Heteroditopic Ru(II)-And Ir(III)-NHC Complexes with Pendant 1,2,3-Triazole/Triazolylidene Groups: Stereoelectronic Impact on Transfer Hydrogenation of Unsaturated Compounds

Article abstract of DOI:10.1021/acs.organomet.9b00156

Imidazol-2-ylidene (ImNHC) and 1,2,3-Traizol-5-ylidene (tzNHC) have been established as important classes of carbene ligands in homogeneous catalysis. To develop Ru(II)/Ir(III) complexes based on these ligand systems considering their electronic as well as steric profiles for hydride transfer reactions, we employed chelating ligands featuring combinations of ImNHC and triazole-N or mesoionic tzNHC donors bridged by a CH₂ spacer with possible modifications at triazole backbone. In general, synthesized Ru(II) complexes were found to perform significantly better than analogous Ir(III) complexes in ketone and aldimine reduction. Among the Ru(II) complexes, electron-rich complexes 8/9 of the general formula [(p-cymene)(ImNHC-CH₂-TzNHC)Ru^II(Cl)]BF₄ with two different carbene donors (ImNHC and tzNHC) were found to perform appreciably better in ketone reduction than analogous complexes with a combination of ImNHC and triazole-N-donor ([(p-cymene)(ImNHC-CH₂-Tz-N)Ru^II(Cl)]BF₄; 4) explaining the electronic fine-Tuning of the catalytic systems. No appreciable variation in activity was observed between complexes 8 and 9 having almost similar electronic profiles. However, less bulky Ru(II) complex 9 with a triazole N-phenyl substituent is more suitable for aldimine reduction than is complex 8, having a triazole N-3,5-dimethylphenyl substituent that explains the steric influence in addition to electronic effect on the reduction process.

Full text of DOI:10.1021/acs.organomet.9b00156

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Heteroditopic Ru(II)− and Ir(III)−NHC Complexes with Pendant
1,2,3-Triazole/Triazolylidene Groups: Stereoelectronic Impact on
Transfer Hydrogenation of Unsaturated Compounds
Praseetha Mathoor Illam, S. N. R. Donthireddy, Sayantan Chakrabartty, and Arnab Rit*
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India
S
* Supporting Information
ABSTRACT: Imidazol-2-ylidene (ImNHC) and 1,2,3-traizol-5-ylidene
(tzNHC) have been established as important classes of carbene ligands in
homogeneous catalysis. To develop Ru(II)/Ir(III) complexes based on
these ligand systems considering their electronic as well as steric profiles
for hydride transfer reactions, we employed chelating ligands featuring
combinations of ImNHC and triazole-N or mesoionic tzNHC donors
bridged by a CH2 spacer with possible modifications at triazole backbone.
In general, synthesized Ru(II) complexes were found to perform
significantly better than analogous Ir(III) complexes in ketone and
aldimine reduction. Among the Ru(II) complexes, electron-rich complexes
8/9 of the general formula [(p-cymene)(ImNHC−CH2−tzNHC)-
RuII(Cl)]BF4 with two different carbene donors (ImNHC and tzNHC)
were found to perform appreciably better in ketone reduction than
analogous complexes with a combination of ImNHC and triazole-N-donor ([(p-cymene)(ImNHC−CH2−tz−N)RuII(Cl)]BF4;
4) explaining the electronic fine-tuning of the catalytic systems. No appreciable variation in activity was observed between
complexes 8 and 9 having almost similar electronic profiles. However, less bulky Ru(II) complex 9 with a triazole N-phenyl
substituent is more suitable for aldimine reduction than is complex 8, having a triazole N-3,5-dimethylphenyl substituent that
explains the steric influence in addition to electronic effect on the reduction process.
oxidation, olefin metathesis, hydro-arylation, transfer hydro-
genations, and so on.4a,d,5b,6b−f
INTRODUCTION
■
The role of N-heterocyclic carbenes (NHCs) as ancillary
ligands in homogeneous catalysis has earned increased
momentum since Arduengo’s report on first isolable carbene
in 1991.1 The ease of synthesis as well as tuning of the
stereoelectronic properties have made them the best alternative
for the classical ligands like phosphines.2 The glorious
advancement of NHC in the field of homogeneous catalysis
can be accredited to the robustness of their metal complexes
toward decomposition and the better catalytic activity as
compared to that of their analogous phosphine based systems
due to strong σ-donor properties.2d,3 A triazole-based congener
of classical Arduengo-type NHCs, which is commonly known
as mesoionic carbene (MIC), became popular in carbene
chemistry in the wake of intriguing reactivities and applications
pioneered by Martin Albrecht.4 Among MICs, 1,2,3-triazol-5-
ylidene (tzNHC) ligands have gained special interest in
transition metal catalysis due to their distinct σ-donor
properties, stronger than most of the classical ImNHCs
(imidazol-2-ylidenes),4d,5 and easy access through simple
Cu(I)-catalyzed Huisgen 1,3-cycloaddition reaction (CuAAC
reaction or click reaction) between an alkyne and an azide
which offers numerous possibilities to fine-tune their stereo-
electronic properties.5c,6a−d Transition metal complexes of
tzNHC ligand have been found to be efficient catalysts for
several important reactions such as C−X bond formation,
Chelating ligands such as classical (bis)ImNHCs,2e,7
ImNHC with secondary N-donor,8a−c,e and ImNHC-based
pincer ligands with phenyl/pyridyl spacer9 are well-known
structural motifs, and their complexes have found prominent
applications in organometallic chemistry/catalysis possibly due
to stability of the intermediate species generated during
catalytic cycles.5b,6d,8c Chelating ligands incorporating ImNHC
coupled with a tzNHC donor are relatively less explored,
although metal complexes of monodentate tzNHC ligand and
tzNHC with a secondary N-pyridyl or C-aryl moiety are
extensively studied.5b,c,6e,10,11 A very recent advancement in
this area is the development of heteroditopic ligands where
imidazol-2-ylidene is coupled to a 1,2,3-triazol-5-ylidine donor
as an alternative to classical chelating (bis)ImNHC ligands.12
A few reports of late transition metal complexes including
hetero(bimetallic) ones containing this type of bidentate
ImNHC∧tzNHC ligands are known and they are mostly
confined to palladium/rhodium metal centers.12,13a Therefore,
catalytic application of metal complexes of these unique
heteroditopic ligands are not well explored12b although this
might lead to efficient catalytic systems due to presence of two
Received: March 9, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00156
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a
Scheme 1. Synthesis of [1-H]Br, [2-H]Br, and [1-H]BF4/PF6 and Molecular Structure of [1-H]PF6
a
(a) Synthesis of imidazolium salts [1-H]Br, [2-H]Br, and [1-H]BF4/PF6: (i) CuI, NEt3, CH3OH/H2O, room temperature (RT), 12 h; (ii)
NH4BF4, CH3OH, RT, 12 h; (iii) KPF6, H2O, RT, 6 h. (b) Molecular structure of [1-H]PF6 with displacement parameters at the 40% probability
level. Hydrogen atoms except H14 and PF6 counterion are omitted for clarity. Selected bond distances (Å) and angles (deg): C14−N1 = 1.330(8),
C14−N2 = 1.308(9); N1−C14−N2 = 110.0(6).
a
Scheme 2. Synthesis of Complexes 3 and 4 and Molecular Structure of Cation in 3b
a
(a) Synthesis of pendant N-coordinated Ir(III)−/Ru(II)−NHC complexes 3 and 4: (i) [1-H]BF4, [(C5Me5)IrIIICl2]2, Cs2CO3, DCM, 65 °C, 36
h; (ii) [1-H]Br, [(C5Me5)IrIIICl2]2, Cs2CO3, NaBr, acetonitrile, 65 °C, 36 h; (iii) [1-H]BF4, [(p-cymene)RuIICl2]2, Cs2CO3, DCM, 65 °C, 36 h.
(b) Molecular structure of the cation in 3b (one of the two molecules in the asymmetric unit is shown; bond parameters of the other molecule are
almost similar) with displacement parameters at the 50% probability level. Hydrogen atoms except H17, counterions and solvent molecules are
omitted for clarity. Selected bond distances (Å) and angles (deg): Ir1−C11 = 2.027(9), Ir1−N3 = 2.097(7), Ir1−Ccentroid(Cp*) = 1.820, Ir1−Br1 =
2.5765(10); C11−Ir1−N3 = 83.9(3), N1−C11−N2 = 103.3(8), N1−C11−Ir1 = 130.7(7), N2−C11−Ir1 = 126.0(7).
strongly σ-donating carbene centers. Elsevier et al. have
described that the catalytic activity of Rh(I)/Ir(I) complexes of
this type of ImNHC∧tzNHC-based chelating ligands in
transfer hydrogenation of unsaturated compound is enhanced
from that of the ligand featuring one ImNHC with a secondary
triazole-N-donor which may be explained by a higher electron
density on the metal centers when a strongly σ-donating
tzNHC donor is present.12b Recently, we have demonstrated
that the fine-tuning of steric and electronic properties of
ancillary NHC ligands of ImNHC∧Cphenyl type can have
significant impact on Ru(II)-catalyzed hydride transfer
reactions.14f Therefore, we became interested in the effect of
stereoelctronic tuning of Ru(II)/Ir(III) complexes containing
this unsymmetrical bidentate ImNHC∧tzNHC chelating ligand
in hydride transfer reactions; herein, we report the syntheses
and characterizations of various stereoelectronically tunable
Ru(II) and Ir(III) complexes of unsymmetrical chelating
ligands containing ImNHC and triazole-N or 1,2,3-tzNHC
donors bridged by a CH2 spacer. The catalytic impact of ligand
stereoelectronic tuning is established by applying these
complexes in transfer hydrogenation of a library of unsaturated
compounds ketones and aldimines.
RESULTS AND DISCUSSION
■
Synthesis of Ligands. Reactions between imidazolium salt
A12a and aromatic azides via well-known copper-catalyzed
azide−alkyne cycloaddition (CuAAC), also known as click
reaction, gave corresponding imidazolium-triazole compounds
[1-H]Br/[2-H]Br as hygroscopic creamy white solids in
excellent yields of 93% (Scheme 1a). The proton signals at δ
= 10.21/10.45 ppm for the acidic imidazolium protons in [1/
2-H]Br are highly deshielded as compared to the triazole ring
protons which are observed at δ = 8.87/9.11 ppm, respectively.
This implies that selective deprotonation of the imidazolium
proton over the triazole proton is feasible. Therefore, the
ligand features two different types of donors, NHC (after
deprotonation of the imidazolium proton) and nitrogen donor,
in close proximity and thereby can act as a chelating ligand.
The bridging methylene group is observed at the expected
range of 5.91−5.93 ppm.15 Hygroscopic [1-H]Br salt was
converted to more stable [1-H]BF4/PF6 salts having non-
coordinating BF4/PF6 counteranions via salt-metathesis in
1
good yield of 62−92% (Scheme 1a). H NMR spectra of [1-
H]BF4/PF6 are consistent with the expected anion exchange as
the characteristic N−C(H)−N proton peak is slightly upfield-
shifted to 8.99 ppm ([1-H]BF4) and 9.24 ppm ([1-H]PF6)
from that of [1-H]Br (δ = 10.21 ppm) possibly due to lesser
hydrogen bonding interaction of BF4/PF6 ion with the
B
DOI: 10.1021/acs.organomet.9b00156
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imidazolium proton as compared to the bromide ion.16a Single-
crystal X-ray diffraction study of a crystal of [1-H]PF6 (Scheme
1b) confirms the expected structure.
cymene moiety which is originated due to loss of symmetry of
the complex via coordination of the triazole N-atom to the
ruthenium center. The other protons and carbons in the
complexes are observed in their expected regions and the
presence of BF4 counterions in the complexes 3a and 4 is
supported by 19F and 11B NMR.8d ESI mass spectra further
reinforce the formation of complexes, showing the major
intensity peak at m/z = 630.1994 (calculated m/z = 630.1968)
for 3a and only one peak at m/z = 538.1317 (calculated m/z =
538.1315) for 4 corresponding to the monopositive ion [M −
BF4]+ with isotopic patterns matching exactly with the
calculated patterns.
Single-crystal X-ray diffraction study of a crystal of 3b,
obtained by slow diffusion of ether into a saturated solution of
3b in DCM, unambiguously confirms the structure of the
complex. Coordination of both the CImNHC and triazole-N
atom to the Ir(III) center results in formation of a six-
membered chelate ring which in turn makes the H atoms
attached to the bridging C atom (C15; Scheme 2b)
diastereotopic,13b,d supporting the conclusion drawn from the
1H NMR spectra of the complexes. The ligand bite angle and
yaw distortion angle at the imidazolylidene ligand in 3b are
found to be around 83.9(3) and 2.4°, respectively. The Ir−
CImNHC bond distance (2.027(9) Å) is significantly shorter
than the Ir−Ntz (2.097(7) Å) implying stronger interaction
between the Ir(III) and NHC in comparison to the N-donor.
Synthesis of (bis)Azolium Salts [5/6-H2](BF4)2. Ligand
[1-H]Br/[2-H]Br has the possibility of triazole N-atom
alkylation to generate the corresponding triazolium salt
which after deprotonation of C5-triazole proton using suitable
base may generate the mesoionic carbene center12 capable of
coordinating to metal center. Therefore, in principle simple
alkylation may easily convert the N-donor coordination in
complexes 3 and 4 into a more strongly donating carbene
center which might have significant impact on catalytic
activities due to shuttle change in the electronic profile of
the metal centers. Keeping this in mind, [1-H]Br and [2-H]Br
were methylated using methylating agent Me3O+BF4− in DCM
under refluxing condition to generate ligand [5-H2](BF4)2 and
Synthesis and Characterization of Ru(II)−and Ir(III)−
NHC Complexes 3 and 4. Complex 3a was obtained by
reacting ligand [1-H]BF4 with [Ir(Cp*)Cl2]2 using Cs2CO3 as
base in DCM (Scheme 2a) and was isolated as yellow air-stable
solid in excellent yield of 89%. Related complex 3b with
different halide ions was also synthesized for crystallization
purpose from ligand [1-H]Br in the presence of excess NaBr
(Scheme 2a). In a similar way, Ru(II)−NHC complex 4 was
isolated as orange-yellow solid using [Ru(p-cymene)Cl2]2 as
the Ru(II) source in DCM (Scheme 2a). Complexes were
well-characterized by multinuclear NMR spectroscopy, mass
1
spectrometry, and X-ray diffraction studies for 3b. The H
NMR spectra of complexes 3 and 4 show the absence of
characteristic imidazolium protons at δ = 10.21 (for [1-H]Br)
and 8.99 ppm (for [1-H]BF4), respectively, confirming the
complete deprotonation and formation of the NHC complexes
as evident from their characteristic metal-bound NHC
signals13b−d at 153.0 (for 3a) and 172.8 (for 4) ppm (Table
1). Coordination of the triazole-N atom to the metal center is
Table 1. 13C{1H} NMR Chemical Shifts of Carbene−
Carbon in Complexes 3a, 4, and 7−9
CImNHC chemical shifts
(ppm)
CtzNHC chemical shifts
(ppm)
entry complex
1
2
3
4
5
3a
7
4
8
9
153.0
150.6
172.8
173.3
173.5
139.1
159.5
160.0
manifested by diastereotopic nature of the bridging CH2
protons (appearance of two doublets at 6.37 and 4.91 ppm,
2JH−H = 16.2 Hz for 3a and at 5.93 and 5.30 ppm, 2JH−H = 15.8
Hz for 4). This is further supported by the presence of four
aromatic protons in the region of 5.75−5.47 ppm for the p-
a
Scheme 3. Synthesis of Ir(III)/Ru(II) Complexes 7−9 and Molecular Structure of [5-H2]2+ in [5-H2](BF4)2
a
(a) Synthesis of Ir(III)/Ru(II) complexes of unsymmetrical imidazolylidene/1,2,3-triazolylidene based chelating dicarbene ligand. (i) Me3OBF4,
DCM, reflux, 24 h; (ii) [(C5Me5)IrIIICl2]2, Cs2CO3, DCM, 65 °C, 36 h; (iii) [(p-cymene)RuIICl2]2, Cs2CO3, DCM, 65 °C, 36 h. (b) Molecular
structure of [5-H2]2+ in [5-H2](BF4)2 with displacement parameters at 40% probability level. Hydrogen atoms except H14/H15 and counterions
are omitted for clarity. Selected bond distances (Å) and angles (deg): C8−N2 = 1.322(5), C8−N3 = 1.315(5), C5−N1 = 1.347(5), C5−C6 =
1.354(5); N2−C8−N3 = 108.7(3), N1−C5−C6 = 105.9(3).
C
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Figure 1. (a) Molecular structure of cations in 7 (left) and 8 (right, one of the two molecules in the asymmetric unit is shown; bond parameters of
the other molecule are almost similar) with displacement parameters at the 50% probability level. Hydrogen atoms and counterions are omitted for
clarity. Selected bond distances (Å) and angles (deg) for 7: Ir1−C14 = 2.030(3), Ir1−C15 = 2.040(3), Ir1−Ccentroid(Cp*) = 1.844, Ir1−Cl1 =
2.4317(8); C14−Ir1−C15 = 85.06(12), N1−C15−N2 = 104.0(3), N1−C15−Ir1 = 129.8(2), N2−C15−Ir1 = 126.3(2); Selected bond distances
(Å) and angles (deg) for 8: Ru1−C3 = 2.045(6), Ru1−C6 = 2.045(5), Ru1−Ccentroid(p-cymene) = 1.726, Ru1−Cl1 = 2.4170(15); C3−Ru1−C6 =
84.0(2), N4−C3−N5 = 103.7(5). (b) Ru(II) complex 10 of symmetrical (bis)ImNHC ligand.
[6-H2](BF4)2, respectively in good yields of 79−81% (Scheme
3a). These compounds feature two different azolium moieties
which are precursors for the generation of carbene. The
compounds are well-characterized by NMR spectroscopy, mass
spectrometry, and elemental analysis as well as by X-ray
The absence of acidic imidazolium (9.49/9.55 ppm) and
triazolium protons (9.25/9.26 ppm) which were present in [5/
6-H2](BF4)2 in the 1H NMR spectra of the complexes and the
presence of characteristic metal-bound carbene signals12,13c at
150.6 (Ir−CImNHC) and 139.1 (Ir−CtzNHC) ppm for 7 and at
1
173.3−173.5 (Ru−CImNHC) and 159.5−160.0 (Ru−CtzNHC
)
diffraction studies for [5-H2](BF4)2. The H NMR spectra
ppm for 8 and 9 (Table 1) reinforce the formation of
corresponding carbene complexes 7−9. The diastereotopic
nature of the bridging CH2 protons is also observed in these
complexes due to chelation similar to that observed in
complexes 3 and 4. Loss of symmetry of complexes 8 and 9
as evidenced by the presence of four different aromatic protons
in the region of 5.43−4.73 ppm for the p-cymene moiety
supports the coordination of unsymmetrical ligands 5 and 6 to
the ruthenium center, and the presence of BF4 counterions in
complexes 7−9 is supported by the 19F and 11B NMR.8d ESI
mass spectrometry further emphasizes the complex formation,
showing peaks at m/z = 644.2170 (calculated m/z = 644.2125
for 7) for the monopositive ion [(5)Ir(Cp*)Cl]+ and at m/z =
552.1477 and 524.1160 (calculated m/z = 552.1472 and
524.1158 for 8 and 9, respectively) for the monopositive ion
[(5/6)Ru(p-cymene)Cl]+ with isotopic patterns matching
perfectly with the calculated patterns.
The structures of the complexes are finally confirmed by X-
ray diffraction analysis of the crystals of 7 and 8 (Figure 1a)
obtained by slow diffusion of diethyl ether/n-pentane into a
saturated solution of the complexes in DCM. The pseudo-
octahedral geometry around the metal center is satisfied by two
carbene donors and one chloride ligand, and the remaining
three sites by Cp* or p-cymene ligands. Coordination of both
the CImNHC and CtzNHC to Ir(III)/Ru(II) center results in a six-
membered chelate ring formation with a bite angle of
85.06(12)° for 7 and 84.0(2) for 8 which fall in the expected
range.17 The Ir−CImNHC bond distance (2.030(3) Å) is slightly
shorter than Ir−CtzNHC bond length (2.040(3) Å), whereas the
Ru−CImNHC bond distance is found to be similar to that of the
Ru−CtzNHC (Figure 1). To the best of our knowledge, this is
the first report of Ru(II)- and Ir(III)-chelating complexes of
this type of heteroditopic bidentate ligand featuring two
different strongly σ-donating carbene centers (ImNHC and
1,2,3-tzNHC).
recorded in DMSO-d6 show the presence of two distinct acidic
azolium protons at 9.49/9.25 ppm ([5-H2](BF4)2) and 9.55/
9.26 ppm ([6-H2](BF4)2). Methylation makes the traizole
proton in [5/6-H2](BF4)2 relatively more acidic, suitable for
deprotonation to generate carbene center, as evident from the
downfield shift of the proton signal to 9.25/9.26 ppm, in
comparison to that observed in [1/2-H]Br (8.87 ppm/9.11
ppm). In addition, appearance of new peaks of three proton
intensity at 3.90−3.92 ppm in 1H NMR and at 38.8−39.2 ppm
in 13C{1H} NMR confirm the presence of N-methyl group on
the triazole ring.
The observed mass fragments corresponding to the
monopositive ion [M−BF4]+ at m/z = 370.1838 and
342.1519 for [5-H2](BF4)2 and [6-H2](BF4)2, respectively,
further support the formation of the compound which is
confirmed by single-crystal X-ray diffraction study of a crystal
of [5-H2](BF4)2 (Scheme 3b). This ligand system offers the
coordination possibility via two carbon centers after generation
of carbene (one of imidazol-2-ylidine type and the other of
1,2,3-triazol-5-ylidine type) in contrast to ligand [1/2-H]Br
which can coordinate via one imidazol-2-ylidine center and the
triazole N atom.
Synthesis and Characterization of Ru(II) and Ir(III)
Bis(Carbene) Complexes 7, 8, and 9. After having these
ligands in hand, Ir(III) complex 7 was synthesized by treating
ligand [5-H2](BF4)2 with [Ir(Cp*)Cl2]2 in the presence of
Cs2CO3 as base at 65 °C (Scheme 3a), and the complex was
isolated as a bright yellow air-stable solid. Corresponding
Ru(II) complexes 8 and 9, having varying steric congestion
around the Ru(II)-center, were also obtained in a similar way
from ligands [5-H2](BF4)2 and [6-H2](BF4)2, respectively,
using [Ru(p-cymene)Cl2]2 in DCM (Scheme 3a). Ir(III)
complex 7 is air- and moisture-stable, whereas Ru(II)
complexes 8 and 9 show slight decomposition under ambient
condition over time which gets accelerated in solvents like
chloroform. Complexes were well-characterized by multi-
nuclear NMR spectroscopy, mass spectrometry, and X-ray
diffraction studies for 7 and 8.
After having Ru(II) complexes of unsymmetrical hetero-
ditopic ligands featuring ImNHC donor coupled either to a tz-
N (complex 4) or a tzNHC donor (complexes 8/9), we also
synthesized a closely related Ru(II) complex of well-known
D
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symmetrical (bis)ImNHC ligand (complex 10; Figure 1b)
following a reported procedure7b,c to systematically study the
impact of different donors on the catalytic efficiency of this
series of structurally related complexes. To have some insight
into the electronic nature of these complexes, electrochemical
analyses were carried out, and it was found that the RuII/RuIII
redox potential for complex 4 (E1/2 = 0.976 V vs Fc) is higher
than those of complexes 8 and 9 (E1/2 = 0.637 V (8) and 0.663
V (9) vs Fc) (Figure S43). However, E1/2(RuII/RuIII) of
complex 10 (0.786 V vs Fc), having two ImNHC donors, falls
in between those of complexes 4 and 8/9.16b,c,17a These values
depict that complexes 8/9 with ImNHC/tzNHC donors
feature more electron-rich metal centers than complex 10
featuring symmetrical (bis)ImNHC donors, whereas complex
4 containing ImNHC/tz−N donors has the least electron
density around the metal center.
Table 2. Comparative Transfer Hydrogenation of
Acetophenone to 1-Phenyl Ethanol Using Different
a
Catalysts
b
entry
catalyst
mol % (time, h)
yield
1
2
3
4
5
6
4
8
9
10
3a
7
0.5 (1)/0.1 (2.5)
0.5 (1)/0.1 (2.5)
0.5 (1)/0.1 (2.5)
0.1 (2.5)
0.5 (1)
0.5 (1)
85/50
96/81
96/84
65
48
23
a
General conditions: ketone (0.5 mmol), catalyst (0.0025/0.0005
mmol), KtBuO (0.05 mmol), PrOH (3 mL), reflux temperature.
i
b
Catalytic Transfer Hydrogenation of Carbonyl Com-
pounds. The transfer hydrogenation process is more
advantageous over direct hydrogenation using molecular
hydrogen as it is more convenient and less hazardous. Besides
transition metal, recently main group element based systems
also have found applications in this field.14a−e Recently we have
demonstrated that the catalytic efficiency of orthometalated
Ru(II)−NHC complexes can be tuned by varying the
electronic profile of ancillary NHC ligands via modification
of p-substituents of the N-Ph ring of the NHC ligand.14f
Elsevier et al. has reported that the Rh(I)/Ir(I) complexes of
chelating dicarbene ligands bearing mixed ImNHC and
tzNHC are more active in transfer hydrogenation of
acetophenone than the corresponding NHC complexes with
N-donor instead of tzNHC donor which is due to presence of
more electron density on metal center in case of dicarbene
complexes.12b To examine the effect of shuttle ligand tuning in
catalytic efficiency of our heteroditopic Ru(II) complexes (4,
8, and 9) and Ir(III) complexes (3a and 7) featuring varying
electronic and steric profiles, the model transfer hydrogenation
of acetophenone in isopropanol to afford 1-phenyl ethanol was
investigated. Among the various bases tested (Table S1),
KtBuO was found to be the most suitable base for the transfer
hydrogenation of acetophenone in isopropanol. No significant
conversion was observed when the reaction was carried out
without any base or in the absence of Ru(II)-precatalysts,
which establishes that both catalyst and base are necessary for
this catalytic transformation (Table S1).
Under the standard transfer hydrogenation condition
(isopropanol as solvent as well as hydrogen source and
KtBuO as base) by deploying 0.5 mol % catalyst, Ru(II)-
dicarbene complexes 8 and 9 were found to exhibit the best
activity (almost quantitative yield in 1 h; Table 2). It is
important to note that the mixed ImNHC-tzNHC-Ru(II)
complexes performed better than their monocarbene analogue
(4) with a secondary triazole-N-coordination instead of
triazolylidene, and the difference in activity becomes more
prominent when the catalyst loading was reduced to 0.1 mol %
(entries 1−3). This may be attributed to the presence of
relatively electron-rich metal centers in complexes 8/9 rather
than in 4 originating from the superior electron-donating
ability of a triazolylidene compared to that of a N-
donor.4a−c,12b It is also worth mentioning that the activity of
complex 10 containing (bis)ImNHC ligand in acetophenone
reduction is in between those of complexes 4 and 8/9 (entries
1−4). As expected based on their similar electronic profiles, no
considerable difference in activity between complex 8 and 9
Determined by GC-MS using mesitylene as internal standard.
was observed (entries 2 and 3). Thus, variation of donor from
nitrogen to carbene (triazolylidene) can be perceived as
systematic approach to finely tune the electronic profile and
thereby optimizing the catalytic efficiency of these Ru(II)−
carbene complexes (4 and 8/9). This type of finding provides
useful information for the development of new efficient catalyst
systems.
In contrast, Ir(III) complexes show inferior activity
compared to their Ru(II) counterparts (entries 1−3 vs 5 and
6; Table 2) which is possibly due to inherent difference in
electronic nature between Ru(II) and Ir(III) metal center-
s.5b,18a Importantly, activity of mixed ImNHC−tzNHC−Ir(III)
complex 7 was found to be less than that of monocarbene
analogue 3a with secondary triazole-N-coordination (entries 5
and 6; Table 2) which has also been observed previously for
related systems.5b,18a The performance of mixed heteroditopic
Ru(II)−carbene complexes 8/9 in acetophenone reduction is
notably better than that of the previously reported structurally
similar complexes although the activity is still lower when
compared with that of most active Ru(II)−NHC catalysts
known.8c,18a,b
On the basis of these observations, complex 8 was chosen as
the precatalyst and applied to a small library of substrates
including (hetero)aromatic, cyclic, and acyclic aliphatic
ketones under the optimized conditions (0.5 mol % catalyst
and 10 mol % base loading; Table S1) to assess the substrate
scope of the present catalytic system, and satisfyingly, the
secondary alcohols were realized in excellent yields (Table 3).
This catalytic system works well for both electron-donating
(methyl, methoxy) and electron-withdrawing (chloro) sub-
stituents (entries 2−4), although the electron-rich carbonyl
substrate requires relatively more time to undergo reduction
than do the electron-poor substrates. This may be attributed to
the electrophilicity of the carbonyl groups as the presence of
electron-donating/-withdrawing substituents alters the electro-
philicity of the carbonyl group which controls the ease of
hydride transfer from the in situ generated metal−hydride
intermediate (see the “Mechanistic Investigations” section) to
carbonyl group and thus affecting the reduction process. No
hydro-dehalogenation of the aryl-halide bond was observed for
the halo-substituted acetophenone (entry 4).
2-Acetonaphthone and (substituted) propiophenone also
worked competently for both complexes 8 and 9, and almost
quantitative yields were observed (entries 5−7) although a
minor amount of dehalogenation was observed for 3-
E
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a
Table 3. Complex 8 Catalyzed Transfer Hydrogenation of Various (Hetero)aromatic and Aliphatic Ketones
a
b
General conditions: ketone (0.5 mmol), catalyst 8 (0.0025 mmol), KtBuO (0.05 mmol), iPrOH (3 mL), reflux temperature. Determined by GC-
c
d
e
f
MS using mesitylene as internal standard, average of two runs. Isolated yield. Complex 9 was used. Rest was dehalogenated product. Using 0.1
mol % catalyst.
chloropropiophenone (entry 7). Sterically crowded carbonyls,
e.g., fluorenone and benzophenone, are also smoothly reduced
to the corresponding alcohols (entries 8 and 9) in contrast to
some of the Ru(II)−NHC-based catalyst10a,19a,b which
perform sluggishly in the reduction of these bulky substrates.
Importantly, reduction of benzophenone can also be
performed with lower catalyst loading of 0.1 mol %, and the
corresponding alcohol was realized in 85% yield in 2 h (entry
9). Next, the relatively less-explored and possibly more
challenging heteroaromatic substrates 2-acetylfuran and 4-
acetylpyridine were also investigated (entries 10 and 11). 4-
Acetylpyridine is readily and efficiently reduced (completed
within 30 min, entry 11) in comparison to (substituted)
acetophenones (entries 1−4) which might be reflecting the
intrinsic electronic difference between the pyridyl and phenyl
rings.14f,19c This catalyst system also delivers competently for
aliphatic ketones (both cyclic and acyclic). Cyclohexanone
requires only 0.5 h for complete reduction (entry 14), whereas
2-hexanone and 1,3-diphenyl-2-propanone were quantitatively
hydrogenated in 1.5 and 4 h, respectively (entries 12 and 13).
Catalytic Transfer Hydrogenation of Imines. Selective
production of amines (known to be important building blocks
for organic synthesis and biologically active molecules in
medical/pharmaceutical sciences as well as for fragrance
industries)20 via transfer hydrogenation of imines is described
to be difficult as compared to that of aldehydes and ketones.
Possibly because of that, a very few reports on ruthenium
based systems are known with broad substrate scope.14f,19d,21
F
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Danopoulos et al. has early reported the reduction of imines
via transfer hydrogenation strategy employing a bis(carbene)−
ruthenium(II) complex which utilizes a tridentate pyridine
based pincer lignad.22 Surprisingly, after that Ru(II)−NHC
complex catalyzed imine reduction did not progress much as
evident from a very limited report in this area with narrow
substrate scope.18a,23 After our success in transfer hydro-
genation of ketones, we then focused on aromatic aldimines
which are comparatively more challenging because of their
stability being lower than that of ketimines.24
To study the suitability of these chelating Ru(II)−carbene
complexes in the aldimines reduction, we have chosen N-
benylideneaniline as model substrate. To our delight, it was
completely hydrogenated to N-benzylaniline in 4 h using
catalyst 8/9 (1 mol %) under transfer hydrogenation
conditions used for ketone reductions (entries 1−3, Table 4)
not show an appreciable influence on the reduction process
(entries 1 vs 7 and 4 vs 8). More sterically demanding and
electronically different substrate α-naphthalene-based aldimine
was reduced slowly, and quantitative reaction was achieved in 8
h (entry 9). In contrast to the aromatic aldimines, the
analogous aliphatic one was found to be a relatively poor
substrate as expected based on its relatively lower electro-
philicity and hydrogenation to a decent amount was attained
with longer reaction time and higher catalyst loading (entry
10).
Mechanistic Investigations. In order to probe the
reaction mechanism, various controlled experiments were
carried out. The presence of excess p-cymene or other arenes
(40 equiv with respect to catalyst) did not result in any
appreciable change of 1-phenyl ethanol formation catalyzed by
complex 8, suggesting that removal of p-cymene may not be
involved in the catalytic cycle; thus, the current transfer
hydrogenation cycle possibly follows an outersphere mecha-
nism.25 The presence of excess elemental mercury in the
reaction mixture (3 equiv with respect to substrate) did not
affect the yield of the reaction (92% yield of 1-phenyl ethanol
using 8) which suggests the homogeneous nature of the
catalytic system.19a,26
Table 4. Comparative Transfer Hydrogenation of N-
Benzylideneaniline to N-Benzylaniline Using Different
a
Catalysts
Ru−H resonances (−9.75 ppm, 8 and −9.74 ppm, 9) in the
1H NMR spectra of the reaction mixture of complexes 8/9
with KtBuO in refluxing isopropanol for 20 min evidences
participation of a Ru(II)−H intermediate (Figures S44 and
S45). This is further validated by the significant conversion of
acetophenone to the corresponding alcohol at ambient
temperature when treated with in situ generated Ru−H
species from the complex 8. On the basis of these basic
mechanistic investigations and the related literature re-
ports,14f,19b,25a,b a plausible mechanism via hydride intermedi-
ate is proposed for the transfer hydrogenation process
(Scheme 4). In the first step, precatalyst 8/9 reacts with
b
entry
catalyst
time (h)
yield
1
2
3
3
5
4
8
9
3a
7
4/2
4/2
4/2
2
90/44
98/87
100/92
14
2
50
a
General conditions: imine (0.3 mmol), catalyst (0.003 mmol),
b
KtBuO (0.06 mmol), PrOH (3 mL), reflux temperature. Deter-
i
mined by GC-MS using mesitylene as internal standard.
although the activity is substantially less compared to that of
ketones (Table 4 vs Table 2). When the reaction time was
reduced to 2 h, an appreciable difference in activity was
perceived between mixed ImNHC-tzNHC-Ru(II) complexes
8/9 and their monocarbene analogue (4) with secondary
triazole-N-coordination instead of triazolylidene (Table 4)
again reflecting the difference in electron richness between
metal centers similar to that observed for ketone. Complex 9
performs significantly better than most of the efficient Ru(II)−
NHC catalysts known for this process.14f,18a,23b,c Electronically
similar complexes 8 and 9 exhibit almost similar activity
(entries 8 and 9, Table 4) as per our expectation for sterically
relaxed aldimines but for bulkier substrates (entries 5 and 9,
Table 5) complex 9 performs significantly better than 8
reflecting the difference in steric profiles between the
complexes. In contrary, Ir(III) complexes 3a and 7 show
relatively less activity as compared to their Ru(II)-analogues
(entries 1−3 vs 5−6; Table 4).
On the basis of these catalytic performances, we proceeded
with precatalyst 9 to assess the scope of this reaction by
applying it to a variety of aldimines featuring variations both at
the C- and N-phenyl rings (Table 5). Aldimines with varying
functionalization unveiled the similar trends as observed for
the ketone substrates: Electron-donating substituents at the C-
phenyl ring slow down the reduction, whereas electron-
withdrawing groups facilitate the reduction process (entries 2−
4). Interestingly, the reaction rate was much affected by the
position of electron-withdrawing substituents at the C-phenyl
ring (entry 4 vs 6), and the substituents on the N-Ph ring did
i
KiPrO (produced via reaction of KtBuO with PrOH) to
generate the ruthenium(II)−isopropoxide complex which then
may undergo β-hydride elimination to form a Ru(II)−H
species along with the elimination of acetone. Carbonyl
substrate is reduced via hydride transfer in next step possibly
with the assistance of isopropanol followed by elimination of
product alcohol under regeneration of catalyst. We believe that
a similar pathway is also followed for imine hydrogenation.
CONCLUSION
■
In conclusion, we have synthesized and well-characterized
several stereoelectronically tunable Ru(II) and Ir(III) com-
plexes containing ImNHC-based chelating ligands and
described that substantial tuning of catalytic efficiency of
these metal complexes in transfer hydrogenation of unsatu-
rated compounds is possible by altering the stereoelectronic
profile of the ancillary carbene ligands. The Ru(II) complexes
(8/9) were found to be superior catalysts as compared to their
structurally similar Ir(III) complex (7). The presence of
chelating ligand with mixed carbene donors (ImNHC and
tzNHC, complex 8/9) is found to be more effective for Ru(II)-
catalyzed reduction of ketones than that of ligand featuring
either only ImNHC donors (complex 10) or an ImNHC
donor coupled with a secondary N-donor (complex 4)
revealing electronic influence of the metal center in the
reduction process. Significant steric influence on the catalytic
activity was observed when sterically different Ru(II) catalysts
8 and 9 were employed in reduction of bulky aromatic
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a
Table 5. Catalytic Transfer Hydrogenation of Various Aromatic Aldimines Using Complex 9
a
b
i
General conditions: imine (0.3 mmol), complex 9 (0.003 mmol), KtBuO (0.06 mmol), degassed PrOH (3 mL), reflux temperature. Reaction
c
d
e
f
time. Determined by GC-MS using mesitylene as internal standard, average of two runs. Isolated yield. Catalyst 8 was used. Using 0.006 mmol
of catalyst 9.
ESI-MS spectra were recorded with a Micromass Q-TOF Mass
spectrometer or an Agilent 6545A Q-TOF Mass spectrometer and
elemental analysis were performed using PerkinElmer 24000 instru-
ment. The starting materials [(p-cymene)RuIICl2]2,27a [(Cp*)-
IrIIICl2]2,27b imidazolium salt A,12a phenyl azide,27c and aldimines27d,e
were prepared according to literature procedures. All other chemicals
were purchased from commercial sources and used as received
without further purification. Caution! Organic azides are high-energy
density materials with explosive properties. Therefore, appropriate care
should be taken while handling these materials.
Synthesis of 3,5-Dimethylphenyl Azide. 3,5-Dimethylaniline
(1 g, 8.25 mmol) was added to 40 mL of 10% HCl solution at 0 °C.
Sodium nitrite (0.68 g, 9.9 mmol) was then added in small portions to
the mixture. The reaction mixture was stirred for 1 h keeping the
temperature constant at 0 °C. Sodium azide (0.7 g, 10.73 mmol) was
then added to the solution in small portions, and the mixture was
stirred for 30 min at room temperature. It was then extracted with n-
hexane, and the hexane layer was washed with saturated brine solution
and dried over anhydrous MgSO4. After filtration, all volatiles from
aldimines. We believe that this kind of stereoelectronic tuning
of the catalytic activity of Ru(II)−NHC complexes might have
potential impact in future catalyst development. Further
applications of these complexes in other organic trans-
formations are underway in our lab.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were performed
under argon/N2 atmosphere using either standard Schlenk line or
Glove box techniques. Glassware was oven-dried at 130 °C overnight
before use. The solvents used for the synthesis were dried, distilled,
and degassed by standard methods and stored over 4 Å molecular
sieves. NMR measurements were performed on Bruker 400 and 500
1
MHz FT-NMR spectrometers. The chemical shifts in the H NMR
spectra were referenced to the residual proton signals of the
1
deuterated solvents (CDCl3, H 7.26 ppm and 13C{1H} 77.16 ppm;
DMSO-d6, 1H 2.50 ppm and 13C{1H} 39.52 ppm) and reported
relative to tetramethylsilane. Coupling constants are expressed in Hz.
H
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MHz, CDCl3) δ 140.7 (Im N−CH−N), 139.7, 137.1, 136.4, 130.7,
123.7, 123.3, 122.8, 118.4, 44.4 (CH2), 36.9 (Im N−CH3), 21.2 (Ar−
CH3). MS (ESI, positive ions): m/z 268.1597 (calculated for [M −
Br]+ 268.1562). Consistent CHN data could not be obtained due to
hygroscopic nature of the compound.
Scheme 4. Plausible Catalytic Cycle for the Transfer
Hydrogenation Reaction
Synthesis of [2-H]Br. Compound [2-H]Br was synthesized
following a procedure similar to that used for [1-H]Br using phenyl
azide (1 g, 10.7 mmol) and compound A (2.2 g, 10.7 mmol). The
compound was obtained as cream-colored hygroscopic solid in
excellent yield of 93% (3.1 g, 9.95 mmol). 1H NMR (400 MHz,
CDCl3) δ 10.45 (s, Im N−CH−N, 1H), 9.11 (s, Trz C−CH−N, 1H),
7.79 (s, 1H), 7.77 (s, 1H), 7.73 (s, Im H4/H5, 1H), 7.50 (t, 3JH−H
=
3
7.7 Hz, 2H), 7.42 (t, JH−H = 7.4 Hz, 1H), 7.23 (s, Im H4/H5, 1H),
5.93 (s, CH2, 2H), 4.02 (s, Im N−CH3, 3H). 13C{1H} NMR (101
MHz, CDCl3) δ 141.0, 137.7, 136.7, 130.0, 129.3, 124.1, 123, 122.9,
120.8, 44.6 (CH2), 37.0 (N−CH3). MS (ESI, positive ions): m/z
240.1274 (calculated for [M − Br]+ 240.1249). Consistent CHN data
could not be obtained due to hygroscopic nature of the compound.
Synthesis of [1-H]PF6. Compound [1-H]Br (1.0 g, 2.88 mmol)
was dissolved in 10 mL of water in a test tube. A solution of potassium
hexafluorophosphate (0.556 g, 3.02 mmol) dissolved in water was
added to the previous solution. The resulting solution was stirred for
2 h, and during that time a white-colored compound started to
precipitate. After the reaction, the precipitated solid was washed with
diethyl ether (3 × 10 mL) and dried in vacuo to give the compound
as creamy white stable solid. Yield: 1.1 g (2.66 mmol, 92%). Single
crystals suitable for X-ray crystal structure determination were
obtained via slow diffusion of diethyl ether into a saturated solution
of the complex in methanol at 4 °C. 1H NMR (500 MHz, DMSO-d6)
δ 9.24 (s, Im N−CH−N, 1H), 8.86 (s, Trz C−CH−N, 1H), 7.83 (s,
Ar−H, 1H), 7.75 (s, Im H4/H5, 1H), 7.51 (s, Ar−H, 2H), 7.17 (s,
Im H4/H5, 1H), 5.64 (s, CH2, 2H), 3.88 (s, Im N−CH3, 3H), 2.38
(s, Ar−CH3, 6H) ppm. 13C{1H} NMR (126 MHz, DMSO-d6) δ 141.5
(Im N−CH−N), 139.5, 136.8, 136.3, 130.3, 123.9, 122.9, 122.5,
117.8, 43.6 (CH2), 35.9 (Im N−CH3), 20.8 (Ar−CH3) ppm. 19F
NMR (471 MHz, DMSO-d6) δ −70.0 (d, 1JP−F = 707.6 Hz) ppm. 31
P
=
the filtrate were removed in vacuo, and a red viscous liquid was
obtained which was further purified by column chromatography using
n-hexane as eluent, giving the desired product as a yellow oil. Purity
was confirmed from the 1H and 13C{1H} NMR spectroscopic studies.
1
NMR (202 MHz, DMSO-d6) δ −133.65 to −154.79 (sept, JP−F
707.6 Hz) ppm. MS (ESI, positive ions): m/z 268.1588 (calculated
for [M − PF6]+ 268.1562).
1
Synthesis of [1-H]BF4. Compound [1-H]Br (0.2 g, 0.58 mmol)
was dissolved in 10 mL of methanol in a test tube. A solution of
ammonium hexafluoroborate (0.067 g, 0.64 mmol) in methanol was
added to the previous solution. The resulting solution was stirred
overnight. After the reaction, the methanol was evaporated, and the
residue obtained was washed with diethyl ether (3 × 10 mL) and
dried in vacuo to yield the compound as creamy white stable solid.
Yield: 0.9 g (6.11 mmol, 74%). H NMR (400 MHz, CDCl3) δ 6.80
(s, Ar−H, 1H), 6.68 (s, Ar−H, 2H), 2.33 (s, Ar−CH3, 6H) ppm.
13C{1H} NMR (101 MHz, CDCl3) δ 139.6, 127.1, 117.1, 21.1 ppm.
Synthesis of 1-Methyl-3-(prop-2-yn-1-yl)-1H-imidazol-3-
ium Bromide (A). A was synthesized following a reported
procedure.12a 1-Methyl-1H-imidazole (2.0 g, 24.36 mmol) was
added to a round-bottomed flask followed by 25 mL of acetonitrile.
To this mixture, was slowly added approximately 2-fold excess of an
80 wt % solution (in toluene) of propargyl bromide (7.30 g, 61.38
mmol). The solution was refluxed for 24 h, after which it was cooled
to the room temperature. All the volatiles were removed in vacuo, and
the resultant product was washed with diethyl ether (2 × 20 mL). The
compound was then dried in vacuo and obtained as a brown oily
liquid. Yield: 3.94 g (19.7 mmol, 81%). Formation of the product was
confirmed by comparing the NMR and mass data with those reported
one.
1
Yield: 0.125 g (0.35 mmol, 60%). H NMR (500 MHz, CDCl3) δ
8.99 (s, Im N−CH−N, 1H), 8.53 (s, Trz C−CH−N, 1H), 7.56 (s,
1H), 7.35 (s, 2H), 7.23 (s, 1H), 7.04 (s, 1H), 5.60 (s, CH2, 2H), 3.93
(s, Im N−CH3, 3H), 2.36 (s, Ar−CH3, 6H). 13C{1H} NMR (126
MHz, CDCl3) δ 140.0 (Im N−CH−N), 137.1, 136.6, 130.9, 123.4,
122.7, 118.5, 44.5 (CH2), 36.6 (Im N−CH3), 21.4 (Ar−CH3) ppm.
19F NMR (471 MHz, CDCl3) δ −150.52 (s), −150.57 (s) ppm. 11B
NMR (160 MHz, CDCl3) δ −0.87 (s) ppm. MS (ESI, positive ions):
m/z 268.1578 (calculated for [M − BF4]+ 268.1562. Anal. Calcd (%)
for C15H18N5BF4·H2O: C 48.28%, N 18.77%, H 5.40%. Found: C
48.54%, N 18.39%, H 4.67%.
Synthesis of [1-H]Br. A 50 mL portion of MeOH and H2O (1:1)
was added to a flask containing 3,5-dimethylphenyl azide (1 g, 6.79
mmol) and compound A (1.50 g, 7.47 mmol). The resulting solution
was stirred for 10 min to make it homogeneous, and CuI (0.065 g,
0.34 mmol) and NEt3 (1.37 g, 13.58 mmol) were added to it. The
solution was then stirred overnight at room temperature, and the
reaction mixture was extracted with chloroform. The organic layer was
dried using anhydrous MgSO4 and filtered, and the solvent was
removed in vacuo. The obtained residue was washed with diethyl
ether (3 × 10 mL) and dried under vacuum to yield the compound as
Synthesis of [IrIII(1)(Cp*)Cl]BF4, 3a. Compound [1-H]BF4 (0.05
g, 0.109 mmol) and Cs2CO3 (0.107 g, 0.328 mmol) were taken in a
predried pressure tube. [Ir(Cp*)Cl2]2(0.044 g, 0.055 mmol) were
added to the pressure tube under inert condition followed by addition
of dry dichloromethane. It was then closed and stirred for 36 h at 65
°C. After that, the solution was filtered through a small pad of Celite,
and the filtrate was concentrated. Diethyl ether was added to get a
pale yellow solid. The powder was washed with diethyl ether (2 × 4
mL) before complete drying to get an air-stable pale yellow solid.
1
cream-colored hygroscopic solid. Yield: 2.2 g (6.34 mmol, 93%). H
1
NMR (400 MHz, CDCl3) δ 10.21 (s, Im N−CH−N, 1H), 8.87 (s,
Trz C-CH−N, 1H), 7.78 (s, Ar−H, 1H), 7.39 (s, Im H4/H5, 1H),
7.31 (s, Ar−H, 2H), 6.99 (s, Im H4/H5, 1H), 5.91 (s, CH2, 2H), 4.01
(s, Im N−CH3, 3H), 2.31 (s, Ar−CH3, 6H). 13C{1H} NMR (101
Yield: 0.070 g (0.098 mmol, 90%). H NMR (500 MHz, CDCl3) δ
8.92 (s, Trz C−CH−N, 1H), 7.58 (d, 3JH−H = 2.0 Hz, Im N−CH−C,
1H), 7.36 (s, 1H), 7.11 (s, 1H), 7.04 (d, 3JH−H = 2.0 Hz, Im N−CH−
2
2
C, 1H), 6.37 (d, JH−H = 16.2 Hz, CH2, 1H), 4.91 (d, JH−H = 16.2
I
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Hz, 1H, CH2), 3.91 (s, Im N−CH3, 3H), 2.39 (s, Ar-CH3, 6H), 1.75
(s, Cp*, 15H). 13C NMR (126 MHz, CDCl3) δ 153.0 (Im CNHC),
141.7, 140.3, 136.1, 132.2, 123.7, 123.5, 123.0, 119.0, 91.2 (Cp*−C),
−1.31 ppm. 19F NMR (471 MHz, DMSO-d6) δ −148.30 (s), −148.36
(s) ppm. MS (ESI, positive ions): m/z 370.1838 (calculated for [M −
BF4]+ 370.1829). Anal. Calcd (%) for C16H21N5B2F8: C 42.05%, N
15.33%, H 4.63%; Found: C 41.69%, N 14.90%, H 4.04%.
44.9 (CH2), 37.6 (Im N−CH3), 21.4 (Ar−CH3), 9.6 (CH3-Cp*). 11
B
NMR (160 MHz, CDCl3) δ −0.82 (s). 19F NMR (471 MHz, CDCl3)
δ −151.89 (s), −151.94 (s). MS (ESI, positive ions): m/z 630.1994
(calculated for [M − BF4]+ 630.1968). Anal. Calcd (%) for
C25H32N5IrClBF4·H2O: C 40.85%, N 9.53%, H 4.66%; Found: C
40.35%, N 8.37%, H 4.24%.
Synthesis of [6-H2](BF4)2. Compound [6-H2](BF4)2 was
synthesized following the same procedure used for [5-H2](BF4)2
using [2-H]Br (0.10 g, 0.312 mmol) and trimethyloxonium
tetrafluoroborate (0.115 g, 0.78 mmol). Yield: 0.106 g (0.247
1
mmol, 79%). H NMR (500 MHz, DMSO) δ 9.57 (s, Im N−CH−
Synthesis of [IrIII(1)(Cp*)Br]Br, 3b. Compound [1-H]Br (0.05 g,
0.144 mmol) and Cs2CO3 (0.094 g, 0.288 mmol) were taken in a
predried Schlenk inside the Glove box. [Ir(Cp*)Cl2]2(0.063 g, 0.079
mmol) and NaBr (0.074 mg, 0.72 mmol) were added to the Schlenk
outside the box under inert condition followed by addition of distilled
CH3CN. The Schlenk was closed and stirred for 24 h at 65 °C. After
completion of reaction, all the volatiles were removed in vacuo, and
the obtained residue was extracted with DCM. The filtrate was dried
and washed with diethyl ether (2 times) before completely drying to
N,1H), 9.26 (s, Trz C−CH−N, 1H), 7.96 (d, Ar−H, 3JH−H = 8.2 Hz,
2H), 7.83 (d, 3JH−H = 8.2 Hz, 2H), 7.77 (m, Ar−H, Im H4/H5, 3H),
5.95 (s, CH2, 2H), 4.45 (s, Im N−CH3, 3H), 3.90 (s, Trz N−CH3,
3H). 13C NMR (126 MHz, DMSO) δ 138.5 (Im N−CH−N), 137.7,
134.5, 132.0, 130.6, 129.2, 124.2, 122.7, 121.4, 40.6 (CH2), 38.8
(CH2), 36.1 (Im N−CH3), 30.7. 19F NMR (471 MHz, DMSO) δ
−148.28, −148.33 ppm. 11B NMR (160 MHz, DMSO) δ −1.31 ppm.
MS (ESI, positive ions): m/z 342.1519 (calculated for [M − BF4]+
342.1516). Anal. Calcd (%) for C14H17N5B2F8: C 39.20%, N 16.33%,
H 3.99%; Found: C 38.84%, N 15.74%, H 3.30%.
1
get a yellow solid. Yield: 0.072 g (0.095 mmol, 66%). H NMR (400
Synthesis of [IrIII(5)(Cp*)Cl]BF4, 7. Compound [5-H2](BF4)2
(0.05 g, 0.11 mmol) and Cs2CO3 (0.106 g, 0.327 mmol) were
transferred to a predried Schlenk under argon atmosphere. [Ir(Cp*)-
Cl2]2(0.048 g, 0.061 mmol) and dichloromethane was added to the
Schlenk. The Schlenk was closed and stirred for 36 h at 65 °C. After
completion of reaction, the all the volatiles were removed in vacuo,
and the obtained residue was dissolved in DCM and filtered through a
small pad of Celite. The filtrate was dried, washed with diethyl ether
(2 times), and then dried in vacuo to get a yellow air-stable solid.
MHz, CDCl3) δ 9.09 (s, Trz C−CH−N, 1H), 7.66 (s, 1H), 7.34 (s,
2H), 7.09 (s, 2H), 6.66 (d, 2JH−H = 15.5 Hz, 1H, CH2), 4.94 (d, 2JH−H
= 15.5 Hz, CH2, 1H), 3.87 (s, Im N−CH3, 3H), 2.37 (s, Ar−CH3,
6H), 1.77 (s, Cp*, 15H). 13C NMR (101 MHz, CDCl3) δ 151.0 (Im
CNHC), 141.9, 140.1, 136.1, 131.9, 124.3, 123.9, 122.4, 118.9, 91.3
(Cp*−C), 46.2 (CH2), 38.4 (Im N−CH3), 21.4 (Ar−CH3), 9.9
(CH3−Cp*). MS (ESI, positive ions): m/z = 674.1438 (calculated for
[M − Br]+ 674.1455). Anal. Calcd (%) for C25H32N5IrBr2·DCM: C
37.20%, N 8.34%, H 4.08%. Found: C 37.40%, N 9.34%, H 3.98%.
Synthesis of [RuII(1)(p-cymene)Cl]BF4, 4. Compound [1-
H]BF4 (0.04 g, 0.11 mmol), Cs2CO3 (0.106 g, 0.327 mmol) and
[Ru(p-cymene)Cl2]2 (0.034 g, 0.056 mmol) were added to a predried
Schlenk flask under argon atmosphere. THF was added to the Schlenk
flask under argon atmosphere, and the reaction mixture was stirred for
36 h at 65 °C. After the reaction, all volatiles were evaporated, and the
residue was extracted with DCM. The solution was filtered through
Celite, and the filtrate was concentrated to 2 mL. Hexane was added
to precipitate the compound. The obtained solid was dried in vacuo
to get dark yellow solid. Yield: 0.072 g (0.095 mmol, 66%). 1H NMR
(500 MHz, CDCl3) δ 8.56 (s, Trz C−CH−N, 1H), 7.41 (s, 1H), 7.35
1
Yield: 0.08 g (0.104 mmol, 95%). H NMR (500 MHz, CDCl3) δ
7.87 (s, Ar−H, 2H), 7.49 (s, Im N−CH−C, 1H), 7.17 (s, Ar−H, 1H),
7.11 (s, Im N−CH−C, 1H), 5.70 (d, 2JH−H = 16.8 Hz, CH2, 1H), 5.50
(d, 2JH−H = 16.8 Hz, CH2, 1H), 4.36 (s, Trz N−CH3, 3H), 3.91 (s, Im
N−CH3, 3H), 2.41 (s, Ar-CH3, 6H), 1.48 (s, 15H, CH3−Cp*) ppm.
13C NMR (126 MHz, CDCl3) δ 150.6 (Im Ccarbene), 139.1 (Trz
C
carbene), 138.9, 138.4, 135.4, 131.9, 124.4, 123.5, 123.3, 92.6 (Cp*−
C), 44.0 (CH2), 38.0 (Trz N−CH3), 37.4 (Im N−CH3), 21.3 (Ar−
CH3), 9.1 (CH3−Cp*) ppm. 11B NMR (160 MHz, DMSO-d6) δ
−0.93 ppm. 19F NMR (471 MHz, DMSO-d6) δ −151.75 (s), −151.81
(s) ppm. MS (ESI, positive ions): m/z 644.2170 (calculated for [M −
BF4]+ 644.2125).
2
(s, 2H), 7.11 (s, 1H), 7.07 (s, 1H), 5.91 (d, JH−H = 15.8 Hz, CH2,
1H), 5.75 (d, 3JH−H = 5.8 Hz, p-cymene−Ph, 1H), 5.64 (d, 3JH−H= 5.8
Synthesis of [RuII(5)(p-cymene)Cl]BF4, 8. Compound [5-
H2](BF4)2 (0.05 g, 0.11 mmol), Cs2CO3 (0.106 g, 0.327 mmol),
and [Ru(p-cymene)Cl2]2 (0.034 g, 0.056 mmol) were added to a
predried pressure tube under Ar atmosphere. DCM was added to the
pressure tube, and the reaction mixture was stirred for 36 h at 65 °C.
The solution was filtered over Celite, and the filtrate was
concentrated. Hexane was added to precipitate the compound. The
obtained solid was dried in vacuo to isolate the compound as dark
yellow solid. Yield: 0.048 g (0.075 mmol, 68%). 1H NMR (500 MHz,
CDCl3) δ 7.72 (s, Ar−H, 2H), 7.45 (s, Im N−CH−C, 1H), 7.28
Hz, p-cymene−Ph, 1H), 5.59 (d, 3JH−H = 5.8 Hz, p-cymene−Ph, 1H),
3
2
5.47 (d, JH−H = 5.8 Hz, p-cymene−Ph, 1H), 5.29 (d, JH−H = 15.8
Hz, CH2, 1H), 3.98 (s, Im N−CH3, 3H), 2.89 (sept, 3JH−H = 7.0 Hz,
p-cymene−iPr, 1H), 2.39 (s, 6H), 2.17 (s, p-cymene−Me, 3H), 1.29
(dd, J = 7.0 Hz, p-cymene−iPr, 6H) ppm. 13C{1H} NMR (126 MHz,
CDCl3) δ 172.8 (Im CNHC), 142.9, 140.2, 136.1, 131.9, 123.8, 123.6,
122.5, 118.8, 112.1, 101.8 (Car−CH3(cym)), 88.1 (Car−H(cym)),
87.2 (Car−H(cym)), 86.8 (Car−H(cym)), 83.5 (Car−H(cym)), 45.1
(CH2), 38.5 (Im−NCH3), 31.5 (CH(CH3)2), 23.7, 21.5, 21.4, 19.3
(CH3−Car(cym)) ppm. MS (ESI, positive ions): m/z 538.1317
(calculated for [M − BF4]+ 538.1315).
2
(Ar−H, 1H), 7.17 (s, Im N−CH−C, 1H), 5.72 (d, JH−H = 16.8 Hz,
CH2, 1H), 5.45 (d, 2JH−H = 16.8 Hz, CH2, 1H), 5.43 (d, 3J H−H = 6.2
3
Synthesis of [5-H2](BF4)2. Compound [1-H]Br (0.10 g, 0.287
mmol), trimethyloxonium tetrafluoroborate (0.148 g, 0.718 mmol),
and dry dichloromethane (4 mL) were mixed in a 25 mL pressure
tube. The tube was closed, and the reaction mixture was heated at 80
°C for 24 h. All the volatiles were removed in vacuo, CH3OH (4 mL)
was added, and the resulting mixture was stirred at room temperature
for 0.5 h in open air to decompose the excess oxonium salt. All the
volatiles were then removed in vacuo, and after washing the residue
with diethyl ether (3 × 5 mL) and a minimal amount of
dichloromethane, the product was obtained as a white stable solid.
Yield: 0.106 g (0.232 mmol, 81%). 1H NMR (400 MHz, DMSO-d6) δ
9.49 (s, Im N−CH−N, 1H), 9.25 (s, Trz C−CH−N, 1H), 7.84 (s,
Ar−H, 2H), 7.60 (s, Ar−H/Im H4/H5, 2H), 7.40 (s, Im H4/H5,
1H), 5.94 (s, CH2, 2H), 4.45 (s, Im N−CH3, 3H), 3.92 (s, Trz N−
CH3, 3H), 2.43 (s, Ar−CH3, 6H) ppm. 13C{1H} NMR (126 MHz,
DMSO-d6) δ 140.4 (Im N−CH−N), 138.5, 137.7, 134.5, 133.2,
128.8, 124.2, 122.7, 118.6, 40.7 (CH2), 38.7 (Trz N−CH3), 36.7 (Im
N−CH3), 20.7 (Ar−CH3) ppm. 11B NMR (160 MHz, DMSO-d6) δ
Hz, p-cymene−Ph, 1H), 5.34 (d, J
= 6.2 Hz, p-cymene−Ph,
H−H
3
3
1H), 5.18 (d, J
= 6.2 Hz, p-cymene−Ph, 1H), 4.76 (d, JH−H =
H−H
6.2 Hz, p-cymene−Ph, 1H), 4.35 (s, Trz N−CH33, 3H), 4.12 (s, Im
N−CH3, 3H), 2.47 (s, Ar−CH3, 6H), 2.20 (sept, JH−H = 7.0 Hz, p-
3
cymene−iPr, 1H), 1.78 (s, p-cymene−Me, 3H), 1.04 (d, JH−H = 6.9
3
Hz, p-cymene-iPr, 3H), 0.73 (d, JH−H = 6.9 Hz, p-cymene-iPr, 3H)
ppm. 13C{1H} NMR (126 MHz, CDCl3) δ 173.3 (Im Ccarbene), 159.5
(Trz Ccarbene), 139.3, 139.0, 132.2, 125.1, 124.0, 123.2, 104.5 (Car−
CH3(cym)), 104.4 (Car−iPr(cym)), 91.1 (Car−H(cym)), 90.7 (Car−
H(cym)), 88.8 (Car−H(cym)), 85.6 (Car−H(cym)), 44.8 (CH2), 39.1
(Trz−NCH3), 37.4 (Im−NCH3), 31.2 (CH(CH3)2), 24.9 (CH-
(CH3)2), 21.4 (Ar−CH3), 20.6 (CH3−Car(cym)), 18.5 (Ar−CH3)
ppm. 11B NMR (160 MHz, CDCl3) δ −1.04 (s) ppm. 19F NMR (471
MHz, CDCl3) δ −152.2 (s), −152.3 (s) ppm. MS (ESI, positive
ions): m/z 552.1477 (calculated for [M − BF4]+ 552.1472).
Synthesis of [RuII(6)(p-cymene)Cl]BF4, 9. Compound 9 was
synthesized following procedure similar to that used for compound 8
using [6-H2](BF4)2 (0.05 g, 0.11 mmol), Cs2CO3 (0.108 g, 0.33
J
DOI: 10.1021/acs.organomet.9b00156
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
mmol), and [Ru(p-cymene)Cl2]2 (0.034 g, 0.056 mmol). Yield: 0.05 g
Accession Codes
1
(0.082 mmol, 75%). H NMR (400 MHz, CDCl3) δ 8.10 (d, Ar−H,
CCDC 1899583−1899586 and 1900769 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
2H), 7.64−7.57 (m, Ar−H, 3H), 7.40 (s, Im N−CH−C, 1H), 7.15 (s,
Im N−CH−C, 1H), 5.63 (d, J = 16.8 Hz, CH2, 1H), 5.42 (d, J = 16.8
3
Hz, CH2, 1H), 5.35 (d, JH−H = 6.6 Hz, p-cymene−Ph, 2H), 5.20 (d,
3
3JH−H = 5.7 Hz, p-cymene−Ph, 1H), 4.73 (d, JH−H = 5.7 Hz, p-
cymene−Ph, 1H), 4.32 (s, Trz N−CH3, 3H), 4.09 (s, Im N−CH3,
3
3H), 2.17 (sept, JH−H = 7.0 Hz, p-cymene−iPr, 1H), 1.75 (s, p-
3
3
cymene-Me, 3H), 1.01 (d, JH−H = 6.9 Hz, 3H), 0.69 (d, JH−H = 6.9
Hz, 3H) ppm. 13C NMR (101 MHz, CDCl3) δ 173.5(Im Ccarbene),
160.0 (Trz Ccarbene), 139.3, 137.5, 130.7, 129.3, 127.8, 124.0, 123.1,
104.9(Car−CH3(cym), 104.2, 91.1 (Car−H(cym)), 91.0 (Car−
H(cym)), 88.8 (Car−H(cym)), 85.5 (Car−H(cym)), 44.5 (CH2),
39.1 (Trz−NCH3), 37.0 (Im N−CH3), 31.1 (CH(CH3)2), 24.9
(CH(CH3)2), 20.6 (Ar−CH3), 18.5 (CH3−Car(cym)) ppm. 11B
NMR (160 MHz, CDCl3) δ −1.0 (s) ppm. 19F NMR (471 MHz,
CDCl3) δ −151.9 (s), −152.0 (s) ppm. MS (ESI, positive ions): m/z
524.1160 (calculated for [M − BF4]+ 524.1158). Anal. Calcd (%) for
C24H29N5RuClBF4: C 47.19%, N 11.47%, H 4.79%; Found: C
46.55%, N 11.47%, H 4.08%.
General Procedure for the Catalytic Transfer Hydrogena-
tion of Ketones. Potassium tert-butoxide (0.05 mmol), and 2-
propanol (3 mL) were added to a pressure tube. After that, catalyst 8
(0.0025 mmol) followed by ketone (0.5 mmol) and mesitylene (0.5
mmol) were added. The reaction mixture was then heated to reflux
for the specified time, and after the reaction, a 50 μL aliquot was taken
and diluted with methanol for GC-MS analysis. For the isolation of
the alcohol, all the volatiles were removed by rotatory evaporator after
completion of the reaction. The obtained compound was then
purified through silica gel column using hexane and ethyl acetate as an
eluent.
General Procedure for the Catalytic Transfer Hydrogena-
tion of Imines. To an oven-dried pressure tube with 4 Å molecular
sieves, potassium tert-butoxide (0.06 mmol) and 2-propanol (3 mL)
were added and degassed by passing argon for a few minutes. Then, to
this solution was added catalyst 8/9 (0.003 mmol), followed by imine
(0.3 mmol) and mesitylene (0.3 mmol). The reaction mixture was
then heated to reflux for the specified time, and after the reaction, 50
μL aliquot was taken and diluted with methanol for GC-MS analysis.
For the isolation of the amines, all the volatiles were removed by
rotatory evaporator after completion of the reaction. The obtained
compound was then purified through silica gel column using hexane
and ethyl acetate as an eluent.
Single-Crystal X-ray Crystallography. X-ray data were collected
on a Bruker AXS Kappa APEX-II CCD diffractometer or a Bruker
APEX-II equipped with graphite-monochromated Mo Kα radiation (λ
= 0.71073 Å).28a The crystal was fixed at the tip of a glass fiber loop,
and after mounting on the goniometer head, it was optically centered.
The APEX2 and APEX2-SAINT/Bruker SAINT programs were used
for the data collection and unit cell determination, respectively.
Processing of the raw frame data was performed using SAINT/
XPREP or Bruker SAINT.28b,c The structures were solved by
SHELXT-2014/4 or SHELXS-97 methods28d and refined against F2
using all reflections with the SHELXL-2014/7 (WinGX) program.28e,f
Non-hydrogen atoms were refined anisotropically. All hydrogen
atoms were placed in calculated positions. The crystal data (CCDC
1899583−1899586 and 1900769) and refinement details are
summarized in Tables S2 and S3. The graphical representations
were performed using the program Mercury.
AUTHOR INFORMATION
Corresponding Author
*E-mail: arnab.rit@iitm.ac.in.
ORCID
Arnab Rit: 0000-0003-0224-8387
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
We gratefully acknowledge SERB, India (ECR/2016/001272)
and IIT Madras (seed grant) for financial support. P.M.I.
acknowledges IIT-Madras for a HTRA fellowship, and
S.N.R.D. acknowledges UGC for JRF. We also thank the
Department of Chemistry and SAIF, IIT Madras for
instrumental facility and Dr. K.R.R.’s lab for CV and DPV
measurements. We thank V. Ramkumar for helping with the
crystal structure solution.
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