N-heterocyclic carbene, high oxidation state molybdenum alkylidene complexes: Functional-group-tolerant cationic metathesis catalysts

Article abstract of DOI:10.1002/anie.201404655

We synthesized the first N-heterocyclic carbene (NHC) complexes of Schrock's molybdenum imido alkylidene bis(triflate) complexes. Unlike existing bis(triflate) complexes, the novel 16-electron complexes represent metathesis active, functional-group-tolera

Full text of DOI:10.1002/anie.201404655

Angewandte
Chemie
DOI: 10.1002/anie.201404655
Molybdenum–NHC Complexes
N-Heterocyclic Carbene, High Oxidation State Molybdenum
Alkylidene Complexes: Functional-Group-Tolerant Cationic
Metathesis Catalysts**
Michael R. Buchmeiser,* Suman Sen, Jçrg Unold, and Wolfgang Frey
Dedicated to Prof. Richard R. Schrock on the occasion of his 70th birthday
Abstract: We synthesized the first N-heterocyclic carbene
NHC) complexes of Schrockꢀs molybdenum imido alkylidene
ucts with unprecedented low metal contaminations, typically
in the ppb regime. Nonetheless, this approach has so far
been limited to cationic Ru alkylidene complexes.
[
4]
(
bis(triflate) complexes. Unlike existing bis(triflate) complexes,
the novel 16-electron complexes represent metathesis active,
functional-group-tolerant catalysts. Single-crystal X-ray struc-
tures of two representatives of this novel class of Schrock
catalysts are presented and reactivity is discussed in view of
their structural peculiarities. In the presence of monomer
Nomura and Zhang reported the NHC-triggered a-
[
5]
hydrogen elimination in vanadium(V) complexes. This
result suggests that rather nucleophilic than basic NHCs
should be suitable for preparing cationic Schrock catalysts.
We anticipated that an NHC complex of one of Schrockꢀs
bis(triflate) progenitors, for example, [Mo(N-2,6-Me C H )-
(
substrate), these catalysts form cationic species and can be
2
6
3
employed in ring-closing metathesis (RCM), ring-opening
metathesis polymerization (ROMP), as well as in the cyclo-
polymerization of a,w-diynes. Monomers containing func-
tional groups, which are not tolerated by the existing variations
of Schrockꢀs catalyst, e.g., sec-amine, hydroxy, and carboxylic
acid moieties, can be used. These catalysts therefore hold great
promise in both organic and polymer chemistry, where they
allow for the use of protic monomers.
(CH-tBu)(OTf) (DME)]
(DME = 1,2-dimethoxyethane)
2
could indeed be a cationic one with the cationic charge
dislocated between the NHC and molybdenum (see meso-
meric structures in Scheme 1). And in fact, reaction of either
1,3-dimesitylimidazolin-2-ylidene (IMesH ) or 1,3-di-tert-
butylimidazol-2-ylidene (I-tBu) with [Mo(N-2,6-Me C H )-
2
2
6
3
(CH-tBu)(OTf) (DME)] resulted in the formation of the
2
desired NHC-complexes 1 and 2 (Scheme 1).
Compound 1 (Figure 1) crystallizes in the orthorhombic
[
1]
V
ery few reports exist on cationic W and Mo alkylidene
system Pbca
,
a = 1824.13(6) pm, b = 2117.49(6) pm, c =
complexes. The Mo alkylidenes include cationic Mo imidoal-
2216.35(5) pm, a = b = g = 908, Z = 8. In the solid state, the
complex exists in its neutral form. The ligands adopt
a distorted square pyramidal (SP) geometry with the Mo
alkylidene unit in the apical position. The distance Mo–C30 is
187.3(3) pm and thus somewhat shorter than that found for
[2]
kylidene b-diketonate and b-diketiminates as well as
cationic Mo imidoalkylidene pyrrolide and Mo monoalkoxide
[
3]
complexes. For cationic Mo imidoalkylidene monoalkoxide
species, a low ring-closing metathesis (RCM) activity was
reported with turnover numbers (TONs) less than 18. Despite
this unsatisfactory status quo, however, cationic Schrock
catalysts are of utmost interest since such catalytic systems
could be used under biphasic or supported biphasic conditions
including supported ionic liquid phase (SILP) conditions.
Particularly the SILP approach has been demonstrated to
allow metathesis reactions under continuous conditions,
where the ionic catalysts are selectively dissolved in a poly-
mer-supported ionic liquid, thereby offering access to prod-
[
*] Prof. M. R. Buchmeiser, M. Sc. S. Sen, Dr. J. Unold
Institut fꢀr Polymerchemie, Universitꢁt Stuttgart
Pfaffenwaldring 55, 70550 Stuttgart (Germany)
E-mail: michael.buchmeiser@ipoc.uni-stuttgart.de
Homepage: http://www.uni-stuttgart.de/ipoc/msf/index_en.html
Dr. W. Frey
Institut fꢀr Organische Chemie, Universitꢁt Stuttgart
Pfaffenwaldring 55, 70550 Stuttgart (Germany)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(
DFG, contract Nr. BU2174/8-1). NHC=N-heterocyclic carbene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201404655.
Scheme 1. Synthesis of catalysts 1 and 2, formation of the cationic
species in the presence of monomer, and trans-selectivity in ROMP.
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is in its neutral form; the ligands adopt a distorted SP
geometry with the Mo alkylidene unit in the apical position.
Unlike in 1 and despite an unfavorable small angle O1-Mo1-
C9 of 147.93(5)8, the triflate group located trans to the N-
arylimido ligand experiences the most pronounced trans-
effect, resulting in a weaker binding of this anionic ligand as
suggested by the different bond lengths (MoÀO1 211.07(11)
and MoÀO2 217.98(12) pm). Particularly the MoÀO2 distance
is significantly larger than the distances found in “standard”
bis(triflate) complexes of the general formula [Mo(NAr’)-
[
6]
(
CH-tBu)(OTf) (DME)]
(208.5 < MoÀO < 210.5 pm).
2
Notably, neither IMesH2 nor I-tBu, though strong bases,
caused any (permanent) deprotonation of the a-hydrogen at
the alkylidene ligand, a reaction that would result in the
formation of Mo alkylidyne complexes. This is because the
NHCs used are more nucleophilic than basic with I-tBu
Figure 1. Single-crystal X-ray structure of 1. Selected bond lengths [pm]
and angles [8]: Mo1–N1 172.9(2), Mo1–C30 187.3(3), Mo1–O4
[7]
[8]
2
1
11.45(18), Mo1–O1 213.52(18), Mo1–C1 223.5(3); N1-Mo1-C30
01.61(11), N1-Mo1-O4 153.17(9), C30-Mo1-O4 105.17(10), N1-Mo1-
[9]
being more nucleophilic than IMesH2, which can be
explained by the Hꢁckel aromaticity of [I-tBu] . This order
+
O1 96.08(9), C30-Mo1-O1 97.82(10), O4-Mo1-O1 78.86(7), N1-Mo1-C1
9
1
6.84(10), C30-Mo1-C1 103.36(11), O4-Mo1-C1 78.59(9), O1-Mo1-C1
52.35(9).
in nucleophilicity is also reflected by the chemical shifts for
the corresponding alkylidene units. Thus, the chemical shift of
the alkylidene in 2 (d = 14.60 ppm) strongly suggests that the
H
partial positive charge at the Mo is larger in 2 than in 1 (d =
H
the Mo alkylidene in [Mo(NAr’)(CH-tBu)(OTf) (DME)]
12.76 ppm). These data are also in line with the single-crystal
X-ray data, which clearly show a more pronounced trans-
effect on one triflate and thus charge separation in 2.
We subjected complexes 1 and 2 to various metathesis
reactions. With diallyl diphenylsilane (Figure 3), 1 allows for
turnover numbers of 480 in 1,2-dichloroethane at T= 808C
2
[
6]
(
1.90/1.93 pm). The distance Mo1ÀO1 with the triflate
being trans to the NHC is somewhat longer than for the one
that is trans to the imido ligand (Mo1ÀO4 213.52(18) vs.
2
11.45(18) pm). Clearly, the mesityl-containing NHC in 1 has
a very unpronounced trans-effect on the triflate ligand, which
is a direct consequence of the distorted geometry of the
complex with a C1-Mo-O1 angle of only 152.35(9)8. In the
solid state, the alkylidene adopts a syn-configuration, which
also predominates in solution as evidenced by the J
CÀH
coupling constant of 118 Hz. Apart from the signal for the
syn-alkylidene, 1 shows approximately 0.1% of the anti-
isomer at d = 14.52 ppm (see Figure S1 in the Supporting
Information).
Compound 2 (Figure 2) crystallizes in the tetragonal
system P42 c, a = 2449.86(11) pm, b = 2449.86(11) pm, c =
1
1
093.68(7) pm, a = b = g = 908, Z = 8. As in 1, the complex
Figure 3. Substrates and monomers used in RCM, ROMP, and cyclo-
polymerization (Cy=cyclohexyl).
[
2,3]
and thus clearly rivals other cationic Mo complexes. With
[10]
1, in the ROMP
of 5,6-bis((pentyloxy)methyl)bicyclo-
[
(
2.2.1]hept-2-ene (Figure 3), the corresponding polymer
M = 9000 gmol , PDI = 1.2), which was isolated in 90%
n
À1
yield, contained 90% trans double bonds (Figure S9). With
catalyst 2, the corresponding polymer had a lower trans
content of 50% (Figure S11).
Similar results were found with poly(7-oxabicyclo-
[
2.2.1]hept-5-ene-2,3-diylbis(methylene) diacetate; Figure 3),
Figure 2. Single-crystal X-ray structure of 2. Selected bond lengths [pm]
and angles [8]: Mo1–C20 188.69(18); N1-Mo1-C20 97.03(8), N1-Mo1-
O1 99.04(6), C20-Mo1-O1 98.13(6), N1-Mo1-O4 167.42(6), C20-Mo1-
O4 95.48(7), O1-Mo1-O4 80.44(5), N1-Mo1-C9 93.43(6), C20-Mo1-C9
which was obtained in quantitative yield by the action of 1 and
also displayed
1
a high trans content of 85% (M =
n
À1
3000 gmol , PDI = 1.7). With catalyst 2, the trans content
1
09.57(7), O1-Mo1-C9 147.93(5), O4-Mo1-C9 81.11(5).
of this polymer was 33%. To our knowledge, these are the
2
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first high-yield metathesis reactions reported for any Mo
imidoalkylidene bis(triflate) complex.
RCM, ROMP, and cyclopolymerization is very surprising,
since all other Mo bis(triflate) complexes of the general
VI
The most striking feature of the novel catalysts, however,
is related to the cyclopolymerization of 4,4-bis(hydroxy-
methyl)-1,6-heptadiyne (Figure 3), a monomer with two
unprotected hydroxy groups. Unlike classic Schrock catalysts,
which are very sensitive towards protic functionalities, this
monomer is quantitatively polymerized within less than 5 min
formula [Mo(NR’)(CHCMe R)(OTf) (DME)] (R = CH3,
2
2
C H ) do not display any metathetical activity and have
6
5
solely served as progenitors for the metathetically active
[
12]
alkoxide, phenoxide, and carboxylate complexes. However,
VI
Schrockꢀs Mo bis(triflate)s are 18 electron complexes in
which decoordination of DME does not occur. Both 1 and 2,
however, are 16 electron complexes with one triflate located
nearly trans to the NHC and the imido ligand. Unlike the
bipyridyl and phenantroyl adducts of Schrockꢀs catalyst
to yield the corresponding purple conjugated polymer (M
n
À1
À1
(
theor.) = 7700 gmol , M = 6500 gmol , PDI = 1.3, l
=
n
max
1
3
5
54, 593 nm). Unfortunately, no high-quality C NMR spec-
[
13]
tra of this polyene could be obtained, probably a result of
severe aggregation of the highly polar and protic polymer.
Furthermore, a dinitrile compound, that is, dipropargylmalo-
dinitrile was cyclopolymerized by the action of 1 and isolated
reported by Fꢁrstner,
no dissociation of the NHC is
required to activate the catalyst. Thus, NMR spectroscopy
clearly shows that the NHC ligand remains bound to the
metal center and no free NHC or imidazol(in)ium salt is
observed. However, what does occur in the course of
polymerization is the dissociation of one triflate ligand,
presumably the one that experiences the strongest trans-effect
(Scheme 1) once monomer has been added. The approach of
the monomer must be expected to occur trans to the Mo
alkylidene unit, thereby forming an octahedral 18-electron
complex. In case this 18-electron complex is characterized by
a more linear orientation of at least one triflate with respect to
the NHC–Mo or arylimido–Mo bond, the trans-effect triggers
the release of the corresponding triflate and a cationic, 16-
electron complex forms (Scheme 1). This complex can either
adopt a (distorted) SP or a trigonal bipyramidal (TBP)
geometry. In view of the reactivity of Mo monoalkoxypyrro-
À1
in 60% yield (M = 1100 gmol , PDI = 1.15, M (theor.) =
1
n
n
À1
420 gmol ). The high functional-group tolerance also holds
for ROMP: catalyst 1 polymerized norborn-5-ene-2,3-dime-
À1
thanol, isolated in 80% yield (M = 2800 gmol , PDI = 1.12,
n
À1
M (theor.) = 1540 gmol , s = 43%). Similar results were
n
trans
obtained with 2-(N,N-dimethylaminomethyl)norborn-2-ene
and 2-(N-cyclohexylaminomethyl)norborn-2-ene giving the
corresponding polymers poly[2-(N,N-dimethylaminomethyl)-
À1
norborn-2-ene], M = 10500 gmol , PDI = 1.21 and poly[2-
n
(
1
N-cyclohexylaminomethyl)norborn-2-ene],
M =
n
À1
3100 gmol , PDI = 1.10, s = 35%, isolated in 80 and
trans
9
0% yield. In view of these results and to gain information on
the regioselectivity of insertion in cyclopolymerization, we
subjected a less reactive 1,7-octadiyne, that is, 4,4,5,5-tetraki-
s(ethoxycarbonyl)-1,7-octadiyne (Figure 3) to cyclopolymeri-
[14]
lide complexes we postulate that either directly, or out of
the SP geometry, a TBP complex forms by rearrangement.
[2+2] Cycloaddition of the monomer to the alkylidene then
starts ROMP. Formation of a polymer with a high trans-
content should then start from the anti-isomer, which is in line
with the chemistry of Schrock catalysts (Scheme 1). Clearly,
the propensity to form a cationic complex strongly depends
on both the nucleophilic character of the NHC and the ability
to arrange at least one triflate almost perfectly trans to either
the NHC or the arylimido ligand.
zation, again using 1. The corresponding polymer (M =
n
À1
1
3200 gmol , PDI = 1.9, l = 484 nm) was isolated in
max
8
1% yield and formed with over 96% a-insertion selectivity
(
Figure S21). NMR and UV/Vis spectroscopy data fit those of
[
11]
independently prepared samples.
Similar results were
obtained for the cyclopolymerization of 4,4-bis[(3,5-diethox-
ybenzoyloxy)methyl]-1,6-heptadiyne. The corresponding
polyene was isolated in 94% yield and with 91% a-selectivity.
The high functional-group tolerance of both 1 and 2 is also
reflected by the fact that the polymerization reactions cannot
be terminated by the addition of an aldehyde, for example,
ferrocene carbaldehyde. Instead, cross-metathesis with an
olefin, for example, styrene is required to terminate the living
polymer chain. Alternatively, termination can be accom-
plished by the addition of a HCl-containing methanol
solution. This unreactivity towards aldehydes in turn allowed
poly(norborn-5-ene-2-yl carbaldehyde) to be synthesized by
1
9
Figure 4 shows the F NMR spectra of the polymerization
of 4,4-bis(hydroxymethyl)-1,6-heptadiyne by the action of 1.
As can be seen, the parent two signals for the two individual
triflate ligands at d = À74.65 and À76.7 ppm (integral ratio
1:1) vanish within less than 5 min and new signals are
observed at d = À76.45 and À79.1 ppm. While the signal at
d = À76.45 ppm can be assigned to a triflate bound to Mo, the
signal at d = À79.1 ppm corresponds to free triflate. Since the
polymer with the initiator attached to it precipitates in the
course of the reaction, the signal of the triflate bound to Mo
appears weaker than the one for the fully soluble free triflate
anion. An analogous release of one triflate is detected in the
the action of 1, and be isolated in 55% yield (M =
n
À1
À1
5
000 gmol , PDI = 2.1, Mn (theor.) = 6100 gmol . Finally,
and again absolutely unprecedented for molybdenum-based
metathesis catalysts, a diyne containing two free carboxylic
acids, that is, 1,7-octadiyne-4,5-dicarboxylic acid could be
cyclopolymerized and the product isolated in 90% yield by
1
9
F NMR spectra recorded during the polymerization of 5,6-
bis[(pentyloxy)methyl]bicyclo[2.2.1]hept-2-ene, 7-
oxabicyclo[2.2.1]hept-5-ene-2,3-diylbis(methylene) diacetate
and tetrakis(ethoxycarbonyl)-1,7-octadiyne by the action of
1 (Figures S14 and 22). Again, the signals for the parent
catalyst vanish, while the one of free triflate and those for
different Mo triflates bound to the polymer (different
initiation products) develop over time. These data clearly
show that in the presence of monomer, but notably not in its
À1
the action of 1 (M = 2600 gmol , PDI = 1.3; M (theor.) =
n
n
À1
2
500 gmol , l = 432 nm).
max
In view of this remarkable metathesis activity and func-
tional-group tolerance of a high oxidation state Mo alkyli-
dene, we were interested in the nature of the active, that is,
propagating species. At first glance the high reactivity in
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and the cyclopolymerization of both 1,6-hepta- and 1,7-
octadiynes. Unprecedented functional-group tolerance was
observed. It is clear that substitution of one of the two
chemically different triflate ligands by, for example, alkoxides,
(chiral bi-) phenoxides, amides, offers access to a new family
of cationic Mo alkylidenes that are chiral at the metal center
and that hold great promise for various metathesis reactions.
Our results on these issues will be reported in due course.
Experimental Section
1
0
1
:
[Mo(N-2,6-Me C H )(CH-tBu)(OTf) (DME)]
(0.300 g,
2
6
3
2
.445 mmol) was dissolved in benzene (8 mL) and stirred for
5 min. Then a solution of 1,3-bis(2,4,6-trimethylphenyl)-2-imidazo-
linylidene (0.136 g, 0.445 mmol) in benzene (5 mL) was added.
Immediately, a color change from yellow to deep red was observed
and a precipitate formed. The mixture was stirred for a further 3 h, the
benzene was decanted and the precipitate was washed with benzene
to obtain a yellow colored solid (0.32 g, 81%). The compound was
dissolved in a minimum amount of CH Cl and the solution was
1
9
Figure 4. F NMR spectra recorded at different time intervals for the
polymerization of 4,4-bis(hydroxymethyl)-1,6-heptadiyne by catalyst 1.
2
2
stored at À308C for 24 h to afford a yellow crystalline material in
1
7
3% yield. H NMR (CD Cl ): (syn-isomer, 99.9%) d = 12.76 (s, 1,
2
2
absence, 1 converts quickly into a cationic 16-electron species
CHCMe , J = 118 Hz), 7.06–6.61 (7H, ArH), 3.98 (4H, CH NC),
+
3
CH
2
catalyst [Mo(NAr’)(IMesH )(monomer) (CH-tBu)(OTf)] -
19
2
x
2
.69–1.71 (24H, Me), 0.93 ppm (s, 9H, CH CMe ); F NMR
2 3
À
[
OTf] ), which then initiates the corresponding metathesis
13
(CD Cl ): d = À74.65 (SO CF ), À76.7 ppm (SO CF ). C NMR
2
2
3
3
3
3
reaction (Scheme 1).
(CD Cl ): d = 320.9 (CH-tBu), 208.7 (CN
), 154.6 (Cipso), 140.4
2
2
carbene
In contrast, catalyst 2 is much more reluctant to release
a triflate (Figure S24), a finding that corroborates with its
lower metathesis activity and with the proposition that
a cationic, 14-electron species is the active catalyst. Nonethe-
less, in view of the more pronounced nucleophilicity of I-tBu,
this is somewhat surprising, since the more nucleophilic NHC
in 2 must stabilize the cationic charge better than the less
nucleophilic one in 1. On the other hand, the bulky character
of I-tBu might well impede the realization of a linear align-
ment of the NHC–Mo–OTf or arylimido–Mo–OTf bond that
is a prerequisite for an effective release of the triflate.
However, to comprehensively comment on that issue, the
current data set, which is based on two complexes, is too
small. Thus, the influence of the ring size and substituents at
the NHC on the formation of the syn- and anti-isomers, their
rates of interconversion and, finally, on the propensity of the
corresponding catalyst to form cationic metal alkylidene
complexes, needs to be investigated.
(Cortho), 137.1 (Caryl), 136.8 (Caryl), 135.7 (Caryl), 131.1 (CHaryl), 130.5
(
(
2
CHaryl), 130.1 (CHaryl), 128.2(Caryl), 120.2 (q, CF , J = 319 Hz), 119.8
3
q, CF , J = 320 Hz), 53.1 (CMe ), 50.7 (CH
), 30.5 (CMe ),
3
3
2-imidazolylene
3
1.3 (CH ), 19.0 (CH ), 18.9 ppm (CH ); elemental analysis calcd (%)
3
3
3
.
for C H F MoN O S CH Cl : C 45.54, H 4.96, N 4.31; found: C
3
6
45
6
3
6
2
2
2
45.52, H 4.75, N 4.37.
2
:
[Mo(N-2,6-Me C H )(CH-tBu)(OTf) (DME)]
(0.100 g,
2
6
3
2
0.148 mmol) was dissolved in benzene (3 mL). Then a benzene
solution (1 mL) of 1,3-di-tert-butylimidazol-2-ylidene (0.027 g,
0.15 mmol) was added under stirring. Immediately, a color change
from yellow to deep red was observed and a precipitate formed. The
mixture was stirred for a further 3 h; then, the benzene was decanted
and the precipitate was washed with benzene to obtain a yellow
colored solid (0.060 g, 65%). The yellow solid was dissolved in
a minimum amount of CH Cl and stored at À308C for 24 h to afford
2
2
1
a yellow crystalline material in 60% yield. H NMR (CD Cl ): d =
2 2
1
4.60 (s, 1H, CHCMe , J = 121 Hz, syn-isomer), 7.12–6.95 (3H,
3 CH
ArH), 2.60 (2H, CHNC), 1.80–1.67 (24H, Me), 1.32 (s, 9H,
1
9
CH CMe ); F NMR (CD Cl ): d = À77.68, À77.69, À77.70, À77.71
2
3
2
2
1
3
(
(
(
CF SO ), À78.06, À78.07, À78.08, À78.09 ppm (CF SO ); C NMR
3
3
3
3
CD Cl ): d = 329.6 (CH-tBu), 175.4 (CN ), 154.3 (Cipso), 142.2
carbene
2
2
It is only possible to speculate about the reasons for the
higher trans-content of polymers prepared by the action of 1.
It might be because this catalyst, in contrast to 2, contains
a small fraction of the anti-alkylidene. If these systems behave
like the existing Schrock catalysts (Scheme 1), then an
involvement of the anti-isomer in conjunction with a fast
syn–anti interconversion should give rise to high trans-
Caryl), 136.9 (Caryl), 129.7 (CHaryl), 129.6 (CHaryl), 128.9 (CHaryl), 121.7
(CC=C), 120.6 (CC=C), 119.8 (q, CF , J = 318 Hz), 119.7 (q, CF , J =
3
3
319 Hz), 61.7 (NCMe
), 61.3 (CMe
), 32.8 (CMe
), 30.5 (CMe
), 30.1
3
3
3
3
(
CMe ), 21.1 (CH ), 18.4 ppm (CH ). Elemental analysis calcd (%)
3
3
3
for C H F MoN O S : C 40.84, H 5.27, N 5.50; found: C 40.88, H
2
6
39
6
3
6 2
5.20, N 5.56.
Received: April 24, 2014
Published online: && &&, &&&&
[
15]
[15a]
contents. As outlined by Schrock et al.,
a fast syn–anti
interconversion requires the presence of electron-donating
anionic ligands, for example, alkoxides. And in fact NHCs are
strongly electron donating and thus fulfill this requirement.
However, at this point, this explanation is only a reasonable
hypothesis and the rates of interconversion need to be
measured.
Keywords: alkylidenes · cyclopolymerization · molybdenum ·
N-heterocyclic carbenes ·
ring-opening metathesis polymerization
.
In summary, we have presented the first 16-electron NHC
molybdenum imidoalkylidene Schrock-type catalysts that are
active in various metathesis reactions including RCM, ROMP,
[
1] a) A. S. Gamble, J. M. Boncella, Organometallics 1993, 12,
2814 – 2819; b) L. L. Blosch, A. S. Gamble, K. Abboud, J. M.
4
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Angewandte
Chemie
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Molybdenum–NHC Complexes
M. R. Buchmeiser,* S. Sen, J. Unold,
To be or not to be (cationic)? In the
presence of monomer, neutral 16-elec-
tron N-heterocyclic carbene adducts of
VI
W. Frey
&&&& — &&&&
high oxidation state Mo N-arylimidoal-
kylidene bis(triflate) complexes convert
into cationic 16-electron complexes that
are active and functional-group tolerant
polymerization catalysts.
N-Heterocyclic Carbene, High Oxidation
State Molybdenum Alkylidene
Complexes: Functional-Group-Tolerant
Cationic Metathesis Catalysts
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!