Synthesis and characterization of the trans(O6) isomer Ni(II) complex containing symmetrical edta-type ligand with mixed carboxylate and diamine rings: Quantum-mechanical evaluation of different isomers

Abstract

The trans(O6) isomer (trans(O6) refer to two six-membered rings in trans position) of the [Ba(H2O)4][Ni(1,3-pddadp)]·4H2O (1,3-pddadp = 1,3-propanediamine-N,N′-diacetate-N,N′-di-3-propionate) was synthesized and spectroscopically characterized (IR, UV–Vis). The trans(O6) geometry was confirmed by X-ray diffraction analysis. Single crystal X-ray diffraction of the complex revealed an octahedral geometry around the Ni(II) centre. Extensive strain analysis of [M(edta-type)]2− complexes (M = Ni, Co, Cu) has been performed and discussed in detail. DFT/NBO analysis (DFT = Density Functional Theory, NBO = Natural Bond Orbital) was performed for all isomers of [Ni(1,3-pddadp)]2− complex. DFT theory shows that trans(O6) isomer is 0.53 kcal mol−1 more stable than trans(O5O6) and 1.95 kcal mol−1 more stable than trans(O5). NBO analysis for all three isomers predicts the existence of a 3-center 2-electron (β system) A: -Ni-: B hyperbonds along with O:-C-:O carboxylate triads. The Second-Order Perturbation Theory Analysis predicts molecular stabilization that comes from delocalization caused by nA → σNiB* or nB → σNiA* charge transfer.