Novel porphyrin-viologen rotaxanes

Article abstract of DOI:10.1039/a708373d

A new porphyrin-containing host has an exceptionally high affinity for viologen guests, with binding constants as high as K_ass = 7 × 106 M^-1 in organic solvents, allowing the construction of porphyrin-viologen rotaxanes.

Full text of DOI:10.1039/a708373d

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Novel porphyrin–viologen rotaxanes
Alan E. Rowan,*† Patrick P. M. Aarts and Koen W. M. Koutstaal
Department of Organic Chemistry, NSR Center, University of Nijmegen, Toernooiveld 6525 ED Nijmegen, The Netherlands
A new porphyrin-containing host has an exceptionally high
ratosylate molecular clip. This U-shaped molecule was con-
verted to the tetraaldehyde derivative and subsequently cyclized
with pyrrole to give the porphyrin clip 1 (6%). NOESY and
COSY 2D spectra revealed that the porphyrin moiety is situated
rigidly and symmetrically, directly above the cavity.5
affinity for viologen guests, with binding constants as high as
Kass = 7 3 106
M
in organic solvents, allowing the
21
construction of porphyrin–viologen rotaxanes.
The design and construction of rotaxanes and catenanes using a
‘self-assembling’ approach is now regularly reported in lit-
erature.1 In general, however, few systems have been assembled
which possess addressable cyclic components, with an eye
toward the construction of functional devices.2 An interesting
functional species for incorporation in a rotaxane is a porphyrin.
Somewhat surprisingly the inclusion of a porphyrin in such a
system has only been preliminarily explored and mainly only as
a photoactive endgroup.3,4 Reported here is a new porphyrin-
clip macrocycle 1 which binds methyl viologen (N,NA-dimethyl-
4,4A-bipyridinium) and a variety of N-substituted derivatives
with the highest binding constants reported to date and is an
ideal basic building block for rotaxane synthesis.
The affinity of this new host for complexation with N,NA-
derivatized 4,4-bipyridinium guests was investigated by UV–
VIS spectroscopy. Host–guest titrations in MeCN–CHCl3 (1:1,
v/v) revealed the formation of exceptionally strong 1:1 charge-
transfer complexes with all the viologen guests 2. The strength
and geometry of binding is strongly dependent upon the
N-functionality of the guest. In the case of the methyl derivative
2a the association constant, Kass = (6.0 ± 0.9) 3 105 m21, is
several orders of magnitude higher than previously reported
association constants for similar complexes.3,7 Complexation of
porphyrin
clip
1
with
N,NA-bis(2-hydroxyethyl)-
4,4A-bipyridinium 2b,8 resulted in one of the strongest host–
guest complexes in organic solvents reported to date
[Kass = (7.4 ± 0.8) 3 106 m21]. The complex between 1 and the
propylamine derivative 2c displayed an association constant,
Kass = (9.0 ± 1) 3 105 m21, which is more similar to that of the
complex between 1 and 2a.
Compound 1 was synthesized as depicted in Scheme 1.5
Catechol was converted in three steps into 1,2-bis(2-p-tolylsul-
fonylethoxy)benzene (in 57% yield) which was then coupled
with tetrachloromethyldiphenylglycoluril,6 to generate the tet-
1H NMR studies on an equimolar solution of 1 and 2b in
CD3CN–CDCl3 (1:1, v/v) revealed large complexation shifts
(see structure 1). In the case of the porphyrin host, the central
NH resonance was shifted significantly upfield by 1.16 ppm
(from d 22.79 to 23.95) implying that this proton is situated
over the centre of an aromatic ring of the guest. Both sets of CH2
protons, which link the porphyrin with the aromatic side-walls
of the cavity, (ArOCHaHb and PorOCHaHb) were shifted
downfield by identical amounts (+0.4 and +0.7 ppm), suggest-
ing that these protons are at the side of an aromatic ring which
is situated symmetrically between the two sets of protons. In the
case of the resonances for the bipyridinium guest 2b even larger
shifts were observed, with the value of the shift diminishing as
one goes from the centre of the bipyridinium ring to the end of
the hydroxyethyl functions (see structure 2b for Dds). The
exceptionally large upfield shifts for the b and a protons (23.51
and 21.99 ppm) confirm that the guest sits in the cavity parallel
to the porphyrin ring.
OTs
OH
OH
O
O
OTs
O
Cl
Cl
N
N
N
Cl
Cl
i
Ph
Ph
N
O
O
O
OTs
OTs
TsO
TsO
O
O
O
N
N
N
N
Ph
Ph
O
The 1H NMR spectra (500 MHz) of an equimolar solution of
2a and porphyrin clip 1 in contrast showed three broad
resonances at d 6.20, 4.13 and 3.35 for the a, b and methyl
protons of the guest, respectively. The complexation shifts are
even larger than those for the complex with 2b: Dda = 22.66,
Ddb = 24.24 and Ddmethyl = 21.05 ppm. In contrast to the
complex of 1 with 2b, upon the addition of 2a to the host only
a very small upfield shift of the central NH proton (20.17 ppm)
was observed. The host protons ArOCH2 of the complex with
2a were now shifted upfield (20.16 and 20.69 ppm) with the
protons PorOCH2 being unaffected by the complexation of the
guest. The geometry of complexation of 2a is such that the
bipyridinium rings sit between the two aromatic walls of 1,
perpendicular with respect to the porphyrin. This binding
geometry is different from that of 2b but the same as that
observed for complexation of this guest in a molecular basket
derived from diphenylglycoluril.7
ii
O
OH
H
O
O
O
O
O
O
O
O
O
O
O
O
O
N
N
N
N
H
H
H
H
Ph
Ph
O
H
N
iii
1
The difference in binding geometry of the two guests is also
reflected in the UV–VIS spectra of the complexes. Upon
Scheme 1 Reagents and conditions: i, SnCl4, ClCH2CH2Cl, 72%; ii, MeCN,
K2CO3, 59%; iii, BF3·Et2O, CH2Cl2, p-chloranil, 6%
Chem. Commun., 1998
611

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–0.18
–0.04
(a)
H
H
–1.16
+0.16
H
–0.08
N
N
N
N
H
O
H
H
+
N
+
–
2a.(PF6
)
2
Me
N Me
H
H
O
O
+0.15
H
O
+0.27
+0.4, +0.7
+0.4, +0.7
H
H
H
–3.51
–1.99
–1.17
O
O
H
O
H
+
N
+
N
I
–
2b.(PF6
)
2
HO
OH
–0.12 H
+0.1
II
O
O
H
H
–0.73
–0.11 H
N
N
H
–0.27
N
+
+
–
N
N
2c.(PF6
)
2
H2N
NH2
1
(b)
O
O
O
O
O
O
O
O
O
O
N
N
N
O
N
O
N
H
N
O
N
H
N
H
O
H
O
O
O
O
N
N
N
N
N
N
N
N
3
Fig. 1 (a) Porphyrin clip 1 and viologen guests 2. The observed 1H NMR complexation shifts for the 1:1 complex of 1 and 2b (CDCl3–CD3CN) are indicated.
(b) Rotaxane 3 assembled by the condensation reaction of 3,3,3-triphenylpropionyl chloride and the pseudo-rotaxane complex of 1 and 2c.
addition of excess guest, the porphyrin Soret band in the UV–
VIS spectrum was shifted to the red by 5 nm in the case of 2b
and only by 2 nm in the case of 2a. The Q-bands remained
unaffected. The addition of 10 equiv. of guest 2b or 100 equiv.
of the methyl derivative 2a to the porphyrin clip caused an
almost complete quenching of the porphyrin fluorescence {I/I0
at 645 nm (excitation 418 nm) = 0.02; [Host] = 2.9 3 1026
mol l21, CH3CN–CHCl3 (1:1, v/v)}, confirming the close
proximity of the guest to the porphyrin.
Toward the construction of [n]rotaxanes assemblies it was
decided to use the 1:1 complex of bis(aminopropyl)bipyridi-
nium 2c and porphyrin clip 1 as the basic building block. NMR
studies showed that guest 2c forms a pseudo-rotaxane complex
with the NH2 functions sticking outside the cavity. Simple
condensation reactions of this 1:1 complex with acid chloride
derivatives should enable a variety of amide rotaxanes to be
formed.9 Using the bulky stopper 3,3,3-triphenylpropionyl
chloride a condensation reaction was carried out with the 1 :1
complex of 1 and 2c, to give molecule 3·(PF6)2 in 30% yield
after column chromatography and counter-ion exchange. The
resulting rotaxane was, surprisingly, very soluble in CHCl3. 1H
NMR studies confirmed that the viologen is bound in the cavity,
in an asymmetric geometry, rapidly exchanging between two
equivalent sites in which the amide function of the thread
hydrogen bonds to the carbonyl of the porphyrin clip (see
structure 3).5
Footnotes and References
† E-mail: rowan@sci.kun.nl
1 D. Philip and J. F. Stoddart, Angew. Chem., Int. Ed. Engl., 1996, 35,
1154; F. Vo¨gtle, M. Handel, S. Meier, S. Ottens-Hildebrandt, F. Ott and
T. Schmidt, Liebigs. Ann., 1995, 739.
2 P. R. Aston, R. Ballardini, V. Balzani, S. E. Boyd, A. Credi,
M. T. Gandolfi, M. Gomez-Lopez, S. Iqbal, D. Philip, J. A. Preece,
L. Prodi, H. G. Ricketts, J. F. Stoddart, M. S. Tolley, M. Venturi,
A. J. P. White and D. J. Williams, Chem. Eur. J., 1997, 3, 152.
3 M. J. Gunter and M. R. Johnston, J. Chem. Soc., Perkin Trans. 1, 1994,
995; M. J. Gunter, D. C. R. Hockless, M. R. Johnston, B. W. Skelton and
A. H. White, ibid., 1994, 1008; M. J. Gunter, D. C. R. Hockless,
M. R. Johnston, B. W. Skelton and A. H. White, J. Am. Chem. Soc.,
1994, 116, 4810.
4 Porphyrins in rotaxanes; D. B. Amabillino and J.-P. Sauvage, Chem.
Commun., 1996, 2441; F. Vo¨gtle, F. Ahuiis, S. Baumann and
J. L. Sessler, Liebigs, Ann., 1996, 921; M. C. Feiters, M. C. T. Fyfe,
M.-V. Martinez-Diaz, S. Menzer, R. J. M. Nolte, J. F. Fraser,
P. J. M. van Kan and D. J. Williams, J. Am. Chem. Soc., 1997, 119,
8119; R. B. Hannak, G. Farber, R. Konrat and B. Krautler, J. Am. Chem.
Soc., 1997, 119, 2313.
5 The full synthesis and characterization of all compounds will be
reported in full in a forthcoming paper.
6 J. N. H. Reek, J. A. A. W. Elemans and R. J. M. Nolte, J. Org. Chem.,
1997, 62, 2234.
7 A. P. H. J. Schenning, B. de Bruin, A. E. Rowan, H. Kooijman,
A. L. Spek and R. J. M. Nolte, Angew. Chem., Int. Ed. Engl., 1995, 34,
2132.
8 P. R. Aston, D. Philip, M. V. Reddington, A. M. Z. Slawin, N. Spencer,
J. F. Stoddart and D. J. Williams, J. Chem. Soc., Chem. Commun., 1991,
1680.
As shown here the strong complexation between porphyrin
clip 1 and bipyridinium guests 2 generates stable pseudo-
rotaxane complexes. The cyclic porphyrin component allows
the system to be potentially addressed by a variety of means:
chemically,3 electrochemically or photochemically.4 Current
studies are directed towards the construction of larger [n]rotax-
ane systems and arrays of porphyrin–viologens and the study of
their properties.
9 A. G. Johnston, D. A. Leigh, A. Murphy, J. P. Smart and M. D. Deegan,
J. Am. Chem. Soc., 1966, 118, 10662; F. Vo¨gtle, T. Duennwald, M.
Haendel, R. Jaeger, S. Meier and G. Harder, Chem. Eur. J., 1996, 2,
640.
We thank Professor R. J. M. Nolte and Professor J. F.
Stoddart for discussions and their interest in this work.
Received in Cambridge, UK, 20th November 1997; 7/08373D
612
Chem. Commun., 1998