Cerium(IV)-Lanthanide(III)-Pyridine-2,6-dicarboxylic Acid System: Coordination Salts, Chains, and Rings

A series of new heterometallic compounds containing cerium(IV) and lanthanide(III) with pyridine-2,6-dicarboxylic acid (dipicH(2)) have been synthesized Depending on the Ln(III) ion and preparation conditions, we obtained four types of compounds. Typed: one-dimensional coordination polymer, [Ln(dipicH)(H2O)(4)Ce(dipic)(3)]center dot 7H(2)O; Ln = La (1), Ce (2), or Pr (3). Type-II: ionic compound, [Ln(dipicH)(H2O)(6)][Ce(dipic)(3)]center dot 7H(2)O; Ln = Nd (4), Sm (5), Eu (6), Gd (7), or Dy (8). Type-III: octanuclear cluster incorporating a six-membered coordination ring, [Ln(H2O)(7)Ln(dipic)(H2O)(4)(Ce(dipic)(3))(2)](2)center dot nH(2)O; Ln = La (9) n = 24, Ce (10) n = 24, Pr (11) n = 24, or Nd (12) n = 26. Type-IV: hexanuclear cluster incorporating a four-membered coordination ring, [(Ln(H2O)(5))(2)(Ce(dipic)(3))(4)(2H(+))]center dot nH(2)O, Ln = Nd (13) n = 34, Sm (14) n = 26, Eu (15) n = 26, or Gd (16) n = 26 Crystal structures were determined for compounds 1-16. Tb and heavier lanthanides all form compounds isomorphous with Type-IV compounds The observed structural variety is a consequence of the interplay of lanthanide contraction and kinetic effects involving water molecules, free as well as clusters, ranging in size from 6 to 26. A metal-centered luminescence sensitized by dipic ligand was observed in compounds containing Eu-III and Dy-III ions Direct chelation, as opposed to bridging coordination, leads to enhanced luminescence. Resolved EPR spectra were observed for the Gd-III compounds which are dependent on the coordination environment.