Three Oxidative Addition Routes of Alkali Metal Aluminyls to Dihydridoaluminates and Reactivity with CO2

Abstract

Three distinct routes are reported to the soluble, dihydridoaluminate compounds, AM[Al(NONDipp)(H)2] (AM=Li, Na, K, Rb, Cs; [NONDipp]2−=[O(SiMe2NDipp)2]2−; Dipp=2,6-iPr2C6H3) starting from the alkali metal aluminyls, AM[Al(NONDipp)]. Direct H2 hydrogenation of the heavier analogues (AM=Rb, Cs) produced the first examples of structurally characterized rubidium and caesium dihydridoaluminates, although harsh conditions were required for complete conversion. Using 1,4-cyclohexadiene (1,4-CHD) as an alternative hydrogen source in transfer hydrogenation reactions provided a lower energy pathway to the full series of products for AM=Li−Cs. A further moderation in conditions was noted for the thermal decomposition of the (silyl)(hydrido)aluminates, AM[Al(NONDipp)(H)(SiH2Ph)]. Probing the reaction of Cs[Al(NONDipp)] with 1,4-CHD provided access to a novel inverse sandwich complex, [{Cs(Et2O)}2{Al(NONDipp)(H)}2(C6H6)], containing the 1,4-dialuminated [C6H6]2− dianion and representing the first time that an intermediate in the commonly utilized oxidation process of 1,4-CHD to benzene has been trapped. The synthetic utility of the newly installed Al−H bonds has been demonstrated by their ability to reduce CO2 under mild conditions to form the bis-formate AM[Al(NONDipp)(O2CH)2] compounds, which exhibit a diverse series of eyecatching bimetallacyclic structures.