Molybdenum and Tungsten Alkylidyne Complexes Containing Mono-, Bi-, and Tridentate N-Heterocyclic Carbenes

Article abstract of DOI:10.1021/acs.organomet.9b00481

New tungsten and molybdenum alkylidyne complexes bearing mono-, bi-, and tridentate N-heterocyclic carbenes (NHCs) have been synthesized. Formation of unprecedented structures in complexes bearing N-tert-butyl substituents on the imidazol(in)-2-ylidene was observed, leading to molybdenum complexes containing an abnormal carbene (Mo-4) and a bridging O,C,C-pincer ligand (Mo-10) and to a tungsten complex containing a cationic imidazolinium-tagged alkoxide forming an inner salt with an anionic tungsten center (W-5). Both the abnormal carbene binding in Mo-4 and the O,C,C-pincer-type structure of Mo-10 were confirmed by single-crystal X-ray analysis, and the proposed structure of W-5 is supported by the single-crystal X-ray structure of a minor byproduct (W-8) formed during the synthesis of W-4, displaying the aforementioned inner-salt-like structure. The novel alkylidyne complexes were also investigated for their capability to form a previously postulated quasi-cationic species with a weakly coordinating anion (WCA) during the alkyne homometathesis of 1-phenyl-1-propyne. Overall, incorporation of bidentate and strongly σ donating NHCs as well as introduction of better leaving groups did not lead to the expected increase in catalytic activity. Despite identical ligand spheres, changing from molybdenum to tungsten led to complete loss of activity in the bidentate systems.

Full text of DOI:10.1021/acs.organomet.9b00481

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Molybdenum and Tungsten Alkylidyne Complexes Containing
Mono‑, Bi‑, and Tridentate N‑Heterocyclic Carbenes
Iris Elser,†,⊥ Jonas Groos,†,⊥ Philipp M. Hauser,†,⊥ Maximilian Koy,†,#,⊥ Melita †van der Ende,†,⊥
and Michael R. Buchmeiser*,†
§
Dongren Wang,† Wolfgang Frey,‡ Klaus Wurst,∥ Jan Meisner,§,∇ Felix Ziegler, Johannes Kastner,
̈
†Institute of Polymer Chemistry, ‡Institute of Organic Chemistry, and §Institute for Theoretical Chemistry, University of Stuttgart,
Pfaffenwaldring 55, 70569 Stuttgart, Germany
∥Institute of General, Inorganic and Theoretical Chemistry, University of Innsbruck, A-6020 Innsbruck, Austria
S
* Supporting Information
ABSTRACT: New tungsten and molybdenum alkylidyne
complexes bearing mono-, bi-, and tridentate N-heterocyclic
carbenes (NHCs) have been synthesized. Formation of
unprecedented structures in complexes bearing N-tert-butyl
substituents on the imidazol(in)-2-ylidene was observed, leading
to molybdenum complexes containing an abnormal carbene
(Mo-4) and a bridging O,C,C-pincer ligand (Mo-10) and to a
tungsten complex containing a cationic imidazolinium-tagged
alkoxide forming an inner salt with an anionic tungsten center
(W-5). Both the abnormal carbene binding in Mo-4 and the
O,C,C-pincer-type structure of Mo-10 were confirmed by single-
crystal X-ray analysis, and the proposed structure of W-5 is
supported by the single-crystal X-ray structure of a minor byproduct (W-8) formed during the synthesis of W-4, displaying the
aforementioned inner-salt-like structure. The novel alkylidyne complexes were also investigated for their capability to form a
previously postulated quasi-cationic species with a weakly coordinating anion (WCA) during the alkyne homometathesis of 1-
phenyl-1-propyne. Overall, incorporation of bidentate and strongly σ donating NHCs as well as introduction of better leaving
groups did not lead to the expected increase in catalytic activity. Despite identical ligand spheres, changing from molybdenum
to tungsten led to complete loss of activity in the bidentate systems.
bene (NHC) alkylidyne complexes deserves an explanation
(Figure 1, right).12 Mechanistic investigations suggested that,
depending on the incorporated NHC, the pentacoordinated
complexes either dissociate the carbene under the formation of
a neutral Schrock-type alkylidyne complex or form a quasi-
cationic species with the alkoxide serving as a weakly
coordinating anion (WCA, Scheme 1, top). Here, strongly σ
donating NHCs, allowing for strong metal−carbene bonds,
promote the latter. However, the proposed quasi-cationic
species was found to form to a very minor extent
(approximately 1%). We assumed this low concentration of
the active species to translate into the observed moderate
activity in alkyne metathesis. Unfortunately, up to this point,
we were not able to isolate the cationic species or
unambiguously prove its existence. Two factors were expected
to foster the formation of the proposed active species: first, the
stabilization of the positive partial charge at the metal complex
and, second, the stabilization of the anionic charge in the
WCA.
INTRODUCTION
■
During the last 30 years, alkyne metathesis has made enormous
progress, starting from “ill-defined” metathesis catalysts to
those whose structure and reactivity in solution can be
controlled with high precision.1 Enormous synthetic efforts
made by a multitude of organometallic chemists paved the way
from the first catalytically active WO3 or MoO3 compounds
supported on silica to the sophisticated catalyst systems
applied today.2 Seminal work by Schrock,3 Furstner,4 Tamm,5
̈
Mortreux,6 Cummins,7 Moore8 and Zhang2e,9 contributed to
the development of well-defined alkyne metathesis catalysts.10
Apart from Schrock-type tris(alkoxy) metal alkylidyne
complexes, catalysts bearing multidentate ligands,2e,9b,11
electronically and sterically flexible silyloxy ligands,4c,d or
imidazolin-2-iminato ligands5a−d have been reported (Figure
1). In all catalyst systems tetracoordinated species have been
identified as the active species. Even the Furstner-type
̈
pentacoordinated complexes dissociate one of the silanolates
to form a neutral tetracoordinated active species prior to
alkyne coordination.4d In this context, the alkyne metathesis
activity of our recently published pentacoordinated 14-VE
molybdenum trishexafluoro-tert-butoxide N-heterocyclic car-
Received: July 18, 2019
© XXXX American Chemical Society
A
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Figure 1. Well-defined alkyne metathesis catalysts developed in the groups of Mortreux,6 Schrock,3a−i Cummins,7 Fu
̈
rstner,4a−d Moore,8
Tamm5b−e and Buchmeiser.12
Scheme 1. (Top) Proposed Active Species (Previous Work) and Measures Taken in This Work to Increase the Concentration
and Stability of the Proposed Active Species and (Bottom) Synthesis of New Molybdenum Alkylidyne NHC Complexes
a
Bearing Monodentate NHCs
a
Isolated yields are given in percent.
In this work, stabilization of the cationic charge on the metal
center was attempted by introduction of strong σ donors and
by incorporation of bi- and tridentate ligands into the catalyst
structures. Stabilization of the anionic charge on the WCA was
addressed by the introduction of more acidic alcohols. Strong
σ donors were additionally expected to result in a stronger
trans effect, thereby lowering the activation energy for the
formation of the catalytically active metal complex. Incorpo-
ration of multidentate NHCs also minimizes the chances of
generating the tetracoordinated active species by NHC
dissociation (Scheme 1, top).
SYNTHESIS
■
Monodentate NHC Alkylidyne Complexes. Several
novel complexes bearing monodentate NHCs based on the
tris(hexafluoro-tert-butoxide) precursor Mo(C-p-OMe-
C6H4)(OCMe(CF3)2)3(DME) (Mo-1) were synthesized.
The applied NHC ligands were chosen for several reasons:
both 1,3-dicyclohexylimidazol-2-ylidene (1) and 1,3-di-tert-
butylimidazol-2-ylidene (2) are strong σ donors (Tolman
electronic parameter (TEP):13 1, 2049.7 cm−1; 2, 2050.6
cm−1). Due to their low TEPs, they were expected to enhance
the formation of the proposed quasi-cationic reactive species
B
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Scheme 2. Synthesis of Molybdenum Alkylidyne Complexes with Bidentate O-Chelating NHCs (Mo-8−Mo-11) by Conversion
a
with NHCs O1−O4
a
NHCs were generated without isolation prior to metal complex synthesis by deprotonation of the respective imidazolinium chlorides HOX·HCl.
Isolated yields are given in percent.
by their trans effect on the weakly coordinating anion
OCMe(CF3)2. Furthermore, strong σ donors can stabilize
the positive partial charge on the metal complex. On the other
hand, 3,4,5-trimethylthiazol-2-ylidene (3), although a weak σ
donor (TEP = 2058.5 cm−1, calculated in analogy to the
literature13), was envisioned to form a strong metal−carbene
bond, due to the decrease in steric bulk resulting from the
missing 1-substituent in the thiazol-2-ylidene. It therefore
seemed unlikely that the NHC would dissociate to form the
proposed neutral tetracoordinated active species (Scheme 1,
top). Consequently, the precursor complex Mo-1 was reacted
with the N-heterocyclic carbenes 1 and 2 and the carbene
silver salt 3-AgI, respectively, in toluene at 40 °C to afford the
complexes Mo(C-p-OMe-C6H4)(OCMe(CF3)2)3(X) (Mo-
3 (X = 1), Mo-4 (X = 2), and Mo-5 (X = 3)) in 67, 44, and
50% isolated yields, respectively. Mo-3 and Mo-5 showed the
expected 1H and 19F NMR spectra in C6D6. Interestingly, Mo-
3 with the sterically demanding cyclohexyl-substituted NHC
showed a rearrangement from a square-pyramidal (SP) to a
trigonal-bipyramidal (TBP) structure in CD3CN (Figures S69
and S70 in the Supporting Information). This change in
geometry was already reported for Mo(C-(p-OMe-C6H4))-
(OCMe(CF3)2)3(5) (Mo-12).12
The two protons on the carbon atom adjacent to the
nitrogen atoms become magnetically equivalent. Also, the
imidazol-2-ylidene backbone protons show up as a singlet at δ
7.43 ppm instead of the two doublets at δ 7.24 ppm (d, 1H,
3JH−H = 2.0 Hz) and δ 7.28 ppm (d, 1H, 3JH−H = 2.0 Hz) in the
corresponding parent complex. In the 19F NMR spectrum only
one new signal for all three OCMe(CF3)2 ligands at δ −76.7
ppm could be observed. Complex Mo-4 shows a surprising 1H
NMR spectrum: the two protons in the backbone of the 1,3-di-
tert-butylimidazol-2-ylidene show up as a doublet at δ 7.86
ppm (d, 4JHH = 1.8 Hz) and a broad singlet at δ 7.09 ppm. The
small coupling constant of 1.8 Hz hints toward a JH−H
4
coupling. In combination with the splitting of the N-tert-
butyl groups into two overlapping singlets at δ 1.64 ppm in the
1H NMR spectrum, we expected a structure with an
abnormally bound imidazol-2-ylidene, which was later
confirmed by single-crystal X-ray measurements (vide infra).
The 19F NMR spectra of Mo-3−Mo-5 indicate a square-
pyramidal (SP) structure in solution, with one of the alkoxide
ligands in the apex, since three distinct resonances for the CF3
groups are observed. This indicates that the CF3 groups are
diastereotopic, in turn indicating that the molybdenum center
is chiral. Chirality in pentacoordinated complexes of the type
Ma3bc is only present in a SP structure with one of the three
equal ligands in the apex. Mo-3−Mo-5 are all based on the
OCMe(CF3)2 ligand; however, a careful choice of the alkoxide
ligand can drastically alter the catalytic performance.
Consequently, we targeted an exchange of the alkoxide ligand
into a better leaving group (weaker base) to foster the
formation of the active species. Pentafluorophenoxide is a
better leaving group than hexafluoro-tert-butoxide (pKa:
HOC6F5, 5.53;14 HOCMe(CF3)2, 9.815). Hence, we first
synthesized the new precursor complex Mo(C-p-OMe-
C6H4)(OC6F5)3(DME) (Mo-2) from Mo(C-(p-OMe-
C6H4))Br3(DME) via reaction with 3 equiv of LiOC6F5 in
60% isolated yield in analogy to the literature.5d Subsequently,
1,3-dimesitylimidazol-2-ylidene (4) was successfully coordi-
nated to Mo-2 in toluene at room temperature, affording
Mo(C-p-OMe-C6H4)(OC6F5)3(4) (Mo-6) in 89% isolated
yield (Scheme 1). Furthermore, Mo-2 was reacted with 1,3-
diisopropylimidazol-2-ylidene (5) to provide Mo(C-p-OMe-
C6H4)(OC6F5)3(5) (Mo-7) in 35% isolated yield as an
analogue to the previously reported complex Mo-12 bearing
C
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Scheme 3. Synthesis of Tungsten Alkylidyne Complexes with Bidentate O-Chelating NHCs (W-3−W-7) by Conversion with
a
NHCs O1−O3
a
NHCs were generated without isolation prior to metal complex synthesis by deprotonation of the respective imidazolinium tetrafluoroborates
HOX·HBF4. Isolated yields are given in percent.
three OCMe(CF3)2 ligands. As judged from the 19F NMR
spectrum, two of the three OC6F5 ligands in Mo-6 are
equivalent, whereas the third occupies another coordination
site. Whether two of the OC6F5 ligands are in the plane of a
TBP structure and the other is in the apex or one of the three
OC6F5 groups experiences a trans effect (for example from the
NHC) in an SP structure with all three OC6F5 ligands in the
plane cannot be distinguished.
C6H4)(OCMe(CF3)2)(O3)]2 (Mo-10) were synthesized from
Mo-1 (Scheme 2). For this purpose, imidazolinium salts HOX·
HCl were deprotonated with LiHMDS in THF and then
reacted with Mo-1 in THF. The corresponding complexes
bearing bidentate NHCs were formed in 47−75% isolated
yield. Mo-8, Mo-9, and Mo-11 showed the expected NMR
spectra. Since the pentacoordinated complexes Mo-8−Mo-11
are all chiral at the metal center, the CF3 groups and the
OCMe(CF3)2 ligands in the complexes are diastereotopic. This
results in four multiplets (Mo-8, Mo-11) or three multiplets
with a ratio of 3:3:6 (Mo-9) in the 19F NMR spectra. Also, the
diastereotopic protons in the backbones of the imidazolin-2-
ylidenes are fully split for all three complexes. In case of Mo-
10, only three methyl groups instead of the expected five
Bidentate NHC Alkylidyne Complexes. Apart from
molybdenum-based alkylidyne complexes with monodentate
NHCs, we were interested in the synthesis of catalysts
containing bidentate ligands. The hypothesis was that the
proposed active, cationic species is stabilized by the chelate
effect, which would in turn result in a better catalytic
performance. Beneficially, kinetic reasons lead to a decreased
propensity for bidentate vs monodentate NHCs to dissociate
from the corresponding metal complexes, thereby preventing
formation of a neutral active species (Scheme 1, top).
Complementary, in the case of tungsten, the alkoxide ligand
was varied to probe the influence of the degree of fluorination
on activity.5j Several O-chelating NHCs were synthesized
according to or in analogy to the literature.16 Ligand properties
of the NHCs were varied by altering the nonchelating N-
substituent from the N-tert-butyl (O3) to the N-2,6-
dimethylphenyl (O1), the N-2,4,6-trimethylphenyl (O2), and
the N-2-phenylphenyl (O4) substituent to span a wide range
of steric hindrance. Also, the 2-phenylphen-1-yl-substituted
NHC (O4) offers flexibility, allowing the phenyl group to
either shield the metal center or open it to attacks. Carbenes
were synthesized directly prior to metal complex formation by
deprotonation of the respective imidazolinium salt with
LiHMDS, KHMDS, or KH. The molybdenum-based com-
plexes Mo(C-p-OMe-C6H4)(OCMe(CF3)2)2(OX) (Mo-8
(O1), Mo-9 (O2), Mo-11 (O4)) and [Mo(C-p-OMe-
1
methyl groups were observed in the H NMR spectrum as
three singlets at δ 1.69, 1.51, and 1.33 ppm (CD2Cl2).
Additionally, at δ 2.46 ppm (br s, 1H) and δ 2.00 ppm (d,
2JH−H = 10.6 Hz, 1H) two unexpected signals were found,
hinting toward a methylene group bearing two diastereotopic
protons. We therefore envisioned a structure containing an
O,C,C-pincer-type NHC, derived from abstraction of one
proton from the tert-butyl substituent on the NHC and
replacement of one OCMe(CF3)2 ligand by the resulting
carbanion (Scheme 2). In line with this proposal, two distinct
signals for the two diastereotopic CF3 groups can be observed
in the 19F NMR spectrum at δ −78.1 and −78.4 ppm due to
the chirality at the metal center. The formation of Mo-10 is
proposed to proceed via an intramolecular C−H activation of
the tert-butyl group followed by release of HOCMe(CF3)2.
Comparable C−H activations on NHC ligands by replacement
of an anionic ligand have been observed in molybdenum imido
alkylidene NHC complexes or in ruthenium NHC alkylidene
complexes.17 The tungsten-based complexes W(C-p-OMe-
C6H4)(OCMe(CF3)2)2(OX) (W-3 (O1) and W-4 (O2)) and
D
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W(C-p-OMe-C6H4)(OCMe(CF3)2)3(O3·H) (W-5) were
synthesized from W(C-p-OMe-C6 H4 )(OCMe-
(CF3)2)3(DME) (W-1). The complexes W(C-p-OMe-
C6H4)(OCMe(CF3)2)2(OX) (W-6 (O1) and W-7 (O2))
w e r e d e r i v e d f r o m W (  C - p - O M e - C 6 H 4 ) -
(OCMe2(CF3))3(THF) (W-2) (Scheme 3). All reactions
employing tungsten precursors were done in toluene, and
the imidazolinium tetrafluoroborates HOX·HBF4 were used
instead of the chlorides. Deprotonation of imidazolinium salts
HOX·HBF4 was performed in toluene with LiHMDS (O1,
O2) or KH (O3). Complexes W-3−W-7 were isolated by
crystallization in 39−90% yield. W-3, W-4, W-6, and W-7
showed regular structures, as judged from the NMR spectra. As
already mentioned for molybdenum, W-3−W-7 are chiral at
the metal center. Therefore, W-3 and W-4 each display four
distinct multiplets in the 19F NMR spectrum, representing two
diastereotopic CF3 groups in two diastereotopic OCMe(CF3)2
ligands. For W-6 and W-7, two singlets can be observed in the
19F NMR spectrum, which is in accordance with two
diastereotopic OCMe2CF3 ligands. Interestingly, for the
reaction of carbene O3 bearing the N-tert-butyl substituent
with the precursor W-1 an irregular structure was observed, as
indicated by the NMR spectra. The resulting complex W-5
converting Mo-1 with O21 in THF (Scheme 4). O21 was
prepared by deprotonation of the inner salt HO21·H
Scheme 4. Synthesis of a Molybdenum Alkylidyne Complex
Bearing a Tridentate NHC in 75% Yield
immediately prior to complex synthesis. Mo-13 was isolated
1
in 75% yield by crystallization from diethyl ether. The H
NMR spectrum shows two doublets for the methylene protons
of the NHC ligand at δ 4.54 and 4.48 ppm (2JH−H = 14.3 Hz).
Mo-13 is not chiral at the metal center (plane of symmetry
through the NHC, the alkylidyne, and the OCMe(CF3)2
group); consequently, the 19F NMR spectrum shows only
three resonances. The two equivalent CF3 groups on distinct
1
shows an unexpected signal at δ 8.63 ppm (s, 1H) in the H
NMR spectrum. This signal is assigned to an imidazolinium
species, pointing toward a structure where only the phenolate
group is coordinated to the metal center, yielding a tungstate
complex (Scheme 3). The existence of an imidazolinium
species is also supported by 13C NMR spectroscopy; the signal
at δ 159.7 ppm can be clearly assigned to the imidazolinium
salt. A structure in which one OCMe(CF3)2 ligand was
replaced by the phenolate can be ruled out, since nine protons
are observed for the OCMe(CF3)2 ligand. The inner-salt-like
4
tethers of the NHC show up at δ −75.5 ppm (q, JF−F = 9.8
4
Hz) and the other two at δ −77.0 ppm (q, JF−F = 9.9 Hz).
Both are quartets, since they couple with the other adjacent
diastereotopic CF3 group.
On the other hand, the CF3 groups at the OCMe(CF3)2
ligand are observed as a singlet at δ −78.6 ppm (s,
OCMe(CF3)2), since those CF3 groups are not diastereotopic.
−
structure is further confirmed by the absence of a BF4 signal
in the 19F NMR spectrum. W-5 shows two multiplets in the 19
F
SINGLE-CRYSTAL X-RAY STRUCTURES
■
NMR spectrum, with a ratio of 12:6. In order to elucidate the
To elucidate the structural peculiarities of the synthesized
complexes, single-crystal X-ray structures of several com-
pounds were measured. As already judged from NMR spectra,
complexes Mo-3, Mo-5, and Mo-7 with monodentate NHCs
showed the expected structures with the NHCs coordinated
via the C2 carbon of the imidazol-2-ylidene (Figure 2). Mo-3
crystallizes in the monoclinic space group P21/n (a =
2274.0(2) pm, b = 1066.42(9) pm, c = 3316.6(3) pm; α =
90°, β = 95.299(4)°, γ = 90°). Mo-5 crystallizes in the
orthorhombic space group Pbca (a = 1034.86(5) pm, b =
1933.02(11) pm, c = 3258.38(19) pm; α = β = γ = 90°).
Comparable to our previously published complexes,12 Mo-3
and Mo-5 show a distorted-square-pyramidal (SP) structure in
the solid state, with τ19 = 0.18 and 0.28, respectively.20 The
benzylidyne ligand occupies the apex of the SP structure in
Mo-3 and Mo-5. Mo-4 crystallizes in the monoclinic space
group P21/c (a = 2074.36(19) pm, b = 3454.3(3) pm, c =
1053.88(9) pm; α = 90°, β = 90.876(3)°, γ = 90°). As
anticipated from the NMR spectra, Mo-4 was shown to
contain carbene 2 bound in the abnormal fashion (vide supra,
Figure 3). Reports on abnormally C4 bound carbene ligands
that are not blocked with an additional substituent at the acidic
C2 position are scarce.21 This rather unusual structural motif
can directly be deduced from the high steric constraint
introduced by the N-t-Bu substituents. The abnormal binding
mode results in a considerably higher TEP for the carbene
(TEP = 2043.9 cm−1, calculated in analogy to the literature;13
see the Supporting Information) and was therefore expected to
formation of the unusual structure of W-5 we investigated the
1
reaction by H NMR spectroscopy. Interestingly, the fact that
the ligand binds only via the phenoxy moiety originates from
exclusive deprotonation of the phenolic moiety of the
imidazolinium tetrafluoroborate HO3·HBF4 even in the
presence of 2 equiv of KH in THF (Figure S1 in the
Supporting Information). A change of the base to LiHMDS
and KHMDS also resulted in exclusive deprotonation of the
phenolic moiety; the imidazolinium proton remained un-
touched in all cases. For the imidazolinium chloride HO3·HCl,
however, complete 2-fold deprotonation was easily achieved
under the same conditions (Figure S2 in the Supporting
Information). Such selective deprotonation could be advanta-
geous, since it offers an attractive access to cationically tagged
alkylidyne complexes that might be used under biphasic
conditions as realized with ionic olefin metathesis catalysts.18
Tridentate NHC Molybdenum Alkylidyne Complex.
Tridentate NHCs, like bidentate NHCs, offer the possibility of
stabilizing metal complexes by the chelate effect. This was
envisioned to be beneficial for the stabilization of the proposed
quasi-cationic active species. Also, NHC ligand dissociation is
highly unlikely in such a system.
The tridentate O,O,C-pincer-type ligand O21 with trifluoro-
methyl groups adjacent to the alkoxides was chosen to ensure
sufficient Lewis acidity at the metal center. Mo(C-(p-OMe-
C6H4))(OCMe(CF3)2)(O21) (Mo-13) bearing the tridentate
O,O,C-pincer-type NHC ligand O21 was synthesized by
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Figure 2. Single-crystal X-ray structures of Mo-3, Mo-5, and Mo-7 displaying regular structures. Bond lengths (pm) and angles (deg): Mo-3, Mo−
C16 174.4(4), Mo−O1 195.5(2), Mo−O3 195.9(3), Mo−O2 196.2(2), Mo−C1 226.5(4), C16−Mo−O1 105.08(14), C16−Mo−O3 106.93(14),
O1−Mo−O3 146.28(10), C16−Mo−O2 105.20(14), O1−Mo−O2 90.98(10), O3−Mo−O2 90.67(11), C16−Mo−C1 97.48(16), O1−Mo−C1
84.03(11), O3−Mo−C1 81.62(12), O2−Mo−C1 157.30(12); Mo-5, Mo−C7 174.0(2), Mo−O4 194.61(15), Mo−O3 195.96(14), Mo−O2
196.17(14), Mo−C1 224.4(2), C7−Mo−O4 106.04(8), C7−Mo−O3 108.42(8), O4−Mo−O3 91.77(6), C7−Mo−O2 106.78(8), O4−Mo−O2
144.54(6), O3−Mo−O2 90.37(6), C7−Mo−C1 90.30(9), O4−Mo−C1 84.14(7), O3−Mo−C1 161.23(7), O2−Mo−C1 82.63(7); Mo-7, Mo−
C10 173.7(2), Mo−O2 195.69(17), Mo−O1 198.17(17), Mo−O3 199.23(18), Mo−C1 220.5(2), C10−Mo−O2 101.84(10), C10−Mo−O1
107.84(9), O2−Mo−O1 95.99(7), C10−Mo−O3 100.45(10), O2−Mo−O3 154.83(7), O1−Mo−O3 88.15(7), C10−Mo−C1 101.41(10), O2−
Mo−C1 83.27(8), O1−Mo−C1 150.20(9), O3−Mo−C1 81.00(8).
Mo-7 crystallizes in the monoclinic space group P21 (a =
1066.04(06) pm, b = 1283.87(9) pm, c = 1296.41(8) pm; α =
90°, β = 98.076(2)°, γ = 90°). In the solid state, Mo-7 displays
an almost perfect SP structure (τ = 0.077) with the alkylidyne
ligand in the apex. In comparison to the previously published
complex Mo(C-(p-OMe-C6H4))(OCMe(CF3)2)3(5) (Mo-
12) based on the OCMe(CF3)2 ligand, the increased steric
bulk of OCMe(CF3)2 vs OC6F5 is only mirrored in the
substantially longer Mo−CNHC bond (225.2(3) pm for Mo-12
vs 220.5(2) pm for Mo-7).12 Generally, the Mo−Calkylidyne
bonds in the complexes bearing monodentate NHCs, Mo-3
(174.4(4) pm), Mo-5 (174.0(2) pm), and Mo-7 (173.7(2)
pm), are shorter than those in the recently reported donor-free
complex Mo(C-(p-OMe-C6H4))(OCMe(CF3)2)3 (175.03
pm) and the doubly THF coordinated complex Mo(C-(p-
OMe-C6H4))(OCMe(CF3)2)3(THF)2 (175.7(2) pm).5i This
could be due to either the change in geometry and
coordination number or to a more positive polarization of
the Mo in the complexes Mo-3, Mo-5, and Mo-7. Two
complexes bearing bidentate NHCs were additionally charac-
Figure 3. Single-crystal X-ray structure of Mo-4. Mo-4 displays an
terized by single-crystal X-ray measurements. Molybdenum-
based Mo-8 crystallizes in the triclinic space group P1 (a =
irregular structure with the 1,3-di-tert-butylimidazol-2-ylidene bond in
an abnormal fashion (small crystals with low diffraction were
obtained). Bond lengths and angles are within experimental accuracy,
but data are not included for further discussion. See the Supporting
Information for details).
̅
1151.90(6) pm, b = 1251.45(7) pm, c = 1417.59(1) pm; α =
98.704(4)°, β = 109.748(3)°, γ = 113.041(2)°). Mo-8 displays
a slightly distorted SP structure (τ = 0.099) with the
benzylidyne in the apical position (Figure 4). Tungsten-
̅
based W-4 crystallizes in the triclinic space group P1 (a =
result in even higher activity in alkyne metathesis. Mo-5
showed a shorter molybdenum−carbene bond of 224.4(2) pm
in comparison to previously published complexes12 based on
rather weak σ donors such as Mo(C-(p-OMe-C6H4))-
(OCMe(CF3)2)3(1,3-dimethyl-4,5-dichloroimidazol-2-ylidene)
and Mo(C-(p-OMe-C6H4))(OCMe(CF3)2)3(1,3-dimethyl-
4,5-dicyanoimidazol-2-ylidene) (Mo−Ccarbene = 225.21(17)
and 226.63(15) pm, respectively).
1524.31(18) pm, b = 1701.61(16) pm, c = 1795.5(2) pm; α =
106.139(6)°, β = 109.867(6)°, γ = 94.334(5)°).
The crystal structure of W-4 reveals two independent
conformers. In both conformers the ligands adopt a distorted
SP geometry with the benzylidyne in the apical position. The
geometry index τ was 0.095 for conformer B and 0.25 for
conformer A, indicating a higher deviation from the SP
structure for conformer A. Since, apart from the metal center,
Mo-8 and W-4 only differ by one methyl group in the
This can be attributed to the decrease in steric demand
derived from replacement of one of the N-Me groups by sulfur.
F
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Article
Figure 4. Single-crystal X-ray structures of Mo-8 and both isomers of W-4. All of the complexes show the expected structure with the bidentate
NHC coordinated via the phenolate and the carbene moiety. Important bond lengths can be found in Table 1.
a
Table 1. Comparison of the Metal−Ligand Bond Lengths and Angles in Mo-8 and the Conformers of W-4
b
c
d
τ
M−Ccarbene (pm) M−Otrans carbene (pm) M−Ochelat (pm) M−Otrans chel (pm) M−Calkylidyne (pm) Otrans NHC−M−CNHC (deg)
Mo-8
W-4B
W-4A
0.099
0.095
0.250
223.4
221.5
225.7
196.2
196.9
196.4
201.75
190.9
198.4
193.25
198.3
191.1
174.38
176.3
162.6
151.1
155.5
151.3
a
b
c
d
Derived from single-crystal X-ray analysis. Oxygen atom trans to the NHC. Chelating oxygen atom of the NHC. Oxygen atom trans to the
chelating oxygen of the NHC.
nonchelating substituent of the imidazolin-2-ylidene, a
comparison between the corresponding Mo and W complexes
in terms of bond lengths and angles seemed appropriate (Table
1). It is noteworthy that the lengths of the M−Otrans NHC bond
are almost identical in Mo-8, W-4A, and W-4B, irrespective of
the differences in the M−CNHC bond length and the
Otrans NHC−M−CNHC angle (Table 1). Conformers A and B
of W-4 show a substantial difference of 13.7 pm in the W−
Calkylidyne bond lengths. Furthermore, the structure of Mo-10
was measured to confirm the proposed O,C,C-pincer-type
structure. Mo-10 crystallizes in the monoclinic space group
P21/n with a = 1248.62(6) pm, b = 2131.16(9) pm, c =
2111.02(9) pm, α = γ = 90°, β = 103.530(2)°, and Z = 4.
Interestingly, the crystal structure revealed a binuclear
structure in the solid state (Figure 5).
The two molybdenum centers are double-bridged via the
two oxygen atoms O1 and O4 of the O,C,C-pincer-type NHC.
The bridging oxygen atom O1 is closer to Mo1 (Mo1−O1 =
202.67(18) pm), whereas O4 is closer to Mo2 (Mo2−O4 =
203.09(18) pm). In comparison, the bond lengths Mo1−O4
and Mo2−O1 are substantially elongated (243.42(17) and
241.22(17) pm). The Mo−C bond lengths Mo1−C13 and
Mo2−C38 in the unexpectedly formed chelate in Mo-10 are
218.2(3) and 218.4(3) pm. Generally, the Mo−Oalkoxide bonds
in all described NHC-coordinated complexes, whether bi- or
monodentate, with bond lengths ranging from 193.25 pm
(Mo-8) to 196.2(2) pm (Mo-3) are substantially longer than
those in Mo(C-(p-OMe-C6H4))(OCMe(CF3)2)3 with bond
lengths between 189.6(2) and 190.8(2) pm.5i
Figure 5. Single-crystal X-ray structure of dimeric Mo-10. Bond
lengths (pm): Mo1−C14 175.8(3), Mo1−O2 197.41(17), Mo1−O1
202.67(18), Mo1−C1 214.7(3), Mo1−C13 218.2(3), Mo1−O4
243.42(17), Mo2−C39 176.2(3), Mo2−O5 197.21(18), Mo2−O4
203.09(18), Mo2−C26 214.4(3), Mo2−C38 218.4(3), Mo2−O1
241.22(17).
We attribute this to the increase in steric congestion induced
by the additional NHC ligands. We furthermore successfully
characterized the crystal structure of a minor byproduct, W(
C-p-OMe-C6H4)(OCMe(CF3)2)3(O3·H) (W-8), formed dur-
ing the synthesis of W-4 (Scheme 3). In W-8, the supposedly
bidentate NHC ligand O2 is exclusively bound via the
phenolate moiety (W-8; Figure 6). The imidazolinium moiety
remained intact and forms a zwitterion with an anionic
tungsten center. W-8 crystallizes in the monoclinic space group
P21/c (a = 1286.93(12) pm, b = 1693.92(13) pm, c =
2151.28(18) pm; α = 90°, β = 102.129(4)°, γ = 90°). W-8
displays an SP geometry (τ = 0.052) with the alkylidyne ligand
in the apex. Anionic W-8 is coordinated by one molecule of
diethyl ether trans to the alkylidyne ligand in the solid state.
The comparable tungstate complex [W(C-p-OMe-C6H4)-
(OSiPh3)4][K]22 with the same alkylidyne ligand has a W−
C
alkylidyne bond length of 176.9(7) pm, which is almost identical
G
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Article
tungsten remains unclear at this point and cannot be explained
by the solid-state structures described above. An increase in the
reaction temperature to 80 °C led to complete loss of activity
for all molybdenum-based complexes. The most reactive
catalyst was Mo-13, with a tridentate NHC, with TONs of
780 after 3 h and 800 after 15 h. To identify the active species,
all complexes were analyzed according to a previously
published reaction sequence12 (Scheme 5): Hexafluoro-tert-
Scheme 5. Determination of the Active Species by NMR
a
Spectroscopy (C6D6)
Figure 6. Single-crystal X-ray structure of zwitterionic W-8, a very
minor byproduct in the synthesis of W-4, with the supposedly
bidentate NHC bound exclusively via the phenolic moiety. Bond
lengths (pm) and angles (deg): W−C19 176.4(9), W−O5 195.4(5),
W−O4 196.1(5), W−O3 198.6(5), W−O1 205.4(5); C19−W−O5
101.8(3), C19−W−O4 103.8(3), O5−W−O4 92.5(2), C19−W−O3
102.4(3), O5−W−O3 90.3(2), O4−W−O3 152.5(2), C19−W−O1
102.5(3), O5−W−O1 155.6(2), O4−W−O1 83.5(2), O3−W−O1
82.7(2).
with the W−C19 bond length of 176.4(9) pm in W-8.
Although providing no final proof, the crystal structure of W-8
corroborates the inner-salt-type structure proposed above for
W-5 (Scheme 3).
REACTIVITY AND ACTIVE SPECIES
■
All novel complexes Mo-3−Mo-11 and Mo-13 and W-3−W-7
were used in the homometathesis (HM) of 1-phenyl-1-
propyne (S1) at 35 °C in toluene in the presence of 5 Å
molecular sieves.4c The complexes showed moderate produc-
tivities with TONs in the range of 0−800 (Table 2).
Pentacoordinated complexes Mo-6 and Mo-7 bearing the
OC6F5 ligand did not exhibit any metathesis activity. At first
glance this was surprising, since pentafluorophenoxide was
expected to be a good leaving group. However, as already
outlined, in addition to a good leaving group a stabilized
molybdenum center is also required for improved formation of
the proposed quasi-cationic active species. Likely, the reduced
steric bulk of the OC6F5 ligands and the reduced donor
strength result in a very unstable cation, thereby counter-
balancing the favorable leaving-group character. Disappoint-
ingly, all tungsten-based complexes were inactive, whereas the
molybdenum-centered complexes showed moderate activity
(Table 2). This becomes particularly interesting for complexes
Mo-8 vs W-3 and Mo-9 vs W-4. These two groups of metal
complexes only differ in the metal. The origin of the complete
loss of activity caused by the replacement of molybdenum by
a
The proposed cationic active species B can be detected in the 19F
NMR spectrum, whereas observation of the imidazolinium salt in the
1H NMR spectrum is indicative for the formation of neutral active
species C.
butyl alcohol and 1-phenyl-1-propyne were added to a solution
of the respective complex in C6D6. The reaction was then
carefully monitored by H and 19F NMR spectroscopy to
1
check for the formation of either a proposed quasi-cationic
active species with a weakly coordinating anion (visible as
partially dissociated OCMe(CF3)2 in the 19F NMR, structure B
in Scheme 5) or the assumed neutral active species indicated
by the observation of imidazolinium salts (characteristic
proton signal in the 1H NMR spectrum, structure C in
Scheme 5), stemming from dissociation of the NHC and
reaction with the acidic alcohol (Figures S66−S68 and S71−
S81 in the Supporting Information). In contrast to our
assumptions we could not observe an enhanced formation of
the active species (maximum: approximately 1% for both Mo-3
and Mo-4) in comparison to the previously reported complex
Table 2. TONs of Mo Catalysts Containing Monodentate and Multidentate NHCs in the HM of 1-Phenyl-1-propyne with
a
Addition of 0, 20, and 500 Equiv of KOCMe(CF3)2
catalyst
Mo-3
Mo-4
Mo-5
Mo-8
Mo-9
Mo-10
Mo-11
Mo-13
TON
TON (20 equiv)
TON (500 equiv)
390 (580)
300 (560)
0 (0)
500 (640)
200 (340)
0 (0)
660 (880)
320 (610)
0 (0)
270*
400 (400)
90 (100)
0 (0)
0
250 (260)
20 (20)
0 (0)
780 (800)
450 (580)
0 (0)
a
Reaction conditions: toluene, 35 °C, 4 h (*3 h), catalyst:substrate = 1:1000, internal standard tert-butylbenzene, molecular sieves 5 Å (according
to Fu
rstner et al.4c). Values in parentheses: TON after 15 h.
̈
H
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Article
(Mo-4), CCDC 1531393 (Mo-5), CCDC 1861323 (Mo-7), CCDC
1861321 (Mo-8), CCDC 1861322 (W-4), CCDC 1897061 (Mo-10),
CCDC 1861324 (W-8) contain supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre by The Cambridge
Crystallographic Data Centre.
bearing the 1,3-diisopropylimidazol-2-ylidene ligand (Mo-
12)12 for all novel synthesized complexes. Gratifyingly, also
no dissociation of the NHC was observed.
To probe the effect of free alkoxide on the reactivity of the
active complexes, the homometathesis of 1-phenyl-1-propyne
with addition of 20 and 500 equiv of KOCMe(CF3)2 was
examined. Under the same conditions the TONs decreased for
all tested complexes when 20 equiv of KOCMe(CF3)2 was
added. The addition of 500 equiv of KOCMe(CF3)2 resulted
in complete loss of reactivity. This decrease in reactivity
supports the existence of a cationic active species, since the
addition of free alkoxide shifts the equilibrium to the
nonreactive neutral pentacoordinated complex. We hold the
tris(hexafluoro-tert-butoxide) precursor accountable for the
observed moderate activity. Probably, insufficient stabilization
of the negative charge on the dissociated hexafluoro-tert-
butoxide prevents the formation of higher amounts of the
proposed cationic species. X-type ligands with an improved
leaving-group character and ample steric demand are required.
Mo(C-(p-OMe-C6H4))(1)(OCMe(CF3)2)3 (Mo-3). 1·HBF4 (35
mg, 0.11 mmol) was suspended in toluene, and KHMDS (22 mg, 0.11
mmol) was dissolved in toluene in 10 mL glass vials. Both mixtures
were cooled to −40 °C. The solution of KHMDS was slowly added to
the solution of 1·HBF4 with stirring. The reaction mixture was stirred
for 1 h at room temperature and then filtered, cooled again to −40
°C, and then added to a cold solution of Mo-1 (100 mg, 0.12 mmol)
in a Schlenk tube equipped with a magnetic stir bar. The reaction
mixture was removed from the glovebox and stirred for 20 min at
room temperature and then for 3 h at 40 °C. In course of the reaction
a color change from brown to red was observed. The reaction mixture
was concentrated in vacuo and brought back into the glovebox. The
remaining solid was crystallized from diethyl ether/n-pentane to give
the product as a red solid (79 mg, 0.080 mmol, 67%). 1H NMR (400
MHz, C6D6): δ 7.42−7.40 (m, 2H), 6.56−6.54 (m, 2H), 6.37−6.36
(m, 2H), 5.01−4.95 (m, 1H), 3.86−3.81 (m, 1H), 3.17 (s, 3H), 2.29
3
3
(d, JH−H = 11.8 Hz, 2H), 2.08 (d, JH−H = 11.6 Hz, 2H), 1.88 (s,
3H), 1.73 (s, 6H), 1.68 (s, 2H), 1.51−1.13 (m, 14H) ppm. 19F NMR
(375 MHz, C6D6): δ −76.19 to −76.27 (m, 6F), −76.41 (s, 6F),
−77.22 (s, 6F) ppm. 13C NMR (100 MHz, CD2Cl2): δ 297.6 (Mo
CONCLUSIONS
■
Mono-, bi-, and tridentate Mo− and W−alkylidene NHC
complexes have been prepared and evaluated for their
propensity to form alkyne-metathesis-productive, presumably
cationic metal alkylidyne NHC complexes, which we believe to
represent the active species in alkyne-metathesis-active
pentacoordinated metal alkylidyne NHC complexes. Stabiliza-
tion of the partial positive charge at the metal was considered
essential. Contrary to our assumptions, the introduction of
strongly σ donating or chelating NHCs did not promote the
formation of the catalytically active cationic species. Evidently,
in analogy to the corresponding metal alkylidene triflate
complexes, which show substantial formation of a cationic
active species,23 the incorporation of good leaving groups with
sufficient steric demand, e.g. triflates, might eventually lead to
higher activity and productivity. Whether the resulting cationic
metal alkylidynes are stable enough to be used in solution or
will have to be immobilized on suitable supports is still to be
evaluated. The results will be reported in due course.
1
C), 186.2 (NCN), 160.5, 138.1, 132.7, 124.7 (q, JC−F = 289.0 Hz,
1
1
CF3), 124.6 (q, JC−F = 290.0 Hz, CF3), 124.4 (q, JC−F = 290.3 Hz,
CF3), 117.6, 117.2, 113.5, 83.5 (hept., 2JC−F = 28.4 Hz, C(CF3)2Me),
82.8 (hept., JC−F = 28.8 Hz, C(CF3)2Me)*, 61.2, 60.4, 55.8, 34.7,
2
33.7, 25.8, 25.7, 25.6, 19.8 ppm. Anal. Calcd for C35H40F18MoN2O4:
C, 42.44; H, 4.07; N, 2.83. Found: C, 42.12; H, 4.16; N, 2.83. *Only
five signals of the septet were observed. Red crystals suitable for
single-crystal X-ray diffraction were obtained by layering an almost
saturated solution of Mo-3 in CH2Cl2 with n-pentane followed by
storage at −40 °C for several days.
Mo(C-(p-OMe-C6H4))(2)(OCMe(CF3)2)3 (Mo-4). Mo-1 (200
mg, 0.24 mmol) and 2 (43 mg, 0.24 mmol) were separately dissolved
in toluene (2 mL each). Both solutions were cooled to −40 °C, and
the solution of 2 was slowly added to that of Mo-1. The resulting
reaction mixture was stirred at 40 °C for 3 h and then concentrated in
vacuo. The remaining solid was crystallized from diethyl ether/n-
pentane to give the product as a red solid (98 mg, 0.104 mmol, 44%).
1H NMR (400 MHz, CDCl3): δ 7.86 (d, 4JH−H = 1.8 Hz, 1H), 7.19−
7.16 (m, 2H), 7.09 (bs, 1H), 6.84−6.80 (m, 2H), 3.81 (s, 3H), 1.65
and 1.64 (2×s, 18H), 1.62 (s, 9H) ppm. 19F NMR (375 MHz,
CDCl3): δ −75.33 to −75.41 (m, 6F), −77.64 to −77.68 (m, 6F),
−77.92 (s, 6F) ppm. 13C NMR (100 MHz, CD2Cl2): δ 292.2 (Mo
EXPERIMENTAL SECTION
■
General Information. Unless stated otherwise, all reactions were
performed under an inert gas atmosphere (N2), either in a glovebox
(LabMaster 130, MBraun, Garching, Germany) or with standard
Schlenk techniques. CH2Cl2, diethyl ether, toluene, n-pentane, and
tetrahydrofuran were dried by a solvent purification system (SPS,
MBraun). NMR measurements were recorded on a Bruker Avance III
400 instrument. Chemical shifts are reported in ppm relative to the
solvent signal; coupling constants are listed in Hz. Single-crystal X-ray
measurements were carried out on a Nonius KappaCCD four-circle
diffractometer equipped with graphite-monochromated Mo Kα
radiation, a Micracol Fiber Optics collimator, and a Nonius FR590
generator at the Institute of General, Inorganic and Theoretical
Chemistry, University of Innsbruck, Austria (structures of Mo-4 and
Mo-5), and on a Bruker Kappa APEXII Duo diffractometer with Mo
Kα radiation at the Institute of Organic Chemistry, University of
Stuttgart (structures of Mo-3, Mo-8, Mo-10, W-4, and W-8). Starting
materials and reagents were purchased from Sigma-Aldrich (Munich,
Germany), Alfa Aesar (Karlsruhe, Germany), or ABCR (Karlsruhe,
Germany) and were used as received unless stated otherwise. The
syntheses of the novel precursor complexes (Mo-2, W-2) and
imidazolinium salts as well as alternative procedures for the synthesis
of W-3 and W-6 are described in the Supporting Information
Literature for previously published compounds is provided in the
Supporting Information CCDC 1531391 (Mo-3), CCDC 1531392
1
C), 170.2, 159.8, 138.5, 131.6, 127.3, 126.1, 124.9 (q, JC−F = 289.7
Hz, CF3), 124.4 (q, 1JC−F = 291.0 Hz, CF3), 113.7, 83.4 (hept., 2JC−F
=
2
28.0 Hz, C(CF3)2Me)*, 81.7 (hept., JC−F = 28.4 Hz, C(CF3)2Me)*,
60.2, 57.8, 55.7, 30.8, 30.2, 19.5, 19.3 ppm. Anal. Calcd for
C31H36F18MoN2O4: C, 39.65; H, 3.87; N, 2.98. Found: C, 39.66;
H, 3.85; N, 3.07. Red crystals suitable for single crystal X-ray
diffraction were obtained by layering an almost saturated solution of
Mo-4 in diethyl ether with n-pentane followed by storage at −40 °C
overnight. *Only 5 signals of the septet were observed.
Mo(C-(p-OMe-C6H4))(3)(OCMe(CF3)2)3 (Mo-5). Under exclu-
sion of light, 3·HI (100 mg, 0.118 mmol), 4 Å molecular sieves (100
mg), Ag2O (55 mg, 0.236 mmol), and CH2Cl2 (4 mL) were placed in
a Schlenk tube equipped with a magnetic stir bar. The reaction
mixture was stirred at room temperature for 1 h under exclusion of
light. Volatile components were removed under reduced pressure.
Toluene (2 mL) and Mo-1 (333 mg, 0.393 mmol) in toluene (2 mL)
were added to the reaction mixture. The reaction mixture was
removed from the glovebox, sonicated for 3 h at 40 °C under the
exclusion of light, concentrated in vacuo, and brought back into the
glovebox. The brownish green solid was extracted with diethyl ether,
filtered, and concentrated in vacuo to give a dark red solid.
Crystallization from diethyl ether/n-pentane at −40 °C gave the
I
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Article
1
product as a red solid (175 mg, 0.198 mmol, 50%). H NMR (400
MHz, C6D6): δ 7.45−7.41 (m, 2H), 6.60−6.56 (m, 2H), 3.20 (s, 3H),
2.96 (s, 3H), 1.98 (s, 3H), 1.80 (s, 6H), 1.36 (s, 3H), 1.17 (s, 3H)
ppm. 19F NMR (375 MHz, C6D6): δ −76.66 to −76.78 (m, 12F),
−77.14 to −77.22 (m, 6F) ppm. 13C NMR (100 MHz, CD2Cl2): δ
300.7 (MoC), 209.0 (NCS), 160.6, 140.2, 138.4, 133.0, 132.5,
124.7 (q, 1JC−1 F = 289.7 Hz, CF3), 124.28 (q, 1JC−F = 290.0 Hz, CF3),
124.1 (q, JC−F = 289.8 Hz, CF3), 113.7, 83.8−82.7 (m,
C(CF3)2Me)*, 55.8, 40.6, 20.0, 19.7, 12.2, 12.0 ppm. Anal. Calcd
for C26H25F18MoNO4S: C, 35.27; H, 2.85; N, 1.58. Found: C, 35.08;
H, 3.09; N, 1.59. *Septet poorly resolved. Red crystals suitable for
single-crystal X-ray diffraction were obtained by layering a saturated
solution of Mo-5 in diethyl ether with n-pentane followed by storage
at −40 °C overnight.
137.8, 136.4, 135.2, 132.5, 132.2, 129.7, 129.3, 128.7, 126.2, 124.7 (q,
1
1JC−F = 287.7 Hz), 124.1 (q, JC−F = 290.3 Hz), 120.3, 118.8, 118.2,
112.9, 83.7−81.6 (m, C(CF3)2Me)*, 55.8, 50.3, 20.0, 19.5, 19.3, 18.9
ppm. Despite numerous efforts, no satisfactory elemental analysis data
could be obtained due to fast decomposition. Nonetheless, its
structure was unambiguously confirmed by single-crystal X-ray
analysis. *Septet poorly resolved.
Mo(C-(p-OMe-C6H4))(O2)(OCMe(CF3)2)2 (Mo-9). HO2·HCl
(56.0 mg, 0.18 mmol) in THF (2 mL) and LiHMDS (59.8 mg, 0.36
mmol) in THF (2 mL) were cooled to −40 °C. The LiHMDS
solution was added dropwise to the HO2·HCl suspension. The
reaction mixture was stirred for 1 h at room temperature and cooled
to −40 °C. The cooled mixture was then added dropwise to a solution
of Mo-1 (150 mg, 0.18 mmol) in THF (2 mL) at −40 °C. After the
reaction mixture was stirred for 16 h at room temperature, all volatiles
were removed in vacuo and the residue was washed with n-pentane (2
mL) and redissolved in CH2Cl2. The mixture was filtered through a
pad of Celite and all volatiles were removed in vacuo. The solid
residue was crystallized from diethyl ether and n-pentane. The
product was isolated in the form of dark red crystals (106 mg, 0.12
Mo(C-(p-OMe-C6H4))(4)(OC6F5)3 (Mo-6). Mo-2 (226 mg, 0.3
mmol) was dissolved in toluene (approximately 15 mL) and cooled to
−40 °C. 4 (81 mg, 0.3 mmol) was added to toluene (approximately 5
mL), and the solution was slowly added to the metal complex
solution. The mixture was stirred for 3 h at room temperature, and the
solvent was removed. The resulting greenish residue was washed with
1
1
diethyl ether to afford Mo-6 (252 mg, 0.23 mmol, 89%). H NMR
mmol, 70%). H NMR (400 MHz, CDCl3): δ 7.12−7.08 (m, 2H),
(400 MHz, CDCl3): δ 7.16 (s, 2H), 6.65 (br s, 4H)*, 6.48 (s, 4H),
3.75 (s, 3H), 2.15 (s, 6H), 1.99 (s, 12H) ppm. 19F NMR (375 MHz,
CDCl3): δ −160.3 (m, 4F), −162.4 (m, 2F), −166.2 (m, 4F), −168.0
(m, 2F), −171.1 (m, 2F), −174.0 (m, 1F) ppm. 13C NMR* (100
MHz, CD2Cl2): δ 310.2 (MoC), 189.9 (NCN), 160.8, 143.3−136.1
(m, CAr, OC6F5), 140.1, 135.5, 133.0, 129.1, 124.1, 111.8, 55.4, 21.0,
17.5 ppm. Anal. Calcd for C47H31F15MoN2O4: C, 52.82; H, 2.92; N,
2.62; Found: C, 52.86; H, 2.959; N. 2.70. *Only seven out of the
expected nine aromatic signals are observed; however, this is in
7.02−6.96 (m, 1H), 6.93−6.86 (m, 1H), 6.79 (s, 1H), 6.51 (m, 4H),
6.31 (s, 1H), 4.29−4.20 (m, 2H), 3.98 (m, 2H), 3.75 (s, 3H), 2.30 (s,
3H), 2.26 (s, 3H), 2.03 (s, 3H), 1.69 (s, 3H), 1.25 (s, 3H) ppm. 19F
NMR (375 MHz, CDCl3): δ −77.09 (q, 4JF−F = 9.6 Hz, 3F), −77.29
4
(s, 3F), −77.47 (q, JF−F = 9.6 Hz, 6F) ppm. 13C NMR (100 MHz,
CDCl3): δ 307.9 (MoC), 208.9 (NCN), 159.8, 138.2, 137.4, 136.5,
135.8, 135.1, 131.8, 131.7, 129.7, 129.4, 129.2, 128.4, 126.3, 125.5,
1
1
124.1 (q, JC−F = 288.8 Hz)*, 123.7 (q, JC−F = 288.0 Hz)*, 120.4,
119.5, 118.1, 112.4, 83.2−81.0 (m, C(CF3)2Me)**, 55.3, 53.2, 49.4,
20.9, 19.9, 19.0, 18.4, 18.1 ppm. Anal. Calcd for C34H32F12MoN2O4:
C, 47.67; H, 3.77; N, 3.27. Found: C, 47.42; H, 4.063; N, 3.29.
*Quartet poorly resolved. **Septet poorly resolved.
1
accordance with the H NMR spectrum, in which only one broad
signal for all aromatic protons of the benzylidyne ligand is visible.
Mo(C-(p-OMe-C6H4))(5)(OC6F5)3 (Mo-7). Mo-2 (101.7 mg,
0.120 mmol) was suspended in toluene and cooled to −40 °C. A
solution of 5 (18.1 mg, 0.12 mmol) in toluene was cooled to −40 °C
and added dropwise to the suspension. The reaction mixture was
stirred for 1 h while it was warmed to room temperature and then
stirred for 3 h at 40 °C. All volatiles were removed under reduced
pressure, and the resulting solid was crystallized in CH2Cl2, diethyl
ether, and n-pentane to yield the product as dark brown crystals (39.0
mg, 0.04 mmol, 35%). 1H NMR (400 MHz, CD2Cl2) δ 7.19 (s, 2H),
6.69 (s, 4H), 5.01 (m, 2H), 3.76 (s, 3H), 1.27 (d, 3JH−H = 6.6 Hz, iPr
12H) ppm. 19F NMR (375 MHz, CD2Cl2): δ −162.44 (s, 6F),
−166.33 (s, 4F), −167.45 (s, 2F), −171.44 (s, 2F), −173.20 (s, 1F)
ppm. 13C NMR (100 MHz, C6D6): δ 304.4 (MoC), 184.6 (NCN),
[Mo(C-(p-OMe-C6H4))(O3)(OCMe(CF3)2)]2 (Mo-10). HO3·
HCl (54.1 mg, 0.21 mmol) in THF (2 mL) and LiHMDS (71.7
mg, 0.43 mmol) in THF (2 mL) were cooled to −40 °C. The
LiHMDS solution was added dropwise to the HO3·HCl solution.
The reaction mixture was stirred for 1 h at room temperature and
cooled to −40 °C. The cooled mixture was then added dropwise to a
solution of Mo-1 (180 mg, 0.21 mmol) in THF (2 mL) at −40 °C.
After the reaction mixture was stirred for 16 h at room temperature,
all volatiles were removed in vacuo and the residue was washed with
n-pentane (2 mL) and redissolved in CH2Cl2. The mixture was
filtered through a pad of Celite, and all volatiles were again removed
in vacuo. The solid residue was crystallized from CH2Cl2. The
product was isolated in the form of a yellow solid (73 mg, 0.10 mmol,
47%). 1H NMR (400 MHz, CD2Cl2): δ 7.19 (d, 3JH−H = 8.0 Hz, 1H),
1
1
161.4, 143.8, 139.7 (d, CF, JC−F = 241.8 Hz), 138.5 (d, CF, JC−F
=
1
248.0 Hz), 137.4, 134.8 (d, CF, JC−F = 245.5 Hz), 131.9, 117.3,
113.6, 54.8, 53.2, 23.2 ppm. Anal. Calcd for C35H23F15MoN2O4: C,
45.87; H, 2.53; N, 3.06. Found: C, 45.55; H, 2.74; N, 3.20.
3
4
7.04−6.96 (m, 1H), 6.82 (s, 1H), 6.82 (dd, JH−H = 5.3 Hz, JH−H
=
3
3
1.0 Hz, 1H), 6.64 (d, JH−H = 8.5 Hz, 2H), 6.57 (d, JH−H = 8.9 Hz,
2H), 4.37−4.22 (m, 1H), 4.14−3.97 (m, 1H), 3.78−3.59 (m, 5H),
Mo(C-(p-OMe-C6H4))(O1)(OCMe(CF3)2)2 (Mo-8). HO1·HCl
(53.5 mg, 0.18 mmol) in THF (2 mL) and LiHMDS (59.8 mg, 0.36
mmol) in THF (2 mL) were cooled to −40 °C. The LiHMDS
solution was added dropwise to the HO1·HCl suspension. The
reaction mixture was stirred for 1 h at room temperature and cooled
to −40 °C. The cooled mixture was then added dropwise to a solution
of Mo-1 (150 mg, 0.18 mmol) in THF (2 mL) at −40 °C. After the
reaction mixture was stirred for 16 h at room temperature, the
volatiles were removed in vacuo and the residue was washed with n-
pentane (2 mL) and redissolved in CH2Cl2. The mixture was filtered
through a pad of Celite, and all volatiles were removed in vacuo. The
solid residue was crystallized from diethyl ether and n-pentane. The
product was isolated in the form of dark violet crystals (72 mg, 0.21
3
2.46 (s, 1H), 2.00 (d, JH−H = 10.6 Hz, 1H), 1.69 (s, 3H), 1.51 (s,
3H), 1.33 (s, 3H) ppm. 19F NMR (375 MHz, CD2Cl2): δ −78.08 (s,
3F), −78.74 (q, JF−F = 9.4 Hz, 3F) ppm. 13C NMR (100 MHz,
4
CD2Cl2): δ 302.4 (MoC), 220.1 (NCN), 159.4, 156.4, 139.9,
1
131.6, 131.4, 124.9 (q, JC−F = 288.3 Hz), 124.0, 119.7, 119.5, 114.3,
2
113.3, 81.4 (sept., JC−F = 28.4 Hz, C(CF3)2Me), 67.8, 63.9, 55.7,
49.8, 43.8, 29.9, 26.7, 20.9 ppm. Despite numerous efforts, no
satisfactory elemental analysis data could be obtained due to
substoichiometric incorporation of tetrahydrofuran and CH2Cl2 (as
confirmed by single-crystal X-ray analysis).
Mo(C-(p-OMe-C6H4))(O4)(OCMe(CF3)2)2 (Mo-11). HO4·HCl
(82.7 mg, 0.24 mmol) in THF (2 mL) and LiHMDS (79.7 mg, 0.48
mmol) in THF (2 mL) were cooled to −40 °C. The LiHMDS
solution was added dropwise to the HO4·HCl solution. The reaction
mixture was stirred for 1 h at room temperature and cooled to −40
°C. The cooled mixture was then added dropwise to a solution of Mo-
1 (200 mg, 0.24 mmol) in THF (2 mL) at −40 °C. After the reaction
mixture was stirred for 16 h at room temperature, all volatiles were
removed in vacuo and the residue was washed with n-pentane (2 mL)
and redissolved in CH2Cl2. The mixture was filtered through a pad of
1
mmol, 48%). H NMR (400 MHz, CDCl3): δ 7.14−7.01 (m, 4H),
6.91 (m, 2H), 6.74 (d, 3JH−H = 7.3 Hz, 1H), 6.54−6.45 (m, 4H), 4.38
3
(d, JH−H = 10.5 Hz, 1H), 4.28−4.18 (m, 1H), 4.18−4.07 (m, 1H),
3
3.87 (q, JH−H = 10.7 Hz, 1H), 3.72 (s, 3H), 2.38 (s, 3H), 2.23 (s,
3H), 1.72 (s, 3H), 1.28 (s, 3H) ppm. 19F NMR (375 MHz, CDCl3): δ
−77.12 to −77.26 (m, 3F), −77.32 (q, 4JF−F = 9.8 Hz, 3F), −77.38 to
−77.51 (m, 3F), −77.72 to −77.88 (m, 3F) ppm. 13C NMR (100
MHz, CDCl3): δ 307.1 (MoC), 208.2 (NCN), 160.3, 158.7, 139.1,
J
DOI: 10.1021/acs.organomet.9b00481
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Celite, and all volatiles were again removed in vacuo. The solid
residue was crystallized from diethyl ether and n-pentane. The
product was isolated in the form of a dark solid (153 mg, 0.18 mmol,
75%). 1H NMR (400 MHz, CD2Cl2): δ 7.52−7.25 (m, 6H), 7.18 (d,
3JH−H = 5.8 Hz, 1H), 6.99 (m, 3H), 6.83 (s, 2H), 6.75−6.64 (m, 1H),
6.41 (d, 3JH−H = 8.9 Hz, 2H), 6.27 (s, 2H), 4.80−4.64 (m, 1H), 4.55−
4.43 (m, 1H), 4.15−3.93 (m, 2H), 3.64 (s, 3H), 1.60 (s, 3H), 1.39 (s,
then cooled to −40 °C. Separately, W-1 (1.03 g, 1.1 mmol) was
dissolved in toluene and added dropwise at −40 °C to the NHC
solution. The reaction mixture was stirred overnight and filtered
through Celite, and the solvent was removed in vacuo. The orange
residue was washed with n-pentane, and the pure product W-4 was
1
crystallized from diethyl ether (0.62 g, 0.66 mmol, 60%). H NMR
(400 MHz, C6D6): δ 7.39 (dd, 3JH−H = 8.1, 4JH−H = 0.8 Hz, 1H), 7.02
(ddd, 3JH−H = 8.1 Hz, 7.5 Hz, 4JH−H = 1.6 Hz, 1H), 6.75 (td, 3JH−H
=
4
3H) ppm. 19F NMR (375 MHz, CD2Cl2): δ −75.77 (d, JF−F = 7.4
7.7 Hz, 4JH−H = 1.4 Hz, 1H), 6.57−6.52 (m, 2H), 6.51−6.42 (m, 5H),
4
Hz, 3F), −76.26 (d, JF−F = 8.2 Hz, 3F), −77.18 to −77.33 (m, 3F),
3.15 (s, 3H), 3.11−3.04 (m, 1H), 2.98 (dd, 2JH−H = 20.7 Hz, 3JH−H
10.2 Hz, 1H), 2.88−2.81 (m, 1H), 2.65 (dd, 2JH−H = 21.2 Hz, 3JH−H
=
=
−77.37 to −77.49 (m, 3F) ppm. 13C NMR (100 MHz, CD2Cl2): δ
302.9 (MoC), 208.1 (NCN), 163.2, 159.8, 139.4, 139.3, 138.2,
137.1, 132.6, 132.0, 128.9, 128.7, 128.1, 127.5, 125.7, 125.0, 124.8 (q,
1JC−F = 289.5 Hz), 118.7, 118.3, 116.1, 112.8, 82.7 (m, C(CF3)2Me)*,
55.7, 49.7, 21.2, 19.4 ppm**. Anal. Calcd for C37H30F12MoN2O4: C,
49.90; H, 3.40; N, 3.15. Found: C, 49.50; H, 3.548; N, 3.23. *Septet
poorly resolved. **Missing signals in the aromatic region are due to
peak overlapping and broad peaks.
10.1 Hz, 1H), 2.18 (s, 3H), 2.12 (s, 3H), 2.01 (s, 3H), 1.99 (s, 3H),
1.69 (s, 3H) ppm. 19F NMR (375 MHz, C6D6): δ −75.98 to −76.15
4
4
(m, 3F), −76.46 (q, JF−F = 9.8 Hz, 3F), −77.01 (q, JF−F = 9.4 Hz,
3F), −77.38 (q, JF−F = 9.2 Hz, 3F) ppm. 13C NMR (100 MHz,
4
C6D6): δ 293.0 (WC), 215.1 (NCN), 160.0, 159.3, 138.1, 138.0,
136.6, 135.3, 135.2, 133.8,133.0, 130.2, 130.0, 126.0, 124.7 (q, 1JC−F
=
288.1 Hz), 120.0, 118.3, 112.2, 82.7 (m, C(CF3)2Me)*, 54.6, 53.1,
49.1, 20.7, 20.3, 19.8, 19.1, 19.0 ppm. Anal. Calcd for
C34H32F12N2O4W: C, 43.24; H, 3.42; N, 2.97. Found: C, 43.53; H,
3.77; N, 2.83. *Septet poorly resolved. Crystals suitable for single-
crystal X-ray analysis were obtained from diethyl ether.
Mo(C-(p-OMe-C6H4))(O21)(OCMe(CF3)2)2 (Mo-13). HO21·H
(131.2 mg, 0.31 mmol) and LiHMDS (103.6 mg, 0.62 mmol) were
suspended/dissolved in THF (each 10 mL) and cooled to −40 °C.
The LiHMDS solution was added to the imidazolinium salt
suspension, and the mixture was stirred at room temperature for 1
h. Mo-1 (260 mg, 0.31 mmol) was dissolved in THF (15 mL) and
cooled to −40 °C. The reaction mixture was cooled to −40 °C and
was then slowly added to the solution of the metal complex. The
reaction mixture was stirred at room temperature for 16 h, and then
all volatiles were removed in vacuo and the residue was washed with
n-pentane (2 × 30 mL). The resulting residue was crystallized from
diethyl ether. The product was isolated in the form of a red solid (189
mg, 0.23 mmol, 75%). 1H NMR (400 MHz, CDCl3): δ 7.20 (s, 2H),
W(C-(p-OMe-C6H4))(O3)(OCMe(CF3)2)3 (W-5). HO3·HBF4
(33 mg, 0.11 mmol, 1 equiv) was dissolved in diethyl ether, and
KH (9 mg, 0.23 mmol, 2.1 equiv) was added. After 3 h of stirring at
room temperature, the potassium salt of the ionically tagged alkoxide
was filtered off, suspended in toluene, and slowly added dropwise at
−40 °C to a toluene solution of W-1 (0.11 g, 0.12 mmol, 1 equiv).
The reaction mixture was stirred at room temperature for 3 h and
filtered through Celite, and the solvent was removed in vacuo. The
red residue was washed with n-pentane and dried (65.6 mg, 0.06
3
6.89 (d, 3JH−H = 8.9 Hz, 2H), 6.72 (d, JH−H = 9.0 Hz, 2H), 4.54 (d,
1
2
2JH−H = 14.3 Hz, 2H), 4.48 (d, JH−H = 14.3 Hz, 2H), 3.76 (s, 3H),
mmol, 53%). H NMR (400 MHz, C6D6): δ 8.63 (s, 1H), 7.92 (d,
3
3JH−H = 8.2 Hz, 1H), 7.23 (d, JH−H = 8.7 Hz, 2H), 7.12−7.04 (m,
1.82 (s, 3H) ppm. 19F NMR (375 MHz, CDCl3): δ −75.46 (q, 4JF−F
=
C
3
4
9.8 Hz, 6F), −77.01 (q, 4JF−F = 9.9 Hz, 6F), −78.55 (s, 6F) ppm. 13
1H), 6.84−6.78 (m, 2H), 6.62 (td, JH−H = 7.9 Hz, JH−H = 1.0 Hz,
1H), 6.48−6.43 (m, 1H), 3.31 (s, 3H), 2.94−2. 87 (m, 2H), 2.45−
2.36 (m, 2H), 2.11 (s, 3H), 1.97 (s, 6H), 0.82 (s, 9H) ppm. 19F NMR
(375 MHz, C6D6): δ −76.87 to −77.01 (m, 12F), −77.13 to −77.20
NMR (100 MHz, CDCl3): δ 310.1 (MoC), 189.4 (NCN), 160.2,
1
1
138.0, 131.3, 123.9 (q, JC−F = 287.0 Hz), 123.0 (q, JC−F = 289.2
1
2
Hz), 122.9 (q, JC−F = 291.4 Hz), 121.8, 113.0, 82.5 (sept., JC−F
=
(m, 6F) ppm. 13C NMR (100 MHz, C6D6): δ 272.3 (WC), 159.7
29.1 Hz), 80.8 (sept., 2JC−F = 28.4 Hz), 55.3, 49.8, 20.2 ppm**. Anal.
Calcd for C23H16F18MoN2O4: C, 33.59; H, 1.96; N, 3.41. Found: C,
33.79; H, 2.139; N, 3.48.
1
(NCN), 157.1, 155.1, 139.2, 134.8, 128.8, 126.0, 125.3 (q, JC−F
=
2
290.0 Hz), 120.5, 120.2, 118.3, 113.1, 82.5 (sept., JC−F, = 27.0 Hz,
C(CF3)2Me)*, 56.8, 54.9, 53.3, 50.0, 43.3, 27.2, 20.2, 19.8 ppm. Anal.
Calcd for C33H34F18N2O5W: C, 37.24; H, 3.22; N, 2.63. Found: C,
37.19; N, 3.34; H, 2.75. *Only five signals of the septet were
observed.
W(C-(p-OMe-C6H4))(O1)(OCMe(CF3)2)2 (W-3). HO1·HBF4
(0.1 g, 0.28 mmol, 1 equiv) was suspended in toluene, and LiHMDS
(99 mg, 0.59 mmol, 2.1 equiv) was added as a solid at −40 °C. The
yellow reaction mixture was stirred for 2 h at room temperature and
then cooled again to −40 °C. Separately, W-1 (0.26 g, 0.28 mmol)
was dissolved in toluene and added dropwise at −40 °C to the NHC
solution. The reaction mixture was stirred overnight and filtered
through Celite, and the solvent was removed under reduced pressure.
The residue was washed with n-pentane, and the clean orange product
W(C-(p-OMe-C6H4))(O1)(OCMe2(CF3))2 (W-6). W-6 was
synthesized analogously to W-3 by the reaction of HO1·HBF4 with
1
W-2 and isolated as a yellow solid (95 mg, 0.12 mmol, 39%). H
NMR (400 MHz, CDCl3): δ 7.17−7.11 (m, 4H), 7.07 (m, 2H),
3
3
6.91−6.83 (m, 1H), 6.56 (d, JH−H = 9.0 Hz, 2H), 6.12 (d, JH−H
=
1
8.9 Hz, 2H), 4.55−4.43 (m, 1H), 4.20−4.04 (m, 2H), 3.69 (s, 3H),
3.62 (m, 1H), 2.42 (s, 3H), 2.21 (s, 3H), 1.62 (s, 3H), 1.50 (s, 3H),
1.35 (s, 3H), 0.74 (s, 3H) ppm. 19F NMR (375 MHz, CDCl3) δ
−80.97 (s, 3F), −82.01 (s, 3F) ppm. 13C NMR (100 MHz, CDCl3) δ
284.9 (WC), 217.7 (NCN), 161.8, 158.5, 139.5, 139.3, 135.1,
W-3 was isolated from diethyl ether (0.11 g, 0.12 mmol, 44%). H
NMR (400 MHz, C6D6): δ 7.38 (dd, 3JH−H = 8.1 Hz, 4JH−H = 0.9 Hz;
3
4
1H) 7.02 (ddd, JH−H = 8.1 Hz, 7.5 Hz, JH−H = 1.6 Hz, 1H), 6.85−
6.70 (m, 4H), 6.56−6.47 (m, 3H), 6.46−6.41 (m, 2H), 3.13 (s, 3H),
3.11−2.93 (m, 2H), 2.80−2.71 (m, 1H), 2.59−2.47 (m, 1H), 2.17 (s,
3H), 2.10 (s, 3H), 1.99 (s, 3H), 1.72 (s, 3H) ppm. 19F NMR (375
MHz, C6D6) δ −75.91 (q, 4JF−F = 8.9 Hz, 3F), −76.46 (q, 4JF−F = 9.8
1
134.6, 134.2, 133.1, 129.9, 129.6, 128.3, 127.9 (q, JC−F = 285.3 Hz,
1
CF3), 126.6 (q, JC−F = 286.9 Hz, CF3)*, 125.2, 118.5, 117.5, 117.0,
2
2
4
4
112.0, 80.7 (q, JC−F = 28.6 Hz, C(CF3)Me2), 98.0 (q, JC−F = 28.3
Hz, C(CF3)Me2), 55.3, 53.6, 49.5, 26.1, 25.6, 25.3, 23.2, 20.0, 19.1
ppm. Anal. Calcd for C33H36F6WN2O4: C, 48.19; H, 4.41; N, 3.41.
Found: C, 48.18; H, 4.50; N, 3.39. *One signal of quartet overlaps
with signal at δ 125.19 ppm. An alternative route to this compound is
provided in the Supporting Information.
Hz, 3F), −76.98 (q, JF−F = 9.1 Hz, 3F), −77.31 (q, JF−F = 9.1 Hz,
3F) ppm. 13C NMR (100 MHz, C6D6): δ 292.9 (WC), 215.1
(NCN), 160.4, 160.1, 138.8, 138.2, 135.4, 135.2, 134.1, 133.1, 129.7,
129.5, 125.9, 122.0 (m, CF3)*, 119.7, 118.3, 117.9, 112.3, 82.9 (m,
C(CF3)2Me)**, 54.6, 53.1, 49.2, 20.7, 19.7, 19.2, 19.1 ppm. Anal.
Calcd for C33H30F12WN2O4: C, 42.60; H, 3.25; N, 3.01. Found: C,
42.75; H, 3.22; N, 3.15. *Quartet signals poorly resolved, overlapping
with the C6D6 signals. **Septet poorly resolved. An alternative
synthetic route to this compound is provided in the Supporting
Information.
W(C-(p-OMe-C6H4))(O2)(OCMe2(CF3))2 (W-7). W-7 was
synthesized analogously to W-4 by the reaction of HO2·HBF4 with
1
W-2 and isolated as a yellow solid (45 mg, 0.05 mmol, 65%). H
NMR (400 MHz, C6D6): δ 7.41 (dd, 3JH−H = 8.1 Hz, 4JH−H = 1.3 Hz,
1H) 7.11 (ddd, 3JH−H = 8.0 Hz, 7.5 Hz, 3JH−H = 1.6 Hz, 1H), 6.83 (td,
W(C-(p-OMe-C6H4))(O2)(OCMe(CF3)2)2 (W-4). HO2·HBF4
(0.401 g, 1.1 mmol, 1 equiv) was suspended in toluene, and LiHMDS
(0.383 g, 2.3 mmol, 2.1 equiv) was added as a solid at −40 °C. The
yellow reaction mixture was stirred for 2 h at room temperature and
3
3JH−H = 7.6 Hz, JH−H = 1.4 Hz, 1H), 6.68−6.44 (m, 7H), 3.18 (m,
1H), 3.13 (s, 3H), 3.10−3.01 (m, 1H), 2.81−2.72 (m, 1H), 2.46 (m,
1H), 2.07 (s, 3H), 2.05 (s, 3H), 2.03 (s, 3H), 1.96 (s, 3H), 1.83 (s,
K
DOI: 10.1021/acs.organomet.9b00481
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
3H), 1.64 (s, 3H), 1.03 (s, 3H; CH3) ppm. 19F NMR (375 MHz,
C6D6): δ −81.50 (s, 3F), −80.65 (s, 3F) ppm. 13C NMR (100 MHz,
C6D6): δ 285.2 (WC), 217.4 (NCN), 162.9, 159.3, 139.8, 137.8,
137.3, 134.7, 134.6, 133.8, 133.6, 130.8, 130.1, 128.8 (q, 1JC−F = 285.5
the rise. Angew. Chem., Int. Ed. 2013, 52, 2794−2819. (c) Schrock, R.
R. Alkyne metathesis by molybdenum and tungsten alkylidyne
complexes. Chem. Commun. 2013, 49, 5529−5531. (d) Zhang, W.;
Moore, J. S. Alkyne metathesis: Catalysts and synthetic applications.
Adv. Synth. Catal. 2007, 349, 93−120. (e) 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., Int. Ed. 2011, 50, 3435−3438.
1
Hz, CF3), 127.6 (q, JC−F = 286.2 Hz, CF3), 125.4, 118.2, 117.8,
2
2
117.3, 112.5, 81.3 (q, JC−F = 28.4 Hz, C(CF3)Me2), 80.4 (q, JC−F
=
27.8 Hz, C(CF3)Me2), 54.6, 53.2, 48.7, 26.7, 26.0, 25.6, 23.6, 20.7,
19.8, 18.8, ppm. Anal. Calcd for C34H38F6N2O4W: C, 48.82; H, 4.58;
N, 3.35. Found: C, 48.83; H, 4.59; N, 3.47.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00481.
■
S
Spectra of all novel compounds/mechanistic experi-
ments/in situ NMR measurements of reaction mixtures,
single-crystal X-ray data of Mo-3, Mo-4, Mo-5, Mo-8,
Mo-10, W-4, and W-8, literature for the synthesis of
known compounds, and information on calculation of
the TEP (PDF)
Cartesian coordinates of the calculated structures (XYZ)
Accession Codes
CCDC 1531391−1531393, 1861321−1861324, and 1897061
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.ca-
m.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
*(M.R.B.): michael.buchmeiser@ipoc.uni-stuttgart.de.
ORCID
■
Hoveyda, A. H.; Tsay, C.; Muller, P. Syntheses of molybdenum oxo
̈
alkylidene complexes through addition of water to an alkylidyne
complex. J. Am. Chem. Soc. 2018, 140, 2797−2800.
Jan Meisner: 0000-0002-1301-2612
(4) (a) Furstner, A.; Mathes, C.; Lehmann, C. W. Mo[N(t-
̈
̈
Johannes Kastner: 0000-0001-6178-7669
Bu)(Ar)]3 complexes as catalyst precursors: In situ activation and
application to metathesis reactions of alkynes and diynes. J. Am. Chem.
̈
Soc. 1999, 121, 9453−9454. (b) Furstner, A.; Guth, O.; Rumbo, A.;
Michael R. Buchmeiser: 0000-0001-6472-5156
Present Addresses
M.K.: Organisch-Chemisches Institut, Westfalische Wilhelms-
Seidel, G. Ring closing alkyne metathesis. Comparative investigation
of two different catalyst systems and application to the stereoselective
synthesis of olfactory lactones, azamacrolides, and the macrocyclic
perimeter of the marine alkaloid nakadomarin A. J. Am. Chem. Soc.
1999, 121, 11108−11113. (c) Heppekausen, J.; Stade, R.; Goddard,
#
̈
̈
̈
̈
Universitat Munster, Correnstrasse 40, 48149 Munster,
Germany.
∇J.M.: Department of Chemistry and The PULSE Institute,
Stanford University, Stanford, California 94305, United States,
and SLAC National Accelerator Laboratory, Menlo Park,
California 94025, United States.
R.; Furstner, A. Practical new silyloxy-based alkyne metathesis
̈
catalysts with optimized activity and selectivity profiles. J. Am.
Chem. Soc. 2010, 132, 11045−11057. (d) Heppekausen, J.; Stade, R.;
Kondoh, A.; Seidel, G.; Goddard, R.; Furstner, A. Optimized
̈
Author Contributions
synthesis, structural investigations, ligand tuning and synthetic
⊥I.E., J.G., P.M.H., M.K., and M.v.d.E. contributed equally to
this paper.
evaluation of silyloxy-based alkyne metathesis catalysts. Chem. - Eur.
̈
J. 2012, 18, 10281−10299. (e) Heppekausen, J.; Furstner, A.
Rendering Schrock-type molybdenum alkylidene complexes air stable:
user-friendly precatalysts for alkene metathesis. Angew. Chem., Int. Ed.
2011, 50, 7829−7832.
(5) (a) 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., Int. Ed. 2007, 46, 8890−8894. (b) 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. (c) Beer, S.; Brandhorst, K.; Hrib, C.
G.; Wu, X.; Haberlag, B.; Grunenberg, J.; Jones, P. G.; Tamm, M.
Experimental and theoretical investigations of catalytic alkyne cross-
metathesis with imidazolin-2-iminato tungsten alkylidyne complexes.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by the Deutsche Forschungsgemeinschaft
(DFG, German Research Foundation) project number
358283783-CRC 1333) is gratefully acknowledged.
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