Preparation Of Cobalt And Nickel Complexes Derived From Nitrogen-Containing Heterocycles And Catalytic Dehydrogenative Coupling Reactions

The transition metal-catalyzed acceptorless dehydrogenative coupling reaction becomes one of the important methods for efficient construction of chemical bonds and synthesis of heterocyclic compounds.This method does not require pre-functionalization of the substrate and reduces the synthetic step.Moreover,the coupling reaction is environmentally friendly because it has high atom economy.Normally,noble metal（including ruthenium,rhodium,iridium etc.）complexes are used as catalysts for acceptorless dehydrogenative coupling reactions of inexpensive and readily available alcohols.Although these noble metal complexes exhibit high catalytic activity,their economical cost is too high to limit their practical application.Therefore,the acceptorless dehydrogenative coupling reactions of alcohols catalyzed by non-noble metal complexes have received widespread attention from synthetic chemists in recent years.Furthermore,cobalt and nickel complexes are low price,low toxicity,and easy coordination with various ligands.They are considered to be a good catalyst in acceptorless dehydrogenative coupling reaction.In this thesis,the work mainly concentrated on the synthesis of novel ligands containing N-oxide units as well as their cobalt and nickel complexes derived from different nitrogen-containing heterocycles（pyridine,thiazole,tetrahydropyrrole,morpholine,piperidine,piperazine）.The catalytic properties of these cobalt and nickel complexes in the acceptorless dehydrogenative coupling reactions of alcohols were further investigated.Four cobalt complexes（Co-1～Co-4）and four nickel complexes（Ni-1～Ni-4）were prepared by reacting N,O-bidentate ligands（L¹～L⁴）with CoCl₂·6H₂O or NiCl₂·6H₂O in ethanol at room temperature,respectively.The N,O-bidentate-coordinated complexes were characterized by infrared spectroscopy,X-single crystal diffraction and elemental analysis.The single crystal structures of Co-1,Co-4 and Ni-4 were grown and obtained by vapor diffusion method.X-single crystal diffraction analysis showed that the metal（Co,Ni）of three complexes were coordinated with ligands in a ratio of 1:2 and associated with two water molecules（or methanol,chlorine atoms）to form a six-coordination complexes with a distorted octahedral geometry.Then we used cobalt complexes（Co-1～Co-4）as catalysts to investigate their catalytic activity in the reaction of benzyl alcohol（1a）and p-bromoaniline（2a）.The results have showed that these cobalt complexes have certain catalytic activity for this reaction,and 10 mol%Co-2 has the best choice of catalyst.After screening other conditions such as base,solvent,temperature,etc.,the optimized conditions of the reaction were obtained:10 mol%Co-2 as the catalyst,1a:2a=2:1,KOH（1.5 equiv）as the base,toluene（1.0 mL）as solvent,100℃,36h.The substrate range of the dehydrogenative coupling reaction was studied,and a total of 22 substrates were studied.The experimental data showed that the substrates containing electron-donating groups had higher reactivity than electron-withdrawing groups.The N,N,N-pincer ligands（L1’～L4’）were synthesized by bromination of 2,6-lutidine as the starting material followed by nucleophilic substitution,and the O,N,O-pincer ligands was further formed by the reaction of the intermediate（L1’～L4’）with m-chloroperoxybenzoic acid.Four pincer cobalt complexes（Co-5～Co-8）were prepared by the reaction of these O,N,O-pincer ligands with CoCl₂·6H₂O in methanol at room temperature.These compounds were characterized by nuclear magnetic resonance,infrared spectroscopy,high-resolution mass spectroscopy and X-single crystal diffraction.X-single crystal diffraction analysis showed that Co-5 belongs to the C2/c space group in the monoclinic system,and the central cobalt atom was respectively coordinated with one nitrogen atom of the pyridine ring and two oxygen atom of two N-oxide units,of the ligand L1,one oxygen atom in the methanol molecule and one chlorine atomto form a five-coordinate compound with a distorted trigonal bipyramid.Eight nickel complexes（Ni-5～Ni-12）were prepared by reacting L1’～L4’or L1～L4 with NiCl₂.6H₂O in methanol at room temperature,respectively.The catalytic activity of these complexes（Ni-5～Ni-12）in the dehydrogenative coupling reaction of benzyl alcohol（1a）with anthranilamide（4a）was investigated.The results indicated that all nickel complexes have good catalytic activity for this reaction and the nickel complex Ni-9 exhibited the highest catalytic activity.Ni-9 was used as the catalyst,and the influence of other factors such as solvent and alkali on the reaction was further studied.Thus the optimal condition of this reaction were established as followed:7 mol%Ni-9 as the catalyst,1a:4a=1.1:1,^t-Bu OK（1.5 equiv）as base,m-xylene（1.0 mL）as solvent,100℃,24h.