Synthesis and structural characterization of uranium ansa-metallocene complexes containing organoimido functional groups; electronic effects of ancillary ligands

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Complexes of U(IV) employing the chelating bis(cyclopentadienyl) ligand sets [Me2Si(η5-C5Me4)2]2− and [Me2Si(η5-C5Me4)(η5-C5H4)]2− have been prepared to examine their utility in generating organoimido complexes of uranium. The chloride complexes [Me2Si(C5Me4)2]UCl2·2LiCl·4(Et2O) (1) and [Me2Si(C5Me4)(C5H4)]-UCl2·2LiCl·4(THF) (2) may be generated at room temperature by reaction of UCl4 with the corresponding bis(cyclopentadienyl) dilithio salt in diethyl ether and tetrahydrofuran, respectively. Complex 1 may be recrystallized from toluene in the presence of TMEDA to yield the complex [Me2Si(C5Me4)2]U(μ-Cl4){Li(TMEDA)}2 (1·TMEDA). The molecular structure of this complex has been determined by single-crystal X-ray diffraction. Compounds 1 and 2 may be alkylated by Grignard reagents. While the dimethyl complex [Me2Si(C5Me4)2]UMe2 (3) may be isolated, the analogous complex and [Me2Si(C5Me4)(C5H4)]UMe2 appears to be thermally unstable. Benzyl derivatives employing both ligand sets (4, 5) have been prepared. These alkyl complexes are protonated by N,N′-diphenylhydrazine to yield phenylimido-containing products. Surprisingly, while reaction of compounds 3 or 4 with N,N′-diphenylhydrazine yields the expected U(VI) complex [Me2Si(C5Me4)2]U(NC6H5)2, reactions employing 5 generate only the U(IV) monoimido dimer [Me2Si(C5Me4)(C5H4)]U(NC6H5)]2 (8). The molecular structure of 8 has been determined; the compound exists as a dimer with asymmetric organoimido bridging ligands. Electrochemical investigations of the chloride compounds 1 and 2 suggest that the ancillary ligands have the capacity to significantly alter the redox activity of the metal center. The use of electron-rich ancillary ligands appears to be important in the isolation of high-valent organouranium complexes.