• Pallepogu Raghavaiah

      Articles written in Journal of Chemical Sciences

    • Synthesis, spectroscopy, thermal studies and supramolecular structures of two new alkali-earth 4-nitrobenzoate complexes containing coordinated imidazole

      Bikshandarkoil R Srinivasan Jyoti V Sawant Pallepogu Raghavaiah

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      The reaction of hydrated magnesium or calcium 4-nitrobenzoate (4-nba) generated in situ, with imidazole (Im) results in the formation of the complexes [Mg(H2O)2(Im)2(4-nba)2] 1 and [Ca(H2O)3(Im)(4-nba)2]$\cdot$Im 2, which exhibit the same metal:4-nba:Im ratio but different degrees of hydration. Complex 1 crystallizes in the triclinic $P_{\bar{i}}$ space group and the Mg atom is located on an inversion centre, while 2 crystallizes in the monoclinic $P2_1/c$ space group and all atoms are located in general positions. In 1 the Im ligands, which are trans to each other, are coordinated to Mg, while 2 contains coordinated as well as free Im. The monodentate 4-nba ligands are disposed trans to each other in 1, while they adopt a cis orientation in 2 resulting in different supramolecular structures. Complex 1 exhibits two types of H-bonding interactions namely O-H𝛬O and N-H𝛬O, while in 2 three varieties of H-bond, viz. O-H𝛬O, N-H𝛬O and O-H𝛬N are observed. The Im ligand functions as a bifurcated H-bond donor in 1 while the O atom of the nitro group functions as a H-bond acceptor. In contrast, the nitro group in 2 is not involved in any H-bonding interactions. The free Im in 2 functions as a bifurcated acceptor and forms an extended chain linking adjacent complex molecules. The chains thus formed are further cross-linked with the aid of H-donor bonds from both the free as well as the coordinated Im. Both 1 and 2 exhibit 𝜋-𝜋 stacking interactions. Complex 1 is thermally more stable as compared to 2, and both complexes can be dehydrated to the corresponding anhydrous complexes by heating at 140 and 100°C respectively. At elevated temperatures, both the complexes can be pyrolysed to the corresponding oxide. The anhydrous complexes can be rehydrated to obtain the starting hydrated materials.

    • catena-Poly[[(pentaaqua)(4-nitrobenzoato-O,O')barium(II)] (𝜇-4-nitrobenzoato-O,O')]: A barium(II) coordination polymer showing O-H$\cdots$O and C-H$\cdots$O interactions

      Bikshandarkoil R Srinivasan Jyoti V Sawant Sarvesh C Sawant Pallepogu Raghavaiah

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      The reaction of barium carbonate with 4-nitrobenzoic acid (4-nbaH) results in the formation of a Ba(II) coordination polymer, catena-poly[[(pentaaqua)(4-nitrobenzoato-O,O')barium(II)](𝜇-4-nitrobenzoato-O,O')] 1. The polymeric compound [[Ba(H2O)5(4-nba-O,O')](𝜇-4-nba-O,O')]$_n$ 1 was characterized by elemental analysis, IR and UV-Vis spectra, weight loss studies, X-ray powder diffraction and its structure determined. In 1 five water molecules are coordinated to the central metal and one of the 4-nba ligands is bonded to Ba(II) in a bidentate manner (4-nba-O,O') through the carboxylate O atoms. The [(pentaaqua)(4-nitrobenzoato-O,O')barium(II)] units are linked into an infinite one-dimensional chain along 𝑏-axis with the aid of the second 4-nba anion, which functions as a bridging bidentate (𝜇-4-nba-O,O') ligand. This results in nine coordination around each Ba(II) ion in the coordination polymer. A long Ba$\cdots$Ba distance of 6.750(1) Å is observed between adjacent Ba(II) ions in the chain and the oxygen atoms of the carboxylate group and the nitro functionalities of the 4-nba ligand are involved in several O-H$\cdots$O and C-H$\cdots$O interactions.

    • A one-dimensional barium(II) coordination polymer with a coordinated nitro group of 2-nitrobenzoate

      Bikshandarkoil R Srinivasan Santosh Y Shetgaonkar Pallepogu Raghavaiah

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      The aqueous reaction of barium carbonate with 2-nitrobenzoic acid (2-nbaH) results in the formation of a one-dimensional coordination polymer, catena-poly[[hexa(aqua)dibarium(II)]bis[($\mu_2$-2-nitrobenzoate-O,O,O-NO2)($\mu_2$-2-nitrobenzoate-O,O,O')]] 1. On heating at 100°C compound 1 is dehydrated to anhydrous barium bis(2-nitrobenzoate) 2. The anhydrous compound can be re-hydrated to 1 on exposure to water vapour. Compounds 1 and 2 were characterized by elemental analysis, IR and UV-Vis spectra, DSC thermograms, weight loss studies and the structure of 1 was determined. 1 and 2 can be thermally decomposed to BaCO3 by heating at 800°C. The polymer [[Ba(H2O)3]2($\mu_2$-2-nba-O,O,O-NO2)2 ($\mu_2$-2-nba-O,O,O')2]$_n$ 1 crystallizes in the centrosymmetric triclinic space group P$_{\bar{i}}$ and all atoms are located in general positions. The polymeric structure is based on a dimeric unit and consists of three water molecules coordinated to a central Ba(II) and two unique 2-nitrobenzoate (2-nba) anions, one of which ($\mu_2$-2-nba-O,O,O-NO2) functions as a tridentate ligand and is linked to a Ba(II) through the oxygen atom of the -NO2 group and forms a monoatomic $\mu_2$-carboxylate bridge between two symmetry related Ba(II) ions with a Ba$\cdots$Ba distance of 4.5726(14) Å. The second unique 2-nba anion ($\mu_2$-2-nba-O,O,O') also functions as a tridentate ligand with the carboxylate oxygen atoms linked to a Ba(II) ion in a bidentate fashion and one of the carboxylate oxygen atoms forming a monoatomic bridge between two symmetry related Ba(II) ions resulting in a Ba$\cdots$Ba separation of 4.5406(15) Å. These differing tridentate 2-nba ligands link {Ba(H2O)3} units into a one-dimensional polymeric chain extending along 𝑏 axis. In the infinite chain each nine coordinated Ba(II) is bonded to three water molecules and further linked to six oxygen atoms of four different 2-nitrobenzoate anions with alternating pairs of Ba(II) ions in the chain bridged by a pair of ($\mu_2$-2-nba-O,O,O-NO2) and ($\mu_2$-2-nba-O,O,O') ligands resulting in alternating Ba$\cdots$Ba distances of 4.5406(15) and 4.5726(14) Å across the chain.

    • Synthesis, characterization and photochemistry of a new heptamolybdate supported magnesium-aqua coordination complex

      Savita S Khandolkar Pallepogu Raghavaiah Bikshandarkoil R Srinivasan

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      The synthesis, crystal structure, redox characteristics and photochemistry of a new heptamolyb-date supported magnesium-aqua coordination complex viz. (hmtH)2 [{Mg(H2O)5}2 {Mo7O24}]·3H2O 1 (hmt = hexamethylenetetramine) is reported. The cyclic voltammogram reveals quasireversible redox behaviour while the degradation of rhodamine B (RhB) by 1 under sunlight irradiation indicates its photocatalytic activity. In the crystal structure of 1, two [Mg(H2O)5]2+ units are linked by a 𝜇2-bridging bidentate hepta-molybdate ligand resulting in a heptamolybdate supported magnesium-aqua coordinated dianionic complex [{Mg(H2O)5}2 {Mo7O24}]2− charge balanced by two (hmtH)+ cations. The cations, anions and the lattice water molecules are linked by several hydrogen bonding interactions.

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