Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Molybdenum Imido Alkylidene N‑Heterocyclic Carbene Complexes Containing Pyrrolide Ligands: Access to Catalysts with Sterically Demanding Alkoxides Iris Elser,† Mathis J. Benedikter,† Roman Schowner,† Wolfgang Frey,‡ Dongren Wang,† and Michael R. Buchmeiser*,† †

Institute of Polymer Chemistry and ‡Institute of Organic Chemistry, University of Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany

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ABSTRACT: Neutral and cationic complexes of the general formulas Mo(NR1)(CHCMe 2 Ph)(NC 4 H 4 ) 2 (NHC) and [Mo(NR 1 )(CHCMe 2 Ph)(NC 4 H 4 )(NHC)][B(ArF)4] (R1 = 2-tBu-C6H4, 2,6-iPr2-C6H3, 2,6-Me2-C6H3, tBu, Ad; B(ArF)4 = tetrakis(3,5-(CF3)2-C6H3)borate; NHC = N-heterocyclic carbene) have been synthesized. Three NHC ligands with different electronic and steric properties, namely, 1,3-diisopropylimidazol-2-ylidene (IPr), 1,3,5-triphenyl-1,3,4triazol-2-ylidene (TPT), and 1,3-dimethyl-4,5-dichloroimidazol-2-ylidene (IMeCl) have been incorporated. The cationic complexes exhibit good activity in olefin metathesis at room temperature, provided no protic functional groups are present. Both neutral and cationic NHC (bis)pyrrolide complexes serve as excellent precursors to molybdenum imido alkylidene NHC complexes with sterically demanding alkoxides. Here, introduction of the alkoxide ligands proceeds upon protonation of pyrrole with the respective alcohol. Application of the sterically demanding alcohols (HO-2,6-(2,4,6-Me3C6H2)2-C6H3 1-H and 5,5′,6,6′,7,7′,8,8′-octahydro-3,3′-dibromo-2-silyloxy-1,1′-binapth-2′-ol 2-H) instead of the corresponding alkoxides circumvents previously described C−H activation reactions at the NHC. The derived sterically encumbered (chiral) complexes were accessible in good yields by simple washing with nonpolar solvents.

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complexes, since they bear a good σ-donor and an acceptor (alkoxide) ligand. We were especially interested whether the inversion of configuration at the metal center, characteristic for MAP-type complexes,8 enabling the synthesis of cis, syndiotactic polymers, would also be present in our systems. However, introduction of bulky basic ligands like O-2,6-(2,4,6-Me3C6H2)2-C6H3, (HMTO, 1) or 5,5′,6,6′,7,7′,8,8′-octahydro3,3′dibromo-2-silyloxy-1,1′-binapth-2′-olate (2) to our molybdenum imido alkylidene NHC bistriflate complexes by salt metathesis with the respective potassium alkoxides turned out to be more challenging than expected. Thus, instead of coordinating to the metal center, the sterically demanding potassium salt (1-K or 2-M, M = Li, K) immediately deprotonated one methyl group of the 1,3-dimesitylimidazolin-2-ylidene ligand (Scheme 2a). Accordingly, we identified C(sp3)H-activation under formation of a complex bearing a Cchelating NHC as competing and dominating reaction pathway.10 Hence, we sought to synthesize molybdenum imido alkylidene NHC bispyrrolide complexes to enable the introduction of sterically demanding, basic alcohols under circumvention of ancillary ligand deprotonation. Here, alkoxides can be introduced by protonation of one pyrrolide ligand with the corresponding alcohol, thereby minimizing chances of

INTRODUCTION Molybdenum- and tungsten-based monoalkoxide pyrrolide (MAP) complexes bearing bulky aryloxide ligands successfully promote stereoselective ring-opening metathesis polymerization,1 (Z)-selective homo- and cross-metathesis (HM/CM),2 and macrocyclization.3 In case chiral ligands are incorporated, asymmetric metathesis reactions, such as asymmetric ringclosing metathesis (ARCM)4 have been reported.5 Catalyst design was based on theoretical calculations by Eisenstein and Copéret et al., who postulated that the combination of two electronically distinct ligands (asymmetric substitution, one acceptor and one donor) is beneficial for catalyst activity.6 Here, the acceptor ligand ensures for a sufficient Lewis acidity at the metal center, whereas the donor ligand facilitates the required geometry change from square pyramidal (SP) to trigonal bipyramidal (TBP). This is necessary for the formation of active metallacyclobutanes and promotes cycloreversion under concomitant formation of product.4a Selectivity stems from steric interactions between the bulky aryloxide ligand in the apex and the substituents in the metallacyclobutane forming the base of the trigonal bipyramid, resulting in a (Z)-double bond (Scheme 1b). Inspired by those results, we were interested if the same mechanism would hold up for our cationic molybdenum and tungsten imido alkylidene N-heterocyclic carbene (NHC) complexes.7 Besides their cationic charge, which should render them more Lewis acidic, they are comparable to MAP-type © XXXX American Chemical Society

Received: March 6, 2019

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DOI: 10.1021/acs.organomet.9b00148 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1a

a

(a) Structural analogy between target complexes and state of the art MAP-type complexes; (b) postulated influence of the sterically demanding alkoxide on the substituents in the metallacyclobutane (MCB);4a,6 and (c) dimeric structure of a molybdenum imido alkylidene bispyrrolide complex.9

Scheme 2. Introduction of Sterically Demanding Ligands to Mo Imido Alkylidene NHC Complexesa

a

(a) Deprotonation of the NHC ligand by the sterically demanding metal alkoxide and isolated complex with a C-chelating NHC,10 and (b) protonation approach applied in this paper.

Scheme 3. Synthesis of Molybdenum Bispyrrolide Complexes Mo-6−Mo-10 from Molybdenum Bistriflate Complexes Mo-1− Mo-5 and Synthesis of Molybdenum Imido Alkylidene Bispyrrolide NHC Complexes Mo-11−Mo-17 by Coordination of Free NHC or Transmetalationa

a

TEP: Tolman electronic parameter.15 In situ: Bispyrrolide complexes were not isolated but immediately reacted with the corresponding NHC.

already been successfully synthesized by our group,12 we first chose a similar approach and investigated reactions between free NHCs and complexes of the general type Mo(NAr)(CHCMe2Ph)(2,5-Me2-NC4H2)2.13 However, in our hands, depending on the basicity and steric demand of the applied carbene, either deprotonation of the alkylidene ligand or no conversion were observed. Consequently, we hypothesized, that similar complexes bearing the parent pyrrolide (NC4H4) instead

undesired deprotonation reactions on either the NHC or the alkylidene ligand (Scheme 2b). Alternative routes to molybdenum imido alkylidene NHC complexes with HMTO and HIPTO ligands have recently been published by our group.11

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RESULTS AND DISCUSSION Synthesis. Mo Bispyrrolide NHC Complexes. Since tungsten imido alkylidene NHC bispyrrolide complexes had B


Article

Organometallics

Scheme 4. Synthesis of Cationic Molybdenum Pyrrolide Complexes Mo-18−Mo-24 from Molybdenum Bispyrrolide Complexes Mo-11−Mo-17a

a

L = MeCN for Mo-22.

intramolecular proton abstraction on one of the N−CH3 groups is highly unlikely, since 4-membered chelates would be formed. Mo-12 was synthesized by reaction of Mo-6 with the free carbene TPT in diethyl ether, whereas Mo-13 was synthesized by reaction of Mo-6 with the silver iodide salt of IMeCl in CH2Cl2 (Scheme 3). Both Mo(N-2-tBu-C6H4)(CHCMe2Ph)(NC 4 H 4 ) 2 (TPT) (Mo-12) and Mo(N-2-tBu-C 6 H 4 )(CHCMe2Ph)(NC4H4)2(IMeCl) (Mo-13) were successfully isolated in 62 and 57% yield by crystallization from mixtures of CH2Cl2, diethyl ether and pentane. To broaden the scope, further molybdenum imido alkylidene bispyrrolide complexes containing different N-aryl (Mo-7, Mo-8) and N-alkyl (Mo-9, Mo-10) imido ligands were reacted with NHC ligands. The Nalkyl imido ligands are of special interest since they have a lower steric demand and were therefore expected to enhance the effect of the bulky aryloxide in the MCB (Scheme 1b) in the prospective molybdenum imido alkylidene NHC complexes. Mo(N-2,6-iPr2-C6H3)(CHCMe2Ph)(NC4H4)2(IPr) (Mo-15, 80% yield) and Mo(N-Ad)(CHCMe2Ph)(NC4H4)2(IPr) Mo17 (73%) with the N-2,6-iPr2-C6H3 and N-Ad imido ligand were synthesized according to the above-described synthetic protocol by reaction of the bispyrrolide complexes Mo-8 and Mo-10 with IPr or IMeCl·AgI, respectively (Scheme 3). In contrast, Mo(NR1)(CHCMe2Ph)(NC4H4)2(IPr) Mo-14 (R1 = 2,6Me2-C6H3) and Mo-16 (R1 = tBu) were directly synthesized in a two-step procedure from the corresponding Mo(NR1)(CHCMe2Ph) (OTf)2(DME) (DME = 1,2-dimethoxyethane) complexes by addition of 2 equiv of lithium pyrrolide followed by 1 equiv of 1,3-diisopropylimidazol-2-ylidene (Scheme 3, in situ), thereby avoiding one purification step. Mo-14 and Mo-16 were isolated in 75 and 54% yield, respectively. All molybdenum imido alkylidene NHC bispyrrolide complexes (Mo-11−Mo17) except Mo-12 with the TPT ligand show sharpened alkylidene and pyrrolide proton signals compared to the precursor bispyrrolide complexes in the 1H NMR spectra. As observed with CD3CN (vide supra), addition of the NHCs breaks up the dimeric structures and renders the complexes less fluxional, most probably with exclusive η1-coordination of the pyrrolide ligands. Complexes Mo-13, Mo-14, Mo-16, and Mo17 all show four distinct resonances for the two pyrrolide ligands. Apparently, the two pyrrolide ligands are diastereotopic due to the occupation of two different coordination sites in the pentacoordinated ligand sphere. Whether they differ due to the occupation of an equatorial and an apical position in a TBP structure, or because one of the pyrrolides experiences a significantly enhanced trans-effect (for example from the NHC) in a cis-pyrrolide SP or slightly distorted SP geometry cannot be deduced from the 1H NMR spectra. The only structures that can be ruled out are TBP structures in which both pyrrolides occupy either axial or equatorial positions or a SP structure with trans-

of the 2,5-dimethyl substituted pyrrolide would be more suitable candidates for the reaction with an additional donor ligand. The Schrock group demonstrated that those complexes are sterically unsaturated and in consequence form dimers (Scheme 1c).9 Also, parent bispyrrolide complexes have been reported to coordinate phosphine and bipyridine ligands.13,14 The bispyrrolide complexes Mo(NR1)(CHCMe2Ph)(NC4H4)2 (Mo-6− Mo-10) were synthesized from the bistriflate complexes Mo(NR1)(CHCMe2Ph)(OTf)2(DME)16 (Mo-1−Mo-5) (R = tBu, Ad, 2-tBu-C6H4, 2,6-Me2-C6H3, 2,6-iPr2-C6H3; Scheme 3). Mo-1−Mo-5 were reacted with 2 equiv lithium pyrrolide in diethyl ether or toluene (Scheme 3). The bispyrrolide complex Mo-10 is literature-known,9 whereas complexes Mo-6, Mo-7 and Mo-8 to the best of our knowledge have not yet been reported. Mo-9 was not isolated, but generated in situ, and then directly reacted with the NHC. The low yields for Mo-9 can be reasoned by the comparably low steric shielding of the Mo center by the NtBu imido ligand, which probably facilitates bimolecular decomposition reactions. Complexes Mo-6, Mo-7, and Mo-8 each show one broad resonance for the alkylidene proton (δ = 13.05 (Mo-6), 13.31 (Mo-7), 13.12 (Mo-8) ppm) and two broad resonances for the protons on the pyrrolide ligands (δ = 6.35, 6.13 ppm (Mo-6); 6.26, 6.10 ppm (Mo-7); 6.23, 6.10 ppm (Mo-8)). The broad signals most probably stem from the above-mentioned fluctuating dimeric structures (Scheme 1b) and the constant change of the pyrrolide coordination mode from η1 to η5 (Figures S1, S4, S6; SI).9 The dimeric structure is also supported by the observation, that upon dissolving Mo-6 in CD3CN a sharpening of the broad pyrrolide proton resonances to two pseudotriplets was observed (δ = 6.68, 5.89 ppm) in the 1H NMR spectrum (Figure S2, SI). Most likely, CD3CN coordinates to the metal center, leading to a breakup of the proposed dimeric structures. This provided further evidence that molybdenum imido alkylidene bispyrrolide complexes are suitable precursors for coordination of an NHC ligand. Indeed, upon reaction of 1,3-diisopropylimidazol2-ylidene (IPr) with Mo(N-2-tBu-C 6 H4 )(CHCMe 2 Ph)(NC4H4)2 (Mo-6) in diethyl ether, a yellow solid precipitated from the reaction mixture and could be isolated by filtration (Scheme 3). Mo(N-2-tBu-C6H4)(CHCMe2Ph)(NC4H4)2(IPr) (Mo-11) was isolated in quantitative yield. We were interested in the influence of the σ-donor strength of a given carbene, characterized by its Tolman electronic parameter (TEP),15 on the reactivity of molybdenum imido alkylidene bispyrrolide NHC complexes and compounds prepared therefrom. Hence, we also introduced 1,3,5-triphenyl-1,3,4-triazol-2-ylidene (TPT, TEP15a = 2057.3 cm−1) and 1,3-dimethyl-4,5-dichloro-imidazol2-ylidene (IMeCl, TEP15b = 2059.0 cm−1), which are both weaker σ-donors than IPr (TEP15b = 2051.5 cm−1, Scheme 3). IMeCl is also charming in that CH-activation through C


Article

Organometallics

Scheme 5. Synthesis of Cationic Molybdenum Imido Alkylidene Pyrrolide (1) NHC Complexes by Protonation of the Residual Pyrrolide Ligand with Either HMTOH or N,N-Dimethyl Anilinium B(ArF)4

washing with nonpolar solvents (pentane or fluorinated ethers) to remove pyrrole and N,N-dimethylaniline. [Mo(NR1)(CHCMe2Ph)(NC4H4)(NHC)][B(ArF)4] Mo-25 (R1 = 2tBu-C6H4), Mo-26 (R1 = 2,6-Me2-C6H4) and Mo-27 (R1 = Ad) were isolated in quantitative yield, 76% and 81% yield, respectively. Mo-27 can be synthesized via two different pathways (Scheme 5). One can either start from the cationic monopyrrolide complex Mo-24 by reaction with 1-H or from the neutral complex Mo-28 by protonation with N,Ndimethylanilinium B(ArF)4 etherate (78 vs 95% yield). Surprisingly, ligand exchange from pyrrolide to HMTO in Mo-24 is hampered by the formation of decomposition products, whereas protonation of Mo-28 proceeds selectively at the pyrrolide ligand, and not at the NHC ligand as could be suspected, rendering the latter synthetic approach more favorable. Finally, we were interested in the synthesis of molybdenum imido alkylidene NHC complexes with chiral alkoxides and sought to introduce more sterically demanding alkoxide ligands. Consequently, a molybdenum imido alkylidene NHC complex with a C2-symmetric chiral alkoxide ligand (5,5′,6,6′,7,7′,8,8′-octahydro-3,3′dibromo-2-silyloxy-1,1′-binapth-2′-olate, 2)4b Mo(N-2-tBu-C6H4)(CHCMe2Ph)(2)(NC4H4)(IMeCl) (Mo-29) was synthesized by reaction of Mo-13 with 1 equiv of the respective alcohol 2-H in quantitative yield in the form of a foam (Scheme 6). Mo-29 is the first molybdenum imido alkylidene NHC complex bearing a C2symmetric chiral alkoxide. The 1H NMR spectrum of Mo-29 shows only one alkylidene signal (δ = 13.14 ppm, CD2Cl2); accordingly, only one set of signals was observed for all ligands, which strongly points toward the formation of only one single

pyrrolides. In Mo-12, the alkylidene signals in the 1H NMR spectrum are still rather broad. Two distinct alkylidene proton resonances are visible, indicating the presence of two isomers in a ratio of 1:1 (δ = 13.86, 13.42 ppm) or a dimeric structure. A dimeric structure seemed unlikely, since the complexes Mo-11− Mo-17 are already pentacoordinated 16-VE complexes. To rule out syn-/anti-isomers, variable temperature NMR between 30 and 62 °C was measured (C6D4Cl2). At 30 °C two distinct alkylidene signals are visible (δ = 13.27 and 13.77 ppm) and converge upon heating until they merge at approximately 62 °C (Figure S56, SI). The two isomeric structures most probably differ due to limited free rotation around the Mo−Ccarbene bond of the TPT ligand and the broad resonances stem from interconversion between the two species. At high temperatures interconversion (rotation around Mo−Ccarbene) is fast on the NMR time scale, resulting in the observation of only one resonance. Cationic Mo Pyrrolide NHC Complexes. Subsequently, the synthesis of the corresponding cationic complexes, in which one pyrrolide ligand is replaced by the weakly coordinating anion (WCA) B(ArF)4 was addressed (Scheme 4, B(ArF)4 = tetrakis(3,5-(CF3)2-C6H3)borate). The pyrrolide ligand, comparable to a cyclopentadienyl ligand, can coordinate either η1 or η5, and thereby formally donate either 2 or 6 electrons, which was envisioned beneficial for the stabilization of a cationic metal center. Also, the targeted introduction of sterically demanding ligands was expected to be more feasible in tetracoordinated vs pentacoordinated complexes due to reduced steric bulk. The cationic complexes [Mo(NR 1 )(CHCMe 2 Ph)(NC 4 H 4 )(NHC)][B(ArF)4] Mo-18−Mo-24 were accessible in 61% to quantitative yields by simple protonation of the bispyrrolide precursors with N,N-dimethylanilinium B(ArF)4 etherate in CH2Cl2. Removal of the solvent and subsequent washing with pentane to remove N,N-dimethylaniline and pyrrole led to analytically pure compounds. Mo NHC Complexes with Sterically Demanding Ligands. Next, we sought to verify the hypothesis, that molybdenum imido alkylidene pyrrolide NHC complexes are precursors to complexes bearing sterically demanding aryloxides. On that account the frequently used sterically demanding alcohol 117 was introduced to a neutral Mo imido alkylidene bispyrrolide NHC complex Mo(N-Ad)(CHCMe2Ph)(NC4H4)2(IMeCl)(Mo-17) and to the cationic Mo imido alkylidene pyrrolide NHC complexes Mo-18 and Mo-21 (Scheme 3). The neutral and cationic precursor complexes were reacted with HMTOH 1-H at room temperature in CH2Cl2. Purification only entailed

Scheme 6. Synthesis of a Molybdenum Imido Alkylidene NHC Complex Mo-29 Containing a Sterically Demanding Chiral Alkoxide by Reaction of the Chiral Alcohol 2-H with the Molybdenum Imido Alkylidene Bispyrrolide NHC Complex Mo-13

D


Article

Organometallics diastereomer. Nonetheless, ultrafast Berry-type or turnstile pseudorotations19 in the pentacoordinated complex cannot be fully ruled out, however, seems in view of the size of the ligands involved very unlikely. It should be noted that the cationic complexes Mo-18−Mo-27 are all chiral at the metal. This is reflected in a splitting of the resonances of the diastereotopic methyl groups in the alkylidene ligand and on IPr (for Mo18,21,22,23) in both, 1H and 13C NMR spectra. Single Crystal X-ray Crystallography. To gain insights into the solid-state structures of the complexes, one representative of each “class” of catalysts was subjected to single crystal X-ray analysis. Most compounds crystallized readily from mixtures of dichloromethane, diethyl ether and pentane at −35 °C. Mo-13 crystallizes in the triclinic space group P1̅ (a = 1199.01(6) pm, b = 1290.23(7) pm, c = 1308.22(12) pm; α = 100.521(4)°, β = 95.608(4)°, γ = 116.104(2)°) and adopts an almost perfect SP structure (τ = 0.002)20 with the alkylidene ligand in the apex (Figure 1). The pyrrolide ligands adopt a cis-

Figure 2. Single crystal X-ray structure of Mo-21.21 The anion B(ArF)4 was omitted for clarity. Bond lengths [pm] and angles [°]: Mo−N3 173.6(2), Mo−C22 192.9(3), Mo−C1 218.0(3), Mo−N4 238.8(2), Mo−C18 234.6(3), Mo−C19 239.4(3), Mo−C20 246.8(3), Mo−C21 242.0(3); N3−Mo−C22 102.10(12), N3−Mo−C1 97.87(11), C22− Mo−C1 99.67(11), N3−Mo−N4 161.17(10), C22−Mo−N4 86.38(11), C1−Mo−N4 97.21(10), C1−Mo−C18 130.66(11), C1− Mo−C19 132.46(11), C1−Mo−C20 99.44(11), C1−Mo−C21 81.33.

Figure 1. Single crystal X-ray structure of Mo-13. A weak disorder of an iodide ligand (2.5% vs pyrrolide 97.5%) was observed; the iodide ligand has been omitted for clarity.18 Bond lengths [pm] and angles [°]: Mo− N3 174.8(2), Mo−C16 186.4(3), Mo−N5 210.4(2), Mo−N4 212.9(4), Mo−C1 222.1(3); N3−Mo−C16 99.32(11), N3−Mo−N5 101.82(9), C16−Mo−N5 100.50(11), N3−Mo−N4 152.98(13), C16−Mo−N4 105.71(13), N5−Mo−N4 83.66(12), N3−Mo−C1 85.70(10), C16−Mo−C1 103.76(11), N5−Mo−C1 153.07(9), N4− Mo1−C1 78.69(12).

Figure 3. Single crystal X-ray structure of Mo-26.22 The anion B(ArF)4 was omitted for clarity. Bond lengths [pm] and angles [°]: Mo−N3 172.2(3), Mo−C42 188.5(3), Mo−O1 189.2(2), Mo−C1 216.2(3); N3−Mo−C42 100.62(13), N3−Mo−O1 123.10(10), C42−Mo−O1 115.92(12), N3−Mo−C1 97.34(12), C42−Mo−C1 100.99(13), O1− Mo−C1 115.14(11).

configuration and the Mo-pyrrolide bond trans to the imido is elongated in comparison to the bond trans to the NHC (Mo− N4 = 212.9(4) pm vs Mo−N5 = 210.4(2) pm). The cispyrrolide distorted SP structure is in accordance with the structural data obtained from 1H NMR spectra. Mo-21 crystallizes in the triclinic space group P1̅ (a = 1263.77(5) pm, b = 2105.87(7) pm, c = 2605.38(9)pm; α = 68.192(2)°, β = 78.314(2)°, γ = 83.222(3)°) (Figure 2). Interestingly, the pyrrolide ligand coordinates η5 to the cationic molybdenum center, generating a piano-stool type half sandwich complex. The observed η5-coordination results in a cationic 18-VE complex, whereas η1-coordination would result in a complex with only 14 valence electrons. Before 18-VE Mo-21 can undergo olefin metathesis, the pyrrolide ligand must change its coordination mode from η5 to η1 to allow for olefin coordination. Mo-26 crystallizes in the triclinic space group P1̅ (a = 127.525 pm, b = 1738.03 pm, c = 2005.57 pm; α = 924.90(5)°, β = 97.197(5)°, γ = 105.341(4)°) and adopts a tetrahedral geometry in the solid state (Figure 3). When compared to MAP-type

complexes, bond lengths in Mo-26 are within the same range as found in Mo−O1 (189.2(2) ppm), Mo−N3 (172.2(3) ppm), and Mo−C42 (188.53(3) pm).2d,14,23 The molybdenum CNHC and the Mo−N imido bond lengths decrease from the pentacoordinated neutral complex Mo-13 over the cationic 18-VE complex Mo-21 to the cationic HMTO complex Mo-26. The imido ligands are bent in all the complexes, with larger Mo− Nimido−Cimido angles for the two cationic complexes Mo-21 (170.5°) and Mo-26 (173.2°) vs neutral Mo-13 (161.9°), indicating a higher triple bond character of the Mo−N bond for the former two catalysts. These differences in bond lengths and angles can be rationalized by the more electron deficient metal centers in Mo-12 and Mo-26 vs Mo-13. The Mo−Calkylidene bond length, however, increases in the order Mo-13 < Mo-26 < Mo-21. All three complexes Mo-13, Mo-21, and Mo-26 adopt a syn-configuration in the solid state, which is in accordance with E


Article

Organometallics

Table 1. Productivities Expressed in TONs of Cationic Molybdenum Imido Alkylidene NHC Complexes Mo-18−Mo-27 in RCM (Ring-Closing Metathesis) and CM (Cross Metathesis) Reactionsa substrate

Mo-18

Mo-19

Mo-20

Mo-21

Mo-22

Mo-23

Mo-25

Mo-26

Mo-27

W-1b

1,7-octadiene diallyl diphenyl silane diallyl ether diallyl sulfide allyltrimethylsilane

1300 900 600 230 100

2400 1000 0 0 500

1200 1200 0 0 200

2500 1800 0 0 500

1800 500 0 0 200

2500 2200 500 500 600

2500 2000 0 2500 1000

2500 2000 0 2500 1200

2500 1900 0 1200 800

200 200 0 0 0

a

Reaction conditions: catalyst:substrate: 1:2500, room temperature, 3 h, 1,2-dichloroethane, 0.5 M respective to substrate, release of ethylene overpressure, internal standard: dodecane, determination of conversion via GC−MS. bW-1: [W(N-2,6-iPr2-C6H3)(CHCMe2Ph)(2,5-Me2NC4H2)(IPr)][B(ArF)4].12

solution 1H NMR data. The syn-configuration was observed for all synthesized complexes that allowed for the annotation of synor anti- isomer by determination of the 1JCH coupling constants of the alkylidene protons in the 1H NMR spectra. Reactivity. Previous results published by our group have shown, that good leaving groups (triflates) are required to render pentacoordinated Mo imido alkylidene NHC complexes active in olefin metathesis.7a Therefore, neutral pentacoordinated complexes (Mo-11−Mo-17, Mo-28, Mo-29) were not extensively studied concerning their activity in olefin metathesis. In fact, in accordance with theory, Mo-6 and Mo-29 did not catalyze the ring-closing metathesis (RCM) of 1,7-octadiene at room temperature and 60 °C (cat: substrate 1:2500; 1,2dichloroethane, 3 h). The cationic molybdenum imido alkylidene pyrrolide NHC complexes Mo-18−Mo-24 were tested in a set of simple olefin metathesis reactions and compared to a structurally related, previously published12 cationic tungsten-based complex [W(N-2,6-iPr 2 -C 6 H 3 )(CHCMe2Ph)(2,5-Me2-NC4H2)(IPr)][B(ArF)4] (W-1). In general, the tungsten-based catalyst was less active then the molybdenum-based complexes (Table 1). All cationic molybdenum-based complexes showed moderate to good activity at room temperature in RCM and cross metathesis (CM) of hydrocarbon-based olefins and olefins with less reactive functional groups, such as ethers and thioethers (Table 1). Heating to 60 °C did not lead to an increase in conversion. In the RCM of 1,7-octadiene complete conversion was observed for catalysts Mo-21 and Mo-23 at room temperature (TON = 2500). This suggests that the change from η5-to η1-coordination for the pyrrolide ligand (vide supra, Mo-21) occurs readily at room temperature and in solution. No correlation between the TEP of the NHCs and the respective metal complexes was observed (Mo-18−Mo-20). Instead, the imido ligand seems to have a bigger impact on reactivity (Mo-18−Mo-23). Notable is the inactivity of Mo-24 with the N-adamantyl imido ligand in all tested olefin metathesis reactions, which might be due to a combination of a small NHC (IMeCl) with a comparably small imido ligand (NAd), potentially resulting in fast bimolecular decomposition reactions. The HMTO-based catalysts Mo-25− Mo-27 showed full conversion (TON = 2500) in the RCM of 1,7-octadiene. Interestingly, Mo-25 and Mo-26 also catalyzed the RCM of diallyl sulfide with TONs of 2500. The increase in activity could result from an increase in stability, since the sterically demanding HMTO ligand is more prone to kinetically stabilize a metallacyclobutane bearing a cationic charge than a pyrrolide ligand. This difference in activity is best illustrated by comparing the inactivity of Mo-24 to the TONs observed for Mo-27 (Table 1). Aldehyde, alcohol, and secondary amine moieties were not tolerated in RCM and HM, most probably

due to coordination of the functional groups to the electrondeficient metal centers.

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CONCLUSIONS A general synthetic route to Mo imido alkylidene bispyrrolide NHC complexes and cationic Mo imido alkylidene pyrrolide NHC complexes has been elaborated. The cationic molybdenum imido alkylidene NHC pyrrolide complexes show good activity in RCM and HM reactions at room temperature. Both neutral and cationic molybdenum imido alkylidene NHC pyrrolide complexes are excellent precursors to complexes containing an NHC and a sterically demanding alkoxide. Beneficially, synthesis proceeds in good to high yields, and purification only requires washing to remove the byproducts pyrrole and N,N-dimethylaniline.

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EXPERIMENTAL SECTION

General Information. Unless stated otherwise, all reactions were performed under inert gas atmosphere (N2), either in a glovebox (LabMaster 130, MBraun, Garching, Germany) or with standard Schlenk techniques. CH2Cl2, diethyl ether, toluene, pentane, and tetrahydrofuran were dried by a solvent purification system (SPS, MBraun). NMR measurements were recorded on a Bruker Avance III 400. Chemical shifts are reported in ppm relative to the solvent signal; coupling constants are listed in Hz. 13C NMR spectra were measured using broadband decoupling. Starting materials and reagents were purchased from Merck (Munich, Germany), Alfa Aesar (Karlsruhe, Germany), or ABCR (Karlsruhe, Germany) and were used as received unless stated otherwise. Mo(NR1)(CHCMe2Ph)(OTf)2(DME) complexes (Mo-1−Mo-5)16 (DME = 1,2-dimethoxyethane, R = 2-tBuC6H4, 2,6-iPr2-C6H3, 2,6-Me2-C6H3, tBu, Ad), Mo-10,9 IPr,24 TPT,25 IMeCl·AgI,26 117 and 24b were synthesized according to published procedures. Crystal data have been deposited with the Cambridge Crystallographic Data Centre (CCDC): Mo-13 CCDC 1916194, Mo21 CCDC 1916195, Mo-26 CCDC 1916196. Synthesis of Mo-6. Mo(N-2-tBuC 6 H 4 )(CHCMe 2 Ph)(OTf) 2 (DME) (1000 mg, 1.3 mmol) was suspended in diethyl ether (20 mL) and cooled to −35 °C. Lithium pyrrolide (191 mg, 2.6 mmol) was added and the reaction was stirred for 1 h at room temperature. The solvent was removed in vacuo and the residue was extracted with CH2Cl2. The resulting suspension was filtered through a pad of Celite and the solvent was removed. The residue was stirred with pentane until crystallization commenced. Filtration yielded Mo-6 as yellow crystals in 64% yield. 1H NMR (400 MHz, CD2Cl2) δ = 13.05 (s, 1H, Mo CH)*, 7.37−7.20 (m, 5H), 7.17 (t, 3JHH = 7.7 Hz, 1H), 7.08 (t, 3JHH = 8.3 Hz, 1H), 6.96 (t, 3JHH = 7.0 Hz, 1H), 6.62 (d, 3JHH = 7.9 Hz, 1H), 6.35 (s, 4H, NC4H4), 6.13 (s, 4H, NC4H4), 1.57 (s, 6H, CMe2Ph), 1.23 (s, 9H, tBu) ppm. 13C NMR (101 MHz, CD2Cl2)** δ = 154.7, 148.5, 146.3, 134.1, 128.9, 128.4, 127.1, 126.7, 126.2, 109.0, 57.6, 35.9, 31.0, 30.8 ppm. Elemental analysis (%) calcd. for C28H33MoN3: C 66.26, H 6.55, N 8.28, Found: C 65.85, H 6.545, N 8.16. *Determination of synor anti-configuration was not possible due to the broad signals. **The alkylidene carbon signal was not visible, although 100 mg of catalyst were used for 13C NMR spectroscopy since all signals were very broad, F


Article

Organometallics

exclusion of light. The resulting suspension was stirred for 6 h and then filtered over Celite to remove AgI. All volatiles were removed, and the resulting brownish residue was treated with pentane and diethyl ether until a yellow solid was obtained. This solid was recrystallized from a mixture of CH2Cl2, diethyl ether and pentane to yield Mo-13 in 57% yield as yellow crystals. 1H NMR (400 MHz, CD2Cl2) δ = 13.62 (s, 1H, 1 JCH = 120.7 Hz, syn, MoCH), 7.44−7.41 (m, 2H), 7.37−7.35 (m, 1H), 7.31−7.27 (m, 2H), 7.23−7.21 (m, 1H), 7.16−7.07 (m, 3H), 6.79−6.76 (m, 1H), 6.59 (br s, 2H, NC4H4), 6.05 (br s, 2H, NC4H4), 5.99 (br s, 2H, NC4H4), 5.90 (br s, 2H, NC4H4), 3.10 (s, 6H, NCH3), 1.95 (s, 3H, CMe2Ph), 1.78 (s, 3H, CMe2Ph), 1.24 (s, 9H, tBu) ppm. 13 C NMR (101 MHz, CD2Cl2) δ = 306.1 (MoC), 306.0 (MoC), 189.9 (NHC-NCN), 154.6, 147.6, 146.0, 131.9, 131.5, 131.2, 129.5, 129.3, 129.2, 127.2, 127.2, 127.0, 126.8, 126.7, 126.6, 119.0, 108.3, 107.6, 57.03, 36.8, 35.6, 31.1, 30.8, 30.5, 30.4 ppm. Despite numerous efforts, only varying and inconsistent elemental analysis data were obtained. However, the structure was indirectly confirmed by the successful synthesis of Mo-20. Synthesis of Mo-14. Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(OTf)2 (DME) (497 mg, 0.68 mmol) was suspended in diethyl ether (10 mL) and lithium pyrrolide (99 mg, 1.36 mmol) was added. The mixture was stirred for 1 h at room temperature. Then, a solution of 1,3diisopropylimidazol-2-ylidene (103 mg, 0.68 mmol) in diethyl ether (5 mL) was added. The reaction mixture was stirred for 3 h at room temperature and the solvent was removed in vacuo. The residue was extracted with toluene (20 mL). The suspension was filtered through Celite and the filtrate was concentrated to dryness. The residue was treated with diethyl ether, resulting in a yellow precipitate. The precipitate was filtered off and crystallized from a mixture of CH2Cl2, diethyl ether and pentane to afford Mo-14 in 75% yield. 1H NMR (400 MHz, CD2Cl2) δ = 13.59 (s, 1H, MoCH), 7.50−7.45 (m, 2H), 7.35−7.29 (m, 2H), 7.24−7.18 (m, 1H), 7.11 (s, 2H), 6.93 (s, 3H), 6.71 (t, 3JHH = 1.8 Hz, 2H, NC4H4), 6.16 (t, 3JHH = 1.8 Hz, 2H, NC4H4), 5.83−5.78 (m, 4H, NC4H4), 4.22 (hept, 3JHH = 6.7 Hz, 2H, CHMe2 iPr), 1.93 (s, 3H, CMe2Ph), 1.89 (s, 6H, CH3 imido), 1.62 (s, 3H, CMe2Ph), 0.99 (d, 3JHH = 6.7 Hz, 6H, CHMe2 iPr), 0.91 (d, 3JHH = 6.7 Hz, 6H, CHMe2 iPr) ppm. 13C NMR (101 MHz, CD2Cl2) δ = 299.1 (MoC), 299.0 (MoC), 185.4 (NHC-NCN), 153.5, 148.3, 136.2, 131.0, 130.9, 129.1, 128.7, 126.9, 126.4, 126.3, 118.6, 107.7, 107.3, 55.1, 32.4, 29.2, 23.3, 22.8, 18.4 ppm. Elemental analysis (%) calcd. for C35H45MoN5: C 66.54, H 7.18, N 11.09, Found: C 66.49, H 7.148, N 11.05. Synthesis of Mo-15. Mo-8 (400 mg, 0.75 mmol) was dissolved in diethyl ether and cooled to −35 °C. A cold (−35 °C) solution of 1,3diisopropylimidazol-2-ylidene (114 mg, 0.755 mmol) in diethyl ether (4 mL) was added, resulting in the formation of a yellow precipitate. The suspension was stirred at room temperature for 1 h and filtered. The solid was washed with diethyl ether to afford Mo-15 in 80% yield. 1 H NMR (400 MHz, CDCl3) δ = 13.55 (s, 1H, MoCH), 7.49−7.42 (m, 2H), 7.32 (t, 3JHH = 7.7 Hz, 2H), 7.26−7.18 (m, 1H), 7.11−7.01 (m, 5H), 6.68 (t, 3JHH = 1.8 Hz, 2H), 6.23 (t, 3JHH = 1.8 Hz, 2H), 5.95 (s, 4H), 4.29 (hept, 3JHH = 6.7 Hz, 2H, CHMe2 iPr), 3.30 (hept, 3JHH = 6.8 Hz, 2H, CHMe2 iPr), 2.07 (s, 3H, CMe2Ph), 1.67 (s, 3H, CMe2Ph), 1.15 (d, 3JHH = 6.8 Hz, 6H, CHMe2 iPr), 1.07 (d, 3JHH = 6.7 Hz, 6H, CHMe2 iPr), 0.78 (d, 3JHH = 6.6 Hz, 6H, CHMe2 iPr), 0.62 (d, 3JHH = 6.7 Hz, 6H, CHMe2 iPr) ppm. 13C NMR (101 MHz, CDCl3) δ = 298.5 (MoC), 185.8 (NHC-NCN), 149.9, 148.3, 147.1, 131.1, 130.8, 128.7, 127.0, 126.4, 125.8, 123.8, 118.3, 107.5, 106.8, 54.0, 54.0, 32.6, 28.8, 27.7, 24.8, 24.7, 23.2, 22.9 ppm. Elemental analysis (%) calcd. for C39H53MoN5: C 68.10, H 7.77, N 10.18, Found: C 67.93, H 7.738, N 10.10. Synthesis of Mo-16. Mo(N-tBu)(CHCMe2Ph)(OTf)2(DME) (500 mg, 0.73 mmol) was dissolved in diethyl ether (15 mL) and lithium pyrrolide (106 mg, 1.45 mmol) was added. The resulting suspension was stirred at room temperature for 1 h. A solution of 1,3diisopropylimidazol-2-ylidene (111 mg, 0.73 mmol) in diethyl ether (5 mL) was added and the reaction mixture was stirred for two more h. The solvent was removed in vacuo, the resulting residue was extracted with toluene (2 × 15 mL) and filtered over Celite to remove lithium pyrrolide. The filtrate was concentrated to dryness and the residue was

most probably due to fast exchange of the coordination modes of the pyrrolide ligand or the formation of dimeric, pyrrolide-bridged structures. When measured in MeCN-d3, the 1H NMR spectrum of Mo-6 showed significantly sharper signals; however, fast decomposition occurred. Synthesis of Mo-8. Mo(N-2,6-iPr2C6H3)(CHCMe2Ph)(OTf)2 (DME) (1110 mg, 1.4 mmol) was suspended in diethyl ether (25 mL) and cooled to −35 °C. Lithium pyrrolide (205 mg, 2.8 mmol) was added and the reaction was stirred for 2 h at room temperature. The solvent was removed in vacuo and the residue was extracted with toluene (30 mL). The resulting suspension was filtered through a pad of Celite and the solvent was removed again. The residue was stirred with pentane (20 mL) at room temperature until crystallization commenced. Filtration yielded Mo-8 as yellow crystals in 81% yield. 1H NMR (400 MHz, CDCl3) δ = 13.12 (s, 1H, MoCH), 7.37−7.27 (m, 4H), 7.27− 7.20 (m, 1H), 7.10 (dd, 3JHH = 8.4, 7.0 Hz, 1H), 6.99 (d, 3JHH = 7.3 Hz, 2H), 6.23 (s, 4H, NC4H4), 6.10 (s, 4H, NC4H4), 3.12 (br s, 2H, CHMe2 iPr), 1.49 (s, 6H, CMe2Ph), 0.93 (d, 3JHH = 6.8 Hz, 12H, CH3 iPr) ppm. 13 C NMR (101 MHz, CDCl3) δ = 304.4 (MoC), 151.8, 149.3, 147.9, 132.7, 128.5, 128.1, 126.4, 126.4, 123.3, 108.0, 57.5, 30.5, 27.7, 24.3 ppm. Elemental analysis (%) calcd. for C30H37MoN3: C 67.28, H 6.96, N 7.85, Found: C 67.60, H 7.293, N 7.55. Synthesis of Mo-11. Mo-6 (467.2 mg, 0.9 mmol) was dissolved in diethyl ether and cooled to −35 °C. A solution of 1,3diisopropylimidazol-2-ylidene (140 mg, 0.9 mmol) in diethyl ether at −35 °C was added to the solution of Mo-6. After 30 min a yellow precipitate formed. After 2 h the solid was filtered off and recrystallized from a mixture of CH2Cl2, diethyl ether and pentane. Mo-11 was isolated in the form of yellow crystals in 93% yield. Mo-11 can also be used as received from the reaction mixture without recrystallization. 1H NMR (400 MHz, CDCl3) δ = 13.87 (s, 1H, 1JCH = 123.7 Hz, syn, Mo CH), 7.40 (br s, 2H), 7.24 (s, 1H), 7.12−7.05 (m, 5H), 6.96 (br s, 2H), 6.87 (t, 3JHH= 7.2 Hz, 1H), 6.79 (t, 3JHH= 7.5 Hz, 1H), 6.52−6.50 (m, 3H), 6.41 (s, 2H), 6.29 (br s, 2H),4.40−4.34 (m, 2H, CHMe2 iPr), 2.04 (s, 3H, CMe2Ph), 1.62 (s, 3H, CMe2Ph), 1.52 (s, 9H, tBu), 0.89 (d, 3 JHH= 6.7 Hz, 6H, CHMe2 iPr), 0.67 (d, 3JHH= 6.6 Hz, 6H, CHMe2 iPr) ppm. 13C NMR (101 MHz, CDCl3) δ = 299.72 (MoC), 185.9 (NHC-NCN), 153.5, 147.7, 145.8, 131.0, 131.0, 128.7, 126.5, 126.2, 125.8, 118.1, 107.1, 107.2, 56.3, 53.9, 35.2, 32.9, 30.4, 29.5, 23.1, 23.0 ppm. Elemental analysis (%) calcd. for C37H49MoN5: C 67.36, H 7.49, N 10.61, Found: C 67.46, H 7.559, N 10.49. Synthesis of Mo-12. Mo-6 (195 mg, 0.4 mmol) was dissolved in diethyl ether and a solution of 1,3,4-triphenyl-1,2,4-triazol-5-ylidene (174.7 mg, 0.4 mmol) in diethyl ether was added at −35 °C. The solution was stirred for 4 h and reduced until a saturated solution was reached. Then, the solution was layered with pentane and crystallized at −35 °C to give Mo-12 in 62% yield. Mo-12 cocrystallizes with one molecule of diethyl ether. For elemental analysis crystallized Mo-12 × Et2O was dissolved in CH2Cl2 and the solution was evaporated to dryness to remove diethyl ether.1H NMR (400 MHz, CDCl3) δ = 13.87 (br s, 0.5H, MoCH)*, 13.41 (br s, 0.5H, MoCH)*, 7.80 (br s, 1H), 7.44−7.41 (m, 1H), 7.34−7.27 (m, 6H), 7.24−7.12 (m, 7H), 7.07−6.99 (m, 6H), 6.54−6.26 (br s, 5H), 6.18−6.14 (m, 3H), 6.05− 5.97 (m, 3H), 1.59 (s, 3H, CMe2Ph), 0.93−0.77 (br s, 12H) ppm. 13C NMR (101 MHz, CDCl3) δ = 310.12 (MoC), 309.15 (MoC), 192.8 (NHC-NCN), 191.5 (NHC-NCN), 155.9, 153.8, 148.3, 146.5, 140.6, 140.0, 136.6, 135.9, 132.0 (br s), 131.8 (br s), 131.6 (br s), 131.4 (br s), 130.6 (br s), 130.2 (br s), 130.0 (br s), 129.7, 129.3, 128.7, 126.7 (br s), 126.4, 126.0, 125.0 (br s), 122.9, 109.2, 108.4, 107.0, 66.2 (Et2O), 56.6 (br s), 35.2, 32.1, 31.9, 30.9, 15.7 (Et2O) ppm. Elemental analysis (%) calcd. for C49H51MoN6: C 71.78, H 6.27, N 10.25, Found: C 71.52, H 6.276, N 9.93. *Mo-12 was isolated as mixture of two isomers (1/1); however, determination of syn- or anti-configuration was impossible due to the broad resonances. The sum of the integrals of both isomers was set to one in the 1H NMR spectrum. Most probably the isomers differ in the coordination mode of the pyrrolide ligands and constantly interchange, thereby explaining the broad signals in both the 1 H and 13C NMR spectrum. Synthesis of Mo-13. Mo-6 (400 mg, 0.8 mmol) was dissolved in CH2Cl2 and IMeCl·AgI (315 mg, 0.8 mmol) was added as a solid under G


Article

Organometallics

B(ArF)4), 123.6, 118.1 (B(ArF)4), 112.0, 61.7, 36.1, 32.1, 30.2, 29.1 ppm. Elemental analysis (%) calcd. for C76H56BF24MoN5: C 56.98, H 3.52, N 4.37, Found: C 65.90, H 3.831, N 4.18. *Determination of synor anti-configuration was impossible due to the broad resonance. Synthesis of Mo-20. Mo-13 (111.4 mg, 0.2 mmol) was dissolved in CH2Cl2 (5 mL), cooled to −35 °C and a chilled (−35 °C) solution of N,N-dimethylanilinium B(ArF)4 etherate (175.5 mg, 0.2 mmol) in a mixture of CH2Cl2 and diethyl ether was added dropwise. The mixture was stirred for 3 h at room temperature. The volatiles were removed in vacuo and the resulting residue was washed with pentane (3 × 5 mL) to afford a dark yellow solid. This solid was recrystallized from a mixture of CH2Cl2, diethyl ether and pentane to afford Mo-20 in 61% yield. 1H NMR (400 MHz, CDCl3) δ = 14.04 (s, 1H, MoCH), 7.76 (br s, 1H), 7.70 (s, 8H, o-CarH, B(ArF)4), 7.52 (s, 4H, p-CarH, B(ArF)4), 7.39 (d, 3 JHH = 8.0 Hz, 1H), 7.25 (d, 3JHH = 10.0 Hz, 6H), 7.14 (t, 3JHH = 7.6 Hz, 1H), 7.08−6.88 (m, 2H), 6.56 (br s, 1H), 6.45 (br s, 1H), 3.22 (s, 6H, NCH3), 1.70 (s, 3H, CMe2Ph), 1.62 (s, 3H, CMe2Ph), 1.45 (s, 9H, tBu) ppm. 19F NMR (376 MHz, CDCl3) δ = −62.33 ppm. 13C NMR (101 MHz, CDCl3) δ = 333.3 (MoC), 173.0 (NHC-NCN), 161.8 (q, 1JCB = 49.8 Hz, Cipso, B(ArF)4), 153.4, 145.3, 145.1, 135.0 (B(ArF)4), 133.2, 129.8, 129.2, 129.0 (qq, 2JCF = 31.3 Hz, 3JCB = 3.1 Hz, B(ArF)4), 127.6, 127.4, 127.3, 124.7 (q, 2JCF = 272.8 Hz, CF3 B(ArF)4), 121.8, 117.6 (B(ArF)4), 62.5, 37.4, 36.1, 30.3, 30.0, 29.9 ppm. Elemental analysis (%) calcd. for C61H47BCl2F24MoN4: C 49.85, H 3.22, N 3.81, Found: C 49.84, H 3.541, N 3.70. Synthesis of Mo-21. Mo-14 (300 mg, 0.48 mmol) was dissolved in CH2Cl2 and cooled to −35 °C. A cold (−35 °C) solution of N,Ndimethylanilinium B(ArF)4 etherate (468 mg, 0.48 mmol) in diethyl ether (5 mL) was added. The solution was stirred at room temperature for 2 h and the solvent was removed in vacuo. The residue was treated with n-pentane. The resulting precipitate was filtered off and crystallized from a mixture of CH2Cl2, diethyl ether and pentane to afford Mo-21 in 72% yield. 1H NMR (400 MHz, CD2Cl2) δ = 13.84 (s, 1H, MoCH), 7.75 (s, 8H, o-CarH, B(ArF)4), 7.58 (s, 4H, p-CarH, B(ArF)4), 7.42−7.26 (m, 8H), 7.19−7.13 (m, 1H), 7.09 (dd, 3JHH = 8.6, 6.4 Hz, 1H), 7.05−7.00 (m, 2H), 6.57 (s, 2H), 4.12 (br s, 2H, CHMe2 iPr), 2.10 (s, 6H), 1.68 (s, 3H, CMe2Ph), 1.63 (s, 3H, CMe2Ph), 1.28 (br s, 6H, CHMe2 iPr), 1.15 (d, 3JHH = 6.6 Hz, 6H, CHMe2 iPr) ppm. 19 F NMR (376 MHz, CD2Cl2) δ = −62.76 ppm. 13C NMR (101 MHz, CD2Cl2) δ = 327.0 (MoC), 326.8 (MoC), 169.8 (NHC-NCN), 162.3 (q, 1JCB = 49.8 Hz, Cipso B(ArF)4), 153.2, 147.4, 135.7, 135.4 (B(ArF)4), 129.5 (qq, 2JCF = 34.3 Hz, 3JCB = 2.8 Hz, B(ArF)4), 129.4, 129.3, 128.8, 128.2, 127.4, 126.4, 125.2 (q, 1JCF = 272.6 Hz, CF3 B(ArF)4), 121.8, 118.3−117.9 (B(ArF)4), 110.0, 59.8, 55.0, 31.0, 29.7, 23.6, 19.8 ppm. Elemental analysis (%) calcd. for C63H53BF24MoN4: C 52.96, H 3.74, N 3.92, Found: C 52.95, H 3.797, N 3.94. Synthesis of Mo-22. Mo-15 (250 mg, 0.36 mmol) was dissolved in CH2Cl2 and cooled to −35 °C. Then, 0.2 mL of acetonitrile and a cold (−35 °C) solution of N,N-dimethylanilinium B(ArF)4 etherate (385 mg, 0.36 mmol) in diethyl ether (5 mL) was added. The solution was stirred for 1 h at room temperature. Then, the solvent was removed in vacuo and the residue was triturated twice with pentane. The resulting yellow solid was dried and crystallized from a mixture of CH2Cl2, diethyl ether and n-pentane to provide Mo-22 in 83% yield. The compound shows broad resonances in the 1H NMR spectrum, most probably due to an equilibrium between bound and dissociated acetonitrile. Therefore, 1 equiv of tris(pentafluorophenyl)borane (BCF) was added for 1H NMR measurements. This resulted in wellresolved signals in the 1H NMR spectrum. 1H NMR (400 MHz, CDCl3) δ = 13.53 (s, 1H, MoCH), 7.71 (s, 8H, o-CarH, B(ArF)4), 7.52 (s, 4H, p-CarH, B(ArF)4), 7.43−7.23 (m, 8H), 7.21 (s, 2H), 7.13 (d, 3JHH = 7.8 Hz, 2H), 6.47 (s, 2H), 4.15 (brs, 2H, CHMe2 iPr), 3.31 (hept, 3JHH = 6.8 Hz, 2H, CHMe2 iPr), 1.76 (d, 3JHH = 8.6 Hz, 6H, CHMe2 iPr), 1.27 (d, 3JHH = 6.8 Hz, 6H), 1.23 (d, 3JHH = 6.7 Hz, 6H, CHMe2 iPr), 0.93 (d, 3JHH = 6.6 Hz, 6H, CHMe2 iPr), 0.87 (s, 6H, CHMe2 iPr) ppm. 19F NMR (376 MHz, CD2Cl2) δ = −62.75 ppm. 13C NMR (101 MHz, CD2Cl2) δ = 319.7, 179.7, 162.2 (q, 1JCB = 49.7 Hz, Cipso, B(ArF)4), 151.5, 145.7, 144.2, 135.2 (B(ArF)4), 129.5, 129.3 (qq, 2 JCF = 31.9 Hz, 3JCB = 3.3 Hz, B(ArF)4), 127.7, 126.5, 125.0 (q, 1JCF = 272.8 Hz, CF3 B(ArF)4), 124.6, 119.4, 117.9 (B(ArF)4), 110.4, 56.0,

treated with diethyl ether to afford a light yellow solid. Mo-16 was isolated in 54% yield. 1H NMR (400 MHz, CDCl3) δ = 13.14 (s, 1H, MoCH), 7.36−7.30 (m, 2H), 7.26−7.21 (m, 2H), 7.18−7.13 (m, 1H), 7.00 (s, 2H), 6.74 (t, J = 1.8 Hz, 2H, NC4H4), 6.22 (t, J = 1.8 Hz, 2H, NC4H4), 5.91 (t, J = 1.8 Hz, 2H, NC4H4), 5.82 (t, J = 1.8 Hz, 2H, NC4H4), 4.31 (brs, 2H), 2.12 (s, 3H), 1.57 (s, 3H), 1.50 (s, 3H), 1.38 (s, 9H, tBu), 1.09−0.31 (m, 9H) ppm. 13C NMR (101 MHz, CDCl3) δ = 298.3 (MoC), 184.9 (NHC-NCN), 147.9, 132.7, 129.2, 128.5, 126.3, 125.8, 117.8, 106.9, 106.8, 71.3, 52.0, 31.7, 31.2, 30.7, 22.7 ppm. Elemental analysis (%) calcd. for C31H45MoN5: C 63.79, H 7.77, N 12.00, Found: C 63.78, H 7.762, N 12.01. Synthesis of Mo-17. Mo-10 (173 mg, 0.339 mmol) was dissolved in 6 mL CH2Cl2 and cooled to −35 °C. IMeCl·AgI (177 mg, 0.44 mmol) was added as a solid in one portion. The suspension was stirred for 3 h. The mixture was filtered, and the filtrate was reduced to dryness. A brown solid was obtained. This solid was recrystallized from CH2Cl2 and pentane (1:1, 3 mL) or Pr2O (1 mL) to obtain colorless crystals in 73% yield. 1H NMR (400 MHz, C6D6) δ = 12.82 (s, 1JCH = 116.7 Hz, 1H, MoCH), 7.13 (t, 3JHH = 1.8 Hz, 2H, pyr), 6.86 (m, 3H, ArH), 6.82 (m, 2H, ArH), 6.78 (t, 3JHH = 1.8 Hz, 2H, pyr), 6.62 (t, 3JHH = 1.8 Hz, 2H, pyr), 6.55 (t, 3JHH = 1.8 Hz, 2H, pyr), 2.65 (s, 6H, NCH3), 2.04 (s, 3H, CMe2Ph), 1.85−1.65 (br m, 9H, Ad), 1.45 (s, 3H, CMe2Ph), 1.38 (br m, 6H, Ad). 13C NMR (101 MHz, C6D6) δ = 303.7 (Mo CH), 191.8 (NHC-NCN), 147.5, 134.7, 130.3, 126.1, 125.4, 117.2 (NHCCClCCl), 108.8 (NC4H4), 108.3 (NC4H4), 74.6 (N-Ad), 51.3 (CMe2Ph), 44.2 (Ad-CH2), 36.0 (Ad-CH2), 35.8 (Ad-CH), 30.5 (NCH3), 30.2 (NCH3), 29.9 (CMe2Ph). Elemental analysis (%) calcd. for C33H41Cl2MoN5: C, 58.76; H, 6.13; N, 10.38. Found: C, 58.40; H, 6.118; N, 10.28. Synthesis of Mo-18. Mo-11 (20 mg, 0.03 mmol) was dissolved in CH2Cl2 and cooled to −35 °C. A solution of N,N-dimethylanilinium B(ArF)4 etherate (31 mg, 0.03 mmol) in diethyl ether at −35 °C was added dropwise. After 3 h the solvent was removed and the resulting residue was washed with pentane to remove aniline and pyrrole. Mo-18 was isolated as yellow solid foam in quantitative yield. Mo-18 can also be crystallized from a mixture of CH2Cl2, diethyl ether and pentane to yield light yellow crystals. 1H NMR (400 MHz, CD2Cl2) δ = 14.05 (s, 1H, 1JCH = 127.3 Hz, syn, MoCH), 7.74 (br s, 8H, o-CarH, B(ArF)4), 7.58 (br s, 4H, p-CarH, B(ArF)4), 7.41−7.31 (m, 9H), 7.26−7.22 (m, 2H), 7.11−7.07 (m, 1H), 6.95−6.93 (m, 1H), 6.59 (s, 2H), 4.18 (s, 4H, NC4H4), 1.72 (s, 3H, CMe2Ph), 1.70 (s, 3H, CMe2Ph), 1.25 (d, 3JHH = 6.4 Hz, 6H, CHMe2 iPr), 1.10 (d, 3JHH = 6.4 Hz, 6H, CHMe2 iPr) ppm. 19 F NMR (376 MHz, CDCl3) δ = −63.37 (s, 24F, B(ArF)4) ppm. 13C NMR (101 MHz, C6D6) δ = 328.8 (MoC), 328.6 (MoC), 169.2 (NHC-NCN), 162.3 (q, 1JCB = 49.8 Hz, Cipso, B(ArF)4), 153.7, 147.3, 145.5, 135.4 (B(ArF)4), 129.8, 129.5 (m*, B(ArF)4), 129.5, 128.9, 127.6, 127.4, 127.3, 126.5, 126.3, 123.8, 122.0, 121.1, 118.1 (m, B(ArF)4), 116.9, 113.1, 61.8, 40.9, 36.4, 32.6, 30.3, 30.2, 23.9 ppm. *Expected: qq, not resolved. Elemental analysis (%) calcd. for C65H57BF24MoN4: C 53.59, H 3.94, N 3.85, Found: C 53.59, H 4.235, N 3.83. Synthesis of Mo-19. Mo-12 (135.2 mg, 0.2 mmol) was dissolved in CH2Cl2 and cooled to −35 °C. N,N-Dimethylanilinium B(ArF)4 (174.7 mg, 0.2 mmol) was dissolved in diethyl ether and cooled to −35 °C. The N,N-dimethylanilinium B(ArF)4 etherate solution was slowly dropped to the solution of the precursor complex and the resulting mixture was stirred at room temperature for 3 h. The solvent was removed, the resulting residue was washed with pentane to remove N,N-dimethyl aniline and pyrrole, again dried and then crystallized from a mixture of CH2Cl2, diethyl ether and pentane to yield light yellow crystals of Mo-19 in 87% yield. 1H NMR (400 MHz, CDCl3) δ = 13.67 (br s, 1H, MoCH)*, 7.71−7.67 (m, 12H), 7.52−7.51 (m, 5H), 7.46−7.40 (m, 7H), 7.31−7.27 (m, 2H), 7.23−7.17 (m, 5H), 7.11− 7.08 (m, 2H), 7.02−7.00 (m, 3H), 6.96−6.94 (m, 2H), 5.79 (br s, 2H), 1.51 (s, 3H, CMe2Ph), 1.11 (s, 9H, tBu), 0.82 (s, 3H, CMe2Ph) ppm. 19 F NMR (376 MHz, CDCl3) δ = −62.38 (s, 24F, B(ArF)4) ppm. 13C NMR (101 MHz, C6D6) δ = 338.1 (MoC), 180.3 (NHC-NCN), 162.3 (q, 1JCB = 49.8 Hz, Cipso B(ArF)4), 157.4, 153.9, 147.9, 145.8, 139.3, 135.6, 135.4, 132.6, 130.8, 130.0, 129.6, 129.5, 129.2, 129.0, 128.4, 127.6, 127.4, 127.1, 126.2, 125.2 (q, 1JCF = 272.4 Hz, CF3 H


Article

Organometallics

127.2, 127.0, 125.16 (q, 1JCF = 272.4 Hz, CF3 B(ArF)4)**, 121.8, 119.72, 118.0 (B(ArF)4), 58.7, 36.0, 31.2, 31.0, 30.2, 29.3, 24.5, 24.2, 21.9, 21.3, 20.6, 3.41 (CH3, MeCN)*** ppm. Elemental analysis (%) calcd. for C85H78BF24MoN3O: C 59.35, H 4.57, N 2.44, Found: C 59.08, H 4.662, N 2.62. *The alkylidene carbon signal was not clearly visible in the 13C NMR spectrum, even though 80 mg of catalyst were used and the spectrum was measured from 0 to 400 ppm. However, a small peak at 324.9 ppm was observed. **The fourth signal of the quartet is not observable due to overlapping with other aromatic signals. ***The signal of the nitrile carbon most probably overlaps with the broad signal at 118.0 ppm. Synthesis of Mo-26. Mo-21 (200 mg, 0.14 mmol) was dissolved in CH2Cl2 (5 mL) and a solution of HMTOH (46 mg, 0.14 mmol) in CH2Cl2 (2 mL) was added at room temperature. The solution was stirred overnight. The solvent was removed, and the residue was washed with n-pentane several times until the washing phase remained colorless. n-Pentane was decanted, and the remaining yellow solid was crystallized from CH2Cl2, diethyl ether and pentane to afford Mo26 in 76% yield. 1H NMR (400 MHz, CD2Cl2) δ = 12.41 (s, 1H, Mo CH), 7.77 (s, 8H, o-CarH, B(ArF)4), 7.59 (s, 4H, p-CarH, B(ArF)4), 7.34−7.16 (m, 8H), 7.12−6.94 (m, 7H), 6.90 (s, 2H), 4.01 (hept, 3JHH = 6.6 Hz, 2H, CHMe2 iPr), 2.31 (s, 6H), 1.98 (s, 6H), 1.92 (s, 6H), 1.70 (s, 6H), 1.51 (s, 6H), 1.31 (d, 3JHH = 6.6 Hz, 6H, CHMe2 iPr), 1.06 (d, 3 JHH = 6.6 Hz, 6H, CHMe2 iPr) ppm. 19F NMR (376 MHz, CD2Cl2) δ = −62.77 ppm. 13C NMR (101 MHz, CD2Cl2) δ = 311.8 (MoC), 173.3 (NHC-NCN), 162.4 (q, 1JCB = 49.8 Hz, Cipso B(ArF)4), 158.5, 156.1, 146.1, 138.3, 137.4, 136.4, 135.9, 135.4, 132.2, 131.4, 129.8, 129.6 (qq, 2JCF = 31.3 Hz, 3JCB = 2.8 Hz, B(ArF)4), 129.5, 129.4, 128.8, 127.7, 125.4, 125.2 (q, 1JCF = 273.0 Hz, CF3 B(ArF)4), 121.3, 118.1, 57.6, 55.4, 32.0, 28.5, 24.0, 23.5, 21.4, 21.4, 20.7, 19.4 ppm. Elemental analysis (%) calcd. for C83H74BF24MoN3O: C 58.91, H 4.41, N 2.48, Found: C 59.20, H 4.692, N 2.53. Synthesis of Mo-27. Mo-28 (27.7 mg, 0.0295 mmol, 1 equiv) was dissolved in CH2Cl2 (3 mL) and cooled to −35 °C. N,NDimethylanilinium B(ArF)4 etherate (30.7 mg, 0.0289 mmol, 0.98 equiv) was added as a cold suspension in CH2Cl2 (1 mL). The reaction mixture was stirred for 30 min at room temperature. The solvent was reduced to ∼1 mL and filtered over Celite to remove small amounts of a colorless solid. The filtrate was reduced to dryness, pentane was added, and the resulting yellow oil was washed with pentane (3 mL) two times. All volatiles were removed. A yellow foam was obtained which was pure by NMR (Yield 49 mg, 95%). All attempts to obtain crystalline material failed. 1H NMR (400 MHz, CDCl3) δ = 12.77 (s, 1JCH = 117.7 Hz, 1H, MoCH), 7.71 (br m, 8H, o-CarH, B(ArF)4), 7.52 (br s, 4H, B, p-CarH, B(ArF)4), 7.23 (m, 4H, ArH), 7.08 (m, 4H, ArH), 6.87 (br s, 2H, MesAr), 6.78 (br s, 2H, Mes-Ar), 3.19 (s, 6H, NCH3), 2.23 (s, 6H, Mes), 2.04 (br m, 3H, Ad), 1.99 (s, 6H, Mes), 1.92 (s, 6H, Mes), 1.71 (s, 3H, CMe2Ph), 1.70−1.55 (br m, 9H, Ad), 1.52 (br m, 3H, Ad), 1.50 (s, 3H, CMe2Ph). 19F NMR (376 MHz, CDCl3) δ = −62.39 (s, 24F, B(ArF)4) ppm. 13C NMR (101 MHz, CDCl3) δ = 309.9 (MoC), 176.3 (NHCNCN), 162.1 (q, B(ArF)4), 156.8 (OAr-ipso), 145.8 (CMe2Ph-ipso), 137.7 (Ar), 136.4 (Ar), 136.0 (Ar), 135.0 (Ar), 134.6 (Ar), 130.5 (m, B(ArF)4), 130.3 (Ar), 129.2 (m, B(ArF)4), 129.0 (Ar), 128.6 (Ar), 128.3 (Ar), 127.5 (Ar), 126.1 (Ar), 125.8 (Ar), 125.0 (Ar), 123.3 (Ar), 121.2 (Ar), 120.6 (Ar), 117.6 (m, B(ArF)4), 79.9 (N-Ad), 54.2 (CMe2Ph), 44.9 (Ad-CH2), 38.2 (Ad-CH), 35.1 (Ad-CH2), 31.1 (NHC-Me), 29.8 (NHC-Me), 29.5 (CMe2Ph), 21.2 (Mes), 20.9 (Mes), 20.6 (Mes). Elemental analysis (%) calcd. for C81H70BCl2F24MoN3O·0.5CH2Cl2: C, 55.07; H, 4.03; N, 2.36. Found: C, 55.14; H, 4.224; N, 2.53. Synthesis of Mo-28. Mo-17 (84 mg, 0.165 mmol, 1 equiv) was dissolved in 5 mL toluene and cooled to −35 °C. A cold toluene solution of HMTO (55 mg, 0.165 mmol, 1 equiv) was added dropwise and the mixture was stirred for 1 h at room temperature. Subsequently, IMeCl·AgI (72 mg, 0.165 mmol, 1.1 equiv) was added as a solid in one portion. The suspension was stirred for 8 h. The mixture was filtered over Celite and the filtrate was reduced to dryness. An oily brown solid was obtained. This solid was recrystallized from CH2Cl2 and pentane (1:3, 5 mL) at −40 °C to obtain a yellow crystalline solid (Yield 124 mg, 81%). 1H NMR (400 MHz, C6D6) δ = 12.77 (s, 1JCH = 120.0 Hz, 1H, MoCH), 7.13 (m, 2H, o-ArH), 7.05−6.95 (m, 5H, ArH), 6.86 (m,

54.8, 30.3, 29.9, 29.3, 25.2, 24.0, 23.2, 22.6, 3.5 ppm. Elemental analysis (%) calcd. for C85H78BF24MoN3O: C 54.31, H 4.23, N 4.59, Found: C 54.29, H 4.297, N 4.52. Synthesis of Mo-23. Mo-16 (150 mg, 0.26 mmol) was dissolved in CH2Cl2. A solution of N,N-dimethylanilinium B(ArF)4 etherate (272 mg, 0.26 mmol) in diethyl ether (4 mL) was added and the mixture was stirred at room temperature for 1 h. The solvent was removed, and the residue was washed several times with n-pentane. The resulting precipitate was removed by filtration and crystallized from a mixture of CH2Cl2, diethyl ether and n-pentane to provide Mo-23 in 80% yield. 1H NMR (400 MHz, CDCl3) δ = 13.64 (s, 1H, MoCH), 7.72 (s, 8H, oCarH, B(ArF)4), 7.54 (s, 4H, p-CarH, B(ArF)4), 7.29−6.80 (m, 9H), 6.44 (s, 2H), 4.59 (hept, 3JHH = 6.6 Hz, 2H, CHMe2 iPr), 1.70 (s, 3H, CMe2Ph), 1.57 (s, 3H, CMe2Ph), 1.38 (d, 3JHH = 6.6 Hz, 6H, CHMe2 iPr), 1.16 (s, 9H, tBu), 1.08 (s, 6H, CHMe2 iPr) ppm. 19F NMR (376 MHz, CDCl3) δ = −62.32 (s, 24F, B(ArF)4) ppm.13C NMR (101 MHz, CDCl3) δ = 324.6 (MoC), 172.0 (NHC-NCN), 161.8 (q, 1JCB = 49.8 Hz, Cipso B(ArF)4), 147.6, 134.9 (B(ArF)4), 129.1 (qq, 2JCF = 31.9 Hz, 3 JCB = 3.0 Hz, B(ArF)4), 128.8, 127.0, 126.5, 124.7 (q, 1JCF = 272.3 Hz, CF3 B(ArF)4), 121.0, 117.7 (B(ArF)4), 108.0, 74.2, 56.0, 32.3, 31.5, 30.3, 23.3, 22.8 ppm. Elemental analysis (%) calcd. for C59H53BF24MoN4: C 51.32, H 3.87, N 4.07, Found: C 51.41, H 3.909, N 4.05. Synthesis of Mo-24. A solution of Mo-17 (100 mg, 0.148 mmol, 1 equiv) in 3 mL CH2Cl2 was cooled to −35 °C. A cold suspension of N,N-dimethylanilinium B(ArF)4 etherate (157 mg, 0.148 mmol, 1 equiv) in CH2Cl2 (2 mL) was added. The reaction mixture was stirred for 30 min at room temperature, and then reduced to ∼1 mL and filtered over Celite to remove small amounts of a colorless solid. The filtrate was reduced to dryness, pentane was added, and the resulting oil was washed with pentane (3 mL) two times. All volatiles were removed again. A colorless foam was obtained which was pure by NMR (Yield 201 mg, 92%). However, the compound decomposed on attempted recrystallization. 1H NMR (400 MHz, CDCl3) δ = 13.61 (s, 1JCH = 125.9 Hz, 1H, MoCH), 7.70 (br m, 8H, o-CarH, B(ArF)4), 7.52 (br s, 4H, p-CarH, B(ArF)4), 7.18 (m, 3H, ArH), 7.01 (m, 2H, ArH), 6.96 (br m, 1H, NC4H4), 6.56 (br m, 1H, NC4H4), 6.40 (br m, 1H, NC4H4), 3.39 (s, 6H, NCH3), 2.10 (br m, 3H, Ad), 1.83 (s, 3H, CMe2Ph), 1.82 (br m, 6H, Ad), 1.64 (br m, 3H, Ad), 1.59 (s, 3H, CMe2Ph), 1.56 (br m, 3H, Ad) ppm. 19F NMR (376 MHz, CDCl3) δ = −62.29 (s, 24F, B(ArF)4) ppm. 13C NMR (101 MHz, CDCl3) δ = 326.8 (MoC), 177.3 (NHC-NCN), 162.1 (q, B(ArF)4), 145.5 (Ar), 134.9 (m, B(ArF)4), 129.2 (m, B(ArF)4), 128.8 (Ar), 127.4 (Ar), 127.3 (Ar), 126.1 (Ar), 126.1 (Ar), 123.3 (NHCCClCCl), 121.2 (NHCCCl CCl), 120.3 (NC4H4), 117.6 (m, B(ArF)4), 107.6 (NC4H), 76.0 (NAd), 55.6 (CMe2Ph), 45.4 (Ad-CH2), 38.3 (Ad-CH), 35.2 (Ad-CH2), 30.4 (NHC-Me), 29.5 (NHC-Me), 29.4 (CMe2Ph). Despite numerous efforts, inconsistent elemental analysis data were obtained. Synthesis of Mo-25. Mo-18 (64.4 mg, 0.04 mmol) was dissolved in CH2Cl2 and a solution of 1-H (14.5 mg, 0.04 mmol) in CH2Cl2 was added at −35 °C. The solution was stirred for 3 h and the solvent was removed. The resulting yellow foam was twice coevaporated with pentane and then stirred with pentane until a solid formed. Pentane was decanted (removal of pyrrole) and the remaining yellow solid was crystallized from CH2Cl2, diethyl ether and pentane to afford Mo-25 in quantitative yield. If acetonitrile was used in the synthesis or during purification, Mo-25 contains 1 equiv of acetonitrile. NMR spectra were measured for Mo-25·MeCN, whereas elemental analysis is for Mo-25. 1 H NMR (400 MHz, CD2Cl2) δ = 12.48 (s, 1H, 1JCH = 122.5 Hz, syn, MoCH), 7.74 (br s, 8H, o-CarH, B(ArF)4), 7.57 (br s, 4H, p-CarH, B(ArF)4), 7.37−7.28 (m, 5H), 7.26−7.22 (m, 1H), 7.15 (s, 2H), 7.09− 6.96 (m, 6H), 6.91−6.66 (m, 3H), 5.97 (d, 3JHH = 7.84 Hz, 1H), 3.93 (hept, 2H, 3JHH = 6.5 Hz, CHMe2 iPr), 2.31 (s, 6H, p-CH3 OHMT), 2.00 (s, 12H, o-CH3 OHMT), 1.76 (s, 3H), 1.68 (s, 3H), 1.46 (s, 3H); 1.28 (s, 9H, tBu), 1.26 (d, 6H, 3JHH = 6.5 Hz, CHMe2 iPr), 0.77 (d, 6H, 3 JHH = 6.5 Hz, CHMe2 iPr) ppm. 19F NMR (376 MHz, CD2Cl2) δ = −65.67 (s, 24F, B(ArF)4) ppm.13C NMR (101 MHz, C6D6)* δ = 177.6 (NHC-NCN), 162.3 (q, 1JCB = 49.8 Hz, Cipso, B(ArF)4), 160.9, 154.9, 145.1, 144.6, 138.0, 137.6, 137.2, 137.1, 137.0, 135.4 (B(ArF)4), 132.1, 131.9, 131.5, 130.1, 129.8, 129.6, 129.2, 129.1, 128.9, 127.5, 127.3, I


Article

Organometallics 1H, p-ArH), 6.84 (br s, 2H, Mes-ArH), 6.79 (br s, 2H, Mes-ArH), 6.60 (t, 3JHH = 1.8 Hz, 2H, NC4H4), 6.51 (t, 3JHH = 1.8 Hz, 2H, NC4H4), 2.96 (s, 6H, NCH3), 2.36 (br s, 6H, Mes), 2.27 (s, 6H, Mes), 1.87 (s, 3H, CMe2Ph), 1.77 (br s, 3H, Ad), 1.68 (s, 6H, Mes), 1.55 (br m, 3H, Ad), 1.45 (s, 3H, CMe2Ph), 1.34 (br m, 9H, Ad). 13C NMR (101 MHz, C6D6) δ = 304.5 (MoCH), 190.4 (NHC-NCN), 161.1 (OAr-ipso), 149.2 (CMe2Ph-ipso), 139.5 (Ar), 138.9 (Ar), 137.0 (Ar), 135.2 (Ar), 131.5 (Ar), 131.2 (Ar), 130.7 (Ar), 128.7 (Ar), 126.8 (Ar), 126.1 (Ar), 118.5 (Ar), 116.8 (NHCCClCCl), 106.9 (pyr), 73.0 (N-Ad), 51.7 (CMe2Ph), 43.1 (Ad-CH2), 37.2 (Ad-CH), 36.1 (Ad-CH2), 31.7 (NHC-Me), 30.1 (CMe2Ph), 29.8 (CMe2Ph), 21.7 (Mes), 21.3 (Mes), 19.9 (Mes). Elemental analysis (%) calcd. for C53H62Cl2MoN5O·0.25 CH2Cl2: C, 66.68; H, 6.57; N, 5.84. Found: C, 67.04; H, 6.439; N, 5.90. Synthesis of Mo-29. Mo-13 (75.4 mg, 0.1 mmol) was dissolved in CH2Cl2 and a solution of 2-H (63.5 mg, 0.1 mmol) in CH2Cl2 was added at −35 °C. The solution was stirred at room temperature for 8 h. The volatiles were removed under reduced pressure and the resulting residue was washed with 3 M Novec 7300 Engineered Fluid to remove pyrrole. The resulting solid was dried. Mo-29 was isolated as yellow solid foam in quantitative yield. 1H NMR (400 MHz, CD2Cl2) δ = 13.14 (s, 1H, 1JCH = 124.9 Hz, syn, MoCH), 7.39−7.34 (m, 3H), 7.30−7.28 (m, 3H), 7.25−7.21 (m, 1H), 7.10−7.00 (m, 3H), 6.64− 6.61 (m, 3H), 5.84 (m, 2H), 3.12 (s, 6H, NCH3), 2.73−2.55 (m, 5H), 2.11−1.99 (m, 3H), 1.84 (s, 3H), 1.67−1.64 (m, 8H), 1.53 (s, 3H), 1,14 (s, 9H, tBu), 0.46 (s, 9H, SitBu TBDMS), 0.23 (s, 3H, SiMe TBDMS), −0.18 (s, 3H, SiMe TBDMS) ppm. 13C NMR (101 MHz, C6D6) δ = 309.5, 188.2, 158.1, 154.6, 148.2, 147.9, 147.0, 139.2, 135.9, 133.2, 133.0, 132.1, 131.4, 131.1, 129.4, 128.2, 127.3, 126.7, 126.6(9), 126.6, 126.4, 117.6, 112.8, 110.9, 106.3, 65.5, 37.6, 35.3, 34.7, 31.1, 30.4, 30.2, 29.5, 28.1, 26.0, 25.9, 24.4, 23.5, 23.4, 22.9, 19.0, 14.4, −1.2, −3.64 ppm. Elemental analysis (%) calcd. for C55H68Br2Cl2MoN4O2Si: C 56.37, H 5.85, N 4.78, Found: C 56.13, H 6.092, N 4.55.

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polymerization of 3-substituted cyclooctenes by monoaryloxide pyrrolide imido alkylidene (MAP) catalysts of molybdenum and tungsten. Organometallics 2013, 32, 4843−4850. (b) Schrock, R. R. Synthesis of stereoregular ROMP polymers using molybdenum and tungsten imido alkylidene initiators. Dalton Trans. 2011, 40, 7484− 7495. (c) Jeong, H.; Ng, V. W. L.; Börner, J.; Schrock, R. R. Stereoselective ring-opening metathesis polymerization (ROMP) of methyl-N-(1-phenylethyl)-2-azabicyclo[2.2.1]hept-5-ene-3-carboxylate by molybdenum and tungsten initiators. Macromolecules 2015, 48, 2006−2012. (d) Benedikter, M. J.; Frater, G.; Buchmeiser, M. R. Regioand stereoselective ring-opening metathesis polymerization of enantiomerically pure Vince lactam. Macromolecules 2018, 51, 2276− 2282. (e) Autenrieth, B.; Jeong, H.; Forrest, W. P.; Axtell, J. C.; Ota, A.; Lehr, T.; Buchmeiser, M. R.; Schrock, R. R. Stereospecific ring-opening metathesis polymerization (ROMP) of endo-dicyclopentadiene by molybdenum and tungsten catalysts. Macromolecules 2015, 48, 2480− 2492. (2) (a) Jiang, A. J.; Zhao, Y.; Schrock, R. R.; Hoveyda, A. H. Highly Zselective metathesis homocoupling of terminal olefins. J. Am. Chem. Soc. 2009, 131, 16630−16631. (b) Meek, S. J.; O’Brien, R. V.; Llaveria, J.; Schrock, R. R.; Hoveyda, A. H. Catalytic Z-selective olefin crossmetathesis for natural product synthesis. Nature 2011, 471, 461−466. (c) Peryshkov, D. V.; Schrock, R. R.; Takase, M. K.; Müller, P.; Hoveyda, A. H. Z-Selective olefin metathesis reactions promoted by tungsten oxo alkylidene complexes. J. Am. Chem. Soc. 2011, 133, 20754−20757. (d) Townsend, E. M.; Schrock, R. R.; Hoveyda, A. H. Zselective metathesis homocoupling of 1,3-dienes by molybdenum and tungsten monoaryloxide pyrrolide (MAP) complexes. J. Am. Chem. Soc. 2012, 134, 11334−11337. (e) Speed, A. W.; Mann, T. J.; O’Brien, R. V.; Schrock, R. R.; Hoveyda, A. H. Catalytic Z-selective cross-metathesis in complex molecule synthesis: a convergent stereoselective route to disorazole C1. J. Am. Chem. Soc. 2014, 136, 16136−16139. (f) Hoveyda, A. H. Evolution of catalytic stereoselective olefin metathesis: from ancillary transformation to purveyor of stereochemical identity. J. Org. Chem. 2014, 79, 4763−4792. (g) Yu, M.; Schrock, R. R.; Hoveyda, A. H. Catalyst-controlled stereoselective olefin metathesis as a principal strategy in multistep synthesis design: a concise route to (+)-neopeltolide. Angew. Chem. Int. Ed. 2015, 54, 215−220. (3) Zhang, H.; Yu, E. C.; Torker, S.; Schrock, R. R.; Hoveyda, A. H. Preparation of macrocyclic Z-enoates and (E,Z)- or (Z,E)-dienoates through catalytic stereoselective ring-closing metathesis. J. Am. Chem. Soc. 2014, 136, 16493−16496. (4) (a) Sattely, E. S.; Meek, S. J.; Malcolmson, S. J.; Schrock, R. R.; Hoveyda, A. H. Design and stereoselective preparation of a new class of chiral olefin metathesis catalysts and application to enantioselective synthesis of quebrachamine: Catalyst development inspired by natural product synthesis. J. Am. Chem. Soc. 2009, 131, 943−953. (b) Malcolmson, S. J.; Meek, S. J.; Sattely, E. S.; Schrock, R. R.; Hoveyda, A. H. Highly efficient molybdenum-based catalysts for enantioselective alkene metathesis. Nature 2008, 456, 933−937. (5) (a) Hoveyda, A. H.; Malcolmson, S. J.; Meek, S. J.; Zhugralin, A. R. Catalytic enantioselective olefin metathesis in natural product synthesis. Chiral metal-based complexes that deliver high enantioselectivity and more. Angew. Chem., Int. Ed. 2010, 49, 34−44. (b) Fürstner, A. Teaching metathesis “simple” stereochemistry. Science 2013, 341, 1357−1365. (6) Poater, A.; Solans-Monfort, X.; Clot, E.; Copéret, C.; Eisenstein, O. Understanding d(0)-olefin metathesis catalysts: which metal, which ligands? J. Am. Chem. Soc. 2007, 129, 8207−8216. (7) (a) Buchmeiser, M. R.; Sen, S.; Lienert, C.; Widmann, L.; Schowner, R.; Herz, K.; Hauser, P.; Frey, W.; Wang, D. Molybdenum imido alkylidene N-heterocyclic carbene complexes: Structure− productivity correlations and mechanistic insights. ChemCatChem 2016, 8, 2710−2723. (b) Sen, S.; Schowner, R.; Imbrich, D. A.; Frey, W.; Hunger, M.; Buchmeiser, M. R. Neutral and cationic molybdenum imido alkylidene N-heterocyclic carbene complexes: reactivity in selected olefin metathesis reactions and immobilization on silica. Chem. Eur. J. 2015, 21, 13778−13787.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00148. 1 H, 19F, and 13C NMR spectra of all new compounds, Crystal data of Mo-13, Mo-21, and Mo-26 (PDF) Accession Codes

CCDC 1916194−1916196 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Michael R. Buchmeiser: 0000-0001-6472-5156 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support provided by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation, project number BU 2174/19-1, BU2174/22-1, 358283783 − CRC 1333) and XiMo AG, Switzerland, is gratefully acknowledged.

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REFERENCES

(1) (a) Jeong, H.; Kozera, D. J.; Schrock, R. R.; Smith, S. J.; Zhang, J.; Ren, N.; Hillmyer, M. A. Z-Selective ring-opening metathesis J


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Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes

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