Studies on lanthanide complexes of the tripodal ligandbis(2-benzimidazolylmethyl)(2-pyridylmethyl)amine. Crystal structures and luminescence properties

Abstract

A new unsymmetric tripodal ligand bis(2-benzimidazolylmethyl)(2-pyridylmethyl)amine (L) bearing two kinds of chromophores benzimidazole and pyridine has been synthesized, and two series of lanthanide(III) complexes [LnL(NO₃)₃]·C₂H₅OH (Ln = La, Sm, Eu, Gd, Tb or Tm) and [LnLCl₃(H₂O)]·C₂H₅OH (Ln = La, Sm, Eu, Tb or Ho) have been characterized by elemental analyses, FAB mass spectra, ¹H NMR, conductivity measurements, and IR spectra. The crystal and molecular structures of the three complexes [SmL(NO₃)₃]·C₂H₅OH 1, [GdL(NO₃)₃]·C₂H₅OH 2, and [SmLCl₃(H₂O)]·C₂H₅OH 3 have been determined by X-ray diffraction analysis. 1 and 2 are isomorphous with the central metal ion co-ordinated by the four nitrogen atoms of the ligand and six oxygen atoms of three chelating nitrate groups. In complex 3, besides the four nitrogen atoms of the ligand and three chlorine atoms, there is one H₂O molecule co-ordinated. Conductivity studies on the [LnL(NO₃)₃]·C₂H₅OH complexes in methanol showed that two of the nitrate anions dissociate to give 2∶1 electrolytes. Comparison of the emission lifetimes of the Eu³⁺ and Tb³⁺ complexes in CH₃OH and CD₃OD gives the average value for q, the number of co-ordinated methanol molecules, of 3.4, suggesting the formation of species [LnL(NO₃)(CH₃OH)₃][NO₃]₂. The emission quantum yields of the nitrate-co-ordinated series are substantially higher than those of the chloride series, as shown by luminescence studies in MeCN.