“This document is the Accepted Manuscript version of a Published Work that appeared in final form in
Inorganic Chemistry, copyright ©2017 American Chemical Society after peer review and technical
editing by the publisher. To access the final edited and published work see:
http://pubs.acs.org/doi/abs/10.1021/acs.inorgchem.7b02375

Synthesis, Structure and Redox Properties of a trans-Diaqua Ru Complex
that Reaches Seven Coordination at High Oxidation States

Roc Matheu,a,b Jordi Benet-Buchholz,a Xavier Sala*,c and Antoni Llobet*,a,c
a

Institute of Chemical Research of Catalonia (ICIQ), Avinguda Països Catalans 16, 43007
Tarragona, Spain.
b

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

Departament de Química, Universitat Autònoma de Barcelona, Cerdanyola del Vallès, 08193
Barcelona, Spain.
Corresponding authors: xavier.sala@uab.cat; allobet@iciq.cat

1

Abstract
In this work we have prepared and characterized two Ru complexes that contain the
pentadenatate tda2- ligand (tda2- = [2,2':6',2''-terpyridine]-6,6''-dicarboxylate) that occupies the
equatorial positions and two monodentate ligands aqua and/or dmso occupy the axial positons:
[trans-RuIII(tda--N3O)(OH2ax)2]+, 3III(OH2)2+ and [RuII(tda--N3O)(dmso)(OH2ax)], 4II. The latter is a
useful synthetic intermediate for the preparation of Ru-tda complexes with different axial
ligands. The two complexes have been characterized in the solid state by single-crystal XRD and
by elemental analysis. In solution complex 4II has been characterized by NMR spectroscopy as
well as the one electron reduction of complex 3III(OH2)2+. The electrochemical properties of
3III(OH2)2+ and 4II have been assessed by cyclic voltammetry (CV) and differential pulse
voltammetry (DPV). Complex 3III(OH2)2+ shows the presence of four redox waves that are
assigned to the VI/V, V/IV, IV/III and III/II redox couples. The variation of the redox potentials is
analyzed as a function of pH and is graphically presented as a Pourbaix diagram. Finally, the
redox potentials displayed by both 3III(OH2)2+ and 4II are compared to related complexes
previously reported in the literature and rationalized based on the electron donating or
withdrawing capacity of the auxiliary ligands as well as with regard to their ability to undergo
seven coordination at high oxidation states.

Keywords: Ru complexes, seven coordination, redox properties, trans-dioxo, trans-aqua, water
oxidation.

2

Introduction
Ruthenium complexes constitute one of the richest families of coordination compounds partly
thanks to the large number of formal oxidation states that Ru can access, ranging from -3 up to
+8.1 These complexes have a myriad of applications in many fields including, photochemistry
and photophysics,2,3 bioinorganic chemistry4,5 and catalysis.6,7 From a catalytic perspective, Ru
complexes are active in a large number of reactions involving C-C coupling,6 C-H insertion and
especially in redox catalysis with organic transformations.7 The latter includes the oxidation of
alkenes to epoxides, sulfides to sulfoxides, alcohols to aldehydes and carboxylic acid.8
Particularly interesting is the application of Ru complexes for the catalytic oxidation of water to
dioxygen, a reaction of interest within the context of new renewable energy conversion
schemes.9,10,11,12
For redox catalysis, one of the unavoidable requirements to display significant redox activity is
the presence of a Ru-OH2 group in the complex.13,14,15,16 This allows to reach reactive high
oxidation states via proton-coupled electron transfer at a relatively low energy. 17,18 For the
particular case of the water oxidation, the best catalysts described so far involve the formation
of seven coordinate complexes at high oxidations states, stabilized by sigma donor groups such
as carboxylates.19,20,21,22,23,24
A good example of this family of catalysts is the [RuIV(tda--N3O)(py)2(OHeq)]+ complex, 1IV(OH)+,
where tda2- is the pentadentate ligand ([2,2':6',2''-terpyridine]-6,6''-dicarboxylate), py is
pyridine and OHeq is a hydroxido ligand bonded to the metal center at the equatorial position,
see Chart 1 and 2. Complex 1IV(OH)+ is today the fastest water oxidation catalysts reported
achieving maximum turnover frequencies (TOFmax) of 50,000 s-1 at pH = 10.0.20 In sharp contrast,
the [RuIV(tda--N3O)(py)(OHax)] complex, 2IV(OH)+, where the hydroxido group is placed at the
axial position, is a poor catalyst.25
Here on we report the synthesis and spectral and redox properties of a new member of the Rutda family of complexes, trans-[RuIII(tda--N3O)(H2Oax)2], 3III(H2O)2+. The properties of the
complex are compared with the rest of the members of the family and with other relevant
complexes previously described in the literature. In addition, we report a new complex [RuII(tda-N3O)(dmso)(H2O)], 4II, that is an excellent synthetic intermediate for the preparation of all the
family of Ru-tda complexes.

Experimental Section
Materials
General Materials
Solvents and products were provided by Sigma-Aldrich unless indicated. [2,2':6',2''-terpyridine]6,6''-dicarboxylic acid (H2tda)26 and [RuIICl2(dmso)4]27 were synthesized and purified according
to the literature. High-purity deionized water was obtained by passing distilled water through a
nanopure Milli-Q water purification system.

3

Synthesis of [Ru(tda--N3O)(dmso)(OH2ax)], 4II. [RuIICl2(dmso)4] (150 mg, 0.31 mmol), 2’,2’’:6’,2’’terpyridine-6’,6’’-dicarboxylic acid (H2tda) (99 mg, 0.31 mmol) and Et3N (0.3 mL) were degassed
in dry methanol (6 mL), refluxed for 6 hours and cooled down to RT. A brown solid (100 mg)
appeared in the reaction mixture and was filtered, washed with methanol and diethyl ether. The
solid was dissolved in water (30 mL) and the mixture was heated at 60 oC until all the solid was
completely dissolved. Then, the solvent was evaporated and the resulting solid was washed with
acetone and diethyl ether and dried under vacuum (90 mg, 50 % yield). Crystals suitable for
single-crystal X-Ray Diffraction (XRD) were collected after water solvent of a 4II solution at pH =
1.0 (~ 1 mM) slowly evaporated. 1H NMR (500 MHz, D2O) δ: 2.63 (6H, s), 8.13 (1 H, t, J = 8.1 Hz),
8.27 (2 H, t, J = 7.8 Hz), 8.32 (2 H, dd, J = 7.8, 1.3 Hz), 8.40 (2 H, d, J = 8.1 Hz), 8.47 (2 H, dd, J =
7.8, 1.3 Hz,). 13C NMR (500 Hz, D2O) δ: 42.7, 115.8, 118.3, 120.8, 123.4, 123.6, 124.4, 126.2,
128.2, 137.6, 140.1, 152.7, 158.8, 160.3, 170.0. HRMS m/z: Calc. for [4-H2O+Na]+
(C19H15N3NaO5RuS+): 521.9668. Found m/z: 521.9672. Anal. Calc. for [4+H2O]: C, 42.7; H, 3.6; N,
7.9; S: 5.9. Found: C, 42.4; H, 3.2; N, 7.7; S, 5.9%.
Synthesis of [trans-Ru(tda--N3O)(OH2ax)2](PF6)0.5(SO3CF3)0.5, 3III(OH2)2(PF6)0.5(SO3CF3)0.5.
Complex 4II (100 mg, 0.19 mmol) was dissolved in 0.3 mL of neat triflic acid and heated to 100
o
C for 4 hours and cooled to room temperature. Then, an aqueous saturated KPF 6 solution (3.6
mL) was added and the mixture was kept in the fridge for 2 hours. During this period, crystals
suitable for XRD appeared as a precipitate (90 mg, 75 %). 1H-NMR (500 Hz, D2O + ascorbic acid)
δ: 7.73 (1 H, t, J=8.1 Hz), 8.06 (4 H, m), 8.34 (2 H, d, J=8.1 Hz), 8.37 (2 H, d, J=7.9 Hz). 13C-NMR
(500 Hz, D2O + ascorbic acid) 122.3, 123.1, 125.3, 132.0, 137.1, 158.6, 160.2, 164.1, 173.3. HRMS
m/z: Calc. for [3III(OH2)2+-H2O] (C17H9N3O4Ru): 420.9637, found m/z: 420.9649 (2.9 ppm). Anal.
Calc. for: [3III(OH2)2++ 0.1·CF3SO3H + H2O] {PF6}0.5{SO3CF3}0.5 (C17.6H15.1F4.8N3O8.8P0.5RuS0.6): C, 33.2;
H, 2.4; N, 6.6; S, 3.0. Found: C, 33.1; H, 2.4; N, 6.4; S, 3.3%.
Synthesis of [Ru(tda--N3O)(py)(SO(CH3)2)], 5II. The synthesis was adapted from the literature
but using complex 4II as starting material.25 Complex 4II (50 mg, 0.1 mmol) was dissolved in a
mixture of water (2.5 mL) and pyridine (7.5 mL). The solution was stirred for 2 days at room
temperature. The brown solution was washed with dichloromethane (3 x 100 mL) and the
organic phase was discarded. Then, the aqueous solution was reduced to dryness under low
pressure. The brown solid obtained in this way was re-dissolved in methanol. Upon addition of
diethyl ether, a brown solid appeared that was filtered and dried under vacuum (44 mg, 0.14
mmol, 75 % yield). The purity of the product was confirmed by 1H NMR and electrochemistry.25
Synthesis of [Ru(tda--N3O)(py)2], 6II. The synthesis was adapted from the literature using
complex 4II as starting material.20 Complex 4II (50 mg, 0.1 mmol) was dissolved in a mixture of
water (2.5 mL) and pyridine (7.5 mL). The solution was stirred overnight at reflux temperature.
The resulting brown solution was washed with dichloromethane (3 x 100 mL) and the organic
phase was discarded. Then, the aqueous solution was reduced to dryness under low pressure.
The brown solid obtained in this way was re-dissolved in methanol. Upon addition of diethyl
ether a brown solid appeared that was filtered and dried under vacuum (40 mg, 70 % yield). The
purity of the product was confirmed by 1H NMR and electrochemistry. 20
Solutions at pH = 1.0, pH = 7.0 and pH = 12.0. pH = 1.0 triflic acid (CF3SO3H) solution (I = 0.1 M):
A sample of neat triflic acid (10 g, 0.0663 mols) was diluted with 662 mL of deionised water. pH
= 7.0 phosphate buffered solution (I = 0.1 M): A sample of NaH2PO4 (2.31 g, 0.0193 M) and
4

Na2HPO4 (3.77 g, 0.0266 M) were dissolved in deionised water up to 1 L. A pH = 12.0 phosphate
buffer solution(I = 0.1 M): A sample of Na2HPO4 (10.293 g, 0.0073 M) and Na3PO4 (2.06 g, 0.0126
M) were dissolved with deionised water up to 1 L.
Instrumentation and Methods
General Instrumentation and Methods. Electrospray ionization (ESI)) mass spectrometry (MS)
experiments were performed on a Waters Micromass LCT Premier equipment. A Bruker Avance
500 MHz were used to carry out NMR spectroscopy. The pH of the solutions was determined by
a pH-meter (CRISON, Basic 20+) calibrated before measurements through standard solutions at
pH = 4.01, 7.00 and 9.21.
Electrochemical methods. All electrochemical experiments were performed in an IJ-Cambria HI730 bipotentiostat, using a three-electrode cell. A 20 mL vial was used as an electrochemical cell.
A Teflon-made with holes for the three electrodes was used as a lid to ensure a reproducible
distance between the electrodes. Glassy carbon disk (ф = 0.3 cm, S = 0.07 cm2) and Pt disk were
used as Working Electrode (WE) and Counter Electrode (CE) respectively. Glassy carbon
electrodes were polished with 0.05 μm alumina (Al2O3) and washed with water. The Reference
Electrode (RE) was Hg/Hg2SO4 (K2SO4 saturated) and potentials were converted to NHE by adding
0.65 V. CVs and DPVs were iR compensated (90%). Cyclic Voltammograms (CV) were recorded
at 100 mV·s−1 scan rate. The DPV parameters were = 4 mV, Pulse width = 5 s, Sampling width
= 0.0167 s, Pulse period = 5 s.
Pourbaix diagram. Three solutions of 3III(OH2)2+ (~ 0.5 mM) were prepared at pH = 1.0, pH = 7.0
and at pH = 12.0. The three solutions contained an ionic strength of 0.1 M. The three solutions
containing complex 3III(OH2)2+ were mixed in order to obtain the desired pH, which was
measured by the use of a pHmeter. CV and DPV experiments were performed to analyze the
solution at each pH. Due to the irreversibility of the VI/V and V/IV redox couple, DPV was used
to extract the E1/2 values for the Pourbaix diagram.
Single-Crystal XRD Methods. The measured crystals were prepared under inert conditions
immersed in perfluoropolyether as protecting oil for manipulation. Crystal structure
determinations for 8(4II+7II)+ and 3III(OH2)2+ were carried out using a Apex DUO diffractometer
equipped with a Kappa 4-axis goniometer, an APEX II 4K CCD area detector, a Microfocus Source
E025 IuS using MoK radiation, Quazar MX multilayer Optics as monochromator and an Oxford
Cryosystems low temperature device Cryostream 700 plus (T = -173 °C). Full-sphere data
collection was used with  and  scans. Data collection with APEX-228, data reduction with
Bruker Saint 29 and absorption correction with SADABS.30 Crystal structure solution was achieved
using direct methods as implemented in SHELXTL31 and visualized using the program XP. Missing
atoms were subsequently located from difference Fourier synthesis and added to the atom list.
Least-squares refinement on F2 using all measured intensities was carried out using the program
SHELXTL. All non-hydrogen atoms were refined including anisotropic displacement parameters.
Complex 3III(OH2)2+ crystallizes with water molecule, half triflate anion and half
hexafluorophosphate anion in the asymmetric unit. The triflate and hexafluorophosphate anions
are located on a mirror plane shared with the neighbouring asymmetric unit. Both anions are
showing large ellipsoids due to a disorder around the mirror plane which could not be properly
refined (CCDC is 1566809). The asymmetric unit in the crystal structure of compound 8(4II+7II)+
5

contains a molecule of 4II and one of 7II,+, where one of the carboxylato groups is protonated
(CCDC is 1566810).
Results and Discussion
Synthesis and solid-state structure
The strategy for the preparation of the Ru-tda complexes is summarized in Scheme 1. Reaction
of [RuIICl2(dmso)4] with tda2- in methanol at reflux for six hours yields a brown precipitate. Upon
dissolving the solid in water it generates the neutral complex [RuII(tda--N3O)(dmso)(OH2)], 4II,
in reasonable good yields (50 %). Complex 4II has been characterized in solution by spectroscopic
and electrochemical techniques and in the solid state by single-crystal X-ray Diffraction (XRD).
Crystals suitable for XRD were collected upon slow evaporation of a 4II solution in water. The
unit cell contains two different complexes that include the initial 4II complex together with its
aquated derivative [RuII(Htda--N3)(dmso)(OH2ax)(OH2eq)]+, 7II,+ (with one craboxylato group
protonated), forming a dinuclear complex 8(4II+7II)+ according to equation 1. Complexes 4II and
7II,+ are strongly interacting via extensive hydrogen bonding described in Figure 1.
2 [RuII(tda--N3O)(dmso)(OH2ax)] + H2O + H+ ->
4II
{[RuII(tda--N3O)(dmso)(OH2ax)][RuII(Htda--N3)(dmso)(OH2ax)(OH2eq)]}+ (1)
8(4II+7II)+
An ORTEP plot32 of 4II is depicted in Figure 1 and shows a pseudo-octahedral geometry around
the Ru metal center expected for a d6 Ru(II) metal ion. In the structure, the tda2- ligand acts in a
tetradentate manner occupying the equatorial positon of the Ru center, with the three pyridyl
moieties and one carboxylate group bonded to the Ru center and one non-bonded dangling
carboxylate. Finally, the axial positons are occupied by an aquo and dmso ligand. An ORTEP plot
of 7II,+ is also illustrated in Figure 1 where the tda2- ligand acts as meridional tridentate ligand
and the two carboxylates are not bonded to the metal center. Instead an aquo ligand is
coordinated to Ru in the free equatorial position and thus is situated cis with regard to the axial
aqua and dmso ligands that complete the octahedral coordination. Here, it is interesting to note
the geometrical distortion imposed by the tda2- ligand. Scheme 2 shows the ORuN and NRuN
angles in the equatorial plane for 4II and 7II,+. For 4II it is striking to see 125o NRuO angle, that
deviates by 35o from the ideal 90o for an octahedral coordination. When the carboxylato
coordination atom is replaced by the monodenate aqua ligand in 7II,+,the geometrical restriction
imposed by the now (tda--N3)2- ligand is released and the two ORuN angles decrease down to
100o and 99o.
The strong geometric distortion imposed by the tda2- ligand has significant consequences in the
Ru-tda complexes depending on the oxidation state of the Ru center. At oxidation state II it
fosters a dynamic behavior in solution whereas at oxidation state III and above it favors the
formation of six and a half and seven coordinated Ru complexes. Both phenomena will be
described below.
While complex 8(4II+7II)+ has been isolated as single-crystals as it will be discussed below,
complex 4II has been isolated as a powder and can used as a versatile synthetic intermediate to
6

prepare the rest of the complexes described in this work. The versatility of this complex as
intermediate arises from the different degree of bonding strength of the three ligands in 4II
namely: the tda2-, the dmso and the aqua ligands. The tda2- ligand here acts in a tetradentate-N3O manner and coordinates in the equatorial position very strongly. In sharp contrast, the axial
aquo ligand is a very labile ligand that will be easily replaced by other monodentate ligands.
Finally, the dmso ligand is situated in an intermediate coordination strength between the tda 2and aqua ligand.
Consequently, treatment of complex 4II with pyridine under mild conditions (stirring at RT),
generates complex 5II according to the following equation,
[RuII(tda--N3O)(dmso)(OH2ax)] + py ->
4II

[RuII(tda--N3O)(dmso)(py)] + H2O

(2)

5II

Where the initial aqua ligand has been substituted by pyridine. Under harsher conditions, at 100
o
C, with excess pyridine both the dmso and aquo ligands are substituted generating the bis
pyridine complex 6II,
[RuII(tda--N3O)(dmso)(OH2ax)] + 2 py -> [RuII (tda--N3O)(py)2] + dmso + H2O
4II

(3)

6II

The isolated yields for 5II and 6II are above 70% although both reactions (equations 2 and 3)
proceed virtually in a quantitative manner as evidenced electrochemically.
Further, the solubilization of 4II in neat triflic acid (CF3SO3H), followed by heating at 100 oC for
four hours in an open atmosphere, produces the removal of the pyridine and an dmso axial
ligands, presumably by triflate anions and the aerial oxidation of Ru(II) to Ru(III), as indicated in
equation 4,
[RuII(tda--N3O)(dmso)(OH2ax)] + 2 CF3SO3- - 1e- ->
4II
[RuIII(tda--N3O)(CF3SO3)2]- + dmso + H2O

(4)

In aqueous solution the labile triflato ligands are easily substituted by aquo ligands forming the
trans-bis-aquo-Ru complex indicated in equation 5,
[RuIII(tda--N3O)(CF3SO3)2]- + 2 H2O -> [trans-RuIII(tda--N3O)(OH2ax) 2]+ + 2 CF3SO33III(OH2)2+

(5)

Addition of PF6- to this solution precipitates the complex with a mixed counter anions
{[3(OH2)2](PF6)0.5(CF3SO3)0.5}, in 75% isolated yield. This complex has also been characterized in
the solid state by XRD and the ORTEP plot of its hydrated cation is illustrated in Figure 1. The
Ru(III) center displays a distorted pentagonal bipyramid geometry where the axial positions are
occupied by the two aqua groups and the five equatorial positions are occupied by the bonding
atoms of the tda2- ligand. Here the three N and one O have the typical Ru(III)-N and Ru(III)-O
7

bonding distances20, 25,33 while one of the Ru-O carboxylates appears at 2.4 Å, and thus is basically
a contact, see Table S1 for the comparison of the distances of different Ru-tda complexes in the
II and III oxidation state. This exotic distance arises because the geometrical nature of the ligand
clearly forces the metal coordination towards a pentagonal bipyramid geometry although the
complex is a d5 Ru(III) and cannot be 7-coordinated. We name the exotic coordination as 6.5 (see
Figure 1).
Solution behavior
Complexes 4II, 5II and 6II are all diamagnetic Ru(II) low spin d6 ions and were characterized by
NMR spectroscopy in a D2O solution containing 0.1 M CF3SO3D (pD = 1.0). Complex 3III(OH2)2+ is
paramagnetic due to its d5 electronic configuration.20,25 The complex was reduced in situ with
ascorbic acid to generate the corresponding diamagnetic complex 3II(OH2)2. All their NMR
spectra are presented in Figure 2.
Complexes 5II and 6II contain axial pyridyl ligands and the resonances of the pyridyl ligands
appear in the 6.5-8.1 ppm region whereas those of the tda2- ligand appear at lower fields.
Besides, it is interesting to realize that the tda2- resonances appear as if the ligand was
coordinated in a symmetrical manner. This is due to the fast dynamic behavior that exists in
these complexes where the bonded and dangling carboxylate synchronically decoordinate and
coordinate respectively, as displayed graphically in the upper part of Scheme 3. This dynamic
behavior is fostered by the geometrical constrains imposed by the tda2- ligand that strongly
distorts from an ideal octahedral geometry at the equatorial zone and at the same time provides
for an additional bonding carboxylate site in very close proximity. Further in water as a solvent,
this dynamic behavior is also in equilibrium with the coordination of a solvent water in the
equatorial position as indicated in the lower part of Scheme 3. This is clearly evidenced in the
case of 4II where in the crystallization process generates 7II,+. However, in solution this species is
clearly very minor as evidenced electrochemically based on their respective redox potentials. It
thus points out that the isolation of 7II,+ is accomplished thanks to a lower solubility of the
crystallized species 8(4II+7II)+ with regard to that of 4II alone.
This behavior in the Ru(II) oxidation state is radically different to the behavior of the Ru-tda
complexes at Ru(IV) oxidation state. In the latter case, the tda2- ligands in low coordinating
organic solvents, favors a seven coordination environment for this d4 ion, with a pentagonal
bipyramid geometry and with the tda2- ligands acting in -N3O2 fashion, as previously
described.20 Further in aqueous solution at pH = 7.0 or in basic solution, one bonded carboxylate
of complex 6II in the IV oxidation state can be replaced by a hydroxido ligand generating the
seven coordinated active catalyst 1IV(OH)+, depicted in Chart 2.20
Electrochemical properties
The electrochemical properties of the Ru-tda complexes were investigated by Cyclic
Voltammetry (CV) and Differential Pulse Voltammetry (DPV) techniques by using a Glassy
Carbon as a working electrode and Hg/Hg2SO4 as a reference electrode. All potentials were
converted to NHE by adding 0.65 V.

8

The CV of complex 3III(OH2)2+ at pH = 1.0 is shown in Figure 3 left, and shows two reversible
waves at E1/2 = 0.52 V (E = 60 mV) and at E1/2 = 0.94 V (E = 60 mV). These redox events
correspond to single outer sphere electron transfer processes associated with the III/II and IV/III
redox couples respectively. No proton transfer is associated to these couples as evidenced by
the Pourbaix diagram shown in Figure 4, right. Figure 3 right shows the DPV of complexes
3III(OH2)2+, 4II, 5II and 6II at pH = 1.0 together with the related complex 2III(OH2)+. The E1/2 for these
complexes are presented in Table 1, together with other redox and thermodynamic data of
related Ru polypyridyl complexes. The DPV for all complexes exhibit a couple of waves
associated with the III/II and IV/III redox couples (see entries 1-5 Table 1). Here, it is interesting
to see that complexes 4II and 5II, which contain the DMSO ligand, have redox potentials about
200-250 mV higher than complexes 2III(OH2)2+, 3III(OH2)2+ and 6II, which only contain py and aquo
ligands. The increase of potential is associated with the electron withdrawing character of the
dmso ligand as has been previously documented.34 On the other hand, the electronic effect
exerted by the pyridyl ligand compared to the aquo ligands can be nicely observed by comparing
complexes 2III(OH2)+, 3III(OH2)2+ and 6II (entries 4, 5 and 3 respectively). where the -acceptor
character of pyridine cathodically shifts the E1/2 values by approximately 30 mV in both couples.
It is also interesting to compare the potentials of the IV/III and III/II redox couples for 3III(OH2)2+
with those of other related Ru complexes containing polypyridyl ligands such as [Ru II(bpy)3]2+,
9II,2+ (E1/2 (III/II) = 1.26 V; bpy is 2,2’-bipyridine; Table 1, entry 6).35 For 9II,2+ the III/II couple is
anodically shifted by 740 mV with regard to that of 3III(OH2)2+ which is due to strong sigmadonation effect exerted by the carboxylates of the tda2- ligands. Further, it is interesting to note
that the potential for the second oxidation to reach Ru(IV) is out of solvent limits and it has never
been measured. In sharp contrast, the second wave of 3III(OH2)2+, the IV/III couple, is 420 mV
more positive than the III/II couple due to the formation of a seven coordinated complex
[RuIV(tda--N3O2)(OH2ax)2]+ where the tda2- acts now as pentadentate k-N3O2 ligand.
Further, it is also interesting to compare the potentials of the IV/III and III/II redox couples for
3III(OH2)2+ with those of other related Ru-aquo complexes containing polypyridyl ligands at pH =
1.0 such as [RuII(trpy)(bpy)(OH2)]2+, 10(OH2)2+ (E1/2 (III/II) = 1.04 V, E1/2 (IV/III) = 1.23 V; trpy is
2,2’:2”,6’-terpyridine; Table 1, entry 7).,36 Here the potentials for 10(OH2)2+ are significantly
lower than 9II,2+ but still more positive than those of 3III(OH2)2+, as a consequence of the absence
of anionic sigma donating ligands and the presence of Ru-aqua groups that enables proton
coupled electron transfer (PCET) processes.
The redox properties of complex 3III(OH2)2+ were also studied as a function of the pH and its
Pourbaix diagram is depicted in Figure 4 right. The CV of 3III(OH2)2+ at pH = 7.0 is shown in Figure
4 left, where four one-electron redox process occur in the 0.4 - 1.2 V range. The degree of proton
content of the different generated species is inferred from its Pourbaix diagram. Thus in this
particular case at pH = 7.0 the III/II couple involves also a proton transfer (approx. 60 mV/pH
slope) whereas the IV/III involves only a single electron transfer (0 mV/pH slope). Further, the
V/IV now involves a one-electron two-protons transfer (approx. 120 mV/pH slope) and finally
the VI/V wave is a one-electron one-proton transfer with an approx. 60 mV/pH slope. The access
to five different reversible oxidation states thanks to PCET mechanisms is exceptional for
complexes containing just a single metal center.17 This extremely rich redox chemistry of
3III(OH2)2+ parallels that of [trans-RuII(bpy)2(OH2ax)2]2+ complex, 11II(OH2)22+, and [cis9

RuII(bpy)2(OH2ax)2)]2+ complex, 12II(OH2)22+, reported earlier (see Table 1, entries 8 and 9 for
selected values and Figure S14 for their Pourbaix diagrams).37 The redox potential values and
the shape of the diagram differ from those of 3III(OH2)2+ due to the different electronic and
geometrical effects exerted by tda2- compared to bpy. The tda2- increases the electron density
around the metal center in comparison to bpy and provides access to seven coordination. In
addition, this increase in electron density is also reflected in an increase of basicity for the Ru
species that contain hydroxido or oxo groups. This is clearly observed when comparing the first
and second pKas for the RuIV(OH2)2 species in 3 and 11 (entries 5 and 8 in Table 1). For 3, the first
pKa = 4 whereas for 11 the first pKa < 1 while the second pKa is higher than 8 for 3 and lower
than 1 for 11. Thus the dicarboxylato ligand tda2-, renders the Ru(IV) species at least 10 million
times more basic than the bpy ligand.
Interestingly, the Pourbaix diagram of complex 3III(OH2)2+ shows that the potentials for the III/II
and IV/III couples are very similar to the ones reported earlier for 2III(OH2)+.25 The similarity is a
consequence of the fact that the replacement of one of the aqua ligands by a pyridyl maintains
almost unchanged the potentials and the pKa at oxidation state II, III and IV (see Table 1 for the
values and Figure S15 for the Pourbaix diagram of 2III(OH2)+). However, the Pourbaix diagram of
complex 3III(OH2)2+ strongly differs from that of 2III(OH2)+ at higher oxidation states than Ru(IV).
While complex 2III(OH2)+ only undergoes a single electron transfer beyond oxidation state IV,
complex 3III(OH2)2+ can access all the way up to Ru(VI) forming the trans-Ru-dioxo species. This
difference is due to the higher number of protons that can be lost in 3III(OH2)2+ with two Ru-aquo
groups as compared to 2III(OH2)+ with only one Ru-aquo group.
Finally, it is interesting to notice that the VI/V couple for 3VI(O)2 is well above the thermodynamic
potential for the oxidation of water to dioxygen and thus could potentially act as a water
oxidation catalyst. Unfortunately, decomposition reactions possibly involving polymeric oxobridged type of species such as the ones described for 10IV(OH)3+ (see Figure S16), can compete
with the catalytic reaction.38,39
Implications for water oxidation catalyst design
Here we report the synthesis and characterization of 3III(OH2)2+ that completes the family of
seven coordinated Ru-tda complexes bearing different aqua/pyridine ligands in the axial
position as drawn in Chart 2. While 1IV(OH)+ is a spectacular catalyst reaching maximum TOF in
the order of 50 000 s-1, the replacement of one pyridine by a hydroxido to generate 2IV(OH)+
renders this complex still a catalyst but very poor due to the lack of access to hydrogen bonding.
Additional replacement of the pyridine ligand by a second hydroxido ligand generates the bishydroxido complex 3IV(OH)2+ that even though has the right redox potential for water oxidation
unfortunately decomposes quickly towards the formation of polymeric oxo-bridged species. It
thus suggests the need of non-aqua ligands or non-labile ligands to block the formation of highly
stable oxo-bridged Ru complexes.
Thus the present work enlarges the landscape of Ru-tda complexes and uncovers additional
factors that influence the water oxidation catalysis namely, the presence of one or two Ru-aqua
groups, the seven coordination and the presence of anionic ligands and their implication in both
pKa and redox potentials, the capacity to generate the proper intramolecular hydrogen bonding
and the avoidance of oxo-bridged formation
10

Associated Content
The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.
Additional characterization data and electrochemical results as well as X-ray data and CIF for
3III(OH2)2+, and 8+.
Acknowledgments
MINECO and FEDER (CTQ2016-80058-R, CTQ2015-64261-R, SEV 2013-0319, ENE2016-82025REDT, CTQ2016-81923-REDC) and AGAUR (2014-SGR-915) are gratefully acknowledge for
financial support. R.M. thanks “La Caixa” foundation for a PhD grant.

11

Scheme 1. Synthetic strategy for the preparation of the complexes described in this work
(colored) and nomenclature used. Key axial and equatorial ligands are indicated with a “ax”
or “eq” super index respectively.

12

Scheme 2. Selected angles for the equatorial first coordination sphere of complexes 4II,
7II,+ and 3III(OH2)2+. Axial ligands are omitted for clarity. Dashed line indicates contact.

4II

7II,+

3III(OH2)2+

Scheme 3. Reaction pathways leading to a combination of equilibria involved in dynamic
behaviour of Ru-tda complexes in water. The dashed lines indicate bonds that are
simultaneously formed and broken in the transition state. L-Ax represents the axial ligands
py, dmso or aquo.

13

Chart 1. Ligands used and/or discussed in this work together with the labelling and numbering
schemes.

1IV(OH)+

2IV(OH)+

3IV(OH)(OH2)+

Chart 2. Ru hydroxido-pyridine complexes based on the tda2- ligand.20,25 Complexes 2IV(OH)+
and 3IV(OH)(OH2)+ are the one-electron oxidized forms of 2III(OH)2+ and 3III(OH2)2+ at pH > 4.

14

Figure 1. ORTEP plot for complexes 3III(OH2)2+, 8(4II+7II)+ and fragments of 8(4II+7II)+: 4II and
7II,+. Ellipsoids are plotted at 50% probability. Dashed lines indicate contacts. Color codes: Ru,
cyan; N, purple; O, red; C, black and H; colorless. Hydrogen bond distances (O-H-O): HBa, dO-H○
O = 2.6 Å, dO-H = 0.7 Å, dO-H = 1.9 Å; angle O-H-O = 186 ; HBb, dO-H-O = 2.5 Å, dO-H = 0.9 Å, dO-H =
○
1.7 Å; angle O-H-O = 153 ; HBc, dO-H-O = 2.8 Å, dO-H = 0.9 Å, dO-H = 2.0 Å; angle O-H-O = 143○;
HBd, 2.5 Å, dO-H = 0.9 Å, dO-H = 1.65 Å; angle O-H-O = 171○; HBe, 2.5 Å. dO-H = 0.9 Å, dO-H = 1.66
Å; angle O-H-O = 152○

15

Figure 2. 1H NMR spectra for 3II(OH2)2, 4II, 5II and 6II at pD = 1.0 solution. See Chart 1 for the
numbering scheme.

16

Figure 3. Left, CV of complex 3III(OH2)2+ at pH = 1.0. Right, DPV of complexes 2III(OH2)+
(black line), 3III(OH2)2+ (pink line), 4II (green line), 5II (grey line), 6II (brown line) at pH =
1.0. E is reported vs. NHE.

17

Figure 4. Left, blank corrected Cyclic Voltammetry (CV) for 0.5 mM 3III(OH2)2+ at pH = 7.0. The
arrows indicate the redox couples involved in the redox events. Right, Pourbaix diagram of
3III(OH2)2+, showing the zone of predominance of the different species as function of potential
and pH (the tda2- ligand is omitted for clarity). Vertical dashed lines indicate pKa values. The
green dashed line indicates the thermodynamic potential for O2/H2O redox couple.

18

Table 1. Thermodynamic properties for complexes 3III(OH2)2+, 4II and other relevant Ru
complexes described previously.

Eo (V vs. NHE)a
Entry
1
2
3
4
5
6

Complex
[RuIII(tda--N3O)(dmso)(OH2ax)]
4II
[RuIII(tda--N3O)(dmso)(py)]
5II
[RuIII(tda--N3O)(py)2]
6II
[RuIII(tda--N3O)(py)(OH2ax)]+
2III(OH2)+
[trans-RuIII(tda--N3O)(OH2ax)2]+
3III(OH2)2+
[RuII(bpy)3]2+
9II,2+

pKa

III/II

IV/III

VI/IV

RuIVOH2

RuIV(OH2)2

RuIV(OH2)(OH)

0.80

1.15

---

---

---

---

t.w.

0.59

1.02

---

---

---

---

t.w., 25

0.80

1.18

---

---

---

---

t.w., 20

0.54

0.97

---

3.9

---

---

25

0.52

0.94

> 1.75

---

4.0

>8

t.w.

Ref
.

1.26

---

---

---

---

---

35

7

[RuII(trpy)(bpy)(OH2)]2+
10II(OH)2+

1.04

1.23

1.80

< 1.0

---

---

36

8

[trans-RuII(bpy)2(OH2ax)2)]2+
11II(OH2)22+

0.69

1.13

1.26 b

---

< 1.0

< 1.0

37

9

[cis-RuII(bpy)2(OH2ax)2]2+
12II(OH2)22+

0.80

1.15

1.42 b

---

< 1.0

< 1.0

37

a

measured at pH = 1.0

b

calculated from the VI/V and V/IV redox couples.

19

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21

TOC
We report a novel trans-dioxo Ru complex containing the pentadentate tda2- ligand (tda2- =
[2,2':6',2''-terpyridine]-6,6''-dicarboxylate). The seven-coordinate trans-di-oxo complex reaches
V and VI oxidation states at relative low potentials as evidenced by electrochemical analysis. The
comparison of its rich electrochemistry with other water oxidation catalysts allows the
rationalization of the factors that influence the water oxidation reaction catalyzed by molecular
complexes.

22