Alkyl–η2-alkene niobocene and tantalocene complexes with the allyldimethylsilyl–η5-cyclopentadienyl ligand: synthesis, NMR studies and DFT calculations

Abstract

Group 5 metal complexes [M(η⁵-C₅H₅){η⁵-C₅H₄SiMe₂(CH₂-η²-CHCH₂)}X] (M = Nb, X = Me, CH₂Ph, CH₂SiMe₃; M = Ta, X = Me, CH₂Ph) and [Ta(η⁵-C₅Me₅){η⁵-C₅H₄SiMe₂(CH₂-η²-CHCH₂)}X] (X = Cl, Me, CH₂Ph, CH₂SiMe₃) containing a chelating alkene ligand tethered to a cyclopentadienyl ring have been synthesized in high yields by reduction with Na/Hg (X = Cl) and alkylation with reductive elimination (X = alkyl) of the corresponding metal(IV) dichlorides [M(η⁵-Cp){η⁵-C₅H₄SiMe₂(CH₂CHCH₂)}Cl₂] (Cp = C₅H₅, M = Nb, Ta, Cp = C₅Me₅, M = Ta). These chloro– and alkyl–alkene coordinated complexes react with CO and isocyanides [CNtBu, CN(2,6-Me₂C₆H₃)] to give the ligand-substituted metal(III) compounds [M(η⁵-Cp){η⁵-C₅H₄SiMe₂(CH₂CHCH₂)}XL] (X = Cl, Me, CH₂Ph, CH₂SiMe₃). Reaction of the chloro–alkene tantalum complex with LiNHtBu results in formation of the imido hydride derivative [Ta(η⁵-C₅Me₅){η⁵-C₅H₄SiMe₂(CH₂CHCH₂)}H(NtBu)]. NMR studies for all of the new compounds and DFT calculations for the alkene-coordinated metal complexes are compared with those known for related group 4 metal cations.