Effect of the inclusion of sodium cations on the binding properties of a switchable diporphyrin receptor

Article abstract of DOI:10.1039/b102904p

The nestling of Na⁺ within the oligooxaethylene frame of the switchable diporphyrin receptor 1Zn results in an allosteric effect on the binding of ditopic amines, and remarkably influences the ligand-induced chiroptic properties of the assembly

Full text of DOI:10.1039/b102904p

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E†ect of the inclusion of sodium cations on the binding properties of a
switchable diporphyrin receptor¤
Donato Monti,*a Laura La Monica,a Anita Scipionib and Giovanna Mancinic
a Dipartimento di Scienze e T ecnologie Chimiche, Universita di Roma ““T or V ergataÏÏ,
via della Ricerca ScientiÐca s.n.c, I-00133 Rome, Italy. E-mail: monti=stc.uniroma2.it
b Dipartimento di Chimica, Universita di Roma ““L a SapienzaÏÏ, Ple. Aldo Moro 3, I-00185 Rome,
Italy
L e t t e r
c Centro C.N.R. di Studio sui Meccanismi di Reazione, Dipartimento di Chimica,
Universita di Roma ““L a SapienzaÏÏ, Ple. Aldo Moro 3, I-00185 Rome, Italy
Received (in London, UK) 29th March 2001, Accepted 3rd April 2001
First published as an Advance Article on the web 17th May 2001
The nestling of Na‘ within the oligooxaethylene frame of the
switchable diporphyrin receptor 1Zn results in an allosteric
e†ect on the binding of ditopic amines, and remarkably inÑu-
ences the ligand-induced chiroptic properties of the assembly
upon inclusion of chiral ditopic guests.
points to a 1 : 1 binding isotherm9 to give the corresponding
values of the association constants. The binding studies have
been carried out with 2Zn for comparison. The results are
reported in Table 1. As expected, receptor 1Zn binds to the
considered ligands more strongly than its monotopic 2Zn
counterpart. On inspection of Table 1 the overwhelming e†ect
of the donor ability of the ligands10 on the strength of the
binding is evident, the relative K values being 7.3 ] 104 and
2.6 ] 105 M~1 for bpy and 1,2-DACy respectively. The corre-
sponding values of e†ective molarity (EM)11a are 2 ] 10~2
Diporphyrin and oligoporphyrin systems are regarded as
excellent tools for the molecular recognition of organic guest
molecules.1 In particular, chiral diporphyrin systems have
been successfully employed in the enantiomeric discrimination
of amino acid derivatives.2 Moreover, diporphyrin tweezers
constitute interesting devices for the determination of the
absolute conÐguration of amines, amino acids and deriv-
atives.3
We recently reported4 that simple oligooxaethylene-
spacered diporphyrin arrays undergo
a conformational
change toward a cofacial geometry upon coordination of
alkali metal ions within the ether framework of the receptor.
We surmised that this geometrical variation would result in a
change of the binding properties of a related zinc derivative
toward the inclusion of bifunctional guests (G). In this paper
we wish to report on the e†ect of the inclusion of Na`5 on the
binding features of the above mentioned diporphyrin receptor
toward some bidentate amines, i.e. trans-1,2-diaminocyclo-
hexane (1,2-DACy, racemic mixture), and 4,4@-bipyridyl (bpy).
The results obtained evidence the presence of an allosteric
e†ect exerted by the complexed metal ion, which promotes the
attainment of a more rigid structure (1Zn É Na=G, Scheme 1).
These studies have been extended to the chiral (])- and ([)-1,
2-DACy stereoisomers evidencing an interesting e†ect on the
ligand-induced chiroptic properties of the resulting supramo-
lecular assemblies.
Scheme 1 Schematic structure of the 1Zn É Na=G supramolecular
ternary complex. Phenyl rings have been emitted for clarity.
Table 1 Binding constant values (K) for the interaction of amine
guests with porphyrin receptors, in CH CNÈCHCl (90 : 10, v/v) at
Diporphyrin derivative 1Zn, and its monotopic counterpart
3
3
2Zn, have been synthesised by following
a previously
T \ 25 ¡C
published procedure.5,6 The derivative 1Zn has been found to
be a good receptor for several ditopic amines.7 We focused
our attention on bpy and 1,2-DACy owing to their di†erent
structural and coordination properties.8 The formation of the
hostÈguest inclusion complexes was carried out in
CH CNÈCHCl (90 : 10, v/v). The formation of 1Zn É Na=G
Entry
Receptor
Guest
K/M~1 (EMa/M)
1
2
3
4
5
6
7
1Zn
bpy
bpyb
bpyc
1,2-DACy
1,2-DACyb
bpyd
7.3 ] 104 (2.0 ] 10~2)
1.1 ] 104 (3.0 ] 10~3)
7.3 ] 104 (2.0 ] 10~2)
2.6 ] 105 (3.0 ] 10~4)
4.5 ] 105 (5.0 ] 10~4)
9.0 ] 102
3
3
adducts was conveniently monitored by following the typical
red-shift of the Q visible bands of the porphyrin macrocycles.”
The titration plots were analysed by Ðtting the experimental
2Zn
1,2-DACyd
1.5 ] 104
a E†ective molarity values are reported in parentheses. b In the pres-
ence of NaClO 0.05 M. c In the presence of Bu NClO 0.05 M. d The
¤ Electronic supplementary information (ESI) available: full experi-
mental details, NMR labelling scheme for the porphyrin derivatives
4
4
4
values of K obtained in the presence of NaClO 0.05 M were identical
4
and UV-visible spectra of 1Zn=G in the presence of added NaClO .
within experimental error.
4
See http://www.rsc.org/suppdata/nj/b1/b102904p/
780
New J. Chem., 2001, 25, 780È782
DOI: 10.1039/b102904p
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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and 3 ] 10~4 M. The EM value estimated° in the case of the
binding of bpy lies fairly well within the calculated range for a
strainless ring closure reaction,11b,c whereas the corresponding
value for the interaction of 1,2-DACy is about two orders of
magnitude lower than that expected. This may indicate that
the formation of the latter pseudo-cyclic assembly, owing to
the shorter NÈN distance of the guest, occurs with some
degree of ring strain.12 The complexation studies were also
performed in the presence of added Na`,Ò which is known to
promote a conformational change of the resulting supramo-
diamines results in the formation of a supramolecular chiral
complex, which features increased ellipticity and stability in
the presence of Na`. This would have promising application
in the construction of receptors and sensors for substrates
with biological activity. Further studies on the e†ect of the
inclusion of di†erent metal ions toward the binding of a larger
class of ditopic guests are currently under investigation in our
laboratories.
Experimental
lecular complex toward
a
pseudo-cyclic structure
Syntheses
(1Zn É Na).4,5,13 This a†ects the binding of a given guest by
virtue of the tuned preorganisation of the receptor. This is
indeed the case for the inclusion of 1,2-DACy, which features
a two-fold increase of the binding constant K in the presence
of the added cation.14 Conversely, in the case of the inclusion
of the ““longerÏÏ guest, i.e. bpy, a negative allosteric e†ect is
found. A decrease in K by nearly one order of magnitude is in
fact observed, and this can be interpreted in terms of a less
favourable interaction of bpy within the less Ñexible cavity of
the host.p The magnitude of these e†ects is in line with other
reports which have recently appeared in the literature,15 in
which the inclusion of some alkali metal ions results in a posi-
tive e†ect on the binding of halides to some ditopic receptors.
The allosteric e†ect is also nicely evident in the CD experi-
ments relative to the binding of chiral diamine guests such as
(1R,2R)-([)-1,2-diaminocyclohexane, and (1S,2S)-(])-1,2-
diaminocyclohexane [(^)G*] stereoisomers. The binding
results in the formation of extrinsically chiral supramolecular
hostÈguest complexes [1Zn=(^)G*] as evidenced by the
exciton-coupled CD of the interacting porphyrin chromo-
phores.16 The chiroptic properties of the resulting assemblies
are modulated by the absolute conÐguration of the guest. The
inclusion of the ([) isomer, for example, results in a negative
bisignate CD spectrum, whereas in the case of the (]) enan-
tiomer a mirrored positive spectrum is observed. Remarkably,
the presence of Na` results in an increased CD amplitude of
the spectra (Fig. 1), as witnessed by the relative *e values for
the complex with ([)G* of [415 and [620 L mol~1 cm~1
for 1Zn and 1Zn É Na, respectively, at 436 nm.17 This Ðnding
conÐrms the hypothesis of the formation of a tighter supramo-
lecular structure, which features a higher degree of ellip-
ticity.18 Finally, the inclusion of Na`, resulting in a more rigid
system, promotes an appreciably increased stability of the
supramolecular complex toward the presence of an excess (up
to 100 equivalents) of the title diamine.**
Zinc[5-(4-hydroxy-3-methoxyphenyl)-10,15,20-triphenyl-
porphyrinato], 2Zn. 1H NMR (CDCl ): d 9.0È8.9 (m, 8H, b-H),
3
8.2 (br s, 6H, o-C H ), 7.7 (br s, 6H, m-C H ), 7.3È7.2 (m, 6H,
6
5
6 5
C H ] aromatics), 3.97 (s, 3H, OCH ). 13C NMR (CDCl ): d
6
5
3
3
150.5 (C-a), 150.2 (C-a), 142.9 (C-1A), 134.5 (C-2A, C-6A), 132.0
(C-b), 127.9 (C-4A), 127.5 (C-3A, C-5A), 126.6 (C-b), 121.1 (C-
meso), 117.5 (C-5@), 112.6 (C-2@), 56.2 (CH O). UV-vis (CHCl ),
3
3
j
/nm (log e): (Soret, 5.8), 490 (4.1), 552 (3.9). FAB-MS
max
(NBA), m/z: 723 [M]`.
1,11-Bis[zinc(2-methoxy-4-(10,15,20-triphenylporphyrin-
ato)phenoxy]-3,6,9-trioxaundecane, 1Zn. 1H-NMR (CDCl ): d
3
9.0È8.8 (m, 8H, b-H), 8.18 (br s, 6H, aromatics), 7.7È7.6 (m,
12H, aromatics), 7.65 (d, J \ 8.4 Hz, 1H, aromatics), 4.33 (br t,
J \ 4.8 Hz, 2H, OCH CH O), 3.96 (br t, J \ 4.8 Hz, 2H,
2
2
OCH CH O), 3.89 (s, 3H, CH O), 3.7 (m, 4H, OCH CH O).
2
2
3
2
2
13C-NMR (CDCl ): d 150.3 (C-a), 150.1 (C-a), 150.0 (C-a),
3
147.9 (C-3@), 147.3 (C-4@), 143.0 (C-1A), 136.1 (C-1@), 134.4 (C-2A),
131.8 (C-b), 127.3 (C-3A), 126.4 (C-b), 122.1 (C-6@), 120.8 (C-
meso), 120.7 (C-meso), 118.7 (C-5@), 111.5 (C-2@), 70.9
(CH CH O), 70.7 (CH CH O), 69.8 (CH CH O), 68.6
2
2
2
2
2
3
2
2
max
2
(CH CH O), 56.0 (OCH ). UV-vis (CHCl ), j /nm (log e):
423 (6.1, Soret), 551 (4.3), 594 (4.1). FAB-MS (NBA), m/z: 1604
[M]`.
3
Acknowledgements
We would like to thank Mr Giuseppe DÏArcangelo for the
FAB-MS spectra. The Ðnancial support of Ministero
dellÏUniversita
e della Ricerca ScientiÐca e Tecnologica
(MURST Project no. 98032774402ÈProgetti di Ricerca
ScientiÐca di Rilevante Interesse Nazionale) is gratefully
acknowledged. Furthermore, we would like to thank Pro-
fessor Gianfranco Ercolani, University of Rome ““Tor
VergataÏÏ, for a critical discussion of this work.
The cation-induced conformational change of a Ñexible
diporphyrin host results in an allosteric e†ect toward the
binding of ditopic amine ligands, which is dependent on the
structural properties of the guests. The interaction with chiral
Notes and references
” Evidence for a 1 : 1 1Zn=G complex formation was given by the
presence of several well-deÐned isosbestic points and by a continuous
variation plot (Job plot) which shows a sharp inÑection point at
s(G) \ 0.5.
° The EM calculation has been made on the assumption that the
cyclic complexes with bpy and 1,2-DACy posses 19 and 20 rotors (i.e.
freely rotating single bonds), respectively.
Ò From the binding constant value for the interaction of Na` with
the porphyrin receptor (K \ 500 M~1, see ref. 4) it can be estimated
ass
that the predominant species is the complexed form (1Zn É Na) at the
chosen cation concentration (0.05 M).
p The fact that, in a separate experiment, the addition of an excess (up
to 0.05 M) of Bu NClO does not inÑuence the strength of the
4
4
binding of 1,2-DACy to 1Zn, safely rules out the occurrence of e†ects
due to the increased ionic strength of the medium. Moreover, the
invariance of the K values for the binding of bpy to the reference 2Zn
receptor in the presence of Na` (see entry 6, Table 1) excludes the
possibility that the reduced degree of binding observed would be a
mere consequence of a Lewis acidÈbase interaction of the ligand with
the sodium cation.
** Related structures, in fact, are known to be unstable, in the pres-
ence of an excess of added ligands, toward the conversion into linear
oligomers [see, for example, ref. 3(a)].
Fig. 1 CD spectra of 1Zn=([)G* ([1Zn] \ 1.8 ] 10~6 M,
CH CNÈCHCl , 90 : 10, v/v), in the presence of added NaClO . (a)
3
3
4
[Na`] \ 0 M, (b) [Na`] \ 0.05 M. The inset shows the CD spectra
for the 1Zn=(^)G* complexes (see text).
New J. Chem., 2001, 25, 780È782
781

View Article Online
1
(a) H. Ogoshi, T. Mizutani, T. Hayashi and Y. Kuroda, Porphy-
rins and Metalloporphyrins as receptor models in Molecular
Recognition, in T he Porphyrin Handbook, ed. K. M. Kadish,
K. M. Smith and R. Guilard, Academic Press, London, 2000, vol.
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(a) M. J. Crossley, L. G. Mackay and A. C. Try, J. Chem. Soc.,
Chem. Commun., 1995, 1925; (b) for a recent overview on the
chiral recognition of porphyrin derivatives see: H. Ogoshi and T.
Mizutani, Acc. Chem. Res., 1998, 31, 81; (c) for a report on the
memory e†ect on chiral recognition of porphyrins see: Y.
Mizuno, T. Aida and K. Yamaguchi, J. Am. Chem. Soc., 2000,
122, 5278.
ligand with the ditopic receptor 1Zn and K
is relative to the
inter
interaction of the considered ligand with the monotopic 2Zn, cor-
rected for the statistical factor of 2. See for example: G. Ercolani,
L. Mandolini, P. Mencarelli and S. Roelens, J. Am. Chem. Soc.,
1993, 115, 3901; (b) for general studies on the e†ect of EM on
cyclisation reactions see: C. Galli and L. Mandolini, Eur. J. Org.
Chem., 2000, 3117; (c) L. Mandolini, Adv. Phys. Org. Chem., 1986,
22, 1.
12 This is di†erent from that observed, for example, in the case of
either covalent or self-assembled diporphyrin cages, in which the
strength of the binding is driven by the good Ðt and rigidity of
the guest. See for example: (a) I. P. Danks, I. O. Sutherland and
C. H. Yap, J. Chem. Soc., Perkin T rans. 1, 1990, 421; (b) I. P.
Danks, T. G. Lane, I. O. Sutherland and M. Yap, T etrahedron,
1992, 48, 7679; (c) H.-J. Schneider and M. Wang, J. Org. Chem.,
1994, 59, 7464; (d) Y. Kuroda, A. Kawashima, Y. Hayashi and H.
Ogoshi, J. Am. Chem. Soc., 1997, 119, 4929.
13 For a recent overview on the hostÈguest chemistry of acyclic
receptors see: G. W. Gokel and O. Murillo, Podands, in Com-
prehensive Supramolecular Chemistry, ed. J. L. Atwood, J. E.
Davies, D. D. MacNicol and F. Vogtle, Elsevier Science Pub-
lications, Oxford, 1996, vol. 1, ch. 1, p. 1.
14 The achievement of a tighter conformation in which the two
porphyrin platforms are brought to a shorter distance is wit-
nessed by the presence of a blue shift of the 1Zn É Na=G(G \ 1,
2-DACy) Soret band, compared to that of the related 1Zn=G
(See Fig. S1, supplementary material). A blue shift of the Soret
band is characteristic of face-to-face diporphyrin systems. See for
example: J. P. Collman, P. S. Wagenknecht and J. E. Hutchin-
son, Angew. Chem. Int. Ed. Engl., 1994, 33, 1537; J. P. Collman,
Inorg. Chem., 1997, 36, 5145.
15 (a) P. D. Beer, P. K. Hopkins and J. D. McKinney, Chem.
Commun., 1999, 1253; (b) J. E. Redman, P. D. Beer, S. W. Dent
and M. G. B. Drew, Chem. Commun., 1998, 231; (c) P. D. Beer
and S. W. Dent, Chem. Commun., 1998, 825; (d) P. D. Beer and
J. B. Cooper, Chem. Commun., 1998, 129; (e) J. B. Cooper,
M. G. B. Drew and P. D. Beer, J. Chem. Soc., Dalton T rans.,
2000, 2721.
2
3
(a) X. Huang, B. Borhan, B. H. Rickman, K. Nakanishi and N.
Berova, Chem. Eur. J., 2000, 6, 216; (b) X. Huang, B. H. Rickman,
B. Borhan, N. Berova and K. Nakanishi, J. Am. Chem. Soc., 1998,
120, 6185; (c) H. Jiang, X. Huang, K. Nakanishi and N. Berova,
T etrahedron L ett., 1999, 40, 7645; (d) H. Tamiaki, N. Matsumoto
and H. Tsukube, T etrahedron L ett., 1997, 38, 4239; (e) for
boronic acid diporphyrin tweezers in the recognition of sugar
derivatives see: M. Takeuchi, T. Imada and S. Shinkai, J. Am.
Chem. Soc., 1996, 118, 10658.
4
5
D. Monti, M. Venanzi, G. Mancini, F. Marotti, L. La Monica
and T. Boschi, Eur. J. Org. Chem., 1999, 1901.
Systematic studies on the complexation-induced conformational
changes of 1Zn indicate that Na` is the ion that induces the most
pronounced conformational variation: L. La Monica, D. Monti,
G. Mancini, M. Montalti, L. Prodi, N. Zaccheroni, G.
DÏArcangelo and R. Paolesse, New J. Chem., 2001, 25, 597.
P. Bhyrappa and V. Krishnan, Inorg. Chem., 1991, 30, 239.
Extensive studies on the binding properties of the receptor 1Zn
and other related diporphyrin structures toward a series of
dinitrogen ligands will be reported elsewhere.
The distances between the nitrogen interaction sites of the
diamines, calculated by AM1 semiempirical molecular orbital
method, are 7.15 and 2.92 A for bpy and 1,2-DACy, respectively:
M. J. S. Dewar, E. G. Zoebisch, E. F. Healy and J. J. P. Stewart,
J. Am. Chem. Soc., 1985, 107, 3902.
K. A. Connors, Binding Constants. T he Measurement of Molecu-
lar Complex Stability, John Wiley & Sons, New York, 1987.
6
7
8
16 G. Snatzke, Circular Dichroism: An Introduction, in Circular
Dichroism, Principles and Applications, ed. K. Nakanishi, N.
Berova and R. W. Woody, VCH Publishers, New York, 1994.
17 It has been reported that the binding of aliphatic a,x-diamines of
9
di†erent length to
a binaphthyl-spacered chiral diporphyrin
10 pKa values for bpy and 1,2-DACy are 4.82 and 9.9, respectively,
in water. Although corresponding data in acetonitrile are not
available in the literature, good estimates would be that of pyri-
dine (12.3) and ethylenediamine (18.5 and 13.0) which have been
reported for that solvent: K. Izutsu, Acid-Base Dissociation Con-
stants in Dipolar Aprotic Solvents, UPAC Chemical Data Series
no. 32, Blackwell ScientiÐc Publications, Oxford, 1990.
receptor results in a modulation of the CD intensities: T. Ayashi,
M. Nonoguchi, T. Aya and H. Ogoshi, T etrahedron L ett., 1997,
38, 1603.
18 It is known that the interaction of cations with non-cyclic crown-
ether-like podands induces a three dimensional conformation of
the inclusion complexes. See for example: G. W. Gokel, Crown
Ethers and Cryptands, in Monographs in Supramolecular Chem-
istry, ed. J. F. Stoddart, The Royal Society of Chemistry, Cam-
bridge, 1991, ch. 4, p. 99.
11 (a) The EM values have been calculated on the basis of the equa-
tion EM \ K/4K2 ; where K is relative to the interaction of the
inter
782
New J. Chem., 2001, 25, 780È782