Lewis Acid Promoted Hydrogenation of CO₂ and HCOO^– by Amine Boranes: Mechanistic Insight from a Computational Approach

Abstract

We employ quantum chemical calculations to study the hydrogenation of carbon dioxide by amine boranes, NMeBH (AB) and NHBH (AB) weakly bonded to a bulkier Lewis acid, Al(CF) (LA). Additionally, computations have also been conducted to elucidate the mechanism of hydrogenation of carbon dioxide by AB while captured between one Lewis base (P(o-tol), LB) and two Lewis acids, Al(CF). In agreement with the experiments, our computational study predicts that hydride transfer to conjugated HCO, generated in the reaction of AB-LA with CO, is not feasible. This is in contrast to the potential hydrogenation of bound HCOH, developed in the reduction of CO with AB-LA, to further reduced species like HC(OH). However, the FLP-trapped CO effortlessly undergoes three hydride (H) transfers from AB to produce a CHO derivative. DFT calculations reveal that the preference for a H abstraction by an intrinsically anionic formate moiety is specifically dependent on the electrophilicity of the 2 e reduced carbon center, which in particular is controlled by the electron-withdrawing capability of the associated substituents on the oxygen. These theoretical predictions are justified by frontier molecular orbitals and molecular electrostatic potential plots. The global electrophicility index, which is a balance of electron affinity and hardness, reveals that the electrophilicity of the formate species undergoing hydrogenation is twice the electrophilicity of the ones where hydrogenation is not feasible. The computed activation energies at M06-2X/6-31++G(d,p) closely predict the observed reactivity. In addition, the possibility of a dissociative channel of the frustrated Lewis pair trapped CO system has been ruled out on the basis of predominantly high endergonicity. Knowledge of the underlying principle of these reactions would be helpful in recruiting appropriate Lewis acids/amine boranes for effective reduction of CO and its hydrogenated forms in a catalytic fashion.

Surface Lewis Acidity of Periphery Oxide Species as a General Kinetic Descriptor for CO2 Hydrogenation to Methanol on Supported Copper Nanoparticles

“…This finding suggests the involvement of these species as key reaction intermediates, in agreement with mechanistic proposals based on the formate route. , In light of this observation, alongside those differences in ionic character inferred for these species as a function of the surface oxide Lewis acidity, it is sensible to hypothesize that an increasing electronic transfer to cus oxide Lewis centers of increasing electron-withdrawing character contributes to the destabilization of these reaction intermediates and thus the decrease in E a observed experimentally for methanol formation. These results are reminiscent of previous computational predictions on the hydrogenation of formate species to methoxy groups, activated by organoaluminum molecular Lewis acids . As inferred from computed reaction energy profiles, the quenching of the electron density around the carbon center in formate intermediates bound to Lewis centers of increasing acidity enhances its electrophilic character and makes it more susceptible to nucleophilic attack by hydride species, as required to undergo hydrogenation into methoxy groups .…”

“…These results are reminiscent of previous computational predictions on the hydrogenation of formate species to methoxy groups, activated by organoaluminum molecular Lewis acids. 42 As inferred from computed reaction energy profiles, the quenching of the electron density around the carbon center in formate intermediates bound to Lewis centers of increasing acidity enhances its electrophilic character and makes it more susceptible to nucleophilic attack by hydride species, as required to undergo hydrogenation into methoxy groups. 42 A similar effect might be at play at Cu-oxide interfaces on solid catalysts.…”

Frustrated Lewis Pair Catalyzed C–H Activation of Heteroarenes: A Stepwise Carbene Mechanism Due to Distance Effect

“…According to the discussions above, the distance effect is of crucial importance in FLP-catalyzed C–H activation. In order to understand the intrinsic relationship between the LA–LB distance and the C–H activation mechanism, the structures and activation distortion–interaction energy decomposition analysis of the corresponding transition states are collected in Figure (see Figures S4 and S5 for details). In the case of FLP1 , the deformation energies of catalysts ( E def(cat.)…”

Theoretical investigation of an acid catalyst for viable release of H2 from BN nanotubes: A local pair natural orbital coupled cluster approach

“… 24 This approach characterizes the turnover‐determining intermediate (TDI) and the turnover‐determining transition state (TDTS) in a multistate catalytic cycle to analyze the efficiency of the process and figure out the most favorable pathway. [ 32,33 ] The energetic difference between the TDTS and TDI, along with the reaction driving force, produces the energetic span (δE) of the overall catalysis. As found from the energy profiles in Figure 1 and Table 3, the proton abstraction by acid anion for catalyst regeneration is the TDTS, and the preceding stable intermediate is the TDI for catalyst types A and B.…”

“…24 This approach characterizes the turnover-determining intermediate (TDI) and the turnover-determining transition state (TDTS) in a multistate catalytic cycle to analyze the efficiency of the process and figure out the most favorable pathway. [32,33] The energetic difference between the TDTS and TDI, along with the reaction driving force, produces the energetic span (δE)…”