Structure of propene in heterogeneous catalysis: II. Isomerization of propene by atomic nickel and magnesium

Abstract

Unstable π-propene-metal complexes are formed when excess propene is co-condensed with atomic nickel or magnesium. During the formation and decomposition of these complexes, the ligand undergoes a double-bond isomerization by a 1,3-hydrogen shift. Infrared and mass spectroscopic data from CD 3CH CH 2/metal complexes show metal-induced, sequential, intramolecular, hydrogen exchange reactions: CD 3CHCH 2 → CH 2DCHCD 2 → cis-CHD 22CHCHD → trans-CHD 2CHCHD. No π-allyl species is observed during these reactions. The low decomposition temperatures of π-propene-nickel and -magnesium complexes (both about −140 °C) and the low isomerization activation energies (less than ∼0.5 kcal/mol) suggest direct formation of isomerization products from the π-propenes without the intervention of any allyl intermediates. The π-propene may also be the only surface species necessary to account for the products in isomerization, hydrogen exchange and hydrogenation reactions of propene on zinc oxide catalyst.