Structural Characterization and Reactivity of UO₂(salophen)L and [UO₂(salophen)]₂:  Dimerization of UO₂(salophen) Fragments in Noncoordinating Solvents (salophen = N,N‘-Disalicylidene-o-phenylenediaminate, L = N,N-Dimethylformamide, Dimethyl Sulfoxide)

The molecular structures of UO₂(salophen)L (L = DMF, DMSO) and a uranyl−salophen complex without any unidentate ligands (L) in solid and solution were investigated using single-crystal X-ray analysis and IR, ¹H NMR, and UV−visible absorption spectroscopies. As a result, it was found that the uranyl−salophen complex without L is a racemic dimeric complex, [UO₂(salophen)]₂, in which the UO₂(salophen) fragments are held together by bridging between one of the phenoxide oxygen atoms in salophen and the uranium in the other UO₂(salophen) unit. Furthermore, it was spectrophotometrically demonstrated that [UO₂(salophen)]₂ retains its dimeric structure even in the noncoordinating solvents such as CH₂Cl₂ and CHCl₃ and is in equilibrium with UO₂(salophen)L {2UO₂(salophen)L ⇄ [UO₂(salophen)]₂ + 2L}. The equilibrium constants and thermodynamic parameters of this equilibrium were evaluated from UV−visible absorption and ¹H NMR spectral changes; log K_dim = −2.51 ± 0.01 for L = DMF and solvent = CH₂Cl₂, log K_dim = −1.68 ± 0.02 for L = DMF and solvent = CHCl₃, log K_dim = −4.23 ± 0.01 for L = DMSO and solvent = CH₂Cl₂, and log K_dim = −3.03 ± 0.02 for L = DMSO and solvent = CHCl₃. The kinetics of L-exchange reactions in UO₂(salophen)L and enantiomer exchange of [UO₂(salophen)]₂ in noncoordinating solvents were also studied using NMR line-broadening method. As a consequence, it was suggested that the DMF-exchange reaction in UO₂(salophen)DMF proceeds through two pathways (dissociative and associative paths) and that the predominant path of DMSO exchange in UO₂(salophen)DMSO is the dissociative one. A sliding motion of the UO₂(salophen) fragments was considered to be reasonable for the enantiomer-exchange mechanism of [UO₂(salophen)]₂. On the basis of the kinetic information for UO₂(salophen)L and [UO₂(salophen)]₂, reaction mechanisms including the L-exchange reaction in UO₂(salophen)L, the formation of [UO₂(salophen)]₂ from UO₂(salophen)L, and the enantiomer exchange of [UO₂(salophen)]₂ are proposed.