Cyclometalated Half-Sandwich Iridium(III) Complexes: Synthesis, Structure, and Diverse Catalytic Activity in Imine Synthesis Using Air as the Oxidant

Article abstract of DOI:10.1021/acs.inorgchem.1c00174

Four air-stable cyclometalated half-sandwich iridium complexes 1-4 with C,N-donor Schiff base ligands were prepared through C-H activation in moderate-to-good yields. These complexes have been well characterized, and their exact structure was elaborated on by single-crystal X-ray analysis. The iridium(III) complexes 1-4 showed good catalytic activity in the imine synthesis under open-flask conditions (air as the oxidant) from primary amine oxidative homocoupling, secondary amine dehydrogenation, and the cross-coupling reaction of amine and alcohol. Substituents bonded on the ligands of the iridium complexes displayed little effect on the catalytic efficiency. The stability and good catalytic efficiency of the iridium catalysts, mild reaction conditions, and substrate universality showed their potential application in industrial production.

Full text of DOI:10.1021/acs.inorgchem.1c00174

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Article
Cyclometalated Half-Sandwich Iridium(III) Complexes: Synthesis,
Structure, and Diverse Catalytic Activity in Imine Synthesis Using Air
as the Oxidant
Rong-Jian Li, Chun Ling, Wen-Rui Lv, Wei Deng, and Zi-Jian Yao*
Cite This: Inorg. Chem. 2021, 60, 5153−5162
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ABSTRACT: Four air-stable cyclometalated half-sandwich iridium
complexes 1−4 with C,N-donor Schiff base ligands were prepared
through C−H activation in moderate-to-good yields. These
complexes have been well characterized, and their exact structure
was elaborated on by single-crystal X-ray analysis. The iridium(III)
complexes 1−4 showed good catalytic activity in the imine synthesis
under open-flask conditions (air as the oxidant) from primary amine
oxidative homocoupling, secondary amine dehydrogenation, and the
cross-coupling reaction of amine and alcohol. Substituents bonded on
the ligands of the iridium complexes displayed little effect on the
catalytic efficiency. The stability and good catalytic efficiency of the iridium catalysts, mild reaction conditions, and substrate
universality showed their potential application in industrial production.
imines with various structures. Various types of metal catalysts
were investigated in these reactions, such as copper,9
ruthenium,10 manganese,11 palladium,12 bismuth,13 niobium,14
titanium,15 platinum,16 gold,17 silver,18 and iron19 metal−
organic frameworks,20 and other nonmetal species, such as
graphene oxide materials,21 carbonaceous materials,22 and
boron-doped compounds.23 These catalytic systems displayed
good activity in the synthesis of imines; however, some
drawbacks limited their extensive application:1b (i) the
selectivity of the reaction is not good because of the easy
oxidation of imines to nitriles; (ii) oxidative reagents used in
the reaction such as TEMPO are not environmentally friendly;
(iii) the reaction temperatures are high and the reaction time is
long. So, the exploration of efficient and active catalysts for the
selective oxidation of amines under mild conditions is highly
desirable.
On the other hand, half-sandwich transition-metal com-
plexes have been largely explored in organometallic and
catalysis chemistry because of their good stability, diverse
reactivity, and catalytic efficiency.24,25 [Cp*MCl2]2 (M = Ir,
Ru, Rh) are often utilized as precursors to prepare a variety of
novel half-sandwich complexes because of their several
advantages, such as the easy preparation of starting metal
INTRODUCTION
■
Schiff bases and their derivatives as useful organic ligands have
attracted large attention because of their extensive application
in organometallic and coordination chemistry,1 medicine
chemistry,2 coordination-driven self-assembly,3 and fine
chemical synthesis.4 On the other hand, cyclometalation
reaction is considerably researched in coordination and
organometallic chemistry, which gives a direct and efficient
route to preparing coordination transition-metal complexes
that contain the metal−carbon σ bond.5 These complexes are
often prepared by a two-step route: the initial reaction between
the metal center and a donor atom of the ligand and
subsequent metal-assisted C−H bond activation.
The unsaturated imine CN group in this ligand showed
diverse reactivity such as cycloadditions, electrophilic addi-
tions, catalytic reductions, aziridinations, and condensation
reactions.6 Imines are generally prepared by the simple
reactions of carbonyl compounds with primary amines through
the condensation and dehydration processes under acidic
reaction conditions. However, the requirement of unstable
substrates such as aldehydes and an acidic reaction environ-
ment limited their large applications.7 Thus, considerable
research has been developed in order to obtain efficient and
mild oxidation processes for the preparation of imine products
starting from simple and low-cost starting materials.8 So far
two synthetic routes have been developed: (i) primary and
secondary amine oxidation processes, which directly sym-
metrically afforded imines with different substituents; (ii)
oxidative cross-coupling reactions of primary amines with
alcohols, for which this protocol unsymmetrically afforded
Received: January 19, 2021
Published: March 24, 2021
https://doi.org/10.1021/acs.inorgchem.1c00174
© 2021 American Chemical Society
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a
Scheme 1. Synthesis of Cyclometalated Half-Sandwich Iridium Complexes 1−4
a
Reaction conditions: (i) 1-tetralone (1.0 equiv), p-TsOH (0.1 equiv), aromatic amines (1.1 equiv), toluene, reflux, 16 h; (ii) [Cp*IrCl2]2 (0.5
equiv), NaOAc (2.0 equiv), CH3OH, reflux, 8 h.
Figure 1. Crystal structures of complexes 3 and 4 with thermal ellipsoids drawn at the 30% level. Selected bond lengths (Å) and angles (deg) for
complex 3: Ir1−N1, 2.095(7); Ir1−C1, 2.019(7); Ir1−Cl1, 2.4015(18); C7−N1, 1.294(9); C11−N1, 1.432(9); C1−Ir1−N1, 78.3(2); C1−Ir1−
Cl1, 88.57(16); Cl1−Ir1−N1, 87.49(19); C6−C7−N1, 114.6(6); C7−C6−C1, 114.8(6). Selected bond lengths (Å) and angles (deg) for complex
4: Ir1−N1, 2.113(6); Ir1−C1, 2.045(7); Ir1−Cl1, 2.4037(18); C7−N1, 1.296(9); C11−N1, 1.410(9); C1−Ir1−N1, 77.5(3); C1−Ir1−Cl1,
86.4(2); Cl1−Ir1−N1, 85.65(17); C8−C7−N1, 124.9(6); C7−C8−C1, 109.9(7).
precursors, high yields of the target products, and the easy
alteration of the properties of such half-sandwich complexes.26
Therefore, the design, synthesis, and application study of
transition-metal complexes bearing half-sandwich moieties has
become one of the hottest research areas in coordination
chemistry.27−32 We report here four half-sandwich C,N-
coordinated cyclometalated iridium complexes bearing Schiff
base ligands. These iridium complexes showed diverse and
good catalytic efficiency in the oxidation reactions of primary
and secondary amines, as well as the cross-couplings of primary
amines and alcohols to imines in open-flask conditions.
CH3OH in the presence of NaOAc for 8 h (Scheme 1). All of
the cyclometalated complexes were isolated as a red powder
and displayed good air stability in solution and the solid state.
Complexes 1−4 are soluble in polar organic solvents such as
tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), CH2Cl2,
acetonitrile, and N,N-dimethylformamide (DMF), affording
red solutions, but their solubility is poor in ether and hexane.
Elemental analysis and high-resolution mass spectrometry
(HRMS) characterization data of the cyclometalated iridium
complexes are consistent with the expected formulas of
complexes 1−4. These cyclometalated complexes showed
good thermal stability, which was confirmed by the results that
the 1H NMR spectra of complexes 1−4 were the same as those
before and after heating in DMF at 100 °C for 1 day.
RESULTS AND DISCUSSION
■
Synthesis of Cyclometalated C,N-Chelate Half-Sand-
wich Iridium Complexes 1−4. Schiff base ligands L1−L4
were prepared in good yields (73−82%) by refluxing 1-
tetralone and the corresponding aromatic amines in toluene
catalyzed by p-toluenesulfonic acid (p-TsOH). Using a Dean−
Stark trap is necessary in the experiment, or the products were
given in low yields. The strong stretch displayed in the range of
1465−1490 cm−1 in their IR indicated formation of the imine
group. New cyclometalated half-sandwich iridium complexes
1−4 containing the ligands were generated with moderate-to-
good yields through C−H bond activation by the refluxing
reaction of L1−L4 and the metal precursor [Cp*IrCl2]2 in
The signals displayed in the 1H and 13C NMR spectra of the
cyclometalated complexes 1−4 confirmed their expected
structures. The singlet observed at the high-field range of
1.40−1.45 ppm was ascribed to protons of the half-sandwich
motifs of the iridium complexes.33 Feature stretching
vibrations of the imine groups shifted from 1465−1490 cm−1
to lower wavenumbers of 1420−1435 cm−1 in the cyclo-
metalated iridium complexes. The red shift of the CN signal
probably stemmed from the coordination interaction between
the Ir center and N donor. The lower wavenumbers of the
imine bond in the cyclometalated iridium complexes were
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caused by the strong back-bonding effects of the Ir atom to the
N donor because the electron density around the metal center
was increased through the coordination interaction.34 All
characterization data of the cyclometalated complexes 1−4
confirmed their analogous structures.
Single-Crystal X-ray Analysis of Cyclometalated
Iridium Complexes. Single-crystal X-ray analysis of the
C,N-chelate cyclometalated iridium complexes 3 (CCDC
2056773) and 4 (CCDC 2056774) was performed with the
aim of confirming their exact structures. Qualified single
crystals of the complexes used for X-ray diffraction were given
through a liquid diffusion method. Complexes 3 and 4
crystallized in the monoclinic space groups P21n and P21c,
respectively (Figure 1). Table 1 show the crystal data of the
Homocouplings of Primary and Secondary Amine
Dehydrogenation. On the basis of our previous study in this
area, we initially investigated the oxidative homocouplings of
primary amines. Benzylamine oxidation was used as the model
reaction to screen the conditions. The iridium complex 1 (1.0
mol %) was used for the homocoupling of benzylamine
oxidized by O2 (1 atm) at 80 °C for 6 h in different organic
solvents. The best results were given when DMSO was used as
the solvent, and the imine product 5a was given in 98%
conversion with 95% yield (Table 2, entries 1−6). A decrease
of the product yield was obviously observed with a shortening
of the reaction time or a lowering of the reaction temperature
to room temperature (Table 2, entries 7 and 8). Decreasing
the catalyst loading of the catalyst 1 from 1.0 to 0.5 mol %
showed little influence on the yields of the product; however,
low yield was furnished when the catalyst loading was lowered
to 0.1 mol % (Table 2, entries 9 and 10). To our delight, good
catalytic activity was observed when the reaction was carried
out under open-flask conditions (air as the oxidant), and 5a
was given in 97% conversion and 94% isolated yield after 10 h
(Table 2, entry 11). Cyclometalated half-sandwich iridium
complexes 2−4 all displayed similar efficiencies in the
homocoupling reactions, and 5a was given in high yield
(93−95%; Table 2, entries 13−15). The catalytic system
exhibited good selectivity because no nitriles were found in the
reaction.12 No products were obtained when the reaction was
performed under an inert atmosphere or in the absence of the
catalyst (Table 2, entries 16 and 17). Oxidative homocouplings
of primary amines catalyzed by [Cp*IrCl2]2 have been carried
out, and only 8% conversion of the reactant was observed
(Table 2, entry 18).
The substrate universality of the reaction has been
investigated to obtain the optimal reaction conditions. The
reactivities of different primary amines were investigated by
using complex 1 as the catalyst, and the corresponding imines
were afforded in excellent yields smoothly under open-flask
conditions. Primary amine substrates with both electron-
donating (Table 3, 5b and 5f) and electron-withdrawing
(Table 3, 5d−5f) groups all furnished corresponding imines in
good yields and selectivity. Halogen groups bonded on the
substrates were intact in the reaction, which indicated mild
reaction conditions (Table 3, 5d−5f). The influence of the
substituent position is not obvious in the catalytic oxidative
reactions because the homocouplings of o-, m-, and p-
methylbenzylamine all gave the corresponding products in
good yields (Table 3, 5b, 5g, and 5h). α-Substituted
benzylamine also afforded imine 5i in good yields, suggesting
few steric effects in the reaction (Table 3, 5i). Additionally, the
oxidation of heterocycle substrates such as 2-aminothiophene
and 2-aminofuran well performed under such conditions
(Table 3, 5j and 5k). Unfortunately, low reactivity of aliphatic
amine was observed, so an elevated reaction temperature and
longer reaction time were used in the reaction for imine
production with good yields (Table 3, 5l). Such reactivity was
also observed in our previous study.37
Table 1. Crystallographic Data and Structure Refinement
a
Parameters for 3 and 4
3
4
chemical formula
C27H31ClNOIr
C26H28ClN2O2Ir·
CH2Cl2
fw
613.18
713.08
T/K
λ/Å
cryst syst
space group
a/Å
173(2)
173(2)
1.34138
monoclinic
P2(1)/c
11.1983(13)
28.908(3)
8.2048(9)
90
93.918(3)
90
2649.9(5)
4
0.71073
monoclinic
P2(1)/n
8.4001(2)
17.5468(4)
16.2533(4)
90
94.9060(10)
90
2386.88(10)
4
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
Z
ρ/(Mg m−3
μ/mm−1
F(000)
)
1.706
11.993
1208
1.787
8.469
1400
θ range/deg
3.714−75.995
27828
4936/30/286
1.143
3.442−57.494
44351
5498/20/331
1.119
reflns collected
data/restraints/param
GOF on F2
final R indices [I >
R1 = 0.0798, wR2 =
0.2065
R1 = 0.0663, wR2 =
0.1931
a
2σ(I)]
largest diff peak/hole/(e 6.034/−4.659
3.955/−5.502
Å−3
)
a
2
R1 = ∑||Fo| − |Fc||/∑|Fo| (based on reflections with Fo > 2σF2).
wR2 = [∑[w(Fo − Fc2)2]/∑[w(Fo )2]]1/2; w = 1/[σ2(Fo ) +
2
2
2
(0.095P)2]; P = [max(Fo ,0) + 2Fc2]/3 (also with Fo > 2σF2).
2
2
two iridium complexes, and the featured bond distances and
angles are concluded in the caption of Figure 1. The Ir center
in the cyclometalated complexes is a distorted-octahedral
geometry, and the metal center of complexes 3 and 4 both
displayed a typical three-legged piano-stool structure, the Ir
center of which was chelated by the Cp* and C, N, and Cl
atoms.35 The bond distances of Ir−N in the two iridium
complexes [2.095(7) Å (3) and 2.113(6) Å (4)] are normal in
the known range of bond lengths in the analogous C,N-chelate
iridium complexes.36 Moreover, the Ir−C(1) and Ir−Cl(1)
bond lengths in complexes 3 and 4 were all located in the
normal range compared with that of the analogous
complexes.36 The cyclohexyl rings in the two complexes are
not planar. Weak intermolecular interaction was not observed
in the crystal stacking structure.
Inspired by the excellent catalytic activity given by the
iridium complex, lower reactive substrate secondary amines
were used in the oxidative reaction under optimal conditions.
The imine product 5a through dibenzylamine dehydrogen-
ation was obtained in low yield, whereas the positive result
(85% yield) was observed when O2 was used as the oxidant.
Other imines were also furnished in good yields through the
O2-oxidized dehydrogenation process. Oxidation of 1,2-
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a
Table 2. Screening of the Catalytic Oxidation of Benzylamine
b
entry
catalyst
solvent
T/°C
oxidant
time/h
conv/%
yield /%
TON
TOF/h−1
1
2
3
4
5
6
1 (1.0 mol %)
1 (1.0 mol %)
1 (1.0 mol %)
1 (1.0 mol %)
1 (1.0 mol %)
1 (1.0 mol %)
1 (1.0 mol %)
1 (1.0 mol %)
1 (0.5 mol %)
1 (0.1 mol %)
1 (0.5 mol %)
1 (0.5 mol %)
2 (0.5 mol %)
3 (0.5 mol %)
4 (0.5 mol %)
1 (0.5 mol %)
DMSO
CH3CN
DMF
80
80
80
80
80
80
80
rt
80
80
80
80
80
80
80
80
80
80
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
air
air
air
air
air
N2
air
air
6
6
6
6
6
98
72
66
51
76
63
72
22
97
63
79
98
97
98
96
95
68
61
47
74
60
68
18
95
60
75
95
93
95
93
95
68
61
47
74
15.8
11.3
10.2
7.8
12.3
10.0
22.7
3.0
31.7
100.0
25.0
19.0
18.6
19.0
18.6
THF
toluene
iPrOH
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
6
60
7
8
9
3
6
6
6
68
18
190
600
150
190
186
190
186
10
11
12
13
14
15
16
17
6
10
10
10
10
10
10
10
c
18
[Cp*IrCl2]2
8
a
b
Reaction conditions: benzylamine (1.0 mmol), iridium catalysts 1−4, O2 or air (1 atm) and solvent (2.0 mL). Conversions and yields were
confirmed by GC−MS (internal standard: n-tridecane). [Cp*IrCl2]2: 0.5 mol %.
c
dihydroquinoline furnished quinoline (Table 3, 5m) in good
yield (88%). Both large-scale homocoupling of benzylamine
and dehydrogenation of 1,2-dihydroquinoline gave positive
results under optimal conditions (Scheme 2).
which could be further transformed into a NH-imine
intermediate C. This intermediate hydrolyzed to furnish the
aldehyde that condensed with amines to give the final imine
products. For the oxidative cross-coupling reaction, the
alcohols first coordinate to species A to form D, which
smoothlyafforded aldehyde. The [M + H]+ peak of the
benzaldehyde intermediate was detected by MS supported the
plausible mechanism. The observed small amount of unsym-
metrical imine in the cross-coupling oxidation reaction also
supported the generation of aldehyde because the nucleophil-
icity of benzylamines is stronger than that of aniline derivatives.
In addition, benzaldehyde was always detected in low yields
and the amount decreased slowly with time, indicating that
benzaldehyde was generated as the intermediate in the reaction
(Figure 2).
Oxidative Couplings between Amines and Alcohols.
The oxidative cross-coupling reactions between amines and
alcohols were carried out under the optimal oxidative
conditions encouraged by the prominent efficiency of the
catalytic system. To our delight, the reaction occurred
smoothly to furnish the expected cross-coupling products in
high yields along with some symmetrical imines as byproducts,
generated from oxidative homocouplings of the amine
substrates (Table 4, entries 1−5). The desired cross-coupling
products were given with lower yields by using substrates with
electron-deficient groups as the reactants possibly because of
less nucleophilicity of the amine group in these substrates
(Table 4, entries 1−6). Heterocyclic alcohol also gave positive
results in the reaction under open-flask conditions (Table 4,
entry 6). The oxidative cross-coupling product was afforded as
the only product in the reaction with lower yield, which
suggested high selectivity of an aliphatic amine (Table 4, entry
7). The survey of the reactivity of a variety of benzyl alcohol
derivatives under optimal conditions indicated the universality
of the reaction, and the corresponding imines were all obtained
in desired yields (Table 4, entries 8−10). Investigation of the
reactivity of the less reactive aniline derivatives was also carried
out. Only cross-coupling products were furnished compared to
that of the reactions of benzylamines (Table 4, entries 11−15).
On the basis of the experimental results and previous
reports,8,10 a tentative mechanism for this catalytic cycle is
illustrated in Figure 2. First, the chloride ligand dissociated
from the metal center, followed by oxidation of Ir(III) to afford
iridium(IV) oxo species A. This rationalized that DMSO is the
best choice among different solvents because the high polarity
of the solvent facilitated dissociation of the chloride ligand.
Then weak coordination between benzylamine and A gave B,
CONCLUSIONS
■
In conclusion, a series of stable half-sandwich cyclometalated
C,N-chelate iridium complexes based on Schiff base ligands
have been synthesized in moderate-to-good yields. These
iridium complexes showed good activity in the catalytic
preparation of imines. Both the oxidative reactions of primary
and secondary amines and the oxidative cross-couplings
between amines and alcohols furnish products with the desired
yields and high selectivity. The highly efficient catalytic systems
were applicable in industrial areas. Investigation of these
iridium catalysts in other organic reactions is currently
underway.
EXPERIMENTAL SECTION
■
General Data. All manipulations were carried out under an inert
atmosphere using standard Schlenk techniques. Chemicals were used
directly as commercial products without further purification. 1H (500
MHz) and 13C (125 MHz) NMR spectra were measured with a
Bruker DMX-500 spectrometer. IR spectroscopy (KBr) spectra were
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a
Table 3. Oxidation Reactions of Primary and Secondary Amines
a
Reaction conditions: complex 1 (0.5 mol %), DMSO (2.0 mL), air or O2 (1 atm), 80 °C, 10h. Yields were confirmed by GC-MS (internal
b
standard: n-tridecane). Isolated yields (%) are provided in parentheses. Reaction conditions: 100 °C, 16 h.
a
7.09 (t, J = 7.4 Hz, 1H), 6.84−6.80 (m, 2H), 2.92 (t, J = 6.1 Hz, 2H),
2.55−2.51 (m, 2H), 1.92 (dd, J = 12.5 and 6.2 Hz, 2H). 13C NMR
(125 MHz, CDCl3): δ 165.6, 151.7, 141.4, 133.9, 130.7, 129.0, 128.8,
126.5, 126.4, 123.1, 119.5, 30.0, 29.9, 23.0. IR (KBr, disk): ν 3422,
1585, 1548, 1472, 1252, 1207, 759 cm−1. Elem anal. Calcd for
C16H15N: C, 86.84; H, 6.83; N, 6.33. Found: C, 86.75; H, 6.90; N,
6.36.
Scheme 2. Gram-Scale Study of the Oxidation Reaction
L2. Yield: yellow solid, 82.6%. 1H NMR (500 MHz, CDCl3): δ 8.28
(d, J = 7.7 Hz, 1H), 7.44 (d, J = 8.6 Hz, 2H), 7.38 (td, J = 7.5 and 1.1
Hz, 1H), 7.29 (t, J = 7.5 Hz, 1H), 7.21 (d, J = 7.6 Hz, 1H), 6.68 (d, J
= 8.6 Hz, 2H), 2.90 (t, J = 6.1 Hz, 2H), 2.51−2.48 (m, 2H), 1.95−
1.90 (m, 2H). 13C NMR (125 MHz, CDCl3): δ 166.4, 150.4, 141.5,
133.5, 131.9, 131.0, 128.8, 126.5, 126.4, 121.4, 116.0, 29.9, 29.9, 22.9.
IR (KBr, disk): ν 3426, 1517, 1545, 1469, 1239, 1015, 766 cm−1.
Elem anal. Calcd for C16H14BrN: C, 64.02; H, 4.70; N, 4.67. Found:
C, 64.11; H, 4.66; N, 4.70.
a
Reaction conditions: complex 1 (0.5 mol %), DMSO (2.0 mL), air
or O2 (1 atm), 80 °C, 10 h.
recorded with a Nicolet FT-IR spectrophotometer. Elemental analysis
was performed on an Elementar Vario EL III analyzer.
Synthesis of Schiff Base Ligands L1−L4. 1-Tetralone (2.2 g,
10.0 mmol), aromatic amines (10.0 mmol), and p-TsOH (1.0 mmol)
were mixed and refluxed in toluene for 16 h using a Dean−Stark
apparatus. After the reaction, the mixture was cooled to room
temperature and the solvent was removed under vacuum. The crude
products were purified through silica gel column chromatography
(PE/EA = 8:1).
L3. Yield: yellow solid, 80.2%. 1H NMR (500 MHz, CDCl3): δ 8.31
(d, J = 7.9 Hz, 1H), 7.36 (t, J = 6.8 Hz, 1H), 7.29 (t, J = 7.5 Hz, 1H),
7.19 (d, J = 7.6 Hz, 1H), 6.90 (d, J = 8.7 Hz, 2H), 6.76 (d, J = 8.7 Hz,
2H), 3.82 (s, 3H), 2.90 (t, J = 6.1 Hz, 2H), 2.58−2.54 (m, 2H),
1.94−1.89 (m, 2H). 13C NMR (125 MHz, CDCl3): δ 165.9, 155.8,
144.7, 141.2, 134.1, 130.5, 128.7, 126.4, 126.3, 120.9, 114.2, 55.5,
30.0, 29.8, 23.0. IR (KBr, disk): ν 3428, 1555, 1512, 1478, 1255,
L1. Yield: yellow solid, 81.6%. 1H NMR (500 MHz, CDCl3): δ 8.34
(d, J = 7.9 Hz, 1H), 7.39−7.31 (m, 4H), 7.22 (d, J = 7.5 Hz, 1H),
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a
Table 4. Oxidative Cross-Coupling Reactions of Benzyl Alcohols and Benzylamines
a
Reaction conditions: complex 1 (0.5 mol %), alcohols (1.0 mmol), amines (1.0 mmol), DMSO (2.0 mL), 80 °C, 10 h. Yields were confirmed by
GC−MS (internal standard: n-tridecane).
1005, 772 cm−1. Elem anal. Calcd for C17H17NO: C, 81.24; H, 6.82;
N, 5.57. Found: C, 81.28; H, 6.87; N, 5.52.
cm−1. Elem anal. Calcd for C16H14N2O2: C, 72.17; H, 5.30; N, 10.52.
Found: C, 72.23; H, 5.36; N, 10.48.
L4. Yield: yellow solid, 79.3%. 1H NMR (500 MHz, CDCl3): δ 8.25
(d, J = 8.8 Hz, 2H), 8.08 (d, J = 9.0 Hz, 1H), 7.43 (dd, J = 10.7 and
4.2 Hz, 1H), 7.34 (t, J = 5.6 Hz, 1H), 6.90 (d, J = 8.8 Hz, 2H), 6.64
(d, J = 9.0 Hz, 1H), 2.95 (t, J = 6.0 Hz, 2H), 2.52−2.49 (m, 2H), 1.99
(t, J = 9.4 Hz, 2H). 13C NMR (125 MHz, CDCl3): δ 166.5, 144.5,
141.9, 133.4, 131.5, 128.9, 128.8, 127.0, 126.6, 125.1, 113.2, 29.7,
29.6, 23.2. IR (KBr, disk): ν 3418, 1540, 1505, 1488, 1220, 1105, 736
Preparation of Half-Sandwich Iridium Complexes 1−4. The
Schiff base ligands (0.2 mmol), [Cp*IrCl2]2 (0.1 mmol), and NaOAc
(0.4 mmol) were mixed and stirred at 60 °C in CH3OH (10 mL) for
12 h under an inert atmosphere. After cooling to room temperature,
the reaction mixture was filtered and the filtrate was concentrated to
give the crude products. Pure iridium complexes were obtained in
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Article
Figure 2. Left: Possible mechanism for imine formation under iridium catalysis. Right: Time dependence for imine formation from benzyl alcohol
and aniline.
moderate yields through silica gel column chromatography (PE/EA =
3:1).
1. Yield: dark-red solid, 65.3%. H NMR (500 MHz, CDCl3): δ
the resulting mixture extracted with diethyl ether and dried over
anhydrous Na2SO4. Solvent was evaporated under vacuum. The
residue was purified by silica column chromatography to give pure
imines (PE:EA:Et3N = 1:6:0.01).
General Procedure for the Oxidative Cross-Couplings of
Benzyl Alcohols and Benzylamines. In a typical run, cyclo-
metalated iridium catalyst 1 (0.5 mol %), benzyl alcohols (1.0 mmol),
and amines (1.0 mmol) were mixed and stirred in DMSO at 80 °C for
10 h under open-flask conditions. Then the reaction was cooled to
room temperature and the resulting mixture extracted with diethyl
ether and dried over anhydrous Na2SO4. Solvent was evaporated
under vacuum. The residue was dissolved in hexane and analyzed by
gas chromatography−mass spectrometry (GC−MS).
X-ray Crystallography. The diffraction data of 3 and 4 were
collected on a Bruker Smart APEX CCD diffractometer with graphite-
monochromated Mo Kα radiation (λ = 1.34138 Å). The structures
were solved by direct methods and subsequently refined on F2 using
full-matrix least-squares techniques (SHELXL). SADABS absorption
corrections were applied to the data; all non-H atoms were refined
anisotropically, and H atoms were located at calculated positions. All
calculations were performed using the Bruker program SMART.
1
7.65 (d, J = 7.5 Hz, 1H), 7.42 (s, 1H), 7.35−7.29 (m, 1H), 7.14 (t, J =
7.5 Hz, 1H), 6.77 (d, J = 7.3 Hz, 1H), 2.94−2.87 (m, 1H), 2.85−2.75
(m, 2H), 2.69−2.60 (m, 2H), 1.92−1.85 (m, 2H), 1.43 (s, 15H). 13C
NMR (125 MHz, CDCl3): δ 182.7, 168.7, 150.1, 144.6, 143.0, 133.4,
132.7, 127.1, 126.1, 121.0, 119.5, 88.8, 30.4, 29.1, 23.8, 8.5. IR (KBr,
disk): ν 1611, 1552, 1536, 1422, 1135, 923, 755 cm−1. Elem anal.
Calcd for C26H29ClNIr: C, 53.55; H, 5.01; N, 2.40. Found: C, 53.63;
H, 4.96; N, 2.44. HRMS. Calcd for [M − Cl]+: m/z 548.1929. Found:
m/z 548.1920.
2. Yield: red solid, 63.0%. 1H NMR (500 MHz, CDCl3): δ 7.64 (d,
J = 7.4 Hz, 1H), 7.54 (d, J = 7.6 Hz, 2H), 7.14 (t, J = 7.4 Hz, 1H),
6.77 (d, J = 7.2 Hz, 1H), 2.95−2.88 (m, 1H), 2.85−2.74 (m, 2H),
2.67−2.61 (m, 1H), 1.93−1.84 (m, 2H), 1.44 (s, 15H). 13C NMR
(125 MHz, CDCl3): δ 183.3, 169.1, 149.1, 144.4, 143.2, 132.8, 132.6,
126.6, 125.0, 121.1, 119.4, 88.9, 30.4, 29.1, 23.8, 8.6. IR (KBr, disk): ν
1605, 1577, 1553, 1433, 1166, 1023, 821 cm−1. Elem anal. Calcd for
C26H28ClBrNIr: C, 47.17; H, 4.26; N, 2.12. Found: C, 47.10; H, 4.29;
N, 2.16. HRMS. Calcd for [M − Cl]+: m/z 626.1034. Found: m/z
626.1039.
3. Yield: red solid, 62.8%. 1H NMR (500 MHz, CDCl3): δ 7.62 (d,
J = 7.5 Hz, 1H), 7.12 (t, J = 7.5 Hz, 1H), 6.92 (d, J = 8.1 Hz, 2H),
6.75 (d, J = 7.4 Hz, 1H), 3.83 (s, 3H), 2.93 (ddd, J = 16.9, 9.6, and 5.7
Hz, 1H), 2.86−2.75 (m, 2H), 2.65 (dt, J = 9.7 and 5.1 Hz, 1H), 1.86
(dd, J = 9.9 and 4.9 Hz, 2H), 1.44 (s, 15H). 13C NMR (125 MHz,
CDCl3): δ 182.8, 168.5, 157.6, 144.6, 143.5, 142.8, 132.7, 132.2,
124.2, 120.9, 114.6, 88.8, 55.5, 30.4, 29.2, 23.8, 8.6. IR (KBr, disk): ν
1609, 1581, 1563, 1425, 1140, 1017, 755 cm−1. Elem anal. Calcd for
C27H31ClNOIr: C, 52.88; H, 5.10; N, 2.28. Found: C, 52.95; H, 5.04;
N, 2.33. HRMS. Calcd for [M − Cl]+: m/z 578.2035. Found: m/z
578.2042.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00174.
■
sı
1H and 13C NMR spectra of ligands L1−L4 and iridium
complexes 1−4 (PDF)
Accession Codes
CCDC 2056773 and 2056774 contain the supplementary
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.
4. Yield: red solid, 63.4%. 1H NMR (500 MHz, CDCl3): δ 8.32 (d,
J = 8.9 Hz, 2H), 7.66 (d, J = 7.6 Hz, 1H), 7.17 (t, J = 7.5 Hz, 1H),
6.80 (d, J = 7.4 Hz, 1H), 3.00−2.93 (m, 1H), 2.89−2.77 (m, 2H),
2.63 (dt, J = 17.3 and 5.3 Hz, 1H), 1.96−1.86 (m, 2H), 1.44 (s, 15H).
13C NMR (125 MHz, CDCl3): δ 183.9, 170.1, 155.5, 145.6, 144.0,
143.9, 133.0, 126.2, 124.5, 121.3, 113.2, 89.0, 30.5, 29.0, 23.8, 8.6. IR
(KBr, disk): ν 1617, 1572, 1536, 1427, 1115, 963, 753 cm−1. Elem
anal. Calcd for C26H28ClN2O2Ir: C, 49.71; H, 4.49; N, 4.46. Found:
C, 49.78; H, 4.55; N, 4.40. HRMS. Calcd for [M − Cl]+: m/z
593.1780. Found: m/z 593.1788.
AUTHOR INFORMATION
Corresponding Author
■
Zi-Jian Yao − School of Chemical and Environmental
Engineering, Shanghai Institute of Technology, Shanghai
201418, China; Key Laboratory of Synthetic Chemistry of
Natural Substances, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, Shanghai 200032, China;
orcid.org/0000-0003-1833-1142; Phone: +86-21-
General Procedure for the Amine Oxidation. In a typical run,
cyclometalated iridium catalyst 1 (0.5 mol %) and amines (1.0 mmol)
were mixed and stirred in DMSO at 80 °C for 10 h under open-flask
conditions. Then the reaction was cooled to room temperature and
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Article
lective Hydrogenation with Chiral Frustrated Lewis Pairs. Angew.
Chem., Int. Ed. 2010, 49, 9475−9478. (d) Kobayashi, S.; Mori, Y.;
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C Bonds by Addition to Imines and Hydrazones: A Ten-Year Update.
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Huang, Z.-T.; Zheng, Q.-Y.; Wang, C. Mn-Catalyzed Three-
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9070.
60877231; Email: zjyao@sit.edu.cn; Fax: +86-21-
60873335
Authors
Rong-Jian Li − School of Chemical and Environmental
Engineering, Shanghai Institute of Technology, Shanghai
201418, China
Chun Ling − School of Chemical and Environmental
Engineering, Shanghai Institute of Technology, Shanghai
201418, China
Wen-Rui Lv − School of Chemical and Environmental
Engineering, Shanghai Institute of Technology, Shanghai
201418, China
Wei Deng − School of Chemical and Environmental
Engineering, Shanghai Institute of Technology, Shanghai
201418, China
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00174
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Grant 21601125), the Chenguang
Scholar of Shanghai Municipal Education Commission (Grant
16CG64), the Shanghai Gaofeng & Gaoyuan Project for
University Academic Program Development, and the research
funding of Shanghai Institute of Technology (Grants ZQ2020-
10 and XTCX2020-24).
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