Ruthenium water oxidation catalysts containing the non-planar tetradentate ligand, biisoquinoline dicarboxylic acid (biqaH2)

Article abstract of DOI:10.1039/c6dt03880h

Two ruthenium complexes containing the tetradentate ligand [1,1′-biisoquinoline]-3,3′-dicarboxylic acid, and 4-picoline or 6-bromoisoquinoline as axial ligands have been prepared. The complexes have been fully characterised and initial studies on their potential to function as molecular water oxidation catalysts have been performed. Both complexes catalyse the oxidation of water in acidic media with Ce^IV as a stoichiometric chemical oxidant, although turnover numbers and turnover frequencies are modest when compared with the closely related Ru-bda and Ru-pda analogues. Barriers for the water nucleophilic attack and intermolecular coupling pathways were obtained from density functional theory calculations and the crucial influence of the ligand framework in determining the most favourable reaction pathway was elucidated from a combined analysis of the theoretical and experimental results.

Full text of DOI:10.1039/c6dt03880h

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Ruthenium water oxidation catalysts containing the non-
planar tetradentate ligand, biisoquinoline dicarboxylic acid
(
biqaH2)
Dominik Scherrer, Mauro Schilling, Sandra Luber,* Thomas Fox, Bernhard Spingler, Roger Alberto and
Craig J. Richmond*
Email: craig.richmond@chem.uzh.ch; sandra.luber@chem.uzh.ch
Abstract
Two ruthenium complexes containing the tetradentate ligand [1,1'-biisoquinoline]-3,3'-dicarboxylic
acid, and 4-picoline or 6-bromoisoquinoline as axial ligands have been prepared. The complexes have
been fully characterised and initial studies on their potential to function as molecular water oxidation
catalysts have been performed. Both complexes catalyse the oxidation of water in acidic media with
IV
Ce as a stoichiometric chemical oxidant, although turnover numbers and turnover frequencies are
modest when compared with the closely related Ru-bda and Ru-pda analogues. Barriers for the
water nucleophilic attack and intermolecular coupling pathways were obtained from density
functional theory calculations and from a combined analysis of the theoretical and experimental
results, the crucial influence of the ligand framework in determining the most favourable reaction
pathway was elucidated.
Introduction
As society strives to wean itself off its reliance on carbon-based fuels, alternative new technologies
1
-5
need to be discovered and developed to facilitate this revolution. Researchers in the field of
Artificial Photosynthesis aim to achieve this by using sunlight to drive the production of so called
2
solar fuels (e.g. H , MeOH) from the oxidation of water and the reduction of Earth-abundant
+
6-8
2
substrates (e.g. H , CO ). Of the key steps involved in this process (light harvesting, charge
9
separation, substrate reduction and water oxidation), water oxidation is considered to be a major
bottle neck and new catalysts capable of facilitating this reaction are therefore constantly in high
1
0-13
demand.
number of molecular water oxidation catalysts (WOCs), including the Ru-bda catalysts (bdaH
The group of Licheng Sun has been particularly active in this area and has published a
2
=
1
4-19
bipyridine dicarboxylic acid),
some of the fastest and most stable molecular WOCs reported to
date. Much work was carried out on varying the axial ligands of these catalysts, which resulted in
2
0-23
catalysts with increased turnover numbers (TONs) and turnover frequencies (TOFs).
However,
In the particular
ligand (pda = phenanthroline
2
4-28
less work has been done with respect to studying equatorial ligand derivatives.
example where the bda ligand was exchanged for the pdaH
2
dicarboxylic acid), a profound effect on the water oxidation mechanism was observed, resulting in a
switch from an intermolecular coupling mechanism (I2M) to water nucleophilic attack (WNA). The
reasoning behind this switch was proposed to be caused by changes in the reorganization energies
throughout the catalytic cycle as a result of the increased rigidity of the ligand. Intrigued by these
results, we decided to study the complementary effect of enforced non-planarity as opposed to
enforced planarity and rigidity in the equatorial ligand plane.
Herein, we report a new variation of the Ru-bda-type WOCs, based on the non-planar ligand [1,1'-
2
biisoquinoline]-3,3'-dicarboxylic acid (biqaH ). The synthesis of the new equatorial ligand and two of
its ruthenium complexes along with their characterisation and evaluation as WOCs is presented
forthwith.

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Results and Discussion
2
Synthesis. The biqaH ligand 7 was prepared in good overall yield over six steps starting from cheap
and readily available precursors (Scheme 1). The first four steps followed previously reported
2
9-31
procedures,
of analogous compounds.
methylisoquinoline and forming the diacid via the chromic acid oxidation of 3,3’-dimethyl-1,1’-
whilst the conditions for the final two steps were adapted from reported procedures
3
2,33
An initial five step synthesis of the biqaH ligand 7, starting from 3-
2
3
4
biisoquinoline, was originally planned, however, this route proved to be unsuccessful due to the
inability to selectively oxidise the methyl groups without affecting the isoquinoline moiety. The six
step procedure without the need for the harsh oxidation step was consequently adopted (Scheme 1).
Coordination of the BiqaH
picoline (pic) or 6-bromoisoquinoline (Br-isoq) was accomplished in a single step to afford complexes
a and 8b respectively. A mixture of compounds was formed under these conditions but the desired
2
ligand to ruthenium and substitution of the axial ligands with either 4-
8
complexes were always the major product and were isolable via simple precipitation or column
chromatography. Attempts to coordinate the ruthenium in a 2-step sequence and first isolate the
2
Ru(biqa)(DMSO) intermediate were unsuccessful and tended to yield mixtures of compounds, none
1
of which were the desired intermediate compound as analysed by HPLC, MS and H NMR. The
isolated complexes 8a and 8b were fully characterised by standard analytical techniques (HPLC,
1
D/2D NMR, MS, UV-vis, EA and voltammetry).
2
Scheme 1. Synthesis of the biqaH ligand 7 and
subsequent ruthenium complexes 8a and 8b.
X-ray structure. Additionally for complex 8b, single crystals suitable for x-ray diffraction were grown
and the absolute structural configuration was confirmed (see Figure 1 and ESI). Interestingly, the
coordination environment of the Ru centre in complex 8b is very similar to analogous Ru-bda-type
1
5,16,35
structures previously reported,
despite the obvious distortion of the biisoquinoline moiety. The
strain imposed by the inherent non-planarity of the biisoquinoline, combined with an almost planar
coordination of the donor atoms to ruthenium, is therefore manifested in a bending of the

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isoquinoline carboxylates rather than a twisted coordination geometry. This is thought to occur
because the octahedral geometry is already distorted due to the widening of the O-Ru-O angle from
the ideal 90° to ca. 120°, similar to Ru-bda structures, and therefore does not permit any further
distortion of the primary coordination sphere that may have been expected as a result of the non-
planarity of the biqa ligand backbone.
Figure 1. ORTEP plot at 50%
probability for compound 8b. Colour
code: C, grey; Br, brown; N, blue; O,
red; Ru, magenta. Hydrogens have
been omitted for clarity.
Electrochemistry. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in both organic
and aqueous solutions were used to probe the electrochemical properties of compounds 8a and 8b.
Not surprisingly, the electrochemical behaviour of the Ru-biqa complexes was found to be very
similar to the analogous Ru-bda complexes. The CVs and DPVs in DCM (0.1 M TBA(PF )) + 20 %
6
III-II
Trifluoroethanol (TFE) indicated a single reversible redox process for the Ru couple at 0.87 V and
3
6
0
.92 V (vs. NHE) for 8a and 8b respectively (Table 1, Figure 2 and ESI). In aqueous solvent (0.1 M
HOTf(aq) + TFE) three redox couples were observed due to the coordination of water permitting
37,38
access to higher oxidation states through proton coupled electron transfer (PCET)
current increase above ca. 1.3 V was attributed to catalytic water oxidation. E1/2 values for Ru , Ru
and a large
III-II
IV-
III
V-IV
,
and Ru are given in Table 1 and their shifts as a function of changes in pH are presented in the
corresponding Pourbaix diagrams in the ESI (Figures S7 and S8). Previous work by Sun et al. showed
that small quantities of TFE had little or no effect on the electrochemistry of Ru-bda complexes in
1
6
aqueous solutions at pH 1 but at higher concentrations the catalytic currents were suppressed. TFE
2
+
2
coordination to ruthenium has also been observed for the Ru(Mebimpy)(bpy)(OH ) WOC previously

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3
9
reported by Meyer et al. In our electrochemical studies we also found that high TFE quantities
could significantly influence the electrochemical behaviour of the Ru-biqa complexes. Due to the
poor solubility of 8b in 0.1 M aqueous HOTf, 33% TFE was required to obtain a 0.3 mM solution and
no effect on the electrochemical response was observed. In the case of 8a only 20 % of TFE was
required to fully dissolve the complex, again without disturbing the electrochemical response,
IV-III
V-IV
however, the Ru
and Ru
waves that were initially observed for catalyst 8a were greatly
diminished at TFE concentrations of 33% and above (ESI Figure S6). These results indicated an
equilibrium between H O and TFE coordination for the Ru-biqa complexes but as the initial Ru-biqa
2
complexes are coordinatively saturated this would require prior decoordination of one of the biqa
ligand donor atoms. A potentially related observation made during the electrochemical experiments
was a change in colour of the acidic aqueous solutions of the catalysts from blue to yellow. UV-vis
spectroscopy was used to probe this further.
Figure 2. Cyclic Voltammograms (top) and Differential
Pulse Voltammograms (bottom) of 8a (dashed black)
and 8b (solid black). [8a] = 0.3 mM in 0.1 M HOTf(aq)
with 20% TFE, [8b] = 0.3 mM in 0.1 M HOTf(aq) with
3
3% TFE. Electrolyte only blank scan (solid grey). CV
-
1
-1
scan rate = 100mV s . DPV sweep rate = 20mV s . GC
disc as working electrode, Pt rod as counter electrode,
Ag/AgCl as reference electrode (values converted to
NHE using E1/2 Ag/AgCl = 0.2 V vs. NHE).

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a
Table 1. Summary of electrochemical data for complexes 8a and 8b.
III-II
IV-III
IV-V
Complex E1/2 Ru
(
ΔEp
)
E1/2 Ru
--
(
ΔEp
)
E1/2 Ru2
b
8
8
8
8
a
a
b
b
0.87 (80)
0.67 (50)
0.92 (80)
0.73 (50)
--
1.30
c
e
1.11 (60)
--
1.15 (40)
b
--
1.31
d
e
3
6
6
a) E1/2 values in V vs. NHE and ΔE in mV; b) 0.5 mM in 0.1 M TBA(PF ) DCM with 20% TFE; c) 0.3
mM in 0.1 M HOTf(aq) with 20% TFE; d) 0.3 mM in 0.1 M HOTf(aq) with 33% TFE; e) Values taken
from DPV.
UV-vis. Changes in the absorbance spectra and a number of other observations were recorded after
dissolving catalysts 8a and 8b in TFE–0.1 M HOTf(aq) mixed solvent (see ESI): The rate of decay at 600
nm was identical for both catalysts, the spectra of the resulting pale yellow solutions contained very
similar absorbance features for both compounds, no change in absorbance was observed for either
compound when the TFE–0.1 M HOTf(aq) mixed solvent was replaced with a TFE-water mix, and
finally, the process could be reversed by the addition of a reducing agent like sodium ascorbate. The
conclusions that can be drawn from these results are; (i) the process involves a reversible oxidation
event, (ii) the oxidation is pH dependent, and (iii) the axial ligands have little effect on the
II
III
2
conversion. The oxidation of the Ru complexes to the Ru -OH species by oxygen is consistent with
each of these statements. The final proof was obtained by carrying out redox titrations of the catalyst
solutions with the strong oxidant, ceric ammonium nitrate (CAN). The same spectra as those
observed after one hour standing in air were obtained after the addition of 1.0 eq of CAN, thus
confirming the 1-electron oxidation of the catalysts as the reason for the observed colour changes.
Chemical water oxidation. Given the structural similarities between the Ru-biqa WOCs and the Ru-
bda WOCs, conditions for assessing the chemical water oxidation were chosen to give the closest
possible comparison. Catalysts 8a and 8b were therefore added to a solution of excess CAN oxidant
in 0.1 M HOTf(aq) and the oxygen concentration was measured in the aqueous phase using a
fluorescence-based oxygen probe. Figure 3 shows the oxygen evolution profiles at various catalyst
concentrations of 8a and 8b. What is evident from these plots is that catalyst 8a and catalyst 8b
behave quite differently under the same reaction conditions: Catalyst 8a has a fairly smooth oxygen
evolution profile that gradually slows down over time due to catalyst deactivation, whereas for
catalyst 8b there appears to be a fast initial phase, abruptly followed by a slow second phase. Despite
these differences, the maximum turnover numbers (TON) of both catalysts were very similar; 87 for
IV
8
a and 80 for 8b (see ESI for all TON and TOF values). The absorbance decay of Ce , which was
proportional to oxygen production, was measured under the same reaction conditions and used to
indirectly estimate the reaction rates (see ESI). Linear fitting of the data from 4 – 20 s gave almost
-
1
-1
identical first order rate constants of 2.51 s for 8a and 2.27 s for 8b, corresponding to turnover
40
-1
-1
frequencies (TOFs) of 0.63 s and 0.57 s respectively. This data fits well with the previous studies
of Sun et al, where they compared the Ru-bda catalyst framework to the more rigid phenanthroline
analogue, Ru-pda, and found that the increased rigidity shifted the preference from the
intermolecular coupling mechanism (I2M) for the Ru-bda’s to water nucleophilic attack (WNA) for
2
4
the Ru-pda’s. Considering the reaction is first order in Ru-biqa catalyst and the TOFs are closer to
-
1
-1
the (pic)
2 2
Ru(pda) analogue than the (pic) Ru(bda) analogue (0.1 s and 3.0 s respectively) measured
1
5,24
under the same conditions, a similar shift towards WNA was indicated here.
The rates recorded
for catalyst 8b over the initial period (0 – 4 s) after catalysts injection were visibly faster than the data
recorded after 4 s but this range is not as accurate because it includes the errors for manual injection
of the catalyst solution and so no reliable correlations to rate order could be extracted from this

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data. It is possible that the energy barriers for the two possible pathways (I2M vs WNA) are perhaps
so close for the Ru-biqa compounds that both pathways are energetically feasible and that
compound 8b begins catalysis via I2M but quickly switches to WNA after only a few cycles. Density
functional theory (DFT) calculations were performed on Ru-biqa catalyst models to determine the
energy barriers of the two pathways and the results are presented in the following computational
section.
Figure 3. Oxygen evolution profiles after addition of 8a
(
top) and 8b (bottom) to CAN measured with
fluorescence probe. [CAN] = 1.5 mM in 0.1 M HOTf(aq)
pH = 1). [8a] = [8b] = 4.0, 2.0, 1.0, 0.5 µM. T = 25°C.
(
Reaction volume = 4.0 mL. Experiments for each [cat.]
st
were carried out in duplicate (1 experiment, solid grey;
nd
2
experiment, dashed black). The red line is the blank
experiment for the addition of solvent with no catalyst
to the CAN solution.
Another possible explanation is that the initial complex is converted to another active species during
the initial phase of catalysis, thus resulting in the more complex behaviour. HPLC analysis of the
catalysis reaction solutions for both compounds 8a and 8b supported this hypothesis to some extent
(see ESI): For catalysis solutions of 8b (16 µM) and CAN (1.5 mM, ca. 94 eq), both the starting
complex and free biqa ligand were identified in the reaction mixture after catalysis. For catalysis
solutions of 8b (8 µM) and CAN (1.5 mM, ca. 188 eq), both the starting complex and free biqa ligand
were again present in the reaction mixture but the relative amount of free ligand had increased. No
starting complex was observed at the end of catalysis reactions with 8a and CAN, free biqa ligand
was the only species identified at both catalyst concentrations. Hence, both catalysts 8a and 8b are

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not stable under the reaction conditions and the majority of the original catalyst has been converted
after only 23 – 47 cycles. What is not entirely clear, is what the other degradation products are,
IV
however, the fact that Ce consumption does not continue after the original catalysts are gone
would indicate that their activity is negligible in comparison to the original molecular species. Work
to determine the fate of the axial ligands and the ruthenium and to identify the exact nature of other
reaction by-products is still ongoing in our labs.
Computational Studies.
Assuming that the molecular catalysts are responsible for catalysis in the initial phases of the
reaction with CAN and that the O-O bond formation is the rate limiting step, as was previously
2
0,41
observed for the analogous Ru-bda compounds,
the activation barriers of the two well-known
were compared for model complexes of the new Ru-biqa compounds
a and 8b in an exhaustive DFT study (see ESI for more details). For both model complexes, Ru-biqa-
2
4,42
I2M and WNA mechanisms
8
py and Ru-biqa-isoq (py = pyridine, isoq = isoquinoline as axial ligands), the barriers for the WNA
pathway were found to be lower than those of the I2M pathway (see Table 2 and Figure 4). The
flexibility of the bda ligand allows it to adapt to the various transition state and intermediate
2
4
structures of the I2M pathway. Both the pda and biqa ligands are restrained in their movement and
are therefore less adaptable, thus increasing the I2M barriers beyond those of WNA. The axial ligand
identity did not appear to play an important role in determining the transition state geometries or
the activation energies for the WNA mechanism (see Table 2, Figure 5 and Figure S33). Transition
state (TS) structures calculated using either one or two explicit water molecules highlighted the
importance of an accurate description of the solvation shell. In particular, the hydrogen bonding
network which shuttles the proton off the attacking water molecule to the carboxylate of the biqa
ligand plays a crucial role. Overall, our findings are in good agreement with previous studies by Tong
2
4
et al, where the necessity of an additional water molecule was discussed in detail.
Figure 4. Energy diagram comparing the energy barriers of oxygen bond
formation for the WNA and I2M mechanisms for Ru-biqa model complexes.

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Table 2. Activation energies for the I2M and WNA mechanism.
Ligands
Ea(l2M-HS)(kcal/mol)
Ea(WNA-H2O)(kcal/mol)
[
a]
[b]
[a]
[b]
[c]
Ru-bda-py
Ru-biqa-py
Ru-biqa-isoq
22.5 (15.0 )
26.9 (18.0 / 32.7 )
[
a]
[a]
23.9
30.7
25.1
[
a]
[a]
23.6
[a] present work, [b] ref 24 [c] ref 43
Figure 5. Top: WNA transition state for Ru-biqa-
py calculated with two explicit water molecules.
Selected bond lengths (in Å) and dihedral angles
are also given. Axial ligands are not shown for
clarity. Bottom: Overlay of TS structure for Ru-
biqa-py (blue) and Ru-biqa-isoq (orange)
highlighting the fact that the axial ligand does
not influence the WNA-TS structure.

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For the I2M mechanism, we observed a significant contribution of π-π stacking that leads to a lower
barrier for Ru-biqa-isoq compared to Ru-biqa-py. This π-stacking effect is in line with observations
2
0
made by Sun and co-workers for Ru-bda complexes with pyridine and isoquinoline axial ligands.
However, this effect would only become relevant in situations where the I2M was favoured over
WNA.
The barriers obtained from the DFT calculations help to explain the observations made during the
catalysis experiments, which showed regular oxygen evolution curves and first order kinetics for the
Ru-biqa-pic complex (8a) but a more complex behaviour for the Ru-biqa-Br-isoq complex (8b): While
the activation energy difference between the two mechanisms for Ru-biqa-py is 6.8 kcal/mol, for Ru-
biqa-isoq it is only 1.5 kcal/mol. This clear preference for the WNA mechanism for Ru-biqa-py agrees
well with the experimentally observed first order kinetics (in catalyst) for compound 8a, whereas for
the Ru-biqa-isoq compound the energy difference of the two pathways is negligible, which indicates
that both pathways are equally energetically feasible. Thus, for compound 8b both mechanisms
could potentially operate, resulting in a more complex kinetic behaviour as the mechanism switches
or the compound begins to degrade. It is also worth pointing out that other mechanisms for O-O
4
4
bond formation with Ru-bda complexes have recently been proposed, which could offer an
alternative theoretical explanation as well and would be worth considering in future studies.
Conclusions
2
In summary, the novel biqaH ligand framework was successfully prepared and complexed with
ruthenium to give a new family of molecular water oxidation catalysts, the Ru-biqa’s. The ruthenium
complexes were shown to be active for chemically induced water oxidation catalysis when added to
CAN oxidant at pH 1, however, their activity was greatly inferior to the analogous Ru-bda compounds
1
5,20,22,35
tested under similar experimental conditions.
DFT studies revealed that for Ru-biqa
complexes, in contrast to Ru-bda analogues, the oxygen-oxygen bond formation is more favourable
via the WNA pathway than the I2M pathway. The slow catalytic rates and relatively lower barriers for
WNA pathway were also in better agreement with previous studies of ruthenium complexes
containing the rigid pda ligand framework, which were proposed to catalyse water oxidation via the
2
4
WNA pathway. It therefore appears that inducing non-planarity in the equatorial coordinating
ligand framework has the same negative effect as inducing rigidity; resulting in a switch from the I2M
mechanism to the WNA mechanism and consequently slower catalysis. Furthermore, the stability
(indicated by TON) of the Ru-biqa complexes was lower than both the Ru-bda’s and the Ru-pda’s.
Replacement of the bda ligand by biqa therefore offers no improvement with regards to catalytic
performance, however, these findings help to further consolidate our understanding of the
mechanisms surrounding catalytic water oxidation and will be invaluable in the design of future
molecular catalysts. For example, future work that may look to investigate substitution effects on the
bda ligand should avoid substituents positioned at the 3 and 3’ positions as they may also lead to
similarly induced non-planarity and a consequent drop in catalytic performance.
Acknowledgements
We would like to thank Peter Deglmann for fruitful discussions concerning the calculations. We also
thank the Swiss National Supercomputing Centre for computational time (project s502) as well as the
University of Zurich for a Forschungskredit (FK-15-101) and the University of Zurich and The
LightChEC program for funding.

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