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Author:    date: 2019-03-01

Hydrogen uptake and release in arene–cycloalkanepairs provide an attractive opportunity for on-board and offboard hydrogen storage. However, the efficiency of arene-cycloalkane pairs currently is limited by unfavorable thermodynamics for hydrogen release. It is shown here that thethermodynamics can be optimized by replacement of H in the-OH group of cyclohexanol and phenol with alkali or alkalineearth metals. The enthalpy change upon dehydrogenationdecreases substantially, which correlates with the delocalization of the oxygen electron to the benzene ring in phenoxides.Theoretical calculations reveal that replacement of H witha metal leads to a reduction of the HOMO-LUMO energy gapand elongation of the C-H bond in theasite in cyclohexanolate, which indicates that the cyclohexanol is activatedupon metal substitution. The experimental results demonstratethat sodium phenoxide–cyclohexanolate, an air- and waterstable pair, can desorb hydrogen at ca. 413 K and 373 K in thesolid form and in an aqueous solution, respectively. Hydrogenation, on the other hand, is accomplished at temperatures aslow as 303 K.

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