Crystal Structure of a Carbanion-Amide Combination with Lithium and Sodium Cations.
In analogy with RLi/MOR' (M = Na, K, Rb, Cs) superbases, metal exchange in mixts. of (RLi)2 and (MNR'2)2 to give (RM)2 and (LiNR'2)2 is exothermic. A small energy difference between complete metal exchange and the formation of mixed aggregates, RM/LiNR'2, suggests a significant role of the latter. The authors prepd. a mixed Li/Na carbanion-amide I.tmeda and studied the structure by single-crystal x-ray diffraction (space group P21/c, Z = 8). The structure of this Li/Na carbanion-amide complex can be described as a tetrameric cluster from units in which Li and Na cations bridge the carbanion and amide functionalities. The carbanion-amide bonding interactions with the Li cations are of major importance, whereas the Na cations are involved in weaker bonding. The different influence of amide anions and alkoxide anions on the structure of mixed aggregates provoked a comparative study on the superbasic properties of such systems.
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