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-H(4)bptc) and biphenyl-3,4,3',4'-tetracarboxylate (3, 4-H(4)bptc) 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-H(4)bptc), [Zn(2,4-H(2)bptc)(4,4'-bipy)center dot H2O](n) (1), {[Zn-3(2,4-Hbptc)(2)(2,2'-bpy)(2)]center dot 2H(2)O}(n) (2), {[Zn-2(2,4-bptc)(2,2'-bpy)]center dot(2,2'-bpy)(0.5)center dot(H2O)}(n) (3), {[Zn-2(2,4- bptc)(2,2'-bpy)(2)](H2O)}(n) (4), {[Cd-2 center dot(2,4-bptc)center dot(2,2-bpy)(2)center dot H2O]center dot H2O}(n) (5), {[Zn-2 center dot(2,4bptc)center dot (phen)center dot H2O](n) (6), {[Co-5(2,4-bptc)(2)(mu(3)-OH)(2)(mu(2)-H2O)(2)(mu(1)-H2O)(2)]center dot 2H(2)O}(n) (7) and {[Co-5(2,4bptc) 2(mu(3)-OH)(mu(2)-H2O)(2)(mu(1)-H2O)(2)]center dot 6H(2)O}(n) (8). Complexes 1 and 2 are 1D chains linked through partially deprotonated H(4)bptc carboxylate oxygen. 2,4-H(2)bptc(2-) in 1 acts as a bidentate ligand while 2,4-Hbptc(3-) in 2 acts as hexadentate ligand. In complexes 3-8, bptc(4-) is fully deprotonated to form 3D coordination polymers. The 2,4-bptc(4-) 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-bptc(4-) ligand have torsion angle varies from 7.83 to 81.4 degrees. When the torsion angle ranges from 61-73 degrees, 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.