Highly Reactive Cationic Molybdenum Alkylidyne N-Heterocyclic Carbene Catalysts for Alkyne Metathesis

Article abstract of DOI:10.1021/acs.organomet.1c00175

The tetracoordinated cationic molybdenum alkylidyne N-heterocyclic carbene (NHC) complexes [Mo(CC6H4-p-OMe)(IMes)(OCMe(CF3)2)2][BPh4] (Mo5) and [Mo(CC6H4-p-OMe)(IMes)(OCMe(CF3)2)2][B(ArF)4] (Mo6, IMes = 1,3-dimesitylimidazol-2-ylidene)) were synthesized from the pentacoordinated progenitor Mo(CC6H4-p-OMe)(IMes)(OCMe(CF3)2)2(OTf) (Mo4). Complexes Mo4-Mo6 were evaluated for their ability to catalyze the self-metathesis of several internal alkynes. The presence of a triflate group facilitates formation of a cationic species while preformation of the cationic molybdenum center in molybdenum alkylidyne NHC complexes indeed results in a strong increase in catalyst productivity and activity, also in the presence of functional groups, compared to previously reported neutral congeners. The most striking feature of this class of tetracoordinate cationic complexes is the excellent catalytic activity in the alkyne metathesis of non-protic substrates, thereby supporting our previously published proposal of a tetracoordinate cationic active species in alkyne metathesis formed from the neutral, pentacoordinate molybdenum alkylidyne NHC progenitors. Catalyst productivity, expressed as turnover number, reached 20 000 in the self-metathesis of 1-phenyl-1-propyne (S1) using Mo(CC6H4-p-OMe)(1,3-dimesitylimidazol-2-ylidene)(OCMe(CF3)2)2[B(ArF)4] (Mo6) and 5-(benzyloxy)-2-pentyne (S2) at catalyst loadings as low as 0.005 mol %.

Full text of DOI:10.1021/acs.organomet.1c00175

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Highly Reactive Cationic Molybdenum Alkylidyne N‑Heterocyclic
Carbene Catalysts for Alkyne Metathesis
Jonas Groos, Philipp M. Hauser, Maximilian Koy, Wolfgang Frey, and Michael R. Buchmeiser*
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ABSTRACT: The tetracoordinated cationic molybdenum alkyli-
dyne N-heterocyclic carbene (NHC) complexes [Mo(CC6H4-p-
OMe)(IMes)(OCMe(CF3)2)2][BPh4] (Mo5) and [Mo(
CC6H4-p-OMe)(IMes)(OCMe(CF3)2)2][B(ArF)4] (Mo6, IMes =
1,3-dimesitylimidazol-2-ylidene)) were synthesized from the
pentacoordinated progenitor Mo(CC6H4-p-OMe)(IMes)-
(OCMe(CF3)2)2(OTf) (Mo4). Complexes Mo4−Mo6 were
evaluated for their ability to catalyze the self-metathesis of several
internal alkynes. The presence of a triflate group facilitates
formation of a cationic species while preformation of the cationic
molybdenum center in molybdenum alkylidyne NHC complexes
indeed results in a strong increase in catalyst productivity and
activity, also in the presence of functional groups, compared to previously reported neutral congeners. The most striking feature of
this class of tetracoordinate cationic complexes is the excellent catalytic activity in the alkyne metathesis of non-protic substrates,
thereby supporting our previously published proposal of a tetracoordinate cationic active species in alkyne metathesis formed from
the neutral, pentacoordinate molybdenum alkylidyne NHC progenitors. Catalyst productivity, expressed as turnover number,
reached 20 000 in the self-metathesis of 1-phenyl-1-propyne (S1) using Mo(CC6H4-p-OMe)(1,3-dimesitylimidazol-2-
ylidene)(OCMe(CF3)2)2[B(ArF)4] (Mo6) and 5-(benzyloxy)-2-pentyne (S2) at catalyst loadings as low as 0.005 mol %.
tarting from “ill-defined” metathesis catalysts, the number
Sof well-defined homogeneous alkyne metathesis catalysts
has grown unceasingly to the point that structure and reactivity
can be precisely controlled.1 The groups of Schrock,2−10
NHC complexes,45 we aimed on the synthesis of a cationic,
preferably tetracoordinate complex, which would represent the
stable active species in alkylidyne metathesis reactions.
We started with molybdenum p-methoxybenzylidyne tris-
(hexafluoro-tert-butoxide) complexes bearing sterically less
demanding NHCs, e.g., 1,3-diisopropylimidazol-2-ylidene
(IiPr); however, their reaction with N,N-dimethylanilinium
tetrakis(3,5-bis(trifluoromethyl)phenyl)borate only resulted in
unstable tetracoordinate cationic species, decomposing in the
course of a few hours. Also, coordination of sterically
demanding carbenes, e.g., 1,3-dimesitylimidazol-2-ylidene
(IMes), to Mo(CC6H4-p-OMe)(DME)(OCMe(CF3)2)3
(Mo1, DME = 1,2-dimethoxyethane) resulted in immediate
decomposition and formation of several byproducts. To
circumvent these issues, we focused on the synthesis of new
precursors with sterically less demanding anionic (X−) ligands.
For these purposes, the triflate ligand was deemed as the target,
additionally serving as an excellent leaving group. Hence, the
Fu
̈
rstner,11−15 Tamm,16−25 Veige,26−28 Zhang,29−31
Moore,32−34 Fischer,35−37 and Cummins38−40 developed
current state-of-the-art catalysts bearing silanolates, amides,
fluorinated alkoxides, iminato or multidentate ligands. A
common feature is the tetracoordinate nature of the active
catalyst systems. In contrast, we recently reported on
pentacoordinate molybdenum alkylidyne complexes, e.g.,
Mo(CC6H4-p-OMe)(NHC)(OCMe(CF3)2)3, bearing
mono-, bi-, and tridentate N-heterocyclic carbene (NHC)
ligands, which showed moderate productivity in alkyne
metathesis reactions with turnover numbers (TONs) <
1000.41 For strongly σ-donating NHCs, the active species
was postulated to be the cationic tetracoordinate species after
dissociation of one fluorinated alkoxide as weakly coordinating
anion (WCA). Encouraged by the successful implementation
of triflate ligands into molybdenum imido alkylidene NHC
catalysts,41−44 we targeted the synthesis of complexes with
better leaving groups than the previously employed fluorinated
alkoxides, namely, triflates, to shift the equilibrium toward the
cationic species, thereby further enhancing the formation of
the active species. In view of the results obtained with tetra-,
penta-, and hexacoordinated, cationic tungsten alkylidyne
Received: March 22, 2021
Published: April 13, 2021
© 2021 The Authors. Published by
American Chemical Society
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(trifluoromethyl)phenyl)borate, (B(ArF)4 ), via reaction of
Mo4 with NaB(ArF)4 to yield the tetracoordinate species
[Mo(CC6H4-p-OMe)(DME)(OCMe(CF3)2)2][B(ArF)4]
Mo6 in 86% isolated yield.
−
trisalkoxide precursor Mo1 was carefully treated with 1 equiv
of triflic acid to obtain the molybdenum alkylidyne
monotriflate bisalkoxide complex Mo(CC6H4-p-OMe)-
(DME)(OCMe(CF3)2)2(OTf) Mo2 in 64% isolated yield.
However, coordination of the NHC toward Mo2 resulted in
the formation of [1,3-dimesityl-2-methylimidazolium][OTf],
most probably formed through carbene-induced DME
activation, followed by transmethylation as previously shown
for tungsten alkylidene complexes.46 The DME ligand was
therefore exchanged by two THF molecules by stirring a
solution of Mo2 in THF for 1 h to yield Mo(CC6H4-p-
OMe)(THF)2(OCMe(CF3)2)2(OTf) Mo3 in 84% isolated
yield (Scheme 1). The monotriflate precursor Mo3 was then
Crystals of the monotriflate complex Mo4 suitable for single
crystal X-ray analysis were obtained from CH2Cl2 and n-
pentane. Complex Mo4 crystallizes in the monoclinic space
group P21/n (a = 1270.26(5) pm, α = 90°, b = 1757.39(8) pm,
β = 106.230(2)°, c = 1922.24(8) pm, γ = 90°, Z = 4) (Figure
1). The metal complex adopts a slightly distorted trigonal
Scheme 1. Synthesis of the Neutral Monotriflate NHC
Complex Mo4 (Middle) and the Respective Cationic NHC
Complexes with B(Ph)4 (Mo5) and B(ArF)4 (Mo6) as
Anion
−
−
Figure 1. Single crystal X-ray structure of complex Mo4. Selected
bond lengths [pm] and angles [deg]: Mo(1)−C(30) 176.75(14),
Mo(1)−O(1) 188.23(10), Mo(1)−O(2) 188.69(10), Mo(1)−C(1)
220.48(13), Mo(1)−O(4) 235.24(11), C(30)−Mo(1)−O(1)
103.03(5), C(30)−Mo(1)−O(2) 102.28(5), O(1)−Mo(1)−O(2)
113.75(5), C(30)−Mo(1)−C(1) 90.36(6), O(1)−Mo(1)−C(1)
117.76(5), O(2)−Mo(1)−C(1) 122.14(5), C(30)−Mo(1)−O(4)
175.08(5), O(1)−Mo(1)−O(4) 79.52(4), O(2)−Mo(1)−O(4)
80.29(4), C(1)−Mo(1)−O(4) 84.72(5).
bipyramidal (TBP) geometry based on the τ5 value of 0.88
with the alkylidyne moiety and the triflate in the apices and the
NHC as well as the alkoxides in the plane.47 The alkoxides are
strongly bound to molybdenum as judged from the Mo−O
bond lengths (Mo−O1: 188.23 pm and Mo−O3: 188.69 pm)
and do not experience any trans influence due to their in-plane
position in the TBP geometry. In contrast, the Mo−Otriflate
bond (235.24 pm) is exceptionally long compared to the
structurally related alkylidene complex Mo(N-2,6-Me2-C6H3)-
(CH(CMe2Ph))(IMesH2)(OTf)(OCMe(CF3)2) in which the
Mo−Oalkoxide and the Mo−Otriflate bonds are 200.9 and 218.5
pm, respectively. It is even substantially longer compared to
the alkylidene complex Mo(N-3,5-Me2-C6H3)(CH-
(CMe2Ph))(IMes)(OTf)2, which is already considered cati-
onic at room temperature in solution with a Mo−Otriflate bond
length of 214.0 pm in the solid state.48,49 The cationic
character of Mo4 with a dissociated triflate in solution is
further vindicated by the triflate signal at δ = −78.92 pm in
CD2Cl2 at 25 °C, virtually identical to the signal of “free”
triflate found for NBu4 OTf− (−78.89 ppm).48 The alkylidyne
+
reacted with IMes to yield 86% of Mo(CC6H4-p-OMe)-
(IMes)(OCMe(CF3)2)2(OTf) Mo4 (Scheme 1). The NHC
coordination toward Mo3 containing THF proceeded
smoothly in contrast to the DME-containing precursors Mo1
and Mo2. Thereupon, the triflate in Mo4 was replaced by the
noncoordinating anion B(Ph)4− via reaction with NaB(Ph)4 to
yield the tetracoordinate species [Mo(CC6H4-p-OMe)-
(DME)(OCMe(CF3)2)2][BPh4] Mo5 in 88% isolated yield.
Alternatively, triflate was exchanged by tetrakis(3,5-bis-
ligand in Mo4 shows an almost perfectly linear arrangement
with a Mo−C(30)−C(31) angle of 179.05°.
The cationic complex Mo6 crystallizes in the triclinic space
̅
group P1 (a = 1575.70(10) pm, α = 97.815(4)°, b =
1618.41(11) pm, β = 95.823(4)°, c = 1650.04(10) pm, γ =
95.894(3)°, Z = 2) and adopts a slightly distorted tetrahedral
structure (τ4 = 0.91, τ4′ = 0.90) (Figure 2). The alkylidyne
bond is shorter (173.90 pm) compared to the neutral species
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(S2), 2-pentyne-5-yl benzoate (S3), 2-pentyne-5-yl 4-nitro-
benzoate (S4), 2-pentyne-5-yl tosylate (S5), and 5-(4-
methylthio-benzoate)-2-pentyne (S6) were chosen as sub-
strates. Substrates S1−S6 (Figure 3) were tested under
optimized reactions conditions with a catalyst loading of 0.1
mol % in 1,2-dichlorobenzene at 35 °C in the presence of 5 Å
MS.
Figure 2. Single crystal X-ray structure of complex Mo6. Selected
bond lengths [pm] and angles [deg]: Mo1−C(30) 173.9(3), Mo1−
O(2) 187.6(2), Mo1−O(1) 187.7(2), Mo1−C(1) 2.132(3), C(30)−
Mo1−O(2) 106.37(12), C(30)−Mo1−O(1) 106.57(12), O(2)−
Mo1−O(1) 114.39(10), C(30)−Mo1−C(1) 95.44(12), O(2)−
Mo1−C(1) 114.01(10), O(1)−Mo1−C(1) 117.26(11). The B-
Figure 3. Substrates S1−S6.
Complexes Mo4, Mo5, and Mo6 were able to reach full
conversion in all metathesis experiments (Table 1), in the case
of Mo6, in less than 3 h. Mo6 turned out to be the most active
catalyst in all alkyne metathesis reactions. Full conversion for
S1, S2, S3, and S5 was reached within 1 h. Reaction with Mo4
and Mo5 proceeded slightly slower than with Mo6; values for
conversion and TONs after 3 h are listed in the Supporting
Information (Table S1). The alkyne metathesis reactions were
repeated with a lower catalyst loading using Mo6 and carried
out at different substrate concentrations. A diluted system with
a substrate concentration of 13 mM, the hitherto used system
with a concentration of 130 mM and a more concentrated
system with a substrate concentration of 660 mM were chosen.
The catalyst loading was reduced to 0.05 mol %, allowing
TONs up to 20 000. All reactions were stirred for 48 h at 35
°C due to the low concentration in the diluted system and the
therefore expected slow conversion rate (Table 1). Values for
the conversion after 3 h can be found in the Supporting
Information (Table S2). For substrate S1, the substrate
concentration was additionally set to 1.3 M as an increase in
substrate concentration usually leads to an increased
conversion. This is particularly the case with S1. In the highly
diluted system (13 mM), only 41% of the starting material was
consumed; however, conversion increased up to 99%, resulting
in a TON of 19 900 for a substrate concentration of 1.3 M.
However, Mo4 and Mo6 are decomposing upon exposure to
air for 1 day (Figures S1−S4) or when reacted with alkynols
such as 3-pentyn-1-ol (Figures S5 and S6).
−
(ArF)4 anion was omitted for clarity.
Mo4 (176.75 pm). The NHC is bound considerably stronger
(213.20 pm) compared to Mo4 (220.48 pm), which is most
probably a result of the substantial charge delocalization
between the NHC and the formally cationic metal center,
rendering the cationic complexes rather “soft” according to the
HSAB principle, as already elaborated for NHC alkylidene
complexes.50 In contrast, the alkoxides are almost unaffected in
the cationic complex and the bond lengths are only negligibly
shorter (Mo6: 187.6 pm, 187.7 pm; Mo4: 188.2 pm, 188.7
pm). Particularly interesting is the comparison between the
neutral and cationic complex regarding the ligand arrangement.
In the neutral monotriflate complex Mo4, the IMes ligand is
orthogonal to the alkylidyne ligand and in the same plane than
the two alkoxides. The triflate is trans to the IMes ligand.
In the cationic species Mo6, one of the mesityl substituents
occupies the now empty former triflate site, whereas the other
mesityl substituent engages in a π−π interaction with the
aromatic para-anisole alkylidyne substituent, therefore shield-
ing and stabilizing the cationic species. The aromatic systems
of the anisole and the mesityl are displaced in a parallel manner
with an interlayer distance of ca. 340 pm, typical for π-stacking.
Complexes Mo4−Mo6 were examined for their activity in
the self-metathesis of 1-phenyl-1-propyne (S1) in three
different solvents (toluene, 1,2-dichloroethane, 1,2-dichlor-
obenzene) and at different temperatures (35 and 80 °C) in the
presence of powdered 5 Å molecular sieves (MS) as 2-butyne
scavenger14 using a catalyst loading of 0.1 mol %. Gratifyingly,
both the neutral monotriflate complex Mo4 and the
tetracoordinate cationic complexes Mo5 and Mo6 showed an
unexpected high level of activity. In the case of Mo6, full
conversion of S1 was reached in less than 1 h with a catalyst
loading of 0.1 mol % in 1,2-dichlorobenzene. The novel triflate
complexes thus outperform the to date most active neutral
molybdenum alkylidyne NHC catalysts51 by far: tested under
similar conditions, Mo(CC6H4OMe)(NHC)(OCMe-
In terms of catalytic performance, the cationic molybdenum
alkylidyne NHC complexes are at least comparable to the
̈
catalysts recently published by Furstner et al., which showed
exceptional tolerance against functional groups and to some
degree against water at high catalyst loadings (>1 mol %),11,52
as well as to the catalysts reported by Tamm et al., which
reached TONs > 180 000, though only for carefully and highly
purified S1,17,18 in that the catalysts presented here allow for
high TONs for various substrates without extensive purifica-
tion. Furthermore, the turnover frequencies after 5 min
(TOF5min) were determined for the conversion of S1−S6
with Mo6 at a catalyst loading of 0.005% and a concentration
of 660 mM (Table 2). For substrate S2, a TON of 3600 was
reached after 5 min, translating into a TOF5min of 12 s−1.
In conclusion, we have synthesized the first cationic
molybdenum alkylidyne NHC complexes with the aim to
increase activity, productivity, and functional group tolerance
51
(CF3)2)3 with NHC = 1,3-diisopropylimidazol-2-ylidene
reached a conversion of 37% after 3 h. These findings are
fully in line with our hypothesis of a cationic tetracoordinate
NHC molybdenum alkylidyne species acting as the active
species.41,51 Complexes Mo4, Mo5, and Mo6 were then
investigated with regard to their activity in self-metathesis
reactions of a wider substrate scope. 5-(Benzyloxy)-2-pentyne
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Table 1. Turnover Numbers (TONs) and Conversion in Alkyne Self-Metathesis of Substrates S1−S6 with Complexes Mo4,
a
Mo5, and Mo6 at Different Concentrations and Catalyst Loadings
a
Solvent: 1,2-dichlorobenzene; internal standard: tBu-benzene; temperature: 35 °C; with the addition of ground 5 Å molecular sieves as 2-butyne
scavenger.14
and demonstrated that they indeed present the catalytic species
that form from neutral pentacoordinate molybdenum
alkylidyne NHC complexes via dissociation of one anionic
ligand. In that regards, particularly the triflate complexes were
found suitable. Tetracoordinate, solvent-free cationic com-
plexes demonstrate high catalytic activity and productivity in
the self-metathesis of several alkyne substrates, do not require
extensive purification of the substrates, and demonstrate
tolerance versus ether, ester-, thioether, and nitro groups.
Table 2. Conversion and Turnover Frequencies after 5 min
(TOF5min) in the Alkyne Self-Metathesis of Substrates S1−
S6 with Complex Mo6 at a Concentration of 660 mM
substrate: catalyst
substrate
conversion [%]
TOF5min [s−1
]
5000
5000
2500
2500
2500
2500
S1
S2
S3
S4
S5
S6
31
72
37
40
34
33
5
12
3
3.3
2.6
2.6
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3JH‑H = 8.9 Hz, 2H), 3.84 (s, 3H), 2.17 (s, 6H), 2.02 (s, 12H), 1.77 (s,
EXPERIMENTAL SECTION
■
4
6H) ppm. 19F NMR (376 MHz, CD2Cl2): δ = −76.92 (q, JF‑F = 9.0
General Information. All operations were performed under an
inert gas atmosphere (N2), either with standard Schlenk techniques or
in a glovebox (LabMaster 130, MBraun, Garching, Germany), unless
stated otherwise. Diethyl ether, n-pentane, CH2Cl2, tetrahydrofuran,
and toluene were dried by a solvent purification system (SPS,
MBraun). All NMR measurements were conducted on a Bruker
Avance III 400 instrument. Chemical shifts are reported in ppm
relative to the solvent signal; coupling constants are listed in Hz. 13C
NMR spectra were measured using broadband decoupling. Single-
crystal X-ray measurements were carried out on a Bruker Kappa
APEXII Duo diffractometer with Mo Kα radiation at the Institute of
Organic Chemistry, University of Stuttgart. Crystal data have been
deposited with the Cambridge Crystallographic Data Centre
(CCDC): Mo4 CCDC 2063125, Mo6 CCDC 2063124.
Reagents and starting materials were purchased from ABCR
(Karlsruhe, Germany), Alfa Aesar (Karlsruhe, Germany), and Merck
(Munich, Germany) and used as received unless stated otherwise. S1
was dried over CaH2 and distilled under N2. All solvents were
purchased anhydrous and stored over 3 Å molecular sieves. 1,3-
Dimesitylimidazol-2-ylidene,53 S2,21 S3,21 S4,21 S5,21 S6,52 and
Mo151 were synthesized according to the literature.
4
Hz), −78.67 (q, JF‑F = 9.2), −78.93 ppm. 13C NMR (100 MHz,
CD2Cl2): δ = 311.0, 185.6, 162.8, 142.9, 137.7, 135.3, 134.0, 132.3,
130.6, 128.2, 122.80 (q, J = 286.7 Hz), 122.76 (q, J = 286.6 Hz),
113.7, 85.25 (hept, J = 30.1 Hz), 56.2, 21.3, 20.9, 17.6 ppm. Anal.
Calcd (%) for C38H37F15MoN2O6S: C, 44.28; H, 3.62; N, 2.72.
Found: C, 44.29; H, 3.74; N, 2.80.
Mo5. Na[B(Ph)4] (33.1 mg, 0.10 mmol, 1 equiv) was slowly added
to a solution of Mo4 (100 mg, 0.10 mmol) in CH2Cl2 (4 mL), cooled
to −40 °C, and the mixture was stirred for 1 h at room temperature.
The resulting suspension was filtered through a pad of Celite, the
solvent was removed in vacuo, and the oily residue was excessively
coevaporated with n-pentane (10 × 2 mL) and diethyl ether (10 × 2
mL). The residue was washed with n-pentane (2 × 6 mL) and
crystallized from CH2Cl2/n-pentane. The product was isolated in the
1
form of dark red crystals (103 mg, 0.09 mmol, 88%): H NMR (400
MHz, CD2Cl2): δ = 7.32 (s, 8H), 6.95 (m, 8H), 6.84 (s, 4H), 6.82−
6.74 (m, 4H), 6.77−6.70 (m, 4H), 6.67−6.60 (m, 2H), 3.82 (s, 3H),
2.18 (s, 6H), 1.91 (s, 12H), 1.76 (s, 6H) ppm. 19F NMR (376 MHz,
CD2Cl2): δ = −76.93 (q, J = 9.0 Hz), −78.74 (q, J = 9.1 Hz) ppm.
13C NMR (100 MHz, CD2Cl2): δ = 310.7, 185.3, 165.53−163.58
(m), 162.8, 142.9, 137.6, 136.4, 135.1, 133.9, 132.1, 130.6, 127.6,
126.2, 122.8 (q, J = 286.9 Hz), 122.7 (q, J = 286.6 Hz), 122.3, 113.7,
85.31 (hept, J = 29.7 Hz), 56.2, 21.3, 20.9, 17.5 ppm. Anal. Calcd (%)
for C61H57BF12MoN2O3·(CH2Cl2)0.5: C, 59.41; H, 4.70; N, 2.25.
Found: C, 59.62; H, 4.79; N, 2.34.
Mo2. A solution of triflic acid (28 mg, 0.19 mmol) in 2 mL of
toluene:DME (5:1) was cooled to −40 °C and slowly added at −40
°C to Mo1 (150 mg, 0.18 mmol) dissolved in 4 mL of toluene. The
reaction mixture was stirred for 4 h at room temperature; then the
volatiles were removed in vacuo. The residue was coevaporated with
n-pentane (8 × 1 mL) and diethyl ether (8 × 1 mL), washed with n-
pentane (2 × 2 mL), and crystallized from diethyl ether/n-pentane.
The product was isolated in the form of red crystals (92 mg, 0.11
Mo6. A suspension of Na[B(ArF)4] (343 mg, 0.39 mmol, 1 equiv)
in CH2Cl2 (12 mL), cooled to −40 °C, was slowly added to a solution
of Mo4 (400 mg, 0.39 mmol) dissolved in CH2Cl2 (12 mL), and the
resulting mixture was stirred for 1 h at room temperature. Then, the
suspension was filtered through a pad of Celite, the solvent was
removed in vacuo, and the oily residue was excessively coevaporated
with n-pentane (10 × 2 mL) and diethyl ether (10 × 2 mL). The
residue was washed with n-pentane (2 × 6 mL) and crystallized from
CH2Cl2/n-pentane. The product was isolated in the form of dark red
crystals (580 mg, 0.24 mmol, 86%): 1H NMR (400 MHz, CD2Cl2): δ
= 7.84−7.63 (m, 10H), 7.56 (s, 4H), 6.86 (s, 4H), 6.77 (d, 3JH‑H = 9.0
Hz, 2H), 6.65 (d, 3JH‑H = 9.0 Hz, 2H), 3.81 (s, 3H), 2.16 (s, 6H), 2.00
(s, 12H), 1.79 (s, 6H) ppm. 19F NMR (376 MHz, CD2Cl2): δ =
−62.87 (24F), −76.95 (q, 4JF‑F = 8.7 Hz), −78.78 (q, 4JF‑F = 9.0 Hz)
ppm. 13C NMR (100 MHz, CD2Cl2): δ = 311.1, 185.0, 162.5, 162.0,
161.5, 161.0, 142.8, 137.2, 134.8, 134.5, 133.5, 131.5, 130.2, 128.7
(qq, J = 30.8 Hz, 5.7 Hz), 126.7, 126.0, 123.2, 122.3 (q, 1JC‑F = 286.7
1
3
mmol, 64%): H NMR (400 MHz, CDCl3): δ = 7.36 (d, JH‑H = 8.9
Hz, 2H), 6.81 (d, 3JH‑H = 9.0 Hz, 2H), 4.41 (s, 3H), 4.29 (td, 3JH‑H
=
11.4 Hz, 2JH‑H = 3.2 Hz, 1H), 3.91−3.83 (m, 2H), 3.80 (s, 3H), 3.57−
3.51 (m, 1H), 3.37 (s, 3H), 2.02 (s, 3H), 1.97 (s, 3H) ppm; 19F NMR
4
(376 MHz, CDCl3): δ = −76.35 (q, JF‑F = 9.7 Hz, 3F), −76.73 to
4
−76.85 (m, 3F), −77.30 (q, JF‑F = 3.4 Hz, 3F), −77.55 to −77.69
(m, 3F), −77.92 to −78.10 (m, 3F) ppm; 13C NMR (100 MHz,
1
CDCl3): δ = 303.2, 161.3, 136.8, 133.7, 123.7 (q, JC‑F = 287 Hz,
CF3), 123.6 (q, 1JC‑F = 287 Hz, CF3), 123.5 (q, 1JC‑F = 287 Hz, CF3),
123.1 (q, 1JC‑F = 287 Hz, CF3), 119.5 (q, 1JC‑F = 318 Hz, CF3), 113.6,
85.6 (hept., 1JC‑F = 29.4 Hz), 84.5 (hept., 1JC‑F = 29.3 Hz), 76.0, 73.2,
69.4, 59.6, 55.6, 18.9, 18.8 ppm. Anal. Calcd (%) for
C21H23F15MoO8S: C, 30.90; H, 2.84. Found: C, 30.83; H, 2.91.
Mo3. Mo2 (500 mg, 0.61 mmol) was dissolved in 10 mL of THF
and stirred for 1 h at room temperature. All volatiles were removed in
vacuo; the residue was coevaporated with n-pentane (5 × 2 mL) and
diethyl ether (5 × 2 mL) and crystallized from diethyl ether/n-
pentane/CH2Cl2. The product was isolated in the form of red crystals
1
Hz), 122.2 (q, JC‑F = 286.7 Hz), 120.5, 117.46, 113.2, 85.0 (hept,
2JC‑F = 30.3 Hz, C(CF3)2Me), 55.6, 20.7, 20.4, 16.9 ppm. Anal. Calcd
(%) for C69H49BF36MoN2O3: C, 47.50; H, 2.83; N, 1.61. Found: C,
47.54; H, 3.02; N, 1.66.
1
(450 mg, 0.52 mmol, 84%): H NMR (400 MHz, CDCl3): δ = 7.39
3
3
(d, JH‑H = 9.0 Hz, 2H), 6.82 (d, JH‑H = 9.1 Hz, 2H), 4.62 (s, 2H),
4.37 (s, 2H), 3.80 (s, 3H), 3.76−3.71 (m, 4H), 2.13−2.05 (m, 4H),
1.99 (s, 3H), 1.90 (s, 3H), 1.82−1.77 (m, 4H) ppm. 19F NMR (376
MHz, CDCl3): δ = −77.16 to −77.48 (m, 9F), −77.54 to −77.94 (m,
6F) ppm. 13C NMR (100 MHz, CD2Cl2): δ = 304.7, 161.8, 137. 7,
134.3, 124.23 (q, J = 287.7 Hz), 123.99 (q, J = 287.2 Hz), 123.86 (q, J
= 287.4 Hz), 120.07 (q, J = 317.5 Hz), 113.8, 86.20 (hept, J = 28.4
Hz), 84.84 (hept, J = 28.2 Hz), 79.9, 69.2, 56.0, 26.5, 25.8, 19.7, 19.1
ppm. Anal. Calcd (%) for C25H29F15MoO8S: C, 34.49; H, 3.36.
Found: C, 34.15; H, 3.41.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00175.
Detailed experimental procedures, NMR data and
spectra for all compounds (PDF)
Mo4. A cold (−40 °C) solution of 1,3-dimesitylimidazol-2-ylidene
(139.9 mg, 0.46 mmol, 1 equiv) in toluene (50 mL) was added to a
solution of Mo3 (400 mg, 0.46 mmol) in toluene (50 mL), cooled to
−40 °C. The reaction was stirred for 3 h; then the solvent was
removed in vacuo and the residue was coevaporated with n-pentane (5
× 4 mL) and diethyl ether (5 × 4 mL). The resulting solid residue
was washed with n-pentane (3 × 10 mL) and crystallized from diethyl
ether/n-pentane. The product was isolated in the form of orange
crystals (405 mg, 0.39 mmol, 86%): 1H NMR (400 MHz, CD2Cl2): δ
Accession Codes
CCDC 2063124 and 2063125 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing 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.
3
= 7.95 (s, 2H), 6.86 (s, 4H), 6.77 (d, JH‑H = 9.0 Hz, 2H), 6.66 (d,
1182
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Organometallics 2021, 40, 1178−1184

Organometallics
pubs.acs.org/Organometallics
Article
structure of W(C3Et3)[OCH(CF3)2]3. A higher order mechanism
for acetylene metathesis. Organometallics 1984, 3 (10), 1563−1573.
(10) Schrock, R. R.; Clark, D. N.; Sancho, J.; Wengrovius, J. H.;
Rocklage, S. M.; Pedersen, S. F. Tungsten(VI) neopentylidyne
complexes. Organometallics 1982, 1 (12), 1645−1651.
AUTHOR INFORMATION
■
Corresponding Author
Michael R. Buchmeiser − Institute of Polymer Chemistry,
University of Stuttgart, 70569 Stuttgart, Germany;
orcid.org/0000-0001-6472-5156;
(11) Hillenbrand, J.; Leutzsch, M.; Yiannakas, E.; Gordon, C. P.;
Wille, C.; Nothling, N.; Coperet, C.; Furstner, A. Canopy Catalysts”
̈
Email: michael.buchmeiser@ipoc.uni-stuttgart.de
for Alkyne Metathesis: Molybdenum Alkylidyne Complexes with a
Tripodal Ligand Framework. J. Am. Chem. Soc. 2020, 142 (25),
11279−11294.
Authors
Jonas Groos − Institute of Polymer Chemistry, University of
Stuttgart, 70569 Stuttgart, Germany
Philipp M. Hauser − Institute of Polymer Chemistry,
University of Stuttgart, 70569 Stuttgart, Germany
Maximilian Koy − Institute of Polymer Chemistry, University
of Stuttgart, 70569 Stuttgart, Germany
(12) Heppekausen, J.; Stade, R.; Kondoh, A.; Seidel, G.; Goddard,
R.; Furstner, A. Optimized synthesis, structural investigations, ligand
̈
tuning and synthetic evaluation of silyloxy-based alkyne metathesis
catalysts. Chem. - Eur. J. 2012, 18 (33), 10281−99.
(13) Heppekausen, J.; Furstner, A. Rendering Schrock-type
̈
Molybdenum Alkylidene Complexes Air Stable: User-Friendly
Precatalysts for Alkene Metathesis. Angew. Chem. 2011, 123 (34),
7975−7978.
Wolfgang Frey − Institute of Organic Chemistry, University of
Stuttgart, 70569 Stuttgart, Germany
(14) Heppekausen, J.; Stade, R.; Goddard, R.; Furstner, A. Practical
̈
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00175
New Silyloxy-Based Alkyne Metathesis Catalysts with Optimized
Activity and Selectivity Profiles. J. Am. Chem. Soc. 2010, 132, 11045−
11057.
Funding
(15) Furstner, A.; Mathes, C.; Lehmann, C. W. Mo[N(t-Bu)(Ar)]3
̈
Financial support by the Deutsche Forschungsgemeinschaft
DFG (grant number 358283783 − CRC 1333) is gratefully
acknowledged.
Complexes As Catalyst Precursors: In Situ Activation and Application
to Metathesis Reactions of Alkynes and Diynes. J. Am. Chem. Soc.
1999, 121 (40), 9453−9454.
(16) Bittner, C.; Ehrhorn, H.; Bockfeld, D.; Brandhorst, K.; Tamm,
M. Tuning the Catalytic Alkyne Metathesis Activity of Molybdenum
and Tungsten 2,4,6-Trimethylbenzylidyne Complexes with Fluoroalk-
oxide Ligands OC(CF3)nMe3−n (n = 0−3). Organometallics 2017, 36
(17), 3398−3406.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Dedicated to Prof. Christian Bruneau for his outstanding
contributions to catalysis.
(17) Estes, D. P.; Gordon, C. P.; Fedorov, A.; Liao, W. C.; Ehrhorn,
H.; Bittner, C.; Zier, M. L.; Bockfeld, D.; Chan, K. W.; Eisenstein, O.;
Raynaud, C.; Tamm, M.; Coperet, C. Molecular and Silica-Supported
Molybdenum Alkyne Metathesis Catalysts: Influence of Electronics
and Dynamics on Activity Revealed by Kinetics, Solid-State NMR,
and Chemical Shift Analysis. J. Am. Chem. Soc. 2017, 139 (48),
17597−17607.
(18) Estes, D. P.; Bittner, C.; Arias, O.; Casey, M.; Fedorov, A.;
Tamm, M.; Coperet, C. Alkyne Metathesis with Silica-Supported and
Molecular Catalysts at Parts-per-Million Loadings. Angew. Chem.
2016, 128 (45), 14166−14170.
(19) Haberlag, B.; Freytag, M.; Jones, P. G.; Tamm, M. Tungsten
and Molybdenum 2,4,6-Trimethylbenzylidyne Complexes as Robust
Pre-Catalysts for Alkyne Metathesis. Adv. Synth. Catal. 2014, 356 (6),
1255−1265.
(20) Haberlag, B.; Freytag, M.; Daniliuc, C. G.; Jones, P. G.; Tamm,
M. Efficient Metathesis of Terminal Alkynes. Angew. Chem. 2012, 124
(52), 13195−13198.
(21) Haberlag, B.; Wu, X.; Brandhorst, K.; Grunenberg, J.; Daniliuc,
C. G.; Jones, P. G.; Tamm, M. Preparation of imidazolin-2-iminato
molybdenum and tungsten benzylidyne complexes: a new pathway to
highly active alkyne metathesis catalysts. Chem. - Eur. J. 2010, 16 (29),
8868−77.
(22) Lysenko, S.; Haberlag, B.; Wu, X.; Tamm, M. Ring-Opening
Metathesis Polymerization of Cyclooctyne Employing Well-Defined
Tungsten Alkylidyne Complexes. Macromol. Symp. 2010, 293 (1),
20−23.
REFERENCES
■
(1) Schrock, R. R. Multiple Metal−Carbon Bonds for Catalytic
Metathesis Reactions (Nobel Lecture). Angew. Chem. 2006, 118 (23),
3832−3844.
(2) Zhai, F.; Schrock, R. R.; Hoveyda, A. H.; Muller, P. Syntheses of
̈
“Phosphine-Free” Molybdenum Oxo Alkylidene Complexes through
Addition of Water to Alkylidyne Complexes. Organometallics 2020, 39
(13), 2486−2492.
(3) Tonzetich, Z. J.; Lam, Y. C.; Muller, P.; Schrock, R. R. Facile
̈
Synthesis of a Tungsten Alkylidyne Catalyst for Alkyne Metathesis.
Organometallics 2007, 26 (3), 475−477.
(4) Schrock, R. R. High-oxidation-state molybdenum and tungsten
alkylidyne complexes. Acc. Chem. Res. 1986, 19 (11), 342−348.
(5) Toreki, R.; Vaughan, G. A.; Schrock, R. R.; Davis, W. M.
Metathetical reactions of rhenium(VII) alkylidene-alkylidyne com-
plexes of the type Re(CR′)(CHR′)[OCMe(CF3)2]2 (R′ = CMe3 or
CMe2Ph) with terminal and internal olefins. J. Am. Chem. Soc. 1993,
115 (1), 127−137.
(6) Freudenberger, J. H.; Schrock, R. R. Multiple metal-carbon
bonds. 42. Formation of tungstenacyclobutadiene complexes contain-
ing a proton in the ring and their conversion to deprotio
tungstenacyclobutadiene complexes. Organometallics 1986, 5 (7),
1411−1417.
(7) Listemann, M. L.; Schrock, R. R. Multiple metal carbon bonds.
35. A general route to tri-tert-butoxytungsten alkylidyne complexes.
Scission of acetylenes by ditungsten hexa-tert-butoxide. Organo-
metallics 1985, 4 (1), 74−83.
(8) Freudenberger, J. H.; Schrock, R. R. Multiple metal-carbon
bonds. 37. Preparation of di-tert-butoxytungsten(VI) alkylidene
complexes by protonation of tri-tert-butoxytungsten(VI) alkylidyne
complexes. Organometallics 1985, 4 (11), 1937−1944.
(9) Freudenberger, J. H.; Schrock, R. R.; Churchill, M. R.;
Rheingold, A. L.; Ziller, J. W. Metathesis of acetylenes by
(fluoroalkoxy)tungstenacyclobutadiene complexes and the crystal
(23) Beer, S.; Brandhorst, K.; Grunenberg, J.; Hrib, C. G.; Jones, P.
G.; Tamm, M. Preparation of Cyclophanes by Room-Temperature
Ring-Closing Alkyne Metathesis with Imidazolin-2-iminato Tungsten
Alkylidyne Complexes. Org. Lett. 2008, 10, 981−984.
(24) Beer, S.; Hrib, C. G.; Jones, P. G.; Brandhorst, K.; Grunenberg,
J.; Tamm, M. Efficient room-temperature alkyne metathesis with well-
defined imidazolin-2-iminato tungsten alkylidyne complexes. Angew.
Chem. 2007, 119 (46), 9047−9051.
̀
̀
(25) Melcher, D.; Arias, O.; Freytag, M.; Jones, P. G.; Tamm, M.
Synthesis of Alkyne Metathesis Catalysts from Tris(dimethylamido)
1183
https://doi.org/10.1021/acs.organomet.1c00175
Organometallics 2021, 40, 1178−1184

Organometallics
pubs.acs.org/Organometallics
Article
tungsten Precursors. Eur. J. Inorg. Chem. 2020, 2020 (47), 4454−
4464.
(26) Roland, C. D.; VenkatRamani, S.; Jakhar, V. K.; Ghiviriga, I.;
Abboud, K. A.; Veige, A. S. Synthesis and Characterization of a
Molybdenum Alkylidyne Supported by a Trianionic OCO3‑ Pincer
Ligand. Organometallics 2018, 37 (23), 4500−4505.
(27) VenkatRamani, S.; Ghiviriga, I.; Abboud, K. A.; Veige, A. S. A
new ONO(3‑) trianionic pincer ligand with intermediate flexibility
and its tungsten alkylidene and alkylidyne complexes. Dalton. Trans.
2015, 44 (42), 18475−86.
(28) McGowan, K. P.; O’Reilly, M. E.; Ghiviriga, I.; Abboud, K. A.;
Veige, A. S. Compelling mechanistic data and identification of the
active species in tungsten-catalyzed alkyne polymerizations: con-
version of a trianionic pincer into a new tetraanionic pincer-type
ligand. Chem. Sci. 2013, 4 (3), 1145.
Heterocyclic Carbene Tungsten Oxo Alkylidene Sites: Highly Active
and Stable Catalysts for Olefin Metathesis. Angew. Chem. 2016, 128
(13), 4372−4374.
(43) Herz, K.; Unold, J.; Hänle, J.; Schowner, R.; Sen, S.; Frey, W.;
Buchmeiser, M. R. Mechanism of the Regio- and Stereoselective
Cyclopolymerization of 1,6-Hepta- and 1,7-Octadiynes by High
Oxidation State Molybdenum−Imidoalkylidene N-Heterocyclic Car-
bene Initiators. Macromolecules 2015, 48 (14), 4768−4778.
(44) Buchmeiser, M. R.; Sen, S.; Unold, J.; Frey, W. Komplexe N-
̈
̈
heterocyclischer Carbene mit Molybdan-Alkylidenen der hochsten
̈
̈
Oxidationsstufe: funktionalitatstolerante kationische Katalysatoren fur
die Olefinmetathese. Angew. Chem. 2014, 126 (35), 9538−9542.
(45) Hauser, P. M.; van der Ende, M.; Groos, J.; Frey, W.; Wang, D.;
Buchmeiser, M. R. Cationic Tungsten Alkylidyne N-Heterocyclic
Carbene Complexes: Synthesis and Reactivity in Alkyne Metathesis.
Eur. J. Inorg. Chem. 2020, 2020, 3070−3082.
(29) Yang, H.; Liu, Z.; Zhang, W. Multidentate Triphenolsilane-
Based Alkyne Metathesis Catalysts. Adv. Synth. Catal. 2013, 355 (5),
885−890.
(46) Imbrich, D. A.; Frey, W.; Buchmeiser, M. R. N-Heterocyclic
carbene-induced transmethylation in tungsten imido alkylidene
bistriflates: unexpected formation of an N-heterocyclic olefin complex.
Chem. Commun. 2017, 53 (88), 12036−12039.
(47) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor,
G. C. Synthesis, Structure, and Spectroscopic Properties of Copper-
(II) Compounds containing Nitrogen-Sulphur Donor Ligands; the
Crystal and Molecular Structure of Aqua[l,7-bis(N-methylbenzimida-
zol-2’-yl)2,6-dithiaheptane]copper(II) Perchlorate. J. Chem. Soc.,
Dalton Trans. 1984, 0, 1349−1356.
(48) Herz, K.; Podewitz, M.; Stohr, L.; Wang, D.; Frey, W.; Liedl, K.
R.; Sen, S.; Buchmeiser, M. R. Mechanism of Olefin Metathesis with
Neutral and Cationic Molybdenum Imido Alkylidene N-Heterocyclic
Carbene Complexes. J. Am. Chem. Soc. 2019, 141 (20), 8264−8276.
(49) Buchmeiser, M. R.; Sen, S.; Lienert, C.; Widmann, L.;
Schowner, R.; Herz, K.; Hauser, P.; Frey, W.; Wang, D. Molybdenum
Imido Alkylidene N-Heterocyclic Carbene Complexes: Structure-
Productivity Correlations and Mechanistic Insights. ChemCatChem
2016, 8 (16), 2710−2723.
(50) Benedikter, M.; Musso, J.; Kesharwani, M. K.; Sterz, K. L.;
Elser, I.; Ziegler, F.; Fischer, F.; Plietker, B.; Frey, W.; Kästner, J.;
Winkler, M.; van Slageren, J.; Nowakowski, M.; Bauer, M.;
Buchmeiser, M. R. Charge Distribution in Cationic Molybdenum
Imido Alkylidene N-Heterocyclic Carbene Complexes: A Combined
(30) Jyothish, K.; Wang, Q.; Zhang, W. Highly Active Multidentate
Alkyne Metathesis Catalysts: Ligand-Activity Relationship and Their
Applications in Efficient Synthesis of Porphyrin-Based Aryleneethy-
nylene Polymers. Adv. Synth. Catal. 2012, 354 (11−12), 2073−2078.
(31) Jyothish, K.; Zhang, W. Introducing a podand motif to alkyne
metathesis catalyst design: a highly active multidentate molybdenum-
(VI) catalyst that resists alkyne polymerization. Angew. Chem. 2011,
123 (15), 3497−3500.
(32) Cho, H. M.; Weissman, H.; Moore, J. S. Synthetic applications
with use of a silica-supported alkyne metathesis catalyst. J. Org. Chem.
2008, 73 (11), 4256−8.
(33) Weissman, H.; Plunkett, K. N.; Moore, J. S. A Highly Active,
Heterogeneous Catalyst for Alkyne Metathesis. Angew. Chem. 2006,
118 (4), 599−602.
(34) Cho, H. M.; Weissman, H.; Wilson, S. R.; Moore, J. S. A
Mo(VI) alkylidyne complex with polyhedral oligomeric silsesquioxane
ligands: homogeneous analogue of a silica-supported alkyne meta-
thesis catalyst. J. Am. Chem. Soc. 2006, 128 (46), 14742−3.
(35) von Kugelgen, S.; Piskun, I.; Griffin, J. H.; Eckdahl, C. T.;
Jarenwattananon, N. N.; Fischer, F. R. Templated Synthesis of End-
Functionalized Graphene Nanoribbons through Living Ring-Opening
Alkyne Metathesis Polymerization. J. Am. Chem. Soc. 2019, 141 (28),
11050−11058.
(36) Bellone, D. E.; Bours, J.; Menke, E. H.; Fischer, F. R. Highly
selective molybdenum ONO pincer complex initiates the living ring-
opening metathesis polymerization of strained alkynes with excep-
tionally low polydispersity indices. J. Am. Chem. Soc. 2015, 137 (2),
850−6.
(37) Paley, D. W.; Sedbrook, D. F.; Decatur, J.; Fischer, F. R.;
Steigerwald, M. L.; Nuckolls, C. Alcohol-promoted ring-opening
alkyne metathesis polymerization. Angew. Chem. 2013, 125 (17),
4689−4692.
(38) Tsai, Y.-C.; Diaconescu, P. L.; Cummins, C. C. Facile Synthesis
of Trialkoxymolybdenum(VI) Alkylidyne Complexes for Alkyne
Metathesis. Organometallics 2000, 19 (25), 5260−5262.
(39) Cummins, C. C. Reductive cleavage and related reactions
leading to molybdenum−element multiple bonds: new pathways
offered by three-coordinate molybdenum(III). Chem. Commun. 1998,
0, 1777−1786.
̈
X-ray, XAS, XES, DFT, Mossbauer, and Catalysis Approach. ACS
Catal. 2020, 10 (24), 14810−14823.
(51) Koy, M.; Elser, I.; Meisner, J.; Frey, W.; Wurst, K.; Kastner, J.;
Buchmeiser, M. R. High Oxidation State Molybdenum N-Hetero-
cyclic Carbene Alkylidyne Complexes: Synthesis, Mechanistic Studies,
and Reactivity. Chem. - Eur. J. 2017, 23 (61), 15484−15490.
(52) Hillenbrand, J.; Leutzsch, M.; Furstner, A. Molybdenum
̈
Alkylidyne Complexes with Tripodal Silanolate Ligands: The Next
Generation of Alkyne Metathesis Catalysts. Angew. Chem., Int. Ed.
2019, 58 (44), 15690−15696.
(53) Arduengo, A. J.; Krafczyk, R.; Schmutzler, R.; Craig, H. A.;
Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Imidazolylidenes,
imidazolinylidenes and imidazolidines. Tetrahedron 1999, 55 (51),
14523−14534.
(40) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.;
Kim, E.; Cummins, C. C.; George, G. N.; Pickering, I. J. Dinitrogen
Cleavage by Three-Coordinate Molybdenum(III) Complexes: Mech-
anistic and Structural Data1. J. Am. Chem. Soc. 1996, 118 (36), 8623−
8638.
(41) Elser, I.; Groos, J.; Hauser, P. M.; Koy, M.; van der Ende, M.;
Wang, D.; Frey, W.; Wurst, K.; Meisner, J.; Ziegler, F.; Kästner, J.;
Buchmeiser, M. R. Molybdenum and Tungsten Alkylidyne Complexes
Containing Mono-, Bi-, and Tridentate N-Heterocyclic Carbenes.
Organometallics 2019, 38 (21), 4133−4146.
(42) Pucino, M.; Mougel, V.; Schowner, R.; Fedorov, A.;
Buchmeiser, M. R.; Coperet, C. Cationic Silica-Supported N-
1184
https://doi.org/10.1021/acs.organomet.1c00175
Organometallics 2021, 40, 1178−1184