Daniel K. Unruh

Symmetry versus Minimal Pentagonal Adjacencies in Uranium‐Based Polyoxometalate Fullerene Topologies

C U soon: Clusters containing 60, 44, and 36 uranyl peroxide hydroxide polyhedra (see picture) adopt fullerene topologies of maximum symmetry. The largest of these, denoted U60, is topologically identical to C(60) with no pentagonal adjacencies and the highest possible symmetry. U44 adopts the topology with maximum symmetry rather than that with the lowest number of pentagonal adjacencies.

Uranyl-peroxide interactions favor nanocluster self-assembly.

Uranyl peroxide polyhedra are known to self-assemble into complex closed clusters with fullerene and other topologies containing as many as 60 polyhedra. Here clusters containing 20 uranyl pentagonal triperoxides have been isolated and characterized that assume the smallest possible fullerene topology consisting only of 12 pentagons. Oxalate has been used to crystallize fragments of larger uranyl peroxide clusters, and these fragments and other known structures indicate that the U-O(2)-U dihedral angle is inherently bent. Such bending is thought to be essential in directing the self-assembly of uranyl peroxide polyhedra into closed clusters.

Uranyl peroxide closed clusters containing topological squares.

Four self-assembling clusters of uranyl peroxide polyhedra have been formed in alkaline aqueous solutions and structurally characterized. These clusters consist of 28, 30, 36 and 44 uranyl polyhedra and exhibit complex new topologies. Each has a structure that contains topological squares, pentagons and hexagons. Analysis of possible topologies within boundary constraints indicates a tendency for adoption of higher symmetry topologies in these cases. Small angle X-ray scattering data demonstrated that crystals of one of these clusters can be dissolved in ultrapure water and that the clusters remain intact for at least several days.

Complex nanoscale cage clusters built from uranyl polyhedra and phosphate tetrahedra.

Five cage clusters that self-assemble in alkaline aqueous solution have been isolated and characterized. Each is built from uranyl hexagonal bipyramids with two or three equatorial edges occupied by peroxide, and three also contain phosphate tetrahedra. These clusters contain 30 uranyl polyhedra; 30 uranyl polyhedra and six pyrophosphate groups; 30 uranyl polyhedra, 12 pyrophosphate groups, and one phosphate tetrahedron; 42 uranyl polyhedra; and 40 uranyl polyhedra and three pyrophosphate groups. These clusters present complex topologies as well as a range of compositions, sizes, and charges. Two adopt fullerene topologies, and the others contain combinations of topological squares, pentagons, and hexagons. An analysis of possible topologies further indicates that higher-symmetry topologies are favored.

Synthesis and Structural Characterization of Hydrolysis Products within the Uranyl Iminodiacetate and Malate Systems

The interplay of hydrolysis and chelation by organic ligands results in the formation of novel uranium species in aqueous solutions. Many of these molecular complexes have been identified by spectroscopic and potentiometric techniques, but a detailed structural understanding of these species is lacking. Identification of possible uranyl hydrolysis products in the presence of organic functional groups has been achieved by the crystallization of molecular species into a solid-state compound, followed by structural and chemical characterization of the material. The structures of three novel molecular complexes containing either iminodiacetate (ida) (Na3[(UO2)3(OH)3(ida)3]·8H2O (1)) or malate (mal) (K(pip)2[(UO2)3O(mal)3]·6H2O (2a) (pip = C4N2H12), (2b) (pip)3[(UO2)3O(mal)3]·H2O, and (pip)6[(UO2)11(O)4(OH)4(mal)6(CO3)2]·23H2O (3)) ligands have been determined by single-crystal X-ray diffraction and have been chemically characterized by IR, Raman, and NMR spectroscopies. A major structural component in compounds 1 and 2 is a trimeric 3:3 uranyl ida or mal species, but different bridging groups between the metal centers create variations in the structural topologies of the molecular units. Compound 3 contains a large polynuclear cluster with 11 U atoms, which is composed of trimeric and pentameric building units chelated by mal ligands and linked through hydroxyl groups and carbonate anions. The characterized compounds represent novel structural topologies for U(6+) hydrolysis products that may be important molecular species in near-neutral aqueous systems.

Uranium(VI) tetraoxido core coordinated by bidentate nitrate.

The synthesis and structural characterization of the compounds K[(UO(2))(2)(UO(4))(OH)(NO(3))(2)]H(2)O (1) and Ba[(UO(2))(4)(UO(4))(2)(OH)(2)(NO(3))(4)]H(2)O (2) have revealed that each contains sheets that are based upon the beta-U(3)O(8)-type topology and that these sheets are linked through low-valence interlayer cations. Consistent with other uranium(VI) compounds that have topologically identical sheets, one of the uranium(VI) sites exhibits a highly unusual (UO(4))(2-) tetraoxido core that is further coordinated by two bidentate (NO(3))(-) groups.

Expanding the Crystal Chemistry of Actinyl Peroxides: μ-η2:η1 Peroxide Coordination in Trimers of U6+ Polyhedra

Uranyl peroxides have been intensively studied recently because they form topologically complex structures including spherical clusters containing tens of uranyl polyhedra. In all uranyl peroxides reported to date, the coordination of U(6+) cations by peroxide is bidentate. The compound K(2)(Mg(H(2)O)(6))(4)[(UO(2))(3)(O(2))(8)].2H(2)O has been synthesized and characterized and contains a trimer of linked uranyl peroxide polyhedra. The central U(6+) cation is linked to two peroxide groups in a mu-eta(2):eta(1) configuration. Inclusion of this mode of linkage could dramatically increase the flexibility and topological complexity of uranyl peroxide nanoscale clusters.

Competitive Pseudopericyclic [3,3]- and [3,5]-Sigmatropic Rearrangements of Trichloroacetimidates

The Woodward-Hoffmann rules predict whether concerted pericyclic reactions are allowed or forbidden based on the number of electrons involved and whether the cyclic orbital overlap involves suprafacial or antarafacial orbital overlap. Pseudopericyclic reactions constitute a third class of reactions in which orthogonal orbitals make them orbital symmetry allowed, regardless of the number of electrons involved in the reaction. Based on the recent report of eight-centered ester rearrangements, it is predicted that the isoelectronic eight-centered rearrangements of imidates would also be allowed. We now report that these rearrangements occur, and indeed, an eight-centered rearrangement is slightly favored in at least one case over the well-known six-centered Overman rearrangements, in a trichloroacetimidoylcyclohexadienone, a molecular system where both rearrangements are possible.

Surface Modification of Al30 Keggin-Type Polyaluminum Molecular Clusters

Keggin-type molecular clusters formed from the partial hydrolysis of aluminum in aqueous solutions have the capacity to adsorb a variety of inorganic and organic contaminants. The adsorptive capability of Keggin-type polyaluminum species, such as Al13 and Al30, lead to their wide usage as precursors for heterogeneous catalysts and clarifying agents for water purification applications, but a molecular-level understanding of adsorption process is lacking. Two model Al30 clusters, whose surface has been modified with chelated metals (Al(3+) and Zn(2+)) have been synthesized and structurally characterized by single-crystal X-ray diffraction. Al32IDA [(Al(IDA)H2O)2(Al30O8(OH)60(H2O)22)](2,6-NDS)4(SO4)2Cl4(H2O)40, IDA = iminodiacetic acid, 2,6-NDS = 2,6 napthalene disulfonate) crystallize in the triclinic space group, P1 with a = 13.952(2) Å, b = 16.319(3) Å, c = 23.056(4) Å, α = 93.31(1)°, β = 105.27(1)°, and γ = 105.52(1)°. Zn2Al32 [(Zn(NTA)H2O)2(Al(NTA)(OH)2)2(Al30(OH)60(O)8(H2O)20](2,6-NDS)5(H2O)64, (NTA = nitrilotriacetic acid), also crystallizes in P1 with unit cell parameter refined as a = 16.733(7) Å, b = 18.034(10) Å, c = 21.925(11) Å, α = 82.82(2)°, β = 70.96(2)°, and γ = 65.36(2)°. The chelated metal centers adsorb to the surface of the Al30 clusters through hydroxyl bridges located at the central belt region of the molecule. The observed binding sites for the metal centers mirror the reactivity predicted by previously reported molecular dynamic simulations and can be identified by the acidity and hydration factor of the water group that participates in the adsorption process.

Synthesis, Structure, and Reactivity of Zwitterionic Divalent Rare-Earth Metal Silanides

The synthesis and structures of the first zwitterionic divalent rare-earth metal silanides of the formula [Si(SiMe2OMe)3-κ(3)]2M (M-3), where M = Eu, Yb, and Sm, are reported. M-3 compounds feature spirocyclic bicyclooctane structures in which the central rare-earth metal ions are being octahedrally coordinated by six methoxy groups. The reaction of Yb-3 with BPh3 and W(CO)6 respectively generated the trinuclear zwitterions [Ph3BSi(SiMe2OMe)3-κ(3)]2Yb (Yb-4) and [(CO)5WSi(SiMe2OMe)3-κ(3)]2Yb (Yb-5) in good yield.