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^- (<b>1</b>^<b>-</b>) and Cl^-·PPh₄⁺Cl^- (<b>2</b>^<b>-</b>)). Negative ion photoelectron spectra at 20 K afford vertical and adiabatic detachment energies of 5.18 and 5.00 eV (<b>1</b>^<b>-</b>) and 5.03 and 4.70 eV (<b>2</b>^<b>-</b>), 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 <b>1</b>^<b>-</b> and <b>2</b>^<b>-</b> have five or six C-H···Cl^- interactions that stabilize these cluster anions by 32 (<b>1</b>^<b>-</b>) and 27 (<b>2</b>^<b>-</b>) 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.