Synthesis and Characterization of a Ruthenium(II) Complex for the Development of Supramolecular Photocatalysts Containing Multidentate Coordination Spheres

Article abstract of DOI:10.1002/ejic.201700565

The synthesis of the new bis-heteroleptic Ru^II complex [(tbbpy)₂Ru(dptpphz)](PF₆)₂ (7), that exhibits a tetradentate coordination sphere for a second metal centre, and its characterization are presented. The complex shows similar photostability and redox properties regarding the first ligand-centred reductions compared to its tpphz analogue. Concentration-dependent NMR investigations were performed and a dimerization constant (KD = 83 ± 5) could be calculated, which was significantly lower than that of other tpphz systems. Photophysical investigations reveal a stabilizing effect of the two electron-withdrawing 2-pyridyl substituents on π–π* transition of the phenazine sphere. The typical H₂O-induced light-switch effects have also been observed. To further highlight the potential of the tetradentate coordination site, the Zn^II adduct of 7 was prepared and preliminary studied.

Full text of DOI:10.1002/ejic.201700565

A Journal of
Accepted Article
Title: Synthesis and characterization of a ruthenium(II) complex for
the development of supramolecular photocatalysts containing
multidentate coordination spheres
Authors: Fabian L. Huber, Djawed Nauroozi, Mengele Alexander, and
Sven Rau
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700565
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10.1002/ejic.201700565
European Journal of Inorganic Chemistry
FULL PAPER
Synthesis and characterization of a ruthenium(II) complex for the
development of supramolecular photocatalysts containing
multidentate coordination spheres
Fabian L. Huber,[a] Djawed Nauroozi,[a] Alexander K. Mengele,[a] and Sven Rau*[a]
Abstract: The synthesis of the new RuII bis-heteroleptic complex
[(tbbpy)2Ru(dptpphz)](PF6)2 (7), that exhibits tetradentate
coordination sphere for second metal centre, and its
characterization are presented. The complex shows similar
towards the catalytic centre. In the resulting PMD
[(tbbpy)2Ru(bmptpphz)PdCl]2+ (bmptpphz = 2,17-bis(4-methoxy-
phenyl)tpphz, tbbpy = 4,4´-di-tert-butyl-2,2´-bipyridine) the Pd
centre coordinated in a tridentate fashion instead of a bidentate
one.[14] This led to a more stable catalytic unit. Although
cyclometalation of the Pd centre led to a higher thermodynamic
stability, the catalytic activity was lower.
a
a
a
photostability and redox properties regarding the first ligand centred
reductions compared to its tpphz analogue. Concentration
dependent NMR investigations were performed and a dimerization
constant (KD = 83 ± 5) could be calculated, which was significantly
lower compared to other tpphz systems. Photophysical
Similar to bmptpphz, 2,2´:6´,2´´:6´´,2´´´-quaterpyridine (qpy) and
its derivatives offer a stable multidentate coordination site for
metal centres. Qpy in general coordinates in a tetradentate way
to a metal centre, forming complexes in trans configuration and
a highly distorted coordination geometry.[19] Currently a large
number of qpy complexes have been developed for electro- and
photocatalysis, mostly based on abundant transition metals such
as cobalt, nickel and iron for H2 formation and CO2 reduction
and ruthenium and cobalt for water oxidation, which showed
remarkable properties regarding stability and catalytic
performance.[19–22] Despite the apparent capabilities of qpy
ligands in stabilising catalytically active metal centres, they can
also act as bridging ligands by twisting around their central bond,
adopting a helical conformation that can bind to two metal
centres. In order to restrict this conformational mobility Thummel
et al. introduced a phenanthroline motif as backbone.[23] On the
basis of the dpp (4, 2,9-di(pyridine-2´-yl)-1,10-phenanthroline)
ligand they prepared a variety of ruthenium(II) complexes
suitable for catalytic water oxidation driven by ceric ammonium
investigations reveal
a stabilizing effect of the two electron
withdrawing 2-pyridyl substituents on π-π* transition of the
phenazine sphere. The typical H2O induced light switch effects have
also been observed. To further highlight the potential of the
tetradentate coordination site, the ZnII adduct of 7 was prepared and
preliminary studied.
Introduction
Since PMDs (photochemical molecular devices) do not rely on
diffusion controlled electron transfer processes, contrary to
intermolecular systems, multiple supramolecular photocatalysts
for the light driven reduction of water and CO2 have been
developed.[1–5] Especially systems based on the tpphz, bridging
ligand (tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine), which
was first introduced by Bolger et al.,[6,7] are well investigated.[4,8,9]
In supramolecular systems it enables the desired vectorial
electron transfer from the photosensitizer towards the catalytic
centre on a sub-nanosecond scale upon light absorption.
Spectroscopic studies relying on ultrafast time resolved
absorption and resonance Raman spectroscopy were carried
out on mono- and dinuclear ruthenium and osmium
oligopyridophenazine complexes, shining light on the
mechanism of excited state electron transfers.[10–13]
nitrate.[24,25] Furthermore Wang et al. recently reported
a
copper(II) based dpp complex, that proved to be an effective
water reduction electrocatalyst. Additionally early photocatalytic
tests showed potential for future use in photocatalytic
systems.[26]
Inspired by these works, our aim was to introduce 2-pyridyl
substituents in 2- and 17-position of [(tbbpy)2Ru(tpphz)]2+,
thereby creating a ruthenium(II) dye with a modified tpphz ligand,
that contains a qpy like binding site for a potential second metal
centre (Figure 1).
Multiple efforts with the aim of tuning the properties and
enhancing the stability of tpphz complexes have been taken by
synthesising and studying various substituted tpphz
derivatives.[14–18] One approach aimed to stabilize the catalytic
centre by introducing electron donating p-anisyl substituents in
2- and 17-position and thereby relocating the electron density
[a]
Institute of Inorganic Chemistry I, Ulm University
Albert-Einstein-Allee 11, 89081 Ulm, Germany
E-mail: sven.rau@uni-ulm.de
http://www.uni-ulm.de/nawi/nawi-anorg1/
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201700565
European Journal of Inorganic Chemistry
FULL PAPER
steps, the complexed phenanthroline species 6[29] was used for
the condensation reaction. Being aware of the heat and light
sensitivity of the diamine complex, the reaction was performed
under inert conditions and exclusion of light. It is noteworthy,
that dione 5 was used in a twofold excess for this reaction in
order to minimize deamination induced condensation reactions
between the amine functionalities. Complex 7 could successfully
be isolated in a moderate yield of 57 % by size exclusion
chromatography. The successful formation of 7 was confirmed
by 1H- and 13C-NMR, as well as by high resolution mass
spectrometry (HRMS). The analysis of H,H-COSY experiments
(Figure S10) further allowed the assignment of proton signals for
the molecule in the aromatic region (Figure 2. and Scheme 2).
According to the C2-symmetry of the molecule, 7 displays two
chemically equivalent molecule sites. Gratifyingly most of the
signals are well separated, while just two sets of signals (e- and
3´- as well as h- and 5-signals) overlap with each other.
Figure 1. Structure of [(tbbpy)2Ru(dptpphz)]2+ (7). The photosensitizing unit
(red) and the qpy motif (blue) are highlighted.
In the present report, we describe the synthesis of
[(tbbpy)2Ru(dptpphz)]2+ (7, dptpphz = 2,17-di(pyridin-2-yl)tetra-
pyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine, its preliminary
photophysical and electrochemical characterization, along with
concentration dependent dimerization studies.
Results and Discussion
The synthesis of the multidentate ligand 4 was performed using
a
Pd-mediated Stille cross-coupling reaction between the
corresponding phenanthroline and pyridine building blocks. The
synthesis route is given in Scheme 1. Chlorinated phen-
anthroline
3 suitable for Stille reaction was obtained by
treatment of the corresponding dione 2 with PCl5 and POCl3
following literature known procedures.[27] The stannylated
pyridine was obtained via a lithium-bromine exchange reaction
and consecutive introduction of the stannyl moiety at the 2-
position of the pyridine.[28] The synthesized precursors 2,9-
dichloro-1,10-phenanthroline (3) and 2-(tributylstannyl)pyridine
were subjected to Stille coupling conditions using
tetrakis(triphenylphosphine)palladium(0) [Pd(PPh3)4] as catalyst
and dry toluene as solvent in order to obtain ligand 4. The
extended reaction time of 43 h as well as evelated temperatures
presumably led to the formation of Pd-colloids making the
purification of 4 tedious. Because of the additional purification
steps the compound could be obtained in moderate yields of
62 %, which is in agreement with the literature yield range of 62
to 91 %.[23,24] The subsequent oxidation of 4 at 5- and 6-position
using KBr and a mixture of nitric and sulfuric acid resulted in 2,9-
(di(pyridine-2´-yl)-1,10-phenanthroline (5). In contrast to
literature, the yellow precipitate could just be obtained after
neutralization with an aqueous NaHCO3 solution and only in
49 % yield compared to the reported 86 %. All compounds were
characterized by 1H-NMR and 4 additionally by 13C-NMR and
high resolution mass spectroscopy. All data were in agreement
with literature.[24]
Scheme 1. Synthesis route towards 2,9-di(pyridin-2-yl)-1,10-phenanthroline-
5,6-dione (5). a: 1,3-dibromopropane, nitrobenzene, 120 °C, 5 h, 94 %; b:
K3[Fe(CN)6], NaOH, H2O, 0 °C, 2 h, 30 %; c: PCl5, POCl3, 115 °C, 22 h,
quantitative; d: 2-(tributylstannyl)pyridine, [Pd(PPh3)4], dry toluene, 115 °C,
43 h, 62 %; e: KBr, H2SO4, HNO3, 85 °C, 2 h, 49 %.
Having dione 5 at hand, a condensation reaction was carried out
between 5 and 5,6-diamine-1,10-phenanthroline (Scheme 2). In
order to enhance solubility and overcome tedious purification
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10.1002/ejic.201700565
European Journal of Inorganic Chemistry
FULL PAPER
formation of the Zn complex within five minutes, evident by the
change from a poorly resolved 1H-NMR spectrum of 7 in CD3CN
to a well resolved one after the addition of Zn(BF4)2 solution
(Figure S12). However a detailed comparison of the spectra of 7
and 7 + Zn is troublesome, since the latter produces poorly
resolved spectra in CD2Cl2 (Figure S13). The 1H-NMR spectrum
in CD3CN of 7 + Zn shows 15 sharp signals in the aromatic and
two in the aliphatic region which could be assigned by H,H-
COSY measurements(Figure S14 and S15). The number of
signals indicates C2-symmetry, similar to [Zn(qpy)(OH2)2](BF4)2
reported by Constable et al..[31] Surprisingly, high resolution
mass spectrometry gave no additional insight into the structure
of the ZnII complex of 7, since no Zn containing species could be
detected.
Dimerization constant
Molecules exhibiting extended π-conjugation tend to form
dimers, a fact that might act detrimental to catalytic activity of the
system.[32] As such tpphz based complexes have been reported
to form dimers due to π-π–stacking.[9,15,32,33] For this reason the
dimerization behaviour of 7 was studied by concentration
1
dependent H-NMR on the basis of the change of the chemical
shift of selected proton signals. Thus 1H-NMR spectra were
recorded in CD2Cl2 in a concentration range from 0.23 mM to
39.89 mM, which are shown in Figure 3. Analysis of the spectra
revealed that signals attributed to terminal tbbpy ligands
underwent only small shifts compared to signals corresponding
to the dptpphz ligand. This is not surprising when taking into
account that π-π–stacking preferably occurs on the dptpphz
sphere due to the rigidity of the bridging ligand. Interestingly, all
signals that can be ascribed to dptpphz moiety are shifted down-
field when increasing the concentration of the complex. The
adduct between ZnII and 7 on the other hand did not show any
concentration dependent shifts, when varying the concentration
(Figure S13). This finding was unsurprising since, ZnII has been
used previously to break up π-π-stacking tpphz complexes.[34]
The concentration dependent shifts can be used to calculate the
equilibrium dimerization constant (KD).[15,33] For this purpose a
set of proton signals was chosen that do not overlap with other
signals and subjected to a non-linear least squares global curve
fitting approach (see ESI for details). Table 1 summarizes the
determined KD values for 7 and the reference compounds
Scheme 2. Condensation reaction between 5 and 6. Signal position notation
of 7 used in 1H-NMR experiments is given in letters and numbers.
Figure 2. Aromatic region of the 1H-NMR spectrum of 7 measured in CD2Cl2
with a complex concentration of 2.95 mM.
To highlight the potential of the tetradentate coordination site of
dptpphz, 7 was treated with a ZnII solution in MeCN at room
temperature for to coordination at the qpy motif. Since ZnII is
known to coordinate to two dpp ligands in a tridentate fashion,[30]
a 10-fold excess of ZnII was used, to ensure the formation of a
dinuclear complex. It is noteworthy, that Zn(BF4)2 was chosen as
the source of ZnII in order to avoid coordinating anions.
Gratifyingly, the formation of the Zn complex could be followed
[(tbbpy)2Ru(tpphz)]2+,
[(tbbpy)2Os(tpphz)]2+
(tpphz(tbp)2 = 3,16-di(tert-butyl-
and
[(tbbpy)2Ru((tbp)2tpphz)]2+
phenyl)-tetrapyrido[3,2-a:2′,3′-c:3″,2″-h:2′′′,3′′′-j]phenazine).
Compared to [(tbbpy)2Ru(tpphz)]2+, the dimerization constant for
7 is significantly lower (about 70 %).
1
1
by H-NMR spectroscopy, Measurement of a H-NMR spectrum
after five minutes upon the addition of Zn followed by additional
measurements at every eight minutes revealed a completed
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10.1002/ejic.201700565
European Journal of Inorganic Chemistry
FULL PAPER
Figure 3. Aromatic region of 1H-NMR spectra of 7 at different concentrations measured in CD2Cl2 at room temperature.
This behaviour might result from rotational freedom of pyridyl-
groups, inhibiting the planarity along the tpphz sphere. Another
possible explanation is steric hindrance between the terminal
tbbpy ligands of the ruthenium center and the 2-pyridyl
substituents, thereby limiting the possible overlap area of the
Photophysical properties
The absorption and emission maxima of 7, 7 + Zn and
[(tbbpy)2Ru(tpphz)]2+ as reference compound in CH2Cl2 and
MeCN are summarized in Table 2. The spectra of 7 are depicted
in Figure 4. In MeCN a broad absorption in the visible region at
441 nm can be observed. Additionally four absorption band were
detected in the UV region at 288, 335, 377 and at 397 nm. In
CH2Cl2 the absorbance underwent a slight bathochromic shift
(4 – 5 nm) with the only exception being the band at 288 nm
which did not shift.
two π–systems.
Table
1.
Dimerization
constants
of
7,
.
[(tbbpy)2Ru(tpphz)]2+
,
[(tbbpy)2Os(tpphz)]2+ and [(tbbpy)2Ru(tpphz(tbp)2)]2+
-1
KD [M ]
7
83 ± 5
[(tbbpy)2Ru(tpphz)]2+[a]
[(tbbpy)2Os(tpphz)]2+[b]
[(tbbpy)2Ru(tpphz(tbp)2)]2+[a]
289 ± 17
400 ± 40
647 ± 46
[a] Value from ref.[15] [b] Value from ref.[30]
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10.1002/ejic.201700565
European Journal of Inorganic Chemistry
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effect of the pyridine groups on the excited states of the
phenazine motif. This effect was expected due to the electron
withdrawing nature of the pyridyl substituents, since similar
effects have been reported for tpphz based ruthenium
complexes upon introduction of electron withdrawing
groups.[15,16] Lastly the broad absorption in the visible region
1
around 445 nm can be attributed to the MLCT (metal to ligand
charge transfer) from the ruthenium center to the tbbpy ligands
and dptpphz. The spectrum of 7 shows an additional absorption
band at 335 nm in MeCN and 339 nm in CH2Cl2, which is not
observable for [(tbbpy)2Ru(tpphz)]2+. This band likely
corresponds to LC (ligand centered) transitions involving the
pyridyl substituents. In case of the ZnII adduct two bands related
to the phenazine sphere are shifted to lower wavelengths,
indicating a destabilizing effect upon coordination of ZnII.
Furthermore, the spectral shape of the band assigned to the
pyridyl substituents became broader and showed a small
shoulder around 323 nm in MeCN and 326 nm in CH2Cl2. This
change might result from restriction of the rotational freedom of
the pyridine groups (Figure S19).
Figure 4. UV/vis (solid) and emission (dashed) spectra of 7 in CH2Cl2 (black)
and MeCN (red). Emission spectra were recorded at a complex concentration
of 10-4
M
For the measurement of the emission spectra the irradiation
wavelength was chosen to match the absorption maximum of
the MLCT band in the respective solvent. Complex 7 exhibits an
emission at 636 nm in MeCN and at 622 nm in CH2Cl2.
Measurement in MeCN led to a much lower intensity and a
bathochromic shift of 17 nm compared to the spectrum
measured in CH2Cl2. These findings were unsurprising as MeCN
is a more polar solvent and stabilizes the phenazine centered
“dark state” due to its higher molecular dipole moment.[17] In
order to investigate the electronic structure of 7 in more detail,
studies towards the water induced light switch effect have been
undertaken.[35] The extend of this effect was studied by
measurement of several emission spectra of 7 in MeCN
containing up to 9 v-% H2O. During this investigation the
Table 2. Absorption and emission maxima of 7 in comparison to reference
systems and its ZnII adduct.
Compound
Solvent
MeCN
DCM
Absorption λmax/nm
(ε/103M-1cm-1)]
Emission
λmax/nm
7
288 (93), 335 (57), 377
(28), 397 (36), 441 (19)
636[a]
622[b]
636[a, e]
639[b, e]
633
288 (87), 339 (60), 381
(27), 402 (33), 445 (17)
7 + Zn
MeCN
DCM
289, sh.[d] 323, 337, 367,
388, 444
emission intensity dropped by 47 % in
a linear fashion
290, sh.[d] 326, 340, sh[d]
367, 389, 448
(Figure S20). Likewise literature investigations showed 59-fold
decrease in emission quantum yield for [(bpy)2Ru(tpphz)]2+
(bpy = 2,2´-bipyridine), upon measurement of emission spectra
in water compared to MeCN.[36] The presence of the pronounced
water induced light switch effect confirmed the assumption that a
so called “dark state” may be populated.[17,36] In addition, 7 + Zn
showed only weak emission, independent of the chosen solvent
(Figure S21), which is in agreement with investigations of Turro
et al., who reported static quenching of the emission of
ruthenium tpphz complexes by various transition metals.[36]
In addition to absorption and emission spectra, excitation
spectra were recorded to investigate whether the absorption
band related to the pyridine groups contributes to the population
of the 3MLCT state and thereby to its emission (Figure S22).
This is apparently the case as the obtained spectra show a
maximum at 336 nm in MeCN and at 339 nm in CH2Cl2 which
correspond to the respective absorption spectra.
[(tbbpy)2Ru(tpphz)]2+[c]
MeCN
DCM
283 (105), sh[d] 314 (45),
379 (31), 442 (20)
285 (130), sh[d] 315 (46),
382 (26), 444 (19)
620
[15]
[a] λex/MeCN = 441 nm. [b] λex/DCM = 445 nm. [c] From ref.
[d]
sh = shoulder. [e] very weak intensity.
Since the absorption spectra of 7 show common features for
ruthenium polypyridine complexes with one tpphz based ligand,
the nature of the observed bands could be mostly clarified via
comparison to [(tbbpy)2Ru(tpphz)]2+(Figure S18).[15,16] The sharp
intense absorption around 288 nm can be assigned to π-π*
transitions centred on the tbbpy ligands and the phenantholine
moiety of dptpphz. The bands between 377 and 402 nm
correspond to π-π* transitions on the phenazine sphere.
Compared to [(tbbpy)2Ru(tpphz)]2+ this two band structure
underwent a redshift of about 17 nm, indicating a stabilizing
Photostability measurements
For ruthenium(II) polypyridyl complexes it is known that they
undergo photodecomposition by loss of one of their bipyridine
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10.1002/ejic.201700565
European Journal of Inorganic Chemistry
FULL PAPER
ligands.[37–39] In comparison ruthenium(II) tpphz complexes
exhibit a recognizable photostability compared to [Ru(bpy)3]2+,
due to the formation of a charge separated excited state located
on the tpphz ligand, hampering population of a destabilizing
ruthenium centered 3MC (metal centered) state.[40]
To gain further insight into the influence of the 2-pyridyl
substituents onto the electronic states available in 7, cyclic
voltammetry (CV) measurements were undertaken (Figure 6).
The obtained redox halfwave potentials of 7 along with reference
compounds are listed in Table 3.
Hence the photostability of 7 was investigated by measuring
absorption spectra of a 10-5 M complex solution under aerated
conditions in MeCN at different times over the course of 22 h.
During the experiment the sample was irradiated with blue light
(470 nm) by a LED stick (45 – 50 mW cm-2) with continuous air
cooling (Figure S26). The resulting spectra are depicted in
Figure S25.
All absorption bands show a slight decline in intensity. Other
than that, the spectral shape remains unchanged upon
irradiation, suggesting a fairly high photostability over 22 h for 7.
To put this findings into perspective [(tbbpy)2Ru(tpphz)]2+
(Figure S24) and [Ru(bpy)3]2+ (Figure S23) were treated the
same way as 7 and the time dependent relative decline of their
respective MLCT bands was observed and compared to 7
(Figure 5).
The obtained CV at RT shows a reversible oxidation at +0.80 V
assignable to the RuII/RuIII couple, and four ligand based
reductions in the studied potential range. For the exact
determination of Ered2 – Ered4 a CV was measured at 0 °C
(Figure S27) due to poor resolution of the respective waves at
RT. The reduction processes can be assigned in the following
order. The first reduction at -1.40 V is located on the phenazine
sphere, the second and third reduction at -1.88 and -2.03 V
respectively are assignable to the tbbpy ligands and lastly the
second reduction of dptpphz at -2.16 V.[6,16] The irreversible
shoulder around +0.57 V could not be assigned, however
diluting the sample led to a less pronounced shoulder relative to
the RuII/RuIII wave, suggesting the irreversible oxidation might
result from π-π-stacking effects in solution (Figure S28). To get
further insight on the nature of this shoulder CV measurements
of the ZnII adduct of 7 were performed (Figure 6).
Table 3. Electrochemical potentials E vs. Fc/Fc+ [V] (ferrocene/ferrocenium)
for 7 and reference compounds.
Eox
Ered1
Ered2
Ered3
Ered4
7
0.80
0.86
0.83
0.86
0.82
-1.40
-1.14
-1.39
-1.35
-1.44
-1.88
-1.64[c]
-1.88
-1.86
-1.90
-2.03
-1.87
-2.05
-2.02
-2.10
-2.16
-2.15
-2.26
-2.26
-2.27
7 + ZnII
[(tbbpy)2Ru(tpphz)]2+[a]
[(tbbpy)2Ru((tbp)2tpphz)]2+[a]
[(tbbpy)2Ru(bmptpphz)]2+[b]
[a] From ref. [15]. [b] From ref. [14]. [c] Estimated value, due to poor
resolution.
Figure 5. Decline of the MLCT absorption band of 7, [Ru(bpy)3]2+ and
[(tbbpy)2Ru(tpphz)]2+ upon irradiation with blue light of a LED stick (λ = 470 nm,
45 - 50 mW cm-2).
The MLCT band of 7 drops to 83 % relative to its initial
absorption after 22 h, thereby the photostability of 7 is on the
same scale as [(tbbpy)2Ru(tpphz)]2+ which was slightly more
photostable and only drops to 91 %. [Ru(bpy)3]2+ on the other
hand underwent
a relatively fast decomposition and its
absorption band droped to 31 % within 5 h of irradiation and
proceeded to drop further to 26 % after 22 h.
This highlights the potential usage of
7 as a dye in
supramolecular photocatalysts, since [(tbbpy)2Ru(tpphz)]2+ has
been reported to enable photocatalytic activity over elongated
time periods ins such PMDs.[4,9]
Electrochemistry
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10.1002/ejic.201700565
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Figure 6. Cyclic voltammograms of 7 and 7 + ZnII in MeCN vs. Fc/Fc+,
containing 0.1 M (nBu)4NPF6 as supporting electrolyte (v = 100 mV s-1).
The complexation of ZnII at the newly created tetradentate
coordination sphere leads to a loss of stacking behaviour and a
significant reduction in emission intensity accompanied by a red
shift of the emission wavelength. There is now significant
influence on the absorption properties detectable. In addition,
irreversible processes in the CV could be inhibited. These
results support the assumption that coordination of a metal at
the new sphere is possible and that the known ligand properties
of the tpphz scaffold are retained.
The obtained CV of the ZnII adduct indeed did not show the
previously observed irreversible shoulder of the RuII/RuIII wave,
further indicating π-π-stacking is probably the cause of this
shoulder in the CV of 7. Additionally the oxidation and reduction
potentials underwent a shift towards more positive potentials,
with the exception of Ered4
In comparison to [(tbbpy)2Ru(tpphz)]2+ the second reduction of
dptpphz (Ered4 ligands proceeds at lower potentials and is
.
The present work provides the foundation for the development of
new supramolecular photocatalysts.
)
shifted to a more positive potential by 0.10 V. At the same time
the potentials of the first reductions (Ered1, Ered2) remain basically
unchanged, which suggests that the available electronic states
in 7 determining light induced electron transfer processes and
population of the excited state located on the phenazine sphere
might proceed in a similar manner and timescale like in
[(tbbpy)2Ru(tpphz)]2+.[10] Therefore one can also assume that 7
has similar redox requirements for potential catalytic centres.
The shift of Ered4 towards more positive potentials upon the
introduction of the 2-pyridine groups may be explainable by their
electron withdrawing effect on the tpphz system. Similar effects
have been reported for other ruthenium-tpphz complexes that
contain electron withdrawing groups.[15,16] Likewise negative
shifts were reported by Kasuga et al. upon the introduction of
electron donating methyl groups.[18]
Experimental Section
Materials and Methods. Commercially available chemicals were used
as received. All solvents of technical grade were redistilled through a
rotary evaporator prior to use. Solvents of higher quality grade were used
without further purification. If not stated otherwise all reactions were
carried out under normal laboratory conditions and without the use of
absolute solvents. For all reactions carried out under inert conditions
standard Schlenk techniques were used with argon as inert gas.
2-(tributylstannyl)pyridine[28] [(tbbpy)2Ru(tpphz)](PF6)2,[4] 1-3[27] and 4,
5[29] were prepared according to literature (see ESI for synthetic details).
1H- and 13C-NMR spectra were recorded on a Bruker DRX400 with a
base frequency of 400 MHz for 1H- and 101 MHz for 13C-measurements.
The chemical shifts were referred to the signal of the respective
deuterated solvent. High resolution mass spectra were measured on a
Bruker solariX using electron spray ionization (HRMS/ESI). UV/vis
absorption spectra were recorded on a JASCO V-670 and emission
spectra on a JASCO FP-8500. In both cases the measurements were
performed at room temperature under aerated conditions using HELMA
OS precision cells made of quartz glass with a path length of 10 mm. For
photostability investigations the samples were irradiated with a LED stick
(470 nm, 45 - 50 mW cm-2) while being in their respective cells. The cells
were cooled during irradiation via a custom made air-cooling apparatus
(Figure S19). Cyclic voltammetry (CV) measurements were conducted on
This apparent ease of reduction of the second, qpy-like,
coordination sphere might be beneficial for applications of 7 in
photocatalytic applications as it could imply a higher driving
force for electron transfers.
Conclusions
a
CHI 620E electrochemical workstation, using a standard three
The mononuclear ruthenium(II) complex 7 was successfully
synthesized via a condensation reaction between the precursor
complex 6 and the 5,6-dione of dpp (5). Introduction of the
2-pyridyl groups was expected to have a stabilizing effect on the
excited states located on the tpphz sphere, which could be
confirmed by the observation of bathochromic shift of the
phenazine band in the absorption spectrum. Further
photostability investigations indicate a similar high stability of 7
compared to [(tbbpy)2Ru(tpphz)]2+, emphasizing its potential
usage as a chromophore in supramolecular photocatalytic
systems. Electrochemical investigations revealed that the first
two reductions occur at nearly the same potentials as for
[(tbbpy)2Ru(tpphz)]2+, indicating similar redox requirements for
possible catalytic centers. At the same time the fourth reduction
is shifted towards more positive potentials. Since the
introduction of aromatic substituents was expected to promote
dimerization, for which tpphz systems are known, concentration
electrode configuration with a glassy-carbon electrode (1.6 mm diameter)
as working electrode, a Pt-wire as counter electrode and a silver
electrode as reference. (n-Bu)4NPF6 (0.1 M) was used as supporting
electrolyte. Flash-column chromatography was performed on a puriFlash
430 from interchim. Alumina neutral 8 gram flash columns from RediSep
Rf were used as stationary phase.
Sample preparation for 1H-NMR dimerization experiments. Complex
7 was directly weighted out into the NMR tubes, using a concentrated
CH2Cl2 stock solution. The solvent was evaporated under slightly
elevated temperatures (around 40 °C) and the dry sample was
redissolved in 0.5 mL CD2Cl2. After addition of CD2Cl2 the tubes were
sealed with parafilm to prevent loss of the solvent.
[(tbbpy)2Ru(dptpphz)](PF6)2 (7). In
a 250 mL Schlenk flask 6
(20.2 mg,17.8 µmol) and 13.1 mg 5 (13.1 mg, 35.9 µmol) were placed
and submitted to three vacuum argon cycles and afterwards suspended
in dry EtOH (90 mL). The resulting suspension was purged for 95 min
with argon before refluxing the mixture under constant stirring for 24 h
under argon atompsphere. After the reaction was cooled to room
temperature the solvent was evaporated under reduced pressure. The
residue was dissolved in a solvent mixture of 47 wt-% chloroform,
30 wt-% acetone and 23 wt-% methanol, and filtrated through a syringe
1
dependent H-NMR studies have been conducted to determine
the dimerization constant KD. Suprisingly
7
showed
a
significantly lower concentration dependent dimerization,
probably due to steric hindrance between the tbbpy ligands and
the 2-pyridyl substituents, thereby limiting the possible overlap
area of two π-systems.
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10.1002/ejic.201700565
European Journal of Inorganic Chemistry
FULL PAPER
Commun. 1995, 20, 1799–1800.
filter, before purification via a sephadex column (sephadex LH-20). The
first red fraction was collected, the solvent removed and the red solid
dried under high vacuum to obtain 7 in 57 % (14.8 mg,10.1 µmol) yield.
1H NMR (400 MHz, CD2Cl2, c = 2.95∙10-4 M): δ = 9.73 (d, J = 8.0 Hz, 2H,
c-H), 9.41 (d, J = 7.9 Hz, 2H, d-H), 8.96 (d, J = 8.0 Hz, 2H, f-H), 8.60 (d,
J = 4.0 Hz, 2H, i-H), 8.55 (d, J = 1.7 Hz, 2H, 3-H), 8.49 (t, J = 4.6 Hz, 4H,
3´,e-H), 8.41 (dd, J = 5.4, 0.9 Hz, 2H,a-H), 8.29 (dd, J = 8.1, 5.1 Hz, 2H,
b-H), 8.06 (td, J = 8.0, 1.3 Hz, 2H, g-H), 8.01 (d, J = 6.5 Hz, 2H, 6-H),
7.92 (d, J = 5.9 Hz, 2H, 6´-H), 7.63 (dd, J = 5.9, 2.1 Hz, 2H, 5´-H), 7.47
(ddd, J = 12.2, 6.1, 2.6 Hz, 4H, 5,h-H), 1.54 (s, 18H, tBu), 1.38 (s, 18H,
tBu´) ppm. 13C NMR (101 MHz, CD2Cl2, c = 3.99∙10-3 M): δ = 163.44,
163.41, 158.03, 157.39, 157.28, 155.08, 153.92, 151.84, 151.68, 149.91,
149.38, 147.28, 140.94, 138.83, 137.54, 133.96, 130.27, 128.29, 126.20,
126.18, 126.08, 125.21, 122.27, 121.50, 121.41, 121.16, 120.74 ppm.
HRMS/ESI (+): calcd. for C70H66N12Ru 588.2292, found 588.2292
[8]
[9]
M. G. Pfeffer, T. Kowacs, M. Wächtler, J. Guthmuller, B. Dietzek, J. G.
Vos, S. Rau, Angew. Chem., Int. Ed. 2015, 6627–6631.
A. K. Mengele, S. Kaufhold, C. Streb, S. Rau, Dalton Trans. 2016, 45,
6612–6618.
[10] S. Tschierlei, M. Presselt, C. Kuhnt, A. Yartsev, T. Pascher, V.
Sundström, M. Karnahl, M. Schwalbe, B. Schäfer, S. Rau, M. Schmitt,
B. Dietzek, J. Popp, Chem. Eur. J. 2009, 15, 7678–7688.
[11] C. Chiorboli, M. T. Indelli, F. Scandola, Top. Curr. Chem. 2005, 257,
63–102.
[12] C. Chiorboli, C. A. Bignozzi, F. Scandola, E. Ishow, A. Gourdon, J.-P.
Launay, Inorg. Chem. 1999, 38, 2402–2410.
[13] C. Chiorboli, M. A. J. Rodgers, F. Scandola, J. Am. Chem. Soc. 2003,
125, 483–491.
[14] M. G. Pfeffer, L. Zedler, S. Kupfer, M. Paul, M. Schwalbe, K. Peuntinger,
D. M. Guldi, J. Guthmuller, J. Popp, S. Gräfe, B. Dietzek, S. Rau,
Dalton Trans. 2014, 43, 11676–11686.
[M-2PF6]2+, 392.4883 [M-2PF6+H]3+
.
[15] K. Ritter, C. Pehlken, D. Sorsche, S. Rau, Dalton Trans. 2015, 44,
8889–8905.
ZnII adduct (7 + ZnII). In a 100 mL flask 7 (15.1 mg, 10 µmol) was
dissolved in a minimal amount of MeCN, to this solution a concentrated
Zn(BF4)2 (24.3 mg, 0.1 mmol) solution in MeCN was added. The resulting
mixture was stirred for 4 h at RT. During the course of these 4 h a white
precipitate formed, which was filtered off. The remaining red solution was
concentrated and the ZnII adduct could be obtained via fractional
crystallization by diffusing diethyl ether into the solution. The resulting red
solid was filtered off and washed with diethyl ether, yielding 63 %
(10.7 mg, 6.3 µmol). 1H NMR (400 MHz, CD3CN): δ = 10.27 (d,
J = 8.6 Hz, 2H, d-H), 10.03 (dd, J = 8.3, 1.3 Hz, 2H, c-H), 9.26 (d,
J = 4.3 Hz, 2H, i-H), 9.14 (d, J = 8.7 Hz, 2H, e-H), 8.81 (d, J = 8.0 Hz, 2H,
f-H), 8.56 (d, J = 1.8 Hz, 2H, 3-H), 8.52 (d, J = 1.8 Hz, 2H, 3´-H), 8.42 (td,
J = 7.9, 1.5 Hz, 2H, g-H), 8.29 (dd, J = 5.4, 1.3 Hz, 2H, a-H), 8.06 (dd,
J = 8.2, 5.3 Hz, 2H, b-H), 7.99 (ddd, J = 7.7, 5.2, 0.9 Hz, 2H, h-H), 7.73
(d, J = 5.9 Hz, 2H, 6-H), 7.63 (d, J = 6.2 Hz, 2H, 6´-H), 7.51 (dd, J = 6.0,
2.0 Hz, 2H, 5-H), 7.25 (dd, J = 6.1, 2.0 Hz, 2H, 5´-H), 1.47 (s, 18H, tBu),
1.36 (s, 18H, tBu´) ppm.
[16] M. Karnahl, S. Tschierlei, C. Kuhnt, B. Dietzek, M. Schmitt, J. Popp, M.
Schwalbe, S. Krieck, H. Görls, F. W. Heinemann, S. Rau, Dalton Trans.
2010, 39, 2359–70.
[17] M. Karnahl, C. Kuhnt, F. Ma, A. Yartsev, M. Schmitt, B. Dietzek, S. Rau,
J. Popp, ChemPhysChem 2011, 12, 2101–2109.
[18] N. Komatsuzaki, R. Katoh, Y. Himeda, H. Sugihara, H. Arakawa, K.
Kasuga, J. Chem. Soc. Dalton Trans 2000, 3053–3054.
[19] Y. Liu, S. M. Ng, S. M. Yiu, W. W. Y. Lam, X. G. Wei, K. C. Lau, T. C.
Lau, Angew. Chem., Int. Ed. 2014, 53, 14468–14471.
[20] C.-F. Leung, S.-M. Ng, C.-C. Ko, W.-L. Man, J. Wu, L. Chen, T.-C. Lau,
Energy Environ. Sci. 2012, 5, 7903.
[21] Z. Guo, S. Cheng, C. Cometto, E. Anxolabéhere-Mallart, S. M. Ng, C. C.
Ko, G. Liu, L. Chen, M. Robert, T. C. Lau, J. Am. Chem. Soc. 2016, 138,
9413–9416.
[22] K. Lam, K.-Y. Wong, S.-M. Yang, C.-M. Che, J. Chem. Soc. Dalt. Trans.
1995, 1103–1107.
[23] R. Zong, R. P. Thummel, J. Am. Chem. Soc. 2004, 126, 10800–10801.
[24] G. Zhang, R. Zong, H. W. Tseng, R. P. Thummel, Inorg. Chem. 2008,
47, 990–998.
[25] R. Zong, B. Wang, R. P. Thummel, Inorg. Chem. 2012, 51, 3179–3185.
[26] Z.-J. Xin, S. Liu, C.-B. Li, Y.-J. Lei, D.-X. Xue, X.-W. Gao, H.-Y. Wang,
Int. J. Hydrogen Energy 2016, 1–6.
Acknowledgements
[27] J. Frey, T. Kraus, V. Heitz, J. P. Sauvage, Chem. Eur. J. 2007, 13,
7584–7594.
[28] C. Braun, E. Spuling, N. B. Heine, M. Cakici, M. Nieger, S. Bräse, Adv.
Synth. Catal. 2016, 358, 1664–1670.
A. M. gratefully acknowledges financial support by
Chemiefonds-Stipendium of Fonds der Chemischen Industrie.
a
[29] E. Ishow, A. Gourdon, J.-P. Launay, C. Chiorboli, F. Scandola, Inorg.
Chem. 1999, 38, 1504–1510.
Keywords: ruthenium • tetradentate ligands • pi interactions •
electrochemistry • photophysics
[30] A. N. Carolan, G. M. Cockrell, N. J. Williams, G. Zhang, D. G.
Vanderveer, H. Lee, R. P. Thummel, R. D. Hancock, Inorg. Chem. 2013,
52, 15–27.
[31] E. C. Constable, S. M. Elder, M. J. Hannon, A. Martin, P. R. Raithby, D.
A. Tocher, Dalton Trans. 1996, 2423–2433.
[1]
[2]
[3]
[4]
B. Gholamkhass, H. Mametsuka, K. Koike, T. Tanabe, M. Furue, O.
Ishitani, Inorg. Chem. 2005, 44, 2326–2336.
[32] M. G. Pfeffer, C. Pehlken, R. Staehle, D. Sorsche, C. Streb, S. Rau,
Dalton Trans. 2014, 43, 13307.
H. Ozawa, M. Haga, K. Sakai, J. Am. Chem. Soc. 2006, 128, 4926–
4927.
[33] J. Habermehl, D. Sorsche, P. Murszat, S. Rau, Eur. J. Inorg. Chem.
2016, 3423–3428.
T. A. White, S. L. H. Higgins, S. M. Arachchige, K. J. Brewer, Angew.
Chem., Int. Ed. 2011, 50, 12209–12213.
[34] M. B. Majewski, N. R. De Tacconi, F. M. MacDonnell, M. O. Wolf, Chem.
Eur. J. 2013, 19, 8331–8341.
S. Rau, B. Schäfer, D. Gleich, E. Anders, M. Rudolph, M. Friedrich, H.
Görls, W. Henry, J. G. Vos, Angew. Chem., Int. Ed. 2006, 45, 6215–
6218.
[35] A. E. Friedman, J.-C. Chambron, J.-P. Sauvage, N. J. Turro, J. K.
Barton, J. Am. Chem. Soc. 1990, 112, 4960–4962.
[5]
[6]
[7]
C. Matlachowski, B. Braun, S. Tschierlei, M. Schwalbe, Inorg. Chem.
2015, 54, 10351–10360.
[36] Y. Liu, A. Chouai, N. N. Degtyareva, D. A. Lutterman, K. R. Dunbar, C.
Turro, J. Am. Chem. Soc. 2005, 127, 10796–10797.
J. Bolger, A. Gourdon, E. Ishow, J.-P. Launay, Inorg. Chem. 1996, 35,
2937–2944.
[37] L. Petermann, R. Staehle, M. Pfeifer, C. Reichardt, D. Sorsche, M.
Wächtler, J. Popp, B. Dietzek, S. Rau, Chem. Eur. J. 2016, 8240–8253.
[38] R. F. Jones, D. J. Cole-Hamilton, Inorg. Chem. 1981, 53, L3–L5.
J. Bolger, A. Gourdon, E. Ishow, J.-P. Launay, J. Chem. Soc. Chem.
This article is protected by copyright. All rights reserved.

10.1002/ejic.201700565
European Journal of Inorganic Chemistry
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[39] P. Vicendo, S. Mouysset, N. Paillous, Photochem. Photobiol. 1997, 65,
647–655.
[40] A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. von
Zelewsky, Coord. Chem. Rev. 1988, 84, 85–277.
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10.1002/ejic.201700565
European Journal of Inorganic Chemistry
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Entry for the Table of Contents
FULL PAPER
The established chromophore
[(tbbpy)2Ru(tpphz)]2+ is modified to
contain a tetradentate coordination
site and first preliminary
Ruthenium chromophore*
Fabian L. Huber, Djawed Nauroozi,
Alexander K. Mengele, Sven Rau*
photophysical, electrochemical and
dimerization studies are undertaken.
Page No. – Page No.
Synthesis and characterization of a
ruthenium(II) complex for the
development of supramolecular
photocatalysts containing
multidentate coordination spheres
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