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

The P-APC ligands (EDTA-like aminopolycarboxylate ligands comprising 1,3-propanediamine backbone) H4pdta, H4pd3ap, H4pddadp and H4pdtp (H4pdta = 1,3-propanediamine-N,N,N′,N′-tetraacetatic acid; H4pd3ap = 1,3-propanediamine-N,N,N′-triacetic-N′-3-propionic acid; H4pddadp = 1,3-propanediamine-N,N′-diacetatic-N,N′-di-3-propionic acid; H4pdtp = 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)]·6H2O (1) and trans(O6)-Ba[Cu(pddadp)]·8H2O (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.