Coordination-driven nanosized lanthanide "molecular lantern" with tunable luminescent properties.

Coordination-driven self-assembly of discrete nanosized molecular cages has become one of the most active areas of supramolecular chemistry.1 Over the past decade, extensive studies have been made on discrete molecular assemblies MxLy with various values of x and y.2 Among these metal-ligand clusters, the assembly of M2L4 tetragonal cage remains quite rare, although a few such d-block metal supramolecular complexes have appeared in the literature recently.3 Canonical symmetric Ln2L4 lanthanide cages have, to the best of our knowledge, never been described. This is clearly due to inherent difficulties in harmonizing the subtle relationship between the versatile coordination modes of the lanthanide metals (coordination numbers usually g8) and the ligand conformations in the synthetic cage systems. Lanthanide discrete cage-like assemblies, however, are of considerable interest in broad scientific areas, especially as the luminescent materials. We have recently reported a series of coordination polymers and supramolecular complexes based on bent five-membered heteroatom-ring-bridged ligands.4 By taking into account the bent geometry of such spacers, we wondered if the five-membered heteroatom ring-bridged 3,3′-biphenylcarboxylate type ligands could be used as an “organic clip”5 to bind lanthanide ions into discrete molecular cages, especially the Ln2L4 tetragonal cages. Depending on this ligand-directed approach,3 a series of lanthanide nanosized tetragonal cages Ln2L14 (Ln(III) ) La (1), Ce (2), Sm (3), Eu (4), and Tb (5)) which encapsulate [Ln(H2O)8] cations based on the bent 3,5-bis(3-benzocarboxylate)-4-amino1,2,4-trizole ligand (L1) and LnCl3 salts were synthesized (Supporting Information). As shown in Scheme 1, the lanthanide molecular cages [Ln3L14(H2O)10Cl]‚2H2O were obtained as colorless cubic crystals with in situ generated carboxylate ligand via hydrolysis of L and LnCl3 under hydrothermal conditions (H2O/ pyridine, 150 °C, 76 h) in high yield.6 The X-ray crystal structure analysis (Supporting Information) revealed that 1-5 are isostructural. They crystallize in the high-symmetry tetragonal space group, I4/m. For example 3, each Sm(2) node lies in a distorted singlecapped square antiprism coordination sphere (Figure 1), which is defined by eight carboxylic oxygen and one aquo oxygen donors and with Sm-O distances range from 2.457 to 2.576 Å.7 For 1-5, the ligand donor to Ln(III) bond lengths simply reflect the ionic radius variation.7 The most important structural feature of 1-5 is their cationic cage-like structure. As shown in Figure 1, four equivalent L1 ligands act as the desired organic clip to bridge two Ln(2) ions to form a tetragonal prismatic cage. It is interesting that the two phenyl rings on the same L1 ligand are basically coplanar, while the central triazole ring rotates by about 40° with respect to the phenyl plane and orients to the tangential direction of the cage. Such organized manner leaves an opening of ∼6.8 Å in the side of the cage. One cubic [Ln(1)(H2O)8] cation (∼2.8 × 2.8 × 2.8 Å3) is trapped inside (Figure 1). Top view of 3 shows that the four ligands crosswise arrange around the Ln(2)‚‚‚Ln(1)‚‚‚Ln(2) axis and the two Ln2L12 planes are perpendicular to each other, which results in the tetragonal cage canonical. There is a crystallographically imposed C4 axis passing through these three Ln(III) ions and σh mirror across the center of the molecule, leading to S4 symmetry of the cage. The Ln‚‚‚Ln distance is ca. 11 Å, and the distance between the two opposite trizole rings is ca. 14 Å. Up to date, only a few examples of M2L4 (M ) Pd, Cu, Ni and Co) cages containing the metal connecting nodes with a square planar, 4 + 1 capped tetragonal planar or octahedral coordination geometry were reported recently.3 To our knowledge, compounds 1-5 is the first family of lanthanide M2L4 nanocages.8 Compounds 1-5 exhibit the same arrangement in the crystal lattice. For example 3, the canonical shape of the tetragonal prisms permits them to be in close proximity, which results in a 2D network driven by intermolecular π-π interactions. Each prism interacts with four neighboring units through three sets of π-π interactions to generate the elliptoid channels (effective cross section of ca. 6.8 × 7.9 Å2), in which the [Sm(H2O)8] cations and Clanions are located alternatively (Figure 2). It is well-known that the lanthanide complexes that are attractive for their luminescent properties contributed to the “antenna effect”.9 Scheme 1. The Synthesis of Ln2L14 Tetragonal Cages