Synthesis, Structures and Recognition Properties of Two Cadmium(II) Phosphonates for Highly Selective Sensing of Cr2O72- and CrO42- Anions

Two cadmium (II) phosphonates, [Cd(MPBP)(2,2-bipy)(H2O)]H2O (1) and [Cd(H2BPDP)(2,2'-bipy)(2)(H2O)]H2O (2) [H2MPBP=4,4-Bis(monoethylphosphonomethyl)biphenyl, H4BPDP=biphenyl-4,4'-diylbis(methylene)diphosphonic acid, 2,2'-bipy=2,2'-bipyridine], had been successfully synthesized under hydrothermal conditions with different reaction temperatures by using H2MPBP as the phosphonate ligand and 2,2'-bipyridine as the second organic ligand. The compound 1 exhibited a 3D supramolecular structure, in which {CdN2O4} polyhedra and {CPO3} tetrahedra were interconnected by MPBP anions to form a 1D double chain via corner-sharing. Neighboring chains were linked into a 2D supramolecular sheet in the ac-plane by hydrogen bonding interactions, which were further assembled through pi-pi stacking interactions to form a 3D supramolecular structure. Compound 2 was assembled through pi-pi stacking and hydrogen bonding interactions to form a 1D supramolecular structure. Furthermore, both compounds exhibited remarkable luminescent properties, and could be applied as the high selective and sensitive luminescent sensors in the sensing of Cr2O72- and CrO42- anions through the luminescence quenching effect with a low-detection limit of 3.5 x 10(-7) M and 4.6 x 10(-7) M (for 1) and 6.1 x 10(-7) M and 4.3 x 10(-7) M (for 2), respectively. Additionally, the probable mechanisms for the quenching behaviour for the two compounds had also been investigated in detail.