Influence of pH on the structures of two Cd phosphonate compounds

Two novel Cd-based coordination polymers Cd(H2BCP)2(phen)2 1 and {[Cd6(BCP)4(phen)3]·2H2O}n 2 (Phen=1,10-phenanthroline) have been hydrothermally synthesized from a combination of 4-(phosphonomethyl) benzoic acid 4-HOOCC6H4CH2PO3H2 (H3BCP) and 1,10-phenanthroline. The X-ray crystal structure analyses revealed that 1 is a 0D mononuclear molecular structure and 2 is a 3D structure existing the novel Cd6 cluster unit. It is worth mentioning that 1 and 2 have the same constitutional units, namely the Cd ion, H3BCP ligand and second ligand- Phen. However, their structures are different and it results from the different solution pH value. In order to explore the structural characteristics, FTIR, photo-luminescent and solid UV-Vis spectra are employed. Synthesis and properties of two novel Cd-based coordination polymers Cd(H2BCP)2(phen)2 1 and {[Cd6(BCP)4(phen)3]·2H2O}n 2 are reported. It is worth mentioning that 1 is zero-dimensional structure and forms super-molecular layers via O–H···O under the condition of solution pH = 2.1, 2 is a three-dimensional framework and contains the novel Cd6 cluster unit under the condition of solution pH = 5. So controlling the pH of the solution plays an important role in the structure of the compound.