• Volume 119, Issue 1

January 2007,   pages  3-51

• Synthesis, structure, redox and spectra of green iridium complexes of tridentate azo-aromatic ligands

Reactions of IrCl$_3 \cdot x$H2O with the ligands, 2-[(phenylamino)phenylazo]pyridine (HL$^{1a}$) and 2-[(𝑝-tolylamino)phenylazo]pyridine (HL$^{1b}$) produce [Ir(L1)2]Cl (L1 = L$^{1a}$, [1]Cl and L1 = L$^{1b}$, [2]Cl) along with many unidentified products. The iridium complexes have been characterized by various techniques such as X-ray crystallography, mass spectrometry, 1H and 13C NMR, cyclic voltammetry and absorption studies. The complex [1]ClO4 crystallises in triclinic space group. The crystallographic data have been determined. Notably, the Ir-N (azo) lengths are short (av. 1.9875(4) Å) as compared to the remaining four Ir-N lengths (av. 2.052(5) Å). There is significant degree of ligand backbone conjugation in the coordinated ligands, which result in shortening of the C-N lengths on the other side of the middle phenyl ring and also in lengthening of the diazo (N=N) lengths. The complexes display multiple low energy transitions ranging between 1010 and 450 nm. These are electro active and show three reversible redox responses in the potential range, +1.5 V to -1.5 V. The cathodic potential responses are ascribed as ligand reductions, while the redox process at the anodic potential occurs at a mixed metal-ligand (HOMO) orbital.

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

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.

• A spiral-like chain from a hydrogen-bonded cyclic dichloride containing a water dimer in a quaternary diammonium dichloride trihydrate

The reaction of benzyl chloride with tetramethylethylenediamine (tmen) results in the formation of the quaternary diammonium dichloride trihydrate (dbtmen)Cl2.3H2O 1 (dbtmen is N,N'-dibenzyl-N,N,N',N'-tetramethylethylenediammonium) in good yields. 1 crystallises in the monoclinic $P2_1/c$ space group and its structure consists of N,N'-dibenzyl-N,N,N',N'-tetramethylethylenediammonium dication, two chloride anions and three crystal water molecules all of which are located in general positions. The organic dication is H-bonded to the chloride anions and the crystal waters with the help of intra- and intermolecular C-H$\cdots$Cl and C-H$\cdots$O interactions, while the chloride anions are linked to the crystal waters via O-H$\cdots$Cl interactions. One of the crystal waters is linked through an intermolecular O-H$\cdots$O bond with another water resulting in the formation of a water dimer. The O-H$\cdots$Cl and O-H$\cdots$O interactions between the chloride anions and water molecules lead to the formation of a five-membered {O3Cl2} cyclic dichloride containing a water dimer. The five-membered rings are linked into a chain with the aid of a O-H$\cdots$Cl interaction. The organic cations are organised in zigzag fashion on either side of the chain and are further linked to the anionic water chain via weak C-H$\cdot$O and C-H$\cdots$Cl interactions, leading to the supramolecular organisation of the rings into a spiral-like of chain.

• Study of the spectroscopic characteristics of methyl (ligand) cobaloximes and their antibacterial activity

Spectroscopic characterization (IR, NMR and electronic spectra) of methyl (ligand) cobaloxime was done, where ligand = pyrazole, dimethyl pyrazole, alanine and alanine methyl ester. The frequency changes in the IR spectra and shifts in the NMR were explained on the basis of basicity of the ligand, steric hindrance, HSAB principle and $d\pi-p\pi$ back-bonding from metal to ligand. Alanine and alanine methyl ester form more stable complexes than pyrazole and dimethyl pyrazole. Based on their IR and 1H NMR spectra it is inferred that pyrazole and dimethylpyrazole bind to Co (III) via N-2 ring nitrogen, i.e. monodentate coordination.

• A comparative study of proton transport properties of metal (IV) phosphates

M(IV) phosphates of the class of tetravalent metal acid (TMA) salts where M (IV) = Zr, Ti, Sn has been synthesized by the sol-gel method. These materials have been characterized for elemental analysis (ICP-AES), thermal analysis (TGA, DSC), X-ray analysis and FTIR spectroscopy. Chemical resistivity of these materials has been accessed in acidic, basic and organic solvent media. The protons present in the structural hydroxyl groups indicates good potential for TMA salts to exhibit solid-state proton conduction. The transport properties of these materials has been explored by measuring specific proton conductance at different temperatures in the range of 30-120°C at 10°C intervals, using a Solartron Dataset impedance analyzer (SI 1260) over a frequency range 1 Hz-10 MHz at a signal level below 1 V. Based on the specific conductance data and the Arrhenius plots, a suitable mechanism has been proposed and conductance performance of ZrP, SnP and TiP compared.

• Solvent-free microwave-assisted synthesis of oxadiazoles containing imidazole moiety

Microwave-assisted as well as conventional synthesis of 5-substituted-2-(2-methyl-4-nitro-1-imidazomethyl)-1,3,4-oxadiazoles containing the nitroimidazole moiety is carried out and their antibacterial, antifungal and anti-inflammatory activity is reported.

• Microwave-assisted addition of azomethines to isatoic anhydride

Diels-Alder addition of azomethines to isatoic anhydride in a solvent-free and eco-friendly condition is investigated using a microwave oven. The product is exclusively 2,3-diaryl-2,3-dihydroquinazolin-4(1H)-ones 4a-d. HOMO-LUMO energy of the iminoketene and the dienophiles were calculated using semi-empirical AM1 calculations.

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