A Thiosemicarbazone–Nickel(II) Complex as Efficient Electrocatalyst for Hydrogen Evolution

Article abstract of DOI:10.1002/cctc.201600967

We report herein the synthesis and characterization of a new mononuclear nickel complex based on a thiosemicarbazone ligand that exhibits an electrocatalytic behavior for H₂ evolution in DMF using trifluoroacetic acid (TFA) as the proton source. Catalysis is observed at quite small overpotential values and a maximum turnover frequency (TOF_max) of 3080 s^?1 was extrapolated for 1 m proton concentration using the foot-of-the wave analysis of cyclic voltammetry data. Gas analysis during controlled potential electrolysis experiments confirmed the catalytic nature of the process and production of dihydrogen with 80 % faradaic yield. Quantum chemical calculations indicate that the catalytic mechanism involves first ligand-based reduction and protonation steps followed by metal-centered processes.

Full text of DOI:10.1002/cctc.201600967

Accepted Article
Title: A thiosemicarbazone-nickel(II) complex as efficient electrocatalyst
for hydrogen evolution
Authors: Tatiana Straistari, Jennifer Fize, Sergiu Shova Shova, Marius
Reglier, Vincent Artero, and Maylis Orio
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10.1002/cctc.201600967
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A thiosemicarbazone-nickel(II) complex as efficient electrocatalyst
for hydrogen evolution
Tatiana Straistari,[a,b] Jennifer Fize,[c] Sergiu Shova,[d] Marius Réglier,[a] Vincent Artero*[c] and Maylis
Orio*[a]
Dedicated in memory of Prof. Constantin Turta for his outstanding contribution to inorganic and coordination chemistry
Abstract: We report herein on of the synthesis and characterization
of new mononuclear nickel complex NiL based on
enhance catalytic activity.[13-16] Transition metal complexes with
bis-thiosemicarbazone ligands have been studied for many
years.[17] Beyond their interest as chemotherapeutics or
radiopharmaceutics,[18] these complexes present some
interesting features (redox properties, nitrogen or sulfur atoms as
potential proton relays) that could be relevant for electrocatalytic
proton reduction. Recently, Haddad et al. have reported a bis-
a
a
thiosemicarbazone ligand that exhibits an electrocatalytic behavior for
H2 evolution in DMF using trifluoroacetic acid (TFA) as the proton
source. A maximum turnover frequency (TOFmax) of 3080 s-1 was
extrapolated for 1 M proton concentration using the foot-of-the wave
analysis of cyclic voltammetry data. Gas analysis during controlled
potential electrolysis experiments confirmed the catalytic nature of the
process and production of dihydrogen with 80% faradaic yield.
Quantum chemical calculations indicate that the catalytic mechanism
involves first ligand-based reduction and protonation steps followed
by metal-centered processes.
thiosemicarbazone zinc complex that functions as
a
homogeneous electrocatalyst for proton reduction,[16] and
analyzed its reactivity in terms of ligand-centered[15] [19,20] proton-
transfer and electron-transfer processes shortcutting traditional
metal-hydride intermediates. In that perspective, we associated
the electroactive bis-thiosemicarbazone ligand with redox-active
transition metal and report here the synthesis and
characterization of a bis-thiosemicarbazone nickel(II) complex
together with its evaluation as an electrocatalyst for proton
reduction.
Introduction
Facing the 21st century energy challenge, dihydrogen production
as an alternative fuel by catalytic water splitting is a central theme
in the field of renewable energy storage.[1] Platinum possesses
the best performances among catalysts able to reduce protons to
dihydrogen.[2] However, due to its scarcity and its cost, efforts to
find alternative non-noble transition metal catalysts have been the
subject of intense research.[3-6] Inspiration by nature has often
guided these researches. For example, some of us recently
Results and Discussion
Synthesis of the ligand and complex
The
4–{bis(4–(p–methoxyphenyl)–thiosemicarbazone)}–2,3-
reported
a heterobinuclear NiFe complex reproducing the
butane ligand (H2L) used in this study was quantitatively prepared
by the reaction of 2,3-dihydrazide-butane with two equivalents of
1-isothiocyanato-4-methoxy-benzene in refluxing ethanol. H2L
precipitated from ethanol as a yellow-orange powder in 93% yield
and was used further without any additional purification, except
washing with ethanol. The addition of one equivalent of nickel
nitrate to a suspension of H2L in methanol, resulted in the
formation of the NiL nickel complex in good yield (65%). The
crude compound precipitated as a dark brown microcrystalline
powder and did not required any purification, except washing with
methanol. H2L and NiL were fully characterized by elemental
analyses, infra-red and 1H NMR spectroscopies and their
structures were confirmed by X-ray crystallography.
structure of the active site of [NiFe]-hydrogenase,[7] and exhibiting
high rates for electrocatalytic H2 evolution in mildly acidic
solutions.[8] Besides such biomimetic approaches, a number of
molecular catalysts based on Earth-abundant metals have been
developed in the recent years and were benchmarked with the so-
called “catalytic Tafel plots”.[9] Together with the introduction of
proton relays in the second coordination sphere of the metal
center,[10,11] redox-active ligands[12] can be exploited to promote or
[a]
T. Straistari, Dr. M. Réglier, Dr. M. Orio
Aix Marseille Univ, CNRS, Centrale Marseille, iSm2, Marseille,
France
E-mail: maylis.orio@univ-amu.fr
[b]
[c]
T. Straistari
Institute of Chemistry, Academy of Sciences of Moldova
3, Academiei str., Chisinau MD 2028, Republic of Moldova
J. Fize, Dr. V. Artero
Univ. Grenoble Alpes, CNRS UMR 5249, CEA, Laboratoire de
Chimie et Biologie des Métaux, 38000 Grenoble, France
E-mail: vincent.artero@cea.fr
[d]
Dr. S. Shova
Institute of Macromolecular Chemistry "Petru Poni", 41A Grigore
Ghica Voda Alley, Iasi-700487, Romania
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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the CV of H2L and its deprotonated form L2-, formed in situ through
reaction with Et3N,revealed no oxidative processes, while an
irreversible wave corresponding to the reduction of the imine
functions is observed at -1.65 V vs Ag/AgCl (Figure SI-4).[21] This
wave is more intense in the deprotonated ligand. To study the
behavior of the ligand in acid medium, we recorded successive
cyclic voltammograms of H2L in DMF in the presence of
trifluoroacetic acid. Scans in the negative direction from 0 to -2 V
vs Ag/AgCl revealed no catalytic wave
Figure 2 shows the CV of NiL in anhydrous DMF under argon. It
revealed two successive reversible one-electron redox systems
at – 1.04 V ((Epc* + Epa*)/2) and –1.67 V vs Ag/AgCl ((Epc** +
Epa**)/2).
Scheme 1. Synthesis of H2L and NiL.
X-ray Crystallography
Crystals of H2L, [NiL]·2.5DMF and [NiL]·2DMSO compounds
suitable for X-ray analysis were grown from DMF. H2L crystallizes
in triclinic P-1 space group and its structure consists of 4–{bis (4–
(p–methoxyphenyl)–thiosemicarbazone)}–2,3-butane molecule
(Figure SI-1 and SI-2) as the asymmetric part of the unit cell. The
results of single crystal X-ray study for [NiL] 2.5DMF and
[NiL]·2DMSO complexes are shown in Figures 1 and SI-3,
respectively. Since both complexes exhibit similar molecular
structures (Table SI-1), only the crystal structure of [NiL]·2.5DMF
will be described. The assymmetric unit contains two
crystallographically distinct but chemically identical NiL neutral
complexes (denoted A and B) and 5 DMSO as solvated molecules.
Each Ni atom has a distorted square-planar N2S2 coordination
geometry provided by tetradentate doubly deprotonated ligand L2-.
Both NiN2S2 fragments are essentially planar within 0.003 and
0.002 Å distortion for A and B molecules, respectively. As a whole,
both independent NiL units are also almost planar being arranged
in parallel fashion with an interplanar angle of 1.33º. Selected
bond lengths are provided in the caption of Figure 1.
Figure 2. Cyclic voltammograms of H2L (black dashed line) and NiL (black plain
line), (1 mM) at a stationary glassy carbon electrode in DMF + 0.1M NBu4PF6.
Scan rate 500mV/s. Potentials are referenced to the Ag/AgCl electrode
A plot of the current peaks versus the square root of the scan
rates for both waves is linear, indicating that is a diffusion-
controlled process (Figure SI-5). In an effort to understand the
redox behavior of NiL, DFT calculations were undertaken. The
structures of NiL, NiL- and NiL2- were subjected to geometry
optimization (Figure SI-9) and their electronic structures were
investigated (Figures 3 and SI-10). Comparison of the DFT-
optimized structure of NiL with the X-ray data showed a pretty
good agreement between the two sets of data which confirmed
that NiL can be best described as a low spin Ni(II) complex ([NiIIL]0,
S = 0). Looking at the one-electron reduced species, a delocalized
ligand-based radical species ([NiIIL•]-, S = ½) is formed upon
reduction of NiL as the singly occupied molecular orbital (SOMO)
of NiL- is essentially distributed over the hydrazone bridge of the
complex. The second reduction process in NiL occurs at the metal
site leading to the formation of Ni(I) as the second SOMO of NiL2-
is mainly distributed on the nickel center and its coordinating
atoms. Due to the orthogonality of the two SOMOs of NiL2- and
an energy gap of 25 kcal.mol-1 with respect to the singlet state,
the ground spin state of this species can be assigned as a triplet
([NiIL•]2-, S =1) (Figure SI-10). These data thus support the
experimental assignment of the reduction loci in NiL.
Figure 1. Asymmetric part in the crystal structure of [NiL]·2.5DMF with atom
labeling scheme and ellipsoids parameters at 50% probability level. H-atoms
not involved in hydrogen bonds are omitted for clarity Selected bond lengths:
.
Ni(1B) − S(1B) 2.170(4) Å, Ni(1B) − S(2B) 2.167(4) Å, Ni(1B) − N(3B) 1.80(1) Å,
Ni(1B) − N(4B) 1.85(1) Å, Ni(1A) − S(1A) 2.165(5) Å, Ni(1A) − S(2A) 2.170(5) Å,
Ni(1A)
− N(3A) 1.82(1) Å, Ni(1A) − N(4A) 1.84(2) Å. Hydrogen bonds
parameters: N1A−H∙∙∙O5 [N1A−H 0.86 Å, H∙∙∙O5 2.09 Å, N1A∙∙∙O5 2.921(3) Å,
N1A−H∙∙∙O5 163.2°]; N6A−H∙∙∙O4 [N6A−H 0.86 Å, H∙∙∙O4 2.11 Å, N6A∙∙∙O4
2.932(3) Å, N6A−H∙∙∙O4 163.3°]; N1B−H∙∙∙O1 [N1B−H 0.86 Å, H∙∙∙O1 1.97 Å,
N1B∙∙∙O1 2.805(3) Å, N1B−H∙∙∙O1 164.9°]; N6B−H∙∙∙O3 [N6B−H 0.86 Å, H∙∙∙O3
2.02 Å, N6B∙∙∙O3 2.848(3) Å, N6B−H∙∙∙O3 160.2°]
Electrochemical Studies
Cyclic voltammograms (CV) were recorded at a glassy carbon
electrode in anhydrous DMF in the presence of 0.1M NBu4PF6 as
supporting electrolyte. Potentials are referred to a Ag/AgCl
reference electrode. The redox behavior of H2L was investigated
first. In the potential range between +0.8 and -2.0 V vs Ag/AgCl,
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NiL2- (Table SI-3 and Figure SI-12). Our calculations also show a
decrease in the intensity of the band for the direduced compound,
consistently with the experimental observation (Figure SI-11).
NiL ([NiIIL]0, S = 0)
NiL- ([NiIIL•]-, S = ½)
Electrocatalysis
To examine the capability of NiL to mediate proton reduction
catalysis, we investigated its electrochemical response in the
presence of trifluoroacetic acid (TFA, pKa = 6.0 ± 0.3 in DMF)[22]
as a protons source. Cyclic voltammograms of NiL recorded in
DMF with increasing concentration of TFA are shown in Figure
SI-6. A large irreversible catalytic wave starting at about – 0.70 V
vs. Ag/AgCl is observed. The current response of this irreversible
cathodic wave is directly proportional to the concentration of
protons in the electrolyte solution while the peak maximum is
gradually shifted of from – 1.57 to – 1.77 V vs Ag/AgCl.
Comparison with a control experiment measured in the absence
of NiL demonstrate the catalytic effect of the nickel complex.. The
wave displays several shoulders indicative several mechanisms
operating at different potentials. We only investigate here the
process occurring at more positive potentials with a mid-wave
potential (퐸푐0푎푡) of -0.8 V vs Ag/AgCl, (Figure 5).
SOMO
HOMO
NiL2- ([NiIL•]2-, S = 1)
SOMO (1)
SOMO(2)
Figure 3. DFT-calculated redox active orbitals for neutral, mono and di-reduced
species of NiL.
UV-Visible Spectroscopy and Spectroelectrochemistry
UV/Vis absorption spectra recorded in DMF for NiL complex
exhibit an intense band at λmax = 470 nm (ε = 17000, Figure 4).
The electrochemical reduction of NiL, at room temperature was
monitored spectro-electrochemically using an optically
transparent electrode under Ar in DMF containing NBu4PF6 (0.1
M) as supporting electrolyte. After applying the potential of the first
reduction band (Epc*), we observed a slight decrease in intensity
of the band associated with the NiL complex (green line), and
after further electrochemical re-oxidation (Epa*), the band returned
to the original shape (red line). Strong decrease in intensity of the
band is observed after second electrochemical reduction (Epc**,
blue line). After total electrochemical re-oxidation (Epa*), the
intensity of the band returned to the initial one. This behavior
strongly confirms the reversibility of the redox electrochemical
processes.
Figure 5. Successive cyclic voltammograms recorded at a glassy carbon
electrode of a 1mM solution of NiL complex in DMF ( 0.1 M NBu4PF6) with 60
mM TFA at 5 different scan rates (left) and increasing concentration of TFA at
500mV/s scan rate (right). Potentials are referenced to the Ag/AgCl electrode.
Bulk electrolysis and gas analysis
To confirm that the catalytic wave corresponds to the reduction of
protons to molecular hydrogen, we performed a bulk electrolysis
experiment on a mercury pool electrode in DMF in the presence
of TFA. This experiment was coupled with in-line GC analysis.
During a 16 h electrolysis experiment of a 1 mM NiL DMF solution
(0.1 M NBu4PF6, 8 mL) added with 100 mM TFA at – 1.2 V vs.
Ag/AgCl, hydrogen was produced with 80 % faradaic efficiency
and a turnover number of 21 (Figure SI-8). A control experiment
performed in the absence of NiL produced significantly less
hydrogen (Figure SI-6). These experiments allow to rule out the
possibility that proton reduction occurs as the result of deposition
of nickel nanoparticles on the electrode surface since they would
amalgamate within the mercury pool electrode.
Figure 4. Electronic absorption spectra for NiL (black dash), one-electron
reduced specie NiL- (green line), two-electron reduced species NiL2- (blue line),
and electrochemical re-oxidized species (red lines).
TDDFT calculations performed on neutral, mono and di-reduced
forms of NiL further supported that the UV-vis spectrum of the
complex is dominated by one main absorption band in all three
redox states. This band could be assigned to (i) a mixed metal-
ligand to metal-ligand transition for NiL; (ii) a metal to ligand
transition in NiL- and (iii) to a ligand-to-ligand charge transfer in
Kinetic analysis and benchmarking of performances
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The foot-of-the-wave analysis (FOWA) was used to quantify the
rates of the hydrogen evolution reaction from CV
measurements.[9,23]
reduced species NiL-: the metal center [Ni-H]-, the sulfur atom
(generating [Ni-SH]-), the nitrogen atom from the hydrazone group
(generating [Ni-HN]-), the internal nitrogen atom from the
thiosemicarbazone motif (generating [Ni-NH]-) and the nitrogen
atom from the phenylthiocarbamide fragment (generating [Ni-
HNH]) (Figures SI-13 and SI-15 to SI-18).
Plotting 푖/푖푃0 (with 푖푃0 being the peak current of a monoelectronic
wave measured in the absence of acid) as a function of
0
(
[
(
)] )
1/ 1 + exp 퐹/푅푇 퐸 − 퐸푐푎푡
near the foot of the wave gave a
linear function at a given scan rate. The observed rate constant
kcat can be extracted from the slope of the linear fit, considering
the formula corresponding to an ECEC mechanism (E
corresponds to an electron transfer step and C to a chemical
reaction, here protonation) as shown in Scheme 2 (see below).
The rate constant kcat for H2 evolution catalyzed by NiL was
determined for 4 TFA concentrations (10, 30, 60 and 80 mM) at 4
different scan rates in the 0.5 - 2 V/s range. Our data indicated
that kcat was proportional to the acid concentration (푘푐푎푡 = 푘표푏푠
×
+
[
]
with kobs the rate constant for the rate-determining step of the
퐻
process and [H+] the concentration of the acid in the bulk solution
(Table SI-2). An average value of kobs = 3080 M-1s-1 (Figure SI-7)
was obtained. From that value a turnover frequency of 3080 s-1
can be extrapolated for 1M acid concentration. Based on this
value, the 퐸푐0푎푡 value of -0.8V vs Ag/AgCl (corresponding to -1.33
V vs Fc+/Fc) and an apparent equilibrium potential of the H+/H2
couple (-1.01 V vs Fc+/Fc[24]) at 1M TFA concentration, we could
derive the bold magenta trace in the catalytic Tafel plot shown in
Figure 6, relating the turnover frequency and driving force of the
Figure 7. Energetic stability of protonated species of NiL-
reaction, that is, the overpotential related to H2 evolution under
25]
the conditions used.[9,
Although not rivalling nickel-
Our results clearly pinpoint that the hydrazine-nitrogen is the
preferential locus and can be best described as a ligand-based
radical species (Figure SI-14). Indeed, the calculated free
energies for the five forms predict the [Ni-HN]- form to be lower in
energy by 4.0, 12.3, 18 and 21.3 kcal/mol-1 than the [Ni-NH]-, [Ni-
HNH]-, [Ni-SH]- and [Ni-H]- ones, respectively (Figure 7). The
preferential locus for the successive reduction of the one-electron
protonated species was also investigated (Figures SI-20 and SI-
22 to SI-25) and the theoretical data show that the favored
species is the di-reduced [Ni-HN]2- complex (Figure SI-19). Our
calculations also indicate that this reduction is a metal-based
process as positive spin population is found at the Ni center
(Figure SI-21). This would support the metal as being the
preferential locus for the second protonation of [Ni-HN]2-,
generating a NiIII-hydride species as the catalytically active
species responsible for H2 evolution.
bisdiphosphine or cobaloxime catalysts, NiL complex displays
significant catalytic activity (log(TOF)/s−1) > 1) for overpotential
values slightly lower than iron tetraphenylporphyrin[9].
N
N
N N
R
R
Ni
N
N
S
S
H
H
NiL
H2
+ e-
+ H+
Figure 6. Catalytic Tafel plots. Comparison of performances for hydrogen
evolution catalyzed by NiL in DMF in presence of 1 M TFA (magenta line) with
those of other catalysts reported in the literature. Black: FeIITPP, DMF, Et3NH+.
Blue: CoII(dmgH)2py, DMF, Et3NH+. Green: [Ni(P2PhNPh)2]2+, MeCN, DMFH+.
H+
H
H
N
N
N
N
N
N
N N
+ e-
Orange: [NiII(P2PhN2C6H4X)2]2+
,
X= CH2P(O)(OEt)2, MeCN, DMFH+. Red :
R
R
R
R
Ni
Ni
N
N
[NiII(P2PhN2C6H4X)2]2+, X= H, MeCN, TfOH..[9]
N
N
S
S
S
S
H
H
H
H
[NiHN]2-
[Ni(I)L•]2-, S = 1.
[NiHN]-
[Ni(II)L•]-, S = 1/2
Mechanistic considerations
Ni(II)
Ni(I)
To gain more insight on the locus of proton addition following the
electrochemical reduction process, we have conducted additional
DFT calculations. For this purpose, we have considered five
potential sites for the addition of one proton on the one-electron
Scheme 2. Proposed mechanism for H2 evolution.
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Conclusions
Synthesis of NiL
To the suspension of ligand H2L (0.225 mmol, 0.1 g) in methanol (5 ml),
wad added drop-wise one equivalent of Ni(NO3)2(H2O)6 (0.225 mmol,
0.065 g) solution in methanol (2 ml). The resulting suspension was stirred
at reflux for 2 hours. The resulting precipitate was filtrated and washed with
methanol. Single crystals suitable for crystallographic analysis were grown
from DMF solution with 65 % yield. Selected data: 1H NMR (400 MHz, [D6]
DMSO-d6, TMS): δ = 9.93 (s, 2H; NH), 7.51 (d, J = 8.7 Hz, 4H; o-CH), 6.87
(d, J = 8.7 Hz, 4H; p-CH), 3.71 (s, 6H; O-CH3), 2.04 ppm (s, 6H; CH3););
FT-IR 3300 cm-1 (N−H), 1600 cm-1 (C=N imine), 1504 cm-1 (N−C), 1240
cm-1 and 1015 cm-1 (O−C), 817 cm-1 (C−S); elemental analysis. Calcd.
C20H24N6O2S2Ni (501.25): C 47.92, H 4.42, N 16.77, S 12.79; found C
47.49, H 4.46, N 16.43, S 11.42.
In summary, we have shown that the mononuclear nickel complex
NiL with bis-thiosemicabazone ligand exhibits
a
high
electrocatalytic activity for proton reduction to dihydrogen. The
benchmarking of the performances of NiL was achieved through
foot-of-the-wave analysis[9] leading to a maximum turnover
frequency 3080 s-1 (TOFmax) placing NiL among the best catalyst
reported in the literature in catalytic Tafel plots. With the help of
DFT calculations, we could proposed a mechanism involving two
steps: 1) the protonation of the N-atom from the hydrazone
function of a mono-reduced species and 2) a second reduction
associated with a nickel centred protonation for subsequent
hydrogen evolution.
X-ray crystallography
Recently; Grapperhaus and coworkers reported
a
bis-
thiosemicabazone zinc complex capable of electrocatalytic proton
reduction through a process entirely centered on the ligand,[16] i.e.
without any occurrence of a zinc hydride species. This was
confirmed by the activity of the ligand alone, which exhibits a quite
high TOF (1320 s-1) compared to the zinc complex (1170 s-1).
Grapperhaus and colleagues argued that in transition-metal
complexes, spin-orbit coupling between ligand-based radical and
unpaired electrons from the metal can reduce reactivity. Our
ligand L2- appears to be deprived of any catalytic activity for H2
evolution. With a TOFmax of 3080 s-1 for NiL, our results however
confirm the interest to combine electroactive ligand such as bis-
thiosemicarbazone with a transition metal ion to obtain a
synergistic effect with both redox moieties.
Crystallographic data for H2L (CCDC-1497213), [NiL]·2.5DMF (CCDC-
1497214) and [NiL]·2DMSO (CCDC-1497215) can be obtained free of
charge from The Cambridge Crystallographic Data Centre via:
www.ccdc.cam.ac.uk/data_request/cif. The molecule of ligand has anti-
spatial orientations towards the substituents in the butane unity. The
thiosemicarbazone substituents are oriented head to head in
approximately of 180º. The dihedral angles between C14/C19 and C2/C6
aromatic rings and almost planar bis–thiosemicarbazone–2,3-butane
fragment are of 17.01(7)º and 64.68(6)º, respectively. In the crystal the
adjacent neutral molecules H2L are interacting through N-H···S hydrogen
bonds to form supramolecular chains in parallel orientation along b
crystallographic axis (Figure SI-2).
Electrochemistry
Cyclic voltammetry experiments were performed using a BioLogic SP300
potentiostat and a three-electrode set-up consisting of a glassy carbon
working electrode, a platinum wire counter electrode and an Ag/AgCl (KCl
3M) reference electrode. Ferrocene was used as an internal standard with
Experimental Section
E0(Fc+/Fc)
= 0.53 V vs Ag/AgCl. All studies were performed in
All solvents and chemicals were purchased from Sigma Aldrich and used
without further purification. Syntheses of 1-isothiocyanato-4-methoxy-
benzene and 2,3-dihydrazide-butane were performed according to
procedures described by Demchenko[26] and Lakshmi.[27] The synthesis of
the ligand H2L is presented in ESI. 1H NMR spectra were recorded on
Bruker 400MHz Avance III nanobay. Chemical shifts for 1H NMR spectra
are referenced relative to TMS or the residual protonated solvent. IR
spectra were recorded with Bruker TENSOR 27 spectrometer equipped
with a single-reflection DuraSamplIR diamond. Elemental analysis was
performed on Thermo Finnigan EA 1112 instrument. The results were
validated by at least two sets of measurements.
deoxygenated DMF containing NBu4PF6 (0.1 M) as supporting electrolyte.
Controlled potential electrolysis experiments were carried out in two-
compartment cell. The volume of solution (DMF, 0.1 M NBu4PF6) used in
the working compartment of the cell was 8 ml. The working electrode used
was a pool of mercury, separated from the coiled platinum wire counter
electrode by a porous frit. Bulk electrolysis solutions were purged witch N2
gas for at least 20 min prior to electrolysis and stirred throughout bulk
electrolysis experiment. During the experiment, the cell was continuously
purged with nitrogen (5 mL.min-1) and the output gas was analyzed at two-
minute intervals in a Perkin Elmer Clarus 500 gas chromatographer using
a previously described setup[28]
.
Synthesis of H2L
DFT calculations
Synthesis of 4–{bis(4–(p–methoxyphenyl)–thiosemicarbazone)}–butane
ligand (H2L). The 2,3-dihydrazide-butane (4.382 mmol; 0.5 g) was
dissolved in ethanol (20 ml), and then added dropwise to two equivalents
of 1-isothiocyanato-4-methoxy-benzene solution in ethanol (10 ml). The
mixture was heated at reflux for 2 h. After the reaction mixture was cooled
to room temperature, the yellow solid was filtered and washed with two
portions of ethanol to give 93% yield of crude compound that was used for
the complexation step whitout additional purification. Selected data for
ligand: 1H NMR (400 MHz, [D6]DMSO, TMS): δ = 10.55 (s, 2H; NH), 9.89
(s, 2H; NH), 7.38 (d, J = 9 Hz, 4H; o-CH), 6.93 (d, J = 9 Hz, 4H; p-CH),
3.76 (s, 6H; O-CH3), 2.30 ppm (s, 6H; CH3). FT-IR: 3209 cm−1 (N−H), 1593
cm−1 (C=N imine), 1510 (N−C) cm−1, 1240 cm−1 and 1026 cm−1 (O−C), 829
cm−1 (C=S). Elemental analysis: calcd. (%) for C20H24N6O2S2 (444.57): C
54.03, H 5.44, N 18.90, S 14.43; found: C 54.10, H 5.37, N 18.89, S 14.36
All theoretical calculations were performed with the ORCA program
package.[29] Full geometry optimizations were undertaken for all
complexes using the GGA functional BP86[30] in combination with the
TZV/P[31] basis set for all atoms, and by taking advantage of the resolution
of the identity (RI) approximation in the Split-RI-J variant[32] with the
appropriate Coulomb fitting sets[33] Increased integration grids (Grid4 in
ORCA convention) and tight SCF convergence criteria were used.
Electronic structures and Molecular Orbital diagrams were obtained from
single-point calculations using the hybrid functional B3LYP[34] together with
the TZV/P[31] basis set. Increased integration grids (Grid4 and GridX4 in
ORCA convention) and tight SCF convergence criteria were used in the
calculations. For according to the experimental conditions solvent effects
were accounted and we used the DMF solvent (= 37) within the
framework of the conductor-like screening (COSMO) dielectric continuum
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10.1002/cctc.201600967
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approach.[35] The relative energies were computed from the gas-phase
optimized structures as a sum of electronic energy, solvation and thermal
corrections to the free energy. Optical properties were predicted from
additional single-point calculations using the hybrid functional B3LYP[34]
together with the TZV/P[31] basis set. Electronic transition energies and
dipole moments for all models were calculated using time-dependent DFT
(TD-DFT)[36] within the Tamm–Dancoff approximation.[37] To increase
computational efficiency, the RI approximation[38] was used in calculating
the Coulomb term, and at least 30 excited states were calculated in each
case. For each transition, difference density plots were generated using
the orca plot utility program and were visualized with the Chemcraft
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The authors gratefully acknowledge the support of this work by
the CNRS and the Academy of Sciences of Moldova (Bilateral
agreement EDC25722 2013-2014), the French National
Research Agency (Labex program, ARCANE, ANR-11-LABX-
0003-01); the COST Actions CM1305 ECOSTBio (Explicit Control
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Keywords: nickel
• redox-active ligand • bioinspiration •
electrocatalyst • hydrogen evolution
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A mononuclear nickel complex NiL
based on a thiosemicarbazone ligand
is an efficient electrocatalyst for H2
evolution in DMF. With a maximum
turnover frequency of 3080 s-1, it
ranks among the best molecular H2-
evolving catalysts in catalytic Tafel
plot. The catalytic mechanism
involves ligand-based reduction and
protonation steps followed by metal-
centered processes.
T. Straistari, J. Fize, S. Shova,
M. Réglier, V. Artero*, M. Orio*
Page No. – Page No.
Title
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