Lanthanide Squarate Complexes Containing Mono- and Trivalent Thallium

The synthesis and structural characterization of 16 new thallium lanthanide squarate complexes and 1 new cerium squarate oxalate complex are presented. These new complexes─Tl[Ln(C4O4)(H2O)5]·C4O4 (Ln = La-Nd) (1), Tl3[Ln3(C4O4)6(H2O)6]·8H2O (Ln = Sm-Lu, Y) (2), Tl[Ce(C4O4)2(H2O)6]·C4O4 (3), and [Ce2(C4O4)2(C2O4)(H2O)8]·2H2O (4)─all contain the squarate ligand bound to the trivalent lanthanides with varying coordination modes and denticities. Of the four new groups of complexes prepared in this work, two groupings contain monovalent thallium and trivalent lanthanides, the most common oxidation states for these metals. One complex (3), however, contains trivalent thallium, which is an unusual and challenging oxidation state to stabilize. The Tl3+ cation is formed from in situ oxidation by way of tetravalent cerium (Ce4+/Ce3+, E° = 1.72 V; Tl3+/Tl+ = 1.252 V), leading to the formation of a Tl3+-Ce3+-squarate complex. Additionally, one complex (4) is unique in this work in that it contains both the squarate and oxalate ligands, the latter of which was formed in situ from squarate. Single-crystal X-ray diffraction analysis reveals that 1 and 2 have a 2D structure constructed from either LnO4(H2O)5 monocapped square antiprismatic (CN = 9) metal centers (for 1) or LnO4(H2O)4 square antiprismatic (CN = 8) metal centers (for 2), 3 is a 1D chain structure constructed from CeO3(H2O)6 monocapped square antiprismatic (CN = 9) cerium centers, and 4 is a 3D framework structure constructed from CeO5(H2O)4 monocapped square antiprismatic (CN = 9) cerium centers. 2 and 4 display rare coordination modes for the squarate ligand. Herein, the synthesis, characterization, and structural descriptions of these new complexes are presented.