Manganese(II) pyrimidine-4,6-dicarboxylates: Synthetic, structural, magnetic and adsorption insights into the system

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(µ-pmdc)(H2O)3]·2H2O}n (1) and {[Mn2(µ-pmdc)2(H2O)5]·2H2O}n (2) to the two-dimensional layers in {[Mn(µ3-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 µ-(κO,κN:κO′′,κN′) 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 µ3-(κO,κN:κO′′,κN′:κO) 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(µ-pmdc)2(H2O)5]·2H2O}n (5), which is isomorphous to 2, was also prepared and fully characterized.