Catalytic Alkyne and Diyne Metathesis with Mixed Fluoroalkoxy-Siloxy Molybdenum Alkylidyne Complexes

Siloxy Molybdenum Alkylidyne Complexes

Article

Catalytic Alkyne and Diyne Metathesis with Mixed Fluoroalkoxy-

Manuel L. Zier, Sophie Colombel-Rouen, Henrike Ehrhorn, Dirk Bockfeld, Yann Trolez, Marc Mauduit,

and Matthias Tamm *

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sı Supporting Information

ABSTRACT: The reaction of the molybdenum alkylidyne complex

MesCMo{OC(CF ) } ] (MoF9, Mes = 2,4,6-trimethylphenyl)

[

3 3

with the potassium siloxides KOSi(OtBu)₃and KOSi(OtBu) -

(

OMes) furnished the mixed ﬂuoroalkoxy-siloxy alkylidyne com-

plexes [MesCMo{OC(CF ) } {OSi(OtBu) }] (MoSiF9) and

[

MesCMo{OC(CF ) } {OSi(OtBu) (OMes)}] (MoSi*F9). A

treatment of MoF9, MoSiF9, and MoSi*F9 with an excess of 3-

hexyne (EtCCEt) aﬀorded labile metallacyclobutadiene (MCBD)

complexes with a (C Et )Mo core, which are in equilibrium with the

corresponding propylidyne (EtCMo) complexes in solution. Thermodynamic parameters for these [2 + 2]-cycloaddition/

cycloreversion reactions were determined by van’t Hoﬀ plots, revealing that the nature of the ancillary siloxide ligand exerts a

signiﬁcant eﬀect on the MCBD stability. An X-ray diﬀraction analysis of MoSi*F9-MCBD provided one of the ﬁrst accurate crystal

structures of a molybdenacyclobutadiene. MoF9, MoSiF9, and MoSi*F9 proved active catalysts for the metathesis of internal

alkynes and diynes, with MoSi*F9 showing unprecendented selectivity in the conversion of sterically encumbered 1,3-pentadiynes

into symmetrical 1,3,5-triynes and 2-butyne.

INTRODUCTION

highest catalytic activity and can even be used for the

metathesis of terminal alkynes (Figure 1).

■

17,18

Alkyne metathesis is an organic reaction that entails the

redistribution of carbon−carbon triple bonds in the presence

of transition-metal alkylidyne complexes through the reversible

formation of metallacyclobutadienes (MCBDs). Shortly after

its initial discovery in 1968, which involved the heteroge-

In the case of tungsten, a lower degree of ﬂuorination is

suﬃcient, and [MesCW{OC(CF )Me } ] (WF3) is argu-

2 3

ably the most active tungsten-based alkyne metathesis catalyst

6,19

reported.

Convenient access to these complexes was

provided by the development of reliable protocols for their

neously catalyzed “disproportionation” of 2-pentyne into 2-

20,21

preparation from Mo(CO) or W(CO) , respectively,

6 6

butyne and 3-hexyne, the ﬁrst homogeneous catalyst for the

which also furnished MoF6-type initiators for ring-opening

metathesis of alkynes was generated from a mixture of

Mo(CO)₆and the phenol reagent resorcinol. With the

22−24

alkyne metathesis polymerization (ROAMP)

as well as N-

heterocyclic carbene (NHC) adducts of related molybdenum

discovery of the ﬁrst SCHROCK-type tungsten and molybdenum

25−28

and tungsten benzylidyne complexes.

Recently, related

alkylidyne complexes, the suitability of molecular catalysts

molybdenum benzylidyne complexes were even used for oleﬁn

such as [tBuCW(OtBu) ] and [tBuCMo(OC(CF ) -

3 2

metathesis reactions.

Me) ] for alkyne metathesis was soon revealed. Ever since,

Siloxide-supported alkylidyne complexes are another im-

ﬂuoroalkoxide-supported alkylidyne complexes have played a

−10

portant class of alkyne metathesis catalysts, and the

prominent role,

and it was demonstrated that the degree of

triphenylsiloxy molybdenum complex I is probably the most

ﬂuorination is crucial for optimum catalytic performance, with

31,32

frequently used species,

in particular, for natural product

the less electrophilic molybdenum requiring a higher degree of

1,12

synthesis through ring-closing alkyne metathesis

ﬂuorination.

With the rise of alkyne metathesis in recent years,

33−43

(RCAM).

Recently, molybdenum alkylidyne complexes

alkylidyne complexes bearing ﬂuoroalkoxide ligands have

Received: May 10, 2021

Published: June 16, 2021

again played an important role in the development of this

4,15

area.

In accordance with the above ﬁndings, the catalytic

activity of the molybdenum 2,4,6-trimethylbenzylidyne com-

plexes [MesCMo{OC(CF ) Me₃₋_n}₃] (MoF0, n = 0;

MoF3, n = 1; MoF6, n = 2; MoF9, n = 3) can be tuned by

varying the degree of ﬂuorination, whereby MoF6 has the

2021 American Chemical Society

008

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Article

with α and β referring to the 1,3 and 2 positions, respectively,

of the carbon atoms in a four-membered metallacycle. These

mechanistic considerations could be substantiated experimen-

57,59

tally and theoretically,

and a ﬂuoroalkoxide-supported α,α-

diethynyl-tungstenacyclobutadiene complex was recently iso-

lated and structurally characterized by single-crystal X-ray

diﬀraction. While diyne cross-metathesis (DYCM) can

certainly be regarded as a convenient new method for the

preparation of unsymmetrical diynes, the selective dispropor-

tion of diynes into monoynes and triynes represents a highly

interesting reaction in its own right, since it would provide a

straightforward way to synthesize compounds with the highly

desirable 1,3,5-hexatriyne core (Scheme 1). In fact, an

expedient synthesis of conjugated triynes from sterically

hindered 1,3-pentadiynes, for example, with R = iPr Si and

R′ = Me, in the presence of catalyst I was recently reported.

Nevertheless, less sterically demanding groups, for example, R

Mes, furnished a considerable drop of selectivity with lower

triyne/diyne ratios, leaving the quest for more eﬃcient

catalysts with higher selectivity and productivity.

Figure 1. Selected ﬂuoroalkoxide- and siloxide-supported alkyne

metathesis catalysts; Mes = 2,4,6-trimethylphenyl.

The tri(tert-butoxy)siloxide ligand was also employed for the

preparation of the mixed ﬂuoroalkoxy-siloxy complex MoSiF6

with arene-bridged tripodal siloxide ligands were introduced

and employed as alkyne metathesis catalysts with improved

(

Figure 1), which served as a model complex for the silica-

supported catalyst MoF6@SiO . The homogeneous catalyst

4−47

functional group tolerance.

This design strategy is

MoF6 and its immobilized form MoF6@SiO₂showed

unprecedented catalytic activity at parts-per-million loadings;

however, a lower activity was observed for the heterogeneous

system, presumably owing to the rigidity of the surface

somewhat reminiscent of molybdenum alkylidyne complexes

with tripodal tris(aryloxide) ligands, which were used

9−53

extensively in polymer and supramolecular chemistry

and, more recently, for alkyne metathesis operating under

species. Noteworthy, the molecular catalysts MoF6 and

open-air conditions.

MoSiF6 showed comparable activity in the self-metathesis of

Another important ancillary ligand is tri(tert-butoxy)siloxide,

-phenyl-1-propyne, revealing that the replacement of one

(

tBuO) SiO, which was used for the preparation of the

ﬂuoroalkoxide by the (tBuO) SiO group does not lead to a

deterioration in the catalytic activity. The X-ray crystal

55,56

tungsten benzylidyne complex II (Figure 1).

This complex

proved active not only in alkyne metathesis but was also

employed for the homo- and cross-metathesis of 1,3-diynes

structure of MoSiF6 showed that the siloxide is bound in a

chelating κ O,O′ fashion in the solid state. This interaction

7−59

(Scheme 1).

The high selectivity toward diyne formation

might be favorable to stabilize the catalyst and also relevant

stationary structures during alkyne metathesis, since similar

interactions were found computationally for the diyne

Scheme 1. Metathesis Reactions with Alkynes and 1,3-

Diynes

metathesis reaction in the presence of II.

We envisaged that an introduction of the (tBuO) SiO group

could be generally used to ﬁne-tune the electronic and catalytic

properties of ﬂuoroalkoxide-supported catalyst systems and

that a further variation of the siloxide ligand, as, for instance, in

unsymmetrical (MesO)(tBuO) SiO, might allow a further

adjustment of the substrate-speciﬁc selectivity. This concept

was ﬁrst applied to MoF9, which had proven to be a valuable

catalyst for the metathesis of internal alkynes. In preliminary

studies, it also demonstrated reactivity toward symmetrical

diynes, however, with a stronger tendency to form a mixture of

diynes, triynes, and even tetraynes. Therefore, we present

herein the preparation and characterization of the siloxy

derivatives MoSiF9 and MoSi*F9 and their application,

together with MoF9, in the metathesis of alkynes and diynes

(

Scheme 1). To rationalize their performance in alkyne

metathesis, we also aimed at the isolation of molybdenacyclo-

butadiene (MoCBD) complexes and have assessed their

stability by variable-temperature NMR and computational

studies.

can be rationalized by the so-called α,α-mechanism that

involves alkynylalkylidyne complexes (MCCCR) as

catalytically active species and α,α-dialkynyl-metallcyclobuta-

diene (MCBD) intermediates. The disproportionation into

monoynes and triynes, which was observed as a very slow side

reaction, can then be explained by deviation from this

mechanism through a formation of α,β-dialkynyl-MCBDs,

RESULTS AND DISCUSSION

■

Preparation and Characterization of Mixed Fluoroal-

koxy-Siloxy Molybdenum Alkylidyne Complexes. The

mixed ﬂuoroalkoxy-siloxy complex MoSiF6 had been prepared

009

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by a one-pot synthesis from [MesCMoBr (dme)] (dme =

dimethoxyethane) by a reaction with 2 equiv of KOC-

(

CF ) Me and 1 equiv of KOSi(OtBu) . This route proved

unfavorable in the case of MoSiF9, aﬀording a mixture of

mono- and disiloxy complexes; see Figure S40 in the

Supporting Information for the X-ray crystal structure of

[

MesCMo{OC(CF ) }{OSi(OtBu) } ]. Alternatively,

3 2

MoSiF9 was prepared following the route established for

imidazolin-2-iminato complexes.

20,66−68

Thus, it was isolated

in a signiﬁcantly higher yield (93%) by a treatment of MoF9

with 1 equiv of KOSi(OtBu) and a substitution of one

OC(CF ) moiety (Scheme 2). Unfortunately, both routes

Scheme 2. Synthesis of Mixed Fluoroalkoy-Siloxy

Molybdenum Benzylidyne Complexes

Figure 2. ORTEP diagram of MoSi*F9 with thermal displacement

parameters drawn at 30% probability; hydrogen atoms are omitted for

clarity.

Table 1. Selected Bond Lengths [Å] and Angles [deg] of

Complexes MoSiFn (n = 0, 3, 6, 9)

M−C1

Mo−O2

M−C1−C2

MoSiF0

MoSiF3

MoSiF6

MoSiF9

1.7523(14)

1.7571(18)

1.755(3)

1.748(5)

1.754(3)

2.7420(10)

2.5734(16)

2.506(3)

2.5376(17)

173.03(12)

172.00(15)

173.5(2)

172.3(4)

173.6(2)

failed to give pure MoSiF0 and MoSiF3, which were only

characterized by X-ray diﬀraction analysis (Figures S37 and

S38 ). Since these complexes are unlikely to act as suitable

6,63

catalysts,

the preparation of MoSi*F9, the unsymmetrical silanol

MesO)(tBuO) SiOH was obtained in four steps from SiCl

we decided not to optimize their synthesis. For

MoSi*F9

(

Disordered OtBu group. From ref 63.

according to the literature. Reaction with KH gave the

potassium salt KOSi(OtBu) (OMes), which was treated with

MoF9 to furnish MoSi*F9 in a moderate yield (56%) after

recrystallization (Scheme 2).

Mo−O1 bond of ca. 1.92 Å, provides a signiﬁcantly longer

Mo−O2 distance, which decreases from 2.742(1) Å in

MoSiF0 to 2.506(3) Å in MoSiF9 with the increasing number

of ﬂuorine atoms. This coordination mode is often found in

The NMR spectra of MoSiF9 and MoSi*F9 were recorded

in C D . The H NMR spectrum of MoSiF9 exhibits one

rare-earth metal siloxides, and only recently, it has been

singlet for the tBu hydrogen atoms together with three signals

for the hydrogen atoms of the Mes substituent, while the

spectrum of MoSi*F9 shows three additional signals for the

established for several complexes derived from the homoleptic

Mo(III) complex [Mo{OSi(OtBu) } ]. Since the additional

coordination through one of the OtBu groups renders

mesityloxy (MesO) group, revealing C symmetry in solution at

MoSi*F9 C -symmetric, a variable-temperature NMR study

room temperature on the NMR time scale. Accordingly, one

was performed. Below a coalescence temperature of ca. −90

singlet is found in each ¹⁹F NMR spectrum at −72.9 ppm

°C, two singlets start to get resolved for the tBu groups by H

(

MoSiF9) and −73.1 ppm (MoSi*F9), respectively, which

NMR spectroscopy, suggesting overall fast dynamics and a

weak secondary molydenum-oxygen interaction (Figures S15 ).

Preparation and Characterization of Molybdenacy-

clobutadiene Complexes. Despite the importance of

molybdenum alkylidyne complexes in alkyne metathesis,

examples of structurally characterized MoCBD are still

falls in the same range as for MoF9 (73.7 ppm). In the

NMR spectra, the carbyne carbon atoms give rise to

characteristic lowﬁeld signals at 320.9 ppm (MoSiF9) and

23.5 ppm (MoSi*F9), which is at a higher ﬁeld compared to

32.7 ppm in MoF9. Likewise, a highﬁeld shift had been

observed from 317.6 ppm in MoF6 to 306.6 ppm in MoSiF6

rare, and early examples were only characterized in solution

7,12,72

upon the introduction of the less electron-withdrawing siloxide

by NMR spectroscopy.

A few 1,3-diamino-substituted

ligands.

MoCBDs were recently obtained, which have diamagnetic or

paramagnetic ground states depending on the amino

The molecular structures of MoSiF9 and MoSi*F9 could be

established by X-ray diﬀraction analysis; an ORTEP diagram of

the latter is shown in Figure 2. The molecular structures of

MoSiFn (n = 0, 3, 9) are presented in Figures S37 −S39 , and

pertinent structural data of all complexes MoSiFn (n = 0, 3, 6,

4,73

substituents.

Related to this study, we reported the

MoCBDs derived from the reactions of MoF6 and MoF9

with an excess of 3-hexyne. Unfortunately, the crystallographic

characterization of the resulting complexes [(C Et )Mo{OC-

) and MoSi*F9 are assembled in Table 1. All complexes show

(CF ) Me} ] (MoF6-MCBD) and [(C Et )Mo{OC(CF ) } ]

3 2 3 3 3 3 3 3

strongly distorted tetrahedral geometries around the metal

atoms with short Mo−C1 bond length of ca. 1.75 Å and large

Mo−C1−C2 angles (≥172°) in a similar range as reported for

(MoF9-MCBD) suﬀered from disorder, precluding an

16,60

accurate discussion of the structural parameters.

Gratify-

ingly, this has recently been possible for [(C Et )Mo-

MoFn (n = 0, 3, 6, 9). In all complexes, the siloxide ligand

(OSiPh ) ], which provided pertinent information about the

3 3

binds in a chelating κ O,O′ fashion and, in addition to a short

structure of an MoCBD in the solid state. Since the presence

010

Organometallics 2021, 40, 2008−2015

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of the siloxide ligands in MoSiF9 and MoSi*F9 might be

favorable to reduce crystallographic disorder associated with

ﬂuoroalkoxide ligands, we aimed at isolating similar MoCBD

complexes from their reaction with an excess of 3-hexyne

1.424(8) Å, respectively, a short−long−short−long alternation

of bond lengths within the MoC ring is observed. In principle,

these structural parameters are similar to those recently

determined for [(C

Et )Mo(OSiPh ) ]; however, with a τ

3 3 3 5

value of 0.37 and a slightly less pronounced bond alternation,

(Scheme 3).

this complex is even more distorted toward TBP. This

structure is usually associated with the transition state for

MCBD interconversion, which involves inverting the SP

structure and swapping the apical positions. It is also

noteworthy that the siloxide ligand is no longer bound in a

Scheme 3. Preparation of Molybdenaclobutadienes

κ fashion, as found in the precursor MoSiF9, but features a

short Mo−O1 bond length of 1.869(4) Å with an almost linear

Mo−O1−Si arrangement of 173.3(2)°, revealing a signiﬁcant

degree of π-donation toward the Mo atom.

Purple crystals of MoSi*F9-MCBD were isolated as

described above by storing a solution of MoSi*F9 with 10

equiv of 3-hexyne in n-pentane at −38 °C (Scheme 3). An X-

ray diﬀraction analysis provided another MoCBD structure;

however, a modulation is observed that aﬀords four

independent molecules in the asymmetric unit. In addition,

disorder could only be fully resolved in one of the four

molecules, and the structural parameters should therefore be

treated with caution. Crystallographic details and ORTEP

presentations together with tentative bond lengths and angles

can be found in the Supporting Information (Figure S41).

An NMR spectroscopic characterization of both MCBD

complexes revealed diﬀerent stabilities in solution. Dissolving

crystalline MoSiF9-MCBD in CD Cl or toluene-d leads to a

In the case of MoSiF9, the addition of 10 equiv of 3-hexyne

to a saturated n-pentane solution at −38 °C produced an

immediate characteristic color change from yellow to purple,

and MoSiF9-MCBD could be isolated as purple crystals by

storing this solution in the refrigerator at −38 °C for a couple

of days. An X-ray diﬀraction analysis conﬁrmed the formation

of the expected metallacycle and provided a second example of

a high-quality MoCBD crystal structure (Figure 3). The

fast decolorization; the characteristic pink color can be

restored by cooling, which suggests the existence of an

equilibrium with the alkylidyne complex MoSiF9 and 3-

hexyne. Accordingly, the H NMR spectra show almost

exclusively the signals for the alkylidyne complex and 3-hexyne

and one signal in the F NMR spectra at room temperature.

At lower temperatures, two F NMR signals are observed, for

example, at −72.9 and −72.0 ppm in toluene-d at −40.7 °C,

which can be assigned to the alkylidyne and MCBD species,

respectively. In contrast, solutions of MoSi*F9-MCBD retain

their characteristic pink color at room temperature, and the F

NMR spectrum in toluene-d shows a major signal at −72.6

ppm for the MCBD complex together with a minor signal at

−

73.1 ppm for the alkylidyne complex in ∼94:6 ratio,

indicating a signiﬁcantly higher stability of MoSi*F9-MCBD

compared to MoSiF9-MCBD. MCBD formation could be

conﬁrmed in both cases by an observation of characteristic

Figure 3. ORTEP diagram of MoSiF9-MCBD with thermal

displacement parameters drawn at 30% probability; hydrogen atoms

are omitted for clarity. Selected bond lengths [Å] and angles [deg]:

Mo−C1 1.877(5), Mo−C3 1.924(6), Mo−O1 1.869(4), Mo−O5

low-ﬁeld C NMR signals at 265.5 and 152.3 ppm (in CD Cl

at −39.6 °C for MoSiF9-MCBD) and at 265.1 and 151.1 ppm

(

in toluene-d at room temperature for MoSi*F9-MCBD).

These chemical shifts can be assigned to the α- and β-carbon

.058(3), Mo−O6 2.053(3), C1−C2 1.480(8), C2−C3 1.424(8),

Mo−O1−Si 173.3(2), O1−Mo−C1 131.9(2), O1−Mo−C3

44.8(2), O5−Mo−O6 162.56(14).

atoms of the MoC fragment and are in good agreement with

the NMR data reported for other MoCBD com-

,12,60,63,72

plexes,

MoF9-MCBD.

The latter complex, MoF9-MCBD, could also be observed

for example, 269.9 and 156.0 ppm for

environment around the Mo atom is best described as

distorted square-pyramidal (SP) with C1 at the apex in

agreement with a τ value of 0.30 based on the two largest

in solution at room temperature, indicating a signiﬁcant

higher stability compared to MoSiF9-MCBD and a similar

stability compared to MoSi*F9-MCBD. To analyze this

surprisingly diﬀerent behavior, variable-temperature NMR

studies were performed. For this purpose, the crystalline

angles, speciﬁcally, O5−Mo−O6 = 162.56(14)° and O1−

Mo−C3 = 144.8(2)°. The alternative description as trigonal-

bipyramidal (TBP) is clearly less appropriate, as the angles

O1−Mo−C1 and O1−Mo−C3 of 131.9(2)° and 144.8(2)°

diﬀer quite signiﬁcantly. In agreement with the SP assignment,

the Mo−C bond lengths of 1.877(5) Å (Mo−C1) and

MCBD complexes were dissolved in toluene-d , and their

reversible [2 + 2]-cycloreversion/cycloaddition reactions

involving equimolar amounts of 3-hexyne and the respective

.924(6) Å (Mo−C3) are markedly diﬀerent, and together

with the C1−C2 and C2−C3 bond lengths of 1.480(8) Å and

propylidyne complex were monitored by F NMR spectros-

011

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Table 2. Experimental Thermodynamic Parameters for the Formation of Metallacyclobutadiene Complexes

alkylidyne

MCBD

OC(CF₃)₃

OSi(OtBu)₃

ΔH° [kcal/mol]

ΔS° [cal/(mol K)]

ΔG° [kcal/mol]

MoF9^E^t

MoF9-MCBD

MoSiF9-MCBD

MoSi*F9-MCBD

−21.4(2)

−18.7(7)

−20.8(3)

−80(1)

−87(3)

−77(1)

−2.4(1)

+7.4(1)

−2.3(1)

MoSiF9^E^t

MoSi*F9

OSi(OtBu) (OMes)

Enthalpies (ΔH°), entropies (ΔS°), and Gibbs free energies at 298.15 K (ΔG°).

copy. The corresponding equilibrium constants K = [MCBD]/

Scheme 4. Alkyne and Diyne Metathesis Reactions

[alkylidyne] were determined in a broad temperature range

(

>50 K), and a van’t Hoﬀ analysis provided the thermody-

namic data shown in Table 2 (see also the Supporting

Information for details). For the pair MoF9 /MoF9-MCBD,

−

the parameters of ΔH° = −21.4(2) kcal mol and ΔS° =

−

−1

−

80(1) cal mol

were obtained, which indicate a fairly

exothermic reaction in agreement with MCBD stabilization in

the presence of three electron-withdrawing OC(CF ) ligands.

However, the reversibility of the MCBD formation is ensured

by the signiﬁcant decrease in entropy, aﬀording an only slightly

−

exergonic reaction with ΔG° = −2.4(1) kcal mol in

agreement with the observation of MoF9-MCBD at room

temperature.

In contrast, the MoSiF9 /MoSiF9-MCBD system gener-

−

ates the parameters of ΔH° = −18.7(7) kcal mol and ΔS° =

−

−1

−

87(3) cal mol

for an appreciably endergonic reaction

−

with ΔG° = +7.4(1) kcal mol , revealing a signiﬁcant

destabilization of MoSiF9-MCBD by an introduction of the

OSi(OtBu) ligand, both by enthalpic and entropic contribu-

tions. These diﬀerences could be ascribed to a weaker electron-

withdrawing nature of the siloxide in comparison with the

ﬂuoroalkoxide ligand and to a more ordered MCBD complex,

in which the siloxide ligand is aligned with the four-membered

metallacycle. The presence of the more asymmetric siloxide

ligand OSi(OtBu) (OMes) in the MoSi*F9 /MoSi*F9-

MCBD system renders MCBD formation favorable again,

Table 3. Yields [%] of Alkyne and Diyne Metathesis

−

with the parameters of ΔH° = −20.8(3) kcal mol and ΔS° =

Reactions

−

−1

−77(1) cal mol

aﬀording an overall exergonic reaction

substrate/product

MoF9

MoSiF9

MoSi*F9

−

with ΔG° = −2.3(1) kcal mol . These ﬁndings indicate that

subtle diﬀerences in the ligand design exert a signiﬁcant impact

on the thermodynamics of MCBD formation, which enables

ﬁne-tuning of the catalytic activity by avoiding unfavorably

strong stabilization or destabilization of the MCBD inter-

mediate. Furthermore, a variation of the ancillary ligands may

aﬀect the selectivity of the alkylidyne complexes in catalytic

alkyne and diyne metathesis, and these studies are presented in

the following.

a/10a

b/10b

Homometathesis: Substrate 0.5 mmol, MS 5 Å 500 mg, catalyst 1

mol %, toluene (2.5 mL), 2 h, rt. RCAM: Substrate 0.25 mmol, MS 5

Å 250 mg, catalyst 2 mol %, toluene (12 mL), 2 h, rt. Diyne

metathesis: Substrate 0.5 mmol, MS 4 Å 400 mg, MS 5 Å 400 mg,

Catalytic Alkyne and Diyne Metathesis. The catalytic

performance of MoSiF9 and MoSi*F9 in alkyne metathesis

was investigated for a couple of standard test substrates

catalyst 1 mol %, toluene (2.5 mL), 4 h (9a), 1 h (9b), rt.

(Scheme 4), which had also been previously employed with

the catalysts MoFn (n = 0, 3, 6, 9). The results are summarized

of molecular sieves (MS 5 Å) as a 2-butyne scavenger,

aﬀording the corresponding products 2 and 4 in almost

quantitative yields within 2 h at room temperature. Similarly,

RCAM of diester 5 gave the cyclic alkyne 6 in ∼95% isolated

yield, which exceeds the results previously reported with MoF9

(76%). 1-Phenylpropyne (7) was also almost completely

converted to tolane (8). For all substrates, the conversion was

also followed by gas chromatography (GC), and the resulting

in Table 3, including also those previously obtained with

MoF9. Note that MoSiF9 and MoSi*F9 proved inactive in

terminal alkyne metathesis and produced polymeric material as

also observed for MoF9. In contrast an excellent catalytic

performance was observed for internal alkynes. Thus, a

homometathesis of the ether 1 and the ester 3 proceeded in

toluene with catalyst loadings of 1 mol % and in the presence

012

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conversion versus time diagrams indicate a higher activity of

MoSiF9 in comparison with the sterically more encumbered

MoSi*F9 (see Figures S43 and S44).

It could be suspected, however, that a steric demand might

aﬀect the metathesis of diynes, which was reported recently to

proceed with a formation of triynes if sterically hindered 1,3-

Accession Codes

CCDC 2074892−2074898 contain the supplementary crys-

tallographic 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.

pentadiynes such as 9a (R = SiiPr = TIPS) and 9b (R = Mes)

are employed. In the presence of I (3 mol %, 40 °C, 3 h), 1,6-

bis(triisopropylsilyl)-1,3,5-hexatriyne (10a) formed with high

selectivity over the corresponding symmetrical 1,3-butadiyne

and could be isolated in 63% yield after silica gel

chromatography. Signiﬁcantly lower selectivity toward triyne

formation was observed for other substrates such as 9b, which

aﬀorded 10b as a complex mixture with the corresponding

diyne, tetrayne, and possibly higher polyynes. Hence, the

diyne disproportionation of 9a and 9b was investigated under

similar conditions in the presence of MoF9, MoSiF9, and

MoSi*F9 (Scheme 4). The reactions were performed at room

temperature in a toluene solution with catalyst loadings of 1

mol %, and the addition of 5 Å as well as 4 Å molecular sieves

AUTHOR INFORMATION

■

Corresponding Author

Matthias Tamm − Institut fur Anorganische und Analytische

Chemie, Technische Universität Braunschweig, 38106

0357 ; Email: m.tamm@tu-bs.de

Authors

Manuel L. Zier − Institut fur Anorganische und Analytische

Chemie, Technische Universität Braunschweig, 38106

Braunschweig, Germany

Sophie Colombel-Rouen − Univ Rennes; Ecole Nationale

Supérieure de Chimie de Rennes, CNRS, ISCR −UMR 6226,

F-35000 Rennes, France; orcid.org/0000-0003-2603-

2783

Henrike Ehrhorn − Institut fur Anorganische und Analytische

Chemie, Technische Universität Braunschweig, 38106

Braunschweig, Germany

Dirk Bockfeld − Institut fur Anorganische und Analytische

Chemie, Technische Universität Braunschweig, 38106

Braunschweig, Germany

(

MS) was found to be favorable in line with the previous

report. In the case of 9a, triyne 10a formed with a high

selectivity for all three catalysts, with the GC traces showing

the complete absence of the diyne iPr SiCCCCSiiPr .

MoSi*F9 showed the best performance, and 10a could be

isolated in 88% yield after a reaction time of 4 h at room

temperature. In the case of 9b, the selectivity toward the

formation of triyne 10b was also signiﬁcantly higher than

previously reported for catalyst I, which allowed its isolation in

1% by application of MoSi*F9. These results clearly prove

Yann Trolez − Univ Rennes; Ecole Nationale Supérieure de

Chimie de Rennes, CNRS, ISCR −UMR 6226, F-35000

Rennes, France; orcid.org/0000-0002-5421-9556

Marc Mauduit − Univ Rennes; Ecole Nationale Supérieure de

Chimie de Rennes, CNRS, ISCR −UMR 6226, F-35000

Rennes, France; orcid.org/0000-0002-7080-9708

the superiority of the catalyst MoSi*F9 and allow us to

envisage the use of less-hindered precursors than described

previously, including highly functionalized diynes.

CONCLUSION

■

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.organomet.1c00290

The preparation of mixed ﬂuoroalkoy-siloxy molybdenum

alkylidyne complexes provides an opportunity to ﬁne-tune

their activity and selectivity in catalytic alkyne and diyne

metathesis reactions by a variation of the ancillary siloxide

ligand, which has a signiﬁcant inﬂuence on the stability of the

intermediate metallacyclobutadiene species. While the intro-

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

duction of the symmetrical siloxide OSi(OtBu) turns MoF9

into the superior alkyne metathesis catalyst MoSiF9, the

introduction of the unsymmetrical siloxide OSi-

■

Y.T. and M.M. are grateful to the CNRS and the Ecole

Nationale supérieure de chimie de Rennes for supporting this

work. M.T. acknowledges support by the Deutsche For-

schungsgemeinschaft through Grant No. TA 189/12-2.

(

OtBu) (OMes) aﬀorded MoSi*F9, which proved to be an

excellent catalyst for the selective conversion of sterically

encumbered 1,3-pentadiynes (RCCCCMe) into sym-

metrical 1,3,5-triynes (RCCCCCCR) and 2-butyne

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