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

Complexes of U(IV) employing the chelating bis(cyclopentadienyl) ligand set s [Me2Si(eta(5)-C5Me4)(2)](2-) and [Me2Si(eta(5)-C5Me4)(eta(5)-C5H4)](2-) h ave been prepared to examine their utility in generating organoimido comple xes of uranium. The chloride complexes [Me2Si(C5Me4)(2)]UCl2. 2LiCl.4(Et2O) (1) and [Me2Si(C5Me4)(C5H4)]-UCl(2.)2LiCl . 4(THF) (2) may be generated at ro om temperature by reaction of UCl4 with the corresponding bis(cyclopent adienyl) dilithio salt in diethyl ether and tetrahydrofuran, respectively. Complex 1 may be recrystallized from toluene in the presence of TMEDA to yi eld the complex [Me2Si(C5Me4)(2)]U(mu-Cl-4){Li(TMEDA)}(2) (1.TMEDA). The mo lecular structure of this complex has been determined by single-crystal X-r ay diffraction. Compounds 1 and 2 may be alkylated by Grignard reagents. Wh ile the dimethyl complex [Me2Si(C5Me4)(2)]UMe2 (3) may be isolated, the ana logous complex and [Me2Si(C5Me4)(C5H4)]UMe2 appears to be thermally unstabl e. Benzyl derivatives employing both ligand sets (4, 5) have been prepared. These alkyl complexes are protonated by N,N'-diphenylhydrazine to yield ph enylimido-containing products. Surprisingly, while reaction of compounds 3 or 4 with N,N'-diphenylhydrazine yields the expected U(VI) complex [Me2Si(C 5Me4)(2)]U(NC6H5)(2), reactions employing 5 generate only the U(IV) monoimi do dimer [Me2Si(C5Me4)(C5H4)]U(NC6H5)](2) (8). The molecular structure of 8 has been determined, the compound exists as a dimer with asymmetric organo imido bridging ligands. Electrochemical investigations of the chloride comp ounds 1 and 2 suggest that the ancillary ligands have the capacity to signi ficantly alter the redox activity of the metal center. The use of electron- rich ancillary ligands appears to be important in the isolation of high-val ent organouranium complexes. (C) 1999 Elsevier Science S.A. All rights rese rved.