Synthesis and properties of some cyclohexadienylrhenium polyhydride complexes

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Abstract
The heptahydride [ReH7(PPh3)2](1) reacts in the presence of 3,3-dimethylbutene with benzene, toluene, p-xylene, and mesitylene to give dihydridocyclohexadienyl complexes. Treatment of these complexes with acid afforded the corresponding trihydridocyclohexadienyl cations, which were isolated and characterised in the case of the benzene and mesitylene derivatives. The cyclohexadienyl complexes prepared all exhibited fluxionality. The most stable conformations of these complexes and the relative energy barriers for cyclohexadienyl-ligand rotation in the neutral complexes were explained by steric interactions between the ring substituents and the bulky phosphine ligands, as was the selectivity of their formation from the heptahydride (1). The reversible migration of a hydrogen atom between the metal and the cyclohexadienyl ligand implied in the observed isomerisation of the latter was detected using spin-saturation-transfer n.m.r. experiments.

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