1H nuclear magnetic resonance studies of the Ni(II) and Co(II) complexes with diaminetetraacetic acids

Abstract

The structures and structural rearrangements of the Ni(II) and Co(II) complexes with diaminetetraacetic acids: tetramethylene-diamine-N,N,N,N′-tetraacetic acid (BDTA), hexamethylenediamine-N,N,N′,N′-tetraacetic acid, ethylene glycol bis(β-aminoethyl ether) N,N,N′,N′-tetraacetic acid (EGTA) were investigated by means of 1H NMR spectroscopy. The 1H NMR spectra were interpreted on the basis of the relationship between the hyperfine coupling constants and dihedral angle. BDTA was found to form complexes with a seven-membered chelate ring. This ring exhibits a skew-boat conformation in equilibrium with a skew-chair conformation. No racemization of the Ni(BDTA) and Co(BDTA) complexes takes place. EGTA does not coordinate with Ni(II) and Co(II) via the ethereal oxygens. The coordination sphere of one metal ion includes two iminodiacetic groups from two different EGTA molecules (nitrogen atoms in trans positions). The structure of the Ni(HDTA) complex is similar to that of Ni(EGTA). The spectral characteristics of the Co(II) system depends on the ligand-metal molar ratio. The existence of dimeric and monomeric forms of the complex ion is discussed. No intramolecular rearrangements were found to occur in the Ni(II) and Co(II) complexes with EGTA and HDTA.