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

摘要

Complexes of U(IV) employing the chelating bis(cyclopentadienyl) ligand sets [Me_2Si(#eta#~5-C_5Me_4)_2]~(2-) and [Me_2Si(#eta#~5-C_5Me_4)(#eta#~5-C_5H_4)]~(2-) have been prepared to examine their utility in generating organoimido complexes of uranium. The chloride complexes [Me_2Si(C_5Me_4)_2]UCl_2 centre dot 2LiCl centre dot 4(Et_2O) (1) and [Me_2Si(C_5Me_4)(C_5H_4)]-UCl_2 centre dot 2LiCl centre dot 4(THF) (2) may be generated at room temperature by reaction of UCl_4 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 [Me_2Si(C_5Me_4)_2]U(#mu#-Cl_4){Li(TMEDA)}_2 (1 centre dot 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 [Me_2Si(C_5Me_4)_2]UMe_2 (3) may be isolated, the analogous complex and [Me_2Si(C_5Me_4)(C_5H_4)]UMe_2 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 [Me_2Si(C_5Me_4)_2]U(NC_6H_5)_2, reactions employing 5 generate only the U(IV) monoimido dimer [Me_2Si(C_5Me_4)(C_5H_4)]U(NC_6H_5)_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.