Functionalized guanidinium chloride based colourimetric sensors for fluoride and acetate: single crystal X-ray structural evidence of -NH deprotonation and complexation

Abstract

A series of new symmetrically functionalized guanidinium chlorides (S1-S10) are synthesized in good yields and their sensing ability toward anions is studied in MeCN-DMF (24 : 1) (v/v). The absorption bands of these molecules in the presence of anions are tuned by varying the functional groups attached to the guanidinium moiety (which resembles urea) with respect to (i) aromaticity (S1-S4), (ii) electron induction effect (S1, S5-S9), (iii) positional isomeric effect (S7-S9), (iv) indole functionality (S10) of the conjugated aryl units. Anions that are above Cl(-) in the Hofmeister series (F(-), AcO(-), H(2)PO(4)(-)) are eligible as an analyte in this series of molecules whereas less basic anions than Cl(-) do not cause any interference. Thus, this series of molecules are suitable for the detection of anions in the narrow window of the Hofmeister series. Out of all the anions, only fluoride causes vivid colour changes from yellow to red to reddish orange and finally to blue, irrespective of the increasing aromaticity, induction and positional isomeric effect of the substituent that is attached to the guanidinium moiety. Interestingly, S9 has shown the ability to sense distinctly both F(-) and AcO(-) colourimetrically. Further S10, a sensor attached with indole functionality shows selective sensing of F(-) colourimetrically with a NIR signature at ∼930 nm though both these outputs are very unstable in nature. Stability constants for complex formation of S1-S10 (except S5) with F(-), AcO(-) are calculated by UV-vis titration experiments. Finally single crystal X-ray structural studies on the species 1 formed upon treating S6 with sodium fluoride confirms -NH deprotonation, whereas the reaction of S6 and S2 with sodium benzoate shows 1:1 host:guest binding that results in complexes 2 and 3 respectively.

Computational study of substituent effects on the gas‐phase stabilities of N‐phenylguanidinium ions

“…Guanidinium ions are interesting not only as cations of ILs but also as conjugate acids of basic organocatalysts utilized in organic syntheses, functionalized molecules, and so on . It will be greatly appreciated if the fundamental investigation presented in this work contributes to such applied fields in guanidinium chemistry.…”

Probing the Recognition of Halide and OxyAnions in the Polarprotic and Polaraprotic Solvent Mediums with Neutral UreaBased Receptors: A Computational Study

“…The recognition of anions with neutral receptors, however, depends on the choice of solvent medium. Anion recognition in aqueous medium has been achieved with the receptors containing multiple positive charges or metal ions . The experimentally observed results for the recognition of anions with artificially host receptors are rationalized with quantum chemical calculations .…”

Switchable Optical Probes for Simultaneous Targeting of Multiple Anions

“…Ghosh et al in 2011, developed a series of symmetrically functionalized guanidinium receptors (24) for selective detection of F À and AcO À ion in acetonitrile medium. [90] Irrespective to the nature of the substitution, larger bathochromic shifts were observed upon addition of F À due to its stronger basic characteristics in comparison to AcO À . Surprisingly, in spite of having higher basicity to that of AcO À , H 2 PO 4 À failed to produce any color change.…”