Organometallic dendrimers based on (tetraphenylcyclobutadiene)cyclopentadienylcobalt modules

Article abstract of DOI:10.1021/ja026462p

The synthesis and characterization of novel organometallic polyphenylene dendrimers containing 24 or 44 phenyl rings and one cyclobutadiene(cyclopentadienyl)cobalt unit is reported. The dendrimers are made by the convergent CpCo(CO)₂-mediated dimerization of di- or tetraethynyltolanes followed by a divergent core extension utilizing tetraphenylcyclopentadienone. The obtained dendrimers are air and water stable, soluble materials that show interesting differences in their hydrodynamic properties as evidenced by gel permeation chromatography. Scanning pulse voltammetry in solution shows that the dendrimers are oxidized at potentials ranging from 0.8 to 0.83 V. The more sterically encumbered the dendrimer, the higher its oxidation potential, that is, the more difficult oxidation is.

Full text of DOI:10.1021/ja026462p

Published on Web 06/26/2002
Organometallic Dendrimers Based on
(Tetraphenylcyclobutadiene)cyclopentadienylcobalt Modules
Shane M. Waybright, Kanika McAlpine, Matthew Laskoski, Mark D. Smith, and
Uwe H. F. Bunz*
Contribution from the Department of Chemistry and Biochemistry, The UniVersity of South
Carolina, Columbia, South Carolina 29208
Received April 8, 2002
Abstract: The synthesis and characterization of novel organometallic polyphenylene dendrimers containing
24 or 44 phenyl rings and one cyclobutadiene(cyclopentadienyl)cobalt unit is reported. The dendrimers
are made by the convergent CpCo(CO)2-mediated dimerization of di- or tetraethynyltolanes followed by a
divergent core extension utilizing tetraphenylcyclopentadienone. The obtained dendrimers are air and water
stable, soluble materials that show interesting differences in their hydrodynamic properties as evidenced
by gel permeation chromatography. Scanning pulse voltammetry in solution shows that the dendrimers
are oxidized at potentials ranging from 0.8 to 0.83 V. The more sterically encumbered the dendrimer, the
higher its oxidation potential, that is, the more difficult oxidation is.
able.5 Their proposed reaction with tetraphenylcyclopentadienone
should furnish organometallic polyphenylene dendrimers. We
Introduction
Dendritic molecules have attracted attention because of their
exciting structures and carefully tailored properties;1 polyphen-
ylene dendrimers have been introduced by Mu¨llen et al., who
reported the reaction of alkynylated aromatic and nonaromatic
cores with tetraphenylcyclopentadienone and its functionalized
derivatives.2 This straightforward, powerful approach has al-
lowed the assembly of numerous polyphenylene dendrimers,
some of which give large graphitic disks upon oxidative ring
closure. Polyphenylene dendrimers are monodisperse rigid
macromolecules of defined shape with significant internal voids.
They are attractive in advanced sensing schemes.3 While organic
polyphenylene dendrimers have been reported by the MPI group,
polyphenylene dendrimers with a central electroactive organo-
metallic4 core are not described to our knowledge.
herein describe the synthetic scheme to and some interesting
properties of simple cyclobutadiene(cyclopentadienyl)cobalt
centered polyphenylene dendrimers.
Results and Discussion
Syntheses. The synthetic sequence starts out with the
preparation of the prerequisite diarylalkynes 2b, 4b, 6b, and
8b (Scheme 1). While these materials have been described in
the literature,2,3 a simplified synthetic access was developed.
Acetylene gas reacts with 1, 3, 5, or 7 under standard
Pd-catalysis to give the intermediate di- and tetrabromo-
(diphenylacetylene)s 2a, 4a, 6a, and 8a. Replacement of the
iodide substituents by acetylene is facile for these substrates,
and yields of the intermediates range from 75 to 99%. The yield
of the dimerization products was highest if the loading of
catalyst was low (0.1 mol % [(Ph3P)2PdCl2]).6c Subsequent
coupling of the intermediates to tri(isopropylsilyl)acetylene
under standard conditions makes the triynes 2b, 4b, and 6b and
the pentayne 8b accessible in yields between 33 and 86% for
both coupling steps. The second coupling replaces the aromatic
bromide by tri(isopropylsilyl)acetylene and is performed in
triethylamine as solvent. A 10-fold higher catalyst loading (1
mol % [(Ph3P)2PdCl2]) at elevated temperatures is necessary to
overcome the decreased reactivity of the aromatic bromides.
Triethylamine is the base and solvent of choice; while piperidine
is excellent as base in the Heck-Cassar-Sonogashira-Hagihara
The synthesis of such organometallic polyphenylene den-
drimers should be facile if terminally alkynylated tetraphenyl-
cyclobutadiene(cyclopentadienyl)cobalt complexes were avail-
* Address correspondence to this author. E-mail: Bunz@mail.chem.sc.edu.
(1) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A. Angew. Chem. 1990, 29,
138. Moore, J. S. Acc. Chem. Res. 1997, 30, 402. Smith, D. K.; Diederich,
F. Chem. Eur. J. 1998, 4, 1353. Archut, A.; Vo¨gtle, F. Chem. Soc. ReV.
1998, 27, 233. Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. ReV.
1999, 99, 1665. Grayson, S. K.; Frechet, J. M. J. Chem. ReV. 2001, 101,
3819. Gorman, C. B.; Smith, J. C. Acc. Chem. Res. 2001, 34, 60.
(2) Watson, M. D.; Fechtenko¨tter, A.; Mu¨llen, K. Chem. ReV. 2001, 101, 1267.
Berresheim, A. J.; Mu¨ller, M.; Mu¨llen, K. Chem. ReV. 1999, 99, 1747. Ito,
S.; Herwig, P. T.; Bo¨hme, T.; Rabe, J. P.; Rettig, W.; Mu¨llen, K. J. Am.
Chem. Soc. 2000, 122, 7698. Do¨tz, F.; Brand, J. D.; Ito, S.; Gherghel, L.;
Mu¨llen, K. J. Am. Chem. Soc. 2000, 122, 7707. Weil, T.; Wiesler, U. M.;
Herrmann, A.; Bauer, R.; Hofkens, J.; De Schryver, F. C.; Mu¨llen, K. J.
Am. Chem. Soc. 2001, 123, 8101. Mu¨ller, M.; Ku¨bel, C.; Mu¨llen, K. Chem.
Eur. J. 1998, 4, 2099.
(3) Schlupp, M.; Weil, T.; Berresheim, A. J.; Wiesler, U. M.; Bargon, J.;
Mu¨llen, K. Angew. Chem. 2001, 40, 4011. Vossmeyer, T.; Guse, B.;
Besnard, I.; Bauer, R. E.; Mu¨llen, K.; Yasuda, A. AdV. Mater. 2002, 14,
238.
(4) Astruc, D.; Chardac, F. Chem. ReV. 2001, 101, 2991. Astruc, D. Acc. Chem.
Res. 2000, 33, 287. Astruc, D.; Blais, J. C.; Cloutet, E.; Djakovitch, L.;
Rigaut, S.; Ruiz, J.; Sartor, V.; Valerio, C. Top. Curr. Chem. 2000, 210,
299.
(5) Bunz, U. H. F. Top. Curr. Chem. 1999, 201, 131. Bunz, U. H. F.; Rubin,
Y.; Tobe, Y. Chem. Soc. ReV. 1999, 28, 131.
(6) (a) Li, C. J.; Slaven, W. T.; John, V. T.; Banerjee, S. Chem. Commun.
1997, 1569. Li, C. J.; Slaven, W. T.; Chen, Y. P.; John, V. T.; Rachakonda,
S. H. Chem. Commun. 1998, 1351. (b) Iyoda, M.; Vorasingha, A.; Kuwatani,
Y.; Yoshida, M. Tetrahedron Lett. 1998, 39, 4701. (c) Wilson, J. N.;
Waybright, S. M.; McAlpine, K.; Bunz, U. H. F. Macromolecules 2002,
35, ASAP.
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A R T I C L E S
Waybright et al.
Scheme 1
coupling of aromatic iodides,7 it gave less satisfying results in
the substitution of bromides 2a, 4a, 6a, and 8a.
after chromatography. It was possible to dimerize the pentayne
8b; however, the cyclobutadiene complex 12 was isolated in
only 12% yield after deprotection with tetrabutylammonium
fluoride and repeated chromatography. Because of the facile
access to the precursor pentayne 8b, the low yield of the
complex 12 did not prohibit the exploration of its reactivity.
The decreasing yield in the dimerization reactions can be
explained by the increasing steric encumbrance of the utilized
diarylalkyne. The least encumbered alkyne, 2b, gives the highest
yields, while the bulky 6b does not undergo the dimerization
reaction at all. The steric bulk of 6b makes it completely
unreactive and it does not undergo cobalt mediated [2 + 2 +
2] cycloaddition reported by Vollhardt10 to occur for the parent
triyne 6, void of the triisopropylsilyl groups.
The reaction of disubstituted alkynes with cyclopentadienyl-
(dicarbonyl)cobalt (CpCo(CO)2) grants facile access to func-
tionalized cyclobutadiene complexes.8 Yields of this dimeriza-
tion step are high and can exceed 90% in the diarylalkynes or
TMS-protected acetylenes. The phenyleneethynylenes 2b, 4b,
6b, and 8b are attractive because they contain two different
types of alkynes. The outer ones are protected by the bulky
triisopropylsilyl groups and should show negligible reactivity
toward CpCo(CO)2, while the inner ones, the diarylalkynes,
should partake in the dimerization. Reaction of 2b with CpCo-
(CO)2 in boiling decane furnished the cyclobutadiene complex
9 in 75% yield (Scheme 2) after removal of the triisopropylsilyl
groups with tetrabutylammonium fluoride. Side products that
could be traced to the reaction of the triisopropylsilyl-protected
alkynes with CpCo(CO)2 were not detected, suggesting clean
reaction of the diarylalkyne unit.
Michl et al.9 dimerized 4,4′-dibromotolane and performed
Heck-Cassar-Sonogashira-Hagihara alkynylation of the in-
termediate [tetrakis(bromophenyl)cyclobutadiene]cyclopenta-
dienylcobalt. They had obtained 9, albeit in lower yield.
Encouraged by the smooth reaction of 2b with CpCo(CO)2, the
triyne 4b was reacted under similar conditions to give the
cyclobutadiene complex 10 in 24% yield. The byproduct was a
dark brown mass of undefined structure and separated from 10
by column chromatography. If the alkyne groups are ortho-
positioned with respect to each other, as in 6b, reaction with
CpCo(CO)2 is unsuccessful and the starting material is reisolated
The cyclobutadiene complexes 9, 10, and 12 are stable to air
and light, are soluble in common organic solvents, and can be
manipulated without problems. Diels-Alder reaction of 9, 10,
and 12 with an excess of tetraphenylcyclopentadienone (Scheme
2) in boiling decaline furnished the organometallic dendrimers
13-15 in good-to-excellent yields (63-91%) as yellow powders
1
after crystallization or chromatography. The H NMR spectra
of the products of the Diels-Alder reaction did not show the
presence of terminal alkyne protons. Instead, in the aromatic
region (Figure 1) 13 and 14 show a diagnostic singlet of Ha at
δ ) 7.63 (4H, 13), 7.63 (4H, 14) that integrate to four protons
and represents the former alkyne protons. In 15 this diagnostic
singlet is not visible at ambient temperature. All of its NMR
signals, including that of the Cp-ring, are significantly upfield
shifted and broadened. This effect must be a consequence of
the high steric pressure the 44 benzene rings exert, indicating a
conformation in which face-to-face interactions of the arenes
(7) Bunz, U. H. F. Chem. ReV. 2000, 100, 1605. Bunz, U. H. F. Acc. Chem.
Res. 2001, 34, 998.
(8) Efraty, A. Chem. ReV. 1977, 77, 691. Fritch, J. R.; Vollhardt, K. P. C.
Organometallics 1982, 1, 590. Fritch, J. R.; Vollhardt, K. P. C. Angew.
Chem. 1979, 18, 409.
(10) Diercks, R.; Vollhardt, K. P. C. Angew. Chem. 1986, 25, 266. Eickmeier,
C.; Junga, H.; Matzger, A. J.; Scherhag, F.; Shim, M.; Vollhardt, K. P. C.
Angew. Chem. 1997, 36, 2103.
(9) Harrison, R. M.; Brotin, T.; Noll, B. C.; Michl, J. Organometallics 1997,
16, 3401.
9
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Organometallic Dendrimers
A R T I C L E S
Scheme 2
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A R T I C L E S
Waybright et al.
Figure 1. 1H NMR spectrum of 13. At δ 7.62 the unique singlet of Ha (part of the pentaphenylbenzene moiety) is visible. The alkyne resonance has
disappeared. At δ 4.42 the singlet of the cyclopentadienyl ring is recorded. The signal at δ 7.23 is due to residual CHCl3 in the solvent CDCl3.
1
Figure 2. Variable temperature H NMR spectrum of 15 taken at 25 °C (bottom) and at 115 °C (top) in tetrachloroethane d-2 (δ 6.00). At δ 6.98 (upper
trace, small singlet) the signal of the Ha protons is visible. The most upfield shifted, unresolved triplet (δ 6.53) represents the four unique para protons of
the innermost phenyl rings. The other resonances give the correct integration but cannot be assigned unequivocally. To higher field the distinct singlet of the
Cp-ring is observed.
are enforced.11 In Figure 2, the 1H NMR spectra of 15 at 25 °C
and at 115 °C are shown. While at ambient temperature the
resonances are unresolved, at 115 °C a well-resolved spectrum
is obtained, suggesting the free rotation of all benzene rings on
the NMR time scale. The signal of the protons Ha in 15 can
now be identified at δ ) 6.98, and a small unresolved triplet at
δ ) 6.53 is assigned to the four unique para protons of the
innermost benzene rings. The temperature dependence of the
1H NMR spectra is consistent with the high steric pressure that
representation is shown in Figure 3.12 As expected, the four
phenyl rings are twisted with respect to each other (Table 1).
The propeller-shaped conformation of 9 is similar to that
observed for the parent (tetraphenylcyclobutadiene)cyclopenta-
dienylcobalt. The twist between the phenyl rings and the
cyclobutadiene core is significant and ranges from 27 to 42°,6,13
while the bond angles and bond lengths are in excellent
agreement with reported values for cyclobutadiene complexes.14
1
the crowding of phenyl groups causes. The H NMR and 13C
(12) Crystallographic data for 9: C41 H25 Co; formula wt 576.54, T ) 180 K, l
) 0.711 A; orthorombic, Pbca, a ) 11.1151(5) Å, b ) 20.7398(10) Å, c
NMR spectra and other analytical data of the Diels-Alder
adducts support smooth formation of the dendrimers 13-15.
Properties and Structures of 9-15. To obtain proof for the
structure of the cyclobutadiene complexes, we grew a single
crystalline specimen of 9 from dichloromethane. An ORTEP
) 25.8240(13) Å, R ) â ) γ ) 90°; V ) 5953.1(5) Å3, Z ) 8, Gcalcd
)
1.287 g cm-3; absorption coefficient ) 0.604 mm-1; crystal size 0.72 ×
0.11 × 0.08 mm3; -9 e h e 13, -22 e k e 24, -30 e l e 30; reflections
collected 33 799, independent reflections 5272 [R(int) ) 0.0790]; semiem-
pirical absorption correction; refinement method: Full-matrix least-squares
on F2; R1 ) 0.0479, wR2 ) 0.1025; largest diffraction peak and hole: 0.342
and -0.323 e‚Å-3
.
(13) Rausch, M. D.; Westover, G. F.; Mintz, E.; Reisner, G. M.; Bernal, I.;
Clearfield, A.; Troup, J. M. Inorg. Chem. 1979, 18, 2605.
(11) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525. Hunter,
C. A. Angew. Chem. 1993, 32, 1584.
(14) Bunz, U. H. F.; Enkelmann, V. Organometallics 1994, 13, 3823.
9
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Organometallic Dendrimers
A R T I C L E S
Figure 3. ORTEP of tetraalkynyated cyclobutadiene complex 9.
Figure 4. Space-filling representations of complexes 13, 14, and 15. The structures were obtained utilizing semiempirical PM3 quantum chemical calculations.
For 14 and 15 it is not clear if the represented structures are local minima or the global minimum. For the conformationally rigid 13 the global minimum
is probably represented correctly.
Table 1. Interplanar Angles (°) of the Phenyl Rings and the
Table 2. GPC Data for Complexes 13, 14, and 15
Cyclobutadiene Core
complex formula weight (FW)a
M a
M a
ratio M /FW M /M retention timeb
n
w
n
w
n
{C1‚‚‚C4}
{C5‚‚‚C10}
{C13‚‚‚C18}
{C21‚‚‚C26}
13
14
15
2000
2000
3522
1490 1600
1250 1320
1820 1860
0.745
0.625
0.517
1.07
1.06
1.02
8.52
8.63
8.45
{C5‚‚‚C10}
{C13‚‚‚C18}
{C21‚‚‚C26}
{C29‚‚‚C34}
36.0
27.5
42.2
30.7
45.1
78.2
46.2
48.9
58.1
a Units in g/mol. b Time in min.
49.0
diffraction pattern is complex and likewise not useful for
structure elucidation.
Unfortunately, attempts to grow single crystalline specimens
from the other cyclobutadiene complexes or the Diels-Alder
adducts 13-15 were unsuccessful and only powders were
obtained.
The molecular size of the para (13), meta (14), and double
meta (15) Diels-Alder adducts (Figure 4) was investigated by
gel permeation chromatography (GPC, Figure 5). All polydis-
persities (Mw/Mn) were between 1.02 and 1.07, giving credibility
to the monodisperse nature of the dendrimers (Table 2). Both
13 and 14 have a formula weight of 2000 Da and 15 has a
formula weight of 3522 Da. Comparing isomeric complexes
13 and 14, 14 has a longer retention time than 13, indicating
that the para complex 13 has a higher hydrodynamic volume
than the meta complex 14. Space-filling models (Figure 4) show
that the rigid dendrimer 13 adopts a more extended structure
than 14 and thus features a higher hydrodynamic volume.
Interestingly, the molecular weights obtained by GPC (Table
2) for 13-15 are underestimated by a factor that ranges from
1.3 to 1.9; the systematic deviation in the measurements is larger
the more compact the dendrimer is. It is remarkable that 13
and 14 can be distinguished so clearly by GPC and that the
difference in their hydrodynamic volumes is so distinct. These
We performed powder X-ray diffraction upon “as obtained”
samples of dendrimer 13; the material was amorphous. Accord-
ing to differential scanning calorimetry (DSC), 13 undergoes
an irreversible exothermic phase transition at 319 °C (22.3 kcal
mol-1). No melting is observed up to 350 °C. Upon cooling,
no endotherm is recorded in the DSC. Bulk samples of 13 heated
to >320 °C are not amorphous but show a distinct but not
interpretable powder XRD pattern. When the annealed material
is dissolved in dichloromethane and the solvent is removed, the
amorphous form of 13 is obtained again. Compared to the heat
of fusion of benzene (2.53 kcal mol-1), this seemingly large
exotherm might be explained by the (possibly cooperative)
ordering of the 24 benzene rings of the dendrimer 13 in the
solid state. The meta-dendrimer 14 is microcrystalline at room
temperature and shows two endothermic transitions at 291 (13.8
kcal mol-1) and 331 °C (2.4 kcal mol-1, melting). Its powder
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Waybright et al.
Figure 5. GPC chromatogram of complexes 13, 14, and 15.
results corroborate observations by Shimizu15 and McGrath16
who demonstrated that more extended oligomers/dendrimers
have significantly greater hydrodynamic volumes than less
extended ones.
Pd-catalyzed couplings, cobalt-mediated [2 + 2] cycloadditions,
and Diels-Alder chemistry allows the synthesis of 13-15. The
pinnacle of the synthetic effort is the high selectivity of the
cyclobutadiene formation of 2b, 4b, and 8b; CpCo(CO)2 leaves
TIPS-substituted alkynes untouched, and only the sterically less
encumbered diarylalkyne units dimerize into the desired cy-
clobutadiene complexes. Desilylation and reaction with tet-
raphenylcyclopentadienone leads to organometallic dendrimers
13-15 with a cyclobutadiene(cyclopentadienyl)cobalt core.
With the facile chemistry developed here, it should be possible
to assemble larger nanoscale organometallic objects entirely
composed of cyclobutadiene complexes and phenyl rings.
Particularly, the functional complex 9 is an excellent starting
point because it is available in multigram quantities. These
organometallic dendrimers will show novel nanoscale structural
and electrochemical properties.
Model studies of encapsulated transition-metal complexes
suggest17 that the local surrounding of a metal significantly
influences its electron-transfer properties and oxidation potential.
The CpCo-ligated cyclobutadiene complexes 13-15 are elec-
troactive18 and undergo oxidation into their 17e cations. We
performed scanning pulse voltammetry upon the complexes 13-
15 and discovered a small but significant variation in their
oxidation potential. While 13 is oxidized at 0.80 V in a
differential pulse voltammetry experiment (ferrocene standard,
nitrobenzene solvent), its meta-isomer 14 is oxidized at 0.82
V, and the larger crowded dendrimer 15 oxidizes at 0.83 V.
These results suggest that oxidation is slightly more difficult in
the more compact, enshrouded dendrimers where the benzene
rings act as an insulating cover. The effects are not dramatic in
solution, and in future we plan to investigate thin films of 13-
15 electrochemically.
Acknowledgment. We thank the University of South Caro-
lina, the USC Nanocenter, and the National Science Foundation
for support (CAREER CHE 9981765) of this project and Dr.
K. Stitzer and Prof. H.-C. zur Loye for help with the powder
diffraction experiments. U.H.F.B. is Camille and Henry Dreyfus
Teacher Scholar.
Conclusions
An access to novel organometallic dendrimers with cyclo-
butadiene cores has been developed. The combination of
(15) Choi, D. S.; Chong, Y. S.; Whitehead, D.; Shimizu, K. D. Org. Lett. 2001,
3, 3757.
(16) Li, S.; McGrath, D. V. J. Am. Chem. Soc. 2000, 122, 6795.
(17) (a) Dandliker, P. J.; Diederich, F.; Gross, M.; Knobler, C. B.; Louati, A.;
Sanford, E. M. Angew. Chem. 1994, 33, 1739. Dandliker, P. J.; Diederich,
F.; Giselbrecht, J.-P.; Gross, M. Angew. Chem. 1995, 34, 2725. Dandliker,
P. J.; Diedrich, F.; Zing, A.; Gisselbrecht, J.-P.; Gross, M.; Louati, A.;
Sanford, E. HelV. Chim. Acta 1997, 80, 1773. (b) Cameron, C. S.; Gorman
C. B. AdV. Mater. 2002, 12, 17.
(18) Geiger, W. E.; Kelly, R. S. Organometallics 1987, 6, 1432. The oxidation
potential of (tetraphenylcyclobutadiene)cyclopentadienylcobalt is +0.90 V
in dichloromethane and +1.07 V in THF vs calomel.
Supporting Information Available: Experimental procedures,
detailed spectroscopic characterization, and crystallographic
information files (CIF) are available free of charge via the
Internet at http://pubs.acs.org.
JA026462P
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