Synthesis and properties of fullerene (C70) complexes of 2,6-bis(porphyrin)-substituted pyrazine derivatives bound to a Pd(II) ion

Article abstract of DOI:10.1080/10610278.2010.487566

2,6-Bis(porphyrin)-substituted 3,5-dimethylpyrazine and its zinc complex bound C₇₀ to yield 1:1 inclusion complexes, which were characterised by ESI-MS, UV-vis, fluorescence and NMR spectroscopies. Association constants of the C₇₀ complexes were determined by fluorescence and NMR spectral analyses. A decrease in absorbance of the Soret band of the pyrazine derivative by the effect of C₇₀ was observed, suggesting the existence of a charge transfer interaction between C₇₀ and porphyrin. Experimentally reliable values for the association constants were obtained by the NMR method and were about six times larger than those of the corresponding C60 complexes. Palladium complexation of the porphyrin-pyrazine ligand was found to enhance the association with fullerene. The association constant of 2,6-bis(porphyrin-Zn)- substituted 3,5-dimethylpyrazine-Pd(II) complex with C₇₀ was determined to be 8400±900M^-1. From the comparison of the association constants, it was found that inclusion room for C₇₀ in the Pd(II) complex was maintained, juxtaposed between porphyrins attached to the opposite sides of the pyrazine ligands.

Full text of DOI:10.1080/10610278.2010.487566

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Synthesis and properties of fullerene (C70) complexes
of 2,6-bis(porphyrin)-substituted pyrazine derivatives
bound to a Pd(II) ion
Yusaku Eda a , Kennosuke Itoh a , Yoshio N. Ito a , Mamoru Fujitsuka b , Tetsuro Majima b
Toshio Kawato a
&
a Department of Chemistry, Faculty of Sciences , Kyushu University , 6-10-1 Hakozaki,
Higashi-ku, Fukuoka, 812-8581, Japan
b The Institute of Scientific and Industrial Research (SANKEN), Osaka University , 8-1
Mihogaoka, Ibaraki, Osaka, 567-0047, Japan
Published online: 09 Jun 2010.
To cite this article: Yusaku Eda , Kennosuke Itoh , Yoshio N. Ito , Mamoru Fujitsuka , Tetsuro Majima & Toshio Kawato (2010)
Synthesis and properties of fullerene (C70) complexes of 2,6-bis(porphyrin)-substituted pyrazine derivatives bound to a Pd(II)
ion, Supramolecular Chemistry, 22:9, 517-523, DOI: 10.1080/10610278.2010.487566
To link to this article: http://dx.doi.org/10.1080/10610278.2010.487566
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Supramolecular Chemistry
Vol. 22, No. 9, September 2010, 517–523
Synthesis and properties of fullerene (C70) complexes of 2,6-bis(porphyrin)-substituted pyrazine
derivatives bound to a Pd(II) ion
Yusaku Edaa, Kennosuke Itoha, Yoshio N. Itoa, Mamoru Fujitsukab, Tetsuro Majimab and Toshio Kawatoa*
aDepartment of Chemistry, Faculty of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan;
bThe Institute of Scientific and Industrial Research (SANKEN), Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
(Received 12 November 2009; final version received 18 April 2010)
2,6-Bis(porphyrin)-substituted 3,5-dimethylpyrazine and its zinc complex bound C70 to yield 1:1 inclusion complexes,
which were characterised by ESI-MS, UV–vis, fluorescence and NMR spectroscopies. Association constants of the C70
complexes were determined by fluorescence and NMR spectral analyses. A decrease in absorbance of the Soret band of the
pyrazine derivative by the effect of C70 was observed, suggesting the existence of a charge transfer interaction between C70
and porphyrin. Experimentally reliable values for the association constants were obtained by the NMR method and were
about six times larger than those of the corresponding C60 complexes. Palladium complexation of the porphyrin–pyrazine
ligand was found to enhance the association with fullerene. The association constant of 2,6-bis(porphyrin-Zn)-substituted
3,5-dimethylpyrazine-Pd(II) complex with C70 was determined to be 8400 ^ 900 M21. From the comparison of the
association constants, it was found that inclusion room for C70 in the Pd(II) complex was maintained, juxtaposed between
porphyrins attached to the opposite sides of the pyrazine ligands.
Keywords: fullerene; porphyrin; pyrazine; palladium(II) complex; association constant
Introduction
the formation of a 1:1 complex of 1 and C70 (Figure 1). For
the sake of comparing C70 with C60 as a guest for 1 and 2,
binding of C70 to 1 and 2 was then studied by the same
methods as in the literature for C60 (4, 8). The fluorescence
spectrum of 1 in toluene (3.27 mM) exhibited a maximum
at 650 nm upon excitation at 550 nm. The fluorescence due
to porphyrin upon excitation at the Q-absorption band was
diminished by the addition of C70 in toluene (Figure S1,
available online). The Stern–Volmer plot for the
fluorescence titration of 1 with C70 was found to be a
straight line by plotting F0/F vs. [C70], where F0 and F are
the fluorescence intensities at 650 nm of 1 without and
with C70, respectively (Figure S2, available online). From
the line-fitting analysis, the association constant Ka was
calculated to be 17550 ^ 170 M21. Analogous fluor-
escence quenching was observed for 2 with C70 in toluene
(Figure S3, available online). However, the Stern–Volmer
plot at 596 nm for the fluorescence titration of 2 with C70
was found to curve upward. Therefore, the association
constant Ka for 2 and C70 was determined by the Benesi–
Once the significant properties of fullerene as an electron
acceptor were recognised (1), many research attempts
focused on the design and construction of host systems
capable of capturing fullerene as a guest (2–7). In a
previous paper, we reported the synthesis of 2,6-
bis(porphyrin)-substituted pyrazine which bound C60 to
form a 1:1 inclusion complex (8). Now we attempt to
construct a supramolecular system by the aid of Pd(II)
complexation with the pyrazine ligands. In order to
develop a better understanding of such a fullerene-
inclusion system, we have extended our investigation
using C70 to show that the pyrazine ligand can embrace a
proper fullerene between the porphyrins to form a stable
inclusion complex. In this article, we will discuss structure
and binding constants of a series of bis(porphyrin)-
pyrazine derivatives with fullerenes.
Results and discussion
Hildebrand equation at 596 nm to be 16400 ^ 600 M21
.
2,6-Bis(porphyrin)-substituted 3,5-dimethylpyrazine (1)
and its Zn(II) complex (2) were prepared according to the
previously reported procedure (8). Inclusion of C70 in the
pyrazine ligand was confirmed first by means of mass
spectrometry. The ESI-MS spectrum of a mixture of 1 and
C70 exhibited a characteristic molecular ionic peak at
2022.4625 ([1 þ C70 þ H]þ), which provided evidence for
The binding isotherm provides a good fit to 1:1
stoichiometry (Figure S4, available online). These values
for C70 are about 10 times larger than those of the
corresponding C60 complexes (8). From these exper-
iments, the association constants of C70 with 1 and 2 were
determined to be larger than those of C60. This result
*Corresponding author. Email: kawato@chem.kyushu-univ.jp
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2010 Taylor & Francis
DOI: 10.1080/10610278.2010.487566
http://www.informaworld.com

518
Y. Eda et al.
Figure 2. UV–vis absorption spectra of 1 (2.03 mM) before
(solid line) and after (broken line) the addition of C70 (27.4 mM)
in dichloromethane at 298 K (path ¼ 1 cm).
carry out a titration experiment to measure accurate
binding data, the association constant of 1 and C70 was
estimated to be 3530 M21 from the difference of two
absorbance measurements (0.0783 at 415 nm) taken of a
solution of 1 (2.03 mM) before and after the addition of C70
(27.4 mM).
Association of 1 and C70 in solution was then
investigated by a 1H NMR titration method (270MHz,
296 K, toluene-d8). With increasing concentrations of C70 in
a mixture of 1 and C70 in toluene-d8, the b-pyrrole proton a
near the pyrazine ring (Scheme 1) and NH proton of 1
shifted considerably upfield and the other b-pyrrole protons
b and c shifted slightly upfield, whereas the protons of the
methyl groups attached to the pyrazine ring shifted
downfield (Figure 3). These experimental results were
similar to the case of C60 (5–7). Thus, the complexation of
C70 was considered to take place in the expected porphyrin–
porphyrin pocket of 1 (Scheme 1). These shifts are larger
than those caused by addition of C60. NMR measurements
of the methyl protons were suitable for the determination of
the association constant Ka. The association constant Ka was
determined to be 4800 ^ 1300M21 by nonlinear curve
fitting of the NMR data with Ka and dmax treated as
parameters (Figure 4). Complexation of 2 with C70 also
takes place in solution as evidenced by changes in the
chemical shifts of the protons of 2 by the addition of C70
(Figure 5). NMR measurements of the b-pyrrole proton a
were suitable for the determination of association constant
Ka. By a similar NMR titration method for 1, the association
constant Ka of 2 with C70 was determined to be
3400 ^ 800 M21 (Figure 6). The amount of the toluene
solution of guest fullerene was more than that of the initial
Figure 1. ESI-MS spectrum of a mixture of 1 and C70.
shows that C70 fits well into the porphyrin–porphyrin
pocket of 1 compared to C60. Such a tendency is consistent
with similar metalloporphyrins reported in the literature
(2, 5, 6).
The UV–vis spectrum of 1 in dichloromethane was
influenced by the addition of C70 (Figure 2). The decrease
in absorbance of the Soret band of 1 by the effect of C70
suggests the existence of a charge transfer interaction
between C70 and 1. As the spectral change was too small to

Supramolecular Chemistry
519
Scheme 1. Possible structure of adducts of 2,6-bis(porphyrin)-substituted pyrazine derivatives (1–4) with fullerene.
host solution; however, we could not improve the NMR
experimental conditions because of the low solubility of
1
fullerene and its complex. Thus, the H NMR titration
The association constant of C70 with Zn porphyrin
complex 2 was smaller than that for 1 with metal-free
porphyrins. This result is consistent with the result of
calyx[4]arene-linked bis(porphyrin) hosts reported in the
literature (6). Electron-rich porphyrin nitrogen atoms are
effective for bond stabilisation by the dispersive forces
associated with p–p interactions between porphyrin
and C70.
method gave much smaller values for the association
constant Ka than the fluorescence spectral method for 1 and
2. In our previous paper (8), we reported that association
constants obtained by fluorescence and the NMR titration
method did not agree within estimated standard deviations.
In this study, the value estimated by UV absorption spectra
seemed to support the value measured by the NMR method.
The error in the fluorescence method might originate
in inaccurately detected fluorescence intensities which are
attributed to the scattering of the incident beam by
coexisting fullerene molecules in solution.
Next, the metal complexation effect on the association
of the pyrazine derivatives with fullerene was investigated.
Complexation of 1 with Pd(II) ion was achieved by
refluxing a solution of 1 and 1/2 molar amount of
[PdCl2(CH3CN)2] to afford 3 (Scheme 1). Comparison of
the 1H NMR peaks of 3 with those of 1 revealed a
remarkable downfield shift (Dd ¼ 1.13 ppm) for the

520
Y. Eda et al.
1
Figure 3. Partial H NMR spectra (270 MHz, 296 K, toluene-
d8) of 1 with C70. Assignments of peaks are shown in Scheme 1.
1
Figure 5. Partial H NMR spectra (270 MHz, 296 K, toluene-
d8) of 2 with C70. Assignments of peaks are shown in Scheme 1.
methyl peak upon complexation. Therefore, two pyrazine
ligands coordinated to a Pd(II) ion at the nitrogen whose
neighbouring carbons were connected to less hindered
methyl groups. From the analogy of Pd(II) complexes
with 2,6-disubstituted N-heterocyclic ligands (8, 9),
cis-coordination is impossible due to the steric repulsion
between the substituents. Therefore, by the support of a
Pd(II) ion to link two trans-pyrazine ligands, the
juxtaposed porphyrin groups on the opposite side of the
ligands can approach each other. Such an arrangement of
the binding sites is essential for the system to work well as
a host molecule (9).
further splitting of the original Soret absorption bands by
exciton couplings that originated from Coulombic
interactions between transition dipole moments of the
four porphyrins in 3. The fluorescence spectrum of 3
exhibited a peak at 648 nm upon excitation by 553 nm light
in dichloromethane (2.0 mM). As the emission from 3
is similar to that from free ligand 1 (emission peak
maximum: 650 nm), the singlet excited state of 1 is not
influenced by complexation with a Pd(II) ion. The UV–vis
spectrum of 3 in dichloromethane was altered by the
addition of C70 (Figure 7). The decrease in absorbance of
the Soret band of 3 by the effect of C70 suggests the
The ground-state absorption spectrum of 3 displayed
one broad Soret absorption peak at 418 nm (Figure 7). The
figure is explained by the overlapping of peaks due to
Figure 4. Changes in the chemical shift of the proton of the
methyl group in 1 by the addition of C70 in toluene-d8 at 296 K.
The solid line is the theoretical isotherm obtained by nonlinear
curve fitting to the experimental data.
Figure 6. Changes in the chemical shift of b-pyrrole proton a in
2 by addition of C70 in toluene-d8 at 296 K. The solid line is the
theoretical isotherm obtained by nonlinear curve fitting to the
experimental data.

Supramolecular Chemistry
521
Figure 8. 1H NMR spectra of 1:2 mixture of 4 (W) and 2 (X)
before (a) and after (b) addition of an overall stoichiometric
deficit of C70 in ratio 1:2:2.1.
of 4 and fullerene are shown in Scheme 1. In the case of
form A in which inclusion room for C70 in the Pd(II)
complex was maintained in the same position as that in 2,
the K1 value should be close to the Ka of 2. The fact that the
value of K1 is much larger than Ka of 2 suggests form B as
the plausible structure of the adduct (10). Association of 4
and C60 in toluene-d8 solution was similarly investigated.
The association constant K was determined to be
4.3(^0.8) £ 106 M22 and the association constant K1
was estimated to be 2100 ^ 200 M21, which was 1/4 of
that for C70. Experimentally reliable association constants
of bis(porphyrin)pyrazine derivatives and fullerene are
summarised in Table 1. The effective enhancement of the
binding with fullerene by Pd(II) complexation of the host
is attributable to the proper bite angle (608 for 4 vs. 1208
for 1 and 2) formed by the two porphyrin rings to capture
fullerene between them.
Figure 7. UV–vis absorption spectra of 3 (2.0 mM) before
(solid line) and after (broken line) addition of C70 (27.5 mM) in
dichloromethane at 298 K (path ¼ 1 cm).
existence of a charge transfer interaction between C70 and
3. However, the degree of spectral change was not enough
to measure an accurate binding constant for the complex.
1
Furthermore, we could not carry out a H NMR titration
to measure the binding constant of 3 and C70 because
of the low solubility of 3 in toluene. Thus, we could not
determine the structure of the inclusion compound of 3
with C70. Complexation of 2 with a Pd(II) ion was then
achieved similarly by refluxing a solution of 2 and 1/2
molar amount of [PdCl2(CH3CN)2] to afford 4 (Scheme 1).
As analytically pure Pd(II) complex was not obtained, the
association of 4 and fullerene was investigated as a
mixture of 2 and 4 by a 1H NMR titration method
(270 MHz, 296 K, toluene-d8). With increasing concen-
trations of C70 in a mixture of 4 and C70 in toluene-d8, the
b-pyrrole proton a shifted considerably upfield and the
other b-pyrrole protons shifted slightly upfield, whereas
the protons of the methyl groups attached to the pyrazine
ring shifted downfield (Figure 8). As probable structures of
the complex of 4 and C70, desired form B as well as form A
was considered (Scheme 1). NMR data measurements of
the b-pyrrole proton a were suitable for the determination
of association constant K, which was found to be 7.1
(^1.5) £ 107 M22. The association constant K1 was then
calculated to be 8400 ^ 900 M21 because the value of K2
would be equal to that of K1ðK ¼ K1K2 ¼ K21Þ due to the
independence of the two sites of the host. This is attributed
to no structural change of 4 after first binding with
fullerene because the NZPdZN bond is not likely to be
flexible due to the steric hindrance of the methyl groups
attached to the pyrazine rings. Possible forms of adducts
In conclusion, a pyrazine derivative with porphyrin
rings at the 2,6-positions was a significantly better host for
C70 than for C60. The association constant of each adduct
was determined successfully by an NMR titration method.
It was found that palladium complexation of the
porphyrin–pyrazine ligand enhanced the association of
the host with fullerene. As photo-excited energy transfer is
an important process in the early stages of photosynthesis
(11), we are now attempting to construct a new system in
Table 1. Association constants of 1 and 2 (Ka/M21) and 4
1
(K1/M21) with fullerene obtained by the H NMR titration
method (270 MHz, 296 K, toluene-d8).
C60
C70
1
2
4
830 ^ 50 (8)
550 ^ 180 (8)
2100 ^ 200
4800 ^ 1300
3400 ^ 800
8400 ^ 900

522
Y. Eda et al.
which photo-excited energy transfer arises from a proper
metal ion to C70 through the bridging pyrazine moiety
connecting the energy donor and acceptor.
1H NMR titration experiment of 1 with C70 in toluene-d8:
general procedure
A solution of 1 in toluene-d8 (600ml, 0.956 mM) was placed
in an NMR sample tube and 1H NMR spectra were recorded
after each addition of 40, 80, 120 and 160 ml of a solution of
C70 in toluene-d8 (1.52 mM). The association constant Ka
was evaluated from the change (Dd) in the chemical shifts
of the b-pyrrole proton a of 1 by applying a nonlinear
curve-fitting method using the equation shown below:
Experimental section
General comments
UV–vis spectra were measured by a Shimadzu UV-2200
spectrophotometer. Fluorescence spectra were measured by
a Hitachi F-2500 spectrophotometer. 1H NMR spectra were
recorded on a JEOL GSX-270 FT NMR spectrometer with
CDCl3 or C6D5CD3 as solvent and TMS as internal standard
(d ¼ 0 ppm). ESI-MS spectra were determined on a Bruker
Daltonics MicroTOF-ks1focus ESI-TOF-MS spectrometer.
Silica gel 60 (MERCK, Whitehouse Station, NJ, USA) was
used for column chromatography. 2,6-Bis(10,15,20-tri
phenylporphyrin-5-yl)-3,5-dimethylpyrazine (1) and its Zn
complex (2) were prepared according to the literature (8).
2
Dd¼Ddmaxð½C70ꢀt þ½1ꢀt þ1=Ka 2ðð½C70ꢀt þ½1ꢀt þ1=KaÞ
1=2
24½C70ꢀt½1ꢀtÞ Þ=ð2½1ꢀtÞ;
where [C70]t and [1]t are total concentrations of C70 and 1,
respectively, and Ddmax is Dd at 100% complexation.
Supplementary material
Supplementary material (fluorescence titration data, full
1
ESI-MS spectrum and full H NMR spectra) associated
with this article can be found online.
Materials
trans-Dichlorobis[2,6-bis(10,15,20-triphenylporphyrin-5-
yl)-3,5-dimethylpyrazine] palladium(II) (3)
Acknowledgements
A solution of trans-PdCl2(CH3CN)2 in benzene (1.7 mM,
0.933 ml) was added dropwise to a solution of 1 (3.8 mg,
3.2 mmol) in benzene (3 ml). After refluxing for 4 h, the
solution was concentrated in vacuo and the residue was
purified by column chromatography on silica gel, eluting
with CH2Cl2 to afford 3 (2.6 mg, 63%) as a purple solid. 1H
NMR (270 MHz, CDCl3): d 22.79 (br s, 8H, NH), 3.77 (s,
12H, Pz-CH3), 7.65–7.80 (m, 36H, PhH), 7.97–8.29
(m, 24H, PhH), 8.77 (s, 16H, b-pyrrole), 8.97–9.13 ppm
(m, 16H, b-pyrrole); UV–vis (CH2Cl2): lmax 418
(1 ¼ 330000), 521 (36000), 553 (14000), 591 (9300),
646 nm (6300 dm3 mol21 cm21); fluorescence (CH2Cl2,
This work was supported by a Grant-in-Aid for Scientific
Research (No. 20550125) from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT) of Japan. We
thank Prof. Tsutomu Katsuki and Dr Tatsuya Uchida of Kyushu
University for their measurements of ESI-MS spectra.
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