Mixed Aggregates of Alkali Metal Compounds:  Structure and Stability of “Superbase” Models

This ab initio study of the structure and stability of mixed alkali metal dimers models the transmetalation reactions which may lead to “superbasic reagents”. Such reagents are mixtures of RLi and MOR‘ (M = Na−Cs) and show strongly enhanced metalating power. The possible existence of mixed dimers RM/LiOR‘ (M = Li−Cs, R = H, CH3, NH2, OH, F, and R‘ = H) has been evaluated at reasonably high levels of ab initio theory by MP2 calculations, using the pseudopotential method for the heavier alkali metals (K−Cs) and the 6-31++G** basis set for all other elements. Structures and energies of dimeric aggregates resulting from possible transmetalation reactions (metal exchange, anion exchange, anion and cation exchange and complete transmetalation) are calculated and compared. In almost every case, mixed aggregates are more stable than other possible combinations. The driving force favoring such mixed aggregates is the formation of stronger Li bonds and the reduction of electrostatic metal−metal repulsion. Additional BECKE3LYP calculations on the (MOH)2/(LiCH3)2 (M = Na, K) equilibria using the 6-311+G* basis sets for H, C, O, Li, and Na, as well as pseudopotentials for K were performed on the RHF optimized geometries. One O(CH3)2 molecule on each of the metals decreases the differences in the relative stabilities of mixed aggregates and transmetalated compounds, relative to the unsolvated equilibria.