Synthetic, Crystallographic, and Computational Study of Copper(II) Complexes of Ethylenediaminetetracarboxylate Ligands

Copper­(II) complexes of hexadentate ethylenediaminetetracarboxylic
acid type ligands H4eda3p and H4eddadp (H4eda3p = ethylenediamine-N-acetic-N,N′,N′-tri-3-propionic
acid; H4eddadp = ethylenediamine-N,N′-diacetic-N,N′-di-3-propionic acid) have been prepared. An octahedral trans­(O6) geometry (two propionate ligands coordinated in axial positions)
has been established crystallographically for the Ba­[Cu­(eda3p)]·8H2O compound, while Ba­[Cu­(eddadp)]·8H2O is proposed
to adopt a trans­(O5) geometry (two axial acetates) on the
basis of density functional theory calculations and comparisons of
IR and UV–vis spectral data. Experimental and computed structural
data correlating similar copper­(II) chelate complexes have been used
to better understand the isomerism and departure from regular octahedral
geometry within the series. The in-plane O–Cu–N chelate
angles show the smallest deviation from the ideal octahedral value
of 90°, and hence the lowest strain, for the eddadp complex with
two equatorial β-propionate rings. A linear dependence between
tetragonality and the number of five-membered rings has been established.
A natural bonding orbital analysis of the series of complexes is also
presented.