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. Author manuscript; available in PMC: 2019 Jan 15.
Published in final edited form as: Organometallics. 2018 May 21;37(11):1641–1644. doi: 10.1021/acs.organomet.8b00168

Synthesis of High-Oxidation-State Mo═CHX Complexes, Where X = Cl, CF3, Phosphonium, CN

Sudarsan VenkatRamami ‡, Richard R Schrock ‡,*, Amir Hoveyda †, Peter Müller ‡, Charlene Tsay ‡
PMCID: PMC6332497  NIHMSID: NIHMS999351  PMID: 30655653

Abstract

Reactions between Z-XCH=CHX where X = Cl, CF3, or CN and Mo(N-t-Bu)(CH-t-Bu)(OHIPT)Cl(PPh2Me) (OHIPT = O-2,6-(2,4,6-i-Pr3C6H2)2C6H3) produce Mo(N-t-Bu)(CHX)(OHIPT)Cl(PPh2Me) complexes. Addition of 2,2’-bipyridyl (Bipy) yields Mo(N-t-Bu)(CHX)(OHIPT)Cl(Bipy) complexes, which could be isolated and structurally characterized. The reaction between Mo(N-t-Bu)(CH-t-Bu)(OHMT)Cl(PPh2Me) (OHMT = O-2,6-(2,4,6-Me3C6H2)2C6H3) and Z-ClCH=CHCl in the presence of Bipy produces a mixture that contains both Mo(N-t-Bu)(CHCl)(OHMT)Cl(PPh2Me) and Mo(N-t-Bu)(CHCl)(OHMT)Cl(Bipy), but the relatively insoluble product that crystallizes from toluene-d8 is the phosphoniomethylidene complex, [Mo(N-t-Bu)(CHPPh2Me)(OHMT)(Cl)(Bipy)]Cl. The Mo(N-t-Bu)(CHX)(OHIPT)Cl(PPh2Me) complexes (X = Cl or CF3) were confirmed to initiate the stereoselective cross-metathesis between Z-5-decene and Z-XCH=CHX.

Monoaryloxide monochloride molybdenum-based metathesis initiators of the type Mo(NR)(CHR’)(OAr)Cl(L) (where OAr is a sterically demanding 2,6-terphenoxide1 and L a 2e donor ligand) have been found to promote stereoselective (E or Z) metathesis reactions between “ordinary” olefins and ClCH=CHCl, BrCH=CHF, or (CF3)CH=CH(CF3).2 Vinyl halides are desirable cross-partners in cross-metathesis reactions because alkenyl halide (X = Cl or Br) products subsequently can be used in other catalytic reactions.2e Ruthenium-catalyzed cross-metathesis reactions that use vinyl chlorides (CH2=CHCl, ClCH=CHCl (E or Z), or E-MeCH=CHCl) or fluorides have been the subject of several investigations periodically since 2000.3 Although alkenyl halide products have been observed, the reactions are not stereoselective and turnovers are limited, in part due to formation of ruthenium carbide complexes.4 The preparation and isolation of Mo=CHX complexes are keys to understanding the stabilities and reactivities of Mo=CHX complexes versus Mo=CHR complexes in cross-metathesis reactions, where R is a carbon-based group, or H. To our knowledge no Mo=CHX intermediate in a cross-metathesis reaction that involves XCH=CHX (e.g., X = Cl or CF3) has been observed. We have now found a way to prepare Mo=CHCl, Mo=CHCF3, and Mo=CHCN complexes, and have structurally characterized 2,2’-bipyridine adducts thereof.

Recently we found that the most successful Mo(N-t-Bu)(CH-t-Bu)(OAr)Cl(PPh2Me) initiators in the test reaction shown in equation 1 are those in which OAr is OHMT (in 1a) or OHIPT (in 1b).5 The reason is that the large OAr ligand encourages dissociation of PPh2Me, which is required to access to the catalytically active 14e Mo(N-t-Bu)(CH-t-Bu)(OAr)Cl core. A low degree of dissociation of phosphine is therefore likely to be the reason why complexes in which OAr = O-2,3,5,6-tetraphenylphenoxide are relatively inactive. We also found that although PPh2Me is fully dissociated when OAr = hexa-t-butylterphenoxide (OHTBT),6 the 14e Mo(N-t-Bu)(CH-t-Bu)(OHTBT)Cl core is simply too crowded to react readily with either cyclooctene or Z-ClCH=CHCl. Therefore, we felt that reactions of 1a and 1b with Z-ClCH=CHCl could provide the opportunity to observe and isolate Mo=CHCl complexes.

graphic file with name nihms-999351-f0001.jpg (1)

graphic file with name nihms-999351-f0002.jpg

graphic file with name nihms-999351-f0003.jpg (2)

Compound 1b reacts in seconds with several equivalents of Z-ClCH=CHCl (Z-DCE) in C6D6 or toluene-d8 at 22 °C to give Z-ClCH=CH-t-Bu and what we propose is Mo(N-t-Bu)(CHCl)(OHIPT)Cl(PPh2Me) (2b). Its alkylidene proton resonance is a doublet at 9.56 ppm in C6D6 with JHP = 5.3 Hz and JCH = 156 Hz. A 1H-13C HSQC NMR experiment locates the alkylidene Cα resonance at 267.7 ppm. The high solubility of 2b prevented its crystallization, so Bipy was added to give Mo(N-t-Bu)(CHCl)(OHIPT)Cl(Bipy) (3b), whose alkylidene resonance is observed at 10.19 ppm in toluene-d8 (JCH = 155 Hz). Removal of solvent in vacuo and trituration of the residue allowed pure 3b to be isolated and recrystallized.

An X-ray structural study (Figure 1) showed 3b to contain a syn alkylidene (Cl points toward the imido ligand) with the Bipy ligand coordinated trans to the alkylidene and chloride ligands. The alkylidene proton was located in the difference Fourier map (Mo1-C1-H1 = 120(2)°). The Mo1-C1-Cl angle (128.92(18)°) and the Mo=C1 distance (1.944 Å) are not unusual for high oxidation state Mo syn alkylidene complexes (see SI).7 The value for JCH (154 Hz) in 2b is high compared to a JCH expected for a syn Mo=CHR analog when R is a carbon-based group (115–130 Hz), but JCH values in vinyl halides are inherently high.8 The structure of Mo(N-t-Bu)(CHCl)(OHIPT)Cl(PPh2Me) (2b, eq 2) is proposed to be analogous to that of Mo(N-t-Bu)(CH-t-Bu)(OHMT)Cl(PPh2Me),5 a square pyramid (τ = 0.24)9 with the alkylidene in the apical position and the N-t-Bu and OHMT ligands trans to one another.

Figure 1.

Figure 1.

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Molecular structure of Mo(N-t-Bu)(CHCl)(OHIPT)Cl(Bipy). All hydrogen atoms (except on C1), lattice solvent, and disordered atoms have been omitted for clarity.

The reaction between 1a and Z-ClCH=CHCl in toluene-d8 at 22 °C is sluggish compared to the rate of the reaction between 1b and Z-ClCH=CHCl because PPh2Me is not dissociated in the OHMT complex to as great an extent in 1a as it is in 1b. Upon heating the reaction mixture to 50 °C for 2 h a doublet alkylidene resonance that we ascribe to Mo(N-t-Bu)(CHCl)(OHMT)Cl(PPh2Me) (2a) appears at 11.08 ppm along with Z-ClCH=CH-t-Bu olefinic proton resonances. Addition of Bipy to 2a and heating the sample to 50 °C led to formation of what we propose is Mo(N-t-Bu)(CHCl)(OHMT)Cl(Bipy) (3a), which has an alkylidene resonance at 10.5 ppm. Continued heating leads to deposition of crystals on the walls of the NMR tube as the intensity of the alkylidene resonance for 3a declines. The isolated crystals (4a) were found to exhibit a doublet alkylidene proton resonance at 12.81 ppm with JHP = 4.1 Hz (in CD2Cl2).

An X-ray structural study (Figure 2) of 4a showed it to be the “phosphoniomethylidene” derivative, [Mo(N-t-Bu)(CHPPh2Me)(OHMT)Cl(Bipy)]Cl. Phosphoniomethylidene complexes were first prepared employing anions of phosphorus ylides.10 Sundermeyer has also published several examples (e.g., for Nb, Ta, W, and Re),11 but phosphoniomethylidene complexes perhaps are best known for Ru complexes of the type that are active for olefin metathesis.12 They usually are formed in a reaction between an intermediate, and sometimes observable, Ru=CHCl complex, and a phosphine originally present on the metal.

Figure 2.

Figure 2.

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Molecular structure of [Mo(N-t-Bu)(CHPPh2Me)(OHMT)Cl(Bipy)]Cl. All hydrogen atoms (except on C1) and lattice solvent have been omitted for clarity.

Metathesis reactions have been reported that use Z-(CF3)CH=CH(CF3) (Z-HFB) as a cross-metathesis partner and Mo(N-t-Bu)(CH-t-Bu)(OHIPT)Cl(MeCN) as the initiator (from which MeCN readily dissociates).2f The reaction between 1b and five equivalents of Z-HFB in C6D6 at 22 °C generates Z-(t-Bu)CH=CH(CF3) and what we propose is Mo(N-t-Bu)(CHCF3)(OHIPT)Cl(PPh2Me) (4b). The reaction at 22 °C requires approximately 36 h to proceed to completion (at a concentration of 0.057 M for 1b). The alkylidene proton resonance in 4b in C6D6 is found at 9.66 ppm as a broad and relatively featureless multiplet that spans 200 Hz (0.4 ppm) as a consequence of coupling of the alkylidene proton to both P and F. A broadband decoupled 1H{31P} NMR spectrum reveals the expected quartet multiplicity for the alkylidene proton with a 3JHF coupling constant of approximately 12.6 Hz. The 19F NMR spectrum in C6D6 shows a doublet centered at −54.3 ppm (3JHF = 10.1 Hz) for Z-(t-Bu)CH=CH(CF3) and a broad multiplet at −55.2 ppm for the CF3 group in 4b. Addition of Bipy to the C6D6 solution of Mo(N-t-Bu)(CHCF3)(OHIPT)Cl(PPh2Me) gave Mo(N-t-Bu)(CHCF3)(OHIPT)Cl(Bipy) (5b) readily, as evidenced by the appearance of a quartet resonance at 11.29 ppm (3JHF =16.1 Hz) in C6D6 for the alkylidene proton. The corresponding 19F resonance for the CF3 group in 5b is found as a doublet at −53.6 ppm in C6D6 (3JHF = 16.2 Hz). An analogous reaction between Mo(N-t-Bu)(CH-t-Bu)(OHMT)Cl(PPh2Me) and Z-HFB is too slow to yield an analogous Mo(N-t-Bu)(CHCF3)(OHMT)Cl(PPh2Me) complex and first metathesis product, Z-(t-Bu)CH=CH(CF3), in any significant yield.

An X-ray structural study of 5b (Figure 3) showed it to be analogous to the other structures described here. The alkylidene proton was located in the difference Fourier map and refined semi-freely (Mo1-C1-Hl = 118.1(15)°). The Mo1-C1-C2 angle (133.77(17)°) is relatively normal for high oxidation state Mo syn alkylidene complexes, as is the Mo=C1 distance (1.948(2) Å; see SI).

Figure 3.

Figure 3.

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Molecular structure of Mo(N-t-Bu)(CHCF3)(OHIPT)Cl(Bipy). All hydrogen atoms (except on C1), lattice solvent, and disordered atoms have been omitted for clarity.

Metathesis of cyano-substituted olefins (acrylonitrile) with molybdenum catalysts was first explored by Crowe,13 who used Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)[OC(CF3)2Me]2 as the initiator. Others have periodically explored similar metathesis reactions, mostly with ruthenium complexes.14 These reports, and the successful reactions between 1b and Z-ClCH=CHCl or Z-(CF3)CH=CH(CF3) just described, encouraged us to try to the reaction between 1b and Z-(CN)CH=CH(CN). Compound 1b reacts with two equivalents of Z-(CN)CH=CH(CN) in C6D6 at a concentration of 0.041 M (for 1b) at 22 °C to give Mo(N-t-Bu)(CHCN)(OHIPT)Cl(PPh2Me) (6b). The doublet alkylidene proton resonance for 6b in C6D6 is centered at 8.73 ppm (3JHP = 7.4 Hz; JCH = 154 Hz). In a one bond 1H-13C correlation (HSQC) NMR experiment the Cα resonance was found to be at 231.1 ppm. Addition of Bipy to the solution of 6b gave Mo(N-t-Bu)(CHCN)(OHIPT)Cl(Bipy) (7b) in a slow reaction that required several hours. In situ NMR monitoring shows the formation of two singlets at 10.73 ppm (25%) and 10.33 ppm (75%) for 7b, which we ascribe to two isomers. The major alkylidene of 7b with the resonance at 10.33 ppm in C6D6 is obtained selectively upon recrystallization of the mixture.

Crystals of 7b suitable for an X-ray study were grown upon addition of one equivalent of Bipy in a benzene solution of 6b which was left to stand without stirring. The X-ray structural study (Figure 4) showed that 7b is analogous to 3b and 5b, with the Bipy ligand coordinated trans to the alkylidene and the chloride ligands. The t-butyl imido and the HIPTO ligands occupy the apical positions. The alkylidene proton was located in the difference Fourier map and refined semi-freely (Mo1-C1-Hl = 115.7(16)°). The Mo1-C1-C2 angle (129.21(18)°) is relatively normal for high oxidation state Mo syn alkylidene complexes, as is the Mo=C1 distance (1.961(2) Å); all are similar to analogous distances and angles found in 3b and 5b (see SI).

Figure 4.

Figure 4.

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Molecular structure of Mo(N-t-Bu)(CH(CN))(OHIPT)Cl(Bipy). All hydrogen atoms (except on C1), lattice solvent, and disordered atoms have been omitted for clarity.

Cross-metathesis reactions between Z-5-decene and either Z-DCE or Z-HFB catalyzed by monomeric complexes analogous to either 1a or 1b have been published;2 these reactions require formation of Mo=CHX (where X = Cl, CF3) intermediates and reaction of them with the cross partner. We have confirmed that 2b and 4b are viable intermediates in reactions between Z-5-decene and a slight excess of XCH=CHX (X = Cl or CF3; eq 3) and that the selectivity for formation of the Z metathesis products approaches 100%. The results are shown in Table 1.

graphic file with name nihms-999351-f0004.jpg (3)

Table 1.

Results of reactions between 2b and 4b with Z-5-Decene and Z-XCH=CHX in C6D6 (X = Cl or CF3).

Equivof A X T (°C) % Conv to B (120 min) % Z of B
3 Cl 22 66a >99b
10c CF3 50 82 >99c
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a

Addition of two equiv of Z-DCE generates >99% of the expected B.

b

Determined by GC.

c

Determined by 19F NMR spectroscopy.

The cross-metathesis reaction between Z-5-decene and Z-DCE catalyzed by 2b proceeds efficiently at 22 °C to generate Z-1-chlorohexene, but >1 equiv of Z-DCE (relative to Z-5-decene) is required for a high conversion to B. The concentration of 2b (monitored by integrating the alkylidene resonance at 9.56 ppm) remains approximately constant throughout the course of the reaction. The rate of reaction begins to slow as Z-DCE is consumed and accelerates upon addition of more Z-DCE. The cross-metathesis experiment between Z-5-decene and Z-HFB initiated by 4b proceeds relatively efficiently only at 50 °C. Although PPh2Me in 4b is rapidly exchanging (as evidenced by the broad alkylidene resonance at 9.66 ppm), the reaction between 4b and Z-5-decene is relatively slow.

Thus far, attempts to promote the cross-metathesis reaction shown in equation 1 through addition of one equiv of B(C6F5)3 to 2b followed by the addition of the olefinic partners have failed. Addition of one equiv of B(C6F5)3 to 2b in the absence of olefin leads to apparent catalyst decomposition (according to proton NMR spectra).

To our knowledge the Bipy derivatives of the Mo=CHCl, Mo=CHCF3, Mo=CHCN, and Mo=CHPPh2Me complexes reported here are the only structurally-characterized examples for molybdenum. Phosphoniomethylidene complexes are relatively well-known, especially for ruthenium, as noted earlier. Two Ru=CHF complexes have been isolated and structurally characterized,3a,e but they are relatively unreactive toward unstrained olefins. An attempt to prepare a high oxidation state W=CHCN complex led to a catalytically inactive tetramer, {W(NAr)(CHCN)[OC(CF3)2Me]2}4 and a tetrameric complex in which acetonitrile has inserted twice into the W=C bond to give a diazatungstanacyclohexadiene complex.15 Addition of CH2=CHX (e.g., X = B(pin), PPh2, O-n-Pr, and SPh, inter alia) to Mo(NAr)(CHR)(Me2Pyr)(OTPP) (Ar = 2,6-i-Pr2C6H3, R = H or CHCMe2Ph, Me2Pyr = 2,5-dimethylpyrrolide, OTPP = O-2,3,5,6-Ph4C6H) complexes led to Mo(NAr)(CHX)(Me2Pyr)(OTPP) complexes,16 but attempts to prepare Mo=CHCl complexes in this manner failed. We could find no examples in the literature of structurally characterized M=CHCF3 or M=CHCl complexes.

We look forward to exploring the synthesis and reactions of the complexes reported here in more detail as well as preparing and exploring those that contain other electron-withdrawing groups in the alkylidene.

ACKNOWLEDGMENT

We are grateful for financial support from the National Institutes of Health (GM-59426) and from the National Science Foundation (CHE-1463707).

Footnotes

Supporting Information

The following Supporting Information is available free of charge on the ACS Publications website: (i) Full experimental details including NMR data and spectra for new compounds; (ii) X-ray crystallographic files for four complexes.

The authors declare no competing financial interests.

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