N Narasimha Murthy

Copper(I)-dioxygen reactivity studies utilizing new binucleating ligands are described. When catalytic quantities of either CuI or Cu II salts were introduced to LH2, which is a dinucleating ligand with two tridentate donors connected via a methylene group at a bis(2-pyridyl)methane junction, dioxygen addition caused complete oxygenation to give bis(2-pyridyl)ketone product L=O. Isolated dicopper(I) complex [Cu I2(LH2)]2+ (1) also reacted with excess O2 to give L=O. Additional observations were consistent with a mechanism that does not involve "oxygen-activation" in this process. Ketone hydration and gem-diol(ate) copper(II)-coordination occurred by addition of copper(II)-perchlorate to L=O, giving [CuII2{L(OH) (O-)}Cu2II(-OClO3)] 2+ (4), with a bridging alkoxide moiety (X-ray). Oxygenation of [CuI2(L=O)(CH3CN)2]2+ (2) (2/O2 = 0.5, i.e., Cu/O2 = 4:1 manometry), gave [Cu II2{L(O-)2}(-OClO 3)]+ (5), where both oxygen atoms of the doubly deprotonated gem-diolate bind separate copper(II) ions; mass spectrometric data showed that one of the two oxygen atoms of the gem-diolate L(O-) 2 came from O2. An oxo-dicopper(II) species is suggested as an important intermediate, which is supported by the observation that reaction of 2 with NO or iodosylbenzene also produced 5 in high yields. Complexes 4 and 5 are acid-base conjugate pairs and can be readily interconverted using Et3N or HClO4(aq) reagents. © 2007 The Chemical Society of Japan.

Treatment of the novel biphenyl-appended N,N′-bidentate ligand, 6-(2-pyridylmethyl)-6,7-dihydro-5H-dibenzo[c,e]azepine with FeX2 (X = Cl-, Br−) affords two stable four-coordinate, pseudo-tetrahedral [Fe(NN′)X2] (where X = Cl−, 1; Br−, 2) complexes. In contrast, the same reaction in the presence of pseudo-halides, KNCS/Se, gives two six-coordinate distorted-octahedral, [trans-Fe(NN′)2(NCE)2] (where E = S, 3; Se, 4) complexes. All the four complexes were characterized by X-ray crystallography, spectroscopic (FTIR, Uv–Vis, paramagnetic 1 H NMR, Mossbauer of 1) methods, ESI-MS and variable-temperature magnetic susceptibility measurements (VSM, 20–300 K). The molecules in the crystal lattice are held together by several strong intermolecular hydrogen-bonding, π⋯π stacking (py-py or py-ph) and C–H⋯π (CH2-ph) interactions. The strong absorption spectral features of 3 and 4 in solution and the paramagnetic 1H NMR spectra of 1–4 are interpreted. Variable-temperature magnetic susceptibility measurements of solids 1–4 show magnetic moment values near 4.9 B.M., characteristic of temperature-independent high-spin (S = 2) state for Fe(II). Mossbauer spectrum of 1 complements this property. The monomeric nature of 1 and 2 is an attractive feature, which may serve as invaluable precursor to model the active-site structure and function of mononuclear non-heme iron oxidase (MNO) enzymes, involved in O2 activation and oxidation catalysis of aromatic organic substrates.

A homologous series of binuclear copper(II) complexes [CuII2(Nn)(Y)2]2+ (1-3) (n = 3-5 and Y = (ClO 4)- or (NO3)-) were studied to investigate the intermediate(s) responsible for selective DNA strand scission in the presence of MPA/O2 (MPA = 3-mercaptopropanoic acid). While the N3 complex does not react, the N4 and N5 analogues show comparable activity with strand scission occurring at a single-strand/double-strand junction. Identical reactivity is also observed in the alternate presence of H2O 2. Spectroscopic and reactivity studies with [CuII2(N4)(Y)2]2+ (2) and H2O2 are consistent with DNA oxidation mediated by formation of a side-on peroxodicopper(II) (Cu2-O2) complex. © 2006 American Chemical Society.

A new series of five-coordinate [(BP)2MX]n+ complexes, (where X = OH2, M = Zn(ii) (1), Cu(ii) (2); X = Cl -, M = Cu(ii) (3), Co(ii) (4)) with a new bidentate chelating ligand [{N,N(1,1′-biphenyl-2,2′-dimethylene)-N(2-pyridyl methyl)} amine] with a biphenyl group (BP), have been synthesized and characterized by X-ray crystal structure and combined spectroscopic methods. They display unique trigonal bipyramidal (TBP) geometry, influenced by the bidentate ligand. The Zn(ii) complex 1 reveals ligand dynamics due to an atropisomeric biphenyl moiety as indicated by variable temperature (VT) proton NMR spectroscopy. The calculated free energy for the inversion of the bridged biphenyl is ∼13.08 kcal mol-1 (Tc = 273 K, Δν = 82.8 Hz, J = 8.7 Hz). The absorption spectra of Cu(ii) complexes 2 and 3, in CH 2Cl2 display greatly enhanced d-d bands (800-950 nm, ε >500 M-1 cm-1). On the other hand, complex 2 in N,N-dimethylformamide (DMF) showed almost 50% reduction in absorption intensity as DMF, a coordinating solvent, displaces the weakly-coordinated tertiary amine-nitrogens of the ligand and this competitive binding was studied by electronic absorption spectroscopy. When the mononuclear copper aqua complex 2 was treated with a base, a dicopper dihydroxide complex, [{(BP)Cu} 2(μ-OH)2]2+, (2a) was obtained. The same phenomenon was also observed with chloro complex 3 when treated with a base. This mono-dicopper equilibrium and conversion of 2→2a was monitored by UV-vis spectroscopy. Copper(ii) complexes 2 and 3 displayed "reverse" EPR spectra consistent with the TBP geometry. Cyclic voltammetry of 2 and 3 in DMF showed an irreversible redox wave owing to Cu(ii)/Cu(i) of five and four-coordinate species. The solution magnetic moment values of 1.76, 1.81 and 4.47 μB for 2, 3 and 4, respectively, are in agreement with Cu(ii) (S = 1/2) and Co(ii) (S = 3/2) high-spin configurations. The 1H NMR of 4 displays sharp but hyperfine shifted signals for the ligand protons between -30 to +220 ppm. The ESI-mass data complement the data obtained from X-ray structure. © The Royal Society of Chemistry 2009.

A mononuclear cobalt(II) complex [Co(BP)(Cl2)], 1 with a new N,N-bidentate ligand, BP ([N,N-(1,1′-biphenyl-2,2′-dimethylene)]-2- aminomethyl) pyridine) was synthesized and characterized by elemental analysis, UV-Vis, paramagnetic NMR, magnetic moment and single crystal XRD. The air and moisture stable Co(II) title complex is a 15 electron species which crystallizes in the monoclinic space group P21/n with cell dimensions a = 10.406(3), b = 12.873(4), c = 13.859(7) Å, β = 96.30(3)Ëš and Z = 4. The cobalt atom has distorted tetrahedral (td) geometry with coordination provided by an amine and pyridyl nitrogen donors of BP and two terminal chloride ions. The packing is stabilized through C- H•••π and van der Waals interactions. Index Abstract: A neutral four coordinate cobalt(II) complex with a new N,N-bidentate ligand capped by a biphenyl appendage was characterized by spectroscopic and single crystal X-ray diffraction technique. The title complex is a 15 e- molecule whose crystal structure is the first report of a neutral tetrahedral Co(II) complex with biphenyl appended bidentate ligand with amino-methyl pyridine.[Figure not available: see fulltext.] © 2010 Springer Science+Business Media, LLC.

Two simple biphenyl-Appended (2-pyridyl)alkylamine N-bidentate ligands, Le and Lm, having ethylene and methylene spacers between donor groups, with bite angles Le â 100° and Lm â 80°, dictate pseudotetrahedral and trigonal-bipyramidal geometries in six high-spin Ni(II)-halide complexes, [Ni(Le)X2] and [Ni(Lm)2X](ClO4) (where X = Cl-, Br-, I-), respectively. The structures in the solid state, determined using X-ray crystallography, and in solution, determined using spectroscopic methods (UV-vis-NIR and paramagnetic 1H NMR), which complement each other, are described.

A penta-coordinate mononuclear cobalt(II) complex, [Co(BP) 2(Cl-)](Br-), 1 with a novel, biphenyl-appended N,N-bidentate ligand, BP ([N,N-(1,1′-biphenyl-2,2′-dimethylene)]-2- aminomethyl) pyridine) was synthesized and characterized by elemental analysis, UV-Vis, electro-spray ionization mass spectrum and single crystal XRD. The title complex crystallizes in the monoclinic space group C2/c with cell dimensions a = 11.389(2), b = 25.627(3), c = 12.276(3) Å, β = 100.419(18)°and Z = 2. The cobalt atom has a distorted trigonal bipyramidal (tbp) geometry and is surrounded by 4Ns of two BP, and a Cl-/Br- at the 5th coordination site. This is the first report of a mononuclear tbp cobalt complex with biphenyl appended bidentate ligand containing amino-pyridyl moiety. © 2006 Springer Science+Business Media, Inc.

μ-1,3-Acetamide or acetate bridged, symmetric and asymmetric dicopper(II) complexes viz [Cu2(P1-O-)(NHAc-)](ClO4)2 (1), [Cu2(P2-O-)(OAc-)](ClO4)2 (2) and [Cu2(P2′-O-)(OAc-)(H2O)](ClO4)2 (3) were synthesized by employing classic dinucleating ligands; P1-OH, P2-OH (symmetric), and P2′-OH (asymmetric) having trivial differences in their ligand frame work. Solid state structures of these complexes were determined by X-ray crystallography. In solution, they were also characterized by various spectroscopic techniques, which includes ESI-MS, FT-IR, optical, solution magnetic moment, paramagnetic 1H NMR and EPR. The solution magnetic moment of these complexes at room temperature suggests a weak magnetic interaction between the two Cu(II) centers.

The title complex, [Ni2(L-O-)(H2O) 2](NO3)2·4H2O, where L-O - is bis{2-[bis(2-pyridyl)ethyl]amino}phenol (C20H 20-N3O), is dimeric with each Ni2+ ion coordinated by two bridging phenolate O atoms and three N atoms of the N3O-tetradentate ligand anion L-O-. A molecule of water on each metal completes the octahedral geometry. Crystallographic inversion symmetry is present at the center of the Ni2O2 plane of the dimeric unit. © 2003 International Union of Crystallography Printed in Great Britain - all rights reserved.

Two bis-tridentate chelated cobalt(ii) complexes, which differ in the ligand structure by a methylene group, activate molecular oxygen (O2), and give different oxidation products. The O2 reaction of [CoII(pepma)2]2+ (1) with unsymmetrical 2-(2-pyridyl)-N-(2-pyridylmethyl)ethanamine (pepma) results in ligand oxidation, to the corresponding Co(ii) imine complex [CoII(pepmi)2]2+ (2). Contrastingly, the Co(ii) complex [CoII(bpma)2]2+ (3) of similar symmetrical bis(2-pyridylmethyl)amine (bpma), undergoes metal oxidation, yielding a cobalt(iii) complex, [CoIII(bpma)2]2+ (4). The reversibility of the amine to imine conversion and the stability of the Co(ii) imine complex (2) are investigated. Furthermore, the solution dynamics of Co(ii) complexes are highlighted with the help of paramagnetic 1H-NMR spectroscopy.