Understanding the Influence of Lewis Acids on CO₂ Hydrogenation: The Critical Effect Is on Formate Rotation

Abstract

Lewis acids (LAs) have been shown to accelerate hydrogenation of CO 2 , but the underlying mechanistic details remain to be elucidated. We have employed computational methods to investigate how LAs affect CO 2 hydrogenation with a range of known metal-hydrides (L n Ir−H, L n Ru−H, L n Mn−H, L n Co−H). Our results show that LAs can alter the nature of the hydride−CO 2 bond formation step, but do not lower its barrier. Instead, the accelerating effect of LAs is on the subsequent step, the rearrangement of the metal-formate σ-intermediate. These insights are essential for understanding the effect of LA additives on metal-mediated hydrogenations of CO 2 .

Kinetic Isotope Effects and the Mechanism of CO2 Insertion into the Metal-Hydride Bond of fac-(bpy)Re(CO)3H

“…The concerted attribute of the GFES deduced in Figure B for the reaction of 1 –Re–H differs from the GFESs reported for CO 2 insertion via TS2 in other octahedral complexes such as 2 –Ir–H and 2 –Mn–H in Scheme . In those systems, TS2 was reported to be the highest point on the GFES of the reaction, but the free energy of intermediate i1 was slightly lower than TS1 , consistent with a stepwise reaction pathway.…”

“…As shown in Scheme A, this TS is often represented in a linear geometry with dashed M–H–C bonds, presumably to convey a hydride transfer step suggestive of a reaction coordinate that couples Re–H bond breaking with H–C bond making. In turn, i1 is commonly represented using a dashed M–H bond and a solid H–C bond, as shown in Scheme A. ,− In their recent investigation, Pavlovic and Hopmann proposed to classify i1 as a η 1 -σ-formate complex in analogy with the experimentally known η 1 -σ-alkane complexes …”

“…28,29 Very recently, Pavlovic and Hopmann provided computational evidence that consideration of TS2 is essential to be able to account for the observed Lewis acid effect. 30 Development of octahedral rhenium and manganese complexes or precursors related to 1−Re−H continue to be of great current interest for use as catalysts, electrocatalysts, and photocatalysts for various transformations of CO 2 . 31−34 Thus, knowledge of the mechanism of the CO 2 insertion step in this system is of fundamental interest and is vital when the calculations are invoked to interpret experimental results or to design new catalysts for CO 2 transformations.…”

“…In turn, i1 is commonly represented using a dashed M−H bond and a solid H−C bond, as shown in Scheme 4A. 28,36−38 In their recent investigation, Pavlovic and Hopmann 30 proposed to classify i1 as a η 1 -σ-formate complex in analogy with the experimentally known η 1 -σ-alkane complexes. 54 TS1 in the reaction of 1−Re−H has a severely bent Re−H− CO 2 reaction center with minimal Re−H bond breaking.…”

“…Soon after the Ahlquist study, Yang calculated the analogue of TS1 in the reaction of other pincer ligated complexes and assumed barrierless formate dissociation and recoordination in water without calculating TS2 . Subsequent theoretical investigations of CO 2 insertion in aprotic solvents diverged in the description of the mechanism: some reporting only TS1 without invoking a role for ion-pair dissociation ,− and some reporting both TS1 and TS2 . ,,, Experimentally, Lewis acid additives are known to accelerate the reaction rates of CO 2 insertion in octahedral metal-hydride complexes. , Very recently, Pavlovic and Hopmann provided computational evidence that consideration of TS2 is essential to be able to account for the observed Lewis acid effect …”