Palladium(II) Acetylide Complexes with Pincer‐Type Ligands: Photophysical Properties, Intermolecular Interactions, and Photo‐cytotoxicity

Two classes of cationic palladium(II) acetylide complexes containing pincer-type ligands, 2,2':6',2″-terpyridine (terpy) and 2,6-bis(1-butylimidazol-2-ylidenyl)pyridine (C^N^C), were prepared and structurally characterized. Replacing terpy with the strong σ-donating C^N^C ligand having two N-heterocyclic carbene units results in the Pd(II) acetylide complexes to display phosphorescence at room temperature and stronger intermolecular interactions in the solid state. X-ray crystal structures of [Pd(terpy)(CΞCPh)]PF6 (1) and [Pd(C^N^C)(CΞCPh)]PF6 (7) reveal that the complex cations are arranged in a one-dimensional stacking structure with pair-like PdII∙∙∙PdII contacts of 3.349 A for 1 and 3.292 A for 7. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were used to examine the electronic properties. Comparative studies on the [Pt(L)(CΞCPh)]+ analogues by 1H NMR spectroscopy were made to shed insight into the intermolecular interactions of these PdII acetylide complexes. The strong Pd-Ccarbene bonds render 7 and its derivative to attain sufficient stability for investigation of photo-cytotoxicity under cellular conditions.