Palladium Complexes of N,N'-Bis(2-aminoethyl)oxamide (H2L): Structural (PdIIL, PdII2L2, and PdIVLCl2), Electrochemical, Dynamic 1H NMR, and Cytotoxicity Studies

Sergey P Gavrish; Yaroslaw D Lampeka; Maria V Babak; Vladimir B Arion

doi:10.1021/acs.inorgchem.7b02732

Palladium Complexes of N,N'-Bis(2-aminoethyl)oxamide (H₂L): Structural (Pd^IIL, Pd^II₂L₂, and Pd^IVLCl₂), Electrochemical, Dynamic ¹H NMR, and Cytotoxicity Studies

Inorg Chem. 2018 Feb 5;57(3):1288-1297. doi: 10.1021/acs.inorgchem.7b02732. Epub 2018 Jan 8.

Authors

Sergey P Gavrish¹, Yaroslaw D Lampeka¹, Maria V Babak², Vladimir B Arion³

Affiliations

¹ L.V. Pisarzhevskii Institute of Physical Chemistry of the National Academy of Sciences of Ukraine , Prospekt Nauki 31, Kyiv 03028, Ukraine.
² Department of Chemistry, National University of Singapore , 3 Science Drive 2, 117543 Singapore.
³ Institute of Inorganic Chemistry, Faculty of Chemistry, University of Vienna , Währinger Straße 42, 1090 Vienna, Austria.

PMID: 29308884
DOI: 10.1021/acs.inorgchem.7b02732

Abstract

The monomeric (PdL·2H₂O) and dimeric (Pd₂L₂·7H₂O) palladium(II) complexes of N,N'-bis(2-aminoethyl)oxamide (H₂L) were isolated, and their structures were established by single-crystal X-ray diffraction. Both compounds display identical cis-(2N_amide + 2N_amine) coordination environments of the metal ion. The dimer, representing a combination of two PdL species with an open lateral chelate ring, has an "open clamshell"-like structure. The intramolecular metal-metal separation in Pd₂L₂ (3.215 Å) is slightly shorter than the sum of the van der Waals radii of the palladium(II) atoms. The dimeric complex is relatively stable to dissociation, and its spectral features in aqueous solutions have been compared to those of the monomeric complex. A ¹H NMR spectroscopic study revealed the presence of the dynamic conformational exchange process assigned to a turning of the dimeric molecule "inside out" with an activation energy of 65 kJ/mol. Cyclic voltammetry of PdL in perchlorate-, chloride-, and sulfate-containing electrolytes revealed two-electron oxidation of the palladium center. For the dimeric complex similar, though irreversible, oxidation to the palladium(IV) state was observed in NaCl electrolyte. At the same time, in NaClO₄ or Na₂SO₄ solutions oxidation of Pd₂L₂ occurs in two distinct steps. The first step is quasi-reversible and can be assigned to the formation of species in an intermediate Pd^IIIPd^III state. Monomeric palladium(IV) complex Pd^IVLCl₂ was generated via chemical oxidation of Pd^IIL by peroxodisulfate in the presence of chloride ions and structurally characterized. The related M^IIL complexes (M = Pd, Ni, Cu) showed low cytotoxicity in human cancer cell lines AGS (gastric adenocarcinoma) and HCT116 (colorectal carcinoma) with IC₅₀ values from 204 to 525 μM, while the proligand H₂L was devoid of antiproliferative activity (IC₅₀ > 1000 μM).