Bis-chelates of nickel(II) and copper(II) with an O,S-donor piperazine ligand

Article abstract of DOI:10.1007/s11243-018-0243-3

Abstract: A bidentate O,S-donor ligand N-(4-benzoyl-piperazine-1-carbothioyl)-benzamide (HL) was synthesized from the reaction of in situ generated benzoylisothiocyanate with piperazine and benzoyl chloride. Reaction of HL with the perchlorate salts of Ni

Full text of DOI:10.1007/s11243-018-0243-3

Transition Metal Chemistry
https://doi.org/10.1007/s11243-018-0243-3
Bis‑chelates of nickel(II) and copper(II) with an O,S‑donor piperazine
ligand
Haridas Mandal1
Received: 17 December 2017 / Accepted: 30 April 2018
© Springer International Publishing AG, part of Springer Nature 2018
Abstract
A bidentate O,S-donor ligand N-(4-benzoyl-piperazine-1-carbothioyl)-benzamide (HL) was synthesized from the reaction
of in situ generated benzoylisothiocyanate with piperazine and benzoyl chloride. Reaction of HL with the perchlorate salts
of NiII and CuII in DMF–MeOH mixture using NEt3 as base aforded complexes 1 and 2 as crystalline solids in moderate
yields. Physico-chemical and spectroscopic studies confrmed the chemical compositions of both the free ligand and its
M(O,S)2 type metal complexes. The ligand undergoes keto-thione to keto-thiol tautomerization during metalation which is
the driving force for the formation of neutral bis-chelates. The X-ray crystal structure of complex 2 shows bis-chelation with
a cis-conformation in an overall square planar environment.
Introduction
bidentate fashion [25] and simultaneously through O,S and
N in monobasic bridging fashion [26]. Of these various pos-
sibilities, monoanionic bidentate binding through neutral O
and anionic S is most common. In general, transition met-
als with +2 oxidation state stabilize bis-chelates in the cis-
conformation, whilst metal ions of +3 oxidation state prefer
tris-chelates in the fac-arrangement. The interesting phe-
nomenon of wavelength-dependent photoinduced cis–trans
isomerization is uncommon [27]. There are various synthetic
strategies propounded by diferent groups, including one pot
multicomponent synthesis, metalation of the pure ligand,
and photochemical reaction [28].
The synthesis of thiourea-based O,S donor chelating ligands
with substituted secondary amines is a mature field of
research [1, 2], which nevertheless continues to draw the
attention of synthetic chemists because of diverse applica-
tions as synthetic intermediates [3, 4], in organocatalysis [5,
6], alcohol oxidation [7] and enantioselective organic trans-
formations [8, 9]. They are also extensively used as anion
transporters [10], anion receptors [11, 12], metal extracting
agents [13, 14], and as precursors for the synthesis of nano-
materials and metal sulfde thin flms [15, 16]. Moreover,
acylthioureas are well known for their in vitro antifungal and
antibacterial activities [17–19], activity towards tumors cells
[20], urease inhibitory action [21] and antioxidant proper-
ties [22].
To date, there have been very limited investigations into
the use of cyclic secondary amines like piperazine and
morpholine in this context, although morpholine-based O,S
ligands and their metal complexes with promising analyti-
cal applications have been reported in recent years [17, 29].
Hoyer et al. reported on a piperazine-based bis-bidentate
O,S ligand and its metal complexes of general formula M3L3
[30]. Monoaryl/acyl-piperazines have been prepared by pro-
tecting one of the N–H groups of piperazine, followed by
synthesis of the corresponding thiocarbamoyl derivatives
[31]. With this in mind, a piperazine-based O,S-donor
ligand, N-(4-benzoyl-piperazine-1-carbothioyl)-benzamide
(HL) has been synthesized by protecting one of the N–H
groups with a benzoyl moiety. Two M(O,S)2 type complexes
1 and 2 were synthesized from this bidentate ligand and
characterized by a combination of spectroscopic techniques.
In addition, the X-ray crystal structure of complex 2 has
Bidentate O,S donor ligands can exhibit versatile coordi-
nation towards metal centers, including through both O and
S in monobasic bidentate fashion [23], only through S in
neutral monodentate mode [24], both by O and S in neutral
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-018-0243-3) contains
supplementary material, which is available to authorized users.
* Haridas Mandal
hmiitkgp@gmail.com
1
Chemical Division, Southern Region, Geological Survey
of India, Hyderabad 500068, India
Vol.:(0123456789)
1 3

Transition Metal Chemistry
been determined. The reported compounds may help con-
tribute to various applications in pharmacology and organic
reaction catalysis.
cm−1): 3446(br), 3303(s), 1678(vs), 1521(vs), 1458(s),
1
1273(m), 1191(vs), 1022(m), 710(vs). HNMR (CDCl3):
δ = 3.66–4.29 (m, 8H, CH2), 7.42–7.50 (m, 7H, Ph), 7.59
(t, J1 =6.8, J2 =7.2 Hz, 1H, Ph), 7.84 (s, 1H, Ph), 8.68 (s,
1H br, NH) ppm. 13CNMR (CDCl3): δ = 41.5 (CH2), 46.5
(CH2), 51.4 (2CH2), 127.1 (2C), 127.8 (2C), 128.6 (2C),
129.0 (2C), 130.2 (C), 132.0 (Cq), 133.3 (C), 134.8 (Cq),
163.3 (CO), 170.8 (CO), 179.8 (CS) ppm.
Experimental
Materials and methods
Hydrated perchlorate salts of copper(II) and nickel(II)
were prepared by treating the corresponding metal carbon-
ates with 1:1 conc. HClO4-water mixture and crystallized
after concentration on a water bath. Other chemicals were
obtained from the following sources: triethylamine from S.
D. Fine-Chem Ltd., benzoyl chloride from Merck, potassium
thiocyanate and piperazine from Aldrich. All other chemi-
cals and solvents were of reagent grade and used as received
without further purifcation. Elemental analysis (C, H, N)
was obtained with a Perkin-Elmer model 240 C elemental
analyzer. FTIR spectra were recorded on a Perkin-Elmer 883
spectrometer. Solution electrical conductivity measurements
were carried out using a Unitech type U131C digital con-
ductivity meter with a solute concentration of about 10−3
M. Electronic spectra were recorded on a Shimadzu 1601
UV–Vis–NIR spectrophotometer using 1 cm quartz cell
pairs. In each measurement, the background was subtracted
by taking reference solvent in one of the cells. NMR spectra
were recorded on a Bruker 400 MHz instrument in CDCl3
solvent and the ESI–MS in positive ion mode was recorded
on a VG Autospec mass spectrometer using sub-millimolar
concentrations in MeCN:H2O (1:1) mixed solvent. A col-
lision energy of 10 eV, cone voltage of 23–30 V, source
temperature of 120 °C, de-solvation temperature of 300 °C,
and an average of total 60 scans were routinely used.
Synthesis of complex 1
A solution of HL (0.35 g, 1 mmol) in DMF (10 mL) was
added dropwise to a stirred solution of nickel(II) perchlo-
rate (0.18 g, 0.5 mmol) in methanol (5 mL). NEt3 (0.13 mL,
1 mmol) was then added to give a brown suspension. Evapo-
ration on a hot water bath gave a solid brown product, char-
acterized as complex 1. Yield: 0.470 g (62%). Anal. Calcd.
for C38H36N6O4S2Ni (763.556 g mol−1): C, 59.77; H, 4.75;
N, 11.01. Found: C, 59.73; H, 4.74; N, 11.06%. Selected
FTIR bands (KBr, cm−1): 3447(br), 1636(vs), 1507(s),
1424(s), 1284(m), 1214(m), 1003(m), 722(s), 711(s). Molar
conductance (ΛM, MeCN solution at 25°C): 13.3 S m2 mol−1.
UV–Vis spectra [λmax, nm (ε, L mol−1 cm−1)]: (MeCN solu-
tion) 505 (145), 293 (13,330), 265 (12,903).
Synthesis of complex 2
Complex 2 was synthesized by a similar procedure as for
1, except that copper(II) perchlorate (0.37 g, 1 mmol) was
used in place of nickel(II) perchlorate. The green pow-
dery product obtained from concentration on a hot water
bath was dissolved in DMSO and kept for crystallization
a room temperature. After about 25 days, block shaped
green crystals were obtained. One of the difraction qual-
ity single crystals was selected for X-ray structure analysis.
Yield: 0.420 g (55%). Anal. Calcd. for C38H36N6O4S2Cu
(768.408 g mol−1): C, 59.40; H, 4.72; N, 10.94. Found:
C, 59.39; H, 4.74; N, 10.97%. Selected FTIR bands (KBr,
cm−1): 3446(br), 1625(vs), 1458(m), 1419(m), 1341(s),
1201(s), 1007(s), 712(s). Molar conductance (ΛM, MeCN
Synthesis of N‑(4‑benzoyl‑pipera‑
zine‑1‑carbothioyl)‑benzamide
Potassium thiocyanate (2.91 g, 30 mmol) was suspended
in dry acetone (80 mL). After stirring for about 15 min,
benzoyl chloride (3.5 mL, 30 mmol) was added dropwise
with continuous stirring and the mixture was boiled under
refux, whereupon a precipitate of KCl appeared. To the
cooled mixture, a solution of piperazine (2.58 g, 30 mmol)
in acetone (20 mL) was added slowly and stirring was con-
tinued for a further 1 h. A second portion of benzoyl chloride
(3.5 mL, 30 mmol) was added, and fnally after adding water
(250 mL), the mixture was stirred overnight. The solid white
precipitate was fltered of and washed several times with
water. The crude product was recrystallized from metha-
nol. Yield: 7.95 g (75%). Anal. Calcd. for C19H19N3O2S
(353.439 g mol−1): C, 64.57; H, 5.42; N, 11.89. Found:
C, 64.61; H, 5.44; N, 11.91%. Selected FTIR bands (KBr,
solution at 25°C): 9.1 S m2 mol−1. UV–Vis spectra [λmax
,
nm (ε, L mol−1 cm−1)]: (MeCN solution) 505 (145), 293
(13,330), 265 (12,903).
Crystal data collection and refnement
X-ray crystallographic difraction data for complex 2 were
collected using a Bruker SMART APEX-II CCD X-ray dif-
fractometer equipped with graphite-monochromated Mo-Kα
radiation (λ=0.71073 Å) at 20 °C, with increasing ω (width
of 0.3° per frame) at a scan speed of 6 s per frame. Informa-
tion concerning X-ray data collection and structure refne-
ment is summarized in Table 1. The structure of complex
1 3

Transition Metal Chemistry
Table 1 Crystal parameters and refnement data for complex 2
Empirical formula
C38H36N6O4S2Cu
Formula weight (M/g mol−1
Space group
Crystal system
a, b, c (Å)
)
768.40
C2/c
Monoclinic
17.150(3), 10.2188(17), 21.161(5)
90, 108.564(5), 90
3515.6(12)
α, β, γ (°)
Volume (Å3)
Temperature (K)
293(2)
4/1.452
7.91
Z/Density (calc.) (g cm−3
)
μ(Mo–Kα)/cm−1
F(000)
1596
Crystal dimension (mm3)
0.39×0.25×0.19
21,791/3548 (Rint =0.0723)
2229/231
Refections collected/unique
Data/parameters
Max. and min. transmission
Final R indices [I>2σ(I)]
Goodness-of-ft on F2
0.789/0.860
R1 =0.0448; wR2 =0.0686
1.426
Largest dif. peak and hole (e.
0.523 and −0.395
Å−3
)
CCDC NO.
1001811
2 was solved by direct methods and refned by full-matrix
least-squares based on F2 with a total of 3548 refections
having Miller indices: hmin = − 20, hmax = 21, kmin = − 12,
kmax = 12, lmin = − 26, lmax = 26 [32]. In the fnal cycles of
the F2 method, all non-hydrogen atoms were assigned ani-
sotropically and hydrogen atoms were introduced in calcu-
lated positions in fxed geometry with respect to their carrier
atoms. The molecular plot was drawn using Diamond V.
2.1e program [33]. CCDC fle 1001811 contains the sup-
plementary crystallographic data in CIF format for complex
2. The data can be obtained free of charge either from www.
ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge
Crystallographic Data Center: 12, Union Road, Cambridge
CB2 1EZ, U.K. (fax: +44-1223/336-033; e-mail: deposit@
ccdc.cam.ac.uk).
Fig. 1 Synthetic scheme of proligand HL and its complexes 1 and 2
at 1273 cm−1 (vide supra) indicates successful formation
of the proligand. Subsequent 2:1 stoichiometric reaction of
HL with M(ClO4)2 (M=NiII or CuII) in DMF in the pres-
ence of NEt3 as base aforded bis-chelate square planar com-
plexes 1 and 2 (Fig. 1). Both the complexes were isolated as
micro-crystalline solids. Complex 2 was crystallized from
a saturated solution of DMSO. The elemental analysis and
solution conductivity measurements confrm the chemical
compositions and non-ionic natures of both complexes.
Comparison of the FTIR spectra (Fig. 2) of the free pro-
ligand with those of the metal complexes confrms the gen-
eration of monoanionic ligands during metal coordination
through keto-thiol tautomerization. The N–H vibration at
Results and discussion
The proligand HL was synthesized in good yield by the reac-
tion of in situ generated benzoylisothiocyanate with pipera-
zine and a slight excess of benzoyl chloride (Fig. 1). The
reaction proceeds via nucleophilic addition of isothiocyanate
to the carbonyl carbon followed by elimination of chloride,
followed by an addition reaction with N-benzoylpiperazine.
A broad resonance in the 1H NMR spectrum at 8.68 ppm,
assigned to the N–H group, together with the presence of an
N–H vibration at 3303 cm−1 in the FTIR, plus the charac-
teristic stretching vibrations of C=O at 1678 cm−1 and C=S
1 3

Transition Metal Chemistry
protonated proligand [H2L]+ (C19H20N3O2S). In addition,
peaks at m/z = 217.055, 376.111 and 729.230 are attrib-
uted to C12H15N3O, [(HL)Na]+ (C19H19N3O2SNa) and
[(HL)2Na]+ (C38H38N6O4S2Na), respectively. Both complex
1 and the free HL give a major fragment due to [(HL)2Na]+
(m/z=376.111) while both complexes and HL show a major
stable fragment assigned to [H2L]+ (m/z=354.126). Com-
plex 1 also shows the molecular ion peak at m/z=763.167
for [Ni(L)2H]+. In contrast to this, complex 2 shows a dif-
ferent and distinguishable major fragment at m/z=416.059
for [Cu(L)]+ (Table 2).
2
1
HL
The electronic spectra (Figures S5 and S6 in the Sup-
porting Information) of both complexes in 10−3 M DMSO
solution in 10 μM HEPES bufer showed strong absorp-
tion bands from charge transfer transitions in the near
UV range. For complex 1, the spin forbidden d–d transi-
tions observed at 507 nm (ε = 145 L mol−1 cm−1) can
be assigned to the A1g → B1g transition [35]. In the UV
3000
2500
2000
1500
1000
-1
Wavenumber (cm )
Fig. 2 Comparative FTIR spectra of ligand HL with complexes 1 and
2 showing shifts of C=O and C=S bond vibrations
region, bands at 283 nm (ε = 14,055 L mol−1 cm−1
)
3303 cm−1 for free ligand is absent from the spectra of both
complexes, confrming the deprotonation of amine nitrogen
[34]. The C=O stretching vibration at 1678 cm−1 and C=S
band at 1273 cm−1 for free HL is shifted to lower wave num-
bers by ca. 42–53 and 59–72 cm−1, respectively, upon com-
plexation, indicative of delocalization of π-electrons within
the chelate rings.
and 250 nm (ε = 10,729 L mol−1 cm−1) for complex 1
and at 275 nm (ε = 16,613 L mol−1 cm−1) and 237 nm
(ε = 13,231 L mol−1 cm−1) for complex 2 can be assigned
to metal-bound intraligand n-π* and π–π* charge trans-
fer transitions, overlapped with alternative MLCT transi-
tions. For complex 2, a low energy transition at 511 nm
(ε=159 L mol−1 cm−1) may be assigned to d–d transitions
of copper(II). The lower energy intraligand charge trans-
fer band observed for free HL is red shifted in the metal
complexes, due to withdrawal of electron density from the
coordinated ligands.
Coordination through both carbonyl-O and thiol-
S is in accordance with the bathochromic shifts of C=O
and C=S bond vibrations compared to the free proligand
HL. Sharp and intense bands in the fngerprint region of
1500–1100 cm−1, assigned to partially double bonded CO,
CN and CS stretching vibrations in the metal complexes, are
absent from the spectrum of free HL. All these observations
are consistent with thionyl to thiol transformation during
coordination to the metal centers [22, 23].
Structure of complex 2
The crystal structure of complex 2 is depicted in Fig. 3
with the partial atom numbering scheme. The monoclinic
complex with C2/c space group bears a C2-symmetry along
the midpoint of the metal center in a square planar S2O2
coordination environment. Each of the deprotonated HL
ligands provides thiolate S and carbonyl O atoms towards
CuII-coordination. Binding of two such bidentate O,S
ESI–MS spectra (Figures S1–S4 in Supporting Infor-
mation) of both free HL and the complexes in MeCN:H2O
(1:1) mixed solvent provide useful information regarding
the stable fragments formed during ionization. A base
peak for the free HL at m/z = 354.128 is assigned to the
Table 2 Selected bond lengths (Å) and bond angles (°) for complex 2
Bond lengths (Å)
Cu1–O1
Cu1–S1
O1–C7
1.9196(17)
2.2313(8)
1.261(3)
N1–C7
N1–C8
S1–C8
1.326(3)
1.332(3)
1.729(2)
Bond angles (°)
O1–Cu1–O1*
O1–Cu1–S1
O1–Cu1–S1*
86.13(10)
94.30(5)
164.27(6)
S1–Cu1–S1*
C7–N1–C8
89.53(4)
126.1(2)
1 3

Transition Metal Chemistry
Fig. 3 Molecular structure of
complex 2 with partial atom
numbering scheme
ligands around each CuII-center gives a neutral Cu(O,S)2
chromophore, with a distorted cis-square planar geometry.
The six-membered chelate rings (NC2OS–Cu) are twisted
about the central N atom (N1) in order to attain an O···S bite
distance of 3.05 Å. The Cu–S distance of 2.231 Å, Cu–O
distance of 1.92 Å, C–O distance of 1.261 Å, C–S distance
of 1.729 Å and C–N distances of 1.32–1.332 Å are indica-
tive of charge delocalization over the six-membered che-
late ring, although the relatively long C–S bond distances
suggest that the negative charge is mainly on sulfur. These
results are in good agreement with the FTIR data. The cis
angles (86.13–94.30°) around the metal are close to the ideal
value of 90°, whereas the trans angle of 164.27° deviates
signifcantly from the ideal angle of 180°. The six-mem-
bered chelate ring deviates somewhat from planarity, as indi-
cated from the O–C–N–C and C–N–C–S torsion angles of
9.24°. The dihedral angle between the two O–Cu–S planes
is 21.37° which is signifcantly higher than a comparable
value from the literature (6.97°) [36].
measurements (θ), being 90° for a tetrahedral geometry
and 0° for a perfectly square planar arrangement. The dihe-
dral angle (θ) between the O1-Cu1-S1 and O1*-Cu1-S1*
planes of complex 2 is 21.37°, which implies the distorted
tetracoordinate copper(II) is closer to a square planar
arrangement [37]. Using another method as described by
Houser [38], the tetracoordinate geometry index was cal-
culated with the formula:
ꢀ4 = [360◦−(ꢁ + ꢂ)]∕141◦
where α and β are the two largest angles in the four-coor-
dination species. The τ4 value is expected to be 1 for tet-
rahedral geometry and zero for a perfectly square planar
geometry. The τ4 value obtained for complex 2 is 0.22, again
indicating a distorted square-planar structure.
Conclusion
The crystal of complex 2 has a one-dimensional supramo-
lecular network structure (Fig. 4), in which discrete mono-
meric units are linked by intermolecular C–H···O hydro-
gen bonds. Atoms C3 and C3* of the phenyl ring from the
ligand backbone serve as hydrogen bond donors via H3 and
H3* to the benzoyl oxygens O2 and O2*, respectively, with
C–H···O hydrogen bonding parameters: C3···O2=1.208 Å,
H3···O2=2.608 Å and C–H···O angles of 120.68°.
A benzoylthiourea-based proligand HL has been synthe-
sized from piperazine as a secondary amine and examined
for its coordination potential towards NiII and CuII. This
anionic unsymmetrical O,S donor ligand binds to both
metal ions in a cis-fashion giving distorted square planar
geometries. The spectroscopic signatures both in solution
and the solid state and the X-ray crystal structure of the
copper complex confrm keto-thiol tautomerization during
metal binding, as well as delocalization of negative charge
over the six-membered chelate rings.
The degree of distortion around a tetracoordinate metal
center can be quantitatively estimated by dihedral angle
1 3

Transition Metal Chemistry
Fig. 4 Formation of 1D supramolecular network through C–H···O hydrogen bonding interactions. Color code: Cu, brown; N, blue; O, red; S, yel-
low; C, ash and H, green. (Color fgure online)
13. Schuster M, Schwarzer M (1996) Anal Chim Acta 328:1–11
Supporting information
14. Bourne S, Koch KR (1993) J Chem Soc, Dalton Trans
13:2071–2072
CCDC 1001811 contains the supplementary crystallo-
graphic data in CIF format for complex 2. The data can be
obtained free of charge from www.ccdc.cam.ac.uk/conts
/retrieving.html, or from the Cambridge Crystallographic
Data Center: 12, Union Road, Cambridge CB2 1EZ, U.K.
(fax: +44-1223/336-033; e-mail: deposit@ccdc.cam.ac.uk).
15. Ramasamy K, Malik MA, Helliwell M, Tuna F, O’Brien P (2010)
Inorg Chem 49:8495–8503
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