Synthesis and Photophysics of the Doubly Cyclometalated Pd(II) Complexes [Pd(C∧N∧C)(L)], L = PPh3, AsPh3, and SbPh3

Abstract

While Pt(II) complexes containing doubly cyclometalated ligands as tridentate luminophores are well studied, the synthetic accessibility of their Pd(II) counterparts was lacking for a long time. Inspired by a recent report on the synthesis of [Pd(dpp)(PPh₃)] involving the C^∧N^∧C coordination mode (with dpp^2- = 2,6-di(phenid-2-yl)pyridine) and following our own work on closely related Pt(II)-based compounds, we produced the series of complexes [Pd(dpp)(PnPh₃)] (Pn = P, As, Sb) by optimizing the synthetic procedure and exploring their reactivity in the process. Our study of the electrochemical (cyclic voltammetry) and photophysical (UV-vis absorption and emission, transient absorption (TA) spectroscopy) properties of the Pd(C^∧N^∧C) complexes represents the first report on their characterization. We observed UV-vis absorption bands down to 450 nm and electrochemical HOMO-LUMO gaps around 3.2 V, which show minimal variation with different PnPh₃ coligands. A more pronounced influence of the coligand was observed in time-resolved emission and TA spectroscopy. The highest photoluminescence quantum yield (Φ_L) in the series was found for [Pd(dpp)(AsPh₃)], reaching 0.06. The interpretation of the spectroscopic data is supported by (TD-)DFT calculations. Additionally, we report structural and spectroscopic data for several dinuclear Pd(II) complexes, including the precursor {[Pd(dppH)(μ-Cl)]}₂ and multiple decomposition products of the sensitive compounds [Pd(dpp)(PnPh₃)].