Mixed-valence μ3-oxo-centered triruthenium cluster [Ru3(II,III,III)(μ3-O)(μ-CH3CO2)6(H2O)3]·2H2O: Synthesis, structural characterization, valence-state delocalization and catalytic behavior

Article abstract of DOI:10.1016/j.ica.2016.05.046

The oxo-centered, trinuclear, mixed valence [Ru₃^(II,III,III)O(CH₃CO₂)₆(H₂O)₃]·2H₂O (2) acetate complex has been prepared with high yield through reduction of [Ru₃^{(III,III,III)}O(CH₃CO₂)₆(CH₃OH)₃]·CH₃CO₂precursor compound in presence of muccic acid under hydrothermal conditions. The crystalline trinuclear oxo-cluster has been obtained as crystalline powder and characterized by single-crystal and powder X-ray diffraction, elemental analysis, SEM, TGA, IR spectroscopy. Complex 2 composes of μ₃-oxocentered trinuclear ruthenium array and exhibits the oxidation state delocalization between three Ru atoms at 293 K. Accurate single-crystal analysis along with valence bond calculations reveal trapped-valence state delocalization at room temperature, whereas three-site relaxation occurs at 100 K leading to Ru(II) and Ru₂(III) formal states. Moreover, the mixed valence of Ru^IIRu₂^IIIunit in compound 2 has been confirmed by XANES spectroscopy. The catalytic behavior of oxo-centered triruthenium complex 2 has been examined in hydration of nitriles and isomerization of allylic alcohols reactions both realized in aqueous media.

Full text of DOI:10.1016/j.ica.2016.05.046

Accepted Manuscript
Mixed-Valence μ3-Oxo-Centered Triruthenium Cluster [Ru3 (II,III,III)(μ3-O)(μ-
CH3CO2)6(H2O)3]·2H2O: Synthesis, Structural Characterization, Valence-State
Delocalization and Catalytic Behavior
Alla Dikhtiarenko, Sergei Khainakov, José R. García, José Gimeno
PII:
S0020-1693(16)30288-2
http://dx.doi.org/10.1016/j.ica.2016.05.046
ICA 17082
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
18 January 2016
18 May 2016
19 May 2016
Please cite this article as: A. Dikhtiarenko, S. Khainakov, J.R. García, J. Gimeno, Mixed-Valence μ3-Oxo-Centered
Triruthenium Cluster [Ru3 (II,III,III)(μ3-O)(μ-CH3CO2)6(H2O)3]·2H2O: Synthesis, Structural Characterization,
Valence-State Delocalization and Catalytic Behavior, Inorganica Chimica Acta (2016), doi: http://dx.doi.org/
10.1016/j.ica.2016.05.046
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Mixed-Valence µ3-Oxo-Centered Triruthenium Cluster
[Ru3(II,III,III)(µ3-O)(µ-CH3CO2)6(H2O)3]·2H2O: Synthesis, Structural
Characterization, Valence-State Delocalization and Catalytic
Behavior
Alla Dikhtiarenko, Sergei Khainakov, José R. García and José Gimeno
Departamento de Química Orgánica e Inorgánica, Universidad de Oviedo – CINN, 33006 Oviedo, Spain
Abstract
The oxo-centered, trinuclear, mixed valence [Ru3(II,III,III)O(CH3CO2)6(H2O)3]·2H2O (2)
acetate complex has been prepared with high yield through reduction of
[Ru3(III,III,III)O(CH3CO2)6(CH3OH)3]·CH3CO2 precursor compound in presence of muccic acid
under hydrothermal conditions. The crystalline trinuclear oxo-cluster has been obtained as
crystalline powder and characterized by single-crystal and powder X-ray diffraction, elemental
analysis, SEM, TGA, IR spectroscopy. Complex 2 composes of µ3-oxocentered trinuclear
ruthenium array and exhibits the oxidation state delocalization between three Ru atoms at 293 K.
Accurate single-crystal analysis along with valence bond calculations reveal trapped-valence
state delocalization at room temperature, whereas three-site relaxation occurs at 100 K leading to
Ru(II) and Ru2(III) formal states. Moreover, the mixed valence of RuIIRu2III unit in compound 2
has been confirmed by XANES spectroscopy. The catalytic behavior of oxo-centered
triruthenium complex 2 has been examined in hydration of nitriles and isomerization of allylic
alcohols reactions both realized in aqueous media.
Keywords: Triruthenium oxo-cluster, mixed-valence delocalization, synthesis, hydration of
nitriles, isomerization of allylic alcohols, catalysis.
1

1. Introduction
Triangular µ-oxo-centered ruthenium-carboxylate clusters of general formula
[Ru3O(RCO2)6L3]n (R = H, CH3, C2H5, C3H7, C3F7, C6H5, C7H15, C8H16; L = H2O, PPh3, CO,
CH3OH, N-heterocycles, OS(CH3)2, etc.) [1-10], where three ruthenium atoms are bridged by the
oxygen atom located at the center of a ruthenium triangle and also by the six carboxylate ligands
in the circumference (Scheme 1), show multiple redox behavior, intriguing mixed-valence
chemistry, and versatile catalytic properties [11-14].
Scheme 1. (single column fitting image) Molecular structure of basic oxo-centered ruthenium carboxylate
complex.
The axial ligands L are comparatively labile and can readily be substituted, thus enhancing
significantly the richness of triruthenium chemistry. Moreover, along axial ligand substitution,
the bridging carboxylates of oxo-centered triruthenium core [Ru3(µ3-O)(µ-RCO2)6]n can be
replaced affording an excellent means to control the chemical and electronic properties by
introducing proper organic ligands [15-17]. Nowadays, metal-oxo clusters gained particular
interest as class of compounds that can be used as predesigned building blocks to elaborate
functional hybrid materials such as metal-organic frameworks (MOFs) [17-21]. Among the
variety of [Ru3O(RCO2)6L3]n clusters, considerable attention has been paid to those that has a
mixed-valence state of ruthenium atoms, e.g. [Ru3II,III,IIIO(RCO2)6L3]0, because their original
spectral and catalytic behavior [11, 22, 23]. The synthetic methodology for preparation of
[Ru3II,III,IIIO(RCO2)6L2L´] complexes (L = L´ = H2O, pyridine; L´ = CO) involves the reduction
of [Ru3(III,III,III)O(CH3CO2)6(CH3OH)3]+ cationic derivatives [6, 7, 9]. Nevertheless, the reduction
process for the synthesis of the aqua-complex [Ru3II,III,IIIO(CH3CO2)6(H2O)3]0 implies rigorous
conditions since a stream of hydrogen at 2 atm pressure through an aqueous solution and in
presence of platinum oxide catalyst is required. In addition to the high cost of the method, the
oxo-centered triruthenium (II,III,III) cluster core frequently undergoes a second step reduction
step leading to binuclear µ-tetraacetate RuIIRuIII product with loss of the central µ-oxo ion [7,
24].
Herein, we report the direct synthesis of [Ru3(II,III,III)O(CH3CO2)6(H2O)3]·2H2O (2) straining
from [Ru3(III,III,III)O(CH3CO2)6(CH3OH)3]·CH3CO2 (1) precursor complex under hydrothermal
conditions which has been isolated as a crystalline powder in high yield using muccic acid as a
one
electron
mild
reduction
agent.
The
structural
characterization
of
[Ru3(II,III,III)O(CH3CO2)6(H2O)3]·2H2O in solid state by X-ray diffraction is also described. In
addition, mixed-valence delocalization in the [Ru3(II,III,III)O(CH3CO2)6(H2O)3] complex at room
2

temperature along with the catalytic activity in the hydration of nitriles and the isomerization of
allylic alcohols in aqueous media, is discussed in details here.
2. Experimental Section
2.1. Materials
The compound [Ru3(III,III,III)O(CH3CO2)6(CH3OH)3]·CH3CO2 (1) was used as starting
material for the preparation of 2 and was obtained by modifying the synthesis method reported
by Spencer and Wilkinson [7]. Muccic acid and other chemicals are commercially available and
were used as purchased.
2.2. Synthesis of [Ru3(III,III,III)O(CH3CO2)6(CH3OH)3]·CH3CO2 (1)
Following the modified procedure described by Baumann et al. [3], 3.0 g (12 mmol) of
RuCl3·3H2O and 6.0 g (44 mmol) of sodium acetate were dissolved in mixture of 75 mL of
ethanol and 75 mL of glacial acetic acid. After being heated at reflux for 4 h or more, the
solution was cooled. In order to separate the product from excess of non-reacted impurities,
resulted solution was centrifuged for about 20 min at a medium speed and filtered. The resulting
filtrate was reduced to green oil by removing the volatiles on a rotary evaporator. A total of 150
mL of CH3OH was added to the oil substance, stirred and then filtered. The filtrate was cooled at
- 40 °C for 4 h, and the precipitate of sodium chloride and sodium acetate was removed by
filtration. Again, the resulting filtrate was evaporated on rotary evaporator, dissolved in
methanol, filtered and evaporated. The pure acetate 1 was obtained by two recrystallizations
from methanol-acetone, and subsequently dried in vacuum giving a 46% yield (basing on Ru).
Anal. Calcd. for C17H30O18Ru3: C, 24.64%; H, 3.62%. Found: C, 24.5%; H, 3.6%. IR (KBr
disks): 1594s, 1511w, 1423s, 1373w, 1332w, 1297m, 1222w, 1045w, 929w, 784m, 767m, 727w,
526m cm-1.
2.3. Synthesis of [Ru3(II,III,III)O(CH3CO2)6(H2O)3]·2H2O (2)
For the preparation, 0.83 g (1 mmol) of 1 and 0.02 g (0.1 mmol) of muccic acid (C6H10O8)
were mixed in 10 mL of distilled water. Then the reaction mixture was stirred at room
temperature to complete dissolving of chemicals, transferred in a Teflon-lined stainless vessel
(40 mL) and heated at 180 °C for 72 h under autogenous pressure. After, the reaction was cooled
down to room temperature, and resulted solid was filtered, washed several times with ice-cold
distillated water and air-dried. The yield of dark green solid lustrous solid was 0.144 g (66%).
Anal. Calcd. for C12H28O18Ru3: C, 18.87 %; H, 3.67%. Found: C, 18.9%; H, 3.7%.
2.4. Single-crystal X-ray diffraction and structure determination of 2
Single-crystal X-ray diffraction data collection was performed at 293 K and 100 K on
Oxford-Diffraction-Gemini diffractometer with a monochromatic CuKα radiation source (λ =
1.5418 Å). The CrysAlisPro program was used for cell refinement and data reduction. Images
were collected at a 55 mm fixed crystal-detector distance, using the oscillation method, with 1°
oscillation and variable exposure time per image. The structures were solved by direct methods
using the SIR92 program [25]. The refinement was performed by SHELX-97 using full-matrix
least squares on F2 [26]. All non-H atoms were anisotropically refined except two crystallization
3

water molecules which were treated isotropically. Position of the H atoms were calculated based
on geometric criteria (C-H = 0.96 Å for methyl groups) than have been placed in their calculated
position and refined isotropically using a rider model with Uiso(H) = 1.5 Ueq(C) for methyl group.
An absorption correction was performed using XABS2 program [27]. All intra- and inter-
molecular interactions were found by PARST [28] and PLATON [29] program packages.
Crystallographic data for 2 have been deposited to Cambridge Crystallographic Data Center with
CCDC# 813218–813219 and can be obtained free of charge upon request at
www.ccdc.cam.ac.uk/data_request/cif. Crystal data and structure refinement details for 2 at 293
K and 100 K are presented in Table 1.
Table 1. Crystallographic data and structure refinement for [Ru3(CH3CO2)6(H2O)3]·2H2O (2)
Temperature
293 K
100 K
Formula
C12H28O18 Ru3
763.55
CuKα (1.54184 Å)
Monoclinic
C2/c
C12H28O18 Ru3
763.55
CuKα (1.54184 Å)
monoclinic
C2/c
Formula weight / g·mol-1
Wavelength
Crystal system
Space group
Unit cell dimensions
a / Å
27.032(6)
26.923(9)
b / Å
c / Å
α / °
13.832(3)
15.257(4)
90
13.841(3)
15.165(6)
90
123.62(3)
123.13(5)
β / °
90
4751(3)
8
2.115
16.01
90
4732(4)
8
2.135
16.07
2928
0.07 × 0.06 × 0.01
3.5 to 71.9
γ / °
Cell volume / Å3
Z
Calc. Density / mg·m-3
Absorption coefficient / mm-1
F(000)
3008
0.03 × 0.06 × 0.08
Crystal size / mm3
Theta range for data collection / ° 3.8 to 71
Index ranges
Reflection collected
-32≤ h ≤33, -16≤ k ≤14, -18≤ l ≤18
19481
-30≤ h ≤33,-17≤ k ≤14,-18≤ l ≤15
8935
Independent reflections
Completeness to θ=70°
Absorption corrections
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I>2σ(I)]
R indices (all data)
4541 [Rint = 0.04]
99 %
Empirical
0.590 and 0.350
Full-matrix least-squares on F2
4541/0/308
4405[Rint = 0.121]
99 %
Empirical
0.373 and 0.852
Full-matrix least-squares on F2
4405/0/288
0.901
0.972
R1 = 0.049, wR2 = 0.117
R1 = 0.084, wR2 = 0.130
1.35 and -0.72 e·Å-3
R1 = 0.088, wR2 = 0.203
R1 = 0.204, wR2 = 0.293
1.17 and -1.05 e·Å-3
Largest diff. peak and hole
− |
R1 = Σ ||F0| Fc|| / Σ |F0| for observed data (I > 2σ (I)); number in parentheses is for all data.
2 2
wR2 = {Σ [w(F02 – Fc2)2]/ Σ [w(F0 ) ]}1/2 for observed data (I > 2σ(I)); number in parentheses is for all data.
2.5. Characterization methods
Microanalyses were carried out using a Perkin-Elmer model 2400B elemental analyzer.
The IR spectra were recorded on a Bruker Tensor-27 spectrophotometer as KBr pellets in the
4000–400 cm-1 region. A Mettler-ToledoTGA/SDTA851 were used for the thermal analyses in
nitrogen dynamic atmosphere (50 mL/min) at a heating rate of 10 °C/min. Approximately 10 mg
of powder sample was thermally treated, and blank runs were performed.
4

X-ray
microanalysis
(SEM/EDX)
confirmed
composition
of
[Ru3(II,III,III)O(CH3CO2)6(H2O)3]·2H2O, by using JEOL JSM-6100 scanning microscopy (SEM)
coupled with an INCA Energy-200 dispersive X-ray microanalysis system (EDX) with a
PentaFET ultrathin window detector. As shown in Fig. S1 (Supplementary Materials), the
lustrous dark green powder of 2 consists of macro-sized crystals exhibiting prismatic habit.
X-Ray powder diffraction patterns were collected with a X´Pert Philips X-ray
diffractometer (CuKα radiation, λ = 1.5418 Å) at room temperature. The powder diffraction
analysis indicate that synthesized compound 2 show analogous pattern as for simulated from the
atomic coordinates of 2 determined by single-crystal X-ray diffraction (Fig. S3, Supplementary
Materials). X-ray near-edge absorption spectra (XANES) at Ru K-edge were recorded in
transmittance mode at European Synchrotron Radiation Facility (ESRF, Grenoble) in Beamline
BM25. The excitation energy was selected using a Si(111) double-crystal monochromator. The
beam size of 1 mm (horizontal) × 1 mm (vertical) was controlled with a slit. The XANES spectra
were measured on pallets containing approximately 40 mg of solid compound 2 in 200 mg of
boron nitride. Each spectrum was recorded with 0.25 eV step size. All spectra processing
procedures (i.e., normalization, smoothing, background subtraction and windowing) including
linear combination calculations were done in the IFEFFIT software package [30, 31].
2.6. Catalytic experiments
Hydration reactions were carried out in Schlenk tube under N2 atmosphere. The reaction
mixture was prepared dissolving 5 mg (6.5 µmol) of catalyst 2 in 3 mL of H2O. The mixture was
degassed and 1.5 mmol of corresponding acetonitrile substrate was added with micropipette to
stirred solution. The reaction was allowed for heating at 110 °C using oil bath or microwave-
assisted heating. The isomerization reactions of allylic alcohols were conducted Schlenk tube
under N2 atmosphere. The reaction mixture was prepared dissolving 3 mg (3.9 µmol) of catalyst
2 in 2 mL of appropriate solvent (DMF, EtOH or H2O). The mixture was degassed and 1 mmol
of corresponding allylic alcohol substrate was added with micropipette to stirred solution. The
reaction was allowed for heating using oil bath
The reaction solutions were analyzed by regular sampling using GC/FID (Hewlett
Packard) equipped with Beta DEX 120 (30 m × 0.25 mm × 0.25 µm) 30 m long column. The
degrees of conversion were calculated on the basis of the ratio of areas of the substrate material
and the products determined from corresponding chromatograms. The optimization of
chromatographic methods and the calibration procedures for detection of products as well as
substrates were realized by injection of authentic commercial samples.
3. Results and Discussion
3.1. Crystal structure of [Ru3O(CH3CO2)6(H2O)3]·2H2O (2)
The single-crystal X-ray diffraction analysis reveals that compound 2 crystalizes in
monoclinic space group C2/c with unit cell parameters listed in Table 1. The molecular structure
of the acetate complex 2 is depicted in Figure 1. The asymmetric unit of 2 contains three
crystallographically independent ruthenium atoms, one µ3-oxygen, six acetate ligands, three
coordinated and two lattice water molecules.
5

Figure 1. (single column fitting image) Molecular structure of complex 2 represented with corresponding atom
labeling scheme. H atoms from the methyl groups were omitted for clarity.
As shown in Fig. 1, the three ruthenium atoms form a near to the equilateral triangle where
pseudo-C3 axis pass through µ3-O perpendicular to the Ru–Ru plane with Ru···Ru separation
distances of 3.322 and 3.274 Å. In similar way, three coordinated water molecules are located in
the same plane that defines for Ru atoms. The central µ3-O1 atom is located in the Ru3 triangular
plane near to the center of the triangle. The acetate ligands deviate from Ru3 triangular plane on
the angle of 45.2°. Furthermore, each ruthenium atom is in distorted octahedral coordination
geometry formed by four carboxyl oxygen atoms originated from four different acetate ligands,
one µ3-O and one oxygen atom coming from coordinated water molecule. The average bond
length of Ru-O is varied from 1.875(6) Å to 2.128(7) Å and depends from the temperature of
measurement. Selected bond distances for ruthenium coordination environments in 2 at 100 K
and 293 K are summarized in Table S1 (Supplementary Materials).
The crystal packing of 2 stabilized through multiple hydrogen bonds involving the
coordinated water in [Ru3O(CH3CO2)6(H2O)3] complex and two crystallization water molecules.
There obviously exists strong intermolecular interaction in the hydrogen-bonding network
associated with the valence state conversion. In the complex 2, a calculation of intermolecular
contacts suggests five hydrogen bonds listed in Table 2.
Table 2. Hydrogen bonds geometry* in structure of 2
d (H···A), Å
d (D···A), Å
D−H···A
d (D−H), Å
∠(D−H···A), °
O14−H14A···O13
O15−H15B···O5
O16−H16B···O9
O18−H18A···O17
O18−H18B···O8
0.90
0.90
0.89
0.89
0.89
2.09
2.09
2.23
1.93
2.57
2.996(12)
3.145(17)
3.044(13)
2.69(4)
179
168
152
148
161
3.39(3)
*The hydrogen bonds were considered according to [32, 33]
The O18−H18B···O8 hydrogen bond holds one crystalized water molecule near to Ru3 site,
while intermolecular O14−H14A···O13, O15−H15B···O5 and O16−H16B···O9 contacts
contribute to the strengthen of cluster molecules packing (Fig. S4). Moreover, the hydrogen
6

bonds between individual cluster molecules connect the neighboring [Ru3O(CH3CO2)6(H2O)3]
complexes to construct a 2D framework. Better insight of resulted packing network can be
achieved by topological analysis using TOPOS4.0 program package [34]. As shown in Fig. S5,
each ruthenium oxo-cluster is linked to six equivalent neighbors via hydrogen bonds and
represents a 6-connected node. The resulted non-interpenetrating 6-connected unimodal net is
simplified as well-known Shubnikov hexagonal plane net topology (hcb) with the Schläfli
symbol of (6,3) and the 6.6.6. vertex symbol.
3.2. Ruthenium mixed valence delocalization in [Ru3(II,III,III)O(CH3CO2)6(H2O)3]·2H2O
The formulation of [Ru3O(CH3CO2)6(H2O)3]·2H2O determined by single-crystal X-ray
study implies that the ruthenium atoms in complex 2 are in mixed oxidation state with RuII:RuIII
ratio equal to 1:2. As have been observed in similar trinuclear-oxo compounds, where transition
metal centers are found in mixed oxidation states, the resonant mixed valence delocalization
between those three centers is usually occurred at room temperature [35-37]. The geometry of
the coordination sphere around the transition metal centers atoms provides much more
information about the oxidation state of these atoms and the degree of valence delocalization in
the M3O core [38-40]. In particular, the Ru–µ3O distance and angles analysis are expected to be
useful for determination of formal valences at each ruthenium center in
[Ru3O(CH3CO2)6(H2O)3]·2H2O complex. Thus, the Ru–µ3O distance and angles in Ru3O
triangular core were analyzed using single-crystal X-ray diffraction data measured at 293 K and
100 K. As shown in Fig. 2a, the Ru2–µ3O bond length (1.875 Å) is significantly shorter than
Ru1–µ3O (1.905 Å) and Ru3–µ3O (1.9931 Å), suggesting that Ru2 center is likely exhibit 3+
oxidation state.
Figure 2. (two column fitting image) The Ru–µ3O bond lengths and Ru2–O–Ru1 angle in triangular Ru3O
core of 2 as determined from single-crystal X-ray diffraction data at (a) 295 K and (b) 100K.
Furthermore, the Ru3–µ3O appears to be longer than Ru1–µ3O that is within the range as
expected for the Ru(2+) atom. Meanwhile, intermediate position of Ru1–µ3O bond points to the
valence delocalization at Ru1 center. To support this, charge ordering calculation performed
using VaList program [41], suggest that Ru2 atom site is mainly Ru(3+) while the Ru1 and Ru3
centers exhibit mixed valence state resembling charges of 2.3+ and 2.7+, respectively. This result
indicate that the mixed oxidation delocalization take place between Ru1 and Ru3 at room
temperature, where Ru1 atom site trend to Ru(2+) approximation. In contrast, as shown in Fig.
2b, at lower temperature Ru2–µ3O and Ru1–µ3O bond are close to be equal with lengths of 1.895

Å and 1.890 Å, respectively, while Ru3–µ3O bond become larger (1.946 Å). Moreover, the Ru2–
µ3O–Ru1 is also depended on the valence state and, as shown in Fig. 2, upon temperature
decreasing became larger on 3°. These observations indicate that oxidation state trapping (3+) on
Ru1 center occurs at lower temperatures. As supported by valence bond calculation at 100 K, the
ruthenium site oxidation for Ru1 and Ru2 metal centers are 3+ whereas for Ru3 center calculated
value became Ru(2+).
In order to confirm the mixed oxidation states of ruthenium atoms in oxo-cluster 2, the
XANES technique was applied. The normalized XANES spectra for solid samples of 2 and
reference compounds measured on Ru K adsorption edge are shown in Fig. 3.
Figure 3. (single column fitting image) XANES part of the Ru K-edge X-ray absorption spectra of compound 2 with
corresponding fitting plot realized using the absorption edges positions of reference compounds Ru(III) and Ru(II)
as a calibration data for determination of Ru formal oxidation states in oxo-cluster 2.
The adsorption edges (determined from the derivatives of normalized spectra) were used to
estimate the formal oxidation state of ruthenium atoms in triruthenium oxo-cluster 2. As follows
from fitting data represented in Fig. 3, the [Ru3(II,III,III)O(CH3CO2)6(H2O)3]·2H2O complex have
the position of the Ru absorption K-edge Eo close to 22142 eV. A combination fit of the
experimental data using the Eo values of corresponding Ru(III) and Ru(II) reference compounds
suggests that the ruthenium centers in complex 2 exhibit II-III mixed valance state. The RuII:RuIII
ratio of formal oxidation states for three ruthenium centers is found to be 0.38:0.62 (calc.
0.33:0.67) which is consistent with expected for RuIIRu2III core.
The magnetic nature of the analogous triruthenium oxo-complex has been previously
discussed by Cotton and Norman [10]. According to this work the mixed-valence RuIIRu2III oxo-
bridged complex exhibits diamagnetic properties even though the Ru(III) centers possess
unpaired electrons. That can be better explained by a molecular orbital treatment of the entire
Ru3O-core rather than considering the Ru(III) centers separately: 16 electrons of Ru3O are paired
in bonding and nonbonding orbitals resulting in diamagnetic complex.
8

3.3. Thermogravimetric analysis
The thermal stability of 2 in nitrogen atmosphere was investigated. As shown in Figure 4,
where corresponding TG/DTG curves are presented, decomposition of ruthenium oxo-cluster
proceeds through three steps with total mass loss of 58.4%. The first step, which take place in
temperature range of 60–200 °C with maximum velocity at 130 °C and has endothermic effect in
SDTA curve, reveals mass loss of 9.4 % and corresponds evacuation of 4 moles (calc. 9.43%) of
water molecules (two crystallization and two coordinated water).
Figure 4. (single column fitting image)Thermogravimetric (TG), derivative (dTG) and simultaneous
differential thermal analysis (SDTA) curves corresponding to decomposition profile of 2 in nitrogen atmosphere.
The second mass loss step is partially overlapped with third stage and occurred between
240 and 280 °C reaching maximum velocity at 260 °C. The mass loss corresponding to this stage
is 2.4% followed by endothermic peak in SDTA graph and attributed to the loss of one
remaining coordinated water molecule (calc. 2.36 %). The third step occurred in the range of
temperatures 290–400 °C with maximum velocity at 300 °C and the mass loss of 46.6% is
associated with decomposition of acetate ligands in resulted anhydrous acetate oxo-cluster (calc.
46.39%). Assuming that decomposition of 2 take place in inert nitrogen atmosphere, metallic
ruthenium is expected as final product of pyrolysis that leads to the residue mass of 41.6 % (calc.
41.07%).
3.4. Infrared spectroscopic characterization
The infrared spectrum of complex 2 acquired from 4000 cm-1 to 400 cm-1 is dominated by
vibrational modes involving the bridging acetate group (Fig. S6a). The high-energy, relatively
weak bands observed ~ 2925 cm-1 are attributed to C–H stretching of acetate ligands, while the
methyl bending mode is represented by peak centered at 1354 cm-1. Other bands occurred at 948
cm-1 and 686 cm-1 apparently attribute to acetate ligands [3]. The intense peaks located at 1630
cm-1 and 1572 cm-1 are associated to asymmetric carboxylate stretching frequencies, whereas the
lower energy band ~1429 cm-1 is due to symmetric stretching of carboxylic group [42].
Furthermore, the bands observed at 1039 cm-1 and 619 cm-1 were assigned to (µ3-O)–Ru
bridging vibrations in complex 2 [43].

Vibration modes of coordinated water molecules occur in the region 3600–3300 cm-1,
among the peak centered at 3550 cm-1 corresponds to crystallized water molecules [42].
Interestingly, when the complex 2 was heated at 140 °C under vacuum during 3 hours, the
dehydration of ruthenium oxo-cluster is occurred as was evidenced by disappearance of the
broad peak in the region of 3600–3300 cm-1 (Fig. S6b). Moreover, the bands attributed to
variations of carboxyl groups are slightly shifted to 1537 cm-1 and 1410 cm-1 indicating that the
polarity of Ru–Ocarboxyl bond increase as a result of releasing coordinated water molecules out of
ruthenium coordination sphere.
3.5. Catalytic activity of 2 in hydration of nitriles to amides in aqueous medium
Hydration of nitriles is one of the most important reaction applied for industrial purposes
since its form inherent link in the large-scale production of amines, which are synthetic
intermediates used in manufacture of pharmacological products, polymers, detergents, etc [44].
To this regard, a variety of ruthenium-based complexes able to selectively and efficiently
catalyze hydration of nitriles to amides in aqueous media have been reported [45-47]. Basing on
these precedents, the trinuclear oxo-cluster 2 was examined in nitrile hydration reaction. In a
typical experiment, 1.5 mmol% of 2 was dissolved in 3 mL of H2O and 1.5 mmol of
corresponding acetonitrile substrate was added. The reaction mixtures were allowed for heating
at 110 °C using oil bath or microwave-assisted heating.
As shown in Table 3, the triruthenium oxo-cluster 2 was found to be active catalyst,
reaching amides formation with yields up to TOF of 248 h-1. It worth to noting that the activity
of complex 2 is higher and more pronounced when reaction was assisted by microwave heating.
Comparing to the nitriles conversions obtained using conventional heating, which are usually
occurred from 5 to 50 h, the microwave-assisted transformation was much faster reaching high
conversion percentage in less than two hours.
In order to explore the effect of hydrophobicity of the substrate on the course of hydration
of nitriles in aqueous media, we focused on the wide array of aliphatic nitriles with different
chain lengths. Thus, as observed in entries 1a-d, the TOF values corresponding to conversions of
different substrates increases as the aliphatic chain length increases. Since the solubility of those
aliphatic nitriles in aqueous media drastically decreases as the aliphatic chain growth, it is
reasonable to conclude that the hydration reaction is favorable biphasic.
As expected, the hydration reaction course should vary somewhat depending on the
reactivity of the nitrile. Using the same reaction conditions a wide range of nitriles substrates
was examined, i.e.: benzonitrile, hexanenitrile, phenoxyacetonitrile, 3-chlorbenzonitrile,
methylbenzonitrile and 3-nitrobenzonitrile. As shown in Fig. S7, the kinetic plots for the studied
substrates exhibit different slopes featuring variable reactivity. Summarizing, the order of
reactivity of nitriles is benzonitrile > 3-nitrobenzonitrile > phenoxyacetonitrile > 3-
chlorbenzonitrile > hexanenitrile > 3-methylbenzonitrile. The same order of reactivity is
observed upon microwave-assisted heating, as shown in and Table 3 (entries 1d, 2a, 2c, 3a, 4b
and 6a-b). Also, the reactivity of benzonitrile as a function of electron-withdrawing groups was
studied. Thus, comparing the conversion rates for methyl-, nitro-, chlor- and methoxy-substituted
in ortho-position benzonitriles (entries 2c, 3a, 4b and 5b), the reactivity of the substrates fall in
order of Cl > NO2 > CH3 > OCH3 that is also confirmed by results obtained in microwave-
assisted experiments. In other hand, beside to consequence of electron-withdrawing power of the
substituents, the position of these groups is also expected to affect the reactivity of the substrates.
10

Table 3. Hydration of nitriles in aqueous mediuam catalyzed by triruthenium oxo-cluster 2 at 110 °C
Conventional heating a
Microwave-assisted heating b
Substrate
Product
time, h
yield, %
TOF, h-1 time, h yield, % TOF, h-1
NH2
1a
47
59
3.1
2
28
35
CN
CN
O
NH2
NH2
1b
1c
13
14
45
87
8.6
2
2
65
92
81
O
CN
15.5
115
O
O
1d
2a
CN
7
92
97
32.9
37.3
1
1
78
97
195
242
NH2
O
CN
CN
NH2
NH2
6.5
CH3
O
CH3
2b
2c
2d
3a
27
11
17
9
64
57
92
94
5.9
2
2
41
76
94
92
51
95
O
CN
H3C
H3C
NH2
12.9
13.5
26.1
O
CN
NH2
1.5
1
156.7
230
H3C
O2N
H3C
O
CN
O2N
NH2
O
Cl
Cl
CN
NH2
4a
4b
4c
18
7
85
87
97
23.6
31.1
40.4
1
1
1
86
99
99
215
O
CN
Cl
Cl
NH2
247.5
247.5
O
CN
NH2
6
Cl
Cl
O
NH2
CN
O
O
CH3
CH3
5a
50
51
2.6
4
61
38.1
O
O
CN
O
H3C
NH2
H3C
5b
5c
40
13
72
99
92
4.5
19
2
1
88
93
110
O
CN
NH2
232.5
H3
C
O
H3C
O
O
CN
O
O
NH2
NH2
6a
6b
12
9
19.2
27.2
1
1
82
99
205
O
O
CN
98
247.5
O
Recycling tests
O
O
CN
CN
NH2
7a
7b
–
–
–
–
–
–
1
1
96
96
222
222
NH2
a Reaction performed under N2 atmosphere at 110 °C in Schlenk tube using 1.3 mmol% (Ru) of catalyst, 1.5 mmol of
substrate and 3 mL H2O.
b Reaction performed under N2 atmosphere at 110 °C in microwave autoclave using the same conditions as for a.
TOF = mmol of product / mmol of catalyst (basing on Ru) / hours.
11

As resulted from the data in Table 3 corresponding to entries 2c-d (for methyl-group), 4a-c
(for chlor-group) and 5a-c (for methoxy-group), ortho-substituted substrates showed a lower
reactivity compared to their meta- or para-substituted counterparts (entry 2b vs 2b-c, entry 4a vs
4b-c, 5a vs 5b-c) which is likely due to steric factors.
In overall, the reactivity of the substrates follows the order of para > meta > ortho
perceiving particular electron-withdrawing effects of each sort of substituent group.
The recyclability tests for complex 2 have been performed using benzonitrile as a model
substrate. The catalytic system contained 2 was allowed to react with benzonitrile for 1 h under
microwave heating at 110 °C, then reaction mixture was cooled to 10°C for quantitative
separation of crystalline benzamide product. After, the aqueous solution contained dissolved
oxo-cluster 2 was removed by decantation and reused for next catalytic run through adding a
new portion of benzonitrile (Fig. S8). As shown in Table 3 (entries 2a, 7a-b), the catalytic
activities of 2 along three consecutive cycles (2a: 1st, 7a: 2nd, 7b: 3rd run) is remain almost
constant leading to conversions of 96% in 1 h (TOF = 222 h-1). These observations suggest that
acetate triruthenium oxo-cluster 2 remain catalytic activity upon at least three consecutive cycles.
3.6. Catalytic activity of 2 in isomerization of allylic alcohols
The high catalytic activities of rhodium oxo-complex selective isomerization of allylic
alcohols early reported by Morrill and Grubbs [48], and similarity in the catalytic behavior of
ruthenium-based compounds [49-51], motivated us for realizing the catalytic tests of ruthenium
oxo-cluster 2 in this organic transformation. In typical experiment, the ruthenium complex 2 (1.2
mmol% Ru) was dissolved in appropriate solvent and 1 mmol of allylic alcohol substrate was
added. The results of catalytic tests performed with ruthenium complex 2 are summarized in
Table 4. A set of variables were examined in order to optimize reaction conditions. The first
variable was a temperature (Table 4, entries 1b, 4a-c). The 1-penten-3-ol was chosen as a model
substrate. It was observed that the catalytic activity of 2 decreases as the temperature increases,
and the higher performance with TOF of 1.7 h-1 is stabilized at 80 °C. Therefore, this
temperature was used as an optimal for the next set of experiments.
The earliest study of Peterson and Larock [52], focused on the isomerization of primary
and secondary allylic alcohols, demonstrates considerable improvement of catalytic activities
upon addition of base, i.e. NaHCO3. Thus, in order to evaluate the effect base, the corresponding
tests were conducted with NaHCO3, KOH and without additive (Table 4: entry 1e, 5a and 5b,
respectively). Unsatisfactory conversion was obtained in experiment 5b, where the isomerization
reaction was realized at 80 °C without the base. Meanwhile, the addition of KOH improves
isomerization of 1-octen-3-ol to 3-octanone reaching TOF of 0.01 h-1. The best catalytic
performance of 2 has been obtained with addition of NaHCO3 (TOF 1.7 h-1).
Using the optimized catalytic conditions, the next task was to determinate the effect of
solvent. A series of substrates (i.e.: 3-buten-2-ol, 1-penten-3-ol, 1-hexen-3-ol, 1-hepten-3-ol and
1-octen-3-ol) was examined with three commonly employed solvents (Table 4: entries 1a-e, 2a-
e, 3a-e). A survey of the results show that the use of water as reaction medium don’t favor the
isomerization reaction and the catalytic activity decreases as the length of aliphatic chain of the
substrate increases. As expected, the hydrophobicity of substrate alcohols increase in order 3-
buten-2-ol > 1-penten-3-ol > 1-hexen-3-ol > 1-hepten-3-ol > 1-octen-3-ol, that stimulate the
aqueous reaction solution to became a inhomogeneous biphasic mixture and lead to worsening of
interactions between substrate and dissolved homogeneous catalyst.
12

Table 4. Isomerization of allylic alcohols catalyzed by triruthenium oxo-cluster 2
Substrate
Product
Solvent
Base
T, °C
t, h
yield, % TOF, h-1
1a
45
97
1.8
1b
1c
47
41
95
86
1.7
1.7
DMF
NaHCO3
80
1d
1e
2a
2b
2c
2d
2e
3a
3b
3c
3d
3e
4a
4b
4c
5a
5b
41
65
23
23
30
71
60
48
24
12
24
24
48
71
26
24
72
96
67
94
96
74
38
95
5
1.9
0.9
3.4
3.5
2.1
0.4
1.3
0.1
0.1
0.1
0
EtOH NaHCO3
80
2
H2O
NaHCO3
80
1.5
0
1
0.03
0.01
0.01
1.6
0.03
0
RT
65
0.6
6.5
51
1
DMF
DMF
NaHCO3
100
80
KOH
–
80
0
Reaction performed under N2 atmosphere in Schlenk tube using 1.2 mmol% (Ru) of catalyst and 1 mmol of substrate. TOF =
mmol of product / mmol of catalyst (basing on Ru) / hours.
In contrast, the use of DMF as the solvent led to almost complete conversion of the started
alcohol and afforded desired ketone with yields up to 97% (TOF of 1.8 h-1). Notably, that effect
of reactivity of alcohol substrate as a function of length of aliphatic chain is not observed with
DMF solution, which is reasonable considering that the DMF is one of the best solvents for
variety of chemical substances. Finally, as expected the highest catalytic performances of 2 were
obtained with ethanol as reaction solvent reaching TOF values up to 3.5 h-1 (Table 4: entries 2a-
e).
13

4. Conclusions
The trinuclear ruthenium-oxo cluster [Ru3(II,III,III)O(CH3CO2)6(H2O)3]·2H2O (2) firstly has
prepared with high yield by one-electron reduction of
been
[Ru3(III,III,III)O(CH3CO2)6(CH3OH)3]·CH3CO2 precursor 1 using muccic acid as reductor under
hydrothermal conditions at 180 °C. Single-crystal X-ray analysis reveals that synthesized
compound 2 is mixed-valence oxo-centered trinuclear ruthenium acetate complex where
oxidation state delocalization at room temperature between one RuIII and one RuII sites are
postulated basing on the precise analysis of crystallographic data and valence bond calculations.
Furthermore, the mixed-valence states in RuII–µ3O–Ru2 structural unit have been confirmed
III
basing on the XANES spectral analysis of [Ru3(II,III,III)O(CH3CO2)6(H2O)3]·2H2O.
The crystal structure of 2 displays 2D supramolecular networks extended through
multiple hydrogen bonds which can be simplified to hexagonal plane (hcb) topological net.
Moreover, triruthenium oxo-cluster 2 is thermally stable up to 250 °C, and its molecular
structure remains unchanged upon dehydration.
The catalytic tests performed for hydration of nitriles in water medium reveal that oxo-
cluster 2 shows to be active homogeneous catalyst in this transformation reaching conversions up
to 99%. Notably, the highest catalytic performances were obtained under experimental sets
where heating have been assisted by microwaves. The evaluation of activity of various nitrile
substrates suggests that hydration of nitriles is likely biphasic and follow the order of
benzonitrile > 3-nitrobenzonitrile > phenoxyacetonitrile > 3-chlorbenzonitrile > hexanenitrile >
3-methylbenzonitrile. In addition, the nature of electron-withdrawing group as well as its
position also shows to affect the reaction efficiency, and the catalytic activity was decreasing in
order Cl > NO2 > CH3 > OCH3 and in sequence para > meta > ortho positions. Moreover, the
recyclability tests for catalyst 2 reveal constant catalytic activity up to three consecutive runs.
Beside remarkable behavior of 2 in hydration of nitriles reaction, the triruthenium oxo-
cluster shows to be active in isomerization of allylic alcohols reaction in presence of NaHCO3.
Under evaluation of reaction conditions we determine that optimal temperature for this organic
transformation is 80 °C. The set of experiments suggest that the best conversions of variety of
substrates were achieved using DMF as a solvent which is expected due to the miscibility
effects. Furthermore, the substrate activity evaluations define order of 3-buten-2-ol > 1-penten-3-
ol > 1-hexen-3-ol > 1-hepten-3-ol > 1-octen-3-ol.
Acknowledgements. The authors thank FEDER and Spanish MINECO for financial support under
projects MAT2013-40950-R, Consolider ORFEO. A.D. also thanks to Spanish Ministerio de Educación,
Cultura y Deporte by their pre-doctoral FPU grant (AP2008-03942). The synchrotron measurement time
was provided by ESRF (Grenoble, France) within the research project No. CH-3428. The authors thank
staff members of BM25 (SpLine): Dr. Germán R. Castro, Dr. Iván Da Silva González and Dr. Pilar Ferrer
Escorihuela for the technical support, guidance and useful suggestions on XANES data collection at the
synchrotron facility.
Supporting Materials containes digital microgrphas of 1−8 crystal habits, SEM images,
observed and calculated X-ray powder diffraction patterns, IR spectra, additional structure plots,
kinetic plots for hydration of nitriles reaction and images of recyclability tests.
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17

18

• The trinuclear ruthenium-oxo cluster was prepared under hydrothermal conditions.
• The crystal structure was determined basing on single-crystal X-ray diffraction data
• Trinuclear ruthenium-oxo array exhibits the trapped-valence state delocalization at room
temperature
• The complex reveals to be active catalyst for hydration of nitriles and isomerization of
allylic alcohols reactions.
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