title: 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.
abstract: 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.
authors.affiliation: Department of Chemistry, Faculty of Sciences, Zanjan University, 45195-313, Zanjan, IR Iran. monfared@znu.ac.ir
authors.forename: Hassan Hosseini
authors.initials: HH
authors.lastname: Monfared
authors.forename: Morteza
authors.initials: M
authors.lastname: Vahedpour
authors.forename: Mahdi Mahdavi
authors.initials: MM
authors.lastname: Yeganeh
authors.forename: Massomeh
authors.initials: M
authors.lastname: Ghorbanloo
authors.forename: Peter
authors.initials: P
authors.lastname: Mayer
authors.forename: Christoph
authors.initials: C
authors.lastname: Janiak
mesh_terms: D056831:Coordination Complexes / Q000737:chemistry*; D003300:Copper / Q000737:chemistry*; D006835:Hydrazones / Q000737:chemistry; D006860:Hydrogen Bonding; D007539:Isonicotinic Acids / Q000737:chemistry; D008024:Ligands; D012545:Schiff Bases / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet
chemical_list: D056831:Coordination Complexes; D006835:Hydrazones; D007539:Isonicotinic Acids; D008024:Ligands; D012545:Schiff Bases; D003300:Copper