Cyclopalladated 3,5-Disubstituted-2-(2’-Pyridyl)Pyrroles Complexed to 8-Hydroxyquinoline or 4-Hydroxyacridine

Four new complexes of general formula [(La)Pd(Qui)] (1), [(Lb)Pd(Acr)] (2), [(La)Pd(Acr)] (3) and [(Lb)Pd(Acr)] (4) containing methyl (1, 3) or phenyl (2, 4) 3,5-disubstituted chelated 2-(2-pyridyl)pyrrole ligands (HLa,b) have been synthesized and characterized. For comparative purposes, the analogous 2-phenylpyridine (HLc) derivatives, [(Lc)Pd(Qui)] (5) and [(Lc)Pd(Acr)] (6), have also been prepared. Mild synthetic conditions and reduced reaction times can be employed with HLa,b ligands to obtain the products in good yields, proving the higher reactivity of HLa,b compared with HLc in cyclopalladation. Depending on the relative dispositions of the NN/NO (14) or CN/NO (5 and 6) chelating rings, all the complexes can in principle exist as two isomers. However, in each case, only one isomer was observed by 1H NMR spectroscopy. The crystal structures of 1, 4, and 5 reveal the presence of different isomers for different complexes, depending on both the cyclometallating and ancillary ligands. The structures of 16 allowed us to hypothesize that the ancillary ligand may play a predominant role in the formation of the trans/cis isomers in these systems and that the pyridylpyrrole definitely produces opposite outcomes compared with the 2-phenylpyridine ligand. Finally, the photophysical properties of 14 were studied in solution. Although 5 and 6 are emissive, no emission was detected for 14 under the same conditions, the emission quenching being most likely caused by the thermal deactivating path of the lowest excited state, which results from a combination of the pyridylpyrrole-localized state with that localized on the NO ligand.