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in Journal of the American Chemical Society, Copyright © 2020 American Chemical Society after peer
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[insert ACS Articles on Request author- directed link to Published Work, see
https://pubs.acs.org/doi/pdf/10.1021/jacs.9b11935

Second Coordination Sphere Effects in an Evolved Ru Complex Based on a
Highly Adaptable Ligand Results in Rapid Water Oxidation Catalysis

Nataliia Vereshchuk1,2, Roc Matheu1, Jordi Benet-Buchholz1, Muriel Pipelier3, Jacques Lebreton3,
Didier Dubreuil3, Arnaud Tessier*,3, Carolina Gimbert-Suriñach1, Mehmed Z. Ertem*,4 and Antoni
Llobet*,1,5

1

Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and Technology
(BIST), Avda. Països Catalans 16, 43007 Tarragona, Spain.
2

Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, Marcel·lí Domingo s/n,
43007 Tarragona, Spain.
3

Université de Nantes, CNRS, CEISAM, UMR 6230, Faculté des Sciences et des Techniques, 2 rue de
la Houssinière, BP 92208, 44322 Nantes, France.
4

Chemistry Division, Energy & Photon Sciences Directorate, Brookhaven National Laboratory,
Upton, New York, 11973-5000, USA.
5

Departament de Química, Universitat Autònoma de Barcelona, Cerdanyola del Vallès, 08193
Barcelona, Spain.

Corresponding authors: arnaud.tessier@univ-nantes.fr; mzertem@bnl.gov and allobet@iciq.es

1

Abstract
A new Ru complex containing the deprotonated 2,2':6',2''-terpyridine,6,6''-diphosphonic acid
(H4tPa) and pyridine (py) of general formula, [RuII(H3tPa--N3O)(py)2]+, 2+, has been prepared and
thoroughly characterized by means of spectroscopic, electrochemical techniques, X-ray diffraction
analysis and with DFT calculations. Complex 2+, presents a dynamic behavior in the solution that
involves the synchronous coordination and the decoordination of the dangling phosphonic groups
of the tPa4- ligand. However, at oxidation state IV complex 2+, it becomes seven coordinated with
the two phosphonic groups now bonded to the metal center. Further, at this oxidation state at
neutral and basic pH, the Ru complex undergoes coordination of an exogenous OH- group from the
solvent that leads to an intramolecular aromatic O-atom insertion into the CH bond of one of the
pyridyl groups forming the corresponding phenoxo-phosphonate Ru complex [RuIII(tPaO-N2OPOC)(py)2]2, 42, where tPaO5- is 3-hydroxo-[2,2':6',2''-terpyridine]-6,6''-diyl)bis(phosphonate)
ligand. This new in situ generated Ru complex, 42-, has been isolated and spectroscopically and
electrochemically characterized. In addition, a crystal structure has been also obtained using single
crystal X-ray diffraction techniques. Complex 42- turns out to be an exceptional water oxidation
catalyst achieving record high TOFmax in the order of 16,000 s-1. A mechanistic analysis
complemented with DFT calculations has also been carried out showing the critical role of
intramolecular second coordination sphere effects exerted by the phosphonate groups in lowering
the activation energy at the rate-determining step.

Keywords
Water oxidation catalysis, Ru complexes, water splitting, redox properties transition metal
complexes

2

1. Introduction
Molecular water oxidation catalysis is a field that has been rapidly developing over the last decade
mainly due to their potential application in new energy conversion schemes based on water splitting
with sunlight.1–7 In particular, Ru complexes have been leading the field and today the amount of
information extracted from these complexes has generated a deep understanding of the different
parameters involved in the catalytic cycle.8,9 This has been possible thanks to the spectroscopic
characterization of the reaction intermediates as well as due to the kinetic characterization of the
different steps involved and complemented with a thorough computational analysis.10–13 This
wealth of information based on Ru complexes as water oxidation catalysts (WOCs) has also been
extended to other transition metals as well as oxides reported as WOCs for their design and
mechanistic proposals.14–17
The best WOCs known today are based on the Flexible Adaptative Multidentate Equatorial (FAME)
ligands18 containing polypyridyl carboxylate groups such as H2tda19 and H2bda 20–22 (see Chart I for
the drawings of the ligands) and their related derivatives where the carboxylate groups have been
partially or totally substituted by phosphonate groups.23–26 In particular, the complex [RuIV(OH)(tda-N3O)(py)2]+ (py for pyridine), 1+, has been recently shown to achieve TOFmax of 8,000 s-1 at pH 7.0.19
Further, the analysis of its performance shows a delicate balance among multiple equilibria at
different oxidation states with diverse coordination number and degree of hydrogen bonding that
is responsible for its performance as a catalyst.19,27 Because of these delicate equilibria, the
replacement of carboxylate groups in H2tda by phosphonate groups leading to the H4tPa ligand
(Chart I), is expected to generate significant differences in its catalytic behavior. The electronic
nature and geometry of the phosphonate with regard to those of the carboxylate group as well as
an increase in steric will be responsible for the different catalytic behavior of the new Ru complexes
based on H4tPa.28 In addition, the increased number of acidic protons with substantially different
pKa’s are expected to not only facilitate remote proton-coupled electron transfer (PCET) events
during redox leveling,29–35 but also give rise to beneficial second sphere coordination effects such as
hydrogen bonding interactions. Further, the phosphonate groups are expected to act as an
intramolecular proton acceptor, at the O-O bond formation step, significantly reducing its activation
energies.
Here on we present a new family of Ru complexes derived from the H4tPa ligand together with a
thorough characterization of their redox and water oxidation catalytic properties.
3

2. Results
2.1 Synthesis and structure of the precursor complex 2+
The synthesis of the H4tPa ligand was carried out in three steps using 2,2':6',2''-terpyridine (trpy) as
the starting material as described in the supporting information. Initially, the trpy ligand is oxidized
to 2,2':6',2''-terpyridine-1,1''-dioxide using metachloroperbenzoic acid. The treatment of the latter
with POEt3 forms tetraethyl 2,2':6',2''-terpyridine-6,6''-diphosphonate that is hydrolyzed with
trimethylbromosilane yielding the H4tPa ligand. The synthetic strategy for the preparation of the
complexes described in this work is outlined in Figure 1. Complex [RuII(H3tPa--N3O)(py)2]+, 2+, is
prepared by refluxing stoichiometric amounts of H4tPa and [Ru(dmso)4Cl2] in n-BuOH, followed by
addition of an excess of pyridine and further refluxing overnight. Proper column chromatography in
silica generates pure 2Cl in moderate yields where the equatorial H3tPa ligand acts in a
monoanionic fashion. Crystals of 2PF6·3H2O sufficiently good for X-ray analysis were obtained by
counteranion exchange crystallization from an acidic aqueous solution of 2Cl and KPF6. The ORTEP
view of its solvated cationic moiety is presented in Figure 1. The Ru(II) center features an octahedral
type of geometry where the H3tPa ligand coordinates in the equatorial plane in a -N3O fashion
leaving a dangling non-coordinated phosphonic acid group. The octahedral geometry is completed
with two pyridyl groups in the axial position. The bonding parameters are unremarkable36 except
for distortion due to the geometrical constraints of the HtPa3 ligand at the equatorial zone, which
produces an N-Ru-O angle of 115o; 25o larger than the ideal Oh geometry. The same angle for the
[RuII(tda)(py)2] is 125o,19 that is 10o higher which is a consequence of the different bonding
parameters associated with the phosphonate and carboxylate moieties.28,37 Finally, there are three
H2O molecules in the unit cell, interacting with the phosphonic groups by hydrogen bonding as can
be observed in Figure 1. Density functional theory (DFT) calculations at the M06 level of theory38 in
conjunction with SMD aqueous continuum solvation model39 (see computational methods for
further details) have been carried out in order to complement the experimental work described
here for 2+. The computed structure for 2+ shows a very good agreement with the experimental
structure in terms of bonding parameters (Figure S62) revealing its good degree of reliability.
Addition of two equivalents of Ce(IV) generates the corresponding seven coordinated Ru(IV)
complex [RuIV(tPa--N3O2)(py)2], 3, where the tPa4 ligand now acts in a pentadentate manner and
the metal center possesses a pentagonal bipyramidal geometry as can be observed in the computed
4

structure shown in Figure 1. This complex is diamagnetic at room temperature (RT) with a high field
low spin d4 electronic configuration, (e1”)4(e2’)0(a1’)0, analogous to related Ru(IV) complexes.19,40,41.
Its 1H, 13C, and 31P NMR are presented in the SI see Figures S27-29.
2.2 Spectroscopy and dynamic behavior of 2
The spectroscopic properties were investigated by means of UV-vis and NMR spectroscopy. The UVvis spectra of [RuII(H2tPa--N3O)(py)2], 2, and its one and two-electron oxidized species [RuIII(H2tPa-

-N3O)(py)2]+ and [RuIV(HtPa--N3O2)(py)2]+ were generated via redox titration using Ce(IV) as the
oxidant at pH 1.0 in aqueous triflic acid (Figures S45-S46). Complex 2 is a diamagnetic low spin d6
ion with a typical (t2g)6(eg)0 electronic configuration and its NMR spectra and assignment are
presented in the Figure 2 and the supporting information (Figures S12-S26 and Figures S35-S36). At
room temperature, the NMR spectrum displays eight resonances as if the complex had C2v
symmetry. However, at low temperature, the resonances of the protons Ha, Hb, Hc and He split
indicating the presence of dynamic behavior (Figure 2). The dynamic behavior is associated with the
synchronic coordination and decoordination of the dangling phosphonate that has an activation
energy of 13.3 kcal/mol based on Eyring plots generated from VT-NMR (Scheme S2). By
computational methods, an estimate activation energy of 7.9 kcal/mol (see Figure S37) was
obtained for this dynamic behavior in line with the experimental observations. The value of the
activation energy is similar to that measured for Ru-bda complexes involving coordination and
decoordination of the dangling carboxylates.42
2.3 Redox properties of 2
The redox properties of 2 were investigated by electrochemical techniques in a three-electrode cell
using glassy carbon as a working electrode and all potentials were measured using a Hg/Hg2SO4 as
a reference electrode and converted to vs NHE by adding 0.65 V to the measured potential. Cyclic
Voltammetry of 2 were performed in aqueous solution as a function of pH that produces a different
degree of protonation at the phosphonate groups. Cyclic voltammetry at pH 7.0 is shown in Figure 3
(red trace) where two chemically reversible and electrochemically quasireversible redox processes
can be observed. The first redox process at E1/2 = 0.83 V is assigned to a PCET process to generate
[RuIII(tPa--N3O2)(py)2] whereas the second wave at E1/2 = 0.92 V is due to a one-electron oxidation
process to form [RuIV(tPa--N3O2)(py)2]. The one-electron nature of the two processes is
corroborated by the spectrophotometric redox titration (Figure S45-S46), and the degree of
5

protonation is deduced from the Pourbaix diagram shown in Figure 3 right. (see SI for the equations
used for the simulation).
The four protons potentially available from the two phosphonic groups in 2, allow obtaining a rich
family of complexes depending on the degree of oxidation and protonation as shown in the Pourbaix
diagram. The pKa’s coincide with the dashed vertical lines and are deduced from the change of slope
in the Eo vs. pH plot. The first and second pKa’s due to the monodeprotonation of each phosphonic
group are expected to occur below pH 1.0.43–45 Further, the pKa for the third and fourth proton loss
are labeled, pKa and pKa’, and are defined in equations 1 and 2 respectively,

[RuII(H2tPa--N3O)(py)2](aq) -> [RuII(HtPa--N3O)(py)2] aq + H+(aq) pKa
[RuII(HtPa--N3O)(py)2](aq) -> [RuII(tPa--N3O)(py)2]2(aq) + H+(aq) pKa’

(1)
(2)

The latter have been independently calculated via spectrophotometric acid-base titration as shown
in Figure S47. It is interesting to see how due to the different proton content of the species involved
at a given pH, the difference between the IV/III-III/II redox couples changes considerably as a
function of pH as can be graphically observed in Figure S49A. Indeed, at pH 7.0 this difference is only
100 mV but it increases up to 330 mV at pH 13.0 (Figure S49B).
2.4 Generation of the active Ru species via O-atom insertion
The synthesis of the active catalyst [RuIII(tPaO--N2OPOC)(py)2]2, 42, (see a drawing of the H5tPaO
ligand in Chart 1; OP refers to oxygen atom coordinated to the Ru center via the oxygen atom from
the phosphonic group and OC from the pyridyl group) was carried out electrochemically using
[RuII(H3tPa--N3O)(py)2]+, 2+, (2+ is predicted to convert to 2 at pH 7.7) as a precursor as outlined in
Figure 1.
A sample of 2+ is dissolved in a pH 7.7 aqueous phosphate buffer solution and placed in a twocompartment electrochemical cell using a Pt mesh as a working electrode. Then a potential of 1.30
V is applied for 110 minutes. During this process, the initial 2+ precursor complex is quantitatively
transformed into the final complex 42, where an O atom insertion between the Ru center and one
of the pyridyl groups takes place. This is reminiscent of the reactivity of dinuclear dicopper
6

complexes at low oxidation states with dioxygen reported earlier.46 One electron reduction of 42
yields the diamagnetic 43 complex that was characterized by 1H, 13C, and 31P NMR spectroscopy and
whose spectra are shown in the (Figure S39-S44). Electrochemically only the initial and final species
are detected (Figure S50).
Single crystals of 43- were obtained by adding excess Cs+ to a solution of 42-in MeOH. A crystal
structure of 43 is displayed in Figure 1 showing the phenoxide bonding and the decoordination of
the initial pyridyl-phosphonic group. The complex was further characterized by UV-vis spectroscopy
using an Optically Transparent Thin-Layer Electrochemical (OTTLE) cell (Figure S48).
In order to get further insight into the potential sequence of reactions that generate 42 from 2 (2
converts to 2- at pH 7 according to the Pourbaix diagram) a computational analysis was carried out
and the results obtained are summarized in Scheme 1. As the current flows at an applied potential
of 1.30 V, the initial Ru(II) complex 2+ is initially converted to seven coordinate (CN7) Ru(IV) species,
[RuIV(tPa--N3O2)(py)2], 3, in agreement with its redox properties described in the previous section.
This complex then reacts with a water molecule with concomitant proton release inducing the
decoordination of one of the pyridyl phosphonate groups to generate [RuIV(O)(HtPa--N2O)(py)2]-.
The latter then undergoes a PCET step to form [RuV(O)(tPa--N2O)(py)2]. This species is responsible
for the intramolecular hydroxylation of the dangling pyridyl group that is highly favored from a
thermodynamic perspective (G = 9.0 kcal/mol) and that finally leads to pure 42following a
deprotonation event (G = 54.6 kcal/mol). A transition state for the electrophilic oxygen insertion
into the CH bond has also been located with the low free energy of activation (G‡) of 14.0 kcal/mol.
The redox and electrocatalytic properties of 42 were investigated by CV and Coulombimetric
techniques. Figure 3 (black trace, left) shows the CV of 42 at pH 7.0 phosphate buffer solution where
an electrochemically and chemically reversible wave appears at E1/2 = 0.35 V that is assigned to the
Ru(III)/Ru(II) couple. At higher anodic potentials a very large electrocatalytic current is observed at
approximately 1.4 V that is due to the oxidation of water to dioxygen. This second wave is associated
with a two-electron process and oxido coordination, reaching the Ru(V)=O species that is
responsible for the reaction with solvent water and O-O bond formation. A two-electron process is
proposed since no other waves are observed besides the III/II wave and the corresponding Ru(IV)=O
species are not sufficiently reactive for fast water oxidation and further supported by DFT.

7

2.5 Water oxidation catalysis and mechanisms
Kinetic characterization of the electrocatalytic water oxidation reaction was carried out for 42,
based on Foot of the Wave Analysis (FOWA)47–49 at pH 7.2 as shown in Figure S54. A TOFmax = 1.6 104
(± 0.2·104) s-1 is obtained that is found to be independent of catalyst concentration between 0.5 and
3.0 mM. This first order kinetics behavior with regard to catalyst concentration points towards a
water nucleophilic attack (WNA)50 type of mechanism where the O-O bond formation between the
Ru(V)=O species and the solvent is the rate-determining step (rds). This is further supported by
theoretical calculations as will be shown in the next section. The performance of 42 as WOC was
further tested by a bulk electrolysis experiments described in detail in the supporting information
(Figure S53). More than 42 million turnovers with Faradaic efficiencies above 93%. were obtained
using Saveant’s methodology that takes into account the concentration of the catalyst close to the
electrode surface and manifests the remarkable stability of this catalyst.21–23 It is also interesting to
point out here that catalyst 42 is also active at lower pH albeit with lower efficiency (Figure S51).
A thorough mechanistic analysis was also carried out based on theoretical calculations and is
summarized in Figure 4 top, where the different reaction intermediates and the computed Gs and
redox potentials associated with each step are presented (see computational methods in the SI for
further details). We propose that once the Ru(III) complex, 42, is generated, water coordination
together with a two-electron oxidation process proceeds to form RuV=O species, [RuV(O)(HtPaO-N3OP)(py)2], which will, in turn, react with a solvent H2O to generate the corresponding
hydroxoperoxido derivative, as indicated in the equation below,

[RuV=O](aq) + H2O(l) -> [Ru-O---OH2]‡ -> [RuIII-OOH](aq) + H+(aq)

(3)

The optimized TS for WNA reaction is presented in Figure 4 bottom. It is interesting to realize here
that the coordination number (CN) for the Ru(V) species is 6 with the tPaO5- ligand acting in a -N2OC
fashion and thus with a dangling phosphonate group. This dangling group is strategically situated so
that it can act as an intramolecular proton acceptor to facilitate the WNA TS and to reduce the
activation free energy (G‡ = 22.1 kcal/mol). Once Ru(III)-OOH is formed then it suffers a PCET that
forms the Ru(IV)-OO that spontaneously evolves dioxygen and generates the initial Ru(II) species
8

completing the catalytic cycle. The computed G‡ of 22.1 kcal/mol for the proposed rate-limiting
WNA step is relatively high compared to experimental TOF of 16,000 s-1 so we further performed
calculations at both M06 and M06-L51 level of theories with additional explicit water molecules to
account for hydrogen bonding effects from the solvent (Table S4). The computed G‡s exhibit
notable decrease with additional water molecules and with four explicit water molecules the
computed activation free energies are 18.7 and 16.3 kcal/mol at M06 and M06-L level of theories
respectively. We also considered O-O bond formation between Ru(V)=O species and an oxygen
atom of the dangling phosphonate group similar to the oxide relay mechanism recently proposed
for Ru-tda complex by Zhan et al52 and located a TS structure featuring a G‡ of 13.2 kcal/mol but
the resulting Ru-O-O-P species is uphill by 6.3 kcal/mol indicating that reverse step has a much lower
G‡ of 6.9 kcal/mol. Moreover, at pH 7, nucleophilic addition of OH- to P atom of Ru-O-O-P moiety
similar to that proposed in the oxide relay mechanism is very unlikely and H2O molecule should
behave as the nucleophile for the hydrolysis of the Ru-O-O-P group, which is expected to result in
higher activation energies. A future direction of our studies will be devoted to a closer inspection of
WNA and alternative O-O bond formation pathways with full atomistic solvation models.”

3. Discussion
Detailed understanding of water oxidation catalysis is challenging due to a large number of reactions
and intermediates involved in this complex process. The electronic demands imposed by the
reaction intermediates accessed at each oxidation state at the different stages of the catalytic cycle
involve a change in metal coordination number. Along this line, high oxidation states will prefer hard
base ligands while lower oxidation states will prefer softer ones. Additional complexity arises
because of the variety of reactions handled by the catalyst including outer-sphere electron transfers
(OSET), PCET and chemical reactions that might not necessarily involve ET such as proton transfer
or O-O bond formation. Moreover, non-desired pathways that derail these reactions from the
catalytic cycle towards non-productive or decomposition products also need to be understood,53 to
be able to avoid them and to obtain robust water oxidation catalysts. To control all the parameters
indicated above, each step should occur with an activation barrier as low as possible so that efficient
water oxidation catalysis can take place.
3.1 Geometrical and electronic consequences due to the presence of the phosphonate groups
9

Complex [RuIV(O)(tda)(py)2], 1, is an example of a catalyst that complies with the above
requirements and up to now appears as the fastest water oxidation catalyst ever reported.54 The
structural consequences of changing carboxylate by phosphonates can be graphically observed by
comparing the structures of the diamagnetic seven coordinated catalyst precursors [RuIV(tda-N3O2)(py)2]2+ and [RuIV(tPa--N3O2)(py)2] (Figures S61 and S63). While the X-ray structure of
[RuIV(tda--N3O2)(py)2]2+ shows all the coordinating atoms of the tda2- ligand nearly on the equatorial
plane, the coordinating oxygen atoms of the phosphonate in [RuIV(tPa--N3O2)(py)2] complex are
significantly off the equatorial plane as observed in the computed structure shown in Figure 1.
Furthermore, this distortion also breaks the planarity of the trpy moiety of the tPa4- ligand
diminishing the aromatic  delocalization. This distortion is due mainly to the different geometric
parameters of the phosphonic acid group (pseudo Td) as compared to the carboxylate (pseudo C2v),
and results in weaker Ru-O bonds for the phosphonic complex. From an electronic perspective the
phosphonate group acts as a stronger sigma donor than the carboxylate, leading in general to lower
redox potentials and higher pKa (pKa’s are 3 and 11 for [RuII(H2tPa)(py)2] and 6.4 for
[RuIII(HtPa)(py)2]).27 The existence of a range of pKa values obtained at different oxidation states
from the auxiliary ligands is interesting because it can promote remote PCET, that is a PCET where
the proton is not associated with the Ru-OH2 or Ru-OH group, but from auxiliary ligands such as the
phosphonic ones. This is important because for typical polypyridyl Ru-aqua complexes such as
[Ru(trpy)(bpy)(H2O)]2+,55 the complex can access RuIV=O species by two consecutive PCET processes
from its aqua derivatives but access to reactive Ru(V) occurs as an ET only process and therefore
requires high energy input and overpotential (1.8 V for [Ru(trpy)(bpy)(H2O)]2+).56–58 Thus, the
possibility to reach Ru(V) via three PCET processes can open up access to a highly active RuV=O
species at much lower overpotentials.
3.2 The importance of the second coordination sphere effects
The two electrons oxidation of the octahedral [RuII(H2tPa--N3O)(py)2], 2, complex generates the
CN7 diamagnetic [RuIV(tPa--N3O2)(py)2]. Here the importance of ligand flexibility and design is
manifested by leaving a non-coordinated dangling phosphonic acid group at oxidation state II where
the Ru complex needs to be CN6. The ligand flexibility also allows for a dynamic behavior where the
phosphonic groups synchronically coordinate and decoordinate fast at room temperature. Upon
reaching Ru(IV) the dangling group coordinates thanks to the flexibility of the ligands achieving CN7.
At oxidation state IV in basic solution, [RuIV(tPa--N3O2)(py)2] undergoes OH substitution to form
10

either [RuIV(OH)(tPa--N2O)(py)2]or [RuIV(O)(HtPa--N2O)(py)2]. Further oxidation leads to the
formation of highly reactive [RuV(O)(tPa--N2O)(py)2]- species. The generation of the RuIV-OH (or
RuIV=O) occurs with the breaking of both the Ru-N and Ru-O bonds of one of the pyridylphosphonato arms of the tPa4 ligand (Scheme 1). This is due to the steric congestion generated by
the phosphonate group that precludes the formation of the CN7 complexes [RuIV(O)(HtPa-N3O)(py)2]and [RuV(O)(tPa--N3O)(py)2]-. Once the highly reactive RuV=O species is generated then
the dangling pyridyl-phosphonato arm has the perfect geometry to undergo intramolecular oxygen
atom transfer to the non-coordinated pyridyl ring. This intramolecular O-atom insertion into the CH
bond generates the catalytically active complex precursor [RuIII(tPaO--N2OPOC)(py)2]2, 42.
Two electron oxidation of 42 forms the RuV=O species that performs O-O bond formation via WNA
and generates the RuIII-OOH intermediate. This step is the rate-determining step (rds) of the catalytic
cycle and is favored by the intramolecular proton transfer from the incoming water molecule to the
phosphonate group that again has the right geometry to promote this step reducing the activation
free energy. This renders 42 as an extremely powerful catalyst giving a TOFmax of 16,000 s-1 with an
overpotential of 530 mV at pH 7.0. It gives a higher TOFmax (8.000 s-1) than 1 at lower overpotential
at (75 mV.)
In summary, the dangling phosphonate group is responsible for low energy pathway of the two key
reactions: (i) The formation of active catalyst precursor [RuIII(tPaO--N2OPOC)(py)2]2, 42, via oxygen
insertion and (ii) the intramolecular proton transfer from the incoming water molecule for the O-O
bond formation step that is the rds in the WNA mechanism. These two key reactions occur
intramolecularly thanks to the right positioning of the RuV=O groups versus the dangling
phosphonato group and thus are entropically highly favored.
The present work highlights the importance of designing catalysts with the right second coordination
sphere environment in the field of redox catalysis and in particular in the catalytic oxidation of water
to dioxygen and the fate of the catalysts during turnover.

4. Supporting Information. Additional experimental details, electrochemical and spectroscopic data
are available free of charge via the Internet at http://pubs.acs.org. CIF files for complexes

11

{[RuII(H3tPa--N3O)(py)2](H2O)3}(PF6) and {[RuII(tPaO--N2OPOC)(py)2](H2O)8.5(CH3OH)}Cs3, with CCDC
numbers 1890782 and 1954776 respectively are available at https://www.ccdc.cam.ac.uk/.
5. Acknowledgments
Support from MINECO, FEDER, “La Caixa” and AGAUR are gratefully acknowledged (CTQ201680058-R, CTQ2015-73028-EXP, SEV 2013-0319, ENE2016-82025-REDT, CTQ2016-81923-REDC, and
2017-SGR-1631). The work at Brookhaven National Laboratory (BNL; M.Z.E) was carried out under
contract DE-SC0012704 with the U.S. Department of Energy, Office of Science, Office of Basic Energy
Sciences, and utilized computational resources at the Center for Functional Nanomaterials, which is
a U.S. DOE Office of Science Facility, and the Scientific Data and Computing Center, a component of
the Computational Science Initiative, at BNL under Contract No. DE-SC0012704.

12

Chart 1. Drawing and labeling of ligands discussed in this work.

H2tda

H2bda

H4tPa

H5tPaO

13

Scheme 1. Computed reaction pathway at pH 7.0 for the generation of the catalytically active
species [RuIII(tPaO--N2OPOC)(py)2]2, 42, from the precursor complex [RuII(H2tPa--N3O)(py)2], 2.
Redox potentials (E) in units of volts (V) vs NHE, Gs, and G‡ in units of kcal/mol. Axial pyridyl
ligands are omitted for clarity.

14

Figure 1. Synthetic scheme and structures of the complexes described in this work. X-ray structure
ORTEP views for {[RuII(H3tPa--N3O)(py)2](H2O)3}+, 2+·3H2O, showing the 3 hydrogen bonded water
molecules and for [RuIII(tPaO--N2OPOC)(py)2]2, 42. Calculated structure for [RuIV(tPa--N3O2)(py)2],
3. Color code: Ru, cyan; P, pink; N, blue; O, red; C, black; H, white.

15

Figure 2. VT 1H-NMR of [RuII(H2tPa--N3O)(py)2], 2 in CD3OD together with labeling.

16

Figure 3. Left, cyclic voltammetry of 0.6 mM [RuII(HtPa--N3O)(py)2], 2, at pH 7.0 in a 0.1 M
phosphate buffer aqueous solution (red trace) and at pH = 1.0 in 0.1 M triflic acid (purple trace).
Solid line black trace, cyclic voltammetry of 0.6 mM [RuIII(tPaO--N2OPOC)(py)2]2, 42, at pH 7.0 in a
0.1 M phosphate buffer aqueous solution, generated from the precursor 2, by a 3h bulk electrolysis
at 1.30 V. Right, Pourbaix diagram for 2. The dominant species as a function of pH and potential are
indicated with the labels such as [RuII(H2tPa)] where the oxidation state is indicated as well as the
degree of protonation of the H4tPa ligand. The dashed vertical lines indicate the pKa of the species
involved. The axial pyridyl ligands are omitted for clarity.

17

Figure 4. Top, calculated catalytic cycle for catalyst [RuV(O)(HtPaO--N2OC)(py)2] at pH 7.0 Redox
potentials (E) (experimental in green and calculated in blue) in units of volts (V) vs NHE, Gs and
G‡ in units of kcal/mol. Axial pyridyl ligands are omitted for clarity. Bottom, optimized TS for the
O-O bond formation step. Color code: Ru, cyan; P, orange; N, blue; O, red; C, black; H, white.

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