Binuclear biscarbene complexes of furan

Article abstract of DOI:10.1039/b419148j

Carbene complexes of chromium and tungsten with a bridging furan substituent were synthesized from lithiated furan precursors and metal hexacarbonyls. The binuclear biscarbene complexes [(CO)₅M{C(OEt)- C₄H₂O-C(OEt)}-M′(CO)₅] (M = M′ = Cr (3), W (4)) were obtained as well as the corresponding monocarbene complexes [M{C(OEt)-C₄H₃O}(CO)₅] (M = Cr (1), W (2)). A method of protecting the carbene moiety during the metal acylate stage was used to increase not only the yields of the binuclear Fischer biscarbene complexes 3 and 4 but to establish a method to synthesize analogous mixed heterobinuclear carbene complexes (M = W, M′ = Cr (5)) in high yields. The binuclear biscarbene complexes 3 and 5 were reacted with 3-hexyne and yielded the corresponding benzannulated monocarbene complexes [M{C(OEt)-C₁₄H ₁₇O₃}(CO)₅] (M = Cr (6), W(7)). Complex 5 reacted regioselectively with the benzannulation reaction occurring at the chromium-carbene centre. The major products from refluxing 3 in the presence of [Pd(PPh₃)₄] were a monocarbene-ester complex [Cr{C(OEt)-C₄H₂O-C(O)OEt}(CO)₅] (8), the 2,5-diester of furan (9) and a carbene-carbene coupled olefin EtOC(O)-C ₄H₂O-C(OEt)=C(OEt)-C₄H₂O-C(O)OEt (10). X-Ray structure analysis of 4 and 6 confirmed the molecular structures of the compounds in the solid state and aspects of electron conjugation between the transition metals and the furan substituents in the carbene ligands were investigated. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b419148j

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F U L L P A P E R
Binuclear biscarbene complexes of furan
Chantelle Crause,a Helmar Go¨rlsb and Simon Lotz*a
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, D-07743, Jena, Germany
Received 21st December 2004, Accepted 3rd March 2005
First published as an Advance Article on the web 4th April 2005
Carbene complexes of chromium and tungsten with a bridging furan substituent were synthesized from lithiated
furan precursors and metal hexacarbonyls. The binuclear biscarbene complexes [(CO)5M{C(OEt)–C4H2O–C(OEt)}-
Mꢀ(CO)5] (M = Mꢀ = Cr (3), W (4)) were obtained as well as the corresponding monocarbene complexes [M{C(OEt)–
C4H3O}(CO)5] (M = Cr (1), W (2)). A method of protecting the carbene moiety during the metal acylate stage was
used to increase not only the yields of the binuclear Fischer biscarbene complexes 3 and 4 but to establish a method
to synthesize analogous mixed heterobinuclear carbene complexes (M = W, Mꢀ = Cr (5)) in high yields. The binuclear
biscarbene complexes 3 and 5 were reacted with 3-hexyne and yielded the corresponding benzannulated monocarbene
complexes [M{C(OEt)–C14H17O3}(CO)5] (M = Cr (6), W(7)). Complex 5 reacted regioselectively with the
benzannulation reaction occurring at the chromium–carbene centre. The major products from refluxing 3 in the
presence of [Pd(PPh3)4] were a monocarbene-ester complex [Cr{C(OEt)–C4H2O–C(O)OEt}(CO)5] (8), the 2,5-diester
=
of furan (9) and a carbene–carbene coupled olefin EtOC(O)–C4H2O–C(OEt) C(OEt)–C4H2O–C(O)OEt (10). X-Ray
structure analysis of 4 and 6 confirmed the molecular structures of the compounds in the solid state and aspects of
electron conjugation between the transition metals and the furan substituents in the carbene ligands were investigated.
metal carbonyl dianions in the presence of chlorotrimethylsilane
Introduction
toprepare aseries ofCr or Fe homonuclear bimetallic biscarbene
The carbene carbon atom in Fischer carbene complexes is a
complexes as well as one mixed Cr,Fe bis(aminocarbene)
highly reactive electrophilic centre, being affected by the metal
complex linked by a benzene moiety in the 1,3-positions.
fragments and the carbene substituents. Binuclear transition
The synthesis of carbene complexes with modified sub-
metal complexes containing conjugated bridges have recently
stituents, the stepwise exploitation of two different transition
attracted considerable interest, since electronic communication
metal carbene centres for different carbon–carbon coupling
between the metal centres could lead to interesting physical
reactions, the resulting regioselectivity determined by a specific
and chemical properties.1 Application of transition metal mono-
metal fragment, and the electronic and structural features of
carbene complexes in organic synthesis is also an exciting and
such binuclear biscarbene complexes are but some of many
already established area of research focussing not only on the
important topics of research that have been neglected and are
carbocationic carbene carbon atom but also on unsaturated
of interest in our laboratories. In this paper we report the
side chains.2 Homonuclear bimetallic carbene complexes with
synthesis of bimetallic biscarbene complexes with a bridging
conjugated even-numbered carbon atoms ranging from C4 to
C14 and heteronuclear bimetallic carbene complexes with C3
furan spacer which was used to highlight the challenge of
synthesizing mixed transition metal biscarbene complexes with
bridges have been reported by using reactive carbene precursors
bridging heteroarene substituents in high yields, the role of
in coupling reactions.3 Bimetallic homonuclear biscarbene com-
the heteroatom in the bridging heteroarene to determine the
plexes withone-dimensionalconjugated phenylene4 andbenzene
conformation of the biscarbene complex in the solid state and the
derivatives5 acting as bridging units are also known. By contrast,
testing of known monocarbene reaction pathways for carbene
similar chain compounds with conjugated heteroarene moieties
modification and carbon–carbon coupling reactions.
instead of carbon units have received far less attention. It has
been reported that thiophene derivatives can be useful compo-
nents of electronic materials,6 have stabilities surpassing that
of polyenes7 and conduct electrons with better efficiency than
their phenyl analogues.8 Consequently, heteroaromatic units are
attractive alternative p-spacers in bimetallic compounds and
we have reported results of homonuclear bimetallic biscarbene
complexes with linking thiophene9 and thiophene derivatives.10
Synthetic methods to prepare mixed transition metal binu-
clear biscarbene complexes with arene units linking carbene car-
bon atoms remain a challenging synthetic problem. Maiorana
and co-workers11 reported the synthesis of a bimetallic (Cr,W)
complex containing a bithiophene unit as bridging substituent
in a mixed metal Fischer biscarbene complex. The objective
of this study was to investigate potential non-linear optical
properties of Fischer carbene complexes. In a second approach,
the groups of Hegedus and Semmelhack12 used acyl halides or
in situ generated chloroimine intermediates that were treated
with nucleophilic transition metal complexes to afford mononu-
clear aminocarbene complexes. Based on this method, Dvorak
and co-workers13 studied the reaction of tertiary diamides and
Results and discussions
Synthesis of the biscarbene complexes
Chromium and tungsten carbene complexes of furan were syn-
thesized from reacting lithiated furan (path A)14 and dilithiated
furan (path C) with metal carbonyls as shown in Scheme 1.
Furan can be monolithiated quantitatively and quickly with
butyllithium15 and Chadwick and Willbe established optimum
conditions for the preparation of 2,5-dilithiofuran.16
Route C shows the synthesis of homonuclear biscarbene
complexes and entails the generation of 2,5-dilithiofuran in
hexane followed by the addition of two equivalents of the
metal carbonyl to form a dilithium diacyl metallate. Subsequent
quenching of the dilithium salt with triethyloxonium tetraflu-
oroborate yielded amongst others the desired heteronuclear
biscarbene complex. This route afforded both the monocarbene,
[M(CO)5{C(OEt)furyl}] (M = Cr 1, M = W 2) and biscarbene
complexes [(CO)5{M{C(OEt)–C4H2O–C(OEt)}M(CO)5] (M =
Cr 3, W 4). A by-product of the reaction was a monocarbene
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 1 6 4 9 – 1 6 5 7
1 6 4 9
©

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Scheme 1 Synthesis of dinuclear biscarbene complexes of furan.
furan complex with an ester group in the 5-position of the
furan substituent.9 The corresponding chromium complex
[Cr(CO)5{C(OEt)–C4H2O–C(O)OEt}] 8 was isolated and fully
characterized. This method characteristically yields significant
amounts of the butyl carbene complexes, [M(CO)5{C(OEt)Bu}],
and furyl monocarbene complexes (1, 2) and low amounts of
the targeted biscarbene complexes (3, 7%; 4, 2%). A possible
explanation for this is the fact that the reaction is carried out
under mild conditions (low temperatures) and that the second
deprotonation of the intermediate furyl anionic complex was
incomplete.
with 3-hexyne was investigated. The chromium complexes had
the highest selectivity to the carbonyl inserted benzannulated
product (phenol) with the optimal conditions being found with
higher metal complex concentrations and with less polar and/or
coordinating solvents.
Very few accounts of the reactions of biscarbene com-
plexes with alkynes exist and as far as we are aware none
with heteroarene substituents that could provide the required
unsaturated bond for the benzannulation reactions. Whereas
the chromium biscarbene complex, [(CO)5Cr{C(OEt)–C6H4–
C(OEt)-p}Cr(CO)5] reacted with one molecule of diphenylacety-
lene in the usual way affording A (Fig. 1), the binuclear diphenyl
biscarbene complexes react with two molecules of dipheny-
lacetylene under similar conditions to afford the anticipated
binaphthol derivative B for chromium and the indene derivative
C for tungsten (Fig. 1).20
A method (path B) based on the one used by Maiorana and
co-workers11 for bithiophene was modified to improve the yield
of the furan biscarbene complexes 3 and 4 and to access the
heteronuclear bimetallic biscarbene complex 5. The method
involves the stepwise generation of acyl metallates from the
reaction with lithiated furan and metal carbonyls. The second
metallation at the 5ꢀ position was achieved with LDA at low
temperatures (Maiorana used nBuLi instead). Method B is based
on a single pot reaction studied by Aoki et al.17 in which they
reacted [M(CO)5{C(OLi)R}] (R = heteroarenes) with LDA
at −78 ◦C followed by an appropriate organic electrophile,
water and finally Et3OBF4. This “anionic protection” strategy
of an electrophilic carbene carbon atom without substantially
reducing the acidity of the proton in the 5-position of the
furyl substituent, accounts for the formation of the dianionic
carbene precursor in high yields. It is during this sequential
addition that a heteronuclear dimetallic biscarbene complex,
[(CO)5Cr{C(OEt)–C4H2O–C(OEt)}W(CO)5] (5), becomes read-
ily accessible by adding a different metal hexacarbonyl in the
second step. Not only was the heteronuclear dimetallic complex
5 isolated in a yield of 47%, but the yields of the homonuclear
dimetallic complexes could also be increased significantly (3,
39%; 4, 40%). The alkylation step needs to be carried out in
dichloromethane as the biscarbene complexes are unstable in
aqueous medium. Alkylation in water of Li2[Cr2(CO)10{C(O)–
C4H2C(O)}] afforded only 8 and none of the corresponding
dichromium biscarbene complex. The reactions also yielded
the butyl- and furyl monocarbene complexes as well as the
diisopropylamino carbene complex, [M(CO)5{C(OEt)NiPr2}],
in low yields. The tungsten biscarbene complex 4 could be stored
for months under argon atmosphere in the solid state, but the
chromium analogue 3 was less stable.
Fig. 1 Benzannulation products from biscarbene complexes with
phenylene and biphenylene bridging substituents.20
Other examples include the double benzannulation of an
annulene derivative21 and the biscarbene from binaphtol that
yielded phenanthrene Cr(CO)3 complexes with axial and planar
chirality.22 Replacement of arene by heteroarene substituents,
could, in the light of these observations, lead to interesting new
coordination complexes and novel heteroarene compounds.
The reaction conditions used by Wulff19 were adapted for
the reactions of the biscarbene complexes 3, 4 and 5 with 3-
hexyne. These complexes presented the opportunity to evaluate a
number of factors that could influence the product distribution.
Not only could the influence of a second carbene functionality
be evaluated but also the effect of a different metal moiety.
The present study deals only with the major products formed
and no attempt was made to isolate intermediates or minor
organic products. Since the reactivity of both chromium and
tungsten complexes was evaluated, reaction conditions were
Benzannulation of biscarbene metal complexes (Do¨tz reaction)
Whilst the reaction of monocarbene complexes containing
heteroaromatic substituents and alkynes has been investigated
extensively18 the same does not hold for the analogous bimetallic
biscarbene complexes. In a study by Wulff et al.19 the reactivity
of aryl monocarbene complexes of chromium and tungsten
1 6 5 0
D a l t o n T r a n s . , 2 0 0 5 , 1 6 4 9 – 1 6 5 7

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chosen which would suit both metals. The reactions of the
biscarbene me◦tal complexes with 3-hexyne was carried out in
toluene at 80 C with a metal concentration of 0.1 mol dm−3
and 4 molar equivalents of alkyne. Higher metal complex
concentrations were not accessible due to the limited solubility of
these compounds in toluene. Reactions were allowed to proceed
for 24 h after which the major product was isolated with column
chromatography and the results are presented in Scheme 2
and Table 1. The monocarbene chromium complex 6 was the
major product isolated from the reaction of 3 with 3-hexyne
and the corresponding binuclear product with a p-coordinated
Cr(CO)3 moiety was not observed. Complex 4 did not react at
all under these conditions. This seems to indicate that either the
bimetallic nature of the metal complex or the furan substituent
contributes to the lower reactivity of the bimetallic complex. For
the mixed Cr,W-binuclear biscarbene complex 5, the formation
of 7 was found and could be expected since it is generally
known that chromium carbene complexes are more reactive
than their tungsten analogues. This result demonstrates the
potential of using heteronuclear dimetal biscarbene complexes
in benzannulation reactions to regioselectively discriminate
between the different metal carbene ligands.
carbene ligand is transferred to another metal centre.24 Fischer
and Beck25 reported the transfer of a carbene ligand from
Cp(CO)(NO)M(CXPh) (M = Cr, Mo, W; X = OMe, NMe2)
to photochemically generated Fe(CO)5. Carbene exchange at
elevated temperatures between alkoxychromium carbene com-
plexes and W(CO)6 to form alkoxytungsten carbene complexes
and Cr(CO)6 has been reported.26 Fischer and co-workers27
studied thermal carbene–carbene coupling reactions of several
monocarbene complexes by refluxing carbene complexes in
hexane. Stone and co-workers28 reported on the stoichiometric
transmetallation of Group VI Fischer carbene complexes to
coordinatively unsaturated metal centres to yield mixed metal
binuclear complexes with l-carbene ligands. Fischer carbene
complexes have also been used to prepare gold carbene com-
plexes by reaction with chloroauric acid.29 Sierra and co-
workers30 have reported results showing that carbene complexes
participate effectively in carbene–carbene coupling reactions
when Pd(0) or Pd(II) catalysts were used. According to the
proposed mechanism one requires the transfer of carbenes to
the palladium centre to facilitate a carbene–carbene coupling
reaction intramolecularly via a pallada-propane intermediate.
For the carbene transfer reaction to a palladium centre a three-
membered heterometallic dimetallapropane is required.
Initial work in this laboratory was aimed at the thermal
coupling of carbene complexes to afford new binuclear bis-
carbene complexes with a growing spacer between the two
identical metal fragments. Important is the retention of p-
conjugation through the spacer ligand in the final product as
this extended conjugated system could facilitate metal-to-metal
communication and impart charge-transfer properties to the
molecule. In the synthesis of bimolybdenum biscarbene complex
with a bridging dithieno[3,2-b:2ꢀ,3ꢀ-d]thiophene substituent,
a molybdenum biscarbene with a carbene–carbene coupled
bridging unit was isolated in low yield (Fig. 2).31
Scheme 2 Monobenzannulation of binuclear biscarbene complexes
with furylene substituents.
Fig. 2 Example of a biscarbene dimolybdenum complex with an
Carbene transfer reactions
extended conjugated spacer unit.31
Early work on the reactivity of Fischer carbene complexes
focussed on the ability of these complexes to act as stable
carbene sources.23 Examples have been cited in which the
Thermal studies in hexane with this complex showed that
the extended spacer was unfortunately more reactive towards
Table 1 Benzannulation reactions
Reagent
Quantity/mmol
1.12
Product
Yield (%)
35
1
No reaction
—
51
1.12
D a l t o n T r a n s . , 2 0 0 5 , 1 6 4 9 – 1 6 5 7
1 6 5 1

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oxygen and subsequent ester formation than the initial precursor
biscarbene dimolybdenum complex. As a result, an oxygen
atom replaced the metal fragment to afford a monocarbene
complex with a terminal ester function. This discouraged us to
further pursue carbene–carbene coupling reactions by refluxing
binuclear biscarbene complexes in hexane. It was anticipated
that the Pd catalyst could catalyse repetitive carbene–carbene
coupling reactions and provide access to extended polyenes
containing chainlike disubstituted furan units in polymeric
materials. The binuclear biscarbene chromium complex 3 was
refluxed in a thf solution in the presence of Pd(PPh3)4 until the
precursor was totally converted. Complex (8) and a mixture of
esters could be isolated (Scheme 3, Table 2). We have observed
previously that the thiophene analogue, [(CO)5Cr{C(OEt)–
C4H2S–C(OEt)}Cr(CO)5] was not stable in ether solutions,
resulting in the formation of 2,5-monocarbene ester complexes
similar to 8.9 The major product of this reaction after 12 h of
refluxing was the 2,5-diester of furan (9) and a cis-trans mixture
of the carbene–carbene coupled diester, 10. The presence of
8 was detectable by thin layer chromatography shortly after
initiation of the reaction in thf but was totally converted into
9 within the first 0.5 h. The experiment was repeated in hexane
and it was possible to isolate and characterize 8 as well as higher
yields of 10. These results indicated that ester formation was in
strong competition with carbene–carbene coupling and that this
reaction was most likely also catalysed by palladium.
Characterization
The carbene complexes were characterised using NMR and
infrared spectroscopy and mass spectrometry. Although anal-
ogous methoxy monocarbene complexes of furan (1, 2) are
known,14 the spectral data of the ethoxy derivatives are given for
comparative purposes. Connor and Jones synthesized a series
of monocarbene complexes of chromium with heteroaromatic
substituents, [Cr(CO)5{C(XR)C4H3Y}] (RX = OMe, NH2; Y =
O, S, NMe) in order to study the extent to which the heteroatom
Y affects the donation of electrons to the empty carbene carbon
p-orbital.14 Final confirmation of the molecular structures of
4 and 6 were obtained from single crystal X-ray diffraction
studies.
The coordination of a metal fragment via a carbene carbon
to furan has a profound effect on the electronic environment
1
of the ring protons and is clearly reflected in the H and 13C
NMR spectra. The 1H NMR data for the mono- and biscarbene
complexes with furan substituents are given in Table 3. The
chemical shift of free furan (7.38 (H2,5) and 6.3 (H3,4) ppm)
and the predicted values for a 2-furylester derivative (7.15 (H3),
6.52 (H4) and 7.63 (H5)) are used as reference values.32 The
Scheme 3 Compounds with extended conjugated spacer units.
Table 2 Palladium catalysed carbene–carbene coupling reactions of 3
Product Yield (%) NMR data
32
1H: d 7.17 (s, 2H, H-2 and H-3), 4.37 (q, 4H, J = 7.2 Hz, OCH2CH3), 1.37 (t, 6H, J =
7.2 Hz, OCH2CH3)
13C: d 146.0 (CO2Et), 118.3 (C1, C4), 117.2 (C2, C3), 60.6 (OCH2CH3), 13.2 (OCH2CH3)
21 (trans) 1H: d 7.21 (d, 2H, H-2 and H-2ꢀ, J = 3.5), 6.85 (d, 2H, H-3 and H-3ꢀ, J = 3.5), 4.34 (q,
4H, J = 7.2 Hz, OCH2CH3), 1.39 (t, 6H, J = 7.2 Hz, OCH2CH3), 3.93 (q, 4H, J =
7.0 Hz, OCH2CH3), 1.38 (t, 6H, J = 7.0 Hz, OCH2CH3)
7 (cis)
1H: d 7.22 (d, 2H, H-2 and H-2ꢀ, J = 3.5), 6.80 (d, 2H, H-3 and H-3ꢀ, J = 3.5), 4.34 (q,
4H, J = 7.2 Hz, OCH2CH3), 1.36 (t, 6H, J = 7.2 Hz, OCH2CH3), 3.71 (q, 4H, J =
7.0 Hz, OCH2CH3), 1.33 (t, 6H, J = 7.0 Hz, OCH2CH3)
See Table 3
13
1 6 5 2
D a l t o n T r a n s . , 2 0 0 5 , 1 6 4 9 – 1 6 5 7

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assignment of the furan protons is clear from the coupling
constants and is also in line with the prediction for a furyl ester.
In all spectra the methylene quartet and the methyl triplet of the
ethoxy substituents of the carbene carbon and ester displayed
the largest downfield shift which is consistent with the strong
electron-withdrawing character of the M(CO)5{C(C4H3O)} and
1
the ester groups. Since H and 13C NMR spectroscopy unam-
biguously discriminates between the various methylene protons
of the ethoxy groups in these compounds, it was the method
of choice to use as a probe to study product formation in
the reactions of biscarbene complexes and [Pd(Ph3)4]. Typical
spectral ranges for 1H NMR (13C NMR) chemical shifts (d, ppm)
=
are: 5.0–5.2 (75–77) for Cr C(OCH2Me)R, 5.0–4.9 (78–79) for
W C(OCH2Me)R, 4.3–4.4 (61–62) ppm for O C(OCH2Me)R,
=
=
=
3.9–4.1 (67–69) ppm for trans-RC(OCH2Me) C(OCH2Me)R
and 3.7–3.8 ppm for cis-RC(OCH2Me) C(OCH2Me)R. The
=
chemical shifts of the methylene protons of the ethoxy carbene
moiety are fairly characteristic for a specific metal and less
sensitive to the number and type of carbene substituents. On the
other hand the number of carbene ligands as well as the nature of
the transition metal involved, determines the chemical shifts of
the ring protons of the furan substituent. Single resonances are
observed for the ring protons in the symmetrical homonuclear
bimetallic biscarbene complexes with d values in between the
highest and lowest values for the corresponding monocarbene
complexes. In the biscarbene complexes two metal moieties
compete to drain electron density from the double bonds in the
furan ring, thus deshielding both H3 and H4 as opposed to H3
and H5 in the monocarbene complexes, a phenomenon that can
be explained by p-resonance effects. The assignment of chemical
shifts for 5 was made by comparison to the homonuclear
bimetallic complexes. An unequal deshielding of H3 and H4
reflects the competitive withdrawal of electron density from 2,5-
disubstituted furan by different carbene carbons in 5 and by
an ester and a carbene in 8. The ester group has a smaller
deshielding effect on the adjacent proton (H4) compared to that
of a carbene substituent on H3. The 13C NMR data supports the
proton data and chemical shifts are determined by the type of
metal, the carbene carbon or furyl substituent. Carbene carbon
and carbonyl carbon resonances are in good agreement with data
reported for other group 6 pentacarbonyl carbene complexes.33
The pentacarbonyl metal fragment has C4v symmetry and of
the three infrared active absorption bands the A1(2) band is by far
the more sensitive for electronic properties of the unique ligand.34
For carbene complexes 1–8, which represent good p-acceptor
ligands, the A1(2) band is found at higher wavenumbers compared
to the E band. The A1(2) band is also found at lower wavenumbers
for W compared to the corresponding Cr complexes. This effect
can be seen when comparing Cr complexes 1 and 3 with W
complexes 2 and 4. The mixed metal complex 5 did not show the
expected doubling of peaks due to the presence of two different
metals. The four peaks observed were however very broad and
the poor resolution indicated that the peaks for the carbonyl
ligands on the different metals overlap to a great extent. A
molecular ion peak, M+, was observed in the mass spectra for
each of the furan complexes and a general fragmentation pattern
was recognized which consisted of the stepwise fragmentation
of the carbonyl ligands and carbene–ethoxy moiety.
X-Ray crystallography
Several attempts were made to obtain crystals of especially
the heteronuclear dimetallic biscarbene complex but suitable
crystals were only obtained for the tungsten biscarbene complex
4. Crystals of this biscarbene complex were afforded from a
hexane–dichloromethane (5 : 1) solution as deep purple needle-
like crystals. Crystals of 6 were formed from a concentrated
hexane solution cooled to 0 ◦C. Single crystal X-ray diffraction
studies confirmed the molecular structure of the complexes as
depicted by the ball-and-stick plots in Figs. 3 and 4.
D a l t o n T r a n s . , 2 0 0 5 , 1 6 4 9 – 1 6 5 7
1 6 5 3

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the hydrogen atoms. The geometrical parameters for furan,
assuming planarity and C2v symmetry as well as selected bond
lengths and angles determined for complexes 4 and 6 are
tabulated in Table 4.
The six ligands, five carbonyl groups and one carbene,
are arranged octahedrally around each metal centre. The W–
C(carbene) bond lengths in 4 differ significantly from each
˚
other, but is comparable to values of approximately 2.1 A
found in literature38 and agrees excellently with the observed
value in the corresponding thiophene biscarbene complex.9 The
thiophene biscarbene tungsten complex does not exhibit such
a discrepancy between the two metal–carbene carbon bonds
˚
(2.154 vs. 2.156 A). This could be due to the orientation of the
tungsten carbene fragments with respect to the furan ring. Fig. 3
shows clearly that the tungsten atoms are on different sides of
the furan ring. In the analogous thiophene compound the metal
moieties are on the same side of the thiophene ring, opposite to
the sulfur atom.9,39 The planarity of the furan ring, carbene
carbons and metal atoms in the solid state would facilitate
electron delocalisation. The torsion angle O(1)–C(5)–C(4)–C(3)
is 0.0(3)◦ indicating that the furan ring is planar. On both sides
of the heterocycle, the furyl ring, carbene carbon and metal
atom are also ◦in the same plane with torsion angles of 180.0(2)
and −180.0(2) for W–C(1)–C(2)–O(1) andWꢀ–C(6)–C(5)–C(4),
respectively. On comparing the bond lengths of the furan ring
in complex 4 with the bond lengths of uncoordinated furan,
evidence is found of electron delocalisation through the ring. All
Fig. 3 Ball-and-stick representation of complex 4.
˚
the bonds were equalized to within 0.08 A with the C(3)–C(4)
bond shorter in the complex while the C(2)–C(3) and C(4)–C(5)
bonds are fractionally longer. The bond angles within the ring
are also within 3◦ of each other. This delocalisation is carried
through to the C(carbene)–C(furyl) bond which is also shorter
˚
than normal C–C single bonds (1.448 and 1.464 vs. 1.51(3) A).
The bond lengths within the fused ring system are not similar,
with large deviations in the C(3)–C(3a) and C(5)–C(6) bonds.
This deformation indicates incomplete electron delocalisation
through the p-system. This may be attributed to electronic rather
than steric factors since the rings are coplanar as shown by
the torsion angle of 179.4(3)◦ through the fused ring system.
Interestingly, the carbene carbon is bent out of the plane formed
by the ring systems as can be seen from the torsion angle of
167.7(3)◦ for Cr–C(1)–C(2)–C(3) thus electron delocalisation
from the aromatic system to the carbene moiety would be
inhibited. This leads to the conclusion that the observed
stabilisation of the carbene moiety by the aromatic substituent
is by inductive effects and not electron delocalisation.
Fig. 4 Ball-and-stick representation of complex 6.
Although the structure of furan in the gas phase has been
determined by microwave spectroscopy,35 the experimental re-
sults for the gas phase differ substantially from those reported
for the crystalline phase.36 Liescheski and Rankin37 determined
the molecular structure of furan by several structural analyses
using various combinations of data from gas-phase electron
diffraction, rotational spectroscopy and liquid crystal NMR
spectroscopy. The best structure obtained confirms the results
of the earlier microwave structures, except for the position of
◦
36
˚
Table 4 Selected bond lengths (A) and angles ( ) for 4, 6 and furan
4 (M = W)
6 (M = Cr)
4 (M = W)
6 (M = Cr)
M–C(1)
2.166(6)
2.139(6)
2.044(7)
2.051(5)
2.046(6)
2.038(5)
1.448(8)
1.382(8)
1.414(7)
1.366(8)
—
2.051(7)
—
1.885(8)
1.884(8)
—
M–C(1)–C(2)
123.0(4)
131.4(4)
105.6(5)
107.4(4)
109.5(5)
107.7(5)
108.7(5)
106.7(5)
102.2(5)
132.4(4)
125.4(4)
125.4(4)
130.2(4)
104.3(5)
105.0(4)
111.3(5)
105.6(5)
110.5(5)
107.5(5)
—
Mꢀ–C(6)
M–C(1)–O(3/4)
C(2)–C(1)–O(3/4)
C(2)–O(1)–C(5/9)
O(1)–C(5/9)–C(4)
C(5/9)–C(4)–C(3)
C(3)–C(2)–O(1)
C(4)–C(3)–C(2)
C(5)–C(6)–O(2)
O(2)–C(6)–W
M–C(trans-CO)
M–C(cis-CO)av
Mꢀ–C(trans-CO)
Mꢀ–C(cis-CO)av
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(4)–C(9)
C(5)–C(6)
O(1)–C(2)
—
1.452(8)
1.371(8)
1.422(8)
—
1.387(8)
—
1.398(7)
1.385(7)
—
—
—
C(5)–C(6)–W
1.464(7)
1.393(6)
—
O(1)–C(9)
O(1)–C(5)
1.370(6)
Furan36
O–C(2)
C(2)–C(3)
C(3)–C(4)
1.364(1)
1.364(1)
1.430(2)
C(2)–O–C(5)
O–C(2)–C(3)
C(2)–C(3)–C(4)
106.74(7)
110.49(7)
1 6 5 4
D a l t o n T r a n s . , 2 0 0 5 , 1 6 4 9 – 1 6 5 7

View Article Online
Route B.
Conclusions
Synthesis of biscarbene metal complexes via monolithiated
furan followed by deprotonation of a metal acylate. 2.0 mmol
of furan was dissolved in 10 ml of thf and 1.25 ml (2 mmol) of
n-butyllithium was added at 0 ◦C and the reaction was stirred for
30 minutes. 2.0 mmol of M(CO)6 was added at −40 ◦C and the
temperature was allowed to rise to room temperature. Stirring
was maintained for a further 15 min during which the colour
of the reaction mixture usually changed to orange–brown. The
reaction mixture was cooled to −78 ◦C and a thf solution of
LDA (2.4 mmol, prepared according to Brandsma)15 was added.
Stirring was maintained at this temperature for 1 h after which
the reaction was warmed to −40 ◦C. 2.0 mmol of M(CO)6
was added and the temperature was allowed to rise to room
temperature. The colour of the reaction mixture usually changed
to dark brown. The solvent was removed in vacuo, the reaction
residue was redissolved in 10 ml of dichloromethane and cooled
to −20 ◦C. A solution of 0.78 g (4.05 mmol) triethyloxonium
tetrafluoroborate in 10 ml of dichloromethane was added. An
immediate colour change to dark purple was observed. The
cooling bath was removed and stirring was continued for a
further hour. The mixture was washed with dichloromethane
through a filter containing silica gel. The solvent was removed in
vacuo and the remainder purified on a silica gel column. Products
were isolated starting withhexane as elutingagent andincreasing
the polarity gradually by adding dichloromethane to the eluting
agent.
We have shown optimised reaction conditions to synthesize
mixed metal binuclear Fischer biscarbene complexes by using an
anionic metal acylate protection strategy and a stepwise addition
of metal carbonyl precursors. We have demonstrated how
one could use [(CO)5Cr{C(OEt)–C4H2O–C(OEt)}W(CO)5] and
[(CO)5Cr{C(OEt)–C4H2O–C(OEt)}Cr(CO)5] as precursors in
benzannulation reaction to synthesize analogous monocarbene
complexes of Cr and W with similar modified carbene ligands.
The tungsten monocarbene complex is not obtainable starting
from [(CO)5W{C(OEt)–C4H2O–C(OEt)}W(CO)5]. This repre-
sents but one example of exploiting the regioselectivity induced
by different transition metal fragments in binuclear biscarbene
complexes and far wider applications in organic syntheses are
obvious. In order to gain deeper insight into the conformational
preferences of binuclear biscarbene complexes and the role
of the heteroatom in the bridging heteroarene moiety, DFT
calculations are presently being carried out. In the solid state a
differerent conformer is found for the bridging furan compare to
bridging thiophene and N-methylpyrrole in binuclear biscarbene
complexes.
Experimental
General remarks
All reactions were performed in an inert atmosphere of either
nitrogen or argon with dry solvents by using standard Schlenk
and vacuum-line techniques. Column chromatography was
carried out under a nitrogen atmosphere using silica gel (particle
size 0.063–0.200 nm) as stationary phase. Solvents were dried
and distilled under an atmosphere of nitrogen before use.
Ether, tetrahydrofuran, hexane and toluene were distilled from
sodium metal, with benzophenone as indicator in the case of
the ethereal solvents. Chloroform and dichloromethane were
distilled from phosphorus pentoxide. TMEDA was distilled
before use. All other reagents were purchased from commercial
suppliers and used without further purification. All NMR spec-
tra were recorded in degassed deuterated chloroform on a Bruker
ARX-300 spectrometer. Chemical shifts were referenced to
chloroform. 1H and 13C NMR spectra were measured at 300.133
and 75.469 MHz, respectively. Infrared spectra were recorded
on a BOMEM Michelson-100 FT-IR spectrophotometer with a
NaCl cell using hexane as solvent. Mass spectra were recorded
at 70 eV on a Perkin-Elmer RMU-6H instrument using the
electron impact method. Melting points were recorded on a hot
stage Gallenkamp melting point apparatus and are uncorrected.
Route C.
Dilithiation of furan16. 0.14 g (2.0 mmol) of furan was
dissolved in 10 ml of hexane. 0.75 ml (5.0 mmol) of TMEDA and
3.1 ml (5 mmol) of a 1.6 mol dm−3 solution of n-butyllithium
in hexane were added at room temperature. The mixture was
refluxed for 30 minutes and then cooled to 0 ◦C.
Synthesis of biscarbene metal complexes via dilithiated furan.
2.0 mmol of furan was dilithiated as specified above. The reaction
mixture was cooled to −40 ◦C and 4.0 mmol of M(CO)6 as well
as 5 ml thf was added. The temperature was allowed to rise to
room temperature. Stirring was maintained for a further 15 min
during which the colour of the reaction mixture usually changed
to dark brown. The solvent was removed in vacuo, the reaction
residue was redissolved in 10 ml of dichloromethane and cooled
to −20 ◦C. A solution of 0.78 g (4.05 mmol) triethyloxonium
tetrafluoroborate in 10 ml of dichloromethane was added. An
immediate colour change to dark purple was observed. The
cooling bath was removed and stirring was continued for a
further hour. The mixture was washed with dichloromethane
through a filter containing silica gel. The solvent was removed in
vacuo and the remainder purified on a silica gel column. Products
were isolated starting withhexane as elutingagent andincreasing
the polarity gradually by adding dichloromethane to the eluting
agent.
Synthesis
Route A.
Synthesis of monocarbene metal complexes via monolithiated
furan. 2.0 mmol of furan was dissolved in 10 ml of hexane and
a slight excess of n-butyllithium 1.27 ml (2.03 mmol) was added
at 0 ◦C and the reaction was stirred for 30 minutes at room
◦
Characterisation of products. 1: Red solid; mp 103–106 C;
C12H8O7Cr (316.25) calc. C 45.57, H 2.57%, found: C 45.81 H
2.76%. Yield: Method A 0.15 g (24%), method B 0.18 g (29%)
and method C 0.48 g (76%); MS (EI): m/z 316 [M+], stepwise
fragmentation of five carbonyl ions to give the principal ion (I =
100%) [M+ − 5CO].
◦
temperature. The reaction mixture was cooled to −10 C and
10ml THF was added. 2.0mmolof M(CO)6 was added at −70 ◦C
and the temperature was allowed to rise to room temperature.
Stirring was maintained for a further 15 minutes during which
the colour of the reaction mixture usually changed to red–brown.
The solvent was removed in vacuo, the reaction residue was
redissolved in 10 ml of dichloromethane and cooled to −20 ◦C. A
solution of 0.40 g (2.08 mmol) triethyloxonium tetrafluoroborate
in 10 ml of dichloromethane was added. The cooling bath
was removed and stirring was continued for a further hour.
The mixture was washed with dichloromethane through a filter
containing silica gel. The solvent was removed in vacuo and the
remainder purified on a silica gel column. Products were isolated
starting with hexane as eluting agent and increasing the polarity
gradually by adding dichloromethane to the eluting agent.
2: Red solid; mp 117–119 ◦C; C12H8O7W (448.09) calc. C
32.16, H 1.81%, found: C 32.55, H 1.97%. Yield: Method A
0.29 g (34%), Method B 0.11 g (13%) and method C 0.67 g
(79%); MS (EI): m/z 448 [M+], stepwise fragmentation of five
carbonyl ligands to give the principal ion (I = 100%) [M+
−
5CO].
3: Purple solid; C20H12O13Cr2 (564.3) calc. C 42.57, H 2.14%,
found: C 42.86, H 2.33%. Yield: Method A 0.08 g (7%) and
Method B 0.44 g (39%); MS (EI): m/z 564 [M+]
4: Purple solid; C20H12O13W2 (828.0) calc. C 28.99, H 1.46%,
found: C 23.17 H 1.66%. Yield: Method A 0.03 g (2%) and
Method B 0.66 g (40%); MS (EI): m/z 828 [M+].
D a l t o n T r a n s . , 2 0 0 5 , 1 6 4 9 – 1 6 5 7
1 6 5 5

View Article Online
Table 5 Crystal data and structure refinement of 4 and 6
4
6
Empirical formula
C20H12O13W2
828.00
183(2)
0.71073
Monoclinic
C2/m
C22H22CrO9
482.40
183(2)
0.71073
Triclinic
¯
P1
M
T/K
˚
k/A
Crystal system
Space group
Unit cell dimensions
˚
a/A
23.8784(9)
7.6038(3)
13.3500(6)
90
97.435(3)
90
2403.5(2)
4
7.569(2)
10.524(3)
14.093(4)
88.52(1)
78.24(2)
86.29(2)
1096.6(5)
2
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
Dc/Mg m−3
2.288
9.629
1.461
0.573
l/mm−1
F(000)
1536
500
Crystal size/mm
h Range for data collection/◦
Index ranges, hkl
Reflections collected
0.10 × 0.10 × 0.08
0.09 × 0.08 × 0.07
2.15–27.50
2.42–21.05
−29 to 30, −9 to 9, −16 to 17
9142
−7 to 7, −10 to 10, −14 to 14
3571
Independent reflections (Rint
Completeness to h = 27.50◦
Absorption correction
Max., min. transmission
Refinement method
)
2775 (0.0735)
93.4
2175 (0.047)
91.6
Semi-empirical
0.286, 0.197
—
—
Full-matrix least-squares on F2
2775/0/187
Full-matrix least-squares on F2
2175/0/289
Data/restraints/parameters
Goodness-of-fit on F2
0.977
1.106
Final R indices [I > 2r(I)]
R Indices (all data)
Largest diff. peak, hole/e A
R1 = 0.0269, wR2 = 0.0586
R1 = 0.0336, wR2 = 0.0599
0.772, −1.397
R1 = 0.0550, wR2 = 0.1181
R1 = 0.0821, wR2 = 0.1373
0.251, −0.295
−3
˚
5: Purple solid; C20H12O13CrW (696.1) calc. C 34.52, H 1.74%,
found: C 34.71 H 1.95%. Yield: Method B 0.68 g (47%); MS (EI):
m/z 696 [M+].
8: Red solid; C15H12O9Cr (388.3) calc. C 46.39, H 3.13%,
found: C 46.61 H 3.26%. Yield: Method A 0.02 g (2%) and
method B 0.17 g (15%); MS (EI): m/z 388 [M+], stepwise
fragmentation of five carbonyl ions to give the principal ion
(I = 100%) [M+ − 5CO].
atoms of the structures were included at calculated positions
with fixed thermal parameters. All non-hydrogen atoms were
refined anisotropically. XP software (SIEMENS Analytical X-
ray Instruments. Inc.) was used for structure representations.
Crystal data and other experimental procedures and refinement
parameters are given in Table 5.
CCDC reference numbers 240666 (4) and 240667 (6).
See http://www.rsc.org/suppdata/dt/b4/b419148j/ for cry-
stallographic data in CIF or other electronic format.
Reactions of metal carbene complexes.
Benzannulation of a biscarbene metal complex and 3-hexyne19.
0.45 ml (4 mmol) 3-hexyne was added to a stirred solution of
1 mmol of the biscarbene metal complex in 10 ml toluene. The
reaction mixture was stirred at 80 ◦C for 24 h. The solvent was
removed under reduced pressure and the remainder purified by
column chromatography on silica gel. Starting with hexane as
eluting agent and increasing the polarity gradually by adding
dichloromethane to the eluting agent isolated products. Several
organic products were formed but no attempt was made to
isolate and characterise non-metal containing compounds.
Palladium catalysed carbene–carbene coupling reactions30.
The carbene complex (1 mmol) and 0.1 molar equivalents of
Pd(PPh3)4 was refluxed in 10 ml of thf for 3 h. The solvent was
removed under reduced pressure and the remainder purified by
column chromatography on silica gel. Products were isolated
starting with hexane as eluting agent and increasing the polarity
gradually by adding dichloromethane. This experiment was
repeated in hexane and refluxed overnight.
Acknowledgements
This material is based upon work supported by the National
Research Foundation under Grant No. 2054240.
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D a l t o n T r a n s . , 2 0 0 5 , 1 6 4 9 – 1 6 5 7
1 6 5 7