Synthesis of some (η-arene)dihydridorhenium cations and their reactions with LiAlH4 and LiAlD4

Abstract

A series of cations [Re(η-arene) H₂(PPh₃)₂]⁺(arene = benzene, toluene, p-xylene, or mesitylene) was prepared from the corresponding neutral dihydridocyclohexadienyl complexes by treatment with CPh₃BF₄. These cations reacted with LiAlH₄ and LiAlD₄ to form dihydridocyclohexadienyl complexes. The unusual selectivity of the hydride attack is explained by steric interactions between the ring methyl groups and the bulky phosphine ligands. The initial product of the reaction of the η-mesitylene cation with LiAlD₄ allows the isomerisation mechanism for the cyclohexadienyl complexes to be clarified.