Lanthanide complexes of 1,3,5-triamino-1,3,5-trideoxy-cis-inositol: evidence for heterotrinuclear species in aqueous solution

Abstract

1,3,5-Triamino-1,3,5-trideoxy-cis-inositol (L) gives trinuclear complexes of trivalent lanthanide cations, [Ln₃L₂H₋₆]³⁺. In this contribution, we demonstrate the capability of this cyclohexanic ligand to form heterotrinuclear complexes of 4f cations in aqueous solutions containing mixtures of rare-earths. Electrospray ionization mass spectra clearly show the presence of heterometallic complexes together with the corresponding homonuclear species. Moreover, these compounds have very characteristic ¹H and ¹³C NMR parameters; paramagnetic chemical shifts and longitudinal relaxation times are typical of the lanthanide cations inserted in the complexes. Potentiometric analysis of solutions containing two different lanthanide salts affords the stability constants of the heterotrimetallic [Ln₂Ln′L₂H₋₆]³⁺ complexes. Furthermore, ¹H NMR titrations show the appearance of the complexes in order of stability as the pH is increased and confirm the potentiometry results. The order of trinuclear complex stabilities deduced from these experiments is: Nd₃ < Nd₂Sm < Nd₂Eu < NdSm₂ < NdSmEu ∼ NdEu₂ < Sm₃ < Sm₂Eu < Eu₃ < SmEu₂. These constants are compared to the statistical introduction of the three lanthanide cations in [Ln₂Ln′L₂H₋₆]³⁺ complexes and the deviations suggest that the ionic recognition between the metallic sites is quite limited when the lanthanide cations are close in the 4f series.