Oxorhenium mixed-ligand complexes with the 2,6-dimercaptomethylpyridine ligand. Crystal structure of [2,6-dimercaptomethylpyridinato][p-methoxy-benzenethiolato]oxorhenium(V)

Article abstract of DOI:10.1016/S0020-1693(00)00056-6

Two novel oxorhenium complexes containing the 2,6-dimercaptomethylpyridine ligand were synthesized and characterized by classical methods of analysis. The [2,6-dimercaptomethylpyridinato][p-methoxybenzenethiolato]oxorhenium complex 1, was produced by simultaneous action of equimolar quantities of 2,6-dimercaptomethylpyridine and p-methoxybenzenethiol on the precursor [(n-C₄H₉)₄N][ReOCl₄] in EtOH. As revealed by spectroscopic data as well as X-ray structure analysis, complex 1 adopts a distorted square pyramidal geometry around the metal with the SNS/S donors forming the basal plane and the oxygen occupying the apex of the pyramid. When the same tridentate ligand reacts with [(n-C₄H₉)₄N][ReOCl₃(PO)] (PO = o-diphenylphosphinophenolato) as a precursor, complex 2a, [2,6-dimercaptomethylpyridinato][o-diphenylphosphinophenolato]-ox-orhenium, is obtained. The latter is a six-coordinate rhenium species to which the distorted octahedral geometry is assigned, according to the analytical findings. In this case, the SNS/P donors occupy the equatorial plane and the two oxygen atoms the apices of the distorted octahedron positioned trans to each other. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(00)00056-6

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Inorganica Chimica Acta 304 (2000) 26–32
Oxorhenium mixed-ligand complexes with the
2,6-dimercaptomethylpyridine ligand. Crystal structure of
[2,6-dimercaptomethylpyridinato][p-methoxy-
benzenethiolato]oxorhenium(V)
Berthold Nock a, Hans-Ju¨rgen Pietzsch b, Francesco Tisato c, Theodosia Maina a,
Peter Leibnitz d, Hartmut Spies b, E. Chiotellis a,*
a Institute of Radioisotopes-Radiodiagnostic Products, National Centre for Scientific Research ‘Demokritos’, PO Box 60228, 15310 Ag. Paraske6i,
Athens, Greece
b Institut fu¨r Bioanorganische und Radiopharmazeutische Chemie, Forschungszentrum Rossendorf, PF 510119, D-01314 Dresden, Germany
c Istituto di Chimica e Tecnologie Inorganiche e dei Materiali A6anzati, Consiglio Nazionale delle Ricerche, Corso Stati Uniti 4,
35020 Padua, Italy
d Bundesanstalt fu¨r Materialforschung, Rudower Chaussee 5, D-12489 Berlin, Germany
Received 2 November 1999; accepted 22 December 1999
Abstract
Two novel oxorhenium complexes containing the 2,6-dimercaptomethylpyridine ligand were synthesized and characterized by
classical methods of analysis. The [2,6-dimercaptomethylpyridinato][p-methoxybenzenethiolato]oxorhenium complex 1, was
produced by simultaneous action of equimolar quantities of 2,6-dimercaptomethylpyridine and p-methoxybenzenethiol on the
precursor [(n-C4H9)4N][ReOCl4] in EtOH. As revealed by spectroscopic data as well as X-ray structure analysis, complex 1 adopts
a distorted square pyramidal geometry around the metal with the SNS/S donors forming the basal plane and the oxygen
occupying the apex of the pyramid. When the same tridentate ligand reacts with [(n-C4H9)4N][ReOCl3(PO)] (PO=o-
diphenylphosphinophenolato) as a precursor, complex 2a, [2,6-dimercaptomethylpyridinato][o-diphenylphosphinophenolato]-ox-
orhenium, is obtained. The latter is a six-coordinate rhenium species to which the distorted octahedral geometry is assigned,
according to the analytical findings. In this case, the SNS/P donors occupy the equatorial plane and the two oxygen atoms the
apices of the distorted octahedron positioned trans to each other. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Oxorhenium complexes; Mixed ligand complexes; Crystal structures
chemistries of these two Group 7 elements are quite
similar — at least as long as no redox reactions are
involved —, the design of novel 99mTc radiopharma-
ceuticals has been very often based on the development
and characterization of the rhenium analogs at macro-
scopic level [6]. Much effort has been dedicated so far
to monooxo [MO]3+ metal chelates (M=Tc, Re), lead-
ing to the development of classical radiopharmaceuti-
cals such as 99mTcꢀECD [7] and 99mTcꢀHMꢀPAO [8]
agents for brain imaging, or 99mTcꢀMAG3 for the study
of renal function [9]. However, the site-directed in vivo
imaging or radiotherapy by means of high specificity
low capacity target systems (receptors, enzymes etc.) is
1. Introduction
The recent spectacular growth of technetium and
rhenium chemistry reflects the continuously expanding
application of important radionuclides of these ele-
ments in Nuclear Medicine either for routine diagnostic
scintigraphic imaging (99mTc) or for cancer radiother-
apy (186/188Re) [1–6]. Due to the fact that only rhenium
possesses stable isotopes, but also because the
* Corresponding author. Tel.: +30-1-6513 793; fax: +30-1-6543
526.
E-mail address: mainathe@mail.demokritos.gr (E. Chiotellis)
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(00)00056-6

B. Nock et al. / Inorganica Chimica Acta 304 (2000) 26–32
27
through membrane filters (0.22 m, Millipore, Milford/
USA) and degassed by helium flux prior to use.
Column chromatography was performed on silica gel
packing material from Merck. Thin layer chromatog-
raphy (TLC) was conducted on 0.25 mm silica gel
coated aluminium F254 sheets from Merck. HPLC
analysis was performed on a Waters Chromatograph
efficient with a Waters 600 solvent delivery system and
coupled to a Waters 996 photodiode array UV detec-
tor. The Millennium Software by Waters was applied
for controlling the HPLC system and processing the
data. For analyses a Lichrospher 100 RP C18 (10 m,
4.0 mm×250 mm) column from Merck was eluted at
a flow rate of 1 ml min−1 MeOH and aqueous or
buffer mixtures of varying composition.
IR spectra were recorded on KBr pellets on a
Perkin–Elmer 1600 FTIR spectrophotometer in the
region 500–4000 cm−1 and using polystyrene for ref-
erence. Proton, 13C and 31P NMR spectra were
recorded on a Bruker AC-200 instrument, using SiMe4
as internal reference (for 1H and 13C) and 85%
aqueous H3PO4 as external reference (for 31P). Sam-
ples were dissolved in deuterated chloroform at a con-
centration of approximately 1–2%. Elemental analyses
for C, H, N and S were conducted on a Perkin–Elmer
2400/II automatic elemental analyzer.
a much more difficult task to accomplish [10,11]. As
the demand for new more sophisticated 99mTc (but
also 186/188Re) probes becomes more pressing, novel
chelating systems with improved characteristics for ei-
ther or both metal nuclides are steadily emerging [9–
12].
Our recent work has been largely focused on mixed
ligand systems for oxorhenium and oxotechnetium
containing primarily the SNS/S or the SSS/S donor
atom sets with potential application in the imaging of
brain perfusion. These are neutral and lipophilic metal
chelates adopting a distorted trigonal bipyramidal or
square pyramidal geometry [13–19]. Despite the fact
that several examples of such mixed ligand metal
chelates coupled to pharmacophore groups, like
tropane, ketanserin or o-methoxyphenylpiperazine,
have been recently proposed for the in vivo mapping
of brain receptors [20–24], complications related to
their in vivo instability prevent their effective applica-
tion for such purposes [25–27]. We have recently pro-
posed the use of mixed ligand complexes, wherein the
monothiolato coligand of the SNS/S ligand system has
been replaced by a bidentate o-diphenylphosphinophe-
nolato ligand. Thus, six-coordinate octahedral com-
plexes, instead of five-coordinate ones, form exhibiting
a closed coordination shell and, consequently, an in-
creased stability [28].
2.2. Synthesis of 2,6-dimercaptomethylpyridine ligand
We report herein on the synthesis and characteriza-
tion of two novel oxorhenium complexes containing
2,6-dimercaptomethylpyridine as the tridentate ligand.
This tridentate ligand, possessing an aromatic nitrogen
donor atom, is for the first time studied in relation to
the above mixed ligand systems. Thus, the complexes
[2,6 - dimercaptomethylpyridinato][p - methoxybenzene-
thiolato]oxorhenium, 1, and [2,6-dimercaptomethyl-
pyridinato][o - diphenylphosphinophenolato]oxorhen-
ium, 2, complexes are prepared and characterized.
Crystal structure analysis of complex 1 is also reported.
To a suspension of 2,6-pyridinedimethanol (14.4 g,
0.1 mmol) in Et2O (100 ml) at 0°C a solution of
SOCl2 (15.8 ml, 0.22 mmol) in Et2O (25 ml) was
added within 15 min. After completion of the addition
the mixture was allowed to reach ambient temperature
and was stirred overnight. The forming precipitate was
filtered off and rinsed with Et2O to afford 2,6-
dichloromethylpyridine hydrochloride (Yield: 21.68 g,
100%).
The chloride (5 g, 24 mmol) was reacted with
thiourea (4.3 g, 57 mmol) in refluxing EtOH (100 ml)
for 15 min. The solvent was expelled under vacuum
and the white residue redissolved in H2O (20 ml). This
solution was refluxed with NaOH (3.1 g, 78 mmol) for
2 h under an Ar atmosphere and the pH then adjusted
to 8 by addition of 1 N HCl. Extraction in CHCl3 and
evaporation of the solvent under vacuum were fol-
lowed by vacuum distillation affording 2,6-dimercap-
tomethylpyridine as a pale yellow oil.
2. Experimental
2.1. General
All chemicals were reagent grade and used as such
without prior purification. p-Methoxythiophenol and
2,6-pyridinedimethanol were purchased from Fluka.
The synthesis and characterization of o-diphenylphos-
phinophenol (POH) was performed according to pub-
lished protocols [29,30]. Rhenium was purchased from
Aldrich as KReO4 and was converted to [(n-
C4H9)4N][ReOCl4] according to published methods
[31]. Solvents for high performance liquid chromatog-
raphy (HPLC) were HPLC grade; they were filtered
Yield: 2.1 g, 50%; B.p.: 84–87°C (0.01 Torr);
HRMS (EI, 70 eV) Calc. for C7H9NS2: 171.0176.
1
Found: 171.0150; H NMR (200 MHz, CDCl3): 2.01
(2H, t, J=7.8 Hz, SꢀH), 3.82 (4H, d, J=7.8 Hz,
CH2SH), 7.20 (2H, d, J=7.7 Hz, 3-H), 7.62 (1H, t,
J=7.7 Hz, 4-H).

28
B. Nock et al. / Inorganica Chimica Acta 304 (2000) 26–32
MeOH the products were isolated as pure red and
2.3. Synthesis of Re(V)O complexes
yellow solids, respectively.
For 2a: Yield: 93.5 mg, 60%; Rf (SiO2; CH2Cl2): 0.10;
tR (Waters Symmetry Shield RP 18 column, 5 m, 150×
3.9 mm; 1 ml min−1; A: MeCN, B: 0.1% TFA; 1–10
min 50 to 80% A): 8.41 min; Anal. Calc. for
C25H21NO2PReS2: C, 46.29; H, 3.26; N, 2.16; S, 9.88.
Found: C, 46.32; H, 3.06; N, 2.32; S, 10.00; IR (KBr,
w/cm−1): 1583, 1456, 1320, 1275, 1097, 1023, 951 (s,
2.3.1. [2,6-Dimercaptomethylpyridinato]-
[p-methoxybenzenethiolato]oxorhenium(V) (1)
A solution of [(n-C4H9)4N][ReOCl4] (146.6 mg, 250
mmol) in EtOH (2 ml) was cooled to 0°C. At this
temperature a solution of 2,6-dimercaptomethylpyridin
(43 mg, 250 mmol) and p-methoxybenzenethiol (34 ml,
275 mmol) in CHCl3 was added under stirring. The
color of the mixture immediately changed to red. Stir-
ring at 0°C was continued for a further 2 h and then
overnight at ambient temperature. The solvent was
removed under vacuum and the residue redissolved in a
small portion of CHCl3. The resulting red solution was
purified over a silica column using CHCl3 as the eluent
affording a pure red solution from which a red solid
was collected. Crystals suitable for X-ray analysis were
obtained by slow evaporation of a CH2Cl2/MeOH solu-
tion of the product or by standing of such a solution at
2°C overnight.
Yield: 61 mg, 48%; m.p.: 202–204°C; tR (RP
C18 Merck Lichrospher 100 column, 10 m, 4.6×
250 mm, 80% MeOH/20% H2O): 4.58 min; Anal. Calc.
for C14H14NO2ReS3: C, 32.88; H, 2.76; N, 2.74; S,
18.77. Found: C, 32.60; H, 2.96; N, 2.56; S, 18.12; IR
(KBr, w/cm−1): 1603, 1561, 1464, 1433, 1023, 968 (s,
Re=O); UV–Vis (u/nm, MeOH/H2O 80/20) 283, 360;
1H NMR (200 MHz, CDCl3): 3.86 (3H, s, OCH3), 4.97
(2H, br, pyr-CH2ꢀSꢀReO), 5.17 (1H, br, pyr-
CH2ꢀSꢀReO), 5.53 (1H, br, pyr-CH2ꢀSꢀReO), 6.97
(2H, d, J=8.8 Hz, ArꢀH), 7.60 (2H, d, J=8.8 Hz,
ArꢀH), 7.80 (2H, br, 3,5-pyrꢀH), 7.97 (1H, t, J=7.8
Hz, 4-pyr-H).
1
w[ReꢁO]), 857; UV–Vis (u/nm, CH2Cl2): 313, 399; H
NMR (200 MHz, CDCl3, ppm): 5.29 (4H, AB system,
pyr-CH2S), 6.34 (1H, dd, J=5.3 Hz, JHP 3.0 Hz,
Ha-PO), 6.65 (1H, t, J=5.3 Hz, Hc-PO), 7.11 (1H, t,
J=5.3 Hz, Hb-PO), 7.31 (1H, t, J=5.3 Hz, HdꢀPO),
7.43–7.90 (10H, PPhꢀH), 7.72 (2H, d, J=7.7 Hz,
3,5-pyr-H), 7.93 (1H, t, J=7.7 Hz, 4-pyr-H). 13C
NMR (200 MHz, CDCl3, ppm): 53.8 (CH2ꢀS), 117.0,
119.2, 119.5, 128.5, 131.0, 131.9, 133.4, 133.5, 133.6,
140.8, 153.8 (Carom); 31P NMR (200 MHz, CDCl3,
ppm): 13.1 (s). The product is soluble in chlorinated
solvents, sparingly soluble in alcohols and insoluble in
Et2O.
For 2b: Yield: 93.5 mg, 20%; Rf (SiO2; CH2Cl2): 0.05;
tR (Waters Symmetry Shield RP 18 column, 5 m, 150×
3.9 mm; 1 ml min−1; A: MeCN, B: 0.1% TFA; 1–10
min 50 to 80% A): 6.63 min; Anal. Calc. for
C43H36NO3P2ReS2: C, 55.66; H, 3.91; N, 1.51; S, 6.90.
Found: C, 55.45; H, 3.95; N, 1.47; S, 6.76; IR (KBr,
w/cm−1): 2923, 1583, 1455, 1438, 1299, 1261, 1228,
1096, 947 (s, w[ReꢁO]), 908, 854; UV–Vis (u/nm,
1
CH2Cl2) 308, 388; H NMR (200 MHz, CDCl3, ppm):
4.70–5.35 (4H, CH2S), 5.70–7.82 (31H, Harom); 31P
NMR (200 MHz, CDCl3, ppm): 4.4 (d, JPP=8 Hz), 9.6
(d, JPP=8 Hz). The product is soluble in alcohols,
sparingly soluble in chlorinated solvents and insoluble
in Et2O.
2.3.2. [2,6-Dimercaptomethylpyridinato]-
[o-diphenylphosphinophenolato]oxorhenium(V) (2a)
By reacting the [(n-C4H9)4N][ReOCl4] precursor with
an equimolar amount of POH, as described previously
[28,32], the emerald [(n-C4H9)4N][ReOCl3(PO)] interme-
diate complex was obtained. This complex (200 mg,
0.24 mmol) was dissolved in CH3CN and a solution of
2,6-dimercaptomethylpyridine ligand (41.04 mg, 0.24
mmol) in MeOH (10 ml) was added under stirring,
whereupon the color of the mixture turned rapidly from
emerald to green. The mixture was then refluxed under
stirring for 30 min giving an orange-brown solution.
The solvent was evaporated to dryness under vacuum
and the residue redissolved in a small portion of
CH2Cl2. The product was purified on a silica gel
column using CH2Cl2 as the eluent. Two bands were
eluted and collected from the column, the major frac-
tion containing a red complex 2a, and a second minor
fraction containing a yellow complex 2b. By concentrat-
ing both fractions to a small volume and by addition of
2.4. X-ray diffraction data and crystal structure
determination and refinement for 1
The X-ray data were collected at room temperature
(293 K) on a SMART-CCD diffractometer (SIEMENS),
using graphite monochromatized Mo Ka radiation
,
(u=0.71073 A). A summary of the crystallographic
data is given in Table 1. The positions of the non-hy-
drogen atoms were determined by the heavy atom
technique. After anisotropic refinement of the positions
of these, the hydrogen positions were calculated accord-
ing to ideal geometries. Empirical absorption correc-
tions were made using psi scans. Most of the
calculations were carried out in the SHELXTL system
with some local modifications.
Relevant bond lengths and angles are contained in
Table 2. Atomic positional and thermal parameters, full
lists of bond lengths and angles, and Fo/Fc values have
been deposited as supplementary material.

B. Nock et al. / Inorganica Chimica Acta 304 (2000) 26–32
29
Table 1
Crystal data and structure refinement for complex 1
3. Results and discussion
In this work oxorhenium mixed ligand complexes
containing the 2,6-dimercaptomethylpyridine ligand
have been synthesized and characterized by classical
analytical methods. Preparation and analytical data of
2,6-dimercaptomethylpyridine are also reported.
Parameter
Complex 1
C14H14NO2S3Re
510.64
orthorhombic
P212121
Empirical formula
Formula weight
Crystal system
Space group
Thus, complex 1 was obtained in good yields by
reaction of equimolar amounts of 2,6-dimercap-
tomethylpyridine and p-methoxybenzenethiol with the
[(n-C4H9)4N][ReOCl4] precursor, as shown in Scheme 1.
The product was purified on a silica column and crys-
tallized by slow evaporation from a CH2Cl2–MeOH
solution. Given that the three thiolate groups of the
tridentate and the monodentate ligand are deproto-
nated upon coordination, complex 1 is a neutral five-
coordinate species. Formation of complex 1 has been
established by elemental analysis, IR, UV–Vis and
NMR spectroscopies. The strong band at 965 cm−1 is
assigned to the w[ReꢁO] stretching vibration and is
consistent with that reported for several other well-
characterized monooxo rhenium complexes [13–
16,18,19,28,31,32]. The UV–Vis spectrum reveals a
maximum at 360 nm, characteristic of Re(V)OꢀS
charge transfer band(s), as reported for analogous oxo-
rhenium species [13–16,18,19].
Upon coordination the proton signals of the thiol
groups disappear from the NMR spectral pattern,
whereas the other signals experience a remarkable
downfield shift, as expected for the strong acid charac-
ter of the [ReO]3+ moiety. In addition, the methylene
protons of the tridentate ligand become diastereotopic
because of the asymmetry introduced in the molecule
by the oxo group (exo and endo protons) and by some
steric constraints,which place these protons in slightly
different magnetic environments.
A crystal suitable for X-ray structure analysis was
obtained for complex 1 and relevant crystallographic
data are summarized in Table 1. As shown in the
ORTEP diagram of Fig. 1, complex 1 adopts the usual
distorted square-pyramidal arrangement of the four
donor atoms (SNS/S) around the [ReꢁO]3+ core. The
tridentate ligand, representing a novel framework of
donor atoms capable of forming ‘3+1’ mixed-ligand
complexes, spans three positions in the basal plane, via
the charged thiolate S atoms and the neutral pyridine
nitrogen. The fourth position is occupied by the p-
methoxybenzenethiolate S atom. The Re atom lies
Unit cell dimensions
,
a (A)
7.249(5)
7.915(4)
28.206(9)
90.00
90.00
90.00
1618.4(15)
4
293(2)
2.096
7.897
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
Volume (A )
Z
3
,
Temperature (K)
z (g cm−3
)
Absorption coefficient (mm−1
)
F(000)
976
0.71073
,
u(Mo Ka) (A)
Crystal size (mm3)
q-range for data collection
Index ranges
0.54×0.27×0.27
2.67–25.95
−85h50
−95k50
−345l534
3625
Reflections collected
Independent reflections
Goodness-of-fit on F2
R [I\2|(I)]
2949
1.064
R1=0.0334,
wR2=0.0870
R1=0.0472 wR2=0.2186
R (all data)
Table 2
,
Selected bond length (A) and angles (°) for complex 1
ReꢀO1
ReꢀN
ReꢀS3
ReꢀS1
ReꢀS2
1.692(7)
2.106(8)
2.279(3)
2.286(3)
2.290(3)
O1ꢀReꢀN
O1ꢀReꢀS3
NꢀReꢀS3
O1ꢀReꢀS1
NꢀReꢀS1
S3ꢀReꢀS1
O1ꢀReꢀS2
NꢀReꢀS2
S3ꢀReꢀS2
S1ꢀReꢀS2
106.3(4)
111.6(3)
81.7(3)
111.0(3)
80.1(2)
136.87(12)
107.3(3)
146.1(2)
89.63(10)
84.37(11)
,
above the basal plane towards the O apex (0.83 A).
In comparison to the ReꢀN bond lengths observed in
the X-ray structures of ‘3+1’ mixed-ligand
‘SN(alkyl)S’ complexes [15,18] the ReꢀN(pyr) distance
,
is significantly shorter by 0.1 A, due to the aromatic
,
character of the pyridine nitrogen (2.10 A for 1 vs. 2.21
,
,
A for Ref. [15] and 2.18 A for Ref. [18]). The distances
of the ReꢀS bonds (2.27–2.29 A) fall in the usual range
,
Scheme 1. Synthesis of complex 1.

30
B. Nock et al. / Inorganica Chimica Acta 304 (2000) 26–32
Fig. 1. An ORTEP diagram of complex 1.
Scheme 2. A two step synthesis of complexes 2a and 2b.
previously reported for a series of oxorhenium(V) com-
plexes [13,15,18,19].
coordination to the ReO3+ core. The IR spectrum
shows an intense band at 951 cm−1 corresponding to
the ReꢁO stretching vibration, found in good agree-
ment with that of analogous six-coordinate oxorhenium
complexes containing the same PO ligand [28,32]. The
UV–Vis spectrum exhibits a maximum at 313, 399 nm,
similar to that of similar oxorhenium compounds [28].
Complex 2a, as depicted in Scheme 2, was obtained
by a two step synthesis: (i) equimolar amounts of
o-diphenylphosphinophenol were reacted with the same
precursor [(n-C4H9)4N][ReOCl4] affording the emerald
intermediate [(n-C4H9)4N][ReOCl3(PO)]; (ii) the latter
was then reacted with 2,6-dimercaptomethylpyridine
affording complex 2a as a red solid. This compound
was purified on a silica column and crystallized by slow
evaporation from a CH2Cl2–MeOH solution. However,
it was not possible to grow crystals of sufficient quality
for X-ray crystallographic studies. Formation and pu-
rity of complex 2a was established by elemental analy-
sis, IR, UV–Vis and NMR spectroscopies. Complex 2a
is a six-coordinate neutral oxorhenium species, given
that the two thiolate groups of the SNS ligand and the
hydroxy group of the PO ligand are deprotonated upon
1
A complete H, 13C and 31P NMR analysis confirms
the formula assigned to complex 2a. In detail, the
proton spectrum displays the expected AB pattern for
the methylene protons of the tridentate ligand centered
at 5.29 ppm, along with signals of the pyridine ring
deshielded with respect to those of the uncoordinated
ligand. The aromatic protons of the phenyl ring inter-
posed between the donors of the PO− ligand display a
four signal pattern consistent with the coordination
spanning a position on the equatorial plane (P) and the
position trans to the oxo linkage (O), as previously

B. Nock et al. / Inorganica Chimica Acta 304 (2000) 26–32
31
shown by the parent complex [ReOCl3(PO)]− [32] and
References
related bis-substituted P,O-oxorhenium compounds
[33]. Analogously, the 13C spectrum shows the aliphatic
carbon at 53.8 ppm and all of the aromatic signals
arising from both the pyridine and arylphosphine in the
range 117.0–153.8 ppm. The 31P spectrum exhibits a
singlet at 13.1 ppm to be compared with the signal of
uncoordinated POH which falls at −31.2 ppm [32].
During the synthesis of the mono-substituted
[ReOCl3(PO)]− emerald complex, always trace amounts
of the bis-substituted [ReOCl(PO)2] green compound
are recovered [32]. The latter reacts also with 2,6-dimer-
captomethylpyridine leading to the formation of a mi-
nor complex 2b, as depicted in Scheme 2. It was
isolated from a silica column during the purification of
the major complex 2a. Complex 2b is more hydrophilic
as compared to 2a. The IR spectrum of 2b reveals a
band at 947 cm−1 attributed to the w[ReꢁO] stretching
vibration [28,32]. The formulation of 2b was assessed
by NMR studies. The 31P spectrum clearly indicates the
presence of two magnetically unequivalent P nuclei
giving a two doublets pattern with coupling constant of
8 Hz, in agreement with a cis-P configuration (see
Scheme 2). These two doublets are centered at 9.6 and
4.6 ppm, values slightly different from those observed
in the same solvent (11.6 and 2.3 ppm) for the parent
complex [ReOCl(PO)2], indicating the substitution of
the chloride ligand by the thiolate function of the
dimercaptomethylpyridine ligand. The proton spectrum
shows two multiplets in the methylene region, along
with a series of overlapped multiplets in the aromatic
region in the correct 4:31 integration ratio. The coordi-
nation of a thiolate group leaving outside a further
uncoordinated dangling thiol is precedented [34].
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