Structural Snapshots of Cluster Growth from {U₆} to {U₃₈} During the Hydrolysis of UCl₄

Abstract

Here we report the assembly of large uranium(IV) clusters with novel nuclearities and/or shapes from the controlled hydrolysis of UCl4 in organic solution and in the presence of the benzoate ligands. {U6}, {U13}, {U16}, {U24}, {U38} oxo and oxo/hydroxo clusters were isolated and crystallographically characterized. These structural snapshots indicate that larger clusters are slowly built from the condensation of octahedral {U6} building blocks. The uranium/benzoate ligand ratio, the reaction temperature and the presence of base play an important role in determining the structure of the final assembly. Moreover, the isolation of different size cluster {U6} (few hours), {U16} (3 days), {U24} (21 days) from the same solution in a chosen set of conditions shows that the assembly of uranium oxo clusters in hydrolytic conditions is time dependent.

Mononuclear to Polynuclear UIV Structural Units: Effects of Reaction Conditions on U‐Furoate Phase Formation

“…Mazzanti and Hixon isolated {An 16 } units for U and Pu, respectively, but these clusters differ significantly from those reported by Loiseau et al. due in part to differences in the U IV coordination environment . Still, it remains unclear how the {U 4 } units isolated in this work relate to these larger oligomers and the {U 38 } cluster, in particular.…”

“…Compound 5 , [U 38 Cl 42 (OH) 2 O 54 (H 2 O) 20 ] ⋅ m H 2 O ⋅ n THF, crystallizes in the tetragonal space group, I 4/ m . Overall, the structure is built from a [U 38 O 54 ] 44+ cluster core related to those previously reported by the groups of Mazzanti and Loiseau for U IV , Loiseau for Np IV , and Soderholm, Burns, and Hixon for Pu IV . Previously reported U 38 cluster cores have been isolated with various carboxylate, chloride, and/or acetamide ligands.…”

“…Further, Mazzanti et al. recently stabilized {U 6 }, {U 16 }, {U 24 }, and {U 38 }‐oxo clusters capped by benzoate ligands from organic solutions . Outside of providing further support for the utility of organic carboxylates in modulating the formation of actinide clusters, this work highlighted the importance of solution conditions on assembly and the time dependence of phase formation.…”

“…Plutonium, for example, has been shown to form large polyoxo clusters ranging from {Pu 16 } to {Pu 38 } oligomers from acidic HCl solutions . Uranium and neptunium similarly form polyoxo clusters; these clusters are often stabilized by organic carboxylates …”

“…For example, many An IV metal cations ligated by carboxylate ligands have been isolated over the past ten years, ranging from mononuclear complexes to large polynuclear clusters capped and/or linked by organic ligands. However, the role of the complexing ligand in the assembly and stabilization of the structural units is somewhat unclear . Until relatively recently, it was generally accepted that organic carboxylates could be used to limit hydrolysis and condensation and thereby “trap” polynuclear species; however, examinations of aqueous Th perchlorate solutions showed that the assembly of hexanuclear units was dependent on the presence of organic carboxylates, with the ligand directing the assembly of the hexameric units from lower order (dinuclear) oligomers .…”