Crystal structures and physicochemical studies of some novel divalent and trivalent transition metal chelates of N-morpholine-N'-benzoylthiourea

Article abstract of DOI:10.1016/j.molstruc.2020.129791

A series of Fe(III), Co(III), Ni(II), Cu(II), Zn(II) and In(III) N-morpholine-N'-benzoylthiourea complexes have been synthesised and characterised by elemental analysis, thermal analysis, infrared spectroscopy, ¹H nuclear magnetic resonance spectroscopy and single crystal X-ray crystallography. Thermogravimetric analysis shows that all the complexes undergo a two-step decomposition process except for the iron(III) complex and the indium(III) complexes, which show three-step and one-step decompositions, respectively. The complexes are thermally stable up to approximately 300 °C. The ligand coordinates the various metal ions in a bidentate (L-kO,S) chelating mode, facilitated by deprotonation of the acidic amide (–C(O) N'HC (S)) moiety. This mode of coordination allows for the facile formation of neutral bis/tris-6-membered chelates of type [M(L-kS,O)_x] where x = 2 or 3 for divalent or trivalent metal ions, respectively.

Full text of DOI:10.1016/j.molstruc.2020.129791

Journal of Molecular Structure 1229 (2021) 129791
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Crystal structures and physicochemical studies of some novel divalent
and trivalent transition metal chelates of
N-morpholine-N’-benzoylthiourea
Kevin I.Y. Ketchemena,b,d, Malik Dilshad Khana, Sixberth Mlowea,f, Matthew P. Akermanc,∗
,
Inigo Vitorica-Yrezabald, George Whiteheadd, Linda D. Nyamenb, Peter T. Ndifonb,∗
,
Neerish Revaprasadua,∗, Paul O’Briend,e
a Department of Chemistry, University of Zululand, Private Bag X1001, KwaDlangezwa 3880, South Africa
b Department of Inorganic Chemistry, University of Yaoundé I, P.O. Box 812, Yaoundé, Cameroon
c School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg, 3209, South Africa
d School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
e School of Materials, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
f Department of Chemistry, University of Dar es Salaam, P.O. Box 2329, Dar es Salaam, Tanzania
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of Fe(III), Co(III), Ni(II), Cu(II), Zn(II) and In(III) N-morpholine-N’-benzoylthiourea complexes have
been synthesised and characterised by elemental analysis, thermal analysis, infrared spectroscopy, 1H nu-
clear magnetic resonance spectroscopy and single crystal X-ray crystallography. Thermogravimetric analy-
sis shows that all the complexes undergo a two-step decomposition process except for the iron(III) com-
plex and the indium(III) complexes, which show three-step and one-step decompositions, respectively.
The complexes are thermally stable up to approximately 300 °C. The ligand coordinates the various metal
ions in a bidentate (L-kO,S) chelating mode, facilitated by deprotonation of the acidic amide (–C(O) N’HC
(S)) moiety. This mode of coordination allows for the facile formation of neutral bis/tris-6-membered
chelates of type [M(L-kS,O)x] where x = 2 or 3 for divalent or trivalent metal ions, respectively.
Received 3 October 2020
Revised 4 December 2020
Accepted 15 December 2020
Available online 24 December 2020
Keywords:
Metal complexes
Benzoylthiourea ligands
Thermal analysis
Single crystal X-ray crystallography
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
ble for any biological properties, which may be further enhanced
through the formation of metal complexes [9]. In addition to bi-
Thiourea and thiobiuret/dithiobiuret compounds have recently
generated interest among scientists due to their potential biolog-
ical applications and their use in the fabrication of metal sulfide
thin films and nanoparticles [1–4]. The main focus of the research
into these compounds centres on their potential biological applica-
tions where they have been shown to possess antibacterial, anti-
fungal, insecticidal, herbicidal, and plant-growth regulator proper-
ties [3–5] Thiourea compounds show great structural diversity and
numerous derivatives can be prepared through facile modification
of the starting materials. Examples of these compounds which in-
clude the N,N-dialkyl-N’-aroyl/acyl-thioureas (HL), are known in lit-
erature and are obtained through a simple one pot reaction which
involves the addition of an acid chloride to KSCN followed by an
ological studies, thiourea compounds are also important in indus-
trial applications, i.e. as corrosion inhibitors and as additives in lu-
bricants [10–12].
The potential of thiourea and its derivatives to act as donor
ligands for transition metal ions due to their carbonyl and thio-
carbonyl groups means they are ubiquitous in the synthesis of
metal sulfide nanomaterials from single-source precursors [13–15].
Thiourea ligands coordinate easily to a wide range of metal cen-
tres as either neutral, monoanionic or dianionic ligands [16]. The
variability of donor atoms of thiourea-derived ligands including
oxygen, nitrogen and sulfur provide a plethora of bonding modes,
bidentate (L-kO,S) chelating groups are, however, dominant [17–
19]. The presence of direct bonds between sulfur and the metal
atom of interest results in better control over stoichiometry and
the phase of nanomaterials. Ramasamy et al. described the syn-
thesis of metal (Co, Cu, Ni) complexes of thiobiurets and dithio-
biurets and their use as precursors for metal sulfide thin films by
AACVD [1]. Akhtar et al. deposited CuS thin films by AACVD us-
– –
amine [6–8]. The N C S moiety in thiourea is generally responsi-
∗
Corresponding author.
E-mail
addresses:
Akermanm@ukzn.ac.za
(M.P.
Akerman),
RevaprasaduN@unizulu.ac.za (N. Revaprasadu).
https://doi.org/10.1016/j.molstruc.2020.129791
0022-2860/© 2020 Elsevier B.V. All rights reserved.

K.I.Y. Ketchemen, M.D. Khan, S. Mlowe et al.
Journal of Molecular Structure 1229 (2021) 129791
Fig. 1. Schematic Molecular diagrams of the ligand and metal complexes.
ꢀ
ing N,N-diethyl-N -(1-naphthoyl)thiourea as a precursor [20]. They
also synthesised a series of acyl thiourea complexes with different
transition metals (Ni, Cu, Co, Zn) and investigated their suitabil-
ity for the deposition of thin films by AACVD [21]. Revaprasadu
et al. have similarly investigated the effects of surfactants on the
phase of copper sulfide via colloidal synthesis and deposited thin
films by AACVD using acyl thiourea copper complexes [22]. They
also prepared PbS and PbS1-xSex solid solutions over wide range of
compositions using chalcogeno urea complexes [23]. There are also
reports of the Group congener of thiourea i.e. selenourea, for the
synthesis of selenide based nanomaterials and thin films [23–25].
The versatile and flexible coordinating ability of thiourea has
also resulted in formation of photo-induced isomers [26,27]. The
photo-isomerisation occurs as a result of reversible protonation
yielding a series of cis- or trans- isomers of Pt(II) and Pd(II)
thiourea complexes. With such a variety of functions for thiourea-
based, there is a need to investigate the structural features of a
series of metal thiourea complexes with different oxidation states,
coordination numbers and coordination geometries.
used as received. The ligand and metal complexes were prepared
using a procedure reported in our previous work [29], with some
modifications. Physical properties, elemental analysis results and IR
data are summarised in Tables S1 and S2.
2.2. Synthesis of N-morpholine-N’-benzoylthiourea, [morthio] (HL)
Benzoyl chloride (2.0 mL; 17 mmol) was dissolved in 50 mL of
acetone and added dropwise to a suspension of potassium thio-
cyanate (1.67 g; 17 mmol) in 30 mL of acetone. The reaction mix-
ture was heated under reflux for 30 min, and then cooled to
room temperature. A solution of morpholine (1.50 mL; 17 mmol)
in 10 mL of acetone was added and the resulting mixture stirred
for 2 h. Hydrochloric acid (300 mL, 0.1 M,) was added to the resul-
tant solution and the solid product filtered off, washed with water
and purified by recrystallization in ethanol: dichloromethane (1:1)
mixture. 1H NMR (400 MHz, acetone): δ 9.68 (s, 1H, NH), 8.02 (m,
2H, C6H5), 7.64 (d, 1H, C6H5), 7.61 (m, 2H, C6H5), 4.21 (s, 4H, CH2),
3.77 (s, 4H, CH2).
Morpholine is an amino ether which is regarded as a versa-
tile chemical with many important applications including optical
brighteners and catalysts [28]. It is commonly used in dithiocarba-
mate metal-organic compounds, which exhibit desirable chemical
stability under ambient conditions. It is for these reasons that mor-
pholine was the preferred moiety in our study involving thiourea
compounds. Herein, the ligand, N-morpholine-N’-benzoylthiourea
is successfully coordinated to Cu(II), Co(III), Fe(III), In(III), Ni(II) and
Zn(II). The metal chelate structures presented in this work have
not been studied by single crystal X-ray crystallography previously,
with the exception of the Co(III) and Ni(II) complexes (Fig. 1). In
these cases, new polymorphs are presented. Our interest in these
compounds stems from their potential use as precursors for the
synthesis of metal sulfide nanomaterials.
2.3. Synthesis of the complexes
2.3.1. Synthesis of tris(N-morpholine-N’-benzoylthioureato)iron(III),
[Fe(morthio)3] (1)
Under ambient conditions iron(III) chloride (0.44 g; 2.00 mmol)
dissolved in water (20 mL) was added dropwise to a stirred
suspension of N-morpholine-N’-benzoylthiourea ligand (1.50 g;
6.00 mmol) in acetone (50 mL). After 1 h of stirring, the resultant
solution was filtered and crystals of suitable quality were grown
from a mixture of ethanol and dichloromethane solvents.
2.3.2. Synthesis of tris(N-morpholine-N’-benzoylthioureato)cobalt(III),
[Co(morthio)3] (2)
Complex (2) was synthesised using the same method as com-
pound (1) with cobalt(II) chloride hexahydrate (0.48 g, 2.00 mmol)
used as the metal salt. The cobalt (II) is oxidised by atmospheric
oxygen during the reaction process to cobalt(III). This is common
practice in cobalt chemistry as the reaction kinetics of the d6
cobalt(III) are slow.
2. Experimental
2.1. Chemicals
Benzoyl chloride 99%, potassium thiocyanate 99%, copper chlo-
ride dihydrate 99%, iron(III) chloride 97%, cobalt(III) chloride hex-
ahydrate 98%, zinc nitrate hexahydrate 98%, nickel chloride 98%,
indium(III) chloride 98%, hydrochloric acid 37%, ethanol 99.5%, ace-
tone 99.5%, dichloromethane 99.9% (Sigma–Aldrich) and morpho-
line 99% (Merck) were all supplied from commercial sources and
2.3.3. Synthesis of bis(N-morpholine-N’-benzoylthioureato)nickel(II),
[Ni(morthio)2] (3)
Complex (3) was synthesized under identical conditions to
complex (1) using 0.26 g; 2.00 mmol of nickel chloride and 1.00 g;
2

K.I.Y. Ketchemen, M.D. Khan, S. Mlowe et al.
Journal of Molecular Structure 1229 (2021) 129791
4.00 mmol of N-morpholine-N’-benzoylthiourea ligand in 50 mL of
ethanol. 1H NMR (400 MHz, CDCl3): δ = 8.13 (m, 2H; 2-C6H5), 7.51
(d, 1H, 2-C6H5), 7.41 (m, 2H, 2-C6H5), 4.22 (m, 4H, 2-CH2), 3.78 (m,
4H, 2-CH2).
3. Results and discussion
3.1. Spectroscopic and gravimetric analysis
The N-morpholine-N’-benzoylthiourea ligand and its corre-
sponding metal complexes synthesized from salts of Fe(III), Co(II),
Ni(II), Cu(II), Zn(II) and In(III) were obtained as high purity com-
pounds in good yields, as confirmed by elemental analysis (ESI Ta-
ble S1). The IR spectroscopy results (ESI Table S2) revealed that
the medium peak observed at 3239 cm−1 for the ligand (HL) is
2.3.4. Synthesis of bis(N-morpholine-N’-benzoylthioureato)copper(II),
[Cu(morthio)2] (4)
Synthesis of complex (4) was carried out using the method for
(3) with copper chloride dihydrate (0.27 g, 2.00 mmol) used as the
metal salt.
–
attributed to the stretching of the N H group adjacent to the car-
bonyl group. The disappearance of this band with the emergence
of the bands observed at 1587-1585 cm−1 (attributed to the C
N
=
functional group) in the complexes confirmed the deprotonation
of the ligand facilitated by the addition of the base, N-morpholine.
The deprotonation is also corroborated by 1H NMR spectroscopy,
where the peak observed at 9.68 ppm (attributed to the amide
proton, N-H) for HL is absent in the spectra of the complexes. The
strong band at 1663 cm−1 for HL is attributed to the vibration of
2.3.5. Synthesis of bis(N-morpholine-N’-benzoylthioureato)zinc(II),
[Zn(morthio)2] (5)
Complex (5) was synthesised using the same procedure as for
(3) with zinc nitrate hexahydrate (0.60 g, 2.0 mmol) used as the
metal salt. 1H NMR (400 MHz, CDCl3): δ = 8.17 (m, 2H; 2-C6H5),
7.50 (d, 1H, 2-C6H5), 7.41 (t, 2H, 2-C6H5), 4.27 (m, 4H, 2-CH2), 3.84
(m, 4H, 2-CH2).
=
the C O carbonyl groups which shifts to lower wavenumbers upon
complexation. This shift to lower frequencies is a result of the elec-
tron withdrawing nature of the ligands and the subsequent lower-
=
ing of the C O bond order. These data show that deprotonation
2.3.6. Synthesis of
=
involves delocalisation of the C O stretching vibration, which is
tris(N-morpholine-N’-benzoylthioureato)indium(III), [In(morthio)2] (6)
Complex (6) was synthesised using the procedure for (1) un-
der the nitrogen with indium chloride (0.44 g, 2.00 mmol) used as
metal salt. 1H NMR (400 MHz, CDCl3): δ = 8.12 (m, 2H; 3-C6H5),
7.47 (d, 1H, 3-C6H5), 7.36 (m, 2H, 3-C6H5), 4.16 (m, 4H, 3-CH2),
3.75 (m, 4H, 3-CH2).
consistent with literature, [34,35] thus confirming coordination to
the central metal ions through the oxygen atom from the carbonyl.
The multiplet in the 1H NMR spectra assigned to the aromatic pro-
tons, are observed in the 8.17-7.41 ppm range in both ligand and
complexes. Two signals assigned to the methylene protons of mor-
pholine are also observed in the 4.27-3.84 ppm range.
The thermal stability and decomposition profile of the syn-
thesised complexes was investigated by TGA and DTA techniques
as presented in Fig. 2 and Fig. S1, respectively. Merdivan et al.
[36] studied the thermal decomposition of many thiourea com-
plexes and suggested two decomposition steps. They proposed that
the first step corresponds to the elimination of dialkylbenzamide,
followed by the loss of SCN• and CN• radicals as a second step
yielding either the metal sulfide or metal. Our complexes showed
similar two-step TGA profiles, with the exception of complex (1)
which showed three-step decomposition processes. In the case of
the two step decomposition, the first step showed weight loss of
73.67% for (4), 68.33% for (3), 74.76% for (5), 72.80% for (2) and
64.42% for (6) corresponding to the loss of 2 × MB (calcd: 68.03%),
2 × MB (calcd: 68.63%), 2 × MB+CN (calcd: 72.43%), 3 × MB
(calcd: 71.11%) and 2 × MB+3 × CN (calcd: 56.86%) respectively,
2.4. Instrumentation
Elemental analysis (EA) using a Thermo Scientific Flash 2000
Organic Elemental analyser and thermal analysis by Mettler Toledo
TGA/DSC1 star system were carried out at the Manchester Univer-
sity Microanalytical Laboratory. The thermal data points were col-
lected at a ramp rate of 10 °C min−1 in a flowing N2 stream. A
400 MHz Bruker Ascend Spectrometer was used to record NMR
spectra at RT using CDCl3 and D2O as solvents.
2.5. X-ray crystal structure analysis
Crystals suitable for single crystal X-ray diffraction studies
were grown using vapour diffusion of a 1:1 solution mixture of
dichloromethane and ethanol. X-ray data for all chelates, except
the Cu(II) chelate, were collected on a dual source Rigaku FR-X ro-
˚
tating anode diffractometer using Cu Kα (λ = 1.5418 A) radiation
at 150 and 293 K, and reduced using CrysAlisPro 171.39.30c. All
non-hydrogen atoms were located in the difference density map
and refined anisotropically with SHELX-2016 [30]. Structures were
solved by direct methods and subsequent refinement was carried
out using SHELX-2016 (Olex2 v1.2.919–21). The Cu(II) chelate was
analysed using a Bruker Apex Duo diffractometer equipped with
an Oxford Instruments Cryojet operating at 100(2) K and an In-
coatec microsource operating at 30 W power. The data were col-
˚
lected with Mo K (λ = 0.71073 A) radiation by omega and phi
α
scans with exposures taken at 30 W X-ray power and 0.50° frame
widths using APEX2 [31]. The data reduction was performed using
Bruker SAINT software [31] which corrects for Lorentz and polari-
sation effects. Absorption corrections for the reflection data were
performed with semi-empirical and multi-scan techniques using
SADABS. All structures were solved by direct methods, SHELX-2016
[32] and WinGX [33] packages. All hydrogen atoms were included
as idealised contributors in the least squares refinement.
Fig. 2. TGA plots of complexes (1 – 6).
3

K.I.Y. Ketchemen, M.D. Khan, S. Mlowe et al.
Journal of Molecular Structure 1229 (2021) 129791
where MB = morpholinylbenzamide. The second step displayed
a mass loss of 9.43%, 16.24%, 6.01%, 13.83% and 8.10% assigned
to 2CN, SCN+CN, CN, 3CN+S and S (calcd: 9.26%, 15.09%, 4.62%,
13.65% and 8.62%), respectively. A similar trend in total weight loss
is observed for complex (1). The masses of the final residue are
23.07% for (1), 13.37% for (2), 15.69% for (3), 16.90% for (4), 19.23%
for (5), and 23.40% for (6), which are in good agreement with the
theoretical values for Fe2S4 (calcd: 25.86%), CoS (calcd: 11.28%),
NiS (calcd: 16.28%), CuS (calcd: 17.00%) and ZnS (calcd: 17.27%),
respectively. The exception is compound (6) which did not corre-
late to either of the stable phases InS or In2S3. The latter can be
attributed to carbon contamination and/or multiple-phase indium
sulfide material. This indicates that the compounds are stable at
ambient conditions, i.e. in air, moisture and at room temperature.
In the DTA profiles (ESI Figure S1), a single endothermic peak is
observed for complexes (4), (2) and (1) whereas two are observed
for complexes (3), (5) and (6). The value of the melting point of
each complex is consistent with the value of the first endothermic
peak in the DTA profiles which is in agreement with the work of
Merdivan et al. [37]. The complete TGA and DTA data of the com-
plexes are summarised in Table S3.
3.2. Determination of crystal structures
The crystallographic refinement data for all crystal structures
used in this work are presented in Table 1 and selected geo-
metrical features of the complexes are summarised in Table 2.
These ligands have been extensively studied by X-ray crystallog-
raphy coordinated to a number of different metal ions, particu-
larly heavy metal ions including, but not limited to: Pb(II) [29],
Pt(II) [38], Hg(II) [39], Re(V), Tc(V) and Tc(III) [40,41]. The ligand
has also reportedly been chelated to Co(III) [42–44], as well as
Ni(II) [31,45,46]. The structures presented herein, with respect to
the Co(III) and Ni(II) compounds, present new polymorphs of these
known structures, the bulk of the compounds (four structures) are
have not been previously reported. The Fe(III) complex (Fig. 3)
crystallises in the monoclinic P21/c space group. Three bidentate
S,O-donor ligands surround the metal centre. This yields a dis-
torted octahedral S3O3 environment with a stereochemical prefer-
ence for a facial geometry common in closely related structures
[47]. This geometry preference is common in related compounds,
as shown by a search of the Cambridge Structural Database (CSD).
The CSD shows that all structures with three bidentate ligands of
Fig. 3. The molecular structure of complex (1) showing the atom naming scheme.
The sulfur and oxygen ligand donor atoms have adopted a facial (fac) coordination
geometry. One of the three morpholine rings is disordered between a ring up and
ring down orientation with a ratio of 53:47. Displacement ellipsoids are shown at
the 50% probability level. Hydrogens are shown as spheres of arbitrary radius.
is the case for the compounds presented herein) have exclusively
adopted a fac-configuration, irrespective of the moieties appended
to the carbon atoms. Considering the ubiquitous nature of this
configuration, it was further explored using DFT methods at the
B3LYP/6-311G (dp) level of theory which showed, for a represen-
tative structure comprising three bidentate S,O-donor ligands co-
ordinated to a metal(III) ion, that the fac-isomer is ca. 9 kJ mol–1
lower in energy than the meridional isomer. Further details on the
computational method used are provided in the ESI.
The ligand structure leads to three, six-membered O,S chelate
˚
–
–
rings (NC2OS Fe). The mean Fe O bond distance (1.98(2) A) is
˚
–
significantly shorter than the Fe S bond length (2.43(1) A). The
phenyl rings of the benzoyl portion of the ligands are approxi-
– – – –
the general structure S C N C O in the coordination sphere (as
Table 1
Crystallographic data of complexes (1–6).
Complexes
Formula
1
C
2
C
3
4
5
6
36H39FeN6O6S3,
36H39CoN6O6S3
C24H26N4NiO4S2
C24H26CuN4O4S2
C24H26N4O4S2Zn
C36H39InN6O6S3,
C3H6O
CH2Cl2
M g mol−1
888.69
806.84
557.32
562.15
920.81
Space group
Crystal system
P 21/c
P 21/c
P -1
P -1
P -1
P 21/c
Monoclinic
13.8376(2)
24.9520(3)
11.8570(2)
90
Monoclinic
13.6419(2)
25.1576(4)
11.5569(2)
90
Triclinic
5.2078(1)
15.2193(2)
15.4417(2)
90.374(1)
99.051(1)
94.185(1)
1205.23(3)
150
Triclinic
4.0786(3)
16.2842(10)
17.7096(11)
81.554(2)
86.833(3)
83.113(3)
1154.21(13)
100
Triclinic
9.5245(2)
11.6012(3)
13.1616(3)
110.724(2)
101.140(2)
107.099(2)
1226.10(5)
293
Monoclinic
14.0804(1)
24.8811(2)
11.9042(1)
90
˚
a (A)
˚
b (A)
˚
c (A)
α (°)
β (°)
γ (°)
98.865(1)
90
98.5570(10)
90
98.172(1)
90
3
˚
V (A )
4045.03(10)
150
3922.15(11)
150.0(2)
4
4128.12(6)
150
T (K)
Z
4
2
2
2
4
Dcalc (g cm−3
)
1.459
1.366
1.536
1.617
1.528
1.482
F(000)
1844.0
1680.0
580.0
582.0
584.0
1896.0
R(F2) (observed
reflections)
wR(F2) (all
reflns)
0.0754(7686)
0.1865(8206)
0.0390
0.0397(4609)
0.1058(4736)
0.0301(3895)
0.0780(4951)
0.0269 (4588)
0.0711 (4771)
0.0627(7203)
0.1677(7843)
0.0996
4

K.I.Y. Ketchemen, M.D. Khan, S. Mlowe et al.
Journal of Molecular Structure 1229 (2021) 129791
Table 2
Selected geometrical parameters describing the coordination spheres of the compounds (1) – (6).
(1)
(2)
Bond
Length
Bond
Angle
Bond
Length
Bond
Angle
–
–
–
–
–
–
Fe S1
2.448(1)
2.414(1)
2.428(1)
1.998(1)
1.976(1)
1.972(1)
S1 Fe O2
168.8(1)
171.34(8)
171.86(8)
86.19(8)
86.6(1)
Co S1
2.2153(6)
2.2066(7)
2.2085(7)
1.917(2)
1.941(2
S1 Co O5
176.61(5)
176.25(5)
176.60(5)
94.98(5)
95.04(5)
94.84(5)
–
–
–
–
– –
S2 Co O1
Fe S2
S2 Fe O1
Co S2
–
–
–
–
– –
S3 Co O3
Fe S3
S3 Fe O3
Co S3
–
–
–
–
– –
O1 Co S1
Fe O1
O1 Fe S1
Co O1
–
–
–
–
– –
O3 Co S2
Fe O2
O2 Fe S3
Co O3
–
–
–
–
–
–
Fe O3
O3 Fe S2
86.31(8)
Co O5
1.922(1)
O5 Co S3
(3)
(4)
Bond
Length
Bond
Angle
Bond
Length
Bond
Angle
a
a
b
a
b
–
–
–
–
–
Ni1 S1
1.833
O1 Ni1–S1
95.48
84.52
95.06
84.94
Cu1 S1
1.906
O1 Cu1 S1
93.77
86.23
93.51
86.49
b
a
b
–
–
–
– –
O1 Cu1 S1
Ni1 O1
2.1733
2.1817
1.827
O1 Ni1–S1
Cu1 O1
2.2812
2.2790
1.909
–
–
–
– –
S2 Cu2 O3
Ni2 S2
S2 Ni2–O3
Cu2 S2
–
–
–
–
–
Ni2 O3
S2 Ni2–O3
Cu2 O3
S2 Cu2 O3
(5)
Bond
(6)
Length
Bond
Angle
Bond
Length
Bond
Angle
a
a
–
–
–
–
– –
Zn S1
2.2730(6)
1.959(1)
2.2700(5)
1.955(2)
O1 Zn S1
98.14(5)
98.99(4)
101.36(6)
116.82(5)
122.25(2)
118.32(5)
In S1
2.526(1)
2.544(1)
2.550(1)
2.140(3)
2.167(3)
2.169(3)
O1 In S2
171.1(1
165.5(1)
169.50(8)
85.4(1)
–
–
–
–
– –
O2 In S3
Zn O1
S2 Zn O3
In S2
b
–
–
–
–
– –
O3 In S1
Zn S2
O1 Zn O3
In S3
b
–
–
–
–
– –
S1 In O1
Zn O3
O1 Zn S2
In O1
b
–
–
–
– –
S2 In O2
S1 Zn S2
In O2
85.9(1)
b
–
–
–
– –
S3 In O3
S1 Zn O3
In O3
84.54(8)
a
intraligand bond angle.
interligand bond angle.
b
dination geometry with an S3O3 environment similar to that of
complex (1). This structure is a polymorph of one previously re-
ported [42–44]. The previously reported structure crystallised in
the P-1 Space Group; similarly with a single molecule in the asym-
metric unit [42,44]. The two structures show comparable molec-
ular geometries in terms of both bond lengths and bond angles
[42,44]. Similar to compound (1), three thiourea ligands coordinate
the metal ion to form three six-membered chelate rings (NC2OS–
–
Co) and a neutral metal complex. The Co S bond distances of
˚
the present structure range from 2.2066(7) to 2.2153(6) A and
˚
–
the Co O bond distances range from 1.917(2) to 1.941(2) A. The
structure shows the same ligand geometry with respect to the co-
planar nature of the phenyl rings and carbonyl groups. Closely re-
lated ligands with fluorine on the phenyl ring have also been co-
ordinated to the Co(III) ion as well as ligands with piperidine in
place of morpholine [43]. It is interesting to note that when the
Co(II) ion is prevented from oxidizing, it forms a nominally tetra-
hedral complex with two ligand molecules (both with fluorine sub-
stitution on the phenyl ring, but the same morpholine moiety)
[43]. When the reaction mixture is allowed to oxidize, the same
fluorine- substituted ligand forms a tri-ligated species with Co(III)
which consequently has an octahedral geometry. In both cases the
metal chelates are neutral overall [43]. Despite the strongly elec-
tron withdrawing effect of the fluorine group, the geometric pa-
rameters compare favourably with those of complex (2) reported
herein. Seemingly, the fluorine substituent is too remote from the
coordination sphere to have any significant impact on the geomet-
ric parameters.
Fig. 4. The molecular structure of complex (2) showing the atom naming scheme.
The sulfur and oxygen ligand donor atoms have adopted a facial (fac) coordination
geometry. One of the three morpholine rings is disordered between a ring up and
ring down orientation with a ratio of 78:23. Displacement ellipsoids are shown at
the 50% probability level. Hydrogens are shown as spheres of arbitrary radius.
mately coplanar with the OCNCS–Fe chelate ring. This conforma-
The Ni(II) chelate has been previously reported by Zhou et al.,
Del Campo et al., and Richter et al. [31,45,46]. Despite the numer-
ous reports of the structure, the present compound is a new poly-
morph, differing in both the unit cell parameters and contents of
the unit cell. The present structure is the only example with in-
version symmetry about the divalent metal ion. Two of the previ-
ously reported compounds also have solvent in the lattice; either
toluene or dichloromethane [45,46]. The Ni(II) and Cu(II) chelates,
compounds (3) and (4), respectively, are isostructural from a crys-
tallographic perspective and will be discussed together. Both com-
pounds crystallise in the triclinic Space Group P-1 with two half-
tion allows for extended π-conjugation from the phenyl ring to
–
–
the carbonyl group. The relatively long C S and short C O aver-
˚
age bond lengths of 1.737(4) and 1.267(5) A for (1), respectively
highlight the single- and double-bond character of these groups.
This pattern of bond lengths in all three ligands of (1) suggests
that the formal negative charge is predominantly localised on the S
atom. The structure crystallised with a disordered dichloromethane
molecule in the asymmetric unit, which was removed in silico
˚
leaving a solvent-accessible void of 174 A [3].
The single crystal X-ray structure of the cobalt complex (Fig. 4)
shows that the cobalt(III) ion has a nominally octahedral coor-
5

K.I.Y. Ketchemen, M.D. Khan, S. Mlowe et al.
Journal of Molecular Structure 1229 (2021) 129791
Fig. 6. [Main] The symmetry-completed structure of a single molecule of complex
(4) showing the atom naming scheme. [Inset] The contents of the asymmetric unit
of complex (4) comprising two half molecules with inversion symmetry on the cop-
per(II) metal centre. Displacement ellipsoids are shown at the 50% probability level.
Hydrogens are shown as spheres of arbitrary radius. Symmetry code: i = -x, -y, -z.
Fig. 5. [Main] The symmetry-completed structure of a single molecule of complex
(3) showing the atom naming scheme. [Inset] The contents of the asymmetric unit
of complex (3) comprising two half molecules with inversion symmetry on the
metal centre. Displacement ellipsoids are shown at the 50% probability level. Hy-
drogens are shown as spheres of arbitrary radius. Symmetry code: i = -x, -y, -z.
˚
˚
–
and short C O mean bond lengths of 1.730(4) A and 1.266(5) A,
–
respectively. These are consistent with predominantly single (C S)
=
and double- (C O) bond character and suggest that the nega-
tive charge of the ligand is located predominantly on the sulfur
atom. Although the Ni(II) chelate is a new polymorph, as with
the Co(III) chelate, the geometric parameters describing the coor-
dination sphere are comparable to the previously reported divalent
nickel chelates [31,45,46].
molecules in the asymmetric unit and inversion symmetry at the
metal centre; as shown in Figs. 5 and 6, respectively. The divalent
nickel and copper ions are each coordinated by two bidentate lig-
ands giving an approximate square planar geometry. In both cases
–
–
the S M O intraligand bond angles are obtuse measuring ca. 95°
and 94° for (3) and (4), respectively. This is a consequence of the
ligand bite and resulting six-membered chelate ring. Subsequently,
A least squares fit of a selection of the non-hydrogen atoms for
the two molecules in the asymmetric unit of (3) shows that the
key difference is the relative orientation of the morpholine rings
(ring up versus ring down). In the case of compound (4), the dif-
ference is again the orientation of the morpholine rings, but the
difference is not as stark as the ring up/ring down orientations of
the half-molecules in (3). These fits are shown in Fig. 7.
–
–
the interligand S M O bond angles are acute, measuring ca. 85°
and 86° for (3) and (4), respectively. The inversion symmetry of
the molecule necessitates that the interligand S M S and S M O
–
–
– –
bond angles measure 180°.
Despite the different coordination geometries compared to
The Zn(II) complex, compound (5), is shown in Fig. 8. This com-
pound is also four-coordinate with two bidentate ligands and crys-
–
compounds (1) and (2), the ligands still show relatively long C S
Fig. 7. Least-squares fits of the two half-molecules in the asymmetric units of (3) and (4) showing the main structural differences. For (3), the half-molecules in the
asymmetric unit differ by the relative orientation of the morpholine ring. A least-square fit of the benzoyl thiourea portion of the molecule (i.e. excluding the morpholine
˚
ring) gives an RMSD of 0.198 A for the 13 atoms used in the structure overlay (molecule A in purple and B in yellow). The half molecules of (4) are more similar as illustrated
˚
by an RMSD of 0.0765 A for the same 13 non-H atoms.
6

K.I.Y. Ketchemen, M.D. Khan, S. Mlowe et al.
Journal of Molecular Structure 1229 (2021) 129791
Table 3
˚
Selected bonds lengths (A) and bonds angles (°) for the complexes with some known structures made from benzoylthiourea (diethyl
amine and piperidine used as secondary amine).
Secondary amine (apiperidine)
Cis-[Ni]
Trans-[Ni]
1.835(2)
Cu
Cis-[Zn]
1.949(2)
˚
Bond
Length (A)
1.853(2)
1.861(2)
2.133(1)
2.140(1)
1.728(4)
1.719(4)
1.264(4)
1.257(4)
1.320(4)
1.325(4)
1.336(4)
1.342(4)
1.339(4)
1.340(4)
Angle (°)
85.4(1)
1.905(3)
M-O1
M-O2
1.968(2)
2.291(1)
2.286(1)
1.742(3)
1.743(3)
1.277(3)
1.271(3)
1.305(4)
1.310(3)
1.341(4)
1.344(4)
1.329(4)
1.326(4)
108.4(9)
98.7(7)
M-S1
2.172(1)
1.716(4)
1.268(4)
1.316(4)
1.341(4)
2.244(8)
1.732(4)
1.268(5)
1.313(5)
1.349(5)
M-S2
S1-C8
S2-C21
O1-C1
O2-C14
N1-C1
N3-C14
N1-C8
N3-C21
N2-C8
1.344(4)
94.4(8)
85.6(8)
[Fe]
1.339(5)
93.8(1)
N4-C21
Bond
86.2(1)
O1-M-O2
O1-M-S1
O1-M-S2
O2-M-S1
O2-M-S2
S1-M-S2
(bdiethylamine)
Bond
1.986(2)
2.4221(8)
1.726(3)
1.353(3)
1.319(3)
1.247(3)
87.52(9)
85.55(6)
99.77(7)
169.70(7)
88.13(3)
125.2(2)
95.6(8)
Fe-O1
117.8(7)
118.1(7)
97.7(6)
168.9(1)
169.9(9)
94.9(8)
Fe-S1
S1-C8
N1-C8
118.1(4)
86.0(4)
C7-N1
[Cu]
C7-O1
˚
Lenght (A)
O1-Fe-O1#
O1#-Fe-S1
O1-Fe-S1∗
O1-Fe-S1#
S1-Fe-S1∗
C7-N1-C8
Cu1-O1
Cu1-O2
Cu1-S1
Cu1-S2
O2-C19
N3-C20
S2-C20
S1-C8
1.918(4)
1.929(4)
2.221(2)
2.233(2)
1.261(5)
1.339(6)
1.742(4)
1.740(5)
1.265(6)
1.338(6)
1.324(6)
1.318(6)
Angle (°)
86.40(16)
93.32(12)
93.87(11)
86.39(6)
179.71(13)
173.04(15)
125.4(4)
125.8(4)
O1-C7
N1-C8
N1-C7
N3-C19
Bond
O1-Cu1-O2
O1-Cu1-S1
O2-Cu1-S2
S1-Cu1-S2
O2-Cu1-S1
O1-Cu1-S2
C8-N1-C7
C19-N3-C20
∗
Symmetry code: = (1-y, x-y, -1+z) and # = (1-x+y, 1-x, -1+z).
a
Data are taken from Xie et al. [49].
b
Data are taken from Mandal et al. [42].
tallises in the triclinic P-1 Space Group. The molecular structure of
(5), however, adopts approximately tetrahedral coordination geom-
etry as opposed to the square planar configuration of (3) and (4).
The tetrahedron is defined by angles around the Zn(II) ion rang-
ing from 98.14(5)–122.25(2)°. The more acute angles of 98.14(5)°
tion environment similar to that of complexes (1) and (2). It sim-
ilarly crystallises in the monoclinic P21/c space group with three
monoanionic, bidentate ligands coordinated. This leads to three
–
six-membered chelation rings (NC2OS–In) with In S bond lengths
˚
–
ranging from 2.526(1) to 2.550(1) A and In O bonds lengths rang-
˚
–
–
and 98.99(4)° are associated with the intraligand S1 Zn1 O1 and
ing from 2.140(3) to 2.169(3) A. The compound crystallised as
–
–
S2 Zn1 O3 bonds, respectively. These angles are in good agree-
ment with the comparable intraligand bond angles of (3) and (4)
as they are constrained by the ligand bite and associated six-
membered chelate rings. The interligand bond angles, which are
not constrained by the ligand geometry, range from 101.36(6) to
122.25(2). The details of these bond angles are available in Table 2.
the acetone monosolvate. The absence of formal hydrogen bond
donors in the ligand structure precludes any intermolecular hy-
drogen bonding. All the structures do, however, show numerous
•••
–
weakly stabilising C H
O and C H•••N interactions in the solid
–
state.
–
–
The data in Table 2 show that the interligand S M O trans bond
angles describing the nominally octahedral geometry of the metal
ion are more acute than the ideal bond angle of 180°. The in-
–
The C N bonds lengths observed for the ligands in this work may
–
suggest a process of imine–enamine tautomerism [48]. The C S
–
– –
and C O bond lengths (1.745(2) and 1.270(2) follow the same trend
traligand S M O bond angles vary in magnitude which is some-
what unexpected considering the limitations imposed by the lig-
and bite. The most notable difference is evident in the three oc-
tahedral chelates, Fe(III), Co(III) and In(III). The intraligand bond
angles of the Fe(III) and In(III) chelates are all slightly acute. In
all other complexes (whether octahedral, tetrahedral or square pla-
as the other compounds in this study and suggest the negative
charge is predominantly located on the sulfur atom.
The solid state structure of the In(III) chelate, compound (6),
depicted in Fig. 9 shows the indium(III) ion to have a slightly dis-
torted octahedral coordination geometry with a fac-S3O3 coordina-
7

K.I.Y. Ketchemen, M.D. Khan, S. Mlowe et al.
Journal of Molecular Structure 1229 (2021) 129791
4. Conclusion
N-morpholine-N’-benzoylthiourea and its novel complexes of
Fe(III), Co(III), Ni(II), Cu(II), Zn(II) and In(III) were synthesized and
characterised using various techniques including single crystal X-
ray diffraction studies. The trivalent compounds all adopt compa-
rable octahedral structures comprising three monoanionic biden-
tate ligands coordinated to the metal centre. The octahedral com-
plexes all showed a preference for an S3O3 facial arrangement of
coordinating atoms. This ubiquitous geometry was found to be ca.
9 kJ.mol–1 lower in energy than the comparable meridional isomer
using DFT methods. The divalent metal chelates are square planar
in the case of Cu(II) and Ni(II), but tetrahedral for Zn(II). The co-
ordination sphere of the divalent compounds is completed by two
of the S,O-donor bidentate ligands. From the TGA and unpublished
data, the complexes have shown a potential usefulness as single
source molecular precursors for the preparation of metal sulfide
nanomaterials.
Fig. 8. The structure of complex (5) showing the atom naming scheme. Displace-
ment ellipsoids are shown at the 50% probability level. One of the morpholine rings
is disordered over two positions with a 1:1 ratio of the two positions. Hydrogens
are shown as spheres of arbitrary radius.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
CRediT authorship contribution statement
Kevin I.Y. Ketchemen: Data curation, Writing - original draft.
Malik Dilshad Khan: Conceptualization, Supervision, Writing
-
original draft. Sixberth Mlowe: Conceptualization, Supervision,
Writing - original draft. Matthew P. Akerman: Data curation, For-
mal analysis, Writing - review & editing. Inigo Vitorica-Yrezabal:
Data curation. George Whitehead: Data curation. Linda D. Nya-
men: Supervision, Writing - original draft. Peter T. Ndifon: Su-
pervision, Funding acquisition, Writing - review & editing. Neerish
Revaprasadu: Conceptualization, Supervision, Funding acquisition,
Writing - review & editing. Paul O’Brien: Supervision, Funding ac-
quisition.
Acknowledgements
Fig. 9. The molecular structure of complex (6) showing the atom naming scheme.
The sulfur and oxygen ligand donor atoms have adopted a facial (fac) coordination
geometry. Two of the three morpholine rings are disordered between a ring up and
ring down orientation with ratios of 76:24 and 69:31. Displacement ellipsoids are
shown at the 50% probability level. Hydrogens are shown as spheres of arbitrary
radius.
The authors thank the Royal Society/Department for Interna-
tional Development (RS-DFID) Africa Capacity Building Initiative
and the National Research Foundation (NRF), South Africa through
the South African Research Chair Initiative (SARChI) for financial
support. The authors also acknowledge Dr. Paul McNaughter of the
School of Chemistry of the University of Manchester for his assis-
tance.
nar), the 6-membered chelate rings and the associated bite of the
–
–
ligand leads to obtuse intraligand S M O bond angles. The triva-
lent iron and indium chelates are also the only solvated com-
pounds, although the reason is not immediately apparent, it is pos-
sible that there is a relationship between the solvation of these
compounds and the acute intraligand bond angles.
Supplementary data
CCDC 2021576, 2021575, 2021578, 1569168, 2021579 and
2021577 contains the crystallographic data for compounds (1) –
(6), respectively.
A comparative study of selected bond lengths and bond an-
gles for the compounds synthesised herein with those of sim-
ilar structures (diethyl amine and piperidine used as secondary
amine) is presented in Table 3. In general, it is observed that
the bond lengths and bond angles of Cu(II) and Fe(III) complexes
of N-piperidine-N’-benzoylthiourea and those of Cu(II), Zn(II) and
Ni(II) complexes of N, N-diethyl-N’-benzoylthiourea are larger than
those of the corresponding morpholine complexes. This is likely at-
tributable to inductive effects, as morpholine contains a more elec-
tronegative oxygen atom and piperidine is more electron donating
as it is a heterocyclic amine, in comparison to diethyl amine, which
is a secondary amine.
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