Aqueous Persistent Noncovalent Ion-Pair Cooperative Coupling in a Ruthenium Cobaltabis(dicarbollide) System as a Highly Efficient Photoredox Oxidation Catalyst

Article abstract of DOI:10.1021/acs.inorgchem.1c00751

An original cooperative photoredox catalytic system, [RuII(trpy)(bpy)(H2O)][3,3′-Co(1,2-C2B9H11)2]2 (C4; trpy = terpyridine and bpy = bipyridine), has been synthesized. In this system, the photoredox metallacarborane catalyst [3,3′-Co(1,2-C2B9H11)2]- ([1]-) and the oxidation catalyst [RuII(trpy)(bpy)(H2O)]2+ (C2′) are linked by noncovalent interactions and not through covalent bonds. The noncovalent interactions to a large degree persist even after water dissolution. This represents a step ahead in cooperativity avoiding costly covalent bonding. Recrystallization of C4 in acetonitrile leads to the substitution of water by the acetonitrile ligand and the formation of complex [RuII(trpy)(bpy)(CH3CN)][3,3′-Co(1,2-C2B9H11)2]2 (C5), structurally characterized. A significant electronic coupling between C2′ and [1]- was first sensed in electrochemical studies in water. The CoIV/III redox couple in water differed by 170 mV when [1]- had Na+ as a cation versus when the ruthenium complex was the cation. This cooperative system leads to an efficient catalyst for the photooxidation of alcohols in water, through a proton-coupled electron-transfer process. We have highlighted the capacity of C4 to perform as an excellent cooperative photoredox catalyst in the photooxidation of alcohols in water at room temperature under UV irradiation, using 0.005 mol % catalyst. A high turnover number (TON = 20000) has been observed. The hybrid system C4 displays a better catalytic performance than the separated mixtures of C2′ and Na[1], with the same concentrations and ratios of Ru/Co, proving the history relevance of the photocatalyst. Cooperative systems with this type of interaction have not been described and represent a step forward in getting cooperativity avoiding costly covalent bonding. A possible mechanism has been proposed.

Full text of DOI:10.1021/acs.inorgchem.1c00751

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Article
Aqueous Persistent Noncovalent Ion-Pair Cooperative Coupling in a
Ruthenium Cobaltabis(dicarbollide) System as a Highly Efficient
Photoredox Oxidation Catalyst
Isabel Guerrero, Clara Vinas, Xavier Fontrodona, Isabel Romero,* and Francesc Teixidor*
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ABSTRACT: An original cooperative photoredox catalytic
system, [RuII(trpy)(bpy)(H2O)][3,3′-Co(1,2-C2B9H11)2]2 (C4;
trpy = terpyridine and bpy = bipyridine), has been synthesized.
In this system, the photoredox metallacarborane catalyst [3,3′-
Co(1,2-C2B9H11)2]− ([1]−) and the oxidation catalyst [RuII(trpy)-
(bpy)(H2O)]2+ (C2′) are linked by noncovalent interactions and
not through covalent bonds. The noncovalent interactions to a
large degree persist even after water dissolution. This represents a
step ahead in cooperativity avoiding costly covalent bonding.
Recrystallization of C4 in acetonitrile leads to the substitution of
water by the acetonitrile ligand and the formation of complex
[RuII(trpy)(bpy)(CH3CN)][3,3′-Co(1,2-C2B9H11)2]2 (C5), struc-
turally characterized. A significant electronic coupling between C2′
and [1]− was first sensed in electrochemical studies in water. The
CoIV/III redox couple in water differed by 170 mV when [1]− had
Na+ as a cation versus when the ruthenium complex was the cation.
This cooperative system leads to an efficient catalyst for the photooxidation of alcohols in water, through a proton-coupled electron-
transfer process. We have highlighted the capacity of C4 to perform as an excellent cooperative photoredox catalyst in the
photooxidation of alcohols in water at room temperature under UV irradiation, using 0.005 mol % catalyst. A high turnover number
(TON = 20000) has been observed. The hybrid system C4 displays a better catalytic performance than the separated mixtures of
C2′ and Na[1], with the same concentrations and ratios of Ru/Co, proving the history relevance of the photocatalyst. Cooperative
systems with this type of interaction have not been described and represent a step forward in getting cooperativity avoiding costly
covalent bonding. A possible mechanism has been proposed.
structural modification upon ET. The challenge is to generate
charge separation by ET and maintain it by avoiding
regeneration of the initial reagent.
INTRODUCTION
■
The development of new photocatalytic methods for organic
transformations is an important challenge given the significant
environmental and economic impact that this entails. Solar
energy combined with water are widely considered to be good
alternative energy sources for the development of nonfossil-
based fuels.1,2 Great effort has been dedicated to the catalysis
activation of organic molecules by light.3−5 The latter can be
considered to be a good reagent for environmentally friendly,
green chemical synthesis; unlike many conventional reagents,
light is an abundant energy form for driving chemical reactions
in a sustainable future.
Cooperative photoredox catalysis supposes a new progress in
this field and refers to a first catalyst that is photochemically
active, while the second is redox-active in the absence of light.9
This strategy allows multielectron photoredox catalysis with
the help of a second redox catalyst, avoiding the single electron
transfer (SET) imposed by the photoredox catalyst. The union
in one single complex of a photoredox catalyst and a transition-
metal catalyst could result in higher efficiency and may lead to
new selective light-induced organic transformations.
The activation of molecules by light depends on the ability
of metal complexes and organic dyes to participate in electron-
transfer (ET) processes with organic molecules upon
excitation by light.6,7 The octahedral compound [Ru(bpy)3]2+
(bpy = bipyridine)8 has been one of the most studied
photoredox catalysts in the literature; it participates in ET by
an outer-sphere mechanism that supposes no significant
Received: March 15, 2021
Published: June 7, 2021
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Currently, systems that are used for the light-induced
oxidation of substrates are based on chromophore−catalyst
dyads of ruthenium polypyridyl compounds,10−12 where one
compound acts as the light-harvesting antenna and the second
metal complex is used as catalyst, to activate a water molecule
or an organic substrate. In these cases, the problem is the
charge trapping between the two moieties influenced by a
bridging ligand that connects both parts, leading to a reduction
of the catalytic activity observed in some systems. Moreover,
the synthesis of these systems can be laborious and expensive.
Oxidized raw materials are needed as bulk chemicals in
different fields, including polymer and fine chemicals, among
others.13 So, the selective oxidation of alcohols is an essential
process in organic chemistry14−17 because this oxidation can be
used to produce aldehydes, acids, ketones, etc.
oxidation catalyst would lead to the formation of a stable and
efficient cooperative photoredox catalyst.
With all this in mind, in this work we present the synthesis
of an air-stable ruthenium cobaltabis(dicarbollide) compound,
[RuII(trpy)(bpy)(H2O)][3,3′-Co(1,2-C2B9H11)2]2 (C4, where
trpy = terpyridine and bpy = bipyridine; Scheme 1), where the
[Ru-OH2] cation belongs to the family of redox oxidation
catalysts,35,36 together with their spectroscopic and electro-
chemical characterization. The recrystallization of C4 in
acetonitrile leads to formation of the complex [RuII(trpy)-
(bpy)(CH3CN)][3,3′-Co(1,2-C2B9H11)2]2 (C5), which has
been structurally characterized. C4 has been tested as a
cooperative photoredox catalyst in the oxidation of aromatic
and aliphatic alcohols in water showing high performance,
using very low catalyst loads.
It is also relevant in hydrogen-based energy technologies
because this reaction involves a two-electron proton-coupled
process that could be recombined on a cathode for hydrogen
production.18,19
Scheme 1. Schematic Representation of the Ruthenium
Cobaltabis(dicarbollide) Complex C4
It is well-known that carboranes interact with light20 and
that they have been studied as catalysts in different processes.21
The use of carboranes in supramolecular chemistry is a field to
be explored for their particular characteristics.22−25 The
metallacarborane sandwich compound [3,3′-Co(1,2-
C2B9H11)2]− ([1]−) has many possibilities of forming
hydrogen bonds, e.g., Cc−H···O or Cc−H···X (X = halogen)
as well as dihydrogen bonding Cc−H···H−B and B−H···H−N
(Cc stands for the cluster C atoms).26−28 It is well-known that
[1]− is highly stable in water but at low concentration forms
aggregates (vesicles and micelles).29,30 Dihydrogen interactions
participate in the self-assembling formation.31 These supra-
molecular interactions appear to be significant in the processes
of ET and therefore in the performance and efficiency of
photocatalytic systems.32 Recently, we have shown that
cobaltabis(dicarbollide), [1]−, and its chloro derivatives, acting
as both catalyst and photosensitizer, are highly efficient in the
photooxidation of alcohols in water, through SET processes.33
A high performance of Na[1] in the photooxidation of alcohols
is observed, using a catalyst load of 0.01 mol % and reaching
TON = 10000, in some cases.33 We have also supported the
cobaltabis(dicarbollide) catalyst on silica-coated magnetite
nanoparticles.34 This system has proven to be a green and
sustainable heterogeneous catalytic system, highly efficient and
easily reusable for the photooxidation of alcohols in water. In
general, the SET processes carry out the transformation of a
limited number of substrates, and in some cases undesirable
byproducts are generated. As we have indicated, the results
offered by [1]− in SET have been excellent, taking into account
the low catalyst loading, the selectivity, and the high degree of
conversion. However, we wanted to know more about their
behavior, whether cooperative action offered any advantage,
whether a cooperative effect really existed, and whether the
noncovalent interactions demonstrated in water had a history.
To this end, the studies with alcohol oxidations already studied
only with [1]− would allow us to prove the features that have
not been demonstrated or found in other related systems.
Once these points are known and demonstrated, we will be in
the position to develop unprecedented systems.
RESULTS AND DISCUSSION
■
Synthetic Strategy, Structure, and Redox Character-
ization. The synthetic process used for preparation of the
compounds is outlined in Scheme 2. The aqua complex C4 is
obtained by dissolving the chloridoruthenium(II) complex
[RuII(trpy)(bpy)(Cl)]Cl (C2), obtained following the method
described in the literature,37 in a mixture of water/acetone
(1:1) in the presence of Ag[1], C3, under reflux. This latter
compound was prepared by a cationic exchange resin from
Cs[1], as described in the literature.38 After filtration of AgCl,
the complex was isolated. The recrystallization of C4 in
acetonitrile led to the substitution of water by the acetonitrile
ligand, and then the formation of complex C5 took place. It is
worth mentioning that, to the best of our knowledge, this is the
first example of a molecular aqua ruthenium(II) complex
containing two cobaltabis(dicarbollide) anions as counterions.
Suitable single crystals of C5 were grown by the
recrystallization of C4 in acetonitrile. All attempts to obtain
crystals of C4 in water or noncoordinating solvents were
unsuccesful.
X-ray diffraction analysis was used to solve the crystal
structure of complex C5 (Figure 1). The main crystallographic
data, together with selected bond distances and angles, are
reported in Tables S1 and S2. The cationic moiety of
compound C5 is formed by a ruthenium(II) complex, where
the Ru center displays an octahedral distorted type of
coordination, in which the trpy ligand is bonded to the RuII
cation in a meridional manner and the bpy ligand is
coordinated in a bidentate fashion. The sixth coordination
site is occupied by the monodentate acetonitrile ligand. All
bond distances and angles are within the expected values for
these types of complexes.39−41 For instance, it is interesting to
As mentioned above, [1]− can form ion-pair complexes
through hydrogen and dihydrogen interactions.31 Then, the
design of a new hybrid system where cobaltabis(dicarbollide)
can be linked by noncovalent interactions to a redox-active
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Scheme 2. Synthetic Strategy for the Preparation of Complexes C4 and C5
The C1A−C2A (1.661 Å) and C1B−C2B (1.642 Å) bond
lengths are different in the cisoid anion, while the C1C−C2C
bond lengths are similar (1.649 Å) in the transoid rotamer
anion. The packing in Figure S1b displays the arrangement
adopted between the metallacarborane anions and cationic
moieties.
The IR spectrum of complex C4 shows vibrations around
2900 cm−1 that can be assigned to the νC−H stretching modes
for the aromatic rings, while the bands around 3030 cm−1
correspond to the νC−H stretching of the Cc−H bonds in the
different rotamers. A band over 3500 cm−1 can be seen that
corresponds to the νO−H stretching of the aqua ligand. We have
also observed a significant vibration around 2530 cm−1 that
corresponds to the νB−H stretching mode for the B−H bond
Figure 1. Crystal structure of complex C5.
note that the Ru−N2 bond length in the bpy ligand, where the
N2 atom is placed trans to the pyridyl N4 atom of trpy, is
longer (2.069 Å) than the analogous distance Ru−N1 (2.033
Å), where the N1 atom is trans to the N6 atom of the
acetonitrile ligand. This evidence highlights the stronger trans
influence of the pyridine ring with respect to the acetonitrile
ligand. The N3−Ru−N4 and N4−Ru−N5 angles are 79.88°
and 79.13°, respectively, less than the 90° expected for an ideal
octahedral geometry. This may be due to the geometry of the
trpy ligand, which defines the equatorial plane of the structure;
as a consequence, the other two equatorial angles, N3−Ru−N2
and N2−Ru−N5, are larger than 90°. The anionic moiety is
formed by two anionic metallabis(dicarbollide) units, which
display different rotamers: one cisoid and the other transoid.
1
(Figure S2). The 1D and 2D H NMR spectra of complexes
C3 and C4 were recorded in acetone-d6 and are displayed in
1
Figures S3 and S4. The H NMR spectrum of C4 shows two
sets of signals (Figure S4a): one in the aromatic region
corresponding to the protons of the bipyridine and terpyridine
ligands of the [Ru−OH2]2+ cation and the second set in the
aliphatic region that can be assigned to the Cc−H of the
cobaltacarborane anions, whose resonances appear near δ =
3.98 ppm. The 1H{11B} NMR spectrum exhibits the
resonances of H atoms bonded to B atoms over a wide
chemical-shift range in the region from δ = 1.50 to 3.5 ppm.
These resonances appear as broad bands (Figure S4b).
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The 11B{1H} NMR resonances featured the typical pattern
of the pristine cobaltabis(dicarbollide) cluster in the range
from δ = 15 to −22 ppm (Figure S4c).42
The UV−vis spectra of Ag[1], C3, and C2′, together with
C4, are displayed in Figure S5a,b. The UV−vis spectrum of
Ag[1], C3 (Figure S5a), shows one strong absorption band at
286 nm and others with less intensity at 210, 333, and 450 nm,
in agreement with the literature.43,44 The spectrum of the aqua
complex C2′ (Figure S5b) shows ligand-based π−π* bands
and dπ(Ru)−π*(L) metal-to-ligand charge-transfer (MLCT)
transitions, as is usual for these complexes.45 In Figures S5b
and 2 is shown the UV−visible spectrum corresponding to
Figure 3. CV for complexes C4 (red line) and Na[1] (blue line)
registered in a phosphate buffer (pH = 7.12) versus Ag/AgCl. The
right red and blue insets show dI/dE to better appreciate the position
of the couple CoIV/CoIII. Further proof of this will be shown later
when dealing with the C4 precedents that we will henceforth call the
C4 history.
Figure 2. UV−vis spectra of complex C4 in CH2Cl2 (dotted line),
phosphate buffer at 7.02 pH (dash-dotted line), and CH3CN (solid
line).
complex C4, which exhibits the ligand-based π−π* bands of
the cationic part below 350 nm that are partially eclipsed by
strong absorptions corresponding to the cobaltabis-
(dicarbollide) anionic moiety. Above 350 nm, the spectra
show less intense bands that correspond to dπ(Ru)−π*(L)
MLCT transitions. Figure 2 displays the UV−vis spectra of C4
in CH2Cl2, phosphate buffer, and CH3CN. It is worth
mentioning the shift to higher energy absorptions observed
for C4 in acetonitrile with regard to the same complex in water
or dichloromethane. This is expected because of substitution of
the aqua ligand by acetonitrile; this ligand shows a higher π-
acceptor capacity with respect to the aqua ligand, which
provokes stabilization of the dπ(Ru) donor orbitals.
The electrochemical behavior of complexes C3, Na[1], and
C4 have been studied by means of cyclic voltammetry (CV)
and differential pulse voltammetry (DPV) (Figures S6−S8).
The CV curve of C3 in acetonitrile shows two reversible one-
electron-redox processes at E1/2 = −1.69 and 1.27 V versus Ag/
AgCl as reference electrode, which can be assigned to CoIII/
CoII and CoIV/CoIII, respectively (Figure S6a). The CV curve
of the aqua complex C4 in dichloromethane exhibits different
redox processes because of the Ru and Co ions. Two are
almost reversible monoelectronic CoIIIRuIII/CoIIIRuII and
CoIIIRuIV/CoIIIRuIII redox waves at E1/2 around 0.68 and
0.96 V versus Ag and two monoelectronic CoIIIRuII/CoIIRuII
and CoIVRuIV/CoIIIRuIV redox waves at E1/2 = −1.57 and 1.38
V Versus Ag/AgCl, respectively (Figure S6b).
Figure 4. DPV for complex C4 registered in a phosphate buffer (pH =
7.12) versus Ag/AgCl.
coupled electron-transfer (PCET) processes (eqs 1 and 2).
These values have been assigned to the RuIII/RuII and RuIV/
RuIII couples of the catalytic unit, based in the values presented
by the mononuclear [RuII(trpy)(bpy)(H2O)]2+ com-
pound.46,47 The CV curve is followed by another two-electron
oxidation wave at 1.19 V (vs Ag/AgCl), which was attributed
to the CoIVRuIV/CoIIIRuIV redox couple. In this case, this value
is assigned to the CoIV/CoIII couple of the photoactive unit
given the similarity with the potential value of Na[1], whose
CV curve presents a CoIV/CoIII redox wave at E1/2 = 1.36 V
(Figure 3, blue line). For C4, this value is 170 mV more
cathodic with respect to the potential of Na[1], which
indicates a certain and significant electronic coupling between
the two units. To check this out, we have measured the redox
potentials of Na[1] after adding different concentrations of
two divalent salts, CaCl2 and ZnCl2 (Figure S9 and Table S3),
to mimic the dipositive charge of the ruthenium complex, and
no substantial change in the redox potential of Na[1] has been
observed. This is consistent with a coupling between the cation
and anion in C4. Equations 1−4 contain the electrochemical
transition reactions corresponding to each redox couple
processes at pH = 7.12 with reference electrode Ag/AgCl.
The electrochemical behavior of C4 was also studied in
aqueous solution. The CV curve of C4 (Figure 3, red line) and
DPV curve (Figure 4) exhibit two successive one-electron-
oxidation waves at 0.55 and 0.67 V (vs Ag/AgCl), which
correspond to CoIIIRuIII/CoIIIRuII and CoIIIRuIV/CoIIIRuIII
redox couples, respectively, and are assigned to two proton-
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a
Table 1. Photooxidation Tests Performed with Ruthenium Cobaltabis(dicarbollide) Complex C4
a
b
Conditions: C4 (0.001 mM), substrate (20 mM), Na2S2O8 (40 mM), 5 mL of a potassium carbonate solution at pH = 7. Ratio 1:20000:40000: 8
c
d
h of reaction, with previous neutralization after 4 h. 4 h of reaction, with previous neutralization after 3 h. 4 h of reaction, without neutralization.
e
Ratio 1:20000:20000; after 4 h of reaction.
II
IV
IV
CopIIIRucIII‐OH + 1e− + 1H+ F CopIIIRuc ‐OH2
CopIVRuc O + 1e− F CopIIIRuc O
E1/2 = 0.55 V
E1/2 = 1.19 V
(1)
(2)
(3)
(4)
CoIV + 1e− F CoIII E = 1.36 V
Therefore, the observed oxidation potentials indicate that
the photogenerated CoIV is able to easily oxidize [RuIII-OH]2+
to [RuIVO]2+, with a driving force of 520 mV, because the
potential for CoIV/CoIII is high enough compared to the RuIV/
CopIIIRucIV=O + 1e− + 1H+ F CopIIIRucIII‐OH
E1/2 = 0.67 V
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a
Table 2. Photooxidation Tests Performed with C4 and C2′ + Na[1]
entry
photocatalyst
substrate
product
TON (yield, selectivity %)
1
2
3
4
5
6
7
8
9
C4
C2′
Na[1]
C2′ + Na[1](1:1)
C2′ + Na[1](1:2)
C4
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenylethanol
acetophenone
acetophenone
acetophenone
acetophenone
16200 (81, ≥99)
4400 (22, ≥99)
6200 (62, ≥99)
12800 (64, ≥99)
15200 (76, ≥99)
19600 (98, ≥99)
5200 (25, ≥96)
17200 (70, 82)
18000 (86, 96)
1-phenylethanol
acetophenone
4-methylbenzyl alcohol
4-methylbenzyl alcohol
4-methylbenzyl alcohol
4-methylbenzyl alcohol
4-methylbenzaldehyde
4-methylbenzaldehyde
4-methylbenzaldehyde
4-methylbenzaldehyde
C2′
C2′ + Na[1](1:1)
C2′ + Na[1] (1:2)
a
Conditions: C4 (0.001 mM); C2′ and Na[1] (0.001 mM) for ratio 1:1 (C2′/Na[1]); C2′ (0.001 mM) and Na[1] (0.002 mM) for ratio 1:2
(C2′/Na[1]), substrate (20 mM), Na2S2O8 (40 mM), 5 mL of a potassium carbonate solution at pH = 7. Light irradiation during 4 h.
RuIII couple. This would be thermodynamically possible in
water via PCET processes.48 Therefore, on the basis of these
studies, it is expected that, for photocatalytic oxidation of
organic substrates, [1]− could act as a good photosensitizer.
Figure 3 shows that there is an ion-pair contact effect, which
causes the CoIV/CoIII redox couple potential to vary near 170
mV under the influence of the ruthenium complex.
1-Hexanol (entry 6), 1-butanol (entry 7), isobutyl alcohol
(entry 8) and 2-ethoxyethanol (entry 9) were converted to the
corresponding acids with yields of 82% (4 h) and 92 % (8 h)
(entry 6), 89% (4 h) and 98 % (8 h) (entry 7), 83% (4 h) and
92 % (8 h) (entry 8), and 76% (4 h) and 93 % (8 h) (entry 9),
respectively. Conversely, cyclohexanol converted to cyclo-
hexanone in 79% yield after 4 h and 95% after 8 h for 10.
When the amount of oxidizing agent was lowered (ratio
applied 1:20000:20000), we observed that the degree of
conversion for 1-phenylethanol and benzyl alcohol, after 4 h, is
on the same order as the values obtained using a ratio
1:20000:40000 (entries 1e and 2e). It should be noticed that
the conversion was a little bit less because of the kinetics of the
Photocatalytic Oxidation. C4 is a [Ru][Co]2 ion-pair
complex in which the photocatalytic system is based on cobalt,
an abundant transition metal, whereas the redox part is the
ruthenium complex. The photocatalytic oxidation experiments
were all carried out by exposing the reaction quartz vials to UV
irradiation (2.2 W, λ ∼ 300 nm), for different times, at
atmospheric pressure and room temperature. The samples
were 5 mL of water, a K2CO3 solution at pH = 7, and a mixture
of C4 as a substrate, Na2S2O8 as a sacrificial oxidant (air could
also do the work),33 and 0.005 mol % catalyst. The blank
experiments were performed in the absence of catalyst,
sacrificial oxidant, or light for 8 h. The results evidenced that
a negligible amount of oxidation product was formed, less than
8% in all cases. After irradiation for a specified time (Table 1),
the reaction products were extracted with dichloromethane
2−
reaction. These results are consistent because 1 mol of S2O8
is needed for each 1 mol of alcohol to achieve the oxidizing
reaction. We have no evidence that the catalyst has undergone
any transformation following catalysis, as has been evidenced
in other cases with ruthenium complexes containing trpy.36 We
performed matrix-assisted laser desorption ionization time-of-
flight mass spectrometry (MALDI-TOF MS) of C4 after the
catalytic experiments, and it can be seen that the catalyst
remains unchanged after catalysis (Figure S10).
1
three times, quantified by means of H NMR spectroscopy,
Relevance of the History of C4. As mentioned in the
Introduction, Na[1] tends to self-aggregate in water, producing
micelles and/or vesicles.29,30 Upon the process of dissolving
C4, the constituent ions would become free; for the vast
majority of salts, this is completely independent. However, this
allegedly would not be the case here; see also the electro-
chemical section in which the [Ru−OH2]2+ affects the redox
couple of [1]−. Persistence even in the water ion-pair coupling
between the cation and anion would be possible, which would
prevent full dissociation and therefore maintain cooperation.
Indeed, no noticeable changes have been observed at the UV−
vis spectra in catalytic conditions near 10−6 M upon a 10-fold
increase in concentration (Figure S11). To evidence the
noncovalent bonding relevance between the photosensitizer
[1]− and the redox catalyst, we took advantage of the
photooxidation process and studied the yields with the same
concentrations of cations and anions but whose origins were
different. Thus, we have studied the behavior of a mixture of
two separate compounds, [RuII(trpy)(bpy)(H2O)](ClO4)2
(C2′) and Na[3,3′-Co(1,2-C2B9H11)2] (Na[1]), after 4 h of
catalysis and using the Co/Ru ratios 1:1 and 2:1, maintaining
the same ruthenium concentrations as that for the C4
photocatalyst. Table 2 displays the results obtained for two
substrates, 1-phenylethanol and 4-methylbenzyl alcohol. Table
2 shows that the noncovalent ion pair C4 displays a better
catalytic performance than the equal 1:2 mixture of C2′ and
and confirmed by gas chromatography with flame ionization
detection (GC-FID) analysis.
Table 1 displays the catalytic results obtained in the
photooxidation of aromatic and aliphatic alcohols, after
different times of reaction. In general, high yields of conversion
and selectivity have been obtained, even when short reaction
times have been used. After 8 h of irraditation and timely
neutralization at 4 h, the yields observed in the photooxidation
of 1-phenylethanol (entry 1) and diphenylmethanol (entry 5),
both secondary aromatic alcohols, were remarkably >99%, with
selectivity values of >99% and turnover numbers equal to
TON = 20000 in both cases. However, the yield values
observed for benzyl alcohol are slightly less than the secondary
ones. It is worth mentioning that, after 4 h of irradiation, the
achieved high selectivity values were to obtain only the
corresponding aldehydes. The conversion values enhanced by
the presence of electron-donating substituents, such as methyl,
on the aromatic ring of the benzyl alcohol (entry 3d) and
decreased with the presence of chloro, an electron-withdrawing
substituent (entry 4d). In the case of both benzyl alcohol and
the substituted substrates, the selectivity decreases after 8 h of
irradiation, observing alcohol overoxidation toward formation
of the corresponding acids. C4 displays high efficiency by
primary and secondary aliphatic alcohols, with yields larger
than 76% after 4 h of irradiation and selectivity values of >99%.
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Na[1], evidencing the advantage of the coupling history
between the two compounds, where nonbonding interactions
between [RuII(trpy)(bpy)(H2O)]2+ and [1]− could facilitate
or stimulate ET and, consequently, the efficiency of the
cooperative system in the alcohol oxidation. For comparison
purposes, the 1:1 ratio of C2′ and Na[1] as well as C2′ and
Na[1] alone was also studied. All of these are reported in
Table 2, showing a distinct and lower yield than C4. Dynamic
light scattering (DLS) measurements of catalytic mixtures were
done, and the results are displayed in Figure S12. The
hydrodynamic radius value of C4 in the catalytic mixture is D
= 140.9 nm, which is of the same order as that presented by
Na[1],30 which indicates the absence of larger aggregates in
the medium, despite the presence of the [Ru−OH2]2+. On the
other hand, the value of D = 128.4 nm, in the case of using a
mixture of a 1:2 ratio of C2′ and Na[1] as catalysts, is less than
that for C4, indicating to what extent the history of the
catalytic medium influences the generated aggregates and
hence, although limited, the results.
This study represents the first work, to the best of our
knowledge, in which a photoredox catalyst based on an
abundant first transition metal (Co3+) is noncovalently bonded
to an active oxidation catalyst ([RuII(trpy)(bpy)(H2O)]2+),
producing a persistent ion-pair interaction even in water and
showing high activity in the photooxidation of alcohols.
The observed values of conversion obtained using C4,
together with the previous electrochemical study, are
consistent and support the proposed mechanism shown in
Figure 5. Under irradiation, photons are absorbed by the
two electrons and two protons takes place in the photo-
oxidation of alcohols.
CONCLUSIONS
■
In this work, an air-stable cooperative ion-pair photoredox
system, C4, formed by cobaltabis(dicarbollide) [1]− as a light
collector, and the aqua ruthenium complex [RuII(trpy)(bpy)-
(OH2)]2+ as an ET agent is reported. Both cobaltabis-
(dicarbollide) and the ruthenium aqua complex are linked by
noncovalent interactions, which to a large degree persist even
after water dissolution. This represents a step ahead in
cooperativity avoiding costly covalent bonding. C4 displays
excellent photoredox catalyst properties in water through
PCET. C4 has been easily made by reaction of the chlorido
ruthenium (II) complex with Ag[1] in water/acetone (1:1)
under reflux. The recrystallization of the aqua complex C4 in
acetonitrile yielded C5 in which the aqua ligand was
substituted by CH3CN, as shown in X-ray diffraction. The
electrochemical studies evidence a significant electronic
coupling between the two units in the noncovalent ion-pair
ruthenium cobaltabis(dicarbollide) compound and that the
photogenerated CoIV can easily oxidize [RuIII-OH]2+ to
[RuIVO]2+ in water via PCET processes; therefore, the
metallacarborane can act as a good photosensitizer for the
photocatalytic oxidation of organic substrates in this
cooperative system.
We have highlighted the capacity of C4 to perform as an
excellent cooperative photoredox catalyst in the oxidation of
alcohols in water, using a catalyst load of 0.005 mol %,
achieving high yields even when short reaction times of
irradiation have been used. In some cases, a high turnover
number (TON = 20000) has been observed.
The CoIV/III redox couple of [1]− in water differed by 170
mV when [1]− had Na+ as the cation versus when
[Ru−OH2]2+ was the cation. In solution, having one or the
other cation should not influence the potential, unless there
were bonds that lasted even after dissolution. This led us to
study two identical solutions in [1]− and [Ru]: one resulting
from dissolving C4 and the other from dissolving the
precursors in water. It was remarkable to find that C4 displays
a better catalytic performance than the mixtures of C2′ and
Na[1], evidencing that the noncovalent bonding existing in C4
facilitates or stimulates ET between the photosensitizer and
catalyst and, consequently, efficiency of the cooperative system
in the photooxidation process. To the best of our knowledge,
cooperative systems with this type of interaction have not been
described and represent a step forward in getting cooperativity
avoiding covalent bonding. In most of the systems displayed in
the literature, both the photoredox catalyst and the catalyst are
connected through an organic linker or the metals of both
share the same ligand. A possible pathway has been proposed.
Figure 5. Proposed mechanism for alcohol photooxidation using C4.
EXPERIMENTAL SECTION
■
Materials, Instrumentation, and Measurements. Na[Co(1,2-
C2B9H11)2] (Na[1]),31 and Ag[Co(1,2-C2B9H11)2] (Ag[1], C3)38
were synthesized from commercial Cs[Co(1,2-C2B9H11)2] (Cs[1];
Katchem Spol.sr.o), following the methods described in the
literature.38 [RuIIICl3(trpy)] (C1),49 [RuIICl(trpy)(bpy)]Cl (C2),
III
photosensitizer Cop that experiences excitation to form
CopIII*, which undergoes oxidative quenching by the oxidizing
agent S2O82−, generating CopIV. This photogenerated strong
oxidizing CoIV is able to oxidize RucII-OH2 to RucIII-OH. Then
the SO4•− radical oxidizes a new CopIII to CopIV, which oxidizes
RucIII-OH to RucIVO species. The RucIVO species reacts
with the corresponding alcohols to afford the oxidized
products, with regeneration of the corresponding catalyst
RucII-OH2. With the proposed mechanism, the exchange of
and [RuII (trpy)(bpy)(OH) ](ClO4)2 (C2′)46,47 were also prepared
2
according to literature procedures. All synthetic manipulations were
performed under a nitrogen atmosphere using vacuum-line
techniques.
All reagents used in the present work were obtained from Aldrich
Chemical Co. and used without further purification. Reagent-grade
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organic solvents were obtained from SDS, and high-purity deionized
water was obtained by passing distilled water through a Nanopure
Milli-Q water purification system.
13C{1H} NMR (acetone-d6, 100 MHz): δ 159.42, 158.98, 153.69,
153.55, 150.66, 138.74, 138.46, 128.32, 127.91, 126.68, 126.50,
124.27, 124.17, 123.63, 123.47 (C Ru trpy-bpy), 51.11 and 50.89
(Cc). IR (ν, cm−1): 3039, 2922, 2854, 2531, 1601, 1446, 1464,1384,
UV−vis spectroscopy was performed on a Cary 50 Scan (Varian)
UV−vis spectrophotometer with 1 cm quartz cells or with an
immersion probe of 5 mm path length. NMR spectra have been
1095, 1016, 980, 884, 761. E1/2 (CH2Cl2 + 0.1 M TBAH): CoIII/II
,
−1.37 V; CoIV/III, 1.38 V; RuIII/II, 0.70 V; RuIV/III, 1.06 V (vs Ag/
AgCl). UV−vis [CH2Cl2, 1.16 × 10−5 M; λmax, nm (ε, M−1 cm−1)]:
279 (37297), 292 (42727), 325 (66493), 392 (9686), 477 (8362).
ESI-MS: m/z 814.4 (100%, [M − cosane − H2O]+), 831.4 (31%, [M
− cosane]+).
1
recorded with a Bruker ARX 300 instrument, and H NMR spectra
were recorded in acetone-d6. Chemical shift values were referenced to
SiMe4. Elemental analyses were performed using a CHNS-O EA-1108
elemental analyzer from Fisons. Electrospray ionization mass
spectrometry (ESI-MS) experiments were performed on a Navigator
liquid chromatography (LC)/MS chromatograph from Thermo
Quest Finnigan, using acetonitrile as the mobile phase. CV or DPV
was performed on an IJ-Cambria 660C potentiostat using a three-
electrode cell. A glassy carbon electrode (3 mm diameter) from BAS
was used as the working electrode and Ag/AgCl as the reference
electrode. All cyclic voltammograms presented in this work were
recorded under a nitrogen atmosphere. The complexes were dissolved
in deoxygenated solvents containing the necessary amount of [n-
Bu4N][PF]6 (TBAH) as the supporting electrolyte to yield a 0.1 M
ionic strength solution. All E1/2 values reported in this work were
estimated from CV experiments as an average of the oxidative and
reductive peak potentials [(Epa + Epc)/2]. Unless explicitly mentioned,
the concentration of the complexes was approximately 1 mM.
GC was performed with a GC-2010 gas chromatograph from
Shimadzu, equipped with an Astec CHIRALDEX G-TA column [30
m × 0.25 mm (i.d.); FID detector, 250 °C; injection, 250 °C; carrier
gas, helium; rate, 1.57 mL min−1; area normalization]. Product
analyses in the catalytic experiments were performed by GC with
biphenyl as the internal standard.
Synthesis of [RuII(trpy)(bpy)(CH3CN)][3,3′-Co(1,2-C2B9H11)2]2
(C5). By recrystallization of C4 in an acetonitrile solution, yellow
needles suitable for X-ray diffraction were obtained corresponding to
complex C5. UV−vis [CH3CN, 1.16 × 10−5 M; λmax, nm (ε, M−1
cm−1)]: 287 (59852), 308 (31314), 333 (11997), 464 (5774).
Photocatalytic Studies. A quartz tube containing an aqueous
solution (5 mL) at pH = 7 (K2CO3) with C4 or C2′ as the catalyst,
alcohol as the substrate, and Na2S2O8 as the sacrificial acceptor was
exposed to UV light (2.2 W, λ = 300 nm) for different times. The
complex/substrate/sacrificial oxidant ratios used (1:20000:40000 and
1:20000:20000 corresponding to concentrations of 0.001:20:40 mM
and 0.001:20:20 mM) were varied according to the study. For each
experiment, a light reactor supplied light illumination with 12 lamps
that produce UVA light at room temperature. The resulting solutions
were extracted with CH2Cl2 three times. The solution was dried with
anhydrous sodium sulfate, and the solvent was evaporated under
reduced pressure. To check the reproducibility of the reactions, all of
the experiments were carried out in triplicate. The reaction products
1
were quantified and characterized by H NMR spectroscopy using
tetramethylsilane as the internal standard in the case of primary
aromatic alcohols and confirmed by gas chromatography.
Crystallographic Data Collection and Structure Determination
of C5. Measurement of the C5 crystals was performed on a Bruker
Smart Apex CCD diffractometer using graphite-monochromated Mo
Kα radiation (λ = 0.71073 Å) from an X-ray tube: data collection,
SMART, version 5.631 (Bruker AXS 1997−2002); data reduction,
SAINT+, version 6.36A (Bruker AXS 2001); absorption correction,
SADABS, version 2.10 (Bruker AXS 2001); structure solution,
SHELXTL, version 6.14 (Bruker 2003); structure refinement,
SHELXL-2018/3 (Sheldrick, 2018). The crystallographic data as
well as details of the structure solution and refinement procedures are
reported in the Supporting Information. CCDC 2058809 for C5
contains the supplementary crystallographic data for this paper.
Synthesis of [RuII(trpy)(bpy)(H2O)][3,3′-Co(1,2-C2B9H11)2]2 (C4). A
90 mg sample of C2 and a 173 mg sample of Ag[1] were dissolved in
60 mL of acetone.water (1:1), and the resulting solution was refluxed
for 3 h. Then, AgCl was filtered off through a frit containing Celite.
The volume of the solution was reduced, and the mixture was chilled
in a refrigerator for 48 h. The dark orange precipitate was collected on
a frit, washed with cold water and anhydrous ethyl ether, and then
vacuum-dried. Yield: 134.69 mg (89.93%). Anal. Found (calcd) for
C33 H65B36N5Co2ORu·1.5H2O·2Et2O: C, 37.04 (36.99); H, 6.24
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00751.
Supplementary crystallographic information, spectro-
scopic characterization (IR, NMR, UV−vis, and ESI-
MS spectra) and additional electrochemical character-
ization, MALDI-TOF-MS of C4 after photooxidation,
and UV−vis and DLS of catalytic mixtures (PDF)
Accession Codes
CCDC 2058809 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
(6.66); N, 5.19 (5.26). H NMR (acetone-d6, 400 MHz): δ 9.87 (d,
1H, 3JH−H = 5.6 Hz, 1H, H1), 8.98 (d, 1H, 3JH−H = 8.1 Hz, 1H, H7),
8.91 (d, 3JH−H = 8.0 Hz, 2H, H17, H19), 8.76 (d, 3JH−H = 8.1 Hz, 2H,
H14, H22), 8.64 (d, 3JH−H = 8.1 Hz, 1H, H4), 8.52 (t, 3JH−H = 8.1 Hz,
1H, H8), 8.43 (t, 3JH−H = 8.1 Hz, 1H, H18), 8.23 (t, 3JH−H = 7.1 Hz,
1H, H9), 8.16 (t, 3JH−H = 8.1 Hz, 2H, H13, H23), 8.04(d, 3JH−H = 5.6
AUTHOR INFORMATION
■
Corresponding Authors
Isabel Romero − Departament de Química and Serveis
3
Hz, 2H, H11, H25), 7.88 (t, JH−H = 8.4 Hz, 1H, H3), 7.55 (ddd,
̀
Tecnics de Recerca, Universitat de Girona, E-17003 Girona,
3JH−H = 7.9 Hz, 8.0 Hz, 4JH−H = 1.3 Hz, 3H, H10, H12, H24), 7.17 (t,
3JH−H = 7.0 Hz, 1H, H2), 5.83 (s, 2H, Ru−OH2), 3.98 (s, 8H, Cc−H).
1H{11B} NMR (acetone-d6, 400 MHz): δ 3.98 (s, 8H, Cc−H), 3.41 (s,
4B−H, B8, B8′), 3.16 (s, 4B−H, B10, B10′), 2.75(s, 8B−H, B4, B4′,
B7, B7′), 1.96 (s, 8B−H, B9, B9′, B12, B12′), 1.68 (s, 4B−H, B6,
B6′), 1.59 (s, 8B−H, B5, B5′, B11, B11′). 11B NMR (acetone-d6, 128
Spain; orcid.org/0000-0003-4805-8394;
Email: marisa.romero@udg.edu
Francesc Teixidor − Institut de Ciencia de Materials de
Barcelona, Consejo Superior de Investigaciones Científicas, E-
08193 Bellaterra, Spain; orcid.org/0000-0002-3010-
2417; Email: teixidor@icmab.es
MHz): δ 6.31 (d, 4B, JB−H = 144.3 Hz, B−H), 1.13 (d, 4B, JB−H
=
143.5 Hz, B−H), −5.87 (m, 16B, B−H), −17.45 (d, 8B, JB−H = 156.0
Hz, B−H), −22.90 (d, 4B, JB−H = 165.3 Hz, B−H). 11B{1H} NMR
(acetone-d6, 128 MHz): δ 6.33 (s, 4B, B8, B8′), 1.19 (s, 4B, B10,
B10′), −5.87 (d, JB−B = 95.4 Hz 16B, B4, B7, B4′, B7′, B9, B12, B9′,
B12′), −17.40 (s, 8B, B5′, B11′, B5, B11), −22.84 (s, 4B, B6′, B6).
Authors
Isabel Guerrero − Institut de Ciencia de Materials de
Barcelona, Consejo Superior de Investigaciones Científicas, E-
08193 Bellaterra, Spain; Departament de Química and
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Clara Vinas − Institut de Ciencia de Materials de Barcelona,
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Consejo Superior de Investigaciones Científicas, E-08193
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Xavier Fontrodona − Departament de Química and Serveis
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Spain
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00751
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research has been financed by MINECO (Grants
PID2019-106832RB-I00 and CTQ2015-66143-P) and Gen-
eralitat de Catalunya (Grant 2017 SGR 1720). The “Severo
Ochoa” Program for Centers of Excellence in R&D 234 (SEV-
2015-0496) is appreciated.
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