Substitution reactions with [ReBr2(CO)2(NCCH 3)2]-: A convenient route to complexes with the cis-[Re(CO)2]+ core

Article abstract of DOI:10.1039/b805410j

Water- and air-stable complexes comprising the cis-[Re(CO) ₂]⁺ core can be synthesized from the (Et ₄N)[ReBr₂(NCCH₃)₂(CO)₂] precursor 1. Complex 1 showed distinctly different chemical and electronic behaviour compared to [ReBr₃(CO)₃]^2-. Substituting the two bromides in 1 with imidazole-like ligands or α,α'-diimines gave new complexes with potential applications in bioinorganic chemistry and photochemistry. The two acetonitrile ligands are very stably bound and could not be replaced. Under CO pressure, the uncommon complex mer-[ReBr(NCCH₃)₂(CO)₃] 2 was formed from 1. The reaction of 1 with the tetradentate ligand bis(2-pyridylmethyl)glycine (BPG) finally induced a four fold substitution at the metal center to form a [Re(CO)₂(L₄)]⁺-type complex.

Full text of DOI:10.1039/b805410j

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Substitution reactions with [ReBr2(CO)2(NCCH3)2]-: a convenient route to
complexes with the cis-[Re(CO)2]+ core†
Lukas Kromer, Bernhard Spingler and Roger Alberto*
Received 1st April 2008, Accepted 4th July 2008
First published as an Advance Article on the web 15th September 2008
DOI: 10.1039/b805410j
Water- and air-stable complexes comprising the cis-[Re(CO)2]+ core can be synthesized from the
(Et4N)[ReBr2(NCCH3)2(CO)2] precursor 1. Complex 1 showed distinctly different chemical and
electronic behaviour compared to [ReBr3(CO)3]2-. Substituting the two bromides in 1 with
imidazole-like ligands or a,a’-diimines gave new complexes with potential applications in bioinorganic
chemistry and photochemistry. The two acetonitrile ligands are very stably bound and could not be
replaced. Under CO pressure, the uncommon complex mer-[ReBr(NCCH3)2(CO)3] 2 was formed from
1. The reaction of 1 with the tetradentate ligand bis(2-pyridylmethyl)glycine (BPG) finally induced a
four fold substitution at the metal center to form a [Re(CO)2(L4)]+-type complex.
In contrast to chemical or biological studies around the fac-
Introduction
[Re(CO)3]+ core, investigations for finding new cores of the type
[Re(CO)x]+ are rare, the [M(CO)2(NO)]2+ core being probably the
only exception.19–21 New cores or synthons, through their distinctly
different electronic properties, would open up new strategies for
the labelling of targeting molecules or for other purposes.22
Drug-discovery and drug-development in medicinal inorganic
chemistry are based on substitution labile metal complex synthons.
Systematic alteration of incoming ligands enables the fine-tuning
of activity and biological behaviour. In coordination chemistry,
such synthons are often solvated metal cations, usually aquo-
complexes, whereas in organometallic chemistry solvated species
rarely exist in low valent oxidation states. Therefore, substitution
chemistry often starts from penta- or hexacarbonyl complexes
such as [ReBr(CO)5], the CO ligands being irreversibly replaced
by an incoming ligand, leading ultimately to substitution stable
complexes with i.e. the fac-[Re(CO)3]+ core. An exception is rep-
resented by the complex [ReBr3(CO)3]2-, which converts rapidly in
coordinating solvents such as H2O into [Re(OH2)3(CO)3]+ or other
solvated species [Re(sol)3(CO)3]+. The considerable lability of the
H2O ligands enabled manifold reactions with the fac-[M(CO)3]+
(M = 99,99mTc, Re) core, following those with “regular” aquo-ions.
Due to water and air stablility, [Re(OH2)3(CO)3]+ has contributed
to various fields such as radiopharmacy,1–3 photocatalysis4,5 and,
more generally, to bioorganometallic chemistry.6 The robust
character of the resulting complexes enabled, for instance, the
labelling of targeting molecules via mixed ligand approaches7
or the labelling of small biologically active molecules such as
amino acid analogues.8 A lot of effort has been invested in the
development of new chelators and labelling strategies for the fac-
[Re(CO)3]+ core.9–16 [ReX(NN)(CO)3]+ (NN = bidentate aromatic
amines) served as photosensitizers and -catalysts in CO2 → CO
conversion.5,17,18 It has been speculated that a cis-[ReI/II(CO)2]
species might be the intermediate but corresponding compounds
could not be prepared due to the lack of versatile cis-[Re(CO)2]+
precursors.
+
A useful {Re(CO)x} precursor must be air and water stable, and
comprise additional substitution labile ligands (such as water). We
recently described the synthesis of (Et4N)[ReBr2(CO)2(NCCH3)2]
(1), a potential source of the cis-[Re(CO)2]+ core.23 Complex 1
possesses four positions, which can potentially be occupied by
a variety of ligands thereby offering convenient access to novel
complexes. In this article, we present stability studies of 1 together
with its reactivities towards mono-, bi- and tetradentate ligands.
Results and discussion
Synthesis of precursor molecules
Complex 1 is accessible via oxidation of (Et4N)2[ReBr3(CO)3]
with Br2 to (Et4N)[ReBr4(CO)2] followed by reduction
with either N1,N1,N1,N1,N2,N2,N2,N2-octamethylethene-1,1,2,2-
tetramine (TDAE) or Mg0 in acetonitrile in 60% yield.23 Depend-
ing on the conditions of purification, this procedure also gave
[ReBr(NCCH3)3(CO)2] (2), in which one of the bromides had been
replaced by the solvent molecule acetonitrile. The procedure can
be adapted to other nitriles. Reduction in e.g. benzonitrile gave
the corresponding complex (Et4N)[ReBr2(NCC6H5)2(CO)2] (3).
Both complexes 1 and 2 are soluble in polar organic solvents
like methanol or acetonitrile and in water. As evidenced by
IR spectroscopy, dissolution in any of these solvents leads to
the exchange of the bromide ligands trans to the carbonyls by
solvent molecules. The two trans-standing acetonitrile ligands
remained untouched. This behaviour resembles, in principle, that
of [ReBr3(CO)3]2- in which the bromide ligands exchanged whereas
the p-acceptors (CO) did not under any conditions. Due to the
reduced numbers of CO ligands in [Re(CO)2]+ vs. [Re(CO)3]+, the
metal centre is more electron rich, which renders the acetonitrile
ligands more tightly bound. Thus, mono- and bidentate ligands
Institute of Inorganic Chemistry, Winterthurerstrasse 190, CH-8057
Zu¨rich, Switzerland. E-mail: ariel@aci.uzh.ch; Fax: +41(0)44 635 6802;
Tel: +41(0)44 635 4631
† Electronic supplementary information (ESI) available: Proton NMR and
liquid IR spectra of complex 10, CV diagram of complex 11. CCDC
reference numbers 683304–683308. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/b805410j
5800 | Dalton Trans., 2008, 5800–5806
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are expected to replace the two bromides readily, whereas strong
tri- or tetradentate ligands are likely to be required for replacing
the two acetonitrile ligands as well.
Substitution reactions
For an insight into the substitution pattern, reactions with differ-
ent types of ligands have been carried out. Imidazole (imz), benz-
imidazole (bzimz), 9-ethylguanine (9-EtG) and carbon monoxide
(CO) are representatives for monodentate ligands, the aromatic
amines 2,2¢-bipyridine (bpy) and 7,7¢-phenanthroline-dione (phd)
are typical bidentate chelators, and bis(2-pyridylmethyl)glycine
(BPG) is a potent tetradentate ligand, which was able to replace
CO in [M(CO)2(NO)]2+.21
Fig. 1 HPLC trace of 6 dissolved in MeOH.
Imidazole containing ligands
Imidazole (imz) is important since its structural motif appears
in many natural biomolecules, e.g. in histidine. This small and
relatively soft ligand offers an aromatic sp2 amine function for
coordination to the metal centre.
Addition of imz to a solution of 1 in MeOH lead to the
formation of [Re(NCCH3)2(CO)2(imz)2]Br (4) (Scheme 1).23 Ac-
cordingly, reaction of 1 with the sterically more demanding imi-
dazole derivative, benzimidazole (bzimz), gave the corresponding
complex [Re(NCCH3)2(bzimz)2(CO)2]Br (5). Imidazole is also
present in guanine, which is a very relevant biological ligand due
to its occurrence in DNA, a target of cisplatin. The reaction of
1 with 9-ethylguanine (9-EtG) was much slower than with bzimz,
mirroring its different electronic properties. In solution, the doubly
substituted product [Re(NCCH3)2(CO)2(9-EtG)2]Br (6) was in
equilibrium with mono-substituted [Re(OH2)(NCCH3)2(CO)2(9-
EtG)]Br. Dissolving crystals of 6 in methanol and analyzing the
solution by HPLC evidenced the equilibrium between the mono-
and the di-substituted products. A typical HPLC trace is given in
Fig. 1. The authenticity of 6 could be confirmed by X-ray structure
analysis and the complex cation is depicted in Fig. 2. Compound
6 crystallised in the monoclinic space group P21/n as colourless
Fig. 2 ORTEP plot of [Re(NCCH3)2(CO)2(9-EtG)2]Br (6). Hydrogen
atoms and counter-ion are omitted for clarity.
Carbon monoxide
˚
plates. At 2.233(5) A, the Re–N(4) bond length is slightly longer
˚
compared to Re–N(3), 2.204(5) A, or to the imidazole complex 4
The low CO stretching frequencies in 1 and throughout the
(2.198(5) and 2.200(5) A). In [Re(OH2)(CO)3(9-MeG)2] (6a),24 the
{Re(CO)2} complexes implied the possibility of replacing Br-
+
+
˚
Re–N bond lengths are substantially shorter and the IR stretching
frequencies of 6a are about 100 cm-1 higher than in 6 (Table 1).
This was expected since 6 has one CO ligand less than 6a, causing
increased backbonding but still higher electron density at the
metal center. Accordingly, the bond to the nitrogen of 9-EtG is
weakened. The IR carbonyl stretching frequencies of all three
[Re(CO)2]+ complexes are equal within 7 cm-1 due to the similarity
of the ligands.
by strong p-acceptors such as CO, thereby labilizing the CH3CN
ligands and rendering them prone to substitution. Thermodynami-
cally, a facial coordination of the third carbonyl would be favoured.
Calculations showed fac-[Re(OH2)3(CO)3]+ to be 103.8 kJ mol-1
more stable than the mer isomer.25 A similar situation is encoun-
tered for technetium where cis-[Tc(OH2)4(CO)2]+ is more stable
than mer-[Tc(OH2)3(CO)3]+ (18.1 kJ mol-1) but less stable than fac-
[Tc(OH2)3(CO)3]+ (91 kJ mol-1).26 As expected from kinetics but
not from thermodynamics, an aqueous solution of 1 under 40 bar
Table 1 Bond lengths and IR frequencies of the imidazole containing
complexes
IR n(CO)/cm-1
˚
Re–N(imz) distances/A
4
2.198(5)
—
2.204(5)
2.186(5)
2.200(5)
—
2.233(5)
2.199(5)
1916
1918
1917
2027
1827
1829
1834
1915, 1895
5
6
Scheme 1 Substitution of 1 with imidazole-like ligands, i) 2–2.2 eq. L in
6a24
H2O, 60 ◦C, 1 hour.
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of CO gave mer-[ReBr(NCCH3)2(CO)3] (7) in good yield. It was
not possible to replace the acetonitrile ligands by CO. Complex 7
1 but in water, the intermediate 8 could not be isolated but the
reaction went straight to [Re(NCCH3)2(CO)2(imz)2]Br (4) even at
a 1 : 1 Re/imz ratio.
+
is one of the rare examples of mer-{Re(CO)3} complexes without
phosphine ligands.27
The neutral complex 7 crystallises from THF–cyclohexane in
the monoclinic space group I2/m. The asymmetric unit contains
only the rhenium with one coordinated acetonitrile, a disordered
CO and Br-. The two-fold rotational axis and the mirror plane
perpendicular to it impose a perfect octahedral geometry (Fig. 3).
Compared to the fac-[ReBr(NCCH3)2(CO)3] (7a),28 the Re–N
bond length in 7 (2.086(8) A) is shorter then in 7a (2.144(5) A) due
to the missing trans-effect of the COs. Four IR carbonyl stretching
frequencies appeared at 2065w, 1953 s, 1887 m and 1827w cm-1.
The 1H-NMR in CDCl3 showed one singlet signal at 2.50 ppm for
the acetonitrile ligands.
Bidentate ligands
Substitution reactions of 1 with neutral or anionic bidentate
ligands L2 in water or organic solvents led to complexes of the
general composition [Re(NCCH3)2(CO)2(L2)]+ (Scheme 3). Even
under harsh conditions, no substitution of acetonitrile could be
observed.
˚
˚
Scheme 3 Substitution reaction of 1 with 2,2¢-bipyridine.
Classical aromatic N-heterocycles such as 2,2¢-bipyridine
(bpy), phenanthroline or 7,7¢-phenanthrolinedione (phd) are of
particular interest since a comparison of their spectroscopic
+
properties to the {Re(CO)3} analogues might be interesting.
Both ligands, bpy and phd reacted rapidly with 1 to yield
the corresponding complexes [Re(NCCH3)2(CO)2(bpy)]Br (9)
and [Re(NCCH3)2(CO)2(phd)]Br (10) (Fig. 4). In contrast to
[ReBr(CO)3(bpy)] or [ReBr(CO)3(phd)], which are yellowish com-
pounds, both 9 and 10 are an intense red colour. In the UV/Vis
spectrum, a typical MLCT absorption was detected at 423 nm
(e = 3000 l mol-1 cm-1) for complex 9 and at 420 nm (e = 5200 l
mol-1 cm-1) for complex 10.
Due to its luminescence properties, [ReBr(CO)3(bpy)] is an
excellent photosensitizer for photocatalytic purposes.17 Neither 9
nor 10 displayed significant luminescence in the visible range with
Fig. 3 ORTEP plot of mer-[ReBr(NCCH3)2(CO)3] (7). Hydrogen atoms
are omitted for clarity.
1
excitation at 420 nm. The H-NMR spectrum after dissolution
For trans-effect reasons, the trans-standing COs are labile
and one is expected to be easily substituted yielding again cis-
[Re(CO)2]+ type complexes. Complex 7 is soluble in organic
solvents like THF, CH2Cl2 or CHCl3 but not in water.
Accordingly, the reaction of 7 with imidazole in THF resulted
in the precipitation of the monosubstituted neutral complex
[ReBr(NCCH3)2(CO)2(imz)] (8) (Scheme 2). When starting from
of monocrystals of complex 10 in MeOD displayed four distinct
sets of signals (see ESI†). All other analytical methods (IR,
HPLC, 1H-NMR in DMSO) clearly indicated the presence of one
single compound. Presumably, hemi-acetal or acetal formation on
the ligand carbonyl groups lead to the observed pattern. Mass
spectrometry of 10 in MeOH, gave a main peak at m/z = 567
[M - Br + CH3OH]+, which is in support of this assumption.
Tetradentate ligands
Tridentate ligands would be very interesting, however, different
isomers are expected. Tetradentate ligands could, in principle,
form isomers as well but the proper choice of a symmetric
ligand might drive the reaction towards one single product. Upon
reaction with pyridine based tetradentate ligands L4, the complex
[Re(OH2)3(CO)2(NO)]2+ formed [Re(CO)(NO)(L4)]. It should be
noted that one CO ligand was substituted during this reaction.
In contrast, the reaction of [ReBr3(CO)3]2- with L4 yielded
tridentate-coordinated complexes [ReL4(CO)3].21,29 The reaction of
an aqueous solution of 1 with 1 eq. of bis(2-pyridylmethyl)glycine
(BPG) caused a yellow precipitate within a short period of time
and the neutral complex [Re(BPG)(CO)2] (11) could be obtained
in 49% yield. Yellow needles of X-ray quality could be grown from
Scheme 2 Substitution reactions of 7 with mono- and bidentate ligands
in THF; i) 1.1 eq. of imidazole and ii) 1.1 eq. of bpy, 1 h at 60 ◦C.
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Fig. 4 ORTEP of [Re(NCCH3)2(CO)2(bpy)]+ (9) and [Re(NCCH3)2(CO)2(phd)]+ (10). Hydrogen atoms and counter-ions were omitted for clarity.
methanol. A representation of the complex molecule is given in
Fig. 5. Complex 11 crystallised in the monoclinic space group
P21/c. The three five-membered chelate rings resulted in angles
well below 90◦, hence the coordination geometry is a strongly
distorted octahedron.
Fig. 6 IR spectra of the mixed cis- and trans-pyridine isomers of the
[Re(BPG)(CO)2] complex.
Fig. 5 ORTEP plot of [Re(BPG)(CO)2] (11). Hydrogen atoms are omitted
for clarity. Selected bond lengths and angles are: Re(1)–N(1) 2.129(6)
the cis-[Re(CO)2]+ and the trans,cis-[Re(NCCH3)2(CO)2]+ core
◦
˚
˚
A, Re(1)–N(2) 2.130(5) A, N(1)–Re(1)–N(2) 155.6(2) , N(1)–Re(1)–N(3)
78.1(2)◦, N(2)–Re(1)–N(3) 78.1(2)◦, O(3)–Re(1)–N(3) 77.4(2)◦.
respectively. The design of novel complexes for e.g. inorganic
medicinal purposes can be achieved by introduction of a variety of
further ligands into [Re(OH2)2(NCCH3)2(CO)2]+. Imidazole type
ligands such as 9-ethylguanine and aromatic N-heterocycles such
as bipyridine are representative examples and reacted straight-
forwardly to give the corresponding complexes. Tetradentate
ligands imposed substitution of water and acetonitrile ligands,
whereas acetonitrile could not be substituted with mono- and
bidentate ligands. We would like to emphasize that this well defined
substitution pattern could be used for building up supramolecular
structures with [Re(OH2)2(NCCH3)2(CO)2]+ representing the cor-
ners. Currently, we are investigating the biological behaviour of
these complexes.
Two isomers are possible with the two pyridine moieties in
the trans or cis positions respectively. The trans pyridine isomer
was obtained in water at 95 ◦C. The IR carbonyl frequencies
appeared at nCO = 1882, 1788, 1774 cm-1. If the reaction was
carried out at room temperature, traces of a second product could
be detected, likely to be the corresponding cis-isomer with nCO
=
1913, 1821 cm-1 (Fig. 6). The blue shift of the nCO absorptions
can be understood as a consequence of the stronger electron
donation of one pyridine and the tertiary amine trans to CO. In
cyclovoltammetry, complex 11 showed a nice and fully reversible
one electron oxidation at E1/2 = 595 mV vs Ag/Ag+.
Conclusion
Experimental
The complex (Et4N)[ReBr2(NCCH3)2(CO)2] (1) represents a water
All reactions were performed under an inert atmosphere. Chem-
stable and substitution labile precursor for complexes comprising
icals were used without further purification. Compounds 1, 2
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Table 2 Crystal data of 6, 7 and 9–11
6
7
9
10
11
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
C22H34BrN12O7Re
844.72
183(2)
Monoclinic
P21/n
10.3611(7)
27.153(2)
11.6030(8)
90
105.975(7)
90
3138.2(4)
4
5.205
C7H6BrN2O3Re
432.25
183(2)
Monoclinic
I2/m
5.9303(4)
7.8984(4)
11.8361(8)
90
102.334(8)
90
541.61(6)
2
14.889
4657
C16H14BrN4O2Re
560.42
183(2)
Monoclinic
P21/c
13.6113(8)
9.9206(5)
14.5778(11)
90
109.508(8)
90
1855.5(2)
4
8.717
19615
C18H12BrN4O4Re
614.43
183(2)
Monoclinic
P2/n
16.7319(14)
7.0306(4)
16.9965(14)
90
93.37(1)
90
1995.9(3)
4
8.121
C17H18N3O5Re
530.54
183(2)
Monoclinic
P21/c
8.7181(6)
13.400(1)
15.7069(11)
90
100.189(8)
90
1806.0(2)
4
6.762
22719
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
Volume/A
Z
Abs. coefficient/mm-1
Reflections collected
33748
12670
Independent refl. [Rint
]
7579 [0.0615]
R1 = 0.0684
wR2 = 0.1663
R1 = 0.0609
wR2 = 0.1590
862 [0.0625]
R1 = 0.0498
wR2 = 0.1114
R1 = 0.0478
wR2 = 0.1066
4936 [0.1399]
R1 = 0.0865
wR2 = 0.1376
R1 = 0.0554
wR2 = 0.1259
3436 [0.0923]
R1 = 0.0816
wR2 = 0.1735
R1 = 0.0674
wR2 = 0.1672
4291 [0.0569]
R1 = 0.0552
wR2 = 0.1428
R1 = 0.0493
wR2 = 0.1387
R indices (all data)a
Final R indices [I > 2s(I)]a
a R1 = |Fo - Fc|/|Fo|. wR2 = [w(Fo - Fc2)]1/2
.
2
and 4,23 (Et4N)[ReBr4(CO)2],30 phd31 and the BPG32 ligand were
synthesised according to the literature. IR spectra were recorded
on a PE Spectrum BX FT-IR spectrometer. NMR spectra were
recorded on a Varian Mercury 200 MHz or a Bruker DRX500
500 MHz spectrometer for compound 10. ESI-MS were performed
on a Merck-Hitachi M-8000 spectrometer for compounds 6, 8, 9,
and 10, on a Bruker esquireTM/LC spectrometer for compound 11
and on a Bruker esquireTM/HCTTM spectrometer for compounds
2 and 5. Values are reported for the 187Re isotope.
Elemental analyses (EA) were performed on a Leco CHNS-
932 elemental analyser, fluorescence measurements on a Perkin
Elmer LS50B spectrometer and UV/Vis spectra on a Cary 50
spectrometer with solution samples in 1 cm quartz cells. Elec-
trochemical measurements were carried out in DMF containing
0.1 M TBA[PF6] as electrolyte.
(Et4N)[ReBr2(NCC6H5)2(CO)2] (3)
(Et4N)[ReBr4(CO)2] (250 mg, 0.4 mmol) was dissolved in 4 ml
benzonitrile and TDAE (126 ml, 1.5 eq.) was added. TDAEBr2
precipitated immediately and was removed by filtration after
stirring for 10 min. The solution was dried in vacuo and the residue
washed with acetone to give 3 (140 mg, 50%) as a brown powder
(Found: C, 38.81; H, 4.16; N, 5.91. Calc. for C24H30Br2N3O2Re:
C, 39.03; H, 4.09; N, 5.69%); nmax(KBr)/cm-1 2226 (CN), 1901
(CO), 1815 (CO); dH (MeOD, 200 MHz) 7.90–7.52 (10 H, m,
NCC6H5), 3.30 (8 H, m, [N(CH2CH3)4]+ + MeOH), 1.29 (12 H,
m, [N(CH2CH3)4]+); m/z (ESI) 481 [M - 2Br + CH3OH]+, 467
[M - 2Br + H2O]+, 449 [M - 2Br]+.
[Re(NCCH3)2(CO)2(bzimz)2]Br (5)
Crystallographic data were collected on a one-circle Stoe IPDS
(Et4N)[ReBr2(NCCH3)2(CO)2] (1) (50 mg, 82 mmol) was dissolved
in 3 ml H2O and benzimidazole (bzimz) (21.2 mg, 2.2 eq.) added
and stirred under N2 for 1 h. The precipitate was filtered off,
washed with water and dried to give an off-white powder 5 (Found:
C, 37.29; H, 3.09; N, 12.96. Calc. for C20H18BrN6O2Re: C, 37.50;
H, 2.83; N, 13.12%); nmax(KBr)/cm-1 2269 (CN), 1917 (CO), 1828
(CO); m/z (ESI) 561 [M - Br]+, 443 [M - Br - bzimz]+.
˚
(Mo Ka radiation, l = 0.71073 A) diffractometer using graphite-
monochromated radiation (Table 2). Suitable crystals were covered
with oil (Infineum V8512, formerly known as Paratone N),
mounted on top of a glass fibre and immediately transferred to
the diffractometer. Data were collected by measuring j-oscillation
scans. A maximum of eight thousand reflections distributed over
the whole limiting sphere were selected by the program SELECT
and used for unit cell parameter refinement with the program
CELL.33 Data were collected for Lorentz and polarisation effects
as well as for absorption (numerical). Structures were solved with
direct methods using SIR9734 and were refined by full-matrix
least-squares methods on F2 with SHELXL-9735 In the case of
7, the symmetry of the space group (I2/m) generates a perfect
disorder of the three CO and one bromide ligands in the equatorial
plane around the rhenium center. Therefore all equatorial ligand
positions are occupied by either a CO or a bromide molecule
at a ratio of 3 : 1. No restraints were required to treat this
disorder. All structures were checked for higher symmetry with
the help of the program Platon.36 CCDC reference numbers are
683304–683308.†
[Re(NCCH3)2(CO)2(9-EtG)2]Br (6)
(Et4N)[ReBr2(NCCH3)2(CO)2] (1) (50 mg, 82 mmol) was dissolved
in 3 ml H2O and 9-EtG (29.2 mg, 2 eq.) added and stirred under
N2 over several days at 60 ◦C. The reaction mixture was cooled
to rt and the resulting precipitate was filtered off. nmax(KBr)/cm-1
2271 (CN), 1917 (CO), 1834 (CO). m/z (ESI) 684 [M - Br]+, 504
[M - Br-9-EtG]+, 463 [M - Br-9-EtG - CH3CN]+.
mer-[ReBr(NCCH3)2(CO)3] (7)
(Et4N)[ReBr2(NCCH3)2(CO)2] (1) (200 mg, 326 mmol) was dis-
solved in a small autoclave in 5 ml H2O and the system was
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degassed. 40 bar of CO was then applied. After 6 h the CO
was removed, and the fine colourless precipitate was centrifuged,
washed with H2O and Et2O and dried to give 7 (0.127 g, 90%)
as a white powder (Found: C, 19.38; H, 1.47; N, 6.44. Calc. for
C7H6BrN2O3Re: C, 19.45; H, 1.40; N, 6.48%); nmax(KBr)/cm-1
2285 (CN), 2065 (CO), 1953 (CO), 1887 (CO), 1827 (CO); dH
(CDCl3, 200 MHz) 2.50 (s).
to reflux. After refluxing overnight, the precipitate was filtered
off, washed with water and dried to yield 11 (40 mg 49%) as
a yellow powder (Found: C, 38.41; H, 2.90; N, 8.32. Calc. for
C16H14N3O4Re: C, 38.55; H, 2.83; N, 8.43%); nmax(KBr)/cm-1 1882
(CO), 1787, 1774 (CO), 1634 (CO); dH (MeOD, 200 MHz) 8.72 (d,
J = 5.5 Hz, 2H), 7.90 (dt, J = 7.5, 1.5 Hz, 2 H), 7.54 (t, J = 8 Hz,
2H), 7.34 (t, J = 6.5, 2 H), 4.90 (d, J = 15 Hz, 2H), 4.39 (d, J =
15 Hz, 2 H), 3.42 (s, 2H). m/z (ESI) 522 [M + Na]+, 500 [M + H]+.
[ReBr(NCCH3)2(CO)2(imz)] (8)
mer-[ReBr(NCCH3)2(CO)3] (7) (47 mg, 110 mmol) was dissolved
Notes and references
in 5 ml THF, imidazole (30 mg, 4 eq.) added and stirred under N2
◦
at 60 C. The precipitate was filtered off, washed with THF and
1 R. Alberto, R. Schibli, R. Waibel, U. Abram and A. P. Schubiger,
Coord. Chem. Rev., 1999, 192, 901–919.
dried to give 8 (42.2 mg, 82%) as a white powder (Found: C, 23.06;
H, 2.19; N, 11.72. Calc. for C9H9BrN4O2Re: C, 22.89; H, 2.13; N,
11.86%); nmax(KBr)/cm-1 2274 (CN), 1910 (CO), 1819 (CO); dH
(MeOD, 200 MHz) 8.11, 7.27 (3H), 2.59 (s, 6H). m/z (ESI) 393
[M - Br]+.
2 S. R. Banerjee, K. P. Maresca, L. Francesconi, J. Valliant, J. W. Babich
and J. Zubieta, Nucl. Med. Biol., 2005, 32, 1–20.
3 R. Alberto, R. Schibli, A. Egli, A. P. Schubiger, U. Abram and T. A.
Kaden, J. Am. Chem. Soc., 1998, 120, 7987–7988.
4 D. J. Stufkens and A. Vlcek, Coord. Chem. Rev., 1998, 177, 127–179.
5 P. Kurz, PhD thesis, University of Zu¨rich, 2005.
6 R. Alberto, J. Organomet. Chem., 2007, 692, 1179–1186.
7 S. Mundwiler, M. Kundig, K. Ortner and R. Alberto, Dalton Trans.,
2004, 1320–1328.
8 Y. Liu, J. K. Pak, P. Schmutz, M. Bauwens, J. Mertens, H. Knight and
R. Alberto, J. Am. Chem. Soc., 2006, 128, 15996–15997.
9 N. Agorastos, L. Borsig, A. Renard, P. Antoni, G. Viola, B. Spingler,
P. Kurz and R. Alberto, Chem.–Eur. J., 2007, 13, 3842–3852.
10 M. M. Saw, P. Kurz, N. Agorastos, T. S. A. Hor, F. X. Sundram,
Y. K. Yan and R. Alberto, Inorg. Chim. Acta, 2006, 359, 4087–
4094.
[Re(NCCH3)2(CO)2(bpy)]Br (9)
Method A. mer-[ReBr(NCCH3)2(CO)3] (7) (50 mg, 117 mmol)
was dissolved in 4 ml THF, 2,2¢-bipyridine (36.1 mg, 2 eq.) added
and stirred under N2 at 60 ◦C. After a few minutes, a red precipitate
formed which was filtered off, washed with Et2O and dried under
vacuum to give 9 (59.8 mg, 94%).
Method B. [ReBr(NCCH3)3(CO)2] (2) (25 mg, 56 mmol) was
dissolved in 3 ml CH3CN, 2,2¢-bipyridine (8.8 mg, 1.1 eq.)
added and stirred overnight at rt. Evaporating the solvent and
recrystallisation from DCM–hexane gave 9 (26 mg, 83%) (Found:
C, 34.27; H, 2.63; N, 9.71. Calc. for C16H14BrN4O2Re: C, 34.29;
H, 2.52; N, 10.00%); nmax(KBr)/cm-1 2273 (CN), 1926 (CO), 1853,
1835 (CO); dH (MeOD, 200 MHz) 9.10 (d, J = 4 Hz, 2H), 8.62
(d, J = 8 Hz, 2H), 8.26 (dt, J = 8, 1.5 Hz, 2H), 7.73 (dt, J = 6,
1.2 Hz, 2H), 2.28 (s, 6H). m/z (ESI) 480 [M - Br - 1]+, 440 [M -
Br - CH3CN]+.
11 P. Hafliger, N. Agorastos, B. Spingler, O. Georgiev, G. Viola and R.
Alberto, ChemBioChem, 2005, 6, 414–421.
12 H. He, M. Lipowska, X. Xu, A. T. Taylor, M. Carlone and L. G.
Marzilli, Inorg. Chem., 2005, 44, 5437–5446.
13 S. James, K. P. Maresca, D. G. Allis, J. F. Valliant, W. Eckelman,
J. W. Babich and J. Zubieta, Bioconjuagte Chem., 2006, 17, 579–
589.
14 S. Kunze, F. Zobi, P. Kurz, B. Spingler and R. Alberto, Angew. Chem.,
Int. Ed., 2004, 43, 5025–5029.
15 S. Top, S. Masi and G. Jaouen, Eur. J. Inorg. Chem., 2002, 1848–
1853.
16 I. Santos, A. Paulo and J. D. G. Correia, Top. Curr. Chem., 2005, 252,
45–84.
17 P. Kurz, B. Probst, B. Spingler and R. Alberto, Eur. J. Inorg. Chem.,
2006, 2966–2974.
[Re(NCCH3)2(CO)2(phd)]Br (10)
18 J. Hawecker, J. M. Lehn and R. Ziessel, Helv. Chim. Acta, 1986, 69,
Method A. mer-[ReBr(NCCH3)2(CO)3] (7) (50 mg, 117 mmol)
was dissolved in 5 ml THF, 7,7¢phenanthrolinedione (phd) (25 mg,
1 eq.) added and stirred under N2 at 60 ◦C. After a few minutes,
a dark red precipitate was formed which was filtered off, washed
with THF and dried under vacuum to give 10 (66.8 mg, 94%).
1990–2012.
19 D. Rattat, P. A. Schubiger, H. G. Berke, H. Schmalle and R. Alberto,
Cancer Biother. Radiopharm., 2001, 16, 339–343.
20 D. Rattat, A. Verbruggen, H. Schmalle, H. Berke and R. Alberto,
Tetrahedron Lett., 2004, 45, 4089–4092.
21 N. Marti, B. Spingler, F. Breher and R. Schibli, Inorg. Chem., 2005, 44,
6082–6091.
Method B. [ReBr(NCCH3)3(CO)2] (2) (25 mg, 56 mmol) was
dissolved in 3 ml CH3CN, phd (14 mg, 1.2 eq.) added and stirred
under N2 for 3 days at rt. The solvent was evaporated under
vacuum and the crude product recrystallised from EtOH–hexane
to give 10 (25 mg, 70%) (Found: C, 35.00 H, 2.12; N, 9.21. Calc.
for C18H12BrN4O4Re: C, 35.19; H, 1.97; N, 9.12%); nmax(KBr)/cm-1
2273 (CN), 1933 (CO), 1850 (CO); dH (DMSO, 300 MHz) 9.19 (dd,
J = 5.4, 1.5 Hz, 2H), 8.73 (dd, J = 8.1, 1.5 Hz, 2 H), 7.99 (dt, J =
5.4, 2.4 Hz, 2H), 2.38 (s, 6H). m/z (ESI) 615 [M + 1]+, 599 [M -
Br + 2CH3OH]+, 567 [M - Br + CH3OH]+, 553 [M - Br + H2O]+,
535 [M - Br]+.
22 R. Alberto, in Technetium, Rhenium and Other Metals in Chemistry
and Nuclear Medicine, ed. U. Mazzi, SGEditorialy, Padova, 2006, pp.
695–698.
23 L. Kromer, B. Spingler and R. Alberto, J. Organomet. Chem., 2007,
692, 1372–1376.
24 F. Zobi, O. Blacque, H. W. Schmalle, B. Spingler and R. Alberto, Inorg.
Chem., 2004, 43, 2087–2096.
25 B. Safi, J. Mertens, F. De Proft and P. Geerlings, J. Phys. Chem. A, 2006,
110, 9240–9246.
26 X. Y. Wang, Y. Wang, X. Q. Liu, T. W. Chu, S. W. Hu, X. H.
Wei and B. L. Liu, Phys. Chem. Chem. Phys., 2003, 5, 456–
460.
27 A. M. Bond, R. Colton and M. E. Mcdonald, Inorg. Chem., 1978, 17,
2842–2847.
28 N. Lazarova, S. James, J. Babich and J. Zubieta, Inorg. Chem. Commun.,
2004, 7, 1023–1026.
[Re(BPG)(CO)2] (11)
29 S. R. Banerjee, M. K. Levadala, N. Lazarova, L. H. Wei, J. F. Valliant,
K. A. Stephenson, J. W. Babich, K. P. Maresca and J. Zubieta, Inorg.
Chem., 2002, 41, 6417–6425.
(Et4N)[ReBr2(NCCH3)2(CO)2] (1) (100 mg, 163 mmol) was dis-
solved in 3 ml H2O, BPG (47 mg, 1.1 eq.) added and heated
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