Using the Singly Deprotonated Triethanolamine to Prepare Dinuclear Lanthanide(III) Complexes: Synthesis, Structural Characterization and Magnetic Studies

Abstract

The 1:1 reactions between hydrated lanthanide(III) nitrates and triethanolamine (teaH3) in MeOH, in the absence of external bases, have provided access to the dinuclear complexes [Ln2(NO3)4(teaH2)2] (Ln = Pr, 1; Ln = Gd, 2; Ln = Tb, 3; Ln = Dy, 4; Ln = Ho, 5) containing the singly deprotonated form of the ligand. Use of excess of the ligand in the same solvent gives mononuclear complexes containing the neutral ligand and the representative compound [Pr(NO3)(teaH3)2](NO3)2 (6) was characterized. The structures of the isomorphous complexes 1∙2MeOH, 2∙2MeOH and 4∙2MeOH were solved by single-crystal X-ray crystallography; the other two dinuclear complexes are proposed to be isostructural with 1, 2 and 4 based on elemental analyses, IR spectra and powder XRD patterns. The IR spectra of 1–6 are discussed in terms of structural features of the complexes. The two LnIII atoms in centrosymmetric 1∙2MeOH, 2∙2MeOH and 4∙2MeOH are doubly bridged by the deprotonated oxygen atoms of the two η1:η1:η1:η2:μ2 teaH2− ligands. The teaH2− nitrogen atom and six terminal oxygen atoms (two from the neutral hydroxyl groups of teaH2− and four from two slightly anisobidentate chelating nitrato groups) complete 9-coordination at each 4f-metal center. The coordination geometries of the metal ions are spherical-relaxed capped cubic (1∙2MeOH), Johnson tricapped trigonal prismatic (2∙2MeOH) and spherical capped square antiprismatic (4·2MeOH). O–H∙∙∙O H bonds create chains parallel to the a axis. The cation of 6 has crystallographic two fold symmetry and the rotation axis passes through the PrIII atom, the nitrogen atom of the coordinated nitrato group and the non-coordinated oxygen atom of the nitrato ligand. The metal ion is bound to the two η1:η1:η1:η1 teaH3 ligands and to one bidentate chelating nitrato group. The 10-coordinate PrIII atom has a sphenocoronal coordination geometry. Several H bonds are responsible for the formation of a 3D architecture in the crystal structure of 6. Complexes 1–6 are new members of a small family of homometallic LnIII complexes containing various forms of triethanolamine as ligands. Dc magnetic susceptibility studies in the 2–300 K range reveal the presence of a weak to moderate intramolecular antiferromagnetic exchange interaction (J = −0.30(2) cm−1 based on the spin Hamiltonian H ^ = - J ( S ^ Gd 1 ⋅ S ^ Gd 1 ′ ) ) for 2 and probably weak antiferromagnetic exchange interactions within the molecules of 3–5. The antiferromagnetic GdIII∙∙∙GdIII interaction in 2 is discussed in terms of known magnetostructural correlations for complexes possessing the {Gd2(μ2-OR)2}4+ core. Ac magnetic susceptibility measurements in zero dc field for 3–5 do not show frequency dependent out-of-phase signals; this experimental fact is discussed and rationalized for complex 4 in terms of the magnetic anisotropy axis for each DyIII center and the oblate electron density of the metal ion.