C-H Activation Mechanisms and Regioselectivity in the Cyclometalation Reactions of Bis(pentamethylcyclopentadienyl)thorium Dialkyl Complexes

This contribution reports on the syntheses, structures, and cyclometalation reactions of a series of bis(pentamethylcyclopentadienyl)thorium dialkyl complexes of the type Cp’₂ThR₂(Cp’ = η⁵-C₅Me₅). Thermal cyclometalation of Cp’₂Th(CH₂CMe₃)₂ proceeds unimolecularly in saturated hydrocarbon solvents to form the thoracyclobutane Cp’₂Th-(CH₂CMe₂CH₂) and CMe₄ with ΔH^‡ = 21.2 (8) kcal mol^-1and AS* = -16 (2) eu. The molecular structure of Cp’₂Th-(CH₂CMe₃)₂has been determined by single-crystal neutron-diffraction techniques. This compound crystallizes in space group P2₁/n with a = 11.206 (4) A, b = 16.670 (6) A, c = 15.742 (5) A, 0 = 93.48 (2)°, and V = 2935 (3) A³. The molecule exhibits a typical Cp’₂ThX₂ bent metallocene structure with highly unsymmetrical bonding of the neopentyl ligands, Th-C(a) = 2.543 (4) and 2.456 (4) A; Th-C(α)-C(β) = 132.1 (3)°, and 158.2 (3)°. Consequences of these distortions include the acute angles Th-C(a)-H(a) = 84.4 (5)° and 87.1 (5)° in the ligand with Th-C(a)-C(0) = 158.2 (3)°. These distortions are proposed to reflect the severe steric congestion around the metal ion. Cyclometalation of Cp’₂Th(CH₂SiMe₃)₂ proceeds unimolecularly in either saturated or aromatic hydrocarbon solvents to yield Cp’₂Th(CH₂SiMe₂CH₂) and SiMe₄ with ΔH^‡ = 25.1 (4) kcal mol^-1and AS* = -10.4 (12) eu. Cyclometalation of Cp’₂Th(CH₂CMe₃)(CH₂SiMe₃) proceeds unimolecularly in saturated and aromatic hydrocarbon solvents to form exclusively Cp’₂Th(CH₂SiMe₂CH₂) and CMe₄ with ΔH^‡ = 19.4 (2) kcal mol^-1and AS* = -20.8 (5) eu. This compound also displays a distorted dialkyl ligand geometry, with Th-C(a) = 2.44 (3) and 2.47 (3) A and Th-C(α)-C(β) = 132 (3)° and Th-C(a)-Si = 150 (3)°. Cyclometalation of Cp^/₂Th(CH₂CMe₂Et)₂ proceeds unimolecularly in saturated hydrocarbon solvents to form exclusively thoracyclobutane Cp’₂Th(CH₂CMeEtCH₂) and CMe₃Et with AH* = 18.5 (7) kcal mol^-1and AS* = -24 (2) eu. Cyclometalation of Cp’₂Th(CH₂SiMe₂Ph)₂ proceeds unimolecularly in saturated or aromatic hydrocarbon solvents to form exclusively Cp’₂Th(CH₂SiMePhCH₂) and SiMe₃Ph with ΔH^‡ = 21.4 (8) kcal mol^-1and AS* = -20 (2) eu. Upon further thermolysis, Cp’₂Th(CH₂SiMePhCH₂) undergoes smooth conversion to Cp’₂Th(CH₂SiMe₂-o-C₆H₄). Thermolysis of Cp’₂Th(CHDSiMe₃)₂ leads to formation of (CH₂D)SiMe₃, and thermolysis of Cp’₂Th(CD₂CMe₃)₂ leads to formation of (CHD₂)CMe₃, ruling out mechanisms involving significant a-hydrogen atom abstraction. Thermolysis of Cp’₂Th[CH₂Si(CD₃)₃]₂ leads to formation of both (CH₂D)Si(CD₃)₃ (process I, ~65%) and (CH₃)Si(CD₃)₃ (process II, ~35%). Process I, which proceeds with AH* = 25.4 (13) kcal mol^-1and AS* = -14 (4) eu, is attributed to the normal γ-hydrogen activating cyclometalation mechanism, while process II, with ΔH^‡ - 28.5 (11) kcal mol^-1and AS* = -3 (3) eu, is attributed to a secondary mechanism involving rate-limiting abstraction of a hydrogen atom from a Cp’ ring, followed by hydrogen (deuterium) atom transfer from the remaining alkyl ligand to an intermediate η⁶-(CH₃)₄C₅CH₂species. Process I exhibits a large deuterium kinetic isotope effect, k_H/k_D= 10.0 (5) at 85 °C and k_H/k_D= 8.5 (6) at 115 °C. Solution and solid state CP-MAS ¹³C NMR data are reported for the dialkyl complexes and are consistent with the structural data. The mechanism for cyclometalation is proposed to involve a concerted, heterolytic process with hydrogen atom abstraction and metallacycle formation occurring in a four-center transition state. Steric factors in the proposed transition state lead to discrimination between methyl and larger substituents on the β-carbon atom, accounting for the high observed regioselectivity. Steric factors also appear to impede intermolecular C-H activation processes involving solvent molecules.