Ruthenium Trichloride Catalyst in Water: Ru Colloids versus Ru Dimer Characterization Investigations

Article abstract of DOI:10.1021/acs.inorgchem.8b03144

An easy-to-prepare ruthenium catalyst obtained from ruthenium(III) trichloride in water demonstrates efficient performances in the oxidation of several cycloalkanes with high selectivity toward the ketone. In this work, several physicochemical techniques were used to demonstrate the real nature of the ruthenium salt still unknown in water and to define the active species for this Csp³-H bond functionalization. From transmission electron microscopy analyses corroborated by SAXS analyses, spherical nanoobjects were observed with an average diameter of 1.75 nm, thus being in favor of the formation of reduced species. However, further investigations, based on X-ray scattering and absorption analyses, showed no evidence of the presence of a metallic Ru-Ru bond, proof of zerovalent nanoparticles, but the existence of Ru-O and Ru-Cl bonds, and thus the formation of a water-soluble complex. The EXAFS (extended X-ray absorption fine structure) spectra revealed the presence of an oxygen-bridged diruthenium complex [Ru(OH)_xCl_3-x]₂(μ-O) with a high oxidation state in agreement with catalytic results. This study constitutes a significant advance to determine the true nature of the RuCl₃·3H₂O salt in water and proves once again the invasive nature of the electron beam in microscopy experiments, routinely used in nanochemistry.

Full text of DOI:10.1021/acs.inorgchem.8b03144

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Ruthenium Trichloride Catalyst in Water: Ru Colloids versus Ru
Dimer Characterization Investigations
†
‡,§
‡
,†
is Lamonier,∥
Anastasia Lebedeva, Brunno L. Albuquerque, Josiel B. Domingos, Jean-Franco
̧
∥
⊥
,†
Jean-Marc Giraudon, Pierre Lecante, Audrey Denicourt-Nowicki,* and Alain Roucoux*
†
Ecole Nationale Super
́
ieure de Chimie de Rennes, CNRS, ISCR-UMR 6226, Universite
LaCBio, Laboratory of Biomimetic Catalysis, Chemistry Department, Universidade Federal de Santa Catarina, Campus Trindade,
Florianopolis 88040-900, Santa Catarina Brazil
LAMOCA, Laboratory of Molecular Catalysis, Chemistry Institute, Universidade Federal do Rio Grande do Sul, Campus do Vale,
Porto Alegre 91501-970, Rio Grande do Su, Brazil
́
de Rennes, F-35000 Rennes, France
‡
́
§
∥
UMR 8181, Unite
́
de Catalyse et Chimie du Solide (UCCS), CNRS, Centrale Lille, ENSCL, Universite
́
de Lille and Universite
́
D’Artois, Lille, 59000, France
⊥
Centre d’Elaboration des Mater
́
iaux et d’Etudes Structurales du CNRS, 9 Rue Marvig, BP 4347, Toulouse Cedex 31055, France
ABSTRACT: An easy-to-prepare ruthenium catalyst obtained from
ruthenium(III) trichloride in water demonstrates efficient perform-
ances in the oxidation of several cycloalkanes with high selectivity
toward the ketone. In this work, several physicochemical techniques
were used to demonstrate the real nature of the ruthenium salt still
3
unknown in water and to define the active species for this Csp -H
bond functionalization. From transmission electron microscopy
analyses corroborated by SAXS analyses, spherical nanoobjects were
observed with an average diameter of 1.75 nm, thus being in favor of
the formation of reduced species. However, further investigations,
based on X-ray scattering and absorption analyses, showed no
evidence of the presence of a metallic Ru−Ru bond, proof of
zerovalent nanoparticles, but the existence of Ru−O and Ru−Cl
bonds, and thus the formation of a water-soluble complex. The EXAFS (extended X-ray absorption fine structure) spectra
revealed the presence of an oxygen-bridged diruthenium complex [Ru(OH) Cl ] (μ-O) with a high oxidation state in
x
3−x 2
agreement with catalytic results. This study constitutes a significant advance to determine the true nature of the RuCl ·3H O
3
2
salt in water and proves once again the invasive nature of the electron beam in microscopy experiments, routinely used in
nanochemistry.
1
1
INTRODUCTION
Despite recent advances in this field, the development of
■
3
more efficient and eco-responsible catalytic methodologies for
The activation and the functionalization of the kinetically inert
3
the oxidation of cycloalkanes remains of interest in regards to
the great demand for the oxyfunctionalized raw materials. Over
the various metals, ruthenium remains a pertinent candidate,
and thus suitable for oxidation
and presents a cheaper cost than other noble
metals with quite stable prices over time.
Very recently, our team has used ruthenium(III) trichloride
hydrate in neat water, as an original and relevant catalyst for
the oxidation of various cycloalkanes in pure biphasic (water/
substrate) conditions. High conversions and nearly complete
selectivities toward the corresponding ketones were obtained.
However, the reaction requires quite long reaction times (9 h),
within fractional additions of oxidant in six times. In the
present paper, easier and practical reaction conditions have
Csp -H bonds constitute one of the main challenges of
1
−3
synthetic chemistry.
In that field, the selective and
controlled oxidation of relatively cheap hydrocarbons con-
stitutes a relevant value-creating synthetic methodology, to
produce pertinent oxyfunctionalized feedstocks for various
industrial branches, ranging from polymer synthesis to
12,13
being an oxophilic metal,
reactions,
1
4,15
4
,5
pharmaceutical chemistry. One of the obvious and large-
scaled industrial application remains the liquid-phase oxidation
of cyclohexane into cyclohexanone and cyclohexanol (KA
16
6
,7
oil), key intermediates for the production of nylon-6 and
8
nylon-6,6. This process, catalyzed by soluble cobalt salts
acting as homogeneous species, is generally performed under
drastic temperature and pressure conditions (413−433 K, 1−2
3
MPa), owing to the high energy required to cleave the Csp -H
9
bond. Furthermore, cyclohexane oxidation presents very low
yields (<5−10%) to limit the overoxidation products and high
Received: November 8, 2018
1
0
waste production.
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b03144
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
been developed and evaluated on the oxidation of various
cyclic alkanes. Moreover, the previous promising results led
our group to ask the following question: What are the real
characteristics of these ruthenium species? Could we define the
structure of this catalyst? To answer these questions and to
better understand the nature of the ruthenium active species,
we carried out some essential spectroscopic and microscopic
techniques, including X-ray absorption and diffusion (scatter-
ing) analyses. In the present paper, the results of all these
investigations are discussed and contributed to solve the
problem of the true nature of the ruthenium species provided
fixed to the sample holder by means of double-sided adhesive tape.
The decomposition of Ru 3d5/2 and Ru 3d3/2 core levels was
performed considering the following methodology: an applied Shirley
background and an asymmetric line shape in the form of a tail-
dampened Lorentzian asymmetric line shape given represented in LF
(
α, β, w, m) form, where α and β define the spread of the tail on either
side of the Lorentzian component, m the width of the Gaussian used
to convolute the Lorentzian curve and w the tail-dampening
parameter. The Casa XPS fitting parameters were LF(0.9, 1.2,
25,280) and LF(0.9, 1.2, 25, 280) for the simulation of the Ru 3d5/2
and Ru 3d3/2 core levels, respectively.
SAXS Experiments. These analyses were performed on the
SAXS1 beamline of the Brazilian Synchrotron Light Laboratory
3
in water for this Csp -H functionalization reaction.
(LNLS−Campinas, SP, Brazil). The solutions were loaded into a
temperature-controlled vacuum flow-through cell composed of two
mica windows separated by 1 mm, normal to the beam. The
collimated beam (k = 1.55 Å) crossed the sample through an
evacuated flight tube and was scattered to a Pilatus 300 K 2D detector
EXPERIMENTAL SECTION
■
Materials. Ruthenium(III) trichloride hydrate, RuCl .3H O, was
provided by STREM Chemicals or Sigma-Aldrich. All cyclic alkanes
and the aqueous tert-butylhydroperoxide solution (Luperox TBH70X,
3
2
(
Dectris) and a 2D CCDmarCCD 165 detector, respectively. The
incident beam was detected at 500 mm distance on the SAXS1
beamline, (silver behenate was used for sample-to-detector distance
calibration), to achieve the scattering vector range q (from 0.1 to 5
7
0% wt. in H O) were purchased from Sigma-Aldrich, Acros Organics
2
or TCI Chemicals. Water was distilled twice before use.
Synthesis of the Colloidal Ruthenium Suspension. The
−
1
RuCl .3H O (0.2 mmol) salt was dissolved in 10 mL of ultrapure
nm ).
The in situ XAS analyses were performed on the XDS Beamline at
3
2
water, resulting in a homogeneous solution with a concentration of
−1
0
.02 mol L . The obtained strongly acidic colloidal solution (pH
the Brazilian Synchrotron Light Source (LNLS) in Campinas. The
spectra were acquired at room temperature in transmission mode with
three ionization chambers using a Si (311) double-crystal
monochromator and a toroidal focusing mirror. A standard Ru foil
(metallic Ru) was used to perform energy calibration in all scans. Four
to six spectra were collected to improve the signal-to-noise ratio. Each
spectrum was acquired in a range of 21.940−22.950 keV with 2−6 s/
point. The data have been processed using the IFEFFIT and Demeter
packages (Athena and Artemis). The data were calibrated using Ru
foil as reference. The Ruthenium K-edge was chosen at 22.117 keV
and the data acquired in transmission mode using a custom designed
Teflon sample holder sealed on both sides with Kapton tape.
Wide-Angle X-ray Scattering (WAXS) Measurements. These
experiments were performed at CEMES Toulouse, on the lyophilized
sample. Measurements of the X-ray intensity scattered by the samples
irradiated with graphite monochromatized molybdenum KR
(0.071069 nm) radiation were performed using a dedicated two-axis
diffractometer. Time for data collection was typically 20 h for a set of
457 measurements collected at room temperature in the range 0° < θ
< 65°. Data were reduced to extract the structure-related component
of WAXS, the so-called reduced intensity function, normalized to a
number of atoms corresponding to the size of the particles, and
Fourier transformed allowing for radial distribution function (RDF)
analysis.
1
.5−1.6) was stirred overnight and could then be directly used for
oxidation reactions.
Catalytic Experiments. The oxidation of various cycloalkanes
was performed at room temperature in the presence of the ruthenium
colloidal suspension. In a typical reaction, a 100 mL round-bottom
flask was charged with 3 mL of ultrapure water, 2.23 mmol (1 equiv.)
of substrate, and 892 μL (0.0178 mmol, 0.008 equiv.) of the
ruthenium colloidal suspension. The flask was closed and the reaction
medium was stirred for 5 min in order to homogenize the solution.
The aqueous solution of tert-butylhydroperoxide (1 + 2 × 0.5 equiv)
was then added using a syringe every hour under stirring. The
adequate amount was withdrawn with a syringe and added in one
time, via the septum. At the end of the reaction, the reaction products
were extracted with diethyl ether (2 × 3 mL). Then, 1.5 mL of the
obtained organic solution was charged in the GC vial. The conversion
and the selectivity were determined using chlorobenzene as internal
standard, using TRACE GC ULTRA apparatus with a FID detector
equipped with a Thermo Fisher HP5-MS capillary column (30 m,
0
2
.25 i.d., 0.5 f.t.). The injector and detector temperatures were set at
50 °C. Products identification was performed by comparison of their
retention times with commercial products.
DLS Analysis. The hydrodynamic size of nanoparticle aggregates
was directly measured in water solution by a dynamic light scattering
(
DLS) technique using a DelsaNano C (Beckman counter).
TEM Experiments. Transmission electron microscopy (TEM)
RESULTS AND DISCUSSION
images were recorded with a JEOL TEM 100CXII electron
microscope operated at an acceleration voltage of 100 kV. The
samples were prepared by the addition of a drop of the stabilized
colloid in water on a copper grid coated with a porous carbon film.
The size distributions were determined for around 200 particles,
through a manual analysis of enlarged micrographs with ImageJ
software using Microsoft Excel to generate histograms of the statistical
distribution and a mean diameter. High resolution TEM analysis was
recorded by a JEOL JEM 2010 UHR electron microscope with
lanthanum hexaboride source (LaB6).
■
Catalytic Oxidation of Cycloalkanes. The catalytic
performances of the ruthenium catalyst were evaluated in the
oxidation of various cyclic alkanes possessing 5 to 12 carbons
at room temperature, using practical and industrially suitable
reaction conditions (shorter reaction times and three-time
oxidant additions). The reaction was performed in the
presence of tert-butylhydroperoxide as a safer and eco-
17
responsible oxidant and in water as a suitable industrial
18
XPS Analysis. X-ray photoelectron spectroscopy measurements
were acquired with VG ESCALAB 220XL spectrometer at room
temperature using Al−Kα radiation (1486.6 eV) as the excitation X-
ray source and calibrated by setting the C 1s peak to 285.0 eV. During
the experiment, the pressure in the analysis chamber was maintained
green solvent. The catalytic results in terms of activity and
selectivity, determined by Gas Chromatography analyses, using
these novel parameters are gathered in Table 1.
First, whatever the size of the substrate is, high selectivity
toward the corresponding ketone ranging from 90 to 100% was
achieved, with negligible or no formation of the respective
alcohol. However, the conversion is dependent on the size of
the ring, reaching an optimum of 60% in 3h for cycloheptane
(Entry 3). For cyclopentane, a 45% conversion was obtained in
−
7
at 1 × 10 Pa. The data processing was performed using the
CasaXPS software. The solid sample for the analysis was prepared by
lyophilization of the initial catalyst solution by means of CryoNext
freeze-dryer. The lyophilization consists of the sublimation of frozen
water in vacuum overnight. The sample prepared as a pellet was then
B
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Article
Table 1. Catalytic Performances of the Ruthenium Catalyst
in the Cycloalkane Oxidation Reaction in Neat Water
catalyst for the selective oxidation of cycloalkanes in neat
water, under mild reaction conditions, compared to recently
a
1
9,20
described catalysts.
Comparing with previous works
(
Table 1, reference), these novel conditions afforded
promising conversions (53% in 3 h vs. 97% in 9 h), with a
comparable selectivity in ketone (90%). Thus, higher
conversions could be achieved in the presence of this
ruthenium catalyst through the modulation of various reaction
parameters (amount of oxidant and reaction time), as already
conversion
selectivity
b
b
entry
cycloalkane
cyclohexane
(%)
(%)
conditions
previous
c
16
reference
97
90
reported by the authors. In the absence of chemical reducing
16
work
agent, which should provide zerovalent species, the real nature
of the active species was investigated, using various character-
ization techniques.
Transmission Electron Microscope (TEM) Studies. The
microscopy (TEM) investigations were carried out to
determine the morphology and the size of the active species
before catalytic oxidation tests. Well-dispersed spherical
metallic particles, with an average diameter of 1.75 ± 0.25
nm, were observed. The distribution histogram (Figure 1)
revealed that 90% of the particles possess a small size between
1
2
3
4
5
6
cyclopentane
cyclohexane
cycloheptane
cyclooctane
cyclodecane
cyclododecane
46
53
61
41
26
29
89
90
100
98
96
90
this work
this work
this work
this work
this work
this work
d
a
Reaction conditions: cycloalkane (2.23 mmol, 1 equiv), t-BHP (1 +
2
× 0.5 equiv), substrate/metal = 125, 3 mL H O, 20 °C, 1 h per t-
2
b
BHP addition, total reaction time = 3 h. Determined by GC analysis
c
using chlorobenzene as internal standard. Reaction conditions:
1
.5 and 2 nm.
cyclohexane (2.23 mmol, 1 equiv), t-BHP (6 × 0.5 equiv), substrate/
The as prepared ruthenium colloids were also identified by
metal = 125, 3 mL H O, 20 °C, 1.5 h per t-BHP addition, total
2
d
reaction time = 9 h. 2 mL of ethyl acetate as cosolvent.
high resolution transmission electron microscopy (HR-TEM)
experiments to better understand the sample atomic structure
(
Figure 2). The HR-TEM micrographs showed the presence of
3
h (Entry 1), affording cyclopentanone, well-known as a key
relatively well-organized crystalline objects with interplanar
distances of 0.234, 0.214, and 0.206 nm corresponding to the
intermediate for the production of Jasmone. The conversion
decreases with the increase in the size of the cycle from C7 to
C10, as already reported in the literature with some
(
(
100), (002) and (101) planes lattice of ruthenium (0)
ICDD-JCPDS no. 06−0663). These observations are in
1
9,20
catalysts.
This difference in reactivity could be explained
agreement with the results obtained in the literature for
by a significant decrease in the solubility of the cycloalkane
within water (from 156 mg L for C5 to 0.33 mg L for
−
1
−1
zerovalent ruthenium nanoparticles synthesized by chemical
reduction of RuCl
microscopic results, in terms of morphologies and sizes of the
particles, are similar to the ones reported in our previous
works, thus showing the good reproducibility of the
methodology for the nanoparticles preparation.
2
1
22,23
C10), and also by a more difficult abstraction of the
cycloalkane hydrogen in the first step of the reaction
mechanism. In the case of the lipophilic cyclododecane, ethyl
acetate was used as a cosolvent for the oxidation reaction,
leading to the formation of cyclododecanone with a 23%
conversion in 3 h (Entry 6). Undoubtedly, the aqueous
solution of ruthenium trichloride proved to be a pertinent
3
with sodium borohydride.
All these
1
6
Small-Angle X-ray Scattering (SAXS). Additionally, the
ruthenium colloids were also analyzed by Synchrotron-based
Figure 1. TEM images (scale bar = 20 nm) and size distribution of Ru colloids before catalytic oxidation applications.
C
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Figure 2. High-resolution transmission electron micrographs of ruthenium colloids (scale bar = 2 nm)
Figure 3. (a) SAXS spectrum of ruthenium colloids and (b) size distribution obtained by “sphere” modeling of SAXS fitting.
small-angle X-ray scattering (SAXS) analysis (Figure 3). The
data were fitted with the “sphere” model, accounting the
particle morphology, as well as the “Beaucage Power-Law”
model, which includes the scattering from agglomerates, as
recently described by Cardoso et al. From the log-normal
size distribution (Figure 3b), the average diameter of the
particles is centered on 0.96 nm with a dispersion (σ) of 0.357,
which is in good agreement with TEM measurements.
Based on this first set of experiments such as Transmission
Electron microscopy and SAXS analysis, we could presume the
formation of reduced metal particles obtained by ligands
displacement and reductive elimination, and stabilized in water
catalyst core was also confirmed by dynamic light scattering
(DLS) analyses, which gives a hydrodynamic diameter of 5 nm,
higher than the one obtained by TEM analyses. This size
difference could probably be attributed to the presence of the
ionic environment around the metal particle. However,
additional characterization techniques (XPS, XAS, WAXS)
were carried out for a better understanding of the catalytic
system through the determination of the metal species nature.
X-ray Photoelectron Spectroscopic (XPS) Analysis.
The surface composition of the catalyst was determined by X-
ray photoelectron spectroscopy. The survey spectrum of
ruthenium colloids is presented on Figure 4a and various
peaks for Ru 3d, Ru 3p, Cl 2p, O 1s, and C 1s (reference) are
observed at binding energies values of 280−284, 460−480,
198, 530, and 285 eV, respectively. Concerning the ruthenium
24
by chloride ions (subshell), themselves surrounded by
+
hydronium ions H O (outer shell) in the strong acidic
3
media. This proposed electronical double-layer around the
D
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Figure 4e, can be decomposed into two peaks. The first one
located at 533.3 eV corresponds to H O adsorbed on the
2
−
surface, while the second one at 531.7 eV is assigned to OH
ions in interaction with Ru . Based on these different results
3
+
and the existence of chloro species under equilibrium in
26
solution, a Ru(OH) Cl
structure may be postulated. This
x
(3‑x)
assumption is supported by the value of the experimental (Cl
p + O 1s (OH))/(Ru 3d) atomic ratio of 3.8 close to the
2
expected one of 3.0.
To resume, XPS information are in favor of the presence of a
high-valent ruthenium complex with a Ru(OH) Cl
type-
x
(3−x)
structure in the colloidal solution (RuCl ·3H O in water),
3
2
instead of zerovalent particles as supposed from our previous
investigations based on analyses usually used in the nano-
catalysis field such as transmission microscopy experiments
(
TEM, TEM-HR).
X-ray Absorption Spectroscopy (XAS). Ruthenium K-
edge XAS analysis were performed in transmission mode on
the colloidal ruthenium solution and compared to surfactant-
stabilized ruthenium zerovalent nanoparticles (Ru@
HEA16Cl), prepared by chemical reduction of the metal
precursor (RuCl ·3H O), in the presence of sodium
3
2
borohydride (NaBH ) as chemical reductant and N,N-
4
dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium chloride
27
(
HEA16Cl surfactant) as protective agent. A ruthenium
metal foil (Ru foil) was also used as reference for zerovalent Ru
species, along with the RuCl ·3H O salt.
3
2
XANES Experiments. First, the XANES (X-ray absorption
near edge structure) spectrum of the ruthenium colloidal
solution was recorded in the range of energies from 22 100 to
2
2 150 eV and compared to those of commercial RuCl ·3H O
Figure 4. X-ray photoelectron spectroscopic (XPS) analysis of
3 2
ruthenium colloids: (a) survey XPS spectrum, (b) Ru 3d, (c) Ru
salt, Ru@HEA16Cl, and Ru foil used as reference materials
(Figure 5).
3
p, (d) Cl 2p, and (e) O 1s regions
On the basis of fingerprinting of the Ru samples, it is
possible to check that RuCl3 is consumed during its
solubilization in water to afford ruthenium colloids as an
evidence of the hydrolysis of the starting metal salt. Studies on
the formation of Ru nanoparticles in aqueous media by
species, the Ru 3d region was first investigated to determine
the ruthenium oxidation state, as generally performed in the
2
5
publications. As shown in Figure 4b, the Ru 3d core level
clearly shows two apparent maxima located at 282.3 and 286.5
eV. Even if these components overlap with those of the C 1s
core level precluding easy identification of Ru species, the
binding energy (BE) value of Ru 3d5 fits well with the value
/2
25
of 282.4 eV reported by D.J. Morgan, considering an average
value of RuCl and Ru(OH) , as well as the Ru 3d
3/2
3
3
component positioned at a distance of +4.17 eV. Moreover,
no contribution of a peak located at 279.9 eV attributed to
Ru(0) was observed (Figure 4b). Furthermore, the absence of
a shoulder in the low BE range precludes the presence of RuO2
as the BE of Ru 3d5/2 is expected to be shifted toward a lower
value to that of RuCl /Ru(OH) , namely at 280.7 eV. To
3
3
conclude, the envelope based on Ru 3d + C 1s contribution is
believed to be composed only of a Ru(III) component. Finally,
the XPS Ru 3p core-level was studied. Figure 4c shows the
well-resolved doublet (3p5/2 and 3p3/2 at 463.9 and 486 eV,
respectively) leading to an energy separation of 22.1.
25
According to the literature, as well as the atomic ratio of
−
−
the OH contribution to that of Cl (O 1s OH)/(Cl 2p) =
.32/9.17 close to one, these binding energies values can be
9
3+
attributed to the presence of Ru species as ruthenium
trichloride salt RuCl and/or ruthenium hydroxide Ru(OH) .
3
3
Figure 5. Normalized XANES spectra for the Ru K-edge of the
ruthenium colloidal solution (red) compared with commercial salt
(blue), Ru@HEA16Cl nanoparticles (black), and metallic ruthenium
foil (green). Inset, the first derivative of the XANES data highlighting
the rising edge feature in our samples.
The Cl 2p signal (Figure 4d) consists of two components,
namely Cl 2p3/2 and 2p1/2, located at 198.4 and 200 eV, with
an energy separation of 1.6 eV. These values are in agreement
with mineral chlorine. Besides, The O 1s envelope, exhibited in
E
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Inorganic Chemistry
Article
2
8
29
26,30
Pantani, Troitskii et al. and Kahn
showed that
Cl atoms coordinated to the Ru, as determined by the EXAFS
fit in Table 2. In the case of the Ru@HEA16Cl, the absence of
the Ru−Cl contribution is evident.
2+
polymerized forms of [RuCl(H O) ] can be formed in
2
5
solution among other species present in the equilibria. Since
the XANES is sensitive to the local coordination and the
chemical environment, our conclusion is that some of the
chlorine on the RuCl structure are replaced by oxygen atoms,
either OH or H O, according to XPS data. Moreover, a rising-
edge shift could be detected for the samples shown in Figure 5,
in the case of ruthenium colloids, thus indicating a higher
oxidation state of the metal species in this sample compared to
the Ru foil or to the Ru@HEA16Cl nanospecies. The
position was determined by the first derivative of the XANES
spectra, Figure 5, inset. The rising edge shifts using Ru foil as
standard was 1.3 eV for Ru@HEA16Cl, 3.3 eV for Ru colloids
The EXAFS spectrum of the ruthenium colloid sample
obtained in transmission mode was fitted and is consisted with
an octahedral dimeric structure, as seen in XANES, with an
oxygen atom in a bridge form and three other oxygen atoms
from hydroxy groups according to XPS investigations. The
3
−
2
−
2
data were fitted in k range from 2.8 to 11 Å and in R range
from 0.75 to 3.8 Å (Figure 7a, b), respectively. Even though
the signal is noisy in high k ranges, probably due to the
aqueous nature of the sample, the data fitted well with R-factor
31
2
of 0.016 and χν of40.25. The Ru−Cl bond in the colloidal
form showed a small expansion in comparison with the RuCl
3
sample (0.05 Å), probably due to the difference in the
coordination structure (2.312 vs. 2.363 Å, respectively). The
bond length of the bridging oxygen (3.648 Å for Ru−O−Ru)
is consistent with a value of 3.724 Å observed in the 2,2′-
and 6 eV for RuCl . The reduced ruthenium nanoparticles,
3
Ru@HEA16Cl, showed a shift close to the ruthenium foil,
which indicates a partial surface passivation of ruthenium
nanoparticles. In fact, a Ru−O contribution can be observed
due to a sample preparation at air. Finally, the shift is similar
bipyridine (bpy) ruthenium aqua-complex [(bpy) (H O)Ru-
2
2
4+
32,37
(
μ-O)Ru(H O)(bpy) ] .
The different chemical environ-
for commercial RuCl , and we could presume that the
2
2
3
ment might be responsible for the slight bond shortening and a
difference was also observed for the Ru−Cl bond length.
The average coordination number gave an insight about the
composition of the compound. From the basic stoichiometry
resulted from the EXAFS fitting, and considering a dimeric
octahedral structure as pointed out in the XANES finger-
printing, the diruthenium complex[Ru(OH) Cl ] (μ-O) (Fig-
oxidation state of the ruthenium in the colloidal solution is
3
+. The XANES spectrum for the colloids shows some similar
32,33
features to those obtained by Ohta et al.
and was attributed
to RuO complex structure. A shoulder is also visible on the Ru
h
colloids sample in the rising edge (pre-edge) of the spectrum.
According to literature, this phenomenon is associated with the
quadrupole transition of electrons from 1s level to the higher
3
2 2
ure 8) could be proposed as a starting precursor of high valent
3
d level and may be presented due to the coordination
3
1,32
34
ruthenium species according to the literature,
which are
chemistry of the ruthenium.
3
19
catalytically active toward Csp -H bond oxidation.
A
EXAFS Experiments. The ruthenium foil was fitted using a
different approach to the data fitting was performed discarding
the presence of a μ-O bridge, (Figure 7c ,d) which resulted in a
poorer quality of the fit. The R-factor increased to 0.021 and
single shell approach with a fixed coordination number of
35,36
1
2,
to obtain the value of the standard amplitude, which
was 0.73 and used for the fitting of the samples. The fitting
2
range for k and R was respectively 3 to 12.63 Å−2 and 1.75 to 3
the χν increased to 76.36. The graph in Figure 7d shows the
fitting discarding the μ-O bridge misses the data points in k > 4
Å. The Debye−Waller factor and the bond length agree with
the crystal standard (ICSD 54236, obtained by X-ray
−
2
Å .
Many studies have been reported to clarify oxidation
diffraction) and are shown in Table 2. The RuCl was also
3
38
mechanisms involving ruthenium-oxospecies. In the case of
fitted and its parameters are summarized in Table 2 using X-
ray diffraction standard (ICSD 22091). From the comparison
between the FT EXAFS spectra for the ruthenium samples and
the oxidation of hydrocarbons by t-BHP, many investigations
prove that the oxidation is not due to tert- butoxyl (tBuO·) and
tert-butylperoxyl (tBuOO·) radicals but to high-valent oxo-
species derived from low valent ruthenium species as Ru-
OOtBu followed by the cleavage of the O−O bond by
the RuCl (Figure 6a, b), the amplitude in the Ru−Cl peak is
3
lower in the ruthenium colloids, characterizing a less amount of
3
8
39
protonolysis. As reported by Thummel et al., these high-
Table 2. EXAFS Parameters Obtained after Fitting of Ru
Colloids and Ru@HEA16Cl
IV
V
valence species (Ru or Ru ) in acid media are often
postulated as intermediate catalysts in water oxidation for
V
V
samples
concomitant O = O formation. In fact, Ru ORu has been
40,41
identified as a key step in solution.
This way could explain
Ru@
parameters
NRu−O
Ru foil
RuCl3
Ru colloids
HEA16Cl
the pivotal role of O discussed in the previous proposed
2
radical mechanism, which is based on (1) the formation of
cyclohexylperoxy intermediate from cyclohexyl radicals and
molecular oxygen, and (2) the decomposition of cyclo-
hexylhydroperoxide in the corresponding ketone as supported
3.1(1)
4.5(2)
R
σ
Ru−O(Å)
1.824(1)
0.0030(2)
1.72(2)
1.782(2)
0.0084(8)
2
(Å2)
Ru−O
a
NRu−Cl
3
42−44
by the recent literature.
Moreover, the observed high
R
σ
Ru−Cl(Å)
2.312(2)
0.002(1)
2.363(3)
0.0070(6)
1.39(9)
3.396(1)
0.0016(2)
2
(Å2)
selectivity toward ketones (Table 1) could also be associated
with oxo ruthenium species, which have already been reported
Ru−Cl
NRu−O2
Ru−O2(Å)
4
5
in alcohol oxidation. Finally, starting from ruthenium
R
46,47
2
(Å2)
materials, single-site ruthenium catalysts
could also be
σ
Ru−O2
a
proposed, even if in our case no proof could consolidate this
hypothesis.
NRu−Ru
Ru−Ru(Å)
12
4.2(3)
2.667(2)
0.0088(6)
R
2.641(1)
0.0031(1)
2
(Å2)
For the sake of comparison, the Ru@HEA16Cl was fitted in
σ
Ru−Ru
−
2
k range from 3 to 11.7 Å and in an R range from 1.22 to 2.82
a
24
Fixed values.
Å (Figure 9a, b, respectively). The statistical parameters of this
F
DOI: 10.1021/acs.inorgchem.8b03144
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 6. For EXAFS experiments, the Ru colloids and Ru@HEA16Cl samples were acquired in solution form and the RuCl as a powder (a)
3
2
Phase-corrected Fourier-transformed k -weighted EXAFS spectra of the Ru samples and (b) scaled-down RuCl data in the sake of comparison.
3
2
2
Figure 7. Fourier-transformed k -weighted EXAFS spectra and k -weighted EXAFS spectra and respective fits for (a,b) ruthenium colloids and (c,
d) ruthenium colloids without the μ-O bridge.
fit are in good agreement with the model, with an R-factor of
and strong reducing agents such as NaBH are required for its
reduction even if passivation, providing an oxidized shell, could
4
2
0
.015 and χν of 23.03. Ruthenium is a high oxophilic metal
G
DOI: 10.1021/acs.inorgchem.8b03144
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Inorganic Chemistry
Article
and Ru@HEA16Cl) were compared to the reference one of
the Ru nanoparticles (Figure 10). The WAXS analyzes of the
lyophilized ruthenium colloids did not show the compact
hexagonal structure (hcp) of metallic ruthenium species and
the presence of the distance of the Ru−Ru bond of 0.267
50
nm. Moreover, these ruthenium-based species are not highly
crystalline and have a rather amorphous structure. Finally, a
51
Ru−O bond (d = 0.195 nm) could be detected, as well as a
Ru−Cl bond (d = 0.230 nm). In contrast, drastic changes were
observed for the Ru@HEA16Cl sample, even if analysis is
complicated due to strong contributions. However, considering
the short distances related to the bonding of Ru, any Ru−Cl
contribution can now be excluded. Moreover, the 0.26−0.30
nm range, where the Ru−Ru metallic bonding distance is
expected, is now consistent with such metallic bonding. The
RDF in this range is however broader than in purely metallic
particles and the shorter distance consistent with Ru−O can
still be observed. All these elements point to a composite
system including small metallic cores and oxidized domains, in
agreement with EXAFS.
Figure 8. Proposed structure for the high-valent oxygen-bridged
diruthenium complex based on the hydrolysis of RuCl ·3H O.
3
2
Hydrogens were omitted for clarity.
not be excluded in water. From the average coordination
number, we can conclude that ruthenium nanoparticles are
formed with a core−shell structure, with a metallic core and
oxidized domains. From EXAFS studies, we can distinguish the
different nature of ruthenium species. In reductive conditions,
ruthenium nanoparticles were obtained and characterized by
the presence of the Ru−Ru metal bond in contrast to the
formation of an oxygen-bridged ruthenium complex for the
colloidal solution easily obtained by dissolving RuCl ·3H O in
CONCLUSION
■
A ruthenium colloidal solution, easily prepared by dissolution
of RuCl .3H O metal precursor in water, proved to be active
3
2
and selective for the oxidation of various cycloalkanes, in the
presence of tert-butylhydroperoxide in water. Based on the
preliminary investigations carried out by means of high
resolution transmission electron microscopy, the formation of
reduced metallic nanospecies of Ru with an average size of 1.75
nm was initially proposed. However, further studies, based on
X-ray scattering and absorption analyses (XPS, EXAFS,
XANES, WAXS), revealed the presence in aqueous solution
of a Ru−O−Ru bridge and a dimeric octahedral structure of
[Ru(OH) Cl ] (μ-O) with a high oxidation state based on
3
2
water.
Wide-Angle X-ray Scattering (WAXS). To support these
results, wide-angle X-ray scattering (WAXS) analyses were
conducted on both samples, the colloidal suspension obtained
by solubilization of RuCl in water and the chemically reduced
3
and surfactant-stabilized ruthenium nanoparticles (Ru@
HEA16Cl). This technique, widely reported in the literature
48,49
for NPs of noble metals,
allows obtaining information on
the distribution of metal−metal distances within a homoge-
neous assembly. Indeed, a well-defined radial distribution
function (RDF) corroborates the presence of well-crystallized
nanoparticles. The comparison with the theoretical data
obtained by modeling helps in determining the crystalline
structure of the particles (position and intensity of the peaks
are compared), as well as the coherent length (longest metal−
metal distance). The RDF of both samples (ruthenium colloids
3
2 2
Ru−O and Ru−Cl bonds which is well-known to provide in
acidic water solution high-valent RuO species in which the
Ru−O bonds have a double-bond character. Thus, the
formation of reduced nanoparticles as suggested by HR-
TEM analyses could be attributed to the electron beam
emitted during the analysis. Synchrotron X-rays can also be a
source for the formation of spherical nano-objects in liquid
2
2
Figure 9. (a) Fourier-transformed k -weighted EXAFS spectra and (b) k -weighted EXAFS spectra and respective fits for Ru@HEA16Cl.
H
DOI: 10.1021/acs.inorgchem.8b03144
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 10. Radial distribution function obtained from WAXS analyzes: ruthenium colloids (red), Ru@HEA16Cl (green), and ruthenium
nanoparticles as reference (blue).
phase due to the high incident energy of the photons of about
ACKNOWLEDGMENTS
■
8
keV (SAXS1-LNLS). This phenomenon, already reported in
The authors thank Patricia Beaunier from Sorbonne Universite
for performing TEM analyses and Simon Pardis for XPS
́
5
2−54
the literature,
allows us to show again the limit of
microscopy experiments and SAXS in the nanocatalysis field.
Synchrotron X-rays can also be a source for the formation of
spherical nano-objects in liquid phase due to the high incident
energy of the photons of about 8 keV (SAXS1-LNLS). This
analysis (UMR 8181− Unite de Catalyse et Chimie du Solide),
́
and MSER for financial support of PhD thesis of A. Lebedeva.
The authors are also grateful to the Brazilian Synchrotron
Light Laboratory (LNLS), under proposals SAXS1-19004 and
XDS- 20160243, for the beam time usage.
52−55
phenomena, already reported in the literature,
allows us
to show again the limit of microscopy experiments and SAXS
in the nanocatalysis field.
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■
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Josiel B. Domingos: 0000-0002-6001-4522
Audrey Denicourt-Nowicki: 0000-0003-2992-8403
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