Watching Hydrogens Migrate: Step by Step from [Re I (η 6 -C 6 H 6 ) 2 ] + to [Re III (η 3 -C 6 H 9 )(η 6 -C 6 H 6 )(NCCH 3 ) 2 ] 2 .

Arene substitution reactions in [M(η 6 -arene) 2 ] 0/2+ are well documented for Groups 6 and 8 but are essentially unknown for the manganese triad. Aiming to replace benzene in [Re I (η 6 -C 6 H 6 ) 2 ] + , we altered the hapticity of one coordinated benzene, which we found to be tunable stepwise from an η 6 to an η 3 -allyl coordination mode. Reduction of [Re I (η 6 -C 6 H 6 ) 2 ] + with hydrides gives [Re I (η 5 -C 6 H 7 )(η 6 -C 6 H 6 )]. Subsequent addition of acid yields [Re III H(η 5 -C 6 H 7 )(η 6 -C 6 H 6 )] + , which converts to [Re I (η 4 -C 6 H 8 )(η 6 -C 6 H 6 )NCCH 3 ] + in acetonitrile. Further protonation gives the title complex [Re III (η 3 -C 6 H 9 )(η 6 -C 6 H 6 )(NCCH 3 ) 2 ] 2+ by a rhenium-mediated, intramolecular hydride shift. Herein, we present a full mechanistic elucidation of these transformations based on NMR studies, isolation of reaction intermediates, and their full characterizations. The structural feature {Re III (η 6 -C 6 H 6 )} is unprecedented. Direct arene exchange from [Re I (η 6 -C 6 H 6 ) 2 ] + to [Re I (η 6 -arene)(η 6 -C 6 H 6 )] + was found only under strongly acidic conditions in neat arene. The analogous chemistry of the lighter homologue technetium ( 99 Tc) is distinctly different. Treatment of [Tc I (η 5 -C 6 H 7 )(η 6 -C 6 H 6 )] with acid in acetonitrile yields only mixtures of [Tc I (η 6 -C 6 H 6 ) 2 ] + and [Tc II (NCCH 3 ) 6 ] 2+ .