DOI: 10.1021/cg100090h
An Unprecedented Eight-Connected Self-Penetrating Coordination Framework Based on Cage-Shaped [Pb6(μ4-O)2(O2C)8] Clusters
2010, Vol. 10 2037–2040
Dong-Sheng Li,*,†,‡ Ya-Pan Wu,†,‡ Peng Zhang,† Miao Du,*,§ Jun Zhao,† Cheng-Peng Li,§ and Yao-Yu Wang‡ †
College of Mechanical & Material Engineering, Functional Materials Research Institute, China Three Gorges University, Yichang 443002, China, ‡College of Chemistry and Life Science, Tianjin Key Laboratory of Structure and Performance for Functional Molecule, Tianjin Normal University, Tianjin 300387, China, and §Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, Department of Chemistry, Northwest University, Xi’an 710069, China Received January 21, 2010; Revised Manuscript Received March 18, 2010
ABSTRACT: A unique self-penetrating coordination framework with (424.64) network topology is constructed from cage-shaped [Pb6(μ4-O)2(O2C)8] clusters as eight-connected nodes and long V-shaped dicarboxylate backbones as spacers, which interestingly can be regarded as the cross-linking of two interpenetrating six-connected pcu networks.
*Corresponding authors. (D.-S.L.) E-mail:
[email protected]. Tel./ Fax: þ86-717-6397516. (M.D.) E-mail:
[email protected]. Tel./Fax: þ86-22-23766556.
conversion, and fluorescence.8 Therefore, it is of great significance to further explore new PbII coordination polymers and develop their potential applications. On the other hand, long V-shaped aromatic polycarboxylate ligands can serve as excellent candidates for building highly connected, self-penetrating, or helical coordination frameworks due to their bent backbones and versatile bridging fashions.9 Taking these points into account, by using Pb(OAc)2 as the metal source and a long V-shaped dicarboxyl ligand, 4,40 -(hexafluoroisopropylidene) bis-(benzoic acid) (H2L) as the connectors, we have fortunately isolated an unprecedented coordination framework [Pb6(μ4-O)2(L)4]n (1) with cage-shaped [Pb6(μ4-O)2(O2C)8] clusters as nodes. As we know, this compound not only has a new 8-connected network topology but also represents the first selfpenetrating example of the 8-connected framework based on PbII-oxo-clusters. Hydrothermal reaction of Pb(OAc)2 3 3H2O with H2L and KOH in CH3OH/H2O at 140 °C for 3 days generates a colorless microcrystalline product [Pb6(μ4-O)2(L)4]n (1).10 The as-synthesized compound was confirmed by elemental analysis, IR, and single crystal X-ray diffraction,11 and the phase purity of the bulk sample was identified by powder X-ray diffraction (PXRD) (see Figure S1, Supporting Information). The asymmetric unit of 1 contains two L2- ligands, one μ4-O2- anion, and three crystallographically independent PbII centers (see Figure 1). Interestingly, the structure of 1 exhibits a three-dimensional (3D) network based on cage-shaped hexa-nuclear PbII-oxo-clusters. The cluster contains a centrosymmetric [PbII6( μ4-O2-)2]8þ octahedral core (see Figure 2a), in which six PbII ions inhabiting the apexes are linked by two equivalent O2- anions with μ4-η4 tetrahedral bridging mode (see Figure 2b). As for the Pb6 octahedron herein, the structural parameters such as Pb 3 3 3 Pb distances, the mean deviations of the planes of PbII, and their dihedral angles (see Table S1, Supporting Information), are similar to those observed for the discrete Pb6 octahedron [Pb6(μ4-O)2(OOCCHdCHFc)8] reported by Hou et al.8b A typical difference is that the PbII ions of 1 have relatively higher coordination numbers that may originate from the coordination features of L2-. For the three crystallographic independent PbII centers of 1, Pb1 and Pb2 are hexa-coordinated by a μ4-O2- anion, five O atoms from four different L2- ligands, resulting in a distorted PbO6 pentagonal-bipyramidal coordination geometry (see Figure S2a, Supporting Information).7b The carboxylate oxygen atoms (O1, O3, O5, O6, and O7 for Pb1; O2A, O3, O4, O5, and
r 2010 American Chemical Society
Published on Web 04/05/2010
The design and synthesis of highly connected coordination frameworks represent a quite active area, not only for their potential applications as functional materials but also for their undisputed beauty, often with complicated architectures and topologies.1 However, owing to the limited coordination sites around metal centers and the steric hindrance of organic ligands, it is difficult to rationally construct such coordination frameworks via a conventional synthetic method. Currently, there are two potential routes to obtain highly connected coordination networks. The first approach is to use lanthanides with high coordination numbers and organic linkers with small steric hindrance. In this regard, Long and Champness et al. have reported a series of highly connected lanthanide coordination frameworks with a flexible 4,40 -bipyridine-N,N0 -dioxide ligand.2 The second approach, probably more general in practice, is to employ metal clusters as secondary building units (SBUs), and some representative examples such as Co43a and Zn113b-based 8-connected bcu networks and a Cd5-based 10-connected γ-Pu topology3c have been reported. Significantly, polynuclear cluster SBUs usually possess more outward-connecting sites and larger surface areas, which can readily meet the steric requirement of the organic linkers to afford the resulting highly connected nets. In this context, another theme of interest is highly connected frameworks with network entanglement such as self-penetration or self-catenation.4 In spite of the great advances in crystal engineering, only a limited number of self-penetrating architectures with highly connected topologies have been known so far,5 and their design and synthesis is still a formidable challenge. Undoubtedly, it is necessary and useful to build more structural paradigms for further demonstrating the nature of such supramolecular systems. As a heavy p-block metal ion, PbII ion, with a lone-pair electron and large ionic radius, can adopt a flexible coordination environment and variable stereochemical activity.6 As a result, it will provide a good opportunity for the construction of unusual coordination frameworks with suitable organic tectons. For example, some highly connected networks such as 8-connected bcu net based on Pb8O8 clusters7a and (4,8)-connected CaF2 net with Pb4O18 subunits7b have been achieved. Moreover, such crystalline systems with structural diversity also exhibit some important properties of electroluminescence, photovoltaic
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Figure 1. Coordination environment of PbII ions in 1 (symmetry code: A -x þ 2, -y þ 2, -z þ 1; all hydrogen atoms omitted for clarity).
Figure 2. Views showing (a) a centrosymmetric [PbII6(μ4-O2-)2]8þ octahedral core and (b) the cage-shaped [Pb6(μ4-O)2(O2C)8] cluster.
O8A for Pb2) make up the basal plane, while the axial positions are occupied by one μ4-O2- anion (O9) and the lone pair of electrons (see Figure S2b, Supporting Information). Also, Pb3 takes a distorted:PbO6 pentagonal-bipyramidal sphere (see Figure S2c, Supporting Information), in which four O atoms (O1, O2, O6A, and O8A) from three different L2- ligands and one μ4-O2- anion (O9) complete the basal plane, and the other μ4-O2anion (O9A) and the lone pair of electrons are located in the axial sites. In this case, all the PbII centers are in the hemidirected geometry, which indicates the electron lone-pairs of PbII are stereochemically active. The Pb-O distances for [Pb4(μ4-O2-)]6þ are in the range of 2.259(9)-2.366(9) A˚, which are significantly shorter than those of Pb-Ocarboxylate (2.4232(10)-2.706(10) A˚, see Table S1, Supporting Information for details). All these structural parameters are comparable to those found in the compounds containing [Pb4(μ4-O2)]4þ units.7a,8b,8c It is interesting to note that two crystallographic independent H2L ligands in 1 display different coordination modes [μ6η2:η2:η2:η1 (A) and μ5-η2:η2:η2:η1 (B)] (see Figure S3a, Supporting Information). First, eight L2- ligands, with three kinds of bridging carboxylate groups of μ3-η2:η2 (2C(1)OO-, 2C(18)OO-), μ3-η2:η1(2C(28)OO-), and μ2-ηη2:η1(2C(11)OO-) (see Figure S3b,
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Figure 3. A description for the evolution of the 2-fold interpenetrating layers (d) from the twisting (4,4) network (c) with alternant helical chains (b) constructed by the {Pb6} SBUs and μ6-L2- ligands (a). L and R indicate the left- and right-handed helical chains, respectively.
Supporting Information), link the PbII centers in [PbII6(μ4O2-)2]8þ core to result in a cage-shaped hexa-nuclear PbII cluster (see Figure 2b). Second, the V-shaped backbones of the μ6-L2ligands (A) extend such {Pb6} SBUs to form a twisting (4,4) layered network along the (010) plane with alternant helical chains by sharing the {Pb6} SBUs (see Figure 3a), in which the center-to-center distances between the adjacent {Pb6} SBUs range from 15.97 to 23.62 A˚. Remarkably, owing to the presence of enough space, two such (4,4) nets are interweaved into a 2-fold interpenetrating array (see Figure 3b). Furthermore, other μ5-L2ligands (B) alternately connect the {Pb6} SBUs of the neighboring 2-fold interpenetrating arrays to yield right-hand helical chains along the b axis (see Figure 4a), with the center-to-center distance between the adjacent {Pb6} SBUs of 16.60 A˚. In this way, these adjoining 2-fold interpenetrating (4, 4) arrays are further extended by such helical pillars into a 3D coordination framework (see Figure 4c), defining a uninodal 8-connected net of (424.64) topology (named net B) with self-penetrating feature (see Figures 4c and 5). Significantly, although this net has the same Schl€ afli symbol with the familiar 8-connected body-centered cubic (bcu) lattice (or CsCl lattice, see Figure 5a) and an eight-connected selfpenetrating net based on the heterometallic {Cu4V4O12} clusters (named net A, see Figure 5b),12a they actually show different network topologies with the long vertex symbols of (4.4.4.4. 4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.66.66.618.618) for 1, (4.4.4. 4.4.4.4.4.4.4.4.4.43.43.43. 43.43.43.43.43.43.43.43.43.*.*.*.*) for bcu, and (4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.830. 8180. *.*) for net A,12a respectively. Apparently, the net B of 1 has six-membered rings and it represents a new 8-connected network topology. On the other hand, the main difference between 1 and bcu is that the single (4,4) sheets (parallel to each other) in bcu (see Figure 5a) are replaced by a set of 2-fold parallel interpenetrating arrays consisting of two twisting (4,4) layers for each (see Figure 5e), which should originate from the bent geometry of the dicarboxylate building block. Thus, it is obvious that the resulting 3-D extended architecture is a self-penetrating network (see Figure 5e), which can also be considered as the cross-linking of two interpenetrating 6-connected pcu nets (see Figure 5f).
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Figure 5. Views showing (a) the bcu net, (b) the self-penetrating (424.64) net A, (c) 8-connecting cage-shaped [Pb6(μ4-O)2(O2C)8] cluster, (d) the topology of eight-connected {Pb6} SBU represented as an eight-connected node (teal), (e) the self-penetrating (424.64) net B and (f) the topological structure of 1 can be viewed as two identical interpenetrating pcu-type nets (yellow and teal) cross-linked by L2connectors (gray). Figure 4. Views showing the 3D pillared coordination framework of 1 (c) constructed by the 2-fold interpenetrating layered networks (b) and the right-hand helical pillars (a). L and R indicate the leftand right-handed helical channel, respectively.
Until now, only several 8-connected self-penetrating coordination nets have been known: (424.5.63) net based on [Zn5(μ3-OH)2(COO)8]12b and [La2Cu(COO)8],12c (420.68) nets based on [Cd3(COO)6(N-donor)2]12d and [Ni2(μ3-H2O)2(COO)4(Ndonor)4]12e (with the same total Schl€afli symbol but different long vertex symbols), a (424.64) net based on [Cu4V4O12(Ndonor)] (net A),12a a (412.56.67.72.8) net based on [Co4(COO)8(N-donor)],12f a (416.58.64) net based on [Co3(COO)6(Ndonor)2],12g and a (416.612) net based on [Mn4(N3)8(COO)4]4-.12h All these networks are based on different d/f-block metallic cluster nodes and the familiar carboxylate and/or N-donor linkers. To aid understanding of this intricate structure, a new approach used to analyze the highly connected frameworks proposed by Champness et al.2c is applied. Namely, visualization of the structures as combinations of interconnected 2D sheets or subnet tectons, instead of considering the 3D structures solely in terms of topology via Schl€afli symbols. In this regard, as for the bcu net, the zigzag chain bridges across the diagonal of a single [2 2] window of the (4,4) layers and thus no self-catenation occurs. For the other three self-penetrating nets (424.5.63), (420.68), and 424.64 (net A), self-penetration derives from the unusual linking modes that the zigzag chain bridges across the diagonal of two, six, or eight [2 2] windows of the (4,4) layers, respectively.12a-e Different from these nets, self-penetration of the (412.56.67.72.8) net or (416.58.64) and (416.612) nets results from a cross-linked R-Po subnets or the (4,4) layers linked by two set of inclined interpenetrating 2D nets.12f-h However, the self-catenation in net B is completely different. As observed in the bcu net, the zigzag chain bridges across the nodes of the (4,4) layers (green in Figure 5a), but in net B, the zigzag chain alternately links the
nodes of 2-fold parallel interpenetrating (4,4) arrays (yellow and teal in Figure 5e). Obviously, such a unique connection inevitably results in the final 3D self-penetrating architecture. In 1, the remarkable self-penetrating and highly connected features, in fact, mainly result from the following two factors. The first is the capability of the p-block PbII ions and carboxylate groups to form polynuclear PbII-clusters, which provides a sound basis for the resulting highly connected net. The second is the use of the long V-shaped dicarboxylate building block, which possibly is a critical factor for the formation of the self-penetrating entanglement structure. In detail, benefiting from the bent conformation, the L2- ligands using mode A link the {Pb6} SBUs into 2-fold interpenetrating arrays. Moreover, the L2- ligands adopting mode B, alternately, bridge across a set of such interpenetrating arrays, furnishing a 3D pillared self-penetrating framework. In a sense, the significance of 1 not only presents a new 8-connected self-penetrating topology but also provides a promising route in the design of highly connected and/or selfpenetrating coordination polymers with long V-shaped polycarboxylate tectons. Complex 1 is stable in air and its thermal stability was analyzed on a crystalline sample from 25 to 800 °C at a heating rate of 10 °C min-1 (see Figure S4, Supporting Information). The coordination framework remains stable upon heating to 310 °C at which decomposition occurs, and the final residue seems to be PbO (observed: 48.0% and calculated: 47.2%),7,8 which has also been further confirmed by temperature-variable PXRD patterns of 1 (see Figure S1, Supporting Information). Solid-state photoluminescent spectrum of 1 is recorded at room temperature (see Figure S5, Supporting Information), which shows a strong emission at around 455 nm (λex = 381 nm). However, the free H2L ligand exhibits an emission maximum at 595 nm upon excitation at 435 nm.9c Herein, the strong emission of 1 is different from those metal-centered transitions involved s and p orbitals of s2
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metal-ion complexes,13 which can be tentatively assigned to ligand to metal charge transfer (LMCT) between the delocalized p bonds of carboxylate groups and p orbitals of PbII centers.7a In summary, we have prepared a unique self-penetrating 3D coordination framework, with cage-shaped hexanuclear PbIIoxo-clusters as the eight-connected nodes. The successful isolation of this crystalline material further enriches our knowledge of structural topology and also confirms the significant potential of the “metallic clusters” strategy for constructing highly connected and/or self-penetrating networks. Further study on new coordination frameworks based on such long V-shaped polycarboxylate tectons is under way. Acknowledgment. This work was financially supported by the NSF of China (20773104), NCET (NCET-06-0891), MOE (208143), the IPHPEO (Z20091301), and the NSF of Hubei Provinces of China (2008CDB030). M.D. also thanks the support from Tianjin Normal University.
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Supporting Information Available: Experimental details, CIF file, TG curve, XRPD pattern, and supplementary tables/figures for 1. This material is available free of charge via the Internet at http:// pubs.acs.org.
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