Inorganic Chemistry Communications
Short Communication
Formation of a methine carbon-to-rhenium σ bond in an
oxorhenium(V)-benzothiazole complex
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X. Schoultz, T.I.A. Gerber , E.C. Hosten
Department of Chemistry, Nelson Mandela Metropolitan University, PO Box 77000, 6031 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 reaction of trans-[ReOCl3(PPh3)2] with 1-(1,3-benzothiazol-2-yl)-3-benzoylthiourea (Hbbt) in methanol
led to the isolation of the complex [ReOCl2(cbt)(PPh3)] (1). It contains the monoanionic bidentate chelate N-
((benzothiazol-2-ylamino)methylene)benzamide (cbt). The new ligand cbt is coordinated via a neutral ketonic
oxygen and an anionic imino-acyl carbon atom, and was formed by the abstraction of the thionyl sulfur of
Hbbt by a triphenylphosphine of the starting complex, with S = PPh3 also been formed as by-product. Compound
1 was characterized by elemental analysis, FTIR, proton NMR and single crystal X-ray diffraction.
© 2016 Elsevier B.V. All rights reserved.
Received 1 February 2016
Received in revised form 18 March 2016
Accepted 23 March 2016
Available online 6 April 2016
Keywords:
Oxorhenium(V)
Benzothiazole
Metal-carbon σ bond
X-ray crystal structure
Due to its central position in the Periodic Table, the metal rhenium is
probably the most versatile of all transition metals, since it occurs in all
oxidation states from −I to +VII [1]. The considerable current interest
in the coordination chemistry of the metal is not only due to the possible
application of its 186/188Re isotopes in nuclear medicine, but also due to
the large variety of the structural, photophysical, redox and photochem-
ical properties of its complexes [2]. These properties are mainly
determined by the coordination sphere of the metal ion [2,3].
Benzothiazole compounds have exhibited a large variety of bio-
logical activities [4]. Previous studies on rhenium complexes of
benzothiazoles have concentrated on the oxidation states +I and +V.
With benzothiazole (bt) the complex [ReI(CO)3(bt)2Cl] was isolated, in
which each bt ligand is coordinated in a monodentate manner via the
thiazole nitrogen atom [5]. With [ReOCl3(PPh3)2] as precursor, the com-
plex [ReOCl2(hbt)(PPh3)] was characterized from the reaction with 2-
(2′-hydroxyphenyl)-2-benzothiazole (Hhbt) [6]. A benzothiazole deriv-
ative conjugated as a monoamine–monoamide was used to synthesize
an oxorhenium(V) complex, which showed selective binding to amyloid
aggregates in the brains of patients with Alzheimer's disease [7]. Our
aim is to synthesize complexes in which the benzothiazole moiety is
uncoordinated, leaving it free to bind to Aβ plaques via intercalation
and π–π interactions [8].
monodentate [9]. It is important to note that these chelation modes
will all lead to the formation of six-membered metallocycles [10].
However, in its reaction with [ReOCl3(PPh3)2] in methanol [11], the
thione sulfur atom was abstracted by a triphenylphosphine group, to
form the chelate cbt, which is coordinated to the metal via a neutral ke-
tonic oxygen atom and monoanionic imino-acyl sp2-hybridized carbon
atom (see Scheme 1) in the complex [ReOCl2(cbt)(PPh3)] (1). Crystals
of S = PPh3 were found in the residue obtained from the evaporation
of the mother liquor, and its identity was established by an X-ray crystal
structure determination [12]. As far as could be established, complex 1 is
the first example of an oxorhenium(V) complex containing a methine
carbon-to-rhenium(V) σ bond in its coordination sphere. It is also a
rare occurrence in high oxidation state coordination chemistry.
The Re_O stretching frequency appears at 947 cm−1 in the IR spec-
trum of 1. A medium intensity and a strong intensity peak at 1694 and
1588 cm−1 are assigned to ν(C_O) and ν(C_N) respectively. The
N\\H stretch occurs at 3055 cm−1, and a strong peak at 508 cm−1 is
tentatively assigned to ν(Re\\C). The UV–vis spectrum in DMF shows
an absorption band at 300 nm, ascribed to the π → π* transition in the
coordinated ligand cbt, and another at 387 nm, which is ascribed to a
combination of the ligand-to-metal charge transfer transitions LMCT
[pπ(O2–) → dπ*(Re), pπ(C–) → dπ*(Re) and pπ(Cl−) → dπ*(Re)] respec-
We considered Hbbt (Scheme 1) as an interesting ligand for the
oxorhenium(V) moiety. Hbbt has a few coordination possibilities,
from dianionic tridentate (NSO), to bidentate (NS or SO) and even
tively [13]. A weak absorption at 663 nm is assigned to a d–d transition
2
[(dxy
)
→ (dxy)1(dxz)1]. The position of these bands and their
assignment are in accordance with other Schiff base complexes of
oxorhenium(V) complexes [14]. The 1H NMR spectrum is complicated
by the signals of the phenyl protons of PPh3, in addition to the multiplets
of the aromatic protons of cbt, which give rise to nine-proton multiplets
in the region 7.64–7.78 ppm. The NH proton occurs as a singlet at
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