Synthesis, Electrochemistry, and Optical Properties of Half-Sandwich Ruthenium Complexes Bearing Triarylamine-Anthracenes

Article abstract of DOI:10.1002/ejic.201701409

The complex [CpRu^II(dppe)(taae)] {Cp = η⁵-C₅H₅, dppe = 1,2-(PPh₂)₂-C₂H₄, taae = 10-ethynyl-N,N-di-p-tolylanthracen-9-amine} has been synthesized in a multistep reactio

Full text of DOI:10.1002/ejic.201701409

A Journal of
Accepted Article
Title: Synthesis, Electrochemistry and Optical Properties of Half-
sandwich Ruthenium Complexes Bearing Triarylamine-
Anthracenes
Authors: Eduard Kovalski, Marcus Korb, and Alexander Hildebrandt
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10.1002/ejic.201701409
European Journal of Inorganic Chemistry
COMMUNICATION
Synthesis, Electrochemistry and Optical Properties of Half-
sandwich Ruthenium Complexes Bearing Triarylamine-
Anthracenes
Eduard Kovalski,[a] Marcus Korb[a] and Alexander Hildebrandt*[a]
Abstract: The complex [CpRuII(dppe)(taae)] (Cp = η5-C5H5, dppe =
1,2-(PPh2)2-C2H4, taae 10-ethynyl-N,N-di-p-tolylanthracen-9-
modified reaction protocol[13] using a solution of NBS (N-
bromosuccinimide) in tetrahydrofuran to give 9-bromo-
anthracene in 84 % yield, which had to be separated from 9,10-
dibromoanthracene by recrystallization from ethanol. 9-
bromoanthracene was converted to di(p-tolyl)aminoanthracene
by a palladium catalyzed Buchwald-Hartwig amination with di(p-
tolyl)amine. The second bromination was achieved by the
reaction of di(p-tolyl)aminoanthracene with NBS in glacial acetic
acid, which gave a faster and cleaner reaction than the literature
described methodologies (see supporting information).[14]
Bromoanthracene 1 was converted to acetylene 2 by
applying a palladium catalyzed Sonogashira C,C cross-coupling
reaction with trimethylsilylacetylene using [PdCl2(PPh3)2] as
catalyst in the presence of CuI (see Scheme 1).[15,16] The
trimethylsilyl protecting group of 2 was removed by the reaction
with tetrabutylammonium fluoride to give anthracenyl-acetylene
3 in almost quantitative yield. In the final reaction step, the
ruthenium was introduced by a chlorine-acetylene exchange
reaction using [CpRu(dppe)Cl] (Cp = η5-C5H5; dppe = 1,2-
bis(diphenylphosphino)ethane) as precursor complex (see
Scheme 1) forming compound 4.
=
amine) has been synthesized in a multistep reaction protocol
including Sonogashira C,C- and Buchwald-Hartwig C,N- cross
coupling reactions. Electrochemical and spectroscopic studies
suggest a high degree of delocalization between the ruthenium and
triarylamine termini through the anthracene bridge, forming a donor-
π-acceptor (D-π-A) dye whose π-π* transitions exhibit charge
transfer character in the monocationic oxidation state.
Introduction
More than 50% of the solar radiation reaching the Earth’s
surface lies within the near-infrared, hence considerable effort is
presently devoted to developing photosensitizers with superior
absorption properties in that energy range.[1–4] Cationic
bis(triarylamine) systems usually display intense NIR
absorptions arising from intervalence charge transfer (IVCT) in
+
addition to the intensive π-π* transitions localized at the NAr3
functionalities. Further oxidation to dications with two NAr3+ sites
quenches the IVCT band.[5–7] The properties of these IVCT
bands can be varied by the choice of substituents and a good
stability of their radical cations once the para positions are
blocked to avoid benzidine-type rearrangements.[8] Properties
similar to the triarylamines can be achieved with ruthenium half-
sandwich compounds, showing one usually reversible one-
electron oxidation per ruthenium subunit at rather low formal
potentials.[9,10] They exhibit a rich electrochemistry with a strong
substituent dependence of the redox potentials. The
combination of triarylamine and Ru-half-sandwich complexes
leads to a donor-π-acceptor (D-π-A) dye whose π-π* transitions
exhibit charge transfer character.[11] Herein, we describe the
synthesis and (spectro-)electrochemical properties of a NIR-dye
combining both triaryl and ruthenium units with anthracene as an
extended π-bridging system.
Scheme 1. Preparation of ruthenium complex 4 from bromoanthracene 1. i)
82 °C, 17 h, 6 mol-% CuI, 1 mol-% [PdCl2(PPh3)2], 6 mol-% PPh3.; ii) 66 °C,
15 h, 1.5 eq. tetrabutylammonium fluoride; iii) 111 °C, 17 h, 1.0 eq.
CpRu(dppe)Cl, 9.8 eq. NaOtBu.
Results and Discussion
Compounds 1 – 4 are stable towards air and moisture, in
the solid state as well as in solution. They have been analyzed
by NMR and IR spectroscopy as well as elemental analysis and
HR-ESI-TOF- mass spectrometry. The molecular structures of 2
(CCDC 1588283) and 3 (CCDC 1588284) in the solid state
could be determined by single crystal x-ray diffraction, however
we were not able to obtain suitable single crystals of 4. The
electrochemical behavior of 2 and 4 have been characterized by
cyclic and square wave voltammetry. In addition the electronic
properties of 4, 4+ and 42+ have been investigated by UV/Vis-NIR
9-Bromo-10-di(p-tolyl)aminoanthracene (1) is accessible
by a three step synthetic procedure including the bromination of
anthracene, subsequent Buchwald-Hartwig amination[12], and
bromination of the resulting anthracenyl-amine in 10-position.
The bromination of anthracene was performed following a
[a]
Technische Universität Chemnitz, Fakultät für Naturwissenschaften,
Institut für Chemie, Anorganische Chemie
09107 Chemnitz, Germany
E-mail: alexander.hildebrandt@chemie.tu-chemnitz.de
https://www.tu-chemnitz.de/chemie/anorg/ah/ah.php
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10.1002/ejic.201701409
European Journal of Inorganic Chemistry
COMMUNICATION
spectroelectrochemisty measurements. Furthermore, 3 and 4
were studied by fluorescence spectroscopy.
band of the 휈̃≡C−H stretching can be observed at 3299 cm-1. For
compound 4 the 휈̃C≡C stretching vibration is found at 2043 cm-1.
Suitable single crystals for X-ray diffraction of 2 and 3 were
obtained by controlled evaporation of saturated solutions of the
respective compounds in hexane. Both compounds crystallize in
a monoclinic space group (2: P21/c; 3: P21/n). The molecular
structures of compounds 2 and 3 in the solid state are depicted
in Figure 1. As common for anthracenyl units the peripherical
benzene units are distorted from the ideal hexagon geometry by
shortening the C8–C9, C10–C11, C15–C16 and C17–C18
bonds for compound 2 to approx. 1.36 Å while the C9–C10, C7–
C12, C19–C14 and C16–C17 bonds are approx. 1.42 Å in length.
The same pattern is observed for compound 3. The anthracene
rings themselves are not perfectly planar. The outer rings of the
anthracene are titled with respect to a mean plane of the middle
ring by 3.5(4) and 1.8(4)° (2) or 3.00(12) and 3.51(12) (3),
respectively. The triaryl amino unit is twisted with respect to the
anthracenyl units by 66.51(19)° (2) and 69.91(7)° (3), while the
tolyl groups themselves adopt a conformation in which the mean
planes are rotated by 53.96(18)° (2) and 53.07(8)° (3).
Conversion of 1 to 2 leads to the appearance of signals for
the SiMe3 group (0.45 ppm 1H; 0.4 ppm 13C{1H}) as well the
resonances for the C,C triple bond carbon atoms at 107.1 and
101.7 ppm respectively. The deprotection of the acetylene from
2 to 3 can be monitored by the disappearance of the signals for
SiMe3 group and the observation of the ≡C-H proton signal at
4.05 ppm. Introduction of the Ru-half-sandwich moiety in
compound 4 is accompanied with the observation of resonance
1
signals for the Cp unit at 4.98 ppm in H NMR as well as 83.1
ppm in 13C{1H} NMR and the dppe ligand (multiplets at 2.26–
2.33 and 2.43–2.55 ppm in 1H; 29.9 13C{1H}; 86.0 ppm 31P{1H}).
The deprotection and subsequent complexation of the acetylene
unit for 2, 3 and 4 can in addition be monitored during the
reaction by IR spectroscopy. Whereby the 휈̃
vibration is
C≡C
observed with a moderate strength at 2140 cm-1 in SiMe3
protected 2 while in 3 this band is shifted to 2093 cm-1 and is
only observed as a very weak absorption, however a strong
Figure 1. ORTEP. (50 % probability level) of the molecular structure of 2 (left) and 3 (right) with the atom numbering scheme. All hydrogen atoms have been
omitted for clarity. Selected bond distances (Å), angles (°) and torsion angles (°) 2: Si1–C4 1.835(6), C4–C5 1.204(8), C5–C6 1.430(8), C13–N1 1.426(7), C7–C8
1.421(8), C8–C9 1.361 (7), C7–C12 1.428(8), C9–C10 1.412(8); C6–C5–C4 178.5(6), C5–C4–Si1 177.3(5), C12–C13–N1 119.4(5), C20–N1–C27 122.4(5); C12–
C13–N1–C20 -66.6(7), C28–C27–N1–C20 -27.8(7). 3: C1–C2 1.177(3), C2–C3 1.442(3), C10–N1 1.433(2), C4–C5 1.426(3), C5–C6 1.357 (3), C4–C9 1.429(3),
C6–C7 1.408(3); C1–C2–C3 178.0(3), C9–C10–N1 118.64(19), C17–N1–C24 123.66(17); C9–C10–N1–C24 -72.2(3), C18–C17–N1–C24 -34.1(3).
For compound 4 three individual redox processes can be
observed of which the first two are reversible one-electron
events, while the 3rd process appears to be irreversible.
Comparison with literature known values for the Ru-half-
sandwich moiety (typically -5 – -595 mV)[10,18] revealed that the
1st redox event is most likely the oxidation of Ru2+ to Ru3+ while
subsequently the triarylamine functionality[19–21] is oxidized.
However, it is noticeable that the amine oxidation occurs at a
much lower potential than in compound 2. Moreover the 3rd
irreversible process, associated with an oxidation process at the
anthracene core, is shifted about 350 mV cathodically compared
to unsubstituted anthracene, despite the fact that 42+ already
carries two positive charges which should result in an anodic
shift of the process. This behavior may be caused by a
delocalization of the electron density and hence the electron
removal of delocalized π-orbitals involving all three functional
groups resulting in a better stabilization of the positive charges.
The electrochemical behavior of compound
4 was
investigated by cyclic voltammetry using an anhydrous
dichloromethane solution of [NnBu4][B(C6F5)4][17] (0.1 mol·L-1) as
supporting electrolyte at 25 °C (see Figure 2). Since compound
4 exhibits three redox active functionalities, for comparison
anthracene as well as compound 2 (bearing the triarylamino and
anthracene moiety) have been measured under the same
conditions. The results at 100 mV/s sweep rates are
summarized in Table 1 (additional information on the reversibility
can be found in the supporting information, see Figure SI3).
ipa
10 µA
ipc
-800 -600 -400 -200
0
200 400 600 800 1000
E / mV (vs FcH/FcH+)
Figure 2. Cyclic voltammogram of
4
in dichloromethane using [NnBu4]
[B(C6F5)4] (0.1 mol·L-1) as supporting electrolyte vs ferrocene/ferrocenium.
Potential range -750 mV – 1000 mV (black), -550 – 350 mV (dotted).
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10.1002/ejic.201701409
European Journal of Inorganic Chemistry
COMMUNICATION
Table 1. Cyclic voltammetry data of 4, anthracene and 2.[a]
using dichloromethane solutions of the analyte (2.0 mmol·L-1)
containing [NnBu4][B(C6F5)4] (0.1 mol·L-1) as supporting
electrolyte.[25,26] During the measurements, the applied cell
potential was increased stepwisely (step width: 15 mV, 50 mV or
100 mV). The potential depending UV/Vis-NIR spectra are
Compound
E1°′ / mV[b]
(ΔEp / mV)[c]
E2°′ / mV[b]
(ΔEp / mV)[c]
E3°′ / mV[b]
(ΔEp / mV)[c]
4
-235 (61)
815 (72)
355 (84)
95 (74)
-
470 (101)[d]
depicted in Figure
3 and the band characteristics are
Anthracene
-
-
summarized in Table 2. The potential increase from -100 to 265
mV vs Ag/AgCl[27] resulted in the mono-oxidation of 4. From the
electrochemical investigations it is concluded that the 1st
oxidation process is mainly located at the ruthenium redox
center, however a partial delocalization with the anthracene π-
system seems plausible. Therefore, an electron transfer
interaction between the amine nitrogen and the ruthenium
moiety via the anthracene backbone is expected in the low
energy region of the spectra of 4+.
2
1030[d]
[a] Potentials versus FcH/FcH+[22–24] (scan rate 100 mV s-1) at a glassy-carbon
electrode of 1.0 mmol L-1 solutions of the analytes in dichloromethane that
contained 0.1 mmol L-1 of [NnBu4] [B(C6F5)4] as supporting electrolyte at 25 °C.
[b] E°′ = formal potential. [c] ΔEp = difference between the cathodic and anodic
peak potentials |Epc – Epa| [d] Epa irreversible process.
To further investigate the redox behavior of 4 spectroelecto
UV/Vis-NIR measurements have been carried out within an
OTTLE[25] (optically transparent thin-layer electrochemical) cell,
5000 L·mol-1·cm-1

5000 L·mol-1·cm-1

42+
43+
4
4+
43+
decomposition
4+
42+
500
1000
1500
2000
500
1000
1500
2000
 / nm
 / nm
Figure 3. Spectroelectrochemical measurements of 4 within an OTTLE cell at 25 °C in dichloromethane (2.0 mmol·L-1) at rising potentials (Left bottom: -100 –
265 mV; left top: 265 to 700 mV; right bottom 700 – 1050 mV; right top 1050 – 1300 mV vs Ag/AgCl); supporting electrolyte 0.1 mmol·L-1 [NnBu4][B(C6F5)4];
arrows indicate rising or decreasing intensities.
41]
Increasing the applied potential up to 700 mV resulted in the
[a]
Table 2. UV-Vis/NIR measurements of 4, 4+, 42+ and 43+
.
formation of 42+. During this process the NIR absorption
disappeared, which also confirms the characteristic of former
band as a Ru/N charge transfer absorption. Interestingly upon
the 3rd oxidation process a new NIR absorption is observed
which resembles the one found in 4+, is however shifted towards
lower energy (1380 nm; 43+) with a lower intensity (3500
Compd. /
λ / nm
origin of transition
oxidation state
(ϵmax / L·mol·cm-1)
π-π* (with charge transfer
4
component from the tolylamine to
505 (17400)
anthracene)[11,14,28]
π-π*[10,11,28]
685 (6600)
L·mol-1·cm-1) and a higher Δ휈̃ value (2350 cm-1). Compound
1/2
4+
43+ was not stable under the measurement conditions and
decomposed slowly over time. This decomposition process was
accelerated upon further potential increase, which reaffirms the
findings during the cyclic voltammetry experiments and
CT-transition (involving the
1195 (4150)
anthracene π system)
42+
LMCT
LMCT
385 (7580)
350 (15200)
740 (14600)
comparable
ruthenium
aryl
alkynes.[10,35] Within
the
π-π*[10,11,28]
spectroelectro-NIR measurements the decomposition was
accompanied with a decrease of the intensity of the charge
transfer excitation at 1380 nm over time.
43+
CT-transition (involving the
anthracene π system)
1380 (3500)
[a] In dichloromethane containing 0.1 mol L-1 of [NnBu4][B(C6F5)4] as
supporting electrolyte at 25 °C.
Within the spectra an absorption band at 1195 nm could
be observed, which is a typical range for charge transfer
excitations. Compared to other IVCT bands in unsymmetrical
mixed valent systems[19,28–32] and ruthenium/iron half-sandwich
aryl alkynes[10,31,33] the observed intensity of the charge transfer
excitation is rather high (4150 L·mol-1·cm-1) with a small
FWHM (full width at half maximum, Δ휈̃ ) of 2090 cm-1 which
½
hints at a considerable degree of delocalization.[15,34–37] The
IVCT band of 4+ is red-shifted compared to related (symmetrical)
bi- and trinuclear complexes and shows lower intensities.[11,20,38–
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10.1002/ejic.201701409
European Journal of Inorganic Chemistry
COMMUNICATION
2
3
4
analogous compounds featuring similar functional units such as
2 (the triarylamine unit is oxidized at 355 mV) and anthracene
(815 mV) which were measured under the same conditions. This
cathodic potential shift hints to a delocalization of the electron
density and thus three fold oxidation of a delocalized π orbital
involving all three redox active units. This was supported by
spectroelectrochemical UV/Vis-NIR measurements which
demonstrated charge transfer excitations within this π system for
4+ and 43+. Fluorescence spectroscopy of 4 on the other hand
only showed a negligible influence of the ruthenium moiety on
the emission wavelength and decreased emission intensities.
1.0
0.8
0.6
0.4
0.2
1.0
0.8
0.6
0.4
0.2
0.0
0.0
400
600
 / nm
Acknowledgements
Figure 4. UV-Vis absorption (left) and fluorescence spectra (right) of 2 (blue),
3 (red) and 4 (green) in dichloromethane (1.0·10-5 mol·L-1).
E. K. thanks the Saxon State Ministry for Science and the
Arts for a Landesgraduierten fellowship. M. K. likes to thank the
Fonds der Chemischen Industrie for their generous financial
support. We are very grateful to Prof. Heinrich Lang for the
supply of laboratory space and equipment. We also wish to
thank Tatiana A. Shumilova and Prof. Evgeny A. Kataev for their
help with the recording of the fluorescence spectra and Prof.
Stefan Spange for providing access to his luminescence
spectrometer.
The optical properties of compounds 2, 3 and 4 were
studied by UV-vis and fluorescence spectroscopy. Figure 4
shows the absorption and emission spectra of the three
substances in CH2Cl2. Absorption peaks of the solutions were
observed at 385, 405 and 470 nm (2) and 380, 400 and 465 nm
(3), as well as 500 nm (4) respectively. The similar absorption
behavior of compounds 2 and 3 can be attributed to the
electronically neutral protective trimethylsilyl group which doesn’t
change the absorption properties of
2 compared to the
unprotected molecule 3. Furthermore, the fluorescence spectra
of 2 (585 nm), 3 (590 nm) and 4 (595 nm) showed a similar
emission behavior in regard to the emission wavelength and
Keywords: alkynyl complexes • electrochemistry • charge
transfer • NIR-spectroelectrochemistry • ruthenium
display
a bathochromic shift compared to 1 .
(535 nm)[14]
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synthesized in multistep reaction protocol involving
complex
[CpRuII(dppe)(taae)]
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