Synthesis, structures and magnetic properties of two 3D 3,4-pyridinedicarboxylate bridged manganese(II) coordination polymers incorporating 1D helical Mn(carboxylate)2 chain or Mn3(OH)2 chain

The hydrothermal reactions of MnCl2·4H2O, 3,4-pyridinedicarboxylic acid (3,4-pydaH2) and triethylamine in aqueous medium yield two 3D metal–organic hybrid materials, [Mn(3,4-pyda)] (1) and [Mn3(OH)2(3,4-pyda)2(H2O)2] (2), respectively. In both complexes, each 3,4-pyda acts as a pentadentate ligand to connect five Mn(II) atoms via the pyridyl group and the two μ2-carboxylate groups (one in syn,anti-mode and one in syn–syn mode for 1 and both in syn,anti-mode for 2). Complex 1 possesses an interesting 3D coordination polymeric structure incorporating 1 D helical Mn(μ2-carboxylate)2 chain units, in which each Mn(II) atom is coordinated in less common square pyramidal geometry to four carboxylato oxygen atoms and one pyridyl nitrogen atom. Each 3,4-pyda links three helical Mn(μ2-carboxylate)2 chains and each Mn(μ2-carboxylate)2 chain is linked by other eight helical Mn(μ2-carboxylate)2 chains via sharing 3,4-pyda bridges. Complex 2 is a 3D coordination network consisting of 1D Mn3(OH)2 chains and 3,4-pyda bridges. The repeating trimeric structural unit in the manganese(II) hydroxide chain consists of two edge-sharing symmetry-related manganese octahedra linked via μ3-OH to a vertex of Mn2 octahedron. Each 3,4-pyda links three Mn3(OH)2 chains and each Mn3(OH)2 chain is linked by other six Mn3(OH)2 chains via 3,4-pyda bridges, resulting in a 3D coordination solid. Magnetic measurements reveal that a weak antiferromagnetic interaction between the MnII ions occurs in complex 1 and a 3D magnetic ordering at about 7.0 K in complex 2.