Synthesis And Application Of Functional Pincer Complexes In Catalytic CO₂ Hydrogenation

The concentration of CO₂ in atmosphere has been increasing rapidly because of the large consumption of fossil fuel since industrial revolution,causing severe greenhouse effect.Transformation of CO₂ into value-added products is one of the most important ways to utilize CO₂ and retard greenhouse effect.Owing to its high thermodynamic stability and kinetic inertness,high temperature and pressure are usually necessary for the selective transformation of CO₂.CO₂ hydrogenation catalyzed by transition metal complexes has attracted much attention.In this system,CO₂ is reduced under relatively mild conditions,and generats formic acid product,which is an important chemical material and good hydrogen storage material.In this study,a series of pincer complexes were developed and applied in the catalytic CO₂ hydrogenation in aqueous solutions,the catalytic activity of pincer complexes and mechanism of the catalytic hydrogenation were explored.1)A series of pincer complexes were prepared,and their structures were characterized by several methods,such as NMR and single crystal X ray diffraction.2)Fuctionlized pincer complex 3 was used to explore optimal conditions.The effects of temperature,pressure,catalyst loading,concentration of base,ratio of organic solvent and water were examined.The optimal conditions were determined by series of controlled experiments.The highest TON of 3382 was reached with 10?M complex 3 at 120 ^oC,4 MPa?H₂/CO₂=1:1?,in 2 mol/L KHCO₃ for 24 h.To our delight,formic acid was also detected using complex 3 when no base was added.3)Complex 1-5 were screened under the optimized conditions,and complex 3 with pendent OH exhibited the best performance.4)The mechanism of CO₂ hydrogenation by 3 was proposed.The pendent OH of 3probably facilitates the heterolytic dissociation of H₂ by forming hydrogen bond,thus hancing the catalytic performance.This study developed a new type of functional pincer complexes for CO₂ hydrogenation with or without base addition in aqueous solution.The experimental results and proposed mechanism provides theoretical basis for design of new catalysts for CO₂ hydrogenation in future.