Alkali metal–1-azaallyl complexes: X-ray crystallographic, NMR spectroscopic and ab initio calculational studies

Abstract

A series of alkali metal–1-azaallyl complexes, [{CH₃CH₂CH₂C(H)C(Bu^t)N(H)Li·HMPA}₂] 1, [{CH₃CH₂CH₂C(H)C(Bu^t)N(H)Na·2HMPA}₂] 2 and [{CH₂C(Bu^t)N(H)Li·HMPA}₂] 3, has been synthesised by treating each appropriate metal alkyl reagent (n-butyllithium, n-butylsodium or methyllithium, respectively) with tert-butyl cyanide in the presence of the Lewis base HMPA [hexamethylphosphoramide, (Me₂N)₃PO]. X-Ray crystallographic studies have established that each structure is dimeric and built around a precisely or approximately centrosymmetric rhomboidal (N–M)₂ ring. However, the nature of the azaallyl–metal bonding differs with 1 and 2 displaying a terminal η¹-N arrangement, while 3 displays a chelating η³-NCC arrangement. ¹H and ¹³C NMR spectroscopic studies suggest that these distinct bonding modes are retained in [²H₈]toluene solution. Long-range (⁴J) “W” coupling (2.4 Hz) is observed for 3 between the NH and one of the α-CH₂ protons, consistent with the trans orientation of the NH and CC linkages seen in the solid state. The preference for this geometry is confirmed by ab initio MO calculations on models of 3, which examine the energetics of the ketimide–azaallyl isomerism involved in the formation of 1–3.