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 (H₈C₂N)₂[Zr(HL)₃] (1), a porous metal-containing hydrogen-bonded organic framework (M-HOF) [Zr(H₂O)₂(HL)₂]·xH₂O (2), and a metal-organic framework (MOF) (H₈C₂N)_2-2[Zr(HL)₂]·x solvent (0 ≤ n ≤ 1) (3) using chelidamic acid (H₃L, H₅C₇NO₅, 4-hydroxypyridine-2,6-dicarboxylic acid) as the ligand (H₈C₂N⁺ = dimethylammonium). High-throughput investigations of the system Zr⁴⁺/H₃L/HCl/DMF/H₂O 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/p₀ = 0.9, 3 is porous against N₂, CO₂, methanol, ethanol, and water with a specific Brunauer-Emmett-Teller (BET) surface area of a_S,BET = 410 m² g^-1 derived from the N₂ adsorption isotherm. Stability upon water adsorption covering 10 cycles between 0.5% < p/p₀ < 90% for 3 is also demonstrated.