Monitoring Ligand Substitution in (Catalytically Active) Metal Complexes with Bodipy-Tagged Diimines and NHC Ligands

The reaction of 2,6-dimethyl-4-(Bodipy-8-yl)­aniline with 1,4-dioxane-2,3-diol provides the respective diimine 3, followed by ring closure with paraformaldehyde, resulting in the imidazolium salt 4·HCl containing two fluorophores. The NHC metal complexes ([IrCl­(cod)­(4)], [Ir­(cod)­(4)­(py)]­OTf, [IrCl­(CO)2(4)], [PdCl­(allyl)­(4)], [PdCl2(Clpy)­(4)], [AuCl­(4)], and [NiCl­(Cp)­(4)]) were prepared. The complexes with (Au, Pd) are fluorescent. Weakly fluorescent complexes with Ir undergo fluorogenic reactions with CO and H2. The oxidative addition of H2 to [Ir­(cod)­(4)]­OTf leads to a fluorescence gain of 8.2 and is suitable for the detection of hydrogen. Following activation of the diimine complex [PdCl­(CH3)­(3)] with NaBArF, the formed [Pd­(CH3)­(3)]­BArF is a competent ethene polymerization catalyst. The addition of (pyridine, CH3CN, CO) or olefins (ethene, 1-hexene, styrene) to the activated complex results in a pronounced increase in the fluorescence because of metal–ligand interactions and the modulation of photoinduced electron transfer quenching. The fluorescence response was used for the determination of the respective association constants with the donor molecules: 1-hexene (K = 9 L·mol–1) and CH3CN (K = 4 L·mol–1); styrene is a poor ligand (K = 0.04 L·mol–1). Based on the ν­(CO) of the [Mo­(CO)4(3)] complex, the Bodipy-8-yl group is a moderately electron-withdrawing group, comparable to the Br substituent.