Electrochemical and Photophysical Study of Homoleptic and Heteroleptic Methylated Ru(II) Bis-terpyridine Complexes

Article abstract of DOI:10.1002/ejic.202100092

In this study, we investigate the impact of N-methylation on the electronic and photophysical properties of both homoleptic and heteroleptic Ru(II) bis-terpyridine complexes based on the recently reported ligand 4’-(4-bromophenyl)-4,4’’’ : 4’’,4’’’’-dipyr

Full text of DOI:10.1002/ejic.202100092

European Journal of Inorganic Chemistry
Accepted Article
Title: Electrochemical and photophysical study of homoleptic and
heteroleptic methylated Ru(II) bis-terpyridine complexes
Authors: Mira Theresa Rupp, Thomas Auvray, Garry S. Hanan, and
Dirk G. Kurth
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202100092
Link to VoR: https://doi.org/10.1002/ejic.202100092
0
1/2020

European Journal of Inorganic Chemistry
10.1002/ejic.202100092
FULL PAPER
Electrochemical and photophysical study of homoleptic and
heteroleptic methylated Ru(II) bis-terpyridine complexes
Mira T. Rupp[a, b], Thomas Auvray , Garry S. Hanan *, Dirk G. Kurth *
[a]
[a]
[b]
[
a]
b]
Dr. T. Auvray, Prof. Dr. G. S. Hanan
Département de Chimie
Université de Montréal
1375 Avenue Thérèse-Lavoie-Roux, Montréal, Québec, H2V-0B3, Canada
E-mail: garry.hanan@umontreal.ca
M. T. Rupp, Prof. Dr. D. G. Kurth
[
Chemische Technologie der Materialsynthese
Julius-Maximilians-Universität Würzburg
Röntgenring 11, 97070 Würzburg, Germany
E-mail: dirk.kurth@matsyn.uni-wuerzburg.de
Supporting information for this article is given via a link at the end of the document. The crystal structures of 2 and 3 are deposited into the CCDC under the
following references: 2034792 and 2034795.
Abstract: In this study, we investigate the impact of N-methylation on
the electronic and photophysical properties of both homoleptic and
heteroleptic Ru(II) bis-terpyridine complexes based on the recently
reported ligand 4’-(4-bromophenyl)-4,4’’’:4’’,4’’’’-dipyridinyl-2,2’:6’,2’’-
terpyridine (Bipytpy), with pyridine substituents in the 4- and 4’’-
position. The first reduction of the methylated complexes takes place
at the pyridinium site and is observed as multi-electron process.
Following N-methylation, the complexes exhibit higher luminescence
quantum yields and longer excited-state lifetimes. Interestingly, the
photophysical properties of the heteroleptic and homoleptic
complexes are rather similar. TD-DFT calculations support the
experimental results. Furthermore, the complexes are tested as
photosensitizers for photocatalytic hydrogen production, as the parent
as well as the redox behavior and the photophysical properties
such as absorption and emission wavelengths and excited-state
lifetimes.
In this study, we investigate the effect of N-methylation on pyridine
substituents in the 4- and 4’’-position of the ligand 4′-(4-
bromophenyl)-4,4‴:4″,4‴′-dipyridinyl-2,2′:6′,2″-terpyridine
(Bipytpy). This ligand was used to prepare complex
1
[Ru(Tolyltpy)(Bipytpy)]2+ (Tolyltpy: 4′-tolyl-2,2′:6′,2′-terpyridine),
which has been recently reported by our group to be a capable
photosensitizer in photocatalytic hydrogen evolution.[4a] Based on
those observations, we were interested in the potential positive
impact of structural variation on the performance in photocatalysis.
We thus investigate here the effect of a homoleptic vs.
heteroleptic design. The structures of the complexes 1 to 4
discussed in this study are shown in Figure 1.
6 2
complex 1 [Ru(Bipytpy)(Tolyltpy)](PF ) (Tolyltpy: 4′-tolyl-2,2′:6′,2′-
terpyridine) was recently shown to be active and highly stable under
photocatalytic conditions. However, the methylated complexes
reported herein are inactive as photosensitizers under the chosen
conditions, presumably due to loss of the methyl groups, converting
them to the non-methylated parent complexes.
Introduction
Due to their high affinity to many transition metal ions, terpyridines
(
2,2‘:6‘,2‘‘-terpyridines) have been widely studied in
[
1]
supramolecular chemistry and material sciences. They can be
used as building blocks in supramolecular assemblies such as
coordination polymers or metal organic frameworks.
[
1a, 2]
With
6
group 8 metals, they form kinetically stable d complexes with a
distorted octahedral geometry, which show reversible redox
behavior as well as a strong absorption in the visible region.[3] The
electrochemical and photophysical properties of these complexes
can be tuned by introducing electron-withdrawing or electron-
accepting substituents.[4] Constable and coworkers have
previously published several studies regarding N-alkylation of
ruthenium, iron and osmium bis-terpyridine complexes and their
application as photosensitizers in water oxidation as well as
molecular switches.[5] For all investigated complexes, the N-
alkylation was introduced at a pyridine ring in the 4’-position of the
terpyridine. By introducing different N-substituents, they could
alter the solubility of the complexes depending on the counter ion
Figure 1. Structures of the complexes 1 to 4 discussed in this paper.
1
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.202100092
FULL PAPER
Results and Discussion
electron-donating ligand in the homoleptic complexes, i.e., the
electron-donating tolyl substituent in the heteroleptic complexes
destabilizes the metal-centered HOMO and thus facilitates the
oxidation of the Ru(II) center while the electron-accepting pyridine
Synthesis and structural analysis
The synthesis of the ligands as well as of complex 1 has been
described previously.[4a] The homoleptic complex 2 is synthesized
in a one-step procedure, reacting two equivalents of the ligand
Bipytpy with a Ru(III) chloride salt using microwave irradiation.
Both the heteroleptic and the homoleptic complexes 1 and 2 are
then methylated using iodomethane, followed by addition of
aqueous potassium hexafluorophosphate to isolate the
and N-methyl pyridinium substituents slightly stabilize the
HOMO.[ The homoleptic complex [Ru(Tolyltpy)
6]
]2+ exhibits an
2
+
oxidation potential of 0.84 V vs. Fc/Fc and is thus slightly easier
to oxidize than complex 1 with an oxidation potential of 0.89 V.[7]
This anodic shift is assumed to be due to the HOMO destabilizing
effect of the electron-donating Tolyltpy ligand, which is less
pronounced in the heteroleptic complex 1. A similar anodic shift
in oxidation potentials can also be observed comparing the non-
methylated complexes 1 and 2 with their methylated counterparts
3 and 4. The methylation of the pyridine substituents makes the
ligands electron poorer and thus more electron-withdrawing,
-
1
6
complexes 3 and 4 as PF salts. Their identity is confirmed by H
and 13C NMR analysis (Figures S.I. 1 – 6) and ESI-MS.
Single crystals of complexes 2 and 3 are obtained either by slow
evaporation of an acetonitrile/water solution for complex 2 or slow
diffusion of diethyl ether into a concentrated acetonitrile solution
of the complex 3. The crystals are analyzed using X-ray diffraction. resulting in a stronger stabilization of the metal-centered HOMO
The structures are shown in Figure 2 with refinement parameters
described in the SI. In both complexes, the ruthenium cation has
a distorted octahedral coordination sphere, resulting from the
restricted bite angle of the two meridionally coordinated
terpyridine ligands. The crystal structure analysis confirms the
expected structures of the complexes.
and therefore an anodic shift of the oxidation potential. This
observation matches well with the results reported by Constable
et al. on related N-methylated and N-alkylated ruthenium bis-
terpyridine complexes.[5b, 8]
The non-methylated complexes 1 and 2 exhibit three reversible
reduction processes in the investigated window, which take place
at the ligands. The methylated complexes 3 and 4 both show one
reversible reduction process (or several very close lying
+
processes) taking place at -1.1 V vs. Fc/Fc . In the case of
complex 3, this first reduction is a two-electron process,
determined by square wave voltammetry (Figure S.I. 10), while in
the case of complex 4, it is a four-electron process (Figure S.I. 11).
The first reduction of the methylated complexes presumably takes
place at the pyridinium ions, indicated both by the multiple
electron processes as well as the strongly anodically shifted
reduction potential.[5b] The strong anodic shift of these redox
processes again matches well with a study conducted by
Constable and Cargill Thompson. However, in a homoleptic
ruthenium bis-terpyridine complex with N-methylated pyridine
substituents, they observed two separate reductions of the
pyridinium sites.[5b] Their observation supports the assumption
that the processes observed around -1.1 V for complexes 3 and
4
are indeed multiple, overlapping one-electron processes, which
also explains the large peak separation (>100 mV). Complexes 3
and 4 furthermore show three and four reversible reduction
processes in the investigated window, respectively. We
previously assigned the reductions occurring in complex 1 by
comparison with the homoleptic [Ru(Tolyltpy)
2
]2+ complex,
Figure 2. Ellipsoid representation of complexes 2 and 3 at 50% probability.
leading to the following assignment: The first reduction process
corresponds to a first reduction of the pyridine substituted ligand;
the second reduction process corresponds to a first reduction of
the Tolyltpy ligand; the third reduction process corresponds to a
second reduction process of the pyridine substituted ligand.[
Following this assignment, we assume that in complex 3, the
reductions occurring at -1.55 V and -2.04 V vs. Fc/Fc correspond
to a first and second reduction of the electron-withdrawing
pyridinium substituted ligand while the reduction process at -
-
Hydrogen atoms, PF
6
counterions and co-crystallized solvent molecules are
omitted for clarity.
Electrochemical properties
The electrochemical properties of complexes
4a]
1 to 4 are
investigated using cyclic voltammetry in deaerated acetonitrile.
The experimental setup is described in the SI. The cyclic
voltammograms of the anodic and cathodic scans are shown in
Figure 3. Cyclic voltammograms of both anodic and cathodic scan
conducted in one single experiment are shown in Figure S.I. 7.
+
1
.70 V occurs at the Tolyltpy ligand. In the homoleptic complex 2,
four reduction processes are observed in the investigated window.
These correspond to both ligands being reduced twice. Complex
+
The potentials are reported vs. ferrocene (Fc/Fc ) as a reference.
All complexes exhibit a reversible oxidation, which takes place at
the metal center.[3a, 6] The non-methylated and methylated
homoleptic complexes 2 and 4 are harder to oxidize, i.e., the
oxidation potential is shifted anodically by +50-100 mV, with
respect to the corresponding heteroleptic complexes 1 and 3. This
shift in oxidation potentials can be explained by the absence of an
4
exhibits four reduction processes below -1.5 V, which
correspond to each ligand being reduced twice. The data are
summarized in Table 1.
2
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.202100092
FULL PAPER
maxima slightly red-shifted for the homoleptic complexes 2 and 4
compared to their heteroleptic counterparts. The methylated
1
2
3
4
complexes
3 and 4 exhibit a broader, red-shifted MLCT
absorption band, with lower absorptivity compared to the non-
methylated analogues. These broader absorption bands overlap
with other transitions at higher energy (around 400 nm), which are
presumably ligand-centered (LC) transitions. Additionally,
complex 3 exhibits a shoulder around 470 nm, which is attributed
to a MLCT transition, likely involving mainly the Tolyltpy ligand,
based on the higher energy of this transition.
5
0 µA
Regarding the emission wavelength, the methylated complexes 3
and 4 again exhibit a red-shifted (25–40 nm) emission compared
to the non-methylated complexes 1 and 2. Furthermore, the
quantum yield of the methylated complexes as well as their
excited-state lifetime are increased by an order of magnitude
compared to complexes 1 and 2. Additionally, the homoleptic
complexes 2 and 4 also exhibit a blue-shifted emission compared
to the heteroleptic complexes 1 and 4. We assume this
observation is due to the lack of the electron-donating Tolyltpy
ligand in the homoleptic complexes. The electron-donating
character of the Tolyltpy ligand leads to a destabilization of the
HOMO and thus to a smaller HOMO-LUMO energy gap[4c], i.e., a
red-shifted emission wavelength.
-
2,5
-2,0
-1,5
-1,0
-0,5
0,0
0,5
1,0
1,5
+
E (vs. Fc/Fc ) / V
Figure 3. Cyclic voltammograms of complexes 1 to 4 (0.5 mM) in dry acetonitrile
under inert gas atmosphere, with 0.1 M TBAPF ; scan rate 100 mV/s; scans
6
both in anodic and cathodic direction start at 0 V vs. reference.
p
Table 1. Electrochemical half-wave redox potentials E1/2 in V (ΔE / mV) for
complexes 1 to 4 in inert-gas purged acetonitrile.a
Oxidation
Reduction E1/2 / V
A study by Constable and coworkers on N-alkylated 4’-pyridyl
substituted bis-terpyridine complexes yielded similar quantum
E
1/2 / V
1
2
0.89 (69)
0.94 (69)
-1.51 (62) -1.81 (75) -2.02 (104)
-4
yields around 40*10 . However, the observed excited-state
lifetimes of these complexes were found to be increased by a
factor of 3–4 compared to the herein reported complexes and the
-1.45 (61) -1.64 (61) -1.92 (83) -2.06 (98)
-1.10 (102) -1.55 (62) -1.70 (71) -2.04(139)
3
4
0.94 (71)
1.04 (73)
emission wavelengths to be red-shifted to around 720 nm.[8]
A
-1.11 (114) -1.58 (66) -1.69 (70) -1.82 (84) -2.09 (142)
longer excited-state lifetime despite a lower HOMO-LUMO energy
gap as indicated by the emission wavelength, indicates that the
non-radiative decay is not governed by direct deactivation from
the excited-state to the ground state and can hence not be
described by the energy gap law, but rather involves the triplet
a
+
Redox potentials are reported vs. ferrocene (Fc/Fc ); the differences between
p
the cathodic and anodic peak potentials ΔE are given in parentheses in
millivolts; measurements are carried out in deaerated acetonitrile solutions
containing 0.1 M tetrabutylammonium hexafluorophosphate at a scan rate of
3
3
metal-centered ( MC) state. Assuming that the energy of the MC
1
00 mV s−1
.
3
state is less affected than the MLCT state by the more electron-
withdrawing character of the ligands due to methylation, a
stronger red-shift of the emission wavelength of the complexes
reported by Constable et al. indicates a larger 3MLCT to 3MC
energy gap and hence a less efficient deactivation of the excited
state, resulting in longer excited-state lifetimes.
Photophysical properties
The complexes are analyzed regarding their photophysical
properties in acetonitrile. The UV-vis absorption and emission
spectra are presented in Figure 4, the data are summarized in
Table 2. All complexes exhibit metal-to-ligand charge transfer
(
MLCT) absorption bands around 500 nm, with absorption
1
0
0
0
0
,0
,8
,6
,4
,2
1
1
00
2
3
4
8
6
4
2
0
0
0
0
0
0,0
900
200
300
400
500
600
700
800
λ / nm
Figure 4. UV-vis absorption spectra (solid lines) and normalized emission spectra (dashed lines) of complexes 1 to 4; spectra measured in acetonitrile.
3
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.202100092
FULL PAPER
Table 2. UV-vis absorption and emission data of complexes 1 to 4; spectra measured in acetonitrile.[a]
3
-1
-1
Φ/ 10-5
λ
abs, max / nm (ε / 10 L mol cm )
λem, max / nm
τ / ns
1
2
3
4
233 (92)
235 (100)
228 (95)
238 (77)
302 (80)
296 (63)
275 (77)
265 (80)
332 (70)
332 (70)
309 (65)
307 (44)
497 (34)
502 (30)
408 (15)
344 (46)
658
74 ± 33
3.8 ± 0.5
2.4 ± 0.2
36 ± 1
649
697
675
78 ± 32
295 ± 128
405 ± 160
511 (21)
377 (19)
519 (22)
39 ± 1
[a]maximum absorption wavelength λabs, max, extinction coefficient ε, maximum emission wavelength λem, max, luminescence quantum yield Φ,
excited-state lifetime τ; emission data is collected in inert-gas purged solutions at room temperature.
Theoretical calculations
the HOMO is localized mainly on the ruthenium center for all four
To further understand the observations in the photophysical and
electrochemical experiments and to support our conclusions, we
conducted theoretical calculations using density functional theory
complexes. In the case of complex 4, the HOMO is described by
three degenerated states. As expected, the electron density in the
LUMO is centered on the terpyridine moieties. In heteroleptic
complexes 1 and 3, most of the electron density is on the pyridine-
substituted ligand, in agreement with its more electron-
withdrawing nature. This non-symmetrical electron density in the
LUMO of complexes 1 and 3 is expected due to the heteroleptic
design. In contrast, for the homoleptic complexes 2 and 4, two
degenerate states exist, with the electron density delocalized
equally over both ligands. The methylation of the peripheral
pyridine substituents increases the contribution of these rings to
the LUMO levels, in agreement with the pyridinium centered
reduction observed in the electrochemical measurements.
(
DFT) and time-dependent DFT (TD-DFT). Figure 5 shows the
calculated HOMO and LUMO energies of the four complexes 1 to
. The LUMO energies of the methylated complexes 3 and 4 are
4
significantly lower than those of the non-methylated complexes 1
and 2. This shift in energy matches well the observed anodically
shifted reduction potentials of complexes 3 and 4.
TD-DFT calculations provide insight into the nature of the excited
state of the reported complexes. The singlet-triplet transition
corresponding to the excited state are presented in Table S.I. 3.
In all cases, they correspond to MLCT states mixed with minor
contributions of LC states. While the methylation in complexes 3
and 4 extends the delocalization on the Bipytpy ligand towards
the pyridines in the 4- and 4”-position, the pyridine rings have no
or only a minor contribution in the excited state with no
contribution from the methyl groups, similarly to what Constable
and coworkers had observed for their system.[8]
Photocatalytic study for hydrogen evolution
We previously reported that complex 1 acts as photosensitizer
(
PS) in hydrogen evolution experiments, exhibiting relatively weak,
yet very long lasting activity.[ The complexes discussed in this
4a]
paper are tested under similar catalytic conditions, using the
catalyst [Co(dmgH)
2
(H
2
O)
2
](BF
4
)
2
,
prepared in-situ from
[
Co(H O) ](BF (1 mM) and dmgH
2
6
4
)
2
2
(6 mM), or colloidal platinum,
Figure 5. Energy diagram of the frontier orbitals of the Ru(II) complexes 1 to 4
2 4
prepared in-situ from K PtCl , as reported in other photocatalytic
studies.[4a,
(degeneracy threshold of 15 meV).
4b]
The tests are conducted in DMF with 1 M
triethanolamine (TEOA) as sacrificial electron donor and 0.1 M
tetrafluoroboric acid (HBF ) as proton source. The instrumental
error of the reported values is found to be within 10 %. Results
are reported in turn-over frequency (TOF) and turn-over number
Furthermore, HOMOs of the homoleptic complexes 2 and 4 are
stabilized compared to the heteroleptic complexes 1 and 3. This
effect is stronger for the methylated complexes, presumably due
to the more electron-withdrawing character of the methylated
ligand. The strong stabilization of the HOMO of complex 4 leads
to a larger calculated HOMO-LUMO gap in comparison to
complex 3. The larger energy gap agrees with the observed
higher energy emission wavelength (Table 2).
The impact of the homoleptic vs. the heteroleptic design of the
complexes can further be visualized when looking at the
calculated contributions to the molecular orbitals (MOs). Table 3
shows the calculated distribution of the electron density in the
HOMOs and LUMOs of complexes 1 to 4. The electron density in
4
(
TON). Complex 2 is active as photosensitizer both with the
cobaloxime complex or the colloidal platinum used as catalyst
Figure 6 and Figure S.I. 16). However, its activity as well as
(
stability are inferior to that of the previously reported heteroleptic
complex 1 (Figure 6). Complex 1 has been reported to be active
as PS for more than 12 days without reaching its half-life time
TOFmax 56 mmolH2 molPS-1 min-1).[4a] Complex 2 on the other hand
(
-
exhibits a half-life time of 5.2 h and a TOFmax of 36 mmolH2 molPS
1
-1
min when using a cobaloxime catalyst and a TOFmax of
-1
-1
3
5 mmolH2 molPS min with a half-life time of 8.0 h using colloidal
4
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.202100092
FULL PAPER
Table 3. Contributions to MOs for complexes 1 to 4.
HOMO
LUMO
1
-
6.53eV
-3.00eV
Ru: 7%
Tolyltpy: 1%; Bipytpy: 92%
Ru: 66%
Tolyltpy: 15%; Bipytpy: 19%
2
-
6.58eV
-3.03 eV
Ru: 7%
Bipytpy: 93%
-
3.03eV
Ru: 64%
Bipytpy: 36%
Ru: 7%
Bipytpy: 93%
3
-
6.73eV
-3.62eV
Ru: 4%
Ru: 52%
Tolyltpy: 32%; Me-Bipytpy: 16%
Tolyltpy: 3%; Me-Bipytpy: 93%
4
-
7.02eV
-3.74eV
Ru: 4%
Me-Bipytpy: 96%
-3.74eV
Ru: 4%
Me-Bipytpy: 96%
-
7.02eV
-7.03eV
Ru: 65%
Me-Bipytpy: 35%
Ru: 55%
Me-Bipytpy: 45%
Ru: 56%
Me-Bipytpy: 44%
platinum. Yet, the UV-vis absorption spectra of the reaction
mixtures recorded before and after blue light irradiation only vary
very slightly (see Figure S.I. 17), which shows that complex 2
does not decompose during the hydrogen evolution reaction and
that other components of the system lead to the loss of hydrogen
production activity over time. Previously, we argued that the
relatively low hydrogen evolution activity of PS 1 is a result of the
reductive quenching being the rate limiting step.[4a] The fact, that
the activity as PS of complex 2 is very similar regardless of the
catalyst used indicates, that here too, the rate limiting step does
not involve the catalyst. However, complex 2 is slightly easier to
reduce, which should make the reductive quenching more
efficient and hence increase the activity in hydrogen evolution
experiments. The fact, that instead of an increased activity rather
a lower activity and decreased longevity of hydrogen production
is observed with complex 2 compared to complex 1, leads to the
conclusion that the heteroleptic design of complex 1, leading to a
non-symmetrical electron density distribution in the excited state,
is beneficial for the electron transfer processes. However, more
detailed studies are needed to further investigate this effect.
Despite its improved photophysical properties, only very low
activity is observed from complex 3 using a cobalt-based catalyst
-1
-1
(TOFmax 22 mmolH2 molPS min , see Figure 6). In contrast, using
colloidal Pt as catalyst, the methylated complex 3 shows an
unusual activity profile where the hydrogen production starts very
slow and increases over time reaching a TOFmax of 45 mmolH2
-1
-1
molPS min (Figure S.I. 16). UV-vis absorption spectra recorded
before and after the experiment (Figure S.I. 18) suggest that the
5
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.202100092
FULL PAPER
Conclusion
In this study, we compare the previously reported heteroleptic
complex 1 with its homoleptic counterpart 2 and prepare the
corresponding methylated complexes 3 and 4. Comparing the
homoleptic and heteroleptic complexes, we find that the
homoleptic complexes 2 and 4 are harder to oxidize but easier to
reduce. As expected, the first reduction of the methylated
complexes 3 and 4 takes place at the pyridinium site and is
observed as multi-electron process. Furthermore, the
photophysical properties are barely affected by changing from
heteroleptic 1 to homoleptic 2. Both methylated complexes
exhibited higher luminescence quantum yields and longer
excited-state lifetimes. Photocatalytic investigations show that
methylation of the complexes makes them unsuitable for the
application as PS in hydrogen evolution experiments. This
observation is assumed to be due to decomposition, e.g., via loss
of the methyl groups, under the catalytic conditions and a lack of
driving force for electron transfer from the reduced PS to the
catalyst. However, the anodically shifted oxidation potentials of
the methylated complexes could render these complexes
interesting candidates for photosensitizers in photocatalytic water
oxidation. In catalytic systems for water oxidation, the excited PS
is usually oxidized by a sacrificial electron acceptor, yielding a
Ru(III) species. Compounds similar to the methylated complexes
Figure 6. Hydrogen photoproduction with complexes 1, 2 and 3 as PS (0.1 mM)
under blue light irradiation (LED centered at 445 nm); with [Co(H
pre-catalyst (1 mM), with dmgH (6 mM), TEOA as sacrificial electron donor (1
as proton source (0.1 M) in DMF; TOF: solid lines; TON: dashed
2 6 4 2
O) ](BF ) as
2
M) and HBF
4
lines.
complex is not stable under the catalytic conditions. The UV-vis
absorption spectra after the hydrogen evolution experiments
show an absorption profile very similar to that of complex 1,
leading to the assumption that complex 3 is demethylated.
Interestingly, the absorption profiles of the reaction mixtures after
irradiation with light differ slightly depending on which catalyst is
used. While the reaction mixture containing a cobaloxime catalyst
leads to a spectrum identical to that of complex 1, the reaction
mixture with colloidal platinum differs slightly, with a small
bathochromic shift of the MLCT absorption band. These small
differences in absorbance are also reflected in the hydrogen
evolution activity, i.e., only with colloidal platinum, the
decomposition product leads to hydrogen evolution activity. It
should however be noted that different batches of freshly in-situ
prepared colloidal platinum can lead to slightly varying hydrogen
evolution profiles, albeit exhibiting similar TOFmax. When a
cobaloxime catalyst is used, a maximum TOFmax of 22 mmolH2
3
and 4, i.e., N-alkylated 4’-pyridyl substituted terpyridine
ruthenium complexes, were previously studied as
photosensitizers in photocatalytic water oxidation. These studies
showed that the N-alkylated bis-terpyridine complexes can lead
to a comparable oxygen evolution activity as the standard
[5j]
ruthenium tris-bipyridine PS.
Experimental Section
Materials All reagents and solvents were obtained from commercial
sources (VWR, Fisher Scientific, Acros or Sigma Aldrich) and used as
3 2
received, unless stated otherwise. RuCl ·3H O was obtained from
[9]
Pressure Chemicals Inc. The synthesis of the ligands Tolyltpy and
Bipytpy[4a] were carried out according to published procedures. The
-1
-1
2+
molPS min , with a half-life time of 2.9 h is found. We assume
that the cobaloxime catalyst is participating in the demethylation
reaction, rendering it inactive for the hydrogen evolution reaction.
Complex 4 is found to be not active as PS, both with the
cobaloxime and the colloidal platinum catalyst. Again, UV-vis
absorption spectra of the reaction mixtures recorded before and
after light irradiation show a change in the absorption profile. This
change could either stem from a decomposition, such as
demethylation, or reduction of the complex. We propose that, due
to the less negative reduction potentials of the methylated
complexes 3 and 4, they are easily reduced by the sacrificial
electron donor, but the reduced species lack driving force to
subsequently transfer the electron to the catalyst. While the cyclic
voltammograms show that the reduction processes are reversible,
the catalytic conditions (light irradiation and presence of other
reagents) apparently lead to a decomposition of the methylated
complexes 3 and 4, e.g., via loss of the methyl groups.
synthesis of the complex [Ru(Tolyltpy)(Bipytpy)] 1 was carried out as a
two-step procedure as described previously.[ For experiments under
microwave irradiation, a Biotage Initiator or a Discover SP microwave
synthesizer were used.
4a]
Synthesis [Ru(Bipytpy)
trihydrate (11.8 mg, 45.0 µmol, 1 eq) and Bipytpy (50.0 mg, 92.2 µmol,
.05 eq) in 7.00 mL ethylene glycol was heated to 150 °C for 40 minutes
2 6 2
](PF ) 2 A suspension of Ru(III) chloride
2
using microwave irradiation. After cooling to room temperature, water and
aqueous KPF solution were added to the solution and the precipitate was
6
filtered off over celite. After washing with water, the solid was dissolved in
acetonitrile, dried over magnesium sulfate and the solvent was removed
under reduced pressure. The crude product was purified by washing with
dichloromethane to yield the product as a dark red solid (62.0 mg,
42.0 µmol, 93.4 %).
1
3
H-NMR (400 MHz, CD CN): δ = 9.24 (s, 4H), 8.99 (s, 4H), 8.77 (d,
3
3
3
J = 5.7 Hz, 8H), 8.24 (d, J = 8.4 Hz, 4H), 8.01 (d, J = 8.3 Hz, 4H), 7.79
3
3
3
(d, J = 5.6 Hz, 8H), 7.61 (d, J = 5.9 Hz, 4H) and 7.54 ppm (d, J = 5.7 Hz,
1
4
1
1
5
H). 13C{ H}-NMR (125 MHz, CD
3
CN): δ = 159.8, 156.5, 154.0, 151.8,
48.4, 148.3, 143.6, 136.8, 133.7, 130.6, 125.9, 125.6, 123.4, 123.1 and
22.5 ppm. ESI-MS: m/z [M]2+ calc. for C62
92.04471; difference: 4.9 ppm.
2
H40Br N10Ru: 592.04183; found:
6
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.202100092
FULL PAPER
[
[
Ru(Tolyltpy)(di-methyl-Bipytpy)](PF
6
)
4
3
To
a
solution
of
Keywords: ruthenium, luminescence, electrochemistry, ligand
Ru(Bipytpy)(Tolyltpy)](PF 1 (35.0 mg, 27.9 µmol, 1 eq) in 10.0 mL
6 2
)
effects, photocatalysis
acetonitrile, iodomethane (79.1 mg, 34.7 µL, 557 µmol, 20 eq) was added.
The reaction mixture was stirred at room temperature for 2 h and
subsequently heated to 40 °C for two days. During the cause of the
reaction, additional iodomethane was added. The product was isolated by
removal of the solvent and excess iodomethane under vacuum. To
exchange the iodide counter ions, the dark red solid was dissolved in
acetonitrile and aqueous potassium hexafluorophosphate solution and
water were added. The precipitate was filtered off and washed with water
[
1]
aJ. Wang, X. Xue, M. Chen, T. Wu, S.-C. Wang, H. Zhao,
Z. Jiang, J. Yan, Z. Jiang, Y.-T. Chan, P. Wang, Inorg.
Chem. 2019, 58, 7662-7666; bT. Mede, M. Jager, U. S.
Schubert, Chem. Soc. Rev. 2018, 47, 7577-7627; cA.
Wild, A. Winter, F. Schlutter, U. S. Schubert, Chem. Soc.
Rev. 2011, 40, 1459-1511.
aE. C. Constable, E. L. Dunphy, C. E. Housecroft, M.
Neuburger, S. Schaffner, F. Schaper, S. R. Batten, Dalton
Trans. 2007, 4323-4332; bM. Elcheikh Mahmoud, H. Audi,
A. Assoud, T. H. Ghaddar, M. Hmadeh, J. Am. Chem.
Soc. 2019, 141, 7115-7121; cS. Chakraborty, G. R.
Newkome, Chem. Soc. Rev. 2018, 47, 3991-4016; dS.
Pai, M. Moos, M. H. Schreck, C. Lambert, D. G. Kurth,
Inorg. Chem. 2017, 56, 1418-1432; eG. Schwarz, I.
Haßlauer, D. G. Kurth, Adv. Colloid Interface Sci. 2014,
[
2]
(33.3 mg, 21.1 µmol, 75.8 %).
1
3
3
H-NMR (300 MHz, CD CN): δ = 9.25 (s, 2H), 9.04 (m, 4H), 8.76 (d,
3
3
J = 6.4 Hz, 4H), 8.69 (d, J = 8.1 Hz, 2H), 8.34 (d, J = 6.4 Hz, 4H), 8.23
3
3
3
(d, J = 8.4 Hz, 2H), 8.15 (d, J = 8.0 Hz, 2H), 7.98 (m, 4H), 7.74 (d, J =
3
5
2
.9 Hz, 4H), 7.61 (d, J = 7.4 Hz, 4H), 7.44 (d, 3J = 5.1 Hz, 2H), 7.21 (m,
H), 4.35 (s, 6H) and 2.56 ppm (s, 3H). 13C{ H}-NMR (75 MHz, CD
1
3
CN):
δ = 160.4, 159.1, 156.4, 156.1, 154.4., 153.7, 152.0, 150.3, 148.2, 147.0,
207, 107-120; fM. Chen, D. Liu, J. Huang, Y. Li, M. Wang,
1
1
43.7, 143.7, 142.2, 139.5, 136.8, 134.7, 133.8, 131.3, 130.6, 128.7, 128.5,
26.9, 126.5, 125.7, 125.6, 123.7, 123.4, 122.7, 49.2 and 21.4 ppm. ESI-
K. Li, J. Wang, Z. Jiang, X. Li, P. Wang, Inorg. Chem.
2019, 58, 11146-11154; gS. Vitale, B. Laramée-Milette, M.
E. Amato, G. S. Hanan, N. Tuccitto, A. Licciardello,
Nanoscale 2019, 11, 4788-4793; hR. Farran, L. Le Quang,
D. Jouvenot, F. Loiseau, R. Pansu, A. Deronzier, J.
Chauvin, Inorg. Chim. Acta 2017, 454, 197-207; iM. K.
Bera, Y. Ninomiya, T. Yoshida, M. Higuchi, Macromol.
Rapid Commun. 2020, 41, 1900384; jM. K. Bera, Y.
Ninomiya, M. Higuchi, ACS Appl. Mater. Interfaces 2020,
2+
MS: m/z [M] calc. for C55
8
H43BrN Ru: 498.09133; found: 498.0926;
difference: 2.6 ppm.
[
Ru(di-methyl-Bipytpy)
2
](PF
6 6 2 6 2
) 4 To a solution of [Ru(Bipytpy) ](PF ) 2
(40.0 mg, 27.1 µmol, 1 eq) in 10.0 mL acetonitrile, iodomethane (385 mg,
167 µL, 2.71 mmol, 100 eq) was added. The reaction mixture was heated
to reflux for 4 h. The product was isolated by removal of the solvent and
excess iodomethane under vacuum. To exchange the iodide counter ions,
the dark red solid was dissolved in acetonitrile and aqueous potassium
hexafluorophosphate solution and water were added. The precipitate was
filtered off and washed with water (41.0 mg, 19.4 µmol, 71.5 %).
1
2, 14376-14385; kS. M. Munzert, S. P. Stier, G. Schwarz,
H. Weissman, B. Rybtchinski, D. G. Kurth, Chem. - Eur. J.
2018, 24, 2898-2912; lJ. Lombard, D. A. Jose, C. E.
Castillo, R. Pansu, J. Chauvin, A. Deronzier, M. N.
Collomb, J. Mater. Chem. C 2014, 2, 9824-9835.
aA. Juris, V. Balzani, F. Barigelletti, S. Campagna, P.
Belser, A. von Zelewsky, Coord. Chem. Rev. 1988, 84, 85-
[
3]
1
3
H-NMR (500 MHz, CD CN): δ = 9.29 (s, 4H), 9.06 (s, 4H), 8.76 (d,
277; bJ. P. Sauvage, J. P. Collin, J. C. Chambron, S.
3
3
3
J = 6.8 Hz, 8H), 8.33 (d, J = 6.8 Hz, 8H), 8.25 (d, J = 8.4 Hz, 4H), 8.02
d, 3J = 8.4 Hz, 4H), 7.74 (d, 3J = 6.0 Hz, 4H), 7.62 (dd, 3J = 6.0 Hz,
Guillerez, C. Coudret, V. Balzani, F. Barigelletti, L. De
Cola, L. Flamigni, Chem. Rev. 1994, 94, 993-1019; cS.
Pai, M. Schott, L. Niklaus, U. Posset, D. G. Kurth, J.
Mater. Chem. C 2018, 6, 3310-3321.
aM. Rupp, T. Auvray, E. Rousset, G. M. Mercier, V.
Marvaud, D. G. Kurth, G. S. Hanan, Inorg. Chem. 2019,
58, 9127-9134; bT. Auvray, R. Sahoo, D. Deschenes, G.
S. Hanan, Dalton Trans. 2019, 48, 15136-15143; cA. K.
Pal, G. S. Hanan, Chem. Soc. Rev. 2014, 43, 6184-6197;
dY.-Q. Fang, N. J. Taylor, F. Laverdière, G. S. Hanan, F.
Loiseau, F. Nastasi, S. Campagna, H. Nierengarten, E.
Leize-Wagner, A. Van Dorsselaer, Inorg. Chem. 2007, 46,
(
4
J = 1.8 Hz, 4H) and 4.34 ppm (s, 12H). 13C{ H}-NMR (125 MHz, CD
δ = 160.3, 156.2, 154.8., 151.9, 149.1, 147.0, 144.2, 136.7, 133.8, 130.6,
26.9, 126.6, 125.8, 123.9, 123.7 and 49.2 ppm. ESI-MS: m/z [M+3H]3+
calc. for 10Ru: 415.73350; found: 415.72972; difference:
.1 ppm.
1
3
CN):
1
[4]
66 2
C H52Br N
9
Computational details The calculations were made with Gaussian16
rev.B.01[10], using the PBE0 hybrid functional[11] with LanL2DZ[12] as basis
set. The optimizations were conducted without symmetry constraints,
followed by frequency calculations to confirm that energy minima had been
reached in all cases. The energy, oscillator strength, and related MO
contributions for the 100 lowest singlet–singlet and 10 lowest singlet–triplet
excitations were obtained from the TD-DFT/singlets and the TD-
2854-2863; eP. Pal, S. Mukherjee, D. Maity, S. Baitalik,
ACS Omega 2018, 3, 14526-14537; fA. K. Pal, S. Serroni,
N. Zaccheroni, S. Campagna, G. S. Hanan, Chem. Sci.
2014, 5, 4800-4811.
DFT/triplets output files, respectively, for the S
0
-optimized geometry.
[5]
aE. C. Constable, A. M. W. C. Thompson, J. Chem. Soc.,
Dalton Trans. 1992, 3467-3475; bE. C. Constable, A. M.
W. C. Thompson, J. Chem. Soc., Dalton Trans. 1994,
GaussView6, GaussSum3.3[13] and Chemissian4.53[14] were used for data
analysis, visualization and surface plots. All calculations were conducted
for acetonitrile solvated complexes using a conductor like polarized
continuum (CPCM) solvation model.[15]
1409-1418; cE. C. Constable, C. E. Housecroft, M.
Neuburger, D. Phillips, P. R. Raithby, E. Schofield, E.
Sparr, D. A. Tocher, M. Zehnder, Y. Zimmermann, J.
Chem. Soc., Dalton Trans. 2000, 2219-2228; dJ. E.
Beves, E. L. Dunphy, E. C. Constable, C. E. Housecroft,
C. J. Kepert, M. Neuburger, D. J. Price, S. Schaffner,
Dalton Trans. 2008, 386-396; eE. C. Constable, C. E.
Housecroft, A. C. Thompson, P. Passaniti, S. Silvi, M.
Maestri, A. Credi, Inorg. Chim. Acta 2007, 360, 1102-
Acknowledgements
G.S.H. thanks the Natural Sciences and Engineering Research
Council of Canada for financial support. T.A. thanks the Faculté
des Arts et Sciences at Université de Montreal for their
Marguerite-Jacques-Lemay scholarship. M.T.R. thanks the DAAD
for their scholarship for bi-nationally supervised doctoral students.
This research was enabled in part by support provided by
Compute Canada. Financial support by the BayFor is appreciated
by M.T.R. and D.G.K.
1110; fS. Silvi, E. C. Constable, C. E. Housecroft, J. E.
Beves, E. L. Dunphy, M. Tomasulo, F. M. Raymo, A.
Credi, Chem. Commun. 2009, 1484-1486; gS. Silvi, E. C.
Constable, C. E. Housecroft, J. E. Beves, E. L. Dunphy,
M. Tomasulo, F. M. Raymo, A. Credi, Chem. - Eur. J.
2009, 15, 178-185; hE. C. Constable, E. L. Dunphy, C. E.
Housecroft, W. Kylberg, M. Neuburger, S. Schaffner, E. R.
Schofield, C. B. Smith, Chem. - Eur. J. 2006, 12, 4600-
4610; iA. L. Kaledin, Z. Huang, Q. Yin, E. L. Dunphy, E. C.
Constable, C. E. Housecroft, Y. V. Geletii, T. Lian, C. L.
7
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.202100092
FULL PAPER
Hill, D. G. Musaev, J. Phys. Chem. A 2010, 114, 6284-
6297; jH. Lv, J. A. Rudd, P. F. Zhuk, J. Y. Lee, E. C.
Constable, C. E. Housecroft, C. L. Hill, D. G. Musaev, Y.
V. Geletii, RSC Adv. 2013, 3, 20647-20654.
[
[
[
6]
7]
8]
M. Maestri, N. Armaroli, V. Balzani, E. C. Constable, A.
Thompson, Inorg. Chem. 1995, 34, 2759-2767.
K. E. Spettel, N. H. Damrauer, J. Phys. Chem. A 2014,
118, 10649-10662.
E. C. Constable, M. Devereux, E. L. Dunphy, C. E.
Housecroft, J. A. Rudd, J. A. Zampese, Dalton Trans.
2011, 40, 5505-5515.
[
[
9]
10]
J. Wang, G. S. Hanan, Synlett 2005, 1251-1254.
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G.
A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. V.
Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B.
Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L.
Sonnenberg, Williams, F. Ding, F. Lipparini, F. Egidi, J.
Goings, B. Peng, A. Petrone, T. Henderson, D.
Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G.
Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R.
Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A.
Montgomery Jr., J. E. Peralta, F. Ogliaro, M. J. Bearpark,
J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov,
T. A. Keith, R. Kobayashi, J. Normand, K. Raghavachari,
A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M.
Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W.
Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B.
Foresman, D. J. Fox, Wallingford, CT, 2016.
[
[
11]
12]
C. Adamo, V. Barone, J. Chem. Phys. 1999, 110, 6158-
6170.
aT. H. Dunning Jr, P. J. Hay, Mordern Theoretical
Chemistry III ed., Vol. 3, Plenum, New york, 1977; bP. J.
Hay, W. R. Wadt, J. Chem. Phys. 1985, 82, 270-283; cP.
J. Hay, W. R. Wadt, J. Chem. Phys. 1985, 82, 299-310;
dW. R. Wadt, P. J. Hay, J. Chem. Phys. 1985, 82, 284-
298.
[
13]
N. M. O'Boyle, A. L. Tenderholt, K. M. Kangner, J.
Comput. Chem. 2008, 29, 839-845.
[
[
14]
15]
S. Leonid, V 4.53 2005-2017, www.chemissian.com.
M. Cossi, N. Rega, G. Scalmani, V. Barone, J. Comput.
Chem. 2003, 24, 669-681.
8
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.202100092
FULL PAPER
Entry for the Table of Contents
Homoleptic and heteroleptic Ru(II) bis-terpyridine complexes and their methylated analogues are investigated regarding their
photophysical and electrochemical properties. The methylated complexes exhibit higher emission quantum yields and longer excited-
state lifetimes. The less negative reduction potentials and reactivity of the reduced species make the methylated complexes unsuitable
as photosensitizers in photocatalytic hydrogen evolution experiments.
9
This article is protected by copyright. All rights reserved.