Chiral bead-like trimer of tris(2,4-pentanedionato)ruthenium(III)

Article abstract of DOI:10.1246/cl.2008.716

A chiral tri-nuclear metal complex, ΔΔΔ- or ΛΛΛ-[Ru(acac)₂(taet)Ru(acac)(taet)Ru(acac) ₂] (acac = acetylacetonato and taet = tetraacetylethanato), was prepared and its monolayer behavior was investigated, leading to the conclusion that the bead-like character of the trimer was an essential factor in achieving stable two-dimensional molecular packing. Copyright

Full text of DOI:10.1246/cl.2008.716

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Chemistry Letters Vol.37, No.7 (2008)
Chiral Bead-like Trimer of Tris(2,4-pentanedionato)ruthenium(III)
1
;2
1
3
4
ꢀ3
Hisako Sato, Jun Kameda, Yutaka Fukuda, Masa-aki Haga, and Akihiko Yamagishi
Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
1
2
PRESTO, Japan Science and Technology Agency, Chiba
Ochanomizu University, 2-1-1 Ohtsuka, Bunkyo-ku, Tokyo 112-8610
3
4
Chuo University, 1-3-27 Kasuga, Bunkyo-ku, Tokyo 112-8551
(Received April 3, 2008; CL-080338; E-mail: yamagishi.akihiko@ocha.ac.jp)
A chiral tri-nuclear metal complex, ꢀꢀꢀ- or ꢁꢁꢁ-
Ru(acac)2(taet)Ru(acac)(taet)Ru(acac)2] (acac = acetylaceto-
[
nato and taet = tetraacetylethanato), was prepared and its
monolayer behavior was investigated, leading to the conclusion
that the bead-like character of the trimer was an essential factor
in achieving stable two-dimensional molecular packing.
Recently, molecular arrays of conjugated organic com-
pounds have attracted extensive attention as possible candidates
1
,2
for conducting wires. The wire-like character of molecular
assemblies is expected to amplify an electrical signal for sens-
3
,4
ing. When such an attempt is extended to metal complexes,
it may open the possibility of coupling the array character with
the electronic or magnetic properties intrinsic to metal ions.
4
–9
Figure 1. CD spectra of methanol solutions of resolved
enantiomers: ꢁꢁꢁ- (solid) and ꢀꢀꢀ-[Ru(acac)2(taet)-
Ru(acac)(taet)Ru(acac)2] (dotted). The vertical axis denotes
In the present work, we report the preparation of a chiral
trimer linked by tetraketonato groups as the bridging ligands
and investigate its monolayer behavior in a Langmuir–Blodgett
ꢀ" per monomer unit.
1
0
(
LB) film. Instead of using an amphiphilic ligand, the present
work employed a bead-shaped multi-nuclear complex. It was
expected that the bead-like properties might help the molecule
align at an air–water interface similar to an amphiphilic
molecule. For this purpose, multi-nuclear complexes were
synthesized by one-pot reactions between a metal complex
monomer and a linking ligand in a solid matrix. It has been dem-
onstrated that the trimer of tris(acetylacetonato)ruthenium(III)
thus prepared forms a reversible monolayer at an air–water
interface. Notably no such stable monolayer forms for a mono-
mer or a dimer. Thus, it was concluded that the bead-like
character was essential to the formation of stable two-dimen-
sional molecular packing. A molecular film consisting of
such bead-shaped molecules might be a first step for forming
molecular wires on a solid surface.
Scheme 1. The molecular structure of a tri-nuclear metal com-
plex (ꢀꢀꢀ-[Ru(acac)(taet)Ru(acac)(taet)Ru(acac)2]).
phenanthroline) and synthetic hectorite.11 The eluting solvent
was 4:1 (v/v) methanol–chloroform. The monomer and dimer
were resolved into pure enantiomers with baseline separation.
The trimer was resolved partially into two enantiomers. The
circular dichroism spectra of the initial and final fractions
are shown in Figure 1. From the spectral shapes and ꢀ", these
fractions were concluded to contain ꢁꢁꢁ- and ꢀꢀꢀ-trimers,
respectively (see Supporting Information) (Scheme 1).12
The crude product obtained after the reaction of [Ru(acac)3]
acac = acetylacetonato) and tetraacetylethane (taetH2) was
(
eluted on a silica gel column with acetonitrile–benzene. As a re-
sult, oligomers up to the tetramer were separated as confirmed
from mass spectral analyses. The fractions corresponding to
the dimer and the trimer were eluted again on a silica gel col-
umn. The dimer fraction was separated into two bands contain-
ing ꢀꢀ=ꢁꢁ and ꢀꢁ (meso) diastereomers as already described
Monolayer behavior of the monomer, the dimer, and the
trimer was investigated at an air–water interface. In the case of
the monomer, a chloroform solution of ꢀ-[Ru(acac)3] was
spread onto pure water. No surface pressure appeared upon com-
7
elsewhere. The trimer fraction was separated into three bands.
From the NMR spectra of these bands, the last band was found
to contain a pure trimer of C2 symmetry (see Supporting
Information).12 Thus, the species in that band was one of the
ꢀꢀꢀ=ꢁꢁꢁ or ꢀꢁꢀ=ꢁꢀꢁ diastereomers. Resolution of the
racemic mixtures of the monomer, the dimer, and the C2-sym-
2
ꢁ1
pressing the surface to less than 0.1 nm molecule . The same
results were obtained for ꢁ-[Ru(acac)3]. These results indicated
that the enantiomeric monomer dissolved in water before the
compression of the surface. This was reasonable since the mono-
ꢂ
metric trimer was carried out on a chiral column packed with
þ
mer dissolved in water to the concentration of 0.4 mM at 20 C.
In the previous experiment, a stable monolayer was formed only
an ion-exchange adduct of ꢀ-[Ru(phen)3]2 (phen = 1,10-
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.7 (2008)
717
(
a)
Figure 2. The ꢀ–A curves when a racemic mixture (solid line)
or a pure enantiomer (dotted and broken line) of trimers was
spread onto the subphase of pure water. The horizontal axis is
the area per monomer unit.
(b)
Scheme 2. Models of molecular packing of racemic (a) and
enantiomeric (b) trimers.
when the acac ligand was made more hydrophobic by replacing
1
of molecular area as seen from their ꢀ–A curves (Figure 2).
Thus, the effect of chirality was manifested in terms of different
molecular orientations in a two-dimensional arrangement. The
possible orientations of the molecule are shown in Scheme 2.
Under these orientations, the area occupied by one monomer unit
0
the methyl groups with longer alkyl chains.
In the case of the dimer, a chloroform solution of ꢀꢀ-
Ru(acac)2(taet)Ru(acac)2] was spread onto a water phase.
Surface pressure appeared upon compressing the surface to
[
2
ꢁ1
2
0.1 nm molecule . The magnitude of the surface pressure
was, however, lacking in reproducibility. It fluctuated between
was estimated to be 0.88 and 0.72 nm for racemic and enantio-
meric trimers, respectively. These values are close to the area
2
ꢁ1
2
1
[
0 and 20 nm molecule . Since the solubility of ꢀꢀ-
ꢁ
per monomer unit (0.6–0.8 nm ) when surface pressure began
to increase from zero in the ꢀ–A curves.
6
Ru(acac)2(taet)Ru(acac)2] in water was less than 1 ꢃ 10
M
ꢂ
at 20 C, the dissolution of the dimer was thought to be
negligible. This suggested that the dimer aggregated to form
a solid particle at the air–water interface instead of forming a
stable molecular film. The same results were obtained for
The present work has demonstrated that the chiral bead-like
metal complex exhibits a chirality effect on molecular packing
in films. This finding may suggest that chirality can be a useful
factor in controlling the molecular arrangement in films.
ꢁ
ꢁ-[Ru(acac)2(taet)Ru(acac)2]. In the case of the trimer, a
chloroform solution of ꢀꢀꢀ-[Ru(acac)2(taet)Ru(acac)(taet)-
Ru(acac)2] was spread onto a water phase. A clear ꢀ–A curve
was obtained as shown in Figure 2, in which the horizontal axis
denotes the area per monomer unit. According to the results, sur-
We thank Dr. Kentaro Tanaka and Mr. Jun Yoshida (The
University of Tokyo) for mass spectrometry experiments. We
thank Mr. Masanori Tsunoda and Mr. Hiroshi Mineo (Chuo
University) for their synthetic experiments.
2
face pressure appeared around 0.7 nm and rose steeply below
2
the region of 0.5 nm . The curve showed a saturating tendency
2
References and Notes
below 0.3 nm . Nearly the same curve was obtained for ꢁꢁꢁ-
1
2
3
T. E. Mallouk, J. A. Gavin, Acc. Chem. Res. 1998, 31, 209.
T. M. Swager, Acc. Chem. Res. 1998, 31, 201.
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Chem. Soc. 2005, 127, 4548.
[
racemic mixture of the trimer, surface pressure had already
Ru(acac)2(taet)Ru(acac)(taet)Ru(acac)2]. In the case of the
2
2
appeared at 1.0 nm . The pressure rose steeply around 0.5 nm ,
remaining higher than that for the enantiomeric trimer across
the entire region. Comparing the ꢀ–A curves between the
enantiomer and the racemic mixture, it was deduced that the
racemic mixture formed a more rigid molecular film within the
same molecular area.
4
5
6
7
H. Sato, A. Yamagishi, J. Photochem. Photobiol., C 2007,
8, 67.
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A floating monolayer of the trimer was deposited onto mica
2
at an area of 0.40 nm per monomer unit. The transfer ratio was
0
:6 ꢄ 0:1 for both enantiomeric and racemic trimers in the up-
ward direction. The low value of the transfer ratio arose from
the fact that the monolayer was hydrophobic and was repelled
from the hydrophilic mica surface. The surface of the deposited
films was observed with an AFM microscope (see Supporting
Information).12 From the AFM observations, the racemic mole-
cules were concluded to be more upright from the substrate
surface than the enantiomeric molecules. This was in accord
with the situation that the racemic monolayer exhibited higher
surface pressure than the enantiomeric one at the same value
8
9
S. Kishi, M. Kato, Inorg. Chem. 2003, 42, 8728.
H. Sato, Y. Furuno, Y. Fukuda, K. Okamoto, A. Yamagishi,
Dalton Trans. 2008, 1283.
10 A. Yamagishi, N. Sasa, M. Taniguchi, Langmuir 1997, 13,
1689.
11 A. Yamagishi, J. Coord. Chem. 1987, 16, 131.
12 Supporting Information is available elecronically on the
CSJ-Journal Web site, http://www.esj.jp/journals/chem-lett/.
Published on the web (Advance View) May 31, 2008; doi:10.1246/cl.2008.716