Synthesis, Structures, and Catalytic Properties of Dinuclear Iridium(I) Complexes with a Hexadentate Macrocyclic Diamine-Tetracarbene Ligand

Article abstract of DOI:10.1002/ejic.201800088

Two dinuclear macrocyclic Ir^I NHC complexes, [Ir₂(L¹)(COD)₂](PF₆)₂ (1, COD = 1,5-cyclooctadiene) and [Ir₂Cl(L¹)(COD)]Cl (2), were prepared by the reaction between [{Ir

Full text of DOI:10.1002/ejic.201800088

A Journal of
Accepted Article
Title: Synthesis, Structures and Catalytic Properties of Dinuclear
Iridium(I) Complexes with a Hexadentate Macrocyclic Diamine-
Tetracarbene Ligand
Authors: Fan Fei, Taotao Lu, Chu-Fan Yang, Xue-Tai Chen, and Zi-
Ling Xue
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800088
Link to VoR: http://dx.doi.org/10.1002/ejic.201800088

10.1002/ejic.201800088
European Journal of Inorganic Chemistry
Synthesis, Structures and Catalytic Properties of Dinuclear
Iridium(I) Complexes with a Hexadentate Macrocyclic
Diamine-Tetracarbene Ligand
Fan Fei,[a] Taotao Lu,[a] Chu-Fan Yang,[a] Xue-Tai Chen*[a] and Zi-Ling Xue[b]
Abstract: Two dinuclear macrocyclic Ir(I)-NHC complexes [Ir2(L1)(COD)2](PF6)2 (1,
COD = 1,5-cyclooctadiene) and [Ir2Cl(L1)(COD)]Cl (2) have been prepared from the
reaction between [IrCl(COD)]2 and macrocyclic N-heterocyclic carbene (NHC) ligand
precursor (H4L1)(PF6)4 containing four benzimidazolium and two secondary amine
groups. Furthermore, 1 and 2 have been carbonylated into [Ir2(L1)(CO)4](PF6)2 (3) and
[Ir2Cl(L1)(CO)2]Cl (4), respectively. Notably, 1 can capture the chloride ion to turn
itself into 2 accompanying with the transformation from cis to trans configuration.2
and 4 are unusual with a five-coordinate Ir(I) center because such five-coordination is
rarely reported for Ir(I)-NHC complexes. The catalytic properties of 1, 2 and a related
mononuclear Ir(I) complex [Ir(L2)(COD)](PF6) (5, L2 = bis-NHC ligand) have been
evaluated in 1,4 conjugate addition reactions of arylboronic acids to
cyclohex-1-en-2-one. These three complexes are the first examples of Ir(I)-NHC
[a]
[b]
Fan Fei, Taotao Lu, Chu-Fan Yang, Xue-Tai Chen*
State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures
School of Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210023, China
E-mail: xtchen@nju.edu.cn.
http://sklcc.nju.edu.cn/showfacu.asp?id=48
Zi-Ling Xue
Department of Chemistry
University of Tennessee
Knoxville, Tennessee 37996, United States
Supporting information for this article is available on the WWW under https:
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10.1002/ejic.201800088
European Journal of Inorganic Chemistry
species showing such catalytic properties for 1,4-addition reactions, which possess
higher catalytic activities than the reported Ir(I) complex
[Ir(hydroquinone)(COD)](BF4).
Introduction
Since the isolation of the first stable N-heterocyclic carbene (NHC) by Arduengo
and co-workers,[1] NHCs have been extensively employed as ancillary ligands in
organometallic chemistry due to the facile tuning of the ligand’s electronic and steric
effects via the introduction of the substituents on the heterocyclic rings.[2] In the past
decades, metal complexes with various type of macrocyclic NHC-containing
ligands[2l] including bidentate,[3] tridentate,[4] tetradentate[5-11] ligands and those
high-dentate with more than four donors[12,13] have been reported, some of which have
found potential applications as luminescent materials,[3a] medicines,[4a] and
catalysts.[8c,10c]
Among the various NHC-containing macrocyclic ligands, the macrocyclic
tetracarbene ligands with different linkers have been considerably studied. The
resulting complexes exhibit diverse conformations due to the flexibility of
macrocyclic ligands.[2l,6-12] They can be roughly classified into two types. The first
type contains the exclusive four NHC groups, which have been substantially
employed to prepare many metal complexes.[6-11] The second type is those with more
than four donor atoms including donor atoms other than the four NHC groups.[12] The
first macrocyclic tetra(NHC) platinum(II) complex (A, Scheme 1) was synthesized
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European Journal of Inorganic Chemistry
via a template controlled reaction by Hahn et al..[6a] Subsequently, a variety of
macrocyclic tetra(NHC) metal complexes have been obtained using in situ
deprotonation or transmetalation method.[6-12] Most of tetracarbene metal complexes
are mononuclear with coordination number 4-6, especially for the first type tetra-NHC
ligands.[6-11] The dinuclear or multinuclear structures are found for Au(I), Ag(I) or
Cu(I) complexes, in which each metal ion is coordinated with two NHC donors in a
linear fashion (example B,Scheme 1).[6b-6d,7a,8b,10a,12] It should be noted that the
combination of the four NHC groups and additional donor atoms such as nitrogen
could give rise to various macrocyclic functionalized NHC ligands. However, the
number of the reported examples is small.[12] Pyridine group has been combined with
four NHC donors to give several mixed pyridine-NHC ligand precursors, from which
very interesting macrocyclic metal-NHC complexes have been synthesized (examples
C, D, Scheme 1).[12a,12b] Recently we have designed a 26-membered macrocyclic
diamine-tetracarbene ligand (L1) and synthesized several metal complexes including
the first example of dinuclear Ni(II) complex with a singly bridging oxo group and
Ni(II) complex with Ni2N2 core formed by two bridging amides (E, Scheme 1).[14]
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European Journal of Inorganic Chemistry
2PF6
NMe2
N
N
N
N
N
N
Me2N
N
N
N
N
N
N
N
M
M
Pt
N
Me2N
NMe2
N
N
M = Ag, Au
B, Chen 2017
A, Hahn 2005
4PF6
N
N
2X
N
N
Ag
Ag
N
N
N
N
N
N
N
N
N
Ag
N
Ag
Ag
N
N
N
Ag
N
N
N
N
N
N
N
N
N
N
N
N
N
C, Hahn 2008
D, Hahn 2010
2
O
O
Ir
O
N
N
N
N
N
N
N
N
N
N
2PF6
Ni
Ni
Au
N
N
N
N
N
N
N
N
O
O
Ir
O
E, Chen 2017
F, Hahn 2015
Scheme 1. Selected examples of metal complexes with macrocyclic tetracarbene
ligands
The Ir-NHC complexes have been extensively investigated as outstanding catalysts
for, e.g., transfer hydrogenation[15] and N-alkylation catalysts.[16] Most of
Ir(I,III)-NHC catalysts are supported by mono-NHC, poly-NHC or functionalized
NHC ligands containing other donor group.[15-17] However, the example of the Ir
complex with macrocyclic NHC-containing ligand is extremely rare. The only known
example is a heterobimetallic complex, in which the Au(I) ion is located in the cavity
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10.1002/ejic.201800088
European Journal of Inorganic Chemistry
of the tetra-NHC ligand and each of the two Ir(I) atom is coordinated with one
carbene donor hanging outside the ligand (F, Scheme 1).[6d]
As an extension of the study of coordination chemistry of our diamine-tetracarbene
ligand L1,[14] we have attempted to prepare Ir(I) complexes with L1. Here we report
dinuclear Ir(I) complexes [Ir2(L1)(COD)2](PF6)2 (1) and [Ir2Cl(L1)(COD)]Cl (2),
[Ir2(L1)(CO)4](PF6)2 (3), and [Ir2Cl(L1)(CO)2]Cl (4). 2 and 4 are unusual binuclear
complexes containing a rare five-coordinate Ir(I) center. 1 can switch its configuration
from cis to trans form by capturing chloride ion to form 2. 1 and 2 display
considerable catalytic activities for 1,4 addition reactions of arylboronic acids to α,
β-unsaturated ketones.
Results and Discussion
Synthesis and Characterization of Metal Complexes.
The macrocyclic salt (H4L1)(PF6)4 was prepared using a reported three-step
procedure.[14] Complex [Ir2(L1)(COD)2](PF6)2 (1) is prepared in 42% yield as the
major product by reacting (H4L1)(PF6)4 with [IrCl(COD)]2 in the presence of CsF at
refluxing in acetonitrile (Scheme 2). A minor product is also isolated as
[Ir2Cl(L1)(COD)]Cl (2) in 15% yield from the same reaction pot. 1 and 2 are
separated by column chromatography.
Ag(I)-NHC complexes are often used as transmetalation reagent to prepare other
transition metal NHC complexes.[18] 1 can also be synthesized by transmetalation
reaction from Ag(I) complex formed in situ, from which a binuclear species was
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European Journal of Inorganic Chemistry
isolated and characterized in our previous work.[14] The reaction was carried out
between [IrCl(COD)]2 and 0.5 equiv of Ag-L1 formed in situ in CH3CN, affording 1 in
56% yield. In this reaction, 1 is the only isolated product because of the absence of
free Cl- ion, which was fully used by Ag+ to form AgCl precipitate.
1 and 2 can be carbonylated by bubbling a stream of CO into an acetonitrile
solution to yield the carbonylated complexes [Ir2(L1)(CO)4](PF6)2 (3) and
[Ir2Cl(L1)(CO)2]Cl (4) in high yield, respectively.
1-4 have been characterized by NMR and IR spectroscopy. No resonance for
2H-benzimidazium protons of (H4L1)(PF6)4 was detected in 1-4, indicating the
coordination of the carbene carbon to the Ir(I) ions. The 1H NMR spectrum of 1
reveals a highly symmetrical structure, which displays four resonances for 16
aromatic protons, one singlet signal at 4.72 ppm for the protons of two equivalent
ethylene linkers between two benzimidazolylidene units, and two signals at 4.84-5.12
ppm and 3.16-3.41 ppm for the ethylene bridges between benzimidazolylidene and
secondary amine. The 13C NMR spectrum of 1 exhibits the characteristic peak of the
carbene carbon at 185.63 ppm, which is similar to those of the reported Ir(I)-NHC
complexes.[15-17] In contrast with 1, the 1H NMR spectrum of 2 is more complicated,
in accordance with lower symmetrical structure. Ten 1H NMR signals appear at
3.32-6.23 ppm, which are due to 12 methylene groups of ethylene linkers. The 13C
NMR spectrum shows two resonances at 185.31 and 186.38 ppm, corresponding to
two magnetically inequivalent carbene carbons. There are also two sets of 13C NMR
resonances at 110.32-135.48 ppm for aromatic carbons, corresponding to two
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10.1002/ejic.201800088
European Journal of Inorganic Chemistry
inequivalent aromatic moieties. No resonances of 1,5-cyclooctadiene at 2.0-2.75 ppm
are found in the 1H NMR spectra of 3 and 4, indicating that COD has been replaced
by CO. The new signals at 174.94 and 168.84 ppm in13C NMR spectra are due to the
carbonyl groups. The IR spectra show infrared stretching frequencies of carbonyl
groups at 2078 cm-1, 1999 cm-1 for 3 and 2092 cm-1, 2010 cm-1 for 4.
Scheme 2. Synthesis of 1-4
It is found that the yield of 2 is enhanced with the addition of n-Bu4NCl at the
beginning of the reaction. When more than 2 equiv n-Bu4NCl is added, 2 is the only
isolated product. Based on this observation, we postulated that 1 can be converted into
2 in the presence of chloride ion. Thus, a reaction between 1 and 2 equiv n-Bu4NCl in
CH3CN was carried out, which indeed yielded 2. This conversion proceeds slowly at
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European Journal of Inorganic Chemistry
ambient temperature, but finishes in half an hour with heating the solution to reflux.
In the conversion from 1 to 2, one COD group in 1 is lost and the simultaneous
coordination of two amine nitrogen atoms and one chloride results in a switch from
trans- to cis-configuration on one half of the molecule.
The solid-state structures of 1 and 2 have been determined by single-crystal X-ray
diffraction on the single crystal grown from slow evaporation of the acetone solution
of each complex (Figs. 1-2 with the selected bond lengths and angles in the captions).
As expected, both 1 and 2 display a dinuclear macrocyclic structure. In the crystal
lattice, there are two crystallographically independent molecules of 1 with similar
structural parameters. Therefore, only structural data for one molecule are discussed
here. 1 exhibits a C2v symmetry, which is in good agreement with the NMR data. Each
Ir(I) center is coordinated with two carbene donors and a COD ligand. Therefore 1
can be regarded as two mono Ir(I) units connected by two diethyleneamino bridges.
The two Ir(I) centers lie on the same side of the macrocyclic ring, adopting a
cis-configuration. The Ir-CNHC distances (2.008(17) Å), Ir-CCOD distances (2.169(17),
2.179(14)Å),and CNHC-Ir-CNHC angle (87.2(8)°) are comparable to the reported
precedents.[15,16]
In 2 (Fig. 2), the two Ir(I) centers are located on the opposite sides of the
macrocyclic ring. Ir2 is four-coordinated with two benzimidazole carbene donors and
a COD ligand, which is similar to 1. Interestingly, the other metal atom Ir1 adopts a
distorted square pyramid geometry (τ = 0.107)[19] with two benzimidazole carbene
donors and two nitrogen atoms in the equatorial plane and one chloride in the apical
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10.1002/ejic.201800088
European Journal of Inorganic Chemistry
position. Despite a large number of Ir(I)-NHC complexes reported, examples of
five-coordinate Ir(I)-NHC complexes remain rare[17] probably due to the high
electron-donor ability of NHC ligands favoring a four-coordinate planar geometry
over five-coordination. The dihedral angle between two benzimidazoles of Ir1
(115.21°) is larger than that in Ir2 (106.84°). The Ir1-CNHC distances are 1.88(2) and
2.022(16) Å, while Ir2-CNHC distances are 1.97(2) and 2.04(2) Å. The Ir1-N distances
are 2.240(13) and 2.243(15) Å. The Ir1-Cl1 distance is 2.513(4) Å, which is similar to
the Ir-Cl distance in other reported Ir(I)-NHC complexes.[17c,17f]
It is found that there are hydrogen bonds between the chloride anions and the
hydrogen atoms. The coordinated Cl1 formed four hydrogen bonds with four
hydrogen atoms. Similar hydrogen bonds are formed between Cl2 and hydrogen
atoms (Figure S12). The π-π stacking interactions of benzimidazole rings exist in the
crystal lattices of 1 and 2, which results in a one-dimensional supramolecular chain
with face to face distance of 3.644 Å in 1 but two neighboring molecules in 2 with
face to face distance of 3.468 Å (Figure S12).
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Figure 1. Structure of 1 showing 30% probability ellipsoids. Hydrogen atoms and the
hexafluorophosphate anions are omitted for clarity. Selected bond lengths (Å) and
angles (°): Ir1-C1 2.008(17), Ir1-C22 2.179(14), Ir1-C23 2.169(17), C1-Ir1-C1
87.2(8), C22-Ir1-C22 80.6(8), C23-Ir-C23 80.5(8).
Figure 2. Structure of 2 showing 30% probability ellipsoids. Hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and angles (°): Ir1-C1 2.022(16),
Ir1-C19 1.88(2), Ir1-Cl1 2.513(4), Ir1-N8 2.240(13), Ir1-N3 2.243(15), Ir2-C12
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European Journal of Inorganic Chemistry
1.97(2), Ir2-C30 2.04(2), C1-Ir1-C19 89.9(7), C1-Ir1-N8 166.2(7), C19-Ir1-N3
172.6(7), C12-Ir2-C30 88.8(9).
Catalytic Performances of 1 and 2.
The construction of carbon-carbon bond has received much attention in organic
chemistry. It has been reported that a large number of NHC-metal complexes can
induce the reactions of C-C bond formation.[20]
Recent research shows that 1,4 addition of arylboronic acids to α, β-unsaturated
ketones are normally catalyzed by Rh[21,22] and Pd complexes.[23] It is found some
Rh(I)-NHC complex show good activities and chemoselectivities.[22] Only few Ir
complexes have been reported to be able to catalyze the 1,4-addition reactions despite
the lower cost of Ir.[24] To the best of our knowledge, there is no report on the
evaluation of Ir-NHC complexes in this addition reaction. Herein, we have evaluated
the catalytic properties of 1 and 2 in 1,4-addition of arylboronic acids to
cyclohex-1-en-2-one.
The general procedure for catalytic reactions refers to the reported Rh-catalyzed
1,4-addition reaction,[22d] and we have optimized the reactuon medium. The substrates
and 1-2 mol% loading of 1 or 2 were dissolved in the solvent at refluxing within 8 h
and KOH was used as the base. Four solvents have been tested and yields are shown
in Table 1. The mixed solvent toluene/water (2:1) is the best medium for our reactions.
Therefore this mixed solvent (Table 1) was used as the solvent to study the catalytic
performances of 1 and 2 towards the addition of six different arylboronic acid to
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10.1002/ejic.201800088
European Journal of Inorganic Chemistry
cyclohex-1-en-2-one. The results are summarized in Table 2. Comparison of the
yields obtained with 1 and 2 under the same catalytic conditions (Entries 1 vs 2, 7 vs 8,
13 vs 14, 19 vs 20, 25 vs 26, 31 vd 32) show that both 1 and 2 exhibit a definite
catalytic activity, and 1 affords better catalytic outcomes than 2. The lower activity of
catalyst 2 could be due to the presence of the five-coordinate Ir(I) center. This
suggests that the four-coordinate Ir(I) center with two NHC units and COD displays
higher activity than the five-coordinate Ir(I) center. When the amount of catalyst 2
was doubled, the yields of reactions catalyzed by 2 reached the level of 1 as expected
(Entries 3, 9, 15, 21, 27, 33; Table 2).
Table 1. Optimization of the Solvent of Arylboronic Acids to cyclohex-1-en-2-one[a]
Entry
Catalyst
Solvent
Yield(%)[b]
1
2
3
4
1
1
1
1
Toluene
28
12
10
70
THF
THF/water(2:1)
Toluene/water(2:1)
[a] Conditions: Cyclohex-1-en-2-one (0.5 mmol), [Ir] (1 mol%), KOH (0.01 mmol),
ArB(OH)2 (0.6 mmol); [b] Isolated yields.
In order to test if the two mononuclear Ir(I) units in 1 show a cooperative
performance in the catalytic reactions, a related mononuclear Ir(I) complex
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European Journal of Inorganic Chemistry
[Ir(L2)(COD)](PF6) (5) has been synthesized and applied to the addition reaction. 5
was prepared by the reaction between [IrCl(COD)]2 and the Ag-L2 species formed in
situ in CH3CN (Scheme 3). The crystals of 5 were obtained by slow evaporation of an
acetone solution of 5. The molecular structure of 5 was found to have a similar
coordination geometry as 1 (Fig. 3) and Ir2 in 2. The Ir-CNHC distances are 2.033(3) Å,
and 2.012(4) Å and CNHC-Ir-CNHC angle is 84.18(13)°, which are close to those in 1
and Ir2 in 2.
Scheme 3. Synthesis of 5
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10.1002/ejic.201800088
European Journal of Inorganic Chemistry
Figure 3. Structure of complex 5 showing 30% probability ellipsoids. Hydrogen
atoms and the hexafluorophosphate anions are omitted for clarity. Selected bond
lengths (Å) and angles (°): Ir1-C1 2.033(3), Ir1-C10 2.012(4), Ir1-19 2.165(4), Ir1-20
2.214(4), Ir1-23 2.189(4), Ir1-24 2.191(4), C1-Ir1-C10 84.18(13).
The catalytic results indicate that the yield of reaction catalyzed by 5 is close to 2
with the same catalyst loading (Table 2, entries 4, 10, 16, 22, 28 and 34), but lower
than 1. When increasing the catalyst loading of 5 to 2 mol%, the yields are similar to
1 (Table 2, entries 5, 11, 17, 23, 29 and 35). These results show that the two Ir(I)
centers in 1 could be regarded as two independent catalytic centers, while the
five-coordinate Ir(I) unit in 2 is probably inactive for this catalytic reaction.
Sweigart’s group have reported a simple mononuclear Ir(I) complex
[Ir(hydroquinone)(COD)](BF4), which has a considerable catalytic activity for the
same reaction.[24] Our Ir(I)-NHC complexes exhibit nearly twice the activity of this
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10.1002/ejic.201800088
European Journal of Inorganic Chemistry
reported complex. The higher yields are tentatively attributed to the π-π stacking
between substrates and macrocyclic ligand.[22d]
The catalytic activity of the iridium precursor [IrCl(COD)]2 is also investigated
under the same reaction condition for comparison. The yields are similar to the
reported mononuclear Ir(I) complex [Ir(hydroquinone)(COD)](BF4).[24]
From Table 2, the arylboronic acids with electron-donating substituent (3-Me,
3-OMe) have higher yields than the electron-donating substituent (3-F, 3-NO2).
However, all the tested Ir(I) complexes 1, 2, 5 and [IrCl(COD)]2 cannot achieve the
efficiency level of Rh(I) complexes.
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European Journal of Inorganic Chemistry
Table 2. 1,4 Addition of Arylboronic Acids to Cyclohex-1-en-2-one [a]
Entry
Catalyst
[Ir] [%]
Arylboronic
Yeild(%)[b]
1
1
1
1
2
1
2
1
1
1
2
1
2
1
1
1
2
1
2
1
1
1
70
50
68
49
65
23
88
48
85
44
85
31
92
59
90
55
89
23
42
26
2
2
3
2
4
5
5
5
6
[IrCl(COD)]2
7
1
8
2
9
2
10
11
12
13
14
15
16
17
18
19
20
5
5
[IrCl(COD)]2
1
2
2
5
5
[IrCl(COD)]2
1
2
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10.1002/ejic.201800088
European Journal of Inorganic Chemistry
21
22
23
24
25
26
27
28
29
30
31
2
2
1
2
1
1
1
2
1
2
1
1
41
23
39
12
84
53
83
49
80
27
38
5
5
[IrCl(COD)]2
1
2
2
5
5
[IrCl(COD)]2
1
32
33
34
35
36
2
1
2
1
2
1
19
35
18
35
10
2
5
5
[IrCl(COD)]2
[a] Conditions: cyclohex-1-en-2-one (0.5 mmol), [Ir] (1 mol%), KOH (0.01 mmol),
ArB(OH)2 (0.6 mmol), toluene/water (2:1) mixed solvent (2 mL).
[b] Isolated yields.
Conclusions
In this work, four dinuclear Ir(I)-NHC complexes 1-4 with macrocyclic
diamine-tetracarbene ligand have been synthesized and characterized by NMR,
ESI-MS and single X-ray diffraction. 1 contains two Ir(I)-centers coordinated by two
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10.1002/ejic.201800088
European Journal of Inorganic Chemistry
NHC groups and one COD ligand, which are in a cis arrangement. 2 exhibits a trans
configuration, which has one four-coordinate Ir(I) center similar to 1 and the other
five-coordinate Ir(I) ion exhibiting a rare tetragonal pyramid geometry. 3 and 4 are the
carbonylation products of 1 and 2, in that the COD moieties are replaced by CO. 1 is
found to convert into 2 in the presence of chloride ions.
1, 2 and related mononuclear complex 5 have been found to exhibit catalytic
performances toward 1,4-addtion of arylboronic acids to cyclohex-1-en-2-one. They
presents the first examples on Ir-NHC complexes with the catalytic activity for this
addition reaction. The four-coordinate Ir(I) center displays higher activity than the
five-coordinate Ir(I) center. There is no cooperative behavior between the two metal
centres probably due to the long metal-metal distance.
Experimental Section
General Procedures. Unless otherwise noted, all reactions and manipulations were
performed under a dry nitrogen atmosphere using the standard Schlenk techniques.
Solvents were dried using conventional methods and freshly distilled before use.
NMR spectra were recorded on a Bruker AM-400 spectrometer. ESI-MS were
recorded on a Bruker MaXis mass spectrometer. Elemental analysis was performed in
a Perkin-Elmer 240C analytic instrument. Macrocyclic tetra-NHC ligand precursor
(H4L1)(PF6)4,[14] (H2L2)(PF6)2[25] and the Ir(I) precursor [IrCl(COD)]2[26] were
synthesized according to the reported procedures.
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10.1002/ejic.201800088
European Journal of Inorganic Chemistry
One-Pot Synthesis of [Ir2(L1)(COD)2](PF6)2 (1) and [Ir2Cl(L1)(COD)]Cl (2).
(H4L)(PF6)4 (0.1246 g, 0.1 mmol), [IrCl(COD)]2 (0.0671 g, 0.1 mmol) and CsF (0.076
g, 0.5 mmol) were mixed and refluxed with vigorously stirring in 5 mL of acetonitrile
for 18 h. After cooling to room temperature, the precipitate was removed by
centrifugation. The red solution was evaporated in vacuum and the residue was
purified by column chromatography (with acetone/DCM = 1:6) to afford the desired
product 1 as red solid (0.069 g, yield: 42.5%) and 2 as orange solid (0.019 g, yield:
12.4%).
1. 1H NMR (400 MHz, acetone-d6, 25 °C): δ 7.67-7.69 (d, 2JH,H = 8 Hz 4H, Ph-H),
7.52-7.54 (d, 2JH,H = 8 Hz 4H, Ph-H), 7.35 (t, 4H, Ph-H), 7.25 (t, 4H, Ph-H), 5.94-5.95
(m, 4H, COD-H), 5.14-5.17 (m, 4H, COD-H), 5.10-5.12 (m, 4H, NCH2), 4.79-4.86
(m, 4H, NCH2), 4.72 (s, 8H, NCH2), 3.39-3.43 (m, 4H, NCH2), 3.67 (br, 4H, NCH2),
2.71-2.74 (m, 4H, COD-H), 2.39-2.47 (m, 8H, COD-H), 2.08-2.12 (m, 4H, COD-H).
13C NMR(100 MHz, acetone-d6, 25°C): δ 185.63 (Ir-Ccarbene), 134.34
(imidazole-ring-C), 123.18 (PhC), 111.34(PhC), 110.14 (PhC), 80.11 (COD-C), 77.12
(COD-C), 50.09 (N-CH2), 49.15 (N-CH2), 44.27 (N-CH2), 31.45 (N-CH2), 30.95
(N-CH2). MS (ESI): m/z [M-PF6]+: 1407.50, [M-2PF6]2+: 632.25, Anal. Calcd for
C56H66N10F12Ir2P2 (1553.55 g mol-1): C, 43.29; H, 4.28; N, 9.02. Found: C, 43.58; H,
3.94; N, 9.32.
2. 1H NMR (400 MHz, acetone-d6, 25 °C): δ 7.69-7.72 (m, 4H, Ph-H), 7.52-7.54 (dt,
2JH,H = 8 Hz, 8 Hz, 4H, Ph-H), 7.36-7.39 (t, 4H, Ph-H), 7.31 (t, 2H, Ph-H), 7.23 (t, 2H,
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Ph-H), 6.18-6.23 (m, 2H, NCH2), 5.96-6.02 (m, 2H, COD-H), 5.46 (m, 2H, NCH2),
5.25 (m, 2H, NCH2), 5.07-5.11 (m, 2H, COD-H), 4.85-4.89 (m, 2H, NCH2), 4.79-4.81
(m, 2H, NCH2) 4.63 (br, 2H, NCH2), 4.52 (br, 2H, NCH2), 4.13-4.18 (br, 2H, NCH2),
4.07-4.10 (m, 2H, NCH2), 3.32-3.45 (m, 4H, NCH2), 2.80-2.90 (m, 2H, COD-H2),
2.28-2.41 (m, 4H, COD-H2), 2.09-2.13 (m, 2H, COD-H2). 13C NMR(100 MHz,
acetone-d6, 25°C): δ 186.30 (Ir-Ccarbene), 185.31 (Ir-Ccarbene), 134.97 (imidazole-ring-
C), 133.70 (imidazole-ring-C), 123.60 (PhC), 123.32 (PhC), 111.66 (PhC), 110.32
(PhC), 81.38 (COD-C), 78.62 (COD-C), 53.53 (N-CH2), 50.29 (N-CH2), 47.94
(N-CH2), 47.74 (N-CH2), 44.81 (N-CH2), 44.48 (N-CH2), 31.16 (N-CH2), 30.07
(N-CH2), 29.45 (COD-C), 22.56 (COD-C). MS (ESI): m/z [M+H+]+: 1227.58,
[M-2Cl-]2+: 596.42, Anal. Calcd for C48H54N10Cl2Ir2 (1226.35 g mol-1): C, 47.01; H,
4.44; N, 11.42. Found: C, 47.33; H, 4.14; N, 11.55.
Synthesis of [Ir2(L1)(COD)2](PF6)2 (1) Using Ag2O. (H4L)(PF6)4 (0.1246 g, 0.1
mmol) and Ag2O (0.0463 g, 0.2 mmol) were suspended in 5 mL of acetonitrile with
vigorously stirring in the dark for 12 h at ambient temperature. The excess Ag2O was
then removed by centrifugation and [IrCl(COD)]2 (0.036 g, 0.053 mmol) was added
into the solution with vigorously stirring for further 3 h at ambient temperature. The
red solution was filtered through Celite to remove the AgCl precipitate. The solvent
was removed in vacuum and the residue was purified by column chromatography
(with acetone/DCM = 1:6) to afford the desired product 1 as red solid (0.046 g, yield:
55.8%).
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European Journal of Inorganic Chemistry
Synthesis of [Ir2Cl(L1)(COD)]Cl (2) in the Presence of n-Bu4NCl. (H4L)(PF6)4
(0.1246 g, 0.1 mmol), CsF (0.076 g, 0.5 mmol), n-Bu4NCl (0.055 g, 0.2 mmol) and
[IrCl(COD)]2 (0.0671 g, 0.1 mmol) were mixed and refluxed with vigorously stirring
in 5 mL of acetonitrile for 18 h. After cooling to room temperature, the resulting
precipitate was removed by centrifugation. The red solution was evaporated in
vacuum and the residue was purified by column chromatography (with acetone/DCM
= 1:3) to afford the desired product 2 as orange solid (0.058g, Yield: 47.2%). The
crystals of 2 suitable for X-ray diffraction studies were obtained by slow evaporated
the acetone solution of 2.
Conversion of 1 into 2. In a round-bottomed flask, 1 (0.046 g, 0.029 mmol, 1 equiv)
was dissolved in 5 mL of CH3CN, followed by addition of n-Bu4NCl (0.0166 g, 0.06
mmol, 2 equiv). The mixture was stirred at 90 °C for 30 min, and the solution
gradually turns from red to orange. Subsequently, the solution was filtered through
Celite and the solvent was removed in vacuum. The residue was redissolved in
CH3CN and purified by flash column chromatography (with acetone/DCM = 1:3) to
afford 2 as orange solid (0.032 g, yield: 88.6%).
Synthesis of [Ir2(L1)(CO)4](PF6)2 (3). In a Schlenk tube, 1 (0.046 g, 0.029 mmol)
was dissolved in 5 mL of CH3CN. A stream of CO was continued to bubble through
the solution for 30 min at room temperature. The red solution rapidly turned into
bright yellow. Then the gas stream was stopped and the solution was stirred for
another 30 min. After that, the solvent was evaporated under reduced pressure. The
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yellow residue was purified by column chromatography (with EA/DCM = 1:4) to
afford 3 as bright yellow solid (0.027 g, yield: 72.6%). 1H NMR (400 MHz,
acetone-d6, 25 °C): δ 7.69-7.71 (d, 2JH,H = 8 Hz, 2H, Ph-H), 7.63-7.65 (d, 2JH,H = 8 Hz,
2H, Ph-H), 7.54-7.59 (m, 4H, Ph-H), 7.29-7.33 (t, 2H, Ph-H), 7.23-7.27 (m, 6H,
Ph-H), 6.06-6.12 (m, 2H, NCH2), 5.52-5.56 (m, 2H, NCH2), 4.99-5.31 (m, 6H, NCH2),
4.66-4.79 (m, 4H, NCH2), 3.94-4.04 (m, 2H, NCH2), 3.30-3.36 (m, 2H, NCH2),
3.14-3.21 (t, 2H, NCH2), 4.52 (br, 2H, NCH2), 4.07-4.18 (m, 4H, NCH2), 3.32-3.45
(m, 4H, NCH2), 2.74-2.82 (m, 2H, NCH2). 13C NMR(100 MHz, acetone-d6, 25°C): δ
180.93 (Ir-Ccarbene), 174.94 (Ir-CO), 133.88 (imidazole-ring-C), 132.50
(imidazole-ring-C), 122.33 (PhC), 123.61 (PhC), 111.34 (PhC), 109.21 (PhC), 55.22
(N-CH2), 52.77 (N-CH2), 48.73 (N-CH2), 47.99 (N-CH2), 46.78 (N-CH2), 44.11
- +
(N-CH2), 43.40 (COD-C). IR (cm-1): 2078, 1999; MS (ESI): m/z [M–PF6 ] ,
- +
- 2+
- +
[M–2CO–PF6 ] : 1303.25, 1247.25, [M–2PF6 ] , [M–2CO–2PF6 ] : 580.25, 551.33,
Anal. Calcd for C44H42N10F12Ir2O4P2 (1449.23 g mol-1): C, 36.47; H, 2.92; N, 9.66.
Found: C, 36.22; H, 3.14; N, 9.96.
Synthesis of [Ir2Cl(L1)(CO)2]Cl (4). In a Schlenk tube, 2 (0.036 g, 0.029 mmol) was
dissolved in 5 mL of CH3CN. A stream of CO was continued to bubble through the
solution for 30 min at room temperature. The red solution rapidly turned into bright
yellow. Then the gas was stopped and the solution was stirred for another 30 min.
After that, the solvent was evaporated under reduced pressure, the yellow-green
residue was purified by column chromatography (with EA/DCM = 1:2) to afford 4 as
green solid (0.022 g, yield: 64.2%). 1H NMR (400 MHz, DMSO-d6, 25 °C): δ
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European Journal of Inorganic Chemistry
7.88-7.90 (d, 2JH,H = 8 Hz, 2H, Ph-H), 7.79-7.81 (dt, 2JH,H = 8 Hz, 8 Hz, 2H, Ph-H),
7.69-7.71 (m, 4H, Ph-H), 7.35-7.45 (m, 8H, Ph-H), 7.31 (t, 2H, Ph-H), 7.23 (t, 2H,
Ph-H), 6.18-6.23 (m, 2H, NCH2), 5.95-6.01 (m, 2H, NCH2), 5.39 (m, 2H, NCH2),
5.17 (m, 2H, NCH2), 4.95-5.03 (m, 8H, NCH2), 4.73 (m, 2H, NCH2), 3.88 (m, 2H,
NCH2), 3.21 (m, 2H, NCH2), 2.75-2.92 (m, 2H, NCH2). 13C NMR(100 MHz,
DMSO-d6, 25°C): δ 181.99 (Ir-Ccarbene), 175.96 (Ir-Ccarbene), 168.84 (Ir-CO) 134.96
(imidazole-ring-C), 133.60 (imidazole-ring-C), 124.76, 124.70, 123.86, 123.43 (PhC),
112.54, 112.42, 110.83, 110.30 (PhC), 68.97 (N-CH2), 56.30 (N-CH2), 53.81 (N-CH2),
49.87 (N-CH2), 49.20 (N-CH2), 47.88 (N-CH2), 45.19 (N-CH2), 44.49 (N-CH2). IR
(cm-1): 2092, 2010; MS (ESI): m/z [M+H+]+: 1175.33, [M-2Cl-]2+: 552.12, Anal.
Calcd for C42H42N10Cl2Ir2O2 (1174.19 g mol-1): C, 42.96; H, 3.61; N, 11.93. Found: C,
42.55; H, 3.92; N, 11.75.
[Ir(L2)(COD)](PF6) (5). (H2L2)(PF6)2 (0.0582 g, 0.1 mmol) and Ag2O(0.0463 g, 0.2
mmol) were suspended in 5 mL of acetonitrile with vigorously stirring in the dark for
12 h at ambient temperature. The excess Ag2O was then removed by centrifugation
and [IrCl(COD)]2 (0.0368 g, 0.05 mmol) was added, the solution was stirred for
further 6 h. The red solution was filtered and the solvent was removed under vacuum.
The red residue was purified by column chromatography (with EA/DCM = 1:5) to
afford 5 as red solid (0.0476 g, yield: 62.1%). 1H NMR (400 MHz, acetone-d6, 25
°C): δ 7.61-7.63 (d, 2JH,H = 8 Hz, 2H, Ph-H), 7.48-7.50 (d, 2JH,H = 8 Hz, 2H, Ph-H),
7.29-7.32 (m, 4H, Ph-H), 5.82-5.88 (m, 2H, COD-H), 5.11-5.16 (m, 2H, COD-H),
4.67-4.70 (m, 2H, CH2), 4.58-4.61 (m, 2H, CH2), 4.18 (s, 6H, CH3), 2.56-2.60 (m, 2H,
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European Journal of Inorganic Chemistry
COD-H2), 2.31-2.39 (m, 4H, COD-H2), 2.05-2.10 (m, 2H, COD-H2). 13C NMR(100
MHz, acetone-d6, 25°C): δ 188.25 (Ir-Ccarbene), 137.17, 136.83 (imidazole-ring-C),
125.00, 124.94 (imidazole-ring-C), 112.32, 111.67 (PhC), 81.69 (COD), 79.28 (COD),
- +
45.92 (COD), 36.90 (N-CH2), 32.78 (N-CH3). MS (ESI): m/z [M–PF6 ] : 591.33, Anal.
Calcd for C26H30F6IrN4P (736.17 g mol-1): C, 42.44; H, 4.11; N, 7.62. Found: C,
44.33; H, 4.52; N, 7.08.
General Procedure for Iridium-Catalyzed 1,4 Addition Reaction.
Cyclohex-1-en-2-one, arylboronic acid, potassium hydroxide and the catalyst (1-2
mol %) were suspended in 5 mL of the solvent (see table 1 and 2) in a 25 mL Schlenk
tube with thick wall. The mixture was stirred at 100 °C for 8 h. The solution was then
filtered and the filtrate was evaporated under reduced pressure. The residue was
purified by flash column chromatography (with EA/DCM = 1:10) to afford the
corresponding desired products.
Crystal Structure Determination. Single crystal X-ray diffraction data were
collected for 1, 2 and 5 on a Bruker APEX DUO diffractometer with a CCD area
detector (Mo Kα radiation, λ = 0.71073 Å).[27] The APEXII program was used to
obtain frames of data and determining lattice parameters. SAINT was used to
integrate the data. The absorption corrections were applied using SADABS.[28] The
structures were solved using SHELXS-97[29] and subsequently completed via Fourier
recycling using SHELXL 2014 program.[30] All non-hydrogen atoms were refined
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10.1002/ejic.201800088
European Journal of Inorganic Chemistry
with anisotropic thermal parameters and hydrogen atoms were set at the calculated
position and generated by the riding model.
CCDC 1589968 (1), 1589969 (2) and 1589970 (5) contain the supplementary
crystallographic data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre.
Acknowledgements
We are grateful for the financial support from the Natural Science Grant of China (No.
21471078 to XTC) and the US National Science Foundation (CHE-1633870 to ZLX).
Keywords: Macrocyclic Ligands • Carbene Ligands • Coordination modes •
Homogeneous Catalysis • Iridium
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