pH-Dependent transfer hydrogenation or dihydrogen release catalyzed by a [(η6-arene)RuCl(κ2- N, N-dmobpy)]+ complex: a DFT mechanistic understanding

Abstract

The reaction mechanism of the pH-dependent transfer hydrogenation of a ketone or the dehydrogenation of formic acid catalyzed by a [(η⁶-arene)RuCl(κ²-N,N-dmobpy)]⁺ complex in aqueous media has been investigated using the density functional theory (DFT) method. The TM-catalyzed TH of ketones with formic acid as the hydrogen source proceeds via two steps: the formation of a metal hydride and the transfer of the hydride to the substrate ketone. The calculated results show that ruthenium hydride formation is the rate-determining step. This proceeds via an ion-pair mechanism with an energy barrier of 14.1 kcal mol^-1. Interestingly, the dihydrogen release process of formic acid and the hydride transfer process that produces alcohols are competitive under different pH environments. The investigation explores the feasibility of the two pathways under different pH environments. Under acidic conditions (pH = 4), the free energy barrier of the dihydrogen release pathway is 4.5 kcal mol^-1 that is higher than that of the hydride transfer pathway, suggesting that the hydride transfer pathway is more favorable than the dihydrogen release pathway. However, under strongly acidic conditions, the dihydrogen release pathway is more favorable compared to the hydride transfer pathway. In addition, the ruthenium hydride formation pathway is less favorable than the ruthenium hydroxo complex formation pathway under basic conditions.

Scheme 1. The metal–ligand bifunctional mechanism for ketone/imine hydrogenation catalyzed by Ru complexes.

Scheme 2. The development of cyclometallated TM complexes.

Scheme 3. (a) Representative reaction modes of formate decarboxylation to produce the metal hydride (b) Representative reaction modes of hydride transfer from the metal to the ketone substrate. (c) Representative reaction modes of H₂ activation/release.

Scheme 4. Reduction of ketones catalyzed by the cyclometallated ruthenium single-site complex.

Fig. 1. The catalytic cycle for the TH of ketones catalyzed by cyclometallated ruthenium single-site complexes.

Fig. 2. The ion-pair mechanism and β-hydrogen elimination mechanism for the formation of metal hydride 5.

Fig. 3. The free energy profiles for the formation of metal hydride 5. All energies are denoted in kcal mol⁻¹ and interatomic distances are shown in Å, the values in parentheses are electronic energies.

Fig. 4. Four possible pathways of hydride transfer from the metal to the ketone substrate. The values of ΔG (in kcal mol⁻¹) indicate the free energy barriers of the four possible pathways.

Fig. 5. The free energy profiles of the hydride transfer pathway (path A) and dihydrogen release pathway. All energies are denoted in kcal mol⁻¹ and interatomic distances are shown in Å, the values in brackets are electronic energies.

Fig. 6. Proposed mechanism for TH of ketones catalyzed by cyclometallated ruthenium single-site complex at different pH values.

Fig. 7. The free energy profiles of the hydride transfer pathway and the dihydrogen release pathway mediated by H₃O⁺. All energies are denoted in kcal mol⁻¹ and interatomic distances are shown in Å, the values in parentheses are electronic energies.

Fig. 8. The free energy profiles of the metal hydride formation pathway and the metal hydroxo formation pathway, the values in parentheses are electronic energies.

Fig. 9. Three typical TM catalysts for HH or TH of ketones and the modes of dihydrogen activation.