Coordination of a Schiff base as an iminium-thiolate zwitterion to rhenium(I)

Article abstract of DOI:10.1016/j.inoche.2014.07.042

The reaction of the potentially tridentate ligand 1-{1-(2-hydroxyphenyl) ethylidene}-4-phenylthiosemicarbazide (H₂hpt) with [Re(CO) ₅Cl] led to the isolation of a complex salt with the formulation [Re(CO)₃(H₂hpt)₂]Cl. One H₂hpt ligand is coordinated via the thione sulfur and imino nitrogen atoms, with the other coordinated in a novel manner as a monodentate iminium-thiolate zwitterion. The crystal structures of 1 and H₂hpt were determined by X-ray single crystal diffraction. In addition, infra-red, ¹H NMR and electronic properties are also reported.

Full text of DOI:10.1016/j.inoche.2014.07.042

Inorganic Chemistry Communications 47 (2014) 159–161
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Coordination of a Schiff base as an iminium-thiolate zwitterion
to rhenium(I)
⁎
G. Habarurema, E.C. Hosten, R. Betz, T.I.A. Gerber
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:
Received 3 July 2014
Received in revised form 30 July 2014
Accepted 31 July 2014
Available online 1 August 2014
The reaction of the potentially tridentate ligand 1-{1-(2-hydroxyphenyl)ethylidene}-4-phenylthiosemicarbazide
(H2hpt) with [Re(CO)5Cl] led to the isolation of a complex salt with the formulation [Re(CO)3(H2hpt)2]Cl. One
H2hpt ligand is coordinated via the thione sulfur and imino nitrogen atoms, with the other coordinated in a
novel manner as a monodentate iminium-thiolate zwitterion. The crystal structures of 1 and H2hpt were deter-
mined by X-ray single crystal diffraction. In addition, infra-red, 1H NMR and electronic properties are also
reported.
Keywords:
Rhenium(I)
© 2014 Elsevier B.V. All rights reserved.
Zwitterion
Iminium-thiolate
X-ray crystal structure
Schiff base complexes of rhenium(I) and (V) have been well studied,
and several comprehensive reports have appeared in the literature dur-
ing the last 30 years [1]. Most of these ligands were of the {2-(hydroxy/
mercapto)phenyl}salicylideneimine type, with a few synthesized from
acetyl-acetone derivatives and amines.
The IR spectrum of 1 is dominated by three very strong absorption
bands at 2024, 1942 and 1918 cm−1, attributed to ν(CO) of the fac-
[Re(CO)3]+ unit [5]. In the 3000–3350 cm−1 region there are three
peaks: a sharp medium-intensity band at 3305 cm−1, which is assigned
to ν(N\H), a broad medium-intensity absorption at 3179 cm−1, which is
ascribed to ν(N\H) of the iminium group, which is involved in strong
intramolecular hydrogen-bonding, and a broad weak absorption at
3114 cm−1 [ν(OH)]. The band of medium strong intensity at 751 cm−1
is assigned to the C_S stretching mode of the coordinated H2hpt ligand
[6]. The three peaks at 1567, 1578 and 1599 cm−1 are assigned to
ν(C_N). In the 1H NMR spectrum of 1 there are four broad singlet ab-
sorptions in the low field region 11.0–12.5 ppm, each corresponding to
one proton. These signals are ascribed to the NH protons, with one at
12.46 ppm assigned to the iminium proton. The electronic absorption
spectrum in dichloromethane exhibits a band at 422 nm, which is as-
cribed to the metal-to-ligand charge transfer dπ(Re) → π*(H2hpt)
transition [7].
Fig. 1 shows an ORTEP illustration of 1. The rhenium(I) is coordi-
nated to three carbonyl donors in a facial orientation, to the imino
nitrogen N(23) and thione sulfur S(2) atoms of H2hpt, and to the
thiolate sulfur S(1) of H2hti. The Re\C bond lengths [average
1.917(5) Å] fall in the range observed [1.900(2)–1.928(2) Å] for
similar complexes [8]. The Re\N(23) bond length of 2.242(3) Å is typ-
ical for Re(I)\N(imine) bonds [9]. The two Re\S bond distances are
different [Re\S(1) = 2.536(1), Re\S(2) = 2.467(1) Å], implying a dif-
ference in their bonding interaction with the metal.
In this report we present the first structural evidence for the coordi-
nation of a Schiff base as a zwitterion to rhenium in any oxidation state.
In fact, we are not aware that structural evidence has ever been pub-
lished for this type of coordination of a Schiff base to any transition
metal. The reaction of the potentially tridentate ligand 1-{1-(2-
hydroxyphenyl)ethylidene}-4-phenylthiosemicarbazide (H2hpt) [2]
with [Re(CO)5Cl] led to the isolation of a complex salt with the formu-
lation [Re(CO)3(H2hpt)2]Cl [3]. The X-ray crystal structure shows that
one H2hpt ligand is coordinated bidentately via the neutral thione sulfur
and neutral imino nitrogen atoms, with the phenolic group uncoordinat-
ed. Surprisingly, the second H2hpt ligand is coordinated monodentately in
the thiolate-iminium zwitterionic form H2hti through the monoanionic
thiolate sulfur atom, to give the salt fac-[Re(CO)3(H2hpt)(H2hti)]Cl (1)
[4].
A consideration of the bonding parameters in the backbones of
the ligands provides information on their coordination modes. In
the bidentate H2hpt chelate the S(2)\C(21) [1.705(4) Å] and
C(22)\N(23) [1.302(5) Å] bonds are double, with the N(22)\N(23)
⁎
Corresponding author.
E-mail address: Thomas.gerber@nmmu.ac.za (T.I.A. Gerber).
http://dx.doi.org/10.1016/j.inoche.2014.07.042
1387-7003/© 2014 Elsevier B.V. All rights reserved.

160
G. Habarurema et al. / Inorganic Chemistry Communications 47 (2014) 159–161
Fig. 1. Crystal structure of 1 with numbered atoms. Ellipsoids are shown at the 50% probability level and the solvent toluene has been omitted for clarity. Selected bond lengths [Å]:
Re\S(1) 2.536(1), Re\S(2) 2.467(1), Re\N(23) 2.242(3), Re\C(3) 1.917(4), N(23)\C(22) 1.302(5), S(2)\C(21) 1.705(4), N(21)\C(21) 1.336(5), N(22)\C(21) 1.342(5), N(22)\
N(23) 1.392(4), S(1)\C(11) 1.753(4), N(11)\C(11) 1.356(6), N(12)\C(11) 1.308(5), N(12)\N(13) 1.374(5), N(13)\C(12) 1.297(5). Selected bond angles [°]: S(2)\Re\N(23)
78.10(8), S(1)\Re\C(3) 179.1(1), C(12)\N(13)\N(12) 120.9(4), N(11)\C(11)\N(12) 117.8(4), C(11)\S(1)\Re 109.9(2), C(21)\S(2)\Re 99.1(2), C(22)\N(23)\N(22)
114.5(3), N(21)\C(21)\N(22) 114.5(4).
[1.392(4) Å], N(22)\C(21) [1.342(5) Å] and N(21)\C(21) [1.336(5) Å]
bonds all single. H2hpt is therefore coordinated in the neutral imino-
thione conformation. The planarity of the ligand backbone is indicated
by the torsion angle S(2)\C(21)\N(22)\N(23) of −6.3(5)°. The dihe-
dral angle between the two phenyl rings equals 32.3(2)°, and it seems
that the hydrogen-bond between O(2)H and the Cl(1) counter-ion
influences the orientation of the C(221)\C(226) phenyl ring. The N(21)
H and N(22)H protons are involved in intermolecular hydrogen-bonds
to Cl(1).
Focussing on H2hti, the S(1)\C(11) bond is single [1.753(4) Å], as
are N(12)\N(13) [1.374(5) Å] and N(11)\C(11) [1.356(6) Å]. The
N(12)\C(11) and N(13)\C(12) bonds are double [1.308(5) and
Fig. 2. Molecular structure of H2hpt. Ellipsoids are shown at the 50% probability level. Selected bond lengths [Å]: S(1)\C(1) 1.680(1), N(3)\C(2) 1.294(2), N(1)\C(1) 1.339(2), N(2)\
C(1) 1.356(2), N(2)\N(3) 1.379(1). Selected bond angles [°]: N(1)\C(1)\N(2) 116.3(1), C(1)\N(2)\N(3) 120.68(9), N(2)\N(3)\C(2) 118.35(9), N(3)\C(2)\C(21) 117.52(9).

G. Habarurema et al. / Inorganic Chemistry Communications 47 (2014) 159–161
J.Y. Kim, Y.J. Ji, H.-J. Ha, H.K. Chae, Bull. Korean Chem. Soc. 24 (2003) 504;
161
1.297(5) Å respectively]. H2hti is therefore coordinated as a neutral
monodentate ligand in the zwitterionic iminium-thiolate form. The li-
gand is essentially planar with a r.m.s. deviation of 0.0941 Å, defined
by the least square plane through all the non-hydrogen atoms, and the
dihedral angle between the two phenyl ring planes is only 5.0(3)°. This
planar conformation of the ligand is mainly determined by hydrogen-
bond interactions. The co-planarity of the phenol ring with the ligand
backbone is due to the N(13)\H⋯O(1) hydrogen-bond. The chloride
Cl(1) also forms a hydrogen-bond with O(1)H, and there is a weak inter-
action between N(11)H and S(2).
The distortion from octahedral ideality is mainly due to the trans
angles S(1)\Re\C(3) [179.1(1)°], S(2)\Re\C(5) [177.7(1)°] and
N(23)\Re\C(4) [168.0(2)°], which are the result of the constraints im-
posed by the bidentate ligand [S(2)\Re\N(23) = 78.10(8)°], which
forms a five-membered metallacycle.
A. Sachse, N.C. Mösch-Zanetti, G. Lyashenko, J.W. Wielandt, K. Most, J. Magull, F. Dall'
Antonia, A. Pal, R. Herbst-Irmer, Inorg. Chem. 46 (2007) 7129;
P. Mayer, T.I.A. Gerber, B. Buyambo, A. Abrahams, Polyhedron 28 (2009) 1174;
P. Mayer, K.C. Potgieter, T.I.A. Gerber, Polyhedron 29 (2010) 1423;
F. Tisato, F. Refosco, U. Mazzi, G. Bandoli, A. Dolmella, Inorg. Chim. Acta 164 (1989)
127;
F. Tisato, F. Refosco, U. Mazzi, G. Bandoli, M. Nicolini, J. Chem. Soc. Dalton Trans.
(1987) 1693.
[2] Synthesis of H2hpt. An equimolar ratio of 2-acetylphenol (3.685 g, 27 mmol) and 4-
phenylthiosemicarbazide (4.513 g) was heated under reflux for 4 h in 50 cm3 of
methanol. After heating was stopped, the volume was reduced under vacuum to
about 20 cm3, and it was left to stand at room temperature. After 2 days colourless
crystals were collected by filtration. They were washed with acetone, and dried
under vacuum. Yield = 5.625 g (73%), m.p. 158 °C. Anal. calcd for C15H15N3OS: C,
63.1; H, 5.3; N, 14.7. Found: C, 62.9; H, 5.3; N, 14.4 %. IR (KBr, cm−1): ν(C_S)
839, ν(C_N) 1618, ν(N\H) 3280, ν(O\H) 3322. 1H NMR (CDCl3, ppm): 8.78 (br
s,1H, OH), 8.46 (s, 2H, NH), 7.65 (d, 2H), 7.44 (d,1H), 7.36 (t, 2H), 7.29 (t, 1H), 7.19
(d, 1H), 6.95 (d,1H), 6.90 (t, 1H), 2.48 (s, 3H). UV–vis (CH2Cl2, λmax (ε, M−1 cm−1)):
399 (1475) nm.
The X-ray crystal structure [10] of the free ligand H2hpt (Fig. 2)
shows that the molecule is in the thione-imino tautomer in the solid
state. The S(1)\C(1) [1.680(1) Å] and N(3)\C(2) [1.294(2) Å] bonds
are double. Both the O(1)H and N(1)H protons are involved in intramo-
lecular hydrogen-bonds to N(3), which mainly determine the confor-
mation of the molecule.
[3] Synthesis of [Re(CO)3(H2hpt)2]Cl: A mixture of [Re(CO)5Cl] (100 mg, 277 μmol) and
H2hpt (170 mg, 595 μmol) was heated under reflux in 25 cm3 of toluene for three
hours. After heating was stopped, the solution was filtered at room temperature
and the filtrate was allowed to evaporate at room temperature. After 3 days X-ray
quality dark yellow crystals were collected by filtration. They were washed with
diethyl ether and dried under vacuum. Yield = 0.179 g (67 % based on Re), m.p.
174 °C. IR (νmax/cm−1): ν(C`O) 1918, 1942, 2024; ν(C`N) 1567, 1578, 1599;
ν(OH) 3114; ν(N\H) 3179, 3305; ν(C_S) 751. 1H NMR (d6-DMSO, 295 K, ppm):
12.46 (br s, 1H, NH), 11.79 (br s, 1H, NH), 11.46 (br s, 1H, NH), 11.02 (br s, 1H, NH),
6.72–7.75 (m, 18H, PhH), 2.76 (s, 3H, CH3), 2.67 (s, 3H, CH3). UV–vis (CH2Cl2, λmax(ε,
The coordination of Schiff base ligands in the iminium [\C_NH+\]
form has never been observed for rhenium/technetium, and is a rare
phenomenon in coordination chemistry. In a study with the Schiff
base N-(2-hydroxy-3-carboxy-1-naphthylidene)-4-methyl-2-sulfonic
acid aniline, spectroscopic data were presented that implied that the li-
gand is coordinated to the metal ions Zn2+, Co2+, Ni2+, Mn2+ and
UO22+ in the solid state in the enol-iminium zwitterionic form [11].
This phenomenon has also been suggested in reactions of pyridine-
2,6-dicarboxylic acid with a variety of divalent metal oxinates [12]. Al-
though free amino acids such as histidine and cysteine exist as zwitter-
ions both in solution and in the solid state, they do not coordinate as
such to metal ions [13].
M
−1 cm−1)): 422 (2250) nm. Conductivity (10−3 M, DMF) = 78 ohm−1 cm2 mol−1
.
[4] Crystallographic data for C33H30N6O5ReS2Cl.C7H8: monoclinic; space group P21/c;
a = 13.8184(8), b = 21.333(1), c = 14.5346(9) Å; β = 107.635(3)°; V = 4083.
3(4) Å3; Z = 4; Dc = 1.576 g cm−3; μ = 3.194 mm−1; data/parameters: 10192/
480; S = 1.27; final R indices [I N 2σ(I)]: R1 = 0.0356, wR2 = 0.0724.
[5] J. Granifo, Polyhedron 18 (1999) 1061;
D.R. Gamelin, M.W. George, P. Glyn, F.W. Grevek, K. Schafner, J.J. Turner, Inorg. Chem.
33 (1995) 3246.
[6] D. Lin-Vien, N.B. Colthup, W.G. Fateley, J.G. Grasselli, The Handbook of Infrared and
Raman Characteristic Frequencies of Organic Molecules, Academic Press Ltd,
London, 1991. 237.
[7] K. Wang, L. Huang, L. Gao, L. Jin, Inorg. Chem. 41 (2002) 3353;
S.-S. Sun, A.J. Lees, Organometallics 20 (2001) 2353.
[8] N.S. Fleur, J.C. Hili, A. Mayr, Inorg. Chim. Acta 362 (2009) 1571;
L. Wei, J.W. Babich, W. Quellette, J. Zubieta, Inorg. Chem. 45 (2006) 3057;
J. Agarwal, R.P. Johnson, G. Li, J. Phys. Chem. A115 (2011) 2877.
[9] M. Chatterjee, G. Achari, S. Das, R. Banerjee, C. Chakrabarti, J.K. Dattagupta, S.
Banerjee, Inorg. Chem. 37 (1998) 5424;
Appendix A. Supplementary material
Supplementary data for 1 (CCDC 1010027) and H2hpt (CCDC
1016107) are available from CCDC, 12 Union Road, Cambridge CB2
IEZ, UK on request. These data can be obtained free of charge from the
Cambridge Crystallographic Data via http://www.ccdc.cam.acuk/data_
request/cif. Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.inoche.
2014.07.042. These data include MOL files and InChiKeys of the most
important compounds described in this article.
T.I.A. Gerber, R. Betz, I.N. Booysen, K.C. Potgieter, P. Mayer, Polyhedron 30 (2011)
1739.
[10] Crystallographic data for C15H15N3OS: triclinic; space group P-1; a = 5.5275(4),
b = 10.6016(8), c = 12.2966(8) Å; α = 74.344(3), β = 107.635(3), γ = 87.
208(3)°; V = 691.66(9) Å3; Z = 2; Dc = 1.370 g cm−3; μ = 0.233 mm−1
;
data/parameters: 3445/191; S = 1.04; final R indices [I N 2σ(I)]: R1 = 0.0295,
wR2 = 0.0843.
[11] J.M. Bellerby, J.H. Morris, W.E. Smith, Inorg. Chim. Acta 96 (1985) 203.
[12] M.M. Aly, Inorg. Nucl. Chem. Lett. 15 (1979) 161.
[13] P.R. Reddy, M. Radhika, P. Manjula, J. Chem. Sci. 117 (2005) 239.
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