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

Authors

  • Timothy J. Brown,

    1. Department of Chemistry, Duke University, French Family Science Center, Durham, NC, 27708 (USA), Fax: (+1) 919-660-1605
    Search for more papers by this author
  • Dr. Atsushi Sugie,

    1. Department of Chemistry, Duke University, French Family Science Center, Durham, NC, 27708 (USA), Fax: (+1) 919-660-1605
    Search for more papers by this author
  • Dr. Marina G. D. Leed,

    1. Department of Chemistry, Duke University, French Family Science Center, Durham, NC, 27708 (USA), Fax: (+1) 919-660-1605
    Search for more papers by this author
  • Prof. Ross A. Widenhoefer

    Corresponding author
    1. Department of Chemistry, Duke University, French Family Science Center, Durham, NC, 27708 (USA), Fax: (+1) 919-660-1605
    • Department of Chemistry, Duke University, French Family Science Center, Durham, NC, 27708 (USA), Fax: (+1) 919-660-1605
    Search for more papers by this author

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[DOUBLE BOND]C bond of the allene and to the allene π face trans to the substituent on the uncomplexed allenyl C[DOUBLE BOND]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 ΔG1=17.4–18.8 and ΔG2=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 η1-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(η2-4,5-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 (ΔG298K=17.4 kcal mol−1), which ruled out the participation of a η1-allylic cation species in the low-energy π-face exchange process for this complex.

Ancillary