Fundamental studies of molybdenum and tungsten methylidene and metallacyclobutane complexes

Article abstract of DOI:10.1021/om100363g

Addition of ethylene to Mo(NAr)(CHCMe₂Ph)(OHIPT)(Pyr) (NAr = N-2,6-i-Pr₂C₆H₃, OHIPT = O-2,6-(2,4,6-i-Pr ₃C₆H₂)₂C₆H₃, Pyr = NC₄H₄) led to the trigonal-bipyramidal metallacyclobutane complex Mo(NAr)(C₃H₆)(OHIPT)(Pyr), in which the imido and aryloxide ligands occupy axial positions. Mo(NAr)(C ₃H₆)(OHIPT)(Pyr) loses ethylene to give isolable Mo(NAr)(CH₂)(OHIPT)(Pyr). W(NAr)(CH₂)(OTPP)(Me ₂Pyr) (OTPP = O-2,3,5,6-Ph₄C₆H, Me ₂Pyr = 2,5-Me₂NC₄H₂) was prepared similarly. Single-crystal X-ray studies of Mo(NAr)(CH₂)(OHIPT)(Pyr) and W(NAr)(CH₂)(OTPP)(Me₂Pyr) show that they are monomers that contain an η¹-pyrrolide ligand and a methylidene ligand in which the M-C-H_anti angle is smaller than the M-C-H_syn angle, consistent with an agostic interaction between CH_anti and the metal. Attempts to prepare analogous Mo(NAd)(CH₂)(OHIPT)(Pyr) (Ad = 1-adamantyl) yielded only the ethylene complex Mo(NAd)(C₂H ₄)(OHIPT)(Pyr). W(NAr^tBu)(CH₂)(OTPP)(Me ₂Pyr) (Ar^tBu = 2-t-BuC₆H₄) was isolated upon loss of ethylene from W(NAr^tBu)(C₃H ₆)(OTPP)(Me₂Pyr), but decomposed in solution over a period of several hours at 22 °C. NMR studies of Mo(NAr)(C₃H ₆)(OHIPT)(Pyr) and W(NAr)(C₃H₆)(OHIPT)(Pyr) species showed them both to be in equilibrium with ethylene/methylidene intermediates before losing ethylene to yield the respective methylidene complexes. Detailed NMR studies of Mo(NAr)(C₃H₆)(OBitet) (Me₂Pyr) (OBitet is the anion derived from (R)-3,3′-dibromo- 2′-(tert-butyldimethylsilyloxy)-5,5′,6,6′,7,7′,8, 8′-octahydro-1,1′-binaphthyl-2-ol) were carried out and compared with previous studies of W(NAr)(C₃H₆)(OBitet)(Me ₂Pyr). It could be shown that Mo(NAr)(C₃H ₆)(OBitet)(Me₂Pyr) forms an ethylene/methylidene intermediate at 20 °C at a rate that is 4500 times faster than the rate at which W(NAr)(C₃H₆)(OBitet)(Me₂Pyr) forms an ethylene/methylidene intermediate. It is proposed that the stability of methylidene complexes coupled with their high reactivity accounts for the high efficiency of many olefin metathesis processes that employ monoaryloxidepyrrolide catalysts.

Full text of DOI:10.1021/om100363g

Organometallics 2010, 29, 5241–5251 5241
DOI: 10.1021/om100363g
Fundamental Studies of Molybdenum and Tungsten Methylidene and
Metallacyclobutane Complexes†
Richard R. Schrock,* Annie J. Jiang, Smaranda C. Marinescu, Jeffrey H. Simpson, and
€
Peter Muller
Department of Chemistry Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received April 28, 2010
Addition of ethylene to Mo(NAr)(CHCMe2Ph)(OHIPT)(Pyr) (NAr = N-2,6-i-Pr2C6H3, OHIPT =
O-2,6-(2,4,6-i-Pr3C6H2)2C6H3, Pyr = NC4H4) led to the trigonal-bipyramidal metallacyclobutane
complex Mo(NAr)(C3H6)(OHIPT)(Pyr), in which the imido and aryloxide ligands occupy axial
positions. Mo(NAr)(C3H6)(OHIPT)(Pyr) loses ethylene to give isolable Mo(NAr)(CH2)(OHIPT)-
(Pyr). W(NAr)(CH2)(OTPP)(Me2Pyr) (OTPP = O-2,3,5,6-Ph4C6H, Me2Pyr = 2,5-Me2NC4H2) was
prepared similarly. Single-crystal X-ray studies of Mo(NAr)(CH2)(OHIPT)(Pyr) and W(NAr)(CH2)-
(OTPP)(Me2Pyr) show that they are monomers that contain an η1-pyrrolide ligand and a methylidene
ligand in which the M-C-Hanti angle is smaller than the M-C-Hsyn angle, consistent with an agostic
interaction between CHanti and the metal. Attempts to prepare analogous Mo(NAd)(CH2)(OHIPT)-
(Pyr) (Ad = 1-adamantyl) yielded only the ethylene complex Mo(NAd)(C2H4)(OHIPT)(Pyr). W(NArtBu)-
(CH2)(OTPP)(Me2Pyr) (ArtBu = 2-t-BuC6H4) was isolated upon loss of ethylene from W(NArtBu)-
(C3H6)(OTPP)(Me2Pyr), but decomposed in solution over a period of several hours at 22 °C. NMR
studies of Mo(NAr)(C3H6)(OHIPT)(Pyr) and W(NAr)(C3H6)(OHIPT)(Pyr) species showed them
both to be in equilibrium with ethylene/methylidene intermediates before losing ethylene to yield the
respective methylidene complexes. Detailed NMR studies of Mo(NAr)(C3H6)(OBitet)(Me2Pyr)
(OBitet is the anion derived from (R)-3,30-dibromo-20-(tert-butyldimethylsilyloxy)-5,50,6,60,7,70,8,80-
octahydro-1,10-binaphthyl-2-ol) were carried out and compared with previous studies of W(NAr)-
(C3H6)(OBitet)(Me2Pyr). It could be shown that Mo(NAr)(C3H6)(OBitet)(Me2Pyr) forms an ethylene/
methylidene intermediate at 20 °C at a rate that is 4500 times faster than the rate at which W(NAr)-
(C3H6)(OBitet)(Me2Pyr) forms an ethylene/methylidene intermediate. It is proposed that the stabi-
lity of methylidene complexes coupled with their high reactivity accounts for the high efficiency of
many olefin metathesis processes that employ monoaryloxidepyrrolide catalysts.
Introduction
have been dominated by bisalkoxide or biphenolate and
related species.1 In the last several years new types of imido
alkylidene complexes that have the formula M(NR)(CHR0)-
(OR00)(Pyr), where Pyr is a pyrrolide or substituted pyrrolide
ligand and OR00 usually is an aryloxide, have been prepared
and explored.2 These MAP (monoaryloxidepyrrolide or
monoalkoxidepyrrolide) species often can be generated in
situ through addition of R00OH to a M(NR)(CHR0)(Pyr)2
species,2a,3 an attribute that allows relatively facile examina-
tion of many MAP variations, including monosiloxide ana-
logues of MAP species4 bound to silica.5 Attributes of homo-
geneous MAP species include their high reactivity in general
and their efficiency for enantioselective metathesis reac-
tions,2c,h for formation of new ROMP polymer structures,2j
for endoselective enyne reactions,2k for Z-selective metathesis
Olefin metathesis reactions catalyzed by high oxidation
state molybdenum and tungsten imido alkylidene complexes
† Part of the Dietmar Seyferth Festschrift.
*To whom correspondence should be addressed. E-mail: rrs@mit.edu.
(1) (a) Schrock, R. R. Chem. Rev. 2002, 102, 145. (b) Schrock, R. R.;
Hoveyda, A. H. Angew. Chem., Int. Ed. 2003, 42, 4592. (c) Schrock, R. R. In
Handbookof Metathesis; Grubbs, R. H., Ed.; Wiley-VCH: Weinheim, 2003;
p 8. (d) Schrock, R. R. Angew. Chem., Int. Ed. 2006, 45, 3748. (e) Schrock,
R. R.; Czekelius, C. C. Adv. Syn. Catal. 2007, 349, 55.
€
(2) (a) Singh, R.; Schrock, R. R.; Muller, P.; Hoveyda, A. H. J. Am.
Chem. Soc. 2007, 129, 12654. (b) Marinescu, S. C.; Schrock, R. R.; Li, B.;
Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131, 58. (c) Malcolmson, S. J.;
Meek, S. J.; Sattely, E. S.; Schrock, R. R.; Hoveyda, A. H. Nature 2008, 456,
933. (d) Sattely, E. S.; Meek, S. J.; Malcolmson, S. J.; Schrock, R. R.;
Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131, 943. (e) Schrock, R. R. Chem.
Rev. 2009, 109, 3211. (f) Ibrahem, I.; Yu, M.; Schrock, R. R.; Hoveyda, A. H.
J. Am. Chem. Soc. 2009, 131, 3844. (g) Jiang, A. J.; Simpson, J. H.; M€uller,
P.; Schrock, R. R. J. Am. Chem. Soc. 2009, 131, 7770. (h) Meek, S. J.;
Malcolmson, S. J.; Li, B.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc.
2009, 131, 16407. (i) Jiang, A. J.; Zhao, Y.; Schrock, R. R.; Hoveyda, A. H.
J. Am. Chem. Soc. 2009, 131, 16630. (j) Flook, M. M.; Jiang, A. J.; Schrock,
R. R.; M€uller, P.; Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131, 7962.
(k) Lee, Y.-J.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131,
10652. (l) Marinescu, S. C.; Schrock, R. R.; M€uller, P.; Hoveyda, A. H.
J. Am. Chem. Soc. 2009, 131, 10840.
(3) (a) Hock, A. S.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem.
Soc. 2006, 128, 16373. (b) Marinescu, S. C.; Singh, R.; Hock, A. S.;
Wampler, K. M.; Schrock, R. R.; M€uller, P. Organometallics 2008, 27, 6570.
(4) Wampler, K. M.; Hock, A. S.; Schrock, R. R. Organometallics
2007, 26, 6674.
ꢀ
(5) (a) Blanc, R.; Berthoud, R.; Salameh, A.; Basset, J.-M.; Coperet,
C.; Singh, R.; Schrock, R. R. J. Am. Chem. Soc. 2007, 129, 8434.
ꢀ
(b) Blanc, F.; Berthoud, R.; Coperet, C.; Lesage, A.; Emsley, L.; Singh, R.;
Kreickmann, T.; Schrock, R. R. Proc. Natl. Acad. Sci. 2008, 105, 12123.
r
2010 American Chemical Society
Published on Web 07/12/2010
pubs.acs.org/Organometallics

5242 Organometallics, Vol. 29, No. 21, 2010
Schrock et al.
of terminal olefins,2i and for efficient and clean ethenolysis
reactions.21 Some of the most interesting catalytic reactions
to date have been carried out with derivatives in which the
aryloxide is relatively large, e.g., the anion derived from (R)-
3,30-dibromo-20-(tert-butyldimethylsilyloxy)-5,50,6,60,7,70,8,80-
octahydro-1,10-binaphthyl-2-ol (HOBitet; see 1; M = Mo or
W), 2,3,5,6-tetraphenylphenol (HOTPP; see 2; M=Mo or
W), or 2,6-(2,4,6-(i-Pr)3C6H2)2C6H3 (hexaisopropylterphenol,
or HOHIPT; see 3; M = Mo or W). As a consequence of the
presence of a stereogenic metal center in MAP species, two
diastereomers are formed when an enantiomerically pure
aryloxide ligand is employed, as in 1. An important feature of
MAP complexes that contain a “large” aryloxide, at least in
terms of long-lived activity and low catalyst loadings, is that
methylidene species, e.g., 2 or 3, often can be observed in
solution and in some cases appear to be relatively stable. For
example, a solution of 2W (i.e., M = W) in toluene-d8 can be
heated to 80 °C and interconversion on the NMR time scale
of the inequivalent methylidene protons (k = 90 s-1 at 20 °C)
thereby observed.2g,6 The stability of these 14-electron MAP
methylidene species contrasts with the low stability of 14e
imido methylidene species of the bisalkoxide or biphenolate
type.1 Bisalkoxide imido methylidene species usually are
observed only when the total metal electron count is >14,
i.e., only when adducts are formed.7
complexes that contain a 2,6-dichlorophenylimido ligand.8
Another possibility is that the pyrrolide is bound in a
η5 manner and that a MAP methylidene complex therefore
is an 18-electron species, even though a pyrrolide is bound
in an η1 manner in MAP species that contain a neopenty-
lidene or neophylidene ligand.2a,c,h,4 Although solid sam-
ples of W(NAr)(CH2)(OBitet)(Me2Pyr) (Ar = 2,6-diiso-
propylphenyl, Me2Pyr = 2,5-dimethylpyrrolide) could be
obtained as a 3:1 mixture of diastereomers,2g no crystals of
either diastereomer could be obtained that were suitable
for an X-ray study.
Highly crystalline unsubstituted tungstacyclobutane com-
plexes can be prepared readily through treatment of a
tungsten MAP species with ethylene, as shown in eq 1.2g
Compound 4W readily exchanges with ethylene and has been
studied in detail through NMR methods.2g The NMR
studies are consistent with the loss of ethylene from 4W to
yield intermediate ethylene/methylidene complexes, (R)-
W(NAr)(Me2Pyr)(OBitet)(CH2)(C2H4) and (S)-W(NAr)-
(Me2Pyr)(OBitet)(CH2)(C2H4). X-ray studies show that 4W
and analogous metallacyclobutane species contain axial imido
and OBitet ligands, as shown in eq 1. All evidence suggests
that an olefin approaches the metal in a MAP species (such as
1W) trans to the pyrrolide and that the configuration at the
metal inverts with each forward metathesis step. The ana-
logous molybdacyclobutane (4Mo), the first example of a
crystallographically characterized TBP molybdacyclo-
butane complex, is structurally virtually identical to 4W.21
Therefore we became especially interested in comparing
and contrasting analogous Mo and W chemistry that con-
cerns methylidene and metallacyclobutane species and their
interconversion.
Results
X-ray Study of W(NAr)(CH2)(OTPP)(Me2Pyr). In a pre-
vious paper2g we showed that W(NAr)(CH2)(OTPP)-
(Me2Pyr) (2W) can be prepared in good yield as a yellow
powder in a reaction between one equivalent of 2,3,5,6-tetra-
phenylphenol (HOTPP) and W(NAr)(CH2)(Me2Pyr)2 in
benzene or by removing ethylene from W(NAr)(C3H6)-
(OTPP)(Me2Pyr) in vacuo. (W(NAr)(C3H6)(OTPP)(Me2Pyr)
can be prepared by treating W(NAr)(CHCMe2Ph)(OTPP)-
(Me2Pyr) with ethylene.2g) Complex 2W (in C6D6) contains a
methylidene Hsyn resonance at 10.24 ppm (JCH = 160 Hz) and
a methylidene Hanti resonance at 8.75 ppm (JCH = 130 Hz).
(The Hsyn proton points toward the imido group, while the
Hanti proton points away from the imido group.9) The JCH
value for Hanti (130 Hz) is typically found to be lower than that
for Hsyn, consistent with a weak CHanti agostic interaction with
The presence of 14-electron methylidene species that are
relatively stable toward bimolecular decomposition, yet
highly reactive toward olefins, compelled us to attempt to
isolate and crystallographically characterize some exam-
ples, since some feature of their structures might be a
reason for their increased stability toward bimolecular
decomposition. For example, the ground-state structure
of a MAP methylidene species could be a dimer that con-
tains two asymmetrically bridging methylidenes, as has
been found for some tungsten biphenolate methylidene
€
(6) The exchange of methylidene protons has been shown to be
independent of concentration, which rules out any intermolecular
scrambling process: King, A. J., unpublished results.
(7) Fox, H. H.; Lee, J.-K.; Park, L. Y.; Schrock, R. R. Organo-
metallics 1993, 12, 759.
(8) Arndt, S.; Schrock, R. R.; Muller, P. Organometallics 2007, 26,
1279.
(9) It should be noted that syn and anti nomenclature in an imido
MdCHR complex refers to the direction in which the R substituent
points, e.g., a syn MdCHR complex contains an Hanti a proton.

Article
Organometallics, Vol. 29, No. 21, 2010 5243
interaction between a syn substituent and the imido group.
Isolation of 2W confirms the proposal that it is relatively stable
toward bimolecular decomposition to give a metal-metal
dimer or a M(IV) ethylene complex.11 Addition of ethylene
to 2W immediately yields the known, crystallographically
characterized TBP tungstacyclobutane complex W(NAr)-
(C3H6)(OTPP)(Me2Pyr).2g
Synthesis of Mo(NAr)(C3H6)(OHIPT)(Pyr) and Mo-
(NAr)(CH2)(OHIPT)(Pyr). Exposure of a pentane solution
of Mo(NAr)(CHCMe2Ph)(OHIPT)(Pyr) to 1 atm of ethy-
lene led to formation of the molybdacyclobutane complex
Mo(NAr)(C3H6)(OHIPT)(Pyr) (5Mo) (OHIPT = hexaiso-
propylterphenoxide, Pyr = NC4H4) and its isolation as a
yellow powder in 51% yield. Compound 5Mo can be recrys-
tallized from a concentrated solution in a mixture of pentane
and ether as fine, feathery crystals. Removal of ethylene in
vacuo from a solution of 5Mo in toluene yielded Mo(NAr)-
(CH2)(OHIPT)(Pyr) (3Mo), which could be isolated from
1
Figure 1. Thermal ellipsoid drawing of W(NAr)(CH2)(OTPP)-
(Me2Pyr) (2W) (50% probability). Hydrogen atoms except for
those on the methylidene are removed for clarity. Selected bonds
pentane as yellow blocks in 88% yield. The H NMR spec-
trum of 3Mo in benzene-d6 showed two HR doublet resonan-
ces at δ 12.18 ppm (Hanti, JCH = 138 Hz, JHH = 5 Hz) and
12.06 ppm (Hsyn, JCH = 163 Hz, JHH = 5 Hz). Note that the
Hanti proton resonance is found downfield of the Hsyn reso-
nance, in contrast to what is found in 2W. A sample of 5Mo in
benzene-d6 was exposed to 13C-ethylene, and then vacuum was
applied to remove ethylene and re-form 3Mo in which the
methylidene carbon atom was partially 13C labeled (∼66%).
The JCH values were confirmed easily in the labeled compound.
The solid-state structure of 3Mo (Figure 2) is analogous to
˚
(A) and angles (deg): W-N1 = 1.758(4), W-N2 = 2.020(4),
W-O1 = 1.913(3), W-C1 = 1.908(4), W-N1-C11 = 176.8(3),
W-O1-C31=148.4(3), W-C1-H1a = 107(4), W-C1-H1b =
136(3), H1a-C1-H1b=117(5). The angle between planes through
H1a-C1-H1b and N1-W-C1 is 7.7°.
the metal.10 Compound 2W decomposes in toluene-d8 to a
significant degree at 60 °C over the course of several days, but
at room temperature decomposition is minimal over a period
of several days at a concentration of ∼30 mM. Both THF
and PMe3 adducts of 2W have been prepared and crystallo-
graphically characterized.2g Each adduct is best described as
a square pyramid with the methylidene in the apical position.
The THF is bound trans to the imido ligand, while PMe3 is
bound trans to the pyrrolide.
˚
that of 2W (Figure 1). The ModC distance (1.892(5) A) is the
˚
same as the WdC distance (1.908(4) A) in 2W. The Mo-
C1-H1a (Mo-C-Hanti) angle is 105(3)°, and the Mo-
C1-H1b (Mo-C-Hsyn) angle is 131(3)°, a difference of
26°, again consistent with a CHanti agostic interaction.10 As
found in 2W, the CH2 plane in 3Mo is turned from the N1-
Mo-C1 plane by about 8°. The structural study of 3Mo is the
first (to our knowledge) of a 14e molybdenum imido methy-
lidene species.
Crystals of 2W suitable for an X-ray study were obtained
from a concentrated toluene solution at -27 °C or a concen-
trated benzene solution at room temperature. As shown in
Figure 1, 2W is a 14e monomeric species that contains an η1-
Interestingly, 3Mo appears to be more stable in benzene
solution than the molybdacyclobutane (5Mo) at the same
concentration. For example, a 20 mM solution of 3Mo
showed little decomposition over a period of 3 days at
room temperature, although over the same time period a
20 mM solution of 5Mo decomposed to a significant degree.
Since only trace amounts of propene are observed in the 1H
NMR spectra, β-hydride rearrangement of the molybda-
cyclobutane complex cannot be the major mode of decom-
position. Therefore we propose that bimolecular decom-
position of 3Mo is the major decomposition pathway and
that bimolecular decomposition is accelerated by ethylene.
(See Discussion section.) Both 5Mo and 3Mo are stable for
weeks in the solid state at -27 °C.
Compound 5Mo appears to be much more stable toward
loss of ethylene than structurally characterized Mo(NAr)-
(C3H6)(Me2Pyr)(OBitet), which had to be isolated from the
reaction between Mo(NAr)(CHCMe3)(Me2Pyr)(OBitet)
and ethylene (1 atm) at -30 °C in a 1:1 mixture of pentane
and tetramethylsilane.2l When ethylene was removed from a
solution of Mo(NAr)(C3H6)(Me2Pyr)(OBitet), mixtures of
˚
dimethylpyrrolide. The WdC distance is 1.908(4) A, which is
similar to MdC bond lengths in other Mo and W neophy-
lidene and neopentylidene MAP species.2a,c,h The methyli-
dene protons in 2W (and other methylidene complexes
described later) were located in the Fourier synthesis and
subsequently refined semifreely (see Supporting Informa-
tion). The CH2 plane is turned approximately 8° out of the
N1-W-C1 plane. The W-C1-H1a angle (H1a = Hanti) is
107(4)°, and the W-C1-H1b angle (H1b=Hsyn) is 136(3)°.
The smaller W-C1-H1a angle (by 29°) is consistent with a
CHanti agostic interaction with the metal.10 In the structure of
the THF adduct (THF trans to the imido ligand), W-C-Hanti
is 118.1(16)° and W-C-Hsyn is 122.5(17)°, while in the
structure of the PMe3 adduct (PMe3 trans to the pyrrolide),
W-C-Hanti is 119.5(17)° and W-C-Hsyn is 133.5(16)°.2g To
our knowledge 2W is the first 14-electron tungsten imido
methylidene species to be crystallographically characterized.
These structural data provide the first direct evidence that in a
methylidene species the alkylidene is distorted from a symme-
trical structure for electronic reasons, not because of steric
(10) (a) Solans-Monfort, X.; Eisenstein, O. Polyhedron 2006, 25, 339.
(b) Poater, A.; Solans-Monfort, X.; Clot, E.; Coperet, C.; Eisenstein, O.
(11) (a) Lopez, L. P. H.; Schrock, R. R. J. Am. Chem. Soc. 2004, 126,
9526. (b) Lopez, L. P. H.; Schrock, R. R.; M€uller, P. Organometallics 2006,
25, 1978. (c) Lopez, L. P. H.; Schrock, R. R.; M€uller, P. Organometallics
2008, 27, 3857.
ꢀ
ꢀ
Dalton Trans. 2006, 3077. (c) Solans-Monfort, X.; Clot, E.; Coperet, C.;
Eisenstein, O. Organometallics 2005, 24, 1586.

5244 Organometallics, Vol. 29, No. 21, 2010
Schrock et al.
Figure 3. Thermal ellipsoid drawing of Mo(NAd)(CHCMe3)-
(OHIPT)(Pyr) (6) (50% probability). Hydrogen atoms (except
˚
for H1) are removed for clarity. Selected bonds (A) and angles
Figure 2. Thermal ellipsoid drawing of Mo(NAr)(CH2)-
(OHIPT)(Pyr) (3Mo) (50% probability). Hydrogen atoms except
for those on the methylidene are removed for clarity. Selected
(deg): Mo-N1 = 1.707(2), Mo-N2 = 2.040(2), Mo-O1 =
1.9124(15), Mo-C1 = 1.886(2), Mo-N1-C6 = 167.54(16),
Mo-O1-C115 = 167.55(15), Mo-C1-C2 = 143.25(18).
˚
bonds (A) and angles (deg): Mo-N1 = 1.737(4), Mo-N2 =
2.035(4), Mo-O1 = 1.911(3), Mo-C1 = 1.892(5), Mo-N1-
C11 = 171.1(3), Mo-O1-C31 = 160.3(3), Mo-C1-H1a =
105(3), Mo-C1-H1b = 131(3), H1a-C1-H1b = 124(5). The
angle between planes through H1a-C1-H1b and N1-Mo-C1
is 7.7°.
could a methylidene species be observed. Therefore the main
conclusion is that in the presence of ethylene Mo(NAd)(CH2)-
(OHIPT)(Pyr) decomposes to Mo(NAd)(C2H4)(OHIPT)-
(Pyr) readily. Two crystallographically characterized neutral
ethylene complexes of this general type are known, Mo(N-
2,6-Cl2C6H3)(CH2CH2)(Biphen)(Et2O)12 and W(N-2,6-Cl2-
C6H3)(CH2CH2)(Biphen)(THF),8 along with one cationic
species, {Mo(NAr)(CH2CH2)[OCMe(CF3)2](THF)3}þ.13
The synthesis of W(NArtBu)(CHCMe2Ph)(OTPP)(Me2Pyr)
(ArtBu = 2-t-BuC6H4) was carried out through addition of
2,3,5,6-tetraphenylphenol to W(NArtBu)(CHCMe2Ph)(Me2-
Pyr)2. Upon exposure of a solution of W(NArtBu)(CHCMe2Ph)-
(OTPP)(Me2Pyr) to ethylene, W(NArtBu)(C3H6)(OTPP)-
(Me2Pyr) (7) formed in high yield and could be isolated as
yellow needles from a mixture of diethyl ether and benzene.
Compound 7 is relatively stable in the solid state, but in ben-
zene it decomposes over the course of 4 h. Dissolution of 7 in
toluene or benzene and removal of the volatiles in vacuo
yielded W(NArtBu)(CH2)(OTPP)(Me2Pyr) (8) after six such
cycles. Compound 8 could not be isolated and analyzed be-
cause it is not stable in solution for more than an hour or two
at a concentration of ∼10 mM. NMR studies suggest that the
rate of rotation of the methylidene ligand in W(NArtBu)-
(CH2)(OTPP)(Me2Pyr) is 230 s-1 at 20 °C, which is ∼2.5
times faster than the rate found in the analogous 2,6-diiso-
propylphenyl imido complex (90 s-1 at 20 °C).2g
the two diastereomers of Mo(NAr)(CH2)(Me2Pyr)(OBitet)
(HR at 12.35 and 12.13 ppm for one diastereomer, 12.94 and
12.24 ppm for the other diastereomer) were observed. The
two diastereomers of Mo(NAr)(CH2)(Me2Pyr)(OBitet) de-
compose to a significant degree in C6D6 at 22 °C in the abse-
nce of ethylene over a period of 1-2 days.2e (NMR studies
involving Mo(NAr)(C3H6)(Me2Pyr)(OBitet) and Mo(NAr)-
(CH2)(Me2Pyr)(OBitet) are described later in this paper.)
Complexes That Contain a N(o-t-BuC6H4) or NAdamantyl
Ligand. We also became interested in whether metallacyclo-
butane and methylidene species could be isolated in which
the imido ligand is smaller than 2,6-diisopropylphenylimido.
Adamantylimido is a good choice since it is an alkylimido
derivative, adamantylimido species have revealed special
reactivities in certain circumstances, and an adamantylimido
ligand is small relative to a 2,6-diisopropylphenylimido lig-
and.2j Mo(NAd)(CHCMe3)(OHIPT)(Pyr) (6), prepared as
described in the literature,2j was shown in an X-ray study
to be a 14e pseudotetrahedral species that contains an η1-
pyrrolide and a syn alkylidene9 (Figure 3). Bond lengths and
angles are similar to other MAP species of this general type.
The Mo-C1-H1 angle (101.6(17)°) should be compared with
the M-C-Hanti angles in 2W (107(4)°) and 3Mo (105(3)°) des-
cribed above (Figures 1 and 2).
Addition of ethylene (1 atm) to 6 afforded initially what we
propose to be one or more metallacyclobutane or metalla-
cyclopentane species (∼80%) and starting material (∼20%).
Multiplet resonances attributed to the metallacycle(s) are
found at 3.81 (JCH = 139 Hz), 2.36 (JCH = 145 Hz), 2.24
(JCH = 146 Hz), and 0.71 (JCH = 154) ppm in a 1:1:1:1 ratio.
The remaining 6 was consumed over a period of 24 h, and an
ethylene complex was formed in a yield of >95%. The four
proton resonances attributed to the ethylene in Mo(NAd)-
(C2H4)(OHIPT)(Pyr) are found at 2.73, 2.32, 2.01, and
0.35 ppm, and the carbon resonances appear at 55.23 ppm
Attempts to prepare W(NArtBu)(CHCMe2Ph)(Me2Pyr)-
(OBitet) by treating W(NArtBu)(CHCMe2Ph)(Me2Pyr)2 with
one equivalent of HOBitet on a ∼0.5 g scale led only to an
oily residue, the 1H NMR spectrum of which suggested the
presence of two syn diastereomers with alkylidene HR reson-
ances at 9.90 ppm (JCH = 116Hz, JWH =15Hz) and9.45ppm
(JCH = 113 Hz, JWH = 15 Hz) in a 2:1 ratio. Addition
of ethylene (1 atm) to crude W(NArtBu)(CHCMe2Ph)-
(Me2Pyr)(OBitet) yielded only an intractable material.
NMR Studies of the M(NAr)(C3H6)(OHIPT)(Pyr) Sys-
1
tems. 2D H-1H NOESY/EXSY experiments were perfor-
med on the Mo and W versions of the M(NAr)(C3H6)-
(OHIPT)(Pyr) complexes. These experiments were analogous
(JCC = 38 Hz, JCH = 154 and 155 Hz) and 46.41 ppm (JCC
=
(12) Tsang, W. C. P.; Jamieson, J. Y.; Aeilts, S. A.; Hultzsch, K. C.;
Schrock, R. R.; Hoveyda, A. H. Organometallics 2004, 23, 1997.
38 Hz, JCH = 158 and 158 Hz). So far we have not been able
to isolate Mo(NAd)(C2H4)(OHIPT)(Pyr) as a crystalline so-
lid. At no time during the formation of the ethylene complex
€
(13) Jiang, A. J.; Schrock, R. R.; Muller, P. Organometallics 2008, 27,
4428.

Article
Organometallics, Vol. 29, No. 21, 2010 5245
to those reported for W(NAr)(C3H6)(Me2Pyr)(OBitet) in the
first paper in this series.2g Unfortunately, it was not possible
to study the Mo(OHIPT) and W(OHIPT) complexes in as
much detail as the W(OBitet) species. (A full discussion and
details can be found in the Supporting Information.)
For W(NAr)(C3H6)(Me2Pyr)(OBitet) the kinetic data could
be fit only when W(NAr)(CH2)(C2H4)(Me2Pyr)(OBitet) was
proposed as an intermediate.2g The same was found to be the
case in the OHIPT systems examined here. The precise struc-
ture of the M(CH2)(C2H4) species is not known. (See Dis-
cussion section.) In the analysis that follows it is arbitrarily
drawn as a TBP species with the ethylene and methylidene
ligands in equatorial positions (eq 2).
molybdacyclobutane (at 20 °C) breaks up 180 times faster than
the tungstacycle (at 40 °C). It was impractical to measure the
rate of methylidene rotation in W(NAr)(CH2)(OHIPT)(Pyr)
because the concentration of W(NAr)(CH2)(OHIPT)(Pyr) was
too low. To our knowledge these are the first data that quan-
titate the “greater stability” of tungstacyclobutane complexes
versus molybdacyclobutane complexes, in terms of both the rate
of loss of ethylene and the position of the equilibrium between
the metallacycle and methylidene species.
In a 20 mM solution of Mo(NAr)(C3H6)(OHIPT)(Pyr)
(5Mo) in benzene-d6 at 20 °C ∼14% of Mo(NAr)(CH2)-
(OHIPT)(Pyr) (3Mo) and 86% of 5Mo are observed (Figure S3
in the Supporting Information). Proton resonances in each
species were assigned on the basis of NOE studies (Figure S4).
In the interconversion of Mo(NAr)(C3H6)(OHIPT)(Pyr) (5Mo
)
and Mo(NAr)(CH2)(C2H4)(OHIPT)(Pyr) (5Mo0) protons d ex-
change exclusively with a, andcmainly with b(eq2). Theassign-
ments of a and b are consistent with the JCH values determined
through partial 13C labeling of 5Mo (JCH = 138 Hz for Ha =
In the W(NArtBu)(C3H6)(OTPP)(Me2Pyr) system the pro-
tons of the metallacyclobutane (four on the CR carbons
and two on the Cβ carbon) were assigned using 2D 1H-1H
NOESY/EXSY NMR spectroscopy. The pattern of beha-
viors and rates of exchange is similar to the 2,6-diisopropyl-
phenyl imido system. (See Supporting Information.)
Hanti and JCH = 163 Hz for Hb = Hsyn
,
When vacuum was applied to a solution of W(NArtBu)-
(C3H6)(OTPP)(Me2Pyr), the methylidene species that was
generated exhibited broad methylidene proton resonances at
10.09 and 9.09 ppm in the proton NMR spectrum in C6D6 at
22 °C. Proton a (δ 10.09 ppm, JCH = 156 Hz, which was
obtained by labeling the CR with 13C) is assigned the methy-
lidene proton syn to the imido ligand and b (δ 9.09 ppm,
J
CH = 128 Hz) to the methylidene proton anti to the imido
ligand, which confirmed the assignments made through 2D
NMR studies. 2D 1H-1H NOESY/EXSY spectra of a 44 mM
sample with 2 ms mixing time at 20 °C (Figures S8 and S10)
illustrated that the syn and anti protons exchange at a rate of
230 s-1, which is about 2 times faster than that in W(NAr)-
(CH2)(OTPP)(Me2Pyr) (90 s-1) and approximately 100 times
faster than that in W(NAr)(CH2)(OBitet)(Me2Pyr) (4-5 s-1).
The rapid rate of exchange of methylidene protons in W-
(NArtBu)(C3H6)(OTPP)(Me2Pyr) accounts for the breadth of
the methylidene proton resonance in room-temperature spectra.
NMR Studies of Mo(NAr)(C3H6)(Me2Pyr)(OBitet). In a
previous paper we reported that Mo(NAr)(CHCMe2Ph)-
(Me2Pyr)2 reacts cleanly with 1 equiv of enantiomerically
pure HOBitet to generate 1Mo as a mixture of two diastereo-
mers.2c,h When ethylene (1 atm) is added to a solution of 1Mo
in toluene-d8 (30 mM), very broad ethylene (δ 5.25 ppm) and
methylidene proton resonances are observed at 20 °C. When
the reaction is cooled under an ethylene atmosphere, Mo-
(NAr)(C3H6)(Me2pyr)(OBitet) (4Mo) was isolated and recry-
stallized from a 1:1 mixture of pentane and tetramethylsilane
at -30 °C in the presence of ethylene (1 atm). An X-ray struc-
tural study of 4Mo revealed it to have a TBP structure in
which the imido and the aryloxide ligands are in the axial
positions.2l The proton NMR spectrum of 4Mo is typical for a
TBP metallacycle with HR resonances at 6.16, 5.69, 5.24, and
5.03 ppm and Hβ resonances at 0.74 and -0.16 ppm at -70 °C.
The isolation of 4Mo allowed us to carry out NMR studies
analogous to those on the tungsten analogue and to compare
tungsten and molybdenum data.
0
vide supra). Pyrrolide resonances in 5Mo and 5Mo were em-
ployed in order to determine the rate of interconversion of
5
0
Mo and 5Mo (Figure S5). An examination of the system at
mixing times of 60, 100, 130, 150, 200, 250, and 300 ms al-
lowed the rate constants for the forward process (1.8 ( 0.3
s-1) and the reverse (9.0 ( 1.5 s-1) at 20 °C to be determined.
The ratio of krMo to kfMo (5.0) is consistent with the ratio of
5
Mo to 3Mo found in the 1D NMR experiment above. Protons
a and b in a sample of Mo(NAr)(CH2)(OHIPT)(Pyr) (free of
ethylene) do not exchange with a mixing time of 200 ms at
20 °C; therefore any rotation of the methylidene ligand about
the Mo-C axis would have to take place at a rate of <0.2
s-1. A rate of <0.2 s-1 contrasts dramatically with a rate of
rotation of the methylidene in W(NArtBu)(CH2)(OTPP)-
(Me2Pyr) (230 s-1 at 20 °C; vide supra) or W(NAr)(CH2)-
(OTPP)(Me2Pyr) (90 s-1 at 20 °C).2g
In a 20 mM solution of W(NAr)(C3H6)(OHIPT)(Pyr)
(C3H6 resonances at δ 4.24, 3.52, -0.78, and -1.11 ppm) in
benzene-d6 at 20 °C the metallacycle protons did not exchange
with each other at any significant rate. At 40 °C approximately
2% W(NAr)(CH2)(OHIPT)(Pyr) (δ 10.30 and 9.38 ppm) could
1
be observed in the H NMR spectrum. The model that was
employed for the Mo system was employed for the W system, as
described in the Supporting Information (see Figures S6 and
S7). The forward rate constant at 40 °C (kfW) was found to be
0.010 ( 0.002 s-1 and the reverse (krW) was found to be 0.50 (
0.05 s-1, for a krW/kfW = 50 (eq 3). The ratio of krW to kfW is
larger by a factor of 10 in the W system (at 40 °C), and the

5246 Organometallics, Vol. 29, No. 21, 2010
Schrock et al.
Scheme 1. Details of Processes Involving Loss of Ethylene from Mo(NAr)(C3H6)(OBitet)(Me2Pyr) in Solution at -20 °C
When 4Mo is dissolved in toluene-d8, an equilibrium is
established between 4Mo, methylidene diastereomers (S)-9
and (R)-9, and ethylene. At -20 °C and 30 mM initial con-
centration of 4Mo, ∼60% 4Mo, ∼40% (S)-9 and (R)-9, and
ethylene (0.6% in solution relative to the total concentration
of metal complexes) are observed. (See Scheme 1 and Sup-
porting Information.) Four methylidene proton resonances
are observed for the diastereomers of 9 (Figure 4a), six metall-
acyclobutane resonances are observed for metallacycle 4Mo
(Figure 4b and c), and a broad resonance is observed for
ethylene (Figure 4b). The ethylene resonance is found at
approximately the chemical shift where it would be expected
in the free form. The ratio of the two diastereomers at-20 °C is
(S)-9:(R)-9 = 1:4. Metallacyclobutane and methylidene reso-
nances have been identified by 2D HSQC and NOESY/EXSY
NMR methods described in the Supporting Information.
A reaction between 4Mo and ∼1 atm of 13C-labeled ethy-
lene showed that 13C is incorporated into the MoC3 ring in
less than 1 min at 20 °C. The CR resonances in 4Mo are found
at 102.2 and 101.2 ppm, while the Cβ resonance is located at
-1.1 ppm, all consistent with the TBP geometry observed in
the solid state. The reaction mixture was placed under vac-
uum to remove the excess ethylene, and (S)-9 and (R)-9 were
observed. The JCH values were established for anti protons a
and c (JCH = 140 and 138 Hz) and for syn protons b and d
(JCH = 163 and 165 Hz; see Supporting Information).
An EXSY experiment of the mixture of (R)-9 and (S)-9 at
20 °C with no ethylene present revealed that a and b, and c
and d, respectively, do not exchange even at 200 ms EXSY
mixing time (see Figure S27). The fact that the methylidene
protons do not exchange suggests that rotation about the
MdC bond must take place at a rate of <0.2 s-1, as was also
found in Mo(NAr)(CH2)(OHIPT)(Pyr) (vide supra). This
result contrasts with several examples of faster rotation
about the WdC bond noted earlier.
Details concerning the breakup and re-formation of 4Mo at
-20 °C (Scheme 1) can be obtained employing the same
methods as those employed in the analogous tungsten system.
(These methods are described in detail in the Supporting
Information.) As in the tungsten system, formation of an
intermediate ethylene/methylidene complex is most consistent
with the data. The ethylene concentration is higher than what
would be expected on the basis of published solubility data in
benzene,14 although different conditions (solvent, etc.) in this
experiment probably limit the reliability of the solubility data.
Data shown in Scheme 1 for S forms are much less accurate
than data for R forms as a consequence of the S forms being
present in much lower concentrations (∼10% of R forms).
Since we do not accurately know the amount of free ethylene in
solution, we do not know accurately the second-order rate for
re-forming the ethylene/methylidene intermediate and the
position of the equilibria between the ethylene/methylidene
intermediates and the methylidene species and free ethylene.
It is not possible to obtain data for 4Mo above -20 °C as a
consequence of rapid exchange processes, but data could be
obtained at -30, -40, and -50 °C (Table S2; see Supporting
Information). Since data for the R forms are the most
accurate, we compare values only for kRfwd and kRrev at dif-
ferent temperatures here. The values of kRfwd (in s-1; see
Table S2) at four temperatures were found to be 34.0 (-20 °C),
5.88 (-30 °C), 0.850 (-40 °C), and 0.0913 (-50 °C). From an
Eyring plot (Figure 5) it was found that ΔH‡=21.6( 0.3 kcal
mol-1 and ΔS‡ = 34.4 ( 1.6 eu. A value for kRfwd for 4Mo at
20 °C was calculated employing these parameters; kRfwd
-
(Mo)20° was found to be 1.45ꢀ104 s-1 (Scheme 2). Since the
value for kRfwd(W)20° = 3.2 ( 0.1 s-1 2g
the ratio of kRfwd-
,
(Mo)20° to kRfwd(W)20° is ∼4500. In a similar manner the
values for ΔH‡ =13.2 (0.3 kcal mol-1 and ΔS‡ =3.4(1.4 eu
(14) Jolley, J. E.; Hildebrand, J. H. J. Am. Chem. Soc. 1958, 80, 1050.

Article
Organometallics, Vol. 29, No. 21, 2010 5247
Figure 4. Proton NMR spectra of Mo(NAr)(C3H6)(OBitet)(Me2Pyr) (4Mo) in toluene-d8 at -20, -30, -40, and -50 °C. (a) Expansion
of the 1H NMR spectrum in the alkylidene region for (S)- and (R)-Mo(NAr)(CH2)(Me2Pyr)(OBitet) (1:4 ratio of (S)-9 and (R)-9)).
(b) Expansion of the 1H NMR spectrum in the HR region for Mo(NAr)(C3H6)(OBitet)(Me2Pyr) (4Mo). (c) Expansion of the 1H NMR
spectrum in the Hβ region for 4Mo. (See Scheme 1 for labels.)
for kRrev(Mo)20° were determined; kRrev(Mo)20° was found
to be 4900 s-1, which is a factor of 71 larger than kRrev(W)20°
relatively stable toward bimolecular decomposition. We
have proposed that bimolecular decomposition involves
(first) formation of an unsymmetrically bridging bis-μ-
methylidene species; one example, heterochiral [W(NArCl)-
(Biphen)(μ-CH2)]2 (where ArCl is 2,6-dichlorophenylimido),
has been crystallographically characterized.8 Homochiral
[W(μ-NArCl)(Biphen)]2(μ-CH2CH2) has also been crystallo-
graphically characterized.8 Both are logical intermediates
in decomposition of a methylidene species to an olefin-free
WdW species11 or to a monomeric ethylene complex.12 One
potentially important unknown detail (since another equiva-
lent of ethylene is required) is the role of ethylene in the con-
version of an intermediate bimetallic species into monomeric
ethylene complexes. There is evidence in the literature that
ethylene accelerates decompositions in W imido alkylidene15
(69 s-1). Therefore at 20 °C the value of kRrev(Mo)20°/kRfwd
-
(Mo)20° =0.34, which should be compared with a value of
kRrev(W)20°/kRfwd(W)20° = 21.56.2g The values for kRfreefwd as a
function of temperature are not well behaved, which suggests
that the rate constants for kfreefwd and kfreerev for both the R and
S forms are not reliable. To our knowledge, these are the first
quantitative data that demonstrate the relative stabilities of a
high oxidation state molybdacyclobutane complex versus the
analogous tungstacyclobutane complex.
Discussion
The work reported here confirms that reactive 14-electron
methylidene species under the right circumstances can be

5248 Organometallics, Vol. 29, No. 21, 2010
Schrock et al.
compared directly with rates of interconversion of syn and
anti isomers. The reason is that if the MdCHanti agostic
interaction is an important stabilizing feature that allows syn
and anti isomers of substituted alkylidenes to be observed,
one could argue that the agostic interaction never fully dis-
appears in the process of rotating the methylidene ligand and
could even act as a means of lowering the energy of the
transition state in which the methylidene has rotated by 90°
and the MdC bond has been formally broken.
A relatively slow rotation of methylidene ligands in moly-
bdenum species was unexpected. So far we can only say that
rotation rates are <0.2 s-1 for the two Mo species that we
have explored. Rotation of tungsten methylidenes may be
significantly faster than molybdenum methylidenes if the
agostic interaction is stronger for tungsten than for moly-
bdenum and the transition state in which the methylidene has
rotated 90° therefore is stabilized to a greater degree than for
molybdenum.
The presence in the kinetic scheme of an intermediate
ethylene/methylidene complex has led to well-behaved tem-
perature dependencies for the rate constants for its format-
ion and for re-formation of the metallacycle in the Mo(NAr)-
(C3H6)(OBitet)(Me2Pyr) system. These data, in combination
with those generated in the W(NAr)(C3H6)(OBitet)(Me2Pyr)
investigation,2g give us confidence that the ethylene/methyl-
idene intermediate exists. The values for ΔS‡ for kRfwd (34.4(
1.6 eu) and for ΔS‡ for kRrev(Mo) (3.4 ( 1.4 eu) are consis-
tent with significant disorder in the transition state lead-
ing to the ethylene/methylidene intermediate and with
more order in the transition state leading back to the metal-
Figure 5. Eyring plots for kRfwd and kRrev at -20, -30, -40,
and -50 °C for cleavage and re-formation of Mo(NAr)(C3H6)-
(OBitet)(Me2Pyr) (see Scheme 1).
and Re alkylidyne alkylidene16 systems. Therefore, the right
combination of sterically bulky ligands can ensure methyl-
idene stability, through either slowing formation of bis-μ-
CH2 species or slowing the reaction of bis-μ-CH2 intermedi-
ates with ethylene to give two ethylene complexes. The stabi-
lity of methylidene species combined with high reactivity are
bound to lead to relatively efficient and long-lived metathesis
catalysts.
It has long been observed qualitatively that TBP tungsta-
cyclobutane complexes are more stable toward loss of olefin
than molybdacyclobutane complexes. We now have the first
direct comparison of the rate of cleavage of an unsubstituted
molybdacycle with the rate of cleavage of an unsubstituted
tungstacycle to an ethylene/methylidene intermediate and
comparison of the subsequent rates of loss of ethylene from the
Mo and W ethylene/methylidene intermediates. The stability of
especially an unsubstituted tungstacyclobutane toward loss of
ethylene early in the development of imido alkylidene catalysts
was one of the reasons why molybdenum catalysts were sought;
that is, ethylene formed in a metathesis process could sequester
tungsten in the form of an unsubstituted metallacyclobutane.17
Relatively more facile loss of an olefin from a metallacyclobu-
tane also should apply to substituted metallacycles. The more
rapid loss of olefin from molybdacyclobutanes could contri-
bute to a relatively high metathesis activity for molybdenum
complexes in a variety of circumstances compared to tung-
sten analogues, as in the recently reported Z-selective coup-
ling of terminal olefins, which are more successful employing
the “less reactive” tungsten catalysts for the substrates exam-
ined so far.2i
We are surprised not only by the stability of certain
methylidene complexes but by the relatively rapid rate of rota-
tion of WdCH2 species about the WdC bond, i.e., inter-
conversion of syn and anti methylidene protons. Confirmed
rates at 20 °C in complexes discussed here vary from 4 to 5 s-1
up to 230 s-1. These rates are summarized in Table 1. Since
we so far have not observed anti isomers of 14-electron sub-
stituted alkylidenes in MAP species, we do not know whether
syn and anti monosubstituted alkylidenes also interconvert
readily. We suspect that methylidene rotation cannot be
lacyclobutane complex, respectively. The value of kRrev
-
(Mo)20°/kRfwd(Mo)20° = 0.34 at 20 °C suggests that more
ethylene/methylidene complex is present at 20 °C than metall-
acyclobutane. It is important to note that the structure of the
ethylene/methylidene intermediate is not known at this
stage, so interconversion of the metallacyclobutane complex
and the ethylene/methylidene complex may include a com-
ponent that consists of a change in geometry at the metal
center and/or even a high degree of fluxionality for the five-
coordinate ethylene/methylidene species. The apparent rela-
tively high stability of metallacyclobutane species toward
loss of ethylene when the aryloxide is OHIPT suggests that
details concerning exactly how the olefin leaves the metalla-
cycle (twisting, sliding, etc.) are likely to be important. To
our knowledge, the only evidence (prior to that reported here
and in a previous paper2g) for an alkylidene/olefin complex
in high oxidation state species is formation of a cycloheptene/
cyclopentylidene species at low temperatures upon addition
of cycloheptene to [W(C5H8)(OCH2CMe3)2Br2]GaBr3.18
Conclusions
Reactive methylidene MAP species under the right cir-
cumstances are relatively stable toward bimolecular decom-
position or as yet poorly defined ethylene-catalyzed
decomposition processes. In the solid state the methylidene
complex is a monomer containing an η1-pyrrolide ligand and
the methylidene ligand is distorted in a manner consistent
with an agostic CHanti interaction. Methylidenes rotate about
(15) Tsang, W. C. P.; Hultzsch, K. C.; Alexander, J. B.; Bonitatebus,
P. J., Jr.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2003, 125,
2652.
the WdC bond at rates that vary from 3.6 s-1 to 230 s-1
,
but rotation about ModC bonds is comparatively slow
(16) Leduc, A.-M.; Salameh, A.; Soulivong, D.; Chabanas, M.;
ꢀ
Basset, J.-M.; Coperet, C.; Solans-Monfort, X.; Clot, E.; Eisenstein,
O.; Boehm, V. P. W.; Roeper, M. J. Am. Chem. Soc. 2008, 130, 6288.
(17) Feldman, J.; Schrock, R. R. Prog. Inorg. Chem. 1991, 39, 1.
(18) Kress, J.; Osborn, J. A. Angew. Chem., Int. Ed. Engl. 1992, 31,
1585.

Article
Organometallics, Vol. 29, No. 21, 2010 5249
Scheme 2. Comparison of the Rate Constants for Interconversion of Mo(NAr)(C3H6)(OBitet)(Me2Pyr) and (R)-Mo(NAr)(CH2)-
(C2H4)(OBitet)(Me2Pyr) with Interconversion of W(NAr)(C3H6)(OBitet)(Me2Pyr) and (R)-W(NAr)(CH2)(C2H4)(OBitet)(Me2Pyr)
at 20 °C
160.32, 151.22, 158.57, 156.07, 148.32, 147.24, 136.58, 132.30,
131.87, 130.76, 124.24, 123.82, 122.33, 119.38, 114.87, 109.26,
100.19 (WCR), 35.15, 31.52 (br s), 29.00 (br s), 26.86, 24.65,
23.43, -0.80 (WCβ). Anal. Calcd for C55H76MoN2O: C, 75.31;
H, 8.73; N, 3.19. Found: C, 75.21; H, 8.76; N, 3.21.
Table 1. Rates of Exchange of Methylidene Protons at 20 °C
methylidene species
rate (s-1
)
Mo(NAr)(CH2)(OHIPT)(Pyr)
<0.2
<0.2
230
90
3.6
5.1
Mo(NAr)(CH2)(OBitet)(Me2Pyr)
W(NArt-Bu)(CH2)(OTPP)(Me2Pyr)
W(NAr)(CH2)(OTPP)(Me2Pyr)2g
(R)-W(NAr)(CH2)(OBitet)(Me2Pyr)2g
(S)-W(NAr)(CH2)(OBitet)(Me2Pyr)2g
Mo(NAr)(CH2)(OHIPT)(Pyr). A 20 mL vial was charged
with a stir bar and a solution of Mo(NAr)(C3H6)(Pyr)(OHIPT)
(0.443 g, 0.505 mmol) in diethyl ether (2 mL) and toluene (2 mL).
The volatiles were removed in vacuo. Toluene was added to the
residue, and vacuum was applied to the dark yellow solution.
This process was repeated once more. Pentane was added to the
residue, and the volatiles were removed in vacuo; this process
was repeated until the sample became a yellow foam. A small
amount of pentane was added, and the solution was allowed to
sit at -27 °C. After 24 h, the mother liquor was decanted, and
the crystals were washed with cold pentane and dried in vacuo
for 2 h; yield 0.375 g (87%): 1H NMR (500 MHz, C6D6) δ 12.21
(d, 1, anti-MoCHR, JHH = 5 Hz), 12.10 (d, 1, syn-MoCHR,
(<0.2 s-1). MAP metallacyclobutane species break up at
widely differing rates to give intermediate ethylene/methy-
lidene intermediates. In M(NAr)(C3H6)(OBitet)(Me2Pyr)
systems, the rate of metallacycle breakup has been found
to be 4500 times faster when M = Mo than when M = W at
20 °C.
Experimental Section
J
J
HH = 5 Hz), 7.35 (s, 2, Ar-H), 7.21 (s, 2, Ar-H), 7.12 (d, 2, Ar-H,
HH = 7 Hz), 6.97 (m, 3, Ar-H), 6.91 (t, 1, Ar-H, JHH = 7 Hz),
General Procedures. General procedures can be found in a
previous paper.2g 2,3,5,6-Tetraphenylphenol (HOTPP),19 3,30-
dibromo-20-(tert-butyldimethylsilyloxy)-5,50,6,60,7,70,8,80-octa-
hydro-1,10-binaphthyl-2-ol (HOBitet),2c,d hexaisopropylterphenol
(HIPTOH),20 W(NAr)(CHMe2Ph)(Me2Pyr)2,21 W(NAr)(CH-
CMe2Ph)(Pyr)2(DME),21 W(NArCl)(CHCMe3)(Pyr)2(DME),21
6.37 (m, 4, Pyr-H), 3.50 (sept, 2, CHMe2), 2.98 (sept, 2 CHMe2),
2.90 (m, 4, CHMe2), 1.31 (d, 6, CHMe2, JHH = 7 Hz), 1.28 (d, 6,
CHMe2, JHH = 7 Hz), 1.22 (d, 6, CHMe2, JHH = 7 Hz), 1.19 (d,
6, CHMe2, JHH = 7 Hz), 1.15 (d, 6, CHMe2, JHH = 7 Hz), 1.14
(d, 6, CHMe2, JHH = 7 Hz), 1.10 (d, 6, CHMe2, JHH = 7 Hz),
1.05 (d, 6, CHMe2, JHH = 7 Hz); 13C NMR (125 MHz, C6D6) δ
278.15 (MoCR), 159.44, 154.83, 149.24, 147.80, 147.70, 145.16,
134.49, 133.42, 132.22, 131.37, 127.52, 123.09, 122.61, 121.80,
121.59, 111.08, 35.24, 31.77, 31.55, 29.35, 26.13, 25.70, 25.07,
25.05, 24.60, 24.55, 23.56, 22.94. Anal. Calcd for C53H72Mo-
N2O: C, 74.97; H, 8.55; N, 3.30. Found: C, 74.65; H, 8.47; N,
3.33.
A sample was degassed via three freeze-pump-thaw cycles
and exposed to 13C-ethylene. A vacuum was applied to remove
ethylene and regenerate the methylidene complex as a mixture of
13C-labeled (∼60%) and unlabeled species: 1H NMR (500 MHz,
C6D6) δ12.18 (JCH = 138 Hz, anti-MoCHR), 12.06 (JCH = 163 Hz,
syn-MoCHR).
Mo(NAr)(CHCMe2Ph)(Me2Pyr)2,2a Mo(NAr)(CHCMe2Ph)-
2c,h
(Pyr)2,3a 1Mo
literature procedures.
,
4
,
21 and 921 were all prepared according to
Mo
Mo(NAr)(C3H6)(OHIPT)(Pyr). A 50 mL Schlenk flask was
charged with a stir bar and a solution of Mo(NAr)(CHCMe2Ph)-
(Pyr)2 (0.580 g, 1.083 mmol) and HOHIPT (0.540 g, 1.083 mmol)
in diethyl ether (2 mL) and pentane (4 mL). The yellow solution
was stirred and heated at 60 °C in a closed system for 24 h. The
mixture was cooled to room temperature and filtered through
glass wool. The filtrate was degassed via three freeze-pump-
thaw cycles and exposed to 1 atm of ethylene. A yellow, fluffy
solid precipitated immediately and was filtered off and dried in
vacuo for 1 h; yield 0.480 g (51%). The product can be recrys-
tallized from a concentrated solution of a 1:1 mixture of pentane
and diethyl ether at -27 °C as a fine, feathery solid. 1H NMR
shows that in solution ∼15% of the sample is the methylidene
species and ∼8% of the expected ethylene is observed: 1H NMR
In Situ Synthesis of Mo(NAd)(C2H4)(OHIPT)(Pyr). Mo-
(NAd)(CHCMe3)(OHIPT)(Pyr) (0.015 g, 0.017 mmol) and
0.6 mL of benzene-d6 was added to a Teflon-seal J-Young tube.
The yellow solution was degassed by three freeze-pump-thaw
cycles. The solution was exposed to 1 atm of ethylene or 13C-
ethylene: 1H NMR (500 MHz, C6D6) δ 7.28 (s, 2, Ar-H), 7.25 (s,
(500 MHz, C6D6) δ 7.32 (m, 2, Pyr-H), 7.26 (d, 2, Ar-H, JHH
=
8 Hz), 7.21 (s, 4, Ar-H), 6.89 (t, 1, Ar-H, JHH = 8 Hz), 6.80 (m, 3,
Ar-H), 6.21 (m, 2, Pyr-H), 4.30 (m, 2, CHR), 3.94 (m, 2, CHR),
3.65 (sept, 2, CHMe2), 3.59-2.24 (br, 4, CHMe2), 2.89 (sept, 2,
CHMe2), 1.32 (d, 12, CHMe2, JHH = 7 Hz), 1.30-1.02 (m, 24,
CHMe2), 1.00 (d, 12, CHMe2, JHH = 7 Hz), -0.15 (br m, 1,
CHβ), -0.65 (br m, 1, CHβ); 13C NMR (125 MHz, C6D6) δ
2, Ar-H), 7.08 (d, 2, Ar-H, JHH = 8 Hz), 6.89 (t, 1, Ar-H, JHH
=
8 Hz), 6.67 (m, 2, Pyr-H), 6.53 (m, 2, Pyr-H), 2.95 (m, 6,
CHMe2), 2.73 (m, 1, MoC2H4), 2.32 (m, 1, MoC2H4), 2.01 (m,
1, MoC2H4), 1.63 (s, 3, Ad-H), 1.47 (s, 6, Ad-H), 1.41 (d, 12,
CHMe2, JHH = 7 Hz), 1.28 (d, 6, CHMe2, JHH = 7 Hz), 1.20 (s,
6, Ad-H), 1.19 (d, 12, CHMe2, JHH = 7 Hz), 1.17 (d, 6, CHMe2,
J
HH = 7 Hz), 0.35 (m, 1, MoC2H4); 13C NMR (125 MHz, C6D6)
(19) Yates, P.; Hyre, J. E. J. Org. Chem. 1962, 27, 4101.
(20) Stanciu, C; Olmstead, M. M.; Phillips, A. D.; Stender, M.;
Power, P. P. Eur. J. Inorg. Chem. 2003, 3495.
δ 55.23 (MoC2H4, JCC = 38 Hz, JCH = 154 and 155 Hz), 46.41
(MoC2H4, JCC = 38 Hz, JCH = 158 and 158 Hz).
W(NArtBu)2(NC5H5)2Cl2. In the glovebox, a 500 mL Schlenk
flask was charged with a stir bar and WO2Cl2 (22.18 g, 77.35 mmol)
€
(21) Kreickmann, T.; Arndt, S.; Schrock, R. R.; Muller, P. Organo-
metallics 2007, 26, 5702.

5250 Organometallics, Vol. 29, No. 21, 2010
Schrock et al.
in 50 mL of dimethoxyethane. 2-tert-Butylaniline (24.31 g,
162.92 mmol) was added. The flask was taken out of the box
and purged with N2 on the Schlenk line. Under N2, 2,6-lutidine
(37 mL, 318 mmol) and trimethylchlorosilane (98 mL, 776
mmol) were added to the solution with vigorous stirring. The
mixture was stirred at 65 °C for 2 days and then cooled to room
temperature. In the glovebox the mixture was filtered through
a frit and the orange salt was extracted with a pyridine (50 mL)
and benzene (300 mL) mixture. The two portions of the filtrates
were combined, and solvents were removed in vacuo. Pentane
was added to the orange-red residue. The precipitated red pow-
der was filtered off and rinsed with pentane; yield 47 g (86%): 1H
NMR (500 MHz, C6D6) δ8.92 (d, 4, Ar-H, JHH = 5 Hz), 7.74 (dd,
2, Ar-H, JHH =8and1Hz), 7.30(dd, 2, Ar-H, JHH = 8and 1Hz),
7.06, (t, 2, Ar-H, JHH = 7 Hz), 6.71 (t, 2, Ar-H, JHH = 7 Hz), 6.66
(t, 2, Ar-H, JHH = 7Hz), 6.27(t, 4, Ar-H, JHH = 7Hz), 1.52(s, 18,
tert-butyl); 13C NMR (125 MHz, C6D6) δ 152.52, 138.15, 127.61,
127.12, 127.03, 125.76, 124.33, 119.16, 118.41, 31.26, 29.97. Anal.
Calcd for C30H36Cl2N4W: C, 50.94; H, 5.13; N, 7.92. Found: C,
50.99; H, 5.17; N, 7.76.
compound could be recrystallized from a concentrated toluene
solution to give yellow blocks, but the powder is pure as judged
by elemental analysis and 1H NMR: 1H NMR (500 MHz, C6D6)
δ 10.90 (s, 1, syn-CHCMe2Ph, JCH = 123 Hz), 7.35 (d, 2, Ar-H,
J
HH = 8 Hz), 7.29 (d, 1, Ar-H, JHH = 8 Hz), 7.15, (m, 3, Ar-H),
7.02 (t, 1, Ar-H, JHH = 8 Hz), 6.83 (m, 2, Ar-H), 5.95 (br s, 4,
Pyr-H), 2.16 (s, 12, Pyr-Me), 1.65 (s, 6, CHCMe2Ph), 1.33 (s, 9,
tert-butyl); 13C NMR (125 MHz, C6D6) δ 281.17 (CHCMe2Ph),
155.06 (JCW = 42 Hz), 151.65, 142.49, 135.03, 128.80, 127.14,
126.85, 126.69, 126.32, 126.21, 106.85 (br s, Pyr), 57.39, 35.84,
33.28, 30.16, 17.80 (br s, Me-Pyr). Anal. Calcd for C32H41-
N3W: C, 58.99; H, 6.34; N, 6.45. Found: C, 59.12; H, 6.33; N, 6.42.
W(NArtBu)(CHCMe2Ph)(OTPP)(Me2Pyr). A 20 mL scintil-
lation vial was charged with a stir bar, W(NArtBu)(CHCMe2-
Ph)(Me2Pyr)2 (0.604 g, 0.927 mmol), and HOTPP (0.369 g,
0.927 mmol). Benzene (10 mL) was added to the mixture. The
mixture was allowed to stir at room temperature overnight and
filtered through glass wool. The solvents were removed from the
filtrate in vacuo. Pentane was added to the residue, and the
yellow precipitate was filtered off; yield 0.759 g (86%): 1H NMR
(500 MHz, C6D6) δ 8.11 (s, 1, syn-CHCMe2Ph, JCH = 113 Hz,
W(NArtBu)2(CH2CMe2Ph)2. A 500 mL flask was charged
with a stir bar, W(NArtBu)2(NC5H5)2Cl2 (11.77 g, 16.64 mmol),
and 200 mL of ethyl ether. The solution was chilled at -30 °C for
2 h, and MgCl(CH2CMe2Ph) (0.5 M, 67 mL, 33.28 mmol) was
added dropwise to the solution. Over the course of half an hour,
the solution changed from red to bright yellow. The mixture was
allowed to stir overnight at room temperature and then filtered
through a bed of Celite. The solvents were removed from the
yellow filtrate in vacuo. Pentane (50 mL) was added to the re-
sulting yellow powder, and the slurry was filtered to afford a fine
yellow powder; yield 9.273 g (72%): 1H NMR (500 MHz, C6D6)
δ 7.38 (d, 4, Ar-H, JHH = 7 Hz), 7.26 (m, 2, Ar-H), 7.16 (t, 4, Ar-
H, JHH = 8 Hz), 7.04, (m, 4, Ar-H, JHH = 7 Hz), 6.80 (m, 4, Ar-
H), 1.75 (s, 4, CH2CMe2Ph), 1.60 (s, 18, tert-butyl), 1.50 (s, 12,
J
WH = 16 Hz), 7.05 (m, 29), 6.11 (br s, 2, Pyr-H), 5.89 (d, 1, Ar-
H, JHH = 8 Hz), 2.61 (br s, 3, Pyr-Me), 1.92 (br s, 3, Pyr-Me),
1.53 (s, 3, CHCMe2Ph), 1.23 (s, 3, CHCMe2Ph), 1.20 (s, 9, tert-
butyl); 13C NMR (125 MHz, C6D6) δ 258.77 (CHMe2Ph),
159.64, 155.76, 152.13, 154.30, 142.60, 142.43, 137.00, 134.40,
132.99 (v br), 131.88 (v br), 130.99 130.53, 128.50, 127.89,
127.85, 127.10, 126.90, 126.72, 126.30, 126.25, 126.07, 110.66,
54.22, 35.64, 34.24, 30.75, 30.66. Anal. Calcd for C56H54-
N2OW: C, 70.44; H, 5.70; N, 2.93. Found: C, 70.60; H, 5.92;
N, 2.95.
W(NArtBu)(C3H6)(OTPP)(Me2Pyr). A 50 mL Schlenk flask
was charged with a stir bar and a solution of W(NArtBu)-
(CHCMe2Ph)(OTPP)(Me2Pyr) (0.759 g, 0.795 mmol) in di-
ethyl ether (3 mL) and benzene (1 mL). The solution was
degassed by three freeze-pump-thaw cycles and exposed to 1
atm of ethylene at room temperature. The yellow cloudy solu-
tion became clear immediately upon the addition of ethylene.
All volatiles were removed from the solution after stirring it at
room temperature overnight. The residue was recrystallized
from ∼2 mL of ether and a few drops of benzene to afford fine
CH2CMe2Ph); 13C NMR (125 MHz, C6D6) δ 154.56 (JCW
=
36 Hz), 151.90, 140.63, 131.84, 129.13, 127.14, 126.63, 126.61,
126.31, 125.46, 92.63 (JCW = 100 Hz), 41.65, 35.80, 33.78, 30.78.
Anal. Calcd for C40H52N2W: C, 64.51; H, 7.04; N, 3.76. Found:
C, 64.24; H, 6.89; N, 3.72.
W(NArtBu)(CHCMe2Ph)(OTf)2(DME).
A sample of W-
(NArtBu)2(CH2CMe2Ph)2 (12.80 g, 17.19 mmol) was dissolved
in DME (30 mL) and pentane (200 mL). The solution was chilled
to -30 °C for 2 h, and then triflic acid (7.74 g, 51.57 mmol) was
added in one portion. The solution changed from bright yellow
to orange immediately upon addition of triflic acid. The solution
was allowed to stir at room temperature for 24 h. All volatiles
were removed in vacuo, the resulting residue was extracted with
toluene (300 mL), and the mixture was filtered through a bed of
Celite. Solvents were removed from the filtrate in vacuo to give a
yellow powder. Pentane was added to the powder, and the mix-
ture was filtered off to afford a yellow powder; yield 8.88 g
(61%). A 1H NMR spectrum shows four isomers in a 0.12, 0.03,
1.00, and 0.45 ratio. Only chemical shifts of the major isomer
and those that are discernible are reported: 1H NMR (500 MHz,
C6D6) δ 12.01 (s, 0.12, CHCMe2Ph), 11.50 (s, 0.03, CHCMe2Ph),
11.12 (s, 1, CHCMe2Ph), 10.70 (s, 0.45, CHCMe2Ph), aryl region
is messy, 3.61 (s, 3, DME), 3.12 (br s, 2, DME), 2.79 (s, 3, DME),
2.75, (br s, 2, DME), too many peaks in alkyl region to pick out
CHCMe2Ph, 1.49 (s, 18, tert-butyl). Anal. Calcd for C26H35F6-
NO8S2W: C, 36.67; H, 4.14; N, 1.64. Found: C, 36.45; H, 3.99;
N, 1.85.
1
yellow needles; yield 0.356 g (52% yield). A H NMR spectrum
shows that in solution 11% of the sample is the methylidene species
1
(vide infra) and ∼2% of ethylene (broad signal) is observed: H
NMR (500 MHz, C6D6) δ 7.05 (m, 25, Ar-H), 6.14 (s, 2, Pyr-H),
4.28 (br s, 2, CHR), 3.35 (br s, 2, CHR), 2.39 (s, 6, Pyr-Me), 1.19 (s, 9,
tert-butyl), -0.99 (br s, 1, CHβ), -1.60 (br s, 1, CHβ); 13C NMR
(125 MHz, C6D6) δ 159.23, 156.07, 144.56, 143.00, 142.61, 141.97,
139.01, 137.30, 133.95, 132.53, 132.14, 131.39, 131.16, 130.48,
129.26, 137.90, 127.22, 126.74, 126.47, 126.14, 110.55, 109.31,
102.00 (WCR), 31.78, 30.85, 17.10, -4.12 (WCβ). Anal. Calcd for
C49H48N2OW: C, 68.06; H, 5.59; N, 3.24. Found: C, 68.18; H, 5.84;
N, 3.21.
A sample was degassed via three freeze-pump-thaw cycles
and exposed to 13C-ethylene. 13C NMR (125 MHz, C6D6) δ 102.00
(WCR,JCH = 159 Hz, JCW =80Hz),-4.12 (WCβ,JCH =154 Hz).
This compound decomposes over the course of ∼4 h in solution
at room temperature.
In Situ Synthesis of W(NArtBu)(CH2)(OTPP)(Me2Pyr). W-
(NArtBu)(C3H6)(OTPP)(Me2Pyr) (0.023 g, 0.027 mmol) and
benzene-d6 (0.6 mL) were added to a 2.9 mL J-Young tube,
and the solvent was removed . Benzene-d6 was added and the
volatiles were removed in vacuo four more times: 1H NMR (500
MHz, C6D6) δ 10.10 (v br s, 1, syn-CHR), 9.09 (v br s, 1, anti-
CHR), 7.32 (s, 1, Ar-H), 7.18 (m, 9, Ar-H), 6.94 (m, 14, Ar-H),
6.16 (m, 1, Ar-H), 6.06 (s, 2, Pyr-H), 2.09 (s, 6, Pyr-Me), 1.30 (s,
9, tert-butyl).
W(NArtBu)(CHCMe2Ph)(Me2Pyr)2. Lithium 2,5-dimethyl-
pyrrolide (1.435 g, 14.200 mmol, 2.5 equiv) and W(NArtBu)(CH-
CMe2Ph)(OTf)2(DME) (4.837 g, 5.680 mmol) were mixed in a
50 mL flask. A stir bar and toluene (20 mL) were added. The
resulting cloudy yellow solution was allowed to stir overnight at
room temperature. The solution was filtered through a bed of
Celite, and solvents were removed from the filtrate in vacuo. Pen-
tane was added to the residue. The precipitated yellow powder
was collected to afford 1.772 g of product (48% yield). The
A sample of W(NArtBu)(C3H6)(OTPP)(Me2Pyr) in ben-
zene-d6 was degassed via three freeze-pump-thaw cycles
and exposed to 13C-ethylene, and then vacuum was applied

Article
Organometallics, Vol. 29, No. 21, 2010 5251
to remove ethylene to generate the methylidene species: 1H
NMR (500 MHz, C6D6) δ 10.10 (syn-CHa, JCH = 156 Hz), 9.09
(anti-CHa, JCH = 128 Hz); 13C NMR (125 MHz, C6D6) δ
242.09 (WCa, JCW = 183 Hz). W(NArtBu)(CH2)(OTPP)(Me2-
Pyr) decomposes over a period of ∼2 h in solution at room
temperature.
Supporting Information Available: Experimental details for
all NMR experiments and X-ray structural studies. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Note Added after ASAP Publication. In the version of this
paper published on the web on July 12, 2010, the grant number
given in the Acknowledgment was incorrect. The grant number
that appears as of August 3, 2010 is correct.
Acknowledgment. We thank the National Science Foun-
dation for research support (CHE-0841187 to R.R.S.).