Computational Study of Reduction Potentials of Th4+ Compounds and Hydrolysis of ThO2(H2O)n, n = 1, 2, 4.

The stability of Th4+ to reduction in water is studied by DFT methods. The standard reduction potential (SRP) of homoleptic complexes including Th(H2O)94+, Th(H2O)104+, Th(NO3)4, Th(NO2)62-, Th(NO3)62-, Th(COT)2, Th(acac)4, ThCp4, ThF4, and ThCl4 have been investigated. The values vary widely (from -3.50 V for Th(OH)4 to -0.62 V for Th(NO3)4 depending on whether the ligands are redox active (noninnocent) or not. Several additional topics of thorium chemistry are explored, including the hydrolysis mechanism of ThO2(H2O)n, n = 1, 2, 4, and the solution phase nonzero dipole moment of ThCp4. Dinuclear complexes are also characterized, including Th2O4, Th2O2(OH)4, Th2O2(H2O)8, Th2(OH)8(H2O)4, and Th2(OH)2(NO3)6(H2O)4 and condensed thorium complexes as [Th4(OH)6(H2O)12]10+ and [Th6(OH)14(H2O)12]10+. For the Th2(OH)2(NO3)6(H2O)4 dinuclear complex, the first SRP is -0.82 V and the second is 1.59 V. The first SRP corresponds to the reduction of the ligand NO3-, and the second SRP corresponds to dissociative electron transfer to the NO32- ligand. The calculated formation constant of Th(EDTA)(H2O)4 is in reasonable agreement with experiment. The different stereochemistries of the bidentate ligands NO2-, NO3-, and acetylacetonate (acac) around the thorium center have very similar stabilities.