A Remarkable Isostructural Homologous Series of Mixed Lithium−Heavier Alkali Metal <i>tert</i>-Butoxides [(<i>t</i>-BuO)₈Li₄M₄] (M = Na, K, Rb or Cs)

Three new mixed lithium−heavier alkali metal tert-butoxides [(t-BuO)8Li4M4] (M = Na, Rb, Cs) are reported which, added to the previously discovered potassium analogue [(t-BuO)8Li4K4], complete the homologous series. Remarkably, X-ray crystallographic studies reveal that all four heterometallic compounds adopt a common structure. This sixteen-vertex O8Li4M4 “breastplate” motif is built around novel (M+)4 planes (M = Na, K, Rb, Cs), both faces of which support chelating (O4Li2)2- dianions. Each such dianion is positioned approximately normal with respect to the other. Bonding within the breastplate structure involves a combination of μ3-Li, μ4-M, μ3-O, and μ4-O centers. Ab initio MO calculations on model systems predict that formation of the heterometallic breastplate from the exclusively μ3-bonded frameworks of its component homometallic structures is a favorable exothermic process. Best regarded as an inherently stable contacted triple ion sandwich comprising a dianion−tetramonocation−dianion arrangement, the breastplate motif is likely to be more widely applicable within heterometallic structural chemistry than so far recognized. This point is discussed with reference to a previously documented series of heterometallic p-block metal-based imide structures of general formula [(CyN)8M14M24], where M1 = Sb or As and M2 = Ag, Cu, or Na.