Syntheses, structures, and catalytic activities of copper(I) complexes with the ligand 2(4,5-diphenyl-1H-imidazol-2-yl)pyridine

Article abstract of DOI:10.1002/zaac.201300223

Two copper(I) complexes of compositions [Cu(HL)I]₂·EtOH (1) and [Cu(HL)₃]I·MeOH (2) were synthesized via the reactions of HL [HL = 2(4,5-diphenyl-1H-imidazol-2-yl)pyridine] and CuI in EtOH and MeOH, respectively, under solvothermal conditions. The complexes were characterized by X-ray single crystal diffraction, IR spectroscopy, and elemental analysis. Compounds 1 and 2 are catalytically active towards ketalization reaction, giving various ketals under mild conditions. Copyright

Full text of DOI:10.1002/zaac.201300223

ARTICLE
DOI: 10.1002/zaac.201300223
Syntheses, Structures, and Catalytic Activities of Copper(I) Complexes with
the Ligand 2(4,5-Diphenyl-1H-imidazol-2-yl)pyridine
Hua Yang,[a,b] Yonglu Liu,[c] and Dao-Dao Hu*[a]
Keywords: Copper; Structures; Catalysis; IR spectroscopy; X-ray diffraction
Abstract. Two copper(I) complexes of compositions [Cu(HL)I]2·EtOH complexes were characterized by X-ray single crystal diffraction, IR
(1) and [Cu(HL)3]I·MeOH (2) were synthesized via the reactions of spectroscopy, and elemental analysis. Compounds 1 and 2 are catalyti-
HL [HL = 2(4,5-diphenyl-1H-imidazol-2-yl)pyridine] and CuI in cally active towards ketalization reaction, giving various ketals under
EtOH and MeOH, respectively, under solvothermal conditions. The mild conditions.
Introduction
The coordination chemistry of CuI complexes has attracted
intense attention in recent years.[1–4] The most important fac-
tors are: (i) development of biologically active CuI precursors,
which serve as 1e shuttles, alternating between CuI and CuII,
and facilitating a series of biological reactions;[5–7] (ii) applica-
tion of CuI complexes as favorable alternatives to the poly-
pyridine RuII compounds that have been extensively utilized in
Scheme 1. The structure of the ligand HL.
molecular solar energy conversion devices and for molecular
sensing;[8–11] (iii) employment of CuI complexes as inexpen-
sive and environmentally benign catalysts to catalyze a variety
Results and Discussion
of organic reactions.[12–14] While pyrazolates,[15] bis(pyr-
azolyl)borate,[16] derivatives of phenanthroline,[17] imid-
azole,[18] and pyridine[19] are widely used as ligands to con-
struct CuI complexes, 2(4,5-diphenyl-1H-imidazol-2-yl)pyr-
idine (HL, Scheme 1),[20] which is a combination of imidazole
and pyridine, has never been employed to prepare CuI com-
plexes. The use of HL has previously given a handful of
PdII,[21] RuII,[22] NiII,[23] and TiIV[20] complexes. It was as-
sumed that a number of CuI complexes might be available and
reactions of CuI and HL were conducted, which yielded the
complexes [Cu(HL)I]2·EtOH (1) and [Cu(HL)3]I·MeOH (2).
Herein, we report the syntheses, structures, and catalytic ac-
tivity of these two complexes.
Synthesis of Complexes 1 and 2
Complexes 1 and 2 were prepared under solvothermal con-
ditions in sealed reaction vessels. Treatment of HL with two
equiv. of CuI in EtOH led to [Cu(HL)I]2·EtOH (1). Complex
1 was readily isolated as yellow needle crystals with moderate
yield.
When the reaction solvent was changed to MeOH, a new
complex [Cu(HL)3]I·MeOH (2) was obtained, suggesting that
the solvent may play an important role in the preparation of 1
and 2. We thus conducted the reaction in a 1:1 mixture of
MeOH and EtOH, and both the microcrystals of 1 and 2 were
found in the reaction vessel.
* Prof. Dr. D.-D. Hu
E-Mail: daodaohu@snnu.edu.cn
[a] School of Materials Science and Engineering
Shaanxi Normal University
Xi’an, Shaanxi, 710062, P. R. China
[b] College of Chemistry and Chemical Engineering
Yan’an University
We have also explored the preparation of 1 and 2 under
atmospheric pressure at room temperature, and failed to obtain
any product. It seems that solvothermal technique is a good
method to prepare 1 and 2.
Yan’an, Shaanxi, 716000, P. R. China
[c] Key Laboratory of Organic Synthesis of Jiangsu Province, College
of Chemistry
Structure Descriptions of Complexes 1 and 2
Chemical Engineering and Materials Science
Soochow University
Suzhou, 215123, P. R. China
Partially labeled plots of complexes 1 and 2 are shown in
Figure 1 and Figure 2, respectively. The crystallographic data
of the complexes are listed in Table 1.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201300223 or from the au-
thor.
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© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 640, (2), 394–397

CuI Complexes with 2(4,5-Diphenyl-1H-imidazol-2-yl)pyridine
Table 1. Crystal data and structure refinements for 1 and 2.
1
2
Empirical formula
Formula weight
Temperature /K
Wavelength /Å
Crystal system
Space group
a /Å
b /Å
c /Å
α /°
C44H40Cu2I2N6O2
1065.70
293(2)
0.71073
monoclinic
P21/n
8.8653(18)
24.173(5)
10.406(2)
90
C61H49CuIN9O
1114.53
293(2)
0.71073
monoclinic
P21/c
11.160(2)
20.155(4)
23.688(5)
90
Figure 1. Solid state structure of 1 showing the atom labeling scheme.
Hydrogen atoms are omitted for clarity.
β /°
γ /°
108.67(3)
90
2112.7(7)
2
1.675
1052
90.53(3)
90
5328.2(18)
4
1.389
Volume /Å3
Z
ρ /mg·m–3
F(000)
2272
Crystal size /mm
Theta range /°
Limiting indices
0.37ϫ 0.28ϫ0.27
2.23 to 28.41
–11 Յ h Յ 11
–32 Յ k Յ 32
–13 Յ l Յ 10
20588 / 5285
0.27ϫ0.25ϫ0.23
1.72 to 28.30
–12 Յ h Յ 14
–26 Յ k Յ 26
–31 Յ l Յ 31
49077 / 13206
Reflections collected
/ unique
Data / restraints
/ parameters
5285 / 0 / 250
1.036
13206 / 7 / 660
0.917
GooF on F2
a pair of iodide bridges and have a similar tetra-coordinate
environment. The arrangement around each central CuI atom
is best described as a distorted tetrahedron. The CuI ion is
bound to one nitrogen atom from imidazole ring, one pyridine
nitrogen atom, and two bridging I– ions. The Cu–N distances
[2.145 (3) and 2.093(3) Å] fall in the normal range of those
previously reported iodide-bridged CuI complexes containing
Cu–N bonds [2.005(3)–2.119(3) Å for Cu–N(imine) bonds and
2.049(3)–2.173(3) Å for Cu–N(pyridyl) bonds].[24–27] The Cu–
I distances [2.6065(10) and 2.6248(7) Å] are congruent with
the published values.[24–27]
Structure of [Cu(HL)3]I·MeOH (2)
X-ray crystallographic analysis reveals that 2 crystallizes in
the monoclinic space group P21/c. It consists of one CuI ion,
three HL ligands, one I– ion to balance the positive charge,
and one solvent methanol molecule (Figure 2a). The CuI ion
displays distorted square-pyramid arrangement. It is coordi-
nated by two pyridine nitrogen atoms (N4 and N7) and three
imine nitrogen atoms (N2, N5, and N8) from three HL ligands,
with Cu–N bond lengths being 1.999(4) and 2.050(4) Å.
Noticeable intermolecular N–H···O contacts from NH
groups of the HL ligands (donors) to the oxygen atoms of
solvent MeOH molecules (acceptors) are determined (Fig-
ure 2b).
Figure 2. (a) Solid state structure of 2 showing the atom labeling
scheme. Hydrogen atoms and solvent molecules are omitted for clarity.
(b) Crystal packing diagram of complex 2. Hydrogen contacts are rep-
resented by dotted lines.
Structure of [Cu(HL)I]2·EtOH (1)
Catalytic Activities of 1 and 2
X-ray crystallographic analysis reveals that 1 crystallizes in
the monoclinic P21/n space group. It has a centrosymmetrical
As the central copper atoms of 1 and 2 show the 1+ oxi-
dimeric structure (Figure 1). The two CuI ions are linked by dation state, they may exhibit Lewis acid catalytic properties.
Z. Anorg. Allg. Chem. 2014, 394–397
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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395

H. Yang, Y. Liu, D.-D. Hu
ARTICLE
We found that 1 and 2 are catalytically active in ketal forma- Table 4. Conversion yields for the ketalization of 2-butanone catalyzed
tion.[28] Ketalization is an important method to protect the
by 2.
carbonyl group in organic synthesis and drug design.[29] The
reaction requires a Lewis acid catalyst to activate the oxygen
of the carbonyl group, allowing glycol to substitute the ketone
group.
The initial experiment was performed for the reaction of 2-
Entry
Time /h
T /°C
Conversion /%
1
2
3
4
12
24
24
24
90
90
100
110
85
90
93
99.8
butanone and ethylene glycol in toluene and catalyst loading
is 0.1%. Our results are summarized in Table 2. As can be
seen from Table 2, when the reaction was performed at 90 °C
for 12 h, the product 2-ethyl-2-methyl-1,3-dioxolane was ob-
tained in 35% yield. When the reaction time was prolonged to
24 h, 42% yield of the product was obtained. It seems that
higher temperature facilitates the reaction. As the reactions
were conducted at 100, 110, and 120 °C, respectively, 52%,
85%, and 90% yields of the products were achieved.
As the reactions were conducted under aerobic conditions,
it is possible that the oxidation of CuI to CuII occurred, and
CuII may also serve as the catalyst. To test this assumption,
the ketalization reaction of 2-butanone with ethylene glycol
catalyzed by HL+Cu(OH)2, was conducted. It was found that
the yield of ketalization reaction catalyzed by CuII is much
lower than that of complex 1 catalyzed reaction, indicating that
CuI species is the main catalyst of the reaction (see for details
Supporting Information).
Table 2. Conversion yields for the ketalization of 2-butanone catalyzed
by 1.
Conclusions
Entry
Time /h
T /°C
Conversion /%
Two CuI complexes were prepared under solvothermal con-
ditions in different solvent. They were able to catalyze the ket-
alization reaction of several ketones, affording various ketals
under mild conditions.
1
2
3
4
5
12
24
24
24
24
90
90
100
110
120
35
42
52
85
90
Experimental Section
Next, the scope of the catalysis was probed and several
ketones were screened. The results are given in Table 3. The
ketalization of cyclohexanone with ethylene glycol proceeded
in 94% yield, whereas only low yields of ketalization products
for acetophenone and benzophenone were observed.
Materials and Instruments: All reagents used in the synthesis were
of analytical grade. Elemental analyses for carbon, hydrogen, and ni-
trogen atoms were performed with a Carlo-Erba EA1110 CHNO-S
microanalyzer. Infrared spectra (4000–400 cm–1) were recorded by
using KBr pellets with a Nicolet MagNa-IR500 FT-IR spectrometer.
Crystal determination was performed with a Bruker SMART APEX II
CCD diffractometer equipped with graphite-monochromatized Mo-Kα
radiation (λ = 0.71073 Å).
Table 3. Conversion yields for the ketalization of various ketones cata-
lyzed by 1.a)
X-ray Crystallography: The single crystals were placed in a Bruker
SMART APEX II CCD diffractometer. The diffraction data were ob-
tained by using graphite monochromated Mo-Kα radiation with a ω–
2θ scan technique at room temperature. The structure was solved by
direct methods with SHELX-97. A full-matrix least-squares refinement
on F2 was carried out by using SHELXL-97. The collected crystal data
for the two structures are shown in Table 1.
Entry
Ketone
Conversion /%
1
2
3
acetophenone
benzophenone
cyclohexanone
18
2
94
Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallo-
graphic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ,
UK. Copies of the data can be obtained free of charge on quoting
the depository numbers CCDC-868930 (1) and CCDC-873502 (2)
(Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://
www.ccdc.cam.ac.uk).
a) Reaction conditions: 110 °C at 24 h.
The catalytic activity of 2 (Table 4) toward ketalization of
2-butanone with ethylene glycol is even better than that of 1.
When the reaction was conducted at 90 °C, 85% yield of the
product was obtained. As the temperature was increased to
120 °C, nearly 100% yield of the product was achieved, which
indicates that the ketone could be completely protected under
the relatively mild condition.
Synthesis of [CuI(HL)]2·EtOH (1): A mixture of CuI (0.0380 g,
0.20 mmol), HL (0.0296 g, 0.1 mmol), and ethanol (1.2 mL) was
sealed in an 8 mL Pyrex-tube, which was heated to 120 °C for 4 d,
and then cooled to room temperature. Purple crystals were collected by
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© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 394–397

CuI Complexes with 2(4,5-Diphenyl-1H-imidazol-2-yl)pyridine
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Published Online: October 2, 2013
Z. Anorg. Allg. Chem. 2014, 394–397
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
397