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-propane-diamine backbone) H4pdta, H4pd3ap, H4pddadp and H4pdtp (H4pdta = 1,3-propane-diamine-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.