Synthesis And Crystal Structure Analysis Of Pyrazine Series Perfume And Aromatic Acid Co-crystal

In recent years,supramolecular chemistry and crystal engineering theory have developed rapidly.A series of new substances can be formed by molecular self-assembly,which can improve the properties of original materials and be widely used in medicine,agriculture,spices,explosives and other fields.Therefore,the research in this field has attracted more and more researchers’attention.Pyrazines are heterocyclic compounds containing two nitrogen atoms at the 1,4positions of benzene ring.Generally,they have roasted sweet taste,nut flavor,coffee aroma and so on.Most of the methylpyrazine is liquid at room temperature,which is limited in application.In this paper,the liquid pyrazine is solidified,the operation is simple,and the application range is expanded.In this paper,2,3-dimethylpyrazine,2,5-dimethylpyrazine,2,6-dimethylpyrazine,2,3,5-trimethylpyrazine and 2,3,5,6-tetramethylpyrazine were used as raw materials,3,5-dichlorosalicylic acid,3,5,6-trichlorosalicylic acid,salicylic acid,m-hydroxybenzoic acid,p-hydroxybenzoic acid,3,5-Dinitrosalicylic acid,4-chlorosalicylic acid and p-toluenesulfonic acid as ligands,co-crystals were prepared by solvent evaporation method.Single crystal X-ray diffraction（SXRD）,Powder X-ray diffraction（PXRD）,Infrared spectroscopy（IR）,Ultraviolet and visible spectroscopy（UV）,Thermogravimetric analysis（TGA）were used to characterize the co-crystal.Hirshfeld surface of co-crystals were analyzed by Crystal Explorer soft.Thirteen co-crystals were obtained in this paper,the results of crystal structure analysis are as follows:Co-crystal 1 and co-crystal 2 were synthesized by the reaction of 2,3-dimethylpyrazine with 3,5-dichlorosalicylic acid and 3,5,6-trichlorosalicylic acid respectively,belong to the triclinic system and P（?）space group;Co-crystal 3 was synthesized by the reaction of 2,3-dimethylpyrazine with salicylic acid,belongs to the monoclinic system and C2/c space group;Co-crystal 4 was synthesized by the reaction of 2,3-dimethylpyrazine with m-hydroxybenzoic acid,belongs to the orthorhombic crystal system and Pna2（1）space group;Co-crystal 5 was synthesized by the reaction of 2,3-dimethylpyrazine with p-hydroxybenzoic acid,belongs to the triclinic system and P（?）space group;Co-crystal 6 and co-crystal 7 were synthesized by the reaction of 2,5-dimethylpyrazine with 3,5-dichlorosalicylic acid and3,5,6-trichlorosalicylic acid respectively,belong to the triclinic system and P（?）space group;Co-crystal 8 and co-crystal 9 were synthesized by the reaction of2,6-dimethylpyrazine with 3,5-Dinitrosalicylic acid and p-toluenesulfonic acid respectively,belong to the same monoclinic system but different space group,the former is P 2₁/c space group,and the latter is Cc space group;Co-crystal 10 was synthesized by the reaction of 2,3,5-trimethylpyrazine with 3,5-dichlorosalicylic acid,belongs to the monoclinic system and P 2₁/n space group;Co-crystal 11 was synthesized by the reaction of 2,3,5,6-tetramethylpyrazine with 3,5-dichlorosalicylic acid,belongs to the triclinic system and P 1space group;Co-crystal 12 was synthesized by the reaction of 2,3,5,6-tetramethylpyrazine with 3,5-Dinitrosalicylic acid,belongs to the orthorhombic crystal system and P 2₁2₁2₁ space group;Co-crystal13 was synthesized by the reaction of 2,3,5,6-tetramethylpyrazine with4-chlorosalicylic acid,belongs to the monoclinic system and C2/c space group.The results of Hirshfeld surface analysis show that the O-H???N hydrogen bond is the strongest intermolecular interaction force in co-crystal 1,2,3,4,5,6,7,8,12 and13,the N-H???O hydrogen bond is the strongest intermolecular interaction force in co-crystal 9,the C-H???O hydrogen bond is the strongest intermolecular interaction force in co-crystal 10,the O-H???O hydrogen bond is the strongest intermolecular interaction force in co-crystal 11;H???H interaction force contributes the most to the co-crystal 1,2,3,4,5,9,10,11 and 13,H???Cl interaction force contributes the most to the co-crystal 6 and 7,O???H interaction force contributes the most to the co-crystal8.By analyzing the crystal structure,optical properties and Hirshfeld surface of the thirteen methylpyrazine co-crystals,it is found that the bonding mode of crystals is basically the same with the number of methyl substitutes changes,but the intermolecular forces will change,and the different stacking forces between the molecules will present different spectra.In most of the co-crystals,O-H???N is the strongest intermolecular force and H???H interaction contributes the most to the co-crystals.The analysis above may provide microscopic insights in the regulation of pyrazine co-crystals.