Watching Hydrogens Migrate: Step by Step from [Re^I(η⁶‑C₆H₆)₂]⁺ to [Re^III(η³‑C₆H₉)(η⁶‑C₆H₆)(NCCH₃)₂]²⁺

Abstract

Arene substitution\nreactions in [M­(η⁶-arene)₂]^0/2+ are well documented for Groups 6 and 8 but\nare essentially unknown for the manganese triad. Aiming to replace\nbenzene in [Re^I(η⁶-C₆H₆)₂]⁺, we altered the hapticity of one\ncoordinated benzene, which we found to be tunable stepwise from an\nη⁶ to an η³-allyl coordination mode.\nReduction of [Re^I(η⁶-C₆H₆)₂]⁺ with hydrides gives [Re^I(η⁵-C₆H₇)­(η⁶-C₆H₆)]. Subsequent addition of acid yields\n[Re^IIIH­(η⁵-C₆H₇)­(η⁶-C₆H₆)]⁺, which converts\nto [Re^I(η⁴-C₆H₈)­(η⁶-C₆H₆)­NCCH₃]⁺ in\nacetonitrile. Further protonation gives the title complex [Re^III(η³-C₆H₉)­(η⁶-C₆H₆)­(NCCH₃)₂]²⁺ by a rhenium-mediated, intramolecular hydride shift.\nHerein, we present a full mechanistic elucidation of these transformations\nbased on NMR studies, isolation of reaction intermediates, and their\nfull characterizations. The structural feature {Re^III(η⁶-C₆H₆)} is unprecedented. Direct arene\nexchange from [Re^I(η⁶-C₆H₆)₂]⁺ to [Re^I(η⁶-arene)­(η⁶-C₆H₆)]⁺ was found only under strongly acidic conditions in neat arene.\nThe analogous chemistry of the lighter homologue technetium (⁹⁹Tc) is distinctly different. Treatment of [Tc^I(η⁵-C₆H₇)­(η⁶-C₆H₆)] with acid in acetonitrile yields only\nmixtures of [Tc^I(η⁶-C₆H₆)₂]⁺ and [Tc^II(NCCH₃)₆]²⁺.