A computational study of the CO dissociation in cyclopentadienyl ruthenium complexes relevant to the racemization of alcohols

Abstract

The formation of an active 16-electron ruthenium sec-alkoxide complex via loss of the CO ligand is an important step in the mechanism of the racemization of sec-alcohols by (η⁵-Ph₅C₅)Ru(CO)₂X ruthenium complexes with X = Cl and O^tBu. Here we show with accurate DFT calculations the potential energy profile of the CO dissociation pathway for a series of relevant (η⁵-Ph₅C₅)Ru(CO)₂X complexes, where X = Cl, O^tBu, H and COO^tBu. We have found that the CO dissociation energy increases in the following order: O^tBu (lowest), Cl, COO^tBu and H (highest). Using the distance between ruthenium and C_CO, r = Ru–C_CO, as a constraint, and by optimizing all other degrees of freedom for a range of Ru–CO distances, we obtained relative energies, ΔE(r) and geometries of a sufficient number of transient structures with the elongated Ru–CO bond up to r = 3.4 Å. Our calculations provide a quantitative understanding of the CO ligand dissociation in (η⁵-Ph₅C₅)Ru(CO)₂Cl and (η⁵-Ph₅C₅)Ru(CO)₂(O^tBu) complexes, which is relevant to the mechanism of their catalytic activity in the racemization of alcohols. We recently reported that exchange of the CO ligand by isotopically labeled ¹³CO in the Ru–O^tBu complex occurs twenty times faster than that in the Ru–Cl complex. This corresponds to a difference of 1.8 kcal mol⁻¹ in the CO dissociation energy (at room temperature). This is in very good agreement with the calculated difference between the two potential energy curves for Ru–O^tBu and Ru–Cl complexes, which is about 1.8–2 kcal mol⁻¹ around the corresponding transition states of the CO dissociation. The calculated difference in the total energy for CO dissociation in (η⁵-Ph₅C₅)Ru(CO)₂X complexes is related to the stabilization provided by the X group in the final 16-electron complexes, which are formed via product-like transition states. In addition to the calculated transition states of CO dissociation in Ru–O^tBu and Ru–Cl complexes, the calculated transient structures with the elongated Ru–CO bond provide insight into how the geometry of the ruthenium complex with a potent heteroatom donor group (X) gradually changes when one of the COs is dissociating.