Isomerization and aggregation of 2-(2-(2-hydroxy-4-nitrophenyl)hydrazono)-1-phenylbutane-1,3-dione: Recent evidences from theory and experiment

Abstract

The title compound potentially can exist as four isomers in solution. Recently Lycka has proposed a protocol for distinguishing two of them based on 15 N NMR. This approach has been confirmed theoretically, in the current study, and further developed into a logical scheme that allows the existence of each of the isomers to be proven in solution. The experimental data, obtained by NMR and UV-Vis spectroscopy, have shown that the studied compound exists as a mixture of two solvent stabilized isomers in diluted solutions of dimethyl sulfoxide. However, the concentration effect also plays a substantial role allowing formation of linear aggregates as the X-ray analysis and theoretical calculations show.

OH Group Effect in the Stator of β-Diketones Arylhydrazone Rotary Switches

“…Based on the NH signals, the ratio between the isomers is 80%/20% and 45%/55%, respectively for 2 and 3 (65%/35% for 1), which corresponds to ∆G values (RT) of 0.36, 0.82 and −0.11 kcal/mol for 1-3. In analogy to 1 [22,24] and following the theoretically predicted relative stabilization, it can be concluded that in DMSO there is an equilibrium between E' (major) and Z' (minor) forms in 2 and between E and Z of 3. The theoretically predicted values for the chemical shifts of the NH proton of the major and minor form at 2 and 3, respectively, are 14.61 ppm (2E') and 13.67 ppm (2Z') and 14.51 ppm (3E) and 13.44 ppm (3Z), i.e., the theoretical results are consistent with the experimental ones.…”

“…No such effect is observed in 3. moiety of 1. The formation of aggregates, as in 1 [22,24], limits the deprotonation, which explains the observed concentration effects. The results above indicate that the association is a possible reason for the observed changes.…”

“…The stability of the E'-E' cyclic dimer in 2 is probably due to the stronger proton acceptor properties of the CH 3 CO group compared to the PhCO moiety of 1. The formation of aggregates, as in 1 [22,24], limits the deprotonation, which explains the observed concentration effects.…”

“…Relative energies (in kcal/mole units) and predicted positions of the long-wavelength bands of the most stable isomers of 2(I) in DMSO. The corresponding relative energies for 1, taken from [24], are given in brackets. We have shown theoretically that, in the case of 1, only tautomer I could be present [22].…”

“…In the solid state, compound 2 exists as an E' conformer, stabilized via a cyclic dimer. The major difference with 1 is in the shape of the aggregate-again, the E' form is stabilized in 1, but in form of a linear aggregate [24]. The stability of the E'-E' cyclic dimer in 2 is Scheme 4.…”