Synthesis and structural characterization of an air and water stable divalent Europium squarate prepared by in situ reduction

A new air and water stable divalent Europium squarate, [Eu(C4O4)(H2O)] (1), has been prepared and structurally characterized. 1 was synthesized by in situ reduction of a trivalent Europium salt using a Zinc amalgam and mild solvothermal heating. The resulting divalent Europium metal centers in 1 are both air and water stable. This demonstrates that this methodology does not require strict anaerobic conditions. A variation on the synthetic conditions used to prepare 1 (sans the Zinc amalgam) yielded two new series of trivalent lanthanide squarates, [Ln(2)(C4O4)2(C4O3OH)(2)(H2O)(12)] (Ln = La - Nd) (2) and [Ln(2)(C4O4)(3)(H2O)(8)] (Ln = Dy - Lu) (3), which have also been structurally and spectroscopically characterized. Single crystal X-ray diffraction analysis reveals that 1 crystallizes in the C2/m space group and is a 3D structure constructed from sheets of EuO7(H2O) polyhedra with an unusual triangular dodecahedral (CN = 8) geometry, 2 crystallizes in the P (1) over bar space group and is a OD structure constructed from LnO(3)(H2O)(6) monocapped square antiprismatic (CN = 9) metal centers, and 3 crystallizes in the P2(1)/n space group and is a 2D sheet structure constructed from LnO(4)(H2O)(4) square antiprismatic (CN = 8) metal centers. In all structures, the squarate ligands bind to the metal centers with varying, and sometimes uncommon, coordination modes and denticities. Herein, the synthesis, structure, and solid-state UV-vis-NIR spectroscopy of 1, 2, and 3 are presented.