Clicking thiourea into a salen scaffold: Structures and cytotoxicity of cobalt(II) and nickel(II) complexes

Article abstract of DOI:10.1016/j.inoche.2014.08.033

A thiourea moiety has been introduced into the known salen-type ligand scaffold to give a new unsymmetric ligand (L) by using a simple CLICK chemistry. The cobalt(II) and nickel(II) complexes of this ligand were prepared by a one-pot procedure and structu

Full text of DOI:10.1016/j.inoche.2014.08.033

Inorganic Chemistry Communications 48 (2014) 127–130
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Clicking thiourea into a salen scaffold: Structures and cytotoxicity of
cobalt(II) and nickel(II) complexes
a,
a
a,
b
b
Guoqi Zhang ⁎, Christine Ta , Shu-Yuan Cheng ⁎, James A. Golen , Arnold L. Rheingold
a
Department of Sciences, John Jay College, The City University of New York, New York, NY 10019, United States
Department of Chemistry, University of California, San Diego, La Jolla, CA 92093, United States
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A thiourea moiety has been introduced into the known salen-type ligand scaffold to give a new unsymmetric
ligand (L) by using a simple CLICK chemistry. The cobalt(II) and nickel(II) complexes of this ligand were prepared
by a one-pot procedure and structurally characterized. X-ray structural analysis confirmed that the two com-
Received 16 May 2014
Accepted 31 August 2014
Available online 1 September 2014
2
plexes are isomorphic and exhibit a tetra-coordinate metal center in a near square planar N OS coordination
environment. The cocrystallized methanol molecules that act as both hydrogen bonding donors and acceptors
are hydrogen bonded to the complexes with both the coordinated O atoms and uncoordinated NH groups to gen-
erate a novel one-dimensional supramolecular helical chain. Both P-helix and M-helix were present in the crystal,
which resulted in an overall racemic mixture of the chiral helix. Both complexes display cytotoxicity towards rat
pheochromocytoma (PC12) and human embryonic kidney (HEK293) cells with the cobalt(II) complex slightly
more toxic than the nickel(II) one.
Keywords:
Click chemistry
Thiourea
Unsymmetric ligand
Crystal structure
Cytotoxicity
©
2014 Elsevier B.V. All rights reserved.
Since its introduction in 2001, the CLICK chemistry has been exten-
sively explored and has applications in many aspects of the current
chemistry including dendrimer, biomacromolecular and supramolecu-
lar materials and pharmaceutical chemistries [1–7]. A CLICK reaction is
generally the one that must be “modular, wide in scope, and give very
high yields” [1], and the Huisgen [3 + 2] dipolar cycloaddition of an
alkyne with an azide, is the most popular CLICK reaction explored to
date [8]. The coupling of organic isothiocyanates and amines is also a
known CLICK reaction that has been applied to the syntheses of asym-
metric organocatalysts and pharmaceutical molecules [9–12].
reaction of isothiocyanates and amines. Since the known capability of
forming both intermolecular hydrogen bonds and coordination bonds
with metals for a thiourea unit, an interesting supramolecular chemis-
try of these unsymmetric ligands is anticipated [21,22]. Although
metal-salen complexes and complexes with thiourea-based ligands
showed promising cytotoxic activity against a variety of cancer cells,
metal complexes composed of the semi-salen ligand L were unexplored
(Scheme 1) [23–26]. Therefore, in this communication we report for the
first time the syntheses and crystal structures of two mononuclear
cobalt(II) and nickel(II) complexes based on L, as well as their cytotoxic
activity towards rat pheochromocytoma (PC12) and human embryonic
kidney (HEK293) cells under physiological environments.
2 2
Salen is a classic bis-Schiff base type ligand that contains a N O bind-
ing cavity, and it and its derivatives have become important building
blocks for generating a variety of transition metal complexes [13–18].
Unsymmetric salen-like ligands are interesting derivatives of this type
Reacting phenyl isothiocyanate with a slightly excess of o-
phenyldiamine in dry CH Cl with the presence of triethylamine at
2 2
that contain different binding sites than traditional C
1
-symmetry salen
room temperature afforded pure monothiourea-substituted amine 1
in high yield by simple filtration of the reaction mixture [27]. 1 is a
known compound in the literature and was prepared by a mechano-
chemical method, the purification of the monosubstituted product,
however, required the use of column chromatography [11,28]. Spectro-
scopic data of 1 is consistent with those reported previously [11]. The
ligands, yet remain strong coordination ability towards metal ions. One
of us has recently investigated the use of chiral 4-pyridyl-containing
unsymmetric salen ligands and their reduced analogs for highly efficient,
enantio- and diasteroselective asymmetric Henry and aza-Henry reac-
tions, highlighting the great potential of metal complexes based on
unsymmetric salen-like ligands in catalytic applications [19,20]. Inspired
by these results, we are now interested in the search of another type
of unsymmetric salen-like ligands through replacing one half of a
salen scaffold with a thiourea moiety, by virtue of the facile CLICK
subsequent one-pot reaction of 1, salicyaldehyde and Co(OAc)
2 2
·2H O
(or Ni(OAc) ·4H O) in a 1:1:1 ratio in CH Cl –MeOH solution led to a
2
2
2
2
II
II
light red-brown solution, from which red blocks of Co or Ni complexes
(2 or 3) formed, respectively, after slow evaporation of the solvent over
several days (Scheme 1). The X-ray quality crystals of 2 and 3 were iso-
lated in 84.8% and 89.0% yields, respectively [29]. Both complexes are
⁎
Corresponding authors.
well soluble in CH
2 2 3
Cl , CHCl and DMSO, but only slightly soluble in
E-mail addresses: guzhang@jjay.cuny.edu (G. Zhang), shcheng@jjay.cuny.edu
(S.-Y. Cheng).
methanol, acetonitrile or water. Their structures were characterized by
http://dx.doi.org/10.1016/j.inoche.2014.08.033
387-7003/© 2014 Elsevier B.V. All rights reserved.
1

128
G. Zhang et al. / Inorganic Chemistry Communications 48 (2014) 127–130
Scheme 1. Synthetic route to the thiourea-amine 1 and cobalt(II) and nickel(II) complexes 2 and 3, and the structure of ligand L formed in situ.
UV–vis, IR, FAB-MS, and elemental analysis and confirmed by X-ray
crystallography. The IR spectra of 2 and 3 revealed a strong absorption
at 1595 and 1596 cm , respectively, assigned to the C_S bonds of
unit adjacent to the phenyl ring was deprotonated and bound to the
cobalt(II) center, resulting in a shorter Co–N2 bond than Co–N1
(1.811(3) vs. 1.857(3)). Atoms of the CoN OS core are essentially copla-
2
−
1
the thiourea moiety, which are remarkably shifted from the peak
observed at 1613 cm in the IR spectrum of 1, due to the metal coordi-
nation. The electronic absorption spectra showed peaks at 361 and
nar. The N2–Co1–S1 angle is 73.64(10)°, considerably more acute than
the N1–Co1–O1 angle (98.49(13)°), due to the formation of a four-
membered Co1–N2–C14–S1 metallocycle (Fig. 1a).
−
1
4
31 nm for 2 and at 364 and 431 nm for 3, indicative of the complexa-
Further inspection of the molecular packing in the cell reveals signif-
icant intermolecular hydrogen bonding interactions between co-
crystallized methanol molecules and the metal complexes. Hydrogen
bonding data for 2 · MeOH and 3 · MeOH is summarized in Table 1.
In 2 · MeOH, each methanol molecule is hydrogen bonded to the coor-
dinated O atom in the complex with an O–H…O hydrogen bond (H…
A = 2.02(3) Å) and to the uncoordinated NH group of thiourea from
another molecule of the complex with an N–H…O hydrogen bond
(H…A = 2.07(3) Å); and acts as both a hydrogen bonding donor and
acceptor. As a consequence, the double hydrogen bonds mediated by
tion of the ligand with metal ions.
X-ray structural analysis unambiguously confirmed the mononucle-
ar tetra-coordinate structures of 2 and 3, and that they crystallized as
methanol solvates with formula of 2 · MeOH and 3 · MeOH, respective-
ly [30]. The crystal structures of both complexes are depicted in Fig. 1
and the relevant bond parameters are given in the caption. Crystals of
2
· MeOH and 3 · MeOH are essentially isomorphic and thus only the
structure of 2 · MeOH is described in detail. 2 · MeOH crystallized in
the orthorhombic space group Pna2(1) with a cocrystallized methanol
molecule in the asymmetric unit. The complex contains a cobalt(II) cen-
II
the methanol molecules assemble the Co complexes into a one-
II
ter and a deprotonated ligand molecule that was formed in situ. Co is
dimensional supramolecular chain as illustrated in Fig. 2a. The complexes
are arranged in two different orientations with an alternate manner so
that a single-stranded helical structure is formed. Interestingly, the he-
lical chain as seen in Fig. 2a runs through the crystallographic a-axis,
having a right-handed helical chirality. The chirality originates from
the supramolecular interactions and can be assigned to the P-helix
tetra-coordinate to two N, O atoms from the Schiff base part of the
ligand, and the N, S atoms from the thiourea moiety and adopts a
pseudo-square planar geometry, mimicking the traditional coordina-
tion mode of Co in Co (salen) complexes [31]. To make the complex
neutral, except for the phenol group, one of the NHs of the thiourea
II
II
Fig. 1. The ORTEP representations of compounds 2 · MeOH (a) and 3 · MeOH (b) with ellipsoids plotted at the 50% probability level. Selected bond parameters: (a) Co1–O1 = 1.826(2),
Co1–N1 = 1.857(3), Co1–N2 = 1.811(3), Co1–S1 = 2.2344(12), N1–C7 = 1.281(5), N1–C8 = 1.437 (4), C1–O1 = 1.316(5), N2–C13 = 1.391(5), N2–C14 = 1.314(5), N3–C14 =
1.328(5), S1–C14 = 1.743(3) Å; N2–Co1–O1 = 174.59(16), N2–Co1–N1 = 86.01(14), O1–Co1–N1 = 98.49(13), N2–Co1–S1 = 73.64(10), O1–Co1–S1 = 102.26(10), N1–Co1–S1 =
158.30(9), C14–S1–Co1 = 76.31(13), N2–C14–N3 = 127.6(3), N2–C14–S1 = 105.6(3), N3–C14–S1 = 126.7(3)°. (b) Ni1–O1 = 1.8302(14), Ni1–N1 = 1.858(2), Ni1–N2 =
1.8200(19), Ni1–S1 = 2.2435(7), N1–C7 = 1.293(3), N1–C8 = 1.434 (3), C1–O1 = 1.330(3), N2–C13 = 1.390(3), N2–C14 = 1.318(3), N3–C14 = 1.338(3), S1–C14 = 1.745(2) Å;
N2–Ni1–O1 = 174.39(10), N2–Ni1–N1 = 85.83(9), O1–Ni1–N1 = 98.81(8), N2–Ni1–S1 = 73.53(7), O1–Ni1–S1 = 102.23(6), N1–Ni1–S1 = 158.11(6), C14–S1–Ni1 = 76.34(8),
N2–C14–N3 = 127.4(2), N2–C14–S1 = 105.80(16), N3–C14–S1 = 126.73(18)°.

G. Zhang et al. / Inorganic Chemistry Communications 48 (2014) 127–130
129
Table 1
Table 2
Hydrogen bonding parameters for 2 · MeOH and 3 · MeOH (Å and °).
Cytotoxic activity of complexes 2 and 3 against PC12 and HEK293 cells in a 24 hour treat-
ment protocol.
Compounds D–H…A
d(D–H)
d(H…A)
d(D…A) b(DHA)
Compounds
LC50 (PC12) [μM]
LC50 (HEK293) [μM]
1147.0 ±
N.D. (≫1147)
2
3
2
3
· MeOH
· MeOH
· MeOH
· MeOH
O(2)–H(2O)…O(1)
O(2)–H(2O)…O(1)
N(3)–H(3N)…O(2)#
0.836(19)
2.02(3)
2.766(3) 148(4)
0.838(10) 2.016(17) 2.774(2) 150(3)
2
3
62.4 ± 25.5
270.9 ± 96.4
0
1
0.879(19)
2.07(3)
2.854(4) 147(4)
N(3)–H(3N)…O(2)#2 0.864(10) 2.099(17) 2.868(3) 148(3)
Symmetry codes: #1x-1/2, −y + 3/2, z; x-1/2, −y + 1/2, z.
#2
complexes. The cobalt(II) and nickel(II) complexes 2 and 3 were charac-
terized and their structures elucidated by X-ray crystallography. Both
structures are isomorphic and in each complex the metal ion resides
in a near square planar coordination environment in the pseudo-salen
with a pitch of 15.6 Å [32]. However, as another non-centrosymmetric
molecule of 2 found in the asymmetric unit cell also forms a helical
chain with the M-chirality through hydrogen bonds, the crystal is over-
all achiral (Fig. 2b). Both helixes are paired closely together through
non-classic CH…π hydrogen bonds with a phenyl-H…π contact of
2
N OS cavity. Intermolecular hydrogen bonding interactions between
methanol solvent molecules and the complexes were found to play a
crucial role in assembling the discrete complexes into one-
dimensional helical chain. Complexes 2 and 3 show weak cytotoxic
activity towards PC12 and HEK cells and 2 is slightly more cytotoxic
than 3.
2
.848 Å between the P- and M-chirality chains.
II
It was of interest to investigate the biological activities of the Co and
NiII complexes, as small molecular metal complexes such as cisplatin
have displayed remarkable pharmacological potential, for example as
II
anticancer drugs [33–35]. Recent studies have indicated that Co and
II
Acknowledgments
Ni complexes with N,O-ligands can be good candidates for anticancer
agents [23–26]. In this work, we wish to report our preliminary results
on the cytotoxic activity of complexes 2 and 3 against the rat pheochro-
mocytoma (PC12) and human embryonic kidney (HEK293) cells. The
LC50 values given in Table 2 show that both complexes are weakly cyto-
toxic against the PC12 and HEK293 cells after a 24 hour treatment [36].
However, they are more active than free cobalt(II) chloride and
nickel(II) sulfate salts [37,38]. It was also revealed that cobalt(II) com-
plex 2 displayed higher cytotoxicity against the PC12 cells than the
HEK293, and it was slightly more cytotoxic than the nickel(II) one
against the PC12 cells. The nickel(II) complex 3 showed almost no cyto-
toxicity against HEK293 cells.
The work is supported by a Seed grant from the Office for the
Advancement of Research and PRISM fund at CUNY John Jay College.
PRISM is the Program for Research Initiatives for Science Majors at
John Jay College and funded by the Title V, HSI-STEM and MSEIP pro-
grams within the U.S. Department of Education; the PAESMEM program
through the National Science Foundation; and New York State's Gradu-
ate Research and Teaching Initiative.
Appendix A. Supplementary data
In conclusion, we have illustrated that the CLICK reaction between
isothiocyanates and amines could be applied to the facile preparation
of a thiourea-containing amine precursor, which was subsequently
converted by one-pot reactions to a new class of transition metal
Crystallographic data for the structural analysis have been deposited
with the Cambridge Crystallographic Data Center as supplementary
publication. CCDC nos. 994757 and 994758 contain the supplementary
crystallographic data for this paper. These data can be obtained free of
Fig. 2. (a) The extended molecular packing in 2 · MeOH showing the 1D hydrogen-bonded helical chain with a P-chirality; and (b) the space-filling representation of a pair of P-helix and
M-helix.

1
30
G. Zhang et al. / Inorganic Chemistry Communications 48 (2014) 127–130
product was collected by decanting the solvent and washed with MeOH (2 × 3 cm )
3
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.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). Supplementary data to this article can be found online at
http://dx.doi.org/10.1016/j.inoche.2014.08.033.
−
5
−3
and dried in vacuo. Yield: 68.5 mg (84.8%). UV–vis λmax/nm (1.86 × 10 mol dm
,
3
3
−1
−1
CH
2
Cl
2
) 263 (ε/10 dm mol
cm
38.3), 361 (11.1), 431 (8.12). FT-IR (solid,
−
1
cm ): 3140br, 1595s, 1549s, 1483s, 1437w, 1417w, 1348w, 1297w, 1276w,
1
4
221 m, 1149s, 1101 m, 1011s, 933 m, 908w, 750 s, 695 s, 641 m. FAB-MS: m/z
04.08 [M]+ (calc. 404.03). Elemental analysis calcd. (%) for C20
3 3
H15CoN OS∙CH OH:
II
C 57.80, H 4.39, N 9.63; found C 57.66, H 4.27, N 9.80%. 3: Ni complex 3 was pre-
pared as for 2 starting with 1 (48.6 mg, 0.200 mmol), salicylaldehyde (24.2 mg, 0.
References
200 mmol) and Ni(OAc)
2 2
·4H O (49.6 mg, 0.200 mmol). 3 was isolated as red
−
5
−3
block-like crystals (71.9 mg, 89.0%). UV–vis λmax/nm (2.16 × 10
mol dm
,
3
3
−1
−1
[
[
[
[
[
[
[
[
[
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1
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3 2
O
3
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−1
5(7) Å , Z = 4, D
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with I N 2σ (I) converged at final R = 0.0420 (R all data = 0.0510), wR = 0.0886
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1
2
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H
3 2
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c = 5.5340(8) Å, U = 1859.9(5) Å , Z = 4, D
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= 1.558 Mg m , μ(Mo-Kα) = 1.
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−
1
1
1
1
2
2
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[
27] 1: o-phenyldiamine (1.38 g, 13.0 mmol) was dissolved in dry dichloromethane
3
atmosphere at 37 °C and 5% CO . Human embryonic kidney (HEK293) cells obtain-
ed from ATCC were cultured within DMEM supplemented with 10% fetal bovine
serum and 2 mM L-glutamine and 50 μg/mL gentamicin in a humidified atmosphere
2
at 37 °C and 5% CO . For chemical treatment, cells were subcultured into 24-well
plate a day prior to the experiment. Cells were about medium density (~80% conflu-
ence) for chemical treatments. Compounds were prepared as a 50 mM stock in
dimethylsulfoxide (DMSO), then diluted in phosphate buffered saline (PBS) to var-
ious concentrations (0.5 to 50 uM) 15 minutes prior chemical treatments. The final
DMSO concentration in the treatment was less than 0.1%, which is not toxic to the
cells. Cytotoxicity of chemicals were determined by measuring the mitochondrial
respiratory damage using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium
bromide (MTT) assay. In brief, cytotoxicity was determined by adding MTT with a
concentration of 0.5 mg/mL in PBS to each well. Cells were incubated with MTT
(10 cm ), to which triethylamine (0.500 g, 5.00 mmol) was added under stirring.
2
A solution of phenyl isothiocyanate (1.35 g, 10.0 mmol) in dichloromethane
3
(
10 cm ) was then added dropwise over 5 min. The reaction mixture was allowed
to stir at room temperature for an additional 4 h, during which time a white precip-
itate formed. The white solid was collected by filtration, washed with CH Cl
5 cm ) and dried in vacuo. Pure compound 1 was isolated as a white powder (1.
2
2
3
(
1
9
(
5 g, 80.2% based on phenyl isothiocyanate). H NMR (400 MHz, DMSO-d ) δ 9.55
6
s, H, NH), 9.04 (s, H, NH), 7.60–7.44 (m, H, H ), 7.38–7.24 (m, 2H, H ), 7.18–
1 1 2 Ar Ar
2
Ar
2
Ar
1
Ar
7.03 (m, H, H ), 7.02–6.88 (m, H, H ), 6.75 (dd, J = 8.1, 1.4 Hz, H, H ), 6.57
1 Ar 2 13
(td, J = 7.5, 1.4 Hz, H, H ), 4.90 (s, H, NH2) ppm. C NMR (101 MHz, DMSO-
d
2
1
6
6
) δ 180.07, 143.96, 139.65, 128.28, 128.08, 127.06, 124.20, 124.04, 123.55, 116.
7, 115.86 ppm. FT-IR (solid, cm ): 3190br, 2995br, 1613s, 1525s, 1500s, 1446s,
−
1
333s, 1312s, 1268s, 1240 m, 1131 m, 1070w, 1023 m, 929w, 750 s, 696 s, 686 s,
32 s. FAB-MS: m/z 244.17 [M + H]+ (base peak, calc. 244.09).
for 2 h at 37 °C and 5% CO . MTT solubilizing solution (acidified isopropanol) was
added to each well with gentle shaking, followed by reading with a Biotek Synergy
2
[
28] V. Štrukil, D. Margetic, M.D. Igrc, M. Eckert-Maksić, T. Friščić, Chem. Commun. 48
2012) 9705.
29] 2:1 (48.6 mg, 0.200 mmol) and salicylaldehyde (24.2 mg, 0.200 mmol) were dis-
Mx microplate reader. Absorbance was read at 570 nm with background at 690 nm.
Data analysis were performed with the differences of OD570 and OD690. The
corrected absorbance values were normalized to that of the PBS treated control.
[37] A. Gürbay, Biol. Trace Elem. Res. 48 (2012) 110.
(
[
3
solved in CH
2 2 3 2 2
Cl /CH OH (9.0 cm , v/v, 2:1), to which a solution of Co(OAc) ·2H O
(42.6 mg, 0.200 mmol) was dropwise added under stirring at room temperature.
The resulting light red-brown solution was allowed to stir for 10 min, and then fil-
tered. Slow evaporation of the filtrate over 5 days resulted in the formation of red
needle-like crystals, which was suitable for single-crystal X-ray analysis. The bulk
[38] S. Carroll, E.J. Wood, Acta Derm. Venereol. 80 (2000) 94.