Cyclometalated Half-Sandwich Iridium Complex for Catalytic Hydrogenation of Imines and Quinolines

Article abstract of DOI:10.1021/acs.organomet.8b00553

Several C,N-chelate cyclometalated half-sandwich iridium-based catalysts for imines and quinoline derivatives reduction have been prepared through metal-mediated C-H bond activation based on benzothiazole ligands. These iridium complexes exhibited high catalytic activity for hydrogenation of various types of imines with high yields. The most active catalyst was obtained from methoxyl substituted complex 2, showing the catalytic TOF value of 975 h^-1 for the reduction of imine 6a. Additionally, these half-sandwich complexes also showed high efficiency for the catalytic hydrogenation of N-heterocyclic quinoline derivatives. Good catalytic activity was displayed for various kinds of substrates with either electron-donating or electron-withdrawing groups. Complexes 1-5 were fully characterized by NMR, IR, and elemental analysis. Molecular structures of complexes 1 and 4 were further confirmed by X-ray diffraction analysis.

Full text of DOI:10.1021/acs.organomet.8b00553

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Cyclometalated Half-Sandwich Iridium Complex for Catalytic
Hydrogenation of Imines and Quinolines
Zi-Jian Yao,*,†,‡ Nan Lin,† Xin-Chao Qiao,† Jing-Wei Zhu,† and Wei Deng*,†
†School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418, China
‡Shanghai Key Laboratory of Molecular Catalysis and Innovative Material, State Key Laboratory of Molecular Engineering and
Polymers, Fudan University, Shanghai 200433, China
S
* Supporting Information
ABSTRACT: Several C,N-chelate cyclometalated half-sand-
wich iridium-based catalysts for imines and quinoline
derivatives reduction have been prepared through metal-
mediated C−H bond activation based on benzothiazole
ligands. These iridium complexes exhibited high catalytic
activity for hydrogenation of various types of imines with high
yields. The most active catalyst was obtained from methoxyl
substituted complex 2, showing the catalytic TOF value of
975 h−1 for the reduction of imine 6a. Additionally, these half-
sandwich complexes also showed high efficiency for the
catalytic hydrogenation of N-heterocyclic quinoline deriva-
tives. Good catalytic activity was displayed for various kinds of substrates with either electron-donating or electron-withdrawing
groups. Complexes 1−5 were fully characterized by NMR, IR, and elemental analysis. Molecular structures of complexes 1 and
4 were further confirmed by X-ray diffraction analysis.
center is perfectly shielded by the Cp# ligands, minimizing the
possibility of undesired side-reactions. (iii) The redox property
and solubility of these transition metal complexes can be tuned
INTRODUCTION
■
Since the discovery in the early 1960s, cyclometalation has
become one of the most studied reactions in organometallic
by introducing various types of substituents to cyclo-
and coordination chemistry, which provides a straightforward
pentadienyl ring.9 Thus, exploring the synthesis and
route to synthesize organometallic complexes that feature a
application of half-sandwich cyclometalated complexes has
M−C σ bond. Meanwhile, cyclometalation allows for the
investigation of the pertinent aspects governing the metal-
become one of the most active and exciting areas of
organometallic chemistry, because of the useful catalytic
mediated activation of unreactive bonds, especially the C−H
reactivity that these ligands impart on complexes.10−23
bond.1−3 The generation of cyclometalated compounds often
Amino compounds (including linear and cyclic structure)
goes through two consecutive reaction steps: The first step is
the initial interaction of the metal center with a donor group,
and subsequent intramolecular C−H bond activation. The
effective bond activation is thus most often a heteroatom-
are widely used as pharmaceuticals and biologically active
molecules.24 One of the simplest methods to obtain these
compounds is the reduction of the corresponding starting
materials containing imine group or nitro group. In the former
assisted process, involving classical donors such as N, P, O, and
case, a common method is to use metal borohydride as the
S atoms. However, the cyclometalation reaction is not very
reducing reagent because of its selectivity for imine
effective until Shaw and co-workers found that the weak base
sodium acetate promotes this process. Thus, various types of
reduction;25 however, the stoichiometric use of reducing
reagent limits their large-scale application due to the waste
cyclometalated compounds were synthesized smoothly by
production.26 Thus, the catalytic hydrogenation represents a
greener synthetic route has gained great consideration
gradually. Thus, various types of catalytic systems based on
different metal catalysts have been reported for the imine
reduction.27−29 Among these catalysts, cyclometalated iridium
complexes attracted our interest because of their high
efficiency in the reduction of carbonyls and imines, as well as
the reductive amination of ketones with various hydrogen
using late transition metal precursors.4−6
In contrast, half-sandwich group 9 metal complexes based on
Cp#M and (arene)M motifs (arene: η6-C6H6, p-cymene; Cp#:
η5-C5H5, η5-C5Me5; M: Co, Ir, Rh, Ru) have been widely used
in many catalytic reactions because of their good stability and
high catalytic activity.7,8 [(η6-arene)MCl2]2 and [Cp#MCl2]2
are often utilized as the metal precursors to prepare diverse
half-sandwich transition-metal complexes due to the following
advantages: (i) The transition metal precursors are easily
synthesized with high yields by reactions of metal chlorides
with conjugated ligands. (ii) The hemisphere of the metal
Received: August 9, 2018
© XXXX American Chemical Society
A
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sources.30,31 We herein report a series of cyclometalated half-
sandwich iridium complexes based on benzothiazole ligands
which are widely used in the preparation of coordination
compounds played important role in catalysis and materials
chemistry.32 The easy modification of the ligands helps us
elaborate the influences of the electron and steric effects on the
catalytic activity of the corresponding iridium catalysts.
Moreover, the planar geometry of the ligands facilitates the
growth of suitable single crystals of the corresponding half-
sandwich iridium complexes. Fortunately, preliminary results
exhibit that these complexes can serve as the catalysts for
hydrogenation of imines and quinoline derivatives in good
yields by using H2 as hydrogen source.
from downfield range of δ 9.00−7.00 ppm suggested the
occurrence of C−H activation in phenyl ring. The chemical
shifts at approximately δ 1.70 ppm of these cyclometalated
complexes are ascribed to the protons of Cp* group. All the
iridium complexes show a characteristic absorption band in the
range 1400−1440 cm−1 in their IR spectra, which can be
assigned to the vibration of the CN bond of the thiazole
ring. These CN absorption bands all shifted to lower
frequencies when compared to those of the free benzothiazole
ligand (1440−1490 cm−1). The redshift reflected the
coordination interaction between the ligand and Cp*Ir moiety.
This is consistent with the increase in the electron density on
the iridium(III) center caused by the coordination of the C
N group, which resulted in increasing the back bonding to the
nitrogen and hence a lower CN stretching vibration.35 The
elemental analysis and spectroscopic data of these complexes
demonstrate that their structures are similar to each other.
Crystal Structures of Cyclometalated Half-sandwich
Iridium Complexes. To elucidate the structures of the
cyclometalated half-sandwich iridium complexes clearly, a
single crystal X-ray analysis is desired. Single crystals suitable
for X-ray diffraction analysis were obtained by the slow
diffusion of hexane into a saturated solution of the iridium
complexes in dichloromethane. Both complexes 1 and 3
crystallized in the monoclinic space group P21c (Figure 1).
Crystallographic data are summarized in Supporting Informa-
tion. Both complexes show a typical three-legged piano-stool
geometry with the iridium center being coordinated by the η5-
Cp* and the C and N atoms, as well as a chloride ligand. The
metal center has a distorted-octahedral environment, assuming
that the η5-Cp* motif occupies three fac coordination sides.36
The air and thermal stability of these half-sandwich iridium
complexes is presumably caused by the formation of the
cyclometalated structure. The bond lengths of Ir−N bonds in
the two complexes (2.105(4) Å (1) and 2.092(5) Å (4)) are
within the range of known values for the bond in similar
iridium complexes.36 The Ir−C bond distances in complexes 1
and 4 are 2.037(6) and 2.061(6) Å, respectively, also
consistent with previous reports.36 The shorter Ir−C bond
length compared with the Ir−N distance suggests that the
stronger interaction between metal center and carbon atom.
The cyclometalated five-membered rings Ir(1)−C(9)−C(8)−
C(7)−N(1) of the two complexes are almost planar with the
dihedral angle of 5.7 and 5.6°, respectively, between the planes
of N(1)−Ir(1)−C(9) and C(9)−C(8)−C(7)−N(1). The sum
of the inner angles of the five-membered ring of approximately
540° further confirmed their planarity. Noncovalent inter-
molecular interactions of Ir−Cl···H(aromatic) in the crystal
packing structure of the two complexes are observed.
RESULTS AND DISCUSSION
■
Synthesis of Cyclometalated Half-Sandwich Iridium
Complexes 1−5. Benzothiazole ligands L1−L5 were
prepared in high yields through cyclocondensation reaction
from aldehydes and thioanilines under air.33 With the ligands
in hand, the target products were synthesized by the reactions
of benzothiazole ligands with half equivalent iridium precursor
[Cp*IrCl2]2 in the presence of sodium acetate (Scheme 1).
Scheme 1. Synthesis of Half-Sandwich Iridium Complexes
a
1−5
a
Reaction condition: (i) toluene, 100 °C, 24 h; (ii) CH3OH, NaOAc,
50 °C, 6 h. Detailed description of the experiments, see the
Experimental Section.
Cyclometalated half-sandwich iridium complexes 1−5 were
obtained in methanol through metal-induced C−H bond
activation34 at elevated temperature in moderate isolated
yields. They form air-stable products that are soluble in THF,
toluene, and CH2Cl2 but are insoluble in nonpolar solvents
such as hexane and diethyl ether. The benzothiazole ligands act
as the chelate through the C,N-coordination mode in the
products which are supported by various techniques and
elemental analysis. No decomposition was observed after
refluxing the cyclometalated complexes in toluene for several
hours suggested the thermal stability of these complexes. The
TGA data further confirmed their thermal stability (see
Supporting Information). No C,S-chelate product was found
in the reaction.
Catalytic Hydrogenation of Imines. Imine hydro-
genation process catalyzed by late transition metal complexes
using clean H2 as hydrogen source represented the green and
sustainable chemical process, when compared with the
traditional reduction methods using stoichiometric metal
hydrides as reduction reagents.36c Thus, the hydrogenation
of imine substrates under catalysis of cyclometalated
complexes 1−5 was explored. Reaction conditions optimiza-
tion of the catalytic hydrogenation process was investigated by
using 6a as model substrate at ambient temperature under 10
atm of H2 under catalysis of complex 1 (Table 1). Solvent
played an important role in the hydrogenation reaction and
results showed that CF3CH2OH was the best choice (Table 1,
entries 1−8). This phenomenon is reasonable because the
Cyclometalated half-sandwich iridium products 1−5 were
1
characterized by their H NMR spectra. In comparison with
the free ligand, the disappearance of one proton signal in 1−5
B
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Figure 1. Molecular structures of 1 (a) and 4 (b) with thermal ellipsoids drawn at the 30% level. All hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Complex 1: Ir(1)−N(1), 2.105(4); Ir(1)−C(9), 2.037(6); Ir(1)−Cl(1), 2.3933(13); C(7)−N(1),
1.309(8); C(7)−C(8), 1.449(8); C(9)−Ir(1)−N(1), 77.0(2); Ir(1)−N(1)−C(7), 116.1(4); N(1)−C(7)−C(8), 117.1(5); C(7)−C(8)−C(9),
112.1(5); Ir(1)−C(9)−C(8), 117.2(4). Complex 4: Ir(1)−N(1), 2.092(5); Ir(1)−C(9), 2.061(6); Ir(1)−Cl(1), 2.3920(13); C(7)−N(1),
1.334(8); C(7)−C(8), 1.409(9); C(9)−Ir(1)−N(1), 77.5(2); Ir(1)−N(1)−C(7), 114.6(4); N(1)−C(7)−C(8), 118.6(5); C(7)−C(8)−C(9),
113.0(5); Ir(1)−C(9)−C(8), 115.9(5).
a
Table 1. Hydrogenation of Imines under Catalysis of Cyclometalated Iridium Complexes
b
entry
cat./mol %
time/h
T/°C
P/atm
solvent
THF
toluene
DMSO
additive
yield/%
TON
TOF/h−1
1
2
3
4
5
6
7
8
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
1 (0.1)
2 (0.1)
3 (0.1)
4 (0.1)
5 (0.1)
2 (0.05)
2 (0.05)
2 (0.01)
2 (0.05)
2 (0.05)
2 (0.01)
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
2
2
2
2
2
2
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
6
trace
trace
trace
trace
trace
trace
12
55
78
80
75
77
88
73
79
74
95
72
43
DMF
1,4-dioxane
CH2Cl2
MeOH
CF3CH2OH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
CF3CH2OH
CF3CH2OH
120
550
780
800
750
770
880
730
790
40
167
260
267
250
257
293
243
263
247
975
720
2150
9
AgOTf
AgBF4
AgPF6
AgSbF6
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
10
11
12
13
14
15
16
17
18
19
740
50
50
50
50
50
50
1950
1440
4300
3
6
6
6
c
20
trace
94
48
21
22
1880
4800
940
2400
6
a
b
Reaction conditions: substrates 6a (1.0 mmol), solvent (2 mL), silver salt/cat. = 1.2:1. Yield was determined by GC analysis; n-tridecane was
c
used as internal standard. PMe3 was added (PMe3/cat. = 1:1 in molar ration).
chloride ligand of the coordination saturated cyclometalated
iridium complex needs to be dissociated in the first step, so a
necessary vacant site is given for the coordination of H2.37 The
high polarity, acidity, and low nucleophilicity of CF3CH2OH
facilitated the dissociation and solvation of the chloride ligand.
Meanwhile, this solvent having minimal interaction with the
cationic iridium intermediate.37 Inspired by these results,
AgOTf was added in the reaction because the chloride ligand
could be easily removed by the silver salt. Fortunately, the
corresponding amine was furnished in high yield by using
cheap and readily MeOH as solvent (Table 1, entry 9). No
counterion effects were observed when other silver salts were
employed in the reactions (Table 1, entries 9−12).
The investigation of complexes 1−5 as catalysts suggests
that complex 2 shows the best catalytic activity for the imine
hydrogenation (Table 1, entries 13−16). Other factors were
also screened by using the most active compound, 2, as the
catalyst. Higher catalytic activity was observed at elevated
temperature, almost quantitative conversion of 6a was found in
2 h with only 0.05 mol % 2 at 6 atm H2 atomsphere (Table 1,
C
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a
Table 2. Scope of the Catalytic Hydrogenation of Imines
a
Reaction conditions: substrates (1.0 mmol), catalyst loading: 0.05 mol %, 50 °C, 2 h, 6 atm, solvent: MeOH (2 mL). Yield was determined by GC
analysis; n-tridecane was used as internal standard. Isolated yields (%) are provided in parentheses.
1
Figure 2. H NMR investigation of the formation of iridium hydride species.
entry 17). Further decreasing the catalyst loading (0.05−0.01
mol %) led to a lower yield of the product (Table 1, entry 18).
In the presence of AgOTf, the yields of the reactions use
CF3CH2OH as solvent are comparable to that of the reaction
by using MeOH as solvent (Table 1, entries 21 and 22).
Having established the optimum conditions for the reaction,
a variety of substrates were employed in the reaction. Table 2
presents the results of the catalytic hydrogenation of imines.
Corresponding amines were furnished in good to high yields
by utilization of complex 2 as the catalyst under standard
conditions (Table 2). The catalytic system tolerates different
kinds of functional groups with either electron-donating or
-withdrawing properties (Table 2, 7a−l). However, for the
reduction of the imines-derived aliphatic precursors, lower
activity of the catalyst was observed compared to that of the
ruthenium carbonyl complex (Table 2, 7m−r).27d In the case
of nitro-substituted compounds (7j and 7k), the intact of the
nitro group indicates the selectivity of this reaction. Although
the catalytic activity is comparable to those of the previously
reported iridium and ruthenium complexes used in transfer
hydrogenation processes, high reaction temperature and toxic
benzene solvent are not essential for the reaction reported
here.27d,38
To elucidate the possible mechanism of the hydrogenation
process, the reactivity between the catalyst and H2 in the
presence of AgOTf was investigated. The NMR tube
containing the solution of 2 (in dry d8-toluene) was
pressurized with H2 (6 atm). The solution was then kept at
1
50 °C and characterized by H NMR after 20 min. A singlet
was observed at δ −9.67 ppm, which indicating that the
formation of metal hydride species (Figure 2).39 This step
involves the heterolytic splitting of H2 with formation of the
hydrido iridium active species. The hydride formation was also
indicated by the disappearance of the high-field singlet because
of the formation of the Ir−D bond, when H2 was replaced by
D2 as starting material. Additionally, the signals of Cp* protons
split into two peaks in the H2 heterolytic splitting step (Figure
D
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S6). This result may suggest that there are two species
([C∧N]Ir)+ and [C∧N]IrH) in the reaction mixture.
The conclusion was further confirmed by the experimental
result that little hydrogenation takes place when PMe3 was
used as additive to poison the catalyst in the hydrogenation
process under standard reaction conditions (Table 1, entry
20). As indicated in the experimental results, the hydro-
genation benefits from higher H2 pressure (Table 1, entries 17
and 18), so the reaction was monitored under different
pressure. Figure 3 shows that there is almost a linear
Figure 4. Possible mechanism of the catalytic hydrogenation process
h when 0.5 mol % catalyst loading was used (Table 3, entry 7).
Further increases of the catalyst loading have little influence on
the reaction (Table 3, entry 8). Decreasing H2 pressure to 3
atm did not lead to a decline in productivity (Table 3, entries 9
and 10).
With the optimal reaction condition for quinoline hydro-
genation in hand, a series of quinoline derivatives were
examined in the hydrogenation reaction. The cyclometalated
iridium complex showed good catalytic activity for all these
substrates employed. The hydrogenation process was found to
be tolerant of different functional groups, such as methyl,
phenyl, and halide groups and so on (Table 4, 9a−j). No
reaction was observed on the ester group of the substrate,
indicating the selectivity of the catalyst for the hydrogenation
of CN bonds (Table 4, 9k). Compared with a number of
waste production of the protocol use NaBH3CN as reducing
reagent,41 this method represents an efficient and greener
route, although the catalytic efficiency is lower than that of the
analogous iridium complexes using HCOOH/HCOONa as
hydrogen source.28 We wonder if the catalytic efficiency could
be enhanced under harsher conditions. However, only slightly
higher yield (48%) was obtained by using 0.1 mol % catalyst at
80 °C and 10 atm (the results have been added in Table 2,
entry 11).
Figure 3. Effect of H2 pressure on the catalytic hydrogenation
process. Reaction conditions: 1.0 mmol of imine 6a, 0.05 mol %
catalyst 2, AgOTf (0.06 mol %), H2, 50 °C for 20 min. The
1
conversion was determined by H NMR.
correlation between the conversions and H2 pressure,
indicating that H2 is probably involved in the rate-determining
step of the hydrogenation. On the basis of the above
experimental results and previous reports, a possible
mechanism has been proposed. At high temperature and
pressure of H2, cyclometalated half-sandwich iridium catalyst
reacted with H2 to give the hydrido iridium intermediate as the
catalytic active species. The catalytic cycle is consisted of three
steps: reversible interaction of cationic metal center with H2,
rate-determining heterolytic cleavage of H2 into a hydride and
a proton to be taken up by the imines, and subsequent transfer
of the hydride to the resulting iminium cation. Mechanistic
studies on the similar ionic pathway have been reported
previously (Figure 4).27d
Catalytic Hydrogenation of Quinolines. Encouraged by
the good activity of the cyclometalated iridium complexes on
imine reduction, we turned our attention on the catalytic
hydrogenation of unsaturated N-heterocycle compounds.40
Initially, we screened different solvents for the hydrogenation
of quinoline 8a by using complex 2 (0.1 mol %) as catalyst at
50 °C (Table 3). To our surprise, the reaction gave the
positive results in the CF3CH2OH (47% yield) without any
additives over 8 h. However, the catalytic efficiency is not
good; only 57% yield of the product was obtained even after 12
h under such conditions (Table 3, entry 6). No reactions were
observed in MeOH and other nonprotic solvents, such as
toluene, DMF, and so on, with the addition of silver salts
(Table 3, entries 1−6). The amount of the catalyst is crucial to
the reaction. The product was obtained in 93% yield only in 3
The recycle of the catalyst was investigated in order to offset
the low catalytic efficiency in the reduction of quinolines. The
catalyst was separated from the reaction mixture by removing
all volatiles under vacuum. Results showed that complex 2
could be reused for at least three cycles without any loss of
activity (93, 92, and 89%; Figure 5).
CONCLUSIONS
■
In summary, we have described a series of air-stable
cyclometalated half-sandwich iridium complexes with the
C,N-coordination mode based on benzothiazole ligands.
These complexes exhibited good catalytic activity for hydro-
genation of imines and quinoline derivatives by using H2 as the
hydrogen source. A survey of various kinds of substrates
suggested the good tolerance of the catalytic system for the
hydrogenation process. In addition, complex 2 could be reused
for at least three cycles without any loss of activity in the
hydrogenation of quinoline. The good stability and prominent
E
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a
Table 3. Optimization for the Catalytic Hydrogenation of Quinoline
b
entry
cat./mol %
P/atm
solvent
MeOH
THF
Toluene
DMF
yield/%
trace
TON
TOF/h−1
1
2
3
4
5
6
7
8
9
2 (0.1)
2 (0.1)
2 (0.1)
2 (0.1)
2 (0.1)
2 (0.1)
2 (0.5)
2 (1.0)
2 (0.5)
2 (0.5)
2 (0.1)
6
6
6
6
6
6
6
6
3
CH2Cl2
d
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
CF3CH2OH
43/57
92
93
430/570
184
93
186
70
54/71
61
31
62
23
c
c
c
92
c
10
1
10
35
48
e
11
480
160
a
b
Reaction conditions: quinoline (1.0 mmol), solvent (2.0 mL), H2, AgOTf/cat. = 1.2:1, reaction time: 8 h. Yield was determined by GC analysis;
c
d
e
n-tridecane was used as internal standard. Reaction time: 3 h, without the addition of silver salt. Reaction time: 12 h. Reaction temperature: 80
°C.
a
Table 4. Scope of the Catalytic Hydrogenation of Quinoline Derivatives
a
Reaction conditions: substrates (1.0 mmol), catalyst loading: 0. Five mol %, 50 °C, 3 h, 3 atm, solvent: CF3CH2OH (2 mL). Yield was determined
by GC analysis, n-tridecane was used as internal standard. Isolated yields (%) are provided in parentheses.
catalytic efficiency of the half-sandwich iridium complexes
proved the possibility of their application in industrial
production.
EXPERIMENTAL SECTION
■
General Data. All manipulations were performed under an
atmosphere of nitrogen using standard Schlenk techniques. Chemicals
were used as commercial products without further purification. H
1
NMR (500 MHz) spectra were measured with a Bruker DMX-500
spectrometer. Elemental analysis was performed on an Elementar
vario EL III analyzer. UV/vis absorption spectra were recorded using
a UV 765 spectrophotometer with 1 cm path length quartz cuvettes.
IR (KBr) spectra were measured with the Nicolet FT-IR
spectrophotometer. Ligands L1−L5 were prepared using the modified
method according to the literature.32
Synthesis of Benzothiazole Ligands L1−L5. The mixture of 2-
aminothiophenol (1 mmol) and corresponding aromatic aldehyde
(1.2 mmol, 1.2 equiv) in toluene (5 mL) in a 20 mL Schlenk tube
Figure 5. Reuse of catalyst 2 in the quinoline hydrogenation process
under optimal conditions.
adapted with an air balloon was heated at 100 °C for 24 h. The
reaction was then monitored by TLC. Column chromatography of the
crude products (PE/EA = 10:1) gave L1−L5 in good yields.
F
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L1. White solid, 85% isolated yield. 1H NMR (500 MHz, CDCl3):
47.79, H 4.18, N 2.32, found: C 47.73, H 4.23, N 2.37. MS (ESI, m/
z): 568 [M − Cl]+.
3
δ 8.11−8.08 (m, Ar−H, 3H), 7.92 (d, JHH = 8.0 Hz, Ar−H, 1H),
1
3
3. Orange red solid, 49.6% yield. H NMR (500 MHz, CDCl3): δ
7.51−7.49 (m, Ar−H, 4H), 7.41 (t, JHH = 8.0 Hz, Ar−H, 1H). IR
8.20 (s, Ar−H, 1H), 8.03 (d, 3JHH = 8.0 Hz, Ar−H, 1H), 7.89 (d, 3JHH
(KBr, disk): υ 1646 (vCC), 1509 (vCC), 1478 (vCN), 1384
(vCC), 1313 (vCC), 1224 (vC−S), 962 (ωC−H), 766 (ωAr−H) cm−1.
Elemental analysis calcd (%) for C13H9NS: C 73.90, H 4.29, N 6.63,
found: C 73.93, H 4.27, N 6.68.
3
= 8.0 Hz, Ar−H, 1H), 7.79 (d, JHH = 7.5 Hz, Ar−H, 1H), 7.60 (t,
3
3JHH = 8.5 Hz, Ar−H, 1H), 7.49 (t, JHH = 8.0 Hz, Ar−H, 1H), 7.32
3
(d, JHH = 8.5 Hz, Ar−H, 1H), 1.73 (s, Cp*, 15H). 13C NMR (100
L2. White solid, 83% isolated yield. 1H NMR (500 MHz, CDCl3):
MHz, CDCl3): 177.5 (CN), 165.7 (CAr(NS)−N = C), 149.2
(Ar(NS)C−S), 147.1 (CAr), 143.8 (CAr), 132.6 (CAr), 132.4 (CAr),
127.3 (CAr), 126.1 (CAr), 125.1 (CAr(NS)), 124.2 (q, JFC = 220 Hz,
CF3), 123.0 (CAr(NS)), 121.5 (CAr(NS)), 119.4.1 (CAr(NS)), 89.1
(C5(CH3)5), 9.6 (C5(CH3)5). IR (KBr, disk): υ 1651 (vCC), 1556
(vCC), 1439 (vCN), 1375 (vCC), 1319 (ωC−H), 1142 (vC−S), 1110
(vC−F), 983 (ωC−H), 765 (ωAr−H) cm−1. Elemental analysis calcd
(%) for C24H22ClF3NIrS: C 44.96, H 3.46, N 2.18, found: C 44.89, H
3.50, N 2.25. MS (ESI, m/z): 606 [M − Cl]+.
3
3
δ 8.05 (t, JHH = 9.0 Hz, Ar−H, 3H), 7.89 (d, JHH = 8.0 Hz, Ar−H,
1H), 7.50 (t, 3JHH = 7.5 Hz, Ar−H, 1H), 7.37 (t, 3JHH = 8.0 Hz, Ar−
H, 1H), 7.01 (d, 3JHH = 9.0 Hz, Ar−H, 2H), 3.89 (s, OCH3, 3H). IR
(KBr, disk): υ 1605 (vCC), 1521 (vCC), 1485 (vCN), 1310
(vCC), 1302 (vCC), 1225 (vC−S), 968 (ωC−H), 833 (ωAr−H) cm−1.
Elemental analysis calcd (%) for C14H11NOS: C 69.68, H 4.59, N
5.80, found: C 69.70, H 4.57, N 5.86.
L3. White solid, 80% isolated yield. 1H NMR (500 MHz, CDCl3):
δ 8.22 (d, 3J (H,H) = 8.0 Hz, Ar−H, 2H), 8.12 (d, 3JHH = 8.0 Hz, Ar−
H, 1H), 7.94 (d, 3JHH = 7.5 Hz,, Ar−H, 1H), 7.76 (d, 3JHH = 8.0 Hz,
1
4. Orange red solid, 56.3% yield. H NMR (500 MHz, CDCl3): δ
7.97 (d, 3JHH = 8.0 Hz, Ar−H, 1H), 7.89 (s, Ar−H, 1H), 7.84 (d, 3JHH
3
3
3
Ar−H, 2H), 7.55 (t, JHH = 8.0 Hz, Ar−H, 1H), 7.45 (t, JHH = 7.5
Hz, Ar−H, 1H). IR (KBr, disk): υ 1618 (vCC), 1593 (vCC), 1484
(vCN), 1323 (vCC), 1253 (vC−S), 1173 (vC−F) 971 (ωC−H), 760
(ωAr−H) cm−1. Elemental analysis calcd (%) for C14H8F3NS: C 60.21,
H 2.89, N 5.02, found: C 60.23, H 2.83, N 5.06.
= 7.5 Hz, Ar−H, 1H), 7.64 (d, JHH = 8.0 Hz, Ar−H, 1H), 7.56 (t,
3
3JHH = 7.5 Hz, Ar−H, 1H), 7.44 (t, JHH = 7.5 Hz, Ar−H, 1H), 7.32
3
(d, JHH = 8.5 Hz, Ar−H, 1H), 1.72 (s, Cp*, 15H). 13C NMR (100
MHz, CDCl3): 177.7 (CN), 167.4 (CAr(NS)−N = C), 149.2
(Ar(NS)C−S), 139.2 (CAr), 137.6 (CAr), 135.7 (CAr), 132.2 (CAr),
127.1 (CAr), 126.4 (CAr), 125.6 (CAr(NS)), 122.9 (CAr(NS)), 122.7
(CAr(NS)), 121.1 (CAr(NS)), 88.9 (C5(CH3)5), 9.7 (C5(CH3)5). IR
(KBr, disk): υ 1655 (vCC), 1550 (vCC), 1436 (vCN), 1354
(vCC), 1329 (ωC−H), 1143 (vC−S), 1069 (vC−Cl), 982 (ωC−H), 772
(ωAr−H) cm−1. Elemental analysis calcd (%) for C23H22Cl2NIrS: C
45.46, H 3.65, N 2.31, found: C 45.53, H 3.70, N 2.22. MS (ESI, m/
z): 572 [M − Cl]+.
L4. White solid, 75% isolated yield. 1H NMR (500 MHz, CDCl3):
δ 8.08 (d, 3JHH = 8.0 Hz, Ar−H, 1H), 8.04 (d, 3JHH = 8.5 Hz, Ar−H,
2H), 7.92 (d, 3JHH = 7.5 Hz, Ar−H, 1H), 7.52−7.46 (m, Ar−H, 3H),
3
7.42 (t, JHH = 7.5 Hz, Ar−H, 1H). IR (KBr, disk): υ 1643 (vCC),
1589 (vCC), 1474 (vCN), 1399 (vCC), 1251 (vC−S), 1090 (vC−Cl),
972 (ωC−H), 756 (ωAr−H) cm−1. Elemental analysis calcd (%) for
C13H8ClNS: C 63.54, H 3.28, N 5.70, found: C 63.55, H 3.30, N 5.60.
L5. White solid, 79% isolated yield. 1H NMR (500 MHz, CDCl3):
1
5. Orange red solid, 49.8% yield. H NMR (500 MHz, CDCl3): δ
3
3
3
δ 8.23 (t, JHH = 5.5 Hz, Ar−H, 1H), 8.15 (d, JHH = 8.0 Hz, Ar−H,
2H), 7.96 (d, 3JHH = 7.5 Hz, Ar−H, 1H), 7.53−7.41 (m, Ar−H, 5H).
IR (KBr, disk): υ 1656 (vCC), 1579 (vCC), 1479 (vCN), 1386
(vCC), 1239 (vC−S), 1088 (vC−Cl), 978, 762 cm−1. Elemental analysis
calcd (%) for C13H8ClNS: C 63.54, H 3.28, N 5.70, found: C 63.59,
H 3.22, N 5.64.
8.05 (d, J (H,H) = 8.0 Hz, Ar−H, 1H), 7.90−7.85 (m, Ar−H, 2H),
7.59−7.53 (m, Ar−H, 1H), 7.48 (t, 3JHH = 7.0 Hz, Ar−H, 1H), 7.15−
7.08 (m, Ar−H, 2H), 1.70 (s, Cp*, 15H). 13C NMR (100 MHz,
CDCl3): 175.2 (CN), 169.2 (CAr(NS)−N = C), 148.0 (Ar(NS)C−S),
138.4 (CAr), 135.0 (CAr), 132.8 (CAr), 132.1 (CAr), 131.3 (CAr), 127.0
(CAr), 125.6 (CAr(NS)), 123.5 (CAr(NS)), 122.5 (CAr(NS)), 121.5
(CAr(NS)), 89.2 (C5(CH3)5), 9.6 (C5(CH3)5). IR (KBr, disk): υ
1671 (vCC), 1538 (vCC), 1429 (vCN), 1352 (vCC), 1328
(ωC−H), 1151 (vC−S), 1065 (vC−Cl), 979 (ωC−H), 762 (ωAr−H) cm−1.
Elemental analysis calcd (%) for C23H22Cl2NIrS: C 45.46, H 3.65, N
2.31, found: C 45.60, H 3.73, N 2.32. MS (ESI, m/z): 572 [M − Cl]+.
General Procedure for the Hydrogenation of Imine. In a
typical run, the substrate (1.0 mmol), catalyst (0.5 mol %), AgOTf
(0.6 mol %) and MeOH (2 mL) were charged in a 5 mL vial with a
magnetic bar. The vial was then transferred to an autoclave. The
autoclave was purged with H2 (6 atm) via three cycles of
pressurization/venting, then pressurized with H2 (6 atm) and
disconnected from the H2 source. The autoclave was heated to the
desired temperature. After stirring for 2 h, the autoclave was cooled
and the pressure was slowly released. The resultant mixture extracted
with diethyl ether (2 × 5 mL) and dried over anhydrous Na2SO4.
Solvent was evaporated under vacuum. The residue was dissolved in
hexane and analyzed by GC-MS.
Synthesis of Half-Sandwich Iridium Complexes 1−5. A
mixture of [Cp*IrCl2]2 (0.1 mmol), NaOAc (0.4 mmol), and
corresponding ligand L1−L5 (0.2 mmol) was stirred at 50 °C in 10
mL of methanol for 6 h. The mixture was filtered and evaporated to
give the crude products which were further purified by silica gel
column chromatography (CH2Cl2/EA = 20:1) to afford pure
cyclometalated half-sandwich iridium complexes in moderate yields.
1
1. Dark red solid, 52.6% yield. H NMR (500 MHz, CDCl3): δ
7.99−7.95 (m, Ar−H, 2H), 7.84 (d, 3JHH = 8.0 Hz, Ar−H, 1H), 7.72
3
3
(d, JHH = 7.5 Hz, Ar−H, 1H), 7.55 (t, JHH = 7.0 Hz, Ar−H, 1H),
3
3
7.43 (t, JHH = 7.5 Hz, Ar−H, 1H), 7.22 (t, JHH = 7.0 Hz, Ar−H,
1H), 7.08 (t, JHH = 7.0 Hz, Ar−H, 1H), 1.72 (s, Cp*, 15H). 13C
3
NMR (100 MHz, CDCl3): 177.9 (CN), 164.9 (CAr(NS)−N = C),
148.3 (Ar(NS)C−S), 139.6 (CAr), 135.2 (CAr), 131.1 (CAr), 130.7
(CAr), 125.9 (CAr), 124.5 (CAr), 124.4 (CAr(NS)), 121.8 (CAr(NS)), 121.3
(CAr(NS)), 120.1 (CAr(NS)), 87.6 (C5(CH3)5), 8.7 (C5(CH3)5). IR
(KBr, disk): υ 1656 (vCC), 1542 (vCC), 1430 (vCN), 1394
(vCC), 1325 (ωC−H), 1230 (vC−S), 982 (ωC−H), 763 (ωAr−H) cm−1.
Elemental analysis calcd (%) for C23H23ClNIrS: C 48.20, H 4.04, N
2.44, found: C 48.25, H 4.03, N 2.50. MS (ESI, m/z): 538 [M − Cl]+.
Recycle of the Catalyst. After GC analysis of the reaction
solution, the reaction mixture was evaporated under vacuum at high
temperature until all volatiles were removed. Then, the residue solid
was used as the catalyst under standard reaction conditions for the
next cycle.
X-ray Crystallography. Diffraction data of 1 and 4 were collected
on a Bruker Smart APEX CCD diffractometer with graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å). All the data
were collected at room temperature, and the structures were solved by
direct methods and subsequently refined on F2 by using full-matrix
least-squares techniques (SHELXL).42 SADABS43 absorption correc-
tions were applied to the data. All non-hydrogen atoms were refined
anisotropically, and hydrogen atoms were located at calculated
positions. All calculations were performed using the Bruker program
Smart.
1
2. Orange red solid, 58.0% yield. H NMR (500 MHz, CDCl3): δ
7.92 (s, Ar−H, 1H), 7.80 (d, 3JHH = 8.0 Hz, Ar−H, 1H), 7.67 (d, 3JHH
= 8.5 Hz, Ar−H, 1H), 7.52−7.48 (m, Ar−H, 2H), 7.37 (t, 3JHH = 7.0
Hz, Ar−H, 1H), 6.65 (dd, 3JHH = 2.5 Hz, 3JHH = 2.5 Hz, Ar−H, 1H),
3.92 (s, OMe, 3H), 1.73 (s, Cp*, 15H). 13C NMR (100 MHz,
CDCl3): 178.1 (CN), 168.3 (CAr(NS)−N = C), 161.9 (C−OMe),
149.4 (Ar(NS)C−S), 133.9 (CAr), 131.8 (CAr), 127.1 (CAr), 126.8
(CAr), 124.8 (CAr), 122.7 (CAr(NS)), 120.6 (CAr(NS)), 120.3 (CAr(NS)),
109.4 (CAr(NS)), 88.6 (C5(CH3)5), 55.2 (OMe), 9.7 (C5(CH3)5). IR
(KBr, disk): υ 1663 (vCC), 1590 (vCC), 1419 (vCN), 1364
(vCC), 1333 (ωC−H), 1245 (vC−S), 1029 (vC−O), 976 (ωC−H), 743
(ωAr−H) cm−1. Elemental analysis calcd (%) for C24H25ClNOIrS: C
G
DOI: 10.1021/acs.organomet.8b00553
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00553.
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1
1H NMR spectra of L1−L5, H and 13C NMR spectra
Complexes 1−5; TGA curves of complex 1−5 (PDF)
Accession Codes
CCDC 1859056−1859057 contain the supplementary crys-
tallographic 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.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zjyao@sit.edu.cn. Tel.: +86-21-60877231. Fax: +86-
21-60873335.
ORCID
Zi-Jian Yao: 0000-0003-1833-1142
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (No. 21601125), the Chenguang Scholar
of Shanghai Municipal Education Commission (No. 16CG64),
Natural Science Foundation of Shanghai (No. 16ZR1435700),
Shanghai Gaofeng & Gaoyuan Project for University Academic
Program Development, Shanghai Science and Technology
Committee (16DZ2270100), Shanghai Young Teacher Train-
ing Program (No. ZZZZyyx16005), the Shuguang Scholar of
Shanghai Municipal Education Commission (No. 16SG49).
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Unprecedented Boron-Functionalized Carborane Derivatives by
Facile and Selective Cobalt-Induced B−H Activation. J. Am. Chem.
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