Donor-Flexible Nitrogen Ligands for Efficient Iridium-Catalyzed Water Oxidation Catalysis

Article abstract of DOI:10.1002/chem.201600875

A pyridylideneamide ligand with variable donor properties owing to a pronounced zwitterionic and a neutral diene-type resonance structure was used as a dynamic ligand at a Cp? iridium center to facilitate water oxidation catalysis, a reaction that requires the stabilization of a variety of different iridium oxidation states and that is key for developing an efficient solar fuel device. The ligand imparts high activity (nearly three-fold increase of turnover frequency compared to benchmark systems), and exceptionally high turnover numbers, which indicate a robust catalytic cycle and little catalyst degradation.

Full text of DOI:10.1002/chem.201600875

DOI: 10.1002/chem.201600875
Communication
&
Ligand Design
Donor-Flexible Nitrogen Ligands for Efficient Iridium-Catalyzed
Water Oxidation Catalysis
[
a, b]
[c]
[b]
[c]
Miquel Navarro,
Mo Li, Helge Müller-Bunz, Stefan Bernhard,* and
[a, b]
Martin Albrecht*
Chem. Eur. J. 2016, 22, 6740 – 6745
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Communication
much better separated (II); however, the neutral L-type bond-
[
3]
Abstract: A pyridylideneamide ligand with variable donor
properties owing to a pronounced zwitterionic and a neu-
tral diene-type resonance structure was used as a dynamic
ligand at a Cp* iridium center to facilitate water oxidation
catalysis, a reaction that requires the stabilization of a vari-
ety of different iridium oxidation states and that is key for
developing an efficient solar fuel device. The ligand im-
parts high activity (nearly three-fold increase of turnover
frequency compared to benchmark systems), and excep-
tionally high turnover numbers, which indicate a robust
catalytic cycle and little catalyst degradation.
ing is less pronounced. Moreover, N-heterocyclic carbenes
[
4]
have typically only relatively low p-acidic properties.
In contrast, N-type ligands can be engineered to adapt varia-
ble donor properties. For example, reversible exocyclic proto-
nation and deprotonation allows a pyridine ligand to adopt
either a neutral, p-acidic imine coordination mode, or an
[
5]
anionic and p-basic amide structure (III). This concept has
been expanded to a variety of other ligands, which can switch
from p-acidic to p-basic properties without the need of
a proton transfer. For example, the guanidine derivative IV can
bind to the metal center either as a formally neutral or anionic
[
6]
donor site. Likewise, pyridylideneamines (PYEs) and pyridyli-
deneamides (PYAs; V) are electronically flexible N-donor sites
that may coordinate to the metal center as a p-acidic imine, or
Oxidation catalysis continuous to be one of the most challeng-
ing domains of homogeneous catalysis. A key issue is the ne-
cessity to stabilize high- and low-valent metal centers on a pu-
tative catalytic cycle. Classically, ligands of homogeneous cata-
lysts have a set of intrinsic and static characteristics such as
either p donor or p acceptor properties, which thus stabilize
[
7]
as a p-basic zwitterionic pyridinium amide. We have recently
demonstrated the adaptiveness of this ligand in response to
the external environment, such as the solvent polarity, and
have exploited this donor flexibility to enhance transfer hydro-
[
8]
genation catalysis. Here we introduce a new, phenyl-substi-
tuted PYA ligand that is easily available from simple starting
materials and that entails substantial synthetic versatility for
further modulation. The donor flexibility and the ensuing stabi-
lization of different metal oxidation states has been exploited
in water oxidation catalysis, providing one of the fastest and
most robust catalytic systems known to date with turnover
numbers close to 100000 for this challenging reaction.
[
1]
either the oxidative or the reductive step in a catalytic cycle.
More efficient would be ligands that can stabilize multiple oxi-
dation states, thus facilitating a variety of transformations rele-
vant to the catalytic process. One such ligand class may be
[2]
heterocyclic carbenes, which display neutral carbonic (L-type)
character as well as ionic (X-type) bonding properties if one
considers an azolium-yl resonance structure (I, Figure 1). This
dynamic bonding is shifted in mesoionic N-heterocyclic car-
benes, in which the positive and the negative charges are
The new PYA ligand precursor 1 was prepared by straight-
forward amide formation and subsequent alkylation with
MeI, which occurred selectively at the pyridine nitrogen
[
9]
[
10]
[11]
(
Scheme 1).
Deprotonation of the amide NH unit
with
common bases such as NaOH afforded the new PYA ligand 2
as a free base in excellent yield and high purity. The free base
was stable for solution analysis as well as X-ray diffraction. In
particular, it was considerably more soluble in apolar solvents
such as CH Cl , suggesting a pronounced neutral diene-type
2
2
1
resonance structure (cf. Figure 1). The H NMR spectrum indi-
cated the disappearance of the NÀH resonance (d =11.52 in
H
1
) and a larger separation of the a and b protons of the pyridyl
Figure 1. Donor-flexible ligands for redox catalysis.
heterocycle (Dd =0.80 in CD Cl ), in agreement with a pro-
H 2 2
nounced diene configuration and substantial loss of aromatici-
[
12]
ty. Addition of the free PYA to [Ir(Cp*)Cl ] induced cyclome-
2
2
[
a] M. Navarro, Prof. M. Albrecht
Department of Chemistry & Biochemistry
University of Bern
Freiestrasse 3, 3012 Bern (Switzerland)
Fax: (+41)31631-4644
talation and afforded, after purification by column chromatog-
raphy, complex 3 as an air-stable solid in 60% yield (Cp*=
À
C5Me , pentamethylcyclopentadienyl). This yield considerably
5
improved to 89% when NaOAc was added as auxiliary, since
E-mail: martin.albrecht@dcb.unibe.ch
NaOAc is known to promote chelate-assisted CÀH bond activa-
[
13]
[
b] M. Navarro, Dr. H. Müller-Bunz, Prof. M. Albrecht
School of Chemistry and Chemical Biology
University College Dublin
tion. Cyclometalation was evidenced by the desymmetriza-
1
tion of the resonances due to the phenyl group in the H NMR
spectrum as well as by X-ray diffraction (Scheme 1). Spontane-
ous cyclometalation at room temperature is in agreement with
the general reactivity pattern observed for iridium(III) when co-
ordinated to strong donor ligands such as N-heterocyclic car-
Belfield, Dublin 4 (Ireland)
[
c] M. Li, Prof. S. Bernhard
Department of Chemistry
Carnegie Mellon University
4
400 Fifth Avenue, Pittsburgh, PA 15213 (USA)
[14]
benes.
E-mail: bern@cmu.edu
Formation of the solvento complexes 4a,b was straightfor-
ward and involved chloride abstraction from complex 3 with
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under http://dx.doi.org/10.1002/
chem.201600875.
KPF in the presence of either MeCN (4a) or DMSO (4b). Both
6
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Scheme 1. a) Synthesis of iridium complexes 3 and 4a,b, and the most relevant limiting resonance structures A (X-type ligand) and B (L-type ligand) and
ORTEP representation of the crystal structures of compounds 1–4a (50% probability ellipsoids, co-crystallized solvents and hydrogen atoms omitted for clari-
ty).
solvento complexes were fully characterized by NMR spectros-
copy, X-ray diffraction, and microanalysis.
vento complex 4b (albeit with a chloride counterion for 3 dis-
solved in DMSO). When the complexes were dissolved in
MeCN, complex 3 and 4a gave identical spectra, again sug-
gesting fast solvolysis of the IrÀCl bond in MeCN. However,
complex 4b is distinctly different and reveals that DMSO re-
mains coordinated. In polar media, therefore the bond stability
in these PYA complexes is IrÀDMSO>IrÀNCMe>IrÀCl, which
has obvious implications for catalysis.
The electronic flexibility of the PYA system owing to variable
contributions of neutral or mesoionic resonance structures was
established from structural comparison of compounds 1–4 in
solution and in the solid state. In the solid state, analysis of the
bond length alterations in the pyridylic ring was accomplished
by X-ray diffraction and indicates marked double bond localiza-
tion in the free base 2, since the CÀC bonds to C are signifi-
Because of these ligand solvolysis effects, complex 4b with
the most robust IrÀL bond was used to probe the predomi-
nant structure in solution. The shift differences between the
pyridylidene a and b proton resonances in CD Cl vary by 0.66
g
cantly longer than the C ÀC bonds (average bond length
a
b
[
15]
1
.427(3) vs. 1.363(5) Š, bond length difference Dd=0.064 Š).
This neutral diene structure is slightly less pronounced if the
amide nitrogen is bound to the iridium center (complex 3,
Dd=0.04 Š) and even less when bound to a proton (com-
pound 1, Dd=0.030 Š). Accordingly, the aromaticity is expect-
ed to be lowest in the free base structure. This trend thus re-
flects the localization of the formal electron pair on nitrogen
and is least in 1 with a rather covalent NÀH bond, more local-
ized in the NÀIr bond, and most pronounced in the free base
2
2
and 0.43 ppm for the free PYA and complex 4b, respectively,
and thus suggest a slightly more pronounced diene structure
for the PYA 2. Complex 3 in contrast shows only a small differ-
ence (0.26 ppm), which insinuates a more zwitterionic reso-
nance form. This notion is in agreement with the structure de-
duced in the solid state. More interesting, however, is the as-
sessment of the flexibility of the PYA in its free, proton- and iri-
dium-bound state by altering the solvent and hence the local
2
without any substituent on nitrogen. In agreement with
1
such a notion, the solvento complex 4a features the smallest
difference between the pyridylidene CaÀCb and the CbÀCg
bonds, in agreement with a higher degree of aromaticity and
a higher preponderance of the zwitterionic resonance struc-
ture. This is readily rationalized by the cationic nature of the iri-
dium center, and the higher electrophilicity, thus inducing
a stronger dipole in the PYA ligand. However, we note that the
DMSO complex 4b shows a very large diene-type structure
with average CaÀCb and CbÀCg bond lengths of 1.360 and
environment. Such H NMR investigations were performed in
a non-polar solvent (CD Cl ; e
=9.08) and in one with high
2
2
CD2Cl2
[
16]
polarity ([D ]DMSO; e
=46.7).
The spectra consistently
6
DMSO
show a shielding of the b protons and a downfield shift of the
a proton resonances upon increasing the polarity (Figure 2).
This shift difference Dd serves as an indicator for the reso-
nance structure flexibility of PYA ligand in solution, and its abil-
ity to toggle between X-type bonding through the zwitterionic
resonance structure A and L-type bonding in the diene struc-
[
15]
[8]
1
.415 Š, respectively.
In solution, the NMR chemical shift differences between the
pyridylidene a and b protons are a diagnostic probe for the
ture B. The shift difference is largest for complex 4b (Dd=
0.69 ppm) and thus suggests substantial bonding dynamics,
and stabilization of p-donor as well as p-acceptor interactions
through preponderance of either X-type bonding (in DMSO)
or the neutral L-type diene resonance form (in CH Cl ).
[
8,12]
prevalence of the mesoionic or the neutral resonance form.
In [D ]DMSO, complexes 3, 4a, and 4b reveal exactly the same
6
2
2
chemical shifts, indicating that chloride and MeCN dissociation
in DMSO are rapid and that the prevailing complex is the sol-
Notably, the flexibility is larger than in the free PYA ligand 2
(Dd=0.50 ppm). Such variable bonding properties are expect-
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Figure 3. a) Oxygen evolution traces of water oxidation reactions catalyzed
by complexes 3, 4a, and complex 5 as benchmark and representative of
previously investigated C,N-chelate systems. Oxygen evolution was mea-
sured by pressure transducer and calibrated by GC using CAN (0.4m) and iri-
dium complex (50Æ0.7 mm) with maximum theoretical turnover 1900 (see
1
Figure 2. Aromatic region of the H NMR spectrum of complex 4b in
a) [D ]DMSO, b) CD CN, and c) CD Cl , indicating the diagnostic shift of the
6
3
2
2
a and b protons of the N-mesoionic PYA ligand.
2
the Supporting Information); inset: log–log plot of maximum O evolution
rates (vmax) at different concentrations of complex 3 (0.9-49.6 mm) and its
linear fit using 0.4m CAN, revealing an apparent 1.5 order on catalyst (slope
ed to facilitate the access of high and low iridium oxidation
states.
2
1
.47, R =0.98), which is in agreement with many previously studied catalyst
[24]
systems. b) Schematic representation of complexes 3–5.
We have exploited these properties in iridium-catalyzed
water oxidation, one of the most pressing challenges towards
the development of artificial photosynthetic processes for the
[a]
Table 1. Catalytic water oxidation with complexes 3, 4a, and 5.
[
17]
sustainable capture and storage of transient energy. Water
oxidation catalysis is supposed to involve high oxidation state
iridium(V) as well as iridium(IV) intermediates that should both
Complex
conc.
mM]
CAN/complex
TON
TOFmax
À1
[
[min
]
[
18]
become more accessible with donor-flexible ligands.
Of
3
49.6
50.7
49.6
4.6
4.6
4.6
7380
7220
7380
79540
79540
79540
406510
406510
1890
1800
1840
18830
19380
18830
85880
75300
26.9
28.5
11.9
6.9
6.2
3.9
4
5
3
4a
5
a
note, these new PYA iridium complexes share structural similar-
ities with phenylpyridine complexes as employed in pioneering
[
19]
[20]
studies by Bernhard and by Crabtree, involving a formally
monoanionic N,C-bidentate chelating system. Such C,N-chelate
iridium complexes, in particular when coordinated to a Cp*
ligand, are amongst the most active molecular water oxidation
3
4
0.9
0.9
2.7
2.5
a
[21]
[a] Measurements were performed in a sealed 40 mL EPA vial containing
0 mL 0.4m CAN solution buffered in 1m HNO and the appropriate com-
plex; O evolution was dynamically monitored by manometry and end
points were calibrated by gas chromatography; TOFmax values were deter-
mined by monitoring oxygen evolution rates (see the Supporting Infor-
mation for further details).
catalysts known to date
and hence constitute valuable
1
3
benchmarks for catalyst evaluation. Unlike in phenylpyridines,
the N-donor in complex 3 is flexible and thus enhances the
electron density at iridium, especially in water as a high dielec-
tric medium (cf. discussion above). The bonding dynamics are
reminiscent of mesoionic carbenes, which have been estab-
lished as reliable spectator ligands for water oxidation cata-
2
[
22]
lysts.
indicating that the oxygen evolution process in the studied
system is similar to those in earlier investigated, structurally di-
Complex 3 is completely soluble in water and, in the pres-
[
23]
ence of (NH ) [Ce(NO ) ] (CAN) as sacrificial oxidant, immediate
verse iridium-based catalysts.
The reaction rate peaks at
4
2
3 6
gas formation was observed, indicating catalytic activity of 3
towards water oxidation. Quantitative analysis revealed that
complex 3 is substantially more active than the previously
a Ce(IV) concentration of 0.05m, which is consistent with previ-
ous observations (Figure S1, Supporting Information). The
effect of increasing Ce(IV) concentrations may point to
[19–22]
[22b,24]
evaluated C,N-chelated iridium complexes.
Whereas those
a change in mechanism for OÀO bond formation.
No de-
systems displayed turnover frequencies (TOFs) around
tectable amounts of CO were produced, which is in agree-
2
À1
1
0 min , complex 3 and its solvento analogue 4a show nearly
ment with related cyclometalated iridium complexes and indi-
cative of a robust coordination of the PYA ligand to the iridium
center. Likewise, DLS measurements do not indicate the forma-
tion of aggregates, neither from iridium nor from cerium pre-
cursors (Figures S3–S6 in the Supporting Information).
threefold higher activity with maximum turnover frequencies
À1
TOFmax =28Æ1 min (Figure 3, Table 1). The maximum turn-
over frequency gradually decreases with decreasing catalyst
loading. For complex 3, a reaction order of 1.5 was detected,
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Stabilization of critical transformative steps on a catalytic
cycle entails a flat energy surface and hence a reduced proba-
bility for side reactions that lead to catalyst deactivation or
degradation. The impact of the PYA ligand has therefore been
probed in long-term experiments under high turnover condi-
tions. Thus, in the presence of 406510 equivalents of CAN per
Acknowledgements
We thank Dr. P. Macchi (Univ. Bern) for solving the X-ray struc-
tures of complexes 4a and 4b. We acknowledge generous fi-
nancial support from the European Research Council (CoG
6
1
1
15653), the Swiss National Science Foundation (200021_
62868, and 206021_128724), Science Foundation Ireland (RC/
074), and U.S. National Science Foundation (CHE-1362629).
[
15]
catalyst, complex 3 was constantly active for over 28 days,
accomplishing up to 86000 turnovers (Table 1, Figure S2 in the
Supporting Information). No CO was detected throughout this
2
long-term run. Despite the large excess of CAN used for these
experiments, blank reactions with CAN only and in the absence
Keywords: donor flexibility · iridium · N-donor ligands · water
oxidation · zwitterion
of complex 3 gave no detectable quantities of O , hence em-
2
phasizing the critical role of the iridium complex. The achieved
TON is one of the largest reported thus far for homogeneous
[
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4
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lution increased significantly for both complexes 3 and 4a
[15]
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conditions, the maximum turnover frequency is around
[
À1
4
0 min , about twice as fast as the analogous CAN-driven re-
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the PYA iridium complex concentration increased the maxi-
À1
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imparts donor properties that can flexibly change without
chemical modulation of the ligand scaffold, that is, no proton
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transient proton or electron scavenging, thus facilitating
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Received: February 24, 2016
Published online on April 5, 2016
Chem. Eur. J. 2016, 22, 6740 – 6745
www.chemeurj.org
6745
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim