Investigations of reactions between chiral molybdenum imido alkylidene complexes and ethylene: Observation of unsolvated base-free methylene complexes, metalacyclobutane and metalacyclopentane complexes, and molybdenum(IV) olefin complexes

Article abstract of DOI:10.1021/om030636+

In this paper we explore reactions between ¹³CH ₂=¹³CH₂ and the imido alkylidene complexes, Mo(NAr)(CHCMe₂Ph)[(S)-Biphen] (3a; NAr = N-2, 6-i-Pr ₂C₆H₃), Mo(NAr_cl)-(CHCMe ₃)[(S)-Biphen] (3b; NAr_cl = N-2,6-Cl₂C ₆H₃), Mo(NAd)(CHCMe₂Ph)[(S)-Biphen] (3c; NAd = N-1-adamantyl), Mo(NAr)(CHCMe₂Ph)[(R)-Benz₂Bitet] (3d), and Mo(NAr_cl)(CHCMe₂-Ph)[(R)-Benz₂Bitet] (3e). (See text for a description of Biphen and Benz₂Bitet.) Under a variety of conditions and for various combinations of imido, alkylidene, and diolate ligands, we have observed α-substituted molybdacyclobutanes Mo(NR)(*CH₂*CH₂CHCMe₂Ph)[diolate] (*C = ¹³C), unsubstituted molybdacyclobutanes Mo(NR)(*CH₂*CH₂*CH₂)[diolate] , olefin complexes Mo(NR)(*CH₂=CHR) [diolate] (R = H or CMe₂Ph), molybdacyclopentane complexes Mo(NR)-(*CH ₂*CH₂*CH₂*CH₂) [diolate], and base-free methylene complexes Mo(NR)(*CH ₂)[diolate]. We also have crystallographically characterized a molybdenum ethylene complex derived from a biphenolate complex, Mo(N-2,6-Cl ₂C₆H₃)(CH₂=CH₂)[rac- Biphen](Et₂O).

Full text of DOI:10.1021/om030636+

Organometallics 2004, 23, 1997-2007
1997
In vestiga tion s of Rea ction s betw een Ch ir a l Molybd en u m
Im id o Alk ylid en e Com p lexes a n d Eth ylen e: Obser va tion
of Un solva ted Ba se-F r ee Meth ylen e Com p lexes,
Meta la cyclobu ta n e a n d Meta la cyclop en ta n e Com p lexes,
a n d Molybd en u m (IV) Olefin Com p lexes
W. C. Peter Tsang,# J ennifer Y. J amieson,# Sarah L. Aeilts,# Kai C. Hultzsch,#,§
Richard R. Schrock,*,# and Amir H. Hoveyda‡
Department of Chemistry, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139, and Department of Chemistry, Merkert Chemistry Center,
Boston College, Chestnut Hill, Massachusetts 02467
Received October 14, 2003
In this paper we explore reactions between 13CH2d13CH2 and the imido alkylidene
complexes, Mo(NAr)(CHCMe2Ph)[(S)-Biphen] (3a ; NAr ) N-2,6-i-Pr2C6H3), Mo(NArCl)-
(CHCMe3)[(S)-Biphen] (3b; NArCl ) N-2,6-Cl2C6H3), Mo(NAd)(CHCMe2Ph)[(S)-Biphen] (3c;
NAd ) N-1-adamantyl), Mo(NAr)(CHCMe2Ph)[(R)-Benz2Bitet] (3d ), and Mo(NArCl)(CHCMe2-
Ph)[(R)-Benz2Bitet] (3e). (See text for a description of Biphen and Benz2Bitet.) Under a variety
of conditions and for various combinations of imido, alkylidene, and diolate ligands, we have
observed R-substituted molybdacyclobutanes Mo(NR)(*CH2*CH2CHCMe2Ph)[diolate] (*C )
13C), unsubstituted molybdacyclobutanes Mo(NR)(*CH2*CH2*CH2)[diolate], olefin complexes
Mo(NR)(*CH2dCHR)[diolate] (R ) H or CMe2Ph), molybdacyclopentane complexes Mo(NR)-
(*CH2*CH2*CH2*CH2)[diolate], and base-free methylene complexes Mo(NR)(*CH2)[diolate].
We also have crystallographically characterized a molybdenum ethylene complex derived
from a biphenolate complex, Mo(N-2,6-Cl2C6H3)(CH2dCH2)[rac-Biphen](Et2O).
In tr od u ction
recently of tungsten,16 that contain an enantiomerically
pure biphenolate or binaphtholate ligand. Often at least
one of the olefins in a metathesis reaction is a terminal
olefin, in which case ethylene usually is generated
during the reaction. During our studies we have found
that the course and extent of some asymmetric meta-
thesis reactions depend on the precise reaction condi-
tions, namely whether ethylene is allowed to accumulate
in the reaction vessel, or is allowed to escape. To
understand what species are formed in the presence of
ethylene, how they decompose, and whether a pathway
back to an alkylidene can be devised, we have turned
to studies that involve the addition of 13C-labeled
ethylene to neopentylidene or neophylidene complexes
of the type Mo(NR)(CHR′)[diolate], where the diolate is
a chiral binaphtholate or biphenolate.
The first investigations involved complexes 1a -c.6
Upon reaction with ethylene these systems quickly
yielded unsubstituted trigonal bipyramidal molybdacy-
clobutane complexes through the reaction of unobserved
intermediate methylene complexes with ethylene (cf.
Scheme 1). These molybdacyclobutane species decom-
posed over time to yield some propylene through â-hy-
dride rearrangement of unsubstituted molybdacyclo-
butane complexes. A deficiency of ethylene led to more
rapid decomposition of molybdacyclobutane complexes
as a consequence of bimolecular decomposition of
In the last several years we have been interested in
asymmetric olefin metathesis reactions1 catalyzed by
imido alkylidene complexes of molybdenum,2-15 or more
#
Massachusetts Institute of Technology.
§ Present address: Institut fu¨r Organische Chemie, Universita¨t
Erlangen Henkestrasse 42, D-91054 Erlangen, Germany.
‡ Merkert Chemistry Center.
(1) Schrock, R. R.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2003, 42,
4592.
(2) Aeilts, S. L.; Cefalo, D. R.; Bonitatebus, P. J ., J r.; Houser, J . H.;
Hoveyda, A. H.; Schrock, R. R. Angew. Chem., Int. Ed. 2001, 40, 1452.
(3) Hoveyda, A. H.; Schrock, R. R. Chem. Eur. J . 2001, 7, 945.
(4) Cefalo, D. R.; Kiely, A. F.; Wuchrer, M.; J amieson, J . Y.; Schrock,
R. R.; Hoveyda, A. H. J . Am. Chem. Soc. 2001, 123, 3139.
(5) La, D. S.; Sattely, E. S.; Ford, J . G.; Schrock, R. R.; Hoveyda, A.
H. J . Am. Chem. Soc. 2001, 123, 7767.
(6) Tsang, W. C. P.; Schrock, R. R.; Hoveyda, A. H. Organometallics
2001, 20, 5658.
(7) Hultzsch, K. C.; Bonitatebus, P. J ., J r.; J ernelius, J .; Schrock,
R. R.; Hoveyda, A. H. Organometallics 2001, 20, 4705.
(8) Schrock, R. R.; J amieson, J . Y.; Dolman, S. J .; Miller, S. A.;
Bonitatebus, P. J ., J r.; Hoveyda, A. H. Organometallics 2002, 21, 409.
(9) Dolman, S. J .; Sattely, E. S.; Hoveyda, A. H.; Schrock, R. R. J .
Am. Chem. Soc. 2002, 124, 6991.
(10) Kiely, A. F.; J ernelius, J . A.; Schrock, R. R.; Hoveyda, A. H. J .
Am. Chem. Soc. 2002, 124, 2868.
(11) Hultzsch, K. C.; J ernelius, J . A.; Hoveyda, A. H.; Schrock, R.
R. Angew. Chem., Int. Ed. 2002, 589.
(12) Tsang, W. C. P.; J ernelius, J . A.; Cortez, G. A.; Weatherhead,
G. S.; Schrock, R. R.; Hoveyda, A. H. J . Am. Chem. Soc. 2003, 125,
2591.
(13) Zhu, S. S.; Cefalo, D. R.; La, D. S.; J amieson, J . Y.; Davis, W.
M.; Hoveyda, A. H.; Schrock, R. R. J . Am. Chem. Soc. 1999, 121, 8251.
(14) Alexander, J . B.; La, D. S.; Cefalo, D. R.; Hoveyda, A. H.;
Schrock, R. R. J . Am. Chem. Soc. 1998, 120, 4041.
(15) Alexander, J . B.; Schrock, R. R.; Davis, W. M.; Hultzsch, K. C.;
Hoveyda, A. H.; Houser, J . H. Organometallics 2000, 19, 3700.
(16) Tsang, W. C. P.; Hultzsch, K. C.; Alexander, J . B.; Bonitatebus,
P. J .; Schrock, R. R.; Hoveyda, A. H. J . Am. Chem. Soc. 2003, 125,
2652.
10.1021/om030636+ CCC: $27.50 © 2004 American Chemical Society
Publication on Web 03/23/2004

1998 Organometallics, Vol. 23, No. 9, 2004
Tsang et al.
methylene complexes. But â-hydride rearrangement of
the molybdacyclobutane also seemed to be promoted by
ethylene. Therefore it appeared that too little or too
much ethylene could lead to more rapid catalyst decom-
position to Mo(IV) species.
to be prepared,14,15 while 3b is a relatively recent
addition to the collection of complexes in this class.8
Complex 3c is the only example of a metathesis catalyst
in this family of imido alkylidene complexes that
contains an alkylimido ligand.12 The adamantylimido
catalyst offers reactivity and selectivity profiles in
asymmetric olefin metathesis that have not been ob-
served with any previous arylimido catalyst. The unique
reactivity of 3c is believed to be the result of reactions
proceeding exclusively through the syn isomer, in which
the alkylidene substituent points toward the imido
group, as opposed to the anti isomer, in which the
alkylidene substituent points away from the imido
group, as shown for a generic complex in eq 1. When
the alkoxides are biphenolates or binaphtholates these
isomers interconvert relatively rapidly (∼100 s-1) at
room temperature.15,17 Complexes 3d and 3e are two
members of yet a third category of useful catalysts that
contain hydrogenated binaphtholate-based ligands.2 We
report here that the biphenolate compounds 3a -e react
with 13CH2d13CH2 to give observable metalacyclobutane
complexes, base-free molybdenum methylene complexes,
olefin complexes, and molybdacyclopentane complexes.
We also report isolation of the first ethylene complex of
a bis(alkoxide)molybdenum imido complex and an X-ray
study of it.
The second investigation involved complexes 2a and
2b.16 Here reactions involving 13C-labeled ethylene
allowed observation of an unsubstituted tungsta-
cyclobutane complex, an ethylene complex, an unsub-
stituted tungstacyclopentane complex, and a dimeric
form of a methylene complex. This was the first time
that a methylene species had been observed that did
not contain a stabilizing base in systems of this general
type, although the methylene ligand itself could be said
to be a base, as the dimer contained asymmetrically
bridging methylenes coupled to two tungsten centers.
In this paper we broaden our studies of this general
type to nonbinaphtholate complexes of molybdenum
through an exploration of reactions that involve the
racemic or [(S)-Biphen]2- complexes 3a , 3b, and 3c, and
the [(R)-Benz2Bitet]2- complexes 3d and 3e. Complex
3a was the first asymmetric catalyst of this general type
Resu lts
Rea ction s of 3d or 3e w ith 13C-La beled Eth ylen e.
Experiments typically consisted of addition of one, two,
several, or many equivalents of 13C-labeled ethylene to
an alkylidene complex in toluene-d8 and following the
reaction progress by 13C NMR over time. Addition of 2
equiv of 13C2H4 to Mo(NAr)(CHCMe2Ph)[Benz2Bitet]
Sch em e 1. F or m a tion of th e In itia l
Molybd a cyclobu ta n e 4d a n d Its Tr a n sfor m a tion
in to Meth ylen e Com p lex 5d a n d Un su bstitu ted
Molybd a cyclobu ta n e 6d (*C ) 13C)
(3d , Ar ) 2,6-i-Pr2C6H3, as
a THF adduct) led
to formation of the first metathesis product
(13CH2dCHCMe2Ph), and a mixture of three molybde-
num complexes (Schemes 1 and 2), according to the 13C
NMR spectra of reaction mixtures. (For convenience the
configuration of the ligand in each enantiomerically
pure complex will not be stated explicitly in formulas
in the text.) The first of these that we discuss is the
unsubstituted molybdacyclobutane complex 6d (Scheme
1). For 6d two very broad resonances are observed at
∼96 and ∼4 ppm at 20 °C (Figure 1). Upon cooling the
sample from 20 to -40 °C, the downfield resonance
sharpened into two doublets at 98.68 ppm (1J CC ) 14.8
1
Hz, J CH ) 158.6 Hz) and 96.44 ppm (1J CC ) 13.8 Hz,
1J CH ) 156.3 Hz), while the upfield resonance sharpened
into a pseudotriplet at 0.45 ppm (1J CC ) 14.8, 13.8 Hz,
1J CH ) 153.4 Hz). On the basis of chemical shift and
coupling values in analogous binaphtholate molybda-
cyclobutane complexes6 and other tungsten-based meta-
(17) Feldman, J .; Schrock, R. R. Prog. Inorg. Chem. 1991, 39, 1.

3CH2d13CH2 and Imido Alkylidene Complex Reactions
Organometallics, Vol. 23, No. 9, 2004 1999
Ta ble 1. Ch em ica l Sh ift a n d Cou p lin g Va lu es for Un su bstitu ted Molybd en u m Meta la cyclobu ta n e
Com p lexesa
13C δ (ppm)
1H δ (ppm)
1J CC (Hz)
1J CH (Hz)
Mo(NAr)(13C3H6)[Benz2Bitet]
98.68 (98.55c)
96.44 (96.41c)
0.45 (0.63c)
4.80,c 4.12c
4.40,c 4.40c
0.22,c -0.31c
4.50,c 4.20c
4.52,c 4.11c
0.17,c -0.25c
2.75, 1.19
14.83
13.82
14.83, 13.82
14.68
13.39
158.6
156.3
153.4
160.9
160.0
153.7
125.6
138.0
129.7
(6d , TBP)b
Mo(NArCl)(13C3H6)[Benz2Bitet]
100.59 (100.05c)
98.90 (98.93c)
0.40 (0.53c)
(6e, TBP)b
14.68, 13.39
Mo(NAd)(13C3H6)[Biphen]
(6c, SP)
42.38 (42.16d)
37.72 (35.80d)
29.75 (31.23d)
31.1d
2.62, 1.03
3.55, 2.77
33.5d
32.1d,e
a
All values are reported at 20 °C in toluene-d8 unless otherwise stated; Ar ) 2,6-i-Pr2C6H3, ArCl ) 2,6-Cl2C6H3, Ad ) 1-adamantyl.
Data are reported at -40 °C. c Data are reported at -20 °C. Data are reported at -80 °C. e The average J CC value was reported due
to the breadth of the triplet resonance.
b
d
1
Sch em e 2. Rea r r a n gem en t of Com p lex 4d by
â-Hyd r id e Elim in a tion (*C ) 13C)
F igu r e 1. Variable-temperature 13C NMR (125 MHz)
spectra of Mo(NAr)(13CH213CH213CH2) [Benz2Bitet] (6d ) in
toluene-d8. (Units are ppm; all temperatures are °C; TBP
) trigonal bipyramidal; SP ) square pyramidal.)
lacyclobutane complexes,16,17 and extensive multidimen-
sional NMR connectivity information, we assign the two
downfield resonances to the R-carbons, and the upfield
resonance to the â-carbon of an unsubstituted trigonal-
bipyramidal molybdacyclobutane complex, Mo(NAr)-
(13CH213CH213CH2)[Benz2Bitet] (TBP-6d , Scheme 1 and
Figure 1; NMR data are summarized in Table 1). At -40
°C, complex 6d is entirely in its trigonal-bipyramidal
(TBP) form. As the sample is warmed we propose that
two things happen: the amount of a square-pyramidal
form (SP-6d ) increases and SP-6d and TBP-6d begin
to interconvert before any significant amount of SP-6d ,
which we would expect to exhibit R- and â-carbon
resonances in the range of 20-50 ppm,17 is formed.
Interconversion of SP-6d and TBP-6d takes place
without loss of ethylene on the basis of the fact that the
resonance for free ethylene (at 123 ppm) remains sharp
between -40 and 20 °C. At 20 °C enough SP-6d is
present to lead to broadening of the TBP-6d resonances
and some shifting of them toward the 20-50 ppm region
where both R- and â-carbon resonances would be found
for SP-6d . These observations are similar to those
observed in a tungsten biphenolate system,16 although
interconversion of fluxional isomers begins at much
lower temperatures (-60 °C) in the tungsten system;
consequently three relatively sharp weighted average
resonances (for two R-carbons and one â-carbon) could
be observed at 20 °C. Again ethylene loss is not part of
the equilibration process.
A second species that can be identified after addition
of 2 equiv of 13C2H4 to 3d is the initial molybdacyclobu-
tane complex, 4d (Scheme 1). Compound 4d is charac-
terized by two sets of sharp doublets at 48.4 and 44.2
ppm (Scheme 1 and Figure 2; see Table 2 for NMR
details). The chemical shift values of the R- and â-carbon
atoms in 4d are more characteristic of a SP molybda-
cyclobutane than a TBP isomer.17
Complex 4d disappears over a period of 36 h to give
a third complex with sharp doublets at 66.4 and 58.6
ppm (Figure 2). On the basis of the chemical shift and
coupling values,18-20 and by comparison with other
ethylene complexes to be described later, including a
crystallographically characterized species, we propose
that the third complex is a molybdenum(IV) ethylene
adduct, 7d (Scheme 2; see Table 3 for all NMR data).
Since no propylene was observed 7d did not form by
rearrangement of 6d to give a propylene complex
followed by exchange of propylene for ethylene. On the
other hand, the set of doublets at 121.5 ppm (1J CC
)
1
43.5 Hz, J CH ) 149.4 Hz) and 18.5 ppm (1J CC ) 43.5
Hz, 1J CH ) 125.6 Hz) suggested that trans-13CH313CHd
CHCMe2Ph (Scheme 2) had formed. (The correlated
(18) Cameron, T. M.; Ortiz, C. G.; Ghiviriga, I.; Abboud, K. A.;
Boncella, J . M. Organometallics 2001, 20, 2032.
(19) Ortiz, C. G.; Abboud, K. A.; Boncella, J . M. Organometallics
1999, 18, 4253.
(20) Robbins, J .; Bazan, G. C.; Murdzek, J . S.; O’Regan, M. B.;
Schrock, R. R. Organometallics 1991, 10, 2902.

2000 Organometallics, Vol. 23, No. 9, 2004
Tsang et al.
Ta ble 4. Ch em ica l Sh ift a n d Cou p lin g Va lu es for
Su bstitu ted Olefin Com p lexesa
13C δ
(ppm)
1H δ
(ppm)
1J CH
(Hz)
Mo(NAr)(13CH2dCHR)-
[Biphen] (8a )
58.2
3.20, 2.06 158, 148
Mo(NAd)(13CH2dCHR)-
[Biphen] (8c)
58.2
55.4
3.31, 2.26 157, 148
2.76, 2.33 155, 152
Mo(NAr)(13CH2dCHR)-
[Benz2Bitet] (8d )
65.4 (65.2b) 3.36, 2.46 159,b 153b
59.1
56.5
2.86, 2.38 161, 152
3.22, 1.71 159, 146
3.06, 2.57 155
Mo(NArCl)(13CH2dCHCMe3) 64.1
[Benz2Bitet] (8e)
55.6
54.9
2.49, 2.19 154
3.60, 1.99 159, 146
a
All values are reported at 20 °C in C6D6 unless otherwise
stated; Ar ) 2,6-i-Pr2C6H3, ArCl ) 2,6-Cl 2C6H3, Ad ) 1-adamantyl,
b
R ) CMe2Ph. Data are reported at 5 °C in C6D6.
via a â-hydride rearrangement process, one possible
version of which is shown in Scheme 2. Approximately
90% of 4d decomposes to give 13CH2dCHCMe2Ph and
5d (Scheme 1); 5d can be observed under some condi-
tions, as explained in a later section. Compound 5d then
either is captured by ethylene to give 6d or decomposes
bimolecularly to give 7d and possibly other unidentified
products.
F igu r e 2. Expanded 13C NMR (125 MHz) spectra of a
sample that contains the initial molybdacyclobutane 4d
and the ethylene adduct 7d . (Units are ppm.)
Addition of only one equivalent of 13C2H4 to 3d led to
formation of the initial molybdacyclobutane 4d , the
ethylene adduct 7d , and three diastereomers of the
substituted olefin adduct, Mo(NAr)(13CH2dCHCMe2Ph)-
[Benz2Bitet] (8d , eq 2; see Table 4 for NMR data). Three
Ta ble 2. Ch em ica l Sh ift a n d Cou p lin g Va lu es for
In itia l Molybd a cyclobu ta n e Com p lexesa
13C δ
(ppm)
1H δ
(ppm)
1J CC 1J CH
(Hz) (Hz)
Mo(NAr)(13CH213CH2CHR)-
[Benz2Bitet] (4d )
48.4
44.2
48.9
45.0
45.0
40.8
3.08, 1.14 10.94 137
2.66, 0.89 10.94 140
2.93, 0.74 11.66 138
2.78, 0.82 11.66 138
2.74, 0.89 11.67 138
2.74, 0.73 11.67 140
Mo(NAr)(13CH213CH2CHR)-
[Biphen] (4a )
Mo(NAd) (13CH213CH2CHR)-
[Biphen] (4c)
a
All values are reported at 20 °C in C6D6; Ar ) 2,6-i-Pr2C6H3,
Ad ) 1-adamantyl, R ) CMe2Ph.
diastereomers of 8d out of the four possible give rise to
three singlets at 65.4, 59.1, and 56.5 ppm in the 13C
NMR spectrum. Coupling values in 8d are similar to
those observed in ethylene complexes (Table 3). Under
these conditions (1 equiv of ethylene) some of the 4d
that is formed decomposes to give either 13CH2dCHCMe2-
Ph and 5d (Scheme 1) or 13CH313CHdCHCMe2Ph
(Scheme 2). Since 13CH2dCHCMe2Ph remains in
solution and the system is starved for ethylene,
13CH2dCHCMe2Ph, the next most strongly binding
olefin present, eventually replaces ethylene (eq 2).
Propylene, the product of rearrangement of 6d , was not
observed in this experiment. In view of the observation
of unsubstituted molybdacyclopentane complexes (to be
described later), compounds 7d and 8d almost certainly
interconvert in reactions that are bimolecular in Mo and
olefin, and that may or may not involve molybdacyclo-
pentane complexes as intermediates.
Ta ble 3. Ch em ica l Sh ift a n d Cou p lin g Va lu es for
Molybd en u m Eth ylen e Com p lexesa
13C δ
(ppm)
1H δ
(ppm)
1J CC
(Hz)
1J CH
(Hz)
Mo(NAr)(13CH2d13CH2)-
[Benz2Bitet] (7d )
66.4
58.6
66.9
60.0
61.2
60.9
65.0
61.4
61.5
59.2
3.11, 2.07 38.6 158.1
2.72, 0.93 38.6 154.4
3.47, 2.14 38.8 159.3
3.12, 1.12 38.8 155.4
3.15, 2.30 37.1 156.0
3.28, 2.30 37.1 156.1
3.82, 3.03 39.7 158.2
3.52, 2.85 39.7 155.8
3.33, 2.41 37.4 154.8
3.33, 2.41 37.4 154.3
Mo(NArCl)(13CH2d13CH2)-
[Benz2Bitet] (7e)
Mo(NAr)(13CH2d13CH2)-
[Biphen] (7a )
Mo(NArCl)(13CH2d13CH2)-
[Biphen](THF) (7b-THF )
Mo(NAd)(13CH2d13CH2)-
[Biphen] (7c)b
a
All values are reported at 20 °C in C6D6 unless otherwise
stated; Ar ) 2,6-i-Pr2C6H3, ArCl ) 2,6-Cl2C6H3, Ad ) 1-adamantyl.
Solvent is toluene-d8.
b
proton for the carbon at 121.5 ppm was located at 5.4
ppm, suggesting that the olefin was internal as opposed
to terminal. The other possible rearrangement product
from 4d , 13CH2d13CHCH2CMe2Ph, was not observed.)
Quantitative analyses, via GLC/FID and GLC/MS, of
the vacuum transferred organic fractions suggested that
the ratio of the first metathesis product (13CH2dCHCMe2-
Ph, Scheme 1) to trans-13CH313CHdCHCMe2Ph (Scheme
2) was 8.7:1. Therefore only ∼10% of 4d decomposes to
give 7d and trans-13CH313CHdCHCMe2Ph, presumably
Addition of 5 equiv of 13C2H4 to 3d immediately led
to a mixture of 6d and 7d . Propylene and what appears
1
1
to be cyclopropane (13C δ -2.4, H δ 0.14, J CH ) 161
Hz)21,22 were both observed and quantitated by 1H NMR
integration. Propylene was observed in ∼16% yield,
while cyclopropane amounted to <1%. The unsubsti-
tuted molybdacyclopentane complex Mo(NAr)(13CH2-
13CH213CH213CH2)[Benz2Bitet] (9d ) was observed in 30%
yield after 36 h. A small amount of 1-butene (∼8%) was

3CH2d13CH2 and Imido Alkylidene Complex Reactions
Organometallics, Vol. 23, No. 9, 2004 2001
of ethylene is consistent with replacement of the olefin
in 8e with ethylene in the presence of sufficient ethyl-
ene.
R ea ct ion s of 3a , 3b , or 3c w it h 13C-La b eled
Eth ylen e. When a stoichiometric amount of 13C2H4 was
added to 3a , two doublets were observed at 48.9 and
45.0 ppm in the 13C NMR spectrum (Figure 3). These
doublets were ascribed to the initial molybdacyclobutane
complex, Mo(NAr)(13CH213CH2CHCMe2Ph)[Biphen] (4a),
on the basis of the similarity of chemical shift and
coupling values with those for 4d (Table 2). Complex
4a disappeared over a period of 2 h to give a mixture of
the ethylene adduct, Mo(NAr)(13CH2d13CH2)[Biphen]
(7a ), and one diastereomer of olefin complex, Mo(NAr)-
(13CH2dCHCMe2Ph)[Biphen] (8a ; Figure 3); unreacted
3a (16%) was still observed by 1H NMR integration. The
singlet at 58.2 (1J CH ) 158, 148 Hz) ppm in the 13C NMR
spectrum was attributed to 8a , and a set of overlapping
doublets at 61.2 and 60.9 ppm to Mo(NAr)(13CH2d13CH2)-
[Biphen] (7a , eq 3). The ethylene resonance for 7a is
F igu r e 3. Expanded 13C NMR (125 MHz) spectra of the
initial molybdacyclobutane 4a , the ethylene adduct 7a , and
the olefin complex 8a . (Units are ppm.)
also observed. Complex 9d was quantitatively converted
to 7d upon removing ethylene from solution in vacuo.
Molybdacyclopentane complexes will be described in
more detail in a later section.
an AB quartet in this case since the resonances for the
two carbons are relatively close to one another. The
intensity of the singlet for 8a gradually increased over
60 h at the expense of 7a and 4a . Olefin complex 8a
was formed in 40% yield eventually, leaving 60% of 7a .
No trace of an unsubstituted molybdacyclobutane com-
plex or propylene was observed.
Addition of 2 equiv of 13C2H4 to 3a yielded only 7a .
Traces of propylene (13C δ 134, 116, and 20 ppm) were
observed. A similar experiment was performed with
unlabeled ethylene in the presence of an internal
standard, 1,4-dimethoxybenzene. On the basis of 1H
NMR integration, we conclude that 7a was formed
quantitatively (>99%) and propylene was formed in
<2% yield. All attempts to isolate 7a failed, in part
because the ethylene ligand is replaced by the first
metathesis product (13CH2dCHCMe2Ph) upon concen-
tration of the mixture in vacuo, resulting in an equi-
librium mixture of 7a and 8a (eq 3). Similar behavior
was observed for 7d and 8d , as noted above (eq 2).
Addition of 2 equiv of 13C2H4 to 3b-THF yielded only
Mo(NArCl)(13CH2d13CH2)[Biphen](THF) (7b-THF). Com-
pound 7b-THF was formed quantitatively (according to
1H NMR integration) in all cases. Compound 7b-THF
is relatively stable in both the presence or absence of
excess ethylene. No substituted olefin complex analo-
gous to 8a or 8d was observed. Well-separated doublets
for the ethylene ligand were observed initially at 60.9
and 64.6 ppm (Figure 4a). After removing all volatile
components and dissolving the sample in fresh benzene-
d6, the doublets shifted downfield in the 13C NMR
spectrum (Figure 4b). The doublets shifted further
downfield when the process was repeated (Figure 4c),
but no further shift was observed when the process was
repeated a third time. We propose that the ethylene
An initial molybdacyclobutane complex (analogous to
4d ) was not observed when 2 equiv of 13C2H4 were added
to complex 3e. Broad resonances characteristic of
the unsubstituted molybdacyclobutane complex, Mo-
(NArCl)(13CH213CH213CH2)[Benz2Bitet] (6e), and sharp
doublets characteristic of the ethylene adduct, Mo-
(NArCl)(13CH2d13CH2)[Benz2Bitet] (7e), were observed.
The broad resonances for 6e sharpen at -40 °C to give
a pattern analogous to that shown in Figure 1 charac-
teristic of a TBP complex. Only the first metathesis
product (13CH2dCHCMe2Ph) was detected; no traces of
trans-13CH313CHdCHCMe2Ph or propylene were found.
The chemical shift and coupling values of 6e and 7e are
similar to those of 6d and 7d , respectively; all NMR data
are summarized in Tables 1 and 3.
When 6 equiv of 13C2H4 were added to 3e, propylene
was observed in ∼5% yield; no cyclopropane was de-
tected. The major product was the ethylene adduct 7e,
with the unsubstituted molybdacyclopentane complex
Mo(NArCl)(13CH213CH213CH213CH2)[Benz2Bitet] (9e)
amounting to <2%. Higher concentrations of ethylene
promoted the formation of more 9e. Molybdacyclopen-
tane complexes will be described in more detail in a later
section.
Three diastereomers of Mo(NArCl)(13CH2dCHCMe3)-
[Benz2Bitet] (8e) were observed along with ethylene
adduct 7e when 1 equiv of 13C2H4 was added to 3e.
Propylene was not observed in this experiment. Forma-
tion of almost exclusively 7e in the presence of 6 equiv
(21) Aitken, R. A.; Hodgson, P. K. G.; Morrison, J . J .; Oyewale, A.
O. J . Chem. Soc., Perkin Trans. 2002, 402.
(22) Rol, N. C.; Clague, A. D. H. Org. Magn. Reson. 1981, 16, 187.

2002 Organometallics, Vol. 23, No. 9, 2004
Tsang et al.
F igu r e 5. Thermal ellipsoid drawing of rac-7b-Et2O.
Thermal ellipsoids are displayed at the 50% probability
level. Hydrogen atoms were omitted for clarity.
relatively slowly over 36 h; a comparable intensity for
8a was reached in 4 h. Only two of the four possible
diastereomers of 8c were observed, with the resonance
for the major diastereomer found at 58.2 ppm in the 13
C
NMR spectrum and the minor diastereomer at 55.4
ppm. Chemical shift and coupling values for both
diastereomers are summarized (along with others) in
Table 4. Propylene was not observed in this experiment.
When 2 equiv of 13C2H4 were added to 3c, four broad
resonances were observed at 60.35, 42.38, 37.72, and
29.75 ppm in the 13C NMR spectrum at 20 °C. The
60.35-ppm resonance was ascribed to Mo(NAd)-
(13CH2d13CH2)[Biphen] (7c) on the basis of its chemical
shift, although its identity could not be confirmed
without further investigation as detailed in the section
on molybdacyclopentane complexes. At -80 °C, two
broad doublets and a well-defined pseudotriplet were
observed at 42.16, 35.80, and 31.23 ppm, respectively.
They were assigned to an SP-unsubstituted molybda-
cyclobutane complex, Mo(NAd)(13CH213CH213CH2)-
[Biphen] (6c, Table 1). Only traces of propylene (<2%)
were observed when 6c was fully converted to 7c, which
suggests that 6c loses ethylene to yield Mo(NAd)(13CH2)-
[Biphen], which then yields 7c upon bimolecular de-
composition and coordination of additional ethylene.
X-r a y Str u ctu r e of Mo(NAr Cl)(13CH2d13CH2)[r a c-
Bip h en ](Et2O) (r a c-7b-Et2O). It should be noted that
3b is the only species discussed here that contains a
neopentylidene ligand instead of a neophylidene ligand.
A subtle but potentially significant difference between
the outcome of reactions involving neophylidene and
neopentylidene complexes and ethylene is the volatility
of the initial metathesis products, CH2dCHCMe2Ph and
CH2dCHCMe3, respectively. The volatility of CH2d
CHCMe3 is perhaps a factor that allows 7b-THF to be
isolated from THF in a 60% yield on a 1.75-g scale by
simply removing all volatile components in vacuo. Slow
diffusion of diethyl ether into a concentrated toluene
solution of rac-7b-THF at -25 °C yielded red single
crystals of the diethyl ether adduct Mo(NArCl)-
(13CH2d13CH2)[rac-Biphen](Et2O) (rac-7b-Et2O). (Since
only 1 equiv of THF is present an excess of ether
resulted in formation and crystallization of the ether
adduct instead of the THF adduct.) An X-ray study was
carried out and revealed the structure shown in Figure
5. Crystal data can be found in Table 5, and selected
F igu r e 4. Effect of solvent coordination and ethylene
exchange on the 13C NMR spectrum of 7b-THF. (Units are
ppm.) (a) Addition of 3 equiv of 13CH2d13CH2; (b) all
volatiles removed (first cycle); (c) all volatiles removed
(second cycle); (d) addition of 3.5 equiv of diethyl ether; (e)
12
addition of 4.2 equiv of THF; (f) exposed to 1 atm of
-
CH2d12CH2.
adduct is free of THF at that point. Addition of 3.5 equiv
of diethyl ether to the sample resulted in only a slight
upfield shift (Figure 4d), whereas addition of 4.2 equiv
of THF shifted the doublets upfield to 64.03 and 59.88
ppm (Figure 4e). The labeled ethylene largely disap-
peared upon exposure of the sample to unlabeled
ethylene (Figure 4f). These results are consistent with
a facile equilibrium between 7b-THF and 7b, and ready
replacement of labeled ethylene with unlabeled ethyl-
ene, as we would expect on the basis of what was
observed as shown in eqs 2 and 3, and molybdacyclo-
pentane formation to be described in detail below.
When 1 equiv of 13C2H4 was condensed into a sample
of 3c, a mixture of a short-lived initial molybdacyclobu-
tane (4c, t1/2 ≈ 30 min), analogous to complexes 4d and
4a , and a complex characterized by a broad resonance
at 60.4 ppm in the 13C NMR spectrum was formed. As
free ethylene was essentially fully consumed in the first
30 min, the broad resonance gradually separated into
two sharp doublets at 61.5 and 59.2 ppm, which we can
ascribe to the ethylene adduct (7c). Broadening of the
resonances for 7c in the presence of ethylene and
sharpening of them as ethylene was consumed is
consistent with dynamic exchange of the coordinated
ethylene with free ethylene, again probably through an
unobservable intermediate bis(ethylene) or an observ-
able (vide infra) metalacyclopentane complex. Reso-
nances for the substituted olefin complex, Mo(NAd)-
(13CH2d13CHR)[Biphen] (8c, R ) CMe2Ph), grew in

3CH2d13CH2 and Imido Alkylidene Complex Reactions
Organometallics, Vol. 23, No. 9, 2004 2003
Ta ble 5. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
for r a c-7b-Et2Oa
empirical formula
fw
MoC36H49Cl2NO3
710.60
temp
183(2) K
wavelength
0.71073 Å
cryst syst
a
orthorhombic, Pbca
20.086(9) Å
b
17.150(7) Å
c
20.086(9) Å
R ) â ) γ
90°
vol, Z
6919(5) Å3, 8
1.364 Mg/m3
0.569 mm-1
density (calcd)
abs coeff
F(000)
2976
cryst size
θ range for data collection
index ranges
0.15 × 0.13 × 0.08
2.03-20.00°
-18 e h e 19, -9 e k e 16,
-19 e l e 18
19184
3229 [Rint ) 0.0834]
99.9%
3229/0/395
1.287
no. of reflns collected
no. of independent reflns
completeness to θ
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
1
F igu r e 6. Expanded H-13C HMQC spectrum of 3a and
Mo(NAr)(13CH2)[Biphen] (5a ).
Ta ble 7. Ch em ica l Sh ift a n d Cou p lin g Va lu es for
Molybd en u m Meth ylen e Com p lexesa
R1 ) 0.0773, wR2 ) 0.1384
R1 ) 0.0837, wR2 ) 0.1407
0.0003(4)
13C δ
(ppm)
1H δ
(ppm)
1J CH
(Hz)
extinction coeff
largest diff peak and hole
0.529 and -0.894 e‚Å-3
a
No absorption correction was applied, and the refinement
Mo(NAr)(13CH2)[Biphen]
(5a )
253.5
253.7
275.3
11.33
11.13
11.27
11.13
12.48
12.22
134
163
137
157
140
161
method was full-matrix least-squares on F2.
Mo(NAd)(13CH2)[Biphen]
(5c)
Ta ble 6. Selected Bon d Dista n ces (Å) a n d An gles
Mo(NAr)(13CH2)[Benz2Bitet]-
(THF) (5d )
(d eg) in r a c-7b-Et2O
Mo(1)-C(1)
Mo(1)-C(2)
Mo(1)-N(1)
Mo(1)-O(1)
Mo(1)-O(2)
Mo(1)-O(3)
C(1)-C(2)
2.204(9) N(1)-Mo(1)-C(1)
2.128(9) N(1)-Mo(1)-C(2)
1.723(7) N(1)-Mo(1)-O(1)
1.958(6) N(1)-Mo(1)-O(2)
2.008(6) N(1)-Mo(1)-O(3)
2.307(6) O(1)-Mo(1)-O(2)
1.400(13) O(1)-Mo(1)-O(3)
95.8(4)
97.8(4)
163.7(3)
101.3(3)
87.4(4)
90.0(2)
77.6(3)
89.2(3)
95.6(3)
154.8(2)
80.2(3)
a
All values are reported at 20 °C in C6D6; Ar ) 2,6-i-Pr2C6H3,
Ad ) 1-adamantyl.
Obser va tion of Molybd en u m Meth ylen e Com -
p lexes. Investigations involving 1H-13C HMQC experi-
ments on complexes 7a , 8a , and 4a revealed a species
with a strong 13C signal at 253.5 ppm, a chemical shift
that is typical for an alkylidene carbon in complexes of
this type.17,24,25 Two protons at 11.33 (1J CH ) 134 Hz)
and 11.13 (1J CH ) 163 Hz) ppm were found to be
through-bond coupled to the carbon giving rise to the
resonance at 253.5 ppm (Figure 6); the only other
alkylidene species observed in these experiments was
the unreacted neophylidene complex (3a ). We propose
that the new species is an unsolvated molybdenum
methylene complex, Mo(NAr)(13CH2)[Biphen] (5a ). The
1J CH coupling values (see Table 7 for NMR data) are
comparable to previously observed base adducts of imido
methylene complexes of molybdenum or tungsten.20,26-28
Unlike base adducts of tungsten methylene complexes,
which are relatively stable, 5a is short-lived and highly
reactive; only a trace amount (∼0.6%) was present
according to 1H NMR integration. The 13C resonance of
C(1)-C(2)-Mo(1) 74.1(6)
C(2)-C(1)-Mo(1) 68.2(5)
O(1)-Mo(1)-C(1)
O(1)-Mo(1)-C(2)
C(3)-N(1)-Mo(1) 169.4(7) O(2)-Mo(1)-O(3)
C(1)-Mo(1)-O(3) 84.7(3)
O(2)-Mo(1)-C(2)
C(2)-Mo(1)-O(3) 122.3(3) C(9)-C(10)-C(20)-C(15) 70.6(12)
bond distances and angles in Table 6. The complex is
essentially a trigonal bipyramid with O(2), O(3), and the
center of the ethylene ligand occupying equatorial
positions. The similarity of the bond distances between
Mo and the two ethylene carbon atoms (Mo-C(1) )
2.204(9) Å; Mo-C(2) ) 2.128(9) Å) suggests that the
ethylene is symmetrically bound, as expected. The C(1)-
C(2) bond distance (1.400(13) Å) is similar to the value
observed in a tungsten ethylene complex, W(NPh)[o-
(Me3SiN)2C6H4](PMe3)2(CH2dCH2) (1.434(6) Å), a rare
related ethylene complex of W(IV).23 The C(9)-C(10)-
C(20)-C(15) dihedral angle in the intact biphenolate
backbone is 70.6(12)°. This value is considerably smaller
than the dihedral angle observed in the parent complex
(3b-THF, 80°);8 it is also smaller than the dihedral
angles typically observed in other four-coordinate imido
5a can be detected only in H-13C HMQC experiments,
with an acquisition time of 9 ms per scan, in the first
2 h immediately after the addition of a stoichiometric
amount of 13C2H4 to rac- or (S)-3a .29 Although the
1
alkylidene complexes (which range from 78°to103° 7,8,12,15
)
or that observed in Mo(NArCl)(CHCMe3)[Biphen](THF)8
(80°), in which the imido and neopentylidene ligands
occupy equatorial positions. Complex 7b-Et2O is the
only crystallographically characterized ethylene complex
derived from a molybdenum imido bisalkoxide alky-
lidene compound.
(24) Schrock, R. R.; Murdzek, J . S.; Bazan, G. C.; Robbins, J .;
DiMare, M.; O’Regan, M. J . Am. Chem. Soc. 1990, 112, 3875.
(25) Oskam, J . H.; Fox, H. H.; Yap, K. B.; McConville, D. H.; O’Dell,
R.; Lichtenstein, B. J .; Schrock, R. R. J . Organomet. Chem. 1993, 459,
185.
(26) Schrock, R. R.; DePue, R.; Feldman, J .; Schaverien, C. J .;
Dewan, J . C.; Liu, A. H. J . Am. Chem. Soc. 1988, 110, 1423.
(27) Fox, H. H.; Lee, J .-K.; Park, L. Y.; Schrock, R. R. Organome-
tallics 1993, 12, 759.
(23) Wang, S. S.; VanderLende, D. D.; Abboud, K. A.; Boncella, J .
M. Organometallics 1998, 17, 2628.
(28) Feldman, J .; Davis, W. M.; Thomas, J . K.; Schrock, R. R.
Organometallics 1990, 9, 2535.

2004 Organometallics, Vol. 23, No. 9, 2004
Tsang et al.
Obser va tion of Un su bstitu ted Molybd a cyclo-
p en ta n e Com p lexes. In previous sections we alluded
several times to the formation of molybdacyclopentane
complexes upon reaction of ethylene complexes with
ethylene. This type of behavior has been observed when
ethylene is added to the tungsten species 2a or 2b.16
Tungsten also was found to be a catalyst for the slow
catalytic formation of 1-butene from ethylene, with
tungstacyclopentane complexes being observed during
this reaction. In this section we elaborate on the
formation of molybdacyclopentane complexes from eth-
ylene complexes and ethylene.
As noted earlier, ethylene complex 7a is formed
quantitatively when 2-4 equiv of ethylene are added
to 3a . Addition of 8 equiv of 13C2H4 to 3a produced a
mixture of 7a and a complex characterized by two sets
1
of doublets at 80.5 ppm (1J CC ) 35.4 Hz, J CH ) 129.5
1
Hz) and 79.1 ppm (1J CC ) 35.0 Hz, J CH ) 129.8 Hz),
and two sets of triplets at 40.0 ppm (1J CC ) 35.0, 33.9
1
Hz, J CH ) 125.5 Hz) and 38.4 ppm (1J CC ) 35.4, 33.9
Hz, 1J CH ) 125.6 Hz), in the 13C NMR spectrum (Figure
7). The doublets were assigned to the R-carbons, and
the triplets to the â-carbons, in the unsubstituted
molybdacyclopentane complex Mo(NAr)(13CH213CH2-
13CH213CH2)[Biphen] (9a , eq 4). These assignments are
F igu r e 7. Conversion of ethylene adduct 7a to unsubsti-
tuted molybdacyclopentane 9a in the presence of increasing
amounts of ethylene. (Units are ppm.)
concentration of 5a was only ∼0.04 times that of
unreacted 3a (16%), the intensity of the HMQC signals
for 5a were significantly stronger than those of un-
reacted 3a at 277 ppm, clearly showing that the meth-
ylene carbon in 5a is 13C-labeled (Figure 6). The low
concentration of 5a is in part a consequence of its rapid
bimolecular decomposition to yield the ethylene adduct
7a . To the best of our knowledge complex 5a is the first
solvent- and base-free molybdenum methylene complex
in the family of imido alkylidene bisalkoxide complexes
to be observed.
consistent with connectivity information obtained from
multidimensional NMR spectroscopy experiments. Pro-
pylene and cyclopropane were both observed in ∼15%
1
yield by H NMR integration. A higher concentration
Addition of a stoichiometric amount of 13C2H4 to rac-
or (S)-3c allowed a second base-free molybdenum-
methylene complex, Mo(NAd)(13CH2)[Biphen] (5c), to be
observed within the first hour. Chemical shift and
coupling values of complex 5c were very similar to those
of 5a (Table 7).
of ethylene promoted further conversion of 7a to 9a . For
example, only 6% of 9a was observed when 8 equiv of
ethylene were added to 3a (Figure 7). In the presence
of 120 equiv of ethylene, complex 9a was obtained in
61% yield and the resonances for ethylene adduct 7a
1
were significantly broadened in both H and 13C NMR
When a sample of 3d was exposed to 1 equiv of
13C2H4, a third molybdenum methylene complex, Mo-
(NAr)(13CH2)[Benz2Bitet](THF) (5d ), was observed. The
molybdenum methylene carbon was located at 275.3
ppm, and its through-bond coupled protons were ob-
served at 12.48 and 12.22 ppm. The proton resonances
were approximately 1 ppm downfield from those ob-
served in 5a or 5c, consistent with the effect of THF
coordination in many previously observed imido alky-
lidene complexes.1,17,30 Complex 5d was slightly longer
lived than base-free methylene complexes; it was still
observable after 3 h with its intensity being strongly
dependent on the initial concentration of 3d .
spectra. We attribute the broadening of the resonances
for 7a to an ethylene dependent exchange between free
and coordinated ethylene. However, since ethylene is
in large excess, no significant broadening of the ethylene
resonance was observed. Conversion of 7a to 9a was
fully reversible. Compound 7a was recovered quantita-
tively upon degassing any mixture of 7a and 9a .
Dimerization of ethylene to 1-butene via 9a was not
observed even in the presence of 120 equiv of ethylene.
In summary, the main points are that (i) the ethylene
complex (7a ) readily adds ethylene reversibly to give a
molybdacyclopentane complex (9a ); (ii) since the reso-
nances for 9a are not broadened in the presence of a
large excess of ethylene, the intermediate in the rela-
tively rapid ethylene dependent exchange between free
and coordinated ethylene is not the molybdacyclopen-
tane complex; and (iii) 1-butene is not formed readily
by a â-hydride rearrangement of the molybdacyclopen-
tane complex. Whether 9a is a square-pyramidal or a
No methylene complexes were observed when 1 equiv,
or a substoichiometric amount, of 13C2H4 was added to
3b-THF or 3e.
(29) For simplicity we will use the rac or S label for the entire
complex, even though it refers to the [Biphen]2- ligand in the complex.
(30) Schrock, R. R. Chem. Rev. 2002, 102, 145.

3CH2d13CH2 and Imido Alkylidene Complex Reactions
Organometallics, Vol. 23, No. 9, 2004 2005
13C label) that contains either ethylene or the
Ta ble 8. Ch em ica l Sh ift a n d Cou p lin g Va lu es for
Un su bstitu ted Molybd en u m Meta la cyclop en ta n e
Com p lexesa
a
relatively nonvolatile primary metathesis product,
*CH2dCHCMe2Ph, if the reaction began with
a
13C δ
(ppm)
1H δ
(ppm)
1J CC
(Hz)
1J CH
(Hz)
ModCHCMe2Ph complex. In contrast, no ethylene or
molybdacyclopentane complexes have been observed in
binaphtholate systems 1a -c. Perhaps binaphtholate
complexes are simply less stable toward irreversible
bimolecular decomposition reactions that ultimately
yield species that contain bridging imido ligands (and
no 13C label).20
Mo(NAr)(13C4H8)-
[Biphen] (9a )
80.5
79.1
40.0
38.4
81.8
69.9
41.7
38.4
84.3
73.5
40.8
37.3
94.5
76.0
41.9
38.0
4.08, 3.10
3.62, 2.86
2.66, 2.66
2.82, 2.57
3.60, 3.27
3.48, 3.03
2.85, 2.85
2.73, 2.53
3.49, 3.38
3.49, 2.85
2.86, 2.48
2.63, 2.34
3.41, 3.12
3.83, 2.91
2.97, 2.40
2.82, 2.19
35.4
35.0
35.0, 33.9
35.4, 33.9
36.0
129.5
129.8
125.5
125.6
129.5
132.8
125.0
125.2
127.6
131.9
127.0
127.0
128.9
133.8
128.2
126.8
Mo(NAd)(13C4H8)-
[Biphen] (9c)
35.4
36.0, 34.7
35.4, 34.7
36.0
Methylene complexes have always been pivotal in-
termediates in any reaction in which ethylene can be
generated. They are of course much more reactive than
any monosubstituted alkylidene complex, and therefore
were found in only barely detectable concentrations.
Methylene complexes can be sequestered through a
reaction with ethylene to form a molybdacyclobutane
complex, although decomposition of methylene com-
plexes bimolecularly to give ethylene or ethylene com-
plexes and unidentified Mo decomposition products still
appears to be rapid and competitive. Molybdenum
methylene complexes have been observed previously
only as adducts that contain PMe3 and DME,20,26-28
which are relatively stable toward bimolecular decom-
position. It should be noted that dimers of these Mo
methylene complexes are not observed, according to
chemical shifts for the methylene protons and carbons,
as was the case for racemic W methylene complexes of
this general type.16 It should be noted that enantio-
merically pure or racemic Mo complexes yield the same
result, but no dimeric W methylene complex was
observed in an analogous enantiomerically pure system.
It was proposed that the dimeric tungsten methylene
complex formed in an enantiomerically pure system
decomposed too readily to be observed, and that the
dimer formed in a racemic system was composed of
enantiomeric halves.
Mo(NAr)(13C4H8)-
[Benz2Bitet] (9d )
35.4
36.0, 33.7
35.4, 33.7
36.3
35.4
36.3, 34.6
35.4, 34.6
Mo(NArCl)(13C4H8)-
[Benz2Bitet] (9e)
a
All values are reported at 20 °C in C6D6; Ar ) 2,6-i-Pr2C6H3,
ArCl ) 2,6-Cl2C6H3, Ad ) 1-adamantyl.
trigonal-bipyramidal species cannot be determined on
the basis of the data in hand. We also have no informa-
tion about the structure of the nonmetalacyclic inter-
mediate, “Mo(NAr)(C2H4)2[Biphen]”, in the ethylene
exchange process.
No molybdacyclopentane complex was detected when
a huge excess of ethylene was added to complex 7b-THF,
presumably because THF is a much better donor than
ethylene, thus preventing coordination of a second
ethylene ligand. Addition of 8 equiv of 13C2H4 to complex
3c led to a mixture of propylene (28%), cyclopropane
(8%), ethylene adduct 7c (75%), and an unsubstituted
molybdacyclopentane complex, Mo(NAd)(13CH213CH2-
13CH213CH2)[Biphen] (9c, 24% yield). Chemical shift and
coupling values of complex 9c are similar to those of
9a (Table 8). Conversion of 7c to 9c was reversible. No
1-butene was observed.
It has been proposed previously that unsubstituted
molybdacyclobutane complexes more readily lose eth-
ylene than do unsubstituted tungstacyclobutane com-
plexes.17 That is still thought to be the case, as judged
qualitatively by the ready decomposition of several of
the molybdacyclobutane complexes reported here to
yield ethylene complexes. Unfortunately, it is difficult
at this stage to quantitate such conversions and to
compare their rates with analogous decomposition reac-
tions of tungstacyclobutane complexes. However, it does
appear that the rate of forming an ethylene complex
increases dramatically upon changing from a N-2,6-i-
Pr2C6H3 to a N-2,6-Cl2C6H3 group. Rearrangement of
molybdacyclobutanes to propylene also seems to be less
common with complexes that contain the N-2,6-Cl2C6H3
group.
As mentioned in an earlier section addition of 5 equiv
of 13C2H4 to 3d immediately led to a mixture of 6d , 7d ,
and the unsubstituted molybdacyclopentane complex
Mo(NAr)(13CH213CH213CH213CH2)[Benz2Bitet] (9d ) in
30% yield after 36 h. A small amount of 1-butene (∼8%)
was also observed. Complex 9d was quantitatively
converted to 7d upon removing ethylene from solution
in vacuo. Similarly, complex 7e was fully converted to
9e in the presence of 125 equiv of ethylene; no 1-butene
was detected. Complex 7e was recovered quantitatively
upon degassing a solution of 9e. Chemical shifts and
coupling constants for 9d and 9e are similar to those of
9a and 9c (Table 8).
Discu ssion
The studies presented here outline reactions that can
take place between molybdenum imido alkylidene com-
plexes that contain biphenolate ligands and ethylene.
The results overall are similar to those found in the
molybdenum hexafluoro-tert-butoxide system in 1991,20
and what has been found for molybdenum binaphtho-
late6 and tungsten biphenolate complexes,16 although
there are some important differences. In the presence
of ethylene the end result in the case of Mo biphenolate
complexes seems to be the generation of a complex of
Mo(IV) (plus unidentified products that do not contain
It has now been demonstrated clearly that even initial
molybdacyclobutane complexes can be observed if the
initial ethylene concentration is low. These species
appear to have square-pyramidal structures, in contrast
to the TBP structures observed for most unsubstituted
molybdacyclobutanes, the exception being adamantyl-
imido molybdacyclobutane complexes. In tungsten sys-
tems, it has been found that more electron-withdrawing
alkoxides stabilize TBP tungstacycles, while more elec-
tron-donating alkoxides stabilize SP tungstacycles.17

2006 Organometallics, Vol. 23, No. 9, 2004
Tsang et al.
Square-pyramidal tungstacycles also appear to be more
stable with respect to olefin loss than TBP tungsta-
cycles.17
also been noted in a rhenium-based system.39 It is not
known at this time whether ring contraction is a feature
of the mechanism of rearrangement of molybdacyclo-
pentane or tungstacyclopentane rings, although that
would now seem to be a more likely possibility than
simple â-hydride elimination/reductive elimination (as
in Scheme 2).
Rearrangement of molybdacyclobutane complexes
does not appear to be a significant mode of their
decomposition, although small amounts of rearrange-
ment products of both the initial molybdacyclobutane
and unsubstituted molybdacyclobutane complexes have
been observed in several systems reported here. Prod-
ucts of rearrangement of molybdacyclobutanes have
been observed in other molybdenum systems that
contain species of this general type.20 Formation of what
appears to be cyclopropane has never been observed
before in systems of this type, or any high oxidation
state alkylidene system to the best of our knowledge.
Cyclopropane would seem to be formed most likely
through reductive elimination in an unsubstituted mo-
lybdacyclobutane complex. The fact that the amount of
cyclopropane that is formed is small is consistent with
the predominant form of metalacyclobutane decomposi-
tion being either rearrangement or metathesis. In
general, formation of cyclopropanes in reactions between
carbene complexes and olefins predominates as metals
become more electron rich. For example, Casey showed
that (CO)5WdCPh2 would react with olefins to yield
both cyclopropanes and metathesis products in substo-
ichiometric quantities31 and that (CO)5WdCHPh would
react with olefins at -78 °C to yield cyclopropanes
exclusively.32 Carbenoid complexes of later transition
metals such as copper yield only cyclopropanes upon
reaction with olefins.33
In the molybdenum systems explored here it appears
that unsubstituted molybdacyclopentane complexes lose
ethylene more readily than tungstacyclopentane com-
plexes, i.e., the equilibrium between ethylene and
metalacyclopentane complexes lies more toward ethyl-
ene complexes when the metal is molybdenum. Catalytic
dimerization of ethylene to 1-butene also appears to be
relatively slow in the molybdenum systems explored
here, either because metalacyclopentane complexes are
less easily formed, or because they rearrange more
slowly, or (most likely) both. In contrast, slow dimer-
ization of ethylene to 1-butene was found in two related
tungsten systems.16 Tungsten systems appear to be less
prone to lose ethylene from a tungstacyclopentane to
give an ethylene complex and more prone toward
rearrangement by a â-hydride rearrangement-based
process. The tendency of W ethylene complexes to react
with ethylene to form tungstacyclopentanes is one
reason tungsten ethylene complexes have not been
isolated. The lower tendency for molybdenum ethylene
complexes to form molybdacyclopentanes allows a mo-
lybdenum ethylene complex to be isolated in one in-
stance. It should be noted that rearrangement of
tantalacyclopentane complexes to olefins involves a
contraction of the TaC4 ring to a TaC3 ring, the so-called
“ring contraction” mechanism.34-38 Ring contraction has
Many of the findings reported here are similar to
those observed in molybdenum and tungsten imido
alkylidene systems that contain the [o-(Me3SiN)2C6H4]2-
supporting ligand.18,19,23 Both tungsten(VI)23 and mo-
lybdenum(VI)18,19 phenylimido dineopentyl and neopen-
tylidene complexes (as PMe3 adducts) were prepared.
The tungsten dineopentyl complex will react slowly with
ethylene at temperatures above 70 °C to give neopen-
tane (from the initial R-hydrogen abstraction reaction),
tert-butylethylene (from the reaction of the intermediate
neopentylidene complex with ethylene), and a structur-
ally characterized (distorted square pyramidal) tung-
stacyclopentane complex. In reactions between W(NPh)-
[o-(Me3SiN)2C6H4](CH-t-Bu)(PMe3) and ethylene the
initial R tert-butyl-substituted tungstacyclobutane com-
plex was observed in solution, as were the unsubstituted
tungstacyclobutane and ethylene complexes. All were
converted into the tungstacyclopentane complex with
time in the presence of ethylene. In [o-(Me3SiN)2C6H4]W
complexes, the tungstacyclobutane did not appear to
rearrange to propylene and the tungstacyclopentane
complex did not appear to rearrange to 1-butene.
We conclude that reactions between ethylene and
enantiomerically pure biphenolate and binaphtholate
complexes proceed through intermediates that have
been proposed in other high oxidation state alkylidene
systems, although these intermediates cannot be iso-
lated and the precise outcome depends on many factors.
Unfortunately, however, precisely how actual meta-
thesis reactions in which ethylene is formed can alter
the outcome of those reactions still cannot be predicted
at this time. Nevertheless, it is clear that the end result
in the presence of ethylene is reduction of the metal to
Mo(IV), either as a simple ethylene complex, or in the
form of some other as yet unidentified species such as
a dimer that contains bridging imido ligands.20 In cases
where the yield of the ethylene complex is high we are
interested in exploring ways of regenerating a molyb-
denum alkylidene complex, e.g., by transferring an
alkylidene fragment from an ylide.40-42
Exp er im en ta l Section
Gen er a l. All reactions were conducted in oven or flame-
dried glassware under an inert atmosphere of nitrogen or
argon. Commercially available chemicals were obtained from
Aldrich Co. or Lancaster Synthesis. Liquid reagents were
distilled from CaH2 under nitrogen and stored over molecular
sieves (4 Å) before use. Ether, pentane, toluene, benzene, and
(37) Schrock, R. R.; McLain, S. J .; Sancho, J . Pure Appl. Chem. 1980,
52, 729.
(31) Casey, C. P.; Burkhardt, T. J . J . Am. Chem. Soc. 1974, 96, 7808.
(32) Casey, C. P.; Polichnowski, S. W.; Shusterman, A. J .; J ones, C.
R. J . Am. Chem. Soc. 1979, 101, 7282.
(33) Kirmse, W. Angew. Chem., Int. Ed. 2003, 42, 1088.
(34) McLain, S. J .; Schrock, R. R. J . Am. Chem. Soc. 1978, 100, 1315.
(35) McLain, S. J .; Wood, C. D.; Schrock, R. R. J . Am. Chem. Soc.
1979, 101, 4558.
(38) Smith, S.; McLain, S. J .; Schrock, R. R. J . Organomet. Chem.
1980, 202, 269.
(39) Yang, G. K.; Bergman, R. G. Organometallics 1985, 4, 129.
(40) Sharp, P. R.; Schrock, R. R. J . Organomet. Chem. 1979, 171,
43.
(41) J ohnson, L. K.; Frey, M.; Ulibarri, T. A.; Virgil, S. C.; Grubbs,
R. H.; Ziller, J . W. J . Am. Chem. Soc. 1993, 115, 8167.
(42) J ohnson, L. K.; Virgil, S. C.; Grubbs, R. H.; Ziller, J . W. J . Am.
Chem. Soc. 1990, 112, 5384.
(36) McLain, S. J .; Sancho, J .; Schrock, R. R. J . Am. Chem. Soc.
1979, 101, 5451.

3CH2d13CH2 and Imido Alkylidene Complex Reactions
Organometallics, Vol. 23, No. 9, 2004 2007
THF were dried with columns of activated alumina. Dimethox-
yethane (DME) was distilled from a purple solution of sodium
benzophenone ketyl. Dichloromethane was distilled from
calcium hydride. Benzene-d6 and toluene-d8 were vacuum
transferred from calcium hydride or sodium benzophenone
ketyl prior to use. All NMR spectra were collected on a Varian
Inova 500 spectrometer unless otherwise stated. Microanalyses
were performed by Kolbe Microanalytical Laboratories (Mu¨l-
heim an der Ruhr, Germany). Mo(NAr)(CHCMe2Ph)[(S)-Bi-
phen] (3a ),15 Mo(NArCl)(CHCMe3)[(S)-Biphen] (3b),8 Mo(NAd)-
(CHCMe2Ph)[(S)-Biphen] (3c),12 Mo(NAr)(CHCMe2Ph)[(R)-
Benz2Bitet] (3d ),8 and Mo(NArCl)(CHCMe2Ph)[(R)-Benz2Bitet]
(3e)8 were prepared as described in the literature.
Rep r esen ta tive P r oced u r e for Rea ction s In volvin g
Eth ylen e. A benzene-d6 solution (0.6 mL) of Mo(NAr)(CHCMe2-
Ph)[Benz2Bitet](THF) (20 mg) was loaded in a J -Young tube,
which was then sealed and attached to a gas transfer setup
equipped with a 760-Torr gauge. The volume of the setup was
calibrated immediately before the experiment (23.73 mL). The
sample was degassed by freeze-pump-thaw and re-sealed
with the J -Young cap. Ethylene (70 Torr, 5 equiv) was passed
into the setup by slowly opening the regulator attached to the
ethylene tank. All the ethylene in the setup was condensed
into the frozen sample (with liquid nitrogen) by opening the
J -Young seal. The sample was then brought to the spectroscopy
facility under liquid nitrogen before it was thawed and
immediately inserted into the NMR probe. Subsequent re-
moval of excess ethylene was achieved by freeze-pump-thaw
using a heavily salted ice bath.
131.5, 126.9, 126.6, 124.9, 122.8, 61.2, 60.9, 35.4, 35.3, 30.7,
29.8, 29.1, 24.0, 23.6, 20.6, 20.2, 16.8, 16.7.
Mo(N-2,6-Cl2C6H3)(CH2dCH2)[r a c-Biph en ](Solven t) (r a c-
7b-Solven t, Solven t ) THF or Eth er ). To a 1-L Teflon-
sealed bomb was added Mo(N-2,6-Cl2C6H3)(CHCMe3)[rac-
Biphen] (2.85 g, 3.80 mmol) and toluene (50 mL). The solution
was degassed three times by freeze-pump-thaw and refilled
with ethylene (1 atm). The container was sealed and the
mixture was stirred for 1 h. The toluene solution was concen-
trated in vacuo to approximately 25 mL and a dark orange
powder was filtered through a fine frit. The isolated powder
was recrystallized from THF to give bright orange Mo(N-2,6-
Cl2C6H3)(CH2dCH2)[rac-Biphen](THF) in a yield of 1.76 g
(59%): 1H NMR (500 MHz, C6D6) δ 7.29 (s, 1, Aryl), 7.20 (s, 1,
Aryl), 6.65 (d, 2, Aryl), 6.07 (t, 1, Aryl), 3.82 (m, 1, CH2CH2),
3.65 (m, 4, OCH2), 3.52 (m, 1, CH2CH2), 3.03 (m, 1, CH2CH2),
2.85 (m, 1, CH2CH2), 2.12 (s, 3, ArCH3), 2.02 (s, 3, ArCH3),
1.90 (s, 3, ArCH3), 1.57 (s, 9, C(CH3)3), 1.55 (s, 9, C(CH3)3),
1.50 (s, 3, ArCH3), 1.34 (m, 4, OCH2CH2); 13C NMR (125 MHz,
CD2Cl2) δ 162.66, 152.74, 149.58, 141.18, 137.16, 134.31,
133.75, 133.32, 132.10, 131.24, 129.27, 128.08, 127.69, 127.10,
126.48, 125.30, 72.19, 63.70, 60.77, 35.80, 34.54, 30.68, 30.23,
29.85, 26.11, 20.43, 20.15, 17.06, 16.48. Anal. Calcd for
MoC32H39NO2Cl2: C, 60.38; H, 6.18; N, 2.20; Cl, 11.14.
Found: C, 60.26; H, 6.25; N, 2.04; Cl, 11.22.
Single crystals of rac-7b-Et2O suitable for X-ray diffraction
were obtained by layering diethyl ether over a concentrated
toluene solution of rac-7b-THF.
At t em p t t o Isola t e Mo(N-2,6-i-P r 2C6H 3)(CH 2dCH 2)-
[Bip h en ] (7a ). A benzene-d6 solution (0.5 mL) of Mo(NAr)-
(CHCMe2Ph)[Biphen] (50 mg, 0.66 mmol) was degassed by
freeze-pump-thaw and exposed to 1 atm of ethylene for 1 h.
A deep red solution was obtained that was shown by integra-
Ack n ow led gm en t. We thank the National Science
Foundation (CHE-0138495 to R.R.S.) for supporting
this research. K.C.H. thanks the Alexander von Hum-
boldt Foundation for partial support through a Feodor
Lynen Research Fellowship. We also gratefully acknowl-
edge the generous assistance of Dr. J effrey H. Simpson
in multidimensional NMR experiments.
tion versus an internal standard to contain
a virtually
quantitative yield of 7a . All volatiles were removed in vacuo
and the oily residue was redissolved in C6D6 (0.5 mL).
Attempts to isolate the complex, even on gram scales, have
not been successful: 1H NMR (400 MHz, C6D6) δ 7.27 (s, 1,
Aryl), 7.24 (s, 1, Aryl), 6.9 (m, 3, Aryl), 3.28 (m, 1, CH2), 3.15
(m, 1, CH2), 3.12 (sept, 2, CHMe2), 2.30 (m, 2, CH2), 2.07 (s, 3,
ArCH3), 1.99 (s, 3, ArCH3), 1.78 (s, 3, ArCH3), 1.66 (s, 9,
C(CH3)3), 1.63 (s, 3, ArCH3), 1.47 (s, 9, C(CH3)3), 1.17 (d, 6,
CH(CH3)2), 0.97 (d, 6, CH(CH3)2); 13C NMR (100 MHz, C6D6)
δ 156.5, 154.2, 150.0, 145.3, 141.7, 138.5, 135.9, 133.8, 131.6,
Su p p or tin g In for m a tion Ava ila ble: Fully labeled ther-
mal ellipsoid drawing, atomic coordinates, bond lengths and
angles, and anisotropic displacement parameters for 7b-Et2O,
and selected multidimensional spectra. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
OM030636+