Coordination of trivalent lanthanum with polyethylene glycol in the presence of picrate anion: Spectroscopic and X-ray structural studies

The effect of polyethylene chain length on molecular structures of lanthanum-picrate-polyethylene glycol (PEG) complexes has been studied. The change in the ability of the inner coordination sphere to accommodate the largest size of La³⁺ has been investigated in complexes with the ligands triethylene glycol (EO3), tetraethylene glycol (EO4), and pentaethylene glycol (EO5). The X-ray studies demonstrated that the complexes [La(Pic)₂(EO3)₂]⁺(Pic)^- (I and II), [La(Pic)₂(OH₂)(EO4)]⁺(Pic)^-·H₂O (III), and [La(Pic)₂(EO5)]⁺(Pic)^- (IV) crystallized in the monoclinic system in space groups P2₁/c (I and IV), Pn (II), and C2/c (III). Compounds I and II are polymorphs. The La³⁺ coordination with the PEG ligand in the presence of picrate anion (Pic) showed a coordination number of 10. The number of oxygen donor atoms dictated the geometry of the inner coordination sphere and the formation of additional inner-sphere ligands through the incorporation of water molecules, especially in III. The result with III is due to the availability of free space in the complex and to the lack of saturation of the inner coordination sphere. All crystals were further stabilized by π-π interactions stacking along the a-axis. No dissociation of La-O bonds in solution was observed in NMR studies conducted at different temperatures.