The pyridine-3,5-bis(benzimidazole-2-yl) ligand (L) with metal salts (Cu(II) or Cd(II)) has been found to gelate
several
aliphatic alcohols to generate multifunctional metal–organic
gels (MOGs). The anions of the metal salt were found to play an important
role in the gelation property; halides (chlorides or bromides) and
sulphates were found to be more effective than other univalent ions
such as nitrate, aceteate, perchlorate, and tetrafluoro borate. The
microscopic investigation with FESEM, TEM, POM, and AFM confirmed
the formation of interwinding 3D gel fibrous networks, which have
immobilized the large volume of solvent. The coordination of the metal
to the ligand L was found to play a vital role in the
construction of gel fibers. Rheology studies on the MOGs revealed
that these MOGs have significantly high mechanical strengths and therefore
exhibit self-sustainability. The porous nature of the MOGs has been
explored by gas sorption studies; xerogels show the type-III N2-sorption isotherm. The MOGs have also shown a tendency as
potent dye removal agents. These gels also exhibited thermo irreversibility
but mechano reversibility. The sol–gel transformations were
observed through applying external chemical stimuli, that is, by adding
metal capturing agents.
Head-to-tail and head-to-head [2+2] photodimerization of an unsymmetrical olefin containing benzimidazole and pyridyl groups was achieved by irradiating Ag(I) complexed olefin in crystalline state and gel state, respectively, in stereoselective manner. The [2+2] reaction indicates that the molecular arrangement in a gel is different from that of a xerogel.
Two new amino acid based low-molecular-mass organic gelators were designed, synthesized, and examined for their ability to gelate various organic solvents. The gelator molecules were found to aggregate via N-H/O hydrogen bonding to form an interwinding 3D network which immobilized a large number of organic solvents. Among the different organic solvents used in the gelation study mesitylene is found to be the best solvent. The microstructure of organogels was studied with FESEM and optical micrographs. The involvement of hydrogen bonding in the aggregation of the gelator molecule was studied using temperature dependent 1 H NMR. The obtained organogels were found to exhibit significant mechanical strength. Scheme 1 Synthetic route to compounds G-1 and G-2. (i) N-Hydroxy succinimide; DCC, dry THF, 0 C, 5 h; (ii) (a) tris(2-aminoethyl)amine, DME, 0 C, 18 h; (b) reflux, 60 C, 6 h.
Single crystal X-ray structures of new perchlorate salts of a series of four xanthine alkaloids, for example, xanthine (xt), caffeine (cf), theophylline (tp), and theobromine (tb) are reported. The treatment of xanthine derivatives with 70% of HClO 4 afforded their salts Hxt 3 ClO 4 3 2(H 2 O), 1; Htp 3 ClO 4 , 2; Htb 3 ClO 4 , 3; and Hcf 3 ClO 4 3 H 2 O, 4. Xanthine form dimers via N(9)-H 3 3 3 O-C( 6) hydrogen bond in the crystal structure. Of the two possible resonance structures of the protonated imidazoles, one of the structures was found to be major in xanthine, theobromine, and caffeine salts, while both structures are of equal contribution in theophylline salts. These observations also have been verified by using the structural information available in the Cambridge Structural Database. Interestingly, xanthine 1 is present as a dihydrate and 4 as a monohydrate, whereas 2 and 3 are crystallized without any water. Thus, proton transfer from the strong acid HClO 4 to the weakly basic imidazole ring of xanthines occurs which leads to the build up of H-bonded supramolecular networks.
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