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)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(η(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. 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.

Bioinspired functional analogs of the active site of molybdenum enzymes: Intermediates and mechanisms

“…The phenomenon that the charge of the ligand backbone controls the rate of a nucleophilic attack had been demonstrated by Wrighton already in 1994 studying nucleophilic attack of azide on the carbonyl ligand of (1,l -bis(dipheny1phosphano)cobaltocene)rhenium(I) carbonyl complexes (Scheme 12b) [118]. This seminal demonstration has led to the concept of redox-switchable catalysis [119][120][121][122][123][124][125][126].…”

Synthesis and study of olefin metathesis catalysts supported by redox-switchable diaminocarbene[3]ferrocenophanes

“…18 In light of these advantages, we have launched a program to explore redox-active NHCs as a general class of ligands for bestowing redox-switchable functions onto a broad range of transition metals. [19][20][21][22][23][24] For example, we recently disclosed a series of redox-switchable Ru-based olefin metathesis catalysts bearing N-ferrocenylated NHCs (Fig. 1).…”

“…1). 24 The activities displayed these catalysts in ring-closing metathesis reactions were found to depend upon the oxidation state of the redox-active ligand: catalysts supported by ferrocenium containing NHCs were significantly less active than their neutral analogues, which was attributed to the relative donating abilities of the respective ligands. Previously, we 19 and others 25 reported the diaminocarbene-…”

Cationic Iridium Complexes Containing Anionic Iridium Counterions Supported by Redox-Active N-Heterocyclic Carbenes

“…Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): C1−N1, 1.377(11); C1−N2, 1.341(11); N1− C2, 1.389(11); C3−N2, 1.383(11); C2−C3, 1.342(13); C1−Ir1, 2.075(8); C24−Ir1, 2.097(8); Ir2−Cl1, 2.331(4); Ir2−Cl2, 2.312(4); Ir2−C49, 1.809(14); Ir2−C50, 1.831(15); Ir1−C47, 1.873(10); Ir1− C48, 1.873(11); C47−O1, 1.135(12); C48−O2, 1.127(12); N1−C1− N2, 104.5(7); C1−Ir1−C24, 90.6(3); N3−C24−N4, 105.2(7); N2− C13−C14, 114.5(7); N3−C24−N4, 105.2(7); N3−C25−C26, 106.4(9); N4−C26−C25, 106.7(9); C47−Ir1−C48, 90.0(5).…”