Rhenium(V) and technetium(V) nitrido complexes with mixed tridentate π-donor and monodentate π-acceptor ligands

Article abstract of DOI:10.1021/ic202605z

Mixed-ligand [M(N)(SNS)(PPh₃)] complexes (M = Tc, Re) (1, 2) were prepared by reaction of the precursor [M(N)Cl₂(PPh ₃)₂] with ligand 2,2′-dimercaptodiethylamine [H ₂SNS = NH(CH₂CH₂

Full text of DOI:10.1021/ic202605z

Article
pubs.acs.org/IC
Rhenium(V) and Technetium(V) Nitrido Complexes with Mixed
Tridentate π-Donor and Monodentate π-Acceptor Ligands
Alessandra Boschi,† Emiliano Cazzola,† Licia Uccelli,† Micol Pasquali,† Valeria Ferretti,‡ Valerio Bertolasi,‡
and Adriano Duatti*,†
‡
†Laboratory of Nuclear Medicine, Department of Radiological Sciences and Department of Chemistry and Centre for Structural
Diffractometry,University of Ferrara, 44121 Ferrara, Italy
S
* Supporting Information
ABSTRACT: Mixed-ligand [M(N)(SNS)(PPh3)] complexes (M = Tc, Re) (1, 2)
were prepared by reaction of the precursor [M(N)Cl2(PPh3)2] with ligand 2,2′-
dimercaptodiethylamine [H2SNS = NH(CH2CH2SH)2] in refluxing dichloromethane/
ethanol mixtures. In these compounds, 2,2′-dimercaptodiethylamine acts as a dianionic
tridentate chelating ligand bound to the [MN]2+ group through the two π-donor
deprotonated sulfur atoms and the protonated amine nitrogen atom. Triphenylphos-
phine completes the coordination sphere, acting as a monodentate ligand.
[M(N)(NS2)(PPh3)] complexes can assume two different isomeric forms depending
on the syn and anti orientations of the hydrogen atom bound to the central nitrogen
atom of the SNS ligand with respect to the MN moiety. X-ray crystallography of the
syn isomer of complex 2 demonstrated that it has a distorted trigonal bipyramidal
geometry with the nitrido group and the two sulfur atoms defining the equatorial
plane, the phosphorus atom of the monophosphine and the protonated amine
nitrogen of the tridentate ligand spanning the two reciprocal trans positions along the axis perpendicular to the trigonal plane.
Synthesis of the analogous Tc derivatives with tris(2-cyanoethyl)phosphine, [Tc(N)(SNS)(PCN)] [(PCN = P(CH2CH2CN)3],
required the preliminary preparation of the new precursor [Tc(N)(PCN)2Cl2]2 (3), which was prepared by reacting [n-
NBu4][Tc(N)Cl4] with a high excess of PCN. The crystal structure of compound 3 consists of a noncrystallographic
centrosymmetric dimer of Tc(V) nitrido complexes having an octahedral geometry. In this arrangement, the apical positions are
occupied by two tris(2-cyanoethyl)phosphine groups and the equatorial positions by the nitrido group whereas the two Cl−
anions and one cyano ligand belong to the other octahedral component of the dimer. By reacting the new precursor
[Tc(N)(PCN)2Cl2]2 with the ligand H2SNS the complex [Tc(N)(SNS)(PCN)] (5) was finally obtained in acetonitrile solution.
The new Tc(III) complex trans-[Tc(PCN)2Cl4][n-NBu4] (4) was also isolated from the reaction solution used for preparing
complex 3 as side product and characterized by X-ray diffraction. The crystal structure of 4 consists of independent trans-
[TcCl4(PCN)2]− anions situated on crystallographic centers of symmetry and tetrabutylammonium cations in general positions.
present, the most popular method, dubbed the ‘bifunctional
approach’, consists of tethering a strong chelating group for the
metal to a point of the bioactive molecule that is irrelevant for
preserving its biological properties. The resulting bifunctional
ligand would act as a bridging backbone between the
biomolecule and the metal.4
The β-emitting radionuclide Re-188 is currently considered a
very attractive candidate for development of therapeutic
radiopharmaceuticals. This nuclide decays through emission
of a β particle (Eβmax = 2.1 MeV, t1/2 = 16.9 h) having high
penetration in soft tissues.5 Re-188 also emits a gamma photon
(Eγ = 0.155 MeV) that can be conveniently used for imaging
the distribution of the radiotherapeutic agent soon after its
administration to the patient. This possibility can be of great
help to determine the response to therapy.
INTRODUCTION
■
Radionuclide therapy still remains one of the potentially most
effective approaches for treatment of cancer.1 In the most
common accomplishment, this therapy involves intravenous
administration of a radiolabeled molecule that was designed to
target cancerous tissue through a specific biological mechanism
selectively expressed by malignant cells. Since the very first
example of this application using 131I-labeled anionic iodide for
the therapy of thyroid malignancies, the molecular design of the
new radiotherapeutic agent has become more and more
sophisticated. A common strategy entails the radiolabeling of
an appropriate biologically active molecule, such as proteins,
peptides, or drugs, which is known to selectively target
cancerous cells. The usual challenge comes from the need to
incorporate the radioactive tag within the structure of the
bioactive molecule without affecting its primitive biological
properties.2 This problem becomes more relevant with metallic
radionuclides, which constitutes the largest set of radioelements
having suitable nuclear properties for therapeutic purposes.3 At
Received: December 3, 2011
Published: February 10, 2012
© 2012 American Chemical Society
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The transition metal rhenium is a congener with technetium
as both belong to group 7 of the periodic table. Though their
chemical properties are not completely superimposable, these
two elements display many similarities. The metastable isomer
99mTc (t1/2 = 6.06 h, Eγ = 0.140 MeV) is still the most widely
used diagnostic radionuclide in nuclear medicine.6 It has been
experimentally demonstrated that a pair of technetium and
rhenium compounds having exactly the same molecular
geometry and ligand composition always exhibit the same in
vivo biodistribution pattern.7 This result implies that a 99mTc
complex can be conveniently employed to assess the biological
properties of the corresponding 188Re analogue. From a
practical point of view, this is of great advantage because it
avoids the need of using high-dose radioactive beta-emitting
radiocompounds in preclinical studies. Moreover, biodistribu-
tion data for a 99mTc analogue allow calculating dosimetry
parameters that are critical to determine the effective lethal
dose at the tumor. In conclusion, current trends in the search
for 188Re therapeutic agents always consider it to be highly
useful to obtain the corresponding 99mTc counterparts
whenever possible.
The chemistry of Tc and Re has been widely investigated in
recent years.8 Studies were devoted to finding strong chelating
systems capable of conferring a high in vivo stability to the
resulting complexes. In particular, our group focused on the
study of five-coordinated complexes characterized by the
presence of a terminal [MN]2+ (M = Tc, Re) multiple
bond.9 An interesting feature of this class of complexes is that
its structural behavior is fairly predictable simply by considering
the nature of the coordinating atoms surrounding this metallic
functional group.10 In our studies, we found that a ligand
system composed of four soft π-donor atoms usually affords
nitrido complexes having a highly distorted umbrella-shaped,
square-pyramidal geometry where the [MN]2+ group lies
above the basal plane. Conversely, when two π-donor and two
π-acceptor atoms compose the coordination set, the preferred
geometry is trigonal bipyrmidal (Scheme 1). In this arrange-
bearing a [S, N, S] π-donor atom set, and the monophosphines,
tris(2-cyanoethyl)phosphine (PCN) and triphenylphosphine
(PPh3), as monodentate π-acceptor ligands for modeling this
category of complexes. The results of this study will be also
compared with data previously described by our group on the
synthesis and structural characterization of the similar nitrido
complex [Tc(N)(ONS)(PPh3)] [H2ONS = S-methyl-3-(2′-
hydroxybenzylidene)dithiocarbazate].12
EXPERIMENTAL SECTION
■
Caution! Tc is a weak β emitter (Eβ = 0.292 MeV, t1/2 = 2.12 ×
105 years). All manipulations were carried out in a laboratory
approved for low-level radioactivity using monitored hoods and
gloveboxes. When handled in milligram amounts, Tc does not
present a serious health hazard since common laboratory
glassware provides adequate shielding. Bremsstrahlung is not a
significant problem due to the low energy of the β particles.
However, proper radiation safety procedures must be used at all
times to avoid contamination and inhalation of radioactive
material.
Reagents. Rhenium was purchased from Sigma-Aldrich
(Milan, Italy) as KReO4. Technetium as [NH4][TcO4] was
obtained from Oak Ridge National Laboratory. Samples were
dissolved in water and treated with excess H2O2 (30% v/v) at
80 °C prior to use to eliminate residual TcO2. Solid samples of
purified [NH4][TcO4] were obtained by slow evaporation of
aqueous ammonia solutions with gentle heating at 40 °C.
General literature methods were employed for preparing the
precursor complexes [Re(N)Cl2(PPh3)2],13 [Tc(N)-
Cl2(PPh3)2],14 and [n-NBu4][Tc(N)Cl4].15 Common reagents
were purchased from Sigma-Aldrich (Milan, Italy) and Fluka
(Milan, Italy) and used without further purification. Tris(2-
cyanoethyl)phosphine [PCN = P(CH2CH2CN)3] and triphe-
nylphosphine [PPh3 = P(C6H5)3] were obtained from Sigma-
Aldrich (Milan, Italy). The ligand 2,2′-iminodiethanethiol
[H2SNS = NH(CH2CH2SH)2] was prepared using literature
procedures.16
Physical Measurements. Elemental analyses (C, H, N, S)
were performed on a Carlo Erba 1106 elemental analyzer. FTIR
spectra were recorded on a Nicolet 510P Fourier transform
spectrometer in the range 4000−400 cm−1 and in KBr mixtures
Scheme 1
1
using a Spectra-Tec diffuse reflectance collector accessory. H
and 31P NMR spectra were collected on a Bruker AC-400
instrument using SiMe4 as internal reference (1H) and 85%
aqueous H3PO4 as external reference (31P). Electron-spray
mass spectra in the positive-ion mode [ESI(+)MS] of rhenium
and technetium compounds (ca. 10−6 mol dm−3 methanol
solutions) were recorded on a LCQ instrument (Finnigan, Palo
Alto, CA). Compounds were injected via a syringe pump at a
flow rate of 5 μL min−1. The spray capillary voltage was set at
4.5 kV, and the entrance capillary temperature and voltage were
set at 240 °C and 13 V, respectively. The nebulizing gas was N2.
High-performance liquid chromatography (HPLC) was per-
formed on a Beckman System Gold instrument equipped with a
programmable solvent model 126, a sample injection valve
210A, a scanning detector Module 166, and a radioisotope
detector model 170. HPLC analyses were carried out using a
reversed-phase Agilent precolumn Zorbax 300SB-C18 (4.6 ×
12.5 mm) and a reversed-phase Agilent column Zorbax 300SB-
C18 (4.6 × 250 mm) at a flow rate of 1.0 mL/min.
ment, the [MN]2+ core lies on the same trigonal plane
occupied by the two π-donor atoms while the two π-acceptor
atoms span a reciprocal trans position along the trigonal
axis.9,11
The aim of this work was to further expand our investigation
on five-coordinated Tc(V) and Re(V) nitrido complexes by
considering the effect on their geometrical features of the
binding of three π-donor and one π-acceptor atoms. This
arrangement can be accomplished by reacting the [MN]2+
group with a combination of one tridentate π-donor ligand and
one monodentate π-acceptor ligand (hence the acronym ‘3 + 1
complexes’ used here). Specifically, we used here the simple
ligand 2,2′-iminodiethanethiol [H2SNS = NH(CH2CH2SH)2],
Syntheses. A summary of the reactions carried out in this
work is reported in Scheme 2.
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NH), 3.15 (m, 2H, CH2N), 3.15−2.87 (m, 2H, CH2S), 2.47
(m, CH2N). 13C NMR (100 MHz, Me4Si, δ, ppm, CDCl3):
134.82−134.71 (6C, CHAr) 130.78 (3C, CHAr), 128.57−128.47
(6C, CHAr), 59.38 (s, CH2S), 37.47 (s, CH2N). 31P NMR (162
MHz, δ, ppm, CDCl3): 35.2 s. ESI(+)MS: 597, 599 [MH]+.
HPLC data are reported in Table 1. Compounds are soluble in
Scheme 2
Table 1. HPLC Data for Selected Complexes
complex
Rf (C18)
retention time (min)
a
syn-[Re(N)(SNS)(PPh3)]
21.04
d
b
b
syn,anti-[99gTc(N)(SNS)PCN]
syn,anti-[99gTc(N)(SNS)PPh3]
0.5
14.09 , 16.02
a
a
c
20.6 , 21.37, 17.01
a
A, H2O [containing trifluoroacetic acid (TFA, 0.1% v/v)]; B, CH3CN
[containing TFA, 0.1% v/v]. Gradient: 0−5 min, B = 30% (isocratic);
5−30 min, B = 90%; 30−35 min, B = 90% (isocratic); 35−40 min, B =
b
30%. A, H2O [containing TFA, 0.1% v/v]; B, CH3CN [containing
TFA, 0.1% v/v]. Gradient: 0−2 min, B = 15% (isocratic); 2−32 min, B
= 20%; 32−37 min, B = 100%; 37−42 min, B = 100% (isocratic); 42−
c
47 min, B = 15%. A, H2O [containing TFA (0.1% v/v)]; B, CH3CN
[containing TFA (0.1% v/v)]. Gradient: 0−5 min, B = 30%
(isocratic); 5−20 min, B = 100%; 20−30 min, B = 100% (isocratic);
d
30−35 min, B = 30%. H2O [containing TFA 0.1% v/v]/CH3CN
[containing TFA (0.1% v/v)] (7:3).
chlorinated solvents, acetonitrile, acetone, and alcohols but
insoluble in diethyl ether and n-hexane.
[Tc(N)(PCN)2Cl2]2 (3). To a red-orange solution containing
0.12 mmol (0.060 g) of [n-NBu4][Tc(N)Cl4] in 20 mL of
CH2Cl2/EtOH (50:50% v/v), 0.48 mmol (0.093 g) of PCN,
dissolved in 5 mL of CH2Cl2/EtOH (50:50% v/v), was added.
The reaction mixture was stirred under a dinitrogen
atmosphere for 1 h. The color of the solution changed to
yellow. After formation of a yellow precipitate, this was filtered
off and washed with diethyl ether (3 mL), ethanol (3 mL), and
chloroform (3 mL). The resulting solid was dried overnight
under vacuum and then dissolved in acetone. Slow diffusion of
ethanol into the acetone solution afforded yellow-orange
crystals suitable for X-ray diffraction. Yield: 30%. Anal. Calcd
for C36H48Cl4N14P4Tc2: C, 37.92; H, 4.24; N, 17.20. Found: C,
37.15; H, 3.99; N, 17.01. IR (cm−1, KBr): 1059 ν(TcN),
[M(N)(SNS)(PPh3)] (M = Tc, 1; M = Re, 2). A slight excess of
the ligand H2SNS (0.15 mmol, 0.021 g dissolved in 5 mL of
EtOH) was added to a suspension containing 0.13 mmol of
[M(N)Cl2(PPh3)2] (0.092 g for Tc; 0.10 g for Re) in 20 mL of
CH2Cl2. The reaction mixture was heated at reflux temperature
and stirred under N2 atmosphere for 2 h. The color of the
solution darkened first and then turned to bright yellow-orange
after addition of excess of Et3N (0.1 mL). After cooling, all
solvents were evaporated and the crude material was washed
with Et2O. A yellow oil was collected and dried by passing a
dinitrogen stream. The resulting yellow solid was crystallized by
slow evaporation from an ethanol/n-hexane mixture yielding
yellow-orange crystals of the final compound. Data for 1. Yield:
70%. Anal. Calcd for C22H24N2PS2Tc: C, 51.77; H, 4.74; N,
4.49; S, 12.56. Found: C, 51.50; H, 4.65; N, 4.50; S, 12.81. IR
(cm−1, KBr): 1057 ν (TcN), 1096 ν (Tc−P), 3067 ν(NH),
1481 ν(Ph), 1435 ν(P−Ph). 1H NMR (δ, ppm, CDCl3): 7.57−
7.38 (m, 15H, CHar), 6.21 (s, 1H, NH), 3.22 (m, 2H, CH2N),
3.20−2.65 (m, 2H, CH2S), 2.27 (m, CH2N). 13C NMR (100
MHz, Me4Si, δ, ppm, CDCl3): 135.17−134.79 (6C, CHAr)
130.71 (3C, CHAr), 128.80−128.66 (6C, CHAr), 59.07 (s,
CH2S), 37.53 (s, CH2N). 31P NMR (162 MHz, −40 °C, δ,
ppm, CDCl3): 52.2 bs. ESI(+)MS: 511 [MH]+. Data for 2.
Yield: 75%. Anal. Calcd for C22H24N2PS2Re: C, 44.20; H, 4.05;
N, 4.69; S, 10.73. Found: C, 44.15; H, 4.11; N, 4.70; S, 10.81.
IR (cm−1, KBr): 1057 ν (ReN), 1093 ν(Re−P), 3081
ν(NH), 1481 ν(Ph), 1434 ν(P−Ph). 1H NMR (δ, ppm,
CDCl3): 7.73 (m, 6H, CHar), 7.43 (m, 9H, CHar), 6.17 (s, 1H,
1
1095 ν(Tc−P). H NMR (δ, ppm, CD3CN): 2.5 (m, 24H,
CH2CN), 2.9 (m, 24H, CH2P). 31P NMR (162 MHz, δ, ppm,
CD3CN): 23 bs. ESI(+)MS: 1139 [MH]+. The compound is
soluble in acetone and CH3CN, slightly soluble in chlorinated
solvents and alcohols, and insoluble in diethyl ether and n-
pentane.
[TcCl4(PCN)2][n-NBu4] (4). Removal of the solvent from the
resulting orange supernatant collected after precipitation of
compound 3 (see above) gave a yellow-orange solid residue
that was filtered off and dissolved in dichloromethane. Slow
evaporation of the solvent afforded orange crystals suitable for
X-ray diffraction. Yield: 20%. Anal. Calcd for C18H24Cl4N6P2Tc:
C, 46.96; H, 6.95; N, 11.27. Found: C, 47.01; H, 7.00; N, 11.82.
IR (cm−1, KBr): 1098 ν(Tc−P), 356, 347 ν(Tc−Cl). 1H NMR
(δ, ppm, CDCl3,): 1.8 (s, 12H, CH2P), 2.6 (s, 12H, CH2CN),
3.4−1.4 (m, 24H, NCH2CH2CH2), 1.0 (t, 12H, NCH3).
ESI(+)MS: 649 [Na+ + MH]+, 591 [MH − Cl]+, 555 [M −
2Cl]+. The compound is soluble in acetone, CH3CN, and
chlorinated solvents, slightly soluble in alcohols, and insoluble
in diethyl ether and n-pentane.
[Tc(N)(SNS)PCN] (5). To a bright orange solution containing
0.045 mmol (0.051 g) of 3 in 20 mL of CH3CN 0.112 mmol
(0.015 g) of the ligand H2NS2, dissolved in 5 mL of CH3CN,
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was added. Suddenly, the color of the solution turned to yellow.
The reaction mixture was stirred under a dinitrogen
atmosphere for 2 h. The solvent was evaporated, and the
crude material was treated with CH2Cl2 (2 × 3 mL). After
evaporation of CH2Cl2, a yellow oil was collected, washed with
Et2O, and dried by passing a dinitrogen stream. Yield: 63%.
Anal. Calcd for C13H21N5PS2Tc: C, 35.38; N, 15.87; H, 4.8.
Found: C, 34.75; H, 4.6; N, 15.32. IR (cm−1, KBr): 1055
Table 2. Selected Bond Distances (Angstroms) and Angles
(degrees) for Compound 2
bond distances
Re1−N1
Re1−N2
Re1−S1
1.669(4)
2.161(4)
2.314(1)
Re1−S2
Re1−P1
2.328(2)
2.385(1)
bond angles
N1−Re1−N2
N1−Re1−S1
N1−Re1−S2
N1−Re1−P1
N2−Re1−S1
H bond
100.9(2)
N2−Re1−S2
N2−Re1−P1
S1−Re1−S2
S1−Re1−P1
S2−Re1−P1
82.9(1)
1
ν(TcN), 1092 ν(Tc−P), 3057 ν(NH). H NMR (δ, ppm,
113.6(1)
114.4(1)
93.6(1)
87.5(1)
165.2(1)
131.63(5)
89.69(4)
93.24(5)
CDCl3,): 3.0 (m, 4H, CH2N), 2.9 (m, 4H, CH2S), 2.2 (m, 6H,
CH2CN), 2.0 (s, 1H, HN), 1.7 (m, 6H, CH2P). 31P NMR (162
MHz, δ, ppm, CDCl3): 49.8 bs. ESI(+)MS: 442 [MH]+. The
compound is soluble in acetone and CH3CN, slightly soluble in
chlorinated solvents and alcohols, and insoluble diethyl ether
and n-pentane.
N2−H2
H2···N1(−x,−y,−z)
0.84(4)
2.04(4)
N2···N1
N2−H2···N1
2.870(5)
170(4)
X-ray Crystallography. Crystal data of compounds 2, 3,
and 4 were collected on a Nonius Kappa CCD diffractometer
using graphite-monochromated Mo Kα radiation (λ = 0.7107
Å) at room temperature (295 K). Data sets were integrated
with the Denzo-SMN package17 and corrected for Lorentz-
polarization and absorption18 effects. Structures were solved by
direct methods (SIR97)19 and refined by full-matrix least-
squares methods with all non-hydrogen atoms anisotropically
and hydrogens included on calculated positions, riding on their
carrier atoms.
2. C22H24N2PReS2, M = 597.72, monoclinic, P21/c (No. 14),
a = 9.3975(1) Å, b = 9.6035(1) Å, c = 25.7530(5) Å, β =
97.4211(6)°, V = 2304.71(6) Å3, Z = 4, Dc = 1.723 g cm−3, μ =
(Mo Kα) = 5.533 mm−1, 5498 independent reflections, θ ≤
28.00°, 4200 observed reflections [I ≥ 2σ(I)], R1 = 0.0324
(observed reflections), wR2 = 0.0792 (all reflections), GOF =
1.008, 257 parameters. Aminic H2 hydrogen atom was refined
isotropically. CCDC No. 855423.
3. C36H48Cl4N14P4Tc2, M = 1140.39, orthorhombic, P212121
(No. 19), a = 12.5552(1) Å, b = 18.1326(2) Å, c = 21.7075(3)
Å, V = 4941.9(1) Å3, Z = 4, Dc = 1.533 g cm−3, μ = (Mo Kα) =
0.948 mm−1, 14 343 independent reflections, θ ≤ 30.00°, 11
849 observed reflections [I ≥ 2σ(I)], R1 = 0.0401 (observed
reflections), wR2 = 0.0962 (all reflections), GOF = 1.028, 537
parameters. Some CN groups of the tris(cyanoethyl)phosphine
ligands were found disordered and refined isotropically over
two positions. CCDC No. 855424.
Table 3. Selected Bond Distances (Angstroms) and Angles
(degrees) for Compound 3
bond distances
Tc1−N1
Tc1−Cl1
Tc1−Cl2
Tc1−P1
Tc1−P2
Tc1−N9
1.630(3)
2.632(1)
2.404(1)
2.484(1)
2.454(1)
2.126(3)
Tc2−N2
Tc2−Cl3
Tc2−Cl4
Tc2−P3
Tc2−P4
Tc2−N3
1.632(3)
2.638(1)
2.406(1)
2.475(1)
2.467(1)
2.126(3)
bond angles
N1−Tc1−Cl1
N1−Tc1−Cl2
N1−Tc1−P1
N1−Tc1−P2
N1−Tc1−N9
Cl1−Tc1−Cl2
Cl1−Tc1−P1
Cl1−Tc1−P2
Cl1−Tc1−N9
Cl2−Tc1−P1
Cl2−Tc1−P2
Cl2−Tc1−N9
P1−Tc1−P2
P1−Tc1−N9
P2−Tc1−N9
170.0(1)
N2−Tc2−Cl3
N2−Tc2−Cl4
N2−Tc2−P3
N2−Tc2−P4
N2−Tc2−N3
Cl3−Tc2−Cl4
Cl3−Tc2−P3
Cl3−Tc2−P4
Cl3−Tc2−N3
Cl4−Tc2−P3
Cl4−Tc2−P4
Cl4−Tc2−N3
P3−Tc2−P4
P3−Tc2−N3
P4−Tc2−N3
170.8(1)
99.1(1)
96.7(1)
98.3(1)
97.0(1)
94.1(1)
95.0(1)
91.9(1)
92.0(1)
90.50(3)
86.48(3)
83.63(3)
78.56(8)
86.41(3)
87.62(3)
168.85(8)
168.40(3)
90.72(8)
93.25(8)
90.86(3)
84.72(3)
83.92(3)
78.87(8)
86.77(4)
88.30(4)
169.54(7)
167.55(4)
90.27(9)
92.53(9)
4. (C18H24Cl4N6P2Tc)− (C16H36N)+, M = 869.56, triclinic,
P-1 (No. 2), a = 11.9436(2) Å, b = 12.5886(2) Å, c =
16.4361(3) Å, α = 71.7489(7)°, β = 72.5811(6)°, γ =
88.9434(7)°, V = 2231.53(7) Å3, Z = 2, Dc = 1.294 g cm−3,
μ = (Mo Kα) = 0.665 mm−1, 10 695 independent reflections, θ
≤ 28.00°, 8264 observed reflections [I ≥ 2σ(I)], R1 = 0.0353
(observed reflections), wR2 = 0.0938 (all reflections), GOF =
1.055, 437 parameters. The asymmetric unit is built up by two
independent anionic complexes situated on centers of
symmetry and a tetrabutylammonium cation in general
position. A number of carbon atoms of the cation moiety
were found disordered and refined isotropically over two
positions. CCDC No. 855425.
high yield from the labile precursors [M(N)Cl2(PPh3)2] via
ligand-exchange reactions using an excess of the H2SNS ligand.
IR spectra exhibit characteristic bands at 1057 cm−1 assigned to
the ν[MN] stretching vibration. Frequencies in the regions
1093−1096 and 3067−3081 cm−1 were assigned to the
stretching vibrations of the M−PPh3 group and of the amine
group of the SNS ligand, respectively. Elemental analyses were
also in good agreement with the proposed formulation. Proton
spectra of complexes 1 and 2 showed relatively complex
patterns consistent with their molecular structures. Complex 2
shows a sharp singlet at 35.2 ppm in the 31P NMR spectrum
arising from the presence of the monophosphine ligand.
Conversely, the same signal was difficult to observe at room
temperature for the Tc complex as a consequence of the wide
broadening caused by the coupling of the quadrupolar 99gTc
nucleus (I = 9/2) with the neighbor 31P nucleus.23 Figure 1a
reports the HPLC chromatogram of the isomer syn-[Re(N)-
(SNS)(PPh3)] (see Discusssion section) eluted as a single peak
at 21.04 min. The HPLC chromatogram of the complex
All calculations were performed using SHELXL-9720 and
PARST21 implemented in the WINGX22 system of programs.
Selected bond distances and angles are reported in Tables 2, 3,
and 4.
RESULTS
■
Synthesis and Characterization. The [M(N)(SNS)-
(PPh3)] complexes [M = Tc (1), Re (2)] were prepared in
[
99gTc(N)(SNS)(PPh3)] (Figure 1b) reveals the presence of
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Table 4. Selected Bond Distances (Angstroms) and Angles (degrees) for Compound 4
molecule A
molecule B
molecule A
2.4571(5)
molecule B
bond distances
Tc1−Cl1
2.4180(5)
2.4242(5)
Tc1−P1
2.4577(7)
Tc1−Cl2
2.3722(7) 2.3542(6)
bond angles
Cl1−Tc1−Cl1′
Cl1−Tc1−Cl2
Cl1−Tc1−Cl2′
Cl1−Tc1−P1
Cl1−Tc1−P1′
180
180
Cl2−Tc1−Cl2′
Cl2−Tc1−P1
Cl2−Tc1−P1′
P1−Re1−P1′
180
180
88.78(2)
91.22(2)
91.69(2)
88.31(2)
88.82(2)
91.18(2)
92.42(2)
87.58(2)
90.01(2)
89.99(2)
180
90.16(2)
89.84(2)
180
the new mixed halogeno−phosphino precursor complex
[Tc(N)(PCN)2Cl2]2 (3) was prepared by reacting [n-NBu4]-
[Tc(N)Cl4] with a high excess of PCN in a CH2Cl2/EtOH
mixture. Compound 3 was isolated by precipitation as a yellow-
orange solid and then further used for preparing the complex
[Tc(N)(SNS)(PCN)] (5) in good yield by a simple exchange
reaction with H2SNS.
Compound 3 was not the only product of the reaction of
[Tc(N)Cl4]− with PCN. Slow evaporation of the residual
orange supernatant left aside after precipitation of 3 yielded
orange crystals of the octahedral monoanionic Tc(III) complex
trans-[Tc(PCN)2Cl4][n-NBu4] (4).
The proposed formulations for complexes 3, 4, and 5 were
confirmed by elemental analyses and spectroscopic measure-
ments. The mass spectrum of 3 showed the parent ions and
various fragments corresponding to loss of chloride ions having
the expected isotopic pattern. No detectable fragmentation was
observed for complex 4, thus indicating the high stability of this
dimeric structure. Whereas complexes 3 and 5 display
observable 31P NMR signals at room temperature in agreement
with the expected phosphine coordination, no distinct 31P
NMR signal was detected for compound 4. The paramagnetism
of octahedral Tc(III) high-spin d3 complexes also accounts for
24
1
the broadening of signals in the H NMR spectrum of 4.
The molecular structure of the isomeric form syn-[Re(N)-
(SNS)(PPh3)] of complex 2 was solved by X-ray diffraction
analysis, and the ORTEP25 view of this complex is reported in
Figure 2a. This five-coordinated complex exhibits a distorted
trigonal bipyramidal geometry with the nitrido group and the
sulfur atoms of the SNS ligand defining the equatorial plane
and the phosphorus of the PPh3 group and the nitrogen of SNS
ligand occupying the axial positions. The Re1−N1 distance of
1.669(4) Å is indicative of a strong ReN triple bond, in
agreement with other X-ray authentic trigonal bipyramidal or
square pyramidal Re(V) nitrido complexes where Re−N bond
lengths fall in the range 1.61−1.67 Å.11,15,26 The overall
geometrical core is very similar to those observed in Re(V)−
oxo complexes containing analogous ligands.26 In the crystal
packing a pair of complexes form dimers by means of N2−
H···N1(nitrido) hydrogen bonds (Figure 2b). A similar
arrangement was found in the Re−oxo complex containing
the same SNS ligand.27
The ORTEP views of complexes 3 and 4 are reported in
Figures 3 and 4, respectively. Compound 3 consists of a
noncrystallographic centrosymmetric dimer of Tc(V) nitrido
complexes with octahedral coordination, where the apical
positions are occupied by two tris(2-cyanoethyl)phosphine
groups and the equatorial positions by the nitrido group, two
Cl− anions, and a cyano ligand belonging to the other
octahedral component of the dimer. The Tc−N distances of
Figure 1. HPLC chromatograms of (a) syn-[Re(N)(SNS)(PPh3)] (2)
and (b) a mixture of syn/anti-[99 gTc(N)(SNS)(PPh3)] (1).
one main peak at almost the same retention time (21.37 min)
of the Re analogue accompanied by another smaller peak (20.6
min). These two peaks were tentatively interpreted as
corresponding to a mixture of syn and anti isomers (see also
Table 1).
Attempts to prepare the analogous complexes with the
monophosphine PCN by simple substitution onto the
precursors [M(N)Cl2(PPh3)2] (M = Tc, Re) with an excess
of H2SNS and PCN did not afford the complexes [M(N)-
(SNS)PCN] (5) in satisfactory yields. Similarly, reaction of the
complex [Tc(N)Cl4]− in the presence of PCN and H2SNS
failed to give the desired product (the same reaction with the
Re analog [Re(N)Cl4]− was not attempted due to the high
instability of this compound). To obtain the PCN derivative,
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Figure 4. ORTEP view of anion complex A of compound 4 displaying
thermal ellipsoids at 30% probability.
2.404(1) and 2.406(1) Å. This result is in close accordance with
the structural data of other Tc(V) complexes where Tc−Cl
bond distances trans to a nitrido group fall in the range 2.59−
2.73 Å, whereas Tc−Cl(cis) distances display values from 2.41
to 2.45 Å.28
The crystal structure of compound 4 consists of independent
trans-[Tc(III)Cl4(PCN)2] anions lying on crystallographic
centers of symmetry and tetrabutyl ammonium cations located
in general positions. Both the independent A and B anions
assume an octahedral geometry where the equatorial plane is
defined by four Cl− ligands and two PCN groups span the
apical positions. Owing to the centrosymmetry, the octahedral
coordination of both anions is rather regular. The Tc−Cl
average bond distance of 2.392 Å is in excellent agreement with
that of 2.386 Å observed in the similar octahedral structure of
the anionic complex trans-[Tc(III)Cl4(Py)2] (py = pyridine).29
Figure 2. (a) ORTEP view of complex 2 displaying thermal ellipsoids
at 30% probability. (b) View of the crystal dimer formed by complex 2
by means of a N2−H2···N1 intermolecular H bond.
DISCUSSION
■
The combination of the tridentate π-donor ligand 2,2′-
iminodiethanethiol (H2SNS) with a monodentate π-acceptor
phosphine (PCN, PPH3) provides a simple model for studying
the behavior of five-coordinate Tc(V) and Re(V) nitrido
complexes comprising a mixed coordination environment
(briefly called here ‘3 + 1’ nitrido complexes). The main
interest for this study originated from the previous observation
that there exists a kind of geometrical control of the structure of
these complexes brought about by the nature of the
coordinating atoms. Specifically, a set composed by four π-
donor atoms favors a highly distorted umbrella-shaped square-
pyramidal (sp) geometry, whereas a set formed by two π-donor
and two π-acceptor atoms promotes the transition to trigonal
bipyramidal geometry (tbp). A simple, qualitative interpretation
of these findings could be obtained by considering the
distribution of frontier molecular orbitals (FMO) of the
metallic fragment [MN]2+ (M = Tc, Re) in a five-coordinate
arrangement as described in standard articles and textbooks.30
In sp symmetry, FMOs belong to the same group formed by
four antibonding orbitals pointing toward the vertexes of the
basal square plane. Conversely, in a tbp symmetry, there exists a
separation between FMOs involved in the π bonding along the
Figure 3. ORTEP view of dimeric complex 3 displaying thermal
ellipsoids at 30% probability.
1.630(3) and 1.632(3) Å are consistent with the length of the
triple TcN bond found for other octahedral Tc(V) nitrido
complexes (in the range 1.60−1.64 Å). The strong trans
influence exerted by the nitrido ligand causes an extreme
lengthening of the Tc−Cl(trans) bond distances up to 2.632(1)
and 2.638(1) Å as compared to the equatorial Tc−Cl bond of
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axial positions and those lying on the trigonal plane. This
suggests that tbp geometry can better accommodate a
combination of π-acceptor and π-donor ligands. A sharp
evidence of this picture comes from the observation that, in
mixed phosphino−thiol Tc(V) nitrido complexes, the two π-
acceptor P atoms always prefer to occupy a reciprocal trans
position along the axis perpendicular to the trigonal plane.11a In
this context, the ‘3 + 1’ chelating system investigated here
constitutes another interesting combination to add to the whole
picture, particularly because it is not immediately apparent
which final coordination arrangement will be the most favored.
The observed results nicely confirmed the theoretical
predictions and the elegant chemical behavior of the MN
(M = Tc, Re) functional group. Reactions of the labile
precursors [M(N)Cl2(PPh3)2] with the ligand H2SNS afforded
the neutral complexes [M(N)(SNS)(PPh3)] (M = Tc, 1; M =
Re, 2). In these complexes, H2SNS binds to the MN moiety
as a dianionic tridentate ligand through two deprotonated thiol
sulfur atoms and the protonated amine nitrogen atom. A PPh3
ligand spans the residual fourth position. Retention of the H
atom bound to the amine nitrogen enable the complexes to
exist in two different isomeric forms depending on the syn and
anti orientations of this hydrogen with respect to the MN
group. The crystal structure of compound 2 corresponds to the
syn isomer isolated in a pure crystalline form. Results showed
that 2 has a distorted tbp structure with the two sulfur atoms of
the SNS ligand and the nitrido group lying on the trigonal
equatorial plane. The phosphorus atom of PPh3 occupies one
axial position trans to the neutral nitrogen atom of the SNS
ligand. The close correspondence between the single HPLC
peak of complex 2 (Figure 1a) and the main peak observed in
the HPLC chromatogram of 1 (Figure 1b) strongly indicates
that both compounds are syn isomers; thus, formation of the
syn arrangement is the most favored. Apparently, the presence
of a small secondary HPLC signal for compound 1 (Figure 1b),
tentatively assigned to the anti isomer, suggests that only a
small fraction of this isomer is formed. No interconversion
between the two isomeric forms was detected in the
temperature range from room temperature to 100 °C.
highly stable nitrido moiety from a Tc(V) ion.11a,31 Compound
3 is remarkable because the expected complex was the five-
coordinate [Tc(N)Cl2(PCN)2] closely analogous to the well-
known precursor [Tc(N)Cl2(PPh3)2]. Formation of a dimeric
species is surprising also in view of the excess of PCN
employed in the reaction. At first sight, 3 might be considered
as an intermediate species formed between the starting Tc(VI)
nitrido precursor and the final Tc(III) reduced compound.
However, after isolation, 3 did not show the tendency to further
react with PCN or convert spontaneously into complex 4 when
dissolved in acetone or CH3CN. This study was partially
hampered by the low solubility of 3 in other solvents. Another
tentative explanation for production of complex 4 could be
attributed to the presence of some residual amount of the
Tc(IV) complex, [TcCl6]2−,8a,32 usually formed during
preparation of the starting [Tc(N)Cl4][n-Bu4]. However,
elemental analysis of the precursor Tc(VI) nitrido complex
reasonably eliminated this possibility. According to these
preliminary observations, formation of complexes 3 and 4
from [Tc(N)Cl4]− seems to follow separate routes. However,
further studies are required to fully determine the exact
mechanism of these reactions.
CONCLUSIONS
■
The new ‘3 + 1’ complexes [M(N)(SNS)(PPh3)] (M = Tc, Re)
and [Tc(N)(SNS)(PCN)] have been prepared and fully
characterized. The structural characteristics of the new category
of compounds could provide a convenient chemical platform
for developing new diagnostic and therapeutic agents labeled
with technetium-99m and rhenium-188. In fact, the relative
flexibility of the coordination environment might allow a fine
tuning of the biological properties of the resulting radiophar-
maceuticals and essential prerequisite for development of
target-specific agents for molecular imaging. Extensive studies
on preparation of the corresponding Re-188 and Tc-99m ‘3 +
1’ complexes using biologically derived SNS-type ligands have
been already carried out and will be published elsewhere.
ASSOCIATED CONTENT
■
As mentioned above, the existence of a tbp geometry for
complex 2, where the amino and phosphino groups share a
reciprocal trans position, indicates that the N and P atoms
overlap with the axial FMOs of the [MN]2+ fragment. It
might be conjectured that the sp3 character of the protonated
NH atom prevents it from behaving as a true π donor. This
precludes its interaction with the π-orbital set of both sp and
tbp geometries and ultimately favors a tbp arrangement.
Previous structural data reported for the complex [99gTc(N)-
(ONS)(PPh3)]12 strongly support this interpretation. In this
compound, the Schiff base H2SNO coordinates to the [M
N]2+ fragment as a tridentate ligand through the negative
hydroxylic oxygen atom, the neutral aldiminic nitrogen atom,
and the negative thiol sulfur atom. The resulting sp geometry is
consistent with the stronger π-donor character of the sp2
nitrogen atom of the Schiff base functional moiety. The
preference of a neutral sp3 nitrogen atom to span a trans
position to a π-acceptor P atom has been recently observed also
in octahedral Tc(V) nitrido complexes with 2-
(diphenylphosphinomethyl)aniline.23c
S
* Supporting Information
X-ray crystallographic data for 2, 3, and 4 in cif format. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Phone: (39) 0532 455354. Fax: (39) 0532 237589. E-mail:
dta@unife.it.
Notes
The authors declare no competing financial interest.
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