Chelating properties of EDTA-type ligands containing six-membered backbone ring toward copper ion: Structure, EPR and TD-DFT evaluation

Abstract The P-APC ligands (EDTA-like aminopolycarboxylate ligands comprising 1,3-propanediamine backbone) H 4 pdta, H 4 pd3ap, H 4 pddadp and H 4 pdtp (H 4 pdta = 1,3-propanediamine- N , N , N ′, N ′-tetraacetatic acid; H 4 pd3ap = 1,3-propanediamine- N , N , N ′-triacetic- N ′-3-propionic acid; H 4 pddadp = 1,3-propanediamine- N , N ′-diacetatic- N , N ′-di-3-propionic acid; H 4 pdtp = 1,3-propanediaminetetra-3-propionic acid) were investigated. The chelating ligands coordinate to copper(II) via five or six donor atoms affording distorted trigonal-bipyramid and octahedral structures that were verified by X-ray analysis for Ba[Cu(pd3ap)]·6H 2 O ( 1 ) and trans (O 6 )-Ba[Cu(pddadp)]·8H 2 O ( 2 ) complexes respectively. The impact of counter-ions on the P-APC complexes is shown in detail together with the analysis of another strain parameters. EPR spectral results confirm the penta-coordination of 1 and hexa-coordination of 2 in aqueous solution, even if several Cu(II) species with different protonation degree exist as a function of pH, and indicate that a hexa-coordinated structure is favored when the two axial COO − donors close five-membered chelate rings. We also present here the results of molecular mechanics (LFMM) calculations based on our previously-developed force field along with results of DFT (Density Functional Theory). On the basis of extensive DFT and TD-DFT calculations the B1LYP/6-311++G(d,p) level has been seen as an accurate theory for calculating and predicting the UV–Vis spectra in case of copper–P-APC compounds.