Chiral Self-Threading Frameworks Based on Polyoxometalate

One is formed by ligands L1 bridging Ni1 atoms with a pitch of 18.577 Å, showing a left-handed screw. The other type is built from alternating linkag...
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Chiral Self-Threading Frameworks Based on Polyoxometalate Building Blocks Comprising Unprecedented Tri-Flexure Helix Chao Qin, Xin-Long Wang, Ling Yuan, and En-Bo Wang* Key Laboratory of Polyoxometalates Science of Ministry of Education, Northeast Normal UniVersity, Changchun 130024, China

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2093–2095

ReceiVed March 4, 2008

ABSTRACT: Two enantiomeric chiral 3D frameworks were constructed by using an achiral ligand, nickel cations, and polyoxometalate ions as building blocks, which not only comprise unusual triflexural helical motifs but also represent the first 3D self-threading examples. Entangled systems have captivated much attention from chemists, not only for their undisputed beauty1 but also for their potential applications2 arising directly from the intertwining nature of the lattices. Although enantioselective separations and catalyzes are important applications of chiral metal-organic frameworks (MOFs),3 most of them are not thermally and chemically stable enough to be useful in practical applications. Therefore, it is a challenge to synthesize robust chiral MOFs. Introduction of an intertwining nature in such framework structures may be a promising strategy to overcome this drawback, because the aspect of entanglement, to some extent, could favor strengthening the interactions between structural motifs,2a consequently enhancing the stability of the whole network. On the other hand, polyoxometalates (POMs),4 as anionic early transition-metal oxide clusters, have become attractive inorganic building blocks by virtue of their intrinsic stability and rich electrical and optical properties.5,6 Using chiral components, POM-containing chiral compounds have been prepared by groups of Pope, Yamase, Hill, Kortz,7 and Wang.8 Nevertheless, to date, no POM-based chiral MOFs possessing entangled feature have been structurally characterized. The synthesis of such compounds will provide actual models for investigating the nature of POMs within chiral entangled system. Given that it is extremely difficult to find appropriate chiral polyoxoanions as precursors, the rational choice of organic ligand is therefore a key factor for achieving this target. The N-heterocyclic ligand 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene (hereafter noted L), synthesized according to the literature method,9 was chosen on the basis of the following considerations. On the one hand, it contains a rigid spacer of phenyl ring and two freely rotating triazolyl arms, which may cause the planes of the triazolyl rings to rotate with respect to the plane of the central phenyl ring with the result that two triazolyl groups significantly deviate from coplanarity. Such “skewing” of coordination sites in L should favor the formation of a helical motif. Within a given crystal, the spontaneous resolution of these helical motifs through weak interactions or more robust metal-ligand interactions is an effective route of introducing chirality into the resulting high-dimensional network.10 On the other hand, long and flexible ligands as such have potential for assembly of uncommon entangled structures.11 In light of these two points, it is highly possible for L to play a “two birds with one stone” role in the network construction. Herein, we report two enantiomeric chiral self-threading frameworks based on strandberg polyanion building blocks, L-1 and D-1: L-[Ni2(H2O)(HL4)][HP2Mo5O23] · 7H2O, L-1 D-[Ni2(H2O)(HL4)][HP2Mo5O23] · 7H2O, D-1 Reaction of L with Ni(NO3)2 · 6H2O, Na2MoO4 · 2H2O, and H3PO4 under hydrothermal conditions resulted in compounds L-1 and D-1 that are separated manually (see the Supporting Information). The formulation of 1 was supported by IR (see Figure S1 in the * To whom correspondence [email protected].

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Figure 1. ORTEP drawing of structural unit in 1 (thermal ellipsoids are drawn at the 50% probability level for P, Mo, Ni; H atoms were omitted for clarity).

Supporting Information), microanalysis, and thermogravimetric analysis (see Figure S2 in the Supporting Information). The phase purity of bulk products was confirmed by powder X-ray diffraction (see Figure S3 in the Supporting Information). Single-crystal X-ray diffraction12 reveals that compounds L-1 and D-1 are enantiomers; their unit-cell dimensions, volumes, related bond distances, and angles are only slightly different. For the convenience of depiction, the structure of L-1 is described representatively. L-1 crystallizes in the chiral space group P212121 with one [HP2Mo5O23]5- cluster, two nickel atoms, four L ligands, and eight water molecules in the asymmetric unit, as shown in Figure 1. The four crystallographically independent L ligands adopt four drastically different conformations, giving four pairs of triazolyl/ phenyl dihedral angles for L1 (61.6 and 84.0°), L2 (67.9 and 86.4°), L3 (46.9 and 72.9°), and L4 (70.2 and 81.8°). Two crystallographically unique NiII ions (Ni1 and Ni2) exhibit distorted octahedral geometry (Ni-N 2.036(9)-2.107(9) Å, Ni-O 2.046(8)-2.078(7) Å). The geometry of the [HP2Mo5O23]5- cluster can be described as a ring of five distorted MoO6 octahedra with two PO4 tetrahedra capped on each side. The intricate chiral net of L-1 can be visualized in terms of a 3D inorganic subnet and 2D sidearm-containing chiral layers, connected together by sharing metal atoms. First of all, taking no account of organic ligands, each [HP2Mo5O23]5- cluster serves as a tetradentate ligand to bridge two Ni1 and two Ni2 atoms by four terminal oxygen atoms into a 3D inorganic backbone with diamondoid topology in which fourconnected nodes are provided by [HP2Mo5O23]5- clusters and the bent connecting rods are provided by the angular two-connected metal atoms, Ni1 and Ni2 (Figure 2a and the Supporting Information, Figure S4). Second, without regard to [HP2Mo5O23]5- clusters, L1, L2, and L3 ligands bridge Ni1 and Ni2 atoms to generate a 2D metal-organic layer in the ab plane (Figure 2b), in which Ni1 and Ni2 atoms act

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Figure 2. (a) 3D diamondlike subnet of L-1, viewed along the b axis. Mo, red; P, yellow; Ni, green. (b) 2D sidearm-containing chiral layer in L-1. Two types of helices and dangling ligands are shown in red, green, and blue. (c) Schematic representation of the connectivity of the two types of helices. Dangling L4 ligands are omitted for clarity. (d) Common helix (left), a meso-helix (middle), and a triflexural helix (right).

as four-connected and two-connected nodes, respectively. It is worth noting that the uncoordinated L4 ligands grafted on Ni2 atoms are disposed in a mutual anti orientation with respect to the layer plane. These dangling L4 ligands provide, obviously, the possibility for the ultimate realization of threading. As expected, there exist two types of homochiral 21 helical chains within the sheet because of the gauche conformation of ligands. One is formed by ligands L1 bridging Ni1 atoms with a pitch of 18.577 Å, showing a left-handed screw. The other type is built from alternating linkages of Ni1 and Ni2 atoms by ligands L2 and L3 and is decorated by pendent L4 ligands at two sides, having identical pitch and handedness with the first type. Surprisingly, a further investigation reveals that the configuration of the second type of helix shows unique feature and the term “tri-flexural helix” is suggested for it, meaning consisting of three flexures in a single strand. A representation of the exceptional triflexural helix, comparing with the only two types of spiral lines presently known (i.e., ordinary single-handed helix and rarely observed meso-helix13), is presented in Figure 2d. Apart from the two points of homoflexure on the edge of the spiral line, the twists at the two CH2 groups of L2 ligand (angles of C-C-Ni 108.7 and 114.8°) lead to the formation of a third contraflexural point between them so that two left-handed helices are seamed by many helical loops to give a chiral triflexural helix (see the Supporting Information, Figure S5). To the best of our knowledge, this new type of helical motif has never been observed prior to this work and may broaden insight into the isomers related to the helices. These two distinct homochiral helices are held together by Ni1 atoms functioning as hinges into a unique chiral metal-organic layer carrying pendent arms (Figure 2c). Being different from L-1, the chiral layers of D-1 are formed by two distinct right-handed helices with a pitch of 18.658 Å (see the Supporting Information, Figure S6). Finally, these sets of chiral layers intersect with the 3D inorganic subnet in such a way that identical metal centers meet at their own nodes. As a result, the overall framework becomes a trinodal (4,6)connected chiral net with an unprecedented (4.53.62)(4.54.6)(42.56.62.75) topology (circuit symbols in sequence of [HP2Mo5O23]5- cluster, Ni2, and Ni1). As illustrated in Figure 3, the integration of these two types of subsets brings another eyecatching result: each dangling arm (L4) with an effective length of about 9.92 Å (N-N distance) threads into the nearest 4-membered ring circled by three Ni atoms and a polyoxoanion (see the Supporting Information, Figure S7), showing the pseudo-rotaxane

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Figure 3. Schematic illustration of the chiral 3D self-threading network of L-1, in which 3D diamond and 2D chiral layer subnets and dangling arms are represented in yellow, blue, and purple, respectively.

nature that exclusively occurred between individual motifs before. Compared with the currently known types of entanglements,14 the topology of entanglement reported herein obviously neither falls in the polythreading nor the self-penetrating categories, although it possesses partially inherent features of these two classes (see the Supporting Information, Figure S8). Considering that it is a structural compromise between a single network and polythreading, a new term “self-threading” is employed to define this new type of entanglement in which closed loops are threaded by components from the network itself. Although many exotic entangled architectures are being documented each year aided by the rapid growth of network-based crystal engineering, they mainly contribute to adding members to already existing sorts of entanglements rather than develop a school of their own. What we describe here, to the best of our knowledge, sets up a new branch for entanglements and represents the first 3D self-threading example as well as a second entangled phenomenon found in single nets besides selfpenetration. The present self-threading framework is stabilized by strong hydrogen bonds involving the protonized nitrogen atoms of dangling ligands and the oxygen atoms of PO4 tetrahedra (N · · · O, 2.59 Å). To examine the chiroptical of the two enantiomeric compounds, the CD spectra of compounds L-1 and D-1 in solid state were investigated (see the Supporting Information, Figure S9). The CD spectra of L-1 and D-1 are mirror images of one another, and conclusively demonstrate that they are enantiomers. Thermogravimetric analysis (TGA) experiment was conducted to examine the thermal stability of compound 1. The TG curve of 1 indicates a 6.46% loss of weight between 50 and 190 °C, which corresponds to the total weight of the coordinated and isolated water molecules (6.75%). At 300 °C, the ligand molecules start to be released. A plateau observed between 190 and 300 °C shows the chiral self-threading framework is stable up to 300 °C, which may be attributed to the occurrence of entanglement as proved by strong hydrogen bonds mentioned above. In conclusion, we have constructed two chiral self-threading frameworks utilizing achiral organic ligand, nickel cations, and [HP2Mo5O23]5- polyoxoanions as building blocks. Further endeavors will focus on the nature of POMs within such a system.

Acknowledgment. This work was financially supported by the National Science Foundation of China (20701006), the Foundation for Excellent Youth of Jilin, China (20070103), Ph.D. Station

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Foundation of Ministry of Education for New Teachers (20070200014/ 20070200015), and Science Foundation for Young Teachers of Northeast Normal University (20070303). Supporting Information Available: Details of the syntheses, IR, TG, PXRD, UV-vis spectra, and complementary drawings for crystal structure (PDF); crystallographic information in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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