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https://pubs.rsc.org/en/content/articlelanding/2019/cy/c9cy01796h#!divAbstract

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(Article)

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A bpp-based dinuclear ruthenium photocatalyst for
visible light-driven oxidation reactions

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Seán Hennessey1, Pau Farràs1,2,*, Jordi Benet-Buchholz2, Antoni Llobet2,3,*

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Received: date; Accepted: date; Published: date

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Abstract:

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A diruthenium dyad molecule consisting of a 2,2-(1H-pyrazole-3,5-diyl)dipyridine (Hbpp) bridging
ligand with the formula out-/in-[(bpy)2Ru(bpp)Ru(L)(tpy)]n+ (L = Cl, CF3COO, H2O or CH3CN; bpy
= 2,2’-bipyridine and tpy = 2,2’:6’,2“-terpyridine) has been prepared and fully characterised. The
complex has been characterized by analytical and spectroscopic techniques and by X-ray diffraction
analysis for two of Cl and CH3CN derivatives. Additionally, full electrochemical characterization
based on cyclic voltammetry and square wave voltammetry has been also performed. The pH
dependence of the redox couples for the aqua complex has also been studied and the corresponding
Pourbaix diagram drawn. Furthermore, the capacity to photo-catalytically oxidize organic
substrates, such as alcohols, alkenes, and sulfides, has been carried out and the overall stability and
selectivity of the catalyst has been analysed.

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Keywords: photocatalysis; ruthenium; photooxidation; solar chemicals; electron transfer

School of Chemistry, Energy Research Centre, Ryan Institute, National University of Ireland, Galway (NUI
Galway), University Road, H91 CF50 Galway, Ireland.
2 Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, 43007 Tarragona, Spain.
3 Departament de Química, Universidad Autònoma de Barcelona, Cerdanyola del Vallès, 08193 Barcelona,
Spain.
* Correspondence: pau.farras@nuigalway.ie; Tel.: +353 91 492765 (P.F.); allobet@iciq.es; Tel.: +34 977 920 000
(A.L.)

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1. Introduction

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Moving away from the earth’s heavy reliance on fossil fuels is one of the greatest challenges
facing researchers today [1]. Due to the abundance and cleanliness of the energy source, using solar
energy to drive chemical reactions for the production of useful materials and fuels is an attractive
alternative to conventional carbon-heavy methods [2]. The area of solar-driven chemistry has become
very promising to the scientific community looking at ways of artificial water splitting and green
chemistry [3]. The opportunity for industrial-scale processes to use 100% renewable energy in the
synthesis of useful chemical material is a highly attractive proposition [4]. Furthermore, removing
the necessity of certain reagents in industrial processes that produce by-products that have a negative
environmental impact, such as permanganate and certain metal oxides, is of significant importance
[5]. To bypass these problematic reagents, the implementation of water as an oxygen source can
provide an inexpensive and environmentally friendly alternative with a very high atom economy
[6,7].

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The production of solar chemicals can be performed using different methods, from purely
heterogeneous and homogeneous catalysis to intermediate hybrids in which molecular catalysts are
immobilised on semiconducting surfaces. However, many of these systems focus on using a
homogenous mixture of a photosensitizer and a (photo)catalyst [8], which has the negative effect of
poor intermolecular electron transfer between the two systems. However, dinuclear systems
containing both a photosensitizer and catalyst in a single molecular system have potential to
overcome these problems. To achieve this, dinuclear transition metal complexes arrayed in a
chromophore-catalyst, or ‘dyad’, arrangement have been considered as an alternative. These
dinuclear species are of particular interest as it has been shown that due to the close proximity of the
two metal centers, intramolecular electron transfer is improved in comparison to their mononuclear
counterparts [9]. These dyads consist of a photosensitizing light-harvesting unit M p, and a catalytic
centre Mc, to give an overall complex MpMc. The use of ruthenium-based polypyridyl complexes has
been widely studied in both the applications of photosensitizers and (photo)catalysts [4]. As a result,
dinuclear ruthenium complexes have been utilized to give complexes of formula, Ru pRuC [10][11],
although other metals and photosensitizing units have also been incorporated with Ru based
polypyridyl units [12,13]. These complexes are coupled through a bridging ligand, through which
intramolecular electron transfer takes place.
The standout feature of these dyad systems is the molecular electronics as a result of the auxiliary
and bridging ligands. These ligands can alter the orbital energy, excited state lifetime and redox
potentials of both the Rup and RuC [10]. The bridging ligand is key to the overall processes of these
dyads, as the strength of the bonding of the systems has been studied to proportionally effect the
speed of the intramolecular electronics of the system [14]. Previous work performed by Jakubivoka
et al. looked at the spatial localization of excited state electrons in a variety of chromophore-catalyst
assemblies. This work used density functional theory (DFT) to determine the effects of modification
of bridging ligands on intramolecular electron transfer in the dyad. Thereby showing how extended
π-conjugated ligand can shift the electronic excitations from the bridging ligand into the terminal
ligand [15].
By basing choice of ligand on the Robin and Day classification of inner-sphere electron transfer,
bridging ligands can be tailored to optimise electron transfer [16,17]. Examples of which are shown
in Figure 1. A class I system, like that shown in Figure 1a, involves weak electronic interaction
between two species in an entirely decoupled independent system. This poor electron transfer can be
improved upon by the introduction of a bridging ligand like that seen in Figure 1b and 1c. This class
II system contains localized valences (oxidation states) and measurable electronic coupling between
the two metal centres, giving rise to two redox sites that are mutually dependent. Work by Chen et
al., showed that a very strong electronic coupling through the bridging ligand (in this case a 2,3,5,6tetrakis(2-pyridyl)pyrazine) results in charge trapping between the chromophore and catalyst centres
[18]. This delocalisation of charge was thought to be responsible for the decreased catalytic
performance.

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Figure 1: Examples of previous ruthenium polypyridyl systems with photosensitizing, Rup, (red) and
catalytic, Ruc, (blue) units highlighted a) unlinked chromophore – catalyst system, Ref. [19] b) Bridged
diruthenium systems through highly decoupled aromatic bridging ligands, Ref. [10] c)
chromophore−catalyst assembly with a saturated alkyl bridge, Ref. [20].

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Several examples of dyad systems have already been demonstrated to have high selectivity
towards the photocatalytic oxidation of organic substrates such as alcohols, sulfides and alkenes
[10,11,18,21–24]. The higher oxidation states required for the oxidation of these substrates is achieved
due to the fast intramolecular electron transfer between the two Ru units [25]. Therefore, building on
this previous work, by using a bridging ligand that enables long-lived charge-separated states, while
ensuring good electron transfer can potentially lead to much more efficient catalysis. Herein we
investigate the use of the bridging ligand, Hbpp, in the framework of the dyad, out-/in[(bpy)2Ru(bpp)Ru(L)(tpy)]n+ (L = Cl, CF3COO, H2O or CH3CN, n = 2 or 3). We have characterized this
dyad by a combination of analytical, photophysical, and electrochemical techniques. Additionally,
we have tested the capacity of the dyad for carrying out photoinduced oxidation of organic
substrates, such as alcohols, sulfides, alkenes and its water oxidation potential. We have used the
data gathered to compare to similar systems used in the photocatalytic oxidation of organics.

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2. Results and Discussion

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2.1. Syntheses

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Both synthetic pathways utilized begin with the synthesis of the bridging ligand 2,2-(1Hpyrazole-3,5-diyl)dipyridine (Hbpp), as seen in Figure 2. From there, complexation of this ligand
with either ruthenium component (Rup or Ruc) is possible.

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Figure 2: Two possible synthesis pathways of the two isomers of the Ru dyad. (i) MeOH, 40 °C,
overnight, NH4PF6 (aq) (ii) EtOH, reflux, 2 hr, NH4PF6 (aq) (iii) NEt3, MeOH, 3 hr, reflux, dark, NH4PF6
(aq) (iv) NEt3, MeOH, 3 hr, reflux, dark, NH4PF6 (aq).

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Complexation of the Hbpp with Ru(tpy)Cl3 leads to the formation of the intermediate
mononuclear complex, out-[Ru(Cl)(Hbpp)(tpy)]2+, out-12+ [26]. Further reaction of out-12+ with
Ru(bpy)2Cl2 produces the out- isomer of the dyad, out-[(bpy)2Ru(bpp)Ru(Cl)(tpy)]2+, out-[3-Cl]2+.
However, this isomer of the dyad was shown to be highly unstable under ambient light, in which a
noticeable colour change from red to purple is observed over short time periods. This has been
tracked by UV-Vis spectroscopy (Figure 3) and cyclic voltammetry (Figure S1), and degradation of
an impure sample is observed via 1H-NMR spectroscopy (Figure S2). The UV-Vis of out-[3-Cl]2+
shows bands mainly corresponding to that of the Ru p unit, Ru(bpy)2. The RuC centre formed from
out-12+ has a weak ε value similar to other Ru-Cl complexes, resulting in these bands showing very
weakly in the 400-600 nm region of out-[3-Cl]2+ [26]. The product obtained after light degradation is
similar to that of the free Ru(bpy)2Cl2, based on the UV-Vis, suggesting that this is reformed after
degradation. This is supported by the CV of the degraded species, which shows the reformation of
Ru(bpy)2Cl2 in addition to the appearance of several new peaks, suggesting the formation of several
individual Ru-based species, which were not distinguishable in the UV-Vis. This is also backed up
by the NMR, which suggests the reformation of out-12+ after light irradiation. It is possible that the
out- isomer is disfavoured due to the geometric constraints between the tpy and bpy of the two
respective Ru units of the dyad. With light excitation the isomerization of the complex to the more
favourable in- isomer is therefore not feasible, resulting in the relatively rapid degradation of the
complex, the speed of which is clearly accelerated by light irradiation. To show this, DFT calculations
were performed on in-3-Cl, out-3-Cl, in-3-CH3CN, out-3-CH3CN and show a clear stabilisation of the
in- isomers in both cases (see SI), and show no available rotation in the complexes. For the chlorido
complexes the energy difference is more pronounced, with the in- complex exhibiting a stability of
9.3 kcal/mol over the out- counterpart. The difference becomes less important for the acetonitrile
complexes, in which the in- isomer is 1.9 kcal/mol more stable. However, in both cases the geometries

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of the complexes in which the tpy and bpy ligands and very close together show that the only way
to see isomerism from the out- to in- isomer would be through bond breaking in the complex, with a
concomitant very high activation barrier.

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Figure 3: UV-Vis absorbance spectra showing degradation of out-[3-Cl]2+ in CH2Cl2 after light
irradiation. Ru(bpy)2Cl2 shown (blue) for comparison.

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The in-isomer of the dyad, in-[3-Cl]2+, can be synthesized via the formation of the intermediate
mononuclear complex, [Ru(Hbpp)(bpy)2]2+, 22+ [27]. From here, further complexation with
[Ru(tpy)Cl3] in excess (12 eq.) of triethylamine leads to the formation of the in-dyad. The final reaction
mixture, when analyzed by mass spectrometry, is shown to contain the dyad, as well as the starting
material, 22+, and a Ru(tpy)2 complex formed as a side product. To obtain the chloro derivative,
successive recrystallisation of the crude reaction using an acetone-diethyl ether mixture leads to the
isolation of in-[3-Cl]2+ in a 24% yield. The chlorido ligand has been shown to be highly labile with
aqua, trifluoroacetate and acetonitrile moieties replacing it readily. We note a clear difference
between the out- and in-isomers in the 1H-NMR (Figure S3). For the out- isomer, a doublet appears at
9.90 ppm, while for in isomer, the most downfield signal is at 9.50 ppm. This main difference is
attributed to the interaction of the pyridine substituent in the bpp ligand interacting with the Cl of
the out isomer, which it is unable to do with the in- counterpart.
The dyad can also be separated effectively using semi-preparative HPLC with a
acetone:water:trifluoroacetic acid eluent (50:50:0.1). The HPLC chromatograms and relevant mass
spectrometry data are shown in the supplementary. After drying of the HPLC fractions, the desired
product is shown by mass spec to be in-[3-OOCCF3]3+, with a trifluoroacetate coordinated to the Ruc
unit, as opposed to the chlorido, in a moderate 29% yield. When dissolved in water, the labile ligand
position is shown to be in equilibrium between the CF3COO- and the OH2 species, as observed via
mass spectrometry and 1H-NMR. In d6-acetone, broad peaks are observed due to the competing
equilibrium of the two ligands and their interaction with the ruthenium centre. This competition can
also be observed in the 19F-NMR, the individual spectra of which are in the supporting information.
The NMR of in-[3-OH2]2+ in D2O show the two PF6 signals at -71.32 and -73.20 ppm respectively
(Figure S4a). However, when performed in acetone-d6 (Figure S4b) we see the expected upfield shift
of the two PF6 ions due to the different solvent, but in addition we see the appearance of a third peak
at -77.73 ppm indicating the presence of the trifluoroacetate ligand bound to the dyad.

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2.2. X-Ray Crystallography

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Two crystal structures of the Ru dyad have been obtained, with the acetonitrile and chloro ligand
bound to the Ru centre (Figure 4a and 4b). It is possible to obtain the acetonitrile derivative by
dissolving up a sample of either in-[3-OH2]3+ or in-[3-Cl]2+ in acetonitrile, drying, and crystalizing by

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slow evaporation of a saturated acetone:water solution. On the other hand, it is possible to obtain the
in-[3-Cl]2+ dyad by successive recrystallization of the final reaction mixture using an acetone-diethyl
ether mixture via a slow diffusion method. The crystallographic data for the two structures is
gathered and shown in Table S1. In the cases of both crystals the cell unit contains two enantiomers
which haven’t been separated. Both crystal structures present a distorted octahedral geometry
around the Ruc centre, with the tpy in each case occupying the meridional plane, and the bpp ligand
coordinating via two nitrogen atoms, N6 and N7, as the bridging ligand. The final coordination
position contains the acetonitrile or chloro ligand facing the Ru P unit. The bond distance between the
coordinating atom and the Ruc does vary with the type of ligand group. However, the bond lengths
and angles of the Ru-L (Ru-Cl: 2.040 Å, Ru-NCCH3: 2.407 Å) are similar to that of previous Ru
complexes in the literature [10,28]. The geometry of the Rup unit is fairly unremarkable and contains
the Ru(bpy)2 unit in the commonly observed cis- fashion in both cases.
a)

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b)

Figure 4: Single-crystal X-ray structure of a) in-[3-CH3CN]3+ (thermal ellipsoid plot drawn at 30%
probability). b) in-[3-Cl]2+ (thermal ellipsoid plot drawn at 20% probability). All hydrogens,
hexafluorophosphate counterions and solvent molecules (acetone) have been omitted for clarity.

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2.3. Spectroscopic Properties

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The dyad, in-[3-OOCCF3]2+, is fully characterized by both 1D and 2D NMR, the spectra of which
are shown in the supporting information. The chloro and acetonitrile derivatives have also been
characterized by 1H-NMR and 13C-NMR as well as their photophysical characteristics and
electrochemical properties.
The photophysical characterization of the aqua derivative has been performed by UV-Vis, the
spectra of which is shown in Figure 5. Three main regions can be distinguished from the UV-vis
spectroscopy. The distinct bands observed between 250 and 400 nm correspond to the intra-ligand
π–π* transitions that arise as a result of the high aromaticity of the bpy, tpy and bpp ligands. The
broad band detected between 400 and 540 nm is assigned to the unsymmetrical Ru(dπ)tpy/bpy/bpp(π*) metal-to-ligand charge transfer (MLCT) bands. The last significant band is the broad
region between 540 and 620 nm, in which d-d transitions are known to be common (highlighted via
inset in Figure 5).

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Figure 5: The UV-Vis absorbance spectra for in-[3-OH2]3+ in CF3SO3H at pH = 1. Transitional d-d
absorption band between 500-700 nm shown inset for clarity.

The UV-Vis redox titration of the aqua complex was studied by the subsequent addition of 1 eq.
of the oxidant cerium (IV) ammonium nitrate (CAN), from which the spectra of the different oxidized
species formed can be observed, which is shown in Figure 6a (the slower additions of 0.1 eq. of CAN
is shown in Figure S5). As has been seen in previous cases, the appearance of a band at 600 nm is due
to the production of an RucIV=O complex as a result of the oxidation of the dyad [29]. In addition, we
observe a significant decrease in the MLCT absorption band at 450 nm after the addition of 1
equivalents of oxidant, which is further oxidized by two further additions of CAN. This is followed
up by the complete disappearance of the band upon the addition of the 4th equivalent of CAN,
indicating oxidation of the RupII to RupIII species. This is further proven by the significant increase in
absorption at 600 nm, which corresponds to the formation of the Ru pIII species after the final addition
of CAN.

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Figure 6: UV-Vis absorbance spectra of oxidized products of in-[3-OH2]3+ upon addition of 1 to 4
equivalents of Ce(IV) under air in 0.1 M CF3SO3H (pH = 1). 6b) UV-Vis absorbance spectra of in-[3OH2]3+ (black line), in-[3-OH2]3+ with 1 eq. of Ce(IV) (red line) and after 900 mins exposed with 1 eq.,
all in 0.1 M CF3SO3H.

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The stability of the aqua complex was tested by addition of the oxidant CAN. Upon the addition
of 1 equivalent of the oxidant we notice a sizeable decrease in the MLCT band as a result of the
oxidation of the RucII-OH2/RucIII-OH (Figure 6b). After the addition of 1 eq. of CAN, the dyad was left
for 900 mins to determine the stability of the RucIII-OH species. The spectrum observed after this time

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period indicate that the initial species is not reformed, indicating a change in structure, although we
do observe the band at 600 nm return, suggesting the presence of some recovered Ru cII.
Due to the need for robust catalysts the stability of the complex over a range of pH was tested
to determine its feasibility under different reactions conditions. To test the robustness, the dyad was
exposed to a harsh environment (pH = 12, RucII-OH) and exposed to air in the absence of any organic
substrate and monitored by UV-Vis spectroscopy (Figure S6). No significant change in the UV-Vis
was observed over time, suggesting good stability of the complex, with the decreasing MLCT over
the time period being attributed to the compound’s precipitation and subsequent decreased
concentration.
A key aspect of this work is the structure/activity relationship affected by the bridging ligand in
dinuclear ruthenium dyads. As discussed in the introduction, Hbpp is a weak bridging ligand but
there was the need to check how weak was it compared to other reported ligands in the literature.
Therefore, to quantify the value the delocalization energy (Hab), the electronic coupling between the
donor and acceptor of a coupled redox site was used, this is related to the integrated absorption band
intensity, and its relationship is given by equation 1;
𝐻

=

.

×

(𝜀

𝜈̅

𝛥𝜈̅

/

)

/

(1)

Where εmax is the molar absorptivity coefficient, 𝜈̅ max is the absorption maximum in cm-1, 𝛥𝜈̅ 1/2 is the
width at half the height in cm-1 and r is the distance between the two Ru centers in angstroms [30] .
From the NIR spectra (Figure S7) and crystal structure data we can characterize the electronic
coupling of the bridging ligand). Under the Robin and Day classification, a class I complex has a Hab
= 0, with class II arising from 0 < Hab < λ/2), and class III arising when 2Hab/λ ≥ 1 [31,32]. In the case of
the dyad analyzed herein, the εmax of 170.6 M-1cm-1 at a wavelength of 8422 cm-1 we obtain a value of
184.6 cm-1 for Hab, indicating that the dyad is a class II system. The small value of Hab suggests that
the dyad, in-[3-OH2]3+ is on the weak side of class II complexes.
The electrochemistry of the complex has been performed on three of the dyads with -Cl, CH 3CN
and H2O moieties. The cyclic voltammetry of the in-[3-Cl]2+ is shown in the supporting information
(Figure S8) and shows the two expected peaks of RuIII/II, for the Rup and Ruc units, at 1.19 and 0.65 V,
respectively. With the in-[3-CH3CN]2+ we observe the disappearance of the peak at 0.65 V, which is
commonly observed for a Ru-Cl species, and the appearance of 2 broad peaks at 1.10 and 1.33 V
respectively, in accordance with two Ru-N6 species present. The CV of in-[3-OOCCF3]2+ in
dichloromethane shows the two peaks for the RuIII/II couple of each Ru unit at 1.22 and 0.78 V, for RuP
and RuC respectively. The electrochemistry of in-[3-OH2]3 has been investigated by CV and SWV in
the pH range 0.0–13.0. The CV performed in aqueous media shows the pH dependence expected for
a Ru-OH2 moiety and this is shown as a Pourbaix diagram, with each electrochemical species labelled,
in Figure 7, the equations corresponding to each transition are shown below.
At pH = 7:
RupIIRucIII-OH + 1e- → RupIIRucII-OH2
RupIIRucIV=O + 1e- + 2H+ → RupIIRucIII-OH

E1/2° = 0.879 V

(4)

RupIIRucIII-OH2 + 1e- → RupIIRucII-OH2

E1/2° = 0.618 V

(5)

RupIIIRucIV=O + 1e- + 2H+ → RupIIRucIII-OH2

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(2)
(3)

RupIIIRucIV=O + 1e- → RupIIRucIV=O

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E1/2° = 0.412 V
E1/2° = 0.481 V

E1/2° = 0.880 V

(6)

At pH = 1:

As shown by the drawn Pourbaix diagram, at pH = 1, the RupIIRucIII-OH2 is present and is further
oxidised to the RupIIIRucIV=O species. Of relevance is the 1e-/2H+ transfer leading to very reactive and
unstable species (see eq. 4 and eq. 6) observed in other Ru pIII complexes [10]. At pH 7 we observe the
formation of a RupIIRucIII-OH species across a very narrow potential window, with a significant

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driving force for RupIII to oxidise RuC to its active catalytic state, RuCIV=O, which is able to oxidise
organic substrates.

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Figure 7: Pourbaix diagram for in-[3-OH2]3+. The stability zones of the different species as a function
of pH and E1/2 (vs. SSCE) are shown and are indicated by the oxidation state of the ruthenium metal
and the degree of protonation of the initial aqua group.

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The CV experiments displayed in Figure 8 show the electrocatalytic nature of in-[3-H2O]3+ in the
presence of increasing concentrations of BzOH. There is a noticeable shift (inset) around 0.9 V as a
direct result of oxidation of the alcohol with concomitant reduction of RucIV=O to RucIII-OH. This
electrochemical change has been observed in previous Ru dyad systems, displaying the fact that the
electrocatalytic peak required for the oxidation of the BzOH species begins at this point [10].

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Figure 8: Cyclic voltammograms of in-[3-OH2]3+ (1.0 mM) at pH 1.0 with nBu4NPF6 (0.1 M) supporting
electrolyte (scan rate 0.01 Vs-1) in a deoxygenated 0.1 M solution of triflic acid after the gradual
additions of 0.05 M of BzOH. A zoomed region between 0.70 - 1.30 V is shown inset.

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2.4. Photocatalytic Properties

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The photocatalytic experiments were all performed by exposing a sealed reaction vial to simulated
visible-sunlight irradiation (λ > 400 nm) for 24 h at room temperature. The samples contained 5 mL
of deoxygenated aqueous solutions at pH 7.0 (50 mM phosphate buffer) containing 20 µM in-[3OH2]3+, 10 mM substrate and 20 mM electron acceptor ([Co(NH3)5(Cl)]Cl2). The reaction products
were characterized and tracked by 1H-NMR spectroscopy, with yields arising from quantitative
analyses of the integrated signal corresponding to the individual reference/substrate, the results of
which are displayed in Table 1. The dyad was shown to display a high selectivity for the
photocatalytic oxidation of benzyl alcohol, with no overoxidation and formation of carboxylic acid
observed, and a turnover number (TON) of 34. This lack of overoxidation was also observed with the
sulfide oxidation, with no sulfone present after catalysis. However, in the case of styrene, due to the
low conversion yield, the disappearance of the substrate by NMR is the only quantitative analysis
possible, as the products of the photocatalysis were not able to be isolated. Dyad in-[3-OH2]3+ has also
been tested under the same conditions without the presence of the organic substrate to check its lightdriven water oxidation properties. However, no O2 evolution could be seen under the same
photocatalytic conditions.

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Table 1: Photocatalytic oxidation of organic substrates using in-[3-OH2]3+ in aqueous solution with
comparative values from similar systems.[a]

Catalyst

Substrate

pH

t [hr]

T [°C]

TN (Conv. [%])

Ref.[c]

in-33+

BzOH
PhSMe
4-HSS[d]
BzOH
PhSMe

7.0
7.0
7.0
7.0
7.0

24
24
24
24
24

25.0
25.0
25.0
25.0
25.0

34 (6.8)
103 (20.6)
13 (2.6)
7 (1.4)
67 (13.4)

tw
tw
tw
[10]
[6]

A[e]

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[a] = Reaction conditions: Deoxygenated aqueous solution, phosphate buffer (50 mM), Catalyst (0.02 mM),
substrate (10 mM), [CoIII(NH3)5Cl]2+ (20 mM), Xe lamp source (150 W – cut-off filter 400 nm) [b] TN = Turnover
number [c] tw = this work. [d] protonated form of sodium 4-styrene sulfonate [e] = (tpy)Ru(µtppz)Ru(OH2)(bpy)]3+

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4.1. Materials

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Ruthenium trichloride trihydrate was supplied by Alfa Aesar and was used as received.
Trifluoromethanesulfonic acid (+99%) was purchased from STREM/CYMIT. Ceric ammonium
nitrate, 2,2′-bipyridine, terpyridine, potassium hexafluorophosphate, tetrabutylammonium
hexafluorophosphate, ammonium hexafluorophosphate, silver perchlorate, benzyl alcohol, phenyl
methyl sulfide, 4-styrene sulfonic acid, pentaamminechloridocobalt(III) dichloride and all organic
solvents were of the highest purity commercially available and were used as received from Aldrich
or Acros/Fisher. All solvents were dried prior to use. Aqueous solutions were prepared by using
deionized water from an Ultra Clear water purifier system from SG Wasseraufbereitung und
Regenerierstation GmbH (conductivity at 25 °C = 0.055 µS cm -1). The compounds Hbpp [33],
[Ru(tpy)Cl3] [34], out-[Ru(bpp)(tpy)Cl] (out-12+) [26], [Ru(bpy)2Cl2] [35] and [Ru(bpp)(bpy)2] (22+) [27]
were all prepared using literature procedures. All synthetic manipulations were routinely performed
under a dry dinitrogen atmosphere by using standard Schlenk and vacuum-line techniques.

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Synthesis of out-[3-Cl]2+, out-[(bpy)2Ru(bpp)Ru(Cl)(tpy)] 2(PF6): In a dark environment, out1 (100 mg, 0.124 mmol) was mixed with sodium methoxide (base, NaOMe, 13.4 mg, 2 eq.) in 50 mL
dry MeOH. The mixture was stirred at rt under N2 for 40 mins. Ru(bpy)2Cl2 (66 mg, 0.136 mmol , 1.1
eq.) was added with a further 80 mL MeOH and refluxed for 3.5 hours. The bright red reaction was
2+

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cooled, solvent removed and precipitated with NH4PF6 (sat aq) to give red-brown precipitate. No
further purification was carried out due to instability.
Synthesis of in-[3-OOCCF3]2+, in-[(bpy)2Ru(bpp)Ru(OOCCF3)(tpy)] 3(PF6): In a dark
environment, Ru(tpy)Cl3 (25 mg, 0.054 mmol) was dissolved in 20 mL of dry MeOH under N2 and
dry NEt3 (23 µL, 0.324 mmol 3 eq.) was added. In a separate flask, [Ru(bpp)(bpy)2]2(PF6) (52 mg, 0.054
mmol) was dissolved in 20 mL of dry MeOH under N2 and dry NEt3 (23 µL, 0.324 mmol 5 eq.) was
added. The two solutions were stirred at room temperature for 30 mins, then combined and refluxed
for 2.5 hr. The reaction was cooled, and the solvent removed, and the red precipitate dissolved in 10
mL of H2O/MeOH (4/1). A saturated solution of NH4PF6 was added and the red precipitate formed
was filtered and isolated. The title compound was isolated as red microcrystals via a semi-prep HPLC
using a acetone:water:trifluoroacetic acid (50:50:0.1) eluent. (29% yield). 1H NMR (500 MHz,
(CD3)2CO) δ: 9.10 (d, J = 5.8 Hz, 1H), 8.97 (d, J = 8.1 Hz, 1H), 8.82 (dd, J = 16.5, 8.2 Hz, 2H), 8.70 (dd, J
= 17.5, 8.1 Hz, 2H), 8.62 – 8.55 (m, 3H), 8.48 – 8.41 (m, 1H), 8.36 – 8.30 (m, 1H), 8.28 (s, 1H), 8.27 – 8.07
(m, 6H), 7.99 (dd, J = 15.5, 6.8 Hz, 2H), 7.92 (t, J = 6.2 Hz, 2H), 7.78 – 7.59 (m, 7H), 7.49 – 7.41 (m, 2H),
7.38 (dd, J = 7.5, 5.7 Hz, 1H), 7.34 (dd, J = 7.5, 5.8 Hz, 1H), 7.27 (d, J = 5.8 Hz, 1H), 7.17 (t, J = 7.5, 5.9
Hz, 1H), 6.88 (dd, J = 7.4, 5.9 Hz, 1H). 13C NMR (126 MHz, (CD3)2CO) δ: 162.33, 161.90, 160.43, 160.21,
159.58, 159.26, 158.28, 158.19, 156.80, 156.77, 156.36, 155.84, 155.58, 155.03, 154.44, 153.67, 152.45,
151.87, 150.89, 150.82, 150.65, 137.81, 137.57, 137.38, 136.44, 136.26, 135.78, 135.48, 134.12, 127.76,
127.49, 127.13, 126.56, 126.42, 125.59, 124.29, 123.83, 123.71, 123.65, 123.49, 123.14, 123.00, 122.51,
121.76, 121.11, 119.66, 115.46. MS (ESI): 541.4 (M3+). HR-MS (EA): C50H36F15N11O2P2Ru2, Found: C, 38.5;
H, 2.5; N, 10.6. Calc C, 43.69; H, 2.64; N; 11.21.

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UV-Vis spectroscopy was performed on a Cary 50 (Varian) UV-Vis spectrophotometer in 1 cm
quartz cuvettes. HPLC analysis was performed using a Waters semipreparative chromatograph (up
to 40 mL/min) equipped with a 600E multisolvent delivery pump, a Rheodyne 7725i injection valve
and 2487 detector. The NMR spectroscopy experiments were performed on a Bruker Avance 400
Ultrashield NMR spectrometer. Samples were run in CDCl3, CH3OD, D2O, or (CD3)2CO with internal
references (residual protons). Elemental analysis was performed by using an EA-1108, CHNS-O
elemental analyzer from Fisons Instruments. ESI-MS analyses were recorded on an Esquire 6000 ESI
ion trap LC/MS (Bruker Daltonics) instrument equipped with an electrospray ion source. CV and
SQWV experiments were performed in a IJ-Cambria IH-660C potentiostat by using a three-electrode
cell. A glassy carbon electrode (2 mm diameter) was used as the working electrode, platinum (2 mm
diameter) as the auxiliary electrode, and SSCE as a reference electrode. Working electrodes were
polished with 0.05 µm alumina paste and washed with distilled water and acetone followed by blowdrying before each measurement. All cyclic voltammograms presented herein were recorded in the
absence of light and inside a Faradaic cage. The complexes were dissolved either in CH2Cl2 or CH3CN
containing the necessary amount of nBu4NPF6 as a supporting electrolyte to yield a 0.1 M ionic
strength solution. In aqueous solution, the electrochemical experiments were carried out in 0.1 M
CF3SO3H (pH 1.0). E1/2 values reported herein were estimated from CV experiments as the average of
oxidative and reductive peak potentials (where [E1/2 = (Ep,a + Ep,c)/2)]) or taken as E(Imax) or SQWV
measurements. For construction of the Pourbaix diagrams, the following buffers were used:
dihydrogen phosphate/phosphoric acid up to pH 4 (pKa = 2.12), hydrogen phosphate/dihydrogen
phosphate up to pH 9 (pKa = 7.67), hydrogen phosphate/sodium phosphate up to pH 13 (pKa = 12.12),
sodium hydroxide for pH 14, and 0.1 M triflic acid was used for pH 1.0. The concentration of the
species was approximately 1 mm. Oxygen generation in solution was measured by using a waterjacketed Clark electrode reactor from Hansatech. In a typical experiment, a 1.0 mM solution of the
complex in CF3SO3H (2 mL, pH 1.0) was degassed with nitrogen until no oxygen could be detected.
Chemical redox spectrophotometric titrations were performed by sequential addition of a small
volume of a solution of CeIV in CF3SO3H (pH 1.0, 50 mL/redox equivalent) to the solution of complex
(2.5 mL) in standard 1 cm quartz cuvettes. All density functional methods calculations were
performed in the Gaussian 16 suite of programs [36] by using the M06 functional [37] and an ultrafine

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integration grid (99 590) in conjugation with the all-electron 6-31G(d) basis set for C, H, N, O, and Cl
atoms [38]. The Stuttgart relativistic effective core potential basis set is used for Ru (ECP28MWB)
[39,40]. All four structures were fully optimized in the gas phase and verified as local minima through
frequency calculations.

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5. Conclusions

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We have prepared a new Ru dyad molecule in which one metal of the dinuclear complex acts as
a light harvester and the other as a catalyst. We have thoroughly characterized the new complex
spectroscopically and electrochemically. In addition, we have shown that a variety of organic
substrates, such as alcohols, alkenes, and sulfides can be selectively photocatalytically oxidized by
the dyad. By comparing to similar systems, we have shown that by tuning the electronic coupling
between the RupRuc we can significantly alter the catalytic activity of the molecule. The future outlook
on these complexes will be the need to calculate the electron transfer rates of the intramolecular
electron transfer between the metal centers to optimize the catalytic output of the dyads.

Kinetics: Experiments were performed on a Cary 50 spectrophotometer equipped with a
temperature-controlled cell holder. In a typical experiment, a solution of CeIV (1 eq) in CF3SO3H (10
mL, pH 1.0) was added at 25 °C to a solution of in-[3-OH2]3+ (3 mL, 0.1 mM) in 0.1 M CF3SO3H.
Light-driven catalytic oxidations: In a typical experiment, a water-jacketed cell containing a
deoxygenated aqueous (H2O, 5 mL) solution at pH 7.04 (50 mM phosphate buffer) with Ru catalyst
(0.02 mM), substrate (10 mM), and [Co(NH3)5(Cl)]Cl2 (acceptor, 20 mM) was exposed to simulated
sunlight (l > 400 nm, UV cut off filter) for 24 h at 25 °C; during which period the mixture was kept
under magnetic stirring. Throughout these photocatalytic experiments, visible-light irradiation was
provided by a 150 W Xe arc lamp. The incident irradiance at the surface of the reaction vessel was
approximately 0.3 W cm2. The reaction product was characterized by 1H-NMR spectroscopy through
quantitative analyses of integrated signal intensities relative to the corresponding
reference/substrate. For water oxidation experiments, oxygen generation in solution was measured
by using a water-jacketed Clark electrode reactor from Hansatech (2 mL solution).
X-ray crystal structure determination: The crystal complex of in-[3-CH3CN](3PF6) was obtained
dissolving up a sample of the Cl ligand in acetonitrile, drying, and crystalizing using a water:acetone
(1:1) mixture with a slow evaporation method. in-[3-Cl](2PF6) was obtained by a successive
recrystallization of the final reaction mother liquor using an acetone-diethyl ether mixture in a slow
diffusion process. The measured crystals were prepared under inert conditions immersed in
perfluoropolyether as a protecting oil for manipulation.
Data collection: Crystal structure determinations were carried out by using a Bruker-Nonius
diffractometer equipped with an APPEX 2 4K CCD area detector, a FR591 rotating anode with MoKa
radiation, Montel mirrors as the monochromator, and an Oxford Cryosystem plus low-temperature
device (T = -173 °C). Full-sphere data collection was used with w and f scans. Programs used: Data
collection APEX-2 [41], data reduction Bruker Saint V/.60A [42], and absorption correction SADABS
[43].
Structure solution and refinement: Crystal structure solution was achieved by using direct methods
as implemented in SHELXTL [44] and visualized by using the program XP. Missing atoms were
subsequently located from difference Fourier synthesis and added to the atom list. Leastsquares
refinement on F2 by using all measured intensities was carried out with the program SHELXTL. All
non-hydrogen atoms were refined, including anisotropic displacement parameters. CCDC-XXX (in[3-Cl]2+) and CCDC-XXX (in-[3-CH3CN]3+) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.

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Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: Cyclic
voltammograms of out-[3-Cl]2+. showing degradation under ambient light over time periods of 2 hours in
CH2Cl2, scan rate 0.1 V/s. out-[1]2+ shown for comparison. Figure S2: Stacked 1H-NMR spectra of impure out-[3Cl]2+ before light irradiation (above) and after light irradiation (below) performed in MeOD. Apodization for
bottom spectra: Exponential: 1.60 Hz, Integral-to-noise effect : 5.129 s. Figure S3: Stacked 1H-NMR spectra of in[3-Cl]2+ in (CD3)2CO (above) and impure out-[3-Cl]2+ before light irradiation performed in MeOD (below). Figure
S4: 19F-NMR comparisons between; a) in-[3-OH2]3+ in D2O b) in-[3-OCOCF3]2+ in acetone-d6. The 19F NMR of free
trifluoroacetate in D2O (c) and free potassium hexafluorophosphate in D 2O (d) are shown for comparison. Figure
S5: UV-vis redox titration of in-[3-OH2]3+ at pH = 1, upon addition of 0.1 eq. (up to 4 eq.) of Ce(IV) over time.
Figure S6: UV-Vis absorbance spectra over time for an aqueous solution of in-[3-OH]+ at pH = 12 under air. The
arrow indicates the decrease in absorption at the MLCT as a result of precipitation of the complex. Figure S7:
Vis-NIR spectra of in-[3-OH2]3+ in CF3SO3H (pH = 1). Values between 1000-2000 nm multiplied by 10 for visibility.
Inset: Vis-NIR spectra of in-[3-OH2]3+ in CF3SO3H (pH = 1) at a concentration of 2.0 mM. vmax (cm-1) = 8422.
Figure S8: Cyclic voltammograms of in-[3-Cl]2+ and in-[3-CH3CN]3+ in a deoxygenated solution of CH2Cl2 with
supporting electrolyte TBAF (0.1 M) scan rate of 0.01 Vs-1. Figure S9: 1H-NMR spectrum of in-[3-Cl]2+ performed
in (CD3)2CO. Figure S10: 13C-NMR spectrum of in-[3-Cl]2+ performed in (CD3)2CO. Figure S11: 1H-NMR spectrum
of in-[3-CH3CN]2+ performed in CD3CN. Figure S12: DEPT spectrum of in-[3-CH3CN]2+ performed in CD3CN.
Figure S13: 1H-NMR spectrum of in-[3-OOCCF3]2+ performed in (CD3)2CO. Figure S14: 13C-NMR spectrum of in[3-OOCCF3]2+ performed in (CD3)2CO. Figure S15: 1H-NMR of in-[3-OH2]3+ in D2O. Figure S16: Full HPLC
chromatogram of dyad mixture after reaction (iv). Fractions assigned by mass spec. Fraction 1 - Ru(tpy)2
derivative, Fraction 2 – 22+, Fraction 3 – in-[3-OCOCF3]3+. Figure S17: ESI-MS of pure fraction from HPLC
dissolved in H2O to give, in-[3-H2O]3+, performed in MeOH with the addition of H2O. m/z (M3+) = 493.2. Figure
S18: ESI-MS of in-[3-CH3CN]2+ performed in MeOH with the addition of H2O. Figure S19: DFT optimized
structures of (a) in-3-Cl (b) out-3-Cl (c) in-3-CH3CN (d) out-3-CH3CN. Hydrogen atoms are omitted for clarity.
Table S1. Crystallographic data for complexes in-[3-L](PF6)2/3 where; L = CH3CN or Cl. Table S2: NMR
spectroscopy data.

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Author Contributions, Methodology, P.F.; software, P.F and J.B-B.; investigation, P.F ; resources, P.F and A.L.;
data curation, P.F.; writing—original draft preparation, S.H.; writing—review and editing, S.H and P.F.;
visualization, P.F and A.L.; supervision, P.F and A.L.; funding acquisition, A.L.

451

Funding:

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Support from MINECO (CTQ2010-21497), Generalitat de Catalunya, and “la Caixa” Foundation are gratefully
acknowledged. SH gratefully acknowledges support from College of Science scholarship, NUI Galway. PF
acknowledges support from Royal Society Newton Alumni programme.

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Conflicts of Interest:

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The authors declare no conflict of interest.

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1.

References
Armaroli, N.; Balzani, V. The future of energy supply: Challenges and opportunities. Angew.
Chemie - Int. Ed. 2007, 46, 52–66.
2.

Balzani, V.; Credi, A.; Venturi, M. Photochemical conversion of solar energy; 2008; Vol. 1; ISBN
0512099456.

3.

Nocera, D.G. Solar fuels and solar chemicals industry. Acc. Chem. Res. 2017, 50, 616–619.

4.

Hennessey, S.; Farràs, P. Production of solar chemicals: gaining selectivity with hybrid
molecule/semiconductor assemblies. Chem. Commun. 2018, 54, 6662–6680.

5.

Garrido-Barros, P.; Funes-Ardoiz, I.; Farràs, P.; Gimbert-Suriñach, C.; Maseras, F.; Llobet, A.
Water as an oxygen source for oxidation reactions. In Catalytic Oxidation in Organic Synthesis;
2017; pp. 63–79.

6.

Hajimohammadi, M.; Safari, N.; Mofakham, H.; Deyhimi, F. Highly selective, economical and
efficient oxidation of alcohols to aldehydes and ketones by air and sunlight or visible light in
the presence of porphyrins sensitizers. Green Chem. 2011, 13, 991–997.

14 of 16

472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514

7.

Kalita, D.; Radaram, B.; Brooks, B.; Kannam, P.P.; Zhao, X. Photocatalytic oxidation of

8.

Kärkäs, M.D.; Verho, O.; Johnston, E. V.; Åkermark, B. Artificial photosynthesis: Molecular

hydrocarbons in water by ruthenium complexes. ChemCatChem 2011, 3, 571–573.
systems for catalytic water oxidation. Chem. Rev. 2014, 114, 11863–12001.
9.

Kaveevivitchai, N.; Chitta, R.; Zong, R.; El Ojaimi, M.; Thummel, R.P. A molecular light-driven
water oxidation catalyst. J. Am. Chem. Soc. 2012, 134, 10721–10724.

10.

Farràs, P.; Maji, S.; Benet-Buchholz, J.; Llobet, A. Synthesis, characterization, and reactivity of
dyad ruthenium-based molecules for light-driven oxidation catalysis. Chem. Eur. J. 2013, 19,
7162–7172.

11.

Guillo, P.; Hamelin, O.; Batat, P.; Jonusauskas, G.; McClenaghan, N.D.; Ménage, S.
Photocatalyzed sulfide oxygenation with water as the unique oxygen atom source. Inorg.
Chem. 2012, 51, 2222–2230.

12.

Iali, W.; Lanoe, P.H.; Torelli, S.; Jouvenot, D.; Loiseau, F.; Lebrun, C.; Hamelin, O.; Ménage, S.
A ruthenium(II)-copper(II) dyad for the photocatalytic oxygenation of organic substrates
mediated by dioxygen activation. Angew. Chemie - Int. Ed. 2015, 54, 8415–8419.

13.

Farràs, P.; Benniston, A.C. Bodipy-ruthenium(II) tris-bipyridyl dyads for homogeneous
photochemical oxidations. Tetrahedron Lett. 2014, 55, 7011–7014.

14.

Haim, A. Role of the bridging ligand in inner-sphere electron-transfer reactions. Acc. Chem.

15.

Jakubikova, E.; Martin, R.L.; Batista, E.R. Systematic study of modifications to ruthenium(ll)

Res. 1975, 8, 264–272.
polypyridine dyads for electron injection enhancement. Inorg. Chem. 2010, 49, 2975–2982.
16.

Browne, W.R.; Hage, R.; Vos, J.G. Tuning interaction in dinuclear ruthenium complexes:
HOMO versus LUMO mediated superexchange through azole and azine bridges. Coord.
Chem. Rev. 2006, 250, 1653–1668.

17.

Parthey, M.; Kaupp, M. Quantum-chemical insights into mixed-valence systems: Within and
beyond the Robin-Day scheme. Chem. Soc. Rev. 2014, 43, 5067–5088.

18.

Chen, W.; Rein, F.N.; Scott, B.L.; Rocha, R.C. Catalytic photooxidation of alcohols by an
unsymmetrical tetra(pyridyl)pyrazine-bridged dinuclear Ru complex. Chem. Eur. J. 2011, 17,
5595–5604.

19.

Farràs, P.; Di Giovanni, C.; Clifford, J.N.; Palomares, E.; Llobet, A. H2 generation and sulfide
to sulfoxide oxidation with H2O and sunlight with a model photoelectrosynthesis cell. Coord.
Chem. Rev. 2015, 304–305, 202–208.

20.

Norris, M.R.; Concepcion, J.J.; Harrison, D.P.; Binstead, R.A.; Ashford, D.L.; Fang, Z.;
Templeton, J.L.; Meyer, T.J. Redox mediator effect on water oxidation in a ruthenium-based
chromophore-catalyst assembly. J. Am. Chem. Soc. 2013, 135, 2080–2083.

21.

Treadway, J.A.; Moss, J.A.; Meyer, T.J. Visible region photooxidation on TiO2 with a

22.

Hamelin, O.; Guillo, P.; Loiseau, F.; Boissonnet, M.F.; Ménage, S. A dyad as photocatalyst for

chromophore−catalyst molecular assembly. Inorg. Chem. 2002, 38, 4386–4387.
light-driven sulfide oxygenation with water as the unique oxygen atom source. Inorg. Chem.
2011, 50, 7952–7954.
23.

Senthil Murugan, K.; Rajendran, T.; Balakrishnan, G.; Ganesan, M.; Sivasubramanian, V.K.;
Sankar, J.; Ilangovan, A.; Ramamurthy, P.; Rajagopal, S. Visible-light activation of the
bimetallic chromophore-catalyst dyad: Analysis of transient intermediates and reactivity

15 of 16

515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557

toward organic sulfides. J. Phys. Chem. A 2014, 118, 4451–4463.
24.

Phungsripheng, S.; Kozawa, K.; Akita, M.; Inagaki, A. Photocatalytic Oxygenation of Sulfide
and Alkenes by Trinuclear Ruthenium Clusters. Inorg. Chem. 2016, 55, 3750–3758.

25.

Nastasi, F.; Santoro, A.; Serroni, S.; Campagna, S.; Kaveevivitchai, N.; Thummel, R.P. Early
photophysical events of a ruthenium (II) molecular dyad capable of performing
photochemical water oxidation and of its model compounds. Photochem. Photobiol. Sci. 2019,
DOI: 10.1039/C8PP00530C.

26.

Sens, C.; Rodríguez, M.; Romero, I.; Llobet, A.; Parella, T.; Benet-Buchholz, J. Synthesis,
structure, and acid-base and redox rroperties of a family of new Ru(II) ssomeric complexes
containing the trpy and the dinucleating Hbpp ligands. Inorg. Chem. 2003, 42, 8385–8394.

27.

Zhang, W.; Liu, J.-H.; Pan, J.-X.; Li, P.; Sun, L.-C. Synthesis and characterization of some new
mononuclear ruthenium complexes containing 4-(un)substituted dipyridylpyrazole ligands.
Polyhedron 2008, 27, 1168–1176.

28.

Sala, X.; Plantalech, E.; Romero, I.; Rodríguez, M.; Llobet, A.; Poater, A.; Duran, M.; Solà, M.;
Jansat, S.; Gómez, M.; et al. Atropisomeric discrimination in new RuII complexes containing
the C2-symmetric didentate chiral phenyl-1,2-bisoxazolinic ligand. Chem. Eur. J. 2006, 12,
2798–2807.

29.

Sens, C.; Romero, I.; Rodríguez, M.; Llobet, A.; Parella, T.; Benet-Buchholz, J. A new Ru
complex capable of catalytically oxidizing water to molecular dioxygen. J. Am. Chem. Soc. 2004,
126, 7798–7799.

30.

Launay, J.-P.; Verdaguer, M. Electrons in molecules: From basic principles to molecular electronics;
2014; ISBN 978–0–19–929778–8.

31.

Demadis, K.D.; Hartshorn, C.M.; Meyer, T.J. The localized-to-delocalized transition in mixedvalence chemistry. Chem. Rev. 2001, 101, 2655–2685.

32.

Roeser, S.; Ertem, M.Z.; Cady, C.; Lomoth, R.; Benet-Buchholz, J.; Hammarström, L.; Sarkar,
B.; Kaim, W.; Cramer, C.J.; Llobet, A. Synthesis, structure, and electronic properties of RuN6
dinuclear Ru-Hbpp Complexes. Inorg. Chem. 2012, 51, 320–327.

33.

Dowling, C.; Dinsdale, D.R.; Lemaire, M.T. Preparation, electrochemical behavior, and
variable-temperature magnetic properties of Co(3,5-DBSQ) 2 complexes of imidazole- or
pyrazole-substituted ligands. Can. J. Chem. 2015, 93, 769–774.

34.

Wasylenko, D.J.; Ganesamoorthy, C.; Koivisto, B.D.; Henderson, M.A.; Berlinguette, C.P.
Insight into Water Oxidation by Mononuclear Polypyridyl Ru Catalysts. Inorg. Chem. 2010, 49,
2202–2209.

35.

Sullivan, B.P.; Salmon, D.J.; Meyer, T.J. Mixed phosphine 2,2’-bipyridine complexes of
ruthenium. Inorg. Chem. 1978, 17, 3334–3341.

36.

Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.;
Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A. V.;
Bloino, J.; Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratch, D.J. Gaussian 16, Revision C.01
2016.

37.

Zhao, Y.; Truhlar, D.G. The M06 suite of density functionals for main group thermochemistry,
thermochemical kinetics, noncovalent interactions, excited states, and transition elements:
two new functionals and systematic testing of four M06-class functionals and 12 other
function. Theor. Chem. Acc. 2008, 120, 215–241.

16 of 16

558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576

38.

Perdew, J.P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple.

39.

Martin, J.M.L.; Sundermann, A. Correlation consistent valence basis sets for use with the

Phys. Rev. Lett. 1996, 77, 3865–3868.
Stuttgart−Dresden−Bonn relativistic effective core potentials: The atoms Ga−Kr and In−Xe. J.
Chem. Phys. 2001, 114, 3408–3420.
40.

Andrae, D.; Haußermann, U.; Dolg, M.; Stoll, H.; Preuß, H. Energy-adjusted ab initio
pseudopotentials for the second and third row transition elements. Theor. Chim. Acta 1990, 77,
123–141.

41.

APEX Data collection with APEX II versions v1.0–22, v2009.1–0 and v2009.1–02. Bruker, 2007.
Bruker AXS Inc., Madison, Wisconsin, USA 2007.

42.

Bruker Data reduction with Bruker SAINT versions V.2.10, 2003, V/.60 A and V7.60 A. Bruker,
2007. Bruker AXS Inc., Madison, Wisconsin, USA 2003.

43.

SADABS SADABS: V.2.10, 2003; V2008 and V2008/1 Bruker, 2001, Bruker AXS Inc., Madison,
Wisconsin, USA; R. H. Blessing, Acta Crystal- logr. Sect. A 1995, 51, 33–38. 2003.

44.

SHELXTL SHELXTL versions V6.12 and 6.14; G. M. Sheldrick, Acta Crystal- logr. Sect. A 2008,
64, 112–122 2008.