Reactions of (Butadiene)tantalocene Cation with Alkyl Isocyanides

Abstract

Treatment of (η4-s-trans-butadiene)tantalocene cation (3, with methyltris(pentafluorophenyl)borane anion) with tert-butyl isocyanide (4 h, at 45 °C in bromobenzene) yields the [(η2-butadiene)(C⋮NCMe3)TaCp2+] complex 4a (70% isolated, ν̃(C⋮NR) 2164 cm-1). Likewise treatment with n-butyl or cyclohexyl isocyanide gives the analogous products 4b and 4c, respectively. The cyclohexyl isocyanide adduct 4c was characterized by X-ray diffraction. It shows the η2-butadiene ligand oriented at the front of the Cp2Ta+ bent metallocene wedge with the noncoordinated butadiene vinyl group located in the “vinyl-inside” position. The Ta−C⋮N−R unit is almost linear (angles Ta−C−N = 177.2(5)°, C⋮N−R = 174.0(8)°; d(C⋮N) = 1.152(6) Å). Photolysis of [(butadiene)TaCp2+] (3) with excess cyclohexyl isocyanide followed by thermal treatment gave rise to the formation of [bis(κC-cyclohexyl isocyanide)TaCp2+] (8, with [CH3B(C6F5)3-] counteranion; >50% isolated, ν̃(C⋮NR) 2134 and 2088 cm-1). Complex 8 was characterized by an X-ray crystal structure analysis. It shows a pseudotetrahedral [Cp2Ta(C⋮NR)2+] cation (R = C6H11). The Ta−C⋮N−R units are both slightly bent at nitrogen (angles C⋮N−R = 169.0(13) and 163.7(8)°; the corresponding Ta−C−N angles are 179.4(9) and 177.4(7)°). The bonding situations of complexes 4 and 8 were analyzed by DFT calculations. The computational study shows that metal to isonitrile back-bonding does not contribute significantly to the Cp2Ta+−C⋮NR bonding interaction in these complexes. The bonding between Cp2Ta+ and the C⋮NR ligands is dominated by electrostatic effects.