Di-tungsten bis-carbene complexes linked by condensed heteroaromatic spacers

Article abstract of DOI:10.1515/znb-2007-0316

The 2,7-dilithiated substrates of 3,6-dimethylthieno[3,2-b]thiophene, N,N′-dimethylpyrrolo[3,2-b]pyrrole and N-methylthieno[3,2-b]pyrrole were reacted with W(CO)₆ to give, after subsequent alkylation with Et ₃OBF₄, the ditungsten biscarbene complexes [(CO) ₅W{C(OEt)XXC(OEt)} W(CO)₅] (XX = condensed heteroaromatic spacers). Sites of attack during the dilithiation of the condensed rings were studied and compared, and the yields of the desired ditungsten biscarbene complexes optimized by changing the reaction conditions according to the role of the heteroatoms in the rings. The crystallographic data of the three ditungsten biscarbene complexes are reported and their structural features compared. The methyl substituents on the condensed heteroaromatic rings play an important role in determining the molecular configurations.

Full text of DOI:10.1515/znb-2007-0316

Di-tungsten Bis-carbene Complexes Linked by Condensed
Heteroaromatic Spacers
Simon Lotza, Marile´ Landmana, Helmar Go¨rlsb, Chantelle Crausea, Hubert Nienabera, and
Andrew Oliviera
a Department of Chemistry, University of Pretoria, Pretoria 0002, South Africa
b Friedrich-Schiller-Universita¨t, Institut fu¨r Anorganische und Analytische Chemie,
Lessingstraße 8, Jena 07743, Germany
Reprint requests to Dr. Simon Lotz. Fax: Int+27 12 362 5297. E-mail: simon.lotz@up.ac.za
Z. Naturforsch. 2007, 62b, 419 – 426; received December 19, 2006
Dedicated to Prof. Helgard G. Raubenheimer on the occasion of his 65th birthday
The 2,7-dilithiated substrates of 3,6-dimethylthieno[3,2-b]thiophene, N,Nꢀ-dimethylpyrrolo[3,2-
b]pyrrole and N-methylthieno[3,2-b]pyrrole were reacted with W(CO)6 to give, after subse-
quent alkylation with Et3OBF4, the ditungsten biscarbene complexes [(CO)5W{C(OEt)XXC(OEt)}
W(CO)5] (XX = condensed heteroaromatic spacers). Sites of attack during the dilithiation of the
condensed rings were studied and compared, and the yields of the desired ditungsten biscarbene
complexes optimized by changing the reaction conditions according to the role of the heteroatoms
in the rings. The crystallographic data of the three ditungsten biscarbene complexes are reported and
their structural features compared. The methyl substituents on the condensed heteroaromatic rings
play an important role in determining the molecular configurations.
Key words: Dinuclear Carbene Complexes, Condensed Heterocycles, Thiophene, Pyrrole, Tungsten
Introduction
Fused heteroaromatic moieties functioning as bridg-
ing spacers in transition metal complexes have not yet
been fully exploited and examples are limited to mostly
main group metal compounds. Even though the co-
ordination chemistry of thiophene and N-substituted
pyrrole with transition metals has been studied exten-
sively, this is not true for condensed heterocycles of
thiophene and pyrrole [1, 2]. The exceptional prop-
erties of condensed thiophene and pyrrole rings are
finding many applications in organic synthesis and
material science. Research in these areas has mainly
been focused on the preparation of potential pharma-
cologically important compounds and polymeric ma-
terials with conducting properties [3, 4]. Incorporat-
ing fused heteroaromatic substrates as linking units
between transition metals is an attractive approach to
study photoelectronic, charge transfer and metal-metal
communication processes [5, 6]. Furthermore, transi-
tion metal fragments acting as templates towards het-
erocyclic ligands can also play an important role in
regio- and stereoselective syntheses in organic chem-
istry and thereby greatly advance heteroaromatic ring
modifications [7 – 9].
Fig. 1. Atom numbering used for 3,6-dimethylthieno[3,2-b]-
thiophene (L1), N,Nꢀ-dimethylpyrrolo[3,2-b]pyrrole (L2)
and N-methylthieno[3,2-b]pyrrole (L3) and the sites of de-
protonation of L3.
Since the properties of thieno[3,2-b]thiophene
derivatives as bridging linkers in transition metal car-
bene complexes have been explored in our laborato-
ries [10, 11], a study of the analogous N-substituted
pyrrolo[3,2-b]pyrrole and the mixed heteroaromatic
complexes, thieno[3,2-b]pyrrole, was undertaken. The
transfer of electron density of these electron exces-
sive substrates to the very electrophilic carbene carbon
atom in Fischer carbene complexes will aid in the sta-
bilization of such multinuclear multicarbene function-
alities.
The isomer of the condensed rings of thienopyrrole
(L3) shown in Fig. 1, represents the most stable iso-
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S. Lotz et al. · Di-tungsten Bis-carbene Complexes Linked by Condensed Heteroaromatic Spacers
mer of this type of compound [12]. When two five-
membered rings of thiophene and pyrrole are fused in
this manner, the two rings display π conjugation from
positions 2 to 7 and hence could support charge trans-
port over the whole molecule. They can act as excellent
links between carbene ligands and hence create a path-
way for metal-metal communication. The atom num-
bering shown in Fig. 1 is unconventional, but repre-
sents the labels used in this study for the reported crys-
tal structures. The biscarbene complexes of tungsten
were prepared with bridging 3,6-dimethylthieno[3,2-
b]thiophene, N,Nꢀ-dimethylpyrrolo[3,2-b]pyrrole and
N-methylthieno[3,2-b]pyrrole units and their struc-
tural features were investigated and compared in the
solid state. This work is an extension of our previ-
ously published work dealing with dinuclear biscar-
bene complexes with bridging thiophene and furan
spacers [9, 13].
Scheme 1.
for the synthesis is shown in Scheme 1. The first
step involves the condensation of thiophene or N-
methylpyrrole carboxaldehyde with ethyl azidoac-
etate to◦form the substituted vinyl azide. Thermolysis
at 130 C afforded the condensed pyrroles. To avoid
N–H deprotonation during metalation, it was necessary
to prepare the N-methyl derivatives of L2 and L3. The
N alkylation could be carried out either directly from
the 4H-pyrolyl- or 4H-thienyl[3,2-b]pyrrole, or as the
last step of the procedure and was accomplished by
deprotonation with potassium tert-butoxide and subse-
quent alkylation with methyl iodide. The last two steps
of the synthesis involved basic hydrolysis of the ester
functionality followed by copper catalyzed decarboxy-
lation to afford L2 and L3, respectively.
Results and Discussion
Since no practical short-step syntheses for thieno-
[3,2-b]thiophene, pyrrolo[3,2-b]pyrrole or thieno[3,2-
b]pyrrole were available, several of the steps required
to construct a second, fused pyrrole ring to the pyr-
role or thiophene substrate, had to be revised to obtain
higher yields of the desired compounds. The most gen-
eral method for the synthesis of pyrrole-condensed het-
erocycles involves an alkyl azido-acrylate as key inter-
mediate which undergoes facile thermal cyclization to
yield the corresponding carboxy derivatives. The Fis-
cher method for carbene synthesis is dependent on de-
protonation steps of the substrates and it was therefore
required to block active sites, e. g. by alkylating the
N–H bond in pyrrole and blocking the β position in
thieno[3,2-b]thiophene.
Synthesis of complexes
The classical Fischer method forms the basis of
the synthesis of the carbene ligands in 1, 2 and 3.
Monometalation studies of heterocyclic systems have
provided a general rule of oxygen > sulfur > nitro-
gen in increasing order of reactivity [17]. This rule
prevails for N-substituted nitrogen compounds, but not
necessarily N–H compounds. Two equivalents of butyl
Synthesis of condensed heteroaromatic precursors
Choi and co-workers reported a one-pot synthesis lithium, W(CO)6 and alkylating agent were used with
of 3,6-dimethylthieno[3,2-b]thiophene [14]. The syn- one equivalent of the heterocycle. Monocarbene com-
thesis was based on the discovery made by Teste plexes are always formed in these reactions as a re-
and Lozac’h that the reaction of 2,5-dimethyl-3- sult of incomplete dilithiation. For L1 – L3 the mono-
hexyne-2,5-diol with elemental sulfur afforded 3,6- lithiated species gave the corresponding monocarbene
dimethylthieno[3,2-b]thiophene (L1) [15]. To improve complexes, [W(CO)5{C(OEt)heterocycle}], but these
the low yield, Choi re-examined this reaction and var- compounds were not isolated and did not form part of
ied the reaction conditions which were used to ob- this study.
tain L1.
It was however unclear how successful double lithi-
The syntheses of L2 and L3 were based on the ation of L1 – L3 could be performed. Like sulfur, ni-
method of Hemetsberger and Knittel [16]. The yields trogen also exhibits an ortho directing effect which
over 5 steps were 20 – 25 % and the route employed would lead to the formation of the 2,7-dilithio deriva-
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S. Lotz et al. · Di-tungsten Bis-carbene Complexes Linked by Condensed Heteroaromatic Spacers
421
◦
n-BuLi in THF at 0 C and treated with W(CO)6 to
form the metal acylate. Under these conditions the first
proton is removed α to the sulfur atom in the thio-
phene ring. A second proton was abstracted by LDA at
low temperature and the resulting dianion was reacted
with a second equivalent of W(CO)6 followed by alky-
lation with triethyloxonium tetrafluoroborate. Again it
was found that metalation β to sulfur was competitive,
but the overall yields of the biscarbene complexes were
improved, while very little of the monocarbene com-
plex was formed. Finally, and parallel with the first ap-
proach, the dilithiated ligand L3 was used as starting
material. In contrast, only one equivalent of W(CO)6
was added and the resulting metal acylate was washed
with several portions of hexane to remove unreacted
starting materials. The residue was redissolved in THF
and reacted with a second equivalent of the tungsten
hexacarbonyl, followed by alkylation. This methodo-
logy improved the yield of the desired ditungsten bis-
carbene complex 3.
Scheme 2.
tive of L2. Whereas lithiation α to sulfur is favoured
and occurs at low temperatures in THF, more strin-
gent conditions are needed to deprotonate the posi-
tion α to nitrogen in a pyrrole moiety. Double lithia-
tion of thieno[3,2-b]thiophene was improved once the
3-positions on the thiophene rings were blocked. Al-
though not the method of choice for this study, both
protons in L1 could be removed in THF at low tem-
peratures. This becomes more difficult for the deproto-
nation of L2, hence hexane is the solvent of choice to
access higher temperatures, and TMEDA is employed
as co-catalyst for the preparation of 2.
It was found that the desired 2,7-dilithio derivative
of L3 was not formed exclusively, but a 2,3-dilithiated
thiophene could also be produced leaving the pyrrole
ring unaffected, and consequently leading to a chelat-
ing cis biscarbene complex of tungsten. It can thus be
concluded that a proton adjacent to the sulfur atom
in thiophene will be removed preferentially during the
first lithiation (a), but during the second lithiation, the
β position of the thiophene ring (bꢀ) becomes a com-
petitive site for lithiation with the position α to the ni-
trogen atom on the pyrrole ring (b). It was noted be-
fore that dilithiation of thieno[3,2-b]thiophene gives a
mixture of mono- and dilithiated species [10]. The di-
tungsten biscarbene complexes 1, 2 and 3 were pre-
pared according to Scheme 2 and are of reasonable sta-
bility in solution under inert atmosphere. These com-
plexes are oxidized with time at one of the metal cen-
tres to give [W(CO)5{C(OEt)XXC(O)OEt}] and other
oxidation products [18]. In the solid state the biscar-
bene complexes can be stored indefinitely under inert
atmosphere.
Characterization of ligands and complexes
Complexes 1 – 3 were characterized using infrared
and 1H NMR spectroscopy as well as mass spectrome-
try. Suitable crystals of 1, 2 and 3 were obtained from
dichloromethane/hexane solutions for X-ray diffrac-
tion studies. No molecular ion peak, M+, was observed
in the mass spectrum of 3. The infrared frequencies
and patterns observed in the carbonyl region confirm
the presence of the W(CO)5 fragment for all the com-
plexes. The coordination of the tungsten fragment via
a carbene carbon atom has a significant effect on the
electronic environment of the remaining ring protons
of the hetero-rings and is reflected in the downfield
chemical shift values recorded in the 1H NMR spectra
of the heterocycles as spacers in the complexes (1 – 3)
compared to the uncoordinated heterocycles (L1 – L3).
Interestingly, the chemical shift values of the methy-
lene protons of the carbene ethoxy groups in 3 attached
to the thiophene ring is the same as in 1, and the corre-
sponding resonances of the carbene attached to pyrrole
are again the same as in 2. Thus, even though there is
a flow of electron density from the hetero-ring to the
carbene carbon atom, there is little evidence of elec-
tron delocalization across the two condensed rings or a
flow of charge from one side of the spacer to the other.
Three procedures were used in the synthesis of 3 and
represent slight modifications of the general method.
The first and general procedure to carbene-complex
formation was based on the generation of a dilithi-
ated nucleophile with n-BuLi/TMEDA in hexane [19].
The second method was tried to improve the yield of 3
and decrease the amount of monocarbene complex for-
mation. This route comprised an adapted method of
Aoki involving a stepwise lithiation approach [20]. N-
Description of crystal structures
Single crystal X-ray diffraction studies confirmed
Methylthieno[3,2-b]pyrrole was mono-lithiated with the molecular structures of 1, 2 and 3. Selected bond
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S. Lotz et al. · Di-tungsten Bis-carbene Complexes Linked by Condensed Heteroaromatic Spacers
˚
Table 1. Selected bond lengths (A), angles (deg), and dihe-
dral angles (deg) for 1, 2 and 3 with estimated standard devi-
ations in parentheses.
Complex
1
2
3
X=Y=S
X=Y=NMe
X=S, Y=NMe
Distances
W–C1(carbene)
W–CO (trans)
W–CO (cis)av
C1–C2
2.221(8)
2.028(8)
2.05(1)
1.46(1)
1.40(1)
1.427(9)
1.39(2)
1.761(7)
2.238(4)
2.038(5)
2.056(5)
1.453(6)
1.402(6)
1.408(5)
1.396(8)
1.422(5)
2.212(6)
2.029(7)
2.040(7)
1.466(8)
1.350(11)
1.287(12)
1.39(2)
1.961(8)
1.267(11)
2.039(12)
1.392(13)
C2–C3
C3–C4
aC4–C4#/5
C2–X
Fig. 3. Ball-and-stick representation of the molecular struc-
ture of 2.
C2#5–Y
C4#/5–X
C4–Y
1.728(7)
1.364(5)
Angles
W–C1–O
W–C1–C2
O–C1–C2
C2#5–X–C4
C2–Y–C4#5
X–C2#5–C3#5
Y–C2–C3
128.2(5)
125.7(5)
105.9(6)
90.9(3)
128.0(3)
123.9(3)
108.0(3)
107.1(3)
128.5(4)
123.4(5)
107.9(5)
87.2(5)
92.0(12)
121.6(5)
97.9(7)
113.2(5)
108.6(3)
C2–C3–C4
C3–C4–C#4/5
C4–C4#/5–X
C4#5–C4A–Y
109.4(6)
115.3(8)
111.3(7)
107.1(3)
108.2(3)
109.7(3)
123.5(10)
96.0(10)
132.4(9)
109.4(11)
Fig. 4. Ball-and-stick representation of the molecular struc-
ture of 3.
Dihedral Angles
W–C1–C2–C3
X–C4#/5–C4–Y
O1–C1–C2–X
O1–C8–C7–Y
152.0(5)
180
159.5(7)
19.9(3)
180
16.5(3)
−18.1(4)
180
14.6(4)
−11.2(6)
−41.5(3)
cycles and the plane of the carbene unit do not coin-
cide with each other. In complexes 1 – 3 these planes
are displaced by 15 – 20◦. Also, the carbene ligand de-
viates from the midpoint (45◦, staggered position) be-
tween two perpendicular planes described by the tung-
sten carbonyl groups. This is ascribed to the bulki-
ness of the spacer and in particular to the methyl sub-
stituents attached to the rings in 1 – 3. Taking into con-
sideration only electronic and packing effects, based
on the many structures of carbene complexes with het-
erocyclic substituents studied in our laboratories, the
heteroatom is preferentially found in a position on the
same side as the ethoxy substituent and on the op-
posite side of the W(CO)5 fragment with respect to
the C(carbene)–C(heterocycle) bond [11, 13]. This we
ascribe to the formation of a six-membered ring that
includes the Li+, the heteroatom of the heterocycle
and the oxygen atom of the intermediate metal acy-
late [21]. In addition, when carbene structures with
C–W–C1–C2
−38.8(6)
−38.9(3)
a
Symmetry transformations used for 1, 2 and 3 to generate equiva-
lent atoms are #1: −x, −y, −z.
Fig. 2. Ball-and-stick representation of the molecular struc-
ture of 1.
lengths and angles are given in Table 1, and Figs. 2, 3 heteroarene rings and their substituents are examined,
and 4 represent ball-and-stick plots of the structures. there is more free space available around the ethoxy
The six ligands, five carbonyl groups and one car- substituent compared with the bulkier W(CO)5 frag-
bene, are arranged octahedrally around each metal cen- ment. In 1 – 3 the methyl substituents determine the
tre. Unlike for carbene complexes with bridging thio- relative positioning of the ethoxy substituent and the
phene or furan units, the plane describing the hetero- W(CO)5 fragment around the carbene carbon atom.
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423
With the methyl groups in the C3 positions of the thio- (1H in CDCl3: 7.24 ppm). IR spectra were recorded on a
Perkin-Elmer Spectrum RXI FT-IR spectrophotometer us-
ing either dichloromethane or hexane as solvent. Only the
vibration bands in the carbonyl stretching region (ca. 1500 –
2200 cm−1) are reported.
3,6-Dimethylthieno[3,2-b]thiophene (L1): The ligand
was prepared by the method of Choi and was used in our
laboratories earlier [14]. – 1H NMR (CDCl3): δ = 2.33 (s,
6H, CH3), 6.92 (s, 2H, H2).
N,Nꢀ-Dimethylpyrrolo[3,2-b]pyrrole (L2): The synthesis
of L2 was adapted from literature and modified in our labo-
ratories [25].
phene ring in 1, the thiophene sulfur atom is now in
the unusual position on the opposite side of the ethoxy
group. The shortest non-bonding (Et)O···H(Me) dis-
˚
tance is 2.10(1) A which falls in the range associ-
ated with some hydrogen-oxygen interaction. For the
methyl substituent on the nitrogen atom of the pyrrole
rings in 2 and 3, the isomeric forms with the nitro-
gen atom on the same side as the ethoxy substituent
of the C(carbene)–C(pyrrole) bonds are found. The
shortest non-bonding (Et)O···N(MeN) distance in 2 is
˚
˚
2.391(8) A, and in 3 2.768(4) A, indicating some hy-
drogen bonding. The bond lengths and angles do not
differ significantly between the three structures, but
one must take into account the differences of inter-ring
angles at the heteroatoms in the thiophene and pyrrole
rings. The carbene carbon atoms exhibit sp2 character
with angles distorted from ideal values as is expected
Condensation: BrCH2COOEt (83.5 g, 0.5 mol), in 30 mL
of EtOH, and NaN3 (35.75 g, 0.55 mol), in 90 mL of H2O,
were added together and heated under reflux for 4 h. A steam
distillation was carried out (to get rid of salts) and the organic
layer was washed twice with water and dried over anhydrous
Na2SO4. After this, a vacuum distillation was carried out
◦
with a water pump vacuum and the fraction between 62 C
for alkoxy carbene complexes. Because the metal frag- and 65 ◦C was isolated.
ment takes up more space, the angles on this side of
the carbene carbon atom and its substituents are greater
than 120◦ [22].
The shortest C–C bond lengths of the heterocyclic
rings of the spacers in all the complexes are at the point
where the rings are fused which supports 1H NMR data
suggesting very little electron delocalization over the
rings. The W–C(carbene) distances are the same, but
comparison with 3 is complicated because of the large
margin of error in the measurements.
To a solution of sodium (2.76 g, 120 mmol) in 55 mL
of ethanol was added, under vigorous stirring at 5 – 10 C,
◦
a mixture of N3-CH2COOEt (15.48 g, 120 mmol) and N-
methylpyrrole aldehyde (4.365 g, 40 mmol) over a period
of 30 min. After the evolution of N2 had subsided (30 –
60 min), half of the solution was distilled at a maximum tem-
perature of 30 ◦C. The brown solution was treated with solid
NH4Cl (6.40 g, 120 mmol) and 500 mL of ice water. After
this the solution was extracted 3 times with diethyl ether, fil-
tered, washed with water until it was neutral, and then dried
over Na2SO4. The solvent was distilled off and the brown
residue dissolved in benzene and filtered through a column
with silica gel. Concentrating the solution led to the forma-
tion of 4.31 g (19.6 mmol, 49 %) of yellow crystals. The final
product was crystallized from an ether/hexane mixture.
Cyclization: The crude product of the previous step was
Experimental Section
General procedures
All manipulations involving organometallic compounds
made use of Schlenk techniques, and operations were car-
ried out under inert atmosphere. Hexane, THF and dichloro-
methane were dried and deoxygenated by distillation prior
to being used. Commercial thiophene was purified [23] and
triethyloxonium tetrafluoroborate prepared according to re-
ported procedures [24]. TMEDA was distilled before use.
The reagents W(CO)6 and butyl lithium (1.6 mmol mL−1
solution in hexane) and all other commercial agents were
◦
placed in a preheated flask with p-xylol (130 – 140 C) in
small portions (6.0 g in 250 mL solvent). The solution was
boiled for an h. The solvent was removed under vacuum and
the dark residue was cleaned by filtration through silica gel
(eluent: CH2Cl2). The product obtained in an almost quanti-
tative yield had a red colour and was pure according to NMR
spectroscopy.
Methylation: In a Schlenk tube with 20 mL of THF the
used as purchased. Column chromatography, using silica product (5 g, 26 mmol) of the cyclization step was dis-
gel 60 (0.0063 – 0.200 mm) as the stationary phase, was solved. The solution was cooled (−25 to −30 ◦C) and KOt-
employed for all separations and purifications. Circulating Bu (4.55 g, 40 mmol) was added. The solution was stirred
cold water through the column jackets served to cool the for 20 min and then the cooling bath was removed and the
columns. NMR spectra were recorded on a Bruker ARX-300 mixture was allowed to warm up to r. t. The solution was
spectrometer and mass spectra on a Finnigan Mat 8200 in- cooled down again (−25 ◦C) and MeI (7.1 g, 50 mmol) was
strument at ca. 70 eV using the electron impact method. added. After 30 min the cooling bath was removed and the
The 1H NMR spectra were recorded at 300.135 MHz and mixture allowed to gradually warm to r. t. After a while the
the signal of the deuterated solvent was used as reference magnetic stirrer was able to stir the suspension. After 1 h
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424
S. Lotz et al. · Di-tungsten Bis-carbene Complexes Linked by Condensed Heteroaromatic Spacers
the mixture was quenched with water. Ether was added and of hexane, and kept for 15 min at that temperature, after
the organic phase was separated. The aqueous phase was ex- which the mixture was allowed to cool down to r. t.
The mixture was first cooled to −40 ◦C and W(CO)6
tracted twice with 20 mL of ether. The organic layers were
combined and dried with anhydrous Na2SO4. The solvent
was removed. The product had a greenish colour, and the
yield was almost quantitative.
Hydrolysis: The solvent was removed under vacuum. The
product was dissolved in H2O, acidified with HCl, extracted
with ether and dried over Na2SO4. Evaporation under vac-
uum gave a white crystalline solid. After two days the colour
turned black (decomposition).
(3.55 g, 10.2 mmol) was added, where after the solution was
allowed to warm up to r. t., until all the W(CO)6 had dis-
solved. The solution was observed to become dark red-brown
in colour. The solvent was removed, a little dichloromethane
was added and the solution was again cooled (−30 ◦C)
and Et3OBF4 (2.0 g, 10.4 mmol – dissolved beforehand in
dichloromethane) was added. The solvent was removed after
the alkylation was completed. The residue was placed on a
silica gel column and the product 2 was obtained after eluting
with a hexane:dichloromethane (2 : 1) mixture and collect-
ing and purifying the dark zone to give 1.02 g (0.95 mmol,
Decarboxylation: The acid product (1.2 g, 6.73 mmol) of
the previous reaction, quinoline (8 mL, 8.74 mmol) and Cu
powder (0.8 g, 12.6 mmol) were mixed together in a tube
1
◦
22.7 %) of 2. – H NMR (CDCl3): δ = 1.69 (t, J = 7.1 Hz,
and heated in a preheated bath to ca. 220 to 230 C. Gas
6H, O–CH2CH3) 3.77 (s, 6H, N–CH3), 4.98 (q, J = 7.1 Hz,
4H, O–CH2CH3), 7.12 (s, 2H, H2). – MS (EI): m/z (based on
184W) = 898 [M]+. – IR (CH2Cl2): νCO = 2070 (m), 1938(s),
evolved. The mixture was heated for 30 min and then al-
lowed to cool down to r. t. The product was purified via col-
umn chromatography with silica gel as stationary phase and
ether or ether/hexane (5 : 2) mixtures as eluants to give 0.42 g
(3.16 mmol, 47 %) of L2. – 1H NMR (CDCl3): δ = 3.68
(s, N–CH3, 6H), 6.59 (d, J = 2.6 Hz, 2H, H2), 5.95 (d, J =
2.6 Hz, 2H, H3).
1932(vs) cm−1
.
Synthesis of [(CO)5W{C(OEt)TPC(OEt)}W(CO)5] (3)
First method: To N-methylthieno[3,2-b]pyrrole (0.69 g,
N-Methylthieno[3,2-b]pyrrole (L3): The same general 5.0 mmol) was added TMEDA (1.52 mL, 10.1 mmol) and
◦
method used to build a condensed pyrrole ring above for L2 then, after cooling to 0 C, n-BuLi (6.4 mL, 10.1 mmol).
was used to synthesize L3. The overall yield over the After 10 min the ice bath was removed and the yellow re-
five steps was 23 %. In accordance with results cited by action mixture was allowed to warm to r. t. The temperature
Aratani and◦co-workers the condensation step was carried was increased to near the boiling point of hexane, and kept
◦
out at −20 C instead of 5 – 10 C, improving the yield of for 15 min, after which the mixture was allowed to cool down
this step substantially [26]. – 1H NMR (CDCl3): δ = 3.80 (s, to r. t.
◦
3H, N–CH3), 7.17 (dd, J = 5.2, 1.3 Hz, 1H, H2), 7.00 (dd,
J = 5.2, 0.8 Hz, 2H, H3), 6.89 (dd, J = 2.85, 0.8 Hz, 1H, H7),
6.49 (dd, J = 2.85, 1.3 Hz, 1H, H6).
The mixture was first cooled to −40 C and after addi-
tion of W(CO)6 (3.55 g, 10.2 mmol) stirred for 30 min and
removed from the cold bath. The solution was observed to
become dark red-brown in colour. The solvent was removed
under reduced pressure and the residue dissolved in 30 mL
of dichloromethane. The solution was again cooled (−30 ◦C)
and then Et3OBF4 (2.0 g, 10.4 mmol), dissolved beforehand
in dichloromethane, was added. The solvent was removed af-
ter the alkylation was completed. The residue was placed
on a silica gel column and the mixture chromatographed
by using a hexane:dichloromethane (5 : 1) mixture as elu-
ant. The red zone containing the monocarbene complex
[W(CO)5{C(OEt)TPꢀH}] was collected but discarded. The
biscarbene product 3 was obtained by eluting with a hex-
ane:dichloromethane (2 : 1) mixture and collecting the dark
zone to give 0.85 g (0.95 mmol, 19 %) of 3. A third blue
band representing [W(CO)4{C(OEt)TꢀC(OEt)PꢀH}] was not
collected [27].
Tungsten biscarbene complexes
Synthesis of [(CO)5W{C(OEt)TꢀTꢀC(OEt)}W(CO)5] (1)
The synthesis affording dark purple crystals in a yield
of 36 % and the characterization of 1 have been reported ear-
lier from our laboratories [10]. – 1H NMR (CDCl3): δ =
1.72 (t, J = 7.1 Hz, 6H, O–CH2CH3), 2.48 (s, 6H, CH3),
5.04 (q, J = 7.1 Hz, 4H, O–CH2CH3). – MS (EI): m/z (based
on 184W) = 928 [M+], fragment ions corresponding to the
stepwise fragmentation of carbonyl ligands. – IR (CH2Cl2):
ν
CO = 2062(m), 1983(vw), 1938(vs) cm−1
.
Synthesis of [(CO)5W{C(OEt)PꢀPꢀC(OEt)}W(CO)5] (2)
To N,Nꢀ-dimethylpyrrolo[3,2-b]pyrrole (0.67 g, 5.0
Second method: To a cooled (0 ◦C) solution of N-methyl-
mmol) was added TMEDA (1.5 mL, 10.1 mmol) and thieno[3,2-b]pyrrole (0.69 g, 5.0 mmol) dissolved in 40 mL
◦
then, after cooling to 0 C, n-BuLi (6.4 mL, 10.1 mmol). of THF was added n-BuLi (6.4 mL, 10.1 mmol) and the re-
After 10 min the ice bath was removed and the yellow action mixture stirred for 30 min and subsequently cooled
reaction mixture allowed to warm to r. t. The temperature of to −40 ◦C and treated with W(CO)6 (1.77 g, 5.1 mmol). Af-
the reaction mixture was increased to near the boiling point ter stirring in the cold for 30 min, the cold bath was removed
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S. Lotz et al. · Di-tungsten Bis-carbene Complexes Linked by Condensed Heteroaromatic Spacers
425
Table 2. Crystal structure data
for 1 – 3.
1
2
3
Formula
Mr
C24H16O12S2W2
928.19
C24H18N2O12W2
894.10
C23H15NO12SW2
897.12
T [K]
183(2)
183(2)
183(2)
Cryst. size [mm3]
Crystal system
Space group
0.32×0.07×0.07
0.02×0.18×0.10
0.10×0.10×0.08
triclinic
triclinic
triclinic
¯
¯
¯
P1
P1
P1
˚
a [A]
6.4190(3)
8.9568(3)
13.1429(4)
71.728(2)
81.682(2)
81.683(2)
706.01(5)
1
6.5306(2)
8.9872(2)
12.6945(4)
72.286(1)
81.923(2)
83.705(2)
700.93(3)
1
6.457(5)
8.619(5)
13.453(5)
71.689(5)
86.395(5)
83.643(5)
706.1(7)
1
˚
b [A]
˚
c [A]
α [deg]
β [deg]
γ [deg]
3
˚
V [A ]
Z
Dcalcd [g cm−3
]
2.183
0.8348
436
2.118
0.8263
420
2.110
0.8273
420
µ(MoKα ) [cm−1
F(000) [e]
]
hkl range
0 ≤ h ≤ 8
−10 ≤ k ≤ 11
−15 ≤ l ≤ 16
3.23 – 26.4
2870
2870
0.0480
−8 ≤ h ≤ 8
−11 ≤ k ≤ 11
−15 ≤ l ≤ 14
3.53 – 26.35
6177
2833
0.0269
−8 ≤ h ≤ 8
−11 ≤ k ≤ 0
−17 ≤ l ≤ 16
2.50 – 27.41
3436
3220
0.0195
θ Range data collection [deg]
Reflexions measured
Reflexions unique
Rint
Parameters refined
R(F)/wR(F2) (all reflexions)
GoF (F2)
181
0.0503/0.1257
1.083
0.986/−0.396
182
0.0278/0.0682
1.051
0.9436/−0.4471
188
0.0562/0.0759
1.044
1.209/−1.521
−3
˚
∆ρfin (max/min) [e A
]
and stirring continued for an additional hour. The reaction X-Ray structure determination
◦
mixture was again cooled to −40 C and freshly prepared
◦
The intensity data were collected at −90 C on a Non-
ius Kappa CCD diffractometer for compounds 1 and 2 and
on a NONIUS CAD4 diffractometer for compound 3, using
graphite-monochromated MoKα radiation. Data were cor-
rected for Lorentz and polarization effects and for absorp-
tion [28 – 31]. The structures were solved by Direct Methods
(SHELXS) [32] and refined by full-matrix least-squares tech-
LDA (3.2 mL, 5.5 mmol) added. After stirring for 30 min,
W(CO)6 (1.77 g, 5.1 mmol) was added and the reaction
mixture removed from the cold and stirred for another h,
followed by the alkylation with triethyloxonium tetrafluoro-
borate (2.0 g, 10.4 mmol) as under the first method. After
column chromatography 0.94 g (1.05 mmol, 21 %) of 3 was
isolated.
2
niques against Fo (SHELXL-97) [33]. The hydrogen atoms
Third method: The dilithiated ligand L3 was prepared by
the first method and treated with W(CO)6 (1.77 g, 5.1 mmol).
The solvent was removed in vacuo and the resulting metal
acylate washed with several portions of hexane to remove
unreacted starting materials. The residue was redissolved in
THF and again reacted with W(CO)6 (1.77 g, 5.1 mmol).
Alkylation was carried out as described for the first method.
This methodology improved the yield of the biscarbene com-
plex affording 1.3 g (1.2 mmol, 24 %) of 3. – MS (EI): m/z
(based on 184W) = [M]+ not observed, fragment ions: 813
([M–3CO]+), 785, 757, 729, 701, 673, 645, etc. represent-
ing stepwise fragmentation by loss of carbonyl ligands. –
1H NMR (CDCl3): δ = 1.68 (t, J = 7.0 Hz, 3H, O–CH2CH3),
1.71 (t, J = 7.0 Hz, 3H, O–CH2CH3) 3.88 (s, 3H, N–CH3),
4.97 (q, J = 7.0 Hz, 2H, O–CH2CH3), 5.01 (q, J = 7.0 Hz, 2H,
of the structures were included at calculated positions with
fixed thermal parameters. All non-hydrogen atoms were re-
fined anisotropically. XP (SIEMENS Analytical X-ray In-
struments, Inc.) was used for structure representations. The
structure of 3 is disordered around the inversion centre. Other
experimental details are given in Table 2.
CCDC 634278 (1), 634279 (2) and 634280 (3) contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data request/cif.
Acknowledgement
This work was supported financially by the University of
O–CH2CH3), 7.59 (s, 1H, H6), 7.89 (s, 1H, H3). – IR (hex- Pretoria and by the National Research Foundation of South
ane): νCO = 2061(m), 1979(vw), 1943(s), 1935(vs) cm−1
.
Africa under Grant No’s: 2047221(SL).
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426
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