Synthesis, X-ray structures, and catalytic applications of palladium(II) complexes bearing N-heterocyclic iminocarbene ligands

Article abstract of DOI:10.1016/j.jorganchem.2008.09.006

Two new Pd(II) N-heterocyclic iminocarbene complexes (C-N)PdCl₂ that contain 5-membered chelate rings have been prepared by carbene transfer from a silver iminocarbene precursor to (COD)PdCl₂. The new Pd imonocarbene complexes, as well as two that have been previously reported (altogether three 5-membered and one 6-membered chelate ring complexes) have been evaluated as catalysts for the Suzuki-Miyaura coupling reaction. The complexes were found to be active in the reaction, but without exceptional catalytic performances. The 5-membered chelate ring complexes appeared to be more robust and remained active for a longer time than the 6-membered ring congener. The catalytic performance of the 5-membered chelate ring complexes appeared to be rather insensitive to the steric demands of the imine-N-aryl group. The X-ray structure of one of the Ag iminocarbene complexes reveals the κ¹(C) bonding of the iminocarbene moiety in a nearly linear Ag(I) complex; two monomeric units are associated through a weak Ag-Ag interaction. The X-ray structures of two new Pd iminocarbene complexes (C-N)PdCl₂ confirm the chelating κ²(C,N) nature of the iminocarbene moiety; in both complexes, the Pd-Cl distances trans to carbene-C are slightly longer than those trans to imine-N.

Full text of DOI:10.1016/j.jorganchem.2008.09.006

Journal of Organometallic Chemistry 693 (2008) 3703–3710
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, X-ray structures, and catalytic applications of palladium(II)
complexes bearing N-heterocyclic iminocarbene ligands
Kjetil Andreas Netland a, Alexander Krivokapic a, Markus Schröder b, Klaus Boldt b, Fredrik Lundvall a,
Mats Tilset a,
*
a Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315 Oslo, Norway
b University of Hamburg, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 June 2008
Received in revised form 1 September 2008
Accepted 2 September 2008
Available online 10 September 2008
Two new Pd(II) N-heterocyclic iminocarbene complexes (C–N)PdCl2 that contain 5-membered chelate
rings have been prepared by carbene transfer from a silver iminocarbene precursor to (COD)PdCl2. The
new Pd imonocarbene complexes, as well as two that have been previously reported (altogether three
5-membered and one 6-membered chelate ring complexes) have been evaluated as catalysts for the
Suzuki–Miyaura coupling reaction. The complexes were found to be active in the reaction, but without
exceptional catalytic performances. The 5-membered chelate ring complexes appeared to be more robust
and remained active for a longer time than the 6-membered ring congener. The catalytic performance of
the 5-membered chelate ring complexes appeared to be rather insensitive to the steric demands of the
Keywords:
Palladium catalysis
NHC complexes
Iminocarbene
Suzuki–Miyaura coupling
X-ray structure
Silver carbene
imine-N-aryl group. The X-ray structure of one of the Ag iminocarbene complexes reveals the j
1(C) bond-
ing of the iminocarbene moiety in a nearly linear Ag(I) complex; two monomeric units are associated
through a weak Ag–Ag interaction. The X-ray structures of two new Pd iminocarbene complexes (C–
N)PdCl2 confirm the chelating
Cl distances trans to carbene-C are slightly longer than those trans to imine-N.
Ó 2008 Elsevier B.V. All rights reserved.
j
2(C,N) nature of the iminocarbene moiety; in both complexes, the Pd–
1. Introduction
[23,25], amide [26], amido [27], ester, keto and ether [28], and
oxazoline [29] donors. Chelating, functionalized NHC ligands in
which the pendant functional group may readily dissociate from
the metal center, a hemilabile NHC system [30,31], are of particular
interest because the opened coordination site can be employed for
substrate binding in catalytic reactions. On the other hand, hemila-
bility also may induce decomposition.
The report of stable N-heterocyclic carbenes (NHCs) of the imi-
dazolin-2-ylidene type by Arduengo et al. [1,2] spurred the imme-
diate development and utilization of such species as ligands in
organometallic complexes [3,4]. It has been firmly established that
these carbenes act as strong
r donors and rather poor p acceptors,
and that they in some respects resemble phosphine ligands rather
than the classical Fisher or Schrock type carbenes [3,5,6].
Herrmann and co-workers early demonstrated that Pd–NHC
complexes could be used to catalyze C–C coupling reactions [7–
9]. Soon thereafter the application of highly diverse metal–NHC
complexes in homogeneous catalysis was initiated. The most
well-known examples include the ruthenium metathesis catalysts
developed by Grubbs and co-workers [10–12] and numerous palla-
dium-catalyzed reactions including Heck, Suzuki–Miyaura, Stille,
and Sonogashira cross-coupling reactions [13–21].
Palladium catalysis is ubiquitous to modern organic chemistry
and a great number of catalytic systems have been designed and
fine-tuned through careful ligand design and choice of experimen-
tal conditions (solvent, temperature, additives, etc). While many of
the efficient catalytic systems that have been discovered operate
by well-defined homogeneous metal–complex catalyzed pathways
(in which the designed metal complex may function as a true cat-
alyst or as a precatalyst), this is not always the case. There is con-
siderable concern that many of the uniquely designed molecular
catalysts or precatalysts undergo reactions or decompositions un-
der true catalysis conditions to generate species such as (but not
limited to) palladium particles, colloids, nanoparticles, ligand-free
or ‘‘naked” catalysts as potentially viable catalytically active spe-
cies in addition to well-defined molecular species [32–36]. A thor-
ough examination of each catalytic system is required in order to
assess the nature of the catalysis. Critical reviews concerning the
problem of distinguishing true homogeneous catalysis from solu-
ble or metal-particle heterogeneous catalysis have been recently
Metal complexes that bear functionalized, chelating NHC
ligands have attracted considerable attention [22]. Commonly
encountered functionalities include phosphine [23,24], pyridine
* Corresponding author. Tel.: +47 22 85 55 02.
E-mail address: mats.tilset@kjemi.uio.no (M. Tilset).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.09.006

3704
K.A. Netland et al. / Journal of Organometallic Chemistry 693 (2008) 3703–3710
system is clearly demonstrated for 1a, but not for 1b. It might be
N
N
N
anticipated that this difference could lead to dramatic differences
in the behavior of the 6-membered and 5-membered ring chelate
structures in catalytic applications.
We herein report the first studies of the catalytic performances
of the palladium complexes 1a–d. The work was undertaken with
the intent to investigate the effect of 5- vs. 6-membered ring che-
lates as well as substituent effects in the 5-membered ring chelates.
The results are discussed in light of the aforementioned questions
regarding whether these palladium-catalyzed reactions are Pd(II)
homogeneously catalyzed or not.
N
N
M
M
N
Scheme 1.
N
N
N
N
N
Cl
Cl
Cl
Cl
Cl
Cl
N
Cl
Cl
N
N
N
Pd
Pd
Pd
Pd
N
N
2. Results and discussion
N
2.1. Ligand synthesis
We recently reported [39] that the iminocarbene ligand system
shown at the right in Scheme 1 is readily prepared by the reaction
between a chloroimine and a monosubstituted imidazole [37–39],
which provides the iminoimidazolium salt as a carbene precursor
in good yields. The chloroimines 3c [40] and 3d [41] were made
from secondary amides 2 by chlorination with SOCl2 or PCl5
[42,43]. The synthesis of the new imidazolium salts 4c and 4d
(4b was previously reported [39]) are illustrated in Scheme 4 and
details of the synthesis and product characterization are given in
the experimental section.
1a
1b
1c
1d
Scheme 2.
published and emphasize that no single test is sufficient to distin-
guish between alternatives [32,35].
We have recently described the synthesis and characterization
of imino-NHC based complexes of the types shown in Scheme 1.
In particular, non-chelated (j )
1) Ag(I) structures and chelated (j2
or non-chelated Pd(II) or Pt(II) complexes have been reported
[37–39]. It was demonstrated that the 6-membered ring chelated
iminocarbene complex 1a (Scheme 2) exhibits dynamic behavior
in solution. The boat conformation undergoes a facile ‘‘boat flip”
at ambient temperature, as demonstrated by variable-temperature
and 1H EXSY NMR. In addition, the molecule undergoes solvent-
dependent chelate ring-opening in which donor solvents
presumably occupy the vacant coordination site. The extent of
side-arm dissociation increases with the solvent coordinating
capacity in the order DMSO > MeCN > THF > MeNO2. The chelating
(r2(C,N))PdCl2 and the non-chelating (r1(C,N))PdCl2(solvent) struc-
tures undergo rapid interconversion, seen by ambient-temperature
1H EXSY NMR and by reversible line broadening at higher temper-
atures when the solvent is DMSO (Scheme 3) [37]. Similar dynamic
processes have not been observed for the 5-membered chelate ring
complexes such as 1b. Thus, the hemilabile nature of the ligand
2.2. Silver iminocarbene complexes
Metal–NHC complexes are frequently prepared either by reac-
tion of the imidazolium salts with appropriate metal salts or by
in situ carbene generation by treatment of imidazolium salts with
strong bases before addition of a metal complex. In situ carbene
generation from compounds 4b–d was unsuccessful, in accord
with a report by Bildstein and coworkers [44]. Treatment of 4b–d
with PdCl2 also did not generate the desired Pd complexes. How-
ever, the by now well-established method that involves carbene li-
gand transfer from Ag(I) NHC complexes [45–47] worked well. The
required Ag(I) NHC complexes are readily prepared by reaction of
the imidazolium salts with Ag2O, which acts as a mild base and
is compatible with the presence of quite acidic functional groups
[45–47]. Accordingly, treatment of the imidazolium salts 4b–d
with Ag2O in dichloromethane (Scheme 5) led to the corresponding
Ag(I) iminocarbene complexes 5b–d in moderate yields (4b and 5b
were previously reported [39]). It should be noted that the reaction
with Ag2O generates water and that the presence of 4 Å molecular
sieves during the reaction limited the partial hydrolysis of the
moisture sensitive species 4 and 5. Complete 1H and 13C NMR
assignments for the new Ag(I) complexes 5c–d are given in the
experimental section. As anticipated, the NMR spectra of the steri-
cally less hindered Ag complex 5d indicated the presence of two
isomers in an approximate 4:1 ratio. These are attributed to the
Ar
N
Pd
N
N
Cl
Cl
N
N
Solvent
N
Solvent Pd Cl
Ar
Cl
κ2
κ1
Scheme 3.
N
-
N
+
N
Cl
SOCl2
or PCl5
O
Cl
O
Ar-NH2
N
Ar
Ar
Ar
Ph
Cl
Ph
N
Ph
N
H
Ph
N
CH2Cl2
Reflux, 10h
THF
b : Ar = 2,6-iPr2C6H3
4b-d
2b-d
3b-d
c
: Ar = 2,6-Me2C6H3
d : Ar = 4-MeC6H4
Scheme 4.

K.A. Netland et al. / Journal of Organometallic Chemistry 693 (2008) 3703–3710
3705
aqueous NaCl in acetone conveniently transformed the cationic
palladium complex back to the desired neutral species 1d as a
finely divided white powder, analytically pure.
N
N
N
+
N
Ag2O
(COD)PdCl2
CH2Cl2
Ag
Cl
Cl
Cl
N
N
-
Cl
N
Pd
High resolution electrospray mass spectrometry of 1d from
dry deoxygenated acetonitrile solutions showed single ions with
m/z values corresponding to the (carbene)PdCl+ and the (car-
bene)Pd(Cl)(CH3CN)+ ions, indicating the monomeric (car-
bene)PdCl2 composition of 1d as depicted in Schemes 5 and 6.
Likewise, elemental analysis data for 1d were in agreement with
a {(carbene)PdCl2}n formulation which again rules out the possibil-
ity of a non-chelating (carbene)2PdCl2 complex. Unfortunately,
NMR and analysis data cannot unambiguously establish whether
the structure of 1d is a chelated monomeric or a non-chelating,
N–C bridged oligomeric one. Attempts at obtaining X-ray quality
crystals of 1d have failed thus far. However, comparison of reported
data to those of nearly identical chelating structures [39,49] sug-
gests that 1d is also a monomeric unit with an iminocarbene che-
lated structure in solution.
CH2Cl2
Ar
N
Ph
Ph
N
Ph
Ar
Ar
5b-d
4b-d
1b-d
i
b
: Ar = 2,6- Pr2C6H3
c : Ar = 2,6-Me2C6H3
: Ar = 4-MeC6H4
d
Scheme 5.
E and Z isomers, respectively, on the basis of NOESY NMR spectros-
copy. The predominance of the E isomer is the opposite of the Z
preference that was observed for a closely related complex,
[3-tBu-1-{C(Ph)@N(Ph)}C3H3N2]AgCl, recently described and crys-
tallographically characterized by Green and coworkers [48]. The
silver complexes 5c–d were analyzed by ES-MS from dry, deoxy-
genated acetonitrile solutions. Interestingly, bis(carbene) ions
(NHC)2Ag+ rather than mono(carbene) ions are observed under
the ES-MS sampling conditions. This contrasts with the mono(car-
bene) structure that was found in the X-ray crystallographic struc-
ture determination of 5c; similar discrepancies between solid-state
structures and ES-MS data have been reported elsewhere [38,47]. It
is not known whether the solid-state mono(carbene) structure also
exists in solution; the available NMR data do not distinguish be-
2.4. X-ray crystal structure determination of silver complex 5c and
palladium complexes 1b and 1c
Crystals of 5c were grown by layering a saturated dichloro-
methane solution with diethyl ether at room temperature. An OR-
TEP drawing of 5c is depicted in Fig. 1. Crystallographic data are
listed in Table 1. Selected bond lengths and angles are shown in Ta-
ble 2 (metric data for the previously reported Ag complex 5b [39]
are included for comparison). The solid-state structure of 5c con-
sists of (carbene)AgCl units nearly linearly coordinated at Ag.
Two units are associated through an Ag–Ag interaction as evi-
denced by the rather short Ag–Ag distance of 3.0577(10) Å, which
is considerably shorter than the sum of the van der Waals radii of
3.44 Å [50]. The interaction leads to a slight bending of the Cl–Ag–
C(carbene) angle to 169.60(15)°. This type of Ag–Ag interaction is
not observed in 5b, for which the corresponding angle is
177.80(4)°. Similar dimeric or even oligomeric structures have
been observed in the solid state for related (carbene)AgCl com-
plexes [47]. The Ag–Cl and Ag–C(carbene) bond distances of
2.3341(14) and 2.081(5) Å, respectively, are approximately the
same as for 5b (2.3355(3) and 2.0776(13) Å). The imine C@N bond
in 5c is nearly coplanar with the plane of the imidazole ring, with a
C(1)–N(2)–C(5)–N(3) torsion angle of 29.5°. This results in a rela-
tively short distance between N(imine) and Ag of 2.9273(41) Å.
These observations are similar to the data reported for 5b, 20.6°
and 2.8253(11) Å [39].
–
tween mono(carbene) (NHC)AgCl or bis(carbene) (NHC)2Ag+AgCl2
formulations.
2.3. Palladium iminocarbene complexes
The (r2-iminocarbene)PdCl2 complex 1c was obtained in good
yields when the silver iminocarbene 5c was reacted with
(COD)PdCl2 in dichloromethane at –70 °C (Scheme 5) [39]. All spec-
troscopic and elemental analysis data are in accord with the 5-
membered ring chelate formulation for 1c, and the structural
assignment was ultimately confirmed by an X-ray crystallographic
structure determination (vide infra). In the 13C NMR spectrum of 1c
the imine carbon resonance at d 163.8 was shifted slightly down-
field compared to the corresponding resonance at d 153.5 in 5c.
The relative positions of the imine-C chemical shifts for Ag and
Pd complexes are in agreement with previously reported data
[39,48,49]. The carbene-C resonance of 1c is located at d 159.6. This
value is comparable to the carbene-C resonances for previously re-
ported (iminocarbene)PdCl2 complexes [39,49].
Suitable crystals of 1b and 1c were by layering saturated dichlo-
romethane solutions at room temperature with pentane and hex-
Palladium complex 1d was practically insoluble in nearly all
solvents (only slightly soluble in acetonitrile) and could be sepa-
rated from impurities and the precipitated AgCl only with great
difficulty. A pure sample could be prepared as depicted in Scheme
6. Chloride abstraction from 1d with silver triflate in the presence
of acetonitrile generated the soluble cationic palladium complex
6d, which was purified by filtration. Further treatment of 6d with
-
OTf
N
N
N
N
N
N
Cl
Cl
Cl
Cl
Cl
+
N
N
N
NaCl
Pd
AgOTf
Pd
Pd
NCMe
Ph
Ph
Ph
Acetone
CH3CN
p-Tolyl - AgCl
p-Tolyl
p-Tolyl
insoluble
insoluble
soluble
6d
ꢀ
1d
1d
Fig. 1. Compound 5c crystallizes in the triclinic space group P with one molecule in
the asymmetric unit. Hydrogen atoms are omitted for clarity, 50% ellipsoid
Scheme 6.
probability is shown. R1 = 6.4%.

3706
K.A. Netland et al. / Journal of Organometallic Chemistry 693 (2008) 3703–3710
Table 1
Crystallographic data for compounds 1b, 1c, and 5c
Compound
1b
1c
5c
Formula
Formula weight
Crystal system
Colour
Space group
a (Å)
C23H27N3PdCl2 C19H19N3PdCl2 C19H19N3AgCl
522.79
Monoclinic
Yellow
P21/c
551.62
Orthorhombic Triclinic
Yellow
Pbca
517.63
White
ꢀ
P1
10.8929(10)
32.012(3)
16.6361(15)
90
105.826(2)
90
11.1695(8)
16.9317(13)
23.6823(18)
90
90
90
9.079(3)
9.358(3)
13.847(4)
84.178(6)
71.321(5)
70.188(6)
1048.5(6)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
5581.2(9)
8
4478.8(6)
8
Z
T (K)
105
105
105
F (000)
2128
2208
520
Radiation
h Range (°)
Reflections measured
Unique reflections
Number of data/restraint/
parameters
Goodness of fit, F
0.71073
1.27–28.3
13172
13172
9282/0/523
0.71073
1.72–28.3
41290
5512
0.71073
1.55–28.42
8309
4705
3420/0/244
3659/0/253
Fig. 2. Compound 1b crystallizes in the monoclinic space group P21/c with two
molecules in the asymmetric unit. Hydrogens are omitted for clarity, 50% ellipsoid
probability is shown. R1 = 3.11%.
1.0977
1.1029
1.0502
R1, wR2 [I > 3
r
(I)]
0.0311,
0.0316
0.0186,
0.0205
0.0640,
0.0699
R1, wR2 (all data)
0.0486,
0.0377
À1.11, 0.45
0.0360,
0.0331
À0.43, 0.37
0.0831,
0.0776
À2.12, 4.04
Largest difference peak (e ÅÀ3
)
Table 2
Selected bond lengths and angles
Bonds
1b
5b [39]
1c
5c
Pd–N(3)
Pd–C(1)
Ag–C(1)
2.064(2)
1.964(2)
–
2.2949(7)
2.3510(6)
–
1.331(3)
1.379(3)
1.401(3)
1.285(3)
1.451(3)
–
–
2.0488(15)
1.9677(18)
–
–
–
2.0776(13)
–
–
2.3355(3)
1.3370(17)
1.3699(16)
1.4320(17)
1.2721(17)
1.4199(17)
2.081(5)
–
–
2.3341(14)
1.327(7)
1.355(8)
1.438(7)
1.259(7)
1.422(7)
Pd–Cl(1)
Pd–Cl(2)
Ag–Cl(1)
N(1)–C(1)
N(2)–C(1)
N(2)–C(5)
N(3)–C(5)
N(3)–C(12)
2.2898(5)
2.3358(5)
–
1.335(2)
1.372(2)
1.401(2)
1.288(2)
1.440(2)
Angles
C(1)–Ag–Cl(1)
N(3)–Pd–Cl(2)
Cl(1)–Pd–Cl(2)
N(1)–C(1)–N(2)
C(1)–N(2)–C(5)–N(3)
–
177.80(4)
–
–
104.46(11)
20.5
–
169.60(15)
–
–
105.0(4)
29.5
94.47(6)
89.39(2)
104.7(2)
5.4
92.66(4)
90.729(17)
104.73(15)
3.7
Fig. 3. Compound 1c crystallizes in the orthorhombic space group Pbca with one
molecule in the asymmetric unit. Hydrogens are omitted for clarity, 50% ellipsoid
probability is shown. R1 = 1.86%.
ane, respectively. The ORTEP drawings are depicted in Figs. 2 and 3.
Selected bond lengths and angles are shown in Table 1. Most bonds
and angles for the two palladium complexes are quite similar, with
the notable exception of the Pd1–N3(imine) and Pd1–Cl2 distances
of 1b which are both elongated by 0.015 Å compared to 1c. This
difference probably is caused by intramolecular steric repulsions
in the former. A slight increase in the N3–Pd1–Cl2 angle and in
the N3–C(aryl) bond length in 1b relative to 1c corroborates this
hypothesis. The Pd–Cl bond distances trans to the carbene, Pd1–
Cl2, are 0.046–0.056 Å longer than the corresponding Pd–Cl dis-
tances trans to the imine, Pd1–Cl1. This trans effect is rather large,
to be compared with 0.027 Å in the closely related r2-(C-N))PdCl2
structure [49] reported by Green and coworkers. Finally, it is note-
worthy that slight elongations of the imine C@N bond distances are
seen when changing from the less electron-rich Ag complexes 5b
and 5c to the more electron-rich Pd complexes 1b and 1c (0.017
and 0.028 Å, respectively). This elongation is probably caused by
improved back bonding from the Pd center to the p* of the imine
bond. Similar trends have been observed for related Pt [39] and
Pd complexes [48,49].
2.5. Catalytic testing of Pd complexes
The Suzuki–Miyaura reaction [15,18,51] offers a powerful and
general methodology for the construction of carbon–carbon bonds
and is currently the most commonly used cross-coupling proce-
dure because of the commercial availability of a great variety of
boronic acids [17]. Considerable improvements have been made
during the last few years with the design of highly active catalysts
that operate at low catalyst loading [17,20,52,53]. The activity of
these catalysts strongly depends on the type of spectator ligand
and its electronic and steric properties. NHCs and tertiary
phosphines have proven to be the most successful ligands in Suzu-
ki–Miyaura reactions. Impressive turnover numbers (TONs) great-
er than 106 have been recently reported by Herrmann and
coworkers [53]. Interestingly, Pd complexes that bear certain steri-

K.A. Netland et al. / Journal of Organometallic Chemistry 693 (2008) 3703–3710
3707
cally demanding NHCs are particularly active [54], and some that
have been around since the early days of intense NHC exploration
[17] are still among the most active.
tested, but were found to give inferior results). Key catalytic results
obtained for the reaction of aryl bromides are summarized in Table
3. The results show that of the four catalysts 1a–d, the 5-membered
ring chelates 1b–1d exhibited the best activities. Among these, the
sterically less hindered 5-membered ring complex 1d demon-
strated a lowering of catalytic activity in the less polar solvents ben-
zene, THF, and dioxane, probably due to the decreased solubility of
1d in less polar solvents. On the other hand, in acetonitrile its per-
formance was comparable to that of 1b and 1c. A further lowering
of the catalyst loading led to poorer results. As a final note, concern-
ing solvent effects, it is noteworthy that the catalysts yielded
All palladium complexes 1a–d (Scheme 2) were found to be ac-
tive catalyst precursors for the Suzuki–Miyaura coupling reaction of
4-bromotoluene with phenylboronic acid. On the other hand, none
of the complexes catalyzed the coupling with the activated aryl
chloride 4-chloroacetophenone. As standard conditions were cho-
sen 0.5 mmol 4-bromotoluene and 0.75 mM phenylboronic acid
in 4 mL solvent, 1 mol% Pd catalyst, and 1 mmol Cs2CO3 as the
added base. (Other bases, such as KOH, tBuOK, or Ba(OH)2 were also
Table 3
Catalytic results from Suzuki–Miyaura coupling reactions
1 mol% atalyst
1mmolCs CO
Br +
B(OH)
2
3
2
0.5 mmol
0.75 mmol
Catalyst
Solvent
Yielda (%)
1 h
3 h
7 h
79
24 h
d
1,4 Dioxaneb
Acetonitrileb
THFc
52
41
44
65
64
51
62
78
–
N
d
d
–
–
N
Cl
Cl
70
75
d
d
Benzeneb
–
–
Pd
N
1a
1b
1c
N
N
1,4 Dioxaneb
Acetonitrileb
THFc
65
46
51
44
77
69
80
57
82
100
d
Cl
Cl
100
100
64
–
–
N
d
Pd
Benzeneb
76
1,4 Dioxaneb
Acetonitrileb
THFc
51
56
28
39
69
73
52
50
88
100
91
100
N
N
d
Cl
Cl
–
N
Pd
100
79
Benzeneb
60
N
d
d
1,4 Dioxaneb
Acetonitrileb
THFc
21
59
20
45
30
86
38
54
–
–
Cl
95
58
59
100
62
66
N
Pd
Benzeneb
Cl
N
1d
a
Yield determined by GC.
Temperature = 80 °C.
Temperature = 60 °C.
b
c
d
No further reaction was observed.

3708
K.A. Netland et al. / Journal of Organometallic Chemistry 693 (2008) 3703–3710
none (1a and 1c) or low yields (1b, 4%) of the coupling products in
DMSO.
100
90
80
70
60
50
40
30
20
10
0
1h
3h
All catalytic systems 1a–1d underwent deactivation at more or
less the same rates in the early phase of the reactions (1–3 h).
However, the 5-membered chelate ring complexes 1b–1d ap-
peared more robust and remained active for a longer time than
their 6-membered ring congener 1a. This tendency was most pro-
nounced when the reactions were carried out in acetonitrile.
Immediate formation of a dark precipitate, presumably metallic
palladium, was observed within a few minutes in all catalytic reac-
tions involving 1a. The mode of deactivation is not known, but we
surmise that the acidic (enolizable) protons in the CH2 group of the
6-membered ring iminocarbene render the system susceptible to
attack by the added base.
7h
24h
The fact that the catalytic results were quite similar (with the
exception of the minor differences described above) led us to sus-
pect that the catalytic action might be caused by species arising
from decomposition of the molecular Pd species rather than by
the molecular complexes themselves. It is conceivable that quite
similar species might arise from the various precursors. Although
we have not exhaustively attempted to distinguish between these
alternative types of catalysis, we did perform the so-called ‘‘mer-
cury test” that has been frequently used in attempts at distinguish-
ing between homogeneous and heterogeneous catalysis. Thus, the
mercury poisoning test [32] was performed on 1a and 1d. The addi-
tion of 300 equiv. of Hg(0) vs. Pd completely inhibited the catalytic
activity. While this appears consistent with a heterogeneous (metal
particle based) catalytic process, this simple test by no means is a
proof [32,35]. Jones and coworkers [35] stated in their recent critical
review that ‘‘. . .the observation that Hg(0) does not affect the catal-
ysis can confirm a mechanism that does not involve unprotected
Pd(0) (soluble or supported), whereas an observation that Hg(0)
quenches the activity may be consistent with a Pd(0) intermediate.”
The effect of the substrate/catalyst ratio (S/C) on the catalytic
behavior has also been promoted as a useful test [34]. Accordingly,
the substrate conversion vs. catalyst loading in the Suzuki–Miya-
ura reaction between 4-bromotoluene and phenylboronic acid
was investigated with the Pd(II) complexes 1a and 1d. For 1d, rel-
atively high reaction rates and excellent substrate conversion (cor-
responding to turnover number of 586) were observed in
acetonitrile with a catalyst loading as low as 0.1 mol% (Fig. 4). A
further lowering of the Pd precatalyst loading to 0.03 mol% re-
sulted in low catalytic activity and ca. 1% yield. This tendency is
consistent with the involvement of competing Pd colloid forma-
tion, as suggested by de Vries [34]. However, we did not observe
the expected [34] rapid catalyst decomposition at high Pd loading
(>1 mol%) of 1d. Contrasting the behavior of 1d, the 6-membered
ring complex 1a was practically inactive as a catalyst at Pd loadings
below 1 mol%. For example, only 1% yield was obtained when the
Pd loading of 1a was reduced to 0.2 mol%.
0.1
0.25
1.0
4.0
0.03
mol% Pd catalyst (1d)
Fig. 4. The effect of different catalyst concentrations on the Suzuki–Miyaura
reaction depicted in Table 3. Experimental conditions: Phenylboronic acid
(0.75 mmol), 4-bromotoluene (0.5 mmol), and Cs2CO3 (1.0 mmol) in acetonitrile
(4.0 mL) at 80 °C.
served insensitivity to steric effects would result. If it is indeed
the presence of Pd(0) colloids and/or non-chelating (r1(C,N))Pd(0)
species that causes an amalgamation with Hg(0), this should be
in good agreement with recent studies published by Danopoulos
and coworkers [36] who presented evidence, backed by kinetic
measurements and EXAFS studies, for an equilibrium between a
molecular Pd(0)–NHC specie and Pd clusters.
3. Experimental section
3.1. General procedures
All reactions involving organometallic compounds were carried
out with use of dry box, vacuum line, syringe, and Schlenk tech-
niques otherwise noted. Solvent for reactions and NMR were dried
according to standard procedures. NMR spectra were recorded on
Bruker Avance DPX 200, DPX 300 and DRX 500 instrument with
QNP or TXI probes. The NMR spectra were recorded at 25 °C.
Assignment of 1H and 13C signals were aided by DEPT45, HMQC,
HMBC, HETCOR, COLOC, COSY 45, and NOESY 1H NMR spectros-
copy. For brevity, the following abbreviations are used for the
assignment: Ph = Phenyl, Aryl = 2,6-dimethylphenyl or 4-methyl-
phenyl. Electrospray ionization Mass spectra were obtained from
acetonitrile solutions on a Micromass QTOF II spectrometer. Ele-
mental analyses were performed by Ilse Beetz Mikroanalytisches
Laboratorium, Kronach, Germany.
Methylimidazole, phenylboronic acid, 4-bromotoluene, Cs2CO3,
PCl5, SOCl2, Ag2O, and (COD)PdCl2 were purchased from Aldrich,
Fluka, or Strem Chemicals, and used as received. The chlorides 3c
and 3d were prepared by chlorination with SOCl2 or PCl5 [42,43].
The palladium complexes 1a and 1b were prepared as described
previously [37,39].
2.6. Concluding remarks
From the presented results above, we find that the circumstan-
tial evidence available, such as the Hg(0) quench of the catalytic
activity, the insensitivity of the catalytic performance to consider-
able changes in the steric bulk of the imine N-aryl group, and some
trends in catalytic activity vs. Pd loading, does suggest that the cat-
alytic action may be caused by Pd(0) species that are closely re-
lated and formed by decomposition of the molecular species
under the operating experimental conditions. For now, the true
nature of the active palladium species in these coupling reactions
is not known. Perhaps being somewhat speculative, one might ex-
pect that a reduction to Pd(0) would weaken the bond to the
imine-N. Ligand side-arm dissociation would then give a species
in which the N-aryl group was far removed from Pd and the ob-
3.1.1. Preparation of [3-Me-1-, and C3H3N2]+ClÀ (4c)
Methylimidazole (0.33 g, 4.0 mmol) was added dropwise to a
solution of the iminoyl chloride 3c (0.960 g, 3.90 mmol) in dry
THF (20 mL) over a period of 5 min. The mixture was stirred at
ambient temperature for 67 h, during which the product slowly
precipitated as a yellow–white solid. After removal of the solvent
by filtration the crude product was washed with dry THF
(3 Â 15 mL) and dried under reduced pressure (1.02 g, 3.13 mmol,
80%). 1H NMR (CD2Cl2, 300 MHz, 25 °C) d 10.79 (s, 1H, NCHN), 7.89
(s, 1H, NCHCHN near N–Me), 7.73 (s, 1H, NCHCHN near imine),

K.A. Netland et al. / Journal of Organometallic Chemistry 693 (2008) 3703–3710
3709
7.32–7.52 (m, 5H, Ph), 6.90–6.99 (t, 3H, Aryl), 4.31 (s, 3H, N–Me),
2.03 (s, 6H, C–Me). 13C{H} NMR (CD2Cl2, 75 MHz, 25 °C) d 143.3
(Cipso, Aryl), 139.0 (Cipso, Ph), 138.7 (NCHN), 132.4 (Cp, Ph), 129.3
(Co or Cm, Ph), 128.7 (Co or Cm, Ph), 128.7 (Cm, Aryl), 128.1 (Cp, Aryl),
126.3 (Co, Aryl), 124.1 (NCHCHN near N–Me), 119.4 (NCHCHN near
imine), 37.3 (N–Me), 18.1 (C–Me). C(imine) was not observed. MS-
ESI: m/z 290 (M+ÀCl, 8%), 208 (M+ÀC4H6N2Cl, 100%). HRMS-ESI:
Calc. for C18H18N3 (M+ÀCl), 290.1650. Found: 290.1651.
microcrystals (0.834 g, 2.67 mmol, 38%). 1H NMR (CD3CN,
500 MHz, 25 °C) d 9.53 (s, 1H, NCHN), 7.82 (m, 1H, NCHCHN near
imine), 7.75 (m, 1H, NCHCHN near N–Me), 7.52 (tt, 3J = 7.4 Hz,
3J = 1.2 Hz, 1H, Hp, Ph), 7.44 (t, 3J = 7.4 Hz, 2H, Hm, Ph), 7.39 (m,
2H, Ho, Ph), 7.03 (d, 3J = 8.1 Hz, 2H, Ar near C–Me), 6.70 (d,
3J = 8.1 Hz, 2H, Ar near imine), 4.01 (s, N–Me), 2.22 (s, C–Me).
13C{H} NMR (CD3CN, 125 MHz, 25 °C) d 150.1 (imine-C@N), 144.3
(Cipso, Ar), 138.5 (NCHN), 136.0 (Cp, Ar), 132.6 (Cp, Ph), 130.7 (Co,
Ph), 130.4 (Cm, Ar), 130.0 (Cm, Ph), 129.0 (Cipso, Ph), 125.4 (NCHCHN
near N–Me), 121.9 (Co, Ar), 120.6 (NCHCHN near imine), 37.4 (N–
Me), 20.8 (C–Me). MS-ESI: m/z 276 (M+ÀCl, 40%), 194
(M+ÀC4H6N2Cl, 100%). HRMS-ESI: Calc. for C18H18N3 ([M]+ÀCl),
276.1495. Found: 276.1491.
3.1.2. Preparation of [3-Me-1-{C(C6H5)N(2,6-Me2C6H3)}C3H3N2]AgCl
(5c)
The imidazolium salt 4c (1.00 g, 3.06 mmol) was suspended in
dry dichloromethane (20 mL) in the presence of activated 4 Å
molecular sieves. Silver oxide (0.926 g, 3.99 mmol) was added
and the reaction mixture was stirred in the absence of light for
4 h at room temperature. The solution was removed by filtration
before the solvent was evaporated under vacuum, yielding the
product as a grey white solid (0.431 g, 0.990 mmol, 34%). 1H
NMR (CD2Cl2, 500 MHz, 25 °C) d 7.84 (m, 1H, NCHCHN near N–
Me), 7.45–7.48 (m, 1H, Hp, Ph), 7.32–7.36 (m, 2H, Hm, Ph), 7.19
(s, 1H, NCHCHN near imine), 7.16–7.17 (m, 2H, Ho, Ph), 6.91–6.92
(m, 2H, Hm, Aryl), 6.83–6.86 (m, 1H, Hp, Aryl), 3.88 (s, 3H, N–Me),
2.05 (s, 6H, C–Me). 13C{H} NMR (CD2Cl2, 125 MHz, 25 °C) d 184.4
3.1.5. Preparation of [3-Me-1-{C(C6H5)N(4-MeC6H4)}C3H3N2]AgCl
(5d)
Silver oxide (0.660 g, 2.82 mmol) was added to a suspension of
the imidazolium salt 4d (1.102 g, 3.534 mmol) in dry dichloro-
methane (55 mL) containing activated 4 Å molecular sieves. The
mixture was stirred in the absence of light for 24 h at room tem-
perature. The solution was removed by filtration before the solvent
was removed under vacuum. The crude product was washed once
with a mixture of dichloromethane/pentane and stored in dichlo-
romethane for 1 day. The precipitate (presumably light-darkened
silver chloride) was removed before addition of ether. The precip-
itated product was removed from solvent and washed with a single
portion of pentane, yielding the product as a yellowish white solid
(0.55 g, 1.3 mmol, 37%).
(NCN), 152.5 (C@N), 144.6 (Cipso, Aryl), 131.6 (Cp, Ph), 130.9 (Cipso
,
Ph), 128.8 (Cm, Ph), 128.8 (NCHCHN near imine), 128.0 (Cm, Aryl),
126.4 (Co, Aryl), 123.9 (Cp, Aryl), 122.4 (Co, Ph), 120.8 (NCHCHN
near N–Me), 40.0 (N–Me), 18.3 (C–Me). MS-ESI: m/z 685
([2 M(107Ag)]+À107AgCl2 (C38H38N6107Ag), 100%). HRMS-ESI: Calc.
*
for C38H38N6107Ag ([2 M(107Ag)]+–107AgCl2), 685.2203. Found:
*
685.2185. Anal. Calc. for C19H19N3AgCl 0.4 CH2Cl2 : C, 50.11; H,
*
3.1.5.1. E-isomer (major). 1H NMR (CD2Cl2, 500 MHz) d 7.79 (m, 1H,
NCHCHN near imine), 7.5 (m, 1H, Hp, Ph), 7.4 (m, 2H, Hm, Ph), 7.2
(m, 2H, Ho, Ph), 7.15 (s, 1H, NCHCHN near N–Me), 7.0 (d,
3J = 8.0 Hz, 2H, Hm, Aryl), 6.6 (d, 3J = 8.0 Hz, 2H, Ho, Aryl), 3.85 (s,
3H, N–Me), 2.24 (s, 3H, C–Me). 13C{1H} NMR (CD2Cl2, 125 MHz,)
d 185 (NCN), 153.5 (C@N), 144.5 (Cipso, Aryl), 134.6 (Cpara, Aryl),
4.30; N, 9.04. Found: C, 49.93; H, 4.45; N, 9.20%.
3.1.3. Preparation of [3-Me-1-{C(C6H5)N(2,6-Me2C6H3)}C3H3N2]PdCl2
(1c)
A cooled solution (–70 °C) of the silver complex 5c (100 mg,
0.230 mmol) in dry dichloromethane (10 mL) was slowly trans-
ferred to a similarly cooled solution of (COD)PdCl2 (0.061 mg,
0.21 mmol) in dichloromethane (15 mL). The mixture was allowed
to slowly warm to ambient temperature before removal of the vola-
tiles under vacuum. The crude product was dissolved in acetonitrile
(15 mL) and filtered to remove precipitated silver chloride. The sol-
vent was removed in vacuo and the crude product was recrystallized
from a pentane/dichloromethane mixture (84 mg, 0.18 mmol, 78%).
1H NMR (CD3CN, 300 MHz, 25 °C) d 7.50–7.56 (m, 1H, Hp, Ph), 7.33–
7.46 (m, 4H, Ho and Hm, Ph), 7.09–7.10 (d, 3J = 2.3 Hz, 1H, NCHCHN
near N–Me), 7.01 (d, 3J = 2.3 Hz, 1H, NCHCHN near imine), 6.97–
7.01 (m, 1H, Hp, Aryl), 6.91–6.93 (m, 2H, Hm, Aryl), 4.26 (s, 3H, N–
Me), 2.31 (s, 6H, C–Me). 13C{H} NMR (CD3CN, 75 MHz, 25 °C) d
163.8 (C@N), 159.6 (NCN), 142.8 (Cipso, Aryl), 133.4 (Cp, Ph), 133.1
(Co, Aryl), 129.8 (Co, Ph), 129.1 (Cm, Ph), 128.4 (Cipso, Ph), 128.1 (Cm,
Aryl), 126.5 (Cp, Aryl), 125.8 (NCHCHN near N–Me), 120.0 (NCHCHN
near imine), 38.8 (N–Me), 19.3 (C–Me). HRMS-ESI: Calc. for
C19H19N3PdCl2Na ([M+Na]+), 486.9898. Found: 486.9884. Anal. Calc.
131.4 (Cpara, Ph), 130.7 (Cipso, Ph), 130.1 (Cortho, Ph), 129.7 (Cmeta
Aryl), 129.3 (Cmeta, Ph), 122.7 (NCHCHN near N–Me), 121.3 (Cortho
Aryl), 120.8 (NCHCHN near imine), 40.3 (N–Me), 20.9 (C–Me).
,
,
3.1.5.2. Z-isomer (minor). 1H NMR (CD2Cl2, 500 MHz) d 7.7 (m, 2H,
Ho, Ph), 7.6 (m, 1H, Hp, Ph), 7.5 (m, 2H, Hm, Ph), 7.07 (m, 1H,
NCHCHN near N–Me), 7.06 (m, 2H, Hm, Aryl), 6.9 (m, 1H,
NCHCHN near imine), 6.7 (m, 2H, Ho, Aryl), 3.81 (s, 3H, N–Me),
2.28 (s, 3 H, C–Me) 13C{1H} NMR (CD2Cl2, 125 MHz,) d 180
(NCN), 148.9 (C@N), 144.4 (Cipso, Aryl), 135.4 (Cpara, Aryl), 134.4
(Cipso, Ph), 132.9 (Cpara, Ph), 130.1 (Cmeta, Aryl), 129.6 (Cortho, Ph),
129.1 (Cmeta, Ph), 122.6 (NCHCHN near imine), 122.3 (NCHCHN
near N–Me), 120.9 (Cortho, Aryl), 39.5 (N–Me), 21.0 (C–Me). MS-
ESI: m/z 657 ([2 M(107Ag)]+À107AgCl2 (C36H34N6107Ag), 19%),
*
194
(100%).
HRMS-ESI:
Calc.
for
C36H34N6107Ag
([2 M(107Ag)]+À107AgCl2), 657.1895. Found: 657.1908. Anal.
*
Calc. for C18H17N3AgCl: C, 51.64; H, 4.09; N, 10.04. Found: C,
51.67; H, 4.18; N, 10.02%.
for C19H19N3PdCl2 0.5CH2Cl2 : C, 45.98; H, 3.96; N, 8.25. Found: C,
*
45.80; H, 4.00; N, 8.25%.
3.1.6. Preparation of [3-Me-1-{C(C6H5)N(4-MeC6H4)}C3H3N2]PdCl2
(1d)
3.1.4. Preparation of [3-Me-1-{C(C6H5)N(4-MeC6H4)}C3H3N2]+ClÀ
(4d)
(COD)PdCl2 (30.6 mg, 0.107 mmol) was added to a degassed
solution of the silver complex 5d (49.0 mg, 0.117 mmol) in
dichloromethane (10 mL), and the mixture was stirred at room
temperature for 20 h. After removal of the solvent in vacuo, the
crude product was washed with dichloromethane (2 Â 5 mL) and
treated with a 0.169 M solution of AgOTf in acetonitrile (0.6 mL,
0.1 mmol). The silver chloride precipitate was separated, the vola-
tiles removed in vacuo, and the residual solid dissolved in acetone
(4 mL). The clear yellow solution was filtered, and aqueous sodium
chloride (0.18 M, 1 mL) was added to precipitate the product as an
1-methylimidazole (0.57 mL, 7.2 mmol) was added to a solution
of iminoyl chloride 3d (1.623 g, 7.065 mmol) in dry THF (20 mL).
The mixture was stirred at ambient temperature for 36 h, during
which slow precipitation of the product as a white material oc-
curred. After removal of the solvent by filtration, the crude product
was washed with several portions of THF and dried under vacuum.
The precipitate was purified by crystallization from a mixture of
dichloromethane and diethylether to yield the product as white

3710
K.A. Netland et al. / Journal of Organometallic Chemistry 693 (2008) 3703–3710
off-white solid. The solid was washed with two portions of aceto-
nitrile (2 Â 4 mL) and dried under vacuum. Yield: 31.8 mg, 66%. 1H
NMR (CD3CN, 300 MHz, 25 °C) d 7.5–7.4 (m, 1H), 7.4–7.3 (m, 4H),
7.06 (d, J = 2.3 Hz, 1H), 7.0 (m, 2H), 6.9 (m, 3H), 4.21 (s, 3H, N–
Me), 2.21 (s, 3H, C–Me). MS-ESI: m/z 418 ([M(107Pd)]+ÀCl, 100%),
m/z 459 ([M(107Pd)]+ÀCl + CH3CN, 50%). HRMS-ESI: Calc. for
C18H17N3PdCl (M+ÀCl), 414.0151. Found: 414.0164. Anal. Calc. for
C18H17N3PdCl2: C, 47.76; H, 3.79; N, 9.28. Found: C, 47.80; H,
3.80; N, 9.25%.
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0.3°, crystal to detector distance 5 cm. Data reduction and cell
determination were carried out with the SAINT and XPREP software
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hydrogen atoms were refined with anisotropic thermal parame-
ters; the H atoms were all located in a difference map, but those
attached to carbon atoms were repositioned geometrically. The H
atoms were initially refined with soft restraints on the bond
lengths and angles to regularize their geometry (C–H in the range
0.93–0.98 Å) and isotropic adps (U(H) in the range 1.2–1.5 Â Uequiv.
of the adjacent atom), after which they were refined with riding
constraints. Table 1 lists the experimental and crystallographic
data. Crystallographic data for all three compounds may be ob-
tained as CIF files (see Appendix A).
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We gratefully acknowledge support from the Norwegian Re-
search Council (stipend to A.K.).
Appendix A. Supplementary material
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CCDC 692557–692559 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2008.09.006.