Full Paper
doi.org/10.1002/chem.202004486
Chemistry—A European Journal
ure S4). The obtained value matches very well with the calcu-
lated distance between the two Ru(bda) centers (d=27.6 ꢂ) of
the geometry-optimized structure of dimer 1 (Figure S3).
Electrochemical studies of dimer 1 were performed by cyclic
voltammetry (CV) and differential pulse voltammetry (DPV) in
aqueous solution under acidic (pH 1, triflic acid) and neutral
conditions (pH 7, phosphate buffer) containing 40% 2,2,2-tri-
fluorethanol (TFE) as a non-coordinating co-solvent for solubili-
zation. The redox properties of dimer 1 under the applied ex-
perimental conditions are summarized in Table S2 and com-
pared with those of the mononuclear reference complex
[Ru(bda)(pic)2], and the voltammograms are displayed in Fig-
ures S5–S8.[22,53] In acidic aqueous solution (pH 1), dimer 1 dis-
played two subsequent oxidation processes which can be as-
III
II
III
signed to the Ru2 /Ru2 and Ru2IV/Ru2 redox couples for each
metal center in accordance to the redox processes reported
for [Ru(bda)(pic)2] (Figure S5 and S6).[22] Due to the overlap
Figure 1. Spectroelectrochemistry of dimer 1 in CH3CN/H2O (4:6; pH 7, phos-
phate buffer) at c=2.4ꢃ10À4 M. The applied voltages for the generation of
V
with the water oxidation current, the oxidation to Ru2 could
III
IV
Ru2II (red), Ru2 (green) and Ru2 (purple) are indicated.
not be observed as it has previously been described for other
dinuclear non-cyclic Ru complexes.[23,24] Under neutral condi-
tions (pH 7), three two-electron oxidation events were ob-
served for dimer 1 which correspond to the oxidation poten-
ther spectral changes were observed upon increasing the po-
tential because a strong catalytic current is associated with the
formation of RuV and subsequent oxidation of water.[39,41]
tials Ru2 /Ru2 , Ru2IV/Ru2 and Ru2 /Ru2 for each metal center
(Figure S7). Compared to the mononuclear complex
[Ru(bda)(pic)2], the incorporation of the axial calix[4]arene in
dimer 1 affected under both acidic and neutral conditions only
III
II
III
V
IV
Chemical water oxidation under acidic conditions
III
II
the Ru2 /Ru2 redox potential, resulting in an approximately
50 mV higher oxidation potential for this dinuclear cyclic
Ru(bda) complex.
The catalytic performance of dimer 1 was first investigated in
the presence of cerium ammonium nitrate (CAN) as a sacrificial
oxidant in aqueous acidic solutions (pH 1, triflic acid) with 40%
II
The UV/Vis absorption spectra of dimer 1 at the Ru2 state
under both acidic and neutral conditions displayed a very
strong absorption band at around 250 nm, which belongs to
an electronic transition along the long molecular axis (1La) for
the axial calix[4]arene ligand (Figure S9).[54] The transition along
the short molecular axis (1Lb) of the calix[4]arene ligand is usu-
ally very weak and is presumably overlapping with the ligand
centered p–p* transitions at around 300 nm.[55] The weak ab-
sorption bands in the visible region can be attributed to sever-
al metal-to-ligand charge transfer (MLCT) absorptions,[56,57]
where the band at around 360 nm is characteristic for the tran-
sition from the Ru d-orbital to the p*-orbital of the axial
ligand, while the less energetic bands at 450 and 500 nm can
be explained by the transition from the Ru d-orbital to the p*-
orbital of the equatorial bda ligand.[41] Compared to the mono-
nuclear [Ru(bda)(pic)2], the high-energy MLCT band at 360 nm
is bathochromically shifted for dimer 1 indicating a less donat-
ing character of the axial calix[4]arene ligand.[57]
acetonitrile as co-solvent due to its oxidative stability.[58,59]
A
large excess of CAN was applied in these experiments to
ensure a direct dependence of the overall catalytic rate on the
catalyst concentration.[19] After the injection of the catalyst so-
lution into the acidic CAN mixture, the subsequent pressure in-
crease due to oxygen evolution was monitored by the at-
tached pressure sensors. The gas composition at the end of
each run was then analyzed by gas chromatography (GC; for
experimental details and reaction conditions see the Support-
ing Information). The catalytic performance of dimer 1 was
then screened in different acetonitrile/water ratios to deter-
mine the optimal experimental conditions (Figure S11a). The
amount of acetonitrile used as co-solvent strongly correlates
with the overall catalytic activity, which can be attributed to its
competitive binding to the seventh coordination site of the
ruthenium center compared with water.[58,60] For dimer 1 the
highest catalytic activity was achieved in aqueous mixtures
containing 40% acetonitrile, whereas higher acetonitrile con-
tent led to lower catalytic activity in the initial first two sec-
onds of catalytic water oxidation (Figure S11b). Hence, all fur-
ther catalytic experiments were performed in CH3CN/H2O 4:6
solvent mixtures and compared to the mononuclear complex
[Ru(bda)(pic)2][22] under the same conditions.
To investigate the spectral changes of dimer 1 upon oxida-
tion, spectroelectrochemistry in CH3CN/H2O 4:6 at pH 7 (phos-
phate buffer) was performed (Figure 1). Upon increasing the
potential from 500 mV to approximately 800 mV, the MLCT
bands at 350 nm and 450–500 nm are bleached with concomi-
tant appearance of a new band at 700 nm which is characteris-
III
tic for the Ru2 state.[41,43] A further increase of the potential to
Concentration-dependent water oxidation experiments were
conducted to determine the turnover number (TON) and the
turnover frequency (TOF) for the dinuclear cyclic complex
dimer 1 and [Ru(bda)(pic)2] as a mononuclear reference under
approximately 960 mV results in a decrease of the transition at
700 nm and a broad absorption at around 520 nm arises for
IV
the formation of the Ru2 state (Figures 1 and S10).[43] No fur-
Chem. Eur. J. 2020, 26, 1 – 8
3
ꢁ 2020 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
&
&
These are not the final page numbers! ÞÞ
Noll, Niklas
