Rhenium(I) tris(carbonyl) complexes with soft scorpionates

Article abstract of DOI:10.1039/b302899b

The novel tris(mercaptoimidazolyl)borate ligands Li[RB(tim^Me)₃] (R = Me (1), Ph (2)) have been synthesized by reaction, in refluxing toluene, of 2-mercapto-1-methylimidazole with lithium methylborohydride or phenylborohydride, respectively. By reacting 1 and 2 with the Re(I) starting material (NEt₄)₂[Re(CO)₃Br₃], the tris(carbonyl) complexes [Re{RB(tim^Me)₃-κ³S,S,S}(CO)₃] (R = Me (3), Ph (4)) have been obtained in moderate yields. Compounds 1-4 have been characterized by IR, ¹H, and ¹¹B NMR spectroscopies, and also by X-ray crystallographic analysis in the case of 3. The X-ray diffraction analysis of 3 showed that the rhenium atom adopts a slightly distorted octahedral coordination with a facial arrangement of the carbonyl ligands. The three remaining coordination positions are occupied by the thione sulfur atoms from the tripodal methyltris(2-mercapto-1-methylimidazoyl)borate, which adopts a typical propeller-like configuration.

Full text of DOI:10.1039/b302899b

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Rhenium(I) tris(carbonyl) complexes with soft scorpionates
Raquel Garcia, António Paulo, Ângela Domingos and Isabel Santos*
Departamento de Química, ITN, Estrada Nacional, 10, 2686-953, Sacavém Codex, Portugal.
E-mail: isantos@itn.mces.pt
Received 13th March 2003, Accepted 23rd May 2003
First published as an Advance Article on the web 9th June 2003
The novel tris(mercaptoimidazolyl)borate ligands Li[RB(timMe)3] (R = Me (1), Ph (2)) have been synthesized by
reaction, in refluxing toluene, of 2-mercapto-1-methylimidazole with lithium methylborohydride or phenylboro-
hydride, respectively. By reacting 1 and 2 with the Re() starting material (NEt4)2[Re(CO)3Br3], the tris(carbonyl)
complexes [Re{RB(timMe)3-κ3S,S,S}(CO)3] (R = Me (3), Ph (4)) have been obtained in moderate yields. Compounds
1–4 have been characterized by IR, 1H, and 11B NMR spectroscopies, and also by X-ray crystallographic analysis
in the case of 3. The X-ray diffraction analysis of 3 showed that the rhenium atom adopts a slightly distorted octa-
hedral coordination with a facial arrangement of the carbonyl ligands. The three remaining coordination positions
are occupied by the thione sulfur atoms from the tripodal methyltris(2-mercapto-1-methylimidazolyl)borate, which
adopts a typical propeller-like configuration.
recorded as KBr pellets on a Perkin-Elmer 577 spectrometer.
Carbon, hydrogen and nitrogen analysis were performed on a
EA110 CE Instruments automatic analyser.
Introduction
Recent studies on the basic coordination chemistry of Tc()
and Re() complexes containing the fac-M(CO)3 moieties
highlighted the potential relevance of these complexes in the
Synthesis of Li[RB(timMe)3] (R ؍ Me (1), Ph (2))
development of radioactive products for diagnostic (99mTc) and
therapeutic (186/188Re) medical applications.1–14 Searching for
novel Tc() and Re() tris(carbonyl) complexes useful for
biomedical applications, our group focused on poly(mercapto-
imidazolyl)borates as ancillary ligands.15 Several Re and 99mTc
complexes have been prepared in high yield and with high
specific activity,15,16 showing that poly(mercaptoimidazoyl)-
borates feature inherent requirements for their application in
the radiopharmaceutical field. Most relevantly, poly(mercapto-
imidazolyl)borates can be easily modified by the controlled
introduction of different substituents, allowing a fine tuning of
the physico-chemical properties of the complexes, such as size
or lipophilicity. In radiopharmaceutical research, this tuning
is a crucial issue, as these properties strongly influence the
transport of the complexes inside the body, namely their ability
to cross biological membranes.17
To a suspension of Li(RBH3) (R = Me, Ph) in toluene (20 ml)
were added three equivalents of solid 2-mercapto-1-methyl-
imidazole, and the resulting mixtures were refluxed for 2 h
(R = Me) or 5 h (R =Ph). After cooling to room temperature,
ligands 1 and 2 precipitate as white solids, which were recovered
by filtration. Further purification of 1 was performed by
recrystallization from a concentrated THF solution, followed
by washing of the white precipitate with chloroform. The
purification of 2 involved just washing with chloroform, to
remove any unreacted 2-mercaptoimidazole. Starting from 100
mg of Li(MeBH3) (2.79 mmol) and from 200 mg (2.05 mmol)
of Li[PhBH3] were obtained 620 mg of 1 (Yield: 60%) and 318
mg of 2 (Yield = 60%), respectively.
1
Compound 1. IR (cmϪ1): 725 (ν(C᎐S)). H NMR (300 MHz,
᎐
CD3CN, δ (ppm)): 1.01 (3H, s, CH3B), 3.42 (9H, s, CH3N), 6.35
(3H, d, JH–H = 2.1 Hz, CH), 6.63 (3H, d, JH–H = 2.1 Hz, CH).
11B NMR (96 MHz, CD3CN, δ (ppm)): 44.5.
Following our efforts to modify poly(mercaptoimidazolyl)-
borates for further application in radiopharmaceutical devel-
opment,15 we started to evaluate the possibility of preparing
tris(mercaptoimidazolyl)borates featuring alkyl or aryl groups
directly attached to the boron atom.
Compound 2. IR (cmϪ1): 730 m (ν(C᎐S)). 1H NMR (300
᎐
MHz, CD3CN, δ (ppm)): 3.41 (9H, s, CH3N), 6.61 (3H, d, JH–H
= 2.1 Hz, CH), 6.80 (3H, br, CH), 7.01 (2H ϩ 1H, m, Ph), 7.20
(2H, br m, Ph). 11B NMR (96 MHz, CD3CN, δ (ppm)): 44.0.
Herein, we report on the synthesis and characterization of
the novel Li[RB(timMe)3] (R = Me (1), Ph (2)) and on their
reactions with the Re() starting material (NEt4)2[ReBr3(CO)3],
which led to the synthesis of the new complexes [Re{RB-
(timMe)3-κ3S,S,S}(CO)3] (R = Me (3), Ph (4)) also described in
this work.
Synthesis of [Re{RB(timMe)3-ꢀ3S,S,S}(CO)3] (R ؍ Me (3),
Ph (4))
To solutions of (NEt4)2[ReBr3(CO)3] (100 mg, 0.246 mmol) in
methanol were added Li[RB(timMe)3] (R = Me (1), Ph(2)) in
approximate 10% molar excess, and the mixtures were stirred
for 1 h at room temperature. Compound 3 and 4 precipitate
from the respective reaction mixtures, and were recovered by
filtration. After drying under vacuum, compounds 3 (82 mg,
yield = 57%) and 4 (68 mg, yield = 40%) were obtained as white
microcrystalline solids.
Experimental
The synthesis of the tris(mercaptoimidazolyl)borates were
performed under a nitrogen atmosphere, using standard
Schlenk techniques and dry toluene, while the synthesis of the
Re complexes were carried out under air. The starting material
(NEt4)2[ReBr3(CO)3]18 and the organoborohydrides Li(RBH3)
(R = Me, Ph)19 were prepared by literature methods. The other
chemicals were used as purchased.
Compound 3. Anal. Calc. for C16H18N6O3S3BRe: C, 30.23; H,
1H and 11B NMR spectra were recorded on a Varian Unity
300 MHz spectrometer; 1H chemical shifts were referenced with
the residual solvent resonances relative to tetramethylsilane,
and the 11B NMR chemical shifts with an external NaBH4
solution. NMR spectra were run in CD3CN. IR spectra were
2.83; N, 13.23%. Found: C, 30.49; H, 2.19; N, 13.11%. IR
1
(cmϪ1): 1895s (ν(C–O)), 1860s (ν(C–O)), 732 m (ν(C᎐S)). H
᎐
NMR (300 MHz, CD3CN, δ (ppm)): 0.70 (3H, s, CH3B), 3.61
(9H, s, CH3N), 6.96 (3H, d, JH–H = 2.4 Hz, CH), 7.03 (3H, br s,
CH). 11B NMR (96 MHz, CD3CN, δ (ppm)): 42.8.
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 2 7 5 7 – 2 7 6 0
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Table 1 Crystallographic data for 3
borates can be quite convenient, as some of these acids are
commercially available and easily derivatised with selected
biomolecules. However, the need of high temperatures is a
potential drawback, which limits the usefulness of boronic
acids in the synthesis of thermally unstable poly(azolyl)borates.
Being aware that 1-methyl-2-mercaptoimidazole derivatives
have a tendency to decompose at high temperatures,25 we
discarded boronic acids as starting materials for the synthesis
of alkyl- or aryl- tris(mercaptoimidazolyl)borates. Instead, we
focused on alkyl or arylborohydrides, which we had already
used for the preparation of [R(H)B(timMe)2]Ϫ (R = Me, Ph).
These bis(mercaptoimidazolyl)borates) are prepared efficiently
by reflux of the desired organoborohydrides (Li[RBH3] = Me,
Ph) and 1-methyl-2-mercaptoimidazole, in tetrahydrofuran and
with an approximate 1 : 2 molar ratio.15 We have also explored
this approach in the synthesis of the corresponding tris-
(mercaptoimidazolyl)borates, using the same solvent but with
an increased concentration of 1-methyl-2-mercaptoimidazole
(1 : 3 molar ratio). However, even after prolonged reflux in THF
(24 h), the bis derivatives were the only products formed. By
changing the solvent to toluene, we succeeded in the synthesis
of the novel Li[RB(timMe)3] (R = Me (1), Ph (2)), which were
obtained after refluxing for 2 and 5 h, respectively (Scheme 1).
Formula
C16H18BN6O3ReS3.OC4H8
707.66
M/g molϪ1
Crystal system
Space group
a/Å
b/Å
c/Å
α/Њ
β/Њ
Triclinic
¯
P1 (no. 2)
14.053(3)
15.030(3)
15.128(3)
74.08(2)
68.28(2)
63.49(1)
2633.3(9)
4
1.785
4.891
9584
γ/Њ
V (Å3)
Z
Dc/g cmϪ3
µ(Mo-Kα)/cmϪ1
Reflections collected
Independent reflections
Parameters
∆ρ/e ÅϪ3
9199 (Rint = 0.0454)
581
0.782 and Ϫ0.745
1.066
Goodness-of-fit
Ra
wR2
0.0592 (0.1222)b
0.0908 (0.1302)b
a
a R = Σ||Fo| Ϫ |Fc||/Σ|Fo|, wR2 = [Σ(w(Fo Ϫ Fc2)2)/[Σ(w(Fo )2)]1/2 [Fo
>
2
2
4σ(Fo)]. b Based on all data.
Compound 4. Anal. Calc. for C21H20N6BO3S3Re: C, 36.15; H,
2.87; N, 12.05%. Found: C, 36.03; H, 1.78; N, 11.34%. IR
(cmϪ1): 1895s (ν(C–O)), 1865s (ν(C–O)), 735 (ν(C᎐S)). 1H NMR
᎐
(300 MHz, CD3CN, δ (ppm)): 3.56 (9H, s, CH3N), 6.88 (3H, d,
JH–H = 2.1 Hz, CH), 6.99 (3H, d, JH–H = 2.1 Hz, CH) 7.34 (2H,
m, Ph), 7.64 (3H, m, Ph). 11B NMR (96 MHz, CD3CN,
δ (ppm)): 43.5.
X-Ray crystallographic analysis
The crystals of compound 3 were obtained by recrystallization
from tetrahydrofuran–n-hexane and mounted in thin-walled
glass capillaries. Data were collected at room temperature
on an Enraf-Nonius CAD-4 diffractometer with graphite-
monochromated Mo-Kα radiation, using an ω–2θ scan mode.
The crystal data are summarized in Table 1.
The data were corrected20 for Lorentz and polarization
effects, for linear decay and empirically for absorption by Ψ
scans. The heavy atom positions were located by Patterson
methods using SHELXS-86.21 The remaining atoms were
located in successive Fourier-difference maps and refined by
least-squares refinements on F 2 using SHELXL-93.22 Complex
3 crystallizes with two independent molecules in the asym-
metric unit, and with one molecule of THF of crystallization
per formula unit. All the non-hydrogen atoms were refined
anisotropically, with the exception of those from the THF of
crystallization; the contributions of the hydrogen atoms were
included in calculated positions, constrained to ride on their
carbon atoms. Geometrical restraints were applied to one of the
THF solvent molecules which is severely disordered. Atomic
scattering factors and anomalous dispersion terms were as in
SHELXL-93.22 The drawings were made with ORTEP-3;23 all
the calculations were performed on a Dec α 3000 computer.
CCDC reference number 205976.
Scheme 1
The formation of 1 and 2 is quite efficient, as shown by the
follow-up of the reactions by 1H and 11B NMR analysis.
However, after work-up, compounds 1 and 2 were obtained
only in moderate isolated yield, since successive recrystal-
lizations are required to remove any traces of unreacted
2-mercapto-1-methylimidazole. Ligands 1 and 2 are hygro-
scopic white solids, which are soluble in most common polar
organic solvents and in water, and are relatively resistant
towards aerobic oxidation and hydrolysis.
As indicated in Scheme 1, treatment of (NEt4)2[Re(CO)3Br3]
with stoichiometric amounts of Li[RB(timMe)3] (R = Me (1),
Ph (2)), in methanol solution and at room temperature,
leads promptly to the novel tris(carbonyl) complexes [Re{RB-
(timMe)3-κ3S,S,S}(CO)3] (R = Me (3), Ph (4)). Complexes 3 and
4 precipitated upon concentration of the respective reaction
mixtures, and were recovered as white microcrystalline solids in
moderate yields (40–50%).
See http://www.rsc.org/suppdata/dt/b3/b302899b/ for crystal-
lographic data in CIF or other electronic format.
[Re{RB(timMe)3-κ3S,S,S}(CO)3] (R = Me (3), Ph (4)) are
air- and water-stable compounds, as observed for the analogous
[Re{HB(timMe)3-κ3S,S,S}(CO)3].15 However, the attachment of
methyl or phenyl groups to the boron atom has a dramatic
influence on the solubility of complexes 3 and 4 which are
soluble in most common polar organic solvents, in contrast to
[Re{HB(timMe)3-κ3S,S,S}(CO)3]. These findings clearly show
that the introduction of different substituents in poly-
(mercaptoimidazolylborates modulates the physico-chemical
properties of the corresponding rhenium tris(carbonyl)
Results and discussion
There are available different synthetic approaches for the
preparation of tris(azolyl)borates containing alkyl or aryl
substituents directly attached to the boron atom, depending
essentially on the boronated starting material. In the case of the
ubiquitous pyrazolyl derivatives, alkyl- or aryl-tris(pyrazolyl)-
borates have been successfully prepared starting from boronic
acids, boronic esters, dihaloboranes or borohydrides.24 The use
of alkyl- or aryl-boronic acids for the synthesis of tris(azolyl)-
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Table 2 Selected bond lengths (Å) and angles (Њ) for 3
complexes, what is quite relevant for their potential application
in radiopharmaceutical development.
Molecule 1
The ligands, 1 and 2, and the respective complexes, 3 and 4,
have been characterized by C, H, N analysis, IR, 1H, and
11B NMR spectroscopies, and also by X-ray crystallographic
analysis in the case of 3. For ligands 1 and 2, it was not possible
to obtain accurate elemental analysis, although 1H and 11B
NMR spectroscopies indicated that we obtained pure samples.
The IR spectra of Li[RB(timMe)3] (R = Me (1), Ph (2)) present
medium intense bands centered at around 730 cmϪ1, which
Re(1)–C(1)
Re(1)–C(3)
Re(1)–S(2)
C(1)–O(1)
C(3)–O(3)
1.894(13)
1.88(2)
2.527(3)
1.155(13)
1.154(15)
Re(1)–C(2)
Re(1)–S(1)
Re(1)–S(3)
C(2)–O(2)
B(1)–C(10)
1.910(12)
2.526(3)
2.509(3)
1.129(12)
1.62(2)
C(1)–Re(1)–C(2)
C(2)–Re(1)–C(3)
C(1)–Re(1)–S(2)
C(2)–Re(1)–S(1)
C(2)–Re(1)–S(3)
C(3)–Re(1)–S(2)
S(1)–Re(1)–S(2)
S(2)–Re(1)–S(3)
89.3(5)
90.5(6)
176.5(4)
177.8(4)
90.0(4)
91.5(4)
88.97(10)
89.76(10)
C(1)–Re(1)–C(3)
C(1)–Re(1)–S(1)
C(1)–Re(1)–S(3)
C(2)–Re(1)–S(2)
C(3)–Re(1)–S(1)
C(3)–Re(1)–S(3)
S(1)–Re(1)–S(3)
91.9(6)
91.6(4)
86.8(4)
90.2(4)
87.5(5)
178.6(4)
92.01(10)
were attributed to ν(C᎐S). The frequencies of these bands are
᎐
almost insensitive to the coordination of the ligands to the
fac-[Re(CO)3]ϩ moiety and appear at 732 and 735 cmϪ1 for
3 and 4, respectively. This kind of behaviour has been already
observed for several coordination complexes with poly-
(mercaptoimidazolyl)borates.25–43 The IR spectra of compounds
[Re{RB(timMe)3-κ3S,S,S}(CO)3] (R = Me (3), Ph (4)) display
two strong bands due to the ν(CO) stretching mode, in the
range 1860–1895 cmϪ1 and with the typical pattern observed
for complexes with the “fac-Re(CO)3” moiety in a C3 environ-
ment.15
Molecule 2
Re(2)–C(4)
Re(2)–C(6)
Re(2)–S(5)
C(4)–O(4)
C(6)–O(6)
1.896(12)
1.876(15)
2.525(3)
1.148(12)
1.156(14)
Re(2)–C(5)
Re(2)–S(4)
Re(2)–S(6)
C(5)–O(5)
B(2)–C(20)
1.89(2)
2.537(3)
2.505(3)
1.161(15)
1.58(2)
The 1H NMR spectra of complexes 3 and 4 are quite simple,
showing two doublets for the methyne protons of the mercapto-
imidazolyl rings and one singlet for the N–CH3 group of the
same rings, in a 3 : 3 : 9 ratio. This pattern is consistent with the
chemical and magnetic equivalence of the coordinated rings,
C(4)–Re(2)–C(5)
C(5)–Re(2)–C(6)
C(4)–Re(2)–S(5)
C(5)–Re(2)–S(4)
C(5)–Re(2)–S(6)
C(6)–Re(2)–S(5)
S(4)–Re(2)–S(5)
S(5)–Re(2)–S(6)
89.9(5)
90.1(6)
92.0(4)
89.4(4)
90.5(4)
88.8(5)
88.73(10)
90.78(11)
C(4)–Re(2)–C(6)
C(4)–Re(2)–S(4)
C(4)–Re(2)–S(6)
C(5)–Re(2)–S(5)
C(6)–Re(2)–S(4)
C(6)–Re(2)–S(6)
S(4)–Re(2)–S(6)
90.1(5)
177.7(3)
87.4(4)
177.8(4)
92.0(4)
177.5(4)
90.41(9)
1
in accordance with the expected C3 symmetry. The H NMR
spectrum of ligand Li[PhB(timMe)3] (2) presents some unique
features, which require some further comments. To the best of
our knowledge, all described poly(mercaptoimidazolyl)borates
1
and respective transition metal complexes present H NMR
spectra with a characteristic pair of doublets for the two CH
protons (H(4) and H(5)) of the mercaptoimidazolyl rings, as we
have found for ligand 1 and for complexes 3 and 4.15,25–43 By
1
for atom numbering). This has been confirmed by a 1D H
NOESY NMR experiment. The selective irradiation of the
mercaptoimidazole N–CH3 protons, resonating at 3.41 ppm,
enhanced the doublet at 6.61 ppm which, therefore, corre-
sponds to the H(4) methyne protons.
1
contrast, in the H NMR spectrum of compound 2 we have
found an unique doublet at 6.61 ppm, showing the usual
coupling constant of the CH protons of mercaptoimidazole
(J = 2.1 Hz) and integrating for three protons, while the remain-
ing mercaptoimidazolyl C–H protons originate a quite broad
signal centered at 6.80 ppm. (see Fig. 1). The attribution of this
broad resonance to the mercaptoimidazolyl C–H protons was
based on a 2D homonuclear [1H, 1H] COSY experiment, which
demonstrated that the broad resonance is coupled with the
doublet appearing at 6.61 ppm, unambiguously assigned to one
of the C–H protons (H(4) and H(5)) of the mercaptoimidazole
rings. The broadening is certainly related with the quadrupolar
moment of 11B,44 and the broad resonance must correspond to
the H(5) proton which is closer to the boron atom (see Fig. 1
For [Re{MeB(timMe)3-κ3S,S,S}(CO)3] (3), the above dis-
cussed IR and 1H NMR spectroscopic data are consistent with
the solid state molecular structure of the complex, which was
obtained by X-ray diffraction analysis. The structure of 3
consists of discrete mononuclear units with the rhenium atom
in a slightly distorted octahedral environment. There are two
molecules per asymmetric unit which are crystallographically
independent but chemically equivalent. The ORTEP view of
one of the molecules is shown in Fig. 2. Selected bond distances
and angles are given in Table 2.
Fig. 1 1H NMR spectrum of Li[RB(timMe)3] (2) in the aromatic
region, displaying an insert with the numbering of mercaptoimidazole
hydrogens.
Fig. 2 ORTEP view of 3. Vibrational ellipsoids are drawn at the 30%
probability level.
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12 J. F. Valliant, P. Morel, P. Schaffer and J. H. Kaldis, Inorg. Chem.,
2002, 41, 628.
13 S. R. Banerjee, M. K. Levadala, N. Lazarova, L. Wei, J. F. Valliant,
K. A. Stephenson, J. W. Babich, K. P. Maresca and J. Zubieta, Inorg.
Chem., 2002, 41, 6417.
14 J. Bernard, K. Ortner, B. Spingler, H.-J. Pietzsch and R. Alberto,
Inorg. Chem., 2003, 42, 1014.
15 (a) R. Garcia, A. Paulo, A. Domingo, I. Santos, K. Ortner and
R. Alberto, J. Am. Chem. Soc., 2000, 122, 11240; (b) R. Garcia,
A. Paulo, A. Domingos and I. Santos, J. Organomet. Chem., 2001,
632, 41; (c) R. Garcia, A. Domingos, A. Paulo, I. Santos and
R. Alberto, Inorg. Chem., 2002, 41, 2422; (d ) R. Garcia, Y. H. Xing,
A. Paulo, A. Domingos and I. Santos, J. Chem. Soc., Dalton Trans.,
2002, 4236.
The carbonyl ligands occupy one face of the coordination
polyhedra, with an average Re–C distance of 1.892(12) Å. The
three remaining coordination positions are occupied by the
thione sulfur atoms, with an average Re–S bond distance of
2.521(12) Å. These metrical parameters are comparable to
those that we have previously reported for the congener [Re-
{HB(timMe)3-κ3S,S,S}(CO)3] (av. Re–C, 1.904(9) Å; av. Re–S,
2.516(2) Å).15 With the exception of the presence of the methyl
group in 3, the molecular structure of both complexes are
almost superimposable and, therefore, the structure of 3 does
not justify a more exhaustive discussion.
16 R. Garcia, A. Paulo, L. Gano and I. Santos, unpublished results.
17 J. R. Dilworth and S. J. Parrott, Chem. Soc. Rev., 1998, 27, 43.
18 A. Alberto, A. Egli, U. Abram, K. Hegetschweiler, V. Gramlich and
P. A. Schubiger, J. Chem. Soc., Dalton Trans., 1994, 2815.
19 B. Singaram, T. E. Cole and C. H. Brown, Organometallics, 1984, 3,
774.
Conclusions
The first examples of tris(mercaptoimidazolyl)borates bearing
alkyl or aryl substituents directly attached to the boron atom
have been prepared. These novel ligands, Li[RB(timMe)3] (R =
Me (1), Ph (2)) were used to prepare the Re() tris(carbonyl)
complexes [Re{RB(timMe)3-κ3S,S,S}(CO)3] (R = Me (3), Ph
(4)), which are quite resistant toward hydrolysis and aerial
oxidation. Compounds 3 and 4 can be seen as valuable models
for the development of specific radiopharmaceuticals. Our
research group is currently evaluating the possibility of
replacing the methyl or phenyl groups in ligands 1 and 2 by
biologically relevant substrates, aiming to explore further these
systems in biomedical applications.
20 C. K. Fair, MOLEN; Enraf-Nonius, Delft, The Netherlands,
1990.
21 G. M. Sheldrick, SHELXS-86: Program for the Solution of Crystal
Structure, University of Göttingen, Germany, 1986.
22 G. M. Sheldrick, SHELXL-93: Program for the Refinement of
Crystal Structure, University of Göttingen, Germany, 1993.
23 L. Farrugia, J. Appl. Crystallogr., 1997, 32, 565.
24 J. L. Kisko, T. Hascall, C. Kimblin and G. Parkin, J. Chem. Soc.,
Dalton Trans., 1999, 1929 and references therein.
25 J. F. Ojo, P. A. Slavin, J. Reglinsli, M. Garner, M. D. Spicer,
A. R. Kennedy and S. J. Teat, Inorg. Chim. Acta, 2001, 313, 15.
26 M. Garner, J. Reglinski, I. Cassidy, M. D. Spicer and A. R. Kennedy,
Chem. Commun., 1996, 19.
27 C. Kimblin, T. Hascall and G. Parkin, Inorg. Chem., 1997, 36, 5680.
28 C. Santini, G. G. Lobbia, C. Pettinari, M. Pellei, G. Valle and
S. Calogero, Inorg. Chem., 1998, 37, 890.
29 C. Santini, C. Pettinari, G. G. Lobbia, R. Spagna, M. Pellei and
F. Vallorani, Inorg. Chim. Acta, 1999, 285, 81.
30 J. Reglinski, M. Garner, I. D. Cassidy, P. A. Slavin, M. D. Spicer and
D. R. Armstrong, J. Chem. Soc., Dalton Trans., 1999, 2119.
31 J. Reglinski, M. D. Spicer, M. Garner and A. R. Kennedy, J. Am.
Chem. Soc., 1999, 121, 2317.
32 C. Kimblin, B. M. Bridgewater, D. G. Churchill and G. Parkin,
Chem. Commun., 1999, 2301.
33 A. F. Hill, G. R. Owen, A. J. P. White and D. J. Williams, Angew.
Chem., Int. Ed., 1999, 38, 2759.
34 C. Kimblin, B. M. Bridgewater, D. G. Churchill, T. Hascall and
G. Parkin, Inorg. Chem., 2000, 39, 4240.
Acknowledgments
This work has been partially supported by the FCT (POCTI/
QUI/48983/2002) and by the Commission of the European
Communities (COST action B12). R. G. thanks the FCT for a
PhD research grant.
References
1 A. Alberto, R. Schibli, A. Egli, A. P. Schubiger, U. Abram and
T. A. Kaden, J. Am. Chem. Soc., 1998, 120, 7987.
2 A. Hoepping, M. Reisgys, P. Brust, S. Seifert, H. Spies, R. Alberto
and B. Johannsen, J. Med. Chem., 1998, 41, 4429.
3 A. Alberto, R. Schibli, R. Waibel, U. Abram and A. P. Schubiger,
Coord. Chem. Rev., 1999, 190–192, 901.
4 A. Alberto, R. Schibli, A. Egli, A. P. Schubiger, U. Abram,
H.-J. Pietzsch and B. Johannsen, J. Am. Chem. Soc., 1999, 121, 6076.
5 A. Egli, A. Alberto, L. Tannahill, R. Schibli, U. Abram,
A. Schaffland, R. Waibel, D. Tourwé, L. Jeannin, K. Iterbeke and
A. P. Schubiger, J. Nucl. Med., 1999, 40, 1913.
35 C. Kimblin, B. M. Bridgewater, T. Hascall and G. Parkin, J. Chem.
Soc., Dalton Trans., 2000, 891.
36 C. Kimblin, B. M. Bridgewater, T. Hascall and G. Parkin, J. Chem.
Soc., Dalton Trans., 2000, 1267.
37 P. A. Slavin, J. Reglinski, M. D. Spicer and A. R. Kennedy, J. Chem.
Soc., Dalton Trans., 2000, 239.
6 R. Waibel, R. Alberto, J. Willuda, R. Finnern, R. Schibli,
A. Stichelberger, A. Egli, U. Abram, J.-P. Mach, A. Plueckthun and
P. A. Schubiger, Nat. Biotechnol., 1999, 17, 897.
7 R. Schibili, R. La Bella, R. Alberto, E. Garcia-Garayoa, K. Ortner,
U. Abram and A. P. Schubiger, Bioconjugate Chem., 2000, 11, 345.
8 H.-J. Pietzsch, R. Schibli, R. La Bella, R. Alberto, E. G. Garayoa,
K. Ortner, A. AbramGupta, M. Reisgys, A. Drews, S. Seifert,
R. Syhre, H. Spies, R. Alberto, U. Abram, P. A. Schubiger and
B. Johannsen, Bioconjugate Chem., 2000, 11, 414.
9 J. Wald, R. Alberto, K. Ortner and L. Candreia, Angew. Chem., Int.
Ed., 2001, 40, 3062.
10 J. Petrig, R. Schibili, C. Dumas, R. Alberto and P. A. Schubiger,
Chem. Eur. J., 2001, 7, 1868.
38 B. M. Bridgewater, T. Filleben, R. A. Friesner and G. Parkin,
J. Chem. Soc., Dalton Trans., 2000, 4494.
39 M. Tesmer, M. Shu and H. Vahrenkamp, Inorg. Chem., 2001, 40,
4022.
40 C. Kimblin, D. G. Churchill, B. M. Bridgewater, J. N. Girard, D. A.
Quarless and G. Parkin, Polyhedron, 2001, 20, 1891.
41 G. G. Lobbia, C. Pettinari, C. Santini, N. Somers, B. W. Skelton and
A. H. White, Inorg. Chim. Acta, 2001, 319, 15.
42 M. Garner, M.-A. Lehmann, J. Reglinski and M. D. Spicer,
Organometallics, 2001, 20, 5233–5236.
43 S. Bakbak, C. D. Incarvito, A. L. Rheingold and D. Rabinovich,
Inorg. Chem., 2002, 41, 998.
44 H. Nöth and B. Wrackmeyer, in Nuclear Magnetic Resonance
Spectroscopy of Boron Compounds, Springer-Verlag, Berlin,
1978.
11 J. D. G. Correia, A. Domingos, I. Santos, R. Alberto and K. Ortner,
Inorg. Chem., 2001, 40, 5147.
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