Crystal Structures and Magnetic and Luminescent Properties of a Series of Homodinuclear Lanthanide Complexes with 4-Cyanobenzoic Ligand

A series of homodinuclear lanthanide(III) complexes with the 4-cba ligand, [La<sub>2</sub>(4-cba)<sub>6</sub>(phen)<sub>2</sub>(H<sub>2</sub>O)<sub>6</sub>] (<b>1</b>) and [Ln<sub>2</sub>(4-cba)<sub>6</sub>(phen)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>] (Ln = Pr (<b>2</b>), Nd (<b>3</b>), Sm (<b>4</b>), Eu (<b>5</b>), Gd (<b>6</b>), and Dy (<b>7</b>); 4-Hcba = 4-cyanobenzoic acid; phen = 1,10-phenanthroline), have been synthesized and structurally characterized by single-crystal <i>X</i>-ray diffraction. In <b>1</b>, two water molecules bridge two nine-coordinated La ions, and six 4-cba ligands coordinate to the two La ions in terminal mode. In the isostructural complexes <b>2</b>−<b>7</b>, two eight-coordinated Ln ions are connected by four bidentate 4-cba ligands, and another two 4-cba ligands terminate the two Ln ions. The variable-temperature magnetic properties of <b>2</b>−<b>7</b> have been investigated. Complex <b>7</b> shows a significant ferromagnetic interaction between Dy(III), while no magnetic interaction exists between Gd(III) ions in <b>6</b>. In<b> 2</b>−<b>5</b>, the value of χ<i><sub>M</sub></i><i>T</i> decreases with decreasing temperature, but the magnetic interactions between the Ln(III) ions cannot definitely be concluded. Notably, the spin−orbit coupling parameters, <i>λ</i>, for Sm(III) (216(2) cm<sup>-1</sup>) and Eu(III) (404(2) cm<sup>-1</sup>) have been obtained in <b>4</b> and <b>5</b>, respectively. The strong fluorescent emissions of <b>4</b>, <b>5</b>, and <b>7</b> demonstrate that ligand-to-Ln(III) energy transfer is efficient and that the coordinated water molecules do not quench their luminescence by the nonradiative dissipation of energy.