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⁺ .