Electrocatalytic hydrogen evolution by an iron complex containing a nitro-functionalized polypyridyl ligand

Article abstract of DOI:10.1016/j.poly.2015.11.023

Iron polypyridyl complexes have recently been reported to electrocatalytically reduce protons to hydrogen gas at -1.57 V versus Fc⁺/Fc. A new iron catalyst with a nitro-functionalized polypyridyl ligand has been synthesized and found to be acti

Full text of DOI:10.1016/j.poly.2015.11.023

Polyhedron xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Electrocatalytic hydrogen evolution by an iron complex containing a
nitro-functionalized polypyridyl ligand
⇑
Carolyn L. Hartley, Ryan J. DiRisio, Trevor Y. Chang, Wanji Zhang, William R. McNamara
Department of Chemistry, College of William and Mary, 540 Landrum Drive, Williamsburg, VA 23185, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Iron polypyridyl complexes have recently been reported to electrocatalytically reduce protons to
hydrogen gas at À1.57 V versus Fc+/Fc. A new iron catalyst with a nitro-functionalized polypyridyl ligand
has been synthesized and found to be active for proton reduction. Interestingly, catalysis occurs at
À1.18 V versus Fc+/Fc for the nitro-functionalized complex, resulting in an overpotential of 300 mV.
Additionally, the complex is active with a turnover frequency of 550 sÀ1. Catalysis is also observed in
the presence of water with a 12% enhancement in activity.
Received 4 September 2015
Accepted 16 November 2015
Available online xxxx
Keywords:
Iron
Electrocatalysis
Hydrogen generation
Artificial photosynthesis
Coordination chemistry
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
ing in highly stable catalysts that can generate hydrogen from
aqueous solutions [5]. Although there are many examples of active
One of the great challenges facing the current generation of
scientists involves limiting our dependence on fossil fuels. In
order to relieve pressure on dwindling fossil fuel reserves,
renewable energy must be pursued. Solar energy appears to be
a viable long-term source of energy because roughly 400 times
the amount of energy needed each year strikes the earth as har-
vestable solar energy [1]. One method of utilizing solar energy
involves the development of systems for artificial photosynthesis
(AP). In general, these systems are designed to use solar energy
to photochemically split water, converting sunlight into both
electricity and fuel in the form of H2 [2]. Although noble metals
can be used to reduce protons to hydrogen gas, the rare nature
of these materials limits their widespread use in devices for AP.
Therefore, it is critical to develop complexes containing earth-
abundant metals that can catalytically reduce protons to hydro-
gen gas.
Many cobalt glyoxime complexes have been studied and can be
tuned to operate at low overpotential [3]. Other cobalt and nickel
complexes have been discovered that are remarkably active with
turnover frequencies (TOFs) as high as 105 sÀ1 [4]. However, many
of the cobalt and nickel catalysts are unstable and operate in only
organic solutions. To circumvent this limitation, polypyridyl
ligands have been coordinated to cobalt and molybdenum, result-
cobalt, nickel, and molybdenum complexes, there are far fewer
examples of molecular iron catalysts that generate hydrogen in
the presence of water.
One approach to developing active iron catalysts involves
mimicking the active site of hydrogenase enzymes [6]. Functional
mimics that are active electrocatalysts for hydrogen generation
often operate at potentials that are more cathodic than À1.6 V
versus Fc+/Fc [6]. Additionally, these catalysts are significantly
less active than [Fe]H2ase [7,8]. In an effort to develop a water-
stable iron catalyst, we have recently reported a mononuclear
iron polypyridyl catalyst (Fig. 1) that is active in aqueous solu-
tions [9]. Although the iron polypyridyl catalysts are active and
stable, catalysis occurs at À1.57 V versus Fc+/Fc, corresponding
to an overpotential of 800 mV. We reasoned that functionalizing
the ligand with an electron withdrawing group would result in
an iron catalyst that operates at a significantly less cathodic
potential.
Herein we report the synthesis and characterization of an iron
complex (2) containing a nitro-functionalized polypyridyl ligand
and its corresponding electrocatalytic activity. The incorporation
of an electron withdrawing nitro group on the phenolate ring of
the complex results in a catalyst that reduces protons at À1.18 V
versus Fc+/Fc, corresponding to an overpotential of just 300 mV.
This represents a significant improvement over complex 1 and is
a promising step towards developing an iron catalyst that is stable,
active, and operates at low overpotential.
⇑
Corresponding author. Tel.: +1 757 221 4868; fax: +1 757 221 2715.
E-mail address: wrmcnamara@wm.edu (W.R. McNamara).
http://dx.doi.org/10.1016/j.poly.2015.11.023
0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.
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2
C.L. Hartley et al. / Polyhedron xxx (2015) xxx–xxx
(118 mg, 41% yield). Crystals suitable for X-ray diffraction were
grown by diffusion of diethyl ether into a solution of 2 in dichlor-
omethane. m/z for C19H17Cl2FeN4O3Na+ expected = 497.991934,
found = 497.992060. Anal. calc. for 2: C, 47.83; H, 3.80; N, 11.74.
Found: C, 47.77, H, 3.83, N, 11.90.
2.3. Instrumentation
1H and 13C NMRs were performed on an Agilent 400MR DD2
instrument operating in pulse Fourier transform mode. Chemical
shifts were referenced to residual solvent. Mass spectrometry
was carried out using positive electrospray ionization on a Bruker
12 Tesla APEX-Qe FTICR-MS with an Apollo II ion source.
Fig. 1. Left: iron polypyridyl monophenolate complex (1). Right: nitro-functional-
ized iron polypyridyl monophenolate complex (2).
2. Materials and methods
2.3.1. X-ray diffractometry
A single crystal was mounted on a glass fiber and data was
2.1. General procedures
collected with graphite-monochromated Cu
K
a
radiation
(k = 1.54187 nm) on a Bruker-AXS three-circle diffractometer using
a SMART Apex II CCD detector. The crystal structure was solved
and refined using SIR2014 and SHELXL-2014/7.
2-Hydroxy-5-nitrobenzaldehyde was purchased from Alfa
Aesar. Bis(pyridin-2-ylmethyl)amine was purchased from Aldrich.
Iron (III) chloride and potassium hydroxide were purchased from
Fisher Scientific. Tetra-n-butylammoniumhexafluorophosphate
(98%), was purchased from Acros Organics. All other reagents were
purchased from Fisher Scientific and used without further
purification.
2.3.2. Cyclic voltammetry (CV)
A CH Instruments 620D potentiostat with a CH Instruments 680
amp booster was used for all experiments. Each experiment was
performed in a standard three-electrode cell with a glassy carbon
working electrode (diameter = 0.30 cm), a Pt auxiliary electrode,
and an SCE reference electrode. Tetrabutylammonium hexafluo-
rophosphate (TBAPF6, 0.1 M) was used as the electrolyte. Ferrocene
was added and used as an internal reference. All electrochemical
experiments were performed under an Ar atmosphere. The work-
ing and auxiliary electrodes were polished with alumina powder
2.2. Syntheses
2.2.1. 2-((Bis(pyridin-2-ylmethyl)amino)methyl)-4-nitrophenol) (L-NO2)
This procedure was modified from a literature method [10].
2-hydroxy-5-nitrobenzaldehyde (500 mg, 3 mmol) was dissolved in
50 mL of methanol and degassed with argon. A degassed solution
of bis(pyridin-2-ylmethyl)amine (0.54 mL, 3 mmol) in 5 mL of
methanol was transferred to the aldehyde solution using a cannula.
Glacial acetic acid was added (3 drops) followed by the dropwise
addition of an air-free solution of sodium cyanoborohydride
(190 mg, 3 mmol) in 5 mL methanol. The resulting clear, red solu-
tion was refluxed for 1 h and then stirred for 24 h at room temper-
ature. 1 M HCl was added to the solution until it reached pH 4. The
solution was evaporated to dryness and dissolved in 25 mL of sat-
urated Na2CO3 solution and extracted with chloroform. The organic
layers were combined, dried with Na2SO4, and filtered through
celite. The solvents were removed under vacuum to yield a red
oil. The ligand was then purified using silica gel chromatography
with 9:1 dichloromethane:methanol. The desired compound
eluted first and the solvent was removed under vacuum to give
540 mg of the purified product (1.85 mmol, 62% yield). 1H NMR
(CDCl3): d 8.58 (d, 2H), d 8.12 (d, 1H), d 8.05 (2, 1H), d 7.66
(t, 2H), d 7.30 (d, 2H), d 7.21 (t, 2H), d 6.95 (d, 1H), d 3.93 (s, 4H),
d 3.85 (s, 2H). 13C NMR (CDCl3): d 164.27, d 157.71, d 148.69, d
139.68, d 137.06, d 126.54, d 125.61, d 123.52, d 123.08, d 122.41,
d 117.17, d 58.67, d 56.10. m/z for C19H18N4O3H+ expected = 351.15,
found = 351.15.
paste (0.05 lm) on a cloth-covered polishing pad and then rinsed
with water and acetonitrile before each scan (unless otherwise
noted). For the acid addition experiments, trifluoroacetic acid
(TFA) was added under argon.
2.3.3. Acid addition study
In an electrochemical cell, 0.5 mg of crystals of 2 were dissolved
in 5.0 mL of 0.1 M TBAPF6 in CH3CN. Cyclic voltammograms (CVs)
were obtained at different concentrations of TFA.
2.3.4. Bulk electrolysis
Controlled-potential coulometry (CPC) experiments were con-
ducted in a closed 500 mL four-neck round-bottom flask. Complex
2 (0.5 mg) was added to 50 mL of 0.1 M TBAPF6 with in CH3CN. The
flask was capped with two vitreous carbon electrodes and a silver
wire reference electrode, all submerged in solution and separated
by VYCOR frits. The solution was purged with argon and TFA was
added resulting in a 65 mM solution. A CPC was run at À1.2 V ver-
sus Fc+/Fc for 1800 s, resulting in a faradaic yield of 98%. No hydro-
gen was observed when the experiment was performed without
catalyst.
2.3.5. Scan rate dependence
In an electrochemical cell, 0.5 mg of 2 was dissolved in 5.0 mL of
0.1 M TBAPF6 in CH3CN. Cyclic voltammograms were taken at scan
rates ranging from 50 mV/s to 700 mV/s.
2.2.2. [FeCl2(L-NO2)] (2)
The complex was synthesized using a modified literature proce-
dure [9]. L-NO2 (100 mg, 0.6 mmol) and triethylamine (83 lL,
0.6 mmol) were dissolved in 10 mL of MeOH and degassed with
Ar. FeCl3Á6H2O (162 mg, 0.6 mmol) was dissolved in 10 mL of
methanol and degassed with argon. The ligand solution was trans-
ferred to the flask containing the iron precursor using a cannula.
The solution immediately turned deep purple with a visible precip-
itate. The reaction was stirred at room temperature for 1 h and was
filtered. The filtrate was evaporated to dryness and was washed
3. Results and discussion
The ligand (L-NO2) was obtained through
a
simple
condensation reaction of 2-hydroxy-5-nitrobenzaldehyde with
bis(pyridin-2-ylmethyl) followed by a reduction using sodium
cyanoborohydride (62% yield). The phenol group of L-NO2 was
deprotonated with triethylamine and the ligand was coordinated
with cold methanol to give the product as
a purple solid
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C.L. Hartley et al. / Polyhedron xxx (2015) xxx–xxx
3
Table 1
to FeCl3 in methanol. X-ray quality crystals were grown from slow
Bond lengths [Å] and angles [°] for 2.
diffusion of diethyl ether into a dichloromethane solution of 2.
Fig. 2 shows the ORTEP diagram of 2 with hydrogen atoms
omitted for clarity. The structure confirms that the ligand binds
to iron to give a distorted octahedral complex. An O–Fe–N bond
angle of 163.7° and an N–Fe–N bond angle of 73.9° delineate from
the 180° and 90°, respectively, that are expected for typical octahe-
dral complexes. The distorted octahedral geometry is likely a result
of the 6-membered chelate ring formed from the bonding of the
phenolate to the iron center. An Fe–O bond length of 1.945 Å is
consistent with what is typically observed for FeIII–phenolate
bonds [10] (see Table 1).
Cyclic voltammograms of 2 (Fig. 3, blue) reveal a reversible
redox couple at À0.45 V versus Fc+/Fc corresponding to the FeIII/FeII
redox couple of the nitro-functionalized complex. A peak separa-
tion of 72 mV is observed under these conditions which is consis-
tent with what is observed for the Fc+/Fc internal standard. The
FeIII/FeII redox couple appears at a potential that is 100 mV more
positive than the corresponding redox couple for complex 1
(Fig. 3, red). Therefore, we reasoned that if complex 2 were an
active catalyst, it would operate at an overpotential of ꢀ100 mV
less than 1. However, upon addition of a proton source (trifluo-
roacetic acid) in an acetonitrile solution, a catalytic wave can be
observed at À1.18 V versus Fc+/Fc (Fig. 4, left). Interestingly, the
catalytic wave of 2 appears at a potential that is 400 mV more pos-
itive than what is observed for 1 (Fig. 4, right). Catalytic activity is
also observed when tosic acid is used as the proton source (see
Supporting information).
Bond length (Å)
Fe(1)–O(1)
Fe(1)–N(2)
Fe(1)–N(1)
Fe(1)–N(3)
Fe(1)–Cl(4)
Fe(1)–Cl(3)
1.945(5)
2.184(6)
2.186(6)
2.227(7)
2.282(2)
2.316(2)
Bond angle (°)
O(1)–Fe(1)–N(2)
O(1)–Fe(1)–N(1)
N(2)–Fe(1)–N(1)
O(1)–Fe(1)–N(3)
N(2)–Fe(1)–N(3)
N(1)–Fe(1)–N(3)
82.8(2)
163.7(2)
93.2(2)
89.8(2)
76.3(2)
73.9(2)
The current associated with this irreversible reduction event
grows linearly upon addition of more acid, suggesting that cataly-
sis is second order with respect to [H+] (Fig. 5). Catalysis becomes
Fig. 3. Cyclic voltammograms of 1 (red) and 2 (blue) in a 0.1 M TBAPF6 solution in
CH3CN. (Colour online.)
independent of scan rate at
m = 10 V/s. By performing the acid addi-
tion experiment at 10 V/s, a turnover frequency can be calculated
using the same method that was used to calculate TOF for 1 (see
Supporting information) [9]. Although the method used to calcu-
late TOF represents simple pseudo-first order system, it is also used
to calculate rates for more complicated systems in order to provide
a point of comparison [4,5,9,11]. Although less active than 1
(TOF = 1000 sÀ1), catalyst
2 is still highly active with a
TOF = 550 sÀ1. The observed catalytic wave at À1.18 V versus
Fc+/Fc corresponds to an overpotential of just 300 mV compared
to 800 mV reported for 1 (see Supporting information) under
similar experimental conditions. Additionally, the activity and
overpotential for
2 are comparable with recently reported
fluorinated diglyoxime-iron complexes that operate with an
overpotential of 300 mV and a TOF ꢀ200 sÀ1 [7b].
Furthermore, when aliquots of catalyst are added to a fixed
amount of TFA, the catalytic wave is still observed at À1.0 V. When
more catalyst is introduced, the current enhancement increases
linearly with [2] (Fig. 6), suggesting that the catalysis is first order
with respect to [2]. This relationship gives rise to an overall rate
expression of rate = k[2][H+]2. These observations are consistent
with what was observed for the previously reported iron polypyr-
idyl complexes [9].
Fig. 2. ORTEP diagram of 2. Ellipsoids are at the 50% probability level with
hydrogen omitted for clarity.
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4
C.L. Hartley et al. / Polyhedron xxx (2015) xxx–xxx
Fig. 4. Left: CVs of 2 without added TFA (black) and with 8.8 mM TFA (blue). Right: CVs of 1 without added TFA (black) and with 8.8 mM TFA (blue). (Colour online.)
Fig. 5. CVs of 2 in CH3CN with 0.1 M TBAPF6 (black), upon addition of 2.2 mM (red),
Fig. 6. CVs in CH3CN with 0.1 M TBAPF6 containing 44 mM TFA with 0.2 mM 2
(black), 0.4 mM 2 (red), 0.6 mM 2 (orange), 0.8 mM 2 (green), 1.0 mM 2 (blue) at
4.4 mM (orange), 6.6 mM (green), and 8.8 mM (blue) TFA at
online.)
m = 200 mV/s. (Colour
m
= 200 mV/s. (Colour online.)
From a mechanistic standpoint, complex 2 appears to behave
similarly to 1. Prior to acid addition, a redox couple is observed
for FeIII/FeII at À0.45 V versus Fc+/Fc. Upon addition of acid, this
redox event shifts to À0.3 V. This shift indicates that the first step
of the catalytic cycle involves protonation of the phenol to give [Fe
(L-H)Cl2]+, where L is the polypyridyl monophenolate ligand. This
protonation is followed by the reduction of [Fe(L-H)Cl2]+ to give
[Fe(L-H)Cl2]. Subsequent steps involve further reduction and pro-
tonation events resulting in a CEEC or CECE mechanism. The proto-
nated form of both 1 and 2 are reduced at the same potential
(À0.3 V versus Fc+/Fc), suggesting that the presence of the nitro
group does not influence the reduction of [Fe(L-H)Cl2]+ to
[Fe(L-H)Cl2]. This implies that upon protonation, the phenol group
becomes hemilabile. Since the catalytic wave for 2 is 400 mV more
positive than what is observed for 1, complex 2 appears to benefit
from the electron withdrawing effects of the nitro substituent. This
suggests that the phenol of 2 likely re-coordinates during the later
stages of catalysis.
Although 2 is active for hydrogen generation at a low overpo-
tential compared to other iron catalysts in acetonitrile, it is of great
interest to develop a system that can reduce protons in aqueous
solutions. To this end, we have examined the activity of the
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C.L. Hartley et al. / Polyhedron xxx (2015) xxx–xxx
5
Acknowledgements
W.R.M. would like to thank R.D. Pike for help solving the crystal
structure. This work was funded by the Research Corporation Cot-
trell College Science Award. Elemental analysis was obtained at the
CENTC Elemental Analysis Facility at the University of Rochester,
funded by NSF CHE 0650456. Mass spectrometry was carried out
at the Cosmic Facility at Old Dominion University in Norfolk, VA.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2015.11.023.
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Previously reported iron polypyridyl monophenolate complexes
have been found to be highly active and stable catalysts for proton
reduction, but are limited by high overpotentials. We have found
that functionalizing the phenol group of a polypyridyl monopheno-
late iron complex results in a catalyst that operates at a substan-
tially lower overpotential. The catalyst is active with a TOF of
550 sÀ1 and operates with an overpotential of just 300 mV (com-
pared to 800 mV for the previously reported iron polypyridyl com-
plex). Catalysis is thought to proceed through a CECE or CEEC
mechanism as has been reported for similar complexes. Hydrogen
generation is first order with respect to [catalyst] and second order
with respect to [H+]. The addition of water resulted in an increase
in catalytic activity, suggesting that the complex is stable in the
presence of water. The incorporation of a nitro group on the ligand
has resulted in an improved catalyst and has helped to elucidate
the mechanism of catalysis with this family of iron catalysts.
Please cite this article in press as: C.L. Hartley et al., Polyhedron (2015), http://dx.doi.org/10.1016/j.poly.2015.11.023