Spectroscopy and electrochemistry of new 6,6′-disubstituted-4, 4′-bipyrimidine molybdenum(O) and tungsten(O) tetracarbonyl complexes

Article abstract of DOI:10.1139/v05-127

A new family of tetracarbonyl molybdenum(O) and tungsten(O) complexes based on new 6,6′-disubstituted-4,4′-bipyrimidine ligands was synthesized and characterized. The visible region of the absorption spectrum of each complex is dominated by a metal-to-ligand charge transfer band significantly lower in energy than the corresponding transition in 2,2′-bipyridine tetracarbonyl metal complexes. The 6,6′-substituents create a larger π-electronic system in the substituted bipyrimidines and are consequently better π acceptors than even the parent 4,4′-bipyrimidine. The absorption bands are shifted bathochromically with a decrease in solvent polarity.

Full text of DOI:10.1139/v05-127

1114
Spectroscopy and electrochemistry of new 6,6′-
disubstituted-4,4′-bipyrimidine molybdenum(0) and
tungsten(0) tetracarbonyl complexes
Elena Ioachim and Garry S. Hanan
Abstract: A new family of tetracarbonyl molybdenum(0) and tungsten(0) complexes based on new 6,6′-disubstituted-
4,4′-bipyrimidine ligands was synthesized and characterized. The visible region of the absorption spectrum of each
complex is dominated by a metal-to-ligand charge transfer band significantly lower in energy than the corresponding
transition in 2,2′-bipyridine tetracarbonyl metal complexes. The 6,6′-substituents create a larger π-electronic system in
the substituted bipyrimidines and are consequently better π acceptors than even the parent 4,4′-bipyrimidine. The ab-
sorption bands are shifted bathochromically with a decrease in solvent polarity.
Key words: molybdenum and tungsten, nitrogen ligands, tetracarbonyl metal complexes, absorption spectroscopy,
electrochemistry.
Résumé : Une nouvelle famille de complexes de type tétracarbonyle de molybdène(0) et tungstène(0), à base des 4,4′-
bipyrimidine substituées dans les positions-6,6′, a été synthétisée et caractérisée. Le spectre d’absorption dans la région
du visible de chaque composé indique un déplacement vers le rouge pour la bande de transfert de charge métal-ligand
comparé à celui du complexe tétracarbonyle de 2,2′-bipyridine. Ceci démontre une meilleure délocalisation du système
π-électronique dans la bipyrimidine substituée qui rend ces ligands des meilleurs accepteurs π. Cette bande d’absorption
est déplacée bathochromiquement avec la diminution de la polarité du solvant.
Mots clés : molybdenum et tungsten, ligands azotés, complexes métal-tétracarbonyle, spectroscopie d’absorption,
électrochimie. Ioachim and Hanan 1119
Introduction
CO dissociation (9), which can be used as the first step in
photocatalytic cycles and photoinitiated redox processes (10).
Polypyridyl complexes of d6 metal ions have been exten-
sively studied over the last two decades owing to their fasci-
nating spectroscopic, redox, and excited-state properties (1,
2). For example, α-diimine complexes of Ru(II) and Os(II)
have been proposed as photosensitizers in the conversion of
solar energy to chemical energy (3, 4) and have been suc-
cessfully applied to the photolysis of water (5, 6). Re(I)
complexes of type [(α-diimine)Re(CO)3] represent another
promising class of d6 metal ion that are used in the photo-
reduction of CO2 (7, 8), where the Re(I) complexes may act
as both photosensitizer and homogenous catalyst.
Another class of compounds, the tetracarbonyl-diimine
complexes of group 6 transitions metals (Cr, Mo, W) of type
M(CO)4L, also provide a unique combination of an electron-
rich d6 metal atom, stabilized by four CO ligands, and an
electron-accepting diazine ligand. The growth in interest for
these compounds stems not only from their role as starting
materials for different classes of complexes, but also from
their interesting spectroscopic properties. Thus, excitation of
the metal-to-ligand charge transfer (MLCT) state can lead to
The structural simplicity of this type of complex also
makes them suitable for experimental and theoretical investi-
gations of their MLCT transitions. It has been demonstrated
that the relative position of the MLCT excited state is de-
pendent on solvent polarity, as well as on the nature of the
π-acceptor ligand (11, 12). The effect of the solvent on the
electronic absorption spectra of transition-metal complexes
is well recognized, and a correlation between the solvent pa-
rameters and electronic properties of this type of complex
has been established (13). In addition, extensive modifica-
tions of the coordinating heterocyclic ligands can tune the
ground- and excited-state properties of these complexes.
Substitution on the pyridine ring (14) and replacement of the
CH group by N, with the formation of a bidiazine ligand that
possesses a more π-electron-deficient system (15), represent
different strategies in which the photophysical properties of
metal carbonyl complexes may be influenced.
In the bidiazine series (Chart 1), 4,4′-bipyrimidine (bpm)
is the ligand with the lowest-lying π* orbitals, which leads
to its metal complexes having the lowest energy MLCT ex-
cited states. Although this property would be favourable for
the development of new red-emitting species for use in or-
ganic light-emitting diodes (16) or as biomarkers (17), no
systematic study of substituted bpm complexes has been pre-
sented, even though considerable research has been done on
2,2′-bipyrimidine metal complexes (15, 18). Suitable sub-
stituents in the 6,6′ positions of bpm may yield bpm-type
metal complexes that display favourable electrochemical and
Received 24 February 2005. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on 15 September
2005.
E. Ioachim and G.S. Hanan.1 Département de Chimie,
Université de Montréal, 2900 Edouard-Montpetit, Montréal,
QC H3T 1J4, Canada.
1Corresponding author (e-mail: garry.hanan@umontreal.ca).
Can. J. Chem. 83: 1114–1119 (2005)
doi: 10.1139/V05-127
© 2005 NRC Canada

Ioachim and Hanan
1115
Scheme 1. Synthesis of (N-N)M(CO)4 complexes. Reagents and
Chart 1. Possible bidiazine ligands.
conditions: (a) toluene, 4 h, yield: 56%–64%; (b) 1 equiv. N-N,
CH2Cl2, reflux 20 min, yield: 46%–71%.
N
N
N
N
N
N
N
N
M(CO)6 + 2C5H10NH
(a)
4,4'-bipyrimidine
(bpm)
2,2'-bipyrimidine
(bpym)
N
N
cis-M(CO)4(C5H10NH)2
N
N
N
N
N N
bipyrazine
(bpz)
bipyridazine
(bpdz)
(b)
dmpb
dpb
Chart 2. The M(CO)4 complexes of Mo(0) and W(0) with 6,6′ -
1
disubstituted-4,4′-bipyrimidine ligands and their H NMR labels.
CO
R
N
CO
CO
cis-(dmpb)M(CO)4
N
N
cis-(dpb)M(CO)4
M
1b, M = Mo(0)
2b, M = W(0)
1a, M = Mo(0)
2a, M = W(0)
R
N
CO
H6'
H5'
a = 6,6'-diphenyl-4,4'-bipyrimidine
(dpb), R = phenyl
H4'
this period, new peaks, corresponding to the coordinated
ligand, appeared and intensified. After about 40 min, these
newly formed peaks started to lose their intensity and the
original peaks for the starting material reappeared. This ob-
servation suggests that a longer reaction time leads to de-
composition of the complexes. The precipitate that formed
after cooling the reaction mixture overnight was recry-
stallized from hexane and further purified by column chro-
matography on silica gel in order to purify the product.
Attempts to use alumina as the support for the column chro-
matography failed as a result of the decomposition of the
compounds during the purification process.
1a, M = Mo
2a, M = W
H2'
H3'
H6'
H5'
b = 6,6'-di(p-methoxyphenyl)-4,4'-bipyrimidine
(dmpb), R = p-methoxyphenyl
OCH3
1b, M = Mo
2b, M = W
H2'
H3'
photophysical properties based on even lower lying π*
orbitals. In this paper, we report on the synthesis of a novel
series of Mo(0) and W(0) complexes of type (N-N)M(CO)4
(Chart 2), where M = Mo or W, and N-N = 6,6′-diphenyl-
4,4′-bipyrimidine (dpb) (a) or 6,6′-bis(p-methoxyphenyl)-
4,4′-bipyrimidine (dmpb) (b). The investigation of their
electrochemical and spectroscopic properties will verify
whether or not they are suitable candidates for incorporation
into photoactive d6 metal systems.
NMR spectroscopy
1
All of the compounds were characterized by H and 13C
1
NMR spectroscopy. The 400 MHZ H NMR data for the
Mo(0) and W(0) complexes are displayed in Table 1 and are
compared with the free ligands. The numbering scheme for
1
the ligands is presented in Chart 2. The H NMR spectra
show that the signal of the H2 proton of the pyrimidine ring
is deshielded with respect to the signal of the free ligand
(Table 1), as well as those of the phenyl substituents of the
pyrimidine rings. The protons H3′,5′ of complexes 1b and 2b
do not follow this trend, indeed, the opposite trend is ob-
served, these protons being slightly more shielded when
compared to the free ligand. The second proton of the py-
rimidine ring, H5, is shifted to higher field in all of the com-
plexes when compared to the free ligand. To coordinate to
the metal ions, the N atoms must move from a trans to a cis
geometry about the pyrimidine–pyrimidine bond. This has
the effect of diminishing the deshielding effect caused by the
trans arrangement of the N atoms about the interannular
bond (20), which leads to an upfield shift in the H5 protons
of the complexes.
Results and discussion
Synthesis
The new (N-N)M(CO)4 complexes, where M = Mo or W,
were obtained via substitution reactions, as outlined in
Scheme 1.
In step (a), the piperidine metal carbonyls were prepared
in good yield by refluxing the corresponding metal hexa-
carbonyl complexes with a large excess of piperidine, fol-
lowing a previously established method (19). The cis-(N-
N)M(CO)4 compounds of Mo and W were prepared in step
(b) in moderate yield by treating cis-M(CO)4(pip)2 with the
appropriate bidiazine ligands in dichloromethane for 20 min
at reflux. The formation of complex 1b was monitored by
1
recording the H NMR spectra every 5 min for 1 h. During
© 2005 NRC Canada

1116
Can. J. Chem. Vol. 83, 2005
1
Table 1. H NMR resonances for compounds 1a, 1b, 2a, 2b, and free ligands.
Pyrimidine
Substituent R
Compound
a
b
2
5
2′
3′
4′
5′
6′
9.42
9.35
9.84
8.93
8.89
8.56
8.27
8.25
8.29
7.58
7.19
7.66
7.58
7.58
7.19
7.66
8.27
8.25
8.29
(dpb)Mo(CO)4 (1a)
7.66
7.65
(dmpb)Mo(CO)4 (1b)
(dpb)W(CO)4 (2a)
(dmpb)W(CO)4 (2b)
9.71
9.95
9.82
8.43
8.61
8.48
8.28
8.31
7.14
7.65
7.13
7.14
7.65
7.13
8.28
8.31
8.26
8.26
1
Note: CDCl3 at 400 MHz, referenced to residual CHCl3. See Chart 2 for H labels.
Fig. 1. Infrared spectra of 1a (dashed line) and 1b (solid line).
Table 2. Carbonyl ligand stretching frequencies of (N-N)M(CO)4
complexes.
Compound
ν(CO) (cm–1)
100
(dpb)Mo(CO)4 (1a)
2010
2007
1901
1879
1880
1830
1848
(dmpb)Mo(CO)4 (1b)
1894
(dpb)W(CO)4 (2a)
(dmpb)W(CO)4 (2b)
2009
2006
1897
1893
1877
1876
1831
1846
a
(bpm)Mo(CO)4
2010
2000
1908
1900
1895
1900
1850
1850
a
(bpm)W(CO)4
80
Note: In KBr pellets.
aIn THF solution from ref. 15.
Mass spectrometry
Even though the structures of the tetracarbonyl metal
complexes have been confirmed by NMR spectroscopy, their
matrix-assisted laser desorption–ionization (MALDI) and
electron impact (EI) mass spectrometry analyses only dis-
played the signal corresponding to the free ligand. A possible
explanation could be the low stability of these compounds
under the conditions used for MALDI and EI mass spec-
trometry, which leads to the decomposition of the com-
plexes. Even the milder electrospray ionization (ES)
technique leads to their decomposition. Typically ES-MS re-
quires the addition of acid to neutral complexes to generate
cations. The ES-MS technique in combination with Ag+ la-
beling, however, proved to be a convenient tool for the mass
spectrometric characterization of this novel series of com-
plexes. It has been demonstrated that this method is a conve-
nient way to generate positively charged species that are
sensitive to acid (21). The signals detected in the MS spectra
of the complexes corresponded to the masses of the Ag+
complexes as calculated from the chemical composition of
the complexes and Ag+. The MS spectra also contained one
intense peak attributed to the ion with one less CO ligand.
This fact supports the low stability of the tetracarbonyl com-
plexes under MS conditions.
2100
2000
1900
1800
Frequency (cm–1)
acceptor ability of the bidiazine ligand as the better back-
donation from metal to bidiazine diminishes the charge
density on the metal center, which in turn diminishes the
back-donation to the carbonyl groups. However, these dif-
ferences in the series of bidiazine complexes are relatively
small. The metal ion also has a slight effect on the energies
of ν(CO), albeit a small one. The ν(CO) energies are similar
to those found for Mo(bpm)(CO)4 and W(bpm)(CO)4 (Ta-
ble 2) (15).
Electronic absorption spectra
The electronic absorption spectral data of complexes 1a
and 1b and 2a and 2b in three different solvents are summa-
rized in Table 3 and are compared to their unsubstituted bpm
analogues.
The absorption spectra in the UV region display charac-
teristic ligand-centered π–π* transitions. In the visible re-
gion, the absorption spectra of the bidiazine tetracarbonyl
complexes consist of two bands. The band at lower energy
has been assigned to a metal-to-ligand charge transfer
d(M) → π*(bidiazine) MLCT transition. The next higher-
energy band near 400 nm is also attributed to a MLCT tran-
sition, from the HOMO orbital localized on the metal center
to the second SLUMO orbital of the bidiazine, by analogy
with similar bidiazine-based complexes (15). In any particu-
lar solvent, the nature of the substituent on the bipyrimidine
influences the position of the lowest MLCT band. Figure 2
shows the absorption spectra of (dpb)W(CO)4 and
IR spectroscopy
All of the complexes that were synthesized in this study
were characterized by vibrational spectroscopy in KBr. The
energies for the vibrational bands are given in wavenumbers
and are listed in Table 2, while the IR spectra of complexes
1a and 1b are illustrated in Fig. 1 for comparison. In their
IR spectra, the tetracarbonyl metal complexes display four
carbonyl stretching bands. The energy of these bands is
shifted to slightly higher energy with the increase in π-
© 2005 NRC Canada

Ioachim and Hanan
1117
Table 3. Metal-to-ligand charge transfer absorption maxima of (N-N)M(CO)4 complexes in three different solvents.
Tetrahydrofuran (0.59)a
Chloroform (0.42)a
Toluene (0.30)a
Compound
λ1, nm; (ε)b
λ2, nm; (ε)b
λ1, nm; (ε)b
λ2, nm; (ε)b
λ1, nm; (ε)b
λ2, nm; (ε)b
(dpb)Mo(CO)4 (1a)
(dmpb)Mo(CO)4 (1b)
(dpb)W(CO)4 (2a)
(dmpb)W(CO)4 (2b)
596 (0.8)
587 (1.5)
621 (1.1)
612 (1.1)
579 (3.5)
607 (3.6)
378 (1.5)
381 (5.2)
382 (2.3)
383 (3.8)
364 (3.6)
364 (3.7)
630 (0.2)
604 (0.5)
653 (2.3)
636 (0.2)
383 (0.5)
390 (5.1)
387 (4.3)
389 (1.3)
645 (1.1)
610 (1.1)
664 (1.1)
642 (0.9)
629
399 (1.3)
385 (4.1)
424 (1.4)
392 (2.9)
380
c
(bpm)Mo(CO)4
c
(bpm)W(CO)4
659
382
a
*
EMLCT derived solvent parameters from ref. 13.
bε × 10–3 (mol/L)–1 cm–1.
cFrom ref.15.
Fig. 2. Absorption spectra at room temperature of (dpb)W(CO)4
Fig. 3. Absorption spectra of (dmbp)Mo(CO)4 (solid line) and
(solid line) and (dmpb)W(CO)4 (dashed line) in THF solution.
(dmbp)W(CO)4 (dashed line) in toluene.
solvent: THF
15000
10000
5000
0
1000
10000
0
600
800
*
instrument
artefact
λ (nm)
5000
400
600
800
0
λ (nm)
400
600
800
λ (nm)
The MLCT absorption band is very sensitive to the nature
of the solvent. All of the (N-N)M(CO)4 complexes present a
negative solvatochromism, translated by a shift of the MLCT
absorption band to higher energy as the solvent polarity in-
creases. The same trend has been observed for other
M(CO)4(α-diimine) related complexes (22, 23) and has been
explained in terms of dipole moment changes between the
ground and excited states (22, 24). The tetracarbonyl metal
complexes are more polar in the ground state than in the ex-
cited state. Hence, the more polar solvents allow an en-
hanced stabilization of the ground state, which in turn leads
to an increase in the transition energy. The results of corre-
lating MLCT energies with solvent polarity are shown in
Fig. 4 for the (dpb)Mo(CO)4 complex in three different sol-
(dmpb)W(CO)4 in THF at room temperature. The presence
of the methoxy donor group destabilizes the empty π*
orbitals of the dmpb ligand and gives rise to absorption at
higher energy when compared to the dpb ligand. However,
for both ligands, the corresponding Mo and W complexes
present MLCT absorption bands at lower energy than the
parent complexes with unsubstituted 4,4′-bipyrimidine (Ta-
ble 3), for which MLCT transitions were observed at
579 nm for (bpm)Mo(CO)4 and 607 nm for (bpm)W(CO)4 in
THF (15). This in accordance with the more extended π
conjugation of the 6,6′-disubstituted-4,4′-bpm ligands and
has the effect of lowering the MLCT energy in the newly
prepared complexes. The planarity of the aryl substituent
was demonstrated in the solid state for a tricarbonyl Re(I)
complex of ligand b.2 The p-methoxyphenyl rings were
twisted with dihedral angles of 4.0° and 5.2° around the
aryl-pyrimidine bond.
*
vents for which the solvent parameter (EMLCT) is included in
parentheses in Table 3. The MLCT band is shifted to lower
energy in going from tetrahydrofuran to toluene, along with
the decrease in solvent polarity.
Electrochemistry
For a given N-N bidiazine ligand and solvent, the energy
of the MLCT absorption follows the order Mo > W, as can
be seen from the data listed in Table 3. This is exemplified
in Fig. 3 for tetracarbonyls of Mo and W, with dmpb as the
bidiazine ligand in toluene.
The electrochemical properties of the newly prepared
complexes have been studied by cyclic voltammetry in
acetonitrile solutions. The redox potentials are collected in
Table 4 along with the corresponding redox potentials of the
tetracarbonyl Mo and W complexes of bpm.
2 E. Ioachim, E.A. Medlycott, and G.S. Hanan. Submitted for publication (2005).
© 2005 NRC Canada

1118
Can. J. Chem. Vol. 83, 2005
Fig. 4. Absorption spectra at room temperature of (dpb)Mo(CO)4
in THF (solid line), chloroform (dotted line), and toluene
(dashed line).
electron donation by the methoxy groups to the bpm ligand,
which raises the π* level of the coordinating diazine.
3000
2000
1000
0
Conclusion
A number of tetracarbonyl bidiazine based complexes of
Mo and W have been synthesized and characterized, and
their electrochemical and photophysical properties have been
investigated. It has been shown that acceptor ligands with
low-lying π* levels can be used to red shift the energy of the
lowest MLCT bands. (N-N)M(CO)4 complexes show a nega-
tive solvatochromism, with a shift to lower energy of the
MLCT absorption in nonpolar solvents. Electrochemical
data was used to identify the lowest energy ligand acceptor.
This study confirms that this new family of substituted bidi-
azine ligands has exceptional π-accepting properties, which
bodes well for their incorporation into other transition-metal
complexes. Further studies with metal carbonyls, such as the
luminescent Re(CO)3Cl (α-diimine) type, and other metal
400
500
600
700
800
2+
complexes, such as the luminescent M(α-diimine)3 type,
are underway and will be discussed in due course.
λ (nm)
Table 4. Half-wave potentials (E1/2) for (N-N)M(CO)4 complexes.
Experimental
Compound
E1/2(ox)a
E1/2(red)
Materials
(dpb)Mo(CO)4 (1a)
(dmpb)Mo(CO)4 (1b)
(dpb)W(CO)4 (2a)
(dmpb)W(CO)4 (2b)
0.67 (irr)
0.59 (irr)
0.65 (irr)
0.58 (irr)
0.66 (irr)
0.64 (irr)
–0.89 (63)
–0.98 (60)
–0.83 (62)
–0.91 (67)
–0.91
–1.48 (103)
–1.55 (74)
–1.30 (68)
–1.25 (91)
–1.65
All substituted bipyrimidine ligands were available in our
laboratory from a previous study (26). M(CO)4(pip)2 (M =
Mo or W) were synthesized according to a literature proce-
dure (19). All metal complexes were purified by column
chromatography on Kiesegel 60 (230–400 mesh) silica gel.
Tetrabutylammonium hexafluorophosphate was purchased
from Aldrich and purified by recrystallization from toluene–
methanol. All of the solvents used for photophysical and
electrochemical experiments were of spectroscopic grade.
Elemental analyses were performed by Laboratoire
d’Analyse Elementaire de l’Université de Montréal (Mon-
treal, Quebec).
b
(bpm)Mo(CO)4
b
(bpm)W(CO)4
–0.88
–1.59
Note: From cyclic voltammetry in acetonitrile (0.1 mol/L in
tetrabutylammonium perchlorate). Potentials are in volts vs. SCE, recorded
at room temperature at a sweep rate of 500 mV/s. The difference between
cathodic and anodic peak potentials (mV) is given in parentheses.
aIrreversible (irr); potential is given for the cathodic wave.
bIn DMF solution, adjusted to acetonitrile as per ref. 15.
Instrumentation
All of the complexes display one oxidation and two reduc-
tion processes in the potential region from +1.50 to –2.00 V.
The oxidation is associated with a metal-centered process
M(0) → M(I) and has proved to be irreversible in all cases,
owing to the known lability of the complexes in the M(I)
state with a 17-electron configuration (25). Indeed, the Mo(CO)4
and W(CO)4 complexes of the parent 4,4′-bipyrimidine,
3,3′-bipyridazine, 2,2′-bipyrimidine, and 2,2′-bipyrazine are
also irreversible (15). The increase in π-acceptor proper-
ties of the bidiazine makes the abstraction of the electron
from the metal center more difficult, and consequently, the
oxidation occurs at more positive potentials for complexes
1a and 2a as compared to 1b and 2b and to the reference
compounds Mo(CO)4bpm and W(CO)4bpm. The donating
methoxyphenyl substituents of complexes 1b and 2b
destabilize the metal-based orbitals, which facilitates the
oxidation of the metal centres. The two reversible or quasi-
reversible reduction waves observed in the cyclic voltammo-
grams of all of the complexes are bidiazine centered. The
reduction potentials depend on the nature of the bidiazine
ligand, while the metal has just a slight influence on the re-
duction potentials. In the case of complexes 1b and 2b, the
slight negative shift of the first reduction potential is due to
NMR spectra were recorded in CDCl3 at room tempera-
ture (rt) on a Bruker AV400 spectrometer at 400 MHz for 1H
NMR and at 100 MHz for 13C NMR. Chemical shifts are re-
ported in parts per million (ppm) relative to residual solvent
protons (7.26 ppm for chloroform-d) and carbon resonance
of the solvent. Routine absorption spectra were measured in
deaerated tetrahydrofuran, chloroform, and toluene solutions
at rt on a Cary 500i UV–vis–NIR spectrophotometer. The IR
spectra were recorded as solid KBr discs on a Bio-Rad IR
spectrophotometer. Electrochemistry data were collected in
deaerated acetonitrile with 0.1 mol/L Bu4NPF6 on a
BAS CV–50W voltammetric analyzer. Redox potentials
were corrected by internal reference ferrocene (395 mV for
acetonitrile vs. SCE).
Preparation of compounds
General procedure for the synthesis of (N-N)M(CO)4 (1a and
2b)
(dpb)Mo(CO)4 (1a)
A solution of 6,6′-diphenyl-4,4′-bipyrimidine (0.032 g,
1 mmol) and Mo(CO)3(pip)2 (0.039 g, 1 mmol) in dichloro-
© 2005 NRC Canada

Ioachim and Hanan
1119
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methane was heated to reflux for 15 min. The reaction mix-
ture was filtered through Celite followed by addition of
hexane. Upon cooling, crystals formed, which were isolated
by filtration. The complex was purified by column chroma-
1
tography (SiO2, ethyl acetate–hexane = 1:3). Yield: 58%. H
NMR (400 MHz, CDCl3, 25 °C) δ: 9.84 (s, 2H, H2), 8.56 (s,
2H, H5), 8.29 (d, J = 3.5 Hz, 4H, H2 ′,6 ′), 7.66 (m, 6H,
H
3 ′,4 ′,5 ′). 13C NMR (75 MHz, CDCl3, 25 °C) δ: 210.1, 166.2,
161.9, 161.2, 159.4, 136.9, 131.8, 129.5, 127.8, 113.9. HR-
MS calcd. for C24H14N4O4Mo-Ag: 626.9114 [M + 1]; found:
626.9147.
(dmpb)Mo(CO)4 (1b)
1
Yield: 51%. H NMR (400 MHz, CDCl3, 25 °C) δ: 9.71
(s, 2H, H2), 8.43 (s, 2H, H5), 8.28 (d, J = 8.5 Hz, 4H,
H
2 ′,6 ′), 7.14 (d, J = 8.5 Hz, 4H, H3 ′,5 ′), 3.93 (s, 6H, O-Me).
13C NMR (75 MHz, CDCl3, 25 °C) δ: 229, 178.8, 165.3,
162.8, 161.7, 159.3, 129.4, 129.3, 114.8, 113.0, 55.9. HR-
MS calcd. for C26H18N4O6Mo-Ag: 686.1497 [M + 1]; found:
686.1502.
(dpb)W(CO)4 (2a)
1
Yield: 46%. H NMR (400 MHz, CDCl3, 25 °C) δ: 9.95
(s, 2H, H2), 8.61 (s, 2H, H5), 8.29 (d, J = 3.5 Hz, 4H,
H
2 ′,6 ′), 7.66 (m, 6H, H3 ′,4 ′,5 ′). 13C NMR (75 MHz, CDCl3,
25 °C) δ: 190.2, 166.2, 161.9, 161.3, 159.4, 136.8, 131.8,
129.9, 127.8, 113.9. HR-MS calcd. for C24H14N4O4W-Ag:
712.9570 [M + 1]; found: 712.9579.
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(dmpb)W(CO)4 1(2b)
Yield: 71%. H NMR (400 MHz, CDCl3, 25 °C) δ: 9.82
(s, 2H, H2), 8.48 (s, 2H, H5), 8.26 (d, J = 8.5 Hz, 4H,
H
2 ′,6 ′), 7.13 (d, J = 8.5 Hz, 4H, H3 ′,5 ′), 3.96 (s, 6H, O-Me).
13C NMR (75 MHz, CDCl3, 25 °C) δ: 218.0, 190.8, 165.6,
162.8, 161.4, 159.3, 129.4, 129.3, 115.3, 114.8, 55.9. HR-
MS calcd. for C26H18N4O6W-Ag: 772.9781 [M + 1]; found:
772.9740.
Acknowledgements
We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC) and l’Université de Montréal
for financial support. François Baril-Robert and Alexandra
Furtos are thanked for assistance with the IR and MS mea-
surements, respectively.
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© 2005 NRC Canada