The reaction of the proton-transfer compound piperazine-1,4-diium pyrazine- 2,3-dicarboxylate 4.5-hydrate, C4H12N2 2+C6H2N2O4 24.5H2O or (pipzH2)- (pyzdc)4.5H2O (pyzdcH2 is pyrazine-2,3-dicarboxylic acid and pipz is piperazine), (I), with Zn(NO3)26H2O and CoCl26H2O results in the formation of bis(piperazine-1,4-diium) bis(-pyrazine-2,3-dicarboxylato)-3N1,O2:O3;3O3:- N1,O2-bis[aqua(pyrazine-2,3-dicarboxylato-2N1,O2)zinc(II)] decahydrate, (C4- H12N2)2[Zn2(C6H2N2O4)4(H2O)2] 10H2O or (pipzH2)2[Zn(pyzdc)2(H2O)]2- 10H2O, (II), and catena-poly[piperazine-1,4-diium [cobalt(II)-bis( -pyrazine- 2,3-dicarboxylato)- 3N1,O2:O3; 3O3:N1,O2] hexahydrate], {(C4H12N2)[Co(C6H2- N2O4)2] 6H2O}n or {(pipzH2)[Co(pyzdc)2] 6H2O}n, (III), respectively. In (I), pyzdcH2 is doubly deprotonated on reaction with piperazine as a base. Compound (II) crystallizes as a dimer, whereas compound (III) exists as a onedimensional coordination polymer. In (II), two pyzdc2 groups chelate to each of the two ZnII atoms through a ring N atom and an O atom of the 2-carboxylate group. In one ligand, the adjacent 3-carboxylate group bridges to a neighbouring metal atom. Awater molecule ligates in the sixth coordination site. The structure of (II) can be described as a commensurate superlattice due to an ordering in the hydrogen-bonded network. In (III), no water is coordinated to the metal atom and the coordination sphere is comprised of two N,O-chelates plus two bridging O atoms. A large number of hydrogen bonds are observed in all three compounds. These interactions, as well as and C O stacking interactions, play important structural roles.
