Structures and Dynamic Solution Behavior of Cationic, Two-Coordinate Gold(I)-p-Allene Complexes

A family of seven cationic gold complexes that contain both an alkyl substituted p-allene ligand and an electron-rich, sterically hindered supporting ligand was isolated in >90?% yield and characterized by spectroscopy and, in three cases, by X-ray crystallography. Solution-phase and solid-state analysis of these complexes established preferential binding of gold to the less substituted C?C bond of the allene and to the allene p face trans to the substituent on the uncomplexed allenyl C?C bond. Kinetic analysis of intermolecular allene exchange established two-term rate laws of the form rate=k1[complex]+k2[complex][allene] consistent with allene-independent and allene-dependent exchange pathways with energy barriers of ?G?1=17.418.8 and ?G?2=15.217.6 kcal?mol-1, respectively. Variable temperature (VT) NMR analysis revealed fluxional behavior consistent with facile (?G?=8.911.4 kcal?mol-1) intramolecular exchange of the allene p faces through ?1-allene transition states and/or intermediates that retain a staggered arrangement of the allene substituents. VT NMR/spin saturation transfer analysis of [{P(tBu)2o-binaphthyl}Au(?2-4,5-nonadiene) ]+SbF6- (5), which contains elements of chirality in both the phosphine and allene ligands, revealed no epimerization of the allene ligand below the threshold for intermolecular allene exchange (?G?298K=17.4 kcal?mol-1), which ruled out the participation of a ?1-allylic cation species in the low-energy p-face exchange process for this complex.