Versatile Reactivity of CH3CN-Coordinated Nickel-Iron Heterodimetallic Complexes with Cp? Ligand on Diazadithiolate (N2S2) or Dithiadithiolate (S4) Platforms

Article abstract of DOI:10.1002/ejic.201500304

By using 3,7-dithianonane-1,9-dithiol (S₄) and N,N′-dimethyl-N,N′-bis(2-mercaptoethyl)ethylenediamine (N₂S₂) as ligands, two CH₃CN-coordinated nickel-iron heterodimetallic complexes, [NiS₄Fe(CH₃CN)Cp?][PF₆] (1a) and [NiN₂S₂Fe(CH₃CN)Cp?][PF₆] (1b) (Cp? = η⁵-C₅Me₅), were synthesized and characterized spectroscopically and crystallographically. Reactivities of 1a and 1b towards CO, CH₂Cl₂, HBF₄, and NOPF₆ have also been investigated. A series of complexes coordinated with small molecules [NiS₄Fe(L)Cp?][PF₆]_n [n = 1, L = Cl (2a), CO (3a); n = 2, L = NO (4a)], [NiN₂S₂Fe(L)Cp?][PF₆] [L = Cl (2b), CO (3b)] were obtained. We also studied the redox properties of 2a, 2b, 3a, and 3b by cyclic voltammetry in CH₂Cl₂ and in CH₃CN solution. Complex 3a was found to be an excellent electrocatalyst for proton reduction from HBF₄.

Full text of DOI:10.1002/ejic.201500304

FULL PAPER
DOI:10.1002/ejic.201500304
Versatile Reactivity of CH3CN-Coordinated Nickel-Iron
Heterodimetallic Complexes with Cp* Ligand on
Diazadithiolate (N2S2) or Dithiadithiolate (S4) Platforms
Dawei Yang,[a] Yang Li,[a] Linan Su,[a] Baomin Wang,[a] and
Jingping Qu*[a]
Keywords: Nickel / Iron / S ligands / Bioinorganic chemistry / Enzyme models / Electrocatalysis
By using 3,7-dithianonane-1,9-dithiol (S4) and N,NЈ-dimeth-
yl-N,NЈ-bis(2-mercaptoethyl)ethylenediamine (N2S2) as li-
gands, two CH3CN-coordinated nickel-iron heterodimetallic
complexes coordinated with small molecules [NiS4Fe(L)-
Cp*][PF6]n [n = 1, L = Cl (2a), CO (3a); n = 2, L = NO (4a)],
[NiN2S2Fe(L)Cp*][PF6] [L = Cl (2b), CO (3b)] were obtained.
We also studied the redox properties of 2a, 2b, 3a, and 3b by
cyclic voltammetry in CH2Cl2 and in CH3CN solution. Com-
plex 3a was found to be an excellent electrocatalyst for pro-
ton reduction from HBF4.
complexes,
[NiS4Fe(CH3CN)Cp*][PF6]
(1a)
and
[NiN2S2Fe(CH3CN)Cp*][PF6] (1b) (Cp* = η5-C5Me5), were
synthesized and characterized spectroscopically and crystal-
lographically. Reactivities of 1a and 1b towards CO, CH2Cl2,
HBF4, and NOPF6 have also been investigated. A series of
[Ni(xbsms)Fe(CO)Cp][BF4] {H2xbsms = 1,2-bis[(4-mer-
capto-3,3-dimethyl-2-thiabutyl)benzene]}, which has not
only an Ni(μ-S)2Fe structural unit but also a Cp ligand on
the iron center.[8c] However, because the Cp ligand occupies
three coordination sites of the metal center, extension of the
reactivity and insights into the mechanism are limited when
strong, small molecule ligands such as CO coordinate to
the metal center. The introduction of labile ligands such as
CH3CN to the {MCp*} fragment can contribute to acti-
vation and conversion of other small molecules. In our pre-
vious work, we reported thiolate-bridged diiron complexes
Introduction
Nickel-iron heteronuclear complexes form the active sites
of many important enzymes,[1] such as [NiFe]-
hydrogenase,[2] carbon monoxide dehydrogenase,[3] and
acetyl coenzyme A synthase (Figure 1).[4] [NiFe]-
hydrogenases are indispensable for hydrogen metabolism by
catalyzing reversibly the oxidation of H2 and reduction of
protons at high rates under mild conditions.[5] Crystallo-
graphic studies of the oxidized inactive form of [NiFe]-
hydrogenase in Desulfovibrio gigas indicate that the Ni cen-
ter is ligated by four cysteinyl thiolates linked to an Fe(CN)2-
(CO) fragment by two bridging cysteinyl thiolates and an
undetermined oxygen ligand (H2O, OH–, O2–).[2a,6] A di-
verse range of structural models for [NiFe]-hydrogenase fea-
turing Ni(SR)2Fe cores complemented by CO, CN, or NO
ligands have been described.[7] In contrast, the construction
of functional model complexes by replacing the
{Fe(CO)(CN)2} moiety with electron-rich {RuCp*},[8a]
{RuCp} (Cp = η5-C5H5),[8b] or {FeCp}[8c] fragments is also
important to develop new electrocatalysts. In this respect,
Fontecave and Artero synthesized an NiFe heteronuclear
complex as an electrocatalyst for hydrogen production,
[a] State Key Laboratory of Fine Chemicals, School of
Pharmaceutical Science and Technology, Faculty of Chemical,
Environmental and Biological Science and Technology, Dalian
University of Technology
2 Linggong Road, Dalian 116024, P. R. of China
E-mail: qujp@dlut.edu.cn
Figure 1. (a) Active site of [NiFe]-hydrogenase; (b) reduced, active
form of the carbon monoxide dehydrogenase C-cluster; (c) cluster
of acetyl coenzyme A synthase.
http://ceb.dlut.edu.cn/index.html
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500304.
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with Cp* bearing two CH3CN ligands [Cp*Fe(μ-SEt)2- CH3CN-coordinated bis(μ-thiolato)NiFe complex reported
(CH3CN)2FeCp*][PF6]2, which can react with N2H4 and by Ogo’s group (2300 cm–1).[18] This result may be due to
C2H2 as nitrogenase substrates.[9] Furthermore, the reactiv- the weaker donor ability of Tm* [Tm* = tris(3,5-dimethyl-
ity with O2 and CO was realized by NiRu complexes with 1-pyrazolyl)methane] compared with the Cp* ligand. The
an {Ru(CH3CN)Cp*} framework.[8a,10] Recently, Ogo and spectral characterizations of 1a described above are consis-
co-workers reported a functional [NiFe]-based model bear- tent with its solid-state structure, which was confirmed by
ing a CH3CN ligand that can heterolytically activate X-ray diffraction analysis (Figure 2).
hydrogen to form a hydride complex.[11] Thus, the synthesis
and reactivity of CH3CN-coordinated NiFe complexes con-
taining Cp* is an interesting topic.
Coordination, activation, and catalytic conversion of
various small molecules by synthetic diiron complexes bear-
ing thiolate and Cp* ligands is one of the research fields in
our group.[12–14] In our previous work, we obtained several
[NiFe] and [FePd] heterodimetallic complexes containing
the Cp*Fe fragment.[13] Recently, we prepared a diiron
nitrogenase model complex [Cp*Fe(μ-bdt)FeCp*] (bdt =
benzene-1,2-dithiolate) that can realize the conversion of
hydrazine into ammonia along the pathway: HN=NH Ǟ
HN–NH2 Ǟ NH2 Ǟ NH3.[14] These results indicate that
the Cp*Fe unit is not only exploited as a building block,
but that it also plays an important role in the process of
small-molecule coordination and conversion. Herein, we
synthesized two [NiFe]-hydrogenase model complexes with
a Cp* bearing a CH3CN ligand [NiS4Fe(CH3CN)Cp*][PF6]
(1a) and [NiN2S2Fe(CH3CN)Cp*][PF6] (1b). A series of
Ni-Fe complexes containing CO, Cl, NO ligands were syn-
Figure 2. Molecular structure of the monocation in 1a. Thermal
ellipsoids are shown at 50% probability. Hydrogen atoms and
–
counteranion PF6 are omitted for clarity.
Crystallographic analysis of 1a reveals that the NiS4 unit
adopts a square-planar geometry, that the Cp*Fe unit has
a piano-stool geometry, and that these are linked through
two bridging thiolate ligands. The Ni···Fe distance of
3.182(2) Å, which is longer than that of isolated oxidized
form [NiFe]-hydrogenase (2.9 Å), suggests the absence of
metal–metal bonding.[2a,6] The Fe center of 1a features a
CH3CN ligand with an Fe–N distance of 1.916(4) Å and
an Fe–N–C angle of 171.2(3)°. The Fe–N distance of 1a is
distinctly shorter than that of the reported CH3CN-coordi-
nated bis(μ-thiolato)NiFe complex with Tm* as ligand
[2.247(4) Å][18] and is close to that of the CH3CN-coordi-
nated complex with three triethyl phosphite ligands
[1.955(3) Å].[11] The Ni1–S1–Fe1 and Ni1–S2–Fe1 angles of
1a are 90.55(5)° and 90.44(5)°, respectively. The Ni(μ2-S)2-
Fe unit of 1a is folded with a dihedral angle of 42.94(5)°,
which is much smaller than those observed in the enzymes
isolated from D. fructosovorans and D. gigas.[6,19,20]
1
thesized and characterized by H NMR, ESI-HRMS, IR,
X-ray crystallographic analysis. From this series, [NiS4-
Fe(CO)Cp*][PF6] (3a) is demonstrated to be a good electro-
catalyst for proton reduction from HBF4 in CH3CN.
Results and Discussion
Synthesis of Complex [NiS4Fe(CH3CN)Cp*][PF6] (1a)
Displacement of two CH3CN ligands of [Cp*Fe-
(CH3CN)3][PF6][15] by one NiS4,[16] which is considered as
a bidentate metallodithiolate ligand,[17] gave [NiS4Fe-
(CH3CN)Cp*][PF6] (1a) as a CH3CN-coordinated thiolate-
bridged heterodimetallic complex (Scheme 1). The 1H
NMR spectrum of 1a exhibits a singlet at δ = 1.55 ppm
for Cp* methyl protons. A sharp singlet at δ = 1.96 ppm
is assigned to methyl protons of coordinated CH3CN. The
resonances of the S4 ligand were distributed in a narrow
range of δ = 2–3 ppm. The IR spectrum of 1a shows a weak
Reactivity of Complex 1a towards Small Molecules
Given the lability of CH3CN as a ligand, we investigated
CϵN absorbance of the CH3CN stretching vibration band the reactivity of complex 1a with some small molecules.
at 2226 cm–1. This value is clearly lower than that of the Complex 1a was unstable in some organic solvents (such as
Scheme 1. Synthesis of complex 1a.
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CH2Cl2, CH3OH, and acetone). Complex 1a was dissolved not react with excess CO to afford the dicarbonyl nickel-
in CH2Cl2 to afford Cl-coordinated heterodimetallic com- iron compound by cleavage of one Fe–S bond. The 1H
pound [NiS4Fe(Cl)Cp*][PF6] (2a) in 85% yield by cleaving NMR spectrum of 3a shows one sharp singlet at δ =
the C–Cl bond of the solvent (Scheme 2). Complex 2a is a 1.67 ppm assignable to Cp* methyl protons. Complex 3a is
1
paramagnetic species at room temperature as judged by H diamagnetic, presumably consisting of low-spin NiII and
NMR spectroscopic analysis. The EPR spectrum of com- FeII ions. The IR spectrum of 3a shows a strong band at
plex 2a displays a characteristic signal (g = 2.16286). An 1897 cm–1 attributed to a terminal CO stretching vibration.
outstanding peak with m/z = 510.0153 (calcd. 509.9512) in This value is distinctly lower than the 2143 cm–1 for a free
the ESI-HR mass spectrum has a diagnostic isotopic distri- CO molecule,[22] which suggests an increased transfer of
bution, which suggests the presence of a chloride atom. back-bonding electron density from iron to π* orbitals of
Crystallographic analysis of 2a is consistent with the afore- CO. This value is distinctly lower than those of some Ni-Fe
mentioned spectroscopic characterization (Figure 3). In our model complexes with a CO ligand.[23] The ESI-HR mass
previous work, we reported that iron-sulfur clusters with spectrum of 3a shows a molecular ion peak at m/z =
Cp* as auxiliary ligand can realize the C–Cl activation of 502.8621 (calcd. 502.9773) for [3a – PF6]+. The solid-state
RCl.[12a] This complex is similar to four chloride-containing structure of 3a was determined by X-ray analysis (Figure 4).
[NiFe]-based complexes with different S4 ligands synthe- The iron center features a CO ligand with an Fe1–C7 dis-
sized by Lubitz and co-workers, which were obtained by tance of 1.753(9) Å. The C–O bond length of 1.155(9) Å is
reaction of [Cp*FeCl] with NiS4 precursors.[21]
comparable to that of free CO (1.128 Å).[22] The Fe–C–O
angle of 179.5(6)° is nearly linear. The Ni–Fe distance
[3.111(5) Å] of 3a is slightly shorter by 0.071 Å compared
with that of complex 1a.
Figure 4. Molecular structure of the monocation in 3a. Thermal
–
ellipsoids are shown at 50% probability. Hydrogen atoms and PF6
Scheme 2. Reactivity of complex 1a towards small molecules.
are omitted for clarity.
Complex 1a reacted rapidly with NOPF6 to give the oxi-
dative addition product [NiS4Fe(NO)Cp*][PF6]2 (4a)
(Scheme 2). The singlet at δ = 1.81 ppm was assigned to
1
Cp* methyl protons of 4a in the H NMR spectrum. The
IR spectrum of 4a shows one intense absorption at
1816 cm–1, which is assigned to the stretching vibration of
the terminal nitrosyl ligand. A slight decrease of this value
in comparison with that of free NO (1870 cm–1)[24] suggests
low electron density from the iron center back-donation to
the NO π* orbitals, which is accompanied by a slight weak-
ening of the NO triple bond. The NO frequency is clearly
higher than the 1725, 1767 cm–1 of [(NO)Ni{(μ-SCH2-
CH2)2S}Fe(NO)2][25] and the 1663, 1624 cm–1 of [Ni(L)-
Figure 3. Molecular structure of the monocation in 2a. Thermal
ellipsoids are shown at 50% probability. Hydrogen atoms and
–
counteranion PF6 are omitted for clarity.
Fe(NO)2] (L
=
N,NЈ-diethyl-3,7-diazanonane-1,9-di-
thiolate).[26] The ESI-HR mass spectrometric data also pro-
When 1a was stirred under 1 atm CO from –40 °C to vides further evidence for 4a, in which a molecular ion peak
ambient temperature in CH3CN, the violet solution grad- with m/z = 252.4912 (calcd. 252.4918) corresponds to [4a –
ually became reddish brown. The analytically pure complex (PF6)2]2+. Figure 5 depicts the molecular structure of 4a de-
of [NiS4Fe(CO)Cp*][PF6] (3a) was obtained in 78% yield termined by X-ray crystallography. The N–O distance of
as red-brown crystalline solid (Scheme 2). Complex 3a did 1.154(4) Å is longer than that in free NO (1.14 Å),[24] but
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shorter than those of the two dinitrosyl Ni-Fe complexes diate increase of the band at 2231 cm–1 (νCN of 1b). The
discussed above,[25,26] which is consistent with the above IR above spectral characterizations of 1b are all consistent with
data. The NO ligand bound to the iron center is slightly its solid-state structure, which was confirmed by X-ray dif-
bent [the Fe–N–O angle is 173.1(4)°], thus NO should be fraction analysis (Figure 6).
best described as NO+ in this nearly linear M–N–O moiety.
Figure 5. Molecular structure of the dication in 4a. Thermal ellip-
–
soids are shown at 50% probability. Hydrogen atoms and PF6 are
omitted for clarity.
Although complex 1a can react with, CO, CH2Cl2, and
NOPF6 as described above, it was unreactive towards H2
and organosilanes (Et3SiH, Ph2SiH2) as hydrogenic surro-
gates. Reactions of 1a with hydrogen transfer agents such
as LiBEt3H or NaBH4 led to decomposition into unknown
products. Upon treatment of complex 1a with proton
Scheme 3. Synthesis and reactivity of complex 1b with N2S2 as li-
gand.
acid HBF4, the restructured trinuclear nickel complex
1
[NiS4NiS4Ni][BF4]2 was obtained. The H NMR spectrum
shows only S4 ligand resonances in the range of δ = 2.5–
4.0 ppm, and no Cp* signal, which indicates that decompo-
sition of 1a had occurred. The ESI-HR mass spectrum
shows a signal at m/z = 313.8992, which confirms that the
product is a dication complex containing three nickel atoms
and two S4 ligands. According to the above data, we specu-
lated that the structure of [NiS4NiS4Ni][BF4]2 should be
similar to that of the previously reported trinuclear nickel
complex,[27] in which three nickel centers are in a line, and
the middle nickel center adopts a distorted square-planar
structure with two NiS4 fragments as ligands.
Figure 6. Molecular structure of the monocation in 1b. Thermal
Synthesis and Reactivity of 1b
ellipsoids are shown at 50% probability. Hydrogen atoms and
–
counteranion PF6 are omitted for clarity.
To examine the effects of the nickel building block on
the physical properties of the NiFe complex, we replaced
Crystallographic analysis of 1b reveals that although the
NiS4 with NiN2S2.[27] Different from the synthesis of com- environments of Ni atoms in 1a and 1b are different, they
plex 1a, the assembly reaction of [Cp*Fe(CH3CN)3][PF6] all contain an NiS2Fe butterfly core with the Fe center in a
and NiN2S2 was reversible, even upon warming to 65 °C distorted octahedral arrangement and the Ni center in a
(Scheme 3). Complex 1b was not successfully isolated, but square-planar arrangement. The Ni···Fe distance of 1b is
1
was obtained in 50% yield as confirmed by H NMR spec- 3.035(2) Å, which is close to that found in the isolated oxid-
troscopic analysis of the products. The H NMR spectrum ized form of [NiFe]-hydrogenase (2.9 Å)[2a,6] but is shorter
1
featured two resonances at δ = 1.46 and 2.43 ppm for Cp* than those in two other CH3CN-coordinated [NiFe]-
methyl protons and NCH3 of the N2S2 ligand of 1b. The hydrogenase mimics[11,18] and complex 1a [3.182(2) Å] (see
synthesis of 1b was also monitored by IR spectroscopy in Table 1). The Fe center of 1b has a CH3CN ligand with
situ. The addition of 1 equiv. of NiN2S2 to a solution of an Fe–N distance of 1.921(4) Å and an Fe–N–C angle of
[Cp*Fe(CH3CN)3][PF6] at ambient temperature resulted in 168.3(4)°, which are similar to those of 1a [1.916(4) Å,
an immediate decrease in the stretching frequency at 171.2(3)°]. However, the Fe–N distance is shorter than
2278 cm–1 {νCN of [Cp*Fe(CH3CN)3][PF6]} and an imme- those of the reported CH3CN-coordinated Ni–Fe com-
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Table 1. Selected bond lengths [Å] and angles [°] of CH3CN-coordinated [NiFe]-heterodinuclear complexes.
Complex
Ni–Fe
Fe–S1
Fe–S2
Fe–N[a]
Fe–N–C[b]
S1–Fe–S2
Ref.
[NiL1Fe(CH3CN){P(OEt)3}3][BPh4]2
3.3189(6)
3.3925(9)
3.182(2)
3.035(2)
2.341(1)
2.465(1)
2.306(2)
2.336(2)
2.340(1)
2.496(1)
2.309(2)
2.351(2)
1.955(3)
2.247(4)
1.916(4)
1.921(4)
170.3(3)
173.7(4)
171.2(3)
168.3(4)
74.49(4)
72.82(4)
77.58(5)
79.32(5)
[c]
[11]
[NiL2Fe(CH3CN)(Tm*)][BF4]2
[d]
[18]
[NiS4Fe(CH3CN)Cp*][PF6]
[NiN2S2Fe(CH3CN)Cp*][PF6]
this work
this work
[a] N represents the nitrogen atom of CH3CN. [b] N–C represents the CN functional group of CH3CN. [c] L1 = N,NЈ-diethyl-3,7-diazanon-
ane-1,9-dithiolato; L2 = N,NЈ-dimethyl-N,NЈ-bis(2,2-dimethyl-2-mercaptoethyl)-1,3-propanediamine. [d] Tm* = tris(3,5-dimethyl-1-pyr-
azolyl)methane.
plexes,[11,18] which suggests that CH3CN binds to the iron
center of 1b more tightly than to the other two complexes.
The Ni1–S1–Fe1 and Ni1–S2–Fe1 angles of 1b are
84.73(5)°, 84.54(6)°, which are smaller than those of 1a
[90.55(5)° and 90.44(5)°]. The dihedral angle of Ni(μ2-S)2Fe
unit of 1b is 51.15(5)°, which is wider by 8.21° compared
that of 1a.
Unfortunately, the reactivity of 1b related to the bio-
logical function of [NiFe]-hydrogenase was not achieved by
changing the auxiliary ligand. Treatment of 1b with a pro-
ton acid such as HBF4 did not afford the desired NiFe
Figure 7. Molecular structure of the monocation in 2b. Thermal
hydride-bridged complex, but instead resulted in the forma-
ellipsoids are shown at 50% probability. Hydrogen atoms and
–
tion of the intermolecular recombination trinuclear com-
counteranion PF6 are omitted for clarity.
1
plex [NiN2S2NiN2S2Ni][BF4]2, which was identified by H
NMR, IR and ESI-HR mass spectra (see the Supporting
Information). The data are consistent with the structure of
the cation reported by Reedijk and co-workers.[27] To our
knowledge, the reported nickel-iron hydride-bridged com-
plexes are all FeIINiII species.[28] Nickel atoms and iron
atoms of 1a and 1b are all formally at the +II oxidation
level. If they can react with HBF4 to give hydride complexes
([1aH]+, [1bH]+), two metal centers of them are all at the
+III oxidation level. To date, no analogous complexes have
been synthesized. Further, protonation of 1b was performed
after a one-electron reduction by CoCp2; however, this re-
sulted in the decomposition of complex 1b in the presence
of reductant.
sity at the two iron centers though variation in the coordi-
nation environments of the two nickel centers. ESI-HRMS
analysis of 3b shows a molecular ion peak with m/z =
483.0735 (calcd. 483.0737) for [3b – PF6]+. Figure 8 depicts
the structure of 3b as determined by X-ray crystallography.
The CO ligand is bound to the iron center with an Fe–C
distance of 1.727(7) Å, which is shorter by 0.026(2) Å com-
pared to that of 3a. The Fe–C–O angle of 172.0(6)° of 3b
is slightly bent, whereas the Fe–C–O arrangement of 3a is
approximately linear [179.5(6)°]. The most striking struc-
tural feature is the Ni–Fe distance of 2.954(1) Å, which
closely resembles those in the oxidized form of [NiFe]-
hydrogenase (2.9 Å).[2a,6] Upon replacing the S4 ligand with
an N2S2 ligand, the Ni–Fe bond length distinctly shortens
[from 3.111(5) Å to 2.954(1) Å]. The Ni–Fe distances of
[NiFe]-hydrogenase and models containing Cp‡ (Cp or
Cp*) are shown in Table 2.
When the synthesis of 1b was carried out in CH2Cl2, Cl-
coordinated complex [NiN2S2Fe(Cl)Cp*][PF6] (2b) was ob-
tained by C–Cl bond activation of CH2Cl2. This complex
1
is also paramagnetic as judged by the H NMR spectrum,
in which several broad or sharp peaks are present over the
range δ = 20 to –50 ppm. In the EPR spectrum of complex
2b, a diagnostic signal at g = 2.07954 appeared. The exis-
tence of a chloride subunit was confirmed by ESI-HRMS
and crystallographic analyses (Figure 7).
When the synthesis of 1b was performed under 1 atm of
CO rather than argon in CH3CN, the monocarbonyl com-
plex [NiN2S2Fe(CO)Cp*][PF6] (3b) formed as a red-brown
1
microcrystalline powder in excellent yield. The H NMR
spectrum of 3b shows a singlet at δ = 1.59 ppm assignable
to Cp* protons. The proton signals of two NCH3 groups of
the N2S2 ligand appear upfield at δ = 2.54 ppm. The solid-
state IR spectrum of 3b exhibits a strong carbonyl stretch
band at 1901 cm–1, which is slightly increased compared
with that observed for 3a (1897 cm–1). A shift of 4 cm–1 in
Figure 8. Molecular structure of the cation in 3b. Thermal ellip-
–
soids are shown at 50% probability. Hydrogen atoms and PF6 are
CO frequencies suggests a slight change in the electron den- omitted for clarity.
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Table 2. Ni–Fe distances of [NiFe]-hydrogenase and [NiFe]-based wave and leads to the appearance of a catalytic process at
complexes with Cp or Cp* ligands.
slightly more positive potentials, which moves to lower po-
tentials for higher acid/catalyst ratios. Remarkably, curve-
crossing was observed for 3a upon reversing the scan fol-
lowing the reduction at –1.52 V, resulting in a current
response at –1.06 V (see Figure 9). Such curve-crossing
anomalies were reported and rationally explained by
Darensbourg.[30] According to the above observations, a
proposed mechanism for the electrochemical proton re-
duction was speculated as reported for a similar system.[8c]
After adding a few equivalents of HBF4, the quasireversible
monoelectronic process at –1.511 V changed into an
irreversible process. This result suggests that 3a underwent
a chemical reaction following an electron reduction. After
elimination of CO, a bridging hydride species should be
generated as the active intermediate in the catalytic cycle,
the potential of which appeared at –1.378 V and shifted to
–1.52 V at high acid/catalyst ratio. The –1.06 V event may
be assigned to a one-electron reduction of diprotonated spe-
cies; after that, dihydrogen is released, and 3a is regener-
ated. Compared with 3a, complex 3b was not a good cata-
lyst, which may be attributed to the difference in ligands.
Complex and hydrogenase
Ni–Fe [Å]
Ref.
[NiL1Fe(CO)Cp][PF6][a]
[NiL2FeCp][I][b]
[NiS4Fe(CO)Cp*][PF6] (3a)
[NiN2S2Fe(CO)Cp*][PF6] (3b)
Oxidized form D. gigas
3.1727(6), 3.1529(7)
2.539(4)
3.111(5)
2.954(1)
2.90
[29]
[29]
this work
this work
[6]
[19]
Oxidized unready form D. fructosovorans 2.92
[a] L1 = 3,7-dithianonane-1,9-dithiol. [b] L2 = N,NЈ-dimethyl-N,NЈ-
bis(2-mercaptoethyl)bis(aminoethyl) sulfide.
Redox Properties and Proton Reduction Electrocatalysis
The redox properties of 2a, 2b, 3a, and 3b in CH2Cl2
solution were studied by cyclic voltammetry (Table 3).
Complex 2a displays one monoelectronic quasireversible re-
duction process at –0.981 V vs. Fc+/0, which is attributed
to the FeIIFeIII redox couple. This result is consistent with
recently reported data (–0.88 to –0.91 V vs. Fc+/0) for ana-
logues of 2a.[21] When replacing the S4 ligand with the N2S2
ligand, the FeIIFeIII redox couple of 2b shifts to slightly
positive potential at –0.927 V. The redox behaviors of com-
pounds 2a and 2b are not affected by changes in the coordi-
nation sphere of the nickel centers. Cyclic voltammetry of
3a in CH2Cl2 solution undergoes a reversible redox wave at
0.404 V and an irreversible redox wave at –1.507 V. How-
ever, complex 3b shows only one reversible process at
0.274 V. Changing the solvent to CH3CN, cyclic voltamme-
try of 3a exhibits an irreversible wave at 0.24 V and a quasi-
reversible wave at –1.468 V. The cyclic voltammetry of 3b
displays a reversible process at 0.118 V vs. Fc+/0 in CH3CN,
followed by another irreversible wave at –1.865 V vs. Fc+/0
.
Table 3. Selected electrochemical properties (V vs. Fc+/0) of 3a, 3b
in CH2Cl2 or CH3CN. Conditions: 0.1 m complex, scan rate
100 mVs–1.
Complex
First redox couple
Second redox couple
Ea
Ec
E1/2
Ea
Ec
E1/2
3a[a]
3b[a]
3a[b]
3b[b]
0.286 0.193 0.240 –1.424
–1.511
–1.865
–1.507
–
–1.468
Figure 9. Cyclic voltammogram of 3a in the presence of varying
equivalents of HBF4. Conditions: ca. 1 mm 3a in CH3CN solution,
0.1 m nBu4NPF6, scan rate 100 mm/s.
0.158 0.077 0.118
0.461 0.347 0.404
0.339 0.208 0.274
–
–
–
–
–
–
[a] CH3CN solution. [b] CH2Cl2 solution.
CH3CN-coordinated complexes 1a and 1b self-assembled
to give trinuclear complexes upon protonation, so no cata-
lytic activity in electrochemical proton reduction for them
Conclusions
We have synthesized two CH3CN-coordinated [NiFe]-
was observed. In addition, Cl-coordinated complexes 2a based complexes (1a, 1b) by employing either S4 or N2S2 as
and 2b were unstable in the presence of proton acids, which ligands. Treatment of 1a or 1b with CO, CH2Cl2, or NOPF6
are also not suitable candidates for electrocatalyst of proton gave the corresponding small-molecule-coordinated com-
reduction. However, complex 3a is an active catalyst for the plexes 2a, 2b, 3a, 3b, and 4a. In addition, we investigated
reduction of protons to H2, as indicated by electrochemical the cyclic voltammetry of 2a, 2b, 3a, 3b in CH2Cl2 and in
measurements. This cyclic voltammetry is not modified in CH3CN. The solvent has a marked effect on the redox
the presence of the weak acid AcOH. Upon addition of properties of complexes 3a and 3b. Complex 3a was shown
1 equiv. of the stronger acid HBF4, the process observed at to be an excellent electrocatalyst for proton reduction. Un-
–1.511 V vs. Fc+/0 becomes irreversible, and a new process fortunately, neither 1a nor 1b activated H2 under mild con-
is observed at –1.378 V vs. Fc+/0. Adding increasing ditions to give hydride-bridged nickel-iron complexes. This
amounts of HBF4 reinforces the irreversibility of the first is because the valences of the metal centers are high, which
Eur. J. Inorg. Chem. 2015, 2965–2973
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1
perature. H NMR (400 MHz, CD3CN, 25 °C): δ = 2.93–2.97 (m,
2 H, S4-H), 2.68–2.76 (m, 4 H, S4-H), 2.56–2.62 (m, 3 H, S4-H),
2.20–2.24 (m, 4 H, S4-H), 1.96 (s, 3 H, CH3CN), 1.55 (s, 15 H,
prevented the formation of bridging hydrides. Future efforts
will focus on the design and synthesis of low-valent nickel-
iron heterodimetallic complexes by modulating ligands for
dihydrogen activation to realize catalytic transformation of
unsaturated small molecules.
Cp*-CH ) ppm. IR (KBr): ν = 2226 (CN) cm–1. HRMS (ESI):
˜
3
calcd. for [C17H29FeNiS4O2]+ [M – CH3CN + O2]+ 506.9722; found
506.9501. C19H32F6FeNNiPS4 (660.98): calcd. C 34.46, H 4.87, N
2.12, S 19.37; found C 34.52, H 4.81, N 2.20, S 19.43.
Preparation of [NiS4Fe(Cl)Cp*][PF6] (2a): A solution of [NiS4-
Fe(CH3CN)Cp*][PF6] (1a) (305 mg, 0.46 mmol) in CH2Cl2 (20 mL)
was stirred at room temperature for 2 h. All volatiles were removed
under vacuum, and the resulting black powder was washed with
diethyl ether. After drying under vacuum, 2a (259 mg, 85%) was
obtained as a purplish microcrystalline powder. Crystals of 2a suit-
able for X-ray diffraction were obtained from a saturated CH2Cl2
solution layered with Et2O at room temperature. HRMS (ESI):
calcd. for [C17H29FeNiS4Cl]+ [M]+ 509.9512; found 510.0153. EPR:
g = 2.16286. C17H29ClF6FeNiPS4 (654.92): calcd. C 31.10, H 4.45,
S 19.53; found C 31.05, H 4.50, S 19.59.
Experimental Section
General Procedures: All manipulations were typically carried out
under argon by using standard Schlenk-line and glovebox tech-
niques. All solvents were dried and distilled from an appropriate
drying agent under argon. Ligand S4 (3,7-dithianonane-1,9-di-
thiol),[31] N2S2 [N,NЈ-dimethyl-N,NЈ-bis(2-mercaptoethyl)ethylene-
diamine],[32] complexes NiS4,[16] NiN2S2,[27] and [Cp*Fe(CH3CN)3]-
[PF6][15] were prepared according to previous reports. HBF4·Et2O
(Aldrich), CoCp2 (Aldrich), and NOPF6 (Aldrich) were used with-
out further purification. 1H NMR spectra were recorded with a
Bruker 400 Ultra Shield spectrometer. IR spectra were recorded
with a NEXVSTM FTIR spectrometer. In situ IR monitoring was
carried out with a Mettler-Toledo ReactIR 15 FTIR spectrometer
equipped with an MCT detector, connected to an AgX fiber con-
duit probe having a SiComp ATR crystal. Elemental analyses were
performed with a Vario EL analyzer. ESI-MS data were recorded
with a UPLC/Q-Tof micro spectrometer. EPR spectra were re-
corded at room temperature with a JEOL JES-FE1bX spectrome-
ter. Electrochemical measurements were recorded with a BAS-
100W electrochemical potentiostat. Cyclic voltammetry experi-
ments were carried out with 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 CH2Cl2 or CH3CN.
All potentials reported herein are quoted relative to the FeCp2/
Preparation of [NiS4Fe(CO)Cp*][PF6] (3a): Under CO, a solution
of [NiS4Fe(CH3CN)Cp*][PF6] (1a) (199 mg, 0.3 mmol) in CH3CN
(20 mL) was stirred from –40 °C to room temperature for 3 h. Dur-
ing this time, the solution changed gradually from violet to brown-
ish red. The solvent was removed under reduced pressure, and the
brown residue was washed with Et2O. The crystalline powder of 3a
was dried under vacuum. Yield: 153 mg (0.23 mmol, 78%). Crystals
of 3a suitable for X-ray diffraction experiment were obtained from
a saturated CH2Cl2 solution layered with Et2O at room tempera-
1
ture. H NMR (400 MHz, CD2Cl2, 25 °C): δ = 3.18–3.24 (m, 2 H,
S4-H), 3.01–3.06 (m, 2 H, S4-H), 2.89–2.92 (m, 2 H, S4-H), 2.74–
2.80 (m, 3 H, S4-H), 2.39–2.43 (m, 2 H, S4-H), 2.26–2.33 (m, 2 H,
S -H), 1.67 (s, 15 H, Cp*-CH ) ppm. IR (KBr): ν = 1897 (CO)
˜
4
3
cm–1. HRMS (ESI): calcd. for [C18H29FeNiS4O]+ [M]+ 502.9773;
found 502.8621. C18H29F6FeNiOPS4 (647.94): calcd. C 33.30, H
4.50, S 19.76; found C 33.22, H 4.45, S 19.80.
+
FeCp2 couple. Electrocatalysis studies were performed by the ad-
Preparation of [NiS4Fe(NO)Cp*][PF6]2 (4a): A solution of [NiS4-
Fe(CH3CN)Cp*][PF6] (1a) (56.2 mg, 0.09 mmol) in CH3CN (5 mL)
was treated with NOPF6 (16 mg, 0.09 mmol), whereupon the solu-
tion immediately changed from violet to orange. The reaction mix-
ture was stirred at room temperature for 2 h. The solvent was re-
moved under reduced pressure, and the orange residue was washed
with Et2O and dried under vacuum to obtain 4a (60 mg, 88%) as
an orange crystalline powder. Crystals of 4a suitable for X-ray dif-
fraction experiment were grown by diffusion of Et2O into a
dition of different amounts of fluoroboric acid by using a microsy-
ringe.
X-ray Crystallography: The data for complexes 1a, 1b, 2a, 2b, 3a,
3b, and 4a were afforded with a Bruker SMART APEX CCD dif-
fractometer with graphite-monochromated Mo-Kα radiation (λ =
0.71073 Å). Empirical absorption corrections were performed with
the SADABS program.[33] Structures were solved by direct methods
and refined by full-matrix least squares based on all data using
F2 using Shelx97.[34] Anisotropic thermal displacement coefficients
were determined for all non-hydrogen atoms. Hydrogen atoms were
placed at idealized positions and refined with fixed isotropic
displacement parameters. CCDC-1032308 (1a), -1032309 (1b),
-1032311 (2a), -1032312 (2b), -1032313 (3a), -1032314 (3b), and
-1032315 (4a) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
1
CH3CN solution of 4a. H NMR (400 MHz, CD3CN, 25 °C): δ =
2.67–3.54 (m, 14 H, S4-H), 1.81 (s, 15 H, Cp*-CH3) ppm. IR (KBr):
ν = 1816 (NO) cm–1. HRMS (ESI): calcd. for [C H FeNiS NO]2+
˜
17 29
4
[M]2+ 252.4918; found 252.4912. C17H29F12FeNNiOP2S4 (794.91):
calcd. C 25.65, H 3.67, N 1.76, O 2.01, S 16.11; found C 25.70, H
3.61, N 1.80, O 2.07, S 16.18.
Preparation of [NiN2S2Fe(CH3CN)Cp*][PF6] (1b): A solution of
[Cp*Fe(CH3CN)3][PF6] (125.2 mg, 0.27 mmol) in CH3CN (5 mL)
was treated with NiN2S2 (72.2 mg, 0.27 mmol), whereupon the
Preparation of [NiS4Fe(CH3CN)Cp*][PF6] (1a): A solution of solution gradually changed from violet to purple red. The reaction
[Cp*Fe(CH3CN)3][PF6] (230 mg, 0.50 mmol) in CH3CN (20 mL) mixture was stirred at room temperature for 2 h. The solvent was
was treated with NiS4 (142 mg, 0.50 mmol), whereupon the solu-
tion changed immediately from violet to purple. The reaction mix-
ture was stirred at room temperature for 2 h. The solvent was re-
moved under reduced pressure, and the purplish black residue was
washed with Et2O (3ϫ 5 mL) and dried under vacuum to obtain
a purplish microcrystalline powder of 1a (305 mg, 92%). Crystals
of 1a suitable for X-ray diffraction experiment were obtained from
a saturated THF solution layered with diethyl ether at room tem-
removed under reduced pressure, and the brown residue was
washed with Et2O (3ϫ 5 mL) and dried under vacuum to obtain
(87.4 mg, 50%) as a brown crystalline powder 1b. Crystals of 1b
suitable for X-ray diffraction experiment were grown by diffusion
of Et2O into a CH3CN solution of 1b. 1H NMR (400 MHz,
CD3CN, 25 °C): δ = 2.05–3.36 (m, 12 H, N2S2-H), 2.43 (s, 6 H, N-
CH ), 1.46 (s, 15 H, Cp*-CH ) ppm. IR (CH CN solution): ν =
˜
3
3
3
2231 (CN) cm–1.
Eur. J. Inorg. Chem. 2015, 2965–2973
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FULL PAPER
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(635.01): calcd. C 33.96, H 5.23, N 4.40, S 10.07; found C 33.91,
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Acknowledgments
This work was supported by the National Natural Science Founda-
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and the “111” project of the Ministry of Education of China.
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Published Online: May 28, 2015
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