Copper(II) hydroxide complexes of N-heterocyclic carbenes and catalytic oxidative amination of arylboronic acids

Article abstract of DOI:10.1021/om100009u

Reactions of the imidazolium salts or their corresponding silver carbene complexes with copper powder afforded unusual copper(II) hydroxide complexes of N-heterocyclic carbenes: [Cu₂(μ-OH)(L1)](PF₆) ₂ (2; L1 = 3,5-bis(N-picolylimidazolylidenylmethy1)pyrazolate), [Cu₄(μ₃-OH)₂(L2)₂](PF ₆)₄ (3; L2 = 3,5-bis(N-pyridylimidazolylidenylmethy1) pyrazolate), and [Cu₄(μ₃-OH)₂(L3) ₂](PF₆)₄ (4; L3 = 3,5-bis(N- pyrimidylimidazolylidenylmethy1)pyrazolate). The same reaction of 3,5-bis(N-thiophenylimidazoliumylmethy1)pyrazole yielded a dinuclear Cu(II)-NHC complex, [Cu₂(L4)₂] (PF₆)₂ (5; L4 = 3,5-bis(N-thiophenylimidazolylidenylmethy1)pyrazolate). The complexes have been fully characterized by X-ray diffraction analysis, elemental analysis, and IR and ESI-MS spectra. A catalytic study shows that complex 4 is highly active in the N-arylation reactions of imidazoles and aromatic amines with arylboronic acids in methanol at room temperature under base-free conditions.

Full text of DOI:10.1021/om100009u

Organometallics 2010, 29, 1457–1464 1457
DOI: 10.1021/om100009u
Copper(II) Hydroxide Complexes of N-Heterocyclic Carbenes and
Catalytic Oxidative Amination of Arylboronic Acids
†
Bo Liu, Bin Liu, Yongbo Zhou, and Wanzhi Chen*
†
†
,†,‡
†
Department of Chemistry, Zhejiang University, Xixi Campus, Hangzhou 310028,
‡
People’s Republic of China, and State Key Laboratory of Elemento-organic Chemistry,
Nankai University, Tianjing, People’s Republic of China
Received January 5, 2010
Reactions of the imidazolium salts or their corresponding silver carbene complexes with copper
powder afforded unusual copper(II) hydroxide complexes of N-heterocyclic carbenes: [Cu (μ-OH)-
2
(
L1)](PF ) (2; L1 = 3,5-bis(N-picolylimidazolylidenylmethyl)pyrazolate), [Cu (μ -OH) (L2) ](PF )
6 2 4 3 2 2 6 4
(
3; L2=3,5-bis(N-pyridylimidazolylidenylmethyl)pyrazolate), and [Cu (μ -OH) (L3) ](PF ) (4; L3=
4 3 2 2 6 4
3,5-bis(N-pyrimidylimidazolylidenylmethyl)pyrazolate). The same reaction of 3,5-bis(N-thiophenyl-
imidazoliumylmethyl)pyrazole yielded a dinuclear Cu(II)-NHC complex, [Cu (L4) ](PF ) (5; L4 =
2
2
6 2
3,5-bis(N-thiophenylimidazolylidenylmethyl)pyrazolate). The complexes have been fully characterized
by X-ray diffraction analysis, elemental analysis, and IR and ESI-MS spectra. A catalytic study shows
that complex 4 is highly active in the N-arylation reactions of imidazoles and aromatic amines with
arylboronic acids in methanol at room temperature under base-free conditions.
Introduction
Cu-NHC complexes, most of the well-defined examples
were prepared from the reaction of free carbenes with
4
a,9
Coinage-metal complexes of N-heterocyclic carbenes
NHCs) have been attracting great attention because of their
diverse structures and wide applications in medicine, nano-
materials, liquid crystals, and organic synthesis as cata-
appropriate copper sources.
Such a route involves the
(
use of strong bases and handling of unstable free carbenes
under air- and moisture-free conditions. Transmetalation
from silver-NHC complexes is practically convenient for
the preparation of many transition-metal-NHC com-
1
-3
lysts.
found wide applications in organic synthesis such as hydro-
Despite the fact that copper NHC complexes have
1
0
plexes; however, we were not successful in preparing Cu(II)
4
silylation, conjugate additions, allylic alkylation reactions,
5
6
complexes by using our previously reported silver-NHC
7
1
1
and cyclopropanation, relatively few Cu(I)- and Cu(II)-
NHC complexes have been structurally characterized. In
complexes as carbene transferring agents. Thus, a practical
synthetic method is required for the development of copper-
NHC chemistry. Most recently, we described a simple syn-
thetic route for the first-row transition-metal-NHC com-
plexes by using commercially available metal powders and
3
many cases, the combinations of copper salts and imidazo-
Among the known
5b,8
lium salts are often used as catalysts.
1
2
*
To whom correspondence should be addressed. Tel and fax: (þ) 86-
imidazolium salts. As an extension of our NHC chemistry,
we report herein the synthesis and characterization of a few
copper(II) complexes of N-heterocyclic carbenes from the
corresponding imidazolium salts and copper powder and
their application to the N-arylation reactions of arylboronic
acids.
5
71-88273314. E-mail: chenwzz@zju.edu.cn.
(
(
(
1) Garrison, J. C.; Youngs, W. J. Chem. Rev. 2005, 105, 3978.
2) Lin, I. J. B.; Vasam, C. S. Coord. Chem. Rev. 2007, 251, 642.
3) Lin, J. C. Y.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A.;
Hwang, W. S.; Lin, I. J. B. Chem. Rev. 2009, 109, 3561.
4) (a) Yun, J.; Kim, D.; Yun, H. Chem. Commun. 2005, 5181.
b) Díez-Gonz ꢀa lez, S.; Kaur, H.; Zinn, F. K.; Stevens, E. D.; Nolan, S. P.
J. Org. Chem. 2005, 70, 4784.
5) (a) Brown, M. K.; May, T. L.; Baxter, C. A.; Hoveyda, A. H.
Angew. Chem., Int. Ed. 2007, 46, 1097. (b) Matsumoto, Y.; Yamada, K.;
Tomioka, K. J. Org. Chem. 2008, 73, 4578.
(
(
(
Results and Discussion
Synthesis and Structural Characterization of [Ag (L1) ](PF )
6 4
(
6) (a) Van Veldhuizen, J. J.; Campbell, J. E.; Giudici, R. E.;
6
2
Hoveyda, A. H. J. Am. Chem. Soc. 2005, 127, 6877. (b) Seo, H.; Hirsch-
Weil, D.; Abboud, K. A.; Hong, S. J. Org. Chem. 2008, 73, 1983.
(1) and [Cu (μ-OH)(L1)](PF ) (2). As shown in Scheme 1,
2 6 2
treatment of the imidazolium salt [H (L1)](PF ) (L1 =
3 6 2
(
7) (a) Fructos, M. R.; Belderrain, T. R.; Nicasio, M. C.; Nolan, S. P.;
Kaur, H.; Dı
az-Requejo, M. M.; P ꢀe rez, P. J. J. Am. Chem. Soc. 2004,
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M. M.; P ꢀe rez, P. J. Organometallics 2006, 25, 2237.
´
1
(10) (a) Wylie, W. N. O.; Lough, A. J.; Morris, R. H. Organometallics
2010, 29, 570. (b) Lee, C.-S.; Sabiah, S.; Wang, J.-C.; Hwang, W.-S.; Lin,
I, J. B. Organometallics 2010, 29, 286. (c) Zhang, X.; Qiu, Y.; Rao, B.; Luo,
M. Organometallics 2009, 28, 3093. (d) Ray, L.; Shaikh, M. M.; Ghosh, P.
Inorg. Chem. 2008, 47, 230.
(
(
8) Lee, Y.; Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131, 3160.
9) (a) Hu, X.; Castro-Rodriguez, I.; Meyer, K. Organometallics
2003, 22, 3016. (b) Gischig, S.; Togni, A. Organometallics 2005, 24, 203.
(
c) Diez-Gonzalez, S.; Stevens, E. D.; Scott, N. M.; Petersen, J. L.; Nolan,
(11) (a) Zhou, Y.; Chen, W. Organometallics 2007, 26, 2742. (b) Liu,
B.; Chen, W.; Jin, S. Organometallics 2007, 26, 3660. (c) Zhou, Y.; Zhang,
X.; Chen, W.; Qiu, H. J. Organomet. Chem. 2008, 693, 205.
S. P. Chem. Eur. J. 2008, 14, 158. (d) Goj, L. A.; Blue, E. D.; Delp, S. A.;
Gunnoe, T. B.; Cundari, T. R.; Petersen, J. L. Organometallics 2006, 25,
4097.
(12) Liu, B.; Xia, Q.; Chen, W. Angew. Chem., Int. Ed. 2009, 48, 5513.
r 2010 American Chemical Society
Published on Web 02/23/2010
pubs.acs.org/Organometallics

1
458 Organometallics, Vol. 29, No. 6, 2010
Liu et al.
Scheme 1. Synthesis of [Ag
6 2 6 4
(L1) ](PF ) and
[
Cu (μ-OH)(L1)](PF )
2
6 2
1
,5-bis(N-picolylimidazolylidenylmethyl)pyrazolate) with
3
3
Ag O in acetonitrile at room temperature led to the isolation
2
Figure 1. ORTEP drawing of the cationic section of [Ag
4þ
6
-
of [Ag (L1) ](PF ) (1) as a colorless crystalline solid in 55%
6
2
6 4
( (1). Thermal ellipsoids are drawn at the 30% probabi-
lity level. Hydrogen atoms, counteranions, and solvent mole-
L1)
2
]
yield. The hexanuclear silver(I)-NHC complex is stable
1
3
toward light and air. In its C NMR spectrum, resonance
signals due to carbenic carbons were not observed. The
absence of a silver-bound carbon resonance has been ascri-
˚
cules have been removed for clarity. Selected bond distances (A)
and angles (deg): Ag(1)-Ag(3) = 2.783(1), Ag(1)-Ag(2) =
2.833(1), Ag(2)-Ag(3) = 3.381(1), Ag(1)-C(15) = 2.231(10),
Ag(1)-C(5) = 2.350(10), Ag(1)-N(5) = 2.397(10), Ag(2)-
N(1A) = 2.166(9), Ag(2)-C(5) = 2.189(11), Ag(3)-N(2A) =
2.174(8), Ag(3)-C(15) = 2.206(11), Ag(3)-N(8) = 2.488(9);
Ag(3)-Ag(1)-Ag(2)=74.03(4), C(15)-Ag(1)-C(5)=169.2(4),
C(15)-Ag(1)-N(5) = 105.3(4), C(5)-Ag(1)-N(5) = 85.4(3),
N(1A)-Ag(2)-C(5)=166.8(4), N(2A)-Ag(3)-C(15)=163.7(3),
N(2A)-Ag(3)-N(8)=109.0(4), C(15)-Ag(3)-N(8)=86.8(4).
Symmetry transformations used to generate equivalent atoms:
(A) -x þ 2, -y þ 2, -z þ 2.
1
bed to the dynamic behavior in solution. The positive ion
ESI-MS analysis shows a m/z peak at 1900.60 amu corres-
þ
ponding to [Ag (L1) (PF ) ] . This result illustrates that the
6
2
6 3
Ag framework is maintained in solution. The compound
6
undergoes extensive fragmentation through loss of ligand or
silver atoms. The most intense peak at 625.15 amu in its ESI-
þ
MS spectrum corresponds to the disilver species [Ag (L1)] .
2
The peaks at 751.56, 879.23, 1157.19, 1394.59, and 1650.09
þ
amu were also observed ascribed to [Ag (L1)(H O)] ,
3
2
þ
þ
þ
[
[
Ag (L1)(PF )] , [Ag (L1) (H O)] , [Ag (L1) (PF )] , and
3
6
3
2
2
4
2
6
þ
Ag (L1) (PF ) ] , respectively.
5
2
6 2
The structure of compound 1 is shown in Figure 1. The
asymmetric unit consists of halves of the molecules that are
basically the same, and thus only one is given in Figure 1. The
complex is composed of two symmetrically related trisilver
units. Among the three independent silver atoms, Ag(1) and
Ag(3) are located in T-shaped coordination environments
having Ag(NHC) (Py) and Ag(NHC)(Py)(Pz) modes, res-
2
pectively, whereas Ag(2) is bicoordinated in a nearly linear
conformation by two NHC carbon atoms. Each of the four
NHC carbons interacts with two neighboring silver atoms.
The three independent silver atoms form a triangular ring
because of the Ag-Ag interaction. The Ag-Ag distances are
˚
found to be 2.783(1) and 2.833(1) A, which are significantly
Figure 2. ORTEP drawing of the cationic section of [Cu (μ-
2
shorter than those of our previously reported multinuclear
1
silver complexes containing NHC ligands. The three sides
are each capped by one pyrazole and two imidazolylidene
rings. The structural analysis shows that the Ag triangle is
nearly coplanar with its attached pyrazole ring. The dis-
tances between the silver atoms of two symmetry-related Ag3
2
þ
1
OH)(L1)
2
]
(2). Thermal ellipsoids are drawn at the 30%
probability level. Hydrogen atoms, counteranions, and solvent
molecules have been removed for clarity. Selected bond dis-
3
˚
tances (A) and angles (deg): Cu(1)-O(1) = 1.931(4), Cu(1)-
C(5)=1.934(5), Cu(1)-N(1)=1.947(4), Cu(1)-N(5)=2.044(4),
Cu(2)-C(15)=1.946(6), Cu(2)-O(1)=1.936(4), Cu(2)-N(2)=
triangles, which are approximately parallel, are longer than
˚
1.972(4), Cu(2)-N(8) = 2.091(4); O(1)-Cu(1)-C(5) = 169.8(2),
3
.44 A. The six silver atoms constitute a six-membered ring
O(1)-Cu(1)-N(1)=88.6(2), C(5)-Cu(1)-N(1)=88.4(2), O(1)-
Cu(1)-N(5) = 96.3(2), C(5)-Cu(1)-N(5) = 89.5(2), N(1)-
Cu(1)-N(5) = 162.5(2), C(15)-Cu(2)-O(1) = 175.2(2),
C(15)-Cu(2)-N(2) = 88.2(2), O(1)-Cu(2)-N(2) = 88.3(2),
C(15)-Cu(2)-N(8) = 92.1(2), O(1)-Cu(2)-N(8) = 92.4(2),
N(2)-Cu(2)-N(8)=160.3(2).
displaying a chair conformation. The pyridine rings are
loosely coordinated to silver atoms, as evidenced by the
remarkably long Ag-N distances (2.397(10) and 2.488(11)
A). For comparison, the Ag-N
pyrazole
and 2.166(9) A) are much shorter.
˚
distances (2.176(9)
˚
Further reaction of the hexanuclear complex either iso-
lated or generated in situ with an excess of commercially
available copper powder at room temperature yielded a deep
(
13) Zhou, Y.; Xi, Z.; Chen, W.; Wang, D. Organometallics 2008, 27,
5
911.

Article
Organometallics, Vol. 29, No. 6, 2010 1459
(μ -OH) (L3) ](PF
Scheme 2. Synthesis of [Cu
4
(μ
3
-OH)
2
(L2)
2
](PF
6
)
4
and [Cu
4
3
2
2
6 4
)
red solution of [Cu (μ-OH)(L1)](PF ) (2). The same com-
2
6 2
plex could also be obtained by reaction of [H (L1)](PF )
and copper powder without any other reagents in a compa-
3
6 2
rable yield. The reactions were conducted in air. Exclusion of
air and moisture significantly retarded the reaction. We
assumed that the hydroxide group might come from mois-
ture. It has been known that copper(I) complexes of NHC
could be prepared by interaction of the corresponding
imidazolium bromides with Cu O in chlorinated solvents
2
1
in the presence of molecular sieves. Complex 2 is probably
4
produced through the reaction of [H (L1)](PF ) with Cu O
3
6 2
2
freshly generated from copper oxidation under weakly acidic
conditions. The presence of a hydroxo group was confirmed
-
1
by the absorption band at 3446 cm in its IR spectrum.
Figure 3. ORTEP drawing of the cationic section of [Cu
4þ
4
(μ-
OH)
2
(L2)
2
]
(3). Thermal ellipsoids are drawn at the 30% pro-
bability level. Hydrogen atoms, counteranions, and solvent mole-
The ESI-MS spectrum of 2 shows the most intense peak at
þ
6
96.85 amu corresponding to [Cu (μ-OH)(L1)(PF )] . Be-
2
6
˚
cules have been removed for clarity. Selected bond distances (A)
cause of the paramagnetic nature of the copper(II) complex, the
compound was essentially confirmed by its diffraction analysis.
Previously we have reported that the reaction of bis-
and angles (deg): Cu(1)-C(6)=1.917(5), Cu(1)-N(1)=1.961(4),
Cu(1)-O(1)=1.982(3), Cu(1)-N(5)=2.116(4), Cu(1)-N(9)=
2
.412(5), Cu(2)-C(14) = 1.911(5), Cu(2)-O(1A) = 1.966(3),
(
(
pyrimidylimidazoliumyl)methane dihexafluorophosphate
H L(PF ) ) with copper powder yielded a dark green mixed
Cu(2)-N(2)=1.968(4), Cu(2)-N(8)=2.132(4), Cu(2)-O(1)=
2.346(3); C(6)-Cu(1)-N(1) = 86.02(18), C(6)-Cu(1)-O(1) =
163.31(18), N(1)-Cu(1)-O(1) = 91.68(14), C(6)-Cu(1)-
N(5) = 78.33(19), N(1)-Cu(1)-N(5) = 162.93(16), O(1)-Cu(1)-
N(5) = 101.34(14), C(6)-Cu(1)-N(9) = 113.43(19), N(1)-
Cu(1)-N(9) = 96.34(18), O(1)-Cu(1)-N(9) = 83.25(15),
C(14)-Cu(2)-O(1A) = 174.16(18), C(14)-Cu(2)-N(2) =
2
6 2
valence Cu(1.5)-Cu(1.5) complex, [Cu (L) ](PF ) , whereas
2
2
6 3
the reaction of [Ag (L) ](PF ) with copper powder affor-
2
2
2
6 2
1
ded a red copper(I) complex, [Cu (L) (CH CN) ](PF ) .
6 3
3
2
3
2
With the same synthetic approach, copper(II), copper(I),
and mixed-valence copper(1.5) complexes could be obtained,
depending on the imidazolylidene ligands.
8
7
1
5.36(19), O(1A)-Cu(2)-N(2)=92.19(14), C(14)-Cu(2)-N(8) =
8.25(19), O(1A)-Cu(2)-N(8)=102.76(14), N(2)-Cu(2)-N(8)=
58.41(16), C(14)-Cu(2)-O(1) = 102.00(17), O(1A)-Cu(2)-
The structure of dinuclear copper complex 2 is depicted in
Figure 2. The asymmetric unit of the structure consists of one
hexadentate L1, two independent copper ions, and one
hydroxide. The structure of 2 is isostructural with our
O(1)=83.71(12), N(2)-Cu(2)-O(1)=102.86(15), N(8)-Cu(2)-
O(1)=94.28(14). Symmetry code: (A) -x, -y þ 2, -z.
1
3
previously reported [Ni (μ-OH)(L1)](PF ) . The two cop-
2
6 2
indicating the more covalent nature of the former Cu-N
per centers in 2 are bridged by a pyrazolate and a hydroxide,
forming a five-membered metallocyclic ring. The two cop-
pers are four-coordinated, located in a CuCN O coordina-
2
tion sphere. The geometry about the metal is distorted square
planar, as evidenced by the diagonal angles ranging from
˚
bonds. The Cu Cu separation at 3.350 A is relatively long,
3
3 3
excluding any metal-metal interaction. The Cu-O bond
distances of the nearly symmetric Cu (μ-OH) unit are con-
2
sistent with those found in the known Cu(II) complexes
1
6
containing N-donating ligands.
Synthesis and Structural Characterization of [Cu (μ -
1
60.2(2) to 175.2(2)°. The coordination planes of two
4
3
copper(II) ions are bisected with a dihedral angle of 10.05°,
and the cation exhibits a beautiful saddle-shaped conforma-
tion. The Cu-C distances, being 1.934(5) and 1.946(6) A, are
relatively longer than those of bicoordinated Cu(I) com-
OH) (L2) ](PF ) (3) and [Cu (μ -OH) (L3) ](PF ) (4). Simi-
2
2
6 4
4
3
2
2
6 4
larly, reactions of copper powder with the white turbid solution
of the silver carbene complexes generated in situ from [H L]-
˚
3
(PF ) (L2 = 3,5-bis(N-pyridylimidazolylidenylmethyl)pyra-
zolate, L3=3,5-bis(N-pyrimidylimidazolylidenylmethyl)pyra-
6 2
9
a,15
plexes bearing NHC ligands.
˚
The two Cu-Npyrazole
13
zolate) and Ag O in acetonitrile at room temperature afforded
distances are ca. 0.1 A shorter than those of Cu-Npyridine,
2
[
Cu (μ -OH) (L2) ](PF ) (3) and [Cu (μ -OH) (L3) ](PF )
4 3 2 2 6 4 4 3 2 2 6 4
(
shown in Scheme 2. Complexes 3 and 4 could also be obtained
4) in good yields as red crystalline solids. The synthetic route is
(
14) Tulloch, A. A. D.; Danopoulos, A. A.; Kleinhenz, S.; Light,
M. E.; Hursthouse, M. B.; Eastham, G. Organometallics 2001, 20, 2027.
15) (a) Arduengo, A. J.III; Rasika Dias, H. V.; Calabrese, J. C.;
(
through direct reaction of [H (L2)](PF ) or [H (L3)](PF )
6 2
3
6 2
3
Davidson, F. Organometallics 1993, 12, 3405. (b) Goj, L. A.; Blue, E. D.;
Delp, S. A.; Gunnoe, T. B.; Cundari, T. R.; Pierpont, A. W.; Petersen, J. L.;
Boyle, P. D. Inorg. Chem. 2006, 45, 9032. (c) Mankad, N. P.; Laitar, D. S.;
Sadighi, J. P. Organometallics 2004, 23, 3369.
(16) Chandrasekhar, V.; Senapati, T.; Sa nꢁ udo, E. C. Inorg. Chem.
2008, 47, 9553.

1
460 Organometallics, Vol. 29, No. 6, 2010
Scheme 3. Synthesis of [Cu
Liu et al.
2 2 6 2
(L4) ](PF ) (5)
with copper powder. The strong absorption band at around
-
1
442 cm indicates the presence of a bridging hydroxide
3
group. In the ESI-MS spectrum of 3, the peak due to the
tetracopper core was not observed. The most intense peak is at
þ
5
10.38 amu, arising from [Cu (L2)] , and another peak at
2
þ
670.90 amu, corresponding to [Cu (OH)(L2)(PF )] . In the
ESI-MS spectrum of 4, the most intense peak at 445.32 amu
corresponds to the [Cu(L3)] species. The peaks at 670.82 and
2
6
þ
1
487.16 amu were also observed and were ascribed to [Cu (μ -
2 3
þ
þ
6 3
OH)(L3)(PF )] and [Cu (μ -OH) (L3) (PF ) ] .
6
4
3
2
2
Single crystals of 3 suitable for structure determination
were obtained by slow evaporation of diethyl ether into an
acetonitrile solution of the complex. A labeled molecular
view of the complex cation of 3 is shown in Figure 3. The
Figure 4. ORTEP drawing of the cationic section of [Cu -
2
2
(L4) ] (5). Thermal ellipsoids are drawn at the 30% prob-
ability level. Hydrogen atoms, counteranions, and solvent mole-
4þ
2þ
complex consists of the cation unit [Cu (μ -OH) (L2) ]
4
3
2
2
and four hexafluorophosphate anions with two acetonitriles
of crystallization. The core is composed of two hydroxide-
˚
cules have been removed for clarity. Selected bond distances (A)
and angles (deg): Cu(1)-N(1)=1.966(4), Cu(1)-C(6)=1.980(6),
Cu(1)-N(2) = 1.984(4), Cu(1)-C(13) = 1.989(5); N(1)-
Cu(1)-C(6) = 89.1(2), N(1)-Cu(1)-N(2) = 103.4(2), C(6)-
Cu(1)-N(2)=148.0(2), N(1)-Cu(1)-C(13)=148.2(2), C(6)-
Cu(1)-C(13) = 95.5(2), N(2)-Cu(1)-C(13) = 89.1(2).
bridged symmetrically related [Cu (OH)(L2)] units, in which
2
the two metals are held together by the hexadentate ligand
and hydroxide. The two subunits display a nearly planar
conformation, and they are parallel to each other. A further
bonding interaction of the copper(II) ion with a hydroxo
group of the second [Cu (OH)(L2)] unit results in aggrega-
2
tion into a tetracopper complex, forming a Cu O ring. The
2
Scheme 3, the reaction of copper powder with a colorless
solution of the silver carbene complex generated in situ
2
planar four-membered ring is approximately perpendicular
to the coordination planes of coppers, as indicated by the
dihedral angle of 84.8°. Each hydroxide binds three Cu
from [H
[Cu (L4)
solid. Complex 5 was also obtained by the direct reaction of
[H (L4)](PF with an excess of copper powder in 45%
(L4)](PF
3
) and Ag O in acetonitrile afforded
6 2 2
](PF (5) in 47% yield as a dark red crystalline
)
6 2
2
2
centers unsymmetrically, and the Cu-O distances are around
˚
.3 A longer than those corresponding values found in 2. The
0
3
)
6 2
˚
short Cu-Oapical distances are 1.966(3) and 1.982(3) A,
whereas the axial distance is 2.348(3) A, which is comparable
yield. In the ESI-MS spectrum of 5, the most intense peak
þ
˚
at 455.33 amu is due to [Cu(L4)] . The peak observed at
þ
to those reported for other hydroxo-bridged tetranuclear
1
1054.85 amu is assigned to [Cu
(L4)
2
2
(PF
6
)] .
7
complexes. Among the four copper ions, the two coppers
forming the Cu O ring sit in a distorted-square-pyramidal
sphere and the other two are square planar, as observed in 2.
Dark red single crystals of complex 5 suitable for an X-ray
diffraction study were grown by slow diffusion of diethyl
ether into its acetonitrile solution. A labeled molecular view
of the complex cation of 5 is shown in Figure 4. X-ray
crystallography revealed that complex 5 is dimeric in the
solid state, residing over a crystallographic inversion center
in the middle of two metal ions. Each copper(II) center is
four-coordinate, N-bound to two pyrazolate and C-bound
to two C2 carbenes of NHC rings. The two coppers are held
together by two 3,5-bis(N-methylimidazolylidenylmethyl)-
2
2
The Cu Cu distance within the unit [Cu (OH)(L2)] is
2
3
3 3
˚
3
.312 A, shorter than that of 2. For both 2 and 3, the Cu-C
distances are comparable to those in the known Cu(II)-
NHC complexes.
4
a,6a,18
Single crystals of complex 4 suitable for structure deter-
mination were obtained by slow diffusion of diethyl ether
into its acetonitrile solution. The structure of 4 is the same as
the structure of compound 3 and thus is given as Figure S1 in
the Supporting Information.
pyrazolate ligands, forming a planar six-membered Cu N
2 4
metallacycle. The copper centers are in a remarkably dis-
torted square planar coordination geometry as a result of the
steric repulsion between the two cis-arranged NHC rings and
Synthesis and Structural Characterization of [Cu (L4) ]-
2
2
(
zolylidenylmethyl)pyrazolate) was obtained from the reaction
PF ) (5). H (L4)(PF ) (L4 =3,5-bis(N-thiophenylimida-
6 2 3 6 2
3
the geometric requirement of sp -hybridized methylene. The
1
of 3,5-bis(chloromethyl)pyrazole with 1-(thiophen-2-yl)-
1a
Cu-C bond distances are normal and quite consistent with
those of copper(II)-NHC complexes displaying a square-
planar geometry, ranging from 1.861(8) to 2.008(13) A.
1
9
H-imidazole in refluxing 1,4-dioxane. As shown in
1
4a,6a,18
˚
The thiophene rings are not coordinated, as indicated by the
˚
(
17) Paital, A. R.; Nanda, P. K.; Das, S.; Aromı
Chem. 2006, 45, 505.
18) Arnold, P. L.; Rodden, M.; Davis, K. M.; Scarisbrick, A. C.;
Blake, A. J.; Wilson, C. Chem. Commun. 2004, 1612.
19) Zhu, L.; Guo, P.; Li, G.; Lan, j.; Xie, R.; You, J. J. Org. Chem.
007, 72, 8535.
´
, G.; Ray, D. Inorg.
long Cu-S distances at 3.428 and 4.866 A to the closer
copper ions.
(
Catalytic Properties. Copper-mediated N-arylation is
an important transformation in organic synthesis, and the
(
2

Article
Organometallics, Vol. 29, No. 6, 2010 1461
a
Table 1. N-arylation of Benzimidazole with Different Arylboronic Acids
a
b
3
Reaction conditions: benzimidazole, 0.5 mmol; arylboronic acid, 1.0 mmol; CH OH, 3.0 mL. Isolated yields.
resulting arylated products have widespread applications in
2
nitrogen-based nucleophiles has become an alternative syn-
thetic strategy because of its milder reaction conditions com-
0
the pharmaceutical and agrochemical industries. Copper-
catalyzed oxidative amination of arylboronic acids with
2
0a,21,22
pared to typical Ullmann coupling.
of mild catalytic procedures for N-arylation of imidazoles
The development
2
and amines still remains an active research area. With the
3
(20) (a) Ley, S. V.; Thomas, A. W. Angew. Chem., Int. Ed. 2003, 42,
5
2
400. (b) Beletskaya, I. P.; Cheprakov, A. V. Coord. Chem. Rev. 2004, 248,
337. (c) Ma, D.; Cai, Q. Acc. Chem. Res. 2008, 41, 1450.
newly prepared Cu(II)-NHC complexes in hand, we studied
(
21) Chan, D. M. T.; Monaco, K. L.; Wang, R. P.; Winters, M. P.
Tetrahedron Lett. 1998, 39, 2933.
22) (a) Collman, J. P.; Zhong, M. Org. Lett. 2000, 2, 1233.
b) Collman, J. P.; Zhong, M.; Zhang, C.; Costanzo, S. J. Org. Chem.
001, 66, 7892. (c) Antilla, J. C.; Buchwald, S. L. Org. Lett. 2001, 3, 2077.
d) Lan, J.; Chen, L.; Yu, X.; You, J.; Xie, R. Chem. Commun. 2004, 188.
(23) (a) Kantam, M. L.; Venkanna, G. T.; Sridhar, C.; Sreedhar, B.;
Choudary, B. M. J. Org. Chem. 2006, 71, 9522. (b) Chen, S.; Huang, H.;
Liu, X.; Shen, J.; Jiang, H.; Liu, H. J. Comb. Chem. 2008, 10, 358. (c) Singh,
B. K.; Stevens, C. V.; Acke, D. R. J.; Parmar, V. S.; Van der Eycken, E. V.
Tetrahedron Lett. 2009, 50, 15.
(
(
2
(

1
462 Organometallics, Vol. 29, No. 6, 2010
Table 2. N-arylation of Azoles and Amines with Phenylboronic Acid
Liu et al.
a
a
b
3
Reaction conditions: azole or amine, 0.5 mmol; phenylboronic acid, 1.0 mmol; CH OH, 3.0 mL. Isolated yields.
the N-arylation of imidazoles and amines at low catalyst
loadings under very mild conditions.
The catalytic activity of 4 for the N-arylation reaction
between the benzimidazole and phenylboronic acid was first
examined (eq 1). We found that the reaction could proceed
phenylboronic acid were used, the corresponding N-aryla-
tion products were obtained in almost quantitative yields
(Table 1, entries 1, 4, and 6). Naphthylboronic acid and
o-tolylboronic acid (Table 1, entries 2 and 3) afforded lower
yields than phenylboronic acid (Table 1, entry 1) because of
greater steric hindrance. It is worth mentioning that all aryl-
boronic acids bearing either electron-donating or electron-
withdrawing groups provided the corresponding N-arylated
benzimidazole in excellent yields, illustrating that the elec-
tronic effect of the substituents is not notable (Table 1,
entries 7-9).
A series of azoles and amines as coupling partners were
also briefly explored under the conditions used above (Table 2).
Different imidazoles such as benzimidazole and imidazole
were successfully coupled with phenylboronic acids to give
N-arylated products in excellent yields (Table 2, entries 1 and 2).
For a sterically hindered substrate such as 2-methylimida-
zole, the reaction yield was sharply reduced to 50% (Table 2,
entry 3). Primary aromatic amines could also be N-arylated,
giving secondary amines in moderate yields (Table 2, entries
smoothly at room temperature in CH OH under an atmos-
3
phere of air, giving the target product in 43% yield. No
additional base was required. A variety of solvents, including
common polar and nonpolar solvents, were screened, and
methanol was found to be the best solvent. The ratio of two
coupling partners is crucial for the reaction. When 1.4 equiv
of benzimidazole was employed at a catalyst loading of
0
.5 mol %, the yield of N-phenylbenzimidazole was decrea-
sed to 34%. The yields could be significantly improved by
increasing the amount of phenylboronic acid. At a phenyl-
boronic acid to benzimidazole ratio of 1.5, N-phenylbenz-
imidazole could be obtained in a yield of 78%. When the
ratio was increased to 2, the target product was obtained
quantitatively.
4
and 5). The formation of the conceivable bis-arylated
products was not observed. Unfortunately, the catalytic
system is totally inactive for N-arylation of aliphatic amines
such as n-butylamine, and no corresponding N-arylated
product was obtained (Table 2, entry 6). To the best of our
knowledge, this is the first example of Cu-NHC catalyzed
oxidative amination reactions of arylboronic acids.
Under the optimized conditions, the catalytic activities
of 2-5 were comparatively tested under the following
conditions: benzimidazole (0.5 mmol) and phenylboronic
acid (1.0 mmol) in CH OH (3.0 mL) at room temperature
3
It has been reported that various N-arylimidazoles can be
prepared in good to excellent yields through the coupling of
benzimidazole with phenylboronic acids in the presence of
copper salts such as Cu(OAc) and copper/diamines.
2
These approaches require a high loading of copper from 5 to
50 mol %. For comparison, we examined the catalytic
for 36 h. At a catalyst loading of 0.5 mol %, complexes 2-4
gave almost quantitative yields of N-phenylbenzimida-
zole. As expected, the catalytic efficiencies of the three
complexes were not significantly different, due to their
similar structures. However, the binuclear copper com-
plex 5 only showed moderate catalytic activity with a yield
of 67%.
21,22
efficiencies of CuCl, Cu(OAc)
Cl under the catalyst loadings and reaction conditions given
, and [Cu(OH)(TMEDA)] -
2
2
2
Subsequently, a number of structurally and electronically
diverse arylboronic acids have been employed for this cata-
lytic coupling, and the results are given in Table 1. When
phenylboronic acid, m-tolylboronic acid, and 3-methoxy-
in Table 1. N-Phenylbenzimidazole was obtained in 40%,
35%, and 71% yields, respectively. The same reaction can be
realized with relatively low loading of the present Cu-NHC
complexes. We tentatively assume that the high catalytic

Article
Organometallics, Vol. 29, No. 6, 2010 1463
OP : C, 32.71; H, 2.75; N, 13.27.
Found: C, 32.72; H, 2.59; N, 12.52. IR (KBr, cm ): 3446 (m),
3165 (w), 1069 (m), 1486 (m), 1446 (m), 1384 (w), 1338 (m), 1310,
activity might arise from the bimetallic cooperativity. The
two copper centers are tightly held together by the dinucleat-
ing ligand with a metal-metal separation of ca. 3.3 A, and
thus the dimetallic core may allow the two metals to activate
the reactants simultaneously.
Calcd for C23
H23Cu
2
F
12
N
8
2
-
1
˚
1
280 (m), 1165 (w), 1117 (w), 1046 (w), 1024 (w), 836 (vs), 766
m), 742 (m), 647 (w), 558 (s), 420 (w). ESI-MS (positive ions,
(
CH
þ
3
CN): m/z 696.85 [Cu
2
(μ-OH)(L1)(PF
6
)] .
The compound could also be obtained by direct reaction of
L1](PF (141 mg, 0.2 mmol) in 10 mL of CH CN and
Conclusion
[H
3
6
)
2
3
copper powder (128 mg, 2 mmol). Yield: 61 mg, 34%. Suitable
crystals for X-ray diffraction analysis were obtained by slow
diffusion of diethyl ether into its CH CN solution at room
We have prepared the rare examples of Cu(II) hydroxide
complexes of NHCs 2-4 and the dinuclear Cu(II)-bis-
(NHC) complex 5 via simple reactions of the corresponding
3
temperature.
silver(I)-NHC complex or imidazolium salts with commer-
cially available copper powder. This procedure avoids the
handling of unstable free carbenes and the use of strong bases.
The complexes 2-4 show an unusual hydroxo-bridged bi-
metallic or tetrametallic structure. These complexes have
proved to be efficient for N-arylation of imidazoles and
aromatic amines under very mild conditions. The simplicity
of the preparation of well-defined copper-NHC complexes
offers more opportunities to explore their applications. We
are currently examining their potential applications in other
organic transformations.
Synthesis of [Cu (μ -OH) (L2)](PF ) (3). The compound was
4
3
2
6 4
obtained as a red solid according to the same procedure as for 2,
starting from [H L2](PF (135 mg, 0.2 mmol), Ag O (116 mg,
.5 mmol), and copper powder (128 mg, 2 mmol). Yield: 128 mg,
3%. Anal. Calcd for C42 : C, 30.85; H, 2.47;
)
3
6
2
2
0
7
H
4 24 16 2 4
40Cu F N O P
-
1
N, 13.71. Found: C, 31.20; H, 2.69; N, 14.25. IR (KBr, cm ):
3
1
442 (s),1617 (s), 1579 (s), 1476 (s), 1443 (s), 1330 (m), 1254 (m),
091 (w), 1040 (w), 966 (w), 848 (vs), 779 (s), 740 (m), 559 (s).
þ
ESI-MS (positive ions, CH
Reaction of [H L2](PF
3
CN): m/z 510.38 [Cu (L2)] .
(135 mg, 0.2 mmol) and copper
2
3
6
)
2
powder (128 mg, 2 mmol) in acetonitrile at room temperature
for 3 days also gave the same compound. Yield: 113 mg, 64%.
Suitable crystals for X-ray diffraction analysis were obtained by
3
slow diffusion of diethyl ether into its CH CN solution at room
temperature.
Experimental Section
All the chemicals were obtained from commercial suppliers
3 6 2
Synthesis of [H L3](PF ) . A solution of 3,5-bis(chloromethyl)-
pyrazole (825 mg, 5 mmol) and 2-(imidazolyl)pyrimidine (2190 mg,
15 mmol) in 1,4-dioxane was refluxed for 4 days. The resulting
white solid was filtered, washed with 1,4-dioxane (10 mL ꢀ 3), and
4 6
then dissolved in 10 mL of water. Subsequent addition of NH PF
(2.5 mg, 15 mmol) to the aqueous solution afforded a white
precipitate, which was collected by filtration, washed with water,
3 6 2
and used without further purification. [H (L2)](PF ) was pre-
1
3
pared according to our previously reported procedure. Ele-
mental analyses were performed on a Flash EA1112 instrument.
1
13
H and C NMR spectra were recorded on a Bruker Avance-
00 (400 MHz) spectrometer. Chemical shifts (δ) are expressed
in ppm downfield from TMS at δ 0 ppm, and coupling constants
J) are expressed in Hz. IR spectra were run on a Perkin-Elmer
4
(
and dried. Yield: 1454 mg, 43%. Anal. Calcd for C19
C, 33.74; H, 2.68; N, 20.71. Found: C, 33.99; H, 2.78; N, 20.93. H
NMR (400 MHz, DMSO-d ): δ 13.28 (s,1H, NH), 10.28 (s, 2H,
H F N P :
18 12 10 2
1
FT-IR Spectrum 100 instrument. ESI-mass spectral data
were acquired using a Waters Micromass ZQ mass spectrometer
6
(
þ mode, ESI source).
NCHN), 9.06 (s, 4H, 4,6-pyrimidine), 8.49 (d, J = 7.2 Hz, 2H,
CHimidazole), 7.95 (s, 2H, CHimidazole), 7.78 (s, 2H, 5-pyrimidine),
Synthesis of [Ag (L1) ](PF ) (1). A slurry of Ag O (194 mg,
6
2
6 4
2
13
0
.8 mmol) in 7 mL of CH
3
CN was treated with [H
3
L1](PF
6
)
2
6.61 (s, 1H, CHpyrazole), 5.67 (s, 2H, CH
NMR (100 MHz, DMSO-d ): δ 160.4, 152.5, 146.2, 137.0, 124.2,
122.9, 119.9, 105.8, 47.2 (CH ), 47.0 (CH ). ESI-MS (positive ions,
CH CN): m/z 530.88 [[H L3](PF )] .
2 2
), 5.57 (s, 2H, CH ). C
(
0
L1=3,5-bis(N-picolylimidazolylidenylmethyl)pyrazolate; 141 mg,
.2 mmol). After the mixture was stirred at room temperature
for 24 h with exclusion of light, the resulting mixture was filtered
through a plug of Celite. Et O was slowly added to the clear
6
2
2
þ
6
3
3
2
4 3 2 2 6 4
Synthesis of [Cu (μ -OH) (L3) ](PF ) (4). The compound
solution, and the resulting precipitate was collected and washed
with methanol. Yield: 101 mg, 55%. Anal. Calcd for C H -
Ag F N P : C, 27.00; H, 2.07; N, 10.95. Found: C, 27.35; H,
was obtained as a dark red solid according to the same proce-
dure as for 2, starting from [H (L3)](PF ) (135 mg, 0.2 mmol),
2
Ag O (116 mg, 0.5 mmol), and copper powder (128 mg, 2 mmol).
4
6
42
3
6 2
6
24 16
4
1
3
.26; N, 11.24. H NMR (400 MHz, DMSO-d ): 8.27 (d, J =
6
Yield: 93.2 mg, 57%. Anal. Calcd for C H Cu F N O P :
4
3
41
4
24 23
2 4
4
each 2H, CHimidazole), 7.70 (d, J=7.6 Hz, 2H, 3-py), 7.52 (t, J=
.8 Hz, 2H, 6-py), 8.03 (t, J = 7.6 Hz, 2H, 4-py), 7.95, 7.80 (s,
C, 30.06; H, 2.35; N, 18.33. Found: C, 30.23; H, 2.54; N, 18.10.
-
1
IR (KBr, cm ): 3442 (s), 2930 (w), 1630 (m), 1586 (m), 1571 (m),
1458 (m), 1434 (s), 1260 (w), 1137 (w), 1098 (w), 854 (vs), 560
(m). ESI-MS (positive ions, CH CN): m/z 1487.16 [Cu (μ -
2
4
1
1
1
.8 Hz, 7.6 Hz, 2H, 5-py), 6.79 (s, 1H, CHpyrazole), 5.64 (d, J=
2
), 5.37 (d, J =
4.4 Hz, 2H, CH
4.4 Hz, 2H, CH
53.6, 151.7, 150.2, 139.7, 128.0, 125.0, 124.4, 99.7, 49.0, 48.6.
2
), 5.52-5.47 (m, 4H, CH
2
3
4
3
1
3
1
þ
þ
6
2
). C{ H} NMR (100.6 MHz, DMSO-d ):
6
OH) (L3) (PF ) ] , 670.82 [Cu (μ -OH)(L3)(PF )] , 445.32
2 2 6 3 2 3
þ
[Cu(L3)] .
The reaction of [H L3](PF ) (135 mg, 0.2 mmol) and copper
þ
ESI-MS (positive ions, CH CN): m/z 1900.60 [Ag
3
6
(L1)
2
(PF
6
3
) ] ,
3
6 2
þ
þ
1
[
650.09 [Ag
Ag (L1) (H
5
(L1)
2
(PF
6
)
O)] , 879.23 [Ag
2
] , 1394.59 [Ag
4
(L1)
2
(PF
6
)] , 1157.19
)] , 751.56 [Ag (L1)] .
powder (128 mg, 2 mmol) in acetonitrile at room temperature
for 3 days also gave the same compound. Yield: 128 mg, 51%.
Suitable crystals for X-ray diffraction analysis were obtained by
þ
þ
þ
3
2
2
3
(L1)(PF
6
3
Crystals suitable for X-ray single diffraction were grown by slow
diffusion of Et O into its CH CN solution.
Synthesis of [Cu (μ-OH)(L1)](PF (2). A solution of
H L1](PF ) (141 mg, 0.2 mmol) in 10 mL of CH CN was
2
3
slow diffusion of diethyl ether into its CH CN solution at room
3
2
6
)
2
temperature.
Synthesis of [H L4](PF ) . A solution of 3,5-bis(chloro-
[
3
6 2
3
3
6 2
treated with Ag O (116 mg, 0.5 mmol). The mixture was allowed
2
to react at room temperature for 24 h and then filtered to remove
a small amount of unreacted Ag O. The filtrate was treated with
2
copper powder (128 mg, 2 mmol). After it was stirred for 24 h at
room temperature, the resulting red solution was concentrated
to ca. 5 mL. Addition of 20 mL of diethyl ether to the solution
led to a red precipitate which was washed with ethanol and
redissolved in 2 mL of CH CN. The red solid was then obtained
3
by addition of 10 mL of diethyl ether. Yield: 74 mg, 41%. Anal.
methyl)pyrazole (0.83 g, 5 mmol) and 1-(thiophen-2-yl)imida-
zole (2.25 g, 15 mmol) in 1,4-dioxane was refluxed for 4 days.
The resulting pale yellow solid was filtered, washed with 1,4-
dioxane (10 mL ꢀ 3), and then dissolved in 10 mL of water.
Subsequent addition of NH PF (2.45 g, 15 mmol) to the
4
6
aqueous solution afforded a pale yellow precipitate, which was
collected by filtration, washed with water, and dried. Yield: 1745 mg,
51%. Anal. Calcd for C H F N P S : C, 33.34; H, 2.65; N,
19 18 12
6 2 2
1
12.28. Found: C, 33.50; H, 2.80; N, 12.45. H NMR (400 MHz,

1
464 Organometallics, Vol. 29, No. 6, 2010
Liu et al.
˚
DMSO-d
1
1
6
): δ 13.29 (s, 1H, NH), 9.85 (s, 1H, NCHN), 9.81 (s,
H, NCHN), 8.25 (d, J=10 Hz, 2H, 5-thiophene), 7.96 (d, J=
0 Hz, 2H, CHimidazole), 7.69 (d, J = 3.6, 2H, CHimidazole), 7.56
area-detector diffractometer with Mo KRradiation (λ=0.710 73 A)
by using an ω-2θ scan mode. Unit-cell dimensions were obtai-
ned with least-squares refinement. Data collection and reduc-
tion were performed using the SMART and SAINT software.
2
4
(
m, 2H, 4-thiophene), 7.19 (s, 2H, 3-thiophene), 6.61 (s, 1H,
1
). C NMR (100
3
CHpyrazole), 5.57 (s, 2H, CH
2
), 5.47 (s, 2H, CH
2
The structures were solved by direct methods, and the non-
hydrogen atoms were subjected to anisotropic refinement by
full-matrix least squares on F using the SHELXTXL pack-
age. Hydrogen atom positions for all of the structures were
calculated and allowed to ride on their respective C atoms with
MHz, DMSO-d
1
6
): δ 160.4, 146.2, 139.3, 137.0, 135.5, 127.3,
26.3, 123.5, 106.0, 46.7 (CH ). ESI-MS (positive ions, CH -
2
3
2
þ
25
CN): m/z 538.79 [[H
3
L4](PF
(L4) ](PF
tained as a dark red solid according to the same procedure as for
, starting from [H (L4)](PF ) (137 mg, 0.2 mmol), Ag O
6
)] .
Synthesis of [Cu
2
2
6
)
2
(5). The compound was ob-
˚
C-H distances of 0.93-0.97 A and U (H) = 1.2-1.5U (C).
iso
eq
2
The hydrogen atom of hydroxide was located from its Fourier
map. The disordered solvent molecule could not be successfully
modeled and was removed from the reflection data of 1 with
the SQUEEZE program with solvent accessible void volume
3
6 2
2
(
Yield: 56.4 mg, 47%. Anal. Calcd for C38
116 mg, 0.5 mmol), and copper powder (128 mg, 2 mmol).
H30Cu
F
2 12
N
12
P
S
2 4
: C,
8.03; H, 2.52; N, 14.01. Found: C, 38.22; H, 2.76; N, 14.12. ESI-
3
MS (positive ions, CH
þ
3 26
˚
3
CN): m/z 1054.85 [Cu
2
(L4)
2
(PF
6
)] ,
234.5 A . Further details of the structural analysis are sum-
marized in Table S1 (Supporting Information).
þ
4
55.33 [Cu(L4)] .
3 6 2
The reaction of [H L4](PF ) (137 mg, 0.2 mmol) and copper
powder (128 mg, 2 mmol) in acetonitrile at room temperature
for 3 days also gave the same compound. Yield: 54.0 mg, 45%.
Suitable crystals for X-ray diffraction analysis were obtained by
Acknowledgment. This project was supported by the
National Natural Science Foundation of China (No.
2
0872129), the Research Fund for the Doctoral Program
slow diffusion of diethyl ether into its CH CN solution at room
3
of Higher Education (No. 2009QNA3004), and the
Chinese Universities Scientific Fund.
temperature.
General Procedure for the N-arylation Reaction. A mixture
of NHC copper catalyst 4 (0.5 mmol %), arylboronic acid
(
1.0 mmol), and azole or amine (0.5 mmol) in 3 mL of CH OH
3
Supporting Information Available: CIF files giving structural
parameters for 1-5. This material is available free of charge via
the Internet at http://pubs.acs.org.
was stirred under an atmosphere of air at room temperature for 36
h. Then the mixture was centrifuged and the centrifugate was eva-
porated to dryness under reduced pressure. The residue was resol-
ved in 20 mL of ethyl acetate and washed with saturated brine
(20 mL ꢀ 3). After the organic layer was dried over anhydrous
MgSO , it was concentrated to ca. 1 mL. The residue was purified by
column chromatography on silica gel to provide the desired product.
X-ray Structural Determination. Single-crystal X-ray diffrac-
tion data were collected at 298(2) K on a Siemens Smart/CCD
(24) SMART-CCD Software, version 4.05; Siemens Analytical X-ray
Instruments, Madison, WI, 1996.
(
25) Sheldrick, G. M., SHELXS-97 and SHELXL-97, Program for
4
X-ray Crystal Structure Refinement; University of G €o ttingen, G €o ttingen,
Germany, 1997.
(
26) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool;
University of Utrecht, Utrecht, The Netherlands, 1998.