Trivalent f-Element Squarates, Squarate-Oxalates, and Cationic Materials, and the Determination of the Nine-Coordinate Ionic Radius of Cf(III)

The synthesis, structure, and solid-state UV-vis-NIR spectroscopy of four new f-element squarates, M-2(C4O4)(3)(H2O)(4) (M = Eu, Am, Cf) and Sm(C4O4)(C4O3OH)(H2O)(2)center dot 0.5H(2)O, four new cationic lanthanide squarate chlorides, [M-4(C4O4)(5)(H2O)(12)]Cl-2 center dot 5H(2)O (M = Eu, Dy, Ho Er), and two new actinide squarate oxalates, M-2(C4O4)(2)(C2O4)(H2O)(4) (M = Am, Cf), are presented. All of the metal centers are trivalent. Singlecrystal X-ray diffraction analysis reveals that M-2(C4O4)(3)(H2O)(4) and Sm(C4O4)(C4O3OH)(H2O)(2) center dot 0.5H(2)O have a two-dimensional sheet structure constructed from MO7(H2O)(2) monocapped square-antiprismatic (coordination number (CN) = 9) metal centers and SmO6(H2O)(2) square-antiprismatic (CN = 8) metal centers, respectively, whereas M-2(C4O4)(2)(C2O4)(H2O)(4) have a three-dimensional (3D) structure constructed from MO7(H2O)(2) monocapped square-antiprismatic (CN = 9) metal centers. Additionally, the cationic framework materials [M-4(C4O4)(5)(H2O)(12)]Cl-2 center dot 5H(2)O have a 3D structure constructed from two crystallographically unique MO5(H2O)(3) square-antiprismatic (CN = 8) metal centers. In these structures, the squarate ligands bind to the metal centers with varying coordination modes and denticities. The results of this study provide another example of the nonparallel chemistry between the lanthanides and transplutonium elements. From the crystallographic data for the isotypic series M-2(C4O4)(3)(H2O)(4) (M = La-Nd, Sm, Eu) and the linear regression fit to a plot of the unit cell volume as a function of the cube of the ionic radius, the nine-coordinate ionic radius of Cf3+ was determined to be 1.127 +/- 0.003 A. Finally, computational analysis of the americium and californium complexes M-2(C4O4)(3)(H2O)(4) and M-2(C4O4)(2)(C2O4)(H2O)(4) reveals three important attributes: (i) the 5f orbitals are nonbonding in all cases, with the bonding differences occurring with the empty 6d orbitals; (ii) the Cf complexes exhibit more covalent character than their Am counterparts; and (iii) there is more covalent character in the squarate-oxalate complexes than in the squarate complexes.