An Insight into Transfer Hydrogenation Reactions Catalysed by Iridium(III) Bis-N-heterocyclic Carbenes

Article abstract of DOI:10.1002/ejic.201500853

A variety of [M(L)₂(L′)₂{κC,C′-bis(NHC)}]BF₄ complexes (M = Rh or Ir; L = CH₃CN or wingtip group; L′ = I^- or CF₃COO^-; NHC=N-heterocyclic carbene) have been tested as pre-catalyst

Full text of DOI:10.1002/ejic.201500853

FULL PAPER
DOI:10.1002/ejic.201500853
An Insight into Transfer Hydrogenation Reactions
Catalysed by Iridium(III) Bis-N-heterocyclic Carbenes
[
a]
[a]
[b]
Nestor García, E. A. Jaseer, Julen Munarriz,
[c]
[b]
[c]
Pablo J. Sanz Miguel, Victor Polo, Manuel Iglesias,* and
[
a,c]
Luis A. Oro*
Keywords: Homogeneous catalysis / Transfer hydrogenation / Nitrogen heterocycles / Carbenes / Iridium
A variety of [M(L)
Rh or Ir; L = CH
2
(LЈ)
2
{κC,CЈ-bis(NHC)}]BF
4
complexes (M
against the concerted Meerwein–Ponndorf–Verley (MPV)
mechanism. The calculated catalytic cycle involves a series
–
–
=
3
CN or wingtip group; LЈ = I or CF COO ;
3
NHC=N-heterocyclic carbene) have been tested as pre-cata- of ligand rearrangements due to the high trans effect of the
lysts for the transfer hydrogenation of ketones and imines.
The conversions and TOF’s obtained are closely related to
the nature of the ligand system and metal centre, more
strongly coordinating wingtip groups yielding more active
and recyclable catalysts. Theoretical calculations at the DFT
level support a classic stepwise metal-hydride pathway
carbene and hydrido ligands, which are more stable when
situated in mutual cis positions. The reaction profiles ob-
tained for the complexes featuring an iodide or a trifluo-
roacetate in one of the apical positions agree well with the
relative activity observed for both catalysts.
Introduction
The catalytic cycle has been proposed to follow the
“
hydridic route” expected for transition-metal-catalysed
The hydrogenation of organic molecules, in particular the
reduction of imines and ketones, is a fundamental reaction
in industrial synthetic chemistry.[1] Transfer hydrogenation
reactions have emerged as an alternative to classic catalytic
hydrogenation for a variety of organic substrates.[ Besides,
this reduction procedure circumvents the need for pressur-
ised systems and its associated safety issues by employing
solid or liquid hydrogen donors instead of molecular
hydrogen.
In this regard, organometallic NHC complexes (NHC =
N-heterocyclic carbene) have shown remarkable activities in
the transfer hydrogenation reaction.[ An interesting class
of pre-catalysts are those reported by Crabtree and co-
workers based on bis-NHC ligands. These iridium(III) com-
[5]
hydrogen transfers.
The Meerwein–Ponndorf–Verley
[6]
(MPV) mechanism, which involves a concerted hydride
transfer to the substrate directly from the alkoxy ligand via
a six-membered transition state, was discarded based on
data obtained from a variation of the deuteration experi-
2]
[7]
ments reported by Pàmies and Bäckvall. Although these
results are not conclusive, several other experimental evi-
dences suggest a metal-hydride route as a more plausible
mechanism. Namely, (i) the reactivity rates of substituted
acetophenones do not compare well with those observed
for MPV reactions; (ii) the reduction reaction is consider-
ably faster than its related oxidation, which deviates from
the expected similar reaction rates for MPV reduction–
3]
[8]
Oppenhauer oxidation reactions; finally, (iii) the reported
plexes of general formula [Ir(bis-NHC)(OAc)I ] are air
stable and moisture insensitive compounds and show activi-
ties that are very dependent on the NHC’s wingtip groups.
2
complexes catalyse the hydrogenation of allylbenzene, this
being unlikely to occur via an MPV mechanism (Figure 1).
[
4]
[
a] Centre of Research Excellence in Petroleum Refining and
Petrochemicals, King Fahd University of Petroleum & Minerals
(
KFUPM),
Dhahran 31261, Saudi Arabia
[
[
b] Departamento Química Física – Instituto de Biocomputación
y Física de Sistemas Complejos (BIFI),
Universidad de Zaragoza, 50009 Zaragoza, Spain
c] Departamento Química Inorgánica – ISQCH, Universidad de
Zaragoza – CSIC,
Figure 1. Bis-NHC iridium transfer hydrogenation catalysts re-
ported by Crabtree et al.
5
0009 Zaragoza, Spain
E-mail: miglesia@unizar.es
Recently, we have reported a variety of [M(L)
(LЈ) -
2 2
oro@unizar.es
{
κC,CЈ-bis(NHC)}]BF (M = Rh or Ir; L = CH CN or
4
3
www.isqch.unizar-csic.es
–
–
wingtip group; LЈ = I or CF COO ; NHC=N-heterocyclic
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500853.
3
carbene) complexes that showed good activities for the
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2
hydrosilylation of alkynes, as well as for the methanolysis Synthesis and Characterisation of [Ir(CH CN) (I) {κ C,CЈ-
3
2
2
and hydrolysis of hydrosilanes.[ The different coordination bis(NHC )}]BF {NHC = methylenebis(N-methyl)
9]
Me
Me
4
ability and steric hindrance of the N-substituents at the imidazole-2-ylidene} (2)
NHC’s have proved to be key factors that determine the
stability and reactivity of the catalyst. Moreover, the nature
of the apical ligands has demonstrated to play a crucial role
2
Me
Complex [Ir(CH CN) (I) {κ C,CЈ-bis(NHC )}]BF (2)
3
2
2
4
was obtained as an orange solid in 78% yield by treatment
of 1 with 1.1 equiv. of HBF ·Et O at 0 °C in dry dichloro-
4
2
on the selectivity of the hydrosilylation of terminal alkynes.
methane, followed by addition of excess acetonitrile
Scheme 1). The protonation and concomitant dissociation
of the acetato ligand was monitored by the disappearance
of the CH COO protons in H-NMR spectroscopy.
I
When the iodide ligands in the parent [Rh {κO,C,CЈ,OЈ-
2
(
bis(NHC)}]BF complex were replaced by trifluoroacetates,
4
a drastic selectivity change, from the anti-Markovnikov
1
3
anti-addition product to the Markovnikov addition prod-
[
9d]
uct, was observed.
Here we report on the catalytic activity in transfer
III
hydrogenation of Ir
complexes featuring methylene
bridged bis-NHC ligands that contain wingtip groups with
different coordination ability and steric impact. Moreover,
we studied the influence of (i) exchanging one of the apical
iodides by a more labile trifluoroacetate ligand, and (ii) the
nature of the metal centre on the activity of the resulting
catalysts, namely Ir vs. Rh . Finally, a detailed mechan-
istic study that discloses the hitherto unknown intermediate
species as well as the role played by the ancillary ligands
is proposed based on DFT calculations and experimental
data.
Scheme 1. Preparation of complex 2.
1
III
III
The H NMR spectra of complex 2 in CD CN shows
3
two doublets (2.0 Hz) that correspond to the CH’s of the
NHC rings at δ values of 7.31 and 7.20 ppm. The protons
of the methylene bridge and the N-methyl groups appear as
a singlet at δ values of 6.39 and 4.04 ppm, respectively. A
singlet at a δ value of 1.96 ppm that integrates ca. 6 protons
could be assigned to free acetonitrile, probably due to dis-
placement of the coordinated acetonitrile ligands by the
deuterated solvent. The low solubility of 2 in CD Cl ,
Results and Discussion
2
2
Complexes 2–7 were tested as catalyst precursors for the CDCl or even [D ]acetone precluded its characterisation
3
6
transfer hydrogenation of a range of ketones and imines in in less coordinating solvents. The most characteristic signal
order to investigate the effect of modifying the ligand sys- in the 13C{ H} NMR spectra is that corresponding to the
tem, i.e. N-substituents or apical ligands, and the nature of carbene carbons at a δ value of 128.0 ppm.
1
III
the metal centre in the activity of Ir or Rh bis(NHC)
complexes (Figure 2). Complexes 3–6 were previously re-
[
9]
ported by us while the preparation of 2 and 7 is described Synthesis and Characterisation of [Ir(κO-CF COO)
3
4
OMe
OMe
in this work (vide infra).
(I){κ O,C,CЈ,OЈ-bis(NHC
)}]BF
4
{NHC
=
methylenebis(N-2-methoxyethyl)imidazole-2-ylidene} (7)
Complex 7 was prepared by treating of 5 with 1 equiv. of
silver trifluoroacetate at room temperature in acetone. The
reaction entails the abstraction of one of the iodides in 5.
Subsequently, the vacant coordination site thus generated is
occupied by a κO-CF COO ligand to give complex 7 as an
3
air-stable, bright-orange solid in good yields (Scheme 2).
1
The H NMR spectrum of 7 is similar to that of 5. The
1
most significant differences in the H NMR spectrum of 7
are the AB system centred at δ values of 6.78 and 6.66 ppm
2
(
JH,H = 13 Hz) that corresponds to the diastereotopic pro-
tons of the methylene group bridging the two NHC rings,
and the multiplet from 4.75 to 4.50 ppm due to the dia-
stereotopic methylenic protons of the wingtip groups
13
(
NCH and OCH ). The C NMR spectra show a singlet
2 2
at a δ value of 120.2 ppm that can be assigned to the carb-
ene carbon atoms coordinated to the iridium centres and
–
two quartets corresponding to the CF COO ligand at δ
3
2
1
values of 161.0 ( J
= 40 Hz) and 113.6 ( J
= 292 Hz)
C,F
C,F
ppm. The 19F NMR presents two peaks at δ values of –74.7
and –151.4 ppm that can be assigned to the BF4–
Figure 2. Complexes tested in this work as transfer hydrogenation
catalysts.
–
counteranion and the CF COO ligand, respectively.
3
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(
–CH CH CH CH ) as N-substituents, is weighed against
2 2 2 3
5, where an oxygen atom substitutes one of the methylene
groups (–CH CH OCH ), a remarkable enhancement of
2
2
3
the activity is observed for the latter groups (for example
see Entry 2 vs. Entry 4 in Table 1). In this regard, heterodi-
Table 1. Transfer hydrogenation of ketones.[a]
Scheme 2. Preparation of complex 7.
Noteworthy, when two equivalents of silver salt were
added to 5 in order to prepare the bis(trifluoroacetato)
complex an unidentified mixture of complexes was ob-
tained.
Suitable crystals of 7 were grown by slow diffusion of
diethyl ether into a saturated acetone solution. The molec-
ular structures obtained by single-crystal X-ray diffraction
analysis (Figure 3) confirm the expected coordination of the
trifluoroacetato ligand at the axial position. The asymmet-
ric unit of 7 encloses two crystallographically different cat-
2
OMe
+
ions [Ir(κO-CF COO)(I) {κ O,C,CЈ,OЈ-bis(NHC
)}]
3
–
and their respective BF4 anions. Geometry of both
enantiomers is roughly identical. Equatorial coordination
OMe
of the tetratopic bis(NHC
) ligand does not display no-
[
9b]
table differences in comparison to analogous complexes.
A difference regarding the trans influence is observable in
the Ir–I bond lengths of the I–Ir–OOCCF system of 7 [Ir1–
3
I1 2.6335(5) Å, Ir2–I2 2.6372(5) Å], which are significantly
shorter when compared to the corresponding I–Ir–I frame
in an analogous complex[
9a]
[Ir1–I1 2.6750(3) Å, Ir1–I2
2.6614(3) Å].
Figure 3. View of cations of 7. Selected bond lengths [Å] and angles
°] of cation 7: Ir1–C12 1.959(7), Ir2–C52 1.948(6), Ir1–C22
.931(7), Ir2–C62 1.935(7), Ir1–O18 2.189(5), Ir2–O58, 2.197(5),
Ir1–O28 2.186(4), Ir2–O68 2.185(4), Ir1–O41 2.054(5), Ir2–O81
.061(5), Ir1–I1 2.6335(5), Ir2–I2, 2.6372(5), C22–Ir1–C12 88.3(3),
[
1
2
C62–Ir2–C52 88.3(3), C12–Ir1–O41 95.8(2), C52–Ir2–O81 91.4(2),
C12–Ir1–O18 91.7(2), C52–Ir2–O58 90.7(2), C12–Ir1–I1 89.15(18),
C52–Ir2–I2 92.3(2), O18–Ir1–I1 91.64(13), O58–Ir2–I2 92.43(13),
O41–Ir1–I1 174.71(13), O81–Ir2–I2 175.42(13).
Catalytic Studies
Initial catalytic studies aimed at comparing the effect of
two sterically similar wingtip groups with and without a
coordinating atom embedded in its structure. In this
[
(
a] Conditions: catalyst (0.01 mmol), ketone (1 mmol), base
tBuOK, 0.05 mmol), iPrOH (5 mL), 80 °C. Conversion deter-
regard, when complex 3, which features butyl groups mined by GC.
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topic NHC ligands have proved highly efficient ancillary (2). Complex 2 is the least active catalysts of the series (2–
ligands for transfer hydrogenation catalysis.[
10]
7); however, it is not clear whether this is due to the reduced
A bis-NHC ligand featuring more labile N-substituents steric shielding of the active site or to the low solubility of
[
11]
than those in 5, namely ethyl phenyl ethers (complex 4),
was explored in order to shed light on whether a weaker
the complex in iPrOH.
Rhodium complex 6 was tested in order to assess the
stabilisation of the metal centre would render a more active influence of the metal centre on the catalytic activity. Anal-
catalyst. Conversely, a screening of the activity of complex ogously to what has been reported previously for NHC
4
for a variety of ketones (Table 1) and imines (Table 2) complexes,[3,12,10d] 6 is markedly less active than its iridium
shows, in general, that its activity is somewhere between 3 counterpart 5, the latter being the most active complex pre-
and 5. Yet, it is worth emphasising the importance of the sented in this work.
stabilisation of the metal centre by the wingtip groups,
Furthermore, the catalytic activity of complexes 5 and 7
which is further supported by the fact that 4 is more reac- was compared with the intention of studying the influence
tive than 3 except for the reaction with p-MeO-aceto- of the apical ligand. According to the monohydride mecha-
phenone (entries 19–20) and, particularly, p-Me-aceto- nism expected for this type of complexes, the catalytic cycle
phenone (entries 24–25).
should be initiated by the attack of the isopropoxide anion
to the metal centre. Two possible coordination positions can
be envisaged for the iPrO , equatorial or axial, i.e. trans or
Table 2. Transfer hydrogenation of imines.[a]
–
cis to one of the carbene carbons, respectively. The next
step would entail the formation of a hydrido ligand, which,
again, can take place at the any of the two positions men-
–
tioned above. Therefore, if the attack of iPrO happened at
the apical position the reaction rate should be improved by
the presence of the more labile trifluoroacetato ligand in 7.
On the contrary, a significant reduction of the catalytic ac-
tivity is observed. On these grounds, the coordination of
the isopropoxide probably occurs at one of the equatorial
positions since the presence of a more labile ligand
–
(
CF COO ) should increase, and never reduce, the catalytic
3
activity, thanks to a more facile access of the isopropoxide
to the metal centre. This behaviour would be in agreement
with the higher trans effect expected for NHC ligands com-
pared to iodides, which strongly labilises the equatorial li-
–
gands, thus favouring the attack of iPrO at these positions.
A plausible explanation for this decrease in the activity
of the catalyst may be that the β-hydrogen elimination step
is somehow hampered by the trifluoroacetato ligand. There-
fore, initial formation of the hydride at the apical position
can be safely discarded because the presence of the iodide,
instead of the more labile trifluoroacetate, should obstruct
this step while, in reality, the bis(iodide) complex 5 is signifi-
cantly more active than 7.
Kinetic experiments performed with catalysts 2–7 for the
transfer hydrogenation of cyclohexanone under the condi-
tions described in Table 1 are shown in Figure 4. The behav-
iour of the catalysts fits well the general trend observed for
other substrates, with the exception of 2, which is slightly
more active than 3 in this case.
The results described above suggest that the activity of
the pre-catalysts may be closely related to the stability of
the active species, as NHCs containing more strongly coor-
dinating wingtip groups lead to better reaction rates. Fig-
ure 5 shows the conversions obtained by catalysts 2–5 for
five consecutive uses in the transfer hydrogenation of cyclo-
hexanone in 2-propanol.
[
0
a] Conditions: catalyst (0.01 mmol), imine (1 mmol), base (tBuOK,
.05 mmol), iPrOH (5 mL), 80 °C. Conversion determined by GC.
The higher stability of 5 compared to that of 3 is sup-
ported by the better performance of the former throughout
The steric protection of the metal centre is reduced to a the recycling studies. The reduction of the steric protection
minimum by introducing methyl substituent on the NHC exerted by the wingtip groups in 2 further decreases the
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Theoretical Calculations and Proposed Catalytic Cycle
The catalytic cycle reported by Crabtree et al. for the
transfer hydrogenation of ketones with the iridium
bis(NHC) complexes presented in Figure 2 has been pro-
posed to follow the monohydride mechanism, thus dis-
carding a MPV type mechanism. Furthermore, it is gen-
erally accepted that metal complexes based on Ru, Mo, Rh
[
13]
and Ir operate by the monohydride route,
ceptions have been reported.
although ex-
[
7,14]
In this work, a computa-
tional study using DFT calculations has been carried out
using complexes 5 and 7 as models in order to clarify the
mechanisms involved in this iridium(III)-catalysed reaction
and the species that determine the overall activation energy
Figure 4. Activity of catalysts 2–7 in the hydrogenation of cyclohex-
anone.
(Figure 6). At the outset of this study we considered that
two main issues needed to be addressed. First of all, we set
off to search for additional support that would confirm a
metal-hydride mechanism, since MPV-type hydrogen trans-
fer is also conceivable –the dissociation of the wingtip
groups in complexes 5 and 7, or the acetonitrile ligands in
2–4, would generate two vacant coordination sites that may
allow for the formation of the six-membered pericyclic tran-
sition state. Secondly, in the case that the former was, as
[
4a]
proposed for related complexes,
the most favourable
mechanism, a detailed theoretical study would be under-
taken in order to explain how and where the attack of the
isopropoxide, the formation of the hydride and hydrogen-
ation of the ketone occur.
Figure 5. Recycling experiments performed with catalysts 2–5
(
1 mol-%), tBuOK (5 mol-%) and cyclohexanone as substrate in 2-
propanol at 80 °C.
The Gibbs free energy profiles for both pathways are
shown in Figure 6. Complexes 5 and 7 may dissociate one
–
of the ether wingtip groups and coordinate the iPrO anion
activity of the catalyst upon successive cycles. Finally, com- to yield neutral complex A. Decoordination of the second
plex 4 exhibits an unexpected behaviour, as a drastic ac- wingtip ether from the metal generates a vacancy cis to the
tivity loss is observed after the first use.
isopropoxide at the metal that permits the formation of a
–1
Figure 6. Gibbs free energy profile (in kcalmol , relative to A and isolated molecules) for the transfer hydrogenation of acetophenone
by 2-propanol catalysed by 5 (X = I, normal values) and 7 (X = CF COO, italic values).
3
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hydride by β-elimination of the C–H bond of the alkoxo monohydride mechanism and, therefore, can be safely dis-
ligand via transition state TSA/B. The energetic barrier for carded.
this step is very similar for catalysts 5 and 7, 11.0 and
Inspection of the calculated energetic profile for the cata-
2.2 kcalmol , respectively. Once the hydride intermediates lytic cycle reveals that hydride species are exergonic with
B) are formed, the isomerisation to BЈ takes place due to respect the catalyst and the reactants. The overall activation
–
1
1
(
the high trans effect of the hydride and the NHC ligand. energies are determined by intermediate CЈ and TSC/D,
Species BЈ present the hydride trans to the iodide ligand yielding values for the energy of 19.9 and 24.3 kcalmol–1
and one of the carbenes trans to the iodido or trifluoroacet- for 5 and 7, respectively. Therefore, higher TOF values are
ato ligand, thus leading to a stabilisation of 4.6 and expected for 5 due to the higher stability of the hydride
–
1
1
1.2 kcalmol , for 5 and 7, respectively. Interestingly, inter- intermediates rendered by catalyst 7, which is in agreement
mediate BЈ is more strongly stabilised for catalyst 7 than for with experimental results.
due to the lower trans influence of the trifluoroacetato
5
ligand compared to that of the iodide. Subsequently, the
acetone ligand (formed after β-elimination) is replaced by
the substrate (acetophenone) to give species CЈ, which re-
Conclusions
A range of iridium(III) bis-NHC complexes with various
sults in an energetic stabilisation of 3.9 and 0.8 kcalmol–1 ligand systems have proved to be active for the transfer
for 5 and 7, respectively.
hydrogenation of ketones and imines using 2-propanol as
Isomerisation to C is required in order to permit the mi- hydrogen source. The activity and stability of these pre-cat-
gratory insertion of the acetophenone C–O bond into the alysts depends to a great extent on the nature of the wingtip
Ir–H bond via transition states TSC/D (see Figure 7), which groups at the NHCs and the apical ligands. Noteworthy,
–
1
have relative energies of 8.8 and 10.7 kcalmol for 5 and the most active catalyst is that featuring a bis-NHC ligand
, respectively. Remarkably, direct migratory insertion from containing strongly coordinating moieties at the wingtip
CЈ leads to a less stable transition state and, consequently, groups (5), which, at the same time, could be postulated
7
to a higher activation energy for the overall process.
also as the most stable complex in the light of the results
obtained from the reusability experiments. Theoretical cal-
culations at the DFT level support that the reaction pro-
ceeds by a stepwise monohydride mechanism. The alterna-
tive Meerwein–Ponndorf–Verley (MPV) pathway has been
discarded by reason of the high energy barrier obtained for
the hydride transfer via the consequent six-membered peri-
cyclic transition state.
The first step of the proposed catalytic cycle requires the
coordination of the isopropoxide at one of the equatorial
positions (trans to the NHCs). β-Elimination affords then
a hydrido and an acetone ligand, both trans to the NHCs.
The high trans effect of the carbenes forces the migration
of the hydride to the apical position and the iodide (or tri-
fluoroacetate) to the equatorial position. Subsequently, the
acetone ligand is replaced by the substrate and the hydride
migrates again to the equatorial plane to give the migratory
insertion of the C=X (X = N, O) bond into the Ir–H bond.
Finally, the resulting alkoxide is protonated according to a
concerted mechanism by a molecule of 2-propanol, which
coordinates at the cis-positioned vacant site generated after
the migratory insertion step.
The proposed mechanism agrees well with the different
reactivity obtained experimentally for complexes 5 and 7,
which present two iodides (5) or an iodide and a trifluoro-
Figure 7. Geometrical representation of the optimised geometries
of TSC/D (up) and TSA/D (down) for catalyst 5. Hydrogen atoms
omitted for clarity.
–
acetate (7) in the apical positions. The replacement of I by
–
Finally, intermediate D undergoes the coordination of an CF COO leads to a stabilisation of the hydride intermedi-
3
iPrOH molecule that transfers the –OH proton to the alk- ates and to the consequent increment of the overall acti-
oxo ligand in a concerted manner through transition state vation energy.
TSD/E to give the corresponding alcohol (1-phenylethanol)
and the active species (A).
As a final remark, this study sheds light on the nature of
the hitherto unknown intermediate species involved in the
Alternatively, the concerted MPV mechanism takes place mechanism that operates in the transfer hydrogenation reac-
via TSA/D (see Figure 7) presenting a relative energy of tion catalysed by iridium(III) Bis-NHC complexes and,
–
1
1
9.2 and 20.0 kcalmol for 5 and 7, respectively. The ener- furthermore, discloses the role played by all the ligands in
getic barrier is substantially higher than that found for the the coordination sphere throughout the catalytic cycle.
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ppm. C15
found C 23.15, H 2.75, N 7.07.
H
20BF
7
IIrN
4
O
4
(783.26): calcd. C 23.00, H 2.57, N 7.15;
Experimental Section
Experimental Details: All the manipulations were performed using
standard Schlenk techniques under an argon atmosphere. All com-
plexes after their formation were treated under aerobic conditions.
2
[
0
Ir(CH
3
CN)
2
(I)
2
{κ C,CЈ-bis(NHCMe)}](BF
4
): HBF
4
Et
2
O (96 μL,
.77 mmol) was added dropwise under argon to a solution of [Ir-
2
Me
2
(κ O,O-CH
dry CH Cl
ture was stirred for 45 min. Subsequently, CH
3
3
COO)(I)
(30 mL) at 273 K, after the addition the reaction mix-
CN (5 mL) was
2
{κC,CЈ-bis(NHC )}] (0.50 g, 0.70 mmol) in
Complex
(
[Ir(COD)Cl]
NHC )}] (1), [Ir(CH CN)
nBu (3), –CH CH OPh (4)}, [Ir
BF ) (OMe = –CH CH
literature methods.
2
,
[Ir(κ O,OЈ-CH
3
COO)(I)
2
{κC,CЈ-bis-
Me
2
R
2
2
3
2
(I)
2
{κ C,CЈ-bis(NHC )}](BF
4
) {R =
I
4
OMe
2
2
2
{κ C,CЈ,O,OЈ-bis(NHC
)}]-
added and the reaction mixture stirred at room temperature for
45 min. The solvent was evaporated under reduced pressure. The
resulting residue was washed with diethyl ether (3ϫ 15 mL) to give
the title compound as an orange solid, yield 78% (432.4 mg,
.55 mmol). H NMR (CD
.0 Hz, 2 H, CHim ext.], 7.20 (d, JH,H = 2.0 Hz, 2 H, CHim int.), 6.39
N), 4.04 (s, 6 H, NCH ), 2.09 (s, 6 H, CH CN) ppm.
C{ H} NMR (CD CN, 100 MHz): δ = 128.0 (NCimN), 125.3
CHim int.), 122.4 (CHim ext.), 64.5 (NCH N), 40.9 (NCH ) ppm.
18BF IrN (791.15): calcd. C 19.74, H 2.29, N 10.62; found
(
4
2
2
OMe) (5) were synthesised according to
2-Propanol of analytical grade used as
[
15]
hydrogen donor was dried with molecular sieves and degassed prior
to use. NMR spectra were recorded on Bruker Avance 400.16 MHz
spectrometer. Chemical shifts are given as dimensionless δ values
and are frequency referenced relative to residual solvent peaks for
1
0
2
3
CN, 400 MHz): δ = 7.31 [d, J(H,H) =
1
13
19
(s, 2 H, NCH
2
3
3
3
H and C and to an external reference of CFCl for F. All cou-
1
3
1
3
pling constants J are given in Hertz and multiplicity of the signals
is indicated as s for single, d for doublet, t for triplet, q for quartet
and m for multiplet. Carbon, hydrogen and nitrogen analysis were
performed using a Perkin–Elmer 2400 microanalyser. The sub-
strates were obtained from common commercial sources and used
as received. Air-sensitive compounds were stored and weighed in
glovebox. All experiments have been carried out under argon atmo-
sphere.
(
C
2
3
13
H
4
I
2
6
C 20.02, H 2.38, N 10.53.
X-ray Crystallography: Crystal data, data collection and refinement
parameters for compound 7 were collected on a Bruker Kappa
APEX2 diffractometer equipped with an area detector and graph-
ite-monochromated Mo-K radiation (0.71073 Å). Data reduction
α
was done with the APEX2 software.[ The structure was solved
16]
General Procedure for Transfer Hydrogenation Catalysis: A solution
of the corresponding catalyst precursor (0.01 mmol) and tBuOK
by direct methods and refined by full-matrix least-squares methods
based on F using SHELXL-97 and WinGX programs.
2
[17]
Non-
(5.6 mg, 0.05 mmol) in dry 2-propanol (5 mL) was prepared under
hydrogen atoms were refined anisotropically. H atoms were posi-
tioned geometrically and refined with isotropic displacement pa-
rameters according to the riding model. Distance and angle calcu-
lations were performed using the SHELXL-97 and WinGX pro-
grams.[ Refinement parameters for compound 7 are as follows:
argon in a 25 mL Schlenk flask. The solution was heated to 353 K
for 15 min in a thermostatted oil bath before the addition of ketone
or imine (1 mmol) and the internal standard (mesitylene, 140 μL,
1
mmol). The resulting mixture was stirred at 353 K for 24 h or
17]
until complete conversion. Aliquots (0.5 mL) were taken at fixed
times, quenched in tetrahydrofuran (1 mL) and filtered through a
short path of SiO , yields were determined by GC analysis of reac-
2
tion mixtures using an Agilent Technologies 7890A. Column: Ag-
ilent J&W HP-5, 0.25 mmϫ30 mϫ0.25 μm. Substrates identities
were determined by NMR spectroscopy and GC–MS analysis
using an Agilent Technologies 7890A system with an Agilent Tech-
nologies 5975C inert MS detector. Column: Agilent J&W DB-1.
¯
[C
15
H
20BF
7
IIrN
4
O
4
], triclinic, P1,
16.5855(13) Å,
a
α
=
=
8.7610(7) Å,
69.8940(10)°, β
b
=
=
1
8
2
1
6.4845(13) Å,
c
=
5.3810(10)°, γ = 80.4010(10)°, Z = 4, M
r
= 783.26, V =
) = 0.71073 Å, T =
00 K, μ = 7.503 mm , 26533 reflections collected, 10173 unique
int = 0.0330), 8558 observed, R1(F ) = 0.0385 [IϾ2σ(I)],
) = 0.0879 (all data), GOF = 1.037.
3
–3
217.1(3) Å , Dcalcd. = 2.347 gcm , λ(Mo-K
α
–
1
(
R
o
2
wR2(F
o
CCDC-1410334 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.
General Procedure for Recycling Experiments: The reactions were
performed analogously to the procedure described above. After ev-
ery cycle a new 1 mmol of ketone was added and 0.05 mmol
tBuOK were added.
Computational Details: All calculations were done at the DFT level
[
Ir(CF
3
COO)I{κ C,CЈ,O,OЈ-bis(NHCOMe)}](BF
4
4
I
): The title com-
[18]
using the B3LYP approximation as implemented in the Gaussian
4
pound was prepared using complex [Ir
2
{κ C,CЈ,O,OЈ-bis-
as starting material. A solution of CF COOAg
45.5 mg, 0.22 mmol; dry acetone, 30 mL) was added under drop-
[19]
program package. Dispersion corrections were added by the D3
OMe
(
(
NHC
)}]BF
4
3
[20]
scheme developed by Grimme
for both energies and gradient
[21]
calculations. The def2-SVP basis set has been selected for geome-
try optimisations and calculation of Gibbs energy corrections at
wise an argon atmosphere at room temperature to a solution of the
cited compound (160.0 mg, 0.20 mmol) in dry acetone (30 mL).
The suspension was stirred for 1 h in the absence of light. The AgI
formed was separated by filtration and the filtrate was partially
concentrated under reduced pressure. Slow addition of diethyl ether
353.15 K and single point energy corrections were added using the
def2-TZVP basis set and the PCM approach for simulation of the
[22]
solvent effect. The nature of the stationary points has been con-
firmed by analytical frequency analysis, and transition states were
characterised by a single imaginary frequency corresponding to the
expected motion of the atoms.
(15 mL) afforded a light brown solid. The new compound was fil-
tered, washed with diethyl ether (3ϫ 15 mL) and dried under vac-
1
6
uum, yield 72% (106.5 mg, 0.14 mmol). H NMR (400 MHz, [D ]-
acetone): δ = 7.61 (d, JH,H = 2 Hz, 2 H, CHim), 7.52 (d, JH,H
=
2
J
4
1
1
6
Hz, 2 H, CHim), 6.77 (d, JH,H = 13 Hz, 1 H, NCH
H,H = 13 Hz, 1 H, NCH N), 4.72–4.49 (m, 8 H, NCH
.14 (s, 6 H, CH ]acetone): δ =
O) ppm. 13C NMR (101 MHz, [D
61.0 (q, 2
C,F = 37 Hz, CF COO), 123.5 (CHim), 121.4 (CHim), of Higher Education, Saudi Arabia, in establishment of the Centre
20.2 (, NCimN), 113.6 (q, JC,F = 293 Hz, CF COO), 75.7 (OCH ), of Research Excellence in Petroleum Refining & Petrochemicals at
3.9 (CH O), 61.7 (NCH N), 47.8 (CH
N) ppm. 19F NMR ([D
), 74.8 (s, 3 F, CF COO)
2
N), 6.64 (d, Acknowledgments
2
2 2
+ CH O),
3
6
The authors would like to acknowledge the support by the Ministry
J
3
1
3
2
3
2
2
6
]
KFUPM (KACST-funded project ART-32-68). The support under
the KFUPM–University of Zaragoza research agreement is also
acetone, 282 MHz): δ = –151.4 (s, 4 F, BF
4
3
Eur. J. Inorg. Chem. 2015, 4388–4395
4394
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
FULL PAPER
highly appreciated. This work was further supported by the Span-
ish Ministry of Economy and Competitiveness (MINECO/
FEDER) (CONSOLIDER INGENIO CSD2009-0050, CTQ2011-
[10] a) A. Binobaid, M. Iglesias, D. J. Beetstra, B. Kariuki, A.
Dervisi, I. A. Fallis, K. J. Cavell, Dalton Trans. 2009, 7099; b)
M. V. Jiménez, J. J. Pérez-Torrente, M. I. Bartolomé, V. Gierz,
F. J. Lahoz, L. A. Oro, Organometallics 2008, 27, 224; c) M. V.
Jiménez, J. Fernández-Tornos, J. J. Pérez-Torrente, F. J. Mod-
rego, S. Winterle, C. Cunchillos, F. J. Lahoz, L. A. Oro, Orga-
nometallics 2011, 30, 5493; d) M. V. Jiménez, J. Fernández-
Tornos, J. J. Pérez-Torrente, F. J. Modrego, P. García-Orduña,
L. A. Oro, Organometallics 2015, 34, 926.
27593 and CTQ2012-35665 projects) and the Diputación General
de Aragón (DGA/FSE-E07). V. P. thankfully acknowledges the re-
sources from the supercomputer “Memento”, technical expertise
and assistance provided by BIFI-ZCAM (Universidad de Zara-
goza).
[
11] As previously reported by us in ref.[6e], phenyl ethyl ether N-
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oxygen atoms of the phenyl ethyl ether wingtip groups are co-
ordinating and dissociating in solution at room temperature.
On the other hand, NHCs featuring methyl ethyl ether on the
peripheral nitrogen atoms are strongly coordinating tetratopic
ligands.
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Received: July 30, 2015
Published Online: August 19, 2015
Eur. J. Inorg. Chem. 2015, 4388–4395
4395
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim