Alkali metal cation cooperative iodide anion recognition by new heteroditopic bis(calix[4]arene) rhenium(I) bipyridyl receptor molecules

Article abstract of DOI:10.1039/a707324k

New heteroditopic bis(calix[4]arene) rhenium(I) bipyridyl receptors are synthesised and shown to simultaneously bind alkali metal cations and iodide anion with positive cooperativity.

Full text of DOI:10.1039/a707324k

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Alkali metal cation cooperative iodide anion recognition by new heteroditopic
bis(calix[4]arene) rhenium(i) bipyridyl receptor molecules
Paul D. Beer* and James B. Cooper
Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, UK OX1 3QR
New heteroditopic bis(calix[4]arene) rhenium(I) bipyridyl
receptors are synthesised and shown to simultaneously bind
alkali metal cations and iodide anion with positive
cooperativity.
neutral transition metal organometallic and coordination amide
containing receptor systems can selectively bind and sense
anions.7 Lower rim ester functionalised calix[4]arenes are
known to coordinate alkali metal cations.8 Incorporating these
two types of recognition sites on to the calix[4]arene building
block will create potential heteroditopic receptors capable of
binding anions and cations. We report here the syntheses of new
heteroditopic bis(calix[4]arene) rhenium(i) bipyridyl receptors
which display positive cooperative upper rim binding of the
iodide anion via lower rim complexation of alkali metal
cations.
The new upper rim mono-amine–lower rim ester substituted
calix[4]arene derivatives 1 and 2 were prepared according to
Scheme 1. Condensation reactions of 2 equiv. of 1 and 2 with
4,4A-bis(chlorocarbonyl)-2,2A-bipyridine 39 followed by com-
plexation with Re(CO)5Cl gave the new receptors L1 and L2 in
good yields (Scheme 2).
The design of new ditopic1 ligands for the simultaneous
complexation of anionic and cationic guest species is a new
exciting area of coordination chemistry of significant relevance
to the selective extraction and/or transportation of metal salts
across lipophilic membranes. Rare examples of receptors
containing appropriate covalently linked binding sites for
anions and cations include Lewis-acidic boron,2 uranyl,3
polyammonium4 centres combined with crown ether moieties
and crown ether or urea functionalised calix[4]arene iono-
phores5,6 which are capable of solubilising alkali metal salts
into organic solvent media. We have shown that charged or
The cation and anion coordination properties of both
receptors were investigated by 1H NMR titration experiments in
CD3CN solution. The addition of LiClO4, NaClO4 and KPF6
salts typically caused the ester methylene receptors’ protons to
initially broaden and sharpen again after 2 equiv. suggesting
complexes of 2M+ :L stoichiometry are being formed in
i
O
OH
OH
OH
OH
OH
OH
OH
ii
O
NH2
Cl
iii
N
O
HO
O
O
O
O
O
O
O O
2
+
O
O
O
RO
O
O
O O
N
O
O
O
OEt
OEt
OEt
OEt
OEt
OEt
Cl
OEt
NO2
OEt
OEt
NO2
iv
ii
O
3
1 R = H
2 R = CH2CO2Et
i
NEt3
ii [Re(CO)5Cl]
HO
O
O
O
O
O
O
O
O
OEt
RO
O
O
O
O
EtOEtO O
O
O
OEt
O
OEt
OEt
OEt
OEt
O
EtO
O
EtO
NO2
NH2
O
O
O
N
H
v
CO
CO
CO
N
N
RO
RO
O
O
O
O
O
O
Re
Cl
O
O
O
O
O
O
H
N
OEt
OEt
OEt
OEt
OEt
OEt
O
1 R = H
2 R = CH2CO2Et
O
O
EtO
EtO
O
RO
O
Scheme 1 Reagents and conditions i, 1 equiv. PhCH2Br, 0.5 equiv. K2CO3,
MeCN reflux, 24 h; ii, BrCH2CO2Et (excess), K2CO3 (excess), MeCN
reflux 48 h; iii, Pd/C, HCO2NH4, EtOH reflux; iv, NH4NO3, HCl, H2O,
acetic anhydride, CH2Cl2; v, Zn (excess), HCl (excess), reflux EtOH,
24 h
O
L1 R = H
L2 R = CH2CO2Et
O
OEt
Scheme 2
Chem. Commun., 1998
129

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Table 1 Stability constants for iodide binding in the presence and absence
of alkali metal cations in CD3CN
bond electrostatic effects of the complexed metal cation may
enhance the relative acidity of the receptors’ amide protons and
lead to stronger hydrogen bonding with the iodide guest anion.
Interestingly Table 1 shows that receptor L2 exhibits the largest
positive cooperative iodide anion binding effect with the
sodium cation, which is known to form highly selective
complexes with lower rim tetrasubstituted ethyl ester cal-
ix[4]arenes.8
In conclusion these new heteroditopic bis(calix[4]arene)
rhenium(i) bipyridyl receptors are capable of simultaneously
binding alkali metal cations and iodide anion with positive
cooperativity.
Receptor
Metal cationb
Ka/dm3 mol21
L1
L1
L1
L1
L2
L2
L2
L2
None
Li+
67
294
202
100
40
305
322
209
Na+
K+
None
Li+
Na+
K+
We thank the EPSRC for a studentship and for use of the
mass spectrometry service at University College, Swansea.
a
b
Errors estimated to be @5%. Titration carried out in the presence of 2
equiv. of alkali metal cation salt, perchlorates for lithium, sodium and
hexafluorophosphate for potassium.
Footnote and References
solution, with the alkali metal cations coordinated at the lower
rim ester recognition sites. The addition of tetrabutylammonium
chloride, iodide and benzoate salts caused substantial downfield
perturbations of the respective receptor’s amide, H3bipyridyl
and aryl calix[4]arene protons indicating anion binding is taking
place at the upper-rim bis(calix[4]arene) vicinity of the
receptor. In all cases the resulting titration curves indicated 1:1
complex stoichiometry. Stability constants were calculated
from the titration data using EQNMR10 for complexation with
iodide (Table 1). Unfortunately the anion complexes with Cl2
* E-mail: paul.beer@icl.ox.ac.uk
1 J.-M. Lehn, Angew. Chem., Int. Ed. Engl., 1988, 27, 89.
2 M. T. Reetz, C. M. Niemeyer and K. Harris, Angew. Chem., Int. Ed.
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3 D. M. Rudkevich, Z. Brzozka, M. Palys, H. C. Visser, W. Verboom and
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5 P. D. Beer, M. G. B. Drew, R. J. Knubley and M. I. Ogden, J. Chem.
Soc., Dalton Trans., 1995, 3117.
2
and PhCO2 are so strong in CD3CN that only a semi-
6 J. Scheerder, J. P. M. van Duynhoven, J. F. J. Engbersen and
D. N. Reinhoudt, Angew. Chem., Int. Ed. Engl., 1996, 35, 1090.
7 P. D. Beer, Chem. Commun., 1996, 689 and references therein.
8 F. Arnaud-Neu, E. M. Collins, M. Deasy, G. Ferguson, S. J. Harris,
B. Kaitner, A. J. Lough, M. A. McKervey, E. Marques, B. L. Ruhl,
M. J. Schwing-Weill and E. M. Seward, J. Am. Chem. Soc., 1989, 111,
8681.
9 C. P. Whittle, J. Heterocycl. Chem., 1977, 14, 191.
10 M. J. Hynes, J. Chem. Soc., Dalton Trans., 1993, 311.
11 In ref. 6, lower rim complexation of alkali metal cations is essential for
upper rim halide anion complexation.
quantitative estimate of the value of K > 104 dm3 mol21 could
be made. The 1H NMR iodide titration experiments were
repeated in the presence of 2 equiv. of alkali metal salt and the
stability constant values are presented in Table 1. Clearly with
both receptors there is a significant increase in the strength of
iodide binding when the alkali metal cations are co-bound by
nearly an order of magnitude in the case of L2 and sodium
cations. This positive cooperative binding of the iodide anion
may be attributed to each lower rim ester complexed metal
cation rigidifying the calix[4]arene structure in such a way as to
preorganise the upper rim for anion binding.11 Also through
Received in Cambridge, UK, 10th October 1997; 7/07324K
130
Chem. Commun., 1998