Photophysical, electrochemical and TD-DFT studies of Ni(II) and Mn(II) complexes of N′-(2-methylfuran-3-carbonyl)hydrazine carbodithioic acid ethyl ester

Article abstract of DOI:10.1016/j.poly.2019.114160

Two new complexes [Ni(Hmfchce)₂(py)₂] (1) and [Mn(Hmfchce)₂(o-phen)] (2) with [N′-(2-methylfuran-3-carbonyl)hydrazine]-carbodithioic acid ethyl ester (H₂mfchce) have been synthesized which contain o-phenanthroline (o-phen)/pyridine (py) as coligand. The ligand and its metal complexes have been characterized by elemental analyses, IR, magnetic susceptibility and single crystal X-ray diffraction data. Complexes 1 and 2 crystallize in orthorhombhic system with space group ‘Pbca’ and ‘Pbcn’, respectively. In complex 1, nickel centre is coordinated through one hydrazine nitrogen atom, one carbonyl oxygen of two units of ligand and two pyridine nitrogen atoms. The manganese center in complex 2 is coordinated in a N₄O₂ core by two uininegative bidentate ligands using hydrazine nitrogen (after loss of proton) and carbonyl oxygen and two nitrogen atoms of o-phen. In both complexes, the metal ion adopts a distorted octahedral geometry. Complexes 1 and 2 are fluorescent materials which exhibit an emission at 291 and 285 nm, respectively upon excitation at 263 and 251 nm. The course of the thermal degradations of complexes 1 and 2 have been investigated by TGA which indicate that metal oxide is formed as the final residue in both complexes. The metal complexes, [Ni(Hmfchce)₂(py)₂] (1) and [Mn(Hmfchce)₂(o-phen)] (2) were immobilized on glassy carbon electrodes using Nafion (Nf). The modified electrodes have been characterized by cyclic voltammetry in 0.1 M KOH. Complexes 1 and 2 have efficient activity towards electrochemical water oxidation in the 0.1 M KOH. The simulated spectra of the two complexes 1 and 2 are characterized by excited states with ligand-to-ligand charge-transfer (LLCT) and ligand-to-metal charge-transfer (LMCT) character.

Full text of DOI:10.1016/j.poly.2019.114160

Journal Pre-proofs
Photophysical, electrochemical and TD-DFT studies of Ni(II) and Mn(II) com-
plexes of N'-(2-furan-3-carbonyl) hydrazine carbodithioic acid ethyl ester
R. Chaurasia, M.K. Bharty, Paras Nath, Piyush Kumar Sonkar, Vellaichamy
Ganesan, B. Maiti, A. Bharti, R.J. Butcher
PII:
S0277-5387(19)30597-2
DOI:
https://doi.org/10.1016/j.poly.2019.114160
POLY 114160
Reference:
Polyhedron
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Revised Date:
Accepted Date:
20 June 2019
19 September 2019
20 September 2019
Please cite this article as: R. Chaurasia, M.K. Bharty, P. Nath, P. Kumar Sonkar, V. Ganesan, B. Maiti, A. Bharti,
R.J. Butcher, Photophysical, electrochemical and TD-DFT studies of Ni(II) and Mn(II) complexes of N'-(2-furan-3-
carbonyl) hydrazine carbodithioic acid ethyl ester, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.
2019.114160
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REVISED MANUSCRIPT
Photophysical, electrochemical and TD-DFT studies of Ni(II) and
Mn(II) complexes of N'-(2-furan-3-carbonyl) hydrazine carbodithioic
acid ethyl ester
R. Chaurasia,a M.K. Bharty,*a Paras Nath,‡a Piyush Kumar Sonkar,a Vellaichamy Ganesan,a B. Maiti,a A. Bharti,b R. J. Butcher c
aDepartment of Chemistry, Banaras Hindu University, Varanasi-221005, India
bDepartment of Chemistry, Kirori Mal College, University of Delhi, Delhi-110007, India
cDepartment of Chemistry, Howard University, 525 College Street NW, Washington, DC 20059, USA
ABSTRACT
Two new complexes [Ni(Hfchce)2(py)2] (1) and [Mn(Hfchce)2(o-phen)] (2) with [N'-(2-
furan-3-carbonyl)hydrazine]-carbodithioic acid ethyl ester (H2fchce) have been synthesized
which contain o-phenanthroline (o-phen)/ pyridine (py) as coligand. The ligand and its
metal complexes have been characterized by elemental analyses, IR, magnetic
susceptibility and single crystal X-ray diffraction data. Complexes 1 and 2 crystallize in
orthorhombhic system with space group ‘P b c a’ and ‘P b c n’, respectively. In complex 1,
nickel centre is coordinated through one hydrazine nitrogen atom, one carbonyl oxygen of
two units of ligand and two pyridine nitrogen atoms. The manganese center in complex 2 is
coordinated in a N4O2 core by two uininegative bidentate ligands using hydrazine nitrogen
(after loss of proton) and carbonyl oxygen and two nitrogen atoms of o-phen. In both
complexes, the metal ion adopts a distorted octahedral geometry. Complexes 1 and 2 are
fluorescent materials which exhibit an emission at 291 and 285 nm, respectively upon
excitation at 263 and 251 nm. The course of the thermal degradations of complexes 1 and 2
have been investigated by TGA which indicate that metal oxide is formed as the final
residue in both complexes. The metal complexes, [Ni(Hfchce)2(py)2] (1) and
[Mn(Hfchce)2(o-phen)] (2) were immobilized on glassy carbon electrodes using Nafion®
(Nf). The modified electrodes have been characterized by cyclic voltammetry in 0.1 M
KOH. Complexes 1 and 2 have efficient activity towards electrochemical water oxidation
in the 0.1 M KOH. The simulated spectra of the two complexes 1 and 2 are characterized
by excited states with ligand-to-ligand charge-transfer (LLCT) and ligand-to-metal charge-
transfer (LMCT) character.
Keywords: Keywords: Ni(II) and Mn(II) complexes, Fluorescence, Electrocatalytic activity,
Crystal structure, TD-DFT.
*Corresponding author E-mail: manoj_vns2005@yahoo.co.in; mkbharty@bhu.ac.in
‡ Present address: Department of Chemistry, Buddha P.G. College, Kushinagar, India
1

1. Introduction
The coordination chemistry of nitrogen, sulphur and oxygen containing ligands is an
interesting and emerging area of research [1-4]. The metal complexes formed with
nitrogen, sulphur/ oxygen-donor ligands have attracted an increasing interest because of
their potential biological and chemical activities [4, 5]. Metal complexes of nitrogen-sulfur
containing ligands have attracted a lot of attention of scientists due to their application in
the synthesis of bioactive compounds [6]. Metals play a crucial role in various biochemical
reactions in therapeutic and diagnostic applications [7]. Metal complexes prepared with
these ligands are very interesting from the viewpoint of their semiconducting behaviour,
electro-optical, molecular magnetism and electrochemical properties [8-15]. The study of
the metal complexes of above types of ligands has profound interdisciplinary relevance
because they have several applications in electroactive materials [16]. The S-alkyl/aryl
esters derived from potassium salts of N- aroylhydrazine carbodithioates have been found
to be more stable towards cyclization as compared to the potassium salt. The potassium
salts of N-(aroyl)-hydrazine carbodithioates are not much more stable since they undergo
intramolecular cyclization with elimination of H2S forming 1,3,4-oxadiazole-2-thiones in
the presence of weak acid or base [17-19]. The major interest in the synthesis of esters is
that they are much more stable than the potassium salt and do not undergo cyclization to
form oxadiazole derivatives. Metal-organic compounds comprised of metal ions and
bridging ligands have received much attention due to their potential applications as
functional materials such as catalysis, gas absorption, molecular recognition and optics [20-
22]. Nitrogen-sulfur donor ligands and their metal complexes are efficient catalysts under
homogeneous and heterogeneous conditions. The catalytic activity of these catalysts
depends on the nature of the substituent as well as the metal centres [23-25]. Therefore,
these compounds have been used as catalysts in reactions such as epoxidation of alkenes
[26], oxidation of alcohols [27], enantioselective oxidation of sulphides [28, 29], and
oxidation of water [30]. Transition metal complexes formed with different metal ions with
various oxidation states of Mn(II, IV & VI), Fe(II & III), Co(II & III), Ni(II), and Cu(I &
II) involve in the electrochemical water oxidation [30, 31-33]. Nickel complexes are
interesting for structural aspects, water oxidation and other important reactions [34-38].
First nickel-based homogeneous water-oxidizing catalyst [Ni(meso-L)](ClO4)2 was
reported which oxidize water at neutral pH and low overpotential in buffer solution [39].
2

The electrocatalytic properties and analytical applications of Ni(II) and Mn(II) complexes
are not intensively explored. Accordingly, in view of the possible electroanalytical
applications, interesting bonding modes, and various other properties exhibited by these
type of Ni(II) and Mn(II) complexes. We have synthesized and studied the crystal
structures, photoluminescence, thermal behaviour and electrochemical properties, of Ni(II)
and Mn(II) complexes of N'-(2-furan-3-carbonyl) hydrazine carbodithioic acid ethyl ester.
2. Experimental Section
2.1 Materials and methods
Commercial reagents were used without further purification and all experiments
were carried out in open atmosphere. Isonicotinic acid hydrazide (Sigma Aldrich),
CS2 (S D Fine Chemicals, India) and KOH (Merk) were used as received. Nafion®
(Nf) was purchased from Sigma-Aldrich, India (5 wt% solution in lower aliphatic
alcohols). N, N'-dimethylformamide (DMF) was procured from S D Fine Chemicals,
India. Solutions for all electrochemical experiments were prepared with triple
distilled water. All the solvents were purchased from Merk Chemicals, India and
used after purification.
2.2. Physical measurements
Carbon, hydrogen and nitrogen contents were estimated on a Carlo Erba 1108 model
microanalyser while sulfur was estimated as BaSO4. Magnetic susceptibility
measurements were performed at room temperature on a Cahn Faraday balance
using Hg[Co(NCS)4] as the calibrant. Electronic spectra were recorded on a
SHIMADZU 1700 UV-Vis Spectrophotometer. Infrared spectra were recorded in
the 4000-400 cm-1 region as KBr pellets on a Varian Excalibur 3100 FT-IR
Spectrophotometer. Thermo-gravimetric analysis (TG-DTA) of complexes 1 and 2
were done using a Perkin Elmer-STA 6000 thermal analyzer, TA Instruments with
o
1
13
heating rate of 10, 20 and 30 C min-1. H and C NMR spectra were recorded in
DMSO-d6 on a JEOL AL 500 FT NMR spectrometer using TMS as an internal
reference.
3

2.3 X-ray crystallography
X-Ray diffraction measurements of complexes 1 and 2 were performed on a Oxford
Gemini diffractometer equipped with a CrysAlis CCD software package using a
graphite mono-chromated Mo Kα (λ= 0.71073 Å) radiation source at 120 (for 1)/293
K (for 2). Multi-scan absorption correction was applied to the X-ray data collection
for both the complexes. The structures were solved by direct methods (SHELXL-
2013) and refined by full matrix least-square on F2 (SHELXL) using anisotropic
displacement parameters for all non-hydrogen atoms. All hydrogen atoms were
included in calculated position and refined with a riding model [40]. The
MERCURY package and ORTEP-3 for Windows program were used for generating
molecular graphics [41, 42].
2.4 Quantum Chemical Calculation
To understand the binding sites and to verify the composition of the complexes, Density
Functional Theory (DFT) calculations have been performed. In these calculations, B3LYP
density functional theory method has been used which is a hybrid version of DFT and
Hartree-Fock (HF) methods [43]. In this method, the exchange energy from Becke's
exchange functional is combined with the exact energy from Hartree-Fock theory [44]. The
three parameters define the hybrid functional, specifying how much of the exact exchange
is mixed in along with the component exchange and correlation funсtionals [45]. The 6-
31G** basis set for all the atoms except metal ions have been used. The LanL2DZ basis set
with an effective core pseudo potential has been used for the metal atom [46]. All the
geometry optimizations and frequency calculations (to verify a genuine minimum energy
structure) were performed using Gaussian 09 program package [47]. The electronic
excitation energies and intensities of the three lowest-energy spin allowed transitions were
calculated using the time dependent density functional theory (TD-DFT). It is worth
pointing out here that the starting geometries were chosen based on X-ray crystallographic
structures with required modification for the computations if needed.
2.5 Cyclic Voltammetry
The electrochemical experiments were performed using CH instruments
electrochemical workstation (CHI-660C, USA). A one compartment-three electrode
4

electrochemical cell setup which includes glassy carbon (GC) electrode (3 mm
diameter), platinum wire and Ag/AgCl (saturated with KCl) electrode as working,
counter and reference electrodes, respectively were used for electrochemical
experiments. The potentials quoted in this manuscript were referenced to reversible
hydrogen electrode (RHE) potential. GC rotating disc electrode (GC-RDE, 5 mm
diameter) and RDE setup from Pine research instruments, USA were used for
hydrodynamic voltammetry studies.
2.6 Preparation of modified electrode
Nf solution (0.01%) was prepared in ethanol and 14.0 μL of 0.01% Nf solution was
coated on a GC electrode and dried for 30 min. The Nf coated GC electrode (GC/Nf)
was dipped in 10 mM Ni-Hfchce solution (in DMF) for 1 h. After 1 h, the GC
electrode was removed from the dipping solution, rinsed with triple distilled water
(abbreviated as GC/Nf/Ni-Hfchce) and used for electrochemical studies. Similar
procedure was followed for the preparation of GC/Nf/Mn-Hfchce electrode also.
2.7 Synthesis
2.7.1 Preparation of N'-(2-furan-3-carbonyl) hydrazine carbodithioic acid ethyl ester
(H2fchce)
N'-(2-furan-3-carbonyl)hydrazinecarbodithioic acid ethyl ester (H2fchce) was
synthesized by a mixture of methyl-2-methyl furancarboxylate (2.51 ml, 20 mmol)
and hydrazine hydrate (1.0 ml, 20 mmol) in absolute ethanol and refluxed for 4 h.
The white crystalline solid obtained upon cooling was filtered off, washed
thoroughly with water and finally with diethyl ether and recrystallized from ethanol.
The above solid was dissolved in alcoholic solution of KOH and carbon disulphide
was added drop wise. After 30 min of stirring, a white precipitate was obtained
which was filtered off washed with diethyl ether and dried in vaccuo. Further the
precipitate was dissolve in absolute ethanol kept on ice bath and ethyl iodide (1.44
ml, 18 mmol) was added. After 8 h of continuous stirring at room temperature, a
yellowish clear solution was obtained which was filtered. The filtrate kept overnight
at room temperature and acidified with dilute acetic acid (50%) yellowish solid was
filtered off, washed with hot water and recrystallize in ethanol, yield a white
ο
crystalline solid. Yield: 60%; m.p. 98 C. Anal. found: C = 44.50, H = 5.80, N =
11.65, S = 26.30 (%) Anal. calc. for C9H12N2O2S2 (244.00) C = 44.26, H = 4.91, N =
5

11.47, S = 26.22%. IR (ν cm-1, KBr): ν(NH) 3294, 3150; ν(C=O) 1645; ν(N–N)
1
1066s; ν(C=S) 974. H NMR (DMSO-d6; δ ppm): 11.38 & 10.59 (2H, NH), 7.59–
13
6.87 (m, 2H, furan ring), 3.18 (s, CH3), 1.38 (q, CH2), 1.24 (t, CH3). C NMR
(DMSO-d6; δ ppm): 203.81 (C=S), 162.33 (C=O), 109.21–141.77 (furan ring
carbons), 29.06 (CH3 carbon attached with furan ring), 28.25, (CH2) 14.86 (CH3).
UV-vis (λmax, MeOH, cm-1) 28515 and 29410.
2.7.2 Synthesis of [Ni(Hfchce)2(py)2] (1)
A solution of freshly prepared ligand N'-(2-furan-3-carbonyl) hydrazine
carbodithioic acid ethyl ester (H2fchce) (0.244 g, 1 mmol) in MeOH (10 ml) was
added to a MeOH solution (10 ml) of NiCl2.6H2O (0.118 g, 0.5 mmol). This mixture
was stirred for 2 h at room temperature. The resulting brown precipitate was filtered
off and washed thoroughly with methanol. Few drops of pyridine (0.081 ml, 1
mmol) were added to the methanol-chloroform suspension of the above compound.
The resulting reddish-brown clear solution was obtained at room temperature,
filtered off and kept for crystallization. Single crystals of the complex suitable for X-
ray analysis were obtained by slow evaporation of the above solution over a period
of 21 days. Yield: 45%; m.p. 200 ᵒC. Anal. found: C = 47.85, H = 4.40, N = 11.70, S
= 18.35%. Anal. calc. for C28H32N6NiO4S4 (703.54) C = 47.75, H = 4.54, N = 11.93,
S = 18.19%. IR (ν cm-1, KBr): ν(N–H) 3067, ν(C=O) 1688; ν(N–N) 1090; ν(C=S)
878, ν(Ni–N) 563, 547; ν(Ni–O) 651. UV-vis (λmax, DMSO, cm-1) 13698 and 15151.
2.7.3 Synthesis of [Mn(Hfchce)2(o-phen) (2)
A solution of freshly prepared ligand N'-(2-furan-3-carbonyl) hydrazine carbodithioic acid
ethyl ester (H2fchce) (0.244 g, 1 mmol) in MeOH (10 ml) was added to a MeOH solution
(10 ml) of Mn(OAc)2∙4H2O (0.123 g, 0.5 mmol). This mixture was stirred for 1 h at room
temperature. The resulting dark-brown precipitate was filtered off and washed thoroughly
with ethanol. A methanol solution of o-phenanthroline (0.09 g, 0.5 mmol) was added to the
methanol-chloroform suspension of the above compound. The resulting yellow clear
solution was obtained at room temperature, filtered off and kept for crystallization. Yellow
crystals were obtained by slow evaporation of the above solution over a period of 10 days,
suitable for single crystal X-ray analysis. Yield: 56%; m.p. 205 οC. Anal. found: C = 49.95,
H = 4.00, N = 11.50, S = 17.85%. Anal. calc. for C30H30MnN6O4S4 (721.78) C = 49.87, H =
6

4.15, N = 11.63, S = 17.73%. IR (ν cm-1, KBr): ν(NH) 3180, ν(C=O) 1622; ν(N–N) 1087;
ν(C=S) 898, ν(Mn–N) 540, 588; ν(Mn–O) 651. UV-vis (λmax, MeOH, cm-1) 28515 and
29410.
O
O
O
S
CH3
O
NH
NH2
N
H
N
H
CS2, KOH
EtOH
NH2NH2.H2O
reflux, 5 h
SK
CH3
CH3
CH3
O
O
O
C2H5I
EtOH
O
H
N
S
CH3
N
H
C
H2
S
O
CH3
Mn(OAc)2.4H2O
1,10 o-phen.
MeOH
NiCl2.6H2O
pyridine
MeOH
O
CH3
N
S
O
H
H2
C
N
N
CH3
CH2
O
H3C
S
N
CH3
O
Mn
S
HN
N
N
N
Ni
N
O
S
S
NH
O
S
H3C
O
N
S
CH3
H3C
H2C
C
H2
N
H
CH3
S
O
Scheme 1. Preparation of the ligand and its complexes 1 and 2
3. Results and discussion
The ligand N'-(2-furan-3-carbonyl)hydrazine]-carbodithioic acid ethyl ester
(H2fchce) ester reacts with NiCl2.6H2O /Mn(OAc)2·4H2O to form brown and dark-
brown precipitates, respectively which dissolve in methanolic solutions of pyridine/
o-phen yield [Ni(Hfchce)2(py)2] (1) and [Mn(Hfchce)2(o-phen)] (2). The
complexes are stable towards air and moisture. The scheme 1 depicts the formation
of the complexes which contain [Hfchce]− as ligand and o-phen as the coligand. The
ligand is soluble in methanol and ethanol while complexes 1 and 2 dissolve in DMF
and DMSO. The ligand and complexes 1 and 2 melt at 371, 473 and 478 K,
respectively. Single crystal X-ray diffraction data indicate that ligand (H2fchce)
behaves as uninegative, bidentate forming a six member chelate ring. Coordination
7

environment of both complexes is fulfilled by hydrazine nitrogen and carbonyl
oxygen of two units of ligand H2fchce and pyridine/o-phen nitrogen donor
secondary ligand.
3.1 IR Spectra
The IR spectrum of the ligand N'-(2-furan-3-carbonyl) hydrazine]carbodithioic acid
ethyl ester (H2fchce) shows absorptions due to the stretching modes of NH (3294,
3150 cm-1), C=O (1645 cm-1), N-N (1066 cm-1) and C=S (974 cm-1). The IR
spectrum of the complex 1 shows a band at 3067 cm-1 indicating the presence of
only one NH group upon complexation. The appearance of two new bands for ν(Ni-
N) at 547 and 563 and ν(Ni-O) at 663 cm-1 suggest bonding of Ni(II) with one
carbonyl oxygen, one hydrazinic nitrogen after loss of a proton and pyridine
nitrogens. The ν(C=O) and ν(N-N) bands suffer negative and positive shifts of 32
cm-1 and 24 cm-1, respectively, indicating that H2fchce is acting as uninegative
bidentate ligand and bonding through carbonyl oxygen and hydrazinic nitrogen in
complex 1. [Mn(Hfchce)2(o-phen)] (2) shows the absence of the ν(NH) band at 3294
cm-1, indicating the loss of hydrogen atom from the nitrogen atom of the thioamide
group which is supported by a small positive shift of 20 cm-1 in ν(N-N). The
appearance of three new bands at 540 and 588 and at 651 cm-1 due to ν(Ni-N) and
ν(Ni-O), suggest bonding through one hydrazinic nitrogen, carbonyl oxygen and
both o-phen nitrogen atoms. Thus, the H2fchce acts as a uninegative bidentate in
complex 2 [48].
1
3.2 H and 13C NMR spectra
1
The H NMR spectrum of N'-(2-furan-3-carbonyl)hydrazine]-carbodithioic acid
ethyl ester (H2fchce) exhibits two signals at δ 11.38 and 10.59 ppm for the amide
and thioamide protons, respectively. Two signals are observed at 1.38 and 1.24 ppm
due to CH2 and CH3 protons. A signal observed at 3.18 ppm due to CH3 protons
attached with furan ring. Two protons of the furan ring appear as multiplet between
δ 7.59-6.87 ppm. The 13C NMR spectrum of H2fchce shows 9 signals for nine
carbons, of which the signals at δ 203.8 and 162.3 ppm are due to the >C=S and
>C=O carbons, respectively. The signal for >CH2 >CH3 and >CH3(attached with
furan ring) carbons are observed at δ 28.2, 14.8 and 29.0 ppm(methyl carbon
8

attached with furan ring). The furan ring carbons appear in the range of 109.2–141.7
ppm.
3.3 Electronic spectra and magnetic moments
A magnetic moment of 2.91 BM for [Ni(Hfchce)2(py)2] (1) and the presence of two
3
3
bands at 729 and 611 nm assigned to the A2g→3T2g (ν1) and T1g(F) (ν2)
transitions, respectively, suggest a distorted octahedral geometry for the complex 1.
Other two high energy bands observed at 317 and 251 nm may be assigned to
intraligand/ charge transfer transitions. [Mn(Hfchce)2(o-phen)] (2) shows a magnetic
moment of 5.9 BM which indicates the presence of five unpaired electrons. It shows
two high intensity absorptions at 325 and 263 nm may be assigned to the intraligand/
charge transfer transition (Fig. 1) [49].
3.4 Photoluminescence studies
In the present work, we have examined the photoluminescence properties of ligand
H2fchce, complexes [Ni(Hfchce)2(py)2] (1) and [Mn(Hfchce)2(o-phen)] (2) at room
temperature. Ligand H2fchce displays an emission at 291 nm upon excitation at 246
nm. Complex 1 shows photoluminiscence at 292 nm upon excitation at 251 nm. The
complex 2 upon excitation of at 263 nm, the corresponding emission was observed
at 285 nm (Fig. 2). Photoluminescence studies indicate that the both complexes are
better fluorescent material as compared to the ligand. The excitation spectra of
complexes 1 and 2 show maxima at 292 nm and 285 nm which are close to those
observed in the absorption spectra.
3.5 Thermal studies
The thermal properties of complexes 1 and 2 were studied by TG-DTA under a nitrogen
atmosphere in the temperature range 25-800 ºC at controlled heating rate of 5 ºC min-1. The
TG-DTA curves of complexes 1 and 2 are shown in Fig. 3. The complexes 1 and 2 are
thermally stable up to 190 °C and 140 °C, respectively. The decomposition of the organic
part of the complex 1 occurs in the temperature range 190-800 °C during which 88.50%
weight loss was observed (calc. 89.39%). The first weight loss at 190 °C corresponds to the
endothermal decomposition of the pyridine molecules (Obs; 20.95%, calc. 20.75 %). The
corresponding endothermic peak of this decomposition was observed in DTA curve at 190
°C. The decomposition of remaining organic moiety was observed in the temperature range
9

200-800 °C with the weight loss of 65.55% (calc. 68.64%). The decomposition of complex
2 occurs at the temperature range of 140-800 °C during which 89.94% weight loss was
observed (calc. 90.16%). The first weight loss of 68.50% at 140 °C corresponds to thermal
decomposition of two ligand moieties (calc. 67.68%) with a significant exothermic effect at
180 °C. In second decomposition, the weight loss of 23.25% (calc. 24.99 %) was observed
corresponds to the release of o-phen molecule. Finally at higher temperature (800 °C), the
metal is converted into the metal oxide in complexes 1 (obs. 11.50%, calc. 10.61%) and 2
(obs. 10.06%, calc. 9.84%). Comparing the TG-DTA curves, it was observed that the major
weight loss process started after the melting temperature of both complexes.
3.6 Crystal structure description of [Ni(Hfchce)2(py)2] (1)
Figure 4 shows an ORTEP diagram of complex 1 with the atomic numbering
scheme. The structural refinement data related to complex 1 are listed in Table 1, the
selected bond distances and bond angles are listed in Table 2. The metal center in
complex 1 is coordinated in an N4O2 core by two uninegative bidentate ligands
using hydrazine nitrogen and carbonyl oxygen. The distances found within the
chelate rings are intermediate between single and double bond lengths. The average
bond lengths in complex 1 are: O1-C6 = 1.252(3), N3-C6 = 1.325(3), N2-N3 =
1.391(3)Å which suggest considerable delocalization of charge [50]. The
nitrogen/oxygen donor sites of the bidentate ligand chelates the Ni(II) center to form
a five-membered CN2ONi ring. The resulting complex has a distorted octahedral
geometry. The arrangement of pyridine is trans which makes less distorted geometry
in complex 1. The Ni-O bond length in the complex is 2.051 Å and the Ni-N bond
lengths are 2.134 and 2.084 Å for Ni-N (hydrazinic) and Ni-N (pyridine),
respectively. The longer Ni-N (hydrazinic) bond length as compared to Ni-N
(pyridine) indicates that the pyridine nitrogen bonds more strongly than the
hydrazine nitrogen. The bond angles O(1)-Ni-N(2) 100.48(8), O(1)-Ni-N(2)
79.52(8) and O(1)-Ni-N(1) 89.32(8)º indicate distortion from an ideal octahedral
geometry [51]. The bond distances C(7)-N(2) and C(6)-N(3) show partial double
bond character due to delocalization of π electrons throughout the whole chelate ring
in the complex [52]. Both chelate rings around Ni(II)centre formed by the (Hfchce)−
are almost planar to each other. In complex, the chelate rings and the pyridine rings
are perpendicular to each other forming a dihedral angle of 83.81º. In the solid state,
10

complex 1 is stabilized by weak intermolecular C-H···S interaction (Table 4)
between thioether sulfur of one molecule and pyridine ring hydrogen atom of nearby
molecule (Fig. 5).
3.7 Crystal structure description of [Mn(Hfchce)2(o-phen)] (2)
Figure 6 shows an ORTEP diagram of complex 2 together with the atom numbering
scheme. The structural refinement data related to complex 2 are listed in Table 1 and
the selected bond distances and bond angles are listed in Table 3. The (N, O) donor
sites of the uninegative bidentate ligand chelates the Mn(II) center to form a five-
membered CN2OMn ring. The average bond lengths in complex 2 are: O2-C6 =
1.301(12), N1-C6= 1.330(3), N1-N2 = 1.387(3), N2-C7 = 1.319(3) Å which suggest
considerable delocalization of charge. The geometry of the resulting complex is a
distorted octahedral. In complex 2, the arrangement of o-phen makes cis chelate
ring, such arrangement possibly makes more distorted geometry. In metal complexes
bonding of the ligand takes place through carbonyl oxygen, nitrogen and also
bonded with N-heterocyclic bases such as o-phen or bpy are prone to exhibit
metalloaromaticity [53], but in both the complexes do not observe such behaviour.
The bond distances for Mn-O, Mn-N and Mn-N(phen) are 2.171(15), 2.260(19) and
2.280(19) Å, respectively which are comparable to the bond lengths reported for
[Mn3(O2CCH3)6(N-N)2] (where N-N = o-phen) [54] and [Mn2(phen)(Hdcbi)]n
(Hdcbi = 4,5-dicarboxyimidazole) [55]. The crystal of complex 2 is stabilized by
weak intramolecular N-H···S interaction between thioether sulfur of one molecule
and hydrazine hydrogen. In addition, an intermolecular C-H···S interaction (Table
5) occurs between CH hydrogen atoms of ethyl group and thioether sulfur atoms of
ligand leading to butter like structure which also stabilizes the structure of complex
2 (Fig. 7).
3.8 DFT calculations
Slight disagreements in the observed and calculated bond lengths, C7-S1=1.698 (1.707)calc;
C6-O1=1.252 (1.279)calc; Ni-N1=2.084 (1.945)calc; Ni-O1=2.051 (1.909)calc for
[Ni(Hfchce)2(py)2] (1) and Mn-O2=2.172 (2.180)calc; Mn-N2= 2.260 (2.402)calc S1-
C7=1.687 (1.669)calc; C6-O2=1.250 (1.254)calc for [Mn(Hfchce)2(o-phen)] (2) may be due to
the fact that the DFT calculations are for an isolated molecule in the gas phase, while the
11

X-ray crystallographic data were obtained from the crystal lattice. Nevertheless, there is
generally good agreement between the geometrical parameters obtained by X-ray
crystallography to those generated by the DFT method (Table 2).
3.8.1 TD-DFT Calculation:
The geometric parameters computed with B3LYP and Hartree-Fock (HF) methods were
found to be in good agreement with the corresponding experimental values. For reference
we have computed absorption bands of the free ligand. Our TD-DFT calculation showed
two bands at 246 and 284 nm, which are very comparable to experimental observed bands
at 246 and 291 nm, respectively. The experimentally observed absorption bands of the
complexes [Ni(Hfchce)2(py)2] (1) [Mn(Hfchce)2(o-phen)] (2) have also been explained
with the help of TD-DFT calculations. As observed, the TD-DFT prediction of the
experimental absorption spectrum matches very well, and from those theoretical
calculations the assignment of the character of these transitions can be proposed. Since
each absorption line in a TD-DFT spectrum is due to several single excitations, a depiction
of the transition character is generally not straightforward. Calculations indicate that
complex [Ni(Hfchce)2(py)2] (1) shows a band at 319 nm which can be compared with
experimentally observed band at 317 nm in MeOH solution. The calculated band at 319 nm
with oscillator strengths (f) 0.2263 is assigned to the ligand to ligand charge transfer
transitions from the coordinated ligand moiety to the five membered furan ring of the
ligand (Fig.8) associated with the transitions HOMO-1 to LUMO+4 (91.6%) and HOMO
to LUMO+3 (4%). In the case of complex [Mn(Hfchce)2(o-phen)] (2), the electronic
absorption spectrum displayed one band at 327 nm which can be compared with the
experimentally observed band at 325 in MeOH solution (Fig.9). Quantum chemical
calculations reveal that the band calculated at 327 nm with oscillator strength (f) 0.1405.
This band is a combination eighteen individual transitions. For clarity, however, we have
shown only three more dominant transitions involving HOMO to LUMO+2 (5%), HOMO-
1 to LUMO+4 (8%) and HOMO to LUMO+4 (2%). These transitions indicate that electron
transfer from the coordinated sulfur atoms of the ligand [N'-(2-furan-3-carbonyl)
hydrazine]-carbodithioic acid ethyl ester (H2fchce) with slight intermingling of the metal d-
electrons to the chelate ring.
12

3.9 Electrochemical characterizations
The modified electrodes, GC/Nf/Ni-Hfchce and GC/Nf/Mn-Hfchce are tested in 0.1
M KOH using cyclic voltammetry (Fig. 10A). GC and GC/Nf electrodes do not
indicate any redox signals in 0.1 M KOH at a scan rate of 20 mVs-1. GC/Nf/Ni-
Hfchce shows oxidation and reduction signals at 1.44 and 1.36 V (vs. RHE),
correspond to Ni(II)/Ni(III) and Ni(III)/Ni(II) electrochemical process, respectively
[56-58]. However, GC/Nf/Mn-Hfchce displays a broad peak at 1.55 V, (vs. RHE)
with very low current indicating a weak oxidation process in the basic medium [59-
61]. The modified electrodes are further tested for water oxidation in the basic
medium (vide infra).
3.9.1 Electrochemical water oxidation
Analysis of electrochemical water oxidation is performed using Linear Sweep
Voltammetry (LSV) with a GC-RDE in 0.1 M KOH at the scan rate of 5 mVs-1 (Fig.
10B). It is observed that the GC-RDE and GC-RDE/Nf electrodes show inefficient
water oxidation at 1.78 and 1.80 V (vs. RHE), respectively. The GC-RDE/Nf/Ni-
Hfchce and GC-RDE/Nf/Mn-Hfchce electrodes represent oxidation signals
attributed to water oxidation at 1.51 and 1.64 V (vs. RHE), respectively. The
oxidation current obtained at GC-RDE/Nf/Ni-Hfchce is higher than GC-
RDE/Nf/Mn-Hfchce, GC-RDE/Nf and GC-RDE electrodes. Efficiency of the water
oxidation can be determined by the potential required to achieve a current density of
10 mA cm-2. The 10 mA cm-2 current has been observed for GC-RDE/Nf/Ni-Hfchce
at 1.72 V (vs. RHE), which is very close to the potential exhibited by commercial
catalysts (20% Pt/C = 2.02, 20% Ru/C=1.62 and Ir/C= 1.61) [62]. and other reported
cobalt based catalysts [63]. These results suggest that the GC-RDE/Nf/Ni-Hfchce
electrode has efficient electrochemical activity towards water oxidation. Thus GC-
RDE/Nf/Ni-Hfchce electrode may be useful for regenerated fuel cells which utilize
oxygen reduction and oxygen evolution reactions, water electrolysis and metal air
batteries [62, 64-66].
13

4. Conclusions
A new ligand [N'-(2-furan-3-carbonyl)hydrazine]-carbodithioic acid ethyl ester
(H2fchce) and its complexes [Ni(Hfchce)2(py)2] (1) and [Mn(Hfchce)2(o-phen)] (2)
have been synthesized. The arrangement of pyridine is trans in complex 1 while in
complex 2 o-phen makes cis chelate ring, such an arrangement possibly makes
complex 2 have much more distorted geometry than complex 1. Complexes 1 and 2
are better fluorescent as compared to free ligand and showed emission band at 292
and 285 nm upon excitation at 251 and 263 nm, respectively. TD-DFT prediction of
the experimental absorption spectrum matches very well. The crystal structure of the
complexes are stabilized through weak intermolecular C-H···S and intramolecular
N-H···S interactions leading to a supramolecular architecture. The Ni-Hfchce and
Mn-Hfchce complexes are successfully immobilized on the GC electrode.
GC/Nf/Ni-L and GC/Nf/Mn--Hfchce have efficient activity for water oxidation. Out
of these electrodes, GC/Nf/Ni-Hfchce has outstanding performance for the water
oxidation in the basic medium. The GC/Nf/Ni-Hfchce could be a suitable alternative
for the commercial OER catalysts used in water splitting, regenerated fuel cells and
rechargeable metal-air batteries.
5. Acknowledgement
Dr. M. K. Bharty is thankful to the Science and Engineering Research Board, India
for the award of young scientist project (No. YSS/2015/001201).
6. Supplementary material
CCDC 1412088 and 1905731 contain the supplementary crystallographic data for the
complexes
1
and 2. These data can be obtained free of 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.
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Fig.1. UV-vis. absorption spectra of H2fchce and complexes 1 and 2 at 10-3 M in MeOH
18

Fig. 2. Emission spectra of H2fchce and complexes 1 and 2 at 10-5 M in MeOH. (Excitation
wavelength for H2fchce and its complexes 1 and 2 are 246, 251 and 263 nm)
Fig. 3. TG-DTA curves showing the thermal degradation of complexes 1 and 2, at 5 oC
min-1 in N2 atmosphere (TGA curves are shown in solid lines & DTA in dotted)
19

Fig. 4. ORTEP diagram of [Ni(Hfchce)2(py)2] (1)
Fig. 5. Showing intermolecular C-H···S hydrogen bonding leading to a linear chain structure in
complex 1
20

Fig.6. ORTEP diagram of [Mn(Hfchce)2(o-phen)] (2) at 30 % probability level.
Fig. 7. Showing intramolecular C-H···S hydrogen bonding leading to a butter like structure in complex
2
21

Fig. 8. Electronic transitions of selected
molecular orbitals for complex 1
Fig. 9. Electronic transitions of selected
molecular orbitals for complex 2
Fig. 10.A. Cyclic voltammetry of GC (a), GC/Nf (b), GC/Nf/Mn-Hfchce (c) and GC/Nf/Ni-
Hfchce (d) electrodes in 0.1 M KOH with scan rate of 20 mVs-1. 10.B. LSV of GC-RDE (a), GC-
RDE/Nf (b), GC-RDE/Nf/Mn-Hfchce (c) and GC-RDE/Nf/Ni-Hfchce (d) electrodes in 0.1 M
KOH with scan rate of 5 mV s-1 and a rotation rate of 1600 rpm.
[67]
22

Table 1. Crystallographic data for complexes 1 and 2
Parameters
Empirical formula
Formula weight
Crystal system
Space group
T ( K)
λ, Mo Kα (Å)
a (Å)
b (Å)
1
2
C28H32N6NiO4S4
703.54
Orthorhombic
P b c a
120(2)
0.71073
15.5285(6)
10.7224(4)
19.5674(9)
90
C30H30MnN6O4S4
721.78
Orthorhombic
P b c n
293(2)
0.71073
18.5123(4)
14.2140(3)
12.6694(2)
90
c (Å)
 (º)
β (º)
90
90
(º)
90
3258.0(2)
4
1.434
0.895
90
V, (Å3)
3333.75(11)
4
1.438
0.691
1728
Z
ρ calcd (g/cm3)
μ (mm-1)
F(000)
1464
θ range for data
collections(º)
Index ranges
2.460-26.668°
3.88- 28.05
-17 ≤ h ≤ 19,
-11 ≤ k ≤ 13,
-24 ≤ l ≤ 23
10083
-24 ≤ h ≤ 25,
-19 ≤ k ≤ 19,
-17 ≤ l ≤ 16
49141
No. of reflections
collected
No. of independent
reflections
3392
(Rint = 0.0270)
4417
(Rint = 0.0335)
4417 / 0 / 204
Data/restrains/paramet 3392 / 0 / 201
ers
Goodness-of-fit on F2 1.091
1.097
a
b
R1 , wR2 [(I>2σ(I)]
0.0493, 0.1464
0.0564, 0.1558
0.482, -1.186
0.0533, 0.1228
0.0698, 0.1303
-0.349, -0.392
a
b
R1 , wR2 (all data)
Largest difference in
peak /hole (e.Å-3)
CCDC
1412088
1905731
aR1 = Σ||Fo| – |Fc||Σ|Fo|;
bR2 = [Σw (|F2 | – |F2 |)2/Σw|F2 |2]1/2
o
c
o
Table 2. Selected interatomic distances & angles for 1
Bond length (Å)
(Exp) (Cal.)
Bond angle (º)
(Exp)
89.3(8)
(Cal.)
90.0
Ni-O(1) 2.051(17) 1.909
Ni-N(1) 2.084(2) 1.945
Ni-N(2) 2.134(2) 2.643
S(1)-C(7) 1.698(3) 1.707
S(2)-C(8) 1.798(3) 1.834
O(1)-C(6) 1.252(3) 1.279
N(2)-N(3) 1.391(3) 1.378
N(2)-C(7) 1.322(3) 1.324
N(3)-C(6) 1.325(3) 1.329
O(2)-C(1) 1.370(5) 1.369
O(2)-C(4) 1.364(4) 1.362
O(1)-Ni-N(1)
O(1)#1-Ni-N(1)
O(1)-Ni-N(2)#1
O(1)#1-Ni-N(2)#1
O(1)-Ni-N(2)#1
N(1)-Ni-N(2)
N(1)#1-Ni-N(2)
C(7)-S(2)-C(8)
C(6)-O(1)-Ni
90.6(8)
90.0
100.4(8)
79.5(8)
100.4(8)
92.0(8)
87.9(8)
104.1(15)
113.0(16)
112.0(2)
142.0(19)
106.4
73.5
106.4
90.2
90.2
104.0
123.6
112.6
151.1
C(7)-N(2)-N(3)
C(7)-N(2)-Ni
23

Table 3. Selected interatomic distances and angles for 2
Bond length (Å)
Bond angle (º)
(Exp) (Cal.)
(Exp)
Mn(01)- O(2) 2.172(15)
Mn(01)- O(2) 2.172(15)
(Cal.)
2.180
2.182
2.400
2.402
2.237
1.669
1.760
1.841
1.254
1.380
1.390
1.327
O(2)- Mn(01)-O(2) 106.2(9) 94.8
O(2)- Mn(01)-N(2) 73.4(6) 69.7
O(2)- Mn(01)-N(2) 92.2(6) 70.6
N(2)- Mn(01)-N(2) 156.3(10) 144.1
O(2)- Mn(01)-N(3) 163.2(7) 169.4
O(2)- Mn(01)-N(3) 90.5(7) 95.7
N(2)- Mn(01)-N(3) 107.8(7) 111.6
Mn(01)- N2
2.260(19)
Mn(01) -N(2) 2.260(19)
Mn(01)- N(3) 2.281(2)
S(1)-C(7)
S(2)-C(7)
S(2)-C(8)
O(2)-C(6)
O(1)-C(1)
C(7)-N(2)
N(2)- N(1)
1.687(3)
1.774(3)
1.797(3)
1.250(3)
1.360(3)
1.319(3)
1.387(2)
N(2)-C(7)-S(1)
C(6)- O(2)- Mn(01) 116.4(14) 122.0
N(2)-C(7)-S(2) 110.5(18) 108.7
N(1)- N(2)- Mn(01) 108.8(13) 108.5
C(6)- N(1)- N(2) 119.0(17) 120.5
126.3(19) 123.8
Table 4. Inter and intramolecular interactions [Å and º] for 1
D-H···A
d(D-H)
0.78(5)
0.95
d(H···A) d(D···A)
2.38(4) 2.834(2)
<(DHA)
119(3)
127.1
157.6
125.5
111.9
145.0
120.8
128.0
110.2
N(3)-H(3N)···S(1)
C(1)-H(1A)···S(1)#2
C(5)-H(5A)···S(2)#1
C(5)-H(5A)···O(1)
C(8)-H(8B)···S(1)
3.02
2.73
2.42
2.66
2.97
2.65
2.79
2.58
3.676(4)
3.656(3)
3.090(3)
3.167(4)
3.783(3)
3.240(4)
3.458(3)
3.046(3)
0.98
0.98
0.99
C(10)-H(10A)···S(2)
C(10)-H(10A)···O(2)#3
C(13)-H(13A)···S(1)#4
C(14)-H(14A)···O(1)
0.95
0.95
0.95
0.95
Symmetry transformations used to generate equivalent atoms:
#1 -x+1,-y+1,-z+1 #2 -x+1/2,y-1/2,z #3 x,y+1,z #4 -x+1/2,-y+1,z-1/2
Table 5. Inter and intramolecular interactions [Å and º] for 2
D-H···A
d(D-H)
0.97
0.78(3)
0.92(3)
d(H···A)
2.76
2.38(3)
2.62(3)
d(D···A)
3.225(3)
2.859(2)
3.452(3)
<(DHA)
110.2
121(2)
151(3)
C(8)-H(8B)···S(1)
N(1)-H(1)...S(1)
C(1)-H(1A)...O(2)#2
Symmetry transformations used to generate equivalent atoms:
#1 -x+1,y,-z+1/2 #2 x,-y+2,z+1/2
[68]
Graphical Abstract (Picture)
24

[69]
Graphical Abstract (Synopsis)
Two new complexes [Ni(Hfchce)2(py)2] (1) and [Mn(Hfchce)2(o-phen)] (2) with [N'-(2-
furan-3-carbonyl)hydrazine]-carbodithioic acid ethyl ester (H2fchce) have been synthesized
containing o-phen/ pyridine as coligand. Both complexes are paramagnetic and have a
distorted octahedral geometry. In complex 1, nickel centre is coordinated through one
hydrazine nitrogen atom, one carbonyl oxygen atoms of two ligand moieties and two
pyridine nitrogen atoms. Coordination environment of Mn(II) center in complex 2 is
fulfilled by two uininegative bidentate ligands using hydrazine nitrogen, carbonyl oxygen
and two nitrogen atoms of o-phen coligand. The arrangement of pyridine is trans in
complex 1 while in complex 2 o-phen makes cis chelate ring, such an arrangement possibly
makes complex 2 have much more distorted geometry than complex 1. Complexes 1 and 2
are fluorescent and upon their excitation at 251 and 2263 nm, exhibit emissions at 292 and
285 nm, cm-1, respectively. TG-DTA of complexes 1 and 2 indicate that the metal is
converted into its metal oxide at very high temperature. Complexes 1 and 2 have efficient
activity towards electrochemical water oxidation in the 0.1 M KOH. TD-DFT prediction of
the experimental absorption spectrum matches very well, and from those theoretical
calculations the assignment of the character of these transitions can be proposed.
[70]
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