Synthesis And Catalytic Properties Of Two Chiral Metal Acyclic Diaminocarbene Decorated Covalent Organic Frameworks

The past few years has seen an increasing interest in and extensive research on covalent organic frameworks(COFs).COFs are crystalline materials with ordered two-dimensional(2D)or three-dimensional(3D)structures formed by C,H,O and other light elements in reversible covalent bonding.In addition to their high crystallinity,porosity and tunability,COFs show high stability in harsh chemical reaction environments(including high temperature,strong acids and bases,and oxidizing and reducing conditions),which has drawn the attention of a large number of researchers.However,COF materials have certain limitations such as reduced catalytic sites in the field of heterogeneous catalysis along with increased stability.Currently,their application in catalysis is mainly realized by modifying catalysts onto the materials through covalent or non-covalent interactions.N-heterocyclic carbene(NHCs),as a type of organic carbon ligands,form stable carbon-metal bonds with metals;they also form metal complexes of different steric and electronic effects through different metal coordination.NHC metal complexes(M-NHCs)are regarded as an important type of organometallic catalysts that can be used to(asymmetric)catalyze various types of organic reactions.Acyclic diaminocarbene metal complexe(M-ADC)is a special class of metal carbene complexe.Owing to the wider N-C-N angle of ADCs relative to NHCs and ADCs’strongerσ-electrons and better spatial tunability for transition metals,ADCs are seen as a promising alternative to N-heterocyclic carbene.Although the first separable ADC was reported more than 20 years ago by Alder et al.,it remains less explored.Today,the recycling and reuse of both M-NHC and M-ADC catalysts is still quite challenging.To meet the requirements of low-cost,resource-conserving and environmentally friendly organic synthesis,molecular M-ADCs are loaded onto solid COF materials for application in heterogeneous catalysis.There are three common methods for the preparation of ADC metal complexes:(1)direct coordination;(2)Li-halogen exchange;(3)nucleophilic addition,which offers a more convenient and novel route for the preparation of chiral ADCs.Therefore,it is a challenging topic to introduce metals and chirality to COF materials and form stable structures for their application and then extended application in organo-catalysis.On such a basis,M-ADC and COF were combined here,and chiral groups were introduced therein to synthesize two M-ADC-modified chiral COFs(CCOFs).The properties of the two CCOFs were then studied.1.First of all,the COF material with chiral secondary amines(S)-SA-COF was synthesized through direct polymerization of organic monomers containing chiral secondary amines and1,3,5-benzenetricarboxaldehyde under solvothermal condition.The material features a stable structure and moderate pores,providing suitable conditions for the introduction of metals.After that,chiral Au-ADC active centers were modified into COFs through addition reaction of Au-isocyanide and chiral secondary amines at atmospheric temperature and pressure,thus obtaining the Au-ADC-modified chiral COF,(S)-Au-ADC-COF,further catalyzed the A~3coupling reaction with high efficient.2.On top of the above research on chiral COF construction,the chiral secondary amines and imidazole salts co-modified COF,(S)-BIm/SA-COF,was synthesized through polymerization of chiral secondary amine-containing organic monomers and benzimidazole salt-modified dialdehyde ligands under solvothermal condition.The material features a stable structure and moderate pores.After that,chiral Au-ADC active centers were modified into the COF in the same way as above at atmospheric temperature and pressure,thus obtaining the chiral Au-ADC and imidazole salts co-modified chiral COF,(S)-BIm/Au-ADC-COF.Its properties were then characterized.In this research,following the same line of thinking,two chiral crystalline porous framework materials were designed and synthesized.The catalytically active chiral Au-ADC was introduced into COFs.The obtained CCOFs and bi-functionalized materials have chiral metal catalytic active sites that can significantly enhance the catalytic activity and selectivity in their catalytic applications.This route of synthesis is simple,and the CCOFs constructed are stable.It offers more possibilities for the application of chiral COF materials in heterogeneous catalysis and lays the research foundation for the disciplinary combination of organometallic chemistry and multi-functionalized COF materials.