A new tricarbonyl fac-[M(acac)(isc)(CO)3] complex (M = Re, 99mTc) with acetylacetonate (acac) and isocyanide (isc) in a 2+1 combination

Article abstract of DOI:10.1016/j.ica.2010.01.004

The synthesis and characterization of the neutral 2+1 mixed ligand complex fac-Re(CO)₃(acac)(isc) (4) with acetylacetonate (acac) as the bidentate ligand and an isocyanide (the isocyanocyclohexane, isc) as the monodentate ligand is described. The synthesis of 4 proceeds through the intermediate formation of the fac-Re(acac)(H₂O)(CO)₃ precursor complex 2. Complex 4 was characterized by elemental analysis, spectroscopic methods, and X-ray crystallography showing a distorted octahedral arrangement of the ligands around Re. At technetium-99m level, the corresponding fac-^99mTc(acac)(isc)(CO)₃ complex 5 was obtained in high yield by reacting the fac-^99mTc(acac)(H₂O)(CO)₃ precursor complex 3 with isocyanocyclohexane and its structure was established by chromatographic comparison with the prototypic rhenium complex using high performance liquid chromatography.

Full text of DOI:10.1016/j.ica.2010.01.004

Inorganica Chimica Acta 363 (2010) 1649–1653
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Inorganica Chimica Acta
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A new tricarbonyl fac-[M(acac)(isc)(CO)3] complex (M = Re, 99mTc) with
acetylacetonate (acac) and isocyanide (isc) in a 2+1 combination
Marina Sagnou a, Charalambos Tsoukalas b, Charalambos Triantis b, Catherine P. Raptopoulou c, Aris Terzis c,
b
Ioannis Pirmettis b, Maria Pelecanou a, , Minas Papadopoulos
*
a Institute of Biology, National Centre for Scientific Research ‘‘Demokritos”, 15310 Athens, Greece
b Institute of Radioisotopes and Radiodiagnostic Products, National Centre for Scientific Research ‘‘Demokritos”, 15310 Athens, Greece
c Institute of Materials Science, National Centre for Scientific Research ‘‘Demokritos”, 15310 Athens, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 October 2009
Received in revised form 16 December 2009
Accepted 5 January 2010
Available online 18 January 2010
The synthesis and characterization of the neutral 2+1 mixed ligand complex fac-Re(CO)3(acac)(isc) (4)
with acetylacetonate (acac) as the bidentate ligand and an isocyanide (the isocyanocyclohexane, isc) as
the monodentate ligand is described. The synthesis of 4 proceeds through the intermediate formation
of the fac-Re(acac)(H2O)(CO)3 precursor complex 2. Complex 4 was characterized by elemental analysis,
spectroscopic methods, and X-ray crystallography showing a distorted octahedral arrangement of the
ligands around Re. At technetium-99m level, the corresponding fac-99mTc(acac)(isc)(CO)3 complex 5
was obtained in high yield by reacting the fac-99mTc(acac)(H2O)(CO)3 precursor complex 3 with isocyano-
cyclohexane and its structure was established by chromatographic comparison with the prototypic rhe-
nium complex using high performance liquid chromatography.
Keywords:
Rhenium
Technetium-99m
Tricarbonyl complexes
Acetylacetone
Isocyanide
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
Our interest in new ligand combinations for the tricarbonyl
fac-[M(CO)3]+ core for potential applications have led us to inves-
The introduction of the air-stable fac-[M(CO)3(H2O)3]+
(M = 99mTc or Re) synthon produced by the gentle reduction of
M(VII) to M(I) in aqueous medium under 1 atm of CO established
the tricarbonyl fac-[M(CO)3]+ core as an easily accessible platform
towards the synthesis of new radiopharmaceuticals labelled with
Tc-99 m (for diagnosis) or Re-188 (for therapy) [1,2]. In the fac-
[M(CO)3(H2O)3]+ synthon three coordination sites are occupied
by CO groups in the stable fac-configuration while the remaining
three coordination sites are occupied by water molecules that
can be easily replaced by a great variety of ligands, or ligand com-
binations [3].
tigate the acetylacetone/isocyanide 2+1 combination. Acetylace-
tone (1) is a well-established bidentate ligand that coordinates
to transition metals through the acetylacetonate (acac) anion
[9] and can be synthetically modified to incorporate a targeting
biomolecule at carbon C-2 and at the edge carbons C-4/C-5
(Fig. 1) [10]. Complexes of 1 with rhenium and technetium at
various oxidation states have been reported [6,11] and recently
Benny et al. described the synthesis of the 2+1 tricarbonyl
Re(acac)(pyridine)(CO)3, proceeding through the intermediate
formation of Re(acac)(H2O)(CO)3 (2) [12]. On the other hand,
the isocyanide (isc) is
a strong, amenable to derivatization,
The 2+1 mixed ligand system, combining a bidentate and a
monodentate ligand, has been successfully applied to the fac-
[M(CO)3(H2O)3]+ synthon [4] leading to stable tricarbonyl com-
plexes with a variety of donor atom groups [5–8]. In principle, both
the monodentate and the bidentate ligand can be tethered to a bio-
molecule rendering the 2+1 combination a versatile system suit-
able for the synthesis of radiopharmaceuticals with desired
properties in terms of target tissue specificity, pharmacokinetic
behavior and labeling yield [5,7,8].
monodentate ligand often used in technetium and rhenium
chemistry [4–7,13], its most acclaimed derivative being
99mTc(I)-MIBI, the myocardial imaging radiopharmaceutical in
clinical practice bearing six isocyanide ligands in its coordination
sphere.
We present herein the synthesis and characterization of the
mixed ligand complex fac-Re(acac)(isc)(CO)3 (4) where isocyano-
cyclohexane was employed as a representative isocyanide ligand.
Complex 4 was isolated in high yield and was characterized by ele-
mental analysis, spectroscopic methods and X-ray analysis. At
technetium-99m level, the corresponding fac-99mTc(acac)(isc)(CO)3
complex 5 was also obtained in high yield.
* Corresponding author. Tel.: +30 210 6503555; fax: +30 210 6511767.
E-mail address: pelmar@bio.demokritos.gr (M. Pelecanou).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.01.004

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M. Sagnou et al. / Inorganica Chimica Acta 363 (2010) 1649–1653
10
7
11
12
9
8
N
C
N C
OH2
4
1
6
(Et4N)2[Re(CO)3Br3]
99mTc(CO)3(H2O)3]+
O
O
OC
OC
O
O
O
OC
OC
2
M
M
[
3
O
CO
CO
5
2 M = Re
3 M = 99mTc
4 M = Re
5 M = 99mTc
1
Fig. 1. Synthetic scheme for complexes 4 and 5.
X-ray analysis were obtained by recrystallization from dichloro-
methane/hexane.
2. Experimental
HPLC: tR = 16.3 min, I.R. (KBr, cmꢀ1): 2212, 2026, 1946, 1911;
Anal. Calc. for C15H18NO5Re: C, 37.65; H, 3.79; N, 2.93. Found: C,
37.29; H, 3.75; N, 2.63%. 1H and 13C NMR data are given in Table 2.
2.1. Materials and methods
All reagents and organic solvents used in this study were pur-
chased from Aldrich and used without further purification. Sol-
vents for high-performance liquid chromatography (HPLC) were
HPLC-grade. They were filtered through membrane filters
2.3. Single-crystal X-ray crystallography
(0.22 lm, Millipore, Milford, MA) and degassed by a helium flux
Crystals of 4 suitable for X-ray analysis were mounted in air on
a Crystal Logic Dual Goniometer diffractometer using graphite
before and during use. [NEt4]2[ReBr3(CO)3] was prepared according
to published procedure [14]. For 99mTc labeling, a kit containing
5.5 mg NaBH4, 4 mg Na2CO3 and 10 mg Na-K tartarate, was purged
with CO gas prior to addition of Na99mTcO4, as described in the lit-
erature [1].
IR spectra were recorded as KBr pellets on a Perkin–Elmer 1600
FT-IR spectrophotometer in the region 4000–500 cmꢀ1. The NMR
spectra were recorded in DMSO-d6 at 25 °C on a Bruker 500 MHz
Avance DRX spectrometer using (CH3)4 Si as the internal reference.
Elemental analysis for C, H and N was conducted on a Perkin–El-
mer 2400 automatic elemental analyzer. HPLC analysis was per-
formed on a Waters 600 chromatography system coupled to both
a Waters 2487 Dual k Absorbance detector and a Gabi gamma
detector from Raytest. Separations were achieved on a C-18 RP
monochromated Mo K
a radiation. Unit cell dimensions were
determined by using the angular settings of 25 automatically cen-
tered reflections in the range 11 < 2h < 23° and they appear in Ta-
ble 1. Intensity data were recorded using a h–2h scan. Three
standard reflections monitored every 97 reflections showed less
than 3% variation and no decay. Lorentz, polarization and psi-scan
absorption corrections were applied using Crystal Logic software.
The structures were solved by direct methods using SHELXS-97
[15] and refined by full-matrix least squares techniques on F2 using
SHELXL-97 [16]. Further crystallographic details of 4: 2hmax = 50°,
scan speed 2.5°/min, scan range 1:6 þ a1a2 separation, reflections
collected/unique/used 3043/2901 [Rint = 0.0206]/2901, 250 param-
eters refined,
[
D
q
]
max/[
D
q
]
min = 1.499/ꢀ1.088 e/Å3,
[
D
/
r
]
=
max
(25.4 cm ꢁ 2.5 cm, 5
lm porosity) column eluted with a binary
0.007, R1/wR2 (for all data) = 0.0306/0.0756. Hydrogen atoms were
located by difference maps and were refined isotropically, except
those of the methyl groups which were introduced at calculated
positions as riding on bonded atoms. All non-H atoms were refined
anisotropically.
gradient system at a 1 mL/min flow rate. Mobile phase A was
methanol containing 0.1% trifluoroacetic acid, while mobile phase
B was water containing 0.1% trifluoroacetic acid. The elution gradi-
ent was 0–1 min 100% B (0% A), followed by a linear gradient to
90% A (10% B) in 9 min; this composition was held for another
15 min. After a column wash with 95% A for 5 min, the column
was re-equilibrated by applying the initial conditions (100% B)
for 15 min prior to the next injection.
2.4. Synthesis of 99mTc(acac)(isc)(CO)3 complex 5
Four hundred micro litres of a freshly prepared solution of the
fac-[99mTc(CO)3(H2O)3]+ precursor (pH 6) were added to a vial con-
taining a 600 lL solution of acetylacetone (2 mg) in water. The vial
2.2. Synthesis of the Re(acac)(isc)(CO)3, complex 4
was sealed, flushed with N2 and heated for 15 min at 80 °C. HPLC
analysis demonstrated the formation of a single complex (radio-
chemical yield > 90%) assigned to fac-99mTc(acac)(H2O)(CO)3 com-
plex 3 by comparative HPLC studies using a sample of the fac-
Re(acac)(H2O)(CO)3 complex 2 as reference (Fig. 3). To the isolated
by HPLC solution (2 mL) of the fac-99mTc(acac)(H2O)(CO)3 complex
3, 1 mg of isocyanocyclohexane was added (concentration of isc in
the reaction mixture was 4.6 ꢁ 10ꢀ3 M) and the mixture was left at
room temperature for 30 min. HPLC analysis of the reaction mix-
ture demonstrated the formation of a single complex (radiochem-
ical yield > 98%). The yield of the reaction remained quantitative
(95%) at concentrations of isc down to 1 ꢁ 10ꢀ3 M, while at isc con-
centration of 2.5 ꢁ 10ꢀ4 M the yield decreased to approx. 80%. The
identity of the 99mTc complex 5 was assigned by comparative HPLC
studies using samples of the well characterized fac-Re(acac)(isc)(-
CO)3 complex 4 as reference (Fig. 3). The radioactivity recovery of
Complex 4 was synthesized through the intermediate formation
of Re(acac)(H2O)(CO)3 (2) according to a published procedure [12]
with slight modifications. Briefly, to the solution of 2,4-acetylace-
tone (1.0 mmol) in 8 mL water (pH 6, by the addition of small ali-
quots of
a 0.1 N sodium bicarbonate solution) 0.6 mmol of
[NEt4]2[ReBr3(CO)3] were added. The solution was heated to
85 °C for 4 hours to yield 2 as a yellowish precipitate that was fil-
tered and washed with water. Yield 50%. HPLC: tR = 14.1 min.
To a stirred solution of 2 (40 mg, 0.1 mmol) in 10 mL methanol,
isocyanocyclohexane (isc, 11 mg, 0.1 mmol) in 2 mL methanol was
added. The solution was stirred for 3 h and the reaction progress
was monitored by HPLC. Subsequently, the solvent was removed
under reduced pressure and the product was dried at high vacuum
to give complex 4 in a yield of 46 mg (98%). Crystals suitable for

M. Sagnou et al. / Inorganica Chimica Acta 363 (2010) 1649–1653
1651
Table 1
methanol to generate the corresponding neutral fac-Re(acac)(isc)(-
CO)3 complex 4 (Fig. 1). HPLC analysis of the reaction mixture
showed the formation of a single product in excellent yield. Com-
plex 4 was collected as yellowish solid and characterized by ele-
Summary of crystal, intensity collection and refinement data for complex 4.
Formula
Formula weight
T (K)
k (Å)
Space group
C15H18NO5Re
478.50
298
mental
analysis,
spectroscopic
methods
and
X-ray
Mo K
P1
a
0.710730
crystallography. It is soluble in methanol, acetone, dichlorometh-
ane and insoluble in n-hexane and water. It is stable in the solid
state and in solution for months as shown by HPLC and NMR.
The infrared spectra of complex 1 demonstrate strong bands at
2026, 1946 and 1911 cmꢀ1 attributed to the C„O stretch of the
fac-[Re(CO)3]+ unit [17] and a sharp resonance at 2212 cmꢀ1 attrib-
uted to the N„C stretch.
ꢀ
a (Å)
b (Å)
c (Å)
9.158(5)
9.754(5)
10.930(5)
100.30(2)
103.99(2)
110.83(2)
846.9(7)
a
(o)
b (o)
c
(o)
V (Å3)
Z
2
The 1H and 13C NMR chemical shifts for complex 4 are given in
Table 2 and its 1H–13C correlation spectrum is shown in Fig. 4. The
chemical shifts of the C-2 proton and carbon as well as those of the
C-4 and C-5 methyl groups of the acac moiety correspond (within
0.1 ppm for the 1H and 1.5 ppm for the 13C) to the chemical shifts
obtained for the enol tautomer of plain acetylacetone in DMSO-d6
([18] and our data]. These data are consistent with coordination of
the acetylacetonate anion to the rhenium core and formation of a
six-membered cyclic enol-like form.
In the isocyanide ligand downfield shifts are noted for the H-7
proton (0.5 ppm) and C-7 carbon (2.8 ppm) upon coordination
compared to the free isocyanocyclohexane in DMSO-d6 ([19] and
our data), while the rest of the protons and carbons of the cyclo-
hexane moiety are affected to a much lesser degree. The C-6 isocy-
anide carbon is shifted upfield by 13.4 ppm by coordination;
apparently, delocalization of the partial negative charge on C-6
through resonance hybrid structures is greatly reduced upon metal
binding, resulting in increased shielding. The increase of the contri-
bution of the –N„C: resonance hybrid upon metal binding may be
DCalc (Mg mꢀ3
)
1.876
7.195
460
1.136
Absorption coefficient
l
(mmꢀ1
)
F (0 0 0)
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
r
(I)]
R1 = 0.0287a, wR2 = 0.0743a
a
For 2775 reflections with I > 2
r
(I).
Table 2
1H and 13C NMR chemical shifts (ppm) for complex 4 in DMSO-d6 at 25 °C. The
numbering of the atoms is shown in Fig. 1.
C-1, C-3
C-2
C-4, C-5
C-6
188.74
101.95
27.03
140.96 J
53.70
30.86
21.17
24.22
193.34, 193.18
H-2
H-4, H-5
5.56
1.94
=
16.3 Hza
14 N—13
C
H-7
4.26
C-7
H-8, H-12
H-9, H-11
H-10
1.76, 1.69
1.47
1.45, 1.35
C-8, C-12
C-9, C-11
C-10
C„O
also reflected in the increase of the J
coupling constant from
14N—13
C
a
The J
was directly measurable from the 1D 13C spectrum of complex 4.
C
14 N—13
4.8 Hz in the free isocyanocyclohexane to 16.3 Hz in complex 4,
in accordance with the increase of J with increasing s character of
the bond between the coupled nuclei [20].
the HPLC column after the injections was monitored and found to
be quantitative.
3.2. Description of the structure
3. Results and discussion
An ORTEP diagram of complex 4 is given in Fig. 2 and selected
bond distances and angles are listed in Table 3. The distorted octa-
hedral environment of the Re atom in the structure of 4 is defined
by the three facially bound CO groups, the nitrilo ligand and the
oxygen atoms of the acacꢀ. The Re-carbonyl bond distances,
1.906(6)–1.963(6) Å, are consistent which those found in other
Re-tricarbonyl complexes [21,12] and substantially shorter than
the Re–C7 bond length (2.094(6) Å). The Re–acac bond distances
are the larger in the coordination sphere at 2.138(4) and
2.145(4) Å. The six-membered ring in the coordination sphere is al-
most planar with C13 being ꢂ0.13 Å out of the best mean plane of
the six atoms. The six-membered ring of the nitrilo ligand adopts
the stable chair conformation with C1 and C4 being 0.62 and
0.66 Å out of the mean plane of the remaining four atoms.
3.1. Synthesis and characterization of fac-Re(acac)(isc)(CO)3
4
Isocyanocyclohexane acts as monodentate ligand and reacts
readily with the fac-Re(acac)(H2O)(CO)3 precursor complex 2 in
3.3. Synthesis of technetium-99m complex
At technetium-99 m level, the complex fac-[99mTc(acac)(isc)(-
CO)3] (5) was obtained quantitatively (>95%) by reacting the fac-
[
99mTc(acac)(H2O)(CO)3] precursor complex 3 at room temperature
with isocyanocyclohexane. The reaction mixture was analyzed by
HPLC and the analysis demonstrated that the reaction produces a
single complex. The identity of complex 2 was established by HPLC
comparison of its retention time to that of the analogous well-
characterized rhenium complex, 1 by applying parallel radiometric
and photometric detection (Fig. 3).
Fig. 2. Labeled ORTEP plot of complex
probability level. Hydrogen atoms have been omitted for clarity.
4 with ellipsoids drawn at the 30%

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M. Sagnou et al. / Inorganica Chimica Acta 363 (2010) 1649–1653
14.1
A
16.3
4
C
B
0.5
0.4
0.3
14.6
D
3
0.5
16.8
4
2
mV
1
mV
0.2
0.1
0.0
2
0
0.0
0
2
4
0
6
8
4
10 12 14 16 18 20 22 24
Minutes
0
2
4
0
6
2
8
4
10 12 14 16 18 20 22 24
Minutes
0
2
6
8
10 12 14 16 18 20 22 24
Minutes
6
8
10 12 14 16 18 20 22 24
Minutes
Fig. 3. Comparative reverse-phase HPLC chromatograms: (A, C): UV recording at 254 nm, fac-Re(acac)(H2O)(CO)3 (2) and fac-Re(acac)(is)(CO)3 (4), retention time: 14.1 and
16.3 min, respectively; (B, D): radiometric detection, fac-99mTc(acac)(H2O)(CO)3 (3) and fac-99mTc(acac)(H2O)(CO)3 (5), retention time: 14.6 and 16.8 min, respectively.
4. Conclusions
The replacement of the H2O ligand in the fac-M(acac)(H2O)(-
CO)3 precursor complex (M = Re, 99m-Tc) by isocyanocyclohexane
proceeds quickly at room temperature to generate a stable neutral
complex both at rhenium and at technetium-99m level. The almost
quantitative yield indicates the high affinity of the isocyanide li-
gand for the tricarbonyl core, and renders the system worth of fur-
ther exploration through the employment of suitably derivatized
pharmacophoric ligands aiming at target specific complexes.
Appendix A. Supplementary material
CCDC 750146 contains the supplementary crystallographic data
for complex 4. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.ica.2010.01.004.
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C(18)–Re(1)–C(7)
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C(18)–Re(1)–O(11)
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90.6(3)
89.5(2)
91.9(2)
92.2(2)
177.0(2)
93.0(2)
176.9(2)
C(17)–Re(1)–O(11)
C(7)–Re(1)–O(11)
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C(18)–Re(1)–O(12)
C(17)–Re(1)–O(12)
C(7)–Re(1)–O(12)
O(11)–Re(1)–O(12)
93.5(2)
84.8(2)
177.1(2)
94.9(2)
91.0(2)
86.4(2)
84.5(2)
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