Base-free transfer hydrogenation of aryl-ketones, alkyl-ketones and alkenones catalyzed by an IrIIICp* complex bearing a triazenide ligand functionalized with pyrazole

Article abstract of DOI:10.1016/j.ica.2020.119551

An Ir^IIICp* complex (2) bearing a triazenide ligand functionalized with pyrazole was synthesized and fully characterized by spectroscopic methods and the structure confirmed by X-ray diffraction studies. The catalytic activity of 2 and the control complex 3, which lacks of pyrazole in its structure, was evaluated in the reduction of aryl-ketones, alkyl-ketones, α,β-unsaturated and γ,δ-unsaturated ketones. The catalytic system, using either 2 or 3, exhibited good to excellent selectivity when tested with ketones and alkenones at 90 °C in 2-propanol as hydrogen source under base-free conditions. Reactivity of 2 in 2-propanol and NaH gave a neutral metal hydride (4) while in the absence of base gave two major cationic hydrides species (5 and 6).

Full text of DOI:10.1016/j.ica.2020.119551

Journal Pre-proofs
Research paper
Base-free transfer hydrogenation of aryl-ketones, alkyl-ketones and alkenones
catalyzed with an IrIIICp* complex bearing a triazenide ligand functionalized
with pyrazole
Layla J. Medrano-Castillo, Miguel Á. Collazo-Flores, Juan P. Camarena-Díaz,
Erick Correa-Ayala, Daniel Chávez, Douglas B. Grotjahn, Arnold L. Rheingold,
Valentín Miranda-Soto, Miguel Parra-Hake
PII:
S0020-1693(19)31800-6
https://doi.org/10.1016/j.ica.2020.119551
ICA 119551
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
21 November 2019
22 February 2020
22 February 2020
Please cite this article as: L.J. Medrano-Castillo, M.A. Collazo-Flores, J.P. Camarena-Díaz, E. Correa-Ayala, D.
Chávez, D.B. Grotjahn, A.L. Rheingold, V. Miranda-Soto, M. Parra-Hake, Base-free transfer hydrogenation of aryl-
ketones, alkyl-ketones and alkenones catalyzed with an IrIIICp* complex bearing a triazenide ligand functionalized
with pyrazole, Inorganica Chimica Acta (2020), doi: https://doi.org/10.1016/j.ica.2020.119551
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Base-free transfer hydrogenation of aryl-ketones, alkyl-ketones and alkenones
catalyzed with an IrIIICp* complex bearing a triazenide ligand functionalized
with pyrazole
Layla J. Medrano-Castillo,1† Miguel Á. Collazo-Flores, 1† Juan P. Camarena-Díaz, 1Erick Correa-
Ayala,1 Daniel Chávez,1 Douglas B. Grotjahn,2 Arnold L. Rheingold,3 Valentín Miranda-Soto,*1
and Miguel Parra-Hake.*1
1 Tecnológico Nacional de México-Instituto Tecnológico de Tijuana/Centro de Graduados e Investigación en
Química, Tijuana, Baja California, 22000, México.
2Department of Chemistry and Biochemistry, 5500 Campanile Drive, San Diego State University, San Diego, CA,
92182-1030.
3Department of Chemistry and Biochemistry, 9500 Gilman Drive, University of California at San Diego, La Jolla,
CA 92093-0385.
†These authors contributed equally to the work.
ABSTRACT
An IrIIICp* complex (2) bearing a triazenide ligand functionalized with pyrazole was synthesized
and fully characterized by spectroscopic methods and the structure confirmed by X-ray
diffraction studies. The catalytic activity of 2 and the control complex 3, which lacks of pyrazole
in its structure, was evaluated in the reduction of aryl-ketones, alkyl-ketones, α,β-unsaturated and
,-unsaturated ketones. The catalytic system, using either 2 or 3, exhibited good to excellent
selectivity when tested with ketones and alkenones at 90 °C in 2-propanol as hydrogen source
under base-free conditions. Reactivity of 2 in 2-propanol and NaH gave a neutral metal hydride
(4) while in the absence of base gave two major cationic hydrides species (5 and 6).
1. Introduction
Due to the capacity of nitrogen donor ligands of stabilizing and modulating the reactivity of
organometallic complexes, the design and development of alternative nitrogen ligands such as
triazenes has been a topic of interest in the coordination and organometallic chemistry [1,2].
Triazenes are versatile compounds of great utility as anticancer agents, in organic synthesis,
synthesis of polymers, and in coordination chemistry as ligand precursors [3]. Deprotonation of
the triazene gives the triazenide ligand, which can easily coordinate to transition metal centers
and stabilize monomeric [4-7] and polymeric complexes [7-10] in different oxidation states. 1,3-
Bis(aryl)triazenide ligands ortho-substituted with aryl groups [11,12] and donor groups as esters,
ethers, and thioethers have been relatively well studied [13-28]. However, coordination
chemistry of triazenide systems substituted with N-heterocycles has been less studied. In the
1

literature few examples of complexes with triazenide ligands substituted with pyrazole [29],
imidazole [30] triazole [31,32], tetrazole [33,34], benzothiazole [35,36] and pyridine [37-40] are
reported. Catalytic processes where the basic nitrogen works as hydrogen acceptor achieving the
activation of the substrate to give way to the chemical transformation have been reported [41-
43].
The transfer hydrogenation (TH) of ketones is a key process for the production of a
variety of alcohols, which are important products and precursors in pharmaceutical,
agrochemical, flavor, fragrances, materials, and fine chemical industries [44,45]. 2-propanol as
solvent and hydrogen source is ideal because is cheap, environmentally friendly and nontoxic
[46,47]. In general, inner and outer coordination sphere with ancillary ligand assistance
mechanisms have been proposed, where the metal hydride is the active species and a base is
needed to generate it through β-H elimination from a metal alkoxide intermediate [48-50].
However, sometimes the presence of a base is undesired, since it can affect the stereo selectivity
of the product, and there are base sensitive ketones [51] and aldehydes, [52] and also produces
corrosion and reduces the life time of the equipment [53-55]. Therefore, metal hydride
complexes have been developed and used directly [51, 56-59]. However, metal hydride
complexes generally are air and water sensitive and in consequence not easy to handle [50].
Alkenones are substrates of certain difficulty due to the possibility to be either selectively
hydrogenated at the C=O, the C=C bond, or non-selectively hydrogenated of both functional
groups. Therefore, hydrogenation of alkenones via catalysis is an attractive strategy to induce
selectivity and there are several examples in the literature of selective hydrogenation at the C=O
or the C=C bond. [60-63]. In this direction, we became interested in the selective hydrogenation
of α,β-unsaturated and ,-unsaturated ketones using protic N-heterocyclic carbene IrIIICp* [64],
triazenide RuII(p-cimene) [18] and triazenide IrIIICp* complexes [30]. Here we report the
synthesis of an IrIIICp* complex bearing a triazenide ligand functionalized with pyrazole (2), and
the evaluation of its catalytic activity in the transfer hydrogenation of aryl-ketones, alkyl-
ketones, α,β-unsaturated and ,-unsaturated ketones. Control experiments with a complex
lacking of pyrazole (3) as precatalyst, were also performed in order to get insight of the pyrazole
role in the catalysis (Figure 1).
2

Ir
N
Ir
N
Cl
Cl
N
N
N
N
N
N
2
3
Figure 1. Molecular structures of complexes 2 and 3.
2. Experimental
2.1 General comments
Catalytic evaluations have been carried out under inert atmosphere in an argon-filled glovebox.
Commercially available reagents and solvents were used as received from Aldrich Chemical Co.
and Strem Chemicals Inc. [IrCp*(μ-Cl)Cl]2 was prepared by published procedures [65]. [Ir{1,3-
bis(4’-methylphenyl)triazenide}Cp*Cl] (3) has been previously synthesized in our laboratory
[30]. NMR spectra were recorded at 400 MHz with a Bruker Avance III spectrometer at 30 °C
1
13
unless otherwise specified. H and C NMR chemical shifts are reported in ppm referenced to
residual solvent resonances (1H NMR: 7.24 for CHCl3 in CDCl3, 7.16 for C6D5H in C6D6, 1.94
13
for CHD2CN in CD3CN. C {1H} NMR: 128.39 for benzene-d6, 77.23 for chloroform-d1 and
118.69 and 1.39 for acetonitrile-d3). Coupling constants J are given in Hertz (Hz). IR spectra
were recorded on a Perking-Elmer FT-IR 1605 spectrophotometer. Melting points were
measured in an Electrothermal GAC 88629 apparatus. Mass spectra were obtained by direct
insertion on an Agilent Technologies 5975C instrument. High Resolution Mass Spectrometry
(HRMS) data were obtained in a micrOTOF-Q III MS instrument with electrospray ionization
using sodium formate as calibrant. X-ray structure of 1 and 2 were determined on a Bruker
APEX II CCD diffractometer.
2.2 Synthesis of ligand
3
5
4
4
2
N
3
1
N
1
N
N
H
N
2
3

2.2.1 1-(1’-Methylpyrazole)-3-(4’-methylphenyl)triazene (1). 3-Amino-1-methylpyrazole
hydrochloride (42.40 mg, 0.2685 mmol) dissolved in water (5.0 mL) and HCl 1 M (15.3 mL,
15.36 mmol, 2.0 eq) were mixed with stirring for 15 min at 0 to −5 °C. Then a 15% solution of
NaNO2 (5.3 mL, 11.52 mmol, 1.5 eq) was added dropwise with stirring for 40 min. A 15%
solution of p-toluidine in ethanol (823.4 mg, 7.686 mmol, 1 eq) was added slowly, and the
resulting solution stirred for 8 h. Then the reaction mixture was neutralized with a solution of
NaHCO3 (3.2272 g, 32.48 mmol, 5 eq) and stored at 15 °C overnight. A solid precipitated, which
was dried under reduced pressure. The isolated product was dissolved in ethyl acetate and
crystallized by vapor diffusion of pentane to obtain yellow crystals (677.3 mg, 84%). MP: 141-
142 °C. IR: 3177, 3014, 2920, 1612, 1523, 1414.7 cm–1. 1H NMR (400.0 MHz, CD3CN) δ: 10.32
(bs, NH), 7.41 (d, 3JHH = 2.3 Hz, 1H, PzH), 7.24 (d, 3JHH = 8.4 Hz, 2H, ArH), 7.17 (d, 3JHH = 8.4
3
13
Hz, 2H, ArH), 6.33 (d, JHH = 2.3 Hz, 1H, PzH), 3.82 (s, 3H, NCH3), 2.31 (s, 3H, ArCH3). C
{1H} NMR (100.0 MHz, CD3CN) δ: 159.2 (C3, Pz), 142.1 (C1, Ar), 134.3 (C4, Ar), 132.7 (C4,
Pz), 130.9 (C3, Ar), 116.7 (C2, Ar), 95.2 (C5, Pz), 39.5 (N-CH3), 20.9 (CH3-Ar). EIMS (70 eV)
m/z (%): [M+] 215(8), [M+ − N2(C3H2N2)CH3] 109(74), [M+ − (C6H4)CH3] 91(100). HRMS
(ESI-TOF) m/z: [M + Na]+ calcd for C11H13N5: 238.1072, found 238.1063. Anal. Calcd. for
C11H13N5 (215.26): C, 61.38; H, 6.09; N, 32.54. Found: C, 61.35; H, 6.55; N, 32.63.
2.3 Synthesis of complexes
2.3.1 [Ir(1-(1’-methylpyrazole)-3-(4’-methylphenyl)triazenide)Cp*Cl] (2). Triazene
1
(272.85 mg, 1.2685 mmol) was dissolved in acetonitrile (2 mL) and triethylamine (214.60 mg,
3.8060 mmol) was added with stirring. Then a solution of [Ir(η5-C5Me5)(μ-Cl)Cl]2 (505.30 mg,
0.6343 mmol) in acetonitrile was slowly added and the mixture stirred for 24 h at room
temperature to give a dark yellow solid. The solid was filtered, washed with diethyl ether (3 × 5
mL) and dried under reduced pressure. The isolated product was dissolved in dichloromethane
and crystallized by vapor diffusion of diethyl ether to obtain 2 as yellow crystals (593.3 mg,
1
81%). MP: 230-232 °C. IR: 3098, 2965, 2919, 1602, 1530, 1505, 1478, 1281 cm–1. H NMR
(400.0 MHz, CD3CN) δ: 7.34 (d, 3JHH = 2.3 Hz, 1H, Pz), 7.14 (d, 3JHH = 8.3 Hz, 2H, Ar), 7.07 (d,
3JHH = 8.3 Hz, 2H, Ar), 6.09 (d, 3JHH = 2.3 Hz, 1H, Pz), 3.79 (s, 3H, NCH3), 2.31 (s, 3H, ArCH3),
13
1.81 {s, 15H, [C5(CH3)5]}. C {1H} NMR (100.0 MHz, CD3CN) δ: 157.4 (C3, Pz), 144.9 (C1,
4

Ar), 134.8 (C4, Ar), 132.4 (C4, Pz), 130.4 (C3, Ar), 117.8 (C2, Ar), 94.1 (C5, Pz), 87.5 (C-Cp*),
39.4 (N-CH3), 21.0 (CH3-Ar), 10.3 [Cp*-CH3]. 1H NMR (400.0 MHz, CDCl3) δ: 7.13 (d, 3JHH
=
=
2.3 Hz, 1H, Pz), 7.11 (d, 3JHH = 8.5 Hz, 2H, Ar), 7.04 (d, 3JHH = 8.5 Hz, 2H, Ar), 6.13 (d, 3JHH
13
2.3 Hz, 1H, Pz), 3.76 (s, 3H, NCH3), 2.28 (s, 3H, ArCH3), 1.83 {s, 15H, [C5(CH3)5]}. C {1H}
NMR (100.0 MHz, CDCl3) δ: 156.6 (C3, Pz), 143.8 (C1, Pz), 133.8 (C4, Ar), 130.7 (C4, Pz),
129.4 (C3, Ar), 117.4 (C2, Ar), 94.4 (C5, Pz), 86.1 [C-Cp*], 39.2 (N-CH3), 21.1 (CH3-Ar), 10.2
[Cp*-CH3]. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C21H27N5ClIrNa: 600.1465, found
600.1469. Anal. Calcd. for C21H27IrClN5 (577.149): C, 43.70; H, 4.72; N, 12.13. Found: C,
43.52; H, 5.43; N, 12.25.
2.3.2 [Ir(1-(1’-methylpyrazole)-3-(4’-methylphenyl)triazenide)Cp*H] (4). Complex 2 (53.7
mg, 0.092 mmol), and NaH (7.4 mg, 0.27 mmol) were dissolved in 2-propanol (8 mL) under
inert atmosphere. After 24 h of stirring, the solvent was removed under vacuum and the residue
extracted with hexanes and filtered through a pad of Celite. Removal of hexanes gave the
1
3
hydride 4 as a yellow powder (42.4 mg, 84%). H NMR (400.0 MHz, C6D6) δ: 7.47 (d, JHH
8.4 Hz, 2H, Ar), 7.02 (d, 3JHH = 8.0 Hz, 2H, Ar), 6.47 (d, 3JHH = 2.4 Hz, 1H, Pz), 6.40 (d, 3JHH
=
=
2.4 Hz, 1H, Pz), 3.13 (s, 3H, NCH3, Pz), 2.15 (s, 4H, CH3, Ar), 1.93 {s, 15H, [C5(CH3)5]}, –7.73
13
(s, 1H). C {1H} NMR (100.0 MHz, C6D6) δ: 160.1 (C3, Pz), 147.5 (C1, Ar), 133.0 (C4, Ar),
130.5 (C4, Pz), 129.7 (C3, Ar), 117.7 (C2, Ar), 93.4 (C5, Pz), 87.3 [C-Cp*], 38.6 (N-CH3), 21.3
(CH3-Ar), 11.5 [Cp*-CH3]. HRMS (ESI-TOF) m/z: [M – H]+ calcd for C21H27N5Ir: 542.1865,
found 542.1891.
2.4 X-Ray crystallography.
The single crystal X-ray diffraction studies were carried out on a Bruker X8 APEX II CCD
diffractometer equipped with Mo Ka radiation (λ = 0.71073). Crystals were mounted on a
Cryoloop with Paratone oil. Data were collected in a nitrogen gas stream at 100(2) K using f and
v scans. The data were integrated using the Bruker SAINT Software program and scaled using
the SADABS software program. Solution by direct methods (SHELXT) produced a complete
phasing model consistent with the proposed structure. All nonhydrogen atoms were refined
anisotropically by full-matrix least-squares (SHELXL). All hydrogen atoms were placed using a
riding model. Their positions were constrained relative to their parent atom using the
appropriate HFIX command in SHELXL. Crystallographic data are summarized in Table S1.
5

1
2.5 General procedure for transfer hydrogenation reactions monitored by H NMR and
Gas Chromatography Mass Spectrometry (GC-MS).
In the glovebox, ketone or alken-2-one (6.3 × 10−5 mol), 1,3,5-trimethylbenzene (2 µL) as
internal standard and 0.7 mL of 2-propanol-d8 were transferred into a J. Young NMR tube, and a
1H NMR spectrum was recorded. The tube was brought back into the glovebox, and precatalyst 2
(1.26 × 10−6 mol in 0.2 mL of 2-propanol-d8) was added. At this point, KOH (0.5 M, 5 µL, 2 eq.)
in 2-propanol-d8 was added only to those catalytic reactions performed in presence of base.
Outside of the glovebox the tube was placed inside an oil bath at 90 °C. The tube was removed
1
from the oil bath at different times, and H NMR spectra were recorded in order to follow the
progress of the reaction. The conversions were determined by the integration of either the
aromatic protons or the aliphatic protons of the ketone. The value of the integral for the singlet
due to the aromatic protons of 1,3,5-trimethylbenzene (internal standard) was set to 10 units. In
addition, the conversion was also determined by GC-MS analysis in order to follow the
deuteration process of the substrates, which is a consequence of using 2-propanol-d8. In addition,
the transfer hydrogenation reactions of some aryl- alkylketones and 5-alken-2-ones were
performed as above using the same quantities of reactants (precatalyst 2 or 3 and KOH if needed)
in regular 2-propanol.
The analytical GC/MS system used was Agilent 7890A GC coupled to 5975C Mass
detector Agilent Technologies, equipped with a HP-5MS capillary column (30 m × 0.25 mm ×
0.25 µm). An Agilent Technologies 7693 auto sampler was used to inject 1 µL of a solution
sample. The ionization energy was 70 eV with a mass range of 30 to 800 m/z. The initial
temperature of the column was set at 80 °C, held for 2 min, and then a ramp of 10 °C/min to 250
°C. The temperature of the injector was set at 250 °C and the detector at 230 °C. The flow rate of
the carrier gas (He) was 1.0 mL/min injected with a gas dilution of 1:50. Identification of the
individual components was based on comparison with the mass spectra library (NIST98).
3. Results and discussion.
3.1 Synthesis and general characterization
3.1.1 Triazene
Triazene 1 functionalized with pyrazole was synthesized with the aim of obtaining one ligand
with a pendant nitrogen base able to promote bifunctional interactions, such as hydrogen
6

bonding, proton abstraction or activation of polar and non-polar bonds. The pyrazole group in the
triazene structure also has a methyl group, which provides enough steric hindrance to prevent the
undesired coordination of the N-heterocycle to the metal center. Triazene 1 was synthesized by
the traditional procedure of N-coupling of diazonium salts [66]. The IR spectrum of triazene 1
shows bands at 3177 cm‒1 for the N‒H group and at 1523 cm‒1 for the N3 system. The 1H NMR
spectrum of 1 shows a broad singlet at 10.32 ppm assigned to the N-H group (Figure S1). In the
aromatic region there are two doublets at 7.24 and 7.17 ppm, characteristic of an A2B2 system,
and two doublets at 7.41 and 6.33 ppm corresponding to the hydrogens in the pyrazole ring that
are coupled to each other. These assignments are supported by gCOSY, 1H-13C gHSQC and 1H-
13C gHMBC correlation experiments (Figures S3 and S4). Two singlets are observed at 3.82 and
13
2.31 ppm assigned to the N‒CH3 and Ar‒CH3, respectively. The C NMR spectrum of triazene
1 shows the typical signals for aromatic carbons from 142.1 to 116.7 ppm. The ipso carbons to
the aryl and the pyrazol groups show signals down field at 142.1 and 159.2 ppm, respectively
(Figure S2). HRMS and EIMS confirmed the molecular weight of triazene 1 (Figure S5). The
adduct [M + Na]+ in the HRMS appears at 238.1072 m/z, whereas the molecular ion in EIMS can
be seen at 215 m/z with an intensity of 8%. In addition, some characteristic fragments of the
compound were also found, one at 119 m/z belonging to the loss of 1-methyl-1H-pyrazol-3-
amine leaving the organic fragment [C6H4(CH3)N2]+. On the other hand, the loss of p-toluidine
leaves the fragment 109 m/z assigned to [C3H2(N2CH3)N2]+. The tropylium ion [C7H7]+ was also
found at 91 m/z.
3.1.2 Iridium triazenide complexes.
Reaction of the ligand precursor 1 with half equivalent of [Ir(η5-C5Me5)(μ-Cl)Cl]2 in acetonitrile
and in the presence of triethylamine resulted in the formation of a new monomeric complex
{IrCl[Me-p-(C6H4)-NNN-(Me-1-C3H2N2)](η5-C5Me5)}, 2 (Equation 1).
Ir
N
1/2 [IrCp*Cl2]2
Cl
N
(1)
N
1
N
N
N
H
N
Et3N, CH3CN, rt
N
N
N
2
7

Complex 2 was isolated as yellow crystals and its formulation is supported by analytical and
spectroscopic data (IR, NMR and HRMS), as well as X-ray crystal structure determination. In
the IR spectrum of complex 2, no significant changes were observed compared to the free
triazene 1, other than the disappearance of the characteristic N-H stretching band at 3177 cm–1 as
an evidence of the formation of the triazenide ligand, and a band at 1526 cm–1 for the coordinated
1
triazenide system (N3). In the H NMR spectrum of 2, the A2B2 system of the tolyl ring gives
signals shifted 0.13 ppm upfield in comparison with the free triazene (Figure S6). The same
behavior is observed for the hydrogens of the pyrazole ring, which appears as doublets at 7.34
and 6.09 ppm (J = 2.3 Hz) in comparison with the free triazene, at 7.41 and 6.33 ppm,
respectively. The aromatic signals of the coordinated ligand are slightly shifted upfield compared
to the free triazene due to coordination to the metal center. In addition, a similar behavior is
13
observed in C NMR in which the aryl ipso carbon to the triazenide system (C1) is slightly
shifted downfield at 144.9 ppm after coordination, while in the free triazene it is at 142.1 ppm
(Figure S7). However, the opposite phenomenon is observed in the pyrazole ipso carbon to
triazenide system (C3), which is shifted upfield at 157.4 ppm after coordination, while in the free
triazene appears at 159.2 ppm. The sp2 and sp3 carbon signals of the ancillary ligand Cp*, show
typical shifts appearing as singlets at 87.5 and 10.3 ppm, respectively [30]. The HRMS (ESI-
TOF) spectrum shows a peak with m/z = 600.1465, consistent with the formation of the adduct
[M + Na]+, which may be a consequence of using sodium formate as calibrant (Figure S10).
In addition, the reaction of complex 2 with NaH in 2-propanol gave a single organometallic
1
product, which was formulated as the metal hydride 4 since its H NMR spectrum exhibited a
high-field signal at –7.73 ppm (Scheme 1).
8

Ir
N
Ir
N
Cl
H
NaH
N
N
2-Propanol
90 oC, 1 h
N
N
N
N
N
N
4
2
2-Propanol
90 oC, 1 h
+
+
Cl
Cl
H
Ir
Ir
H
H
+
N
N
N
N
H
N
N
N
N
N
6
5
N
Scheme 1. Reactivity of complex 2 in 2-propanol.
Interestingly, complex 2 was also able to react with 2-propanol in the absence of NaH to form
two major metal hydrides species (5 and 6 as proposed structures), which gave 1H NMR signals
at –14.67 and –15.44 ppm (ratio 6 to 4). We hypothesized that these chemical shifts may be
consistent with cationic complexes where the triazenide has been protonated and converted into
the neutral triazene. Therefore, the hydrogen can be located either at the nitrogen substituted with
the pyrazole or at the nitrogen substituted with the aryl group. The ability to form neutral or
cationic hydrides species has been observed in related complexes bearing a triazenide
functionalized with imidazole and also in the complex 3 [30]. Moreover, neutral metal hydrides
have been proposed as transient species to explain the mechanism of transfer hydrogenation
performed under basic conditions; conversely, cationic metal hydrides have been proposed as
transient species to explain the mechanism of transfer hydrogenation under base-free conditions
[67].
3.3 Description of the crystal structures
Crystals of 1 suitable for X-ray diffraction studies were obtained. The triazene
crystallized in the monoclinic system, with space group P2(1)/n. The ORTEP diagram of 1 is
shown in Figure 2 with selected bond distances and angles, while crystal data is given in the
9

supporting information (Table S1). Triazene 1 adopts a trans configuration about the N=N
double bond and is quasi-planar with the dihedral angle between the p-tolyl and pirazol planes at
4.72°. The N(3)-N(4)=N(5) bond angle is 113.68(14)°, whereas the N(3)-N(4) and N(4)=N(5)
bond distances of 1.2757(18) and 1.333(2) Å, respectively, indicate the presence of single and
double bonds. These bond distances and angles are in agreement with those reported for related
triazenes with N-heterocycles [33, 35, 37, 38].
Figure 2. ORTEP diagram for triazene 1 with thermal ellipsoids drawn at 50% probability level.
Bond distances (Å): N(1)-N(2), 1.356(2); N(3)-N(4), 1.2757(18); N(4)-N(5), 1.333(2); N(5)-
H(5N), 0.884(9); C(5)-N(5), 1.407(2); N(3)-C(4), 1.408(2). Bond angles (⁰): N(3)-N(4)-N(5),
113.68(14); N(4)-N(5)-C(5), 119.06(14); N(4)-N(3)-C(4), 110.60(14).
The crystal arrangement exhibits intermolecular N(5)–H(5N)∙∙∙N(3) and C(6)–H(6A)∙∙∙N(2)
hydrogen bonds [2.290(10) and 2.454(10) Å, respectively], forming a dimer with three 8-
membered supramolecular rings (Figure S15). This dimer polymerized via intermolecular
hydrogen bonds with a bifurcated geometry C(2)∙∙∙∙∙H(2B)∙∙∙∙∙N(1) [2.883 and 2.643 Å,
respectively, <27.84º] resulting in a two-dimensional network of triazene molecules. This
arrangement contrasts with the dimers formed by related triazenes with N-heterocycles in which
the triazene hydrogen (N-H) interacts with a N-heterocycle instead of a second triazene system,
forming stronger hydrogen bonds (ca. 2.01 Å) [30, 33].
The structure of compound 2 was determined by X-ray diffraction methods. Compound 2
crystallizes in the monoclinic crystalline system, space group (P2(1)/c. Crystallographic data are
included in Table S1. The ORTEP diagram of compound 2 is shown in Figure 3 with selected
10

bond distances and angles displayed in the caption. The molecular structure has pseudo-
octahedral three-legged piano-stool geometry around the metal center. The triazenide ligand is
coordinated to the metal ion through the N(3) and N(5) atoms, forming a four-membered chelate
ring, while the pendant 1-methylpyrazol group remains unbound, as a result of the steric
hindrance provided by the methyl substituent. This behavior is in contrast to the coordination
mode of related triazenide ligands previously reported, where the N-heterocycle coordinates to
the metal center [31,34-40].
The octahedral distortion in compound 2 is caused primarily by the small-bite angle of
the triazenide [N(3)-Ir-N(5)] of 58.61(16)º, whereas the N-Ir-Cl angle is close to the expected
90º. The equivalent N-N bond distances of ~1.32 Å, the Ir-N bond distances of ~2.10 Å and the
near planarity of the chelate ring, show a characteristic π delocalization over the metal-triazenide
system [21, 68-71]. On the other hand, the distance Ir-CT (CT = centroid) of the Cp* ligand in
compound 2 at 1.773 Å is within the range of centroid distances in related IrCp* complexes [21].
Figure 3. ORTEP diagram of compound 2 with thermal ellipsoids drawn at 50% probability
level. Bond distances (Å): N(2)-C(4), 1.333(6); N(1)-N(2), 1.373(6); N(1)-C(1), 1.432(7); N(1)-
C(2), 1.332(7); Ir-N(3), 2.100(4); Ir-N(5), 2.091(4); N(3)-N(4), 1.316(6); N(4)-N(5), 1.320(5);
11

N(3)-C(4), 1.397(6); N(5)-C(5), 1.401(6); Ir-Cl(1), 2.3850(12); Ir-CT, 1.773. Bond angles ():
N(2)-N(1)-C(1), 120.4(4); N(2)-N(1)-C(2), 112.1(4); (N3)-N(4)-N(5), 102.2(4); N(3)-Ir(1)-N(5),
58.61(16); Cl(1)-Ir(1)-N(3), 84.89(11); Cl(1)-Ir(1)-N(5), 85.79(12).
3.4 Transfer hydrogenation catalysis
Complex 2 was used as precatalyst (2% mol) in the transfer hydrogenation of aryl and alkyl-
ketones to give the corresponding alcohol using either 2-propanol-d8 or regular 2-propanol as
hydrogen source (Table 1). All substrates can be hydrogenated in high yields in the absence of
base except the acetylpyridine. Acetophenone can be hydrogenated either in the absence or
presence of base with 93 and 83% yields in 39 h, respectively (Entries 1 and 2). In contrast, the
hydrogenation of acetylpyridine is slow and required the presence of base to be hydrogenated in
high yield in just 18 h (>99%, Entry 2). It is assumed that, in the case of acetylpyridine, the
catalytic species is the neutral metal hydride, which formation requires the presence of base.
Interestingly, no base was needed when alkyl-ketones are the substrates, for example 4-tert-
butylcyclohexanone and -tetralone were hydrogenated in short times of reaction in high yields
(98 to >99%, entries 3 and 5). Moreover, in the case of acetophenone and alkyl-ketones, the
addition of base slows down the speed of the catalysis (entries 1, 3 and 5). On the other hand,
base-free hydrogenation of 4-tert-butylcyclohexanone gives a mixture of diastereomers cis and
trans due to the existence of a chiral carbon at position 4 and the generation of a second chiral
center after hydrogenation of the pro-chiral carbonyl group (entry 5). The ratio between
diastereomers cis and trans is 27 and 73%, respectively (Figures S32, S33 and S34). In contrast,
control experiments using precatalyst 3 lacking of pyrazole gives high yields in the
hydrogenation of -tetralone and 4-tert-butylcyclohexanone. These results are comparable to
those observed for precatalyst 2 at the same reaction time (entries 4 and 6). Therefore, the
presence of the pendant pyrazole in complex 2 seems not to be relevant for the substrates shown
in Table 1.
12

Table 1. Transfer hydrogenation of aryl and alkyl-ketones with 2% of precatalyst loading.
Entry
Substrate
Product
preCat/base
Time (h)
Yield (%)b
O
OD
2/-
39
39
93
83
1
D
2/KOH
O
OD
2/-
18
18
6
N
N
2
3
D
2/KOH
>99
2/-
7
98
28c
99
D
O
OD
OH
2/KOH
2/KOH
16
6 d
O
O
4
5
3/-
7
>99
OD
D
2/-
6
6
>99
24
2/KOH
O
OH
6
3/-
6
>99
b
aReaction conditions: 2-propanol-d8, 90 °C, the amount of base if added was 2% mol. The
c
reaction progress was followed by NMR, yields were determined by GC-MS analysis. Yield
was determined by 1H NMR using trimethylbenzene as internal standard.
In the reduction of α,β-unsaturated ketones and aldehydes the main drawback of a catalyst
operating by inner sphere hydride transfer is that they are often not very selective for C=O bonds
over C=C bonds, presumably due to a competition between C=O and C=C bonds for the vacant
coordination site [50], although it has been reported that hydrogenation of the C=C bond will
require less energy [72]. In this context, the ability of complex 2 to be a selective precatalyst in
the transfer hydrogenation of α,β-unsaturated ketones was explored with trans-chalcone, 4-
phenyl-3-buten-2-one, methyl acrylate and butyl acrylate (Table 2). Indeed, we have found that
the base-free hydrogenation of trans-chalcone is very selective and gives the deuterated analogue
of either product B or C using complex 2 as precatalyst (Equation 2). An increase of 20 °C,
decreased the reaction time considerably maintaining high conversion. Thus, in the absence of
base the olefinic bond is preferably hydrogenated in the first place to give product B in 4 to 32 h
13

of reaction time (94 to 95% yield, entries 1, 4, 8 and 9). Once the substrate has been
hydrogenated to B, the reduction of the carbonyl group proceeded smoothly to give C in 4 to 7 d
(>99%, entries 2 and 5). On the other hand, addition of base slows down the reaction to give the
trideuterated analogue of B in 45% yield (entry 3). Interestingly, no traces of product A were
detected after performing the reaction either in the presence or the absence of base.
O
O
OD
R'
OD
R'
2
(2% mol)
+
+
(2)
R
R'
R
R'
R
R
2-propanol-d8
B
90 oC
A
C
7a
7b
7c
7d
: R = R' = Ph
: R = Ph, R' = Me
: R = H, R' = OMe
: R = H, R' = OBu
Table 2. Transfer hydrogenation of α,β-unsaturated ketones with 2% of precatalyst loading.
Entry
Substrate
Product
preCat/base
Time (h)
Yield (%)b
O
O
D
1
2/-
32
95
D
D
O
O
OD
O
D
D
D
2
3
4
5
6
7
2/-
2/KOH
2/-
4 d
32
4
>99
45
D
D
D
D
O
O
95
D
O
O
O
OD
2/-
7 d
>99
D
O
D
2/-
3/-
1
1
77
51
2/-
3/-
20
20
42/58
39/61
B + C
14

O
O
D
O
8
2/-
23
94
O
O
O
D
O
2/-
3/-
2/-
3/-
6
6
6
6
>99
56
9
O
O
O
81
10
O
O
57
b
aReaction conditions: 2-propanol-d8, 90 °C, the amount of base if added was 2% mol. The
reaction progress was followed by 1H NMR, yields and the ratio between products A/B/C as well
as deuteration were determined by GC-MS analysis.
In addition, a series of control experiments for the TH of 7b, 7c and 7d were performed in
regular 2-propanol, using as precatalyst complexes 2 and 3 (entries 6, 7, 9 and 10). The results
showed that both complexes are very selective in the TH of 7b, 7c and 7d to give B in 1 to 6 h.
However, yields are higher in the reactions where the complex 2 was used as precatalyst.
It has been proposed that chemoselective reduction of C=C double bond of ,-unsaturated
ketones involves metal hydride complexes and follow a 1,4-addition pathway [62b]. In this case,
the TH is a stepwise process with transfer of the hydride to the -carbon and transfer of the NH
proton to the carbonyl oxygen to give the enol species, which will isomerize to the ketone B.
Hydrogenation experiments of the α,β-unsaturated ketones 7a-c, carried out under base free
conditions in 2-propanol-d8, gave the addition of two deuterium atoms to the double bond, to
form exclusively the product of entries 1, 4 and 8 (Table 1). The level of deuteration in these
compounds was examined by 1H NMR and Mass Spectrometry. In the 1H NMR spectrum of the
product of entry 4, the integration values for hydrogens at the alpha and beta positions are
consistent with the presence of one hydrogen, as well as the presence of three hydrogens at the
methyl group (Figure S54). Moreover, the spectrum of the product in entry 4 showed the
molecular ion (M+) at 150.1 atomic mass unit (amu), which is consistent with the molecular
formula of C10H10D2O and their expected isotope pattern (Figure 4). Furthermore, the spectrum
showed the sequential fragmentation of M+ giving fragments at 135.0, 107.0 and 92.1 amu, from
the loss of methyl, carbonyl and methylene (CHD) groups, respectively (Figure 5).
15

Figure 4. Mass spectrum of the hydrogenation product of 4-phenyl-3-buten-2-one.
D
O
D
D
O
CH
CO


3
D
D
D
150.1 m/z
135.0 m/z
107.1 m/z
Figure 5. Sequential fragmentation of M+.
The hydrogenation of 5-alken-2-ones (8a-b) was also studied using complexes 2 and 3 as
precatalyst in regular 2-propanol at 90 °C (Equation 3). 5-Alken-2-ones having more functional
groups amenable to be hydrogenated, more products would be obtained. Therefore, high
selectivity is the main challenge in these experiments.
R
O
R
OH
R
O
R
OH
2
3
or (2% mol)
+
+
(3)
R
R
R
2-propanol
90 oC
R
8a
B
C
: R = H
8b
A
: R = Me
Table 3 shows that hydrogenation with base of 8a at 70 and 90 °C (entries 1 and 3) gave good
selectivity toward the carbonyl obtaining 5-hexen-2-ol (A) as the main product, but hexan-2-ol
(C), the product of double reduction, was also observed along with unreacted substrate.
16

Table 3. Transfer hydrogenation of 5-alken-2-ones (8a-b) with 2% of precatalyst loading.a
Entry
1
substrate
8a
preCat/baseb
2/KOH
2/-
Time (h)
36
Yield (%)c
ratio A/B/C
75/0/6
0/98/0
84/0/10
0/98/0
0/12/0
-
81
98
94
98
12
-
2
8a
27
3
8a
2/KOH
2/-
39
4
8a
1.33
39
5
8a
-/KOH
-/-
6
8a
39
7
8a
3/KOH
3/-
36
95
100
100
19
20
100
57
0
16-16*/58/5
0/76/24
0/80/20
0/9/10
18/2/0
0/87/13
0/57/0
-
8
8a
1.33
6
9
8a
3/-
10
11
12
13
14
15
8a
3/-
0.16
24
8b
2/KOH
2/-
8b
23
8b
2/-
1.33
1.33
1.33
8b
3/KOH
3/-
8b
33
0/33/0
aReaction conditions: 2-propanol, 90 °C, entries 1 and 2 were performed at 70 °C, the amount of
b
c
base if added was 10% mol. 10% mol of KOH. Yields were determined by GC-MS.*This
species correspond to 4-hexen-2-ol, which is an isomer of product A.
In the experiments performed without base (KOH) at 70 and 90 °C, a conversion of 98% to
hexan-2-one (B) was achieved with only 2% of substrate remaining (entries 2 and 4). An
increase of 20 °C made the reaction 20 times faster maintaining the selective hydrogenation to
the olefin. The same reaction without catalyst in the presence of KOH gave 12% of B in 39 h
(entry 5). The hydrogenation of 8a without precatalyst nor KOH does not proceed, even after 39
h (entry 6). These results provide an easy access to the two products of mono reduction: selective
carbonyl hydrogenation when a base is used (entry 3); selective reduction of the olefinic double
bond when no base is used (entry 4). Therefore, it is apparent that these reactions need the
precatalyst 2 to improve their performance. For comparison purposes, complex 3, with a
triazenide ligand without the N-heterocycle was also tested as precatalyst in the same reaction.
Interestingly, the results show that complex 3 is not as fast nor selective in the presence of base
17

giving a mixture of products A, B, C and 4-hexen-2-ol, which is an isomer of A (entry 7).
However, the hydrogenation of 8a using 3 in the absence of base was faster and required only
1.33 h to give B and C in 76 and 24%, respectively (entry 8). In this reaction, the ratio did not
change too much after 6 h giving B and C in 80 and 20%, respectively (entry 9). Interestingly,
analysis of the reaction mixture of 8a and 3 after 10 min of heating at 90 °C showed only 9 and
10% of B and C, respectively. On the other hand, hydrogenation of 8b in the presence of base
proceeded very slowly (entry 11). However, without base the hydrogenation of 8b is faster and
gives a mixture of products B and C in a ratio of 87 to 13%, respectively (entry 12). A
1
competition experiment followed by H NMR (Figures S91-S95), using acetophenone (9), 5-
hexen-2-one (8a) and the cationic metal hydrides 5 and 6, resulted in the isomerization of the
double bond from the ,-position to the ,-position (10) prior to the double bond
hydrogenation (Scheme 2).
O
O
O
O
5
6
and
+
+
5
6
+
and
C6D6
24 h, rt
8a
8a
9
9
C6D6
90 oC, 1.5 h
O
O
O
3 Eq. of 2-propanol
90 oC, 1 h
O
+
8a
9
+
+
9
11
10
11
Scheme 2. Competition experiment of hydrogenation of 5-hexen-2-one (8a) and acetophenone
(9).
These results show that the pyrazole group in the triazenide ligand of complex 2 induces
selectivity in the hydrogenation of 5-alken-2-ones, being A the main product in the presence of
base or being B the main product without added base. In contrast, the hydrogenation of 8a is not
selective when complex is used as precatalyst in presence of base (entry 7).
Comparing the results between an α,β-unsaturated ketones and ,-unsaturated ketones, it was
found that at 90 °C both reactions are selective to the reduction of the olefin without base.
However, when base is added the reaction is slower for the α,β-unsaturated ketones. In contrast,
18

the reduction of the carbonyl group in ,-unsaturated ketones is favored by addition of base
giving A as main product, which was not observed when the substrate is an α,β-unsaturated
ketone.
4. Conclusions
A precursor of the triazenide ligand functionalized with pirazol and its IrIIICp* complex was
synthesized and fully characterized. X-ray diffraction studies confirmed the molecular structures
of both ligand and complex. In compound 2 the triazenide binds to the metal through nitrogen 1
and nitrogen 3 forming a four member chelate. Reactivity of 2 in 2-propanol allows the
formation of a neutral metal hydride (4) and cationic metal hydrides (5 and 6) when base is
added. Complexes 2 and 3 were evaluated as precatalysts in the reduction by transfer
hydrogenation of aryl-ketones, alkyl-ketones, α,β-unsaturated and ,-unsaturated ketones with
good to excellent yields and high selectivity. Reactions carried out in the absence of an external
base turned out to be faster, which can be attributed to the ability of the triazenide to be
protonated by the solvent giving a cationic metal hydride species with a neutral triazene ligand,
which may be deprotonated by the substrate after transferring the hydride and the proton of the
NH group. Both complexes (2 and 3) were excellent precatalysts in TH of aryl- and alkyl-
ketones; the presence of the pyrazole in complex 2 does not seem to be relevant for these
substrates (Table 1). Nevertheless, for the TH of α,β-unsaturated ketones the presence of the
pyrazole in the structure of the precatalyst is relevant since higher yields of the product can be
obtained. Moreover, there is an improvement in the chemoselectivity when the TH of ,-
unsaturated ketones is catalyzed by complex 2, which can be switched from C=C to C=O bond
by addition of base. In conclusion, the introduction of an N-heterocycle as pyrazole in the
structure of the ligand improves the selectivity and the reaction rate in the transfer hydrogenation
of α,β-unsaturated and ,-unsaturated ketones. Further reactivity and theoretical studies along
these lines will be necessary to state the role of the pyrazole.
Acknowledgements
19

This work was supported by Consejo Nacional de Ciencia y Tecnología (CONACyT,
grant 162005) and Tecnológico Nacional de México (TecNM, grant 6253.19-P ). LJMC
(258854), ECA (238891), MACF (311948) and JPCD thank CONACyT for graduate
fellowships. We thank CONACyT for ITT NMR and HRMS facilities (Grants INFR-2011-3-
173395 and INFR-2012-01-187686).
Appendix A. Supplementary material
CCDC 1966546 (1) and 1966548 (2) contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in the online version, at
http://xxxxxxxxxxx
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Highlights:
 Iridium triazenide complex as catalysts for selective hydrogenation of α,β-unsaturated
and ,-unsaturated ketones.
 Base free transfer hydrogenation by an iridium triazenide complex bearing a pyrazole
with high yields and selectivity.
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