Liberating the Anion: Evaluating Weakly Coordinating Cations
Stephen H. Dempsey, Steven R. Kass
Abstract
A series of commonly used weakly coordinating cations (WCCs) including tetraalkylammonium, bis(triphenylphosphine)iminium (PPN), P2 and P5 phosphazenium, and tetraphenylphosphonium ions were investigated along with five additional tetraarylphosphonium ions [Ar = 4-Me, 4-t-Bu, 3,5-Me2, 4-Me2N, and 3,5-(MeO)2 phenyl derivatives]. Dissociation enthalpies of their chloride complexes were computed in the gas phase and in dichloromethane. These results are compared to infrared spectra of these salts in chloroform-d which provide free and bound C–D stretches, 19F NMR spectra of the corresponding tetrafluoroborate salts, ion-pair equilibrium constants, and reaction rates of the chlorides and acetates with 1-iodooctane. The latter transformation was also carried out under catalytic conditions. Substituted tetraphenylphosphonium ion derivatives are found to be surprisingly good WCCs that function as well or better than the P2 and P5 phosphazenium ions, the current WCC gold standard.