Sterically Hindered N-Heterocyclic Salts Utilized as Antimicrobial Agents

By the adoption of annulated ring systems for their steric bulkiness, a new series of symmetric N-heterocyclic imidazolium and perimidium salts were synthesized. Also, corresponding asymmetric ferrocenyl N-functionalized series were prepared as salts. All the reported salts were fully characterized. The reported X-ray structure of 4,5-diphenyl-1,3-dimethyl-1H-imidazol-3-ium iodide (2a) shows that it crystallized in the orthorhombic space group P212121. Low to moderate antimicrobial activities were observed with both sets of salts against important clinical isolates of Staphylococcus aureus, Bacillus subtilis, and Enterococcus faecalis and Escherichia coli, Pseudomonas aeruginosa, and Salmonella enterica. The results were benchmarked against meropenem. Both 1,3-propyl-1H-phenanthro[9,10-d]imidazol-3-ium iodide (3b) and 1-ferrocenyl-3-propyl-1H-perimidin-3-ium iodide (9b) showed high antimicrobial activities against all tested Gram-positive bacterial strains, with minimum inhibitory concentration values ranging from 8 to 4 μg/mL (meropenem = 0.5–0.125 μg/mL), while all the salts showed little or no activity against Gram-negative bacterial strains. In general, the asymmetric ferrocenyl-containing salts exhibited higher activities than the symmetric ones.