Cyclisation of diferrocenylbutadiyne dicobalt hexacarbonyl mono-adduct to novel ferrocenotropones

Article abstract of DOI:10.1016/j.jorganchem.2004.08.050

Treatment of 1,4-diferrocenyl-1,3-butadiyne dicobalt hexacarbonyl complex (1) [CARN: 178388-65-3] with trifluoroacetic acid resulted in cyclisation with concomitant CO insertion to give the green 2,3-ferroceno-7-ferrocenyl-4,5- dehydrotropone dicobalt hexacarbonyl (2), which in turn was converted by trifluoroacetic acid to red 2,3-ferroceno-7-ferrocenyltropone (3). The first step implied the involvement of a second dicobalt hexacarbonyl complex as a CO source. The molecular structures of 2 and 3 were determined by single crystal X-ray analysis, and pairs of enantiomers with different conformations were observed in 3.

Full text of DOI:10.1016/j.jorganchem.2004.08.050

Journal of Organometallic Chemistry 690 (2005) 691–695
www.elsevier.com/locate/jorganchem
Cyclisation of diferrocenylbutadiyne dicobalt hexacarbonyl
mono-adduct to novel ferrocenotropones
a
b,
b
b
Gerhard Laus , Herwig Schottenberger *, Josef Lukasser , Klaus Wurst ,
c
Johannes Schutz , Karl-Hans Ongania , Laszlo Zsolnai
¨
d
b
´
´
a
Immodal Pharmaka GmbH, Bundesstrasse 44, 6111 Volders, Austria
Institute of Inorganic Chemistry, University of Innsbruck, Innrain 52a, 6020 Innsbruck, Austria
b
c
Department of Pharmaceutical Chemistry, Institute of Pharmacy, University of Innsbruck, Innrain 52a, 6020 Innsbruck, Austria
d
Institute of Organic Chemistry, University of Innsbruck, Innrain 52a, 6020 Innsbruck, Austria
Received 15 May 2004; accepted 17 August 2004
Abstract
Treatment of 1,4-diferrocenyl-1,3-butadiyne dicobalt hexacarbonyl complex (1) [CARN: 178388-65-3] with trifluoroacetic acid
resulted in cyclisation with concomitant CO insertion to give the green 2,3-ferroceno-7-ferrocenyl-4,5-dehydrotropone dicobalt
hexacarbonyl (2), which in turn was converted by trifluoroacetic acid to red 2,3-ferroceno-7-ferrocenyltropone (3). The first step
implied the involvement of a second dicobalt hexacarbonyl complex as a CO source. The molecular structures of 2 and 3 were deter-
mined by single crystal X-ray analysis, and pairs of enantiomers with different conformations were observed in 3.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Alkyne dicobalt hexacarbonyl; Alkynyl ferrocene; Crystal structure; HPLC; Tropone
1. Introduction
ferrocenylvinyl cations, the rate of solvent capture was
found to be sensitive to alkyl substitution in the ferro-
cene rings, and when the b-vinyl position was sterically
obstructed the cation was extremely resistant to nucleo-
philic addition [4]. 1H NMR spectroscopical observation
of vinyl cations and their TFA adducts was reported [4].
Moreover, inter- and intra-molecular trapping may
occur. Thus, intra-ionic cyclisation of ferrocenyl-
stabilised vinyl cations was observed when a neighbour-
ing p-system was available [5]. As an alkyne dicobalt
hexacarbonyl group is known to possess less cation-
stabilising capacity than a ferrocene group [6], it was
anticipated that, in a disubstituted alkyne with compet-
ing donors, a-ferrocenyl cationic sites would prevail
over Co-complexed propargyl cations [7]. In view of
possible Pauson–Khand reactions [8] and other cobalt-
mediated cyclisations with multiple carbon–carbon
bond formations [9], it seemed to be of interest to
explore the chemistry of a ferrocenyl-stabilised vinyl
In earlier work, we reported on allyl, allenyl and
fluorenyl cations stabilised by adjacent organotransi-
tion-metal donor groups, and even polyferrocenyl-
substituted cumulenic carbocations [1]. Another area
of interest were ferrocene-derived alkynes [2], and it ap-
peared compelling to expand our research into the field
of vinyl cations. Vinyl cations are readily generated in
trifluoroacetic acid (TFA) by protonation of alkynes
[3]. Previously, it was shown that these cations can be
captured by solvent molecules to give trif-
luoroacetoxycarbenium ions, and the corresponding ke-
tones were isolated after hydrolysis [3]. In the case of
*
Corresponding author. Tel.: +43 512 507 5118; fax: +43 512 507
2934.
E-mail address: herwig.schottenberger@uibk.ac.at (H. Schotten-
berger).
0022-328X/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.08.050

692
G. Laus et al. / Journal of Organometallic Chemistry 690 (2005) 691–695
[Co]
[Co]
cation with an additional neighbouring cobalt carbonyl
cluster.
Fe
2. Results and discussion
Fe
1,4-Diferrocenyl-1,3-butadiyne dicobalt hexacarbo-
nyl (1) was treated with TFA to create the correspond-
ing vinyl cation and to study the subsequent reactions.
Formation of the cation adjacent to the ferrocene group
was expected [6], but all attempts to observe the vinyl
1 [Co] = Co(CO)3
TFA
[Co]
1
cation or its TFA adduct by H NMR were futile. The
NMR signals collapsed within a few minutes, possibly
due to the formation of intermediate paramagnetic spe-
cies. Obviously, no solvent addition took place, since no
ketones were formed upon immediate hydrolysis, and
only starting material was recovered, confirming that
this cation was sterically shielded from nucleophilic at-
tack. However, when the reaction was terminated after
3 hours, the green dicobalt hexacarbonyl complex of
2,3-ferroceno-7-ferrocenyl-4,5-dehydrotropone (2) was
isolated. Thus, a formal [6 + 1] cyclisation with incorpo-
ration of carbon monoxide has occurred, with another
molecule acting as the source of the CO. This explained
the rather modest yield, but the structure of the product
also confirmed the expected site of protonation. The rate
of reaction was found to be quite variable, and therefore
the transformation had to be monitored by TLC to get
acceptable preparative results.
[Co]
Fe
Fe
1. CO source
2. NaHCO3/H2O
[Co]
[Co]
Fe
O
Fe
2
A different product was obtained after prolonged
reaction time. The dicobaltatetrahedrane completely
decomposed in TFA to give the red ferrocenotropone
3 (Scheme 1). Such a decomplexation usually requires
reductive conditions to afford alkenes.
1. TFA
2. NaHCO3/H2O
The X-ray diffraction data of the products revealed
some notable features. In compound 2, the average tor-
sion angle of the fused ferrocene subunit is 26ꢁ, whereas
that of the r-bonded subunit is 12ꢁ (Fig. 1); the Co–Co
Fe
O
Fe
˚
bond length is 2.473(1) A, and the C(30)–C(31) bond
3
˚
length in the tetrahedrane is 1.337(8) A, the shortest of
Scheme 1.
all ring bonds. The pivotal angles C(29)–C(30)–C(31)
and C(30)–C(31)–C(32) are 130.7(5)ꢁ and 135.0(5)ꢁ,
respectively, showing extreme deviation from sp hybrid-
isation. Both compounds display planar chirality. Inter-
estingly, in the crystal of compound 3, with two
molecules in the asymmetric unit, one enantiomer dis-
plays an average torsion angle of the fused ferrocene
subunit of 19ꢁ and that of the r-bonded of 7ꢁ; in the
other enantiomer the fused subunit is virtually eclipsed
(<2ꢁ), and the r-bonded unit is twisted by only 3ꢁ
(Fig. 2). Such enantiomers with different conformations
have been also called conformational diastereomers.
Details of the data collection, cell dimensions and
structure refinement are given in Table 1.
solubility of 1 in TFA. However, the conversion of iso-
lated 2 in TFA to 3 clearly followed a pseudo-first order
rate law, as derived from a linear plot of lnc vs. t up to a
degree of reaction of 0.95. It was also found that HBF4
or concentrated H2SO4 were not effective for this trans-
formation, giving inseparable mixtures.
A related TFA-mediated cyclisation of alkyne dicobalt
hexacarbonyl complexes with concomitant CO insertion
to the corresponding Pauson–Khand cyclopentenone
has been reported previously [10]. Dicobalt hexacarbonyl
complexes of silylalkynes have been also reported to give
alkenes upon TFA treatment in a protonolysis of C–Co
bonds [10]. Only one other ferrocene-anellated tropone
was found in the literature, i.e., 4,5-ferrocenotropone,
A kinetic study of the reaction starting with 1 by
HPLC was not conclusive, most likely due to the poor

G. Laus et al. / Journal of Organometallic Chemistry 690 (2005) 691–695
693
Fig. 1. ORTEP View of 2 drawn with 50% displacement ellipsoids.
Fig. 2. ORTEP View of 3 drawn with 50% displacement ellipsoids; the fused Fc subunits of the two enantiomers are (a) twisted and (b) nearly
eclipsed.
which was obtained by a trivial condensation of ferro-
cene-1,2-dicarbaldehyde with acetone [11].
Although the mechanistic picture is only tentative at
the moment, the reactions described here should be of
interest for further mechanistic studies. Certainly, the
use of a CO atmosphere or, possibly, a metal carbonyl
catalysed version are among the synthetic variations
worthwhile to be explored. In a preliminary experiment,
the conversion of 1 to 2 was attempted in the presence of
gaseous CO, but the expected product was accompanied
by at least two new byproducts. It was decided not to
pursue this matter at this stage.

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G. Laus et al. / Journal of Organometallic Chemistry 690 (2005) 691–695
Table 1
Crystal data and structure refinement for compounds 2 and 3
2
3
Molecular formula
Mr
C31H18Co2Fe2O7
732.0
C25H20Fe2O
448.1
Crystal system
Space group
Unit cell dimensions
a (pm)
Monoclinic
P21/c
Monoclinic
P21/c
1636.1(2)
1311.6(1)
1379.3(2)
90
1922.2(8)
2137.3(9)
1325.9(5)
90
b (pm)
c (pm)
a (ꢁ)
b (ꢁ)
108.64(1)
90
137.92(2)
90
c (ꢁ)
V (nm3)
Z
2.8046(6)
4
293
3.6505(26)
8
200
T (K)
Dcalc (g cmÀ3
Absorption coefficient (mmÀ1
F (000)
)
1.734
2.224
1464
1.631
1.602
1840
)
Color, habit
Crystal size (mm)
Green prism
0.7 · 0.3 · 0.25
4.05–24.77
Red prism
0.2 · 0.3 · 0.3
1.58–24.06
h Range for data collection (ꢁ )
Index ranges
À18 6 h 6 18,
À15 6 k 6 1,
À1 6 l 6 15
0 6 h 6 21,
0 6 k 6 24,
À14 6 l 6 14
Reflections collected
5340
4296(Rint = 0.0453)
2582
5722
5541 (Rint = 0.0588)
3905
Independent reflections
Reflections with I > 2r(I)
Absorption correction
Refinement method
w-scan
w-scan
Full-matrix least-squares on F2
4295/0/379
1.028
Full-matrix least-squares on F2
4990/0/509
1.055
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2(I)]
R indices (all data)
Largest difference peak and hole (e nmÀ3
R1 = 0.0500, wR2 = 0.0785
R1 = 0.1099, wR2 = 0.0965
362 and À318
R1 = 0.0521, wR2 = 0.1190
R1 = 0.0876, wR2 = 0.1395
631 and À615
)
3. Experimental
easy workup by precipitation compared with earlier
protocols.
All reactions were carried out using standard Schlenk
techniques. Solvents were deoxygenated, purified and
3.1. rac-2 3-Ferroceno-7-ferrocenyl-4,5-dehydrotropone
dicobalt hexacarbonyl (2)
1
dried prior to use. H and 13C NMR spectra were re-
corded on Bruker AC 200 and Varian Gemini 200 spec-
trometers. Chemical shifts are quoted relative to Me4Si.
Infrared spectra were measured as KBr pellets on a Nic-
olet 510 FT-IR spectrometer. High resolution mass
spectra were obtained with a Finnigan MAT 95 instru-
ment. Diffraction data were collected on a Siemens P4
diffractometer using graphite-monochromated Mo Ka
radiation. The HPLC equipment consisted of a Merck-
Hitachi L-6200A pump and an L-4250 UV–Vis detector.
1,4-Diferrocenyl-1,3-butadiyne dicobalt hexacarbo-
nyl (1) [CARN: 178388-65-3] [12–14] was obtained from
1,4-diferrocenyl-1,3-butadiyne [CARN: 1273-18-3]
along with 1,4-diferrocenyl-1,3-butadiyne bis(dicobalt
hexacarbonyl) [CARN: 178388-66-4] [12,14,15]. 1,4-Dif-
errocenyl-1,3-butadiyne was prepared by strictly follow-
ing the procedure for the synthesis of the analogous 1,4-
diphenyl-1,3-butadiyne [16], providing an extremely
1,4-Diferrocenyl-1,3-butadiyne dicobalt hexacarbo-
nyl (1) (1.0 g, 1.42 mmol) was suspended in degassed
TFA (25 ml) and stirred under argon at room tempera-
ture. The reaction was monitored by TLC (silica, diethyl
ether–pentane 1:2). After approximately 3 h, the solvent
was evaporated, saturated sodium bicarbonate solution
was added, and the mixture was extracted with diethyl
ether. Column chromatography (silica gel 60) of the
crude product using diethyl ether–pentane (1:2) as elu-
ent yielded 220 mg (21%) of the green product. M.p.
145ꢁC (dec.). IR (KBr): 2087, 2052, 2023, 1997, 1607
1
cmÀ1. H NMR (CDCl3, TMS): 4.16 (s, 5H), 4.20 (s,
5H), 4.40 (m, 2H), 4.75 (m, 1H), 4.82 (m, 2H), 4.94
(m, 1H), 5.19 (m, 1H), 7.94 (s, 1H) ppm. 13C NMR
(CDCl3, TMS): 68.8, 69.3, 69.6, 69.8, 70.6, 70.7, 71.4,
71.8, 73.7, 84.2, 87.9, 129.0, 131.0, 133.3, 141.7, 192.8,

G. Laus et al. / Journal of Organometallic Chemistry 690 (2005) 691–695
695
198.2, 199.6 ppm. HRMS (FAB): 732.8502 (M + H),
C31H19Co2Fe2O7 requires 732.8488.
Data Centre, CCDC No. 216576 for compound 2 and
CCDC No. 216577 for compound 3. These data can
be obtained free of charge via www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallo-
graphic Data Centre, 12, Union Road, Cambridge CB2
1EZ, United Kingdom; fax: +44 1223 336033; or
deposit@ccdc.cam.ac.uk).
3.2. rac-2 3-Ferroceno-7-ferrocenyltropone (3)
1,4-Diferrocenyl-1,3-butadiyne dicobalt hexacarbo-
nyl (1) (1.0 g, 1.42 mmol) was suspended in degassed
TFA (25 ml) and stirred under argon at room tempera-
ture for 24 h. The reaction was monitored by TLC (silica,
diethyl ether–pentane 1:2). The solvent was evaporated,
saturated sodium bicarbonate solution was added, and
the mixture was extracted with diethyl ether. Column
chromatography (silica gel 60) of the crude product using
diethyl ether–pentane (1:2) as eluent yielded 150 mg
(24%) of the red product. M.p. 122ꢁC. IR (KBr): 1620,
References
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1
1603 cmÀ1. H NMR (CDCl3, TMS): 4.05 (s, 5H), 4.13
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(s, 5H), 4.34 (m, 2H), 4.56 (m, 1H), 4.74 (m, 1H), 4.80
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449.0254 (M + H), C25H21Fe2O requires 449.0286.
The same product 3 was obtained by treatment of 2
with TFA (10 mg/ml). The reaction was monitored by
HPLC analysis of aliquots (10 ll) sampled from the mix-
ture (containing 10 mg/ml formylferrocene as internal
standard) in suitable time intervals. After hydrolysis (1
ml sat. aq. NaHCO3), the sample was subjected to sol-
id-phase extraction on a preconditioned (2 ml CH3CN,
2 ml H2O) RP-18 LiChrolut (200 mg, endcapped) col-
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CH3CN). The eluate was diluted (1 ml H2O) and chro-
matographed on a LiChrospher 100 RP-18 column
(125 · 4 mm, 5 lm, Merck) using a gradient program
(0–1 min CH3CN–H2O 60:40, 1–2 min to 95:5, 2–7
min 95:5) with a flow of 2 ml minÀ1 at 25 ꢁC and detec-
tion at 220 nm. Retention time of 2 was 5.0 min, of 3 3.7
min. Formylferrocene was eluted at 1.2 min, and traces
of 1 could be observed at 6.0 min.
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4. Crystallographic data
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