Synthesis of Ether-Functionalized and Sterically Demanding Molybdenum Alkylidyne Complexes

Article

pubs.acs.org/Organometallics

Cite This: Organometallics XXXX, XXX, XXX −XXX

Synthesis of Ether-Functionalized and Sterically Demanding

Molybdenum Alkylidyne Complexes

̀

̈

Oscar Arias, Henrike Ehrhorn, Johanna Hardter, Peter G. Jones, and Matthias Tamm*

Institut fur Anorganische und Analytische Chemie, Technische Universitat Braunschweig, Hagenring 30, D-38106 Braunschweig,

̈

Germany

S

* Supporting Information

ABSTRACT: The synthesis of ether-functionalized molybdenum benzylidyne

complexes [ArCMo{OC(CF₃)₂Me}₃] (6, Ar = para-methoxyphenyl; 7, Ar =

2,4,6-trimethoxyphenyl) and of the sterically demanding benzylidyne complex

[{2,4,6-(i-Pr)₃C₆H₂}CMo{OC(CF₃)₂Me}₃] are presented, together with

their spectroscopic characterization, molecular structures, and catalytic activity

in alkyne metathesis. Complexes 6 and 7 feature intermolecular contacts

between the para-methoxy group and the molybdenum center that give rise to

1D-polymeric structures in the solid state. The preparation of other

functionalized alkylidyne complexes, [ArCMo{OC(CF₃)₂Me}₃] (Ar = 2-(i-

PrO)C₆H₄, 8-MeO-Naph, 2,6-(i-Pr)₂C₆H₃), was also attempted, but only the

acyl precursors [ArC(O)Mo(CO)₅]⁻could be isolated. The synthesis of the

molybdenum acyl complexes was challenging, and appropriate alternative protocols were developed.

Scheme 1. Application of the Low-Oxidation-State Route

for the Synthesis of Molybdenum Alkylidyne Complexes

INTRODUCTION

■

a

Since the discovery of the disproportionation of acetylenes 50

years ago,¹great progress has been achieved in the develop-

ment of eﬃcient, well-deﬁned catalysts (and precursors) for

the alkyne metathesis reaction.²The most prominent examples

essentially comprise high oxidation state tungsten- and

molybdenum-based alkylidyne complexes of the Schrock

type^2a,3bearing alkoxide⁴(e.g., ﬂuorinated alkoxides,⁵

silanolates,⁶and multidentate ligands),^2d,7amido⁸or iminato

ligands,^5d,6c,9among others, and combinations thereof. The

synthesis of these complexes initially followed two dominant

strategies: (i) The cross-metathesis of binuclear complexes of

the type [(RO)₃WW(OR)₃]¹⁰with alkynes (R′CCR′) to

give 2 equiv of the corresponding alkylidyne complex [R′C

W(OR)₃], and (ii) the so-called “high-oxidation-state”

route,^5c,11starting from WCl₆or MoO₂Cl₂to aﬀord, in several

steps, neopentylidyne complexes such as [Me₃CCM-

(dme)_n(OR)₃] (M = Mo, W; dme = 1,2-dimethoxyethane; n

= 0, 1) via the versatile precursor mer-[Me₃CCM-

(Cl)₃(dme)]. A major improvement was the development of

the “low-oxidation-state” route^2c,9c,12(Scheme 1), which for

both tungsten and molybdenum provides access to mer-

[R′CM(Br)₃(dme)] (M = Mo, W) in two main steps

starting from the metal hexacarbonyls. Because of higher atom

economy, more convenient operability, and less restricted

access to other alkylidyne ligands (R′C), this synthetic

strategy is certainly superior; it has found extensive

applications in the preparation of several new alkylidyne

complexes that are suitable alkyne metathesis promoters,

reaching in some cases high performance and outstanding

functional group compatibility. Very recently, a new cross-

metathesis protocol has been reported for the synthesis of

a

DME = 1,2-dimethoxyethane.

selectively functionalized molybdenum alkylidynes.¹³This

methodology takes advantage of versatile benzylidyne-transfer

reagents derived from 1-phenyl-3,3,3-triﬂuoropropyne¹³

(ArCCCF₃) or phenyl(trimethylsilyl)ethyne¹⁴(ArC

CSiMe₃), which react with [EtCMo(dme)(OR)₃] (R =

C(CF₃)₂Me) to give the corresponding [ArCMo(dme)-

(OR)₃] complexes and the highly volatile 1,1,1-triﬂuoro-2-

pentyne or 1-(trimethylsilyl)-1-butyne, respectively. However,

the alkyne reagents are not always readily accessible.

Some recent examples of homogeneous catalysts are

highlighted in Figure 1 and comprise (i) the highly active

alkylidyne complex [t-BuC≡W(Im^t‑BuN){OC(CF₃)₂Me}₂] (1,

Im^t‑BuN = 1,3-di-tert-butylimidazolin-2-iminato)^5dsupported

by an asymmetric push−pull ligand environment; (ii) the

triphenylsiloxy-based complex [{4-(MeO)C₆H₄}CMo-

(OSiPh₃)₃] (2),^6awhich has been extensively used in natural

product synthesis;¹⁵(iii) the trialkoxysiloxy-substituted tung-

Received: October 26, 2018

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a

Scheme 2. Activation of Bench-Stable Precatalysts

a

Conditions (a): MnCl₂or ZnCl₂, toluene, 80−100 °C; abbrevia-

tions: phen = 1,10-phenanthroline.

(Scheme 2) requires a non-negligible additional step, namely,

dissociation of the chelating ligand. One possibility to

overcome this drawback consists in increasing the coordination

number through formation of intra- or intermolecular

interactions without the need for additional ligands. This can

be accomplished if proper functional groups are installed at

selected positions in the already existing ligand environment,

including the alkylidyne moiety. These functionalities should

have the ability, at least in the solid state, to promote a

chelating binding mode or engage in weak covalent

interactions with a neighboring molecule. For example,

molybdenum complexes of the types 6−8 (Figure 2a) can

Figure 1. Selection of carefully designed alkyne metathesis catalysts.

sten alkylidyne [PhCW{OSi(O-t-Bu)₃}₃] (3),^6bwhich

emerged as a versatile catalyst able to metathesize not only

acetylenes but also conjugated 1,3-diynes;¹⁶(iv) the chelate

complex 4^7a(and derivatives),¹⁷largely employed in polymer¹⁸

and supramolecular chemistry;¹⁹and (v) the most active

catalyst so far in alkyne metathesis,²⁰the 2,4,6-trimethylbenzyl-

idyne complex [MesCMo{OC(CF₃)₂Me}₃] (5),²¹which

performs eﬃciently even in terminal alkyne metathesis.²⁰⁻²²

Moreover, 5 and related complexes are also excellent initiators

in ring-opening alkyne metathesis polymerization

(ROAMP).^13,23

In spite of these signiﬁcant developments in catalyst and

ligand design, the applicability and the potential of the alkyne

metathesis reaction is limited to some extent by the air

sensitivity of the catalysts. Nevertheless, there are a few

reported strategies that circumvent this shortcoming. Mor-

treux-type catalyst systems,²⁴although quite obsolete and ill-

deﬁned, are robust and user-friendly and exhibit catalytic

activity under noninert conditions,²⁵but at the expense of

eﬃciency and functional group compatibility. In this context, a

major milestone was reached in 2010 with the ﬁrst bench-

stable precatalysts,^6achelated derivatives of the molybdenum

complex 2 (and its benzylidyne congener)²⁶that act as

reservoirs of the active species.^2bAccordingly, the increase in

coordination number at the metal center by chelation of a

nucleophilic ligand such as 1,10-phenanthroline (occupation of

free coordination sites) results in air-stable complexes, which

upon dissociation assisted by more electrophilic metal salts

(such as MnCl₂and ZnCl₂) at elevated temperatures liberate

the active catalysts (Scheme 2).

Figure 2. (a) Ether-substituted benzylidyne complexes showing

proposed intermolecular coordination (complexes 6 and 7). (b)

Proposed intramolecular stabilization of the MCBD complex. (c)

Initiation step of the alkyne metathesis after addition of a typical

substrate such as RCCMe (R = alkyl, aryl). [Mo] = [Mo{OC-

(CF₃)₂Me}₃].

potentially form inter- (6, 7) or intramolecular (7, 8)

interactions between the ether substituents at the alkylidyne

moiety and the metal center. Obviously, because of the

linearity of the MoC−Ar units, chelation in complexes 7 and

8 is not possible. Instead, stabilization of the metal-

lacyclobutadiene (MCBD) structures formed by addition of

an alkyne to the alkylidyne complex, e.g., 2-butyne to 8 (see

Figure 2b), is envisaged, the structure of which resembles the

robust and well-established Hoveyda−Grubbs catalysts^14,27for

oleﬁn metathesis. Since 8 and the corresponding MCBD

complex are in equilibrium in solution, the release of 2-butyne

Inspired by this previous work, we planned various strategies

to improve the robustness and user-friendliness of the

alkylidyne complexes. The activation of air-stable precatalysts

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Scheme 3. Overview of All Complexes Prepared and Discussed in This Work Including the Modiﬁed Synthetic Protocol for

Acyl Complexes Li[13], Li[17], and Li[19]

to activate the catalyst should be driven by its adsorption in

molecular sieves or by distillation under reduced pressure.

Although a decrease in the catalytic performance can be

anticipated, the strength (or weakness) of these interactions is

crucial to avoid complete suppression of the activity, perhaps

in solution or at elevated reaction temperatures. In particular, if

the substituents at the alkylidyne moiety are involved in this

process, then substitution of the alkylidyne in the initiation

step of the alkyne metathesis reaction (Figure 2c), which is

expected to proceed at slow rates, would liberate the binding

site and generate the catalytically active species, so that

complete conversion would presumably be reached, as usual,

within a few minutes. It should be mentioned here that a

similar strategy (albeit with diﬀerent intentions) has been

applied by Fischer and co-workers,²⁸in which the addition of

functionalized ynamines to a molybdenum alkylidyne catalyst

generates MCBD complexes stabilized intramolecularly by

secondary metal binding sites, thus irreversibly deactivating the

catalytically active center. This approach grants control over

the termination of living ring-opening alkyne metathesis

polymerizations and gives access to telechelic polymers.²⁸

Alternatively, an increase in the steric bulk at the metal

center can hinder undesirable decomposition reactions and

enhance the stability of the complex toward air or moisture.

We have already explored this concept by replacement of the

phenyl group of the alkylidyne moiety by the sterically more

demanding 2,4,6-trimethylphenyl (mesityl, Mes) substituent in

complexes of the type [MesCM(X)(Y)₂] (M = Mo, W; X =

Im^t‑BuN, Y = OC(CF₃)₂Me or X = Y = OC(CF₃)₂Me (5, M =

Mo), OC(CF₃)₃, OSi(O-t-Bu)₃; Im^t‑BuN = 1,3-di-tert-butyli-

midazolin-2-iminato).^6cWhereas the molybdenum complexes

were not air-stable, some of the tungsten analogs, namely

[MesCW(Im^t‑BuN){OC(CF₃)₂Me}₂] and [MesCW{OSi-

(O-t-Bu)₃}₃], proved suﬃciently robust to be handled in air for

at least 2 or 3 h without perceptible loss of their catalytic

performance. The question addressed in the present work is

whether even more crowded alkylidyne complexes bearing

isopropyl or tert-butyl groups (i.e., 2,6-disubstituted or 2,4,6-

C

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trisubstituted benzylidynes, vide infra) can improve the

stability of the precatalysts in comparison with their less

encumbered congeners.

observation, our NMR spectroscopic analyses (see Figures

S20a,b) showed one signal set, indicating the presence of only

one species in solution. In addition, the purity of the

compound was veriﬁed by elemental analysis (which is missing

in the original publication; for experimental details, see

Supporting Information p S3), and since its solid-state

structure has not been reported to date, we attempted to

grow single crystals. Orange crystals were obtained from a

concentrated dichloromethane solution at −30 °C, and the

crystal structure of compound [Me₄N][12] was established by

X-ray diﬀraction analysis (Figure 3); it crystallizes in the

In principle, all envisaged complexes are accessible from the

Schrock carbyne complexes mer-[ArCMo(Br)₃(dme)],

which in turn can be conveniently prepared, as mentioned

above, through the low-oxidation-state route (Scheme 1) from

the corresponding lithium aryls (LiAr) and [Mo(CO)₆]. In

most cases, however, the preparation of these precursors

proved challenging; therefore, it was crucial to review the

established synthetic protocols and develop alternative

strategies, which will be discussed throughout this work.

RESULTS AND DISCUSSION

■

We initially attempted the synthesis of complexes of the type

6−8, containing para-methoxyphenyl, 2,4,6-trimethoxyphenyl

or ortho-isopropoxyphenyl in place of the aryl (Ar) moiety. In

para-substituted arylalkylidyne 6, intermolecular interactions

are intended to stabilize the alkylidyne complex, whereas ortho-

substitution in 8 aims at the stabilization of the MCBD

through intramolecular coordination (Figure 2b). In the

intermediate case (7), both strategies apply because of

substitution at both ortho- and para-positions. As for the

second concept outlined in the introduction, the synthesis of

bulky alkylidyne complexes is discussed in a separate section.

Finally, preliminary catalytic studies with the new alkylidyne

complexes are presented at the end of this contribution.

Synthesis of (para-Methoxybenzylidyne)-

molybdenum Complexes. Aiming at the preparation of

complex 6, the para-anisole moiety was selected based on the

established use of 2, for which a multi-gram-scale synthesis has

been reported.²⁶In addition, closely related complexes to 6

such as [{4-(MeO)C₆H₄}CMo(L){OC(CF₃)₂Me}₃] (L =

DME or N-heterocyclic carbene) have been successfully

prepared and tested as alkyne metathesis initiators.^13,29

However, these complexes are not suitable for our purpose,

as the free binding site is already blocked by the additional

coordinating ligand (L). In contrast, mesityl complex 5 is

obtained without coordinating solvent via the low-oxidation-

state route with [Me₄N][MesC(O)Mo(CO)₅], [Me₄N][9],

and mer-[MesCMo(Br)₃(dme)] (10) as key intermediates

(see Scheme 3). Therefore, we aimed at the synthesis of DME-

free alkylidyne complex 6, which can be accomplished in an

analogous fashion by reacting mer-[{4-(MeO)C₆H₄}CMo-

(Br)₃(dme)]²⁶(11) with 3 equiv of K[OC(CF₃)₂Me] in THF

Figure 3. Molecular structure of the anion of [Me₄N][12] with

thermal displacement parameters drawn at the 50% probability level.

For selected bond lengths and angles, see Figure S2.

monoclinic space group P2₁/n. The molybdenum atom resides

in a distorted octahedral environment and is coordinated to

ﬁve carbonyl ligands (average Mo−C bond distances of

2.0391(18) Å and C−O distances of 1.142(2) Å) and to the

alkylidene carbon atom with a signiﬁcantly elongated Mo−C

bond length of 2.2907(18) Å (Table 1). Indeed, this distance is

Table 1. Comparison of the Mo−C(O)Ar Bond in the Acyl

Complexes Reported in This Work

compound

Mo−C [Å]

a

[Me₄N][9]

2.268(4)

[Me₄N][12]

[Li(13)·Et₂O]₂

[Li(13)·thf]₂

2.2907(18)

2.2470(13)

2.251(2)

[Li(14)·Et₂O]₂, polymorph A

[Li(14)·Et₂O]₂, polymorph B

[Li(19)·(thf)₃]

2.196(3)

2.183(2)

2.2467(19)

2.238(2)

a

[Li₄(25)₄·(Et₂O)₂]

[Me₄N][{2,6-(i-Pr)₂Ph}C(O)Mo(CO)₅]

2.2819(14)

a

(Scheme 3). Although already reported by Fu

̈

rstner et al.,²⁶the

Average value.

preparation of 11 following the low-oxidation-state route

deserves some attention here, especially with regard to its

precursor [Me₄N][{4-(MeO)C₆H₄}C(O)Mo(CO)₅],

[Me₄N][12]. Starting with 4-methoxyphenyl lithium and

[Mo(CO)₆], acyl complex [Me₄N][12] is obtained in a two-

step synthesis (Scheme 3), in which the intermediate lithium

comparable to the longest Mo−C distance of 2.293(2) Å³⁰

found in complexes of the type [(R)C(O)Mo(CO)₅].³¹We

have also determined the structure of analogous mesityl

complex [Me₄N][9] and present this in Figure S1; it features

an Mo−C bond length of 2.268(4) Å (Table 1).

To complete the synthesis of the alkylidyne complex 6,

reaction of the tribrominated precursor 11 with K[OC-

(CF₃)₂Me] in THF, followed by extraction with n-pentane,

delivered a pale brown solid in good yields. As shown by NMR

spectroscopy (Figures S22a −c) and elemental analysis, the

isolated product was the THF solvate of the expected (p-

methoxybenzylidyne)molybdenum complex, [6(thf)]. Storing

of a concentrated n-pentane solution at low temperature

aﬀorded yellow crystals, which were analyzed by X-ray

diﬀraction, but the structure could not be reﬁned satisfactorily

because of twinning and severe disorder. However, the data

salt Li[12] is not isolated. Fu

̈

rstner and co-workers found by

NMR spectroscopy that two isomers of [Me₄N][12] were

present in solution in an approximately 9:1 ratio, but no

discussion of the nature and origin of the minor isomer was

presented. Moreover, the reported NMR resonances are

inconsistent with two conﬁgurational isomers of the same

compound (for example, only one downﬁeld signal at δ_C

=

294.5 ppm is listed for the alkylidene carbon atom) and can be

better explained if the minor constituent is in fact a

degradation product (presumably the decarbonylated deriva-

tive analogous to 15 and 18, vide infra). In contrast to this

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(see structure in Figure S3) qualitatively conﬁrmed the

presence of one THF molecule trans to the alkylidyne group.

In order to obtain better crystals of [6(thf)], recrystallization

was attempted from a cold solution of [6(thf)] in n-pentane, to

which a drop of THF was added. In addition to the known

yellow crystals, red crystals were obtained and subjected to X-

ray diﬀraction analysis. The molecular structure (Figure 4)

in [6(thf)], whereas they appear at δ_C= 114.1 and δ_C= 131.9

ppm in 6. Finally, the alkylidyne carbon atom is shifted

downﬁeld for 6 (δ_C= 299.2 ppm) compared to its THF solvate

(δ_C= 294.3 ppm).

The structure of 6 (Figure 5) was conﬁrmed by X-ray

diﬀraction analysis of pale yellow, platy crystals; it crystallizes

Figure 4. Molecular structure of cis,mer-[6(thf)₂] with thermal

displacement parameters drawn at the 50% probability level. Only one

position of the disordered THF molecule (at O5) is shown; hydrogen

atoms are omitted for clarity. For selected bond lengths and angles,

see Figure S4.

Figure 5. Molecular structure of one of the two independent

monomers of 6 with thermal displacement parameters drawn at the

50% probability level. A second independent molecule in the

asymmetric unit, a disordered C(CF₃)₂Me group (at O3), and

hydrogen atoms are omitted for clarity. Relevant parts of neighboring

molecules are indicated using paler colors. For selected bond lengths

and angles, see Figure S5a.

shows cis,mer-[6(thf)₂], in which two THF molecules are

coordinated to parent compound 6. The compound crystallizes

in the monoclinic space group P2₁/n and the molybdenum

atom resides in a distorted octahedral environment (the bond

angles around the metal center are in the range of 150.80−

177.79° (trans) and 76.51−102.47° (cis)). Furthermore, the

Mo−C1 triple bond length (1.757(2) Å) is comparable to

those of similar molybdenum alkylidyne complexes.³¹

in the noncentrosymmetric monoclinic space group P2₁with

two independent molecules in the asymmetric unit. Each

molecule is connected to its neighbor through the para-

methoxy group at the alkylidyne moiety, forming zigzag

polymeric chains (cf. Figure S5b) with bridging angles Mo2···

O1−C5, 128.67(16)° and Mo1···O5#−C25#, 118.41(16)°

(operator x − 1, y, z − 1). The overall chain direction is

parallel to [101]. The geometry around the pentacoordinated

molybdenum atom is trigonal-bipyramidal (τ₅= 0.94 and 1.03

for the two independent molecules),³²with an axial angle of

174.28(10)° (C1−Mo1···O5#) and 179.44(11)° (C21−

Mo2···O1) and equatorial mean angles of 114.72(10) and

115.39(9)°. The alkylidyne bond distances of 1.750(3) Å

(Mo−C1) and 1.753(3) Å (Mo2−C21) are in the expected

range for alkylidyne molybdenum complexes,³¹whereas the

intermolecular contacts of 2.7241(19) Å (Mo1···O5#) and

2.4723(19) Å (Mo2···O1) are clearly below the sum of the van

der Waals radii of molybdenum and oxygen (3.65 Å),³³

although much longer than the covalent Mo−O bonds

(1.896(2)−1.9052(19) Å and 1.905(2)−1.908(2) Å), indicat-

ing the weak character of the interactions.

Synthesis of a (2,4,6-Trimethoxybenzylidyne)molyb-

denum Complex. Complex 7 was prepared following the

low-oxidation-state route (see Scheme 3). However, the

synthesis of this ortho- and para-methoxy-substituted benzy-

lidyne complex was unusually challenging, and the protocol

was adapted to accomplish this goal. First, the synthesis of

[Me₄N][{2,4,6-(MeO)₃C₆H₂}C(O)Mo(CO)₅] ([Me₄N]-

[13]) starting from 2,4,6-trimethoxyphenyl lithium and

[Mo(CO)₆] in diethyl ether, followed by salt metathesis with

[Me₄N]Br in degassed water, aﬀorded a pale yellow solid,

In contrast to the closely related DME derivative [{4-

(MeO)C₆H₄}CMo(dme){OC(CF₃)₂Me}₃] (vide supra),

which was prepared in diethyl ether²⁹or toluene,¹³the

reaction of 11 and K[OC(CF₃)₂Me] in THF aﬀorded THF

complex [6(thf)] (Scheme 3). This distinction is crucial,

considering that, as reported previously,²¹THF is much more

easily removed than DME to generate the solvent-free

alkylidyne complex (cf. 5, Scheme 3). Indeed, recrystallization

of [6(thf)] from a cold mixture of toluene and n-pentane or,

alternatively, repeated dissolution of [6(thf)] in toluene

followed by evaporation of the solvent at 50 °C and

crystallization from n-pentane aﬀorded the desired THF-free

complex 6 in moderate yields. The NMR spectra of 6 (Figures

S23a −c) show signiﬁcant diﬀerences in comparison with the

spectra of [6(thf)]. For instance, the methyl groups of the

alkoxide ligands are found at δ_H= 1.68 ppm in the former

compound and are more deshielded (δ_H= 1.84 ppm) in the

latter. Similarly, an increase of the resonances for the aryl

protons (δ_H= 6.41 ppm and δ_H= 7.04 ppm for 6) is observed

for [6(thf)] (δ_H= 6.44 ppm and δ_H= 7.16 ppm). In the ¹⁹F

NMR spectrum, the ﬂuorine atoms give rise to a single peak,

which is slightly shifted from δ_F= −77.81 ppm in [6(thf)] to

δ_F= −77.92 ppm in the THF-free complex. Furthermore, the

¹³C NMR signals of the ortho and meta positions at the

aromatic ring resonate at δ_C= 113.8 ppm and δ_C= 132.6 ppm

E

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Scheme 4. Decarbonylation Pathway of ortho-

Functionalized Complex 13

which was assumed to contain molybdenum species 13 and, as

judged by NMR spectroscopy (see Figures S25a −d),

signiﬁcant amounts of 1,3,5-trimethoxybenzene and [Mo-

(CO)₆] that increased with time. In addition, not all the

expected signals for [Me₄N][13] were observable; the signal of

the acyl carbon atom (ArC(=O)Mo), usually found at δ_C>

300 ppm, was missing in the ¹³C{¹H} NMR spectrum. These

ﬁndings suggested the formation of the decarbonylated species

[Me₄N][{2,4,6-(MeO)₃C₆H₂}Mo(CO)₅], [Me₄N][15] (see

Scheme 3, bottom left), a hypothesis that was conﬁrmed

later by solid-state structural analysis (vide infra). Presuming

that the desired compound is unstable in water, the isolation of

the intermediate lithium complex Li[{2,4,6-(MeO)₃C₆H₂}C-

(O)Mo(CO)₅] (Li[13]) was attempted, which resulted in a

mixture of Li[13], its decarbonylated counterpart (Li[{2,4,6-

(MeO)₃C₆H₂}Mo(CO)₅], Li[15]), and minor amounts of

1,3,5-trimethoxybenzene, as revealed by NMR spectroscopy

(see Figures S26a,b). Fortunately, our eﬀorts to separate the

complexes through fractional crystallization yielded several

yellow-orange crystals from an Et₂O/n-hexane solution at −40

°C. Interestingly, inspection under the microscope revealed

three diﬀerent types of crystals. In all three cases single crystals

suitable for X-ray diﬀraction analysis were available and the

determined molecular structures are presented in Figure 6.

Remarkably, the structures unfold the mechanism of the CO

deinsertion (Scheme 4) and show three species involved in this

reaction: (i) desired complex Li[13], (ii) aryl complex Li[15],

and (iii) intermediate species Li[{2,4,6-(MeO)₃C₆H₂}C(

O)Mo(CO)₄] (Li[14]), in which a CO ligand has been

released but the carbonyl group in the acyl moiety has not yet

been deinserted. In the latter structure, the vacant coordination

site arising at the metal center is occupied by a chelating

methoxy group of the aryl unit, giving additional stability to the

unsaturated tetracarbonyl complex.

Compound Li[13] crystallizes as pale yellow prisms in the

triclinic space group P1 as the diethyl ether solvate and is

̅

composed of centrosymmetric dimers of the type [Li(13)·

Et₂O]₂in which the lithium atoms bridge the two halves of the

molecule. The molybdenum atom resides in a distorted

orthogonal environment with average angles of 173.51(6)°

(trans) and 90.07(6)° (cis). The Mo−C1 distance of

2.2470(13) Å (Table 1) is particularly long and characteristic

of a Mo−C single bond, underlining the acyl character of the

substituent (Ar−C(O)−Mo) rather than a carbene structure

(Ar−C(O)Mo). The lithium cation is tetracoordinated by

the carbonyl group of the acyl moieties (O1 and O1#, operator

1 − x, 1 − y, 1 − z), one ortho-methoxy group of the aromatic

ring (O2) and one diethyl ether molecule. Calculation of the

structural index parameter τ₄= (360 − α − β)/141 =

0.80(19)³⁴indicates a distorted trigonal pyramidal geometry at

Figure 6. (a) Molecular structure of [Li(13)·Et₂O]₂with thermal

displacement parameters drawn at the 50% probability level. The

second molecule in the inversion-symmetric dimer (except for atoms

labeled with #) and hydrogen atoms are omitted for clarity. (b)

Molecular structure of one polymorph (A) of [Li(14)·Et₂O]₂with

thermal displacement parameters drawn at the 50% probability level.

The second molecule in the inversion-symmetric dimer (except for

atoms labeled with #) and hydrogen atoms are omitted for clarity. (c)

Molecular structure of [Li(15)·(Et₂O)₂]_nwith thermal displacement

parameters drawn at the 50% probability level. Contacts to the next

units of the polymeric structure are indicated by dashed bonds.

Hydrogen atoms and one position of the disordered ethyl group at

O10 are omitted for clarity. For selected bond lengths and angles, see

Figures S6 −S8.

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the lithium atom, in which the apex (O1) is tilted away from

O10 (O1···Li−O10, 107.27(13)).

ΔG^⧧= 25.51(19) kcal mol⁻¹(106.7(8) kJ mol⁻¹) at T = 293

K.

The highly air-sensitive, yellow-orange dichroic crystals of

In view of these results, we tried to circumvent the

decarbonylation problem. On the basis of the calculated half-

life, the deinsertion process can be assumed to be very slow in

solution at ambient temperature, which does not explain the

observed mixtures of complexes. Hence, the deinsertion

reaction must be accelerated during isolation of the product,

probably because of removal of the solvent under dynamic

vacuum, which promotes the release of carbon monoxide and

governs the outcome of the reaction. For this reason, we

decided to avoid the solvent evaporation step and explored

new conditions to isolate the acyl complex. Accordingly, we

isolated the complex from the reaction mixture by precipitation

with n-hexane (Scheme 3), giving Li[13] as a yellow powder in

moderate yields, the purity and homogeneity of which was

conﬁrmed by NMR spectroscopy (Figures S27a,b) and

elemental analysis and powder X-ray diﬀraction (Figure

S10b). It should be mentioned here that the solvent choice

(diethyl ether, tetrahydrofuran, and n-hexane) and ratio

(8:1:14) were critical to avoid the precipitation of byproducts

or unreacted [Mo(CO)₆]. The latter is moderately soluble in

all three solvents, so small amounts remain in solution after

precipitation. In addition, large block-shaped crystals of the

THF solvate [Li(13)·thf]₂precipitated from the ﬁltrate and

were analyzed by X-ray diﬀraction (Figure S10a); structural

parameters are similar to those discussed for [Li(13)·Et₂O]₂.

Subsequently, the alkylidyne complex mer-[{2,4,6-

(MeO)₃C₆H₂}CMo(Br)₃(dme)] (16) was prepared directly

from Li[13] (Scheme 3) to avoid decarbonylation during the

classical salt metathesis step with [Me₄N]Br. Thus, oxidation

to the alkylidyne complex was accomplished by treatment with

oxalyl bromide and bromine at −80 °C, although the reaction

did not proceed as cleanly as usual, and removal of some

unidentiﬁed impurities was unsuccessful (see Figures S28a −c).

Nevertheless, a crude product containing indeterminate

amounts of 16 was obtained in moderate yields and was

used without further puriﬁcation in the next step. Fortunately,

the reaction of crude 16 with three equivalents of K[OC-

(CF₃)₂Me] in THF (Scheme 3), followed by extraction with n-

hexane and recrystallization from n-pentane, aﬀorded yellow-

orange dichroic plates of pure [7(thf)], as determined by X-ray

diﬀraction analysis (Figure 7). The compound crystallizes in

̅

complex Li[14] are triclinic with space group P1 and, as

observed for Li[13], reveal a dimeric structure of its diethyl

ether solvate ([Li(14)·Et₂O]₂) symmetrically bridged by two

tetracoordinated lithium atoms (to O1, O1#, O4#, and one

solvent molecule, operator 1 − x, 1 − y, 1 − z), which adopt a

distorted trigonal pyramidal geometry (τ₄= 0.80(4))³⁴with

the apex (O1#) tilted away from O9 (O1#···Li−O9,

110.3(3)°). The molybdenum atom resides in a distorted

orthogonal environment with bond angles in the range of

168.81−174.59° (trans) and 72.97−102.15° (cis) and mean

angles of 171.09(14)° (trans) and 90.12(12)° (cis). Notably, as

observed for a similar structure,³⁵the C1−Mo−O2 angle is

particularly acute (72.97(9)°) because of the chelating nature

of the acyl ligand, which forms a ﬁve-membered ring upon

coordination of one methoxy group (through O2) to the metal

center. This ring is almost planar, but exhibits a slight envelope

conformation in which the Mo atom lies 0.40 Å out of the

plane of the other four atoms. The Mo−C1 bond of 2.196(3)

Å is 5.1(4) pm shorter than that in complex Li[13] (Table 1),

but is still in the range of a single bond. Interestingly,

successive experiments reproduced the concomitant crystal-

lization of the diﬀerent species and on one occasion also

produced orange crystals of a second polymorph (B) of Li[14],

which crystallized in the monoclinic space group P2₁/n (see

Table S9 and Figure S9). The coordination geometry and

structural parameters are very similar to those discussed for

polymorph A.

Finally, aryl complex Li[15] crystallizes in the monoclinic

space group P2₁/n as a diethyl ether disolvate, forming highly

air-sensitive, blade-shaped, colorless crystals. In the crystal

structure, the complex is a 1D-coordination polymer [Li(15)·

(Et₂O)₂]_norganized in chains parallel to the b axis (cf. Figure

S8b), in which the lithium cations bridge the molybdenum

fragments at O2 and O4, the para-methoxy substituent of the

aryl ring and the carbonyl ligand coordinated trans to this ring,

respectively. The hexacoordinated molybdenum core resides in

an orthogonal environment (average angles: 177.92(8)°

(trans) and 90.00(8)° (cis)), whereas the four-coordinate

geometry around the lithium atom is close to tetrahedral (τ₄=

0.90(3)),³⁴with a particularly wide angle of 124.89(19)° for

O9−Li−O10. The Mo−C_Ardistance (2.312(2) Å) is indicative

of a single bond.

Carbonyl deinsertions are a classical type of organometallic

reaction and have already been described for Fischer carbene

complexes of chromium,³⁶molybdenum³⁵and tungsten,³⁷

which led to the formation of carbene(tetracarbonyl) chelate

complexes similar to Li[14]. In order to obtain some kinetic

information on the decarbonylation of Li[13], the reaction

progress was monitored by NMR spectroscopy. Assuming that

the concentration of intermediate Li[14] is negligible (it is not

observed in the NMR spectra) and the CO deinsertion

accordingly occurs rapidly to give Li[15], the loss of CO in

Li[13] must be the rate-determining step, and its consumption

can be followed. The natural logarithm of the peak integral (ln

̅

the triclinic space group P1 with two independent molecules in

the asymmetric unit; these diﬀer in the orientation of the

ligand at O4 but are otherwise similar. The molybdenum

atoms are pentacoordinated and reside in a distorted trigonal-

bipyramidal geometry, as indicated by the structural index

parameters τ₅= 0.89 and τ₅′ = 0.82.³²The alkoxide ligands

occupy the equatorial positions, whereas the THF molecule is

attached trans to the alkylidyne group with axial angles of

174.63(7)° (C1−Mo−O7) and 173.55(7)° (C1′−Mo′−O7′).

The Mo−O distances in the alkoxide ligands range from

1.9005(13) Å to 1.9226(13) Å, which is commonly found for

covalent bonds; in contrast, the THF molecule is weakly

coordinated, as can be concluded by the considerably longer

Mo−O bond lengths of 2.4256(13) Å (Mo−O7) and

2.4007(13) Å (Mo′−O7′).

A_Li[13]) against reaction time shows a linear dependence

(Figure S39), which suggests that the rate law is ﬁrst-order in

13 (r = k[13]; k, ﬁrst-order rate constant). The observed rate

constant can be derived from the slope of the regression line

and has a value of k₂₉₃= 1.26(6) 10⁻⁶s⁻¹, which corresponds

to a half-life of t_1/2= 6.4(3) days and a free activation energy of

In addition, the structure of the compound was conﬁrmed

by NMR spectroscopy. The experimental elemental composi-

tion, however, indicated a partial loss of THF (∼0.5 equiv,

probably because of the thorough drying of the sample), which

supports the assumption that the THF is weakly coordinated.

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Figure 7. Molecular structure of [7(thf)] with thermal displacement

parameters drawn at the 50% probability level. Only one independent

molecule is shown; all hydrogen atoms are omitted for clarity. For

selected bond lengths and angles, see Figure S11.

As expected, a subtle but nontrivial modiﬁcation of the workup

(extraction of the product with n-pentane, then evaporation of

the solvent at 50 °C followed by several cycles of dissolution/

evaporation) aﬀorded THF-free complex 7 in modest yields.

As discussed above for the para-anisole analogs, compounds

[7(thf)] and 7 can be distinguished by NMR spectroscopy

(Figures S29a −c and S30a −c). For example, a decrease of all

¹H shifts (1.87, 3.14, 3.18, 5.64 ppm) can be observed for the

latter compared to its THF counterpart (1.94, 3.17, 3.21, 5.66

ppm). Moreover, in the ¹⁹F NMR spectrum, the ﬂuorine

resonance for 7 (−78.1 ppm) is slightly high-ﬁeld-shifted in

comparison to that for [7(thf)] (−77.9 ppm). The most

relevant diﬀerences in the ¹³C NMR spectrum are found for

the signals of the alkoxide ligands (18.0, 84.8, and 123.9 ppm

for [7(thf)] vs 18.2, 84.5, and 123.6 ppm for 7) and the

alkylidyne carbon atom, which resonates at 292.6 ppm in

[7(thf)] and at a lower ﬁeld in 7 (296.73 ppm).

Single crystals of 7 were obtained from a cold n-pentane

solution at −35 °C, which allowed conﬁrmation of the

structure (Figure 8a) by X-ray diﬀraction analysis. The yellow-

orange dichroic prisms crystallize in the monoclinic space

group P2₁/n. Analogous to 6, the complex is a 1D-

coordination polymer in the solid state (Figures 8b and

S12b), in which the para-substituent of the alkylidyne moiety

is connected via the n glide plane to the molybdenum atom of

the neighboring molecule, forming zigzag chains with a

bridging angle of 124.16(12)° (C5−O2···Mo#) and overall

direction parallel to [101]. The molybdenum center features a

nearly ideal trigonal-bipyramidal geometry (τ₅= 0.97),³²

wherein the ﬂuorinated ligands occupy the equatorial positions

but are slightly tilted out of the plane toward O2#. The

molybdenum−carbon triple bond of 1.758(2) Å is in the

expected range for alkylidyne complexes and the axial angle

C1−Mo···O2# between the alkylidyne and the para-methoxy

group of the next molecule is nearly linear (177.66(7)°). This

intermolecular contact is characterized again by an elongated

Mo···O2# bond of 2.5664(15) Å, which conﬁrms the weak

nature of the interaction. Interestingly, the unprecedented

Figure 8. (a) Molecular structure of 7 with thermal displacement

parameters drawn at the 50% probability level. Hydrogen atoms are

omitted for clarity. Contacts to neighboring molecules are shown

using paler colors. For selected bond lengths and angles, see Figure

S12a. (b) Representation as a wireframe model of a polymer string of

7 seen along the a axis.

formation of these alkylidyne-based, single-chain coordination

polymers is not aﬀected by the ortho-substitution at the

benzylidyne ring, but depends strongly on the nature of the

ancillary ligands. Hence, such intermolecular contacts are not

observed, for example, in complex 2,²⁶which bears

triphenylsilanolate ligands, certainly more sterically demanding

than the ﬂuorinated alkoxides. Unfortunately, as will be

discussed in the last section, the bridging of single molecules

in the solid state by the ether functional group did not lead to

any signiﬁcant enhancement of the stability toward air or

moisture.

The question remains whether the ether functionalities at

the ortho positions could promote the stabilization of the

MCBD generated upon addition of a small alkyne to

molybdenum alkylidyne 7. In our hands, the only presumably

identiﬁed products after reacting 7 with 3-hexyne were the

propylidyne complex [EtCMo{OC(CF₃)₂Me}₃] and the

mixed acetylene (ArCCEt) formed by cross metathesis.

Crystallization attempts of the same reaction applying low-

temperature conditions (as described for recently isolated

MCBD complexes)^5e,38aﬀorded no crystals of the desired

MCBD complex. These unsuccessful attempts underline the

fact that the MCBD intermediates in the alkyne metathesis

catalytic cycle, formed by [2 + 2] cycloaddition of an alkyne

with an alkylidyne complex, feature a low energy barrier,^5e

which makes it extremely diﬃcult to kinetically trap and isolate

the MCBD species. This is particularly true if the complexes

perform as highly active catalysts, such as 5 and similar

molybdenum alkylidynes bearing the hexaﬂuorinated alkoxides.

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2-Isopropoxybenzoyl and 8-Methoxynaphthoyl

Complexes. In spite of the unsuccessful stabilization of the

MCBD species involving the ortho-substituted benzylidyne

molybdenum complex 7, we focused on the preparation of the

envisaged isopropoxy-functionalized benzylidyne complex 8

(see Figure 2). As usual, the synthetic pathway began with the

reaction of 2-isopropoxyphenyl lithium (generated in situ by

metal−halogen exchange of 2-bromo-1-isopropoxybenzene

and n-butyl lithium at −20 °C) with [Mo(CO)₆] in diethyl

ether to furnish the anionic benzoyl complex Li[{2-(i-

PrO)C₆H₄}C(O)Mo(CO)₅], Li[17], which was converted

to [Me₄N][17] via salt metathesis in degassed water. However,

step of the reaction, as revealed by an NMR sample of

intermediate Li[17], which presented the characteristic pattern

with duplicated resonances besides, again, the downﬁeld signal

in the carbon NMR spectrum. Instead, the expedient protocol

modiﬁcations introduced into the synthesis of Li[13],

involving its precipitation with n-hexane (Scheme 3), were

applied successfully in the isolation of Li[17] in pure form (see

Figures S33a,b and the Experimental Section for details).

Indeed, the precipitation approach gives access to function-

alized acyl complexes (Fischer carbene precursors) for which

suitable synthetic strategies were missing. To illustrate the

applicability of the method, the synthesis of the unreported

Li[(8-MeO-Naph)C(O)Mo(CO)₅] (Li[19], Naph = 1-

naphthyl; for details, see Scheme 3, Supporting Information

p S4, and Figures S34 and S35) was achieved in good yield

upon treatment of [Mo(CO)₆] with 8-methoxynaphthyl

lithium in diethyl ether and precipitation with n-hexane. The

molecular structure was additionally determined by X-ray

diﬀraction analysis of pale yellow acicular single crystals of

[Li(19)·(thf)₃] (Figure S14), conﬁrming the formation of the

acyl complex, with structural parameters similar to those found

for Li[13] (Table 1).

Notwithstanding, the present approach faces some limi-

tations. For example, in an attempt to improve the synthesis of

the p-anisole derivative presented above, benzylidyne complex

11, the isolation of acyl precursor Li[12] proved to be more

diﬃcult than expected (characterization of the complex was

only possible in solution by NMR spectroscopy; for details, see

Supporting Information p S4). Thus, direct oxidation omitting

salt metathesis to access 11 was not considered further, and it

was obtained as described above.²⁶In addition, it has to be

noted that this procedure is only straightforward if the lithium

aryl compounds are available as pure substances. Otherwise,

the use of, e.g., n-BuLi or t-BuLi to generate the lithium

organyl in situ can yield LiBr as a byproduct, which could

interfere in the isolation of the acyl complex by coprecipitation.

Finally, with acyl complexes Li[17] and Li[19] in hand, the

preparation of the alkylidyne complexes [ArCMo-

(Br)₃(dme)] (Ar = 2-(i-PrO)C₆H₄(20), 8-MeO-Naph (21))

via treatment with oxalyl bromide and bromine in the presence

of DME at subambient temperatures was attempted (Scheme

5). We were surprised to ﬁnd that the reaction repeatedly

resulted in a mixture of decomposition products,⁴⁰suggesting

that the introduced substituents may be incompatible with the

conditions of the low-oxidation-state route. We anticipate at

this point that the cross-metathesis approach using benzyli-

dyne-transfer reagents (ArCCCF₃or ArCCSiMe₃),^13,14as

mentioned in the introduction, may be a suitable alternative to

generate the projected alkylidynes [ArCMo{OC-

(CF₃)₂Me}₃] (Ar = 2-(i-PrO)C₆H₄(8), 8-MeO-Naph (22)),

although in such case the removal of DME may be

problematic.

1

the H NMR spectrum (Figure S31a) revealed two sets of

resonances for the isopropyl moiety and for the aromatic

protons, again suggesting (cf. [Me₄N][13], vide supra) the

presence of two molybdenum species in solution. Accordingly,

the signal duplication was also observed in the ¹³C{¹H} NMR

spectrum (Figure S31b), except for the most low-ﬁeld-shifted

signal at about δ_C= 303 ppm, which correlates with the acyl

carbon atom (ArC(O)Mo). Colorless prisms precipitated

from the NMR solution and their molecular structure was

established by X-ray diﬀraction analysis, conﬁrming the nature

of the second component in the mixture, the anionic

arylpentacarbonylmolybdenum complex [Me₄N][{2-(i-PrO)-

C₆H₄}Mo(CO)₅] ([Me₄N][18], Figure 9). As discussed above

Figure 9. Molecular structure of one anion of [Me₄N][18] with

thermal displacement parameters drawn at the 50% probability level.

The second independent anion in the asymmetric unit and the two

cations ([Me₄N]⁺) are omitted. For selected bond lengths and angles,

see Figure S13.

for 13, acyl complex 17 underwent a decarbonylation process,

in which the released CO ligand was replaced by the

deinserted carbonyl group of the acyl unit to give 18.

Compound [Me₄N][18] crystallizes with two independent

molecules per unit cell in the monoclinic, noncentrosymmetric

space group P2₁and adopts essentially an octahedral geometry

around the zero-valent molybdenum atom (average angles for

each independent molecule, respectively: trans, 176.2(4) and

172.8(5)°; cis, 90.0(4) and 90.1(5)°). The Mo−C_Arbond

lengths of 2.275(9) and 2.299(9) Å, slightly shorter than that

of complex 15, are indicative of a Mo−C single bond and are

in the expected range for aryl polycarbonyl molybdenum

complexes³¹(cf. structure of [(C₆F₅)Mo(CO)₅]⁻).³⁹

Bulky Benzylidyne Molybdenum Complexes. The

second strategy to improve the stability of the alkylidyne

complexes consists in increasing the steric bulk of the

benzylidyne moiety. Hence, we attempted ﬁrst the preparation

of the (2,4,6-triisopropyl)benzylidyne complex [ArCMo-

{OC(CF₃)₂Me}₃] (Ar = (2,4,6-triisopropyl)phenyl, 23) from

the corresponding tribromide species mer-[ArCMo-

(Br)₃(dme)] (24), which in turn can be synthesized from

[Mo(CO)₆] and (2,4,6-triisopropyl)phenyl lithium via the

low-oxidation-state route (Scheme 3). Our precipitation

approach, however, was not employed here because of

Because of similar properties of both compounds, their

separation failed. Moreover, we realized that the decarbon-

ylation already takes place (at least partially) during the ﬁrst

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The solid-state structures of alkylidyne complex 23 and its

precursors 24 and Li[25] were additionally determined by X-

ray diﬀraction analysis. Acyl species Li[25] crystallizes from an

n-pentane solution in the noncentrosymmetric orthorhombic

space group P2₁2₁2₁and consists of tetramers of diethyl ether

hemisolvate [Li₄(25)₄·(Et₂O)₂] (Figure 10). The molecule has

Scheme 5. Attempted Synthesis of Alkylidyne Complexes 8

and 20−22

diﬃculties in the isolation of the lithium organyl, and instead,

the acyl complex [Me₄N][(ArC(O)Mo(CO)₅], [Me₄N]-

[25], was obtained in rather low yields after salt metathesis in

deoxygenated water (for details, see Supporting Information p

S5). Because of the low yields, we explored again the

possibility of isolating lithium derivative Li[25]; therefore,

the following changes were introduced in the workup

procedure. Combination of [Mo(CO)₆] and in situ generated

(2,4,6-triisopropyl)phenyl lithium followed by solvent removal

under reduced pressure yielded a mixture of acyl complex

Li[25] and lithium bromide; this mixture was subsequently

extracted with dichloromethane to remove the inorganic salt.

Precipitation of the extract with n-pentane aﬀorded the desired

product in acceptable yields, which was then successfully

oxidized using standard conditions to give complex 24 in

excellent yields. In the ﬁnal step, bromide exchange with

K[OC(CF₃)₂Me] in THF, followed by exhaustive drying at

slightly elevated temperatures and extraction with n-pentane,

produced desired alkylidyne complex 23 after recrystallization

in dichloromethane at low temperatures. Characterization by

means of NMR spectroscopy (Figures S38a −c) conﬁrmed the

Figure 10. (a) Molecular structure of one Li(25) unit of [Li₄(25)₄·

(Et₂O)₂] with thermal displacement parameters drawn at the 50%

probability level. Neighboring atoms of other units are included using

paler colors. Hydrogen atoms are omitted for clarity. (b)

Representation of the tetrameric structure as a wireframe model.

For selected bond lengths and angles, see Figure S15a.

approximate-C₂symmetry along the b axis (see Figure S15b for

details) and can be divided virtually into two halves. Hence,

considering only one half of the molecule, two Li(25)

fragments are bridged by both lithium cations through the

oxygen atoms at the acyl groups forming a pair, Li₂(25)₂·Et₂O.

The second pair, rotated 180° around the pseudo-C₂axis, is

joined to the ﬁrst through interactions of one carbonyl ligand

at each molybdenum core with the inner lithium atom of the

other pair. This arrangement results in a [5.5.5.5]paddlane-like

structure, in which the four O−C−Mo−C−O bridges intersect

at the interior lithium cations (Li1 and Li3). These

tetracoordinated atoms are best described as distorted

tetrahedral (Li1, τ₄= 0.877(3); Li3, τ₄= 0.899(3)),³⁴whereas

the outer lithium ions (Li2 and Li4) display a distorted trigonal

planar or Y-shaped geometry with the smallest O−Li−O bond

angle approximately equal to 90°. The molybdenum centers

show essentially octahedral geometries, and the Mo−C(O)

bond lengths (2.231(2)−2.245(2) Å, Table 1) and the Mo−

CO distances (1.981(2)−2.098(3) Å) are all in the expected

range.

1

structure of the complex; for example, the H NMR spectrum

displays the expected doublets (1.20, 1.25 ppm) and septets

(2.98, 3.86 ppm) for the isopropyl groups at the para and ortho

positions of the aryl moiety, respectively. In addition, the

singlet at 1.86 ppm attributable to the methyl groups of the

alkoxide ligands integrates to nine protons, whereas the

resonance for the aromatic protons appears at 6.96 ppm. In

the ¹³C{¹H} NMR spectrum the isopropyl substituents give

rise to four signals at 23.9, 24.7, 30.7, and 34.8 ppm, whereas

the four expected resonances for the aryl ring are found in the

aromatic region (121.5, 141.4, 152.8, and 153.4 ppm).

Furthermore, the carbon atoms in the alkoxide ligands are

observed as a singlet (CH₃, 19.3 ppm), a quartet (CF₃, 123.3

ppm), and a poorly resolved septet (quaternary C, 83.8 ppm).

Finally, the most downﬁeld shift (320.2 ppm) can be assigned

to the alkylidyne carbon atom. Naturally, the ﬂuorine NMR

spectrum consists of only one singlet at −78.2 ppm.

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Brown-orange dichroic needles of 24 were grown from

dichloromethane/n-pentane solution at −30 °C and the

molecular structure is illustrated in Figure 11. The compound

range (1.745(2) Å). The Mo−C1−C2 bond angle of

174.41(15)° deviates slightly from linearity.

Our eﬀorts to prepare further bulky derivatives of the

benzylidyne complex failed for various reasons. For example,

the synthesis of the 2,6-diisopropylphenyl analog was

prevented by unselective lithiation and miserable yields of

the benzoyl precursor [Me₄N][{2,6-(i-Pr)₂C₆H₃}C(O)Mo-

(CO)₅] (for further details, see Supporting Information p S6),

for which solely the molecular structure could be established

by X-ray diﬀraction analysis (Figure S18). The attempted

preparation of the even more sterically hindered tris(tert-

butyl)benzoyl complex [Li][{2,4,6-(t-Bu)₃C₆H₂}C(O)Mo-

(CO)₅] unexpectedly produced the already known η⁶-(1,3,5-

tri-tert-butylbenzene)tricarbonylmolybdenum(0) complex⁴¹as

revealed by comparison of the cell parameters of single crystals

obtained at −35 °C from a CH₂Cl₂/n-pentane solution of the

product.

Activity in Alkyne Metathesis. Both complexes [6(thf)]

and 6 and complex 7 are excellent initiators of the alkyne

metathesis reaction and achieve 90, 94, or 90% conversion,

respectively, of our standard substrate^9cBnO(CH₂)₂CCMe

(26a) into BnO(CH₂)₂CC(CH₂)₂OBn (27) in 10 min

(Figure 13) with a catalyst loading of 1 mol % (not optimized;

Figure 11. Molecular structure of 24 with thermal displacement

parameters drawn at the 50% probability level. Hydrogen atoms are

omitted for clarity. For selected bond lengths and angles, see Figure

S16.

crystallizes in the monoclinic space group P2₁/n and shows, as

expected, the structure of the mer-isomer, in which the

hexacoordinated molybdenum complex exhibits a distorted

octahedral geometry (angles: trans, 178.6(3)−164.169(19)°;

cis, 72.86(12)−97.11(14)°) in which the Mo atom lies 0.26 Å

out of the plane of Br1, Br2, Br3, and O2 in the direction of

C1. Thus, the bromide ligands are arranged in a T-shape

around the metal center; the two oxygen atoms of the DME

molecule are chemically inequivalent and force a particularly

acute angle O1−Mo−O2 of 72.86(12)° by chelation. The

alkylidyne bond (Mo−C1, 1.740(4) Å) is unremarkable and

forms a virtually linear angle of 178.6(3)° with the aryl moiety.

Finally, yellow crystals of 23 were grown from n-pentane

solution at −30 °C, and the molecular structure is presented in

Figure 12. The complex crystallizes in the monoclinic space

group P2₁/n and features a distorted tetrahedral geometry (τ₄

= 0.9338(9)),³⁴in which the molybdenum atom is coordinated

by the three ﬂuorinated alkoxide ligands and the benzylidyne

unit with typical Mo−O bond distances (1.8892(14)−

1.9237(14) Å) and a Mo−C1 bond length in the expected

Figure 13. GC conversion−time plot of 26a (0.25 mmol) into 27 in

toluene (0.2 M) with 1 mol % catalyst loading in the presence of

molecular sieves 5 Å (250 mg) and n-decane (0.25 mmol) as internal

standard. The inset shows an expansion of the diagram for clarity.

TOF_{10 min}([6(thf)]) = 9.0 min⁻¹, TOF_{10 min}(6) = 9.4 min⁻¹,

TOF_{10 min}(7) = 9.0 min⁻¹) in the presence of molecular sieves.

The presence of the attached THF molecule does not interfere

signiﬁcantly with the catalytic performance, suggesting that

THF is displaced easily by the alkyne substrate. In the same

way, it is reasonable to aﬃrm that the intermolecular

coordination in 6 and 7 disappears upon dissolution or alkyne

binding. Related to this phenomenon, weak coordination of

Figure 12. Molecular structure of 23 with thermal displacement

parameters drawn at the 50% probability level. Only one position of

the disordered isopropyl group (at C7) is shown; hydrogen atoms are

omitted for clarity. For selected bond lengths and angles, see Figure

S17.

K

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Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

are described in Supporting Information pp S3 −S6. All other reagents

were obtained commercially. Bromine (Acros), 1,1,1,3,3,3-hexaﬂuoro-

2-methyl-2-propanol (Apollo Scientiﬁc), 2-bromo-1,3-diisopropylben-

zene (Sigma-Aldrich), 2-bromo-1,3,5-triisopropylbenzene (Sigma-

Aldrich), n-butyl lithium (Acros), t-butyl lithium (Sigma-Aldrich),

2-isopropoxyphenyl bromide (TCI), molybdenum hexacarbonyl

(ABCR), 1,3,5-trimethoxybenzene (Acros), 1-methoxynaphthalene

(TCI), and oxalyl bromide (Sigma-Aldrich) were used without further

puriﬁcation. Bromanisole (Fluka) was distilled under reduced

pressure (68 °C/5.2 mbar) before use. n-Decane (Alfa Aesar) and

1,2-dimethoxyethane (Alfa Aesar) were dried over Na/benzophenone

and stored under argon over molecular sieves. The molecular sieve 5

Å MS (powder < 50 μm, Sigma-Aldrich) for metathesis reactions was

dried for 24 h at 180 °C under vacuum prior to use. Potassium

hydride dispersion in mineral oil (30%, Acros) was washed several

times with n-hexane on a frit and dried under vacuum before use.

Equipment. Gas chromatography (GC) was executed on a

Hewlett-Packard 5890 SERIES II using a DB5-HT column (l = 30 m,

d = 0.25 mm) and FID detection (310 °C). The sample (1 μL) was

injected at 250 °C with a split/splitless ratio of 1:10 and heated in the

column from 50 to 300 °C with a heating rate of 10 °C min⁻¹. For

calibration, n-decane was used as an internal standard. Elemental

analyses (C, H, N) were determined by combustion and gas

chromatographic analysis on a VarioMICRO Tube instrument

equipped with WLD and IR detectors.

heteroatoms (such as in Ph₃SiOH or CH₃CN) to the

molybdenum atom in alkylidyne complexes has been brieﬂy

discussed,^26,29focusing on the geometrical implications and

how this circumstance aﬀects the binding of the alkyne

substrate. To test our hypothesis that intermolecular contacts

can increase the stability of the complexes, at least in the solid

state, a sample of 7 was exposed for 1 h to air before the

catalytic test was repeated (for details, see Supporting

Information p S7). Unfortunately, the results fell short of our

expectations, as the metathesis conversion decreased dramat-

ically, and after 1 h only a marginal conversion of 4% was

determined by gas chromatography. Finally, the ability of

complex 23 to promote alkyne cross-metathesis was tested in

the conversion of both the internal alkyne 26a and the terminal

alkyne BnO(CH₂)₂CCH (26b) into 27 applying standard

conditions (for details, see Supporting Information p S7). Even

though the activity (97 or 92% conversion after 10 min,

respectively) was indeed comparable with that of 5,²¹6, or 7,

the bulky benzylidyne presented no signs of air stability, and

after exposure of the complex to air for just 10 min, the

conversion was negligible.

CONCLUSIONS

■

NMR Spectroscopy. NMR spectra were recorded on Bruker DPX

200, AV II 300, AV III HD 300, AV III 400, AV III HD 500, or AV II

600 spectrometers. Chemical shifts (δ) are reported in ppm (parts per

million) and are referenced relative to internal TMS (tetramethylsi-

The portfolio of molybdenum alkylidyne complexes bearing

ﬂuorinated alkoxides has been extended by the installation of

ether substituents or sterically demanding isopropyl groups on

the benzylidyne moiety, a strategy that may contribute to

increasing the air stability of the complexes. While the

synthesis of complexes 6, 7, and 23 was successful, the

preparation of other benzylidyne derivatives (8, 22) was not

feasible under the applied conditions. However, after the

established synthetic protocol was deliberately modiﬁed, it was

possible to isolate and characterize the corresponding acyl

precursors. The principal problem was the undesired

deinsertion of the carbonyl fragment at the acyl moiety.

Thus, the lithium acyl complexes were isolated directly by

precipitation, before salt metathesis in water, to avoid

decarbonylation during the solvent evaporation step, giving a

more straightforward access to acylpentacarbonylmolybdenum

complexes featuring challenging functional groups.

1

lane, δ_H0.00 ppm), to residual solvent H signals (C₆D₆, δ_H7.16;

THF-d₈, δ_H1.72; CD₂Cl₂, δ_H5.32; toluene-d₈, δ_H2.08), to the ¹³C

resonance of the solvents (C₆D₆, δ_C128.06; THF-d₈, δ_C25.31;

CD₂Cl₂, δ_C53.84; toluene-d₈, δ_C20.43) or to virtual internal CFCl₃

(the observation frequencies of a dilute solution of CFCl₃in the

deuterated solvents were determined earlier). The number of protons

(n) for a given resonance is indicated by nH. All ¹³C and ¹⁹F NMR

spectra are proton-decoupled, and the number of protons attached to

each carbon atom was determined by ¹³C-DEPT135 experiments. If

required, then signals were assigned by 2D H,H-COSY, H,H-NOESY,

H,H-ROESY, H,C-HSQC, and H,C-HMBC NMR experiments. The

following abbreviations are used for spin multiplicity: s, singlet; d,

doublet; t, triplet; q, quartet; sep, septet; m, multiplet; app, apparent;

br, broad.

X-ray Diﬀraction Studies. Single crystals were examined and

mounted in perﬂuorinated inert oil and transferred to the cold gas

stream of the diﬀractometer. Data were recorded on an Oxford

Diﬀraction Nova A diﬀractometer, using mirror-focused Cu Kα

radiation (compounds [6(thf)], 7, [7(thf)], [Me₄N][12], [Li(13)·

thf]₂, [Li(14)·Et₂O]₂(polymorphs A and B), [Li(15)·(Et₂O)₂]_n,

[Me₄N][18], 23, 24, [Me₄N][{2,6-(i-Pr)₂C₆H₃}C(O)Mo(CO)₅]·

CH₂Cl₂, and Li[OMoBr{OC(CF₃)₂Me}₃]·(thf)₂) or an Oxford

Diﬀraction Xcalibur E diﬀractometer using graphite-monochromated

Mo Kα radiation (compounds 6, cis,mer-[6(thf)₂], [Me₄N][9],

[Li(13)·Et₂O]₂, [Li(19)·(thf)₃], and [Li₄(25)₄·(Et₂O)₂]). Absorption

corrections were performed on the basis of multiscans. All structures

were solved by direct methods (SHELXS-97) and reﬁned anisotropi-

cally by full-matrix least-squares procedures on F²using the SHELXL-

97 program.⁴³Hydrogen atoms were included as idealized methyl

groups allowed to rotate but not tip (all methyls with the exception of

disordered methyl groups in 6 and [Li(15)·(Et₂O)₂]_n) or placed

geometrically and allowed to ride on their attached carbon atoms (all

other H atoms). Numerical details are summarized in Tables S1 −S19.

For special features and exceptions, see Supporting Information pp

S8 −S34. The program Diamond⁴⁴was used for graphical

representations.

Remarkably, complexes 6 and 7 feature in the solid state a

polymeric structure, in which the methoxy group at the para-

position is bridged to the neighboring molecule forming zigzag

chains. This is, to the best of our knowledge, the ﬁrst example

of an intermolecular coordination involving metal alkylidynes.

Unfortunately, neither the presence of intermolecular contacts

in the solid state nor the increase of the steric bulk in 23

positively aﬀected the stability of the complexes. Whether

more Lewis basic functionalities such as amines or phosphines

may be more successful in fulﬁlling this challenge is currently

under investigation in our group.

EXPERIMENTAL SECTION

■

General Considerations. All operations were performed under

an atmosphere of dry argon using Schlenk and vacuum techniques.

Solvents (except THF) were puriﬁed and dried by an MBraun GmbH

solvent puriﬁcation system and degassed by argon-bubbling for at

least 15 min prior to use. THF and deuterated solvents were dried by

conventional methods⁴²and stored under argon over molecular

sieves. Celite diatomaceous earth (D.E.) was stored at least 3 days in

an oven at 120 °C.

Experimental Procedures. Preparation of Tris((1,1,1,3,3,3-

hexaﬂuoro-2-methyl-2-propanyl)oxy)(4-methoxybenzylidyne)-

(tetrahydrofuran)molybdenum(VI), [6(thf)]. A solution of K[OC-

(CF₃)₂Me] (0.66 g, 3.00 mmol, 3 equiv) in THF (7 mL) was added

to a stirred solution of 11 (0.54 g, 1.00 mmol) in THF (3 mL), and

Starting Materials. Compounds 11,²⁶26a,^9cand 26b²¹were

synthesized according to the literature. The syntheses of K[OC-

(CF₃)₂Me], 8-methoxynaphthyl lithium and 4-methoxyphenyl lithium

L

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the reddish brown mixture was stirred overnight at ambient

temperature. The volatiles were removed under reduced pressure,

and the brown residue was dried under dynamic vacuum. It was then

extracted repeatedly with n-pentane (total amount 10 mL), ﬁltered

through D.E., and the frit was washed with more n-pentane to give a

yellow ﬁltrate. The solvent was removed under high vacuum and the

product was isolated as a pale brown solid in 70% yield (0.58 g, 0.70

mmol). Crystallization attempts from concentrated n-pentane

solutions at −35 °C aﬀorded yellow crystals of [6(thf)], but the

structure could not be reﬁned satisfactorily (for details, see Figure

S3). Single crystals of [6(thf)₂] were obtained from a concentrated n-

pentane/THF (∼20:1) solution of [6(thf)] at −35 °C. Elemental

analysis. Found: C, 34.62; H, 3.04. Calcd for C₂₄H₂₄F₁₈MoO₅: C,

34.71; H, 2.91%. NMR spectroscopy. δ_H(C₆D₆, 300.1 MHz, 297 K)

1.38 (m, 4H, THF), 1.83 (s, 9H, 3 × C(CH₃)(CF₃)₂), 3.17 (s, 3H,

OCH₃), 3.82 (m, 4H, THF), 6.44 (d, 2H, ³J_HH= 8.9 Hz, 2 × Ar−H),

second crop of product precipitated from the ﬁltrate as large block-

shaped orange crystals (0.91 g). Total yield: 44% (2.07 g, 4.06 mmol).

Elemental analysis. Found: C, 44.45; H, 4.07. Calcd for

C₁₉H₁₉LiMoO₁₀: C, 44.72; H, 3.75%. NMR spectroscopy. δ_H

(THF-d₈, 200.1 MHz, 300 K) 1.73 (m, THF), 3.57 (m, THF),

3.62 (s, 6H, 2 × o-OCH₃), 3.67 (s, 3H, p-OCH₃), 6.01 (s, 2H, 2 × m-

H), (Figure S27a); δ_C(THF-d₈, 75.5 MHz, 299 K) 26.4 (THF), 55.1

(o-OCH₃), 55.3 (p-OCH₃), 68.2 (THF), 91.0 (m-CH), 134.5 (i-C_q),

153.4 (o-C_qO), 159.8 (p-C_qO), 211.0 (cis-CO), 218.6 (trans-CO),

313.9 (Ar−CO) (Figure S27b).

Preparation of Tris(bromido)(1,2-dimethoxyethane-κ²O,O′)-

(2,4,6-trimethoxybenzylidyne)molybdenum(VI), 16. To a stirred

and precooled (−80 °C) suspension of Li[13·thf] (1.16 g, 2.27

mmol) in dichloromethane (30 mL) was added a precooled (−80 °C)

solution of oxalyl bromide (0.59 g, 2.72 mmol, 1.2 equiv) in

dichloromethane (5 mL). The brown mixture was allowed to warm

up gradually to −20 °C (the cooling bath was not removed) during

which time the color darkened slightly and evolution of gas was

observed. The mixture was cooled back to −80 °C and ﬁltered at this

temperature through D.E. on a jacketed frit to separate the

precipitated LiBr. Then, 1,2-dimethoxyethane (2.35 mL, 2.04 g,

22.60 mmol, 10.0 equiv) was added to the stirred ﬁltrate at −70 °C,

followed by slow addition of a precooled (−70 °C) solution of

bromine (0.12 mL, 0.37 g, 2.31 mmol, 1.02 equiv) in dichloro-

methane (5 mL). The resulting orange-brown solution was allowed to

warm up gradually to approximately 15 °C, and the volatiles were

removed under high vacuum. The deep brown residue was dissolved

in dichloromethane (10 mL), and after addition of n-pentane (45

mL), a brown solid precipitated. The mother liquor was decanted oﬀ

and the solid dried under high vacuum to give 16 as a crude product

as judged by NMR spectroscopy (any attempts to recrystallize or

further purify the compound resulted in additional decomposition).

Yield (crude product): 73% (996 mg, 1.65 mmol). NMR spectros-

copy. δ_H(CD₂Cl₂, 300.1 MHz, 297 K) 3.48 (br s, DME), 3.65 (br s,

DME), 3.82 (br s, DME), 3.87 (s, 3H, p-OCH₃), 3.91 (s, 6H, 2 × o-

OCH₃), 3.97 (br s, DME), 6.03 (s, 2H, 2 × m-H) (Figure S28a); δ_C

(CD₂Cl₂, 75.5 MHz, 299 K) 56.0 (p-OCH₃), 56.2 (o-OCH₃), 60.0

(DME−CH₃), 70.0 (DME−CH₂), 73.4 (DME−CH₃), 78.6 (DME−

CH₂), 88.9 (m-CH), 117.1 (i-C_q), 165.1 (p-C_qO), 168.4 (o-C_qO),

331.4 (CMo) (Figure S28b).

3

7.16 (d, 2H, J_HH= 8.9 Hz, 2 × Ar−H) (Figure S22a); δ_F(C₆D₆,

188.3 MHz, 300 K) −77.81 (CF₃) (Figure S22b); δ_C(C₆D₆, 75.5

MHz, 299 K) 19.4 (C(CH₃)(CF₃)₂), 25.5 (THF), 54.9 (OCH₃), 68.6

2

(THF), 84.7 (sep, J_CF= 29 Hz, C(CH₃)(CF₃)₂), 113.8 (m-CH),

124.1 (q, ¹J_CF= 288 Hz, CF₃), 132.6 (o-CH), 139.2 (i-C_q), 160.7 (p-

C_q), 294.4 (CMo) (Figure S22c).

Preparation of Tris((1,1,1,3,3,3-hexaﬂuoro-2-methyl-2-

propanyl)oxy)(4-methoxybenzylidyne)molybdenum(VI), 6. A solu-

tion of [6(thf)] (47 mg, 57 μmol) in toluene (1 mL) and n-pentane

(1 mL) was stored for 5 days at −35 °C. From that solution pale

brown crystals precipitated, which were suitable for X-ray diﬀraction

analysis. After decantation and drying under dynamic vacuum the

product was isolated in 43% yield (20 mg, 26 μmol). Alternatively,

[6(thf)] can be repeatedly dissolved in toluene and the solvent

subsequently evaporated at 50 °C to obtain THF-free complex 6.

Elemental analysis. Found: C, 31.70; H, 1.86. Calcd for

C₂₀H₁₆F₁₈MoO₄: C, 31.68; H, 2.13%. NMR spectroscopy. δ_H

(C₆D₆, 300.1 MHz, 298 K) 1.68 (s, 9H, 3 × C(CH₃)(CF₃)₂), 3.16

(s, 3H, OCH₃), 6.41 (d, 2H, ³J_HH= 9.0 Hz, 2 × Ar−H), 7.04 (d, 2H,

³J_HH= 9.0 Hz, 2 × Ar−H) (Figure S23a); δ_F(C₆D₆, 188.3 MHz, 300

K) −77.92 (CF₃) (Figure S23b); δ_C(C₆D₆, 75.5 MHz, 299 K) 19.6

2

(C(CH₃)(CF₃)₂), 55.2 (OCH₃), 84.6 (sep, J_CF= 29 Hz, C(CH₃)-

(CF₃)₂), 114.1 (m-CH), 123.5 (q, ¹J_CF= 288 Hz, CF₃), 131.9 (o-CH),

139.3 (i-C_q), 161.1 (p-C_q), 299.2 (CMo) (Figure S23c).

Preparation of 2,4,6-Trimethoxyphenyl Lithium. A published

procedure⁴⁵was modiﬁed as follows. In a 100 mL Schlenk tube, n-

BuLi (1.6 M in hexanes, 20 mL, 32.0 mmol, 1.01 equiv) was added to

a stirred solution of 1,3,5-trimethoxybenzene (5.35 g, 31.8 mmol, 1.0

equiv) in diethyl ether (12 mL). A reﬂux condenser was attached, and

the reaction mixture was reﬂuxed at 40 °C for 2.5 h, inducing the

rapid formation of a white precipitate. The mixture was allowed to

cool down and then stirred overnight at ambient temperature. The

white solid was collected on a frit, washed with n-hexane (3 × 5 mL),

and dried under high vacuum. Yield: 93% (5.13 g, 29.5 mmol).

Elemental analysis. Found: C, 62.06; H, 6.61. Calcd for C₉H₁₁LiO₃:

C, 62.08; H, 6.37%. NMR spectroscopy. δ_H(THF-d₈, 300.1 MHz,

297 K) 3.68 (s, 3H, p-OCH₃), 3.69 (s, 6H, 2 × o-OCH₃), 5.98 (s, 2H,

2 × m-H) (Figure S24a); δ_C(THF-d₈, 75.5 MHz, 298 K) 54.2 (o-

OCH₃), 54.8 (p-OCH₃), 90.2 (m-CH), 136.0 (i-C_q), 161.3 (p-

C_qOCH₃), 169.8 (o-C_qOCH₃) (Figure S24b).

Preparation of Tris((1,1,1,3,3,3-hexaﬂuoro-2-methyl-2-

propanyl)oxy)(tetrahydrofuran)(2,4,6-trimethoxybenzylidyne)-

molybdenum(VI), [7(thf)]. To a stirred solution of K[OC(CF₃)₂Me]

(132 mg, 600 μmol, 3.3 equiv) in THF (1.5 mL) was added the crude

molybdenum complex 16 (111 mg, 183 μmol) in portions through a

funnel, and the funnel was rinsed with THF (2 × 0.5 mL). The

mixture immediately turned deep brown and was stirred for 16 h at

ambient temperature. The volatiles were removed under vacuum, and

the deep brown residue was dried for 1 h at 50 °C under dynamic

vacuum. The residue was extracted several times with n-hexane,

ﬁltered through D.E., and the frit was washed with more n-hexane,

leaving a black residue and a yellow ﬁltrate. Removal of the solvent

under high vacuum aﬀorded the title compound as a yellow solid in

62% yield (101 mg, 113 μmol). Single crystals suitable for X-ray

diﬀraction analysis were obtained by storing a concentrated n-pentane

solution at −30 °C. Elemental analysis. Found: C, 33.52; H, 2.81.

Calcd for C₂₆H₂₈F₁₈MoO₇: C, 35.07; H, 3.17%. Combustion analysis

repeatedly produced low carbon and hydrogen values, which we

ascribe to partial loss of THF. NMR spectroscopy. δ_H(C₆D₆, 300.1

MHz, 297 K) 1.43 (m, THF), 1.94 (s, 9H, 3 × C(CH₃)(CF₃)₂), 3.17

(s, 6H, 2 × o-OCH₃), 3.21 (s, 3H, p-OCH₃), 3.86 (m, THF), 5.66 (s,

2H, 2 × m-H) (Figure S29a); δ_F(C₆D₆, 188.3 MHz, 300 K) −77.9

(CF₃) (Figure S29b); δ_C(C₆D₆, 75.5 MHz, 298 K) 18.0

(C(CH₃)(CF₃)₂), 25.6 (THF), 54.4 (o-OCH₃), 55.0 (p-OCH₃),

68.4 (THF), 84.8 (sep, ²J_CF= 30 Hz, C(CH₃)(CF₃)₂), 89.8 (m-CH),

120.3 (i-C_q), 123.9 (q, ¹J_CF= 288 Hz, CF₃), 162.8 (o-C_qO), 163.2 (p-

C_qO), 292.6 (CMo) (Figure S29c).

Preparation of Lithium (2,4,6-Trimethoxybenzoyl)-

pentacarbonylmolybdate(0), Li[13]. A suspension of 2,4,6-trime-

thoxyphenyl lithium (1.60 g, 9.20 mmol, 1.0 equiv) in diethyl ether

(15 mL) was added to a 250 mL Schlenk ﬂask charged with a

suspension of [Mo(CO)₆] (2.43 g, 9.20 mmol, 1.0 equiv) in diethyl

ether (25 mL), and residual 2,4,6-trimethoxyphenyl lithium in the ﬁrst

ﬂask was rinsed with THF (2 × 2.5 mL) and added to the reaction

ﬂask. The mixture immediately turned yellow, and stirring for 15 min

at ambient temperature resulted in a clear orange solution. Then, n-

hexane (70 mL) was added, precipitating a yellow powder that

remained in suspension, and a gluey dark orange solid that stuck to

the walls of the ﬂask. The suspension was immediately ﬁltered before

the yellow powder started to settle; the yellow solid was then dried on

the frit under dynamic vacuum to give 1.16 g of the title compound. A

Preparation of Tris((1,1,1,3,3,3-hexaﬂuoro-2-methyl-2-

propanyl)oxy)(2,4,6-trimethoxybenzylidyne)molybdenum(VI), 7. K-

[OC(CF₃)₂Me] (228 mg, 1035 μmol, 0.97 equiv) was dissolved in

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THF (3 mL) and added to a stirred solution of crude 16 (216 mg,

357 μmol) in THF (1 mL), which immediately turned dark brown.

The mixture was stirred overnight at ambient temperature. The

volatiles were removed under vacuum, and the brown residue was

dried for 90 min at 50 °C under dynamic vacuum. It was then

extracted several times with n-pentane, ﬁltered through D.E., and the

frit was washed with more n-pentane, leaving a black residue and a

yellow ﬁltrate. The solvent was removed under high vacuum, and the

residue was dried for 90 min at 40 °C under dynamic vacuum. n-

Pentane (5 mL) was added and then removed quickly at 40 °C twice.

After additional drying for 15 min at 40 °C under dynamic vacuum,

the product was isolated as a brown solid in 36% yield (101 mg, 123

μmol). Single crystals suitable for X-ray diﬀraction analysis were

obtained from a concentrated n-pentane solution at −30 °C.

Elemental analysis. Found: C, 32.29; H, 2.29. Calcd for

C₂₂H₂₀F₁₈MoO₆: C, 32.29; H, 2.46%. NMR spectroscopy. δ_H

(C₆D₆, 600.1 MHz, 303 K) 1.87 (s, 9H, 3 × C(CH₃)(CF₃)₂), 3.14

(s, 6H, 2 × o-OCH₃), 3.18 (s, 3H, p-OCH₃), 5.64 (s, 2H, 2 × m-H)

(Figure S30a); δ_F(C₆D₆, 470.7 MHz, 298 K) −78.14 (CF₃) (Figure

S30b); δ_C(C₆D₆, 150.9 MHz, 303 K) 18.19 (C(CH₃)(CF₃)₂), 54.54

Preparation of Lithium (2,4,6-Triisopropylbenzoyl)-

pentacarbonylmolybdate(0), Li[25]. A solution of 1-bromo-2,4,6-

triisopropylbenzene (3.0 mL, 12.0 mmol) in diethyl ether (30 mL)

was treated with t-BuLi (1.7 M in pentane, 14 mL, 24 mmol, 2 equiv)

at −30 °C. After stirring at low temperature for 2 h, the solution was

transferred to a precooled (0 °C) suspension of [Mo(CO)₆] (3.17 g,

12.00 mmol) in diethyl ether (20 mL), and the reaction mixture was

stirred at ambient temperature for 2 h. The solvent was removed

under reduced pressure, and the residue was dried for several hours

under high vacuum. The yellow solid was dissolved in dichloro-

methane (100 mL) and ﬁltered through a small pad of D.E. to remove

LiBr. The orange ﬁltrate was concentrated to 20 mL, and the product

was precipitated with n-pentane (80 mL) to give Li[25·Et₂O] as an

intensely yellow solid. Single crystals suitable for X-ray diﬀraction

analysis were obtained from a concentrated n-pentane solution at −35

°C. Yield: 52% (3.42 g, 6.24 mmol). Elemental analysis. Found: C,

53.53; H, 5.47. Calcd for C₉₂H₁₁₂Li₄Mo₄O₂₆: C, 54.02; H, 5.52%.

Although the carbon content is outside the range viewed as

establishing analytical purity, it is provided to illustrate the best

values obtained to date. NMR spectroscopy. δ_H(CD₂Cl₂, 300.1 MHz,

2

3

(o-OCH₃), 55.01 (p-OCH₃), 84.52 (sep, J_CF= 30.1 Hz, C(CH₃)-

300 K) 1.05 (d, 2 × 3H, J_HH= 6.8 Hz, p-CH(CH₃)₂), 1.12 (t, 2 ×

1

3H, ³J_HH= 7.0 Hz, Et₂O−CH₃), 1.20 (d, 2 × 3H, ³J_HH= 6.8 Hz, 2 ×

(CF₃)₂), 89.75 (m-CH), 120.35 (i-C_q), 123.62 (q, J_CF= 286.5 Hz,

3

CF₃), 163.02 (o-C_qO), 163.19 (p-C_qO), 296.73 (CMo) (Figure

o-CHCH₃), 1.33 (d, 2 × 3H, J_HH= 6.8 Hz, 2 × o-CHCH₃), 2.82

(sep, 1H, ³J_HH= 6.8 Hz, p-CH), 3.12 (sep, 2 × 1H, ³J_HH= 6.6 Hz, 2 ×

o-CH), 3.43 (m, 2 × 2H, Et₂O−CH₂), 6.93 (m, 2H, m-H) (Figure

S36a); δ_C(CD₂Cl₂, 75.5 MHz, 300 K) 14.4 (Et₂O-CH₃), 23.2 (2 ×

CH₃), 24.2 (2 × CH₃), 26.6 (2 × CH₃), 28.9 (o-CH), 34.5 (p-CH),

65.7 (Et₂O-CH₂), 121.6 (m-CH), 139.2 (o-C_q), 147.5 (p-C_q), 155.7

(i-C_q), 209.5 (4 × cis-CO) (Figure S36b).

S30c).

Preparation of 2-Isopropoxyphenyl Lithium. A solution of n-BuLi

(1.6 M in hexanes, 8.88 mL, 14.2 mmol, 1.0 equiv) was added to a

stirred solution of 2-isopropoxyphenyl bromide (3.06 g, 14.2 mmol)

in diethyl ether (20 mL) at ambient temperature, which resulted in

the formation of a white precipitate. The mixture was stirred at

ambient temperature for 12 h, then cooled in an ice bath, the solid

collected by ﬁltration, washed with n-hexane (5 × 20 mL), and dried

under high vacuum to give the title compound as a white solid. Yield:

87% (1.76 g, 12.4 mmol). NMR spectroscopy. δ_H(THF-d₈, 600.1

Preparation of Tris(bromido)(1,2-dimethoxyethane-κ²O,O′)-

(2,4,6-triisopropylbenzylidyne)molybdenum(VI), 24. A precooled

(−80 °C) solution of oxalyl bromide (0.9 mL, 1.37 g, 6.26 mmol,

∼1.2 equiv) in dichloromethane (20 mL) was added dropwise to a

precooled (−80 °C) solution of Li[25·Et₂O] (2.90 g, 5.28 mmol) in

dichloromethane (100 mL). After stirring for 2 h at −80 °C, the

reaction mixture was slowly warmed to −35 °C and then instantly

cooled back to −80 °C. The suspension was ﬁltered through D.E. in a

jacketed frit at low temperature (−80 °C) and then treated with 1,2-

dimethoxyethane (5.5 mL, 4.76 g, 52.8 mmol, 10 equiv) followed by

addition of a precooled (−80 °C) solution of bromine (0.27 mL, 0.84

g, 5.28 mmol, 1 equiv) in dichloromethane (40 mL). The resulting

red-brown solution was allowed to warm up and stirred for 12 h at

ambient temperature. The volatiles were evaporated under reduced

pressure and the resulting brown residue was dissolved in dichloro-

methane (7 mL) and precipitated with n-pentane (100 mL) three

times. The brown solid thus obtained was separated from the

supernatant by decantation and dried under high vacuum. Yield: 87%

(2.96 g, 4.62 mmol). Single crystals suitable for X-ray diﬀraction

analysis were obtained from a dichloromethane/n-pentane solution at

−35 °C. Elemental analysis. Found: C, 38.02; H, 5.29. Calcd for

C₂₀H₃₃Br₃MoO₂: C, 37.47; H, 5.19%. Although the carbon content is

outside the range viewed as establishing analytical purity, it is

provided to illustrate the best values obtained to date. NMR

spectroscopy. δ_H(CD₂Cl₂, 600.1 MHz, 303 K) 1.22 (d, 2 × 3H,

³J_HH= 6.9 Hz, p-CH(CH₃)₂), 1.37 (d, 4 × 3H, ³J_HH= 6.8 Hz, 2 × o-

3

MHz, 303 K) 1.25 (d, 6H, J_HH= 6.1 Hz, 2 × CH₃), 4.50 (sep, 1H,

³J_HH= 6.1 Hz, OCH), 6.55 (app d, 1H, ^3,appJ_HH= 8.0 Hz, Ar−H), 6.66

3,app

(app tm, 1H,

J

= 6.7 Hz, Ar−H), 6.84 (app tm, 1H,

J

HH

=

HH

7.6 Hz, Ar−H), 7.70 (app dd, 1H, ^3,appJ_HH= 6.3 Hz, ^4,appJ_HH= 1.7 Hz,

Ar−H) (Figure S32a); δ_C(THF-d₈, 150.9 MHz, 303 K) 22.78 (2 ×

CH₃), 69.49 (OCH), 109.94 (Ar−CH), 121.36 (Ar−CH), 125.19 (br,

ν_1/2= 19.5 Hz, Ar−CH), 143.21 (Ar−CH), 167.82 (C_qO), 169.2 (br,

ν_1/2= 113 Hz, C_qLi) (Figure S32b).

Preparation of Lithium (2-Isopropoxybenzoyl)-

pentacarbonylmolybdate(0), Li[17]. A 250 mL Schlenk ﬂask was

charged with [Mo(CO)₆] (3.70 g, 14.0 mmol), 2-isopropoxyphenyl

lithium (2.00 g, 14.1 mmol, 1.01 equiv), and diethyl ether (15 mL),

and the yellow suspension was stirred for 2 h at ambient temperature.

After addition of diethyl ether (20 mL) and n-hexane (40 mL) a

brown precipitate began to deposit from solution. The mixture was

placed in an ice bath (0 °C) for 2 h to facilitate complete precipitation

of the product. The supernatant was decanted, and the residue was

washed with n-hexane (3 × 10 mL) and dried under dynamic vacuum

to give a dark yellow solid. A second crop of product was obtained

after the mother liquor had stood for several hours. Total yield: 87%

(1.93 g, 12.1 mmol). Elemental analysis. Found: C, 44.31; H, 2.95.

Calcd for C₁₅H₁₁LiMoO₇: C, 44.36; H, 2.73%. NMR spectroscopy. δ_H

3

(THF-d₈, 399.8 MHz, 298 K) 1.29 (d, 2 × 3H, J_HH= 6.1 Hz, 2 ×

CH(CH₃)₂), 2.98 (sep, 1H, J_HH= 6.9 Hz, p-CH), 3.86 (br s, 3H,

CH₃), 4.53 (sep, 1H, ³J_HH= 6.1 Hz, OCH), 6.62−6.65 (m, 1H, 6-H),

6.73−6.79 (m, 2H, 3-H, 5-H), 6.91−6.96 (m, 1H, 4-H) (Figure

S33a); δ_C(THF-d₈, 100.5 MHz, 299 K) 22.4 (2 × CH₃), 70.0

(OCH), 113.5 (3-CH), 120.3 (5-CH), 122.1 (6-CH), 126.5 (4-CH),

150.1 (2-C_qO), 154.1 (i-C_q), 210.9 (4 × cis-CO), 218.2 (trans-CO),

316.3 (Ar−CO) (Figure S33b).

DME−CH₃), 3.93 (br s, 3H, DME−CH₃), 3.99−4.07 (br m, 2 × 2H,

2 × DME−CH₂), 4.76 (sep, 2 × 1H, ³J_HH= 6.8 Hz, 2 × o-CH), 7.11

(s, 2 × 1H, 2 × m-H) (Figure S37a); δ_C(CD₂Cl₂, 150.9 MHz, 303 K)

24.29 (p-CH(CH₃)₂), 26.02 (o-CH(CH₃)₂), 30.41 (o-CH), 34.47 (p-

CH), 60.37 (DME−CH₃), 69.96 (DME−CH₂), 73.29 (DME−CH₃),

78.70 (DME−CH₂), 120.60 (m-CH), 136.85 (i-C_q), 154.15 (p-C_q),

157.12 (o-C_q), 339.81 (CMo) (Figure S37b).

Preparation of tris((1,1,1,3,3,3-hexaﬂuoro-2-methyl-2-

propanyl)oxy)(2,4,6-triisopropylbenzylidyne)molybdenum(VI), 23.

A solution of K[OC(CF₃)₂Me] (628 mg, 2.85 mmol, 3.2 equiv) in

THF (25 mL) was treated with a suspension of benzylidyne complex

24 (579 mg, 0.90 mmol) in THF (10 mL). After stirring the reaction

mixture at ambient temperature for 24 h the solvent was evaporated at

50 °C under reduced pressure. The residue was extracted with n-

N

DOI: 10.1021/acs.organomet.8b00783

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

pentane (7 × 7 mL) and ﬁltered through a small pad of D.E. to give a

yellow ﬁltrate. The solvent was evaporated under reduced pressure,

and the residual yellow solid was dissolved in a small amount of

dichloromethane. Storing the solution for several days at −80 °C gave

compound 23 as yellow crystals. Yield: 34% (265 mg, 0.31 mmol).

Elemental analysis. Found: C, 39.25; H, 3.97. Calcd for

C₂₈H₃₂F₁₈MoO₃: C, 39.36; H, 3.77%. NMR spectroscopy. δ_H

ACKNOWLEDGMENTS

■

̀

O.A. thanks the Ph.D. program Catalysis for Sustainable

Synthesis (CaSuS), and Richard Boser, Daniel Becker, and

̈

Dominik Korner for experimental support in the synthesis of

Li[17] and Li[19]. H.E. thanks the Chemical Industry Fund

(FCI) for a Chemiefonds Fellowship. We are thankful to Dr.

Matthias Freytag for assistance in the determination of the

crystal structures, to Ludwig Hackl and Tobias Schnabel for

providing experimental data on bulky benzoyl molybdenum

complexes, to Dr. habil. Christian Kleeberg for powder X-ray

diﬀraction analysis of [Li(13)·thf]₂, and to Celine Bittner and

Dirk Bockfeld for the X-ray structure determination of

[Me₄N]-[9]. We acknowledge ﬁnancial support by the

Deutsche Forschungsgemeinschaft (DFG) through grant Ta

189/12-1.

3

(CD₂Cl₂, 600.1 MHz, 303 K) 1.20 (d, 2 × 3H, J_HH= 7.0 Hz, p-

CH(CH₃)₂), 1.25 (d, 4 × 3H, ³J_HH= 6.7 Hz, 2 × o-CH(CH₃)₂), 1.86

3

(s, 3 × 3H, 3 × C(CH₃)(CF₃)₂), 2.87 (sep, 1H, J_HH= 7.0 Hz, p-

CH), 3.86 (sep, 2 × 1H, ³J_HH= 6.7 Hz, 2 × o-CH), 6.96 (s, 2 × 1H, 2

× m-H) (Figure S38a); δ_F(CD₂Cl₂, 188.3 MHz, 298 K) −78.2 (CF₃)

(Figure S38b); δ_C(CD₂Cl₂, 150.9 MHz, 303 K) 19.26 (C(CH₃)-

(CF₃)₂), 23.88 (p-CH(CH₃)₂), 24.75 (o-CH(CH₃)₂), 30.73 (o-CH),

34.75 (p-CH), 83.81 (sep, ²J_CF= 30 Hz, C(CH₃)(CF₃)₂), 121.55 (m-

1

CH), 123.35 (q, J_CF= 287 Hz, CF₃), 141.43 (i-C_q), 152.82 (p-C_q),

153.44 (o-C_q), 320.23 (CMo) (Figure S38c).

Metathesis of 5-(Benzyloxy)-2-pentyne (26a). A mixture of 26a,

(0.25 mmol), n-decane (0.25 mmol), molecular sieves (250 mg), and

toluene (1.25 mL, 200 mm) was stirred for 10 min before adding the

catalyst (1 mol %, 2.5 μmol) as a solid. Aliquots (0.05 mL) for GC

analysis were taken at t = 0 min (prior to addition of catalyst) and at

speciﬁed time intervals (see Figure 13). The samples were ﬁltered

through a small pad of neutral alox, washed with diethyl ether (1 mL)

and analyzed by GC. RT(n-decane) = 5.1 min, RT(26a) = 12.2 min,

RT(27) = 22.5 min. To determine isolated yields, the procedure

described above was repeated without n-decane, and the mixture was

stirred for 1 h at ambient temperature, followed by ﬁltration through

neutral alox and evaporation of the solvent under reduced pressure.

The crude reaction product was puriﬁed by ﬂash chromatography on

silica gel with 1:8 ethyl acetate/n-hexane.

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Accession Codes

CCDC 1875368−1875385 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.

AUTHOR INFORMATION

■

Corresponding Author

*Fax: (+49) 531-391-5309. E-mail: m.tamm@tu-bs.de.

ORCID

̀

Oscar Arias: 0000-0001-5403-3316

Matthias Tamm: 0000-0002-5364-0357

Present Address

J.H.: Abteilung fur NMR-Spektroskopie, Fachbereich Chemie,

̈

Universitat Hamburg, Martin-Luther-King-Platz 6, D-20146

Hamburg, Germany.

(6) (a) Heppekausen, J.; Stade, R.; Goddard, R.; Furstner, A.

̈

Notes

Practical New Silyloxy-Based Alkyne Metathesis Catalysts with

The authors declare no competing ﬁnancial interest.

Optimized Activity and Selectivity Profiles. J. Am. Chem. Soc. 2010,

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Q

DOI: 10.1021/acs.organomet.8b00783

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