Electrocatalytic Reduction of Nitrogen to Hydrazine Using a Trinuclear Nickel Complex

Article abstract of DOI:10.1021/jacs.0c08785

Activation and reduction of N2 have been a major challenge to chemists and the focus since now has mostly been on the synthesis of NH3. Alternatively, reduction of N2 to hydrazine is desirable because hydrazine is an excellent energy vector that can release the stored energy very conveniently without the need for catalysts. To date, only one molecular catalyst has been reported to be able to reduce N2 to hydrazine chemically. A trinuclear T-shaped nickel thiolate molecular complex has been designed to activate dinitrogen. The electrochemically generated all Ni(I) state of this molecule can reduce N2 in the presence of PhOH as a proton donor. Hydrazine is detected as the only nitrogen-containing product of the reaction, along with gaseous H2. The complex reported here is selective for the 4e-/4H+ reduction of nitrogen to hydrazine with a minor overpotential of ～300 mV.

Full text of DOI:10.1021/jacs.0c08785

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Electrocatalytic Reduction of Nitrogen to Hydrazine Using a
Trinuclear Nickel Complex
Paramita Saha, Sk Amanullah, and Abhishek Dey*
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ABSTRACT: Activation and reduction of N2 have been a major challenge to chemists and the focus since now has mostly been on
the synthesis of NH3. Alternatively, reduction of N2 to hydrazine is desirable because hydrazine is an excellent energy vector that can
release the stored energy very conveniently without the need for catalysts. To date, only one molecular catalyst has been reported to
be able to reduce N2 to hydrazine chemically. A trinuclear T-shaped nickel thiolate molecular complex has been designed to activate
dinitrogen. The electrochemically generated all Ni(I) state of this molecule can reduce N2 in the presence of PhOH as a proton
donor. Hydrazine is detected as the only nitrogen-containing product of the reaction, along with gaseous H2. The complex reported
here is selective for the 4e−/4H+ reduction of nitrogen to hydrazine with a minor overpotential of ∼300 mV.
ne of the most important biogeochemical processes is
phosphine catalyst for selective catalytic chemical reduction of
N2 to N2H4.40 Here we describe a small-molecule catalyst that
can selectively reduce N2 electrochemically to form N2H4
without any other nitrogenous side product.
O
the fixation of atmospheric nitrogen.1,2 Nitrogen is an
essential element in several biological molecules such as
nucleic acids, amino acids, hormones, vitamins, cofactors,
etc.3−5 Fertilizers rich in nitrogen are crucial for plant growth,
which in turn helps in sustenance of an ever-growing global
population. Although nitrogen is available in abundance as
dinitrogen gas, which composes 78% of the earth’s atmosphere,
its conversion to fixed forms is rendered difficult owing to the
kinetic inertness of the molecule.4,6,7 Nitrogen fixation mainly
occurs via (i) geochemical processes such as lightning,8 (ii)
biologically through an enzyme named nitrogenase,9−11 and
(iii) industrially by the Haber−Bosch process.12,13
While there has been substantial focus on the chemical and
electrochemical conversion of N2 to NH3, hydrazine (N2H4) is
a lucrative avenue for energy storage.14 The heats of formation
(ΔG°f (298 K)) of NH3 and N2H4 (from N2 and H2) are −16.4
and +159.2 kJ mol−1, respectively.15,16 Thus, unlike NH3, the
formation of N2H4 is considerably endothermic, rendering its
direct synthesis from N2 an avenue for energy storage, albeit a
challenging task. Unlike other energy storage materials like H2
and hydrocarbons, release of energy from N2H4 is almost
barrierless, and the stored energy can be released efficiently by
both chemical and electrochemical methods. The energy
storage density of N2H4 is 1.5368 × 1010 J m−3/4269 Wh L−1
(chemical/electrochemical).17 The high energy storage density
makes N2H4 the choice of fuel to propel rockets, and it is
considered to be a viable alternative to H2.18,19 Currently N2H4
is synthesized by oxidation of NH3 through several different
chemical routes.20
N2 is difficult to activate because of its strong π and σ
bonds.6,7 A multimetallic system is envisaged where binding of
N2 to a metal center in an end-on/side-on fashion6 will induce
π back-donation from the filled d orbitals of other electron-rich
metal centers like Co/Ni/Cu in their low-valent states to the
N2 π* orbitals. This back-bonding could be effective if these
metals are specifically oriented to form a T-shape (Scheme 1).
The π back-donation should result in weakening of the NN
bond, i.e., N2 activation. Shilov and later Nishibayashi
demonstrated the advantage of a bimetallic system in activating
N2.25,26,32 A trimetallic system should have an additional
advantage and was indeed discussed by Shilov,26 and later Ti3
and Cr3 cluster hydrides were reported by Hou to activate
N2.42,43 Thiolate auxiliary ligands are logical choices for these
metal centers because, unlike O/N donors, they can increase
the electron density on the metal and upon protonation may
assist with H+ transfer required for the conversion, and they
have been demonstrated by Holland to be effective in N2
activation.44 Similar roles of thiolate have been reported in the
active sites of metalloenzymes like Ni superoxide dismu-
tase45,46 and Ni−Fe hydrogenase47−49 and in molecular
cobalt/nickel−dithiolato complexes that reduce CO2 to
CO.50,51 Here we disclose a trimetallic Ni complex designed
as discussed above.
Several elegant molecules have been reported to catalyze the
reaction of dinitrogen to ammonia as the terminal product,
with N2H4 as a side product in some cases21−34 A few reports
of reduction of N2 to N2H4 in low yields appeared in the
1970s.35,36 In addition, a number of well-defined molecular
systems have been reported to cleave the strong triple bond of
dinitrogen.37−39 Ashley’s group reported a molecular iron
Received: August 15, 2020
https://dx.doi.org/10.1021/jacs.0c08785
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
© XXXX American Chemical Society
A

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Scheme 1. Schematic Representations of Bimetallic
Systems25,26,32 (L = Linker), Previous Trimetallic
Systems,38,41 and the Proposed Trimetallic Design of the
Molecular Framework (X = Linker) for N2 activation
Figure 1. Crystal structure of the Ni3S8 complex. Ellipsoids are drawn
with 50% probability. Color code: C, gray; O, red; S, yellow; Ni,
green. Hydrogen atoms have been omitted for clarity. Abbreviations:
ST, terminal thiolate; SB, bridging thiolate; NiT, terminal nickel; NiC,
central nickel.
A solution of 5 mmol of NiCl2·6H2O in 6 mL of water was
slowly added to a 10 mmol solution of [Na2(S2C3H6)] (0.84
mL of 1,3-propanedithiol and 800 mg of NaOH) in 15 mL of
water (Scheme 2). The resulting dark-green solution was
are the NiII/I processes and are similar to those observed for
the Darensbourg complexes.53 The intensity of the redox wave
at −2.20 V vs Fc+/Fc is almost twice that of the process at
−2.40 V vs Fc+/Fc (as is also evident from differential pulse
voltammetry), which matches the expectations based on the
structure, where two Ni centers have a S4 environment with
two bridging S and one Ni center has a S4 environment in
which all four S atoms are bridging (Figure 2). The CV of
Ni3S8 under N2 shows the same two reduction processes at
−2.20 and −2.40 V vs Fc+/Fc.
Scheme 2. Schematic Representation of the Synthetic
Protocol for the Ni3S8 Complex
In a N2 atmosphere, the CV of Ni3S8 is similar to that in an
Ar atmosphere. With the addition of an external proton source
like PhOH, the NiII/I redox waves are replaced by catalytic
currents at −2.35 and −2.85 V vs Fc+/Fc (Figures 3 and S9).
These catalytic currents could represent either the nitrogen
stirred for 2−3 h and filtered to remove an insoluble brown
residue (small amount). The filtrate was treated with 3.2 g (10
mmol) of (n-Bu4N)Br in 8 mL of water, and the mixture was
stirred for 3 h, during which time dark-green microcrystals
appeared. These crystals were filtered, washed with cold water,
1
and dried in vacuum [yield 74%; H NMR (THF-d8) δ 0.92
(dd, 4H), 1.05 (t, 24H), 1.15 (t, 4H), 1.33 (s, 8H), 1.49 (q,
16H), 1.79 (16H), 2.51 (dd, 8H), 3.53 (t, 16H); calcd for
C44H100N2Ni3O2S8 C 47.11, H 8.99, N 2.50; found C 47.07, H
8.23, N, 2.39].
The complex was characterized by single-crystal X-ray
diffraction. It crystallizes in a monoclinic symmetry with the
centrosymmetric space group P21/n. Structural analysis
revealed that two tetrabutylammonium cations are present
per unit cell. A similar trimetallic cluster was reported by
Darensbourg et al., whose trinickel cluster attained a typical
slant chair structure.52 The structure of the Ni3S8 complex, on
the other hand, is boat-shaped as a result of the hydrogen
bonds between the terminal thiolates from the two different
arms with the water molecules observed crystallographically
(Figure 1 and Table S1). The presence of the water molecules
implies that this cavity (NiT−NiT = 4.9 Å) is open and wide
enough to fit small molecules like N2.
Figure 2. Homogeneous differential pulse voltammograms and cyclic
voltammograms of Ni3S8 in THF solution under N2 (red) and Ar
(blue) (1 mM solution, 0.1 V/s, glassy carbon (GC) working
electrode (diameter 3 mm) and GC counter electrode, 0.1 M n-
Bu4NPF6 as the supporting electrolyte).
Cyclic voltammetry (CV) of Ni3S8 under Ar showed two
reduction processes at −2.20 and −2.40 V vs Fc+/Fc. These
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(Figure 5). The headspace gas was investigated using gas
chromatography, which showed only a peak for hydrogen due
Figure 3. CV of a 0.5 mM solution of Ni3S8 in N2-saturated THF with
increasing concentrations of phenol (0.1 V/s, GC working electrode).
Figure 5. Controlled-potential electrolysis under N2 at −2.35 V vs
Fc+/Fc.
reduction reaction (NRR) or the hydrogen evolution reaction
(HER). Under an Ar atmosphere in the presence of PhOH,
only a minor catalytic current was observed at −2.35 V vs Fc+/
Fc (Figure 4). In contrast, the catalytic response at −2.85 V vs
to minor background proton reduction by Ni3S8 (Figure S13).
The soluble product was investigated using ion-exchange
chromatography (IC), in which only N2H4 with a retention
time of 9.4 min was detected relative to the control
background sample where no catalyst was present (Figures 6
Figure 4. Overlay of cyclic voltammograms of Ni3S8 (0.5 mM) under
N2 (red), N2 in the presence of phenol (green), and Ar in the
presence of phenol (blue) (0.1 V/s, GC working electrode).
Figure 6. Overlay of ion chromatograms. The shaded box
corresponds to hydrazinium (retention time = 9.4 min).
Fc+/Fc was observed, indicating that the HER proceeds at
−2.85 V vs Fc+/Fc under both nitrogen and argon, whereas the
process at −2.35 V vs Fc+/Fc observed only under the N2
atmosphere represents the electrochemical NRR. Thus, N2 can
be reduced electrocatalytically with Ni3S8 at −2.35 V vs Fc/Fc+
in the presence of PhOH as the proton donor. This inference is
further confirmed from analysis of the products after
controlled-potential electrolysis (CPE) at −2.35 V vs Fc+/Fc.
A rinse test and CV after electrolysis did not evidence
deposition or decomposition of the Ni3S8 complex during the
reaction (Figure S17).
and S15).54 Comparison of the IC data with NH3, N2H4, and
NH2OH under the same conditions (20% acetone and 4 mM
HNO3 as the mobile phase55) further confirmed that only
N2H4 was produced. Hydrazine was further confirmed using
the p-(dimethylamino)benzaldehyde method (Figure S4).56
Both IC and the spectrophotometric method gave Faradaic
yields of ∼8−12% (Figures S3, S5, and S6) Thus, the Ni3S8
complex can catalyze selective 4e−/4H+ reduction of N2 to
N2H4. A previously reported mononuclear Ni(II) tetrathiolate
complex57 was synthesized to evaluate its reactivity under the
same conditions (Scheme S1 and Figure S10). CPE was
performed at both −2.4 and −2.8 V vs Fc+/Fc, and product
analysis did not reveal hydrazine (Figure S16). The HER was a
major process, as indicated by gas chromatography analysis of
CPE of Ni3S8 in THF was carried out at −2.35 V vs Fc+/Fc
in an H-cell inside a N2 glovebox in the presence of 100 mM
phenol (Figure S1). The charge consumed in the presence of
Ni3S8 was substantially higher than that in a blank experiment
without the catalyst, consistent with an electrocatalytic NRR
C
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the headspace gas (Figure S14). Thus, the trinuclear geometry
is indeed effective in N2 activation as envisaged.
to be delivered to the vacant π* orbitals of N2. The Ni3S8
complex seems to achieve that, as three of the four electrons
needed can be obtained from the reduced Ni(I) centers and
one is obtained from the electrode, whereas a mononuclear
NiS4 complex cannot. While the water molecules trapped in
the molecule support the possibility of N2 coordination as
envisaged, they also suggest possible proton delivery pathways
via hydrogen bonding to the thiolates, but direct spectroscopic
evidence will be needed to ascertain such binding. The details
of the mechanism of action are currently being investigated
with the goal of enhancing the stability of Ni3S8 and further
enhancing the catalytic efficiency of the process.
Hydrazine is easily oxidized back to N2 at −0.25 V vs Fc+/Fc
in a Pt/glassy carbon (GC) electrode (Figure S12).58 Thus,
rotating ring−disk electrochemistry (RRDE) was used to
detect N2H4 in situ and confirm the inference from IC. In
RRDE, the product of the electrochemical reaction at the disc
electrode diffuses to the encircling Pt electrode because of the
hydrodynamic force created by rotation of the electrode shaft.
As the applied potential at the graphite disk working electrode
(which was dipped in a solution of Ni3S8 in N2-saturated THF
containing 100 mM PhOH) was lowered below −2.35 V vs
Fc/Fc+, the Pt ring (held at −0.25 V vs Fc/Fc+) registered the
reoxidation of the N2H4 produced at the adjacent working
electrode (Figure 7). Even for a GC electrode, the N2H4
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c08785.
Additional NMR, mass, and FTIR data (PDF)
Crystallographic data for Ni3S8 (CIF)
AUTHOR INFORMATION
■
Corresponding Author
Abhishek Dey − School of Chemical Sciences, Indian Association
for the Cultivation of Science, Kolkata 700032, West Bengal,
India; orcid.org/0000-0002-9166-3349; Email: icad@
iacs.res.in
Authors
Paramita Saha − School of Chemical Sciences, Indian Association
for the Cultivation of Science, Kolkata 700032, West Bengal,
India; orcid.org/0000-0002-3877-3418
Sk Amanullah − School of Chemical Sciences, Indian Association
for the Cultivation of Science, Kolkata 700032, West Bengal,
India; orcid.org/0000-0003-3288-5942
Figure 7. Overlay of RRDE responses of Ni3S8 vs a blank solution
(without Ni3S8) with the Pt ring held at −0.25 V vs Fc/Fc+.
Conditions: 50 mV/s, N2-saturated THF, 100 mM PhOH, 100 mM
nBu4NPF6, 300 rpm. The dashed line shows the corresponding in situ
oxidation current from N2H4.
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c08785
produced can be detected by a Pt loop held at −0.25 V vs Fc/
Fc+ (Figure S2). It should be noted that H2 and PhOH were
not oxidized by the Pt electrode at this potential (Figure S11).
The electrochemical NRR was achieved at −2.35 V vs Fc/
Fc+. The thermodynamic potential for the process can be
estimated from the following equation:59
Notes
The authors declare no competing financial interest.
CCDC 2002886 (Ni3S8) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, U.K.; fax: + 44 1223336033.
E°(N2(g)/N2H4(THF)) = −0.398 V − 0.059pKa
With a pKa of 27.8 for phenol in THF,59 the formal E° was
calculated to be −2.04 V vs Fc/Fc+. Thus, the Ni3S8 complex
works with an overpotential of approximately 300 mV, which is
very encouraging. For comparison, the other molecular systems
reported to convert N2 to N2H4 use a strong reducing agent
like CoCp* or KC8.60−62 The E° values for KC8 and CoCp*
are approximately −3.5 and −2.0 V vs Fc/Fc+, respectively,
where the former is substantially lower than the thermody-
namic potential for hydrazine synthesis.
Herein, selective electrochemical reduction of N2 to N2H4
by an easy-to-synthesize Ni3S8 complex has been demonstrated
using PhOH as a proton source. While the complex is readily
synthesized, its reduced state is very air-sensitive, and it is
unstable in most protic solvents as well as chlorinated solvents,
akin to most N2-activating molecular systems. Reduction of N2
to N2H4 requires 4e−/4H+, and specifically, the electrons need
ACKNOWLEDGMENTS
■
This research is funded by Department of Science and
Technology (DST) Grant SERB/EMR-II/008063. P.S.
acknowledges UGC-SRF.
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