Thermal Decomposition of Alkyl Halides on Aluminum. 2. The Formation and Thermal Decomposition of Surface Metallacycles Derived from the Dissociative Chemisorption of Dihaloalkanes

Surface metallacycles are formed on aluminum single-crystal substrates when dihaloalkanes of the general structure RCHX(CH₂)_nCH₂X (R = Me, H; n = 1, 2; X = Br, I) are dissociatively adsorbed in ultrahigh vacuum. All members of this class thermally decompose to evolve hydrocarbon products between 450 and 550 K. The energetics of the reaction pathways are similar to those reported in the preceding paper for β-carbon-hydrogen bond activation on Al surfaces. The C₃ metallacycles derived from 1,3-dihalopropanes yield propylene, while the C₄ species derived from the corresponding 1,4-dihalobutanes yield predominantly butadiene. The metallacycle derived from 1,3-dibromobutane partitions selectively along the olefin yielding reaction channel. A unified mechanism consistent with isotopic labeling experiments and surface vibrational spectroscopy studies is presented along with a discussion of the nature and structure of the intermediates involved.