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Hosseini Monfared, Hassan; Vahedpour, Morteza; Mahdavi Yeganeh, Mahdi; Ghorbanloo, Massomeh; Mayer, Peter and Janiak, Christoph (2011): Concentration dependent tautomerism in green [Cu(HL1)(L2)] and brown [Cu(L1)(HL2)] with H2L1 = (E)-N’-(2-hydroxy-3-methoxybenzylidene)- benzoylhydrazone and HL2 = pyridine-4-carboxylic (isonicotinic) acid. In: Dalton Transactions, Vol. 40, No. 6: pp. 1286-1294 [PDF, 1MB]

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The in situ formed hydrazone Schiff base ligand (E)-N’-(2-hydroxy-3-methoxybenzylidene)-benzoylhydrazone (H2L1) reacts with copper(II) acetate in ethanol in the presence of pyridine-4-carboxylic acid (isonicotinic acid, HL2) to green-[Cu(HL1)(L2)]・H2O・C2H5OH (1) and brown-[Cu(L1)(HL2)] (2) complexes which crystallize as concomitant tautomers where either the mono-anion (HL1)- or di-anion (L1)2- of the Schiff base and simultaneously the pyridine-carboxylate (L2)- or the acid (HL2) (both through the pyridine nitrogen atom) function as ligands. The square-planar molecular copper(II) complexes differ in only a localized proton position either on the amide nitrogen of the hydrazone Schiff base in 1 or on the carboxyl group of the isonicotin ligand in 2. The proportion of the tautomeric forms in the crystalline solid-state can be controlled over a wide range from 1:2 = 95 : 5 to ~2 : 98 by increasing the solution concentration. UV/Vis spectral studies show both tautomers to be kinetically stable (inert), that is, with no apparent tautomerization, in acetonitrile solution. The UB3LYP/6-31+G* level optimized structures of the two complexes are in close agreement with experimental findings. The solid-state structures feature 1D hydrogen-bonded chain from charge-assisted O(-) … H–N and O–H … (-)N hydrogen bonding in 1 and 2, respectively. In 1 pyridine-4-carboxylate also assumes a metal-bridging action by coordinating a weakly bound carboxylate group as a fifth ligand to a Cu axial site. Neighboring chains in 1 and 2 are connected by strong π-stacking interactions involving also the five- and six-membered, presumably metalloaromatic Cu-chelate rings.

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