Facile C-N bond cleavage promoted by cuprous oxide: Formation of C-C-coupled biimidazole from its methylene-bridged congener

Article abstract of DOI:10.1021/om9010406

The methylenebis(N-alkylimidazolium) halides are converted to dialkylbiimidazoles by cuprous oxide with concurrent C-N bond cleavage and C-C bond formation. This unusual C-N bond cleavage is proposed to involve a Cu(I)-NHC (N-heterocyclic carbene) complex. The reaction is unique, as it involves C-H bond activation, C-N bond cleavage, and C-C bond formation.

Full text of DOI:10.1021/om9010406

290 Organometallics 2010, 29, 290–293
DOI: 10.1021/om9010406
Facile C-N Bond Cleavage Promoted by Cuprous Oxide: Formation of
C-C-Coupled Biimidazole from Its Methylene-Bridged Congener
Shahulhameed Sabiah, Chen-Shiang Lee, Wen-Shu Hwang,* and Ivan J. B. Lin
Department of Chemistry, National Dong Hwa University 1, Section 2, Da-Hsueh Road, Hualien 974,
Taiwan, Republic of China
Received December 3, 2009
Summary: The methylenebis(N-alkylimidazolium) halides are
converted to dialkylbiimidazoles by cuprous oxide with con-
current C-N bond cleavage and C-C bond formation. This
unusual C-N bond cleavage is proposed to involve a Cu-
(I)-NHC (N-heterocyclic carbene) complex. The reaction is
unique, as it involves C-H bond activation, C-N bond
cleavage, and C-C bond formation.
method be devised.7 In our efforts to study the metal-
catalyzed C-C coupling reactions, we have found that solid
cuprous oxide can promote C-C coupling in methylenebis-
(N-alkylimidazolium) halides through unusual C-N bond
cleavage under ambient conditions. The reaction is unique,
as it involves C-H bond activation, C-N bond cleavage,
and C-C bond formation which are, to our knowledge,
being reported for the first time.
Carbon-nitrogen bond cleavage by transition-metal com-
plexes has been the recent focus in organometallic chemis-
try.1 Catalytic C-N bond cleavage by a Rh complex with a
phosphine ligand2 and model C-C coupling reactions cata-
lyzed by Ru and Ni with organoborates via C-N bond
cleavage are known.3 However, direct observation or isola-
tion of a C-C-coupled biimidazole type product in metal-
catalyzed coupling reactions is very rare.4 This is quite
surprising, since transition-metal-catalyzed C-C coupling
reactions have broad applications in organic synthesis and
related disciplines.5 In addition, the C-C-coupled dialkyl-
biimidazoles are versatile ligands for deriving metal com-
plexes with interesting structural, luminescent, and catalytic
properties.6 These ligands exhibit different properties, and
their commercial nonavailability demands that a synthetic
Treatment of L1 or L2 with Cu2O in acetonitrile resulted
in reactive, air-sensitive copper(I) complexes ([Cu2(μ-Me-
mbim)2](Cl)2 (1), [Cu2(μ-Bu-mbim)2](Cl)2 (2)) which on ex-
posure to air were transformed to the copper(II) complexes
Cu2(Me2biim)2Cl4 (3; 72%) and Cu2(Bu2biim)2Cl4 (4; 71%
yield), respectively. These two copper(II) complexes were
characterized by MALDI-MS, UV-vis spectroscopy, and
elemental analysis, and the molecular structures of 3 and 4
(see Supporting Information) were confirmed by X-ray crys-
tallography. The structures clearly show carbon-carbon-
coupled dialkylbiimidazoles (R2-biim) through which the
coppers are coordinated. The R2-biim ligands were quanti-
tatively extracted from the copper(II) complexes by simple
treatment with ammonia (Scheme 1).
The obtained C-C-coupled product gave us a hint that the
imidazolium C2 protons of L1/L2 were probably abstracted
by basic cuprous oxide to produce initially a Cu(I) carbene
species. Hence, we tried to analyze the cuprous products
before exposure to air. As predicted, the successive forma-
tion of the copper(I)-carbene intermediate [Cu2(μ-Bu-
mbim)2](Cl)2 (2) in the case of L2/Cu2O is indicated by an
examination of the 1H NMR spectra in Figure 1, which show
the disappearance of C2-imidazolium protons at 10.16 ppm
upon coordination to Cu(I). The imidazole-ring protons
appear at 7.59 and 7.47 ppm, bridging methylene appears
at 6.5 ppm, and n-butyl protons appear at 4.05, 1.73, 1.19,
and 0.81 ppm, respectively. The disappearance of bridging
methylene at 6.5 ppm was clear when the C-C-coupled
product L4 was formed. The 13C NMR spectrum of complex
2 shows a chemical shift at 177.18 ppm characteristic of
carbenoid carbons bonded to Cu(I) (Figure S2, Supporting
Information).
*To whom correspondence should be addressed. Fax: þ886 3
8632000. Tel: þ886 3 8632001. E-mail: hws@mail.ndhu.edu.tw.
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pubs.acs.org/Organometallics
Published on Web 12/28/2009
2009 American Chemical Society

Communication
Organometallics, Vol. 29, No. 2, 2010 291
1
Figure 1. Comparative H NMR spectra of L2, [Cu2(μ-Bu-mbim)2](Cl)2 (2), and C-C-coupled L4 in DMSO-d6 (asterisks denote
solvent and moisture peaks).
Scheme 1
Scheme 2
oxide under identical reaction conditions (Scheme 2) af-
forded complex 3 along with benzaldehyde. The identity of
reaction failed to yield the C-C-coupled L3/L4 in the
absence of a copper source. Also, [Cu2(μ-Bu-mbim)2](Cl)2
(2) is stable in solution compared to [Cu2(μ-Me-mbim)2](Cl)2
(1). The extreme air sensitivity of the cuprous carbene
complex 1 prevents us from obtaining the NMR data, as it
changes color to blue instantly. This difference implies that
the steric hindrance might control the stability of the carbene
intermediate.9a Recently, a Cu(I)-NHC bearing an N-sub-
stituted bulky tert-butyl group has been isolated.9b
To understand the fate of bridging methylene during C-N
bond cleavage, we have synthesized phenylbis(3-methylimi-
dazolium-1-yl)methane dichloride (L5) from R,R0-dichloro-
toluene and 1-methylimidazole. Reaction of L5 with cuprous
1
benzaldehyde was confirmed by TLC, H NMR, and GC-
MS analyses. Alternatively, treatment of the formaldehyde
produced in Scheme 1 with 2,4-dinitrophenylhydrazine led
to its quantitative conversion to formaldehyde 2,4-dinitro-
phenylhydrazone and complex 3.
Further, the yield of C-C-coupled product relies on
methylenebis(N-methylimidazolium) halide sources. The re-
action of Cu2O with bromide and iodide salts (L 2Br or L 2I
3
3
where L is methylenebis(N-methylimidazolium dication)
afforded 42% and 15% of L3, respectively, along with an
uncharacterizable material. It is also observed that the use of
L 2PF6 with Cu2O under identical reaction conditions did
not afford the C-N-cleaved product, whereas L1/L2 readily
gives the C-N-cleaved products.
3
(9) (a) 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. (b) Shishkov, I. V.;
Rominger, F.; Hofmann, P. Dalton Trans. 2009, 1428.
To substantiate the involvement of a Cu(I)-NHC
complex during C-N bond cleavage, parallel synthesis of

292 Organometallics, Vol. 29, No. 2, 2010
Sabiah et al.
Figure 3. ORTEP plot (30% probability ellipsoids) of Cu2(Me2-
biim)2Cl4 (3). Hydrogens are omitted for clarity. Selected interato-
˚
mic distances (A) and angles (deg) are as follows: Cu1-Cu2, 3.594;
Cu1-N4, 1.981(2); Cu1-Cl3, 2.2456(5); Cu2-N3, 1.965(1);
Cu2-Cl4, 2.2346(6); Cl4-Cu2-Cl4, 135.46(4); Cl3-Cu1-Cl3,
135.69(4); N3-Cu2-N3, 140.72(9); N4-Cu1-N4, 141.37(9).
Figure 2. ORTEP plot (30% probability ellipsoids) of the dica-
tionic part of [Cu2(μ-Me-mbim)2](PF6)2 (5). Hydrogens are omit-
˚
ted for clarity. Selected interatomic distances (A) and angles (deg)
that the active methylene acidic protons of L 2X (X =
3
are as follows: Cu1-C8, 2.112(8); Cu2-C9, 2.083(8); Cu1-Cu2,
anion) play a significant role during the course of the
reaction. In the present study, this has been substantiated
by the formation of aldehyde products. In general, palla-
dium, rhodium, and ruthenium catalysts routinely used for
the activation of C-H bonds are expensive.12 The use of
active metal catalysts that are cheaper and less exotic (e.g.,
copper, iron, and manganese) is clearly attractive.
3.350; C8-Cu1-C80, 167.6(4); C9-Cu2-C90, 170.3(4).
Scheme 3
The C-C-coupled copper(II) complexes Cu2(Me2biim)2-
Cl4 (3) and Cu2(Bu2biim)2Cl4 (4) were characterized by
single-crystal XRD. They exhibit similar structural and spec-
tral features. As shown in Figure 3, complex 3 consists of two
copper centers connected by two Me2biim ligands with four
terminal chloride ions. The ORTEP plot of complex 4 is
shown in Figure S4 (Supporting Information). Me2biim/
Bu2biim acts as a bridging ligand rather than a bidentate
chelate. Of special note, the alkyl substituent causes the two
imidazole rings to be out of planarity to give an unfavorable
bite angle suitable for bidentate chelation.13 The coppers are
[Cu2(μ-Me-mbim)2](PF6)2 (5) by an Ag transfer route10 was
performed. Reaction of [Ag2(μ-Me-mbim)2](PF6)2 with cu-
prous iodide under an inert atmosphere afforded the copper-
(I)-carbene complex 5, which was characterized by spectral
analysis and single-crystal XRD (Figure 2). The complex
was crystallized from an acetonitrile-acetone mixture and
crystallizes in the orthorhombic system with the Pnma space
group. The structure shows two copper centers separated at a
˚
separated at a distance of 3.594 A in 3 and 4. Each copper(II)
adopts a highly distorted tetrahedral coordination with an-
gles of 96.98(5), 97.66(5), 140.72(9), 141.37(9)° and 93.95(3),
95.87(5), 139.87(3), 144.60(7)° in 3 and 4, respectively, which
deviate significantly from 109°.
˚
distance of 3.350 A. Each copper is bridged by a bis-N-
heterocyclic carbene ligand with Cu-C distances of 2.083(8)
and 2.112(8) A. The copper atom adopts a slightly bent
The tetrahedral Cu(II)-Me2biim complex 3 with unu-
sual geometric distortion exhibits interesting luminescent
behavior in solution.14 The UV-vis absorption spec-
trum of complex 3 in water shows electronic transitions
at 210 (18 666), 270 (1714), 390 (952), 580 (ε 5048 M-1
cm-1), and 880 nm due to π-π*, charge transfer, and d-d
transitions, respectively. When the aqueous solution of
complex 3 is excited at 580 nm, an emission band at 630 nm
(Supporting Information) is observed. Such fluorescent
behavior of L3 with Cd and Zn is known,6a whereas this
is the first report of copper(II) with L3. Its fluorescent
behavior in the visible region is a promising feature to
apply in materials.
˚
geometry (C-Cu-C = 170.3(4), 167.6(4)°), in comparison
to the linear geometry (∼180°) seen in many NHC-Cu(I)
complexes.11 The molecule has Cu-F interactions at 2.9 A
˚
and intermolecular C-H F interactions (see the packing
3 3 3
diagram given by Figure S3 in the Supporting Information).
The reaction of a 1:2 ratio of [Cu2(μ-Me-mbim)2](PF6)2 (5)
and lithium chloride afforded complex 3 in 65% yield
(Scheme 3).
The C-N bond cleavage of methylenebis(N-alkylimida-
zolium) halides was found to be linker dependent. The
ethylene (-CH2CH2-)-bridged imidazolium salt, in con-
trast with the salt containing a single methylene bridge, failed
to afford a C-C-coupled product, which strongly suggests
In summary, we have observed an unprecedented C-N
bond cleavage promoted by cuprous oxide in methylene-
bridged bis-imidazolium salts. The active methylene group is
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I. J. B. Organometallics 1998, 17, 972.
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1074.
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2006, 25, 2355. (b) Arduengo, A. J.; Dias, H. V. R.; Calabrese, J. C.;
Davidson, F. Organometallics 1993, 12, 3405. (c) Hu, X.; Castro-Rodriguez,
I.; Olsen, K.; Meyer, K. Organometallics 2004, 23, 755. (d) Kaur, H.; Zinn, F.
K.; Stevens, E. D.; Nolan, S. P. Organometallics 2004, 23, 1157. (e) Diez-
(13) Goulle, V. R.; Thummel, P. Inorg. Chem. 1990, 29, 1767.
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2004, 25, 1959. (b) Solomon, E. I. Inorg. Chem. 2006, 45, 8012. (c)
Solomon, E. I.; Szilagyi, R. K.; DeBeer George, S.; Basumallick, L. Chem.
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Communication
Organometallics, Vol. 29, No. 2, 2010 293
converted to formaldehyde during the C-N bond cleavage.
The reaction works well in gram-scale synthesis. The reac-
tion is proposed to involve a Cu(I)-carbene intermediate, as
Acknowledgment. We acknowledge the National Science
Council (Taiwan, ROC) and National Dong Hwa Univer-
sity for financial support. We sincerely thank Dr. Babu
Varghese, Sophisticated Analytical Instruments Facility
(SAIF), IIT Madras, India, for the structure solution of 5.
1
evidenced by H NMR. The isolated [Cu2(μ-Me-mbim)2]-
(PF6)2 complex 5 and its reaction with chloride anion further
supports the involvement of a Cu(I)-NHC complex. The
Cu(II) complexes 3 and 4 show promising fluorescent beha-
vior and are the first examples of fluorescent Cu(II) com-
plexes with R2biim ligands. The novel Cu-mediated
transformation leading to the formation of 3 and 4 involves
a sequence of C-H bond activation, C-N bond cleavage,
and C-C bond formation steps. The role of chloride anion,
the reaction sequences, and the exact mechanism by which
this unusual C-N bond cleavage occurs is currently under
investigation.
Supporting Information Available: Text giving experimental
details, figures giving 1H and 13C NMR spectra of complex 2, an
ORTEP plot of 4, the emission spectrum of 3, and a packing
diagram of 5 showing the cooperative C-H F interactions,
3 3 3
and Table 1 and CIF files giving crystallographic data for 3-5.
This material is available free of charge via the Internet at http://
pubs.acs.org. Crystal data are also available from the Cam-
bridge Crystallographic Database as file numbers 714906,
714908, and 739121.