Redox-switchable ring-closing metathesis: catalyst deisgn, synthesis, and study

Arumugam, K., Varnado, C.D., Sproules, S. , Lynch, V. M. and Bielawski, C. W. (2013) Redox-switchable ring-closing metathesis: catalyst deisgn, synthesis, and study. Chemistry: A European Journal, 19(33), pp. 10866-10875. (doi:10.1002/chem.201301247)

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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 (kobs) 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.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Sproules, Dr Stephen
Authors: Arumugam, K., Varnado, C.D., Sproules, S., Lynch, V. M., and Bielawski, C. W.
College/School:College of Science and Engineering > School of Chemistry
Journal Name:Chemistry: A European Journal
Publisher:Wiley-VCH Verlag
ISSN (Online):1521-3765

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