Ligand-based electronic effects on the electrocatalytic hydrogen production by thiosemicarbazone nickel complexes

Article abstract of DOI:10.1039/c9dt04775a

This work reports on the synthesis and characterization of a series of mononuclear thiosemicarbazone nickel complexes that display significant catalytic activity for hydrogen production in DMF using trifluoroacetic acid as the proton source. The ligand framework was chemically modified by varying the electron-donating abilities of the para substituents on the phenyl rings, which was expected to impact the capability of the resulting complexes to reduce protons into hydrogen. Over the four nickel complexes that were obtained, the one with the thiomethyl substituent, NiSCH₃, was found to overtake the catalytic performances of the parent complex NiOCH₃ featuring lower overpotential values and similar maximum turnover frequencies. These results confirm the electronic effects of the ligand on HER when using thiosemicarbazone nickel complexes and support that chemical modifications can tune the catalytic performances of such systems.

Full text of DOI:10.1039/c9dt04775a

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and M. Orio, Dalton Trans., 2020, DOI: 10.1039/C9DT04775A.
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ARTICLE
Ligand-Based Electronic Effects on the Electrocatalytic Hydrogen
Production by Thiosemicarbazone Nickel Complexes
Michael Papadakis,a,b Alexandre Barrozo,a Tatiana Straistari,a Nicolas Queyriaux,c Anisa Putri,a
Received 00th January 20xx,
Accepted 00th January 20xx
Jennifer Fize,d Michel Giorgi,e Marius Réglier,a Julien Massin, a Renaud Hardré,a and Maylis Orio *a
DOI: 10.1039/x0xx00000x
This work reports on the synthesis and characterization of a series of mononuclear thiosemicarbazone nickel complexes that
display significant catalytic activity for hydrogen production in DMF using trifluoroacetic acid as the proton source. The ligand
framework was chemically modified by varying the electron-donating abilities of the para substituents on the phenyl rings,
which was expected to impact the capability of the resulting complexes to reduce protons into hydrogen. Over the four
nickel complexes that were obtained, the one with the thiomethyl subsituent, NiSCH3, was found to overtake the catalytic
performances of the parent complex NiOCH3 featuring lower overpotential values and similar maximum turnover
frequencies. These results confirm the electronic effects of the ligand on HER when using thiosemicarbazone nickel
complexes and support that chemical modifications can tune the catalytic performances of such systems.
Besides these biomimetic approaches,
a large number of
Introduction
molecular electrocatalysts have been developed in recent
years. They involve mainly the conversion of chemical into
electrical energy through the electrochemical oxidation of
Along with economic growth, the demand and use of fuel has
increased. The risk of depletion of fossil resources combined
with environmental problems also reflects the unsustainability
of the current energy model.1 The development of renewable
energy sources that are environmentally friendly and affordable
is of critical importance facing the 21st century energy
challenge.1 Hydrogen is regarded as a promising molecular fuel
to address such a problem.2,3 Up to now, platinum-based
catalysts possess the best performances among catalysts able
to reduce protons to hydrogen.2 However, due to the scarcity
and cost of this noble element, intense research has been
performed to find alternative non-noble transition metal
catalysts.4 Nature can serve as a guide for such quest, as it offers
efficient H2 evolution catalysts in the form of hydrogenases,
organometallic enzymes containing nickel and/or iron sites
whose catalytic performances rival commonly used platinum
catalysts for hydrogen production.5,6 Hydrogenases thus offer
fascinating blueprints for the design of new molecular catalysts
based on Earth-abundant metals, and several studies indeed
report homo-7-9 and hetero-binuclear10-12 metal complexes as
electrocatalysts for hydrogen evolution reactivity (HER)
reproducing the structure of the active site of hydrogenase.
hydrogen using
a metal. Particularly, such molecular
electrocatalysts are the subject of a proper chemical design for
the efficient hydrogen production at low overpotential.13-19 In
this context, diverse ligand skeletons with cobalt and nickel-
based complexes were found to generate molecular hydrogen
in organic and aqueous media.20 Metal complexes based on
thiosemicarbazone ligands are emerging as a new class of
electrocatalyst for HER.21-26 Indeed, it was suggested that the
introduction of proton relays in the second coordination sphere
of the metal center combined with the use of redox-active
ligands can be exploited to enhance or promote catalytic
activity.27-29 These complexes thus present some interesting
features relevant to electrocatalytic proton reduction: the
thiosemicarbazone ligand has already been shown to be redox
active while the presence of S-donors and several N-atoms
allows for protonation of the ligand and can serve as proton
relays. The ability to store electrons in the ligand rather than in
the metal renders a variety of mechanisms for HER. Those will
give freedom to design electrocatalysts in terms of flexibility
and thermodynamics (reduction potentials and pKa values).30 In
2016, our group reported the synthesis and characterization of
a
mononuclear nickel complex, NiOCH3, an efficient
electrocatalyst for HER.24 The use of catalytic Tafel plot31 to
benchmark its catalytic performances with other catalysts
reported in the literature demonstrated that this complex is
among the best catalysts reported so far with respect to
overpotential requirement. A ligand-assisted metal-centered
a.
Aix Marseille Univ, CNRS, Centrale Marseille, iSm2, Marseille, France.ꢀ
E-mail: maylis.orio@univ-amu.fr
b. Univ. of Crete, Department of Chemistry, Laboratory of Bioinorganic chemistry,
Voutes campus, Heraklion, Crete.
c. CNRS, LCC (Laboratoire de Chimie de Coordination), 31077 Toulouse, France
d. Univ. Grenoble Alpes, CNRS, CEA, IRIG, Laboratoire de Chimie et Biologie des
Métaux, 17 rue des Martyrs, 38000 Grenoble, France.
reactivity was proposed for this system with
a two-step
e. Aix Marseille Univ, CNRS, Spectropole FR1739, Marseille, France.
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mechanism involving initial ligand-centered reduction and of isothiocyanate derivative in ethanol heated at reflux as
protonation followed by metal-centered reduction leading to previously reported for the parent ligand with methoxy
the formation of a stable hydride intermediate.24,32-34
substituents.24 The products were obtained as yellow solids in
good yields (> 56%) and were further used to generate the
metal complexes. The NiL complexes were obtained in good
yield (> 80 %) by adding one equivalent of nickel nitrate to a
suspension of H2L ligands in methanol. The crude compounds
precipitated as solid powders after washing with methanol and
did not require any further purification (Scheme 1). The sets of
ligands and complexes were fully characterized by HRMS ESI, IR
and NMR spectroscopy (see Supporting Information).
Fig.1 Thiosemicarbazone nickel complexes under investigation with R1, R2 = OCH3,
SCH3, N(CH3)2 or CN
Following this work, our goal is now to improve the catalytic
performances for hydrogen production of the nickel complex
bearing methoxy substituents (R1=R2=OCH3). For that purpose,
we have prepared and characterized a series of mononuclear
complexes by varying the chemical nature of the R1 and R2
phenyl ring substituents (Figure 1). Based on Hammett 
Constants, we modified our parent thiosemicarbazone ligand
H2L by tuning the electron withdrawing/donating character of
both substituents. Our study also included the synthesis of an
asymmetric ligand that combines at the same time the electron
withdrawing or donating abilities of two different R1 and R2
Scheme 1 Synthetic route for both the ligands H2L and the metal complexes NiL.
groups. The resulting complexes display
a wide range of
electron-donating abilities of their para substituents. Their
capability to mediate hydrogen evolution was evaluated to
determine the impact of the electronic substitution on the two
parameters that are key for catalyst evaluation: the overvoltage
and the rate of catalysis.
Our attempts to obtain suitable crystals for X-ray diffraction
analysis of the NiL complexes remained unsuccessful. As the
PdL complex with OCH3 substituent was previously reported to
be isostructural with the nickel analogue37 the palladium was
also used for a comparative analysis of the structural features
of NiL. The PdL complexes were prepared in refluxing
acetonitrile by interaction of one equivalent of H2L ligands in
DMF with one equivalent of palladium chloride. A series of four
PdL complexes was obtained as crystalline precipitates and only
the one having SCH3 as R1 and R2 substituents was successfully
crystallized.
Results and Discussion
Synthesis and Characterization
In this work, two types of new ligands were considered: (i)
symmetric ligands built with three different substituents
including thiomethyl (SCH3); dimethylamino (N(CH3)2) and
cyano (CN) groups and (ii) an asymetric ligand which combines
dimethylamino and cyano (N(CH3)2, CN) substituents mixing the
two opposite strongest mesomeric effects in a molecule and
therefore presenting an electronic “push-pull effect”.
These ligands were designed such that their properties were a
function of the substituents at the para position of the phenyl
ring. Indeed, the Hammett constants of the chosen substituents
combine mesomeric and inductive effects induced by para-
substitutions. The mesomeric effect, which is a property of
functional groups used in a qualitative way, describes the
electron withdrawing or donating properties of substituents
based on relevant resonance structure,35 while the inductive
effect is an experimentally observed effect of the transmission
of charge through a chain of atoms in a molecule, resulting in a
permanent dipole in a bond.36
Single crystal X-ray diffraction
Crystals suitable for X-ray diffraction analysis were obtained
from DMF solution by slow evaporation of the solvent. Details
of data collection and structure refinement parameters for
[PdSCH3].2DMF are shown in Supporting Information (Table SI-
1). Compound [PdSCH3].2DMF crystallizes with 2 H-bonded
DMF molecules in the asymmetric unit. Each Pd atom has a
distorted square-planar N2S2 coordination geometry provided
by the doubly deprotonated tetradentate ligand L2-. Both
Å distortion.
PdN2S2 fragments are essentially planar with 0.006
However the overall complex is not flat as shown by the angle
between the two phenyl of the methylsulfanylphenyl moieties
equal to 27°. Selected bond lengths are provided in the caption
of Figure 2.
The protonated ligands, referred here as H2L , were synthesized
by the reaction of 2,3-dihydrazide-butane with two equivalents
2 | J. Name., 2012, 00, 1-3
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previously attributed to a mixed metal-ligand to metal-ligand
transition24 while the low-energy one is compatible with a d-d
transition. The spectral features of the complex with the
asymmetric ligand relate with the presence of different phenyl
rings substituents. They are expected to affect individually the
electron density distribution in the ligand upon excitation
resulting in two high-energy absorption bands. Based on the
,
that the
spectral features reported in Figure 3 it is worth noting
different
R1 and R2 substitutions have an electronic impact on
the optical properties that influences the position of the main
absorption band. Indeed, we observe sequential
a
bathochromic shift of the band when going from the CN
substituent to the N(CH3)2 one which reflects the electron-
donating abilities of the para substituents (N(CH3)2
>OCH3>SCH3> CN). This trend was further supported by TD-DFT
calculations performed on the NiL series of complexes. Indeed,
the predicted spectroscopic data provide calculated spectra
that compare well with the experimental observations (Figure
SI-17). Our computations adequately reproduce the energy of
the key features of the experimental spectra, which highlight
the electronic influence of the phenyl ring substituent on the
optical properties of the systems.
Fig.2 X-ray crystal structure of [PdSCH3].2 DMF with atom labelling scheme and
ellipsoids parameters at 50 % probability level. Selected bond lengths: Pd1-S2
2.28(10) Å, Pd1-S3 2.275(9) Å, Pd1-N3 1.98(3) Å, Pd1-N4 1.98(3) Å, N2-N3 1.36(4)
Å, N2-C8 1.33(4) Å, N3-C9 1.31(4) Å, N4-N5 1.36(4) Å, N4-C10 1.31(4) Å, N5-C11
1.33(4) Å, C9-C19 1.49(4) Å, C10-C20 1.49(4) Å. Hydrogen bonds parameters: N1-
H···O1 [N1-H 0.86 Å, H-O1 1.94 Å, N1···O1 2.785 Å, N1-H···O1 167.5°]; N6-H···O2
[N6-H 0.86 Å, H-O2 1.97 Å, N6···O2 2.81 Å, N6-H···O2 165.14°].
PdSCH3 with its methoxy analogue24 shows
Comparison of
that the two palladium complexes have similar structures. At
first glance, the electronic properties of R1 and R2 substitutions
do not seem to greatly influence the geometry of the complex.
However, they may likely affect their electronic structures and
their spectroscopic parameters.
Electrochemical studies
UV/Vis spectroscopy
Cyclic voltammograms (CV) were recorded at room
temperature at a glassy carbon electrode in Ar-deaerated
anhydrous DMF containing NBu4PF6 as a supporting electrolyte.
The redox behaviour of the ligands was investigated in the
potential range between +0.5 and -2.5 V vs. Fc+/0. We recorded
the successive cyclic voltammograms of the H2L ligands with
and without 40 mM of trifluoroacetic acid (Figure SI-4). In the
absence of TFA, the CV show one irreversible wave in the
cathodic region while in its presence, no catalytic wave was
detected for the whole series. This is consistent to what was
previously observed, confirming that the ligands themselves
cannot perform catalytic proton reduction.24
The optical properties of the ligands and the nickel complexes
were followed by UV/Vis spectroscopy in DMF (Figures 3 and SI-
1). The evolution of the spectra upon ligand metalation allowed
us to confirm the nickel binding to the ligands for the whole
series of systems.
The CVs of the five NiL complexes recorded in anhydrous DMF
under argon are shown in Figure 4. We can observe two quasi-
reversible processes in the cathodic region from 0 to -2.5 V vs.
Fc+/0. The values of the processes in cathodic directions for the
four NiL complexes are presented in Table 1 and compared with
the NiOCH3 reference. The plot of the peak currents versus the
square root of the scan rates for both cathodic and anodic
waves were found to be linear, indicating that it is a diffusion-
controlled process (Figure SI-5). A detailed analysis of the data
presented in Figure 4 and Table 1 show that the R1 and R2
substitutions have an electronic effect influencing the redox
properties of the NiL complexes. Indeed, the more positive is
the σpara Hammett constant, the more positive is the potential
for the first cathodic wave. These results thus support that the
redox properties of the systems can be tuned by the ligand,
Fig.3 Electronic absorption spectra for the series of NiL complexes, concentration
of 0.05 mM in DMF.
Allligandsdisplay one main absorption band centered at 350 nm highlighting the importance of the substituent at the para
while the NiL complexes display one main electronic transition position of the phenyl ring.
in the 445-515 nm range and one shoulder around 700-750 nm
(Table SI-2). The absorption band found at high energy has been
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reduction process, we assessed all spin electronic
configurations for the doubly reduced species by considering
the closed-shell singlet state (S =0), the open-shell triplet state
(S=1) and the open-shell singlet state (Ms=0), the latter being
evaluated using the Broken Symmetry (BS) approach. For all
complexes, the open shell singlet (Ms=0) was found to be the
lowest in energy and the predicted redox potentials compare
well with experiment. Analysis of the Unrestricted
Corresponding Orbitals confirm that the second reduction
process is metal-centered (Figure SI-16).
Table 2 Comparison of calculated and experimental shifts of redox potentials with
respect to NiOCH3.
∆E (V) vs. NiOCH
3 (BP86/def2-TZVP)
N(CH3)2CN
SCH3
Calc.
0.02
N(CH3)2
CN
Exp.
-0.07
Calc.
Exp.
Calc.
Exp.
Calc.
-0.33
Exp.
-0.26
NiIIL/NiIIL·–
0.11
0.04
-0.11
-0.09
NiIIL·–/NiIL2–
-0.08
-0.07
- 0.03
-0.09
-0.20
-0.12
-0.31
-0.21
Fig.4 Cyclic voltammograms of the series of NiL complexes, concentration 1 mM
at a stationary glassy carbon electrode in DMF + 0.1M NBu4PF6. Scan rate 500
mV/s.
The qualitative trend for the different substituents and their
The trend observed for the first reduction process of the four relative order is well reproduced for both reduction steps. This
NiL derivatives matches the relative electron-donating ability of serves as a validation of the choice of theoretical approach, in
the para-substituents. This is of interest as it provides order to understand how substituents can modulate redox
qualitative information about the degree of electronic potentials.
delocalization in the complex and gives insight into the radical
When comparing the relative redox potentials (Figures SI-
distribution within the ligand framework. Indeed, our results 20-23) with spin density distributions, we notice that less spin
show that electron withdrawing groups tend to diminish the density in the Ni center yields less negative redox potentials,
potential required for reduction of the system which is likely thus reducing the potential necessary for reduction. This is
due to stabilization of electron attachment energy and can be evidenced by the CN substituent, which seems to draw
correlated to larger delocalization within the complex.
electronic density away from the metal center (Figure 5).
Conversely, for N(CH3)2 we see the most negative redox
potential, and it is the substituent that causes a subsequent
localization of the spin density at Ni center. Thus, addition of
electron-withdrawing groups tends to decrease the reduction
potential, which will have interesting implications for catalysis
as HER with our systems is supposedly initiated with the
electrochemical step (electron transfer).
Table 1 Electrochemical data for NiL complexes. Potentials (V) refer to Fc+/0
.
∆E (V) vs. NiOCH3
Complex
NiIIL/NiIIL·–
NiIIL·–/NiIL2–
Epc*
-0.21
-0.09
-
Epc**
NiCN
NiN(CH3)2CN
NiOCH3
-1.40
-1.48
-1.57
-1.50
-1.62
-1.99
-2.09
-2.20
-2.13
-2.11
-0.21
-0.13
-
NiSCH3
- 0.07
0.04
-0.07
-0.08
NiN(CH3)2
DFT calculations
In previously published studies on thiosemicarbazone nickel
complexes,24,32,34 DFT calculations were performed to
understand their HER mechanism. The results offered
important insight into the protonation sites of the complex, and
also the electronic structure change upon reduction, the latter
confirming the non-innocence of the ligand. In this work, we
chose to focus solely on the two reduction processes, in order
to infer the impact of the substituent on the electronic
structures of the complexes (Figure SI-15). Our computational
results confirm that the first reduction process remains ligand-
based and show
a fair agreement when comparing our
calculated values with those from electrochemical
measurements (Tables 2 and SI-5-9). Regarding the second
Fig.5: Localized SOMOs and spin density plots for the one-electron reduced
species of the NiL series.
4 | J. Name., 2012, 00, 1-3
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Electrocatalytic proton reduction
Fc+/0 was applied to the mercury-pool working electrode
resulting in the immediate formation of hydrogen. 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. Consequently,
inorganic coating cannot be responsible for catalysis with our
NiL complexes and the catalysis is expected to be homogeneous
in nature.
To examine the capability of the NiL complexes to mediate
proton reduction catalysis, their electrochemical behaviour was
studied by cyclic voltammetry in the presence of trifluoroacetic
acid (TFA, pKa = 6.00 ± 0.3 in DMF)38 as proton source. Cyclic
voltammograms of the NiL complexes were recorded under
argon in DMF solutions with increased concentrations of TFA
and are shown in Figures 6, SI-6 and SI-7.
Focusing on NiSCH3, the addition of 20 mM of TFA triggers the
appearance of two proton-dependent waves with peak
potentials at -1.32 and -2.18 V vs. Fc+/0, suggesting the existence
of potential-dependent mechanisms (Figure SI-6). The current
response of these peak-shaped waves is directly proportional to
the concentration of protons in the electrolyte solution. While
the peak potential of the more negative process is gradually
shifted of from -2.18 V to -2.39 V vs. Fc+/0 (Figure SI-6), the first
catalytic process potential remains mostly unaffected (Figure SI-
7). Comparison with control experiments performed in the
absence of NiSCH3 clearly indicates the role played by the nickel
complex to in the observed current enhancement (Figure 6). As
for NiOCH3[24], we only investigated the process occurring at the
more positive potentials with a midwave potential = -1.32 V.
To verify that the observed catalytic response in presence of
TFA can be attributed to the complexes in solution and not due
to adsorbed species on the surface electrode or degradation
products, we performed a series of dip and rinse tests. After
each electrochemical measurement, the working electrode was
removed from the solution and immersed into a fresh DMF
solution. 60 mM TFA was then added and the electrochemical
response was recorded between 0 to -2.5 V vs. Fc+/0. For all
complexes, only a residual background current was observed
upon sweeping the potential in the cathodic region and upon
addition of TFA, only the acid blank was observed (Figure SI-8).
These control experiments confirm that no active adsorbed
species is formed during electrocatalysis as no catalytic current
could be detected indicating that the HER activity is due to
dissolved NiL complexes. It is also worth mentioning that our
systems displaying ligand-based reduction steps, it will limit the
During the 16 hours electrolysis experiments, the four new NiL
complexes were active in catalytic proton reduction. Their
catalytic performances are summarized in Table
3 and
graphically represented in Figure SI-9. It is worth mentioning
that the maximum TON that can be obtained in our case is 50.
In similar condition, our reference catalyst (NiOCH3 Hammett
constant = -0.27) mediated hydrogen evolution during a 16
hours electrolysis with 80 % faradaic efficiency and a turnover
number of 21. These data confirm that NiSCH3 is more efficient
to catalyze proton reduction when compared to the NiOCH3
complex which would support that chemical substitution of the
ligand can help in enhancing the catalytic activity of our
thiosemicarbazone nickel complexes. However, it is not possible
at this stage to correlate the catalytic performances of the
complexes with the electron-donating properties of their
phenyl ring substituents even though we could observe a nice
trend when looking at their redox and spectroscopic properties.
formation of Ni(0) species, preventing from
degradation pathway.
a possible
Table 3 HER parameters of the series of NiL complexes.
Hammett
constant
Faradaic
efficiency
(FY, %)
94
Complex
Q (C)
TON
NiSCH3
NiN(CH3)2CN
NiN(CH3)2
NiCN
0
61075
26169
26065
18617
37
14
10
8
-0.83/0.66
-0.83
86
56
0.66
63
To confirm that the observed catalytic waves correspond to
the electrocatalytic reduction of protons into hydrogen, we
performed bulk electrolysis experiments of DMF solutions of
TFA (100 mM) and NiL (1 mM). Gas products were monitored
by continuous in-line GC analysis. A potential of -1.70 V versus
Fig.6 Successive cyclic voltammograms of a 1 mM solution of NiSCH3 in DMF (0.1
M NBu4PF6) recorded at a glassy carbon electrode with increasing amounts of TFA
at 500 mV/s (top) and with 60 mM TFA at five different scan rates (bottom).
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Mechanistic considerations and benchmarking of performances
precluding any insight regarding the nature of the rate
determining step. Such situation thus prevents any
UV/Vis measurements performed in the absence and presence unambiguous determination of the rate constant from FOWA
of trifluoroacetic acid (Figure SI-2) display almost identical (see Supporting Information).
features, indicating that the structure of NiSCH3 is unaffected
To circumvent this limitation, the TOF values were
by the addition of a proton source of such strength. Associated determined from the bulk electrolyses experiments as
with the electrochemical behaviour of the complex in the previously reported.51,52 For heterolytic processes, the
presence of TFA, this suggests an ECEC mechanism (where E following equation was used:
퐹
푖2푒푙(1 + 푒푥푝
퐸푎푝푝푙 ― 퐸1/2
)
(
)
]
[
corresponds to an electron transfer step and C to a chemical
reaction, here a protonation) where a first reduction is required
to trigger the initial protonation step. Such mechanism has been
classically found for many Ni- and Co-based HER
electrocatalysts.14,20,40-47
The foot-of-the-wave analysis (FOWA) was tentatively used
in this work to estimate the relevant HER rate constants from
CV measurements.31,47-49 Linearization of the catalytic wave was
achieved by applying the equation corresponding to an ECEC
mechanism with the second electron transfer being easier than
the first one. The slope of the linear portion is expected to
provide helpful kinetic information.49 From this point, the
푅푇
푇푂퐹 =
(1)
퐹2퐴2퐷푐푎푡[퐶푎푡]2
with F is Faraday’s constant, A the surface area of the mercury-
pool working electrode, D the diffusion coefficient, [Cat] the
concentration of the catalyst, Eappl the applied potential during
electrolyses, E1/2 the midwave potential for the catalytic wave
and iel the current density during bulk electrolysis (see
Supporting Information for the detailed analysis). The maximal
turnover frequencies (TOFmax) of the series of complexes were
then evaluated using the following relation:
푇푂퐹푚푎푥
푇푂퐹 =
(2)
퐹
(
)
]
1 + 푒푥푝
퐸푎푝푝푙 ― 퐸1/2
[
푅푇
nature of the rate-determining step (RDS) must be examined. TOFmax values of 115.9, 18.8, 14.3 and 10.2 s–1 were thus
Considering fast and non-limiting electron transfers, kinetic obtained for NiSCH3; NiN(CH3)2CN; NiN(CH3)2 and NiCN,
limitations may arise either from the first (k1<<k2) or the second respectively. For comparison, the TOFmax value of the parent
(k2<<k1) protonation step. In the present case, the mid-wave complex was also evaluated following the same procedure and
potentials of the series of complexes are less negative than the a value of 89.7 s–1 was obtained for NiOCH3. Based on these
potential of the redox process triggering the catalytic wave, values, the mid-wave potentials for each complex (-1.32, -1.33,
suggesting that k2 is more likely the rate-determining step. To -1.31, -1.28 and -1.27
V
vs. Fc+/Fc for NiSCH3; NiOCH3,
confirm these findings, we estimated the maximum value of k2 NiN(CH3)2; NiN(CH3)2CN and NiCN, respectively) and an
as approximately k1/1050 to conform to a regime where catalysis apparent equilibrium potential of the H+/H2 couple (-1.01 V vs.
is kinetically-controlled by the second protonation step. Under Fc+/Fc24) at 1 M TFA concentration, we could generate catalytic
such condition, theoretical values of the catalytic enhancement Tafel plots31 (Figures 8 and SI-14), connecting the turnover
ratio i/i0 can be estimated in regard to the nature of the rate frequency (TOF) for the H2-evolution and the overpotential, ,
p
determining step. These estimates were made for the NiL series an essential kinetic parameter for catalyst evaluation.39
and are reported in Figures 7 and SI-11-13 at various scan rates.
Fig.8 Catalytic Tafel plots: comparison of performances for HER catalyzed by
NiSCH3 in DMF in the presence of 1 M TFA (red line) and NiOCH324 in DMF in the
푖푝0
Fig.7 Theoretical values for ic/ for NiSCH3 in the presence of 60 mM TFA and
presence of 1 M TFA (green line) with other catalysts reported in the literature.
31,51 31,54
various scan rates plotted using equation (8). Red circles represent values were
kcat = k1 = 2812 M-1 s-1. Green triangles represent values were kcat = k2 = k1/10 = 281 [NiII(P2PhN2C6H4X)2]2+, X=CH2P(O)(OEt)2, MeCN, DMFH+.31,55
M-1 s-1
Black: FeIITPP, DMF, Et3NH+
Blue: [CoII(dmgH)2py], DMF, Et3NH+
Orange:
.
Focusing on NiSCH3 (red trace, Figure 7), we observe that this
catalyst displays significant catalytic activity (log(TOF) > 1) for
low overpotential values. It compares well with benchmark
molecular catalysts like cobaloximes14 (blue trace, Figure 8) or
Upon comparing the experimental data (blue squares) to
the theoretical boundaries established for k1 = kcat (red circles)
and k2 = kcat (green triangles), it is clearly observed that the
experimental values for all complexes fall in the last region,
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nickel bisdiphosphine catalysts43,45,55 (orange trace, Figure 8) as and 300 MHz Avance III nanobay. Chemical shifts for 1H NMR
far as the overpotential requirement is concerned. spectra refer to TMS or the residual protonated solvent. IR
Interestingly, NiSCH3 features similar intrinsic catalytic spectra were recorded with Bruker TENSOR27 spectrometer
performances when compared to NiOCH3. However, when equipped with a single-reflection DuraSamplIR diamond. ESI-MS
looking at the macroscopic scale, the HER parameters (FY and analyses were performed using a SYNAPT G2 HDMS (Waters)
TON) suggest that NiSCH3 displays an increased long-term spectrometer equipped with
a
pneumatically assisted
stability with respect to the parent thiosemicarbazone complex. Atmospheric Pressure Ionization (API) source. Samples were
Taken together, these results support the fact that thiomethyl ionized by positive electrospray mode as follows: ion-spray
substitution on the ligand influences the electronic and redox voltage: 2.8 kV; sampling cone: 20 V for NiSCH3, NiCN, NiN(CH3)2
properties of the system which directly impact its catalytic and 40 V for NiN(CH3)2CN; nitrogen flux (nebulization): 100 L h-
behavior. However, it seems unclear whether there is an actual 1. High resolution mass spectra (HRMS) were performed on a
relationship between the catalytic observables and the QStar Elite (Applied Biosystems SCIEX) spectrometer equipped
electronic properties of the phenyl ring substituent within the with atmospheric pression ionization source (API)
NiL series of complexes. While the overpotential of the four pneumatically assisted. The samples were placed in
a
derivatives increases when going from the CN substituent to the methanol/3 mM ammonium acetate. The results were
N(CH3)2 one and matches the relative electron-donating ability validated by three measurements. UV/Vis spectra were
of the para-substituents, the TOFmax values seem to go in the recorded on a Varian Cary 50 spectrophotometer.
opposite direction with smaller values being obtained for the
more electron-withdrawing substituents (Figure SI-14).
Single crystal X-ray diffraction
A suitable crystal for compound [PdSCH3].2DMF was measured
Conclusions
on a Rigaku Oxford Diffraction SuperNova diffractometer at
room temperature at the CuKα radiation (λ=1.54184 Å). Data
collection reduction and multiscan ABSPACK correction were
performed with CrysAlisPro (Rigaku Oxford Diffraction). Using
Olex260 the structures were solved by direct methods with
SHELXT61 and SHELXL62 was used for full matrix least squares
refinement. All H-atoms were found experimentally except
those for the methyl groups that were introduced at
geometrical positions. Their coordinates and Uiso parameters
were constraint to 1.5Ueq (parent atoms) for the methyl groups
and to 1.2Ueq (parent atom) for the other carbons and the
amines. The crystallographic and refinement parameters are
given in supporting information. CCDC number 1971128
We have reported a new series of thiosemicarbazone nickel
complexes for which the R1 and R2 phenyl ring substituents were
chosen to cover a wide range of electron-donating properties.
This approach was meant to modulate the electronic properties
of the catalysts based on the different Hammett Sigma
Constants at the para position of the benzene. The complexes
were experimentally and theoretically characterized, and
shown to exhibit from moderate to high electrocatalytic activity
for proton reduction to dihydrogen. Detailed analysis of the
electrocatalytic data allowed to benchmark the performances
of the series of complexes and catalytic Tafel plots showed that
NiSCH3 compares well with benchmark molecular catalysts as
far as the overpotential requirement is concerned. Our results
indicate that the chemical nature of the substituent in the para
position of the ligand indeed influences the electrochemical and
catalytic behaviour of the system. Further work will be
necessary to decipher the actual relationship between the
electron-donating ability of the phenyl ring substituent of the
complexes and the HER parameters as opposite trends were
obtained when looking at the overpotential and the TOFmax
values. This encourages us to pursue our research on improving
the catalytic performances of our nickel thiosemicarbazone
complexes and on unravelling the key elements that both affect
and control the redox and electronic properties of these HER
catalysts.34, 56, 57
contains the supplementary crystallographic data for this paper
.
Electrochemistry
Cyclic voltammetry experiments were performed using
a
BioLogic SP300 potentiostat and three-electrode set-up
a
consisting of a glassy carbon working electrode, a platinum wire
counter electrode and a Leakless (Ag/AgCl) reference electrode.
Ferrocene was used as an internal standard with E0(Fc+/Fc) =
0.52 V vs. Ag/AgCl for each measurement. All studies were
performed in 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
electrolysis experiments were carried out in two-compartment
cell. The volume of solution (DMF, 0.1 M NBu4PF6) used in the
2 ml. Controlled potential
Experimental Section
All solvents were purchased from Sigma Aldrich and used
without further purification, and for reagents chemical were
purchased from Alfa Aesar for isothiocyanate derivative.
Syntheses of isothiocyanate derivative were performed
according to procedures described by Demchenko et al.58 and
Lakshmi et al.59 1H NMR spectra were recorded on Bruker 400
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
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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
Graduate school of Chemistry, Biology and Health of Univ. Grenoble
Alpes, CBH-EUR-GS, ANR-17-EURE-0003).
Notes and references
DFT calculations
All theoretical calculations used ORCA 4.1.2.63,64 Geometry
optimizations and numerical frequency calculations were
performed for all intermediates using B3LYP and BP86, with
def2-TZVP as the basis set.65–72 We chose to use ’Grid4’ and
’TightSCF’ for integration and convergence criteria, following
ORCA notation, and CPCM as the implicit solvation model for
the solvent effect of DMF.69 Single-point Broken-Symmetry DFT
calculations were performed to evaluate all possible spin
configurations using the “FlipSpin” feature of ORCA.72,73 Free
energy differences for the redox potential calculations were
extracted from the Gibbs free energies obtained from numerical
frequencies calculations. Since experimental values are
reported with respect to the ferrocene electrode in DMF, we
need to shift our calculated absolute potentials. We calculated
the Gibbs free energy difference for the ferrocene reduction in
DMF, obtaining ∆G(BP86) = 4.87 eV and ∆G(B3LYP) = 4.36 eV.
While those values differ by 0.5 eV, they inherit the behaviours
of each functional, thus offering the possibility of cancelling
systematic errors. Optical properties were predicted from
additional single-point calculations using the CAM-B3LYP
functional74,75 and the def2-TZVP basis set. Electronic transition
energies and dipole moments for all models were calculated
using time-dependent DFT (TD-DFT)76-78 within the Tamm-
Dancoff approximation.79,80 To increase computational
efficiency, the RI approximation80 was used in calculating the
Coulomb term and at least 40 excited states were calculated in
each case. All chemical structure images were generated using
Chemcraft.82
1
2
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Author contributions
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production and M.G. performed X-ray analysis. J.M., R.H. and
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Acknowledgements
The authors gratefully acknowledge research support of this work by
the French National Research Agency (ANR JCJC, CODEC, ANR-19-
CE05_0030_01 and Labex program, ARCANE, ANR-11-LABX-0003-01,
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