Synthesis and crystal structure of N-heterocyclic carbene complexes of bis(η5-cyclopentadienyl)molybdenum

Article abstract of DOI:10.1246/bcsj.76.991

The one-pot reaction of [Mo(η⁵-C₅H₅)₂(H)(OTs)] (1) with 1,3-diisopropylimidazolium chloride (IⁱPr·HCl, 2a) in the presence of two equivalents of KO′Bu in THF gave a bis(η⁵-cyclopentadienyl)molybdenum derivative bearing the 1,3-diisopropylimidazol-2-ylidene ligand, [Mo(η⁵-C₅H₅)₂ (IⁱPr)] (3a). 1,3-Dimethyl- and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene coordinated molybdenocene complexes (3b, 3c) were prepared in a similar fashion (Method A). The treatment of Mo(η⁵-C₅H₅)₂(H) Li]₄ (6-Mo) with the corresponding imidazolium salt in THF under photo-conditions also gave carbene complexes 3a-3c (Method B). X-ray crystal structure analyses of 3a and 3b revealed that the N-heterocyclic carbene ligand is bonded to a bent molybdenocene skeleton. The Mo-C(carbene) bond distances are 2.219(7) A for 3a and 2.212(6) A for 3b. The five-membered carbene ligand in both complexes is located on the bisecting plane between two cyclopentadienyl rings.

Full text of DOI:10.1246/bcsj.76.991

Ó 2003 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 76, 991–997 (2003)
991
Synthesis and Crystal Structure of N-Heterocyclic Carbene Complexes
of Bis(ꢀ5-cyclopentadienyl)molybdenum
ꢀ
Yoshitaka Yamaguchi, Ryoji Oda, Katsunori Sado, Kimiko Kobayashi,y Makoto Minato,
ꢀ
and Takashi Ito
Department of Materials Chemistry, Graduate School of Engineering, Yokohama National University,
79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501
yThe Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198
(Received November 5, 2002)
The one-pot reaction of [Mo(ꢀ5-C5H5)2(H)(OTs)] (1) with 1,3-diisopropylimidazolium chloride (IiPr HCl, 2a) in
ꢁ
the presence of two equivalents of KOtBu in THF gave a bis(ꢀ5-cyclopentadienyl)molybdenum derivative bearing the
1,3-diisopropylimidazol-2-ylidene ligand, [Mo(ꢀ5-C5H5)2(IiPr)] (3a). 1,3-Dimethyl- and 1,3-bis(2,4,6-trimethylphen-
yl)imidazol-2-ylidene coordinated molybdenocene complexes (3b, 3c) were prepared in a similar fashion (Method
A). The treatment of [Mo(ꢀ5-C5H5)2(H)Li]4 (6-Mo) with the corresponding imidazolium salt in THF under photo-con-
ditions also gave carbene complexes 3a–3c (Method B). X-ray crystal structure analyses of 3a and 3b revealed that the
N-heterocyclic carbene ligand is bonded to a bent molybdenocene skeleton. The Mo–C(carbene) bond distances are
ꢀ
2.219(7) A for 3a and 2.212(6) A for 3b. The five-membered carbene ligand in both complexes is located on the bisect-
ꢀ
ing plane between two cyclopentadienyl rings.
Since the first preparation of bis(ꢀ5-cyclopentadienyl)mo-
lybdenum and -tungsten derivatives by Green, Wilkinson,
and co-workers in 1961,1 much effort has been made on the
synthesis and reaction of their derivatives.2 Studies on metal-
lacyclobutane derivatives of MCp2 (M = Mo, W; Cp = ꢀ5-
C5H5), which were prepared by treating [MCp2(ꢀ3-allyl)]þ
with nucleophiles, such as NaH, have shown important aspects
regarding the olefin metathesis process.3 The photo-decompo-
sition of the thus-formed metallacyclobutane complexes of
tungsten evolved ethylene, suggesting a concomitant forma-
tion of the methylidene derivative of WCp2, although such a
carbene species has been neither isolated nor detected spectro-
scopically due to its high reactivity.3 Although there have
been numerous studies on the carbene complexes of various
transition metals to date, the isolation of MCp2 (M = Mo
and W) compounds bearing a carbene ligand is rather
sparse.4 In this paper we will report on the synthesis and crys-
tal structure of bis(cyclopentadienyl)molybdenum derivatives
bearing an N-heterocyclic carbene compound.
(L)]þ[OTs]ꢂ, where L represents PPh3, PEt3, PnBu3, PCy3,
and P(OEt)3. The treatment of the cationic complexes with
one equivalent of NaOH as a base afforded neutral divalent
molybdenocene complexes having a trivalent phosphorus li-
gand (Eq. 1).
ð1Þ
Recently, stable crystalline carbene compounds, i.e., an imi-
dazol-2-ylidene and an imidazolin-2-ylidene (N-heterocyclic
carbene), have been widely used as alternatives to phosphines
in organometallic chemistry.7 Therefore, we attempted to pre-
pare a cationic hydrido(N-heterocyclic carbene) complex,
[MoCp2(H)(N-heterocyclic carbene)]þ[OTs]ꢂ, by the reaction
of complex 1 with an imidazol-2-ylidene generated in situ by
the reaction of an imidazolium salt with a base. However,
every trial was unsuccessful, presumably due to the highly re-
active nature of N-heterocyclic carbenes toward air and moist-
ure, even under an inert atmosphere (Eq. 2). We next exam-
ined a one-pot reaction for preparing [MoCp2(N-heterocyclic
carbene)] complexes under a vacuum condition.
Results and Discussion
Synthesis of [MoCp2(carbene)] Complexes from Hydri-
do(tosylato) Complex (Method A). In the course of our sys-
tematic research on the preparation, structure, and reactivity of
bis(ꢀ5-cyclopentadienyl)molybdenum and -tungsten com-
pounds,5 we successfully isolated a highly reactive hydrido-
(tosylato) complex, [MoCp2(H)(OTs)] (1), and demonstrated
that compound 1 is a useful starting material for preparing
of divalent MoCp2 derivatives.6 In the reaction of complex
1 with two electron donor compounds (L), such as phosphines
and/or phosphites, the OTs ligand in 1 is easily displaced by L
to give cationic complexes formulated as [MoCp2(H)-
ð2Þ

992 Bull. Chem. Soc. Jpn., 76, No. 5 (2003)
N-Heterocyclic Carbene Complexes of Molybdenocene
formed by the deprotonation of an imidazolium salt (2) with
KOtBu as a base. The free carbene (4) generated in the system
coordinates to the molybdenum center to produce a cationic
hydrido(carbene) complex (5).11 This intermediary cationic
complex 5 may be easily neutralized by a second KOtBu to af-
ford the final product 3.12
We next examined the preparation of a molybdenocene
complex having an imidazolin-2-ylidene (CC-saturated N-het-
erocyclic carbene) instead of an imidazol-2-ylidene. A hydri-
do(tosylato) complex 1 was treated with one equivalent of 1,3-
Scheme 1. Synthesis of [MoCp2(imidazol-2-ylidene)] com-
plexes (Method A).
A hydrido(tosylato) complex 1 was treated with one equiva-
dimesitylimidazolinium chloride (InMes HCl,13 2d) in the
ꢁ
lent of 1,3-diisopropylimidazolium chloride (IiPr HCl,8 2a) in
presence of two equimolar amounts of KOtBu as a base in
THF under vacuum. However, although the desired reaction
did not occur, unidentified products were formed (Eq. 3). It
has been reported that, in the reaction of an imidazolinium salt
with KOtBu as a base, an alcoholate adduct of the imidazol-
ꢁ
the presence of two equimolar amounts of KOtBu as a base at
ꢂ78 ꢃC in THF under a vacuum; the reaction mixture was al-
lowed to warm to room temperature. A clean reaction took
place and, after a work-up, a 1,3-diisopropylimidazol-2-yl-
idene coordinated molybdenocene compound [MoCp2(IiPr)]
(3a) was isolated as a dark-red solid in good yield (81%)
and characterized by 1H and 13C{1Hg NMR spectroscopies.
In a similar fashion, 3b and 3c, a methyl and 2,4,6-trimethyl-
phenyl (mesityl) analogues, respectively, of 3a, were success-
fully prepared and characterized (Method A, Scheme 1).
Complexes 3a and 3b were characterized by a single-crystal
X-ray diffraction study (vide infra).
t
inium salt forms (Eq. 4).14;15 In this case, the BuO adduct
of InMes HCl may be predominantly formed, which thus pre-
vents the formation of the carbene complex.
ꢁ
ð3Þ
1
In the H NMR spectra, products 3a–3c showed no signal
assignable to the CH-acidic proton in imidazolium salts 2,
whose resonance usually is known to appear at a low magnetic
field (ꢁ ¼ 8{10).7;9 Complex 3a showed the characteristic
ð4Þ
i
septet signal assignable to the methine protons of the Pr
groups, which was observed at 6.86 ppm with the coupling
3
constant JHH ¼ 6:6 Hz. This resonance is lower than that
Synthesis of [MoCp2(carbene)] Complexes from Lithi-
¨
ated Complex (Method B). In 1968, Ofele reported the first
for the parent imidazolium salt 2a (ꢁ ¼ 4:70)9 by 2.16 ppm
and, moreover, about 2 ppm lower than that for the free IiPr
(ꢁ = ca. 4.90).10 Presumably, this may be due to a magnetic
anisotropy of the Cp ligands and/or to an influence of the cen-
tral transition metal. In the 13C{1Hg NMR spectrum, complex
3a showed five signals and a resonance due to the carbene car-
bon bonded to molybdenum was observed at 193.6 ppm (in
THF-d8). In cases of complexes 3b and 3c, the carbene car-
bons were observed at 197.3 and 201.8 ppm (in benzene-d6),
respectively. These resonances for the coordinating carbene
carbon are slightly higher than that for the free imidazol-2-yl-
idenes (ꢁ ¼ 210{220); a tendency normally seen in a series of
these sorts of carbene complexes.7
preparation of a transition-metal complex having an imidazol-
2-ylidene as a ligand using a deprotonation method of the CH-
acidic proton on the imidazolium salt by an anionic metal
precursor.16 We employed a similar strategy in our approach
to prepare [MoCp2(carbene)]. We thus examined the reaction
of a lithiated molybdenocene compound, [MoCp2(H)Li]4 (6-
Mo),17 instead of a hydrido(tosylato) compound 1, with an
imidazolium salt.
A tetrameric lithiated bis(ꢀ5-cyclopentadienyl)molybdenum
compound, 6-Mo, was treated with four equivalents of
IiPr HCl (2a) in THF under irradiation with high-pressure
ꢁ
mercury lamp. After a work-up, complex 3a was isolated in
moderate yield (55%). In a similar fashion, complexes 3b
and 3c were successfully prepared (Method B, Scheme 3).
This method would be applied to the preparation of molyb-
denocene derivatives of an imidazolin-2-ylidene, CC-saturated
We propose the reaction pathway of the formation of imida-
zol-2-ylidene complexes (3a–3c; Method A), as shown in
Scheme 2. In this reaction, an imidazol-2-ylidene (4) is first
N-heterocyclic carbene. In the reaction of InMes HCl (2d)
ꢁ
Scheme 3. Synthesis of [MoCp2(imidazol-2-ylidene)] com-
plexes (Method B).
Scheme 2. Reaction pathway for Method A.

Y. Yamaguchi et al.
Bull. Chem. Soc. Jpn., 76, No. 5 (2003)
993
may be formed either via hydrido(alkyl) complex 8a or by di-
rect abstraction of the CH-acidic proton in 2a by the anionic
molybdenum compound 6-Mo as a base, although the detec-
tion of 8a has so far been unsuccessful. After the formation
of a dihydrido compound 7-Mo and a free carbene 4a, irradia-
tion of the resulting mixture led to the formation of a deep-red
carbene complex 3a.
In order to obtain further information regarding the reaction
mechanism, we examined the reaction of a tungsten analogue
of 6-Mo, [WCp2(H)Li]4 (6-W), with 2a. Even in this reaction,
the compound corresponding to 8a was not observed, but the
formation of a tungsten analogue of 7-Mo, [WCp2(H)2] (7-
W), and 4a was observed.
Scheme 4.
The acyclic carbene, bis(diisopropylamino)carbene, was
prepared and isolated by Alder and co-workers in 1996,19
and its first transition-metal complexes were recently reported
by Herrmann et al.15 The reaction of the acyclic diaminocar-
bene precursor 2f with a lithiated tetramer 6-Mo resulted in the
formation of a free diaminocarbene 4f and [MoCp2(H)2] (7-
Mo), as is observed by 1H NMR spectroscopy; also the molyb-
denocene derivative of the diaminocarbene was not formed by
this reaction (Eq. 6). It has been reported that the N–C–N an-
gle in the free diaminocarbene 4f is much larger than that in
any cyclic diaminocarbene derivative (imidazolin-2-ylidene)
and that the N–C–N angle is almost intact in their transition-
metal complexes in the solid state.15;19 The steric hindrance
may be the main reason for preventing its coordination to
the MoCp2 fragment, although the electronic feature of the
diaminocarbene15 might not be ruled out.
with 6-Mo, InMes coordinated molybdenocene compound 3d,
which was unable to be prepared by method A, was success-
fully isolated as a dark-green solid in 45% yield (Scheme 4).
Complex 3d showed similar 1H and 13C{1Hg NMR spectral
data to those of complex 3c. The carbene carbon in 3d was
observed at 234.5 ppm (in benzene-d6), which is a slightly
higher magnetic field than that for the free carbene (243.8
ppm in benzene-d6).18 In the case of 1,3-bis(2,6-diisopropyl-
phenyl)imidazolinium chloride (InDipp HCl, 2e), in contrast,
ꢁ
the desired carbene complex was not formed (Scheme 4). In
the reaction, the formation of free InDipp (4e) and
[MoCp2(H)2] (7-Mo) was seen before irradiation in the
1H NMR spectrum of the reaction mixture (Eq. 5). After irra-
diation of this solution, however the carbene complex was not
formed, presumably due to a steric problem between the
MoCp2 fragment and bulky substituents (2,6-diisopropylphe-
nyl) on the carbene nitrogens.
ð6Þ
Crystal Structures of Molybdenocene-Carbene Com-
plexes 3a and 3b. The molecular structures of 3a and 3b
were determined by X-ray crystallography. ORTEP drawings
of 3a and 3b are displayed in Figs. 1 and 2, respectively. The
crystal data and the selected bond distances and angles are
summarized in Tables 1–3.
A single-crystal X-ray diffraction study confirmed that 3a
and 3b possess a bent molybdenocene skeleton bearing a
1,3-imidazol-2-ylidene as a carbene ligand. The Mo–C(car-
ð5Þ
In order to obtain some insight into the pathway of the for-
mation of 3 (Method B), we monitored the reaction of 6-Mo
with 1,3-diisopropylimidazolium chloride 2a by 1H NMR
spectroscopy, and now propose a plausible reaction path, as
shown in Scheme 5. Before irradiation, the detectable pro-
ducts of this reaction were [MoCp2(H)2] (7-Mo) and a free
imidazol-2-ylidene 4a, as mentioned above. These compounds
ꢀ
ꢀ
bene) bond distances are 2.219(7) A for 3a and 2.212(6) A
for 3b, which are in good agreement with the Mo–C(N-hetero-
cyclic carbene) bond distances reported so far; the reported
Mo–C(carbene) bond distances are in the range from
20
ꢀ
2.152(5) to 2.35(1) A.
The bond distances of C1–N1 (3a;
ꢀ
ꢀ
ꢀ
1.366(6) A, 3b; 1.359(5) A) and angles of N1–C1–N1 (3a;
103.9(6)ꢃ, 3b; 103.5(5)ꢃ) are similar to those of free imida-
zol-2-ylidenes reported by Arduengo et al.7 The carbene car-
bons of the ligand have a trigonal-planar geometry; the sum of
the angles at the carbon is 360.1ꢃ for 3a and 3b. The nitrogen
atoms on both carbene ligands also have a trigonal-planar
structure; the sum of angles is 359.9ꢃ for 3a and 360.0ꢃ for 3b.
It is worth discussing the orientation of the carbene ligand in
the complexes. The carbene ligand in both complexes (3a and
Scheme 5. Reaction pathway for Method B.

994 Bull. Chem. Soc. Jpn., 76, No. 5 (2003)
N-Heterocyclic Carbene Complexes of Molybdenocene
Fig. 2. Molecular structure of 3b (normal view (upper) and
side view (under)) with thermal ellipsoids drawn at the
50% probability level. All hydrogen atoms are omitted
for clarity.
Fig. 1. Molecular structure of 3a (normal view (upper) and
side view (under)) with thermal ellipsoids drawn at the
50% probability level. All hydrogen atoms are omitted
for clarity.
paration of molybdenocene-carbene complexes (Method A).
A tetrameric lithiated molybdenocene compound 6-Mo was
also found to be a useful starting material for the preparation
of molybdenocene-carbene complexes (Method B). In addi-
tion, the reaction of 6-Mo with an imidazolinium salt afforded
an imidazolin-2-ylidene coordinated molybdenocene complex,
which could not be prepared by a treatment of complex 1 with
an imidazolinium salt. Hydrido(tosylato) and tetrameric-
lithiated complexes of MoCp2 can be considered to be a catio-
nic and anionic molybdenocene compounds, respectively
(Chart 1). In this work, we demonstrated the effective pre-
paration of molybdenocene-carbene complexes starting from
both cationic and anionic compounds. Molybdenocene deriva-
tives of imidazol-2-ylidene were characterized by an X-ray
diffraction study. To our best knowledge, this is the first crys-
3b) is located on the bisecting plane between two Cp rings
(Figs. 1, 2). Since the HOMO of the MoCp2 fragment also lies
on the same plane,21 the empty p orbital of the carbene carbon
lies perpendicular to the bent MoCp2’s HOMO. Although the
HOMO (metal fragment) orbital has a strong electron-donating
character, there must be no interaction between it and the
LUMO (carbene carbon) in these complexes. It is known that
the divalent carbon compound exhibits both ꢂ-donor and ꢃ-
acceptor properties upon binding to a transition metal. In fact,
this HOMO–LUMO overlap is seen in a bent tungstenocene–
carbene complex, [W(Cp2)=C(H)OZr(H)(ꢀ5-C5Me5)2].4
However, there have been several studies recently which re-
veal that imidazol-2-ylidene acts only as a ꢂ-donor to a metal,
and that its ꢃ-acceptability is negligible.7 In the solid state,
the orientation of the carbene ligand in 3a and 3b mentioned
above may come from a steric hindrance between two Cp rings
and substituents on both nitrogen atoms, resulting in a loss of
its ꢃ-acceptor ability. The same geometrical arrangement has
been seen in a related divalent silicon compound coordinated
to the Cp2Mo fragment, [MoCp2{SiN(tBu)CHCHNtBug].22
tallographical study on
a bent molybdenocene-carbene
complex. A further investigation of the reactivity of these
compounds is now in progress.
Conclusion
We examined the preparation of the molybdenocene deriva-
tives of an N-heterocyclic carbene as a ligand. A preparative
method utilizing a hydrido(tosylato) complex 1 as a starting
material proved to be simple, easy, and effective for the pre-
Chart 1.

Y. Yamaguchi et al.
Table 1. Summary of Crystal Data for 3a and 3b
Bull. Chem. Soc. Jpn., 76, No. 5 (2003)
995
3a
3b
Empirical formula
Formula weight
Crystal color, Habit
Crystal size/mm
Crystal system
C19H26MoN2
378.37
red, prismatic
0:40 ꢄ 0:40 ꢄ 0:25
orthorhombic
Fdd2 (#43)
C15H18MoN2
322.26
red, prismatic
0:20 ꢄ 0:10 ꢄ 0:10
orthorhombic
Fdd2 (#43)
Space group
Lattice parameters
ꢀ
a/A
23.777(3)
8.690(3)
17.370(3)
3588(2)
8
12.465(3)
22.357(3)
9.519(3)
2652(1)
8
ꢀ
b/A
ꢀ
c/A
V/A
Z value
ꢀ 3
Dcalc/g cmꢂ3
1.400
1.614
Fð000Þ
1568.00
7.29
1456
1342
152
1312.00
9.70
1091
1022
118
ꢄ(Mo Kꢅ) cmꢂ1
Reflections measured
Independent reflections
No. variables
Reflection/parameter ratio
Residuals: R; Rw
Residuals: R1
8.83
0.052; 0.086
0.028
8.66
0.045; 0.061
0.021
No. of Reflections to calcd R1
Goodness of fit indicator
Maximum peak in final
1015 (I > 2:0ꢂðIÞ)
0.92
884 (I > 2:0ꢂðIÞ)
0.93
0.54
0.19
ꢀ ꢂ3
diff. map/e A
Minimum peak in final
diff. map/e A
ꢂ0:75
ꢂ0:68
ꢀ ꢂ3
ꢀ
Table 2. Selected Bond Distances (A) and Angles (deg) for 3a
ꢀ
Table 3. Selected Bond Distances (A) and Angles (deg) for 3b
Bond distances
Bond distances
Mo1–C1
Mo1–C6
Mo1–C7
Mo1–C8
Mo1–C9
2.219(7)
2.222(7)
2.292(6)
2.300(7)
2.289(6)
Mo1–C10
N1–C1
N1–C2
2.222(7)
1.366(6)
1.383(8)
1.474(9)
1.34(2)
Mo1–C1
Mo1–C4
Mo1–C5
Mo1–C6
Mo1–C7
2.212(6)
2.219(4)
2.286(4)
2.295(4)
2.287(5)
Mo1–C8
N1–C1
N1–C2
N1–C3
2.228(4)
1.359(5)
1.370(6)
1.463(8)
1.38(2)
N1–C3
C2–C2
ꢀ
ꢀ
C2–C2
Bond angles
Bond angles
C1–N1–C2
C1–N1–C3
C2–N1–C3
110.9(6)
125.8(5)
123.2(6)
Mo1–C1–N1
ꢀ
128.1(3)
103.9(6)
107.1(4)
C1–N1–C2
C1–N1–C3
C2–N1–C3
112.3(5)
125.3(4)
122.4(5)
Mo1–C1–N1
ꢀ
128.3(3)
103.5(5)
106.0(4)
N1–C1–N1
N1–C2–C2
N1–C1–N1
N1–C2–C2
ꢀ
ꢀ
diisopropylphenyl)imidazolinium chloride (InDipp HCl, 2e),18
ꢁ
Experimental
and N,N,N0,N0-tetraisopropylformamidinum chloride (2f),19 [Mo-
Cp2(H)Li]4 (6-Mo),17 and [WCp2(H)Li]4 (6-W)17 were prepared
according to literature methods.
General Procedures. All manipulations involving air and
moisture-sensitive organometallic compounds were carried out
under an atmosphere of dry argon by using a standard Schlenk
tube or high vacuum techniques. All solvents were distilled over
appropriate drying agents prior to use. MoCl5 and WCl6 were
purchased from Mitsuwa Pure Chemicals and Kanto Chemical
Co., Inc., respectively, and used as received. nBuLi in a hexane
solution was purchased from Kanto Chemical Co., Inc. and ti-
trated prior to use. Other reagents employed in this research were
used without further purification. [MoCp2(H)(OTs)] (1),6 1,3-di-
1H and 13C NMR spectra were recorded on a JEOL EX-270
spectrometer at ambient temperature. 1H and 13C NMR chemical
shifts were reported in ppm relative to Me4Si. All coupling con-
stants were reported in Hz. A photo-reaction was performed with
a Riko Kagaku Sangyo 100 W high-pressure Hg lamp placed in a
water-cooled Pyrex jacket. Complexes 3a–3d were so unstable
toward air and moisture that correct elemental analysis data could
not be obtained, though satisfactory spectroscopic data were ob-
tained (vide infra).
isopropylimidazolium chloride (IiPr HCl, 2a),9 1,3-dimethylimi-
ꢁ
dazolium iodide (IMe HI, 2b),23 1,3-bis(2,4,6-trimethylphenyl)-
Preparation of [MoCp2(IiPr)] (3a). Method A: [MoCp2-
ꢁ
imidazolium chloride (IMes HCl, 2c),24 1,3-bis(2,4,6-trimethyl-
(H)(OTs)] (1) (710 mg, 1.78 mmol), IiPr HCl (2a) (351 mg,
ꢁ
ꢁ
phenyl)imidazolinium chloride (InMes HCl, 2d),18 1,3-bis(2,6-
1.86 mmol), and KOtBu (413 mg, 3.38 mmol) were put into a
ꢁ

996 Bull. Chem. Soc. Jpn., 76, No. 5 (2003)
N-Heterocyclic Carbene Complexes of Molybdenocene
Schlenk tube, which was attached to a high-vacuum line. THF
(ca. 15 mL) was added by a trap-to-trap-transfer technique at
(s, 4H, H2CCH2), 3.77 (s, 10H, C5H5), 6.79 (s, 4H, m-(CH3)3-
C6H2); 13C{1Hg NMR (in benzene-d6) ꢁ 19.3 (o-(CH3)3C6H2),
20.9 (p-(CH3)3C6H2), 52.5 (NCC), 73.0 (C5H5), 130.1 (m-
(CH3)3C6H2), 135.8 (o-(CH3)3C6H2), 136.3 (p-(CH3)3C6H2),
140.5 (ipso-(CH3)3C6H2), 234.5 (NCN).
ꢃ
ꢂ78 C, and the reaction mixture was allowed to warm to room
temperature. The solution was then stirred over-night to give a
dark-red solution. After removing the volatiles under reduced
pressure, the residual solid was extracted with toluene. The fil-
trate was evaporated off under high vacuum, and spectroscopically
pure 3a was obtained (yield, 546 mg, 1.44 mmol, 81%). 1H NMR
NMR Tube Reaction of 6-Mo with Salt 2. A typical proce-
dure for the tube reaction was as follows. [MoCp2(H)Li]4 (6-Mo)
(ca. 20 mg, 0.021 mmol) and four equivalents of corresponding
salt 2 were placed in an NMR tube, which was attached to a
high-vacuum line. THF-d8 (ca. 0.4 mL) was introduced onto
the mixture by a trap-to-trap-transfer technique at ꢂ78 ꢃC, and
then the NMR tube was flame-sealed. The reaction mixture was
allowed to warm to room temperature. The resulting solution
was subjected to a 1H NMR measurement, which indicated the
formation of [MoCp2(H)2] (7-Mo) and the free carbene (4).
1H NMR data for 7-Mo: ꢁ ꢂ9:24 (s, 2H, MoH), 4.54 (s, 10H,
3
(in THF-d8) ꢁ 1.24 (d, JHH ¼ 6:6 Hz, 12H, NCCH3), 3.65 (s,
3
10H, C5H5), 6.86 (sept, JHH ¼ 6:6 Hz, 2H, NCHCH3), 7.15 (s,
3
2H, HCCH); (in benzene-d6) ꢁ 0.89 (d, JHH ¼ 6:4 Hz, 12H,
NCCH3), 3.99 (s, 10H, C5H5), 6.24 (s, 2H, HCCH) 6.89 (sept,
3JHH ¼ 6:4 Hz, 2H, NCHCH3); 13C{1Hg NMR (in THF-d8) ꢁ
23.5 (NCCH3), 55.2 (NCCH3), 70.2 (C5H5), 118.4 (NCC),
193.6 (NCN); (in benzene-d6) ꢁ 23.1 (NCCH3), 54.3 (NCCH3),
70.4 (C5H5), 117.0 (NCC), 193.7 (NCN).
3
Method B: [MoCp2(H)Li]4 (6-Mo) (355 mg, 0.38 mmol) and
2a (286 mg, 1.52 mmol) were put into a Schlenk tube, which was
attached to a high-vacuum line. THF (ca. 15 mL) was added by a
trap-to-trap-transfer technique at ꢂ78 ꢃC, and the reaction mixture
was allowed to warm to room temperature. The solution was then
irradiated with a 100 W high-pressure Hg lamp at ambient tem-
perature for 3 h to give a dark-red solution. After removing the
volatiles in vacuo, the residual solid was extracted with
benzene. The filtrate was evaporated off; the removal of a conco-
mitantly formed [MoCp2(H)2] (7-Mo) by sublimation (60 ꢃC,
10ꢂ2 mmHg) afforded 3a (yield, 316 mg, 0.84 mmol, 55%).
C5H5). For 4a: ꢁ 1.34 (d, JHH ¼ 6:2 Hz, 12H, NCCH3), 4.95
3
(sept, JHH ¼ 6:2 Hz, 2H, NCHCH3), 6.99 (s, 2H, HCCH). For
3
3
4e: ꢁ 1.19 (d, JHH ¼ 6:9 Hz, 12H, CHCH3), 1.28 (d, JHH
¼
3
6:9 Hz, 12H, CHCH3), 3.22 (sept, JHH ¼ 6:9 Hz, 4H, CHCH3),
3.78 (s, 4H, H2CCH2), 7.15–7.27 (m, 6H, Ph). For 4f: ꢁ 0.98
3
3
(d, JHH ¼ 6:4 Hz, 24H, CHCH3), 3.30 (sept, JHH ¼ 6:4 Hz,
4H, CHCH3).
NMR Tube Reaction of 6-W with 2a. [Cp2W(H)Li]4 (6-W)
(28 mg, 0.022 mmol) and IiPr HCl 2a (17 mg, 0.090 mmol) were
ꢁ
placed in an NMR tube, which was attached to a high-vacuum
line. After THF-d8 (ca. 0.4 mL) was introduced onto the mixture
by a trap-to-trap-transfer technique at ꢂ78 ꢃC, the NMR tube was
flame-sealed. The reaction mixture was allowed to warm to room
ꢃ
Complex 3a can be further purified by sublimation at 120 C in
vacuo (10ꢂ2 mmHg).
1
Preparation of [MoCp2(IMe)] (3b). Method A: Complex
3b was prepared from [MoCp2(H)(OTs)] (1) (245 mg, 0.62
temperature. The resulting solution was subjected to a H NMR
measurement, which indicated the formation of [WCp2(H)2] (7-
W) and IiPr (4a). 1H NMR data for 7-W: ꢁ ꢂ12:77 (s, 2H,
mmol), IMe HI (2b) (202 mg, 0.90 mmol), and KOtBu (158
ꢁ
1
mg, 1.41 mmol) in the same manner as that for 3a. 3b was iso-
lated as a dark-red solid (yield, 157 mg, 0.49 mmol, 79%).
1H NMR (in benzene-d6) ꢁ 3.55 (s, 6H, NCH3), 3.89 (s, 10H,
C5H5), 6.06 (s, 2H, HCCH); 13C{1Hg NMR (in benzene-d6) ꢁ
39.8 (NCH3), 70.6 (C5H5), 120.4 (NCC), 197.3 (NCN).
Method B: This complex was prepared from [MoCp2(H)Li]4
(6-Mo) (260 mg, 0.28 mmol) and 2b (260 mg, 1.16 mmol) in the
same manner as that for 3a. 3b was isolated as a dark-red solid
(yield, 181 mg, 0.56 mmol, 50%).
WH, JWH ¼ 73:3 Hz), 4.44 (s, 10H, C5H5). For 4a: ꢁ 1.34 (d,
3JHH ¼ 6:4 Hz, 12H, NCCH3), 4.85 (sept, JHH ¼ 6:4 Hz, 2H,
3
NCHCH3), 6.96 (s, 2H, HCCH).
X-ray Structure Analyses for 3a and 3b. Single crystals of
ꢃ
3a and 3b were obtained from a toluene solution at ꢂ30 C and
sealed under Ar in a thin-walled glass capillary, respectively.
X-ray crystallography was performed on a Rigaku AFC-7R dif-
fractometer with graphite monochromated Mo Kꢅ radiation
ꢀ
(ꢆ ¼ 0:71069 A). The diffraction data for 3a and 3b were col-
Preparation of [MoCp2(IMes)] (3c). Method A: Complex
3c was prepared from [MoCp2(H)(OTs)] (1) (355 mg, 0.89 mmol),
lected at 296(2) K using the !–2ꢇ scan technique to a maximum
2ꢇ value of 60.0ꢃ. Cell constants and an orientation matrix for
data collection were determined from 25 reflections with 2ꢇ an-
gles in the ranges 28.81–29.77ꢃ (for 3a) and 28.72–29.93ꢃ (for
3b), respectively. The structure was solved by direct methods
(SIR 92)25 and expanded using Fourier techniques.26 The non-hy-
drogen atoms were refined anisotropically. All hydrogen atoms
were located from difference Fourier maps and refined
IMes HCl (3c) (303 mg, 0.89 mmol), and KOtBu (201 mg, 1.79
ꢁ
mmol) in the same manner as that for 3a. 3c was isolated as a
brown solid (yield, 336 mg, 0.63 mmol, 71%). 1H NMR (in benz-
ene-d6) ꢁ 2.08 (s, 6H, p-(CH3)3C6H2), 2.12 (s, 12H, o-
(CH3)3C6H2), 3.69 (s, 10H, C5H5), 6.16 (s, 2H, HCCH), 6.76
(s, 4H, m-(CH3)3C6H2); 13C{1Hg NMR (in benzene-d6) ꢁ 19.2
(o-(CH3)3C6H2), 20.9 (p-(CH3)3C6H2), 71.0 (C5H5), 123.2
(NCC), 129.6 (m-(CH3)3C6H2), 135.4 (o-(CH3)3C6H2), 137.6
(p-(CH3)3C6H2), 138.7 (ipso-(CH3)3C6H2), 201.8 (NCN).
Method B: This complex was prepared from [MoCp2(H)Li]4
(6-Mo) (238 mg, 0.25 mmol) and 3c (341 mg, 1.00 mmol) in the
same manner as that for 3a. 3c was isolated as a brown solid
(yield, 265 mg, 0.50 mmol, 50%).
isotropically.
All calculations were performed using the
teXsan27 crystallographic software package of Molecular Struc-
ture Corporation.
Crystallographic data have been deposited at the CCDC, 12
Union Road, Cambridge CB2 1EZ UK and copies can be obtained
on request, free of charge, by quoting the publication citation and
the deposition numbers 188521 for 3a and 188522 for 3b.
Preparation of [MoCp2(InMes)] (3d). Method B: This
complex was prepared from [MoCp2(H)Li]4 (6-Mo) (208 mg,
0.22 mmol) and InMes HCl (3d) (304 mg, 0.89 mmol) in the
same manner as that for 3a. 3d was isolated as a dark-green solid
(yield, 213 mg, 0.40 mmol, 45%). 1H NMR (in benzene-d6) ꢁ
2.14 (s, 6H, p-(CH3)3C6H2), 2.26 (s, 12H, o-(CH3)3C6H2), 3.04
The authors are grateful to Prof. Kohtaro Osakada (Tokyo
Institute of Technology) for his kind help in the elemental
analyses. This work was supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
ꢁ

Y. Yamaguchi et al.
Bull. Chem. Soc. Jpn., 76, No. 5 (2003)
997
12 A referee suggested that, in method A, [MoCp2] may be in-
itially formed in the reaction of compound 1 with KOtBu and then
it reacts with carbene 4 generated in situ to give final product 3
(Eqs. 8 and 9). Although such reaction pathway cannot be ruled
out completely, the proposed mechanism shown in Scheme 2
seems to be more plausible taking into account our previous re-
sults on the reaction of 1 with phosphorus compounds (Eq. 1).
We wish to thank a referee for insightful comments.
Sports, Science and Technology (No. 10450340) and by
Grant-in-Aid for Encouragement of Young Scientists for
Y. Y. from Japan Society for the Promotion of Science
(No. 13740413).
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1,3-Disubstituted imidazol-2-ylidenes are abbreviated as
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9
10 See Experimental Section.
11 We now investigate the reactivity of molybdenocene deri-
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ð7Þ