Electronic/substituents influence on imidazole ring donor–acceptor capacities using 1H-imidazo[4,5-f][1,10]phenanthroline frameworks

Eight imidazole-based compounds 4-methyl-2,6-bis(4,5-diphenyl-1H-imidazol-2-yl)phenol ( A-dp), 2-(1H-phenanthro[9,10-d]imidazol-2-yl)phenol ( B-2H), 5-methoxy-2-(1H-phenanthro[9,10-d]imidazol-2-yl)phenol ( B-2H4M), 3-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenol ( C-3H), 2-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)-4-methoxyphenol ( C-2H5M), 4-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)-2-methoxyphenol ( C-4H3M), 2-(2-methoxyphenyl)-1H-imidazo[4,5-f][1,10]phenanthroline ( C-2M) and 2-(2,4-dimethoxyphenyl)-1H-imidazo[4,5-f][1,10]phenanthroline ( C-2,4M) were synthesized and characterized by elemental, spectroscopic and X-ray single crystal analyses. Two different crystals of A-dp were grown from ethanol and THF, which revealed that A-dp crystallizes in a monoclinic (P2(1)/c) space group while A-dp·2THF crystallizes in a triclinic (P) space group. A-dp·2THF was devoid of any kind of networks whereas the absence of solvent adducts in the ethanol sample produces 1-dimensional single-stranded helices. Contrary to the reported literature conclusion that 1H-imidazole alkylation should increase the donor strength of the imidazole N-base, protonation–deprotonation equilibrium studies on the compounds suggest that push/pull of electron density on the 2-carbon of the imidazole ring by electron-rich/electron-withdrawing substituents is necessary to influence donor capacity of the N-base electrons. Furthermore, the notable increase in pKa,N: values due to ortho/para-directing methoxy substituents supports the conclusion that electron density push towards the 2-position of the imidazole ring is important for improving N-base donor strengths. DFT calculation results using the B3LYP/6-311+G level of theory were conducted to explore possible theoretical explanations.