Sensing of chiral fullerenes by a cyclic host with an asymmetrically distorted π-electronic component

Article abstract of DOI:10.1021/ja063828f

Upon inclusion complexation with a racemic mixture of chiral fullerene C₇₆ ((±)-C₇₆), a novel cyclic host (1_Rh) composed of a chiral N-methylporphyrin (P_NMe) and a methylrhodium porphyrin (P_Rh) exhibi

Full text of DOI:10.1021/ja063828f

Published on Web 07/28/2006
Sensing of Chiral Fullerenes by a Cyclic Host with an Asymmetrically
Distorted π-Electronic Component
Yoshiaki Shoji, Kentaro Tashiro,* and Takuzo Aida*
Department of Chemistry and Biotechnology, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan
Received May 31, 2006; E-mail: aida@macro.t.u-tokyo.ac.jp; tashiro@macro.t.u-tokyo.ac.jp
Design of host molecules for recognizing asymmetrically dis-
torted π-electronic surfaces of carbon nanoclusters is one of the
most challenging issues in relation to the discrimination of metallic
and semiconducting carbon nanotubes.1 Unlike ordinary asymmetric
compounds, carbon nanoclusters are devoid of any appropriate
functionalities for point recognition so that their chirality sensing
certainly requires a different strategy. Here we report a novel
heterocyclic porphyrin dimer (1Rh, Chart 1), containing an asym-
metrically distorted N-alkylporphyrin as the first host molecule
capable of sensing chiral fullerene C76 by means of 1H NMR. Chiral
2
fullerenes such as C76, C78, and C84 are all-carbon spheroidal
clusters, and designing of sensing hosts for these discrete chiral
objects would give a new rationale for the recognition of asym-
metrically curved π-electronic surfaces.
Figure 1. CD spectra of the enantiomers of (a) 1Rh ((-)-1Rh and (+)-1Rh
)
and (b) 12H ((-)-12H and (+)-12H) in toluene at 20 °C.
Newly designed cyclic host 1Rh bears a C1-symmetric N-
methyldiarylporphyrin (PNMe) moiety on the other side of a
methylrhodium diarylporphyrin (PRh) unit. N-Alkylporphyrins are
nonplanar because porphyrins cannot sterically accommodate
N-alkyl groups in their nitrogen core. Thus, as reported previously,3
N-alkylporphyrins are chiral depending on the arrangement of the
peripheral substituents, and their enantiomers are separable by
means of chiral HPLC. We chose PRh as the counterpart of this
chiral porphyrin, because cyclic dimers of methylrhodium porphy-
rins can accommodate fullerenes to form highly stable inclusion
complexes.4b,c,f The molecular design of 1Rh was based on an
expectation that the asymmetrically distorted π-electronic surface
of PNMe could fit those of chiral fullerenes trapped in the
Chart 1
P
Rh-containing host cavity. A racemic mixture of 1Rh ((()-1Rh)
was obtained by alkaline-mediated coupling of a N-methylporphyrin
bearing bromoalkyl pendants with a methylrhodium porphyrin
having phenolic functionalities and unambiguously characterized
to one another, were obtained, respectively. Similar to cyclic dimers
of other porphyrins, (()-1Rh alone in toluene-d8 at 20 °C showed
multiple 1H NMR signals due to, for example, the N-Me (δ ) -4.61
to -4.95 ppm) and RhMe (δ ) -6.07 to -6.54 ppm) groups
because of the presence of conformational isomers.4,5 However,
upon mixing (()-1Rh with an equimolar amount of (()-C76, the
1H NMR spectrum became much simplified, displaying single
signals due to the N-Me and RhMe groups at downfield regions, δ
) -4.57 and -5.82 ppm, respectively.5 Molecular models predicted
that better fitting of the π-electronic cavity of 1Rh to C76 requires
unfavorable conformational isomers of 1Rh to rotate PNMe and PRh
to point their N-Me and RhMe groups outward. At a closer look,
one may notice that the NH proton of PNMe shows two split signals
at δ ) -2.76 and -2.79 ppm (Figure 2a). When (+)-C76 was
allowed to complex with (-)-1Rh and (+)-1Rh under identical
conditions to the above, either of the two NH signals was observed.
Hence, the characteristic signals at δ -2.76 and -2.79 ppm are
assignable to diastereoisomers (-)-1Rh⊃(+)-C76/(+)-1Rh⊃(-)-C76
and (+)-1Rh⊃(+)-C76/(-)-1Rh⊃(-)-C76, respectively. We confirmed
that the integral ratio of these two signals remains unity, even upon
increment of the guest/host ratio from 1 up to 4.5 Therefore, chiral
1
by MALDI-TOF-MS and H NMR analyses.5 Optical resolution
of 1Rh was achieved by preparative chiral HPLC on CHIRALPAK
IA with hexane/CH2Cl2/Et2NH (87.5/12.5/0.1, v/v/v) as an eluent,5
where the two well-separated fractions showed mirror-image
circular dichroism (CD) spectra of one another (Figure 1a). The
first and second fractions displayed the most intensive CD bands
at 420 nm, with negative and positive signs, respectively, and are
denoted as (-)-1Rh and (+)-1Rh. As a metal-free reference of 1Rh
,
we synthesized 12H (Chart 1) and successfully separated its
enantiomers (-)-12H and (+)-12H by preparative chiral HPLC
(Figure 1b).5 As the guest fullerene, C76 was chosen for simplicity
because other chiral fullerenes, unlike C76, possess more than two
geometrical isomers.2
Host 1Rh can include C76 in its cavity. Spectroscopic titration of
(()-1Rh with (()-C76 in toluene at 20 °C resulted in a decrease in
absorbances at 423 and 406 nm due to PNMe and PRh, respectively,5
indicating that the two porphyrin moieties in 1Rh both interact with
C76. From the absorbance changes at 423 and 406 nm, association
constants (Kassoc) of 1.5 × 107 and 1.4 × 107 M-1, virtually identical
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10.1021/ja063828f CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. 1H NMR (500 MHz) spectra (NH signals of PNMe) of equimolar
mixtures of (+)-host/(+)-C76 (blue curves), (-)-host/(+)-C76 (red curves),
and (()-host/(()-C76 (black curves) in toluene-d8 at 20 °C. Host ) (a) 1Rh
and (b) 12H. [(+)-C76] ) [(()-C76]/2 ) 7.7 × 10-5 M.
Figure 3. (a) 1H NMR (500 MHz) spectra (NH signals of PNMe) of
equimolar mixtures of (-)-1Rh and C76 of different ∆ꢀ values ([C76] ) 1.5
× 10-4 M) in toluene-d8 at 20 °C. (b) Plots of enantiomeric purities of C76,
as determined by the integral ratio of the NH signals of (-)-1Rh⊃C76, versus
∆ꢀ values of C76 in toluene at 20 °C.
cyclic host 1Rh is nonenantioselective in the guest binding but can
spectroscopically discriminate the enantiomers of C76.
In conclusion, by using a C1-symmetric N-methylporphyrin as
an asymmetrically distorted π-electronic component, we have
developed the first chiral host 1Rh capable of discriminating the
enantiomers of C76 by means of 1H NMR and also determining its
enantiomeric purity by visualization of the relative enantiomeric
abundance. By virtue of this direct method, an accurate ∆ꢀ value
of enantiomerically pure C76 has been obtained. Elaboration of the
obtained rationale into the molecular design of host molecules for
enantiomeric separation of chiral fullerenes and larger carbon
nanoclusters is one of the subjects worthy of further investigation.
The PRh unit as well as the chiral PNMe moiety of 1Rh contributes
to the chiral discrimination of C76. Metal-free reference 12H, upon
titration with C76 in toluene at 20 °C, displayed an absorption
spectral change similar to that observed for 1Rh, where Kassoc
obtained for 12H⊃C76 (2.5 × 106 M-1) was an order of magnitude
smaller than that for 1Rh⊃C76.5 While the inclusion complexation
of (()-12H with (()-C76 in toluene-d8 at 20 °C showed an analogous
1H NMR spectral change profile to that of (()-1Rh with (()-C76,
none of the characteristic signals due to the N-Me and NH groups
of PNMe displayed diastereoisomeric splitting (Figure 2b).5 We then
lower the temperature for the measurement down to -20 °C.
Although the NH signal of PNMe remained single, the N-Me signal
started to split at -10 °C into two signals.5 Hence, metal-free 12H
is not as practical as 1Rh for the chirality sensing of C76. This is
due to the coalescence of diastereoisomerically split signals caused
by the dynamics in guest exchange of (-)-C76 with (+)-C76, because
the combination of enantiomerically pure 12H and C76, that is, (-)-
12H⊃(+)-C76 and (+)-12H⊃(+)-C76, gave signals with different
chemical shifts both for the N-Me and for the NH groups at 20 °C
Acknowledgment. This work was supported by Industrial
Technology Research Grant Program in ‘04 from New Energy and
Industrial Technology Development Organization (NEDO) of Japan.
Y. S. thanks the JSPS Young Scientist Fellowship.
Supporting Information Available: Synthesis and optical resolu-
tion of 1Rh, 12H, and C76 and analytical data of mixtures of 1Rh or 12H
with C76. This material is available free of charge via the Internet at
http://pubs.acs.org.
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