Iridium(I) versus ruthenium(II). A computational study of the transition metal catalyzed transfer hydrogenation of ketones

We present a density functional theory based computational study comparing simplified models for the ruthenium(II)- and iridium(I)-catalyzed transfer hydrogenation of ketones. For the ruthenium compound our results confirm earlier findings that the hydrogenation involves a ruthenium hydride and occurs via a concerted hydrogen transfer mechanism with no direct ruthenium-ketone binding along the reaction path. In contrast, for the iridium compound our calculations suggest that the reaction proceeds via direct hydrogen transfer between simultaneously coordinated ketone and alcohol. We find that for both metal complexes the formation of a very stable metal-alkoxide complex plays an important role. For the ruthenium-catalyzed reaction it constitutes a resting state that does not take an active part in the transfer hydrogenation, while for the iridium-catalyzed reaction it is an important intermediate along the reaction path.