DFT Calculation Study Of RuPNP Type Molecular Complex Catalyzed Hydrogenation Of CO₂ To Alcohol Phosphotungstic Acid

[（^RPN^HP）Ru HCl（CO）]（^RPN^HP=HN{CH₂CH₂（PR₂）}₂）complexes can directly capture CO₂ in the air under the auxiliary conditions of organic amines,and generate CH₃OH through catalytic hydrogenation,which has huge application prospec.However,the reaction conditions are harsh,requiring higher reaction temperature and hydrogen pressure.How to complete the above-mentioned catalytic reaction under mild conditions has become a current research hotspot.A large number of theoretical and experimental studies have shown that in the presence of H₂,ruthenium,manganese and iron pincer complex catalysts can directly or indirectly reduce CO₂ to CH₃OH at lower temperature and pressure.Although these key intermediates have been clearly characterized by using in-situ spectroscopy and single crystal X-ray diffraction techniques,the thermodynamic and kinetic parameters associated with these steps are still unknown.With the aid of organic amines,the mechanism of the homogeneous hydrogenation of CO₂ to CH₃OH catalyzed by PNP-type complexes is still controversial.Based on the above reasons,this article has carried out the following two parts of research work:（1）Using density functional theory calculations,the potential ability of M-PNP（M=Ru,Mn,Fe,Co,Ni,Cu）type complexes to adsorb H₂ molecules is analyzed,and screen the M-PNP type complexes that can better adsorb H₂ molecules.Calculation studies have shown that the adsorption of H₂ molecules by M-PNP（M=Ru,Mn,Fe,Co,Ni,Cu）type complexes is an endothermic process.Due to the small adsorption energy,the WBI bond sequence and bond length of H₂ change significantly,the M-PNP（M=Ru,Fe）type complex has a strong adsorption effect on H₂ molecules relatively,and the adsorption energy of the remaining M-PNP（M=Mn,Co,Ni,Cu）type complexes with H₂ molecules is very weak.Through the NBO charge,orbital analysis showed that interacted with the 4d_z2 orbital of the Ru center and theσ_1sorbital of the H₂ molecule,which led to the transfer of electrons from the coordinated H₂ molecule to the Ru center.This electron transfer process helps to activate H₂molecules,thereby increasing the activity of H₂ molecules.（2）The mechanisms of ruthenium pincer catalysts[Ru HCl PNPR（CO）]（R=Ph,i-Pr,Cy,t-Bu）-catalyzed synthesis of methanol from hydrogen and CO₂ in the presence of amine have been carried out using density functional theory（DFT）calculations at the B3LYP level.The results show that the whole catalytic cycle includes four stages:CO₂ hydrogenation,formic acid and amine dehydration and condensation to dimethylformamide,the hydrogenation of dimethylformamide to semi-amino compounds and the hydrogenation of formaldehyde to methanol.In terms of kinetics,the hydrogenation stage of CO₂,the dehydration and condensation stage of formic acid and amine to produce dimethylformamide and the hydrogenation stage of formaldehyde to produce methanol may be relatively easy to occur,and the kinetic energy barrier is small.The rate-limiting step of the whole catalytic cycle is the hydrogenation of dimethylformamide into semi-amino compounds,and the high energy barrier is explained by the transition state energy decomposition.It is found that the strain energyΔE_s,S of the substrate is the main reason to inhibit the reaction.For the ruthenium biscarbonyl cation complex[Ru HPNPR（CO）₂]⁺formed by the catalyst deactivation pathway in the experiment,its structure was characterized by fitted infrared spectroscopy,and a reasonable reaction path to transform the dicarbonyl complex back to active substance was determined.In addition,the mechanism of pincer complexes with different substituents（Ph,i-Pr,Cy,t-Bu）catalyzed by amine-assisted hydrogenation of CO₂ to methanol was discussed and compared,and it was found that the PNP（R=Ph）type complex with a strong electron withdrawing group has the same catalytic performance as the experiment,showing better catalytic performance,which further shows that the electron withdrawing ability and steric hindrance effect will affect the catalytic performance of the catalyst.