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*2Zr(η4-1,2,3,4-RC4R) (Cp* = η5-pentamethylcyclopentadienyl, R = Me3Si 1a, Me 1b, Ph 1c) each react differently with B(C6F5)3. For
1a (R = Me3Si) a bond cleavage of the central carbon−carbon double bond of the
cyclocumulene was found, and the bis-σ-alkynyl complex Cp*2Zr(σ-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 β-C atom of the starting cumulene and
the C4 chain remains intact, forming Cp*2Zr{η3-1,3,4-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 α-C atom
of the starting cumulene and yields Cp*2Zr{η3-1,2,3-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.