A molecular-clip-based approach to cofacial zinc-porphyrin complexes

Article abstract of DOI:10.1016/j.jorganchem.2009.09.035

We have synthesized molecular-clip-based cofacial zinc-porphyrin complexes. The complexes are shown to be new efficient receptors for the complexation of 1,4-diazabicyclo[2,2,2]octane, acridinium ions, 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA) and 1,2,4,5-benzenetetracarboxylic dianhydride (BTDA). Large binding constants (K_asso) in the range of 2.0 × 10⁴-1.1 × 10⁸ M^-1 were obtained for the 1:1 complexes of molecular tweezers (3) and rectangle (8) by inserting the hosts between the two porphyrin rings. The values of K and the spectroscopic results suggest that 3 and 8 serve as effective building blocks to capture the hosts. Semi-empirical calculations show that the guests position themselves within the cleft of the bis-porphyrins.

Full text of DOI:10.1016/j.jorganchem.2009.09.035

Journal of Organometallic Chemistry 695 (2010) 111–119
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
A molecular-clip-based approach to cofacial zinc–porphyrin complexes
Duckhyun Kim a, Sewon Lee a, Guohua Gao b, Hong Seok Kang c, Jaejung Ko a,
*
a Department of Chemistry, Korea University, Jochiwon, Chungnam 339-700, Republic of Korea
b Pohl Institute of Solid-State Physics, Tongji University, Shanghai 200092, PR China
c Department of Nano and Advanced Materials, College of Engineering, Jeonju University, Hyoja-dong, Wansan-ku, Chonju, Chonbuk 560-759, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We have synthesized molecular-clip-based cofacial zinc–porphyrin complexes. The complexes are shown
to be new efficient receptors for the complexation of 1,4-diazabicyclo[2,2,2]octane, acridinium ions,
1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA) and 1,2,4,5-benzenetetracarboxylic dianhydride
(BTDA). Large binding constants (Kasso) in the range of 2.0 ꢀ 104–1.1 ꢀ 108 Mꢁ1 were obtained for the 1:1
complexes of molecular tweezers (3) and rectangle (8) by inserting the hosts between the two porphyrin
rings. The values of K and the spectroscopic results suggest that 3 and 8 serve as effective building blocks
to capture the hosts. Semi-empirical calculations show that the guests position themselves within the
cleft of the bis-porphyrins.
Received 14 August 2009
Received in revised form 16 September
2009
Accepted 25 September 2009
Available online 3 October 2009
Keywords:
Molecular clip
Porphyrin
Ó 2009 Elsevier B.V. All rights reserved.
Rigid spacer
Self-assembly
1. Introduction
force a cofacial arrangement of the porphyrin moieties by the
introduction of a rigid spacer. We recently initiated a study to ex-
For over two decades, intensive research focused on studying
the synthesis and properties of cofacial porphyrins has generated
many self-assembly [1,2]. Such porphyrin complexes have at-
tracted considerable attention due to their light harvesting [3],
catalytic [4], chirality sensing [5], and molecular recognition
properties [6]. The cooperative interaction between the two por-
phyrins plays a crucial role in exerting these functions. Therefore,
the structural modifications for the cooperative effect may be
useful for the design of molecular recognition. Efficient molecular
recognition of the bis-porphyrin host for unsaturated molecules
[7] or ions [8] requires high structural and binding-site comple-
mentarity between the host and the guest. Cofacial bis-porphyrins
plore their potential as a new generation of preorganized scaf-
folds [12]. The methods we have used to synthesize such
porphyrins have all utilized a well-defined molecular clip [13] be-
tween the porphyrin units. We herein describe how an aromatic
molecular-clip-based bis-zinc porphyrin is used to efficiently di-
rect the self-assembly for a series of guests.
2. Experimental
2.1. General procedures
All reactions were carried out under an argon atmosphere. All
solvents were distilled prior to use. All reagents were purchased
from Sigma–Aldrich. 2,12-Diethynyl-[7-(3,5-di-tert-butylphenyl)-
dibenzo[c,h]acridine] (1) [14], [5-(4-iodophenyl)-10, 15,20-tris(3,
5-di-tert-butylphenyl)porphinato]zinc(II) (2) [15], [5,15-bis(4-me-
thoxy-carbonylphenyl)-10,20-dimesitylporphinato]zinc(II) (4) [16]
and 2,12-bis[trans-Pt(PEt3)2NO3]-[7-(3,5-di-tert-butylphenyl)- di-
benzo[c,h]acridine] (7) [12] were synthesized via previously
reported methods. 1H, 13C, and 31P NMR spectra were recorded on a
Varian Mercury 300 spectrometer operating at 300.00, 75.44, and
121.44 MHz, respectively. MALDI-MS spectra were recorded on a
Voyager-DE STR MALDI-TOF mass spectrometer. Elemental analyses
wereperformedusingaCarloErbaInstrumentsCHNS-OEA1108ana-
lyzer. Mass spectra were recorded on a JEOL JMS-SX102A mass spec-
trometer. Absorption spectra were recorded on a Perkin–Elmer
Lambda 2S UV–Vis spectrometer.
have been shown to form
p-complexes with acridinium ions [9]
or fullerenes [10], which are inserted between the two porphyrin
rings. On the other hand, metalloporphyrins can act as acceptor
building blocks if the metal atom inside the porphyrin has one
site available for coordination [11]. Due to their preorganized
U-shaped configurations and the cooperative interaction of their
two binding sites, such porphyrin units are able to coordinate
with bridging donor ligands such as 1,4-diazabicyclo[2,2,2]octane
(DABCO) [11]. A significant advance in this field would be the
development of an efficient synthetic method for preparing cofa-
cial porphyrin complexes that allows control over the orientation
and interfacial distances of the porphyrins. One approach is to
* Corresponding author. Tel.: +82 41 860 1337; fax: +82 41 867 5396.
E-mail address: jko@korea.ac.kr (J. Ko).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.09.035

112
D. Kim et al. / Journal of Organometallic Chemistry 695 (2010) 111–119
2.2. Synthesis of porphyrin tweezers (3)
128.2, 127.2, 125.5, 125.1, 124.6, 122.7, 122.6, 122.4, 122.2,
120.8, 120.6, 119.8, 91.8, 91.2, 35.3, 35.2, 34.9, 31.9, 31.7, 31.6.
Anal. Calc. for C175H181N9Zn2: C, 81.51; H, 6.23. Found: C, 81.19;
H, 6.12%.
Triethylamine (30 mL) was added to a mixture of 2,12-diethy-
nyl-[7-(3,5-di-tert-butylphenyl)-dibenzo[c,h]acridine] (1) (0.045 g,
0.087 mmol),
[5-(4-iodophenyl)-10,15,20-tris(3,5-di-tert-butyl-
phenyl)porphinato]zinc (2) (0.3 g, 0.26 mmol), tetrakis-(triphenyl-
phosphine) palladium (0.01 g, 0.0087 mmol), and copper(I) iodide
(0.001 g, 0.005 mmol). The reaction mixture was stirred under
nitrogen at 80 °C for 36 h. The solvent was then removed under re-
duced pressure. The resulting residue was extracted with dichloro-
methane and then purified by chromatography on silica gel using
dichloromethane/hexane (1:2, v/v) as an eluent. This afforded
0.18 g of product 3. Yield: 81%. M.p.: >400 °C. 1H NMR (CDCl3): d
10.21 (s, 2H), 8.94 (d, 4H, J = 4.8 Hz), 8.90 (d, 4H, J = 4.8 Hz), 8.88
(d, 4H, J = 4.8 Hz), 8.83 (d, 4H, J = 4.8 Hz), 8.25 (d, 4H, J = 8.1 Hz),
8.16 (d, 4H, J = 8.1 Hz), 8.13 (d, 2H, J = 9.3 Hz), 8.04 (m, 6H), 7.85
(s, 8H), 7.80 (s, 4H), 7.76 (s, 1H), 7.73 (s, 2H), 7.59 (m, 4H), 7.43
(s, 2H), 1.54 (s, 36H), 1.51 (s, 18H), 1.26 (s, 72H). 13C{1H} NMR
(CDCl3): d 151.2, 150.5, 150.4, 150.3, 149.8, 148.6, 148.5, 148.4,
147.8, 145.6, 143.5, 142.0, 141.8, 135.6, 134.5, 133.5, 132.6,
132.4, 132.3, 132.2, 132.1, 131.5, 130.1, 129.7, 129.5, 128.6,
2.3. Synthesis of [5,15-bis(4-carboxyphenyl)-10,20-
dimesitylporphinato]zinc (5)
A mixture of THF/MeOH/water (2/1/1, 200 mL) containing
[5,15-bis(4-methoxy-carbonylphenyl)-10,20-dimesitylporphina-
to]zinc(II) (4) (0.13 g, 0.148 mmol) and NaOH (0.9 g, 23 mmol) was
refluxed for 0.5 h. After it was allowed to cool to room tempera-
ture, the reaction mixture was acidified with AcOH (5 mL) and then
washed with water. The separated organic phase was dried over
Na2SO4 and then evaporated to dryness under reduced pressure.
The resulting residue was purified by recrystallization (CH2Cl2/
hexane) to afford 0.11 g of product 5. Yield: 87%. M.p.: 321 °C. 1H
NMR (DMSO-d6): d 13.19 (s, 2H, COOH), 8.73 (d, 4H, J = 4.2 Hz),
8.60 (d, 4H, J = 4.2 Hz), 8.34 (m, 8H), 7.32 (s, 4H), 2.56 (s, 6H),
1.77 (s, 12H). 13C{1H} NMR (DMSO-d6): d 177.1, 158.6, 158.2,
156.7, 148.5, 147.9, 146.4, 143.9, 141.4, 139.9, 139.2, 137.1,
N
N
N
N
Zn
Zn
Pd(PPh3)4
CuI, NEt3
N
N
N
N
I
N
Zn
N
+
2
N
N
N
N
1
2
3
Scheme 1. Synthesis of porphyrin tweezer 3.
N
N
N
N
N
N
N
N
(a)
N
N
O
O
N
N
(b)
O
O
O
O
O
Zn
Zn
Zn
O
OH
ONa
HO
NaO
6
4
5
Pt ONO2
PEt3
N
PEt3
Pt ONO2
PEt3
PEt3
Pt
PEt3
PEt3
Pt
PEt3
O
O
N
N
N
O
Zn
N
O
7
N
N
(c)
PEt3
Pt
PEt3
PEt3
Pt
PEt3
N
N
N
N
O
O
O
O
Zn
8
Scheme 2. Synthesis of porphyrin rectangle 8: (a) NaOH, THF, MeOH, and H2O (b) NaHCO3, acetone, and H2O (c) acetone and H2O.

D. Kim et al. / Journal of Organometallic Chemistry 695 (2010) 111–119
113
Fig. 1. MALDI-mass spectrum of compound 3 with isotopic distribution pattern.
Fig. 2. NMR spectra of molecular rectangle 8 at 298 K in chloroform-d1: (a) structure, (b) 31P NMR and (c) 1H NMR spectra.

114
D. Kim et al. / Journal of Organometallic Chemistry 695 (2010) 111–119
137.0, 128.1, 127.9, 30.9, 30.5. Anal. Calc. for C52H40N4O4Zn: C,
73.45; H, 4.74. Found: C, 73.27; H, 4.66%.
tively. The synthesis of noncyclic metalloporphyrin
3
is
conveniently prepared from molecular clip 1. The palladium-
catalyzed cross-coupling of 3 equiv. of [5-(4-iodophenyl)-
10,15,20-tris(3,5-di-tert-butylphenyl)porphinato]zinc (2) [15] with
2,12-diethynyl-[7-(3,5-di-tert-butylphenyl)dibenzo[c,h]acridine] (1) [14]
afforded 3 in 81% yield. The spectroscopic data for 3 were com-
pletely consistent with its proposed formulation. Fifteen distinct
resonances in the 10.21–7.40 ppm region of the 1H NMR spectrum
and thirty-eight peaks in the 151.2–119.8 ppm aromatic region of
the 13C NMR spectrum were observed. Additional evidence for the
formation of 3 was obtained from MALDI-TOF mass spectroscopy
(Fig. 1). The parent ion in the mass spectrum was observed at m/
z 2541. The isotopic distribution patterns matched the calculated
patterns well. The cyclic platinum-based macrocycle 8 was synthe-
sized in three steps from the rigid oxygen donor building block 4
[16] (Scheme 2). The linear dicarboxylate dianion 6 was prepared
from 4 by a hydrolysis reaction, followed by a salt exchange reac-
tion that converted 5–6. The 1:1 stoichiometric combination of 7
with the linear dicarboxylate 6 produced 8 as a red solid in 97%
yield. Similar Pt(II)–O bond-containing macrocycles have been re-
ported by Stang and co-workers [17]. The NMR spectrum of 8 indi-
cated the formation of a highly symmetrical structure. This was
evidenced by the number of resonances in aromatic region of the
1H NMR spectrum and the 13C NMR spectrum being reduced (from
15–38, respectively, for 3) to 10 and 28, respectively (Fig. 2). The
31P NMR spectrum of 8 was also consistent with the formation of
a highly symmetrical species, as indicated by the appearance of a
sharp singlet with concomitant 195Pt satellites (Jp–pt = 2910 Hz),
shifted upfield by 1.70 ppm relative to that observed for 7. The
small change in the phosphorous resonance of 8 relative to that
of the molecular clip is attributable to the structural similarity of
the Pt–O coordinate bond.
2.4. Synthesis of porphyrin rectangle (8)
A 50 mL flask was charged with [5,15-bis(4-carboxyphenyl)-
10,20-dimesityl-porphinato]zinc(II) (5) (0.05 g, 0.059 mmol) and
NaHCO3 (0.015 g, 0.177 mmol). After adding an acetone–H2O mix-
ture (1:1, 8 mL) to the flask, the mixture was stirred for 1 h. This
resulted in the formation of compound 6. Compound 7 (0.085 g,
0.059 mmol) in acetone (4 mL) was then slowly added to this mix-
ture, which was subsequently stirred overnight at 60 °C. Recrystal-
lization from CH2Cl2/hexane yielded pure
8 as a dark red
crystalline solid. Yield: 97%. M.p.: >400 °C. 1H NMR (CDCl3): d
8.70 (s, 4H), 8.65 (d, 8H, J = 4.5 Hz), 8.41 (d, 8H, J = 4.5 Hz), 8.32
(d, 8H, J = 7.8 Hz), 7.97 (d, 8H, J = 7.8 Hz), 7.52 (d, 4H, J = 7.5 Hz),
7.46 (s, 2H), 7.11 (s, 4H), 6.98 (s, 8H), 6.82 (d, 4H, J = 7.5 Hz), 2.74
(s, 24H), 2.54 (s, 12H), 1.76 (m, 48H), 1.38 (s, 36H), 1.29 (m,
72H). 13C{1H} NMR (CDCl3): d 171.7, 151.2, 150.8, 149.6, 149.4,
148.3, 144.4, 143.4, 139.1, 137.9, 137.6, 137.3, 137.1, 136.9,
134.6, 134.3, 134.0, 132.1, 131.6, 130.2, 128.4, 127.4, 127.3,
126.4, 123.4, 120.8, 119.9, 118.5, 35.1, 31.7, 30.1, 29.6, 28.1, 26.7,
14.2, 8.2. 31P{1H} NMR (CDCl3): d 10.5 (JPt–P = 2910 Hz). Anal. Calc.
for C222H266N10O8P8Pt4Zn2: C, 61.14; H, 6.15. Found: C, 61.01; H,
6.10%.
3. Results and discussion
Our strategy for the synthesis of the cofacial bis-zinc porphyrin
utilizes a molecular clip with two parallel reactive sites facing in
the same direction. The synthetic procedures for noncyclic and cyc-
lic metalloporphyrins are outlined in Schemes 1 and 2, respec-
Scheme 3.

D. Kim et al. / Journal of Organometallic Chemistry 695 (2010) 111–119
115
Fig. 3. Spectral changes of the Soret band of (a) 3 (4.7 ꢀ 10ꢁ7 M), (c) 8 (4.7 ꢀ 10ꢁ6 M) and the Q-band of (b) 3 (5.9 ꢀ 10ꢁ7 M), (d) 8 (4.7 ꢀ 10ꢁ6 M) upon the addition of DABCO.
The concentrations of DABCO were: (a) 0–2.9 ꢀ 10ꢁ6 M, (b) 0–1.4 ꢀ 10ꢁ6 M, (c) 0–1.4 ꢀ 10ꢁ5 M and (d) 0–8.6 ꢀ 10ꢁ6 M in CHCl3. Inset: the plot of absorbance change at
421 nm versus 3 and 8.
Table 1
Association constants (Ka (Mꢁ1)) and Gibbs enthalpies (
D
G° (kcal/mol)) for the formation of host–guest complexes in CHCl3 at the Soret band.
Rectangle (8)
Substrate
Tweezers (3)
k (nm)
Kasso (Mꢁ1
)
D
G° (Kcal/mol)
k (nm)
Kasso (Mꢁ1
)
DG° (Kcal/mol)
DABCO
Acridinium
NTDA
421.2
420
420.8
421.4
1.1 ꢀ 108
1.6 ꢀ 107
6.5 ꢀ 104
4.0 ꢀ 104
ꢁ10.85 0.2
ꢁ9.72 0.1
ꢁ6.50 0.1
ꢁ6.21 0.1
421.4
421.3
417.5
417.4
4.5 ꢀ 106
3.1 ꢀ 107
1.3 ꢀ 105
4.0 ꢀ 104
ꢁ8.98 0.2
ꢁ10.11 0.1
ꢁ6.90 0.1
ꢁ6.21 0.1
BTDA
Fig. 4. 1H NMR spectra of 3 (3.9 ꢀ 10ꢁ2 mol Lꢁ1) in the presence of DABCO in CDCl3 at 298 K. The concentrations of DABCO were: (a) 0.0, (b) 2.4 ꢀ 10ꢁ2, and (c)
3.9 ꢀ 10ꢁ2 mol Lꢁ1
.

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D. Kim et al. / Journal of Organometallic Chemistry 695 (2010) 111–119
The coordination of DABCO to 3 was probed by UV–Vis spec-
troscopy in chloroform (Scheme 3). The addition of incremental
amounts of DABCO to the solution of receptor 3 gave a series of
UV-vis spectra with one isosbestic point. The Soret band of 3 was
significantly shifted from 421.2 to 429.4 nm with a half bandwidth
of 12.1 nm. The Q-bands were also significantly red-shifted with
isosbestic points. This red-shift and the isosbestic point of the Soret
and Q-bands absorption are characteristic of a DABCO-porphyrin
sandwich complex [2,18,19]. Fig. 3a shows the data for the titra-
tions up to 25 equiv. of DABCO. Analysis of the binding isotherm
for 3 with a 1:1 binding model [20] gave an association constant
Kasso of 1.1 ꢀ 108 Mꢁ1. This is comparable with previously reported
values for other zinc porphyrins and DABCO [2,19]. The UV–Vis
spectrum of rectangular macrocycle 8 in CHCl3 at 25 °C changed
as the DABCO was added. The Soret band and Q-bands underwent
a significant hypochromic and bathochromic shift in the same
manner as the host 3. The titration curve for 8, shown in the inset
of Fig. 3c, is essentially a straight line with an abrupt endpoint after
the addition of 1 equiv. of DABCO to 8. Analysis of this binding iso-
therm yields an association constant Kasso of 4.5 ꢀ 106 Mꢁ1 (Table
1). The relatively small constant for 8 compared with that for 3
may result from the rigidity of macrocycle 8, which causes the
shrinkage of bond distance between the two porphyrins. The 1:1
complex formation is further confirmed by the 1H NMR spectra
shown in Fig. 4. When 0.6 equiv. of DABCO were added to 3, a
new signal appeared at ꢁ4.5 ppm. This signal is indicative of the
formation of the binary complex and can be assigned to the meth-
ylene protons of a DABCO molecule situated between two zinc–
porphyrin complexes. The unusual upfield shift of the signal is
attributable to the influence of the porphyrin ring current, which
is diagnostic of a sandwich complex. When a 1:1 concentration ra-
tio of 3 and DABCO was added, a new set of resonances corre-
sponding to the 1:1 complex formation were observed and the
resonances due to 3 completely disappeared. The 1H NMR reso-
nances of the 1:1 complex exhibited upfield shifts of around 0.2–
0.5 ppm upon complexation relative to the porphyrin signals for
Fig. 5. Molecular modeling of the bis-porphyrin tweezer 3: (a) in the absence of a
guest and (b) with DABCO, thereby forming a 1:1 complex formation. The 3,5-di-
tert-butylphenyl in the molecular clip was omitted in order to simplify the
calculation.
free 3. This was due to the proximity of the two porphyrin p-sys-
Fig. 6. 1H NMR spectra of 3 (3.9 ꢀ 10ꢁ2 mol Lꢁ1) in the presence of acridinium ions (b) in CDCl3 at 298 K. The acridinium ion concentrations were (a) 0.0, (c) 1.9 ꢀ 10-2, and (d)
3.9 ꢀ 10ꢁ2 mol Lꢁ1
.

D. Kim et al. / Journal of Organometallic Chemistry 695 (2010) 111–119
117
tem in the complex, which caused the protons to experience a large
ring-current-induced shift. On the other hand, the proton reso-
nances of the molecular clip framework exhibited a slightly down-
field shift or remained unchanged.
lene tetracarboxylic dianhydride (NTDA) and 1,2,4,5-benzenetetra-
carboxylic dianhydride (BTDA), are complexed in solution. The
binding behavior between BTDA and host 3 was investigated in
chloroform (Fig. 7). Upon adding BTDA, the position of the Soret
band of 3 showed a characteristic change from 421.4 to 426.3 nm
with decreasing of intensity. A new, broad charge transfer band
was observed at 426.3 nm, indicating that the guest is inserted in
the cleft of 3. The UV–Vis titration experiments and the fitting of
As crystals of suitable quality for X-ray crystallographic analysis
could not be obtained, calculations were required to visualize the
size and shape of the molecule. We carried out ab initio calcula-
tions to obtain reliable structural information. The geometry of 3
was optimized by Hartree–Fock (HF) calculations using a suite of
GAUSSIAN 98 programs. Stereoplots of the optimized structures of 3
and DABCOꢂ3 are shown in Fig. 5. The optimized structures show
the data yielded an association constant Kasso of 4.0 ꢀ 104 Mꢁ1
.
Similar behavior was observed when 3 was titrated with NTDA.
Following the above procedure, it was determined that 3 and NTDA
that the bis-porphyrin tweezer 3 has a cofacial arrangement of por-
form a 1:1 complex with an association constant of 6.5 ꢀ 104 Mꢁ1
.
0
phyrins with a centre-to-centre distance of 7.551 ÅA. Therefore, the
molecular modeling indicates that the bis-porphyrin tweezer hav-
ing such a distance is a suitable as a host for molecular recognition.
This relatively large constant indicates that 3 experiences appre-
ciable interactions with the NTDA through
p–p interactions and
charge-induced interactions. The 1H NMR experiment also re-
vealed that 3 could bind BTDA within the cleft of the bis-porphy-
rins to form a 1:1 complex. The stepwise addition of BTDA (0.1,
0.5, and 1.0 equiv.) to 3 caused upfield shifts of all of the signals
of the porphyrin in 3 and those of BTDA (Fig. 8). Of particular note
Based on these sizes, the guest, DABCO was chosen as a suitable
0
one because the N–N distance in these bidentate ligands is 2.7 ÅA
0
and the Zn in the porphyrin to nitrogen bond distance is 2.2 ÅA. As
expected, these guests form 1:1 host–guest complexes with 3.
The optimized model reveals a porphyrin centre-to-centre distance
is the resonance at 1.26 ppm, which is assigned to the H of the
a
0
of 7.210 ÅA, showing that the host shrinks slightly to accommodate
BTDA located between the two porphyrins. The unusual upfield
the guest. This configuration is a very reasonable one in view of the
shift (7.40 ppm) of the H signal is caused by the ring-current ef-
a
0
total summation of the three bond distances (7.1 ÅA). Since the dis-
fect of the porphyrin rings. This shift supports the 1:1 complexa-
tion (see Supplementary material). Such an unusual upfield shift
0
tance between the two porphyrins of the two hosts is 7.551 ÅA, the
acridinium ion is suitably positioned for stacking interactions
within the cleft of the bis-porphyrins. An electronic interaction be-
tween 3 and the acridinium ion was evidenced by the UV–Vis titra-
tion experiment. The gradual addition of acridinium ions to a
chloroform solution of 3 slightly decreased the absorbance of the
Soret band at 420 nm. Fitting the data to a 1:1 binding mode gave
rise to an association constant Kasso of 1 ꢀ 107 Mꢁ1 (Table 1). Sim-
ilar results were also obtained for the rectangular macrocycle 8.
Upon the addition of the acridinium ions to the solution of the
receptor 8 in CHCl3, the Soret band of 8 in the UV–Vis spectrum
significantly shifted from 421.3 to 427.1 nm and we calculated
the Kasso of the 1:1 complex to be 3.1 ꢀ 107 Mꢁ1. The Kasso values
of the acridinium ions for 3 and 8 are much larger than those of
other cofacial bis-porphyrins. This demonstrates that the two hosts
3 and 8 form strong
p-complexes with acridinium ions via p–p
interactions due to the rigid molecular clip spacer. The presence
of a 1:1 complex is also evidenced by the 1H NMR spectra shown
in Fig. 6. The titration of 3 with 0.5 equiv. of acridinium ions caused
upfield shifts in the proton signals of both the acridinium and 3. Of
particular note is the largest upfield shift of the acridinium ion rel-
ative to that of 3. In particular, the signals of the Ha and Hb protons
of the acridinium ion underwent an apparent upfield shift of 0.26–
0.28 ppm. This is thought to be due to the ring-current effect of the
porphyrin rings. When a 1:1 concentration ratio of 3 and acridini-
um ion was combined, the proton resonances of the acridinium
ions slightly downfield shift relative to 1:0.5 titration. The proton
signals of the porphyrin unit in the 1:1 complex showed no detect-
able shift, indicating that the guest only weakly interacts with the
two porphyrins. Similar behavior was observed when 8 was ti-
trated with acridinium ions. All of these observations indicate that
hosts 3 and 8 each experience some interaction with the acridini-
um ion and that the ion is located between the two porphyrin rings
of each host.
Molecular clips and rectangular macrocycles such as 3 and 8,
which have two porphyrins connected by a spacer, may be suitable
hosts for a variety of aromatic guests since they can retain the
guest between the two porphyrin units via
p–p stacking interac-
tions. When simple aromatics, such as naphthalene and anthra-
cene, are added to host 3, they show no propensity for binding.
This was deduced from the fact that the absorption spectra did
not show any characteristic Soret and Q-band changes. On the
Fig. 7. UV-visible titration data showing the Soret band shift for (a) 3 and (b) 8
upon adding BTDA. The concentrations of BTDA were: (a) 0–1.4 ꢀ 10ꢁ4 M in CHCl3
and (b) 0–7.1 ꢀ 10ꢁ5 M in CHCl3. Inset: the plot of absorbance change at 421.4 nm
and 417.5 nm versus 3 and 8, respectively.
other hand,
p-electron-deficient species, such as 1,4,5,8-naphtha-

118
D. Kim et al. / Journal of Organometallic Chemistry 695 (2010) 111–119
Fig. 8. 1H NMR spectra of 3 (3.9 ꢀ 10ꢁ2 mol Lꢁ1) in the presence of BTDA (a) in CDCl3 at 298 K. The BTDA concentrations were: (b) 0.0, (c) 3.9 ꢀ 10ꢁ3, (d) 1.9 ꢀ 10ꢁ2, and (e)
3.9 ꢀ 10ꢁ2 mol Lꢁ1
.
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Chem. Soc. 108 (1986) 417–424;
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Yu, J. Org. Chem. 72 (2007) 2897–2905;
of the BTDA has been previously observed for a similar bis-porphy-
rin [21].
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(2006) 701–712.
4. Conclusions
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In conclusion, we have described
a molecular-clip-based
approach to synthesizing cofacial zinc–porphyrin complexes. The
new preorganized zinc–porphyrin complexes represent new
efficient receptors for the complexation of 1,4-diazabicy-
clo[2,2,2]octane (DABCO), acridinium ions, 1,2,4,5-benzenetetra-
carboxylic dianhydride (BTDA), and 1,4,5,8-naphthalene
tetracarboxylic dianhydride (NTDA). Their preorganized configura-
0
tion and the 7.551 ÅA centre-to-centre distance between their cofa-
cial bis-porphyrins mean that guests can readily position
themselves within the new receptors. The association constants
and spectroscopic results obtained for these new complexes show
that the bis-porphyrin receptors are suitable hosts for molecular
recognition. The results demonstrate the great potential of molec-
ular-clip-based cofacial porphyrin complexes as building blocks for
developing novel receptors.
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Appendix A. Supplementary material
1H NMR spectra (1.8–1.0 ppm) of 3 in the presence of BTDA.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2009.09.035.
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