New Group 6 metal carbonyl derivatives of 2-(2'(-pyridyl)benzimidazole: synthesis and spectroscopic studies.

Article abstract of DOI:10.1016/S1386-1425(02)00330-X

Reaction of Cr(CO)(6) with 2-(2'-pyridyl)benzimidazole (pbiH) under reduced pressure resulted in the formation of the dinuclear complex [Cr(2)(CO)(6)(pbiH)(2)]. Infra-red (IR) spectroscopy revealed the presence of terminal and bridge Cr-CO bonds. Interact

Full text of DOI:10.1016/S1386-1425(02)00330-X

Spectrochimica Acta Part A 59 (2003) 1341Á1347
/
www.elsevier.com/locate/saa
New Group 6 metal carbonyl derivatives of
2-(2?-pyridyl)benzimidazole: synthesis and spectroscopic
studies
Mostafa M.H. Khalil a, Hassan A. Mohamed b, Samir M. El-Medani b,
Ramadan M. Ramadan a,*
a Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt
b Chemistry Department, Faculty of Science, Cairo University, El-Faiyum Branch, El-Faiyum, Egypt
Received 20 June 2002; received in revised form 5 September 2002; accepted 13 September 2002
Abstract
Reaction of Cr(CO)6 with 2-(2?-pyridyl)benzimidazole (pbiH) under reduced pressure resulted in the formation of the
dinuclear complex [Cr2(CO)6(pbiH)2]. Infra-red (IR) spectroscopy revealed the presence of terminal and bridge CrꢀCO
bonds. Interaction of M(CO)6, MꢁCr, Mo and W, with pbiH in the presence of 2,2?-bipyridine (bpy) gave the
tetracarbonyl complexes [M(CO)4(pbiH)]×bpy. Spectroscopic studies of the complexes indicated the presence of
/
/
/
hydrogen bonding between the bpy nitrogen and the NH group of pbiH. Reactions of M(CO)6 with pbiH in the
presence of PPh3 gave the tricarbonyl monosubstituted derivatives [M(CO)3(PPh3)(pbiH)]. The spectroscopic studies of
the complexes suggested the proposed structures.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Metal carbonyls; Benzimidazole complexes; PPh3 complexes; Spectra; Charge transfer; Hydrogen bonding; Proton transfer
1. Introduction
and LHꢁ/benzotriazole or benzimidazole) were
characterized by electrochemical and spectro
electro-chemical methods. The E1/2 versus pH
curves were used to illustrate the relationship
Interest in transition metal complexes of imida-
zoles and benzimidazoles stems from their abilities
of proton, metal-to-ligand or ligand-to-metal
between the redox and acidÁbase species, and
/
charge transfers [1Á
ruthenium complexes of the type [Mꢀ
(where Mꢁ
/
4]. For example, iron and
their interconversion equilibrium. The dependence
of the MIII/MII couple on the pH was taken into
account in order to evaluate the usefulness of such
simple complexes as models for proton-coupled
electron transfer. The results were interpreted in
terms of the acceptor/donor electronic character of
/LH]n
/
[Ru(NH3)5]2ꢁ,3ꢁ or [Fe(CN)5]3ꢀ,2ꢀ
* Corresponding author. Tel.: ꢁ
/
20-966-7-2294069.
the ligands and s, p-metalÁ
/
ligand interactions in
E-mail
address:
r_m_ramadan@yahoo.com
(R.M.
Ramadan).
both redox states of the metal ion [5]. On the other
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 3 3 0 - X

1342
M.M.H. Khalil et al. / Spectrochimica Acta Part A 59 (2003) 1341Á1347
/
hand, the binuclear ruthenium complexes
[(NH3)5RuÁLÁ benzotriazo-
Ru(edta)]n (where Lꢁ
late or benzimidazolate; edtaꢁethylenediaminete-
traacetate; nꢂ1, 0, ꢀ1) exhibited intramolecular
complexes were also characterized by elemental
analysis (PerkinÁElmer 2400 CHN elemental
analyzer) and mass spectroscopy (Finnigan MAT
/
/
/
/
/
/
/
SSQ 7000).
electron transfer phenomena with a distinct redox
and metal-to-ligand and ligand-to-metal charge
transfer behaviour [6]. Previous reports showed
2.3. Syntheses of complexes
the synthesis of [M(CO)4(pbiH)], Mꢁ
/
Mo and W,
2.3.1. Synthesis of [Cr2(CO)6(pbiH)2]
from the reactions of M(CO)6 with 2-(2?-pyri-
dyl)benzimidazole (pbiH) [7]. Also, the complexes
A mixture of Cr(CO)6 (0.10 g, 0.45 mmol) and
pbiH (0.09 g, 0.45 mmol) in a sealed tube contain-
ing approximately 30 ml THF was degassed and
then heated for 18 h at which the colour of
solution changed from yellow to deep red. The
reaction mixture was cooled and the solvent was
removed on a vacuum line. The residue was
washed several times with boiling petroleum ether
and then recrystallized from hot ethanol to give
fine reddish brown crystals (yield 53%). Analysis
of C30H18O6N6Cr2 gave: 54.4% C, 2.7% H, 12.7%
[Ru(CO)3(pbiH)],
[Ru(CO)3(pbiH)]×
isolated from the reactions of the cluster com-
pounds M3(CO)12, MꢁRu and Os, with pbiH
[Ru(CO)3(pbiH)]×
bpy and [Os(CO)2(pbiH)2] were
/py,
/
/
alone or in presence of pyridine (py) or 2,2?-
bipyridine (bpy) [8]. Spectroscopic studies of these
Group 6 and 8 complexes revealed the presence of
intramolecular metal-to-ligand charge transfer in-
teractions. In addition, the ruthenium complexes
with py or bpy exhibited intramolecular hydrogen
bonding between the NH proton of the imidazole
group and the hetero nitrogen of py or bipyridine
[8].
N; found: 54.1% C, 2.6% H, 12.8% N. Mwꢂ
/
662.25, value of m/zꢂ635 [Pꢀ
/
/
CO]ꢁ.
2.3.2. Synthesis of [Cr(CO)4(pbiH)]×
/
bpy
In this paper, we report the synthesis and
spectroscopic studies of the Group 6 metal com-
plexes of pbiH with either bpy or triphenyl
phosphine.
Cr(CO)6 (0.11 g, 0.50 mmol), pbiH (0.10 g, 0.50
mmol) and bpy (0.08 g, 0.51 mmol) were mixed
together in a sealed tube containing THF solvent.
The reaction mixture was degassed and heated for
20 h. The colour of the solution changed from
yellow to red and finally to dark red. The reaction
mixture was cooled and the solvent was removed
on a vacuum line. The residue was washed several
times by boiling petroleum ether followed by cold
benzene. The solid was then recrystallized from
hot benzene to give fine reddish brown crystals
(yield 73%). Analysis of C26H17O4N5Cr gave:
60.6% C, 3.3% H, 13.6% N; found: 60.3% C,
2. Experimental
2.1. Reagents
M(CO)6, Mꢁ/Cr, Mo and W; PPh3 and bpy were
supplied from Aldrich. pbiH was purchased from
Across. All the solvents were of analytical reagent
grade and were purified by standard methods.
3.5% H, 13.7% N. Mwꢂ
/
515.45, value of m/zꢂ360
/
[Pꢀ
/
bpy]ꢁ.
2.2. Instruments
2.3.3. Synthesis of [Cr(CO)3(PPh3)(pbiH)]
Infra-red (IR) measurements, KBr discs, were
carried out on a Unicam-Mattson 1000 FT-IR
spectrometer. Electronic absorption spectra were
A mixture of Cr(CO)6 (0.10 g, 0.45 mmol), pbiH
(0.09 g, 0.45 mmol) and PPh3 (0.11 g, 0.45 mmol)
were mixed together in a sealed tube containing
approximately 25 ml THF. The solution was
degassed and heated to reflux for 18 h. The colour
of the solution changed from yellow to red and
finally to dark red. The reaction mixture was
cooled and the solvent was then removed on a
measured on a Unicam UV2-300 UVÁvis spectro-
/
photometer. Proton NMR measurements were
performed on a Spectrospin-Bruker AC 200
MHz spectrometer. Samples were dissolved in
(CD3)2SO with TMS as internal reference. The

M.M.H. Khalil et al. / Spectrochimica Acta Part A 59 (2003) 1341Á
/
1347
1343
vacuum line. The residue was washed several times
with boiling petroleum ether and then recrystal-
lized from hot benzene/diethyl ether mixture to
give reddish brown crystals (yield 76%). Analysis
of C33H24O3N3PCr gave: 66.8% C, 4.1% H, 7.1%
3. Results and discussion
3.1. IR and NMR studies
In contrast to the behaviour of molybdenum
and tungsten [7], reaction of Cr(CO)6 with 2-(2?-
pyridyl)benzimidazole under reduced pressure in
THF yielded a dinuclear complex with a molecular
formula [Cr2(CO)6(pbiH)2]. It gave a molecular
N; found: 66.5% C, 4.2% H, 7.0% N. Mwꢂ
/593.57,
value of m/zꢂ
594 [Pꢁ].
/
ion peak at m/zꢂ
was corresponding to the [Pꢀ
/
635 in the mass spectrum which
CO]ꢁ ion peak. The
2.3.4. Synthesis of [Mo(CO)4(pbiH])×
/
bpy
/
A similar procedure was performed as in the
IR spectrum of the complex displayed two asym-
metric and symmetric CO stretching frequencies in
the terminal metal carbonyl region (Table 1). Also,
the spectrum showed a strong band at 1790 cmꢀ1
with a shoulder at 1759 cmꢀ1. This band could be
assigned for bridging carbonyl groups [9]. In
addition, the IR spectrum of the complex exhibited
case of [Cr(CO)4(pbiH)]×bpy complex. The reac-
/
tion period was 10 h and the complex was reddish
brown (yield 68%). Analysis of C26H17O4N5Mo
gave: 55.8% C, 3.1% H, 12.5% N; found: 55.6% C,
3.2% H, 12.7% N. Mwꢂ
/
559.39, value of m/zꢂ404
/
[Pꢀ
/
bpy]ꢁ.
characteristic bands due to the Cꢁ
/
N, Cꢁ/C and
NH groups of the ligand moiety with the corre-
sponding shifts (Table 1). These shifts indicated
coordination through the py-type nitrogen of the
imidazole ring of pbiH. Furthermore, the in-plane
ring deformation bands of py part were shifted to
higher frequencies suggesting the bonding through
the pyridyl nitrogen [10].
2.3.5. Synthesis of [Mo(CO)3(PPh3)(pbiH)]
A similar procedure was carried out as in the
case of [Cr(CO)3(PPh3)(pbiH)] complex. The reac-
tion period was 15 h and the complex was reddish
brown (yield 73%). Analysis of C33H24O3N3PMo
gave: 62.2% C, 3.8% H, 6.6% N; found: 61.9% C,
3.6% H, 6.7% N. Mwꢂ
/
637.48, value of m/zꢂ638
/
The 1H NMR spectrum of pbiH in DMSO
displayed two multiplets and two doublets due to
the protons in the benzimidazole and pyridyl rings
(Table 2). It also showed a broad singlet at 12.70
ppm corresponded to the NH proton. On the other
hand, the proton NMR spectrum of the chromium
complex displayed a similar pattern similar to that
of ligand (Table 2). The NH groups of imidazole
rings displayed a slightly broad singlet and exerted
down field shift (Table 2). According to the
spectroscopic data, the complex might consist of
two chromium atoms bridged by two CO groups.
Each chromium is also bonded to two terminal
carbonyl groups in the trans positions and a pbiH
ligand through the nitrogen of the pyridyl ring and
the py-type nitrogen of the imidazole moiety in the
cis positions (Scheme 1).
[Pꢁ].
2.3.6. Synthesis of [W(CO)4(pbiH)]×
A similar procedure was performed as in the
case of [Cr(CO)4(pbiH)]×bpy complex. The reac-
/bpy
/
tion period was 24 h and the complex was brown
(yield 54%). Analysis of C26H17O4N5W gave:
48.2% C, 2.7% H, 10.8% N; found: 47.9% C,
2.6% H, 10.8% N. Mwꢂ
/
647.30, value of m/zꢂ464
/
[Pꢀ(bpyꢁ
/
/
CO)]ꢁ.
2.3.7. Synthesis of [W(CO)3(PPh3)(pbiH)]
A similar procedure was carried out as in the
case of [Cr(CO)3(PPh3)(pbiH)] complex. The reac-
tion period was 24 h and the complex was orange
red (yield 62%). Analysis of C33H24O3N3PW gave:
54.6% C, 3.4% H, 5.8% N; found: 54.8% C, 3.5%
When the reactions of M(CO)6, Mꢁ
W, with pbiH were carried out in the presence of
bpy, the [M(CO)4(pbiH)]×bpy complexes were
isolated. The IR spectra of the three complexes
exhibited characteristic bands due to the CꢁN,
/
Cr, Mo and
/
H, 5.9% N. Mwꢂ
/
725.39, value of m/zꢂ
/
698 [Pꢀ
/
CO]ꢁ.
/

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M.M.H. Khalil et al. / Spectrochimica Acta Part A 59 (2003) 1341Á1347
/
Table 1
Important IR data for 2-(2?-pyridyl)benzimidazole and its complexes
Compound
IR data (cmꢀ1 a
)
n(NH)
n(NH
/
Á Á Á/N)
n(CꢁN)
n(CꢁC)
n(CO)
d(py)
PbiH
3061(s)
1600(m)
1568(w)
1612(s)
1566(m)
1445 (s)
Á
/
620(mw), 590(w), 547(mw)
[Cr2(CO)6(pbiH)2]
3079(s)
1520(m)
1450(s)
1890(s)
1867(m)
1790(s)
1759(sh)
2006(s)
1920(s)
1875(vs)
1813(vs)
1890(s)
1865(s)
1805(s)
2014(m)
1911(sh)
1867(s)
1813(s)
2014(m)
1890(s)
1867(s)
1813(s)
2006(m)
1902(sh)
1962(s)
1811(s)
2006(m)
1902(sh)
1859(s)
1805(s)
648(m), 540(m), 509(m)
640(m), 548(m)
[Cr(CO)4(pbiH)]×
/
bpy
3015(m)
1605(s)
1566(m)
1520(m)
1443(m)
[Cr(CO)3(PPh3)(pbiH)]
3055(m)
1612(s)
1520(m)
1450(s)
648(m), 540(s)
[Mo(CO)4(pbiH)]×
/bpy
3022(m)
1597(m)
1465(m)
1443(m)
640(mw), 579(m)
[Mo(CO)3(PPh3)(pbiH)]
3078(m)
1609(w)
1597(m)
1465(m)
1442(m)
640(w), 540(s)
625(m), 569(m)
632(mw), 540(m)
[W(CO)4(pbiH)]×
/
bpy
3019(m)
1605
1465
[W(CO)3(PPh3)(pbiH)]
3071(m)
1605(m)
1466(m)
1443(m)
a
s, strong; m, medium; w, weak; sh, shoulder
Cꢁ
/
C groups of the ligands moieties with the
probably through hydrogen bonding between the
nitrogen of the base and the NꢀH group of the
imidazole part (Scheme 2). Solvent effect, for
example, in the charge-transfer transition of
some ruthenium-amine complexes was interpreted
by the hydrogen bonding type interaction invol-
corresponding shifts (Table 1). In addition, the
in-plane ring deformation bands of the py moiety
of bipyridine were shifted to higher frequencies
due to coordination of the pyridyl nitrogen. The
IR spectra of the complexes also showed similar
/
IR patterns to that of [M(CO)4(pbiH)], Mꢁ
/
Mo
ving electronÁ
/
pair donation from a solvent mole-
H bond [12].
The H NMR spectra of [M(CO)4(pbiH)]×
and W, in the terminal metal carbonyl region [7]
with shifts toward the low frequency range (Table
1). On the other hand, the NH frequency exhibited
shift to lower frequency range to give a medium
cule to the Nꢀ
/
1
/bpy
complexes exhibited signals due to both the pbiH
and bpy moieties (Table 2). On the other hand, the
spectra of the three complexes displayed some
discrepancies with respect to the signal due to the
NH proton. While the chromium complex did not
show any signals due to the NH group, the
weak band at around 3015Á
could be assigned to the streching frequency of
Nꢀ Á Á ÁN bond [11]. Therefore, the bonding of
bpy to the complex moiety [M(CO)4(pbiH)] was
/
3022 cmꢀ1. This shift
/
H
/
/

M.M.H. Khalil et al. / Spectrochimica Acta Part A 59 (2003) 1341Á
/
1347
1345
Table 2
Important 1H NMR data for 2-(2?-pyridyl)benzimidazole and
its complexes
tungsten derivative showed a signal at 14.49 ppm.
The molybdenum complex, on the other hand,
exhibited two broad signals at 13.00 and 14.19
ppm. These results can be explained as the
complex persisted in equilibrium between the two
isomers (A) and (B) (Scheme 2). In case of the
chromium complex, the exchange between A and
B was very fast and the signals were collapsed to
the base line [13,14]. For the tungsten derivative,
isomer B was expected to be the predominant
species at that condition. The molybdenum com-
plex showed two signals due to both two isomers
(Scheme 2).
Compound
pbiH
1H NMR data (ppm)a
7.12(m), 7.61(m), 8.15(d), 8.48(d),
12.70(s)
[Cr2(CO)6(pbiH)2]
7.20(m), 7.59(m), 7.96(d), 8.32(m),
8.69(m), 13.01(s)
[Cr(CO)4(pbiH)]×
/
bpy
[Cr(CO)3(PPh3)(pbiH)] 7.21(m), 7.59(m), 7.98(m), 8.32(m),
7.48(m), 8.03(m), 8.52(m), 9.06(m)
8.72(m), 13.02(s)
[Mo(CO)4(pbiH)]×
/bpy
7.21(m), 7.48(m), 7.64(m), 7.94(m),
8.17(m), 8.64(dd), 8.71(d), 8.97(d),
13.00(s), 14.19(s)
Heating a mixture of equimolar of M(CO)6, Mꢁ
/
[Mo(CO)3(PPh3)(pbiH)] 7.34(m), 7.57(m), 8.26(m), 8.44(dd),
9.03(d), 13.04(bs), 14.34(bs)
Cr, Mo or W, and pbiH in presence of triphenyl
phosphine in THF under reduced pressure resulted
in the formation of the tricarbonyl monosubsti-
tuted derivatives [M(CO)3(pbiH)(PPh3)]. The IR
spectra of the complexes were almost similar,
which could indicate that they have identical
structures. The IR spectra of the complexes
showed, besides the pbiH ligand bands, two bands
at 1165 and 1119 cmꢀ1 characteristic for triphenyl
phosphine [9]. Also, the spectra showed a set of
bands in the terminal metal carbonyl range due to
the CO groups (Table 1). The 1H NMR spectra of
[M(CO)3(pbiH)(PPh3)] complexes showed multi-
[W(CO)4(pbiH)]×
/
bpy
7.33(m), 7.62(m), 7.91,8.2(m),
8.62(d), 8.77(d), 9.08(d), 9.15(d),
14.49(s)
[W(CO)3(PPh3)(pbiH)] 7.65(m), 7.79(m), 8.23(m), 8.39(m),
8.67(dd), 9.08(d), 14.34(s)
a
s, singlet; d, douplet; dd, douplet of doublets; m, multiplet;
b, broad.
plets in the range 7.2Á9.2 ppm corresponding to
/
the protons of PPh3 and pbiH moieties (Table 2).
In addition, the spectra of the chromium and
tungsten complexes displayed a singlet at 13.02
and 14.34, respectively, due to the proton of the
NH group. On the other hand, the 1H NMR
spectrum of [Mo(CO)3(pbiH)(PPh3)] complex
showed two broad singlets for the NH proton at
13.04 and 14.34 ppm. The appearance of two NH
Scheme 1.
1
signals in the H NMR spectrum of molybdenum
derivative indicated the possibility of the presence
of two equilibrated isomeric forms (C and D,
Scheme 3) depending on the position of PPh3
ligand.
3.2. UVÁvis studies
/
The absorption spectra of pbiH and its com-
plexes were measured in ethanol. PbiH displayed
three bands at 297, 310 and 322 nm due to pꢀ
/
p*
Scheme 2.
and n ꢀp* transitions (Table 3). The electronic
/

1346
M.M.H. Khalil et al. / Spectrochimica Acta Part A 59 (2003) 1341Á1347
/
Scheme 3.
spectra of the chromium complex showed a new
shoulder at 377 nm and a weak band at 450 nm.
The band at 450 nm could be attributed to metal-
to-ligand charge transfer transitions (pbiHp*1
/
Mdp). Other mono- and bimetallic tetracarbonyl
centers bound to 2,3-bis(2’-pyridyl)pyrazine and
its derivatives were found to be highly absorbing in
the visible region of the spectrum, and this was
attributed to the lower energy of MLCT versus LF
Fig. 1. The UVÁ
bpy. ((* ) chromium complex, (Á
plex, (
Á Á Á Á Á Á) tungsten complex).
/
vis spectra of the complexes [M(CO)4(pbiH)]×
/
/
*/
/
×
/
Á
/
×
/
Á
/
) molybdenum com-
/
complexes [Mo(CO)4(pbiH)]×
/bpy and [W(CO)4-
bands [15Á17]. The charge transfer bands for the
/
(pbiH)]×bpy were appeared at longer wavelengths
/
than the complexes without bpy due to the
hydrogen-bond formation between bipyridine
and the NH group [7] (Fig. 1). Furthermore, it
Table 3
Electronic absorption spectral data for 2-(2?-pyridyl)benzimi-
dazole and its complexes
Compound
pbiH
l (nm)a
297 (20200), 310 (22500), 322
(19000)b
[Cr2(CO)6(pbiH)2]
241(19017), 296 (20763)b, 312
(21300), 324 (17660)b, 377 (9420)b,
450 (1035)
[Cr(CO)4(pbiH)]×
/
bpy
244(23910), 297(22839), 325 (13830),
370 (6960), 497 (2068)c
[Cr(CO)3(PPh3)(pbiH)] 265 (10729), 272 (10950), 310 (19550),
323 (19540), 353 (13345)b, 378
(8260)b, 450 (640)c
[Mo(CO)4(pbiH)]×
/bpy
257 (18490), 296 (20800), 325 (7820)b,
385(2820)b, 468 (2694)c
[Mo(CO)3(PPh3)(pbiH)] 259 (30145), 264 (31240), 271 (317
(24340), 335 (14610)b, 358 (4870)b,
453 (3530)
[W(CO)4(pbiH)]×
/
bpy
254 (21960), 299 (21480), 325
(15130)b, 484 (3660)
[W(CO)3(PPh3)(pbiH)] 259 (28310), 311 (19450), 324 (19540),
350 (10820)b, 440 (3660)c
a
Molar extinction coefficients, o , are given in parentheses.
Shoulder.
Weak and broad.
Fig.
[M(CO)3(PPh3)(pbiH)]. ((*
molybdenum complex, (
Á Á Á Á Á Á) tungsten complex).
2. The
UVÁ
/
vis
spectra
of
the
complexes
b
/
*
/
) chromium complex, (Á
/
×
/
Á/
× Á)
/
/
c
/

M.M.H. Khalil et al. / Spectrochimica Acta Part A 59 (2003) 1341Á
/
1347
1347
[6] R.C. Rocha, H.E. Toma, Inorg. Chim. Acta 310 (2000) 65.
[7] M.M.H. Khalil, Trans. Met. Chem. 25 (2000) 358.
[8] M.M.H. Khalil, S.A. Ali, R.M. Ramadan, Spectrochim.
Acta 57 (2001) 1017.
can be seen that the CT bands for these complexes
appeared at longer wavelengths than the com-
plexes [M(CO)3(pbiH)(PPh3)] (Fig. 2), due to the
differences in the electronic structure of the two
ligands.
[9] K. Nakamoto, Infrared and Raman Spectra of Inorganic
and Coordination Compounds, 4th ed., Wiley, New York,
1986.
[10] R.J.H. Clark, C.S. Williams, Inorg. Chem.
350.
4 (1965)
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