Synthesis and Structural Characterization of Heterometallic Thorium Aluminum Polynuclear Molecular Clusters

Aluminum can undergo hydrolysis in aqueous solutions
leading to
the formation of soluble molecular clusters, including polynuclear
species that range from 1 to 2 nm in diameter. While the behavior
of aluminum has been extensively investigated, much less is known
about the hydrolysis of more complex mixed-metal systems. This study
focuses on the structural characteristics of heterometallic thorium–aluminum
molecular species that may have important implications for the speciation
of tetravalent actinides in radioactive waste streams and environmental
systems. Two mixed metal (Th4+/Al3+) polynuclear
species have been synthesized under ambient conditions and structurally
characterized by single-crystal X-ray diffraction. [Th2Al6(OH)14(H2O)12(hedta)2]­(NO3)6(H2O)12 (ThAl1) crystallizes in space group P21/c with unit cell parameters of a = 11.198(1) Å, b = 14.210(2) Å, c = 23.115(3) Å, and β = 96.375° and [Th2Al8(OH)12(H2O)10(hdpta)4]­(H2O)21 (ThAl2) was modeled in P1̅ with a = 13.136(4) Å, b = 14.481(4) Å, c = 15.819(4) Å,
α = 78.480(9)°, β = 65.666(8)°, γ = 78.272(8)°.
Infrared spectra were collected on both compounds, confirming complexation
of the ligand to the metal center, and thermogravimetric analysis
indicated that the thermal degradation of these compounds resulted
in the formation of an amorphous product at high temperatures. These
mixed metal species have topological relationships to previously characterized
aluminum-based polynuclear species and may provide insights into the
adsorption of tetravalent actinides on colloidal or mineral surfaces.