Fluorescent orthopalladated complexes of 4-aryliden-5(4H)-oxazolones from the Kaede protein: Synthesis and characterization

This article belongs to the Special Issue Influencers in Organometallic Chemistry—A Themed Issue Honoring Dr. Michel Pfeffer.The goal of the work reported here was to amplify the fluorescent properties of 4-aryliden-5(4H)-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 ortho carbon of the arylidene ring. Two different 4-((Z)-arylidene)-2-((E)-styryl)-5(4H)-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 (1a OMe; 1b F), were used as starting materials. Oxazolones 1a and 1b were reacted with Pd(OAc)2 to give the corresponding dinuclear orthometalated palladium derivates 2a and 2b by regioselective C–H activation of the ortho-position of the arylidene ring. Reaction of 2a (2b) with LiCl promoted the metathesis of the bridging carboxylate by chloride ligands to afford dinuclear 3a (3b). Mononuclear complexes containing the orthopalladated oxazolone and a variety of ancillary ligands (acetylacetonate (4a, 4b), hydroxyquinolinate (5a), aminoquinoline (6a), bipyridine (7a), phenanthroline (8a)) were prepared from 3a or 3b 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 7a 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 6a). This fact shows that the coordination of the oxazolone to the palladium efficiently suppresses the hula-twist deactivation pathway.This research was funded by the Aragón Government (Spain, Project LMP144_18: Programa Operativo FEDER Aragón 2014–2020, “Construyendo Europa desde Aragón”, and DGA-FSE research group Aminoácidos y Péptidos E19_20R); the Spanish Government (Project No. PID2019-106394GB-100); the European Cooperation in Science and Technology (COST) program under CA15106 grant (CHAOS: CH Activation in Organic Synthesis); the Romanian Ministry of Education and Research through the grant PN-III-P1-1.1-MC-2018-2580; the Ministerio de Economía y Competitividad (Spain) PID2019-104379RB-C21 and DGA-FSE (E07_20R).Peer reviewe