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Structures and Dynamic Solution Behavior of Cationic, Two‐Coordinate Gold(I)–π‐Allene Complexes
Author(s) -
Brown Timothy J.,
Sugie Atsushi,
Leed Marina G. D.,
Widenhoefer Ross A.
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201103289
Subject(s) - allene , chemistry , intramolecular force , ligand (biochemistry) , intermolecular force , cationic polymerization , substituent , crystallography , stereochemistry , photochemistry , molecule , catalysis , organic chemistry , biochemistry , receptor
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 CC bond of the allene and to the allene π face trans to the substituent on the uncomplexed allenyl CC 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 Δ G ≠ 1 =17.4–18.8 and Δ G ≠ 2 =15.2–17.6 kcal mol −1 , respectively. Variable temperature (VT) NMR analysis revealed fluxional behavior consistent with facile (Δ G ≠ =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( t Bu) 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 (Δ G ≠ 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.