Anionic metal–organic and cationic organic layer alternation in the coordination polymers [{M(BTEC)(OH2)4}·{C4H12N2}·4H2O]n (M = Co, Ni, and Zn; BTEC = 1,2,4,5-benzenetetracarboxylate)

Abstract

New coordination polymers [{M(BTEC)(OH₂)₄}·{C₄H₁₂N₂}·4H₂O]_n (BTEC = 1,2,4,5-benzenetetracarboxylate) (M = Co 1; Ni 2; Zn 3) have been synthesized starting from the respective transition metal salts, 1,2,4,5-benzenetetracarboxylic acid and piperazine hexahydrate. The highly-crystalline compounds, which are insoluble in water as well as common organic solvents, have been characterized in the solid-state with the aid of elemental analysis, IR and diffuse reflectance UV-visible spectroscopy, and TGA/DTA measurements. The molecular structures of all the compounds have been determined in the solid-state by X-ray diffraction studies; all the three compounds are isomorphous and crystallize in centerosymmetric triclinic space group. The molecular structures are made up of extensively hydrogen-bonded alternating layers of anionic {M(BTEC)(OH₂)₄}_n coordination polymer and piperazinium dications. The extended structure formed in the solid-state due to extensive inter-layer O–H⋯O and N–H⋯O hydrogen bonds incorporate four water molecules per unit cell. The water molecules present in the polymeric network can be easily removed at fairly low temperatures. Heating 1 or 2 to approximately 60–70 °C in a vacuum (0.1 mmHg) for 12 h results in the elimination of three water molecules as evidenced by elemental analysis. Similar results were obtained from the thermogravimetric studies. The dehydration process is accompanied by a colour change in the case of compounds 1 and 2. Exposure of the dehydrated sample to ammonia results in the uptake of three equivalents of ammonia molecules. However, the methane gas adsorption studies carried out on dehydrated sample did not show the presence of a porous structure.