Influence of the Metal Ion on the Electrocatalytic Hydrogen Production by a Thiosemicarbazone Palladium Complex

Article abstract of DOI:10.1002/ejic.201800120

We describe the synthesis and characterization of a new mononuclear palladium complex (PdL) based on a thiosemicarbazone ligand. X-ray diffraction data collected on single crystals reveal that the complex is a neutral mononuclear entity with symmetric square-planar geometry around the metal center. The complex exhibits an electrocatalytic behavior for proton reduction in DMF solvent using trifluoroacetic acid (TFA) as a proton source. Gas analysis under controlled electrocatalytic conditions during 4 h at a pool mercury working electrode showed a moderate production of dihydrogen. By combining experimental techniques to theoretical calculations, the results are analyzed and compared to those obtained for the previously reported NiL complex in an effort to rationalize the influence of the metal center on the catalytic performance of PdL to mediate hydrogen evolution.

Full text of DOI:10.1002/ejic.201800120

A Journal of
Accepted Article
Title: Influence of the metal ion on the electrocatalytic hydrogen
production by a thiosemicarbazone palladium complex.
Authors: Tatiana Straistari, Renaud Hardré, Julien Massin, Mireille
Attolini, Bruno Faure, Michel Giorgi, Marius Réglier, and
Maylis Orio
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10.1002/ejic.201800120
European Journal of Inorganic Chemistry
FULL PAPER
Influence of the metal ion on the electrocatalytic hydrogen
production by a thiosemicarbazone palladium complex.
Tatiana Straistari,[a,b] Renaud Hardré,[a] Julien Massin,[a] Mireille Attolini,[a] Bruno Faure,[a] Michel
Giorgi,[c] Marius Réglier[a] and Maylis Orio*[a]
hydrogenases have allowed to define a number of important
features for the design of efficient catalysts for hydrogen
evolution. Among these features, the presence of (i) a metal
center to generate a hydride intermediate and (ii) thiolate ligands
that can represent electron-donating groups to bring a proton in
the vicinity of the metal-hydride species seems to be of great
importance. Taking into account these features, Dubois' group[23]
has shown that ligands exhibiting bases in the second
coordination sphere of nickel complexes can act as protons
relays, accelerating the rates of intra and intermolecular proton
transfer in the corresponding electrocatalytic systems. Transition
metal complexes with bis(thiosemicarbazone) ligands have been
studied for many years[24] and are now emerging as a new class
of electrocatalyst for HER catalysts.[16,21] These complexes
present some interesting features that are relevant for
electrocatalytic proton reduction: the thiosemicarbazone ligand
has already been shown to be redox active[25-27] while the
presence of S-donors and several N-atoms allows protonation of
the ligand.[7,8] In this context, we report here on the synthesis
and the characterization of a bis(thiosemicarbazone) palladium
complex that was evaluated as electrocatalyst for hydrogen
evolution. Its electronic and redox properties as well as its
reactivity are discussed and further compared to our previously
reported nickel complex.[28]
Abstract: We describe the synthesis and characterization of a new
mononuclear palladium complex (PdL) based on
a
thiosemicarbazone ligand. X-ray diffraction data collected on single
crystals reveal that the complex is a neutral mononuclear entity with
symmetric square planar geometry around the metal center. The
complex exhibits an electrocatalytic behavior for proton reduction in
DMF solvent using trifluoroacetic acid (TFA) as a proton source. Gas
analysis under controlled electrocatalytic conditions during 4 h at a
pool mercury working electrode showed a moderate production of
dihydrogen. By combining experimental techniques to theoretical
calculations, the results are analyzed and compared to those
obtained for the previously reported NiL complex in an effort to
rationalize the influence of the metal center on the catalytic
performances of PdL to mediate hydrogen evolution.
Introduction
In the context of continued growth in energy demand, hydrogen
can be a pertinent energy vector.[1] Over the time, significant
effort has been made in the field of renewable energy storage to
design new molecular catalytic systems by focusing onto the
synthesis of transition metal complexes active for electrocatalytic
proton reduction.[2-8] This approach was proven to be a good
alternative to the classical method used for hydrogen production
based on scarce and expensive platinum catalyst.[9] Inspiration
by nature with the [NiFe][10] - or [FeFe][11] - hydrogenases has
often guided these researches. Over the years several groups
have developed and studied synthetic complexes with good
performances for electrocatalytic hydrogen production. These
metal complexes that include nickel[12-14]
, , ,
iron[15] zinc[16]
cobalt[17-19] copper[21,22] or palladium[22] ions, represent structural
and/or functional bioinspired models of hydrogenases.[10,11]
Thanks to structural biology, spectroscopy, electrochemistry,
synthetic chemistry and quantum chemistry, significant
progresses in the understanding of the mechanisms of [NiFe]-
Scheme 1: Transition metal complexes ML based on the H2L ligand.
Results and Discussion
[a]
Dr. T. Straistari, Dr. R. Hardré, Dr. J. Massin, Dr. M. Attolini, Dr. B.
Faure, Dr. M. Réglier, Dr. M. Orio
Synthesis and single-crystal X-ray diffraction analysis
Aix Marseille Univ, CNRS, Centrale Marseille, iSm2, Marseille,
France
E-mail: maylis.orio@univ-amu.fr
Homepage URL: http://ism2.univ-amu.fr/fr/biosciences/biosciences
Dr. T. Straistari
Institute of Chemistry, Academy of Sciences of Moldova 3,
Academiei str., Chisinau MD 2028, Republic of Moldova
Dr. M. Giorgi
The palladium(II) complex was synthetized using the previously
reported 4–{bis(4–(p–methoxyphenyl)–thiosemicarbazone)}–2,3-
butane ligand (H2L, Scheme 1).[28] The PdL complex was
prepared in refluxing acetonitrile by interaction of one equivalent
of ligand H2L with one equivalent of palladium chloride. The PdL
complex was obtained in good yield (92%) as a brown crystalline
precipitate.
[b]
[c]
Aix Marseille Univ, CNRS, Fédération de chimie, Spectropole,
Marseille, France
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201800120
European Journal of Inorganic Chemistry
FULL PAPER
waves is linear which is indicative of a diffusion-controlled
process (Figures SI-1 and SI-2). As in the case of NiL, we
propose that the first redox process is ligand-centered whereas
the second redox process is attributed to the reduction of PdII
into PdI.
Figure 1: Asymmetric part in the crystal structure of [PdL]•(DMF)2.5 with atom
labeling scheme and ellipsoids parameters at 50% probability level. Selected
bond lengths: Pd(1)-S(1) 2.2887(8) Å, Pd(1)-S(2) 2.2914(6) Å, Pd(1)-N(1)
1.980(2) Å, Pd(1)-N(4) 1.981(3) Å; Pd(2)-S(3) 2.2792(8) Å, Pd(2)-S(4)
2.2767(6) Å, Pd(2)-N(7) 1.974(2) Å, Pd(2)-N(10) 1.978(3) Å. Selected bond
angles: S(1)-Pd(1)-S(2) 111.03(3), S(1)-Pd(1)-N(1) 83.87(6)º, S(1)-Pd(1)-N(4)
164.95(6)º, S2-Pd(1)-N(1) 165.10(6)º, S(2)-Pd(1)-N(4) 84.02(6)º, N(1)-Pd(1)-
N(4) 81.09(9)º, and S(3)-Pd(2)-S(4) 110.07(3)º, S(3)-Pd(2)-N(7) 84.31(7)º,
S(3)-Pd(2)-N(10) 165.58(7)º, S(4)-Pd(2)-N(7) 165.62(7)º, S(4)-Pd(2)-N(10)
84.35(7)º, N(7)-Pd(2)-N(10) 81.27(9)º.
Figure 2: Cyclic voltammograms of H2L (dashed line) and PdL (solid line),
concentration 1 mM at a stationary glassy carbon electrode in DMF with 0.1 M
NBu4PF6. Scan rate 500 mV.s-1. Potentials are referenced to the Ag/AgCl
electrode.
In an effort to understand the redox behavior of PdL, DFT
calculations were undertaken and the structure of the complex
was subjected to geometry optimization. Comparison of the
DFT-optimized structure of PdL with the X-ray data showed a
good agreement between the two sets of data which confirmed
that PdL can be best described as a low spin Pd(II) complex
([PdIIL]0, S = 0, Table SI-2 and Figure SI-7). The theoretical
structure obtained upon geometry optimization of the one-
electron reduced species of PdL shows that only the C–C and
N–N bond lengths of the bridge are affected. It suggests that the
first reduction of PdL is a ligand-based process leading to the
formation of a paramagnetic species with a Pd(II) ion bound to a
ligand radical ([PdIIL•]-, S = 1/2). The calculated structure for the
two-electron reduced species shows that the second reduction
mainly affects the Pd–S and Pd–N bond lengths that are
elongated. It is indicative of a metal-based process consistent
with the formation of a coupled Pd(I)-ligand radical species
([PdIL•]-, S = 0 or 1).
Inspection of the electronic structure of the one-electron reduced
species supports that a delocalized ligand-based radical species
is formed upon reduction of PdL as the SOMO of PdL- is a π-
orbital essentially distributed over the hydrazone bridge of the
complex (Pd, 3%; S, 1%; N, 46%; remaining, 50%, Figures 3
and SI-8). The second reduction process in PdL is similar to that
of NiL as it occurs at the metal site leading to the formation of
Pd(I) with the second SOMO of PdL2- being mainly distributed
on the palladium center and its coordinating atoms (Pd, 71%; S,
14%; N, 12%; remaining, 3%, Figures 3 and SI-8). Due to the
orthogonality of the two SOMOs of PdL2- and an energy gap of
13.8 kcal.mol-1 with respect to the singlet state, the ground spin
state of this direduced species can be assigned as a triplet S =1
(Table SI-3). Redox potential calculations were conducted to
fully assign the electrochemical events (Tables SI-4 and SI-5).
Crystals suitable for X-ray diffraction analysis were obtained
from DMF solution by slow evaporation of the solvent. The
palladium(II) complex PdL is isostructural with the previously
reported [NiL]2.5(DMF) complex,[28] which crystallizes in
a
triclinic P-1 space group (Table SI-1). It consists of two
crystallographically independent units, [Pd(1)L] and [Pd(2)L],
and five molecules of DMF solvent. The interplanar angle
between both independent molecules is 2.03°. Both neutral
mononuclear symmetric units [Pd(1)L] and [Pd(2)L] have a four-
coordinated stereochemistry with a little difference in bond
lengths and angles values. The ligand is in thiolate form as
shown by the increase in the C–S bond distances (1.774; 1.775
Å for [Pd(1)L] and 1.774; 1.778 Å for [Pd(2)L]) compared to the
neutral ligand H2L (1.674 and 1.673 Å). The Pd ion is four-
coordinated and lies perfectly in the plane of the donor system
(the out of N2S2 plane is 0.01 Å, Figure 1).
Electrochemistry
Cyclic voltammograms (CV) were recorded at room temperature
at a glassy carbon electrode in anhydrous DMF under argon
containing NBu4PF6 as background electrolyte. Potentials are
referred to a Ag/AgCl reference electrode. While the CV of H2L
only showed an irreversible wave due to the reduction of imine
functions, the CV of PdL revealed two successive reversible
one-electron redox systems at E*1/2 = -1.02 V and E**1/2 = -1.64 V
vs. Ag/AgCl (where E1/2 = [Epc + Epa]/2, Figure 2). The plot of the
current peaks versus the square root of the scan rates for both
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10.1002/ejic.201800120
European Journal of Inorganic Chemistry
FULL PAPER
The computed values for the two reductive processes leading to
the formation of PdL- and PdL2- are of -1.33 V and -1.89 V vs.
Ag/AgCl, respectively. The calculated potentials are in fair
agreement with the experimental data (-1.02 and – 1.64 V) and
confirm that both redox systems correspond to monoelectronic
processes. Based on the good correlation between theory and
experiment, we can thus support our previous assignment of the
reduction loci in PdL.
Figure 4: Electronic absorption spectra for the neutral PdL (black lines), one-
electron reduced species PdL- (green line) and electrochemically re-oxidized
species (red line).
Electrocatalytic proton reduction
To examine a possible catalytic activity of PdL towards proton
reduction, its electrochemical behavior was studied by cyclic
voltammetry in the presence of acid under argon in DMF
solutions with increasing concentrations of trifluoroacetic acid
(TFA, pKa = 6.0 ± 0.3 in DMF)[29] as proton source. With the
addition of 20 mM of TFA a new irreversible peak appears at a
potential of -0.6 V vs. Ag/AgCl, before the first reduction wave of
the PdL complex. The current intensity of the new peak
increases with sequential increments of the acid concentration
and was attributed to the electrocatalytic proton reduction. The
catalytic potential average value was determined to Ecat/2 = -1.2
V vs. Ag/AgCl (Figure 5). A control experiment was performed
to determine if an active adsorbed species is formed during
electrocatalysis. After the working electrode was rinsed and
immerged into a fresh solution of DMF in the presence of TFA, a
non-negligeable catalytic current was detected indicating that
the HER activity is due to both dissolved and adsorbed PdL
complex (Figure SI-5).
Figure 3: DFT-calculated redox-active orbitals for the neutral, mono and di-
reduced species of PdL.
UV-Visible Spectroscopy and Spectroelectrochemistry
UV-vis absorption spectra recorded in DMF for PdL are very
similar to those of the previously reported NiL complex and
exhibit one intense absorption band at ca. λmax = 450 nm (ε =
19400 L.mol-1.cm-1) and another less intense feature at ca. 625
nm (ε = 3140 L.mol-1.cm-1, Figure 4). After applying the potential
of the first reduction (Epc*), a slight decrease in intensity of the
main absorption band is observed (green line) which then
returns to its original shape (red line) after further
electrochemical re-oxidation (Epa*). A similar behaviour was
observed after second electrochemical reduction (Epc**) and re-
oxidation (Epa**) but with a stronger variation of the intensity of
the 450 nm band (Figure SI-3). This behavior strongly suggests
a quasi-reversibility of the redox processes.
TDDFT calculations were conducted on the neutral, mono and
di-reduced forms of PdL and further supported that the UV-vis
spectra are dominated by one intense absorption band in all
three redox states. The calculated spectra also confirm the
presence of another electronic transition found at a lower energy
(Figure SI-9 and Table SI-6). The dominant absorption band was
assigned to (i) to a ligand-to-ligand charge transfer for PdL (ii) a
a mixed ligand to metal-ligand transition in PdL- and (iii) a mixed
metal-ligand to ligand transition in PdL2- (Table SI-6 and Figure
SI-10). Our calculations finally show a decrease in the intensity
of the main absorption band upon successive reduction of PdL,
consistently with the experimental observation (Figure SI-9).
Figure 5: Successive cyclic voltammograms recorded at a glassy carbon
electrode of a 1 mM solution of PdL complex in DMF (0.1 M NBu4PF6) with
increasing concentration of TFA.
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10.1002/ejic.201800120
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calculations. We have considered five potential sites for the
addition of one proton to the one-electron reduced species PdL-:
the metal center, the sulfur atom, the hydrazino nitrogen, the
coordinated nitrogen and the phenylthiocarbamide nitrogen,
generating [PdH], [Pd-SH], [Pd-NH], [Pd-HN], and [Pd-HNH],
respectively. Our results indicate that the hydrazino nitrogen is
the preferential protonation site as [Pd-NH] is found to be the
lowest energy species lying 3.8, 9.3, 10 and 20 kcal.mol-1 lower
than [Pd-HN], [Pd-SH], [Pd-H] and [Pd-HNH], respectively
(Figure 7). Electronic structure calculations show that [Pd-NH]
can be best described as a ligand-based radical species
(Figures SI-11, SI-12 and SI-13).
To confirm that the catalytic wave corresponds to the
reduction of protons to molecular hydrogen we performed a
bulk electrolysis experiment in DMF in the presence of TFA
with in-line GC analysis. Applying a potential of -1.2 V vs.
Ag/AgCl electrode to a mercury pool working electrode
during 4 h of flow electrolysis in presence of PdL produced
hydrogen with 34 % faradaic efficiency and a turnover
number of 2. This experiment allows to rule out the
possibility that proton reduction occurs as the result of
deposition of palladium nanoparticles on the electrode
surface since they would amalgamate within the mercury
pool electrode. Figure 6 shows the evolution of the charge
passed during the bulk electrolysis. We can observe that a
plateau is reached after 1h of experiment and
electrocatalytic hydrogen production is no longer observed
which strongly suggests decomposition of our complex
(Figure SI-6). In similar conditions the NiL complex
produced hydrogen during 16 h of electrolysis with 80 %
faradaic efficiency and a turnover number of 21. These data
confirm that PdL catalyzes the reduction of protons to
molecular hydrogen but its performances are pretty
moderate compared to those of the nickel complex NiL and
to any other reported electrocatalysts based on
thiosemicarbazone ligands like the zinc and the copper
analogues.[16,21]
Figure 7. Energetic stability of protonated species of PdL-.
The stabilities of the doubly reduced and singly protonated
species of PdL were evaluated and our results show that the
electron transfer event did impact the location of the proton as
the coordinating nitrogen becomes the preferential protonation
site leading to [Pd-HN]- being the most energetically favored
species (Figures
8 and SI-14). The calculated electronic
structure of [Pd-HN]- supports a metal-based reduction process
leading to the formation of a Pd(I)-ligand radical species as
positive spin population is found at the Pd center (Figure SI-15).
Owing to the orthogonality of the two SOMOs and an energy
gap of 19 kcal.mol-1 with respect to the singlet state, the ground
spin state of [Pd-HN]- can be assigned as a triplet (Figure SI-16
and Table SI-3).
Figure 6: Coulometry during a bulk electrolysis experiment at -1.2 V vs.
Ag/AgCl using a mercury pool electrode. The electrolytic solution contains 0.1
M NBu4PF6 in DMF, 100 mM TFA and no (red line) or 1 mM (black line) of PdL.
Mechanistic considerations
The catalytic mechanism for H2 evolution that we propose is
based on the ECEC sequence, as classically found for many
transition metal complexes.[13,14,18,19] Such a mechanism is in line
with the fact that the electrocatalytic mid-wave potential is equal
to that of the PdL/PdL– couple. This is also supported by
additional UV-vis measurements conducted on PdL in the
absence and presence of trifluoroacetic acid. A similar behavior
was observed in the both cases as the two spectra display
identical features. These data confirm that the structure of the
complex does not change in the presence of protons so that
protonation occurs upon the first reduction process (Figure SI-4).
To gain more insight on the locus of proton addition following the
electrochemical reduction of PdL, we have conducted DFT
Figure 8. Energetic stability of protonated species of PdL2-.
The proposed mechanism involving
a second protonation
process, we finally investigated the stabilities of the doubly
reduced and doubly protonated species of PdL. Our results
show that the hydrazino nitrogen is the preferential locus for the
addition of a second proton to the system leading to the
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10.1002/ejic.201800120
European Journal of Inorganic Chemistry
FULL PAPER
energetically favored [Pd-NH-HN] complex as the catalytically
active species (Figures SI-17 and SI-18). [Pd-NH-HN] is best
described as a Pd(I)-ligand radical species having a triplet
ground spin state with an energy gap of 11.3 kcal.mol–1 with
respect to the singlet state S = 0 (Figures SI-19 and SI-20, Table
SI-3).
protonation at the hydrazino nitrogen for PdL and protonation at
the coordinating nitrogen for NiL. The third event is an
electrochemical step (E) corresponding to a metal-centered
reduction, which leads to a coupled metal-radical species for
both systems. It is to be noted that the coordinating nitrogen is
now protonated in both two-electron reduced complexes
showing that the second reduction process affects the locus of
protonation in PdL which is reflected in its catalytic
performances. Eventually, the fourth event is the last chemical
(C) step leading to the formation of the hydrogen evolving
complex. H2 will thus evolve through coupling between the
protons from the adjacent nitrogen atoms of PdL (Scheme 2).
These results are in great contrast with those obtained for the
nickel analogue NiL as [Ni-HN] was found to be the most stable
species with protonation of the coordinating nitrogen. Besides,
the location of the proton in NiL- was not impacted by the
second reduction process as [Ni-HN]- was found to be
energetically preferred. The impact of the metal is hereby
manifested in the initial protonation step and the low efficiency of
the palladium complex compared to the nickel one can be
attributed to the stability difference of their respective protonated
species. This is in line with recent results our group obtained on
a bis(thiosemicarbazone) cobalt complex, CoL.[30] The catalytic
activity of this complex for hydrogen evolution was found to be
as moderate as that of PdL and does not compare either with
that of NiL. Interestingly, the hydrazino nitrogen is also the
favoured protonation site for the one-electron species of CoL
and this locus is affected by the second reduction. This event
being a metal-based process, it also leads to the coordinating
nitrogen as being the preferential location for the proton in the
two-electron reduced species of CoL.
Conclusions
We have shown that mononuclear metal complexes ML (M = Ni
and Pd) with bis(thiosemicarbazone) ligand exhibit very different
electrocatalytic activities for hydrogen production. Although NiL
appears to be one of the best catalysts reported in the literature,
the performances of PdL for proton reduction are pretty
moderate as a 4h hydrogen production is observed with 34 %
faradaic efficiency and a turnover number of 2 while in similar
conditions NiL produced hydrogen during 16 h with 80 %
faradaic efficiency and a turnover number of 21. The differences
relative stability upon protonation of the singly and doubly
reduced species of the NiL and PdL help to rationalize their
respective efficiency to mediate proton reduction. These results
highlight the notable influence of the metal center that strongly
impacts the nature and the locus of both the protonation and
redox processes which will affect the efficiency of the chemical
reaction under investigation. Further studies are currently
undergoing in our group to investigate the influence of the first
coordination sphere of the bis(thiosemicarbazone) ligand on the
catalytic properties of the metal complexes for proton reduction
and hydrogen evolution.
Experimental Section
All solvents and chemicals were purchased from Sigma Aldrich and used
without further purification. The synthesis of the ligand H2L was
performed according to procedures described previously.[28] 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
ATR. Elemental analysis was performed on Thermo Finnigan EA 1112
instrument. The results were validated by at least two sets of
measurements.
Scheme 2: Proposed mechanism for H2 evolution with PdL.
To summarize, the reactivity of PdL is of metal-assisted ligand-
centered type, similarly to what was reported in the case of a
bis(thiosemibarbazone) copper complex.[21] The group of
Grapperhaus studied the electrocatalytic behavior of a neutral
and monomeric Cu(II) complex of diacetyl-bis(N-4-methyl-3-
thiosemicarbazonato) that exhibited a high TOF of 5100 s-1 but
with large overpotential values limiting its use as HER catalyst.
The complex was found to display a less common type of
mechanism with redox noninnocent ligands such as the metal
serves as the auxiliary redox site, which facilitates two-electron
chemistry at the ligand.[21]
Synthesis of PdL
The ligand H2L (0.3 mmol, 0.133 g) was dissolved in DMF (5 ml), and
then one equivalent of PdCl2 (0.3 mmol, 0.052 g), dissolved in
acetonitrile (5 ml), was added dropwise. The suspension was stirred at
reflux for 2 hours and left at room temperature for cooling. Single crystals
suitable for crystallographic analysis were obtained in 92 % yield. Anal.
Calcd. for 2(C20H22N6O2PdS2) 5(C3H7NO): C, 45.14; H, 5.40; N, 16.28; S,
8.75. Found: C, 45.48; H, 5.30; N, 16.34; S, 8.92 FT-IR, ν (cm−1): 3288
(N−H), 1616 (C═N imine), 1506 (N−C), 1240 and 1017 (O−C), 1170 and
817 (C−S). 1H-NMR (400 MHz, DMSO-d6), δ (ppm): 10.00 (s, 2H, NH),
7.96 (s, 2,5H, C–H(DMF)), 7.55 (d, J = 9 Hz, 4H, o-CH), 6.89 (d, J = 9 Hz,
The reaction mechanism involving PdL is also slightly different
from that of NiL even though they both follow an ECEC
mechanism. The first step is an electrochemical (E) event
corresponding to
a
ligand-based reduction for the two
complexes. The second step is a chemical (C) process involving
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10.1002/ejic.201800120
European Journal of Inorganic Chemistry
FULL PAPER
4H, m-CH), 3.72 (s, 6H, O-CH3), 2.89 and 2.73 (2 s, 15H, CH3 (DMF)),
2.15 (s, 6H, CH3). 13C NMR (101 MHz, DMSO-d6), δ (ppm): 162.77 (C=S),
159.37 (C=N), 155.96 (C–O), 134.30 (o-C), 122.24 (m-C), 114.23 (p-C),
55.63 (O–CH3), 15.01 (C–CH3).
set. Vertical electronic transitions were calculated using time-dependent
DFT (TD-DFT)[46-48] within the Tamm–Dancoff approximation.[49,50] To
increase computational efficiency, the RI approximation[51] 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 program.
X-ray crystallography
Crystallographic data for [PdL]•2.5DMF (CCDC-1548060) can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via: www.ccdc.cam.ac.uk/data_request/cif. Suitable crystals of
Acknowledgements
PdL were measured on
a Rigaku Oxford Diffraction SuperNova
diffractometer at 223K at the CuKα radiation (λ=1.54184 Å). Data
collection reduction and multiscan ABSPACK correction were performed
with CrysAlisPro (Rigaku Oxford Diffraction). The structures were solved
by direct methods with SHELXS[31] and SHELXL[31] was used for full
matrix least squares refinement. PdL crystallized with two independent
molecules in the asymmetric unit and five DMF solvent molecules. All H-
atoms were found experimentally and were refined with riding
coordinates to their parent atoms and their Uiso parameters constraint to
1.5Ueq (parent atoms) for the methyls and to 1.2Ueq (parent atom) for
the other carbons and amines.
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 COST Actions CM1305
ECOSTBio (Explicit Control Over Spin-States in Technology and
Biochemistry) and CM1202 Perspect-H2O (Supramolecular
photocatalytic water splitting). The authors would like to thank
Jennifer Fize and Dr. Vincent Artero for assistance in the
electrocatalytic measurements and fruitful discussions.
Keywords: palladium • redox-active ligand • quantum chemistry
Electrochemistry
• electrocatalyst • hydrogen evolution reaction
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
[1]
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FULL PAPER
How does the metal centre influence
the reaction mechanism and the
catalytic properties for hydrogen
production of thiosemicarbazone-
based complexes?
T. Straistari, R. Hardré, J. Massin,
M. Attolini, B. Faure, M. Giorgi, M.
Réglier, M. Orio*
Page No. – Page No.
Influence of the metal ion on the
electrocatalytic hydrogen
production by a
thiosemicarbazone palladium
complex.
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