Coordination polymers of biphenyl-2,4,2′,4′-tetracarboxylic acid—synthesis, structures and adsorption properties

Abstract

Coordination polymers, especially porous coordination polymers have attracted much attention due to novel structures and potential applications. Biphenyl-3,5,3′,5′-tetracarboxylate (3, 5-H₄bptc) and biphenyl-3,4,3′,4′-tetracarboxylate (3, 4-H₄bptc) have been used in construction of porous coordination polymers. In this paper, a series of metal–organic framework polymers constructed from biphenyl-2,4,2′,4′-tetracarboxylate (2,4-H₄bptc), [Zn(2,4-H₂bptc)(4,4′-bipy)·H₂O]_n (1), {[Zn₃(2,4-Hbptc)₂(2,2′-bpy)₂]·2H₂O}_n (2), {[Zn₂(2,4-bptc)(2,2′-bpy)]·(2,2′-bpy)_0.5·(H₂O)}_n (3), {[Zn₂(2,4-bptc)(2,2′-bpy)₂](H₂O)}_n (4), {[Cd₂·(2,4-bptc)·(2,2-bpy)₂·H₂O]·H₂O}_n (5), {[Zn₂·(2,4-bptc)·(phen)·H₂O]_n (6), {[Co₅(2,4-bptc)₂(μ₃-OH)₂(μ₂-H₂O)₂(μ₁-H₂O)₂]·2H₂O}_n (7) and {[Co₅(2,4-bptc)₂(μ₃-OH)(μ₂-H₂O)₂(μ₁-H₂O)₂]·6H₂O}_n (8). Complexes 1 and 2 are 1D chains linked through partially deprotonated H₄bptc carboxylate oxygen. 2,4-H₂bptc²⁻ in 1 acts as a bidentate ligand while 2,4-Hbptc³⁻ in 2 acts as hexadentate ligand. In complexes 3–8, bptc⁴⁻ is fully deprotonated to form 3D coordination polymers. The 2,4-bptc⁴⁻ can form 6–9 coordination bond with metal ions. There are free 2,2′-bpy fill in the porous channel in complex 3. Complexes 7 and 8 were obtained under the same condition except reaction temperature. Using a higher temperature tends to form 7 with a lower water content. In complexes 7 and 8, the Co ions form Co₂O₂ diamond-core ribbon. In all the complexes, the two benzene rings in the 2,4-bptc⁴⁻ ligand have torsion angle varies from 7.83 to 81.4°. When the torsion angle ranges from 61–73°, the two 2-carboxylate coordination to a metal ion to form 9-membered coordination rings. The coordination rings have stereoisomers. This phenomena did not exist in 3,5-H₄bptc and 3,4-H₄bptc complexes. The water molecules in all complexes can be removed by heating. The water molecules in 7 and 8 continually lost without discernable difference between coordination water and crystalline water molecules. The dehydrated sample of 7 and 8 still keep crystallinity. Dehydrated 7 can adsorbs 10% methanol corresponding to all water molecules replaced by methanol. Fully dehydrated 8 can adsorbs 20% ethanol molecules. All the complexes, except 7 and 8, have similar fluorescence to that of 2,4-H₄bptc, therefore, all the fluorescence can be attributed intra-ligand emission.