Monomeric/dimeric complexes of fac-[Re(CO)3]+ with benzoylthiourea derivatives

Article abstract of DOI:10.1016/j.inoche.2012.08.008

The reactions of [Re(CO)₅Br] with the potentially bidentate ligands N,N-dialkylbenzoylthiourea (HL1) and its N,N-diphenyl derivative (HL2) were studied. In the complex fac-[Re(CO)₃(HL1)₂Br] (1) the two neutral HL1 ligands are coordinated in a monodentate manner via the thiourea sulfur atom. Coordination of the monoanionic chelate L2 occurs through the neutral benzoyl oxygen atom and the anionic sulfur atom, which also acts as a bridge to the second metal centre, in the dimeric complex [Re ₂(CO)₆(L2)₂] (2). The crystal structures of 1 and 2 were determined by X-ray single crystal diffraction. UV-vis absorption spectra of the complexes show the intraligand and MLCT transitions.

Full text of DOI:10.1016/j.inoche.2012.08.008

Inorganic Chemistry Communications 24 (2012) 136–139
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Monomeric/dimeric complexes of fac-[Re(CO)3]+ with benzoylthiourea derivatives
⁎
B. Schmitt, T.I.A. Gerber , E. Hosten, R. Betz
Department of Chemistry, Nelson Mandela Metropolitan University, Port Elizabeth, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
The reactions of [Re(CO)5Br] with the potentially bidentate ligands N,N-dialkylbenzoylthiourea (HL1) and its N,
N-diphenyl derivative (HL2) were studied. In the complex fac-[Re(CO)3(HL1)2Br] (1) the two neutral HL1 ligands
are coordinated in a monodentate manner via the thiourea sulfur atom. Coordination of the monoanionic chelate
L2 occurs through the neutral benzoyl oxygen atom and the anionic sulfur atom, which also acts as a bridge to
the second metal centre, in the dimeric complex [Re2(CO)6(L2)2] (2). The crystal structures of 1 and 2 were deter-
mined by X-ray single crystal diffraction. UV–vis absorption spectra of the complexes show the intraligand and
MLCT transitions.
Received 24 May 2012
Accepted 11 August 2012
Available online 18 August 2012
Keywords:
Rhenium(I)
Benzoylthiourea
Thiolic bridge
Dimer
© 2012 Elsevier B.V. All rights reserved.
The current interest in the coordination chemistry of rhenium and
technetium is mainly the result of the application of their 186/188Re and
99mTc radionuclides in radiopharmacy [1]. Since these two metals are
congeners, rhenium is often used as a non-radioactive substitute for
technetium in experiments. Metal-based radiopharmaceuticals require
thermodynamically and kinetically stable complexes, and therefore the
focus has recently mainly been on compounds containing the robust
rhenium(I) (d6) core fac-[Re(CO)3]+ with a large variety of ligands
containing a combination of nitrogen, oxygen and sulfur donor atoms
[2–4].
In the past few years there has been considerable interest in
benzoylthiourea derivatives as ligands for transition metals in general
[5], and also for rhenium and technetium in particular [6]. These
derivatives have shown in vitro activity against a large range of tumours
[7], and have exhibited antibacterial [8], antiviral [8], herbicidal [9] and
antifungal [10] activities.
In this account the products of the reactions of the potentially
bidentate ligands, N,N-dialkyl-(HL1) and N,N-diphenylbenzoylthiourea
(HL2) with [Re(CO)5Br] are reported [11]. With HL1, the complex
fac-[Re(CO)3(HL1)2Br] (1) was isolated, in which HL1 is coordinated
as a neutral monodentate ligand with coordination via the thiocarbonyl
sulfur atom. The reaction with HL2 led to the dimeric complex
[Re2(CO)6(L2)2] (2), in which L2 acts as a monoanionic chelate
with coordination via the benzoyl oxygen and the sulfur atoms as a
bridge between the two rhenium(I) centres.
Both complexes were prepared by the heating under reflux of
[Re(CO)5Br] with a twofold molar excess of the ligands HL in toluene
[12]. They were characterised by elemental analysis, FTIR, 1H NMR
and single crystal X-ray diffraction. All the data are consistent with
their formulation.
The infrared spectrum of 1 displays three strong bands in the
carbonyl stretching region at 2024, 1943 and 1884 cm−1. In 2 the
corresponding absorptions are at 2050, 1932 and 1902 cm−1. This
pattern is typical for three carbonyls in a facial isomer arrangement
around the rhenium [13]. The ν(C_O) of the ligands HL1 in 1 is
assigned to the peak at 1703 cm−1, with ν(C_S) and ν(NH) at
1535 and 3119 cm−1 respectively. For 2 the ν(C_O) of L2 occurs
as a medium intensity peak at 1697 cm−1, with a strong absorption
at 1577 cm−1 being assigned to ν(C_N).
In the 1H NMR spectrum of 1, the signal of the NH proton occurs the
furthest downfield as a singlet at 10.50 ppm, with the four protons on
each phenyl ring appearing as two doublets at 7.77 and 7.58 ppm. The
presence of the four ethyl chains in 1 is confirmed by four-proton
quartets at 3.94 and 3.42 ppm and two triplets at 1.36 and 1.21 ppm.
The spectrum of 2 is not informative, with a 26 proton multiplet in
the range 7.11–7.83 ppm.
The electronic absorption and luminescence spectra of 1 and 2
were recorded at room temperature. The absorption spectra of both
complexes exhibit two peaks. On the basis of previously reported
⁎
Corresponding author. Tel.: +27 41 5044285; fax: +27 41 5044236.
E-mail address: thomas.gerber@nmmu.ac.za (T.I.A. Gerber).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2012.08.008

B. Schmitt et al. / Inorganic Chemistry Communications 24 (2012) 136–139
137
As is shown in Fig. 1, the photo-excitation of the complexes at
422 nm (for 1 in acetone) and 420 nm (for 2 in dichloromethane)
leads to ligand-centred emissions at 503 and 489 nm respectively.
In fact, both ligands HL1 and HL2 exhibit an emission at 501 nm
after being excited at 425 nm (for HL1 in acetone) and 420 nm (for
HL2 in chloroform). The nitrogen-equilibrated samples of 1 and 2
gave quantum yields of 0.012 and 0.008 respectively. The lifetimes
of the complexes could not be measured due to the limitations of
the spectrofluorometer. A comparison of the photophysical data of
the complexes with other [Re(CO)3]+ complexes containing thiourea
derivatives as ligands was not possible, since no such data were
reported [6a,15,16]. However, for the complexes fac-[Re(CO)3N3],
containing the tridentate derivatives of benzimidazole, quinoline
and tryptophan (N3), emission lifetimes of the order of 16 μs were
found for MLCT emissions [17]. Interestingly, for complexes in
which the emissions are not related to MLCT, the lifetimes were also
too short to be measured [17].
Fig. 1. Emission spectra of complexes 1 (in acetone) and 2 (in dichloromethane) after
equilibration under nitrogen.
Â
Ã
Structure of f ac‐ ReðCOÞ3ðHL1Þ2Br
ð1Þ
Crystals of 1 were obtained from the slow evaporation of the
preparative solution at room temperature. The packing in the unit
cell shows discrete monomeric and neutral units, with no intermolecular
contacts shorter than the Van Der Waals' radii sum [18]. Each
rhenium(I) atom lies in a distorted octahedral environment, with
the bromide and two sulfur atoms in a facial arrangement, imposed
by the fac-[Re(CO)3]+ core (Fig. 1). Both HL1 ligands are neutral
and are coordinated monodentately through the thiocarbonyl sulfur
atoms. The three Re\C bond distances [average 1.910(3) Å] fall in
the range observed [1.900(2)–1.928(2) Å] for similar complexes
spectroscopic studies of Re(I) tricarbonyl complexes [14], we assigned
the intense absorption around 350 nm (355 nm for 1, 343 nm for 2)
to the intraligand transition (π→π*), since similar absorptions are
observed for the uncoordinated ligands HL1 and HL2. The less
intense and lower-energy absorptions around 445 nm (441 nm for
1, 448 nm for 2) are assigned to the metal-to-ligand charge transfer
(MLCT) dπ(Re) → π* (ligand) transitions. The latter absorption is
more intense for complex 2, which is reasonable since it has a more
conjugated π system.
Fig. 2. ORTEP diagram of the molecular structure of fac-[Re(CO)3(HL1)2Br] (1). Thermal ellipsoids are drawn at 40% probability. Selected bond lengths [Å]: Re–Br(3) 2.6690(3), Re–S(1)
2.5027(7), Re–S(2) 2.5234(8), Re–C(92) 1.890(3), Re–C(90) 1.922(3), Re–C(91) 1.919(3), C(1)–S(1) 1.703(3), C(3)–S(2) 1.709(3), O(1)–C(2) 1.210(4), O(2)–C(4) 1.211(3), C(1)–N(2)
1.324(4), C(3)–N(4) 1.320(4), C(3)–N(3) 1.382(4). Bond angles [°]: S(1)–Re–S(2) 85.63(3), Br(3)–Re–S(1) 92.99(2), Br(3)–Re–S(2) 93.91(2), Br(3)–Re–C(91) 90.00(9), S(1)–Re–C(90)
175.4(1), C(1)–S(1)–Re 115.22(8), N(1)–C(1)–N(2) 119.2(3).

138
B. Schmitt et al. / Inorganic Chemistry Communications 24 (2012) 136–139
Fig. 3. Molecular diagram and atom numbering scheme for [Re2(CO)6(L2)2] (2). Selected bond lengths [Å]: Re–S(1) 2.514(2), Re–S(1i) 2.530(2), Re–O(1) 2.154(2), Re–C(43)
1.901(4), C(1)–O(1) 1.251(4), S(1)–C(2) 1.782(4), N(1)–C(2) 1.308(4), N(1)–C(1) 1.333(4), C(2)–N(2) 1.354(5). Bond angles [°]: Re(1)–S(1)–Re(1i) 97.89(3), Re–S(1)–C(2)
98.8(1), S(1)–Re–O(1) 85.61(6), C(1)–N(1)–C(2) 127.5(3), O(1)–Re–C(42) 174.9(1), S(1)–Re–C(43) 175.1(1), S(1)–Re–C(41) 91.9(1).
[14,19]. The two Re\S bond lengths are similar [average of
2.5131(8) Å] and are typical for Re(I)-thiocarbonyl bonds [19,20].
The distortion from octahedral ideality is mainly the result of the trans
angles S(1)–Re–C(90)=175.4(1)°, S(2)–Re–C(91)=176.09(9)° and
Br(3)–Re–C(92)=178.3(1)°.
In the neutral coordinated ligands HL1 the oxygen atoms O(1) and
O(2) are not coordinated, and are doubly bonded to C(2) and C(4)
respectively [O(1)–C(2)=1.210(4) Å, O(2)–C(4)=1.211(3) Å].
Enolisation of HL1 has not occurred. The C(1)–S(1) and C(3)–S(2)
bonds are double [1.706(3) Å average], and all the C\N bonds are
single. The coordinated bromide Br(3) is involved in intramolecular
hydrogen bonds with N(1)H and N(3)H.
In a previous study the complex salt fac-[Re(CO)3(tu)3](NO3) was
isolated from the reaction of fac-[Re(CO)3Br3]2− and thiourea (tu)
[20]. The three tu ligands are coordinated monodentately via the
neutral sulfur atoms, with an average Re\S bond length of 2.529(2)
Å.
(Fig. 3). These distances fall in the range normally observed for thiolate
sulfur atoms coordinated to a fac-[Re(CO)3]+ core [22]. The Re–Re dis-
tance across the rhombus is 3.803(2) Å, implying no Re–Re bonding.
The Re–C(carbonyl) distances [1.905(4) Å] fall in the observed range.
The Re–O(1) distance [2.154(2) Å] is considerably longer than the ob-
served distance of a Re–O(alcoholate) bond, which occurs around
2.00(2) Å [22], implying that O(1) is a neutral benzoyl oxygen atom.
The distortion from an ideal octahedral geometry around the
rhenium is mainly the result of the constraints imposed by the
coordination of the ligand L2, which forms a five-membered chelate
ring with Re(1) [O(1)–Re(1)–S(1)=85.61(6)°].
The bonding parameters in the ligand L2 indicate that it is coordinated
in the deprotonated enolate form. The C(1)–O(1) bond is double, the
C(2)–S(1) bond is single and C(2)–N(1) is a methinyl double bond
[C_N=1.308(4) Å], compared to the single C(2)–N(2) bond with a
length of 1.354(5) Å.
In a previous study the rhenium(I) complex fac-[Re(CO)3Br3]2−
has been reacted with thiourea (tu) to produce the complex salt
fac-[Re(CO)3(tu)3](NO3), in which each tu ligand is coordinated
monodentately via the neutral sulfur atom, with a mean Re\S bond
length of 2.529(2) Å [16]. With N,N-diethylbenzoylthiourea (Hbtu), the
monomeric complex fac-[Re(CO)3(Hbtu)Br] was isolated, in which
Hbtu acts as a neutral bidentate chelate with an oxygen and
sulfur as donor atoms [16]. With the related ligand N,N-
diethylthiocarbamoylbenzamidine (Htcb) the dimeric complex
[Re2(CO)6(tcb)2] was formed, with bridging through the neutral sulfur
atom and coordination via the deprotonated aminate nitrogen atom
[16]. The rhenium(V) compound trans-[ReOCl3(PPh3)2] reacts with
Hbtu to form the complex [ReOCl2(PPh3)(btu)], in which the benzoyl
oxygen is coordinated trans to the oxo oxygen atom [6c].
Â
Ã
Structure of Re2ðCOÞ6ðL2Þ2
ð2Þ
Yellow crystals with the formulation 2 toluene were obtained by
the slow evaporation of the preparative solution [21]. The dimer is
produced by inversion, of which the centre is positioned in the centre
of the nearly square Re2S2 ring. Each rhenium(I) atom lies in a
distorted octahedral environment, with an oxygen and two bridging
thiolic sulfur atoms coordinated in a facial arrangement to the
fac-[Re(CO)3]+ moiety (Fig. 2). Both L2 ligands act as monoanionic
tridentate ligands with coordination through a neutral carbonyl
oxygen and an anionic bridging thiolate sulfur atom.
The dinuclear molecule is characterised by a rhombic (μ-S)2Re2 unit
at the centre. Each sulfur-bridge is practically symmetrical, with very sim-
ilar Re–S distances [Re(1)–S(1)=2.514(2) Å, Re(1)–S(1′)=2.530(2) Å]
The different coordination behaviour of the two ligands used in
this study, HL1 and HL2, is peculiar. The only structural difference

B. Schmitt et al. / Inorganic Chemistry Communications 24 (2012) 136–139
139
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[12] General method of synthesis: Two equivalents of the ligand HL1 (equimolar quantities
of HL2) and 100 mg (246 μmol) of [Re(CO)5Br] in 15 cm3 of toluene were heated
under relux for 3 h under nitrogen. After the heating was stopped, the solution was fil-
tered at room temperature and the filtrate was left to evaporate at room temperature.
After 3 days, crystals which were suitable for X-ray diffraction studies, were collected
by filtration, and they were washed with acetone and diethyl ether.[Re(CO)3(HL1)2Br]
(1): Yellow. Yield: 153 mg (63%), m.p. 148 °C. Anal. Calcd. for C27H30Br3N4O5ReS2: C,
33.1; H, 3.1; N, 5.7. Found: C, 32.8; H, 3.2; N, 5.5%. IR (cm−1): ν(NH) 3119 m,
ν(CO)fac 2024vs, 1943vs, 1884vs; ν(C_O) 1703 s; ν(C_S) 1535 s. 1H NMR (ppm):
10.50 (s, 2H, NH), 7.77 (d, 4H), 7.58 (d, 4H), 3.94 (q, 4H, CH2) 3.42 (q, 4H, CH2),
1.36(t, 6H, CH3), 1.21 (t, 6H, CH3). UV–vis (DMF, λmax (ε, M−1 cm−1)): 355 (7700),
441(800).[Re2(CO)6(L2)2].C7H8 (2): Yellow. Yield: 101 mg (57%), m.p. 201 °C. Anal.
Calcd. for C46H28Br2N4O8Re2S2.C7H8: C, 44.4; H, 2.5; N, 3.9. Found: C, 44.6; H, 2.7; N,
3.7%. IR (cm−1): ν(CO)fac 2050vs, 1932vs, 1902vs; ν(C_N) 1577 s; ν(C_O)
1697 m. 1H NMR (ppm): 7.87 (d, 2H), 7.11–7.83 (m, 26H). UV–vis (CH3CN, λmax (ε,
M−1 cm−1)): 343 (15200), 448 (1100).
Acknowledgements
This work was supported financially by the Nuclear Technologies in
Medicine and the Biosciences Initiative (NTeMBI), a national technology
platform developed and managed by the South African Nuclear
Energy Corporation and supported by the Department of Science and
Technology.
Appendix A. Supplementary material
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Supplementary data for 1 (CCDC 879917) and 2 (CCDC 879918)
are available from the CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK on request. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via http://www.ccdc.cam.
ac.uk/data_request/cif. Supplementary data associated with this arti-
cle can be found, in the online version, at http://dx.doi.org/10.1016/
j.inoche.2012.08.008.
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