Anion-Cation-Anion Ion Triplet Characterization by Computation and Photoelectron Spectroscopy

Ion triplets of the chloride salts of two commonly used weakly coordinating cations are reported (i.e., Cl^-·NMe₄⁺Cl^- (1) and Cl^-·PPh₄⁺Cl^- (2)). Negative ion photoelectron spectra at 20 K afford vertical and adiabatic detachment energies of 5.18 and 5.00 eV (1) and 5.03 and 4.70 eV (2), respectively. These results are well reproduced by coupled cluster calculations with single, double, and perturbative triple excitations (CCSD(T)) whereas M06-2X is systematically too small by ∼0.3 eV (i.e., 7 kcal mol^-1). The structures of both 1 and 2 have five or six C-H···Cl^- interactions that stabilize these cluster anions by 32 (1) and 27 (2) kcal mol^-1 as given by their chloride dissociation enthalpies. These values drop to 7.4 and 3.8 kcal mol^-1 in dichloromethane based up conductor-like polarizable continuum model calculations and suggest that X^-·M⁺X^- ion triplets with a weakly coordinating cation maybe the reactive form of salts under some conditions.