Experimental and theoretical characterization of Co(III) and Zn(II) complexes with phenol-based hexa-dentate macro-acyclic ligands: synthesis, density functional theory and time-dependent density functional theory studies

Abstract

Macro-acyclic complexes with two dissimilar coordination sites: one includes six coordination set (N4O2) and the other a tetradentate donor set (N2O2) of the type [CoIIILen]ClO4 and [CoIIILtn]ClO4 (where Len = N,N′-bis(pyridine-2-ylmethyl)-N,N′-bis(5-bromo-3-formyl-2-hydroxybenzyl)-1,2-diaminoethane and Ltn = N,N′-bis(pyridine-2-ylmethyl)-N,N′-bis(5-bromo-3-formyl-2-hydroxybenzyl)-1,3-diaminoepropane) have been synthesized and characterized. Characterization result revealed that in the synthetic procedure of [CoIIILen]ClO4 only an isomer (trans-pyridine/cis-phenolate) among three possible geometrical isomers is formed. However, synthesis of its counterpart, [CoIIILtn]ClO4 resulted in the formation of a mixture of geometrical isomers (trans-pyridine/cis-phenolate and cis-pyridine/cis-phenolate). The computational studies of these complexes demonstrated that in [CoIIILtn]+ an equilibrium exists between two possible geometrical isomers as result of a small energy difference (0.72 kcal mol−1), whereas in [CoIIILen]+ the difference is (2.71 kcal mol−1). Furthermore, [ZnIILen] has been studied computationally and experimentally by IR, UV–Vis spectroscopy techniques. The results showed that it has a similar structure to [CoIIILen]+ complex. Electronic spectra of the complexes were analyzed and the absorption bands were assigned through the density functional theory (DFT) and time-dependent density functional theory (TD–DFT) studies procedures. The molecular orbital diagrams of the complexes were also determined.