Synthetic, kinetic and mechanistic studies in early transition metal systems. I. α- and β-hydrogen elimination reactions in derivatives of permethyltitanocene, -zirconocene, and -hafnocene. II. The reactions of aluminum alkyls with derivatives of permethylniobocene

Abstract

The thermal decomposition of Cp*2Ti(CH3)2 (Cp* ≡ ƞ5-C5Me5) toluene solution follows cleanly first-order kinetics and produces a single titanium product Cp*(C5Me4CH2)Ti(CH3) concurrent with the evolution of one equivalent of methane. Labeling studies using Cp*2Ti-(CD3)2 and (Cp*- d 15)2Ti(CH3)2 show the decomposition to be intramolecular and the methane to be produced by the coupling of a methyl group with a hydrogen from the other TiCH3 group. Activation parameters, ΔH‡ and ΔS‡, and kinetic deuterium isotope effects have been measured. The alternative decomposition pathways of α-hydrogen abstraction and a-hydrogen elimination, both leading to a titanium-methylidene intermediate, are discussed. The insertion of unactivated acetylenes into the metal-hydride bonds of Cp*2MH2 (M = Zr, Hf) proceeds rapidly at low temperature to form mono- and/or bisinsertion products, dependent upon the steric bulk of the acetylene substituents. Cp*2M(H)(C(Me) = CHMe), Cp*2M(H)(CH = CHCMe3), Cp*2M(H)-(CH = CHPh), Cp*2M(CH = CHPh)2, Cp*2M(CH = CHCH3)2 and Cp*2Zr-(CH = CHCH2CH3)2 have been isolated and characterized. To extend the study of unsaturated-carbon ligands, Cp*2M(C ≡ CCH3)2 have been prepared by treating Cp*2MCl2 with LiC ≡ CCH3. The reactivity of many of these complexes with carbon monoxide and dihydrogen is surveyed. The mono(2-butenyl) complexes Cp*2M(H)(C(Me) = CHMe) rearrange at room temperature, forming the crotyl-hydride species Cp*2M(H)(ƞ3-C4H7). The bis(propenyl) and bis(l-butenyl) zirconium complexes Cp*2Zr(CH = CHR)2 (R = CH3, CH2CH3) also rearrange, forming zirconacyclopentenes. Labeling studies, reaction chemistry, and kinetic measurements, including deuterium isotope effects, demonstrate that the unusual β-hydrogen elimination from an sp2-hybridized carbon is the first step in these latter rearrangements but is not observed in the former. Details of these mechanisms and the differences in reactivity of the zirconium and hafnium complexes are discussed. The reactions of hydride- and alkyl-carbonyl derivatives of permethylniobocene with equimolar amounts of trialkylaluminum reagents occur rapidly producing the carbonyl adducts Cp*2Nb(R)(COAlR'3) (R = H, CH3, CH2CH3, CH2CH2Ph, C(Me) = CHMe; R' = Me, Et). The hydride adduct Cp*2NbH3•AlEt3 has also been formed. In solution, each of these compounds exists in equilibrium with the uncomplexed species. The formation constants for Cp*2Nb(H)(COAlR'3) have been measured. They indicate the steric bulk of the Cp* ligands plays a deciding factor in the isolation of the first example of an aluminum Lewis acid bound to a carbonyl-oxygen in preference to a metalhydride. Reactions of Cp*2Nb(H)CO with other Lewis acids and of the one:one adducts with H2, CO and C2H4 are also discussed. Cp*2Nb(H)(C2H4) also reacts with equimolar amounts of trialkyl-aluminum reagents, forming a one:one complex that 1H NMR spectroscopy indicates contains a Nb-CH2CH2-Al bridge. This adduct also exists in equilibrium with the uncomplexed species in solution. The formation constant for Cp*2Nb+(H)(CH2CH2Al-Et3) has been measured. Reactions of Cp*2Nb(H)(C2H4) with other Lewis acids and the reactions of Cp*2Nb+(H-(CH2CH2Al-Et3) with CO and C2H4 are described, as are the reactions of Cp*2Nb(H)(CH2 = CHR) (R = Me, Ph), Cp*2Nb(H)(CH3C ≡ CCH3) and Cp*2Ti-(C2H4) with AlEt3.