Amides and related nitrogen derivatives

One of the first solvent-free alkali metal amides to be investigated was potassium diethylamide, 149 [122]. In the solid state, rings of (KNEt2)2 units form a complicated network. An interesting ladder strueture containing amide bridges and n-coordination was found for sodium 2,3,4,5-tetramethylpyrrolide, 150 [123]. 

A remarkable structural detail found in [KNPr 2(tmeda)]2, 102, should be mentioned [92b]. In the crystal the compound forms dimers containing a central four-membered K2N2 ring in which the potassium ions are primarily coordinated by four N atoms. The overall coordination number around potassium is, however, higher than 4, because the structure also includes several secondary intramolecular 

K- H(C) interactions involving the methyl groups of the diisopropylamido ligand. This type of interaction has been compared with those found in the alkali metal hypersilanides [MSi(SiMe3)3] , 73-75 [66], 

Self-organization as a result of the formation of amide-bridged dimers is not limited to rfiorganoamides of the alkali metals. Occasionally alkah metal derivatives of primary amines have been shown to be dimeric in the solid state. Structurally characterized examples are [LiNHPh(thf)2]2, 79a [77a], [LiNHC6F5(thf)2]2, 79b, and [K NHC6H2(CF3)3-2,4,6 (thf)3]2, 105 [77b]. These three examples nicely illustrate how the composition of the solvates depends on the size of the alkali metal ion. In 79a and 79b two additional THF ligands are sufficient to saturate the coordination sphere of lithium (coordination number 4), whereas the potassium ions in 105 are coordinated by three THF ligands and by two weak K- - F interactions. 

Another special example of self-assembly to give a dimer is the lithiation product of a diaza crown ether, 82. In the product the two lithium ions are bridged by the 

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