Solution structure and behavior of dimeric uranium(III) metallocene halides

The variable-temperature 1H NMR behavior of the uranium(III) dimers [Cp double-prime 2UX]2 and [Cpdouble-dagger2UX]2, where X is F, Cl, Br, or I, Cp double-prime is 1,3-(Me3Si)2C5H3, and Cpdouble-dagger is 1,3-(Me3C)2C5H3, has been examined. At low temperature, the number of inequivalent CMe3 or SiMe3 groups implies that the solution structure is the same as the solid-state structure in all of these complexes. The barriers to ring rotation in the Cp double-prime series are strongly dependent upon the U-X distance, but all of the barriers to ring rotation in the Cpdouble-dagger series are the same. The trends in ring rotation barriers are explained by the different conformations of the Cp ligands in the dimers. In addition to the homo-halide dimers, the variable-temperature NMR behavior of the hetero-halide dimers Cp'4(μ-X)(μ-Y), where Cp' is Cp double-prime or Cpdouble-dagger and X and Y are halides where X ≠ Y, was examined. Above room temperature, the halide atoms exchange sites rapidly on the NMR time scale