N,N′-Ethylenebis(pyridoxylideneiminato) and N,N′ -Ethylenebis(pyridoxylaminato): Synthesis, Characterization, Potentiometric, Spectroscopic, and DFT Studies of Their Vanadium(IV) and Vanadium(V) Complexes

Isabel Correia, Joao Costa Pessoa, M. Teresa Duarte, Rui T. Henriques, M. Fátima M. Piedade, Luís F. Veiros, Tamás Jakusch, Tamás Kiss, Ágnes Dörnyei, M. Margarida C.A. Castro, Carlos F.G.C. Geraldes, Fernando Avecilla

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Abstract

The Schiff base N,N′-ethylene-bis(pyridoxylideneiminato) (H 2pyr2en, 1) was synthesized by reaction of pyridoxal with ethylenediamine; reduction of H2pyr2en with NaBH 4 yielded the reduced Schiff base N,N′ -ethylenebis-(pyridoxylaminato) (H2Rpyr2en, 2); their crystal structures were determined by X-ray diffraction. The totally protonated forms of 1 and 2 correspond to H6L4+, and all protonation constants were determined by pH-potentiometric and 1H NMR titrations. Several vanadium(IV) and vanadium(V) complexes of these and other related ligands were prepared and characterized in solution and in the solid state. The X-ray crystal structure of [VvO2(HR-pyr2en)] shows the metal in a distorted octahedral geometry, with the ligand coordinated through the N-amine and O-phenolato moieties, with one of the pyridine-N atoms protonated. Crystals of [(VVO2)2(pyren) 2]2H2O were obtained from solutions containing H 2pyr2en and oxovanadium(IV), where Hpyren is the "half" Schiff base of pyridoxal and ethylenediamine. The complexation of VIVO2+ and VVO2+ with H2pyr2en, H2Rpyr2en and pyridoxamine in aqueous solution were studied by pH-potentiometry, UV/Vis absorption spectrophotometry, as well as by EPR spectroscopy for the V IVO systems and 1H and 51V NMR spectroscopy for the VVO2 systems. Very significant differences in the metal-binding abilities of the ligands were found. Both 1 and 2 act as tetradentate ligands. H2Rpyr2en is stable to hydrolysis and several isomers form in solution, namely cis-trans type complexes with VIVO, and α-cis- and β-cis-type complexes with V VO2. The pyridinium-N atoms of the pyridoxal rings do not take part in the coordination but are involved in acid-base reactions that affect the number, type, and relative amount of the isomers of the V IVO-H2Rpyr2en and VVO 2-H2Rpyr2en complexes present in solution. DFT calculations were carried out and support the formation and identification of the isomers detected by EPR or NMR spectroscopy, and the strong equatorial and axial binding of the O-phenolato in VIVO and VVO 2 complexes. Moreover, the DFT calculations done for the [V IVO(H2Rpyr2en)] system indicate that for almost all complexes the presence of a sixth equatorial or axial H2O ligand leads to much more stable compounds.

Original languageEnglish
Pages (from-to)2301-2317
Number of pages17
JournalChemistry - A European Journal
Volume10
Issue number9
DOIs
Publication statusPublished - May 3 2004

Keywords

  • Coordination modes
  • Geometric isomers
  • N, O ligands
  • Speciation
  • Vanadium

ASJC Scopus subject areas

  • Catalysis
  • Organic Chemistry

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