Synthesis and characterization of oxorhenium-'3 + 1' mixed-thiolate complexes. Crystal and molecular structures of [ReO{η3-(SCH2CH2)2S}(C 6H4X-4-CH2S)] (X = F, Cl, Br, OMe) and o

Article abstract of DOI:10.1016/s0020-1693(98)00296-5

The reaction of [(n-C₄H₉)₄N][ReOBr₄(OPPh ₃)] with bis(2-mercaptoethyl) sulfide produces [ReOBr{η³-(SCH₂CH₂)₂S}] (1), which can be reacted in turn to replace

Full text of DOI:10.1016/s0020-1693(98)00296-5

Inorganica Chimica Acta 284 (1999) 252±257
Synthesis and characterization of oxorhenium-`3 ‡ 1' mixed-thiolate
complexes. Crystal and molecular structures of
[ReO{h3-(SCH2CH2)2S}(C6H4X-4-CH2S)] (X ˆ F, Cl, Br, OMe)
and of the pendant thiolate compounds [ReO{h3-(SCH2CH2)2S}-
(h1-SCH2CH2SCH2CH2SH)] and [ReO{h3-(SCH2CH2)2S}-
{h1-SCH2CH(OH)CH(OH)CH2SH}]
Kevin P. Marescaa, Grant H. Bonaviaa, John W. Babichb, Jon Zubietaa,*
a Department of Chemistry, Syracuse University, Syracuse, NY 13244-4100, USA
b Department of Radiology, Massachusetts General Hospital, Boston, MA 02114, USA
Received 12 May 1998; accepted 6 July 1998
Abstract
The reaction of [(n-C4H9)4N][ReOBr4(OPPh3)] with bis(2-mercaptoethyl) sul®de produces [ReOBr{h3-(SCH2CH2)2S}] (1), which can
be reacted in turn to replace the labile bromine atom with various monodentate thiolate ligands. The reactions of [ReOBr{h3-
(SCH2CH2)2S}] (1) with (C6H4X-4-CH2S) where X ˆ Br, Cl, F and OMe in alcohol treated with triethylamine has led to the isolation of a
series of rhenium complexes of the type [ReO{(SCH2CH2)2S}(C6H4X-4-CH2S)]. The reaction of [ReOCl3(DMS) (OPPh3)] with 2-
mercaptoethyl sul®de and base results in the isolation of [ReO{h3-(SCH2CH2)2S}(h1-SCH2CH2SCH2CH2SH)] (6) while the reaction of 1
with dithioerythritol and triethylamine results in the formation of [ReO{h3-(SCH2CH2)2S}{h1-SCH2CH(OH)CH(OH)CH2SH}] (7),
unusual examples of `3 ‡ 1' coordination chemistry with a pendant thiol arm. The X-ray structures of [ReO{h3-(SCH2CH2)2S}(C6H4X-4-
CH2S)] (X ˆ Br (2), Cl (3), F (4), and OMe (5)), [ReO(h3-(SCH2(CH2)2S}{h1-SCH2CH2SCH2CH2SH)] (6) and [ReO{h3-
(SCH2CH2)2S}{h1-SCH2CH(OH)CH2CH(OH)CH2SH}] (7) have been determined. # 1999 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Rhenium complexes; Oxo complexes; Thiolate complexes
1. Introduction
size oxo-metal±mixed-thiolate complexes is important for
the systematic exploration of the size, shape, molecular
weight, and charge of the metal-thiolate complexes with
respect to the biodistribution patterns exhibited. However,
the inherent complexities of thiolate chemistry have
imposed limitations on the synthetic manipulation of the
organic thiolates, as well as on the consistency of preparing
the metal complexes. In an attempt to overcome such
dif®culties, the `3 ‡ 1' concept of ligand addition to a Group
VII metal-oxo species has been recently developed to
explore the chemistry of such complexes [10±12]. The
concept is based on creating a tridentate `3' and a mono-
dentate `1' mixed-thiolate core. The technique uses triden-
tate dithiolates ( S±S±S ) with a 2-charge in combination
with a monothiol ligand of 1-charge disposed about a Group
VII metal-oxo core, so as to occupy four coordination sites
on the metal with a total formal charge of 3. The `3 ‡ 1'
Thiolate ligands have been extensively explored in the
development of both technetium and rhenium coordination
chemistry with radiopharmaceutical applications [1±5]. The
nuclear properties of 99mTc (t1/2 ˆ 6 h, ꢀ ˆ 140 keV) are
ideal for diagnostic imaging, while the b-emitting isotopes
186Re and 188Re (t1/2 ˆ 90.64 h, Emax ˆ 1.1 MeV; t1/2
ˆ
17 h, Emax ˆ 2.1 MeV, respectively) are promising can-
didates for therapeutic applications [6]. In the speci®c case
of rhenium coordination chemistry, the focus has been on
polydentate ligands with sulfur and nitrogen donors [7±9]. In
an attempt to design radiopharmaceutical agents incorpor-
ating thiols as coligands, the ability to consistently synthe-
*Corresponding author. Tel.: +1-315-443-2925; fax: +1-315-443-4070.
0020-1693/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(98)00296-5

K.P. Maresca et al. / Inorganica Chimica Acta 284 (1999) 252±257
253
ligand combination preserves the oxometal core and the
2.2. Synthesis
formal ‡5 oxidation state of the metal, thus conforming to
the metal coordination and oxidation preferences. Recently,
Megalla and others derivatized a tropane ring with a term-
inal-free thiol in a `3‡1' 99mTc coordination regime [13],
allowing the preparation of numerous derivatives with a
range of biodistribution properties [14].
2.2.1. Preparation of [(n-C4H9)4N][ReOBr4(OPPh3)] [16]
All steps were performed open to the atmosphere. To a
solution of PPh3 (2.12 g, 8.10 mmol) in 20 ml of CH2Cl2,
cooled in a water bath was added dropwise, via pipette, a
solution of Br2 (0.410 ml, 8.10 mmol) in 20 ml of CH2Cl2.
The orange±red phosphorane solution (Ph3PBr2) which was
produced was then stirred for 2 h before being added to a
colorless solution of [(n-C4H9)4N][ReO4] (1.00 g,
8.10 mmol) in 20 ml of CH2Cl2. To this dark reaction
mixture were added 100 ml of ether, 1.50 ml of cyclohex-
ene, and 10 ml of pentane, resulting in precipitation of a
pink solid. The product was ®ltered and air-dried (yield:
1.775 g, 22%). Anal. Calc. for C52H66NO2P2ReBr4 (mol.
wt. 1303.74): C, 47.9; H, 5.06; N, 1.07. Found: C, 47.6; H,
5.56; N, 1.37%. IR (KBr, cm 1): 2961 (m), 2872 (m), 1458
(s), 1438 (s), 1146 (s), 1120 (s), 1090 (m), 994 (s), 932 (m),
878 (m), 724 (m), 693 (m), 537 (s).
In an effort to expand `3 ‡ 1' chemistry to the oxorhe-
nium core, our work has concentrated on the reactions of the
potentially tridentate thiol (HSCH2CH2)2S with the recently
described [(n-C4H9)4N][ReOBr4(OPPh3)], as well as a
related oxorhenium halide derivative, [ReOCl3(DMS)-
(OPPh3)], so as to prepare families of compounds with
mixed thiol ligands, starting with the systematic exploration
of the para-substituted benzylmercaptan series. This study
continues our development of the coordination chemistry of
the Group VII metal rhenium with thiol-containing ligands
in order to provide new materials for the radiolabeling of
chemotactic peptides [15]. We report the synthesis and
structural characterization of a series of oxorhenium
`3 ‡ 1' thiolate complexes [ReO{h3-(SCH2CH2)2S}-
(C6H4X-4-CH2S)] (X ˆ Br (2), Cl (3), F (4), OMe (5)),
as well as `3 ‡ 1' thiolate species with pendant donor sites
[ReO{h3-(SCH2CH2)2S}(h1-SCH2CH2SCH2CH2SH)] (6)
and [ReO{h3-(SCH2CH2)2S}{h1-SCH2CH(OH)CH(OH)-
CH2SH}] (7).
2.2.2. Preparation of [ReOBr{ꢂ3-SCH2CH2}2S] (1)
A solution of [(n-C4H9)4N][ReOBr4(OPPh3)] (0.0500 g,
0.0493 mmol) in a mixture of 10 ml of chloroform and 1 ml
of methanol was placed in an ice bath. To this solution was
added dropwise
a 5 ml chloroform solution of 2-
(SCH2CH2)2S (0.0068 g, 0.0444 mmol). The solution color
changed from pink to gray±green. The solution was stirred
for 30 min and then evaporated to a blue±green residue,
which was treated with 1 ml of chloroform. The mixture was
®ltered and the blue precipitate was allowed to air dry
(yield: 0.016 g, 82%). IR (KBr, cm 1): 1654 (m), 1541
(s), 1522 (m), 1508 (w), 1421 (m), 1269 (w), 964 (s), 847
(w). 1H NMR (CD3CN, ppm): 2.51 (m, 2H), 3.21 (m, 2H),
4.07 (m, 2H), 4.15 (m, 2H).
2. Experimental
2.1. General considerations
NMR spectra were recorded on a Bruker DPX 300 (1H
300.10 MHz) spectrometer in CD2Cl2 (ꢁ 5.32) or CD3CN
(ꢁ 1.93). IR spectra were recorded as KBr pellets with a
Perkin-Elmer Series 1600 FTIR. Electrochemical experi-
ments were performed by using a BAS CV-27 Cyclic
Voltammograph and a BAS Model RXY recorder. A
three-compartment, gas adapted cell was used with a glassy
carbon working electrode, a platinum wire auxiliary elec-
trode and a silver/silver chloride reference electrode. Cyclic
voltammograms were recorded under N2 in dry CH2Cl2
which was 0.1 M in supporting electrolyte (tetrabutylam-
monium hexa¯uorophosphate [TBAP]) and 1 Â 10 3 M
sample. Elemental analysis for carbon, hydrogen, and
nitrogen were carried out by Oneida Research Services,
Whitesboro, NY. Ammonium perrhenate (Aldrich), 2-mer-
captoethyl sul®de (Aldrich), 4-chlorobenzylmercaptan
(Lancaster), 4-¯ourobenzylmercaptan (Lancaster), 4-meth-
oxybenzylmercaptan (Lancaster), dithiothreitol (Aldrich),
and triethylamine (Aldrich) were used as received without
further puri®cation. The 4-bromobenzylmercaptan was a
gift from Massachusetts General Hospital. The [(n-
C4H9)4N][ReOBr4(OPPh3)] [16] was synthesized by
published procedures as was the [ReOCl3(DMS)(OPPh3)]
[17].
2.2.3. General procedure for the preparation of [ReO{(ꢂ3-
SCH2CH2)2S}(C6H4X-4-CH2S)] where X ˆ Br (2),
Cl (3), F (4) and OMe (5)
To
a re¯uxing solution of [ReOBr(SCH2CH2)2S]
(0.010 g, 0.0230 mmol) in 5 ml of acetonitrile was added
dropwise a solution of ®ve equivalents of the 4-halobenzyl
mercaptan in 1 ml of acetonitrile. The solution remained
blue until the addition of NEt3 (0.020 g, 0.198 mmol)
whereupon the reaction mixture immediately changed from
blue to orange±brown. The solution was stirred and re¯uxed
for an additional 15 min and then evaporated to dryness. The
brown residue was washed with 2 ml of methanol to give an
orange±brown precipitate which was ®ltered and allowed to
air dry. X-ray suitable crystals were grown by slow diffusion
of pentane into a solution of the compound in CH2Cl2.
2.2.3.1. [ReO{(ꢂ3-SCH2CH2)2S}(C6H4Br-4-CH2S)] (2).
(Yield: 0.005 g, 39%). IR (KBr, cm 1): 1560 (w), 1508
1
(m), 1248 (s), 1184 (m), 1032 (m), 964 (s), 830 (w). H
NMR (CD2Cl2, ppm): 2.77 (m), 3.20 (m), 3.85 (m), 4.21 (m)
4.93 (s), 6.80 (dd), 7.1±7.5 (mm).

254
K.P. Maresca et al. / Inorganica Chimica Acta 284 (1999) 252±257
2.2.3.2. [ReO{(ꢂ3-SCH2CH2)2S}(C6H4Cl-4-CH2S)] (3).
(Yield: 0.006 g, 51%). Calc. for C11H14OS4ClRe (mol.
wt. 512.11): C, 25.80; H, 2.74. Found: C, 25.94; H,
2.90%. IR (KBr, cm 1): 1508 (m), 1489 (m), 1421 (w),
1247 (s), 1185 (m), 1091 (m), 1016 (w), 964 (s), 836 (m). 1H
NMR (CD2Cl2, ppm): 2.75 (m, 2H), 3.14 (m, 2H), 3.90 (m,
2H), 4.25 (m, 2H), 4.95 (s, 2H), 6.80 (dd, 1H), 7.13 (dd, 1H),
crystals were grown by slow diffusion of pentane into a
solution of 7 in CH2Cl2. IR (KBr, cm 1): 3463 (w), 2678
(w), 1640 (m), 1429 (m), 1032 (w), 959 (s), 855 (w).
2.3. X-ray crystallography
The crystals of 27 were studied using a Siemens SMART
diffractometer system with graphite-monochromated Mo
7.27 (dd, 1H), 7.40 (dd, 1H). Cyclic voltammetry (E1/2
CH2Cl2): ‡1.58 V, 1.19 V (ÁEp ˆ 110 mV).
,
Ê
Kꢃ radiation (ꢄ (Mo Kꢃ) ˆ 0.71073 A). All the data col-
lections were carried out at 150 K except for the collection
of 5 which was carried out under ambient conditions. The
crystal parameters and other experimental details of the data
collections are summarized in Table 1. A complete descrip-
tion of the details of the crystallographic methods is given in
Section 5. The structures were solved by direct methods
[18]. Neutral atom scattering factors were taken from
Cromer and Waber [19] and anomalous dispersion correc-
tions were taken from those of Creagh and McAuley [20].
All calculations were performed using SHELXTL [21]. Non-
hydrogen atoms were re®ned anisotropically. No anomalies
were encountered in the re®nements of any of the structures.
2.2.3.3. [ReO{(ꢂ3-SCH2CH2)2S}(C6H4F-4-CH2S)] (4).
(Yield: 0.006 g, 44%). Calc. for C11H14OFS4Re (mol. wt.
495.70): C, 26.66; H, 2.83. Found: C, 27.03; H, 3.07%. IR
(KBr, cm 1): 1623 (m), 1508 (m), 1421 (m), 1222 (s), 1218
1
(w), 1083 (s), 963 (s). H NMR (CD2Cl2, ppm): 2.83 (m),
3.29 (m), 4.07 (m), 4.44 (m), 5.09 (s), 6.98 (m), 7.14 (m),
7.25 (m), 7.54 (m). Cyclic voltammetry (E1/2, CH2Cl2):
‡1.58 V, 1.16 V (ÁEp ˆ 90 mV).
2.2.3.4. [ReO{(ꢂ3-SCH2CH2)2S}(C6H4OMe-4-CH2S)] (5).
(Yield: 0.007 g, 60%). X-ray suitable crystals were grown
by slow diffusion of pentane into a solution of 5 in CH2Cl2
whereupon it lost the CH3OH. Calc. for C13H22O3S4Re
(mol. wt. 539.7): C, 30.30; H, 4.16. Found: C, 29.69; H,
3.71%. IR (KBr, cm 1): 1608 (m), 1512 (s), 1420 (w), 1303
3. Results and discussion
1
3.1. Synthesis and general properties
(w), 1257 (s), 1176 (m), 1036 (m), 960 (s), 830 (m). H
NMR (CD2Cl2, ppm): 3.12 (m), 3.61 (s), 3.78 (m), 3.93 (m),
4.30 (m), 4.93 (s), 6.83 (m), 6.86 (m), 7.19 (m), 7.34 (m).
Cyclic voltammetry (E1/2, CH2Cl2): ‡1.56 V, 1.22 V
(ÁEp ˆ 110 mV).
The choice of [n-(C4H9)4N][ReOBr4(OPPh3)] as starting
material was predicated on its potential for clinical applica-
tions. The more commonly employed oxorhenium (V)-
halide starting material ReOCl4 is extremely moisture
sensitive and thus an unlikely starting material for radio-
pharmaceutical applications. Although the synthesis of the
[n-(C4H9)4N][ReOBr4(OPPh3)] requires several steps it is
facile and high yield. The IR spectrum of [n-
(C4H9)4N][ReOBr4(OPPh3)] displays ꢅ(C±H) for the
2.2.4. Preparation of [ReO{(ꢂ3-SCH2CH2)2S}-
(ꢂ1-SCH2CH2SCH2CH2SH)] (6)
To a re¯uxing solution of [ReOCl3(DMS)(OPPh3)]
(0.050 g, 0.0765 mmol) in 30 ml of ethanol was added
dropwise 2-mercaptoethyl sul®de (0.035 g, 0.229 mmol)
whereupon the reaction mixture was stirred and re¯uxed
for 1 h and then evaporated to orange±brown residue. Red,
X-ray suitable crystals of 6 were grown by slow diffusion of
pentane into a solution of the residue in CH2Cl2 (yield:
0.003 g, 10%). IR (KBr, cm 1): 3433 (m), 1654 (w), 1438
(s), 1122 (s), 962 (s), 727 (m), 693 (m), 539 (s).
‡
[Bu4N] cation at 2961 and 2872 cm 1, ꢅ(C±C) at
1466 cm 1, and a strong band assigned to ꢅ(Re=O) at
993 cm 1. The pink solid was inde®nitely air and moisture
stable making it suitable for probing the reactivities of a
variety of ligand types.
The synthesis of [ReOBr{(SCH2CH2)2S}] (1) employs
the route of Fietz et al. [11], but instead of using ReOCl4
the less moisture sensitive [ReOBr4(OPPh3)] was used.
The starting material was placed in an ice-bath to prevent
possible dimerization. The tridentate bis(2-mercaptoethyl)
sul®de was added dropwise in 5 ml of chloroform over a
period of 20 min. The gray±blue solution was stirred in the
ice-bath for an additional 30 min at which time it was
vacuumed down to residue. To the residue was added
1 ml of chloroform which produced [ReOBr{(SCH2-
CH2)2S}] (1) as a blue precipitate. The IR spectrum of 1
2.2.5. Preparation of [ReO{(ꢂ3-SCH2CH2)2S}-
{ꢂ1-SCH2CH(OH)CH(OH)CH2SH}] (7)
To
a re¯uxing solution of [ReOBr(SCH2CH2)2S]
(0.010 g, 0.0230 mmol) in 5 ml of acetonitrile was added
dropwise a solution of dithiothreitol (0.004 g, 0.0250 mmol)
in 1 ml of acetonitrile. The solution remained blue until the
addition of NEt3 (0.009 g, 0.092 mmol) whereupon the
reaction mixture immediately changed from blue to
orange±brown. The solution was stirred and re¯uxed for
an additional 15 min and then evaporated to residue. The
brown residue was washed with 2 ml of ethanol which
produced an orange precipitate which was ®ltered and
allowed to air dry (yield: 0.005 g, 43%). X-ray suitable
1
is dominated by a strong peak at 964 cm attributed to
1
ꢅ(Re=O). The H NMR spectrum contains multiplets at
2.51, 3.21, 4.07 and 4.15 ppm assignable to each of the CH2
moiety comprising backbone of the ligand. All four of the

K.P. Maresca et al. / Inorganica Chimica Acta 284 (1999) 252±257
255
Table 1
Summary of the crystallographic data for the compounds [ReO{h3-(SCH2CH2)2S}(C6H4Br-4-CH2S)] (2), [ReO{h3-(SCH2CH2)2S}(C6H4Cl-4-CH2S)] (3),
[ReO{h3-(SCH2CH2)2S}(C6H4F-4-CH2S)] (4), [ReO{h3-(SCH2CH2)2S}(C6H4OMe-4-CH2S)] (5), [ReO{h3-(SCH2CH2)2S}(h1-SCH2CH2SCH2CH2SH)] (6)
and [ReO{h3-(SCH2CH2)2S}{h1-SCH2CH(OH)CH(OH)CH2SH}] (7)
2
3
4
5
6
7
Chemical formula
Ê
a (A)
Ê
b (A)
C
11H14OS4BrRe
C11H14OS4ClRe
7.3462(2)
C11H14OFS4Re
10.9845(6)
C12H17O2S4Re
7.5461(1)
C8H16OS6Re
21.650(1)
8.0836(4)
18.0528(8)
90
C8H14O3S2Re
9.5362(4)
11.6653(4)
26.809(1)
90
7.3160(1)
24.6003(4)
9.452(1)
19.9973(3)
10.6182(2)
13.9134(7)
10.3565(5)
22.1678(1)
9.439(1)
Ê
c (A)
ꢃ (8)
ꢆ (8)
ꢀ (8)
111.252(1)
95.286(1)
113.41(1)
109.102(1)
90
90
90
90
Ê 3
V (A )
1585.48(1)
4
1553.22(6)
4
1452.5(1)
4
1492.03(1)
4
3159.5(1)
8
2982.3(1)
8
Z
FW
Space group
T (K)
556.57
P21/n
512.11
P21/n
495.66
P21/c
507.70
P21/n
506.77
Pca21
504.69
Pcab
150(1)
0.71073
2.332
150(1)
0.71073
2.190
150(1)
0.71073
2.267
293(2)
0.71073
2.260
150(1)
0.71073
2.131
150(1)
0.71073
2.248
Ê
ꢄ (A)
1
)
Dcalc (g cm
1
ꢇ (mm
Ra
)
10.696
0.0464
0.1169
8.517
8.936
8.697
8.464
8.841
0.0312
0.0606
0.0473
0.1225
0.0880
0.2311
0.0375
0.1003
0.0996
0.2482
b
wR2
aÆ|Fo| |Fc||/Æ|Fo|.
2
b[Æ[w(Fo Fc2)2]/Æ[w(Fo2)2]]1/2
.
multiplets integrate to two protons, however, individual
assignments of the resonances is tenuous at best due to
the multiplicity of the peaks and the ¯exibility of the ligand
in solution, as noted by others [22]. As one might expect the
signals of the tridentate ligand are very sensitive to the
identities of the ligands used to replace the bromine atom.
The syntheses of compounds [ReO{h3-(SCH2CH2SCH2-
CH2S)}(h1-C6H4X-4-CH2S)], where X ˆ Br (2), Cl (3), F
(4) and OMe (5) were straightforward once the optimal
conditions were realized for the substitution of Br in
[ReOBr{(SCH2CH2)2S}] with the appropriate thiolates.
Reactions in acetonitrile of a mixture of [ReOBr{(SCH2-
CH2)2S}] with thiol in a 1:0.9 mol ratio, followed by
addition of excess triethylamine, resulted in a color change
from the characteristic blue of the starting material to
orange±brown. After stripping the solvent and washing
the residues with ethanol, air-stable orange±brown precipi-
tates in yields of 40±50% were isolated.
The cyclic voltammograms of compounds 3±5 display an
irreversible oxidation at approximately 1.57 V and a
pseudo-reversible one electron reduction in the region of
1.20 V, with ÁEp ranging from 90 to 110 mV, attributed to
the reduction of the neutral [ReO{(SCH2CH2)2S}(C6H4X-
4-CH2S)] complex to the Re (IV) species [ReO{(SCH2-
CH2)2S}(C6H4X-4-CH2S)]1 . The narrow range of reduc-
tion potentials make any direct comparisons between the
electronic effect of the para-substituent and ease of reduc-
tion tenuous at best.
Replacing the oxorhenium(V)±halide starting material
[Bu4N][ReOBr4(OPPh3)] with [ReOCl3(DMS)(OPPh3)]
led to the isolation of the unusual `3 ‡ 1' compound
[ReO{(SCH2CH2)2S}(h1-SCH2CH2SCH2CH2SH)] (6) as
a minor product of the reaction between [ReOCl3(DMS)-
(OPPh3)] and three equivalents of bis(2-mercaptoethyl)
sul®de. The IR spectrum of 6 exhibited a strong peak at
1
962 cm attributable to ꢅ(Re=O). The analogous `3 ‡ 1'
The infrared spectra of the products 25 all displayed
characteristically sharp ꢅ(Re=O) stretches at approximately
964 cm 1. The remainder of the spectra were dominated by
pendant thiolate [ReO{h3-(SCH2CH2)2S}{h1-SCH2CH-
(OH)CH(OH)CH2SH}] (7) was prepared in a similar man-
ner from [ReOBr{h3-(SCH2CH2)2S}] (1). The IR spectrum
1
1
ꢅ(C=C) stretching between 1400±1600 cm . The 1H NMR
of 7 displays a strong ꢅ(Re=O) stretch at 959 cm
.
spectra displayed resonances assigned to the CH2 backbone
of the tridentate ligand at approximately 2.10, 2.75, 3.15 and
3.90 ppm with integration con®rming that the peaks were
attributable to two protons. The monodentate thiolates
displayed methylene protons at approximately 5.00 ppm
as a single peak, as anticipated from Sadtler spectra [23],
as well as the aromatic protons in the range of 6.80±
3.2. Description of the structures
The compounds [ReO{h3-(SCH2CH2)2S}(C6H4X-4-
CH2S)] (X ˆ Br (2), Cl (3), F (4) and OMe (5)) are
structurally identical with the exception of para-substituent
of the mercaptobenzyl group. The core geometry of the ®ve-
coordinate oxorhenium±thiolate complexes is shown in
Fig. 1 for [ReO{h3-(SCH2CH2)2S}(C6H4Cl-4-CH2S)] (3)
and seen to consist of distorted square pyramidal geometry.
The distorted ®ve-coordinate geometry may be quanti®ed
1
7.40 ppm. The H NMR spectrum of [ReO{(SCH2CH2)2-
S}(C6H4OMe-4-CH2S)] (5) exhibited an additional singlet
at 3.61 ppm attributed to the three protons of the methoxy
group, as expected.

256
K.P. Maresca et al. / Inorganica Chimica Acta 284 (1999) 252±257
Table 3
Comparison of selected bond lengthsa for rhenium-mixed thiolate
complexes
Complex
Re=O
h1-thiolate h3-donor (S1±S2±S3)
Re±S4
Re±S1
Re±S2
Re±S3
2
3
4
5
6
7
1.695(5)
1.687(3)
1.696(5)
1.665(7)
1.699(8)
1.69(2)
2.310(2)
2.313(1)
2.318(2)
2.267(2)
2.315(2)
2.293(8)
2.309(2)
2.312(1)
2.309(2)
2.247(3)
2.307(2)
2.289(8)
2.373(2)
2.389(1)
2.382(2)
2.341(2)
2.382(2)
2.384(8)
2.311(2)
2.301(1)
2.299(2)
2.250(3)
2.315(2)
2.299(7)
Fig. 1. A view of the structure of 3, showing the atom-labeling scheme
and 50% thermal ellipsoids.
a
Ê
Distances reported in A with e.s.d.s in parentheses.
using the ꢈ index described by Addison [24]. The measure-
ment uses the two largest angles contained in the basal
plane, which expressed in the form (ꢆ ꢃ)/60 give a
unitless value ranging from 0 to 1, with a value 0 for the
square pyramidal limit and a value of 1 for the idealized
trigonal bipyramid. The ꢈ indices for the rhenium mixed
thiolate compounds of this study are listed in Table 2. The
nature of the ligands, speci®cally the inability of the tri-
dentate ligands to occupy the idealized equatorial positions
while sterically blocking the sixth potential coordination
position, as well as the strong trans in¯uence of the terminal
oxo-group, constrain the complexes towards the square
pyramidal geometry.
Fig. 2. A view of the structure of 6, showing the atom-labeling scheme
and 50% thermal ellipsoids.
The apical position is occupied by a terminal oxo-group
Ê
with typical Re=O distances ranging from ca. 1.67±1.70 A.
The four equatorial positions are occupied by the three
sulfur donor atoms of the bis(2-mercaptoethyl) sul®de
and the lone sulfur donor of the derivatized mercaptobenzyl
ligands. The rhenium is displaced from the basal plane in the
direction of the apical oxygen. The sulfur distances are all
The structure of 6, shown in Fig. 2, has similar metrical
parameters to the other `3 ‡ 1' complexes. The bis(2-mer-
captoethyl) su®de ligand adopts two different binding
modes: one is the tridentate binding mode, with the sul®de
Ê
donor (S2) slightly elongated at 2.382(2) A, while the other
Ê
fairly unexceptional ranging from 2.247±2.389 A, well
ligates through a single sulfur. The ®fth position is occupied
by the terminal oxo-group. The terminal thiol sulfur of the
pendant, monodentate ligand is protonated. While this
projecting thiol group suggests a method for bridging metal
centers into unique binuclear bridged complexes, this attrac-
tive chemistry has yet to be realized.
The analogous `3 ‡ 1' pendant thiolate complex
[ReO{h3-(SCH2CH2)2S}{h1-SCH2CH(OH)CH(OH)CH2-
SH}] (7) incorporates dithiothrietol as the monodentate
ligand. The ®ve-coordinate rhenium core is similar to that
of 2±6. Although the quality of X-ray data did not directly
reveal the hydrogen positions, the dithiothrietol must be
protonated at O2, O3, and S5 to maintain charge balance for
this neutral Re(V) complex.
within the range of those previously reported [25±27].
The sul®de donor (S2) of the h3-bis(2-mercaptoethyl) sul-
®de ligands gives consistently the longest Re±S distances,
an observation consistent with general trends for sul®de
(R2S) versus thiolate (RS) coordination. Comparisons of the
relevant bond lengths for compounds 2±7 are presented in
Table 3.
Table 2
ꢈ index for the five-coordinate rhenium complexes of this study, as a
measure of geometric distortion from idealized geometries
Compound
ꢃa
ꢆb
ꢈc
[ReO(S±S±S)(C6H4Br-4-CH2S)] (2)
[ReO(S±S±S)(C6H4Cl-4-CH2S)] (3)
[ReO(S±S±S)(C6H4F-4-CH2S)] (4)
[ReO(S±S±S)(C6H4OMe-4-CH2S)] (5)
[ReO(S±S±S)(SCH2CH2SCH2CH2SH)] (6)
134.1 149.3 0.253
131.1 153.1 0.367
129.8 153.3 0.392
130.4 153.9 0.392
133.0 150.6 0.293
4. Conclusions
Exploring the thiolate chemistry of the oxorhenium(V)
core has led to the isolation and characterization of a
series of rhenium-mixed thiolate complexes which are
directly analogous to 99mTc materials used in medicinal
applications. The syntheses employed the `3 ‡ 1' method
of preparing oxorhenium(V)±thiolate complexes using
[ReO(S±S±S){SCH2(CH(OH))2CH2SH}] (7) 129.0 154.4 0.423
aꢃ ꢀ the second largest basal angle, S1±Re±S3.
bꢆ ꢀ the largest basal angle, S2±Re±S4.
cꢈ ꢀ (ꢆ ꢃ)/60.
S±S±S ˆ h3-(SCH2CH2SCH2CH2S).

K.P. Maresca et al. / Inorganica Chimica Acta 284 (1999) 252±257
257
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[TBA][ReOBr4(OPPh3)] and [ReOCl3(DMS)(OPPh3)] as
the starting materials.
[6] J. Ehrhardt, A.R. Ketring, T.A. Turpin, M.S. Razavi, J.L.
Vanderheyden, F.M. Su, A.R. Fritzberg, in: M. Nicolini, G. Bandoli,
U. Mazzi (Eds.), Technetium and Rhenium in Chemistry and Nuclear
Medicine 3, Cortina International, Verona, 1990.
By an analogous method for that used by us for the
preparation of [ReOCl(SCH2CH2SCH2CH2S)], we have
synthesized [ReOBr(SCH2CH2SCH2CH2S)] (1) in good
yield for use in subsequent substitution reactions replacing
the labile bromine atom with monothiols so as to form
[ReO{h3-(SCH2CH2)2S}{C6H4X-4-CH2S}] (X ˆ Br (2),
Cl (3), F (4), OMe (5)). The unusual `3 ‡ 1' compound
[ReO{h3-(SCH2CH2)2S}{h1-SCH2CH2SCH2CH2SH}] (6)
was isolated as a minor product of the reaction between
[ReOCl3(DMS)(OPPh3)] and (HSCH2CH2)2S. Finally, an
analogous `pendant' thiolate [ReO{h3-(SCH2CH2)2S}{h1-
SCH2CH(OH)CH(OH)CH2SH}] (7) was synthesized using
[ReOBr(SCH2CH2SCH2CH2S)] (1) as the starting material.
This route appears to be a quite general, facile method for
high yield synthesis of materials with the {ReOS4} core.
The geometry and charge of the monodentate thiol ligand
may be easily manipulated to produce complexes of various
sizes, shapes and charges for clinical development.
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5. Supplementary material
Atomic positional parameters for the structures have been
deposited with the Cambridge Structural Database.
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Acknowledgements
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WI, 1990.
This work was supported by a grant from the Department
of Energy Of®ce of Health and Environmental Research
DE-FG02-93RG1571.
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