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 sigma-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 angstrom for 1 and 3.292 angstrom 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 H-1 NMR spectroscopy shed insight on the intermolecular interactions of these Pd-II acetylide complexes. The strong Pd-C-carbene bonds render 7 and its derivative sufficiently stable for investigation of photo-cytotoxicity under cellular conditions.