Coordination–Insertion of Norbornene at Gold: A Mechanistic Study

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.