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 2HfH 2 (CP = (η 5-C 5Me 5)) equilibrates with Cp 2Hf(C 6H 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 6H 5BDE. Relative ScC 6H 5 and Scalkyl BDEs have been estimated from the equilibration of the metallated complex Cp(η 5,η 1-C 5Me 4CH 2CH 2CH 2)Sc, C 6H 6 and Cp(η 5-C 5Me 4CH 2CH 2CH 3)ScC 6H 5, the ScC 6H 5 BDE being 16.6(3) kcal mol −1 stronger than the ScCH 2CH 2CH 2C 5Me 4 BDE. From a similar reversible intramolecular metallation of Cp(η 5- C 5Me 4CH 2CH 2CH 3)HfH 2 to give Cp(η 5,η 1-C 5Me 4CH 2CH 2CH 2)HfH and dihydrogen, the HfH BDE is estimated to be 23.0(3) kcal mol −1 stronger than the HfCH 2CH 2CH 2C 5Me 4 BDE. The equilibration of Cp(η 5-C 5Me 4CH 2C 6H 5)ScCCCMe 3 with metallated scandocene derivative Cp(η 5,η 1-C 5Me 4CH 2- o-C 6H 4)Sc and tert-butylacetylene lies very far towards Cp(η 5- C 5Me 4CH 2C 6H 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 5Me 4CH 2- o-C 6H 3- p-X)(η 5-C 5Me 4CH 2C 6H 4- m- CH 3)Sc (η 5- C 5Me 4CH 2C 6H 4- m-X)(η 5,η 1-C 5Me 4CH 2- o-C 6H 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.