EPR spectroscopic study of the reaction of aluminium and gallium atoms with acetylenes in a rotating cryostat

Abstract

An electron paramagnetic resonance spectroscopic study of the reaction of ground-state Al atoms with acetylene in cyclohexane and adamantane at 77 K has demonstrated that transη¹-aluminovinyl, AICHCH is the initial paramagnetic product and that it disappears on warming to 100 K. The spectra of cyclohexyl and 2-adamantyl, produced by hydrogen abstraction from the matrix, appear at this temperature. A second aluminium species is formed in adamantane with, magnetic parameters which indicate that it is not the π-complex η²-Al(CHCH) or aluminovinylidene, AlCCH₂, but may be a hydrogen-bridged isomer. Substituted acetylenes give either a trans vinyl, a π-complex, or a polyvinyl while propyne gives a substantial yield of propargyl.

Ga atoms and acetylene do not give an η¹-gallovinyl but instead give a species, Ga(CHCH) with two magnetically equivalent H and C nuclei that is either a η²-, π- or σ-complex. Again in this system a significant yield of cyclohexyl and 2-adamantyl is produced byH-atom abstraction from the matrix.

When the matrix is removed from samples of Al and acetylene an Al-substituted polyacetylene remains but in the case of Ga only the metal is left.