Biscarbene palladium(II) complexes. reactivity of saturated versus unsaturated N-heterocyclic carbenes

Article abstract of DOI:10.1021/ic9025188

A series of designed palladium biscarbene complexes including saturated and unsaturated N-heterocyclic carbene (NHC) moieties have been prepared by the carbene transfer methods. All of these complexes have been characterized by ¹H and ¹³C NMR spectroscopy as well as X-ray diffraction analysis. The reactivity of Pd-C(_{saturated NHC}) is distinct from that of Pd-C_{(unsaturated NHC)}. The Pd-C_{(saturated NHC)} bonds are fairly stable toward reagents such as CF₃COOH, AgBF₄ and I₂, whereas Pd-C_{(unsaturated NHC)} bonds are readily cleaved under the similar conditions. Notably, the catalytically activity of these palladium complexes on Suzuki-Miyaura coupling follows the order: (sat-NHC)₂PdCl₂ > (sat-NHC)(unsat-NHC)PdCl₂ > (unsat-NHC)₂PdCl₂.

Full text of DOI:10.1021/ic9025188

Inorg. Chem. 2010, 49, 3011–3018 3011
DOI: 10.1021/ic9025188
Biscarbene Palladium(II) Complexes. Reactivity of Saturated Versus Unsaturated
N-Heterocyclic Carbenes
†
†
†
†
‡
‡
Ching-Feng Fu, Chun-Chin Lee, Yi-Hung Liu, Shie-Ming Peng, Stefan Warsink, Cornelis J. Elsevier,
,
†
,†
Jwu-Ting Chen,* and Shiuh-Tzung Liu*
†
‡
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan and, and Van’t Hoff Institute of
Molecular Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam,
The Netherlands
Received December 17, 2009
A series of designed palladium biscarbene complexes including saturated and unsaturated N-heterocyclic
carbene (NHC) moieties have been prepared by the carbene transfer methods. All of these complexes have
been characterized by H and C NMR spectroscopy as well as X-ray diffraction analysis. The reactivity of
1
13
Pd-C(saturated NHC) is distinct from that of Pd-C(unsaturated NHC). The Pd-C(saturated NHC) bonds are fairly stable toward
reagents such as CF COOH, AgBF and I , whereas Pd-C
bonds are readily cleaved under the similar
3
4
2
(unsaturated NHC)
conditions. Notably, the catalytically activity of these palladium complexes on Suzuki-Miyaura coupling follows the
order: (sat-NHC) PdCl > (sat-NHC)(unsat-NHC)PdCl > (unsat-NHC) PdCl .
2
2
2
2
2
Introduction
complexes that show other remarkable catalytic activities
have also been reported.
4-7
The chemistry of palladium complexes stands as an im-
portant class of catalysts in useful organic reactions, parti-
cularly in the coupling reactions. It is known that the
properties of ligands can influence critically the activity of
metal complexes. Recently, the uses of N-heterocyclic car-
benes (NHCs) with palladium complexes often show better
stability and reactivity than those of the TM-phosphine
Among this context, most known palladium biscarbene
complexes contain two identical carbene ligands except a
1
8
dicoordinated palladium(0) species. Less investigation has
been endeavored to the complexes with different NHC ligands,
(4) (a) Hahn, F. E.; Jahnke, M. C.; Gomez-Benitez, V.; Morales-Morales,
D.; Pape, T. Organometallics 2005, 24, 6458. (b) Tu, T.; Bao, X.; Assenmacher,
W.; Peterlik, H.; Daniels, J.; D €o tz, K. H. Chem.;Eur. J. 2009, 15, 1853. (c)
Zhang, T.; Shi, M. Chem.;Eur. J. 2008, 14, 3759. (d) Buscemi, G.; Biffis, A.;
Tubaro, C.; Basato, M. Catal. Today 2009, 140, 84.
2
counterparts in analogous catalytic reactions. For instance,
Herrmann and co-workers have demonstrated that the bis-
carbene palladium complexes are not only stable compounds
but also serve as excellent catalysts in Heck coupling reac-
(5) (a) Han, Y.; Hong, Y.-T.; Huynh, H. V. J. Organomet. Chem. 2008,
6
93, 3159. (b) G €o k c- e, A. G.; T €u rkmen, H.; Ayg €u n, M.; C- etinkaya, B.;
3
tions. And, many examples with stable palladium biscarbene
B €u y €u kg €u ng €o r, O. Struct. Chem. 2008, 19, 57. (c) T €u rkmen, H.; S- ahin, O.;
B €u y €u kg €u ng €o r, O.; C- etinkaya, B. Eur. J. Inorg. Chem. 2006, 4915. (d) Cavell,
*
Corresponding author. E-mail: jtchen@ntu.edu.tw (J.-T.C); stliu@
ntu.edu.tw (S.-T.L).
1) There are a number of reviews and books for palladium-catalyzed
K. J.; Elliott, M. C.; Nielsen, D. J.; Paine, J. S. Dalton Trans. 2006, 4922. (e)
Boydston, A. J.; Rice, J. D.; Sanderson, M. D.; Dykhno, O. L.; Bielawski, C. W.
Organometallics 2006, 25, 6087.
(
reaction, for example: (a) Palladium in Organic Synthesis; Tsuji, J., Ed.;
Springer: Berlin, Germany, 2005. (b) Handbook of Organopalladium Chemistry
for Organic Synthesis; Negishi, E., De Meijere, A., Eds.; Wiley: New York, NY,
(6) (a) Huynh, H. V.; Jothibasu, R.; Koh, L. L. Organometallics 2007, 26,
6852. (b) Stylianides, N.; Danopoulos, A. A.; Pugh, D.; Hancock, F.; Zanotti-
Gerosa, A. Organometallics 2007, 26, 5627. (c) Weiss, R.; Kraut, N. Angew.
Chem., Int. Ed. Engl. 2002, 41, 311. (d) Bertani, R.; Mozzon, M.; Michelin, R. A.
J. Organomet. Chem. 1992, 431, 117. (e) Michelin, R. A.; Zanotto, L.; Braga, D.;
Sabatino, P.; Angelici, R. J. Inorg. Chem. 1988, 27, 93. (f) Khramov, D. M.;
Rosen, E. L.; Er, J. A. V.; Vu, P. D.; Lynch, V. M.; Bielawski, C. W. Tetrahedron
2008, 64, 6853.
(7) (a) Hahn, F. E.; Jahnke, M. C.; Pape, T. Organometallics 2007, 26, 150.
(b) Huynh, H. V.; Ho, J. H. H.; Neo, T. C.; Koh, L. L. J. Organomet. Chem. 2005,
690, 3854. (c) Nielsen, D. J.; Cavell, K. J.; Skelton, B. W.; White, A. H.
Organometallics 2006, 25, 4850. (d) Metallinos, C.; Barrett, F. B.; Chaytor, J. L.;
Heska, M. E. A. Org. Lett. 2004, 6, 3641. (e) Chamizo, J. A.; Morgado, J.; Castro,
M.; Bernes, S. Organometallics 2002, 21, 5428. (f) Hahn, F. E.; Foth, M. J.
Organomet. Chem. 1999, 585, 241. (g) Hahn, F. E.; von Fehren, T.; L €u gger, T.
Inorg. Chim. Acta 2005, 358, 4137.
2
002.
(
2) (a) Liddle, S. T.; Edworthy, I. S.; Arnold, P. L. Chem. Soc. Rev. 2007,
3
6, 1732. (b) K €u hl, O. Chem. Soc. Rev. 2007, 36, 592. (c) Dragutan, V.; Dragutan,
I.; Delaude, L.; Demonceau, A. Coord. Chem. Rev. 2007, 251, 765. (d) Arnold,
P. L.; Pearson, S. Coord. Chem. Rev. 2007, 251, 596. (e) Hahn, F. E.; Jahnke, M.
C. Angew. Chem., Int. Ed. 2008, 47, 3122. (f) Jacobsen, H.; Correa, A.; Poater,
A.; Costabile, C.; Cavallo, L. Coord. Chem. Rev. 2009, 253, 687. (g) Lin, I. J. B.;
Vasam, C. S. Coord. Chem. Rev. 2007, 251, 642. (h) Lin, J. C. Y.; Huang,
R. T. W.; Lee, C. S.; Bhattacharyya, A.; Hwang, W. S.; Lin, I. J. B. Chem. Rev.
2
009, 109, 3561.
(
3) (a) Herrmann, W. A.; Elison, M.; Fischer, J.; K o€ cher, C.; Artus,
G. R. J. Angew. Chem., Int. Ed. 1995, 34, 21. (b) Herrmann, W. A.; Elison, M.;
Fischer, J.; K €o cher, C.; Artus, G. R. J. Angew. Chem., Int. Ed. 1995, 34, 2371. (c)
Muehlhofer, M.; Strassner, T.; Herrmann, W. A. Angew. Chem., Int. Ed. Engl.
(8) Titcomb, L. R.; Caddick, S.; Cloke, F. G. N.; Wilson, D. J.;
McKerrecher, D. Chem. Commun. 2001, 1388.
2
002, 41, 1745.
r 2010 American Chemical Society
Published on Web 02/09/2010
pubs.acs.org/IC

3
012 Inorganic Chemistry, Vol. 49, No. 6, 2010
Fu et al.
Scheme 1
although they are expected to exhibit interesting properties due
to distinguishable trans influence. In this work, we report the
preparation of a series of designed biscabene palladium
complexes, particularly comprising both saturated and
unsaturated NHCs. With such species, the differences of
donating ability between saturated and unsaturated NHCs
to the palladium(II) centers as well as the reactivity of
these complexes have been compared by means of spectro-
scopic analyses, single crystal structures, and chemical
reactions.
for 72 h generated a mixture of bis-NHC palladium
complexes trans- and cis-4 in a ratio of 2:1.
Silver NHCs have been successfully used in transmeta-
lation reactions for the preparation of a wide range of
2
g
metal carbene complexes. Thus, the carbene transfer
reaction between unsaturated silver-carbene 6a and 2a
takes place to provide trans-5a, bearing both saturated
and unsaturated NHCs as the kinetic product that slowly
isomerizes to yield a mixture of trans- and cis-5a in a ratio
of 1:1. Complex trans-5b that contains sterically bulky N-
substitutents on NHC did not transform into its cis-
isomer, presumably energetically unfavored. For the
purpose of comparison, the homo unsaturated NHC
palladium complex 7 was prepared by the similar method.
Reaction of [(COD)PdCl ] with excess of 6a provides a
2
mixture of cis- and trans-7 in a ratio of 1:1.
Results and Discussion
Synthesis and Characterization. Both mono- and
bis-(saturatedNHC) palladium complexes (Scheme1) were
prepared according to the previously reported procedure
via a carbene transfer reaction of tungsten carbene com-
plexes with Pd(II) ions. Reactions of 1 with [(COD)PdCl ]
9
2
in dichloromethane always result in a mixture of 2 and 3.
However, carrying out the reaction by a slow addition of 1
intoadichloromethanesolutionof[(COD)PdCl ] leads to2
2
as the sole product. In contrast, a procedure with reverse
addition yields the homobiscarbene species trans-3 exclu-
sively. Conversion of trans-3 into thermodynamically more
stable cis-3 is a fairly slow process at room temperature but
is accelerated in the presence of silver salt in an acetonitrile
solution (see Scheme 1).
All palladium complexes, which are air stable, were
isolated as solids; complexes 2c, trans- and cis-3a, and cis-
7
were even in crystalline forms. The spectral data of
complexes 2a-b and 3a-b are consistent with the re-
ported data. The newly prepared species could be easily
9
b
1
characterized by the determination of their H and
13
C
It is noted to mention that complexes 2 do not convert
into the corresponding biscarbene complexes 3 either
thermally or even by the addition of silver salts. This
observation clearly indicates that the carbene does not
transfer from one palladium metal center to the other.
Mixed bis(NHC) complexes of palladium(II) can be
acquired by double carbene transfer reactions: the trans-
NMR spectral data. The coordination of the NHC
ligands to the palladium is confirmed by the appearance
of a downfield shift in the C NMR spectra for the
carbenic carbon (Table1). It is noticed that there is an
average difference of 20-30 ppm between the chemical
shifts due to unsaturated and saturated carbenic carbons
1
3
(such as trans-3a vs trans-7, cis-3a vs cis-7, or cis-4 vs
cis-5a), in agreement with the reported data.
1
0,11
fer of a carbene moiety either from (NHC)W(CO) or
5
(NHC)AgX to the palladium center (Scheme 2). Thus,
reaction of 2a with an excess of 1b at room temperature
(10) Huynh, H. V.; Han, Y.; Jothibasu, R.; Yang, J. A. Organometallics
(
9) (a) Liu, S.-T.; Hsieh, T.-Y.; Lee, G.-H.; Peng, S.-M. Organometallics
2009, 28, 5395 and references therein.
1
998, 17, 993. (b) Ku, R.-Z; Huang, J.-C.; Cho, J.-Y.; Kiang, F.-M.; Reddy, K. R.;
(11) (a) Fantasia, S.; Jeffrey, L.; Petersen, J. L.; Jacobsen, H.; Cavallo, L.;
Nolan, S. P. Organometallics 2007, 26, 5880–5889. (b) Hillier, A. C.; Sommer,
W. J.; Yong, B. S.; Petersen, J. L.; Cavallo, L.; Nolan, S. P. Organometallics 2003,
22, 4322.
Chen, Y.-C.; Lee, K.-J.; Lee, J.-H.; Lee, G.-H.; Peng, S.-M.; Liu, S.-T. Organo-
metallics 1999, 18, 2145. (c) Fu, C.-F.; Chang, Y.-H.; Liu, Y.-H.; Peng, S.-M.;
Elsevier, C. J.; Chen, J.-T.; Liu, S.-T. Dalton Trans. 2009, 6991.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 3013
Scheme 2
13
Table 1. C Shifts of the Carbene Carbons for Complexes
1
3
13
complex
C NMR C(NHC)-Pd complex
C NMR C(NHC)-Pd
a
2
2
2
a
b
c
173.3
174.5
182.9
trans-4
cis-4
trans-5a 197.1, 170.1
cis-5a 198.2, 168.8
trans-5b 195.8, 178.1
198.7, 197.9
198.2, 197.8
trans-3a 198.1
cis-3a 189.1
trans-3b 199.1
cis-7
trans-7
168.9
169.1
a
Ref 9b.
Crystallography. To further confirm the geometrical ar-
rangement and the bonding around the metal center,
X-ray diffraction studies were carried out on complexes 2c,
trans-3a, cis-3a, and cis-7. The complexes were crystallized by
a slow vaporization from saturated solutions in CH Cl /
2
2
Figure 1. ORTEP plot of complex trans-3a (drawn with 30% probabi-
lity ellipsoids).
hexanes at ambient temperature. The crystal structures of 2c,
trans-3a, cis-3a, and cis-7 are shown in Figures 1-4, respec-
tively, and the selected bond lengths are shown in Table 2.
The C-C distances of the imidazole rings in 3a lie in the
˚
range of 1.50-1.52 A, typical for a single bond, whereas
˚
those in cis-7 appear to be 1.378(4) A, typical for a double
bond, showing the difference between saturated and un-
saturated NHCs. The nonplanarity of the heterocycle in
cis-3a, caused by the two sp carbons in the NHC backbone,
3
is also different from the structure of cis-7, in which the
heterocycle is in a planar arrangement. Other than these
observations, the structural difference between cis-3a and -7
(caused by saturated or unsaturated NHC) is negligible.
There is a rather small difference in the Pd-C(carbene)
as well as Pd-Cl bond lengths in complexes cis-3a of
the saturated NHC and cis-7 of the unsaturated NHC,
that is consistent with the reported data. The slightly
1
0b
Figure 2. ORTEP plot of complex cis-3a.
longer Pd-C(carbene) bond distance in trans-3a than that
in cis-3a (ca. 0.06 A) is attributed to the good trans
˚
Reaction of Pd-NHC Complexes with AgBF . In a
4
influence from the strong donating NHC ligands. Generally
speaking, little difference in the carbene character between
the saturated and unsaturated NHCs in the investigated
biscarbene palladium system may be drawn from the crys-
tallographic data. In another words, the structural and
NMR data appear not able to differentiate the electronic
power between the saturated and unsaturated NHCs.
previous work, we have found that reaction of trans-3a
with AgBF4 in CH Cl (or CHCl ) with a trace of
2
2
3
moisture yielded the corresponding imidazolium 9
(Scheme 3, path a). It is believed that the NHC moiety
is first transferred from Pd to Ag and followed by the
protonation. However, treatment of trans-3a with AgBF4
in an acetonitrile solution under refluxing conditions

3
014 Inorganic Chemistry, Vol. 49, No. 6, 2010
Fu et al.
the fragment of [(saturated NHC)PdCl ] readily under-
2
went the dimerization via the chloride bridging to form
2a. The formation of the 1,3-diethylimidazolium salt is
presumably due to the protonation of the carbene, and
the source of the proton is probably from a trace of water
in the reaction medium. Similarly, reaction of 5b with
9
b
AgBF under the same conditions also resulted in the
4
formation of the corresponding imidazolium salt 10. One
would expect that complex 7, which has two unsaturated
NHC moieties, might also undergo Pd-C cleavage upon
treatment with silver ions. Indeed, reaction of complex 7
with an excess of AgBF provided 1,3-diethylimidazo-
4
lium salt (71%) and the chloride-bridged dimer 11 (18%)
(eq 2).
Figure 3. ORTEP plot of complex cis-7.
Figure 4. ORTEP plot of complex 2c.
Reactivity Toward I . Complexes 2a, 3a-b, and 4 are
2
stable toward iodine in the dichloromethane solution
even under refluxing conditions. But, complex 5a reacts
with iodine at room temperature for 8 h to provide 2a and
Table 2. Selected Bond Distance and Bond Angles of NHC-Pd Complexes
complex
trans-3a
cis-3a
cis-7
2c
2
-iodoimidazolium salt 12 quantitatively (eq 3). Complex
Pd-C(carbene)
Pd-C(carbene)
Pd-Cl(1)
2.051(5)
2.046(5)
2.307(2)
2.315(2)
1.50(1)
1.519(9)
1.319(7)
1.314(6)
1.303(6)
1.321(6)
178.1(3)
1.988(3)
1.988(3)
2.3653(7)
2.3654(7)
1.519(5)
1.519(5)
1.330(4)
1.335(4)
1.330(4)
1.335(4)
91.13(15)
1.986(3)
1.986(3)
2.3842(6)
2.3843(6)
1.378(4)
1.378(4)
1.345(4)
1.346(4)
1.345(4)
1.346(4)
88.7(1)
1.943(3)
1.943(3)
2.3319(9)
2.2980(9)
1.524(5)
1.524(5)
1.325(4)
1.320(4)
1.325(4)
1.320(4)
-
7 behaves similarly, and it reacts with iodine to produce
the iodo-substituted imidazolium salt 12 and the chloride-
bridged complex 11 in a ratio of 2:1 by the NMR
integration.
Pd-Cl(2)
C-C(imi-ring)
C-C(imi-ring)
C
C
C
C
(carbene)-N
(carbene)-N
(carbene)-N
(carbene)-N
C-Pd-C
resulted the isomerization of a trans-cation I to a
cis one (Scheme 3, path b). Presumably, the abstrac-
tion of chloride by the silver ion generates an aceto-
nitrile coordinating intermediate I that may facilitate the
isomerization. From these observations, it appears that
the coordinating ability of the solvent molecules plays a
remarkable effect on the stabilization of NHC palladium
complexes.
Since the NHC palladium complexes 2a, 3a-b, and 4
are inert toward I , the decomposition of 5a or 7 through
the oxidative addition of I toward the metal center,
2
2
which demands a Pd(IV) intermediate, is quite unlikely.
As discussed in the previous section, the formation of 2a is
due to the dimerization of [(saturated NHC)PdCl2],
generated by the dissociation of the unsaturated NHC
moiety from 5a. A plausible mechanism for the forma-
tion of 12 is shown in Scheme 4. The unsaturated NHC
moiety is dissociated from the metal center to yield the
intermediate 13 and the free carbene, which then reacts
Unlike trans-3a, treatment of 5a with AgBF4 in
CH CN at 80 °C for 48 h caused the selective cleavage
3
of Pd-C(
, yielding 2a, silver-carbene, and
unsaturated NHC)
imidazolium salt (eq 1). Obviously, the unsaturated NHC
moiety may be transferred from Pd(II) back to Ag(I) but
not the saturated NHC. Upon the addition of chloride,
with I directly to form 12. It has been demonstrated
2
that the reaction of free NHC with I would provide
2

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 3015
Scheme 3
Scheme 4
accompanied with a trace amount of the chloride-bridged
palladium species 11.
1
-iodo-imidazolium salt easily. Compound 12 is stable
2
2
toward the Pd(II) species, and it is not likely to regenerate
the carbene species under this reaction conditions. We
were not able to detect any free carbene species by NMR
or MS analysis of the crude reaction mixture. It may be
that the concentration of these species, if formed, was low
in the presence of iodine. From these observations, it
indicates that the unsaturated NHC moiety on these
biscarbene palladium complexes could dissociate from
the metal center.
These results clearly demonstrate the difference of
the reactivity between Pd-C(
and Pd-
saturated NHC)
C NHC). It is known that the basicity of
unsaturated
saturated NHCs is stronger that that of the correspond-
(
13
ing unsaturated one. One would expect that proton-
ation should occur at the site of the saturated NHCs.
However, Pd-C(saturated NHC) is stable toward acid,
indicating that ligands of saturated NHCs do not
dissociate from the metal center. On the other hand,
the dissociation of unsaturated NHCs from [(NHC)2-
Reactivity Toward CF COOH. All studied carbene
3
palladium complexes are stable toward air and
water. In order to investigate further the stability
of the Pd-C(carbene) bonds, all palladium complexes
were subjected to react with trifluoroacetic acid. The
PdCl ] takes place, then yielding the imidazolium salt in
2
the presence of acid. Again, the Pd-C(carbene) cleavage
by CF COOH reveals a similar trend, as illustrated
in the cleavage of these bonds in the presence of I2
3
reaction of trans-5b with an excess of CF COOH in
3
CDCl in a sealed NMR tube was monitored by
3
or silver ions, suggesting that the strength of Pd-
1
H NMR spectroscopy (eq 4). After 12 h, both 2a
and 1,3-bis(2,6-diisopropylphenyl)imidazolium salt
were obtained quantitatively (eq 4). These two pro-
C
C(unsaturated carbene)
is more stable than that of Pd-
.
(
saturated carbene)
These NHC complexes are stable toward common
organic bases, such as pyridine and amines. However,
the reactions of bis(NHC)palladium complexes with
hydroxide cause their decomposition, as evidenced by
the generation of palladium black.
1
13
ducts were confirmed by both H and C NMR
spectroscopies. Obviously, the unsaturated NHC
moiety is readily protonated by acid to yield the
imidazolium ion, and the residual palladium frag-
ment undergoes the dimerization via the bridging
chloride. Similarly, reaction of a mixture of trans- and
Catalysis. Suzuki-Miyaura coupling is one of the
most efficient methods for the construction of C-C
bonds through Pd(II)-involved catalysis. The auxiliary
ligands in such processes are known to play important
cis-5a with excess of CF COOH showed the same reaction
3
pattern, i.e., the reaction products were 2a and 1,3-diethy-
limidazolium salt. In contrast to unsaturated NHCs, com-
plexes trans- and cis-3a and 3b were retained in the solution
of CF COOH/CDCl over a temperature range (room
14
roles. The Pd-NHC complexes indicate that they can
better stabilize the Pd(II)-center as well as enhance their
3
3
temperature to 50 °C) for an even longer period. As
(
13) (a) Herrmann, W. A.; K o€ cher, C. Angew. Chem., Int. Ed. Engl. 1997,
36, 2162. (b) Dixon, D. A.; Arduengo, A. J., III. J. Phys. Chem. 1991, 95, 4180.
14) Recent reviews: (a) Negishi, E.-I.; Huang, Z.; Wang, G.; Mohan, S.;
Wang, C.; Hattori, H. Acc. Chem. Res. 2008, 41, 1474. (b) Catellani, M.; Motti,
E.; Della Ca, N. Acc. Chem. Res. 2008, 41, 1512. (c) Martin, R.; Buchwald, S. L.
Acc. Chem. Res. 2008, 41, 1461. (d) Weng, Z.; Teo, S.; Hor, T. S. A. Acc. Chem.
Res. 2007, 40, 676. (e) Yin, L.; Liebscher, J. Chem. Rev. 2007, 107, 133. (f)
Christmann, U.; Vilar, R. Angew. Chem., Int. Ed. Engl. 2005, 44, 366. (g) Miura,
M. Angew. Chem., Int. Ed. Engl. 2004, 43, 2201.
expected, complex 7 is also sensitive toward CF COOH.
Thus, complex7 wasreadilydecomposedinthe presenceof
trifluoroacetic acid to yield 1,3-diethylimidazolium salt
3
(
(
12) Liu, Q.-X.; Song, H.-B.; Xu, F.-B.; Li, Q.-S.; Zeng, X.-S.; Leng,
X.-B.; Zhang, Z.-Z. Polyhedron 2003, 22, 1515.

3
016 Inorganic Chemistry, Vol. 49, No. 6, 2010
Fu et al.
Table 3. Results of the Coupling Reaction of Aryl Halides with Phenylbonoric
Acid
Table 4. Results of the Suzuki-Miyaura coupling catalyzed Various Pd(II)
a
a
Complexes
b
b
complex
2a
trans-3a
cis-3a
4
5a
trans-5b
7
yield (%)
50
>99
>99
95
81
74
47
a
Reaction conditions: p-chloroacetophenone (0.2 mmol), phenylbo-
-
3
t
-3
noric acid (0.3 mmol), Pd(II) complex (2 ꢀ 10 mmol), Bu P (4 ꢀ 10
3
mmol) in water (5 mL), refluxing temperature for 24 h, yields given based
on the average of two runs. Mixture of cis- and trans-isomers.
b
b
entry
substrates
additives
base
Cs CO
yield
t
t
t
t
1
2
3
4
5
6
7
8
9
p-chloroacetophenone
p-chloroacetophenone
p-chloroacetophenone
p-chloroacetophenone
p-chloroacetophenone
p-chloroacetophenone
o-chlorotoluene
Bu
3
3
3
3
P, TBAB
2
3
23%
33%
33%
74%
63%
>99%
57%
42%
20%
complex 3a. This might explain the more stable complex
providing a better activity.
Bu
Bu
Bu
P, TBAB
P, TBAB
P, TBAB
K
KF
2
CO
3
K
K
K
K
K
K
3
PO
3
PO
3
PO
3
PO
3
PO
3
PO
4
TBAB
4
4
4
4
4
Summary
t
Bu
3
Bu
3
Bu
3
Bu
3
P
P
P
P
t
t
t
In this study, we demonstrate the synthetic approach to
prepare both symmetrical and unsymmetrical biscarbene
palladium complexes. The comparison of spectroscopic and
structural and catalytic activities of these palladium com-
plexes has been presented. As compared to unsaturated NHC
complexes, saturated NHC palladium counterparts show
better stability toward acid as well as iodine. From these
investigations, it reveals that the unsaturated NHC moiety
could dissociate from the biscarbene palladium complexes
but not from the saturated NHCs. In terms of catalytically
reactivity, bis-saturated NHC palladium complexes appear
to be better precatalysts in the Suzuki-Miyaura coupling
reaction.
chlorobenzene
p-chlorobenzoic acid
a
Reaction conditions: p-chloroacetophenone (0.2 mmol), phenylbo-
-
3
noric acid (0.3 mmol), trans-3a (2 ꢀ 10 mmol), additives, base (0.4
b t
mmol) in water (5 mL), and refluxing temperature for 24 h. Bu
3
P (4 ꢀ
-3
10
mmol), TBAB = tetrabutylammonium bromide (0.5 mmol).
catalytic activity, comparing to their TM-phosphine
1
5
counterparts. Aiming to explore if the electronic
modification would have any significant effect on
the catalytic activity, the Suzuki-Miyaura reactions
with the use of our new Pd-carbene complexes were
examined.
The reaction of p-chloroacetophenone and phenylbono-
ric acid, with loading of 1 mol % trans-3a and the added
Bu P and/or tetrabutylammonium bromide (TBAB), was
3
Experimental Section
t
General Information. All reactions, manipulations, and pur-
ification steps were performed under a dry nitrogen atmosphere.
Tetrahydrofuran was distilled under nitrogen from sodium/
benzophenone. Dichloromethane and acetonitrile were dried
over CaH and distilled under nitrogen. Other chemicals and
2
solvents were of analytical grade and were used after a degassed
found to achieve a quantitative yield of the coupling
product in the presence of K PO in water at 100 °C for
3
4
2
4 h, as listed in entry 6 of Table 3. Entries 7-9 of Table 3
show the feasibility of other aryl halides in the comparable
reactions. The correlation of yields versus the substituted
aryl halides appears to be similar to that of other catalytic
9a,b
17
process. Tungsten carbene complexes 1a-c,
6a-b were
prepared accordingly to the method reported previously.
Nuclear magnetic resonance spectra were recorded in CDCl3
on a Bruker either AM-300 or AVANCE 400 spectrometer.
t
systems. It may be noticed that the addition of Bu P is
3
essentially to acquire the high conversion presumably due
to the further stabilization of the active palladium species
upon the catalysis.
Chemical shifts are given in parts per million relative to Me Si
4
1 13
for the H and C NMR.
In order to explore to NHC ligand effect on the
catalysis, we selected the coupling of p-chloroacetophe-
none with phenylboronic acid and the optimized reaction
conditions that were used for trans-3a. All palladium
complexes are subjected to test their catalytic activity on
the Suzuki-Miyaura coupling reaction, and the results
are summarized in table 4.
General Procedure for Complexes 2a-c. A solution of 1 (0.09
mmol) in CH
(COD)PdCl]
Cl
(5 mL) was added slowly to a solution of
2
2
[
2
(0.09 mmol) in CH Cl (5 mL) with stirring. The
2 2
resulting solution turned into a dark color immediately. After
stirring at room temperature for 8 h, the reaction mixture was
filtered through Celite. The filtrate was concentrated, and the
residue was recrystallized to yield the desired complex.
1
Complex 2a. Yellow solids (61%): HNMR(CDCl
3
,300MHz):
Gratifyingly, higher reactivity due to both trans- and
cis-3a in these coupling reactions was confirmed by as our
prediction. The complexes 5 and 7 with unsaturated
NHCs appear to be less active as the analogues with
saturated NHCs, again supporting that the higher stabi-
lity of the Pd-center possesses a higher catalytically
3
δ 4.22 (q, 8H, -CH
2
-, JHH = 7.3 Hz), 3.63 (s, 8H, imi-H), 1.37
3
, JHH = 7.3 Hz), which is identical to the reported
(
t, 12H, -CH
3
9b
data.
1
Complex 2b. Yellow solids (42%): H NMR (CDCl , 300
3
MHz): δ 7.46-7.50 (m, 8 H, Ar-H), 7.36-7.28 (m, 12 H,
Ar-H), 5.41 (s, 8 H, -CH Ph), 3.37 (s, 8 H, imidazole-H);
2
1
3
1
6
C NMR (CDCl
28.7, 128.2 (Ph), 54.7, 47.7. Anal. calcd. for C H Cl N Pd :
3
, 100 MHz): δ 174.5 (MdC), 134.3, 128.8,
activity. In fact, we did observe the precipitation of
palladium black out the reaction mixture with the use of 7
as the precatalyst during the catalytic reactions but not
1
C, 47.74; H, 4.24; N, 6.55. Found: C, 47.39; H, 4.08; N, 6.39.
3
4
36
4
4
2
1
Complex 2c. Yellow solids (36%): H NMR(CDCl
3
, 400MHz):
δ 4.22 (q, J = 7.3 Hz, 8H, -CH -), 3.63 (s, 8H, imidazole-H),
2
(
15) (a) Viciu, M. S.; Germaneau, R. F.; Nolan, S. P. Org. Lett. 2002, 4,
053. (b) Viciu, M. S.; Navarro, O.; Germaneau, R. F.; Kelly, R. A.; Sommer, W.;
Marion, N.; Stevens, E. D.; Cavallo, L.; Nolan, S. P. Organometallics 2004, 23,
629. (c) Marion, N.; Navarro, O.; Mei, J.; Stevens, E. D.; Scott, N. M.; Nolan,
S. P. J. Am. Chem. Soc. 2006, 128, 4101.
16) Winkelmann, O. H.; Riekstins, A.; Nolan, S. P.; Navarro, O.
Organometallics 2009, 28, 5809 and references therein.
4
(17) (a) Lee, C. K.; Vasam, C. S.; Huang, T. W.; Wang, H. M. J.; Yang,
R. Y.; Lee, C. S.; Lin, I. J. B. Organometallics 2006, 25, 3768. (b) De Fremont,
P.; Scott, N. M.; Stevens, E. D.; Ramnial, T.; Lightbody, O. C.; Macdonald, C. L.
B.; Clyburne, J. A. C.; Abernethy, C. D.; Nolan, S. P. Organometallics 2005, 24,
6301.
1
(

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 3017
1
3
1
.37 (t, J = 7.3 Hz, 12H, -CH
MHz): δ 182.9 (PddC), 156.9, 129.2, 128.9, 122.8, 120.2, 110.8,
5.4, 48.8, 47.9. Anal. calcd. for C38 Pd : C, 46.79; H,
.55; N, 5.74. Found: C, 46.68; H, 4.75; N, 5.58.
General Procedure for Complexes 3a-b. A solution of
(COD)PdCl] (0.16 mmol) in CH Cl (15 mL) was added slowly
to a solution of 1 (0.33 mmol) in CH Cl (15 mL) with stirring at
3
). C NMR (DMSO-d
6
, 100
CH
as light-yellow solids (61 mg, 87%). Complex trans-5: H NMR
2
Cl
2
/ether to give the desired products (trans-5a þ cis-5a)
1
5
4
H44Cl
4
N
4
O
4
2
(CDCl
3
, 400 MHz): δ 6.79 (s, 2H, imidazole-H), 4.50 (q, J =
7.3 Hz, 4H, -CH CH ), 4.11 (q, J = 7.3 Hz, 4H, -CH CH ),
2
3
2
3
3.58 (s, 4H, imidazole-H), 1.60 (t, J = 7.3 Hz, 6H, -CH
3
), 1.40
, 100 MHz): δ
197.1 (MdC), 170.1 (MdC), 119.7, 47.9, 45.5, 44.4, 16.5, 13.7.
1
3
[
2
2
2
3 3
(t, J = 7.3 Hz, 6H, -CH ); C NMR (CDCl
2
2
1
room temperature. The resulting solution turned into a dark
color immediately. After stirring for 8 h, the reaction mixture
was filtered through Celite. The filtrate was concentrated, and
the residue was recrystallized from CH
desired complex.
Complex trans-3a. Colorless crystalline solids (55%):
Complex cis-5: H NMR (CDCl , 400 MHz): δ 6.83 (d, J = 3.9
3
Hz, 1H, imidazole-H), 6.80 (d, J = 3.9 Hz, 1H, imidazole-H),
4.47-4.56 (m, 4H, -CH
2
CH
3
), 4.00-4.13 (m, 4H, -CH
2
CH
3
),
),
2
Cl /hexane to yield the
3.55 (s, 4H, imidazole-H), 1.58-1.67 (m, 6H, -CH
3
2
13
); C NMR (CDCl
98.2 (MdC), 168.8 (MdC), 119.7, 47.9, 45.5, 44.4, 16.5, 13.7.
1.33-1.42 (m, 6H, -CH
3
3
, 100 MHz): δ
1
H
1
3
2 4
Anal. calcd. for C14H26Cl N Pd: C, 39.31; H, 6.13; N, 13.10.
NMR (CDCl , 400 MHz): δ 4.03 (q, 4H, -CH -, J
= 7.2
3
2
HH
3
Found: C, 39.02; H, 5.89; N, 12.99.
Complex trans-5b. A mixture of 2a (10.0 mg, 1.65 ꢀ 10
Hz), 3.53 (s, 4H, imidazole-H), 1.34 (t, 6H, -CH
3
, JHH = 7.2
1
3
-2
Hz); C NMR (CDCl , 100 MHz): δ 198.1 (MdC), 47.9, 44.3,
3
3
5
106
þ
-2
1
3
3.7. HR-FAB-MS calcd. m/z for C14
93.1037. Found: 393.1027. Anal. calcd. for: C14
H
28
N
4
Cl Pd [M-Cl] :
Pd:
mmol) and 6b (20.5 mg, 3.3 ꢀ 10 mmol) in CHCl
60 °C for 36 h. After filtration of silver salt, the solution was treated
with excess of Et NCl. The reaction mixture was then concen-
trated, and the residue was recrystallized from CH Cl /ether to
3
was heated at
H
2 4
28Cl N
C, 39.13; H, 6.57; N, 13.04. Found: C, 39.29; H, 6.82; N, 12.80.
Complex cis-3a. A solution of trans-3a (4.7 mg) in CH CN
1 mL) was added to a flask loaded with AgBF (5.5 mg). The
mixture was stirred at refluxing temperature for 24 h, and then
LiCl (3.1 mg) was added. After stirring for another 24 h, the
solvent was removed, and the H NMR spectrum of the reaction
4
2
2
3
1
give the desired product as light-yellow solids (16.1 mg, 71%): H
NMR (CDCl , 400 MHz): δ 7.44 (t, J = 7.7 Hz, 2H, Ar-H), 7.31
d, J = 7.7 Hz, 2H, Ar-H), 7.05 (s, 2H, imidazole-H), 3.45 (q,
J = 7.2 Hz, 4H, -CH CH ), 3.29 (s, 4H, imidazole-H),
3.09-3.16 (m, 4H, -CH(Me) ), 1.37 (d, J = 6.8 Hz, 12H, -CH-
(CH )(CH )), 1.05 (d, J = 6.8 Hz, 12H, -CH(CH )(CH )), 0.89
(t, J = 7.2 Hz, 6H, -CH , 100 MHz):
δ 195.8 (MdC), 178.1 (MdC), 147.1, 135.8, 129.5, 123.8, 123.4,
47.5, 43.7, 28.6, 26.4, 22.7,13.1. Anal. calcd. for C34 Pd:
(
4
3
(
1
2
3
product showed only a single species cis-3a presented. This
residue was recrystallized from CH Cl /hexane to give the desired
2
2
2
3
3
3
3
1
complex cis-3a as white solids (2.1 mg, 45%): H NMR (CDCl
13
3
,
CH ); C NMR (CDCl
2 3 3
4
4
00 MHz): δ 4.07 (q, J = 7 Hz, 4H, -CH -), 3.87 (q, J = 7 Hz,
2
H, -CH -), 3.55-3.61 (m, 8H, imidazole-H), 1.20 (t, J = 7
H50Cl N
2 4
2
13
Hz, 12H, -CH
4
6
3
); C NMR (CDCl
3
, 100 MHz): δ 189.1 (MdC),
C, 59.00; H, 7.28; Cl, 10.24; N, 8.10. Found: C, 58.76; H, 7.04;
N,7.88.
7.2, 45.3, 13.1. Anal. calcd. for C14H28Cl N Pd: C, 39.13; H,
2 4
.57; N, 13.04. Found: C, 38.83; H, 6.86; N, 12.94.
Complex trans-3b. Light-yellow solids (80%): H NMR
CDCl , 400 MHz): δ 7.45-7.41 (m, 12 H), 7.19 (m, 8 H), 5.25
Complexes trans-7 þ cis-7. A mixture of 6a (50 mg, 016 mmol)
1
and [(COD)PdCl] (23 mg, 0.08 mmol) in CH Cl (3 mL) was
2 2 2
(
(
stirred at room temperature for 8 h. A solution of LiCl (50 mg) was
added to the reaction mixture. After filtration of salts, ether was
slowly added to the reaction solution, and yellow solids precipi-
3
9
b
s, 8 H), 3.31 (s, 8H), which is identical to the reported data.
1
Complex cis-3b. Light-yellow solids (78%): H NMR
CDCl , 400 MHz): δ 7.32-7.38 (m, 8H, Ar-H), 7.24-7.39
m, 12H, Ar-H), 5.57 (d, J = 14.1 Hz, 4H, -CHHPh), 4.76 (d,
(
(
3
tated (25 mg, 73%), which was identified as a mixture of trans- and
1
cis-isomers. Complex trans-7: H NMR (CDCl
3
, 400 MHz): δ 6.84
CH ), 1.59-1.66
, 100 MHz): δ 169.1, 119.95,
J = 14.1 Hz, 4H, -CHHPh), 3.33-3.42 (m, 4H, imidazole-H),
(s, 4H, imidazole-H), 4.49-4.60 (m, 8H, -CH
2
3
1
3
13
3
.14-3.25 (m, 4H, imidazole-H); C NMR (CDCl
MHz): δ 190.2 (MdC), 134.9, 128.8, 128.3, 128.1 (Ph), 54.7,
7.9. Anal. calcd. for C H Cl N Pd: C, 60.23; H, 5.35; N, 8.26;
3
, 100
(m, 12H, CH
119.8, 45.6, 16.4. Complex cis-7: H NMR (CDCl
6.82 (s, 4H, imidazole-H), 4.40-4.58 (m, 8H, -CH
3
); C NMR (CDCl
3
1
, 400 MHz): δ
CH ),
, 100 MHz): δ
68.9, 120.1, 119.92, 45.7, 16.3. Anal. calcd. for C14 Pd:
3
4
2
3
3
4
36
2
4
1
3
Found: C, 59.94; H, 5.29; N, 7.88.
Complexes trans-4 þ cis-4. A mixture of 2a (5.0 mg, 8.24 ꢀ
3 3
1.53-1.59 (m, 12H, CH ); C NMR (CDCl
1
2 4
H24Cl N
-
3
-2
C, 39.50; H, 5.68; N, 13.16. Found: C, 38.92; H, 5.64; N, 12.96.
Typical Procedure Reaction of Biscarbene Palladium with
1
0
mmol) and 1b (18.9 mg, 3.3 ꢀ 10 mmol) in dichloro-
methane (4 mL) was stirred at room temperature for 72 h. The
excess of 1b was removed by chromatography. A mixture of
trans- and cis-4 was obtained as white solids (5.3 mg, 58%).
However, neither trans- nor cis-4 could be obtained in pure form
-
2
AgBF
(1 mL) was added to a flask loaded with AgBF
4
. A solution of 5a (20 mg, 4.7 ꢀ 10 mmol) in CD
3
CN
4
(18.2 mg, 9.5 ꢀ
-
2
10 mmol). The mixture was stirred at refluxing temperature
for 48 h, and then LiCl (10 mg) was added. After stirring for
another 24 h, the reaction mixture was filtrated to remove all
1
not even by chromatography.Complex trans-4: H NMR
(
CDCl , 400 MHz): δ 7.58-7.60 (m, 4H, Ar-H), 7.21-7.37
3
1
(
-
m, 6H, Ar-H), 5.28 (s, 4H, -CH
2
Ph), 4.00 (q, J = 7 Hz, 4H,
solids, and the H NMR spectrum of the solution was taken. The
CH CH ), 3.55 (s, 4H, imidazole-H), 3.31 (s, 4H, imi-
2
3
signals of spectrum were identified as a mixture of 2a, 1,3-diethyl-
imidazolium salt, and 6a, which are essentially identical to the
authentic samples.
1
); C NMR (CDCl
3
dazole-H), 1.28 (t, J = 7 Hz, 6H, -CH
3
3
,
1
1
00 MHz): δ 198.7 (MdC), 197.2 (MdC), 136.0, 128.6, 128.5,
1
27.5, 54.1, 47.9, 47.9, 44.4, 13.6. Complex cis-4: H NMR
, 400 MHz): δ 7.51-7.53 (m, 4H, Ar-H), 7.21-7.37 (m,
H, Ar-H), 5.24 (s, 4H, -CH Ph), 3.70-4.08 (m, 4H,
Typical Procedure for Reaction of Biscarbene Palladium with
-
2
(
CDCl
3
I
2
. A solution of 5a (18 mg, 4.2 ꢀ 10 mmol) in CDCl
3
(1 mL)
-
2
6
-
was added to a NMR tube loaded with I (22 mg, 8.6 ꢀ 10
2
2
CH CH ), 3.55 (s, 4H, imidazole-H), 3.31 (s, 4H, imi-
mmol). The mixture was stirred at room temperature for
10 h. After that, the reaction mixture was filtrated, and the
H NMR spectrum of the solution was taken. The signals
2
3
1
3
dazole-H), 1.34-1.39 (m, 6H, -CH
3
); C NMR (CDCl
00 MHz): δ 198.2 (MdC), 197.8 (MdC), 135.9, 128.6, 128.5,
27.5, 54.1, 47.9, 47.9, 44.3, 13.7. Anal. calcd. for
Pd: C, 52.04; H, 5.82; N, 10.12. Found: C, 51.72;
3
,
1
1
1
of spectrum were identified as a mixture of 2a and 2-iodo-1,
3-diethylimidazolium salt 10, which are essentially identical to
the authentic samples.
24 2 4
C H32Cl N
H, 5.59; N, 9.82.
Complexes trans-5a þ cis-5a. A mixture of 2a (50.0 mg, 8.24 ꢀ
-
3
Compound 10. Complex 7 (7.3 mg, 7.8 ꢀ 10 mmol) and I
2
-
2
-2
1
0
mmol) and 6a (51.4 mg, 0.165 mmol) in CHCl
3
was heated
2 2
(3.5 mg, 1.4 x10 mmol) were dissolved in CH Cl (1 mL). The
at 60 °C for 12 h. After filtration of silver salt, the solution
was treated with excess of Et NCl. The reaction mixture was
then concentrated, and the residue was recrystallized from
resulting mixture was stirred at room temperature for 10 h. The
reaction mixture was filtrated through Celite, and the filtrate
was chromatographed on silica gel to separate 10 from 2a.
4

3
018 Inorganic Chemistry, Vol. 49, No. 6, 2010
Fu et al.
Table 5. Crystal Data of 2c, trans-3a, cis-3a and cis-7
complex
trans-3a
28Cl Pd
cis-3a
28Cl
cis-7
24Cl
425.67
monoclinic
C2/c
10.1228(2)
11.1158(2)
15.4894(3)
90
91.205(2)
90
2c
formula
fw
C
14
H
2
N
4
C
429.70
14
H
2
N
4
Pd
C
14
H
2
N
4
Pd
C
40
H
48Cl
1145.22
monoclinic
P2 /n
8 4 4 2
N O Pd
429.70
orthorhombic
Pna2
crystal system
space group
monoclinic
C2/c
10.0204(3)
11.0892(3)
16.3857(5)
90
94.360(3)
90
1815.48(9), 4
1.572
880
1
1
˚
a, A
17.8170(3)
8.3450(1)
12.5420(2)
90
10.1320(1)
13.9446(2)
17.1902(2)
90
106.787(1)
90
2325.25(5), 2
1.636
1152
0.30 ꢀ 0.20 ꢀ 0.15
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
90
90
3
˚
V, A ; Z
1864.78(5), 4
1.595
920
1742.53(6), 4
1.623
864
3
d (calc.), Mg/m
F(0,0,0)
crystal size, mm
rflns collected
independent rflns
3
0.25 ꢀ 0.20 ꢀ 0.10
0.20 ꢀ 0.15 ꢀ 0.10
0.20 ꢀ 0.15 ꢀ 0.10
10 779
4230
10 834
2088
11 807
2003
16 626
5332
[
R(int) = 0.0489]
[R(int) = 0.0610]
2.74-27.49
[R(int) = 0.0355]
3.00-27.49
[R(int) = 0.0324]
1.91-27.48
θ range, deg
refined method
goodness of fit on F
2.29-27.46
2
full-matrix least-squares on F
0.807
2
1.229
0.886
1.112
R indices [I > 2σ(I)]
R indices (all data)
R1 = 0.0694
wR2 = 0.1485
R1 = 0.0817
wR2 = 0.1555
R1 = 0.0386
wR2 = 0.1029
R1 = 0.0416
wR2 = 0.1049
R1 = 0.0266
wR2 = 0.0914
R1 = 0.0309
wR2 = 0.0945
R1 = 0.0386
wR2 = 0.1004
R1 = 0.0571
wR2 = 0.1204
1
8
1
Compound 10 was obtained as light-yellow solids (2.3 mg,
8
4-Acetylbiphenyl
:
3
H NMR (CDCl , 400 MHz): δ 8.01
1
0%): H NMR (CDCl
3
, 400 MHz): δ 7.58 (s, 2H, imi-
(d, J = 8 Hz, 2H, Ar-H), 7.65 (d, J =
8 Hz, 2H, Ar-H), 7.59 (d, J = 7 Hz, 2H,
Ar-H), 7.44-7.38 (m, 3 H), 2.60 (s, 3H,
dazole-H), 4.28 (q, J = 7 Hz, 4H, -CH -), 1.51 (t, J = 7
2
1
3
Hz, 6H, -CH
3
); C NMR (CDCl
3
, 100 MHz): 123.9, 96.0, 48.0,
m/z = 251.00. Found
50.94.
Complex 11. A mixture of 6a (100 mg, 0.32 mmol),
(COD)PdCl]2 (135 mg, 0.35 mmol), and LiCl (50 mg) in
þ
1
5.5. ESI-MS calcd for [M-I] C
7
H
12IN
2
3
-CH ).
1
8
1
2
2-Methylbiphenyl
:
H NMR (CDCl
7.40-7.31 (m, 5H, Ar-H), 7.25-7.20-
(m, 4H, Ar-H), 2.25 (s, 3H, -CH ).
H NMR (CDCl , 400 MHz): δ 7.46 (d,
3
, 400 MHz): δ
[
3
1
8
1
dichloromethane (1 mL) was stirred for 8 h. The reaction
mixture was filtered, and the filtrate was concentrated. The
residue was chromatographed on silica gel with elution of ethyl
Biphenyl
:
3
J = 7 Hz, 4H, Ar-H), 7.48 (m, 4H), 7.39 (m,
2H).
Biphenyl-4-carboxylic acid : H NMR (CDCl , 400 MHz):
3
1
8 1
acetate. A yellow band was collected and concentrated to yield
1
1
1 as yellow solids (54 mg, 56%): H NMR (CDCl
, 400 MHz):
-),
δ 8.06 (d, J = 8 Hz, 2H, Ar-H),
7.78-7.73 (m, 4H, Ar-H),
7.53-7.42 (m, 3H, Ar-H).
3
δ 6.91 (s, 4 H, imidazole-H), 4.67 (q, J = 7 Hz, 8H, -CH
2
1
3
1
1
.63 (t, J = 7 Hz, 12H, -CH3); C NMR (CDCl , 100 MHz): δ
3
39.8(MdC), 121.5, 46.1, 16.1. Anal. calcd. for C H Cl -
14
24
4
Crystallography. Crystals suitable for X-ray determination
were obtained for 2c, trans-3a, cis-3a, and cis-7 by recrystalliza-
tion at room temperature. Cell parameters were determined by a
Siemens SMART CCD diffractometer. Crystal data of these com-
plexes are summarized in Table 5. The structure was solved using
the SHELXS-97 program and refined using the SHELXL-97
program by full-matrix least-squares on F2 values. Other crystal-
lographic data are deposited as Supporting Information.
N
4 2
Pd : C, 27.88; H, 4.01; N, 9.29. Found: C, 27.36; H, 4.21;
N, 9.26.
Typical Procedure for Reaction of Biscarbene Palladium with
-
2
CF COOH. A solution of 5a (18 mg, 4.2 ꢀ 10 mmol) in CDCl3
3
(
1 mL) was added to a NMR tube loaded with an excess of
CF COOH. The tube was immersed into a sonication bath for
2 h. The spectrum of the sample showed signals corresponding
19
3
20
1
to 2a and 1,3-diethylimidazolium salt, which are essentially
identical to the authentic samples.
Acknowledgment. We thank the National Science Council
NSC97-2113-M-002-013-MY3) and NSC-NWO Project for
Catalysis-General Procedure. A mixture of aryl halide (0.2
mmol), phenylboronic acid (0.3 mmol), palladium complex
(
their financial support.
(
0.002 mmol), phosphine (0.004 mmol), and K
in flask under nitrogen atmosphere. Then, degassed water
5 mmol) was syringed into the reaction mixture. The resulting
3 4
PO was placed
(
Supporting Information Available: Complete description of
the X-ray crystallographic structural determination of 2c, trans-
mixture was heated for 24 h. The reaction mixture was extracted
with dichloromethane (5 mL x 2). The organic extracts were
dried and concentrated. The residue was chromatographed on
silica gel to give the desired organic product. Products obtained
in this work were characterized by spectral methods particularly
3
a, cis-3a, and cis-7 including: tables of atomic coordinates,
isotropic and anisotropic thermal parameters, and bond dis-
tances and angles are given as cif files. This material is available
free of charge via the Internet at http://pubs.acs.org.
1
with H NMR, and the data were consistent with those reported.
(
19) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990,
46, 467.
(20) Sheldrick, G. M. SHELXL-97; University of G €o ttingen: G €o ttingen,
Germany, 1997.
(
18) (a) Chen, C.-L.; Liu, Y.-H.; Peng, S.-M.; Liu, S.-T. Organometallics
005, 24, 1075. (b) Bei, X.; Turner, H. W.; Weinberg, W. H.; Guram, A. S.;
Petersen, J. L. J. Org. Chem. 1999, 64, 6797.
2