Inorganic−Organic Hybrid Coordination Polymers: A New Frontier for

Inorganic-Organic Hybrid Coordination Polymers: A New Frontier for Materials Research Maochun Hong* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 1 10-14

ReceiVed NoVember 13, 2006

ABSTRACT: This paper presents a short review of our recent advances in the design and syntheses of inorganic-organic hybrid functional materials and nanometer-scale metallosupramolecules. Perspectives in these two fields are provided. Introduction Design and construction of inorganic-organic hybrid materials with tunable physical properties have attracted extensive interest in materials chemistry.1-3 The study in this field has provided numerous examples of rationally designed onedimensional (1D), two-dimensional (2D), and three-dimensional (3D) coordination polymers possessing interesting structural motifs and significant properties through the assembly of molecular building units.4-6 The number of papers that feature the terms “coordination polymer”, “coordination frameworks”, “metal-organic frameworks”, etc., has increased dramatically over the past few years. Therefore, the design and construction of coordination polymers on the basis of the different topology are a great challenge for material chemists. Recently, we have carried out studies on such types of polymers through the assembly of tailored ligands and transition metal and/or lanthanide ions under different reaction conditions. This paper presents a short review of our recent advances in this field, and perspectives are provided. Inorganic-Organic Hybrid Functional Materials Recently, a great deal of work has been devoted to inorganicorganic hybrid framework assemblies such as multilayer perovskites. We have demonstrated that metal-thiolate polymers with graphite-like arrays of metal atoms can be formed that possess semiconducting properties. Different from those of other metal-thiolate polymers, the semiconducting properties observed from the above-mentioned metal-thiolate polymers result from their M6 hexagonal arrays and metal-metal interactions. For instance, the [Ag(SPy)]n we prepared has a 2D lamellar structure, wherein the silver atoms are linked by thiolate ligands to form inorganic layers with organic groups protruding into the interlayer region (Figure 1).7 All silver atoms of each layer are nearly coplanar and arranged to form a graphite-like hexagonal motif. Conductivity measurements show that the compound is a semiconductor, which provides the evidence for interaction or bonding between the formally closed-shell d10 cations in extended solid structure. Because hexagonal structural units of MoS2 can be used to construct nested inorganic fullerene and nanotubes, a goal of our research is to assemble analogue aggregates by using M6 hexagons as building blocks. Chiral coordination polymers can be prepared by either enantioselective synthesis using enantiopure chiral species, which yields enantiopure samples, or spontaneous resolution * To whom correspondence should be addressed. E-mail: hmc@ fjirsm.ac.cn. Tel: 86-591-83792460. Fax: +86 591/8379 4946.

Professor Maochun Hong graduated from Fuzhou University in 1978 and received his M.S. degree from the Fujian Institute of Research on the Structure of Matter (FJIRSM), Chinese Academy of Sciences (CAS), in 1981. He worked as a research assistant at FJIRSM, CAS, until 1984. In 1985−1987, he was a research fellow at the Department of Chemistry of the University of Michigan and the University of Illinois at Chicago in the United States. He was a research associate professor at FJIRSM in 1988−1991. In 1992−1993, he worked as a visiting scientist at the Department of Chemistry of Newcastle University, United Kingdom. Since 1994, he has been a Full Professor of Chemistry at the FJIRSM, CAS. During this period, he worked as a JSPS visiting professor in Japan in 1998 and received his Ph. D degree from Nagoya University. He was promoted to the Executive Director of the FJIRSM in 2000 and to the Director of the FJIRSM, CAS, in 2002. He is a member of the Chinese Academy of Sciences. His research interests are focused on nanomaterials, crystal design and growth, supramolecular chemistry, and inorganic−organic hybrid functional materials.

upon crystallization without any enantiopure chiral auxiliary.8,9 Research in our laboratory has led to an interest in the design and construction of helical metal-organic assemblies and chiral coordination polymers on the basis of helical topology.10,11 Concerning that a helix is one of the most attractive and evocative expressions of chirality, our interest in chiral coordination polymers is focused on the preparation of helices with homochirality. Thus, both construction of chiral coordination polymers combining asymmetric catalysis, enantioselective separation, and optical activity and an understanding of how to induce homochiral packing of helices in crystals have been systematically investigated in our group. The introduction of chemical interaction sites in asymmetric bridging ligands is a challenging subject in the development of new polar and chiral systems.12 However, the helical and other recurring coordination networks used in NLO materials still remain relatively unexplored. Toward the goal of designing NLO

10.1021/cg068015h CCC: $37.00 © 2007 American Chemical Society Published on Web 12/13/2006

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Figure 3. Ln24O24 charged cage with two trapped [Cu(bpy)2]+ guests. Figure 1. Two-dimensional lamellar graphite-like motif structure.

Figure 2. (a) View of the two types of homochiral helices in [Zn(spcp)(OH)]n and (b) a schematic showing the regular structure.

materials with new noncentrosymmetric MOFs containing chemical interaction sites, we are currently taking an approach of self-assembly reaction between metal ions and sulfurcontaining asymmetrical linking ligands, where sulfide moieties are well-known as redox active functional groups that could enhance electronic asymmetry. By reacting 4-sulfanylmethyl4′-phenylcarboxylate pyridine (Hspcp) and Zn(NO3)2 in the presence of triethylamine, we obtained a novel 2-D tubular coordination polymer [Zn(spcp)(OH)]n complex. A striking structural feature of this polymer is the alternating assembly of the two distinct homochiral helices (Figure 2). One helix is formed by hydroxo bridging Zn(II) atoms, and the other is constructed by an spcp bridge between the Zn(II) centers, displaying an opposite helical orientation to the former helix. These two distinct homochiral helices are in an orderly arrangement, with the zinc atoms functioning as hinges. Although the bulk product is racemic because of the complex obtained by spontaneous resolution from achiral components without any chiral sources, each crystal has a chirality that could be applicable for functional materials. Second harmonic generation experimental results show that the complex displays a modest powder SHG efficiency that is approximately 5 times higher than that of technologically useful potassium dihydrogen phosphate (KDP), and the complex represents the first NLOactive bulk solid based on a two-dimensional tubular coordination polymer with alternating assembly by two types of homochiral helices with sulfide sites. The stability of the complex makes it a potential candidate for practical applications. Heterometallic lanthanide-transition metal compounds have the nature of a magnetic exchange interaction between the 3d and 4f metal ions of magnetic materials. Recently, we have begun to explore the syntheses and characterizations of Ln-M coordination polymers by using ligands with mixed N- and O-donor atoms. Systematic studies have been carried out on the assembly of tailored ligands with both lanthanide and

transition metal ions under different reaction conditions. By employing a hydrothermal reaction method, we successfully isolated a series of lanthanide(III)-transition metal coordination polymers with fascinating structures and unusual properties in our laboratory.13,14 By assembling lanthanide and transition metal ions with N-donor atoms of the 2,5-pydc ligand, we prepared a series of Ln(III) and Cu(II) coordination polymers with 2D wavelike structure [{Ln4Cu2(2,5-pydc)8(H2O)12}‚4H2O]n (Ln ) Sm, Gd, Er).14 The 2D wavelike units can also be viewed as being Sm-Cu ladders, in which the rungs are formed by [Cu(2,5pydc)2] species and the side pieces by Sm(III) chains. The side pieces of the neighboring ladders are linked by pydc via Sm-O bonds to yield the wavelike units. If the oxygen-donor and nitrogen-donor ligands are introduced separately in reactions with lanthanide and transition metal ions, coordination polymers with more diverse structures are expected to form. Thus, by the hydrothermal reaction of lanthanide and transition metal ions with mixed ligands of isophthalate (ip) and 2,2′-bipyridine (bpy) in a different molar ratio, a series of compounds [{Ln4(ip)7(H2O)2}{Cu(I)(bpy)2}2]n (Ln ) Sm, Gd, Nd, Er) consisting of charged cages containing two encapsulated [Cu(bpy)2]+ cations were prepared (Figure 3).14 In these structures, eight Ln2O2 units are linked by ip ligands to generate a large Ln24O24 charged cage (ca 11.5 × 14.9 × 16.5 Å3), in which two [Cu(bpy)2]+ cations in a distorted tetrahedral geometry are trapped as chargecompensating guests. Unusually, two trapped guests (about 11.0 Å diameters) adopt the encapsulated modes because they are larger than the largest effective pore of cage (ca 9.78 Å diameter). The cationic guests are further stabilized by π-π stacking interactions and van der Waals forces. The inclusion cages are connected by ip ligands to form a 3D cavity framework. The compound represents the first 3D framework containing multiencapsulated complex cations within a charged cage, which is different from other inclusion complexes. In an attempt to synthesize a Gd(III)-Cu(II) analogue by using a relatively small amount of bpy, we successfully yielded [{Ln3(ip)5(Hip)}{Cu(II)(bpy)}]n (Ln ) Gd, Nd, Er) in a similar reaction.14 As shown in Figure 4, ip ligands link Ln(III) and Cu(II) ions to form a 3D open framework containing irregular cavities (ca 11.4 × 8.10 Å2). Two [Cu(bpy)]2+ cations are alternately bonded to the inner backbone of the Ln-ip cavity. The magnetic studies imply the presence of global ferromagnetic-antiferromagnetic interactions. The result indicates that the Gd(III)-Gd(III) interaction is weakly antiferromagnetic, whereas the Gd(III)-Cu(II) interaction is ferromagnetic. The weak ferromagnetic Gd-Cu interactions are observed in

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Figure 6. View of 1D chain structure in [Ag7(tpst)4(ClO4)2(NO3)5]n. Figure 4. Open framework containing cavities with [Cu(bpy)]2+ cations.

Figure 5. Nanometer-sized cage [M6(tpst)8Cl12] with Oh symmetry.

[{Gd3(ip)5(Hip)}{Cu(II)(bpy)}]n, which include two spincoupled Gd-Cu dimers. Nanometer-Scale Metallosupramolecules Supramolecular chemistry is an emerging interdisciplinary field of science covering the chemical, physical, and biological features of complex chemical species held together and organized by means of intermolecular binding interactions. We have combined the power of molecular self-assembly and suprainteractions and developed several methods to prepare nanometerscale metallosupramolecules, including nanowire, nanocages, nanotube, macrocycles, and nanoclusters, by using selforganization and self-assembly techniques. The properties of the nanometer-scale metallosupramolecules in some scientifically interesting and commercially attractive reactions have been extensively explored. Modification of the organic ligands through the adjustment of their electronic and steric properties to make them more effective for specific reactions has been carried out. We designed a multidentate ligand, 2,4,6-tri[(4-pyridyl)sulfanylmethyl]-1,3,5-triazine (tpst), which possesses exo-tridentate bonding sites. By using metal ions with square planar or octahedral coordination geometries, such as Pt(II), Pd(II), or Ni(II), to link the three pyridyl groups of tpst, we may expect the nanometer-sized supramolecular cages. In fact, the [M6(tpst)8Cl12] we obtained is a supramolecular cube with a nanometersized inner cage and Oh symmetry (Figure 5).15 The volume of the inner cavity is ca. 1000 Å3, implying that the compound can host many solvent molecules. Furthermore, the other interesting feature of this structure is its flexible windows. Because the sulfanylmethyl spacer groups result in the flexibility of the ligand, the cube opens 12 large windows. 1H NMR studies of solvent exchange for this compound indicate that DMF molecules located in each cage can be exchanged by other solvent molecules through the windows.

Examining the symmetry and stereochemistry of the tpst ligand, the coordination polymers with nanometer-sized tubes of different lengths are expected, if the metal ions possessing linear coordination geometries, such as Au(I), Ag(I), and Cu(I), are provided. Thus, the reaction of AgNO3 with tpst gave rise to a single stranded 1D coordination polymer [Ag7(tpst)4(ClO4)2(NO3)5]n containing nanometer-sized tubes.16 The basic nanometer-sized tube unit is built from two [Ag3(tpst)2] nanocycles and accommodates two DMF molecules and two perchlorate anions. The tube units share silver atom to form a 1D chain nanostructure, as shown in Figure 6. The water-soluble p-sulfonatocalix[4]arene possesses a cone cavity that can act as a host for small substrate molecules of relevance in materials science, such as separation technology, biomimetics, and structural biology.17 Furthermore, as a multifunctional supramolecular synthon, p-sulfonatocalix[4]arene can form capsules, nanoscale spheres, tubes, and bilayer structures with cavities (Figure 7).18 Recently, we have begun to explore p-sulfonatothiacalix[4]arene (H4TCAS4-) as a polydentate ligand to synthesize polynuclear clusters and expected to link the clusters into novel nanostructures with special properties. Employing some kinds of aromatics, such as 1,2bis(imidazol-1′-yl)ethane (BIME) or 2,2′-bipyridine (BIPY), as guest molecules to induce molecular capsule formation, we obtained a series of new molecular capsules or chain nanostructures based on water-soluble p-sulfonatothiacalix[4]arenas.19 For instance, several new compact molecular capsules of p-sulfonatothiacalix[4]arene with bis(imidazol-1′-yl)ethane or 2,2′-bipyridine guests were isolated. These supramolecular compounds possess the open or closed capsule conformation in a chain array, and their guest molecules in both types of compounds are well-accommodated by p-sulfonatothiacalix[4]arene capsules via a variety of supramolecular interactions (Figure 8). Polyoxovanadate clusters are some of the most interesting members of the polyoxometalate family. Most polyoxovanadates have been prepared in aqueous media or under hydrothermal conditions. In our research work in this field, we have recently developed new synthetic routes and prepared a series of polyoxovanadates clusters in organic media (Figure 9). The first tetradecaoxovanadate compound, [Et4N]5[V14O36Cl], was synthesized from organic solution and is air-stable both in the solid state and in solution at room temperature.20 The above new mixed-valence vanadium cluster has a novel structure and unique properties compared with its congeners prepared by conventional methods. It has an unprecedented half-open basket framework, and the {V14O36} clusters display the {ABAB...} type of closely packed layers that form hexagonal close packing (Figure 10). The six clusters run along the 3-fold axis to form infinite onedimensional channels with considerable accessible volume in

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Figure 7. Bilayer structures, nanoscale spheres, and tubes.

Figure 10. Hexagonal close packing of {V14O36} cluster units. Figure 8. View of capsules via a variety of supramolecular interactions.

Figure 9. Polyoxovanadates clusters isolated from organic media.

the crystalline state. Upon photoexcitation at 302 nm, its aqueous solution emits an intense blue (422 nm) light, which has rarely been reported for vanadium oxide clusters. Conclusion and Perspective In the ongoing research toward the design and development of nanosupramolecular materials and inorganic-organic hybrid coordination polymers, some potential applications, such as asymmetric catalysis, enantioselective separation, and optical activity, have been investigated.15 A continuing challenge may be to form a coordination polymer film at the appropriate interface. Monolayer and Langmuir-Blodgett (LB) films are

sophisticated supramolecular systems, in which molecules can be orderly arranged. Besides, on the supramolecular level, the spontaneous formation of chiral aggregate from achiral molecules is still an important research project and has received much attention recently. The knowledge of the chemistry of well-defined coordination polymers is necessary for the understanding of the detection and amplification of chirality. Increasing interest will be drawn to the dynamic helical coordination polymers, the most important feature of these being high sensitivity to a chiral environment; therefore, such systems might provide the basis to construct a novel chirality-sensing probe. Moreover, the rational design of chiral networks can be possible by using helical coordination polymers combined with functional groups as the pendants, which target particular chiral hostguest molecules for developing a highly efficient chiralitysensing system. It is a scientific challenge to develop bistable systems (switches) and construct the necessary molecular architectures and “molecular machines” capable of servicing such systems. Our approach allows the construction of extended structures on the basis of coordination bonds between building blocks and metal ions to direct the assembly of complex molecular architectures and nanostructures. The combination of supramolecules with nanotechnology is a new frontier and is expected to give interesting results. The hollow tube can act as a nanoscale test tube for doing chemistry or a mold for making nanorods of other materials. We will demonstrate that the internal volume of nanotubes can be filled with a wide variety of materials using solutions of their precursors, hence opening new avenues in nanomaterial research for magnetic and electronic applications. These studies could open promising lines, allowing for development of nanodevices.

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