Tuning the Catalytic Alkyne Metathesis Activity of Molybdenum and Tungsten 2,4,6-Trimethylbenzylidyne Complexes with Fluoroalkoxide Ligands OC(CF3)nMe3-n (n = 0-3)

Article abstract of DOI:10.1021/acs.organomet.7b00519

The molybdenum and tungsten 2,4,6-trimethylbenzylidyne complexes [MesC=M{OC(CF₃)_nMe_3-n}₃] (M = Mo: MoF0, n = 0; MoF3, n = 1; MoF6, n = 2; MoF9, n = 3; M = W: WF3, n = 1; Mes = 2,4,6-trimethylphenyl) were prepared by the reaction of the tribromides [MesC - MBr₃(dme)] (dme = 1,2-dimethoxyethane) with the corresponding potassium alkoxides KOC(CF₃)_nMe_3-n. The molecular structures of all complexes were established by X-ray diffraction analysis. The catalytic activity of the resulting alkylidyne complexes in the homometathesis and ring-closing alkyne metathesis of internal and terminal alkynes was studied, revealing a strong dependency on the fluorine content of the alkoxide ligand. The different catalytic performances were rationalized by DFT calculations involving the metathesis model reaction of 2-butyne. Because the calculations predict the stabilization of metallacyclobutadiene (MCBD) intermediates by increasing the degree of fluorination, MoF9 was treated with 3-hexyne to afford the MCBD complex [(C₃Et₃)Mo{OC(CF₃)₃}₃], which was characterized spectroscopically.

Full text of DOI:10.1021/acs.organomet.7b00519

Article
pubs.acs.org/Organometallics
Tuning the Catalytic Alkyne Metathesis Activity of Molybdenum and
Tungsten 2,4,6-Trimethylbenzylidyne Complexes with
Fluoroalkoxide Ligands OC(CF3)nMe3−n (n = 0−3)
Celine Bittner,† Henrike Ehrhorn,† Dirk Bockfeld, Kai Brandhorst, and Matthias Tamm*
Institut fur Anorganische und Analytische Chemie, Technische Universitat Braunschweig, Hagenring 30, 38106 Braunschweig,
̈
̈
Germany
S
* Supporting Information
ABSTRACT: The molybdenum and tungsten 2,4,6-trimethyl-
benzylidyne complexes [MesCM{OC(CF3)nMe3−n}3] (M =
Mo: MoF0, n = 0; MoF3, n = 1; MoF6, n = 2; MoF9, n = 3; M
= W: WF3, n = 1; Mes = 2,4,6-trimethylphenyl) were prepared
by the reaction of the tribromides [MesCMBr3(dme)] (dme
= 1,2-dimethoxyethane) with the corresponding potassium
alkoxides KOC(CF3)nMe3−n. The molecular structures of all
complexes were established by X-ray diffraction analysis. The
catalytic activity of the resulting alkylidyne complexes in the
homometathesis and ring-closing alkyne metathesis of internal
and terminal alkynes was studied, revealing a strong depend-
ency on the fluorine content of the alkoxide ligand. The
different catalytic performances were rationalized by DFT calculations involving the metathesis model reaction of 2-butyne.
Because the calculations predict the stabilization of metallacyclobutadiene (MCBD) intermediates by increasing the degree of
fluorination, MoF9 was treated with 3-hexyne to afford the MCBD complex [(C3Et3)Mo{OC(CF3)3}3], which was characterized
spectroscopically.
Since the early 1980s, fluorinated and nonfluorinated
alkoxides (ORF) of the type OCH(CF3)2 and OC(CF3)nMe3−n
(n = 0−3) have been the most widely used ligand systems for
the preparation of high oxidation state complexes with metal−
carbon multiple bonds.17,47,48 Hence, neopentylidyne com-
plexes [Me3CCM(ORF)3] (M = Mo, W), or solvated species
such as [Me3CCM(ORF)3(dme)] (dme = 1,2-dimethoxy-
ethane), have been shown to be highly active, e.g., in the
metathesis of 3-heptyne.49−52 These studies clearly revealed a
strong dependency of the alkyne metathesis activity on the
metal−ligand combination; for instance, comparison of the
unfluorinated species [Me3CCM(OCMe3)3] indicated supe-
rior performance of the tungsten complex.49,52,53 The reverse
order was found for the hexafluoro-tert-butoxide species
[Me3CCM{OC(CF3)2Me}3] with the Mo system represent-
ing one of the best alkyne metathesis catalysts studied by the
Schrock group.50−52
The closely related 2,4,6-trimethylbenzylidyne complex
[MesCMo{OC(CF3)2Me}3] (MoF6) was prepared via the
so-called low-oxidation-state route from Mo(CO)6.11 The steric
demand of the Mes substituent allowed for easy removal of
coordinating solvents (THF or DME), which was found to be
beneficial for the catalytic performance.54 In fact, MoF6
exhibits unprecedented catalytic activity,55 and it is even
INTRODUCTION
■
The number of well-defined homogeneous alkyne metathesis
catalysts that are capable of efficiently promoting the reversible
cleavage and formation of carbon−carbon triple bonds has
steadily grown over the past decade.1−7 Recent developments
were commenced with imidazolin-2-iminato tungsten alkyli-
dyne complexes of type I (Figure 1),8−15 which were developed
drawing on the structure of the most active alkene metathesis
catalysts, i.e., Schrock-type alkylidene complexes.2,16−18 Besides
these asymmetrically substituted alkylidyne complexes, all other
recent additions to the group of highly active alkyne metathesis
catalysts feature a symmetric set of ancillary ligands X as in
[RCMX3]. Thereby, molybdenum alkylidyne complexes such
as II with triphenylsiloxide ligands (X = OSiPh3)19,20 represent
the most frequently used catalysts in natural product synthesis,
predominantly through ring-closing alkyne metathesis
(RCAM).21−28 Siloxide ligands, namely X = OSi(OtBu)3,
were also employed for the preparation of the tungsten
alkylidyne complex III, which proved catalytically active not
only in alkyne metathesis29 but also in the metathesis of
conjugated diynes.30,31 Molybdenum alkylidyne complexes such
as IV supported by chelating phenoxide ligands represent
another important class of well-defined catalysts32−35 that have
found application in polymer synthesis36−39 and, most notably,
in the construction of supramolecular aryleneethynylene
macrocycles and cages through alkyne metathesis.40−46
Received: July 11, 2017
© XXXX American Chemical Society
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accessible from the tribromides [MesC≡MBr3(dme)] (1a, M =
Mo; 1b, M = W) synthesized through a two-step protocol
starting with molybdenum or tungsten hexacarbonyl and
mesityllithium (MesLi).54 Treatment of 1a or 1b with three
equivalents of the potassium alkoxides KOC(CF3)nMe3−n (n =
0−3) resulted in the formation of the trisalkoxide complexes
MoF0−MoF9 and WF3 (Scheme 1). As described for the
Scheme 1. Synthesis of 2,4,6-Trimethylbenzylidyne
Complexes
synthesis of MoF6,54 the preparations of MoF0 and WF3 were
accomplished by reaction of 1a or 1b with the respective
alkoxides in THF. After stirring for 16 h, evaporation at slightly
elevated temperature, extraction of the residual with pentane,
and recrystallization at −40 °C afforded the products as yellow
crystalline solids in 45 and 72% yields, respectively. When
treating 1a with three equivalents of the trifluoro-tert-butoxide
KOC(CF3)Me2 in THF, significant amounts of the correspond-
ing dimolybdenum hexaalkoxide were formed. This complex
featuring a Mo−Mo triple bond has been reported previously;66
details of its spectroscopic characterization and of an X-ray
diffraction analysis can be found in Figure S23 and Table S3.
The use of toluene as solvent and diethyl ether for
recrystallization furnished sufficient amounts of MoF3, albeit
in comparatively low yield (31%). Treatment of 1a with the
perfluoro-tert-butoxide KOC(CF3)3 in THF gave exclusively
the solvate MoF9·thf with a strongly bound THF ligand; this
complex had previously proved inactive in catalytic alkyne
metathesis.13 Performing the same reaction in toluene solution
with longer reaction times (3 d) afforded the desired THF-free
product in 45% yield as a red crystalline solid.
Figure 1. Well-defined catalysts for alkyne metathesis.
capable of efficiently metathesizing terminal alkynes,54−56
a
reactivity that was recently also uncovered for II.23,57 It should
be noted that previous attempts to promote terminal alkyne
metathesis (TAM), e.g., with [Me3CCW(OCMe3)3], proved
significantly less efficient,58−61 which was ascribed to
deprotonation of intermediate metallacyclobutadiene species
and formation of deprotiometallacycles.52,62,63 The latter
species are expected to be responsible for the observed high
activity toward alkyne polymerization. Lately, MoF6 and
related species have also been successfully employed in ring-
opening alkyne metathesis polymerization (ROAMP).64,65
To affirm whether the optimum degree of fluorination in
combination with the metal atom (Mo) has already been
realized in MoF6, we wish to present herein the synthesis and
structural characterization of the complete series of molybde-
num complexes [MesCMo{OC(CF3)nMe3−n}3] (MoF0, n =
0; MoF3, n = 1; MoF6, n = 2; MoF9, n = 3) together with a
combined experimental and theoretical study of their catalytic
alkyne metathesis activity. From the corresponding series of
tungsten complexes, [MesCW{OC(CF3)nMe3−n}3], we
aimed exclusively at the synthesis of WF3 (n = 1) because
WF6 (n = 2) and WF9 (n = 3) were already shown to be
inactive in alkyne metathesis,13,54 which was attributed to the
higher electrophilicity of tungsten alkylidynes in comparison
with their molybdenum relatives.48 WF0 (n = 0) was also not
included in this study as its neopentylidyne congener
[Me3CCW(OCMe3)3] has been widely used in alkyne
metathesis, albeit usually at elevated temperature and with
higher catalyst loadings.1,49,53,58,59
In the 13C NMR spectra, the carbyne carbon atoms generate
characteristic lowfield signals. As anticipated, the chemical shift
of the alkylidyne carbon atom increases with the degree of
alkoxide fluorination. Thus, the alkylidyne resonance of
nonfluorinated MoF0 can be found at 296.9 ppm, whereas
the resonances of the fluorinated alkylidynes MoF3, MoF6, and
MoF9 appear at gradually lower fields at 307.1, 317.6, and
332.7 ppm, respectively. These values fall in the range typically
observed for molybdenum alkylidyne complexes.13,52 The 13C
NMR signal for the carbyne carbon atom of WF3 is found
slightly more upfield at 282.3 ppm in comparison to the
molybdenum species, which can be ascribed to the higher
electrophilicity of the tungsten metal.50,54 This signal shows the
expected satellites owing to coupling with the 183W isotope, i.e.,
1JCW = 147 Hz. The 19F NMR spectra of the fluorinated
complexes show only one singlet each at −82.7, −78.1, and
−73.1 ppm for MoF3, MoF6, and MoF9 and at −82.7 ppm for
RESULTS AND DISCUSSION
■
Preparation and Characterization of 2,4,6-Trimethyl-
benzylidyne Molybdenum and Tungsten Complexes.
Following the low-oxidation-state route,11 the 2,4,6-trimethyl-
benzylidyne complexes of molybdenum and tungsten are easily
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WF3, which indicates fast rotation of the mesityl group at room
temperature on the NMR time scale.
The molecular structures of all complexes were established
by X-ray diffraction analysis; ORTEP diagrams of MoF3 and
WF3 are shown in Figure 2, and the molecular structures of
the pseudomirror plane. In contrast, a perpendicular orientation
of the mesityl group is observed for WF3. Short metal−carbon
bonds together with almost linear M−C1−C2 axes are found
for all complexes. Within experimental error, the Mo−C1
distances of ∼1.75 Å are identical in MoF0, MoF3, MoF6, and
MoF9, revealing a negligible impact of the degree of
fluorination. As observed for other Mo/W pairs,11,13,54 the
W−C1 bond in WF3 of 1.7689(19) Å is slightly longer
compared to MoF3 and the other molybdenum complexes.
The M−C1−C2 angles range from 173.64(14)° in WF3 to
177.40(16)° in MoF6 (Table 1).
Catalytic Alkyne Metathesis with 2,4,6-Trimethylben-
zylidyne Molybdenum and Tungsten Complexes. To
examine the catalytic potential of the alkylidyne complexes and
to compare their performance with theoretically predicted
activities (vide infra), we investigated the homometathesis of
various internal and terminal alkynes. Initially, 3-pentynyl
benzyl ether (2a) and 3-butynyl benzyl ether (2b) were used as
well-established test substrates (Scheme 2).54,55 Toluene
Scheme 2. Homometathesis of the Benzyl Ethers 2a and 2b
solutions of the substrates with catalyst loadings of 1 mol %
were stirred at room temperature in the presence of molecular
sieves (5 Å MS) as 2-butyne or acetylene scavenger. The
reactions were monitored by gas chromatography with n-
decane as internal standard, and the resulting conversion versus
time diagrams are shown in Figures 3 and 4. Figure 3 reveals
Figure 2. ORTEP diagram of MoF3 (top) and WF3 (bottom) with
thermal displacement parameters drawn at 50% probability; hydrogen
atoms are omitted for clarity.
MoF0 and MoF9 can be found in Figure S22. Pertinent
structural data are assembled in Table 1. All complexes show
distorted tetrahedral geometries around the metal atom. As
observed for MoF6,54 the complexes adopt approximate Cs
symmetry with the mesityl group in a coplanar alignment with
Figure 3. Conversion versus time diagram for the metathesis of 3-
pentynyl benzyl ether (2a, 0.5 mmol) in toluene (2.5 mL) catalyzed by
1 mol % of MoF0−MoF9 and WF3 in the presence of 5 Å MS at
room temperature.
Table 1. Selected Bond Lengths, Angles, and 13C NMR Shifts
of Complexes MoF0−MoF9 and WF3
that all catalysts are active in the metathesis of 2a at room
temperature. Even the least active system MoF0 achieves a
moderate conversion of 35% after 2 h. All other catalysts,
MoF3, MoF6, MoF9, and WF3, accomplish almost full
conversion within 120 min with MoF6 showing the highest
activity. For better comparison of the initial rates, plots of
conversion against a logarithmic timeline can be found in
M−C1 [Å]
M−C1−C2 [deg]
δ [ppm]
MoF0
MoF3
MoF6
MoF9
WF3
1.747(3)
176.3(2)
296.9
307.1
317.6
332.7
282.3
1.747(2)
175.28(18)
177.40(16)
173.86(13)
173.64(14)
1.74738(16)
1.74729(16)
1.7689(19)
C
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Figure S1. Metathesis product 3 could be isolated in good
yields by filtration through alumina and evaporation of the
solvent. MoF6 provides the highest isolated yield of 96%,
whereas MoF3, MoF9, and WF3 afforded yields of 94, 89, and
93%, respectively (Table 2).
Similar results were obtained using the corresponding
terminal alkyne 2b at significantly lower substrate concentration
to avoid side reactions such as polymerization (Figure 4).
Under these conditions, MoF9 proved completely inactive in
terminal alkyne metathesis (TAM), and polymeric material was
isolated instead and analyzed by gel permeation chromatog-
raphy (Mn = 802.1 g mol−1, Mw = 1803.3 g mol−1). Low
conversion of only 12% was found for MoF0 after 2 h. In the
same period, 88% conversion and 84% isolated yield were
obtained with MoF3. Surprisingly, MoF6 and WF3 proved to
be almost equally active, and very fast initiation rates >80 min−1
were observed for both catalysts (see Figure S2). From these
reactions, 3 was isolated in 90 and 94% yields, respectively
(Table 2). It should be emphasized that WF3 represents the
first tungsten alkylidyne complex that is able to promote the
metathesis of terminal alkynes in an efficient manner.58−62
Figure 4. Conversion versus time diagram for the metathesis of 3-
butynyl benzyl ether (2b, 0.5 mmol) in toluene (24 mL) catalyzed by
1 mol % of MoF0−MoF6 and WF3 in the presence of 5 Å MS at
room temperature; MoF9 gave around 10% conversion with exclusive
formation of polymeric material.
a
Table 2. Alkyne Metathesis of Internal and Terminal Alkynes
a
Homometathesis: substrate (0.5 mmol), catalyst (1 mol %), toluene (internal alkynes: 2.5 mL, 200 mM; terminal alkynes: 24 mL, 21 mM), 5 Å MS
(500 mg), 25 °C, 2 h. RCAM: substrate (0.5 mmol), catalyst (2 mol %), toluene (internal alkynes: 24 mL, 21 mM; terminal alkynes: 112 mL, 4.5
mM), 5 Å MS (1 g), 25 °C, 2 h. ACM: substrates (0.25 mmol each), catalyst (1 mol %), toluene (12 mL, 21 mM), 5 Å MS (250 mg), 25 °C, 2 h.
b
c
GC yield after 2 h; all other yields are averaged isolated yields based on at least two runs. No yield obtained due to polymerization of the substrate.
d
Unexpectedly, several runs with different substrate and catalyst samples did not result in any conversion.
D
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Encouraged by these results, we further studied the substrate
scope for all catalysts (Table 2). Thus, we attempted
homometathesis of benzoic esters 4 and RCAM of α,ω-diynes
6 and 8 bearing either 3-pentynyl (a: R = Me) or 3-butynyl (b:
R = H) groups. For the benzoic esters 4, the metathesis
reaction was performed analogously to the benzyl ethers 2,
whereas the RCAM reactions with 6 and 8 were performed at
lower substrate concentration of 21 mM for internal and 4.5
mM for terminal substrates with catalyst loadings of 2 mol % in
the presence of molecular sieves (5 Å). In principle, the results
resemble the situation established with substrates 2; MoF6 and
WF3 show the best performance with high isolated yields of
almost all products. For unknown reasons, however, RCAM of
8b with WF3 repeatedly gave no conversion at all. MoF3 also
afforded good yields despite its overall lower activity. As
observed for substrates 2, MoF9 can only accomplish the
metathesis of internal alkynes in a satisfactory manner but fails
completely in the metathesis of all terminal alkynes. MoF0 as
the least active catalyst gave only moderate or low conversions
as indicated by GC analysis.
diene intermediate IN) were computed. Enthalpic and entropic
contributions were calculated by statistical thermodynamics as
implemented in the Gaussian09 set of programs.70 A full
catalytic cycle additionally includes the interconversion
between two metallacyclobutadiene (MCBD) intermediates.
However, this rearrangement has a considerably lower barrier,8
and therefore, no attempts were made to locate this second
transition state. Furthermore, it should be mentioned that
entropic effects for the associative mechanism at work are
expected to be smaller in the condensed phase, which should
lead to significantly lower free activation barriers in comparison
with these gas-phase computations.71 The following discussion
will therefore be based on the calculated enthalpies, which are
listed in Tables 3 and 4 along with additional thermodynamic
data.
Table 3. Relative B3LYP Energies and Enthalpies (kcal
mol−1) for the Model Metathesis Reaction of 2-Butyne with
a
MoF0−MoF9
b
b
TS
IN
The best catalysts MoF3, MoF6, and WF3 were also
employed for the alkyne cross metathesis (ACM) of the
terminal benzyl ether 2b with trimethylsilylacetylene. Mixtures
of both substrates (1:1, 0.25 mmol each) were stirred in
toluene for 2 h at room temperature with catalyst loadings of 1
mol %. Workup afforded unsymmetrical alkyne 10 in excellent
yields in the cases of MoF6 (98%)55 and WF3 (95%), whereas
only 32% GC conversion into 10 was observed for MoF3 after
2 h. At this point, homometathesis product 3 was present in
58%. After 24 h, however, GC analysis indicated 75 and 17%
conversion into products 10 and 3, respectively. For conversion
versus time diagrams, see Figures S3−S5. To the best of our
knowledge, MoF6 and WF3 represent the first catalysts that
can promote ACM of two terminal alkynes efficiently, whereas
several ACM reactions involving one terminal and one internal
alkyne, e.g., 1-trimethylsilylpropyne, were reported with II.57
We assume that this process is driven thermodynamically with
silylated product 10 being (slightly) more stable than
homocoupled alkynes 3 and bis(trimethylsilyl)acetylene.
Theoretical Investigation of Alkyne Metathesis with
Molybdenum and Tungsten Alkylidyne Complexes. To
explain the different activities of the Mo and W alkylidyne
complexes depending on the degree of alkoxide fluorination, we
performed a series of theoretical calculations using the B3LYP
density functional.67,68 For this purpose, the metathesis of 2-
butyne was selected as a model reaction (Scheme 3), which had
ΔE‡0
ΔH‡298
ΔG‡298
ΔE0
ΔH298
ΔG298
MoF0
MoF3
MoF6
MoF9
23.28
18.57
7.18
22.47
17.72
6.29
38.82
34.93
24.44
21.93
16.19
12.68
1.42
15.39
11.83
0.57
30.89
28.39
18.00
15.38
5.14
4.28
−2.19
−3.26
a
b
For more details, see Supporting Information. ΔE0: relative energies
at 0 K, ΔH298: enthalpies at 298 K, ΔG298: Gibbs free energies at 298
K; all values are stated relative to the isolated catalyst and 2-butyne.
Table 4. Relative B3LYP Energies and Enthalpies (kcal
mol−1) for the Model Metathesis Reaction of 2-Butyne with
a
WF0−WF9
b
b
TS
IN
ΔE‡0
ΔH‡298
ΔG‡298
ΔE0
6.31
ΔH298
ΔG298
WF0
WF3
WF6
WF9
17.26
16.58
10.04
9.43
16.63
16.03
9.65
32.92
32.70
26.24
25.41
5.51
1.68
21.00
18.31
10.76
6.26
2.55
−7.70
−11.32
−8.82
−12.29
9.02
a
b
For more details, see Supporting Information. ΔE0: relative energies
at 0 K, ΔH298: enthalpies at 298 K, ΔG298: Gibbs free energies at 298
K; all values are stated relative to the isolated catalyst and 2-butyne.
The calculations predict an increasing relative stability of the
metallacyclobutadienes (IN) for both sets of molybdenum and
tungsten complexes with an increasing degree of fluorination
on the alkoxide ligands. For the same degree of fluorination, the
tungsten metallacyclobutadienes are significantly more stabi-
lized (by at least 9 kcal mol−1) compared to their molybdenum
congeners, which can be ascribed to the greater electrophilic
character of tungsten alkoxides.48 A similar trend can be
observed for the transition states, suggesting that the formation
of the metallacyclobutadienes is not only favored thermody-
namically but also kinetically with increasing the degree of
fluorination. Because these metallacyclobutadienes are crucial
intermediates in alkyne metathesis (Scheme 3),72−75 it is
expected that the catalytic activity increases in the same order.
A full catalytic cycle, however, also involves a [2 + 2]
cycloreversion, which could also become the rate-determining
step if the metallacyclobutadiene is more stable than the
respective alkylidyne complex and alkyne. This is most likely
Scheme 3. Model Reaction of 2-Butyne with Trialkoxide
Ethylidyne Complexes (M = Mo, W; X = OCMe3,
OC(CF3)Me2, OC(CF3)2Me, OC(CF3)3)
previously been used to rationalize the relative activities of Mo
and W alkyne metathesis catalysts in a reliable fashion.10,11,69
For the model alkylidynes [MeC ≡ MX3] (M = Mo, W; X =
OCMe3, OC(CF3)Me2, OC(CF3)2Me,, OC(CF3)3), the first
three relevant stationary points of the rate-determining step
(catalyst plus 2-butyne, transition state TS, metallacyclobuta-
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the case for the intermediates derived from MoF9, WF6, and
WF9 for which our computations predict negative enthalpies.
In agreement with the experimental findings, the increase in
catalytic activity in the order MoF0 < MoF3 < MoF6 correlates
with the decreasing activation barrier, whereas the lower
catalytic activity of MoF9 compared to that of MoF6 can be
ascribed to an overly stabilized MCBD intermediate. Alkylidyne
complexes of the type WF0, e.g., Me3CC≡W(OCMe3)3, have
tylenes and structurally characterized, but theoretical calcu-
lations revealed that these systems are best described as
Mo(IV) species containing anionic diaminodicarbene ligands of
the type [(R2N)CC(Mes)C(NR2)]−.84
CONCLUSIONS
■
Comparison of the series of molybdenum alkylidyne complexes
[MesCMo{OC(CF3)nMe3−n}3] (n = 0−3) with varying
fluorine content reveals that MoF6 (n = 2) shows the overall
best catalytic performance in the metathesis of internal and
terminal alkynes. Lower or higher fluorine content diminishes
the catalytic activity, and MoF0 (n = 0), MoF3 (n = 1), and
MoF9 (n = 3) clearly exhibit inferior performance. This trend
can be rationalized by DFT calculations on the metathesis
model reaction of 2-butyne, which predict higher activation
barriers for the formation of the metallacyclobutadiene
(MCBD) intermediate for MoF0 and MoF3, whereas the
lower catalytic activity of MoF9 can be ascribed to an overly
stabilized MCBD. This conclusion is supported by isolation of
the stable MCBD complex 11 by reaction of MoF9 with 3-
hexyne. These results indicate that the optimum level of
fluorination for the application of molybdenum alkylidynes in
alkyne metathesis catalysts is realized in MoF6.
been studied intensely by R. R. Schrock and A. Furstner and
̈
showed good catalytic activity in homometathesis and RCAM
reactions, usually at elevated temperature.17,50,76−79 Although
we did not directly compare WF0 and WF3, the catalytic
performance of WF3 appears to be superior, and unlike the
WF0 derivative Me3CC≡W(OCMe3)3,58−60 it can even
transform terminal alkynes efficiently at room temperature.
Further increasing the degree of fluorination is counter-
productive, and as previously reported, WF6 and WF9 show
no catalytic activity in alkyne metathesis.54,13 This observation
can again be ascribed to a significantly exothermic MCBD
formation.
Formation of an Isolable Molybdenacyclobutadiene
by Stoichiometric Reactions. Because the calculated
enthalpies suggest a potentially stable MCBD derived from
MoF9, we attempted to isolate such an intermediate.
Therefore, a saturated CH2Cl2 solution of MoF9 was treated
with ten equivalents of 3-hexyne, which resulted in an
immediate color change from red to purple, and upon
refrigeration at 3 °C, a purple crystalline product formed.
The crystals were suitable for X-ray diffraction analysis;
however, the structure could not be refined because of severe
disorder. An ORTEP plot revealing the connectivity and further
information can be found in Figure S24 and Table S3. NMR
analysis of the crystalline product in CD2Cl2 solution revealed
the formation of metallacyclobutadiene 11. The 1H NMR
spectrum shows signals corresponding to two different ethyl
groups in a 2:1 ratio. The 13C NMR spectrum also shows the
lowfield chemical shifts expected for molybdenacycles at 269.9
ppm for the α carbon atoms and at 156.0 ppm for the β carbon
atom, which is in good agreement with the NMR data reported
for other molybdenacyclobutadienes.51,52,80 The 19F NMR
spectrum exhibits two multiplets at −72.6 (18F) and −72.7
ppm (9F), indicating the presence of two sets of alkoxide
ligands. The formation of 11 can be explained by conversion of
the 2,4,6-trimethylbenzylidyne (MesC≡Mo) into a propylidyne
moiety (EtC≡Mo) and subsequent cycloaddition of a second
equivalent of 3-hexyne (Scheme 4).
Owing to the higher electrophilicity of their tungsten
alkylidyne congeners [MesCW{OC(CF3)nMe3−n}3], the
calculations predict significantly stabilized MCBD intermedi-
ates for WF6 and WF9 in agreement with their catalytic
inefficiency. Therefore, the optimum degree of fluorination in
this series of tungsten-based alkyne metathesis catalysts is
already attained in WF3, which showed excellent catalytic
performance in the metathesis of internal and even terminal
alkynes. These results confirm that the development of efficient
alkyne metathesis catalysts based on molybdenum and tungsten
alkylidyne complexes requires an elaborate adjustment of the
metal/ligand combination, which considers that more electron-
withdrawing, or more electronegative, ligands are needed for
the Mo systems than for the more electrophilic W systems.
EXPERIMENTAL SECTION
■
General Experimental Considerations. All operations with air-
and moisture-sensitive compounds were conducted in a glovebox
(MBraun 200B) under a dry and oxygen-free argon atmosphere or on
a Schlenk line using standard Schlenk techniques. Solvents were
purified and dried by a Braun Solvent Purification System and stored
over a molecular sieve (3−4 Å). Complexes 1a, 1b, and MoF6, the
substrates 2, 4, 6, and 8,11,54,9 and the alkoxides (except for KOCMe3)
were synthesized according to literature methods. All other
compounds were obtained from commercial sources and used without
further purification. The molecular sieve 5 Å MS (Sigma-Aldrich,
powder <50 μm) for metathesis reactions was dried for 24 h at 180 °C
under vacuum prior to use. Sodium and potassium hydride dispersions
in mineral oil were washed several times with hexane and dried under
vacuum directly before use.
Scheme 4. Preparation of Metallacyclobutadiene 11 with
MoF9
Analytical Methods. 1H, 19F, and 13C NMR spectra were
recorded on Bruker AV II-300, DRX-400, and AV II-600 instruments
at room temperature. Chemical shifts (δ) are expressed in ppm (parts
per million). 1H and 13C NMR spectra are referenced to residual
solvent signals (C6D6: δ(H) 7.16 ppm, δ(C) 128.06 ppm; CDCl3:
δ(H) 7.26 ppm, δ(C) 77.16 ppm; CD2Cl2: δ(H) 5.32 ppm, δ(C)
53.84 ppm). For 19F NMR, an external calibration with CFCl3 was
used. Coupling constants (J) are reported in hertz (Hz). Multiplicities
are expressed with s (singlet), d (doublet), t (triplet), q (quartet), or m
(multiplet). The number of hydrogen atoms (n) for a signal is
indicated by nH. When necessary, signal assignment was confirmed by
two-dimensional H,C-HSQC NMR and H,C-HMBC NMR experi-
It should be noted that 11 represents a rare example of an
isolable molybdenacyclobutadiene,51,52,80 whereas significantly
more tungstenacyclobutadienes of the type [(C3R3)WX3] (R =
alkyl, aryl; X = alkoxide, phenoxide, amide, halide) have been
reported.50,81−83 Paramagnetic MCBD-type complexes were
recently isolated from the reaction of MoF6 with diaminoace-
F
DOI: 10.1021/acs.organomet.7b00519
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
ments. Elemental analyses were accomplished by combustion and gas
chromatographic analysis using a VarioMICRO Tube and WLD
detection. Gas chromatography (GC) was executed on a Hewlett-
Packard 5890 SERIES II using a DB5-HT column and FID detection.
For calibration, n-decane was used as an internal standard. Gel
permeation chromatography (GPC) was conducted on an Agilent
Series 1200 instrument, which consisted of a PSS SDV (Lux) column
(5 μm, 100 Å × 1000 Å). The flow-rate (THF) was maintained at 1
mL/min with the column temperature at 35 °C and elution monitored
by an evaporative light-scattering detector.
Synthesis of MoF0. Compound 1a (400 mg, 0.72 mmol) was
added in small portions to a solution of KOCMe3 (242 mg, 2.15
mmol) in THF (8 mL). The reaction mixture was stirred for 16 h at
ambient temperature. After removal of the solvent at 50 °C under
reduced pressure, the residue was extracted with small amounts of
pentane. The combined extracts were filtered through a short pad of
Celite, and the filtrate was concentrated and stored at −40 °C,
affording the product as light yellow crystals. Yield: 144 mg (45%). 1H
NMR (600.1 MHz, C6D6, 298 K): δ 6.69 (m, 2H, m-CH), 2.85 (s, 6H,
o-CH3), 2.10 (s, 3H, p-CH3), 1.46 (s, 27H, OC(CH3)3). 13C{1H}
NMR (150.9 MHz, C6D6, 298 K): δ 297.4 (s, Mo≡C), 143.5 (s, I-C),
139.3 (s, o-C), 136.5 (s, p-C), 128.2 (s, m-CH), 78.6 (s, OC(CH3)3),
32.3 (s, OC(CH3)3), 21.4 (s, o-CH3), 21.1 (s, p-CH3). Anal. Calcd for
C22H38MoO3: C 59.18, H 8.58. Found: C 58.98, H 8.34.
Synthesis of MoF3. Compound 1a (200 mg, 0.36 mmol) was
added in small portions to a solution of KOC(CF3)Me2 (179 mg, 1.08
mmol) in toluene (4 mL). The reaction mixture was stirred for 16 h at
ambient temperature. After removal of the solvent under reduced
pressure, the residue was extracted with small amounts of pentane and
Et2O. The combined extracts were filtered over a short pad of Celite,
and the filtrate was dried under reduced pressure. Crystallization from
Et2O at −40 °C afforded the product in sufficient purity as yellow
crystals. Yield: 68 mg (31%). 1H NMR (600.1 MHz, C6D6, 298 K): δ
6.57 (m, 2H, m-CH), 2.64 (s, 6H, o-CH3), 2.03 (s, 3H, p-CH3), 1.44
(s, 18H, OC(CH3)2(CF3)). 13C{1H} NMR (150.9 MHz, C6D6, 298
K): δ 307.1 (s, Mo≡C), 143.1 (s, i-C), 140.2 (s, o-C), 138.9 (s, p-C),
128.2 (s, m-CH), 126.9 (q, 1JCF = 284 Hz, OC(CH3)2(CF3)), 81.3 (q,
2JCF = 29 Hz, OC(CH3)2(CF3)), 24.5 (s, OC(CH3)2(CF3)), 21.1 (s, p-
CH3), 20.8 (s, o-CH3). 19F{1H} NMR (282.5 MHz, C6D6, 298 K): δ
−82.7 (s, CF3). Anal. Calcd for C22H29F9MoO3: C43.43, H 4.80.
Found: C 43.26, H 4.75.
OC(CH3)2(CF3)), 23.9 (s, OC(CH3)2(CF3)), 20.4 (s, p-CH3), 20.3 (s,
o-CH3). 19F{1H} NMR (282.5 MHz, C6D6, 298 K): δ −82.8 (s, CF3).
Anal. Calcd for C22H29F9WO3: C 37.95, H 4.20. Found: C 37.87, H
4.23.
Synthesis of [(C3Et3)Mo(OC(CF3)3)3] 11. Ten equiv of 3-hexyne
(88 mg, 1.07 mmol) was added to a solution of MoF9 (100 mg, 0.11
mmol) in CH2Cl2. The color of the reaction mixture immediately
changed from red to purple. After cooling the solution to 3 °C for 2
1
days, purple crystals were obtained. Yield: 53 mg (55%). H NMR
(600.1 MHz, CD2Cl2, 298 K): δ 4.01 (q, 2H, 3JHH = 7.6 Hz, CβCH2),
3
3
3.75 (q, 4H, JHH = 7.3 Hz, CαCH2), 1.86 (t, 6H, JHH = 7.3 Hz,
CβCH2CH3), 1.71 (t, 3H, 3JHH = 7.6 Hz, CαCH2CH3). 13C{1H} NMR
(150.9 MHz, CD2Cl2, 298 K): δ 269.9 (s, Cα), 156.0 (s, Cβ), 121.3 (q,
1JCF = 295 Hz, equatorial-OC(CF3)3), 120.8 (q, JCF = 290 Hz, axial-
1
OC(CF3)3), 83.1 (m, 2JCF = 29 Hz, OC(CF3)3), 34.7 (s, CαCH2), 32.3
(s, CβCH2), 12.7 (s, CβCH2CH3), 12.6 (s, CαCH2CH3). 19F{1H} NMR
4
(376.1 MHz, CD2Cl2, 298 K): δ −72.6 (m, 18F, JFF = 2.8 Hz, CF3),
−72.7 (m, 9F, 4JFF = 2.8 Hz, CF3). Anal. Calcd for C21H15F27MoO3: C
27.29, H 1.64; Found: C 27.24, H 1.26.
Comparison of Catalytic Activities. A 25 mL Schlenk flask
under an Ar atmosphere was charged with 5 Å MS (250 mg), 0.05 mL
of n-decane, and a solution of the substrate (0.25 mmol) in toluene
(internal alkynes: 1.25 mL, terminal alkynes: 12 mL). Then, the
catalyst (1 mol %) was added, and samples (internal alkynes: 0.05 mL,
terminal alkynes: 0.25 mL) were taken for gas chromatographic
analysis in certain intervals.
General Procedure for Homometathesis. A 50 mL Schlenk
flask under an Ar atmosphere was charged with 5 Å MS (500 mg) and
a solution of the substrate (0.5 mmol) in toluene (internal alkynes: 2.5
mL, terminal alkynes: 24 mL). Then, the catalyst (1 mol %) was
added, and the mixture was stirred for 2 h at room temperature. The
catalyst and the molecular sieve were removed by filtration through
alumina, and the solvent was evaporated. The crude reaction product
was purified by flash chromatography on silica gel with 1:8 ethyl
acetate/hexane.
General Procedure for RCAM. A Schlenk flask under an Ar
atmosphere was charged with 5 Å MS (1 g) and a solution of the
substrate (0.5 mmol) in toluene (internal alkynes: 24 mL, terminal
alkynes: 112 mL). Then, the catalyst (2 mol %) was added, and the
mixture was stirred for 2 h at room temperature. The catalyst and the
molecular sieve were removed by filtration through alumina, and the
solvent was evaporated. The crude reaction product was purified by
flash chromatography on silica gel with 1:8 ethyl acetate/hexane.
General Procedure for ACM. A 50 mL Schlenk flask under an Ar
atmosphere was charged with 5 Å MS (250 mg) and a solution of the
substrate (0.25 mmol) and trimethylsilyl-acetylene (0.25 mmol) in 12
mL of toluene. Then, the catalyst (1 mol %) was added, and the
mixture was stirred for 2 h at room temperature. The catalyst and the
molecular sieve were removed by filtration through alumina, and the
solvent was evaporated. The crude reaction product was purified by
flash chromatography on silica gel with 1:8 ethyl acetate/hexane.
Synthesis of MoF9. A suspension of KOC(CF3)3 (1.0 g, 3.78
mmol) in toluene (10 mL) was added to 1a (700 mg, 1.26 mmol) in
toluene (10 mL). The reaction mixture was stirred for 3 days at
ambient temperature. After removal of the solvent under reduced
pressure, the red residue was extracted with small amounts of pentane.
The combined extracts were filtered over a short pad of Celite, and the
filtrate was concentrated and stored at −40 °C, affording the product
as red crystals. To achieve purity suitable for catalysis, the product was
recrystallized three times. Yield: 618 mg (53%). 1H NMR (600.1
MHz, CD2Cl2, 298 K): δ 6.78 (m, 2H, m-CH), 2.50 (s, 6H, o-CH3),
2.33 (s, 3H, p-CH3). 13C{1H} NMR (150.9 MHz, CD2Cl2, 298 K): δ
332.7 (s, Mo≡C), 143.7 (s, o-C), 143.3 (s, i-C), 143.2 (s, p-C), 128.9
1
2
ASSOCIATED CONTENT
(s, m-CH), 120.7 (q, JCF = 293 Hz, OC(CF3)3), 85.5 (m, JCF = 31
Hz, OC(CF3)3), 21.3 (s, p-CH3), 19.7 (s, o-CH3). 19F{1H} NMR
(376.1 MHz, CD2Cl2, 298 K): δ −73.69 (s, CF3). Anal. Calcd for
C22H11F27MoO3: C 28.34, H 1.19. Found: C 28.44, H 1.56.
■
S
* Supporting Information
The Supporting Information is available free of charge via the
Internet at . The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.organomet.7b00519.
Synthesis of WF3. Compound 1b (200 mg, 0.31 mmol) was
added in small portions to a solution of KOC(CF3)Me2 (155 mg, 0.93
mmol) in THF (4 mL). The reaction mixture was stirred for 16 h at
ambient temperature. After removal of the solvent under reduced
pressure at 50 °C, the residue was extracted with small amounts of
pentane. The combined extracts were filtered over a short pad of
Celite, and the filtrate was concentrated and stored at −40 °C
Spectroscopic characterization of all new compounds,
details of the X-ray measurements of complexes MoF0
(CCDC 1560020), MoF3 (CCDC 1560021), MoF9
(CCDC 1560022), WF3 (CCDC 1560023), Mo2{OC-
(CF3)2Me}6 (CCDC 1560024), and 11 (CCDC
1566270) (PDF)
1
affording the product as yellow crystals. Yield: 156 mg (72%). H
NMR (600.1 MHz, C6D6, 298 K): δ 6.78 (m, 2H, m-CH), 2.74 (s, 6H,
o-CH3), 2.23 (s, 3H, p-CH3), 1.41 (s, 18H, OC(CH3)2(CF3)).
13C{1H} NMR (150.9 MHz, C6D6, 298 K): δ 282.3 (t, 1JCW = 147 Hz,
W≡C), 141.3 (s, i-C), 141.0 (s, o-C), 137.3 (s, p-C), 127.3 (s, m-CH),
Optimized geometries of the molybdenum (MoF0−
MoF9) and tungsten (WF0−WF9) complexes and their
1
2
126.7 (q, JCF = 284 Hz, OC(CH3)2(CF3)), 81.2 (q, JCF = 29 Hz,
G
DOI: 10.1021/acs.organomet.7b00519
Organometallics XXXX, XXX, XXX−XXX

Organometallics
corresponding transition states and intermediates during
Article
(21) Persich, P.; Llaveria, J.; Lhermet, R.; de Haro, T.; Stade, R.;
Kondoh, A.; Furstner, A. Chem. - Eur. J. 2013, 19, 13047−13058.
(22) Valot, G.; Mailhol, D.; Regens, C. S.; O’Malley, D. P.; Godineau,
̈
the alkyne metathesis with 2-butyne (MOL)
Accession Codes
E.; Takikawa, H.; Philipps, P.; Furstner, A. Chem. - Eur. J. 2015, 21,
̈
2398−2408.
CCDC 1560020−1560024 and 1566270 contain the supple-
mentary 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_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(23) Willwacher, J.; Heggen, B.; Wirtz, C.; Thiel, W.; Furstner, A.
̈
Chem. - Eur. J. 2015, 21, 10416−10430.
(24) Ungeheuer, F.; Furstner, A. Chem. - Eur. J. 2015, 21, 11387−
̈
11392.
(25) Cromm, P. M.; Schaubach, S.; Spiegel, J.; Furstner, A.;
̈
Grossmann, T. N.; Waldmann, H. Nat. Commun. 2016, 7, 11300.
(26) Cromm, P. M.; Wallraven, K.; Glas, A.; Bier, D.; Furstner, A.;
̈
AUTHOR INFORMATION
Corresponding Author
*E-mail: m.tamm@tu-bs.de.
ORCID
Ottmann, C.; Grossmann, T. N. ChemBioChem 2016, 17, 1915−1919.
■
(27) Ahlers, A.; de Haro, T.; Gabor, B.; Furstner, A. Angew. Chem.,
̈
Int. Ed. 2016, 55, 1406−1411.
(28) Schaubach, S.; Michigami, K.; Furstner, A. Synthesis 2016, 49,
̈
202−208.
Matthias Tamm: 0000-0002-5364-0357
(29) Lysenko, S.; Haberlag, B.; Daniliuc, C. G.; Jones, P. G.; Tamm,
M. ChemCatChem 2011, 3, 115−118.
Author Contributions
†C.B. and H.E. contributed equally to this work.
(30) Lysenko, S.; Volbeda, J.; Jones, P. G.; Tamm, M. Angew. Chem.,
Int. Ed. 2012, 51, 6757−6761.
Notes
(31) Li, S. T.; Schnabel, T.; Lysenko, S.; Brandhorst, K.; Tamm, M.
Chem. Commun. 2013, 49, 7189−7191.
The authors declare no competing financial interest.
(32) Jyothish, K.; Zhang, W. Angew. Chem., Int. Ed. 2011, 50, 3435−
3438.
ACKNOWLEDGMENTS
We are grateful to the group of Prof. Christophe Coper
■
(33) Jyothish, K.; Wang, Q.; Zhang, W. Adv. Synth. Catal. 2012, 354,
́
et
2073−2078.
(ETH Zurich) for helpful discussions. H.E. thanks the Fonds
der Chemischen Industrie (FCI) for a Chemiefonds Fellow-
ship. This work was funded by the Deutsche Forschungsge-
meinschaft (DFG) through project TA 189/12−1 (“Mecha-
nistic studies on the catalytic metathesis of internal and
terminal alkynes and diynes”).
(34) Yang, H.; Liu, Z.; Zhang, W. Adv. Synth. Catal. 2013, 355, 885−
890.
(35) Du, Y.; Yang, H.; Zhu, C.; Ortiz, M.; Okochi, K. D.; Shoemaker,
R.; Jin, Y.; Zhang, W. Chem. - Eur. J. 2016, 22, 7959−7963.
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DOI: 10.1021/acs.organomet.7b00519
Organometallics XXXX, XXX, XXX−XXX