Increasing Olefin Metathesis Activity of Silica‐Supported Molybdenum Imido Adamantylidene Complexes through E Ligand σ‐Donation

Abstract

Molybdenum imido adamantylidene complexes with different substituents on the imido ligand (dipp = 2,6diisopropylphenyl, Ar F5 = C 6 F 5 , and t Bu) having distinct electron donating abilities were investigated for the metathesis of internal and terminal olefins, for both molecular and silica-supported species using standardized protocols. Here we show that surface immobilization of these compounds results in dramatically increased activity compared to their molecular counterparts. Additionally, we show that electron withdrawing imido groups increase the activity of the compound towards terminal olefins while they simultaneously decrease the ability to metathesize internal olefins. Furthermore, these systems also show high stability when used as initiators in olefin metathesis, although the species that display higher initial activity deactivate faster than those that show more a more moderate reaction rate at first. Our catalytic studies, augmented by DFT calculations, show that all investigated compounds have a remarkably small energy difference between the trigonal bipyramidal (TBP) and square planar (SP) configurations of the metallacyclobutane intermediates, which has previously been linked to high activity.

Olefin Metathesis Catalysts Generated In Situ from Molybdenum(VI)‐Oxo Complexes by Tuning Pendant Ligands

“…Metathesis from these systems initiates rapidly through cross-metathesis of the starting alkylidene (usually a neopentylidene (R = t Bu), neophylidene (R = CMe 2 Ph) or, more recently, adamantylidene) and the olefin substrate. [16][17][18][19][20][21] These catalysts display very high turnover frequencies (TOFs) and turnover numbers (TONs) when the right ligand set is chosen, with often even higher activities upon immobilization on an oxide support. [17,19,22,23] Detailed studies have shown that, even with well-defined high oxidation state alkylidenes, low valent Mo(IV) olefin complexes or dimeric compounds are generated during metathesis; [24,25] these have been shown to display poor albeit measurable activity.…”

“…The involvement of these high‐oxidation state Mo(VI) alkylidenes parallels what is proposed in molecular chemistry: in fact, all efficient Mo‐based catalysts are based on well‐defined Mo(VI) oxo or imido alkylidenes, the so‐called Schrock metathesis catalysts, which have the same general formula, (X)(Y)Mo(E)(=CHR) with E=oxo or imido and X,Y=anionic ligands. Metathesis from these systems initiates rapidly through cross‐metathesis of the starting alkylidene (usually a neopentylidene (R= t Bu), neophylidene (R=CMe 2 Ph) or, more recently, adamantylidene) and the olefin substrate [16–21] . These catalysts display very high turnover frequencies (TOFs) and turnover numbers (TONs) when the right ligand set is chosen, with often even higher activities upon immobilization on an oxide support [17,19,22,23] .…”

W‐oxo Adamantylidenes: Stable Molecular Precursors for Efficient Silica‐Supported Metathesis Catalysts

“…Interestingly, there is not as much of a difference in activity between compounds with weakly donating (perfluoro tertbutoxy) and strongly donating (pyrrolylide) X-type ligands, as previously observed for the molybdenum analogs. [13]…”

“…Detailed investigations of the influence of the ligands on the metal center have shown that the highest activities have been achieved with a combination of strongly donating E ligands along with weakly donating X/Y ligands or vice versa (weaker donating E ligands along with at least one stronger donating X ligand) [13–18] . This principle also applies for cationic alkylidene compounds, [19,20] where the σ‐donating mono‐anionic X ligand is replaced by a σ‐donating neutral NHC ligand.…”

Cationic molybdenum oxo alkylidenes stabilized by N-heterocyclic carbenes: from molecular systems to efficient supported metathesis catalysts

“…Since the discovery of olefin metathesis more than half a century ago, 1 the development of novel catalyst families has been at the core of extensive research efforts, with the goal to increase catalytic performance, from functional group tolerance to increased activities, selectivities and stabilities. 2 Apart from Ru-based “Grubbs-type” catalysts, 3 d 0 early-transition-metal metathesis catalysts mostly based on group 6 metals (Mo/W), i.e. Schrock-type catalysts of the general formula M(E)(CHR)(X)(Y) with E = oxo or imido and X, Y = anionic ligands of various types (alkyls, alkoxys, amidos), have emerged as a central class of olefin metathesis catalysts.…”