Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Catalysts

Article abstract of DOI:10.1002/anie.201913322

The origin of hydroxyl group tolerance in neutral and especially cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes has been investigated. A wide range of catalysts was prepared and tested. Most cationic complexes can be handled in air without difficulty and display an unprecedented stability towards water and alcohols. NHC complexes were successfully used with substrates containing the hydroxyl functionality in acyclic diene metathesis polymerization, homo-, cross and ring-opening cross metathesis reactions. The catalysts remain active even in 2-PrOH and are applicable in ring-opening metathesis polymerization and alkene homometathesis using alcohols as solvent. The use of weakly basic bidentate, hemilabile anionic ligands such as triflate or pentafluorobenzoate and weakly basic aromatic imido ligands in combination with a sterically demanding 1,3-dimesitylimidazol-2-ylidene NHC ligand was found essential for reactive and yet robust catalysts.

Full text of DOI:10.1002/anie.201913322

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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International Edition
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Accepted Article
Title: Origin and Use of Hydroxyl Group Tolerance in Cationic
Molybdenum Imido Alkylidene N-Heterocyclic Carbene Catalysts
Authors: Roman Schowner, Iris Elser, Mathis Benedikter, Mohasin
Momin, Wolfgang Frey, Tanja Schneck, Laura Stoehr, and
Michael Rudolf Buchmeiser
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201913322
Angew. Chem. 10.1002/ange.201913322
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10.1002/anie.201913322
Angewandte Chemie International Edition
RESEARCH ARTICLE
Origin and Use of Hydroxyl Group Tolerance in Cationic
Molybdenum Imido Alkylidene N-Heterocyclic Carbene Catalysts
Roman Schowner[a], Iris Elser[a], Mathis Benedikter[a], Mohasin Momin[a], Wolfgang Frey[b], Tanja
Schneck[a], Laura Stöhr[a], Michael R. Buchmeiser*[a]
Dedicated to Prof. Richard R. Schrock on the occasion of his 75th birthday
Abstract: The origin of hydroxyl group tolerance in neutral and
especially cationic molybdenum imido alkylidene N-heterocyclic
carbene (NHC) complexes has been investigated. A wide range of
catalysts was prepared and tested for their stability and reactivity.
Most cationic complexes can be handled in air without difficulty and
display an unprecedented stability towards water and alcohols. NHC
complexes were successfully used with substrates containing the
hydroxyl functionality in acyclic diene metathesis polymerization,
homo-, cross and ring-opening cross metathesis reactions. It was
found that these catalysts remain active even in wet 2-PrOH and are
applicable in ring-opening metathesis polymerization and alkene
homometathesis using alcohols as a solvent. The use of weakly basic
bidentate, hemilabile anionic ligands such as triflate or
pentafluorobenzoate and weakly basic aromatic imido ligands in
combination with a sterically demanding 1,3-dimesitylimidazol-2-
ylidene NHC ligand was found essential for reactive and yet robust
catalysts.
not address the challenge of dealing with protic groups in
substrates or with traces of water, since the active species is
structurally not changed and thus remains very sensitive.
Therefore, costly purification and drying protocols still must be
followed, even for unfunctionalized substrates or solvents. First
studies on neutral, pentacoordinate Mo imido alkylidene NHC
complexes revealed their remarkable stability towards functional
groups including alcohols, carboxylic acids, aldehydes or
amines,[4] which in general deactivate Schrock-type catalysts by
protonation or via Wittig-type reactions with the alkylidene
ligand.[5] So far, this enhanced functional group tolerance has only
been exploited in the preparation of polymers via ring opening
metathesis polymerization (ROMP) or the cyclopolymerization of
,-diynes by neutral, pentacoordinate molybdenum imido
alkylidene NHC bistriflate species.
Results and Discussion
Introduction
We were interested in the question, whether the reported
tolerance towards hydroxyl groups and general reactivity would
be retained under more harsh conditions and employed the
previously reported[4a] neutral pentacoordinate complex Mo(N-
2,6-Me2-C6H3)(CHCMe2Ph)(IMesH2)(OTf)2 (1, IMesH2 = 1,3-
dimesitylimidazolin-2-ylidene, OTf = CF3SO3) in the acyclic diene
metathesis (ADMET) polymerization of 6-hydroxy-1,10-
undecadiene (M1)[6] and diphenylbis(pent-4-en-1-yl)silane (M2) at
80 °C and 20 mbar,[7] resulting in the formation of high molecular
weight poly-M1 (Mn = 11,000 g.mol-1, Đ = 2.0, 16% trans) and
poly-M2 (Mn = 433,000 g.mol-1, Đ = 1.8, 77% trans), both in 95%
isolated yield. Encouraged by these results, we set out to explore
whether the potentially more active cationic catalysts also display
enhanced stability in the presence of protic groups. This seemed
not unlikely since the pentacoordinate complexes have been
identified as precursors to the actual olefin metathesis-active four
coordinate cationic catalysts.[4a] We particularly tried to identify the
structural motifs that are responsible for their unique properties.
Although NHC-free cationic molybdenum alkylidene complexes
have successfully been prepared over a decade ago,[8] their
moderate stability and low reactivity led to their abandonment
quickly after. We started our investigations by revisiting these
compounds in form of 2a-c (Figure 1) bearing electron-
withdrawing terphenoxide and alkoxide ligands or a basic
pyrrolide. Complex 2b features a strongly electron-donating,
nucleophilic pyrazole, which stabilizes the cationic metal center
considerably more than (previously reported) coordinated THF or
lutidine.[9] Even though these compounds are stable in dry
Olefin metathesis catalysts based on high-oxidation state
molybdenum or tungsten alkylidene species, usually referred to
as “Schrock catalysts”, have been under development for
decades and are a cornerstone for a vast number of regio- and
stereoselective transformations today.[1] However, their inherent
oxophilicity and sensitivity to air and moisture limits the user-
friendliness that competing ruthenium alkylidenes can offer in
many cases. Several attempts have been made to circumvent this
challenging situation. By the reversible chelation of the active
catalysts by bipyridine or phenanthroline, Fürstner et al. showed
that Mo(VI) alkylidenes could indeed be converted into air-stable,
storable but unreactive pre-catalysts, which can be reactivated by
the addition of zinc salts to regain their original reactivity.[2]
A
different approach was the formulation of a catalyst/paraffin
mixture that displays significantly lowered sensitivity to air and
offers the possibility of handling these catalysts without a
glovebox.[3] These admittedly elegant solutions, however, still do
[a]
M.Sc. R. Schowner, Dr. I. Elser, M.Sc. M. Benedikter, M.Sc. M.
Momin, M.Sc. T. Schneck, Dr. L. Stöhr, Prof. Dr. M. R. Buchmeiser
Institut für Polymerchemie, Universität Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart
E-Mail: michael.buchmeiser@ipoc.uni-stuttgart.de
Dr. W. Frey
[b]
Institut für Organische Chemie, Universität Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201913322
Angewandte Chemie International Edition
RESEARCH ARTICLE
Subsequent salt metathesis of the remaining triflate provides
access to other cationic monoalkoxide catalysts in high yield,
usually in a one-pot reaction from the bistriflate precursor. We
employed various imido ligands, including the previously not
reported 2,6-difluorophenylimido ligand in 4d (Scheme S1, S.I.).
Additional variations comprised different NHC ligands and the
presence of coordinating acetonitrile. Coordination of polar
molecules to cationic monotriflate species can easily be probed
by 19F NMR spectroscopy, since cationic metal centers are
coordinated in a 2-fashion by the triflate ligand in case no other
polar molecule is present. This results in chemical shifts of   -
73.5 ppm in the 19F NMR spectrum. The single crystal X-ray
structure of 3a (Figure 3) confirms this binding situation. 3a
crystallizes in the monoclinic space group P21/n with a =
1892.45(17) pm, b = 1653.39(15) pm, c = 2273.5(2) pm,  =  =
90 °,  = 94.994(3) ° (Z = 4). The geometry at the metal is distorted
square pyramidal (5 = 0.26)[11] with the alkylidene in the syn
orientation forming the apex (Mo-C28 = 186.7 pm). Notably, the
triflate ligand is bound unsymmetrically, as evidenced by the Mo-
O1 bond being weaker compared to Mo-O2 (226.9 vs. 217.9 pm),
an effect that is likely caused by the NHC, which is coordinated
fairly trans to O1 of the triflate. However, in solution, the broad
solvents (2b at least for two weeks in CD2Cl2 at room
temperature), they immediately react with air, moisture and 2-
PrOH to unknown species that no longer contain an alkylidene
ligand.
Figure 1. Cationic Schrock-type catalysts 2a-c prepared for this study. B(ArF)4
= tetrakis[3,5-bis(trifluoromethyl)phenyl]borate.
We have already shown that cationic molybdenum imido
alkylidene NHC complexes are highly active catalysts in various
olefin metathesis reactions.[4b, 10] Here we prepared a wide range
signals for both the triflate and alkylidene ligand in the 1H and 19
F
of
complexes
having
the
general
formula
NMR spectra imply a degree of fluctuation in the triflate-metal
bonding.
[Mo(NR’)(CHCMe2R)(NHC)(X)][B(ArF)4] (Figure 2, X = anionic
ligand). The emphasis was placed on monotriflate species since
they exhibit high reactivity and are prepared in a straightforward
manner from the corresponding bistriflate complexes by reaction
with Na(BArF)4 and elimination of NaOTf (usually in >90% isolated
yield).
Figure
3.
Single
crystal
X-ray
structure
of
[Mo(N-2,6-Cl2-
C6H3)(CHCMe3)(IMes)(OTf)][B(ArF)4], 3a. Relevant bond lengths [pm] and
angles [°]: Mo-N3 = 173.4, Mo-C28 = 186.7, Mo-O2 = 217.9, Mo-C1 = 218.2,
Mo-O1 = 226.9, Mo-S1 = 280.2; N3-Mo-C28 = 104.81, N3-Mo-O2 = 136.15,
C28-Mo-O2 = 115.56, N3-Mo-C1 = 99.87, C28-Mo-C1 = 101.72, O2-Mo-C1 =
88.45, N3-Mo-O1 = 98.45, C28-Mo-O1 = 94.12, O2-Mo-O1 = 63.52, C1-Mo-O1
= 151.75, C22-N3-Mo = 160.5, C29-C28-Mo = 144.3. One CH2Cl2 molecule, the
anion and hydrogens have been omitted for clarity. Thermal ellipsoids are set
at a 50% probability level.
Other potential neutral ligands like acetonitrile coordinate trans to
the NHC and consequently break up 2 coordination of the triflate
resulting in chemical shifts of   -75 ppm and below. This can be
observed in the single crystal X-ray structure of 4d (Figure 4).
Coordination trans to the NHC is also observed with monodentate
ligands, as is seen in the structures of 5d and its acetonitrile
adduct (Figure S128-129, S.I.). 4d crystallizes in the triclinic
space group P¯1 with a = 1279.17(5) pm, b = 1815.54(7) pm, c =
1860.54(8) pm,  = 64.059(2) °,  = 70.333(2) °,  = 73.654(2) °
Figure 2. Cationic molybdenum imido alkylidene NHC catalysts. IMes = 1,3-
dimesitylimidazol-2-ylidene, IMesCl2
= 1,3-dimesityl-4,5-dichloroimidazol-2-
ylidene, NC4H4 = pyrrolide, 5-iPr = 1,3-diisopropylimidazol-2-ylidene.
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10.1002/anie.201913322
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RESEARCH ARTICLE
(Z = 2). The molybdenum center has a slightly distorted square
pyramidal coordination sphere (5 = 0.14) with the alkylidene in
the apex. Here the Mo-O(triflate) bond is considerably shorter
(208.9 pm) compared to the one in 3a (217.9 pm) with the triflate
being trans to the imido ligand.
imidazolium). On the other hand, complex 3d, which features the
less basic, air stable 1,3-dimesityl-4,5-dichloroimidazol-2-ylidene
(IMesCl2) NHC ligand,[13] did not show any signs of protonation. In
almost the same manner, 4c and complexes 7 and 8, containing
acetonitrile, did not decompose. Notably complex 8 lost one
acetonitrile in favor of water. 4d with the 2,6-difluorophenylimido
ligand remained virtually unchanged after 12 h. 4a also did not
show any decomposition even after 24 hours in air. The solid
material contained ~0.5 equiv. of water after this time. In solution,
the broad 1H and 19F NMR signals for the triflate and the
alkylidene ligand imply a substantial degree of fluctuation in the
triflate-metal bonding. We attribute this to the reversibility of water
coordination. To finally prove the inherent stability of these
catalysts against moisture, we were able to crystallize 4a as the
dihydrate complex (Figure 5). 4a.H2O crystallizes in the triclinic
space group P¯1 with a = 1544.78(14) pm, b = 1631.48(15) pm, c
= 1722.14(16) pm,  = 96.014(5) °,  = 110.634(5) °,  = 97.988 °
(Z
= 2). The molybdenum center adopts an octahedral
coordination sphere with the triflate being trans to the imido ligand.
The Mo-NHC bond is comparably long (224.1 pm). The water
molecules coordinate trans to the NHC and the alkylidene ligand.
With the NHC causing the stronger trans-effect, the Mo-O(water)
bond is significantly longer than the Mo-O(water) bond trans to
the alkylidene (232 vs. 219 pm).
Figure
4.
Single
crystal
X-ray
structure
of
[Mo(N-2,6-F2-
C6H3)(CHCMe2Ph)(IMes)(OTf)(MeCN)][B(ArF)4], 4d. Relevant bond lengths
[pm] and angles [°]: Mo-N3 = 173.1, Mo-C28 = 187.1, Mo-O1 = 208.9, Mo-N4 =
217.2, Mo-C1 = 218.3; N3-Mo-C28 = 102.03, N3-Mo-O1 = 151.93, C28-Mo-O1
= 105.03, N3-Mo-C1 = 97.06, C28-Mo-C1 = 102.72, N4-Mo-C1 = 160.29, N3-
Mo-O1 = 151.93, C28-Mo-O1 = 105.03, N4-Mo-O1 = 81.88, C1-Mo-O1 = 84.21,
C22-N3-Mo = 166.38, C29-C28-Mo = 143.56. The anion and hydrogens have
been omitted for clarity. Thermal ellipsoids are set at a 50% probability level.
In contrast to 2a-c, most cationic NHC-stabilized complexes
shown here do not react readily with protic groups. NMR
measurements (Figure S90-92, S.I.) suggest that complex 3a is
coordinated by 2-PrOH as indicated by a shift of the triflate
resonance in the 19F NMR from  = -73.7 ppm (2-bound) to -74.8
ppm (1-bound) and a shift of the alkylidene proton in the 1H NMR
from  =12.59 to 13.02 ppm. The broadening of signals in the
presence of 0.5 equiv. 2-PrOH indicates that coordination is highly
reversible. Likewise, if 2 equiv. of 2-PrOH are present, several
broad 1H NMR signals between  =3.5 and 4.7 ppm suggest that
2-PrOH molecules exchange readily.
Recently, we have shown that tailored six-coordinate neutral
Mo-imido alkylidene NHC catalysts can serve as latent pre-
catalysts in the polymerization of dicyclopentadiene (DCPD) and
are stable to air for at least 12 h.[12] Here we tested the air stability
of our cationic complexes for a minimum of 12 hours. While those
complexes that contained monodentate or basic ligands
decomposed during that time, the monotriflate complexes did not.
However, the monotriflate complexes proved to be hygroscopic,
absorbing various amounts of water (Figure S93-115, S.I.). Thus,
complexes 3a and 3c contained approx. 2-3 equiv. of water after
12 h. Accordingly, new alkylidene species, which we attribute to
the corresponding water-coordinated complexes, are observed.
In addition, the appearance of imidazolium signals (~10%, relative
to the anion as judged by NMR) and free triflate implied slow
decomposition. Similar observations were made for 5a (~10%
Figure
5.
Single
crystal
X-ray
structure
of
[Mo(N-2-tBu-
C6H4)(CHCMe2Ph)(IMes)(OTf)(H2O)2][B(ArF)4], 4a*H2O. Relevant bond lengths
[pm] and angles [°]: Mo-N3 = 173.2, Mo-C32 = 191.8, Mo-O2W = 219.2, Mo-C1
= 224.1, Mo-O1 = 226.8, Mo-O1W = 232.11; N3-Mo-C32 = 99.47, N3-Mo-O2W
= 99.39, C32-Mo-O2W = 88.23, N3-Mo-C1 = 95.05, C32-Mo-C1 = 96.37, O2W-
Mo-C1 = 163.92, N3-Mo-O1 = 172.82, C32-Mo-O1 = 87.69, O2W-Mo-O1 =
81.21, C1-Mo-O1 = 83.58, N3-Mo-O1W = 99.06, C32-Mo-O1W = 154.92, O2W-
Mo-O1W = 72.15, C1-Mo-O1W = 98.69, O1-Mo-O1W = 74.25, C33-C32-Mo =
142.38, C22-N3-Mo = 172.02. One CH2Cl2 molecule, anion and hydrogens have
been omitted for clarity. Thermal ellipsoids are set at a 50% probability level.
In the solid state, the more tightly bound water molecule shows a
weak interaction between one of its hydrogens and one of the
oxygens of the triflate ligand and is consequently slightly rotated
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10.1002/anie.201913322
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RESEARCH ARTICLE
around the C32-Mo-O1W axis. The distance of the hydrogen atom
tilted towards the triflate ligand and the oxygen is 263.1 pm and
thus markedly shorter than the other H(water)-O(triflate)
distances, which are around 330 pm for the second water
molecule (trans to the NHC). To learn about the stability of
cationic monotriflate NHC complexes towards hydroxyl groups in
the presence of double bonds, 5a was reacted with 1.5 equiv 2-
allyloxyethanol (Scheme 1). After a few minutes the stable,
Next, we were interested, whether the stability of cationic
molybdenum imido alkylidene NHC monotriflate species could be
further enhanced by introducing other bidentate ligands. We
therefore prepared a series of carboxylate-ligated cationic
complexes, that were expected to fit our requirements (Scheme
2). Previously, dicarboxylate-based molybdenum olefin
metathesis catalysts have successfully been prepared and
employed in the regioselective cyclopolymerization of ,-
diynes.[14] However, because of their saturated coordination
sphere, these compounds were rather poor catalysts for other
metathesis reactions. Until now, monocarboxylate species of
Schrock-type catalysts have not been reported. However, the
crystalline
alkylidene
complex
[Mo(N-2,6-Me2-
formed
C6H3)(CHCH2O(CH2)2OH)(IMes)(OTf)][B(ArF)4]
quantitatively.
6
monotriflate-monocarboxylate
species
Mo(N-
tBu)(CHCMe2Ph)(IMes)(O2CC6F5)(OTf) 10 can readily be
prepared from 9a[12] (Scheme 2). Subsequent transformation into
the cationic monocarboxylate species 11 is facile and high-
yielding employing NaB(ArF)4.
tBu
Ag[O2CC6F5], CH2Cl2,
-35 °C to rt, 1 h
CMe2Ph
Mes
N
F
F
F
F
O
O
Mo
OTf
Scheme
1.
Formation
of
[Mo(N-2,6-Me2-C6H4)(CHCH2O(CH2)2OH)
F
N
- AgOTf
(IMes)(OTf)][B(ArF)4] (6).
N
Mes
CMe2R'
10
R
TfO
TfO
N
In 6, the hydroxyl group at the alkylidene ligand is coordinated to
molybdenum (Figure 6). This implies that the metallacyclobutanes
of these complexes are stable towards hydroxyl groups, which
makes productive metathesis possible. 6 crystallizes in the
triclinic space group P 1¯ with a = 1333.10(6) pm, b =
1734.49(7) pm, c = 1819.79(9) pm,  = 78.865(2) °,  =
88.052(2) °,  = 88.528(2) ° (Z = 2). The allyl ether oxygen forms
a four-membered ring by coordinating trans to the imido ligand.
The hydroxyl group is trans to the NHC. Both in the solid state and
in solution the alkylidene ligand is in the anti-configuration (Mo-
C30 = 192 pm) ( = 13.38 ppm, 1JCH = 167 Hz, CDCl3).
Mes
N
Mo
NaB(ArF)4
N
Mes
- NaOTf
CH2Cl2, 30 min
-35 °C to rt
9a: R = tBu, R' = Ph
9b: R = 2,6-Me2-C6H3, R' = Ph
9c: R = 2,6-Cl2-C6H3, R' = Me
9d: R = 3,5-Me2-C6H3, R’ = Ph
CMe2R'
A
1.) Li[Al(OC(CF3)3)4], 15 min
2.) M[O2CR''], CH2Cl2,
-35 °C to rt, 1 h
R
O
N
Mes
Mo
R''
N
- 2 MOTf
O
N
M = Li or Tl
Mes
11: R = tBu, R' = Ph, R'' = C6F5, A = B(ArF)4
12: R = 2,6-Me2-C6H3, R' = Ph, R'' = C6F5, A = Al(OC(CF3)3)4
13: R = 3,5-Me2-C6H3, R' = Ph, R'' = C6F5, A = Al(OC(CF3)3)4
14: R = 2,6-Cl2-C6H3, R' = Me, R'' = C6F5, A = B(ArF)4
15a: R = 2,6-Cl2-C6H3, R' = Me, R'' = 2,6-(CF3)2-C6H3, A = Al(OC(CF3)3)4
15b: R = 2,6-Cl2-C6H3, R' = Me, R'' = 2,6-(CF3)2-C6H3, A = B(ArF)4
1. Li[Al(OC(CF3)3)4], 15 min
Ph
Ph
CF3
Al(OC(CF3)3)4
OTl
2.
CF3
O
Cl
Cl
Ad
N
O
N
N
CF3
Mo
R
Mo
R
R
N
CH2Cl2, -35 °C to rt, 1 h
O
N
N
R
- LiCl, TlOTf
CF3
16a: NHC = IMesH2
16b: NHC = 5-iPr
17a: NHC = IMesH2
17b: NHC = 5-iPr
Figure 6. Single crystal X-ray structure of 6. Selected bond lengths [pm] and
angles [°]: Mo-N3 = 171.8, Mo-C30 = 192.2, Mo-O4 = 217.7, Mo-O1 = 218.9,
Mo-C1 = 220.5, Mo-O5 = 229.3; N3-Mo-C30 = 98.63, N3-Mo-O4 = 94.36, C30-
Mo-O4 = 97.11, N3-Mo-O1 = 117.38, C30-Mo-O1 = 143.97, O4-Mo-O1 = 79.88,
N3-Mo-C1 = 96.24, C30-Mo-C1 = 95.16, O4-Mo-C1 = 162.38, O1-Mo-C1 =
82.75, N3-Mo-O5 = 157.22, C30-Mo-O5 = 64.87, O4-Mo-O5 = 73.49, O1-Mo-
O5 = 80.10, C1-Mo-O5 = 100.64, C22-N3-Mo = 168.47. The anion, hydrogens
(except H on C30 and O4) and one Pr2O molecule coordinating to the hydrogen
on O4 have been omitted for clarity. Thermal ellipsoids are set at a 50%
probability level.
Scheme 2. Synthesis of cationic monocarboxylate species 11 – 15 from
bistriflates 9a-d[12, 15] via mixed monotriflate/monocarboxylate complex 10 or in
situ generation of cationic monotriflate species and subsequent salt metathesis.
Synthesis of cationic monocarboxylate complexes 17a-b from dichloride NHC
alkylidenes 16a-b.[16] Ad = adamantyl.
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The synthesis of cationic carboxylates can be accomplished in an
even more straightforward manner in a one-pot reaction of the
bistriflate or dichloride precursors with NaB(ArF)4 or
LiAl(OC(CF3)3)4, respectively,[17] followed by salt metathesis of the
triflate or chloride, usually in ~90% yield. It is interesting to note,
that we did not observe bridged dimers as reported for neutral
carboxylate species with small ligands,[18] probably due to the
repulsion of the cationic metal centers. The single crystal X-ray
structure of complex 15b confirms the monomeric structure
(Figure 7). Solution NMR experiments and the solid-state
structure suggest a significantly more tightly bound carboxylate
ligand, compared to the triflate in 3a. This results in sharp
functionality. Table 1 contains the corresponding maximum
turnover numbers (TONmax) for catalysts 3 – 8 and the Schrock-
type catalysts Mo(N-2,6-Me2-C6H3)(CHCMe2Ph)(OC(CF3)3)2 SF9
and 2a - c, which were employed for comparison. Several
important observations were made: 1) Electron-withdrawing imido
ligands increase productivity. 2) X-type ligands with lower pKa
values for the corresponding acid, increase productivity. 3) In X-
type ligands with comparable pKa values, higher steric demand is
more favorable (5b vs. 5d). 4) The IMes ligand seems to be the
best choice, however, it is questionable that only its donor
strength is the decisive measure. 5) Coordinated acetonitrile does
not play a significant role in reactivity. Results for the benchmark
reactions for carboxylate substituted catalysts are summarized in
Table 2. We essentially observed the same trends as in Table 1:
Catalysts with electron-withdrawing imido ligands outperform
more basic imido ligands. Pentafluorobenzoate is favored over
the slightly more basic 2,6-bis(trifluoromethyl)benzoate but is not
favored over the triflate ligand in terms of reactivity. These
findings are counterintuitive since one might anticipate that
1
resonances in both the H and 19F NMR spectra and an almost
symmetrically bound carboxylate ligand (Mo-O1 = 219.1 pm, Mo-
O2 = 218.6 pm) and a Mo-C28 (carboxylate carbon) distance of
255.3 pm in the solid state (compared to Mo-S1 = 280.2 pm, in
3a). The overall structure is a slightly distorted square pyramid (
= 0.1) with the alkylidene ligand forming the apex (Mo-C37 =
184.1 pm) and one of the carboxylate oxygens roughly trans to
the NHC.
substitution of
a cationic metal alkylidene with electron-
withdrawing substituents would further enhance the electrophilic
character and thus lead to more active but also more unstable
catalysts. However, while electrophilicity is indeed enhanced, the
propensity of the catalysts to decompose in the presence of protic
functional groups is reduced. Our data suggest that the
predominant decomposition pathway is initiated by the
coordination of the protic group to the metal center and
subsequent proton transfers. Coordination always occurs trans to
the NHC. This is supported by the single crystal X-ray structures
reported here and several other already reported crystal
structures of similar compounds,[4c] recently published
mechanistic studies from our group[19] and mechanistic studies for
MAP (monoalkoxide pyrrolide) type Schrock-catalysts, which
indicate that coordination occurs preferably trans to the strongest
-donor ligand.[20] Hence the NHC dictates the coordination of
molecules to the metal center. This results in a maximum distance
between the NHC and the protic groups as compared to cis
coordination, which in turn makes protonation of the NHC less
probable. Recently, we reported on oxo-bridged dimers that
formed in the reaction of a Mo-imido alkylidene NHC complex
containing a basic aryloxide with water.[16] Moreover, it was shown
Figure
7.
Single
crystal
X-ray
structure
of
[Mo(N-2,6-Cl2-
C6H3)(CHCMe3)(IMes)(2,6-(CF3)2-C6H3)][B(ArF)4], 15b. Selected bond lengths
[pm] and angles [°]: Mo-N3 = 172.5, Mo-C37 = 184.1, Mo-O2 = 218.6, Mo-C1 =
218.9, Mo-O1 = 219.1, Mo-C28 = 255.3; N3-Mo-C37 = 103.4, N3-Mo-O2 =
142.33, C37-Mo-O2 = 109.69, N3-Mo-C1 = 101.58, C37-Mo-C1 = 101.41, O2-
Mo-C1 = 89.29, N3-Mo-O1 = 100.69, C37-Mo-O1 = 95.22, O2-Mo-O1 = 59.42,
C1-Mo-O1 = 148.17, C22-N3-Mo = 165.1, C38-C37-Mo = 145.9. Thermal
ellipsoids are set at a 50% probability level. The B(ArF)4 anion and hydrogens
except for H on C37 have been omitted for clarity.
that
protonation
of
Mo(N-
Ad)(CHCMe2Ph)(NHC)(pyrrolide)(aryloxide)
selectively
eliminates pyrrole.[21] Also, other examples in which pyrrole was
eliminated from metal imido alkylidene NHC bispyrrolide
complexes or cationic metal imido alkylidene NHC monopyrrolide
complexes were reported.[15] All this evidence indicates that X-
type ligands are the first and preferred target for proton transfer.
Notably, so far, we have not observed direct protonation of the
alkylidene- or the imido ligand, which theoretically would result in
amido alkylidene or imido alkyl species. Indeed, the imido
alkylidene motif is remarkably stable considering the fact that it is
generated via protonation of a bisimido species, often with excess
acid or alcohols.[22] Amido alkylidyne complexes, however, seem
to be close in energy and form reversibly from imido alkylidenes
by formal proton transfer.[16, 23] Data published by Schrock et al.
also suggest that the alkylidene ligand is protonated preferably by
strong acids in case two alkyl ligands (strong  donors) are
present, but not in the presence of one alkyl and one alkoxide
As anticipated and evidenced by the structure of 15b, the
carboxylate complexes exhibit remarkable air stability. 11, 13, 15a,
17a and 17b were left open to air for 5 days. After this time only
17b with the small 1,3-diisopropylimidazol-2-ylidene ligand
decomposed. All other complexes did not show any sign of
decomposition and remained virtually unchanged. Also, no water
absorption was observed (Figure S116-119, S.I.). This shows that
beside electronic stabilization, steric protection by the NHC ligand
seems to play a crucial role in the prevention of decomposition in
air. In addition to that, more tightly bound bidentate ligands are a
key factor for stability.
Next, we subjected our catalysts to several benchmark olefin
metathesis reactions with substrates containing the alcohol
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10.1002/anie.201913322
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RESEARCH ARTICLE
ligand.[24] In the same report it was surmised that Mo alkylidenes
might be stable in alcohols under certain conditions. Therefore,
we propose that if the most probable decomposition pathways are
inhibited by ligands with low basicity, by steric hindrance and
cationic metal centers (no bimolecular decomposition), olefin
coordination and subsequent olefin metathesis is possible
(Scheme 3).
Table 1. Maximum turnover numbers (TONmax) for the homometathesis of hydroxyl-substituted substrates with cationic NHC-type
catalysts 3 - 8 and the Schrock-type catalysts SF9 and 2a - c.
3a
3b
3c
3d
4a
4b
4c
4d
5a
5b
5d
7
8
SF9
2a
2b
2c
4-penten-1-
ol[a]
4000
(1100[c]
2900
3300
3000
3000
3300
3300
4000
2100
1100
600
1900
2000
100
0
0
0
)
5-hexene-1-
ol[a]
2300
500
200
900
500
0
150
0
100
0
1200
0
1500
0
2300
0
2000
0
100
0
0
0
0
1600
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
(500[c]
)
7-octene-1-
ol[a]
3200
(300[c]
)
2-
3800
(0[c]
)
3300
3800
0
400
600
2900
100
0
1300
allylphenol[b]
[a] CH2Cl2, 2 h, room temperature, 3M in substrate, cat : substrate 1 : 4000, internal standard for GC-MS dodecane. [b] Internal
standard for GC-MS: cyclooctane. [c] Reaction in 2-PrOH.
The use of ligands with low basicity, e.g. of perfluorinated
Table 2. Maximum turnover numbers (TONmax) for the homometathesis of
alkoxides, triflate and carboxylates impedes proton transfer from
the alcohol. Notably, while coordination of an alcohol to the
cationic metal center will certainly increase its acidity, pKa is still
considered a useful tool for choosing potentially suitable ligands.
This low-basicity concept is also in line with reports by Hohloch et
al. on the protolytic stability of Mo-imido NHC complexes.[25] We
chose triflate (pKa,HOTf(H2O) = -12)[26], nonafluoro-tert-butoxide
(pKa,HOR(H2O) = 5.4, R = C(CF3)3)[26] and pentafluorobenzoate
(pKa,HOOCR(H2O) = 1.48, R = C6F5)[27] as the most promising X-type
ligands. We also decided to employ 1,3-dimesitylimidazol-2-
ylidene as the NHC-type ligand, since its excellent -donating
properties (TEP = 2050.5 cm-1)[28] and the ability to delocalize the
cationic charge on molybdenum over its aromatic system led us
to believe that it would engage in strong metal carbene bonds and
provide more productive catalysts. However, while IMes-based 3a
proved to be the most productive in our benchmark reactions,
IMesCl2-based 3d (TEP = 2052.6 cm-1)[29] was significantly more
stable in air, which probably is a consequence of the lower
basicity of the NHC. Then again, IMes-based 4d with the
fluorinated imido ligand was robust and highly productive, which
shows that careful consideration of all ligands is crucial. It became
evident that the imido ligand has a huge impact on productivity
(Table 1 and 2). Catalysts containing the more basic alkyl-
substituted imido ligands (8, 11, 17a) displayed decreased
reactivity compared to complexes with aromatic imido ligands.
This was also observed earlier.[12, 15, 21] To demonstrate that this
is a reactivity issue we mixed catalyst 11 with 10 equiv. 10-
hydroxyl-substituted substrates with cationic carboxylate NHC catalysts 11 -
17.
11
12
13
14
15a
17a
4-penten-1-ol[a]
5-hexene-1-ol[a]
7-octene-1-ol[a]
2-allylphenol[b]
2500
700
0
1500
200
0
3100
750
2800
550
1500
0
0
0
0
0
0
0
200
150
50
500
3300
2300
[a] CH2Cl2, 2 h, room temperature, 3M in substrate, cat : substrate 1 : 4000,
internal standard for GC-MS dodecane. [b] Internal standard for GC-MS:
cyclooctane.
NHC
NHC
R
R
TfO Mo
TfO Mo
N
N
H
O
H
O
R'
R'
R''
R''
path I - decomposition
olefin
NHC
R
TfO Mo
H
NHC
N
R
R'' O Mo
R'
N
R'''
OTf
R'
1
undecenoic acid and monitored the reaction by H NMR (Figure
S122, S.I.). Even after 80 minutes, most of the catalyst was
unchanged, clearly showing that proton transfer to either ligand
was very slow. After 8 hours all catalyst initiated as indicated by
the formation of CH=CH2CMe2Ph but only little metathesis
product was formed before decomposition took place (formation
of the imidazolium species was verified by NMR). We therefore
conclude that even basic imido ligands are not protonated readily
path II - decomposition
path III - olefin metathesis
Scheme 3. Proposed initial decomposition reactions competing with olefin
metathesis.
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10.1002/anie.201913322
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RESEARCH ARTICLE
in metal imido alkylidene NHC complexes but that electron-
withdrawing imido ligands increase reactivity by increasing
electrophilicity. Additionally, the formation of stable chelates as in
6 is a factor that lowers productivity. When comparing the triflate-
to the nonafluoro-tert-butoxide or the carboxylate ligands in
complexes 3a, 3b, 14, and 15a, respectively, the triflate ligand
outperforms the other in accordance with its weaker basicity. This
is understandable when revisiting the crystal structure of 4a*H2O
(Figure 5), where a weak interaction between one of the hydrogen
atoms of water and a triflate ligand shows the way to X-type ligand
protonation. On top, in stark contrast to catalysts bearing weakly
basic ligands, [Mo(N-2,6-Me2-C6H3)(CHCMe2Ph)(NC4H4)(5-
catalyst loading of 1 mol% with respect to the NBE derivative. For
both cross partners, the formation of two products was observed,
formed by either one or two cross metathesis events. In both
cases approximately equal amounts of double-substituted
product and mono-substituted product were observed. 3a was
also employed in the ROMP of 2-exo-norborn-5-enemethanol
(87% yield) and 2-exo,3-exo-norborn-5-ene-2,3-dimethanol (68%
yield) in the polar protic solvent 2-PrOH. Finally, we successfully
employed 3a in cross metathesis reactions of 4-penten-1-ol, 5-
hexen-1-ol, 7-octen-1-ol and 2-allylphenol in 2-PrOH and still
observed activity, although somewhat reduced compared to the
TON in CH2Cl2 (Table 1, values in brackets). This is nonetheless
remarkable since the olefinic substrate competes here in
coordination with a large excess of alcohol.
iPr)][B(ArF)4] (5c),[15] with
a
basic pyrrolide ligand
(pKa,pyrrole(H2O) = 16.5) exhibited no productivity in any of the
investigated reactions. Instead, when 5c was reacted with five
equiv. of 4-penten-1-ol, the formation of pyrrole was immediately
observed in the 1H NMR spectrum, accompanied by the emerging
Conclusion
3
of a new alkylidene signal ( = 14.67 ppm, t, JHH = 5.7 Hz,
In summary, we extended the concept of functional group-
tolerant Mo-imido alkylidene NHC complexes. We have shown
that these highly active catalysts clearly stand out from all known
high oxidation state molybdenum and tungsten olefin metathesis
catalysts in terms of (air) stability and in this regard rival ruthenium
based alkylidene complexes. The applicability in HM, RCM,
ROCM, ADMET polymerization and ROMP reactions with
hydroxyl functionalized olefins even more closes the gap between
Mo- and competing Ru-catalysts. We successfully demonstrated
a correlation between ligand basicity and hydroxyl group
tolerance in olefin metathesis employing cationic molybdenum
imido alkylidene NHC complexes. Work on extending this unique
reactivity to (cationic) W imido and W oxo alkylidene NHC
complexes is under way.
CD2Cl2), which we tentatively assign to [Mo(N-2,6-Me2-
C6H3)(CH(CH2)3O)(5-iPr)][B(ArF)4], in which the neophylidene
ligand was replaced by substrate; the alkylidene proton therefore
couples to the adjacent methylene (Figure S120-121, S.I.). The
results in Table 1 and Table 2 not only demonstrate that the
correct ligand combination is decisive for catalytic activity, but also
that the choice of a catalyst is crucial for a given substrate.
Table 3. HM, RCM, ROCM and ROMP with catalyst 3a.
[a]
HM (TONmax
)
oleyl alcohol
1600
1400
[a]
RCM (TONmax
)
1,6-heptadiene-4-ol
ROMP in 2-PrOH (Yield [%])[b]
Acknowledgements
2-exo-norborn-5-enemethanol
87
68
Financial
support
provided
by
the
Deutsche
Forschungsgemeinschaft (DFG, German Research Foundation,
project number 358283783 - CRC 1333) and XiMo AG,
Switzerland, is gratefully acknowledged.
2-exo,3-exo-norborn-5-enedimethanol
ROCM with 2-endo,3-endo-norborn-5-enedimethanol[c]
cross partner
conversion
[%][d]
yield
[%][e]
double-
subst. [%]
mono-
subst. [%]
Keywords: Olefin Metathesis • Alcohols • Molybdenum • N-
Heterocyclic Carbenes
1-hexene
100
100
76
65
54
50
46
50
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RESEARCH ARTICLE
Sometimes less is more: reduction of
electron density in cationic Mo imido
alkylidene N-heterocyclic carbene
complexes substantially enhances
tolerance versus hydroxyl groups in
olefin metathesis reactions including
acyclic diene metathesis
R. Schowner, I. Elser, M. Benedikter, M.
Momin, W. Frey, T. Schneck, L. Stöhr,
M. R. Buchmeiser*
Page No. – Page No.
Title
polymerization, cross-metathesis and
ring-opening cross metathesis.
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