Synthesis, Characterization, and Ligand Rearrangement of Tungsten

53 mins ago - In an effort to study the β-H abstraction/1,3-addition mechanism for CH bond activation, the syntheses of tungsten organometallic compl...
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Communication Cite This: Organometallics XXXX, XXX, XXX−XXX

Synthesis, Characterization, and Ligand Rearrangement of Tungsten Cyclopropyl Complexes Kyle D. J. Parker,†,‡ Laure Vendier,†,‡ and Michel Etienne*,†,‡ †

CNRS, LCC (Laboratoire de Chimie de Coordination), 205 Route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France Université de Toulouse, UPS, INPT, LCC, F-31077 Toulouse Cedex 4, France

‡

S Supporting Information *

ABSTRACT: In an effort to study the β-H abstraction/1,3addition mechanism for CH bond activation, the syntheses of tungsten organometallic complexes featuring a cyclopropyl ligand were undertaken. Attempts to prepare the chloro cyclopropyl complex Cp*W(NO)Cl(c-C3H5) instead afforded the previously reported allyl complex Cp*W(NO)Cl(η3-CH2CHCH2), due to a ring-opening rearrangement of the cyclopropyl ligand. The alkyl cyclopropyl complex Cp*W(NO)(CH2SiMe3)(c-C3H5) (1TMS) was prepared and characterized in solution via NMR spectroscopy but also undergoes a ligand rearrangement process to afford the allyl complex Cp*W(NO)(CH2SiMe3)(η3CH2CHCH2) (2TMS), which was characterized by NMR spectroscopy and an X-ray diffraction study. Attempts to inhibit the ligand rearrangement or isolate 1TMS in the solid state have been unsuccessful. In order to investigate the mechanism of the conversion of 1TMS to 2TMS, a kinetic study was undertaken, and some preliminary results are discussed.

T

he activation of CH bonds by early-transition-metal complexes can proceed through a variety of mechanisms. In select cases, this transformation can be achieved via oxidative addition, similar to the case for their late-metal counterparts.1 In complexes containing carbon- or nitrogen-based ligands, σbond metathesis 2 and 1,2-CH addition at an imide,3 alkylidene,4 or alkylidyne5 moiety are the most prominent mechanisms by which this reaction occurs. While less common, CH bond activation can also be achieved via 1,3-addition across the bond between an early-transition-metal center and an unsaturated alkyne/aryne,6 allene/diene,7 or alkene8 ligand. Previous work from our group has focused on studying this 1,3-CH addition mechanism with early-transition-metal complexes that feature a cyclopropyl ligand. As shown in Scheme 1,

In this work, we describe our efforts to extend these investigations to group 6 organometallic complexes. To this end, we took inspiration from the seminal work done by Legzdins and co-workers, investigating the activation of CH bonds with tungsten nitrosyl dialkyl complexes, Cp*W(NO)R2.4d,14 This system was attractive for several reasons: in addition to being a capable ligand framework for the synthesis of a wide variety of mixed dialkyl and diaryl complexes,15 several congeners have been observed to undergo β-H abstraction to form η2-alkene or -allene type intermediates similar to the type generated by cyclopropyl complexes, as in the case of Cp*W(NO)(cyclohexyl)(neopentyl) (Scheme 2).16 Furthermore, many of these tungsten nitrosyl complexes, such as Cp*W(NO)(η3-1-methylallyl)(neopentyl), are capable of cleaving CH bonds, including those of methane, via the 1,3addition mechanism (Scheme 2).17 Thus, our first task was to synthesize new tungsten nitrosyl complexes that feature cyclopropyl ligands, with the ultimate goal of investigating their ability to activate CH bonds via the 1,3-addition mechanism. A retrosynthetic analysis of our approach is shown in Scheme 3. Using the tungsten dichloride complex A18 as a starting point, one may install a cyclopropyl group first, to afford the tungsten cyclopropyl chloride B, and then proceed to add a cyclopropyl, or generic alkyl group, to generate the dicyclopropyl derivative D or the mixed dialkyl species 1, respectively. Conversely, it is also possible to synthesize 1 via addition of a cyclopropyl group to the alkyl chloride complex C.

Scheme 1

a transient metallabicyclobutane/η2-cyclopropene intermediate is formed via the intramolecular abstraction of a β-H from the cyclopropyl ligand. Reports from our laboratory have described the reactivity of group 4 and 5 cyclopropyl complexes, such as Cp2Zr(c-C3H5)29 and TpMe2NbMe(c-C3H5)(MeCCMe),10 which are capable of cleaving a variety of CH bonds, including furan and thiophene,9 benzene,10 xylenes,11 and other unsaturated hydrocarbons,12 and even the strong and relatively inert CH bonds of methane.13 © XXXX American Chemical Society

Received: January 12, 2018

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DOI: 10.1021/acs.organomet.8b00019 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics Scheme 2

Scheme 4

(NO)(CH2TMS)(c-C3H5) (1TMS). The 13C-gated NMR spectrum features, in addition to Cp* and CH2 TMS resonances, a doublet at δ 65.0 (1JCH = 145 Hz) attributed to Cα of the cyclopropyl ring and two doublets of doublets at δ 15.1 (1JCH = 167, 158 Hz) and δ 9.7 (1JCH = 163, 162 Hz) for the two inequivalent Cβ. A 1H−13C HMQC experiment correlates these carbon resonances to three poorly resolved multiplets in the 1H NMR spectrum corresponding to the five inequivalent protons of the cyclopropyl ring: δ 1.98 (Hα), 1.80−1.65 (overlapping resonances for 3 Hβ), and 0.55 (1 Hβ). Coupling between 183W (14.3%, I = 1/2) and the Cα of the cyclopropyl (1JWC = 123 Hz) and methyltrimethylsilyl (1JWC = 89 Hz) ligands is observed in the 13C{1H} NMR spectrum, which reflects the greater degree of sp2 hybridization in the strained C3H5 ring. Unfortunately, 1TMS is thermally sensitive, and at room temperature THF solutions gradually change color from redpurple to brown-amber over a period of several hours. When this system is monitored by 1H NMR spectroscopy, we observe the loss of resonances attributed to 1TMS and the appearance of resonances corresponding to the alkyl allyl complex Cp*W(NO)(CH2 TMS)(η3-CH 2CHCH2) (2TMS), as shown in Scheme 5. The decomposition of 1TMS occurs over the course of approximately 30 h and only gives rise to resonances attributable to 2TMS (see the Supporting Information).

Scheme 3

We began our investigations by focusing on synthesizing the mono- and dicyclopropyl derivatives B and D. Initial attempts, where cooled (−78 °C) THF solutions of A were treated with either 1 or 2 equiv of a cyclopropyllithium (c-C3H5Li) THF solution, produced an intractable mixture of products. Mindful of the success that Legzdins and co-workers have had preparing tungsten alkyl complexes using solid dialkylmagnesium dioxane (1,4-C4H8O2) adducts prepared from the corresponding Grignard reagents,19 we attempted these analogous reactions. Vacuum-transferring THF into a Schlenk flask containing an intimate mixture of solid A and 1 equiv of Mg(c-C3H5)2(1,4C4H8O2)x at −196 °C, in the hope of generating D, ultimately produced a complicated mixture of products. However, when only 0.5 equiv of Mg(c-C3H5)2(1,4-C4H8O2)x was used, we were able to isolate an organometallic product in fair yield. Unfortunately, and to our surprise, NMR spectroscopic analysis revealed that instead of the expected cyclopropyl complex Cp*W(NO)Cl(c-C3H5) (B), this protocol generates the (previously reported20) allyl complex Cp*W(NO)Cl(η3CH2CHCH2), as shown in Scheme 4. We then investigated the reactivity of the W alkyl chloride complexes C, in the hope that they might better stabilize the cyclopropyl ligand. Indeed, starting from the previously reported 15c methyltrimethylsilyl chloride Cp*W(NO)(CH2TMS)Cl (CTMS), we succeeded in synthesizing the corresponding cyclopropyl complex. Treatment of a cooled (−35 °C) THF-d8 solution of CTMS with 0.5 equiv of Mg(cC3H5)2(1,4-C4H8O2)x led to an immediate color change from indigo to red-purple. NMR spectroscopic analysis indicated the quantitative formation of the cyclopropyl complex Cp*W-

Scheme 5

In THF-d8, the 13C-gated NMR spectrum of 2TMS features a doublet at δ 112.2 (1JCH = 164 Hz) and two apparent triplets at δ 79.4 (1JCH = 158 Hz) and δ 40.8 (1JCH = 157 Hz), corresponding to the CH and two inequivalent CH2 groups of the allyl ligand, respectively. A 1H−13C HMQC experiment correlates these carbon resonances to five broad singlets in the 1 H NMR spectrum, corresponding to CH (δ 5.19) and two diastereotopic CH2 groups (δ 3.52 and 2.19, δ 2.29 and 0.74) of the allyl ligand. NMR experiments conducted in C6D6 produce similar spectra (see the Supporting Information). These data are in good agreement with similar previously reported tungsten allyl complexes (cf. Cp*W(NO)(CH2CMe3)(η3CH2CHCH2),17a Cp*W(NO)Cl(η3-CH2CHCH2)20). Thus far, attempts to isolate 1TMS as a solid have been unsuccessful. In addition to being thermally unstable in solution, the in vacuo removal of solvent from roomB

DOI: 10.1021/acs.organomet.8b00019 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

time required for complete conversion to 2TMS (and free PMe3) from approximately ∼30 h to several days. Furthermore, the addition of radical trapping agents such as TEMPO and 2,6-ditert-butylphenol (∼5 equiv) does not noticeably affect the rate of the rearrangement. For early-transition-metal complexes, two pathways for cyclopropyl to allyl ring-opening rearrangements can be identified, depending on which C−C bonddistal Cβ−Cβ′ or adjacent Cα−Cβis cleaved, where the nature of the metal as well as the substituents on the Cβ seem to be important determining factors.21 There are also examples for latetransition-metal cyclopropyl complexes, involving reductive elimination and oxidative addition steps.22 In an effort to better understand the nature of the cyclopropyl to allyl rearrangement process, we undertook a kinetic study of the conversion of 1TMS to 2TMS, via 1H NMR spectroscopy. As shown in Figure 2, data

temperature THF solutions of 1TMS results in complete and rapid conversion to 2TMS, as evidenced by the characteristic color change to brown-amber. While removal of the solvent at −78 °C yields a red-purple residue indicative of 1TMS, decomposition occurs rapidly at temperatures above −50 °C, making further handling difficult. In all cases, NMR spectroscopic analyses of the resultant brown-amber residues indicate clean conversion to 2TMS, with the absence of any unidentified decomposition byproducts. Consequently, 2TMS can be deliberately synthesized by treating a cooled THF solution of CTMS with 0.5 equiv of Mg(c-C3H5)2(1,4-C4H8O2)x, followed by the removal of solvent in vacuo. After the removal of residual Mg salts via cannula filtration of a CH2Cl2 solution, yellow-amber crystals of 2TMS were obtained from a concentrated pentane−CH2Cl2 mixture left for several days at −35 °C. Additionally, 2TMS can be prepared directly by treating CTMS with 0.5 equiv of the allyl reagent Mg(CH2CHCH2)2(1,4-C4H8O2)x. The molecular structure of 2TMS, shown in Figure 1, is similar to those of other previously reported W alkyl allyl complexes

Figure 2. First-order kinetics plots for a 47.7 mM solution of 1TMS in THF-d8, reflecting the loss of intensity, via 1H NMR, of the Cp* (◆) and SiMe3 (■) resonances.

from a freshly prepared sample in THF-d8 indicate that the process is first order with respect to 1TMS (k298 = (1.38 ± 0.06) × 10−5 s−1). An Eyring plot of kobs (T = 288−318 K) gives the activation parameters: ΔH⧧ = 84 ± 8 kJ/mol and ΔS⧧ = −55 ± 10 J/(mol K) (see the Supporting Information). This suggests that the isomerization is an intra-monomolecular process with an ordered transition state, possibly due to a Cα−Cβ agostic interaction between the cyclopropyl ring and the W center. Similar agostic interactions have influenced the reactivity of other zirconium and niobium cyclopropyl complexes.9,10 We are currently exploring the effect that different alkyl substituents, changes in concentration, and the presence of trace amounts of dissolved MgCl2 (which, due to the intolerance of 1TMS to the removal of solvent, is difficult to completely exclude from the sample) may have on the rate of the rearrangement.

Figure 1. Solid-state molecular structure of 2TMS rendered with CYLVIEW, with hydrogen atoms omitted for clarity. Selected bond lengths (Å) and angles (deg): W1−C18 2.254(5), W1−C17 2.328(5), W1−C19 2.413(5), W1−C1 2.238(4), C18−C17 1.424(7), C17−C19 1.371(7), W1−N1 1.776(4), N1−O1 1.219(5); W1−C1−Si1 120.6(2), C18−C17−C19 118.5(4), W1−N1−O1 171.2(3).

(cf. Cp*W(NO)Cl(η 3 -CH 2 CHCH 2 ), 2 0 Cp*W(NO)(CH 2 CMe 3 )(η 3 -CH 2 CHCH 2 ), 1 7 a and Cp*W(NO)(CH2TMS)(η3-CH2CHCHMe)17a). The metal−nitrosyl fragment is essentially linear (W−N−O = 171.2(3)°), and the three W−Callyl distances (2.254(5), 2.328(5), and 2.413(5) Å) support an η3-bonding description. The allyl ligand adopts an endo conformation and exhibits a σ−π distortion,17a wherein the C−C linkage trans to the nitrosyl ligand (C17−C19 1.371(7) Å) is shorter, and has more double-bond character, than the C−C linkage cis to NO (C17−C19 1.424(7) Å). Attempts to inhibit the rearrangement of the 16-electron 1TMS to the 18-electron 2TMS via the addition of a Lewis base have been unsuccessful. Treating a THF-d8 solution of freshly prepared 1TMS with ∼5 equiv of PMe3 results in a red-purple to brown-amber color change and gives rise to a 31P{1H} NMR spectrum containing a broad resonance at δ −56 (cf. free PMe3 at δ −62), possibly indicative of a phosphine adduct. However, this does not halt the rearrangement and only increases the

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00019. Experimental details, NMR and FT-IR spectra for compounds 1TMS and 2TMS, data and analysis from kinetic experiments, and crystallographic data for compound 2TMS (PDF) Accession Codes

CCDC 1812989 contains 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 data_ [email protected], or by contacting The Cambridge C

DOI: 10.1021/acs.organomet.8b00019 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for M.E.: [email protected]. ORCID

Michel Etienne: 0000-0002-1099-3232 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Agence Nationale de la Recherche for support (ANR RICH, contract ANR-14-CE06-0012-01). REFERENCES

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DOI: 10.1021/acs.organomet.8b00019 Organometallics XXXX, XXX, XXX−XXX