A remarkable isostructural homologous series of mixed lithium-heavier alkali metal tert-butoxides [(t-BuO)(8)Li4M4] (M = Na, K, Rb or Cs)

Three new mixed lithium-heavier alkali metal tert-butoxides [(t-BuO)(8)Li4M4] (M = Na, Rb, Cs) are reported which, added to the previously discovered potassium analogue [(t-BuO)(8)Li4K(4)], 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 bees 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 mu (3)-Li. mu (4)-M, mu (3)-O, and mu (4)-O centers. Ab initio MO calculations on model systems predict that formation of the heterometallic breastplate from the exclusively mu (3)-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)(8)(M4M42)-M-1], where M-1 = Sb or As and M-2 = Ag, Cu, or Na.