POCOP Pincer-Ligated Nickel Complexes:Syntheses,Reactivity And Catalytic Application In Carbon Dioxide Reduction

Human demand for energy is increasing.Therefore,developing new energy sources is among the priorities for modern scientists.On the other hand,human activities have made the concentration of carbon dioxide in the atmosphere increase every year,leading to global warming.In order to find new energy sources and reduce the concentration of carbon dioxide in the atmosphere,scientists are trying to generate fuel from carbon dioxide.Catalytic reduction of carbon dioxide under transition-metal complexes is one of the most active subjects in this field.Unfortunately,there are many problems such as low efficiency and high cost still need to be solved.Finding out new catalyst systems is still one of the main tasks in this field.Pincer ligated transition metal complexes are catalytic active in many types of reactions.The fact that this type of complex has received a great deal of attention in the catalysis field is largely attributed to their high catalyst thermal stability,well balanced ligand properties and modified electronic and steric properties.Recently,significant effort has focused on the elucidation of structure-reactivity relationships with the goal of developing more active catalysts.Furthermore,this type of complexes has been successfully used in carbon dioxide reaction and methanol generation.In this thesis,a series of pincer ligated nickel chloride complexes [4-Z-2,6-?R₂PO?₂C₆H₂]NiCl?R = ^tBu,ⁱPr,Ph;Z = H,CO₂Me?has been synthesized.Further reactions of the nickel chloride complexes with KSCN,NaN₃,NaSPh and NaSCH₂ Ph have led to the isolation of pincer ligated nickel isothiocyanate,azide,aromatic thiolate and aliphatic thiolate complexes [4-Z-2,6-?R₂PO?₂C₆H₂]NiX/Y?R = ^tBu,ⁱPr,Ph;Z = H,CO₂Me;X = NCS,N₃;Y= SPh,SCH₂Ph?respectively.The nickel complexes have been fully characterized by multinuclear NMR,FTIR,X-ray crystallography,HRMS and elemental analysis.Metathesis reactivity of pincer ligated nickel isothiocyanate and azide complexes and ligands exchange reactions between the nickel isothiocyanate and nickel azide complexes have been investigated.The catalytic application of the thiolate complexes in carbon dioxide reaction and methanol generation has been explored.It was found that the thiocyanate ion binds to the nickel center through the nitrogen.The metathesis reactions with thiocyanate/azide complexes are faster with a less electron rich and more sterically accessible nickel center.The thiolate complexes are highly efficient in catalytic carbon dioxide hydroboration and methanol generation.A turnover frequency?TOF?of 2142 h^-1 was observed at room temperature and under 1 atm of CO₂.Up to date this is the most efficient homogeneous catalytic carbon dioxide hydroboration and methanol generation process under mild condition.In summary,we have synthesized a series of pincer ligated nickel complexes and studied their structure characteristics,reactivity and catalytic application in carbon dioxide reaction.The results obtained in this work are of great importance to pincer complex chemistry as well as carbon dioxide reaction and methanl generation.