Solution conformation of aqueous lanthanide(III)–antipyrine complexes

Abstract

The conformations of lanthanide(IIII)–antipyrine complexes in aqueous medium have been studied by analysis of their ¹H and ¹³C n.m.r. spectra. The 1 : 1 Ln³⁺–antipyrine complexes are proposed to be isostructural (Ln = Pr, Eu, Tb, Dy, Ho, Er, Tm, or Yb). The potentially monodentate ligand does not co-ordinate to the metal via the ketone oxygen atom as is expected for a classical co-ordination compound. Instead, one antipyrine ligand is proposed to be weakly bonded to the hydrated Ln³⁺ ion via a second co-ordination sphere interaction with the ligand in a non-classical ‘metallocene’ type of orientation. Anomalous Tm³⁺ data are interpreted in terms of the assumption of axial symmetry being invalid; all the other Ln³⁺ complexes are proposed to have effective axial symmetry.