Investigation of Porous Ni-Based Metal–Organic Frameworks Containing Paddle-Wheel Type Inorganic Building Units via High-Throughput Methods

In the search of Ni based metal–organic frameworks (MOFs) containing paddle-wheel type building units, three chemical systems Ni2+/H n L/base/solvent with H n L = H3BTC (1,3,5-benzenetricarboxylic acid), H3BTB (4,4′,4′′,-benzene-1,3,5-triyl-tris(benzoic acid)), and H2BDC (terephthalic acid) were investigated using high-throughput (HT) methods. In addition to the conventional heating, for the first time HT microwave assisted synthesis of MOFs was carried out. Six new compounds were discovered, and their fields of formation were established. In the first system, H3BTC was employed and a comprehensive HT-screening of compositional and process parameters was conducted. The synthesis condition for the Ni paddle-wheel unit was determined and two compounds [Ni3(BTC)2(Me2NH)3]·(DMF)4(H2O)4 (1a) and [Ni6(BTC)2(DMF)6(HCOO)6] (1b) were discovered (Me2NH = dimethylamine, DMF = dimethylformamide). In the second system, the use of the extended tritopic linker H3BTB and the synthesis conditions for the paddle-wheel units led to the porous MOF, [Ni3(BTB)2(2-MeIm)1.5(H2O)1.5]·(DMF)9(H2O)6.5 (2), (2-MeIm = 2-methylimidazole). This compound shows a selective adsorption of H2O and H2 with a strong hysteresis. In the third system, H2BDC was used, and the base (DABCO) was incorporated as a bridging ligand into all structures. Thus, two pillared layered porous MOFs [Ni2(BDC)2(DABCO)]·(DMF)4(H2O)1.5 (3a) and [Ni2(BDC)2(DABCO)]·(DMF)4(H2O)4 (3b) as well as a layered compound [Ni(BDC)(DABCO)]·(DMF)1.5(H2O)2 (3c) were isolated. The 3a and 3b polymorphs of the [Ni2(BDC)2(DABCO)] framework can be selectively synthesized. The combination of microwave assisted heating, low overall concentration, stirring of the reaction mixtures, and an excess of DABCO yields a highly crystalline pure phase of 3b. The fields of formation of all compounds were established, and scale-up was successfully performed for 1b, 2, 3a, 3b, and 3c. All compounds were structurally characterized. In addition to IR, elemental and TG analyses, gas and vapor sorption experiments were carried out.