Alkali-metal guanidinates: solid-state structural diversity in solvent-free derivatives

Reaction of 1,3-dicyclohexylcarbodiimide with alkali-metal amides, MN(SiMe3)(2) (M = Li, Na or K), in non-co-ordinating solvents yielded the guanidinate salts {M[CyNC(N(SiMe3)(2))NCy]}(x) (M = Li, x = 2; Na, x = 3; K, x = 2) in high yield. All three species were characterized by X-ray crystallography and found to be free from interaction with the solvent. These compounds represent the first base-free examples of alkali-metal guanidinate or amidinate species to be examined by X-ray crystallography. Although the structures of the lithium and potassium salts have precedent in amidinate and diazaallyl chemistry, the sodium salt represents a new mode of aggregation for species containing an anionic N-C-N linkage.