Molybdenum and tungsten monoalkoxide pyrrolide (MAP) alkylidene complexes that contain a 2,6-dimesitylphenylimido ligand

Article abstract of DOI:10.1021/om4000693

Molybdenum and tungsten bispyrrolide alkylidene complexes that contain a 2,6-dimesitylphenylimido (NAr) ligand have been prepared, in which the pyrrolide is the parent pyrrolide or 2,5-dimethylpyrrolide. Monoalkoxide pyrrolide (MAP) complexes were prepared through addition of 1 equiv of an alcohol to the bispyrrolide complexes. MAP compounds that contain the parent pyrrolide (NC ₄H₄^-) are pyridine adducts, while those that contain 2,5-dimethylpyrrolide are pyridine free. Molybdenum and tungsten MAP 2,5-dimethylpyrrolide complexes that contain O-t-Bu, OCMe(CF₃) ₂, or O-2,6-Me₂C₆H₃ ligands were found to have approximately equal amounts of syn and anti alkylidene isomers, which allowed a study of the interconversion of the two employing ¹H-¹H EXSY methods. The K_eq values ([syn]/[anti]) are all 2-3 orders of magnitude smaller than those observed for a large number of Mo bisalkoxide imido alkylidene complexes, as a consequence of the destabilization of the syn isomer by the sterically demanding NAr* ligand. The rates of interconversion of syn and anti isomers were found to be 1-2 orders of magnitude faster for W MAP complexes than for Mo MAP complexes.

Full text of DOI:10.1021/om4000693

Article
pubs.acs.org/Organometallics
Molybdenum and Tungsten Monoalkoxide Pyrrolide (MAP)
Alkylidene Complexes That Contain a 2,6-Dimesitylphenylimido
Ligand
Laura C. H. Gerber, Richard R. Schrock,* and Peter Muller
̈
Department of Chemistry 6-331, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
S Supporting Information
*
ABSTRACT: Molybdenum and tungsten bispyrrolide alkyli-
dene complexes that contain a 2,6-dimesitylphenylimido
(
NAr*) ligand have been prepared, in which the pyrrolide is
the parent pyrrolide or 2,5-dimethylpyrrolide. Monoalkoxide
pyrrolide (MAP) complexes were prepared through addition
of 1 equiv of an alcohol to the bispyrrolide complexes. MAP
−
compounds that contain the parent pyrrolide (NC H ) are
4
4
pyridine adducts, while those that contain 2,5-dimethylpyrro-
lide are pyridine free. Molybdenum and tungsten MAP 2,5-
dimethylpyrrolide complexes that contain O-t-Bu, OCMe-
(
CF ) , or O-2,6-Me C H ligands were found to have
3 2 2 6 3
approximately equal amounts of syn and anti alkylidene
isomers, which allowed a study of the interconversion of the two employing H− H EXSY methods. The K values ([syn]/
1
1
eq
[
anti]) are all 2−3 orders of magnitude smaller than those observed for a large number of Mo bisalkoxide imido alkylidene
complexes, as a consequence of the destabilization of the syn isomer by the sterically demanding NAr* ligand. The rates of
interconversion of syn and anti isomers were found to be 1−2 orders of magnitude faster for W MAP complexes than for Mo
MAP complexes.
INTRODUCTION
imido ligand. The theory of metathesis by MAP complexes,
which are members of the large class of four-coordinate
stereogenic-at-metal (SAM) complexes, M(NR)(CHR′)(X)-
■
Bulky alkoxides and imido ligands in Mo- and W-based olefin
metathesis catalysts of the type M(NR)(CHR′)(OR″) slow or
2
(
Y), continues to be explored through theoretical calcula-
prevent intermolecular decomposition and/or ligand scram-
12
tions. A large N-heterocyclic carbene and a stereogenic metal
bling reactions, which allows the monomeric nature of these1
13
four-coordinate imido alkylidene complexes to be maintained.
center also are found in Z-selective ruthenium catalysts.
Chiral versions that contain biphenolates or binaphtholates
have been employed for enantioselective metathesis reactions
Although it has been established that the electronic and steric
natures of the imido ligand play a significant role in
determining the reactivity and selectivity of metathesis catalysts,
no catalysts have been prepared in which the imido ligand is an
analogue of a 2,6-terphenoxide. Anilines analogous to the large
2,6-terphenonols that we have employed have been prepared,
namely 2,6-(2,4,6-i-Pr C H ) C H NH (2,6-Trip C H NH )
(
when the biphenolate or binaphtholate is enantiomerically
2
pure) or in order to control the structure of polymers prepared
in ROMP reactions (usually when the biphenolate or
3
binaphtholate is racemic). The most recent development has
been the synthesis of monoalkoxide (or monoaryloxide)
pyrrolide (MAP) complexes of the type M(NR)(CHR′)-
3
6
2 2
6
3
2
2
6
3
2
and 2,6-(2,4,6-Me C H ) C H NH (2,6-Mes C H NH =
3
6
2
2
6
3
2
2
5
3
2
(
OR″)(Pyr), where Pyr is usually the parent pyrrolide or a 2,5-
Ar*NH ), as have several transition-metal complexes that
1c
2
dimethylpyrrolide (Me Pyr). MAP complexes have proven to
14
2
contain amido or imido derivatives of these large anilines. In
be more efficient in many olefin metathesis reactions in terms
of higher turnover numbers, but more interestingly, they can
provide high Z selectivities. Z-selective MAP catalysts have now
been developed for ring-opening metathesis polymerization
6
anticipation of an NAr* ligand being less sterically demanding
than the 2,6-Trip C H N ligand, we targeted NAr* imido
2
5
3
alkylidene catalysts of Mo and W. The NAr* ligand is the
approximate steric equivalent of the OHMT ligand. A shorter
MN bond as opposed to a M−O bond (MN is expected
to be ∼0.12 Å shorter) would suggest that the steric demand of
a NAr* ligand may be greater than that of an OHMT ligand.
4
5
(
ROMP), homocoupling, ring opening/cross metathesis,
7
ethenolysis, and formation of natural products through ring-
closing reactions. Z-Selective reactions have been possible
8
when one large OR″ ligand is present, e.g., a terphenoxide such
9
as 2,6-(2,4,6-i-Pr C H ) C H O (HIPTO), 2,6-(2,4,6-
3
6
2
2
6
3
1
0
Me C H ) C H O (HMTO),
or 2,6-(C F ) C H O
Received: January 30, 2013
Published: April 4, 2013
3
6
2
2
6
3
6
5
2
6
3
1
1
(
DFTO), especially in combination with a relatively small
©
2013 American Chemical Society
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Article
However, this effect may be counteracted by an essentially
other MAP compounds except upon irradiation of syn isomers
4
b,d
1
1
linear M−N−C angle in most circumstances.
at low temperatures.
Preliminary 2D H− H EXSY
ipso
15
15
In our initial communication we reported the synthesis of
MoNAr* alkylidene complexes. A synthetic route had to be
devised (Scheme 1) that did not require formation of
experiments suggested that the syn and anti forms of
Mo(NAr*)(CHCMe Ph)(Me pyr)(O-t-Bu) interconvert at a
2
2
−1
rate of ∼0.05 s at 22 °C. Since the rate of interconversion of
syn and anti isomers and the equilibrium between them is a
crucial feature of many metathesis reactions with imido
Scheme 1. Synthesis of Mo Alkylidenes That Contain the
NAr* Ligand
17
alkylidene complexes, and since little is known about syn
4b
and anti isomers in MAP species, we set out to expand the
chemistry of MoNAr* MAP species and to prepare WNAr*
MAP complexes.
RESULTS AND DISCUSSION
■
Synthesis of W(NAr*) Compounds. We chose W(N-t-
1
8
Bu) Cl (py)
as the starting point for expanding NAr*
2
2
2
chemistry to tungsten. The approach (Scheme 2) is the same as
Scheme 2. Synthesis of W Complexes That Contain the
NAr* Ligand
Mo(NAr*) (CH R) precursors (R = t-Bu, CMe Ph) analo-
2
2
2
2
gous to those employed for synthesizing virtually all imido
alkylidene complexes of Mo and W to date. The synthetic route
shown in Scheme 1 was inspired by observations published by
1
6
Gibson. Mo(N-t-Bu) Cl (dme) was treated with LiNHAr* to
2
2
give Mo(N-t-Bu) (NHAr*)Cl, which was then transformed
2
into Mo(N-t-Bu)(NAr*)(NH-t-Bu)Cl upon treatment with
NEt . Addition of 2,6-lutidinium chloride to Mo(N-t-Bu)-
3
(
NAr*)(NH-t-Bu)Cl yielded Mo(N-t-Bu)(NAr*)(NH -t-Bu)-
2
2
Cl , which was then alkylated to give Mo(N-t-Bu)(NAr*)-
(
CH CMe Ph) . Upon addition of pyridinium chloride or 3,5-
2 2 2
that employed to prepare the Mo(NAr*) species (Scheme 1).
Upon addition of LiNHAr* to W(N-t-Bu) Cl (py) , W(N-t-
dimethylpyridinium chloride to Mo(N-t-Bu)(NAr*)-
2
2
2
(
CH CMe Ph) the tert-butylimido group was protonated
2
2
2
Bu) Cl(NHAr*) is formed, which without isolation is treated
2
selectively and Mo(NAr*)(CHCMe Ph)Cl (L) complexes (L
2
2
with NEt to give W(NAr*)(N-t-Bu)Cl(NH-t-Bu) (5) in 74%
3
=
pyridine, 3,5-dimethylpyridine) were isolated in high yield.
yield. W(N-t-Bu) Cl (py) did not react with Ar*NH after 16
2
2
2
2
This was the first time we were able to use a form of HCl
instead of triflic acid to generate the alkylidene complex.
Addition of triflic acid to Mo(N-t-Bu)(NAr*)(CH CMe Ph)
2
19
h at 80 °C. Attempts to use [(t-BuN) WCl (NH -t-Bu)]
2
instead of W(N-t-Bu) Cl (py) as a starting material were also
2
2
2
2
2
2
2
2
unsuccessful.
W(NAr*)(N-t-Bu)Cl (NH -t-Bu) (6) was synthesized
in the presence of 1,2-dimethoxyethane led only to
2
2
decomposition instead of formation of Mo(NAr*)-
through addition of 2,6-lutidine-HCl to 5 (Scheme 2). Rather
than reduce the yield of 6 as a consequence of a lengthy
purification, W(NAr*)(N-t-Bu)(CH CMe Ph) (7) was syn-
(
CHCMe Ph)(OTf) (dme).
2 2
The only Mo(NAr*) MAP compound that has been
1
5
2
2
2
reported is Mo(NAr*)(CHCMe Ph)(Me pyr)(O-t-Bu). It
2
2
thesized through the addition of 2 equiv of MgClCH CMe Ph
2
2
was synthesized either by treating Mo(NAr*)(CHCMe Ph)-
2
to crude 6. W(NAr*)(N-t-Bu)(CH CMe Ph) was isolated in
2
2
2
(
Me pyr)2 (2Mo) with tert-butyl alcohol or by treating
2
6
8% yield.
Addition of 1 equiv of pyridine to 7 followed by 3 equiv of
Mo(NAr*)(CHCMe Ph)Cl(O-t-Bu)(py) with LiMe pyr. Mo-
2
2
(
NAr*)(CHCMe Ph)(Me pyr)(O-t-Bu) was found to have
2 2
HCl in diethyl ether gave W(NAr*)(CHCMe Ph)Cl (py) (8)
2
2
two alkylidene resonances in the proton NMR spectrum (at
1.861 and 11.695 ppm in C D ) in approximately a 1:1 ratio.
1
in 63% yield. Only the anti alkylidene isomer is visible in the H
NMR spectrum of 8 ( J = 144 Hz). Compound 8 could not
1
6
6
1
1
CH
The J
value for the downfield resonance (118 Hz) is
CH
be prepared employing 3 equiv of pyridine-HCl. Employing
HCl in diethyl ether or triflic acid also resulted in
decomposition with little or no identifiable alkylidene species
consistent with it being a syn alkylidene and that for the upfield
proton resonance (152 Hz) an anti alkylidene; the substituent
on a syn alkylidene points toward the imido ligand, and the
substituent on an anti alkylidene points away from the imido
ligand (eq 1). The syn isomer is the only one observed in all
1
being observed in H NMR spectra of the crude reaction
product.
An alkylidene species, W(NAr*)(CHCMe Ph)Cl (bipy) (9),
2
2
was also synthesized through a reaction between W(NAr*)(N-
t-Bu)(CH CMe Ph) and 2,2′-bipyridine (bipy) followed by 3
2
2
2
equiv of HCl (eq 2). Compound 9 is insoluble in pentane,
diethyl ether, benzene, and toluene. Its low solubility in CH Cl
2
2
allows W(NAr*)(CHCMe Ph)Cl (bipy) to be extracted and
2
2
thereby separated from t-BuNH Cl. Bipy adducts have been
3
19,20
employed as synthetic intermediates previously,
in part
2
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Article
spectrum at −40 °C reveals that all pyrrolide protons and
1
methyl groups are inequivalent. A J value of 126 Hz, which
CH
can be observed at −40 °C, suggests that the alkylidene is in the
1
syn orientation. (The J value for Mo(NAr*)(CHCMe Ph)-
CH
2
15
(
Me pyr) (2 ) is 130 Hz. ) We do not know whether one
pyrrolide ligand is bound in an η fashion and the other in an η
fashion in both 2 and 2 or both pyrrolides are bound in an
η fashion. In either case, restricted rotations of ligands at −40
C could result in the observed lack of symmetry for 2 at that
2 2 Mo
1
5
W
Mo
because their low solubility allows them to be obtained readily
in pure form.
1
°
W
ZnCl or ZnCl (1,4-dioxane) can be employed to remove
2
2
temperature.
2
,2′-bipyridine or 1,10-phenanthroline from Mo imido
19−21
Synthesis of M(NAr*) MAP Compounds. Addition of 2
alkylidene complexes.
When ZnCl (dioxane) is added to
2
equiv of LiPyr to Mo(NAr*)(CHCMe Ph)Cl (py) led to
2
2
W(NAr*)(CHCMe Ph)Cl (bipy) suspended in CD Cl , a new
2
2
2
2
1
formation of Mo(NAr*)(CHCMe Ph)(Pyr) (py) (1 ) in
2 2 Mo
product and free dioxane are observed by H NMR
spectroscopy, but bipyridine resonances are still visible. An X-
ray structure showed that the product is [W(NAr*)-
good yield. Pyridine is retained in the coordination sphere in
1Mo simply for steric reasons. The synthesis of 1Mo completes
the syntheses of M(NAr*)(CHCMe Ph)(Pyr) (py) (M = Mo
2
2
(
CHCMe Ph)Cl(bipy)][Zn Cl ] (10): i.e., ZnCl abstracts
2 2 6 0.5 2
(
1 ), W (1 )) and M(NAr*)(CHCMe Ph)(Me Pyr) (M =
Mo W 2 2 2
a chloride ligand to form a cationic W bipyridine complex in
2−
Mo (2 ), W (2 )) and sets the stage for the synthesis of MAP
Mo W
which [Zn Cl ] is the (di)anion (eq 2).
2
6
complexes.
[
W(NAr*)(CHCMe Ph)Cl(bipy)][Zn Cl ] crystallizes in
2 2 6 0.5
with one [W(NAr*)(CHCMe Ph)Cl-
Complexes 3a−g are formed upon addition of 1 equiv of
the space group P1
bipy)][Zn Cl ]
̅
2
alcohol to 1 (Scheme 3). The pyridine ligand remains bound
Mo
(
unit and one toluene molecule per
2
6 0.5
asymmetric unit (Figure 1). The alkylidene ligand is in the
Scheme 3. Synthesis of Mo and W MAP Complexes from
Bispyrrolides
Figure 1. Crystal structure of [W(NAr*)(CHCMe Ph)Cl(bipy)]-
2
[
Zn Cl ] in a thermal ellipsoid representation at the 50% probability
2 6 0.5
2−
level. Only half of the Zn Cl6 anion is present in the asymmetric
2
unit, but the whole unit is pictured. Hydrogen atoms, the toluene
solvent molecule, and a minor component of disorder are omitted for
clarity. Selected bond angles (deg): C11−N1−W1 = 153.03(15), C2−
C1−W1 = 148.42(17).
1
to the metal in 3a−g. In all cases in solution, according to H
syn orientation. The bipyridine ligand is disordered over two
positions. The geometry about W is best described as a
distorted square pyramid (τ = 0.34, where τ = 0 for a perfect
NMR spectroscopy, the alkylidene is in the anti form. This is
unusual for Mo or W imido alkylidene complexes in general if
they are four-coordinate, but less so when the complex is five-
coordinate. Several five-coordinate NAr* monochloride mono-
alkoxide species reported previously were also anti alkylidenes
2
2
square pyramid ) with the alkylidene ligand at the apical site.
The W1−N1−C11 angle (153.03(15)°) is relatively small
compared to the 175−180° found in many other imido
alkylidene complexes, possibly as a consequence of some
attractive π interaction between one of the mesityl rings and the
bipy ring system, but otherwise the bond lengths and angles
15
in solution. Other anti group 6 imido alkylidenes that are
23
base-stabilized species have been known for some time.
Since pyridine essentially blocks a coordination site and
thereby reaction of the alkylidene complex with olefins, Lewis
acids were employed with the goal of removing pyridine and
isolating base-free alkylidene complexes. It was found that upon
addition of 1 equiv of B(C F ) to 3a−g, B(C F ) (NC H )
formed immediately, according to H and F NMR spectra.
Unfortunately, the similar solubilities of any base-free species
and B(C F ) (NC H ) prevented isolation of the base-free
(
see the Supporting Information) are those expected for W
imido alkylidene complexes. The coordination geometry
around each zinc is a slightly distorted tetrahedron, as expected.
Since bipyridine could not be removed easily from
W(NAr*)(CHCMe Ph)Cl (bipy), possibly in part because of
6
5 3
6
5 3
5
5
1
19
2
2
the π interaction noted above, further syntheses focused on
W(NAr*)(CHCMe Ph)Cl (py) as a starting material. W-
6
5 3
5
5
alkylidene complexes in pure form. Also, clean conversion to
one base-free alkylidene species, which we expected to have two
alkylidene resonances, was not observed when B(C F ) was
2
2
(
NAr*)(CHCMe Ph)(Pyr) (py) (1 ) can be synthesized
2 2 W
through addition of excess LiC H to a solution of 8 in diethyl
4
4
6 5 3
ether. Addition of LiMe C H to 8 gives W(NAr*)-
added to 3a−e; thus, further characterization of the target MAP
2
4
2
(
CHCMe Ph)(Me pyr) (2 ). Proton NMR spectra of 2
species in situ did not seem feasible. However, only two
2
2
2
W
W
1
show free pyridine unless the product is left under a good
alkylidene resonances are observed in H NMR spectra of base-
1
vacuum for several hours. The resonances in the H NMR
free 3f,g (3f′,g′, respectively). In each case the two resonances
spectrum of 2 are broad at room temperature. A proton NMR
were confirmed as being those of syn and anti alkylidenes on
W
2
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Organometallics
Article
the basis of the 1JCH values. For 3f′ K = 2.0, and for 3g′ K =
viewed along the C21−N1−W1 axis, one mesityl group of the
NAr* ligand is seen to be located over the alkoxide, while the
other mesityl group falls between the pyrrolide and alkylidene
ligands.
eq
eq
2
.3 (where Keq = [syn]/[anti]).
In order to isolate pyridine-free MAP compounds, attention
shifted toward pyridine-free 2Mo as a precursor. Four
representative alcohols were chosen in order to prepare MAP
Study of Alkylidene Rotation. Compounds 4a−d for Mo
and W (Table 1) are a mixture of syn and anti alkylidene
species. The MAP complexes Mo(NAr*)(CHCMe Ph)-
2
(
Me pyr)(O-t-Bu) (4a ), Mo(NAr*)(CHCMe Ph)(Me pyr)-
2 Mo 2 2
[
(
(
OCMe(CF ) ] (4b ), Mo(NAr*)(CHCMe Ph)(Me pyr)-
Table 1. Rate and Equilibrium Constants for MAP Species at
3
2
Mo
2
2
OSiPh ) (4c ), and Mo(NAr*)(CHCMe Ph)(Me pyr)-
21 °C for Mo and W M(NAr*)(CHCMe Ph)(Me Pyr)(OR)
3
Mo
2
2
2
2
OAr′) (4d ) (Ar′ = 2,6-Me C H ) could all be synthesized
(“M(OR)”) Compounds
Mo
2
6
3
through addition of 1 equiv of the appropriate alcohol to 2
(
found to be extremely soluble in solvents that have previously
been employed for recrystallization (e.g., pentane or diethyl
ether) and therefore not isolable in crystalline form from such
Mo
kf (s−1)
kr (s−1)
compd
Keq
Scheme 3). Like many MAP complexes, 4a −d were all
Mo
Mo
Mo(O-t-Bu) (4aMo
)
0.9
2.7
26
0.05(0.01)
0.029(0.006)
∼0.5
0.06(0.01)
0.011(0.004)
∼0.02
Mo[OCMe(CF ) ] (4bMo)
3
2
Mo(OSiPh ) (4cMo)
3
Mo(O-2,6-C H ) (4dMo)
2.2
1.8
12
0.10(0.02)
1.4(0.6)
1.8(1.1)
∼50
0.05(0.01)
0.8(0.4)
0.15(0.2)
∼0.5
6
3
solvents. However, 4a −d
could be isolated in pure
Mo
Mo
W(O-t-Bu) (4aW)
crystalline form from acetonitrile and isolated as acetonitrile-
free species; there was no evidence for any reaction between
the MAP species and acetonitrile at room temperature.
W[OCMe(CF ) ] (4b )
3
2
W
W(OSiPh ) (4c )
100
5.6
3
W
W(O-2,6-C H ) (4d )
2(2)
0.4(0.4)
6
3
W
Compounds 4a −d
alkylidene isomers, according to H NMR studies.
are all mixtures of syn and anti
Mo
Mo
1
1
Addition of 1 equiv of an alcohol to W(NAr*)-
isomers (eq 1) in C D solution, according to H NMR studies.
6
6
(
(
[
(
(
CHCMe Ph)(Me Pyr) (2 ) led to W(NAr*)(CHCMe Ph)-
The syn isomer is often the major isomer in imido alkylidene
complexes of Mo and W, due to an agostic interaction that
stabilizes it relative to the anti isomer. Significant additional
factors include the counteracting steric demands of the
alkylidene, imido, and other ligands. Compounds similar to
those reported here, but with a relatively smaller 2,6-
diisopropylphenyl imido ligand, such as Mo(NAr)-
2
2
2
W
2
Me pyr)(O-t-Bu) (4a ), W(NAr*)(CHCMe Ph)(Me pyr)-
2
W
2
2
OCMe(CF ) ] (4b ), W(NAr*)(CHCMe Ph)(Me pyr)-
3
2
W
2
2
OSiPh3 ) (4c ), and W(NAr*)(CHCMe Ph)-
W
2
Me pyr)(OAr′) (4d ) (Scheme 3). All could be isolated in
2
W
pure form through crystallization from acetonitrile.
An X-ray study of W(NAr*)(CHCMe Ph)(Me pyr)(O-t-
2
2
Bu) (4a ) revealed a whole molecule disorder, with the major
(CHCMe
2
Ph)(Me
2
pyr)(OCMe ) and Mo(NAr)-
3
W
(
CHCMe Ph)(Me pyr)[OCMe(CF ) ], only show the syn
2 2 3 2
24
component representing approximately 90% of the electron
density (Figure 2). The alkylidene ligand in the major
component is in the syn orientation and is slightly twisted
with an N1−W1−C1−C2 dihedral angle of 12.25°. The W1−
C1 bond length is 1.875(2) Å, the W1−N1−C21 angle is
isomer in solution.
The Keq values in Table 1 are all 2−3 orders of magnitude
smaller than those observed for a large number of Mo
bisalkoxide imido alkylidene complexes in benzene or
17
17
toluene. Keq values in Mo bisalkoxide complexes are
approximately the same order of magnitude as those given in
1
73.4(3)°, and the C2−C1−W1 angle is 147.33(19)°, all
typical of group 6 MAP complexes. When the molecule is
Table 1 when data are obtained in THF-d , as a consequence of
8
THF binding more strongly to the anti form and therefore
shifting the equilibrium in that direction. We propose that the
“
low” Keq values in Table 1 are simply a consequence of the
demanding steric bulk of the NAr* ligand and therefore
destabilization of the syn isomer relative to the anti isomer. The
values for Keq for Mo(OSiPh ) (4c ) and W(OSiPh ) (4c )
3
Mo
3
W
(
26 and 100, respectively) are consistent with the greater steric
demand of the OSiPh ligand in comparison with the three
3
other OR ligands.
1
1
H− H EXSY studies were conducted to obtain the rate
constants for alkylidene rotation for 4a,b,d for Mo and W
Table 1). The relatively large values of K for Mo(OSiPh )
4c ) and W(OSiPh ) (4c ) (26 and 100, respectively) did
25
(
(
eq 3
Mo
3
W
not allow us to obtain rate constants for interconversion of syn
1
1
and anti isomers using the H− H EXSY method. Therefore,
1
7
4c
and 4c were photolyzed at 350 nm at −78 °C to
Mo
W
generate a greater proportion of the anti isomer. Rate constants
Figure 2. Crystal structure of W(NAr*)(CHCMe Ph)(Me pyr)(O-t-
2
2
were obtained over a 20 °C range (−20 to −40 °C for 4c and
Mo
Bu) (4a ) in a thermal ellipsoid representation at the 50% probability
w
−
40 to −60 °C for 4c ) for the rotation of the anti alkylidene
W
level. Hydrogen atoms and a minor disorder component are omitted
for clarity. Selected bond lengths (Å): C1−W1 = 1.875(2), W1−N1 =
to the syn form, and the data were extrapolated to give k at 21
f
°
C. Because rate constants can only be obtained over a small
1
.750(2), W1−O1 = 1.8682(19), W1−N2 = 2.033(2). Selected bond
temperature range, k at 21 °C is not highly accurate and is
angles (deg): C2−C1−W1 = 147.33(19), N1−W1−O1 = 115.59(14),
N1−W1−C1 = 106.04(16), O1−W1−C1 = 109.35(10), N1−W1−N2
=
f
therefore given with only one significant figure in Table 1.
The values for k for the four Mo complexes (4a , 4b ,
111.05(13), O1−W1−N2 = 110.03(10), C1−W1−N2 =
f
Mo
Mo
104.07(11), C21−N1−W1 = 173.4(3).
4c , and 4d ) vary from 0.029 to 0.10, while the values for k
Mo Mo
r
2
376
dx.doi.org/10.1021/om4000693 | Organometallics 2013, 32, 2373−2378

Organometallics
Article
vary from 0.011 to 0.06. For comparison, k for Mo(NAr)-
CONCLUSIONS
f
■
(
CHCMe Ph)(OTPP)(Pyr) (OTPP = 2,3,5,6-tetraphenyl-
2
Molybdenum and tungsten alkylidene compounds that contain
the 2,6-dimesitylphenylimido (NAr*) ligand have been
synthesized and several MAP species for both Mo and W
prepared. The demanding steric bulk of the NAr* ligand is
reflected in the relatively low Keq values ([syn]/[anti]) along
with a slower rate of conversion of the anti to the syn alkylidene
isomers in NAr* complexes relative to complexes that contain a
smaller imido ligand, even NAr. Alkylidene rotation in four-
coordinate MAP species was found to be at least an order of
magnitude larger in W(NAr*) complexes than in Mo(NAr*)
complexes. It remains to be seen how the steric bulk of the
NAr* ligand will effect the reactivity of M(NAr*) MAP species,
the stability of metallacyclobutanes, and the performance of
M(NAr*) MAP species in a variety of olefin metathesis
reactions.
−1
4b
phenoxide) at 21 °C has been found to be 0.67 s , roughly
an order of magnitude larger. Another example is k for
f
Mo(NAd)(CHCMe Ph)(OHIPT)(Pyr) (Ad = 1-adamantyl) at
2
−1
4d
2
98 K, which is 0.96 s . For Mo(NAd)(CHCMe Ph)-
2
(
OHIPT)(Pyr) the equilibrium constant is estimated to be on
the order of 4000 or more and the value for k therefore is 2.5 ×
r
−4
−1
10
s
or less. Although few data are available, we can
tentatively draw the conclusion that the NAr* ligand not only
destabilizes the syn isomer but also restricts the rate of anti to
syn alkylidene rotation. Both are consistent with the unusually
large steric demand of the NAr* ligand.
The data in Table 1 can be compared to data for
1
7
Mo(NAr)(CHR)(OR′) complexes.
For Mo(NAr)-
2
(
CHCMe Ph)(OR) complexes in toluene the k values at
2
2
f
2
98 K for OR = O-t-Bu, OCMe (CF ), OCMe(CF ) ,
2 3 3 2
−1
OC(CF ) are ∼500 (estimated), 6.8, 0.10, and 0.0015 s ,
ASSOCIATED CONTENT
Supporting Information
3
3
■
respectively. This is a dramatic trend that spans approximately 5
orders of magnitude. Since the Keq values for this series of
bisalkoxides (in toluene at 298 K) are 1200, 1800, 1400, and
S
*
Text and figures giving experimental details for the synthesis of
all compounds and characterization data, along with tables and
CIF files that provide crystallographic details. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
90, the k values are 2−3 orders of magnitude smaller than k .
r
f
The most obvious reason why the rates of interconversion vary
more dramatically in bisalkoxides than in the MAP species in
Table 1 is that MAP species contain only one alkoxide;
therefore, the “alkoxide effect” is diluted in MAP species.
Another possibility is that in a MAP species, in which the metal
is a stereogenic center, the alkylidene might rotate in only one
direction, one that is regulated largely by the pyrrolide ligand,
which is the same in all the MAP species in Table 1. The
AUTHOR INFORMATION
Corresponding Author
E-mail for R.R.S.: rrs@mit.edu.
■
*
Notes
The authors declare no competing financial interest.
“
alkoxide effect” would again be diluted, perhaps dramatically.
ACKNOWLEDGMENTS
■
Finally, it should be noted that a “bending” of the NAr* ligand
17
We thank the National Science Foundation (CHE-1111133 to
R.R.S.) for supporting this research. The Department of
Chemistry thanks the NSF for funds to purchase a Varian 500
NMR instrument (CHE-9808061) and an X-ray diffractometer
in bisalkoxide complexes was proposed to stabilize the
intermediate alkylidene that has rotated by 90°. One might
expect that the NAr* ligand would not bend as readily as (e.g.)
the 2,6-diisopropylphenyl ligand, which also could contribute to
a less dramatic variation in the MAP species than in the
(
CHE-0946721).
bisalkoxide complexes. What is required are k data for
f
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dx.doi.org/10.1021/om4000693 | Organometallics 2013, 32, 2373−2378