Combining bifunctional chelator with (3 + 2)-cycloaddition approaches: Synthesis of dual-function technetium complexes

Article abstract of DOI:10.1021/ic202212e

A new concept for the synthesis of dual-functionalized technetium (Tc) compounds is presented, on the basis of the reactivity of fac-{Tc ^VIIO₃}⁺ complexes. The concept combines the "classical" bifunctional chelator (BFC) a

Full text of DOI:10.1021/ic202212e

Article
pubs.acs.org/IC
Combining Bifunctional Chelator with (3 + 2)-Cycloaddition
Approaches: Synthesis of Dual-Function Technetium Complexes
Henrik Braband,* Sebastian Imstepf, Michael Benz, Bernhard Spingler, and Roger Alberto*
Institute of Inorganic Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland
S
* Supporting Information
ABSTRACT: A new concept for the synthesis of dual-
functionalized technetium (Tc) compounds is presented, on
the basis of the reactivity of fac-{TcVIIO3}+ complexes. The
concept combines the “classical” bifunctional chelator (BFC)
approach with the new ligand centered labeling strategy of fac-
{TcO3}+ complexes with alkenes ((3 + 2)-cycloaddition
approach). To evidence this concept, fac-{99TcO3}+ model
complexes containing functionalized 1,4,7-triazacyclononane
(tacn) derivatives N-benzyl-2-(1,4,7-triazonan-1-yl)acetamide
(tacn-ba) and 2,2′,2″-(1,4,7-triazonane-1,4,7-triyl)triacetic acid (nota·3H) were synthesized and characterized. Whereas
[99TcO3(tacn-ba)]+ [2]+ can be synthesized following a established oxidation procedure starting from the TcV complex
[99TcO(glyc)(tacn-ba)]+ [1]+, a new synthetic pathway for the synthesis of [99TcO3(nota)]2‑ [5]2‑ had to be developed, starting
from [99Tc(nota·3H)(CO)3]+ [4]+ and using sodium perborate tetrahydrate (NaBO3·4H2O) as oxidizing reagent. While
[99TcO3(nota)]2‑ [5]2‑ is a very attractive candidate for the development of trisubstituted novel multifunctional radioprobes, (3 +
−
2)-cycloaddition reactions of [99TcO3(tacn-ba)]+ [2]+ with 4-vinylbenzenesulfonate (styrene-SO3 ) demonstrated the suitability
of monosubstituted tacn derivatives for the new mixed “BFC-(3 + 2)-cycloaddition” approach. Kinetic studies of this reaction
lead to the conclusion that the alteration of the electronic structure of the nitrogen donors by, e.g., alkylation can be used to tune
the rate of the (3 + 2)-cycloaddition.
is achieved solely at the ligands (ligand-centered reactivity).
Thus, for synthesizing dual-functionalized imaging agents, a
first “function” is introduced by modification of the stabilizing
tripodal ligand, in this particular case the 1,4,7-triazacyclono-
nane (tacn) ligand, and the second “function” via cycloaddition
to the oxo ligands of the fac-{99mTcO3}+ core ((3 + 2)
approach) (Scheme 1). We showed recently how different
bioactive functionalities can be introduced via (3 + 2)-
cycloaddition.4
INTRODUCTION
■
Molecular imaging has gained enormous importance over the
past decade for visualizing biological events in noninvasive
diagnostics.1,2 99mTc-compounds are among the most favorable
radiotracers due to their physical properties (low energy γ-rays
140.5 keV, half-life time 6 h) and ready availability (generator
nuclide). The bifunctional chelator approach (BFC), in which a
ligand for a 99mTc precursor is conjugated to a targeting
molecule, is a powerful tool in the development of new
radiopharmaceuticals.2 As an approach complementary to the
BFC principle, we have introduced the ligand-centered (3 + 2)-
cycloaddition, based on the high-valent, fac-{TcVIIO3}+-core.
This labeling method has been established as an effective
synthetic pathway toward novel 99(m)Tc complexes.3,4 Thus, a
combination of the BFC and the cycloaddition strategies is an
excellent opportunity for the development of site specific dual-
functional imaging agents. Conceptually, the synthesis of a site
specific, dual-functional technetium probe is based on “func-
tional” groups which can be selectively linked to a biovector for
targeting and a second, which can be used to introduce an
imaging modality (e.g., fluorescence marker) or a second
targeting moiety, e.g., for cell nucleus targeting.
In this report, we investigated derivatized 1,4,7-triazacyclo-
nonane complexes of the fac-{99TcO3}+-core in order to
exemplify the suitability for the dual-functionality approach.
Complexes of the general form [99TcVIIO3(tacn-R)]+ were
obtained from oxidation of [99TcVO(glyc)(tacn-R)]+ (glyc =
ethylene glycol) type complexes by sodium hypochlorite
(NaOCl) under acidic conditions.3,6 We report selected
coordinating properties of the 1,4,7-triazacyclononane (tacn)
derivatives N-benzyl-2-(1,4,7-triazonan-1-yl)acetamide (tacn-
ba) and 2,2′,2″-(1,4,7-triazonane-1,4,7-triyl)triacetic acid
(nota·3H) and the synthesis of their respective fac-{99TcO3}+
complexes (Scheme 2).
Whereas [99TcO3(tacn-ba)]+ ([2]+) could be synthesized by
the established procedure, a new synthesis for [99TcO3(nota)]2‑
([5]2‑) is described, starting from [99TcI(nota)(CO)3]2‑ and
Current labeling strategies are based on ligand substitution
reactions at the 99mTc-center (metal-centered reactivity).5
Thereby, some weakly bound ligands are replaced by the
BFC. In contrast, labeling with the fac-{99mTcO3}+ core does
not change the ligand atoms in the first coordination sphere but
Received: October 12, 2011
Published: March 14, 2012
© 2012 American Chemical Society
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Scheme 1. Concept for the Synthesis of Site Specific Dual-Functionalized fac-{99mTcO3}+ Probes Based on the BFC Approach
(First Function) and (3 + 2)-Cycloaddition (Second Function)
Scheme 2. Synthesis of [99TcO3(tacn-ba)]+ ([2]+) and [99TcO3(nota)]2‑ ([5]2‑)
solution of [TcO(glyc)(tacn-ba)]Br. Yield: 44.6 mg (84%). IR
sodium perborate tetrahydrate (NaBO3·4H2O) as the oxidizing
reagent.
1
(νTcO, KBr): 952 (s) cm−1. H NMR (500 MHz, D2O): δ =
7.35 (m, 5H, arom), 5.55 (m, 1H, glycol), 5.41 (m, 1H, glycol),
5.08 (m, 1H, glycol), 4.56 (m, 1H, glycol), 4.26 (m, 7H, tacn-
CH2-CONH/CONH-CH2-Ph/tacn), 3.64 (m, 1H, tacn), 3.36
(m, 1H, tacn), 3.27 (m, 1H, tacn), 2.98 (m, 1H, tacn), 2.91 (m,
1H, tacn), 2.60 (m, 2H, tacn), 2.36 ppm (m, 2H, tacn). 13C
NMR (125 MHz, D2O): δ = 170.25 (CONH), 139.03 (ipso-
Carom), 130.34 (Carom), 129.13 (Carom), 128.95 (Carom), 84.92
(2C, glycol), 68.01 (tacn-CH2-CONH), 64.96 (tacn), 57.01
(tacn), 54.67 (tacn), 50.29 (tacn) 46.69 (tacn), 45.10 (tacn),
44.56 ppm (CON-CH2-Ph). Tc analysis: calcd 18.62%; found
18.04%.
EXPERIMENTAL SECTION
■
Caution: 99Tc is a weak β− emitter. All experiments have to be done in
appropriate laboratories for low-level radioactive materials. All reactions
were carried out under an inert N2 atmosphere. The ligand 1,4,7-
triazacyclononane-N,N′,N″-triacetic acid (nota·3HCl)7,8 as well as the
common precursor complexes (NEt4)2[MX3(CO)3] (M = Re, 99Tc;
X= Cl, Br)9,10 and (NBu4)[TcO(glyc)2]3,11 (glyc = ethyleneglycol)
were synthesized according to published procedures. 1,4,7-Triazacy-
clononane, free base (≥97%), and nota, free base (≥97%), were
purchased from CheMatech; (NH4)[99TcO4] (Oak Ridge) and all
other chemicals were of reagent grade and used without further
purification.
[99TcO3(tacn-ba)]Br ([2]Br). This compound was prepared accord-
ing to a modification of a synthetic procedure reported previously.3
[99TcO(glyc)(tacn-ba)]Br (26.56 mg, 0.05 mmol) was dissolved in 1
mL of H2O, and a freshly prepared acidic solution of NaOCl (14%; 0.1
mL) was added. After about 30 min the dark blue solution turned
yellow. Slow evaporation of the solvent yielded yellow crystals that
were filtered from the mother liquor and washed with a small amount
of saturated KBr solution. Yield: 20.25 mg (80%). IR (νTcO3, KBr):
896 (s), 889 (s) cm−1. 1H NMR (500 MHz, MeOH-d4): δ = 7.28 (m,
5H, arom), 4.39 (s, 2H, CONH−CH2−Ph), 4.32 (s, 2H, tacn-CH2-
CONH), 3.35 ppm (m, 12H, tacn). 13C NMR (125 MHz, MeOH-d4):
δ = 169.19 (1C, CONH), 139.68 (1C, ipso-Carom), 129.81 (Carom),
128.87 (2C, Carom), 128.57 (Carom), 62.63 (tacn-CH2−CONH), 55.64
(2C, tacn), 49.44 (2C, tacn), 48.75 (2C, tacn), 44.18 ppm (CONH-
CH2−Ph). 99Tc NMR (113 MHz, MeOH-d4): δ = 392 ppm (s, Δν1/2
= 1200 Hz). Tc analysis: calcd 18.98%, found 19.35%.
Elemental analyses (EA) were performed on a Leco CHNS-932 and
a Leco TruSpec Micro elemental analyzer. H, 13C, and 99Tc NMR
1
spectra were recorded on a BrukerDRX500 500 MHz spectrometer.
13C NMR spectra were proton decoupled recorded. ESI-MS were
performed on a Bruker esquire/HCT spectrometer. UV−vis spectra
and kinetic studies were recorded with an Agilent Cary 50
spectrometer with solution samples in 1 cm quartz cells. During the
measurement the temperature was controlled to maintain 293 K.
HPLC conditions are described in the Supporting Information. For
technetium content measurements, pure compounds were dissolved in
water. The measurements were carried out with a scintillation cocktail
(Packard Ultimate Gold XR) and a liquid scintillation counter (TRI-
CARB 2200CA, Packard).
Synthesis. [99TcO(glyc)(tacn-ba)]Br ([1]Br). To a solution of
(NBu4)[99TcO(glyc)2] (0.1 mmol) in 4 mL of THF was added
tacn-ba·3HCl (38.4 mg, 0.1 mmol). The resulting suspension
was refluxed for 2 h. During this time the formation of a light
blue precipitate was observed. The blue precipitate was filtered,
washed twice with fresh THF, and dried in vacuo to yield a blue
powder. This product was suspended in a saturated KBr
solution and stirred for some minutes. The dark blue powder
was filtered off and dried in vacuo. X-ray quality crystals (blue
needles) were grown by slow evaporation of an aqueous
[Re(nota·2H)(CO)3] ([3]). (NEt4)2[ReBr3(CO)3] (65.3 mg, 0.08
mmol) was dissolved in 2 mL of MeOH. To the colorless solution was
added a solution of nota (32.8 mg, 0.1 mmol) and sodium acetate
(58.0 mg, 0.7 mmol) in 2 mL of MeOH. The mixture was stirred at
room temperature overnight. The solvent was then removed under
reduced pressure, and the remaining colorless powder was dissolved in
2 mL of water. Upon acidification with 1 M HCl to pH 2−3, the
colorless product precipitated from the aqueous solution. Crystals,
suitable for X-ray diffraction analysis (colorless sticks), were obtained
by slow evaporation of the MeOH reaction solution. Yield: 29.7 mg
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Table 1. Crystallographic Data for [TcO(glyc)(tacn-ba)]Br ([1]Br), [TcO3(tacn-ba)]Br·H2O ([2]Br·H2O),
Na2[Re(nota)(CO)3]·2.5H2O (Na2[3]·2.5H2O), [Tc(nota·3H)(CO)3]Cl ([4]Cl), and Na1.5[TcO3(nota·0.5H)]·4H2O
(Na1.5[5]·4H2O)
[TcO(glyc)(tacn-ba)]Br [TcO3(tacn-ba)]Br·H2O
Na2[Re(nota)(CO)3] ·2.5H2O
Na2[3] ·2.5H2O
[Tc(nota·3H)(CO)3]Cl Na1.5[TcO3(nota·0.5H)]·4H2O
[1]Br
[2]Br·H2O
[4]Cl
Na1.5[5]·4H2O
formula
C17H28BrN4O4Tc
530.34
C15H26BrN4O5Tc
520.31
C15H23N3Na2O11.5Re
661.54
C15H21ClN3O9Tc
520.80
C12H26.5N3Na1.5O3Tc
553.35
Mw
space
group
P1
̅
P21/n
P1
P1
̅
P1
̅
̅
a/Å
b/Å
7.8503(3)
10.8590(3)
12.3046(4)
106.501(3)
93.887(3)
96.816(3)
2.768
10.1014(2)
11.4750(8)
17.935(2)
90
7.7667(5)
11.9159(8)
12.1265(6)
109.724(5)
95.710(5)
90.975(5)
5.897
8.794(2)
8.958(2)
13.342(3)
94.55(2)
98.59(2)
107.66(2)
0.92
6.4835(2)
14.2176(5)
14.3398(5)
66.044(4)
87.183(3)
89.303(3)
0.685
c/Å
α/deg
β/deg
γ/deg
μ/mm−1
Z
95.344(4)
90
2.66
2
4
2
2
2
V/Å3
992.92(6)
1.774
2069.9(4)
1.670
1049.7(1)
2.093
981.6(4)
1.762
1206.47(7)
1.523
ρcalcd/g
cm−3
a,c
R1
0.0375
0.0476
0.0480
0.0916
0.0334
0.0640
847665
0.0369
0.0457
847661
0.0450
0.0762
847662
b,c
wR2
CCDC
847663
847664
a
b
c
2
2
R1 = |Fo − Fc|/|Fo|. wR2 = [w(Fo − Fc2)2/(wFo )]1/2. I > 2σ(I).
(65%). IR (νTc(CO)3, KBr): 2030 (s), 1924 (s), 1910 (s) cm−1. ESI-MS
(MeOH): m/z = 574.1 [M + H]+. 1H NMR (500 MHz, MeOH-d4): δ
= 4.28 (s, 6H, tacn-CH2-COO−), 3.76 (m, 6H, tacn), 3.58 ppm (m,
6H, tacn). 13C NMR (125 MHz, MeOH-d4): δ = 196.23 (3 C, Re-
CO), 172.24 (3 C, tacn-CH2-COO‑), 67.19 (3 C, tacn-CH2-COO−),
58.61 ppm (6 C, tacn). Anal. Calcd for C15H20N3O9Re (%): C, 31.47;
H, 3.52; N, 7.34. Found: C, 31.28; H, 3.73; N, 7.34.
purged). Initial concentration of [99TcO3(tacn-ba)]+ [2]+ was 1.7 ×
10−4 M and 4.53 × 10−3 M for sodium 4-vinylbenzenesulfonate (≈26-
fold excess). Changes of absorption were observed at two different
wavelengths (200 and 375 nm) for the first 2400 s (collection of 20
data points for each wavelength). The data was linearearized and fitted
linearly to yield kobs. The kinetic parameter k1 was then calculated with
the initial concentration listed above.
[99Tc(nota·2H)(CO)3] ([4]). (NEt4)2[99TcCl3(CO)3] (23 mg, 0.05
mmol) was dissolved in 6 mL of MeOH and nota (25.1 mg, 0.08
mmol) added to the solution. After addition of triethylamine (10
drops), the solution was refluxed for 5 h under stirring. The solvent
was removed under reduced pressure and the remaining colorless
powder taken up in 2 mL of water. Upon acidification to pH 2−3, a
colorless product precipitated. Crystals of [99Tc(nota·3H)(CO)3]Cl,
suitable for X-ray diffraction analysis, were obtained by slow
evaporation of an acidic aqueous solution (pH 1) of [Tc(nota·2H)-
(CO)3]. Yield: 15.2 mg (72%). IR (νTc(CO)3, KBr): 2034 (s), 1926 (s),
1892 (s) cm−1. 1H NMR (500 MHz, MeOH-d4): δ = 4.09 (s, 6H, tacn-
CH2-COO−), 3.65 (m, 6H, tacn), 3.50 ppm (m, 6H, tacn). 13C NMR
(125 HMz, MeOH-d4): δ = 169.35 (3 C, tacn-CH2-COO‑), 68.4 (3 C,
tacn-CH2-COO−), 57.5 ppm (6 C, tacn). 99Tc NMR (113 MHz,
MeOH-d4): δ = −912 ppm (s, Δν1/2 = 1320 Hz).
X-ray Diffraction. Crystallographic data were collected at 183(2)
K (compounds [1]Br, Na2[3], [4]Cl, Na1.5[5]) or 298(2) K
(compound [2]Br) with Mo Kα radiation (λ = 0.7107 Å) that was
monochromated with help of a graphite on either a Stoe IPDS
diffractometer ([Tc(tacn)(CO)3]Cl and [2]Br) or an Oxford
Diffraction Xcalibur system ([1]Br, Na2[3], [4]Cl, Na1.5[5]) with a
Ruby detector. Suitable crystals were covered with oil (Infineum
V8512, formerly known as Paratone N), mounted on top of a glass
fiber and immediately transferred to the diffractometer. In the case of
the IPDS, a maximum 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.12 Data were
corrected for Lorentz and polarization effects as well as for absorption
(numerical). In case of the Oxford system, the program suite CrysAlis
Pro was used for data collection, semiempirical absorption correction,
and data reduction.13 Structures were solved with direct methods using
SIR9714 or SHELXL-97 (Na2[3]) and were refined by full-matrix least-
squares methods on F2 with SHELXL-97.15 For the structure of
Na1.5[5], the disordered solvent molecules had to be treated with the
SQUEEZE procedure within Platon.16 More details on data collection
and structure calculation are contained in Table 1 ([Tc(tacn)(CO)3]-
Cl, Table S1).
Na2[99TcO3(nota)] Na2[5]. Perborate tetrahydrate (46.16 mg, 0.3
mmol) was added to an aqueous solution of [99Tc(nota·2H)(CO)3]
(16 mg, 0.03 mmol), and the reaction mixture was heated to 55 °C for
2 h. After this time the HPLC trace of the reaction solution (TEAP
buffer) showed a quantitatively conversion of the starting compound
to the desired product. The basic reaction mixture was acidified by the
addition of four drops of 1 M HCl (destruction of excess perborate)
and afterward neutralized with 1 M NaOH. The product was purified
by column chromatography (SPE-Cartridge, C18 ec (L), Chromafix)
and was dried under vacuum. Yield: 12.35 mg (82%). IR (νTcO3, KBr):
906 (s) cm−1. 1H NMR (500 MHz, D2O): δ = 4.15 (s, 6H, tacn-CH2-
COO−), 3.55 (m, 6H, tacn), 3.24 ppm (m, 6H, tacn). 13C NMR (125
HMz, D2O): δ = 175.17 (3 C, tacn-CH2-COO‑), 66.32 (3 C, tacn-CH2-
COO−), 56.07 ppm (6 C, tacn). 99Tc NMR (113 MHz, D2O): δ = 479
ppm (s, Δν1/2 = 6800 Hz).
RESULTS AND DISCUSSION
■
Syntheses. The oxidation of water stable 99TcV complexes
by hypochlorite (OCl)− is a synthetic pathway for fac-
[99TcO3(tacn)]+ type complexes.3 Conjugation of biologically
active functions to tacn will enable labeling opportunities
according to the BFC approach. Accordingly, 1,4,7-triazacyclo-
nonane was derivatized with N-benzyl-2-bromoacetamide to
yield the bioconjugate N-benzyl-2-(1,4,7-triazonan-1-yl)-
acetamide (tacn-ba) as a model. The complex [99TcO(glyc)-
(tacn-ba)]Br ([1]Br) was then synthesized from in situ
prepared [99TcO(glyc)2]− with 1 equiv of (tacn-ba)·3HCl.
Crystals of Na1.5[99TcO3(nota·0.5H)] (Na1.5[5]) suitable for crystal
structural analysis were obtained by fast evaporation of a neutralized
reaction mixture.
Kinetic Measurements. The kinetics of reduction were measured
under pseudo-first-order conditions in doubly distilled H2O (N2
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Scheme 3. Synthesis of [99TcO(glyc)(tacn-ba)]+ ([1]+)
Compound [1]Cl precipitated as blue powder after 2 h. Anion
metathesis with a saturated KBr solution gave the correspond-
ing crystalline [1]Br in 84% yield (Scheme 3).
[1]Br is stable under basic to neutral conditions in aqueous
solution and as a solid but slowly decomposes under acidic
conditions. Blue, plate-shaped crystals were obtained by slow
evaporation of a methanol solution of [1]Br. Figure 1 shows
the molecular structure of the cation [1]+.
comparable complexes [99TcO(glyc)(tacn)]+ (949 cm−1),6
[99TcO(glyc)(tacn-bz)]+ (949 cm−1).3
The oxidation of [1]Br with a small amount of NaOCl in
acidic solution rapidly afforded [99TcO3(tacn-ba)]+ ([2]+,
Scheme 4).
HPLC monitoring indicated nearly quantitative conversion
after one day with a minor amount of [99TcO4]− as a side
product. [99TcO3(tacn-ba)]Br ([2]Br) formed readily and was
isolated as yellow stick-shaped crystals from the aqueous
reaction solution in good yield (80%). [2]Br is water stable
over a wide pH range (1−10) at ambient temperature and
partially soluble in methanol. Thus, 2-(1,4,7-triazonan-1-yl)
acetic acid represents a good ligand system for stabilizing the
fac-{99TcO3}+ core even after conjugation to biomolecules via
amide bond formation and to act as a bifunctional chelator for
the first function. An ORTEP representation of the cation [2]+
is given in Figure 2.
Figure 1. ORTEP17 representation of the [99TcO(glyc)(tacn-ba)]+
[1]+ cation. Thermal ellipsoids represent 50% probability. Hydrogen
atoms are omitted for clarity. Selected bond lengths [Å] and angles
[deg]: Tc−O1 1.664(2), Tc−O2 1.905(2), Tc−O3 1.922(2), Tc−N1
2.256(2), Tc−N2 2.142(2), Tc−N3 2.229(2), O1−Tc−O2 111.44(9),
O1−Tc−O3 107.99(9), O2−Tc−O3 84.06(8), N1−Tc−N2 74.98(9),
N1−Tc−N3 76.21(8), N2−Tc−N3 80.39(8).
[1]Br crystallizes in the triclinic space group P1 with one
̅
molecule per asymmetric unit. A structural feature of [1]+ are
the two elongated Tc−N bonds (Tc−N1 2.256(2), Tc−N3
2.229(2) Å), in comparison to [99TcO(glyc)(tacn)]+ where
only one Tc−N bond (trans-oxo) is elongated (2.164(2),
2.174(2), 2.260(2) Å).6 The extended bond lengths of the Tc−
N1 bond can be understood by the trans-influence of the
terminal oxo ligands and elongation of the Tc−N3 bond in [1]+
may arise from sterical reasons. The elongation of two Tc−N
bonds in [99TcO(glyc)(tacn-R)]+ type complexes was equally
observed in [99TcO(glyc)(tacn-bz)]+ (bz = benzyl).3 Bond
angles in [1]+ do not differ significantly from [99TcO(glyc)-
(tacn-bz)]+. The O1−Tc−O2 and O1−Tc−O3 angles are
107.99(9)° and 111.44(9)°, respectively, to minimize inter-
ligand repulsions between the terminal oxo and the glycolato
ligand. An interesting feature of the IR spectrum of compound
[1]+ is the ν(TcO) stretch at 952 cm−1, similar to the
Figure 2. ORTEP17 representation of the [99TcO3(tacn-ba)]+ [2]+
cation. Thermal ellipsoids represent 50% probability. Hydrogen atoms
are omitted for clarity. Selected bond lengths [Å] and angles [deg]:
Tc−O1 1.690(4), Tc−O2 1.694(4), Tc−O3 1.693(4), Tc−N1
2.223(5), Tc−N2 2.237(4), Tc−N3 2.360(4), O1−Tc−O2
106.7(2), O1−Tc−O3 107.8(2), O2−Tc−O3 106.9(2), N1−Tc−N2
73.7(2), N1−Tc−N3 75.0(2), N2−Tc−N3 74.5(2).
[2]Br crystallizes in the monoclinic space group P21/n with
one cation and one anion per asymmetric unit. The bromide
atom is disordered over two positions with occupancies of 50%.
In [2]+, a distorted octahedral coordination sphere is observed
around the Tc center. The N−Tc−N angles are between
73.7(2)° and 75.0(2)° and the O−Tc−O angles between
106.7(2)° and 107.8(2)° wide. This feature is common to all
fac-{MO3}+ (M = Re, Tc) structures and mirrors the spatial
demand of terminal oxo ligands with “M” in its highest
Scheme 4. Synthesis of [99TcO3(tacn-ba)]+ ([2]+)
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Scheme 5. (3 + 2)-Cycloaddition Reaction of [2]+ with 4-Vinylbenzenesulfonate (Styrene-SO3 ), Kinetic Measurements
−
Scheme 6. Synthesis of [M(nota)(CO)3]2‑ (M = Re, Tc)
oxidation state +VII. Ligand field effects play, if ever, a minor
role, and the terminal oxo ligands assume a minimal energy
conformation by minimizing electrostatic repulsion. In analogy
to the structurally characterized fac-{99TcO3}+ complexes
[99TcO3(tacn-bz)]+ and [99TcO3(tacn-bz-COOH)]+,3 the
Tc−N3 bond (2.360(4) Å) in complex [2]+ is elongated by
about 6% relative to the remaining two Tc−N bonds (2.223(5)
and 2.237(4) Å, respectively). The 99Tc-NMR signal of [2]+
appeared at 392.39 ppm (Δν1/2= 1200 Hz) relative to
[99TcO4]− at 0 ppm, thus, in the expected range for fac-
{99TcO3}+ complexes.18 IR spectra show additional interesting
features. The absorption of the very characteristic symmetric
TcO stretch is split into two bands at 896 and 889 cm−1,
respectively, due to a lowered symmetry (C3v to C2v) as
imposed by the insertion of one substituent on the tacn
scaffold.
[99TcO(glyc)2]− with nota·3H did not lead to the desired
[99TcO(glyc)(nota)]2‑ complex. A similar synthesis published
in the literature was not successful as well.19 Hence, (OCl)−
oxidation of [99TcO(glyc)(nota)]2‑ was not found to be a
successful pathway to higher oxidation states. The correspond-
ing rhenium complex [ReO3(tacn)]+ was originally prepared by
an oxidative decarbonylation of [Re(tacn)(CO)3]+.20 To adopt
this route, [M(nota)(CO)3]2‑ (M = Re, 99Tc) was synthesized
by reaction of [MX3(CO)3]2‑ (M = Re, X = Br; M = 99Tc, X =
Cl) with 1 equiv of (nota·3H) under alkaline conditions which
gave [M(nota)(CO)3]2‑ (M = Re [3]2‑, 99Tc [4]2‑; Scheme 6)
in very good yields.
Complexes [3] and [4] precipitated from an acidic aqueous
solution as colorless powders after double protonation. Both
compounds [3] and [4] are stable in solution and as solids for
weeks. Depending on their protonation grade, they are soluble
in water as [M(nota)(CO)3]2‑ ([3]2‑, [4]2‑) and in methanol as
[M(nota·2H)(CO)3] ([3], [4]). The IR spectra of complexes
[3] and [4] show strong CO absorption bands at 2030, 1924,
1910 and 2034, 1926, 1892 cm−1, respectively. These
frequencies indicate much weaker donation of nota in
comparison to the cationic [Re(tacn)(CO)3]+ (2014 and
1881 cm−1).21 The 99Tc NMR of compound [4]2‑ showed a
broad signal (Δν1/2= 1320 Hz) at −912.3 ppm, shifted toward
lower field as compared to [99Tc(tacn)(CO)3]+ (−1012
ppm).22 In contrast to the 13C NMR spectrum of the Re
compound [3], which shows a signal for the carbonyl carbon
atoms at 196.23 ppm, the 13C NMR signal of the Tc compound
[4] cannot be observed in the 13C NMR spectrum, due to the
Cycloaddition. According to the dual-functionality concept,
the first function is introduced at the tacn ligand and the
second by cycloaddition at the fac-{99TcO3}+ core. Kinetic (3 +
2)-cycloaddition studies were thus performed with [2]+ to
assess convenience for labeling reactions at the tracer level. The
kinetic measurements for the reaction of [2]Br with 4-
−
vinylbenzenesulfonate (styrene-SO3 ) were performed under
the same conditions as for [99TcO3(tacn-R)]+ (R = H, bz, bz-
COOH) (Scheme 5).
The second-order rate constant at 20 °C in water for this
reaction was found to be 0.243 0.004 M−1 s−1 and is in good
−
agreement with the reaction of styrene-SO3 with
[99TcO3(tacn)]+ (0.107
0.001 M−1 s−1), [99TcO3(tacn-
bz)]+ (0.42 0.01 M−1 s−1), and [99TcO3(tacn-bz-COOH)]+
scalar coupling to the 99Tc quadrupole nucleus (spin /2). The
9
(0.26
0.01 M−1 s−1).3 This data indicate that 99TcVII
weaker coordination properties of nota·3H in comparison to
the tacn ligand are evident from the X-ray crystal structures of
Na2[Re(nota)(CO)3] (Na2[3]) and [99Tc(nota·3H)(CO)3]Cl
([4]Cl), respectively. Since both compounds (Na2[Re(nota)-
(CO)3] and [99Tc(nota•3H)(CO)3]Cl) have very similar
structural features, only the structure of [99Tc(nota·3H)-
(CO)3]Cl will be discussed in detail. Crystallographic data for
Na2[Re(nota)(CO)3] (Na2[3]) can be found in the Supporting
Information (Table SI1). A representation of the triply
protonated cation [99Tc(nota· 3H)(CO)3]+ [4]+ is given in
Figure 3.
complexes with a monosubstituted tacn-ligand react faster in
(3 + 2)-cycloadditions. For [2]+, the rate constant for the
cycloaddition is about twice as high as for unsubstituted
[99TcO3(tacn)]+. This leads to the conclusion that the
alteration of the electronic structure of the nitrogen donors,
by, e.g., alkylation, substantially influences the (3 + 2)-
cycloaddition rate. This observation is of relevance for tuning
the rate of the (3 + 2)-cycloaddition by introducing not only
one but two or three additional substituents.
Accordingly, in a next step we studied the multisubstituted
tacn-based ligand derivative 2,2′,2″-(1,4,7-triazonane-1,4,7-
triyl)triacetic acid (nota·3H). Unexpectedly, reaction of
Comparing the bond lengths in Na2[Re(nota)(CO)3] and
[Tc(nota·3H)(CO)3]Cl with the unsubstituted [M(tacn)-
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Figure 4. ORTEP17 representation of the [99TcO3(nota·0.5H)]1.5‑
anion ([5]1.5‑). Thermal ellipsoids represent 50% probability. Hydro-
gen atoms are omitted for clarity. Selected bond lengths [Å] and
angles [deg]: Tc−O1 1.702(3), Tc−O2 1.713(3), Tc−O3 1.706(3),
Tc−N1 2.302(4), Tc−N2 2.281(3), Tc−N3 2.296(4), O1−Tc−O2
106.5(2), O1−Tc−O3 106.6(2), O2−Tc−O3 106.2(2), N1−Tc−N2
76.2(1), N1−Tc−N3 76.4(1), N2−Tc−N3 76.6(1).
Figure 3. ORTEP17 representation of the [99Tc(CO)3(nota·3H)]+
cation [4]+. Thermal ellipsoids represent 50% probability. Noncarboxy
hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and
angles [deg]: Tc−C1 1.906(3), Tc−C2 1.907(3), Tc−C3 1910(4),
Tc−N1 2.237(2), Tc−N2 2.236(2), Tc−N3 2.238(2), C1−O1
1.146(3), C2−O2 1.154(3), C3−O3 1.151(4), C1−Tc−C2 87.8(1),
C1−Tc−C3 86.4(1), C2−Tc−C3 87.6(1), N1−Tc−N2 79.35(8),
N1−Tc−N3 79.69(9), N2−Tc−N3 79.98(8).
are acceptors of hydrogen-bridges. There is a residual electron
density at O7 in a reasonable geometry, but this would mean
that H10d (hydrogen atom of a solvent molecule) would have
to be pointing away from O7 in 50% of the cases and this
position could not be found. There is also the possibility that
the missing half positive charge is located in the disordered
solvent. The crystal packing of Na1.5[5] shows an interesting
channel structure with [5]1.5‑ and Na+ forming a two-
dimensional network (Figure SI3). These layers are packed in
a way that channels are formed along the [100] direction of the
crystal. The channels are filled with disordered water molecules
(solvent accessible void: 19(2) Å3, 2%vol). The Tc−N bond
lengths in Na1.5[5] are all in the same range (2.281(3)−
2.302(4) Å) and are significantly longer than in [99TcO3(tacn)]
(2.239(4) Å).6 The elongation of one Tc−N bond after
substitution of one tacn nitrogen atom was observed previously
in fac-{99TcO3}+ complexes [99TcO3(tacn-bz)]+ (2.343(3) Å),3
[99TcO3(tacn-bz-COOH)]+ (2.286(5) Å),3 and complex [2]+
(2.360(4) Å). The substitution of all three nitrogen atoms in
nota·3H leads to the elongation and weakening of all three
bonds. Substituted nitrogen atoms are usually better σ-donors,
and stronger binding to a metal center was expected. Obviously,
sterical components overcompensate for strong donation, and
overall stability is reduced as it is indicated in the crystal
structure of Na1.5[5]. Since the tacn backbone is distorted upon
coordination to the metal center and forced to assume a specific
conformation, the substituents might keep the tacn ligand from
attaining the preferred coordination conformation. However,
compound [5]2‑ is stable in the solid state and in solution over
time (days). As all other fac-{TcO3}+ complexes, the yellow
compound [5]2‑ reacts with 4-vinylbenzenesulfonate (styrene-
(CO)3]+ complexes provides an insight into binding properties.
The structure of [Re(tacn)(CO)3]+ is known for over two
decades,21,23,24 but the one of its homologue [99Tc(tacn)-
(CO)3]+ was not. To complete this fundamental series, we
synthesized and structurally characterized [99Tc(tacn)(CO)3]+
(SI). Within standard variations, the structures of [99Tc(tacn)-
(CO)3]+ and [Re(tacn)(CO)3]+ are identical.
[99Tc(nota·3H)(CO)3]Cl [4]Cl was obtained from an
acidified aqueous solution of [4]2‑. It crystallizes in the triclinic
space group P1. All acetate groups in nota·3H are protonated,
̅
yielding a cationic complex with chloride as counterion. In the
crystal structure of [4]+, the metal center is in a distorted
octahedral coordination sphere due to sterical constraints of the
aza-macrocycle. Interesting structural features are the elongated
Tc−N bonds (2.236(2), 2.237(2), and 2.238(2) Å). These
bonds are significantly longer than found in [M(tacn)(CO)3]+
(M = Re, 2.189(8)−2.222(8),23 2.169 − 2.213,21 2.195(4) −
2.203(4) Å;24 M = Tc, 2.195(5)−2.206(5) Å) and confirm
nota·3H to be a weaker ligand than tacn.
Starting from [4], different oxidizing reagents such as H2O2,
(OCl)−, and sodium perborate tetrahydrate (NaBO3·4H2O)
were employed. With NaBO3·4H2O, the best results were
obtained, and the reaction of a colorless aqueous solution of
[4]2‑ with this oxidizing reagent gave quantitatively
[99TcO3(nota)]2‑ ([5]2‑) after 2 h at 55 °C (proven by
HPLC monitoring). The 99Tc NMR of compound [5]2‑ shows
a very broad signal (Δν1/2 = 6800 Hz) at 488 ppm. This is
remarkable, since the 99Tc NMR shifts of other [99TcO3(tacn-
R)]+ (R = H, bz, bz-COOH, ba) are in the range 358−392 ppm
(Δν1/2 1200−4800 Hz),3,6 which suggest a different binding
situation in complex [5]2‑. The broad signal is a hint for a
weakly bound nota ligand, which enables dynamics in solution.
In addition, the shift to lower field also indicates a weaker
donating property of the nota ligand, which is in agreement
with observations made with the fac-{M(CO)3}+ (M = Re,
99Tc) core (vide infra, compounds Na2[3] and [4]Cl). The
crystal structure analysis of yellow crystals of
Na1.5[99TcO3(nota·0.5H)] (Na1.5[5]), which were isolated
from a fast evaporated reaction mixture, confirms this
hypothesis. A representation of the molecular structure of
[5]1.5‑ is given in Figure 4.
−
SO3 ) to form a blue (3 + 2)-cycloadduct ([TcO(nota)-
(styrene-SO3)]3‑). On the basis of these observations, the first
negatively charged fac-{99TcO3}+-complex [5]2‑ is a very
attractive candidate for the development of novel multifunc-
tional radioprobes. Experiments with the nuclear isomer 99mTc
and kinetic measurements with compound [5]2‑ are currently
under study.
CONCLUSION
■
The coordination of functionalized tacn-derivatives with the
fac-{99TcO3}+ core according to the BFC approach leads to
new model compounds for radiopharmacy based on high valent
99TcVII chemistry. The coordination properties of the
functionalized tacn derivatives N-benzyl-2-(1,4,7-triazonan-1-
[99TcO3(nota·0.5H)]1.5‑ ([5]1.5‑) crystallizes with 1.5 Na+
cations. The residual positive charge is compensated by 0.5
H+, which could not be localized. All carboxylate oxygen atoms
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yl)acetamide (tacn-ba) and 2,2′,2″-(1,4,7-triazonane-1,4,7-triyl)-
triacetic acid (nota·3H) were evaluated in this respect. The
former ligand stabilizes the {99TcVO}3+ as well as the fac-
{99TcVIIO3}+ core. Corresponding complexes are water stable,
and tacn-ba is a model ligand for conjugating targeting
functions to 2-(1,4,7-triazonan-1-yl) acetic acid (noma).
Noma derivatives therefore enable the BFC approach with
fac-{99(m)TcO3}+ cores. Additional (3 + 2)-cycloadditions are
faster than with unsubstituted tacn; thus, a second bioactive
function can be introduced enabling the access to dual-
functional agents.
The nota ligand does not coordinate as strong as the tacn or
monosubstituted derivatives to the metal center. However, fac-
{99TcO3}+ complexes containing the nota ligand are water
stable, and corresponding 99 mTc compounds are very attractive
candidates for the development of novel multifunctional
radioprobes. Beside the new opportunities nota·3H enables
for the high oxidation states of technetium, it is also an
excellent ligand for the fac-{MI(CO)3}+ core (M = Re, 99mTc).
Nota containing fac-{MI(CO)3}+ (M = 186/188Re, 99mTc)
complexes are hydrophilic complexes with a high potential
for radiopharmacetutical imaging or therapy.
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(22) Alberto, R.; Herrmann, W. A.; Bryan, J. C.; Schubiger, P. A.;
Baumgartner, F.; Mihalios, D. Radiochim. Acta 1993, 63, 153−61.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Additional figures, tables, and details. Crystal data in CIF
format. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Phone: (+41) 44 63 546 31. Fax: (+41)446356803. E-mail:
braband@aci.uzh.ch (H.B.), ariel@aci.uzh.ch (R.A.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
H.B. acknowledges financial support from the Swiss National
Science Foundation, Ambizione Project PZ00P2_126414.
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