Ruthenium (II) complexes with C2- and C1-symmetric bis-(+)-camphopyrazole ligands and their evaluation in catalytic transfer hydrogenation of aldehydes

Article abstract of DOI:10.1016/j.ica.2021.120429

Ruthenium (II) piano-stool complexes with bis-(+)-camphopyrazole ligands of C₂ and C₁ symmetry were prepared in good yields (66–98%). New C₂-C₁ ligands and complexes were characterized by multinuclear NMR spectroscopy, FT-IR and elemental analysis. The catalytic performance of the Ru(II)-bis-(+)-camphopyrazole complexes in the transfer hydrogenation of benzaldehyde and valeraldehyde using isopropanol/potassium carbonate and formic acid/triethylamine mixtures as hydrogen donors, was evaluated, resulting in moderate yields (>54%) for the reduction to the desired primary alcohols. The system with isopropanol as hydrogen source proved to be more selective than the analogous system using the azeotropic formic acid/triethylamine mixture, allowing benzyl alcohol to be obtained in quantitative yield (>99%) for a particular catalyst precursor. Furthermore, complexes with C₂ symmetry ligands showed higher yields than those with C₁ symmetry ligands in all of the evaluated systems.

Full text of DOI:10.1016/j.ica.2021.120429

Inorganica Chimica Acta 524 (2021) 120429
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Ruthenium (II) complexes with C2- and C1-symmetric
bis-(+)-camphopyrazole ligands and their evaluation in catalytic transfer
hydrogenation of aldehydes
Christian O. Blancoa, Ligia Lloverab, Alberto Herrerac, Romano Dortac, Giuseppe Agrifoglioa,
d
a
,
*
a,*
´
´
Domenico Venuti , Vanessa R. Landaeta , Jesús Pastran
a
´
Departamento de Química, Universidad Simon Bolívar, Caracas, Venezuela
b Centro de Química, Instituto Venezolano de Investigaciones Científicas, Miranda, Venezuela
c
¨
Department Chemie und Pharmazie, Anorganische und Allgemeine Chemie, Friedrich–Alexander–Universitat Erlangen–Nürnberg, Egerlandstraße 1, 91058 Erlangen,
Germany
d Escuela de Química, Universidad Central de Venezuela, Caracas, Venezuela
A R T I C L E I N F O
A B S T R A C T
Ruthenium (II) piano-stool complexes with bis-(+)-camphopyrazole ligands of C2 and C1 symmetry were pre-
pared in good yields (66–98%). New C2-C1 ligands and complexes were characterized by multinuclear NMR
spectroscopy, FT-IR and elemental analysis. The catalytic performance of the Ru(II)-bis-(+)-camphopyrazole
complexes in the transfer hydrogenation of benzaldehyde and valeraldehyde using isopropanol/potassium car-
bonate and formic acid/triethylamine mixtures as hydrogen donors, was evaluated, resulting in moderate yields
(>54%) for the reduction to the desired primary alcohols. The system with isopropanol as hydrogen source
proved to be more selective than the analogous system using the azeotropic formic acid/triethylamine mixture,
allowing benzyl alcohol to be obtained in quantitative yield (>99%) for a particular catalyst precursor.
Furthermore, complexes with C2 symmetry ligands showed higher yields than those with C1 symmetry ligands in
all of the evaluated systems.
This contribution is dedicated to Dr. Maurizio
Peruzzini on the occasion of his 65th birthday.
Keywords:
Transfer hydrogenation
Ruthenium (II)
C
2 and C1 bis-(+)-camphopyrazole ligands
Aldehydes
1. Introduction
Nevertheless, the carbonyl group and, in particular, ketones constitute,
–
by far, a main focus of study of TH reactions. The hydrogenation of C
O
–
Transfer hydrogenation (TH) of unsaturated compounds is one of the
most important synthetic reactions, not only from the academic point of
view, but also for its industrial importance. Given its operational
simplicity, sustainability, and low impact on the environment, as well as
its atom-economy, TH has emerged as a suitable candidate to obtain
saturated compounds beyond the bench-scale [1,2]. This reaction in-
volves the transfer of hydrogen from an organic donor molecule (e.g.
formic acid or isopropanol) to an unsaturated bond using a metal cata-
lyst, in most cases in the presence of a base. It is considered a reaction
superior to conventional hydrogenation since it avoids the risks and
restrictions associated with the use of molecular hydrogen [3]. It has
also proven to be more selective and often requires milder working
conditions in terms of temperature and pressure. Successful TH exam-
ples include functional groups such as: imines [4], nitroarenes [5], al-
kenes [6] and the partial reduction of alkynes to obtain alkenes [7].
moieties is challenging, being thermodynamically disadvantaged if
compared, for example, to the hydrogenation of alkenes [8]. However,
significantly fewer examples involve the TH of the formyl group [9–18],
despite the fact that the reduction of aldehydes to obtain primary
alcohol building-blocks is vital for the fragrances and flavors industries
[19], and that other industries, such as alkene-based polymer
manufacturing, could likewise benefit from these systems [20]. Due to
the higher redox potentials of aldehydes compared to ketones, their TH
is thermodynamically favored [21]. In addition to this, the lower steric
hindrance of the formyl group compared to the ketone motif should
facilitate the access of the substrate to the active species of the catalyst
[22]. The difficulty to perform catalytic reduction of aldehydes lies in
the side reactions that can occur during the reduction, which are
generally favored by the basic media [12]. These transformations
include Cannizzaro dismutation [23], Tischenko dimerization [24] and
´
* Corresponding authors at: Departamento de Química, Universidad Simon Bolívar, Apartado 89000, Caracas 1020-A, Venezuela.
´
E-mail addresses: vlandaeta@usb.ve (V.R. Landaeta), jpastran@usb.ve (J. Pastran).
https://doi.org/10.1016/j.ica.2021.120429
Received 25 November 2020; Received in revised form 12 April 2021; Accepted 26 April 2021
Available online 30 April 2021
0020-1693/© 2021 Elsevier B.V. All rights reserved.

C.O. Blanco et al.
Inorganica Chimica Acta 524 (2021) 120429
aldol condensations [25]. Also, the decarbonylation of the aldehyde
substrate can deactivate the catalytically active species [26]. Many of
the catalysts used in TH are sophisticated ruthenium-based complexes
[27] or other precious-metal based systems [28,29]. Non-precious
metals such as iron [30], cobalt [31] or manganese [32] have recently
been used, and their complexes reach speeds that exceed, by far, those
obtained by heterogeneous catalysts at lower temperatures. Although
these are cheaper and more Earth-abundant metals than ruthenium,
their complexes bear ligands which can be even more expensive than the
metal used. Ruthenium (II)-arene piano stool complexes with a wide
variety of ancillary ligands have been extensively studied in TH. The
arene ligands are relatively inert towards substitution and act as spec-
tator ligands, stabilizing and protecting the metal center and thereby
preventing the rapid oxidation of the Ru (II) to Ru (III) [33]. On the
other hand, ancillary ligands can modulate the activity of the complexes
depending on their electronic and steric nature in any catalytic process
[17,19,34]. To the best of our knowledge, no examples of metal com-
plexes with ancillary chiral bis-(+)-camphopyrazole ligands have been
reported in transfer hydrogenation reactions. Such fact is rather sur-
prising, considering that bis-(+)-camphopyrazole ligands have been
previously reported and exhibit a number of intrinsically useful char-
acteristics for catalytic purposes, as previously shown [35–38]. Among
these, their ability to modulate electronic and steric effects and their
simple, high-yielding preparation. Furthermore, the size of the ligand
would inhibit the discoordination of one of the nitrogen atoms of the
chelate and the rotation out of the plane, avoiding possible dimerization
processes of the catalyst or of the catalytically active species. Likewise,
the substituents on the backbones and side structures would increase
solubility in most of the conventional solvents. Lastly, they exhibit good
thermal stability compared to ordinary N-monodentate ligands [39].
Similarly, the use of ligands that contain pyrazole ring is interesting
singlets, doublets, triplets, quartets, or multiplets, respectively. The
abbreviation br. is given for broadened signals. All the NMR spectra
were recorded at room temperature (25 ◦C) unless otherwise stated.
Melting points were determined on an Electrothermal Mel-Tem appa-
ratus in open capillary tubes and are not corrected. Elemental analyses
were performed at Department Chemie und Pharmazie, Anorganische
¨
und Allgemeine Chemie, Friedrich–Alexander–Universitat, on a Euro EA
3000 analyzer and samples were handled in air (hygroscopic compounds
are corrected for water content). IR spectra were performed at Uni-
´
versidad Simon Bolívar and recorded in KBr medium on a Thermo Sci-
entific Nicolet iS10 spectrophotometer.
2.2. General procedure for the preparation of C2 and C1 ligands
(+)-(1R,1′R)-3,3′-(1,2-Dihydroxyethane-1,2-diylidene)bis[(1,7,7-
trimethyl-bicyclo[1,2,2]-heptan-2-one] (Tetraketone I), (+)-3,3′-[(1,2-
Dihydroxyethane-1,2-diylidene)(1,7,7-trimethyl-bicyclo[1,2,2]-heptan-
2-one)]-(1-phenyl-camphopyrazo-5-ol) or (+)-3,3′-[(1,2-dihydroxy-
ethane-1,2-diylidene)(1,7,7-trimethyl-bicyclo[1,2,2]-heptan-2-one)]-
((4′S,7′R)-7′,8′,8′-trimethyl-4′,6′,7′-trihydro-5′-ol-4′,7′-methano-1′-
phenyl-indazol) (diketone intermediate II), C2: L1, L2 and C1: L4 sym-
metry ligands used in this work have been previously reported [35,36].
Tetraketone I: A mixture of (+)-camphor and diethyl oxalate in THF
was added to a slurry of NaH in THF. The mixture was refluxed for 48 h,
after which the solvent was removed by rotary evaporation. The reac-
tion crude was added to an ice/HCl mixture and extracted with chlo-
roform. The organic layer was dried over MgSO4, filtered, and stripped
to yellow oil. Slurrying and washing with methanol resulted in a yellow
powder.
For C2 ligand L3: Tetraketone I (5.00 g, 13.9 mmol) was added over
ethanol forming slurry and concentrated hydrochloric acid (12.0 M,
0.50 mL) was dropped slowly. The mixture was stirred for 15 min and
then was added a solution of 4-bromophenyl hydrazine hydrochloride
(6.21 g, 27.8 mmol) in ethanol. The resulting mixture was heated under
reflux for 48 h. The solvent was then removed by filtration and the solid
dried in vacuo. White solid product (6.74 g, 73%). M.P.: >320 ◦C (d). IR
given its π-donor ability [40]. This could increase the effectiveness of
ruthenium complexes by allowing TH of aldehydes with high efficiency.
The use of complexes with ligands containing pyrazole rings has been
proven in different catalytic processes in the last decade, such as: car-
bon–carbon coupling [41], epoxidation [42,43], oxidation [44], hy-
drogenation/dehydrogenation [45–47] and other transformations [48].
However, only few examples of their use as ancillary ligands in the
transfer hydrogenation of ketones can be found [49–54]. Considering
the above, this work presents the synthesis and characterization of
ruthenium (II) piano-stool complexes with chiral bis-(+)-camphopyr-
azole ligands of C2 and C1 symmetry, and their application in the
reduction of benzaldehyde and valeraldehyde using isopropanol/po-
tassium carbonate and formic acid/triethylamine as hydrogen sources.
(KBr) (
ν
, cmꢀ 1): 3062 (w), 2955 (s), 2871 (m), 1587 (m), 1507 (s), 1465
(m), 1277 (m), 1044 (m), 999 (m), 777 (s), 723 (m), 652 (m). 1H NMR
(300 MHz, CDCl3) (δ, ppm): 0.83 (s, 6H, 2CH3); 0.93 (s, 6H, 2CH3); 1.16
(s, 6H, 2CH3); 1.21–1.44 (m, 4H, 2CH2); 1.76–1.95 (m, 2H, CH2);
2.02–2.24 (m, 2H, CH2); 3.02–3.03 (d, J = 3.6 Hz, 2H, 2CH); 7.30–7.46
(d, J = 4.0 Hz, 4H, 4CH); 7.46–7.58 (d, J = 4.0 Hz, 4H, 4CH). 13C{1H}
NMR (75 MHz, CDCl3) (δ, ppm): 12.54 (2CH3); 19.79 (2CH3); 20.43
(2CH3); 27.42 (2CH2); 33.80 (2CH2); 48.00 (2CH); 53.64 (2C); 63.37
(2C); 120.49 (2CH); 125.82 (4CH); 129.55 (2C); 131.71 (4CH); 139.34
(2C); 140.08 (2C); 154.17 (2C). Elemental analysis for C34H36N4Br2:
Calculated: C, 61.83%; H, 5.49%; N, 8.48%. Found: C, 61.83%; H,
5.50%; N, 8.35%.
2. Experimental
2.1. Materials and physical measurements
Diketone intermediate II: Acetic acid was added dropwise to a so-
lution of tetraketone I in ethanol and allowed to react for 15 min. A
solution of phenyl hydrazine in ethanol was then added dropwise to the
above mixture and refluxed overnight. The solid crude product was
isolated by filtration, and recrystallization from methanol at low tem-
perature followed by high vacuum drying yielded a yellow powder.
For C1 ligand L5: Following the same procedure before described for
C2 ligands. Diketone intermediate II (2.64 g, 5.89 mmol) and 4-bromo-
phenyl hydrazine hydrochloride (1.97 g, 8.82 mmol). White solid
The reactions with air-sensitive compounds were carried out under
anaerobic and anhydrous conditions, using standard Schlenk tech-
niques. Tetrahydrofuran was distilled under nitrogen from purple so-
dium/benzophenone solution; triethylamine was dried with sodium
followed by distilled under nitrogen. Chloroform was dried with Linde
type 4 Å molecular sieves followed by distillation under atmospheric
pressure [55]. Methanol, ethanol, isopropanol, and hexane were used
without prior purification. NMR spectra were recorded on a Bruker 300
´
MHz spectrometer at Laboratorio Nacional de Resonancia Magnetica
product (2.47 g, 72%). M.P.: >320 ◦C (d). IR (KBr) (
ν
, cmꢀ 1): 3062 (w),
Nuclear, Instituto Venezolano de Investigaciones Científicas (IVIC) or on
a Bruker Avance 300 spectrometer at Laboratorium für Anorganische
Chemie, ETH Zürich. Peak positions are relative to tetramethylsilane, for
1H and 13C{1H}. The chemical shifts (δ) are measured according to
IUPAC [56], expressed in parts per million (ppm) and were calibrated
against the residual solvent resonance (1H) or the deuterated solvent
multiplet (13C). Coupling constants J are given in Hertz (Hz) as absolute
values. The multiplicity of the signals is indicated as s, d, t, q, or m for
2948 (s), 2872 (w), 1594 (m), 1503 (s), 1417 (m), 1328 (w), 1280 (m),
1266 (m), 1067 (m), 998 (s), 909 (m), 834 (m), 765 (m), 699 (m). 1H
NMR (300 MHz, CDCl3) (δ, ppm): 0.84 (s, 6H, 2CH3); 0.85 (s, 6H, 2CH3);
0.93 (s, 6H, 2CH3); 1.14–1.15 (m, 2H, 2CH); 1.20–1.46 (m, 2H, 2CH);
1.77–1.90 (m, 2H, 2CH); 2.09–2.20 (m, 2H, 2CH); 3.05–3.06 (d, J = 3.5
Hz, 1H, CH); 3.08–3.09 (d, J = 3.5 Hz, 1H, CH); 7.26–7.36 (t, J = 7.2 Hz,
1H, CH); 7.37–7.44 (d, J = 4.0 Hz, 4H, 4CH); 7.47–7.59 (d, J = 4.0 Hz,
4H, 4CH). 13C{1H} NMR (75 MHz, CDCl3) (δ, ppm): 12.25 (CH3); 12.49
2

C.O. Blanco et al.
Inorganica Chimica Acta 524 (2021) 120429
(CH3); 19.75 (2CH3); 20.45 (CH3); 20.46 (CH3); 27.43 (2CH2); 33.79
(2CH2); 47.99 (CH); 48.02 (CH); 53.60 (C); 53.69 (C); 63.31 (C); 63.41
(C); 124.71 (2CH); 125.92 (2CH); 127.47 (C); 128.64 (CH); 129.01
(CH); 129.60 (CH); 131.75 (4CH); 139.11 (2C); 139.68 (2C); 154.74
(2C). Elemental analysis for C34H37N4Br: Calculated: C, 70.22%; H,
6.41%; N, 9.63%. Found: C, 70.10%; H 6.40%; N, 9.32%.
(C); 160.52 (C); 161.13 (C). Elemental analysis for C44H52ClN4PF6Ru:
Calculated: C, 57.54%; H, 5.71%; N, 6.10%; F, 12.41%. Found: C,
57.16%; H, 5.58%; N 6.14%; F, 12.41%.
Complex 3: [(p-cymene)RuCl2] (100 mg, 163
μ
mol) in dichloro-
mol) in dichloromethane
(5.0 mL). Ammonium hexafluorophosphate (164 mg, 4.12 mmol). Or-
ange solid product (140 mg, 80%). M.P.: 192–195 ◦C (d). IR (KBr) (
methane (5.0 mL) and ligand L3 (217 mg, 326
μ
ν
,
cmꢀ 1): 3050 (w), 2958 (m), 2875 (w), 1625 (w), 1597 (w), 1534 (w),
1507 (w), 1454 (m), 1438 (m), 1398 (m), 1134 (m), 854 (s), 741 (m), 705
(m), 557 (s). 1H NMR (300 MHz, CDCl3) (δ, ppm): 0.60 (s, 3H, CH3); 0.74
(s, 6H, iPr-2CH3); 0.88 (s, 3H, CH3); 0.93 (s, 3H, CH3); 0.97 (s, 3H, CH3);
1.01 (s, 3H, CH3); 1.13 (s, 3H, CH3); 1.61 (s, 3H, cymene-CH3);
1.69–1.86 (m, 4H, 2CH2); 1.91–2.17 (m, 4H, 2CH2); 2.20–2.37 (m, 1H,
iPr-CH); 2.93–2.94 (d, J = 3.7 Hz, 1H, CH); 3.04–3.05 (d, J = 3.7 Hz, 1H,
CH); 3.78–4.05 (s, 2H, cymene-2CH); 4.80–5.02 (d, 2H, cymene-2CH);
7.79 (m, 8H, 8CH). 13C{1H} NMR (75 MHz, CDCl3) (δ, ppm): 12.05
(CH3); 12.56 (CH3); 19.07 (CH3); 19.34 (CH3); 19.81 (CH3); 20.18
(CH3); 20.30 (CH3); 21.62 (CH3); 21.80 (CH3); 26.64 (CH2); 27.31
(CH2); 31.16 (CH); 33.23 (CH2); 33.53 (CH2); 47.83 (CH); 48.07 (CH);
54.55 (C); 55.25 (C); 62.47 (C); 65.19 (C); 78.42 (2CH); 84.47 (2CH);
101.50 (C); 104.97 (C); 124.94 (C); 125.84 (C); 125.10 (4CH); 126.81
(CH); 127.01 (CH); 132.83 (4CH); 136.23 (2C); 138.13 (C); 139.08 (C);
160.96 (C); 161.54 (C). Elemental analysis for C44H50Br2ClN4RuPF6:
Calculated: C, 49.11%; H, 4.68%; N, 5.21%. Found: C, 49.34%; H,
4.79%; N, 5.17%.
2.3. General procedure to the synthesis of ruthenium complexes
The ligand was dissolved in dichloromethane or chloroform, and
slowly added to a solution of the precursor [(p-cymene)RuCl2] dissolved
in the same solvent. The solution was stirred for 24 h at room temper-
ature. The formed suspension was filtered to separate a solid identified
as impurities. The resulting solution was rotavaporated, obtaining a
solid that was dried under vacuum. This solid was dissolved in ethanol
and ammonium hexafluorophosphate was added and stirred at room
temperature. After 24 h of stirring, the solution was rotavaporated
obtaining a solid which was dried under vacuum. This solid was dis-
solved in chloroform and filtered using a Pasteur pipette with silica gel.
The resulting solution was rotavaporated to obtain a solid as the final
product. The recrystallization of the complexes was carried out in an
ethanol-hexane mixture. The mixture was cooled to ꢀ 5 ◦C during an
appropriate period, obtaining crystalline solids which were dried under
vacuum.
Complex 1: [(p-cymene)RuCl2] (100 mg, 163
μmol) in chloroform
Complex 4: [(p-cymene)RuCl2] (100 mg, 163
μ
mol) in dichloro-
(5.0 mL), ligand L1 (114 mg, 326 mol) in chloroform (5.0 mL).
μ
methane (5.0 mL) and ligand L4 (141 mg, 331
μmol) in dichloromethane
Ammonium hexafluorophosphate (181 mg, 1.11 mmol). Orange solid
product (146 mg, 70%). M.P.: 190–194 ◦C (d). IR (KBr) (
ν
, cmꢀ 1): 3264
(5.0 mL). Ammonium hexafluorophosphate (227 mg, 1.39 mmol). Or-
ange solid product (152 mg, 66%). Reddish orange crystals were ob-
(s), 2964 (s), 2877 (w), 1554 (w), 1538 (w), 1472 (m), 1438 (m), 1392
(m), 1285 (m), 1135 (w), 848 (s), 558 (s). 1H NMR (300 MHz, CDCl3) (δ,
ppm): 0.62 (s, 3H, CH3); 0.71 (s, 3H, CH3); 0.85–0.86 (d, J = 7.0 Hz, 3H,
iPr-CH3); 0.87–0.89 (d, J = 6.8 Hz, 3H, iPr-CH3); 0.91 (s, 3H, CH3); 0.93
(s, 3H, CH3), 1.05–1.12 (m, 2H, CH2); 1.18–1.34 (m, 2H, CH2); 1.37 (s,
3H, CH3); 1.39 (s, 3H, CH3); 1.81–1.93 (m, 2H, CH2); 2.04–2.16 (m, 2H,
CH2); 2.20 (s, 3H, cymene-CH3); 2.35–2.41 (m, 1H, iPr-CH); 2.86–2.87
(d, J = 3.7 Hz, 1H, CH); 2.88–2.89 (d, J = 3.7 Hz, 1H, CH); 5.74–5.82
(dd, J = 5.9 Hz, 2H, cymene 2CH); 6.04–6.13 (dd, J = 5.9 Hz, 2H,
cymene 2CH); 12.01 (s, 1H, NH); 12.18 (s, 1H, NH). 13C{1H} NMR (75
MHz, CDCl3) (δ, ppm): 10.23 (CH3); 10.48 (CH3); 19.19 (CH3); 19.26
(CH3); 19.39 (CH3); 20.07 (CH3); 20.37 (CH3); 21.81 (CH3); 21.83
(CH3); 26.83 (CH2); 27.38 (CH2); 31.06 (CH); 33.19 (CH2); 33.25 (CH2);
47.89 (2CH); 53.42 (C); 53.45 (C); 63.39 (C); 63.66 (C); 79.35 (CH);
79.59 (CH); 83.80 (CH); 83.99 (CH); 101.06 (C); 105.10 (C); 124.45 (C);
124.91 (C); 137.76 (C); 137.81 (C); 161.37 (C); 161.44 (C). Elemental
analysis for C32H44ClN4PF6Ru⋅1.2H2O: Calculated: C, 48.79%; H,
5.94%; N, 7.11%; F, 14.47%. Found: C, 48.41%; H, 5.66%; N, 7.01%: F,
14.88%.
tained (84 mg, 36%). M.P.: 256–260 ◦C (d). IR (KBr) (
ν
, cmꢀ 1): 3356 (m),
3056 (w), 2962 (m), 2926 (m), 2874 (w), 1624 (w), 1534 (w), 1506 (m),
1455 (m), 1435 (m), 1391 (m), 1275 (m), 1133 (m), 841 (s), 737 (w), 701
(m), 557 (s). 1H NMR (300 MHz, CDCl3) (δ, ppm): 0.64–2.31 (m, 72H,
18CH3, 18CH); 2.86–3.02 (m, 4H, 4CH); 4.14 (s, 2H, cymene 2CH); 4.81
(s, 2H, cymene 2CH); 5.45 (s, 2H, cymene 2CH); 5.87 (s, 2H, cymene
2CH); 7.65–7.73 (m, 10H, 10CH); 11.47 (s, 1H, NH); 11.54 (s, 1H, NH).
13C{1H} NMR (75 MHz, CDCl3) (δ, ppm): Isomer A: 9.85 (CH3); 10.29
(CH3); 19.09 (CH3); 19.24 (CH3); 20.16 (CH3); 20.26 (CH3); 21.53
(CH3); 22.00 (2CH3); 26.63 (CH2); 27.40 (CH2); 31.05 (CH); 32.96
(CH2); 33.55 (CH2); 47.71 (CH); 48.07 (CH); 53.76 (C); 53.80 (C); 62.51
(C); 63.74 (C); 78.15 (CH); 78.86 (CH); 82.87 (CH); 84.84 (CH); 97.19
(C); 104.50 (C); 125.72 (C); 125.93 (C); 129.61 (2CH); 130.90 (2CH);
131.08 (CH); 137.68 (C); 137.72 (C); 139.03 (C); 160.23 (C); 161.74 (C).
Isomer B: 10.28 (CH3); 11.87 (CH3); 19.20 (CH3); 19.27 (CH3); 20.31
(CH3); 20.40 (CH3); 21.75 (CH3); 22.07 (2CH3); 26.87 (CH2); 27.44
(CH2); 31.10 (CH); 33.21 (CH2); 33.73 (CH2); 47.97 (CH); 48.07 (CH);
54.27 (C); 54.92 (C); 63.63 (C); 64.79 (C); 79.43 (CH); 79.76 (CH);
82.98 (CH); 84.98 (CH); 101.78 (C); 105.10 (C); 125.08 (C); 125.75 (C);
129.61 (2CH); 130.90 (2CH); 131.08 (CH); 137.68 (C); 137.47 (C);
138.58 (C), 160.98 (C); 161.93 (C). Elemental analysis for
Complex 2: [(p-cymene)RuCl2] (274 mg, 448
methane (5.0 mL), ligand L2 (450 mg, 895 mol) in dichloromethane
(5.0 mL). Ammonium hexafluorophosphate (722 mg, 4.42 mmol). Or-
ange solid product (802 mg, 98%). M.P.: 242–244 ◦C (d). IR (KBr) (
μmol) in dichloro-
μ
C
38H48ClN4PF6Ru⋅0.4H2O: Calculated: C, 53.73%; H, 5.79%; N, 6.60%;
F, 13.42%. Found: C, 53.72%; H, 5.67%; N, 6.47%; F, 13.53%.
Complex 5: [(p-cymene)RuCl2] (100 mg, 163 mol) in dichloro-
methane (5.0 mL) and ligand L5 (190 mg, 326 mol) in dichloromethane
(5.0 mL). Ammonium hexafluorophosphate (263 mg, 1.17 mmol). Or-
ν,
cmꢀ 1): 3056 (w), 2965 (m), 2875 (w), 1593 (m), 1492 (m), 1456 (m),
1432 (m), 1392 (m), 1275 (m), 1125 (m), 841 (s), 774 (m), 687 (m), 558
(s). 1H NMR (300 MHz, CDCl3) (δ, ppm): 0.63 (s, 3H, CH3); 0.70 (s, 6H,
iPr-2CH3); 0.87 (s, 3H, CH3); 0.93 (s, 3H, CH3); 0.97 (s, 3H, CH3); 1.02
(s, 3H, CH3); 1.09 (s, 3H, CH3); 1.39–1.75 (m, 2H, CH2); 1.73–1.81 (m,
2H, CH2); 2.01–2.14 (m, 4H, 2CH2); 2.27–2.31 (m, 1H, iPr-CH); 2.62 (s,
3H, cymene CH3); 2.95–2.96 (d, J = 3.7 Hz, 1H, CH); 3.05–3.06 (d, J =
3.7 Hz, 1H, CH); 3.73 (s, 2H, cymene 2CH); 4.74–4.79 (d, 2H, cymene
2CH); 7.62–7.96 (m, 10H, 10CH). 13C{1H}NMR (75 MHz, CDCl3) (δ,
ppm): 10.19 (CH3); 11.89 (CH3); 19.03 (CH3); 19.29 (CH3); 20.18 (CH3);
20.29 (CH3) 21.52 (CH3); 21.77 (2CH3); 26.62 (CH2); 27.37 (CH2);
31.07 (CH); 33.40 (CH2); 33.51 (CH2); 47.80 (CH); 48.03 (CH); 54.35
(C); 55.03 (C); 62.37 (C); 64.94 (C); 78.18 (CH); 78.32 (CH); 84.41 (CH);
84.61 (CH); 100.70 (C); 105.31 (C); 126.28 (C); 126.55 (C); 129.63
(4CH); 130.76 (2CH); 130.92 (4CH); 137.31 (2C); 137.94 (C); 138.73
μ
μ
ange solid product (203 mg, 70%). M.P.: 298–300 ◦C (d). IR (KBr) (
ν,
cmꢀ 1): 3060 (w), 2963 (m), 2930 (w), 2872 (w), 1621 (w), 1594 (w),
1489 (m), 1455 (w), 1456 (w), 1391 (w), 1129 (m), 841 (s), 776 (m), 701
(m), 557 (s). 1H NMR (300 MHz, CDCl3) (δ, ppm): 0.58–2.46 (m, 72H,
18CH3, 8CH2, 2CH); 2.91 (s, 2H, 2CH); 3.01 (s, 2H, 2CH); 3.50–3.98 (d,
4H, cymene 4CH); 4.50–5.16 (d, 4H, cymene 4CH); 7.42–8.20 (m, 18H,
18CH). 13C{1H} NMR (75 MHz, CDCl3) (δ, ppm): Isomer A: 10.28 (CH3);
12.01 (CH3); 19.09 (CH3); 19.36 (CH3); 20.21 (CH3); 20.34 (CH3); 21.57
(CH3); 21.84 (2CH3); 26.69 (CH2); 27.35 (CH2); 31.15 (CH); 33.23
(CH2); 33.54 (CH2); 47.87 (CH); 48.12 (CH); 54.47 (C); 55.16 (C); 62.42
(C); 65.08 (C); 78.43 (2CH); 84.55 (2CH); 101.19 (C); 105.23 (C);
3

C.O. Blanco et al.
Inorganica Chimica Acta 524 (2021) 120429
124.90 (C); 126.52 (C); 126.80 (C); 129.70 (4CH); 130.87 (CH); 131.02
(2CH); 132.87 (2CH); 136.33 (2C); 137.88 (C); 138.71 (C); 160.65 (C);
161.26 (C). Isomer B: 10.37 (CH3); 12.01 (CH3); 19.11 (CH3); 19.36
(CH3); 20.26 (CH3); 20.35 (CH3); 21.65 (CH3); 21.82 (2CH3); 26.69
(CH2); 27.43 (CH2); 31.18 (CH); 33.51 (CH2); 33.60 (CH2); 47.87 (CH);
48.12 (CH); 54.56 (C); 55.24 (C); 62.56 (C); 65.15 (C); 78.43 (2CH);
84.55 (2CH); 101.19 (C); 105.23 (C); 125.07 (C); 126.69 (C); 126.88 (C);
129.70 (4CH); 130.87 (CH); 131.02 (2CH); 132.87 (2CH); 137.32 (2C);
138.33 (C); 139.20 (C); 161.01 (C); 161.56 (C). Elemental analysis for
C
44H51BrClN4RuPF6∙0.29C6H14: Calculated: C, 53.77%; H, 5.44%; N,
5.48%. Found: C, 53.61%; H, 5.45%; N, 5.33%.
2.4. General procedure for the transfer hydrogenation of aldehydes
With either one of the procedures for the catalytic studies described
below, the products obtained were identified by a Buck Scientific 910
gas chromatograph (FID detector) fitted with a capillary column MTX-1
(30 m × 0.52 mm × 1.0 mm). The retention times of benzaldehyde,
benzyl alcohol and biphenyl under an isothermal run at 180 ◦C were:
0.816; 0.950 and 3.050 min, respectively. Yield and conversion values
were determined by FID coupled gas chromatography using the internal
standard method. Yield was determined based on the amount of a benzyl
alcohol obtained, while conversion was determined based on the
amount of benzaldehyde consumed.
Using potassium carbonate and isopropanol:
Complex 2 (10.0 mg, 10.9 µmol), potassium carbonate (576 mg,
4.36 mmol) and biphenyl (111 mg, 727 µmol), used as internal standard,
were placed in a Schlenk. Isopropanol (5.0 mL) was added under an inert
atmosphere. Benzaldehyde (220 µL, 2.18 mmol) was added and the
system was immersed in a silicone bath with stirring. It was heated at the
reflux temperature of the solvent for 2 h. On completion of the reaction,
hexane (5.0 mL) was added, and the mixture was filtered using a column
of silica gel to later be analyzed.
Scheme 1. Synthesis of C2 and C1 (+)-camphopyrazole ligands. (i) Ethanol,
reflux, 48 h; For L1: Hydrazine hydrate, concentrated hydrochloric acid; For L2:
Phenyl hydrazine, concentrated hydrochloric acid; For L3: 4-Bromophenyl hy-
drazine hydrochloride. (ii) Acetic acid, ethanol, phenyl hydrazine, reflux,
overnight. (iii) Ethanol, reflux, 48 h; For L4: Hydrazine hydrate, concentrated
hydrochloric acid; For L5: 4-Bromophenyl hydrazine hydrochloride.
Using formic acid/triethylamine azeotrope as hydrogen source:
Complex 2 (5.0 mg, 5.4 µmol) and biphenyl (55.3 mg, 364 µmol)
were placed in a Schlenk. Chloroform (2.5 mL) was added as a co-
solvent. Dry and deoxygenated HCOOH/NEt3 azeotrope 5:2 (2.5 mL)
was added under an inert atmosphere. Benzaldehyde (110 µL, 1.09
mmol) was added and the system was immersed with stirring in a sili-
cone bath. It was heated at reflux temperature for 2 h. Upon completion,
the temperature was allowed to reach room temperature and the crude
was filtered using a column of silica gel to later be analyzed.
spectra correspond to half of the total number of expected signals. On
the other hand, for C1 ligands: L4-L5 two doublet signals appear corre-
sponding to bridge-head protons in each camphor moieties present in
the ligands, Table S1 (Supplementary information). Most of the signals
in the 1H NMR spectra for C1 ligands are not equivalent for each frag-
ment camphor-pyrazole, since these lack of symmetry elements. Similar
features are observed on the corresponding 13C spectra of the C2 and C1-
symmetric ligands.
3. Results and discussion
3.1. Synthesis and characterization of ligands and complexes
The coordination chemistry of the prepared (+)-camphopyrazole
ligands toward Ru(II) was studied. The route shown in Scheme 2 de-
scribes the general procedure used for the synthesis of the ruthenium (II)
complexes [(p-cymene)RuClLn]PF6 (Ln = L1-L5, 1–5). The reaction of
two equivalents of the respective (+)-camphopyrazole ligand with one
equivalent of the ruthenium precursor in chloroform or dichloro-
methane, followed by anion metathesis in ethanol at room temperature
afforded complexes 1–5 in good to excellent yields (66–98%). These
complexes are air-stable for some weeks and highly soluble in most of
the common polar hydrocarbon solvents. The characterization of the
complexes was carried out via multinuclear NMR and IR spectroscopy. A
good correlation between experimental and calculated elemental anal-
ysis values for the ruthenium complexes 1–5 and for the new ligands
prepared was found, thus confirming their analytical purity.
The new C2 and C1-symmetric (+)-camphopyrazole ligands, L3 and
L5, respectively, were prepared following a modification of the pub-
lished procedure for L1, L2 and L4 [35,36]. For the C2-symmetric ligand:
L3, the synthesis involves a single step procedure in which the reported
tetraketone I is treated with two equivalents of 4-bromophenyl hydra-
zine hydrochloride, Scheme 1 (i). The new C1-symmetric ligand L5, was
prepared following a two-step procedure, in which the reported dike-
tone intermediate II was initially obtained from tetraketone I, and then
condensed with 4-bromophenyl hydrazine hydrochloride. This proced-
ure afforded the respective ligand, as shown in Scheme 1 (ii) and (iii).
The set of ligands was prepared in good yields (72–73%). Characteristic
1H NMR signals of C2 and C1 ligands corresponding to bridge-head
protons in each camphor unit depend on its symmetry. In the case of
the C2-symmetric ligands, L2-L3, their symmetric environment is re-
flected in the fact that the number of signals and the integrals in their 1H
1H NMR characterization of complexes 1–5 featured the signals
4

C.O. Blanco et al.
Inorganica Chimica Acta 524 (2021) 120429
Scheme 2. Synthesis of ruthenium (II) complexes 1–5 with C2 and C1 (+)-camphopyrazole ligands. i) CHCl3 or CH2Cl2, room temperature, 24 h. ii) Ethanol, NH4PF6,
room temperature, 24 h.
corresponding to the C2 or C1 symmetric ligands and the coordinated p-
cymene ligand. The spectra of complexes 1–3, bearing C2-symmetric
ligands, display similar features in general. Thus, considering the simi-
larities observed, only the diagnostic signals of complex 1 will be herein
discussed. In the 1H NMR spectrum of 1 the methyl groups of the
(+)-camphor motif are observed as individual singlets (6 singlets, be-
tween 0.62 and 1.39 ppm), while those of the isopropyl group on the p-
cymene ligand appear as doublets (0.85 and 0.89 ppm). Two doublet
signals between 2.86 and 2.89 ppm correspond to the bridge-head
protons in each camphor unit on the methinic carbon atoms of bicy-
clo. Further, singlet signals appear at 12.01 and 12.18 ppm, assigned to
solution becomes more evident upon analysis of the 13C NMR spectra of
complexes 4 and 5. For these, all of the signals have been unequivocally
assigned. However, while a single signal was expected for each carbon
atom, given the fact that their structures lack of symmetry elements, two
signals are observed for each carbon atom instead (for example, twelve
signals between 9.85 and 20.40 ppm, corresponding to the carbon atoms
of the methyl groups of the (+)-camphor bicyclo).
To discuss the general trends of the NMR results, the chemical shifts
of the characteristic bridge-head protons of the (+)-camphopyrazole
ligands in the synthesized complexes have been summarized in Table S2
(Supplementary information). For complexes 1–3 with C2 symmetric
ligands, the characteristic signals of the bridge-head protons of the co-
ordinated ligand appears as two doublet signals with coupling constants
of 3.7 Hz. In the free C2 ligands, these protons appear as unique doublets
(Table S1). The presence of double the signals with respect to the free
ligands in the 1H NMR spectra is also observed in the aliphatic region for
the rest of the prepared complexes, which indicates that the protons of
each (+)-camphopyrazole fragment are in different chemical environ-
ments. The coupling constant in the free ligands varies slightly with
respect to coordinated ligands. This suggests that the angles of these
protons with their neighboring protons change when the ligand is cis-
oriented, reducing the orbital interaction of the spins, as indicated by the
Karplus diagrams [57]. In Fig. 1, the C2-symmetry axis in the free li-
gands, and the C1 axis in the Ru(II) complexes, can be observed. The loss
of the C2 symmetry in the ligands when cis-coordinated creates a non-
equivalent environment for the bridge-head protons. This fact has
been reported in the literature for other piano-stool complexes with
different C2 symmetric ligands [58–61]. This loss of C2-symmetry was
also evidenced from the duplicity of most of the 13C{1H} NMR signals, as
previously discussed.
–
the protons of the N H groups of the pyrazole ring in the coordinated
ligand. The rest of the protons on the complex were also assigned via 1H
NMR. For complexes 2 and 3, the main difference observed in the NMR
characterization corresponds to the signals of the aromatic protons on
the unsubstituted and para-Br substituted phenyl groups on the pyrazole
rings, respectively.
Similarly, analysis of the 13C{1H} NMR spectrum of complex 1,
allowed for the assignment of nine signals (between 10.23 and 21.83
ppm) as those corresponding to the carbon atoms of the methyl groups of
the (+)-camphor bicyclo and p-cymene ligand. The rest of the methy-
lene, methine and quaternary carbon atoms of the camphopyrazole and
p-cymene ligands have been unequivocally assigned. For complexes 2
and 3, the spectra featured similar characteristics regarding the general
framework of the ligands, with additional signals corresponding to the
carbon atoms on the phenyl substituents. With respect to the free C2
symmetric ligands, the main feature to highlight is the fact that, for
complexes 1–3 most of the 13C{1H} NMR signals appear duplicated,
except for the aromatic rings.
The 1H NMR spectra of complexes 4 and 5, with C1 symmetric li-
gands, features broad, low-resolution signals, which overlap each other
in the region of the aliphatic protons. Analogously, since both complexes
present similar NMR features, only the spectrum of complex 4 will be
discussed. In such a case, as mentioned above, a broad multiplet (be-
tween 0.20 and 2.45 ppm) was assigned to the protons of the
(+)-camphor and the aliphatic protons of the p-cymene ligand. Two
broad doublets (between 2.64 and 3.18 ppm) correspond to the bridge-
head protons in the bicyclo. The rest of the signals of the p-cymene
ligand and phenyl substituents on the pyrazole ligand, were also
assigned. Broad singlet signals at 11.47 and 11.54 ppm, assigned to the
protons on the nitrogen atom of the pyrazole ring in the coordinated
ligand, could be indicative of the existence of an isomeric mixture in
On the other hand, for complexes 4–5, the bridge-head protons on
the ligands appear as broad multiplets, unlike the free C1 ligands where
–
solution for this complex. These N H proton signals appear at lower
fields than in the respective free ligand L4 (11.32 ppm), evidencing that,
upon coordination to the metallic center, the electronic deficiency of the
pyrazole ring in the ligand increases. The possible existence of isomers in
Fig. 1. Structure and axes of rotation in the C2 symmetry bis-(+)-camphopyr-
azole ligands and their ruthenium (II) complexes. Bridge-head protons are
marked in color.
5

C.O. Blanco et al.
Inorganica Chimica Acta 524 (2021) 120429
these appear as two well-defined doublets (Table S1). This observation
was attributed to the possible presence of isomers in solution, as
mentioned above. The isomerism of this type of complexes can have
different origins. In Fig. 2, two possible isomers for complexes with C1
symmetric ligands can be seen, which are based on the different orien-
tation of the substituents R and R’ in the bis-(+)-camphopyrazole ligand,
and correspond to the existence of diastereoisomers. It has been reported
that piano-stool ruthenium (II) complexes with C1 symmetry ligands can
generate diastereoisomers in solution [59,60]. In complexes with C2
symmetry ligands, this isomerism is not possible since the substituents
on both sides of the ligand are identical.
Table 1
Chemical shift ranges (δ) of the olefinic protons in the p-
cymene ligand on the dimeric precursor and ruthenium
complexes 1–5 with bis-(+)-camphopyrazole ligands.
Complex
δ (ppm)
[(p-cymene)RuCl2]2
5.29–5.45
5.74–6.13
3.73–4.79
4.08–5.91
3.78–5.02
3.50–5.16
1
2
3
4
5
Hence, the 13C NMR spectra of complexes 1–3 confirm the presence
of a single species in solution and indicates that the origin of isomerism
in complexes 4 and 5 is mainly due to the spatial arrangement of the
substituents on the ligand. This also excludes the possible observation of
other types of isomerism, which could originate due to rotation of the
bond between the metal center and the p-cymene ligand.
Table 2
Values of stretching frequencies of the C N and C C bonds of the pyrazole ring
–
–
–
–
for complexes 1–5 with C2 and C1 symmetry bis-(+)-camphopyrazole ligands.
Complex
Stretching frequencies (cm¡1
)
–
–
–
–
C C (pz)
C
N (pz)
Finally, a comparison between the chemical shift ranges of the sig-
nals assigned to the p-cymene ligand in the 1H NMR spectrum of com-
plexes 1–5 with those of the dimeric precursor [(p-cymene)RuCl2]2
could give information about the electronic effect of ligands on the metal
center. These are shown in Table 1.
1
2
3
4
5
1554
1593
1597
1596
1594
1472
1492
1534
1455
1456
The signals corresponding of the olefinic protons of the p-cymene
ligand in complex 1 appear at lower fields, relative to the dimeric pre-
cursor. This could indicate that bis-(+)-camphopyrazole ligand in the
complex 1 attracts electron density of the metal center, disfavoring the
metal-arene backdonation more than in the precursor. This low-field
intense bands around 850 cmꢀ 1 (P-F stretching frequencies), confirm the
presence of the hexafluorophosphate counterion for all the complexes,
1–5 [70].
–
–
shift has been reported for piano-stool complexes with π-acceptor li-
The trend found in the values of the frequencies of the C N and
–
–
gands, such as bipyridines and phosphines [62–65]. In the dimeric
precursor, without bidentate ligands, the arene ligand orbitals have
double bond characteristics and appear more shielded. On the other
hand, for the complexes 2–5 chemical shifts of protons on the methine
carbon atoms of the p-cymene ligands appear at higher fields than in the
dimer precursor. This phenomenon has been reported in the literature
for piano-stool complexes with bidentate ligands with S, N and O donor
atoms [66–68]. Such observation suggests that the backdonation from
the metal to the p-cymene ligand is more favored in these complexes,
probably since the substituted phenyl ligands donate electron density to
C
C bonds in the complexes with C symmetric ligands is: 3 > 2 > 1.
This can be attributed to a modifica2tion of the electron density of the
pyrazole rings, due to the different substitution of the nitrogen atoms in
the pyrazole rings of the ligands. The values of the stretch frequencies of
–
–
–
–
the C N and C C bonds in the complexes 4–5 with C1 symmetric li-
gands, do not vary significantly from each other. This suggests that
electronic effects of the substituents are not significant in this case.
–
–
Comparing the values of the stretch frequencies in the C C bonds in
the pyrazole rings of complexes 1–3 with the free ligands of C2 sym-
metry: L1 (1471 cmꢀ 1), L2 (1452 cmꢀ 1) and L3 (1465 cmꢀ 1), it is found
that the former are higher. This suggests that C2 ligands are electron
the metallic center. Thus, ruthenium (II) can populate the π-antibonding
orbitals of arene ligand with this electron density, making its orbitals
have single bond characteristics [69].
density donors through their π system in the complexes, that is, pyrazole
rings donate electron density to the metal center, increasing the fre-
Additional characterization was carried out via IR spectroscopy for
complexes 1–5. Characteristic bands of the corresponding ligands in the
prepared complexes. The bands of greatest interest correspond to the
quency values. Likewise, the values of the stretching frequencies of the
–
–
C
N bond in complex 3 also increase with respect to free ligand L
3
(1587 cmꢀ 1). However, it is observed that the values of the stretching
–
–
–
–
–
stretching frequencies of the C N and C C bonds in the pyrazole rings,
–
frequencies of the C N bonds decrease for complexes 1 and 2 with
respect to free ligands L1 (1571 cmꢀ 1) and L2 (1596 cmꢀ 1), respectively.
since these can interact with the metal center through the
π system.
–
–
Values of the stretch frequencies for the complexes with C2 and C1
This could indicate that the C N bonds in complex 1 receive electron
symmetry ligands are shown in Table 2. Furthermore, characteristic
density via backdonation from the metal center.
Fig. 2. Proposed structure for possible diastereoisomers: 4′ and 5′ in ruthenium (II) complexes with bis-(+)-camphopyrazole ligands of C1 symmetry: 4 and 5.
6

C.O. Blanco et al.
Inorganica Chimica Acta 524 (2021) 120429
with C2 and C1-symmetric bis-(+)-camphopyrazole ligands, 1–5, in the
TH of benzaldehyde (as model substrate) using isopropanol as hydrogen
source, was evaluated. The results obtained are shown in the Table 3.
The best substrate/complex (catalyst)/base (S/C/B) ratio found was
100/1/400.
In the absence of catalyst, or by using the precursor [(p-cymene)
RuCl2]2 as catalyst no conversion toward the expected product was
observed (entry 1). In the presence of 1.0 mol% of complex 3, with a C2-
symmetric ligand, the TH of benzaldehyde proceeded to completion,
affording benzyl alcohol quantitatively. By contrast, attempts to use a
smaller catalyst load, such as 0.5 mol% were unsuccessful, leading only
to moderate yield of the product (entry 4). It was observed that the in-
fluence of the catalyst load in the yield and conversion values was not as
significant when complexes 1 and 2 were used as catalysts, although a
slight decrease were also noted (entries 3 and 2). With complex 3
(catalyst load = 1.0 mol%) the TH reaction is more selective, with
conversion occurring only to the desired product without formation of
by-products (entry 4). By contrast, complex 2 (1.0 mol%) seems to
promote side reactions, since the differences between the yield and
conversion values become more evident in such a case (entry 3). In
general, the trend in the yield and conversion values for the complexes
with C2 symmetric ligands is 3 > 2 > 1, regardless of the catalytic load.
On the other hand, for the complexes bearing ligands with C1 symmetry,
it was observed that the yield and conversion values using complexes 4
and 5 do not vary significantly with an increase in catalyst loading
(entries 5 and 6).
Fig. 3. TH of benzaldehyde with iPrOH/K2CO3 using complex 2.
–
–
A comparison of the values of the stretch frequencies for the C
N
–
and C C bonds in complexes 4 and 5 with C symmetry free ligands: L
–
(1596 and 1454 cmꢀ 1) and L5 (1596 and 11454 cmꢀ 1), reveals no sig4-
nificant differences. This could be attributed to the fact that the metal-
–ligand orbital interaction in these complexes with C1 symmetry ligands
is less than in the complexes with C2 symmetry ligands. However, to
determine the reason behind the lack of variation between frequencies,
additional in-depth studies should be carried out with the aid of
computational tools.
3.2. Catalytic studies
The initial evaluation of the transfer hydrogenation reaction was
performed using complex 2 as catalyst precursor, benzaldehyde as
model substrate and isopropanol as the hydrogen source, and varying
reaction conditions such as: temperature, time, base/substrate and
substrate/catalyst ratios, see Fig. 3. A comparison between catalytic
runs carried out under an inert atmosphere and in air revealed no sig-
nificant differences in the yield values obtained. Therefore, the reactions
were conducted in the presence of air. Complex 2 was the precursor of
choice among the prepared complexes since a larger quantity of this
compound was more available, being the one obtained with a higher
synthetic yield. Potassium carbonate was selected as base for these re-
actions, since it is an easy-to-handle, cheap, and air-stable compound.
Considering that the variation of reaction conditions resulted in low
yield and conversion values (<15%), the influence of the amount of
solvent in the TH reaction was studied. The results obtained are shown
in the Table S3 (Supplementary information). As the amount of solvent
is increased, yield and conversion values are observed to increase
significantly to a maximum of 57% and 66%, respectively. To explain
this result, it must be considered that the TH of benzaldehyde with
isopropanol as hydrogen source is a thermodynamically neutral reaction
[71]. This implies that the reaction is quite close to the equilibrium and
that the formation of the product can be favored using Le Chatelier
principle. By increasing the amount of isopropanol, which is at the same
time one of the reagents, the increase in the equilibrium constant dis-
places the reaction towards the formation of products. Furthermore, an
increase in the amount of dissolved potassium carbonate could be an
additional benefit of using a larger amount of solvent. To avoid excessive
solvent expenditure, successive reactions were carried out using an in-
termediate amount of isopropanol (15 mL).
To expand the scope of the complexes studied we consider evaluating
substituted benzaldehydes, however recent studies have shown that the
electronic nature of the substituents affects very little their yields
[17,18] and therefore we chose valeraldehyde, an aliphatic aldehyde.
This was studied under the best conditions previously determined for
benzaldehyde. In general, this type of substrate is usually more sensitive
to the basic conditions of TH due to the presence of hydrogen atoms in
α
position to the carbonyl group. As a result, such substrate is susceptible
to participating in aldol condensation reactions generating different by-
products in addition to the one of interest. The results of the TH re-
actions of valeraldehyde are also shown in Table 3. The reaction in
absence of complex shows a high conversion value (entry 1), which
indicates that under the conditions used, the formation of by-products
other than the product of interest, n-pentanol, is favored. The trend in
the behavior of complexes with C2 symmetry ligands in TH of valer-
aldehyde is similar as in TH of benzaldehyde: 3 > 2 > 1. However,
complex 1 does not generate the product in appreciable amounts (entry
2), probably due to their instability in solution and the π-acceptor nature
of ligand in the complex, as previously explained. Conversion using
complex 1 is complete, while with complexes 2 and 3 the conversion
values are moderate (entries 2–4). This indicates that complex 1 favors
the transformation of the substrate through side reactions. The catalytic
behavior of complex 3 is better, with respect to the rest of the complexes,
probably due to the electro-donating effect of the bromine atoms, as
mentioned above. This effect can increase the electron density in the
The catalytic potential of the prepared ruthenium (II) complexes
Table 3
TH of benzaldehyde and valeraldehyde with iPrOH using the ruthenium (II) complexes 1–5.
Entry
Complex
Benzaldehyde
Valeraldehyde
Yield (%)
Yield (%)
Conversion (%)
Conversion (%)
1
2
3
4
5
6
–
0i,ii
0i,ii
0ii
<1
18
45
8
58ii
>99
53
1
2
3
4
5
24(16)
34(34)
>99(62)
20(23)
35(30)
28(20)
44(36)
>99(69)
23(28)
40(32)
65
41
<1
55
Reaction conditions: Reflux; Time = 4 h; V(iPrOH) = 15 mL; Base = K2CO3; S/C/B = 100/1/400; Yields and conversions in parentheses are using S/C/B = 200/1/400.
i
Reactions in presence of [(p-cymene)RuCl2]2;
ii
Reactions in absence of metal complex.
7

C.O. Blanco et al.
Inorganica Chimica Acta 524 (2021) 120429
coordinated pyrazole rings, which in turn increases the electron density
of the metal center and therefore their ability to mediate TH reactions.
This complex 3, also shows the highest selectivity since the difference
between the yield and conversion values is less than in the rest of the
complexes, which implies the formation of smaller amounts of by-
products. Complexes 2–3, with C2 symmetric ligands afford higher
yield and conversion values than complexes 4–5, with C1 symmetric
ligands (entries 3–4 vs 5–6). This is consistent with the behavior pre-
viously shown by the complexes with the C2 symmetric ligand used in
the TH of benzaldehyde. The complexes with the C1 symmetric ligands
lead to lower yields, however, their conversion values are moderate.
This suggests that by-product formation is favored over the reduction
reaction to the alcohol of interest. The trend according to the catalytic
behavior of the complexes is 4 > 5. Complex 4 affords a slightly higher
yield within this group, probably since it is more selective towards TH
than to the possible side-reactions, as established for the TH benzalde-
hyde. For complex 5, the alcohol of interest is not observed, however,
the conversion value is moderate, indicating that side reactions are
favored.
interest and to reduce the amount of side products by effect of the base
(entries 7–8). Considering only the conversion values it can be noted
that the stability of benzaldehyde using NaH was lower than using
K2CO3.
On the other hand, it was found that the yield and conversion values
increase when complex 1, 2 and 4 were used with a strong base with less
nucleophilic character and greater solubility such as sodium hydride,
contrasting with the behavior of the same complexes using potassium
carbonate (Table 4, entries 9–11 versus Table 3, entries 2, 3 and 5). This
is attributed to the increase in the concentration of the isopropoxide ion,
which participates in the formation of the intermediates in the catalytic
process. Using NaH, complex 4, with the ligand of C1 symmetry, showed
higher yield and conversion values than complexes 1–2 with C2 sym-
metric ligands.
Finally, the TH reaction of benzaldehyde was evaluated using formic
acid as hydrogen source and triethylamine as base. The general reaction
is observed in Fig. 4. A brief evaluation of the reaction conditions: inert
atmosphere or air, temperature, time, substrate/catalyst ratio, formic
acid/triethylamine ratio and co-solvent was carried out using complex
2. The results are shown in Table 5.
The evaluated complexes afford moderate yield and conversion
values, compared to some of the systems reported in the literature
[12,14,15,17,18]. However, in most of these reports involving Ru (II)
complexes, the effect of the base in the absence of the complex on the
yield values obtained was not fully understood.
The stability of the system was determined without an inert atmo-
sphere. For this, comparative catalytic reactions were carried out in air
and under a nitrogen atmosphere using the best conditions found in TH
with isopropanol. No significant differences were found in conversion
and yield values when performing catalytic tests under inert atmosphere
and in air (entries 1–2), as previously observed for the system using
isopropanol as hydrogen source. Therefore, subsequent reactions were
carried out in air. The effect of catalytic loading was also studied. As the
catalytic loading decreases, a considerable decrease in the yield and
conversion values occurs (entries 2 vs 3 and 4 vs 5). A catalytic loading
of 1.0 mol% was then selected since it produces the highest yield value
(entry 2). Likewise, the effect of the temperature on the TH of benzal-
dehyde was evaluated. The yield and conversion values are higher when
the reaction is carried out at the reflux temperature of chloroform (entry
4 vs 6) and, therefore, it was established as the best condition. Unlike the
isopropanol system, this reaction generates benzyl alcohol at room
temperature, demonstrating the exergonic character of the reduction
reaction using formic acid compared to isopropanol as hydrogen source
[3]. Subsequently, the effect of the reaction time on the benzaldehyde
TH was determined. By decreasing the reaction time, a decrease in the
yield and conversion values was obtained (entry 3 vs 4). The difference
between yield and conversion values decreases with decreasing reaction
time, indicating either that long reaction times decrease selectivity or
that longer reaction times favor the transformation of the substrate or
product(s) into by-products. This is an expected phenomenon since
other TH aldehyde systems require short reaction times to avoid de-
creases in selectivity [73]. Also, the effect of the formic acid/triethyl-
amine ratio on the TH of benzaldehyde was evaluated. The 5:2
azeotropic mixture was compared with the best ratio reported in the
literature for the carbonyl TH reaction, 1:5 azeotropic mixture [74]. By
using the mixture of formic acid/triethylamine in 1:5 M ratio, a low
yield value is obtained, but the conversion value increases (entry 7 vs 4).
This can probably be attributed to the excess base present, which gen-
erates side reactions that convert aldehyde into by-products. When THF
was used as co-solvent in a TH reaction, exceptionally low yield and
conversion values were obtained (entry 8 vs 4). This is probably due to
To determine the effect of the base strength on TH of benzaldehyde,
catalytic reactions were carried out using potassium tert-butoxide and
sodium hydride with complexes 1, 2 and 4. The results obtained are
shown in the Table 4.
TH reactions carried out in absence of complex could assess on the
stability of benzaldehyde in the reaction medium. When 200 mol% of
potassium carbonate was used no product was obtained (entry 1), while
that when the same amount of potassium tert-butoxide was used it was
possible to demonstrate that strongly basic conditions lead to the for-
mation of the product of interest with good yield and conversion values
(entry 2). This is probably because alkali metals can catalyze hydrogen
transfer reactions through of the Meerwein-Ponndorf-Verley (MPV)
mechanism. It has been shown that potassium tert-butoxide can act as a
reducing agent in the presence of isopropanol with good yields [72]. The
yield and conversion values using the complexes 1 and 2 in the presence
of this base do not vary significantly with respect to the reaction in the
absence of the complex (entries 2–4), while using complex 4 the yield of
the product was lower (entry 5).
Taking into account the lower stability and availability of potassium
tert-butoxide, we turned our attention to sodium hydride as a base in the
reactions of TH. In this way, the yield and conversion values in the re-
actions carried out in absence of complex with this base can also be
attributed to the same base-mediated effect described before (entry 6).
Therefore, the amount of sodium hydride used was reduced from 200 to
5 mol% to favor the ruthenium-mediated formation of the product of
Table 4
TH of benzaldehyde using ruthenium complexes 1, 2 and 4 as catalyst precursors
with NaH and KOtBu in isopropanol.
Entry
Complex
Base
S/C/B
Yield (%)
Conversion (%)
1
*
*
1
2
4
*
*
*
1
2
4
K2CO3
KOtBu
KOtBu
KOtBu
KOtBu
NaH
200/0/400
200/0/400
200/1/400
200/1/400
200/1/400
200/0/400
200/0/50
200/0/10
200/1/10
200/1/10
200/1/10
0
0
2
62
67
68
53
70
14
0
>99
>99
>99
>99
72
3
4
5
6
7
NaH
22
8
NaH
14
9
NaH
45
53
69
51
10
11
NaH
58
NaH
77
Reaction conditions: Reflux; Time = 4 h; ViPrOH = 15 mL; * Reactions in absence
of metal complex.
Fig. 4. TH of benzaldehyde with formic acid/triethylamine using complex 2.
8

C.O. Blanco et al.
Inorganica Chimica Acta 524 (2021) 120429
by potassium carbonate [75]. It is likely that in complex 1, the ligand is
de-protonated, generating a pyrazolate which can donate higher elec-
tron density to the ruthenium (II) metal center. This would favor
oxidation processes to ruthenium (III) and therefore its instability. On
the other hand, with formic acid the medium is essentially acidic, being
the acid in excess. Thus, it was proposed that the nitrogen atoms of the
pyrazole rings in the ligand in complex 1 remain protonated under these
conditions, which would explain the greater stability in solution in the
formic acid/triethylamine mixture, when compared to the system in
isopropanol.
Table 5
TH of benzaldehyde with formic acid/triethylamine: Screening of reaction
conditions using complex 2 and evaluation of the ruthenium (II) complexes 1,
3–5.
Entry
Complex
S/C
Time (h)
Yield (%)
Conversion (%)
1i
2
2
2
2
2
2
2
2
–
100/1
100/1
200/1
200/1
400/1
200/1
200/1
200/1
100/1
100/1
100/1
100/1
100/1
100/1
4
4
4
2
2
2
2
2
4
4
4
4
4
4
54
55
37
23
18
9
>99
>99
66
2
3
4
35
5
40
6ii
7iii
8iv
9v
10vi
11
12
13
14
24
Complexes 4–5 with C1 symmetric ligands, afford low yield and
conversion values comparable to those obtained with isopropanol. The
trend in catalytic behavior found for these complexes is 5 > 4. Complex
5 shows the highest yield in this series probably due to the effect of the
bromine atom in para position, similar to complex 3. After comparison
of both types of complexes, it becomes evident that by using complexes
1–3 higher yield values are obtained than with compounds 4–5. As
described above, this could be attributed to the fact that the metal-
–ligand interaction for complexes with C2 symmetric ligands is stronger,
as indicated by IR analyses. Conversion values using complexes 1–3 with
the C2 symmetry ligands are almost quantitative, while in complexes
4–5 with C1 symmetric ligands these values do not exceed a 46% con-
version. This again indicates that complexes with C2 symmetric ligands
show a better catalytic behavior and that the number of by-products
increases with the amount of alcohol of interest present in the mixture.
The preparation and characterization of these Ru (II) complexes with
bis-(+)- camphopyrazole ligands of C1 and C2 symmetry opens a window
of possibilities to study asymmetric transfer hydrogenation of prochiral
substrates in future works to be carried out by our research group.
Similarly, reported complexes in this work will be considered in more
detail to be studied in transfer hydrogenation reactions with a wide
variety of substituted benzaldehydes and other substrates, such as nitro
compounds, in an extensive work in progress.
14
1
52
14
0
10
–
0
9
1
3
4
5
45
65
16
30
>99
>99
26
46
Reaction condition: Air; Reflux; V(HCOOH/NEt3 5:2) = 2.5 mL;V(CHCl3) = 2.5 mL;
i
N2;
ii
iii
Room temperature;
V(HCOOH/NEt3 1:5) = 2.5 mL;
iv
V(THF) = 2.5 mL;
v
vi
Reaction in absence of complex;
Reaction in presence of [(p-cymene)RuCl2]2
the coordinating nature of this solvent, which could be deactivating the
catalytically active species by occupying a vacant coordination site
necessary for the substrates.
Once the best reaction conditions have been established using
complex 2: reflux temperature, 1.0 mol% catalytic loading, 4 h of re-
action time, chloroform as co-solvent and a formic acid/triethylamine
mixture with a molar ratio 5:2, the ruthenium (II) complexes 1 and 3–5
with bis-(+)-camphopyrazole ligands were evaluated in the TH of
benzaldehyde using formic acid/triethylamine as hydrogen source. The
results are also shown in Table 5. In absence of complex or using the
precursor [(p-cymene)RuCl2]2 as catalyst, the formation of benzyl
alcohol is not observed (entries 9 and 10). This highlights the role of the
ruthenium complexes 1–5 as catalysts in TH under these conditions, and
the effect of the bis-(+)-camphopyrazole ligands to generate/stabilize
the catalytically active species. However, the low conversion values
obtained indicate the formation of by-products mostly because of the
reaction conditions used.
Considering the effect of the ligand in the complexes on the studied
TH reactions, an internal sphere mechanism might be proposed. This
pathway involves the formation of metal-hydride intermediate, which is
generated through coordination of the donor and hydrogen acceptor to
the metal center in separate steps. Fig. 5 shows the internal sphere
mechanism proposed for the ruthenium (II) complexes that contain bis-
(+)-camphopyrazole ligands using isopropanol/potassium carbonate
and formic acid/triethylamine, based on reports from the literature
[17,76].
The yield values in the TH of benzaldehyde with formic acid/trie-
thylamine mixture as hydrogen source using the prepared complexes
1–3, with C2 symmetric ligands, are greater than those obtained with
isopropanol/potassium carbonate. By contrast, the conversion values
are higher than those found with isopropanol, indicating that the reac-
tion with the formic acid/triethylamine mixture is less selective (Table 5
vs Table 3). This could be because the alcohol produced reacts to
generate by-products. Complex 3 was the most active under these TH
conditions (entry 12) probably due to the resonance electro-donating
effect of bromine atoms in the para position, as previously described.
Interestingly, complex 1 turned out to be more active with this hydrogen
source than with isopropanol (Table 5, entry 11 versus Table 3, entry 2)
suggesting a greater stability in the HCOOH/NEt3 mixture. The trend in
catalytic behavior found for these complexes is 3 > 2 > 1. The lower
On step I of the mechanism using isopropanol/potassium carbonate,
the formation of an isopropoxide-metal species as catalytic intermedi-
ate, generated by exchange with the chloride after deprotonation of
isopropanol by the base, is proposed. Then, a β- elimination reaction
generates acetone and a hydride complex, step II. This intermediate is
electronically saturated to continue with the catalytic cycle, and there-
fore an available coordination site is required. This can be achieved
through the change in hapticity of the p-cymene ligand. The hapticity
change has been proposed by some authors for internal sphere mecha-
nisms based on the conformational rigidity [77], and the donor nature of
the ligands [78]. Therefore, it is postulated that step III of this mecha-
nism is the change in hapticity from
η
6 to
η
4 followed by simultaneous
coordination of the substrate. Then, in the step IV, migratory insertion of
the formyl group at the Ru-H bond allows for the formation of the pri-
mary alkoxide. Finally, an isopropanol-mediated proton transfer process
occurs, in which the alkoxide of interest leaves the coordination sphere
as benzyl alcohol to regenerate the catalytically active species with a
coordinated isopropoxide ligand, step V.
performance of complex 1 could be attributed to the π-acceptor nature of
ligand, as demonstrated by NMR analysis. In the TH using formic acid/
triethylamine azeotrope, no color changes were observed during the
course of the reaction when complex 1 was used as catalyst precursor. In
such a case, the solution remained yellowish-orange during the whole
period of time. By contrast, in the TH using isopropanol, the solution of
complex 1 is yellow, when it reaches the reflux temperature it changes to
red and after 5 min it turns brown. This seems to indicate that under TH
conditions with isopropanol/potassium carbonate, complex 1 is unsta-
ble. The pyrazole ring has a pKa = 2.48, low enough to be deprotonated
The reaction in formic acid/triethylamine occurs probably through
step I′ to obtain a hydride complex. Similar steps: III, IV would allow the
formation of the primary alkoxide, while V′ would lead to the formation
of the product, regenerating the catalytically active species. The
experimental evidence obtained indicates that, upon addition of the
complexes to the formic acid/triethylamine mixture in the absence of
9

C.O. Blanco et al.
Inorganica Chimica Acta 524 (2021) 120429
Fig. 5. Proposed internal sphere mechanism for transfer hydrogenation of benzaldehyde with iPrOH/K2CO3 and HCOOH/NEt3 using Ru (II) complexes with bis-
(+)-camphopyrazole ligand.
aldehyde, a gas release is observed. Such gas evolution was attributed to
the release of carbon dioxide, product of the dehydrogenation of formic
acid. This observation could confirm that the formation of the hydride
intermediate in such hydrogen source is favored, and that the reaction
rate is controlled by the steps following it.
symmetric ligand to those obtained using isopropanol as solvent/
hydrogen source.
CRediT authorship contribution statement
Christian O. Blanco: Investigation, Formal analysis, Writing -
original draft, Visualization. Ligia Llovera: Investigation, Data cura-
tion, Resources. Alberto Herrera: Investigation, Data curation, Re-
sources. Romano Dorta: Conceptualization, Investigation, Resources.
4. Conclusions
Five piano-stool ruthenium (II) complexes with new bis-(+)-cam-
phopyrazole bidentate ligands were prepared with good yields and
characterized. The complexes were evaluated in the TH of benzaldehyde
and valeraldehyde in air using potassium carbonate as a less expensive
and easy-to-handle base, and isopropanol as hydrogen source, affording
moderate yields (>54%). One of the catalytic precursors, bearing the C2-
symmetric ligand with a 4-bromophenyl substituent, afforded quanti-
tative yields of benzyl alcohol (>99%). The use of a strong base with less
nucleophilic character and better solubility increases the yield and
conversion values of the synthesized complexes. Also, the TH of benz-
aldehyde was evaluated with a formic acid/triethylamine mixture,
finding yields and conversion values higher for complexes with C2-
´
Giuseppe Agrifoglio: Conceptualization, Resources. Domenico Ven-
uti: Investigation, Data curation, Resources. Vanessa R. Landaeta:
Data curation, Formal analysis, Investigation, Writing - original draft,
´
Writing - review & editing. Jesús Pastran: Conceptualization, Formal
analysis, Methodology, Writing - original draft, Writing - review &
editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
10

C.O. Blanco et al.
Inorganica Chimica Acta 524 (2021) 120429
[31] T.J. Korstanje, J.I. Van Der Vlugt, C.J. Elsevier, B. De Bruin, Science 350 (2015)
298–302, https://doi.org/10.1126/science.aaa8938.
the work reported in this paper.
[32] S. Elangovan, M. Garbe, H. Jiao, A. Spannenberg, K. Junge, M. Beller, Angew.
Chem. Int. Ed. 55 (2016) 15364–15368, https://doi.org/10.1002/anie.201607233.
Angew. Chem. 128 (2016) 15590–15594.
Acknowledgments
[33] J.M. Gichumbi, B. Omondi, H.B. Friedrich, Z. Kristallogr, NCS 232 (2017)
309–311, https://doi.org/10.1515/ncrs-2019-0793.
´
´
We thank Dr. Marisol Ortega (Laboratorio de Catalisis Homogenea
Dr. Orlando Leal - UCV) for the use of the gas chromatograph. V.R.L.
thanks Prof. Dr. H. Grützmacher and Prof. Dr. A. Togni for the oppor-
tunity to perform a sabbatical stage at Laboratorium für Anorganische
Chemie, ETH Zürich, during which the characterization of some of the
compounds was carried out.
[34] P. Kumar, R.K. Gupta, D.S. Pandey, Chem. Soc. Rev. 43 (2014) 707–733, https://
doi.org/10.1039/C3CS60189G.
[35] M.J. Spallek, S. Stockinger, R. Goddard, F. Rominger, O. Trapp, Eur. J. Inorg.
Chem. (2011) 5014–5024, https://doi.org/10.1002/ejic.201100694.
˜
´
[36] A. Herrera, A. Briceno, T. Gonzalez, A. Linden, F.W. Heinemann, G. Agrifoglio,
´
J. Pastran, R. Dorta, Tetrahedron Asymmetry 27 (2016) 759–767, https://doi.org/
10.1016/j.tetasy.2016.06.013.
[37] M. Sarkara, P. Pandeya, J.K. Bera, Inorg. Chim. Acta 486 (2019) 518–528, https://
doi.org/10.1016/j.ica.2018.11.014.
Appendix A. Supplementary data
[38] C. Groß, A. Omlor, T. Grimm, B. Oelkers, Y. Sun, V. Schünemann, W.R. Thiel,
Z. Anorg. Allg. Chem. 646 (2020) 968–977, https://doi.org/10.1002/
zaac.201900310.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2021.120429.
[39] M.J. Spallek, S. Stockinger, R. Goddard, F. Rominger, O. Trapp, Adv. Synth. Catal.
354 (2012) 1466–1480, https://doi.org/10.1002/adsc.201100953.
[40] E.C. Constable, P.J. Steel, Coord. Chem. Rev. 93 (1989) 205–223, https://doi.org/
10.1016/0010-8545(89)80016-5.
References
[41] Stephen O. Ojwach, James Darkwa, Inorg. Chim. Acta 363 (2010) 1947–1964,
https://doi.org/10.1016/j.ica.2010.02.014.
[1] H. Shimizu, I. Nagasaki, K. Matsumura, N. Sayo, T. Saito, K. Acc, Chem. Res. 40
(2007) 1385–1393, https://doi.org/10.1021/ar700101x.
[2] M. Rueping, A.P. Antonchick, T. Theissmann, Angew. Chem. Int. Ed. 45 (2006)
3683–3686, https://doi.org/10.1002/anie.200600191. Angew. Chem. 118 (2006)
3765–3768.
[42] J.A. Schachner, P. Traar, C. Sala, M. Melcher, B.N. Harum, A.F. Sax, M. Volpe,
¨
F. Belaj, N.C. Mosch-Zanetti, Inorg. Chem. 51 (2012) 7642–7649, https://doi.org/
10.1021/ic300648p.
`
[43] E. Manrique, A. Poater, X. Fontrodona, M. Sola, M. Rodríguez, I. Romero, Dalton
[3] S. Gladiali, E. Alberico, Chem. Soc. Rev. 35 (2006) 226–236, https://doi.org/
10.1039/B513396C.
Trans. 44 (2015) 17529, https://doi.org/10.1039/c5dt02787j.
[44] A. Maurya, C. Haldar, J. Coord. Chem. 74 (2021) 885–904, https://doi.org/
10.1080/00958972.2020.1857747.
¨
[4] J.S.M. Samec, J.-E. Backvall, Chem. Eur. J. 8 (2002) 2955–2961, https://doi.org/
10.1002/1521-3765(20020703)8:13<2955::AID-CHEM2955>3.0.CO;2-Q.
[5] D. Formenti, F. Ferretti, F.K. Scharnagl, M. Beller, Chem. Rev. 119 (2019)
2611–2680, https://doi.org/10.1021/acs.chemrev.8b00547.
[45] N. Onishi, R. Kanega, E. Fujita, Y. Himeda, Adv. Synth. Catal. 361 (2019) 289–296,
https://doi.org/10.1002/adsc.201801323.
[46] D. Hong, Y. Shimoyama, Y. Ohgomori, R. Kanega, H. Kotani, T. Ishizuka, Y. Kon,
Y. Himeda, T. Kojima, Inorg. Chem. 59 (2020) 11976–11985, https://doi.org/
10.1021/acs.inorgchem.0c00812.
[6] M.R. Friedfeld, M. Shevlin, J.M. Hoyt, S.W. Krska, M.T. Tudge, P.J. Chirik, Science
342 (2014) 1076–1080, https://doi.org/10.1126/science.1243550.
[7] P. Hauwert, G. Maestri, J.W. Sprengers, M. Catellani, C.J. Elsevier, Angew. Chem.
Int. Ed. 47 (2008) 3223–3226, https://doi.org/10.1002/anie.200705638.G.
[8] G. Zassinovich, G. Mestroni, S. Giadiali, Chem. Rev. 92 (1992) 1051–1069, https://
doi.org/10.1021/cr00013a015.
[47] W. Wang, H. Wang, Y. Yang, X. Lai, Y. Li, J. Wang, Y. Himeda, M. Bao,
ChemSusChem 13 (2020) 5015–5022, https://doi.org/10.1002/cssc.202001190.
˜
´
[48] J.M. Munoz-Molina, T.R. Belderrain, P.J. Perez, Coord. Chem. Rev. 390 (2019)
171–189, https://doi.org/10.1016/j.ccr.2019.03.013.
[9] J.R. Miecznikowski, R.H. Crabtree, Organometallics 23 (2004) 629–631, https://
doi.org/10.1021/om034393x.
[49] W. Jin, L. Wang, Z. Yu, Organometallics 31 (2012) 5664–5667, https://doi.org/
10.1021/om300602w.
[10] S. Naskar, M. Bhattacharjee, J. Organomet. Chem. 690 (2005) 5006–5010, https://
doi.org/10.1016/j.jorganchem.2005.08.006.
[50] M.N. Magubane, G.S. Nyamato, S.O. Ojwach, O.Q. Munro, RSC Adv. 6 (2016)
65205–65221, https://doi.org/10.1039/C6RA12788F.
˜
[11] R. Castarlenas, M.A. Esteruelas, E. Onate, Organometallics 27 (2008) 3240–3247,
[51] A.G. Nair, R.T. McBurney, D.B. Walker, M.J. Page, M.R.D. Gatus, M. Bhadbhadec,
B.A. Messerle, Dalton Trans. 45 (2016) 14335–14342, https://doi.org/10.1039/
C6DT02459A.
https://doi.org/10.1021/om800183k.
[12] P.N. Liu, K.D. Ju, C.P. Lau, Adv. Synth. Catal. 353 (2011) 275–280, https://doi.
org/10.1002/adsc.201000667.
[52] T. Toda, K. Saitoh, A. Yoshinari, T. Ikariya, S. Kuwata, Organometallics 36 (2017)
1188–1195, https://doi.org/10.1021/acs.organomet.7b00065.
[53] I.D. Alshakova, B. Gabidullin, G.I. Nikonov, ChemCatChem 10 (2018) 4860–4869,
https://doi.org/10.1002/cctc.201801039.
[13] S.S. Bhosale, K.S. Singh, Synth. Commun. 45 (2015) 1411–1420, https://doi.org/
10.1080/00397911.2015.1022793.
[14] F. Saleem, G.K. Rao, S. Kumar, M.P. Singh, A.K. Singh, Dalton Trans. 44 (2015)
19141–19152, https://doi.org/10.1039/C5DT02926K.
[54] P. Weingart, Y. Sun, W.R. Thiel, ChemCatChem 12 (2020) 6223–6233, https://doi.
org/10.1002/cctc.202001370.
[15] S.A. Talouki, G. Grivani, P. Crochet, V. Cadierno, Inorg. Chim. Acta 456 (2017)
142–148, https://doi.org/10.1016/j.ica.2016.10.041.
[55] W.F.L. Armarego, D.D. Perrin, Purification of Laboratory Chemicals, fourth ed.,
Pergamon Press, Oxford, 1996.
[16] R. Wang, Y. Tang, M. Xu, C. Meng, F. Li, J. Org. Chem. 83 (2018) 2274–2281,
https://doi.org/10.1021/acs.joc.7b03174.
[56] R.K. Harris, E.D. Becker, S.M.C. de Menezes, R. Goodfellow, P. Granger, Pure App.
Chem. 73 (2001) 1795–1818, https://doi.org/10.1351/pac200173111795.
[57] M.J. Minch, Concepts Magn. Reson. 6 (1994) 41–56, https://doi.org/10.1002/
cmr.1820060104.
[17] J. Karmakar, S. Bhattacharya, Polyhedron 172 (2019) 39–44, https://doi.org/
10.1016/j.poly.2019.02.046.
[18] R. Saha, A. Mukherjee, S. Bhattacharya, Eur. J. Inorg. Chem. (2020) 4539–4548,
https://doi.org/10.1002/ejic.202000859.
[58] K. Mashima, K.-H. Kusano, N. Sato, Y. Matsumura, K. Nozaki, H. Kumobayashi,
N. Sayo, Y. Hori, T. Ishazaki, S. Akutagawa, H. Takaya, J. Org. Chem. 59 (1994)
3064–3076, https://doi.org/10.1021/jo00090a026.
[19] E. Corey, H. Ensley, W.E. Suggs, J. Org. Chem. 41 (1976) 380–381, https://doi.
org/10.1021/jo00864a047.
[20] R.A. Farrar-Tobar, Z. Wei, H. Jiao, S. Hinze, J.G. De Vries, Chem. Eur. J. 24 (2018)
2725–2734, https://doi.org/10.1002/chem.201705423.
[59] H. Kurosawa, H. Asano, Y. Miyaki, Inorg. Chim. Acta 270 (1998) 87–94, https://
doi.org/10.1016/S0020-1693(97)05829-5.
[21] H. Adkins, R. Elofson, A.G. Rossow, C. Robinson, J. Am. Chem. Soc. 71 (1949)
3622–3629, https://doi.org/10.1021/ja01179a012.
[60] A.J. Davenport, J. Fawcett, L. Lad, D.R. Russell, Chem. Commun. (1997)
2347–2348, https://doi.org/10.1039/A706287G.
[22] S. Baldino, S. Facchetti, H.G. Nedden, A. Zanotti-Gerosa, W. Baratta,
ChemCatChem 8 (2016) 3195–3198, https://doi.org/10.1002/cctc.201600892.
[23] J.J. Li, Cannizzaro reaction, in: Name Reactions, Springer, Cham, 2014,
pp. 106–107, https://doi.org/10.1007/978-3-319-03979-4_51.
[24] L. Claisen, Ber. Dtsch. Chem. Ges. 20 (1887) 650, https://doi.org/10.1002/
cber.188702001148.
[61] D.L. Davies, J. Fawcett, S.A. Garratt, D.R. Russell, Organometallics 20 (2001)
3029–3034, https://doi.org/10.1021/om010092g.
[62] O. Dayan, S. Dayan, I. Kani, B. Çetinkaya, Appl. Organomet. Chem. 26 (2012)
663–670, https://doi.org/10.1002/aoc.2901.
´
[63] M. Płotek, R. Starosta, U.K. Komarnicka, A. Skorska-Stania, P. Kołoczek, A. Kyzioł,
J. Inorg. Biochem. 170 (2017) 178–187, https://doi.org/10.1016/j.
[25] D. Klein, Organic Chemistry, third ed., John Wiley & Sons Inc, 2017.
[26] C.M. Beck, S.E. Rathmill, Y.J. Park, J. Chen, R.H. Crabtree, L.M. Liable-Sands, A.
L. Rheingold, Organometallics 18 (1999) 5311–5317, https://doi.org/10.1021/
om9905106.
jinorgbio.2017.02.017.
¨
¨
[64] W. Lackner-Warton, S. Tanaka, C.M. Standfest-Hauser, O. Oztopcu, J. Hsieh,
K. Mereiter, K. Kirchner, Polyhedron 29 (2010) 3097–3102, https://doi.org/
10.1016/j.poly.2010.08.014.
[27] J. Zhang, E. Balaraman, G. Leitus, D. Milstein, Organometallics 30 (2011)
5716–5724, https://doi.org/10.1021/om200595m.
[65] J.M. Gichumbi, H.B. Friedrich, B. Omondi, J. Organomet. Chem. 808 (2016)
87–96, https://doi.org/10.1016/j.jorganchem.2016.02.015.
[66] D. Pandiarajan, R. Ramesh, J. Organomet. Chem. 723 (2013) 26–35, https://doi.
org/10.1016/j.jorganchem.2012.10.003.
[28] V.R. Landaeta, A.D. Salazar-La Rosa, R.E. Rodríguez-Lugo, Inorg. Chim. Acta 470
(2018) 303–311, https://doi.org/10.1016/j.ica.2017.08.014.
[29] P. Dubey, S. Gupta, A.K. Singh, Organometallics 38 (2019) 944–961, https://doi.
org/10.1021/acs.organomet.8b00908.
[67] R.K. Rath, M. Nethaji, A.R. Chakravarty, Polyhedron 20 (2001) 2735–2739,
https://doi.org/10.1016/S0277-5387(01)00894-4.
[30] S. Fleischer, S. Zhou, K. Junge, M. Beller, Angew. Chem. Int. Ed. 52 (2013)
5120–5124, https://doi.org/10.1002/anie.201301239. Angew. Chem. 125 (2013)
5224ꢀ 5228.
[68] F. Ding, Y. Sun, F. Verpoort, Eur. J. Inorg. Chem. (2010) 1536–1543, https://doi.
org/10.1002/ejic.200900926.
11

C.O. Blanco et al.
Inorganica Chimica Acta 524 (2021) 120429
[69] G.L. Miessler, P.J. Fischer, in: Inorganic Chemistry, fifth ed., Pearson, 2014,
pp. 555–556.
[74] X. Zhou, X. Wu, B. Yang, J. Xiao, J. Mol. Catal. A: Chem. 357 (2012) 133–140,
https://doi.org/10.1016/j.molcata.2012.02.002.
[70] A.M. Heyns, Spectrochim. Acta 33 (1977) 315–322, https://doi.org/10.1016/
0584-8539(77)80036-6.
[75] S. Olivella, J. Vilarrasa, J. Heterocycl. Chem. 18 (1981) 1189–1196, https://doi.
org/10.1002/jhet.5570180625.
[71] D.A. Alonso, P. Brandt, S.J.M. Nordin, P.G. Andersson, J. Am. Chem. Soc. 121
(1999) 9580–9588, https://doi.org/10.1021/ja9906610.
[76] R. Noyori, M. Yamakawa, S. Hashiguchi, J. Org. Chem. 66 (2001) 7931–7944,
https://doi.org/10.1021/jo010721w.
[72] A. Ouali, J.-P. Majoral, A.-M. Caminade, M. Taillefer, ChemCatChem 1 (2009)
504–509, https://doi.org/10.1002/cctc.200900237.
[77] Y.K. Ghebreyessus, J.H. Nelson, J. Organomet. Chem. 669 (2003) 48–56, https://
doi.org/10.1016/S0022-328X(02)02216-7.
[73] S. Baldino, S. Facchetti, A. Zanotti-Gerosa, H.G. Nedden, W. Baratta,
ChemCatChem 8 (2016) 2279–2288, https://doi.org/10.1002/cctc.201600420.
[78] A. Grabulosa, A. Mannu, A. Mezzetti, G. Muller, J. Organomet. Chem. 696 (2012)
4221–4228, https://doi.org/10.1016/j.jorganchem.2011.09.015.
12