Syntheses of new imidazole ligand series and evaluation of 1-, 2- and 4,5-imidazole substituent electronic and steric effects on N-donor strengths

A series of new imidazole based heterocycles (5-(4,5-diphenyl-1H-imidazol-2-yl)furan-2-yl)methyl acetate (Him-dp), (5-(1H-phenanthro[9,10-d]imidazol-2-yl)furan-2-yl)methyl acetate (HIm-pt), (5-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)furan-2-yl)methyl acetate (HIm-phen), 2-(2-nitrophenyl)-4,5-diphenyl-1H-imidazole (HIm-n), 1-methyl-2-(2-nitrophenyl)-4,5-diphenyl-1H-imidazole (MeIm-n), N-(2-(1-ethyl-4,5-diphenyl-1H-imidazol-2-yl)phenyl)benzamide (EtIm-ba) and 2,4-di-tert-butyl-6-(8-(1-ethyl-4,5-diphenyl-1H-imidazol-2-yl)-1,4-dihydroquinolin-2-yl)phenol (EtIm-q) were synthesized and studied for the dependence of their azole donor characteristics on substituent factors by means of experimentally determined ionization constant data (derived as pKas), spectroscopic analyses and calculated properties of their DFT optimized molecular geometries performed at the B3LYP/6-311 + G* level. Results showed that the lowest donor strength recorded for HIm-pt (pKa = 2.67 ± 0.07) could be traced to the extensive electronic conjugation of the azole π-electrons with 4,5- and 2-substituents. On the other hand, the strongest imidazole donor strength in the series was obtained from EtIm-q (pKa = 4.61 ± 0.04) for which the substituents possessed negligible π-overlap with the azole ring. The experimental results and theoretical calculations lead to conclusions that effective conjugation between the imidazole ring and substituent aromatic groups is accountable for significant withdrawal of charge densities on the imidazole N-donor atom and vice versa. Furthermore, observed donor strengths in the series suggest that electronic inductive effects of the substituents provided lesser impact on donor strength modification of imidazole base and that alkylation of 1-imidazole position did not yield the anticipated push of electron density in favour of the N-donor atom. It is anticipated that the results should promote the understanding of azole-containing bio-macromolecular species and reactions as well as tuning and application of azole functions in molecular science.