Aqueous synthesis of derivatized cyclopentadienyl complexes of technetium and rhenium directed toward radiopharmaceutical application

Article abstract of DOI:10.1021/ic0204575

Half-sandwich complexes of the type [(RCOCp)M(CO)₃] with M = Re and ^99(m)Tc were synthesized from [M(OH₂)₃-(CO)₃]⁺ in water. The R group can be an organic residue or a receptor binding biomolecule with a spacer to cyclopentadienyl (Cp). This provides a general route to Cp complexes of technetium without the need for starting from [TcBr(CO)₅]. The X-ray structure of [{C₆H₅CH₂COC₅ H₄}Tc(CO)₃] has been elucidated. The compound crystallizes in the monoclinic space group P2_1/c with a = 16.1454(9), b = 7.6300(6), and c = 12.3922(7) A and β = 107,792(6)°, We have chosen a serotonergic receptor ligand (WAY) as an example for the derivatization of Cp with a bioactive molecule. WAY is linked to Cp by an aliphatic chain of variable length, The half-sandwich complexes were prepared from water and organic solvents. The structure of [(WAY4-Cp)Re(CO)₃] could be elucidated. The compound crystallizes in the monoclinic space group P2_1/c with a = 15.7112(6), b = 6.8775(3), and c = 25,5217(12) A and β = 103.778(5)°. Quantification of inhibition constants gave a clear structure-activity relationship. A single methylene group between the receptor binding site and the half-sandwich complex gave an IC₅₀ of 217 nM for HT_1A, whereas a butylene linker resulted in retention of the inhibition constant with an IC₅₀ of 6 nM with respect to underivatized WAY. For use as radiopharmaceuticals, the compounds have also been prepared with ^99mTc in quantitative yield.

Full text of DOI:10.1021/ic0204575

Inorg. Chem. 2003, 42, 1014−1022
Aqueous Synthesis of Derivatized Cyclopentadienyl Complexes of
Technetium and Rhenium Directed toward Radiopharmaceutical
Application
,†
Jonathan Bernard,† Kirstin Ortner,† Bernhard Spingler,† H.-J. Pietzsch,‡ and Roger Alberto*
Institute of Inorganic Chemistry, UniVersity of Zu¨rich, CH-8057 Zu¨rich, Switzerland, and Research
Center Rossendorf, Institute of Bioinorganic and Radiopharmaceutical Chemistry,
D-01314 Dresden, Germany
Received July 15, 2002
Half-sandwich complexes of the type [(RCOCp)M(CO)3] with M ) Re and 99(m)Tc were synthesized from [M(OH ) -
2 3
(CO)3]+ in water. The R group can be an organic residue or a receptor binding biomolecule with a spacer to
cyclopentadienyl (Cp). This provides a general route to Cp complexes of technetium without the need for starting
from [TcBr(CO)5]. The X-ray structure of [{C H CH COC H }Tc(CO)3] has been elucidated. The compound crystallizes
6
5
2
5 4
in the monoclinic space group P2 /c with a ) 16.1454(9), b ) 7.6300(6), and c ) 12.3922(7) Å and â ) 107.792(6)°.
1
We have chosen a serotonergic receptor ligand (WAY) as an example for the derivatization of Cp with a bioactive
molecule. WAY is linked to Cp by an aliphatic chain of variable length. The half-sandwich complexes were prepared
from water and organic solvents. The structure of [(WAY4−Cp)Re(CO)3] could be elucidated. The compound
crystallizes in the monoclinic space group P21/c with a ) 15.7112(6), b ) 6.8775(3), and c ) 25.5217(12) Å and
â ) 103.778(5)°. Quantification of inhibition constants gave a clear structure−activity relationship. A single methylene
group between the receptor binding site and the half-sandwich complex gave an IC of 217 nM for HT , whereas
50
1A
a butylene linker resulted in retention of the inhibition constant with an IC of 6 nM with respect to underivatized
50
WAY. For use as radiopharmaceuticals, the compounds have also been prepared with 99mTc in quantitative yield.
Introduction
metal complexes, in particular those with a d6 electronic
configuration, can be very useful due to their robust nature.
They introduced organometallic complexes to radiopharmacy
with the synthesis of [99mTc(CNR)6]+ which became an
important class of imaging agents.1,2 While most radiophar-
maceutical complexes in use today are perfusion agents, the
challenge for the future lies in preparing 99mTc radiophar-
maceuticals with specific receptor binding properties. This
requires a 99mTc fragment that can coordinate easily to ligands
attached to targeting molecules. Following this idea we
introduced the low-valent mixed aquo-carbonyl complex
Technetium and rhenium are the subject of great interest
as radiopharmaceuticals for diagnostic (99mTc) and therapeutic
(
188Re, 186Re) purposes. 99mTc is still the most widely used
isotope in diagnostic nuclear medicine. It is readily available
and inexpensive, has good decay characteristics, and typically
results in a low dose burden to the patient. Most complexes
investigated in research and applied in clinical practice to
this date are of the “Werner type”, comprising tetradentate
NxSy ligands or polyaminocarboxylates with an intermediate
oxidation state (i.e. +5). Some recent reviews cover these
aspects of technetium chemistry.1 Davison and co-workers
have demonstrated for the first time that low oxidation state
[
99mTc(OH2)3(CO)3]+ as a versatile precursor for the labeling
of biomolecules.3,4 Whereas Davison’s compound is inher-
ently stable under reasonable chemical and biological condi-
* To whom correspondence should be addressed. E-mail: ariel@
aci.unizh.ch.
(2) Jain, D. Semin. Nucl. Med. 1999, 29, 221-236.
(3) Alberto, R.; Schibli, R.; Schubiger, P. A.; Abram, U.; Kaden, T. A.
Polyhedron 1996, 15, 1079-1089.
(4) Alberto, R.; Schibli, R.; Egli, A.; Schubiger, P. A.; Abram, U.; Kaden,
T. A. J. Am. Chem. Soc. 1998, 120, 7987-7988.
† University of Zu¨rich.
‡ Institute of Bioinorganic and Radiopharmaceutical Chemistry.
(1) Jurisson, S. S.; Lydon, J. D. Chem. ReV. 1999, 99, 2205-2218.
Dilworth, J. R.; Parrott, S. J. Chem. Soc. ReV. 1998, 27, 43-55.
1014 Inorganic Chemistry, Vol. 42, No. 4, 2003
10.1021/ic0204575 CCC: $25.00 © 2003 American Chemical Society
Published on Web 01/23/2003

DeriWatized Cp Complexes of Tc and Re
tions, the aquo ligands in [99mTc(OH2)3(CO)3]+ are easily
exchanged for a variety of mono-, bi-, and tridentate ligands,
affording complexes of very high kinetic stability.5 Until now
these ligands were essentially histidine, histamine, and related
ones. Typical organometallic ligands such cyclopentadienyl
(Cp) are normally excluded from this list due to their
instability and insolubility in water even though the utility
of half-sandwich complexes with rhenium combined with
biomolecules has been demonstrated by several groups and
has pioneered the field of bioorganometallic chemistry.6-9
The tridentate Cp ligand offers many advantages for ap-
plication in nuclear medicine because of its small size and
low molecular weight. With these properties in mind, robust
complexes of the [RCOCpRe(CO)3] (R ) steroid hormone,9
antibody8) have been prepared by classical organometallic
methods and it could be shown that the bioactivity was
reasonably maintained, whereas larger complexes lead to loss
of affinity for the corresponding receptor. For the first time,
Wenzel10 has shown that it is possible to prepare half-
sandwich complexes [(Cp)99mTc(CO)3] by a so-called double
ligand transfer approach starting with radiopharmaceutically
useful [99mTcO4]-.11 Though there have been further im-
provements toward a general synthesis of [(RCOCp)Re-
(CO)3], each approach still suffers from unacceptable reaction
conditions for routing use.7,12
Scheme 1
opens a synthetic approach to labeling biomolecules with
half-sandwich complexes that is radiopharmaceutically rea-
sonable and can also be adapted to more basic aqueous
chemistry related to the water-soluble acetyl-Cp ligand.
Results and Discussion
Synthetic Aspects. Various methods are used in organo-
metallic Re and Tc chemistry to prepare “piano stool”
complexes, as depicted in Scheme 1. Rhenium carbonyl
complexes [Re2(CO)10] (1) are prepared at high temperature
and pressure in autoclaves. From [ReBr(CO)5] (2), [NEt4]2-
[ReBr3(CO)3] (3) may be produced in organic solvents by
the substitution of two CO’s with two bromides.14 From 1
half-sandwich complexes are prepared by oxidative addition
or from 2 by reaction with M[Cp]. More recently it was
shown that the reaction of [Re(NCCH3)3(CO)3]+, prepared
from 3, reacts with diazocyclopentadiene or with stannylated
cyclopentadienes to form [(Cp)Re(CO)3].7,15 Due to the
difficulty of preparing the [99TcBr(CO)5] analogue under
high-pressure and high-temperature conditions, a different
method has been devised. Addition of BH3‚THF and CO to
[99TcO4]- results in reduction to Tc(I) and carbonyl coor-
dination. [NEt4]2[99TcCl3(CO)3] (4) may then be isolated after
the addition of excess [NEt4]Cl.3 When 4 is dissolved in
water, the three chloride ligands are replaced by H2O and
[99Tc(OH2)3(CO)3]+ (5) is formed.
We have recently demonstrated that the cyclopentadienyl
ligand can be coordinated to [99mTc(OH2)3(CO)3]+ in water
by introducing the electron-withdrawing acetyl group in
cyclopentadiene to give acetyl-Cp. This has a significantly
lowered the pKa value and allowed a one-step preparation
of model complexes [(RCOCp)Tc(CO)3] directly from
[
99mTcO4]-. In the way of classical coordination chemistry,
cyclopentadienyl is introduced by substituting the three water
ligands.13 In this study we will present a general synthesis
of [(RCOCp)99(m)Tc(CO)3] type half-sandwich complexes
from water and [99(m)TcO4]- in which R can be different
organic functionalities or CNS receptor ligands. This study
(5) Schibli, R.; Bella, R. L.; Alberto, R.; Garcia-Garayoa, E.; Ortner, K.;
Abram, U.; Schubiger, P. A. Bioconjugate Chem. 2000, 3, 345-351.
Schibli, R.; Katti, K. V.; Higginbotham, C.; Volkert, W. A.; Alberto,
R. Nucl. Med. Biol. 1999, 26, 711-716.
(6) Jaouen, G. Chem. Br. 2001, 36-38. Jaouen, G.; Top, S.; Vessie`res,
A.; Alberto, R. J. Organomet. Chem. 2000, 600, 23-36. Jaouen, G.;
Top, S.; Vessie`res, A.; Pigeon, P.; Leclercq, G.; Laios, I. Chem.
Commun. 2001, 383-384. Le Bideau, F.; Salmain, M.; Top, S.;
Jaouen, G. Chem.sEur. J. 2001, 7, 2289-2294. Severin, K.; Bergs,
R.; Beck, W. Angew. Chem. Int. Ed. 1998, 37, 1634-1654. Top, S.;
Lehn, J.-S.; Morel, P.; Jaouen, G. J. Organomet. Chem. 1999, 583,
63-68. Top, S.; Lescop, C.; Lehn, J.-S.; Jaouen, G. J. Organomet.
Chem. 2000, 593-594, 167-174.
While convenient for the preparation of macroscopic
quantities, these procedures are not feasible for radiophar-
maceutical application which requires rapid reaction under
aqueous conditions. To meet these requirements, BH4- was
used as an aqueous reductant in the presence of CO and NaCl
affording fac-[99mTc(OH2)3(CO)3]+ (6) from [99mTcO4]-
eluted from a generator at low concentrations (10-6-10-8
M). As a further improvement, we have used potassium
boranocarbonate (K2H3BCO2), the properties of which were
reported by us, as an in-situ reducing agent and CO source.
This approach allows for the rapid formation of 6 in >95%
(7) Minutolo, F.; Katzenellenbogen, J. A. J. Am. Chem. Soc. 1998, 120,
4514-4515.
(8) Salmain, M.; Gunn, M.; Gorfti, A.; Top, S.; Jaouen, G. Bioconjugate
Chem. 1993, 4, 425-433.
(9) Top, S.; Hafa, H. E.; Vessie`res, A.; Quivy, J.; Vaissermann, J.; Hughes,
D. W.; McGlinchey, M. J.; Mornon, J.-P.; Thoreau, E.; Jaouen, G. J.
Am. Chem. Soc. 1995, 117, 8372-8380.
(10) Wenzel, M.; Klinge, C. J. Labelled Compd. Radiopharm. 1994, 34,
981-987.
(11) Wenzel, M. J. Labelled Compd. Radiopharm. 1992, 31, 641-650.
(12) Minutolo, F.; Katzenellenbogen, J. A. Angew. Chem. Int. Ed. 1999,
38, 1617-1620. Spradau, T. W.; Edwards, W. B.; Anderson, C. J.;
Welch, M. J.; Katzenellenbogen, J. A. Nucl. Med. Biol. 1999, 26, 1-7.
(13) Wald, J.; Alberto, R.; Ortner, K.; Candreia, L. Angew. Chem. Int. Ed.
2001, 40, 3062-3066.
(14) Alberto, R.; Egli, A.; Abram, U.; Hegetschweiler, K.; Gramlich, V.;
Schubiger, P. A. J. Chem. Soc., Dalton Trans. 1994, 2815-2820.
(15) Cesati, R. R.; Katzenellenbogen, J. A. J. Am. Chem. Soc. 2001, 123,
4093-4094.
Inorganic Chemistry, Vol. 42, No. 4, 2003 1015

Bernard et al.
Scheme 2
Scheme 4
Scheme 3
ality and the introduction of Cp can be conveniently
monitored by 1H NMR spectroscopy. This flexible approach
allows for the introduction of a wide variety of (bio)-
molecules, linked by an alkyl chain of variable length, to
the carbonyl-Cp residue. The synthetic method is shown
in Scheme 4.
yield and fulfills the requirements for routine application
(Scheme 2).13
Initial attempts to coordinate Cp or Cp derivatives to the
fac-[M(CO)3]+ moiety directly in water failed or resulted in
very low yields. Cyclopentadienyl is readily protonated due
to its very high pKa of about 14 and quickly decomposes by
polymerization. Alkene coordination as an initial step is
unlikely since this type of ligand is uncharged and does not
compete for water. Considering coordination chemistry with
other ligands, the key step that allows metal coordination to
Cp is the introduction of an electron-withdrawing carbonyl
group to the Cp ring to yield the Cp derivative of general
formula [RCOC5H5], referred to as carbonyl-Cp. The
resulting keto-enol tautomerism stabilizes the deprotonated
form and increases the acidity by about 5 orders of
magnitude. For example, the pKa value for acetyl cyclopen-
tadiene was found to be 8.62(1) in 0.1 M NaCl (Scheme 3).
The methyl group in the acetyl functionality can be taken as
a linking group for model groups or directly to receptor
binding biomolecules.13
The introduction of an acetyl group in [Cp]- results in
so-called pentafulvenes. Such derivatization of [Cp]- with
double-bonded functional groups have been performed
among many pathways and with a multitude of functional-
ities. The reaction of Na[Cp] with an ester functionality
attached to a biomolecule or an organic model residue in
THF allows for the straightforward formation of various
differently substituted carbonylcyclopentadienyl compounds
Na[RCOCp].16 Cyclopentadienyl attacks the carbonyl carbon
with concomitant cleavage of the corresponding [R-O]-
group. The alcoholato group is sufficiently alkaline to
deprotonate the cyclopentadiene. Loss of this ester function-
To explore this synthetic route to substituted cyclopenta-
dienyls and to investigate the subsequent aqueous coordina-
tion chemistry with 5 or 6, we have chosen the phenyl group
as a future model for steroids and 1-(2-methoxyphenyl)-
piperazine (WAY) as a serotonergic receptor binding ligand
which would allow the assessment of relative binding
affinities (RBA). The Cp derivative of phenylacetyl, Na-
[PhCH2COCp] (7), was prepared in 70% yield by the reaction
of Na[Cp] in THF with the ethyl ester of phenylacetic acid.
The crude product from this step was of sufficient purity
for the reaction with the Re- or the 99,99mTc precursors. For
the affinity studies with receptor binding ligands, further
purification by preparative HPLC was performed. As an
example of a more complicated and receptor binding
molecule, a lead structure, 1-(2-methoxyphenyl)piperazine,
was chosen. Derivatives of this bioligand are among the most
thoroughly studied ligands for the 5-HT1A subclass of
serotonergic receptors.17 After alkylation of the piperazine
ring with ethyl bromoacetate to yield 11, the Cp derivative
was prepared by the same method as above to give
Na[WAY1-Cp] (8). Since it has been reported that small
changes in the length of the linker between the metal center
and (bio)molecule can have a large effect on the relative
binding affinity, we decided to prepare a similar Cp-
derivatized biomolecule differing only in the length of the
alkyl chain between the (bio)molecule and Cp ligand. The
(17) Wilson, A. A.; Inaba, T.; Fischer, N.; Dixon, L. M.; Nobrega, J.;
DaSilva, J. N.; Houle, S. Nucl. Med. Biol. 1998, 25, 769-776.
Kulkarni, S. K.; Aley, K. O. Drugs Today 1988, 24, 175-183.
Glennon, R. A.; Westkaemper, R. B.; Bartyzer, P. Serotonin Receptor
Subtypes; Wiley: New York, 1991. Cliffe, I. A.; Fletcher, A. Drugs
Future 1993, 18, 631-642.
(16) Neuenschwander, M. In The Chemistry of Double-bonded Functional
Groups; Patai, S., Ed.; John Wiley & Sons Ltd.: London, 1989; pp
1131-1268.
1016 Inorganic Chemistry, Vol. 42, No. 4, 2003

DeriWatized Cp Complexes of Tc and Re
synthetic procedure for an ethyl linker Na[WAY2-Cp] (9)
was again the same as above. However, NMR analysis of
the product from the reaction of Na[Cp] with 12 indicated
the formation of a neutral fulvene rather than the anionic
compound 9 (Scheme 4). This implies that the fulvene form
is stabilized by the formation of a six-membered ring with
the proton chelated by the piperazine nitrogen and the enol
oxygen atom. In principle, this does not prevent the Cp ligand
from coordinating. Still, this assumption is in agreement with
the observation that all reaction attempts with 4 or 5
respectively with 6 failed or produced unacceptably low
yields of [η5-(WAY2-Cp)Tc(CO)3] (18). To avoid fulvene
stabilization by intramolecular hydrogen bridge formation,
we alkylated 1-(2-methoxyphenyl)piperazine with ethyl
5-bromovalerate to get 13 to generate a butylene linker and
a terminal ester function. Reaction of 13 with Na[Cp]
produced the substituted cyclopentadienyl compound Na-
[WAY4-Cp] (10) in about 60% yield after 2-3 days at 70
Figure 1. ORTEP presentation of 15. Ellipsoids are drawn at the 50%
level.
lengths of 2.30 Å and M-CO carbon bond lengths of 1.14
Å. The X-ray structures of 14 and 15 are essentially identical.
All their corresponding cell constants differ by less than
0.7%, and the equivalent metal carbon bond lengths are not
significantly different. The 99Tc NMR spectra confirms nicely
the Tc(I) nature of the complex since the chemical shift fits
1
°C. The reaction was terminated when a H NMR sample
well in the region usually found for Tc(I) compounds.18
A
did not show any remaining resonance of the ester func-
tionality. Product formation was directly indicated by the
growth of two doublets from carbonyl-bound Cp centered
near 6.1 and 6.6 ppm.
single and relatively broad signal at -2502 ppm relative to
[99TcO4]- was observed. In comparison to other half-
sandwich complexes of 99Tc such as [CpTc(CO)3] at -1716
or [(Cp*)Tc(CO)3] at -1874 ppm, respectively,19 the reso-
nance is shifted toward stronger shielding which might be a
consequence of the additional carbonyl group. The relatively
large ∆ν1/2 of 720 Hz is typical for asymmetric coordination
around this spin 9/2 nucleus.
Half-sandwich complexes of the general form [(RCOCp-
)M(CO)3] with M ) Re or 99Tc can be prepared along two
pathways. The classical reaction of 7 with 2 in benzene
proceeded smoothly overnight. 1H NMR was used to monitor
the shift in the resonance from free to coordinated 7. Due to
the water solubility of all Cp derivatives presented herein,
any excess ligand can easily be washed away while the high
lipophilicity allows the isolation of the product [(PhCH2-
COCp)Re(CO)3] (14) into hexane by repeated extraction.
Purification by column chromatography gives a clean product
from which X-ray-quality crystals could be grown by
crystallization from CH2Cl2/hexane. Since these organic
conditions are not appropriate for the requirements in future
application, we assessed the possibility of using Cp deriva-
tives directly in water, comparable to “normal” ligands.
Routine application in a hospital setting must rely on the
preparation of the complexes [(RCOCp)Tc(CO)3] directly
from water. The reaction of 7 with 4, which readily produces
[99Tc(OH2)3(CO)3]+ (5) when dissolved in water, proceeds
within a few hours at 75 °C or overnight at rt (room
temperature), generating [(PhCH2COCp)99Tc(CO)3] (15) in
modest yield. Since the chlorides do not compete in
complexation, it was not necessary to remove them by
precipitation with a silver salt. The modest yield is not due
to uncontrolled side reactions or decomposition of the ligand
(which is nicely stable in water or buffer over this time
period) or precursor but by the competing hydrolytic reaction
of 5 to form the stable cluster compound [M(µ-OH)(CO)3]4.
The complex 15 could be purified by extraction, water
washing, and a small silica gel column. From the pale yellow
product, crystals suitable for X-ray analysis were obtained,
and an ORTEP presentation of the molecule is given in
Figure 1. The crystal structures of the rhenium and techne-
tium compounds are isomorphous and reveal the typical
piano-stool arrangement with average M-Cp carbon bond
Reactions with both 2 in benzene or 4 in water with 8
proceeded at a similar rate to give [(WAY1-Cp)M(CO)3]
in reasonable yield (15-25% for M ) Re (16) and 40% for
M ) Tc (17), respectively). The structure of 16 has already
been described.13 As with ligand 7, the modest yield is due
to cubic cluster formation as confirmed by HPLC analysis.
For the analogue reactions with 99mTc, this side reaction is
not expected to play an important role since the 99mTc
concentration is too low to afford di- or even tetramerization.
Preparing Cp complexes with the ligands 7 and 8 in water
leads to the possibility of applying this chemistry to other
metal centers in a comparable synthetic strategy. The in vitro
binding affinity has here been determined for complex 16.
It was found that the inhibition constant (IC50) against the
5-HT1A receptor was 217 nM and that against the 5-HT2A
receptor 8753 nM. The former value is decisive but unac-
ceptably low. This number is not surprising since it has been
shown by structure-activity relationships (SAR’s) that,
independent of the nature of the complex attached to the
lead structure, one methylene group imposes significant
sterical interactions with the binding pocket, resulting in a
low inhibition constant as observed in this case.20 This
inhibition constant is still among the smallest known so far
(18) O’Connell, L. A.; Pearlstein, R. M.; Davison, A.; Thornback, J. R.;
Kronauge, J. F.; Jones, A. G. Inorg. Chim. Acta 1989, 161, 39-43.
(19) Alberto, R.; Schibli, R.; Egli, A.; Abram, U.; Abram, S.; Kaden, T.
A.; Schubiger, P. A. Polyhedron 1998, 17, 1133-1140.
(20) Johannsen, B.; Pietzsch, H.-J. Eur. J. Nucl. Med. Mol. I. 2002, 29,
263-275. Drews, A.; Pietzsch, H.-J.; Syhre, R.; Seifert, S.; Varnas,
K.; Hall, H.; Halldin, C.; Kraus, W.; Karlsson, P.; Johnsson, C.; Spies,
H.; Johannsen, B. Nucl. Med. Biol. 2002, 29, 389-398.
Inorganic Chemistry, Vol. 42, No. 4, 2003 1017

Bernard et al.
Scheme 5
Figure 2. ORTEP presentation of 19. Ellipsoids are drawn at the 50%
level.
for this type of a (too) short linker. We emphasize that the
small number encountered with a system far from optimal
supports the hypothesis of designing a complex as small and
lipophilic as possible. To extend the spacer length, we
attempted to apply the fulvene compound 9 which is also
expected to provide a η5-coordination to the metal center.
Surprisingly, only minimal amounts of complex could be
detected analytically under any of the reaction conditions
successfully applied to 8 or 7. Although the compound was
not isolated in analytically pure form, it was evident from
spectroscopic data that the major product is a complex in
which the tertiary piperazine amine and the deprotonated
enolate form a six-membered chelate to the metal center (18
in Scheme 5), thus preventing the ring slippage to η5
coordination.
Figure 3. HPLC trace after the synthesis of 20 with 99Tc. The lower trace
is monitored with a â- detector, and the upper trace, with a UV detector at
250 nm for 19 for comparison (delay due to detector separation).
identical geometry in the complexes 14 and 19. The tris-
(carbonyl) moiety, however, is twisted by about 57° com-
pared between the two structures. The charge of the
diprotonated piperazine is compensated by a chloride and a
nitrate anion, both of which form a hydrogen bond to the
nitrogen atoms: the distance from the nitrogen atoms to the
chloride/oxygen O30 of the nitrate is 3.02/2.80 Å, respec-
tively.
To confirm that [WAY4-Cp99Tc(CO)3] (20) can be
prepared directly from water as well, 5 was reacted with 10
in an analogous way to the reaction with 8 at rt or at 70 °C.
The reaction progress was monitored by means of HPLC
with parallel UV and â- detection. After 2 h at 70 °C, two
new product peaks were found in the â- trace at 21.3 and
22.33 min along with a larger peak of unreacted 5. After
continued reaction overnight, only the two product peaks
remained. The HPLC â- trace of 20 and comparison to 19
are depicted in Figure 3.
The first peak was identified as 20 (30% yield) by compar-
ison of the HPLC retention time with 19. Furthermore, ESI
LC-MS gave the correct mass of this product for the molecule
cation (m/e 522.73; theoretical for C24H27O5N299Tc, m/e
522.44). The second â- active peak was identified as
the expected tetrameric [99Tc(µ-OH)(CO)3]4 cluster in about
70% yield. For the rhenium reaction in benzene and water
and the Tc reaction in water, no evidence was found for
the formation of a stable intermediate as detected with
This mechanistic result clearly implies that the major
pathway of coordination is by initial binding to the enolate
and subsequent intramolecular rearrangement, affording the
desired half-sandwich complex. Obviously, the carbonyl
group in carbonyl-Cp does not only acidify the cyclopen-
tadiene ring but acts the same time as an initial coordinating
site. Chelation to nitrogen and oxygen hampers this route
for 9. A comparable failure in Cp coordination was also
found for other biomolecules providing an acac donor set
including the -CO- group from carbonyl-Cp. We did not
observe this behavior with 8 which would have a five-
membered chelate. Obviously, the relatively large Tc(I)
center prefers six-membered over five-membered rings. To
further support this hypothesis, we reacted 10 with 2 in
benzene or [Re(H2O)3(CO)3]+ in water generating [(WAY4-
Cp)Re(CO)3] (19) in reasonable yield. The progress of the
reaction is conveniently monitored by 1H NMR spectroscopy
or by HPLC. Product purification by silica gel chromatog-
raphy generated 19 in 25-40% yield. Crystals of X-ray
quality were prepared from acidified MeOH/hexane. An
ORTEP presentation of 19 is given in Figure 2. The Cp ring,
rhenium center, and the ketone moiety possess an almost
1018 Inorganic Chemistry, Vol. 42, No. 4, 2003

DeriWatized Cp Complexes of Tc and Re
19. The retention times are identical, and the slight delay of
radioactive over optical detection is due to separation of the
respective detectors. The steroid model 7 was labeled by the
same method to yield [(PhCH2COCp)99mTc(CO)3] (22) in
95% yield. Preliminary animal studies in mice showed a
relatively high first pass brain uptake of 1.2% ID/g with the
expected washout from the brain due to the lack of a trapping
mechanism. The compound showed very high lung uptake
(>20% ID/g) which even increased over time. The study of
the biological mechanism responsible for this behavior in
lung is currently underway.
Although the straightforward synthesis of derivatized half-
sandwich complexes has been demonstrated, the concentra-
tion of the biomolecule still remains a concern. It is known
from other approaches that relatively high concentration of
unlabeled brain receptor binding molecules may not interfere
with potential application since the unlabeled material might
be too polar to pass the blood brain barrier.21 This is probably
also the case with 10 since it is zwitterionic to a large extent
at physiological pH. We observed in our case that excess
RCOCp precipitated under the conditions applied during the
reaction with 99mTc, probably as dimers or oligomers. This
precipitate can easily be removed by filtration through a
syringe, and the UV trace of the corresponding solution did
not show the presence of any Cp derivatives. For in vivo
application, this would act to increase the specific activity
of the complex by preventing saturation of the target receptor
with “cold” substrate.
To further improve the system, a true one-pot synthesis
was finally performed, producing the half-sandwich com-
plexes directly from [99mTcO4]- in saline and in the presence
K2[H3BCO2] as a reducing agent and CO source. The
combination of conditions described above in one single vial
and 40 min at 90 °C was sufficient to form the corresponding
half-sandwich complex in >95% yield. This method can
generally be applied to other biomolecules derivatized with
a carbonyl-Cp unit, provided that the biomolecule is not
susceptible for reduction. We have found in some cases that
the carbonyl functionality was reduced to a hydroxy group
but optimized conditions should prevent this undesirable side
reaction.
Figure 4. Formation of 21 and 22 with 99mTc in water from 6. The lower
HPLC traces show the products with γ detection, and the upper trace shows
the product with UV detection for 19 with Re for comparison.
9. The reaction proceeds clearly from the educt to the two
described products without the formation of any undesirable
side products. The successful reaction with 10 supports the
hypothesis that the formation of six-membered chelates
interferes with the desired formation of the half-sandwich
complex. In vitro binding studies confirmed the SAR from
other studies. Complex 19 displays remarkable affinity
toward the 5-HT1A receptor (IC50: 6 nM) and a lower affinity
for the 5-HT2A receptor (IC50: 300 nM). Animal studies with
this compound and 99mTc are currently underway.
Labeling Studies. To further examine the novel reactions
of Cp compounds in water and to demonstrate their versatility
for radiopharmaceutical purposes, we carried out reactions
at the no-carrier-added (nca) level with [99mTcO4]- (10-6-
10-8
M
99mTc). These are conditions typically encountered
in routine application and will demonstrate the practical
convenience of this approach. At low concentration, [M(µ-
OH)(CO)3]4 cluster is not formed for kinetic reasons. In
addition and in contrast to the synthetic methods on the
macroscopic level, there is normally several orders of
magnitude ligand (or biomolecule) excess though the con-
centration should still be as low as possible. Thus, the
reaction rate is expected to be much lower assuming a
second- or pseudo-first-order kinetic substitution reaction.
The synthesis of [(RCOCp)99mTc(CO)3] can be performed
along two pathways. As previously reported, the precursor
6 was prepared from [99mTcO4]- eluted in saline from a
generator and using potassium boranocarbonate K2[H3BCO2]
as an air-stable, in-situ reducing agent and CO source.
Addition of 1-4 mL of [99mTcO4]- to a vial containing
K2[H3BCO2] and borate generates 6 in greater than >98%
yield after 15 min at 95 °C. After phosphate buffering of
this solution to pH 7.4-8 aqueous solutions of 8 or 10,
respectively, were added to 1 mL of 6 to produce a final
concentration of RCOCp between 0.1 and 1 mM. Heating
to 90 °C for 15-30 min affords [(RCOCp)99mTc(CO)3], i.e.,
[WAY4-Cp99mTc(CO)3] (21), in high yield. Traces of HPLC
γ-detection are given in Figure 4. The identity of the product
was confirmed by co-injection of the corresponding Re
complex and comparison of the retention time. In Figure 4
the lower trace represents the γ-detection and the upper trace
the UV absorption at 250 nm for the rhenium compound
Conclusions
We have demonstrated that cyclopentadienes carrying a
carbonyl functionality acidify the ring protons sufficiently
to be used as aqueous ligands. This allows for the general
preparation of half-sandwich complexes [(RCOCp)Tc(CO)3]
directly from [99Tc(OH2)3(CO)3]+ and the corresponding Na-
[RCOCp]. Presumably, initial coordination is by the depro-
tonated enolate with subsequent ring slippage to η5-
coordination. We want to emphasize that this pathway can
(21) Kung, M. P.; Stevenson, D. A.; Plossl, K.; Meegalla, S. K.; Beckwith,
A.; Essman, W. D.; Mu, M.; Lucki, I.; Kung, H. F. Eur. J. Nucl. Med.
1997, 24, 372-380. Meegalla, S. K.; Plossl, K.; Kung, M. P.;
Chumpradit, S.; Stevenson, D. A.; Kushner, S. A.; McElgin, W. T.;
Mozley, P. D.; Kung, H. F. J. Med. Chem. 1997, 40, 9-17. Meegalla,
S. K.; Plossl, K.; Kung, M. P.; Stevenson, D. A.; Mu, M.; Kushner,
S.; Liable-Sands, L. M.; Reingold, A. L.; Kung, H. F. J. Med. Chem.
1998, 41, 428-436.
Inorganic Chemistry, Vol. 42, No. 4, 2003 1019

Bernard et al.
1
yellow oil (yield >95%). HPLC retention time ) 17.09 min. H
probably be applied to other transition metal cations, opening
pathways to new mixed Cp and water complexes. Attachment
of a biomolecule to carbonyl-Cp, followed by reaction with
NMR (DMSO): 6.83-6.93 (aromatic, 4H), 4.02 (quartet, 2H), 3.75
(s, 3H), 2.93 (br, 4H), 2.47 (br, 4H), 2.31 (t, 4H), 1.35-1.65 (m,
4H)), 1.17 (t, 3H). 13C NMR (DMSO): 172.8, 151.9, 141.2, 122.2,
120.7, 117.7, 111.8, 59.6, 57.4, 55.2, 52.9, 50.0, 33.3, 25.6, 22.4,
14.1.
[
99mTc(OH2)3(CO)3]+ at the nca level, proceeds rapidly and
in good yield to generate [(RCOCp)99mTc(CO)3] complexes.
This procedure has been successfully demonstrated with a
serotonergic receptor binding ligand. In addition, this route
represents a general synthetic method for the production of
radiopharmaceutically relevant technetium and also rhenium
complexes, suitable for application in, e.g., CNS receptor
imaging. Considering basic SAR’s, this method underlines
the possibility of introducing the very small and highly
lipophilic [(RCOCp)99mTc(CO)3] moiety into a variety of
receptor binding molecules.
Synthesis of 10. NaCp (12.48 mmol from a 2 M solution in
THF) was added dropwise in a glovebox to 13 (3.12 mmol) in THF
(10 mL). The reaction mixture was then transferred to a fume hood
1
and heated to 70 °C for 2 days or until a sample removed for H
NMR showed no trace or ester remaining (yield 60-80%). Samples
were used as is or further purified by preperative HPLC, taking
care to exclude oxygen as much as possible. HPLC retention time
1
) 18.8 min. H NMR (D2O): 6.85-7.11 (aromatic, 4H), 6.53 (d,
2H), 6.06 (d, 2H), 3.74 (s, 3H), 2.79-3.02 (br, 4H), 2.57 (t, 4H),
2.36 (t, 4H), 1.39-1.66 (m, 4H). 13C NMR (D2O): 152.3, 140.0,
124.5, 121.0, 119.2, 118.8, 113.4, 111.6, 57.6, 55.6, 52.0, 50.0,
48.8, 26.0, 20.6. HPLC-MS, (M + 2H)+: m/e 341.07 (calcd for
C21H27O2N2, m/e 339.21).
Synthesis of 14: Method a. [ReBr(CO)5] (0.739 mmol) and 7
(2.18 mmol) were placed in a round-bottom flask containing 10
mL of benzene and gently refluxed overnight. Evaporation of the
benzene left a cream colored residue which was washed with water.
The crude product was then extracted with hexane and purified by
silica gel chromatography (3:1 CH2Cl2-hexane). The yield was
42%. Crystals suitable for X-ray analysis were obtained by slow
evaporation of a solution of 14 in hexane. HPLC retention time )
22.37 min. 1H NMR (MeOH): 7.20-7.36 (aromatic, 5H), 6.27 (t,
2H), 5.62 (t, 2H), 3.96 (s, 2H). 13C NMR (MeOH): 195.5, 193.5,
135.9, 130.4, 129.7, 128.1, 95.4, 90.0, 87.4, 46.5. HPLC-MS, (M
+ H)+: m/e 453.46 (calcd for C16H11O4Re, m/e 454.60).
Synthesis of 15: Method b. Compound 7 (0.2 mmol) in 3 mL
of H2O was added to 1 mL of an aqueous solution of 4 (0.13 mmol)
and warmed to 65 °C overnight although HPLC analysis indicated
the reaction was mostly complete within a few hours. The crude
product was extracted with hexane (3 × 5-10 mL) and dried under
vacuum or with N2 flow. The yellow residue was washed several
times with water, filtered on a fritted glass filter, and washed with
acetonitrile. After dissolution in a minimal volume of CH2Cl2/
hexane, the product was transferred to a silica gel column and eluted
with CH2Cl2. Thin layer chromatograpyh (4:1 CH2Cl2-hexane)
gave one spot with an Rf ) 0.5 (yield ) 17%). Crystals suitable
for X-ray analysis were obtained by slowly cooling a solution of
15 in hexane from room temperature to -27 °C. HPLC retention
time ) 23.08 min. Analytical data for 15 are as follows. 99Tc NMR
(D3COD): -2502 (740 Hz). 1H NMR (MeOH): 7.34-7.21
(aromatic, 5H), 6.19 (t, 2H), 5.51 (t, 2H), 3.95 (t, 2H). HPLC-
MS, (M + H)+: m/e 366.60 (calcd for C16H11O499Tc, m/e 365.16).
Synthesis of 19: Method a. 2 (0.155 mmol) and 10 (0.221
mmol) were placed in a round-bottom flask containing 5 mL of
benzene and gently refluxed overnight. Evaporation of the solvent
left a cream colored residue which was dissolved in a minimal
amount of CH2Cl2-MeOH and transferred to a silica gel column.
Elution with 20:1 CH2Cl2-MeOH gave one spot by TLC (Rf (7:1
CH2Cl2-MeOH) ) 0.7). The yield varied between 25 and 40%.
Crystals suitable for X-ray analysis were obtained by diffusion of
hexane into acidified methanol. HPLC retention time ) 21.15 min.
1H NMR (DMSO): 6.86-7.05 (aromatic, 4H), 6.25 (t, 2H), 5.62
(t, 2H), 3.83 (s, 3H), 2.97-3.14 (br, 4H), 2.59-2.74 (br, 4H), 2.50
(t, 4H),1.51-1.74 (m, 4H). 13C NMR (DMSO): 195.5. 193.7, 152.0,
139.5, 123.7, 121.0, 118.4, 112.1, 96.5, 89.3, 87.3, 55.4, 55.2, 51.1,
46.8, 37.5, 22.5, 21.0. MS, (M + H)+: m/e 609.74 (calcd for
C24H27O5N2Re, m/e 608.74).
Experimental Section
Chemicals and solvents of reagent grade were purchased from
Fluka AG Buchs and used without further purification. All
manipulations were carried out using standard Schlenk techniques
unless otherwise noted. NaCp was freshly prepared in situ from
Na and the corresponding amount of freshly distilled dicyclopen-
tadiene, or purchased as a 2 M solution in THF and stored in a
glovebox. NMR spectra were recorded on a Varian Gemini 300
MHz or Varian Mercury 200 MHz instrument. Chemical shifts
(ppm) are reported relative to residual solvent. HPLC were
measured on two different Merck Hitachi LaChrom D-7000
instruments. One of these was interfaced with an EG&G Berthold
LB 508 radioflow detector while the other was interfaced with a
Merck Hitachi M-8000 LCMS. A Varian Prostar was used for
preparative HPLC. [99mTcO4]- was eluted from a Mallinckrodt Med.
Inc. generator. The amount of radioactivity varied from 370 MBq
to 3.7 GBq, and no dependence of the total 99mTc concentration on
yield was noted. The complexes 3, 4, and 6 and K2[H3BCO2] were
prepared as described elsewhere.22 HPLC system: RP-18 column
(A ) 0.1% CF3COOH in H2O, B ) MeOH): 0-3 min 100% A;
3-9 min 75% A; 9.1 min 66% A; 9.1-20 min 66% f 0% A;
20-25 min 0% A; 25.1-30 min 100% A.
Caution! 99Tc is a weak â- emitter (t1/2 ) 2.13 × 105 yrs, â- )
294 keV). Therefore, all manipulations were carried out in specially
equipped (C-type) laboratories to aVoid contamination or ingestion.
Synthesis of 7. Phenylacetic acid ethyl ester (0.15 mol) was
added dropwise to a solution of Na[Cp] (0.10 mol) in dry THF (50
mL) over 45 min. The wine red solution was then heated for 5 h
at 75 °C, during which time the solution color changed to orange/
brown. After solvent removal, the light tan colored residue was
rinsed with hexane and ether and dried under vacuum (yield 90%).
The product was used as is for further reactions or purified by
preparative HPLC in small quantities taking with care to exclude
1
oxygen. HPLC retention time ) 20.34 min. H NMR (DMSO):
7.05-7.30 (aromatic, 5H), 6.22 (d, 2H), 5.63 (d, 2H), 3.68 (s, 2H).
13C NMR (DMSO): 185.5, 140.2, 129.4, 127.8, 125.3, 123.0, 114.8,
113.3, 111.2, 45.2.
Synthesis of 13. 1-(2-Methoxyphenyl)piperazine (15.6 mmol),
ethyl 5-bromovalerate (23.4 mmol), and potassium carbonate (31.2
mmol) were added to 150 mL of acetonitrile under an ambient
atmosphere, and the mixture was refluxed for 3 days. After filtration
on a fritted glass filter, the yellow filtrate was concentrated to an
oil on a rotary evaporator. The product was purifed be silica gel
chromatography (19:1 ethyl acetate-methanol), giving a pure (TLC)
(22) Alberto, R.; Ortner, K.; Wheatley, N.; Schibli, R.; Schubiger, A. P. J.
Am. Chem. Soc. 2001, 123, 3135-3136.
1020 Inorganic Chemistry, Vol. 42, No. 4, 2003

DeriWatized Cp Complexes of Tc and Re
Table 1. Summary of X-ray Crystallographic Data
14
15
19
formula
molecular mass
T, K
C16H11O4Re
453.45
183(2)
C16H11O4Tc
365.25
183(2)
C24H29ClN3O8Re
709.15
183(2)
space group
a, Å
b, Å
P21/c
P21/c
P21/c
16.0665(9)
7.6797(4)
107.172(8)
12.3656(11)
1457.73(17)
4
2.066
8.347
856
0.15 × 0.14 × 0.04
colorless plate
2.65-28.17
19 852
3529, 0.0510
2632
16.1454(9)
7.6300(6)
107.792(6)
12.3922(7)
1453.57(16)
4
1.669
1.002
728
0.20 × 0.17 × 0.16
colorless block
2.65-28.12
20 643
3390, 0.0369
2634
15.7112(6)
6.8775(3)
103.778(5)
25.5217(12)
2678.4(2)
4
1.759
4.688
1400
0.45 × 0.07 × 0.06
colorless needle
2.35-28.0
25 279
6444, 0.0780
4893
â, deg
c, Å
V, Å3
Z
F
calcd, g/cm3
µ(Mo KR), mm-1
F(000)
cryst size, mm3
cryst description
θ range, deg
tot. no. of data
no. of unique data, Rint
obsd dataa
max/min transm
data/params
0.7165/0.3104
3529/190
0.980
0.0367
0.0889
0.8561/0.8248
3390/234
0.892
0.0271
0.0663
0.7657/0.3054
6444/343
0.891
0.0361
0.0848
goodness of-fit on F2
Ra,b
wR2 a,c
max, min peaks, e/Å3
0.977, -1.504
0.519, -0.331
1.745, -1.347
b
c
2
2
2
a Observation criterion: I > 2σ(I). R ) Σ||Fo| - |Fc||/Σ|Fo|. wR2 ) {Σ[w(Fo - Fc )2]/Σ[w(Fo )2]}1/2
.
Synthesis of 20: Method b. Compound 10 (0.2 mmol) in 3
mL of H2O was added to 1 mL of an aqueous solution of 4 (0.13
mmol), and the solution was warmed to 65 °C overnight although
HPLC analysis indicated the reaction was mostly complete within
a few hours. The product was identified by HPLC with â- detection
and HPLC-MS (30% yield by â- detection for 20). Due to the
low amount of starting material used, confirmation of 20 was done
only by HPLC comparison and by LC-MS (m/e): 522.73 (calcd
for C24H27O5N299Tc, 522.44). HPLC retention time ) 20.97 min
(UV detection) and 21.42 min (â- detection). The rhenium
compound 19 can be produced accordingly with 3 as a starting
material. The fulvene complex 18 was prepared the same way but
not isolated and purified. 1H NMR gave evidence for the presence
of complex 18 as depicted in Scheme 5.
were dissolved in DMSO up to 1 mM and then diluted with
buffer.
Nonspecific binding was defined as the amount of [3H]8-OH-
DPAT bound in the presence of 10 µM serotonin (Sigma) and
ranged from 5% to 9% of total binding. The samples were incubated
in triplicates at 20 °C for 60 min. The incubation was terminated
by rapid filtration through GF/B glass fiber filters (Whatman) using
a 30-port Brandel Cell Harvester. The filters were rapidly washed
with four 4 mL portions of ice-cold buffer, transferred into 10 mL
scintillation fluid (Ultima-Gold, Packard), and analyzed for radio-
activity.
(b) 5-HT2A Receptor Binding Assay. The cortex of rat brain
was prepared as described above and stored at -20 °C until used
in binding studies. [3H]ketanserin (3148.7 GBq/mmol from NEN)
was used as a radioligand for 5-HT2A receptor binding. The binding
assay was carried out in a final volume of 5 mL of Tris-HCl buffer,
pH 7.6, containing 0.12 nM [3H]ketanserin, membrane homogenate
(about 20 µg/mL protein), and various concentrations of the Re
complexes dissolved and diluted as described above. Nonspecific
binding was defined as the amount of [3H]ketanserin bound in the
presence of 1 µM mianserin (Sigma) and ranged from 17% to 24%
of total binding. The samples were incubated in triplicates at 20
°C for 120 min. Filtration and counting of the samples were the
same as described above.
Labeling Procedure. A vial was charged with 4 mg of
boranocarbonate (K2H3BCO2), 7 mg of Na2B4O7, and 7 mg of
sodium tartrate, sealed, and flushed with N2. Generator eluate (1-4
mL) containing [99mTcO4]- was added and the vial heated to 90
°C for 15-30 min to generate 6 in >95% yield (HPLC). The pH
of this solution was then set to 7.4 with phosphate buffer. An aliquot
of this solution was then added to an aqueous solution of RCOCp.
Further heating at 90 °C for 30 min generates the labeled product
of the form [(RCOCp)99mTc(CO)3], i.e., 21 or 22.
In Vitro Binding Studies with 5-HT1A and 5-HT2A Receptors.
(a) 5-HT1A Receptor Binding Assay. The hippocampus of rat brain
was homogenized in 10 volumes of ice-cold buffer (50 mM Tris-
HCl, pH 7.6), centrifuged at 20000g for 10 min. The resulting pellet
was resuspended and centrifuged again at 20000g for 10 min. After
the same procedure was repeated, the pellet was resuspended in
10 volumes of buffer and stored at -20 °C until used in binding
studies.
[3H]8-OH-DPAT (5494.5 GBq/mmol from NEN) was used as
radioligand for the 5-HT1A receptor binding. The binding assay was
carried out in a final volume of 2.5 mL Tris-HCl buffer (50 mM,
pH 7.4, 0.1% ascorbic acid, 2 mM CaCl2) containing 0.10 nM
[3H]8-OH-DPAT, membrane homogenate (about 20 µg/mL protein),
and various concentrations of the Re complexes. The complexes
X-ray Crystallography. Crystal data and experimental details
are listed in Table 1. Suitable crystals were covered with Paratone
N oil, mounted on top of a glass fiber, and immediately transferred
to a Stoe IPDS diffractometer. Data were collected at 183(2) K
using graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å).
A total of 8000 reflections distributed over the whole limiting sphere
were selected by the program SELECT and used for unit cell
parameter refinement with the program CELL.23 Data were cor-
rected for Lorentz and polarization effects as well as for absorption
(numerical). Structures were solved with direct methods using
(23) CELL, 2.87 5, 1998 ed.; STOE & Cie, GmbH: Darmstadt, Germany,
1998.
Inorganic Chemistry, Vol. 42, No. 4, 2003 1021

Bernard et al.
SHELXS-9724 or SIR9725 and were refined by full-matrix least-
squares methods on F2 with SHELXL-97.26 Further experimental
details are available as Supporting Information.
project (C99.001) within the COST action B12. BS thanks
the Swiss National Science Foundation for financial support.
Supporting Information Available: Tables of X-ray data,
ORTEP diagrams, and complete X-ray data in CIF format. This
material is available free of charge via the Internet at http://pubs.
acs.org.
Acknowledgment. We thank the “Bundesamt fu¨r Bildung
und Wissenschaft (BBW)” for financially supporting this
(24) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467-473.
(25) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115-119.
IC0204575
(26) Sheldrick, G. M. SHELXL-97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
1022 Inorganic Chemistry, Vol. 42, No. 4, 2003