Lanthanum(III) and uranyl(VI) diglycolamide complexes: synthetic precursors and structural studies involving nitrate complexation.

The synthesis and structural characterization of lanthanum(III) and uranyl(VI) complexes coordinated by tridentate diglycolamide (DGA) ligands O(CH2C(O)NR2)2[R=i-Pr (L1), i-Bu (L2)] are described. Reaction of L with UO2Cl2(H2O) n forms the uranyl(VI) cis-dichloride adducts UO2Cl2L [L=L1 (1a), L2 (1b)], while reaction of excess L with the corresponding metal nitrate hydrate produces [LaL3][La(NO3)6] [L=L1 (2a), L2 (2b)] for lanthanum and UO2(NO3)2L [L=L1 (3a), L2 (3b)] for uranium. Compounds 2b and 3a have been structurally characterized. The solid-state structure of the cation of 2b shows a triple-stranded helical arrangement of three tridentate DGA ligands with approximate D3 point-group symmetry, while the counteranion consists of six bidentate nitrate ligands coordinated around a second La center. The solid-state structure of 3a shows a tridentate DGA ligand coordinated along the equatorial plane perpendicular to the OUO unit as well as two nitrate ligands, one bidentate and oriented in the equatorial plane and the other monodentate and oriented parallel to the uranyl unit with the oxygen donor atom situated above the mean equatorial plane. Ambient-temperature NMR spectra for 3a and 3b indicated an averaged chemical environment of high symmetry consistent with fluxional nitrate hapticity, while spectroscopic data obtained at -30 degrees C revealed lower symmetry consistent with the slow-exchange limit for this process.

[Na5(THF)10Ce(mnt)4]infinity: a honeycomb network polymer that yields homoleptic cerium(III) tetrakis(dithiolene) complexes in donor solvents.

The first example of a lanthanide tetrakis(dithiolene) complex, [Na5(THF)10Ce(mnt)4] (1) (mnt = 1,2-maleonitrile-1,2-dithiolate), has been synthesized and characterized by X-ray crystallography and spectroscopic methods. In the solid state, 1 exists as a 2-D corrugated honeycomb network polymer in which the monomeric units comprising the trigonal nodes are knitted together by interlocking dative Na-N bonds extended from nitrile groups of bifunctional mnt ligands coordinated through the sulfur atoms to adjacent cerium centers. Individual honeycomb sheets are separated by 14.8 A. Compound 1 dissolves in donor solvents such as THF and acetonitrile to give soluble [Ce(mnt)4]5- units that exhibit spectroscopic features (i.e., NMR, luminescence, UV-vis) that are consistent with the 4f1 Ce(III) ion. In the first examination of the redox chemistry of a lanthanide dithiolene complex, cyclic voltammetry measurements conducted on 1 reveal a single irreversible oxidation wave that is likely attributable to ligand-centered oxidation.

Fluoride abstraction and reversible photochemical reduction of cationic uranyl(VI) phosphine oxide complexes.

The syntheses, structural and spectroscopic characterization, fluoride abstraction reactions, and photochemical reactivity of cationic uranyl(VI) phosphine oxide complexes are described. [UO2(OPPh3)4][X]2 (1a, X = OTf; 1b, X = BF4) and [UO2(dppmo)2(OPPh3)][X]2 (2a, X = OTf; 2b, X = BF(4)) are prepared from the corresponding uranyl(VI) chloride precursor and 2 equiv each of AgX and phosphine oxide. The BF4- compounds 1b and 2b are prone to fluoride abstraction reactions in methanol, leading to dinuclear fluoride-bridged uranyl(VI) complexes. Fluoride abstraction of 2b in methanol generates two structural isomers of the fluoride-bridged uranyl(VI) dimer [(UO2(dppmo)2)2(mu-F)][BF4]3 (4), both of which have been structurally characterized. In the major isomer 4C, the four dppmo ligands are all chelating, while in the minor isomer 4B, two of the dppmo ligands bridge adjacent uranyl(VI) centers. Photolysis of 2b in methanol proceeds through 4 to form the uranium(IV) fluoride complex [UO2F2(dppmo)3][BF4]2 (5), involving another fluoride abstraction step. X-ray crystallography shows 5 to be a rare example of a structurally characterized uranium(IV) complex possessing terminal U-F bonds. Complex 5 reverts to 4 in solution upon exposure to air.

Anhydrous photochemical uranyl(VI) reduction: unprecedented retention of equatorial coordination accompanying reversible axial oxo/alkoxide exchange.

In a dramatic reversal of the normal trend of observed reactivity in uranyl(VI) coordination chemistry, an unprecedented retention of the normally labile equatorial coordination plane accompanies facile and reversible axial oxo/alkoxide exchange during both the photochemical reduction of cationic uranyl(VI) phosphine-oxide complexes with organic substrates and subsequent hydrolysis of the uranium(IV) alkoxide complexes to regenerate the uranyl(VI) starting complex.

Contrasting solvent and capping ligand effects directing the photochemistry of uranyl(VI) Schiff base complexes.

Photolysis of the uranyl(VI) Schiff base complex UO2(tBu4-salphen)(THF) (1a) with cobaltocene in THF affords [Cp2Co][UO2(tBu4-salphen)(OH)] (2) in high yield while irradiation in toluene yields no reaction. Electronic emission spectra of 1a reveal a large Stokes' shift in toluene similar to that observed in the free ligand, while in THF the structural rearrangement responsible for this shift is blocked. Instead, the ligand-centered excited state is redirected to the uranyl(VI) center by way of energy transfer, thus generating 2 from the intramolecular activation of a coordinated THF molecule.

Synthesis and structural characterization of a uranyl(VI) complex possessing unsupported unidentate thiolate ligands.

The synthesis and structural characterization of the first example of a uranyl(VI) complex possessing unsupported unidentate thiolate ligands, UO2(S-2,6-Cl2C6H3)2L2 (2b, L=N,N-diisobutylisopropylamide), are reported. Isolation of 2b as a stable mononuclear complex is provided by the alkyl substituents of the organic amide ligands, which offer enhanced solubility, electron-releasing properties, and steric protection to help saturate the uranyl(VI) coordination sphere.

Versatile new uranyl(VI) dihalide complexes supported by tunable organic amide ligands.

A series of uranyl(VI) dihalide complexes UO2X2L2 (X = Cl, Br) supported by organic amide ligands (L = R'C(O)NR2; R' = i-Pr; R = i-Pr, i-Bu, s-Bu) offers the versatile combination of facile synthesis using benchtop methods, air-stable crystalline solids obtained in high yield, high solubility in common organic solvents and tunable steric/electronic properties.

Palaeoclimatology: the record for marine isotopic stage 11.

The marine isotopic stage 11 (MIS 11) is an extraordinarily long interglacial period in the Earth's history that occurred some 400,000 years ago and lasted for about 30,000 years. During this period there were weak, astronomically induced changes in the distribution of solar energy reaching the Earth. The conditions of this orbital climate forcing are similar to those of today's interglacial period, and they rendered the climate susceptible to other forcing--for example, to changes in the level of atmospheric carbon dioxide. Here we use ice-core data from the Antarctic Vostok core to reconstruct a complete atmospheric carbon dioxide record for MIS 11. The record indicates that values for carbon dioxide throughout the interglacial period were close to the Earth's pre-industrial levels and that both solar energy and carbon dioxide may have helped to make MIS 11 exceptionally long. Anomalies in the oceanic carbonate system recorded in marine sediments at the time, for example while coral reefs were forming, apparently left no signature on atmospheric carbon dioxide concentrations.