An iridium-SPO complex as bifunctional catalyst for the highly selective hydrogenation of aldehydes

Article abstract of DOI:10.1039/c7cy01953j

A secondary phosphine oxide (SPO) ligand (tert-butyl(phenyl)phosphine oxide) was employed to generate an Ir-SPO complex which shows a particular ability to activate dihydrogen under mild conditions without the help of an external base or additive. Such an iridium(i) complex serves as a precursor for homogeneous catalysis since under H₂ it is converted to a mixture of several iridium(iii) hydride species that are the active catalysts. This system was found to be a highly active catalyst for the hydrogenation of substituted aldehydes, giving very high conversions and chemoselectivities for a wide range of substrates. The SPO ligand presumably plays a key role in the catalytic process through heterolytic cleavage of H₂ by metal-ligand cooperation. In addition, an exhaustive characterization of the different iridium hydride species was performed by 1D and 2D NMR spectroscopy. The oxidative addition of H₂ to the Ir(i)-SPO complex is highly stereoselective, as all generated Ir(iii) hydrides are homochiral. Finally, the crystal structure, as determined by X-ray diffraction, of a dinuclear iridium(iii) hydride complex is described.

Full text of DOI:10.1039/c7cy01953j

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An Iridium–SPO Complex as Bifunctional Catalyst for the Highly
Selective Hydrogenation of Aldehydes†
Israel Cano,*ab Luis M. MartínezꢀPrieto,a Laure Vendier,c and Piet W. N. M. van
Leeuwen*a
ABSTRACT: A secondary phosphine oxide (SPO) ligand (tertꢀbutyl(phenyl)phosphine oxide)
was employed to generate an Ir–SPO complex which shows a particular ability to activate
dihydrogen under mild conditions without the help of an external base or additive. Such an
iridium (I) complex serves as a precursor for homogeneous catalysis since under H2 it is
converted to a mixture of several iridium (III) hydride species that are the active catalysts. This
system was found to be a highly active catalyst for the hydrogenation of substituted aldehydes,
giving very high conversions and chemoselectivities for a wide range of substrates. The SPO
ligand presumably plays a key role in the catalytic process through heterolytic cleavage of H2 by
metal–ligand cooperation. In addition, an exhaustive characterization of the different iridium
hydride species was performed by 1D and 2D NMR spectroscopy. The oxidative addition of H2
to the Ir(I)–SPO complex is highly stereoselective, as all generated Ir(III) hydrides are
homochiral. Finally, the crystal structure, as determined by XꢀRay Diffraction, of a dinuclear
iridium (III) hydride complex is described.
INTRODUCTION
The selective hydrogenation of an aldehyde function in the presence of other reducible groups is
an important step in synthetic chemistry. As an example, the chemoselective reduction of α,βꢀ
unsaturated aldehydes to their corresponding allylic alcohols has tremendous industrial
importance, since these compounds are relevant intermediates and end products in the
preparation of fine chemicals, flagrances and pharmaceutical compounds.1 An interesting
pathway to reduce polarized C=X bonds is the heterolytic cleavage of dihydrogen into H+ and Hꢀ
and the subsequent transfer of hydrogen atoms to substrates such as C=O bonds.2 In this
,
a
Laboratoire de Physique et Chimie des Nano Objets, LPCNO, UMR5215 INSA-UPS-CNRS, Institut
National des Sciences Appliquées, 135 Avenue de Rangueil, 31077 Toulouse, France
b
GSK Carbon Neutral Laboratory for Sustainable Chemistry, University of Nottingham, NG7 2GA,
Nottingham, UK
c
CNRS, LCC (Laboratoire de Chimie de Coordination) 205 Route de Narbonne, BP44099, F-31077
Toulouse Cedex 04, France; Université de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 04, France
† Electronic supplementary information (ESI) available: Synthesis, experimental procedure and
supporting data.
1

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approach H2 is frequently cleaved by metalꢀligand cooperation; that is, a ligand containing basic
sites and a coordinated metal center operate in tandem to activate the hydrogen molecule.3
In the field of homogeneous catalysis, this type of process is accomplished by numerous
transition metal complexes, leading to the hydrogenation of aldehydes, ketones, and imines.4
However, iridiumꢀbased complexes have shown a limited efficiency in the chemoselective
hydrogenation of substituted aldehydes and only a few systems performing this transformation
have been reported in the literature.5,6 Additionally, some of these complexes are not able to act
as bifunctional catalysts and require the help of an external base to promote the heterolytic
splitting of H2.5h,i On the other hand, in the area of metallic nanoparticle (MNP) catalysis, this
process is rarely achieved by a heterolytic cleavage mechanism involving the ligand or
stabilizing agent. Indeed, the described systems based on iridium produce the H2 activation and
its later transfer to aldehydes by the use of heterogeneous catalysts consisting of iridium
nanoparticles (IrNPs) immobilized on oxide supports or oxygenated surfaces,7 for which the
process takes place by a strong metal–support interaction.8
Along this line, secondary phosphine oxides (SPOs) form an interesting group of phosphorus
ligands.9 Once coordinated (via P) as the phosphinous(III) tautomer10 to a suitable transition
metal, the resulting complexes display an ability to cleave H2 heterolytically across M and O, as
long as there is a vacancy on the metal. Then, the complex can transfer the hydrogen atoms to
an appropriate substrate.11 This SPO–metal cooperative effect has been widely utilized in
hydrogenation catalysis, in which such reactivity is particularly notorious.12 In that regard, our
group has a longstanding experience in the use of this type of ligands, both in homogeneous11
and MNP13 catalysis.
Inspired by these works, we recently reported the synthesis and characterization of an Ir–SPO
complex, for which two coordinated phosphine oxide ligands selfꢀassemble after loss of one
proton into a monoanionic bidentate ligand held together by an intramolecular hydrogen bond.14
In a preliminary catalytic study, the system showed a very high activity and selectivity in the
chemoselective hydrogenation of cinnamaldehyde and pꢀnitrobenzaldehyde. The complex acts
as precursor for homogeneous catalysis, since under H2 it is converted to a mixture of several
hydrides.
Herein we describe the characterization and catalytic applications of such an Ir–SPO hydride
system. This catalyst is very active for the chemoselective hydrogenation of substituted
aldehydes, providing exceptionally high conversions and selectivities. The SPO ligand
presumably plays a crucial double role, as modifying ligand, and as functional ligand acting as
heterolytic activator for dihydrogen, since its oxygen atom operates as a basic site and takes a
H+ from H2, leaving a H− bound to the metal center.11,13
2

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RESULTS AND DISCUSSION
Synthesis and Characterization of Catalytic System
Treatment of (1,5ꢀcyclooctadiene)(methoxy)iridium(I) dimer ([Ir(OMe)(COD)]2) with 4
equivalents of tertꢀbutyl(phenyl)phosphine oxide (1) and 2 equivalents of H2O affords complex
2 (Scheme 1, for further details see ESI†).14
Scheme 1 Formation of iridium SPO complex 2.
The structure of 2 was elucidated unambiguously by single crystal Xꢀray structure analysis and
1
solid state fast magic angle spinning (MAS) H NMR.14 Complex 2 adopts a square planar
molecular geometry around the metal center with double coordination to cyclooctadiene (COD)
and the SPO ligands coordinated to the iridium center as a hydrogen bonded pair of the two, for
which SPOs have a strong preference in metal complexes thus obtaining a monoanionic
bidentate ligand.15
This complex is a precursor instead of an active catalyst for aldehyde hydrogenation. To
investigate the active catalytic species, we treated iridium complex 2 with 5 bar of H2 pressure
in acetonitrile (AN) at room temperature (R.T.) for 30 min (Scheme 2, for further details see
ESI†).
3

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tBu
He
tBu
AN
tBu
Hd
tBu
Ph
Ph
Ph
Ph
P
P
P
O
O
O
P
O
H
O
H2 (5 bar)
H
O
H
O
AN
AN
AN
Ha
AN
Hc
Ir
Ir
Ir
H
P
Ir
P
P
Ph
Ph
Ph
O
P
CH3CN, R.T.
Ph
tBu
tBu
tBu
Hb
AN
X
tBu
2
3
4
5
tBu
AN
tBu
AN
Ph
Ph
P
P
O
O
Hg
Ph
Hg
tBu
H
H
O
Ph
tBu
Hf
Hf
Hf
Ir
Ir
P
P
O
Ph
Ph
O
H
O
H
P
P
Ir
Ir
Hf
tBu
tBu
Hg
Hg
O
O
P
P
tBu
Ph
AN
AN
Ph
tBu
6
7
tBu
Hg
Ph
P
O
Hg
tBu
H
Ph
Hf
Ir
P
O
Ph
O
H
P
Ir
Hf
tBu
AN
O
P
tBu
AN
Ph
8
Scheme 2. Formation of Iridium (III) hydride species under substrateꢀfree hydrogenation
conditions.
Under H2, the Ir(I) complex is converted, via H2 oxidative addition, to a mixture of one
monohydride
Ir((SPO)2H)H(solvent)2X
(5),
two
diastereomeric
dihydrides
Ir((SPO)2H)H2(solvent)2 (3, 4) and three bridging dihydride dimers (6–8) thereof after loss of a
solvent molecule (Scheme 2). The reaction is very fast with an instantaneous change of colour
from redꢀorange to light yellow. The NMR analyses in tetrahydrofuran (THF–d8) led to rapid
decomposition and therefore we conducted the experiments in acetonitrile (CD3CN), which has
a stabilizing effect on the hydrides.16
The oxidative addition of H2 to Ir(I) complex 2 was studied by 1H and 31P NMR spectroscopy.
1
The hydride region of the H NMR spectrum acquired 30 min after introducing H2 into the
solution shows several iridium dihydride species (Fig. 1). The hydride resonances with highest
intensity correspond to diastereomer 3 and appear as a double doublet of doublets at –10.38
ppm (Ha) due to cis and trans phosphorus couplings (19.1 and 143.3 Hz, respectively) and one
hydrogen coupling (4.8 Hz), and as a double doublet of doublets at –21.40 ppm (Hb) attributable
to the coupling with two cis phosphorus (14.5 and 20.7 Hz) and one hydrogen (4.8 Hz).
Similarly, the hydride resonances for the minor diastereomer 4 appear as a couple of double
doublet of doublets at –9.77 ppm (Hc, JPꢀH = 140.9, 19.8 Hz and JHꢀH = 4.8 Hz) and –21.52 ppm
4

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(Hd, JPꢀH = 22.7, 12.6 Hz and JHꢀH = 4.8 Hz), since both diastereomers exhibit the same
coordination geometries.17 The ratio between the diastereomers is 2.23:1.
Fig. 1 1H NMR spectrum (500 MHz) in the hydride region after reaction of 2 with H2 (5 bar) in
CD3CN.
The identity of the major diastereomer 3 was established by 2D NOESY NMR experiments
(Fig. S3†). As the tertꢀbutyl groups are nonꢀequivalent, both diastereomers display 2 signals for
these substituents in the 1H NMR spectrum (Fig. S2†), being the most intense peaks those that
belong to 3 (0.71 and 0.94 ppm). Only the tertꢀbutyl group at 0.71 ppm shows NOESY
correlations with both hydrides Ha and Hb (Fig. S3†) in this species, whereas the other tertꢀbutyl
substituent does not give signal with the axial hydride Hb. This suggests that the tertꢀbutyl
groups of 3 are placed in trans position. Indeed, no interaction between the tertꢀbutyl groups of
3 was detected in the NOESY experiment (Fig. S4†). In addition, the tertꢀbutyl resonances for 4
appear at 0.80 and 0.82 ppm (Fig. S2†). The 31P{H} NMR spectrum (Fig. S5†) exhibits two
doublet signals for each diastereomer (70.2 and 77.2 ppm with JPꢀP = 14.6 Hz for 3, and 62.5
and 84.2 ppm with JPꢀP = 14.9 Hz for 4). The assignation of signals was performed through a
1H–31P HMBC 2D experiment (Fig. S6† and S7†), which also enabled us to confirm the identity
of the tertꢀbutyl resonances corresponding to 4. Both iridium dihydride complexes 3 and 4 are
homochiral (Scheme 2 shows the RR isomers), which is the most stable configuration, and there
is no meso isomer.
The 1H NMR spectrum in the hydride region also shows a signal attributable to a species 5 with
one hydride (He) and one nonꢀidentified anion (X) generated by decomposition, both located in
axial position (Scheme 2). The hydride resonance appears at –24.70 ppm as a wellꢀresolved
triplet with JPꢀH of 13.6 Hz (Fig. 1) due to the coupling with two cis phosphorus nuclei (the
signal would be located at –5ꢀ(–10) ppm in case of coupling with one trans phosphorus
nucleus), whereas the phosphorus signals arise as two doublets (AB pattern) at 77.6 (JPꢀP = 12.7
5

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Hz) and 78.1 ppm (JPꢀP = 13.2 Hz) in the 31P{H} NMR spectrum (Fig. S5† and S6†). This AB
system indicates that the molecule is homochiral (RR/SS), since a meso compound would give a
singlet signal for the two phosphorus in the 31P{1H} NMR spectrum. The integration of hydride
signals in the 1H NMR spectrum reveals that the iridium monohydride complex is the third most
abundant species. Consequently, the peaks for the tertꢀbutyl substituents are those observable at
0.64 and 0.97 ppm in the 1H NMR spectrum, which was corroborated by analysis of the 1H–31P
HMBC 2D experiment in the tertꢀbutyl zone (Fig. S7†).
On the other hand, we observed the formation of three dimers 6, 7 and 8 with bridging and
1
terminal hydrides as minor species (Scheme 2 and Fig. 1). The hydride region of the H NMR
spectrum displays three triplets at δ –8.59 (JPꢀH = 60.5 Hz), –9.01 (JPꢀH = 61.5 Hz) and –9.37
ppm (JPꢀH = 62.3 Hz) corresponding to the bridging hydrides (Hf) of each dimer, and small
signals around –19.5 and –20.8 ppm attributable to the terminal hydrides (Hg).18 Each dimeric
species exhibits two different phosphorus environments, giving a pair of signals in the 31P{H}
NMR spectrum (Fig. S5†). As was previously described for the mononuclear iridium complexes
3–5, all the dimers are homochiral and there are no meso isomers because all the dimeric species
generate two peaks with cis phosphorus coupling in the 31P{H} NMR spectrum (Fig. S5†). The
homochiral nature of all species as caused by the bulky tertꢀbutyl groups reduces the number of
possible diastereoisomers and allows the present interpretation.
Evaporation of solvent favors a displacement of the equilibrium between the hydride species
toward the formation of the dimers. As a result, crystals of the dinuclear iridium(III) hydride
complex 7 suitable for Xꢀray Diffraction were obtained. The crystal structure of 7 is shown in
Figure 2. The Ir–Ir distance is 2.77 Å, in good agreement with the values reported in the
literature for dinuclear Ir(III)/Ir(III) complexes containing an (IrH)2(ꢀꢀH2) unit.16,18,19 The Ir2
core possesses a 36–electron configuration with no M–M bond. The two acetonitrile molecules
are orientated mutually trans. However, the two terminal hydrides were not located by Fourier
differences, but one vacant site in the coordination sphere of each iridium atom trans to the
acetonitrile molecule were assumed to be their positions. Fourier differences revealed a Ir1–Hy2
distance of 1.75 Å and an Ir1ꢀIr1iꢀHy2 angle of 42.9o, the latter very close to the 45o required for
the bridging hydrides being equidistant to both iridium atoms. Interestingly, Fourier differences
also showed that the intramolecular hydrogen bond O–H–O contained in the monoanionic
bidentate ligand is essentially linear (O….O is 2.398 Å) as was also found by DFT calculations
for related Rh complexes,11e and as was already proposed by Palenik et al. on the basis of the
short O….O distance of 2.5 Å in Pd–SPO complexes,20 although routinely the structures are
drawn showing the O–H–O atoms as part of a regular 6ꢀmembered ring.11a–d
6

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Fig. 2 Xꢀray structure of 7 showing thermal ellipsoids set at 40% probability level. Selected
bond lengths (Å): Ir1–P1, 2.2930(13); Ir1–P2, 2.2794(12); Ir1–N1, 2.120(4); Ir1–Hy2, 1.75(6);
P1–O1, 1.555(4); P2–O2, 1.558(4); O1–Hy1, 0.97(6); O2–Hy1, 1.43(6). Selected angles (deg):
N1–Ir1–P2, 91.61(11); N1–Ir1–Hy2, 89(2); P2–Ir1–P1, 89.01(5); P2–Ir1–Hy2, 90.3(19).
As concerns the symmetry, the crystal structure of 7 contains an inversion center but has neither
a plane of symmetry (due to the chiral ligands) nor a 2ꢀfold axis due to the acetonitrile
molecules and hydrides in trans axial positions. The tertꢀbutyl groups are placed in trans
position and the pairs of SPOs exhibit an R configuration at the phosphorus atoms in one
monoanionic bidentate ligand and S in the other one. Thus, one side of the dimer is RR and the
other side is SS, giving two doublets at approximately 72 (JPꢀP = ~22 Hz) and ~82 ppm (JPꢀP
~22 Hz) in the 31P{1H} NMR spectrum (Fig. S5†).
=
Catalytic Hydrogenation of Substituted Aldehydes
This encouraging demonstration of hydrogen activation prompted us to evaluate the ability of
the Ir–SPO system as hydrogenation catalyst. The hydrogenation of a range of substituted
aldehydes was investigated using catalytic quantities of 2, which was found to be the precursor
for a highly active and almost exclusively selective catalyst. Firstly, we decided to evaluate the
activity of the catalytic system in a screening set of experiments for the hydrogenation reaction
of cinnamaldehyde (Table 1). At R.T. and 5 bar H2 pressure, complete selectivity toward the
unsaturated alcohol was observed in 1 h, albeit with low conversion (entry 1, 25 %). In 2.5 h,
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97% conversion and 99% selectivity to the expected allylic alcohol were obtained (entry 2).
Increasing the time to 4.5 h, quantitative conversion of the substrate with >99% selectivity was
achieved (entry 4). With the pressure maintained at 5 bar but increasing the temperature to 60
ºC, a decrease in the conversion was observed (entry 11, 45%), which points to some
decomposition and/or deactivation process of the catalyst. Furthermore, a loss of selectivity to
the α,β-unsaturated alcohol was produced. We observed hydrogenation of the C=C bond and
both 3ꢀphenylpropanal (6%) and 3ꢀphenylpropanol (4%) were generated in addition to the
expected product (cinnamyl alcohol, 90%). The solvent was found to be relatively important in
that THF consistently provided good results (entry 2–5), while other solvents gave much lower
conversions (entry 6, toluene; entry 7, CH2Cl2). Moreover, the use of methanol led to a decrease
in the selectivity, since the acetal derivative was generated as byꢀproduct (entry 8).
It is worth noting that the use of higher pressures or longer reaction times involved only a slight
reduction in the chemoselectivity (entries 5, 9 and 10), which highlights the preference of the
catalytic system toward the aldehyde functionality. With these optimized reaction parameters
based on 2.5 h as reaction time, R.T., 5 bar of hydrogen pressure and THF as solvent, a TON of
2910 and a TOF of 1164 hꢀ1 (entry 2) were achieved.
Table 1 Optimization Parameters for the Hydrogenation of Cinnamaldehyde.a
Entry
Solvent T (K) Time (h) P (bar) Conversion (%)b Selectivity (%)c
1
2
THF
THF
295
295
295
295
295
295
295
295
295
295
333
1
5
5
25
97
>99
99:1 (UA:A)
99:1 (UA:Al+A)
>99
2.5
3
3
THF
5
99
4
THF
4.5
5
5
>99
>99
2
5
THF
5
99:1 (UA:A)
6
Toluene
CH2Cl2
CH3OH
THF
4.5
4.5
4.5
18
18
4.5
5
>99
7
5
4
>99
56d
8
5
50
9
5
>99
>99
45
96:4 (UA:A)
98:2 (UA:A)
90:6:4 (UA:Al:A)
10
11
THF
10
5
THF
a Reagents and conditions: 2 (0.00125 mmol), cinnamaldehyde (3.75 mmol), solvent (0.75 mL).b
1
Conversions determined by H NMR spectroscopy and refer to the selective conversion of
cinnamaldehyde (average of two runs).c UA = Unsaturated Alcohol, A = Saturated Alcohol, Al
= Saturated Aldehyde.d The formation of the acetal product was observed.
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We studied the rate dependence on the reaction time (Fig. 3). The profile clearly shows an
incubation time of ca. 1 h, during which 2 generates the catalytically active hydrides species.
Since the hydrides are formed in acetonitrile on the timescale of the NMR sample preparation,
we cannot say what this incubation time involves. Nanoparticle formation can be excluded on
the basis of rate –IrNP being >50 times slower catalysts for entry 1– and selectivity, vide infra.14
Indeed, the formation of nanoparticles requires more than 12 h under 5 bar of H2 pressure,
whereas no nanoparticles generation was observed under these conditions in the NMR tube
employed for the characterization of hydrides.14 From the profile we deduced a maximum TOF
of 2040 h–1 at 1.5–2 h of reaction for the hydrogenation of cinnamaldehyde. To the best of our
knowledge, the catalytic system described herein performs as one of the best catalysts in terms
of rate and selectivity compared to iridiumꢀbased systems reported to date.5,7 The oxidative
addition of H2 observed is the same as that described for diphosphine complexes17 and thus
there is no indication that in this instance we are dealing with a heterolytic cleavage.21 Since the
present Ir–SPO catalyst shows poor activity for alkenes compared to iridium catalysts
containing neutral ligands (monophosphines, bisphosphines, Phox ligands), mechanistically the
SPO function might be involved in the hydrogenation, but firm evidence is lacking.
Fig. 3 Profile of the evolution of the reaction with the time for the hydrogenation of
cinnamaldehyde. Reagents and conditions: 2 (0.00125 mmol), cinnamaldehyde (3.75 mmol),
THF (0.75 mL), 5 bar, 295 K.
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With optimized conditions in hand, we were keen to study the substrate scope and functional
groups tolerated by 2 (Table 2). The catalyst showed a very high activity and selectivity in the
hydrogenation of aldehydes over other functional groups. In terms of activity, the reaction
appears to be very general and, in nearly all cases, very high conversions were obtained.
Nevertheless, we observed differences in the TOF depending on the reactant, since some
substrates required longer reaction times to complete the catalytic process (Table 2 and Section
4†).
In all cases, high selectivities were observed for several α,βꢀunsaturated aldehydes (entries 1–
4), including some that are of particular interest in the production of perfumes and fragrances.22
The complex was very selective to the carbonyl functionality in cinnamaldehyde and prenal
(entries 1–2). However, a slight reduction in the selectivity was produced in the hydrogenation
of transꢀ2ꢀhexenꢀ1ꢀal in comparison with the previous substrates (entry 3). The steric
impediment in the former probably avoids a higher reduction of the C=C bond. Of particular
importance is the selective hydrogenation of citral, which proceeded with complete
chemoselectivity (entry 4). Interestingly, in contrast to other systems based on Ru,23 no reaction
was observed in the reduction of 2ꢀoctynal. Indeed, this substrate poisoned the catalyst, as we
reported in a preliminary communication.14
In addition to this selectivity to C=O over alkenes, the catalyst is highly tolerant to several other
functional groups. For example, the hydrogenation of pꢀnitrobenzaldehyde yielded the
corresponding nitrobenzyl alcohol with perfect retention of the nitro group (entry 5). This is the
second indication that nanoparticles are not responsible for the catalytic activity, because IrNPs
gave formation of aminoaldehyde and aminoalcohol when used as the catalyst.14 In addition, we
found poisoning of the catalytic system for the reactions with pꢀcyanobenzaldehyde and 2ꢀ
octynal, while IrNPs on the contrary showed high conversions and chemoselectivities in the
hydrogenation of these substrates.14 Esters were also tolerated excellently and the aldehyde
group was selectively reduced to alcohol (entry 6). Finally, complete chemoselectivity was
observed in compounds containing reducible heteroaromatic substituents (entries 7–8), such as
furfural (compound derived from biomass) and 2ꢀthiophenecarboxaldehyde.
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Table 2 Catalytic Hydrogenation of Aldehydes with 2.a
Entry
Substrate
Product
t (h) Conv. (%)b Selectivity (%)c TOF (hꢀ1)
1
2.5
5
97
38
94
99:1 (UA:A)
99:1 (UA:A)
97:3 (UA:A)
2040d
228
2
3
5
564
OH
O
OH
O
4
5
5
>99
>99
600
O
5
>99
>99
>99
>99
600
150
O2N
6
20
7
8
5
5
96
98
>99
>99
720
588
a Reagents and conditions: 2 (0.00125 mmol), substrate (3.75 mmol), THF (0.75 mL), 295 K, 5
1
bar of H2.b Conversions and product identities were determined by H NMR spectroscopy
(average of two runs).c UA = Unsaturated Alcohol, A = Saturated Alcohol, Al = Saturated
Aldehyde.d Maximum TOF.
CONCLUSIONS
In summary, we have successfully used an Ir–SPO complex to activate dihydrogen under mild
conditions. The treatment of the iridium (I) complex with H2 led, via H2 oxidative addition, to
the formation of a mixture of one monohydride Ir((SPO)2H)H(solvent)2X, two diastereomeric
dihydrides Ir((SPO)2H)H2(solvent)2, and three bridging dihydride dimers as minor species. Such
a process undergoes a high degree of diastereoselectivity, since a thorough characterization of
the different iridium (III) hydride complexes by 1D and 2D NMR spectroscopy showed that all
monomeric species are homochiral. In addition, the crystal structure of a dinuclear iridium (III)
hydride complex was determined by XꢀRay Diffraction and discussed. This dimer shows the
preferred homochiral coordination for each half of the molecule, with opposite chirality
resulting in an inversion center for the dimer.
This catalytic system was highly efficient for the chemoselective hydrogenation of substituted
aldehydes, displaying exceptionally high activities and selectivities for a wide variety of
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substrates. For instance, the hydrogenation of cinnamaldehyde proceeded with a TOF of 2040 h–
1
and a selectivity of 99%. The catalyst works without the help of an external base or additive
presumably through a ligandꢀmetal cooperative mechanism in which the SPO ligand might play
a crucial double role, as modifying ligand, and as functional ligand acting as heterolytic
activator for dihydrogen. Finally, it is worth noting that with the study reported herein, SPO
ligands have shown their merit in homogeneous hydrogenation catalysis, which may inspire the
design of new homogeneous SPOꢀbased catalysts that incorporate earthꢀabundant metals and
exhibit similar catalytic properties.
CONFLICTS OF INTEREST
There are no conflicts of interest to declare.
ACKNOWLEDGEMENTS
We thank UPSꢀToulouse and INSA for financial support. Dr. Israel Cano acknowledges
financial support from the European Community through a Marie SkłodowskaꢀCurie Individual
Fellowships (IFꢀEF; Programme/Call: H2020ꢀMSCAꢀIFꢀ2015; Proposal No: 704710–
Sdchirnanocat). Prof. P. W. N. M. van Leeuwen thanks the Université Fédérale Toulouse Midiꢀ
Pyrénées for an IDEX Chaire d´Attractivité.
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14

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isomers
An Ir(III) hydride catalytic system shows very
high activities and selectivities in the
chemoselective hydrogenation of various
substituted aldehydes.