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

Abstract

The reaction of the diprotonated linker precursor in the form of the salt [H 4 bpe 2+ ](TFA − ) 2 (H 2 bpe = 1,2-bis(1Hpyrazol-4-yl)ethyne; HTFA = trifluoroacetic acid) with Zn(NO 3 ) 2 • 4H 2 O 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 P4 2 /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 m 2 /g (0.77 cm 3 /g at p/p 0 = 0.95) and 1904 m 2 /g (0.73 cm 3 /g at p/p 0 = 0.95) when an optimized activation is employed (1380 m 2 /g was reported previously for tetra-1). Reaction of the boc-protected linker precursor Boc 2 bpe with Ni(OAc) 2 • 4H 2 O in DMF/water under reflux yielded the MOF [Ni 8 (OH) 4 (H 2 O) 2 (bpe) 6 ]•nSolv (2•nSolv, HHUD-6), which crystallized in the cubic space group Fm m 3 established from PXRD. Compound 2 is the newest member of the isoreticular series of [Ni 8 (OH) 4 (H 2 O) 2 L 6 ] (L = bis-pyrazolate or carboxylate-pyrazolate) and exhibits a surface area of 1415 m 2 /g and a pore volume of 0.78 cm 3 /g at p/p 0 = 0.80. The CO 2 uptake at 273 K and 1 bar was 3.99 mmol/g for ortho-1 and 5.84 mmol/g for 2; the CH 4 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 CO 2 , and above 6 bar for CH 4 , the uptake in ortho-1 surpasses that in 2 due to the higher surface area of ortho-1. Both MOFs show high H 2 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 CO 2 , CH 4 , and H 2 uptake in the analogous MOFs with the slightly longer 4,4′-(1,4phenylene)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 sorption and stability tests which also show a hydrophobicity of the bpe 2− linker.