COMPETITIVE CH ACTIVATION IN (PENTAMETHYLCYCLOPENTADIENYL)(CYCLOOCTATETRAENE)ALKYLZIRCONIUM COMPOUNDS CP-ASTERISK-ZR(COT)R - SELECTIVE FORMATION OF FULVENE OR CYCLOOCTATRIENYNE LIGANDS - MOLECULAR-STRUCTURE OF CP-ASTERISK-ZR(MU-ETA-8-ETA-2-C8H6)ZRCP-ASTERISK(ETA-4-C8H8), A DINUCLEAR ZWITTERIONIC ZIRCONIUM COMPLEX WITH A BRIDGING CYCLOOCTARIENYNE LIGAND

Thermolysis of Cp*Zr(COT)R (Cp* = eta5-C5Me5, COT = eta8-C8H8, R = CH2SiMe3, Me) proceeds via a double hydrogen abstraction from a cyclooctatetraene ligand to give RH and Cp*Zr(mu-eta8:eta2-C8H6)ZrCp*(eta4-C8H8) as the kinetic product. Cp*Zr(mu-eta8:eta2-C8H6) undergoes a thermally induced hydrogen transfer from the pentamethylcyclopentadienyl ligands to the cyclooctatetraenediyl ligand to give the thermodynamic product FvZrCOT (Fv - eta6-C5Me4-CH2). 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 me Cp*Zr(mu-eta8:eta2-C8H6)ZrCp*(eta4-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 sps carbon of the pentamethylcyclopentadienyl ligand. Cp*Zr(mu-eta8:eta2-C8H6)ZrCp*(eta4-C8H8) crystallizes in the monoclinic space group P2(1)/n with a = 8.324 (2) angstrom, b = 15.966 (3) angstrom, c = 22.647 (3) angstrom, beta = 92.56 (1)-degrees, V = 3007 (1) angstrom3, and Z = 4. The molecular structure shows an unprecedented cyclooctatrienyne ligand, bridged asymmetrically between the Zr atoms. The formulation as a cyclooactatrienyne complex is formal; the actual bonding of the ligand is as a dimetalated eta8-cyclooctatetraene-1,2-diyl. The diamagnetism of Cp*Zr(mu-eta8:eta2-C8H6)ZrCp*(eta4-C8H8) implies a zwitterionic complex in which a 16-electron zirconium center [Cp*Zr(COT)]+ is connected to another 16-electron center [Cp*Zr(eta4-C8H8)R2]- through a bridging eta8-cyclooctetraene-1,2-diyl ligand.