Manganese(II) pyrimidine-4,6-dicarboxylates: synthetic, structural, magnetic, and adsorption insights

A series of manganese(II) coordination polymers containing the bridging ligand pyrimidine-4,6-dicarboxylate (pmdc) have been prepared. The stoichiometries and structural features of these materials, which range from the one-dimensional (1D) chains in ([Mn(mu-pmdc)(H2O)3].2H2O)n (1) and ([Mn2(mu-pmdc)2(H2O)5].2H2O)n (2) to the two-dimensional layers in ([Mn(mu3-pmdc)(H2O)].H2O)n (3) or the three-dimensional porous network in ([Mn(pmdc)].2H2O)n (4), are extremely dependent on the synthetic conditions (i.e., temperature and solvent). In spite of the structural diversity of these systems, crystallographic studies revealed that the pmdc ligand typically displays a tetradentate mu-(kappaO,kappaN:kappaO'',kappaN') coordination mode with the carboxylate groups almost coplanar with the pyrimidine ring [as in compounds 1 and 2 and compound 5 described below)]. In compound 3, the pmdc moiety adopts a pentadentate mu3-(kappaO,kappaN:kappaO'',kappaN':kappaO) coordination mode. The thermal, magnetic, and adsorption properties of these systems were also studied. The results showed that these compounds behave as antiferromagnets as a consequence of efficient magnetic exchange through the pmdc bridges. Compound 4 possesses permanent porosity, as proved by gas sorption data (N2 at 77 K and CO2 at 293 K). Finally, the heteronuclear iron(II)/manganese(II) compound ([FeMn(mu-pmdc)2(H2O)5].2H2O)n (5), which is isomorphous to 2, was also prepared and fully characterized.