Synthesis, structural characterization, and activity on the transfer hydrogenation reaction of dimesitylacetonate piano stool organometallic complexes of Ru(II), Rh(III), and Ir(III)

Article abstract of DOI:10.1016/j.molstruc.2019.04.029

A new small family of Ru, Rh, and Ir piano-stool complexes partially containing the bulky acac^diMes, p-Cymene and Cp* ligands was synthetized and characterized. Their catalytic activity in transfer hydrogenation reaction was tested. The turn ov

Full text of DOI:10.1016/j.molstruc.2019.04.029

Accepted Manuscript
Synthesis, structural characterization, and activity on the transfer hydrogenation
reaction of dimesitylacetonate piano stool organometallic complexes of Ru(II), Rh(III),
and Ir(III)
M. Fernanda Ibarra-Vázquez, José G. Alvarado-Rodríguez, Ana C. Esqueda, I. Idalia
Rangel-Salas, Oracio Serrano
PII:
S0022-2860(19)30424-7
DOI:
https://doi.org/10.1016/j.molstruc.2019.04.029
MOLSTR 26401
Reference:
To appear in: Journal of Molecular Structure
Received Date: 12 October 2018
Revised Date: 14 March 2019
Accepted Date: 5 April 2019
Please cite this article as: M.F. Ibarra-Vázquez, José.G. Alvarado-Rodríguez, A.C. Esqueda, I.I. Rangel-
Salas, O. Serrano, Synthesis, structural characterization, and activity on the transfer hydrogenation
reaction of dimesitylacetonate piano stool organometallic complexes of Ru(II), Rh(III), and Ir(III), Journal
of Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.04.029.
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ACCEPTED MANUSCRIPT
TOC
Ru, Rh and Ir β-Dimesitylaceonate piano stool complexes were used as catalytic precursors on the transfer hydrogenation reaction; the
microwave irradiation heating raise the TOF values in two order of magnitude comparing with the thermal conditions.
TON-MW
TON-H
TOF- MW
TOF-H
2
00
00
0
1
3-A
2-A
1-A
3-B
2-B
1-B
3-C
2-C
1-C

ACCEPTED MANUSCRIPT
Journal of Molecular Structure
Synthesis, structural characterization, and activity on the transfer hydrogenation
reaction of dimesitylacetonate piano stool organometallic complexes of Ru(II),
Rh(III), and Ir(III)
M. Fernanda Ibarra-Vázquez,a,d José G. Alvarado-Rodríguez, Ana C. Esqueda, I. Idalia Rangel-
b
c
a* d*
Salas and Oracio Serrano
a) Universidad de Guadalajara, Centro Universitario de Ciencias Exactas e Ingenierías,
Departamento de Química, Blvd. Marcelino García Barragán, 1421, esq. Calzada Olímpica, C.P.
4
4430 Guadalajara, Jalisco, México.
b) Universidad Autónoma del Estado de Hidalgo, Unidad Universitaria, km 4.5 Carretera Pachuca-
Tulancingo, C.P. 42184, Mineral de la Reforma, Hidalgo, México.
c) Escuela de Nivel Medio Superior de León, Universidad de Guanajuato, Boulevard Hermanos
Aldama S/N, San Miguel, 37000, León, Gto., México.
d) Departamento de Química, Sede Pueblito de Rocha, Universidad de Guanajuato, Cerro de la
Velada s/n, Pueblito de Rocha, C.P. 36040, Guanajuato, México.
Corresponding Author
*
E-mail for I. I. R.-S.: iirangel@yahoo.com.mx
E-mail for O.S.: oraciosinh@ugto.mx
*

ACCEPTED MANUSCRIPT
Abstract
A new small family of Ru, Rh, and Ir piano-stool complexes partially containing the bulky
diMes
acac
, p-Cymene and Cp* ligands was synthetized and characterized. Their catalytic activity in
transfer hydrogenation reaction was tested. The turn over frequency (TOF) values for aromatic- and
alkyl-ketones giving the corresponding desired alcohols were increased in two orders of magnitude
by using microwave heating compared with conventional thermal method.
Keywords: Transfer hydrogenation reaction; Piano stool complexes; Microwave heating; Iridium,
Rhodium, Ruthenium

ACCEPTED MANUSCRIPT
1
. Introduction
Hydrogenation reaction is a valuable and fundamental tool in Modern Chemistry in both industrial
and laboratory scales [1]. Two approaches can be visualized to produce chemical hydrogenate
compounds. The first one is by means of direct activation of the dihydrogen molecule by transition
metals under either homogenous or heterogenous catalysis [2], and, more recently, by frustrated
Lewis pairs [3]. The other approach is via transfer hydrogenation (TH) where the hydrogen source
is other than H and an organometallic complex can act as catalyst [4-5]. This methodology can be
2
carried out with economical hydrogen sources under low pressure and mild temperature conditions
[
6]. Consequently, TH reaction has been considered as a good choice in Green Chemistry [7].
On the other hand, it is well-known that organometallic catalysts containing Rh, Ir, Pd, Pt, or Ru
metallic centers provide greater selectivity in homogeneous catalysis and are effective under mild
conditions with high conversion rates. Thus, in seminal papers, Vidal, Krause, and Brown have
independently reported that Ru, Rh, and Ir complexes having β-acetylacetonate ligands can be used
as catalyst in TH protocols [8-10]. In the same vein, Ikariya and Noyori published also a remarkable
work related to TH using organoiridium complexes linked to cyclopentadienyl ligands [11-12]. In
addition, cymene-ruthenium complexes showed acceptable activities in the TH reaction [13]. It is
noteworthy that organometallic heteroleptic complexes of the VIIIB group comprising both
aromatic rings and O,O’-chelating β-diketonate ligands have gained increasing interest in recent
years mainly due to their notable features such as antimicrobial [14] and cytotoxic activities [15-
20].
6
II
III
III
Herein we report the synthesis and structural characterization of three d Ru , Rh , and Ir piano-
diMes
diMes
stool type organometallic complexes of general formula [LM(acac
)Cl] [acac
= 3-oxo-1,3-
dimesitylprop-1-en-1-olato; L= cymene, M= Ru (1); L= pentamethylcyclopentadienide (Cp*), M=
Rh (2); L= Cp*, M= Ir (3)] and their activity as catalyst in homogeneous transfer hydrogenation
under different heating conditions (conventional vs. microwave).
2
. Experimental section
2.1 Materials and methods
All reactions were carried out in oven-dried glassware under nitrogen atmosphere unless otherwise
noted. The dimeric compounds [LMCl ] (M = Rh, Ir, L= Cp*; M = Ru, L= p-cymene) and
2
2

ACCEPTED MANUSCRIPT
diMes
Hacac
3]. All solvents and Silica Gel, 70-230 mesh, 60 Å, for column chromatography where purchased
to Sigma-Aldrich. NMR spectra were recorded on a Bruker Ultra Shield 500 MHz spectrometer at
(3-oxo-1,3-dimesitylprop-1-en-1-ol) were prepared according to literature methods [21-
2
1
13
room temperature. H and C chemical shifts (δ) are given in ppm relative to TMS. The solvent
signal was used as reference and the chemical shifts converted to the TMS scale. Elemental analysis
experiments were carried out on 2400 CHNS/O Series II System PerkinElmer. X-ray diffraction
data for all complexes were collected on an Oxford Diffraction Gemini CCD diffractometer.
Melting points were recorded in a Fisher-Johns apparatus. Anton-Parr microwave reactor model
Monowave EDU was used for the catalysis experiments. UV-Visible spectra were acquired in a
Shimadzu UV-2401PC spectrophotometer. IR spectra were acquired in a Bruker Tensor27
spectrophotometer (KBr pellets, 99% of purity).
2.2. General procedure for the synthesis of complexes 1-3.
The corresponding dimeric complex [LMCl ] (0.4 mmol) was dissolved in dichloromethane (5
2
2
diMes
mL) and added dropwise to a mixture of Hacac
(0.8 mmol) and triethylamine (0.3 mL) also
dissolved in dichloromethane (10 mL). The mixture was stirred for 1 h; then, was vacuum
evaporated to give a solid. Next, diethyl ether (20 mL) was added to the solid at room temperature.
The solution was filtered, concentrated, and cooled to ca. −15 °C overnight to give crystalline
compounds.
2.2.1 Selected data for complex 1.
Deep orange crystals, Yield 83%. M.p. 225 °C. Anal. Calcd. for C H ClO Ru: C, 64.45; H, 7.26. Found: C,
3
5
47
3
1
3
6
1
4.51; H, 6.67. H NMR (500 MHz, CDCl ): 6.78 (s, 4H, H-5); 5.49 (d, J = 6.4 Hz, 2H, H-11); 5.29 (s,
3 HH
3
3
H, OCCHCCO, H-1); 5.23 (d, J = 6.4 Hz, 2H, H-10); 2.95 (sept, J = 6.9 Hz, 1H, H-12); 2.27 (s, 12H,
HH
HH
3
13
1
H-7); 2.24 (s, 6H, H-8); 1.60 (s, 3H, H-14); 1.33 (d, J = 6.9 Hz, 6H, H-13). C{ H} NMR (125.7 MHz,
HH
CDCl ): 187.0 (C-2); 138.1 (CH, CqAracac); 137.7 (CH, CqAracac); 134.5, 128.2 (CH, CHAracac); 103.7 (C-1);
3
1
00.6; 97.7; 82.3 (CH, CHArCym); 79.2 (CH, CHArCym); 30.7 (C-12); 22.7 (C-13); 21.2 (C-8); 19.7 (C-7);
−
1
1
8.3 (C-14). FT-IR (ATR, cm ): 2917.1 ν( C-H), 1613.0 ν(C=O), 1519.0 ν(C-C). UV/Visible (CH CN) 289
3
nm.
2.2.2. Selected data for complex 2.
Deep red crystals, Yield 95%. M.p. 200 °C. Anal. Calcd. for C H ClRhO ⋅C H O: C, 64.17; H, 7.39.
3
1
38
2
4
10
1
Found: C, 64.16; H, 7.98. H NMR (500 MHz, CDCl ): 6.78 (s, H-5); 5.20 (s, 1H, H-1); 2.30 (s, 12H, H-7);
3
1
3
1
2
.24 (s, 6H, H-8); 1.66 (s, 15H, C Me ). C{ H} NMR (125.7 MHz, CDCl ): 186.9 (C-2); 139.0 (CH,
5 5 3
CqAracac); 137.3 (CH, CqAracac); 128.0 (CH, CHAracac); 103.3 (C-1); 92.3 (d, JC-Rh = 9.2 Hz, C Me ); 21.2 (C-
5
5

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−
1
8
); 19.6 (C-7); 8.8 (C Me ). FT-IR (ATR, cm ): 2918.0 ν(C-H), 1612.1 ν(C=O), 1523.0 ν(C-C). UV/Visible
5 5
(
CH CN) 291 nm.
3
2.2.3. Selected data for complex 3.
Yellow crystals, Yield 84%. M.p. 197 °C. Anal. Calcd. for C H ClIrO : C, 55.55; H, 5.71. Found: C, 55.38;
3
1
38
2
1
H, 6.19. H NMR (500 MHz, CDCl ): 6.78 (s, 4H, H-5), 5.35 (s, 1H, H-1); 2.33 (s, 12H, H-7); 2.25 (s, 6H,
3
1
3
H-8); 1.63 (s, 15H, C Me ). C NMR (125.7 MHz, CDCl ): 184.6 (C-2); 138.3(CH, CqAracac); 137.3 (CH,
5
5
3
CqAracac); 128.6 (CH, CHAracac); 105.3 (CH, OCCHCCO); 83.6 (C Me ); 21.2 (C-8); 19.4 (C-7); 9.2
5
5
−
1
(
C Me ). FT-IR (ATR, cm ): 2916.0 ν( C-H), 1611.0 ν(C=O), 1520.1 ν(C-C). UV/Visible (CH CN) 285
5 5 3
nm.
2.3. Structural determination of compounds 1-3.
X-ray diffraction data of complexes 1-3 were collected at room temperature on an Oxford
Diffraction Gemini CCD diffractometer with either graphite-monochromatic Cu-K (1.54184 Å) or
Mo-K radiation (0.71073 Å). Data were integrated, scaled, sorted, and averaged using the CrysAlis
software package. By using Olex2 [24] the structures were solved with the ShelXT [25] structure
solution program using intrinsic phasing and refined with the ShelXL [25] refinement package
using Least Squares minimization. All nonhydrogen atoms were refined anisotropically. The
positions of the hydrogen atoms were kept fixed with a common isotropic displacement parameter.
The crystal structures of isostructural compounds 2 and 3 displayed positional disorder in one of the
mesityl groups and in the Cp* ring; the main components have the occupancies of 0.61(3) and
0.72(2) for 2, and 0.60(3) and 0.62(3) for 3, respectively. SAME, SIMU, RIGU, and FLAT
instructions were used during the refinement of the disorder.
2.4. General procedure for the transfer hydrogenation reactions.
2.4.1. Conventional heating.
A Schlenk flask with a Teflon stopper was charged with the substrate desired (2 mmol), isopropanol
5 mL), KOH solution (20 ppm, 0.4 mL), and the corresponding organometallic complex (0.02
mmol). The mixture reaction was warmed to 80 °C. The best results achieved are collected in table
. Yields were obtained after a chromatographic column purification using hexanes as eluent. The
proposed structure is based by NMR experiments analysis using CDCl as solvent.
(
3
3
2.4.2. Microwave heating.
A microwave reactor with a Teflon stopper was charged with the substrate desired (2 mmol),
isopropanol (2 mL), KOH solution (20 ppm, 0.2 mL), and the corresponding complex (0.02 mmol).
The mixture reaction was warmed to 80 °C. After the screening time, the best results achieved are
collected in table 4. Yields were obtained after a chromatographic column purification using
hexanes as eluent. The proposed structure is based by NMR experiments analysis using CDCl as
3
solvent.

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2
.4.3. Chromatographic purification.
For each reaction mixture, a chromatographic purification was carried out in a column of 3.0
cm of diameter and 10.0 cm of height, packed with silica gel (technical grade, pore size 60Å,
70-230 mesh, 63-200 µm from Sigma-Aldrich). The mixture was eluted with hexanes.
3
. Results and discussion
3.1 Synthesis of the complexes
6
diMes
diMes
The novel d organometallic complexes with general formula [LM(acac
)Cl] [acac
= 3-oxo-
1,3-dimesitylprop-1-en-1-olato; L= cymene, M= Ru (1); L= Cp*, M= Rh (2); L= Cp*, M= Ir (3)]
were obtained as orange, red and yellow solids, respectively, in good yields by the reaction of
diMes
Hacac
(3-oxo-1,3-dimesitylprop-1-en-1-ol) with 0.5 equivalents of the corresponding dimeric
species [LMCl ] , and a slight excess of triethylamine (TEA) in dichloromethane at room
2
2
temperature (Scheme 1).
diMes
Scheme 1. Synthesis of heteroleptic complexes [LM(acac
)Cl]; NMR numbering in gray color.
Complexes 1-3 were characterized by spectroscopic and analytical techniques as elemental analysis,
m.p.; IR, UV-Vis and NMR in 1D and 2D experiments. Their structures were corroborated by
single crystal X-ray diffraction experiments. Additionally, they can be stored in air atmosphere at
room temperature for a year without decomposition and their solutions do not show changes after a
week, according to the proton NMR spectra.
3.2. NMR spectra
The analysis of the proton NMR for complex 1 does not offers difficulty, thus, the singlet signals
observed at 6.78 (4H), 5.29 (1H), 2.27 (12H), and 2.24 (6H) ppm are assigned to the aromatic
diMes
(CHMs), methine (OC-CH-CO), orto- and para-methyl protons of the acac
fragment,
respectively, in good agreement with the spectra data for analogous complexes [26-29].
3
3
Furthermore, the signals observed at 5.49 (d, 2H, J = 6.4 Hz, CH ) and 5.23 (d, 2H, J = 6.4
HH
Ar
HH
3
3
Hz, CH ); 2.95 (sept, 1H, J = 6.9 Hz, CH(CH ) ), 1.60 (3H, CH ) and 1.33 (d, 6H, J = 6.9 Hz,
Ar
HH
3 2
3
HH

ACCEPTED MANUSCRIPT
CH(CH ) ) ppm, are assigned to the protons from the Cymene ring (Figure 1). In agreement, the
3
2
1
3
1
C{ H} spectra show six signals at 187.0 (C , CO), 138.1 (C ), 137.7 (C ), 128.2 (CH ), 103.7
q
q
q
Ar
(
CH, OC-CH-CO), 21.2 (CH , para-CH ) and 19.7 (CH , orto-CH ) ppm, which are assigned to
3 Ar 3 Ar
DiMes
the Acac
fragment. Furthermore, the remaining signals at 100.6 (C , Cym), 97.7 (C , Cym), 82.3
q q
(
CH, Cym), 79.2 (CH, Cym), 30.7 (CH, CH(CH ) ), 22.7 (CH , CH(CH ) ) and 18.3 (CH ) ppm, are
3 2 3 3 2 3
assigned to the cymene ring. Later, the NOESY spectra shows a NOE between the orto-CH groups
3
6
(from the mesityl ring) with the isopropyl and methyl groups from the
η
-cymene ring, suggesting a
free rotation in solution at room temperature (See S.I.).
1
Figure 1. H NMR spectra for complex 1 (CDCl , 25 °C).
3
On other hand, the proton NMR spectra for complex 2 shows four singlet signals at 6.78 (4H,
CHMs), 5.20 (1H, OC-CH-CO), 2.30 (12H, orto-CH ), 2.24 (6H, para-CH ) ppm, which are
3
3
diMes
assigned to the acac
fragment. Additionally, a singlet observed at 1.66 (15H) ppm is assigned to
1
3
1
1
the methyl groups on the Cp* ring. In the C{ H} NMR spectra the signals observed at 92.3 (d, J
C-
Rh= 9.2 Hz) and 8.6 ppm are assigned to the carbons from the Cp* ring. Down field, a singlet
observed at 186.9 and four signals from 139.0 to 137.3 ppm, are assigned to the carbonyl and
diMes
quaternary carbon atoms of the acac
fragment, respectively. Finally, the signals at 128.0, 103.3
and 21.2-19.6 ppm are assigned to the CH , OC-CH-CO and CH groups. Compound 3 shows a
Ar
3
very similar 1D and 2D NMR spectroscopic data, compared with 2, and will not be discussed.
3.3. IR data
−
1
IR spectra for complexes 1–3 show an intense band around 1600 cm which is assigned mainly to
−
1
the C-O bond stretches. The band around 1400 cm has been assigned to multiple vibrations
−
1
arising for CH bending. The band around 1500 cm is assigned to C–C stretching. Finally, around
3

ACCEPTED MANUSCRIPT
−
1
2
900 cm intense bands are assigned to symmetrical and asymmetrical stretches of C–H of methyl
and methylenes, in good agreement with other analogous complexes previously reported.
3.4. X-Ray diffraction studies
Suitable crystals of compounds 1–3 were grown by slow evaporation at room temperature.
Crystallographic data are displayed in Table 1 and selected bond lengths and angles are given in
Table 2. The molecular structures are showed in Figures 2 and 3, where only the main components
of the disorder observed for 2 and 3 are displayed. The piano-stool structure for compounds 1–3
was corroborated by means of X-ray diffraction studies at room temperature. The coordination
sphere on the metallic cations is formed by the coordination of one chloro ligand and by one
diMes
2
acac
moiety, displaying a κ -O,O’-donor mode, besides the linking of one aromatic ring
6
5
exhibiting either
η
(in 1) or η (in 2 and 3) coordination mode. The molecular structures of these
complexes are directly comparable with others having an analogous coordination entity; in table 2
we listed relevant structural parameters for some selected complexes, and a more complete data set
is tabulated in Tables S.I.1 and S.I.2 (see E.S.I). From the data, it can be noted that the M–O and the
centroid-to-metallic center distances are like those reported. In the same vein, the angles between
the “legs” of the piano-stool structure are in the 80.2–89.6° range for all analogous complexes [14-
29,30], indicating a common feature in this type of complexes.
C2
C3
C1
O2
O1
Cl
Ru
6
II
diMes
Figure 2. Molecular structure of [
distances (in Å): Ru–Cl, 2.4187(8); Ru–O1, 2.092(2); Ru–O2, 2.079(2); O1–C1, 1.266(3); O2–C3, 1.272(3); Ru–C,
.151(4) to 2.189(3).
η
-(cymene)Ru (acac
)Cl] (1). ORTEP at 40% probability. Selected bond
2

ACCEPTED MANUSCRIPT
C2
C1
C3
C2
C1
O1
C3
O2
Cl
O2
Cl
O1
Rh
Ir
5
III
diMes
Figure 3. Molecular structures of [
Selected bond distances (in Å): For 2: Rh–Cl, 2.403(3); Rh–O1, 2.068(6); Rh–O2, 2.099(7); O1–C1, 1.28(1); O2–C3,
.28(1); Rh–C, 2.11(2) to 2.16(2). For 3: Ir–Cl, 2.404(2); Ir–O1, 2.102(4); Ir–O2, 2.088(5); O1–C1, 1.270(7); O2–C3,
.265(9); Ir–C, 2.13(2) to 2.17(2).
η
-(Cp*)M (acac
)Cl] [M = Rh (2) left; Ir (3) right]. ORTEP at 40% probability.
1
1
III
III
III
In particular, the close similitude of the M –C, M –O, and M –Cl distances in the isostructural
Group 9 complexes 2 and 3 can be attributed to the lanthanide contraction effect present in the 5d
iridium(III) cation.
Table 1. Crystallographic data and refinement parameters for compounds 1–3.
Compound
1
2
3
Empirical formula
C H O ClRu
C H O ClRh
C H O ClIr
3
1
37
2
31 38
2
31 38
2
Formula weight
Temperature (K)
Crystal system
Space group
578.12
294
580.97
294
670.26
294
orthorhombic
monoclinic
C2/c
triclinic
P−1
P2 2 2
1 1
1
a(Å)
b(Å)
c(Å)
α(°)
β(°)
γ(°)
11.76380(10)
13.37800(10)
18.3449(2)
90
31.862(6)
8.8057(9)
21.998(3)
90
8.8927(4)
11.2412(6)
16.7404(9)
78.760(5)
74.961(4)
73.502(5)
1536.26(15)
90
95.682(16)
90
90
3
Volume(Å )
2887.05(5)
6141.7(15)
Z
4
8
2
3
ρcalcg/cm
1.330
1.257
1.449

−
1
ACCEPTED MANUSCRIPT
5.425 0.666
ꢀ
(mm )
9.381
F(000)
1200.0
2416.0
668.0
3
Crystal size/mm
0.5 × 0.4 × 0.2
0.4 × 0.2 × 0.2
0.4 × 0.3 × 0.2
Reflections collected
42058
5895 [Rint = 0.0308,
Rsigma = 0.0150]
5895/0/325
39014
6265 [Rint = 0.2268,
Rsigma = 0.1737]
6265/0/327
18324
6480 [Rint = 0.0770,
Rsigma = 0.0736]
6480/0/327
Independent reflections
Data/restraints/parameters
2
Goodness-of-fit on F
1.050
1.016
1.095
Final R indexes [I>=2σ (I)]
R = 0.0235, wR = 0.0605
R = 0.1006, wR = 0.1759
R = 0.0711, wR = 0.1764
1
2
1
2
1
2
Final R indexes [all data]
R = 0.0238, wR = 0.0610
R = 0.2143, wR = 0.2227
R = 0.0834, wR = 0.1893
1
2
0.30/−0.55
1566701
1
2
1
2
5.34/−2.88
1566699
−3
Largest diff. peak/hole (e Å )
1.02/−0.74
15666700
Data CCDC
On the other hand, one of the subtle structural differences between the complexes previously
reported [14-15,18-20], and the complexes herein described comes from the use of the mesityl
group. The unsubstituted and substituted phenyl groups in the reported complexes are practically
coplanar to the C(O)CHC(O) moiety; we used the torsional angles O-C-C-C to analyze the
structural data. In the structural data displayed in the Tables S.I.1 and S.I.2, the torsional angles
range from 2.0 to 32.3°. It has been stated [17] that the coplanarity of the aromatic rings is due to
the presence of intramolecular hydrogen bonding in some complexes, where the H-acceptors are the
oxygen atoms of the acac ligand. It was also commented that the presence of bulkier groups in para
position does not increase the angle between the phenyl groups [17]. In contrast, in 1–3 the angles
vary from 63.8 to 84.0° due to the bulkiness of the Mes groups, where the methyl groups are,
evidently, in ortho- and para-positions.
Table 2. Structural data for heteroleptic complexes [(cymene)M(acac)X] (left) and [(Cp)M(acac)X] (right), where X =
atom directly bonded. Bond distances in Å and angles in degrees.
Me
iPr
M
M
1
1
14
11 14
O
1
O
X
X
13
13
12
1
21
21
O
O
1
2
2
4
24
2
3
23
2
2
22
2
2
Angles
Torsion
angles
11-12-13-14 M–centroid
11
Distance
M–O
O -M-X
Distance
M
X
Ring 1
Ring 2
21
O -M-X
11
21
O -M-O
4.53(6)
86.13(6)
7.80(8)
21-22-23-24
8
II
2.092(2)
63.8
1.647
80.4
1
Ru
Cl
Mes
Mes
2
.079(2)
8

ACCEPTED MANUSCRIPT 8
7.6(2)
86.3(2)
9.5(2)
III
2.099(6)
.068(6)
77.6
84.0
2
Rh
Cl
Cl
P
Mes
Mes
Ph
Mes
Mes
Ph
1.773
1.776
1.700
1.649
1.653
2
8
8
5.2(1)
III
2.103(5)
2
71.7
70.6
3
Ir
84.7(2)
8.4(2)
7.1
86.1
.088(5)
8
8
II
2.071
11.9
32.3
Ref19
Ref30
Ref18
Ru
Ru
Ru
2
.078
8
8.3
8
5.4
II
II
2.065
.064
14.4
2.2
Cl
Cl
Ph
Ph
84.9
2
8
7.7
8
4.3
2.076
.058
15.3
23.3
OMe
tBu
85.0
2
8
8.1
HO
8
86.1
8
4.2
II
2.070
.078
15.2
2.8
Ref20
Ru
Cl
Cl
Ph
1.654
1.756
2
6.5
HO
8
89.6
8
7.7
III
III
2.103
.097
2.0
17.4
Ref15 Rh
Ref15 Rh
2
6.0
8
87.9
8
8
85.6
8
8
5.3
2.087
.076
14.9
16.0
OMe
tBu
1.748
1.752
1.753
Cl
Cl
Cl
2
8.1
6.3
III
2.103
.083
16.0
8.6
Ref15
Ir
Ph
Ph
Ph
Ph
2
7.1
8.7
III
2.100
.082
15.1
8.0
Ref14 Rh
87.9
7.8
2
8
3.5. Transfer hydrogenation studies
Once the complexes 1–3 were properly characterized, we tested their role as catalyst in transfer
hydrogen reaction (TH) of ketones to alcohols under conventional heating. We chose the initial
conditions considering previous reports [5,13] (Scheme 2 and Table 3).
Scheme 2. Transfer hydrogenation reaction catalyzed by complexes 1-3.
As initial study, acetophenone was tested as substrate in a model reaction (see E.S.I.). Thus,
complex 1 shows an excellent conversion of acetophenone to 1-phenyl-ethanol in high yields,
isolated after chromatographic purification, with a TON and TOF values of 97 and 5, respectively

ACCEPTED MANUSCRIPT
(
Table 3, entry 1, Figure 3). Compounds 2–3 also gave acceptable conversions, with similar TON
and TOF values (Table 3, entries 2–3). When cyclohexanone was used as substrate, a quantitative
conversion to cyclohexanol was obtained (Table 3, entries 4–6). It was found that the reduction of
the reaction time from twenty hours to five does not affect the conversion, giving similar TON
values with a considerable increase on the TOF values (ca. 20, Table 3, entries 7–9). Notably, the
TH reaction of benzophenone can also occur to provide the desired diphenylmethanol in excellent
yields, when compounds 1 and 2 were used as catalyst (Table 3, entries 11–12). However, when
iridium complex was used, a dramatic drop in conversion to 32 % was observed (Table 3, entry 10).
This can be attributed to the deactivation via the formation of a stable iridacycle [31-32] or by
losing the Cp* ring [6].
Table 3. Hydrogen transfer catalysis by conventional conditions.
a
b
c
Entry Catalyst Time (h)
Yield (%)
97
Substrate
Product
TON
97
TOF
5
1
2
3
4
5
6
7
8
9
1
2
20
20
5
67
67
3
3
90
91
4
1
2
3
1
2
3
1
2
3
100
99
100
99
5
5
OH
99
99
5
100
100
100
98
100
100
100
97
20
20
20
5
1
0
1
2
O
OH
1
1
20
94
93
4
32
32
1
–1
a) Isolated product after chromatographic purification. b) TON = mol product/mol cat. c) TOF = (mol product/mol cat.) × h
On the other hand, it is well known that microwave (mw) irradiation has become an outstanding
alternative for conventional heating on different synthesis proposes [33]. In independent works,
Zhang and Aguirre showed in seminal papers on TH reaction that Ru, Rh, or Ir complexes partially
supported by Cp´s or acac ligands can be used as catalyst on the reduction of nitrobenzene or
acetophenone [34-35]. Thus, having in mind that just a few acetylacetonate complexes have been
tested under mw conditions, we carried out a comparative set of TH reactions of ketones under this
approach, by using 1–3 as catalyst. The results are collected in table 4 and figure 3. Interestingly,
acetophenone can be converted to 1-phenyl-ethanol in yields up to 80 % in two hours, increasing
the TOF values in one order of magnitude, from ca. 4 to 40 (Table 4, entries 1-3, Figure 3).
Similarly, higher TOF values were obtained for the TH reaction of cyclohexanone, giving a striking
increment of two orders of magnitude, from ca. 4 to 200 (Table 4, entries 4–6). Similar results were
obtained when benzophenone was used as substrate in the TH reaction (Table 4, entries 7–9).

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Unfortunately, our attempts to perform the nitro-benzene reduction under similar conditions were
unsuccessful.
Table 4. Hydrogen transfer catalysis by microwave heating conditions.
a
b
c
Entry Catalyst Time (h) Yield (%)
Substrate
Product
TON
80
TOF
40
1
2
3
4
5
6
7
8
1
2
3
1
2
3
1
2
80
100
80
OH
2
100
80
50
40
100
100
100
98
100
100
100
98
200
200
200
50
OH
0.5
OH
95
93
47
2
9
3
50
50
25
-1
a) Isolated product after chromatographic purification. b) TON = mol product/mol cat. c) TOF = (mol product/mol cat.) × h .
TON-MW
TON-H
TOF- MW
TOF-H
2
00
00
0
1
3-A
2-A
1-A
3-B
2-B
1-B
3-C
2-C
1-C
Figure 3. Comparative analysis of TON and TOF values for TH reaction under thermal and microwave heating by 1-3.
4
. Conclusions

ACCEPTED MANUSCRIPT
6
In summary, herein we report the synthesis and characterization of three d organometallic
diMes
complexes of Ru, Rh, and Ir of piano-stool half-sandwich type containing the bulky acac
ligand. The complexes can be used as excellent catalyst on the TH reaction of aromatic- and alkyl-
ketones giving the corresponding desired alcohols. Moreover, when microwave irradiation was used
as heating source, the TOF values were improved by two orders of magnitude, compared with the
conventional heating process. Finally, Ru and Rh complexes shows slightly activities compared
with the Ir specie, this behavior is attributed to their low reactivity and lability.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the website at DOI: Experimental
procedures, characterization data and optimization attempt for TH, including NMR spectra of new
compounds, and single crystal X-ray diffraction data for compounds 1–3.
Notes
The authors declare no competing financial interest.
Acknowledgements
M. F. I.-V. is grateful to the CONACYT for a Doctoral Fellowship (273965). O. S. is grateful to the
Universidad de Guanajuato (DAIP-UG-2013, Excelencia Académica 2015, DAIP-UG-2018) and
FOMIX (GTO-2012-C04-195264) for partial support to carry out this research. Authors thanks the
Laboratorio Nacional UG-CONACYT.
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5
III
diMes
Figure 3. Molecular structures of [
η
-(Cp*)M (acac
)Cl] [M = Rh (2) left; Ir (3) right]. ORTEP at 40%
probability. Selected bond distances (in Å): For 2: Rh–Cl, 2.403(3); Rh–O1, 2.068(6); Rh–O2, 2.099(7); O1–
C1, 1.28(1); O2–C3, 1.28(1); Rh–C, 2.11(2) to 2.16(2). For 3: Ir–Cl, 2.404(2); Ir–O1, 2.102(4); Ir–O2,
2
.088(5); O1–C1, 1.270(7); O2–C3, 1.265(9); Ir–C, 2.13(2) to 2.17(2).