Synthesis and Structures of New Pyromellitate Coordination Polymers with Piperazine as a Ligand

Two new coordination polymers of the pyromellitic acid, [Zn2(H2O)(C10H2O8)(C4N2H8)]∞, I, and [Cd4(H2O)2(C10H2O8)2(C4N2H8)3]∞, 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).