Synthesis of rhodium(III) complexes with tris/tetrakis-benzimidazoles and benzothiazoles - quick identification of cyclometallation by nuclear magnetic resonance spectroscopy

Reactions of rhodium(III) halides with multidentate N, S-heterocycles, (LH3) 1,3,5-tris(benzimidazolyl)benzene (L1H3; 1), 1,3,5-tris(N-methylbenzimidazolyl) benzene (L2H3; 2) and 1,3,5-tris(benzothiazolyl)benzene (L3H3; 3), in the molar ratio 1 : 1 in methanol-chloroform produced mononuclear cyclometallated products of the composition RhX2 (LH2)(H2O) (X = Cl, Br, I; LH2 = L1H2, L2H 2, L3H2). When the metal to ligand (1-3 or 1,2,4,5-tetrakis(benzothiazolyl)benzene L4H2; 4) molar ratio was 2 : 1, the reactions yielded binuclear complexes of the compositions Rh2Cl5 (LH2)(H2O) 3 (LH2 = L1H2, L2H2, L 3H2) and Rh2X4 (L 4)(H2O) 2 (X= Cl, Br, I). Elemental analysis, IR and 1H nuclear magnetic resonance (NMR) chemical shifts supported the binuclear nature of the complexes. Cyclometallation was detected by conventional 13C NMR spectra that showed a doublet around �190 ppm. Cyclometallation was also detected by gradient-enhanced heteronuclear multiple bond correlation (g-HMBC) experiment that showed cross-peaks between the cyclometallated carbon and the central benzene ring protons of 1-3. Cyclometallation was substantiated by two-dimensional 1H- 1H correlated experiments (gradiant-correlation spectroscopy and rotating frame Overhauser effect spectroscopy) and 1H-13C single bond correlated two-dimensional NMR experiments (gradient-enhanced heteronuclear single quantum coherence). The 1H-15N g-HMBC experiment suggested the coordination of the heterocycles to the metal ion via tertiary nitrogen. © 2009 John Wiley & Sons, Ltd

Reactions of organo-rhodium complexes with multidentate N,N and N,S-heterocycles and exchange studies by NMR

Dihalobridged binuclear complexes Rh(diolefin)(μ-X)2 {diolefin = 1,5-cyclooctadiene (cod), X = Cl or Br; diolefin = norbornadiene (nbd), X = Cl}, undergo halide bridge cleavage reactions with multidentate N,N-heterocycles 1,3,5-tris(benzimidazolyl)benzene (L1H3), 1,3,5-tris(N-methylbenzimidazolyl)benzene (L2H3) and N,S-heterocycle 1,3,5-tris(benzothiazolyl)benzene (L3H3) to yield trinuclear heterocycle bridged complexes {RhX(cod)}3(μ-LH3) and {RhCl(nbd)}3(μ-LH3) (LH3 = L1H3, L2H3, L3H3). 1H NMR exchange measurements have shown resonances for olefinic protons 1â³, 2â³, 5â³ and 6â³ of cod at different chemical shifts, perhaps due to restricted Rh-N bond rotation. The olefinic and aliphatic protons would undergo exchange with each other and also with intermediate species. The exchange mechanism may be visualized to involve Rh-N bond breaking, rotation of the cod ligand of the T-shaped (three-coordinate) intermediate species followed by recomplexation. An alternate mechanism may be Rh-cod bond breaking at olefin positions 5â³ and 6â³, isomerisation of the T-complex such that 5â³/6â³ moves trans to X coupled with rotation of the heterocycle about the Rh-N bond (made easier by the reduced coordination number of the intermediate), followed by recoordination of 1â³/2â³ trans to N, followed by recomplexation. NMR signals from the intermediate species in one dimensional 1H, 13C and 2D NMR spectra have supported the exchange of protons. © 2009 Elsevier Ltd. All rights reserved

Cationic complexes of platinum and palladium with p-tolyl isocyanide

Cationic complexes of platinum and palladium of the type [MCl(p-CH3C6H4NC)L2]ClO4 (M = Pt, Pd; L = p-tolyl3P, o-tolyl3P, Cy3P, Ph2MeAs, Ph2EtAs, Ph2PrAs, Cy3As; Cy = cyclohexyl) have been isolated. These show a v(CN) band at ca. 2200 cm-1 compared with 2130 cm-1 for the free isocyanide ligand, suggesting weak π-character in the metal-carbon bond. The PMR spectra of the cationic complexes indicate trans configurations for the complexes. © 1977

Ruthenium catalyzed oxidative conversion of isatins to anthranilic acids: Mechanistic study

Oxidation of isatins (isatin, 5-methylisatin, 5-bromoisatin, and 5-nitroisatin) to their corresponding anthranilic acids was performed with sodium N-bromo-p-toluenesulfonamide or bromamine-T (BAT) as an oxidant and ruthenium trichloride (Ru(III)) as a catalyst in HCl medium at 30 +/- 0.1 degrees C. The four reactions follow identical kinetics with a first-order dependence each on [BAT](o) and [Ru(III)], zero-order on [Isatin], and inverse fractional-order on [H(+)]. Activation parameters have been deduced for the composite reaction. The rates satisfactorily correlate with the Hammett U relationship and the reaction constant p is -0.36 signifies that the electron donating groups accelerate the reaction while the electron withdrawing groups retard the rate. An isokinetic relationship is observed with beta = 360 K, indicating that enthalpy factors control the reaction rate. Oxidation products of isatins were identified as their corresponding anthranilic acids by GC-MS analysis and the yields were found to be > 90%. Under similar experimental conditions, the kinetics of Ru(III)-catalyzed oxidation of isatins with BAT has been compared with that of uncatalyzed reactions, revealing that the catalyzed reactions are three to fourfold faster. The observed results have been explained by a plausible mechanism and the related rate law has been deduced. The method adopted for the oxidation of isatins to anthranilic acids in the present work offers several advantages and can be scaled up to industrial operation. (c) 2008 American Institute of Chemical Engineers

Reactions of rhodium and iridium salts with multidentate N-heterocycles

Complexes of rhodium and iridium of the types MX3L, MX(CO)2L and MX3(CO)L (X = halide) containing multidentate N-heterocycles (L), 2,6-bis(benzimidazolyl)pyridine (bBzlH2py) and 2,6-bis(N-methyl-benzimidazolyl)pyridine (bBzlMe2py) have been prepared and characterized by IR, electronic and 1H and 13C NMR spectral data. RhX(CO)2L, on treatment with alcoholic solvents or DMF undergoes reversible decarbonylation to produce RhXL·2H2O. Passage of NO or O2 through the carbonyl suspended in hot 2-methoxyethanol releases CO2. Copyright © 1997 Elsevier Science Ltd

Transition metal chelates as accelerators for epoxy resin systems—studies with cobalt (III) acetylacetonate

Cobalt(III)acetylacetonate serves as an accelerator for anhydride curable epoxy resin system and the rate of curing is found to increase with enhanced concentrations of the metal chelate. There is also an appreciable reduction in the cure gel time. Kinetic studies based on thermal analytical techniques reveal that the overall curing process follows first order kinetics. Based on the kinetic results a cure schedule has been proposed. It is also observed that the electrical, mechanical, and thermal properties of the cured epoxy system are not altered by the presence of the metal chelate at the concentration studied

Cyclometallation of bis-benzimidazole derivatives with rhodium(III) halides

Treatment of rhodium(III) halides with the N-heterocycles (LH), 1,3-bis(benzimidazolyl)benzene (bBzlH2bzH; Ia) and its N-methyl derivative (bBzlMe2bzH; Ib) in methanol gave halobridged binuclear cyclometallated products of the composition RhX2L2 (X=Cl, Br or I). The chloro complex undergoes halobridge cleavage reactions to yield several new mononuclear complexes of the types RhCl2(bBzlH2bz)(AsPh3), RhCl(bBzlH2bz)(OClO3)(Lâ²/N-N) (Lâ²=AsPh3; N-N=2,2â²-bipyridine or 1,10-phenanthroline) and the heterocycle bridged binuclear complexes of the composition RhCl2(bBzlH2bz)2(μ-N-N) (N-N=pyrazine or 4,4â²-bipyridine). Passage of CO through RhCl2(bBzlH2bz)2 in DMF yielded mononuclear carbonyl complex RhCl2(CO)(bBzlH2bz)·2H2O. Treatment of carbonylated solution of rhodium trichloride with Ia produced non-cyclometallated mononuclear complex of the type Rh(CO)2(bBzlH2bzH)Cl. The complexes are characterised by 1H, 13C NMR, IR, Far-IR, electronic and FAB-mass spectral studies. © Elsevier Science Ltd

Synthesis, Characterization and Biological Activity Studies on 6-p-Dimethylaminophenyl-5,6-dihydrobenzoimidazo[1,2-c]quinazoline: Crystal Structure of the Title Compound and Comparative Study with Related Derivatives

Reaction of o-aminophenylbenzimidazole with p-dimethylaminobenzaldehyde yielded 6-p-dimethylaminophenyl-5,6-dihydrobenzoimidazo[1,2-c]quinazoline, which was characterized by elemental analysis, IR, UV-Vis, H-1 NMR, C-13 NMR, mass spectral studies and X-ray crystal structure analysis. Studies on the antimicrobial activity of the compound revealed that it is active against fungus Yeast but not Bacillus subtilis. The compound crystallized in the space group P2(1)/n with the unit cell parameters a = 10.652(2) , b = 11.002(2) , c = 15.753(2) , beta = 109.29(2)A degrees and the structure was refined to an R-factor of 0.0479. The hydropyrimidine ring in the quinazoline moiety is in skew-boat conformation. The dimethylamino group attached to phenyl ring is in conjugation with it. The structure was stabilized by intermolecular C-H-N interactions. A few of the related quinazolines (6-p-hydroxyphenyl-5,6-dihydrobenzoimidazo[1,2-c]quinazoline; 6-phenyl-5,6-dihydrobenzoimidazo[1,2-c]quinazoline; 6-pyridyl-5,6-dihydrobenzoimidazo[1,2-c]quinazoline; 6-furyl-5,6-dihydrobenzoimidazo[1,2-c]quinazoline) were also examined for their biological activity, in addition to their characterization by IR, UV-Vis, H-1 and C-13 NMR spectral studies along with structural comparison

Cadmium complexes stabilized by bis-benzimidazolyl derivatives

Cadmium halides/perchlorate react with bis-benzimidazolyl ligands (1) 1,3-bis(benzimidazol-2-yl)benzene (L1), 1,3-bis(1-methylbenzimidazol- 2-yl)benzene (L2) and 2,6-bis(1-methylbenzimidazol-2-yl)pyridine (L3) in ethanol to produce complexes of the compositions CdX 2L1.nH2O (X = Cl, n = 1, X = Br or I, n = 2), (CdCl2)2L2, CdX2L (X = Br or I, L = L2; X = Cl, Br or I, L = L3), Cd2(L 1)3(ClO4)4.6H2O and Cd(L)2(ClO4)2.nH2O (L = L 2, n = 2; L = L3, n = 0). The complexes have been characterized by elemental analyses, conductance measurements, IR, 1H and 13C NMR spectral studies as well as thermal analysis