C-H activation mechanisms and regioselectivity in the cyclometalation reactions of bis(pentamethylcyclopentadienyl)thorium dialkyl complexes

Joseph W. Bruno, Gregory M. Smith, Tobin J. Marks, C. Kay Fair, Arthur J. Schultz, Jack M. Williams

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Abstract

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-1 and ΔS = -16 (2) eu. The molecular structure of Cp′2Th(CH2CMe3)2 has been determined by single-crystal neutron-diffraction techniques. This compound crystallizes in space group P21/n with a = 11.206 (4) Å, b = 16.670 (6) Å, c = 15.742 (5) Å, β = 93.48 (2)°, and V = 2935 (3) Å3. The molecule exhibits a typical Cp′2ThX2 bent metallocene structure with highly unsymmetrical bonding of the neopentyl ligands, Th-C(α) = 2.543 (4) and 2.456 (4) Å; Th-C(α)-C(β) = 132.1 (3)°, and 158.2 (3)°. Consequences of these distortions include the acute angles Th-C(α)-H(α) = 84.4 (5)° and 87.1 (5)° in the ligand with Th-C(α)-C(β) = 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-1 and ΔS = -10.4 (12) eu. Cyclometalation of Cp′2Th(CH2CMe3)(CH2SiMe 3) proceeds unimolecularly in saturated and aromatic hydrocarbon solvents to form exclusively Cp′2Th(CH2SiMe2CH2) and CMe4 with ΔH = 19.4 (2) kcal mol-1 and ΔS = -20.8 (5) eu. This compound also displays a distorted dialkyl ligand geometry, with Th-C(α) = 2.44 (3) and 2.47 (3) Å and Th-C(α)-C(β) = 132 (3)° and Th-C(α)-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 ΔH = 18.5 (7) kcal mol-1 and ΔS = -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-1 and ΔS = -20 (2) eu. Upon further thermolysis, Cp′2Th(CH2SiMePhCH2) undergoes smooth conversion to Cp′2Th(CH2SiMe2-O-C6H 4). 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 α-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 ΔH = 25.4 (13) kcal mol-1 and ΔS = -14 (4) eu, is attributed to the normal γ-hydrogen activating cyclometalation mechanism, while process II, with ΔH = 28.5 (11) kcal mol-1 and ΔS = -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)4C5CH2 species. Process 1 exhibits a large deuterium kinetic isotope effect, kH/kD = 10.0 (5) at 85°C and kH/kD = 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.

Original languageEnglish
Pages (from-to)40-56
Number of pages17
JournalJournal of the American Chemical Society
Volume108
Issue number1
Publication statusPublished - 1986

ASJC Scopus subject areas

  • Chemistry(all)

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