| The covalent triazine skeleton(CTF)is a special and emerging COF material.Covalently bonded CTF has good stability and low density,and has many practical applications in the fields of optics,electrochemistry and catalysis.These materials are generally synthesized by a trimerization reaction of a nitrile compound.A more common monomer is 1,4-dicyanobenzene.The synthesis is generally carried out by trimeric reaction of a nitrile compound with trifluoromethanesulfonic acid or hydrochloric acid to prepare a crystalline CTF.However,the catalyst of this method is costly.It is essential to find the intermediate structure of the catalytic reaction and to determine the mechanism of its trimerization in order to find a safe and economical catalyst to catalyze the trimerization reaction.However,the theoretical study of the mechanism of trimerization of nitrile compounds have not yet reported.In this paper,the process of trimerization of 1,4-dicyanobenzene and 1,3,5-tricyanobenzene monomer by trifluoromethanesulfonic acid and hydrochloric acid was studied.The most likely mechanism of trimerization was studied by density functional theory at the M06L/cc-pVTZ level.The specific research contents of this paper are as follows:1.Trimerization of 1,4-dicyanobenzene monomer and 1,3,5-tricyanobenzene monomer and 1,4-dicyanobenzene trimerization product by trifluoromethanesulfonic acid under solvent model Mechanism research.The mechanism of the trimerization of the three nitriles is similar.First,the oxygen atom of the trifluoromethanesulfonic acid attacks the cyano group of the nitrile to form an imine group.Next,with the addition of a molecule of nitrile,the lone pair of electrons in the cyano nitrogen atom attack the carbon atom of the imine group to form a chain dimer structure.Another molecule of nitrile again attacks the imine moiety of the dimer to form a ring-opening trimer.Finally,a triazine ring is formed by ring closure.The reaction energy barrier of the imine-based carbon atom undergoing the nucleophilic attack of the second nitrile RCN to form a chain-shaped dimer intermediate is 25.9 kcal/mol,which is the highest reaction energy barrier in the whole trimerization process.Therefore,this step is the final step of the entire trimerization reaction.Trimerization of 1,3,5-tricyanobenzene monomer The energy barrier of the first reaction in the whole trimerization reaction is up to 29.6 kcal/mol,which is the reaction of 1,3,5-tricyanobenzene trimerization.Quick step.Compared with the reaction energy barrier of trifluoromethanesulfonic acid catalyzed 1,4-dicyanobenzene trimerization reaction,trifluoromethanesulfonic acid catalyzes the trimerization of 1,3,5-tricyanobenzene monomer The energy barrier is slightly higher.Mainly due to electronic effects and steric hindrance,that is,1,3,5-tricyanobenzene increases an electron withdrawing group(C?N)compared to 1,4-dicyanobenzene,resulting in a benzene ring.The electron density is lowered and the steric hindrance is increased.Therefore,the reaction energy barrier increases and the reactivity decreases.The reaction energy of the reaction of the 1,4-dicyanobenzene trimerization product in the reaction trimerization step was 25.1 kcal/mol.The reaction energy barrier is almost the same as that of the 1,4-dicyanobenzene trimerization step.Therefore,it is feasible to catalyze the polymerization of aromatic nitrile compounds with trifluoromethanesulfonic acid to obtain CTFs.2.The mechanism of the trimerization of 1,4-dicyanobenzene monomer by hydrochloric acid under solvent model.The results show that the mechanism of the trimerization of 1,4-dicyanobenzene monomer with trifluoromethanesulfonic acid is similar to that of the solvent model.The reaction energy barrier of the trimerization process is 35.5 kcal/mol,which is 9.6 kcal/mol higher than the reaction energy of the trimerization of 1,4-dicyanobenzene monomer by trifluoromethanesulfonic acid.Mainly due to the difference in acid strength of the catalyst,the catalytic performance is different.3.Study on the mechanism of trimerization of 1,4-dicyanobenzene monomer by hydrochloric acid(gas phase).The calculation results show that the mechanism of the trimerization reaction of 1,4-dicyanobenzene monomer with hydrochloric acid is similar to that of the solvent model.The difference is that the rate of the trimerization process is the process of forming a trimer in the second step,the reaction energy barrier is 35.9 kcal/mol,and the trimeric reaction of 1,4-dicyanobenzene monomer is catalyzed by hydrochloric acid under a solvent model.The energy barrier is slightly higher.Therefore,the state of hydrochloric acid has no effect on the catalytic effect.Through the above research,we have explained the mechanism of the trimerization of nitrile compounds by trifluoromethanesulfonic acid and hydrochloric acid under solvent model.The different effects of the two acid catalysis in the experiment are explained.In addition,it provides a very valuable theoretical reference for finding a better catalyst to catalyze the trimerization of nitrile compounds to form a covalent triazine skeleton(CTF). |
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