Unique Dinuclear, Tetrakis(nitrato-O,O′)-Bridged Lanthanide(III) Complexes from the Use of Pyridine-2-Amidoxime: Synthesis, Structural Studies and Spectroscopic Characterization

Abstract

The initial use of pyridine-2-amidoxime (NH₂paoH) in lanthanide(III) [Ln^III] chemistry is described. The reactions of Ln(NO₃)₃ · xH₂O (x = 6 for Eu and Tb; x = 5 for Dy) and NH₂paoH led to the isolation of complexes [Ln₂(NO₃)₆(NH₂paoH)₂] (Ln = Eu, 1; Ln = Tb, 2; Ln = Dy, 3) in low yields. The structure of 3 was solved by single-crystal, X-ray crystallography. Elemental analyses and IR spectroscopy provide strong evidence that 1 and 2 are isostructural with 3. The structure of 3 consists of dinuclear molecules in which the two Dy^III ions are bridged by two bidentate bridging (η¹:η¹:μ) and two chelating-bridging (η¹:η²:μ) nitrato groups. The Dy^III centers are each chelated by a N(2-pyridyl), N′(oxime)-bidentate NH₂paoH ligand and a near-symmetrical bidentate nitrato group. The coordination geometry of each Dy^III can be best described as a muffin-type (MFF-9). The 2D lattice (crystal structure) of 3 is built through H-bonding and π–π interactions. IR spectra of the complexes are discussed in terms of the coordination modes of the ligands involved (NO^– ₃, NH₂paoH). The Eu(III) complex 1 displays an intense red, metal-centered emission in the solid state which is achieved by an indirect process (antenna effect). The Tb^III ion in complex 2 emits in the same region with the ligand.