Carbon-Hydrogen Bond Activation by Peralkylhafnocene and Peralkylscandocene Derivatives

Citation

Bulls, Al Ray (1988) Carbon-Hydrogen Bond Activation by Peralkylhafnocene and Peralkylscandocene Derivatives. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ZSJW-KC36. https://resolver.caltech.edu/CaltechTHESIS:01172013-130132630

Abstract

Chapter 1

Thermal decomposition of Cp*₂Hf(CH₂C₆H₅)₂ (Cp* = (η⁵-C₅Me₅)) in benzene-d₆ cleanly affords toluene and hafnabenzocyclobutene [chemical symbol; see abstract in scanned thesis for details]. Deuterium labeling of the benzyl ligands indicates that decomposition of Cp*₂Hf(CY₂C₆H₅)₂ (Y = H, D) proceeds primarily by α-H abstraction to form a permethylhafnocene benzylidene intermediate [Cp*₂Hf=CHC₆H₅], which rapidly rearranges to the metallated-cyclopentadienyl, or "tucked-in" benzyl complex Cp*(η⁵,η¹-C₅Me₄CH₂)HfCH₂C₆H₅. The observed product arises from rearrangement of Cp*(η⁵,η¹-C₅Me₄CH₂)HfCH₂C₆H₅ to its tautomer [chemical symbol; see abstract in scanned thesis for details]. A series of meta substituted benzyl derivatives of the proposed metallated cyclopentadienyl intermediates, Cp*(η⁵,η¹-C₅Me₄CH₂)HfCH₂C₆H₄X (X = H, CH₃, CF₃, NMe₂), has therefore been prepared. The kinetics of their conversion to [chemical symbol; see abstract in scanned thesis for details] have been examined in order to probe the nature of the transition state for aryl C-H bond activation which occurs in the final steps of the rearrangement. The rates are found to be insensitive to the nature of X, suggesting that the benzyl π system is not attacked by the electrophilic hafnium center along the reaction coordinate for C-H bond activation. The structure of Cp*(η⁵,η¹-C₅Me₄CH₂)HfCH₂C₆H₅, as determined by single crystal X-ray diffraction techniques, indicates that the complex is best described as a Hf(IV) derivative containing an η⁵,η¹-C₅Me₄CH₂ Iigand, rather than a Hf(ll) η⁶-fulvene adduct. Cp*(η⁵,η¹-C₅Me₄CH₂)HfCH₂C₆H₅ crystallizes in the triclinic space group P1̅ (a= 9.084(2), b = 10.492(2), c = 12.328(1) Å; α = 95.81(1), β = 96.60(1), γ = 91.15(2); Z = 2). Least-squares refinement led to a value for R of 0.048 (I > 3σI) and a goodness-of-fit of 4.37 for 4029 independent reflections.

Chapter 2

Thermolysis of the singly metallated complex Cp*(η⁵,η¹-C₅Me₄CH₂)Hf(CH₂CMe₃) in toluene affords neopentane and the doubly metallated complex Cp*(η⁵,η¹,η¹-C₅Me₃(CH₂)₂)Hf. The structure of Cp*(η⁵,η¹,η¹-C₅Me₃(CH₂)₂)Hf, as determined by single-crystal X-ray diffraction techniques, confirms that metallation has occurred at adjacent methyl groups of the same pentamethylcyclopentadienyl ring. Cp*(η⁵,η¹,η¹-C₅Me₃(CH₂)₂)Hf crystallizes in the space group P2/c (a= 13.775(4) Å, b = 9.516(1) Å, c = 14.183(6) Å; β = 103.965(31)°, z = 4). Least squares refinement led to a value for R of 0.036 (I > 3σ_i) and a goodness-of-fit of 2.66 for 1984 independent reflections. Cp*(η⁵,η¹,η¹-C₅Me₃(CH₂)₂)Hf and Cp*(η⁵,η¹-C₅Me₄CH₂)Hfl cleanly insert one equivalent of ethylene into the hafnium methylene carbon bond to form the propyl bridged species Cp*(η⁵,η¹,η¹-C₅Me₃(CH₂)(CH₂CH₂CH₂))Hf and Cp*(η⁵,η¹-C₅Me₃CH₂CH₂CH₂)Hfl, respectively. Cp*(η⁵,η¹,η¹-C₅Me₃(CH₂)(CH₂CH₂CH₂))Hf reacts with excess ethylene to cleanly afford [chemical symbol; see abstract in scanned thesis for details]. Deuterium labeling experiments suggest that this reaction occurs via an α-H abstraction to generate the alkylidene intermediate Cp*(η⁵,η¹-C₅Me₄CH₂CH₂CH=)Hf, which rapidly traps ethylene to form the observed product. The metallated phenyl complex Cp*(η⁵,η¹-C₅Me₃CH₂)HfC₆H₅ has been shown to be in equilibrium with the benzyne complex Cp*₂Hf(η²-C₆H₄). Heating benzene-d₆ solutions of Cp*(η⁵,η¹-C₅Me₃CH₂)HfC₆H₅ in the presence of ethylene traps the benzyne intermediate and generates the hafnacyclopentene [chemical symbol; see abstract in scanned thesis for details].

Chapter 4

Relative bond dissociation energies (BDEs) have been obtained by equilibrating early transition metal alkyls and hydrides with H₂ or the C-H bonds of hydrocarbons. Thus, in benzene solution Cp*₂HfH₂ (Cp* = (η⁵-C₅Me₅)) equilibrates with Cp*₂Hf(C₆H₅)H and dihydrogen. From the enthalpy of the reaction, ΔH° = +6.0(3), the Hf-H (BDE) is calculated to be 0.8(3) kcal·mol^-1 stronger than the Hf-C₆H₅ BDE. Relative Sc-C₆H₅ and Sc-alkyl BDEs have been estimated from the equilibration of the metallated complex Cp*(η⁵,η¹-C₅Me₄CH₂CH₂CH₂)Sc, C₆H₆ and Cp*(η⁵-C₅Me₄CH₂CH₂CH₃)Sc(C₆H₅), the Sc-C₆H₅ BDE being 16.6(3) kcal·mol^-1 stronger than the Sc-CH₂CH₂CH₂C₅Me₄ BDE. From a similar reversible intramolecular metallation of Cp*(η⁵-C₅Me₄CH₂CH₂CH₃)HfH₂ to give Cp*(η⁵,η¹-C₅Me₄CH₂CH₂CH₂)HfH and dihydrogen, the Hf-H BDE is estimated to be 23.0(3) kcal·mol^-1 stronger than the Hf-CH₂CH₂CH₂C₅Me₄ BDE. The equilibration of Cp*(η⁵-C₅Me₄CH₂C₆H₅)Sc-C≡CCMe₃ with the metallated scandocene derivative Cp*(η⁵,η¹-C₅Me₄CH₂-o-C₆H₄)Sc and tert-butylacetylene lies very far towards Cp*(η⁵-C₅Me₄CH₂C₆H₅)Sc-C≡CCMe₃, so that only a lower limit for the relative Sc-alkynyl and Sc-aryl BDEs may be determined: BDE(Sc-alkynyl) - BDE(Sc-aryl) ≥ 29(5) kcal·mol^-1. These early transition metal-hydrocarbyl (M-R) BDEs correlate with the corresponding H-R BDEs (i.e. M-alkynyl > M-aryl > M-alkyl); however, the M-R BDEs increase more rapidly with s character than does the H-R BDEs. The origin of this effect is not known, but it is undoubtedly also responsible for the characteristically high M-H BDEs for transition metal hydride compounds. In an effort to probe the polarity of Sc-aryl bonds a series of scandocene derivatives capable of reversibly metallating at either of two differently substituted benzyl groups was prepared. The equilibrium constants for these metallated derivatives: (η⁵,η¹-C₅Me₄CH₂-o-C₆H₃-p-X)(η⁵-C₅Me₄CH₂C₆H₄-m-CH₃)Sc ⇌ (η⁵-C₅Me₄CH₂C₆H₄-m-X)(η⁵,η¹- C₅Me₄CH₂-o-C₆H₃-p-CH₃)Sc (X= H, CF₃, NMe₂) were determined. The small dependence of Keq on the nature of X suggests that the Sc-aryl bond is essentially covalent with only a slight ionic character.

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