Metallic coordination selectivity effect in the trinuclear M3(RCOO)6 secondary building units of three layer metal–carboxylate frameworks

Abstract

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(L_T)_2/4 clusters where the coordination terminal ligand (L_T) 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 L_T. 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.