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

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.