Synthesis Of Cobalt,Iron Hydrides Supported By Novel Pincer Ligands And Their Application In Alkene Hydrosilylation Reactions

With the rapid development of organometallic chemistry,the design and synthesis of new complexes are still the focus of researchers.The synthesis of new organometallic complexes and the study of their properties will not only help to overcome the scientific problems,but also create great economic benefits for industrial production.Among the many factors affecting the properties of complexes,the structure of ligands has always been the most important.Since the first successful separation of stable free NHCs by professor Arduengo in 1991,carbene ligands have been widely used in homogeneous catalysis as an ideal substitute for phosphine ligands.To take advantage of carbenes' strong ?-electron donating ability and flexible tunable steric hindrance properties,scientists have recently applied it to the Non-symmetric pincer ligand framework with modular design ideas.This interesting design concept can give full play to the advantages of carbene ligands,and also consider the stability and activity of complexes,which has great application potential in homogeneous catalysis.Like carbene,N-heterocyclic silylene also has high reactivity and stability.Especially in 2015,Professor Szilvasi predicted the bright application prospects of silylene ligands through theoretical calculations.Unfortunately,the development of silylene chemistry has been greatly influenced by fewer studies on this field.The lack of this research has greatly affected the development of silylene chemistry.In recent years,the hydrosilylation of alkenes promoted by inexpensive transition metal complexes have become the focus of scientific research.Currently,the field is also experiencing problems with reaction mechanism,selectivity control,industrialization improvement,and so on.Therefore,this paper will focus on the above areas,mainly divided into the following four parts:1.Two kinds of asymmetric pincer NHC ligand precursors 2a,2b were designed and synthesized.Cobalt hydrides 3a,3b and trans-bischelate complexes 4a,4b were synthesized by the reaction of 2a,2b with CoMe(PMe3)4.The experimental results show that cobalt hydride 3a can effectively catalyze the Markovnikov addition of aryl terminal alkenes with Ph2SiH2.For alkyl terminal alkenes,3a can effectively catalyze the anti Markovnikov addition reaction.In this system,pyridine N-oxide is a suitable additive,which can effectively reduce the reaction temperature.We also explored the reaction mechanism and thought that the catalytic system might follow the classic Chalk-Harrod mechanism.2.By using well-defined low valent cobalt complexes Co(PMe3)4 and CoCl(PMe3)3,the precise control of selectivity of the hydrosilylation have preliminarily realized.The former promoted Markovnikov-type hydrosilylation of the aryl alkenes while the latter catalyzed anti-Markovnikov-type hydrosilylation of the alkyl alkenes.These two catalytic systems tolerate a variety of functional groups and provide high selectivity and medium to high yield.In the exploration the reaction mechanism,a dinuclear silyl cobalt(?)complex 10 from Co(PMe3)4 system and a silyl cobalt dihydride 11 from CoCl(PMe3)3 system were obtained.The separation of these two complexes is helpful to understand the possible mechanism of the reaction,which have been proposed.3.On the basis of the previous work,we improved the precursor of asymmetric NHC ligand and synthesized three kinds of asymmetric pincer NHC ligand precursors 13a-13c.Three kinds of ionic cobalt hydrides,14a-14c,were prepared by reacting with n-BuLi and then with Co(PMe3)4.After changing the reaction solution to acetonitrile with stronger polarity,the ionic bischelate complex 15 was obtained.Experiment results show that complex 15 can catalyze the dehydration of amides to nitriles.In addition,the NHC precursor 13a-13c reacted with(DME)NiCl2 to obtain three nickel chloride complexes 17a-17c.The preliminary experimental results show that complex 17a can effectively promote the Markovnikov addition reaction of styrene with PhSiH3,and the detailed study of this system is still in progress.4.On the basis of ligand 5-A,we have synthesized diphosphine ligands 19 and 20 with PPh2 and piPr2 side arms,respectively.By the reaction of 19 and 20 with Fe(PMe3)4,we have obtained iron hydride 21 and iron nitrogen complex 22.These results indicate that the difference of electron donating ability of the ligands partly affects the formation of nitrogen complexes.The difference is that the reaction of 5-A with Fe(PMe3)4 achieves a higher activation of nitrogen,which shows the difference between 5-A and 20.In addition,similar to 5-A,two cobalt hydrides 23 and 24 were obtained by the reaction of 19 and 20 with CoCl(PMe3)3.The preliminary experimental results show that cobalt hydride 23 supported by PPh2 ligand can promote the anti Markovnikov addition reaction of styrene with Ph2SiH2,while the cobalt hydride 5-C supported by silylene ligand can promote the Markovnikov addition reaction of styrene with Ph2SiH2.The different results reflect the unique properties of bis(silylene)ligands,and the detailed study of this reaction is also in progress.