Planar or Bent? Redox Modulation of Hydrogenase Bimetallic Models by the [Ni2(μ-SAr)2] Core Conformation

Article abstract of DOI:10.1002/ejic.202100105

Five neutral nickel(II) bimetallic models of the active site of [NiFe]-hydrogenase supported by tridentate sulfur-rich RNS₂ ligands, were synthesized and tested as electrocatalysts for proton (H⁺) reduction. Complexes were classified according to the ?NR substituent (1: 1-methylpyrene; 2: 2-methylthiophene; 3: phenyl) and as type a for those without bulky substituents and type b for the analogues with voluminous groups. Solid state structures were determined for three dimers, revealing [Ni₂(μ-SAr)₂] frameworks, in which the two coordination planes around the Ni centres define a dihedral angle (θ) that is influenced by the substituents on the ligands (2 a: θ=180.0°, Ni ??? Ni=3.356 ?; 2 b: θ=98.55°, Ni ??? Ni=2.760 ?; 3 a: θ=107.32°, Ni ??? Ni=2.825 ?). Using CF₃COOH as H⁺ source, 1 b and 2 b exhibit catalytic activity at ?1.72 V (i_cat/i_p≈2.40) and ?1.80 V (i_cat/i_p≈2.89) vs the ferrocenium/ferrocene couple (Fc⁺/Fc), respectively. In contrast, type a complexes were not viable catalysts. This behaviour suggests a relationship between the dimer conformation and its activity, due to a Ni ??? Ni cooperative effect, which is favoured in angular molecules and appears to assist during electrocatalytic H⁺ reduction.

Full text of DOI:10.1002/ejic.202100105

European Journal of Inorganic Chemistry
Accepted Article
Title: Planar or Bent? Redox Modulation of Hydrogenase Bimetallic
Models by the [Ni2(μ-SAr)2] Core Conformation
Authors: Brenda Angélica Murueta Cruz, Armando Berlanga Vázquez,
Diego Martínez Otero, Luis Norberto Benitez, Iván Castillo,
and Alexander Mondragón Díaz
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202100105
Link to VoR: https://doi.org/10.1002/ejic.202100105
01/2020

10.1002/ejic.202100105
European Journal of Inorganic Chemistry
FULL PAPER
Planar or Bent? Redox Modulation of Hydrogenase Bimetallic
Models by the [Ni2(μ-SAr)2] Core Conformation
Brenda A. Murueta-Cruz,[a] Armando Berlanga-Vázquez,[b] Diego Martínez-Otero,[c] Luis Norberto
Benitez,[a] Ivan Castillo*[b] and Alexander Mondragón-Díaz*[a]
[a]
[b]
[c]
Mtr. B. A. Murueta Cruz, Dr. L. N. Benitez, Dr. Alexander Mondragón Díaz
Departamento de Química, Facultad de Ciencias
Universidad del Valle
Ciudad Universitaria Meléndez, Calle 13 # 100-00, Cali, Colombia
E-mail: mondragon.alexander@correounivalle.edu.co
Prof. Iván Castillo P., M.Sc. A. Berlanga-Vázquez
Instituto de Química
Universidad Nacional Autónoma de México
Ciudad Universitaria, 04510, Ciudad de México, México
E-mail: joseivan@unam.mx
Dr. Diego Martínez-Otero
Centro Conjunto de Investigación en Química Sustentable UAEM-UNAM
Carretera Toluca-Atlacomulco km 14.5
Toluca, 50200, Estado de México, México
Supporting information for this article is given via a link at the end of the document.
Abstract: Five neutral nickel(II) bimetallic models of the active site
of [NiFe]-hydrogenase supported by tridentate sulphur-rich RNS2
ligands, were synthesized and tested as electrocatalysts for proton
(H+) reduction. Complexes were classified according to the -NR
substituent (1: 1-methylpyrene; 2: 2-methylthiophene; 3: phenyl) and
as type a for those without bulky substituents and type b for the
analogues with voluminous groups. Solid state structures were
determined for three dimers, revealing [Ni2(μ−SAr)2] frameworks, in
which the two coordination planes around the Ni centres define a
dihedral angle (θ) that is influenced by the substituents on the
ligands (2a: θ = 180.0º, Ni···Ni = 3.356 Å; 2b: θ = 98.55º, Ni···Ni =
2.760 Å; 3a: θ = 107.32º, Ni···Ni = 2.825 Å). Using CF3COOH as H+
source, 1b and 2b exhibit catalytic activity at −1.72 V (icat/ip ≈ 2.40)
and −1.80 V (icat/ip ≈ 2.89) vs the ferrocenium/ferrocene couple
(Fc+/Fc), respectively. In contrast, type a complexes were not viable
catalysts. This behaviour suggests a relationship between the dimer
conformation and its activity, due to a Ni···Ni cooperative effect,
which is favoured in angular molecules and appears to assist during
electrocatalytic H+ reduction.
to explore the proposal that the soft nature of sulphur is related
to the redox flexibility of metal centres.[1g,4,5] In this work, we
explore the relationship between the redox activity and the
conformation in a series of [Ni2(RNS2)2] complexes. By varying
the -R group on the central amine (1: 1-methylpyrene; 2: 2-
methylthiophene; and 3: phenyl), along with the degree of steric
hindrance with unsubstituted a ligands (R1 = H; R2 = H) and the
ortho-, para- substituted analogue
(CF3)2C6H3; R2 = CH3), five nickel complexes were synthesized
as homodimeric models of the [NiFe]-ase active site (Figure 1).
b = 3,5-
ligands (R1
R2
R1
R1
R2
S
R
N
Ni
S
S
Ni
N
R
R1
S
R2
R1
R2
1a
2a
3a
1b
2b
*
*
*
*
*
S
S
R
=
R
=
=
R
=
R
=
R
=
=
Introduction
R1
Ar(CF3)2
R1
Ar(CF3)2
R1 = H
R2 = H
R1 = H
R2 = H
R1 = H
R2 = H
Production of H2 as a green fuel from discrete inorganic systems
as bioinspired models of the active site of [NiFe]-hydrogenases
and their enzymatic activity is the subject of intense research.[1]
These enzymes catalyse the interconversion between molecular
hydrogen and protons, thus being capable of generating H2.[2]
And based on numerous studies, it has been established that
the nickel centre plays a key role in the catalytic activity of these
metalloenzymes,[3] since its position is directly related to the
electron/proton transfer pathway in the enzyme, suggesting that
its reactivity is focused on the Ni-S moiety of the active site,
where dihydrogen activation takes place.[3g] Additionally, the
conformation of the binuclear active site seems to be an
important factor for the activity of the enzyme.[2] Therefore, the
development of thiolato-bridged bimetallic nickel complexes is of
relevance to decipher the contribution of the sulphur donors, and
R2 = CH3
R2 = CH3
Figure 1. General structure of the bioinspired [Ni2(RNS2)2] dimeric models.
Ar(CF3)2 = 3,5-(CF3)2C6H3.
Results and Discussion
Synthesis and Characterization
Ligand precursors. The use of three different primary amine
sources (1: 1-pyrenemethylamine hydrochloride; 2: 2-
thiophenemethylamine; and 3: aniline hydrochloride) in the
reaction with the building blocks a: S-2-(chloromethyl)phenyl-
1
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10.1002/ejic.202100105
European Journal of Inorganic Chemistry
FULL PAPER
N,N-dimethylthiocarbamate and the related ortho-, para-
Complex 1a is sparingly soluble in most organic solvents, so
substituted
b:
S-2-(chloromethyl-4-methyl-6-(aryl-bis-3,5-
derivatization was undertaken to obtain a more soluble
trifluoromethyl)phenyl-N,N-dimethylthiocarbamate, leads to the
formation of the corresponding ligand precursors without bulky
groups (Lna) and its sterically hindered analogues (Lnb),
according to the previously reported method[4a] (Scheme 1). The
five ligand precursors synthesized, denoted as L1a, L1b, L2a, L2b
and L3a were purified by recrystallization and characterized by
the conventional spectroscopic techniques. See synthetic
procedures in the Experimental Section and full characterization
in the Supporting Information, SI (Figures S1-S23). Additionally,
the single crystal structure of L1a is illustrated in Figure S24; and
the crystallographic data is presented in Table S1.
compound for characterization. Reactions with PPh3 using
solvents such as dichloromethane, tetrahydrofuran and
dimethoxyethane were not productive. Cleavage of the putative
dimer was achieved by stirring a suspension of 1a in CH2Cl2 with
2-naphthylisocyanide, resulting in the new compound 1a*, which
corresponds to a soluble monomeric complex (Scheme 3). Thus,
1a* could be characterized by 1H and 13C NMR spectroscopy, as
well as the aforementioned techniques (Figures S35-S39).
S
N Ni
S
S
1/2
i
N
Ni
C
N
S
2
1a
1a*
Scheme 3. Synthesis of monomeric complex 1a* derived from 1a, in which 0.5
equiv. of the proposed dimer 1a react with 1.0 equiv. of the exogenous ligand.
(i) CN(C10H7), CH2Cl2, r. t., overnight.
Structural analysis
Attempts to grow X-ray quality crystals of the pyrene-containing
complexes 1a, 1a* and 1b were unsuccessful. In contrast, dark
green crystals of 2a and 2b were grown by slow diffusion of
acetonitrile into saturated chloroform solutions of each complex,
and dark wine-coloured crystals of 3a were obtained by slow
evaporation of a saturated 1,1,2,2-tetrachloroethane solution of
this compound. Diffraction studies confirmed that 2a, 2b and 3a
consist of dimeric [Ni2(RNS2)2] structures, in which the Ni(II)
centres are in a slightly distorted square planar environment.
Thus, each metal ion is tetracoordinated by the nitrogen and
both sulphur atoms of one tridentate ligand and a bridging
thiophenolate supplied by the second ligand. Each monomeric
unit displays a bridging thiolato (μ−SAr) and a terminal one (SAr),
where the terminal thiolatos can be considered more electron
rich than the bridging ones.[5] 2a crystallized in the triclinic space
group Pī, whereas 2b and 3a in the monoclinic space group
P21/c. The selected bond distances (Å) and trans angles (º) in
2a, 2b, and 3a confirm the coordination sphere around each Ni
centre in the dimers (Table 1). Supplementary crystallographic
data is presented in Table S2. The solid-state structure of 2a
(Figure 2, left) shows a planar-anti[6] conformation in the
[Ni2(μ−SAr)2] core with a slight distortion, in which the labelled
atom S1 is below and the crystallographically related S1* above
the plane defined by the two Ni(II) centres. Due to the particular
ligand spatial arrangement, 2a displays an inversion centre,
such that the molecule belongs to the Ci symmetry point group.
The coplanarity of both monomeric units produces a separation
of 3.3564(6) Å between the metal centres, labelled as Ni1 and
Ni1*; the Ni−N bond length is 1.983(4) Å and the mean Ni−S
distance is 2.195(1) Å. Complex 2b (Figure 2, centre) belongs to
the C2 point group. In this case, the [Ni2(μ−SAr)2] framework has
a bent-endo[6] configuration, in which the coordination planes
form a hinge angle (θ) of 98.55º, with the planes joined along the
edge outlined by the sulphur bridging atoms S3 and S5,
resulting in a shorter Ni1···Ni2 distance of 2.760(3) Å; the
Ni1−N54 and Ni2−N53 bond lengths are 1.94(1) Å and 2.00(1) Å,
separately; while the average Ni−S distance is 2.205(4) Å. The
Scheme 1. Synthesis of ligand precursors Ln(a/b) displaying three different -R
groups on the central amine (L1: 1-methylpyrene; L2: 2-methylthiophene; L3:
phenyl), without (a) and with the bulky Ar(CF3)2 = 3,5-(CF3)2C6H3 substituents
(b). (i) NaI/K2CO3, THF, 65 ºC.
Bioinspired models. Preparation of the bimetallic complexes
with general formula [Ni2(RNS2)2] required treatment of the
proper Ln(a/b) precursor with an excess of sodium methoxide
under inert atmosphere. Once deprotection of each precursor
was completed, in situ addition of solid NiCl2·6H2O afforded the
desired complex (Scheme 2). See the Experimental Section for
synthetic details. 1a, 1b, 2a, 2b and 3a are indefinitely stable to
air and moisture in the solid state, and they were characterized
by mass spectrometry, combustion analysis and IR
spectroscopy. See full characterization in Figures S25-S34.
Scheme 2. Synthesis of [Ni2(RNS2)2] dimeric complexes 1a, 1b, 2a, 2b and 3a.
(i) MeONa, THF, 6 d, 70 ºC. (ii) NiCl2·6H2O, r. t., 3h.
2
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10.1002/ejic.202100105
European Journal of Inorganic Chemistry
FULL PAPER
proximity between the metal centres in 2b indicates the possible
existence of a Ni···Ni interaction.[7] Finally, 3a (Figure 2, right) as
well exhibits a bent-endo[6] conformation despite the absence of
the bulky substituents. As a result of its arrangement, this
molecule similarly displays C2 symmetry. The distance between
the two nickel centres in 3a is 2.825(1) Å and the coordination
planes joined by the edge formed with the S3 and S7 sulphur
bridges describe a hinge angle θ = 107.32º. The average Ni−S
distance is 2.197(2) Å, while the Ni1−N2 and Ni2−N1 bond
lengths are 2.004(5) Å and 2.007(5) Å, respectively. The
previously reported complex,[4a] denoted here as 3b (analogue of
3a including the 3,5-(CF3)2C6H3 bulky groups in the ortho-
position of the arylthiolato donors), also exhibits a bent-endo[6]
configuration at the [Ni2(μ−SAr)2] core, but the Ni···Ni distance of
2.730 Å and θ = 81.3º confirm that 3b has a considerably more
folded structure than 3a. In this context, despite the angular
arrangement of 3a, the Ni···Ni distance and the hinge angle are
larger than those observed for 2b and 3b, suggesting that the
high degree of steric hindrance imparted by the 3,5-(CF3)2C6H3
substituents and the occurrence of N-aromatic groups are the
two main factors that promote more folded structures in this type
of complexes, emphasizing the importance of the substituents in
the ligands to favour a particular conformation. Moreover, it was
formerly determined through DFT calculations,[4a] that for the
related [Ni2(RNS2)2] dimer with non-bulky thiophenolate ligands
and N-isobutyl groups, there is a small energetic difference of
2.3 kcal·mol−1 between the planar and bent (θ
= 102º)
configurations. These results allow us to propose the possibility
that, in solution, complex 3a could interconvert between both
conformations or merely exist in the planar arrangement, where
this fluctuation may be modulated by the presence of N-aromatic
groups in combination with the lack of bulky substituents.
Table 1. Selected distances (Å) and trans angles (º) for 2a, 2b and 3a.
2a
2b
Distances (Å)
2.760(3)
3a
Ni1···Ni1*
Ni1−S1
Ni1−S1*
Ni1−S2
3.3564(6)
2.199(1)
2.191(1)
2.183(1)
1.983(4)
2.191(1)
2.199(1)
2.183(1)
1.983(4)
Ni1···Ni2
Ni1−S3
Ni1−S4
Ni1−S5
Ni1−N54
Ni2−S3
Ni2−S5
Ni2−S6
Ni2−N53
Ni1···Ni2
Ni1−S3
Ni1−S7
Ni1−S5
Ni1−N2
Ni2−S3
Ni2−S7
Ni2−S4
Ni2−N1
2.825(1)
2.215(2)
2.192(2)
2.180(2)
2.004(5)
2.211(2)
2.204(2)
2.179(2)
2.007(5)
2.189(4)
2.200(4)
2.219(4)
1.94(1)
2.210(4)
2.229(4)
2.182(4)
2.00(1)
Ni1−N1
Ni1*−S1
Ni1*−S1*
Ni1*−S2*
Ni1*−N1*
Angles (º)
S2−Ni1−S1
169.56(3)
169.05(8)
169.05(8)
169.56(3)
S6−Ni2−S3
N53−Ni2−S5
S5−Ni1−S4
S3−Ni1−N54
164.1(2)
169.7(3)
166.0(2)
170.8(4)
S5−Ni1−S3
N2−Ni1−S7
S7−Ni2−S4
S3−Ni2−N1
166.33(9)
170.67(2)
161.32(8)
166.37(2)
N1−Ni1−S1*
S1−Ni1*−N1*
S1*−Ni1*−S2*
θ = 180.0 º
θ = 107.32 º
θ = 98.55 º
Figure 2. Solid-state structures and hinge angles (θ) of dimeric complexes 2a (left), 2b (centre) and 3a (right), emphasizing the coordination planes with ellipsoids
at 30% probability level. Hydrogen atoms and solvent molecules are omitted for clarity. Colour code: C, grey; N, blue; S, yellow; Ni, green; F, lime green.
3
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10.1002/ejic.202100105
European Journal of Inorganic Chemistry
FULL PAPER
Optical studies
nm (S2 ← S0), 272 nm (S3 ← S0) and 243 nm (S4 ← S0), with the
S1 ← S0 excitation as dipole forbidden.[8] The absorptions bands
observed in the UV-vis spectra of 1a* (purple trace in Figure 3a),
1b (red trace in Figure 3b); as well as for 2a, 2b and 3a,
obtained in anhydrous THF (Figure 3c) are summarized in Table
2. Intraligand (L* ← L) an ligand to metal charge transfer (LMCT)
bands were assigned according to prior reports.[4a,9,10] Lastly, it is
important to notice that the high intensity and low energy of the
charge transfer absorptions observed, reflect the high degree of
covalent character of the Ni−S bond in these complexes.[10]
The absorption spectra of 1a* and 1b, acquired in CHCl3, display
very sharp absorption bands in the UV region, several of which
can be attributed to the pyrene moiety, as they contain the
electronic transitions observed for the ligand precursors L1a and
L1b (magenta trace in Figure 3a and blue trace in Figure 3b,
respectively), shown in Table 2. Moreover, the energy values of
those absorptions are very similar to the previously reported for
the frontier orbitals (FO) of pyrene at λ = 372 nm (S1 ← S0), 334
Figure 3. Absorption spectra of (a) complex 1a* (purple) with precursor L1a (magenta), both [0.025 mM] in CHCl3; (b) complex 1b (red) with precursor L1b (blue),
both [0.025 mM] in CHCl3; and (c) complexes 2a (green), 2b (purple) and 3a (red), all [0.01 mM] in anhydrous THF.
Table 2. Absorption bands of the UV-vis spectra acquired for L1a, L1b, 1a*, 1b (at [0.025 mM] in CHCl3); and 2a, 2b, 3a (at [0.01 mM] in anhydrous THF).
Electronic transitions
Pyrene moiety (FO)
l / nm
e / M-1 cm-1
L*
¬
L[a]
LMCT (Ni
l / nm
¬
S)[a]
Compound
l / nm
e / M-1 cm-1
e / M-1 cm-1
245
268
331
376
5.89 ´ 104
2.84 ´ 104
3.15 ´ 104
6.76 ´ 102
279
304
316
347
4.24 ´ 104
5.52 ´ 103
1.30 ´ 104
4.71 ´ 104
L1a
L1b
1a*
-
-
-
-
244
268
331
375
6.22 ´ 104
2.84 ´ 104
2.33 ´ 104
6.76 ´ 102
279
303
316
348
3.66 ´ 104
5.38 ´ 103
1.02 ´ 104
3.43 ´ 104
243
268
331
Hidden[c]
7.56 ´ 104
4.82 ´ 104
4.12 ´ 104
Hidden[c]
278
304
316
348
5.12 ´ 104
3.17 ´ 104
3.80 ´ 104
4.49 ´ 104
395[b]
469
» 3.84 ´ 103
1.69 ´ 103
245
268
332
Hidden[c]
1.35 ´ 105
7.12 ´ 104
6.50 ´ 104
Hidden[c]
279
303
318
348
7.83 ´ 104
3.66 ´ 104
4.49 ´ 104
8.20 ´ 104
380[b]
437
» 2.44 ´ 104
8.24 ´ 103
1b
2a
2b
213
234[b]
304
4.31 ´ 104
» 2.95 ´ 104
8.18 ´ 103
377
426
2.62 ´ 103
1.95 ´ 103
-
-
-
-
213
224
288[b]
314
1.06 ´ 105
1.05 ´ 105
» 2.39 ´ 104
1.85 ´ 104
367
423
1.68 ´ 104
6.39 ´ 103
213
4.54 ´ 104
» 1.90 ´ 104
1.27 ´ 104
1.59 ´ 104
336
434
1.24 ´ 104
3.47 ´ 103
232[b]
256
3a
-
-
303
[a] Assigned according to previous reports. [4a,9,10] [b] Shoulder. [c] Masked under the intense charge transfer bands in the complex.
4
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10.1002/ejic.202100105
European Journal of Inorganic Chemistry
FULL PAPER
Electrochemical studies
no other event limits the current,[14] since the peak currents (ip)
revealed a linear relationship with the square root of the applied
sweep rate (Figure S42).
To evaluate the redox behaviour of 1a*, 1b, 2a, 2b and 3a,
cyclic voltammetry (CV) was performed. See the Experimental
Section for trial details. All redox potentials are referenced
relative to the ferrocenium/ferrocene (Fc+/Fc) redox couple.[11]
Figure 4 shows the cathodic cyclic voltammograms (CVs)
obtained for the aforementioned complexes.
Electrocatalytic proton reduction
To gauge the catalytic activity of 1a*, 1b, 2a, 2b and 3a towards
the electrochemical hydrogen evolution reaction, trifluoroacetic
acid (TFA) was used as H+ source. By means of CV, to 1.0 mM
THF solutions of each complex, TFA aliquots were successively
added, and the current response was assessed. 1b and 2b
displayed the best performance when [H+] increased, with 2b
having the highest catalytic current increase, while 1a*, 2a and
3a did not display suitable catalytic behaviour. In this way, for 2b,
a total of 20 equivalents of acid were added (Figure 5). Initially, a
linear increment of the cathodic current response was observed
for 2b with increasing [H+] (inset in Figure 5). Above 20.0 mM of
TFA, negligible changes in the current were observed, i.e., the
catalytic current became independent of acid concentration,
indicating saturation of the catalyst.[15] Thus, from the maximum
catalytic current (icat) and the reduction peak current of the
catalyst in the absence of substrate (ip), the catalytic half-wave
potential (Ecat/2) for 2b was determined at −1.80 V at a scan rate
of 100 mVs−1 (Figure 6). For 1b, the maximum icat was also
identified with [H+] = 20.0 mM and Ecat/2 was found at −1.72 V at
the same sweep rate (Figures S43 and S44). On the other hand,
for 2a and 3a, a poor current response was observed, reaching
acid saturation with merely 2.0 and 6.0 mM of TFA, respectively,
evidencing an inefficient mechanism for H+ reduction (Figures
S45 and S46).[16] These results are directly related to the lack of
appropriate ligand substituents in 2a and 3a, and consequently
an arrangement that is not optimal due to their total and near
planarity, respectively; which prevents the occurrence of a
Ni···Ni stabilizing effect to improve their catalytic response.
Finally, for complex 1a*, at low TFA concentrations (0.4 mM)
new cathodic events began to appear, evidencing the generation
of other electrochemical species as side-phenomena (Figure
S47).[17] This result revealed that 1a* is not viable as a catalyst
for H+ reduction.
Figure 4. CVs of complexes 1a* (blue), 1b (red), 2a (green), 2b (purple) and
3a (magenta). Conditions: 1.0 mM in 0.1 M n-Bu4NPF6 THF solutions; scan
rate of 100 mV s−1; glassy carbon (GC) working electrode.
In the forward scan, the processes are assigned to the reduction
of one Ni(II) centre, on the basis of prior reports for analogous
inorganic systems.[4a,9c] At more negative potentials, no further
reduction events were observed. For complexes 1a*, 1b, and 2b
the half-peak potentials determined at -1.67, -1.70 and -1.68 V,
respectively, are reversible processes; since the ratio ipa/ipc ≈ 1
and the peak-to-peak separation (∆Ep) is smaller than the ∆Ep
observed for the Fc+/Fc couple under the same conditions.[11] In
this context, the redox reversibility of 1b and 2b may be
favoured by two factors: the high covalency of the Ni−S bond,
which provides redox modulation;[10] and the occurrence of a
donor-acceptor
type
Ni···Ni
interaction
for
electron
delocalization.[7] The combined effects contribute to stabilize
different oxidation states. For 1a*, the reversible process may be
aided by back-bonding to the π-acceptor CN group of the 2-
naphthylisocyanide ligand.[12] In contrast, 2a and 3a displayed
quasi-reversible behaviour at -1.69 and -1.77 V, which was
evident by varying the scan rate; suggesting that at slower
sweep rates the reduced species undergoes an irreversible
chemical reaction before reoxidation can occur.[13] By reversing
the scan direction, the oxidation events were assigned to the
Ni2II,II/II,III couple. For 1b and 2b, quasi-reversible processes were
identified at 0.22 and 0.28 V, separately (Figure S40); whereas
1a*, 2a and 3a, presented irreversible waves at 0.40, 0.25 and
0.25 V, respectively (Figure S41).[13] These results prove that the
bulky 3,5-(CF3)2C6H3 groups within 1b and 2b afford some steric
protection and favour the occurrence of the Ni···Ni interaction,[7]
stabilizing the electrochemically formed species. Thus, the redox
performance of 2a and 3a can be related to its negligible steric
protection and to the lack of a Ni···Ni assistance when the bulky
substituents are absent. For 3a it was proposed that in solution it
could exist in the planar conformation, removing the possibility of
a Ni···Ni cooperative effect. Finally, analysis of the current
response at several scan rates demonstrate that electrochemical
events observed in the CVs were free-diffusion processes, i.e.,
Figure 5. Cathodic CVs of a THF solution of 2b (1.0 mM in 0.1 M n-Bu4NPF6)
with successive additions of TFA (0.0 - 20.0 mM). Inset: icat/ip dependence of
2b with increasing [TFA]. Conditions: scan rate of 100 mVs−1; GC working
electrode.
5
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10.1002/ejic.202100105
European Journal of Inorganic Chemistry
FULL PAPER
The first two steps in the catalytic response of 1b and 2b seem
to follow an EC sequence, where E is the electro-reduction step
of the complex, while C denotes its protonation. Assuming that
the rate-determining step is catalyst protonation after initial
reduction, followed by a second electron transfer occurring in
solution (recalling that towards more negative potentials, no
further reduction events were observed for these complexes); eq.
1 allows estimation of the global rate constant (kobs) from the
CVs; where n represents the electron transfer processes at the
electrode and n’ the catalyst equivalents used per turnover.[17]
Thus, the pseudo-first-order apparent rate constants kobs were
estimated at 3.14 s−1 and 3.29 s−1 for 1b and 2b, respectively.
And given that acid is present in large excess, [H+] >> [cat], so
that its concentration is essentially unaffected during the CV
experiments, proton reduction can be considered a pseudo-first-
order reaction.[13b,18]
Figure 6. Cathodic CVs of a THF solution of 2b with [TFA] = 0.0 and 20.0 mM.
The maximum icat is shown for H+ reduction with 2b. Conditions: see Figure 5.
Kinetic studies
ꢀ!"#
1
ꢃꢄ
ꢁ
=
ꢂ
ꢈ ꢅ’ꢉ%&'
The kinetics of electrocatalytic proton reduction was analysed for
ꢀ$
0.4463 ꢅꢆꢇ
Eq. 1
1b and 2b from the ratio of the maximum catalytic current (icat
)
obtained for each model relative to the reduction peak current of
the catalyst in the absence of acid (ip). For 2b, initially the icat/ip
ratio increased linearly, until get to 4.0 mM of TFA; acid
saturation point was reached at [H+] = 20.0 mM with a maximum
icat/ip » 2.89 (inset in Figure 5). For 1b, a similar trend was
observed; and saturation was reached at the same [H+] with a
maximum icat/ip » 2.40 (Figure S48). The initial slope of icat/ip vs
successive additions of TFA is indicative of the acid-dependent
rate determining step, i.e., protonation of the catalyst, where this
linear dependence denotes that proton reduction is second-
order in [H+] (Figure S49).[18] In addition, it was confirmed that
the substrate exhibits no redox processes at the electrode within
the potential window and under the same conditions used in
electrocatalytic studies, which served as a control experiment
that allowed to attribute the increase in current intensity, induced
by the additions of TFA, to the catalytic response of 1b and 2b,
and not to the direct reduction of H+ at the electrode surface.[19]
In this regard, a 1.0 mM TFA solution in THF displayed H+
reduction upon the addition and progressive increase of [1b]
(Figure S50) and [2b] (Figure 7). Maintaining [TFA] constant
allowed analysis of the H+ reduction dependence for each
complex, resulting to be first-order in [cat] (Figure S51 and inset
in Figure 7). Hence, the data obtained for electrocatalytic H+
reduction with 1b and 2b, corresponds to the overall expression:
rate = kobs[cat][H+]2, using TFA as proton source.[18,20]
Additionally, chemical stability of the complexes was monitored
through UV-vis spectroscopy in the presence of excess acid in
anhydrous THF at room temperature. Their integrity was verified
since no variations in the absorption bands were observed under
increasing concentrations of TFA (Figure S52). These results
evidence that 1b and 2b are robust complexes and can only be
protonated under precise conditions, i.e., after undergoing the
reduction process, reinforcing the postulate that the first steps in
the mechanism of proton reduction with these systems involves
an EC sequence. Based on DFT calculations in previous[4a] and
related work,[15a,21] it is reasonable to suggest that the most
favourable site of protonation in 1b and 2b is the electron-rich
terminal sulphur atom directly attached to the reduced Ni centre.
This observation allowed us to propose a mechanism for
electrocatalytic proton reduction reaction with 1b and 2b,
consistent with previous reports.[17,22] The dimeric complex
denoted as D in Scheme 4 is first reduced at the electrode
surface to afford the NiINiII catalytically active species [D]−.
Subsequent proton attachment to the active species takes place
to form [DH] in step 2. Since this step is the rate-determining,
[DH] may react with a second equivalent of [D]− that diffuses
away from the electrode. Homogeneous electron transfer from
[D]− to [DH] could generate the protonated NiINiI species,
followed by a concerted intramolecular proton migration from
sulphur to the electron-rich metal centres,[15a] thus step 3 would
result in [DH]− with the concomitant regeneration of one
equivalent of D. In step 4, the addition of the second proton may
occur to form the species [DHH], and would produce H2 in step 5
along with regeneration of the second equivalent of D. Therefore,
this proposal involves an ECEC mechanism for HER (E =
electron transfer, C = chemical reaction, i.e., protonation of the
catalyst), although an ECCE pathway cannot be ruled out. In
either case, the second electron transfer step is proposed to
occur in solution as a homogeneous process, since only one
reduction event was confirmed within the potential window
employed throughout the electrochemical studies performed for
these complexes. And as stated before, dinuclear models with
an optimal angular conformation that favour electron
delocalization through a Ni···Ni interaction would give rise to
efficient electrocatalysts.
Figure 7. Cathodic CVs of a THF solution of TFA (1.0 mM in 0.1 M n-
Bu4NPF6) with successive additions of 2b (0.0 - 1.0 mM). Inset: dependence of
the current response of the TFA solution with increments in [2b].
6
This article is protected by copyright. All rights reserved.

10.1002/ejic.202100105
European Journal of Inorganic Chemistry
FULL PAPER
S
Infrared spectra were obtained using
a
Shimadzu Affinity
1
e
E (V)
N
S
H2
NiII
NiII
spectrophotometer with ATR accessory. Absorption spectra were
performed using a Jasco V-750 spectrophotometer with W and D2 lamps
in anhydrous THF and CHCl3 solutions. To acquire electrospray
S
S
D
N
1
H+
H
S
5
N
NiII
NiI
S
N
S
S
ionization mass spectra (ESI-IT) an Amazon
X (Bruker Daltonics)
N
N
S
S
NiII
NiII
spectrometer was used with electro-nebulization ionization and ionic
tramp mass analyser, operating in full scan mode with positive
polarization. Fast atom bombardment mass spectra (FAB+-MS) were
obtained in a JEOL SX 102A spectrophotometer with an accelerating
voltage of 10 keV, nitrobenzyl alcohol matrix and xenon atoms at 6 keV.
Electronic impact mass spectra (EI-MS) were acquired using a Shimadzu
GCMS-QP 2010 spectrometer. EPR spectra were recorded in anhydrous
THF solutions in a JEOL JES-TE300 X-band spectrometer at 77 K.
Combustion analysis of carbon, hydrogen, nitrogen and sulphur were
conducted on a Thermo Scientific Flash 2000 elemental analyser at an
oven temperature of 950 ºC and the amount of each sample was
measured on a Mettler Toledo XP6 microbalance.
S
[D] -
H+
S
2
[DHH]
H
4
H
S
N
S
S
H+
NiII
NiI
N
S
S
S
NiI
NiI
3
N
S
S
N
[DH]
[DH] -
D
Scheme 4. Proposed mechanism for the electron and proton transfers in the
electrocatalytic proton reduction with 1b and 2b.
X-Ray Crystallography. Single crystal of ligand precursor L1a and
complexes 2a, 2b and 3a were chosen and mounted on glass fiber for
data collection. Data were collected on an Oxford Diffraction Gemini “A”
diffractometer with charge-coupled device (CCD) area detector with Mo
Kꢀ radiation (ꢁ = 0.71073 Å) at 130 K, in a Bruker Smart Apex X-ray
diffractometer with Mo Kꢀ radiation (ꢁ = 0.71073 Å) at 150 K and in a
Bruker D8 Venture with Cu Kꢀ radiation (ꢁ = 1.54178 Å) at 200 K.
Structures were solved using SHELXS-97 and were refined by full matrix
least squares against F2 using SHELXL-2018/3 (Sheldrick, 2015).[24] The
CCDC Mercury 4.3.0 software (Build 270010) was used to prepare
material for publication.[25] Additional data collection can be found in
Table S1 for ligand precursor L1a and Table S2 for complexes 2a, 2b and
3a. All the non-hydrogen atoms were refined anisotropically. H atoms of
the ligands attached to C atoms were placed in geometrically idealized
positions. Complete crystallographic data have been deposited in the
Cambridge Crystallographic Data Centre (CCDC): 2057972 (2a),
2057973 (2b), 2039671 (3a) and 2057971 (L1a). These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.
Conclusion
For the series of nickel complexes reported herein, 1b and 2b
are active for electrocatalytic proton reduction. In contrast, the
corresponding dinuclear 2a, 3a and the monomeric 1a* exhibited
deficient catalytic performance. These results indicate that the
electrochemical activity of dimeric complexes 1b and 2b is
related to two factors: the high covalency of the Ni-S bond, and
a suitable structural conformation that generates an optimal
hinge angle between the coordination planes, favouring the
proximity of metal centres and allowing the occurrence of a
Ni···Ni cooperative effect to support electronic delocalization
during HER. On the other hand, the functionality at the central
amine of the ligands in 1b and 2b did not have a determining
effect with regard to the electrochemical activity of the
complexes, since the methylene linker between the
pyrene/thiophene groups and the N donor prevented a direct
electronic effect of these groups. Nonetheless, solid state
structural analysis evidences that N-aromatic groups favour a
greater degree of folding in the dimer conformation, as seen for
3a. Finally, in the catalytic mechanism suggested for HER, the
second electron transfer is postulated as a homogeneous
process based on the results of electrochemical studies.
Furthermore, the proton is proposed to be able to migrate from
the terminal thiolato sulphur atom, as a proton relay, to metal
centres due to the highly nucleophilic character of the reduced
NiI ions. Subsequent studies will be aimed at corroborating the
formation of some of the proposed intermediate species.
Electrochemical Measurements. Cyclic voltammetry experiments were
performed using a CH Instruments 600E Series potentiostat in a glass
cell with a three-electrode set-up: glassy carbon (GC) working electrode,
silver wire pseudo-reference electrode (Ag) and platinum wire counter
electrode (Pt). Same study conditions were used in all cases, working
with 1.0 mM anhydrous THF and N2-saturated complex solutions
containing 0.1
M of tetrabutylammonium hexafluorophosphate (n-
Bu4NPF6) as the supporting electrolyte. Electrochemical cell was
exhaustively degassed with N2 for at least 20 minutes before each
experiment. All solutions were freshly prepared prior each analysis. Inert
atmosphere was maintained in systems through continuous N2 flow.
Ferrocene was added at the end of experiments as the internal standard.
Synthesis of ligand precursors.
Common synthetic procedure. The reactions were performed in oven-
dried two-neck and round bottom flasks connected to a Schlenk line. The
flask was evacuated and refilled with N2 three times in each case. All
reactants were added to the flask against a positive pressure of N2. Thus,
2.0 equivalents of the unsubstituted S-2-(chloromethyl)phenyl-N,N-
dimethylthiocarbamate (a) or the analogous ortho-, para- substituted S-2-
(chloromethyl-4-methyl-6-(aryl-bis-3,5-trifluoromethyl)phenyl-N,N-
dimethylthiocarbamate (b) building blocks were dissolved in
approximately 30 mL of anhydrous THF. An equimolar amount of solid
NaI was incorporated, the flask was protected from light and the mixture
was stirred for 1 h at room temperature. Afterwards, an excess of solid
K2CO3 and 1.0 equiv. of the appropriate primary amine (1: 1-
pyrenemethylamine hydrochloride, 2: 2-thiophenemethylamine or 3:
aniline hydrochloride) were added to the flask and the mixture was stirred
and refluxed for 48 h. After this time, the mixture was cooled to room
temperature and the solvent was evaporated to dryness under vacuum,
Experimental Section
General procedures. All reactions were carried out under N2
atmosphere using standard Schlenk techniques. Solvents were purified
from appropriate drying agents and freshly distilled for synthetic
procedures and electrochemical studies. All other chemicals and solvents
were reagent grade and used without further purification, unless
otherwise indicated. The unsubstituted S-2-(chloromethyl)phenyl-N,N-
dimethylthiocarbamate and the corresponding ortho-, para- substituted S-
2-(chloromethyl-4-methyl-6-(aryl-bis-3,5-trifluoromethyl)phenyl-N,N-
dimethylthiocarbamate building blocks used for the synthesis of ligand
precursors were formerly obtained according to the previously published
method.[23] NMR spectra were acquired using Bruker spectrometers
operating at 400 MHz (1H) and 100 MHz (13C). Applying the residual
solvent peaks or tetramethylsilane as internal references, chemical shifts
are reported in parts per million (TMS δ = 0.00, CHCl3 δ = 7.26 ppm).
7
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10.1002/ejic.202100105
European Journal of Inorganic Chemistry
FULL PAPER
the solid residue was dissolved in ethyl acetate, the organic phase was
washed with brine solution (3 x 15 mL) and treated with Na2SO4, then it
was filtered and concentrated under reduce pressure. Finally, the product
was purified by recrystallization to obtain the desired ligand precursor.
the common synthetic procedure. Yield = 72.0 %. NMR−1H (400 MHz,
CDCl3, r. t.): δ 7.54–6.62 (m, 13H, Ar), 4.78 (s, 4H, -N(CH2)Ar), 3.11 (s,
6H, -N(CH3)2), 3.01 (s, 6H, -N(CH3)2) ppm. NMR−13C {1H} (100 MHz,
CDCl3, r. t.): δ 165.86 (C=O), 148.55, 142.73, 137.88, 130.24, 129.32,
127.38, 127.29, 127.16, 116.55, 112.39, 53.62, 37.15 ppm. EI-MS m/z =
479 [M]+, 285 [CH2CHN(ArS)2(CO)]+, 194 [CH2ArS(CO)N(Me)2]+, 72
[(CO)N(Me)2]+ (100.0%).
[NPyr{CH2C6H4S(CO)N(Me)2}2] - L1a: This ligand precursor was prepared
from the unsubstituted building block a (0.5500 g, 2.39 mmol) and 1-
pyrenemethylamine hydrochloride (0.2670 g, 1.00 mmol) following the
common synthetic procedure. Yield = 86.0 %. X-ray quality crystals were
grown from a concentrated CHCl3 solution by slow diffusion of acetonitrile
at room temperature. NMR−1H (400 MHz, CDCl3, r. t.): δ 8.14–7.92 (m,
9H, Pyr), 7.55 (d, 2H, J = 8 Hz, Ar), 7.48 (d, 2H, J = 8 Hz, Ar), 7.35 (t, 2H,
J = 8 Hz, Ar), 7.26 (t, 2H, J = 8 Hz, Ar), 4.18 (s, 2H, -N(CH2)Pyr), 3.80 (s,
4H, -N(CH2)Ar), 2.74 (s, 6H, -N(CH3)2), 2.42 (s, 6H, -N(CH3)2) ppm.
NMR−13C {1H} (100 MHz, CDCl3, r. t.): δ 166.57 (C=O), 142.92, 137.32,
132.84, 131.67, 131.18, 130.96, 130.65, 130.35, 129.53, 129.14, 129.07,
127.53, 127.49, 126.91, 126.48, 125.73, 125.07, 124.88, 124.80, 124.75,
124.68, 124.19, 57.50, 57.43, 36.47 ppm. EI-MS m/z = 617 [M]+, 513 [M -
{S(CO)N(Me)2}]+, 402 [M - Pyr]+, 215 [Pyr]+ (100.0%), 72 [(CO)N(Me)2]+.
Elemental analysis (EA) calcd. (%) for C37H35N3O2S2: C 71.93, H 5.71, N
6.80, S 10.38; found: C 73.09, H 5.76, N 5.72, S 9.64.
Synthesis of biomimetic models.
Common synthetic procedure. The reactions were performed in oven-
dried two-neck and round bottom flasks connected to a Schlenk line.
Each flask was evacuated and refilled with N2 for three times. All
reactants were added to the flask against a positive pressure of N2.
Hence, under N2 atmosphere the corresponding ligand precursor (1.0
equiv.) was dissolved in 30 mL of anhydrous THF, then an excess of
sodium methoxide, MeONa (4.0 equiv.) was added and anhydrous
methanol (3−5 mL) until clearness of solution. The reaction mixture was
stirred and refluxed for 6 d. After this time, the flask was cooled to room
temperature, and once the complete deprotection of the ligand precursor
was verified by means of mass spectrometry (EI-MS) through the
disappearance of the 72 m/z peak characteristic of the carbamate
fragment in Ln(a/b); in situ was added 1.0 equiv. of solid NiCl2·6H2O. The
reaction mixture was stirred for 3 h at room temperature. Solvent was
evaporated to dryness under reduce pressure, the solid residue was
dissolved in CHCl3 or CH2Cl2; impurities and by-products were removed
by extraction with brine solution (3 x 15 mL). The organic solution was
treated with Na2SO4, filtrated and dried under vacuum. Finally, to remove
remaining impurities, each complex was washed with organic solvents of
different polarities: MeOH, ACN and/or hexane.
[NPyr{CH2(MeC6H2R')S(CO)N(Me)2}2] (R' = 3,5-(CF3)2C6H3) - L1b: This
ligand precursor was synthesized from the ortho-, para- substituted
building block b and 1-pyrenemethylamine hydrochloride following the
common synthetic procedure. Yield = 74.0 %. NMR−1H (400 MHz, CDCl3,
r. t.): δ 8.37–7.99 (m, 9H, Pyr), 7.81 (s, 2H, -Ar(CF3)2), 7.68 (s, 4H, -
Ar(CF3)2), 7.50 (s, 2H, Ar), 6.88 (s, 2H, Ar), 4.34 (s, 2H, -N(CH2)Pyr),
4.01 (s, 4H, -N(CH2)Ar), 2.72 (s, 6H, -N(CH3)2), 2.58 (s, 6H, -N(CH3)2),
2.25 (s, 6H, -Ar(CH3)) ppm. NMR−13C {1H} (100 MHZ, CDCl3, r. t.): δ
164.47 (C=O), 143.89, 143.31, 142.87, 138.22, 135.20, 131.47, 131.31,
129.99, 129.71, 129.63, 129.55, 129.22, 129.13, 128.66, 128.39, 127.86,
126.26, 125.96, 125.33, 124.75, 124.06, 123.85, 123.76, 123.69, 123.57,
123.03, 121.07, 119.49, 119.46, 119.42, 63.06, 58.14, 36.22, 20.98 ppm.
EI-MS m/z = 855 [M − Pyr]+, 215 [Pyr]+ (100.0%), 72 [(CO)N(Me)2]+.
[Ni2(
ꢂ
-N,S,S,S'-NPyr{CH2C6H4S}2)2] – 1a. This complex was prepared
from ligand precursor L1a (0.500 g, 0.809 mmol), MeONa (25 %, 0.74 mL,
3.237 mmol) and NiCl2·6H2O (0.192 g, 0.809 mmol). After several
minutes, the formation of 1a as a reddish solid was observed in the
reaction mixture. The solid was filtered and washed with brine solution,
ethanol, CHCl3 and diethyl ether. 1a is an air stable reddish-brown solid,
which was found to be insoluble in all the solvents used, including hot
DMF, DMSO and CHCl3. Yield = 67.0 %. 1a was formulated as a dimeric
species based on the results of elemental analysis and mass
spectrometry. EA calcd. (%) for C62H46N2Ni2S4·CHCl3: C 63.91, H 4.00, N
2.37, S 10.83; found: C 65.25, H 4.52 N 2.47, S 10.19. MS (ESI+-IT) m/z
= 1065.1 [M]+, 474.0 [NPyr(SAr)2]+.
[NThiop{CH2C6H4S(CO)N(Me)2}2]
- L2a: This ligand precursor was
prepared from the building block a (1.000 g, 4.3529 mmol) in the reaction
with 2-thiophenemethylamine (212.7 µL, 1.103 g/mL, 2.0728 mmol)
according to the common synthetic procedure. Yield = 79.0 %. NMR−1H
(400 MHz, CDCl3, r. t.): δ 7.79 (d, 2H, J = 8 Hz, Ar), 7.51 (d, 2H, J = 8 Hz,
Ar), 7.42 (t, 2H, J = 8 Hz, Ar), 7.25 (t, 2H, J = 8 Hz, Ar), 7.24–6.92 (m, 3H,
Thiop), 3.84 (s, 4H, -N(CH2)Ar), 3.83 (s, 2H, -N(CH2)Thiop), 3.09 (s, 6H, -
N(CH3)2), 3.05 (s, 6H, -N(CH3)2) ppm. NMR−13C {1H} (100 MHZ, CDCl3, r.
t.): δ 166.57 (C=O), 142.92, 137.32, 132.84, 131.67, 131.18, 130.96,
130.65, 130.35, 129.53, 129.14, 129.07, 127.53, 127.49, 126.91, 126.48,
125.73, 125.07, 124.88, 124.80, 124.75, 124.68, 124.19, 57.50, 57.43,
[Ni(
ꢂ
-N,S,S'-NPyr{CH2C6H4S}2)(CNC10H7)] – 1a*. This compound was
derived from complex 1a. 2-naphthylisocyanide (0.115 g ,0.752 mmol)
was added to a suspension of 1a (0.400 g, 0.376 mmol) in 30 mL of
CH2Cl2. The reaction mixture was stirred overnight and the obtention of
1a* was evidenced by the formation of a reddish-brown soluble product.
Solvent was evaporated to dryness under reduce pressure, the solid
residue was purified by washing with hexane, acetonitrile and methanol.
1a* is an air stable reddish-brown solid soluble in THF, CHCl3 and
CH2Cl2. Yield = 83.0 %. NMR−1H (400 MHz, CDCl3, r. t.): δ 9.82 (d, 1H, J
= 8 Hz, Ar), 8.825 (d, 1H, J = 12 Hz, Ar), 8.32 (d, 1H, J = 8 Hz, Naph),
8.17–8.10 (m, 4H, Naph), 8.07 (s, 1H, Naph), 7.96 (t, 1H, J = 8 Hz, Naph),
7.87–7.81 (m, 4H, Ar), 7.63 (d, 2H, J = 8 Hz, Ar), 7.56–7.04 (m, 9H, Pyr),
5.29 (CH2Cl2), 4.57 (s, 2H, -N(CH2)Pyr), 3.795 (d, 2H, J = 12 Hz, -
N(CH2)Ar), 3.66 (d, 2H, J = 16 Hz, -N(CH2)Ar) ppm. NMR−13C {1H} (100
MHZ, CDCl3, r. t.): δ 140.96, 136.47, 135.61, 133.07, 132.70, 131.88,
131.71, 131.37, 131.32, 130.64, 130.27, 129.77, 129.06, 128.23, 128.17,
128.08, 128.00, 127.84, 127.71, 126.06, 125.75, 125.68, 125.28, 125.03,
124.89, 124.59, 123.89, 123.39, 122.93, 64.83, 59.00, 53.56 ppm. EA
calcd. (%) for C42H30N2NiS2: C 73.59, H 4.41, N 4.09, S 9.35; found: C
70.60, H 4.21, N 4.08, S 8.57. MS (ESI+-IT) m/z = 685.0 [M]+, 531.9 [M −
CNC10H7]+, 474.0 [NPyr(SAr)2]+, 214.7 [Pyr]+.
36.47 ppm. EI-MS m/z
= −
500 [M]+, 402 [M Thiop(CH2)]+, 194
[CH2SAr(CO)N(Me)2]+, 97 [Thiop(CH2)]+, 72 [(CO)N(Me)2]+ (100.0%).
[NThiop{CH2(MeC6H2R')S(CO)N(Me)2}2] (R' = 3,5-(CF3)2C6H3) - L2b: This
ligand precursor was synthesized from the building block b and 2-
thiophenemethylamine following the common synthetic procedure. Yield
= 83.0 %. NMR− 1H (400 MHz, CDCl3, r. t.): δ 7.82 (s, 2H, -Ar(CF3)2),
7.74 (s, 4H, -Ar(CF3)2), 7.69 (s, 2H, Ar), 7.24 (t, 1H, J = 3 Hz, Thiop),
7.05 (s, 2H, Ar), 6.935 (d, 2H, J = 4 Hz, Thiop), 3.92 (s, 4H, -N(CH2)Ar),
3.91 (s, 2H, -N(CH2)Thiop), 2.88 (s, 12H, -N(CH3)2), 2.43 (s, 6H, -
Ar(CH3)) ppm. NMR−13C {1H} (100 MHZ, CDCl3, r. t.): δ 165.98 (C=O),
145.37, 144.84, 144.12, 142.76, 140.00, 131.21, 131.10, 130.76, 130.43,
130.11, 129.80, 129.59, 126.35, 126.15, 124.83, 124.78, 123.86, 122.12,
120.61, 120.56, 56.56, 53.57, 36.79, 21.40 ppm. EI-MS m/z = 952 [M]+,
855 [M – Thiop(CH2)]+, 531 [NThiopCH2(MeC6H2R')S(CO)N(Me)2]+, 420
[CH2(MeC6H2R')S(CO)N(Me)2]+ (100.0%), 347 [CH2(MeC6H2R')S]+, 97
[Thiop(CH2)]+ (100.0%), 72 [(CO)N(Me)2]+.
[NPh{CH2C6H4S(CO)N(Me)2}2] - L3a: This ligand precursor was obtained
from the unsubstituted building block a (0.3850 g, 1.6759 mmol) in the
reaction with aniline hydrochloride (0.1034 g, 0.7980 mmol) according to
[Ni2(
This complex was synthesized from ligand precursor L1b (0.400 g, 0.3738
ꢂ
-N,S,S,S'-NPyr{CH2(MeC6H2R')S}2)2] (R' = 3,5-(CF3)2C6H3) – 1b.
8
This article is protected by copyright. All rights reserved.

10.1002/ejic.202100105
European Journal of Inorganic Chemistry
FULL PAPER
mmol), MeONa (25 %, 0.34 mL, 1.4952 mmol) and NiCl2·6H2O (0.089 g,
0.3738 mmol) following the common synthetic procedure. The product
was washed with hexane, acetonitrile and methanol. 1b is an air stable
brown-brassy solid slightly soluble in THF, CHCl3 and CH2Cl2. Yield =
61.0 %. 1b was formulated as a dimeric complex based on the results of
elemental analysis and mass spectrometry. EA calcd. (%) for
C98H62F24N2Ni2S4: C 59.77, H 3.17, N 1.42, S 6.51; found: C 59.52, H
Sci. 2008, 1, 15–31. (c) H. Ogata, W. Lubitz, Y. Higuchi, Dalton Trans.
2009, 7577–7587. (d) M. Frey, ChemBioChem 2002, 3, 153–160. (e) S.
Kamali, H. Wang, D. Mitra, H. Ogata, W. Lubitz, B. C. Manor, T. B.
Rauchfuss, D. Byrne, V. Bonnefoy, F. E. Jenney, M. W. Adams, Y.
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4.02,
[NPyr(SAr(Ar(CF3)2))2]+.
N 1.24, S =
5.53. MS (ESI+-IT) m/z 1969.1 [M]+, 926.1
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[Ni2(
ꢂ
-N,S,S,S'-NThiop{CH2C6H4S}2)2] – 2a. This complex was prepared
from ligand precursor L2a (0.300 g, 0.5889 mmol) MeONa (25 %, 0.549
mL, 2.40 mmol) and NiCl2·6H2O (0.143 g, 0.600 mmol) according to the
common synthetic procedure. The product was washed with hexane and
methanol. 2a is an air stable reddish-brown solid soluble in THF, CHCl3
and CH2Cl2. Yield = 70.0 %. X-ray quality crystals were grown by slow
diffusion of ACN into a saturated CHCl3 solution of this compound. EA
calcd. (%) for C38H34N2Ni2S6·2CHCl3: C 45.02, H 3.40, N 2.63, S 18.02;
found: C 45.98, H 3.43, N 3.10, S 18.11. MS (FAB+) m/z = 828 [M]+, 356
[NThiopSAr2]+, 77 [C6H5]+.
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This complex was prepared from ligand precursor L2b (0.300 g, 0.3151
mmol), MeONa (25 %, 0.288 mL, 1.261 mmol) and NiCl2·6H2O (0.075 g,
0.3151 mmol)) following the common synthetic procedure. Product was
washed with hexane, acetonitrile and methanol. 2b is an air stable dark-
green solid soluble in THF, CHCl3 and CH2Cl2. Yield = 68.0 %. X-ray
quality crystals were grown by slow diffusion of ACN into a saturated
CHCl3 solution of this complex. EA calcd. (%) for C74H50F24N2Ni2S6: C
51.29, H 2.91, N 1.62, S 11.10; found: C 51.10, H 3.22, N 1.90, S 10.82.
MS (FAB+) m/z = 1732 [M]+, 97 [C5H6S]+ (100.0%).
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the common synthetic procedure. The product was washed with hexane,
acetonitrile and methanol. Complex 3a is an air stable dark-wine solid
soluble in THF, CHCl3 and CH2Cl2. Yield = 65.0 %. X-ray quality crystals
were grown by slow evaporation of a saturated 1,1,2,2-tetrachloroethane
solution of this complex. EA calcd. (%) for C40H34N2Ni2S4·CH3CN·CH2Cl2:
C 56.49, H 4.30, N 4.60, S 14.03; found: C 56.94, H 4.84, N 4.02, S
13.70. MS (FAB+) m/z = 788 [M]+, 77 [C6H5]+.
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Keywords: bimetallic complexes • electrocatalysis •
hydrogenase • nickel • tridentate ligands
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European Journal of Inorganic Chemistry
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10
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10.1002/ejic.202100105
European Journal of Inorganic Chemistry
FULL PAPER
Entry for the Table of Contents
Insert graphic for Table of Contents here.
Ni···Ni = 2.760 Å
θ = 98.55 º
Ni···Ni = 2.825 Å
θ = 107.3 º
Dihedral angle (θ) tuned by
NS2 ancillary ligands
Ni···Ni = 3.356 Å
θ = 180.0 º
Insert text for Table of Contents here.
The redox modulation and the conformation of a series of [Ni2(RNS2)2] homodimeric models of the active site of [NiFe]-ase, were
analysed by modifying the R group attached to the central amine of the tridentate sulphur-rich RNS2 ligands, as well as the effect of
changing between bulky ligands for the analogues without voluminous substituents. The results indicate that the conformation is
related to the potential catalytic activity of the complex.
11
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