Fluorescent Orthopalladated Complexes of 4-Aryliden-5(4 H )-oxazolones from the Kaede Protein: Synthesis and Characterization

The goal of the work reported here was to amplify the fluorescent properties of 4-aryliden-5(4 )-oxazolones by suppression of the hula-twist non-radiative deactivation pathway. This aim was achieved by simultaneous bonding of a Pd center to the N atom of the heterocycle and the carbon of the arylidene ring. Two different 4-(( )-arylidene)-2-(( )-styryl)-5(4 )-oxazolones, the structures of which are closely related to the chromophore of the Kaede protein and substituted at the 2- and 4-positions of the arylidene ring ( OMe; F), were used as starting materials. Oxazolones and were reacted with Pd(OAc) to give the corresponding dinuclear orthometalated palladium derivates and by regioselective C-H activation of the -position of the arylidene ring. Reaction of ( ) with LiCl promoted the metathesis of the bridging carboxylate by chloride ligands to afford dinuclear ( ). Mononuclear complexes containing the orthopalladated oxazolone and a variety of ancillary ligands (acetylacetonate ( , ), hydroxyquinolinate ( ), aminoquinoline ( ), bipyridine ( ), phenanthroline ( )) were prepared from or through metathesis of anionic ligands or substitution of neutral weakly bonded ligands. All species were fully characterized and the X-ray determination of the molecular structure of was carried out. This structure has strongly distorted ligands due to intramolecular interactions. Fluorescence measurements showed an increase in the quantum yield (QY) by up to one order of magnitude on comparing the free oxazolone (QY < 1%) with the palladated oxazolone (QY = 12% for ). This fact shows that the coordination of the oxazolone to the palladium efficiently suppresses the hula-twist deactivation pathway.