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.