Competitive CH Activation in (Pentamethylcyclopentadienyl)(cyclooctatetraene)alkylzirconium Compounds Cp*Zr(COT)R. Selective Formation of Fulvene or Cyclooctatrienyne Ligands. Molecular Structure of Cp*Zr(μ-η8:η2-C8H6)ZrCp*(η4-C8H8), a Dinuclear Zwitt

Thermolysis of Cp*Zr(COT)R (Cp* = η5-C5Me5, COT = η8-C8H8, R = CH2SiMe3, Me) proceeds via a double hydrogen abstraction from a cyclooctatetraene ligand to give RH and Cp*Zr(μ-η8:η2-C8H6)ZrCp*(η4-C8H8) as the kinetic product. Cp*Zr(μ-η8:η2-C8H6)ZrCp*(η4-C8H8) undergoes a thermally induced hydrogen transfer from the pentamethylcyclopentadienyl ligands to the cyclooctatetraenediyl ligand to give the thermodynamic product FvZrCOT (Fv = η6-C5Me4CH2). For Cp*Zr(COT)R (R = CH2Ph) a different thermolysis reaction was observed, in which FvZrCOT was the major product at low temperatures, together with some Cp*Zr(μ-η8:η2-C8H6)ZrCp*(η4-C8H8). The latter does not convert into the thermodynamic product at the temperature of thermolysis, indicating an alternative, direct, thermal decomposition pathway to FvZrCOT for R = CH2Ph, with preferred hydrogen abstraction from the sp3 carbon of the pentamethylcyclopentadienyl ligand. Cp*Zr(μ-η8:η2-C8H6)ZrCp*(η4-C8H8) crystallizes in the monoclinic space group P21/n with a = 8.324 (2) Å, b = 15.966 (3) Å, c = 22.647 (3) Å, β = 92.56 (1)° , V = 3007 (1) Å3, and Z = 4. The molecular structure shows an unprecedented cyclooctatrienyne ligand, bridged asymmetrically between the Zr atoms. The formulation as a cyclooctatrienyne complex is formal; the actual bonding of the ligand is as a dimetalated η8-cyclooctatetraene-1,2-diyl. The diamagnetism of Cp*Zr(μ-η8:η2-C8H6)ZrCp*(η4-C8H8) implies a zwitterionic complex in which a 16-electron zirconium center [Cp*Zr(COT)]+ is connected to another 16-electron center [Cp*Zr(η4-C8H8)R2]- through a bridging η8-cyclooctetraene-1,2-diyl ligand