possible 4,6-conneced nets, such as the fsg, iac, ifi,nets that are based on 4,6-connected square planar, rectangular,tetrahedral, hexagonal, trigonal prismatic, and octahedral nodes.12

Of these 32 nets, only 12 have been found in inorganic compoundsor metal-organic frameworks; the remaining 20 may eventuallybe discovered. Because the 4,6-connected net reported herein for

1-2 contains very unusual and unexpected distorted hexagonalnodes it is not one of the above 32 possibilities, and we believe itis unique and unprecedented. Moreover, the survey on the EPINETdatabase suggests that this is the first occurrence of sqc502 in crystalstructures.13

Thermogravimetric studies show that there is no weight lossbelow 315 °C, its decomposition temperature of 1 (SupportingInformation, Figure S2).

Due to the luminescent properties of Tb(III), the solid-stateluminescence of 2 has been studied at room temperature byexcitation at 330 nm (Figure 3). The observed emission bands of3, which are characteristic of Tb(III), are the 5D4f

7F6 band at 490nm, the 5D4f

7F5 band at 544 nm, the 5D4f7F4 band at 584 nm,

and the 5D4f7F3 band at 620 nm; the most intense transition is at

544 nm, which corresponds to a green emission. In addition, theselection of an excitation wavelength of 330 nm is based on the

Figure 1. VLPP description of PtS net and the expanded 4,6-connected net, as well as their nodes and connectivity.

Scheme 1. Schematic Description of the in Situ Generated H2C2O4 from HCO2CH2OH and the Coordinated Mode of C2O42-

and CO2CH2OH

Figure 2. Schematic representation of the 2-fold interpenetrating 4,6-connected net.

Figure 3. Luminescence spectra of compound 2 in the solid state atroom temperature.

3512 Crystal Growth & Design, Vol. 8, No. 10, 2008 Communications

most intense transition, a wavelength that is also comparable withthose observed in various Tb(III)-containing compounds.8

Taking into account the structural features of polymers 1-2,further inspection of the magnetic behavior is worthwhile, as it isbelieved that the equilateral triangle of three antiferromagneticallycoupled spins is inherently frustrated.14 As illustrated in S3, themost encountered frustrating lattice is kgm, and the hca and kgd-like lattice are relatively rare.15 Similarly, polymers 1-2 alsocontain the approximate equilateral triangle of three magnetic ionsfor which magnetic exchange coupling could potentially lead tospin-frustration. Thus, because of both its unusual structuralproperties and the possibility of magnetic spin-frustration, themagnetic properties of 1 have been measured upon cooling from290 to 2 K in a 0.1 T applied field (Figure 4). The inverse molarmagnetic susceptibility of 1 is perfectly linear and thus 1 isseemingly paramagnetic from 2 to 290 K. The inverse molarmagnetic susceptibilty yields a very small Weiss temperature of-0.32(3) K, a Curie constant of 7.879(5) emu K/mol Gd, and acorresponding µeff of 7.938(5) µB per Gd, exactly the effectivemagnetic moment expected of a paramagnetic compound containingisolated Gd(III) ions. It seems most likely that there is virtually noGd(III)-Gd(III) magnetic exchange interaction in 1 because theGd(III) ions are separated by at least 6.4 Å. It is well-known thatmagnetic frustration can suppress or significantly reduce long-rangeordering. The degree of frustration can be estimated via the ratioof Weiss temperature to the ordering temperature, viz. f ) |θ|/TN.16

Typically, a non-frustrated lattice exhibits an f value in the range2-5, whereas a frustrated lattice exhibits much larger values. For1, the small Weiss temperature and the absence of long-rangeordering down to 2 K cannot give the direct evidence for theabsence or presence of spin frustration below 2 K.

In summary, herein we report a unique and unprecedented 4,6-connected net that contains highly rare rectangular, distorted,hexagonal rare-earths based nodes. In addition, the in situ formedC2O4

2- ligands from HCO2CH2OH with possible Cu(II) catalysis,and the strong luminescence properties of 2, as well as a possiblyspin-frustration lattice that exhibits perfect paramagnetic behaviorof 1 are also notable. We believe that the present work has bothdemonstrated a new example of a 4,6-connected net and provides

further incentive for the exploration of HCO2CH2OH-based coor-dination chemistry.

Acknowledgment. This work was supported by the NationalNatural Science Foundation of China (50572040).

Supporting Information Available: Crystal data in CIF format;synthesis and crystal data of 1-2; view of the tetrahedral La nodes inPtS net connected by C2O4

2- linkers; TG plot of 1; schematicdescription of the former observed spin-frustration lattice and the presentwork. This material is available free of charge via the Internet at http://pubs.acs.org.

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CG800640U

Figure 4. The molar magnetic susceptibility of 1; the solid points arethe observed values; the line corresponds to the values expected for aparamagnetic complex with µeff ) 7.938 µB. Inset: The inverse molarmagnetic susceptibility of 1; the solid line is a Curie-Weiss law bestfit.

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