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.

The application of ionic liquid-based system in the extraction of palladium: synthesis, characterization and computer calculation of palladium complexes

“…The trans congurations of palladium are in the majority as reported elsewhere [42][43][44] and are more energetically stable in view of the smaller steric hindrance. 45 Nevertheless, the cis conguration was obtained according to the results of X-ray crystallography. However, the powder pattern does not completely match with the XRD result of the sediment washed out of the [EMIm]NTF 2 with high levels of extracted complexes, which indicates that the trans conguration may also exist in the extracted complexes.…”

Synthesis, structural and theoretical studies of dithiodiglycolamide compounds of palladium(ii)

“…Indeed, if there is bonding between the metal and the carbamoyl group, then it should be reflected in the IR spectrum by showing a fre-quency difference (Δν) of 40-80 cm −1 for the carbamoyl group with respect to the free ligand. 11a The slight difference observed in the reported spectrum 10 has been explained recently as being due to the protonation of the carbamoyl group during contact with HNO 3 . 12 The observed Pd-O distance of 2.04 (±5%) Å in the EXAFS study may be due to the oxygen atom from water, nitrate or carbamoyl groups.…”

“…6 Recent studies using Extended X-ray Absorption Fine Structure (EXAFS) 9 and theoretical studies of a palladium nitrate compound with dithiodiglycolamide show that the ligand forms a 1 : 1 compound by bonding to the metal through both the thioether and carbamoyl groups in a tetradentate fashion (chelate binding mode). 10 However, there is no spectroscopic evidence provided to support that conclusion. Indeed, if there is bonding between the metal and the carbamoyl group, then it should be reflected in the IR spectrum by showing a fre-quency difference (Δν) of 40-80 cm −1 for the carbamoyl group with respect to the free ligand.…”

Effect of HNO₃ Concentration on the Pd(II) Extraction Properties using a Thiodiglycolamide Compound

“…Among the sulfur-containing extractants, thiodiglycolamide (TDGA) extractants have exhibited both good selectivity and rapid extraction of Pd(II), and they are very durable in strongly acidic environments [15][16][17][18][19]. Some studies have used Fourier-transform infrared spectroscopy (FT-IR), extended X-ray absorption fine structure, and computational methods to investigate the extraction properties of TDGA-HNO 3 systems and have shown that TDGA molecules probably act as multidentate ligands through both the thioetheric sulfur atom and the carbonyl oxygen atom [20,21].…”