Characterization and performance of electrostatically adsorbed Ru-Hbpp water oxidation catalysts

Article abstract of DOI:10.1039/c3cy00643c

A new family of Ru-Hbpp dinuclear complexes containing the positively charged terpyridine derivative ligand 4-(-p-(pyridin-1-ylmethyl)phenyl)-2, 2′:6′,2′-terpyridine of general formula {[Ru(L1 ⁺)]₂(μ-bpp)(L-L)}^m+ (L-L = μ-Cl, μ-acetato, or (H₂O)₂; m = 4 or 5) have been synthesized and fully characterized, both in the solid state (X-ray diffraction) and in solution (1D and 2D NMR spectroscopy, UV-vis spectroscopy and electrochemical techniques). New hybrid materials have been prepared by the electrostatic interaction of these complexes with several oxidatively rugged solid supports such as SiO₂, FTO-TiO₂ and FTO-Nafion. These new hybrid materials were prepared and catalytically evaluated with regard to their capacity to chemically and electrochemically oxidize water to dioxygen.

Full text of DOI:10.1039/c3cy00643c

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Characterization and performance of
electrostatically adsorbed Ru–Hbpp water
oxidation catalysts†
Cite this: Catal. Sci. Technol., 2014,
4, 190
Joan Aguiló,a Laia Francàs,b Hai Jie Liu,a Roger Bofill,a Jordi García-Antón,a
b
ab
a
a
*
*
*
Jordi Benet-Buchholz, Antoni Llobet,
Lluís Escriche and Xavier Sala
A new family of Ru–Hbpp dinuclear complexes containing the positively charged terpyridine derivative
ligand 4-(-p-(pyridin-1-ylmethyl)phenyl)-2,2′:6′,2′-terpyridine of general formula {[Ru(L1+)]2(μ-bpp)(L-L)}m+
(L-L = μ-Cl, μ-acetato, or (H2O)2; m = 4 or 5) have been synthesized and fully characterized, both in the
solid state (X-ray diffraction) and in solution (1D and 2D NMR spectroscopy, UV–vis spectroscopy and
electrochemical techniques). New hybrid materials have been prepared by the electrostatic interaction of
these complexes with several oxidatively rugged solid supports such as SiO2, FTO–TiO2 and FTO–Nafion®.
These new hybrid materials were prepared and catalytically evaluated with regard to their capacity to
chemically and electrochemically oxidize water to dioxygen.
Received 28th August 2013,
Accepted 29th October 2013
DOI: 10.1039/c3cy00643c
www.rsc.org/catalysis
dates from 2008 with the electropolymerization of a pyrrole-
modified Ru–Hbpp system (Chart 1, B) onto vitreous carbon
Introduction
The use of stable redox-active catalysts such as ruthenium
mono- or dinuclear complexes is an interesting strategy in
order to succeed in water oxidation catalysis.1 Our group
has developed several complexes of this type (mainly the
so-called Ru–Hbpp family, see Chart 1) presenting a relatively
good performance with regard to this challenging reaction.
Furthermore, most of these oxygen-evolving species have
been mechanistically investigated.2 In terms of catalyst
stability, one of the common deactivation pathways operating
in the above mentioned systems is the catalyst–catalyst
intermolecular oxidative degradation, which generates non-
active species and/or CO2 as ligand degradation products.2b,3
The immobilization of these catalysts onto conductive/
semiconductive solid supports has been one of the used
strategies to overcome their self-degradation. In addition,
the use of conductive materials allows the electrochemical
activation of the catalysts and represents a step forward
towards the feasible construction of a fuel-cell for the photo-
production of hydrogen.3 Our first approach towards the prep-
aration of heterogeneous water oxidation catalysts (WOCs)
sponges (VCS) and fluorinated tin oxide electrodes (FTO).4 This
system confirmed the feasibility of the solid-state approach,
since it was able to perform the catalytic oxidation of water
to dioxygen through electroactivation at 1.17 V vs. SSCE.
However, despite the observed increase in robustness,
these hybrid materials suffered from the oxidation of the
polymeric matrix within the harsh catalytic conditions
employed.
Soon afterwards, we further modified the Ru–Hbpp
catalyst through the introduction of a carboxylate functional
group into the bridging ligand (Chart 1, C). That allowed its
anchoring onto a more rugged, inorganic solid support such
as nanoparticulated TiO2.5 The chemical (Ce(IV)) mediated
activation of this system ended up with the concomitant
generation of O2 and CO2, the latter due to the inter-
molecular ligand oxidation beginning at the benzylic position
of the modified Hbpp ligand.
Taking into account all the aforementioned results
presented above we envisaged a new strategy based on the
electrostatic interaction between different solid supports/
electrodes and positively charged Ru–Hbpp catalytic species.
With this aim, herein we present the synthesis, characteriza-
tion and catalytic activity of a new catalyst belonging to the
Ru–Hbpp family containing extra and positively charged
pyridylic rings on the trpy ligands (Chart 1, L1+). Further-
more, we report its anchoring onto TiO2, SiO2 and Nafion®
surfaces, the characterization of the new hybrid materials
generated and their catalytic evaluation with regard to the
oxidation of water to dioxygen.
a Departament de Química, Universitat Autònoma de Barcelona, Cerdanyola del
Vallès, 08193, Barcelona, Spain. E-mail: lluis.escriche@uab.cat, xavier.sala@uab.cat;
Fax: + 34 93 581 3101; Tel: +34 93 586 8295
b Insititute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans, 16,
E-43007, Tarragona, Spain. E-mail: allobet@iciq.es; Fax: +34 977 920 222;
Tel: +34 977 920 201
† Electronic supplementary information (ESI) available. CCDC 949999
&
950000. For crystallographic data in CIF or other electronic format see DOI:
10.1039/c3cy00643c
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Chart 1 Drawing of previously reported Ru–Hbpp complexes (A, B and C)
and Hbpp and trpy ligands together with their modified counterparts,
including the L1+ ligand used in this work.
Results and discussion
Synthesis and structural characterization
The synthesis of the L1+ ligand (Chart 1, bottom-right) was
carried out by slightly modifying a two-step procedure previ-
ously reported.6,7 After bromination of 4′-(4-methylphenyl)-
2,2′:6′,2′-terpyridine with N-bromosuccinimide (NBS), the
bromomethyl derivative obtained was subjected to a nucleo-
philic attack with pyridine, which generated the cationic L1+.
The use of a microwave reactor in the last step allowed
shorter reaction times (from 48 h to 30 min) and better
yields (from 70% to 80%). After solvent evaporation and
redissolution in water, the pure ligand was obtained by
precipitation with NH4PF6(aq) and characterized by 1D and
2D NMR (Fig. S1, ESI†).
Fig. 1 ORTEP plot (ellipsoids at 50% probability) of the X-ray crystal
structure of the cationic moiety of (top) 1+ and (bottom) 24+ (molecule A)
and their corresponding labelling scheme.
Reaction of RuCl3·nH2O with L1+ in refluxing methanol for
four hours afforded [RuCl3(L1+)](PF6), 1(PF6), as a brown solid.
Further combination of the latter with the anionic tetra-
N-dentate bpp− bridge (bpp− = 3,5-bis(2-pyridyl)pyrazolate)
under the conditions shown in Scheme 1 leads to the binuclear
ruthenium complexes here reported, 2(PF6)4, 3(PF6)4 and 45+.
Under excess of acetate and the presence of stoichiometric Ag+
ions, the Cl-bridging ligand of 24+ can be easily replaced by an
acetato-bridging ligand, leading to 34+. The latter is then
replaced by two aqua ligands under acidic conditions to yield
45+. All of the isolated ruthenium complexes were characterized
by elemental analysis and spectroscopic (UV–vis and NMR),
spectrometric (MS) and electrochemical (CV, DPV) techniques.
X-ray diffraction analysis was also carried out for complexes
1(PF6) and 2(PF6)4. Their crystallographic and acquisition
parameters are reported in Tables S1 and S2 (ESI†), and ORTEP
views of their cationic moieties are presented in Fig. 1. Single
crystals of 1(PF6) were obtained by the slow evaporation of a
saturated solution of 1+ in nitric acid. A selection of the more
relevant bond distances and angles is reported in Table S3.† As
expected, the ruthenium center is meridionally coordinated by
the trpy-based ligand L1+, and three chloride anions occupy
the three remaining coordination sites. The ruthenium ion
adopts a distorted octahedral geometry with bond distances
and angles comparable to analogous complexes reported
earlier in the literature.8 The constrain imposed by the geo-
metry of the L1+ ligand is clearly detected in the N3–Ru–N1
angle, reduced from the ideal 180° value to the observed 159°.
1+ crystallizes with one nitrate anion and one nitric acid
molecule linked by a hydrogen bond. Both molecules are
situated next to the pyridinium positive charge. In contrast to
what was reported for the parent [RuCl3(trpy)] complex,8
almost no hydrogen bonding interactions and a less ordered
packing are observed now. The unit cell of the structure is
shown in Fig. S2 (ESI†) containing a total of eight 1+ cations.
A head to tail orientation between the two central molecules of
the unit cell is also observed. Concerning 2(PF6)4, because of
the packing effect imposed by the pyridinium moiety of L1+,
this complex crystallizes in an extremely large cell containing
−
four independent complex molecules, sixteen PF6 anions
Scheme 1 Synthetic strategy for the complexes described in this work.
and several disordered acetone and toluene molecules in the
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asymmetric unit (Fig. S3†). In order to avoid the disordered
solvent molecules the SQUEEZE program9 was applied, leading
to a refined model with a R1 value of 7.43%. The four cationic
24+ units described in the asymmetric cell display slightly
different metric parameters due to the different orientation
of the pyridinium moieties, and therefore only the so-called
“A” complex will be described here. A selection of the more
relevant bond distances and angles for all four independent
molecules is reported in Table S4.† Each ruthenium atom
adopts a pseudo-octahedral coordination geometry with two
positions occupied by the bpp− ligand, three by the meridional
L1+ ligand and the last one by a Cl-bridged ligand (Fig. 1,
bottom). Bond distances and angles show no significant differ-
ences with regards to related complexes previously described
in the literature.10 1D and 2D NMR spectroscopy allowed
the structural characterization in solution of the isolated dia-
magnetic complexes (see Fig. 2, the Experimental section, and
Fig. S4 and S5†). All of the resonances observed in the NMR
spectra can be unambiguously assigned based on their
integrals, symmetry and multiplicity. 24+ displays C2v symmetry
in solution, with one symmetry plane containing the bpp−
ligand, the two Ru atoms, both central terpyridine nitrogen
atoms and the bridging chlorido moiety. A second plane
(perpendicularly bisecting the former) passes through the
chlorido bridge and the central pyrazolic carbon and bisects
the N–N bond of the same pyrazolic ring, thus interconverting
the two terpyridine ligands. The downfield shift of the singlet
corresponding to H22 (see Fig. 2) is in accordance with the
high electron-withdrawing effect of the closer pyridinium
moiety, as also previously reported for related compounds.11
When the acetato-bridged complex 34+ is analyzed in solution,
the NMR resonances of the external pyridyls of the L1+ ligands
appear as magnetically symmetric at room temperature. In
the solid state, the accommodation of the acetato bridging
ligand should provoke a further distortion of the Ru pseudo-
octahedral geometry, as previously reported for the parent
{[RuII(trpy)]2(μ-bpp)(μ-AcO)}2+,12 (resulting in one Ru center
moving above the equatorial plane together with its L1+ ligand,
whereas the other metal center does the opposite). However, in
solution at room temperature, these two moieties display a
dynamic behavior, with the two Ru centers synchronically
moving very fast below and above the equatorial plane.
Consequently, the NMR resonances appear as if the complex
had C2v symmetry. Furthermore, the downfield shift of the
methylenic singlet with regards to 24+ (from 6.2 to 6.3 ppm)
and the presence of the acetate singlet peak at 0.45 ppm
(Fig. S5a†) are experimental evidences of the successful
exchange of the chlorido bridge by the acetato moiety.
Spectroscopic and redox properties
The UV–vis spectral features in acetone for the complexes
described in this work are listed in the Experimental section,
and the UV–vis spectra for 24+, 34+ and 45+ are displayed in
Fig. 3. Two main regions can be distinguished: one between
350 and 550 nm, in which there are mainly broad unsymmetrical
Ru(dπ)–trpyPyr/bpp(π*) metal-to-ligand charge-transfer bands;
and the region above 550 nm, in which d–d transitions are
observed.13 The MLCT absorption features shown in Fig. 3
nicely fit with those of previously reported chlorido, acetato
and bis-aqua Ru–Hbpp dinuclear complexes.2b,14
The redox properties of the complexes described in the
present work were investigated by means of CV and DPV and
are reported in Table 1, Fig. 4 and Fig. S7–S8 in the ESI.†
Fig. 3 UV-spectra for 24+ (blue line) and 34+ (green line) at 26 mM
concentration in acetone and 45+ (96 mM, red line) in acetone : water
(pH = 1) (20 : 80).
Table 1 Redox potentials in V (vs. SSCE) at a scan rate of 100 mV s−1
for 1+, 24+
, ,
34+ 45+ and their respective trpy homologues for
comparison purposes
III/II
[RuIIICl3(trpy)]a
0.01
0.05
III,II
/
1+a
III,III
/
IV,III
/
III,III
—
—
—
—
0.81
0.90
IV,IV
/
IV,III
—
—
—
—
1.10
1.00
II,II
0.71
0.79
0.73
0.76
0.54
0.57
III,II
1.12
1.20
1.05
1.09
0.61
0.63
{[RuII(trpy)]2(μ-bpp)(μ-Cl)}2+c
24+b
{[RuII(trpy)]2(μ-bpp)(μ-AcO)}2+c
34+b
{[RuII(H2O)(trpy)]2(μ-bpp)}3+d
45+d
a
b
In acetonitrile using 0.1 M of TABH as the electrolyte. In acetone
using 0.1 M of TABH as the electrolyte. c In CH2Cl2 using 0.1 M of TABH
as the electrolyte. d Aqueous solution at pH = 1 (0.1 M triflic acid).
Fig. 2 Drawn structure with an atom labelling scheme and 1H NMR
(acetone-d6) for 24+
.
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These have been tentatively assigned, taking into account
previous results on related complexes,10 to a total of four
redox processes:
½RuIII–RuIIꢀ þ 1e− → ½RuII–RuIIꢀ ð0:57VÞ
½RuIII–RuIIIꢀ þ 1e− → ½RuIII–RuIIꢀ ð0:63VÞ
½RuIV–RuIIIꢀ þ 1e− → ½RuIII–RuIIIꢀ ð0:90VÞ
½RuIV–RuIVꢀ þ 1e− → ½RuIV–RuIIIꢀð1:00VÞ
ð4Þ
ð5Þ
ð6Þ
ð7Þ
As can be observed in Table 1, in this case no significant
changes are observed when comparing the potentials of
45+ with regard to those of the related Hbpp complex
{[RuII2(H2O)2(trpy)2](μ-bpp)}3+, particularly when comparing
the high oxidation states. When the potential is increased
further up to 1.3 V a large anodic current is observed in the
DPV, which can be associated with a further one electron
oxidation of the complex together with the concomitant
electrocatalytic oxidation of water to dioxygen in agreement
with eqn (8) and (9).
6þ
5þ
fORuV–RuIVOg þ 1e− → fORuIV–RuIVOg
ð8Þ
Fig. 4 Cyclic voltammogram for the chlorido-bridged complex 24+
in 0.1 M n-Bu4NPF6 in acetone at a 100 mV s−1 scan rate (top) and
differential pulse voltammogram (red line) and cyclic voltammogram
(blue line) for the aqua complex 45+ at pH = 1 in a 0.1 M triflic acid
aqueous solution (bottom). In both cases glassy carbon is used as the
working electrode and the potential is measured vs. SSCE.
6þ
6þ
fORuV–RuIVOg þ H2O → fHO–RuIV–RuIII–OOHg ð9Þ
The hydroperoxide intermediate is then responsible for
the subsequent reactions that end up generating dioxygen.
The oxidation process depicted in eqn (8) is not observed in
the DPV (Fig. 4, bottom) given the concomitant and fast elec-
trocatalytic current corresponding to the oxidation of water.
The CV of 1+ in MeCN (Fig. S6†) exhibits a unique reversible
wave at E1/2 = 0.05 V (ΔE = 71 mV), corresponding to the
following process:
Attachment and characterization of Ru catalysts onto SiO2,
½RuIIICl3ðL1þÞꢀþ þ 1e− → ½RuIICl3ðL1þÞꢀ ð0:05VÞ
ð1Þ
FTO–TiO2 and FTO–Nafion®
The charged pyridinium rings of complexes 24+ and 34+ have
been employed to electrostatically interact with several solid
supports and thus generate the following new hybrid
materials: SiO2–24+/34+: the attachment of 24+ and 34+ onto
SiO2 was carried out by introducing a sample of SiO2 (3 g) to
0.548 mM (10 mL) acetone solutions of 24+/34+. In just 5 min,
the solutions decolorized indicating the total adsorption
of the complexes into SiO2, thus generating SiO2–24+ and
SiO2–34+. These new materials were thoroughly washed with
fresh acetone and water and finally air-dried at room temper-
ature. The absence of UV–vis signal in the washing acetone
revealed the stability of the new hybrid materials (see Fig. S9
in ESI†) and the diffuse reflectance UV–vis spectra shown
in Fig. S10† confirm the structural analogy of the new hybrid
materials with regards to their homogeneous counterparts.
FTO–TiO2–24+: FTO–TiO2 films were prepared as usual
(see the Experimental section) and then soaked overnight in
Comparison with the related complex [RuIIICl3(trpy)]
(Fig. S7,† E1/2 = 0.01 V, ΔE = 69 mV) revealed an up shift of the
E1/2. This behavior can be assigned to the electron-withdrawing
effect of the extra pyridinium group of L1+. For the dinuclear
Cl− and AcO− complexes that do not contain aqua groups,
the voltammograms in organic solvents show two chemi-
cally reversible and electrochemically quasi-reversible redox
waves. Fig. 4, top, shows the CV of complex 24+ in acetone
(see Fig. S8† for the CV of 34+). These two processes are
assigned to the following electrochemical reactions (the L1+
and bpp− ligands are not shown for the sake of clarity):
5þ
4þ
½RuIIIðμ - ClÞRuIIꢀ þ 1e− → ½RuIIðμ - ClÞRuIIꢀ ð0:79VÞ ð2Þ
6þ
5þ
½RuIIIðμ - ClÞRuIIIꢀ þ 1e− → ½RuIIIðμ - ClÞRuIIꢀ ð1:20VÞ ð3Þ
The electrochemistry of 45+ has been investigated after its
“in situ” generation in a pH 1 aqueous solution (0.1 M triflic
acid) using 34+ as precursor (see Scheme 1). From the CV and
DPV of 45+ (Fig. 4, bottom) a total of four waves are observed.
a
0.3 mM acetone solution of 24+. The FTO–TiO2–24+
films were then thoroughly washed with fresh acetone and
air-dried. The amount of anchored catalyst was confirmed
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by means of UV–vis spectroscopy. The use of films previously
activated at pH = 12 (soaked overnight on a 10 mL aqueous
solution at pH 12) ended up in higher amounts of anchored
catalyst (see Fig. S11 in the ESI†). We propose that, under these
conditions, TiO− residues are generated on the surface of the
solid support, thus allowing a better support–catalyst ionic
interaction. The electrochemical properties of FTO–TiO2–24+
have been also investigated by means of CV in DCM. The
corresponding voltammograms at different scan rates are
shown in Fig. S12 (ESI†). This new hybrid material presents
similar redox potentials (0.81 V and 1.18 V vs. SSCE) to those
found for complex 24+ (0.79 and 1.20 V vs. SSCE), revealing
that no changes in the electrochemical (and thus structural)
properties of the catalyst have occurred during the anchoring
process. Later on, the stability of the FTO–TiO2–24+ films was
evaluated under acidic (pH 1), neutral (pH 7) and basic (pH 12)
conditions by monitoring the potential catalyst leaching via
UV–vis spectroscopy. The spectra of those solutions along
several hours/days are displayed in Fig. S13.† The observed
absorbance increase with time of the acidic and neutral
solutions containing soaked FTO–TiO2–24+ films points out
the instability of the ionic interaction at these pH ranges.
Surprisingly, a detachment/reattachment process of 24+ is
observed when the electrode is soaked at pH 12 (Fig. S13c†).
The early absorbance increase reveals an initial leaching
that, after three days, is clearly reversed. The latter behavior
can be due to the activation of the TiO2 surface (generation
of TiO− anions at basic pH) as previously indicated.
Fig. 5 CV for the chlorido-bridged complex 24+ (blue line, signal
increased 50 times for comparison purposes) and FTO–Nafion–24+
(red line) in a 0.1 M n-Bu4NPF6 acetone solution at 100 mV s−1 scan
rate (top). CV for the acetato-bridged complex 34+ (blue line, signal
increased 10 times for comparison purposes) and FTO–Nafion–34+
(red line) in a 0.1 M n-Bu4NPF6 acetone solution at 100 mV s−1 scan
rate (bottom). A glassy carbon electrode for 24+ and 34+ and FTO for
FTO–Nafion–24+ and FTO–Nafion–34+ were used as working electrodes.
The potential was measured vs. SSCE.
FTO–Nafion–24+/34+: the absence of easily oxidizable –CH
groups and its terminal and deprotonable sulfonate groups
converts Nafion® into an excellent polymeric material to elec-
trostatically interact with the positive residues of complexes
24+ and 34+ and thus generates rugged hybrid materials use-
ful as catalysts for the oxidation of water. FTO–Nafion films
have been prepared depositing a known volume of the
Nafion® solution (Nafion 5% w/w in a mixture of water and
low-weight alcohols) on a piece of FTO film, as shown in
Scheme S1 in the ESI.† The films are then oven-dried at
100 °C for 30 min. After cooling at room temperature, the
FTO–Nafion supports are soaked overnight into acetone
solutions of 24+ or 34+. The final colorless nature of the
solution after that time clearly points out to a complete
attachment of the catalyst onto the Nafion® polymer. The
new hybrid materials FTO–Nafion–24+ and FTO–Nafion–34+
have been electrochemically characterized by means of CV
and DPV (Fig. 5) and their redox potentials have been
compared with their homogeneous counterparts. The CV of
FTO–Nafion–24+, shown in Fig. 5 (top), displays two chemically
reversible redox waves at E1/2 = 0.64 V (ΔEp = 113 mV) and
at E1/2 = 1.05 V (ΔEp = 113 mV) that are assigned to the
RuIII/RuII → RuII/RuII and RuIII/RuIII → RuIII/RuII couples.
A downshift of about 100 mV in E1/2 is observed when
comparing FTO–Nafion–24+ with its homogeneous counter-
part, 24+. This behavior can be explained by the less electron-
withdrawing effect of the pyridinium salts when its positive
charge ionically interacts with the Nafion sulfonate residues
(see a drawing of this interaction in Scheme S2 in the ESI†).
The UV–vis spectrum of FTO–Nafion–24+ (Fig. S14, ESI†)
perfectly matches that of its homogeneous counterpart
(see Fig. 3 above) and further confirms the preservation of
the Ru coordination sphere along the anchoring process. On
the other hand, the CV of FTO–Nafion–34+ shown in Fig. 5
(bottom) displays two chemically reversible redox waves at
E1/2 = 0.10 V (ΔEp = 115 mV) and at E1/2 = 0.79 V (ΔEp = 200 mV)
that are assigned to the RuIII/RuII → RuII/RuII and RuIII/RuIII
→
RuIII/RuII couples. The clear and large downshift of both redox
waves (660 mV and 300 mV, respectively) when compared
with the ones observed for its homogeneous counterpart 34+
suggests changes in the first coordination sphere of the
ruthenium metal ions during the anchoring process towards
a significantly stronger electron-donating environment. Thus,
the coordination of at least one sulfonate residue to the Ru
metal center is proposed.
Chemically triggered oxidative catalysis
Complex 45+ and the hybrid materials SiO2–45+ and FTO–
Nafion–45+ have been tested as WOCs upon the addition of a
strong sacrificial oxidant such as Ce(IV). All these bis-aqua
species have been generated “in situ” from 34+, SiO2–34+ and
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Table 2 Catalytic WO efficiencies and O2 : CO2 ratios for 45+, silica–45+, FTO–Nafion–45+ and other previously reported homogeneous and
heterogeneous systems for comparison purposes. Catalytic conditions: Ce(IV) as oxidant. Ratio 1/100 cat./Ce(IV)
Entry
System
[Cat.] (mM)
TN O2
TN CO2
[O2]/[CO2]
TN total
Eff.c
Ref.
1
2
3
4
5
6
7
8
9
{[Ru(H2O)(trpy)]2(μ-bpp)}3+
{[RuII(H2O)(trpy)]2(μ-bpp-Bz)}3+
{[RuII2(H2O)2(trpy)2]2(μ-(bpp)2-o-xyl)}6+
{[RuII2(H2O)2(trpy)2]2(μ-(bpp)2-m-xyl)}6+
{[RuII2(H2O)2(trpy)2]2(μ-(bpp)2-p-xyl)}6+
45+
1.0a
18
—
—
16
72.0
29.2
46.7
45.5
48.9
44
7.2
55
4
9
1.0a
5.8
18.4
15.6
14.8
12
1.8
3.5
1
5.8
6.1
6.2
5.8
—
14.2
3
1.0
3.0
2.5
2.5
—
0.125
1.2
—
11.7
20.8
19.9
21.9
12
16
6.5
1
1b
1b
1b
1b
tw
tw
4
0.5a f
0.5a f
0.5a f
1.0a
SiO2–45+
0.3%a d
0.5b
TiO2–{[RuII(H2O)(trpy)]2(μ-bpp-Ra)}3+
FTO–Nafion–45+
0.06e
—
tw
a
b
Total volume of the reaction: 2 mL at pH = 1.0 in 0.1 M triflic acid. Total volume of the reaction: 4 mL at pH = 1.0 in 0.1 M triflic acid.
c
Eff. = efficiency. d 0.350 g of SiO2–34+ (0.3%). e Total volume of the reaction: 3 mL at pH = 1.0 in 0.1 M triflic acid. f 1/200 cat./Ce(IV). tw = this work.
FTO–Nafion–24+, respectively, when poured into the acidic
the bimolecular oxidative degradation in this particular case.
Contrarily, in the SiO2–45+ system this positive charge is
delocalized due to the ionic interaction with the SiO− residues
of the solid support. Consequently, the methylene group in
alpha position to the pyridinium nitrogen atom is here again
more prone to oxidation, thus decomposing and evolving CO2
within the catalytic conditions (Fig. S15c–d,† Table 2, entry 7,
and Scheme S3†). The chemically triggered catalytic activity
of the bis-aqua FTO–Nafion–45+ system has been analyzed
after activation of FTO–Nafion–24+ films for 24 h at pH = 1.0 in
0.1 M triflic acid (see below for the neat conversion to the
bis-aqua derivative under these conditions). Surprisingly, a very
low TN value of about 1 (Table 2, entry 9, and Fig. S16†) was
obtained. Given the polymeric nature of the Nafion-based
hybrid material, the encapsulation of the active sites within
the inter-winkled polymeric chains and therefore their
difficult interaction with the Ce(IV) ions could be invoked
here in order to explain its poor catalytic activity when
chemically activated.
catalytic conditions (see the Experimental section). The total
gas evolved has been manometrically measured and its
composition in terms of O2 : CO2 ratio analyzed by means of
mass spectrometry (45+ and SiO2–45+) or a Clark electrode in
solution (FTO–Nafion–45+). The overall catalytic results,
together with the ones corresponding to related systems for
comparison purposes, are displayed in Table 2.
When Ce(IV) is added to a pH 1 (0.1 M triflic acid) solution
of 45+, a TN value of 12 is achieved (Table 2, entry 6 and
Fig. S15a of the ESI†). Comparison with the parent
{[RuII(H2O)(trpy)]2(μ-bpp)}3+ complex (entry 1, 18 TN)10
revealed a slight decrease in the catalytic activity of the
modified complex. This decrease in the catalytic activity is in
agreement with previous results attained by our research
group.15 The electronic modification of Ru–Hbpp WOCs
by introducing electron-withdrawing substituents into their
surrounding ligands (as here is the case with the L1+ ligand)
resulted in low substitution reaction rates and higher redox
potentials, thus complicating the attainment of high oxidation
states. Therefore, a slow down of the catalytic processes and a
reduction of their efficiency are observed.15 On the other hand,
no CO2 generation is detected when the catalytic process with
45+ is on-line monitored by mass spectrometry (Fig. S15b† and
Table 2, entry 6), which is in sharp contrast with the significant
CO2 values reported for several structurally related complexes
such as {[RuII(H2O)(trpy)]2(μ-bpp-Bz)}3+ (Table 2, entry 2)
and their tetranuclear {[RuII2(trpy)2(H2O)2]2(μ-(bpp)2-L-xyl)}6+
(L = ortho, meta or para) and heterogenous derivatives
TiO2–{[RuII(H2O)(trpy)](μ-bpp-Ra)}3+ (Table 2, entries 3–5 and 8),
all of them containing methylenic units on the modified bpp−
ligands (for a drawing of several of these complexes, see
Scheme S3 in the ESI†). The CO2 generation on these kinds of
complexes is associated with a bimolecular reaction where
oxidation of the methylenic unit of the ligands is carried out
by an active Ru–O group of another molecule of the catalyst.
This is the only possible pathway because Ce(IV) does not
react with the free ligands at room temperature and the
geometrical disposition of the Ru–O units prevents an intra-
molecular ligand oxidation.5 Therefore, the proximity of the
positively charged pyridinium rings to the methylenic groups
of the L1+ ligand of complex 45+ is key to electronically disfavor
Electrochemically triggered oxidative catalysis
FTO–TiO2–24+ and FTO–Nafion–24+/34+ have been investi-
gated in terms of their electrocatalytic capacity to oxidize
water to dioxygen. The instability of the FTO–TiO2–24+ films
prepared under the usual catalytic conditions at low (pH = 1)
or neutral (pH = 7) conditions (see above) moved us to
prepare modified electrodes previously activated at pH 12. In
order to test their electrocatalytic activity through bulk
electrolysis, the correct potential to be applied has been
electrochemically evaluated by means of DPV and estimated
to be about 1.1 V vs. SSCE (Fig. S17†). The catalytic reaction
has been carried out in an electrochemical cell (sketched
in Scheme S4†) formed by two different compartments
(A for oxidation and C for reduction) separated by a frit
membrane (B). FTO–TiO2–24+ was used as working electrode
in the above-mentioned electrochemical cell applying a
potential of 1.1 V vs. SSCE for 8 h (Fig. S18†). On-line O2 mea-
surement on the headspace of the cell compartment using a
Clark electrode revealed no oxygen evolution (Fig. S19†).
The electrochemical characterization of the electrode after the
CPE experiment revealed leaching of 45+ from the FTO–TiO2–45+
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coordination of at least one sulfonate group to the
Ru metal center and the consequent blocking of its
putative catalytic activity (see above in the Electrochemical
Characterization section).
Conclusions
Ru–Hbpp complexes constitute a wide family of efficient
WOCs extraordinarily well studied both from a structural and
a mechanistic approach.2b–e,10,12,15–17 The accurate under-
standing of the limitations and consequences of anchoring
such family of WOCs onto solid surfaces is a must when
attempting to include them into photoelectrochemical cells
(PECs).18 Therefore, research in order to explore and optimize
the different types of catalyst–support interactions is a
mandatory step. With this leit motif in mind, our group has
explored several anchoring strategies for the Ru–Hbpp family
(covalent anchoring, electropolymerization, etc.) but so
far none of them has been proven to be fully satisfactory.4,5
In this work, the ionic interaction between Ru–Hbpp
WOCs and several rugged solid supports has been analyzed.
For this purpose a new family of Ru–Hbpp complexes
(24+ and 34+) containing the positively charged ligand
4′-(-p-(pyridin-1-ylmethyl)phenyl)-2,2′:6′,2′-terpyridine L1+ has
been synthesized and fully characterized, both in the solid state
and in solution. Furthermore, their in situ conversion (aqueous
media, pH = 1) into its bis-aqua derivative 45+ has been electro-
chemically demonstrated. The anchoring of the new catalysts
onto TiO2, SiO2 and Nafion® has been achieved and the new
hybrid materials prepared have been characterized by means
of electrochemical and spectroscopic techniques.
Fig. 6 DPV of FTO–Nafion–24+ (bottom) and FTO–Nafion–34+ (top) in
aqueous solutions at pH = 1.0 in 0.1 M triflic acid. FTO was used as
working electrode and the potential measured vs. SSCE.
material to the basic aqueous solution. Effectively, Fig. S17b†
displays the electrode CV after CPE, where the total disappear-
ance of the redox waves can be appreciated. In addition, the
UV–vis spectra of the solution after CPE (Fig. S20†) displayed
the typical bands for 45+, further confirming this hypothesis.
FTO–Nafion–24+ and FTO–Nafion–34+ films have been
investigated in terms of their potential electrocatalytic capac-
ity to oxidize water to dioxygen at pH = 1.0 in a 0.1 M triflic
acid aqueous solution, as displayed in Fig. 6. The DPV of
FTO–Nafion–24+ (Fig. 6, bottom) shows the “activation” of the
film, characterized by the growing of an electrocatalytic wave
at around 1.4 V, assignable to an electrochemically triggered
water oxidation reaction. These results are in agreement with
the exchange of the chlorido-bridged ion for two water mole-
cules, as shown in Scheme S5 in the ESI.† As reported here,
the “in situ” generation of Ru–OH2 species from their Ru–Cl
counterparts in acidic media usually provokes a decrease of
the intensity of their redox waves and the convergence of
several oxidation states in a narrow potential range, as well
as the appearance of intense electrocatalytic currents due
to WO catalysis. These phenomena have been extensively
studied for several related systems.2c,16,17 In contrast, the
DPV of FTO–Nafion–34+ (Fig. 6, top) shows neither a shift
of the redox waves nor an increase of the electrocatalytic
current. The latter is in agreement with the above-proposed
The catalytic evaluation of 45+ with Ce(IV) leads to a stable
catalyst that evolved only O2 (and no CO2) as reaction prod-
uct, potentially due to the electron-withdrawing effect of the
positive pyridinium groups of the L1+ ligands close to the
easily oxidizable –CH2 groups. However, the catalytic evalua-
tion of SiO2–45+ has shown high CO2 : O2 ratios, thus reveal-
ing the oxidative degradation of the catalyst under the harsh
reaction conditions. This result, compared with the clean
O2 evolution observed for the homogenous 45+ counterpart,
points out that the activation of the methylene groups in
alpha position to the pyridinium residues happens when the
positive charge of the ligand interacts with the SiO− residues.
On the other hand, the interaction of the positively charged
pyridines of 24+ with TiO2 in FTO–TiO2–24+ seems to be weak,
since clear leaching of the catalyst was observed when a CPE of
the electrode was performed. Finally, despite the poor perfor-
mance of FTO–Nafion–45+ as
a chemically-driven WOC,
yielding only stoichiometric amounts of O2, the evaluation of
the FTO–Nafion–45+ by DPV has revealed its clear electro-
catalytic activity and, therefore, its potential use as a modified
electrode for the oxidation of water. In conclusion, the work
herein presented demonstrates the feasibility of using ionic
interactions to generate stable and active Ru–Hbpp hybrid
materials, such as FTO–Nafion–45+, able to electrochemically
oxidize water to dioxygen.
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(1.5 mL) were connected to the apparatus capillary tubing.
Subsequently, the previously degassed solution of Ce(IV)
(0.5 mL) at pH = 1 (triflic acid, 200 equiv.) was introduced
using a Hamilton gastight syringe, and the reaction was
dynamically monitored. A response ratio of 1 : 2 was observed
when equal concentrations of dioxygen and carbon dioxide,
respectively, were injected, and was then used for calculation
of their relative concentrations.
Experimental
Materials
All reagents used in the present work were obtained from
Aldrich Chemical Co. and were used without further purifica-
tion. Reagent-grade organic solvents were obtained from
Scharlab. RuCl3·3H2O was supplied by Alfa Aesar and was
used as received. Titanium dioxide paste (100% anatase)
was supplied by Solaronix. Both the silica and the Nafion
polymer were provided by Sigma Aldrich. The starting
ligands bis-(2-pyridyl)pyrazole (from now on Hbpp)19 and
4′-(-p-(bromomethyl)phenyl)-2,2′:6′,2′-terpyridine20 were pre-
pared as described in the literature. All synthetic manipula-
tions were routinely performed under nitrogen atmosphere
using Schlenk tubes and vacuum-line techniques.
X-ray crystal structure determination
Crystal structure determination for 1+ was carried out using a
Bruker-Nonius diffractometer equipped with an APPEX II 4K
CCD area detector, a FR591 rotating anode with Mo Kα
radiation, Montel mirrors as a monochromatic and a Kryoflex
low-temperature device (T = −173°C). Crystal structure deter-
mination for 24+ was carried out using a Apex DUO Kappa
4-axis goniometer equipped with an APPEX 2 4K CCD area
detector, a microfocus source E025 IuS using MoKα 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 w and f scans. Programs used: data collection APPEX II,21
data reduction, Bruker Saint V/.60A,22 absorption corrections,
SADABS.23 CCDC 949999 & 950000 contains supplementary
data for this paper.
Instrumentation and measurements
UV–Vis spectroscopy was performed using a HP8453 spectro-
meter using 1 cm quartz cells. NMR spectroscopy was
performed on a Bruker DPX 250 MHz, DPX 360 MHz or a
DPX 400 MHz spectrometer. Samples were run in CD3CN or
acetone-d6 with internal references. Elemental analyses were
performed using a Carlo Erba CHMS EA-1108 instrument pro-
vided by the Chemical Analysis Service of the Universitat
Autònoma de Barcelona (CAS-UAB). Electrospray Ionization
Mass Spectrometry (ESI–MS) measurements were carried out
on a HP298s gas chromatography (GC–MS) system from the
CAS-UAB. UV–vis absorption spectra of the solid samples
were recorded by diffuse reflectance measurement with a
8 cm integrating sphere (ISR-240 A) using BaSO4 as reference
in a Shimadzu UV-2401 PC spectrophotometer. Cyclic voltam-
metry and differential pulse voltammetry experiments were
performed on an Ij-Cambria HI-660 potentiostat using a
three-electrode cell. A glassy carbon electrode (2 mm diameter)
was used as a working electrode, platinum wire as an auxiliary
electrode and a SSCE as a reference electrode. Working
electrodes were polished with 0.05 micron alumina paste
washed with distillated water and acetone before each
measurement. The complexes were dissolved in acetone
containing the necessary amount of n-Bu4NPF6 (TABH) as a
supporting electrolyte to yield 0.1 M ionic strength. E1/2
values reported in this work were estimated from CV experi-
ments as the average of the oxidative and reductive peak
potentials (Ep,a + Ep,c)/2. On-line manometry measurements
were carried out on a Testo 521 differential pressure mano-
meter with an operating range of 1–100 hPa and accuracy
within 0.5% of the measurement, coupled to thermostatted
reaction vessels for dynamic monitoring of the headspace
pressure above each reaction. The secondary ports of the
manometers were connected to thermostatically controlled
reaction vessels that contained the same solvents and head-
space volumes as the sample vials. On-line monitoring of the
gas evolution was performed on a Pfeiffer Omnistar GSD
301C mass spectrometer. Typically, 16.04 mL degassed vials
containing a suspension of the catalysts in 0.1 M triflic acid
Structure and refinement
The SHELXTL24 software was used. The crystal data para-
meters are listed in Tables S1 and S2.† For the structure
of {[RuII(L1+)]2(μ-bpp)(μ-Cl)}(PF6)4 (2(PF6)4), the program
SQUEEZE9 implemented in Platon was used in order to
avoid the highly disordered solvent molecules (8 acetone and
13 toluene molecules could be detected). Applying the pro-
gram SQUEEZE the R1 value could be lowered from 11.45%
to 7.43%.
Synthetic preparations
4′-(-p-(Pyridin-1-ylmethyl)phenyl)-2,2′:6′,2′-terpyridine (L1(PF6)):
4′-(-p-(bromomethyl)phenyl)-2,2′:6′,2′-terpyridine
(0.200
g,
0.497 mmol) was dissolved in pyridine (30 mL). The mixture
was allowed to react in a microwave reactor at 150 °C and
300 W for 30 minutes. The volume was reduced until dryness
and the solid residue was redissolved in water (30 mL). The
final suspension was filtered through celite and a saturated
aqueous NH4PF6 solution (1 mL) was added to the colorless
solution to obtain a white precipitate. The mixture was filtered
after stirring for two hours to yield 0.2 g (85%) of the desired
powder. 1H NMR (400 MHz, acetonitrile-d3): δ = 8,79 (d, 2H23-27
,
J23-24 = 6,40 Hz) 8,73 (s, 2H9-7), 8,73 (d, 2H1-15), 8,69 (d, 2H4-12
J3-4 = 8,30 Hz), 8,55 (t, 1H25, J24-25 = 8,10 Hz), 8,06 (t, 2H24-26
,
,
J24-25 = 8,10 Hz, J23-24 = 6,40 Hz ), 7,99 (ddd, 2H3-13, J3-4
8,30 Hz, J3-2 = 7,60 Hz, J1-3 = 1,46 Hz), 7,99 (d, 2H17-21, J17-18
=
=
8,10 Hz), 7,63 (d, 2H18-20, J17-18 = 8,10 Hz), 7,47 (t, 2H2-14
,
J2-3 = 7,60 Hz, J1-2 = 6,0 Hz), 5,81 (s, 2H22). 13C{1H} NMR
(100 MHz, acetonitrile-d3): δ = 157,45 (C6-10), 156,65 (C5-11),
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150,56 (C1-15), 150,47 (C8), 147,78 (C25), 145,99 (C23-27), 141,17
(C16), 138,89 (C3-13), 135,16 (C19), 131,47 (C18-20), 130,09 (C24-26),
129,57 (C17-21), 125,86 (C2-14), 122,54 (C4-12), 119,94 (C9-7),
65,40 (C22).
8.47 (d, 4H , J1-2 = 5.70 Hz; H1), 8.37 (t, 4H, J24-25,23 = 6.90 Hz;
H24), 8.35 (d, 4H, J17-18 = 8.15 Hz; H17), 8.22 (d, 2H, J28-29
=
7.97 Hz; H28), 8.05 (t, 4H, J3-4,2 = 7.85 Hz; H3), 7.94 (d, 4H,
J17-18 = 8.15 Hz; H18), 7.76 (t, 2H, J29-28,30 = 7.90 Hz; H29),
7.55 (t, 4H, J2-1,3 = 6.50 Hz; H2), 7.45 (d, 2H, J30-31 = 5.70 Hz;
H31), 6.83 (t, 2H, J30-31 = 7.73 Hz; H30), 6.30 (s, 4H, H22),
0.45 (s, 3H, H36). 13C{1H} NMR (100 MHz, acetone-d6): δ =
191.59 (C35), 160.34 (C6), 159.77 (C5), 156.47 (C32), 153.81
(C1), 152.80 (C31), 151.89 (C33), 146.59 (C25), 145.02 (C23),
144.77 (C8), 138.83 (C16), 137.32 (C3), 135.99 (C29), 135.17
(C19), 130.19 (C18), 128.95 (C24), 128.59 (C17), 127.46 (C2),
123.89 (C4), 122.24 (C30), 120.36 (C7), 119.61 (C28), 103.89
(C34), 64.20 (C22), 25.25 (C36). UV–vis (acetone): lmax (ε) =
367 (27615), 497 (17119), 525 (15923). ESI–MS (MeOH): m/z =
788.09 ([M−2PF6]2+). Elemental analysis (%) found: C, 43.92;
H, 2.75; N, 8.95. Calcd. for C69H55F24N12O2P4Ru2: C, 44.21;
H, 2.97; N, 9.01.
[RuCl3(L1+)](PF6) (1(PF6)): L1(PF6) (0.915 g, 1.617 mmol)
and RuCl3·3H2O (0.422 g, 1.617 mmol) were dissolved in dry
MeOH (130 mL). The mixture was stirred and heated at reflux
temperature for 4 h. Then the solution was kept cool until a
brown precipitate appeared. The solid was filtered, washed
with cold water (3 × 5 mL) and diethyl ether (3 × 5 mL) and
finally dried under vacuum to afford complex 1(PF6) (0.949 g,
77%). ESI–MS (MeOH): m/z = 610 ([M−PF6]+). Elemental
analysis (%) found: C, 43.02; H, 2.81; N, 7,43. Calcd. for
C27H21Cl3F6N4PRu: C, 42.89; H, 2.96; N, 7.22.
{[RuII(L1+)]2(μ-bpp)(μ-Cl)}(PF6)4 (2(PF6)4): 1(PF6) (0.942 g,
1.77 mmol) and LiCl (0.113 g, 2.65 mmol) were dissolved in a
solution of NEt3 (492 mL, 3.54 mmol) and dry MeOH
(180 mL). The mixture was stirred at room temperature for
20 min, and then Hbpp (0.197 g, 0884 mmol) and 0.6684 M
MeONa (1.32 mL, 0.844 mmol) in dry MeOH (20 mL) were
added. The resulting solution was heated for 4 h and then
stirred in the presence of a 100 W tungsten lamp for 8 h. The
reaction mixture was filtered and then a saturated aqueous
NH4PF6 solution (1 mL) was added to obtain a brown precipi-
tate. The solid was collected, washed with cold water (3 × 5 mL)
and diethyl ether (3 × 5 mL) and finally dried under vacuum to
afford complex 2(PF6)4 (0.860 g, 75%). 1H NMR (400 MHz,
acetone-d6): δ = 9.34 (d, 4H, J23-24 = 6.08 Hz; H23), 9.01 (s, 4H;
Preparation of FTO–TiO2–24+: on clean FTO films, anatase
TiO2 paste was spread uniformly. Then the films were heated
for 10 min at 100 °C in order to reduce the surface irregulari-
ties. The films were calcinated following the appropriated
temperature ramps (see Fig. S26†). The anchoring process
was carried out by soaking overnight every film into 5 mL of
an acetone solution (0.305 mM) of 24+.
Preparation of FTO–Nafion–X4+ (X = 24+ or 34+): on clean
FTO films, Nafion 5% w/w in water and low weight alcohols
(50 mL) was uniformly deposited. Then the films were heated
for 30 min at 100 °C in order to remove water and low weight
alcohols. After cooling down until room temperature, the
films were soaked overnight into an acetone solution of X4+
(10 mL, 0.0201 mM).
H7), 8.80 (t, 2H, J25-24 = 7.80 Hz; H25), 8.72 (d, 4H, J3-4
=
8.00 Hz; H4), 8.54 (s, 1H; H8), 8.41 (d, 4H , J1-2 = 5.76 Hz; H1),
8.35 (t, 4H, J24-25 = 7.80 Hz; H24), 8.31 (m, 6H; H28-17), 7.97
(t, 4H, J3-4,2 = 8.00 Hz; H3), 7.89 (d, 4H, J17-18 = 8.20 Hz; H18),
7.83 (t, 2H, J29-28,30 = 8.00 Hz; H29), 7.64 (t, 4H, J2-1,3 = 6.57 Hz;
Preparation of silica–X4+ (X = 24+ or 34+): silica (3 g) was
poured into 10 mL of a solution of 24+ or 34+ (0.548 mM) in ace-
tone. The mixture was stirred for some minutes until the solu-
tion became colorless. Then the pink solids were washed several
times with acetone and diethyl ether and were finally air-dried.
H2), 7.50 (d, 2H, J30-31 = 5.90 Hz; H31), 6.82 (t, 2H, J30-31
=
5.90 Hz; H30), 6.20 (s, 4H, H22). 13C{1H} NMR (100 MHz,
acetone-d6): δ = 159.39 (C6), 159.05 (C5), 158.78 (C32), 153.89
(C31), 153.56 (C1), 148.48 (C33), 146.57 (C25), 145.06 (C23),
145.00 (C8), 138.44 (C16), 137.07 (C3), 136.92 (C29), 135.15
(C19), 130.14 (C18), 128.95 (C24), 128.57 (C17), 127.39 (C2),
123.87 (C4), 122.19 (C30), 120.47 (C28), 120.20 (C7), 103.29
(C34), 64.16 (C22). UV–vis (acetone): lmax (ε) = 366 (26775), 480
(19187), 509 (20235). ESI–MS (MeOH): m/z = 1697.2 ([M−PF6]+).
Elemental analysis (%) found: C, 43.60; H, 2.91; N, 9.00. Calcd.
for C67H51ClF24N12P4Ru2: C, 43.70; H, 2.79; N, 9.13.
Acknowledgements
Support from MINECO (CTQ2010-21497, CTQ2010-21532-C02-02
and CTQ2011-26440) and ICIQ is gratefully acknowledged.
JA is grateful for a PIF pre-doctoral grant from UAB.
Notes and references
{[RuII(L1+)]2(μ-bpp)(μ-O2CMe)}(PF6)4 (3(PF6)4):
a sample
of 2(PF6)4 (0.225 g, 0.120 mmol), sodium acetate (0.054 g,
0.660 mmol) and AgBF4 (0.023 g, 0.120 mmol) was dissolved
in acetone–water (3 : 1, 40 mL), and the solution was heated
at reflux overnight in the dark. The resulting solution was
filtered, and a saturated aqueous NH4PF6 solution (1 mL)
was added. A solid precipitated appeared upon reducing the
volume. The solid was collected and washed with cold water
(3 × 5 mL) and diethyl ether (3 × 5 mL) and finally dried
under vacuum to afford complex 3(PF6)4 (0.166 g, 73%). 1H
NMR (400 MHz, acetone-d6): δ = 9.35 (d, 4H, J23-24 = 6.90 Hz;
H23), 9.10 (s, 4H; H7), 8.82 (m, 6H; H25-4), 8.56 (s, 1H; H34),
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