Bipyridyl Ruthenium Complexes Featuring P-Ylide Ligands: A Comparative Study of Their Redox and Photophysical Properties

The electrochemical and photophysical properties of two series of ruthenium bis(bipyridine) complexes featuring either a pyridine-phosphonium ylide (PC) or a pyridine-iminophosphorane (PN) ligand were synthesized. Three different bipyridines were employed allowing the synthesis of six P-ylide RuII complexes. All the complexes were fully characterized by multinuclear NMR spectroscopy, HR-mass spectrometry, as well as X-ray crystallography, and no major difference was observed between them. In cyclic voltammetry, the measured potentials agree with the electronic properties of the bipyridine ligands and underline the strong electron donation of the phosphonium ylide. Indeed, the oxidation is facilitated and the reductions become more difficult compared to the corresponding [Ru(Rbpy)₃] complexes. The absorption spectra of the PC and PN RuII complexes are similar, with a bathochromic shift compared to polypyridine RuII complexes, which is amplified in the presence of electron-withdrawing substituents on the bipyridine. Regarding luminescence, the PC complexes emit at a lower energy than the PN analogues. In addition, the associated lifetime depends on both the nature of the P-ylide and the substituents on the bipyridine ligand. This was related to the different adiabatic ³MC/³MLCT gap calculated at the DFT level, which is larger for two iminophosphorane complexes exhibiting a lifetime of the order of a few hundred nanoseconds.