Structures and Dynamic Solution Behavior of Cationic, Two‐Coordinate Gold(I)–π‐Allene Complexes

Abstract

A family of seven cationic gold complexes that contain both an alkyl substituted π-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 π 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=k(1)[complex]+k(2)[complex][allene] consistent with allene-independent and allene-dependent exchange pathways with energy barriers of ΔG(≠)(1)=17.4-18.8 and ΔG(≠)(2)=15.2-17.6 kcal mol(-1), respectively. Variable temperature (VT) NMR analysis revealed fluxional behavior consistent with facile (ΔG(≠)=8.9-11.4 kcal mol(-1)) intramolecular exchange of the allene π faces through η(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)(2)o-binaphthyl}Au(η(2)-4,5-nonadiene)](+)SbF(6)(-) (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 (ΔG(≠)(298K)=17.4 kcal mol(-1)), which ruled out the participation of a η(1)-allylic cation species in the low-energy π-face exchange process for this complex.

Intriguing mechanistic labyrinths in gold(i) catalysis

“…Gold( i ) forms stable monomeric two coordinate π-complexes with alkenes, 20–22 1,3-dienes, 23 allenes, 24 and substituted alkynes. 19a,25,26 Variations of the bond lengths as well as the ligand–metal-substrate angle suggest that the alkene orientation is controlled largely by steric factors.…”

Supra-molecular synthons based on gold…π(arene) interactions

“…4a, have been developed [27], as such species are crucial in many catalytic applications of gold [28]. X-ray crystal structure determination shows the π-allene ligand to be coordinated at the least congested side (Fig.…”

Mechanistic insights and functionalization of alkynes in homogeneous gold catalysis.

“…As expected, hydrogen bond acceptors with lower gold affinity (e.g., DMPU, DMF) [141][142][143][144][145][146][147][148] enhanced the rate of reaction (Scheme 14b), and ionic hydrogen bond acceptors (e.g., Bu4N Other research groups reported that certain compounds improved the chemical yields in selected gold catalyzed reactions. 153,154 One notable example is the …”