Synthesis and crystal structure of N-heterocyclic carbene complex of silver

Article abstract of DOI:10.1016/S0022-328X(02)01411-0

New imidazolium salt, monocarbene silver dinulear complex, [m-C₆H₄(CH₂ImMeAgCl)₂] (3) was obtained by reacting 1 and Ag₂O in CH₂Cl₂. The structure of 3 was determined by X-ray cr

Full text of DOI:10.1016/S0022-328X(02)01411-0

Journal of Organometallic Chemistry 654 (2002) 233ꢀ236
/
www.elsevier.com/locate/jorganchem
Short communication
Synthesis and crystal structure of N-heterocyclic carbene complex of
silver
Wanzhi Chen, Biao Wu, Kazuko Matsumoto *
Department of Chemistry, Advanced Research Institute for Science and Engineering, Waseda University, Japan Science and Technology Corp. (JST),
Okubo, Shinjuku Ku, Tokyo 169-8555, Japan
Received 30 January 2002; received in revised form 5 March 2002; accepted 25 March 2002
Abstract
New imidazolium salt, [m-C6H4(CH2ImMe)2]Cl2 (1) was prepared by quaterization of N-methylimidazole with m-xylylene
dichloride. A novel monocarbene silver dinulear complex, [m-C6H4(CH2ImMeAgCl)2] (3) was obtained by reacting 1 and Ag2O in
CH2Cl2. The structure of 3 was determined by X-ray crystallography. The geometry around the Ag atoms in 3 is linear, and each Ag
atom is bi-coordinated by a carbon and chloride ion. # 2002 Published by Elsevier Science B.V.
Keywords: Carbene; Imidazolium salts; Silver complex; X-ray structures
1. Introduction
culties arising from the use of strong base to yield free
heterocyclic carbenes.
It is noted that transition metal complexes containing
Since the discovery of stable imidazoline-2-ylidenes,
which was isolated and structurally characterized by
Arduengo et al. in 1991 [1], much interest has been
generated in the chemistry of both free heteroatom
carbenes and the metal complexes of these ligands. Most
recently, the synthesis and application of N,N?-dialky-
XCX-coordinating pincer-type ligands (XꢁP, N, O, S,
/
C coordinating atoms) have been receiving great atten-
tion, since these complexes are efficient catalysts for the
CÃ
alkanes [26], and the CÃ
1).
/
C bond activation [25], the dehydrogenation of
/
C coupling reactions [27] (Plate
limidazolium salts have been reviewed [2ꢀ4]. Hetero-
/
We report here the preparation and characterization
of new imidazolium salts [m-C6H4(CH2ImMe)2]Cl2, and
cyclic carbenes derived from imidazolium ions form
complexes with many transition metals; heterocyclic
carbene complexes of Pd, Ni, Pt, Rh, Ru, Ag, and Au
the
silverꢀ
/
carbene
complex
[m-C6H4(CH2Im-
MeAgCl)2]. [m-C6H4(CH2ImMe)2]Cl2 is a precursor of
a novel carbene ligand, which may potentially acts as a
pincer bis-carbene ligand.
have been reported [3ꢀ24]. The late transition metal
/
complexes have been employed as catalysts for Heck
and Suzuki coupling reactions [5,6], copolymerization of
ethene and CO [7], olefin metathesis [8], hydrogenation,
hydroformylation, and hydrosilylation [9,10]. The use of
silver and tungsten carbene complexes as carbene
transfer reagent for the synthesis of Pd, Pt, Rh, and
Au carbene complexes has been described [11,12]. Such
a route affords a convenient method for the preparation
of metal carbene complexes, and overcomes the diffi-
2. Results and discussion
1
2.1. Synthesis and H-NMR spectra
The imidazolium chloride, 1 was prepared by quater-
ization of N-methylimidazole with m-xylylene dichlor-
ide, and was obtained as white hygroscopic solid in a
high yield. No satisfactory elemental analysis was
obtained due to its severe hygroscopic property. The
corresponding tetraphenylborate salt was obtained by
mixing 1 and sodium tetraphenylborate in acetone or
* Corresponding author. Tel.:
52733489.
E-mail address: kmatsu@mn.waseda.ac.jp (K. Matsumoto).
ꢂ/
81-3-32034141; fax:
ꢂ/81-3-
0022-328X/02/$ - see front matter # 2002 Published by Elsevier Science B.V.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 4 1 1 - 0

234
W. Chen et al. / Journal of Organometallic Chemistry 654 (2002) 233ꢀ236
/
solution of the complex. Complex 3 crystallizes in the
monoclinic unit cell of dimensions aꢁ13.7315(19), bꢁ
92.128(3)8 with the
/
/
˚
8.7175(12), cꢁ
/
15.763(2) A and bꢁ
/
space group of C2/c. The dinulear silver complex is
symmetric with a C2 axis passing through the C6H4
ring, and the asymmetric unit contains half of [m-
C6H4(CH2ImMeAgCl)2]. An ORTEP drawing of the
molecule and the numbering scheme is shown in Fig.
1. Selected bond lengths and angles are collected in
Table 1.
Plate 1. Pincer ligand.
water. These two salts show very similar 1H-NMR
spectra, and the chemical shifts are consistent with those
of the reported imidazolium salts [1] (Scheme 1).
The metal complexes of N-heterocyclic carbenes can
be prepared by ligand substitution reactions by using
free carbenes, which are accessible via deprotonation of
The silver atoms are bi-coordinated with carbene
carbon atoms and chloride ion, with the C1Ã
Cl1ÃAg1 bond distances of 2.068(3) and 2.3187(8) A.
The geometry at the Ag atom is virtually linear, with a
Cl1ÃAgÃC1 bond angle of 176.25(8)8. These values are
quite normal when compared to the corresponding
values of other silverꢀcarbene and silverꢀchloride com-
/Ag1 and
˚
/
the corresponding imidazolium salts [1ꢀ3,8]. A more
/
/
/
convenient alternative is the direct reaction of the
imidazolium salts with the metal compound containing
basic ligands. By using the latter approach, a number of
Ag, Ni, and Pd complexes have been prepared. A few N-
/
/
plexes [11,22]. The planes of the imidazolato rings are
approximately perpendicular to the phenyl ring with the
dihedral angle of 83.888. Generally, the central silver
heterocyclic carbeneꢀsilver complexes have been re-
/
cently reported [11,22]. Indeed, the imidazolium chloride
1 was added to an Ag2O suspension in CH2Cl2 at a ratio
of 1:1 at room temperature to afford 3. While the
mixture was stirred overnight, the black Ag2O solid
atoms in the reported silverꢀ
linear conformation of Ag(carbene)2, the structurally
characterized silverꢀcarbene complexes consist of a
linear [bis(carbene)Ag]ꢂ cation and a [AgX2]ꢃ anion,
which are linked via AgꢀAg interaction in their solid
/carbene complexes have a
/
disappeared and the silverꢀcarbene complex was iso-
/
/
lated in 94.5% yield after evaporation of the solvent. The
solubility of compound 3 is poor in common organic
solvents, but 3 is stable in air, and is only slightly light-
sensitive.
Attempts to prepare the free carbene by deprotona-
tion of the imidazolium salt 1 with either t-BuOK or
KH was not successful, probably due to the high acidity
of the methylene protons linking phenyl and imidazole
rings.
state [11,22]. It was suggested that the transfer of a
carbene and a bromide bridge to form neutral [Ag(car-
bene)Br] in solution may occur [11,22]. Our successful
characterization of 3 affords direct evidence. A few
related compounds, [o-C6H4(CH2ImRAgCl)2] (Rꢁt
Bu,
/
i
2,4,6-Me3C6H2, and Pr), have been reported recently
[22b].
As shown in Fig. 2, the molecule displays a chair-like
conformation, with the intermolecular AgÁ Á ÁAg contacts
˚
of 4.054 and 4.377 A, and intermolecular AgÁ Á ÁCl of
2.2. Structure of [m-C6H4(CH2ImMeAgCl)2] (3)
˚
3.690 A. Thus these weak interactions led to the
formation of three-dimensional networks. The AgÃ
Ag
˚
distance in Na[Ag(CN)2] (3.71 A) [28] and
/
Ag
Crystals suitable for X-ray crystallography were
obtained by slow evaporation of the dichloromethane
separations in 3 are remarkably longer than the AgÃ
/
˚
K2Na[Ag(CN)2]3 (3.52 A) [29]. It is important to note
Fig. 1. An ORTEP view of [m-C6H4(CH2ImMeAgCl)2] (3) with the
atom numbering scheme (30% probability thermal ellipsoids). Hydro-
gen atoms are omitted for clarity.
Scheme 1.

W. Chen et al. / Journal of Organometallic Chemistry 654 (2002) 233ꢀ
/236
235
Table 1
was obtained. Yield: 3.1 g (91.4%). Satisfactory ele-
mental analysis was not obtained due to its heavily
hygroscopic property. 1H-NMR (270.5 MHz, D2O,
˚
Selcted bond lengths (A) and angles (8) for [m-C6H4(CH2ImMeAgCl)2]
(3)
ppm): d 8.65 (s, NCHN, 2H), 7.23ꢀ7.30 (m, C6H4 and
/
Bond lengths
Ag(1)ÃC(1)
Ag(1)ÃCl(1)
N(1)ÃC(1)
N(1)ÃC(2)
N(1)ÃC(4)
N(2)ÃC(1)
N(2)ÃC(3)
N(2)ÃC(5)
C(2)ÃC(3)
2.068(3)
2.3187(8)
1.335(3)
1.376(4)
1.461(4)
1.349(3)
1.375(4)
1.468(4)
1.332(5)
NCHCHN, 8H), 5.23 (s, CH2, 4H), 3.70 (s, CH3, 6H).
3.2. Preparation of [m-C6H4(CH2ImMe)2](BPh4)2
(2)
A solution of NaBPh4 (1.1 g, 3.21 mmol) in 10 ml of
C3H6O was added to
a
suspension of [m-
Bond angles
C6H4(CH2Im)2]Cl2 (0.5 g, 1.47 mmol) in 30 ml of
C3H6O. The resulted colorless solution was filtered
through cellite and the filtrate was concentrated to 10
ml. The white solid was obtained by addition of 20 ml
Et2O. Yield: 1.21 g (91.7%). Anal. Calc. for C64H60BN4:
C, 84.77; H, 6.67; N, 6.18. Found: C, 84.69; H, 6.56; N,
C(1)ÃAg(1)ÃCl(1)
N(1)ÃC(1)ÃAg(1)
N(2)ÃC(1)ÃAg(1)
C(1)ÃN(1)ÃC(2)
C(1)ÃN(1)ÃC(4)
C(2)ÃN(1)ÃC(4)
C(1)ÃN(2)ÃC(3)
N(1)ÃC(1)ÃN(2)
C(3)ÃC(2)ÃN(1)
C(2)ÃC(3)ÃN(2)
176.25(8)
126.60(19)
128.6(2)
110.8(2)
124.1(2)
125.1(3)
110.8(2)
104.8(2)
107.1(3)
106.5(3)
1
6.19%. H-NMR (270.5 MHz, C3H6O-d6, ppm): d 8.26
(s, NCHN, 2H), 7.48ꢀ6.74 (m, NCH and phenyl, 48H),
/
5.32 (s, NCH2, 4H), 3.76 (s, NCH3, 6H).
3.3. Preparation of [m-C6H4(CH2ImMeAgCl)2] (3)
To a suspension of [m-C6H4(CH2ImMe)2]Cl2 (0.339
g, 1.0 mmol) in 50 ml of CH2Cl2 was added Ag2O ( 0.231
g, 1.0 mmol). The mixture was stirred until Ag2O
dissolved. The resulted colorless solution was filtered
and concentrated to 3 ml. The white solid was afforded
by addition of 20 ml of Et2O. Yield: 0.52 g (94.5%).
Single crystals suitable for X-ray diffraction were
obtained by slow evaporation of the CH2Cl2 solution.
Anal. Calc. for C16H18Ag2Cl2N4: C, 34.75; H, 3.28; N,
1
10.13. Found: C, 34.66; H, 3.09; N, 10.01%. H-NMR
(270.5 MHz, CDCl3, ppm): d 6.76, 6.64 (both d, NCH,
Fig. 2. Packing diagram of [m-C6H4(CH2ImMeAgCl)2] (3) viewed
down the b axis.
JHH
(m, phenyl, 3H), 4.53 (s, NCH2, 4H), 3.00 (s, NCH3,
ꢁ
/
1.62 Hz, each 2H), 6.72 (s, phenyl, 1H), 6.55ꢀ6.49
/
that AgꢀAg interactions have been emphasized in
/
6H).
several recent articles in the literature, and the term
argentophilicity has been adopted [30,31]. The argento-
philicity, namely, the d10ꢀd10 closed-shell attraction
/
promotes the aggregation of silver(I) centers, which
has been supported by recent spectroscopic and struc-
tural evidence.
3.4. X-ray structure determination
Prismatic single crystal with dimensions 0.26ꢄ
/
0.17ꢄ
/
0.12 mm for 3 was covered with polyfluoroether, and
mounted on a glass fiber. Data were collected on a
3. Experimental
Siemens SMART CCD diffractometer by using graphite
˚
Ka radiation (lꢁ0.71073 A) at
monochromated Moꢀ
/
/
room temperature. The structure was solved by direct
method and refined by full-matrix least-squares techni-
ques [32,33]. All hydrogen atoms were geometrically
fixed and allowed to ride on their attached atoms. Table
2 summarizes the crystal data collection and refinement
parameters.
3.1. Preparation of [m-C6H4(CH2Im)2]Cl2 (1)
A mixture of m-xylylene dichloride (1.75 g, 10 mmol)
and N-methylimidazole (3.28 g, 40 mmol) was heated at
60 8C for 2 h, giving white waxy product. It was treated
with 20 ml of C3H6O, and the white hygroscopic solid

236
W. Chen et al. / Journal of Organometallic Chemistry 654 (2002) 233ꢀ236
/
[6] L. Xu, W. Chen, J.F. Bickley, A. Steiner, J. Xiao, J. Organomet.
Chem. 598 (2000) 409.
Table 2
Crystal data and structure refinement for [m-C6H4(CH2ImMeAgCl)2]
(3)
[7] M.G. Gardiner, W.A. Herrmann, C.-P. Reisinger, J. Schwarz,
M.J. Spiegler, J. Organomet. Chem. 572 (1999) 239.
[8] J. Huang, E.D. Stevens, S.P. Nolan, Organometallics 19 (2000)
1194.
Compound
3
Empirical formula
Formula weight
Temperature (K)
C
552.98
16H18Ag2Cl2N4
[9] W.A. Herrmann, C. Kocher, Angew. Chem. Int. Ed. Engl. 36
¨
(1997) 2162.
293.2(2)
0.71073
[10] W.A. Herrmann, L.J. Goosen, C. Kocher, G.R.J. Artus, Angew.
¨
Chem. Int. Ed. Engl. 35 (1996) 2805.
˚
l (A)
Crystal system
Space group
Monoclinic
C2/c
[11] H.M.J. Wang, I.J.B. Lin, Organometallics 17 (1998) 972.
[12] S. Liu, T. Hsieh, G. Lee, S. Peng, Organometallics 17 (1998) 993.
[13] H.M.J. Wang, C.Y.L. Chen, I.J.B. Lin, Organometallics 18 (1999)
1216.
˚
a (A)
13.7315(19)
8.7175(12)
15.763(2)
92.128(3)
1885.6(5)
4
˚
b (A)
˚
c (A)
[14] W.A. Herrmann, M. Elison, L.J. Fischer, C. Kocher, G.R.J.
¨
Artus, Chem. Eur. J. 2 (1996) 772.
b (8)
3
V (A )
˚
[15] L. Jafarpour, E.D. Stevens, S.P. Nolan, J. Organomet. Chem. 606
(2000) 49.
Z
Dcalc (Mg mꢃ3
Crystal size (mm)
)
1.948
0.28ꢄ0.19ꢄ0.16
[16] D.S. Mcguinnes, K.J. Cavell, B.W. Skelton, A.H. White, Orga-
nometallics 18 (1999) 1596.
u range (8)
Reflections collected
Reflections unique (Rint
2.59ꢀ
5348
/
27.49
[17] M. Prinz, M. Grosche, E. Herdtweck, W.A. Herrmann, Organo-
metallics 19 (2000) 1692.
)
2145 (0.0206)
2145/0/112
0.0290, 0.0797
0.0360, 0.0835
1.065
[18] D.S. Mcguinnes, K.J. Cavell, Organometallics 18 (1999) 1596.
[19] W.A. Herrmann, J. Schwarz, M.G. Gardiner, Organometallics 18
(1999) 4082.
Data/restraints/parameters
R1, wR2 [I ꢀ2d(I)]
R1, wR2 (all data)
[20] W.A. Herrmann, G. Gerstberger, M. Spiegler, Organometallics 16
(1997) 2209.
Goodness-of-fit
[21] L. Xu, W. Chen, J. Xiao, Organometallics 19 (2000) 1123.
[22] (a) D.S. McGuinness, K.J. Cavell, Organometallics 9 (2000)
741A;
4. Supplementary material
(b) A.D. Tulloch, A.A. Danopoulos, S. Winston, S. Kleinhenz, G.
Eastham, J. Chem. Soc. Dalton Trans. (2000) 4499;
(c) M.V. Baker, B.W. Skelton, A.H. White, C.C. Williams, J.
Chem. Soc. Dalton Trans. (2001) 111.
Additional data of the X-ray structures reported in
this paper have been deposited with the Cambridge
Crystallographic Data Centre, CCDC no. 169826 for
compound 3. Copies of the data can be obtained free of
charge on application to The Director, CCDC, 12
[23] J.C. Garrison, R.S. Simons, J.M. Talley, C. Wesdemiotis, C.A.
Tessier, W.J. Youngs, Organometallics 20 (2001) 1276.
[24] V.P.W. Bohm, C.W.K. Gstottmayr, T. Weskamp, W.A. Herr-
mann, J. Organomet. Chem. 595 (2000) 186.
Union Road, Cambridge CB2 1EZ, UK (Fax: ꢂ44-
1223-336033, e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
/
[25] A. Sundermann, O. Uzan, D. Milstein, J.M.L. Martin, J. Am.
Chem. Soc. 122 (2000) 7095.
[26] S. Li, M.B. Hall, Organometallics 20 (2001) 2153.
[27] J.A. Loch, M. Albrecht, E. Peris, J. Mata, J.W. Faller, R.H.
Crabtree, Organometallics 21 (2002) 700.
[28] K.J. Range, H. Kuhnel, M. Zabel, Acta Crystallogr. Sect. C 45
¨
(1989) 1419.
References
[29] M. Zabel, H. Kuhnel, K.J. Range, Acta Crystallogr. Sect. C 45
¨
(1989) 1619.
[1] A.J. Arduengo, III, R.I. Harlow, M. Kline, J. Am. Chem. Soc.
113 (1991) 361.
[30] M.A. Omary, T.R. Webb, Z. Assefa, G.E. Shankle, H.H.
Patterson, Inorg. Chem. 37 (1998) 1380.
[2] T. Welton, Chem. Rev. 99 (1999) 2071.
[3] D. Bourissou, O. Guerret, F.P. Gabbai, G. Bertrand, Chem. Rev.
100 (2000) 39.
[31] Q. Wang, T.C.W. Mak, J. Am. Chem. Soc. 123 (2001) 7594.
[32] Saint Software Reference Manual, Siemens Analytical X-ray
Instruments Inc., Madison, Wisconsin, USA, 1995.
[33] G.M. Sheldrick, SHELXL-97, An Integrated System for Sovling
and Refining Crystal Structures from Diffraction Data, Univer-
[4] T. Weskamp, V.P.W. Bohm, W.A. Herrmann, J. Organomet.
Chem. 600 (2000) 12.
[5] D.S. McGuinness, K.J. Cavell, Organometallics 19 (2000) 741.
sity of Gottingen, Germany, 1997.
¨