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 strongly ?-donating C^N^C ligand with two N-heterocyclic carbene (NHC) units results in the Pd(II) acetylide complexes displaying phosphorescence at room temperature and stronger intermolecular interactions in the solid state. X-ray crystal structures of [Pd(terpy)(C?CPh)]PF6 (1) and [Pd(C^N^C)(C?CPh)]PF6 (7) reveal that the complex cations are arranged in a one-dimensional stacking structure with pair-like Pd(II) ???Pd(II) contacts of 3.349?Å for 1 and 3.292?Å for 7. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were used to examine the electronic properties. Comparative studies of the [Pt(L)(C?CPh)](+) analogs by (1) H?NMR spectroscopy shed insight on the intermolecular interactions of these Pd(II) acetylide complexes. The strong Pd-Ccarbene bonds render 7 and its derivative sufficiently stable for investigation of photo-cytotoxicity under cellular conditions.