Photoluminescent and Magnetic Properties of a Series of Lanthanide Coordination Polymers with 1H-Tetrazolate-5-formic Acid

Abstract

Reaction of lanthanide oxides and 1H-tetrazolate-5-ethyl formate in water at 50 °C in the presence of pyridine afforded a series of lanthanide coordination polymers, {(Hpy)2[Pr4(tzf)7(H2O)11]·10H2O}n (1) and {[Ln2(tzf)3(H2O)6]·4H2O}n, where Ln = Sm (2), Eu (3), Gd (4), Tb (5), Dy (6), Hpy = protonated pyridine, and H2tzf = 1H-tetrazolate-5-formate acid. Polymer 1 features a three-dimensional (3-D) anion framework built up by cross-linkage of tetrameric [Pr4(tzf)4]4+ units and tzf2− ligands, showing a six-connected primitive cubic (pcu) topology (412·63). Isomorphous polymers 2−6 present a two-dimensional (2-D) neutral structure with a four-connected (4,4) topology. Four new coordination modes of the tzf2− ligand are first observed in tetrazolate-5-carboxylate complexes. The lanthanide contraction effect and coordination flexibility of tzf2− ligand play a key role in governing the formation of coordination polymers with different polymeric architectures. The photoluminescent analyses for 2, 3, 5, and 6 suggest that ligand-to-Ln(III) energy transfer is efficient, especially for Eu(III) and Tb(III) ions, and the tzf2− ligand may act as a good “antenna molecule” to sensitize Ln(III) emission. The variable-temperature magnetic study shows that magnetic interactions between the Ln(III) ions in 1−6 are mainly ascribed to the antiferromagnetic coupling as well as the depopulation of the Stark levels. The spin−orbit coupling parameters (λ) for Sm(III) in 2 and Eu(III) in 3 are 259(2) and 344(1) cm−1, respectively, based on the free-ion approximation.