Reactions of five-membered zirconacyclocumulenes with tris(pentafluorophenyl)borane: Carbon-carbon double bond cleavage and formation of novel zwitterionic complexes

The five-membered zirconacyclocumulenes Cp*Zr-2(eta(4)-1,2,3,4-RC4R) (Cp* = eta(5)-pentamethylcyclopentadienyl, R = Me3Si 1a, Me 1b, Ph 1c) each react differently with B(C6F5)(3). For la (R = Me3Si) a bond cleavage of the central carbon-carbon double bond of the cyclocumulene was found, and the bis-c-alkynyl complex Cp*Zr-2(sigma-C=CSiMe3)(2) (2) was formed. This reaction occurred with 10% B(C6F5)(3), giving after 6 days a quantitative yield of complex 2. With 1b (R = Me) B(C6F5)3 attacks the P-C atom of the starting cumulene and of the C-4 chain remains intact, forming Cp*Zr-2{eta(3)-1,3,4-C(Me)-C[B(C6F5)(3)]-C=-CMe} (3) with a hex-2-en-4-yn-2-yl-3-[tris(pentafluorophenyl)borate] ligand at the permethylzirconocene center. With 1c (R = Ph) the C4 chain also is not cleaved, but B(C6F5)3 attacks the alpha-C atom of the starting cumulene and yields Cp*Zr-2{eta(3)-1,2,3-C(Ph)=C=C=C(Ph)[B(C6F5)(3)]} (4) with a 1,4-diphenylbuta-1,2,3-trien-1-yl-4-[tris(pentafluorophenyl)borate] ligand, coordinated at the permethylzirconocene center. The molecular structure of complex 4 was confirmed by X-ray crystallography.