Synthesis Of Iron,Cobalt And Nickel Complexes Supported By NHC Pincer Ligands And Their Applications In Catalytic Reactions

As N-heterocyclic carbene(NHC)ligands own high stability and strong electron donor ability,the common electron-rich phosphine ligands have been gradually replaced by NHC ligands in the field of organometallic chemistry.In recent years,more and more researchers are interested in introducing NHC into the structural framework of pincer ligands.The strong metal-carbene bonds make the metal complexes stable in the environment of air,moisture and even strong acids.In addition,the structure of NHC is easily regulated and functionalized,which makes the corresponding complexes more flexible.With the increase of NHC pincer metal complexes,the types of organic reactions effectively catalyzed by these complexes are also expanding.Here,the base transition metal iron,cobalt and nickel complexes supported by NHC pincer ligands were synthesized and their applications in catalytic reactions were explored.1.Complex 4 was obtained from the reaction of L1 with Fe(PMe3)4.The iron center was coordinated by two bis-chelate[C,N]ligands,and the-NMe2 groups were uncoordinated.The central Fe atom of 4 is in a distorted tetrahedral coordination geometry.Complex 6 was obtained from the reaction of L1 with Co(PMe3)4.Its molecular structure was similar as that of 4.A paramagnetic complex 7 was obtained from the reaction of L1 with CoCl2.The central Co atom is in a distorted trigonal bipyramid coordination geometry.Complexes 1-3 could be hydrogenated by NaBH4 to afford nickel hydrido complexes 8-10.8-10 were characterized by NMR and IR spectroscopy as well as X-ray diffraction.Nickel hydrido complexes 8-10 are relatively active and can participate in a variety of property reactions with small molecules,such as the substitution reaction with methyl iodide and the insertion reaction with phenylacetylene.2.The catalytic application of complexes 8-10 for hydrodehalogenation reactions was investigated.The optimized reaction condition for this catalytic system was explored. In a combination of 3 mol%catalyst loading of 8,(EtO)3SiH/NaOtBu/toluene/80?and different reaction time,organic halides were successfully reduced to the related alkanes.99%GC yields of aliphatic halides and 57%-93%isolated yields of the aromatic halides could be achieved.The mechanism of this catalytic reduction reaction was discussed and supported by experiments.3.The catalytic performance of complexes 1-3 for Sonogashira cross-coupling reactions was explored.In a combination of 5 mol%catalyst loading of 1,CuI/Cs2CO3/NaI/DMSO at different temperatures for the coupling reactions between alkyl iodides,bromides,chlorides and terminal alkynes,the corresponding coupling products were obtained in moderate to excellent yields.Especially for alkyl chlorides 44-82%isolated yields could be achieved.The mechanism of Sonogashira reaction was discussed and partially confirmed by experiments.4.On the basis of the structure of ligands L1a-Llb,new type of C(carbene)N(amido)N(pyrrolidine)pincer ligands L2a-L2b and corresponding nickel complexes 17 and 18 were synthesized.The complex 17 with pyrrolidine group exhibited a remarkable effect on the transfer hydrogenation(TH)of ketones.The catalytic performance of 17 for TH reactions is better than that of complex 1.With a combination of NaOtBu/iPrOH/80? and 2%catalyst loading of 17,77-98%yields of aromatic alcohols could be achieved.