Influence of sterically non-hindering methyl groups on adsorption properties of two classical zinc and copper MOF types

The metal-organic frameworks (three-dimensional porous coordination polymers) Zn₄O(Me₄BPDC)₃] × 9 DMF, 2 · 9 DMF and Cu₂(Me₄BPDC)₂] × 9 DMF, 3 · 9 DMF are representatives of the classical Zn-IRMOF series and Cu paddle-wheel complexes with H₂Me₄BPDC = 2,2′,6,6′-tetramethyl-4,4′-biphenyldicarboxylic acid, 1. The dicarboxylate linker of 1 is a representative of the non-planar biphenyl ligand family, known as an efficient scaffold for chiral molecules. There is a 90° twist angle between the phenyl rings in 1, dictated by the methyl groups, which leads to assembly of doubly interpenetrated pcu-a (in 2) and nbo-a (in 3) nets under low temperature solvothermal conditions in dimethylformamide (DMF). Activation by degassing (to yield 2), exchange with methanol or tetrahydrofuran and subsequent evacuation at elevated temperatures (to yield 3^I) gave materials with BET surface areas of 1735 m²/g (2) and 1041 m²/g (3^I). Adsorbed quantities of H₂ were 1.26 wt% (2) and 1.02 wt% (3^I) (77 K, 1 bar), CO₂ 30.8 cm³/g (2) and 50 cm³/g (3^I) (273 K, 1 bar) and CH₄ 12.9 cm³/g (2) and 11.4 cm³/g (3^I) (273 K, 1 bar). The H₂ and CO₂ sorption values for 2 are similar to those of MOF-5 (IRMOF-1) with its almost doubled BET surface area. An increase is found concerning the adsorbed amounts of N₂, H₂, and CO₂ for 3^I compared to related doubly interpenetrated nbo-a-type MOF-601, MOF-602, MOF-603 (Cu₂L₂] with L = 2,2′-R₂-4,4′-biphenyldicarboxylate, R = CN, Me, I, respectively).