Watching Hydrogens Migrate: Step by Step from [Re(η-CH)] to [Re(η-CH)(η-CH)(NCCH)].

Arene substitution reactions in [M(η-arene)] are well documented for Groups 6 and 8 but are essentially unknown for the manganese triad. Aiming to replace benzene in [Re(η-CH)], we altered the hapticity of one coordinated benzene, which we found to be tunable stepwise from an η to an η-allyl coordination mode. Reduction of [Re(η-CH)] with hydrides gives [Re(η-CH)(η-CH)]. Subsequent addition of acid yields [ReH(η-CH)(η-CH)], which converts to [Re(η-CH)(η-CH)NCCH] in acetonitrile. Further protonation gives the title complex [Re(η-CH)(η-CH)(NCCH)] 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(η-CH)} is unprecedented. Direct arene exchange from [Re(η-CH)] to [Re(η-arene)(η-CH)] was found only under strongly acidic conditions in neat arene. The analogous chemistry of the lighter homologue technetium (Tc) is distinctly different. Treatment of [Tc(η-CH)(η-CH)] with acid in acetonitrile yields only mixtures of [Tc(η-CH)] and [Tc(NCCH)].