Reactivity of 2-pyridine-aldehyde and 2-acetyl-pyridine coordinated to [Re(CO)3]+ with alcohols and amines: Metal mediated Schiff base formation and dimerization

Article abstract of DOI:10.1016/j.ica.2003.08.001

The coordination of an aldehyde or a ketone to Lewis acidic metal centers activates the carbonyl carbon atom towards nucleophilic attack. We studied the coordination chemistry of [ReBr₃(CO)₃]^2- (1) in water and in organic solvents with 2-acetyl-pyridine (2) and 2-pyridine-aldehyde (3) and investigated its reactivity towards aromatic and aliphatic amines. Rapid imine formation in water between coordinated organic carbonyls and pendant amine groups of biomolecules is useful to directly link the latter with the fac-[Re(CO)₃]⁺ or fac-[ ^99mTc(CO)₃]⁺ moiety. The reaction of [ReBr ₃(CO)₃]^2- with 2 in methanol produced the mononuclear complex [ReBr(CO)₃(C₅H₄NCOCH ₃)] (4), in which the carbonyl group is coordinated to the Re(I) center. The reaction of 1 with 3 is different and gave the dinuclear complex [Re₂(μ-OCH(OCH₃)C₅H₄N) ₂(CO)₆] (5) in which a semiacetal is formed. The coordinated aldehyde in the mononuclear complex [ReBr(CO)₃(C ₅H₄NCHO)] (6) is highly electrophilic and reacts with methanol to form the dinuclear complex 5. The reaction of 4 with aniline affords very rapid formation of [ReBr(CO)₃(C₅H ₄NC(CH₃)NC₆H₅)] (7). In situ preparation of 6 and reaction with aniline or 1-aminopropane gave the corresponding Schiff base complexes [ReBr(CO)₃(C₅H ₄NCHNC₆H₅)] (8) and [ReBr(CO) ₃(C₅H₄NCHNCH₂CH₂CH ₃)] (9), respectively. Compounds 4, 5, 8 and 9 are characterized by X-ray structure analysis. The Schiff base formation occurs in water at a very high rate. The rate constant for the formation of 7 was determined at -50°C in methanol. The rate law is second-order with k=1.36±0.19×10 ^-2 M^-1 s^-1. In the absence of rhenium, no formation of Schiff base is observed at 20°C over the same time period. The formation of the ^99mTc analogues of complexes 8 and 9 was confirmed by HPLC comparison with the corresponding Re complexes.

Full text of DOI:10.1016/j.ica.2003.08.001

Inorganica Chimica Acta 355 (2003) 386Á393
/
www.elsevier.com/locate/ica
Reactivity of 2-pyridineÁ
/
aldehyde and 2-acetylÁ
/
pyridine coordinated
to [Re(CO)3]ꢀ with alcohols and amines: metal mediated Schiff base
formation and dimerization
Wenwu Wang, Bernhard Spingler, Roger Alberto *
Institute of Inorganic Chemistry, University of Zurich, Winterthurerstr. 190, CH-8057 Zurich, Switzerland
¨ ¨
Received 5 August 2003; accepted 11 August 2003
Abstract
The coordination of an aldehyde or a ketone to Lewis acidic metal centers activates the carbonyl carbon atom towards
nucleophilic attack. We studied the coordination chemistry of [ReBr3(CO)3]2ꢁ (1) in water and in organic solvents with 2-acetylÁ
pyridine (2) and 2-pyridineÁaldehyde (3) and investigated its reactivity towards aromatic and aliphatic amines. Rapid imine
/
/
formation in water between coordinated organic carbonyls and pendant amine groups of biomolecules is useful to directly link the
latter with the fac-[Re(CO)3]ꢀ or fac-[99mTc(CO)3]ꢀ moiety. The reaction of [ReBr3(CO)3]2ꢁ with 2 in methanol produced the
mononuclear complex [ReBr(CO)3(C5H4NCOCH3)] (4), in which the carbonyl group is coordinated to the Re(I) center. The
reaction of 1 with 3 is different and gave the dinuclear complex [Re2(m-OCH(OCH3)C5H4N)2(CO)6] (5) in which a semiacetal is
formed. The coordinated aldehyde in the mononuclear complex [ReBr(CO)3(C5H4NCHO)] (6) is highly electrophilic and reacts with
methanol to form the dinuclear complex 5. The reaction of 4 with aniline affords very rapid formation of [Re-
Br(CO)3(C5H4NC(CH3)NC6H5)] (7). In situ preparation of 6 and reaction with aniline or 1-aminopropane gave the corresponding
Schiff base complexes [ReBr(CO)3(C5H4NCHNC6H5)] (8) and [ReBr(CO)3(C5H4NCHNCH2CH2CH3)] (9), respectively. Com-
pounds 4, 5, 8 and 9 are characterized by X-ray structure analysis. The Schiff base formation occurs in water at a very high rate. The
rate constant for the formation of 7 was determined at ꢁ
/
50 8C in methanol. The rate law is second-order with kꢂ
/
1.369
/
0.19ꢃ
/
10ꢁ2 Mꢁ1 sꢁ1. In the absence of rhenium, no formation of Schiff base is observed at 20 8C over the same time period. The
formation of the 99mTc analogues of complexes 8 and 9 was confirmed by HPLC comparison with the corresponding Re complexes.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Rhenium; Technetium; Schiff bases; Radiopharmaceuticals; Carbonyl; Organometallics
1. Introduction
linking functionality must be activated, i.e. carboxylato
group by ester formation. Hydrolysis of the activated
group with water competes labeling, hence, labeling
yields are in general not very good. It would be
intriguing to bypass this problem by activating the
linking group through the metal center. A dynamic
equilibrium with water would then not deactivate the
linking group and quantitative labeling can be expected.
It is well known from organometallic catalysis that
coordinated aldehydes or ketones are very active, i.e. for
reduction with H2 and it can be expected that they are
also activated towards nucleophilic attack with amines
for Schiff base formation. These coordinate much
stronger to the metal center and render the process
irreversible. If the amine is a functionality of the
biomolecule, this method can be used for a direct
The complexes [M(OH2)3(CO)3]ꢀ (Mꢂ188
have attracted interest as precursors for the labeling of
targeting biomolecules [1Á6]. The labeling of a biomo-
lecule with a metallic radionuclide requires in general a
bifunctional chelator [7Á9]. One function is for con-
jugation to a biomolecule and the second function for
coordination to the metal. In the prelabeling concept,
complex formation is the first step followed by coupling
of the complex to the biomolecule. For that purpose, the
/
Re, 99mTc)
/
/
* Corresponding author. Tel.: ꢀ
6803.
E-mail address: ariel@aci.unizh.ch (R. Alberto).
/
41-1-635-4631/54; fax: ꢀ41-1-635-
/
0020-1693/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2003.08.001

W. Wang et al. / Inorganica Chimica Acta 355 (2003) 386Á
/393
387
labeling without ‘deactivation’ of the metal complex.
Investigations about activating carbonyls by coordina-
tion as presented in this paper are rare if ever and have
never been performed in water for biomolecule conjuga-
tion purposes [10]. We exemplify this strategy with 2-
monitored by UV absorbance at 254 nm or g-ray
detection for the 99mTc complexes.
2.3. X-ray crystallographic data collection and
refinement of structures
acetylÁ
/
pyridine and 2-pyridineÁaldehyde coordinated
/
to the ‘fac-[Re(CO)3]ꢀ’ core as depicted in Scheme 1.
Pyridine coordinates strongly, imposing concomitantly
coordination and activation of the carbonyl group. The
reaction with a primary amine leads to Schiff base
formation. We compare the rate of reaction of the
coordinated carbonyl group to the free carbonyl group
to assess the amount of activation.
Single crystals of 4 and 8 were obtained by slow
diffusion of hexane into a CH2Cl2 solution, in the case
of 5 and 9 by slow evaporation of CH2Cl2Á
/pentane or
CH2Cl2Áhexane solutions, respectively. Suitable crystals
/
were covered with Paratone N oil, mounted on top of a
glass fibre and immediately transferred to a Stoe IPDS
diffractometer. Data was collected at 183(2) K using
We report herein about the preparation of Re(I) and
Tc(I) complexes containing a coordinated aldehyde or
ketone and their reactivity with aliphatic or aromatic
amines. This will allow a direct labeling of biomolecules
with 99mTc by imine formation but also to introduce
heavy metals selectively into proteins for solving the
phase problems in protein crystallography.
graphite-monochromated Mo Ka radiation (lꢂ
/
˚
0.71073 A). 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 [11]. Data was corrected for Lorentz
and polarization effects as well as for absorption
(numerical). Structures were solved with direct methods
using SHELXS-97 [12] or SIR-97 [13] and were refined by
full-matrix least-squares methods on F2 with SHELXL-97
[14]. The crystallographic data and experimental details
of compounds 4, 5, 8 and 9 are listed in Table 1. Further
experimental details are available as supplementary
material.
2. Experimental
2.1. Materials
The precursor complexes [NEt4]2[ReBr3(CO)3] and
99mTc(H2O)3(CO)3]2ꢁ were prepared as previously
reported [1]. The preparation procedure were monitored
with HPLC by UV absorbance at 254 nm or g-ray
detection for the 99mTc complexes.
All chemicals and solvents were of reagent grade and
used without further purification. HPLC analyses were
carried out using a gradient eluant of methanol and
0.1% TFA solution. Sodium pertechnetate was obtained
from a commercial 99Mo/99mTc generator (Mallinck-
rodt).
[
2.3.1. [Bromo(2-acetylÁ
rhenium(I)][ReBr(CO)3(C5H4NCOCH3)] (4)
One equiv. of 2-acetylÁpyridine (2) (0.018 ml, 0.16
/pyridine)tricarbonyl-
2.2. Instruments and methods
/
mmol) in methanol (2 ml) was added to [NE-
t4]2[ReBr3(CO)3] (0.120 g, 0.16 mmol) in methanol (5
ml) at r.t., resulting in instantaneous formation of a
orange-red solution. After 2 hours the solvent was
reduced to 2 ml. The mixture was left to slow evapora-
The reactions were carried out by standard Schlenk
techniques. IR spectra were recorded on a Perkin Elmer
BX II. Elemental analyses were performed on a Leco
CHN(S)-932 analyzer. NMR spectra were recorded on
the following spectrometers: Varian Gemini-200 instru-
ment, 1H at 199.98 MHz; Varian Gemini-300 instru-
ment, 1H at 300.08 MHz. All NMR spectra were
recorded at room temperature (r.t.). HPLC analyses
tion, during this procedure an orangeÁred precipitate
/
formed. The precipitate was filtered and washed with
cold water to give complex 4 (0.066 g, 90%). Slow
diffusion of hexane into a CH2Cl2 solution of 4 at r.t.
afforded orange block crystals. Anal. Calc. (Found) for
C10H7NO4BrRe: C, 25.49 (25.17); H, 2.97 (2.85); N, 1.50
(1.50)%. IR (cmꢁ1, KBr): 2022 (vs), 1929 (vs), 1897 (s).
were carried out with a Nucleosil EC column (250ꢃ3
/
mm) using a gradient eluant of methanol and 0.1%
trifluoracetic acid aqueous solution and a flow rate of
0.5 ml minꢁ1. The effluent from the column was
1H NMR (d, CD3CN): [9.08Á
/
9.06 (d, 1H), 8.83Á8.50 (d,
/
Scheme 1. Labeling of biomolecules by activated aldehyde and ketone.

388
W. Wang et al. / Inorganica Chimica Acta 355 (2003) 386Á393
/
Table 1
Summary of X-ray crystallographic data
4
5
8
9
Formula
Fw
C
10H7BrNO4Re
C20H16N2O10Re2
816.74
183(2)
C15H10BrN2O3Re
532.36
183(2)
C12H12BrN2O3Re
498.35
183(2)
471.28
183(2)
¯
P1
T (K)
Space group
¯
P1
C2/c
P21/n
Unit cell dimensions
˚
a (A)
˚
6.3358(7)
8.6054(9)
11.6427(13)
91.015(13)
94.877(13)
102.005(13)
618.25(12)
2
16.9234(12)
8.9103(4)
14.9769(11)
12.4036(19)
9.1681(8)
14.3146(17)
8.0767(8)
10.0835(10)
10.1519(11)
63.780(11)
78.968(12)
79.411(11)
723.37(13)
2
b (A)
˚
c (A)
a (8)
b (8)
91.633(9)
99.912(16)
g (8)
3
V (A )
˚
2257.5(3)
4
1520
1603.5(3)
4
992
Z
F(000)
Goodness-of-fit on F2
432
0.927
464
1.055
0.986
0.0378
0.0926
0.849
0.0344
0.0646
a,b
R
0.0457
0.1090
0.0338
0.0864
wR2 a,c
a
Observation criterion: I ꢀ2s(I).
/
b
c
Rꢂ
/
ajjFojꢁ
/
jFcjj/ajFoj.
wR2ꢂ
/
{a[w(Fo2ꢁ
/
Fc2)2]/a[w(Fo2)2]}1/2
.
1H), 8.33Á
(s, 3H; CH3); (d, CD3OD): [9.11Á
8.65 (d, 1H), 8.39Á8.36 (t, 1H), 7.96Á
C5H4N], 3.01 (s, 3H; CH3).
/
8.28 (t, 1H), 7.89Á
/
7.85 (t, 1H); C5H4N], 2.95
9.10 (d, 1H), 8.68Á
7.94 (t, 1H);
removed under vacuum, and the orange solid was
redissolved in CH2Cl2, hexane was added to precipitate
the complex 7 in quantitative yield. IR (cmꢁ1, KBr):
/
/
/
/
1
2024 (vs), 1926 (vs), 1897 (vs). H NMR (d, CD3CN):
7.73Á/7.16 (m, 9H; C5H4N, C6H5), 2.39 (s, 3H; CH3).
2.3.2. [Bromo(2-pyridineÁ
rhenium(I)][ReBr(CO)3(C5H4NCHO)] (6) and
[bis(m-2-pyridine-hemiacetal)hexacarbonylÁ
dirhenium(I)][Re2(OCH(OCH3)C5H4N)2(CO)6] (5)
As described for the preparation of 4, a solution of 2-
pyridineÁaldehyde (3) (0.009 ml, 0.1mmol) in MeOH
/
aldehyde)tricarbonylÁ
/
2.3.4. {Bromo[N-(2-
/
pyridinylmethylene)phenylamine]tricarbonylÁ
/
rhenium(I)}[ReBr(CO)3(C5H4NCHNC6H5)] (8)
A solution of aniline (0.01 ml, 0.1 mmol) in methanol
(2 ml) was added in situ prepared [Re-
/
(1.5 ml) was added to [NEt4]2[ReBr3(CO)3] (1) (0.077 g,
0.1 mmol) in MeOH (3 ml). The reaction mixture was
stirred for 7 h under r.t., HPLC trace of the final
reaction mixture gave one single peak at 16.9 min
representing the intermediate complex 6. Then water
was added until no more off-white precipitate formed.
The precipitate was filtered and washed with cold water
to give complex 5 (0.029 g, 73%). Slowly evaporation of
Br(CO)3(C5H4NCHO)] (6) (Â0.1 mmol) in methanol
/
(6 ml). After stirring overnight, the solvent was removed
under vacuum, and the orange solid was extracted with
THF in order to remove the [NEt4]Br. Slow diffusion of
hexane into a CH2Cl2 solution of 8 at r.t. afforded
orange plate crystals. Yield: 69%, 0.037 g. Anal. Calc.
(Found) for C15H10N2O3BrRe: C, 33.84 (34.14); H, 1.89
(1.70); N, 5.26 (5.25)%. IR (cmꢁ1, KBr): 2021 (vs), 1915
1
CH2Cl2Á
/
pentane solution of 5 at r.t. gave colorless
crystals. Anal. Calc. (Found) for
C20H16N2O10Re2: C, 29.41 (29.22); H, 1.97 (2.09); N,
(vs), 1884 (vs). H NMR (d, CD3OD): 9.13Á
/7.49 (m,
needle
9H; C5H4N, C6H5). HPLC Rtꢂ21.1 min.
/
3.43 (3.46)%. IR (cmꢁ1, KBr): 2012 (vs), 1893 (vs), 1876
2.3.5. {Bromo[N-(2-
pyridinylmethylene)propylamine]tricarbonylÁ
rhenium(I)}[ReBr(CO)3(C5H4NCHNCH2CH2CH3)]
(vs). HPLC Rtꢂ/17.3 min.
/
2.3.3. {Bromo[N-(2-
(9)
pyridinylmethylmethylene)phenylamine]tricarbonylÁ
rhenium(I)}[ReBr(CO)3(C5H4NC(CH3)NC6H5)] (7)
One equiv. of aniline (0.005 ml, 0.05 mmol) in
methanol (1 ml) was added to 4 (0.0246 g, 0.05 mmol)
in methanol (4 ml). The color of the solution changed
/
This compound was prepared as described for 8,
starting from complex 6 prepared in situ and after
addition of 1 equiv. of 1-aminopropane. Isolated yield:
75%. Slow evaporation of a CH2Cl2Áhexane solution at
/
r.t. gave orange plate crystals. Anal. Calc. (Found) for
C12H12N2O3BrRe: C, 28.92 (28.94); H, 2.43 (2.52); N,
5.62 (5.62)%. IR (cmꢁ1, KBr): 2018 (vs), 1934 (vs), 1889
from orangeÁ
/
red to orangeÁyellow. After 1 h, HPLC
/
revealed one single peak at 21.27 min. The solvent was

W. Wang et al. / Inorganica Chimica Acta 355 (2003) 386Á
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389
(vs). 1H NMR (d, CD3OD): 9.03Á
4.05 (m, 2H; CH2), 2.05 (m, 2H; CH2), 1.02 (t, 3H;
CH3). HPLC Rtꢂ21.2 min.
/7.69 (m, 4H; C5H4N),
1620 cmꢁ1, thus, clearly shifted compared to the free
ligand 2 where nCÄO appears at 1698 cmꢁ1. The 13C
NMR spectrum of ligand 2 (methanol-d4) displays seven
signals. The 13C chemical shifts of 4 are shifted slightly
downfield due to complex formation. Complex 4 is air
and moisture stable and is soluble in polar organic
/
2.3.6. [Chloro(2-picolinaldehyde)tricarbonylÁ
/
technetium(I)][TcCl(CO)3(C5H4NCHO)]
The pH value of the water solution of
solvents but not in water. From a 4/1 mixture MeOHÁ
/
99mTc(CO)3(H2O)3]ꢀ was adjusted to 4 by HCl (1M)
and phosphate (1M) buffer. A vial was sealed and
H2O 4 precipitates quantitatively.
[
X-ray structure analysis of 4 confirmed the composi-
tion of the complex 4. An ORTEP presentation of 4 is
given in Fig. 1. As shown in Scheme 1, compound 2
coordinates with N and O to the rhenium center.
degassed with a stream of nitrogen gas for 10 min. 2-
aldehyde (3) (0.1 ml, 10ꢁ1 M) in water and
PyridineÁ
/
[
99mTc(CO)3(H2O)3]ꢀ (1 ml) were added to the vial via
The behavior in solution of 4 was investigated by
means of 1H NMR spectroscopy. Interestingly, the
NMR spectrum of 4 in methanol-d4 and acetonitrile-d3
revealed solvent dependent equilibria. In the aromatic
region a set of multiplets are assigned to the pyridine
protons. In methanol-d4, one singlet at d 3.01 was
assigned to methyl protons of the acetyl group. This
singlet completely disappeared after a few minutes
whereas the spectrum did not change in the aromatic
region. Obviously, the methyl protons exchange rapidly
with deuterium from the protic solvent. The spectrum
remained then unchanged overnight. The behavior is
different in the aprotic solvent acetonitrile-d3. The
spectrum of a fresh solution showed the methyl protons
at d 2.95 as shown in Fig. 2(a). After 3 h, the intensity of
the singlet at d 2.95 was significantly reduced and a new
singlet appeared at d 2.63 (Fig. 2(b)). The ratio of these
two singlets is about 7:2. Addition of a base such as
aniline (6 equiv. relative to Re), moved this ratio to 2:5
(Fig. 2(c)). As depicted in Scheme 2 we assume that
(slow) deprotonation of the methyl group occurs to yield
the complex with a coordinated enolate. Deprotonation
is slow since it probably requires decomplexation of the
Brꢁ ligand from the Re-center in order to accommodate
the negative charge from the enolate. A DEPT NMR
spectrum of this mixture confirmed that the signal at
2.63 ppm is a CH2 group. The two protons are
chemically not equivalent but still show one signal.
syringe and the vial was heated to 95 8C for 30 min.
Formation of [99mTcCl(CO)3(C5H4NCHO)] was con-
firmed by radiochemical/HPLC analysis. Rtꢂ18.5 min
/
(Yield: 84%).
3. Results and discussion
When the complex [ReBr3(CO)3]2ꢁ (1) is dissolved in
a coordinating solvent such as methanol or water, the
fully solvated complexes [Re(HOCH3)3(CO)3]ꢀ or
[Re(OH2)3(CO)3]ꢀ immediately form. The solvent li-
gands can be replaced by a wide variety of ligands and
the formed complexes are of high kinetic stability.
Aldehyde or ketone coordinated to the [Re(CO)3]ꢀ
moiety are rare [15,16]. Isolated aldehydes or ketones
are not very good ligands, therefore we have chosen 2-
acetylÁ
/
pyridine (2) and 2-pyridineÁaldehyde (3) to
/
afford bidentate coordination through the very well
coordinating heterocyclic nitrogen and the less well
coordinating carbonyl oxygen. As evident from HPLC
monitoring of the solution, the Re(I) complex [Re-
Br(CO)3(C5H4NCOCH3)] (4) formed rapidly in the
reaction of stochiometric amounts of 1 and 2 at room
temperature as depicted in Scheme 2 in quantitative
yields. The reaction runs equally well in water and the
product 4 precipitates quantitatively from this solvent.
As expected, the carbonyl group of 2 is coordinated to
the Re(I) center and the sixth position is occupied by a
bromide. The IR spectrum (KBr) of 4 showed a nCÄO at
Scheme 2. (a) Preparation of 4 and 6; (b) equilibrium of 4 and enolate
in CH3CN.
Fig. 1. ORTEP plot of 4. Ellipsoids are drawn at the 50% probability
level.

390
W. Wang et al. / Inorganica Chimica Acta 355 (2003) 386Á393
/
The reaction of 1 with 3 in methanol did not give the
corresponding complex [ReBr(CO)3(C5H4NCHO)] (6)
but
the
dinuclear
complex
[Re2(m-
OCH(OCH3)C5H4N)2(CO)6] (5) was isolated in quanti-
tative yield. The formation of 5 was already evident
from the i.r. spectra where no nCÄO could be observed
anymore. Compared to the Á
electrophilic character of the coordinated Á
/
COCH3 group of 4 the
CHO group
/
is obviously strongly enhanced in the intermediate 6.
The lack of electron density at the carbonyl carbon in 4
can be compensated by donation from the deprotonated
ÁCH3 group which is not the case for 6. Aldehydes are in
/
equilibrium with hemiacetals in alcohols which is also
the case for coordinated 3 where a coordinated hemi-
acetal is formed. Since the alcoholato group is a very
strong donor, the electron density at the Re center is
high and is compensated by bridging of the oxygen to a
second rhenium center and essentially all Re(I) com-
plexes with alcoholato ligands are of di- or higher
nuclearity [17,18]. The x-ray structure could be eluci-
dated and an ORTEP is given in Fig. 3. In 5 two ligands
3 are coordinated to one Re center each, the alcoholato
serves as m-ligand. Bond lengths and angles are within
the usual range found in related compounds [5].
As depicted in Scheme 4 and obvious from the
physico-chemical data, complex 4 is activated for further
reaction with amines to form Schiff bases but electro-
philicity of the enolate carbon is strongly reduced
compared to the coordinated ketone. Since the two
forms are in equilibrium compound 4 still does react
reasonably fast with amines. The same is true for 6
which is also in equilibrium with 5 as depicted in Scheme
3.
Fig. 2. 1H NMR study of complex 4 in acetonitrile-d3: (a) spectrum
was recorded within 5 min; (b) spectrum was recorded after 3 h; (c)
spectrum was recorded after addition of the base aniline (six times
concentration) and left it overnight.
Probably the double bond is significantly delocalized
enabling rotation and thus equivalence of these two
protons. It is worthwhile to mention that no Schiff base
formation could be detected in acetonitrile. Addition of
aniline to 4 only shifts the equilibrium to the right side,
thus blocking the Schiff base formation with aniline.
Due to the lack of a methyl group adjacent to the
carbonyl, an aldehyde group will not be involved in such
an equilibrium leading to deactivation of the carbonyl
group towards nucleophilic attack. We therefore chose
To show the versatility of compounds 4 and 5 (6 in
situ) for metal assisted formation of Schiff base com-
2-pyridineÁaldehyde (3) for similar complexation stu-
/
dies and the reaction was found to be rather different.
The reactions involved are depicted in Scheme 3.
Scheme 3. (a) Equilibrium of 6 and semiacetal complex in MeOH; (b)
formation of 5 by dimerization of semiacetal complex.
Fig. 3. ORTEP plot of 5. Ellipsoids are drawn at the 50% probability
level.

W. Wang et al. / Inorganica Chimica Acta 355 (2003) 386Á
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391
pyridine occurred very slowly and no reaction could
be observed at ꢁ50 8C or even at room temperature
/
over the same time scale. The assumption that free
Schiff base formation is the rate-determining step can,
thus, be excluded since the reaction of free aldehyde
would occur with at least the same rate than observed in
the presence of the metal. The fact that Re(I) is a low d6
system furthermore makes a fast ligand release from the
metal center unlikely. The reaction between free ketone
and amine was also performed according to literature,
which required refluxing of 2 and aniline (1:1) in toluene
for about 12 h [19]. This comparison confirms the strong
activation of the coordinated carbonyl group towards
nucleophilic attack. It also implies that imine formation
on the complex is versatile and superior to initial
preparation of the Schiff base and subsequent labeling.
In full analogy to the reaction with aniline, the
condensation of 6 and 1-aminopropane afforded the
Scheme 4. Schiff base formation of the complexes 7, 8, and 9 from 4
and 6, respectively.
plexes, reactions with a variety of aromatic and aliphatic
amines have been performed in water and in methanol.
The reaction of 4 and 5 with aniline or 1-aminopropane
in methanol or methanolÁ/water (v:v, 4:1) produced
rapidly the Schiff
base
complexes
(7),
(8)
[Re-
[Re-
and
Br(CO)3(C5H4NC(CH3)NC6H5)]
Br(CO)3(C5H4NCHNC6H5)]
[ReBr(CO)3(C5H4NCHNCH2CH2CH3)] (9), respec-
tively, in quantitative yields. The reactions are shown
in Scheme 4.
corresponding
imine
complex
[Re-
Br(CO)3(C5H4NCHNCH2CH2CH3)] (9) at even faster
rate since sp3 nitrogen atoms are more reactive than the
sp2-nitrogen in aniline. X-ray quality crystals of com-
pounds 8 and 9 could be grown and the structures be
elucidated. The ORTEP plots of 8 and 9 along with the
corresponding atom-numbering schemes are given in
Fig. 4. In complex 8, the Re(I) center exhibits a distorted
octahedral coordination sphere with a facial arrange-
ment of the three carbonyl groups. The coordinated
imine forms a five-membered ring with the metal center.
The bromine atom and the carbonyl group trans to it
Aprotic solvents such as CH3CN are not suitable for
this reaction since deprotonation as described in Scheme
2 blocks the formation of the Schiff base to a significant
extent. Very fast reactions were found exclusively in
protic organic solvents or in water. The HPLC and MS
studies indicate that mono-dimer equilibrium is shifted
to the left side to give 6 when adding aniline or 1-
aminopropane. Complex 6 then reacts further to give
the imine complexes and after a short time period the
reaction goes to completion.
One could assume that an alternative mechanism for
the Schiff base formation on coordinated aldehyde or
are disordered in a 91:9 ratio. The N1Á
has value of 74.64(19)8, which deviates from the
expected ideal 908 angle for an octahedron. The CÁ
Re1ÁC angles have average values of 89.1(3)8. The
ReÁC bond lengths have a mean distance of 1.939(9) A
and that of ReÁ
/
Re1Á/N2 angle
ketone consists first in the release of 2-pyridineÁ
/
/
aldehyde from the [Re(CO)3]ꢀ core, Schiff base forma-
tion with aniline or 1-aminopropane and subsequent
recoordination of the Schiff base to the [Re(CO)3]ꢀ
core. However, as outlined below, these multiple step
pathway would take place at a much slower rate than
observed. To assess the proposed metal mediated Schiff
base formation mechanism, the rate constant for this
reaction has been determined. The reaction rate for the
formation of 7 from 4 and aniline was determined under
pseudo first-order conditions by monitoring the de-
/
˚
/
˚
N is 2.179(5) A [19]. More detailed data
/
is given in the supplementary material.
Due to the insolubility of 4 and 5 (6) in water, these
Schiff base formations at the [Re(CO)3]ꢀ core have been
performed in methanol. The same reaction was per-
formed in water to rationalize the applicability of the
procedure for the direct labeling of biomolecules with
pendant aliphatic amino groups and 99mTc. To improve
water solubility, Brꢁ in 4 was removed by precipitation
with Agꢀ in methanol. Methanol was evaporated and
the residue found to be perfectly soluble in H2O. This
yields the complex [Re(H2O)(CO)3(C5H4NCOCH3)]ꢀ
(10) with non-coordinating NO3ꢁ as the counter-ion.
The formulation could be confirmed by ESI-MS spec-
1
crease of H signals in 4 in methanol-d4. Due to the
exchange of ÁCH3 group of 4 with deuterium, this
/
group could not be used for the calculation. The rate of
formation at room temperature is so fast that the
reaction had to be performed at low temperature.
Analysis of the concentration of the starting material
versus time allowed the calculation of the rate constant
trometry where the signal of [Mꢀ ꢁ
m/z 391.73. It is notable that 10 reacts, comparable to 4,
extremely rapid with aniline to give
[Re(H2O)(CO)3(C5H4NCHNC6H5)]ꢀ (11), which was
confirmed by MS spectrum (m/z 466.53, [Mꢀ ꢁ
H2O]).
/
H2O] was found at
k which was found at ꢁ
/
50 8C to be 1.369
/
0.19ꢃ
10ꢁ2
/
Mꢁ1 sꢁ1 for the second-order reaction. The experi-
mental points (under pseudo first-order conditions) are
indeed compatible with first-order kinetics as suggested
by the linearity of the plot ln[4] versus time. The
/
As in methanol, 10 also reacted with 1-aminopropane at
a very high rate and the corresponding Schiff base
corresponding reaction with uncoordinated 2-acetylÁ
/

392
W. Wang et al. / Inorganica Chimica Acta 355 (2003) 386Á393
/
Fig. 4. ORTEP plot of 8 and 9. Ellipsoids are drawn at the 50% probability level.
complex [Re(H2O)(CO)3(C5H4NCHNCH2CH2CH3)]ꢀ
(12) was formed and confirmed by ESI MS (m/z
aliphatic or aromatic amines. This strategy was success-
fully used to conjugate [Re(OH2)(pyrCOR)(CO)3]ꢀ
432.5, [Mꢀ ꢁ
/
H2O]) and by HPLC. Evaporation of
(pyrCORꢂ
/
pyridineÁ
/
aldehyde or pyridineÁketone) to
/
water gives the complex in which NO3ꢁ is coordinated
to the Re-center. All the complexes are stable in water
for days without any decomposition.
(bio)molecules containing primary amines. Although
the corresponding reactions with 99mTc are slow (due
to the low concentration of 99mTc) this concept allows a
direct labeling of biomolecules containing primary
amines if they are stable at elevated temperature. On
the macroscopic level the reaction is sufficiently fast at
room temperature to allow introduction of heavy metals
in, i.e. proteins. Further studies for the labeling of
biomolecules with 99mTc and for the introduction of Re
in proteins are on the way.
To show the potential of this concept for radio-
pharmaceutical purposes, we prepared the correspond-
ing [99mTc(H2O)3(CO)3]ꢀ and did the reaction with 2-
acetylÁ
pyridine. This afforded the 99mTc analogous of 4
/
in 90% yield which could be confirmed by HPLC
comparison with g-detection. The retention time was
found at 18.5 min. Obviously at the very low concentra-
tion of 99mTc (B10ꢁ6M) dimerization should not occur
/
and was not observed indeed. Further reaction with
aniline or 1-aminopropane to afford the 99mTc analo-
gues of 8 and 9 occurred very slowly but could clearly be
observed. Although the reaction is fast at the macro-
scopic level (with rhenium), the low concentration of
99mTc rationalizes this result for a reaction at pseudo
5. Supplementary material
The supplementary material mentioned in this text is
available from the authors upon request.
first-order conditions (i.e. 10ꢁ7 M in 99mTc and 10ꢁ3
M
in amine). Although rate constants are in general ten
times higher for Tc than for Re, a rapid reaction can not
be expected from the observed k at room temperature
for 99mTc. Still, if the biomolecule is stable at, i.e. 100 8C
as is the case for many peptides, this approach provides
a reasonable route to the direct labeling as proposed in
the beginning. Moreover, from the data with rhenium it
is clear that the concept can be applied to selectively
introduce the heavy metal in proteins which will help for
instance to solve the phase problem.
Acknowledgements
The authors thank Mallinckrodt Medical B.V. (NL)
and the Swiss National Science Foundation for financial
support.
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