Studies on Molybdena- and Tungstenacyclobutadiene Complexes

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

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Studies on Molybdena- and Tungstenacyclobutadiene Complexes Supported by Fluoroalkoxy Ligands as Intermediates of Alkyne Metathesis Henrike Ehrhorn,† Dirk Bockfeld,† Matthias Freytag,† Thomas Bannenberg,† Christos E. Kefalidis,‡ Laurent Maron,‡ and Matthias Tamm*,†

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Institut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, 38106 Braunschweig, Germany ‡ Institut National des Sciences Appliquées, Université de Toulouse, CNRS, INSA, UPS, 135 Avenue de Rangueil, 31077 Toulouse, France S Supporting Information *

ABSTRACT: The molybdenum and tungsten 2,4,6-trimethylbenzylidynes [MesCM{OC(CF3)2Ph}3] (12a, M = Mo; 12b, M = W) were prepared and structurally characterized as related complexes to already known [MesC M{OC(CF3)2Me}3] (MoF6, M = Mo; WF6, M = W). While treatment of 12a with 3-hexyne yielded the propylidyne complex [EtCMo{OC(CF3)2Ph}3] (13), the tungsten congener 12b formed isolable metallacyclobutadiene (MCBD) species 14−16 by reaction with 3-hexyne, 1-phenyl-1-propyne, and 2,4-hexadiyne, which can be correlated with the higher electrophilicity of the tungsten complex. Furthermore, the labile MCBD [(C3Et3)Mo{OC(CF3)2Me}3] (17) was isolated at low temperature from the reaction of the highly active MoF6 catalyst with 3-hexyne and could be characterized by X-ray diffraction analysis. At room temperature, the same reaction afforded [EtCMo{OC(CF3)2Me}3] (18), and the equilibrium reaction with 3-hexyne to form 17 was additionally studied by variable temperature NMR spectroscopy, which allowed determining ΔH° and ΔS° for the formation of MCBD 17. The experimental thermodynamic data were used to set the benchmark for DFT calculations. Moreover, the deprotiometallacyclobutadiene complex (DPMCBD) [{C3(Mes)(Ph)}Mo{OC(CF3)3}2] (19), prepared from [MesCMo{OC(CF3)3}3] (MoF9) and phenylacetylene, was isolated and structurally characterized as a decomposition product of terminal alkyne metathesis and employed in the polymerization of phenylacetylenes.

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oxidation state” route),29,34,35 which was adapted from the work of Mayr.36,37 Fürstner and his co-workers introduced molybdenum complex 2 with triphenylsilanolate ligands as a highly active alkyne metathesis catalyst with outstanding functional group tolerance.5,16,17,38 The application of 2 in natural product synthesis has been studied thoroughly during recent years.39−42 Inspired by silica-supported alkyne metathesis catalysts,43,44 the homogeneous analogue 3 with tris(tertbutoxy)silanolate ligands was synthesized.18 Complex 3 not only did show good activity in internal alkyne metathesis but also was found to selectively promote the metathesis 1,3diynes.45−47 Chelate complexes such as 4 were reported by Zhang and have shown great success in polymer22,48−53 and supramolecular chemistry.54−61 Among the most active catalysts are also the 2,4,6-trimethylbenzylidyne complexes MoF6 (Figure 1, M = Mo, n = 2) and WF3 (M = W, n = 1), which even perform terminal alkyne metathesis efficiently.11−13,62,63 Additionally, MoF6 and related complexes have been applied in ring-opening alkyne metathesis polymerization (ROAMP)64−67 and were transformed into heteroge-

INTRODUCTION Since its initial discovery in 1968,1 alkyne metathesis has become a useful tool for the formation of CC triple bonds. The development of efficient, well-defined catalysts especially during the past decade has greatly benefited the growth of this method.2−6 Most catalysts are high oxidation state molybdenum and tungsten alkylidyne complexes of the Schrock-type [RCMX3] (M = Mo, W)7 with ancillary ligands X such as fluoroalkoxides,8−15 silanolates,16−19 multidentate phenoxides,20−23 and amides.24−26 A selection of highly active catalysts is given in Figure 1. While early alkyne metathesis catalysts usually required elevated temperatures or high catalyst loadings, the tungsten neopentylidyne complex 1a was among the first catalysts operating efficiently at room temperature with low catalyst loadings.10,27−29 The ligand design of 1 was inspired by Schrock-type olefin metathesis catalysts and combines electron-withdrawing fluoroalkoxides with electrondonating imidazolin-2-iminato ligands.30 Initially, the so-called “high oxidation state” protocol afforded tungsten neopentylidynes such as Schrock’s original catalyst [tBuC W(OtBu)3] or 1a in several steps.31−33 Recently, benzylidyne complexes such as 1b have been prepared from M(CO)6 (M = Mo, W) via a more convenient two-step protocol (“low © XXXX American Chemical Society

Received: February 1, 2019

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DOI: 10.1021/acs.organomet.9b00068 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 2. Selected structurally characterized metallacyclobutadiene (MCBD) and deprotiometallacyclobutadiene (DPMCBD) complexes.

Figure 1. Examples of well-defined alkyne metathesis catalysts.

neous catalysts by immobilization onto dehydroxylated silica.12,14,68 The proposed mechanism of alkyne metathesis first suggested by Katz proceeds via metallacyclobutadiene (MCBD) intermediates, which form by [2+2] cycloaddition of the metal alkylidyne and an alkyne.69 Stable metallacyclobutadienes such as 5 and 6 were isolated from active catalysts with electron-withdrawing alkoxides including OCH(CF3)2, OC(CF3)2Me, or OiPr2C6H3 (Figure 2).8,27,70−78 A low activation barrier for the [2+2] cycloaddition and an energetically labile intermediate are believed to be optimal for high catalytic activity. Both computational79−83 and experimental studies13,14 have shown that the metathesis activity can be controlled by an elaborate adjustment of ancillary ligands tailored to the metal atom. For fluoroalkoxy-supported, molybdenum-based catalysts, the optimum degree of fluorination was realized in MoF6. Higher fluorine content presumably leads to overly stabilized MCBD complexes, and their [2+2] cycloreversion becomes the rate-determining step. This assumption is supported by the isolation of stable MCBDs 7 from complex MoF9 (Figure 2, M = Mo, n = 3).13,14 While tungsten-based MCBDs have been studied thoroughly, complexes 7 belong to rare examples of structurally characterized molybdenacyclobutadienes. A detailed study on the structure−activity relationship revealed that, for tungstenbased catalysts, the optimum level of fluorination is already achieved in WF3 (M = W, n = 1), while the catalytic activity is diminished for WF6 (n = 2) and WF9·thf (n = 3). The difference between the metals can be attributed to the higher intrinsic electrophilicity of tungsten in comparison to molybdenum.8,84 Unusual, paramagnetic MCBD complexes such as 8, which are better described as diaminodicarbene complexes, were prepared from our most active catalyst MoF6 and diaminoacetylenes.85 Similarly, Fischer used ynamides to form stable metallacycles and applied them as selective

termination reagents for propagating species in the ROAMP.66 Classical MCBD complexes of MoF6, however, have not been isolated until now. In this context, we planned to further investigate the alkyne metathesis reaction of MoF6 and a related complex bearing the hexafluoro-2-phenyl-2propoxy ligands (OC(CF3)2Ph), whose potential as ancillary ligands has not been studied. For the latter, we anticipated not only a similar catalytic activity to MoF6 but also more favorable crystallization properties, since fluoro-tert-butoxy ligands have shown severe disorder in the past, which made crystal structure refinement impossible. Herein, we present the synthesis and structural characterization of tris(hexafluoro-2phenyl-2-propoxy) molybdenum and tungsten complexes and their reactivity toward internal alkynes and 1,3-diynes. Furthermore, we wish to report the successful isolation of an MCBD complex derived from MoF6. As expected for an active catalyst, the MCBD intermediate is highly labile and decomposes rapidly. Therefore, we intended to study the metathesis reaction by variable temperature NMR spectroscopy and were able to detect the intermediate at low temperature. Additionally, the NMR studies allowed us to gain a more detailed insight into the thermodynamics of the cycloaddition reaction. Finally, the stoichiometric reaction of nonafluoro-tert-butoxy complexes MoF9 and WF9·thf with alkynes will be discussed in the last part of this work, which includes the formation of a decomposition product, a deprotiometallacyclobutadiene (DPMCBD) complex similar to complexes 98 and 10.17

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RESULTS AND DISCUSSION Preparation and Characterization of 2,4,6-Trimethylbenzylidyne Complexes with Hexafluoro-2-phenyl-2propoxy Ligands. Treatment of the well-established precursors [MesCMBr3(dme)] (11a, M = Mo; 11b, M = W; Mes = mesityl, dme = 1,2-dimethoxyethane)29 with 3 equiv B


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butyne scavengers. The molybdenum complex 12a accomplished an excellent conversion of 97% of the test substrate after 60 min, and the product was isolated in 94% yield. The corresponding tungsten complex 12b only reached 5% conversion after 2 h. The GC conversion plots can be found in Figure S31. These results are in good agreement with the previously reported high activity of MoF6, which achieved 99% conversion of the test substrate under the same conditions, and the diminished potential of the tungsten analogue WF6.11 The superior performance of 12a can be ascribed to the higher electrophilicity of tungsten and presumably MCBD stabilization for 12b.8,13,14 While the related catalyst MoF6 was shown to achieve 90% conversion of the corresponding terminal 3butynyl benzyl ether,11 12a proved incapable of terminal alkyne metathesis. In conclusion, having a phenyl instead of a methyl substituent attached to the hexafluoro alkoxide has a large impact on the catalytic activity. Besides steric reasons, the lower activity of 12a compared to MoF6 could be attributed to a slightly more electron-withdrawing property of OC(CF3)2Ph compared to the related OC(CF3)2Me. Owing to the higher electrophilicity of sp2 carbon atoms, the phenyl group is generally considered as an inductively withdrawing substituent, while alkyl groups act as inductively donating substituents.86 This hypothesis is further supported by the carbyne carbon chemical shifts as the resonance of complex 12a is slightly more low-field shifted than for the OC(CF3)2Me analogue (vide supra). Stoichiometric Reactions of Complexes 12 with Different Alkynes. The reactivity of complexes 12 toward alkynes was further investigated by attempting to trap catalysis intermediates in stoichiometric reactions. Treatment of an nhexane solution of 12a with a 10-fold excess of 3-hexyne afforded pale yellow crystals at −35 °C, which were subjected to X-ray diffraction analysis (Scheme 2).

of KOC(CF3)2Ph in THF at ambient temperature afforded the trisalkoxy complexes 12 after workup and three-fold recrystallization from n-pentane at −35 °C in yields of 28% and 25%, respectively (Scheme 1). The solid-state structures of the Scheme 1. Synthesis of 2,4,6-Trimethylbenzylidyne Complexes 12 with Hexafluoro-2-phenyl-2-propoxy Ligands

complexes were determined by X-ray diffraction analysis, and the ORTEP diagram of 12a is shown in Figure 3. The molecular structure of 12b exhibits similar properties and is shown in Figure S36 of the Supporting Information (SI). Both compounds feature distorted tetrahedral geometries, in which the metal atom is coordinated by three OC(CF3)2Ph ligands and one 2,4,6-trimethylbenzylidyne moiety. As expected, complexes 12 exhibit short metal−carbon bonds together with almost linear M−C1−C2 bond angles. Like observed for other Mo/W pairs, the Mo−C1 bond in 12a is 1.7440(13) Å, slightly shorter than the W−C1 bond in 12b with 1.7588(19) Å. Carbyne carbon atoms generate characteristic low-field resonances in the 13C{1H} NMR spectrum. For 12a, the carbyne signal appears at δ = 321.5 ppm and for 12b at δ = 298.0 ppm. Both resonances are slightly more low-field shifted than their tert-butoxy analogues MoF6 (δ = 317.6 ppm) and WF3 (δ = 293.7 ppm).11 The 19F{1H} NMR spectra each constitutes of one singlet at δ = −72.7 ppm. To determine the catalytic activity of complexes 12, the homometathesis of the well-established test substrate 3pentynyl benzyl ether was attempted. At room temperature, toluene solutions of the substrate were treated with 1 mol % of 12a or 12b in the presence of molecular sieves (MS 5 Å) as 2-

Scheme 2. Reaction of 12 with 3-Hexyne and Formation of Propylidyne Complex 13 and MCBD Complex 14

The molecular structure of complex 13 is displayed in Figure 4. 13 adopts a slightly distorted tetrahedral geometry around the metal atom. The Mo−C1 bond is 1.7299(19) Å shorter and the Mo−C1−C2 angle is 175.40(16)°; both values are smaller than in complex 12a. The 13C{1H} NMR spectrum shows the carbyne chemical shift at δ = 324.5 ppm, which is slightly more low-field compared to 12a. Additionally, the 19 1 F{ H} NMR spectrum exhibits one singlet at δ = −73.0 ppm. Metallacyclobutadiene formation was also not observed for the reaction of 12a with 1-phenyl-1-propyne and 2,4-hexadiyne, which is in line with the fact that molybdenacyclobutadienes

Figure 3. ORTEP diagram of 12a with thermal displacement parameters drawn at 50% probability; hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Mo−C1 1.7440(13), Mo−O1 1.9015(9), Mo−O2 1.8859(9), Mo−O3 1.9390(9), Mo−C1−C2 178.13(12), C1−Mo−O1 106.00(6), C1− Mo−O2 106.21(5), C1−Mo−O3 97.68(5). C


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Table 1. Selected Bond Lengths [Å] and Angles [deg] of MCBD Complexes W−C1 W−C2 W−C3 C1−C2 C2−C3 W−O1 W−O2 W−O3 C2−C1−W C1−W−C3 C3−C2−C1 C2−C3−W O2−W−O3 C1−W−O1 C2−W−O1 C3−W−O1 τ589

Figure 4. ORTEP diagram of 13 with thermal displacement parameters drawn at 50% probability; hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Mo−C1 1.7299(19), Mo−O1 1.9051(12), Mo−O2 1.8979(12), Mo−O3 1.9080(12), Mo−C1−C2 175.40(16), C1−Mo−O1 102.32(8), C1− Mo−O2 108.74(7), C1−Mo−O3 105.03(8).

have rarely been isolated and mainly studied by NMR spectroscopy.8,29,74,87,88 Treatment of the corresponding tungsten complex 12b with a 10-fold excess of 3-hexyne furnished tungstenacyclobutadiene 14 as light pink crystals at −35 °C in 52% yield. The compound crystallizes with two independent molecules in the asymmetric unit; an ORTEP diagram of molecule 14 is displayed in Figure 5. Detailed structural parameters of both

14

14′

15

16

1.870(4) 2.145(4) 1.940(4) 1.508(5) 1.403(5) 1.940(2) 2.003(2) 1.975(2) 78.0(2) 83.21(15) 120.7(3) 78.1(2) 159.37(10) 128.54(13) 171.53(12) 148.15(13) 0.19

1.875(4) 2.139(4) 1.914(4) 1.504(5) 1.417(5) 1.962(2) 1.998(2) 1.995(2) 77.7(2) 83.82(16) 120.1(3) 78.3(2) 162.00(10) 136.01(13) 178.59(12) 140.16(14) 0.36

1.8580(12) 2.1495(12) 1.9352(12) 1.5167(17) 1.4003(18) 1.9503(8) 1.9913(9) 1.9894(9) 78.41(7) 83.38(6) 119.76(11) 78.45(8) 162.07(4) 130.14(5) 173.82(4) 146.47(5) 0.26

1.898(3) 2.157(3) 1.917(3) 1.460(4) 1.447(4) 1.9142(18) 1.9909(17) 1.9812(18) 78.73(18) 82.68(13) 120.2(2) 78.37(17) 164.18(8) 129.69(11) 171.30(10) 147.63(11) 0.28

of 159.37(10)°. The 1H NMR spectrum exhibits signals for the two different ethyl groups in the α and β positions of the metallacycle, and the protons of the phenyl groups appear between δ = 7.37 and 7.86 ppm. Additionally, 13C{1H} NMR analysis revealed the low-field chemical shifts characteristic for C3-MCBD fragments. The Cα and Cβ atoms can be found at δ = 248.9 ppm and δ = 140.6 ppm, respectively, which is consistent with previously reported shifts of tungstenacycles.27,72,73,77The 19F{1H} NMR spectrum features two multiplets at δ = −70.8 ppm and −71.3 ppm corresponding to the axial and equatorial alkoxides, respectively. The formation of the ethyl-substituted MCBD 14 can be rationalized by a metathetical substitution of the 2,4,6trimethylbenzylidyne moiety for a propylidyne moiety with simultaneous elimination of MesCCEt (Mes = mesityl) and subsequent [2+2] cycloaddition of a second equivalent of 3hexyne. Recently, we have also shown that, while 1a furnished MCBD complex 6, the molybdenum congener [PhC Mo(OC(CF3)2Me)2(NImtBu)] (NImtBu = 1,3-di-tert-butylimidazolin-2-iminato) was simply transformed into the corresponding propylidyne complex.27,29 After isolating 14, tungsten complex 12b was treated with other alkynes (Scheme 3). The reaction of 12b with an excess of 1-phenyl-1-propyne in n-hexane yielded orange crystals at room temperature, which were suitable for X-ray diffraction

Figure 5. ORTEP diagram of 14 with thermal displacement parameters drawn at 50% probability; hydrogen atoms and the second independent molecule (14′) of the asymmetric unit are omitted for clarity.

Scheme 3. Reaction of 12b with 1-Phenyl-1-propyne and 2,4-Hexadiyne and Formation of MCBD Complexes 15 and 16

molecules 14 and 14′ are listed in Table 1. The solid-state structure of 14 shows a pentacoordinated tungsten atom in a distorted square-pyramidal environment (τ5 = 0.19).89 The long-short-long-short bond lengths alternation within the fourmembered WC3 ring, which is characteristic for MCBD complexes, is also observed for 14. The W−C1 and W−C3 bond distances are 1.870(4) Å and 1.940(4) Å, respectively, and the C1−C2 and C2−C3 bond distances are 1.508(5) Å and 1.403(5) Å. These are similar to previously reported MCBDs such as 6.27,72,73,77,78 The oxygen atoms O2 and O3 are tipped away from the WC3 ring with an O2−W−O3 angle D


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at −35 °C furnished purple crystals, which were suitable for Xray diffraction analysis. The molecular structure is shown in Figure 7 and proves the formation of α,α′-dipropynyl

analysis. The molecular structure, displayed in Figure 6, revealed the formation of the unsymmetrically substituted

Figure 6. ORTEP diagram of 15 with thermal displacement parameters drawn at 50% probability; hydrogen atoms are omitted for clarity.


tungstenacycle 15 with a phenyl group in the α position of the metallacycle. The pentacoordinated tungsten atom adopts a distorted square pyramidal geometry (τ5 = 0.26).89 Overall, the WC3 ring has similar structural features as the ethyl-substituted MCBD 14. The long-short-long-short alternation of bond lengths is clearly distinct with W−C1 and W−C3 distances of 1.8580(12) Å and 1.9352(12) Å, respectively. The C−C distances are 1.5167(17) Å and 1.4003(18) Å for C1−C2 and C2−C3, respectively. The formation of 15 likely proceeds with the exchange of the 2,4,6-trimethylbenzylidyne moiety by an ethylidyne fragment and concomitant elimination of the alkyne MesCCPh. Subsequent addition of a second 1-phenyl-1propyne molecule furnished MCBD 15. The 1H NMR spectrum exhibits two singlets for the methyl groups in the α′ and β positions at δ = 2.31 ppm and δ = 3.00 ppm, respectively, as well as several multiplets in the aromatic region for the different phenyl protons. The 13C{1H} NMR spectrum shows two signals for Cα and Cα′ at δ = 252.0 and 234.0 ppm, respectively, and Cβ is found at δ = 136.7 ppm. Finally, the 19 1 F{ H} NMR spectrum confirms the low symmetry of the complex by featuring three multiplets at δ = −68.9 ppm, −71.2 ppm, and −75.2 ppm for the different alkoxide ligands. Under the same reaction conditions, a crystal of the symmetrically substituted MCBD [(C3Me3)W{OC(CF3)2Ph}3] was also isolated by coincidence and subjected to Xray diffraction analysis. The molecular structure and structural parameters are displayed in Figure S37. However, the selective synthesis of this compound failed, and all subsequent attempts exclusively afforded complex 15. Previously, the reaction of [MesCMo{OC(CF3)3}3] (MoF9) with an excess of 1phenyl-1-propyne produced only the all-methyl substituted MCBD 7 (Figure 2).14 In recent years, the method of diyne metathesis in the presence of catalyst 3 has been established and the mechanism has been studied thoroughly.45−47 However, an MCBD as a diyne metathesis intermediate with alkynyl substituents at the metallacycle was not isolated until now. Therefore, the formation of such a species was attempted with tungsten complex 12b by treating the latter with an excess of 2,4hexadiyne (Scheme 3). Storing the n-hexane reaction mixture

tungstenacyclobutadiene complex 16. Similar to 14 and 15, the tungsten atom is pentacoordinated and resides in a distorted square pyramidal environment (τ5 = 0.26).89 The two propynyl groups in the α and α′ positions of the WC3 fragment are slightly tipped away from the plane of the metallacycle. The long-short-long-short bond lengths alternation in the WC3 moiety is significantly less pronounced for 16 with W−C1 and W−C3 distances of 1.898(3) Å and 1.917(3) Å and C1−C2 and C2−C3 distances of 1.460(4) Å and 1.447(4) Å, respectively. A more detailed list of bond lengths and angles is given in Table 1. Despite several attempts to isolate 16 in pure form to collect more complete analytical data, the purification of 16 failed. In all cases, the NMR spectra displayed at least two species in the product mixture, which could not be separated. Dialkynyl-MCBD 16 was likely formed through the replacement of the 2,4,6-trimethylbenzylidyne moiety by an alkynylalkylidyne fragment (WC−CCMe) in a [2+2] cycloaddition and reversion sequence. Subsequently, the alkynylalkylidyne intermediate underwent a second [2+2] cycloaddition with 2,4-hexadiyne to form 16. Even though 16 is not an intermediate of an active diyne metathesis catalyst, the position of the propynyl groups on the Cα atoms supports the proposed α,α-mechanism of diyne metathesis.45−47 Mechanistic Study of the Alkyne Metathesis Reaction with the Well-Established MoF6 Catalyst. Detailed insights into the kinetics and thermodynamics of the metathesis reaction of MoF6 are rare and mainly comprise the study of the structure−activity relationship of fluoroalkoxyand silica-supported 2,4,6-trimethylbenzylidyne complexes.12−14 Therefore, we initially attempted to explore the metallacyclobutadiene formation of MoF6 with stoichiometric amounts of 3-hexyne. For this purpose, an n-pentane solution of MoF6 was treated with an excess of the alkyne at room temperature, which did not lead to an immediate color change as usually observed upon MCBD formation (Scheme 4). However, storing the reaction mixture at −35 °C afforded purple, highly temperature sensitive crystals of metallacycle 17, which decomposed immediately at room temperature. Therefore, these crystals were carefully kept and handled at low E


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starting material MoF6, the 19F{1H} NMR exhibits one singlet at δ = −78.0 ppm. Having the suitable starting material at hand, the equilibrium reaction with 3-hexyne was investigated (Scheme 6). Initially,

Scheme 4. Reaction of MoF6 with 3-Hexyne and Formation of MCBD Complex 17 at Low Temperature

Scheme 6. Equilibrium of Propylidyne Complex 18 and 3Hexyne with the MCBD Complex 17 temperature, and a crystal was mounted on the diffractometer at −80 °C. Due to the extreme sensitivity of the crystals, the resulting molecular structure suffers from low quality. Thus, the ORTEP plot displayed in Figure 8 only confirms the the 1:1 reaction of 18 and 3-hexyne was analyzed by NMR spectroscopy at room temperature. The 1H NMR spectrum showed broad signals for the ethyl substituents and a sharp, unaltered alkoxide signal. Similarly, the 19F{1H} NMR resonance for the CF3 groups seemed unaffected by the addition of 3-hexyne at room temperature. Progressively lowering the temperature to −90 °C resulted in a sharpening of the resonances in the 1H NMR spectrum starting at −50 °C. When the temperature was lowered to −70 °C, well separated sets of signals emerged in the 1H NMR spectrum. New signals appeared in the 19F{1H} NMR below −30 °C. In addition to the initial signal of alkylidyne 18 at δ = −77.7 ppm, the 19F{1H} NMR spectrum showed two singlets at δ = −75.8 ppm and δ = −76.3 ppm in a 1:2 ratio, which is in line with previous reports on fluoroalkoxy MCBDs including 7.13,14 Hence, the spectra at low temperature strongly suggested the presence of not only alkylidyne 18 and 3hexyne but also metallacyclobutadiene 17 in high quantity. Accordingly, 2D NMR experiments were conducted at −70 °C, which further substantiated the existence of 17. For the metallacycle, the 1H NMR spectrum exhibits two sets of signals for the ethyl groups in the α (δ = 2.64 and 1.48 ppm) and β positions (δ = 2.44 and 0.72 ppm). Additionally, the resonance for the axial and equatorial alkoxides can be found at δ = 1.93 ppm and δ = 0.86 ppm, respectively. Detailed analyses of the 1 H NOESY and ROESY correlations at −70 °C showed an interconversion between the alkylidyne 18 and the MCBD 17 (Figures S25 and S26). The characteristic low-field 13C{1H} NMR shifts were assigned indirectly through H,C coupled 2D experiments (Figures S27 and S28). The Cα atoms of the MoC3 fragment appear at δ = 258 ppm and the Cβ atom can be found at δ = 142 ppm, which is in good agreement with previously reported shifts of molybdenacyclobutadienes.13,14,87 A detailed list of 13C{1H} NMR shifts of MCBD 17 can be found in Table S1. The formation of an MCBD complex at low temperature has been observed before by NMR spectroscopy by the group of Schrock for [EtCMo(O-2,6-iPr2C6H3)3].8 Additionally, the variable temperature NMR study was used to derive thermodynamic data for the alkyne metathesis reaction. Determination of equilibrium constants K at different temperatures by integration of the 19F{1H} NMR resonances allowed us to evaluate ΔH° and ΔS° via van’t Hoff analysis (Figure 9). More detailed information on the exact procedure are included in the SI. This yielded parameters of ΔH° = −5.4 ± 0.1 kcal mol−1 and ΔS° = −25 ± 5 cal mol−1 K−1. In contrast to our previous calculations on the relative energies of the series of tert-butoxy alkylidynes, which suggested a slightly endothermic MCBD formation enthalpy for MoF6,13 the


connectivity of this molecule, and a discussion of bond lengths and angles is not meaningful. The labile nature of 17 is in good agreement the high catalytic activity of MoF6. Since further studies on MCBD 17 could not be carried out at room temperature, we envisaged to apply low temperature NMR spectroscopic analysis for this purpose. Therefore, the corresponding propylidyne complex 18 was synthesized as a suitable alkylidyne complex to avoid scrambling of substituents and consequently complicated NMR spectra. We attempted the formation of [EtCMo{OC(CF3)2Me}3] (18) through a stoichiometric alkyne metathesis reaction: MoF6 was treated with 0.5 equiv of 3-hexyne and stirred in toluene (Scheme 5). Scheme 5. Synthesis of Propylidyne Complex 18 from MoF6

The crude reaction mixture contained both the desired product 18 as well as 0.5 equiv of bis(mesitylacetylene). 18 was isolated in 61% yield by careful sublimation under reduced pressure at 45 °C. The 1H NMR spectrum exhibits signals corresponding to the ethyl group of the propylidyne ligand and the methyl groups of the alkoxide ligands at δ = 1.48 ppm. The carbyne carbon resonance can be found in the low-field of the 13 C{1H} NMR spectrum at δ = 313.5 ppm and, like the F


Article

Organometallics

experiment (−5.5 vs −5.4(1) kcal mol−1), and the activation barrier is also just slightly increased (7.2 vs 4.7 kcal mol−1). Therefore, the association of the B3PW91 functional with solvent in a simple double-ζ + polarization basis set for all atoms except Mo (relativistic effective core potential with its adapted basis set) appears to be a good choice to describe this reaction. Since most recent theoretical calculations were performed on model alkyne metathesis reactions involving ethylidyne complexes and 2-butyne,10,13,27,29,80 the enthalpy profile was also calculated for this reaction, which affords the MCBD [(C3Me3)Mo{OC(CF3)2Me}3] (Figure 10, right). Quite unexpectedly, gas phase calculations using a simple double-ζ basis set with only polarization of the heavy atoms and a relativistic effective core potential with its adapted basis set for Mo leads to exothermic formation of the MCBD intermediate in excellent agreement with the experiment (−3.9 vs −5.4(1) kcal mol−1). The associated barrier is 8.3 kcal mol−1, in agreement with the one computed for the real reaction (4.7 kcal mol−1). Interestingly, improving the basis set by the inclusion of polarization functions to describe the hydrogen atoms induces an extra-stabilization of the MCBD intermediate by 8.7 kcal mol−1, yielding a worse comparison with the experiment (−12.6 vs −5.4(1) kcal mol−1) than with a smaller basis set. On the other hand, the activation barrier is slightly decreased but remains in the same range (6.1 vs 8.3 kcal mol−1) considering the precision of the method. Using the larger basis set but reoptimizing in solvent (using the SMD continuum method, see SI) marginally destabilizes the MCBD by 1.9 kcal mol−1, making a fair comparison with the experimental value (−10.7 vs −5.4(1) kcal mol−1). Thus, our computational approach allows an excellent comparison with the experiment and double-ζ basis sets appear to be sufficient in this case, but the choice of the model is in fact very important for the comparison. For the model reaction with the ethylidyne complex, we have also localized alkylidyne-alkyne van der Waals (vdW) complexes as intermediates prior to cycloaddition. These complexes do not show any direct alkyne−metal interaction; their structures together with the full potential energy surfaces are displayed in Figure S38 (see SI also for detailed information about the DFT calculations).

Figure 9. van’t Hoff plot derived from 19F{1H} NMR spectroscopic data in toluene-d8 from 225 to 170 K in 5 K intervals.

experimentally obtained enthalpy is clearly exothermic. To estimate the activation barrier of the metathesis reaction, we further aimed at identifying the rate constants at different temperatures using dynamic exchange line-shape analysis (DNMR). Unfortunately, however, we could not establish a suitable model, since other dynamic processes such as alkoxide rotation and/or exchange seem to interfere with the simulation of the alkyne metathesis reaction. The experimentally derived enthalpy for the cycloaddition reaction between the propylidyne complex 18 and 3-hexyne (Scheme 6) was further used for the calibration of computational methods. Therefore, the enthalpy profile of MCBD formation was established by DFT calculations (B3PW91 functional, Figure 10, left). In order to check the accuracy of the computational method, calculations were carried out both in gas phase and in solvent (using an implicit model). In gas phase, the use of a simple double-ζ + polarization basis set for all atoms except Mo (relativistic effective core potential with its adapted basis set) leads to an enthalpy of MCBD formation in excellent agreement with the experiment (−8.0 vs −5.4(1) kcal mol−1). The associated barrier is 4.7 kcal mol−1, indicating a facile reaction. The inclusion of solvent effects affords a less exothermic reaction enthalpy in perfect agreement with the

Figure 10. Computed enthalpy profiles for the reaction of propylidyne [EtCMo{OC(CF3)2Me}3] with 3-hexyne (left) and of ethylidyne [MeCMo{OC(CF3)2Me}3] with 2-butyne (right). G


Article

Organometallics Stoichiometric Reactions with Nonafluoro-tert-butoxy 2,4,6-Trimethylbenzylidyne Complexes MoF9 and WF9·thf. The known deactivation mechanisms of alkyne metathesis catalysts include bimolecular alkyne elimination13,90−92 and the formation of metallatetrahedrane and cyclopentadienyl species.93,94 In the presence of terminal alkynes, Schrock alkylidyne complexes also form deprotiometallacyclobutadienes such as 9 and 10, which is accompanied by elimination of 1 equiv of alcohol. DPMCBDs are believed to form alkynylalkylidene complexes, which are able to initiate alkyne polymerization via metallacyclobutene intermediates.8,9,17,75,76,95−97 Recently, the preparation and catalytic activity of 2,4,6-trimethylbenzylidyne MoF9 with nonafluoro-tert-butoxy ligands was reported.13,14 MoF9 efficiently catalyzed the metathesis of internal alkynes but proved inactive in terminal alkyne metathesis and polymers were formed. Therefore, the isolation of a DPMCBD was attempted by treating a CH2Cl2 solution of MoF9 with 1 equiv of phenylacetylene at −20 °C (Scheme 7).

only slightly shorter than the Mo−Cβ distance (2.012(2) Å). The Mo−Cβ bond length is noticeably shorter than in the MCBD complexes 14−16, which fall in the range of 2.139(4)−2.157(3) Å. The carbon−carbon distances C1−C2 (1.364(4) Å) and C2−C3 (1.390(4) Å) are almost identical and shorter compared to 14−16. The C1−C2−C3 angle of 133.3(2)° is significantly more obtuse than in classical MCBD complexes. Complex 19 displays a pseudotetrahedral geometry (τ4 = 0.88)98 with a large O1−Mo−O2 angle of 119.30(8)° opposing the naturally acute C1−Mo−C3 angle of 81.51(9)°. Similar characteristics have been observed for Schrock’s structurally characterized DPMCBDs [(tBu2C3)W(η5-C5H5)Cl]75,95 and [(tBu2C3)Mo{OCH(CF3)2}2(py)2] (9, py = pyridine),8 which were described as metallacycloallene complexes.99,100 The 13C{1H} NMR shifts of the ring carbon atoms are also in good agreement with previously reported DPMCBD complexes.8 The signal of the Cα atom bearing the phenyl substituent is observed at δ = 233.6 ppm, while the signal at δ = 229.5 ppm can be assigned to the mesitylsubstituted Cα atom. The Cβ atom gives rise to a resonance at δ = 195.8 ppm that is significantly more low-field shifted compared to related MCBDs. The 19F{1H} NMR spectrum displays one singlet at δ = −75.2 ppm. Inspired by the work of Veige and co-workers, who applied the alkylidyne complexes [tBuCW{OCOtBu}(thf)2] and [MesCMo{OCOtBu}(thf)2] containing a trianionic phenyldiphenolate pincer ligand (OCOtBu3−) as active acetylene polymerization catalysts,101,102 we also envisaged the use of 19 for this purpose. Poly(phenylacetylene)s (PPA) are interesting materials with unique properties including electrical conductivity, optical nonlinearity, photoconductivity, and paramagnetic susceptibility.103−107 The polymerization activity was studied for complexes 19 and MoF9 by treatment of phenylacetylene and para-methoxyphenylacetylene with 0.02 mol % of the potential catalysts (Scheme 8) at room temperature.

Scheme 7. Reaction of MoF9 with Phenylacetylene and Formation of DPMCBD Complex 19 (Mes = mesityl)

19 was isolated as a brown crystalline solid in 72% yield. An ORTEP plot of the X-ray crystal structure is shown in Figure 11. Schrock reported the formation of a similar DPMCBD

Scheme 8. Polymerization of Phenylacetylenes by Complexes 19 and MoF9 (PPA = polyphenylacetylene)

Characterization of the resulting red polymers included 1H NMR and IR spectroscopy as well as gel-permeation chromatography (GPC). Details regarding the isolated polymers are given in Table 2. Terminal alkynes typically show IR absorption for C−H and CC stretching vibrations at around 3300 and 2100 cm−1, respectively. These bands are absent in the IR spectra of the isolated polymers. 1H NMR spectra of the poly(phenylacetylene)s displayed in Figures S32 and S33 were recorded in CDCl3 and show the characteristic broad signals at δ = 6.96 and 5.85 ppm. The 1H NMR spectrum of para-methoxy-PPA also reveals the expected signals at δ = 6.64 and 5.76 ppm. Furthermore, the cis/trans ratio of double bonds in poly(phenylacetylene) was determined by using the integral of the peak at δ = 5.85 ppm.108−110 The cis content was estimated to be between 75% and 80% for PPA produced by 19 and MoF9, respectively, which is in agreement with other polymers obtained from carbene or carbyne initiators.97,101,111 The molecular weights range from 10339 g mol−1 to 33488 g mol−1, depending on the

Figure 11. ORTEP diagram of 19 with thermal displacement parameters drawn at 50% probability; hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Mo−C1 1.940(3), Mo−C2 2.012(2), Mo−C3 1.932(3), C1−C2 1.364(4), C2−C3 1.390(4), Mo−O1 1.9515(18), Mo−O2 1.9338(18), Mo− C1−C2 72.71(12), Mo−C3−C2 72.49(14), C1−Mo−C3 81.51(9), C1−C2−C3 133.2(2), O1−Mo−O2 119.30(8), C1−Mo−O1 107.57(9), C1−Mo−O2 111.68(9), C3−Mo−O1 116.63(10), C3− Mo−O2 113.45(9).

complex [(tBu2C3)Mo{OC(CF3)3}2] upon treatment of [tBuCMo{OC(CF3)3}3(dme)] with tert-butylacetylene. However, a solid-state structure was not determined.8 The molybdenum atom in 19 is pentacoordinated with a planar MoC3 ring and a C3R2 fragment bound in an η3-fashion. The Mo−Cα distances of 1.940(3) Å and 1.932(3) Å are equal and H


Article

Organometallics Table 2. Results of Catalytic Phenylacetylene Polymerization with Complexes 19 and MoF9a cat.

R

volume (mL)

yield (%)

19 19 19 MoF9

H H OMe H

1.00 1.25 1.25 1.00

98 94 96 96

MW (g/mol) 1.0339 3.3488 1.0853 2.3413

× × × ×

104 104 104 104

Mn (g/mol) 3.5593 1.4666 5.6810 8.0038

× × × ×

103 103 103 103

PDI 2.90 2.28 1.85 2.92

a

Conditions: Under an argon atmosphere, the substrate (1.0 mmol) was treated with a toluene solution of the catalyst (0.02 mol %) at room temperature. The reaction was quenched after 2 h, and the polymer was washed with methanol.

solvent volume, substrate, and catalyst. The PDIs are relatively high (1.85−2.92) and indicate that the polymerization occurred under nonliving conditions. Finally, 19 proved inactive in both the polymerization and metathesis of 1phenyl-1-propyne. In conclusion, DPMCBD 19 and MoF9 are good polymerization catalysts for phenylacetylene and paramethoxyphenylacetylene. The corresponding nonafluoro-tertbutoxy tungsten analogue WF9·thf was also treated with phenylacetylene to produce a tungsten-based DPMCBD, but only a mixture of products was obtained. Furthermore, no single crystals suitable for X-ray diffraction analysis could be isolated. Following the synthesis of α,α′-dialkynyl-substituted MCBD 16 and the ability to trap MCBDs with the electrophilic MoF9 complex, we treated a CH2Cl2 solution of MoF9 with an excess of 2,4-hexadiyne (Scheme 9), which was accompanied by a

Figure 12. ORTEP diagram of 20 with thermal displacement parameters drawn at 50% probability; hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]: W−C1 1.8937(19), W−C2 2.1350(19), W−C3 1.9134(19), C1−C2 1.447(3), C2−C3 1.471(3), W−O1 1.9507(13), W−O2 2.0110(13), W−O3 2.0136(13), C14−C15 1.199(3), C2−C1−W 78.20(13), C1−W−C3 83.73(9), C2−C3−W 76.99(11), C3−C2− C1 121.09(17), O3−W−O2 166.72(5), C1−W−O1 130.81(8), C2− W−O1 172.37(8), C3−W−O1 145.46(9), C3−C14−C15 177.0(2).

Scheme 9. Reaction of MoF9 and WF9·thf with 2,4Hexadiyne and Formation of MCBD Complexes 7 and 20 (Mes = mesityl)

lengths alternation within the WC3 ring can be observed. The W−C distances are 1.8937(19) Å and 1.9134(19) Å, in the range of other tungstenacyclobutadienes.27,70,72,73,77 However, similar to 16, the difference of the W−Cα bonds is less distinctive compared to 14 and 15. Interestingly, the C1−C2 and C2−C3 bond lengths of 1.447(3) Å and 1.471(3) Å, respectively, are nearly equal and inversed in comparison to other MCBDs.27 1H NMR spectroscopic characterization showed the expected chemical shifts for complex 20 with four singlets for the different methyl groups of the mesityl and propynyl substituents and in the β position of the metallacycle. The 13C{1H} NMR spectrum displays the ring carbon resonances at δ = 245.8 and 226.4 ppm for Cα/Cα’ and at δ = 148.8 ppm for C β, respectively. Moreover, signals corresponding to two different sets of alkoxide ligands were found in the 13C{1H} NMR spectrum. The 19F{1H} NMR spectrum shows two multiplets at δ = −71.6 ppm and δ = −73.1 ppm in a 2:1 ratio. Compound 20 is the [2+2] cycloaddition product of WF9·thf and 2,4-hexadiyne. All other MCBD complexes discussed in this paper (14−16) underwent at least one cycloaddition-cycloreversion sequence before forming the final MCBD. The different reactivity could be ascribed to the high electrophilicity of the tungsten atom in WF9·thf and hence strong stabilization through MCBD formation. Interestingly, the isomer with the propynyl group at Cα is the only observed product in this reaction, which again supports the α,α-mechanism of diyne metathesis.45−47

characteristic color change from red to purple. Interestingly, the reaction yielded exclusively the previously reported methylsubstituted metallacycle 7 at low temperature (−35 °C).14 The formation of [(C3Me3)Mo{OC(CF3)3}3] from MoF9 and 2,4hexadiyne can be rationalized by exchange of the 2,4,6trimethylbenzylidyne moiety for an ethylidyne fragment with concomitant formation of mesityl-1,3-pentadiyne (MesCC− CCMe). Subsequent addition of 2-butyne, which must have formed from metathesis of 2,4-hexadiyne, furnished MCBD complex 7. In contrast, the same reaction with the tungstenbased analogue WF9·thf furnished dark red crystals of MCBD 20 in 84% yield at −35 °C (Scheme 9). The crystals were suitable for X-ray diffraction analysis, and the resulting molecular structure is displayed in Figure 12. The solid-state structure features an almost planar, fourmembered metallacycle. The metal resides in a distorted square-pyramidal environment (τ5 = 0.35),89 and similar to compound 16, no pronounced long-short-long-short bond I


Article

Organometallics

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CONCLUSION In a recent study on the optimum degree of fluorination for fluoroalkoxy-supported alkyne metathesis catalysts, the hexafluoro-tert-butoxy molybdenum complex [MesCMo{OC(CF3)2Me}3] (MoF6) showed the best overall catalytic alkyne metathesis performance.12−14 Comparison of this hexafluorotert-butoxy complex with its hexafluorobenzyloxy congener [MesCMo{OC(CF3)2Ph}3] (12a) revealed that this complex is in fact a highly active catalyst for the metathesis of internal alkynes, whereas terminal alkyne metathesis was not achieved. However, the stoichiometric reaction of 12a with 3hexyne yielded exclusively alkylidyne 13 and no MCBD formation was observed. As expected, the corresponding tungsten complex [MesCW{OC(CF3)2Ph}3] (12b) does not promote alkyne metathesis efficiently and reacted with alkynes to yield stable metallacyclobutadienes. Among the isolated MCBDs was the unusual, unsymmetrically substituted complex [(C3PhMe2)W{OC(CF3)2Ph}3] (15) prepared from 1-phenyl-1-propyne. The difference in reactivity of complexes 12a (M = Mo) and 12b (M = W) was attributed to the higher electrophilicity of tungsten compared to molybdenum. Additionally, the reaction of 12b with a 2,4-hexadiyne furnished the first dialkynyl-substituted MCBD 16. The formation presumably proceeded via an alkynylalkylidyne complex (WC−C C−R), which is believed to be the active species in diyne metathesis.45−47 In agreement with the proposed α,αmechanism of diyne metathesis, the propynyl groups in 16 were found in the α positions of the metallacycle. Therefore, we regard this finding as a valuable validation for the proposed mechanism of the metathesis of diynes. Furthermore, as anticipated, improved crystallization properties with missing disorder for the alkoxides were found for complexes bearing the OC(CF3)2Ph ligand. Thus, the molecular structures are of better quality compared to those of MCBDs bearing nonafluoro- and hexafluoro-tert-butoxides such as 7 and 17. The latter complex, [(C3Et3)Mo{OC(CF3)2Me}3] (17), was isolated as a labile MCBD complex from the reaction of MoF6 with 3-hexyne at low temperature. The instability of this intermediate is in line with the high catalytic activity of MoF6. However, its NMR spectroscopic observation allowed conducting a variable temperature NMR study of the equilibrium alkyne metathesis reaction of 3-hexyne involving the propylidyne complex [EtCMo{OC(CF3)2Me}3] (18) and MCBD 17. For the first time, we could obtain experimental thermodynamic values for ΔH° and ΔS° as benchmarks for further theoretical investigations. DFT calculations were conducted, and the association of the B3PW91 functional and implicit treatment of solvent effects led to a perfect comparison with experiment. We expect that this benchmarking of computational studies with experimentally obtained enthalpies can be used to predict the catalytic activities of alkyne metathesis catalysts more reliably in the future. Additionally, stoichiometric reactions of nonafluoro-tertbutoxy complexes MoF9 and WF9·thf with alkynes were investigated. The deprotiometallacyclobutadiene (DPMCBD) 19 was isolated from the reaction of MoF9 with phenylacetylene, which further substantiates that catalyst deactivation in terminal alkyne metathesis proceeds with formation of DPMCBD species. In line with previous findings, 19 acts as a polymerization catalyst for arylacetylenes. In contrast, the tungsten congener WF9·thf did not afford isolable DPMCBD

complexes, while its reaction with 2,4-hexadiyne yielded the simple [2+2] cycloaddition product 20.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00068. Details of the X-ray measurements of complexes 12−17, 19, 20 and (C3Me3)W{OC(CF3)2Ph}3, full experimental details, catalytic data and NMR spectra, computational details (PDF) Optimized geometries of all calculated structures (MOL) Accession Codes

CCDC 1892541−1892550 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

Christos E. Kefalidis: 0000-0002-1380-4337 Laurent Maron: 0000-0003-2653-8557 Matthias Tamm: 0000-0002-5364-0357 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.E. wishes to thank the Fonds der Chemischen Industrie (FCI) for a Chemiefonds Fellowship. The authors are thankful to Dr. Kerstin Ibrom and Dr. Christian Kleeberg for helpful discussions on NMR spectroscopy. This work was funded by the Deutsche Forschungsgemeinschaft (DFG) through project TA 189/12−1 (“Mechanistic studies on the catalytic metathesis of internal and terminal alkynes and diynes”).

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REFERENCES

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