Establishing the Hydride Donor Abilities of Main Group Hydrides

Interest in reductions with main group hydrides has been reinvigorated with the discovery of frustrated Lewis pairs. Computational analysis showed that the borohydride of the commonly used Lewis acid B(C6F5)3 was determined to be 15 kcal/mol less reducing than borohydride ([BH4]−), 22 kcal/mol less reducing than aluminum hydride ([AlH4]−), and 41 kcal/mol less reducing than superhydride ([HBEt3]−). In addition to [HB(C6F5)3]−, a hydride donor ability scale with an estimated error of ∼3 kcal/mol includes 132 main group hydrides with gradually changing reducing capabilities spanning 160 kcal/mol. The scale includes representatives from organosilanes, organogermanes, organostannanes, borohydrides, boranes, aluminum hydrides, NADH analogues, and CH hydride donors. The large variety of reducing agents and the wide span of the scale (ranging from 0.5 to 160 kcal/mol in acetonitrile) make the scale a useful tool for the future design of metal-based or main group reducing agents.