Metallic coordination selectivity effect in the trinuclear M3(RCOO)6 secondary building units of three layer metal–carboxylate frameworks†
Three two-dimensional metal–carboxylate frameworks (MCFs), namely Zn3(BDC)3(py)2·2DMF (1), Mn3(BDC)3(DMF)4 (2) and Co3(BDC)3(DMF)2(py)2 (3) [H2BDC = 1,4-benzene dicarboxylic acid, py = pyridine, DMF = N,N′-dimethylformamide], have been synthesized by reaction of the M(NO3)2 and H2BDC with py as base and DMF as solvent, under the microwave-assisted solvo-thermal conditions. Three MCFs were characterized by single crystal X-ray diffraction and mensurated its fluorescent or magnetic behaviors. The structural analyses reveal that 1–3 are constructed of the slightly dissimilar carboxylate bridged linear M3(RCOO)6(LT)2/4 clusters where the coordination terminal ligand (LT) are py in 1, DMF in 2 and py/DMF in 3, respectively. The results demonstrated the different metal ions in two side of every SBU inclined to coordinate their respectively selected LT. The time-dependent density functional theory (TDDFT) calculations of 1 show that the emission peak of 423–428 nm (the experimental value is 425 nm) might be derived from the energy transition on the ligand-to-ligand charge transfer (LLCT) where electron densities residing on the π-bonding orbital of BDC2− are transferred to the π*-anti-bonding orbital of pyridine. Variable-temperature magnetic susceptibility studies indicate the presence of antiferromagnetic exchange coupling within the M3-units of 2 and 3.