Synthesis and oxidation of (benzimidazolylidene)Cr(CO)5 complexes

Article abstract of DOI:10.1016/j.jorganchem.2005.01.030

A series of (benzimidazolylidene)pentacarbonylchromium(0) complexes were synthesized by the addition of lithium 1,2-phenylenediamides to Cr(CO) ₆ in the presence of 12-crown-4, followed by the treatment of chlorotrimethylsilane. X-ray crystal s

Full text of DOI:10.1016/j.jorganchem.2005.01.030

Journal of Organometallic Chemistry 690 (2005) 1750–1755
www.elsevier.com/locate/jorganchem
Synthesis and oxidation of (benzimidazolylidene)Cr(CO)5 complexes
1
Hidehiro Sakurai , Koichi Sugitani, Toshiyuki Moriuchi, Toshikazu Hirao
*
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565-0871, Japan
Received 26 November 2004; accepted 20 January 2005
Available online 2 March 2005
Abstract
A series of (benzimidazolylidene)pentacarbonylchromium(0) complexes were synthesized by the addition of lithium 1,2-phenyl-
enediamides to Cr(CO)6 in the presence of 12-crown-4, followed by the treatment of chlorotrimethylsilane. X-ray crystal structural
analysis revealed a rather single-bond-like character between Cr and the carbene carbon. One reversible oxidation process was
observed in the cyclic voltammograms of these complexes. Chemical oxidation using (4-BrC6H4)3NSbCl6 was studied by ESR to
afford a hyperfine structure derived not from Cr but from N and H, indicating that the spin is delocalized on the benzimidazolidene
moiety.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Cr(0) complex; Benzimidazolylidene; Carbene complex; Oxidation
1. Introduction
ple carbene-transferring reagent [3]. Use of the more sta-
ble carbene ligands, which may coordinate to chromium
with the r donor character, is expected to provide the
more stable complexes with chromium pentacarbonyl.
On the other hand, these type of carbenes show a very
little p-acceptor character because it is rather difficult
to induce back donation from d-orbital to pp-orbital.
The ratio of the back donation can be increased if the
mesomeric stabilization of the heterocyclic moiety is re-
duced. Thus, the oxidized form of the imidazolylidene li-
gand is considered to increase the contribution of the
back donation from metal center. Although some reac-
tions of group 6 metal carbene complexes induced by
oxidation and/or reduction have been reported [4], there
are few reports concerning with the study of oxidized/re-
duced intermediates [5,6]. To clarify the oxidized state,
the benzimidazolylidene ligand was chosen partly be-
cause the radical cation species might be stabilized by
delocalization through the enlarged p-conjugation. In
this paper, we describe a general route to a series of
(benzimidazolylidene)Cr(CO)5 complexes, and their
structural and electronic characterization.
N-Heterocyclic carbenes such as imidazolylidenes
possess advantageous characters as organometallic li-
gands, to broaden the scope of applications accessible
to phosphanes [1]. N,N0-Heterocyclic carbenes exist in
a singlet (r2) electronic ground state. Inductive effect
of the nitrogen atoms induces a large r-pp gap by stabi-
lizing the non-bonding r orbital while leaving the pp
orbital energetically unchanged. Additional mesomeric
stabilization of the heterocyclic carbene by the lone pairs
at the nitrogen substituents results formally in a three-
center, four-electron p-system [2]. The heteroatom-
stabilized group 6 metal carbene complexes have
received considerable attention due to the higher stabil-
ity than the corresponding free carbenes. The carbene
complexes have been employed as far beyond as a sim-
*
Corresponding author. Tel.: +81 6 6879 7413; fax: +81 6 6879 7415.
E-mail address: hirao@chem.eng.osaka-u.ac.jp (T. Hirao).
1
Present address. Research Center for Molecular-Scale Nanosci-
ence, Institute for Molecular Science, Myodaiji, Okazaki 444-8787,
Japan.
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.01.030

H. Sakurai et al. / Journal of Organometallic Chemistry 690 (2005) 1750–1755
1751
2. Results and discussion
a slightly pale yellow crystal. As expected, these com-
plexes are stable enough to be handled under air.
2.1. Synthesis and characterization of chromium
complexes
Me
N
∆
HCr(CO)5
+
H
¨
Since the first report by Ofele in 1968 [7], chro-
N
mium complexes bearing the imidazolylidene ligand
have been synthesized usually from the free imidazol-
ylidene and coordinatively unsaturated ‘‘Cr(CO)5’’
species (Eq. (1)). However, not a few imidazolylidenes
have been isolated so far. Although several synthetic
methods without isolation of imidazolylidenes have
been developed [8,9], a more convenient route for
the direct preparation from the corresponding 1,2-
phenylenediamine derivatives was investigated. A new
synthetic pathway to the benzimidazolylidene com-
plexes is shown in Scheme 1. 4-Dimethylaminophenyl
and mesityl (2,4,6-trimethylphenyl) groups are selected
as a substituent on the nitrogen. 1,2-Dibromobenzene
was treated with N,N0-dimethyl-1,4-phenylenediamine
in the presence of t-BuONa, 10 mol% of Pd(OAc)2,
and 2,20-bis(diphenylphosphino)-1,10-binaphthyl (BI-
NAP) in toluene [10,11], to give 3 in 90% yield. The
mesityl derivatives 4 was also obtained in the similar
manner in 86% yield. Thus-obtained diamine 3 or 4
was deprotonated with BuLi in the presence of 12-
crown-4, followed by the addition of Cr(CO)6, to gen-
erate the carbamoyl chromate intermediate A [12].
Chlorotrimethylsilane promoted the second intra-
molecular nucleophilic attack [13] of the lithium amide
B. After the silica-gel column chromatography and
recrystalization from MeCN or hexane, (1,3-diaryl-2-
benzimidazolylidene)Cr(CO)5 1 or 2 was obtained as
Me
Me
N
Me
N
Cr(CO)5
+
(1)
Cr(CO)5
N
Me
N
Me
Chemical shifts of the carbene carbons in the 13C
NMR spectra and IR streching bands of the trans-car-
bonyl groups for representative Fischer-type chromium
carbene complexes, together with 1 and 2 are listed in
Table 1 [7,14,15]. As for 13C NMR, the more p-donating
substituent such as the dialkylamino group tends to
cause the higher-field shift of the carbene carbon, as
exemplified by complexes 5 and 6, 7 and 8. The chemical
shifts of the carbene carbons of 1 and 2 are 209.8 and
210.0 ppm, respectively, indicating that the N,N0-di-
aryl-1,2-phenylenediamine units possess very strong
p-donating character. This inclination is in good agree-
ment with the chemical shifts of the related imidazolilyd-
ene complexes 9 and 10.
The frequency assignable to the trans-carbonyl group
reflects on the p-back donation character of the ligands.
The wavelength of mCOtrans of Fischer-type chromium
carbene complexes ranges from 1930 to 1960 cmꢀ1
.
The imidazolilydene complexes 9 and 10 showed the
RNH2
10 mol% Pd(OAc)2
10 mol% BINAP
t-BuONa
12-crown-4
BuLi
Cr(CO)6
R
R
N
O
Br
Br
NH
Cr(CO)5
toluene, reflux, 12 h
NH THF, –78 ˚C, 15 min,
N
R
then 0 ˚C, 30 min
Li(12-crown-4)
R
3
90%
4
86%
A
OSiMe3
Cr(CO)5
R
N
R
N
Me3SiCl
Cr(CO)5
N
–78 ˚C, 15 min,
then 0 ˚C, 30 min,
then rt, 3 h
N
R
R
Li(12-crown-4)
1
2
(from 3 or 4)
31% 13%
B
Me
Me
Me
NMe2
1, 3
R =
2, 4
Scheme 1. Preparation of the (benzimidazolilydene)Cr(CO)5 complexes 1 and 2 via double nucleophilic attack route.

1752
H. Sakurai et al. / Journal of Organometallic Chemistry 690 (2005) 1750–1755
Table 1
Selected data for the representative chromium carbene complexes
including 1 and 2 [7,14,15]
wavelength around 1925 cmꢀ1 due to the lack of their p-
back donation character. On the other hand, vCOtrans of
1 and 2 was further shifted to 1912 and 1916 cmꢀ1
,
respectively. These results indicate that the benzimidaz-
olilydene complexes 1 and 2 exhibit very strong p-donat-
ing and very weak p-back donating characters. It should
be noted that the values of mCOtrans are not attributed
only to p-back donation abilities [16].
2.2. X-ray crystallography
Fig. 1. ORTEP diagram of 1 and 2 and the crystal packing of 1.
Hydrogen atoms are omitted for clarity.
The structures of 1 and 2 were furthermore deter-
mined by X-ray crystallography (Fig. 1). The bond
lengths of Cr-C1(carbene) in 1 and 2 are 2.125 and
˚
2.131 A, respectively, representing medium bond char-
acters between a single bond and a double bond one
dimethylaminobenzene ring of 1. The torsion angles of
N1, Cl, Cr1, C44 in 1 and 2 were 116.5ꢁ and 92ꢁ, respec-
tively, suggesting that the degree of d-pp orbital overlap
is sterically affected by the steric effect of the N-substitu-
ent attached to the benzimidazolidene ring.
The electronic repulsion between the p-electrons of
the N-attached benzene ring and the carbonyl group is
considered to cause the distortion of coordination geom-
etry of Cr center, resulting in the angles C41–Cr1–C43
˚
˚
(cf. 8, 2.116 A; 10, 2.139 A). Little d-pp electronic back
donation is present in both complexes. The plane angles
between the N-attached benzene ring and center im-
idazolidene ring were 79.1ꢁ and 99.8ꢁ for 1, and 96.2ꢁ
and 91.6ꢁ for 2, respectively. Due to the steric effect of
the methyl group of 2, the mesitylene ring of 2 was more
perpendicular to the central imidazolidene ring than the

H. Sakurai et al. / Journal of Organometallic Chemistry 690 (2005) 1750–1755
1753
of 169.56(10)ꢁ for 1 and 164.7(1)ꢁ for 2. Furthermore,
the benzimidazolidene rings of 1 are intermolecularly
overlapped in a face-to-face manner to form the p-stack
dimmer in the crystal packing.
2.3. Oxidation of the chromium complexes
Electrochemical behavior of 1 and 2 was next investi-
gated. In the cyclic voltammogram of 1 in CH2Cl2, one
reversible oxidation wave and one irreversible oxidation
wave were observed at the potentials of 330 and 770 mV
vs. Fc/Fc+, respectively, being in contrast to one revers-
ible oxidation wave at 450 mV with 2 (Fig. 2). These
reversible oxidation waves might correspond to the
one-electron process attributable to the oxidation of
the benzimidazolylidene ring, suggesting that the radical
cation species is stable.
To confirm the radical cation species, chemical
oxidation of 2 was conducted. As an oxidant, tris-
(4-bromophenyl)aminium hexachloroantimonate [17]
(E1/2 = 0.70 V vs. Fc/Fc+) was chosen. Chemical oxida-
tion of 2 with one equivalent of (4-BrC6H4)3NSbCl6 in
CH2Cl2 at 233 K yielded a reddish purple solution,
which might correspond to a radical species of the imi-
dazolylidene ring, and the color did not change when
the solution was warmed up to 297 K. The ESR spec-
trum at 297 K showed no peak derived from the cou-
pling of 53Cr, but a hyperfine structure resulting from
14N and 1H nuclei at the benzimidazolylidene ring
(g = 2.005, AN = 5.0 G, AH = 1.3 G) (Fig. 3). These cou-
pling constant suggests that the radical cation is delocal-
ized at the benzimidazolylidene ring. The very similar
ESR spectrum was also observed in the oxidation of 1
(g = 2.005, AN = 5.0 G, AH = 1.3 G) with (4-BrC6H4)3-
NSbCl6. In contrast, in LappertÕs imidazolylnylidene
chromium radical cation complexes [5], these radicals
are not coupled with nitrogen but coupled with only
Cr and coordinated phosphines (Fig. 4). These differ-
Fig. 3. ESR spectra of radical cation species of 1 and 2 and simulation.
Et
–
BF4
N
Cr(CO)3(dmpe)
Me2P PMe2
N
Et
2
dmpe =
g = 2.019
AP = 2.2 mT
ACr = 1.3 mT
10 µA
1
Fig. 4. LappertÕs radical cation complex of (imidazolynylidene)-
Cr(CO)3(dmpe).
-2.5 -2.0 -1.5 -1.0 -0.5
V vs Fc / Fc+
0
0.5 1.0
E1ox(V)
E2ox(V)
Complex
ences suggest that the electronic character of chromium
carbene complexes is dependent on the ligands. The
benzimidazolylidene ligand, which possesses larger p-
conjugation, effectively stabilizes the radical cation spe-
cies, diminishing the donation effect from d-orbital of Cr
to pp-orbital.
1
2
0.33
0.45
0.77
–
Fig. 2. Cyclic voltammograms of 1 and 2. 5.0 · 10ꢀ4 M in CH2Cl2
(0.1 M Bu4NPF6) with a platinum working electrode at 100 mV/s scan
rate.

1754
H. Sakurai et al. / Journal of Organometallic Chemistry 690 (2005) 1750–1755
3. Summary
4.2. Synthesis of (benzimidazolylidene)penta-
carbonylchromium(0) complexes
As described above, (benzimidazolylidene)pentacar-
bonylchromium(0) complexes were synthesized and
characterized. Spectroscopic and X-ray crystal struc-
tural analysis of the neutral carbene complex revealed
that the back-donation effect from d-orbital of Cr to
pp-orbital of carbene carbon is limited. Chemical oxi-
dation of 1 and 2 afforded the radical cation com-
plexes, which were detected by ESR spectra. Since
the hyperfine structure resulting from the coupling of
53Cr was not observed, the d-pp back donation effect
is also restricted even in the oxidized state. It is prob-
ably because the expansion of p-conjugation in the li-
gand affects the characters of the chromium
complexes. These phenomena indicate that the ligand
design of carbene complexes might be important to
control the characteristics and reactivities of the metal
center.
4.2.1. N,N0-Bis(4-dimethylaminophenyl)-1,2-
phenylenediamine (3)
A well-dried three-necked round bottomed flask
equipped with a condenser, a magnetic stirring bar,
and
a
septum, was charged with (rac)-BINAP
(250 mg, 0.4 mmol), Pd(OAc)2 (90 mg, 0.4 mmol),
and toluene (20 mL). The mixture was heated to
60 ꢁC with stirring, then 1,2-dibromobenzene
(242 lL, 2.0 mmol) was added, followed by N,N0-di-
methyl-1,4-phenylenediamine (637 mg, 4.7 mmol) and
t-BuONa (480 mg, 5.0 mmol). The reaction mixture
was heated to reflux until the 1,2-dibromobenzene
was consumed (monitored by TLC). After cooling to
room temperature, the reaction mixture was filtered
through Celite and silica-gel pad and was washed with
AcOEt, then the solvent was evaporated under vac-
uum. Column chromatography (hexane/AcOEt = 3/2,
Rf = 0.26) of the residue afforded 624 mg (90%) of 3;
m.p. 108 ꢁC (from hexane); IR (KBr) 3358, 2878,
2359, 1517, 1339, 1286 cmꢀ1 1H NMR (300 MHz,
;
4. Experimental
CDCl3) d = 2.89 (12H, s), 5.35 (2H, br), 6.74 (4H, d,
J = 9.0 Hz), 6.82 (2H, dd, J = 3.3 and 5.7 Hz), 6.92
(4H, d, J = 9.0 Hz), 7.04 (2H, dd, J = 3.3 and
5.7 Hz); 13C NMR (75 MHz, CDCl3) 41.5, 114.4,
117.7, 120.8, 121.3, 134.1, 135.9, 146.3 ppm. Found:
C, 76.19; H, 7.56; N, 16.03%. Calc. for C22H26N4:
C, 76.27; H, 7.56; N, 16.17%.
4.1. General procedures
All melting points are uncorrected. 1H (300 MHz)
and 13C (75 MHz) NMR spectra were measured on
a Varian Mercury 300 spectrometer. CDCl3 was used
as a solvent and residual chloroform (d = 7.24 ppm;
13C, 77.0 ppm) or Me4Si was used as an internal stan-
dard. Infrared spectra were recorded on JASCO FT/
IR-480plus. Mass spectra were measured on a JEOL
JMS-DX-303 spectrometer using either electron im-
pact (EI) or chemical ionization (CI) modes. ESR
spectra were measured with Bruker ESP 300 electron
spin resonance spectrophotometer. The cyclic volta-
mmetry measurements were performed on a BAS
CV-50W voltammetry analyzer in deaerated CH2Cl2
containing 0.1 M Bu4NClO4 as a supporting electro-
lyte at 298 K with a three-electrode system consisting
of a stationary platinum working electrode (BAS), a
platinum auxiliary electrode (BAS), and an Ag/AgCl
(0.01 M) reference electrode (BAS) at 100 mV/s scan
rate. Potentials are given vs. Fc/Fc+. Elemental analy-
ses were carried out at the Analytical Center, Gradu-
ate School of Engineering, Osaka University. Column
chromatography was conducted on silica gel (Wakogel
C-200). Toluene was freshly distilled from CaH2. THF
was purchased from Kanto Chemical Co., Inc. as
dehydrated stabilizer free grade, and were collected
with argon stream introduced directly into the appara-
tus. All reagents were purchased from commercial
sources and were further purified prior to use with
the standard methods. All of the operations were per-
formed under argon atmosphere.
4.2.2. N,N0-Bis(mesityl)-1,2-phenylenediamine (4)
Compound 4 was prepared from 2,4,6-trimethylani-
line according to the similar procedure as the prepara-
tion of 3. Yield 86%; m.p. 180 ꢁC (from hexane); IR
(KBr) 3333, 3015, 2964, 2915, 2854, 1598, 1507, 1498,
1482, 1457, 1399 cmꢀ1 1H NMR (300 MHz, CDCl3)
;
d = 2.19 (12H, s), 2.31 (6H, s), 5.11 (2H, br), 6.29 (2H,
dd, J = 3.3 and 5.7 Hz), 6.66 (2H, dd, J = 3.3 and
5.7 Hz), 6.95 (4H, s); 13C NMR (75 MHz, CDCl3)
18.2, 20.9, 114.2, 119.9, 129.2, 133.3, 133.9, 135.1,
136.8 ppm. Found: C, 83.28; H, 8.06; N, 8.09%. Calc.
for C24H28N2: C, 83.68; H, 8.19; N, 8.13%.
4.2.3. (1,3-Bis(40-dimethylaminophenyl)benzimidazol-2-
ylidene)pentacarbonyl chromim(0) (1)
A well-dried three-necked round bottomed flask
equipped with a condenser, a magnetic stirring bar,
and a septum, was charged with 12-crown-4 (867 lL,
5.36 mmol), which was azeotropically dehydrated with
toluene. After the addition of THF (12 mL) solution
of the 1,2-phenylenediamine 3 (928 mg, 2.68 mmol),
the mixture was cooled to ꢀ78 ꢁC and BuLi
(4.53 mL, 5.36 mmol) was added dropwise via syringe.
A THF (20 mL) solution of Cr(CO)6 (590 mg,
2.68 mmol) was added to the reaction mixture via can-

H. Sakurai et al. / Journal of Organometallic Chemistry 690 (2005) 1750–1755
1755
nula. The solution was stirred for 15 min at ꢀ78 ꢁC
and for 30 min at 0 ꢁC, then was cooled to ꢀ78 ꢁC
again. Chlorotrimethylsilane (400 lL, 3.15 mmol) was
added dropwise. The thus-obtained reaction mixture
was stirred for 15 min at ꢀ78 ꢁC, for 30 min at 0 ꢁC,
and then for 1 h at room temperature. The solvent
was removed under vacuum. The crude product was
purified by silica-gel column chromatography
(CH2Cl2) to afford 451 mg (31%) of 1; m.p. 172 ꢁC
(dec.) (from MeCN); IR (KBr) 2051, 1972, 1912,
graphite monochromated Mo Ka radiation. The struc-
tures of 1 and 2 were solved by direct methods and ex-
panded using Fourier techniques. The non-hydrogen
atoms were refined anisotropically. The H atoms were
placed in idealized positions and allowed to ride with
the C atoms to which each was bonded. Crystallo-
graphic details are given in Table 2. Crystallographic
data (excluding structure factors) for the structure re-
ported in this paper have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary
publication no. CCDC-256806 for 1 and no. CCDC-
256807 for 2. Copies of this information may be ob-
tained free of charge from: The Director, CCDC, 12 Un-
ion Road, Cambridge, CB2 1EZ, UK (fax: +44 1223 336
033; email: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
1895 cmꢀ1
;
1H NMR (300 MHz, CD2Cl2) d = 3.07
(12H, s), 6.88 (4H, d, J = 8.4 Hz), 6.89 (2H, dd,
J = 3.0 and 6.0 Hz), 7.17 (2H, dd, J = 3.0 and
6.0 Hz), 7.31 (4H, d, J = 8.4 Hz); 13C NMR
(75 MHz, CDCl3) 40.0, 110.3, 112.1, 122.5, 127.2,
129.5, 137.2, 151.0, 209.8 (Ccarbene), 217.0 (COcis),
222.1 (COtrans) ppm. Found: C, 61.05; H, 4.42; N,
10.20%. Calc. for C28H24CrN4O5: C, 61.31; H, 4.41;
N, 10.21%.
Acknowledgements
4.2.4. (1,3-Bis(20,40,60-trimethylphenyl)benzimidazol-2-
ylidene)pentacarbonyl chromium(0) (2)
The use of the facilities of the Analytical Center,
Graduate School of Engineering, Osaka University is
acknowledged. This work was supported by Grant-
in-Aid for Scientific Research on Priority Areas (No.
15036240, ‘‘Reaction Control of Dynamic Complexes’’)
from Ministry of Education, Culture, Sports, Science
and Technology, Japan.
Compound 2 was prepared from 4 according to the
similar procedure as the preparation of 1 except hex-
ane/EtOAc = 95/5 was used as an eluent for chromatog-
raphy. Yield 13%; m.p. 161 ꢁC (dec.) (from hexane); IR
(KBr) 2052, 1950, 1916, 1883 cmꢀ1
;
1H NMR
(300 MHz, CD2Cl2) d = 2.00 (12H, s), 2.42 (6H, s),
6.77 (2H, dd, J = 3.3 and 6.3 Hz), 7.13 (4H, s), 7.21
(2H, dd, J = 3.3 and 6.3 Hz); 13C NMR (75 MHz,
CD2Cl2) 17.1, 20.7, 110.1, 123.3, 129.4, 134.0, 135.4,
136.2, 139.7, 210.0 (Ccarbene), 215.7 (COcis), 220.9
(COtrans) ppm. Found: C, 65.97; H, 4.83; N, 5.11%.
Calc. for C30H26CrN2O5: C, 65.93; H, 4.80; N, 5.13%.
References
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¨
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Table 2
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1
2
Formula
C
548.12
28H24CrN4O5
C30H26CrN2O5
546.12
Formula weight
Crystal system
Space Group
[11] F.M. Rivas, U. Riaz, A. Giessert, J.A. Smulik, S.T. Diver, Org.
Lett. 3 (2001) 2673.
Triclinic
ꢀ
Monoclinic
P21/c (#14)
9.8901(3)
P1 (#2)
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461 (1993) 99.
[13] R. Imwinkelried, L.S. Hegedus, Organometallics 7 (1988) 702.
˚
a (A)
10.886(1)
13.728(1)
10.2393(8)
101.392(3)
109.249(4)
67.020(4)
1325.8(2)
2
˚
b (A)
17.6812(6)
16.6486(7)
˚
c (A)
¨
[14] K. Ofele, W.A. Herrmann, D. Mihalios, M. Elison, E. Herdtweck,
a (ꢁ)
b (ꢁ)
c (ꢁ)
W. Scherer, J. Mink, J. Organomet. Chem. 459 (1993) 177.
[15] A. Hafner, L.S. Hegedus, G. deWeck, B. Hawkins, K.H. Do¨tz,
J. Am. Chem. Soc. 110 (1988) 8413.
101.949(1)
3
˚
Volume (A )
2848.2(2)
4
[16] For more discussion, see: M.-T. Lee, C.-H. Hu, Organometallics
23 (2004) 976, and references cited therein.
Z
R1
wR2
0.046
0.157
0.051
0.181
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