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

Abstract

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)Cl-2(CH-o-O-iPrC(6)H(4))] (Cy=cyclohexyl) afforded 3 ([Ir(2)(cod)Cl]) and 5 ([Ru(2)Cl-2(CH-o-O-iPrC(6)H(4))]), respectively. Complex 4 ([Ir(2)(CO)(2)Cl]) 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 ( (-1); 5 with [Fe((5)-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. Mossbauer, 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 3.1x10(-4) and 1.2x10(-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 (0.30-0.36V vs. 0.56-0.62V, respectively). Redox switching experiments using [Ru(10)Cl-2(CH-o-O-iPrC(6)H(4))] revealed that greater than 94% of the initial catalytic activity was restored after an oxidation-reduction cycle.