Coordination-Insertion of Norbornene at Gold: A Mechanistic Study
Résumé
A detailed experimental/theoretical study of the migratory insertion of norbornene into gold(III)-carbon bonds of discrete (P,C)-cyclometalated gold(III) alkyl complexes is reported. The generation of cationic gold(III) methyl complex 2 by methide abstraction with B(C6F5)3 has been confirmed spectroscopically at low temperature. The DFT-optimized structure of this highly electrophilic species indicates a three-coordinate gold(III) species with T-shaped geometry. In the presence of norbornene, the corresponding gold(III)-olefin complex 3 is formed, and it has been fully characterized by NMR spectroscopy at low temperature. Bonding analysis of this key intermediate confirmed the significantly weaker contribution of metal-to-olefin back-donation with gold compared with the other late transition metals. Upon warm-up, the π complex 3 evolves by insertion of the olefin into the Au(III)-Me bond to form the cationic (P,C)gold(III)-norbornyl complex 4. The mechanism of the double addition has been extensively explored by DFT calculations, and the results strongly support a two-step coordination-migratory insertion pathway. The influence of the unsymmetrical (P,C) bidentate ligand on the kinetic and thermodynamic parameters of the reaction is also discussed. This comprehensive organometallic study provides new important insights into the coordination, structure, and reactivity of gold(III) complexes. It highlights some fundamental similarities but also points out some significant differences with isolobal Pd(II) alkyl complexes. © 2016 American Chemical Society.
Mots clés
Bidentate ligands
Carbon
Coordination reactions
Electrophilic species
gold
Gold compounds
Kinetic and thermodynamic parameters
Late transition metals
Ligands
Mechanistic studies
Migratory insertion
Nuclear magnetic resonance spectroscopy
Olefins
Optimized structures
Organometallics
Palladium
Palladium compounds
Temperature
Transition metals
T-shaped geometry