Iron polypyridyl complex adsorbed on carbon surfaces for hydrogen generation

Article abstract of DOI:10.1039/d1cc02131a

A series of homogeneous Fe(iii) complexes were recently reported that are active for electrocatalytic hydrogen generation. Herein we report a naphthalene-terminated Fe(iii) complex for use in the functionalization of glassy carbon surfaces for electrocata

Full text of DOI:10.1039/d1cc02131a

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Iron polypyridyl complex adsorbed on carbon
surfaces for hydrogen generation†
Cite this: Chem. Commun., 2021,
57, 7697
Caroline M. Margonis,a Marissa Ho,a Benjamin D. Travis,a William W. Brennessel
b
a
Received 21st April 2021,
Accepted 1st July 2021
and William R. McNamara
*
DOI: 10.1039/d1cc02131a
rsc.li/chemcomm
A series of homogeneous Fe(III) complexes were recently reported further highlighted by catalytic activity achieved in solutions
that are active for electrocatalytic hydrogen generation. Herein containing local pond water.5
we report a naphthalene-terminated Fe(III) complex for use in the
Although highly active, hydrogen evolution by homogeneous
functionalization of glassy carbon surfaces for electrocatalytic systems is limited by diffusion.6 A promising method to over-
hydrogen generation with retention of catalytic activity.
come this issue and expedite the electron-transfer process is to
anchor the catalyst to a wide band gap semiconductor, such as
Through artificial photosynthesis (AP), solar energy can be TiO2, or SrTiO2.6 However, fast photo-generated electron–hole
harvested and used to split water into H2 and O2.1 The reductive pair recombination before participation in the redox reaction
side of AP focuses on the conversion of protons to hydrogen can limit the activity of these systems.7 Therefore, it is critical to
gas.1 Many transition metal complexes have been demon- examine a wide range of semiconductor surfaces. Carbon
strated to be active electrocatalysts for hydrogen generation.2 nanotubes (CNTs) have shown great promise as a semiconduc-
However, development of systems integrating inexpensive tor support for hydrogen generation catalysts.8 Several of such
materials is essential for wide-spread applications.3 As the most systems incorporating a CNT-enhanced semiconductor and a
Earth-abundant metal, iron is widely accessible and relatively Pt-loaded catalyst have also been found to be active for photo-
low in cost, which makes incorporation of iron into large-scale catalytic hydrogen generation.9
AP systems more economically feasible.
One method of immobilizing iron catalysts on CNT surfaces
To this end, we have recently reported a series of iron involves adsorption to the surface through p–p stacking
polypyridyl monophenolate complexes that are active for elec- interactions.10 As a non-covalent interaction, these forces are
trocatalytic hydrogen generation (Fig. 1).4 Electrocatalysis by typically much weaker than covalent bonds, but the magnitude
complex 1 occurs at ꢀ1.57 V vs. Fc+/Fc in CH3CN with a of attraction can be modified by altering the size of the pi–
turnover frequency up to 1000 sꢀ1 and a 660 mV over- stacking network; calculations on the strengths of p–p-stacking
potential.4 Catalytic activity is enhanced in the presence of interactions between aromatic molecules and CNTs suggest
water, with 1 achieving turnover frequencies of 3000 sꢀ1 and that binding energies are in the range of 10–25 kJ molꢀ1 per
operating with an 800 mV overpotential.4 The complex is also is benzene ring, signifying that the strength of the attach-
active for photocatalysis when paired with fluorescein (chromo- ment increases with the size of the polyaromatic network.11
phore) and triethylamine (sacrificial donor) in 1 : 1 ethanol : water
mixtures.5 This highly active and stable system achieves TONs
with respect to catalyst of 42100 after 24 hours of irradiation.5
A second addition of the sacrificial donor after 24 hours restored
hydrogen production activity.5 The robustness of this system is
a College of William and Mary, 540 Landrum Drive, Williamsburg, VA 23185, USA.
E-mail: wrmcnamara@wm.edu
b University of Rochester, 252 Elmwood Ave, Rochester, NY 14611, USA
† Electronic supplementary information (ESI) available: Crystallographic infor-
mation files and experimental details. CCDC 2078140. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/d1cc02131a
Fig. 1 Left: Iron polypyridyl monophenolate parent catalyst (1). Right:
Naphthalene-terminated iron polypyridyl monophenolate parent catalyst (2).
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Furthermore, adsorption of p–p stacking molecules is reversible, linearly with the potential sweep rate.13 This contrasts the
suggesting that the structure of the catalyst may remain intact. redox behaviour of a freely-diffusing material, in which there
In order to develop an iron polypyridyl monophenolate is a linear relationship between the peak current and the square
catalyst capable of adsorption to carbon surfaces, a ligand root of the scan rate.13 The linearity (R2 4 0.99) of the peak
was designed that contains both the polypyridyl ligand and a current versus potential scan rate plot for 2 indicates that the
pendant naphthalene group (Fig. 1). The naphthalene group observed electrochemical response arises from material
was selected based on its previously demonstrated capacities adsorbed onto the surface of the glassy carbon electrode rather
for carbon surface adsorption and functionalization.10 The than from material freely diffusing in solution (Fig. 3). Quanti-
naphthalene-terminated ligand was synthesized and subse- tatively, the relationship between peak current and potential
quently coordinated to FeCl3 to give 2, and crystals suitable scan rate for an electroactive adsorbate is given to be:13
for X-ray diffraction were grown through the slow diffusion of
n2F2
ip ¼
uAGꢁ
toluene into a concentrated solution of 2 in dichloromethane.
4RT
The complex presents itself as a disoriented octahedral, with
the Fe(III) centre bound to two chlorides and the ligand (Fig. 2).
Distortion is evident based on deviation from the expected
octahedral-bond angles, which likely occurs to minimize strain
arising from the 6-membered chelate ring, resulting from
coordination of the phenolate and N(3) to the iron centre.
Additionally, disorder within the crystal arises from a planar
flip of the naphthalenyl group, which is modelled as being
disordered over two positions (0.80 : 0.20); because the two
orientations do not fill the exact same relative special volume,
the co-crystallized solvent is disorganized to accommodate
them (see ESI†).
Cyclic voltammetry was used to observe and characterize the
adsorption behaviour of 2 on the surface of glassy carbon
electrodes. Though not as large or pristine as the graphene
surfaces found in CNTs, the smaller domains of sp2 hybridized
carbon in glassy carbon allow this electrode to act as a con-
venient model system.12 Electrodes were soaked overnight in a
0.5 mM solution of 2 before being removed, rinsed with
CH3CN, and added to an electrochemical cell containing
0.1 M TBAPF6 in CH3CN. The working electrode was cycled
between +0.2 and ꢀ0.95 V vs. Fc+/Fc to observe only the Fc+/Fc
(internal standard) and Fe(III)/Fe(II) redox couples and to mini-
mize the possibility of reductive desorption.
where ip is the peak current, n is the number of electrons
transferred in the redox event, F is the Faraday constant, R is
the gas constant, T is the temperature, u is the potential sweep
rate, A is the surface area of the electrode, and G* is the surface
coverage of the adsorbed species.13 As such, the relationship
between peak current and potential sweep rate can be used to
calculate the surface coverage of 2, which was found to have a
value of 7.7 ꢂ 10ꢀ11 mol cmꢀ2 at 298 K. This value is similar to a
previously reported naphthalene-terminated cobalt complex.10
Another expected characteristic of an electroactive adsorbate
is a separation less than 58 mV between the peak of the anodic
wave and the peak of the cathodic wave (DEp).13 This behaviour
is observed for the 200 and 400 mV sꢀ1 scans, in which DEp is
42 and 43 mV respectively, of 2. However, DEp continues to
increase with the scan rate and reaches upwards of 104 mV with
a potential sweep rate of 1000 mV sꢀ1. This deviation from the
ideal behaviour indicates that a kinetic barrier to electron
transfer exists. Despite these variations, DEp for the naphtha-
lene complex is still significantly less than the homogeneous
complex, 1, at all potential scan rates (Fig. 4). The smaller DEp
of 2 compared to 1 indicates that 2 is adsorbed to the electrode
surface and is not freely diffusing in solution.
To confirm that the adsorption behaviour observed was a
specific result of pi–stacking interactions between the naphtha-
lene moiety and the electrode surface, analogous iron complexes
with different functional groups were examined. These complexes
Cyclic voltammograms of surface-adsorbed 2 taken at var-
ious potential scan rates show
a reversible Fe(III)/Fe(II)
reduction at ꢀ0.60 V vs. Fc+/Fc. This potential is 100 mV more
positive than the homogeneous parent complex 1 (Fig. 4).
For an electroactive adsorbate, the peak current increases
Fig. 3 Left: Cyclic voltammograms of 2 adsorbed to the surface of a
glassy carbon electrode in 5 mL of 0.1 M TBAPF6 in CH3CN taken at scan
rates of 200 (red), 400 (blue), 600 (black), 800 (green), and 1000 (orange)
mV sꢀ1. Right: Cathodic peak current versus potential sweep rate for 2
(R2 = 0.99972).
Fig. 2 ORTEP diagram of 2 with Fe (orange), O (red), N (blue), Cl (green),
and C (gray). Hydrogen atoms are omitted for clarity.
7698
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Fig. 5 Cyclic voltammograms of surface adsorbed 2 in 5 mL of 0.1 M
Fig. 4 Cyclic voltammograms of 2 (red) adsorbed to the surface of a
glassy carbon electrode and 0.1 mg of 1 (blue) in 5 mL of 0.1 M TBAPF6 in
TBAPF6 in CH3CN in the presence of no acid (red), 0.4 mM (blue), 0.8 mM
(black), 1.2 mM (green), and 1.6 mM (orange) at 200 mV sꢀ1
.
CH3CN taken at 200 mV sꢀ1
.
observed for 1. Furthermore, when a potential of ꢀ1.6 V vs. Fc+/Fc
is applied in a bulk electrolysis experiment, hydrogen evolution is
observed with a faradaic yield of 99% (see ESI†). With an
ic/ip = 5.46, complex 2 exhibits similar activity to 1 (see ESI†), but
with an overpotential of 480 mV compared to 660 mV for 1 (see
ESI†).’’
have the same basic structure, but lack the naphthalene anchoring
group. Following the same overnight electrode soaking procedure,
no electrochemical response was observed using electrodes soaked
overnight in individual solutions of these complexes (see ESI†). This
indicates that the naphthalene moiety plays a critical role in surface
adsorption.
In addition to TFA, tosic acid was examined as a proton source
for hydrogen generation catalyzed by 2. In the presence of 0.4 mM
tosic acid, an irreversible catalytic wave corresponding to proton
reduction is observed at ꢀ1.43 V vs. Fc+/Fc (see ESI†). However, in
contrast to the TFA additions, the current of the proton reduction
peak decreases with increasing tosic acid concentrations. This
decrease in current can be attributed to desorption of 2 from the
electrode surface at higher acid concentrations.
Following confirmation of adsorption behaviour, the adsorp-
tion kinetics of 2 were examined by soaking the glassy carbon
electrode in a 0.5 mM solution of 2, rinsing, and collecting CVs.
The measured current, which is linearly related to the surface
coverage of the catalyst, initially grows with soak time. The most
rapid growth is seen within the first 60 minutes, in which the
cathodic peak current reaches 83% of its maximum value. The
current peaks at a soak time of 720 minutes, which marks
the point at which the adsorbed material is at equilibrium with
the freely diffusing material in the solution (see ESI†).
Since an ideal catalyst for AP is active in aqueous solutions, the
catalytic activity of 2 was examined in aqueous buffer solutions
(Fig. 6). Catalytic reduction waves were observed for each pH. A 52
With adsorption behaviour characterized, its ability to function
as an electrocatalyst for proton reduction was examined. CVs of
the electrode-adsorbed catalyst were obtained upon the addition
of known concentrations of trifluoroacetic acid (TFA) in acetoni-
trile. In the presence of TFA, an irreversible reduction event is
observed at ꢀ1.53 V vs. Fc+/Fc (Fig. 5). At the same [TFA], this
catalytic wave for 2 occurs at a potential 120 mV more positive
than that of 1. The peak current of this proton reduction wave
increases linearly with [TFA], suggesting a second order depen-
dence on [H+], which is consistent with what is observed for 1 (see
ESI†). Upon addition of a proton source, the Fe(III)/Fe(II) redox
couple shifts 340 mV to a more anodic potential. This behavior is
consistent with what is observed for the homogeneous catalyst, 1.
The positive shift of the Fe(III)/Fe(II) redox couple suggests that the
first step in the mechanism is a chemical step (C), or more
specifically, the protonation of the phenol group. The complex
then undergoes electron transfer, (E), to be reduced to Fe(II). The
subsequent increase in current observed at ꢀ1.53 V corresponds to
another reduction and protonation event to liberate hydrogen gas.
This suggests a CECE or CEEC mechanism, similar to what is
Fig. 6 Cyclic voltammograms of surface adsorbed 2 in 5 mL of citrate-buffered
aqueous solutions at pH 3.8 (red), 4.6 (blue), 5.4 (orange) and 6.2 (black).
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(h) H. I. Karunadasa, C. J. Chang and J. R. Long, Nature, 2010, 464,
1329–1333; (i) B. Stubbert, J. C. Peters and H. B. Gray, J. Am. Chem.
Soc., 2011, 133, 9212; ( j) X. Zhao, I. P. Georgakaki, M. L. Miller,
J. C. Yarbrough and M. Y. Darensbourg, J. Am. Chem. Soc., 2001, 123,
9710–9711; (k) G. A. N. Felton, R. S. Glass, D. L. Lichtenberger and
D. H. Evans, Inorg. Chem., 2006, 45, 9181–9184; (l) M. E. Carroll,
B. E. Barton, T. B. Rauchfuss and P. J. Carroll, J. Am. Chem. Soc.,
2012, 134, 18843–18852; (m) S. Kaur-Ghuman, L. Schwartz,
R. Lomoth, W. Stein and S. Ott, Angew. Chem., Int. Ed., 2010,
49, 8033; (n) A. D. Nguyen, M. D. Rail, M. Shanmugam,
J. C. Fettinger and L. A. Berben, Inorg. Chem., 2013, 52, 12847–12854.
3 (a) S. C. Eady, M. M. MacInnes and N. Lehnert, Inorg. Chem., 2017,
56, 11654–11667; (b) R. J. DiRisio, J. E. Armstrong, M. A. Frank,
W. R. Lake and W. R. McNamara, Dalton Trans., 2017, 46,
10418–10425.
4 (a) C. L. Hartley, R. J. DiRisio, T. Y. Chang, W. Zhang and
W. R. McNamara, Polyhedron, 2016, 114, 133–137; (b) A. C. Cavell,
C. L. Hartley, D. Liu, C. S. Tribble and W. R. McNamara,
Inorg. Chem., 2015, 54, 3325–3330; (c) G. P. Connor, K. J. Mayer,
C. S. Tribble and W. R. McNamara, Inorg. Chem., 2014, 53,
5408–5410.
and 64 mV variation in Fe(III)/Fe(II) redox couple is observed per pH
unit (see ESI†).
In summary, we have synthesized a naphthalene-terminated
iron polypyridyl monophenolate complex capable of adsorbing
to glassy carbon electrodes. Additionally, this complex is elec-
trocatalytically active for proton reduction in the presence of
TFA with catalysis occurring at ꢀ1.53 V vs. Fc+/Fc and operating
with an overpotential of 480 mV. The catalyst is also active in
purely aqueous buffer solutions of pH = 3.8–6.2. This result
underscores the versatility of using p–staking interactions
to immobilize proton reduction catalysts on sp2-hybridized
carbon surfaces.
We are grateful to the National Science Foundation
(CHE-1749800) for funding. WRM also thanks the Henry Drey-
fus Foundation for funding.
5 C. L. Hartley, R. J. DiRisio, M. E. Screen, K. J. Mayer and
W. R. McNamara, Inorg. Chem., 2016, 55, 8865–8870.
Conflicts of interest
6 N. A. Race, W. Zhang, M. E. Screen, B. A. Barden and
W. R. McNamara, Chem. Commun., 2018, 54, 3290–3293.
7 (a) C. D. Jaeger and A. J. Bard, J. Phys. Chem., 1979, 83, 3146–3152;
(b) X. Chen and S. Mao, Chem. Rev., 2007, 107, 2891–2959.
8 (a) Y. Yau, G. Li, S. Ciston, R. Lueptow and K. Gray, Environ. Sci.
Technol., 2008, 42, 4952–4957; (b) I. Robel, B. A. Bunker and
P. V. Kamat, Adv. Mater., 2005, 17, 2458; (c) A. Le Goff, V. Artero,
B. Jousselme, P. D. Tran, N. Guillet, R. Metaye, A. Fihri, S. Palacin
and M. Fontecave, Science, 2009, 326, 1384–1387.
9 (a) Y. Kim and H. Park, Energy Environ. Sci., 2011, 4, 685–694;
(b) K. Dai, T. Peng, D. Ke and B. Wei, Nanotechnology, 2009, 20, 12.
10 H. L. Smith, R. L. Usala, E. W. McQueen and J. I. Goldsmith,
Langmuir, 2010, 26, 3342–3349.
11 T. Kar, H. F. Bettinger, S. Scheiner and A. K. Roy, J. Phys. Chem.,
2008, 112, 20070–20075.
12 P. J. F. Harris, Philos. Mag., 2004, 84, 3159–3167.
13 (a) N. Elgrishi, K. Rountree, B. McCarthy, E. Rountree, T. Eisenhart
and J. Dempsey, J. Chem. Educ., 2018, 95, 197–206; (b) A. J. Bard and
L. R. Faulkner, Electrochemical Methods, J. Wiley & Sons, New York,
2nd edn, 2001.
There are no conflicts to declare.
Notes and references
1 (a) D. G. Nocera, Acc. Chem. Res., 2012, 45, 767–776; (b) N. S. Lewis
and D. G. Nocera, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 15729;
(c) R. Eisenberg, Science, 2009, 324, 44; (d) J. L. Dempsey,
B. S. Brunschwig, J. R. Winkler and H. B. Gray, Acc. Chem. Res.,
2009, 42, 1995–2004; (e) D. Kim, K. K. Sakimoto, D. Hong and
P. Yang, Angew. Chem., Int. Ed., 2015, 54, 3259–3266.
2 (a) P. Connolly and J. H. Espenson, Inorg. Chem., 1986, 25, 2684;
(b) X. Hu, B. S. Brunschwig and J. C. Peters, J. Am. Chem. Soc., 2007,
129, 8988; (c) C. Baffert, V. Artero and M. Fontecave, Inorg. Chem.,
2007, 46, 1817; (d) P. A. Jacques, V. Artero, J. Pecaut and
M. Fontecave, Proc. Natl. Acad. Sci. U. S. A., 2009, 106, 20627;
(e) D. L. Du Bois, Inorg. Chem., 2014, 53, 3935–3960;
( f ) R. M. Bullock, A. M. Appel and M. L. Helm, Chem. Commun.,
2014, 50, 3125–3143; (g) Y. Sun, J. P. Bigi, N. A. Piro, M. L. Tang,
M. J. R. Long and C. J. Chang, J. Am. Chem. Soc., 2011, 133, 9212;
7700 | Chem. Commun., 2021, 57, 7697–7700
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