Investigation of the extraction complexes of palladium(ii) with novel thiodiglycolamide and dithiodiglycolamide ligands by EXAFS and computational methods

Abstract

The structure of the extraction complexes of palladium(II) with novel ligands, namely, N,N,N',N'-tetra-(2-ethylhexyl) thiodiglycolamide (T(2EH)TDGA) and N,N,N',N'-tetra-(2-ethylhexyl) dithiodiglycolamide (DTDGA), have been determined by extended X-ray absorption fine structure spectroscopy (EXAFS). The interpretation of the EXAFS data is well supported by theoretical calculations of the complex geometry based on density functional theory (DFT). T(2EH)TDGA, having one thioetheric 'S' atom, forms a square planar complex with the Pd(II) ion, exhibiting 2: 1stoichiometry with one sulphur atom and one carbonyl oxygen atom from each T(2EH)TDGA molecule interacting with Pd(II) at distances of 2.24 and 2.04 Å, respectively. On the other hand, DTDGA, having two 'S' atoms in addition to two carbonyl groups, forms a square planar complex with Pd(II), exhibiting 1: 1stoichiometry, wherein both the sulphur atoms and the carbonyl oxygen of DTDGA interact with Pd(II) at distances of 2.29 and 2.05 Å, respectively. The slight distortion from the perfect square planar geometry could be attributed to the steric hindrance imposed by the bulky alkyl group of the amidic moieties. DFT calculations for the Pd-ligand complexes show that the Pd(II)-DTDGA complex with 1: 1stoichiometry is energetically more stable than the Pd(II)-T(2EH)TDGA complex with 1: 2stoichiometry. Among the two possible Pd(II)-T(2EH)TDGA complex geometries, the cis configuration appears more favorable. The methodology of fitting the EXAFS data has been validated by fitting the EXAFS data of a Pd(II)-aquo complex which showed square planar geometry with two axial water molecules constituting the secondary hydration sphere.