Preparation of star-shaped pentapyridyl ligands for the formation of giant fullerene-like molecules by coordination chemistry

Article abstract of DOI:10.2533/chimia.2007.184

We present the syntheses of two new star-shaped penta-ligands containing five pyridyls, which may be considered as starting materials for the preparation of fullerene-like molecules by coordination chemistry. One synthetic strategy is similar to that used

Full text of DOI:10.2533/chimia.2007.184

LAUREATES: AWARDS AND HONORS, SCS FALL MEETING 2006
184
doi:10.2533/chimia.2007.184
CHIMIA 2007, 61, No. 4
Chimia 61 (2007) 184–185
© Schweizerische Chemische Gesellschaft
ISSN 0009–4293
Preparation of Star-Shaped Pentapyridyl
Ligands for the Formation of Giant
Fullerene-Like Molecules by Coordination
Chemistry
Thibaut Jarrosson§*, Olivier Oms, Gérald Bernardinelli, and Alan F. Williams
§SCS Poster Prize Winner
Abstract: We present the syntheses of two new star-shaped penta-ligands containing five pyridyls, which may be
considered as starting materials for the preparation of fullerene-like molecules by coordination chemistry. One
synthetic strategy is similar to that used for pentaphenylferrocenyl di-tert-butylphosphine. The penta-4-pyridyl
derivative was prepared in two steps from ferrocene. A second strategy involving a Suzuki cross-coupling reaction
with a penta-bromoaryl scorpionate is presented.
Keywords: Coordination chemistry · Ligand synthesis
1. Introduction
One of the major objectives in supramo-
lecular chemistry is the preparation, the
design and the synthesis of molecular
spheroids.[1,2] After the first preparation of
a molecular container (carcerand) by Cram
in 1985, a variety of molecular cages have
been prepared through supramolecular self-
assembly using multiple hydrogen bonds or
metal–ligand coordination.
The basic problem in generating a sphe-
Scheme 1.
roidal structure is to induce a curvature of
the surface and to avoid the formation of an
extended structure in two dimensions. One
approach is to use the well-known fact that
one cannot assemble pentagons into a pla-
ene C60 which may be regarded as twelve copy and showed the presence of the tetra
pentagons linked by the vertices (Scheme and penta-adducts (Fig. 2). The separa-
1).
tion of the two products was therefore
Recently, fullerene-like nanoballs were envisaged, but the penta-adduct proved to
nar structure. Thus if one of the hexagons
of a regular hexagonal net is transformed
into a pentagon, the sheet will buckle. This
fact was used in the construction of Buck-
minster Fuller’s geodesic domes, and is the
basis of the structure of buckminsterfuller-
synthesized from pentaphospha-ferrocene be highly unstable towards oxygen form-
and copper derivatives.[1] The structures are
formed by linking cyclo-P5 rings (pentago- suggested that the ligand should be sta-
ing penta(4-pyridyl)cyclopentadienol. This
nal units) by coordination of copper ions. bilized by complexation of a transition
This provides the formation of five- and metal to reduce the electron density, but
six- membered rings in a similar manner to several attempts starting from FeCl2 and
that observed in C60.
CpNa (or CpFe(CO)2I and t-BuOK) failed
to give the desired pentapyridyl ferrocene.
In the last case only the tetrapyridyl fer-
rocene could be isolated.
2. Synthesis of Pentapyridyl
Cyclopentadiene
*Correspondence: Dr. J. Jarrosson
Department of Inorganic,
Analytical and Applied Chemistry
University of Geneva
30 Quai Ansermet
CH-1211 Geneva 4
Tel.: +41 22 379 64 32
Fax: +41 22 379 68 30
E-Mail: thibaut.jarrosson@chiam.unige.ch
We initially attempted to synthesise 3. Synthesis of the Pentapyridyl
free
penta(4-pyridyl)cyclopentadiene Ferrocene Derivative 3b
(Scheme 2) using the procedure of M.
Nomura et al.[3] (Fig. 1). The reaction was
Hartwig et al. have reported the effi-
followed by electrospray mass spectros- cient incorporation of five phenyl groups

LAUREATES: AWARDS AND HONORS, SCS FALL MEETING 2006
185
CHIMIA 2007, 61, No. 4
4. Synthesis of the Star-shaped
Ligand 5
examined the MM3 minimized structures of
the predicted systems as shown in Scheme
3. In the presence of Ag+ or Cu+, ligand 2b
should form a nano-sphere with a diameter
Another approach consisting of adding
the pyridyl moieties to a scorpionate deriva-
tive was also considered. In the literature,
˚
of around 32A. With the extended penta py-
ridyl ligand 4, the diameter of the assembly
the preparation of a penta(4-bromophenyl)-
˚
would be even larger (52 A).
Scheme 2.
cyclopentadiene ruthenium scorpionate 4
Received: December 18, 2006
was described by G. Rapenne et al.[5] After
starting from ferrocenyl di-tert-butylphos-
reproducing the synthesis with slight modi-
fication (Fig. 4), the coupling reaction of
the pyridyl boronic acid ester under Suzuki
conditions produced the penta-aryl system
5 in good yield after purification on silica
gel (CH2Cl2/MeOH 90:10).
[1] J. Bai, A. V. Virovets, M. Scheer, Science
2003, 300, 781–783.
[2] M. Scheer, J. Bai, B. P. Johnson, R. Merk-
le, A. V. Virovets, C. E. Anson, Eur. J. In-
org. Chem. 2005, 4023–4026.
[3] G. Dyker, J. Heiermann, M. Miura, J.-I.
Inoh, S. Pivsa-Art, T. Satoh, M. Nomura,
Chem. Eur. J. 2000, 6, 3426–3433.
[4] N. Kataoka, Q. Shelby, J. P. Stambuli, J. F.
Hartwig, J. Org. Chem. 2002, 67, 5553–
5566.
[5] A. Carella, J. Jaud, G. Rapenne, J.-P. Lau-
nay, Chem. Commun. 2003, 2434–2435.
phine (FcP(t-Bu)2)[4] and we investigated
the possibility of introducing the five py-
ridyl groups using this methodology. The
synthesis consists in a palladium-catalysed
coupling reaction between FcP(t-Bu)2 and a
4-pyridyl halide in the presence of t-BuOK
in toluene at reflux. In the first attempts,
Pd(OAc)2 was used as catalyst and 4-py-
5. Modelling Studies
ridyl chloride as reactant (Fig. 3). Tetra-4-
pyridylferrocenyl di-tert-butylphosphine
While coordination studies are currently
under investigation and have given promis-
ing results for ligand 3b with Cu+, we have
was the major product, and incorporation of
the fifth cyclopentadienyl moiety was not
observed. When five equivalents of triphe-
nylphosphine per mol of Pd(OAc)2 were
added to the solution, the (PPh3)2Pd(OAc)2
complex is formed and gave the forma-
tion of the penta-4-pyridylferrocenyl di-
tert-butylphosphine 3a which was now the
major product of the reaction. However,
even if 4-pyridyl chloride is used in large
excess, a minor quantity of tetra-adduct is
still detected. In order to avoid competition
between the phosphine group and the py-
ridine moieties in metal ion coordination,
the phosphine oxide derivative 3b was pre-
pared using 4-methylmorpholine N-oxide
monohydrate as the oxidizing reagent in
75% yield.
Cl
N
Ar
Pd(OAc) 2
PPh3
Ar
Ar
Ar
Ar
Fe
t- BuOK
Fe
-
BF4
Toluene
110 °C
PH(t -Bu )2
R
3 a : R = P(t -Bu )2
O-
O
N
Ar = 4-pyridyl
3 b : R = PO(t -Bu )2
Fig. 3. Synthesis of the pentapyridyl ferrocene derivative 3b
Br
R
Br
R
Ar
Br
Fe
I
Br
Ar
Ar
Ar
Ar
Ar
Ar
Br
Br
Br
OC
Br
CO
Ar
Ar
Cs2CO3
i
Br
ii, iii
Na+
Ar
Ar Br
+
Br
iv
Ru
N
Ru
N
Ar
Fe
N
N
N
Pd(OAc) 2
P( t- Bu) 3
N
N
t- BuOK
N
B
H
N
B
H
N
N
N
Ar
Ar
THF
Br
Br
66 °C
2a, R = H
2b R = Ar
N
4
5
Ar =
1a, R = H
1b, R = Ar
Ar = 3 or 4-Pyridyl
i) B r2, 2 5 °C, 94 % , ii) R u 3(CO)12, Tolu ene, 110 ° C, 7 5 %, iii) S odiu m tris(1-pyra zolyl)b o rohyd ride, THF, 66 ° C, 3 8 %,
iv) (PPh 3)2P d C l2, PP h 3, K2CO3, 4-(4, 4 , 5 , 5-Te tra met h yl-1, 3 , 2 -dioxabo rola n-2 -yl)pyridine, Tolu ene/ E t OH/ H 2O (10 : 4 : 3 ),
1 00 ° C, 48 %.
Fig. 1. The synthesis of pentapyridyl ferrocene 2b
Fig. 4. Synthesis of the star-shaped ligand 5
Fig.2.TypicalES-MSspectrumofthereactionmixtureduringthepreparation
Scheme 3.
of the pentapyridyl cyclopentadiene 1b