Twisted coordination mode of bis(N-heterocyclic carbene) ligands in octahedral geometry of group 6 transition metal complexes: Synthesis, structure, and reactivity

Article abstract of DOI:10.1016/j.ica.2012.04.027

On treatment of bis(imidazolium) salts bound by o-xylylene, propylene, and ethylene linkers with two moles of LiBEt₃H, the corresponding BEt₃-adducts of bis-NHCs, (Et₃B·ImR)₂E (Im = imidazole; R = Me, ⁱPr; E = o-xylylene, propylene, ethylene) (2), were obtained. Reaction of [Mo(CO)₆] with compound 2 afforded the carbene complex, [Mo(CO)₄(bis-NHC)] (3-Mo), in a good yield. Tungsten and chromium analogs of 3-Mo were obtained from [M(CO) ₄(η⁴-norbornadiene)] (M = W, Cr). The X-ray analyses and NMR measurements of these complexes revealed that the bis-NHC ligand adopts a twisted conformation in an octahedral geometry and thus complexes 3 showed a C₂-symmmetric structure. In a reaction of 3-Mo with trimethylphosphite, a CO/P(OMe)₃ substitution reaction took place to give fac-[Mo(CO)₃(bis-NHC){P(OMe)₃}] (4-Mo). The formation of the fac-form was found to be caused by a strong electron donor ability of the NHC ligand. The electronic features of the bis-NHC ligand were investigated by X-ray analysis, CO stretching frequency, and cyclic voltammetry of the complex 3-Mo. Furthermore, we estimated the donor ability of the bis-NHC ligand by comparing with those of 2,2′-bipyridine and 1,2-bis(diphenylphosphino) ethane. Density functional calculations (B3LYP/DGDZVP) showed that the C ₂-symmetric structure of o-xylylene-bridged 3-Mo having N-methyl azole rings was more stable than a C_s-symmetric structure by ΔG = 6.69 kcal mol^-1.

Full text of DOI:10.1016/j.ica.2012.04.027

Inorganica Chimica Acta 390 (2012) 199–209
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Inorganica Chimica Acta
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Twisted coordination mode of bis(N-heterocyclic carbene) ligands in octahedral
geometry of group 6 transition metal complexes: Synthesis, structure, and reactivity
a
a
b
Kenichi Ogata a, Yoshitaka Yamaguchi a, , Youji Kurihara , Kazuyoshi Ueda , Hirotaka Nagao ,
⇑
Takashi Ito a,
⇑
a Department of Advanced Materials Chemistry, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
b Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioicho, Chiyoda-ku, Tokyo 102-8554, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
On treatment of bis(imidazolium) salts bound by o-xylylene, propylene, and ethylene linkers with two
moles of LiBEt3H, the corresponding BEt3-adducts of bis-NHCs, (Et3BꢀImR)2E (Im = imidazole; R = Me,
iPr; E = o-xylylene, propylene, ethylene) (2), were obtained. Reaction of [Mo(CO)6] with compound 2
afforded the carbene complex, [Mo(CO)4(bis-NHC)] (3-Mo), in a good yield. Tungsten and chromium ana-
Received 25 February 2012
Accepted 18 April 2012
Available online 25 April 2012
logs of 3-Mo were obtained from [M(CO)4(g
4-norbornadiene)] (M = W, Cr). The X-ray analyses and NMR
Keywords:
N-Heterocyclic carbene
Bis-NHC ligand
C2-symmetric structure
Group 6 transition metals
DFT calculations
measurements of these complexes revealed that the bis-NHC ligand adopts a twisted conformation in an
octahedral geometry and thus complexes 3 showed a C2-symmmetric structure. In a reaction of 3-Mo
with trimethylphosphite, a CO/P(OMe)3 substitution reaction took place to give fac-[Mo(CO)3(bis-
NHC){P(OMe)3}] (4-Mo). The formation of the fac-form was found to be caused by a strong electron donor
ability of the NHC ligand. The electronic features of the bis-NHC ligand were investigated by X-ray anal-
ysis, CO stretching frequency, and cyclic voltammetry of the complex 3-Mo. Furthermore, we estimated
the donor ability of the bis-NHC ligand by comparing with those of 2,20-bipyridine and 1,2-bis(diphenyl-
phosphino)ethane. Density functional calculations (B3LYP/DGDZVP) showed that the C2-symmetric
structure of o-xylylene-bridged 3-Mo having N-methyl azole rings was more stable than a Cs-symmetric
structure by
D .
G = 6.69 kcal molꢁ1
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
rings in (or close to) the xy plane, while long linkers (n = 3, 4) allow
the azole rings to align more closely with the z-axis (perpendicular
to the xy plane). On the basis of many studies on the coordination
chemistry of bis-NHC ligands binding to late transition metals, it
has been demonstrated that the orientation of the azole rings
adopts either the in-plane conformation (A-1 in Chart 1) or perpen-
dicular orientation (A-2 in Chart 1). That is, these complexes have a
Cs-symmetric structure.
On the other hand, introducing a rigid chiral linker as a binder of
two azole rings in bidentate NHCs effectively led to chiral C2-sym-
metric complexes. A novel family of axially chiral metal complexes
with bidentate NHCs such as 1,10-binaphthyl-2,20-diamine (BI-
NAM) or H8-BINAM framework has been developed by Shi and
co-workers [5]. They also reported chiral bidentate NHC ligands
with biphenyl framework [6]. Their systematic studies on chiral
bidentate NHC complexes of Rh, Ir, and Pd revealed that these com-
plexes operate as excellent catalysts for asymmetric organic
transformations.
N-Heterocyclic carbenes (NHCs) are now fully established as an
important class of ligands for organometallic chemistry, because
NHCs enhance the electron density at the metal center with a ro-
bust metal–ligand bond. In particular, NHCs emerged as versatile
ligands in late transition metal chemistry, proving their high po-
tential in homogeneous catalysis [1]. The use of polydentate li-
gands containing NHC(s) allowed the preparation of new
complexes whose stability is entropically improved by the chelate
effect [2]. Most of these polydentate NHCs reported so far are
bidentate NHC ligands and there are numerous studies on their late
transition metal complexes of Rh, Ir, Ni, Pd, and so on [3,4].
The systematic study on the chelated bis-NHC complexes of late
transition metals revealed that the structural properties of coordi-
nated bis-NHC ligands depend on the length and rigidity of the lin-
ker unit [3b]. In this study, (CH2)n chains (n = 1–4) were used as the
aliphatic linkers to connect two azole rings. For short linkers (n = 1,
2), the NHC ligand tends to prefer a conformation with the azole
Compared with numerous reports on bis-NHC complexes of late
transition metals, only a limited number of reports has been pub-
lished on bis-NHC complexes of group 6 transition metals. In most
of them, the methylene group was the linker of the bidentate
⇑
Corresponding authors. Tel./fax: +81 45 339 3932 (Y. Yamaguchi).
E-mail address: yyama@ynu.ac.jp (Y. Yamaguchi).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.04.027

200
K. Ogata et al. / Inorganica Chimica Acta 390 (2012) 199–209
techniques. All solvents were distilled over appropriate drying
E
agents prior to use. All reagents employed in this research were
commercially available and used without further purification.
1,2-Bis(imidazolylmethyl)benzene [4c], 1a [4c], 1d [13], 2a
N
N
N
N
N
N
R
M
[12a], [W(CO)4(g g
4-C7H8)] [14], [Cr(CO)4( 4-C7H8)] [15], [Mo(CO)4
M
R
N
N
(bpy)] [16], and [Mo(CO)4(dppe)] [17] were prepared according to
literature methods.
R
R
A-2 (perpendicular)
A-1 (in-plane)
IR spectra were recorded on a HORIBA FT-730 spectrometer. 1H,
13C{1H}, 31P{1H} and 11B{1H} NMR spectra were recorded on a JEOL
EX-270 spectrometer at ambient temperature, unless otherwise
mentioned. 95Mo NMR spectra were recorded on a JEOL ECA-600
spectrometer. 1H and 13C{1H} NMR chemical shifts were recorded
in ppm relative to internal Me4Si. 31P{1H}, 11B{1H}, and 95Mo
NMR chemical shifts were recorded in ppm relative to external
H3PO4, BF3ꢀOEt2, and Na2MoO4, respectively. All coupling constants
were recorded in Hz. Cyclic voltammograms were recorded on
HECS 317S, 321, and 326 in CH3CN containing 0.1 M Bu4NBF4 as
a supporting electrolyte by using a conventional three-electrode
Cs-symmetric
z
R
y
N
x
N
N
M
E
N
R
system, platinum (U = 1.6 mm, working electrode), platinum wire
B
(counter electrode), and Ag/AgCl (reference electrode) at 100 mV/
s scan rate. Potentials are given vs. Fc/Fc+. Elemental analyses were
performed on a Perkin Elmer 240C.
C2-symmetric
R = alkyl, aryl; E = (CH2)n (n = 0, 1, 2, etc.)
2.2. Preparation of bis(imidazolium) salts 1
Chart 1. Coordination modes of bis-NHC ligand.
2.2.1. Preparation of 1b
ligand [7]. Hahn and co-workers reported the preparation of the
molybdenum complex coordinated by a propylene-bridged biden-
tate NHC and revealed that the complex adopts the twisted confor-
mation of two azole rings, i.e., the C2-symmetric coordination
mode in an octahedral geometry (B in Chart 1) [8]. However, to
our best of knowledge, there is no systematic study on the coordi-
nation modes of bis-NHC ligands in group 6 transition metals.
We have recently reported the preparation of chiral o-xylylene-
bridged bis-NHC ligands, derived from chiral 1,4-diol,1,2-bis-(1-
hydroxypropyl)benzene, and their molybdenum complexes. The
report revealed that these molybdenum complexes show the chiral
C2-symmetric structure in an octahedral geometry [9]. In this pa-
per, we describe the results of the systematic investigation on
the synthesis, structure, and reactivity of group 6 transition metal
complexes having chelated bidentate NHC with o-xylylene, propyl-
ene, and ethylene linkers.
In the preparation of NHC complexes, the use of free NHCs,
either isolated or generated in situ by the deprotonation of the cor-
responding salts, is one of the most common methods. However,
manipulation of free NHCs is often difficult due to their highly
reactive nature toward air and moisture. Therefore employing
NHC adducts as protected forms of free NHC attracted much atten-
tion for the preparation of NHC complexes [10]. The most promis-
ing procedure for the preparation of the NHC complex is
transmetalation of NHC from a silver NHC complex to such late
transition metals as Rh, Ir, Pd, and so on [11]. We have already re-
ported the preparation of the efficient NHC transfer reagent, the
BEt3-adduct of NHC [12]. In this paper, we also report the prepara-
tion of the BEt3-adducts of bis-NHCs and their application for the
synthesis of bis-NHC complexes of group 6 transition metals.
1,2-Bis(imidazolylmethyl)benzene (215 mg, 0.90 mmol), DME
(15 mL), and 2-iodopropane (2.0 mL, 3.4 g, 20.0 mmol) were put
in a Schlenk tube. The reaction mixture was refluxed for 8 h, and
then the volatiles were removed under reduced pressure. The
residual solid was washed with hexane (3 ꢂ 5 mL) and dried in va-
cuo to yield 1b as a white solid (446 mg, 0.77 mmol, 86%). Anal.
Calc. for C20H28I2N4: C, 41.54; H, 4.88; N, 9.69. Found: C, 41.31;
H, 4.97; N, 9.58%. 1H NMR (in CDCl3) d 1.65 (d, J = 6.6 Hz, 12H,
iPr–CH3), 4.81 (sept, J = 6.6 Hz, 2H, Pr–CH), 6.11 (s, 4H, –CH2–),
i
7.25–7.29 (m, 2H, Ph), 7.42–7.46 (m, 2H, Ph), 7.45 (s, 2H, CH@CH),
7.71 (s, 2H, CH@CH), 10.00 (s, 2H, NCHN). 13C{1H} NMR (in CDCl3) d
23.1 (iPr–CH3), 51.0, 53.8 (iPr–CH, –CH2–), 120.4, 123.0, 129.9,
130.2, 131.8, 134.7 (CH@CH, Ph, NCHN).
2.2.2. Preparation of 1c
1,3-Dibromopropane (1743 mg, 0.88 mL, 8.63 mmol), 1-meth-
ylimidazole (2.0 mL, 2.0 g, 24.4 mmol) and ethanol (15 mL) were
put in a Schlenk tube. The reaction mixture was refluxed for
10 h, and then the volatiles were removed under reduced pressure.
The residual solid was washed with toluene (4 ꢂ 10 mL) and dried
in vacuo to yield 1c as a white solid (2798 mg, 7.64 mmol, 89%).
Anal. Calc. for C11H18Br2N4: C, 36.09; H, 4.96; N, 15.30. Found: C,
36.00; H, 5.07; N, 15.28%. 1H NMR (in DMSO-d6) d 2.40 (quint,
J = 7.3 Hz, 2H, –CH2CH2CH2–), 3.86 (s, 6H, CH3), 4.25 (t, J = 7.3 Hz,
4H, –CH2CH2CH2–), 7.31 (s, 2H, CH@CH), 7.80 (s, 2H, CH@CH),
9.24 (s, 2H, NCHN). 13C{1H} NMR (in DMSO-d6)
(–CH2CH2CH2–), 35.8 (CH3), 45.6 (–CH2CH2CH2–), 121.9 (CH@CH),
123.4 (CH@CH), 136.6 (NCHN).
d 29.5
2.3. Preparation of BEt3-adducts of bis-NHC compounds 2
2.3.1. Preparation of 2b
2. Experimental
Compound 1b (155 mg, 0.27 mmol) was put in a Schlenk tube,
which was attached to a high-vacuum line. THF (ca. 10 mL) was
added by a trap-to-trap-transfer technique at ꢁ78 °C. At this tem-
perature, LiBEt3H (0.54 mL of its 1.0 M THF solution, 0.54 mmol)
was added by syringe. Then the reaction mixture was allowed to
warm to room temperature. After being stirred for 16 h at room
temperature, the volatiles were removed under reduced pressure.
The residual solid was washed with distilled water (3 ꢂ 10 mL)
2.1. General procedures
All manipulations involving air- and moisture-sensitive orga-
nometallic compounds were carried out under an atmosphere of
dry argon or nitrogen, which was purified by SICAPENT (Merck
Co., Inc.), by using a standard Schlenk tube or high vacuum

K. Ogata et al. / Inorganica Chimica Acta 390 (2012) 199–209
201
and dried in vacuo to yield 2b as a white solid (133 mg, 0.26 mmol,
96%). Anal. Calc. for C32H56B2N4: C, 74.13; H, 10.89; N, 10.81.
Found: C, 73.94; H, 11.39; N, 10.78%. 1H NMR (in CDCl3) d 0.47
(quint, J = 7.3 Hz, 12H, BCH2CH3), 0.61 (t, J = 7.3 Hz, 18H, BCH2CH3),
4.71; N, 10.82%. IR (KBr) mCO 1998, 1877, 1849, 1802 cmꢁ1
.
1H
NMR (in CDCl3) d 1.40 (d, J = 6.6 Hz, 6H, iPr–CH3), 1.52 (d,
i
J = 6.6 Hz, 6H, Pr–CH3), 4.45 (d, J = 14.5 Hz, 2H, –CH2–), 5.74 (d,
i
J = 14.5 Hz, 2H, –CH2–), 5.96 (sept, J = 6.6 Hz, 2H, Pr–CH), 6.67 (d,
i
1.43 (d, J = 7.3 Hz, 12H, Pr–CH3), 5.44 (s, 4H, –CH2–), 5.50 (sept,
J = 2.0 Hz, 2H, CH@CH), 6.96 (d, J = 2.0 Hz, 2H, CH@CH), 7.3 (m,
4H, Ph). 13C{1H} NMR (in CDCl3) d 22.7 (iPr–CH3), 23.8 (iPr–CH3),
51.1, 52.3 (iPr–CH, –CH2–), 119.2, 121.6, 128.4, 131.3, 136.7
(CH@CH, Ph), 189.6 (NCN), 210.4 (CO), 218.5 (CO). 95Mo NMR (in
DMSO-d6) d –1637 (W1/2 = 6 Hz).
i
J = 7.3 Hz, 2H, Pr–CH), 6.60 (d, J = 2.0 Hz, 2H, CH@CH), 6.91 (dd,
J = 5.3, 3.3 Hz, 2H, Ph), 6.98 (d, J = 2.0 Hz, 2H, CH@CH), 7.29 (dd,
J = 5.3, 3.3 Hz, 2H, Ph). 13C{1H} NMR (in CDCl3) d 11.5 (BCH2CH3),
14.5 (br, BCH2CH3), 23.8 (iPr–CH3), 49.0, 50.3 (iPr–CH, –CH2–),
116.2 (CH@CH), 120.9 (CH@CH), 128.0, 128.2, 134.3 (Ph), 175.6
(br, NCN). 11B{1H} NMR (in CDCl3) d ꢁ12.2 (BEt3).
2.4.3. Preparation of 3c-Mo
This compound was prepared from Mo(CO)6 (85 mg, 0.32
mmol) and 2c (129 mg, 0.32 mmol) using heptane (10 mL) as a sol-
vent in the same manner as that for 3a-Mo. Complex 3c-Mo was
isolated as a yellow solid (113 mg, 0.27 mmol, 84%). Anal. Calc.
for C15H16MoN4O4: C, 43.70; H, 3.91; N, 13.59. Found: C, 43.38;
2.3.2. Preparation of 2c
This compound was prepared form 1c (545 mg, 1.49 mmol) and
LiBEt3H (3.00 mL of its 1.0 M THF solution, 3.00 mmol) using THF
(ca. 15 mL) as a solvent in the same manner as that for 2b. Com-
pound 2c was isolated as a white solid (560 mg, 1.40 mmol, 94%).
Anal. Calc. for C23H46B2N4: C, 69.02; H, 11.58; N, 14.00. Found: C,
68.53; H, 11.63; N, 14.03%. 1H NMR (in CDCl3) d 0.44 (quint,
J = 7.3 Hz, 12H, BCH2CH3), 0.63 (t, J = 7.3 Hz, 18H, BCH2CH3), 2.25
(m, 2H, –CH2–), 3.85 (s, 6H, CH3), 4.30 (t, J = 7.9 Hz, 4H, –CH2–),
6.74 (d, J = 2.0 Hz, 2H, CH@CH), 6.76 (d, J = 2.0 Hz, 2H, CH@CH).
13C{1H} NMR (in CDCl3) d 11.4 (BCH2CH3), 14.1 (br, BCH2CH3),
37.9, 46.2, 68.1 (CH3, –CH2–), 119.5 (CH@CH), 122.4 (CH@CH),
175.9 (br, NCN). 11B{1H} NMR(in CDCl3) d ꢁ12.5 (BEt3).
H, 4.16; N, 13.53%. IR (KBr) mCO 1993, 1879, 1844, 1792 cmꢁ1 1H
.
NMR (in acetone-d6) d 1.81 (quint, J = 4.6 Hz, 2H, –CH2CH2CH2–),
3.96 (t, J = 4.6 Hz, 4H, –CH2CH2CH2–), 4.06 (s, 6H, CH3), 7.22 (d,
J = 1.3 Hz, 2H, CH@CH), 7.32 (d, J = 1.3 Hz, 2H, CH@CH). 13C{1H}
NMR (in acetone-d6)
d 35.0, 40.4, 46.0 (–CH2CH2CH2–, –
CH2CH2CH2–, CH3), 121.1 (CH@CH), 124.8 (CH@CH), 191.7 (NCN),
211.4 (CO), 220.3 (CO). 95Mo NMR (in DMSO-d6) d –1649 (W1/2
=
7 Hz).
2.4.4. Preparation of 3d-Mo
This compound was prepared from Mo(CO)6 (47 mg,
0.18 mmol) and 2d (70 mg, 0.18 mmol) using heptane (10 mL) as
a solvent in the same manner as that for 3a-Mo. Complex 3d-Mo
was isolated as a yellow solid (50 mg, 0.13 mmol, 72%). Anal. Calc.
for C14H14MoN4O4: C, 42.22; H, 3.54; N, 14.07. Found: C, 42.05; H,
2.3.3. Preparation of 2d
This compound was prepared from 1d (200 mg, 0.57 mmol) and
LiBEt3H (1.15 mL of its 1.0 M THF solution, 1.15 mmol) using THF
(ca. 10 mL) as a solvent in the same manner as that for 2b. Com-
pound 2d was isolated as a white solid (210 mg, 0.54 mmol,
96%). Anal. Calc. for C22H44B2N4: C, 68.41; H, 11.48; N, 14.51.
Found: C, 68.15; H, 11.95; N, 14.50%. 1H NMR (in CDCl3) d 0.50
(quint, J = 6.6 Hz, 12H, BCH2CH3), 0.67 (t, J = 6.6 Hz, 18H, BCH2CH3),
3.88(s, 6H, CH3), 4.59 (s, 4H, –CH2–), 6.71 (d, J = 2.0 Hz, 2H,
CH@CH), 6.73 (d, J = 2.0 Hz, 2H, CH@CH). 13C{1H} NMR (in CDCl3)
d 11.5 (BCH2CH3), 14.5 (br, BCH2CH3), 38.0 (CH3), 49.7 (–CH2–),
120.4 (CH@CH), 122.4 (CH@CH), 176.5 (br, NCN). 11B{1H} NMR(in
CDCl3) d ꢁ12.5 (BEt3).
3.67; N, 14.02%. IR (KBr) m
CO 1991, 1874, 1844, 1804 cmꢁ1. 1H NMR
(in DMSO-d6) d 3.80 (s, 6H, CH3), 4.62 (s, 4H, –CH2CH2–), 7.26 (s,
2H, CH@CH), 7.29 (s, 2H, CH@CH). 13C{1H} NMR (in CD3CN) d
40.2 (CH3), 50.5 (–CH2CH2–), 122.8 (CH@CH), 123.7 (CH@CH),
191.7 (NCN), 212.2 (CO), 220.6 (CO).
2.4.5. Preparation of 3a-W
This compound was prepared from [W(CO)4(g
4-nbd)] (71 mg,
0.18 mmol) and 2a (88 mg, 0.19 mmol) using heptane (5 mL) as a
solvent in the same manner as that for 3a-Mo. Complex 3a-W
was isolated as a yellow solid (45 mg, 0.08 mmol, 44%). IR (KBr)
2.4. Preparation of [M(CO)4(bis-NHC)] complexes 3
mCO 1994, 1885, 1839, 1780 cmꢁ1 1H NMR (in DMSO-d6) d 3.30
.
2.4.1. Preparation of 3a-Mo
Mo(CO)6 (91 mg, 0.34 mmol), 2a (162 mg, 0.35 mmol), and hep-
tane (10 mL) were put in a Schlenk tube. After being refluxed for
6 h, the volatiles were removed under reduced pressure. The resid-
ual solid was washed with hexane (3 ꢂ 10 mL) and dried in vacuo
to give 3a-Mo as a yellow solid (138 mg, 0.29 mmol, 85%). Anal.
Calc. for C20H18MoN4O4: C, 50.64; H, 3.83; N, 11.81. Found: C,
50.51; H, 3.87; N, 11.87%. IR (KBr) mCO 1999, 1896, 1847, and
(s, 6H, CH3), 4.72 (d, J = 13.8 Hz, 2H, –CH2–), 5.34 (d, J = 13.8 Hz,
2H, –CH2–), 6.90 (d, J = 2.0 Hz, 2H, CH@CH), 7.31 (d, J = 2.0 Hz,
2H, CH@CH), 7.34 (dd, J = 5.3, 3.3 Hz, 2H, Ph), 7.47 (dd, J = 5.3,
3.3 Hz, 2H, Ph). 13C{1H} NMR (in DMSO-d6) d 52.2 (–CH2–), 120.3,
124.4, 128.5, 131.3, 136.4 (CH@CH, Ph), 183.7 (NCN), 203.7 (CO),
210.7 (CO). Methyl carbons on the NHC rings were not able to be
detected due to an overlap with solvent signals. Correct elemental
analysis data of this complex could not be obtained, though satis-
factory spectroscopic data were obtained.
1788 cmꢁ1 1H NMR (in DMSO-d6) d 3.96 (s, 6H, CH3), 4.71 (d,
.
J = 13.9 Hz, 2H, –CH2–), 5.42 (d, J = 13.9 Hz, 2H, –CH2–), 6.90 (d,
J = 1.3 Hz, 2H, CH@CH), 7.29 (d, J = 1.3 Hz, 2H, CH@CH), 7.34 (dd,
J = 5.3, 3.3 Hz, 2H, Ph), 7.48 (dd, J = 5.3, 3.3 Hz, 2H, Ph). 13C{1H}
NMR (in DMSO-d6, at 60 °C) d 51.5 (–CH2–), 120.1, 124.1, 128.2,
131.1, 136.3 (CH@CH, Ph), 191.5 (NCN), 210.3 (CO), 218.8 (CO).
Methyl carbons on the imidazole rings were not able to detect
due to an overlap with solvent signals. 95Mo NMR (in DMSO-d6)
d –1657 (W1/2 = 6 Hz).
2.4.6. Preparation of 3a-Cr
This compound was prepared from [Cr(CO)4(g
4-nbd)] (59 mg,
0.23 mmol) and 2a (116 mg, 0.25 mmol) using heptane (10 mL)
as a solvent in the same manner as that for 3a-Mo. Complex 3a-
Cr was isolated as a yellow solid (55 mg, 0.13 mmol, 57%). IR
(KBr) mCO 1986, 1926, 1845, 1791 cmꢁ1 1H NMR (in DMSO-d6) d
.
3.99 (s, 6H, CH3), 4.70 (d, J = 13.8 Hz, 2H, –CH2–), 5.18 (d,
J = 13.8 Hz, 2H, –CH2–), 6.86 (d, J = 2.0 Hz, 2H, CH@CH), 7.28 (d,
J = 2.0 Hz, 2H, CH@CH), 7.31 (dd, J = 5.3, 3.3 Hz, 2H, Ph), 7.44 (m,
2H, Ph). 13C{1H} NMR (in DMSO-d6) d 51.1 (–CH2–), 121.1, 125.0,
128.3, 131.0, 136.6 (CH@CH, Ph), 196.4 (NCN), 222.8 (CO), 228.1
(CO). Methyl carbons on the NHC rings were not able to be de-
tected due to an overlap with solvent signals. Correct elemental
2.4.2. Preparation of 3b-Mo
This compound was prepared from Mo(CO)6 (42 mg, 0.16
mmol) and 2b (85 mg, 0.16 mmol) using heptane (5 mL) as a sol-
vent in the same manner as that for 3a-Mo. Complex 3b-Mo was
isolated as a yellow solid (70 mg, 0.13 mmol, 81%). Anal. Calc. for
C24H26MoN4O4: C, 54.34; H, 4.94; N, 10.56. Found: C, 54.45; H,

202
K. Ogata et al. / Inorganica Chimica Acta 390 (2012) 199–209
analysis data of this complex could not be obtained due to highly
hygroscopic nature, though satisfactory spectroscopic data were
obtained.
J = 15.7 Hz, NCN), 219.2 (d, J = 59.2 Hz, CO), 220.5 (d, J = 12.3 Hz,
CO), 221.1 (d, J = 13.4 Hz, CO). 31P{1H} NMR (in CDCl3) d 168.8
(P(OCH3)3).
2.5. Preparation of [Mo(CO)3(bis-NHC)(phosphite)] complexes 4-Mo
2.5.1. Preparation of 4a-Mo
2.5.3. Preparation of 4c-Mo
This compound was prepared from 3c-Mo (52 mg, 0.13 mmol)
and trimethylphosphite (22 lL, 23 mg, 0.19 mmol) using toluene
3a-Mo (55 mg, 0.12 mmol), trimethylphosphite (30 lL, 32 mg,
(10 mL) as a solvent in the same manner as that for 4a-Mo. Com-
plex 4c-Mo was isolated as a white solid (63 mg, 0.12 mmol,
92%). Anal. Calc. for C17H25MoN4O6P: C, 40.17; H, 4.96; N, 11.02.
Found: C, 40.11; H, 5.04; N, 10.95%. IR (KBr) mCO 1900, 1797,
0.26 mmol), and toluene (5 mL) were put in a Schlenk tube. After
being refluxed for 3 h, the volatiles were removed under reduced
pressure. The residual solid was washed with hexane (3 ꢂ 5 mL)
and dried in vacuo to give 4a-Mo as a white solid (40 mg,
1752 cmꢁ1 1H NMR (in acetone-d6) d 1.73 (m, 2H, –CH2CH2CH2–
.
0.070 mmol, 58%). IR (KBr) mCO 1913, 1806, 1765 cmꢁ1 1H NMR
.
), 3.34 (d, J = 10.6 Hz, 9H, P(OCH3)3), 3.58 (m, 4H, –CH2CH2CH2–),
4.03 (s, 3H, CH3), 4.14 (s, 3H, CH3), 7.11 (d, J = 1.3 Hz, 1H, CH@CH),
7.15 (d, J = 1.3 Hz, 1H, CH@CH), 7.19 (d, J = 1.3 Hz, 1H, CH@CH),
7.26 (d, J = 1.3 Hz, 1H, CH@CH). 13C{1H} NMR (in DMSO-d6) d
34.3, 44.6, 45.1, 50.7, 50.9 (–CH2CH2CH2–, CH3), 50.0 (d,
J = 2.2 Hz, P(OCH3)3), 120.1, 120.6, 123.7, 123.9 (CH@CH), 194.1
(d, J = 16.2 Hz, NCN), 194.4 (d, J = 12.9 Hz, NCN), 220.9 (d,
J = 57.6 Hz, CO), 221.8 (d, J = 11.7 Hz, CO), 222.4 (d, J = 14.0 Hz,
CO). 31P{1H} NMR (in CDCl3) d 170.9 (P(OCH3)3).
(in DMSO-d6) d 3.33 (d, J = 10.6 Hz, 9H, P(OCH3)3), 3.92 (s, 3H,
CH3), 4.03 (s, 3H, CH3), 4.44 (d, J = 14.5 Hz, 1H, –CH2–), 4.58 (d,
J = 13.8 Hz, 1H, –CH2–), 5.54 (d, J = 14.5 Hz, 1H, –CH2–), 5.65 (d,
J = 13.8 Hz, 1H, –CH2–), 6.8–7.6 (m, 8H, CH@CH, Ph). 31P{1H} NMR
(in CDCl3) d 167.9 (P(OCH3)3). 13C{1H} NMR data of this complex
were not obtained due to its low solubility and correct elemental
analysis data of this complex were not able to be obtained.
2.5.2. Preparation of 4b-Mo
This compound was prepared from 3b-Mo (50 mg, 0.094 mmol)
and trimethylphosphite (15
l
L, 16 mg, 0.13 mmol) using toluene
2.6. Experimental procedure for X-ray crystallography
(10 mL) as a solvent in the same manner as that for 4a-Mo. Com-
plex 4b-Mo was isolated as a yellow solid (30 mg, 0.048 mmol,
51%). Anal. Calc. for C26H35MoN4O6P: C, 49.85; H, 5.63; N, 8.94.
Suitable single crystals were obtained by recrystallization from
hexane (2b), from CH3CN (3a-Mo), from toluene–hexane (3b-Mo),
or from CH2Cl2 (4b-Mo, 4c-Mo), in a refrigerator and are individu-
ally mounted on glass fibers. Diffraction measurements of all com-
pounds were made on a Rigaku AFC-7R automated four-circle
Found: C, 50.03; H, 5.41; N, 8.56%. IR (KBr) mCO 1917, 1814,
i
1775 cmꢁ1
.
1H NMR (in CDCl3) d 1.36 (d, J = 6.6 Hz, 3H, Pr–Me),
1.42 (d, J = 6.6 Hz, 3H, iPr–Me), 1.50 (d, J = 6.6 Hz, 6H, iPr–Me),
3.43 (d, J = 10.6 Hz, 9H, P(OCH3)3), 4.29 (d, J = 14.5 Hz, 1H, –CH2–),
diffractometer by using graphite-monochromated Mo K radiation
a
i
4.38 (d, J = 14.5 Hz, 1H, –CH2–), 5.75 (sept, J = 6.6 Hz, 1H, Pr–CH),
(k = 0.71069). The data collections were carried out at –50 2 °C
5.86 (d, J = 14.5 Hz, 1H, –CH2–), 5.87 (d, J = 14.5 Hz, 1H, –CH2–),
using the
x
ꢁ 2h scan technique to a maximum 2h value of 55.0°
6.32 (sept, J = 6.6 Hz, 1H, Pr–CH), 6.66, 6.89, 6.92, 7.29, 7.30 (m,
for all crystals. Cell constants and an orientation matrix for data
collection were determined from 25 reflections with 2h angles in
the range 22.39–25.01° for 2b, 29.75–29.96° for 3a-Mo, 29.58–
29.99° for 3b-Mo, 26.92–29.72° for 4b-Mo, and 29.58–29.98° for
4c-Mo. Three standard reflections were monitored at every 150
i
8H, CH@CH, Ph). 13C{1H} NMR (in CDCl3) d 23.4, 23.6, 25.0 (iPr–
CH3), 50.4 (d, J = 2.2 Hz, P(OCH3)3), 52.3, 52.5, 52.8, 53.1 (iPr–CH,
–CH2–), 117.7, 118.0, 120.0, 120.3 (CH@CH), 128.5, 130.7, 131.1,
137.5, 137.7 (Ph), 195.7 (d, J = 13.4 Hz, NCN), 196.7 (d,
Table 1
Summary of crystal data for compounds 2b, 3a-Mo, 3b-Mo, 4b-Mo and 4c-Mo.
2b
3a-Mo
3b-Mo
4b-Mo
4c-Mo
Empirical formula
Formula weight
Crystal color, habit
Crystal size/mm
Crystal system
Space group
Lattice parameters
a (Å)
C
32H56B2N4
C20H18MoN4O4
474.33
yellow, plate
0.50 ꢂ 0.25 ꢂ 0.08
monoclinic
C24H26MoN4O4
530.43
yellow, needle
0.58 ꢂ 0.10 ꢂ 0.10
monoclinic
C26H35MoN4O6P
626.50
colorless, plate
0.25 ꢂ 0.25 ꢂ 0.08
monoclinic
C17H25MoN4O6P
508.32
yellow, needle
0.25 ꢂ 0.13 ꢂ 0.13
monoclinic
518.44
colorless, plate
0.25 ꢂ 0.20 ꢂ 0.13
monoclinic
P21/c (No. 14)
P21/c (No. 14)
C2/c (No. 15)
P21/c (No. 14)
P21/n (No. 14)
18.129(9)
13.20(2)
14.472(9)
106.89(4)
3314(5)
4
1.039
1144.00
0.597
13.900(5)
9.108(4)
16.917(4)
109.52(2)
2018(2)
4
1.561
960.00
6.825
16.590(9)
12.579(7)
12.574(9)
114.94(4)
2379(3)
4
1.481
1088.00
5.876
14.32(1)
12.49(2)
16.642(8)
92.79(5)
2974(3)
4
1.399
1296.00
5.377
9.633(7)
14.182(9)
15.50(2)
96.52(7)
2104(3)
4
1.605
1040.00
7.394
b (Å)
c (Å)
b (°)
V (Å3)
Z
Dc (g cmꢁ3
F(000)
)
l
(Mo K
a
) (cmꢁ1
)
Index ranges
ꢁ23 6 h 6 22
0 6 k 6 17
0 6 l 6 18
7931
7627 (0.0787)
399
0 6 h 6 18
ꢁ11 6 k 6 0
ꢁ21 6 l 6 20
4811
4625 (0.0323)
280
0 6 h 6 21
0 6 k 6 16
ꢁ16 6 l 6 14
2832
2742 (0.0361)
163
ꢁ18 6 h 6 0
0 6 k 6 16
ꢁ21 6 l 6 21
7073
6808 (0.0624)
378
0 6 h 6 12
ꢁ18 6 k 6 18
ꢁ20 6 l 6 20
10018
4839 (0.1116)
287
Reflections measured
Independent reflections (Rint
No. variables
)
Reflection/parameter ratio
Residuals: R; RW
19.12
0.2350; 0.1723
0.0703
1.079
1.30, ꢁ1.22
16.52
0.0397; 0.0898
0.0299
1.039
0.62, ꢁ1.29
16.82
0.0400; 0.0968
0.0333
1.026
0.45, ꢁ1.18
18.01
0.1260; 0.1834
0.0632
1.051
1.68, ꢁ1.98
16.86
0.0952; 0.1883
0.0579
0.896
2.09, ꢁ2.44
Residuals: R1 [I > 2
Goodness-of-fit (GOF) on F2
(e Åꢁ3
r](I)]
d
qmax,
)
min

K. Ogata et al. / Inorganica Chimica Acta 390 (2012) 199–209
203
measurements. The data were corrected for Lorentz and polariza-
tion effects.
Crystallographic data and the results of measurements are sum-
marized in Table 1. The structures were solved by direct methods
(SIR 92) [18] for 2b and 4b-Mo or by direct methods (SIR 97)
[19] for 3a-Mo, 3b-Mo, and 4c-Mo and expanded using Fourier
techniques. The non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were introduced at the ideal positions and refined
by using the riding model. All calculations were performed using
the CrystalStructure [20,21] crystallographic software package.
2.7. DFT calculations
Density functional theory (DFT) [22] calculations were per-
formed with the hybrid Becke’s three-parameter exchange func-
tional [23] and the Lee–Yang–Parr nonlocal correlation functional
[24] (B3LYP) implemented in the GAUSSIAN03 program package
[25]. Double-zeta valence plus polarization (DGDZVP) basis set
[26] was used on all atoms. All geometries were optimized without
constraints, and vibrational frequencies were computed.
Fig. 1. ORTEP drawing of compound 2b (30% probability of thermal ellipsoids). All
hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg):
B1–C1 = 1.677(6), B2–C15 = 1.662(6), C2–C3 = 1.330(5), C16–C17 = 1.329(5), N1–
C1–N2 = 103.6(3),
N1–C1–B1 = 131.9(3),
N2–C1–B1 = 124.4(3),
N3–C15–
N4 = 102.1(3), N3–C15–B2 = 131.6(3), N4–C15–B2 = 126.2(3).
3.2. Synthesis and structures of [M(CO)4(bis-NHC)] complexes of group
6 transition metals
3. Results and discussion
3.1. Preparation of BEt3-adducts of bis-NHC compounds
We have recently reported that the BEt3-adduct of monodentate
NHC can act as an efficient NHC transfer reagent to molybdenum
and tungsten complexes [12]. In this study we investigated the
NHC transfer ability of the BEt3-adducts of bis-NHCs 2 toward
group 6 transition metals. We first examined the reaction of
[Mo(CO)6] with 2a under toluene-refluxing conditions, which re-
ferred to the reaction conditions for the preparation of [Mo
(CO)5NHC] [12a]. The desired bis-NHC complex of molybdenum
We have already reported the preparation of the triethylborane-
adduct of bis-NHC 2a, which was prepared by the reaction of the
corresponding bis(imidazolium) salt 1a with two moles of LiBEt3H
[12a]. In this study, we examined the preparation of the isopropyl
analog of 2a and propylene- and ethylene-bridged BEt3-adducts of
bis-NHCs, 2c and 2d (Scheme 1).
The treatment of bis(imidazolium) salt 1b with two moles of Li-
BEt3H at ꢁ78 °C afforded a homogeneous reaction mixture, from
which a white solid of 2b was isolated in 96% yield. In cases of
1c and 1d, the propylene- and ethylene-bridged analogs of 1a, a
similar reaction took place to give white solids of 2c and 2d in good
yields. Elemental analyses and 1H, 13C{1H}, and 11B{1H} NMR spec-
tra indicated the formation of BEt3-adducts of bis-NHCs (2b–2d). In
the 1H NMR spectrum of 2b, the signal of imidazolium proton
atoms observed at 10.00 ppm for 1b disappeared and the signals
of triethylborane proton atoms were observed at 0.47 and
0.61 ppm as quartet and triplet, respectively. The 13C{1H} NMR
spectrum showed the broad signal assignable to the carbene car-
bon atoms at 175.6 ppm. The broadening may be caused by the
influence of the binding boron. A signal of the boron atom of the
BEt3 group was observed at ꢁ12.2 ppm in the 11B{1H} NMR. These
spectroscopic data are consistent with those of 2a [12a]. Com-
pounds 2c and 2d also showed similar spectroscopic features to
those of compounds 2a and 2b. The structure of 2b obtained by
the X-ray analysis is shown in Fig. 1 and is consistent with the
spectroscopic data. The B–C(carbene) bond lengths are 1.677(6)
and 1.662(6) Å and similar to that of reported BEt3ꢀNHC com-
pounds [12a].
3a was obtained as a mixture containing [Mo(CO)3(g
6-toluene)]
as a by-product. After screening the reaction conditions, we found
that heptane was a suitable solvent. On treatment of [Mo(CO)6]
with 2a in heptane under refluxing conditions, a substitution reac-
tion cleanly took place to give the bis-NHC complex of molybde-
num 3a-Mo as a yellow solid in 83% yield. Complex 3b-Mo as
well as the propyrene- and ethylene-bridged homologs 3c-Mo
and 3d-Mo were also obtained in good yields by the same proce-
dure as that for 3a-Mo (Scheme 2). In cases of tungsten and chro-
mium complexes, by refluxing [M(CO)6] (M = W, Cr) with 2a,
desired complexes were obtained in low yields. In these cases, nor-
bornadiene (nbd) complexes formulated as [M(CO)4(g
4-nbd)]
(M = W, Cr) were used as starting materials and thus the bis-NHC
complexes 3a-W and 3a-Cr were obtained (Scheme 3).
The structures of o-xylylene-bridged bis-NHC complexes 3a-Mo
and 3b-Mo were determined by the X-ray analyses. The ORTEP
drawings of 3a-Mo and 3b-Mo are shown in Fig. 2. The selected
bond lengths and angles for 3a-Mo and 3b-Mo are listed in
Table 2. Complexes 3a-Mo and 3b-Mo have a pseudo-octahedral
geometry around the central metal and show C2-symmetric
structures in which the bis-NHC ligand coordinates in a twisted
2X
H
H
BEt3
N
BEt3
N
2eq. LiBEt3H
E
E
R
R
R
R
N
N
N
N
N
N
THF, - 78 °C → r.t.
- 2H2, - 2LiX
2a: R = Me, E = o-xylylene (y. 83%)
2b: R = iPr, E = o-xylylene (y. 96%)
2c: R = Me, E = propylene (y. 94%)
2d: R = Me, E = ethylene (y. 95%)
1a: R = Me, E = o-xylylene, X = Br
1b: R = iPr, E = o-xylylene, X = I
1c: R = Me, E = propylene, X = Br
1d: R = Me, E = ethylene, X = Br
Scheme 1. Preparation of BEt3-adducts of bis-NHCs.

204
K. Ogata et al. / Inorganica Chimica Acta 390 (2012) 199–209
of the methylene-bridged bis-NHC complex of tungsten (79.2(2)°
2
[Mo(CO)6]
and 80.6(3)°) [7b], and slightly larger than those of late transition
metal complexes (83.2–92.2°) reported by Crabtree and co-work-
ers [3b].
heptane, reflux
- 2CO
In IR spectra of complexes 3a-Mo and 3b-Mo, both complexes
showed four stretching bands at 1999, 1896, 1847, and
1788 cmꢁ1 for 3a-Mo and 1998, 1877, 1849, and 1802 cmꢁ1 for
3b-Mo. These observations indicate that the bis-NHC ligand coordi-
nates in the cis position [28]. In 13C{1H} NMR spectra, signals assign-
able to the carbene carbon atoms were observed at 191.5 ppm for
3a-Mo and 189.6 ppm for 3b-Mo. In the case of 3a-Mo, the signals
due to the carbonyl carbon atoms were observed at 210.3 and
218.8 ppm with almost equivalent intensities. Similar spectro-
scopic data were obtained in complexes 3b-Mo, 3a-W, and 3a-Cr.
These observations clearly demonstrate that the bis-NHC ligands
in these complexes coordinate to the metals in the twisted mode
to form the C2-symmetric structure in a solution state.
R
N
OC
OC
N
N
Mo
E
OC
OC
N
R
3a-Mo: E = o-xylylene, R = Me (y. 85%)
3b-Mo: E = o-xylylene, R = iPr (y.81%)
3c-Mo: E = propylene, R = Me (y. 84%)
3d-Mo: E = ethylene, R = Me (y. 72%)
Scheme 2. Preparation of bis-NHC complexes of molybdenum.
Suitable crystals of propylene- and ethylene-bridged bis-NHC
complexes, 3c-Mo and 3d-Mo, were not obtained and thus we
could not confirm the structures of these complexes in a solid
state. However spectroscopic data of these complexes indicated
that these complexes adopt the C2-symmetric structure in a solu-
tion. In 13C{1H} NMR spectra, two signals with almost equivalent
intensities were observed in the region of the carbonyl carbon
atoms; 211.4 and 220.3 ppm for 3c-Mo and 212.2 and 220.6 ppm
for 3d-Mo as in the case of complexes 3a-Mo and 3b-Mo. Therefore
the bis-NHC ligand in those complexes most probably adopts a
twisted conformation in a solid state.
2a
[M(CO)4(η4-nbd)]
heptane, reflux
- nbd
Me
N
OC
OC
OC
N
N
M
OC
N
Me
3.3. Reactivity of [Mo(CO)4(bidentate ligand)] toward phosphorus
donors under thermal conditions
3a-W: M = W (y. 44%)
3a-Cr: M = Cr (y. 57%)
We were interested in the estimation of steric and/or electronic
properties of the bis-NHC ligand in molybdenum complexes, com-
paring with other bidentate ligands such as 2,20-bipyridine (bpy)
and 1,2-bis(diphenylphosphino)ethane (dppe), by the reactivity
with the phosphorous compound. We examined the reaction of
[Mo(CO)4(bis-NHC)] (3) with trimethylphosphite (P(OMe)3)
(Scheme 4).
On treating complex 3a-Mo with P(OMe)3 under toluene-reflux-
ing conditions, a CO/P(OMe)3 substitution reaction took place to
give complex 4a-Mo as a white solid in 58% yield. Complex
4b-Mo was also obtained by the reaction of complex 3b-Mo with
P(OMe)3 under the same conditions. Spectroscopic measurements
and X-ray analysis (vide infra) revealed that complexes 4a-Mo
and 4b-Mo show a facial geometry around the metal center. In
Scheme 3. Preparation of [M(CO)4(bis-NHC)] complexes (M = W, Cr).
conformation. It was reported that, in cases of late transition metal
complexes such as Rh [3a] and Pd [4c,4h,4p], o-xylylene-bridged
bis-NHC ligands adopt a Cs-symmetric coordination mode in these
complexes. The C2-symmetric coordination mode in complex 3
might come from flexibility of the linker unit, which prevents the
steric repulsion between the carbene ligand and the apical ligands
in an octahedral geometry. The bond lengths of Mo–C(carbene)
(2.305(3) and 2.293(3) Å for 3a-Mo and 2.290(3) Å for 3b-Mo)
are similar to those of zero-valent Mo–NHC complexes [7,8,27].
Bite angles defined by C(carbene)–Mo–C(carbene) are 94.2(1)° for
3a-Mo and 94.6(1)° for 3b-Mo, which are quite larger than those
Fig. 2. ORTEP drawings of complexes 3a-Mo (left) and 3b-Mo (right) (30% probability of thermal ellipsoids). All hydrogen atoms are omitted for clarity.

K. Ogata et al. / Inorganica Chimica Acta 390 (2012) 199–209
205
Table 2
Selected bond lengths (Å) and angles (°) for complexes 3a-Mo, 3b-Mo, 4b-Mo, and 4c-Mo.
3a-Mo
3b-Mo
4b-Mo
4c-Mo
Mo1–C1
Mo1–C2
Mo1–C3
Mo1–C4
Mo1–C5
Mo1–C17
O1–C1
O2–C2
O3–C3
2.044(3)
1.968(3)
1.969(3)
2.036(3)
2.294(2)
2.306(2)
1.141(3)
1.161(3)
1.156(3)
1.141(4)
88.9(1)
87.0(1)
173.6(1)
91.1(1)
93.4(1)
88.3(1)
Mo1–C1
Mo1–C2
Mo1–Mo1–C3
O1–C1
2.035(4)
1.976(3)
2.290(3)
1.140(4)
1.157(4)
Mo1–P1
Mo1–C1
Mo1–C2
Mo1–C3
Mo1–C4
Mo1–C18
C1–O1
2.468(3)
1.962(6)
1.942(7)
1.972(6)
2.312(6)
2.304(6)
1.169(8)
1.182(8)
1.157(8)
Mo1–P1
Mo1–C1
Mo1–C2
Mo1–C3
Mo1–C4
Mo1–C11
C1–O1
2.487(3)
1.967(5)
1.969(6)
1.970(5)
2.331(5)
2.321(5)
1.166(6)
1.169(7)
1.162(7)
O2–C2
C2–O2
C3–O3
C2–O2
C3–O3
O4–C4
C1–Mo1–C2
C1–Mo1–C3
C1–Mo1–C4
C1–Mo1–C5
C1–Mo1–C17
C2–Mo1–C4
C3–Mo1–C4
C4–Mo1–C5
C4–Mo1–C17
C5–Mo1–C17
C1–Mo1–C20
C1–Mo1–C2
C1–Mo–C10
C1–Mo1–C3
C1–Mo1–C30
C20–Mo1–C10
C2–Mo1–C10
C10–Mo1–C3
C10–Mo1–C30
C3–Mo1–C30
93.4(1)
88.0(1)
178.1(1)
87.4(1)
91.3(1)
88.0(1)
93.4(1)
91.3(1)
87.4(1)
94.6(1)
P1–Mo1–C1
P1–Mo1–C2
P1–Mo1–C3
P1–Mo1–C4
P1–Mo1–C18
C1–Mo1–C3
C2–Mo1–C3
C3–Mo1–C4
C3–Mo1–C18
C4–Mo1–C18
100.8(2)
P1–Mo1–C1
P1–Mo1–C2
P1–Mo1–C3
P1–Mo1–C4
P1–Mo1–C11
C1–Mo1–C3
C2–Mo1–C3
C3–Mo1–C4
C3–Mo1–C11
C4–Mo1–C11
92.9(1)
88.9(2)
177.8(2)
90.1(2)
91.4(1)
85.0(2)
90.2(2)
92.0(2)
89.1(2)
95.1(2)
85.9(2)
171.9(2)
85.3(1)
94.3(1)
85.0(3)
89.2(3)
88.6(3)
91.4(3)
94.1(2)
86.9(1)
91.5(1)
92.4(1)
94.2(1)
Mo–P bond lengths are 2.468(3) Å for 4b-Mo and 2.487(3) Å for
4c-Mo. These lengths are normal Mo–P dative bond lengths which
fall in the range 2.40–2.57 Å of previously reported complexes [29].
The Mo–C(carbene) bond lengths (2.312(6) and 2.304(6) Å for 4b-
Mo and 2.311(5) and 2.321(5) Å for 4c-Mo) are similar to those of
parent bis-NHC complexes of molybdenum (3a-Mo, 3b-Mo). Bite
angles of C(carbene)–Mo–C(carbene) are 94.1(2)° for 4b-Mo and
95.1(2)° for 4c-Mo, which are quite similar to those of parent com-
plexes 3a-Mo and 3b-Mo. In complex 4b-Mo, steric repulsion be-
tween the phosphorus ligand and the isopropyl substituent on
the imidazole ring was observed. Angles of P1–Mo1–C18
(94.3(1)°) and P1–Mo1–C1 (100.8(2)°) are larger than those of
P1–Mo1–C4 (85.3(1)°) and P1–Mo1–C2 (85.9(2)°) and thus the an-
gle of P1–Mo1–C3 (171.9(2)°), which is slightly distorted from an
ideal octahedral geometry, is smaller than that of the correspond-
ing angle in 4c-Mo (177.8(2)°).
P(OMe)3
3-Mo
toluene, reflux
- CO
R
N
(MeO)3P
OC
N
N
Mo
E
OC
OC
N
R
4a-Mo: E = o-C6H4, R = Me (y. 58%)
4b-Mo: E = o-C6H4, R = iPr (y.51%)
4c-Mo: E = CH2, R = Me (y. 92%)
Scheme 4. Reaction of [Mo(CO)4(bis-NHC)] with P(OMe)3.
The reactivity of tetracarbonyl complexes of molybdenum bear-
ing a bidentate ligand such as bpy or dppe with phosphorus com-
pounds has been reported [30,31]. In the reaction of [Mo(CO)4
(bpy)] with various phosphorus compounds (PR3) under thermal
conditions, a CO/PR3 substitution reaction cleanly took place to give
fac-[Mo(CO)3(bpy)(PR3)]. On the other hand, in the case of [Mo
(CO)4(dppe)], the geometrical isomers, i.e., fac- and mer-[Mo
(CO)3(dppe)(PR3)], have been obtained. The different reactivity be-
tween bpy and dppe might have come from the difference of the
the IR spectra, three CO stretching bands were observed at 1913,
1806, and 1765 cmꢁ1 for 4a-Mo and at 1917, 1814, and
1775 cmꢁ1 for 4b-Mo. In 31P{1H} NMR, the singlet signal in the
region of the coordinated P(OMe)3 was observed at 167.9 ppm for
4a-Mo and 168.8 ppm for 4b-Mo. Although the 13C{1H} NMR
spectrum of 4a-Mo was not obtained due to its low solubility, sub-
stantial data of 4b-Mo was obtained. The 13C{1H} NMR spectrum of
4b-Mo showed two doublets due to the carbene carbon atoms with
2
similar coupling constants (d = 195.7 (d), JPC = 13.4 Hz, and
p
-acceptability of the bidentate ligands [28,32]. Bpy, which is a
weaker -acceptor ligand than dppe, may produce only fac isomer,
whereas the phosphorus ligand which has greater -acceptor
d = 196.7 (d), 2JPC = 15.7 Hz). This result indicates that the phospho-
rus ligand locates in the cis position toward the NHC ligands. In the
region of the carbonyl carbon atoms, three doublet signals were ob-
served; two doublet showed small coupling constants (2JPC = 12.3
and 13.4 Hz) and one showed a relatively large coupling constant
(2JPC = 59.2 Hz). This coupling pattern is indicative for a fac arrange-
ment of the CO ligands. The CO ligand with the large coupling
constant is located trans to the P(OMe)3 ligand. We also examined
the reaction of complex 3c-Mo having propylene-bridged bis-NHC
with P(OMe)3 and thus fac-[Mo(CO)3(bis-NHC){P(OMe)3}] (4c-Mo)
was obtained. The 13C{1H} NMR spectrum of 4c-Mo in the region
of carbene and carbonyl carbon atoms showed similar patterns to
those of 4b-Mo. Furthermore, X-ray analysis revealed that the
structure of 4c-Mo shows a fac-form in an octahedral geometry.
The ORTEP drawings of complexes 4b-Mo and 4c-Mo are shown
in Fig. 3. The selected bond lengths and angles are listed in Table 2.
These complexes show slightly distorted fac-form in an octahedral
geometry and a twisted configuration of the bis-NHC ligand. The
p
p
strength might have formed both fac and mer isomers. In the case
of [Mo(CO)4(bis-NHC)], the fac isomer was obtained. Although the
steric influence of the bidentate ligands can not be ruled out, these
results may indicate that the electronic feature of bis-NHC ligand
is similar to that of a nitrogen donor, bpy, rather than a phosphorus
donor, dppe.
3.4. Electronic and steric properties of [Mo(CO)4(bidentate ligand)]:
experimental and theoretical investigations
In order to obtain further information about electronic and
steric properties of the bis-NHC ligand, we investigated (i) bond
lengths of Mo–CO in complexes 3a-Mo and 3b-Mo in detail, (ii)
the comparison of CO stretching frequencies of a series of [Mo
(CO)4(bid)] (bid = bis-NHC, bpy, dppe), (iii) oxidation potentials

206
K. Ogata et al. / Inorganica Chimica Acta 390 (2012) 199–209
Fig. 3. ORTEP drawings of complexes 4b-Mo (left) and 4c-Mo (right) (30% probability of thermal ellipsoids). All hydrogen atoms are omitted for clarity.
Table 3
IR spectra (
m
CO/cmꢁ1) of complex 3-Mo, [Mo(CO)4(bpy)] and [Mo(CO)4(dppe)].
3a-Mo
3b-Mo
3c-Mo
3d-Mo
[Mo(CO)4(bpy)]
[Mo(CO)4(dppe)]
Founda
1999
1896
1847
1788
1998
1877
1849
1802
1991
1874
1844
1804
1993
1879
1844
1792
2009
1918
1869
1814
2017
1922
1894
1877
Averageb
1883
1882
1877
1878
1903
1928
a
Experimental data (KBr disk).
The mean value of four CO stretching frequencies.
b
of [Mo(CO)4(bid)] by cyclic voltammograms, and (iv) 95Mo NMR
spectroscopy of the bis-NHC complex 3-Mo. The DFT calculations
were also carried out from a viewpoint of the relative stability
between C2-symmetric and Cs-symmetric structures for 3a-Mo.
Table 4
Cyclic voltammetric data of 2a, 2c, 3a-Mo, 3c-Mo, and [Mo(CO)4(bid)] (bid = bpy,
dppe) in CH3CN (vs. Fc/Fc+).
Compound
Epa1/V
Epa2/V
Epa3/V
Epa4/V
2a
2c
3a-Mo
3c-Mo
[Mo(CO)4(bpy)]
[Mo(CO)4(dppe)]
0.43
0.42
0.44
0.47
0.72
0.70
0.70
0.73
0.71
3.4.1. Structural features in bond lengths of Mo–CO
ꢁ0.07
ꢁ0.15
0.19
1.09
0.95
The strong donor ability of the carbene ligands is clearly illus-
trated by the different Mo–CO bond lengths of the carbonyl ligands
that are cis and trans to the NHC ligands. Hahn and co-workers
have reported that the Mo–C(carbonyl) bond length in the trans
position of the NHC ligand is shorter than that in the cis position
of the NHC ligand [8]. Similar tendencies were observed in com-
plexes 3a-Mo and 3b-Mo (Table 2). The Mo–C(carbonyl) bond
lengths in the trans position to the NHC ligand (1.968(3),
1.968(3) Å for 3a-Mo and 1.976(3) Å for 3b-Mo) are shorter than
those in the cis position (2.035(3), 2.043(3) Å for 3a-Mo and
2.035(4) Å for 3b-Mo). Furthermore, the C–O bond lengths of the
CO ligand trans to the NHC ligand (1.158(4), 1.161(3) Å for 3a-
Mo and 1.157(4) Å for 3b-Mo) are somewhat longer than those
of cis (1.140(4), 1.141(3) Å for 3a-Mo and 1.140(4) Å for 3b-Mo).
0.55
for 3a-Mo (1883 cmꢁ1) was 20 and 45 cmꢁ1 lower in frequency
than those of [Mo(CO)4(bpy)] and [Mo(CO)4(dppe)], respectively.
These results clearly demonstrate that bis-NHC ligand shows
stronger electron donor ability and weaker
p-acceptor than bpy
or dppe.1
3.4.3. Cyclic voltammograms of molybdenum complexes
In order to investigate electronic properties of the central metal,
the cyclic voltammograms (CVs) of molybdenum complexes were
measured in CH3CN. The results are listed in Table 4. The CV of
3a-Mo showed irreversible oxidation waves at the potentials of
ꢁ0.07, 0.44, 0.73, and 1.09 V vs. Fc/Fc+. The oxidation potentials
of 0.44 and 0.73 V were similar to those of the BEt3-adducts of
NHC 2a and thus these waves were assigned to the oxidation
caused by the NHC ligand [34]. The oxidation wave at ꢁ0.07 V
3.4.2. CO stretching frequency in IR spectra
Comparison of the IR stretching frequency of the CO ligand is
one of the reliable methods to estimate the electron donor ability
of NHC ligands [33]. The IR data of complexes 3-Mo were com-
pared with those of [Mo(CO)4(bid)] (bid = bpy, dppe). The data
are summarized in Table 3. In a series of complexes 3-Mo, there
are a little influence of the electron donor ability of NHC ligands
concerning the substituent on the nitrogen atom or the linker bind-
ing two imidazole rings. On the other hand, the mean value of mCO
1
The bis-NHC ligand shows a stronger electron donor ability than bpy or dppe. This
tendency was supported by the DFT calculations (B3LYP/DGDZVP) of 3a-Mo,
[Mo(CO)4(bpy)], and [Mo(CO)4(dppe)]. See the Supplementary material.

K. Ogata et al. / Inorganica Chimica Acta 390 (2012) 199–209
207
O
C
O
C
N
C
N
N
C
C
C
Mo
C
O
Mo
C
O
C
N
C
C
O
C
O
Fig. 4. Optimized structures of the twisted bis-NHC complex (C2-symmetric structure) (left) and the parallel bis-NHC complex (Cs-symmetric structure) (right) calculated by
B3LYP/DGDZVP. All hydrogen atoms are omitted for clarity.
was corresponded to the one-electron oxidation of the molybde-
num center of 3a-Mo. The one-electron oxidation wave of 3c-Mo,
the propylene-bridged analog of 3a-Mo, showed slightly lower po-
tential of ꢁ0.15 V than that of 3a-Mo. The CVs of [Mo(CO)4(bid)]
(bid =
bpy, dppe) in CH3CN were measured under the same conditions.
These complexes showed one-electron oxidation wave at 0.19 V
for [Mo(CO)4(bpy)] and 0.55 V for [Mo(CO)4(dppe)] vs. Fc/Fc+.
Based on the one-electron oxidation potentials in those complexes,
3.5. Conformations of bis-NHC ligand in octahedral geometry
As mentioned above, we revealed that the bis-NHC ligand bear-
ing the hydrocarbon linker such as o-xylylene, propylene, or ethyl-
ene showed the twisted conformation in an octahedral geometry of
group 6 transition metal complexes. This result is reverse tendency
to the late transition metal complexes, in which the orientation of
the azole rings adopts a perpendicular orientation (Cs-symmetric
structure in Chart 1). To investigate the possibility of the formation
of a Cs-symmetric structure in an octahedral geometry, we applied
the DFT calculations to both isomers of molybdenum complexes
bearing the o-xylylene-bridged bis-NHC ligand. The results of the
optimized structures are shown in Fig. 4.
it was conceivable that p-acceptability of the ligands in molybde-
num complexes increases in order of NHC < bpy < dppe. In other
words, the electron density accumulated at the Mo center in-
creases in the reverse order. This tendency is in good agreement
with that observed in the CO stretching frequencies of molybde-
num complexes (Table 3).
In the C2-symmetric structure, the calculated Cap–Mo–Cap angle
(179.77°), in which Cap denotes the carbonyl carbon atom in the
apical position, is somewhat larger than the experimental value
(173.6(1)°) observed by X-ray analysis of 3a-Mo. On the other
hand, in the Cs-symmetric structure, the Cap–Mo–Cap angle was cal-
culated to be 160.32°, which is highly distorted from an ideal octa-
hedral geometry. In the case of the C2-symmetric geometry, two
methyl groups on the imidazole rings directed toward the open
space between apical and equatorial CO ligands and thus this con-
formation adopts an ideal octahedral geometry. In the Cs-symmet-
ric structure, two methyl groups and two methylene groups are
oriented to the CO ligands in the apical positions and thus there
is the steric repulsion between the CO ligands and these groups at-
tached on the NHC ligand in this conformation. The Gibbs energy
difference between these two isomers is estimated to be
6.69 kcal molꢁ1 by the DFT calculations. The C2-symmetric struc-
ture is found to be more stable than the Cs-symmetric one in an
octahedral geometry.
On scanning between ꢁ0.45 and 0.15 V in a CH3CN solution,
carbene complex 3a-Mo showed a quasi-reversible redox wave
(Epa = ꢁ0.07 V, Epc = ꢁ0.16 V). On the other hand, bpy and dppe
complexes showed an irreversible oxidation wave under the same
conditions. Similar results have been already reported by Morse
and Ackermann, independently [35]. In their reports, the irrevers-
ible wave shows the formation of seven-coordinate complexes by
coordination of a solvent (CH3CN) in the oxidized state. Therefore,
we considered that, in complex 3a-Mo, the strong donor ability of
the bis-NHC ligand stabilizes the oxidized molybdenum center by
preventing the solvent molecule from coordination to the metal.
Furthermore, from a steric point of view, the NHC ligand might
give a significant influence on the stability of the oxidized form.
3.4.4. 95Mo NMR spectroscopy of molybdenum complexes
In order to estimate the electronic feature at the Mo center,
95Mo NMR measurements of molybdenum complexes, 3a-Mo,
3b-Mo, and 3c-Mo, were undertaken. These complexes showed
similar chemical shifts; ꢁ1657 ppm for 3a-Mo, ꢁ1637 ppm for
3b-Mo, and ꢁ1649 ppm for 3c-Mo. These results suggested that
the substituent on the nitrogen atom or the linker group of the
bis-NHC ligand have a little influence on the chemical shift in
95Mo NMR. We compared the 95Mo chemical shift of 3-Mo with
those of [Mo(CO)4(bid)] (bid = bpy, dppe). It has been reported that
the resonances of [Mo(CO)4(bpy)] and [Mo(CO)4(dppe)] are ob-
served at ꢁ1190 ppm and ꢁ1782 ppm, respectively [35b,36]. The
95Mo shielding increases in the order [Mo(CO)4(bpy)] < 3-Mo
< [Mo(CO)4(dppe)], whereas this fact did not reflect the tendency
observed in the IR and CV spectra (vide supra). Therefore it might
be difficult to discuss the electronic feature at the Mo center of a
bis-NHC complex comparing with bpy or dppe complexes by
95Mo NMR spectroscopy, because of difference of the coordinating
atoms (C, N, and P) to the metal.
4. Conclusion
The BEt3-adducts of bis-NHCs 2, prepared by the reaction of
imidazolium pro-ligand with two moles of LiBEt3H, were used effi-
ciently to prepare bis-NHC complexes of group 6 transition metals.
The systematic preparations of bis-NHC complexes bearing o-
xylylene, propylene, and ethylene linkers were achieved using this
method. These complexes are found to possess the C2-symmetric
structure in an octahedral geometry. This conformation of the
bis-NHC ligand in the group 6 transition metal complexes is
remarkably different from those in late transition metal com-
plexes. This tendency observed in group 6 transition metal com-
plexes is found to come from the steric repulsion between the
bis-NHC ligand and the CO ligands coordinating in the apical
positions.

208
K. Ogata et al. / Inorganica Chimica Acta 390 (2012) 199–209
(h) S.K. Schneider, J. Schwarz, G.D. Frey, E. Herdtweck, W.A. Herrmann, J.
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num complexes, [Mo(CO)4(bis-NHC)] (3-Mo), as well as by the
reaction of 3-Mo with P(OMe)3 to give fac-form of molybdenum
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Society for the Promotion of Science (Grant-In-Aid for Scientific Re-
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