Ligand Excess "Inverse-Defected" Zr6 Tetrahedral Tetracarboxylate Framework and Its Thermal Transformation.

A new porous coordination polymer (PCP/MOF), ZRTE-10, based on a tetrahedral 1,3,5,7-tetra(carboxyphenyl)benzene ligand (H4L4) was synthesized using formic or acetic acids as modulators. The low symmetry (C2/c) framework, [Zr6(μ3-O)4(μ3-OH)4(L4)(HL4)2(OH)2(H2O)2], is built upon a rare 10-connected Zr6 cluster. Two-thirds of the ligands bear one nondeprotonated carboxy group, and the framework has a complex trinodal 3,4,10-c, {414.624.87}{43}2{45.6}, underlying net. Supercritical CO2 drying and mild degassing at 120 °C yielded a porous material with SBET = 1190 m2 g-1. When heated up to ∼200 °C, ZRTE-10 converts to another crystalline framework, ZRTE-11. The latter was identified to be identical to the expected fluorite (flu) observed previously for other tetrahedral ligands. The high symmetry (I4/m) framework is built upon 8-connected Zr6 clusters and has a formula of [Zr6(μ3-O)4(μ3-OH)4(OH)4(L4)2]. The complicated trinodal network of ZRTE-10 and the simple flu net in ZRTE-11 are topologically interrelated via the operation of the merging of two neighbor three-connected nodes to one four-connected one. The thermally induced conversion of ZRTE-10 proceeds with expulsion of one ligand per Zr6 node in the pores of the framework, resulting in a relatively low SBET = 585 m2 g-1 for the activated H4L4@ZRTE-11. A mixed ligand approach for ZRTE-10,11 was attempted using 1,3,5-tetra(carboxyphenyl)benzene (H3L3), which is a truncated analog of H4L4 with one missing branch. The monocrystalline sample of ZRTE-10 obtained in small yields demonstrated only minor inclusion of H3L3. However, the high-yielding (∼80%) procedure with HCl as a modulator allows near proportional incorporation of the ligands. The formed materials are semiamorphous with powder XRDs intermediary between pure ZRTE-10 and -11. Thermal treatment of the semiamorphous materials increases their crystallinity and allows SBET = 400-550 m2 g-1 surface areas to be reached for pure H4L4 and H3L3 or their mixture alike. The approach proposes a viewpoint on the H3L3 trifunctional ligand as a model of a ligand platform, suitable for bearing a large functionality on the fourth "truncated" branch. The significance of ZRTE-10 as a material for postsynthetic introduction of metal-based cluster functionality and as a model of functionality encapsulation, an alternative to the ship-in-the-bottle method, is discussed.