Redox non-innocent ligand controls water oxidation overpotential in a new family of mononuclear cu-based efficient catalysts

Article abstract of DOI:10.1021/jacs.5b03977

A new family of tetra-anionic tetradentate amidate ligands, N₁,N₁′-(1,2-phenylene)bis(N₂-methyloxalamide) (H₄L1), and its derivatives containing electron-donating groups at the aromatic ring have been prepared a

Full text of DOI:10.1021/jacs.5b03977

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Communication
Redox Non-Innocent Ligand Controls Water Oxidation Overpotential
in a New Family of Mononuclear Cu-Based Efficient Catalysts
Pablo Garrido-Barros, Ignacio Funes-Ardoiz, Samuel Drouet,
Jordi Benet-Buchholz, Feliu Maseras, and Antoni Llobet
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b03977 • Publication Date (Web): 18 May 2015
Downloaded from http://pubs.acs.org on May 21, 2015
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Redox Non-Innocent Ligand Controls Water Oxidation Overpoten-
tial in a New Family of Mononuclear Cu-Based Efficient Catalysts
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Pablo Garrido-Barros†, Ignacio Funes-Ardoiz†, Samuel Drouet†, Jordi Benet-Buchholz†, Feliu
Maseras†,‡,* and Antoni Llobet†,‡,*
† Institute of Chemical Research of Catalonia (ICIQ), Avgda. Països Catalans, 16, 43007 Tarragona, Spain
‡Departament de Química, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
Supporting Information Placeholder
ligand liberation from the metal complex is an additional
ABSTRACT: A new family of tetraanionic tetradentate ami-
driving force towards the formation of metal oxides and/or
mixed oxo-hydroxides that will be highly dependent on
working pH. On the other hand, an interesting feature of a
water oxidation catalyst design is the use of redox non-
innocent ligands that can help on the difficult task of manag-
ing the multiproton-multielectron needed to carry out the
water oxidation reaction.6 This feature would be particularly
useful if the ligand based redox processess are tied to the rate
determining step of the catalytic process and is not linked to
unwanted radical based reactions leading to fast decomposi-
tion.7
date ligands N1,N1'-(1,2-phenylene)bis(N2-methyloxalamide),
(H4L1), and its derivatives containing electron donating
groups at the aromatic ring have been prepared and charac-
terized together with their corresponding anionic Cu(II)
complexes [(LY)Cu]2-. At pH 11.5 the latter undergoes a re-
versible metal based III/II oxidation process at 0.56 V and a
ligand based pH-dependent electron transfer process at 1.25
V, associated with a large electrocatalytic water oxidation
wave (overpotential of 700 mV). Foot-of-the-wave analysis
gives a catalytic rate constant of 3.6 s-1 at pH = 11.5 and 12 s-1
at pH 12.5. As the electron donating capacity at the aromatic
ring increases the overpotential is drastically reduced down
to a record low of 170 mV. In addition, DFT calculations
allows proposing a complete catalytic cycle that uncovers an
unprecedented pathway where crucial O-O bond formation
occurs in a two-step one electron process where the peroxo
intermediate generated has no formal M-O bond, but is
strongly hydrogen bonded to the auxiliary ligand
.
Figure 1. (left) Ligand structures and (right) ORTEP figure of com-
plex [(L2)Cu]2-.
Molecular water oxidation catalysis by transition metal
complexes1 is a highly active field of research at present due
to its implications in new energy conversion schemes based
on water splitting with sunlight.2 In addition water oxidation
is also of interest in biology because it is the reaction that
takes place at the oxygen evolving complex of photosystem II
in green plants and algae.3 The very high thermodynamic
potential needed for water oxidation (1.23 V vs NHE at pH =
0.0) implies necessarily the use transition metal complexes
containing oxidatively rugged ligands, in order to come up
with long lasting systems that can have potential commercial
applications.2c,4 In addition these complexes need to be
working in water as a solvent imposing and additional re-
quirement for the auxiliary ligands that have to be substitu-
tionally inert at the pH of action since otherwise they end up
generating the corresponding aqua/hydroxo complexes and
the free ligand. This is especially critical for first row transi-
tion metal complexes as has been previously shown in the
literature5 because there will only be a limited pH range were
the integrity of the complex is maintained. Furthermore
In order to explore the options of water oxidation catalysis
based on oxidatively rugged but redox active ligands we have
prepared a family of four Cu(II) complexes containing the
tetradentate amidate acyclic ligands H4LY (Y = 1-4) with
different substituent groups at the aromatic ring (see Figure
1). The H4LY (Y = 2-4) ligands are new compounds that have
been prepared following related procedures already de-
scribed for the unsubstituted ligand H4L1.8
The
new
copper
complexes
reported
here,
[(LY)Cu](NMe4)2 (Y = 2-4), have been characterized by the
usual spectroscopic techniques and by monocrystal X-ray
diffraction analysis. An Ortep view of the cationic part of
complex [(L2)Cu](NEt4)2, is depicted in Figure 1 while for the
other methoxy derivatives is presented in the supporting
information (SI). It is interesting to observe here that the d9
Cu(II) ion is four coordinate with a basically square planar
geometry manifesting the existence of π-delocalization over
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the phenyl and amidate moieties of the ligand ensuring a
and thus in agreement with the model proposed. These ki-
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strong ligand bonding to the metal center.
netic values compare well with the ones reported in the
literature for related complexes (See Table 1). However, since
different methods are used for this type of calculation the
comparative exercise should be done with caution.
The redox properties of the anionic complexes [(LY)Cu]2-
(Y = 2-4), were investigated by CV and amperometric tech-
niques using the mercury sulfate reference electrode saturat-
ed with K2SO4 (MSE) unless explicitly indicated. All redox
potentials in the present work are reported versus NHE by
adding 0.65 V to the measured potential. Figure 2 shows the
cyclic voltammetric experiments carried out for [(L1)Cu]2- in
the pH range 11.5-12.5.
To confirm the stability of the water oxidation catalysts
spectroelectrochemical experiments were performed using
an OTTLE cell scanning from 0.4 V to 1.2 V and back to the
original 0.4 V at a very low scan rate of 2 mV·s-1, that allows
for complete transformations during the redox events. Under
these conditions the initial complex is fully recovered as
ascertained by both UV-vis spectroscopy and by charge inte-
gration under the III/II wave (see SI for details). Bulk elec-
trolysis experiments for complex [(L1)Cu]2- (Eapp = 1.3 V at pH
11.5 with a 2.5 cm2 ITO working electrode) shows a current
density of 0.11 mA/cm2 that slowly decays to 0.06 mA/cm2.
Simultaneous measurement of the oxygen gas generated by a
Clark electrode confirms the generation of dioxygen with a
Faradaic efficiency close to 100%. The decrease of current
intensity over time is a consequence of lower activity of the
catalyst as the pH decreases as has been shown by FOWA
analysis. Furthermore, both CV and UV-vis spectroscopy (see
SI) show that the initial species are totally retained and in
addition restoring the initial pH by base addition restores the
initial activity. Similarly sequential base addition during bulk
electrolysis experiments maintains the catalyst activity over
long periods of time (1 h) without apparent losses (see SI).
Further no copper oxide adsorption at the ITO electrode
surface could be detected under the present conditions as
confirmed by CV, UV-vis and EDX analysis evidencing the
molecular nature of the electrocatalytic water oxidation.
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Figure 2. Left, CV experiments of 1 mM [(L1)Cu]2-, in phosphate
buffer (0.1 M of ionic strength) at different pH values. Right, CV and
DPV for complexes [(LY)Cu]2- (Y= 1-4). The green vertical dashed
line indicates the thermodynamic Eo for the 4e- oxidation of water to
dioxygen at pH = 11.5. CVs are run at a scan rate of and 100 mV/s.
A first pH-independent, chemically reversible and electro-
chemically quasi-reversible wave at Eo = 0.56 V vs. NHE (ΔE
= 71 mV) eq. 1)
[(L1)CuIII]- (B) + 1e- -> [(L1)CuII]2- (A)
Eo = 0.56 V (1)
is associated with the formation of a d8 Cu(III) square planar
ion, with very low reorganizational energy as inferred from
the very small differences in their respective geometries, and
with a relatively low potential due the tetra-anionic nature of
the L1 ligand (the A and B labels indicated here are also used
in Figure 3). A second pH-dependent wave (approx. 59
mV/pH unit) is observed at more anodic potentials,
[(L1·)CuIII(OH)]-(C) + 1e- -> [(L1)CuIII]-(B) + OH- Eo=1.25 V (2)
associated with a ligand based-aryl oxidation, forming for-
mally a phenyl radical cation, together with the coordination
of a hydroxido ligand. This ligand based oxidation had been
previously proposed based on electrochemical grounds for
related complexes,9 and is further supported by the strong
inductive effects exerted by the phenyl substituents as it will
be shown further below and also supported by DFT calcula-
tions (vide infra). In the presence of water this wave is associ-
ated with a large electrocatalytic anodic current due the
catalytic oxidation of water to dioxygen.
Figure 3. Calculated catalytic cycle. Free energy changes for steps at
the electrode indicated explicitly in Volts (red) and for steps in
solution indicated in kcal/mol (blue).
The nature of the species generated in the catalytic cycle
was also investigated by DFT calculations (B3LYP-D3 calcula-
tions with implicit SMD solvation, see SI for Computational
Details) and nicely complement the results obtained experi-
mentally. A complete catalytic cycle is presented in Figure 3
including the energies involved in the different steps. At
oxidation state II the catalyst remains in its resting state and
is activated by two consecutive one electron transfers as
shown by CV techniques and described in equations 1 and 2.
The computed oxidation potentials were 0.53 V and 1.26 V
that are in good agreement with experimental 0.56 V and 1.25
V respectively. Calculations confirmed the radical cationic
In order to obtain kinetic information about the catalytic
process, a foot of the wave analysis (FOWA) was carried out
to calculate the apparent rate constant kcat. We followed the
methodology described in the literature,10 assuming that the
rate determining step (rds) is the last electron transfer step
coupled to a chemical reaction. The largest slope at the very
beginning of the catalytic process gives an impressive value
of kobs= 3.56 s-1 that is basically independent of the scan rate
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ligand nature of species C, as the attempts to obtain a Cu(IV)
species always reverted to the Cu(III) complex with internal
electron transfer from the ligand (Figure S27).
Species C, [(L1.)Cu(III)(OH)], reacts with an additional
OH- from the media to ultimately produce intermediate E
(See Figure 3), with a peroxo unit [(L1)Cu(II)(HO-OH)], as
usually assumed in this type of chemistry following a con-
certed two electron step.5b,11 However it follows an unusual
path that deserves some comment.
Precedent for intramolecular ligand based hydrogen bonding
has been reported recently for related Fe complexes.12 In our
case, the SET-WNA mechanism has the important conse-
quence of allowing the O-O bond formation with a very low
barrier of 5.5 kcal/mol, thus practically instantaneous after
species C is formed by oxidation at the electrode in presence
of a basic solution. The evolution from E to dioxygen release
and recovery of catalyst A follows a more conventional path-
way, which is discussed in detail in the SI, although it is
worth mentioning that the [(L1)Cu(II)(HOOH)] complex E is
first oxidized to the corresponding [(L1)Cu(III)(HOOH)]
species by a metal based electron transfer at 0.63 V. Then a
proton-coupled electron transfer happens at a potential of
0.42 V, resulting into the formation of a [(L1)Cu(III)(HOO.)]
complex which evolves dioxygen and a free proton. These
values indicate that those two processes experimentally oc-
cur in a single step involving two electrons and one proton.
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Table 1. Kinetic and Electrochemical Data of Com-
plexes [(LY)Cu]2- (Y = 1-4) and Related Cu Complexes
Described in the Literature that Have Been Reported
to Act as WOCS.
Ent.a
1tw
2tw
Catalystb
pH
11.5
11.5
11.5
11.5
8.0
kobs,s-1,d
3.56e
3.58
0.43
0.16
20
η, mVc
700
[(L1)Cu]2-
[(L2)Cu]2-
[(L3)Cu]2-
[(L4)Cu]2-
[(Py3P)Cu(OH)]-
[(dhbp)Cu(OH2)2]
[(bpy)Cu(OH)2]
Figure 4. Potential energy relaxed scan for the O-O bond formation.
The energy barrier for the second ISET is estimated from changes of
potential energy in the coordinate scan.
400
3tw
270
4tw
514c
614g
714a
814d
170
In the usual water nucleophilic attack (WNA) pathway, the
OH- would directly attack the O and form the O-O bond in a
single step through a moderate energy transition state. Our
attempts to locate such transition state failed, and revealed
instead the more complex picture indicated in Figure 4.
There is not a direct connection between species C and E
(the two curves in red) through a two electron transfer. In-
stead, they are connected through an additional electronic
state, in blue in the figure. This electronic state, which we
have labelled as species D, results from C by transfer of one
electron from the incoming OH- to the [(L1·)Cu(III)] moiety
(initially in a doublet state) to form a (HO---OH)·,- radical
anion fragment with a partial O-O bond (doublet state; with
and O---O distance of 2.3 Å) and hydrogen bonded to the
[(L1)Cu(III)] complex (H-bonding distances between 2.5-2.9
Å, see SI). D then evolves to E through a second electron
transfer, generating the HO-OH unit to a singlet state and
the [(L1)Cu(II)] unit to a doublet. In species E the HO-OH
fragment is again strongly hydrogen bonded to the
[(L1)Cu(II)] moiety (see SI). We propose to label this type of
mechanism as single electron transfer water nucleophilic
attack (SET-WNA), differing from the traditional WNA,
where the two electrons are transferred in a single step. Of
course, this SET-WNA mechanism would be impossible in
the absence of a transition metal able to undergo a fast single
electron transfer towards the HOOH moiety formation. The
generation of hydroxyl radical species would otherwise be
prohibitively high. Thus in this new SET-WNA mechanism,
the O-O bond formation step can be considered to involve
~500
12.4 ~540
12.5 750
0.4
100
[Cu2(BPMAN)(μ-OH)]3+ 7.0
~1050
0.6
(a) tw stands for this work. (b) Py3P is N,N-bis(2-(2-
pyridyl)ethyl)pyridine-2,6-dicarboxamidate; dhbp is 6,6’-dihydroxy-
2,2’-bpy;
bpy
is
2,2’-bpy;
bpman
is
2,7-[bis(2-
pyridylmethyl)aminomethyl]-1,8-naphthyridine. (c) Measured by
DPV for entries 1-4 and 8 and from the initial foot of the electrocata-
lytic wave or the half-peak potential for CVs for the rest. (d) Meas-
ured by FOWA in complexes [(LY)Cu] and other methodologies for
the other complexes. (e) kobs = 11.96 at pH = 12.5.
As evidenced by CV and confirmed by DFT, the rate de-
termining step involves the generation of the radical cation
species C and its reaction with OH-. It is thus reasonable that
the exertion of electronic perturbation on the aromatic ring
should strongly influence both thermodynamics and kinetics
of the water oxidation reaction catalyzed by this type of
complexes. For this purpose we have prepared a family of
copper complexes containing electron donating groups such
as Me and OMe in the aromatic ring as indicated in Figure 1.
The CV depicted in Figure 2 clearly shows how the onset on
catalytic wave is shifted to the cathodic region as a function
of the strength of the electron donating group. In particular
it is impressive to see that for complex [(L4)Cu]2- the overpo-
tential at which water oxidation occurs is 530 mV lower than
for [(L1)Cu]2- and is situated at only 170 mV above the ther-
modynamic value. This is the first example in the literature
where a rational ligand variation allows exerting such a de-
gree of control over the electrocatalytic water oxidation
overpotential, driving it to a record low for first row transi-
tion metal complexes.13
two consecutive single intramolecular electron transfer (ISET
)
steps. Another interesting particularity of our SET-WNA
mechanism proposed here is the absence of a direct Cu-O
bond in species D and E. Instead the (HO---OH)- and (HO-
OH)2- moieties are bonded to the [(L1)Cu]2- metal complex
via hydrogen bonding as has been already indicated above.
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A set of electrochemical parameters and kinetic data is
allobet@iciq.es; fmaseras@iciq.es
Notes
1
2
3
4
5
6
7
8
presented in Table 1, together with related data for other Cu
complexes described previously in the literature.8b,14 It is
interesting to observe that for [(L1)Cu]2-and [(L2)Cu]2-
(entries 1-2, Table 1) the rate constants are 3.56 and 3.58 s-1
respectively whereas for [(L3)Cu]2- and [(L4)Cu]2-(entries 3-4,
Table 1) the rate constants decrease by one order of magni-
tude suggesting an important involvement of the electron
transfer process at the rds and a significant stabilization of
the radical cation active species. On the other hand increas-
ing the pH from 11.5 to 12.5 increases the rate constant up to
11.96 s-1.It is also important to realize here that as the
strength of the electron donating group increases the oxida-
tive ruggedness of the radical cation species decreases mani-
festing the existence of a decomposition pathway coupled to
the water oxidation catalysis for complexes [(L3)Cu]2- and
[(L4)Cu]2-. We are focusing at present at ligand design to
improve oxidative stability. Nevertheless the [(L1)Cu]2- is the
most oxidatively rugged Cu-based water oxidation catalyst
reported to date as judged electrochemically by the charge
under the III/II redox wave before and after the electrocata-
lytic wave (see SI). The fastest Cu-based water oxidation
catalysts (WOCs) complex reported, [(bpy)Cu(OH)2], works
under an overpotential of 750 mV at pH =12.5 (see entry 8,
Table 1) whereas the complex [(Py3P)Cu(OH2)] (entry 5,
Table 1) has been reported to work at pH = 8.0 with a rate
constant of 20 s-1 and an overpotential of approx. 500 mV.
Finally a dinuclear complex [Cu2(BPMAN)(μ-OH)]3+ (entry 8,
Table 1) has been reported that works at pH 7, is relatively
slow, 0.6 s-1 and works under an overpotential of 1050 mV.
Clearly more molecular Cu based water oxidation catalysts
are needed in order to understand the factors that allow
rationally building fast complexes with oxidatively rugged
ligands and working ideally at pH 7 and with low overpoten-
tials.
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We thank MINECO (F.M.: Grants CTQ2014-57761-R, A.L.:
CTQ-2013–49075-R, Severo Ochoa ICIQ (SEV-2013-0319)) and
the ICIQ Foundation. I.F-A. also thanks the Severo Ochoa
predoctoral training fellowship (Ref: SVP-2014-068662). P.G-
B. thanks “La Caixa” foundation for a Ph.D. grant.
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Zeller, M.; Papish, E. T. Inorg. Chem. 2014, 53, 12689. (g) Zhang, T.;
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273.
In conclusion we have prepared a family of Cu complexes
that are capable of oxidizing water to dioxygen and whose
rate determining step involves the redox activity of the lig-
and. Further fine tuning of the ligand backbone allows re-
ducing the overpotential for water oxidation in this family of
complexes by more than 500 mV, all the way to a record low
overpotential of 170 mV. In addition DFT analysis puts for-
ward an unprecedented pathway where the O-O bond for-
mation occurs in a two-step one electron processes and
where the peroxo intermediate generated has no formal M-O
bond in sharp contrast with the previous mechanism de-
scribed in the literature.11c The interplay between electrons
being removed from the metal and/or the ligands opens up
new avenues for molecular water oxidation catalyst design.
We are at present focusing our attention on developing fur-
ther families of molecular water oxidation catalysts based on
redox non-innocent and oxidatively rugged ligands.
ASSOCIATED CONTENT
Supporting Information
Procedures and crystallographic data. Cartesian coordinates.
This material is available free of charge via the Internet at
http://pubs.acs.org..
AUTHOR INFORMATION
Corresponding Author
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