Lanthanide Complexes of 3-Diphenylamino-4-hydroxycyclobut-3-ene-1,2-dione (Diphenylaminosquarate)

Complexation of the ligand 3-diphenylamino-4-hydroxycyclobut-3-ene-1,2-dione (diphenylaminosquarate) with metal salts of La, Eu, Gd and Tb produced the first set of lanthanide complexes with a monosubstituted aminosquarate ligand. The lanthanum complex La[(C₆H₅)₂NC₄O₃]₃(H₂O)₆·5H₂O (1) is monomeric and crystallizes in the triclinic space group P1̄, with a nine-coordinate La center bonded to three pendant diphenylaminosquarate groups and six aqua ligands. The Eu, Gd, and Tb complexes [M(μ-(C₆H₅)₂NC₄O₃)((C₆H₅)₂NC₄O₃)(NO₃)(OH₂)₄]₂·4H₂O (2−4) are isomorphous (also P1̄), each metal atom being nine-coordinate and bridged to its neighbor by two diphenylaminosquarate ligands in a μ-1,2- fashion; the coordination polyhedron around each metal is completed by a pendant diphenylaminosquarate ligand, four aqua ligands, and a chelating nitrate ion. The resistance of the diphenylamino substituent to hydrolysis, vis-à-vis the hydrolysis of dialkylamino substituents, is discussed.