Cyclic dimer of a fused porphyrin zinc complex as a novel host with two π-electronically coupled binding sites

Article abstract of DOI:10.1039/b501689d

Upon complexation with 4,4′-bipyridine, a cyclic dimer of a fused porphyrin zinc complex, having two π-electronically coupled binding sites, shows a strong negative cooperativity in the second guest binding, to allow stepwise formation of 1:1 and 1:2 incl

Full text of DOI:10.1039/b501689d

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Cyclic dimer of a fused porphyrin zinc complex as a novel host with two
p-electronically coupled binding sites{
Hiroshi Sato,a Kentaro Tashiro,a Hideyuki Shinmori,b Atsuhiro Osukab and Takuzo Aida*a
Received (in Cambridge, UK) 3rd February 2005, Accepted 4th March 2005
First published as an Advance Article on the web 15th March 2005
DOI: 10.1039/b501689d
plot profile,5 these titration characteristics indicate the stepwise
Upon complexation with 4,49-bipyridine, a cyclic dimer of a
formation of highly stable 1 : 1 (1[bpy) and 1 : 2 (1[bpy2) host–
guest complexes. The 1H NMR spectrum of a 1 : 2.5 mixture of 1
and bpy in toluene-d8 at 20 uC showed at d 5.58 and 4.93 ppm two
fused porphyrin zinc complex, having two p-electronically
coupled binding sites, shows a strong negative cooperativity in
the second guest binding, to allow stepwise formation of 1 : 1
and 1 : 2 inclusion complexes.
Cyclic host molecules composed of extra large p-conjugated
systems are interesting in that their host–guest chemistry can be
markedly affected by p-electronic interactions. However, examples
of these molecules are only very limited. Mu¨llen et al. have
synthesized a cyclic dimer of a hexa-peri-hexabenzocoronene
(HBC) derivative with 42 p-electrons, and reported that the two
HBC units strongly interact electronically, thereby making the
conformational change dynamics extremely slow.1 Here we report
a novel cyclic dimer of a fused porphyrin bearing 44 p-electrons
(1). Fused porphyrins2 are a new class of p-conjugated molecules,
whose metal complexes are expected to provide two electronically
coupled binding sites for guest molecules. Thus, the first guest
binding may electronically affect the second guest binding. Despite
numerous examples of steric effects on multiple guest-binding
events,3 cooperative binding phenomena due to such a p-electronic
conjugation have never been explored to date. In the present
communication, we report results of a coordination interaction
between a nitrogenous base such as 4,49-bipyridine (bpy) and cyclic
dimer 1,4 along with its monomeric precursor (3), and highlight a
strong negative cooperativity in the second guest inclusion into the
p-electronic cavity of 1.
Scheme 1 Reagents and conditions: (i) DDQ, Sc(OTf)3, toluene, r.t.; (ii)
Br(CH2)6Br, KF, 18-crown-6, acetone, 50 uC; (iii) TBAF, THF, r.t.; (iv)
K2CO3, DMF, r.t.
Cyclic dimer 1 was obtained according to Scheme 1.5 Oxidative
ring closure of 2 with DDQ/Sc(OTf)3, followed by bromoalkyl-
ation at the phenolic functionalities, afforded a zinc complex of
fused porphyrin 4. On the other hand, deprotection of the silyl
groups after the oxidative ring closure gave a fused porphyrin zinc
complex 5. Then, alkaline-mediated coupling of 4 with 5 yielded
cyclic dimer 1, which was unambiguously characterized by
MALDI-TOF-MS and 1H NMR analyses. The absorption
spectrum of 1 was nearly identical to that of its monomeric
precursor (3), suggesting that the two fused zinc porphyrin units in
1 hardly interact with one another.
Spectroscopic titration of 1 with bpy in toluene at 20 uC clearly
displayed a two-step spectral change profile with isosbestic points
at 958, 984 and 1072 nm at [bpy]/[1] 5 0–0.7 (a) and 962, 1015 and
1107 nm at [bpy]/[1] 5 1.4–2.0 (b) (Fig. 1). When [bpy]/[1] exceeded
2.0, no further spectral change took place. Together with the Job’s
{ Electronic supplementary information (ESI) available: synthesis of 2–6,
1H NMR of 1 and 1[bpy2 and spectroscopic titration of 1/bpy, 3/py and
6/py. See http://www.rsc.org/suppdata/cc/b5/b501689d/
*aida@macro.t.u-tokyo.ac.jp
Fig. 1 Spectroscopic titration of 1 with 4,49-bipyridine (bpy) in toluene
at 20 uC. [1]0 5 2.0 6 1026 M; [bpy]/[1] 5 0–0.7 (a) and 1.4–2.0 (b).
2324 | Chem. Commun., 2005, 2324–2326
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upfield-shifted signals due to included bpy, whose integral ratios to
the OCH2 signal (8H) of 1, for example, were in excellent
agreement with those expected for 1[bpy2.5
(6) having two p-electronically separated binding sites was titrated
with py under identical conditions to the above.5 As expected, the
ratio K1/4K2 obtained from the spectral change profile was nearly
unity (Table 1), indicating the absence of any cooperativity
between the two binding sites. From these observations, it is clear
that the negative cooperativity observed for the complexation of 1
with bpy and that of 3 with py, is due to the electronic coupling
between the binding sites. The fact that the complexation of 1 (K1/
4K2 5 12) shows a much greater negative cooperative effect than
that of 3 (K1/4K2 5 1.8) is interesting and is understandable in that
the first ligand for 1 is very strongly coordinated and its electronic
effect on the second binding site can therefore be pronounced.
The association constants for the first (K1) and second (K2)
complexation events of 1 with bpy were too large for accurate
evaluation by spectroscopic titration methods. However, the
relative ratio K1/4K2, generally used for the evaluation of
cooperativity in complexation with two guest molecules,6 was
successfully obtained from the populations of uncomplexed and
complexed 1 in toluene. Namely, changes in absorbance of 1 at the
isosbestic points for the first (1072 nm) and second (1107 nm) guest
binding events, upon titration with bpy, allowed for the evaluation
of the populations of 1, 1[bpy and 1[bpy2 at [bpy]/[1] 5 1.0 as
18, 76 and 6%, respectively (Fig. 2). Using the equation K1/
4K2 5 [1[bpy]2/4[1][1[bpy2], the ratio K1/4K2 was estimated as
12 (Table 1). Since this value is much greater than unity, it is
obvious that the binding of 1 with the first bpy molecule results in
considerable lowering of the complexation activity of the second
binding site (negative cooperativity).
Titration of monomeric fused porphyrin zinc complex 3 with
pyridine (py) in toluene at 20 uC displayed a spectral change profile
similar to the case with 1. When the molar ratio [py]/[3] became
greater than 50, the isosbestic points initially observed at 946, 983
and 1076 nm shifted toward 968, 1017 and 1116 nm, respectively.5
The association constants K1 and K2, as evaluated by a nonlinear
curve-fitting method using a program LSPE,7 were 1.4 6 104 and
2.0 6 103 M21, respectively (Table 1), and the ratio K1/4K2 was
calculated to be 1.8. This ratio is again larger than unity but much
smaller than that observed for the cyclic dimer 1 (12). For
comparison, the zinc complex of a phenylene-bridged diporphyrin
In conclusion, we have developed the first host molecule having
p-electronically coupled binding sites (1), by cyclodimerization of a
fused porphyrin zinc complex. Although the host molecule can
accommodate two molecules of 4,49-bipyridine in its large
p-electronic cavity, it shows a strong negative cooperativity in
the complexation events due to an electronic coupling between the
two binding sites. The highly enhanced cooperativity of 1,
compared with its monomeric reference (3), enables the stepwise
guest inclusion to form 1 : 1 and 1 : 2 host–guest complexes
selectively. Hybridization of 1[bpy with different guest molecules
to form heterodimeric inclusion complexes is an interesting subject
worthy of further investigation.{
Hiroshi Sato,a Kentaro Tashiro,a Hideyuki Shinmori,b Atsuhiro Osukab
and Takuzo Aida*a
aDepartment of Chemistry and Biotechnology, School of Engineering,
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656,
Japan. E-mail: aida@macro.t.u-tokyo.ac.jp; Fax: +81-3-5841-7310;
Tel: +81-3-5841-7251
bDepartment of Chemistry, Graduate School of Science, Kyoto
University, Kyoto 606-8502, Japan
Notes and references
{ Cyclic dimer 1: To a DMF suspension (50 mL) of K2CO3 (100 mg,
0.72 mmol), being stirred under Ar, was added dropwise a DMF solution
(15 mL) of a mixture of 4 (84 mg, 0.042 mmol) and 5 (70 mg, 0.042 mmol)
at a rate of 0.4 mL/h using a microfeeder. After the addition was
completed, the mixture was stirred for a further 24 h. Then, the reaction
mixture was poured into toluene, washed with water, dried over Na2SO4
and evaporated to dryness. The residue was subjected to preparative size
exclusion chromatography with toluene as an eluent, where the second
fraction was collected and evaporated to dryness. The residue was
recrystallized from CHCl3/cyclohexane to give 1 as dark purple powder
Fig. 2 Mole fractions of 1 (blue), 1[bpy (red) and 1[bpy2 (green)
upon titration of 1 with 4,49-bipyridine (bpy) in toluene at 20 uC. [1]0 5
2.0 6 1026 M.
Table 1 Association constants (K1, K2) for the complexation of 1
with 4,49-bipyridine (bpy), and 3 and 6 with pyridine (py) in toluene at
20 uC
1
K1/M21
K2/M21
K1/4K2
(22 mg, 15%); H NMR (CDCl3, 20 uC, ppm): 7.62 (d, 8H, pyrrole-b-H),
7.60 (d, 8H, pyrrole-b-H), 7.55 (br, 8H, Ar), 7.48 (s, 8H, Ar), 7.42–7.34 (m,
16H, Ar), 7.14 (s, 8H, pyrrole-b-H), 7.10 (br, 4H, Ar), 7.04 (d, 4H, Ar), 3.69
(t, 8H, OCH2), 2.34–2.32 (m, 8H, OCH2CH2), 1.87–1.84 (m, 8H,
OCH2CH2CH2), 1.37 (s, 72H, t-Bu), 1.19 (s, 72H, t-Bu); MALDI-TOF-
MS m/z calcd. for C228H232N16O4Zn4 (M+) 3520.6, found 3520.9; UV-vis
1/bpy
3/py
6/py
. 108
. 108
12
1.8
0.97
1.4 6 104
9.3 6 103
2.0 6 103
2.4 6 103
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(toluene, 20 uC): lmax (log e) 418 (5.50), 461 (5.06), 561 (5.42), 928 (4.64),
1050 (4.80) nm.
4 J. K. M. Sanders, in The Porphyrin Handbook; K. M. Kadish, K. M.
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2326 | Chem. Commun., 2005, 2324–2326
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