Effect of Structure on Charge Distribution in the Isatin Anions in Aprotic Environment: Spectral Study

Pavol Tisovský, Róbert Šandrik, Miroslav Horváth, Jana Donovalová, Klaudia Jakusová, Marek Cigáň, Róbert Sokolík, Anton Gáplovský, Martin Gáplovský, Juraj Filo
2017 Molecules  
Five isatin anions were prepared by deprotonation of initial isatins in aprotic solvents using basic fluoride and acetate anions (F − and CH 3 COO − ). The F − basicity is sufficient to deprotonate isatin NH hydrogen from all the studied compounds. This process is reversible. In the presence of proton donor solvents, the anions form the corresponding isatins. The isatin hydrogen acidity depends on the overall structure of the isatin derivatives. The anions were characterized by
more » ... by ultraviolet-visible (UV-Vis), Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy. Interestingly, the anions form aggregates at concentrations above 10 −3 mol·dm −3 . Further, the effect of cations on the UV-Vis spectra of the studied anions was studied. Charge transfer and its distribution in the anion depends on the radius and the cation electron configuration. The alkali metal cations, tetrabutylammonium (TBA + ), Mg 2+ and Ag + , interact with the C-2 carbonyl oxygen of the isatin anion. The interaction has a coulombic character. On the other hand, Cd 2+ , Zn 2+ , Hg 2+ , Co 2+ , and Cu + cations form a coordinate bond with the isatin nitrogen. not show a simple dependence on the environment pH. There are changes in mechanism and the rate-determining step, and the hydrolysis mechanism depends on the environment polarity. The hydrolysis of isatin and its derivatives is a complex process in which, depending on the conditions, several reaction steps or intermediates compete with each other. One of the intermediates predicted by some authors [26] is the conjugated isatin anion (III). The physicochemical anion properties differ from the neutral molecule physicochemical properties. Anions, due to weaker bonded valence electrons, make stronger van der Waals interactions with surrounding molecules than more compact and less polarizable neutral molecules. These interactions can also significantly affect the anion reactivities. The imide and amide anion nucleophilic reactivity was tested by the reaction with various electrophilic substrates [33] . Bunnett and Beale studied the reaction kinetics of several imide and sulfonamide anions with methyl iodide and methyl methanesulfonate in methanol. They reported that the nucleophilic reactivity of these anions correlates with their basicity [34, 35] . The ease of anion formation, their stability or reactivity, and likely the biological activity, depend on the anion structure, environment, and so on. Even though the isatin anion or its structural modification may play an important role in the metabolism process of isatin and its derivatives, not enough attention has been paid to the study of their properties so far. Fourier transform infrared (FTIR) and Raman spectra of the isatin anion itself were published by Binev et al. [36] . The photophysics of the isatin anion were described in the work of Berci-Filho et al. [37]. As mentioned above, isatin has high application potential in various areas of industry. An isatin structural fragment is often a part of functional materials. The isatin structure is found in several compounds whose properties have been designed in the area of chemical anion sensors [38, 39] , or materials usable in electronics [40] . For this area of functional material applications, we have recently used the isatin fragment as the carrier structure in the development of anion sensors, or signal switches. The isatin fragment, depending on the overall structure of the molecule, can enter into tautomeric equilibria that affect the entire molecule functionality. When designing structures of such functional materials, it is necessary to know the spectral, physical, photophysical, and chemical properties of the molecules with the isatin structure as well as their azanions. Therefore, in this work, we have focused on the preparation and spectral properties study (ultraviolet-visible (UV-Vis), FTIR, nuclear magnetic resonance (NMR) of isatin azanions depending on their structure. Organic anions can be generated in various ways. For compounds that have acidic hydrogen in their molecule, the following reaction is often used to generate anions:
doi:10.3390/molecules22111961 pmid:29135954 fatcat:dvwgb2u4tnheld4bkqnmocwhn4