Metal–Organic Azolate Frameworks with an Acetylene-bis-pyrazolate Linker: Assessing the Role of the Triple-Bond Spacer in Gas and Vapor Sorption

• Published in: Crystal growth & design 2024-05, Vol.24 (9), p.3837-3854
• Main Authors: Jordan, Dustin N., Müller, Tim, Oestreich, Robert, Strothmann, Till, Boldog, István, Janiak, Christoph
• Format: Article
• Language: English
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/acs.cgd.4c00165

The reaction of the diprotonated linker precursor in the form of the salt [H4bpe2+]­(TFA–)2 (H2bpe = 1,2-bis­(1H-pyrazol-4-yl)­ethyne; HTFA = trifluoroacetic acid) with Zn­(NO3)2·4H2O in DMF at 75 °C for 9 days led to single crystals of the metal–organic framework (MOF) [Zn­(bpe)]·1.8DMF (ortho-1·1.8DMF) (HHUD-5). The compounds crystallize in the orthorhombic space group Fddd with rhombic channels, and is a polymorph of the previously reported structure of [Zn­(bpe)]·1.2DMF (tetra-1·1.2DMF) that crystallizes in the tetragonal space group P42 /mmc and features square channels as established from powder X-ray diffraction (PXRD) data. Microporous ortho-1 and tetra-1 demonstrate similar porosities with surface areas (pore volumes) of 2135 m2/g (0.77 cm3/g at p/p 0 = 0.95) and 1904 m2/g (0.73 cm3/g at p/p 0 = 0.95) when an optimized activation is employed (1380 m2/g was reported previously for tetra-1). Reaction of the boc-protected linker precursor Boc2bpe with Ni­(OAc)2·4H2O in DMF/water under reflux yielded the MOF [Ni8(OH)4(H2O)2(bpe)6]·nSolv (2·nSolv, HHUD-6), which crystallized in the cubic space group F m 3 ̅ m established from PXRD. Compound 2 is the newest member of the isoreticular series of [Ni8(OH)4(H2O)2L6] (L = bis-pyrazolate or carboxylate-pyrazolate) and exhibits a surface area of 1415 m2/g and a pore volume of 0.78 cm3/g at p/p 0 = 0.80. The CO2 uptake at 273 K and 1 bar was 3.99 mmol/g for ortho-1 and 5.84 mmol/g for 2; the CH4 uptake was 1.11 mmol/g for ortho-1 and 1.64 mmol/g for 2, all in line with the higher heat of adsorption of the gases in 2, where open metal sites are possible after activation. Above 2 bar for CO2, and above 6 bar for CH4, the uptake in ortho-1 surpasses that in 2 due to the higher surface area of ortho-1. Both MOFs show high H2 uptakes of 11.6 mmol/g for ortho-1 and 8.7 mmol/g for 2 at 77 K and 1 bar. Comparison to the CO2, CH4, and H2 uptake in the analogous MOFs with the slightly longer 4,4′-(1,4-phenylene)­bis­(pyrazolate) linker (having phenylene instead of acetylene spacer) suggests an advantage of the −CC– triple bond. Vapor adsorption experiments with volatile organic compounds at 293 K for ortho-1 yielded uptakes of 8.2 mmol/g for benzene, 6.6 mmol/g for cyclohexane, and 5.7 mmol/g for n-hexane with type I isotherms. Compound 2 gave uptakes of 10.4 mmol/g for benzene, 10.7 mmol/g for cyclohexane, and 8.8 mmol/g for n-hexane with type II isotherms. Both MOFs are water stable as indicated by water vapor sorptio