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.