Cationic Group VI Metal Imido Alkylidene N‐Heterocyclic Carbene Nitrile Complexes: Bench‐Stable, Functional‐Group‐Tolerant Olefin Metathesis Catalysts

Abstract Despite their excellent selectivities and activities, Mo‐and W‐based catalysts for olefin metathesis have not gained the same widespread use as Ru‐based systems, mainly due to their inherent air sensitivity. Herein, we describe the synthesis of air‐stable cationic‐at‐metal molybdenum and tungsten imido alkylidene NHC nitrile complexes. They catalyze olefin metathesis reactions of substrates containing functional groups such as (thio‐) esters, (thio‐) ethers and alcohols without the need for prior activation, for example, by a Lewis acid. The presence of a nitrile ligand was found to be essential for their stability towards air, while no decrease in activity and productivity could be observed upon coordination of a nitrile. Variations of the imido and anionic ligand revealed that alkoxide complexes with electron‐withdrawing imido ligands offer the highest reactivities and excellent stability compared to analogous triflate and halide complexes.


Introduction
While the first observations on olefin metathesis were already reported in the middle of the 20 th century,interest in the topic increased dramatically upon discovery of the first well-defined (pre-) catalysts.C urrently,w ell-defined olefin metathesis catalysts are mainly based on ruthenium-based Grubbs-catalysts as well as molybdenum-and tungsten-based Schrock-type catalysts.
Grubbs-catalysts have become popular for many organic and polymer chemists,p articularly because of their air stability [1] and thus easy handling,a sw ell as their excellent stability towards many functional groups,i ncluding protic ones such as alcohols. [2] Schrock catalysts allow for impressive activities,e xcellent stereo-and enantioselectivity [3] and can tolerate an umber of different functional groups.H owever, due to their inherent sensitivity to air and protic functional groups, [4] their use essentially requires the use of ag lovebox. Therefore,they yet have not gained the same widespread use as their ruthenium-based counterparts.Boncella et al. reported on several molybdenum and tungsten alkylidene complexes bearing hydridotris(pyrazolyl)borate ligands,t hat are tolerant versus air,m oisture and heat for al ong time. However,t hese 18-electron complexes were not active in olefin metathesis and required the addition of AlCl 3 to result in ROMP (ring-opening metathesis polymerization) active species.A lso,t he mechanism of activation and the active species itself could not be identified. [5] Fürstner et al. reacted Schrock-type bisalkoxide catalysts with 2,2'-bipyridine and 1,10-phenanthroline leading to octahedral complexes that are stable in air for several weeks.T or egenerate the parent compounds,these complexes require activation, for example, by treatment with anhydrous ZnCl 2 in toluene at up to 100 8 8C for 30 minutes (Scheme 1). [6] This offers an elegant solution for the creation of air-stable Schrock pre-catalysts,h owever, due to the hygroscopic nature of ZnCl 2 ,t his process most likely still requires aglovebox. Also,during activation, an airand moisture sensitive Schrock catalyst is reformed.
In recent years,molybdenum imido and, to alesser extent, tungsten oxo/imido alkylidene NHC complexes have been developed by our group,w hich exhibit outstanding activities and excellent functional group tolerance. [7] Furthermore, excellent stereoselectivities in ring-opening metathesis polymerization (ROMP) as well as ring-opening-cross metathesis (ROCM) could be achieved. Upon introduction of achelating carboxylate ligand, air-stable (for at least 5days) cationic molybdenum imido alkylidene NHC complexes were obtained, however, at the cost of as ignificantly reduced Scheme 1. Protection and deprotection of Schrock-typeb isalkoxide catalysts as reported by Fürstner et al. [6] productivity compared to other, monodentate ligands. [8] In molybdenum chemistry,the universal bistriflate complexes of the general formula [Mo(NR)(CHCMe 2 Ph)(DME)(OTf) 2 ] (R = tBu, Ad, 2,6-Me 2 C 6 H 3 ,2 ,6-iPr 2 C 6 H 3 ,2 ,6-Cl 2 C 6 H 3 ,3 ,5-Me 2 C 6 H 3 ,2 -tBuC 6 H 4 ,2 -CF 3 C 6 H 4 ;D ME = 1,2-dimethoxyethane) are typically used as starting material for the coordination of NHCs. [7b,e] However,t hese reactions cannot be transferred to tungsten chemistry,b ecause transmethylation of the DME ligand is observed upon addition of the free NHC to the precursor (Scheme 2). [9] Forthis reason, most of the work reported so far focused on molybdenum-based complexes.O nly af ew examples of tungsten imido alkylidene NHC complexes have been reported, all of which contain the 2,6-diisopropylphenylimido or the 1,3-diisopropylimidazol-2-ylidene (IiPr)l igand as the NHC. [10] Since it was previously shown that both, the imido and the NHC ligand have ap ronounced influence on the reactivity and selectivity of ac atalyst, we were eager to find aw ay to prepare tungsten imido alkylidene NHC complexes containing tailored imido-and NHC ligands. [8a,c] Herein, we report the routes we discovered for the synthesis of such compounds,a sw ell as their productivity, activity and stability.Most important, we were able to transfer the results found for tungsten to molybdenum to create aset of bench-, that is,a ir-stable,f unctional-group-tolerant Moand W-based olefin metathesis catalysts that do not require activation by aLewis acid.

Synthesis of Metal Complexes
After unveiling the reaction cascade that commences upon treating tungsten imido alkylidene bistriflate complexes with 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene (IMesH 2 ), which presumably also applies to other NHCs,w ea ssumed that these reactions are only possible because the triflate ligand represents an excellent leaving group. [9] Therefore,the triflate ligands were exchanged for alkoxide and 2,5-dimethylpyrrolide ligands,r espectively,w hich indeed enabled the coordination of IiPr. [10] However, coordination of more sterically demanding NHCs was still not possible,presumably for steric reasons.T herefore,weset out to find other anionic ligands that were sterically less demanding but provided sufficient stability.W ea lready reported that some molybdenum bistriflate complexes [Mo(NR)(CHCMe 2 Ph)(DME)-(OTf) 2 ]c an undergo salt metathesis with KBr to yield the corresponding dibromide complexes,w hich even react with highly basic 1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene (6-Mes)t of orm the desired NHC complex. [8b] Encouraged by these results,w er eacted [W(NDipp)(CHCMe 2 Ph)-(DME)(OTf) 2 ]w ith KBr in CH 2 Cl 2 ,w hich resulted in the formation of the dibromide complex W-01 (Scheme 3). The use of excess KBr as well as finely powdering the reagent were found to speed up the reaction significantly.H owever, on al arger scale of several grams,r eaction times of up to Scheme 2. The use of [Mo(NR)(CHCMe 2 Ph)(DME)(OTf) 2 ]complexes as precursorsf or the coordinationo fNHCs and the products of DME activation in the case of the analogous tungsten compounds.
Scheme 3. Synthesis of the dibromide complex W-01 and coordination of different NHC ligands to the precursor. Reaction conditions:( i) toluene, r.t.,1h; (ii)Et 2 O, À35 8 8Ct or .t.,1h; (iii)CH 2 Cl 2 ,r .t.,20min;(iv) benzene, r.t.,15min. 4days were sometimes necessary to achieve full conversion due to the heterogeneous nature of the reaction. However,no side-products were formed and the dibromide complex could be stored at À35 8 8Cu nder aN 2 -atmosphere for months.D ue to the apparent equivalency of the DME protons in 1 H-and 13 C-NMR, an octahedral structure with the bromide ligands trans to each other, similar to the corresponding bistriflate complexes,c an be assumed. Reaction of the dibromide complex with 1,3-dimesitylimidazol-2-ylidene (IMes)i nb enzene led to displacement of DME by the NHC,y ielding the tungsten imido alkylidene NHC dibromide complex W-02. Ther eaction again proceeded cleanly without formation of any side products.
Interestingly,incase the reaction of W-06 with NaB(Ar F ) 4 was conducted in the presence of PivCN,d ecomposition was observed, likely due to steric overcrowding of the metal center. Thea ttempted reaction of W-07 with NaB(Ar F ) 4 also led to decomposition of the compound, even in the presence of anitrile.However,if[Ag(MeCN) 3 B(Ar F ) 4 ]was employed, the reaction proceeded cleanly,t hough the product did not exhibit an alkylidene signal in the 1 H-NMR. Instead, an ew signal with an integral of one appeared at d = 5.49 ppm, indicating the presence of an olefinic proton. This can be attributed to nitrile metathesis,t hat is,t he insertion of acetonitrile into the alkylidene-bond via an azatungstacyclobutene leading to the formation of the mixed bisimido complex W-13 (Scheme 5).
Similar observations have previously been made for tungsten complexes by Schrock et al. [12] We were able to obtain asingle crystal X-ray structure of the insertion product ( Figure 1) that confirms its proposed structure.
To determine the influence of the imido ligand on selectivity and reactivity,w ea ttempted to prepare the dibromide complexes from other bistriflates.R eactions with KBr were successful for the 2,6-dimethylphenylimido and the 2-trifluoromethylphenylimido ligand, but not for the 2-tertbutylphenylimido and 2,6-dichlorophenylimidoligand (Scheme 6a).
However,t he tungsten 2,6-dichlorophenylimido and 3,5dimethylphenylimido alkylidene dichloride complexes were prepared following our recently published alternative synthetic procedure, [13] in which the imido ligand is introduced by reaction WOCl 4 with an isocyanate (Scheme 6b). [14] Thet ungsten 2-tert-butylphenylimido alkylidene dichloride complex W-19 was obtained by protonation of the corresponding bispyrrolide complex with HCl in diethyl ether in the presence of DME (Scheme 6c). All tungsten imido alkylidene dihalide DME complexes were then reacted with IMes (Scheme 7). In all cases,clean formation of the desired NHC complexes and no DME activation was observed. Notably,t he dihalide DME complexes do not necessarily need to be isolated and purified prior to the reaction with IMes,a ss uccessfully demonstrated for the tungsten 2trifluoromethylphenylimido alkylidene IMes dibromide comlex W-21.
Theresulting IMes complexes W-02 and W-20-W-24 were then successfully transformed into their cationic counterparts by salt metathesis with NaB(Ar F ) 4 (Scheme 8). Based on our previous findings (vide supra), PivCN was added to all reactions except the one employing W-02.T he cationic monohalide complexes W-08 and W-25-W-29 were reacted with AgOTft oy ield the triflate complexes W-36-W-41; reaction with LiOC 6 F 5 yielded the pentafluorophenoxide complexes W-30-W-35 (Scheme 8). In the case of W-08,t he addition of pivalonitrile was necessary to obtain crystalline products.T he triflate anion was chosen because the related molybdenum complexes have proven highly reactive and tolerant towards alcohol-containing substrates and, in some cases,a ir-stable for several hours. [7a] Thep entafluorophenoxide was chosen in view of an earlier observation that cationic tungsten imido alkylidene NHC complexes contain-  ing the pentafluorophenoxide ligand by far outperformed all other tungsten compounds in terms of productivity. [10] Furthermore,a lkoxide ligands allow for the immobilization of catalysts on silica, analogously to what has been published for molybdenum. [15] Reactivity All cationic complexes were employed in as et of benchmark olefin metathesis reactions,typically ring-closing metathesis (RCM) or homometathesis (HM), using substrates containing ether, ester,t hioester, thioether,a nd alcohol groups.S ubstrates are displayed in Figure 3. Thep roductivities of the catalysts expressed in turnover numbers (TONs) are listed in Table 1.
Theproductivities of the monohalide complexes containing the 2,6-diisopropylphenylimido ligand (W-08-W-12, W14) are summarized in Table S1 (Supporting Information). Generally,t hese complexes show rather low productivities.A possible explanation could be,t hat the bromide ligand does not withdraw enough electron density from the metal center. 1-Octene was the only substrate that was metathesized by all complexes,w ith complex W-08 displaying the highest productivity. W-14 is the only complex that displays notable activity in the metathesis of other substrates such as diallyl sulfide,methyl oleate and pent-4-en-1-ol. Gratifyingly,variation of the imido ligand as well as exchange of the halide ligand for am ore electron-withdrawing ligand increased productivities (Table 1). Productivities for the other halide complexes W-25-W-29 were still moderate,b ut in all cases exceeded those of the previously examined complex W-08 bearing the 2,6-diisopropylphenylimido ligand.
Most halide compounds showed no activity in the metathesis of ethers,esters and alcohols.The corresponding triflate complexes behaved similarly,however, significantly increased TONs for the metathesis of alcohols were observed, likely due to the more electron-withdrawing nature of the triflate ligand. In line with our concept, complexes containing the pentafluorophenoxide ligand were found to be the most active ones,a sjudged by the significantly higher TONs for allyl sulfide and the ether containing substrate S3,a sw ell as activity in the metathesis of methyl oleate.The productivity in the metathesis of pent-4-en-1-ol was similar or slightly lower   than for the triflate complexes,t his is in line with what has been observed for the analogous molybdenum complexes. [7a] Overall, tungsten complexes based on electron-withdrawing imido ligands,that is,2,6-dichlorophenylimido and 2-trifluoromethylphenylimido offered the highest productivities,likely due to the more electrophilic metal centers.

Air Stability
Encouraged by the air stability of cationic molybdenum imido alkylidene NHC complexes, [7a] we set out to explore the stability of their tungsten analogs.F or this purpose,asample of the solid complex was exposed to air for up to two weeks. Thesample was then dissolved in dry deuterated solvent and the stability was assessed by 1 H-and 19 F-NMR. After 16 hours,a lmost none of the complexes showed signs of decomposition. Themost notable exception was W-08,which decomposed completely,f orming one equivalent of imidazolium salt, with B(Ar F ) 4 acting as the anion, [16] and further, unknown side products.T his is surprising for two reasons: first, all analogous cationic halide complexes containing the IMes ligand (W-25 to W-29)s howed no or only very minor signs of decomposition after storage in air for two weeks (longer periods were not tested);s econd, W-12 did not decompose either,e ven though all ligands except the NHC are identical to that of W-08.This indicates that air stability is most likely dependent on the steric environment of the metal center. Thus,the IMes complexes W-25 to W-29 are sterically protected by the additional nitrile ligand and coordinatively saturated, while W-12 bears the sterically significantly more demanding IDipp.T his poses the question, whether addition of anitrile ligand to four-coordinate cationic complexes could potentially present ag eneral strategy to obtain air-stable olefin metathesis (pre-) catalysts and, if yes,w ould this increased stability be realized at the expense of activity, productivity or stereoselectivity.I ndeed, the pentafluorophenoxide complexes,a ll of which are pentacoordinate and contain anitrile ligand, showed no (W-31, W-32, W-33, W-35) or very minor (W-30 and W-34)s igns of decomposition after exposure to air overnight. After two weeks in air, W-30 and W-34 were 80 %a nd completely decomposed, respectively. Theo ther alkoxide complexes W-31, W-32, W-33, W-35, however, were at least 95 %i ntact. This illustrates the importance of exploring different imido ligands.I nt he case of W-34,t his significantly reduced stability might be explained by an insufficient steric shielding of the metal center due to the sterically less demanding imido ligand. The instability of W-30 compared to W-31,however, appears very counterintuitive and cannot be explained with the available data. Thetriflate complexes were found to be less stable than the halide and alkoxide compounds.T hus, W-36 exhibited minor signs of decomposition and changes in chemical shift after exposure to air overnight, both of which can be explained by uptake and coordination of water. After two weeks,h owever,c omplete decomposition and formation of one equivalent of imidazolium B(Ar F ) 4 was observed. Similar observations were also made for the other triflate complexes. Interestingly,f or W-36 and W-41 one equivalent of the corresponding anilinium triflate could be identified as coproduct of the reaction with air.F or the other triflate complexes,n os ignals originating from the imido fragment or from the triflate could be detected, which can be explained by the insolubility of the corresponding anilinium triflates in CDCl 3 .T os ubstantiate this hypothesis,w ep repared 2,6diisopropylanilinium triflate and 2-trifluoromethylanilinium triflate,and indeed, the solubility of the former was sufficient to record NMR-spectra in CDCl 3 ,w hile the latter was completely insoluble in CDCl 3 .T he reduced stability of the triflate complexes might be caused by the excellent leaving group character of the triflate anion. While the triflate complexes should not be stored in air,they can be handled in air for limited time without significant decomposition. The NHC ligand also influences the stability of these cationic complexes.C omplexes W-10, W-09, W-11 bearing the sterically less demanding IMe, IMeCl 2 and IiPr,respectively,were significantly less stable.Alarge fraction of W-09 decomposed after one night in air,w hile W-10 and W-11 showed only minor signs of decomposition. After two weeks,h owever, complete decomposition was observed. Contrarily, W-14 bearing the sterically demanding 6-Mes was still intact after two weeks.Despite the fact that it is only tetracoordinate, W-12 was more stable than the complexes bearing small NHCs, being 90 %intact after 16 hand 15 %intact after two weeks in air.

Extended Air-Stability of Cationic Molybdenum Imido Alkylidene NHC Nitrile Complexes
Having realized the influence of the nitrile ligand on the air stability of cationic tungsten imido alkylidene NHC complexes,w ee xplored whether this concept could eventually be transferred to the analogous molybdenum compounds.F irst, we reevaluated the results of our previously conducted experiments on air stability of cationic molybdenum imido alkylidene NHC complexes. [7a] Interestingly,upon storage in air for 16 h, the nitrile-free complexes all showed the uptake and coordination of water, accompanied by varying amounts of decomposition. Then itrile-coordinated complex Mo-01 (Figure 4), however, showed no signs of decomposition or uptake of water after exposure to air overnight. Therefore,anumber of molybdenum imido alkylidene NHC complexes with and without an itrile ligand were prepared, the structures of which are depicted in Figure 4. Introduction of an itrile ligand was accomplished by simply adding an excess of nitrile,typically acetonitrile,to as olution of the complex. Usually,n op urification except removal of the solvent and excess nitrile under reduced pressure was necessary.A se xpected, the tetracoordinate compounds Mo-02, Mo-03 and Mo-05 all showed signs of water uptake in the 1 H-NMR spectrum after storing samples in air overnight. Furthermore,p artial decomposition was observed for all compounds except for Mo-04.B yc ontrast, the nitrile-coordinated analogues Mo-02-MeCN, Mo-03-MeCN and Mo-04-MeCN displayed excellent stability and showed no signs of decomposition after being exposed to air overnight. After two weeks,however, Mo-04-MeCN and Mo-03-MeCN had decomposed while Mo-02-MeCN was still intact.

Influence of the Nitrile Ligand on Activity,Productivity and Selectivity
Evidently,i ntroduction of an itrile ligand reduces the reactivity of these complexes towards water and oxygen. This posed the question whether this also leads to ar educed productivity in olefin metathesis.T his seemed not unlikely, since dissociation of the nitrile ligand at some stage might be part of the reaction mechanism. However, it is currently not known, whether dissociation of the nitrile takes place before or after coordination of the substrate,t hat is,w hether the mechanism is associative or dissociative.T oi nvestigate the influence of the nitrile ligand on productivity,t he TONso f complexes Mo-02, Mo-03 and Mo-04 were compared to those of the nitrile-coordinated analogues Mo-02-MeCN, Mo-03-MeCN and Mo-04-MeCN in as et of benchmark olefin metathesis reactions ( Figure 4). As ummary of the results obtained is provided in Table 2.
In general, the molybdenum complexes appear to be more active than their tungsten counterparts.S urprisingly,n itrile coordination does not appear to significantly reduce productivities of the molybdenum complexes.T his is attributable to the large excess of substrate employed in the reaction, that is, even aw eakly coordinating substrate such as 1-octene could displace the nitrile ligand for stoichiometric reasons.T o simulate the influence of as ubstrate bearing ac oordinating functional group,1 00 equivalents of CD 3 OD were added to as olution of W-33 in CDCl 3 ( Figures S239 and S240, S.I.). Changes in chemical shifts and abroadening of all signals was observed. Thec hemical shift of PivCN changed from d = 0.87 ppm in W-33 to d = 1.30 ppm, almost the shift of free PivCN (CDCl 3 , d = 1.36 ppm). Together with the observed broadening of the signal, this suggests coordination of CD 3 OD and an equilibrium between loosely bound and free PivCN,w hich could easily be displaced by an olefin. The productivities of the cationic molybdenum and tungsten imido alkylidene NHC were also compared to the productivities obtained with the 2 nd -generation Grubbs catalyst (G2, Table 2). Form ost substrates, G2 exhibited lower productivities than the pentafluorophenoxide complexes.H owever, comparable productivities were obtained for 1-octene and G2 markedly outperformed the tungsten and molybdenum complexes in the self-metathesis of methyl oleate.T his is aprime example for how different catalyst systems complement each other.
In order to compare the activity of nitrile-containing complexes to their nitrile-free analogues,k inetic measurements for the metathesis of 1-octene and pent-4-en-1-ol were conducted ( Figure 5). Compounds Mo-03, Mo-04, Mo-03-MeCN and Mo-04-MeCN were employed. In the case of tungsten, the nitrile-free complex was obtained from nitrilecontaining complex W-31 by reaction with six equivalents of the strong Lewis-acid tris(pentafluorophenyl)borane (BCF). This led to abstraction of the nitrile ligand, that is,formation of the nitrile free complex, and formation of the PivCN adduct of BCF,a ss hown by 1 H-NMR ( Figure S193-S196, S.I.). While the nitrile could be removed completely from W-31,insome cases,for example, W-33,only partial removal was possible,e ven upon adding al arge excess of borane. Interestingly,o nly very minor differences for the activity of nitrile-ligated and nitrile-free complexes could be observed. So far,wecannot fully rationalize this similar behavior. Only  Mo-03-MeCN showed reduced activity in the metathesis of 1octene compared to the nitrile-free Mo-03.N op ronounced differences in activity for the metathesis of pent-4-en-1-ol were observed. We do not know yet, whether the nitrile ligand dissociates prior to or after coordination of the substrate,t hat is,i fthe reaction proceeds via adissociative or associative mechanism with the chosen substrates. [17] Investigations regarding this topic are currently underway.

Conclusion
As ynthetic route to cationic tungsten imido alkylidene NHC complexes has been elaborated. It is applicable to alarge variety of imido-and NHC ligands and allows for both, the preparation of the tungsten analogues of previously reported cationic molybdenum imido alkylidene NHC compounds and the investigation of the influence of the metal on the catalytic performance in olefin metathesis.Introduction of an itrile ligand to cationic molybdenum and tungsten complexes results in air-stable complexes,u sually without any loss in activity,p roductivity or selectivity.U nlike previously reported air-stable 18-electron Schrock-type alkylidene complexes,n oa ctivation with Lewis acids is required. Cationic complexes containing an itrile,a ne lectron-withdrawing imido and an alkoxide ligand appear particularly promising,s ince they display high productivity while also being very stable towards air.O wing to their excellent stability,t hey represent user-friendly group VI metal-based olefin metathesis initiators.Furthermore,they offer access to metathesis products with ah igh E-content and form transisotactic poly(norbornene)s and are therefore complementary to commonly used ruthenium-based initiators.