The synthesis of an aryl alkyl ionic liquid and its application in catalyzing Suzuki-Miyaura coupling reaction

Article abstract of DOI:10.1134/S0036024414080111

An aryl alkyl ionic liquids with a tertiary amino group (N(CH ₃)₂) in the cation were synthesized. As a ligand, the synthesized ionic liquids can coordinate with palladium to form N-heterocyclic carbene (NHC)-Pd complex which exhibited high efficiency in catalyzing Suzuki-Miyaura coupling reaction.

Full text of DOI:10.1134/S0036024414080111

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2014, Vol. 88, No. 8, pp. 1317–1322. © Pleiades Publishing, Ltd., 2014.
CHEMICAL KINETICS
AND CATALYSIS
The Synthesis of an Aryl Alkyl Ionic Liquid and Its Application
in Catalyzing Suzuki–Miyaura Coupling Reaction1
Guochang Chen*, Mingfu Ye, Hongbin Qiao, and Xiaoning Qiu
College of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan, 243032 China
*eꢀmail: chenguochang@iccas.ac.cn
Received June 26, 2013
Abstract—An aryl alkyl ionic liquids with a tertiary amino group (N(CH3)2) in the cation were synthesized.
As a ligand, the synthesized ionic liquids can coordinate with palladium to form
Nꢀheterocyclic carbene
(NHC)–Pd complex which exhibited high efficiency in catalyzing Suzuki–Miyaura coupling reaction.
Keywords: ionic liquid, coupling reaction,
DOI: 10.1134/S0036024414080111
Nꢀheterocyclic carbene, palladiumꢀligand catalyst.
1
INTRODUCTION
donors with negligible
π
character. For the past few
years, NHCs have enjoyed increasing popularity as
ligands in Pdꢀmediated crossꢀcoupling and related
transformations because of their superior perforꢀ
mance compared to the more traditional tertiaryꢀ
Ionic liquids (ILs) have attracted an increasing
amount of interest, owing to their low volatility, nonꢀ
flammability, high chemical and thermal stabilities,
high ionic conductivity, and broad electrochemical
windows [1]. Initial investigations concerning ILs
focused on employing them as “green” solvents in
chemical synthesis, catalysis, separation, and electroꢀ
chemistry [2–8]. In the last decades, ILs have
emerged as templates or stabilizers for nanostructures
[9–11], and as supported catalysts for organic and
electrochemical reactions [12–l6]. Some task specific
ILs have also been designed [17, 18], because the
structures and properties of ILs can be easily tuned by
selecting the appropriate combination of organic catꢀ
ions and anions [1]. It is also possible to utilize one
ionic component to deliver a unique function and the
other to deliver a different and independent function.
This concept would provide a facile and promising
method to prepare multifunctional compounds.
Recently, aryl alkyl ionic liquids (AAILs) [19] were
developed, which combine of sp3 alkyl and sp2 aryl
substituents at those nitrogen atoms of the imidazoꢀ
phosphanes. The strong ꢀelectron donating ability
σ
of NHCs renders oxidative insertion even in chalꢀ
lenging facile substrates, while the steric factors and
particular topology is responsible for fast reductive
elimination. The strong Pd–NHC bonds contribute
to the high stability of the active species, even at low
ligand/Pd ratios and at high temperatures [21–27].
The design and preparation of commercially availꢀ
able, stable, userꢀfriendly, and powerful NHC–Pd
catalysts for Pdꢀmediated crossꢀcoupling reaction, is
highly desirable, and consistent with the requireꢀ
ments of green chemistry.
In the present study, an aryl alkyl ionic liquids with
a tertiary amino group (N(CH3)2) in the cation was
synthesized. In the presence of the synthesized ionic
liquids as a ligand, NHC–Pd catalyst was prepared
and exhibited high performances in catalyzing
SuzukiꢀMiyaura coupling reaction.
lium core. Not only
σ
ꢀbased but also ꢀsystemꢀbased
π
electronic effects can be used to tune the properties.
AAILs are not restricted to van der Waals interactions;
EXPERIMENTAL
Chlorobenzene, bromobenzene, iodobenzene,
imidazole, tetrahydrofuran (THF), 1ꢀbromobutane,
dioxane, ethyl acetate, petroleum ether, methanol,
ethanol, CH2Cl2, MgSO4, NaCl, Et3N, piperazine,
formic acid, formaldehyde, toluene, and DME were
provided by Beijing Chemical Company. Cesium
carbonate, 2ꢀbromoethylamine hydrobromide were
purchased from Aladdin. DBU, 2ꢀbromonaphthaꢀ
lene, 1ꢀbromoꢀ3,5ꢀbis(trifluoromethyl)ꢀbenzene, and
1ꢀbromoꢀ3,5ꢀdiꢀtertꢀbutylbenzene were provided by
Aldrich. THF and toluene were freshly distilled over
they also allow
π
– interactions, which will be imporꢀ
π
tant for applications in the separation of compounds
as well as for the stabilization of catalytically active
metals.
N
ꢀHeterocyclic carbenes (NHCs) are a very
prominent class of ligands in homogeneous catalysis
[20]. They are generally more stable than phosphane
ligands and have been described as predominantly
σ
1
The article is published in the original.
1317

1318
GUOCHANG CHEN et al.
sodium prior to use. Dioxane and DME were freshly was subsequently purified by column chromatography
1
distilled over calcium hydride prior to use. Other (CH3OH/CH2Cl2 = 3/5) and recrystallization. H
chemicals were used as received. All the chemicals NMR (400 MHz, D2O)
were A.R. grade.
δ
: 7.93 (1H, s), 7.77 (1H, s),
7.67 (5H, s), 4.49–4.52 (2H, t), 2.98–3.01 (2H, t),
2.33 (6H, s). 13C NMR (D2O)
: 134.83, 133.99,
δ
1H and 13C NMR spectra were recorded in D2O,
deuterated chloroform or dimethylsufoxide (DMSO)
on a Bruker AVꢀ400 and DMXꢀ300 spectrometers.
1H NMR data are reported as follows: chemical shift,
multiplicity (s = singlet, d = doublet, t = triplet, q =
130.01, 129.88, 122.50, 122.27, 121.87, 55.13, 43.86,
42.41. Calcd. for C13H19Br2N3:C, 41.40; H, 5.08; N,
11.14. Found: C, 42.05; H, 5.09; N, 11.19.
Synthesis of 1ꢀbutylꢀ3ꢀphenylimidazolium bromide.
quartet, qn = quintet, and m = multiplet). The eleꢀ 1ꢀButylꢀ3ꢀphenylimidazolium bromide was syntheꢀ
mental analyses were performed on Flash EA 112 sized according to literature [19]. Typically, comꢀ
instrument.
pound 1 (3.04 g), 1ꢀbromobutane (2.72 mL), and
THF (5 mL) were added to a twoꢀnecked flask
equipped with a magnetic stirrer and the mixture was
Synthesis of aryl imidazole (compound 1). Aryl imiꢀ
dazole was synthesized according to the literature [28].
Under an argon atmosphere a flask was charged with
imidazole (4.08 g, 60 mmol), 1,10ꢀphenanthroline
(0.71 g, 3.6 mmol), CuI (0.57 g, 3 mmol), K2CO3
(16.58 g, 120 mmol), bromobenzene (12 mL,
90 mmol) and DMSO (10 mL). The mixture was
heated under reflux in an oil bath at 100 C for 19 h
°
under nitrogen. The precipitate is collected, washed
several times with THF, and dried in vacuo. The prodꢀ
uct was subsequently purified by column chromatogꢀ
raphy (CH3OH/CH2Cl2 = 1/8). 1H NMR (400 MHz,
DMSO) : 9.81 (1H, s), 8.33 (1H, s), 8.05 (1H, s),
δ
reftuxed for 24 h at 100 C before cooling to room temꢀ
°
7.77–7.79 (2H, d), 7.65–7.69 (2H, t), 7.57–7.61 (1H,
t), 4.24–4.26 (2H, t), 1.84–1.91 (2H, m), 1.31–
1.36 (2H, m), 0.91–0.95 (3H, t). 13C NMR (DMSO)
perature. The product is extracted with ethylacetate
and washed with sodium chloride saturated solution,
the organic layers were dried over MgSO4, and the solꢀ
vent was removed in vacuo. The product is subseꢀ
quently purified by column chromatography (ethyl
acetate/petroleum ether = 3/1). 1H NMR (400 MHz,
δ
: 135.81, 135.23, 120.60, 130.14, 123.80, 122.25,
121.52, 49.53, 31.58, 19.30, 13.78.
Typical Suzuki–Miyaura coupling reaction proceꢀ
DMSO) : 8.25 (1H, s), 7.74 (1H, s), 7.64–7.66 (2H, dure. Under an atmosphere of argon, a Schlenk tube
δ
d), 7.49–7.53 (2H, t), 7.34–7.37 (1H, t), 7.11 (1H, s). was charged with Pd(OAc)2 (6.7 mg, 0.03 mmol),
13C NMR (DMSO)
120.29, 117.95.
δ
: 136.88, 135.47, 129.81, 126.82, [mapmim][Br] (18.8 mg, 0.05 mmol), and Cs2CO3
(684 mg, 2.10 mmol). Dioxane (2 mL) was added via
syringe. After 30 min at 100 C, the reaction mixture
°
Synthesis of 1ꢀaminoethylꢀ3ꢀphenylimidazolium
bromide hydrobromide (compound 2). Compound
(8.64 g, 60 mmol), 2ꢀbromoethanamine hydrobroꢀ
mide (8.16 g, 40 mmol), and toluene (15 mL) were
added to a twoꢀnecked flask equipped with a magnetic
stirrer and the mixture was heated under reflux in an
was cooled to room temperature, chlorobenzene
(102 mL, 1.0 mmol) and phenylboronic acid
(0.18 mg, 1.5 mmol) was added to the reaction mixꢀ
ture. The argon inlet was removed, the septum was
covered with parafilm and the reaction vessel was
1
placed in an oil bath at 100 C. Stirring was continued
°
oil bath at 100 C for 24 h under nitrogen. After this
°
for 36 h. The reaction mixture was then cooled to
room temperature, diluted with dichloromethane (50
mL), filtered through a pad of celite and volatiles were
evaporated under reduced pressure.
time the liquid was separated, the remaining solid was
washed three times with toluene and dried under vacꢀ
uum. The product was subsequently purified by colꢀ
umn chromatography (CH3OH/H2O = 5/1) and
recrystallization. 1H NMR (400 MHz, D2O)
δ
: 7.98–
Purification by column chromatography (petroꢀ
7.99 (1H, d), 7.81–7.82 (1H, d), 7.66 (5H, s), 4.70– leum ether) yielded biphenyl as a white solid (140 mg,
4.73 (2H, t), 3.62–3.66 (2H, t). 13C NMR (D2O)
δ
:
0.91 mmol, 91%). All coupling products were found to
135.03, 134.33, 129.96, 129.87, 122.61, 122.18, be identical by NMR.
121.86, 46.34, 38.37. Calcd. for C11H15Br2N3: C,
37.85; H, 4.33; N, 12.04. Found: C, 37.59; H, 4.36;
N, 11.95.
RESULTS AND DISCUSSION
The synthesis of this AATL was shown in Fig. 1.
Synthesis of 1ꢀN,Nꢀdimethylaminoethylꢀ3ꢀphenyl
imidazolium bromide hydrobromide (compound 3, Aryl imidazole can be synthesized by copper(I)ꢀcataꢀ
named as [mapmin][Br]). Compound (3 g), H2O lyzed Ullmann coupling reaction with 1,10ꢀphenanꢀ
2
(6 mL), formic acid (3.4 mL), and aqueous formaldeꢀ throline as an ligand. The synthesis is only one of sevꢀ
hyde solution (8 mL) were added to a twoꢀnecked flask eral possible ways to construct the imidazole core;
equipped with a magnetic stirrer and the mixture was many others are known [19]. Compound
heated under reflux in an oil bath at 100 C for 36 h pared by nucleophilic substitution reaction with aryl
under nitrogen. Formic acid, formaldehyde, and water imidazole and 2ꢀbromoethylamine hydrobromide,
were then removed under reduced pressure and the and compound was synthesized with compound
remaining solid was washed with ethanol. The product and aqueous formaldehyde solution in the presence of
2 was preꢀ
°
3
2
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THE SYNTHESIS OF AN ARYL ALKYL IONIC LIQUID AND ITS APPLICATION
1319
N
Br
N
N
N
1
NH2HBr
Br
⊕
⊕
HBr
NH2
N HBr
N
N
N
N
HCHO
ꢀ
Brꢀ
Br
3
2
Fig. 1. Synthesis of the IL [mapmim][Br] (3).
formic acid. The structures of compound 2 and 3 were was found an effective base for Suzuki–Miyaura couꢀ
all confirmed by 1H NMR and 13C NMR. Herein the
AAIL with a tertiary amino group (N(CH3)2) in the
cation ([mapmim][Br], 3) was obtained.
pling reaction in room temperature in the presence of
the synthesized NHC–Pd complex as a catalyst, and
other bases tested like piperazine, Et3N and DBU, etc.
were less effective (Table 1, entries 1–4).
NHC palladacycles are often prepared by displaceꢀ
ment of the bridging chloride ligand of the correꢀ
sponding dimeric palladacycle by the free NHC, as
demonstrated in a recent study by Herrmann and coꢀ
workers [29]. It is well known that amine groups bind
very strongly to nanoparticles [30, 31]. So the syntheꢀ
A further investigation of the substrate scope was
carried out under the given reaction conditions in
Table 2. Substrates containing electronꢀdonating
groups provided higher yields. The yields for 2ꢀbroꢀ
monaphthalene is up to 97% with Cs2CO3 as the base
after the reaction of 23 h (Table 2, entry 5). Under
optimized conditions, other substituted substrates
such as 1ꢀbromoꢀ3,5ꢀdimethylbenzene and 1ꢀbromoꢀ
3,5ꢀbis(trifluoromethy])benzene and 1ꢀbromoꢀ3,5ꢀ
diꢀtertꢀbutylbenzene smoothly reacted with boronic
acid and furnished corresponding products in satisfacꢀ
tory yields in desired reaction times. When compound
sized AAIL
3 could be a ligand to form NHC–Pd
complex, which can be easily prepared from pallaꢀ
dium(II) acetate and the corresponding imidazolium
salts [32]. For instance, Lebel et al. [33] obtained
NHC–Pd species from
nyl)ꢀimidazolium chloride and palladium(II) acetate
under the reaction conditions (dioxane, 80 C, 6 h). In
light of these results, NHC–Pd complex (named as
N,Nꢀbis(2,4,6ꢀtrimethylpheꢀ
°
3
was replaced by compound 2 or 1ꢀbutylꢀ3ꢀpheꢀ
nylimidazolium bromide to prepare NHC–Pd comꢀ
plexes to catalyze Suzuki–Miyaura coupling reaction
no product was obtained. So the tertiary amino group
(N(CH3)2) play a key role to stabilize [mapmim][Br]–
Pd resulting in well catalyzing Suzuki–Miyaura couꢀ
pling reaction.
[mapmim][Br]–Pd) was prepared from compound
and palladium(II) acetate.
3
The catalytic activity of the synthesized NHC–Pd
complex was evaluated by catalyzing Suzuki–Miyaura
reactions. Table 1 shows the yields of Suzuki–Miyaura
reactions under different conditions over the resulting
catalyst. As can be seen, the yield of product was up to
91% under the optimized reaction conditions using aryl
chlorides as the coupling partner (Table 1, entry 5).
Factors including ligand, base, solvent and reaction
temperature would influence the interaction of the
NHC–Pd complexes with the C–H bond. Typical
bases used at elevated reaction temperatures for C–H
activation processes are Cs2CO3, piperazine, Et3N and
DBU, etc. which are not strong enough to remove a
proton on an sp3ꢀhybridized carbon atom at room
The possible structure of [mapmin][Br]–Pd was
proposed in Fig. 2. In the structure, the carbine and
the tertiary amino group are both coordinated with
Pd2+. Complex
4 is an example of airꢀstable divalent
palladium compounds with dissymmetric bidentate
Nꢀheterocyclic carbeneꢀamine ligands to constitute
an organometallic complex containing one NHC and
one amine functionality [33, 37–39].
In summary, an aryl aikyl ionic liquids with a terꢀ
tiary amino group (N(CH3)2) on the cation was synꢀ
thesized. Used as a ligand, the synthesized ionic liqꢀ
temperature [34–36]. However, in this study Cs2CO3 uids was coordinated with noble metal Pd to form
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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GUOCHANG CHEN et al.
Table 1. Palladiumꢀcatalyzed Suzuki–Miyaura reactions under different conditionsa
Cl
B(OH)2
IL
+
Pd(OAc)2
R
R
Entry
1
Aryl chlorides
Cl
Solventb
Dioxane
Basec
T
, °C
t
, h
Yield, %
78
Cs2CO3
Piperazine
Et3N
100
100
100
100
100
80
23
23
23
23
36
23
23
23
23
36
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
2
3
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
DME
n.r
Trace
<3%
91
4
DBU
5
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
6
47
7
100
120
120
100
60
8
47
9
60
10
Dioxane
70
a
b
c
Reaction conditions: Aryl halide (1.0 equiv.), arylboronic acid (1.5 equiv.), palladium catalyst (3 mol %), base (2.1 equiv.), solvent (2 mL).
DME = dimethoxyethane.
DBU = 1,8ꢀDiazabicyclo[5.4.0]undecꢀ7ꢀene.
4
N
⊕
2
N HBr
N
N
Pd(OAc)2
Dioxane
N
Pꢀd
ꢀ
⊕
Br
AcO
N
OAc
).
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
4
Fig. 2. The proposed structure of [mapmim][Br]–Pd (
4
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THE SYNTHESIS OF AN ARYL ALKYL IONIC LIQUID AND ITS APPLICATION
1321
Table 2. Palladiumꢀcatalyzed Suzuki–Miyaura reactions: Variation of the aryl halide derivativesa
X
B(OH)2
Pd(OAc)2 IL
+
Cs2CO3 Dioxane
R
R
Entry
1
Substrate
I
Product
t
, h
Yield, %
98
2
Br
Cl
2
3
4
2
95
91
70
36
36
Cl
Br
5
6
23
23
97
83
Br
F
F
F
F
F
F
F
F
F
Br
7
8
23
23
93
80
F
F
F
Br
a
Reaction conditions: Aryl halide (1.0 equiv.), arylboronic acid (1.5 equiv.), palladium catalyst (3 mol %), base (2.1 equiv.), solvent (2 mL), 100°C.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 88 No. 8 2014

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GUOCHANG CHEN et al.
17. A. Mohajeri and A. Ashrafi, J. Phys. Chem. A 115, 6589
NHC–Pd complex which was proved to be a good catꢀ
alyst for SuzukiꢀMiyaura coupling reaction.
(2011).
18. C. Zhao, A. M. Bond, and X. Lu, Anal. Chem. 84, 2784
(2012).
ACKNOWLEDGMENTS
19. S. Ahrens, A. Peritz, and T. Strassner, Angew. Chem.
Int. Ed. 48, 7908 (2009).
The authors thank the Key Project of Scientific
Research Foundation of Education Department of
Anhui Province, China (nos. KJ2013Z026,
KJ2013A064) and the Jiangsu Planned Projects for
Postdoctoral Research Funds (no. 1202012C) for proꢀ
viding financial support to this project.
20. T. Strassner, Top. Organomet. Chem. 22, 125 (2007).
21. Xi. Chen, H. Ke, Y. Chen, C. Guan, and G. Zou,
J. Org. Chem. 77, 7572 (2012).
22. Z. Wang, G. Chen, and L. Shao, J. Org. Chem. 77
,
6608 (2012).
23. E. A. B. Kantchev, C. J. O’Brien, and M. G. Organ,
REFERENCES
Angew. Chem. Int. Ed. 46, 2768 (2007).
1. J. P. Hallett and T. Welton, Chem. Rev. 111, 3508
24. O. Navarro, R. A. Kelly, and S. P. Nolan, J. Am. Chem.
(2011).
Soc. 125, 16194 (2003).
2. H. R. Shaterian, M. Mohammadnia, and F. Moradi,
25. W. A. Herrmann, Angew. Chem., Int. Ed. 41, 1290
J. Mol. Liq. 172, 88 (2012).
(2002).
3. F. V. Rantwijk, R. M. Lau, and R. A. Sheldon, Trends
26. G. Altenhoff, R. Goddard, C. W. Lehmann, and
Biotechnol. 21, 131 (2003).
F. Glorius, J. Am. Chem. Soc. 126, 15195 (2004).
4. C. F. Weber, R. Puchta, N. J. R. V. E. Hommes, P. Wassꢀ
27. A. E. Wang, J. H. Xie, L. X. Wang, and Q. L. Zhou, Tetꢀ
erscheid, and R. V. Eldik, Angew. Chem. Int. Ed. 44
,
rahedron 61, 259 (2005).
6033 (2005).
28. G. Evano, N. Blanchard, and M. Toumi, Chem. Rev.
5. W. Qian, E. Jin, W. Bao, and Y. Zhang, Angew. Chem.
108, 3054 (2008).
Int. Ed. 44, 952 (2005).
29. G. D. Frey, J. Schttz, E. Herdtweck, and W. A. Herꢀ
6. X. Sun, H. Luo, and S. Dai, Chem. Rev. 112, 2100
rmann, Organometallics 24, 4416 (2005).
(2012).
30. S. Mandal, D. Roy, R. V. Chaudhari, and M. Sastry,
7. J. Li, Y. Shen, Y. Zhang, and Y. Liu, Chem. Commun.,
p. 360 (2005)
Chem. Mater. 16, 3714 (2004).
31. R. M. Crooks, M. Zhao, L. Sun, V. Chechnik, and
8. A. Makowska, P. Papis, and J. Szydlowski, J. Mol. Liq.
L. K. Yeung, Accounts Chem. Res. 34, 181 (2001).
176, 86 (2012).
32. W. A. Herrmann, M. Elison, J. Fischer, C. Kocher, and
9. M. Antonietti, D. Kuang, B. Smarsly, and Y. Zhou,
Angew. Chem. Int. Ed. 43, 4988 (2004).
G. R. J. Artus, Angew. Chem., Int. Ed. 34, 2371 (1995).
33. H. Lebel, M. K. Janes, A. B. Charette, and S. P. Nolan,
10. J. D. Scholten, B. C. Leal, and J. Dupont, ACS Catal.
J. Am. Chem. Soc. 126, 5046 (2004).
2, 184 (2012).
34. X. Chen, K. M. Engle, D. H. Wang, and J. Q. Yu,
11. S. Miao, Z. Liu, B. Han, J. Huang, Z. Sun, J. Zhang,
Angew. Chem. Int. Ed. 48, 5094 (2009).
and T. Jiang, Angew. Chem. Int. Ed. 45, 266 (2006).
35. T. S. Mei, X. Wang, and J. Q. Yu, J. Am. Chem. Soc.
12. H. R. Shaterian and M. Mohammadnia, J. Mol. Liq.
131, 10806 (2009).
177, 162 (2013).
36. D. H. Wang, M. Wasa, R. Giri, and J. Q. Yu, J. Am.
13. T. Sasaki, C. Zhong, Mizuki Tada, and Y. Iwasawa,
Chem. Commun., p. 2506 (2005).
Chem. Soc. 130, 7190 (2008).
37. S. Grundemann, A. Kovacevic, M. Albrecht,
J. W. Faller, and R. H. Crabtree, J. Am. Chem. Soc.
124, 10473 (2002).
14. A. Riisager, R. Fehrmann, S. Flicker, R. V. Hal,
M. Haumann, and P. Wasserscheid, Angew. Chem. Int.
Ed. 44, 815 (2005).
38. J. Huang, E. D. Stevens, S. P. Nolan, and
15. D. W. Kim and D. Y. Chi, Angew. Chem. Int. Ed. 43
,
J. L. Petersen, J. Am. Chem. Soc. 121, 2674 (1999).
483 (2004).
39. S. Warsink, S. Y. de Boer, L. M. Jongens, C. Fu, S. Liu,
J. Chen, M. Lutz, A. L. Spek, and C. J. Elsevier, Dalton
Trans., 7080 (2009).
16. Y. Shen, Y. Zhang, Q. Zhang, L. Niu, T. You, and
A. Ivaska, Chem. Commun., p. 4193 (2005).
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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No. 8
2014