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

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)(3)P=O]. 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 eta(1)-N arrangement, while 3 displays a chelating eta(3)-NCC arrangement. H-1 and C-13 NMR spectroscopic studies suggest that these distinct bonding modes an retained in [H-2(8)]toluene solution. Long-range ((4)J) "W" coupling (2.4 Hz) is observed for 3 between the NH and one of the alpha-CH2 protons, consistent with the trans orientation of the NH and Cchemical anionC 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.