Synthesis and structures of new pyromellitate coordination polymers with piperazine as a ligand

Two new coordination polymers of the pyromellitic acid, [Zn-2(H2O)(C10H2O8)(C4N2H8)](infinity), I, and [Cd-4(H2O)(2)(C10H2O8)(2)(C4N2H8)(3)](infinity), II, have been prepared employing hydrothermal methods in the presence of piperazine. The structures have three-dimensional connectivity involving both the carboxylate and the amine molecule. The amine molecule acts as a ligand connecting two different metal centers giving rise to a simple dimer in I, and a two-dimensional layer structure in II. There are two types of pyromellitates in I and only one type in II. Of the two types of acids in I, acid-1 has all the oxygen atoms connected to Zn with monodendate connectivity and acid-2 has only four monodendate connectivity and possesses four terminal C-O linkage. In II, two of the carboxylates have bidendate connectivity and the other two have monodendate connectivity and possess one terminal C-O bond. The projection of the bound amine molecules, in II, into the large elliptical channels is noteworthy. These amine molecules may be amenable for chemical manipulations. Both the coordination polymers exhibit photoluminescence at room temperature, the main emission being at 380 and 410 nm, respectively, for I and II, which is due to the ligand-to-metal charger transfer (LMCT).