183W NMR Spectroscopy Guides the Search for Tungsten Alkylidyne Catalysts for Alkyne Metathesis

Article abstract of DOI:10.1002/anie.202009975

Triarylsilanolates are privileged ancillary ligands for molybdenum alkylidyne catalysts for alkyne metathesis but lead to disappointing results and poor stability in the tungsten series. ¹H,¹⁸³W heteronuclear multiple bond correlation spectroscopy, exploiting a favorable ⁵J-coupling between the ¹⁸³W center and the peripheral protons on the alkylidyne cap, revealed that these ligands upregulate the Lewis acidity to an extent that the tungstenacyclobutadiene formed in the initial [2+2] cycloaddition step is over-stabilized and the catalytic turnover brought to a halt. Guided by the ¹⁸³W NMR shifts as a proxy for the Lewis acidity of the central atom and by an accompanying chemical shift tensor analysis of the alkylidyne unit, the ligand design was revisited and a more strongly π-donating all-alkoxide ligand prepared. The new expanded chelate complex has a tempered Lewis acidity and outperforms the classical Schrock catalyst, carrying monodentate tert-butoxy ligands, in terms of rate and functional-group compatibility.

Full text of DOI:10.1002/anie.202009975

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Accepted Article
Title: 183 W NMR Spectroscopy Guides the Search for Tungsten
Alkylidyne Catalysts for Alkyne Metathesis
Authors: Alois Fürstner, Julius Hillenbrand, Markus Leutzsch,
Christopher P. Gordon, and Christophe Copéret
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183W NMR Spectroscopy Guides the Search for Tungsten Alkylidyne Catalysts for Alkyne Metathesis
Julius Hillenbrand, Markus Leutzsch, Christopher P. Gordon, Christophe Copéret, and Alois Fürstner*
Abstract: Triarylsilanolates are privileged ancillary ligands for molybdenum alkylidyne catalysts for
1
183
alkyne metathesis but lead to disappointing results and poor stability in the tungsten series. H, W
5
Heteronuclear Multiple Bond Correlation (HMBC) spectroscopy exploiting a favorable J-coupling
between the 183W center and the peripheral protons on the alkylidyne cap revealed that these ligands
upregulate the Lewis-acidity to an extent that the tungstenacyclobutadiene formed in the initial [2+2]
cycloaddition step is over-stabilized and the catalytic turnover brought to a hold. The metallacycle itself
does exist in two tautomeric forms and must not be mistaken for a mesomeric resonance extreme, as
evident from spectroscopic and crystallographic data. Guided by the 183W NMR shifts as a proxy for the
Lewis acidity of the central atom and an accompanying chemical shift tensor analysis of the alkylidyne
unit, the ligand design was revisited and a more strongly -donating all-alkoxide ligand prepared. The
new “expanded” chelate complex has a tempered Lewis-acidity and outperforms the classical Schrock
catalyst carrying monodentate tert-butoxy ligands in terms of rate and functional group compatibility.
Introduction
Molybdenum alkylidyne complexes endowed with triarylsilanolate ligands such as 1, the
corresponding ate-complex 2 and the derived bench-stable phenanthroline adduct [1(phen)] set the
standards in the field of alkyne metathesis (Figure 1).1,2,3,4,5 Their functional group compatibility is
largely unrivaled;6,7,8,9,10 it has recently been further improved by the development of a second catalyst
generation distinguished by a tripodal silanolate ligand framework.11,12 Specifically, the “canopy
complex” 3 and relatives maintain the virtues of the parent complex 1, yet allow the chelate effect to
*
J. Hillenbrand, Dr. M. Leutzsch, Prof. A. Fürstner
Max-Planck-Institut für Kohlenforschung
4
5470 Mülheim/Ruhr
E-Mail: fuerstner@kofo.mpg.de
C. P. Gordon, Prof. C. Copéret
Department of Chemistry and Applied Biosciences, ETH Zürich
Vladimir-Prelog-Weg 1-5, 8093 Zürich (Switzerland)
Supporting Information and the ORCID identification numbers for the author(s) of this article can be found
under ###############
1
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be harnessed in form of an improved stability towards protic sites; in conjunction with the well-
balanced electrophilic character and proper steric protection of the operative MoCR unit, this results
in an excellent overall application profile.11
Figure 1. Selected alkylidyne catalysts for alkyne metathesis.
13
From a historic perspective, however, tungsten alkylidyne complexes had taken the lead: Complex
4
a developed by Schrock and coworkers was the first molecularly well-defined catalyst for alkyne
14
15
metathesis; it played a quintessential role in deciphering the mechanism of this transformation and
16
empowered early applications, even though these examples also witnessed that the functional group
tolerance is limited.16,17,18,19 Related tungsten alkylidynes, most notably complexes of types 5 and 6
carrying more electron-withdrawing (partly) fluorinated alkoxide ligands, have also been thoroughly
studied from the organometallic viewpoint,20,21,22,23 but the relatively few available reactivity data do
not convincingly prove that their scope is significantly broader than that of 4a.24
Results and Discussion
Tungsten Alkylidynes with a Tripodal Silanol Ligand Architecture. When seen against this backdrop,
it is perhaps surprising that tungsten alkylidynes with silanolates as ancillary ligands are conspicuously
underrepresented in the literature (Scheme 1). The neopentylidyne complex 8a is accessible from 7a
by salt metathesis but was found to degrade rapidly in solution as well as in the solid state; the derived
phenanthroline adduct [8a(phen)] is stable but kinetically inert and hence no adequate precatalyst
either.25,26 Unligated 8a succumbs to a CH activation process that converts the alkylidyne into an
alkylidene unit: while this transformation is clearly manifested in the NMR data,27 the resulting
2
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25
complex could not be obtained in crystalline form and hence remains structurally undefined. In this
22
context, reference is made to a similar CH activation observed for 5a on treatment with 11. This
decomposition pathway may be the reason why yet another complex carries siloxide ligands devoid of
aromatic substituents in the periphery; once again, the fairly small number of applications that 10a
has found so far, mostly to conjugated 1,3-diyne substrates, make an accurate assessment difficult.28,29
Scheme 1. Known tungsten alkylidynes with silicon-based ligands; decomposition by arene C-H
activation with concomitant formation of an alkylidene unit.
On top of this stability issue comes a practical problem: other than the trichloro alkylidyne 7a, the
30
much more readily available tribromo complexes of type 7b with an aryl substituent on the alkylidyne
invariably led to formation of the corresponding ate-complexes such as 9 on treatment with Ph SiOK
3
in toluene.1a,31 For the intrinsically high Lewis-acidity of tungsten, they do not lose the fourth silanolate
to vacate the necessary coordination site for the incoming substrate under conditions that work well
1
in the molybdenum series. Although 9 exhibits modest catalytic activity at high temperature in the
homo-metathesis of 1-phenyl-1-propyne, it cannot compete at all with its molybdenum counterpart
2
.1,25 Interestingly, however, 9 and relatives cleave the triple bond of aryldiazonium salts [Ar-NN]+
with surprising ease at a rate that outcompetes loss of dinitrogen.32
These issues notwithstanding, we conjectured that the tripodal silanolate ligand framework of the new
“
canopy catalysts” such as 3 might provide an opportunity for the development of truly competent
3
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tungsten catalysts. The phenyl ring forming the basal plane blocks the coordination site trans to the
alkylidyne and should hence prevent detrimental C-H activation as observed for 5a and 8a from
occurring. Moreover, the problem with competing ate-complex formation can likely be solved by a
11
new synthesis route that avoids salt metathesis (Scheme 2): Specifically, it was found that 3 is best
prepared on treatment of [(tBuO) MoCAr] with trisilanole 15; the ligand exchange is entropically
3
driven and benefits from the different acidity of alcohols and silanols. Since deprotonation of the ligand
is unnecessary, irreversible ate-complex formation is effectively prevented.
4
Scheme 2. a) SiCl , EtOH, 0°C  RT, 90% (X = H, 20 g scale); b) see ref. 11; c) 4b, 15, toluene, 79% (16a),
9
8% (16b), quant. (16c), 99% (16d); d) 4c, 15 (R = Ph), toluene.
In consideration thereof, the analogous complex 4b33 was chosen as the starting point for the
preparation of a series of new tungsten alkylidynes; it differs from the classical Schrock catalyst 4a14
only in the substituent of the alkylidyne but is easier to make on scale. In line with our expectation,
34
reaction of 4b with 15a (R = Ph) in toluene afforded 16a in 79% yield (Scheme 2), whereas 4c lacking
the 2,6-dimethyl substitution on the benzylidyne furnished a complex mixture. The structure of 16a in
the solid state (Figure 2) shows great resemblance to the analogous molybdenum canopy complexes,11
even though the short WO bonds bear testimony to the higher Lewis-acidity of tungsten. The catalytic
activity, however, was disappointing and the likely reason readily identified. Thus, addition of 3-hexyne
4
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(5 equiv.) to a solution of 16a in [D
8
]-toluene caused an instant color change from yellow to deep red:
Upon cooling to 20°C, a sharp signal set was recorded that allowed the new species to be identified
as the mixed metallacyclobutadiene complex 17 formed in the initial [2+2] cycloaddition step (Scheme
35
2
). Although 3-hexyne was present in excess, it took one week at ambient temperature for 17 to
convert into metallacyclobutadiene 18 carrying three ethyl substituents by reaction with a second
equivalent. The striking inertia of 17 as the “snapshot” of the “initiation” step of the catalytic cycle
36
implies that cycloreversion is not only rate-determining, as expected, but is much too slow for a
competent catalyst. Such hyper-stabilization of the metallacycle is well known to be the result of an
overly Lewis-acidic metal fragment.20,37,38,39 In this context, it needs to be emphasized that treatment
of an analogous molybdenum canopy complex with 3-hexyne unexpectedly furnished a
metallatetrahedrane complex;11,12 such disparate behavior of tungsten and molybdenum alkylidynes
with the same ancillary ligand sphere is striking and unprecedented.
Figure 2. Representation of the truncated structure of the tungsten alkylidyne complex 16a with a
tripodal ligand framework in the solid state, in which the lateral phenyl rings on the silicon atoms were
removed for clarity (for the full structure, see the SI); likewise, co-crystallized Et O and all H-atoms are
2
not shown. Selected bond lengths (Å) and angles (°): W1-C1 1.759(3), C1-C2 1.453(5), W1-O1 1.880(2),
W1-O2 1.884(2), W1-O3 1.873(2), W1-C1-C2 176.5(3), Si1-O1-W1 171.4 (2), Si2-O2-W1 159.1(2), Si3-
O3-W1 167.0(2)
5
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Figure 3. Representation of the truncated structure of the metallacyclobutadiene complex 17 with a
tripodal ligand framework in the solid state, in which the lateral phenyl rings on the silicon bridges and
all H-atoms were removed for clarity. Selected bond lengths (Å) and angles (°): W1-C1 1.877(2), W1-
C3 1.965(2), W1-C2 2.152(2), W1-O1 1.988(1), W1-O2 1.903(1), W1-O3 1.952(1), W1-C1-C2 78.2(1),
W1-C3-C2 77.2(1), C1-C2-C3 121.5(1), Si1-O1-W1 153.9(1), Si2-O2-W1 155.3(1), Si3-O3-W1 149.8(1).
The coordination geometry about the tungsten center of 17 (Figure 3) is much closer to square
pyramidal (
5
 0.14) than to the trigonal bipyramidal arrangement, which basically all other known
tungstenacyclobutadiene complexes adopt;14b,20,23,24b,37-40 this difference is likely a consequence of the
41
chelate ligand framework and may contribute to the low reactivity of 17. The equatorial siloxide
serves as a notably better -electron donor to tungsten, since the W1-O2 bond (1.903(1) Å), which is
essentially coplanar with the metallacycle, is significantly shorter than the orthogonal W1-O1 (1.988(1)
Å) and W1-O3 (1.952(1) Å) bonds. The metallacyclobutadiene ring itself is highly distorted, in that all
bond lengths and angles are uneven. This finding is in accord with the spectroscopic data which show
that the C

/C’ atoms of the metallacycle in 17 are inequivalent (Figure 4). Most notable are the
different 1JC,W coupling constants: although a detailed interpretation is difficult, we note that the
42
shorter W1C1 “double” bond features the much larger 1JC,W coupling constant. In the derived complex
1
13
1
8, each ethyl substituent gives rise to distinct H and C NMR signals. Importantly, ROESY-NMR
experiments show a dynamic process that mutually interconverts the C

/C’ positions and their ethyl
6
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43
substituents on the NMR timescale, which is interpreted as interconversion of the two tautomers
8a and 18b.44 The observation of two different valence isomers implies that the
1
metallacyclobutadiene formed by [2+2] cycloaddition and the metallacyclobutadiene releasing the
product in the [2+2] cycloreversion step are discrete entities that must not be mistaken for a
mesomeric resonance form.45
Figure 4. Selected spectral and crystallographic data of the metallacyclobutadiene subunits of 17 and
1
8; the insert shows the satellite peaks for the C

/C ’ atoms of 1

Ligand Design Revisited: Guidance by 183W NMR Spectroscopy. The exceptional kinetic stability of 17
and 18 shows that a simple extrapolation of the successful ligand design from molybdenum to tungsten
95
is to no avail. We had previously recognized that a direct look at the central metal via Mo NMR
spectroscopy was tremendously useful for catalyst design.11 Therefore it was hoped that similar
advantage could be taken from 183W NMR in our quest for truly competent tungsten alkylidynes; the
very large spectral range of this spin-½ nucleus and the pleasingly narrow signal line widths suggest so.
7
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However, the dramatically poor receptivity is a serious handicap in that the low- 183W isotope is 49
times less sensitive than the already insensitive 95Mo nucleus.46 This issue had previously been
addressed in various ways, of which 2D shift correlation spectroscopy is arguably the most successful
47
approach. Unfortunately, many tungsten complexes show very small scalar couplings to protons
hydrides), even when directly bound. It is hence truly remarkable that the 183W NMR signal of 16a
4
8
(
1
183
(
 = 517 ppm) can be detected with ease by H, W HMBC measurements using long-range couplings
W
(5
J) between the tungsten center and the protons on the ortho-methyl groups of the benzylidyne unit
Figure 5).49 The fact that such J-coupling does exist is testament to an efficient orbital overlap
5
(
between the alkylidyne and its aryl cap.
1
183
Figure 5. H, W HMBC spectrum of complex 16a.
3
Scheme 3. a) Ph SiOH, toluene, 41%; b) H
2
O, toluene, 50%; c) (tBuO)
3
SiOLi, CH
2
Cl , quant.
2
We are aware of a single prior study into tungsten alkylidynes based on 1D 183W NMR spectroscopy.50
Surprisingly though, the reported 183W NMR shift of [(tBuO)
WCPh] (
= 2526 ppm) is drastically
8
3
W
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1
183
different from what we record by the H, W HMBC pulse sequence for the closely related complex
4
b ( = 166 ppm) carrying two remote methyl substituents on the phenyl ring. Struck by the
W
discrepancy, we sought to rigorously confirm our data point by a number of control experiments. To
this end, spectra were recorded with different offset and sweep widths to exclude possible folding of
the signals. In addition, a 1D 183W NMR spectrum of 4b was recorded using an INEPT magnetization
51
transfer, which proved that the inversely detected and the directly recorded 
W
are identical within

0.2 ppm.
23: δ = 114 ppm
4
b: δ = 166 ppm
5bTHF: δ = 396
1
0b: δ = 458 ppm
1
6b: δ = 460 ppm
1
6c: δ = 475 ppm
8
b: δ = 548 ppm
16a: δ = 517 ppm
Figure 6. Comparison of the 183W NMR shifts ([D
]-toluene) of different tungsten alkylidyne complexes,
8
as determined by H, W HMBC experiments (for 4b, the INEPT-based 1D 183W NMR spectrum referred
to in the text is shown); siloxide containing complexes are color-coded in blue, alkoxide complexes in
red, and fluoroalkoxide in green; Ar = 2,6-dimethylphenyl
1
183
9
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Figure 7. Structure of the unusual benzyl tungsten-oxo complex 19 in the solid state; all hydrogen
atoms except for the benzylic ones were removed for clarity. Selected bond length (Å) and angles (°):
W1C1 2.180(4), W1-O4 1.703(3), W1-O1 1.877(3), W1-O2 1.889(3), W1-O3 1.886(3), W1-C1-C2
1
22.7(3), Si1-O1-W1 159.9(2), Si2-O2-W1 150.5(2), Si3-O3-W1 165.8(2)
1
183
After this validation, H, W HMBC experiments were used to record spectra with an excellent signal-
to-noise ratio in less than 10 min on a Bruker AVIIIHD 400 MHz NMR spectrometer. Figure 6 shows that
the catalytically poorly active podand complex 16a is much more deshielded than the traditional
Schrock-type complex 4b from which it is derived. Since the over-stabilization of the
metallacyclobutadienes 17 and 18 had shown that the metal center in 16a is too Lewis-acidic, it is
tempting to take the deshielding of the 183W NMR signal as a proxy. To test this hypothesis more
closely, additional tungsten alkylidyne complexes were prepared by the standard methods (Schemes
52
2
and 3). It is of note that solvolysis of 4b by Ph SiOH furnished complex 8b bearing three monodentate
3
3
Ph SiO- ligands, which had been beyond reach of salt metathesis because of instant ate-complex
formation. Since the 183W NMR signal of 8b (
= 548 ppm) is even more deshielded than that of 16a,
W
53
this complex is no promising candidate for catalytic alkyne metathesis. Crystals suitable for X-ray
diffraction could not be grown but repeated attempts furnished a different crop that turned out to be
0
the tungsten d oxo alkyl complex 19 (Figure 7), inadvertently formed by addition of moisture to the
alkylidyne unit of 8b. Indeed, this complex can be deliberately made from 8b and one equivalent of
water in toluene; the strong deshielding of the tungsten center ( = 758.1 ppm, see the SI) is well
W
VI
befitting a W =O unit. To the best of our knowledge, 19 is only the second example for the formation
1
0
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of a alkyl ligand by controlled double-protonation of an alkylidyne and the first reported case in which
a monomeric oxo-complex is obtained in this way.54,55,56,57
Next, the phenyl groups on the silicon bridges of the tripodal framework were replaced by more
electron-donating alkyl substituents (note that complex 3 as the currently best canopy catalyst of the
11
molybdenum series carries methyl rather than phenyl groups). As expected, the spectral response is
a shift of the 183W signal to higher frequencies, even though complexes 16b (R = Et) and 16c (R =
2
CH CHMe
2
) are still much more deshielded than the parent complex 4b (Figure 6). The same is true for
33
28
1
0b carrying permethylated siloxide rather than silanolate ligands, which differs from complex 10a
as the only previously disclosed tungsten alkylidyne with a silicon-based ancillary ligand sphere solely
in the substitution pattern of the aryl cap on the alkylidyne. The expectation that the almost identical
183W NMR shifts of 10b, 16b and 16c might translate into similar performance proved correct: all of
them effect the homo-metathesis of 1-methoxy-4-(prop-1-yn-1-yl)benzene at ambient temperature in
58
toluene, but the mass balance was invariably poor because of substantial competing polymerization,
independent of whether MS 5Å was added or not.59
Scheme 4. a) tBuLi, Et
2
O, benzophenone, 125°C  RT, 78%; b) see ref. 11, 79% (Ar = p-MeOC
6
H ); c)
4
4
b, toluene; d) tBuLi, Et
2
O, 125°C  RT, 61% (2.6 g scale); e) 4b, toluene, quant. (1.6 g scale)
1
1
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To complete the picture, Figure
6
also includes the 183W NMR spectrum of
3 3
[ArCW{OC(CH )(CF )
2
}
3
]THF (5b, Ar = 2,6-dimethylphenyl) for comparison. Despite the presence of
THF as an additional neutral donor ligand, the observed signal at 
W
= 396 ppm speaks for a highly
Lewis-acidic alkylidyne. This conclusion fits well to the previously reported finding that tungsten
alkylidynes with hexafluoroalkoxide ligands lead to (over)stabilized metallacycles and are hardly
adequate for alkyne metathesis because they lack activity38,60 or cause competing polymerization.20,61
We hence conclude that silanolates (and highly fluorinated alkoxides) as poor - and -donors
synergize exceedingly well with molybdenum alkylidynes, but the rather disappointing results obtained
with 8b, 10b and 16a-c in concord with their strongly deshielded 183W NMR signals indicate a
fundamental mismatch for the tungsten series that cannot be rectified by peripheral modifications.
Based on these data, it became increasingly clear that the Si-linker strategy had to be abandoned
altogether and the search re-directed toward a ligand with stronger (rather than weaker) donor
VI
properties than tBuO- that the inherently more electropositive W seems to mandate; if one
overshoots, however, catalytic activity will also get lost as a certain Lewis-acidity is necessary for the
[
2+2] cycloaddition to proceed.3,4,24b,24c,36 Once again, we reasoned that a chelate structure might
provide an adequate balance: the WOC angles are expected to be more obtuse and the O-atoms
therefore more sp-hybridized than those of monodentate tBuO-, which should gently upregulate their
-donor character; an improved stability of a chelate architecture comes on top. However, triol 20 as
the carbinol variant of trisilanole 15 is inadequate for this purpose because it is incapable of engaging
VI
VI
11,62
all three –OH groups with a single Mo or W atom (Scheme 4).
As a consequence, the ligand
design was revisited and the expanded C -symmetric triol 22 prepared from 14 in two high-yielding
3
steps. Despite the additional rotatable bond and the resulting higher conformational freedom, this
ligand adopts a favorable “all-up” conformation in the solid state (see the SI); treatment with 4b
furnished the targeted chelate complex 23 in almost quantitative yield (Scheme 4).63
Using the H, W HMBC technique, a 183W resonance at 
upfield of the signal of the parent complex 4b ( = 166 ppm). In line with our expectation that a
chelate carbinol ligand is a somewhat better donor for the geometrically enforced substantial widening
1
183
= 114 ppm was recorded for 23, which is
W
W
64
of the WOC angles, this data point suggests that the tungsten center of 23 is indeed less Lewis-
1
2
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acidic than that of the classical Schrock-type complex 4b, even though the first coordination sphere is
formally made up of (substituted) “tert-butoxy” groups in either case. This view is corroborated by
computational data. Specifically, the calculated energies of the canonical MO’s are notably different:
the tripodal ligand as the better donor raises the energy of the -symmetric orbitals. This effect is
slightly manifested in the (WC) orbitals (HOMO, HOMO-1) but particularly notable in the energy of
the *(WC) orbitals (for details, see the SI). An important consequence of this uplifting of the
*(WC) levels is the fact that the canonical LUMO of 23 is almost entirely ligand-based, with the
largest lobes on the phenyl group that forms the basal plane (Figure 8).
Figure 8. a) Canonical LUMO and LUMO+2 of the tripodal chelate complex 23; b) Graphical
representation of the shielding tensors of the Schrock complex 4b and 23, which is more shielded in
all three principle components; c) the three most relevant orbital couplings of a metal alkylidyne; d)
Chemical Shift Tensor (CST) analysis for the alkylidyne C-atom of both complexes based on Natural
Localized Molecular Orbitals (NLMOs).
1
3
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Table 1. Tabulation of recorded and computed NMR data ([D ]-toluene) of the alkylidyne
8
unit of different tungsten 2,6-dimethylbenzylidyne complexes
Complex

W
(ppm)

C
(ppm)
computed
1JW,C (Hz)
2JW,C (Hz)
exp.
287
283
281
273
264
exp.
270
273
274
297
293
computed
exp.
37.1
30.1
38.7
44.8
44.4
computed
1
1
6a
517
548
458
166
114
285
274
280
281
262
282
289
n.d.
310
304
37.3
38.9
n.d.
43.4
43.4
8
b
0b
4
b
2
3
n.d. = not determined
A detailed analysis of the 13C NMR data by 2-component ZORA DFT calculations65 confirms the
conclusions (for full computational details, see the SI): the alkylidyne C-atom of 23 is clearly more
shielded than that of 4b (Table 1). Because the computed isotropic shifts (iso) and 1JW,C and 2JW,C
42
coupling constants nicely reproduce the experimental values, one can confidently break the data
down to the individual components ii of the chemical shift tensor (iso = (11 + 22 + 33)/3), which
themselves are linked to the computed shielding tensor  (ii  iso,ref  ii). Details of chemical shift
66
tensor analysis (CST) apart, it has been shown that magnetic shielding reflects an increasing energy
gap between high-lying occupied and the low-lying vacant orbitals. For metal alkylidynes, the key
contributions come from the pairs (MC)*(MC), (MC)*(MC), and (CAr)*(MC)
(
Figure 8c).67
The computed data shown in Figure 8 and the results of a Natural Chemical Shielding (NCS) analysis of
all complexes compiled in Table 1 (see also the Supporting Information Table S-13 and Figure S-35) can
be summarized as follows: The contributions arising from (MC) orbitals (and to a lesser extent those
of the (MC) orbitals) decrease upon going from 4b to 23. This reflects the higher-lying *(MC)
orbitals that render 23 less electrophilic, as well as the equally higher-lying *(MC) orbital induced
by the tripodal ligand. It is also interesting to note that the lower-lying of the *(MC) orbitals in 4b
and 23 is associated with the less-deshielding 22 component. This presumably results from the
delocalization of the (MC) system over the dimethylphenyl moiety, which decreases the efficiency
of the orbital coupling. The component oriented along the MC axis (33) is particularly shielded in 23
and points to a highly cylindrical electron density distribution around this M–C bond, as is observed for
1
4
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68
acetylene and molecules with a C axis, making it difficult to draw a direct relation to the HOMO-
∞
LUMO gap for this component.
One can therefore safely conclude that complex 23 must not be mis-taken for just a tethered version
of the classical Schrock catalyst; rather, the tripodal ligand topology itself constitutes an intrinsic
electronic determinant. With the canonical LUMO delocalized on the ligand framework and a relatively
high lying metal-centered *(WC) orbital (LUMO+2), over-stabilization of a derived metallacycle is
improbable; a good level of activity and selectivity may therefore be expected.
Figure 9. Comparison of the intrinsic reactivity of the tripodal complex 23 with that of the ordinary
Schrock catalyst 4b with monomeric tert-butoxy ligands in the homo-metathesis of 1-methoxy-4-
(
prop-1-yn-1-yl)benzene to 4,4’-dimethoxytolane ([D ]-toluene, 0.1 M, 27°C, 5 mol% catalyst loading).
8
Benchmarking. Indeed, the direct comparison shows that 23 outperforms the Schrock catalyst 4b in
terms of rate; moreover, polymerization has essentially ceased, as evident from the mass balance data
69
70
(
Figure 9). When performed in the presence of silanized MS 5Å to sequester the released 2-butyne,
the reaction furnished the desired homo-metathesis product in 85% yield (NMR). Good results were
also achieved in a number of demanding ring closing alkyne metathesis (RCAM) reactions (Scheme 5).
Since acid-sensitive epoxides and aldol subunits as well as many heteroatom donor sites are known to
be incompatible with the parent Schrock catalyst 4a,16,17a,17,71,72 it is notable that complex 23 opens
new opportunities. The successful formation of cycloalkyne 30 bears witness of this notion, since this
compound comprises a very acid- and base-sensitive vinylepoxide subunit, a labile allylic ether as well
73
as a skipped and hence non-thermodynamic array of double and triple bonds. The formation of
1
5
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cycloalkyne 31 is equally indicative: for the presence of a highly elimination-prone aldol subunit and
the additional donor site of the thiazole ring in the side chain, this particular compound had previously
been reported to lay beyond the reach of the classical Schrock catalyst 4a.74
Scheme 5. Representative RCAM reactions; unless stated otherwise, all reactions were performed with
5
mol% of catalyst; [a] with 30 mol%
Conclusions
Although the new prototype complex 23 does not yet rival the best molybdenum alkylidynes known
to date, it constitutes a new lead in the tungsten series. From the conceptual viewpoint, we are inclined
to believe that future ligand design must try to better balance rather than simply increase the Lewis
acidity of the operative tungsten alkylidyne unit; this conclusion, however, is very much at odds with
much of what has been practiced during the last decades. Any such endeavor will likely benefit from
1
183
reverse 2D H, W spectroscopy exploiting long-range coupling, which helped us to uncover the
reasons why the poorly - and -donating silanol ligands that excel in the molybdenum series are by
and large inadequate for tungsten. This insight then resulted in the development of a new catalyst
endowed with an expanded tripodal alkoxide ligand, which outperforms the classical Schrock catalyst.
We plan to take further advantage of 183W NMR as a particularly relevant analytical technique in our
ongoing quest for evermore competent catalysts.
1
6
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Acknowledgements
Generous financial support by the MPG and the Swiss Chemical Industry (Scholarship for C.P.G) is
gratefully acknowledged. We thank Dr. J. Heppekausen for exploratory studies into this topic and for
the preparation of 8a and [8a(phen)], J. Rust and Prof. C. W. Lehmann for solving the X-ray structures,
and the analytical departments of our Institute for excellent support.
For the Table of Contents
Rerouting: Silanolate ligands excel with molybdenum alkylidynes but are found to be inadequate for
tungsten in that they impart undue Lewis acidity; this effect is manifested in the 183W NMR shifts, which
1
183
are surprisingly easy to record by H, W HMBC experiments. As a consequence, the ligand scaffold
was redesigned and an unprecedented chelate alkoxide complex developed which is a new lead in the
quest for competent tungsten-based alkyne metathesis catalysts.
Keywords: Alkyne Metathesis  Alkylidyne Complexes  NMR  Tungsten
1
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1
0
2
V
Only very recently, a new d Re alkylidyne complex was published which shows a remarkable
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J. H. Freudenberger, R. R. Schrock, M. R. Churchill, A. L. Rheingold, J. W. Ziller, Organometallics
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3
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For other types of tungsten alkylidynes with little or no activity in alkyne metathesis, see: a) I.
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J. Heppekausen, Dissertation, TU Dortmund, 2012.
2
0
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26
The structure of [8a(phen)] is contained in the SI. All attempts to remove the phen ligand under
1
conditions that work well in the molybdenum series basically met with failure.
2
7
8
Signals characteristic for a tungsten alkylidene were observed (
H
= 9.83 ppm;  = 296.2 ppm).
C
2
a) S. Lysenko, B. Haberlag, C. G. Daniliuc, P. G. Jones, M. Tamm CatChemCat 2011, 3, 115-118;
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9
022-9032.
2
9
0
For a silica-supported catalyst, see: N. Merle, M. Taoufik, M. Nayer, A. Baudouin, E. Le Roux, R.
M. Gauvin, F. Lefebvre, J. Thivolle-Cazat, J.-M. Basset, Organometallics 2008, 693, 1733-1737.
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Dorries, A. Mayr, Organometallics 1987, 6, 925-931.
3
31
32
33
34
The ate-complex prevails even if less than three equivalents of Ph SiOK are used.
3
A. Lackner, A. Fürstner, Angew. Chem. Int. Ed. 2015, 54, 12814-12818.
The structures of 4b and 10b in the solid state are contained in the SI.
Small amounts of a second alkylidyne species (
C
= 276 ppm) of unknown composition is formed
O.
but easily removed because of its higher solubility in Et
2
35
36
37
ROESY-NMR experiments prove that 17 undergoes dynamic exchange with 3-hexyne.
J. Zhu, G. Jia, Z. Lin, Organometallics 2006, 25, 1812-1819.
M. R. Churchill, J. W. Ziller, J. H. Freudenberger, R. R. Schrock, Organometallics 1984, 3, 1554-
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562.
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8
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H. Ehrhorn, D, Bockfeld, M. Freytag, T. Bannenberg, C. E. Kefalidis, L. Maron, M. Tamm,
Organometallics 2019, 38, 1627-1639.
3
For an instructive study in the molybdenum series, see: D. P. Estes, C. P. Gordon, A. Fedorov, W.-
C. Liao, H. Ehrhorn, C. Bittner, M. L. Zier, D. Bockfeld, K. W. Chan, O. Eisenstein, C. Raynaud, M.
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4
0
1
For a discussion of structure and (transannular) bonding in metallacyclobutadiene complexes,
see: C. H. Suresh, G. Frenking, Organometallics 2010, 29, 4766-4769.
4
A trigonal-bioyramidal geometry is thought to facilitate the retro-[2+2] step.
2
1
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42
The 1JW,C and 2JW,C coupling constants are rather difficult to interpret. The by far dominant terms
to J-coupling originate from Fermi contact interaction, which couples two nuclear spins via s-
electron density; the most important contributions originate form orbitals of s(C), s(W), (WC)
and (CC) character. J-coupling hence gives primarily information about -bonding, whereas
the chemical shifts provide insights into the -interactions as well. For the alkylidyne complexes
(Table 1), one might see a trend that the J values increase for more shielded alkylidynes.
Although care must be taken in making any generalization at this point because of the small data
set, it is of note that the very deshielded alkyl oxo complex 19 (
W
= 758.1 ppm) shows a very
small J-value, which is at least consistent with such a trend.
4
3
4
According to the ROESY experiments, there is no exchange on the NMR timescale between the
metallacyclobutadiene 18 and free 3-hexyne.
4
ǂ
The activation parameters deduced from line-shape and Eyring analyses (H = 13.60.1

1
ǂ
1 1
kcalmol , S = 1.240.29 calmol K ) fit well to such an intramolecular reorganization of the
-system and hence corroborate this interpretation.
45
Several studies noticed the uneven bond lengths of the tungstenacyclobutadienes in the solid
state, but the two different valence isomers have not been observed in solution, cf. ref.
1
4b,20,22a,23,24b,37,38; the only exception is a strongly puckered tungstenacyclobutadiene,
5
approximately halfway en route to a metallatetrahedrane, wherein the metal carries an  -Cp
ring; it showed no catalytic activity, see: M. R. Churchill, J. W. Ziller, L. McCullough, S. F.
Pedersen, R. R. Schrock, Organometallics 1983, 2, 1046-1048.
46
47
48
49
B. E. Mann, Ann. Rep. NMR Spectroscopy 1991, 23, 141-207.
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We know of only two previous studies that took advantage of proton-detected correlation
spectra using long-range couplings, in which bimetallic W/Pd -alkylidene complexes or low-
valent Fischer-type alkenylvinylidene and alkenylcarbyne complexes were studied, see: a) A.
Macchioni, P.S. Pregosin, P. F. Engel, S. Mecking, M. Pfeffer, J.-C. Darran, J. Vaissermann,
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1
998, 36, 807-814.
2
2
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5
0
1
C. G. Young, E. M. Kober, J. H. Enemark, Polyhedron 1987, 6, 255-259.
5
We assume that the shift of the three complexes reported in ref. 50 are caused by folding in the
direct dimension, an artefact that was much more probable with older generations of NMR
95
spectrometers. In ref. 11, we had already voiced concerns about some of the Mo NMR shifts
of analogous molybdenum alkylidynes also reported in ref. 50.
52
0
The chemical shift of d transition metals is mostly affected by the electronegativity and the
relative location of the anionic ligands, compare: L. Latsch, E. Lam, C. Copéret, Chem. Sci. 2020,
1
1, 6724-6735. The tungsten chemical shift in the presence case hence depends on the -
donating ability and the structure of the alkoxide/siloxide ligand (tethered or not-tethered),
leading to an overall correlation with Lewis acidity.
5
3
4
In the homo-metathesis of 1-methoxy-4-(prop-1-yn-1-yl)benzene, the formation of significant
amounts of polymeric material was noticed by NMR, independent of whether the reaction was
performed at ambient temperature or at 80°C in toluene (see the SI).
5
Hydrolysis of [(Me
3 2
CCH )
3
WCCMe
3
] gives dimeric [W
2
O
3
(CH
2
CMe ) ] with a linear
3
6
O=WOW=O core, see: a) I. Feinstein-Jaffe, S. F. Pedersen, R. R. Schrock, J. Am. Chem. Soc.
983, 105, 7176-7177; b) I. Feinstein-Jaffe, D. Gibson, S. J. Lippard, R. R. Schrock, A. Spool, J. Am.
1
Chem. Soc. 1984, 106, 6305-6310; for a case in which the protonation stops at the stage of the
alkylidene, see: S. M. Rocklage, R. R. Schrock, M. R. Churchill, H. J. Wasserman, Organometallics
1
982, 1, 1332-1338.
For a computational study, see: P. Chen, L. Zhang, Z.-L. Xue, Y.-D. Wu, X. Zhang, Inorg. Chem.
017, 56, 2111-2119.
5
5
6
2
5
Related reactions of molybdenum alkylidynes also stop at the stage of the corresponding oxo
alkylidene complexes, see: a) F. Zhai, K. V. Bukhryakov, R. R. Schrock, A. H. Hoveyda, C. Tsay, P.
Müller, J. Am. Chem. Soc. 2018, 140, 13609-13613; b) F. Zhai, R. R. Schrock, A. H. Hoveyda, P.
Müller, Organometallics 2020, 39, 2486-2492.
57
0
A small number of tungsten d oxo alkyl complex are known, which were prepared by different
routes, see: a) J. R. M. Kress, M. J. M. Russell, M. G. Wesolek, J. A. Osborn, J. Chem. Soc., Chem.
Commun. 1980, 431-432; b) . R. M. Kress, M. G. Wesolek, J.-P. Le Ny, J. A. Osborn, J. Chem. Soc.,
Chem. Commun. 1981, 1039-1040; c) C. Zhang, E. O. Schlemper, G. N. Schrauzer,
2
3
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10.1002/anie.202009975
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2
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795-2800; e) D. C. Rosenfeld, D. S. Kuiper, E. B. Lobkovsky, P. T. Wolczanski, Polyhedron 2006,
5, 251-258
58
6d is hardly reactive, likely because the Si(iPr) groups shield the tungsten center; a similar
2
observation was made for the analogous molybdenum alkylidyne, see ref. 11
For the use of molecular sieves as butyne-sequestering agents, see ref. 1
5
9
0
6
B. Haberlag, M. Freytag, C. D. Daniliuc, P. G. Jones, M. Tamm, Angew. Chem. Int. Ed. 2012, 51,
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We confirm this latter observation in that 5bTHF led to massive competing polymerization when
applie to the model reaction (homo-metathesis of 1-methoxy-4-(prop-1-yn-1-yl), cf. Figure 9);
for details, see the SI
The wider bindings site in between the three –OH groups of 20 is not ideal for binding to a single
VI
VI
Mo or W center; moreover, the unfavorable conformation adopted by 20 is a kinetic handicap
for chelate complex formation.
1H NMR spectra recorded in [D
spectra of dilute samples of 23 in C
in addition to the signals assigned to the major C
]-THF are fully consistent with a C
or [D ]-toluene show trace amounts of a second species
-symmetric complex; because the amount of
-symmetric species in solution;
8
3
6
D
6
8
3
this second complex increases upon increasing the concentration, it is assumed to be a dimeric
or oligomeric entity; this conclusion is consistent with the recorded DOSY data, see the SI.
The computed WOC angles are as follows: 4b: 125.4/142.7/144.0°; 23: 147.0/150.6/152.3°;
for computational details, see the SI.
64
65
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When the concentrated solutions are kept overnight at ambient temperature, small amounts of
polymer are visible in the NMR spectra of the crude material.
70
71
72
2
3 seems to react with the surface SiOH groups of ordinary molecular sieves; therefore the
powder was pre-treated with Me SiCl to cap the reactive surface sites.
3
For the incompatibility of 4a with epoxides, see: V. Hickmann, A. Kondoh, B. Gabor, M. Alcarazo,
A. Fürstner, J. Am. Chem. Soc. 2011, 133, 13471-13480.
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2
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