N-Heterocyclic olefins as ancillary ligands in catalysis: A study of their behaviour in transfer hydrogenation reactions

Article abstract of DOI:10.1039/c6dt02571d

The Ir(i) complexes [Ir(cod)(κP,C,P′-NHO^PPh₂)]PF₆ and [IrCl(cod)(κC-NHO^OMe)] (cod = 1,5-cyclooctadiene, NHO^PPh₂ = 1,3-bis(2-(diphenylphosphanyl)ethyl)-2-methyleneimidazoline) and NHO^OMe = 1,3-bis(2-(methoxyethyl)-2-methyleneimidazoline), both featuring an N-heterocyclic olefin ligand (NHO), have been tested in the transfer hydrogenation reaction; this representing the first example of the use of NHOs as ancillary ligands in catalysis. The pre-catalyst [Ir(cod)(κP,C,P′-NHO^PPh₂)]PF₆ has shown excellent activities in the transfer hydrogenation of aldehydes, ketones and imines using ⁱPrOH as a hydrogen source, while [IrCl(cod)(κC-NHO^OMe)] decomposes throughout the reaction to give low yields of the hydrogenated product. Addition of one or two equivalents of a phosphine ligand to the latter avoids catalyst decomposition and significantly improves the reaction yields. The reaction mechanism has been investigated by means of stoichiometric studies and theoretical calculations. The formation of the active species ([Ir(κP,C,P′-NHO^PPh₂)(ⁱPrO)]) has been proposed to occur via isopropoxide coordination and concomitant COD dissociation. Moreover, throughout the catalytic cycle the NHO moiety behaves as a hemilabile ligand, thus allowing the catalyst to adopt stable square planar geometries in the transition states, which reduces the energetic barrier of the process.

Full text of DOI:10.1039/c6dt02571d

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ARTICLE
N-Heterocyclic Olefins as Ancillary Ligands in Catalysis: A Study of
their Behaviour in Transfer Hydrogenation Reactions
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
b
c
a
Amaia Iturmendi, Nestor García, E. A. Jaseer, Julen Munárriz, Pablo J. Sanz Miguel, Victor
c
a
a,b
Polo, Manuel Iglesias,* and Luis A. Oro*
DOI: 10.1039/x0xx00000x
PPh2
OMe
PPh2
The Ir(I) complexes [Ir(cod)(κP,C,P'-NHO )]PF
6
and [IrCl(cod)(κC-NHO )] (cod = 1,5-cyclooctadiene, NHO
= 1,3-bis(2-
www.rsc.org/
OMe
(diphenylphosphanyl)ethyl)-2-methyleneimidazoline) and NHO
= 1,3-bis(2-(methoxyethyl)-2-methyleneimidazoline),
both featuring an N-heterocyclic olefin ligand (NHO), have been tested in the transfer hydrogenation reaction, this
PPh2
representing the first example of the use of NHOs as ancillary ligands in catalysis. Pre-catalyst [Ir(cod)(κP,C,P'-NHO )]PF
6
i
has shown excellent activities in the transfer hydrogenation of aldehydes, ketones and imines using PrOH as hydrogen
OMe
source, while [IrCl(cod)(κC-NHO )] decomposes throughout the reaction to give low yields of the hydrogenated product.
Addition of one or two equivalents of a phosphine ligand to the latter avoids catalyst decomposition and significantly
improves the reaction yields. The reaction mechanism has been investigated by means of stoichiometric studies and
PPh2
i
theoretical calculations. The formation of the active species ([Ir(κP,C,P'-NHO )( PrO)]) has been proposed to occur via
isopropoxide coordination and concomitant COD dissociation. Moreover, throughout the catalytic cycle the NHO moiety
behaves as a hemilabile ligand, thus allowing the catalyst to adopt stable square planar geometries in the transition states,
which reduces the energetic barrier of the process.
Moreover, they can act as ligands for transition metal
2
complexes, namely, Au, Rh, Ir and Mo have been reported.
,1d
Introduction
In this regard, it has been recently proved that NHCs are more
strongly coordinating ligands than NHOs. In an elegant
experiment that makes use of IR spectroscopy, structural data
N-Heterocyclic olefins (NHOs) feature strongly polarised
double bonds due to the presence of two geminal amine
substituents incorporated into a five-membered heterocycle.
The aromatisation of the ring favours charge separation at the
olefin, hence, ylidic mesomeric forms gain more weight
(
X-ray crystallography) and DFT calculations, Rivard et al.
compared the nature of the M–C bond and the donor
capability of N-heterocyclic carbene (NHC) and N-heterocyclic
(Scheme 1). As a result of this unique electronic structure, the
2
olefin (NHO) ligands in [RhCl(CO) L] complexes (where L = NHC
terminal carbon of the NHO possesses a lone pair of electrons,
which accounts for the nucleophilic nature of this type of
molecules. NHOs, consequently, are able to react with Lewis
or NHO). NHO ligands bring about lower carbonyl stretching
frequencies, which is indicative of a higher electronic density
at the metal centre. However, such behaviour is not the
1
acids such as, boranes, carbon dioxide and Group 14 adducts.
consequence of NHOs being more strongly
σ
than NHCs, but it has been attributed to a reduced
-donating ligands
-acceptor
π
ability in the case of NHOs; which, overall, results in weaker
3
M–C bonds in the case of NHOs.
The restricted number of transition metal complexes featuring
NHO ligands so far described and the limited knowledge of
their coordination chemistry has probably precluded their use
in catalysis, since, to our best knowledge, no study of the
catalytic activity of this class of complexes has been hitherto
reported. Conversely, they have been employed as
organocatalysts for a variety of reactions, often outperforming
Scheme 1 Extreme resonance structures of a generic NHO.
a.
4
Departamento Química Inorgánica – ISQCH, Universidad de Zaragoza – CSIC,
0009 Zaragoza, Spain. E-mail: miglesia@unizar.es, oro@unizar.es
www.isqch.unizar-csic.es.
the widely successful NHCs.
5
2c
According to recent theoretical calculations at the DFT level
b.
Centre of Research Excellence in Petroleum Refining and Petrochemicals, King
Fahd University of Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia.
Departamento Química Física – Instituto de Biocomputación y Física de Sistemas
Complejos (BIFI), Universidad de Zaragoza, 50009 Zaragoza, Spain.
Electronic Supplementary Information (ESI) available: Computational calculations
and X-ray data. CCDC 1487555. See DOI: 10.1039/x0xx00000x
the HOMO orbital of the NHO corresponds mainly to the lone
pair at the p atomic orbital of the terminal carbon atom;
whereas the HOMO–1 orbital would largely correspond to the
double bond. The coordinated NHO shows values of the
c.
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Wiberg bond index (WBI) between 1.14 and 1.22, which
indicates that the ligand would be better represented by the
ylidic mesomeric structure depicted in Scheme 1, i.e. the
donation to the metal centre would take place by the HOMO
orbital. Notwithstanding, the fact that the WBI depends on the
nature of the complex points out that intermediate situations,
where a certain degree of electron density is donated by the
HOMO–1 orbital, are also conceivable. This unusual attribute
allows NHO ligands to adapt to different coordination
geometries, which may be of interest in catalytic processes. In
this regard, we recently published various Ir complexes
featuring
diphenylphosphanyl)ethyl)-2-methyleneimidazoline, which is
able to adjust to facial or meridional coordination modes
the
PC(NHO)P
ligand
1,3-bis(2-
Figure 1 View of the cation of
Ir1–C5A, 2.088(7); Ir1–C5F, 2.157(7); Ir1–C6F, 2.162(8); Ir1–C1F, 2.232(7); Ir1–
C2F, 2.239(7); Ir1–P1, 2.3160(18); Ir1–P2, 2.4107(19); Ir2–C5G 2.075(6); Ir2–C5L,
.160(7); Ir2–C6L, 2.165(7); Ir2–C1L, 2.230(6); Ir2–C2L, 2.244(7); Ir2–P3,
.3113(17); Ir2–P4, 2.4153(18); C5A–Ir1–P1, 90.26(19); C5A–Ir1–P2, 91.59(19);
P1–Ir1–P2, 98.71(6);C5G–Ir2–P3, 90.58(18); C5G–Ir2–P4, 90.86(18); P3–Ir2–P4,
8.20(6).
3. Selected bond lengths [Å] and angles [º] of 3:
(
2
2
2
c
depending on the steric requirements of the complex.
In this work we explore the performance of Ir-NHO complexes
in catalysis using the transfer hydrogenation reaction as a test We have recently observed that above 40 C
bench. Moreover, theoretical study of the reaction CH Cl to give binuclear complex
(Scheme 2), which
mechanism, supported by experimental evidences, was crystallises from the reaction mixture (Figure 1). The unit cell
9
o
1
dimerises in
a
3
2
2
5
performed in order to shed light on the behaviour of the NHO of
ligand throughout the catalytic cycle.
3 encloses two different centrosymmetric cations, which
exhibit minor differences due to the accommodation of the C-
rings (Ir1···Ir1', 8.6082(7) Å; Ir2···Ir2', 8.6430(7) Å).
The dimerisation process requires the dissociation of one of
Results and discussion
the wingtip groups in complex
iridium atom in complex , thus acting as a bridging ligand in
the new complex. Remarkably, the NHO ligand in undergoes
1 to coordinate to a second
The catalytic activity of the Ir(I) complexes [Ir(cod)(κP,C,P'-
3
PPh2
OMe
NHO )]PF
6
(2) and [IrCl(cod)(κC-NHO )] (5) (cod = 1,5-
1
PPh2
cyclooctadiene; NHO
= 1,3-bis(2-(diphenylphosphanyl)ethyl)-2-
an isomerization process to give a C4 bound imidazolylidene
ligand (“abnormal N-heterocyclic carbene”) featuring a methyl
OMe
methyleneimidazoline); NHO
=
1,3-bis(2-(methoxyethyl)-2-
methyleneimidazoline) was tested in the transfer hydrogenation of group in the C2 position.
i
aldehydes, ketones and imines employing PrOH as a hydrogen
source. This work represents the first study of the behaviour of N-
heterocyclic olefins (NHOs) as an ancillary ligand in catalysis. In
order to evaluate the impact of the coordinating ability of the
NHO’s wingtip groups we tested two different complexes (2 and 5),
which feature PPh2 and OMe as potential donor groups,
respectively.
31
The P NMR spectra of
3
3 in CD CN at temperatures above 313
K show two doublets for the two inequivalent phosphine
2
ligands mutually coupled ( JP-P = 29.7 Hz) at
δ values of –18.6
and –26.1 ppm. However, at lower temperatures a new pair of
2
doublets emerges at
δ values of –17.4 and –20.7 ppm ( JP-P =
29.7 Hz) that can be attributed to a different conformer of
3.
3
The P NMR spectra in CD Cl show again the presence of two
1
2
2
The preparation and characterisation of complexes
PPh2
1
and
2
pairs of doublets but, in this case, the minor conformer is
(
Scheme 2), where NHO
acts as a tridentate ligand, was present in higher proportions and, at temperatures above 323
K, the peaks disappear due to the decomposition of the
2
c
previously reported by us.
1
complex. The H NMR spectra of
3
show a complicated mixture
values of 4.50 and
.50 ppm that correspond to the COD ligands and the
of broad multiplets between approximate
δ
1
methylenic protons of the wingtip groups of both conformers.
Similarly, the broad multiplets that appear in the aromatic
region are assigned to the phenyl rings of the phosphine
moieties. The CH protons of the imidazole ring come about in a
cleaner region, at
fluxionality of precluded its characterisation by C NMR
since the spectra in CD CN or CD Cl show only some of the
expected resonances as broad peaks even at low temperature.
3
δ values of ca. 6 ppm in CD CN. The
1
3
3
3
2
2
–
In sharp contrast with 1, complex 2, which presents a PF
6
–
conterion instead of a Cl , does not experience any apparent
modification of its structure at high temperatures. When a
o
solution of
2
in CD
3
CN is heated to 80
31
C
no changes are
1
observed in the H or P NMR spectra of the complex apart
Scheme 2 Synthetic route to complexes
2 and 3.
2
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from the presence of small amounts of free COD. Hence, Variable temperature
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1
H
NMR in toluene-d8 shows a
complex
2
was tested in transfer hydrogenation instead of
1
in broadening of the methylenic and aromatic protons of the
ligand at low temperatures, which suggests an unrestricted
rotation around the Ir–C bond at room temperature that
becomes more hindered at lower temperatures.
order to ensure the molecular integrity of the catalyst.
A related iridium complex containing the new ligand of formula
OMe
IrCl(cod)(κC-NHO )] (NHO
OMe
[
= 1,3-bis(2-(methoxyethyl)-2-
OMe
The olefinic protons of the COD ligand appear as two broad
singlets at values of 4.59 and 3.27 ppm, evidencing the
methyleneimidazoline) (5), was prepared NHO
in order to
δ
presence of two different ligands in trans position, namely, the
assess the effect of the potential coordinating ability of the ligand’s
wingtip groups in the transfer hydrogenation of polar bonds.
NHO and Cl ligands. Noteworthy, the CH group of the ligand
3
OMe
The synthesis of the ligand precursor NHO ·HCl (4) was
precursor (
4
) disappears and a new resonance, a singlet that
value of
show as the most
representative resonances those assigned to the Ir–CH and
corresponds to the Ir–CH protons, emerges at a
δ
achieved in two steps. First, 2-methylimidazole was
deprotonated with sodium hydroxide in acetonitrile and
reacted with 1 equivalent of 2-chloroethyl methyl to give the
substituted imidazole, which was reacted neat with excess 2-
2
1
3
2.18 ppm. The C NMR spectra of 5
2
the NCN carbon atoms of the NHO ligand at
and 163.2 ppm, respectively.
δ values of 17.9
chloroethyl methyl ether to afford the product as an off-white
1
solid in good yields. The H NMR spectra of
4
show two singlets In order to test the stability of the monodentate NHO complex,
in toluene-d8 was placed in an NMR
at
protons, respectively, which correspond to the CH
moieties. The CH N and CH O protons appear as triplets ( JH-H
.7 Hz) at
only one singlet at a
aromatic CH protons due to the two-fold symmetry of the The new complex, [IrCl(cod)(aNHC )] (
molecule.
bis(2-(methoxyethyl)-2-methylimidazole-4-ylidene),
identified as the result of an isomerisation NHO
δ
values of 3.20 and 2.67 ppm that integrate 6 and 3 a solution of
5
3
O and CH
3
spectrometer and the temperature increased progressively. At
3
o
2
2
=
temperatures above 60 C the formation of a new complex
4
δ values of 4.45 and 3.67 ppm, respectively. Finally, was observed. An increase of the reaction temperature to 70
o
δ
value of 7.74 ppm is observed for the
C permits to achieve total conversion after 3 h (Scheme 4).
OMe
OMe
6) (aNHC
= 1,3-
was
OMe
→
aNHC
The ligand, NHO , was generated in situ by deprotonation of
with 1 equivalent of potassium tert-butoxide in THF.
Subsequently, the metal precursor [Ir( -Cl)(COD)] was
dissolved in toluene and added dropwise to the solution of
6
(
aNHC = abnormal N-heterocyclic carbene ) while the
4
fragment “IrCl(cod)” remained unaffected, which agrees with
the fact that NHCs are more strongly coordinating ligands than
ꢀ
2
3
1
OMe
NHOs. The H NMR spectra of
resonances for the methylenic protons of the wingtip groups,
which appear as apparent triplets at values of 4.56, 3.93,
6 show four different
NHO
off, the solvent was evaporated and the crude was washed
repeatedly with hexane to afford complex as a yellow solid in
good yields (Scheme 3). In the H NMR of complex , two
values of 3.66 and 3.36 ppm are
N and CH O groups, respectively. Besides,
the two aromatic CH and OCH protons of the NHO ligand
come about as singlets at values of 6.26 and 2.92 ppm, which
suggests that, in contrast to complexes and , the wingtip
previously prepared. The inorganic salts were filtered
δ
5
1
1
2.98 and 2.81 ppm. This contrasts with the H NMR spectra of
5
3
5, since only two resonances are observed for the side arms
triplets ( JH-H = 4.8 Hz) at
observed for the CH
δ
due to the two-fold symmetry of the complex. Analogously,
the singlet resonance observed for the CH O protons in
2
2
5
3
3
disappears and two new singlets emerge at
and 2.77 ppm.
δ values of 3.03
δ
1
2
groups are not coordinated to the iridium centre. Therefore, Further proof for the proposed isomerisation process lies on
the NHO ligand may rotate without barrier around the Ir–C the integration of the low field peak at a
value of 6.15 ppm
δ
bond at room temperature or adopt a fixed position below the that corresponds to the imidazole CH, which confirms the
chlorine atom (or the COD olefin), with the imidazolium ring presence of only one aromatic proton. Besides, a new singlet
being bisected by the Ir–Cl bond.
that integrates 3 protons, corresponding to the methyl group
at the imidazole ring, comes up at a value of 1.73 ppm, with
concomitant loss of the Ir–CH peak of complex C
δ
1
3
5. The
2
NMR spectra present as most representative resonances those
corresponding to the Ir–CIm and CIm–H carbon atoms that
appear at δ values of 161.0 and 120.8 ppm, respectively.
Scheme 3 Synthetic route to complex
8
5
2 2 3 3
; i) NaOH and ClCH CH OCH in CH CN at
in THF at room temperature for 18 h.
o
0 C for 18 h; ii) K BuO and [IrCl(COD)]
t
Scheme 4 Thermal isomerisation of 5 to yield 6 (NHO → aNHC).
2
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It is worth mentioning that the isomerisation process is not Noteworthy, no relationship between the substitution of this
reversible and complex is not obtained again when the substrate and the catalytic activity can be drawn from these results.
reaction mixture returns to room temperature; moreover, The use of aliphatic ketones increases the reaction times compared
complex remains unaltered at least until temperatures as to their aromatic counterparts; namely, 3-decanone required 8.5 h
5
6
o
–1
high as 90 C in a toluene solution.
of reaction to achieve a 79% conversion with a TOF value of 157 h
Table 1, Entry 8).
(
Attempts to abstract the chloride ligand with AgPF
6
in
dichloromethane in order to form the O-coordinated chelate The reduction of aldehydes by transfer hydrogenation presents
complex have been unsuccessful, probably due to the weak inherent complications, such as, aldehyde decarbonylation followed
coordination of the ether functions, which precluded its by deactivation of the catalysts due to carbon monoxide
isolation as a result of complex decomposition.
coordination or to the formation of condensation by-products via
aldehyde deprotonation. Remarkably, the transfer hydrogenation of
Catalysis
benzaldehyde to benzyl alcohol using catalyst 2 was completed in
The transfer hydrogenation of carbonyl compounds and imines
is a sustainable alternative to stoichiometric reduction
reagents, and a less hazardous route to amines and alcohols
–1
4
0 s with a TOF value of 16071 h (Table 1, Entry 15).
Table 1 Transfer hydrogenation of ketones and aldehydes with catalyst 2.
7
than the catalytic hydrogenation with molecular hydrogen.
–1
Entry
Substrate
T (min) Conv. (%)
TOF10% (h )
Therefore, owing to the importance of the reduction of C=O
and C=N bonds to the pharmaceutical industry, the
development of more active catalysts for transfer
O
1
7
100
2222
a
2
300
100
18750
hydrogenation, as well as the search for
a
better
O
O
understanding of the mechanisms involved therein, are still
ongoing endeavours. Moreover, the large number of iridium
catalysts hitherto screened for this reaction makes it an
excellent test bench to measure the activity of Ir-NHO
3
4
60
90
100
100
779
8
,7c
complexes.
The catalytic activity of complex [Ir(PCP)(cod)]PF6
1053
(2) in
OMe
transfer hydrogenation was studied for a variety of substrates,
including, ketones, aldehydes and imines. The reaction
conditions entailed the use of isopropanol as solvent,
O
5
6
180
180
91
2143
594
MeO
Me
potassium tert-butoxide as a base and complexes
2 or 5 as
o
O
catalysts at 80 C (a 1/5 ratio precatalyst/base was employed).
Before addition of the substrate, the mixture precatalysts/base
was preactivated in isopropanol by placing the mixture for 15
100
o
min in a thermostated oil bath at 80 C (Scheme 5).
O
Subsequently, the reactions were monitored by GC analysis
taking 0.1 mL aliquots at adequate reaction times using
mesitylene as internal standard.
7
8
60
510
10
100
79
319
157
145
F
O
Under the conditions described above, we found that the catalytic
transfer hydrogenation of cyclohexanone to cyclohexanol using 1
–1
O
mol % of 2 is completed in 7 min with a TOF value of 2222 h (Table
, Entry 1). When the catalyst loading was reduced to 0.1 mol % the
reaction time required to achieve total conversion was 5 h but,
b
1
9
100
–1
remarkably, a TOF value of 18750 h was obtained (Table 1, Entry
O
2
). These results compare well with the most active catalyst
1
0
120
100
236
8
b,f,j,k
hitherto reported for the hydrogenation of cyclohexanone.
For
example, nanotube-supported [Ir(Cl)(COD)(NHC)] complexes give
c
–
1
8k
11
30
420
180
60
100
100
100
100
100
166(B)
259(A)/59(B)
1948(A)/46(B)
375(A)/31(B)
16071
initial TOF values up to 5500 h and 94% conversion in 80 min.
c,a
c,d
c,e
O
1
1
1
2
3
4
Among the variety of substituted acetophenone derivatives
screened for transfer hydrogenation, the unsubstituted
acetophenone experienced the fastest transformation, rendering
O
O
1
5
0.6
–
1
total conversion after 1 h (TOF = 779 h ) (Table 1, Entry 3).
H
a
1
6
1440
63
28571
Conditions: catalyst (0.01 mmol), substrate (1 mmol), base (tBuOK, 0.05 mmol),
o
a
b
iPrOH (5 mL), 80 C. Conv. determined by GC. catalyst 0.001 mmol. During the
c
reaction mixtures of partially hydrogenated products are observed. Mixtures of
Scheme 5 Transfer hydrogenation of polar bonds using isopropanol as hydrogen
source.
reduction products are obtained, 2-hydroxy-1,2-diphenylethanone (A) and 1,2-
d
e
o
Diphenylethane-1,2-diol (B). Catalyst (0.002 mmol). 60 C.
4
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–
1
A TOF value of 28571 h was achieved when the catalyst loading different catalyst loadings, the effect of the temperature on the
was reduced to 0.1 mol %; however, total conversion to the alcohol selectivity was also studied. For that, the temperature was reduced
o
was not accomplished (Table 1, entry 16), which suggests catalyst to 60 C (Figure 2d), with a 1 mol % of catalyst. After 15 min, the
deactivation during the course of the reaction. To the best of our reaction mixture contains 25% of starting material and 75% of
knowledge, the remarkable activity of this catalyst in the transfer mono-hydrogenated product, benzoin. However, before total
hydrogenation of bezaldehyde is only surpassed by the Ru and Ir conversion to benzoin occurs, the mono-hydrogenated product
9
complexes reported by Baratta’s and Xiao’s groups, respectively.
starts to convert into the di-hydrogenated product. After 1 h,
complete conversion to benzoin takes place. The transfer
hydrogenation of imines usually proceeds more sluggishly than that
The hydrogenation of diketone 1,2-diphenylethane-1,2-dione
(
benzil) to the corresponding diol was completed in 30 min (TOF =
1
0
–1
of carbonyl compounds and significantly less examples are known.
1
66 h ) using 2 as catalyst (Figure 2a). This reaction may afford two
Catalyst 2 shows good activities for the reduction of imines by
transfer hydrogenation, for example, N-benzylideneaniline was
transformed into N-benzylaniline using a 1 mol % of catalyst 2 in 45
hydrogenation products, i.e the mono-hydrogenated product, 2-
hydroxy-1,2-di(phenyl)ethanone (benzoin), and dihydrogenated
product 1,2-diphenyl-1,2-ethanediol (hydrobenzoin), the latter
being formed by hydrogenation of the former. After 5 min, the
reaction mixture contains 21% starting material, 64% benzoin and
–1
min with a TOF value of 1118 h (Table 2, Entry 1). Better TOF
values were only achieved by the Ir(bis-NHC) catalysts described by
7
c
Crabtree and Miecznikowski, and improves the results obtained by
1
5% hydrobenzoin. After 30 min, the starting material has been
11,8b
Shvo’s catalyst and other iridium catalysts.
like m-methoxy-N-benzylideneaniline
Substituted imines
and p-methoxy-N-
completely consumed, with a product distribution of 6% and 94% of
benzoin and hydrobenzoin, respectively. After 90 min only
hydrobenzoin is observed. The amount of catalyst was reduced to
benzylideneaniline were also converted to their corresponding
amines under the same reaction conditions, featuring TOF values of
0
.2 mol % with the intention of reducing the reaction rate and, thus,
–
1
–1
5
84 h and 110 h , respectively (Table 2, Entries 3 and 4).
allowing for the selective formation of benzoin (Figure 2b). In this
case, after 45 min of reaction, there is a 36 % of benzil and a 64 % Complex 5 was tested in order to achieve a better understanding of
benzoin with no formation of hydrobenzoin. However, after 45 min, the role played by the wingtip groups, and whether the presence of
the formation of hydrobenzoin is initiated, which precludes total strongly coordinating functional groups would improve or hamper
conversion to the mono-hydrogenated product. After 3 h, the the activity of the catalyst. Initial catalytic tests were performed
starting material was completely consumed, with benzoin and with cyclohexanone as the substrate. Complete conversion occurs
–
1
hydrobenzoin being obtained in 31% and 69% yield, respectively. after 2 h and the reaction shows a TOF value of 60 h (Table 3,
Reduction of the catalyst loading to 0.1 mol % afforded similar entry 1), which contrasts sharply with the results obtained with 2,
–1
results to those obtained for 0.2 mol % (Figure 2c). After screening total conversion after 7 min and TOF = 2222 h (Table 1, Entry 1).
Figure 2 Hydrogenation of 1,2-diphenylethane-1,2-dione (blue) with catalyst
2 to the mono-hydrogenated product 2-hydroxy-1,2-di(phenyl)ethanone
(red), and dihydrogenated product 1,2-diphenyl-1,2-ethanediol (green).
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–1
complete conversion in 15 min with a TOF value of 822 h (Table 3,
entry 2).
Table 2 Transfer hydrogenation of imines
Conv
%)
TOF10%
Entry
Substrate
Cat
2
T (min)
–1
When the amount of phosphine was increased to 2 equivalents, the
reaction proceeded even faster, as it required only 10 min to reach
complete consumption of cyclohexanone (Table 3, entry 3). With
the intention of screening the effect of different phosphines in the
(
(h )
1
45
98
1118
70
N
a
2
5
1350
100
activity of the in situ generated catalyst, P(p-F-Ph)
3 3
, P(p-MeO-Ph) ,
P(o-MeO-Ph) , P(p-Me-Ph) , P(o-Me-Ph) and PCy (Cy = Cyclohexyl)
3
3
3
3
N
were employed as additives (Table 3, Entries 3 to 9). Among them,
the highest activity was obtained for tris(4-fluorophenyl)phosphine,
3
2
90
96
68
584
110
–1
P(p-F-Ph)
the least reactive catalytic system was that resulting from the use of
tris(o-methoxyphenyl)phosphine (P(o-MeO-Ph) ), which features a
TOF value of 99 h (Table 3, Entries 7). Hence, we used 2
equivalents of P(p-F-Ph) with complex 5 for the screening of the
Conditions: catalyst (0.01 mmol), imine (1 mmol), base (tBuOK ,0.05 mmol), transfer hydrogenation reaction with various substrates. In the case
3
, with a TOF value of 3158 h (Table 3, Entries 5), whilst
OMe
N
3
4
2
120
–1
MeO
3
o
a
iPrOH (5 mL), 80 C. Conv. determined by GC. 2 eq P (p-F-C
6
H
4
)
3
were used as an of acetophenone or benzaldehyde, the activity drops significantly
additive.
compared to catalyst 2. Complete conversion was achieved after 60
–1
min with 2 (TOF = 779 h ) for acetophenone, while 5 required 240
Table 3. Transfer hydrogenation of ketones and aldehydes with catalyst 5.
–1
min (TOF = 178 h ). The case of benzaldehyde is even more
pronounced since catalyst 2 affords total conversion after 40 s (TOF
Conv.
TOF10%
–1
Entry
Substrate
Additive
T (min)
–1
=
16071 h ), being remarkably more active than 5 (with two
(
%)
(h )
–1
3
equivalents of P(p-F-Ph) ), which requires 10 min (TOF = 2222 h )
1
2
3
4
-
120
15
100
100
100
100
60
(
Table 3, Entry 14). Similarly, N-benzylideneaniline is totally
PPh
3
3
(1 eq)
(2 eq)
(2 eq)
822
462
513
–1
PPh
10
hydrogenated in 45 min with catalyst 2 (TOF = 1118 h ) while 5
with 2 equivalents of P(p-F-Ph) as additive necessitates almost 24 h
PCy
3
25
3
–1
P(p-F-Ph)
3
(TOF = 70 h ) (Table 2, Entry 2).
5
6
7
8
9
5
100
100
100
100
100
3158
2222
99
O
(2 eq)
Mechanism
P(p-MeO-C
6
H
4
4
)
3
10
120
10
30
A theoretical study at the DFT level, using the B3LYP-D3 method,
has been carried out in order to clarify some key points of the
reaction mechanism for catalyst 2 employing acetophenone as a
model substrate. First, the energetic cost for the decoordination of
the different donor moieties of the PCP ligand or COD ligand
(phosphine, NHO or olefin) upon coordination of tert-butoxide to 2
(
2 eq)
P(o-MeO-C
2 eq)
P(p-Me-C
6
H
)
3
(
6
H
4
)
)
3
3
984
181
(2 eq)
6 4
P(o-Me-C H
(2 eq)
(
structure A) has been evaluated (see Figure 3). Decoordination of
O
P(p-F-C
6
H
4
)
3
one phosphine ligand to form a square planar Ir complex (A1) raises
the relative energy 29.9 kcal/mol, and is, therefore, an unfeasible
process. Alternatively, dissociation of the coordinated NHO leads to
the coordination of the second olefin of the COD ligand, forming the
trigonal bipyramid A2 complex, which increases the energy 21.1
kcal/mol. Finally, complete decoordination of the COD ligand
forming the square planar metallic complex A3 is nearly
isoenergetic (1.8 kcal/mol) than complex A. Therefore, COD
dissociation is the most energetically favourable process. This is in
agreement with experimental observations, since addition of 1
1
1
0
1
240
120
100
100
178
171
(2 eq)
O
6 4 3
P(p-F-C H )
(2 eq)
F
O
1
1
2
3
PPh
3
(2 eq)
10
10
100
100
1905
2222
H
P(p-F-C
6 4 3
H )
(2 eq)
Conditions: catalyst (0.01 mmol), substrate (1 mmol), base (tBuOK ,0.05 mmol),
o
a
equivalent of sodium isopropoxide to a solution of 2 in methanol-d
4
iPrOH (5 mL), 80 C. Conv. determined by GC. Catalyst 0.001 mmol.
o
affords the free COD upon heating to 80 C. Besides, a mixture of
Moreover, the formation of colloidal iridium is observed, which
suggests that the catalyst progressively decomposes during the
course of the reaction. With the intention of studying whether the
addition of phosphine would improve the stability and activity of
the catalyst, 1 equivalent of triphenylphosphine was added to the
reaction mixture. Indeed, the new catalyst generated in situ led to
31
two unidentified complexes, which show one singlet each in the
P
NMR at δ values of 8.9 and –17.3 ppm, is also observed. In this
regard, it is worth mentioning that none of the peaks corresponds
to the free PCP ligand (–22.2 ppm) or the starting complex (2) (a
broad singlet at –24.4 ppm). Following this line of thought,
attempts to prepare a Rh complex analogous to 2 resulted always in
6
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1
COD dissociation. The H NMR resonances that correspond to the The stepwise mechanism starts by β-elimination of the C–H bond at
free COD ligand appear at δ values of 5.55 and 2.35 ppm. The the alkoxo ligand via transition state TSB/C to yield the hydride
unsaturated complex thus obtained can be stabilised in a THF intermediate C, this step presents an activation energy of 22.3
solution by solvent coordination. However, attempts to isolate the kcal/mol. Remarkably, TSB/C features a dissociated NHO ligand,
3
1
THF adduct have been unsuccessful. The P NMR spectrum in which allows the metal centre to adopt a square planar geometry.
CD Cl2 confirms that the PCP ligand remains coordinated to the Exchange of acetone by acetophenone gives complex D, thus
2
rhodium centre, as it shows a doublet resonance due to P-Rh leading to the migratory insertion of the ketone into the Ir–H bond
1
coupling ( JRh-P = 92.5 Hz) at a δ value of 2.43 ppm. Moreover, the through transition state TSD/E. The activation energy of this step is
methylenic protons of the wingtip groups are diastereotopic, 4 29.0 kcal/mol and, analogously to TSB/C, requires the
multiplets are observed centred at δ values of 4.56, 4.11, 3.62 and decoordination of the NHO ligand from the metal centre. Finally, 1-
3
.44 ppm, which confirms the coordination of the NHO moiety. The phenylethanol is released through a proton transfer mechanism
higher tendency of the Rh complex to eject the COD ligand can be characterised by transition state TSE/F upon coordination of a
i
explained in terms of an increased steric hindrance about the metal molecule of PrOH to the metal. Alternatively, the concerted MPV
centre due to the smaller size of rhodium compared to iridium.
mechanism occurs via TSB/E, showing a relative energy of 34.9
Another evidence that supports the decoordination of the 1,5- kcal/mol.
cyclooctadiene ligand (COD) to form the active species is that The DFT calculated energetic profile reveals that the hydride
neither catalysts 2 nor 5 catalyse the transfer hydrogenation of intermediates are exergonic and the overall activation energy (29.0
olefins, namely, cyclooctene, COD or styrene, which proves that the kcal/mol) is determined by intermediate D and TSD/E. Noteworthy,
required vacant coordination sites need to be generated by a the NHO moiety acts as a hemilable ligand coordinating to the
process different from COD hydrogenation. In fact, addition of metal at the intermediates but decoordinating at the transition
excess COD (1 mL) to the reduction of cyclohexanone to states. A plausible explanation for this behaviour would be that the
cyclohexanol using 2 as catalyst completely inhibits its catalytic reversible coordination of the NHO makes it possible for the iridium
activity, probably due to the impossibility to generate the active centre to preserve the more favourable square planar geometry
species in significant amounts under these conditions.
throughout the reaction pathway, thus decreasing the energy of the
The transfer hydrogenation of ketones may proceed via a stepwise corresponding transition states.
mechanism by means of a hydride intermediate or a concerted one,
also called the MPV (Meerwein–Ponndorf–Verley) mechanism.
Conclusion
Although Ir complexes usually follow the monohydride route, both
possible pathways have been calculated in order to clarify the role In summary, we have evaluated for the first time the behaviour of
1
2
of the NHO ligand in the elementary steps of the catalytic cycle.
NHOs as ancillary ligands in catalysis. Several conclusions may be
The calculated Gibbs free energy profile is shown in Figure 4 drawn from the experiments described here. Firstly, the NHO–Ir
assuming the decoordination of the COD ligand (starting from A3). bond is labile and requires the coordination of the wingtip groups
An alternative energetic profile starting from A was also calculated; to stabilize this coordination mode. In fact, in this work we present
however, a significantly higher overall energy barrier was obtained the only examples so far reported an isomerization NHO → aNHC.
(
Supporting Information).
+
-
Figure 3 DFT calculated relative Gibbs free energy (to A, in kcal/mol) for the possible isomers of complex 2 and iPrO .
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Figure 4 DFT calculated Gibbs free energy profile (relative to B and isolated molecules, in kcal/mol) for the stepwise and concerted transfer hydrogenation of ketones
by 2-propanol catalysed by 2.
Secondly, the decoordination of the COD ligand appears to be recorded on Bruker Avance 300 MHz spectrometer. Chemical shifts
energetically more favourable than NHO dissociation in complex 2 are given as dimensionless δ values and are frequency referenced
1
13
and its rhodium analogue, both featuring two strongly coordinating relative to residual solvent peaks for H and C. All coupling
side arms (two PPh
moieties). As a consequence of this behaviour, constants J are given in Hertz and multiplicity of the signals is
under catalytic conditions, i.e. in the presence of isopropoxide at 80 indicated as s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet.
2
o
i
Carbon, hydrogen and nitrogen analysis were performed using a
Perkin-Elmer 2400 microanalyzer. The substrates were obtained
from common commercial sources and used as received. Air
sensitive compounds were stored and weighed in a glovebox.
C complex 2 is transformed into the active species [Ir(PCP)( PrO)].
via isopropoxide coordination and concomitant COD dissociation.
Finally, theoretical calculations at the DFT level support this
postulation while, at the same time, show that the PCP ligand
behaves as a hemilable ligand. The NHO moiety is able to dissociate General Procedure for Transfer Hydrogenation Catalysis. A
from the iridium centre in order to allow the catalyst to adopt solution of the corresponding catalyst precursor (0.01 mmol) and
t
BuOK (5.6 mg, 0.05 mmol) in dry 2-propanol (5 mL) was prepared
under argon in a 25 mL Schlenk flask. The solution was heated to 80
square planar geometries at the transition states, which are more
stable than the related tpb geometries, thus reducing the overall
energetic barrier of the process.
o
C for 15 min in a thermostated oil bath before the addition of
substrate (1 mmol) and the internal standard (mesitylene, 140 µL, 1
o
mmol). The resulting mixture was stirred at 80 C and monitored by
gas chromatography. Aliquots (0.3 mL) were taken at fixed times,
quenched in tetrahydrofuran (1 mL) and filtered through a short
Experimental
General Procedures
2
path of SiO . Yields were determined by GC analysis of reaction
All the manipulations were performed using standard Schlenk mixtures using an Agilent Technologies 7890A. Column: Agilent
techniques under an argon atmosphere. 2-Propanol of analytical J&W HP-5, 0.25 mm × 30m × 0.25 µm. Substrates identities were
grade used as hydrogen donor was dried with Molecular Sieves and determined by NMR spectroscopy and GC-MS analysis using an
degassed prior to use. Complex [Ir(COD)Cl] was synthesised Agilent Technologies 7890A system with an Agilent Technologies
2
13
according to literature methods. The substrates were obtain from 5975C inert MS detector. Column: Agilent J&W DB-1. Experiments
common commercial sources and used as received. Air sensitive at different temperature using 1,2-diphenylethane-1,2-dione
compounds were stored and weighed in a glovebox. All experiences (benzil) were performed in the same manner.
have been carried out under argon atmosphere. NMR spectra were
8
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1
,3-bis(2-(methoxyethyl)-2-methylimidazolium chloride (4). 2- employed including the D3 dispersion correction scheme developed
1
6
methylimidazole (3.3 g, 40 mmol) was added over a solution of by Grimme for both energies and gradient calculations and the
1
7
sodium hydroxide (3.2 g, 80 mmol) and 2-chloroethyl methy ether “ultrafine” grid. The def2-SVP basis set has been selected for all
1
8
(40 mmol, 4 mL) in acetonitrile (20 mL). This solution was refluxed atoms and the PCM method was chosen to simulate solvent
o
at 80 C for 18 h. Then, the solution was filtered and the solvent effects (2-propanol, ε=19.264), both have been used for geometry
evaporated under reduced pressure. The resulting oil was optimizations and calculation of Gibbs energy corrections at 298 K.
redissolved in 2-chloroethyl methyl ether (7 mL) and refluxed for 3 Single-point energy corrections using the def2- TZVP basis set have
o
days at 90 C. Evaporation of the volatiles afforded a brown oil that been included. The nature of the stationary points has been check
was washed with diethyl ether (3 x 10 mL) to give the product as an by analytical frequency analysis and transition states were
1
off-white solid (6.4 g, 27 mmol, 68% yield). H NMR (300 MHz, characterized by a single imaginary frequency corresponding to the
CDCl
3
): δ 7.74 (s, 2H, NCH), 4.45 (t, JH-H = 4.7, 4H, CH
4.7, 4H, CH O), 3.20 (s, 6H, OCH ), 2.67 (s, 3H, CH
): δ 144.9 (s, NCN), 122.0 (s, NCH), 70.8 (s, CH
), 48.9 (s, CH N), 10.7 (s, CH ). HRMS (ESI) m/z calcd. for
2
N), 3.67 (t, JH-H expected motion of the atoms.
1
3
=
2
3
3
). C(75 MHz,
O), 59.0 (s,
CDCl
3
2
Acknowledgements
OCH
3
2
3
-
C
10
H
H
19
19
N
N
2
2
O
O
2
Cl (M – Cl ) 199.1441, found, 199.1443. Anal. Calcd. for
Cl (234.72): C, 51.17; H, 8.16; N, 11.93. Found: C, 50.95;
The authors would like to acknowledge the support by the
Ministry of Higher Education, Saudi Arabia, in establishment of
the Centre of Research Excellence in Petroleum Refining &
C
10
2
H, 8.05; N, 11.15.
Complex 5. Potassium tert-butoxide (225 mg, 2 mmol) was added Petrochemicals at KFUPM (KACST-funded project ART-32-68).
to
solution of 1,3-bis(2-methoxyethyl)-2-methylimidazolium The support under the KFUPM–University of Zaragoza research
a
chloride (400 mg, 1.7 mmol) in tetrahydrofuran (10 mL). After 1 h a agreement is also highly appreciated. This work was further
solution of bis(1,5-cyclooctadiene)diiridium(I) dichloride (600 mg, supported by the Spanish Ministry of Economy and
0
.89 mmol) in tetrahydrofuran (10 mL) was added dropwise to the Competitiveness (MINECO/FEDER) (CONSOLIDER INGENIO
first solution and stirred for 18 h at room temperature. The solvent CSD2009-0050, and CTQ2015-67366-P projects) and the
was removed under reduced pressure and the solid dissolved in Diputación General de Aragón (DGA/FSE-E07). J. M. and V. P.
toluene (10 mL). The solution was filtered through Celite and the thankfully
acknowledge
the
resources
from
the
solvent removed under reduced pressure. The oil was washed with supercomputer “memento”, technical expertise and assistance
hexane (2 x 10 mL) and the product was obtained as a yellow solid provided by BIFI-ZCAM (Universidad de Zaragoza). J. M.
1
(
363 mg, 0.68 mmol, 40% yield). H NMR (300 MHz, C
6
D
6
): δ 6.26 (s, acknowledges the financial support from the Ministry of
N), 3.36 (t, Education Culture and Sports (FPU14/06003).
O), 3.27 (bs, 2H, CHCOD), 2.92 (s, 6H, CH ), 2.45-
.27 (m, 4H, CH2 COD), 2.18 (s, 2H, CH Ir), 1.72–1.58 (m, 4H, CH2 COD
): δ 163.2 (s, NCN), 117.6 (s, NCH), 70.9 (s,
O), 58.4 (s, CH ), 52.7(s, CHCOD), 47.4 (s, CH
N), ‡ Intermediates
COD), 31.5 (s, CH COD), 17.9 (s, CH Ir). HRMS (ESI) m/z
calcd. for C18 30IrN Cl (M – Cl ) 499.1932, found, 499.1928. Anal.
Calcd. for C18 30IrN Cl (534.16): C, 40.48; H, 5.66; N, 5.24. Found:
2
H, NCH), 4.59 (bs, 2H, CHCOD), 3.66 (t, JH-H = 4.8, 4H, CH
2
JH-H = 4.8, 4H, CH
2
3
2
2
)
1
3
Notes and references
C NMR (75 MHz, C
CHCOD), 70.5 (s, CH
3.9 (s, CH
6 6
D
C and D are adducts of the square planar Ir(I)
2
3
2
PPh2
complex [IrH(κP,C,P'-NHO )] with acetone and acetophenone,
respectively.
3
2
2
2
-
H
2 2
O
1
a) Y. Wang, M. Y. Abraham, R. J. Gilliard, Jr., D. R. Sexton, P.
H
2 2
O
Wei and G. H. Robinson, Organometallics 2013, 32
,
C, 39.96; H, 5.64; N, 5.37.
−6642; b) S. M. I. Al
-Rafia, A. C. Malcolm, S. K. Liew, M.
6
639
J. Ferguson, R. McDonald and E. Rivard, Chem. Commun.,
011, 47, 6987–6989; c) N. Kuhn, H. Bohnen, J. Kreutzberg,
D. Bläser and R. Boese, J. Chem. Soc., Chem. Commun., 1993,
136–1137; d) Y.-B. Wang, Y.-M. Wang, W.-Z. Zhang and X.-
Complex 6. A toluene solution of 5 (60 mg, 0.11 mmol) was heated
2
o
for 3 h at 70 C. Then, the solvent was removed under reduced
pressure and the solid washed with hexane (3 x 10 mL). The product
1
was obtained as a dark yellow solid (45 mg, 0.08 mmol, 75% yield).
B. Lu, J. Am. Chem. Soc. 2013, 135, 11996−12003; e) S.
Kronig, P. G. Jones and M. Tamm, Eur. J. Inorg. Chem. 2013,
1
7 8
H NMR (300 MHz, C D ): δ 6.08 (s, 1H, NCH), 4.92-4.83 (bs, 2H,
2301–2314.
a) A. Fürstner, M. Alcarazo, R. Goddard and C. W. Lehmann,
Angew. Chem. 2008, 120, 3254–3258; Angew. Chem. Int. Ed.
CHCOD), 4.52 (t, JH-H=4.9, 2H, CH
2
N), 3.89 (t, JH-H=5.0, 2H, CH
bs, 2H, CHCOD), 2.98 (s, 3H, OCH ), 2.94 (t, JH-H=5.1, 2H, CH
t, JH-H=5.1, 2H, CH O), 2.73 (s, 3H, OCH
COD), 1.83-1.72 (m, 4H, CH2 COD), 1.69 (s, 3H, CH
MHz, C ): δ 161.8 (s, IrC), 141.8 (s, NCN), 121.5 (s, NCH), 80.4 (s,
CHCOD), 72.6 (s, OCH ), 71.3 (s, OCH ), 59.0 (s, OCH ), 58.9 (s, OCH ),
1.7 (s, CHCOD), 49.9 (s, NCH ), 47.0 (s, NCH ), 35.1 (s, CH2 COD), 31.0
s, CH2 COD), 10.2 (s, CH ). HRMS (ESI) m/z calcd. for C18
M – Cl ) 499.1932, found, 499.1927. Anal. Calcd. for C18
2
O), 3.09
2
(
(
3
2
N), 2.76
2
3
), 2.45-2.30 (m, 4H,
2
008, 47, 3210–3214; b) M. Viciano, M. Feliz, R. Corberán, J.
A. Mata, E. Clot and E. Peris, Organometallics 2007, 26
304–5314; c) M. Iglesias, A. Iturmendi, P. J. Sanz Miguel, V.
Polo, J. J. Pérez-Torrente and L. A. Oro, Chem. Commun.,
015, 51, 12431–12434.
1
3
CH
2
3
). C NMR (75
,
5
7 8
D
2
2
3
3
2
5
2
2
3
4
5
K. Powers, C. Hering-Junghans, R. McDonald, M. J. Ferguson,
E. Rivard Polyhedron 2016, 108, 8–14
R. D. Crocker and T. V. Nguyen, Chem. Eur. J. 2016, 22, 2208–
(
(
(
3
H
30IrN
30IrN
2
O
O
2
Cl
Cl
–
H
2
2
2213 (and references therein).
Crystal data of compound 3
534.16): C, 40.48; H, 5.66; N, 5.24. Found: C, 40.03; H, 5.25; N,
.41.
2 4 2 4
: [C84H100Cl10Ir N O P ], triclinic,
5
P–1, a = 13.9013(7) Å, b = 17.4906(9) Å, c = 20.0339(11) Å, α
= 93.6950(10)º, β = 105.6320(10)º, γ = 110.4170(10)º, Z = 2,
Computational details
3
–3
M = 2060.46, V = 4327.9(4) Å , Dcalcd = 1.581 g cm , λ(Mo
r
All DFT theoretical calculations have been carried out using the
–1
Kα) = 0.71073 Å, T = 100 K, µ = 3.502 mm , 52738
1
4
15
Gaussian program package. The B3LYP method has been
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reflections collected, 20117 unique (Rint = 0.0355), 16485
Journal Name
G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P.
Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.
Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.
Nakai, T. Vreven, J. A. Montgomery Jr, J. E. Peralta, F.
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