Intramolecular Thermolytic C-H Activation Processes: Solid-State Structural Characterization of a Mononuclear η⁶–Me₄C₅CH₂ Zirconium Complex and a Mechanistic Study of Its Formation from (Me₅C₅)₂Zr(C₆H₅)₂

The complex Cp′₂Zr(C₆H₅)₂ (1, Cp′ = η⁵-Me₅C₅) undergoes unimolecular thermolysis in solution (ΔH* = 22.5 (3) kcal/mol, ΔS* = -11.8 (3) eu) to yield the ring-metalated complex Cp′Zr(C₆H₅)(Me₄C₅CH₂) (2) and benzene. Complex 2 crystallizes in the monoclinic space group P2₁/c with four molecules in a unit cell of dimensions a = 13.550 (2) Å, b = 9.651 (2) Å, c - 17.613 (3) Å, and β= 94.94 (1)° at 0 °C. Least-squares refinement led to a value for the conventional R index (on F) of 0.043 for 2594 reflections having 2θ_MoKa < 55° and I > 3σ(I). The molecular structure of 2 consists of η⁶-Cpʹ and η⁶-Me₄C₅CH₂ligands in a distorted “bent sandwich” Cpʹ₂ZrX₂configuration (⁁ring centroid-Zr-ring centroid = 143.6 (3)°) with the X groups being an unexceptional phenyl ligand (Zr-C = 2.285 (5) Å) and a metalated cyclopentadienyl methyl group (Zr-C = 2.388 (7) Å). The latter carbon atom is displaced 0.88 Å (37°) from the C₅mean plane in the direction of the zirconium atom and is bonded to the C₅ring at a C-C distance of 1.468 (9) Å. The zirconium atom is displaced slightly from the center of the methyl-metalated C₅ring toward the metalated carbon atom. It appears that the η⁶ligand has little fulvene character and that a metalated σ,η⁵-Cpʹ bonding description is more appropriate. Labeling and kinetic studies with Cpʹ₂Zr(C₆D₅)₂ show that thermolysis occurs via rate-limiting phenyl ortho hydrogen atom abstraction(kh/kd =6.5 (10) at 70 °C) to yield a benzene/ortho phenylene intermediate which can be trapped with ethylene. Benzene elimination is followed by hydrogen atom abstraction from a Cp′ ligand to yield 2.