Synthesis and reactivity of cationic niobium and tantalum methyl complexes supported by imido and β-diketiminato ligands

摘要

The synthesis and reactivity of the cationic niobium and tantalum monomethyl complexes [(BDI)MeM(N~tBu)][X] (BDI = [Ar]NC(CH _3)CHC(CH_3)N[Ar], Ar = 2,6-~iPr_2C _6H_3; M = Nb, Ta; X = MeB(C_6F_5) _3, B(C_6F_5)_4] was investigated. The cationic alkyl complexes failed to irreversibly bind CO but formed phosphine-trapped acyl complexes [(BDI)(R_3PC(O)Me)M(N ~tBu)][B(C_6F_5)_4] (R = Et, Cy) in the presence of a combination of trialkylphosphines and CO. Treatment of the monoalkyl cationic Nb complex with XylNC (Xyl = 2,6-Me_2-C _6H_3) resulted in irreversible formation of the iminoacyl complex [(BDI)(XylNC(Me))Nb(N~tBu)][B(C_6F_5) _4], which did not bind phosphines but would add a methide group to the iminoacyl carbon to provide the known ketimine complex (BDI)(XylNCMe _2)Nb(N~tBu). Further stoichiometric chemistry explored i) migratory insertion reactions to form new alkoxide, amidinate, and ketimide complexes; ii) protonolysis reactions with Ph_3SiOH to form thermally robust cationic siloxide complexes; and iii) catalytic high-density polyethylene formation mediated by the cationic Nb methyl complex.