Lanthanide Complexes of Macrocyclic Polyoxovanadates by VO4 Units: Synthesis, Characterization, and Structure Elucidation by X-ray Crystallography and EXAFS Spectroscopy

Reactions of a tetravanadate anion, [V4O12]4–, with a series of lanthanide­(III) salts yield three types of lanthanide complexes of macrocyclic polyoxovanadates: (Et4N)6[LnIIIV9O27] [Ln = Nd (1), Sm (2), Eu (3), Gd (4), Tb (5), Dy (6)], (Et4N)5[(H2O)­HoIII(V4O12)2] (7), and (Et4N)7[LnIIIV10O30] [Ln = Er (8), Tm (9), Yb (10), Lu (11)]. Lanthanide complexes 1–11 are isolated and characterized by IR, elemental analysis, single-crystal X-ray diffraction, and extended X-ray absorption fine structure spectroscopy (EXAFS). Lanthanide complexes 1–6 are composed of a square-antiprism eight-coordinated LnIII center with a macrocyclic polyoxovanadate that is constructed from nine VO4 tetrahedra through vertex sharing. The structure of 7 is composed of a seven-coordinated HoIII center, which exhibits a capped trigonal-prism coordination environment by the sandwiching of two cyclic tetravanadates with a capping H2O ligand. Lanthanide complexes 8–11 have a six-coordinated LnIII center with a 10-membered vanadate ligand. The structural trend to adopt a larger coordination number for a larger lanthanide ion among the three types of structures is accompanied by a change in the vanadate ring sizes. These lanthanide complexes are examined by EXAFS spectroscopies on lanthanide LIII absorption edges, and the EXAFS oscillations of each of the samples in the solid state and in acetonitrile are identical. The Ln–O and Ln···V bond lengths obtained from fits of the EXAFS data are consistent with the data from the single-crystal X-ray studies, reflecting retention of the structures in acetonitrile.