Platinum(II) complexes bearing asymmetrically substituted pivaloylthioureas: Synthesis, crystal structures, DFT and antioxidant studies

Three new platinum(II) complexes derived from asymmetrically di-substituted pivaloylthiourea ligands (L1–3) are synthesized and characterized by 1H, and 195Pt{1H} NMR. The ligands and PtL1–3 complexes display configurational E,Z isomerism evinced by distinct sets of 1H, 195Pt{1H} NMR resonances observed in dichloromethane‑d2. Single-crystal X-ray diffraction studies reveal a bidentate cis-PtL1–3 coordination mode, in which two sulfur and oxygen donor atoms bind to Pt(II) forming a six-membered chelate. In addition, the EE isomers were preferentially isolated in acetonitrile for cis-PtL1 and cis-PtL2, while ZZ was obtained for cis-PtL3 with N-phenyl substituent. The relative energies of the cis-ZZ-Pt-L1–3, cis-EZ-Pt-L1–3, cis-EE-Pt-L1–3 configurational isomers are obtained by DFT analysis at the M06-2X/def2-SVP level of theory using an implicit acetonitrile solvent. Moreover, the structural studies indicate a potential intra-electron transfer from dz2 (HOMO) to dx2-y2 (LUMO), with a reduced electron affinity at the LUMO level for the cis-ZZ/ZE/EE-Pt-L1–3 structures. The antioxidant capacity of the L1–3 ligands and cis-PtL1–3 were evaluated using the ORAC, DPPH radical scavenging and FRAP methods. The free L1–3 ligands showed higher antioxidant activities in comparison to their coordinated cis-PtL1–3 complexes.