Systematic Study on Zirconium Chelidamates: From a Molecular Complex to a M-HOF and a MOF.

We report the synthesis and in-depth characterization of three zirconium chelidamates, a molecular complex (H 8 C 2 N) 2 [Zr(HL) 3 ] ( 1 ), a porous metal-containing hydrogen-bonded organic framework (M-HOF) [Zr(H 2 O) 2 (HL) 2 ]· x H 2 O ( 2 ), and a metal-organic framework (MOF) (H 8 C 2 N) 2-2 n [Zr(H n L) 2 ]· x solvent (0 ≤ n ≤ 1) ( 3 ) using chelidamic acid (H 3 L, H 5 C 7 NO 5 , 4-hydroxypyridine-2,6-dicarboxylic acid) as the ligand (H 8 C 2 N + = dimethylammonium). High-throughput investigations of the system Zr 4+ /H 3 L/HCl/DMF/H 2 O were carried out, which resulted in highly crystalline compounds. The crystal structures of 1 and 2 were determined by single-crystal X-ray diffraction. Single-crystal three-dimensional (3D) electron diffraction and Rietveld refinements of powder X-ray diffraction (PXRD) data had to be used to elucidate the crystal structure of 3 since only very small single crystals of about 500 nm in diameter could be obtained. In all structures, chelidamate ions act as anionic palindromic pincer ligands, and in 3 , a coordinative bond is additionally formed by the aryloxy group. While dense packing of the molecular complexes is found in 1 , hydrogen bonding of the molecular complexes in 2 leads to a porous network that shows flexibility depending on the water content. The three-dimensional framework structure of the Zr-MOF 3 contains a mononuclear inorganic building unit (IBU), which is very uncommon in Zr-MOF chemistry. The three compounds are stable in several organic solvents, and thermal decomposition starts above 280 °C. While the hydrogen-bonded framework 2 is only porous toward water with a water uptake of almost 3.75 mol mol -1 at p / p 0 = 0.9, 3 is porous against N 2 , CO 2 , methanol, ethanol, and water with a specific Brunauer-Emmett-Teller (BET) surface area of a S,BET = 410 m 2 g -1 derived from the N 2 adsorption isotherm. Stability upon water adsorption covering 10 cycles between 0.5% < p / p 0 < 90% for 3 is also demonstrated.