Carbon-Hydrogen Bond Activation by Peralkylhafnocene and Peralkylscandocene Derivatives

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 P1 (a= 9.084(2), b = 10.492(2), c = 12.328(1) A; 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) A, b = 9.516(1) A, c = 14.183(6) A; β = 103.965(31)°, z = 4). Least squares refinement led to a value for R of 0.036 (I > 3σi) 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.