Redox‐Switchable Ring‐Closing Metathesis: Catalyst Design,Synthesis, and Study

High yielding syntheses of 1‐(ferrocenylmethyl)‐3‐mesitylimidazolium iodide ( 1 ) and 1‐(ferrocenylmethyl)‐3‐mesitylimidazol‐2‐ylidene ( 2 ) were developed. Complexation of 2 to {Ir(cod)Cl}₂] (cod=cis,cis‐1,5‐cyclooctadiene) or Ru(PCy₃)Cl₂(?CH‐o‐O‐iPrC₆H₄)] (Cy=cyclohexyl) afforded 3 (Ir( 2 )(cod)Cl]) and 5 (Ru( 2 )Cl₂(?CH‐o‐O‐iPrC₆H₄)]), respectively. Complex 4 (Ir( 2 )(CO)₂Cl]) was obtained by bubbling carbon monoxide through a solution of 3 in CH₂Cl₂. Spectroelectrochemical IR analysis of 4 revealed that the oxidation of the ferrocene moiety in 2 significantly reduced the electron‐donating ability of the N‐heterocyclic carbene ligand (ΔTEP=9 cm^?1; TEP=Tolman electronic parameter). The oxidation of 5 with Fe(η⁵‐C₅H₄COMe)Cp]BF₄] as well as the subsequent reduction of the corresponding product 5 ]BF₄] with decamethylferrocene (Fc*) each proceeded in greater than 95 % yield. Mössbauer, UV/Vis and EPR spectroscopy analysis confirmed that 5 ]BF₄] contained a ferrocenium species, indicating that the iron center was selectively oxidized over the ruthenium center. Complexes 5 and 5 ]BF₄] were found to catalyze the ring‐closing metathesis (RCM) of diethyl diallylmalonate with observed pseudo‐first‐order rate constants (k_obs) of 3.1×10^?4 and 1.2×10^?5 s^?1, respectively. By adding suitable oxidants or reductants over the course of a RCM reaction, complex 5 was switched between different states of catalytic activity. A second‐generation N‐heterocyclic carbene that featured a 1′,2′,3′,4′,5′‐ pentamethylferrocenyl moiety ( 10 ) was also prepared and metal complexes containing this ligand were found to undergo iron‐centered oxidations at lower potentials than analogous complexes supported by 2 (0.30–0.36 V vs. 0.56–0.62 V, respectively). Redox switching experiments using Ru( 10 )Cl₂(?CH‐o‐O‐iPrC₆H₄)] revealed that greater than 94 % of the initial catalytic activity was restored after an oxidation–reduction cycle.