Iron Complexes of Square Planar Tetradentate Polypyridyl-Type Ligands as Catalysts for Water Oxidation

Article abstract of DOI:10.1021/jacs.5b08856

The tetradentate ligand, 2-(pyrid-2′-yl)-8-(1″,10″-phenanthrolin-2″-yl)-quinoline (ppq) embodies a quaterpyridine backbone but with the quinoline C8 providing an additional sp² center separating the two bipyridine-like subunits. Thus, the four

Full text of DOI:10.1021/jacs.5b08856

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Communication
Iron Complexes of Square Planar Tetradentate
Polypyridyl-type Ligands as Catalysts for Water Oxidation
Lanka D. Wickramasinghe, Rongwei Zhou, Ruifa Zong,
Pascal Vo, Kevin J Gagnon, and Randolph P. Thummel
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b08856 • Publication Date (Web): 01 Oct 2015
Downloaded from http://pubs.acs.org on October 4, 2015
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Iron Complexes of Square Planar Tetradentate Polypyridyl-type
Ligands as Catalysts for Water Oxidation
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Lanka D. Wickramasinghe,† Rongwei Zhou,† Ruifa Zong,† Pascal Vo,† Kevin J. Gagnon,‡ and
Randolph P. Thummel†*
†Department of Chemistry, 112 Fleming Building, University of Houston, Houston, TX 77204-5003, United States
‡Advanced Light Source, Lawrence Berkeley National Lab, 1 Cyclotron Rd., Berkeley, CA 94720, United States
Supporting Information Placeholder
ly feasible, earth-abundant first row transition metals began
ABSTRACT: The tetradentate ligand, 2-(pyrid-2'-yl)-8-(1'',
to increase. Situated just above ruthenium on the periodic
chart, iron became a target for oxidation catalyst design. This
approach is given further impetus by the fact that iron acts as
an important oxygen transfer agent in physiological respira-
tion.
10''-phenanthrolin-2''-yl)-quinoline (ppq) embodies a quater-
pyridine backbone but with the quinoline C8 providing an
additional sp2 center separating the two bipyridine-like sub-
units. Thus the four pyridine rings of ppq present a neutral,
square planar host that is well suited to first row transition
metals. When reacted with FeCl3, a μ-oxo-bridged dimer is
formed having a water bound to an axial metal site. A simi-
lar metal-binding environment is presented by a bis-
phenanthroline amine (dpa) which forms a 1:1 complex with
FeCl3. Both structures are verified by X-ray analysis. While
the FeIII(dpa) complex shows two reversible one-electron
oxidation waves, the FeIII(ppq) complex shows a clear two
Several different types of ligands have been used to form
iron-based water oxidation catalysts. Collins and coworkers
reported on a series of Fe complexes with tetraamido macro-
cyclic ligands.3 It was suggested that these catalysts might
involve an FeV=O intermediate.4 Although several of these
systems were quite active, unfortunately this activity was
rather short-lived. A recent study on the FeIII complex of
N,N-dimethyl-2,11-diaza[3,3](2,6)pyridinophane shows good
water oxidation activity and suggests the involvement of
FeIV=O or FeV=O intermediates.5
electron oxidation associated with the process H2O-FeIIIFeIII
-
> H2O-FeIVFeIV -> O=FeVFeIII. Subsequent disproportiona-
tion to an Fe=O species is suggested. When the FeIII(ppq)
complex is exposed to a large excess of the sacrificial electron
acceptor ceric ammonium nitrate at pH 1, copious amounts
of oxygen are evolved immediately with a turnover frequency
TOF = 7920 h-1. Under the same conditions the mononuclear
FeIII(dpa) complex also evolves oxygen with TOF = 842.
Other non-macrocyclic pyridine-containing tetra- and
pentadentate ligands provided Fe complexes that showed
modest water oxidation activity.6 In several cases a quinoline
moiety was incorporated into the ligand backbone.7 It was
suggested by Costas and coworkers that iron complexes hav-
ing cis-oriented labile sites were more active than similar
complexes with trans-oriented labile sites.8 Several different
groups have reported on the unusual reactivity of μ-oxo
bridged dinuclear iron complexes9 especially in water oxida-
tion.10 Lau and coworkers have suggested that at low pH
multidentate N-donor iron complexes accomplish water oxi-
dation through a molecular iron-oxo intermediate.11 At high
pH, however, they claim that ligand dissociation occurs to
generate Fe2O3 nanoparticles that catalyze water oxidation.
As the world’s economies expand and the global demands
for energy increase, a strong mandate has emerged for a shift
away from fossil-based fuels to a hydrogen-based economy.
The ideal source for the massive amounts of hydrogen that
will be needed is water and thus the realization of solar ener-
gy promoted decomposition of water into its elements has
become a holy grail for the twenty-first century. A significant
breakthrough in this area occurred about 10 years ago with
the discovery that mono-nuclear RuII complexes could cata-
lyze the oxidation of water in the presence of a sacrificial
chemical oxidant.1 Because of the attractive electro- and pho-
tochemical properties of its polypyridyl complexes, RuII was
initially the metal of choice for water oxidation studies and
very soon a variety of catalysts based on this metal were re-
ported.2 The fact that complexes of RuII are air-stable, dia-
magnetic with well-behaved redox properties and absorp-
tion/emission behavior made them ideal choices for initial
water decomposition studies. More recently, as the field pro-
gressed, the demand for catalysts based on more economical-
Recently we reported a new tetradentate polypyridine lig-
and, 2-(pyrid-2'-yl)-8-(1'', 10''-phenanthrolin-2''-yl)-quinoline
(ppq) that embodies a quaterpyridine backbone but with the
quinoline C8 providing an additional sp2 center separating
the two bipyridine-like sub-units.12 The result is a more
square planar arrangement of the four pyridine binding
units. The smaller first row transition metal CoII readily
formed a complex with ppq that demonstrated a comfortable
5-6-5 chelate ring pattern. We found that in the presence of
[Ru(bpy)3]Cl2, ascorbic acid, and a blue LED at pH = 4.5 the
Co-ppq complex was an excellent proton reduction catalyst.12
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Scheme 1
1
O
AlCl3
POBr3
2
3
4
5
6
7
∆, - C6H6
N
N
O
H
H
Br
Br
N
1
2
(
55%)
1 equiv.
Sn(n-Bu)3
1. n-BuLi, THF
N
Br
(41%)
N
2. B(OCH3)3
Br
4
N
Br
PdCl2(PPh3)2
3
(60%)
8
Figure 1. X-ray crystal structure of FeIII(ppq) (left) and
9
N
N
N
N
FeIII(dpa) (right) showing ellipsoids at 50% probability with
hydrogens omitted for clarity.
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N
Cl
N
B(OH)2
N
Pd(PPh3)4
N
ppq
(65%)
5
(78%)
Several interesting geometric features for the two com-
plexes can be pointed out (Table S2). The Fe-N bond lengths
fall into a rather narrow range for each complex. For the ppq
complex they are slightly longer at 2.11-2.16 Å while for the
dpa complex they are 2.09-2.11 Å. The Fe-Cl bond for the ppq
complex is 0.11-0.15 Å longer than the the Fe-Cl bonds in the
dpa complex. This increased Fe-Cl bond length implies more
polar bonds that are more easily broken. For the ppq com-
plex the interior Fe-O bonds are 0.38-0.35 Å shorter than the
exterior Fe-OH2 bond. The 5-membered chelate rings have
N-Fe-N angles that are more acute at 78.4-79.9 Å while the
N-Fe-N angles for the 6-membered chelate ring are closer to
the ideal 90° at 85.3°, 87.8° and 88.0°. The exterior N-Fe-N
angle is considerably expanded at 112.1-115.6°. All this data
supports the fact that the complexes have rather similar ge-
ometries. We suspect that it is the N-isopentenyl group in
the dpa complex that may inhibit formation of a μ-oxo-
bridged dimer.
An impediment to the study of complexes based on ppq is
the difficulty in accessing 2-nitro-3-bromobenzaldehyde
which is reduced to the corresponding amine and then used
in the initial Friedländer condensation to afford 8-bromo-2-
(pyrid-2'-yl)-quinoline (4). We have developed a four step
alternate synthesis of 4 starting with the amide 1 derived
from the reaction of 2-bromoaniline and cinnamoyl chlo-
ride13 (scheme 1). This material undergoes an intramolecular
Friedel-Crafts acylation followed by the thermal loss of ben-
zene to give the bromoquinolone 2. Treatment of this mate-
rial with POBr3 gives 3.14 The Stille coupling of 3 with 2-
pyridyl-tri-n-butylstannane occurs selectively at the 2-
position to provide 4. In the final step, we have replaced the
Friedländer condensation with a Suzuki coupling of 5 with 2-
chloro-1,10-phenanthroline to afford ppq in 78% yield.
Scheme 2
The electrochemical behavior of both Fe complexes was
measured in CH3CN and the results are recorded in Table 1
and illustrated in Figure 2. FeIII(ppq) showed a reversible
oxidation at 0.01 V (∆Ep = 75 mV) and another quasi-
reversible oxidation at 0.49 V (∆Ep = 55 mV) vs. Ag/AgCl. The
current amplitude observed for the first oxidation is more
than twice as great as that observed for the second oxidation
process, suggesting that the first oxidation is likely a two-
electron process. This process can be tentatively attributed to
the simultaneous oxidation of the two FeIII-centers (FeIIIFeIII
→ FeIVFeIV). The observation of a single two-electron wave
can be explained in two ways. In one case the two Fe centers
do not interact at all and thus both oxidize at the same po-
tential but this explanation seems unlikely since the two Fe
centers are separated by only one atom. More likely, the
FeIII(ppq) core (Cl-FeIII(ppq)-O-FeIII(ppq)-OH2) could behave
as a single species and undergo a two electron oxidation.
This explanation would agree with electrochemical theory
which states that the current passed for a one electron pro-
cess (A) is equal to n1/2 which equals 1.0 when n = 1. Similar-
ly, the current passed for a two electron process (B) is equal
to n3/2 which is 2.82 for n = 2.17 The B/A ratio from Figure 2 is
2.71. The direct two electron oxidation would afford a
FeIVFeIV-OH2 species that could then disproportionate with
the loss of two protons to afford a FeIIIFeV=O species. The
oxidation process observed at 1.3 V was confirmed as the
oxidation of chloride by the enhancement of this wave upon
the addition of KCl (Figure S1).
The geometry of ppq is rather unique in that the binding
cavity, in its all-cisoid conformation, involves four pyridine
rings in a square planar arrangement. In surveying the litera-
ture, we find only a very few systems with a similar geometry.
These are mainly bis-phenanthrolines linked at their 2-
position with either a carbonyl group15 or a tertiary amine.
Following a reported procedure, we prepared the known di-
phen amine (dpa)16 in order to compare its metal binding
properties to ppq. When a CH2Cl2 solution of either ppq or
dpa was treated with a MeOH solution of FeCl3, a complex
readily formed (scheme 2) and was characterized by X-ray
crystallography. Interestingly, while dpa formed a 1:1 complex
with FeCl3, ppq formed a μ-oxo-bridged dimer (Figure 1).
Pertinent bond lengths and bond angles are summarized in
Table S2.
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to destruction of the catalyst and activity can be restored by
1
the addition of more catalyst (Figure 4).
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Figure 3. CV scans of FeIII complexes (ppq, left and dpa,
right) measured in pH 1 buffer with carbon working elec-
trode, Pt wire auxiliary electrode, and Ag/AgCl reference
electrode at a scan rate of 100 mV/s.
Figure 2. Cyclic voltammetry data of FeIII complexes meas-
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ured in CH3CN with carbon working electrode, Pt wire auxil-
iary electrode, and Ag/AgCl reference electrode at a scan rate
of 100 mV/s with TBAPF6 as the supporting electrolyte.
Figure 4. (Left) Plot of O2 evolution vs time for FeIII(ppq)
(2.4 μM, red) and FeIII(dpa) (2.5 μM, green) upon addition of
CAN (0.1 M) in HNO3 solution (10 mL, pH 1). (Right) The
regeneration of O2 evolution by addition of FeIII(ppq) cata-
lyst (50 μL of 1.29 mM solution) to 0.1 M HNO3 (10 mL) con-
taining CAN (0.1 M) and FeIII(ppq) (10 μM) after O2 evolution
had diminished.
The electrochemical behavior of the mononuclear
FeIII(dpa) complex is quite different. This system shows a
clearly reversible one electron oxidation at almost the same
potential as the FeIII(ppq) complex, 0.01 V (∆Ep = 76 mV). A
second reversible oxidation is observed at 0.45 V (∆Ep = 86
mV). These waves are thus assigned to the FeIII/IV and FeIV/V
couples. Both complexes show a reversible one electron re-
duction. This reduction is most likely metal-based FeIII
→
FeII and indicates that ppq in the dinuclear FeIII(ppq) com-
plex is a somewhat better electron acceptor than FeIII(dpa).
The electrochemical and structural analysis of the two Fe
complexes suggests that they might owe their catalytic activi-
ty to quite different mechanisms. An obvious question was
whether the dimeric complex might dissociate to a mono-
meric species in solution. We examined the UV-Vis spectra
of both complexes in aqueous solution and saw no change
after more than 6 hours (Figure S2). We also studied the
kinetics of the water oxidation process by measuring the
amount of oxygen generated versus time for both FeIII(ppq)
and FeIII(dpa) added to a CAN solution in nitric acid (Figure
S3). From these plots we were able to measure initial rates
and when these rates were plotted against catalyst concen-
tration, clear first order behavior was revealed (Figure 5). It
appears most likely therefore that the dimeric catalyst re-
mains intact during the oxidation process.
Table 1. Cyclic voltammetry data for FeIII complexesa
Complex
E , (ΔEp) (V)
½
E , (ΔEp) (V) E , (ΔEp) (V)
½
½
FeIII(ppq) -0.87 (62 mV)
FeIII(dpa) -1.20 (94 mV)
0.01 (75 mV)
0.49 (55 mV)
0.01 (76 mV) 0.45 (86 mV)
aMeasured in CH3CN with carbon working electrode, Pt
auxiliary electrode, and Ag/AgCl reference electrode at a
scan rate of 100 mV/s with TBAPF6 as the supporting elec-
trolyte.
The two complexes were also studied in pH 1 aqueous
buffer where they both evidenced the onset of an electrocata-
lytic wave at about +1.25 V vs. Ag/AgCl (Figure 3). The impli-
cation that these complexes could serve as water oxidation
catalysts prompted us to study their behavior in the presence
of the strong chemical oxidant ceric ammonium nitrate
(CAN). When CAN (0.1 M) was added to a nitric acid solu-
tion (pH 1) of either FeIII(ppq) or FeIII(dpa) (approx. 2.5 μM),
copious amount of oxygen were generated immediately (Fig-
ure 4). It was also very clear that the dimeric ppq complex
was considerably more active than the monomeric dpa com-
plex. Interestingly, the ppq complex shows a fast initial rate
(7920 h-1) but looses most of its activity after one hour while
the slower dpa complex continues to show activity after one
hour. The diminished activity of FeIII(ppq) is apparently due
Figure 5. Initial rate of O2 formation vs. catalyst conc for
FeIII(ppq) (left) and FeIII(dpa) (right).
In other related systems workers have reported the ap-
pearance of an absorption band at 613 nm that they attribute
to the formation of a FeV=O species.4 When we add CAN to
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an aqueous solution of FeIII(ppq) we observe the immediate
ment of Energy under Contract No. DE-AC02-05CH11231, and
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formation of a green color and band at 675 nm that quickly
disappears after six minutes (Figure 6). To address the per-
sistent concern about the possible formation of iron oxide
nano-particles, we have examined the reaction solutions for
both FeIII(ppq) and FeIII(dpa) after the cessation of oxygen
generation using dynamic light scattering. No evidence was
found for the formation of any nano-particles (Figure S4).
the Robert A. Welch Foundation (Grant E-621) for financial
support of this work. We also thank Professor Karl Kadish for
a helpful discussion and Ms. Maria A. Vorontsova for con-
ducting dynamic light scattering experiments.
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Figure 6. Electronic absorption spectrum of FeIII(ppq) (0.1
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ASSOCIATED CONTENT
Supporting Information
X-ray crystallographic data for FeIII(ppq) and FeIII(dpa) in CIF
format. Synthetic details and electrochemical data. This ma-
terial is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
thummel@uh.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
This material is based upon work supported by the U.S. De-
partment of Energy, Office of Science, Office of Basic Energy
Sciences under Award Number DE-FG02-07ER15888, the
Advanced Light Source supported by the Director, Office of
Science, Office of Basic Energy Sciences, of the U.S. Depart-
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60
5
ACS Paragon Plus Environment