Synthesis and Structural Characterization of Novel Organometallic Dehydroannulenes with Fused CpCo-Cyclobutadiene and Ferrocene Units Including a Cyclic Fullerenyne Segment

Article abstract of DOI:10.1021/ja992262a

The synthesis of fused organometallic dehydroannulenes containing cyclobutadiene(cyclopentadienyl)-cobalt or ferrocene units separated by butadiyne linkers is presented. Starting from the corresponding 1,2-diethynylated sandwich complexes, either Eglinton, Hay, or Voegtle-type coupling furnishes the desired dehydroannulenes. The choice of the coupling variant is critical for the success of this reaction. It was found that for diethynylferrocene a variant of the classic Eglinton system utilizing a Cu₂Cl₂/CuCl₂ mixture in pyridine works successfully, while for the case of the cyclobutadiene(cyclopentadienyl)cobalt complexes the Voegtle coupling utilizing Cu(OAc)₂ in acetonitrile gave the most impressive results, leading to a 87% yield of cyclized material. To obtain perethynylated organometallic dehydroannulenes, a novel synthesis of tetraethynylated cyclobutadiene complexes was developed. Flash vacuum pyrolysis of a bisbutadiynylated CpCo-ligated cyclobutadiene furnishes the desired tetraethynylated cyclobutadiene complex via a Bergman-type rearrangement. The two TMS and two TIPS substituents of the cyclobutadiene complex are oriented pairwise ortho with respect to each other. Removal of the TMS groups accesses a monomer, which under the conditions of the Voegtle coupling cyclizes to novel trimeric and tetrameric, perethynylated, cyclobutadieno fused, organometallic dehydroannulenes.

Full text of DOI:10.1021/ja992262a

J. Am. Chem. Soc. 1999, 121, 10719-10726
10719
Synthesis and Structural Characterization of Novel Organometallic
Dehydroannulenes with Fused CpCo-Cyclobutadiene and Ferrocene
Units Including a Cyclic Fullerenyne Segment
Uwe H. F. Bunz,*,† Gaby Roidl,†,‡ Markus Altmann,‡ Volker Enkelmann,‡ and
Ken D. Shimizu†
Contribution from the Department of Chemistry and Biochemistry, The UniVersity of South Carolina,
Columbia, South Carolina 29208, and Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg 10,
55021 Mainz, Germany
ReceiVed June 30, 1999
Abstract: The synthesis of fused organometallic dehydroannulenes containing cyclobutadiene(cyclopentadienyl)-
cobalt or ferrocene units separated by butadiyne linkers is presented. Starting from the corresponding 1,2-
diethynylated sandwich complexes, either Eglinton, Hay, or Vo¨gtle-type coupling furnishes the desired
dehydroannulenes. The choice of the coupling variant is critical for the success of this reaction. It was found
that for diethynylferrocene a variant of the classic Eglinton system utilizing a Cu2Cl2/CuCl2 mixture in pyridine
works successfully, while for the case of the cyclobutadiene(cyclopentadienyl)cobalt complexes the Vo¨gtle
coupling utilizing Cu(OAc)2 in acetonitrile gave the most impressive results, leading to a 87% yield of cyclized
material. To obtain perethynylated organometallic dehydroannulenes, a novel synthesis of tetraethynylated
cyclobutadiene complexes was developed. Flash vacuum pyrolysis of a bisbutadiynylated CpCo-ligated
cyclobutadiene furnishes the desired tetraethynylated cyclobutadiene complex via a Bergman-type rearrangement.
The two TMS and two TIPS substituents of the cyclobutadiene complex are oriented pairwise ortho with
respect to each other. Removal of the TMS groups accesses a monomer, which under the conditions of the
Vo¨gtle coupling cyclizes to novel trimeric and tetrameric, perethynylated, cyclobutadieno fused, organometallic
dehydroannulenes.
Introduction
dehydroannulenes forming C60-ions under mass spectrometric
conditions. Both, novel carbon allotropes and their smaller
segments, come with the promise of exciting optical, electronic,
conducting, and supramolecular properties.
While the field of organic dehydroannulenes is flourishing,
less is known about the organometallic ”arm” of this enterprise.
Diederich13 accomplished the preparation of a dicobalt complex
of C18. More recently, Youngs14a,b reported titanium- or
platinum-containing cyclynes,14c while Tilley15 accessed large-
ring zirconacyclopentadienes. However, organometallic deriva-
tives of dehydroannulenes in which a π-sandwich, such as
ferrocene or cyclobutadiene Cp-cobalt, is fused to the dehy-
droannulene part are not described. This class of compounds
Fused dehydroannulenes have attracted the attention of
chemists for almost half a century. One of the first reported
examples was the cyclic dimer1 of 1,2-diethynylbenzene,1a,2 and
cyclooligomerization of terminal diynes by copper salts3,4
remains the preferred tool for the synthesis of dehydroannu-
lenes.5
Dehydroannulenes are fascinating due to their structural
relationship to graphite and its segments,6 fullerenes,7 fulleren-
ynes,8 fullerene fragments,9 and other proposed allotropes of
carbon.10 Recently, Rubin11 and Tobe12 have made cage-like
† The University of South Carolina.
‡ Max-Planck-Institut.
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1999, 5, 263. (c) Guo, L.; Bradshaw, J. D.; Tessier, C. A.; Youngs, W. J.
J. Chem. Soc., Chem. Commun. 1994, 243. (d) Zhou, Q.; Caroll, P. J.;
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10.1021/ja992262a CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/30/1999

10720 J. Am. Chem. Soc., Vol. 121, No. 46, 1999
Bunz et al.
Scheme 1
would be the organometallic equivalent of dehydrobenzannu-
lenes. Vollhardt and Youngs14d-f demonstrated that dehydrobenz-
annulenes are precursors for carbon-onions and novel conjugated
polymers. Extrapolation suggests that organometallic dehy-
drobenzannulenes could show similar properties. They are
structures of intrinsic beauty and important in a fundamental
way for the study of through-bond vs through-space interactions
in intramolecular electron transfer.14d,e
We are interested in multiply ethynylated π-complexes of
iron, manganese, and cobalt16 as modules for larger organome-
tallic targets. Linear,17 star,8 and dumbbell shaped18 representa-
tives have been reported. Here we present the synthesis and
structure of a cyclic organometallic fullerenyne segment and
of novel CpCo-cyclobutadieno-fused dehydroannulenes.19
amounts of poly-1 may have formed, but we were never able
to detect any. This is in contrast to the case of 1,2-diethynyl-
cymantrene, where oxidative coupling gave a well-defined and
fully characterized polymer.8a As for 1, the formation of a dark
and insoluble material is attributed to partial oxidation/degrada-
tion of the ferrocene nucleus. Oxidative degradation is probable,
if it is taken into account that even the dimerization of
ethynylferrocene to 1,4-diferrocenylbutadiyne proceeds only in
modest yield (36%).21 Obviously the ferrocene nucleus is not
particularly stable toward the conditions of the copper-catalyzed
couplings. The two trimers 3a and 3b were identified according
to their 13C NMR spectra. While the highly symmetric syn-
cycle 3b displays only six signals, in anti-3a the signals are
split due to the loss of the 3-fold axis.
How much does the positional isomerism of the CpFe residues
in 3a and 3b influence their electronic and electroactive22
properties? The UV/vis spectra of 3a/b are virtually superim-
posable. This suggests that the electronic ground state in the
neutral molecules 3a,b is quite similar. We investigated the
electrochemistry of the two cycles 3 and here we do find a
difference in their respective cyclic voltammograms. While anti-
3a gives rise to three oxidation waves at 0.61, 0.74, and 0.83
V, syn-3b displays only two waves at 0.61 and 0.78 V. For
comparison, monomer 1 oxidizes at a potential of 0.60 V. The
observed splits in 3 are relatively small, which suggests that
both through-bond and through-space interactions are weak in
these cycles.
Structural proof for 3a was obtained by a single-crystal X-ray
structure analysis of a very small specimen grown from
dichloromethane. The data set was recorded on a CCD instru-
ment, due to the insufficient number of reflections acquired on
a conventional single-crystal diffractometer. The molecular
structure of 3a is shown in Figure 1. The bond lengths and
angles of 3a are in excellent agreement with reported values
for similar ethynylated cyclobutadiene complexes and cyman-
trenes.7,16-18 The hydrocarbon ligand in 3a is flat; the butadiyne
bridges are close to linear and appear almost unstrained. The
larger bond angles (72°; the optimum would be 60°) are easily
accommodated by the three butadiyne bridges.
Results and Discussion
Ferrocene-Fused Dehydroannulenes. Treatment of 1,2-
diethynylferrocene 120 under Hay conditions (Cu2Cl2, acetone,
25 °C, O2; Scheme 1) furnished large amounts of polymeric
material which showed extremely broad lines in both its 1H and
13C NMR spectra. However, traces of a single compound, 3a,
could be isolated. Attempts to use Cu(OAc)2 in acetonitrile at
60 °C led to decomposition of 1. A mixture of Cu2Cl2 and CuCl2
in pyridine provided the cycles 3 in yields of 13.1% (3a) and
3.6% (3b). In addition to 3, 1, a small amount of the open dimer
2, and a mixture of higher cycles, predominantly tetramer 4,
were isolated. To obtain 3 it was necessary to add the solution
of 1 in pyridine over of 14 h via syringe pump to the suspension
of Cu2Cl2/CuCl2. After complete product analysis the fate of
25% of 1 could be detected. The first assumption was that large
(14) (a) Zhang, D. M.; McConville, D. B.; Hrabusa, J. M.; Tessier, C.
A.; Youngs, W. J. J. Am. Chem. Soc. 1998, 120, 3506. (b) AlQaisi, S. M.;
Galat, K. J.; Chai, M. H.; Ray, D. G.; Rinaldi, P. L.; Tessier, C. A.; Youngs,
W. J. J. Am. Chem. Soc. 1998, 120, 12149. (c) Scott, L. T.; Cooney, M. J.
In Modern Acetylene Chemistry; Stang, P. J., Diederich, F., Eds.; VCH
Weinheim: Weinheim, Germany, 1995. (d) Boese, R.; Matzger, A. J.;
Vollhardt, K. P. C. J. Am. Chem. Soc. 1997, 119, 2052. (e) Baldwin, K. P.;
Matzger, A. J.; Scheiman, D. A.; Tessier, C. A.; Vollhardt, K. P. C.; Youngs,
W. J. Synlett 1995, 1215.
(15) (a) Mao, S. S. H.; Tilley, T. D. J. Am. Chem. Soc. 1995, 117, 5365.
(b) Mao, S. S. H.; Tilley, T. D. J. Am. Chem. Soc. 1995, 117, 7031. (c)
Lucht, B. L.; Mao, S. S. H.; Tilley, T. D. J. Am. Chem. Soc. 1998, 120,
4354.
CpCo(cyclobutadiene)-Fused Dehydroannulenes. In the
first experiment we subjected 524b to the conditions of the Hay
coupling reaction at a temperature of 40 °C. After 2 h, all of 5
(16) (a) Bunz, U. H. F. Top. Curr. Chem. 1999, 201, 131. (b) Mu¨ller, T.
J. J.; Lindner, H. J. Chem. Ber. 1996, 129, 607. (c) Fink, H.; Long, N. J.;
Martin, A. J.; Opromolla, G.; White, A. J. P.; Williams, D. J.; Zanello, P.
Organometallics 1997, 16, 2646.
(17) Altmann, M.; Enkelmann, V.; Beer, F.; Bunz, U. H. F. Organo-
metallics 1996, 15, 394.
(18) (a) Bunz, U. H. F.; Wiegelmann-Kreiter, J. E. C. Chem. Ber. 1996,
129, 785. (b) Bunz, U. H. F.; Enkelmann, V. Angew. Chem. 1993, 32, 1653.
(c) Bunz, U. H. F.; Enkelmann, V. Organometallics 1994, 13, 3823. (d)
Roidl, G.; Enkelmann, V.; Bunz, U. H. F. Organometallics 1999, 18, 4.
(19) Parts of the herein presented material have been communicated
earlier. (a) Altmann, M.; Friedrich, J.; Beer, F.; Reuter, R.; Enkelmann,
V.; Bunz, U. H. F. J. Am. Chem. Soc. 1997, 119, 1472. (b) Altmann, M.;
Roidl, G.; Enkelmann, V. Angew. Chem. 1997, 36, 1107.
(21) Yuan, Z.; Stringer, G.; Jobe, I. R.; Kreller, D.; Scott, K.; Koch, L.;
Taylor, N. J.; Marder, T. B. J. Organomet. Chem. 1993, 452, 115.
(20) Bunz, U. H. F. J. Organomet. Chem. 1995, 494, C8.

NoVel Organometallic Dehydroannulenes
J. Am. Chem. Soc., Vol. 121, No. 46, 1999 10721
The statistical distribution of diastereomers can be interpreted
by the fact that the monomer units in the cycles do not interact
with each other, despite the considerable bulk of both the CpCo
as well as the SiMe3 groups. Butadiynes are sufficiently long
spacers to separate the monomer units so that free rotation is
possible in the open oligomeric precursors.
1
How are the diastereomers of 8 and 9 discerned? The H
NMR spectra of all of the cycles show one signal for the TMS
groups and, depending upon symmetry, up to three signals for
the Cp ring (9b). Diastereomer 9b has three different Cp rings,
thus three different Cp signals are recorded. The Cp signals all
fall in the range of 4.99-5.06 ppm, and it is possible to develop
an increment system: If the Cp ring under consideration has
two adjacent Cp rings oriented syn, its signal is shifted downfield
to 5.06 ppm. This holds for all Cp signals in 8b, one Cp signal
in 9b, and all of the Cp signals in 9d. If the Cp ring under
consideration has two neighboring anti Cp rings, its signal is
recorded at 4.99 ppm (8a and 9a,c). If the Cp ring under
consideration has one syn and one anti positioned Cp ring as
neighbor, its shift is observed in the range of 5.02-5.04 ppm
(8a, 5.03; 9b, 5.04; 9c, 5.02). This assignment is supported by
the correct values obtained for the integration of the corre-
sponding Cp signals. Likewise, the 13C NMR spectroscopic data
uphold this assignment. The reason for the small but distinct
downfield shift in the presence of a syn-oriented Cp-ring lies
in its aromaticity. A proton in the vicinity and in the same plane
as the Cp ring experiences deshielding, resulting in a downfield
Figure 1. ORTEP plot of 3a.
was consumed, but no defined compound could be isolated,
suggesting that ring formation is not efficient. The propensity
toward ring formation in aliphatic systems can be enhanced by
the introduction of geminal dimethyl substituents, described as
the Thorpe-Ingold effect.23 It is entropic in nature. Introduction
of large substituents seems to favor conformations in open
oligomers, which are prone to ring closure. A transfer of this
concept to 5 suggested that the bulky complex 6 might be a
better choice as monomer than 5. Hay reaction of 624 (boiling
TMEDA, 1 h reaction time, introduction of O2) under conditions
developed for the oligomerization of 717,25 led to a mixture of
products. Column chromatography combined with preparative
gel permeation chromatography separated the cyclotrimers (8)
from the cyclotetramers (9) and the cyclopentamers (10)
(Scheme 2). The crude yield of all cyclized material was 63%.
The two trimers 8a/8b were obtained in 10.1% (anti) and
4.4% (syn) yield after repeated column chromatography. The
crude mixture of the tetramers 9a-d was separated by prepara-
tive HPLC. A yield of 4.0% of 9a, 17.8% of a mixture of 9b,c,
and 4.7% of 9d was isolated. While the stereoisomers 9b,c were
not completely separable, single fractions enriched in either 9b
or 9c were obtained by repeated HPLC. This protocol allowed
spectroscopic identification of 9b and 9c. Subjecting 6 to the
conditions of the Vo¨gtle4-type coupling (Cu(OAc)2, boiling
acetonitrile) leads to a crude yield of 87% of cycles 8-10.
Chromatography furnished 16.6% 8a, 7.9% 8b, 6.6% 9a, 26.8%
9b,c, 7.5% 9d, and 13% of the mixture of the four pentamers
10. Analytical HPLC of the diastereomeric mixture of 10
showed that three diastereomers form in a ratio of approximately
1:1:1.
1
shift of its H NMR signal.
Treatment of 8a by Me4N+F- 24b leads to the deprotected
cycle 8c in almost quantitative yield (mp 156). The unsatisfying
cyclization of 5, as a consequence, can not be attributed to the
instability of the formed products. Trimer 8c is stable, further
supporting the notion that the steric environment (i.e., bulky
TMS groups), may exert a similar effect toward ring closure of
6 to 8-10, as the gem-dimethyl groups do in the Thorpe-Ingold
case. 23
A cyclic voltammogram of 8c shows one irreversible oxida-
tion wave at 0.91 V suggesting that the CpCo fragments do not
interact. To get more information on the electronic structure of
8-10, UV/vis spectra of 8a,b, 9d, and the isomeric mixture of
10 were recorded. Both 8a and 8b show superimposable UV/
vis spectra, suggesting analogy to the ferrocene case. The
relationship (syn/anti) of the organometallic fragments with
respect to each other does not play a role for the electronic
structure of the complexes in their ground state. With this
proviso we are confident that 9d appropriately represents the
UV/vis spectra of all of the tetramers. The UV/vis spectra of
the pentamers 10 were obtained from the diastereomeric mixture.
The absorption spectra of 9 and 10 resemble each other (Figure
3). They are similar to the ones obtained for the linear oligomers
and polymers of 7.17 However, the trimer 8a reveals an
additional band at 390 nm that is absent in the larger cycles.
The presence of this additional transition in the trimer is not
too surprising. We expect 8 to be strained with a slightly altered
hybridization and concomitantly a changed electronic environ-
ment. However, there is no conclusive evidence for enhanced
conjugation or specific aromatic stabilization/destabilization in
these cycles. It seems much more that 8 or 9 are similar to
Mitchell’s ferroceno-fused dihydropyrenes.26 In these, ferrocenes
or Cr(CO)3-ligated benzenes strongly localize the aromatic units
and diminish the aromaticity of the large perimeter. The
cyclobutadiene complexes in 8-10 and the ferrocene units in
3 appear to have the same effect on their dehydroannulenic
cores.
Using a simple model, the statistic stereoisomer distribution
for all cycles can be calculated. For the trimer 8 two isomers,
syn and anti in a ratio of 1:2, are expected. The tetramer 9 should
form four stereoisomers with a distribution of 1:4:2:1, and the
pentamer 10 should likewise form four diastereomers but in a
ratio of 3:3:3:1. If the yields of the respective stereoisomers
are compared to the statistical model, a good agreement is
reached (see Figure 2). In the case of the pentamer 10 one
diastereomer (the minor one) is missing. It likely co-elutes with
one of the three major diastereomers.
(22) Mueller-Westerhoff, U.-T. Angew. Chem. 1986, 25, 702.
(23) (a) Beesley, R. M.; Ingold, C. K.; Thorpe, J. F. J. Chem. Soc. 1915,
107, 1080. (b) Schleyer, P. v. R. J. Am. Chem. Soc. 1961, 83, 1368. (c)
Milstein, S.; Cohan, L. A. J. Am. Chem. Soc. 1972, 94, 9166. (d) Lightstone,
F. C.; Bruice, T. C. J. Am. Chem. Soc. 1994, 116, 10789.

10722 J. Am. Chem. Soc., Vol. 121, No. 46, 1999
Bunz et al.
Scheme 2
The molecular structure of 12 is shown in Figure 4, parts a
and b. There are several noticeable features about this X-ray
crystal structure. (a) Only the dl pair is present in the crystalline
specimen examined. That is most likely a consequence of
differing propensities of the dl vs meso diastereomers to
crystallize. (b) While only one diastereomer of 12 is present in
the crystal, it occurs in two different conformations in the same
single crystalline specimen, i.e. a U- and a Z-shaped one.
Figure 2. Distribution of diastereomers for 9. Data for the Hay type
and the Vo¨gtle type coupling are compared to the statistic ratio.
To access 11 via an alternative route, 6 can be reacted with
1-bromopropyn-3-ol under the conditions of the Cadiot-
Chodkiewicz coupling. The bisdiyne 11a is isolated in 83%
yield.28 The hydroxymethyl group in 11a is removed by a
mixture of MnO2/KOH in benzene to furnish a solution of 11c.
Attempts to sequester 11c led to immediate polymerization under
formation of a dark tarry residue. A similar observation was
made upon removal of the TIPS groups in 11b. The solution of
11c was added to a mixture of Cu(OAc)2 in acetonitrile and
stirred for 24 h at ambient temperature. Aqueous workup led
to the isolation of the orange polymer 13 (87%) (Scheme 4),
only partially soluble in dichloromethane or chloroform. The
polymeric nature of 13 was evidenced by GPC of the soluble
extract. Cycles could not be detected.
Perethynylated Cyclobutadiene-Fused Dehydroannulenes.
An important class of compounds for organometallic all-carbon
networks would be derivatives of 8-9 in which the trimethyl-
silyl groups were replaced by alkynyl substituents. These would
be the cycles 15 and 16. To access them, a route to a partially
deprotected, tetraethynylated cyclobutadiene complexes, viz.
14c, was necessary. A Stille-type route to tetraethynylated
cyclobutadiene complexes has been developed for Fe(CO)3-
stabilized representatives,18 but these do not form cycles upon
oxidative couplings.18a The unavailability of iodinated CpCo-
(cyclobutadiene) complexes renders the Stille reaction useless
for the synthesis of 14c.
Figure 3. UV/vis spectra of 8a, 9d, and 10 overlaid (chloroform).
Extended Monomers. A possible extension of this chemistry
would involve the synthesis of dehydroannulenes with octatet-
rayne bridges. To that end, the monomers 11 would have to be
accessed. Scott14c has recently described a method for directed
alkyne-alkyne cross coupling utilizing a preformed copper
acetylide. Treatment of 6 with BuLi in THF and addition of
CuI, propylamine, and TIPS-CtC-Br furnishes 11b in a 53%
yield (Scheme 3). A second product 12 is formed in this reaction.
Only one set of signals is found in the 13C NMR spectrum of
12. We assume that the dimer exists as a mixture of diastere-
omers with accidentally unresolved NMR shifts. The alternative,
involving the diastereoselective coupling of the monomers,
seems less probable. To obtain proof of topology and stereo-
chemistry of 12, a single-crystal X-ray structure was obtained.
Vollhardt24b,29 discovered that silyl-transposition in 6 is
possible by flash vacuum pyrolysis, furnishing 5b (Scheme 5).
This rearrangement probably proceeds by a Bergman-type30
reaction. Its preparative value would be limited, if only TMS
(24) (a) Roidl, G.; Enkelmann, V.; Adams, R. D.; Bunz, U. H. F. J.
Organomet. Chem. 1999, 578, 144. (b) Fritch, J. R.; Vollhardt, K. P. C.
Organometallics 1982, 1, 590.
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1994, 15, 785.
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Chem. Soc. 1998, 120, 1785. (b) Rubin, Y.; Kahr, M.; Knobler, C. B.;
Diederich, F.; Wilkins, C. L. J. Am. Chem. Soc. 1991, 113, 495.
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112, 1607.
(28) (a) Enkelmann, V.; Steiert, M. Personal communication. (b) Cadiot,
P.; Chodkiewicz, W. In Chemistry of Acetylenes; Viehe, H. G., Ed.;
Dekker: New York, 1969; Chapter 9. (c) See reference 14c, particularly p
321.
(29) Fritch, J. R.; Vollhardt, K. P. C. J. Am. Chem. Soc. 1978, 100, 3643.

NoVel Organometallic Dehydroannulenes
J. Am. Chem. Soc., Vol. 121, No. 46, 1999 10723
Scheme 3
Figure 4. (a) ORTEP plot of 12, U-shaped conformation. (b) ORTEP plot of 12, Z-shaped conformation.
groups were to be transposed. If this rearrangement could be
extended to other substituents, the transposition of alkyne groups
might likewise be feasible. Retrosynthetic analysis suggested
11b as potential precursor to 14c. Indeed, flash vacuum pyrolysis
of 11b furnishes 14a (Scheme 6). After optimization with regard
to temperature and rate of sublimation an 87% yield of 14a
was obtained.19b The star is the only isolable compound of this
reaction, besides unconverted 11b. Attempts to extend this
rearrangement by utilizing a phenyl instead of the triisopropyl-
(30) Bergman, R. G. Acc. Chem. Res. 1973, 6, 25.

10724 J. Am. Chem. Soc., Vol. 121, No. 46, 1999
Bunz et al.
Scheme 4
by mass spectroscopy and are in accord with our assessment.
15 as well as 16 gave the expected molecular ions (+2 mass
units, EI mode) with the correct isotopic distribution.
Conclusion
The synthesis of dehydroannulenes fused to an organometallic
sandwich can be performed by cyclization of diethynylated
π-complexes. The key to obtain the organometallic dehydroan-
nulenes 3, 8-10, and 15-16 was the careful selection of the
coupling reaction, dependent upon the organometallic precursor.
The separation of different diastereomers presented a challenge.
The diastereomers arise from the orientation of the organome-
tallic fragments with respect to each other and the plane of the
polycyclic hydrocarbon ligand. These diastereomers allow, at
least in principle, through-bond versus through-space interactions
to be separated in intramolecular electron transfer of sandwich
complexes. Surprisingly, the absorption spectra are dominated
by the transitions of the organometallic fragments and their
respective relationship (syn/anti) seems to be of minute influence
on the electronic structure of these complexes. The UV/vis
spectra of different diastereomers of the same ring size are
superimposable in all of the examined cases.
The presented results depict a major step in the direction
toward larger and closed segments of organometallic all-carbon
networks. The use of the stable ferrocenes and CpCo-ligated
cyclobutadienes (in comparison to cymantrenes and tricarbonyl
iron cyclobutadiene complexes) has rewarded our efforts to
make these novel rings. With the synthesis of compound 14b,
access to the structurally spectacular cycles 15 and 16 has been
achieved. In the future, we will exploit the chemistry of the
star-shaped tetraethynycyclobutadiene 14.
Scheme 5
silyl substituent in 11b were unsuccessful and led to the isolation
of a ceramic-graphitic material of unknown structure.
It suffices to use a high-temperature heat gun to sublime 11b
in substance into the heated quartz spiral; 11b is stable in the
solid state at high temperatures, obviously due to its bulky
substituents. Approximately 50 mg of 11b can be pyrolized in
several hours. The reaction is not particularly sensitive to
moderate pressures in the pyrolysis spiral, and it was sufficient
to employ a vacuum of 0.01 mmHg.
In 14a both the TMS and the TIPS groups are ortho to each
other. Potassium carbonate in methanol removes only the TMS
groups. The semi-deprotected 14b is stable to ambient conditions
and can be stored for extended periods of time. Complete
deprotection of 14a occurred (89%) when it was treated with
tetramethylammonium fluoride in DMSO for 1 h. Chromatog-
raphy (flash silica gel/pentane) furnished 14c, which displayed
only four signals (δ 59.9, 77.1, s; 82.3, 84.5, d) in its 13C NMR
Experimental Section
1
spectrum and two singlets (δ 3.33, 4.97; ratio 4:5) in its H
Oxidative Coupling of 1. A mixture of Cu2Cl2 (5.41 g, 60.8 mmol)
and CuCl2 (10.1 g, 81.0 mmol) in 100 mL of pyridine was prepared in
a 250 mL flask under nitrogen. Compound 1 (948 mg, 4.05 mmol, in
30 mL of pentane) was added over 14 h using a syringe pump. Aqueous
workup (dichloromethane) followed by chromatography over flash silica
gel (pentane/dichloromethane, 80:20) gave 1 (26 mg, 2.7%), dimer 2
(19 mg, 2.0%), 3a, 3b, and higher cycles 4 (34 mg, 3.6%). 3a (124
mg, 13.1%): Mp 165 °C dec. IR (KBr): ν 3097, 2925, 2215, 2152,
NMR spectrum. The bright yellow 14c is stable in the crystalline
state for a couple of hours at ambient temperature and can be
stored at 0 °C for several days. The sterically demanding CpCo
group is responsible for the substantially enhanced stability when
14c is compared to its Fe(CO)3-stabilized cousin.18 A single-
crystal X-ray structure of 14c has been obtained and discussed.19b
Treatment of 14b with Cu(OAc)2 in acetonitrile4 furnishes a
mixture of the trimers 15 and the tetramers 16 in a combined
yield of 76% (Scheme 7). The residual 24% of the starting
material must be incorporated into higher oligomers and
polymers that were not mobile under the chromatographic
conditions employed. Repeated flash chromatography and
preparative TLC enabled the separation of the two diastereomers
of 15 and the isolation of one diastereomer (all anti) of 16. The
three remaining diastereomers of 16 were inseparable from each
other but one fraction was sufficiently enriched (all-syn-16d)
1
1413, 1108, 1003, 825 cm-1. H NMR (CDCl3): δ 4.34 (bs, 3 H),
4.35, 4.40 (2 s, 15 H), 4.60 (bs, 6 H). 13C NMR (CD2Cl2): δ 68.63,
68.68, 68.83, 69.37, 71.64, 71.70, 72.26, 72.37, 74.15, 74.20, 77.86,
78.03, 78.07. UV/vis (CH2Cl2): λmax (ꢀ) 296 (449370), 319 (494230),
383 (103620), 464 (61910). MS (FD) m/z 696.1. C42H24Fe3 (696.2).
Calcd C 72.47, H 3.48; found C 72.38, H 3.54. 3b (31 mg, 3.3%): Mp
176 °C dec. IR (KBr): ν 3099, 2925, 2212, 2150, 1106, 1001, 825
cm-1. 1H NMR (CDCl3): δ 4.30 (t, 3JHH ) 2.70 Hz, 3 H), 4.41 (s, 15
H), 4.56 (d, 2JHH ) 2.70 Hz, 6 H). 13C NMR (CD2Cl2): δ 69.71, 70.22,
72.37, 72.72, 74.93, 78.53. UV/vis (CH2Cl2): λmax (ꢀ) 272 (382210),
296 (420510), 319 (475080), 382 (103620), 465 (61910). MS (FD):
m/z 696.1. C42H24Fe3 (696.2). Calcd C 72.47, H 3.48; found C 72.18,
H 3.65.
Hay Coupling of 6. A solution of 6 (1.08 g, 2.93 mmol) and Cu2-
Cl2 (0.594 g, 6.00 mmol) in 200 mL of TMEDA was heated to reflux
(122 °C). For 1 h, O2 was bubbled through the solution. The TMEDA
was removed at 0.1 Torr/40 °C, the residue was hydrolyzed (ammonia
solution) and extracted with dichloromethane. Filtration over aluminum
oxide (pentane/dichloromethane, 80:20) furnished crude product (683
mg, 63%). The separation of 8a,b was achieved by repetitive chroma-
tography of the residue over aluminum oxide with pentane as eluent
to give 8a (109 mg, 10.1%) as a first fraction followed by 8b (48 mg,
4.4%). Compounds 9 and 10 were isolated as last fraction. They were
preseparated by preparative gel permeation chromatography (GPC) into
a fraction containing only 9 and one containing only 10. The single
fractions were separated into their diastereomers by preparative HPLC.
1
to obtain its H NMR spectrum. All of the diastereomers are
yellow crystalline powders stable under ambient conditions. The
dehydroannulenes decompose above 245 °C (observed under a
polarizing microscope) without melting. As expected, the two
diastereomers of 15 show different 13C NMR spectra. The Cs-
symmetrical 15a displays six signals for the three cyclobutadiene
carbons, six signals for the butadiyne bridges, three signals for
the alkyne substituents (unresolved), and two signals for the
Cp rings in a ratio of 2:1.
The syn-isomer 15b is C3h-symmetric and six resonances are
expected in its 13C NMR spectrum (hydrocarbon ligand). Five
are observed and we conclude from the position of the signals
that two singlets for the cyclobutadiene ring are unresolved.
One signal for the cyclopentadienyl rings is observed. The
molecular weights for the cycles 15 and 16 were determined

NoVel Organometallic Dehydroannulenes
J. Am. Chem. Soc., Vol. 121, No. 46, 1999 10725
Scheme 6
Scheme 7
1
The following yields were obtained: 9a (43 mg, 4.0%), a mixture of
9b,c (192 mg, 17.8%) and 9d (51 mg, 4.7%). The pentamers 10 were
isolated as a mixture of diastereomers (130 mg, 12.0%) but not
separated on a preparative scale.
811 cm-1. H NMR (CD2Cl2): δ 0.20 (s, 72 H), 5.06 (s, 20 H). 13C
NMR (CD2Cl2): δ 0.20, 67.69, 75.08, 78.08, 80.16, 81.81. UV/vis (CH2-
Cl2): λmax (ꢀ) 266 (81480), 401 (36470). MS (FD): m/z 1465.1. C76H92-
Si8Co4 (1466.1). Calcd C 62.27, H 6.33; found C 61.91 H 6.50. 10 (61
mg, 13.0%), mixture of diastereomers.
Vo1gtle Coupling of 6. A solution of 6 (470 mg, 1.28 mmol) and
Cu(OAc)2 (3.10 g, 17.0 mmol) in 300 mL of acetonitrile was heated
under nitrogen to reflux (80 °C) for 3 h. The acetonitrile was removed
under reduced pressure. Aqueous workup with dichloromethane fol-
lowed by filtration over aluminum oxide (pentane/dichloromethane, 80:
20) furnished crude product (409 mg, 87%). The separation of the cycles
was performed by a combination of preparative GPC and HPLC. 8a
(78 mg, 16.6%): Mp >230 °C dec. IR (KBr): ν 3107, 2956, 2900,
Desilylation of 8a. To a suspension of Me4N+F- (3.8 mg, 41 µmol)
in 5 mL of DMSO, was added 8a (22 mg, 20 µmol) in 5 mL of ethyl
ether under N2. The reaction mixture was stirred for 16 h at 21 °C.
Aqueous workup and chromatography (silica gel, pentane/dichlo-
romethane, 80:20) affords 8c (12 mg, 90%): Mp 156 °C. IR (KBr): ν
3104, 2178, 2117, 1004, 816 cm-1. H NMR (CD2Cl2): δ 4.19 (s, 6
1
H), 5.05 (s, 5 H), 5.09 (s, 10 H). 13C NMR (CD2Cl2): δ 59.46, 59.73,
59.93, 61.77, 61.82, 62.05, 78.36, 78.47, 78.52, 80.23, 80.54, 80.59,
81.64, 81.70. UV/vis (CH2Cl2): λmax (ꢀ) 261 (39570), 315 (30400),
383 (41210). MS (FD): m/z 666.3. C39H21Co3 (666.5). Calcd C 70.28,
H 3.16; found C 70.17, H 3.75.
1
2178, 2120, 1259, 1247, 841, 811 cm-1. H NMR (CD2Cl2): δ 0.20
(s, 18 H), 0.21 (s, 36 H), 4.99 (s, 5 H), 5.03 (s, 10 H). 13C NMR (CD2-
Cl2): δ 0.14, 0.17, 68.70, 68.87, 69.03, 73.68, 73.72, 74.04, 78.57,
78.70, 78.87, 80.91, 81.18, 81.26, 81.68, 81.79. UV/vis (CH2Cl2): λmax
(ꢀ) 264 (45050), 308 (29470), 391 (33280). MS (FD): m/z 1099.1.
C57H69Si6Co3 (1099.5). Calcd C 62.27, H 6.33; found C 61.93, H 6.46.
8b (37 mg, 7.9%): Mp >230 °C dec. IR (KBr): ν 3107, 2955, 2900,
Synthesis of 11a. At 0 °C a 100 mL Schlenk flask was charged
under N2 with Cu2Cl2 (109 mg, 1.00 mmol), hydroxylamine hydro-
chloride (150 mg, 2.16 mmol), aqueous ethylamine solution (5 mL,
70% in H2O), methanol, and a solution of 6 (403 mg, 1.09 mmol).
Subsequently 1-bromopropyn-3-ol (386 mg, 2.86 mmol) was added at
0 °C. The mixture was stirred for 1 h at ambient temperature,
hydrolyzed with dilute HCl, and extracted with ether. Chromatography
of the crude product (silica gel/dichloromethane) furnishes 11a (431
mg, 83%): Mp 98 °C. IR (KBr): ν 3285, 3102, 2958, 2932, 1462,
1245, 847 cm-1. 1H NMR (CDCl3): δ 0.20 (s, 18 Η), 4.26, 4.28 (2 s,
4 Η), 5.05 (s, 5 H). 13C NMR (CD2Cl2): δ 0.31, 51.25, 67.26, 70.33,
75.16, 75.66, 76.61, 82.40, 84.12. C24H27Si2CoO2 (462.1). Calcd C
62.32, H 6.13; found C 62.48, H 6.53.
Synthesis of 11c via MnO2/KOH. Powdered KOH (560 mg, 10.0
mmol), activated MnO2 (1.74 g, 20.0 mmol), and 11a (431 mg, 904
µmol) in 20 mL of benzene were stirred for 1 h at ambient temperature.
The resulting suspension was filtered over silica gel and eluted with
pentane. According to TLC control, the material was homogeneous.
Attempts to isolate 11c were unsuccessful, due to its spontaneous
polymerization upon concentration of the pentane/benzene solutions.
1H NMR (CDCl3): δ 0.15 (s, 18 H), 2.63 (s, 2 H), 4.94 (s, 5 H).
1
2176, 2126, 1259, 1247, 841, 812 cm-1. H NMR (CD2Cl2): δ 0.22
(s, 54 H), 5.06 (s, 15 H). 13C NMR (CD2Cl2): δ 1.56, 70.38, 75.17,
80.31, 82.54, 83.01. UV/vis (CH2Cl2): λmax (ꢀ) 264 (49560), 392
(29890). MS (FD): m/z 1099.1. C57H69Si6Co3 (1099.5). Calcd C 62.27,
H 6.33; found C 62.56, H 5.90. 9a (31 mg, 6.6%): Mp >240 °C dec.
IR (KBr): ν 3108, 2954, 2925, 2904, 2366, 2179, 2124, 1259, 1247,
1
841, 812 cm-1. H NMR (CD2Cl2): δ 0.21 (s, 72 H), 5.01 (s, 20 H).
13C NMR (CD2Cl2): δ 0.19, 67.51, 75.24, 81.95, 77.91, 80.07. UV/vis
(CH2Cl2) λmax (ꢀ) 266 (75400), 360 (45750). MS (FD) m/z 1465.1.
C76H92Si8Co4 (1466.1). Calcd C 62.27, H 6.33; found C 61.99 H 6.34.
Mixture of 9b and 9c (126 mg, 26.6%). 9b: IR (KBr): ν 3107, 2955,
2925, 2898, 2364, 2178, 2129, 2125, 2050, 1259, 1247, 842, 812 cm-1
.
1H NMR (CD2Cl2): δ 0.21 (s, 54 H), 0.22 (s, 18 H), 4.99 (s, 5 H),
5.04 (s, 10 H), 5.06 (s, 5 H). 13C NMR (CD2Cl2): δ 0.27, 67.68, 67.83,
75.05, 75.16, 75.28, 78.02, 78.17, 80.10, 80.20, 81.84, 81.94, 82.00.
1
MS (FD): m/z 1465.1 C76H92Si8Co4 (1466.1). 9c: H NMR (CD2Cl2):
δ 0.21 (s, 72 H), 5.02 (s, 20 H). 9d (35 mg, 7.5%): Mp >240 °C dec.
IR (KBr): ν 3106, 2955, 2924, 2367, 2180, 2127, 1260, 1247, 842,

10726 J. Am. Chem. Soc., Vol. 121, No. 46, 1999
Bunz et al.
Polymerization of 11c to 13. A solution of 11c in pentane/benzene,
Cu(OAc)2 (3.63 g, 20.0 mmol), and 200 mL of acetonitrile were
combined. The mixture was stirred for 24 h at ambient temperature,
filtered, and concentrated under reduced pressure. Precipitation into
methanol furnished 13 as an insoluble orange material (350 mg, 87%
with respect to 11a). 1H NMR of soluble fraction (CD2Cl2): δ 0.20 (s,
18 H), 5.02 (s, 5 H). End groups can be seen as a weak signal at δ
2.73.
H), 4.97 (s, 5 H). 13C NMR (CDCl3): δ 59.86, 77.05, 82.30, 84.52.
UV/vis (CH2Cl2): λmax (ꢀ) 268 (24290), 324 (8200). MS (FD): m/z
272.26. C17H9Co (272.2). Calcd. C 75.02, H 3.33; found C 75.20, H
4.21.
Synthesis of 15 and 16. A mixture of Cu(OAc)2 (1.00 g, 5.51 mmol),
14b (100 mg, 0.171 mmol) in 50 mL of acetonitrile was heated to
reflux for 8 h. Aqueous workup and column filtration (1:4 dichlo-
romethane/hexane) led to the isolation of 76 mg (76%) of cycles. The
mixture was subjected to column chromatography (Merck silicagel 60,
pure hexane, gradually increasing to a 1:49 mixture of dichloromethane/
hexane) to furnish 39 mg of the trimers 15 and 37 mg of the tetramers
16. Repeated thick-layer chromatography of the isomeric mixtures of
15 (Merck thick-layer plates, 1:49 dichloromethane/hexane) furnishes
15a and 15b. The same procedure was applied for the tetramers 16
(1:19 dichloromethane/hexane) to furnish 16a, a mixture of 16b,c, in
Synthesis of 11b. A flame-dried Schlenk flask was charged with 6
(398 mg, 1.08 mmol) and 50 mL of THF and cooled to -78 °C. BuLi
(1.38 mL, 2.20 mmol, hexanes) was added and the reaction mixture
warmed to 0 °C. CuI (419 mg, 2.20 mmol) was introduced. After 15
min the mixture was recooled to -78 °C and 10 mL of dry propylamine
and bromo(triisopropylsilyl)acetylene (577 mg, 2.21 mmol) were
injected by syringe. After warming to 21 °C, aqueous workup, and
chromatography (flash silica gel/pentane) 11b and as a second band
12 were isolated. 11b (421 mg, 53%): Mp 103 °C. IR (KBr): ν 3105,
1
which two isomers were identified by their H and 13C NMR spectra,
and as a last fraction 16d. All of the obtained cycles decompose above
245 °C without melting. 15a (15 mg, 15%): IR (KBr): ν 3078, 2958,
1
2955, 2938, 2180, 1247, 846 cm-1. H NMR (CD2Cl2): δ 0.17 (s, 18
H), 1.09 (s, 42 H), 4.98 (s, 5 H). 13C NMR (CD2Cl2): δ 0.00, 11.49,
18.51, 66.69, 73.32, 75.39, 81.69, 88.62, 90.75. C41H65Si4Co (729.2).
Calcd C 67.53, H 8.98; found C 67.68, H 8.31. 12 (125 mg, 22%):
Mp 119 °C. IR (KBr): ν 3107, 2955, 2939, 2182, 1248, 848 cm-1. 1H
NMR (CD2Cl2): δ 0.21 (s, 36 H), 1.10, 1.11 (2 s, 42 H), 5.02 (s, 10
H). 13C NMR (CD2Cl2): δ 0.27, 11.76, 18.77, 66.55, 67.76, 73.75,
75.55, 75.77, 77.36, 77.95, 80.17, 81.95, 88.82, 91.08.
1
2925, 2866, 2140, 1261, 1020, 801 cm-1. H NMR (CD2Cl2): δ 5.05
(s, 10 H), 5.02 (s, 5 H), 1.26 (s, 18 H), 1.12 (s, 108 H). 13C NMR
(CD2Cl2): δ 12.21, 14.76, 61.82, 62.08, 62.34, 62.57, 79.92, 80.12,
80.24, 80.83, 80.91, 81.00, 84.81, 84.87, 98.85, 98.91, 98.94, 100.34.
UV/vis (CH2Cl2): λmax 278, 333, 412. MS (FD): m/z 1748.2. C105H141
-
Co3Si6 (1747.8). 15b (11 mg, 11%): IR (KBr): ν 2924, 2864, 2136,
1
1463, 996, 884. H NMR (CDCl3): δ 5.03 (s, 15 H), 1.24 (s, 18 H),
1.09 (s, 108 H). 13C NMR (CDCl3): δ 12.21, 19.79, 62.58, 62.60, 80.37,
81.49, 84.78, 98.76, 100.57. UV/vis (CH2Cl2): λmax 281, 333, 408. MS
(FD): m/z 1748.2. C105H141Co3Si6 (1747.8). 16a (13 mg, 13%): 1H NMR
Pyrolysis of 11b. A coiled quartz tube (1 m × 8 mm) was held at
550 °C under 1.5 × 10-1 to 10-4 mmHg (dynamic) while 11b (50 mg,
68.6 µmol) was sublimed/distilled with a heating gun at 170-250 °C
through the quartz tube. The product was collected in a U-tube held at
ambient temperature. Chromatography (flash silica gel/pentane) fur-
nished back 11b (12 mg, 24%) and as a second band 14a (33 mg,
66%; 87% with respect to consumed starting material). 14a: Mp 88
(CDCl3): δ 4.99, 4.96 (2 s, 20 H), 1.24 (s, 24 H), 1.09 (s, 144 H). 13
C
NMR (CDCl3): δ 11.20, 18.77, 60.05, 62.81, 77.80, 79.45, 83.87, 83.98,
97.76, 99.67. MS (FD): m/z 2330.3. C140H188Co4Si8 (2331.02). 16b
and 16c (24 mg, 24%): 1H NMR (CDCl3): δ 5.08, 5.06, 5.05, 5.02 (4
s, 20 H), 1.27 (s, 24 H), 1.13 (s, 144 H). 13C NMR (CDCl3): δ 12.25,
19.72, 61.40, 61.49, 61.66, 63.64, 78.86, 81.01, 84.60, 84.72, 84.75,
°C. IR (KBr): ν 3104, 2955, 2948, 1450, 847 cm-1 1H NMR
.
(CDCl3): δ 0.21 (s, 18 H), 1.11 (s, 42 H), 4.89 (s, 5 H). 13C NMR
(CDCl3): δ -0.10, 11.51, 18.86, 60.61, 61.80, 84.37, 97.49, 98.75,
100.68, 100.70. UV/vis (CH2Cl2): λmax (ꢀ) 275 (23220), 335 (4120).
MS (FD): m/z 728.2. C41H65Si4Co (729.2). Calcd. C 67.53, H 8.98;
found C 68.49, H 8.75.
1
84.93, 98.51, 101.42. 16d (7 mg, 7%): H NMR (CDCl3): δ 5.04 (s,
20 H), 1.24 (s, 24 H), 1.09 (s, 144 H).
Acknowledgment. We wish to acknowledge experimental
help from Jan Friedrich, Ronald Reuter (electrochemistry), and
Frank Beer (HPLC separations). This work was supported by
the Deutsche Forschungsgemeinschaft (1994-1997) and the
Petroleum Research Fund, administered by the American
Chemical Society (1998-2000). U.H.F.B. thanks the University
of South Carolina for Startup Funds. G.R. thanks Prof. K.
Mu¨llen (Mainz). This contribution is dedicated to Professor
Emanuel Vogel.
Deprotection of 14a to 14b or 14c. (a) 14a (20.0 mg, 27.4 µmol)
was dissolved in 10 mL of dichloromethane. A suspension of potassium
carbonate (500 mg, 3.67 mmol) in 3 mL of methanol was added and
the reaction mixture stirred for 10 min. Aqueous workup and chroma-
tography (flash silica gel, pentane) furnished 14b (15.6 mg, 98%): Mp
53 °C. IR (KBr): ν 3301, 3102, 2956, 1456, 1248, 848 cm-1. 1H NMR
(CDCl3): δ 1.12 (s, 42 H), 3.33 (s, 2 H), 4.97 (s, 5 H). 13C NMR
(CDCl3): δ 11.51, 18.86, 59.87, 61.61, 77.69, 81.90, 84.33, 97.73,
100.17. MS (FD): m/z 584.1. C35H49Si2Co (584.9). (b) Tetramethyl-
ammonium fluoride (300 mg, 3.22 mmol) was dissolved in 15 mL of
DMSO at 140 °C. After the mixture was cooled to 21 °C, 14a (45.0
mg, 61.7 µmol) in 10 mL of ethyl ether was added and the mixture
was stirred for 2 h at 21 °C. Aqueous workup and chromatography
(flash silica gel/pentane) gave 14c (15.0 mg, 89%): Mp 105 °C dec.
Supporting Information Available: Details and tables for
X-ray crystal structures of 3a, 8a, and 12 (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
1
IR (KBr): ν 3302, 3103, 1245 cm-1. H NMR (CDCl3): δ 3.33 (s, 4
JA992262A