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)].