Aqueous one-pot synthesis of derivatized cyclopentadienyl-tricarbonyl complexes of 99mTc with an in situ CO source: Application to a serotonergic receptor ligand

Article abstract of DOI:10.1002/1521-3773(20010817)40:16<3062::AID-ANIE3062>3.0.CO;2-O

Instant radiopharmaceuticals - just add [^99mTcO₄]^- in water! Half-sandwich complexes of the type [(RCOCp)^99mTc(CO)₃] (e.g. 1) were synthesized in a single step in water from [^99mTcO₄

Full text of DOI:10.1002/1521-3773(20010817)40:16<3062::AID-ANIE3062>3.0.CO;2-O

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[10] Typical procedure for the improved phenolic oxidative coupling: To a
solution of 3d (2.0 g, 4.0 mmol) in 2,2,2-trifluoroethanol (100 mL) was
added a solution of PIFA (1.9 g, 4.4 mmol) in 2,2,2-trifluoroethanol
(10 mL) at 408C under nitrogen, and the resulting mixture was
stirred for 1 h. After the evaporation of the solvent under reduced
pressure, the product 4d (1.29 g, 65%) was isolated by crystallization
of the residue from ethyl acetate. The additional crop of 4d (341.5 mg,
17%) was obtained by column chromatography (chloroform/meth-
anol 20:1) of the condensed mother liquor, followed by crystallization
from ethyl acetate. 4d: colorless needles; m.p. 187 ± 1898C (dec.)
(ethyl acetate); 1H NMR spectra of a mixture of two conformational
isomers were measured. The signals for the minor isomer are shown in
parentheses. 1H NMR (400 MHz, CDCl3, 258C, TMS): d ˆ (8.21) 8.19
(s, 1H; CHO), 7.47 ± 7.22 (m, 10H), (7.05) 6.98 (d, J ˆ 10.2 Hz, 2H),
(6.76) 6.57 (s, 1H; Ar-H), 6.14 (d, J ˆ 10.2 Hz, 2H), 5.15 (5.14) (s, 2H),
4.88 (4.84) (s, 2H), (4.69) 4.60 (s, 2H), 3.75 (3.74) (s, 3H), 3.71 (3.65) (t,
J ˆ 6.2 Hz, 2H), (2.29) 2.26 (t, J ˆ 6.2 Hz, 2H); IR (CHCl3): nÄ ˆ 1663
Scheme 2. a) 1) Tf2O, pyridine, 99%, 2) Pd(OAc)2, PPh3, Et3N, HCO2H,
97%; b) ethylene glycol, PPTS, 92%; c) 1) LiAlH4, 2) HCl, 83% (2 steps);
d) 1) L-Selectride, 2) LiAlH4, 61% (2 steps). PPTS ˆ pyridinium p-tolu-
enesulfonate.
carbonyl group, as previously reported.[5g] (Æ)-Galanthamine
has been optically resolved previously,[5g] and (Æ)-narwedine
(2) has been converted into ( )-galanthamine (1).[5i,j, 8c]
Therefore, the described efficient route to (Æ)-narwedine
(2) and (Æ)-galanthamine implies a formal total synthesis of
( )-galanthamine (1).
In conclusion, we were able to improve the pivotal phenolic
oxidative coupling reaction of norbelladine-type derivatives,
which had been limited to moderate yields over 40 years, and
have provided an efficient synthetic route for the industrial
production of ( )-galanthamine (1).
(C O, dienone), 1620 and 1593 (C O, formyl), 1123 cm 1 (C O); EI-
ˆ
ˆ
‡
MS (70 eV): m/z (%): 465 (7) [M ], 404 (6), 91 (100), 65 (7); HRMS
‡
calcd for C31H29NO5 [M ]: 495.2045, found: 495.2052; elemental
analysis: calcd (%) for C31H29NO5: C 75.13, H 5.90, N 2.83; found: C
75.26, H 5.98, N 2.92.
[11] D. A. Whiting in Comprehensive Organic Synthesis, Vol. 3 (Eds.: B. M.
Trost, I. Fleming, G. Pattenden), Pergamon, Oxford, 1991, pp. 803 ±
820.
[12] a) Y. Kiso, S. Nakamura, K. Ito, K. Ukawa, K. Kitagawa, T. Akita, H.
Moritoki, J. Chem. Soc. Chem. Commun. 1979, 971 ± 972; b) Y. Kiso,
K. Ukawa, S. Nakamura, K. Ito, T. Akita, Chem. Pharm. Bull. 1980,
28, 673 ± 676; c) M. Node, K. Nishide, M. Sai, K. Fuji, E. Fujita, J. Org.
Chem. 1981, 46, 1991 ± 1993.
[13] L. Jacob, M. Julia, B. Pfeiffer, C. Rolando, Synthesis 1983, 451 ± 452.
[14] a) S. Allen, T. G. Bonner, E. J. Bourne, N. M. Saville, Chem. Ind. 1958,
630; b) T. G. Bonner, E. J. Bourne, S. McNally, J. Chem. Soc. 1960,
2929 ± 2934; c) A. B. Foster, D. Horton, N. Salim, M. Stacey, J. M.
Webber, J. Chem. Soc. 1960, 2587 ± 2596.
Received: March 26, 2001 [Z16843]
[1] a) O. Hoshino in The Alkaloids, Vol. 51 (Ed.: G. A. Cordell),
Academic Press, New York, 1998, pp. 323 ± 424; b) S. F. Martin in
The Alkaloids, Vol. 30 (Ed.: A. Brossi), Academic Press, New York,
1987, pp. 251 ± 376.
[15] S. Cacchi, P. G. Ciattini, E. Morera, G. Ortar, Tetrahedron Lett. 1986,
27, 5541 ± 5544.
[2] R. L. Irwin, H. J. Smith III, Biochem. Pharmacol. 1960, 3, 147 ± 148.
[3] a) H. A. M. Mucke, Drugs Today 1997, 33, 251 ± 257; b) M. Rainer,
Drugs Today 1997, 33, 273 ± 279.
[4] W.-C. Shieh, J. A. Carlson, J. Org. Chem. 1994, 59, 5463 ± 5465.
[5] a) D. H. R. Barton, G. W. Kirby, Proc. Chem. Soc. 1960, 392 ± 393; J.
Chem. Soc. 1962, 806 ± 817; b) T. Kametani, K. Yamaki, H. Yagi, K.
Fukumoto, J. Chem. Soc. C 1969, 2602 ± 2605; c) K. Shimizu, K.
Tomioka, S. Yamada, K. Koga, Heterocycles 1977, 8, 277 ± 282; d) K.
Shimizu, K. Tomioka, S. Yamada, K. Koga, Chem. Pharm. Bull. 1978,
26, 3765 ± 3771; e) J. Szewczyk, A. H. Lewin, F. I. Carroll, J. Hetero-
cycl. Chem. 1988, 25, 1809 ± 1811; f) R. Vlahov, D. Krikorian, G.
Spassov, M. Chinova, I. Vlahov, S. Parushev, G. Snatzke, L. Ernst, K.
Kieslich, W.-R. Abraham, W. S. Sheldrick, Tetrahedron 1989, 45,
3329 ± 3345; g) J. Szewczyk, J. W. Wilson, A. H. Lewin, F. I. Carroll, J.
Heterocycl. Chem. 1995, 32, 195 ± 199; h) Y. Kita, M. Arisawa, M.
Gyoten, M. Nakajima, R. Hamada, H. Tohma, T. Takada, J. Org.
Chem. 1998, 63, 6625 ± 6633; i) L. Czollner, W. Frantsits, B. Küenburg,
U. Hedenig, J. Fröhlich, U. Jordis, Tetrahedron Lett. 1998, 39, 2087 ±
2088; j) B. Küenburg, L. Czollner, J. Fröhlich, U. Jordis, Org. Process
Res. Dev. 1999, 3, 425 ± 431.
[6] a) B. M. Trost, F. D. Toste, J. Am. Chem. Soc. 2000, 122, 11262 ±
11263; see also a synthesis of the galanthamine ring system: b) P. J.
Parsons, M. D. Charles, D. M. Harvey, L. R. Sumoreeah, A. Shell, G.
Spoors, A. L. Gill, S. Smith, Tetrahedron Lett. 2001, 42, 2209 ± 2211; a
formal synthesis of (Æ)-lycoramine by means of the Heck reaction:
c) E. Gras, C. Guillou, C. Thal, Tetrahedron Lett. 1999, 40, 9243 ± 9244.
[7] A. R. Battersby, W. I. Taylor, Oxidative Coupling of Phenols, Dekker,
New York, 1967.
Aqueous One-Pot Synthesis of Derivatized
Cyclopentadienyl ± Tricarbonyl Complexes of
99mTc with an In Situ CO Source: Application to
a Serotonergic Receptor Ligand**
Joachim Wald, Roger Alberto,* Kirstin Ortner, and
Lukas Candreia
The radionuclide 99mTc is among the most widely used
isotopes in diagnostic nuclear medicine.[1] It is cheap, exhibits
good decay characteristics, and typically burdens the patient
with a low radiation dose. Whereas in the past 99mTc-
complexes were preferentially applied as perfusion agents,[2]
a challenge now lies in combining a 99mTc complex with a
targeting molecule such as a tumor-specific peptide or a small
[*] Prof. Dr. R. Alberto, Dr. J. Wald, Dr. K. Ortner, L. Candreia
Institute of Inorganic Chemistry, University of Zürich
Winterthurerstrasse 190, 8057 Zürich (Switzerland)
Fax : (‡41)1-635-68-03
[8] a) R. A. Holton, M. P. Sibi, W. S. Murphy, J. Am. Chem. Soc. 1988, 110,
314 ± 316; b) D. A. Chaplin, N. Fraser, P. D. Tiffin, Tetrahedron Lett.
1997, 38, 7931 ± 7932; c) D. A. Chaplin, N. B. Johnson, J. M. Paul, G. A.
Potter, Tetrahedron Lett. 1998, 39, 6777 ± 6780.
[9] The norbelladine-type derivatives 3a ± h were easily prepared from
methyl gallate and tyramine in high yields, according to ref. [5d] and
[5e].
E-mail: ariel@aci.unizh.ch
[**] This work was supported by BBW (Bundesamt für Bildung und
Wissenschaft) and Mallinckrodt Med. Inc., Petten (The Netherlands).
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CNS receptor-binding molecule, as recently highlighted.[3]
The numerous technetium or rhenium complexes that have
been investigated in either case are essentially of the Werner
type, comprising tetradentate N,S-ligands or polyamino poly-
carboxylates.[4] One such complex, which has become the
most important myocardial imaging agent, is an organo-
metallic compound.[5] Since any clinically useful diagnostic
agent ultimately has to be prepared in water, the difficulty in
using sensitive organometallic ligands such as cyclopenta-
dienyl (Cp) is evident.
found that the pKa value of 4 is 8.62(1) in 0.1m NaCl
(potentiometric titration), which means an increase in acidity
by about 5 orders of magnitude relative to cyclopentadiene.
Thus, about 5% of 4 is dissociated at physiological pH. The
enolate form of 4 should be able to coordinate efficiently to
3a or its Re analogue, either through the alkoxide group, with
subsequent intramolecular rearrangement, or directly to the
negatively charged cyclopentadienyl ring, to afford the
desired half-sandwich complex as the thermodynamic prod-
uct.
Apart from these feasibility considerations, the Cp ligand
offers very attractive properties as a ligand for radiopharma-
ceutical purposes. Its inherent advantages are the small size,
the low molecular weight, and the stability of half-sandwich
complexes, [CpM(CO)3] (1; Me ˆ Re and Tc). These highly
crucial prerequisites minimize steric interference with the
receptor binding part of a labeled biomolecule. Complexes 1
are highly lipophilic, which makes them particularly interest-
ing as labeled, blood ± brain barrier-crossing molecules.
The fact that 1 (M ˆ Re) can be coupled to biomolecules by
classical organometallic methods without affecting the bio-
activity has been demonstrated by several groups.[6] The
physiological stability of the complexes [(R-Cp)Re(CO)3] (2;
R ˆ steroid hormone,[7] antibody,[8] etc.) initiated investiga-
tions into the preparation of [(R-Cp)99mTc(CO)3] (2a).
Wenzel reported a double ligand-transfer reaction, which
ultimately led to the formation of 2a starting from
The preparation of differently substituted carbonyl cyclo-
pentadienyl compounds (Na[R-Cp]) by the reaction of NaCp
with an ester functionality is straight forward and gives
typically 50 ± 60% yields. As a model for steroids, we
introduced a phenyl group instead of the methyl group to
get benzoyl-Cp (BCp) 5. To assess the possibility of attaching
Cp to more complicated (bio)molecules, we have chosen a
lead structure of the class of aryl piperazines, 1-(2-methoxy-
phenyl)piperazine 6, which are among the most thoroughly
studied ligands for the 5-HT1A subclass of serotonergic
receptors (Scheme 2).[16] Its Cp derivative 7 was prepared in
[
99mTcO4] .[9] Based on this approach, further improvements
towards a general synthesis of 2 and 2a have recently been
published[10] and octreotide was labeled in a five-step proce-
dure.[11] Both approaches still suffer from unacceptable
reaction conditions for routine use.
We recently reported the synthesis of fac-[99mTc-
‡
(OH2)3(CO)3] (3a) directly from [99mTcO4] in water.[12]
Precursor 3a is
[
a very versatile source of the ªfac-
‡
99mTc(CO)3] º moiety which accepts different chelators for
the remaining three positions. Attempts to coordinate cyclo-
pentadienyl or fulvene derivatives to 3a failed or resulted in
very low yields. This lack of reactivity is in agreement with the
low tendency of neutral alkenes to compete with water in
Scheme 2. Derivatization of the cyclopentadienyl group with different
organic (bio)molecules.
‡
[M(OH2)3(CO)3] (M ˆ 99mTc, Re). Alkyl h1 coordination of
deprotonated cyclopentadiene as the synthetic key step,[13]
preceding ring slippage to h5-coordination, is unlikely owing
to its prohibitively high pKa of about 14.
The key step in the preparation of 2a
the same way, after alkylation of the piperazine ring in 6 with
ethyl bromoacetate to give 6a. The pKa values for 7 were
found to be >12.5, 8.71(1), and 6.37(1). The dissociation
constants pKa1 and pKa3 were assigned to the tertiary amine
groups, and pKa2 represents the dissociation constant of
cyclopentadiene. In contrast to NaCp, 4, 5, and 7 are
reasonably soluble in water and are stable for hours, even
when exposed to air.
To demonstrate the possibility of preparing complexes with
the general composition 2 (M ˆ Tc or Re) directly in water, 3
was treated in phosphate buffer at pH 7.4 with 4. Within 2 h at
858C, [(ACp)M(CO)3] (8) formed in about 15 ± 25% yield for
Re, and 40% for Tc. This is one of the first examples in which
a Cp derivative reacts in water with an organometallic
precursor to yield the corresponding half-sandwich complex.
We anticipate that this convenient strategy can be extended to
the preparation of other half-sandwich complexes, probably
in water lies in the ability of a Cp-
derivative to be deprotonated to a
reasonable extent at physiological
pH. This can be achieved by introduc-
ing electron-withdrawing substituents.
The requirements of delocalizing the
resulting negative charge over the Cp
system prompted us to investigate the
well-known acetyl cyclopentadiene (4;
ACp),[14] in which the keto ± enol tau-
Scheme 1. Acidifica-
tion of acetyl cyclo-
tomerism stabilizes the deprotonated
form (Scheme 1).
The pKa value of formyl cyclopenta-
diene is about 8.[15] More accurately, we
pentadiene
4 by ke-
to ± enol tautomerism.
Several isomers are
possible.
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containing water ligands. As depicted in Scheme 3, the
preparation of 2 directly in water is not restricted to 4;
compound 5 and the receptor binding molecule 7 also reacted
comparably in water with 3 to yield the corresponding
complexes [(Cp')M(CO)3] (9, Cp' ˆ 5; 10, Cp' ˆ 7).
since the concentration of 99mTc is very low (nm ± mm range).
The precursor 3a was prepared as previously reported,[12]
buffered (phosphate buffer, pH 7.4, 1 ± 2 cm3), and then mixed
with an aqueous solution containing, for example, 0.1 ± 1 mmol
of 4. The reaction was carried out for 15 min at 908C to afford
the corresponding complex 8(99mTc) in quantitative yield. Its
identity was confirmed by coinjecting 8(Re) and 8(99mTc) into
the HPLC (high-pressure liquid chromatrograph) (Figure 2),
thus allowing the simultaneous detection by UV absorption
and radiodetection (retention times 21.5 min for the Re-
species and 21.9 min for the 99mTc-species; the delay is a result
of the separation of the two detectors).
Scheme 3. Synthesis of cyclopentadienyl complexes [(R-Cp)M(CO)3]
(M ˆ Re) in water. Reagents and conditions: a) 5, phosphate buffer,
pH 7.4, 2 h, 858C.
The observed dimerization of the ligands at elevated
temperatures and higher concentrations can be compensated
by a slight ligand excess. The limitation of the yield essentially
arose from the competing hydrolytic reaction of 3 with the
very stable cluster compound [M(m3-OH)(CO)3]4, which
represents the only rhenium-containing by-product.[17] The
rhenium compounds of 9 and 10 were crystallized, and their
structures were elucidated by X-ray studies. A SCHAKAL
representation of 10 is given in Figure 1.[18, 19]
Figure 2. The upper trace shows the UV absorption at 250 nm for 8 (Re);
the lower trace shows the radioactive trace for the corresponding 99mTc
complex, proving their identity (delay due to detector separation).
This synthetic pathway was also applied to the Cp-
derivatized serotonergic receptor ligand 7 with identical
results. Excess 7 dimerizes during the reaction and the
precipitate can be removed by filtration, thus increasing the
specific activity efficiently, which may be of importance for
preventing a saturation of in vivo receptors with a ªcoldº
substrate. This unique reaction clearly reveals the high
potential behind the chosen approach, which allows the
preparation and use of half-sandwich 99mTc-complexes in
physiological solutions for the first time, in quantitative yield
and convenient for routine use.
At this stage, the reaction consisted of two steps, the initial
preparation of 3a and subsequent reaction with the respective
Cp-derivatized (bio)molecule. To achieve a true one-pot
synthesis for [(R-Cp)99mTc(CO)3], we introduced potassium
boranocarbonate (11, K2H3BCO2), the preparation and prop-
erties of which were recently reported by us.[22] Boranocar-
bonate 11 is an air-stable reducing agent, and acts as an in situ
CO source in water at elevated temperatures. The simple
addition of [99mTcO4] in a saline solution to a vial that
contained a small amount of 11, is sufficient to give 3a in
quantitative yield. If, in addition to 11, a small amount of the
Cp-derivative 4 or 7 was present, 40 min at 908C was
sufficiently long to produce the corresponding half-sandwich
complexes 8 or 10 (99mTc) in a single step in >95% yield
(Scheme 4).
Figure 1. SCHAKAL representation of complex 10 (Re) with protonated
piperazine shown without the counterion Cl .[19]
Complex 9 (M ˆ Re) has been prepared by Friedel ± Crafts
acylation of 1 by Fischer et al.[20] The crystal structures of 9
and 10 exhibit the typical piano-stool arrangements. The
average bond lengths of Re C(h5) are 2.31 Š in both cases.
The preparation of [(R-Cp)Re(CO)3] directly in water
offers a convenient alternative to the classical approach,
which starts from [ReBr(CO)5] in benzene.[21] This is partic-
ularly important for the syntheses of half-sandwich complexes
with a pendant biomolecule, since the latter are often
insoluble in common organic solvents. The availability of a
ªcoldº compound is essential for radiopharmaceutical appli-
cation of the labeled ªhotº biomolecules, as their character-
ization is only possible by chromatographic comparison.
To examine the potential of this novel approach for routine
preparations of Cp-derivatized biomolecules such as 7 labeled
with 99mTc, we transferred the reaction conditions to the no-
carrier-added (n.c.a.) level. For kinetic reasons, cluster
formation as the major competing reaction is not expected,
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J ˆ 4.8 Hz, 2H), 2.65 (t, J ˆ 4.8 Hz, 2H); 13C NMR (300 MHz, CD3CN):
d ˆ 193.7, 153.3, 142.4, 124.1, 121.9, 119.2, 112.0, 96.1, 90.2, 89.7, 87.4, 86.9,
65.6, 54.2, 50.9. FAB-MS (fast-atom bombardment mass spectrometry):
‡
[M ] 567.8 (567).
‡
[(R-Cp)99mTc(CO)3]: The starting material [99mTc(OH2)3(CO)3] (3a) was
prepared as described elsewhere.[12, 22] An aqueous solution of 7, for
example, in phosphate buffer was added to 3a (final concentration of 7 ˆ
10 3 ± 10 4 m). Subsequent heating at 958C for 15 min afforded
[(R-Cp)99mTc(CO)3] in overall yields >95%. The HPLC retention times
for 8 and 10 are 21.9 and 20.5 min, respectively.
For the one-pot synthesis of 8 or 10 (M ˆ 99mTc), a vial was charged with 11
(4 mg, reducing agent and CO source) and Na2B4O7 (5.5 mg), sealed and
flushed with dry N2. Generator eluate (2 cm3) containing [99mTcO4] was
added, followed by an aqueous solution of 4. Heating at 908C for about
40 min gave 8 or 10 in high yields (ꢀ80% and ꢀ20% reduced form). The
yield and time course of the reaction were not influenced by the total
amount of radioactivity (up to 20 GBq of 99mTc).
Scheme 4. One- and two-step synthesis of complexes 8 and 10 (99mTc)
directly in water and [99mTcO4] . Reagents and conditions: a) CO (1 atm),
NaBH4, pH 12 or K2[H3BCO2] only, >98%. b) 4 or 7 (0.1 ± 1 mmol), 15 min
908C, >98%. c) 0.1 ± 1 mmol 4 or 7, K2[H3BCO2] (11; 4 mg), Na2B4O7,
pH 10, 40 min 908C, >95%.
Besides the major product, a second complex was formed in
minor amounts, with a similar retention time. It is likely that
11 reduced the carbonyl group in 8 to a hydroxy group.
We assume that the one-pot reaction is consecutive
Received: April 3, 2001 [Z16895]
[
99mTcO4] !3a !8 or 10. All the compounds labeled with
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without any significant decomposition, thus confirming the
highly robust nature of [(R-Cp)M(CO)3].
We demonstrated herein that half-sandwich complexes
[(R-Cp)M(CO)3] can easily be synthesized if the acid
dissociation constant of the cyclopentadiene ring is increased.
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The reaction of acetyl cyclopentadiene and derivatives with
[
‡
99mTc(OH2)3(CO)3] directly yielded the radiopharmaceuti-
cally relevant complexes [(R-Cp)99mTc(CO)3] in good yields.
The major impact of this work emerges from the general
possibility of introducing the very small and highly lipophilic
[Cp99mTc(CO)3] moiety in a wide variety of small receptor-
binding biomolecules. Furthermore, the direct reaction of
acidic and water-soluble cyclopentadiene compounds with
aqua ions could lead to interesting and novel species in
aqueous organometallic chemistry.
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Experimental Section
7: Alkylation of 1-(2-methoxyphenyl)piperazine (Fluka) with ethyl bro-
moacetate for 24 h at room temperature in the presence of Hünigꢁs base,
and purification on silica gel (CH2Cl2/ethylacetate 4:1 v/v) afforded 6a in
65 ± 70% yield. NaCp was freshly prepared in situ from Na (0.3 g,
12.7 mmol) and the corresponding amount of freshly distilled dicyclopen-
tadiene. Compound 6a in THF (1.8 g, 6.4 mmol) was added to the NaCp
solution and heated at about 608C for 6 h to afford a precipitate, which was
isolated by filtration under N2. The solid was washed several times with n-
hexane to leave an almost colorless powder, which was recrystallized from
hot THF to give analytically pure 7. 1H NMR (200 MHz, D2O): d ˆ 7.0 (m,
4H), 6.60 (brs, 2H), 6.15 (brs, 1H), 6.06 (brs, 1H), 3.74 (s, 3H), 3.50 (s,
2H), 2.95 (brs, 4H), 2.70 (brs, 4H); 13C NMR (300 MHz, D2O): d ˆ 182.4,
150.4, 138.2, 122.5, 122.2, 119.1, 116.9, 116.5, 115.9, 114.2, 111.3, 109.7, 59.8,
53.0, 50.5, 48.1.
[15] K. Hafner, G. Schulz, K. Wagner, Justus Liebigs Ann. Chem. 1964, 678,
39 ± 48.
8: Compound 8 can be synthesized according to published routes, starting
from [ReBr(CO)5] or directly from an aqueous buffer solution. (NEt4)2-
[ReBr3(CO)3] (130 mg, 0.15 mmol) in phosphate buffer (15 mL) was added
dropwise over a period of 2 h to compound 7 (70 mg, 0.18 mmol) dissolved
in phosphate buffer (15 mL, pH 7.4). The mixture was heated for 3 h at
758C, causing a brownish precipitate containing the product, as shown by
using HPLC. The precipitate was filtered and dissolved in CH2Cl2.
Separation of the product was performed by silica-gel column chromatog-
raphy. Elution with CH2Cl2 gave nonpolar side products, whereas
subsequent elution with CH2Cl2/2-propanol (80:1 v/v) gave compound 8
(20 mg, 23%). The product was recrystallized from an acidic mixture of
EtOH/water by slow evaporation of ethanol. 1H NMR (200 MHz, CD3CN):
d ˆ 6.9 (m, 4H), 6.24 (t, 2H), 5.53 (t, 2H), 3.79 (s, 3H), 3.48 (s, 2H), 3.05 (t,
[16] S. K. Kulkarni, K. O. Aley, Drugs Today 1988, 24, 175 ± 183; R. A.
Glennon, R. B. Westkaemper, P. Bartyzer in Serotonin Receptor
Subtypes (Ed.: S. J. Peroutka), Wiley, New York, 1991, pp. 19 ± 64;
I. A. Cliffe, A. Fletcher, Drugs Future 1993, 18, 631 ± 642; A. A.
Wilson, T. Inaba, N. Fischer, L. M. Dixon, J. Nobrega, J. N. DaSilva, S.
Houle, Nucl. Med. Biol. 1998, 25, 769 ± 776.
[17] R. Alberto, R. Schibli, A. Egli, U. Abram, S. Abram, T. A. Kaden,
P. A. Schubiger, Polyhedron 1998, 17, 1133 ± 1140.
[18] Crystallographic analysis: 10 (C21H22ClN2O5Re), colorless column,
0.19  0.08  0.03 mm3, monoclinic, space group P21/c, a ˆ
22.8795(14), b ˆ 14.0402(11), c ˆ 7.1351(4) Š, b ˆ 94.495(7)8, Vˆ
2285.0(3) Š3, Z ˆ 4, 1calcd ˆ 1.756 gcm 3, R1 (I ꢁ 2s(I)) ˆ 0.0325, wR2
(F 2) ˆ 0.1090 for 22230 data (5468 independent, 1263 observed
Angew. Chem. Int. Ed. 2001, 40, No. 16
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COMMUNICATIONS
(I ꢁ 2s(I))), 275 parameters,
6 restraints, numerical absorption
with a colorimetric or fluorescent indicator, namely, a highly
colored or light-emitting substrate. Two main requirements
have to be fulfilled: the receptor/indicator interaction must
not be too strong and the indicator must show significantly
different optical properties when bound to the receptor and
when dispersed in solution. Thus, the indicator is displaced
from the host cavity on titration with the desired analyte and
released to the solution, where it displays drastically different
optical features. Hence, the occurrence of the recognition
event is communicated by either a substantial color change or
a dramatic modification of the light emission. In their
competition studies Anslyn and co-workers considered anion
receptors capable of interacting with the analyte through
formation of hydrogen bonds. Typical investigated analytes
were citrate, tartrate, malate, and inositol triphosphate. The
procedure of competitive spectrophotometry has been treated
in detail by Connors.[3] Ueno and co-workers later extended
the approach by covalently linking the dye to the receptor.[4]
We now extend the competition approach to the fluorescent
sensing of anions by making use of a different type of
receptor± analyte interaction: metal ± ligand (coordinative)
interactions. Coordinative interactions present some substan-
tial advantages compared to hydrogen bonding and, in
general, to electrostatic interactions.[5] In fact, they can be
highly energetic as a result of the strong ligand-field
stabilization energy effects that may be induced by coordina-
tion. Moreover, transition metal ions present definite stereo-
chemical preferences which can be addressed to impart
selective binding tendencies towards anions of a given shape.
In order to benefit from these features, metal ± ligand
interactions have been utilized in the design of anion
receptors operating in highly polar media, including pure
water. In this connection, we have shown that the dicopper(ii)
cage complex 1 is capable of including polyatomic anions
correction, m ˆ 5.468 mm 1, Tmin ˆ 0.5802, Tmax ˆ 0.8524. Single crys-
tals of 10 were obtained by dissolving the product in a slightly acidic
EtOH/water mixture and slow evaporation of ethanol. Data were
collected on a Stoe IPDS diffractometer using graphite-monochro-
mated MoKa (l ˆ 0.71073 Š) radiation, and were corrected for
Lorentz, polarization, and absorption effects. The structure was
solved by direct methods with SHELXS97 and refined with
SHELXL97 on F 2 using all data with all non-hydrogen atoms
anisotropically defined.[23] The hydrogen atoms were placed in
calculated positions and isotropically refined with thermal parameters
at 1.5 Â U(eq) of the parent carbon atom, except for the piperazyl-
N(H) which was found in the difference Fourier map and isotropically
refined. Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publica-
tion nos. CCDC-160745 (9) and CCDC-160746 (10). Copies of the
data can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB21EZ, UK (fax: (‡44)1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk).
[19] E. Keller, SCHAKAL97, University of Freiburg, 1997.
[20] E. O. Fischer, W. Fellmann, J. Organomet. Chem. 1963, 1, 191 ± 199.
[21] W. P. Hart, D. W. Macomber, M. D. Rausch, J. Am. Chem. Soc. 1980,
102, 1296 ± 1298.
[22] R. Alberto, K. Ortner, N. Wheatley, R. Schibli, A. P. Schubiger, J. Am.
Chem. Soc. 2001, 123, 3135 ± 3136.
[23] G. M. Sheldrick, SHELXS97, University of Göttingen, 1990; G. M.
Sheldrick, SHELXL97, University of Göttingen, 1997.
A Chemosensing Ensemble for Selective
Carbonate Detection in Water Based on
Metal ± Ligand Interactions**
Luigi Fabbrizzi,* Antonella Leone, and
Angelo Taglietti
The classical approach to the design of a fluorescent sensor
involves the covalent linking of a fluorescent fragment (the
ªsignaling unitº) to a receptor, which displays specific binding
tendencies towards a given analyte. In order to communicate
the occurrence of the recognition event to the outside world,
the receptor/analyte interaction must affect, to some extent,
the emission properties of the signaling unitÐeither quench-
ing the fluorescence through a defined mechanism (usually
energy or electron transfer) or enhancing it by suppressing a
pre-existing quenching process.[1]
Recently, Anslyn and co-workers[2] have taken inspiration
from antibody-based biosensors in immunoessays and devel-
oped an efficient ªcompetitionº approach to the design of
chemosensors: according to the so-called ªchemosensing
ensembleº approach, the plain receptor interacts in solution
within its intermetallic cavity in an aqueous solution buffered
at pH 8. The ion selectivity is determined by the bite length of
the anion, that is, the distance between two consecutive donor
atoms.[6] In particular, receptor 1 showed a definite preference
towards the triangular HCO3 ion[7] and the two linear
triatomic ions N3 and NCO , whose bite length range
between 2.28 and 2.42 Š. The intrinsic limit of receptor 1 as
a proper molecular sensor is that only small changes in the
absorption spectra and in the color are observed following
anion recognition. For example, in the case of HCO3 ions, the
color turns from the pale blue of the void receptor to the pale
[*] Prof. L. Fabbrizzi, Dr. A. Leone, Dr. A. Taglietti
Dipartimento di Chimica Generale
Á
Universita di Pavia, viale Taramelli 12
27100 Pavia (Italy)
Fax : (‡39)0382-528544
E-mail: luigi.fabbrizzi@unipv.it
[**] This work was supported by the CNR ªProgetto Finalizzato ªBio-
tecnologieºº and MURST ªProgramma Chimicaº Contract
no. 99.03356.PF28.
3066
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1433-7851/01/4016-3066 $ 17.50+.50/0
Angew. Chem. Int. Ed. 2001, 40, No. 16