Lanthanide and actinide based coordination polymers and metal-organic framework materials present a number of interesting opportunities with respect to their syntheses and properties. Ln elements typically display large coordination numbers and roughly spherical bonding environments whereas An elements (specifically U(VI)) tend to form triatomic species with terminal oxo groups. As such, these features may be perceived by some as obstacles to the development of designed architectures of f-metal containing hybrid materials. We argue that these features, when coupled to the luminescent behavior of these elements actually give rise to a diverse family of compounds with a range of interesting topologies. Presented herein is an overview of the structural features and luminescent behavior of these materials, as well as some synthetic strategies toward producing both homo-and heterometallic compounds.

The hydrothermal synthesis and structures of [UO2(PDA)] (1) and [Th(PDA)2(H2O)2].H2O (2) (PDA = 1,10-phenanthroline-2,9-dicarboxylic acid) are reported. 1 is orthorhombic, Pnma, a = 11.1318(7) A, b = 6.6926(4) A, c = 17.3114(12) A, V = 1289.71(14), Z = 4, R = 0.0313; 2 is triclinic, P1, a = 7.6190(15) A, b = 10.423(2) A, c = 17.367(4) A, alpha = 94.93(3) degrees , beta = 97.57(3) degrees , gamma = 109.26(3) degrees , V = 1278.3(4) A (3), Z = 2, R = 0.0654. The local geometry around the U in 1 is a pentagonal bipyramid with the two uranyl oxygens occupying the apical positions. The donor atoms in the plane comprise the four donor atoms from the PDA ligand (average U-N = 2.558 and U-O = 2.351 A) with the fifth site occupied by a bridging carboxylate oxygen from a neighboring UO2/PDA individual. The PDA ligand in 1 is exactly planar, with the U lying in the plane of the ligand. The latter planarity, as well as the near-ideal U-O and U-N bond lengths, and O-U-N and N-U-N bond angles within the chelate rings of 1 suggest that PDA binds to the uranyl cation in a low-strain manner. In 2, there are two PDA ligands bound to the Th (average Th-N = 2.694 and Th-O = 2.430 A) as well as two water molecules (Th-O = 2.473 and 2.532 A) to give the Th a coordination number of 10. The PDA ligands in 2 are bowed, with the Th lying out of the plane of the ligand. Molecular mechanics calculations suggest that the distortion of the PDA ligands in 2 arises because of steric crowding. UV spectroscopic studies of solutions containing 1:1 ratios of PDA and Th(4+) in 0.1 M NaClO4 at 25 degrees C indicate that log K1 for the Th(4+)/PDA complex is 25.7(9). The latter result confirms the previous prediction that complexes of PDA with metal ions of higher charge and an ionic radius of about 1.0 A such as Th(IV) would have remarkably high log K1 values with PDA. The origins of this very high stability are discussed in terms of a synergy between the pyridyl and the carboxylate donor groups of PDA. Metal ions of high charge normally bond poorly with pyridyl donors in aqueous solution because such metal ions require donor groups that are able to disperse charge to the solvent via hydrogen-bonding, which pyridyl groups are unable to do. In PDA, the carboxylates fulfill this need and so enable the high donor strength of the pyridyl groups of PDA to become apparent in the high log K1 for Th(IV) with PDA.

Self-assembly under hydrothermal conditions has been employed to synthesize several novel uranium-containing polymeric materials in the pyridinedicarboxylic acid (pydc) system. Uranium containing coordination polymers were synthesized utilizing 2,3-pyridinedicarboxylic acid (2,3-pydc), 2,4-pyridinedicarboxylic acid (2,4-pydc) and 2,6-pyridinedicarboxylic acid (2,6-pydc) as the organic linker. Furthermore, several bimetallic compounds were also synthesized, U-M-2,6-pydc (M = Cu, Ag, Pb). A new secondary building unit for uranium(vi) compounds has also been realized in compound 4 [(UO(2))(2)(C(7)H(3)NO(4))(O)(H(2)O)] through tetramer building units edge shared to form one-dimensional chains. Presented herein will be the syntheses, crystal structures and fluorescent properties of these uranium-containing compounds.

Two novel coordination polymers, UO2(C5H2N2O4)(H2O) (1) and (UO2)Cu(C5H2N2O4)2(H2O)2 (2), have been prepared by the hydrothermal reaction of uranyl nitrate hexahydrate [(UO2(NO3)2.6H2O], 3,5-pyrazoledicarboxylic acid (H3pdc) and copper(II) nitrate hemipentahydrate (Cu(NO3)2.2.5H2O) and characterized by single-crystal X-ray diffraction, thermogravimetric analyses (TGA) and fluorescence spectroscopy. Compound 1 (monoclinic, P2(1)/c, a=6.9556(6)A, b=11.302(1)A, c= 10.5288(9)A, beta=90.057(2) degrees and Z=4) consists of a two-dimensional sheet containing uranyl hexagonal bipyramids. Compound 2 (triclinic, P-1, a=5.1014(7)A, b=7.6067(11)A, c=10.2910(15)A, alpha=72.380(3) degrees, beta=86.796(3) degrees, gamma=84.447(3) degrees and Z=1) consists of two-dimensional sheets. Both structures contain the linear UO2(2+) moiety and have extended networks built up from the H3pdc ligand. Compound 1 exhibits the characteristic UO(2)2+ emission spectra when it is excited at the ligand or uranium excitation wavelength. With the addition of the copper metal center in compound 2, the uranium emission is absent regardless of the excitation wavelength.