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 eac h appropriate metal alkyl reagent (n-butyllithium, n-butylsodium or me thyllithium, respectively) with tert-butyl cyanide in the presence of the Lewis base HMPA [hexamethylphosphoramide, (Me2N)(3)P=O]. X-Ray cry stallographic studies have established that each structure is dimeric and built around a precisely or approximately centrosymmetric rhomboid al (N-M)(2) ring. However, the nature of the azaallyl-metal bonding di ffers 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 spectro scopic studies suggest that these distinct bonding modes an retained i n [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, con sistent with the trans orientation of the NH and Cchemical anionC link ages seen in the solid state. The preference for this geometry is conf irmed by ab initio MO calculations on models of 3, which examine the e nergetics of the ketimide-azaallyl isomerism involved in the formation of 1-3.