Synthesis and explosive decomposition of organometallic dehydro[18] annulenes: An access to carbon nanostructures

Article abstract of DOI:10.1021/ja026809o

The synthesis of eight new cyclobutadiene or ferrocene-fused organometallic dehydroannulenes is reported. Cadiot-type coupling of a 1-bromoethynyl-2-silylethynylbenzene derivative to an organometallic diyne (1,2-diethynyl-3,4-bis(trimethylsilyl)cyclobutad

Full text of DOI:10.1021/ja026809o

Published on Web 10/24/2002
Synthesis and Explosive Decomposition of Organometallic
Dehydro[18]annulenes: An Access to Carbon Nanostructures
Matthew Laskoski, Winfried Steffen, Jason G. M. Morton, Mark D. Smith, and
Uwe H. F. Bunz*
Contribution from the USC NanoCenter and the Department of Chemistry and Biochemistry,
The UniVersity of South Carolina, Columbia, South Carolina 29208
Received May 6, 2002
Abstract: The synthesis of eight new cyclobutadiene or ferrocene-fused organometallic dehydroannulenes
is reported. Cadiot-type coupling of a 1-bromoethynyl-2-silylethynylbenzene derivative to an organometallic
diyne (1,2-diethynyl-3,4-bis(trimethylsilyl)cyclobutadiene(cyclopentadienyl )cobalt or 1,2-diethynylferrocene)
is followed by deprotection and Cu(OAc)2-promoted ring closure. Five of the organometallic dehydroan-
nulenes were structurally characterized. Three of the novel cycles explode at temperatures from 196 to
293 °C and form insoluble carbon materials. The soot produced from 13a shows a high abundance of
onion-like carbon nanostructures. The nanostructures were characterized by high-resolution transmission
electron microscopy.
Introduction
We describe the synthesis, structural characterization, and
explosive decomposition of organometallic dehydro[18]annulenes
containing ferrocene or cyclobutadiene(cyclopentadienyl)cobalt
units. The explosive decomposition of the dehydroannulene 13a
gives rise to the formation of onion-type carbon nanostructures.
Since the beginning of the nineties, the chemistry of “un-
natural” carbon allotropes has flourished. Fullerenes, carbon
onions, nanotubes, and all-carbon ropes are of great interest as
hydrogen storage containers, cloaking layers, and catalysts.1-5
Nanotubes have been made mostly by pyrolytic and high-energy
ablation routes of small molecules or arc evaporation of solid
graphite. The conversion of small molecules into carbon
nanomaterials is promoted by transition-metal catalysts.6 How-
ever, high temperatures and suitable equipment are necessary
for these experiments. Examples of all-carbon molecules from
polyynes are Rubin’s7a and Tobe’s7b access to C608 by pyrolysis
of cyclophane precursors.
Oligoynes and polyynes are known to explosively decompose
upon heating or rubbing.9,10 Vollhardt demonstrated elegantly
that the products of these reactions can be closed-shell carbon
structures, such as carbon onions or metal-filled carbon nano-
tubes.10 The authors examined different exploding oligoynes and
found that structures 1-3 gave closed-shell carbon materials.
They contended that the explosion of carbon-rich materials may
be a general way to make carbon nanostructures. From our
experience and the lack of other literature references, it seems
* Address correspondence to this author. E-mail: Bunz@mail.chem.
sc.edu.
(1) (a) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes
and Carbon Nanotubes; Academic Press: San Diego, CA, 1996. (b) For
carbon nanotubes formation see: Terrones, M.; Hsu, W. K.; Kroto, H. W.;
Walton, D. R. M. Top. Curr. Chem. 1999, 199, 189-234. (c) Sinnott, S.
B.; Andrews, R. Crit. ReV. Solid State Mater. Sci. 2001, 26, 145-249. (d)
Iijima, S. Nature 1991, 354, 56-58. Ajayan, P. M. Chem. ReV. 1999, 99,
1787-1799. (e) Ugarte, D. Carbon 1995, 33, 989-993. (f) Ugarte, D.
Carbon 1994, 32, 1245-1248.
(7) (a) Rubin, Y.; Parker, T. C.; Pastor, S. J.; Jalisatgi, S.; Boulle, C.; Wilkins,
C. L. Angew. Chem., Int. Ed. 1998, 37, 1226-1229. (b) Tobe, Y.;
Nakagawa, N.; Naemura, K.; Wakabayashi, T.; Shida, T.; Achiba, Y. J.
Am. Chem. Soc. 1998, 120, 4544-4545.
(2) (a) Che, G. L.; Lakshmi, B. B.; Martin, C. R.; Fisher, E. R. Langmuir
1999, 15, 750-758. (b) Dahn, J. R.; Zheng, T.; Liu, Y. H.; Xue, J. S.
Science 1995, 270, 590-593.
(8) For a direct synthesis of C60 see: Scott, L. T.; Boorum, M. M.; McMahon,
B. J.; Hagen, S.; Mack, J.; Blank, J.; Wegner, H.; de Meijere, A. Science
2002, 295, 1500-1503.
(3) Rao, A. M.; Richter, E.; Bandow, S.; Chase, B.; Eklund, P. C.; Williams,
K. A.; Fang, S.; Subbaswamy, K. R.; Menon, M.; Thess, A.; Smalley, R.
E.; Dresselhaus, G.; Dresselhaus, M. S. Science 1997, 275, 187-191.
(4) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D.
S.; Heben, M. J. Nature 1997, 386, 377-379.
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Cooney, M. J.; Scott, L. T. Chem. Eur. J. 1995, 1, 124-131. (b) Zhou, Q.;
Carroll, P. J.; Swager, T. M. J. Org. Chem. 1994, 59, 1294-1301. (c) Bunz,
U. H. F.; Enkelmann, V. Chem. Eur. J. 1999, 5, 263-266.
(5) Tans, S. J.; Devoret, M. H.; Dai, H. J.; Thess, A.; Smalley, R. E.; Geerligs,
L. J.; Dekker, C. Nature 1997, 386, 474-477.
(6) See, for example: Rao, C. N. R.; Satishkumar, B. C.; Govindaraj, A.; Nath,
M. Chem. Phys. Chem. 2001, 2, 78-105.
(10) (a) Boese, R.; Matzger, A. J.; Vollhardt, K. P. C, J. Am Chem. Soc. 1997,
119, 2052-2053. (b) Dosa, P. I.; Erben, C.; Iyer, V. S.; Vollhardt, K. P.
C.; Wasser, I. M. J. Am. Chem. Soc. 1999, 121, 10430-10431. (c) Faust,
R. Angew. Chem., Int. Ed. 1998, 37, 2825-2828.
9
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J. AM. CHEM. SOC. 2002, 124, 13814-13818
10.1021/ja026809o CCC: $22.00 © 2002 American Chemical Society

An Access to Carbon Nanostructures
A R T I C L E S
Scheme 1 a
a The synthesis of 9a uses a slightly different route, not via 8a. See Supporting Information for details.
Table 1. Substituent Key for 7-10 and Yields for 9-10
Table 3. DSC Data of the Macrocycles 10 and 13a
overall
yield 9 yield 10 yield
compound
temperature (°C)
∆Hrxn (kcal/mol-1
)
1
2
3
R for 8a,9
R for 7, 9, 10
R
R
10a
10b
10c
10d
10e
13a
13b
13c
234
202
210, 354
293
243
196
183, 286
190
-53
-153
a
b
methyla
SiMe3
H
H
H
40%
9%b 4%b
isopropyl 35% 57% 20%
isopropyl 7, 9, R1 ) SiMe3;
-15, -5
-136
10b, R1dH
-93
-118
c
d
e
isopropyl SiMe3
isopropyl SiMe3
H
H
tert-butyl 73% 13%c 9%
butyl butyl
tert-butyl
57% 58% 33%
-8, -13
-144
H
17%c 12%
a For 9a, the bromoyne 8a (R ) isopropyl) is not utilized. See Supporting
Information for a modified procedure. For simplicity, 9a is included in Table
1. b Cycle 10a is only marginally soluble. c Combined yield ) 30%. Entries
c and e represent one experiment.
a 10b, 10d, and 13a contain solvent in their crystal lattices. These three
are the exploding cycles. The other cycles turn dark and form insoluble
materials without explosive decomposition. Vollhardt reported less exo-
thermic reactions (-47 to -53 kcal mol-1) for his exploding alkynes.10
Table 2. Substituent Key and Yields for 12-13
3
R for 8,12
R
yield 12
yield 13
overall yield
a
b
c
isopropyl
isopropyl
isopropyl
H
45%
62%
56%
75%
44%
57%
34%
27%
32%
isopropyl
tert-butyl
though that only very few oligoynes give defined carbon
nanostructures upon explosion.10 The organometallic dehy-
droannulene 13a is one of them.
Figure 1. ORTEP representation of the crystal structure of dehydroannulene
13a.
Results and Discussion
A larger number of butadiynes per dehydroannulene ring
might increase their propensity to explode. An access to
nanostructured forms of carbon may result. We anticipated that
such explosive transformations could be feasible; Fe, Co, and
Ni are known to catalyze nanotube formation,6 and the projected
dehydroannulenes 10 and 13 do contain either Fe or Co.
Syntheses. Vollhardt’s diethynylcyclobutadiene complex 714
was metalated with butyllithium and treated with CuI to furnish
a cuprate that was coupled to bromo-ynes 8b-d (Scheme 1).
This approach is similar to that reported by Rubin,15 Haley,16
and Scott.17 It furnished 9 in yields from 35 to 73% (Table 1).
Our interest in carbon-rich organometallic materials11 led us
to prepare the organometallic dehydroannulenes 4-6 to quantify
the aromaticity of the cyclopentadienyl rings in ferrocene and
the cyclobutadiene rings in cyclobutadiene(cyclopentadienyl)-
cobalt.12 When heated, we found 4-6 either to be thermally
stable or to polymerize to an amorphous but infusible, insoluble
dark material.13 No explosive decomposition was evidenced.
(11) (a) Bunz, U. H. F. Top. Curr. Chem. 1999, 201, 131-161. (b) Bunz, U. H.
F.; Rubin, Y.; Tobe, Y. Chem. Soc. ReV. 1999, 28, 107-119.
(12) (a) Laskoski, M.; Steffen, W.; Smith, M. D.; Bunz, U. H. F. Chem. Commun.
2001, 691-692. (b) Laskoski, M.; Roidl, G.; Smith, M. D.; Bunz, U. H. F.
Angew. Chem., Int. Ed. 2001, 49, 1460-1463. (c) Laskoski, M.; Smith,
M. D.; Morton, J. G. M.; Bunz, U. H. F. J. Org. Chem. 2001, 66, 5174-
5181.
(13) (a) Baldwin, K. P.; Matzger, A. J.; Scheiman, D. A.; Tessier, C. A.;
Vollhardt, K. P. C.; Youngs, W. J. Synlett 1995, 1215-1218. (b)
Enkelmann, V. AdV. Polym. Sci. 1984, 63, 91-136. Wegner, G. Z.
Naturforsch. 1969, 24b, 824-832. (c) Ha¨dicke, E.; Mez, E. C.; Krauch,
C. H.; Wegner, G.; Kaiser, J. Angew. Chem., Int. Ed. Engl. 1971, 10, 266-
267.
(14) (a) Fritch, J. R.; Vollhardt, K. P. C. Organometallics 1982, 1, 590-602.
(b) Roidl, G.; Enkelmann, V.; Adams, R. D.; Bunz, U. H. F. J. Organomet.
Chem. 1999, 578, 144-149.
(15) Rubin, Y.; Parker, T. C.; Khan, S. I.; Holliman, C. L.; McElvany, S. W. J.
Am. Chem. Soc. 1996, 118, 5308-5309.
(16) (a) For organic dehydroannulenes see: Pak, J. J.; Weakley, T. J. R.; Haley,
M. M. J. Am. Chem. Soc. 1999, 121, 8182-8192. (b) Haley, M. M.; Pak,
J. J.; Brand, S. C. Top. Curr. Chem. 1999, 121, 81-130.
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A R T I C L E S
Laskoski et al.
Figure 2. Packing diagram of 13a. Disordered solvent molecules (dichloromethane and hexanes) are omitted. Crystallographic axes are included for orientation.
Top: centroid-centroid (shown with dotted lines) and Calkyne-Calkyne distances are in the range of 3.59 (distance I) and 3.68 Å (distance II). The interplanar
angles between neighboring phenyl rings are close to 7°. Bottom: alternate view of the π-π stacks (horizontal and into the figure).
The open chain precursors are obtained after desilylation with
either Me4N+F- or K2CO3 (if R ) Me). Crude solutions of the
diynes are cyclized in situ by addition of Cu(OAc)2 and
acetonitrile. This coupling18 leads to cycles 10 in yields ranging
from 9 to 58%. The low yield of 10a is due to its marginal
solubility that hindered isolation and column chromatography.
In the deprotection/cyclization of 9c, we find the formation of
the expected cycle 10c in a 13% yield. The main product is the
dehydroannulene 10e (17%) that has lost the trimethylsilyl
groups off the cyclobutadiene ring under the conditions of the
deprotection reaction.
reduced yields may be due to oxidation of the organometallic
moieties. The formation of a significant amount of an insoluble
and intractable dark material was always observed. Open chain
or cyclic dimers or higher oligomers were not detected. It has
been reported that alkynylated ferrocenes19 do not oxidatively
dimerize in high yields under the Glaser-Eglinton conditions.
The dehydro[18]annulenes can be stored under ambient condi-
tions for days and are indefinitely stable in the freezer. While
determining the melting points of 10 and 13, we noticed that
10b,d and 13a explode. Ferrocenes 13b,c undergo a solid-state
reaction. They turn black and amorphous at 183 and 190 °C,
respectively. To quantitate this behavior, we performed DSC
analysis of the cycles (Table 3), all of which undergo an
exothermic transformation between 183 °C and 293 °C.
Structural Characterization of the Dehydroannulenes.
Suitable crystalline specimens of 10b,d,e and 13a,b were ob-
tained at room temperature by slow evaporation from a di-
chloromethane/hexane mixture. Details of the structure deter-
mination and the CIF files are given in the Supporting
Information.
Likewise, 9b furnishes the desilylated cycle 10b in 57% yield.
The synthetic sequence is applicable to make the ferrocene-
based dehydro[18]annulenes 13a-c. Diethynylferrocene 11 is
transformed into its cuprate and coupled to 8a-c furnishing
12a-c in yields ranging from 45 to 62% (Table 2). Deprotection
and ring closure are performed analogously to the experiments
described in the cyclobutadiene series. The dehydroannulenes
13a-c formed in 44-75% yield. This two-step process gave
the organometallic dehydro[18]annulenes 10 and 13, in oVerall
yields from 4 to 34%. The copper-promoted ring closure
proceeds fairly well, but not as easily16 as in the dehydro[14]-
annulenes 4-6, where yields of up to 93% are realized. The
The molecular geometries of the cycles are similar. The
structure of one representative compound, 13a, is displayed
(Figure 1, ORTEP). The dehydro[18]annulene system is struc-
turally undisturbed by the fusion of ferrocene to the perim-
(17) Scott, L. T. In Modern Acetylene Chemistry; Stang, P. J., Diederich, F.,
Eds.; VCH: Weinheim, 1996.
(18) (a) Berscheid, R.; Vo¨gtle, F. Synthesis 1992, 58-62. (b) Siemsen, P.;
Livingston, R. C.; Diederich, F. Angew. Chem., Int. Ed. 2000, 39, 2632-
2657.
(19) For the sensitivity of ferrocenes to the conditions of the copper-catalyzed
alkyne-alkyne couplings see: Yuan, Z.; Stringer, G.; Jobe, I. R.; Kreller,
D.; Scott, K.; Koch, L.; Taylor, N. J.; Marder, T. B. J. Organomet. Chem.
1993, 452, 115-120.
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An Access to Carbon Nanostructures
A R T I C L E S
eter. The bond lengths and bond angles are in agreement with
literature values for ferrocenes and dehydrobenzoannu-
lenes.11,12,16,20 The packing of 13a in the solid state is displayed
in Figure 2. The packing is different from the packing of the
other cycles. There are numerous close contacts13,21 between
the diyne units in 13a (see Figure 2) that are in the range of the
sum of the van der Waals radii (3.6 Å) of sp-carbons. Such
interactions of the diyne groupings are observed only in the
packing of 13a.
Thermal Decomposition of 10 and 13: Formation of
Carbon Nanostructures.22,23 The exploding cycles were identi-
fied from the DSC and melting-point data. All of the exploding
cycles contained disordered crystal solvent. The cycles 10b,d
and 13a were subjected to a semipreparative (20-50 mg scale)
explosion under vacuum in a Schlenk flask. Despite the large
amount of energy released during the explosion, larger quantities
(100-200 mg) of the cycles could be exploded in a vacuum-
sealed thick-walled quartz tube. Black magnetic powders
resulted. The explosion of 10b,d gave amorphous materials with
no discerning features on the nanoscale. Only the ferrocene cycle
13a exploded to form large amounts of distinct carbon nano-
structures.24 According to 57Fe Mo¨ssbauer spectroscopy, the soot
obtained by the explosion of 13a contains iron hydroxide, iron
oxide, and some elemental iron. Iron carbides were not detected
in these samples. Washing and digeration with dilute HCl (2
N) removed the iron-containing contaminants.
Transmission electron microscopy was performed on the HCl-
treated soot. Figure 3 shows representative examples of onion-
shaped carbon nanostructures. These are obtained from 13a;
the ring-shaped structures are abundant in the soot. Onion
structures are visible on all examined grids and on single grids
in different places. At low magnification, these onion-shaped
materials do not display an internal structure and we initially
assumed that the soot was amorphous carbon. However, at
higher magnification, the onion-type rings exhibit a layered
structure (width 1.8-2.0 nm). Even though layers/tubes can be
distinguished, we were not able to see the graphitic spacings
that are usually recognizable in carbon onions. A possible reason
for the lack of fine structure is the low temperature at which
these nanostructures form.
Upon annealing to 900 °C, we found that the order in the
nanostructures increased (Figure 3, bottom). The classification
of these structures22 is not completely clear. Probably, these
are carbon onions similar to those described by Ugarte.22e
Figure 3. High-resolution transmission electron micrographs of the carbon
nanostructures formed by explosion of 13a. The bottom picture shows the
TEM of an annealed sample (to 900 °C).
(20) (a) Bunz, U. H. F.; Enkelmann, V. Organometallics 1994, 13, 3823-3833.
(b) Bunz, U. H. F.; Enkelmann, V.; Ra¨der, J. Organometallics 1993, 12,
4745-4747. (c) Altmann, M.; Roidl, G.; Enkelmann, V.; Bunz, U. H. F.
Angew. Chem., Int. Ed. Engl. 1997, 36, 1107-1109.
(21) (a) Jaufmann, J. D.; Case, C. B.; Sandor, R. B.; Foxman, B. M. J. Solid
State Chem. 2000, 152, 99-104. (b) Song, F.; Snook, J. H.; Foxman, B.
M.; Snider, B. B. Tetrahedron 1998, 54, 13035-13044.
(22) (a) Subramoney, S. AdV. Mater. 1998, 10, 1157-1171. (b) Kroto, H. W.
Nature 1992, 359, 670-671. (c) Kutznetsov, V. L.; Chuvilin, A. L.; Moroz,
E. M.; Kolomiichuk, V. N.; Shaikhutdinov, S. K.; Butenko, Y. V.; Malkov,
I. Y. Carbon 1994, 32, 873-882. (d) Thess, A.; Lee, R.; Nikolaev, P.;
Dai, H. J.; Petit, P.; Robert, J.; Xu, C. H.; Lee, Y. H.; Kim, S. G.; Rinzler,
A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fischer, J. E.; Smalley,
R. E. Science 1996, 273, 483-487. (e) Ugarte, D. Nature 1992, 359, 707-
709. (f) Liu, J.; Dai, H. J.; Hafner, J. H.; Colbert, D. T.; Smalley, R. E.;
Tans, S. J.; Dekker, C. Nature 1997, 385, 780-781. (g) Selvan, R.;
Unnikrishnan, R.; Ganapathy, S.; Pradeep, T. Chem. Phys. Lett. 2000, 316,
205-210.
(23) De Heer, W. A.; Ugarte, D. Chem. Phys. Lett. 1993, 207, 480-486.
(24) It is difficult to estimate yields by TEM but the ring-shaped nanostructures
are almost everywhere. For a low magnification picture that gives an
impression of the abundance of the nanostructures, see the Supporting
Information.
Alternatively, they could be multiwalled-bent carbon nanotubes,
carbon tori,1b or ribbon polymers (Figure 3, middle left) formed
by the initial topochemical polymerization13 of the 3-fold diyne
13a.
Explosive decomposition with the loss of crystal solvent
(dichloromethane/hexanes) seems to be coupled. No discrete
DSC signal was recorded for the evaporation of crystal solvent.
Explosive decomposition commences with single crystalline
material but not with precipitated powders, suggesting that the
preorganization of the π-system is important.21 The supramo-
lecular order and not the intrinsic strain within the molecular
structure plays a major role in the explosive formation of the
nanostructures.10,21 The likely reason for the explosion of 13a
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Laskoski et al.
is the uniquely close contact of the butadiyne units in the solid
state because of the absence of bulky ring substituents.21 These
contacts tighten upon loss of crystal solvent (Figure 2) with
considerable compaction/collapse of the lattice, ultimately
leading to the observed carbon nanostructures.
Acknowledgment. We thank the National Science Foundation
(Bunz, CAREER, CHE 9981765) and the USC NanoCenter for
generous funding. UB is a Camille Dreyfus Teacher-Scholar
(2000-2004). We thank Dr. D. Dunkelberger for help with the
electron microscopy, Dr. J. Selegue and Dr. F. E. Huggins (U
of Kentucky) for valuable discussions and a Mo¨ssbauer spectrum
of the soot formed from 13a.
Conclusions
A series of eight novel organometallic dehydro[18]annulenes
10 and 13 was prepared by a combination of standard Pd- and
Cu-catalyzed couplings. These compounds pyrolyze from 183
to 293 °C. In the cycle 13a, the explosive thermal decomposition
furnishes carbon nanomaterials with a rope- or onion-like
internal structure. Currently, we are investigating the introduc-
tion of heteroatoms into organometallic dehydro[18]annulenes.
We will study how the presence of heteroatoms will affect the
formation and structures of the explosion-generated carbon
nanostructures.
Supporting Information Available: Experimental procedures,
detailed spectroscopic characterization, and crystallographic
information files [CIFs for 10b (CCDC 162440), 10d (CCDC
162301), 10e (CCDC 162305), 13a (CCDC 162304), 13b
(CCDC 162303)] are available free of charge via the Internet
at http://pubs.acs.org.
JA026809O
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