A bioinspired thiolate-bridged dinickel complex with a pendant amine: Synthesis, structure and electrocatalytic properties

Article abstract of DOI:10.1039/c9dt04493k

By employing X(CH₂CH₂S^-)₂ (X = S, tpdt; X = O, opdt; X = NPh, npdt) as bridging ligands, four thiolate-bridged dinickel complexes supported by phosphine ligands, [(dppe)Ni(μ-13κSSS′:22κSS-tpdt)Ni(dppe)][PF₆]₂ (1[PF₆]₂, dppe = Ph₂P(CH₂)₂PPh₂), [(dppe)Ni(μ-13κSSN:22κSS-npdt)Ni(dppe)][PF₆]₂ (2[PF₆]₂) and [(dppe)Ni(t-Cl)(μ-13κSSX:22κSS-C₄H₈S₂X)Ni(dppe)][PF₆] (3[PF₆], X = S; 4[PF₆], X = O) were facilely obtained by the salt metathesis reaction. These four thiolate-bridged dinickel complexes have all been fully characterized by spectroscopic methods and X-ray crystallography. In 2[PF₆]₂, elongation of the Ni-N bond distance, possibly caused by steric hindrance, indicates that the pendant nitrogen group shuttles between the two nickel centers in solution, which is evidenced by ³¹P{¹H} NMR spectroscopic results. Furthermore, these thiolate-bridged dinickel complexes have all been proved to be electrocatalysts for proton reduction to hydrogen. Notably, complex 2[PF₆]₂ featuring a pendant amine group in the secondary coordination sphere exhibits the best catalytic activity at a relatively low overpotential.

Full text of DOI:10.1039/c9dt04493k

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DOI: 10.1039/C9DT04493K
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ARTICLE
A bioinspired thiolate-bridged dinickel complex with a pendant
amine: synthesis, structure and electrocatalytic properties
Puhua Sun,a Dawei Yang,*a Ying Li,a Baomin Wanga and Jingping Qu*ab
Received 22th November 2019,
Accepted 00th January 2020
By employing X(CH2CH2S−)2 (X = S, tpdt; X = O, opdt; X = NPh, npdt) as bridging ligands, four thiolate-bridged dinickel
DOI: 10.1039/x0xx00000x
3
2
complexes supported by phosphine ligands, [(dppe)Ni(μ-1κ SSS':2κ SS-tpdt)Ni(dppe)][PF6]2 (1[PF6]2, dppe = Ph2P(CH2)2PPh2),
www.rsc.org/
2
2
3
2
[(dppe)Ni(μ-1κ SS:2κ SS-npdt)Ni(dppe)][PF6]2 (2[PF6]2) and [(dppe)Ni(t-Cl)(μ-1κ SSX:2κ SS-C4H8S2X)Ni(dppe)][PF6] (3[PF6], X =
S; 4[PF6], X = O) were facilely obtained by the salt metathesis reaction. These four thiolate-bridged dinickel complexes
have all been fully characterized by spectroscopic methods and X-ray crystallography. In 2[PF6]2, elongation of the Ni−N
bond distance, possibly caused by steric hindrance, indicates that the pendant nitrogen group shuttles between the two
nickel centers in solution, which is evidenced by 31P{1H} NMR spectroscopic results. Furthermore, these thiolate-bridged
dinickel complexes have all been proved to be electrocatalysts for proton reduction to hydrogen. Notably, complex 2[PF6]2
featuring a pendant amine group in the secondary coordination sphere exhibits the best catalytic activity at relatively low
overpotential.
Introduction
Hydrogen generation has attracted considerable attention,
particularly since hydrogen is a clean and sustainable energy
carrier when it is derived from proton reduction, which is
commonly regarded as a promising alternative to increasingly
exhausted fossil fuels.1 To overcome the major barrier of
conversion between chemical energy such as H−H bond of
hydrogen and electricity for the storage and utilization of Fig. 1 Structures of active sites of [NiFe]-hydrogenase (a) and
energy, much effort has been devoted to the development of acetyl coenzyme A synthase (b).
active catalysts for hydrogen production during the past
decades.2 In nature, hydrogenases can effectively catalyse the high catalytic activity similar to the function of the secondary
reversible reduction of protons to release hydrogen under coordination sphere of [FeFe]-hydrogenase.3 For example,
ambient conditions using earth-abundant iron and nickel as Helm et al. reported
a synthetic mononuclear nickel
active centres.3 Inspired by biological systems, versatile electrocatalyst supported by PPh2NPh (PPh2NPh = 1,3,6-triphenyl-
thiolate-bridged diiron and iron-nickel complexes as structural 1-aza-3,6-diphosphacycloheptane) ligands with high TOF for
and functional models were designed and synthesized.4 hydrogen evolution.6 Subsequently, a series of more efficient
However, these biomimetic bimetallic model complexes mononuclear nickel electrocatalysts were synthesized by the
usually exhibit relatively low turnover frequency (TOF) toward same group using PR NR’ (1,5-diaza-3,7-diphosphacyclooctane
2
2
proton reduction to hydrogen compared with some with alkyl or aryl groups on the P and N atoms) ligands, which
mononuclear nickel complexes.5 That is because these can catalyze the proton reduction to hydrogen with higher TOF
mononuclear nickel complexes commonly possess one or more and lower overpotential.7 These successful catalysts have
pendant amine groups, which can play as proton relays for
suggested the importance of the nickel center and its
surrounding environment which are both essential during
hydrogen evolution process.
In some Ni-containing enzymes such as [NiFe]-hydrogenase
and acetyl coenzyme A synthase (Fig. 1), the coordination
sphere of the nickel center usually contains several cysteine
residues as sulfur donors.3,8 To get deep insight into how the
sulfur-rich environment facilitates corresponding redox
aState Key Laboratory of Fine Chemicals, Dalian University of Technology, 2
Linggong Road, Dalian, 116024, P.R. China.
bKey Laboratory for Advanced Materials, East China University of Science and
Technology, Shanghai, 200237, P.R. China.
Email: qujp@dlut.edu.cn
yangdw@dlut.edu.cn
Electronic Supplementary Information (ESI) available: Experiment section, spectra
and crystal structures. CCDC 1899923–1899925, 1476294. For ESI and
crystallographic data in CIF or other electronic format See DOI:10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 2019
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reactions, plenty of thiolate-bridged dinickel complexes were
synthesized and examined toward modelling the related
enzymatic activity.9 In contrast, there are only a few thiolate-
bridged dinickel complexes constructed with respect to [NiFe]-
hydrogenase mimicry.10 For example, DuBois et al reported
several bidentate thiolate-bridged dinickel complexes
supported by bidentate phosphine ligands and investigated
their electrochemical properties and reactivities.10a However,
there is no report on the electrocatalytic hydrogen generation
with this system. Recently, Bouwman and co-workers adopted
thiolate-functionalized carbene ligand to construct a thiolate-
bridged dinickel complex, which can realize electrocatalytic
proton reduction to hydrogen.10c Notably, when the ligand set
contains the pyridine group as a potential proton shuttle,
relatively higher catalytic activity was also observed. In this
context, it is an interesting and challenging topic to construct
biomimetic thiolate-bridged dinickel functional complexes
featuring pendant amine with the catalytic capacity of
hydrogen production.
View Article Online
DOI: 10.1039/C9DT04493K
Scheme 1 Synthesis of thiolate-bridged dinickel complexes.
Reagents and conditions: (i) 0.5 eq. Na2(tpdt), 1 eq. NH4PF6,
acetone, rt, 2 h, 68%; (ii) 0.5 eq. Li2(npdt), 1 eq. NH4PF6, THF,
−78 °C to rt, 73%; (iii) (a) 0.5 eq. Na2(tpdt), 0.5 eq. NH4PF6,
acetone, rt, 2 h, 70%; (b) 0.5 eq. Li2(opdt), 0.5 eq. NH4PF6, THF,
−78 °C to rt, 70%.
In our preliminary work,
a series of thiolate-bridged
homodinuclear metallic complexes were constructed for small
molecule activation and catalytic transformation.11-13 Recently,
utilizing tridentate S(CH2CH2S−)2 (tpdt) or O(CH2CH2S−)2 (opdt),
0.5 equiv. of Na2(tpdt) or Li2(npdt) (npdt = N(Ph)(CH2CH2S−)2)16
in the presence of 1 equiv. of NH4PF6, dicationic thiolate-
a
family of homo- and heteronuclear di/multi-metallic
complexes containing iron were synthesized and
characterized.14 As an extension of this work, herein we adopt
three different tridentate ligands containing sulfur to
synthesize several novel thiolate-bridged dinickel complexes.
Notably, these complexes all act as good promoters for
electrocatalytic proton reduction to hydrogen, among which
the dinickel complex featuring a pendant amine in the
secondary coordination sphere shows the best catalytic
activity.
3
2
bridged
tpdt)Ni(dppe)][PF6]2
npdt)Ni(dppe)][PF6]2
dinickel
complexes
1[PF6]2 and [(dppe)Ni(μ-1κ SS:2κ SS-
2[PF6]2) were facilely obtained. In the
[(dppe)Ni(μ-1κ SSS':2κ SS-
2
2
(
(
)
presence of 0.5 equiv. of NH4PF6, cationic thiolate-bridged
3
2
dinickel chloride complexes [(dppe)Ni(t-Cl)(μ-1κ SSS':2κ SS-
3
2
tpdt)Ni(dppe)][PF6] (3[PF6]) and [(dppe)Ni(t-Cl)(μ-1κ SSO:2κ SS-
opdt)Ni(dppe)][PF6] (4[PF6]) were smoothly generated under
similar conditions. Notably, complex 1[PF6]2 can interact with
nBu4NCl to give 3[PF6] in moderate yield. Reversibly, complex
3[PF6] can also be facilely transformed into 1[PF6]2 in the
presence of NH4PF6. These complexes are all stable whether in
the solid-state or in solution under inert gas atmosphere.
These complexes were all fully characterized by various
spectroscopic methods. The 1H NMR spectroscopic results
clearly indicate these complexes are all diamagnetic species
consisting of two low-spin NiII ions. In the 1H NMR spectrum of
1[PF6]2, two multiplets and a very broad peak at around 2.60
ppm with integral area for 12H are assigned to the four CH2
Results and discussion
Synthesis and characterization of thiolate-bridged
dinickel complexes
As illustrated in Scheme 1, four thiolate-bridged NiIINiII
complexes were successfully synthesized in good yields by the
salt metathesis reaction. Upon treatment of mononuclear
nickel precursor [NiCl2(dppe)] (dppe = Ph2P(CH2)2PPh2)15 with
a
b
c
d
Fig. 2 Molecular structures of thiolate-bridged dinickel complexes 1[BPh4]2 (a), 2[PF6]2 (b), 3[PF6] (c) and 4[PF6] (d). Thermal ellipsoids are
shown at 50% probability level. Hydrogen atoms and counter anions PF6− or BPh4− are omitted for clarity.
2 | Dlton Trans., 2019, 48, 1-8
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Table 1 Selected bond lengths [Å] and angles [°] of complexes 1[BPh4]2
ARTICLE
,
2[PF6]2
,
3[PF6] and 4[PF6].
View Article Online
DOI: 10.1039/C9DT04493K
Complex
1[BPh4]2
3.1093(8)
2.238(2)
2.247(2)
2.183(1)
2.174(2)
2.391(2)
2.217(2)
2.237(2)
2.178(2)
2.168(2)
−
2[PF6]2
3.2048(6)
2.2574(9)
2.2185(9)
2.1757(10)
2.1731(10)
2.475(3)
2.2378(9)
2.2283(8)
2.1754(9)
2.1872(10)
−
3[PF6]
4[PF6]
Ni1···Ni2
Ni1−S1
Ni1−S2
Ni1−P1
Ni1−P2
Ni1−Xa
Ni2−S1
Ni2−S2
Ni2−P3
3.2528(10)
2.250(1)
2.242(1)
2.158(2)
2.180(2)
2.392(2)
2.239(2)
2.244(1)
2.170(1)
2.188(2)
2.614(2)
86.55(5)
86.77(5)
9.70(4)
3.2276(4)
2.2462(7)
2.2365(8)
2.1790(8)
2.1701(8)
2.377(2)
2.2533(8)
2.2390(7)
2.1934(7)
2.1750(8)
2.5955(8)
87.34(3)
87.10(3)
13.18(1)
15.26(2)
Ni2−P4
Ni2−Cl1
S1−Ni1−S2
S1−Ni2−S2
Ni1S1S2/Ni2S1S2
Ni1S1S2/Ni1P1P2
aX = S, N, O
87.86(6)
88.64(5)
28.57(5)
25.55(6)
87.86(3)
88.11(3)
9.71(3)
25.55(6)
22.07(6)
groups of the tpdt ligand and two CH2 groups of the dppe nickel centers in the solution state. Only in this case, it is
ligands. The other two multiplets at 1.15 and 0.18 ppm at a 1:1 reasonable that there is only one resonance observed for two
ratio are attributed to the other two CH2 groups of the dppe dppe ligands in the 31P{1H} NMR spectrum. Furthermore, the
ligands. These assignments were in good agreement with the electrospray ionization high-resolution mass spectrometry
1H,1H-COSY spectroscopic results.
(ESI-HRMS) and elemental analysis provide further strong
Differently, the 1H NMR spectrum of 2[PF6]2 shows two evidence for their molecular compositions.
broad peaks with equal integral area at 2.22, 2.68 ppm for the
X-ray crystallographic analysis of thiolated-bridged
CH2 groups of the npdt ligand and two singlets at 1.35, 2.98
ppm with 1:1 integral ratio for the methylene groups of the
two equivalent dppe ligands. Notably, the proton signals of the
dinickel complexes
The crystal structures of these complexes were all well-
phenyl group in the npdt ligand appear at 6.49, 7.14 and 7.23 defined by X-ray diffraction analysis except for 1[PF6]2, whose
ppm, which is up-field shifted compared to the corresponding structural information was provided by counter anion
phenyl resonances of the dppe ligands. Distinct from the exchange product 1[BPh4]2. The ORTEP of 1[BPh4]2
,
2[PF6]2
,
1
obvious overlap of signals in the H NMR spectrum of 1[PF6]2
,
3[PF6] and 4[PF6] are shown in Fig. 2 and the selected bond
complex 3[PF6] displays six separate resonances in the region distances and angles are collectively given in Table 1. The
from 0.05 to 3.26 ppm, which are clearly assigned to the molecular structures of these complexes all contain
a
methylene groups of the tpdt and dppe ligands, respectively. butterfly-shaped {Ni2(μ-S)2} core geometric framework. The
Similar to 2[PF6]2, the 1H NMR spectrum of 4[PF6] displays two Ni1S1S2/Ni2S1S2 dihedral angle of 1[BPh4]2 is 28.57(5)°, which
sets of signals at a 1:1 ratio for the CH2 groups of opdt and is obviously bigger than those of the coordinatively saturated
1
dppe ligands. All assignments of above H NMR data are fully complexes 3[PF6] and 4[PF6] (9.70(4) and 13.18(1)°). Notably,
consistent with the 13C NMR spectra.
this dihedral angle of structurally similar complex 2[PF6]2 is
also small (9.71(3)°), which is due to the steric hindrance
between the phenyl rings of npdt and dppe ligands. The
distances between the two nickel centers in these complexes
are 3.1093(8)−3.2528(10) Å, which exclude the existence of
the metal−metal bond interaction. These values fall in the
range of those of reported thiolate-bridged dinickel complexes
supported by phosphine or Cp ligands.10a,10d,18 In these
The 31P{1H} NMR spectra of 1[PF6]2 and 3[PF6] both show
two singlets and one septet. Using 1[PF6]2 as an example, the
two singlets at δ 57.8 and 63.1 ppm are attributed to
phosphorus atoms of two inequivalent dppe ligands. The
septet at δ −144.3 ppm is attributed to the phosphorus atoms
of two PF6 anions.17 Different from 1[PF6]2 and 3[PF6], the
31P{1H} NMR spectra of 2[PF6]2 and 4[PF6] both exhibit only
one singlet and one septet. These results suggest the
geometric arrangements of complexes 2[PF6]2 and 4[PF6]
should be symmetric in solution. For 2[PF6]2, one possible
explanation is that the pendant amine is dynamic in solution,
which can shuttle between the two nickel centers. For complex
4[PF6], similar phenomenon should be also observed at the
pendant oxygen site. Meanwhile, the terminal chloride group
of 4[PF6] should shift to the bridging site between the two
complexes, the five-coordinate Ni1 centres all adopt
a
distorted tetragonal pyramid configuration with the
Ni1S1S2/Ni1P1P2 dihedral angles of 15.26(2)−25.55(6)°, and
the pendant S, O or N atom of the tridentate ligands occupies
the apical site. The Ni2 center of complexes 1[BPh4]2 and
2[PF6]2 both adopt
a square-planar arrangement with
Ni2S1S2/Ni2P3P4 dihedral angles of 3.65(6) and 8.45(4)°.
Differently, due to the presence of the terminal chloride group,
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Table 2 Electrochemical data of 1[PF6]2, 2[PF6]2, 3[PF6] and 4[PF6].a
View Article Online
DOI: 10.1039/C9DT04493K
1st E1/2red (V)
2nd E1/2red or Epred (V)
Overpotential (mV)c
b
Complex
icat/ip
CH2Cl2
−1.13
−1.10
−1.15
−1.19
MeCN
−1.23
CH2Cl2
MeCN
−1.49
1[PF6]2
2[PF6]2
3[PF6]
4[PF6]
−1.56
−1.40
−1.56
−1.42
6.9
500
420
510
490
−0.75, −0.95
−1.24
−1.17, −1.31
−1.47
11.6
4.6
−1.25
−1.41
8.9
aDinickel complexes (1 mM) recorded at scan rate of 100 mV/s with nBu4NPF6 (0.1 M) as supporting electrolyte. bicat is denoted as
c
the catalytic current at the 10 mM TFA in MeCN. Overpotentials were estimated by the standard potential for hydrogen
evolution from TFA (10 mM) in MeCN. All potentials are referred to the Fc+/0 redox couple.
the Ni2 centers of complexes 3[PF6] and 4[PF6] are in a
distorted tetragonal pyramid geometry with
Ni2S1S2/Ni2P3P4 dihedral angles of 21.54(4) and 21.65(2)°.
In addition, all Ni−P and Ni−Sbridge bond lengths in these
complexes locate in the common range of reported
complexes with a NiP2S2 subunit.19
The Ni1−S3 bond lengths of 2.391(2) and 2.392(2) Å in
1[BPh4]2 and 3[PF6] are significantly longer than those of
the Ni1−S1 and Ni1−S2 bonds. However, the Ni1−S3 bond
lengths in these two complexes still fall in the range of
common Ni−S bond length.20 It suggests the existence of
the strong bonding interaction between the nickel center
and the thioether sulfur atom, which means it is difficult
that the Ni1−S3 bond spontaneously breaks in the solution
state. This proposal is fully consistent with the experimental
fact that there are two inequivalent resonances with similar
intensity for dppe ligands in the 31P{1H} NMR spectra of
is irreversible. Differently, the two reduction waves of
complex 2[PF6]2 at Epred = −1.10 and −1.40 V versus Fc+/0 are
both irreversible. These complexes all undergo two
successive one-electron reduction processes assigned to
the NiIINiII/NiIINiI and NiIINiI/NiINiI couples,10a which are in
good agreement with the experimental fact that the
reduction peak currents of the two redox couples are close
to that of equivalent ferrocene (Fc) as an internal standard.
Notably, reduction potentials of complex 2[PF6]2 featuring
pendant amine are shifted positively compared with other
three complexes. Interestingly, when the solvent was
changed from CH2Cl2 to MeCN, there are three quasi-
reversible and an irreversible reduction processes observed
(Figure S35). These significant changes of the reduction
events in different solvents may be due to the formation of
a new species with coordinated MeCN. However, when we
revisited the 1H NMR spectrum of complex 2[PF6]2 in CD3CN,
no expected new species was detected compared with its
1H NMR spectrum in CD2Cl2. Based on above results, we
considered MeCN can reversibly bind to the nickel center
under electrochemical conditions, which generates an
equilibrium mixture of 2[PF6]2 and potential complex
2(MeCN)[PF6]2. That is why the CV curve of 2[PF6]2 in
CD3CN appears four reduction waves. The first and third
1[PF6]2 and 3[PF6]
.
In complex 2[PF6]2, the Ni−N distance is 2.475(3) Å, which
is obviously longer than common Ni−N bond lengths in
some nickel complexes21 and other metal-nitrogen bond
lengths in mononuclear metal complexes possessing
pendant nitrogen group.22 However, this bond distance is
remarkably shorter than the sum of the van der Waal’s radii
(3.18 Å). Therefore, there should be
a weak bond
interaction between the nickel center and the pendant
nitrogen atom in 2[PF6]2. The elongation of the Ni−N
distance is likely caused by steric hindrance between the
dppe ligand and the phenyl of the pendant amine group.
This weak coordination mode in this system provides the
possibility of the dynamic transformation in solution as
above mentioned. Similar situation should be also observed
in 4[PF6], because the Ni−O distance of 2.377(2) Å is beyond
the common range of the Ni−O bond length in previously
reported nickel complexes.23
Electrochemical studies of thiolated-bridged
dinickel complexes
Firstly, the redox properties of four thiolate-bridged
dinickel complexes in MeCN and CH2Cl2 were explored by
cyclic voltammetry and their electrochemical data were
summarized in Table 2 (see ESI for CV traces). In the cyclic
voltammograms of 1[PF6]2, 3[PF6] and 4[PF6] in CH2Cl2, the
first reduction event is quasi-reversible and the second one
Fig. 3 Cyclic voltammograms of 2[PF6]2 (1 mM in 0.1 M nBu4NPF6
in MeCN under Ar) with increments of TFA (0, 1, 2, 4, 8, 10, 20,
40 and 60 mM). Inset: icat/ip dependence with increasing acid
concentration at 100 mV s–1.
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ARTICLE
the overpotential is the other important metrics Vfoierw eArvtiaclleuOatnilninge
the catalytic activity of hydrogen eDvoOlIu: t1i0o.1n0.39D/Cu9eDTt0o449th3Ke
homoconjugation effect, TFA exhibits a significantly low pKa
value in MeCN, which obviously impacts the half-wave potential
of TFA reduction. Based on this understanding, we calculated
the overpotentials of these thiolate-bridged dinickel complexes
by the updated method reported by Helm and Appel.28 At 10
+
mM TFA in MeCN, ETFA = −0.68 V versus Fc+/0 29
,
and the
overpotential for 2[PF6]2 was calculated to be 0.42 V using Ecat/2
= −1.10 V versus Fc+/0 (Fig. 4). The overpotential of 2[PF6]2 is
slightly lower than those of complexes 1[PF6]2, 3[PF6] and 4[PF6].
Although inferior to some mononuclear nickel complexes
supported by phosphine ligands,30 complex 2[PF6]2 is obviously
superior to other previously reported thiolate-bridged dinuclear
complexes,10b,10c,10e,23c,27c,31 Superior catalytic performance of
2[PF6]2 suggests that the pendant amine group may behave as a
proton relay similar to some reported functional complexes for
hydrogen formation.32
Fig. 4 Determination of the overpotential of 2[PF6]2 (1 mM) in
the presence of 10 mM TFA in MeCN.
reduction waves are assigned to complex 2[PF6]2, and the
other two reduction processes belong to the proposed
species 2(MeCN)[PF6]2. Furthermore, a plot of the peak
current of reduction wave (ip) vs square root of scan rate
shows a linear correlation, implying diffusion-controlled
electrochemical reduction events.24
Conclusions
In summary, a series of biomimetic thiolate-bridged dinickel
complexes were designed and synthesized, which were all
proven as promoters for electrocatalytic hydrogen production.
Remarkably, the incorporation of a pendant amine into the
bridging dithiolate ligand as proton relay facilitates
enhancement of the catalytic activity at relatively low
overpotential in this dinickel system. It bodes well for future
work on catalytic transformations of other small molecules such
as N2, which may involve complicated multi-proton, multi-
electron reduction processes.
One step further, the electrocatalytic activity of these
complexes for proton reduction was explored by cyclic
voltammetry. As shown in Fig. 3, complex 2[PF6]2 was chosen as
a
representative electrocatalyst for the reduction of
MeCN
trifluoroacetic acid (TFA, pKa
= 12.65).25 Upon addition of 2
equiv. of TFA, a broad reductive peak appeared at −1.19 V. As
the acid concentration increased, peak current of this reduction
event increased linearly and the potential is shifted toward
more negative cathodic values. These observations are all
diagnostic to the electrocatalysis of proton reduction.
Furthermore, we carried out the bulk electrolysis of 1 mM
2[PF6]2 in the presence of 60 mM TFA at −1.20 V. After
electrolysis over a 0.5 h period, about 1.60 μmol of H2 was
detected by gas chromatography and 0.32 C charges had
passed through the cell corresponding to the theoretical H2
production of 1.68 μmol. Hence, the Faraday efficiency for
H2 was calculated to be 95%. In addition, the control
experiment that acid reduction in the absence of catalyst
was also conducted, however, no catalytic peak was
observed at corresponding potential. Moreover, the rinse
test was performed in which the working electrode was
rinsed several times with MeCN following the catalytic
cyclic voltammetry and then transferred to a fresh solution
of TFA in MeCN. No observation of catalytic peak suggests
the active species is molecule in solution not nickel deposit
on the electrode.
Experimental
General Procedures.
All manipulations were routinely conducted under inert gas
atmosphere in a glovebox or by standard Schlenk techniques. All
solvents were dried and distilled over an appropriate drying
agent under argon. Precursor complex [NiCl2(dppe)]15 and
16
ligand N(Ph)(CH2CH2SH)2 were prepared according to
literature procedures. NH4PF6 (Aldrich), NaBPh4 (Aldrich),
S(CH2CH2SH)2 (Aldrich), O(CH2CH2SH)2 (Aldrich), CF3COOH
(Aldrich), nBuLi (Aldrich), NaH (Aldrich), nBu4NCl (Aldrich)
and nBu4NPF6 (Aldrich) were used without further
purification.
Spectroscopic Measurements.
1
The H, 13C{1H} and 31P{1H} NMR spectra were recorded on a
To further assess the catalytic activity, we next explored some
important evaluating parameters.26 Current enhancement of the
reductive peak is an essential indicator for comparison of
catalytic capacity of metal complexes.27 Hence, we first
calculated the ic/ip ratio value in the presence of 10 mM TFA,
which means the ratio of catalytic current (ic) to reductive peak
current (ip). As listed in Table 2, complex 2[PF6]2 shows the best
catalytic activity, whose ic/ip ratio value is up to 11.6. In addition,
Brüker 400 Ultra Shield spectrometer. The 1H, 1H-COSY spectrum
was recorded on a Brüker 500 Ultra Shield spectrometer. The
chemical shifts (δ) are given in parts per million relative to
1
CD2Cl2 (5.32 ppm for H, 53.84 ppm for 13C). Infrared spectra
were recorded on a NEXVSTM FT-IR spectrometer. ESI-HRMS
were recorded on a HPLC/Q-Tof micro spectrometer. Elemental
analyses were performed on a Vario EL analyzer.
This journal is © The Royal Society of Chemistry 2019
Dlton Trans., 2019, 48, 1-8 | 5
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Dalton Transactions
X-ray Crystallography Procedures.
of 3[PF6] (125 mg, 0.10 mmol) in MeCN (10 mVLi)ewwAartsicleaOddnleinde
NH4PF6 (16 mg, 0.10 mmol) with vigorous stirring for 1 h. The
DOI: 10.1039/C9DT04493K
The data were obtained on a Brüker SMART APEX CCD
diffractometer with graphite-monochromated Mo Kα radiation
solution was evaporated to dryness under reduced pressure.
The residue was washed with n-hexane for three times,
extracted with CH2Cl2 (20 mL) and then dried in vacuo. The
(λ
= 0.71073 Å). Empirical absorption corrections were
performed using the SADABS program.33 Structures were solved
by direct methods and refined by full-matrix least-squares based
on all data using SHELX97.34 All of the non-hydrogen atoms were
refined anisotropically. All of the hydrogen atoms were
generated and refined in ideal positions and refined with fixed
isotropic displacement parameters.
3
2
product, [(dppe)Ni(μ-1κ SSS':2κ SS-tpdt)Ni(dppe)][PF6]2 (1[PF6]2,
111 mg, 0.08 mmol, 80%), was obtained as a brownish-yellow
powder.
Using the similar synthetic method except for the replacement
of NH4PF6 by NaBPh4, 1[BPh4]2 was smoothly obtained in 65%
yield. Crystals suitable for X-ray diffraction were obtained from a
saturated CH2Cl2 solution layered with n-hexane at room
Electrochemistry.
1
temperature. H NMR (400 MHz, CD2Cl2, ppm): δ 0.01-2.05 (m,
Electrochemical measurements were recorded using a BAS-
100W electrochemical potentiostat at a scan rate of 100 mV/s.
Electrochemical experiments were carried out in a three-
electrode cell under argon at room temperature. The working
electrode was a glassy carbon disk (diameter 3 mm), the
reference electrode was a nonaqueous Ag/Ag+ electrode, the
auxiliary electrode was a platinum wire, and the supporting
electrolyte was 0.1 M nBu4NPF6 in MeCN. Electrocatalysis studies
were performed by the stepwise addition of different amounts
of CF3COOH (TFA) with microsyringe. Controlled potential
electrolysis experiments were carried out in a two-compartment,
gas-tight, H-type electrolysis cell under argon at room
temperature. The working electrode was a glassy carbon disk
(diameter 3 mm), the reference electrode was a nonaqueous
Ag/Ag+ electrode, and the auxiliary electrode was a platinum
sheet. All potentials reported herein are quoted relative to the
8H, Ph2P(CH2)2PPh2), 2.15-2.68 (m, 8H, tpdt-H), 6.82-7.24 (m,
40H, BPh4-H), 7.32-7.93 (m, 40H, Ph-H). 13C{1H} NMR (100 MHz,
CD2Cl2, ppm): δ 26.3 (PCH2), 29.9 (PCH2), 34.6 (SCH2), 36.6 (SCH2),
122.2 (BPh4-C), 126.0 (BPh4-C), 129.8 (Ph-C), 130.4 (Ph-C), 132.8
(Ph-C), 133.6 (Ph-C), 134.0 (Ph-C), 136.3 (BPh4-C). 31P{1H} NMR
(162 MHz, CD2Cl2, ppm): δ 58.5(s), 63.4(s). IR (film; cm−1): 3059,
1432, 1101, 702, 524. ESI-HRMS: Calcd. For [1]2+ 532.0601;
Found 532.0604. Anal. Calcd. For C104H96B2Ni2P4S3: C, 73.26; H,
5.68. Found: C, 73.59; H, 5.76.
2
2
Preparation of [(dppe)Ni(μ-1κ SS:2κ SS-npdt)Ni(dppe)][PF6]2
(2[PF6]2)
At −78 °C, a suspension of Li2(npdt) (npdt = N(Ph)(CH2CH2S−)2) in
n
THF (10 mL), prepared by the reaction of BuLi (0.36 mL, 2.2 M
solution in n-hexane, 0.80 mmol) and N(Ph)(CH2CH2SH)2 (85 mg,
0.40 mmol) at 0 °C , was transferred via a cannula to a cooled
solution of [NiCl2(dppe)] (422 mg, 0.80 mmol) in THF (15 mL)
and then NH4PF6 (130 mg, 0.80 mmol) was added to the reaction
solution. The mixture was vigorously stirred at −78 °C to room
temperature. After all volatiles were removed under vacuum,
the residues were extracted with CH2Cl2 (3×10 mL) and then
dried in vacuo. Finally, the residues were washed with THF (3×10
+
FeCp2/FeCp2 couple. Gas analysis was performed with a
Techcomp GC7900 gas chromatography instrument with argon
as the carrier gas and a thermal conductivity detector.
3
2
Preparation of [(dppe)Ni(μ-1κ SSS':2κ SS-tpdt)Ni(dppe)][PF6]2
(1[PF6]2)
Method A: At room temperature, a suspension of Na2(tpdt) (tpdt
= S(CH2CH2S−)2) in acetone (10 mL), prepared by the reaction of
NaH (12 mg, 0.50 mmol) and S(CH2CH2SH)2 (39 mg, 0.25 mmol),
was transferred via a cannula to a solution of [NiCl2(dppe)] (264
mg, 0.50 mmol) in acetone (15 mL) and then NH4PF6 (82 mg,
0.50 mmol) was added to the reaction solution. The mixture was
vigorously stirred for 2 h. After all volatiles were removed under
vacuum, the residues were extracted with CH2Cl2 (3×10 mL) and
then dried in vacuo. Finally, the residues were washed with THF
2
2
mL). The product, [(dppe)Ni(μ-1κ SS:2κ SS-npdt)Ni(dppe)][PF6]2
(2[PF6]2, 408 mg, 0.29 mmol, 73%), was obtained as a brownish-
red powder. Crystals suitable for X-ray diffraction were obtained
from a saturated CH2Cl2 solution layered with n-hexane at room
1
temperature. H NMR (400 MHz, CD2Cl2, ppm): δ 1.35 (br, 4H,
Ph2P(CH2)2PPh2), 2.22-2.68 (br, 8H, npdt-CH2), 2.98 (br, 4H,
Ph2P(CH2)2PPh2), 6.49-7.23 (m, 5H, NPh-H), 7.32-8.09 (m, 40H,
PPh-H). 13C{1H} NMR (100 MHz, CD2Cl2, ppm): δ 27.0 (PCH2), 31.0
(SCH2), 32.7 (NCH2), 119.3 (NPh-C), 123.8 (NPh-C), 130.0 (PPh-C),
130.1 (PPh-C), 132.4 (PPh-C), 132.8 (PPh-C), 133.4 (PPh-C), 134.1
(PPh-C), 151.5 (NPh-C). 31P{1H} NMR (162 MHz, CD2Cl2, ppm): δ
3
2
(3×10
mL).
The
product
[(dppe)Ni(μ-1κ SSS':2κ SS-
tpdt)Ni(dppe)][PF6]2 (1[PF6]2, 231 mg, 0.17 mmol, 68%), was
obtained as a brownish-yellow powder. 1H NMR (400 MHz,
CD2Cl2, ppm): δ 0.18-2.65 (m, 8H, Ph2P(CH2)2PPh2), 2.51-2.63 (m,
8H, tpdt-H), 7.36-8.01 (m, 40H, Ph-H). 13C{1H} NMR (100 MHz,
CD2Cl2, ppm): δ 27.0 (PCH2), 30.0 (PCH2), 34.8 (SCH2), 36.5 (SCH2),
129.7 (Ph-C), 130.3 (Ph-C), 133.1 (Ph-C), 133.4 (Ph-C), 133.6 (Ph-
C), 133.9 (Ph-C). 31P{1H} NMR (162 MHz, CD2Cl2, ppm): δ −144.3
−
−144.3 (septet, PF6 ), 54.4 (s). IR (film; cm−1): 1438, 1101, 840,
693, 526. ESI-HRMS: Calcd. For [2]2+ 561.5952; Found 561.5931.
Anal. Calcd. For C62H61F12NNi2P6S2: C, 52.61; H, 4.34; N, 0.99.
Found: C, 52.69; H, 4.79; N, 1.29.
3
2
Preparation
of
[(dppe)Ni(t-Cl)(μ-1κ SSS':2κ SS-
−
(septet, PF6 ), 57.8 (s), 63.1 (s). IR (film; cm−1): 3060, 2927, 1437,
tpdt)Ni(dppe)][PF6] (3[PF6])
1109, 841, 695, 517. ESI-HRMS: Calcd. For [1]2+ 532.0601; Found
532.0626. Anal. Calcd. For C56H56F12Ni2P6S3·THF: C, 50.45; H, 4.52.
Found: C, 50.61; H, 4.62.
Method A: At room temperature, a suspension of Na2(tpdt) in
acetone (10 mL), prepared by the reaction of NaH (10 mg, 0.40
mmol) and S(CH2CH2SH)2 (31 mg, 0.20 mmol), was transferred
via a cannula to a solution of [NiCl2(dppe)] (211 mg, 0.40 mmol)
Method B: At room temperature, to a brownish-yellow solution
6 | Dlton Trans., 2019, 48, 1-8
This journal is © The Royal Society of Chemistry 2019
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Dalton Transactions
Dalton Transactions
ARTICLE
in acetone (10 mL) and then NH4PF6 (33 mg, 0.20 mmol) was
added to the reaction solution. The mixture was vigorously
stirred for 2 h. After all volatiles were removed under vacuum,
the residues were extracted with CH2Cl2 (3×10 mL) and then
This work was supported by the National Natural Science
Foundation of China (Nos. 21690064, 21571026), the
Program for Changjiang Scholars and Innovative Research
Team in University (No. IRT13008), the “111” project of the
Ministry of Education of China and the Fundamental
View Article Online
DOI: 10.1039/C9DT04493K
3
2
dried in vacuo. The product, [(dppe)Ni(t-Cl)(μ-1κ SSS':2κ SS-
tpdt)Ni(dppe)][PF6] (3[PF6], 175 mg, 0.14 mmol, 70%), was
obtained as a brownish-yellow powder. Crystals suitable for X-
ray diffraction were obtained from a saturated MeCN solution
Research
Funds
for
the
Central
Universities
(DUT19RC(3)013).
1
layered with Et2O at room temperature. H NMR (400 MHz,
Notes and references
CD2Cl2, ppm): δ 0.05-3.26 (m, 8H, Ph2P(CH2)2PPh2), 2.21-2.79 (br,
8H, tpdt-H), 7.25-8.33 (m, 40H, Ph-H). 13C{1H} NMR (100 MHz,
CD2Cl2, ppm): δ 27.9 (PCH2), 29.5 (PCH2), 34.5 (SCH2), 35.6 (SCH2),
127.9 (Ph-C), 129.3 (Ph-C), 129.6 (Ph-C), 130.8 (Ph-C), 131.5 (Ph-
C), 133.7(Ph-C), 134.2 (Ph-C), 134.5 (Ph-C). 31P{1H} NMR (162
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Calcd. For [3]+ 1099.0890; Found 1099.0873. Anal. Calcd. For
C56H56ClF6Ni2P5S3·Et2O: C, 54.55; H, 5.04. Found: C, 54.61; H,
4.84.
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of 1[PF6]2 (136 mg, 0.10 mmol) in MeCN (10 mL) was added
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extracted with THF (20 mL) and then dried in vacuo. The product,
[(dppe)Ni(t-Cl)(μ-1κ SSS':2κ SS-tpdt)Ni(dppe)][PF6] (3[PF6]2, 50
mg, 0.04 mmol, 40%), was obtained as a brownish-yellow
powder.
3
2
5
3
2
Preparation
of
[(dppe)Ni(t-Cl)(μ-1κ SSO:2κ SS-
opdt)Ni(dppe)][PF6] (4[PF6])
6
7
At −78 °C, a suspension of Li2(opdt) (opdt = O(CH2CH2S−)2) in THF
(10 mL), prepared by the reaction of nBuLi (0.45 mL, 2.2 M
solution in n-hexane, 1.0 mmol) and O(CH2CH2SH)2 (69 mg, 0.50
mmol) at 0 °C , was transferred via a cannula to a cooled solution
of [NiCl2(dppe)] (528 mg, 1.0 mmol) in THF (15 mL) and then
NH4PF6 (82 mg, 0.50 mmol) was added to the reaction solution.
The mixture was vigorously stirred at −78 °C to room
temperature. After all volatiles were removed under vacuum,
the residues were extracted with CH2Cl2 (3×10 mL) and then
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8
9
3
2
dried in vacuo. The product, [(dppe)Ni(t-Cl)(μ-1κ SSO:2κ SS-
opdt)Ni(dppe)][PF6] (4[PF6], 437 mg, 0.35 mmol, 70%), was
obtained as a brownish-red powder. Crystals suitable for X-ray
diffraction were obtained from a saturated CH2Cl2 solution
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1
layered with n-hexane at room temperature. H NMR (400 MHz,
CD2Cl2, ppm): δ 1.45-2.18 (br, 8H, opdt-CH2), 2.72-3.57 (br, 8H,
Ph2P(CH2)2PPh2), 7.35-7.95 (m, 40H, Ph-H). 13C{1H} NMR (100
MHz, CD2Cl2, ppm): δ 27.4 (PCH2), 28.1 (PCH2), 31.8 (SCH2), 67.2
(OCH2), 128.8 (Ph-C), 129.3 (Ph-C), 131.5 (Ph-C), 132.1 (Ph-C),
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−
δ −144.4 (septet, PF6 ), 53.8 (s). IR (film; cm−1): 3059, 2921, 1431,
1101, 840, 695, 524. ESI-HRMS: Calcd. For [4]+ 1083.1118; Found
1083.1136. Anal. Calcd. For C56H56ClF6Ni2OP5S2: C, 54.64; H, 4.59.
Found: C, 54.75; H, 4.42.
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Acknowledgements
This journal is © The Royal Society of Chemistry 2019
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DOI: 10.1039/C9DT04493K
A table of content entry
A bioinspired thiolate-bridged dinickel complex featuring a pendant amine realizes
electrocatalytic hydrogen evolution at a relatively low overpotential.