Metal-centered oxidations facilitate the removal of ruthenium-based olefin metathesis catalysts

Highlights

• The catalytic activities of Ru complexes may be switched using redox-driven processes.

• Chemical oxidation reduces the solubilities of Ru-based metathesis catalysts.

• A new approach for removing commercial catalysts using has been established.

Abstract

Commercially available catalysts (SIMes)(PCy₃)Cl₂Ru(CHPh) (2) and (SIMes)Cl₂Ru(CH-o-O-i-PrC₆H₄) (3) (SIMes = 1,3-dimesitylimidazolin-2-ylidene) were found to display reversible Ru oxidations via a series of electrochemical measurements. The redox processes enabled the catalysts to be switched between two different states of activity in ring opening metathesis polymerizations and ring closing metathesis reactions, primarily through changes in catalyst solubility. Moreover, treating a solution of 2 dissolved in C₆H₆/CH₂Cl₂/[1-butyl-3-methylimidazolium][PF₆] (6:1:1.1 v/v/v) with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone was found to remove >99.9% of the catalyst, as determined by UV/vis spectroscopy. The methodology described herein establishes a new approach for controlling the activities displayed by commercially available olefin metathesis catalysts and for removing residual Ru species using redox-driven processes.

Introduction

The removal and reuse of precious metal based catalysts from reaction media has been a long-standing challenge in the field of homogeneous catalysis. Indeed, catalysts have been attached to various types of phase tags [1], [2] including fluorine containing derivatives [3], [4], [5], photoresponsive groups [6], [7], redox active groups [8], [9], and ionic liquids [10] to facilitate separation. The use of redox active phase tags to recover and reuse catalysts is a particularly attractive approach as redox processes are often diffusion controlled and operate in a manner that is orthogonal to other stimuli, such as light or heat. One area where redox active phase tags [8], [9], [11] have found utility is in the recovery of ruthenium-based olefin metathesis catalysts [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. For example, Plenio and co-workers [9] elegantly designed a Ru-based olefin metathesis catalyst featuring two ferrocenyl groups attached to an N-heterocyclic carbene ligand [22], [23], [24], [25], [26], [27], [28], [29]. Upon the addition of two equivalents of [FcCOCH₃][CF₃SO₃], the ferrocenyl moieties underwent oxidation to their ferrocenium derivatives and caused the catalyst to precipitate from solution. The oxidized catalyst could then be recovered via filtration and reused upon reduction to its neutral form. A related catalyst which featured a ferrocenyl group appended to the 2-isopropoxybenzylidene moiety of a Hoveyda–Grubbs type catalyst was recently reported by Wang and co-workers [11]. Similar to the above mentioned system, exposing the catalyst to iodine resulted in the oxidation of the ferrocenyl group and facilitated extraction of the corresponding complex from non-polar media into an ionic liquid; subsequent reduction with decamethylferrocene (Fc*) restored the complex's solubility as well as its catalytic activity [11]. Alternatively, a number of other approaches have been employed to remove ruthenium residues upon completion of metathesis reactions [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], including attachment to silica gel [30], [31], [32], [33], [34], [35], [36] or other heterogeneous supports [37], [38], [39], [40], [41], [42], selective chemical degradation [43], [44], [45], [46], conversion to water soluble derivatives through ligand exchange [47], [48] or quaternization of a pendant amino groups [49], and selective extraction [49], [50], [51], [52].

The primary drawbacks to the aforementioned redox-based approaches are that either specially designed redox-active ligands are needed or the metal is recovered in a form that is not easily reusable. Since Ru-based olefin metathesis catalysts contain redox active metal centers, we envisioned finding suitable conditions that utilizes the Ru^II/III couple to induce solubility changes and to facilitate catalyst removal. An ability to selectively oxidize the metal center without irreversible chemical degradation of the complex should also facilitate reuse upon subsequent recovery and reduction, or enable external control over polymerizations [55] and other types of redox-mediated applications [56].