Exploiting sigma/pi coordination isomerism to prepare homologous organoalkali metal (Li, Na, K) monomers with identical ligand sets

Tetraamine Me6TREN has been used as a scaffold support to provide coordinative saturation in the complexes PhCH2M center dot Me6TREN (M = Li, Na, K). The Li derivative displays a Li-C sigma interaction with a pyramidalized CH2 both in the solid state and in solution, and represents the first example of eta(4) coordination of Me6TREN to lithium. In the sodium derivative, the metal cation slips slightly towards the delocalized pi electrons whilst maintaining a partial sigma interaction with the CH2 group. For the potassium case, coordinative saturation successfully yields the first monomeric benzylpotassium complex, in which the anion binds to the metal cation exclusively through its delocalized pi system resulting in a planar CH2 group.