Ni-alkyl Complexes Supported By P-S Tetradentate Ligand:Synthesis And Reactivity Studies

Ni-alkyl complexes play an important role in enzyme catalysis and organic synthesis.The active site of acetyl-CoA synthetase(ACS)catalyses the methylation of CO through a nickel methyl(Ni-CH3)intermediate in which CO insertion into the Ni-C bond is the essential step.In nickel-catalyzed C-C bond coupling reactions,Ni-alkyl complexes are also key species.Therefore,the synthesis and reactivity studies of Ni-alkyl complexes is of great significance in modelling the methylation process of CO in nature,understanding the mechanism of C-C coupling reactions,and further optimizing the structure of related catalysts.The tetradentate ligand containing P atom and S atom has the soft and flexible coordination properties.Its coordination mode can adjust according to the oxidation states of the metal center to stabilize the the complex.In this thesis,a series of nickel complexes bearing P2S2 and PSNP ligands were designed and synthesized.Their reactivity towards CO insertion,coupling of alkyl halides with Grignard reagents and B-H bond activation of pinacoborane was investigated.The results were obtained as follows:1.The nickel methyl complex[(P2S2)Ni-CH3]+([1-CH3]+)supported by P2S2 tetradentate ligand was synthesized and CO insertion into the Ni-C bond resulted in the transformation of CO to valuable chemicals,such as CH3CHO and CH3CH2OH.[[P2S2)Ni-CH3]+([1-CH3])was synthesized by the transmetallation reaction of the[(P2S.2)Ni(NCMe)2]2+ and CH3MgBr,or by the reaction of[1]2+ and CH3MgBr alternatively.CO inserted into Ni-C bond of[1-CH3]+ affording a nickel-acetyl complex[(P2S2)Ni-COCH3]+([1-COCH3]+).[1-COCH3]+ was then reduced by NaBH4 to form CH3CHO,CH3CH2OH and Ni(0)complexes([1]0)and[1]0 can react with CH3I to regenerate[1-CH3]+.By conducting deuterium labeling experiments,we have demonstrated that CH3CHO is the major product from CH3CH2OH in such CO transformation reactions.The results have established fundamental steps in the exploration of nickel-mediated CO transformation to valuable chemicals.2.The reaction of the Ni(O)complex[(P2S2)Ni]°([1]0)with the alkyl halides RX(R = Bn,Et,iPr,allyl)was investigated and a variety of Ni—alkyl compounds([Ni-R]+)were obtained.The stability of[Ni-R]+ is closely related to the type of alkyl group.When[1]0 reacted with C2H3CH2Br,a stable Ni(?)-allyl complex was provided.When[1]0 reacted with C6H5CH2Br,Ni(?)complex and an organic homo-coupled product 1,2-diphenylethane were formed through a radical pathway.When[1]° reacted with C2H5I or(CH3)2CHI,the corresponding nickel-alkyl compounds were formed through oxidative addition accompanied by the generation of[1]+ and alkyl dimer through by a radical pathway.The study found that the[Ni(?)-Et]+ complex underwent ?-H elimination to generate Ni-H and ethylene.Also,[1]0 was shown to be an active catalyst for the coupling reaction of primary halides and Grignard reagents.3.Based on the study of chapter ? and ?,we replaced one S atom in the P2S2 ligand to N atom,and a nickel(?)complex[(PSNP)Ni]+([2]+)supported by PSNP anionic tetradentate ligand was designed and synthesized;the reaction of[2]+ with HBpin was studied.It was found that the N atom acts as a basic site to help cleave the B-H bond in cooperation with the Ni center to give[(PSNBpinP)Ni-H]+.This study lays the foundation of Ni-based metal-ligand synergistic catalyst for the reduction of organic compounds containing unsaturated bonds.Moreover,in this chapter,the nickel-methyl complex([(PSNP)Ni-CH3]0)based on PSNP tetradentate ligand was synthesized and characterized.