Sanjeev P. Mahanta

Colorimetric sensing of fluoride ion by new expanded calix[4]pyrrole through anion-π interaction.

Three new expanded calix[4]pyrroles were synthesized, where the two dialkylldipyrromethane units are linked via C-C double bonds. One of them, calix[2]bispyrrolylethene, colorimetrically senses fluoride ion only, owing to anion-π interaction in polar aprotic solvents.

Oxidative coupling of 2-naphthol by zeolite-Y supported homo and heterometallic trinuclear acetate clusters

Two trinuclear acetate clusters of iron and cobalt of general formula [Fe3O(O2CCH3)6(H2O)3]NO3·2H2O and Fe2Co(O)[(OOCC6H4NO2)6]NO3·2H2O are synthesized and characterized. The synthesized trinuclear clusters are supported on zeolite-Y via an ion exchanged method. FTIR study reveals that the two complexes are tethered via formation of Si–O–H⋯O–H hydrogen bond linkages with a zeolite-Y matrix. Homogeneous and heterogeneous trinuclear catalysts are found to be efficient catalysts for oxidative coupling of 2-naphthol. Compared to homometallic oxo-clusters, bimetallic complexes are found to show better catalytic activity. Besides obtaining BINOL as a major product, these metal clusters also lead to formation of the tautomeric form of BINOL. The crystal structure of the by-product indicates the formation of a tetrahedral chiral centre in the molecule via the attachment of the solvent. Density Functional Theory (DFT) calculations have been performed to elucidate the structural and electronic properties of both homogeneous and heterogeneous complexes.

Naphthalene strapped fluorescent calix[4]pyrrole isomers: halide ion selectivity based on strap topography

Two novel isomeric strapped calix[4]pyrroles were synthesized using diether straps derived from naphthalene as the fluorescent reporting unit for the first time. The receptor derived from 2,3-dihydroxynaphthalene displays highest sensitivity towards Cl− ion, whereas that derived from 2,7-analogue shows peak sensitivity for F− ion.

Cation–π interaction in cofacial molecular dyads: a DFT and TDDFT study

In this study we proposed a cofacial molecular dyad or a tweezer (4,5-biphenyl acridine, BPA) with the acridine moiety serving as the spacer and two phenyl rings as the wings. Density functional theory (DFT) calculation was implemented to illustrate the pivotal role of the phenyl rings in stabilizing the cation (Li+, Na+ or K+) at the tweezers via cation–π interaction. Our calculations reveal that apart from the covalent interaction, the cation–π interaction energy contributes to the stabilization of cations inside the molecular pocket. The effect of substituents on the structure and interaction energy of the cation–π complexes was also exemplified. An electron donating group favored the cation–π interaction while an electron withdrawing group lowered the strength of cation–π interaction. Solvent polarity was found to play a critical role in stabilizing the cation–π interaction. Solvents with a low dielectric constant mostly favored such interaction while highly polar solvents resulted in repulsive interaction energy. TDDFT (time dependent density functional theory) calculations were performed to understand the effect of the cation–π interaction on the absorption spectra of such molecular dyads.

A Cu(II) mediated approach for colorimetric detection of aqueous fluoride in ppm level with a Schiff base receptor

Inspired by the importance of fluoride detection in aqueous environment, a new methodology is demonstrated by employing the synergistic reaction of fluoride and Cu(ii) salts with a designed Schiff base receptor. The cooperative ion binding behaviour of the ligand SR involves the reduction of Cu(ii) centre to Cu(i) in the presence of fluoride. In chloroform solution, the sensor shows equivalent reversible colorimetric change from amber yellow to red irrespective of the order of the addition of copper and fluoride. However, the change is equivalent but reversibility cannot not be attained in DMSO. The ion binding event can be easily recycled by adding EDTA and the resulting reversible colorimetric change mimics AND and a combinatorial logic gate. The mechanism of interaction is thoroughly analyzed by the UV-Vis, H-1 NMR, F-19 NMR, ESR spectroscopy and cyclic voltammetry techniques, which is further supported by DFT calculations. The methodology is applied in the detection of fluoride in real life samples such as ground water and toothpaste extract. The method successfully detects fluoride at concentrations as low as 5 ppm.

Condensation of cyclohexanediones with pyrrole under acidic conditions: unusual products and interesting structural features

Reaction of the three isomers of cyclohexanediones (1,2-, 1,3- and 1,4-diones) with pyrrole under acidic conditions are explored. The rigid cyclohexane scaffold led to the formation of the bisdipyrromethanes in case of both 1,4- and 1,3-cyclohexanedione, not observed in case of their acyclic analogues (2,5-hexanedione and 2,4-pentanedione respectively). While the former exclusively forms the desired bisdipyrromethane, the latter produced in addition the mono-condensed product along with a 2-alkenylpyrrole derivative and a highly unexpected 5,5-dipropyldipyrromethane, albeit in low yield. Surprisingly, 1,2-cyclohexanedione produced a tripyrrole derivative and a pyrrole substituted bicyclic dihydroindole (a ring annulated product) along with the mono-condensed product. The structure of these products was comprehensively established by various spectroscopic means including single crystal X-ray diffraction analysis.