Rhenium(I) organometallic complexes with novel bis(mercaptoimidazolyl)borates and with hydrotris(mercaptoimidazolyl)borate: Chemical and structural studies

Article abstract of DOI:10.1016/S0022-328X(01)00838-5

Lithium salts of novel poly(mercaptoimidazolyl)borate anions [H(R)B(tim^Me)₂]^- have been synthesised under mild conditions by reaction of the corresponding lithium organoborohydrides with 2-mercapto-1-methylimidazole. Treatment of the Re(I) starting materials [Re(CO)₅Br] or (NEt₄)₂[Re(CO)₃Br₃] with the stoichiometric amount of Li[H(R)B(tim^Me)₂] (R=Me (1), Ph (2)) or Na[HB(tim^Me)₃] gave the tricarbonyl complexes [Re{κ³-R(μ-H)B(tim^Me)₂}(CO) ₃] (R=Me (3), Ph (4)) and [Re{κ³-HB(tim^Me)₃}(CO)₃] (5). These complexes were also prepared under aqueous and aerobic conditions, in almost quantitative yield, using (NEt₄)₂[ReBr₃(CO)₃] as starting material. Compounds 1-5 have been characterised by the usual analytical techniques and by X-ray crystallographic analysis in the case of 3-5. The X-ray diffraction analysis of 3-5 showed that the rhenium atom adopts a slightly distorted octahedral coordination with a facial arrangement of the carbonyl ligands. In complex 5 the three remaining coordination positions are occupied by the three thione sulphur atoms from the tripodal hydrotris(2-mercapto-1-methylimidazolyl)borate, and in 3 and 4 these positions are occupied by the two thione sulphur atoms and by one hydrogen atom, which is involved in a strong agostic B-HRe interaction.

Full text of DOI:10.1016/S0022-328X(01)00838-5

Journal of Organometallic Chemistry 632 (2001) 41–48
www.elsevier.com/locate/jorganchem
Rhenium(I) organometallic complexes with novel
bis(mercaptoimidazolyl)borates and with
hydrotris(mercaptoimidazolyl)borate: chemical and structural
studies
.
Raquel Garcia, Anto´nio Paulo, Angela Domingos, Isabel Santos *
Departamento de Qu´ımica, Instituto Tecnologico e Nuclear, Estrada Nacional 10, 2686-953 Saca6em Codex, Portugal
Received 18 February 2001; accepted 29 March 2001
Dedicated to Professor Alberto Roma˜o Dias on the occasion of his 60th birthday in recognition of his friendship and outstanding
contribution to inorganic chemistry
Abstract
Lithium salts of novel poly(mercaptoimidazolyl)borate anions [H(R)B(timMe)2]− have been synthesised under mild conditions
by reaction of the corresponding lithium organoborohydrides with 2-mercapto-1-methylimidazole. Treatment of the Re(I) starting
materials [Re(CO)5Br] or (NEt4)2[Re(CO)3Br3] with the stoichiometric amount of Li[H(R)B(timMe)2] (R=Me (1), Ph (2)) or
Na[HB(timMe)3] gave the tricarbonyl complexes [Re{k3-R(m-H)B(timMe)2}(CO)3] (R=Me (3), Ph (4)) and [Re{k3-
HB(timMe)3}(CO)3] (5). These complexes were also prepared under aqueous and aerobic conditions, in almost quantitative yield,
using (NEt4)2[ReBr3(CO)3] as starting material. Compounds 1–5 have been characterised by the usual analytical techniques and
by X-ray crystallographic analysis in the case of 3–5. The X-ray diffraction analysis of 3–5 showed that the rhenium atom adopts
a slightly distorted octahedral coordination with a facial arrangement of the carbonyl ligands. In complex 5 the three remaining
coordination positions are occupied by the three thione sulphur atoms from the tripodal hydrotris(2-mercapto-1-methylimida-
zolyl)borate, and in 3 and 4 these positions are occupied by the two thione sulphur atoms and by one hydrogen atom, which is
involved in a strong agostic BꢀH···Re interaction. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Rhenium; Carbonyl; Poly(mercaptoimidazolyl)borates; Agostic interaction; X-ray diffraction analysis
for diagnostic (99mTc) and therapeutic (186/188Re) medi-
cal applications [2]. Recently, these synthons have been
1. Introduction
successfully applied on the labelling of some
biomolecules, such as peptides or serotoninergic recep-
tor (5HT1A) antagonists [3]. To further apply these new
labelling tools it is of great importance to introduce
novel chelator systems and to evaluate their coordinat-
ing capability to the fac-M(CO)3 cores (M=Re, Tc).
By introducing new chelators one can tune the physi-
cal–chemical properties of the final complexes (e.g.
charge, size and lipophilicity) which determine their
potential biological applications.
Rhenium(I) carbonyl complexes are one of the most
studied class of compounds in organometallic rhenium
chemistry. In the past, the interest on these low-valent
complexes was driven mainly by their interesting be-
haviour in different chemical, electrochemical or photo-
chemical processes, including catalytic ones [1]. More
recently, studies on the basic coordination chemistry of
Tc(I) and Re(I) complexes containing the fac-M(CO)3
moieties highlighted the potential relevance of these
complexes in the development of radioactive products
Recently, the tripodal [HB(timMe)3]− and the poten-
tially bidentate [H2B(timMe)2]− (we use the abbreviation
timMe, from the term ‘thioimidazolyl’, to represent the
2-mercapto-1-methylimidazolyl fragment) have been in-
* Corresponding author. Tel.: +351-2199-46201; fax: +351-2199-
41455.
E-mail address: isantos@itn1.itn.pt (I. Santos).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)00838-5

42
R. Garcia et al. / Journal of Organometallic Chemistry 632 (2001) 41–48
troduced and their coordination chemistry studied with
a few late transition metals and with ruthenium [4].
These soft ligands were considered as analogues of the
ubiquitous poly(pyrazolyl)borates, largely used in
organometallic and inorganic chemistry, namely for
rhenium [5,6].
external NaBH4 solution. NMR spectra were run in
CD3CN or in DMSO-d6. IR spectra were recorded as
KBr pellets on a Perkin–Elmer 577 spectrometer. C, H
and N analyses were performed on an EA110 CE
Instruments automatic analyser.
As part of our ongoing research on low-valent Tc
and Re complexes for biomedical applications, we have
recently described the complexes [M{k3-H(m-
H)B(timMe)2}(CO)3] (M=Re, 99Tc), and we have
shown that the analogous 99mTc complex can be pre-
pared with high radiochemical yield and with high
specific activity [7]. An unprecedented and quite robust
BꢀH···M agostic interaction was formed and, consis-
tently, the 99mTc complex was remarkably stable, even
under aqueous and aerobic conditions. These findings
indicated that poly(mercaptoimidazoyl)borates features
inherent requirements for developing novel radiophar-
maceuticals. Moreover, the tuning of the physico-chem-
ical properties of the complexes is facilitated by the
easy modification of these ligands and selected
biomolecules can be coupled to the chelator framework
either by attachment to the imidazole residue or by
linkage to the boron atom. Following this approach,
our group is currently investigating the chemistry of Re
and Tc carbonyl complexes stabilised with poly(mer-
captoimidazolyl)borates bearing different substituent
groups, including biologically active ones. In this re-
search, it is of crucial importance to acquire knowledge
about the influence of the substituents on the coordina-
tion chemistry of the ligands. Herein, we report the
synthesis and characterisation of the novel
[H(R)B(timMe)2]− (R=Me (1), Ph (2)) and reactions of
Re(I) starting materials with these ligands and with the
already available [HB(timMe)3]−. These reactions led to
the synthesis of the new complexes [Re{k3-R(m-
H)B(timMe)2}(CO)3] (R=Me (3), Ph (4)) and [Re{k3-
HB(timMe)2}(CO)3] (5), which are also described in this
work.
2.1. Synthesis of Li[H(Me)B(timMe)2]·THF (1)
To a solution of Li(MeBH3) (174 mg, 4.86 mmol) in
THF was added 2-mercapto-1-methylimidazole (1.120
g, 9.81 mmol) dissolved in the minimum volume of
THF, and the resulting mixture was stirred at room
temperature (r.t.). The course of the reaction was
checked by 1H- and 11B-NMR analyses of aliquots
from the reaction mixture. After 2 h there was complete
conversion of the borohydride and the solvent was
removed under vacuum. The solid residue was washed
with C6H5CH3 to remove any unreacted 2-mercapto-1-
methylimidazole. The recovered white insoluble solid
(440 mg, yield=34%) was formulated as 1 based on
1
IR, H- and 11B-NMR spectroscopies.
1
IR (cm−1): w(BꢀH) 2438 w. H-NMR (CD3CN, 300
MHz): l=0.11 (d, 3H, JHꢀH=5.7 Hz, CH3ꢀB), 3.40 (s,
6H, CH3ꢀN), 6.64 (d, 2H, JHꢀH=2.1 Hz, CH), 6.76 (d,
2H, JHꢀH=2.1 Hz, CH). 11B-NMR (CD3CN, 96 MHz):
l=39.35.
2.2. Synthesis of Li[H(Ph)B(timMe)2]·THF (2)
The preparation and recovery of 2 was done as
described above for 1, except that the reaction mixture
was refluxed during 2 h. Starting from 200 mg (2.05
mmol) of Li[PhBH3] were obtained 440 mg of 2 in the
form of a microcrystalline white solid (yield: 67%).
1
IR (cm−1): w(BꢀH) 2440 w. H-NMR (CD3CN, 300
MHz): l=3.48 (s, 6H, CH3ꢀN), 6.44 (d, 2H, JHꢀH
=
2.1 Hz, CH), 6.72 (d, 2H, JHꢀH=2.1 Hz, CH), 6.97–
7.13 (br m, 5H, Ph). 11B-NMR (CD3CN, 96 MHz):
l=41.12.
2.3. Synthesis of [Re{s3-R(v-H)B(timMe)2}(CO)3]
(R=Me (3), Ph (4))
2. Experimental
The reactions were carried under a N2 atmosphere
using standard Schlenk techniques, unless otherwise
indicated. Solvents were dried and distilled prior to use
according to described procedures. The starting materi-
als [ReBr(CO)5] [8] and (NEt4)2[ReBr3(CO)3] [9], the
ligand Na[HB(timMe)3] [4e] and the organoborohydrides
Li(RBH3) (R=Me, Ph) [10] were prepared by the
literature methods. The other chemicals were used as
purchased.
To solutions of [Re(CO)5Br] (100 mg, 0.25 mmol) in
THF were added Li[H(R)B(timMe)2] (R=Me, Ph), at
low temperature and in ca. 1:1 molar ratio (10% molar
excess). The respective reaction mixtures were slowly
warmed to r.t., then refluxed for 2 h. The reaction
mixtures were filtered to remove any insoluble materi-
als. The filtrates were concentrated under vacuum; on
addition of n-hexane pale-yellow microcrystalline solids
precipitated which were formulated as 3 (95 mg,
yield=74%) and 4 (144 mg, yield=76.4%).
1H- and 11B-NMR spectra were recorded on a Varian
1
Unity 300 MHz spectrometer; H and chemical shifts
Compound 3. Anal. Found: C, 30.20; H, 2.21; N,
10.71. Calc. for C12H14N4O3S2BRe: C, 27.53; H, 2.68;
N, 10.71%. IR (cm−1): w(CꢀO) 1880 s, w(CꢀO) 1905 s,
were referenced with the residual solvent resonances
relative to Me4Si and the 11B-NMR chemical shifts with

R. Garcia et al. / Journal of Organometallic Chemistry 632 (2001) 41–48
43
w(CꢀO) 2020 s, w(BꢀH···Re) 2160 w. 1H-NMR
2.5. X-ray crystallographic analysis
(CD3CN, 300 MHz): l= −6.36 (br, 1H, BꢀH), 0.50
(3H, CH3ꢀB), 3.50 (s, 6H, CH3ꢀN), 6.91 (d, 2H,
The crystals were obtained by recrystallisation from
THF/n-hexane (3 and 4) or from CH2Cl2/n-hexane (5),
and mounted in thin-walled glass capillaries. Data were
collected at r.t. on an Enraf–Nonius CAD-4 diffrac-
tometer with graphite-monochromated Mo–Ka radia-
tion, using a ꢀ–2q scan mode. The crystal data are
summarised in Table 1.
JHꢀH=2.1 Hz, CH), 7.09 (br m, 2H, CH).
Compound 4. Anal. Found: C, 34.60; H, 2.29; N,
10.25. Calc. for C17H16N4BO3S2Re: C, 34.87; H, 2.73;
N, 9.57%. IR (cm−1): w(CꢀO) 1900 s, w(CꢀO) 1970 s,
w(CꢀO) 2020 s, w(BꢀH···Re) 2160 w. 1H-NMR
(CD3CN, 300 MHz): l= −5.20 (br, 1H, BꢀH), 3.56 (s,
6H, CH3ꢀN), 6.74 (d, 2H, JHꢀH=2.1 Hz, CH), 7.13 (d,
2H, JHꢀH=2.1 Hz, CH), 7.29 (br m, 2H, Ph), 7.36 (br
m, 3H, Ph).
The data were corrected [11] for Lorentz and polari-
sation effects, for linear decay (no decay was found in
5) and empirically for absorption by c scans. The
heavy atom positions were located by Patterson meth-
ods using SHELXS-86 [12]. The remaining atoms were
located in successive Fourier-difference maps and
2.4. Synthesis of [Re{s3-HB(timMe)3}(CO)3] (5)
Complex 5 was prepared by reacting [Re(CO)5Br]
with Na[HB(timMe)3], as described above for 3 and 4.
The only difference was that in the case of 5 the
reaction mixture was refluxed during 6 h. Starting from
100 mg (0.25 mmol) of [ReBr(CO)5] were obtained 120
mg (yield: 72%) of 5.
refined by least-squares refinements on F2 using SHEXL
-
93 [13]. For compound 5 on the basis of the systematic
absences and of the threefold symmetry expected for
(
the molecule, the possible space groups are R3 and R3.
(
Attempts to solve the structure in R3 with the Re and
B atoms on a crystallographic threefold axis and admit-
ting disorder of the ligands led to unsatisfactory refine-
ment and was unsuccessful. So, the solution of the
structure was attempted in the non-centrosymmetric
space group R3, with two molecules per asymmetric
unit, and with the two Re and the two B atoms on a
crystallographic threefold axis. The refinement con-
Anal. Found: C, 28.64; H, 2.53; N, 13.34. Calc. for
C15H16N6BO3S3Re: C, 28.98; H, 2.58; N, 13.53%. IR
(cm−1): w(CꢀO) 1863 s, w(CꢀO) 1987 s, w(BꢀH) 2470 w.
1H-NMR (DMSO-d6, 300 MHz): l=3.57 (s, 9H,
CH3ꢀN), 7.06 (d, 3H, JHꢀH=1.5 Hz, CH), 7.09 (d, 3H,
JHꢀH=1.5 Hz, CH).
Table 1
Crystallographic data for 3, 4 and 5
Compound
3
4
5
Empirical formula
Formula weight
Crystal system
C
523.40
Triclinic
12H14BN4O3ReS2
C17H16BN4O3ReS2
585.47
Monoclinic
C15H16BN6O3ReS3
621.53
Trigonal
(
Space group
Unit cell dimensions
P1 (no. 2)
C2/c (no. 15)
R3 (no. 146)
,
a (A)
9.039(1)
13.7441(13)
13.0203(10)
22.960(3)
90
105.056(11)
90
3967.7(7)
8
1.960
14.302(2)
14.302(2)
17.956(3)
90
90
120
3180.8(8)
6
,
b (A)
9.2910(1)
12.453(1)
73.84(1)
75.41(1)
61.64(1)
875.0(2)
2
1.987
7.198
,
c (A)
h (°)
i (°)
k (°)
V (A )
3
,
Z
Dcalc (g cm−3
)
1.947
6.055
v(Mo–Ka) (cm−1
)
6.362
Reflections collected
Independent reflections
Parameters
3528
4008
3600
3313 [Rint=0.0227]
209
3905 [Rint=0.0658]
254
3404 [Rint=0.0299]
178
−3
,
Largest difference peak and hole (e A
)
0.749 and −0.557
1.058
1.090 and −1.064
1.066
0.623 and −0.759
1.043
Goodness-of-fit on F2
a
R
0.0389 (0.0608) b
0.0632 (0.0813) b
0.0803 (0.1968) b
0.1050 (0.1829) b
0.0341 (0.0523) b
0.0671 (0.0879) b
a
wR2
a R=ꢀꢁꢁFoꢁ−ꢁFcꢁꢁ/ꢀꢁFoꢁ, wR2=[ꢀ(w(F2o−Fc2)2)/ꢀ(w(F2o)2)]1/2; [Fo\4h(Fo)].
b Based on all data.

44
R. Garcia et al. / Journal of Organometallic Chemistry 632 (2001) 41–48
3. Results and discussion
3.1. Syntheses
The work of Reglinski and coworkers [4a] proved
that poly(mercaptoimidazolyl)borate ligands can be
easily obtained by reacting alkaline salts of BH−4 with
2-mercapto-1-methylimidazole, with procedures similar
to those used for the related poly(pyrazolyl)borates.
Following this approach, we have evaluated the possi-
bility of preparing ligands of the type [H(R)B(timMe)2]−
, starting from the organoborohydrides Li[RBH3]
(R=Me, Ph) which were prepared by reaction of the
corresponding boronic acids RB(OH)2 with LiAlH4,
according to procedures described in the literature [10].
As shown in Scheme 1, Li[RBH3] (R=Me, Ph) react
readily, in THF, with two equivalents of 2-mercapto-1-
methylimidazole, giving Li[H(R)B(timMe)2] (R=Me
(1), Ph (2)). The kinetics of the reaction depends on the
nature of the substituent attached to the boron. Indeed,
ligand 1 is easily prepared at r.t. while the preparation
of 2 requires reflux in THF. Obviously, this difference
reflects the higher basicity of the hydrides in [MeBH3]−
in comparison with [PhBH3]−, as a consequence of the
electronic withdrawing properties of the phenyl group.
The formation of 1 and 2 is quite efficient, as shown
by the follow-up of the respective reaction mixtures by
1H- and 11B-NMR analyses. However, after work-up
compounds 1 and 2 are obtained only in moderate to
low isolated yield, since the removal of traces of unre-
acted 2-mercapto-1-methylimidazole is only possible by
washing with C6H5CH3 and the ligands are also slightly
soluble in this solvent.
Scheme 1.
Ligands 1 and 2 are hygroscopic white solids, which
are soluble in most common polar organic solvents and
in water and are relatively resistant towards aerobic
oxidation and hydrolysis. These characteristics are com-
patible with their further use in the development of
organometallic complexes potentially relevant in bio-
logical and biomedical applications that usually involve
aqueous medium.
Scheme 2.
As shown in Scheme 2 reaction of [Re(CO)5Br] with
stoichiometric amounts of Li[H(R)B(timMe)2] (R=Me
(1), Ph (2)) or Na[HB(timMe)3], in THF solution and
upon reflux, gives [Re{k3-R(m-H)B(Simz)2}(CO)3] (R=
Me (3), Ph (4)) and [Re{k3-RB(timMe)3}(CO)3] (5).
Compounds 3–5 are recovered as pale-yellow (3 and 4)
or white (5) solids in moderate to high yield, upon
concentration of the respective reaction mixtures fol-
lowed by addition of n-hexane.
verged consistently to R1=0.0432 [for I\2|(I)], with
all non-hydrogen atoms refining anisotropically. At this
stage the refinement program flagged a possible racemic
twin. The refinement as a racemic twin (including
TWIN and BASF parameters) proved to be acceptable,
decreasing the wR2 value from 0.0955 to 0.0672 (R1=
0.0341). The BASF value converged to 0.60(1). For the
structures of 3–5 all the non-hydrogen atoms were
refined anisotropically; the contributions of the hydro-
gen atoms were included in calculated positions, con-
strained to ride on their carbon atoms with group Uiso
values assigned. Atomic scattering factors and anoma-
lous dispersion terms were as in SHELXL-93 [13]. The
drawings were made with ORTEP-II [14a] and ORTEP-3
[14b]; all the calculations were performed on a 3000
Dec a computer.
Owing to our interest in biomedical applications, we
studied using 1H-NMR the possibility of preparing
3–5, using (NEt4)2[Re(CO)3Br3] as starting material. In
fact this compound in coordinating solvents forms
cations of the type [Re(CO)3L3]+ (L=solvent
molecule), which are analogous to the synthons
[M(CO)3L3]+ (M=99mTc, 186 188Re) available in radio-
/
pharmaceutical preparations [2]. We observed that

R. Garcia et al. / Journal of Organometallic Chemistry 632 (2001) 41–48
45
(NEt4)2[Re(CO)3Br3] reacts with Li[H(R)B(timMe)2] or
with Na[HB(timMe)3] in CD3CN at 40°C, leading quan-
titatively to the complexes 3–5, within 15 min.
in a typical pattern for complexes with the ‘fac-
Re(CO)3’ moiety in a distorted C3 environment. In the
case of 3 and 4 the IR spectra show three w(CO) bands,
as a consequence of the lower symmetry (Cs) of these
complexes [3g]. The values of the carbonyl stretching
frequencies in [Re{k3-HB(timMe)3}(CO)3] (1863 and
1987 cm−1) are lower than those previously reported
for the congener [Re{k3-HB(pz)3}(CO)3] (1896 and
2020 cm−1) [16b], confirming the better donor capabil-
ity of [HB(timMe)3]−. An important feature of the IR
spectra of 3 and 4 is the presence of weak to medium
intensity bands centred at 2160 cm−1, which were
assigned to w(BꢀH···Re). These w(BꢀH···Re) frequencies
are strongly red shifted compared to the w(BꢀH)
stretching frequency in the free ligands (2438 cm−1 for
1 and 2440 cm−1 for 2). These data indicated that we
are in the presence of strong w(BꢀH···Re) agostic inter-
actions, as previously observed for [Re{k3-H(m-
H)B(timMe)2}(CO)3] [7]. This type of interaction is
absent in complex 5 which contains a tripodal ligand
coordinated through the three thione sulphur atoms. In
fact for 5 the w(BꢀH) stretching band appears at 2470
cm−1, only slightly shifted from the corresponding
band of the free ligand (2480 cm−1). These spectro-
scopic features are consistent with the molecular struc-
tures of complexes 3–5 which were determined by
X-ray diffraction analysis (see Section 3.3).
Compounds 3 and 4 are analogous to the previously
described [Re{k3-H(m-H)B(timMe)2}(CO)3] [7] and are
relatively rare examples of rhenium complexes which
contain BꢀH···Re agostic interactions. These agostic
interactions also remain intact in the presence of coor-
dinating solvents and, therefore, their strength is not
affected strongly by the alkyl or aryl groups attached to
the boron. The coordinative behaviour of the ligands
[H(R)B(timMe)2]− (R=H, Me (1), Ph (2)) towards the
electron-rich Re(I) centre markedly contrasts with that
previously described for the harder congener
[H2B(pz)2]−. In fact, when [Re(CO)5Br] was treated
with Na[H2B(pz)2] no compound containing a BꢀH···Re
agostic interaction was detected. In this reaction some
degradation of the ligand was observed, and the only
complex isolated has been [Re{H2B(pz)2}(CO)2(pzH)]
[15]. It is not clear whether these differences are deter-
mined by steric factors or by a better match between
the electronic properties of the sulphur donor ligands
and the soft d6 rhenium centre. In fact, in the case of
the
hydrotris(2-mercapto-1-methylimidazolyl)borate,
the isolated complex [Re{k3-HB(timMe)3}(CO)3] (5) is
analogous to the previously described [Re{k3-
HB(pz)3}(CO)3] [16], both tripodal ligands displaying a
similar behaviour.
1
The H-NMR spectra obtained for 3–5 are in accor-
dance with the Cs (3 and 4) and C3 (5) symmetries
found in the solid state. The most striking feature of the
1H-NMR spectra of complexes 3 and 4 is the presence
of highfield shifted resonances at −6.36 and −5.20
ppm, respectively, which are due to the protons in-
volved in the BꢀH···Re agostic interactions. These re-
sults confirm the stability of the agostic interactions in
solution, even in coordinating solvents, as previously
observed for [Re{k3-H(m-H)B(timMe)2}(CO)3] (l= −
6.40 ppm) [7]. It is interesting to note that the chemical
shifts observed for the BꢀH···Re protons are in the
range where the hydrides normally appear in rhenium
polyhydride complexes [17], indicating that the coordi-
nated hydrogen has an almost hydridic character.
3.2. Spectroscopic data
The IR spectra of [Re{k3-R(m-H)B(Simz)2}(CO)3]
(R=Me (3), Ph (4)) and [Re{k3-HB(timMe)3}(CO)3] (5)
present very strong bands in the range 1860–2020
cm−1, which were attributed to the w(CꢀO) stretching
mode. The IR spectra of compound 5 display two
bands associated with the E1 and A1 vibration modes,
3.3. Molecular structures of complexes 3–5
The structures of complexes [Re{k3-R(m-H)B-
(timMe)2}(CO)3] (R=Me (3), Ph (4)) and [Re{k3-HB-
(timMe)3}(CO)3] (5) consist of discrete mononuclear
units with the rhenium atom in a slightly distorted
octahedral environment. Complex 5 possesses a crystal-
lographically imposed threefold rotation symmetry,
with the Re and the BꢀH atoms located in the crystallo-
graphic axis. There are two molecules per asymmetric
unit which are crystallographically independent but
chemically equivalent. The ORTEP views of the molecu-
lar structures are shown in Figs. 1–3. Fig. 4 presents a
Fig. 1. ORTEP view of 3. Vibrational ellipsoids are drawn at the 40%
probability level.

46
R. Garcia et al. / Journal of Organometallic Chemistry 632 (2001) 41–48
of [HB(timMe)3]− due to the presence of a third mercap-
toimidazolyl ring. The ReꢀS distances found for 3–5
are in the range reported for Re(I) tricarbonyl com-
plexes with chelating or unidentate thiourea derivatives
,
(2.466–2.536 A) also coordinated through thione sul-
phur atoms [18a]. The CꢀS bond distances span from
,
1.68 to 1.73 A, with average values of 1.704(7), 1.71(2)
,
and 1.723(8) A for 3, 4 and 5, respectively. These CꢀS
lengths are intermediate between single and double
bonds and indicate a partial reduction of the p-charac-
ter of the CꢀS bond if compared with the same bond
Fig. 2. ORTEP view of 4. Vibrational ellipsoids are drawn at the 30%
probability level.
Fig. 4. ORTEP view of the two independent molecules of 5, down the
c axis (along the BꢀRe axis). Vibrational ellipsoids are drawn at the
20% probability level.
Table 2
,
Selected bond lengths (A) and angles (°) for 3 and 4
Fig. 3. ORTEP view [14b] of molecule 1 of 5. Vibrational ellipsoids are
drawn at the 30% probability level.
3
4
ReꢀC(1)
ReꢀC(2)
ReꢀC(3)
ReꢀS(1)
ReꢀS(2)
ReꢀB
C(1)ꢀO(1)
C(2)ꢀO(2)
C(3)ꢀO(3)
C(4)ꢀS(1)
C(8)ꢀS(2)
1.878(7)
1.906(10)
1.921(8)
2.478(2)
2.491(2)
2.908(8)
1.162(8)
1.148(9)
1.149(8)
1.711(7)
1.696(7)
1.89(2)
1.87(2)
1.92(2)
2.493(5)
2.462(6)
2.92(2)
1.16(2)
1.18(2)
1.16(2)
1.73(2)
1.68(2)
BꢀRe view of the two independent molecules of 5.
Selected bond distances and angles are given in Tables
2 and 3, respectively.
The carbonyl ligands occupy one face of the coordi-
nation polyhedra, with average ReꢀC distances of 1.902
,
(10), 1.89 (2) and 1.904(9) A for 3, 4 and 5, respectively.
,
These distances are in the range (1.89–1.94 A) found
for other Re(I) tricarbonyl complexes containing sul-
phur donor ligands, such as thioethers or thioureas
[3g,18]. The three remaining coordination positions are
occupied by two thione sulphur atoms and by the
hydrogen from the BꢀH···Re agostic interactions in the
case of 3 and 4, while for 5 those positions are defined
by the three thione sulphur atoms of the tripodal
ligand. The average ReꢀS bond distance found in com-
C(1)ꢀReꢀC(2)
C(1)ꢀReꢀC(3)
C(2)ꢀReꢀC(3)
C(1)ꢀReꢀS(1)
C(1)ꢀReꢀS(2)
C(2)ꢀReꢀS(1)
C(2)ꢀReꢀS(2)
C(3)ꢀReꢀS(1)
C(3)ꢀReꢀS(2)
S(1)ꢀReꢀS(2)
88.1(3)
89.6(3)
92.1(3)
93.3(2)
91.9(2)
177.7(2)
90.3(2)
89.7(3)
177.2(2)
87.85(8)
88.1(9)
89.1(10)
90.9(8)
92.2(7)
93.4(7)
177.6(6)
88.6(6)
91.6(6)
177.4(7)
88.9(2)
,
plex 5 (2.516(2) A) is larger than the values of 2.485(2)
,
and 2.478(6) A found in complexes 3 and 4, respec-
tively. This is justified by the greater steric requirements

R. Garcia et al. / Journal of Organometallic Chemistry 632 (2001) 41–48
47
Table 3
consequence of the increase in size (from six to eight) of
the chelate rings upon complexation. In [Re{k3-
HB(pz)3}(CO)3] the ligand bite angles are rather less
than 90° (avg. NꢀReꢀN: 81.0(7)), while in compound 5
these angles are close to 90° (avg. SꢀReꢀS: 91.15(7)°).
These data show the larger flexibility of the hydrotris-
(mercaptoimidazolyl)borate proportionate to a less dis-
torted octahedral coordination environment.
,
Selected bond lengths (A) and angles (°) for 5
Molecule 1
Re(1)ꢀC(1)
Re(1)ꢀB(1)
C(2)ꢀS(1)
C(1)ꢀRe(1)ꢀC(1) a
C(1)ꢀRe(1)ꢀS(1) b
S(1)ꢀRe(1)ꢀS(1) b
1.901(9)
4.20(1)
1.721(8)
Re(1)ꢀS(1)
C(1)ꢀO(1)
2.521(2)
1.146(10)
89.9(4)
88.7(3)
90.48(7)
C(1)ꢀRe(1)ꢀS(1)
C(1)ꢀRe(1)ꢀS(1) a
90.9(3)
178.4(3)
Molecule 2
Re(2)ꢀC(6)
Re(2)ꢀB(2)
C(7)ꢀS(2)
4. Concluding remarks
1.906(8)
4.20(1)
1.725(7)
Re(2)ꢀS(2)
C(6)ꢀO(2)
2.510(2)
1.145(9)
The soft poly(mercaptoimidazolyl)borates [H(R)B-
(timMe)2]− (R=H, Me (1), Ph (2)) and [HB(timMe)3]−
are remarkably stable ligands which are very efficient in
the stabilisation of complexes with the ‘fac-Re(CO)3’
moiety. We have demonstrated that the attachment of
alkyl or aryl substituent to the boron atom has no
notorious effect on the coordination behaviour of the
[H(R)B(timMe)2]− ligands, which coordinate to the
metal through a remarkably robust BꢀH···Re agostic
interaction, retained even in coordinating solvents. By
introducing a third mercaptoimidazolyl ring no agostic
interaction is formed and the [HB(timMe)3]− ligand
coordinates in a tripodal fashion through the three
thione sulphur atoms. Again, the resulting rhenium
tricarbonyl complex is quite resistant towards hydroly-
sis and aerial oxidation.
Our results indicate that poly(mercaptoimida-
zolyl)borates are expected to provide new building
blocks for the labelling of biomolecules, based on
organometallic complexes containing the donor sets
k3-(H,S2) or k3-(S3). Noticeably, these ligands can
provide easy control of the size and lipophilicity of the
complexes, which are very important characteristics in
their potential biomedical applications.
C(6)ꢀRe(2)ꢀC(6) a
90.9(3)
90.1(3)
91.82(7)
C(6)ꢀRe(2)ꢀS(2)
C(6)ꢀRe(2)ꢀS(2) a
177.8(3)
87.2(3)
C(6)ꢀReꢀS(2) b
S(2)ꢀRe(2)ꢀS(2) a
a Equivalent atoms generated by the symmetry operation −x+y,
−x, −z.
b Equivalent atoms generated by the symmetry operation −y,
x−y, z.
,
distance (1.685(2) A) in free 2-mercapto-1-methyl-imi-
dazole (timMeH) [19].
The structures of complexes 3 and 4 are analogous to
those
previously
described
for
[M{k3-H(m-
H)B(timMe)2}(CO)3] (M=99Tc, Re) [7], all presenting
3-centre–2-electron BꢀH···Re agostic interactions. The
,
B···Re distances of 2.91(1) (3) and 2.92(2) A (4) are
comparable and slightly longer than the B···M (M=
99
,
Tc, Re) distances of 2.834(12) and 2.832(12) A found
in [M{k3-H(m-H)B(Simz)2}(CO)3] (M=99Tc, Re) [7]. If
this difference should arise from electronic factors we
would expect a stronger interaction in 3 but a weaker
one in 4, due to different inductive effects of the methyl
(−I) and phenyl (+I) groups. Apparently, these dif-
ferences reflect mainly the greater steric requirements of
the uncoordinated methyl and phenyl groups in com-
parison with the uncoordinated hydrogen atom.
5. Supplementary material
The analogy between the complex [Re{k3-
HB(timMe)3}(CO)3] (5) and the previously described
[Re{k3-HB(pz)3}(CO)3] gives the opportunity of a direct
comparison between these tripodal ligands. The most
striking difference is that the pyrazole rings in the latter
are almost parallel to the ReꢀB axis giving approxi-
mately C36 symmetry, while in 5 the mercaptoimida-
zolyl rings lie at an average angle of 60.3° to the BꢀRe
axis, resulting in a lower C3 symmetry. Additionally, in
complex 5 the eight-membered chelating rings are
highly asymmetric, presenting two very different
ReꢀSꢀC and CꢀNꢀB angles (avg. values of 107.7(2) and
132.6(7)°). The ligand adopts a ‘propeller-like’ confor-
mation of the rings with an average ReꢀSꢀCꢀN torsion
angle of 73° (62.5° for the imaginary torsion angle
SꢀRe···BꢀN The ReꢀB distance found in 5 (avg. 4.20(1)
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC nos 157756, 157757 and 157758
for compounds 3, 4 and 5, respectively. Copies of this
information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (Fax: +44-1223-336033; e-mail: de-
posit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.
ac.uk).
Acknowledgements
This work has been partially supported by PRAXIS/
P/QUI/10047/98 and by the Commission of the Eu-
ropean Communities (COST action B12). R.G. thanks
the FCT for a BIC research grant.
,
A) is considerably longer than that reported for the
,
hydrotris(pyrazolyl)borate complex (3.36(3) A), as a

48
R. Garcia et al. / Journal of Organometallic Chemistry 632 (2001) 41–48
(j) C. Kimblin, B.M. Hascall, T. Bridgewater, G. Parkin, J.
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