Reactions of the Five-Membered Hafnacyclocumulene Cp₂Hf(η⁴-<i>t</i>-Bu-C₄-<i>t-</i>Bu) with the Lewis Acids Tris(pentafluorophenyl)borane and Diisobutylaluminum Hydride

The previously described hafnacyclocumulene Cp2Hf(η4-t-Bu-C4-t-Bu) (1) reacts with B(C6F5)3 to give the dinuclear complex 2, which consists of a cationic part with two hafnocene centers bridged unsymmetrically by butadiyne and acetylide moieties and an alkynyl boranate representing the anionic part. In the reaction of 1 with i-Bu2AlH the central CC bond of the hafnacyclocumulene ring is weakened, resulting in the formation of an intermediate zwitterion, which then forms the hafnium bis-acetylide Cp2Hf(CC-t-Bu)2. Subsequent hydroalumination of one of the CC bonds with i-Bu2AlH yields the final complex Cp2Hf(CC-t-Bu)[C(Al-i-Bu2)=CH(t-Bu)] (3). Reaction of the titanacyclocumulene 4 with catalytic amounts of B(C6F5)3 results in the formation of the previously described dinuclear complex 5, which then undergoes further reaction with B(C6F5)3 to give the known zwitterionic Ti(III) species 6 and the ene-yne t-BuCHCHCC-t-Bu.