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

Group 5 metal complexes [M(η5-C5H5){η5-C5H4SiMe2(CH2-η2-CH[double bond, length as m-dash]CH2)}X] (M = Nb, X = Me, CH2Ph, CH2SiMe3; M = Ta, X = Me, CH2Ph) and [Ta(η5-C5Me5){η5-C5H4SiMe2(CH2-η2-CH[double bond, length as m-dash]CH2)}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(CH2CH[double bond, length as m-dash]CH2)}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(CH2CH[double bond, length as m-dash]CH2)}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(CH2CH[double bond, length as m-dash]CH2)}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.The authors acknowledge MCyT (project MAT2001-1309), the Funds of the University of Zürich and COST (project COST-D12/0016/98) for financial suppor