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

Abstract

The synthesis and reactivity of the cationic niobium and tantalum monomethyl complexes [(BDI)MeM(N^tBu)][X] (BDI = [Ar]NC(CH₃)CHC(CH₃)N[Ar], Ar = 2,6-ⁱPr₂C₆H₃; M = Nb, Ta; X = MeB(C₆F₅)₃, B(C₆F₅)₄] was investigated. The cationic alkyl complexes failed to irreversibly bind CO but formed phosphine-trapped acyl complexes [(BDI)(R₃PC(O)Me)M(N^tBu)][B(C₆F₅)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-Me₂-C₆H₃) resulted in irreversible formation of the iminoacyl complex [(BDI)(XylNC(Me))Nb(N^tBu)][B(C₆F₅)₄], which did not bind phosphines but would add a methide group to the iminoacyl carbon to provide the known ketimine complex (BDI)(XylNCMe₂)Nb(N^tBu). Further stoichiometric chemistry explored i) migratory insertion reactions to form new alkoxide, amidinate, and ketimide complexes; ii) protonolysis reactions with Ph₃SiOH to form thermally robust cationic siloxide complexes; and iii) catalytic high-density polyethylene formation mediated by the cationic Nb methyl complex.