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)

Abstract

Three new mixed lithium−heavier alkali metal tert -butoxides [( t -BuO) 8 Li 4 M 4 ] (M = Na, Rb, Cs) are reported which, added to the previously discovered potassium analogue [( t -BuO) 8 Li 4 K 4 ], complete the homologous series. Remarkably, X-ray crystallographic studies reveal that all four heterometallic compounds adopt a common structure. This sixteen-vertex O 8 Li 4 M 4 “breastplate” motif is built around novel (M + ) 4 planes (M = Na, K, Rb, Cs), both faces of which support chelating (O 4 Li 2 ) 2 - dianions. Each such dianion is positioned approximately normal with respect to the other. Bonding within the breastplate structure involves a combination of μ 3 -Li, μ 4 -M, μ 3 -O, and μ 4 -O centers. Ab initio MO calculations on model systems predict that formation of the heterometallic breastplate from the exclusively μ 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 M 1 4 M 2 4 ], where M 1 = Sb or As and M 2 = Ag, Cu, or Na.