2D and 3D lanthanide coordination polymers constructed from benzimidazole-5,6-dicarboxylic acid and sulfate bridged secondary building units

Abstract

Hydrothermal reaction of Ln(NO₃)·6H₂O with benzimidazole-5,6-dicarboxylic acid (H₃bidc) and KSCN afforded a series of new two- and three-dimensional coordination polymers, namely [Ln₂(Hbidc)₂(SO₄)(H₂O)₃]_n (Ln = La (1), Pr (2), Sm (3), Gd (4)) and [Ln₄(Hbidc)₄(SO₄)₂]_n·2nH₂O (Ln = Eu (5), Tb (6), Dy (7), Er (8)). The sulfates were in situ generated by catalytic oxidation of KSCN. These complexes were characterized by single crystal X-ray diffraction, IR spectroscopy, thermal analysis, fluorescent spectra and magnetic measurements. Complexes 1–4 are isomorphous and comprise a 2D network based upon planar tetranuclear lanthanide secondary building units. The 2D layers extend to a 3D framework by π–π stacking between imidazole rings and hydrogen bonding interactions. The complexes 5–8 are also isomorphous and have similar 3D frameworks fabricated through Hbidc²⁻ and SO₄²⁻ bridging dinuclear lanthanide secondary building units, and the guest water molecules are trapped in the lattice. The luminescent properties of complexes 5 and 6 exhibit characteristic europium and terbium luminescence, while 4 involves ligand-to-metal charge transfer. The magnetic properties of four complexes were also investigated.