A Spectroscopic Study of Tautomeric Equilibrium of Salicylideneaniline in ZSM-5 Zeolites

Abstract

Salicylideneaniline (SA) sorbed in cation-exchanged M-ZSM-5 (M = H+, Li+, Na+, K+, Rb+, Cs+ and Zn2+) zeolites was studied by spectroscopic techniques assisted by quantum-chemical calculations. The nature of extra-framework cations present in the zeolite void was found to affect the spectral signature of the sorbed SA molecule that points to the shift of tautomeric equilibrium between the enol and keto forms. Small size cations, such as H+ and Li+, stabilize a cis-keto SA tautomer along with a enol one in the zeolite structure. The calculations indicate that the sorbed cis-keto tautomer may have the dipole large enough to be considered as a zwitterion. New features appearing in the spectra with the increase of the cation size were attributed to the presence of trans-keto SA tautomer, which up to now has been observed only in time-resolved spectroscopic experiments. A strong interaction of the molecule with cations in Zn-ZSM-5 zeolite results in the chelation of enol SA with the divalent Zn2+ ions. The results of the study suggest that the tautomeric equilibrium of molecules belonging to the Schiff base family can be tuned by the confinement in the nanoporous materials via a choice of topology of zeolite framework and the nature of extra-framework cations.

Incorporation of Photo- and Thermoresponsive N-Salicylidene Aniline Derivatives into Cobalt and Zinc Layered Hydroxides

“…This behavior of partial tautomerization is commonly observed in N-salicylidene anilines where some conformers can get stabilized in the lattice either by solvent molecules or the matrix effect. 97,98 Upon raising back the temperature, although the peaks related to the low-temperature "enol phase" are far less intense, they can still be distinguished for the higher harmonics, indicating a incomplete reverse tautomerization. The temperature effects in Co-N-Sali-SO 3 can be summed up as follows: Zn-N-Sali-COO presents a noticeable evolution (−0.2 Å), however, very small and hardly significant given the broadness of the 00l peaks.…”

“…This behavior of partial tautomerization is commonly observed in N-salicylidene anilines where some conformers can get stabilized in the lattice either by solvent molecules or the matrix effect. 97,98 Upon raising back the temperature, although the peaks related to the low-temperature "enol phase" are far less intense, they can still be distinguished for the higher harmonics, indicating a incomplete reverse tautomerization. The temperature effects in Co-N-Sali-SO 3 can be summed up as follows: (i) enol and keto forms coexist in equilibrium at 25 °C with a d-spacing of 2.15 nm; (ii) lowering the temperature to −180 °C perturbs the enol−keto equilibrium by favoring the enol form, which leads to the emergence of a new phase in the diffractogram with a d-spacing of 2.40 nm at −180 °C; meanwhile, the d-spacing of the starting phase also increases, in a lower extent, to 2.19 nm at −180 °C; and (iii) raising the temperature to 60 °C, keto forms get more populated, which leads to the disappearance of the phase which had emerged at low temperatures and to the overall contraction of the d-spacing to 2.10 nm at 60 °C.…”