Relative bond dissociation energies for early transition metal alkyl, aryl, alkynyl and hydride compounds. Equilibration of metallated cyclopentadienyl derivatives of peralkylated hafnocene and scandocene with hydrocarbons and dihydrogen

Abstract Relative bond dissociation energies (BDEs) have been obtained by equilibrating early transition metal alkyls and hydrides with H 2 or the CH bonds of hydrocarbons. Thus, in benzene solution Cp 2 HfH 2 (CP = (η 5 -C 5 Me 5 )) equilibrates with Cp 2 Hf(C 6 H 5 )H and dihydrogen. From the enthalpy of the reaction, Δ H O = +6.0(3), the HfH (BDE) is calculated to be 0.8(3) kcal mol −1 stronger than the HfC 6 H 5 BDE. Relative ScC 6 H 5 and Scalkyl BDEs have been estimated from the equilibration of the metallated complex Cp(η 5 ,η 1 -C 5 Me 4 CH 2 CH 2 CH 2 )Sc, C 6 H 6 and Cp(η 5 -C 5 Me 4 CH 2 CH 2 CH 3 )ScC 6 H 5 , the ScC 6 H 5 BDE being 16.6(3) kcal mol −1 stronger than the ScCH 2 CH 2 CH 2 C 5 Me 4 BDE. From a similar reversible intramolecular metallation of Cp(η 5 - C 5 Me 4 CH 2 CH 2 CH 3 )HfH 2 to give Cp(η 5 ,η 1 -C 5 Me 4 CH 2 CH 2 CH 2 )HfH and dihydrogen, the HfH BDE is estimated to be 23.0(3) kcal mol −1 stronger than the HfCH 2 CH 2 CH 2 C 5 Me 4 BDE. The equilibration of Cp(η 5 -C 5 Me 4 CH 2 C 6 H 5 )ScCCCMe 3 with metallated scandocene derivative Cp(η 5 ,η 1 -C 5 Me 4 CH 2 - o -C 6 H 4 )Sc and tert -butylacetylene lies very far towards Cp(η 5 - C 5 Me 4 CH 2 C 6 H 5 )ScCCCMe 3 , 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 for R than do 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 order 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: (η 5 ,η 1 -C 5 Me 4 CH 2 - o -C 6 H 3 - p -X)(η 5 -C 5 Me 4 CH 2 C 6 H 4 - m - CH 3 )Sc (η 5 - C 5 Me 4 CH 2 C 6 H 4 - m -X)(η 5 ,η 1 -C 5 Me 4 CH 2 - o -C 6 H 3 - p -CH 3 )Sc (X = H, CF 3 , NMe 2 ) were determined. The small dependence of K eq on the nature of X suggests that the Scaryl bond is essentially covalent with only a small ionic contribution.