Oxidorhenium(V) complexes with tetradentate thiourea derivatives

Article abstract of DOI:10.1016/j.poly.2012.06.016

Potentially tetradentate, binegative thiocarbamoylbenzamidines derived from o-phenylenediamines (H₂L or H₃L) are shown to be suitable ligand systems for oxidorhenium(V) cores. They readily react with (NBu ₄)[ReOCl₄] or [ReOCl₃(PPh₃) ₂] under formation of monoxido complexes of the composition [ReO{(H)L}(Y)] with various co-ligands (Y = ReO₄^-, F ₃CCO₂^-, Cl^- or methanol) or μ-oxido dimers depending on the reaction conditions applied. Representative products were isolated and studied spectroscopically and by X-ray diffraction. Substitutions in the periphery of the ligands allow the introduction of a carboxylic substituent, which may serve as anchor group for future bioconjugation of appropriate rhenium (or technetium) complexes.

Full text of DOI:10.1016/j.poly.2012.06.016

Polyhedron 43 (2012) 123–130
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Oxidorhenium(V) complexes with tetradentate thiourea derivatives
Juan Daniel Castillo Gomez a, Hung Huy Nguyen b, Adelheid Hagenbach a, Ulrich Abram a,
⇑
a Freie Universität Berlin, Institute of Chemistry and Biochemistry, Fabeckstr. 34–36, D-14195 Berlin, Germany
b Department of Chemistry, Hanoi University of Science, 19 Le Thanh Tong, Hanoi, Viet Nam
a r t i c l e i n f o
a b s t r a c t
Article history:
Potentially tetradentate, binegative thiocarbamoylbenzamidines derived from o-phenylenediamines
(H2L or H3L) are shown to be suitable ligand systems for oxidorhenium(V) cores. They readily react with
(NBu4)[ReOCl4] or [ReOCl3(PPh3)2] under formation of monoxido complexes of the composition
Received 17 May 2012
Accepted 5 June 2012
Available online 19 June 2012
[ReO{(H)L}(Y)] with various co-ligands (Y = ReO4ꢀ, F3CCO2ꢀ, Clꢀ or methanol) or
l-oxido dimers depend-
ing on the reaction conditions applied. Representative products were isolated and studied spectroscopi-
cally and by X-ray diffraction.
Substitutions in the periphery of the ligands allow the introduction of a carboxylic substituent, which
may serve as anchor group for future bioconjugation of appropriate rhenium (or technetium) complexes.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Rhenium
Oxido complexes
Tetradentate ligands
Synthesis
X-ray structure
1. Introduction
equivalents of the corresponding benzimidoyl chloride are coupled
to diamines. H2L1 and H2L2 can act as
Tri-, tetra- or multiple-dentate ligands, which form stable or
kinetically inert complexes with rhenium and technetium are of
permanent interest for modern nuclear medical labeling proce-
dures, since previous studies have shown that mono- and biden-
tate ligand systems may suffer from insufficient in vivo stability
due to rapid ligand exchange reactions with plasma components
[1–9]. For common technetium(V) and rhenium(V) cores, particu-
larly ligands with ‘medium’ and ‘soft’ donor atoms are recom-
mended [1,2]. Thus, chelators with a mixed sulfur and nitrogen
donor sphere should be very suitable and some of them have been
found application in routine nuclear medical procedures. One focus
of current research in this field is the search for suitable chelating
systems for bioconjugation procedures. Such ligands must (i) form
thermodynamically stable and/or kinetically inert complexes with
O
O
N
N
N
N
N
N
N
N
O
NH
S
S
NH
S
S
NH
NH
S
S
-O
NH
NH
Et3NH+
N
N
N
N
O
O
H2L1
H2L2
(Et3NH)(H2L3)
tetradentate, binegative ligands and form stable complexes with
metal ions, which can adopt square-planar or pyramidal coordina-
tion spheres. Keeping in mind the structures of such ligands with
metal ions like Ni2+ or Cu2+ [20,21], the tetradentate chelators
should also be suitable for the coordination of the equatorial coor-
dination spheres of oxidotechnetium(V) and oxidorhenium(V)
complexes.
one or more of the common metal cores (e.g. {M = O}3+, {MN}2+
,
{M(CO)3}+; M = Tc, Re) and (ii) possess a suitable anchor group,
which does not contribute to the coordination of the metal, but
is able to form stable bioconjugates (e.g. carboxylates, aldehydes,
alkynes).
Thiourea derivatives have been shown to be excellent bi- and
tridentate ligands for Re(V) oxido-, nitrido-, and phenylimido cores
[10–20]. Particularly thiocarbamoylbenzamidines are highly flexi-
ble ligands [13–19]. They are prepared from benzimidoyl chlorides
and amines, which allows access to a large number of ligands with
various donor sites. Tetradentate ligands are formed when two
In the present paper, we report about the coordination chemis-
try of H2L1 and H2L2 with oxidorhenium(V) centers as models for
further studies with technetium, as well as the synthesis and coor-
dination chemistry of a novel SNNS proligand with an additional
carboxylic group for future bioconjugation, (Et3NH)(H2L3).
2. Results and discussion
⇑
N,N-[(Dialkylamino)-N0-(thiocarbonyl)]benzamidines can read-
ily be varied in their periphery. This has been demonstrated with
Corresponding author.
E-mail address: ulrich.abram@fu-berlin.de (U. Abram).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.06.016

124
J.D. Castillo Gomez et al. / Polyhedron 43 (2012) 123–130
a number of bi- and tridentate examples before. Such modifica-
tions help to tune their properties or couple them to biomolecules.
With regard to the molecular building blocks, which are used for
the synthesis of the ligands, benzoyl chloride, ammonium thiocya-
nate, secondary amines and a second (functionalized) amine, there
exist several positions, where functional groups for bioconjugation
can be introduced. For the potentially tetradentate ligands under
study, we have chosen the central phenylendiamine unit for sub-
stitution with an additional carboxylic group (Scheme 1). This
has the advantage, that only one molecular position will contribute
in future bioconjugation procedures, while substitution of the
benzoyl or amine units would result in two possible coupling
positions, which might cause problems in order to produce one un-
ique coupling product.
observed signals are similar to those of H2L2 and shall not be dis-
cussed here in detail. Further support for the composition of
(Et3NH)(H2L3) is given by the ESI mass spectra of the compound.
The ESI(ꢀ) spectrum is clearly dominated by the molecular ion of
the (H2L3)ꢀ at m/z = 615.1888 (Calc. 615.1854), while the positive
mode spectrum shows the (Et3NH)+ cation as base peak together
with a less intense peak at m/z = 617.2014 (Calc. 617.2004), which
can be assigned to the doubly protonated (H4L3)+ ion.
Reactions of the potentially tetradentate proligands with the
common precursor [ReOCl3(PPh3)2] gave insoluble red or brown sol-
ids, from which no crystalline products could be isolated. More con-
trolled reactions are possible starting from (NBu4)[ReOCl4]. Thus,
H2L1 reacts with (NBu4)[ReOCl4] and Et3N as supporting base in
MeOH under formation of a red crystalline precipitate of the compo-
sition [ReO(L1)(OReO3)]. The yield is only about 20%, which can be
explained by the rapid formation of perrhenate. Such side-reactions
are not unusual in the chemistry of oxidorhenium(V) complexes
and were previously found as results of hydrolysis followed by dis-
proportion or oxidation of the anionic complex [ReOCl4]ꢀ (to ReO2
and ReO4ꢀ or ReO4ꢀ exclusively) [2,22–28]. The IR spectrum of [Re-
O(L1)(OReO3)] shows no band in the region above 3100 cmꢀ1, which
The proligands H2L1 and H2L2 were prepared as almost colorless
solids from the corresponding benzimidoyl chlorides and o-phen-
ylenediamine. Previous attempts to prepare these compounds
ended in the isolation of crude, oily products, which have directly
been used for the syntheses of the corresponding Cu(II) and Ni(II)
complexes [20,21]. Some slight modifications during the ligand
synthesis, particularly the use of THF instead of acetone, improve
the yields and allow the isolation of H2L1 and H2L2 in pure form.
They were characterized by elemental analysis and spectroscopic
methods. IR spectra of the compounds exhibit medium absorptions
around 3250 cmꢀ1 and very strong bands in the region between
could be assigned to an NH stretch, and the intense mC@N absorption
in the spectrum of H2L1 at 1640 cmꢀ1 is shifted by about 100 cmꢀ1 to
longer wavelengths. This indicates the expected double deprotona-
tion and chelate formation of the ligand. While the terminal {Re = O}
core is confirmed by a medium absorption at 983 cmꢀ1, the pres-
ence of coordinated ReO4ꢀ is indicated by a very strong absorption
at 921 cmꢀ1 [2]. The 1H NMR spectrum of the product reveals its
symmetric structure, in which two benzamidine parts are magnet-
ically equivalent and, thus, a planar coordination mode of {L1}2ꢀ is
suggested. As the consequence of hindered rotation around the
C–N bonds in the C(S)–NEt2 moieties and the inflexible structure,
two well resolved triplets and four multiplets are observed for the
ethyl protons. The FAB+ MS spectrum of [ReO(L1)(OReO3)] does
not show the molecular ion peak, but exposes an intense fragment
at m/z = 745 with the isotopic pattern of a mononuclear rhenium
complex, which can be assigned to [ReO(L1)]+. Such a fragmentation
pattern is not unusual and confirms the weakness of the bond to
ReO4ꢀ and the ready dissociation of this ligand.
1590 and 1640 cmꢀ1, which are assigned to
mNH and mC@N stretches
respectively. 1H NMR spectra confirm the symmetric structure of
the products. Thus, the resonances of the aromatic protons of the
o-phenylenediamine residue appear as two doublets at around
6.40 and 6.80 ppm. Two series of signals corresponding to alkyl
groups of the NR1R2 residues are also observed, but are less re-
solved, which reflects the hindered rotation of the thiourea moiety.
The carboxylate-substituted proligand (H2L3)ꢀ was prepared
analogously to H2L2. After removal of a brownish solid, it can be
isolated from the remaining solution as triethylammonium salt.
The 1H NMR spectrum of the products confirms the ionic nature
of the compound, since the signals of the (Et3NH)+ can clearly be
detected with a correct ratio besides those which can be assigned
to the thiocarbamoylbenzamidine. Chemical shifts and ratio of the
R
N
R
H
N
O
HN
KSCN +
+
R
R
N
Cl
O
S
1) NiCl2
2) SOCl2
R
N
R
O
Cl
S
NH2
NH2
NH2
NH2
HOOC
R
N
O
O
N
N
NH
N
R
NH
S
S
S
-O
NH
S
NH
Et3NH+
R
N
R
N
N
N
H L1:
H L2:
(Et3NH)(H2L3)
R=Et
NR2=Mor
2
2
Fig. 1. Molecular structure of [ReO(L1)(OReO3] [35]. H atoms have been omitted for
Scheme 1. Synthesis of the ligands used in this paper.
clarity.

J.D. Castillo Gomez et al. / Polyhedron 43 (2012) 123–130
125
Table 1
Selected bond lengths (Å) and angles (°) in the molecular structures of [ReO(L1)(OReO3], [{ReO(L2)}2O], [ReO(L3)(MeOH)] and [ReO(L3)(TFA)].
[ReO(L1)(OReO3)]
[{ReO(L2)}2O]
[ReO(L3)(MeOH)]
[ReO(HL3)(TFA)]
Re–O10
Re–O20
Re–S1
1.669(4)
2.350(4)
2.364(2)
2.024(5)
2.360(2)
2.019(5)
1.748(7)
1.341(8)
1.298(8)
1.367(8)
1.761(6)
1.344(8)
1.302(7)
1.356(8)
1.711(7)
1.919(1)
2.379(3)
2.062(8)
2.382(3)
2.112(7)
1.75(1)
1.32(2)
1.31(1)
1.37(1)
1.73(1)
1.32(1)
1.32(1)
1.34(1)
1.669(7)
2.355(6)
2.339(3)
2.033(9)
2.334(3)
2.002(9)
1.77(1)
1.32(2)
1.32(1)
1.35(1)
1.74(1)
1.34(1)
1.31(1)
1.38(1)
1.665(5)
2.272(5)
2.360(2)
2.036(5)
2.342(2)
2.019(6)
1.740(7)
1.356(8)
1.304(8)
1.345(8)
1.753(8)
1.343(9)
1.307(8)
1.365(8)
Re–N5
Re–S11
Re–N15
S1–C2
C2–N3
N3–C4
C4–N5
S11–C12
C12–N13
N13–C14
C14–N15
O10–Re–O20
O10–Re–S1
O10–Re–N5
O10–Re–N15
O10–Re–S11
S1–Re–O20
S1–Re–N5
S1–Re–N15
S1–Re–S11
N5–Re–O20
N5–Re–N15
N5–Re–S11
N15–Re–O20
N15–Re–S11
S11–Re–O20
Re–O20–Xa
178.5(2)
100.5(2)
100.9(2)
102.0(2)
101.5(1)
78.8(2)
93.6(2)
157.2(1)
84.45(6)
77.8(2)
79.5(2)
157.5(1)
78.6(2)
93.7(1)
79.8(2)
164.8(3)
177.4(4)
94.5(3)
93.4(3)
91.7(3)
94.8(3)
87.7(3)
94.8(2)
171.9(2)
90.5(1)
85.3(2)
79.6(3)
169.8(2)
85.8(4)
94.2(2)
86.20(9)
174.7(6)
178.0(3)
101.1(3)
100.8(4)
100.8(3)
102.0(3)
80.6(2)
92.9(2)
158.0(2)
85.7(1)
77.9(3)
80.1(3)
156.9(2)
77.5(3)
92.6(2)
79.1(2)
129.2(7)
175.4(2)
99.3(2)
100.7(2)
100.1(2)
101.6(2)
78.6(1)
94.9(2)
160.5(2)
85.39(6)
75.6(2)
79.5(2)
157.4(2)
81.9(2)
92.7(2)
82.4(2)
132.1(5)
a
X = Re2 for [ReO(L1)(OReO3)], X = Re0 for [{ReO(L2)}2O], X = C21 for [ReO(L3)(MeOH)] and [ReO(HL3)(TFA)].
The spectroscopic analysis of [ReO(L1)(OReO3)] is confirmed by
tion environment. The Re1–O20–Re10 angle is 175.5(7)°. Expect-
edly, the Re–O20 bond of 1.918(1) Å is clearly longer than the
bond to the terminal oxido ligand (1.738(1) Å), but reflects some
double bond character. The donor atoms of the tetradentate ligand
are planar within 0.005 Å, and the rhenium atom is situated out-
side this plane by 0.141(4) Å towards O10.
the results of an X-ray structure determination. Fig. 1 depicts the
molecular structure of the complex and selected bond lengths
and angles are presented in Table 1. The rhenium atom is coordi-
nated in a distorted octahedral environment with a terminal oxido
ligand and a perrhenato unit in axial positions. The {L1}2ꢀ ligand is
arranged in the equatorial plane and binds symmetrically to the
rhenium atom as an {N2S2} tetradentate ligand. The Re atom is
placed 0.425(2) Å above this plane towards the oxido ligand. In this
arrangement, all phenyl rings are bent out of the equatorial plane.
While the Re1–O10 bond length of 1.669(4) Å falls within the
common range of rhenium–oxygen double bonds, the Re1–O20
distance of 2.350(2) Å is much longer than a typical rhenium–oxy-
gen single bond and reflects only weak interactions between the
perrhenato ligand and the Re atom of the chelate. Consequently,
the Re2–O20 distance is only a little longer than those of the other
Re–O bonds in the perrhenato unit.
In order to prevent the undesired formation of [ReO4]ꢀ, which is
frequently observed, when the removal of chlorido ligands from
[ReOCl4]ꢀ by the addition of a supporting base under atmospheric
and hydrous conditions is faster than the stabilization of the
{ReO}3+ center by incoming ligands, the synthetic procedure was
slightly modified. The supporting base was just added after heating
the mixture of (NBu4)[ReOCl4] and one equivalent of H2L2 in MeOH
for a period of 5 min (reactions under consequently anhydrous and
anaerobic conditions have not been undertaken with regard to the
nuclear medical background of the present study). A red solid of
the composition [{ReO(L2)}2O] precipitated directly from the reac-
tion mixture and was isolated in high yield. The compound was
recrystallized from CH2Cl2/acetone and characterized spectroscop-
ically and by X-ray diffraction. Fig. 2 shows a structural plot and se-
lected bond lengths and angles are summarized in Table 1. A
central oxido ligand links two {ReO(L2)}+ units. Thus, the rhenium
atoms in the symmetry-related subunits have a distorted coordina-
Fig. 2. Molecular structure of [{ReO(L2)}2O] [35]. H atoms have been omitted for
clarity.

126
J.D. Castillo Gomez et al. / Polyhedron 43 (2012) 123–130
Fig. 3. Molecular structure of [ReO(L3)(MeOH)] [35]. H atoms on carbon atoms have
been omitted for clarity.
The 1H NMR spectrum of [{ReO(L2)}2O] is complex. Expectedly,
the CH2 signals of the morpholine moieties appear as an overlap-
ping array which is poorly resolved. But also the protons of the
central phenylene diamine ring show four different signals. This
indicates that the magnetic inequivalence of the phenyl rings in
the solid state structure of the complex is also present in solution.
Obviously, a hindered rotation around the Re–O20–Re0 bonds is
responsible for this result. The FAB+ MS spectrum does not show
the molecular ion peak of the dimeric compound, but a peak of
high intensity at m/z = 774.9, which can be assigned to the frag-
ment cation [ReO(L2)]+. A less intense signal at m/z = 790.8 corre-
sponds to a fragment of the composition [ReO(H2O)(L2)]+.
The reaction of the carboxyl-substituted proligand (Et3NH)
[H2L3] with (NBu4)[ReOCl4] in a chloroform/methanol mixture pro-
ceeds at room temperature without the addition of any base. Single
crystals of [ReO(L3)(MeOH)] with co-crystallized CHCl3 and water
were obtained after a couple of days by slow evaporation of the
reaction mixture. The X-ray structure analysis of these crystals
confirm the formation of a six-coordinate rhenium complex with
a general coordination environment as was observed before for
[ReO(L1)](OReO3)] with the axial coordination position trans to
the oxido ligand being occupied by a methanol ligand instead of
perrhenate. A structural plot is given in Fig. 3, and selected bond
lengths and angles are collected in Table 1. The coordination of a
methanol ligand instead of a methanolato one is strongly sug-
gested by the relatively long Re–O20 bond length of 2.355(6) Å
and the Re–O20–C21 angle of 129.2(7) Å. In all hitherto structur-
ally characterized complexes with trans-{O@Re–OMe}2+ cores,
the Re–O–Me angles are higher [29], which is the result of signifi-
cant transfer of electron density from the terminal oxido ligand to
the trans-situated Re–O bond and is also reflected by a shortening
of this bond in comparison to Re–O single bonds in the equatorial
coordination sphere of such complexes [16]. The carboxylic group
in the periphery of the tetradentate ligand is deprotonated in the
solid state structure under study. This can be deduced by almost
equal C–O bond lengths of 1.259(16) and 1.262(16) Å, respectively.
Additional support for the coordination of a neutral methanol li-
gand and the deprotonation of the carboxylic group is given by
the formation of an extended network of hydrogen bonds, in which
they are involved together with the co-crystallized water mole-
cules. The bonding situation is depicted in Fig. 4 and details of
Fig. 4. Hydrogen bonds between [ReO(L3)(MeOH)] [35] and the solvent water
combining each two molecules of the complex to dimeric units. Symmetry
operations: (0) ꢀx, ꢀy, 1 ꢀ z; (00) x ꢀ 1, y ꢀ 1, z; (000) ꢀx, ꢀy, ꢀz; (IV) 1 ꢀ x, 1 ꢀ y, ꢀz;
(V) x, y, 1 + z.
the established hydrogen bonds are given in Table 2. The hydrogen
bonds organize each two complex molecules to dimeric units. Due
to their deprotonation, the negatively charged carboxylate residues
cannot undergo direct interactions, but by means of water mole-
cules, which act as primary H atom donors for this bonding.
The spectroscopic data of [ReO(L3)(MeOH] are in the accordance
with the results of the X-ray diffraction study. The IR spectrum of
the single crystals (measured in the ATR mode on a Nicolet-FT-IR
670 spectrometer) shows a strong band at 3407 cmꢀ1, which is
caused by the co-crystallized water. The mC@N vibrations of the or-
ganic ligand can be identified at 1671 cmꢀ1 and two strong bands
at 1532 and 1517 cmꢀ1 belong to the vibrations of the carboxylate
anion. The mRe@O stretch appears as a band at 970 cmꢀ1. The 1H
NMR spectrum of [ReO(L3)(MeOH)] in CDCl3 shows a doublet at
6.54 ppm, which is caused by the hydrogen atom in meta position
to the carboxylic function. All other aromatic signals can be found
between 6.98 and 7.88 ppm. A broad multiplet between 3.86 and
4.51 ppm is assigned to the CH2 groups of the morpholine substit-
uents and is complex due to the hindered rotation of these resi-
dues. The ESI(+) spectrum of the substance shows no molecular
peak [M+H]+, but an intense peak at m/z = 817.1294, which corre-
sponds to the [ReO(HL3)]+ fragment.
All complexes reported above have been prepared starting from
the readily soluble complex (NBu4)[ReOCl4]. Analogous reactions
with the common, but sparingly soluble oxidorhenium(V)

J.D. Castillo Gomez et al. / Polyhedron 43 (2012) 123–130
127
In the case of the reaction of [ReOCl3(PPh3)2] with H3L3, the
product could be purified and isolated in crystalline form. Recrys-
tallization of the initially also sparingly soluble red compound
from trifluoroacetic acid (HTFA) gave red single crystals of the
composition [ReO(HL3)(TFA)]ꢁHTFA, which were suitable for X-
ray structure analysis. As in all previous examples, this complex
also shows a distorted octahedral coordination around the rhe-
nium atom, with an oxido and a trifluoroacetato ligand in trans-po-
sition to each other. The tetradentate organic ligand occupies the
equatorial coordination plane of the molecule (Fig. 5a). An addi-
tional trifluoroacetic acid molecule is co-crystallized in the asym-
metric unit and forms hydrogen bonds to the trifluoroacetato
ligand.
Table 2
Hydrogen bonding in [ReO(L3)(MeOH)ꢁ2.6CHCl3ꢁ2H2O and [ReO(HL3)(TFA)]ꢁHTFA. For
symmetry designators see Figs. 4 and 5.
D–Hꢁ ꢁ ꢁA
d(D–H)
d(Hꢁ ꢁ ꢁA)
d(Dꢁ ꢁ ꢁA)
<(DHA)
[ReO(L3)(MeOH)ꢁ2.6CHCl3ꢁ2H2O
O20000–H20000ꢁ ꢁ ꢁO90000
O910–H91A0ꢁ ꢁ ꢁO58
O910–H91B0ꢁ ꢁ ꢁO9000
O9000–H90A00ꢁ ꢁ ꢁO59
0.93
0.85
0.85
0.85
2.25
2.324(11)
2.23
2.933(7)
129.9
2.999(11)
2.9112(3)
2.888(10)
136.7(4)
137.361(6)
161.2(3)
2.071(10)
[ReO(HL3)(TFA)]ꢁHTFA
O58–H58ꢁ ꢁ ꢁO590
0.82
0.82
0.82
1.76
1.75
2.51
2.574(9)
2.558(9)
2.907(12)
172.8
167.2
111.2
O8200–H8200ꢁ ꢁ ꢁO22
O8200–H8200ꢁ ꢁ ꢁF25
The Re–O10 length of 1.665(5) Å is in the expected range for a
rhenium–oxygen double bond. This bond exerts a strong structural
trans influence which weakens the Re–O bond to the trifluoroace-
tato ligand (2.272(5) Å). The carboxylic group of the ligand {HL3}2ꢀ
is protonated in this complex, which can only partially be derived
from a bond lengths consideration (C57–O58: 1.31(1) Å, C57–O59:
1.28(1) Å), but clearly be seen by the hydrogen bonding situation,
which is shown in Fig. 5b. Two adjacent [ReO(HL3)(TFA)] molecules
are arranged to dimers via the hydrogen bonding of the carboxylic
group. A comparison of the hydrogen bonds in [ReO(L3)(MeOH)]
and [ReO(HL3)(TFA)] strongly indicate the different bonding
precursor [ReOCl3(PPh3)2] with H2L1 and H2L2 in refluxing CH2Cl2
or CH3CN delivered almost insoluble red solids of unsatisfactory
purity. Elemental analysis and IR spectra of the products confirm
the presence of the organic ligand together with oxidorhenium(V)
core(s). Most probably, the products represent mixtures of
[ReO(L)Cl] complexes with mixtures of oligomeric compounds.
Unfortunately, NMR or MS studies were not possible due to their
low solubility. This is also the reason that we did not follow this
synthetic approach further for the unsubstituted ligands H2L1
and H2L2.
Fig. 5. Molecular structure [35] of [ReO(HL3)(TFA)] (a) and hydrogen bonding in the solid state structure of [ReO(HL3)(TFA)]ꢁHTFA (b).

128
J.D. Castillo Gomez et al. / Polyhedron 43 (2012) 123–130
Table 3
X-ray structure data collection and refinement parameters.
[ReO(L1)(OReO3]
[{ReO(L2)}2O]ꢁCH2Cl2
[ReO(L3)(MeOH)]ꢁ2.6CHCl3ꢁ2H2O
[ReO(HL3)(TFA)]ꢁHTFA
Formula
Formula weight
Crystal system
a (Å)
b (Å)
c (Å)
C
30H34N6O5Re2S2
C61H62Cl2N12O7Re2S4
1646.77
tetragonal
19.549(5)
19.549(5)
17.943(5)
90
90
90
6857(3)
P41212
4
C34.6H39.6Cl7.8N6O8ReS2
1197.34
triclinic
12.246(1)
13.8131)
16.484(1)
108.88(1)
106.59(1)
98.26(1)
C35H31F6N6O9ReS2
1043.99
triclinic
11.359(1)
12.148(1)
16.022(2)
91.51(1)
995.15
monoclinic
14.109(1)
11.160(1)
21.779(2)
90
102.96(1)
90
3341.9(5)
P21/n
a
(°)
b (°)
105.82(1)
111.96(1)
1952.3(3)
c
(°)
V (Å3)
2441.3(3)
ꢀ
ꢀ
Space group
P1
P1
Z
4
2
2
Dcalc. (g cmꢀ3
)
1.978
7.410
1.595
3.785
1.623
3.056
1.776
3.309
l
(mmꢀ1
)
No. of reflections
No. of independent
No. of parameters
R1/wR2
35020
8954
434
0.0441/0.0781
0.924
7291
5240
404
0.0463/0.0780
0.950
18340
8557
546
0.0703/0.16878
0.975
20541
10432
532
0.0503/0.1118
0.863
Goodness-of-fit (GOF) on F2
modes of the carboxylic group of the organic ligand in these two
complexes.
in the form: m/z, assignment. Elemental analysis of carbon, hydro-
gen, nitrogen, and sulfur were determined using a Heraeus Vario EL
elemental analyzer. The elemental analyses of the rhenium com-
pounds showed systematically too low values for hydrogen and
sometimes carbon (in some cases in a significant extent). This
might be caused by an incomplete combustion of the metal com-
pounds and/or hydride formation, and does not refer to impure
samples. Similar findings have been observed for analogous oxo-
rhenium(V) complexes with the same type of ligands before
[13,19]. We left these values uncorrected. Additional proof for
the identity of the products is given by high-resolution mass spec-
tra for selected representatives. NMR-spectra were taken with a
JEOL 400 MHz multinuclear spectrometer.
3. Conclusions
The tetradentate SNNS ligands under study are well suitable to
form stable complexes with oxidorhenium(V) cores. They occupy
the equatorial coordination spheres of the resulting complexes.
Substitution in the molecular periphery allow the introduction of
anchor groups for bioconjugation, which do not contribute to the
coordination of the transition metal as has been demonstrated by
an carboxylic group at the central phenyl ring of the ligand.
For reproducible syntheses of uniform rhenium (or technetium)
compounds under conditions which are required for nuclear med-
ical applications, however, the combination with oxidometal(V)
cores seems to be inappropriate. The combination of the {MO}3+
core with the tetradentate, binegative SNNS ligands obviously
causes significant problems with the charge compensation in the
formed complexes, particularly with the occupation of the sixth
coordination site. This results in the formation of various complex
species depending on the reaction conditions applied, including di-
4.3. Syntheses
4.3.1. H2L1 and H2L2
Solid thiocarbamoylbenzimidoyl chloride (5 mmol) was added
to a stirred solution of o-phenylenediamine (252 mg, 2.5 mmol)
and triethylamine (1.01 g, 10 mmol) in 10 mL of dry THF. The mix-
ture was stirred for 4 h and then cooled to 0 °C. The formed precip-
itate was filtered off and the solvent was removed under vacuum.
The resulting residue was recrystallized from diethyl ether.
H2L1: Yield: 815 mg (30%). Anal. Calc. for C30H36N6S2: C, 66.14;
H, 6.66; N, 15.43; S, 11.77. Found: C, 66.01; H, 6.45; N, 15.29; S,
meric l-oxo compounds and complexes with perrhenato ligands.
Such undesired side-reactions may be avoided by choosing a
better appropriate metal core, such as the {M„N}2+ units
(M = Re, Tc), which should be able to form neutral, five-coordinate
rhenium or technetium complexes with the title ligands. Accordant
studies are currently underway in our laboratories.
11.89%. IR (
m
in cmꢀ1): 3055(w), 2927(m), 2860(w), 1640(vs),
1423(s), 1353(s), 1253(m), 1095(m), 1064(m), 995(m), 744(m),
694(s). 1H NMR (CDCl3; d, ppm): 1.32 (m, 12H, CH3), 3.67 (m, 8H,
NCH2), 6.34 (d, br, 2H, C6H4), 6.56 (d, br, 2H, C6H4), 7.23 (t,
J = 7.1 Hz, 4H, Ph), 7.48 (t, J = 7.2 Hz, 2H, Ph), 7.50 (d, J = 7.4 Hz,
4H, Ph).
4. Experimental
H2L2: 1.57 g (55%). Anal. Calc. for C30H32N6O2S2: C, 62.91; H,
5.63; N, 14.67; S, 11.20. Found: C, 63.10; H, 5.35; N, 14.16; S,
4.1. Materials
11.08%. IR (
m
in cmꢀ1): 3417(m), 3335(m), 3209(m), 3060(w),
All reagents used in this study were reagent grade and used
without further purification. The syntheses of corresponding N,N-
dialkylamino-N0-(thiocarbonyl)benzimidoyl chlorides followed
the standard procedures [11,30,31]. (NBu4)[ReOCl4] [32] and [Re-
OCl3(PPh3)2] [33] were prepared by published methods.
2962(m), 2912(w), 2858(s), 2731(w), 2599(m), 2495(m),
2380(w), 2337(w), 2052(w), 1963(w), 1674(w), 1624(s), 1569(w),
1423(s), 1350(m), 1276(s), 1226(s), 1110(s), 1064(m), 999(s),
925(w), 837(m), 748(s), 694(s). 1H NMR (CDCl3; d, ppm): 3.66 (t,
br, J = 4.8 Hz, 4H, NCH2), 3.70 (t, br, J = 4.9 Hz, 4H, NCH2), 4.02 (t,
br, J = 4.8 Hz, 4H, OCH2), 4.10 (t, br, J = 4.8 Hz, 4H, OCH2), 6.90 (m,
2H, C6H4), 7.16–7.22 (m, 10H, Ph + C6H4), 7.31 (t, J = 7.8 Hz, 2H, Ph).
4.2. Physical measurements
Infrared spectra were measured from KBr pellets on a Shimadzu
FT-IR-spectrometer or an Nicolet FT-IR 670 instrument between
400 and 4000 cmꢀ1. ESI mass spectra were measured with an Agi-
lent 6210 ESI-TOF (Agilent Technologies). All MS results are given
4.3.2. [Et3NH][H2L3]
Solid morpholinylthiocarbonylbenzimidoyl chloride (4 g,
15 mmol) was added to a stirred solution of 3,4-diaminobenzoic

J.D. Castillo Gomez et al. / Polyhedron 43 (2012) 123–130
129
acid (1.13 g, 7.5 mmol) and triethylamine (3.03 g, 30 mmol) in
20 mL of dry THF. The mixture was stirred for 4 h and then cooled
to 0 °C. The formed precipitate was filtered off and the solvent was
removed under vacuum. The resulting residue was recrystallized
from diethyl ether. Yield: 5.19 g (88%). Anal. Calc. for
1164(w), 1136(w), 1111(s), 1080(w), 1062(w), 1024(s), 978(s),
970(s), 963(s), 921(s), 896(s), 881(s), 822(s). 1H NMR (CDCl3; d,
ppm): 3.11 (s, 3H, OCH3), 3.37 (s, br, 1H, OH), 3.86–3.97 (m, 8H,
OCH2), 4.25–4.57 (m, 8H, NCH2), 6.54 (d, J = 8.3 Hz, 1H, C6H3),
6.98–7.88 (m, 12H, Ph + C6H3). ESI TOF(+) (m/z): 817.1294
[MꢀMeOꢀ]+ (Calc. 817.1276).
C
37H47N7O4S2: C, 61.90; H, 6.60; N, 13.66; S, 8.93. Found: C,
59.99; H, 6.59; N, 13.15; S, 8.89%. IR (
m
in cmꢀ1): 3321(m),
4.3.6. [ReO(HL3)(TFA)]ꢁHTFA
3205(m), 2974(s), 2854(s), 2627(w), 2496(m), 1701(m), 1697(w),
1597(s), 1470(m), 1427(m), 1280(s), 1223(s), 1026(s), 837(w),
783(s), 698(s). 1H NMR (CDCl3; d, ppm): 1.27 (t, J = 7.3 Hz, 9H,
CH3), 3.05 (q, J = 7.3 Hz, 6H, ethyl-CH2), 3.68–3.77 (m, 8H,
morph-NCH2), 3.97–4.29 (m, 8H, OCH2), 6.67 (d, J = 8.24 Hz, 1H,
C6H3), 7.15–8.32 (m, 12H, Ph + C6H3), 10.81(s, br, 1H, NH). ESI(+)
TOF-MS (m/z): 102.1288 ([Et3NH]+, Calc. 102.1283), 617.2014
([H4L3]+, Calc. 617.2004). ESI(ꢀ) TOF-MS (m/z): 615.1877
([H2L3]ꢀ, Calc. 615.1848).
[ReOCl3(PPh3)2] (83 mg, 0.1 mmol) was suspended in 5 mL THF.
Solid [Et3NH][H2L3] (0.11 mmol) and 3 drops of NEt3 were added.
The suspension was stirred for 4 h at room temperature. The
resulting red precipitate was filtered off, washed with acetone
and diethyl ether and redissolved in pure trifluoroacetic acid
(HTFA). Orange-red crystals were obtained by slow evaporation
of this solution. Yield: 23 mg (22 %). IR (
m
in cmꢀ1): 3367(w),
2995(m), 2809(w), 2707(m), 2517(m), 2359(w), 1775(w),
1759(w), 1730(w), 1673(s), 1634(m), 1597(w), 1582(w), 1528(s),
1492(m), 1480(w), 1465(w), 1446(m), 1428(s), 1353(s), 1309(s),
1274(s), 1262(m), 1196(s), 1174(s), 1139(s), 1116(m), 1064(w),
1025(m), 966(m), 799(m), 769(m), 720(m), 672(m), 649(w),
597(w), 533(w). 1H NMR (CDCl3; d, ppm): 3.68–3.99 (m, 8H,
OCH2), 4.22–4.64 (m, 8H, NCH2), 6.51 (d, J = 8.7 Hz, 1H, C6H3),
6.95–8.06 (m, 12H, Ph + C6H3), 9.17 (s, br, 1H, COOH), 11.49 (s,
br, 1H, COOH). ESI TOF(+) (m/z): 817.1299 [MꢀTFAꢀ]+ (Calc.
817.1276).
4.3.3. [ReO(L1)(OReO3)]
H2L1 (54 mg, 0.1 mmol) and three drops of NEt3 were added to a
solution of (NBu4)[ReOCl4] (58 mg, 0.1 mmol) in MeOH (3 mL). This
solution was heated under reflux for 30 min and finally the solvent
was removed under vacuum. The residue was dissolved in acetone.
The resulting clear red solution was slowly evaporated at room
temperature to give red crystals. Yield: 15 mg (15%). Anal. Calc.
for C30H34N6O5S2Re2: C, 36.21; H, 3.44; N, 8.44; S, 6.44. Found: C,
36.51; H, 3.22; N, 8.59; S, 6.63%. IR (
m
in cmꢀ1): 3055(w),
2970(w), 2936(w), 1543(vs), 1477(s), 1443(m), 1346(s), 1280(m),
1242(m), 1141(m), 1076(m), 983(m), 921(s), 875(s), 767(m). 1H
NMR (acetone-d6; d, ppm): 1.43 (t, J = 7.2 Hz, 6H, CH3), 1.49 (t,
J = 7.1 Hz, 6H, CH3), 4.05 (m, 4H, CH2) 4.37 (m, 2H, CH2), 4.45 (m,
2H, CH2), 6.53 (m, 2H, C6H4), 6.60 (m, 2H, C6H4), 7.52 (t,
J = 7.2 Hz, 4H, Ph), 7.53 (d, J = 7.1 Hz, 4H, Ph), 7.58 (t, J = 7.2 Hz,
2H, Ph). FAB+ MS (m/z): 745.4 [MꢀReO4ꢀ]+.
4.4. X-ray crystallography
The intensities for the X-ray determinations were collected on a
STOE IPDS 2T instrument with Mo K
Standard procedures were applied for data reduction and absorp-
tion correction. Structure solution and refinement were performed
with SHELXS and SHELXL [34]. Hydrogen atom positions were calcu-
lated for idealized positions and treated with the ‘riding model’ op-
a radiation (k = 0.71073 Å).
4.3.4. [{ReO(L2)}O]
tion of SHELXL
.
Solid H2L2 (57 mg, 0.1 mmol) was added to a solution of
(NBu4)[ReOCl4] (58 mg, 0.1 mmol) in MeOH (3 mL). The reaction
mixture was heated under reflux for 5 min, before 3 drops of
Et3N were added. The heating was continued for 30 min and the
solvent was removed under vacuum. The resulting residue was
recrystallized from a CH2Cl2/acetone mixture giving red crystals.
Yield: 54 mg (69%). Anal. Calc. for C60H60N12O7S4Re2: C, 46.14; H,
3.87; N, 10.76; S, 8.21. Found: C, 46.12; H, 3.95; N, 10.51; S,
More details on data collections and structure calculations are
contained in Table 3. Additional information on the structure
determinations has been deposited with the Cambridge Crystallo-
graphic Data Centre.
Appendix A. Supplementary data
m
in cmꢀ1): 3055(w), 2962(w), 2916(w), 2854(w),
CCDC 881796, 881797, 881798 and 881799 contain the supple-
mentary crystallographic data for [ReO(L1)(OReO3)], [{ReO(L2)}2O],
[ReO(L3)(MeOH)] and [ReO(L3)(TFA)]. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
or e-mail: deposit@ccdc.cam.ac.uk.
8.06%. IR (
1527(vs), 1477(vs), 1438(m), 1420(vs), 1361(s), 1265(m),
1226(m), 1172(w), 1114(m), 1026(m), 941(w), 767(m), 744(w),
694 (w). 1H NMR (CDCl3; d, ppm): 3.5–3.7 (m, br, 4H, NCH2), 3.8–
4.0 (m, br, 4H, NCH2), 4.1–4.3 (m, br, 4H, OCH2), 4.64 (d, br, 2H,
OCH2), 4.80 (d, br, 2H, OCH2), 5.95 (d, J = 8.3 Hz, 1H, CH2), 6.22
(d, J = 7.8 Hz, 1H, C6H4), 6.37 (t, J = 7.7 Hz, 1H, C6H4), 6.47 (t,
J = 7.7 Hz, 1H, C6H4), 7.11 (m, 4H, Ph), 7.29 (m, 4H, Ph), 7.53 (d,
J = 7.1 Hz, 2H, Ph). FAB+ MS (m/z): 790.8 [ReO(H2O)(L2)]+, 774.9
[ReO(L2)]+.
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(NBu4)[ReOCl4] (58 mg, 0.1 mmol) was dissolved in 20 mL of a
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