Self-assembly of a bis-porphyrinic supramolecular rectangle using two orthogonal binding strategies

Article abstract of DOI:10.1039/b608315c

A bis-porphyrinic rectangle is created via a double self-assembly algorithm using two orthogonal coordination themes, i.e. first a 90° building block composed of a dynamic heteroleptic Cu(i) or Ag(i) bisphenanthroline followed by a coordinative dimerizati

Full text of DOI:10.1039/b608315c

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Self-assembly of a bis-porphyrinic supramolecular rectangle using two
orthogonal binding strategies{
Ravuri S. K. Kishore, Venkateshwarlu Kalsani and Michael Schmittel*
Received (in Cambridge, UK) 13th June 2006, Accepted 29th June 2006
First published as an Advance Article on the web 28th July 2006
DOI: 10.1039/b608315c
formation of supramolecular structures under thermodynamic
equilibration.9
A bis-porphyrinic rectangle is created via a double self-assembly
algorithm using two orthogonal coordination themes, i.e. first a
90u building block composed of a dynamic heteroleptic Cu(I) or
Ag(I) bisphenanthroline followed by a coordinative dimeriza-
tion using the pyridine–zinc porphyrin motif; the spectroscopic
and electrochemical properties as well as the dynamic nature of
the supramolecule were tested.
The phenanthroline linked porphyrin 1 (Scheme 1) was
synthesised by the Sonogashira method as based on an earlier
report.6d Ligand 4 was prepared readily in 48% yield using
3-ethynyl phenanthroline and 4-bromopyridine, again by a
Sonogashira cross-coupling. Copper(I) mediated complexation of
1 with 1,10-phenanthroline afforded the heteroleptic complex 3 as
an exclusive product in quantitative yield (detected by ESI-MS and
1H NMR). A characteristic upfield shift to d 6.06 ppm is noticed
for the 39,59-mesitylene protons (39,59-MesH) of 1 due to the
shielding from the second phenanthroline p system.
Self-assembly, at its very best, involves the spontaneous and
thermodynamically controlled formation of a single, well-defined,
discrete supramolecular entity from a collection of components.1,2
Emplacement of double or multiple binding modes in dynamic
self-assembly processes, although desirable, is not readily to hand.3
While the majority of known supramolecular structures to date are
restricted to a single binding algorithm,2 those that do utilise
double or multiple binding interactions resort to multistep
processes for introduction of diverse binding modes.4 Especially,
in the case of metal based supramolecular grids, squares and
rectangles, cases illustrating the use of two different dynamic metal
coordination interactions are sparse. Even so, a one-pot approach
seems to have little precedent other than the example of Lehn et al.5
Ligand 4, when combined with 1 and Cu(I) in dichloromethane,
immediately furnished a deep red-coloured solution. In the 1H
NMR spectrum of 5a/6a most peaks are shifted 0.1 ppm upfield
compared to those of complex 3 (Fig. 1). Moreover, the 1H NMR
displayed a characteristic upfield shift of the signals corresponding
to the 3-,5--mesitylene protons (3-,5--MesH). However, as
against a single signal noticed for the enantiotopic 39,59-MesH
A
dynamic, hence thermodynamically controlled one-pot
approach using a double binding algorithm is only feasible if the
constituent binding interactions are pre-programmed to follow a
non-interfering and self-contained pathway, thus ensuring the
desired disposition of the metal ions in the final assembly.
Consequently, compatibility and orthogonality of the metal-
interactions are the most important factors to consider. We have,
therefore, carried out several detailed metal and ligand exchange
studies on dynamic heteroleptic bisphenanthroline assemblies.6
In the present communication we report on our success in
generating dynamic heterometallic rectangles based on a dual
binding algorithm allowing the use of a facile one-pot self-
assembling process.
Our strategy towards a dynamic supramolecular heterometallic
rectangle is based on two well-established binding motifs: (a) the
Cu(I) or Ag(I) based heteroleptic complexation of modified 1,10-
phenanthrolines as developed recently;7 and (b) the zinc(II)
porphyrin–pyridine coordination. While other metal porphyrins,
e.g. Ru(II)CO porphyrins, also strongly bind to nitrogen ligands,8
the Zn(II) counterparts are much more labile, thus enabling
Center of Micro and Nanochemistry and Engineering, Organische
Chemie I, Universita¨t Siegen, Adolf-Reichwein-Str., D-57068 Siegen,
Germany. E-mail: schmittel@chemie.uni-siegen.de;
Fax: (+49) 271 740 3270; Tel: (+49) 271 740 4356
{ Electronic supplementary information (ESI) available: Experimental
section. See DOI: 10.1039/b608315c
Scheme 1 The bis-porphyrinic supramolecular assembly 6a,b with the
two orthogonal binding modes highlighted.
3690 | Chem. Commun., 2006, 3690–3692
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present in solution. The finding that the diffusion coefficient
measured for the complex 5a/6a is much lower than that measured
for 3 (12.2 6 10210 m2 s21) suggests that 5a exists in CD2Cl2 as
the supramolecular dimer 6a. In the case of negligible association,
the diffusion coefficient of 6a would have remained similar to that
of 3. As it is reasonable to assume similar viscosities in both
solutions, one can additionally estimate the hydrodynamic radius r
from the diffusion coefficients by the equation Dx/Dy = ry/rx.
Accordingly, r6a, the hydrodynamic radius of 6a is 1.74 6 r3.12
This value is within the range expected for 6a vs. 3.
Formation of an analogous complex was also observed with
AgPF6 in dichloromethane affording 6b. The diffusion coefficient
was measured to be 6.1 6 10210 m2 s21 also confirming that 5b
exists as its dimer 6b.
Metal exchange makes it possible to interrogate the dynamic
nature of the supramolecular rectangles. When [Cu(CH3CN)4]PF6
was added to 6b, the rectangle fully exchanged Ag+ to Cu+
affording 6a within a few minutes of mixing as seen from the
ESI-MS data. In contrast, when rectangles 6a and 6b were mixed
together in a 1 : 1 ratio, no exchange of metal ions was noticed
after 24 hours. Hence, although the rectangles exist in dynamic
equilibrium, they do not show any ‘‘cross talk’’.
Fig. 1 Comparison of the aromatic region in the 1H NMR of complexes
5a/6a and 3 measured in CD2Cl2.
protons of complex 3, complex 5a/6a exhibited two sets at d
6.06 ppm and d 5.89 ppm, due to the non-equivalence of the
mesitylene protons. The two sharp doublets of the pyridyl protons
Py50 and Py30 in 4 at d 7.48 ppm and d 8.64 ppm are drastically
shifted upfield in 5a/6a and appear as two broad singlets at d 7.07
and 6.12. Such shifts are characteristic of pyridyl protons bound to
zinc porphyrin.10 A H–H COSY revealed that the 29 and 49
protons of residue 4 in 5a/6a also experience shielding from the p
system of the porphyrin leading to their upfield shift from 8.44 and
8.52 ppm to 7.76 and 7.88 ppm, respectively. Hence, NMR
evidence strongly supports the formation of 6a.{
The ESI-MS of 5a/6a showed a signal of 100% intensity at m/z
1681 corresponding to the molecular weight of 5a. Also visible,
however, is a signal at m/z 3397 (100%) corresponding to 6a + Cl2.
The isotopic splitting of the signal at 1681 was dominated by the
signals of a monocation, while the isotopic splitting for the dication
could not be isolated due to the limits of the instrument.
The thermodynamic driving force of the formation of rectangle
6a was probed by UV–vis titrations in CH2Cl2 at 25 uC." Titration
of 4 against 1 provided an association constant of 4.4 6 103 M21
(log K(1+4) = 3.6 ¡ 0.6) in agreement with known binding
constants of pyridine derivatives with zinc porphyrin.9i The
association constant of complex 6a was determined to be 2.3 6
1021 M22 (log K6a = 21.3 ¡ 0.3) (Table 1).
The global association constant for formation of 3 is thus log
b3 = log K(1+Cu) + log K3 = 9.7. The formation constant for 6a
would be log b6a = 21.3 + (2 6 3.6) = 28.5. From the individual
constants, log b6a can be calculated as 2 (log b3 + log K(1+4)) =
26.7. Thus a small cooperative effect is noticed in the formation of
6a. Using log b6a = 28.5 makes it possible to estimate K(1+Cu+)+4
=
19.1 by a thermochemical cycle calculation (Fig. 3). This value may
To achieve further proof of the identity of the proposed
rectangle 6a formed through axial interaction of the pyridyl unit
with the Zn(II) porphyrin in 5a, NMR diffusion-ordered spectro-
scopy (DOSY) experiments were performed (Fig. 2).11 This
technique allows diffusion coefficients to be correlated with
molecular composition by observing chemical shifts.
Table 1 Binding constants for the various equilibrium processes
involved in the dynamic rectangle
Reaction
log K (M21
)
1 + Cu+ = (1 + Cu+)
(1 + Cu+) + 2 = 3
1 + 4 = (1 + 4)
4.7 ¡ 0.2
5.0 ¡ 0.3
3.6 ¡ 0.6
21.3 ¡ 0.3
DOSY experiments were performed in CD2Cl2 for the
complexes 3 and 5a/6a.§ One single diffusion coefficient (7.0 6
10210 m2 s21) was found for 5a/6a, indicative of a single species
2(1+4) + 2Cu+ = 6a
Fig. 3 Association constants of individual complexation steps leading to
3 and 6a, as determined experimentally, except for log K(1+Cu+)+4 = 19.1
(obtained from a thermochemical cycle calculation).
Fig. 2 DOSY plots of 6a (left) and 3 (right) in CD2Cl2 at 298 K plotted
for log D showing a marked difference in the diffusion coefficients.
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" UV–vis titrations were carried out by addition of a 1 mM solution of the
titrant (either Cu(I) or 4 ) at 298 K in CH2Cl2 to a 10 mM solution of the
other constituent(s) by a micro liter syringe. UV–vis titrations were
analyzed by fitting the whole series of spectra at 0.5 nm intervals using the
software SPECFIT.
a
Table 2 Redox potentials E1/2 of 3 and 6a
E1/2/Va (oxidation)
Cu+/Cu2+
Por/Por2+
E1/2/Va (reduction)
21.15
3
0.70
0.64
0.64
0.63
1.14, Epa = 1.21
Epa = 1.47c
1 (a) J.-M. Lehn, Supramolecular Chemistry: Concepts and Perspectives,
VCH, Weinheim, 1995; (b) J. W. Steed and J. L. Atwood,
Supramolecular Chemistry, Wiley, Chichester, 2000; (c) Encyclopedia of
Supramolecular Chemistry, ed. J. L. Atwood and J. W. Steed, Marcel
Dekker Ltd., New York, 2004.
3 + pyb
6a
21.11
6a + pyd
a
E1/2 vs. SCE in CH2Cl2 with n-Bu4NPF6 (0.1 M); scan rate 100 mV.
Addition of 1 equiv. of pyridine. Irreversible wave. Addition of
2 equiv. of pyridine.
b
c
d
2 M. Schmittel and V. Kalsani, Top. Curr. Chem., 2005, 245, 1 and
references therein.
3 Attempts towards a bi-metallic grid led to two exclusive grids:
M. Barboiu, E. Petit, A. van der Lee and G. Vaughan, Inorg. Chem.,
2006, 45, 484.
4 (a) D. M. Bassani, J.-M. Lehn, S. Serroni, F. Puntoriero and
S. Campagna, Chem.–Eur. J., 2003, 9, 5936; (b) L. H. Uppadine and
J.-M. Lehn, Angew. Chem., Int. Ed., 2004, 43, 240; (c) L. H. Uppadine,
J.-P. Gisselbrecht, N. Kyritsakas, K. Na¨ttinen, K. Rissanen and
J.-M. Lehn, Chem.–Eur. J., 2005, 11, 2549.
be compared to 2 6 (log K(1+4) + log K3) = 17.2, again hinting at a
small cooperative effect.
Electroanalytical studies of 6a showed a marked difference in
the redox potentials as against those of 3. An anodic shift in the
oxidation and reduction potentials of the porphyrin unit of 6a was
noticed which arises due to coordination of the pyridine to the zinc
metal in the centre of the porphyrin (Table 2). In addition, a
cathodic shift in the oxidation potential of the Cu(I) complex was
noticed. Control experiments with 3 and 6a in the presence of
pyridine clearly indicated that the shift of the Cu(I)/Cu(II) wave in
6a vs. 3 cannot be ascribed to a supramolecular effect, but that it is
due to the electronic influence of the pyridine–Zn binding on the
remote copper complex. Hence, the electroanalytical data confirm
that 5a/6a exists as a dimer with pentacoordinated Zn centres.
In conclusion, we have demonstrated that by appropriate choice
of two non-interfering binding algorithms dynamic supramolecular
heterometallic rectangles can be constructed. The dynamics of
these assemblies was established by metal exchange experiments.
The spectroscopically determined complexation constants suggest
that the two binding modes in the rectangle exhibit a cooperative
influence in the formation of the complex.
5 A. Petitjean, N. Kyritsakas and J.-M. Lehn, Chem. Commun., 2004, 24,
1168.
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We are indebted to the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie for financial support.
Moreover, we would like to thank Dr Paululat (Siegen) for
support with the DOSY NMR measurements.
Notes and references
{ 1H NMR experiments were carried out on 2 mM solutions of the
samples. With the binding constants being in the order of 1020 it is expected
that the concentration range makes it possible to maintain a 99% yield of
the supramolecular squares.9h
§ Diffusion experiments were performed on the Bruker Avance 400 MHz
NMR spectrometer, with a 5 mm BBI probe head, equipped with a pulsed
field gradient unit capable of producing magnetic field gradients in the
z-direction of about 5.35 G cm21. All experiments were carried out at 298 K
in a 5 mm NMR tube at 2 mM concentration. The bipolar magnetic field
pulse gradients (d) were of 2.5–4.5 ms duration, and the diffusion time (D)
was 50 ms. The pulse gradients were incremented from 0.10 to 5.08 G cm21
in 32 steps. Signals were averaged over 30–45 scans. In each experiment the
peaks were analyzed using an inbuilt intensity fit function ‘‘simfit’’ which
utilizes the formula
10 (a) A. Tsuda, T. Nakamura, S. Sakamoto, K. Yamaguchi and A. Osuka,
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S. Geremia, R. Graff, B. Kieffer and E. Alessio, Chem.–Eur. J., 2002, 8,
4670.
11 (a) W. S. Price, Concepts Magn. Reson., 1997, 9, 299; (b) W. S. Price,
Concepts Magn. Reson., 1998, 10, 197; (c) For a recent review on the use
of DOSY experiments in supramolecular chemistry, see: Y. Cohen,
L. Avram and L. Frish, Angew. Chem., Int. Ed., 2005, 44, 520.
12 M. S. Kaucher, Y.-F. Lam, S. Pieraccini, G. Gottarelli and J. T. Davis,
Chem.–Eur. J., 2005, 11, 164.
I = I0 exp[D(2c2 G2 d2)(D 2 d/3)]
where c is the gyromagnetic radius (rad s21 G21), d = length of the
diffusion gradients (G cm21) and D = time of separation between the
gradients, G is the pulsed gradient strength and D = diffusion coefficient.
3692 | Chem. Commun., 2006, 3690–3692
This journal is ß The Royal Society of Chemistry 2006