| Abstract
| - The relative stabilities of trans-[Ir(XMe)(CO)(PR3)2] species (X = O, CH2) toward β-H eliminationhave been studied via combination of density functional and hybrid density functional/Hartree−Fockcalculations. For both small (R = H) and full (R = Ph) model systems β-H elimination from the methoxidespecies is found to be disfavored both kinetically and thermodynamically compared to that from theanalogous ethyl complexes. This is consistent with the greater stability of alkoxide species seenexperimentally (R = Ph). In all cases the major contribution to the activation barrier is phosphinedissociation, and for the alkyl systems this leads directly to an agostically stabilized intermediate fromwhich β-H transfer readily occurs. In contrast, with the trans-[Ir(OMe)(CO)(PR3)2] species a π-stabilizedintermediate is formed and a further isomerization barrier must be overcome before β-H transfer can beaccessed. Further calculations were performed on the acetophenone complex [Ir(H)(η2-OC(Me)Ph)(CO)(PPh3)], and a low-energy pathway for face exchange of the metal-bound ketone has beencharacterized. This involves an η1-intermediate and provides a mechanism for facile racemization of theprecursor alkoxide. Selected calculations using alternative hybrid calculations showed the sensitivity ofPPh3 binding energies to the methodology employed. This is especially the case for the final step in theβ-H elimination reaction, the formation of [Ir(H)(CO)(PPh3)3] from [Ir(H)(CO)(PPh3)2] and free PPh3,where the use of the UFF approach appears to be particularly unreliable.
- Calculations have shown that the greater stability of trans-[Ir(OMe)(CO)(PR3)2] species towards β-H elimination compared to their alkyl analogues arises from the ability of the alkoxide ligand to act as a π-donor in the unsaturated intermediate formed upon phosphine loss. In addition, a low-energy pathway resulting in exchange of the η2-bound face of ketone ligands has been characterized, accounting for the racemization of chiral alkoxides in species such as trans-[Ir{OCH(Me)Ph}(CO)(PPh3)2].
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