Catalytic hydrogenation of olefins over 1:1 and 1:2 electron donor-acceptor complexes of polynuclear aromatic hydrocarbons with alkali metals

Abstract

Hydrogenation of olefins like ethylene and propylene proceeds at temperatures between –20 and 180°C over stoichiometric (1:1 and 1:2) electron donor-acceptor (EDA) complexes of various polynuclear aromatic hydrocarbons, such as pyrene, tetracene and violanthrene, with alkali metals. Higher catalytic activities for olefin hydrogenation are obtained over complex films of the aromatic hydrocarbons which have the smaller “ortho-bond-localization energies” with potassium, rubidium or caesium as a strong donor. Kinetic and spectroscopic measurements show that olefin hydrogenation takes place via direct hydrogen transfer from the ortho-dihydro complexes of the corresponding aromatic acceptors, which were formed during hydrogen adsorption, to the carbon-carbon double bond of the olefin by the reaction scheme: A^2–2M⁺+ H₂→ AH^2–₂2M⁺(1), [graphic omitted] (2) where A denotes the aromatic acceptor and M the alkali metal.

Microwave spectroscopic data suggest that the propylene-deuterium hydrogen exchange reaction by which but-1-ene isomerizes preferentially to cis-but-2-ene (cis/trans-but-2-ene = 5–10 at 120–180°C), proceed via the π-allyl adsorbed species over EDA complex films such as tetracene^2–(caesium)⁺₂.