Towards a Rational Design of Ruthenium CO₂ Hydrogenation Catalysts by Ab Initio Metadynamics

Abstract

Complete reaction pathways relevant to CO2 hydrogenation by using a homogeneous ruthenium dihydride catalyst ([Ru(dmpe)2H2], dmpe=Me2PCH2CH2PMe2) have been investigated by ab initio metadynamics. This approach has allowed reaction intermediates to be identified and free‐energy profiles to be calculated, which provide new insights into the experimentally observed reaction pathway. Our simulations indicate that CO2 insertion, which leads to the formation of formate complexes, proceeds by a concerted insertion mechanism. It is a rapid and direct process with a relatively low activation barrier, which is in agreement with experimental observations. Subsequent H2 insertion into the formateRu complex, which leads to the formation of formic acid, instead occurs via an intermediate [Ru(η2‐H2)] complex in which the molecular hydrogen coordinates to the ruthenium center and interacts weakly with the formate group. This step has been identified as the rate‐limiting step. The reaction completes by hydrogen transfer from the [Ru(η2‐H2)] complex to the formate oxygen atom, which forms a dihydrogen‐bonded RuH⋅⋅⋅HO(CHO) complex. The activation energy for the H2 insertion step is lower for the trans isomer than for the cis isomer. A simple measure of the catalytic activity was proposed based on the structure of the transition state of the identified rate‐limiting step. From this measure, the relationship between catalysts with different ligands and their experimental catalytic activities can be explained.

Carbon Dioxide Hydrogenation Catalyzed by a Ruthenium Dihydride: A DFT and High‐Pressure Spectroscopic Investigation

“…[27] Field and co-workers reported that the trans isomer also exists at a ratio of cis:trans = 12:1 at 240 K in toluene. [28] We also confirmed the minor presence of the trans isomer based on 31 P and 1 H NMR data (quintet at d = À10.3 ppm in 1 H and d = 49.0 ppm in 31 P, see Supporting Information and reference [28]) and their 2D correlation. We obtained a ratio of cis:trans = 18:1 at 300 K in toluene.…”

“…[31] The Ru À H bond of TS 2-3 is elongated to 2.30 and the Ru···O distance is 2.92 . Interestingly, the formate unit of TS 2-3 undergoing rotational movement has nearly the same bond lengths and angles as the formate ion COOH ) to give 4, in which the formate hydrogen atom points away from the Ru center and the formate oxygen atom is in the vicinity of the methyl hydrogen atoms of the dmpe ligand.…”

“…This route is nearly impossible to analyze by using static models; therefore, we studied the pathway by ab initio metadynamics, which indicated that formation of 9 via 4 is possible but unlikely. [31] Another possible route is via trans-[RuH 2 ] (7). As shown before, 7 is in equilibrium with 1 and exists in a small fraction.…”

“…Based on the ab initio metadynamics study, [31] the most likely pathway is 3). The gradual change in concentration from 6 to 9 is probably due to the high energy barriers for CO 2 dissociation from 6 (19.4 kcal mol …”

Recent advances in catalytic hydrogenation of carbon dioxide

“…295,299 Simulations indicate that CO 2 insertion, which leads to the formation of formate complexes, is a rapid process with a relatively low activation barrier. 299 The activation energy of the H 2 insertion step is lower for the trans isomer than that for the cis isomer. 299 Complex cis-(PMe 3 ) 4 RuCl(OAc) (1, OAc is acetate) has performed favorable activity for CO 2 hydrogenation as shown in Table 4.…”