title: Catalyst activity or stability: the dilemma in Pd-catalyzed polyketone synthesis.
abstract: A series of Pd-complexes containing nonsymmetrical bis(aryl-imino)acenaphthene (Ar-BIAN) ligands, characterized by substituents on the meta positions of the aryl rings, have been synthesized, characterized and applied in CO/vinyl arene copolymerization reactions. Crystal structures of two neutral Pd-complexes have been solved allowing comparison of the bonding properties of the ligand. Kinetic and mechanistic investigations on these complexes have been performed. The kinetic investigations indicate that in general ligands with electron-withdrawing substituents give more active, but less stable, catalytic systems, although steric effects also play a role. The good performance observed with nonsymmetrical ligands is at least in part due to a compromise between catalyst activity and lifetime, leading to a higher overall productivity with respect to catalysts based on their symmetrical counterparts. Additionally, careful analysis of the reaction profiles provided information on the catalyst deactivation pathway. The latter begins with the reduction of a Pd(II) Ar-BIAN complex to the corresponding Pd(0) species, a reaction that can be reverted by the action of benzoquinone. Then the ligand is lost, a process that appears to be facilitated by the contemporary coordination of an olefin or a CO molecule. The so formed Pd(0) complex immediately reacts with another molecule of the initial Pd(II) complex to give a Pd(I) dimeric species that irreversibly evolves to metallic palladium. Mechanistic investigations performed on the complex with a nonsymmetrical Ar-BIAN probe evidence that the detected intermediates are characterized by the Pd-C bond trans to the Pd-N bond of the aryl ring bearing electron-withdrawing substituents. In addition, the intermediate resulting from the insertion of 4-methylstyrene into the Pd-acyl bond is a five-member palladacycle and not the open-chain η(3)-allylic species observed for complexes with Ar-BIANs substituted in ortho position.
authors: Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Trieste, Via Licio Giorgieri 1, 34127 Trieste, Italy. milaniba@units.it.
authors: Francesco
authors: F
authors: Amoroso
authors: Ennio
authors: E
authors: Zangrando
authors: Carla
authors: C
authors: Carfagna
authors: Christian
authors: C
authors: Müller
authors: Dieter
authors: D
authors: Vogt
authors: Mohamed
authors: M
authors: Hagar
authors: Fabio
authors: F
authors: Ragaini
authors: Barbara
authors: B
authors: Milani
mesh_terms: D000073:Acenaphthenes / Q000737:chemistry*; D002248:Carbon Monoxide / Q000737:chemistry; D002384:Catalysis; D007659:Ketones / Q000138:chemical synthesis* / Q000737:chemistry; D007700:Kinetics; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D010165:Palladium / Q000737:chemistry*; D011108:Polymers / Q000138:chemical synthesis* / Q000737:chemistry
chemical_list: D000073:Acenaphthenes; D007659:Ketones; D009942:Organometallic Compounds; D011108:Polymers; D010165:Palladium; D002248:Carbon Monoxide