Coordination Polymers Based on Inorganic Lanthanide(III) Sulfate Skeletons and an Organic Isonicotinate N-oxide Connector:  Segregation into Three Structural Types by the Lanthanide Contraction Effect

Fourteen three-dimensional coordination polymers of general formula [Ln(INO)(H₂O)(SO₄)]_n, where Ln = La, 1·La; Ce, 2·Ce; Pr, 3·Pr; Nd, 4·Nd; Sm, 5·Sm; Eu, 6·Eu; Gd, 7·Gd; Tb, 8·Tb; Dy, 9·Dy; Ho, 10·Ho; Er, 11·Er; Tm, 12·Tm; Yb, 13·Yb; and Lu, 14·Lu; INO = isonicotinate-N-oxide, have been synthesized by hydrothermal reactions of Ln³⁺, MnCO₃, MnSO₄·H₂O, and isonicotinic acid N-oxide (HINO) at 155 °C and characterized by single-crystal X-ray diffraction, IR, thermal analysis, luminescence spectroscopy, and the magnetic measurement. The structures are formed by connection of layer, chain, or dimer of Ln−SO₄ by the organic connector, INO. They belong to three structural types that are governed exclusively by the size of the ions:  type I for the large ions, La, Ce, and Pr; type II for the medium ions, Nd, Sm, Eu, Gd, and Tb; and type III for the small ions, Dy, Ho, Er, Tm, Yb, and Lu. Type I consists of two-dimensional undulate Ln−sulfate layers pillared by INO to form a three-dimensional network. Type II has a 2-fold interpenetration of “3D herringbone” networks, in which the catenation is sustained by extensive π−π interactions and O−H···O and C−H···O hydrogen bonds. Type III comprises one-dimensional chains that are connected by INO bridges, resulting in an α-Po network. The progressive structural change is due to the metal coordination number decreasing from nine for the large ions via eight to seven for the small ions, demonstrating clearly the effect of lanthanide contraction. The sulfate ion acts as a μ₄- or μ₃-bridge, connecting two, three, or four metals, and is both mono- and bidentate. The INO ligand acts as a μ₃- or μ₂-bridge with carboxylate group in syn−syn bridging or bidentate chelating mode. The materials show considerably high thermal stability. The magnetic properties of 4·Nd, 6·Eu, 7·Gd, and 13·Yb and the luminescence properties of 6·Eu and 8·Tb are also investigated.