Alkali metal reduction of alkali metal cations

Counter to synthetic convention and expectation provided by the relevant standard reduction potentials, the chloroberyllate, [{SiN^Dipp}BeClLi]₂ [{SiN^Dipp} = {CH₂SiMe₂N(Dipp)}₂; Dipp = 2,6-i-Pr₂C₆H₃)], reacts with the group 1 elements (M = Na, K, Rb, Cs) to provide the respective heavier alkali metal analogues, [{SiN^Dipp}BeClM]₂, through selective reduction of the Li⁺ cation. Whereas only [{SiN^Dipp}BeClRb]₂ is amenable to reduction by potassium to its nearest lighter congener, these species may also be sequentially interconverted by treatment of [{SiN^Dipp}BeClM]₂ by the successively heavier group 1 metal. A theoretical analysis combining density functional theory (DFT) with elemental thermochemistry is used to rationalise these observations, where consideration of the relevant enthalpies of atomisation of each alkali metal in its bulk metallic form proved crucial in accounting for experimental observations.