Structural Systematics of Lanthanide(iii) Picrate Solvates: Hexamethylphosphoramide and Octamethylpyrophosphoramide Adducts.

Crystalline products of the reactions of lanthanide picrates, Ln(pic)₃ (pic = 2,4,6-trinitrophenoxide), with hexamethylphosphoramide (hmpa) and octamethylpyrophosphoramide (ompa) have been characterised by single-crystal X-ray diffraction studies. With hmpa and lighter lanthanides (La, Sm, Eu), isomorphous species (monoclinic, P 2₁/ c , Z 4) of stoichiometry [Ln(pic)₃(hmpa)₃]·0.5H₂O, have been defined where the molecular units in the lattice contain 9-coordinate Ln with tricapped trigonal-prismatic coordination geometry. The picrate ligands are bidentate through phenoxide-O and 2-nitro-O, with the latter occupying the capping positions, while the hmpa ligands are singly O-bound to one trigonal face. Heavier lanthanides (Gd, Lu) and Y have been found to give isomorphous (monoclinic, P 2₁/ n , Z 4) species of stoichiometry [Ln(pic)₃(hmpa)₂], with 8-coordinate Ln of an irregular geometry best considered as close to that of a bicapped trigonal-prism. The picrate ligands chelate in the same manner as in the lighter Ln complexes but with one spanning a trigonal edge, and the hmpa-O donors occuping two apices of the other trigonal face. The ligand ompa has been found to act as a bidentate chelate in all isolated species, displacing one picrate from the metal ion coordination sphere to give ionic complexes. For La, Nd, and Gd, isomorphous (monoclinic, P 2₁/ n , Z 4) complexes of stoichiometry [Ln(pic)₂(ompa)₂(OH₂)](pic)·0.5H₂O containing 9-coordinate, tricapped trigonal-prismatic Ln with a single aqua ligand have been defined, while for Er, Yb, Lu, and Y, both the coordinated and lattice water molecules are lost in isomorphous (monoclininc, P 2₁/ c , Z 8) 8-coordinate, square-antiprismatic species of stoichiometry [Ln(pic)₂(ompa)₂](pic). For Er, further polymorphs, one monoclinic, P 2₁/ c , and the other triclinic, CH19251_IE1.gif , have also been characterised. Structural studies of adducts of the lanthanide picrates with hexamethylphosphoramide (hmpa) and octamethylpyrophosphoramide (ompa) provide significant contrasts with those of analogous adducts with dimethylsulfoxide and trimethylphosphate. Adducts of hmpa show a transition from 9-coordinate (hmpa)₃ to 8-coordinate (hmpa)₂ species, while ompa displaces one picrate from the primary coordination sphere. [ABSTRACT FROM AUTHOR]