Fast guest exchange of a 1:1 zinc porphyrin-amine host-guest complex via a six-coordinated zinc porphyrin

Full text of DOI:10.1246/bcsj.20100007

950
Bull. Chem. Soc. Jpn. Vol. 83, No. 8, 950–952 (2010)
Short Articles
Fast Guest Exchange of a 1:1 Zinc
Porphyrin-Amine Host-Guest
Complex via a Six-Coordinated
Zinc Porphyrin
Yutaka Hitomi,*1 Junya Ohyama,2
Minori Takegoshi,2 Akira Ando,2
Takuzo Funabiki,1 Masahito Kodera,1
and Tsunehiro Tanaka2
1Department of Molecular Chemistry and Biochemistry,
Faculty of Science and Engineering, Doshisha University,
Kyotanabe, Kyoto 610-0321
2Department of Molecular Engineering, Graduate School of
Engineering, Kyoto University, Kyoto Daigaku Katsura,
Nishikyo-ku, Kyoto 606-8501
Figure 1. Synthetic scheme and crystal structure of Zn-1.
a) 1) Pyrrole, BF3¢Et2O, EtOH, CHCl3, 2) DDQ, and
3) silica gel chromatography. b) Zn(OAc)2, MeOH, CHCl3.
Received January 13, 2010
E-mail: yhitomi@mail.doshisha.ac.jp
The receptor Zn-1 was synthesized by the condensation of
pyrrole with benzaldehyde 3.5 Due to the sterically demanding
substituents, the desired ¡¢¡¢-atropisomer was preferentially
obtained (¡¢¡¢:¡¡¢¢:¡¡¡¢:¡¡¡¡ = 55:27:18:0)6,7 and
easily separated from the other isomers by silica gel chroma-
tography (Figure 1). As designed, the solid structure of Zn-1
shows two pockets surrounded by six phenyl rings on both
sides of the porphyrin plane, in which one pocket accommo-
dates one methanol and one chloroform, while the other pocket
is occupied with the aromatic rings of another crystallo-
graphically identical Zn-1 (Figure S1).8 Furthermore, ROE
(rotating frame NOE) measurements support the fact that the
two equivalent pockets exist in solution (Figure S2). In order to
understand the effect of the created pockets, titration of small
amine guests with Zn-1 was followed by UV-visible absorp-
tion spectroscopy (Figure S3). A nonlinear least-square analy-
sis of the spectral changes showed a simple 1:1 complexation
in all cases. The estimated binding energies are listed in
Table 1, together with those for the zinc-tetraphenylporphyrin
(ZnTPP) as a reference. Among the amine guests, 1,4-
diazabicyclo[2.2.2]octane (DABCO) shows a preference for
Zn-1 over ZnTPP by 5.3 kJ mol¹1. A molecular modeling
study indicated that the bound DABCO closely contacts the
aromatic walls of the pocket (Figure S4), which accounts for
the tight binding of DABCO to Zn-1.
A novel zinc porphyrin receptor has been synthesized
that has two identical binding pockets surrounded by six
phenyl rings on both sides of the porphyrin plane. The
binding of amine guests to the zinc porphyrin receptor was
studied by UV-vis titration experiments. Among the amine
guests, 1,4-diazabicyclo[2.2.2]octane (DABCO) showed the
highest binding affinity (¦G = ¹36.6 kJ mol
¹1
at 298 K
in toluene) thanks to close contacts of DABCO with the
aromatic walls of the binding pocket. The binding of
DABCO was further investigated by dynamic NMR experi-
ments. DABCO was tightly bound in one binding pocket
when less than 1 equivalent of DABCO was added, but it
started a rapid exchange between the two binding pockets
when exceeding 1 equivalent.
Porphyrins have been frequently employed as a synthetic
host in host-guest chemistry due to its rich photophysical
properties and well-established synthetic routes.1 In particular,
amine coordination to zinc porphyrin has been widely utilized
as a major driving force to capture small molecules in various
host-guest systems,2 and also as an organizing tool in
supramolecular porphyrin assemblies such as molecular
squares, boxes, circles, and coordination polymers.3 In spite
of the important roles of amine coordination to zinc porphyrins,
studies on its dynamic aspects are limited.4 Understanding
host-guest systems and more complex supramolecular systems
in terms of not only static, but also dynamic perspectives would
provide valuable insights into the design of more stable and/or
more dynamic supramolecular systems. Herein, we report a
unique dynamic aspect of amine coordination to a novel zinc
porphyrin Zn-1, in which a tightly bound amine guest is
released by coordination of a second amine guest from the
opposite face.
To investigate the dynamic aspects of the DABCO binding,
the titration of DABCO with Zn-1 was further investigated by
1H NMR spectroscopy (Figures 2 and S5). Upon the binding
of DABCO to Zn-1, two diagnostic signals, A and B, are
observed for the methylene protons of the bound DABCO
(¦¤A = ¹3.22, ¦¤B = ¹5.90), caused by the shielding effects
of the porphyrin plane and phenyl groups. The two sets of
signals do not shift during the titration of DABCO up to ca. 0.9
equiv, indicating that the DABCO-Zn-1 complex is kinetically
stable on the NMR time scale. Two sets of singlets (C and D)
Published on the web July 24, 2010; doi:10.1246/bcsj.20100007

Y. Hitomi et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 8 (2010)
951
Table 1. Binding Constants K (M¹1) and the Energies ¦G
(kJ mol¹1) for the Complexation of the Amine Guests to
Zn-1 or ZnTPP in Toluene at 298 K
Host
Guest
K
¦Ga)
¦¦Gb)
¹0.6
o
o
o
o
Zn-1
Pyridine
5.6 © 104 ¹21.3
4.3 © 104 ¹20.7
1.9 © 104 ¹24.4
1.0 © 104 ¹22.9
ZnTPP Pyridine
Zn-1 4-Picoline
ZnTPP 4-Picoline
Zn-1
¹1.5
2.6
1-Methylimidazole 2.1 © 104 ¹24.6
ZnTPP 1-Methylimidazole 6.0 © 104 ¹27.2
Figure 3. Chemical exchange rates of signals C and D with
various concentrations of DABCO.
Zn-1
ZnTPP DABCO
DABCO
2.6 © 106 ¹36.6
3.0 © 105 ¹31.3
¹5.3
a) Estimated errors within 5%. b) ¦¦G = ¦G(Zn-1) ¹
¦G(ZnTPP).
exchange of the bound DABCO with the free DABCO, and the
latter shows that the two pockets of the 1:1 DABCO-Zn-1
complex became indistinguishable on the NMR time scale.
Thus, the two pockets of the 1:1 DABCO-Zn-1 complex are
distinguishable at first and then become indistinguishable
beyond 1 equiv of DABCO added. This unique behavior has
never been reported in host-guest chemistry utilizing zinc
porphyrin receptors to our best knowledge. This observation
means that DABCO must stay in one pocket longer than the
NMR time scale until the ratio of DABCO/Zn-1 reach one;
that is, the chemical exchange between species A and B shown
in Scheme S1 is slow in terms of the NMR time scale, due to
slow dissociation rate of DABCO from Zn-1, and the equi-
librium between species A and B becomes fast beyond 1 equiv
of DABCO added. Dynamic NMR study provided more
information about the chemical exchange between species A
and B. The chemical exchange rates of signals C and D are
effectively constant (ca. 9 s¹1) at first and then dramatically
increase when the ratio of DABCO/Zn-1 exceeds ca. one
(Figure 3). The rising curve of the chemical exchange rate
suggested the involvement of unbound DABCO in accelerating
the chemical exchange between species A and B, because the
concentration of unbound DABCO also increases in a similar
way, see Figure S6. Therefore, we concluded that unbound
DABCO facilitates the chemical exchange between species A
and B via a 2:1 DABCO-Zn-1 intermediate or transition state,
that is, a second DABCO occupies the unbound pocket and
facilitates the dissociation of the first DABCO ligand. One may
imagine that occupation of a second DABCO in the unbound
pocket of the 1:1 DABCO-Zn-1 complex deforms the porphy-
rin plane and/or the pockets to facilitate the dissociation of the
first DABCO without forming six-coordinated Zn-1 species,
since six-coordinated zinc porphyrin complexes have never
been reported in solution. However, the above-mentioned
structural communication between the two pockets is highly
unlikely judged from the crystal structure of Zn-1 as well as the
optimized structure of the 1:1 DABCO-Zn-1 complex. There-
fore, we propose that the associative chemical exchange
proceeds via a six-coordinated transition state of Zn-1. Six-
coordinated zinc porphyrin complexes can be seen in crystal
structures,9 and therefore, the formation of six-coordinated 2:1
DABCO-Zn-1 species could be possible under certain con-
ditions, especially as a transition state. In fact, we have never
detected the binding of a second DABCO by NMR and UV-vis
absorption spectroscopies. Moreover, our theoretical calcula-
tions support that the six-coordinated Zn-1 species exists as a
Figure 2. 1H NMR spectra from a titration experiment with
the equivalents of added DABCO indicated on the left of
the spectra. The labels correspond to those in the molecular
models shown below.
also appear close to the singlet signal (E) of eight methoxy
groups of Zn-1. The signals C and D are assignable to the
methoxy groups on the DABCO-bound and DABCO-free
sides, respectively. Upon successive addition of DABCO
beyond 1 equiv, the DABCO methylene signals broaden and
then disappear, while signals C and D coalesce to give a sharp
singlet signal. The former phenomenon indicates the fast

952
Bull. Chem. Soc. Jpn. Vol. 83, No. 8 (2010)
© 2010 The Chemical Society of Japan
This research was supported through a Grant-in-Aid for
Scientific Research (No. 17750156 to Y.H.) from the Ministry
of Education, Culture, Sports, Science and Technology Japan.
We are grateful to T. Mizutani for valuable discussion, and to S.
Kitagawa, H. Chang, and M. Higuchi for kind access and
assistance to X-ray crystallography analysis.
Supporting Information
Crystal structural data, ROE spectrum, and synthetic
procedure of Zn-1 and experimental details of NMR and
UV-vis titration experiments. This material is available free of
charge on the web at http://www.csj.jp/journals/bcsj/.
Scheme 1. Associative and dissociative mechanisms for
DABCO exchange between the two pockets of Zn-1.
References
1
H. Ogoshi, Y. Mizutani, T. Hayashi, Y. Kuroda, in The
transition state, but not as an intermediate (Figure S7). The six-
coordinated transition state of Zn-1 has a flat porphyrin plane.
In the association mechanism, the bound DABCO disso-
ciates from the zinc center in synchronization with the binding
of an unbound DABCO to the DABCO-Zn-1 complex
(Scheme 1), and the chemical exchange rate is proportional
to the concentration of unbound DABCO. This associative
process is also in agreement with observed negative activation
entropies for the chemical exchange of signals C and D in the
presence of more than 1 equiv of DABCO (Table S1). In
addition to the associative mechanism, a dissociative guest
exchange process should be operative, although this process
should be slower than the associative guest exchange in the
host-guest system studied here. In the dissociative mechanism,
the bound DABCO dissociates form the zinc center of the 1:1
DABCO-Zn-1 complex to form a guest-free Zn-1 and then an
unbound DABCO binds in the another pocket of the guest-free
Zn-1 (Scheme 1). Thus, this process involves the formation of
guest-free Zn-1 by the dissociation of DABCO from the zinc
center of the 1:1 DABCO-Zn-1 complex. Therefore, the
exchange rate must be equal to the dissociation rate of the
bound DABCO from the zinc center of the 1:1 DABCO-Zn-1
complex, and cannot be accelerated by unbound DABCOs.
Thus, the change in the chemical exchange is interpreted as a
combination of these two mechanisms, that is, the associative
guest-exchange becomes dominant over the dissociative mech-
anism when the amount of DABCO exceeds 1 equiv. As shown
in Figure 3, the rapid chemical exchange between the peaks for
C and D already started in the presence of 1 equiv of DABCO,
which cannot be explained only by the dissociative mechanism.
At this point, the concentration of unbound DABCO is
calculated to be ca. 19 ¯M. This illustrates how effectively
the unbound DABCO can accelerate the chemical exchange of
species A and B.
Porphyrin Handbook, ed. by K. M. Kadish, K. M. Smith, R.
Guilard, Academic Press, San Diego, 2000, Vol. 6, pp. 279-346,
and references therein.
2
For a recent review, see: J. Weiss, J. Inclusion Phenom.
Macrocyclic Chem. 2001, 40, 1, and references therein.
V. Heitz, J.-C. Chambron, J.-P. Sauvage, in The Porphyrin
3
Handbook, ed. by K. M. Kadish, K. M. Smith, R. Guilard,
Academic Press, San Diego, 2000, Vol. 6, pp. 1-41, and refer-
ences therein; B. G. Maiya, J. Porphyrins Phthalocyanines 2004,
8, 1118; A. Satake, Y. Kobuke, Tetrahedron 2005, 61, 13.
4
C. A. Hunter, M. N. Meah, J. K. M. Sanders, J. Am. Chem.
Soc. 1990, 112, 5773; H. L. Anderson, C. A. Hunter, M. N. Meah,
J. K. M. Sanders, J. Am. Chem. Soc. 1990, 112, 5780; T. Mizutani,
T. Murakami, H. Ogoshi, Tetrahedron Lett. 1996, 37, 5369; V. V.
Borovkov, J. M. Lintuluoto, H. Sugeta, M. Fujiki, R. Arakawa, Y.
Inoue, J. Am. Chem. Soc. 2002, 124, 2993.
5
J. S. Lindsey, R. W. Wagner, J. Org. Chem. 1989, 54, 828;
K. Tamao, K. Sumitani, Y. Kiso, M. Zembayashi, A. Fujioka, S.
Kodama, I. Nakajima, A. Minato, M. Kumada, Bull. Chem. Soc.
Jpn. 1976, 49, 1958; A. Iwasaki, Y. Kondo, K. Maruoka, J. Am.
Chem. Soc. 2000, 122, 10238.
6
The ratio of the atropisomers was determined from the
integrated area of 1H signals of the pyrrole ¢-protons of each
atropisomer.
7
R. Song, A. Robert, J. Bernadou, B. Meunier, Analusis
1999, 27, 464; T. Hayashi, T. Asai, H. Hokazono, H. Ogoshi,
J. Am. Chem. Soc. 1993, 115, 12210.
8
Crystallographic data have been deposited with Cambridge
Crystallographic Data Centre: Deposition number CCDC-775279
for Zn-1. Copies of the data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge, CB2 1EZ, UK; Fax: +44 1223 336033; e-mail:
deposit@ccdc.cam.ac.uk).
9
For recent examples, see: J.-Q. Wang, C.-X. Ren, L.-H.
Weng, G.-X. Jin, Chem. Commun. 2006, 162; A. W. Kleij, M.
Kuil, D. M. Tooke, A. L. Spek, J. N. H. Reek, Inorg. Chem. 2005,
44, 7696; D. J. Ring, M. C. Aragoni, N. R. Champness, C. Wilson,
CrystEngComm 2005, 7, 621; E. Deiters, V. Bulach, M. W.
Hosseini, Chem. Commun. 2005, 3906; M. Vinodu, I. Goldberg,
New J. Chem. 2004, 28, 1250.
10 K. Tashiro, Y. Hirabayashi, T. Aida, K. Saigo, K. Fujiwara,
K. Komatsu, S. Sakamoto, K. Yamaguchi, J. Am. Chem. Soc. 2002,
124, 12086; T. Mizutani, S. Yagi, A. Honmaru, T. Goldacker, S.
Kitagawa, M. Furusyo, T. Takagishi, H. Ogoshi, Supramol. Chem.
1999, 10, 297.
In summary, DABCO undergoes a rapid exchange between
the two binding pockets of the zinc porphyrin receptor Zn-1
when more than 1 equiv of DABCO is added. The dissociation
of DABCO is facilitated by binding of another molecule of
DABCO to the opposite face of the zinc porphyrin receptor.
This phenomenon should be general for the complexation of
amine guests to zinc porphyrin receptors. Therefore, this new
finding could provide valuable insight into understanding
the stability of supramolecular porphyrin or related systems
utilizing amine coordination as an organizing tool.10