Alkali metal salts of dianions: A theoretical and experimental study of (C₆H₄)^2-M⁺ (M = Li and Na)

Ab initio molecular orbital calculations have been carried out on the lithium and sodium salts of doubly deprotonated benzene. Full geometry optimizations of the three isomeric bare dianions and their corresponding lithium and sodium adducts were performed at the HF/6-31+G(d) and MP2/6- 311+G(d,p) levels. The resulting structures are discussed. Deprotonation energies for phenyllithium and phenylsodium are reported. Phenyllithium is more acidic than benzene, while phenylsodium is only 3 kcal/mol less acidic. Alkali metal bridging and the formation of ion triplets can be extremely stabilizing and can lead to enhanced second deprotonations. Electrospray ionization mass spectrometry of terephthalic acid (p-C₆H₄(CO₂H)₂) followed by collision-induced dissociation of the resulting alkali metal salts of the dicarboxylate (I) affords C₆H₄^2-Li⁺ and C₆H₄^2-Na⁺. Formation of these species corroborates the theoretical prediction that alkali metal salts of dianions should be stable in the gas phase.