Hydrogenation of Carbon Dioxide Using Half-Sandwich Cobalt, Rhodium, and Iridium Complexes: DFT Study on the Mechanism and Metal Effect

Abstract

The hydrogenation of carbon dioxide catalyzed by half-sandwich transition metal complexes (M = Co, Rh, and Ir) was studied systematically through density functional theory calculations. All metal complexes are found to process a similar mechanism, which involves two main steps, the heterolytic cleavage of H2 and the hydride transfer. The heterolytic cleavage of H2 is the rate-determining step. The comparison of three catalytic systems suggests that the Ir catalyst has the lowest activation free energy (13.4 kcal/mol). In contrast, Rh (14.2 kcal/mol) and Co (18.3 kcal/mol) catalysts have to overcome relatively higher free energy barriers. The different catalytic efficiency of Co, Rh, and Ir is attributed to the back-donation ability of different metal centers, which significantly affects the H2 heterolytic cleavage. The highest activity of an iridium catalyst is attributed to its strong back-donation ability, which is described quantitatively by the second order perturbation theory analysis. Our study indicates that the functional group of the catalyst plays versatile roles on the catalytic cycle to facilitate the reaction. It acts as a base (deprotonated) to assist the heterolytic cleavage of H2. On the other hand, during the hydride transfer, it can also serve as Brønsted acid (protonated) to lower the LUMO of CO2. This ligand assisted pathway is more favorable than the direct attack of hydride to CO2. These finds highlight that the unique features of the metal center and the functional ligands are crucial for the catalyst design in the hydrogenation of carbon dioxide.

Catalytic oxidation of primary aromatic alcohols using half sandwich Ir(III), Rh(III) and Ru(II) complexes: A practical and theoretical study

“…Gibbs free-energy profile calculations rationalize the stability and reactivity of various transformations of catalyst 2 as per the proposed 26 mechanism. In the reaction mechanism, the Rh-H step was processed via β-hydride elimination Kcal/mol) was in good agreement with a reported value for a similar species [53]. The calculated activation free energy barrier of the 16e -Rh complex (D)TS 3 was higher (31.85 Kcal/mol) than the respective Rh complex (C)TS 2 (20.34 Kcal/mol), which indicates that the proposed reaction mechanism is more favorable through the (C)TS 2 transition state than the (D)TS 3 16e -Rh complex (D)TS 3 .…”

“…The electron densities of the Cl group attached to the iridium (0.212) and ruthenium (0.057) are less than that of the Cl group attached to the rhodium (0.273). These electron density values confirm that the Rh-Cl (2) bond is stronger and this would account for it being a less reactive complex compared to the latter two complexes [53].…”

Control in the Rate-Determining Step Provides a Promising Strategy To Develop New Catalysts for CO2 Hydrogenation: A Local Pair Natural Orbital Coupled Cluster Theory Study

“…The same holds true for half-sandwich complexes [Cp*M III (6,6′-O − -bpy)(H 2 O)] (M = Co, Rh, and Ir; bpy = 2,2′-bipyridine). 12 However, CO 2 conversion mediated by [(η 5 -C 5 Me 5 )Ir III (bpy)-(H)] + was dissected experimentally to traverse a hydride transfer RDS, while for its Ru(II) analogue, [(η 6 -C 6 Me 6 )-Ru II (bpy)(H)] + , the RDS was identified to be H 2 splitting. 13 Additionally, the recent experimental study on the [Co I (dmpe) 2 H] (dmpe = 1,2-bis(dimethylphosphino)ethane) has suggested the hydride transfer to be the RDS in the presence of a strong base.…”

Thermal-reductive transformations of carbon dioxide catalyzed by small molecules using earth-abundant elements

“…The highest TON and TOF achieved are 39 and 39 h 1 , respectively. DFT calculations on a related system reported by Zhao et al [17] suggested that the formation of the monohydride might be the ratedetermining step.…”