Synthesis And Structural Characterization Of Guanidinium Salt Ionic Liquids

N,N,N',N'-tetramethyl-N"-butylguanidine (BuTMG) was synthesized from tetramethylurea (TMU) and n-BuNH₂ in the presence of POCl₃. According to the same method, N,N,N',N'-tetramethyl-N"-cyclohexylguanidine (CyTMG) , N,N,N',N'-tetramethyl-N"-octylgua- nidine (OctTMG), and N,N,N',N'-tetramethyl-N"-phenylguanidine (PhTMG) were prepared from TMU and corresponding amine,i.e. cyclohexylamine, octylamine and aniline. Among these pentaalkylguanidine compounds, CyTMG, OctTMG and PhTMG were first synthesized in this paper and their structures were determined by means of IR , ¹H-NMR and ESI-MS analysis.N,N,N',N'-tetramethyl-N"-methyl-N"-butylguanidinium iodide ([MBTMG]I) was prepared by N-methylation of BuTMG with CH₃I and the corresponding guanidinium salt ionic liquids with different anions, including BF₄^-, PF₆^-, CH₃COO^-, CF₃COO^-, CF₃SO₃^-, and CH₃CH(OH)COO-, were synthesized by metathesis reaction for the first time in this paper. The structures of the guanidinium salts ionic liquids, including N,N,N',N'-tetramethyl-N"-methyl-N"-butylguanidinium tetrafluoroborate ([MBTMG]BF₄) , N,N,N',N'-tetramethyl-N"-methyl-N"- butylguanidinium hexafluorophosphate ([MBTMG]PF₆) , N,N,N',N'-tetramethyl-N"-methyl-N"- butylguanidinium trifluoroacetate ([MBTMG]CF₃COO) , N,N,N',N'-tetramethyl-N"-methyl-N"- butylguanidinium trifluoromethanesulfonic ([MBTMG]CF₃SO₃) , were determined by means of IR , ¹H-NMR , ESI-MS and elemental analysis, and the structures of the two kinds of guanidinium salt ionic liquids containing anions CH₃COO^- and CH₃CH(OH)COO^- were confirmed by IR , ¹H-NMR and ESI-MS analysis.According to the above-mentioned synthesis route, N,N,N',N'-tetramethyl-N"-methyl-N"- cyclohexylguanidinium iodide ([MCyTMG]I) was prepared by N-methylation reaction of CyTMG and its corresponding ionic liquids, N,N,N',N'-tetramethyl-N"-methyl-N"-cyclohexylg- uanidinium tetrafluoroborate ([MCyTMG]BF₄) and N,N,N',N'-tetramethyl-N"-methyl-N"-cyclo- hexylguanidinium hexafluorophosphate ([MCyTMG]PF₆) , were synthesized by the metathesis reaction with sodium tetrafluoroborate and potassium hexafluorophosphate, respectively. The structures of both [MCyTMG]I and the two ionic liquids were confirmed by IR , ¹H-NMR and ESI-MS analysis. N,N,N',N'-tetramethyl-N"-methyl-N"-octylguanidinium salt ionic liquids, namely N,N,N',N'-tetramethyl-N"-methyl-N"-octylguanidinium iodide ([MOTMG]I) and N,N,N',N'-tetrameth- yl-N"-methyl-N"-octylguanidinium hexafluorophosphate ([MOTMG]PF₆) , were synthesized by similar method to [MCyTMG]I and [MCyTMG]PF₆, and their structures were confirmed by the characterization methods of IR , ¹H-NMR , ESI-MS and elemental analysis.The melting points of all the solid guanidinium iodides and guanidinium salt ionic liquids at room tempreture, including [MBTMG]I, [MOTMG]I, [MCyTMG]I, [MBTMG]PF₆, [MOTMG]PF₆, [MCyTMG]PF₆ and so on, were measured, and the correlations between the melting points and the structures of the guanidinium salt ionic liquids were studied. The results were outlined as following:(1) The kind of the anion can effectively influence the melting points of the guanidinium salt ionic liquids containing the same cation.(2) Along with the increase of the the carbon numbers (C₄^C₈) of the alkyl substituted groups on the nitrogen atom of the guanidinium salt ionic liquids with the uniform anions (except PF₆^-), the melting points of the guanidinium iodide rise, while the melting points of the hexaalkylguanidinium salt ionic liquids with anion PF₆^- drop evidently.(3) When varying the substituted group on the nitrogen atom from saturated alkyl chain to cyclic saturated alkyl with the same carbon numbers, both the melting points of the guanidinium iodide and the hexaalkylguanidinium salt ionic liquids with anion PF₆^- decrease significantly.N,N,N',N'-tetramethylguanidinium-based compounds with different substituted groups , including pentaalkylguanidine and hexaalkylguanidinium salt ionic liquids, were synthesized and characterized in this paper. This will lay the foundation of the studies of the physical-chemical properties and the applications.