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

Abstract

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 σ-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-C_carbene bonds render 7 and its derivative sufficiently stable for investigation of photo-cytotoxicity under cellular conditions.

Keywords: intermolecular interactions; palladium; phosphorescence; photo-cytotoxicity.