Crystal engineering and sorption studies on CN- and dipyridyl-bridged 2D coordination polymers

Abstract

Two new cyano-bridged coordination polymers, {[Ni(bpe)(H₂O)₂][Ni(CN)₄]·2H₂O}_n (1) and {[Cu(bpe)(H₂O)₂][Ni(CN)₄]·ethanol}_n (2) (bpe = 1,2-bis(4-pyridyl)ethane) were synthesized and characterized. This was part of a study attempting to rationalize the formation of Hofmann coordination polymers over their aqua-derivatives, in which we found that the length of the ligand does not control the outcome of the synthesis. In the initial phase of thermogravimetric analysis, compound 1 lost the guest water molecule while compound 2 lost its guest ethanol. Single crystal X-ray diffraction revealed that both compounds crystallize in P with similar unit cell parameters. In both compounds, the Ni(II) ion is coordinated with four cyanide–carbon atoms in a square planar geometry and Cu(II) or the second Ni(II) ion adopts a distorted octahedral coordination by two bpe ligands, two bridging cyano groups, and two water molecules. In both structures, chains composed of adjacent [Ni(C₁₂H₁₂N₂)(H₂O)₂]²⁺ (for 1) or [Cu(C₁₂H₁₂N₂)(H₂O)₂]²⁺ (for 2) are further linked together via [Ni(CN)₄]²⁻ units and extend to generate a two-dimensional framework. The presence of disordered terminal cyanide moieties in 1 could be attributed to a larger volume available to this moiety in the structure of 1. The BET surface areas of the activated phases 1′ of 1 and 2′ of 2 are 28 m² g⁻¹ and 5 m² g⁻¹, respectively, and originally, they have no pores to accommodate gas molecules at low pressure (1 atm). 1′ does not adsorb carbon dioxide at 195 K (0.30 mmol g⁻¹), 273 K (0.086 mmol g⁻¹) and 298 K (0.056 mmol g⁻¹), or hydrogen at 77 K, but it shows an affinity to adsorb nitrogen (around 2.5 mmol g⁻¹) at 77 K and water vapor molecules (around 0.95 mmol g⁻¹) at 298 K with typical type-II isotherms. 2′ adsorbs a small quantity of water vapor (around 0.20 mmol g⁻¹) at 298 K and does not adsorb carbon dioxide at 195 K (0.125 mmol g⁻¹), 273 K (0.069 mmol g⁻¹) and 298 K (0.062 mmol g⁻¹), or hydrogen at 77 K, but shows more affinity to adsorb nitrogen (around 1.2 mmol g⁻¹) at 77 K.