Bipyridine adducts of molybdenum imido alkylidene and imido alkylidyne complexes

Article abstract of DOI:10.1021/om300353e

Seven bipyridine adducts of molybdenum imido alkylidene bispyrrolide complexes of the type Mo(NR)(CHCMe₂R′)(Pyr)₂(bipy) (1a-1g; R = 2,6-i-Pr₂C₆H₃ (Ar), adamantyl (Ad), 2,6-Me₂C₆H₃ (Ar′), 2-i-PrC ₆H₄ (Ar^iPr), 2-ClC₆H₄ (Ar^Cl), 2-t-BuC₆H₄ (Ar^tBu), and 2-MesitylC₆H₄ (Ar^M), respectively; R′ = Me, Ph) have been prepared using three different methods. Up to three isomers of the adducts are observed that are proposed to be the trans- and two possible cis-pyrrolide isomers of syn-alkylidenes. Sonication of a mixture containing 1a-1g, HMTOH (2,6-dimesitylphenol), and ZnCl₂(dioxane) led to the formation of MAP species of the type Mo(NR)(CHCMe₂R′)(Pyr) (OHMT) (3a-3g). DCMNBD (2,3-dicarbomethoxynorbornadiene) is polymerized employing 3a-3g as initiators to yield >98% cis,syndiotactic poly(DCMNBD). Attempts to prepare bipy adducts of bisdimethylpyrrolide complexes led to the formation of imido alkylidyne complexes of the type Mo(NR)(CCMe ₂R′)(Me₂Pyr)(bipy) (Me₂Pyr = 2,5-dimethylpyrrolide; 4a-4g) through a ligand-induced migration of an alkylidene α proton to a dimethylpyrrolide ligand. X-ray structures of Mo(NAr)(CHCMe₂Ph)(Pyr)₂(bipy) (1a), Mo(NAr ^iPr)(CHCMe₂Ph)(Pyr)(OHMT) (3d), Mo(NAr)(CCMe ₂Ph)(Me₂Pyr)(bipy) (4a), the NAr′ analog of 4a (4c), and Mo(NAr^T)(CCMe₃)(Me₂Pyr)(bipy) (Ar ^T = 2-(2,4,6-i-Pr₃C₆H₂)C ₆H₄; 4g) showed normal bond lengths and angles.

Full text of DOI:10.1021/om300353e

Article
pubs.acs.org/Organometallics
Bipyridine Adducts of Molybdenum Imido Alkylidene and Imido
Alkylidyne Complexes
Alejandro G. Lichtscheidl, Victor W. L. Ng, Peter Muller, Michael K. Takase, and Richard R. Schrock*
̈
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
Steven J. Malcolmson, Simon J. Meek, Bo Li, Elizabeth T. Kiesewetter, and Amir H. Hoveyda*
Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02467, United States
S
* Supporting Information
ABSTRACT: Seven bipyridine adducts of molybdenum imido alkylidene bispyrrolide
complexes of the type Mo(NR)(CHCMe2R′)(Pyr)2(bipy) (1a−1g; R = 2,6-i-Pr2C6H3
(Ar), adamantyl (Ad), 2,6-Me2C6H3 (Ar′), 2-i-PrC6H4 (AriPr), 2-ClC6H4 (ArCl), 2-t-
BuC6H4 (ArtBu), and 2-MesitylC6H4 (ArM), respectively; R′ = Me, Ph) have been prepared
using three different methods. Up to three isomers of the adducts are observed that are
proposed to be the trans- and two possible cis-pyrrolide isomers of syn-alkylidenes.
Sonication of a mixture containing 1a−1g, HMTOH (2,6-dimesitylphenol), and
ZnCl2(dioxane) led to the formation of MAP species of the type Mo(NR)-
(CHCMe2R′)(Pyr)(OHMT) (3a−3g). DCMNBD (2,3-dicarbomethoxynorbornadiene)
is polymerized employing 3a−3g as initiators to yield >98% cis,syndiotactic poly-
(DCMNBD). Attempts to prepare bipy adducts of bisdimethylpyrrolide complexes led to
the formation of imido alkylidyne complexes of the type Mo(NR)-
(CCMe2R′)(Me2Pyr)(bipy) (Me2Pyr = 2,5-dimethylpyrrolide; 4a−4g) through a ligand-
induced migration of an alkylidene α proton to a dimethylpyrrolide ligand. X-ray
structures of Mo(NAr)(CHCMe2Ph)(Pyr)2(bipy) (1a), Mo(NAriPr)(CHCMe2Ph)(Pyr)(OHMT) (3d), Mo(NAr)(CCMe2Ph)-
(Me2Pyr)(bipy) (4a), the NAr′ analog of 4a (4c), and Mo(NArT)(CCMe3)(Me2Pyr)(bipy) (ArT = 2-(2,4,6-i-Pr3C6H2)C6H4;
4g) showed normal bond lengths and angles.
Mo(NAd)(CHCMe2Ph)(NC4H4)2 has been employed as a
precursor to Mo(NAd)(CHCMe2Ph)(NC4H4)(OAr) com-
plexes that are especially useful as Z-selective olefin metathesis
catalysts,3b,c,e,g−j,5 where OAr is a large 2,6-disubstituted
phenoxide. Only 1 equiv of a large 2,6-terphenol adds to the
metal in bispyrrolide complexes for steric reasons, a circum-
stance that allows the MAP species to be generated and/or
isolated in relatively high yields.
It has long been known that 14-electron bisalkoxide catalysts
will form 16- or 18-electron adducts with donor ligands.1b
Bipyridine was first employed as a ligand in imido alkylidene
chemistry in order to isolate the methylidene complex, yellow
crystalline Mo(NAr)(CH2)[OC(CF3)2Me]2(bipy) in high
yield.6 Eighteen-electron Mo(NAr)(CH2 )[OC-
(CF3)2Me]2(bipy) is essentially inactive as a metathesis catalyst
and stable toward bimolecular decomposition reactions.
INTRODUCTION
■
High oxidation state molybdenum and tungsten imido
alkylidene complexes1 were discovered approximately 25
years ago.2 In the last several years, new types of imido
alkylidene complexes with the formula M(NR)(CHR′)(OR″)-
(Pyr), where Pyr is a pyrrolide or substituted pyrrolide ligand
and OR″ usually is an aryloxide, have been prepared and
explored;1a,3 we refer to these monoaryloxide monopyrrolide
complexes as MAP species. MAP species were discovered in the
process of adding alcohols or phenols to bispyrrolide complexes
4
with the general formula M(NR)(CHR′)(Pyr)2 in order to
prepare bisalkoxide or biphenolate catalysts in situ and screen
them for olefin metathesis activity.
Bispyrrolide complexes have been prepared that contain
pyrrolide,4a 2,5-Me2pyrrolide,4b 2,3,4,5-Me4pyrrolide,4c 2,5-i-
Pr2pyrrolide,4c 2,5-Ph2pyrrolide,4c and 2-Mesitylpyrrolide.4d
The majority of MAP species that have been prepared contain
2,5-dimethylpyrrolide. The relatively small number of MAP
compounds that contain an unsubstituted pyrrolide is a
consequence of the often poor crystallinity and instability of
Mo(NR)(CHR′)(pyrrolide)2 species over a period of days,
even in the solid state. Two exceptions are compounds in which
the imido substituent (R) is 2,6-i-Pr2C6H3 (Ar) or adamantyl
(Ad), which are stable for many days at −35 °C under nitrogen.
Furstner has reported that bipyridine adducts of several related
̈
molybdenum species are relatively stable to air and can be
activated toward metathesis in the absence of air in solution
through addition of ZnCl2 to remove bipyridine;7a apparently,
little exchange of alkoxide for chloride on the molybdenum is
Received: April 27, 2012
Published: June 6, 2012
© 2012 American Chemical Society
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observed during the activation process. He also has reported
examples of 18-electron alkylidyne complexes that are relatively
stable in air and can be activated through addition of Lewis
acids.7b Other types of 18-electron alkylidene complexes that
are activated upon addition of Lewis acids are known.8
In this paper, we report the synthesis of relatively air-stable
bipyridine adducts of bispyrrolide complexes that contain a
variety of different imido groups and their use as catalyst
precursors for the preparation of Mo(NR)(CHCMe2Ph)-
(NC4H4)(OAr) species; we have been able to prepare several
of these MAP species only in this manner. We also report that
attempts to prepare bispyridine adducts of bis-2,5-dimethyl-
pyrrolide complexes lead to formation of imido alkylidyne
complexes of the type Mo(NR)(CCMe2R′)(Me2Pyr)(bipy)
through sterically induced α abstraction of the alkylidene
proton by one of the dimethylpyrrolide ligands.
RESULTS AND DISCUSSION
Figure 1. Drawing of the solid-state structure of Mo(NAr)-
(CHCMe2Ph)(Pyr)2(bipy) (1a; 50% probability ellipsoids). The
solvent molecule, hydrogen atoms, and the disorder are omitted for
clarity. Selected bond lengths (Å) and angles (deg): Mo(1)−C(11) =
1.932(3), Mo(1)−N(1) = 2.330(3), Mo(1)−N(2) = 2.354(3),
Mo(1)−N(3) = 1.730(2), Mo(1)−N(4) = 2.135(3), Mo(1)−N(5)
= 2.143(2); Mo(1)−C(11)−C(12) = 138.3(2), Mo(1)−N(3)−C(21)
= 171.0 (2), N(5)−Mo(1)−N(4) = 155.75(10).
■
Addition of 1 equiv of bipyridine to Mo(NR)(CHCMe2Ph)-
(Pyr)2 in diethyl ether led to precipitation of complexes with
the general formula Mo(NR)(CHCMe2Ph)(Pyr)2(bipy) (R =
Ar, 1a; R = Ad, 1b) in good yields (eq 1). This procedure will
Bispyrrolide species also can be prepared in situ from
Mo(NR)(CHCMe2Ph)(OTf)2(DME) complexes and treated
with 0.8−0.9 equiv of bipyridine to produce the insoluble
compounds of type 1 shown in eq 2 (R′ = t-Bu or CMe2Ph).
be referred to as method A. Mo(NR)(CHCMe2Ph)-
(Pyr)2(bipy) species are relatively insoluble, a property that
allows them to be isolated readily. A proton NMR spectrum of
1b could be obtained in CD2Cl2, but no high-quality 13C NMR
spectrum could be obtained readily as a consequence of
insolubility of 1b. The three alkylidene isomers of 1b are
proposed to arise from one adduct with trans-pyrrolide ligands
and two adducts that contain cis-pyrrolide ligands; all are
proposed to be syn-alkylidene isomers. Compound 1a dissolves
more readily in CD2Cl2 than 1b, so both H and 13C NMR
1
spectra could be obtained. Only one isomer of 1a is observed.
An X-ray study of 1a shows it to have a structure (Figure 1)
in which the pyrrolide ligands are trans to one another and bipy
is bound trans to the alkylidene and imido ligands. In contrast,
Mo(NAr)(CHCMe2Ph)[OC(CF3)2Me]2(bipy)7 adopts a cis
configuration in which bipy is bound trans to the alkylidene and
one of the alkoxide ligands. The Mo−Nbipy bond lengths in 1a
(2.330(3) and 2.354(3) Å), therefore, are similar, whereas the
two Mo−Nbipy bond lengths in Mo(NAr)(CHCMe2Ph)[OC-
(CF3)2Me]2(bipy) (2.3503(11) and 2.2462(10) Å)7 differ
significantly, with the latter bond length (trans to the alkoxide)
being the shorter of the two.
This method will be referred to as method B. It is an effective
way to make six of the seven bipyridyl adducts of type 1.
Compounds 1b−1g were obtained in analytically pure form
simply through filtration. The yield of 1f suffers from some
solubility in toluene.
Only one alkylidene resonance is present in the alkylidene
region in the 1H NMR spectrum (in CD2Cl2) of 1c and 1f; two
are present for 1e and 1d, whereas three are present for 1g. All
isomers are presumed to arise from cis/trans isomerism of the
pyrrolide ligands, as noted earlier, although, in the case of 1g,
restricted rotation of the NArM imido ligand could give rise to
the third isomer. Unfortunately, because of the insolubility of
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samples 1b−1g, no JCH coupling could be obtained from 13C
NMR spectra in order to identify syn or anti isomers.
slightly soluble in toluene, whereas the MAP complexes are
highly soluble. The MAP complexes 3a−3g are obtained in
crystalline form by filtering off any remaining insoluble
material(s), removing the solvent from the filtrate, and
recrystallizing the solid products from pentane at −35 °C (eq
5). Proton NMR and carbon NMR spectra of 3a−3g are
Mo(NR)(CHCMe2Ph)(OTf)2(bipy) complexes can be
synthesized from Mo(NR)(CHCMe2Ph)(OTf)2(dme) com-
plexes by suspending the latter in benzene that contains 1 equiv
of bipyridine at room temperature (R = 2,6-i-Pr2C6H3 (Ar), 1-
adamantyl (Ad), 2,6-Me2C6H3 (Ar′), and 2-MesC6H4 (ArM), or
in diethyl ether (R = 2-ClC6H4 (ArCl), 2-i-PrC6H4 (AriPr), 2-t-
BuC6H4 (ArtBu)) and stirring the mixtures for 12 h at 22 °C (eq
3). In all cases, the relatively insoluble Mo(NR)(CHCMe2Ph)-
(OTf)2(bipy) complexes can be collected by filtration in good
yields. All Mo(NR)(CHCMe2Ph)(OTf)2(bipy) complexes are
soluble in CD2Cl2, with the exception of 2a and 2b, which are
only sparingly soluble and for which 13C NMR spectra could
not be obtained. Proton NMR spectra of the complexes in
CD2Cl2 show either one or two alkylidene resonances, which
arise from cis and trans disposition of the triflates, a proposal
that is corroborated by the 19F NMR spectra of each
compound. Two isomers are observed when the imido group
has only one ortho substituent.
consistent with the presence of only one isomer (syn) in
solution. Apparently, exchange of pyrrolide (on Mo) for
chloride (on Zn) is not a significant problem in the reaction
shown in eq 5. The in situ synthesis of 3b directly from
Mo(NAd)(CHCMe2Ph)(NC4H4)2 has been reported else-
where.5
The structure of 3d was obtained through an X-ray study.
The complex crystallized in the space group P1with both the R
̅
Complexes 2a, 2c, and 2d react with 2 equiv of LiNC4H4 to
generate the bispyrrolide species, 1a, 1c, and 1d (method C; eq
4). These compounds can be isolated in moderate to good
and S enantiomers present (Figure 2a,b). Details are available in
the Supporting Information.
The efficacies of 3a−3g for polymerization of 50 equiv of
2,3-dicarbomethoxynorbornadiene (DCMNBD) over a period
of 1−2 h to give cis-poly(DCMNBD) were explored with each
as an initiator at 22 °C. The cis content of poly(DCMNBD)
1
was determined through H and 13C NMR spectroscopy. All
reactions are relatively fast and all give >98% cis polymer that
we presume is syndiotactic on the basis of the similarity of NMR
spectra to those for poly(DCMNBD) samples prepared with
Mo(NAd)(CHCMe2R′)(Pyr)(OHIPT) as an initiator and
polymers prepared from menthoxy analogues of DCMNBD.3i
Full details can be found in the Supporting Information. Similar
behavior had been observed for the analogous Mo(NR)-
(CHCMe2R′)(Me2Pyr)(OHMT) initiators in which the phenyl
imido ligands were monosubstituted in the ortho positions with
Cl, CF3, or i-Pr groups, although t-Bu, mesityl, or 2,4,6-
triisopropylphenyl groups in the ortho position of the
phenylimido ligand led to the formation of poly(DCMNBD)
samples that contained some trans linkages.9 Therefore, at least
for polymerization of DCMNBD and similar diesters, the MAP
species that contains the parent pyrrolide is preferred.
In contrast to the results presented so far concerning
bipyridine adduct formation, attempted syntheses of bipyridine
adducts of Mo(NR)(CHCMe2Ph)(Me2Pyr)2 or Mo(NR)-
(CHCMe3)(Me2Pyr)2 complexes led to the imido alkylidyne
complexes 4a−4g shown in eq 6. Only the reaction between
Mo(NAriPr)(CHCMe2Ph)(Me2Pyr)2 and 1 equiv of bipy or
Mo(NAd(CHCMe2Ph)(Me2Pyr)2 and 5 equiv of bipy can be
carried out to completion at 22 °C in a 1:1 mixture of toluene
and pentane or diethyl ether, respectively. Other reactions
yields by running the reaction in diethyl ether for 12 h, filtering
off the precipitated product, and washing the precipitate with
diethyl ether.
When complexes 2b and 2e−2g are treated with 2 equiv of
LiNC4H4 under the same conditions, impure products are
obtained as a consequence of what is proposed to be
incomplete substitution. The reaction is not driven to
completion when longer times (1−2 days) are employed, and
complications arise in subsequent reactions if these impure
compounds are employed.
In general, bipyridine adducts of type 1 are easy to isolate,
handle, and store for long periods of time, but they would be
useful only if bipy could be removed and MAP species
prepared. Complexes 1a−1g were mixed with 1 equiv of
ZnCl2(dioxane) and 1 equiv of 2,6-dimesitylphenol (HMTOH)
in 10−15 mL of toluene in a Teflon-sealed Schlenk flask. The
flask was placed in a conventional ultrasonic cleaner for 3−5 h
at 22 °C. The choice of solvent is key because the reagents,
except for HMTOH and the ZnCl2(bipy) byproduct, are only
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and 4g crystallized in the monoclinic space group P2(1)/c. The
structures are shown in Figures 3−5. In the case of 4g, two
Figure 2. (a) A drawing of the solid-state structure of (R)-
Mo(NAriPr)(CHCMe2Ph)(Pyr)(OHMT) (R-3d; 50% probability
ellipsoids). Selected bond lengths (Å) and angles (deg): Mo(1)−
C(1) = 1.8769(15), Mo(1)−N(1) = 1.7300(12), Mo(1)−N(2) =
2.0198(13), Mo(1)−O(1) = 1.9186(10); Mo(1)−C(1)−C(2) =
145.61(11), Mo(1)−N(1)−C(11) = 178.12(11), Mo(1)−O(1)−
C(31) = 143.14(9). (b) A drawing of the solid-state structure of
(S)-Mo(NAriPr)(CHCMe2Ph)(Pyr)(OHMT) (S-3d; 50% probability
ellipsoids). Selected bond lengths (Å) and angles (deg): Mo(2)−
C(101) = 1.8759(15), Mo(2)−N(3) = 1.7263(12), Mo(2)−N(4) =
2.0294(13), Mo(2)−O(2) = 1.9168(10); Mo(2)−C(101)−C(102) =
143.31(11), Mo(2)−N(3)−C(111) = 177.45(11), Mo(2)−O(2)−
C(131) = 150.15(9).
Figure 3. Drawing of the solid-state structure of (NAr)(CCMe2Ph)-
(Me2Pyr)(bipy) (4a; 50% probability ellipsoids). Selected bond
lengths (Å) and angles (deg): Mo(1)−C(29) = 1.764(3), Mo(1)−
N(1) = 2.326(3), Mo(1)−N(2) = 2.209(3), Mo(1)−N(3) = 1.804(3),
Mo(1)−N(4) = 2.098(3); Mo(1)−C(29)−C(30) = 161.5(2),
Mo(1)−N(3)−C(11)
= 159.6(2), N(1)−Mo(1)−N(3) =
144.12(10), N(2)−Mo(1)−N(4) = 153.05(10).
independent molecules were present in the asymmetric unit
along with six benzene molecules. The phenyl imido ligand in
one of the complexes is disordered (disorder not shown),
whereas in the other, it is not disordered. Compounds 4a, 4c,
and 4g can best be regarded as distorted square pyramids with
the alkylidyne ligand located in the apical position. The most
striking features are the bond lengths Mo(1)−C(29) in 4a
(1.764(3)Å), Mo(1)−C(1) in 4c (1.7643(17) Å), and Mo(1)−
C(1) in 4g (1.780(5)Å), and the relatively large Mo(1)−
C(29)−C(30) bond angle in 4a (161.5(2)°), the Mo(1)−
C(1)−C(2) bond angle in 4c (159.05(14)°), and the Mo(1)−
C(1)−C(2) bond angle in 4g (167.1(4)°); all are consistent
with formation of alkylidyne complexes. The MoNR bond
lengths of 4a (1.804(3)Å), 4c (1.7958(14)Å), and 4g
(1.823(4)Å) are longer than in analogous alkylidene complexes,
as expected in view of competition between the imido and
require heating in toluene at 60 °C. In all cases, the color of the
reaction mixture changes from orange-brown to red-purple.
Upon completion of the reaction, 1 equiv of Me2PyrH can be
observed in solution in proton NMR spectra. Upon removing
the solvent from the reaction mixture and washing the resulting
solids with pentane, compounds 4a−4g can be obtained as
purple or red-purple solids in good to very good yields (eq 6).
The relatively low solubility of 4a−4g along with the low
sensitivity of alkylidyne carbon atoms in general in natural
abundant carbon NMR spectra prevented confirmation that
compounds 4 were in fact alkylidyne complexes.
X-ray quality crystals of 4a and 4c were grown from a
mixture of dichloromethane and pentane at −45 °C, whereas
crystals of 4g were grown from benzene at 22 °C. Complex 4a
crystallized in the monoclinic space group P2(1)/n, whereas 4c
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(CF3)2]2}−. Although adducts analogous to 1a−1g are not
formed readily upon addition of bipyridine to bisdimethylpyr-
rolide complexes, we propose that adducts are likely
intermediates in the process of forming 4a, 4c, and 4g and
that steric crowding leads to an alkylidene with a larger Mo−
C−C angle, which activates that alkylidene’s α proton toward
migration, ultimately to a pyrrolide, and generation of
dimethylpyrrole (eq 7).
Figure 4. Drawing of the solid-state structure of Mo(NAr′)-
(CCMe2Ph)(Me2Pyr)(bipy) (4c; 50% probability ellipsoids). Selected
bond lengths (Å) and angles (deg): Mo(1)−C(1) = 1.7643(17),
Mo(1)−N(1) = 1.7958(14), Mo(1)−N(2) = 2.1228(14), Mo(1)−
N(3) = 2.3165(13), Mo(1)−N(4) = 2.2100(13); Mo(1)−C(1)−C(2)
= 159.05(14), Mo(1)−N(1)−C(11) = 162.64(12), N(1)−Mo(1)−
N(3) = 137.44(6), N(2)−Mo(1)−N(4) = 153.09(15).
High oxidation state alkylidyne complexes of tantalum,
tungsten, molybdenum, and rhenium have been synthesized
through several routes in the last 30 years.11 Formation of a
neopentylidyne ligand through deprotonation of a neo-
pentylidene ligand was first demonstrated in a reaction between
Ta(CHCMe3)(CH2CMe3)3 and butyllithium to give a lithium
salt of {Ta(CHCMe3)(CH2CMe3)3}−.12 Dehydrohalogenation
of W(NPh)(CHCMe3)(PEt3)2Cl2 by Ph3PCH2 led to
W(NPh)(CCMe3)(PEt3)2Cl,13 a compound that is most
similar to 4a−4g, although no X-ray structure of W(NPh)-
(CCMe3)(PEt3)2Cl was determined. Deprotonation of Mo-
(NAr)(CHCMe3)[OCMe(CF3)2]2 by Ph3PCH2 led to the
alkylidyne complex, {Ph3PMe}{Mo(NAr)(CCMe3)[OCMe-
(CF3)2]2},10 as noted earlier. Attempts to prepare certain
types of imido neopentylidene complexes have resulted in
formation of amido neopentylidyne complexes as a conse-
quence of migration of a proton from an alkylidene to an imido
ligand, or as a consequence of an actual deprotonation/
readdition sequence.14 Neopentylidyne ligands have been
generated from neopentyl ligands early in the development of
high oxidation state organometallic chemistry of tungsten and
molybdenum, a circumstance that has allowed tungsten and
molybdenum alkylidyne complexes of the type M(C-t-Bu)-
(CH2-t-Bu)3 to be synthesized;15 α deprotonation of a
neopentyl ligand to give an intermediate neopentylidene ligand
is probably the rate-limiting deprotonation step in these
reactions. Rhenium neopentylidyne complexes have been
prepared through deprotonation of neopentylidene ligands,
by transfer of a proton either to a neopentyl group or to an
imido or amido group.16 A proton also has been added to an
alkylidyne α carbon atom in several circumstances to give an
alkylidene complex. For example, transfer of a proton from an
amido ligand to a neopentylidyne α carbon atom was the first
general method of preparing imido alkylidene complexes of
tungsten.2 Intramolecular α proton migration has been known
to be promoted in more sterically crowded circumstances, the
first being ligand-induced formation of tantalum neopentyli-
dene complexes from tantalum dineopentyl complexes through
addition of various phosphines or simply THF.17 On the basis
Figure 5. Drawing of the solid-state structure of Mo(NArT)(CCMe3)-
(Me2Pyr)(bipy) (4g; 50% probability ellipsoids). Solvent molecules
and the second independent molecule, which shows some disorder, as
well as the hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Mo(1)−C(1) = 1.780(5), Mo(1)−N(1)
= 2.105(4), Mo(1)−N(2) = 2.306(3), Mo(1)−N(3) = 2.225(4),
Mo(1)−N(4) = 1.823(4); Mo(1)−C(1)−C(2) = 167.1(4), Mo(1)−
N(4)−C(31) = 152.6(3), N(1)−Mo(1)−N(3) = 153.28(14), N(2)−
Mo(1)−N(4) = 140.98(15).
alkylidyne ligands for π-type d orbitals. The Mo(1)−N(3)−
C(11) bond angle in 4a (159.6(2)°), the Mo(1)−N(1)−C(11)
bond angle in 4c (162.64(12)°), and the Mo(1)−N(4)−C(31)
bond angle in 4g (152.6(3)°) are relatively small, consistent
with a Mo−N double bond more than a triple bond in the bent
imido ligands. However, the imido ligand in {Mo(NAr)(C-t-
Bu)[OCMe(CF3)2]2}− is more bent10 (Mo−N−C =
141.16(17)°) than any in 4a, 4c, or 4g. We propose that steric
interactions between ligands in five-coordinate 4a, 4c, and 4g
prevent the imido ligands being bent as much as the imido
ligand in four-coordinate {Mo(NAr)(C-t-Bu)[OCMe-
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of all of the above reports, it is fair to say that there is ample
precedent for intramolecular migration of a proton from a
neopentylidene ligand to a dimethylpyrrolide ligand, even
though there is no example of this particular reaction in the
literature to our knowledge. It is not possible to determine on
the basis of data in hand whether the proton migrates to the
pyrrolide nitrogen directly or first to a pyrrolide α or β carbon
atom.1a
We felt that phenols might add across the metal−carbon
triple bond in complexes 4a−4g to regenerate MAP species.
Although preliminary studies show promise for reactions of this
type, Mo(NR)(CCMe2R′)(Me2Pyr)(bipy) complexes probably
would have to be accessible more directly in order for this
approach to be competitive with existing approaches to MAP
species.
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CONCLUSION
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7770. (g) Marinescu, S. C.; Levine, D.; Zhao, Y.; Schrock, R. R.;
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bispyrrolide complexes of the type Mo(NR)-
(CHCMe2R′)(Pyr)2(bipy) (1a−1g) have been prepared using
three different methods. The adducts are isolated readily and
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Mo(NR)(CHCMe2R′)(Pyr)2 complexes themselves. They can
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of the type Mo(NR)(CHCMe2R′)(Pyr)(OHMT) (3a−3g)
through sonication of a mixture containing 1a−1g, HMTOH,
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ASSOCIATED CONTENT
■
S
* Supporting Information
(6) Fox, H. H.; Lee, J.-K.; Park, L. Y.; Schrock, R. R. Organometallics
1993, 12, 759.
Details of the synthesis and characterization of all complexes,
crystallographic details, fully labeled thermal ellipsoid diagrams
for all crystallographically characterized species, and crystallo-
graphic information files in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
Data for the X-ray structures are also available to the public at
http://www.reciprocalnet.org.
(7) (a) Heppekausen, J.; Furstner, A. Angew. Chem., Int. Ed. 2011, 50,
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AUTHOR INFORMATION
■
(9) Lichtscheidl, A. G.; Ng, V. W. L.; Muller, P.; Takase, M. K.;
̈
Corresponding Author
*E-mail: rrs@mit.edu (R.R.S.).
Schrock, R. R. Organometallics 2012, 31, 2388.
(10) Tonzetich, Z. J.; Schrock, R. R.; Muller, P. Organometallics 2006,
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Notes
(11) (a) Schrock, R. R. Acc. Chem. Res. 1986, 19, 342. (b) Murdzek, J.
S.; Schrock, R. R. In Carbyne Complexes; VCH Publishers: Weinheim,
Germany, 1988; p 147. (c) Schrock, R. R. In Handbook of Metathesis;
Grubbs, R. H., Ed.; Wiley-VCH: Weinheim, Germany, 2003; p 173.
(12) Guggenberger, L. J.; Schrock, R. R. J. Am. Chem. Soc. 1975, 97,
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(13) (a) Rocklage, S. M.; Schrock, R. R.; Churchill, M. R.;
Wasserman, H. J. Organometallics 1982, 1, 1332. (b) Schaverien, C.
J.; Dewan, J. C.; Schrock, R. R. J. Am. Chem. Soc. 1986, 108, 2771.
(14) Schrock, R. R.; Jamieson, J. Y.; Araujo, J. P.; Bonitatebus, P. J.,
Jr.; Sinha, A.; Lopez, L. P. H. J. Organomet. Chem. 2003, 684, 56.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
R.R.S. thanks the National Science Foundation (CHE-0841187
and CHE-1111133) and the Department of Energy (DE-FG02-
86ER13564) for supporting this research. R.R.S. and A.H.H
also thank the National Institutes of Health (GM-59426) for
support. We also thank Dr. Margaret M. Flook for polymerizing
DCMNBD with 3b.
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