Zn-MOFs containing flexible α,ω-alkane (or alkene)-dicarboxylates with 1,2-bis(4-pyridyl)ethylene: comparison with Zn-MOFs containing 1,2-bis(4-pyridyl)ethane ligands

Abstract

Flexible ditopic α,ω-alkane (or alkene)-dicarboxylate bridging ligands provided the following six Zn-MOFs with a 1,2-bis(4-pyridyl)ethylene (bpe) pillar (malonate (1-bpe), succinate (2-bpe), fumarate (3-bpe), glutarate (4-bpe), adipate (5-bpe), and muconate (6-bpe)). Newly-prepared 5-bpe and 6-bpe were structurally characterized. Both 3-bpe and 6-bpe formed three-dimensional (3-D), diamond-like frameworks with a 4-connected uninodal net and a Schläfli symbol of 6⁶. By contrast, 5-bpe formed a 3-D framework of a 6-connected uninodal net with a Schläfli symbol of 4¹²·6³. 2-bpe formed a 2-D framework with a 4-connected uninodal net with a Schläfli symbol of 4⁴·6². 3-bpe and 6-bpe showed potential void spaces after solvent removal. Gas sorption analysis with N₂, H₂, and CO₂ at a suitable temperature indicated that 6-bpe was an adsorbent with selectivity for CO₂ adsorption at 196 K over N₂ and H₂. The CO₂ uptake at 196 K was 86.68 cm³ g⁻¹ (3.87 mmol g⁻¹). 3-bpe did not exhibit appreciable uptake levels for N₂, H₂, and CO₂, which was possibly due to the relatively rigid nonporous activated form of the framework after solvent removal. Both 3-bpe and 6-bpe exhibited a good encapsulating ability for molecular iodine in cyclohexane. The crystal structure of I₂-containing 6-bpe_I₂ was elucidated. A comparison with Zn-MOFs containing 1,2-bis(4-pyridyl)ethane ligands is discussed.