Syntheses,Reactivity And Catalytic Application Of Novel Palladium Pincer Complexes

Since the first report in the late 1970s,pincer ligated transition metal complexes have been extensively investigated.This type of complexes has already been widely applied in many research fields,including catalysis,due to their high stability and tunable performance for structure and reactivity.So far a huge number of pincer complexes with different types of pincer backbones,such as POCOP,PCP,PBP,PNP,NNN etc.,have been successfully synthesized,and almost all transitional metals are involved in.The objective of this project is to synthesize novel palladium pincer complexes,and explore their structure,reactivity and catalytic application in the hydroboration of carbon dioxide.In this thesis,palladium chloride complexes with POCOP pincer ligand[2,6-?R₂PO?₂C₆H₃]PdCl?R=^tBu,ⁱPr,Ph?were synthesized first.Then further reactions of these chloride complexes with sodium thiolate resulted in the formation of a series of POCOP-pincer ligated palladium thiolate complexes[2,6-?R₂PO?₂C₆H₃]PdSY?R=^tBu,ⁱPr,Ph;Y=Ph,CH₂Ph,H?.This is the first time that a pincer ligated palladium thiolate complex is synthesized.In order to explore the structure and coordination mode of transition metal pincer complexes more extensively,two PBP pincer ligands?C₆H₄{N?CH₂PR₂?}₂BH??R=^tBu,^tBu-PBP;Ph,Ph-PBP?were synthesized,and their complexation with palladium was investigated.The reaction of ^tBu-PBP with Pd?cod?Cl₂ produced the PBP pincer ligated palladium chloride complex,while the reaction of Ph-PBP with Pd?cod?Cl₂ or Pd₂dba₃ afforded the tetrametallic or bimetallic palladium complexes.From the reactions of the POCOP-pincer ligated palladium thiolate complexes with BH₃·THF,the corresponding borohydride complexes were obtained.From the reactions of the ^tBu-PBP-PdCl palladium pincer complexes with KSCN,NaN₃ and NaBH₄,the PBP pincer ligated palladium isothiocyanate,azide and borohydride complexes were isolated respectively.These new palladium complexes were fully characterized by multinuclear NMR,X-ray crystallography,elemental analysis and HRMS.Finally,the catalytic application of the POCOP-pincer ligated palladium thiolate complexes in the hydroboration of carbon dioxide was investigated.It turned out that the bis?phosphinite?pincer ligated palladium thiolate complexes are good catalysts for the reduction of CO₂ to methanol with catecholborane at room temperature under an atmospheric pressure of CO₂.Turnover frequencies?TOFs?up to 1780 h^-11 were observed.The results also indicated that less bulky substitution on the phosphorous donor favors the catalytic reaction,and aromatic thiolate complexes are more active than the related aliphatic thiolate complexes.In summary,we have synthesized a series of palladium complexes with POCOP or PBP pincer ligand and investigated their reactivity and catalytic application.A highly efficient chemical transformation from carbon dioxide to methanol derivative under very mild conditions has been achieved.This work is of significant importance to the chemistry of pincer ligated transition metal complexes.