Heterodinuclear nickel(ii)-iron(ii) azadithiolates as structural and functional models for the active site of [NiFe]-hydrogenases

Article abstract of DOI:10.1039/d0ra04344c

To develop the biomimetic chemistry of [NiFe]-H₂ases, the first azadithiolato-bridged NiFe model complexes [CpNi{(μ-SCH₂)₂NR}Fe(CO)(diphos)]BF₄(5, R = Ph, diphos = dppv;6, 4-ClC₆H₄, dppv;7,

Full text of DOI:10.1039/d0ra04344c

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Heterodinuclear nickel(II)–iron(II) azadithiolates as
structural and functional models for the active site
of [NiFe]-hydrogenases†
Cite this: RSC Adv., 2020, 10, 32069
ab
Bei-Bei Liu,a Wen-Bo Liua and Zheng-Lei Tana
*
Li-Cheng Song,
To develop the biomimetic chemistry of [NiFe]-H2ases, the first azadithiolato-bridged NiFe model
complexes [CpNi{(m-SCH2)2NR}Fe(CO)(diphos)]BF4 (5, R ¼ Ph, diphos ¼ dppv; 6, 4-ClC6H4, dppv; 7, 4-
MeC6H4, dppv; 8, CO2CH2Ph, dppe; 9, H, dppe) have been synthesized via well-designed synthetic
routes. Thus, treatment of RN[CH2S(O)CMe]2 with t-BuONa followed by reaction of the resulting
intermediates RN(CH2SNa)2 with (dppv)Fe(CO)2Cl2 or (dppe)Fe(CO)2Cl2 gave the N-substituted
azadithiolato-chelated Fe complexes [RN(CH2S)2]Fe(CO)2(diphos) (1, R ¼ Ph, diphos ¼ dppv; 2, 4-
ClC6H4, dppv; 3, 4-MeC6H4, dppv; 4, CO2CH2Ph, dppe). Further treatment of 1–4 with nickelocene in
the presence of HBF4$Et2O afforded the corresponding N-substituted azadithiolato-bridged NiFe model
complexes 5–8, while treatment of 8 with HBF4$Et2O resulted in formation of the parent azadithiolato-
bridged model complex 9. While all the new complexes 1–9 were characterized by elemental analysis
and spectroscopy, the molecular structures of model complexes 6–8 were confirmed by X-ray
crystallographic study. In addition, model complexes 7 and 9 were found to be catalysts for H2
production with moderate icat/ip and overpotential values from TFA under CV conditions.
Received 15th May 2020
Accepted 20th August 2020
DOI: 10.1039/d0ra04344c
rsc.li/rsc-advances
reforming, the utilized catalyst is the expensive and relatively
scarce noble metal Pt. To develop the biomimetic chemistry of
Introduction
[NiFe]-H2ases and to search for the water splitting H2-producing
catalysts that contain the earth-abundant and cheap metals
such as iron and nickel present in the active site of [NiFe]-
H2ases, synthetic chemists have prepared a wide variety of
biomimetic models for [NiFe]-H2ases.9–29 However, among the
prepared models no one bears a bridging azadithiolato ligand
between their NiFe centers. In view of the important catalytic
role played by the central N atom of the bridging azadithiolato
ligand in the active site of [FeFe]-H2ases,30–32 we recently
launched a study on the preparation of the rst azadithiolato
ligand-containing [NiFe]-H2ase models. Fortunately, we have
[NiFe]-hydrogenases ([NiFe]-H2ases) are a class of biological
enzymes that catalyze the reversible redox reaction between
molecular H2 and protons in a variety of microorganisms such
as bacteria, archaea, and cyanobacteria.1–3 An X-ray crystallo-
graphic study revealed that the active site of [NiFe]-H2ases
features a Ni(tetracysteinate) moiety, in which the two cys-
teinate atoms are connected to the Fe atom of an Fe(CN)2(CO)
moiety. In addition, the oxidized state of [NiFe]-H2ases includes
an additional bridging oxygenous ligand, while the reduced
state of [NiFe]-H2ases contains an additional bridging hydride
ligand or a Ni–Fe metal–metal bond (Fig. 1).4–6
successfully prepared and characterized such
a type of
In recent years, molecular H2 has been considered as an
ideal energy source to replace fossil fuels to solve the current
energy shortage and environmental pollution problems. This is
because dihydrogen is a high energy-dense, clean-burning, and
sustainable energy.7–9 In addition, although H2 production can
be carried out in an industry scale from fossil fuel by petroleum
azadithiolato-bridged models in which each contains an aza-
dithiolato RN(CH2S)2 ligand bridged between their CpNi and
(diphos)(CO)Fe units (Fig. 2). In addition, the two representative
aDepartment of Chemistry, State Key Laboratory of Elemento-Organic Chemistry,
College of Chemistry, Nankai University, Tianjin 300071, China. E-mail: lcsong@
nankai.edu.cn
bCollaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300072, China
† Electronic supplementary information (ESI) available: Some electrocatalytic
plots/data (Fig. S1–S3/Table S1) and some IR/NMR spectra (Fig. S4–S23). CCDC
2001290–2001292. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/d0ra04344c
Fig. 1 Active site of [NiFe]-H2ases.
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addition, the IR spectra of 1–4 showed two absorption bands in
the range 2035–1968 cmꢀ1 for their two terminal carbonyls.
Synthesis and characterization of targeted [NiFe]-H2ase
models [CpNi{(m-SCH2)2NR}Fe(CO)(diphos)]BF4 (5–9)
Interestingly, the targeted N-substituted dithiolato-bridged NiFe-
based models 5–8 were found to be prepared by treatment of the
mononuclear Fe complexes 1–4 with nickelocene/HBF4$Et2O in
a mixed solvent CH2Cl2/Et2O at room temperature in 21%–62%
yields (Scheme 2). Further treatment of the N-substituted
dithiolato-bridged model complex 8 with HBF4$Et2O in a mixed
solvent CH2Cl2/PhOMe at room temperature gave rise to the
parent azadithiolato-bridged model 9 in 42% yield (Scheme 3). As
shown in Schemes 2 and 3, while model 9 can be formally viewed
as produced through the selective acidolysis of the N–C(]O)
bond rather than O–C(]O) bond of model 8, model complexes 5–
8 might be regarded as produced via two elementary reaction
steps. The rst step involves protonation of one anionic cyclo-
pentadienyl ligand in Cp2Ni to give cyclopentadiene and cyclo-
pentadienylnickel tetrauoroborate.35,36 The second step involves
coordination of the two S atoms in complexes 1–4 to Ni atom of
the in situ formed intermediate CpNi(BF4) followed by loss of one
CO ligand from 1–4 to give the nal products.
Fig. 2 Models of [NiFe]-H2ases reported in this article.
models have been found to be catalysts for proton reduction to
hydrogen under electrochemical conditions. Herein, we report
the interesting results obtained by this study.
Results and discussion
Synthesis and characterization of precursor complexes
[RN(CH2S)2]Fe(CO)2(diphos) (1–4)
The N-substituted azadithiolato ligand-chelated mononuclear
Fe complexes 1–4 utilized for the preparation of the targeted
[NiFe]-H2ase models could be prepared by one-pot reactions of
the bis(thioester) compounds RN[CH2S(O)CMe]2 (R ¼ Ph, 4-
ClC6H4, 4-MeC6H4, CO2CH2Ph) with tert-butoxysodium in THF
at ꢀ78 ꢁC followed by treatment of the resulting disodium
mercaptides RN(CH2SNa)2 with (dppv)Fe(CO)2Cl2 or (dppe)
Fe(CO)2Cl2 generated in situ by reaction of an acetone solution
of FeCl2 with the corresponding diphosphine and bubbling CO
gas33,34 in 18–35% yields (Scheme 1).
Model complexes 5–9 are also air-stable, brown-red solids.
They were characterized by the IR, 1H NMR, 13C{1H} NMR, and
Precursor complexes 1–4 are air-stable, brown-red solids and
have been characterized by elemental analysis and various
spectroscopic methods. For example, the 1H NMR spectra of 1–3
each displayed one singlet in the region 4.56–4.63 ppm for the
four H atoms in their azadithiolato methylene groups, whereas
1
the H NMR spectrum of 4 displayed a multiplet in the range
4.89–5.30 ppm for the four H atoms in its azadithiolato meth-
ylene groups. The 13C{1H} NMR spectra of 1–4 exhibited one or
two singlets in the range 200–214 ppm for their two terminal
carbonyl C atoms, whereas the 31P{1H} NMR spectra of 1–3
displayed one singlet at about 79 ppm for the two P atoms in
their dppv ligands and the 31P{1H} NMR spectrum of 4 exhibited
one singlet at 44.3 ppm for the two P atoms in its dppe ligand. In
Scheme 2 Synthesis of targeted models 5–8.
Scheme 1 Synthesis of precursor complexes 1–4.
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Table 1 Selected bond lengths (A˚) and angles (ꢁ) for 6–8
6
Ni(1)–Fe(1)
Ni(1)–S(1)
Ni(1)–S(2)
N(1)–C(28)
Ni(1)–S(1)–Fe(1)
Ni(1)–S(2)–Fe(1)
S(2)–Ni(1)–S(1)
P(1)–Fe(1)–P(2)
2.5033(9)
2.1783(13)
2.1770(11)
1.433(5)
68.87(4)
69.10(4)
90.34(4)
86.10(4)
Fe(1)–S(1)
Fe(1)–S(2)
Fe(1)–P(1)
Fe(1)–P(2)
S(2)–Fe(1)–S(1)
S(2)–Fe(1)–Ni(1)
S(1)–Fe(1)–Ni(1)
S(1)–Ni(1)–Fe(1)
2.2472(12)
2.2361(11)
2.2198(11)
2.2215(12)
87.09(4)
54.34(3)
54.26(3)
56.86(4)
Scheme 3 Synthesis of targeted model 9.
7
Ni(1)–Fe(1)
Ni(1)–S(1)
Ni(1)–S(2)
N(1)–C(33)
Ni(1)–S(1)–Fe(1)
Ni(1)–S(2)–Fe(1)
S(2)–Ni(1)–S(1)
P(1)–Fe(1)–P(2)
2.5071(9)
2.1826(16)
2.1795(14)
1.429(7)
68.88(5)
69.15(4)
90.31(5)
86.13(5)
Fe(1)–S(1)
Fe(1)–S(2)
Fe(1)–P(1)
Fe(1)–P(2)
S(2)–Fe(1)–S(1)
S(2)–Fe(1)–Ni(1)
S(1)–Fe(1)–Ni(1)
S(1)–Ni(1)–Fe(1)
2.2497(14)
2.2374(13)
2.2268(15)
2.2249(14)
87.15(5)
54.33(4)
54.30(4)
56.83(4)
31P{1H} NMR spectroscopic techniques and elemental analysis.
For instance, the IR spectra of 5–9 showed one strong absorp-
tion band in the region 1956–1939 cmꢀ1 for their terminal
carbonyls, which are close to that (1944 cmꢀ1) displayed by the
[NiFe]-H2ase from D. Vulgaris Miyazaki F.37 The 1H NMR spectra
of 5–9 displayed one singlet in the range 4.42–4.61 ppm for the
ve H atoms in their Cp rings. In addition, the 13C{1H} NMR
spectra of 5–9 exhibited one singlet in the region 206–220 ppm
for their terminal carbonyl C atoms, whereas the 31P{1H} NMR
spectra of 5–7 displayed one singlet in the region 82.7–83.9 ppm
for the two P atoms in their dppv ligands and those of 8 and 9
exhibited the corresponding signals in the range 78.1–79.1 ppm
for the two P atoms in their dppe ligands.
The molecular structures of the azadithiolato-bridged
complexes 6–8 were conrmed by X-ray crystallographic anal-
ysis. While Fig. 3–5 show their molecular structures, Table 1
lists their selected bond lengths and angles. As shown in
Fig. 3ꢀ5, the three biomimetic models are indeed the azadi-
thiolato ligand-containing complexes that are composed of one
monocation [CpNi{(m-SCH2)2NC6H4Cl-4}Fe(CO)(dppv)]+, [CpNi
{(m-SCH2)2NC6H4Me-4}Fe(CO)(dppv)]+, or [CpNi{(m-SCH2)2-
NCO2CH2Ph}Fe(CO)(dppe)]+ and one monoanion BF4ꢀ. Since
the molecular structures of 6 and 7 are virtually the same as that
of 8 (except that the 4-ClC6H4/dppv in 6 and 4-MeC6H4/dppv in 7
8
Ni(1)–Fe(1)
Ni(1)–S(1)
Ni(1)–S(2)
N(1)–C(34)
Ni(1)–S(1)–Fe(1)
Ni(1)–S(2)–Fe(1)
S(1)–Ni(1)–S(2)
P(1)–Fe(1)–P(2)
2.5036(9)
2.1807(13)
2.1819(12)
1.434(5)
69.04(4)
69.13(4)
89.93(4)
87.08(4)
Fe(1)–S(1)
Fe(1)–S(2)
Fe(1)–P(1)
Fe(1)–P(2)
S(2)–Fe(1)–S(1)
S(2)–Fe(1)–Ni(1)
S(1)–Fe(1)–Ni(1)
S(1)–Ni(1)–Fe(1)
2.2367(11)
2.2306(12)
2.2433(12)
2.2242(11)
87.28(4)
54.52(4)
54.43(3)
56.54(4)
are replaced by the CO2CH2Ph/dppe in 8), we just discuss the
structure of 8. In the monocation of 8, the Ni–Fe distance
˚
˚
(2.5036 A) is shorter than that (2.57 A) of the [NiFe]-H2ase from
D. Vulgaris Miyazaki F38 and the sum (2.56 A) of the Ni and Fe
˚
covalent radii,39 indicative of the presence of a Ni–Fe metal–
metal bond. In addition, the Ni–Fe distance of 8 is very close to
Fig. 3 Molecular structure of model complex 6 with ellipsoids drawn Fig. 4 Molecular structure of model complex 7 with ellipsoids drawn
at a 50% probability level. All hydrogen atoms and the anion BF4ꢀ are at a 50% probability level. All hydrogen atoms and the anion BF4ꢀ are
omitted for clarity.
omitted for clarity.
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studied by cyclic voltammetric techniques,40–46 there isn't any
studied result regarding the electrochemical and electro-
catalytic properties of the azadithiolato-bridged [NiFe]-H2ase
models, so far. In order to see whether the prepared
azadithiolato-bridged models 5–9 could have the electro-
catalytic H2-producing ability, we chose models 7 and 9 (note
that they have the same type of structures as 5, 6 and 8) as
representatives to study their electrochemical properties by CV
techniques using the supporting electrolyte n-Bu4NPF6 in MeCN
at a scan rate of 100 mVsꢀ1. As shown in Table 2 and Fig. 6,
complex 7 displayed one quasi-reversible reduction wave at
–1.07 V (due to its |Epc ꢀ EPa| ¼ 82 mV and ipc/ipa ¼ 1.05),47 one
irreversible reduction wave at ꢀ1.98 V and one irreversible
oxidation wave at 0.54 V, while complex 9 displayed one quasi-
reversible reduction wave at ꢀ1.15 V (due to its |Epc ꢀ EPa| ¼
103 mV and ipc/ipa ¼ 1.33),47 one irreversible reduction wave at
ꢀ1.99 V, and one irreversible oxidation wave at 0.45 V, respec-
tively. Previously, the electrochemical properties of the two pdt-
bridged analogues of 7 and 9, namely complexes [CpNi(pdt)
Fe(dppv)(CO)]BF4 (A) and [CpNi(pdt)Fe(dppe)(CO)]BF4 (B) (see
Table 2) were studied by cyclic voltammetry.40 Similar to this
case, the three redox events of 7 and 9 could be assigned to the
NiIIFeII/NiIFeII, NiIFeII/NiIFeI and NiIIFeII/NiIII/FeII couples,
respectively.
Fig. 5 Molecular structure of model complex 8 with ellipsoids drawn
at a 50% probability level. All hydrogen atoms and the anion BF4ꢀ are
omitted for clarity.
Having determined the electrochemical properties of 7 and
9, we further determined their cyclic voltammograms in the
presence and absence (for comparison) of triuoroacetic acid
(TFA) (pKMa eCN ¼ 12.7, EHꢁ A ¼ ꢀ0:89 V),48,49 in order to see
whether they could catalyze the proton reduction of TFA to give
H2. As shown in Fig. 7 and 8, when TFA was sequentially added
from 2 to 10 mM to MeCN solutions of 7 and 9, the current
heights of their original rst reduction waves were basically
unchanged. However, in contrast to this, the current heights of
their original second reduction waves were remarkably
increased. Apparently, such an observation features an elec-
trocatalytic proton reduction process from TFA to hydrogen.41,43
Note that through comparison of the reduction potentials Epc
and peak currents ip in the CVs of 7/9 with those in the CVs of
TFA without 7/9 (see Fig. S1 and Table S1 in the ESI†), models 7/
˚
that (2.5145 A) of the reported 1,3-propanedithiolato(pdt)-
bridged analogue [CpNi(pdt)Fe(dppe)(CO)]BF4.40 Owing to the
presence of a Ni–Fe metal–metal bond in 8, the Ni center could
be regarded as taking a “piano stool” geometry and the Fe
center adopts a pseudo-octahedral geometry. In addition, the
azadithiolato ligand in 8 is bridged between Ni and Fe atoms to
construct two fused six-membered rings. The six-membered
ring Fe1S1C33N1C34S2 shown in Fig. 3 possesses a boat
conformation,
whereas
the
six-membered
ring
Ni1S1C33N1C34S2 possesses a chair conformation. While the
N-substituted CO2CH2Ph group is attached to the common N1
atom of the two fused six-membered rings with an axial type of
bond, the terminal carbonyl ligand is bound to Fe1 atom in an
axial position of the pseudo-octahedral geometry of the Fe
center.
Electrochemical properties and electrocatalytic H2 production
catalyzed by model complexes 7 and 9
Although the electrochemical properties and electrocatalytic H2
production for some [NiFe]-H2ase models were previously
Table 2 Electrochemical data of 7/9a and A/Bb
Complex
Epc1/V
Epc2/V
Epa/V
7
9
ꢀ1.07
ꢀ1.15
ꢀ1.15
ꢀ1.16
ꢀ1.98
ꢀ1.99
ꢀ2.18
ꢀ2.15
0.54
0.45
0.72
0.70
A40
B40
a
All potentials are versus Fc/Fc+ in 0.1 M n-Bu4NPF6/MeCN at a scan rate
Fig. 6 Cyclic voltammograms of 7 and 9 (1.0 mM) in 0.1 M n-Bu4NPF6/
MeCN at a scan rate of 0.1 V sꢀ1. Arrows indicate the starting potential
and scan direction.
of 0.1 V sꢀ1
.
All potentials are versus Fc/Fc+ in 0.1 M n-Bu4NPF6/CH2Cl2
b
at a scan rate of 0.5 V sꢀ1
.
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reduction waves caused by their Fe(II)Fe(I) couples (see Fig. S2
and S3 in the ESI†). Obviously, such an observation demon-
strates an electrocatalytic proton reduction process catalyzed by
complexes 3 and 4. So, we might conclude that for mononuclear
Fe complexes 3 and 4, only Fe makes contribution to their
electrocatalytic proton reduction reactions caused by their
Fe(II)/Fe(I) couples, while for dinuclear models 7 and 9, both Ni
and Fe make contribution to the electrocatalytic proton reduc-
tion reactions occurred near their original second reduction
waves caused by their Ni(I)Fe(II)/Ni(I)Fe(I) couples.
To evaluate the electrocatalytic H2-producing ability of 7 and
9, we determined the ratio of the catalytic current (icat) to
reductive peak current (ip) in the absence of the added acid
(note that a higher icat/ip value indicates a faster catalysis).53 At
a concentration of 10 mM TFA, the icat/ip values of 7 and 9 were
calculated to be 6.0 and 5.3, respectively. It follows that the H2-
producing ability catalyzed by complex 7 (with a 4-MeC6H4N
group and a dppv ligand) is higher than that catalyzed by 9 (with
an NH group and a dppe ligand). However, in order to compare
the electrocatalytic H2-producing ability of the bridgehead N-
substituted complex 8 with its parent complex 9, we further
determined its icat/ip value by CV techniques to be 6.4 under the
same conditions. Therefore, the H2-producing ability of
complex 8 with a stronger electron-attracting PhCH2CO2N
group is higher than that of complex 9 with a weaker electron-
attracting NH group. In addition, at a 10 mM TFA concentra-
tion, the icat/ip values (5.3–6.4) of complexes 7–9 are higher than
those (ca. 4) of the previously reported analogues
[HNiFe(pdt)(dppe)(L)(CO)2]BF4 (L ¼ P(OPh)3, PPh3).54
Fig. 7 Cyclic voltammograms of 7 (1.0 mM) with TFA (0–10 mM) in
0.1 M n-Bu4NPF6/MeCN at a scan rate of 0.1 V sꢀ1
.
Since overpotential is also an important parameter for eval-
uating the electrocatalytic ability for a catalyst (note that a better
catalyst has a lower overpotential), we used Evans' method to
calculate the overpotentials of 7 and 9 according to the
following two equations:49,55
ꢀ
ꢀ
ꢀ
ꢀ
ꢁ
cat=2
ꢁ
ꢀ
Overpotential ¼ E
ꢀ EHA
(1)
ꢀ
ꢁ
ꢂ
2:303RT
EHꢁ A ¼ EHꢁ
ꢀ
pKa;HA
(2)
Fig. 8 Cyclic voltammograms of 9 (1.0 mM) with TFA (0–10 mM) in
0.1 M n-Bu4NPF6/MeCN at a scan rate of 0.1 V sꢀ1
þ=H2
F
.
where Ecat/2 is the catalytic half-wave potential and EHꢁ A is the
standard potential of the acid; R is the ideal gas constant 8.314 J
molꢀ1 Kꢀ1; T is the absolute temperature 298 K; F is the Fara-
day's constant 9.65 ꢂ 104 C molꢀ1; in addition, the pKa value of
9 were further proved to be catalysts for proton reduction. In
addition, during the proton reductions catalyzed by 7 and 9, the
original second reduction waves were positively shied by ca. 15
and 45 mV, respectively; this is most likely due to protonation of
the central N atoms in their bridged azadithiolato ligands.50
Recently, we51 and others52 reported that some mononuclear
Fe complexes can serve as catalysts for proton reduction to
hydrogen. In order to observe whether the mononuclear Fe
precursors of model complexes 5–8 (namely complexes 1–4)
could catalyze the proton reductions, we chose complexes 3 and
4 as representatives to determine their cyclic voltammograms in
the presence of TFA under the same conditions as those used
for dertermination of models 7 and 9. We found that when TFA
was sequentially added from 2 to 10 mM to MeCN solutions of 3
and 4, the current heights of the acid-induced reduction waves
were considerably increased near their only one original
TFA in MeCN and the standard redox potential EHꢁ þ
have been
=H2
previously determined to be 12.7 (ref. 48) and ꢀ0.14 V,49
respectively. According to eqn (2), the standard potential EHꢁ A
can be calculated to be ꢀ0.89 V and then according to eqn (1),
the overpotentials of 7 and 9 can be calculated to be 0.78 and
0.54 V, respectively (see Fig. 7 and 8). At a 10 mM acid
concentration, overpotentials of our azadithiolato-bridged
dinuclear NiFe complexes 7 (0.78 V) and 9 (0.54 V) by using
TFA as proton source are higher than those (ca. 0.5 V) of the
previously reported analogues [HNiFe(pdt)(dppe)(L)(CO)2]BF4
(L ¼ P(OPh)3, PPh3) using TFA as proton source.54 Particularly
worth noting is that in terms of the studied results regarding the
structure and catalytic function described above, complexes 7
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Table 3 Crystal data and structure refinement details for 6–8
6
7
8
Formula
C
P2S2
908.57
40H35BClF4FeNNiO
C41H38BF4FeNNiO
P2S2$CH2Cl2
973.08
C42H40BF4FeNNiO3
P2S2$CH2Cl2
1019.10
Mw
Cryst syst
Space group
Monoclinic
P121/c1
17.861(4)
11.063(2)
22.252(5)
90
108.15(3)
90
4178.2(16)
4
Monoclinic
P121/c1
17.8793(11)
11.0429(5)
22.4895(10)
90
108.615(6)
90
4208.0(4)
4
Triclinic
P1
ꢀ
˚
a (A)
10.113(2)
12.365(3)
18.383(4)
75.55(3)
87.96(3)
77.76(3)
2175.1(8)
2
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
g (ꢁ)
3
˚
V (A )
Z
Crystal size (mm)
0.05 ꢂ 0.04 ꢂ 0.03
0.06 ꢂ 0.05 ꢂ 0.03
0.20 ꢂ 0.18 ꢂ 0.12
Dc (g cmꢀ3
)
1.444
1.536
1.556
m (mmꢀ1
F(000)
)
1.088
1856
1.148
1992
1.117
1044
Rens collected
Rens unique
qmin/max (ꢁ)
Final R
49 278
9958
2.078/27.863
0.0693
43 090
8586
1.905/26.372
0.0712
26 271
10 291
1.144/27.834
0.0567
Final RW
0.1648
1.127
0.536/ꢀ0.576
0.1987
1.061
1.000/ꢀ1.039
0.1575
1.060
0.738/ꢀ1.255
GOF on F2
ꢀ3
˚
Drmax/min (e A
)
and 9 might be regarded as not only the structural models but anhydrous FeCl2 and other chemicals were commercially
also the functional models for the active site of [NiFe]-H2ases.
available and used as received. RN[CH2S(O)CMe]2 (R ¼ Ph,56 4-
ClC6H4,57 CO2CH2Ph,58 (dppv)Fe(CO)2Cl2 (dppv ¼ 1,2-bis(di-
phenylphosphino)ethene)35 and (dppe)Fe(CO)2Cl2 (dppe ¼ 1,2-
bis(diphenylphosphino)ethane)36) were prepared according to
the published procedures. While 1H, 13C{1H}, and 31P{1H} NMR
spectra were obtained on a Bruker Avance 400 NMR spectrom-
eter, IR spectra were recorded on a Bio-Rad FTS 135 infrared
spectrophotometer. Elemental analyses were performed on an
Elementar Vario EL analyzer. Melting points were determined
on a SGW X-4 microscopic melting point apparatus and were
uncorrected.
Summary and conclusion
We have synthesized the rst azadithiolato-bridged dinuclear
NiFe model complexes 5–8 and 9 by reactions of the N-
substituted azadithiolato-chelated mononuclear Fe complexes
1–4 with Cp2Ni/HBF4$Et2O and by reaction of the N-substituted
azadithiolato-bridged complex 8 with HBF4$Et2O, respectively.
Particularly worth noting is that the spectroscopic character-
ization of all the new model complexes 5–9 and the X-ray crys-
tallographic study on their three representative complexes 6–8
have proved that models 5–9 indeed contain an azadithiolato
ligand that is bridged between their NiFe centers to form
a buttery Ni(II)S2Fe(II) cluster core with a Ni–Fe metal–metal
bond. In addition, from the electrochemical and electrocatalytic
studies, the two representative models 7 and 9 have been found
to be catalysts for proton reduction to hydrogen using TFA as
the proton source under electrochemical conditions. Therefore,
according to the structural and functional similarity with the
active site of [NiFe]-H2ases, complexes 5–9 might be regarded as
the structural and functional models of [NiFe]-H2ases.
Preparation of [PhN(CH2S)2]Fe(CO)2(dppv) (1). A 250 mL
Schlenk ask was charged with FeCl2 (0.634 g, 5.0 mmol) in
100 mL acetone and dppv (1.98 g, 5.0 mmol) in 20 mL of THF.
The mixture was stirred at room temperature and bubbled with
CO gas for 4 h to give a (dppv)Fe(CO)2Cl2-containing brown-red
solution. Additionally, a 100 mL Schlenk ask was charged with
thioester PhN[CH2S(O)CMe]2 (2.02 g, 7.5 mmol), sodium tert-
butoxide (1.44 g, 15.0 mmol), and 40 mL of THF. The mixture
was stirred at ꢀ78 ꢁC for 4 h to give a PhN(CH2SNa)2-containing
pale yellow solution. To the in situ prepared (dppv)Fe(CO)2Cl2-
containing solution cooled to ꢀ78 ꢁC was slowly added the
prepared PhN(CH2SNa)2-containing solution and then the
ꢁ
Experimental section
General comments
mixture was stirred at ꢀ78 C for 5 h. Solvent was removed at
reduced pressure and the residue was subjected to column
chromatography (silica gel). Elution with CH2Cl2 developed
a red band, from which 1 (0.622 g, 18%) was obtained as
All reactions were carried out using standard Schlenk and
vacuum-line techniques under an atmosphere of argon. While
tetrahydrofuran (THF) and Et2O were distilled under Ar from
sodium/benzophenone ketyl, CH2Cl2 was distilled from CaH2. t-
BuONa, (h5-C5H5)2Ni, HBF4$Et2O (50–55% w/w in Et2O),
ꢁ
a brown-red solid, mp 93–94 C. Anal. calcd for C36H31FeNO2-
P2S2: C, 62.52; H, 4.52; N, 2.03. Found: C, 62.35; H, 4.65; N, 2.00.
IR (KBr disk): nC^O 2018 (w), 1973 (vs.) cmꢀ1 1H NMR (400
.
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MHz, CDCl3): 4.63 (s, 4H, CH2NCH2), 6.69–7.91 (m, 27H, CH] 5C6H5) ppm. 13C{1H} NMR (100 MHz, CDCl3): 24.6–57.0 (m,
CH, 5C6H5) ppm. 13C{1H} NMR (100 MHz, CDCl3): 48.6 (s, PCH2CH2P, OCH2), 66.7–68.0 (m, CH2NCH2), 127.2–154.7 (m,
CH2NCH2), 114.7–134.7 (m, C6H5), 146.3–150.8 (m, CH]CH), C6H5), 210.8 (s, C]O), 213.0, 213.3 (2s, C^O) ppm. 31P{1H}
211.8, 212.2 (2s, C^O) ppm. 31P{1H} NMR (162 MHz, CDCl3): NMR (162 MHz, CDCl3): 44.3 (s, FeP) ppm.
79.9 (s, FeP) ppm.
Preparation of [CpNi{(m-SCH2)2NPh}Fe(CO)(dppv)]BF4 (5).
Preparation of [4-ClC6H4N(CH2S)2]Fe(CO)2(dppv) (2). The While stirring, nickelocene (0.038 g, 0.2 mmol) was dissolved in
same procedure was followed as for the preparation of 1, except diethyl ether (5 mL) and then HBF4$Et2O (27.2 mL, 0.2 mmol)
that 4-ClC6H4N[CH2S(O)CMe]2 (2.28 g, 7.5 mmol) was used was added. Aer the mixture was stirred at room temperature
instead of PhN[CH2S(O)CMe]2. 2 (0.835 g, 23%) was obtained as for 15 min, a brown-red suspension was formed. To this
a brown-red solid, mp 145–146 ꢁC. Anal. calcd for C36H30- suspension was added a solution of 1 (0.138 g, 0.2 mmol) in
ClFeNO2P2S2: C, 59.56; H, 4.17; N, 1.93. Found: C, 59.47; H, CH2Cl2 (5 mL). The new mixture was stirred at room tempera-
1
4.21; N, 1.69. IR (KBr disk): nC^O 2020 (w), 1973 (vs.) cmꢀ1. H ture for 2 h, solvent was removed at reduced pressure to give
NMR (400 MHz, CDCl3): 4.57 (s, 4H, CH2NCH2), 6.61–7.78 (m, a residue, which was subjected to column chromatography
26H, CH]CH, 4C6H5, C6H4) ppm. 13C{1H} NMR (100 MHz, (silica gel), elution with CH2Cl2/acetone (v/v ¼ 10 : 1) developed
DMSO-d6): 48.0 (s, CH2NCH2), 115.5–144.1 (m, C6H5, C6H4), a major red band, from which 5 (0.052 g, 30%) was obtained as
150.2–150.6 (m, CH]CH), 212.2 (s, C^O) ppm. 31P{1H} NMR brown-red solid. Mp 220–221 ꢁC. Anal. calcd for C40H36BF4-
(162 MHz, CDCl3): 80.0 (s, FeP) ppm.
FeNNiOP2S2: C, 54.96; H, 4.15; N, 1.60. Found: C, 54.86; H,
Preparation of [4-MeC6H4N(CH2S)2]Fe(CO)2(dppv) (3). In 3.98; N, 1.62. IR (KBr disk): nC^O 1956 (m) cmꢀ1. 1H NMR (400
order to prepare 3, we should rst prepare its precursor, the new MHz, acetone-d6): 4.43 (s, 5H, C5H5), 4.68, 5.47 (dd, J ¼ 14.6 Hz,
bis(thioester) 4-MeC6H4N[CH2S(O)CMe]2. (i) Preparation of 4- 4H, CH2NCH2), 6.77–8.13 (m, 25H, 5C6H5), 8.71, 8.86 (2s, 2H,
MeC6H4N[CH2S(O)CMe]2: a 250 mL three-necked ask was CH]CH) ppm. 13C{1H} NMR (100 MHz, DMSO-d6): 54.4 (s,
charged with 4-MeC6H4NH2 (10.7 g, 100 mmol), 37% aqueous CH2NCH2), 94.5, 95.9 (2s, C5H5), 115.4–142.2 (m, C6H5), 149.3–
formaldehyde (24.2 mL, 320 mmol), thioacetic acid (14.6 mL, 149.9 (m, CH]CH), 205.9 (s, C^O) ppm. 31P{1H} NMR (162
200 mmol), and ethanolꢁ(100 mL). While stirring, the mixture MHz, CDCl3): 82.7 (s, FeP) ppm.
was heated to about 60 C and stirred at this temperature for
Preparation of [CpNi{(m-SCH2)2NC6H4Cl-4}Fe(CO)(dppv)]BF4
3 h. To the resulting mixture was added 100 mL of ice-water to (6). The same procedure was followed as for the preparation of
give some precipitates. The precipitates were collected by 5, except that 2 (0.145 g, 0.2 mmol) was utilized instead of 1.
ltration, washed with some cold ethanol, and nally dried Complex 6 (0.113 g 62%) was obtained as a brown-red solid, mp
ꢁ
under vacuum to give 4-MeC6H4N[CH2S(O)CMe]2 (25.6 g, 90%) 197–198 C. Anal. calcd for C40H35BClF4FeNNiOP2S2: C, 52.88;
ꢁ
as a white solid, mp 66–67 C. Anal. calcd for C13H17NO2S2: C, H, 3.88; N, 1.54. Found: C, 52.84; H, 4.01; N, 1.62. IR (KBr disk):
1
55.10; H, 6.05; N, 4.95. Found: C, 55.35; H, 6.30; N, 4.66. IR (KBr nC^O 1952 (m) cmꢀ1. H NMR (400 MHz, acetone-d6): 4.47 (s,
disk): nC]O 1688 (vs.) cmꢀ1. 1H NMR (400 MHz, CDCl3): 2.27 (s, 5H, C5H5), 4.64, 5.49 (dd, 4H, J ¼ 15.4 Hz, CH2NCH2), 7.32–8.13
3H, CH3C6H4), 2.36 (s, 6H, 2CH3C]O), 5.10 (s, 4H, CH2NCH2), (m, 24H, 4C6H5, C6H4), 8.73, 8.88 (2s, 2H, CH]CH) ppm. 13C
6.68–7.10 (m, 4H, C6H4) ppm. 13C{1H} NMR (100 MHz, CDCl3): {1H} NMR (100 MHz, CD2Cl2): 52.6, 54.1 (2s, CH2NCH2), 94.8,
20.5 (s, CH3C6H4), 31.4 (s, CH3C]O), 53.1 (s, CH2NCH2), 114.7– 96.5 (2s, C5H5), 117.0–142.4 (m, C6H5, C6H4), 149.4–151.3 (m,
141.6 (m, C6H4), 196.1 (s, C]O) ppm. (ii) Preparation of 3: the CH]CH), 210.9 (s, C^O) ppm. 31P{1H} NMR (162 MHz, CDCl3):
same procedure was followed as for the preparation of 1, except 83.9 (s, FeP) ppm.
that 4-MeC6H4N[CH2S(O)CMe]2 (2.13 g, 7.5 mmol) was
Preparation of [CpNi{(m-SCH2)2NC6H4Me-4}Fe(CO)(dppv)]
employed in place of PhN[CH2S(O)CMe]2. 3 (0.741 g, 21%) was BF4 (7). The same procedure was followed as for the preparation
ꢁ
obtained as a brown-red solid, mp 113–114 C. Anal. calcd for of 5, except that 3 (0.141 g, 0.2 mmol) was employed in place of
C
37H33FeNO2P2S2: C, 62.98; H, 4.71; N, 1.99. Found: C, 62.72; H, 1. Complex 7 (0.038 g, 21%) was obtained as a brown-red solid,
4.95; N, 2.05. IR (KBr disk): nC^O 2035 (w), 1973 (vs.) cmꢀ1. H mp 210–211 ꢁC. Anal. calcd for C41H38BF4FeNNiOP2S2: C, 55.45;
NMR (400 MHz, acetone-d6): 2.18 (s, 3H, CH3), 4.56 (s, 4H, H, 4.31; N, 1.58. Found: C, 55.23; H, 4.28; N, 1.34. IR (KBr disk):
CH2NCH2), 6.92–7.80 (m, 24H, 4C6H5, C6H4), 8.17–8.31 (m, 2H, nC^O 1955 (s) cmꢀ1. 1H NMR (400 MHz, acetone-d6): 2.12 (s, 3H,
CH]CH) ppm. 13C{1H} NMR (100 MHz, DMSO-d6): 17.7 (s, CH3), 4.42 (s, 5H, C5H5), 4.67, 5.41 (dd, 4H, J ¼ 13.0 Hz,
CH3), 45.9 (s, CH2NCH2), 113.1–131.6 (m, C6H5, C6H4), 144.7– CH2NCH2), 7.14–8.13 (m, 24H, 4C6H5, C6H4), 8.69–8.84 (m, 2H,
145.8 (m, CH]CH), 200.1 (s, C^O) ppm. 31P{1H} NMR (162 CH]CH) ppm. 13C{1H} NMR (100 MHz, CD3CN): 20.1 (s, CH3),
1
MHz, CDCl3): 77.9 (s, FeP) ppm.
54.8 (s, CH2NCH2), 96.1 (s, C5H5), 117.0–141.5 (m, C6H5, C6H4),
Preparation of [PhCH2O2CN(CH2S)2]Fe(CO)2(dppe) (4). The 150.5–151.2 (m, CH]CH), 219.5 (s, C^O) ppm. 31P{1H} NMR
same procedure was followed as for the preparation of 1, except (162 MHz, CDCl3): 83.7 (s, FeP) ppm.
that PhCH2O2CN[CH2S(O)CMe]2 (2.46 g, 7.5 mmol) was used in
Preparation of [CpNi{(m-SCH2)2NCO2CH2Ph}Fe(CO)(dppe)]
place of PhN[CH2S(O)CMe]2. 4 (1.32 g, 35%) was obtained as BF4 (8). The same procedure was followed as for the preparation
ꢁ
a brown-red solid, mp 91–92 C. Anal. calcd for C38H35FeNO4- of 5, except that 4 (0.150 g, 0.2 mmol) was used instead of 1.
P2S2: C, 60.73; H, 4.69; N, 1.86. Found: C, 60.42; H, 4.94; N, 1.66. Complex 8 (0.052 g, 28%) was obtained as a brown-red solid, mp
IR (KBr disk): nC^O 2013 (vs.), 1968 (vs.); nC]O 1700 (vs.) cmꢀ1
.
121–122 ꢁC. Anal. calcd for C42H40BF4FeNNiO3P2S2: C, 54.00; H,
1H NMR (400 MHz, CDCl3): 3.02–4.56 (m, 6H, PCH2CH2P, 4.32; N, 1.50. Found: C, 54.27; H, 4.43; N, 1.58. IR (KBr disk):
OCH2); 4.89–5.30 (m, 4H, CH2NCH2); 7.30–7.76 (m, 25H, nC^O 1940 (vs.); nC]O 1704 (vs.) cmꢀ1 1H NMR (400 MHz,
.
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CDCl3): 3.03–4.05 (m, 6H, PCH2CH2P, OCH2), 4.54 (s, 5H, C5H5), method. Details of crystal data, data collections, and structure
4.98–5.31 (m, 4H, CH2NCH2), 7.12–7.86 (m, 25H, 5C6H5) ppm. renements are summarized in Table 3.
13C{1H} NMR (100 MHz, CD3CN): 29.5, 29.8 (2s, PCH2CH2P),
50.7 (s, CH2NCH2), 69.5 (s, OCH2), 96.5 (s, C5H5), 128.9–154.1
Conflicts of interest
(m, C6H5), 211.0 (s, C]O), 216.4 (s, C^O) ppm. 31P{1H} NMR
(162 MHz, CDCl3): 78.3, 79.1 (2s, FeP) ppm.
There are no conicts of interest to declare.
Preparation of [CpNi{(m-SCH2)2NH}Fe(CO)(dppe)]BF4 (9). To
a solution of 8 (0.093 g, 0.1 mmol) in CH2Cl2 (5 mL) were added
PhOMe (0.5 mL) and HBF4$Et2O (408 mL, 3.0 mmol). The Acknowledgements
mixture was stirred at room temperature for 4 h to give a brown-
We are grateful to the National Natural Science Foundation of
China (21772106) and the Ministry of Science and Technology
of China (973 program 2014CB845604) for nancial support.
red solution. Solvents were removed to give a residue, which was
subjected to column chromatography (silica gel). Elution with
CH2Cl2/acetone developed a major red band, from which 9
(0.034 g, 42%) was obtained as a brown-red solid, mp 170 ꢁC
(dec). Anal. calcd for C34H34BF4FeNNiOP2S2: C, 51.04; H,
4.28; N, 1.75. Found: C, 50.81; H, 4.56; N, 1.70. IR (KBr disk):
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