N-Heterocyclic Carbenes and Charge Separation in Heterometallic s-Block Silylamides

Addition of the N-heterocyclic carbene (NHC), 1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene (IPr), to equimolar quantities of group 1 and group 2 bis(trimethylsilyl)-amides results in the isolation of charge separated species, [M(IPr2](+)[M'{N(SiMe3)(2)}3](-) (M = Li, Na, K; M' = Mg, Ca, Sr, Ba). Although these systems were found to be prone to the separation of oily, most likely liquid clathrate, materials, either slow cooling or careful diffusion of the less polar solvent hexane into toluene solutions yielded nine crystalline heterobimetallic complexes in which the coordination sphere of the cationic group 1 center was found by X-ray diffraction analysis to be provided by two IPr ligands. These derivatives are the first examples of any compounds in which coordination at the central alkali metal cation is provided exclusively by NHC ligands and, for the cases where M = Na, are the first instances of any type in which an NHC ligand is bound to sodium. The anionic group 2-containing component of each compound was found to comprise three bis(trimethylsilyl)amido ligands coordinated in an approximately trigonal array about the divalent metal center. The bonding within the unusual cationic components of the compounds has been investigated by density functional theoretical (DFT) methods. Natural Bond Orbital (NBO) analyses have revealed that the coordination is provided by donation of the sp-hydridized IPr lone pair into the valence s-orbital of the alkali metal cation and are consistent with weaker binding, and consequently more labile solution behavior, as group 1 is descended.