Switching between cis and trans anions of 2-(2′-hydroxyphenyl)benzimidazole: a molecular rotation perturbed by chemical stabilization

Abstract

Despite the fact that 2-(2'-hydroxyphenyl)benzimidazole (HPBI) exists as the cis and trans enol in neutral form, it was reported to exist only in the trans form upon deprotonation in an aqueous medium. It was also shown that the dianion formed in the ground state can be re-protonated to form the trans-anion in the excited state. In the present study, the cis-anion and dianion could be obtained upon proper stabilization in suitable environments. Switching between the cis and trans-anion was demonstrated to be possible by changing the environment. Theoretical calculations were performed to substantiate the existence of the cis-anion and dianion. In contrast to a literature report, it was shown that the 'OH' group deprotonates before the 'NH' group to form a monoanion not only in protic solvents but also in aprotic solvents.

An unusual deprotonation trend in 2-(2′-hydroxyphenyl)pyridoimidazoles

“…A similar emission from the cis-anion was observed upon excitation of the cis-H-bond complex of HPBI in the presence of fluoride ions. 20 The theoretically predicted emission energy (Table 1) is also in agreement with the experimental emission energy. The calculated absorption maximum for the cis-anion is 410 nm.…”

“…A similar shift is observed in the absorption spectra of HPBI upon addition of fluoride due to the formation of such cis-Hbond complexes. 20 On the other hand, the absence of a similar shift in the main absorption band of HPIP-c clearly indicates a weaker interaction between HPIP-c and the added anions in the ground state. This is labeled as a weak H-bond complex.…”

“…31 Such shifts were also observed when HPBI forms the cis-H-bond complex. 20 Negative charge accumulates on the oxygen atom either due to the hydrogen bonding interaction of the negatively charged ions or by the formation of anionic forms. This charge generates a high shielding effect.…”

“…Due to electrostatic repulsion between the phenolate anion and the azole nitrogen, the cis-anion of HPBI is twisted. 20 Surprisingly, like the trans-anions, the cis-anions of both the nitrogenous analogues possess a planar geometry with a negligible dihedral angle between the phenolate anion and pyridyoimidazole moiety (Tables S1-S5, ESI †). Nonetheless, like the trans-anion of HPBI, the trans-anions of the nitrogenous analogues are also found to be more stable than their respective cis-anions (Table 1).…”

“…However, recently we showed the existence of cis-anions (Chart 1) of HPBI in a polar aprotic solvent and also demonstrated that their survival depends upon their relative chemical stability. 20 In the present work, the effect of nitrogen substitution (i) on the existence of cis-anions and (ii) the anionic sensing ability are explored. Contrary to the expectation that pyridyl nitrogen substitution facilitates the formation of anions, a different trend is observed.…”