Transition Metal Cp~*Co Complex Catalyzed Hydrogen Transfer And Acceptorless Alcohol Dehydrogenate Reaction

Liquid organic hydrogen storage technology offers great advantages in hydrogen storage density and flexibility for storage and transportation.In this regard,it has become particularly promising hydrogen storage system.The hydrogen transfer(HT)reaction and acceptorless alcohol dehydrogenate(AAD)reaction have relatively mild conditions in terms of reaction pressure and reaction temperature,and they are suitable for industrial production equipment requirements.Therefore,they have great potential in the hydrogenation process and dehydrogenation process of liquid organic hydrogen storage technology.However,noble metal complexes are mostly used as catalysts for the above-mentioned two reactions,and phosphine ligands,which are expensive and sensitive to air and water,are mainly used as the complex framework,which restricts its industrial operation.Based on the good stability of pentamethylcyclopentadienemetal(Cp*M)complexes and excellent performance of cobalt complexes in the hydrogenation/dehydrogenation reaction,this research proposes to design and synthesis of a series of phosphine-free non-noble metal cobalt complexes[Cp*Co]and apply them to both HT and AAD reactions simultaneously for the first time.This research aim to promote the catalytic performance through coupling the ligands types and coordination methods.Based on the analysis of key intermediate through reaction pathways and control steps,the regulatory mechanism is obtained between the molecular structure and the catalytic performance in the HT and AAD reactions.This research will give direction for catalyst rational design for non-precious metal complexes.Firstly,the monometallic complex A[Cp*Co(CO)I2]and the bimetallic complex B[Cp*CoI2]2 were synthesized with Cp*Co as framework.Through systemic characterization and calculation of the molecular structure,electronic properties and surface forces,the influence of the metal coordination modes on the electronic properties of the central metal and reactive sites was studied.The results showed that the bimetallic complex B had a lower central metal reduction potential,and was more inclined to show the electron transfer process inside the metal atom.The coordination modes did not change the distribution of redox sites,and the reactive sites of the two complexes are mainly concentrated on the metal.When applying the two complexes to the HT reaction,complex B exhibited better catalytic performance.Through screening different conditions such as the type of base and catalyst loading,a yield of 99.8%for the corresponding product,1-phenylethanol,was obtained under optimized conditions.The yield was better than most of the non-noble metal complexes used in HT reactions that had been reported so far,and even comparable with some noble metal complexes.The results of the substrate expansion study showed that complex B had good catalytic activity for aromatic ketones,aliphatic ketones and alicyclic ketones,among which aromatic ketones with stronger electron donating ability were obtained the best yield.Secondly,based on complexes A and B,the proton-responsive ligand 6DHBP(6,6’-dihydroxy-2,2’-bipyridine)was introduced to synthesis complexes C[Cp*Co(6DHBP)(Ⅰ)PF6]and D[Cp*Co(6DHBP)(OH2)(PF6)2].Through systemic characterization and calculation of the molecular structure,electronic properties and surface forces,the effect of different auxiliary ligands on the electronic properties and reactive sites of the central metal was investigated.The results showed that the central metal reduction potential was reduced after the introduction of 6DHBP.In addition,the Cp*Co group in complex D had a stronger affinity with the ligand 6DHBP,and the energy required for electron transfer is lower.During the redox reaction,the calculated result showed that oxidation site was mainly concentrated on the 6DHBP ligand,and the reduction site was mainly on the Co center for complexes C and D.When applying the synthesized complexes A-D to the AAD reaction,the complex D showed better catalytic performance.Through the investigation of reaction temperature,reaction solvent,catalyst loading and other conditions,the yield of the corresponding product benzaldehyde could reach 43.8%,and its selectivity was almost close to 100%.This high selectivity was even better than some precious metal complexes.Finally,through density functional theory(DFT)calculations,this study provided further insight into the mechanism of HT and AAD reactions with Cp*Co complexes.Also,the reaction pathways and control steps were discussed in both reaction process.The results showed that in the HT reaction,the initial active state of complex B under alkaline conditions was[Cp*CoI2]-iPrO,and the reaction proceeded through the innersphere pathway,in which the β-H elimination process was the rate-determining step with a ΔG value of 29.5 kcal/mol.In the AAD reaction,the initial active state of complex D was Cp*Co(6DHBP-2H+),and the calculated result indicated that the outersphere pathway was more plausible,which required both the metal center and the ligand participate in the bond forming and bond breaking process.The formation step of dihydrogen complex was the rate-determining step with a ΔG value of 15.6 kcal/mol.Through the analysis of the charge transfer process in the reaction path of the outersphere pathway,it was indicated that the Cp*played a key role in stabilizing the corresponding translation states.This research combines experiments and theoretical calculations to systematically investigated the application of Cp*Co complexes in the hydrogen transfer reaction and acceptorless alcohol dehydrogenation reaction.It not only expanded the application of non-precious metal Co in the hydrogenation and dehydrogenation reaction system,but also provided insight in the influence of the coordination mode on the reaction path in different reactions.This research provided theoretical guidance for the design and optimization of more non-precious metal catalysts.