Homogeneous hydrogen absorption and hydrogen exchange reactions by 1:1 and 1:2 electron donor–acceptor complexes of various aromatic hydrocarbons with alkali metals

Abstract

The mechanism of hydrogen absorption and hydrogen exchange reaction (D₂–HZ and H₂–D₂) on the stoichiometric ion salts of various aromatic hydrocarbons with alkali metals (A^–M⁺ and A^2–2M⁺; M = Li, Na, K, Rb and Cs) has been studied in homogeneous systems between 0 and 50°C. Markedly different behaviours of hydrogen activation, such as hydrogen absorption and the reversible hydrogen exchange reactions, were examined in relation to the electronic properties of aromatic mono- and di-anions and also to the nature of the alkali metal cation. Hydrogen absorption took place preferentially on the complexes of aromatic anions with Li and Na. On the other hand, the reversible hydrogen exchange reaction occurred without hydrogen absorption on those complexes with K, Rb and Cs. The relative activities of the D₂–HZ exchange reaction by 1:1 and 1:2 aromatic hydrocarbon-potassium electron donor-acceptor complexes were dependent linearly upon the “one-electron localization energy”(–E+½L_π) of the corresponding aromatic mono- and dianions. Solvent effects on the reactivity of the aromatic ion salt solutions is examined using various polyethers as polar solvents.