Spectroscopic, electrochemical and X-ray crystallographic properties of a novel palladium(II) complex of thioamide deprotonated di-2-pyridyl ketone thiosemicarbazone (dpktsc-H)⁻

<p>The reaction between di-2-pyridyl ketone thiosemicarbazone (dpktsc) and PdCl₂(CH₃CN)₂, generated <i>in situ</i> from the reaction between PdCl₂ and CH₃CN, gave the unprecedented [Pd₂Cl₃(κ⁵-N_py,N_im,S,N_py,N_am-dpktsc-H)]·2CH₃CN (<b>1</b>) complex (py = pyridine, im = imine and am = amide). The identity of <b>1</b> was confirmed via its elemental analysis and spectroscopic properties. Infrared and ¹H-NMR spectra confirmed the coordination of (dpktsc-H)⁻ to the palladium ions. The electronic absorption spectra measured in dmso and dmf and density functional theory (DFT) calculations revealed metal-to-ligand charge-transfer (MLCT), d–d and intra-ligand charge-transfer (ILCT) electronic transitions. X-ray structural analysis on a crystal of [Pd₂Cl₃(κ⁵-N_py,N_im,S,N_py,N_am-dpktsc-H)]·H₂O (<b>2</b>) grown from dmf solution of <b>1</b> confirmed its formulation and showed the solid-state structure contains a web of molecules locked via a network of non-covalent interactions. Electrochemical measurements on <b>1</b> in dmf revealed metal- and ligand-based redox processes. In contrast to the electrochemical decomposition of uncoordinated dpktsc, coordinated (dpktsc-H)⁻ in <b>1</b> does not undergo electrochemical decomposition. Electrochemical titrations of <b>1</b> with <i>p</i>-toluenesulfonic acid monohydrate (<i>p</i>-TSOH) revealed electro-catalytic proton reduction. Over-potential (<i>η</i>) of 180 mV for the H₂ evolution was observed and is comparable to several molecular electro-catalysts for proton reduction. Controlled-potential electrolysis confirmed the electro-catalytic proton reduction by the Pd-complex. Electrochemical reactions of CO₂ in the presence of <b>1</b> exhibited a proton dependence, and metal- and ligand-based electrochemical reaction.