MIXED AGGREGATES OF ALKALI-METAL COMPOUNDS - STRUCTURE AND STABILITY OF SUPERBASE MODELS

This ab initio study of the structure and stability of mixed alkali me tal dimers models the transmetalation reactions which may lead to ''su perbasic reagents''. Such reagents are mixtures of RLi and MOR' (M = N a-Cs) and show strongly enhanced metalating power. The possible existe nce 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 theor y by MP2 calculations, using the pseudopotential method for the heavie r alkali metals (K-Cs) and the 6-31++G* basis set for all other eleme nts. Structures and energies of dimeric aggregates resulting from poss ible transmetalation reactions (metal exchange, anion exchange, anion and cation exchange and complete transmetalation) are calculated and c ompared. In almost every case, mixed aggregates are more stable than o ther possible combinations. The driving force favoring such mixed aggr egates is the formation of stronger Li bonds and the reduction of elec trostatic 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 e ach of the metals decreases the differences in the relative stabilitie s of mixed aggregates and transmetalated compounds, relative to the un solvated equilibria.