Versatile behavior of 2-guanidinobenzimidazole nitrogen atoms toward protonation, coordination and methylation

The structure, dynamic behavior, protonation, methylation, and coordination sites of 2-guanidinobenzimidazole 1a were investigated. Structures of compounds [2-guanidinium-1,3,10-trihydrobenzimidazole]sulfate 1b, [2-guanidinium-1,3-dihydro-benzimidazole]sulfate 1c1d, [2-guanidinium-1,3-dihydro-benzimidazole]tetrafluoroborate 1e, [2-guanidinium-1,3-dihydro-benzimidazole]chloride 1f, [2-guanidinium-1,3-dihydro-benzimidazole] perchlorate 1g, 2-guanidino-1-methyl-benzimidazole 2a, [2-guanidinium-1,3-dimethyl-benzimidazole]iodide 2b, [2-guanidinium-1-methyl-3-hydro-benzimidazole]chloride 2c, [2-guanidinium-1,10-dihydro-benzimidazole]acetate 3, 2-guanidino-1-hydro-3-borane-benzimidazole 4a, 2-guanidino-1-methyl-3-borane-benzimidazole 4b, (2-guanidino-benzimidazole)dimethyltin 5, [bis(2-guanidino-10-hydro-benzimidazole)nickel(II)] 6, and [bis(2-guanidino-1,10-dihydro-benzimidazole)zinc(II)]nitrate 7 were determined based on 1H, 13C, and 15N NMR spectroscopy. The X-ray diffraction structures of 2a, 2b, 3, 6, and 7 were obtained. The results show that 1a has an open structure without an intramolecular hydrogen bond in DMSO or DMF. The imidazolic N-3 is the preferred basic site in solution for protonation, methylation, and coordination and not N-10 as was suggested from semiempirical calculations. Under strong acidic conditions, diprotonation occurs at N-3 and N-10. In the solid state, 3 and 6 were protonated preferently at N10 rather than at N-1.