Practical one-pot, three-component synthesis of N-heterocyclic carbene (NHC) ligated palladacycles derived from n,n-dimethylbenzylamine

Article abstract of DOI:10.1021/om8008475

Further explorations of the catalytic potential of N-heterocyclic carbene (NHC) ligated palladacycles as catalysts for Pd-mediated transformations have been hampered by the lack of general and practical methods for their synthesis. In this work, we descri

Full text of DOI:10.1021/om8008475

Organometallics 2009, 28, 289–299
289
Practical One-Pot, Three-Component Synthesis of N-Heterocyclic
Carbene (NHC) Ligated Palladacycles Derived from
N,N-Dimethylbenzylamine
Eric Assen B. Kantchev and Jackie Y. Ying*
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669
ReceiVed September 2, 2008
Further explorations of the catalytic potential of N-heterocyclic carbene (NHC) ligated palladacycles
as catalysts for Pd-mediated transformations have been hampered by the lack of general and practical
methods for their synthesis. In this work, we describe a novel, practical approach to NHC-ligated
palladacycles by a three-component, one-pot reaction of imidazolium salts, PdCl2, and N,N-dimethyl-
benzylamine in the presence of excess K2CO3 under reflux in reagent-grade acetonitrile in air.
1,3-Diarylimidazolium salts afford the corresponding NHC-Pd(dmba)Cl (dmba ) N,N-dimethylbenzy-
lamine-κ2N,C) complexes in >80% yield. The conversion of 1,3-diaryl-4,5-dihydroimidazolium and 1,3-
dialkylimidazolium or benzimidazolium salts requires the use of stronger base (Cs2CO3) and/or higher
temperature (100 °C). The Pd-bound chloride anion can be exchanged with silver salts or sodium salts.
The NHC-palladacycle adducts have been characterized by single-crystal X-ray crystallography.
catalyst generation. Such precatalysts usually require additional,
sacrificial ligands for the stabilization of the active species, a
monoligated NHC-Pd complex. In the past few years, IPr-Pd(0)
or IPr-Pd(II) derivatives with p-quinone,10 π-allyl,11 carboxy-
late,12 3-chloropyridine,9,13 and acetylacetonate (acac)14 stabiliz-
ing ligands have been shown to exhibit high activity in a number
Introduction
N-heterocyclic carbenes (NHCs)1-3 have recently emerged
as high-performance ligands or organocatalysts in a variety of
organic reactions,4 including Pd-mediated cross-couplings.5 In
particular, monoligated Pd complexes of bulky N,N-diaryl-
substituted carbenes6 (1-4, Figure 1) have shown high levels
of activity and wide applicability. Usually, NHCs 1-4 are
prepared by deprotonation of imidazolium or 4,5-dihydroimi-
dazolium salts with strong bases1,3 and then captured by suitable
Pd precursors. As many Pd-catalyzed cross-coupling reactions
involve either basic organometallic reagent or external base,
active catalysts can be prepared directly in the reaction mixture
starting from common Pd precursors and imidazolium salts,
usually in a 1:1 or 1:2 molar ratio.7,8 However, since these
conditions are not optimized for the complexation of NHC to
Pd, often the yield of the active catalyst is low and its chemical
composition is hard to control.8,9 Therefore, the preparation of
well-defined NHC-Pd precatalysts that can be easily activated
when introduced into the reaction in high yields under optimized
conditions represents a significant improvement over the in situ
(7) (a) Burstein, C.; Lehmann, C. W.; Glorius, F. Tetrahedron 2005,
61, 6207–6217. (b) Lerma, I. S.; Cawley, M. J.; Cloke, F. G. N.; Arentsen,
K.; Scott, J. S.; Pearson, S. E.; Hayler, J.; Caddick, S. J. Organomet. Chem.
2005, 690, 5841–5848. (c) Hadei, N.; Kantchev, E. A. B.; O’Brien, C. J.;
Organ, M. G. Org. Lett. 2005, 7, 3805–3807. (d) Song, C.; Ma, Y.; Chai,
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Lehmann, C. W.; Glorius, F. Angew. Chem., Int. Ed. 2003, 42, 3690–3693.
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1445. (k) Sato, Y.; Yoshino, T.; Mori, M. Org. Lett. 2003, 5, 31–33. (l)
Yang, C.; Nolan, S. P. Organometallics 2002, 21, 1020–1022. (m) Yang,
C.; Nolan, S. P. J. Org. Chem. 2002, 67, 597–593. (n) Grasa, G. A.; Viciu,
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2002, 21, 2866–2873. (o) Yang, C.; Nolan, S. P. Synlett 2001, 1539–1542.
(p) Grasa, G. A.; Viciu, M. S.; Huang, J.; Nolan, S. P. J. Org. Chem. 2001,
66, 7729–7737. (q) Lee, S.; Hartwig, J. F. J. Org. Chem. 2001, 66, 3402–
3415. (r) Shauffer, S. R.; Lee, S.-W.; Stambuli, J. P.; Hauck, S. I.; Hartwig,
J. F. Org. Lett. 2000, 2, 1423–1426. (s) Cheng, J.; Trudell, M. L. Org.
Lett. 2001, 3, 1371–1374. (t) Zhang, C.; Trudell, M. L. Tetrahedron Lett.
2000, 41, 595–598. (u) Huang, J.; Grasa, G.; Nolan, S. P. Org. Lett. 1999,
1, 1307–1309. (v) Huang, J.; Nolan, S. P. J. Am. Chem. Soc. 1999, 121,
9889–9890.
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Chem. 2005, 70, 8503–8507.
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O’Brien, C. J.; Valente, C. Chem. Eur. J. 2006, 12, 4749–4755.
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Nolan, S. P. J. Am. Chem. Soc. 2006, 128, 4101–4111. (b) Viciu, M. S.;
Navarro, O.; Germaneau, R. F.; Kelly, R. A., III; Sommer, W.; Marion,
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* To whom correspondence should be addressed. E-mail: jyying@ibn.a-
star.edu.sg.
(1) General reviews on NHC’s and their coordination to metals: (a) Hahn,
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Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290–1309.
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F. P.; Bertrand, G. Chem. ReV. 2000, 100, 39–91. (c) Herrmann, W. A.;
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(4) For comprehensive treatment of the application of NHC-based
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10.1021/om8008475 CCC: $40.75
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Publication on Web 12/16/2008

290 Organometallics, Vol. 28, No. 1, 2009
KantcheV and Ying
of Pd-mediated processes. Some of these complexes are now
commercially available. Palladacycles15 are also suitable for the
stabilizing ligand role. Under the reaction conditions, pallada-
cycles behave as well-defined precatalysts16 that release catalyti-
cally active, ligandless Pd species in a controlled manner,
depending on the temperature, solvent, coreactants, and nature
of the cyclopalladated ligand.17 Ligation of dimeric palladacycles
with a single spectator ligand (e.g., a bulky phosphane) enhances
the usefulness of the corresponding precatalysts because a more
active monoligated phosphane-Pd complex18 is formed when
the precatalyst is activated. For instance, the palladacycle derived
from N,N-dimethylbenzylamine (5; dmba)19-22sa commercially
available, inexpensive palladacycle precursorsand its phosphane
adducts19,23 have been shown to be highly active precatalysts
for Suzuki-Miyaura, Buchwald-Hartwig, and Heck-Mizoroki
reactions, even with such challenging substrates as nonactivated
aryl chlorides. NHC-ligated palladacycles have been somewhat
less explored. Nolan et al. showed that N,C-palladacycles
derived from N,N-dimethyl-2-aminobiphenyl ligated with IPr
or IMes efficiently mediated Suzuki-Miyaura, Buchwald-
Hartwig, enolate arylation, and haloarene dehalogenation reac-
tions even at room temperature.24 The precatalysts were prepared
by simple substitution of a bridging chloride in the palladacyclic
dimer with the isolated carbenes 1 and 2. In a similar fashion,
a catalyst was prepared very recently by the reaction of the free
Figure 1. Highly active, versatile NHC ligand for catalytic
applications.
IPr carbene and cyclopalladated ferrocenylimine25 donor ligands.
The resulting NHC-palladacycles were excellent catalysts for
Buchwald-Hartwig amination and Suzuki-Miyaura coupling.
Iyer et al. explored N,C-palladacycles derived from N,N-
dimethylbenzylamine, acetophenone, and benzophenone oximes
with simple 1,3-diphenyl-4,5-dihydroimidazolyl-2-ylidene as
catalysts for Heck-Mizoroki and Suzuki-Miyaura couplings
of activated substrates at low catalyst loadings.26 Remarkably,
these complexes were first prepared by Hiraki et al. as early as
1978 via thermal decomposition of the corresponding saturated
carbene dimer in situ.27 As that was almost two decades before
the potential of NHCs as spectator ligands in Pd-catalyzed
organic transformations was recognized in the seminal paper
by Herrmann et al.,28 no catalytic studies were reported at that
time. Herrmann et al. have prepared a number of NHC-ligated
N,C- and P,C-palladacycles in a similar manner.29 Among them,
excellent precatalysts for Heck-Mizoroki reaction were found.
Very recently, the same group disclosed a more practical
procedure for NHC-palladacycle preparation relying on the in
situ generation of the carbene in the presence of purified,
preformed palladacycles.30 Whereas NaOAc was a suitable base
for the unsaturated NHCs, cyclic diaminocarbene precursors still
demanded the use of tBuOK. Bedford et al. prepared phosphite-
derived P,C-palladacycles ligated with SIPr (3) and SIMes (4)
via µ-chloride displacement with the saturated carbene prepared
in situ from the corresponding chloroform adduct.31 These
precatalysts showed moderate activity in the Suzuki couplings
of aryl bromides. However, aryl chlorides and alkyl bromides32
failed to couple. With the above in mind, we reasoned that
hitherto unknown complexes of [Pd(dmba)Cl]2 (6) with carbenes
1-4 have the potential to be highly active and versatile cross-
coupling catalysts. However, all the methods discussed above
suffer from practical drawbacks, such as handling of isolated
moisture- and air-sensitive carbenes and the strong bases
required for NHC preparation (for instance, in a glovebox),
narrow scope, and/or medium-to-low yields. Thus, the first step
(12) (a) Singh, R.; Viciu, M. S.; Kramareva, N.; Navarro, O.; Nolan,
S. P. Org. Lett. 2005, 7, 1829–1832. (b) Singh, R.; Nolan, S. P. J.
Organomet. Chem. 2005, 690, 5832–5840. (c) Viciu, M. S.; Stevens, E. D.;
Petersen, J. L.; Nolan, S. P. Organometallics 2004, 23, 3752–3755. (d)
Jensen, D. R.; Schultz, M. J.; Mueller, J. A.; Sigman, M. S. Angew. Chem.,
Int. Ed. 2003, 42, 3810–3813.
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12, 4743–4748.
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Tetrahedron 2005, 61, 9716–9722.
(15) General overview on palladacycles: Dupont, J.; Consorti, C. S.;
Spencer, J. Chem. ReV. 2005, 105, 2527–2571.
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I. P.; Cheprakov, A. V. J. Organomet. Chem. 2004, 689, 4055–4082. (b)
Bedford, R. B. Chem. Commun. 2003, 1787–1796. (c) Dupont, J.; Pfeffer,
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P. N.; Hursthouse, M. B.; Light, M. E. Organometallics 2003, 22, 987–
999.
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2003, 5, 1479–1482.

N-Heterocyclic Carbene Ligated Palladacycles
Organometallics, Vol. 28, No. 1, 2009 291
Scheme 1. One-Pot, Three-Component Synthesis of NHC-Pd(dmba)Cl from Imidazolium Salts
Table 1. Yields of NHC-Ligated Palladacycles from Scheme 1
Table 2. Optimization of the Conditions for Synthesis of the
SIPr-Pd(dmba)Cl Complex (18)
entry
NHC · HCl
none
R1
R2
R3
yield, %
1
2
3
4
5
6
98 (6)
82 (12)
81 (13)
96 (14)
81 (15)
86 (16)
IPr · HCl (7)
iPr
Et
Me
tBu
iPr
iPr
Et
Me
H
H
H
Me
H
H
IEt · HCl (8)
IMes · HCl (9)
ITbp · HCl (10)
IPrp · HCl (11)
H
toward successful catalysts33 would be to develop a practical
and general synthetic method for preparing a range of
NHC-palladacycle adducts.
yield, %
Results and Discussion
entry
base (amt, equiv)
additive
18
19
1
2
3
K2CO3 (5.0)
K2CO3 (5.0)
Cs2CO3 (2.5)
none
4 Å MS
4 Å MS
34
74
96
26
7
<2
Synthesis of NHC-Palladacycles from 1,3-Diarylimidazo-
lium Salts and Their 4,5-Dihydro Analogues. We reasoned that
the addition of an equimolar amount of N,N-dimethylbenz-
ylamine (5) to PdCl2 would lead to formation of a 1:1 adduct,
which could be converted to the corresponding dimeric palla-
dacycle 6 upon addition of a weak base. Moreover, if the same
base could also be used to generate NHC in situ from
imidazolium salts, then the desired NHC-ligated palladacycles
would be formed directly, without any intermediate isolation.
Our initial attempts to carry out the palladacycle formation using
PdCl2/K2CO3 or Pd(OAc)2 in a number of solvents (e.g., CH2Cl2,
toluene, THF, dioxane, and neat 5) resulted in rapid decomposi-
tion of the reactants and formation of Pd black. However,
heating of PdCl2 and the amine 5 (1.00 and 1.05 equiv,
respectively) in CH3CN led to the formation of a dark orange,
clear solution, which turned canary yellow within 5 min upon
the addition of 5 equiv of K2CO3 at that temperature (Scheme
1). The known palladacyclic dimer 620 was isolated in 98% yield
(Table 1, entry 1). Encouraged by these results, we added the
commercially available IPr · HCl (7; 1.10 equiv) to the crude
palladacyclic solution at 80 °C. After the mixture was stirred
for 18 h, complex 12 was isolated in 82% yield (Table 1). The
reaction also proceeded well with other imidazolium salts. The
known IEt · HCl (8)8,34 and the commercially available IMes · HCl
(9) led to the corresponding NHC-ligated palladacycles 13 and
14 (81% and 96% isolated yields, respectively). Hitherto
unknown, less sterically hindered ligands carrying only one ortho
substituent on the N-bound aryl group, tBu (ITbp · HCl, 10) or
iPr (IPrp · HCl, 11), also gave the corresponding complexes in
Table 3. Synthesis of NHC-Pd(dmba)X (X ) Cl, Br) Complexes
from 1,3-Diaryl-4,5-dihydroimidazolium Salts
entry
PdX2
NHC · HBF4
R1
R2
R3
yield, %a
1
2
3
4
PdCl2
PdBr2
PdCl2
PdCl2
SIMes · HBF4 (20)
SIMes · HBF4 (20)
SITbp · HBF4 (21)
SIPrp · HBF4 (22)
Me
Me
tBu
iPr
Me
Me
H
Me
Me
H
98 (23)
67 (24)
90 (25)
80 (26)
H
H
a The reactions were conducted in the presence of 4 Å molecular
sieves.
similarly high yields (81% for 15 and 86% for 16). We further
extended the scope of the reaction to the analogous saturated
carbenes. When the commercially available SIPr · HCl (17) was
used under the standard conditions, the corresponding SIPr-Pd-
(dmba)Cl complex (18) was obtained in moderate yield, which
was contaminated with significant amounts of unreacted carbene
precursor and N,N′-bis(2,6-diisopropylphenyl)-N-formylethyl-
enediamine (19), a product of base-promoted hydrolytic ring
opening of the carbene precursor (Table 2). This result can be
explained by taking the lower acidity of the 4,5-dihydroimida-
zolium salts (compared to that of the unsaturated analogues)
and their susceptibility to hydrolysis into account. The addition
of powdered 4 Å molecular sieves at the beginning of the
reaction suppressed the undesirable 4,5-dihydroimidazolium salt
hydrolysis without introducing major operational hurdles into
the process. The use of a stronger base (Cs2CO3) yielded the
targeted product 18 in almost quantitative yield (96%). Under
these conditions, the commercially available 1,3-bis(2,4,6-
trimethylphenyl)-4,5-dihydroimidazolium tetrafluoroborate (20;
SIMes · HBF4) produced the corresponding NHC-ligated palla-
dacycle 23 in similarly high yield (Table 3, entry 1) without
(33) Kantchev, E. A. B.; Peh, G.-R.; Zhang, C.; Ying, J. Y. Org. Lett.
2008, 10, 3949–3952.
(34) Sprengers, J. W.; Wassenaar, J.; Clement, N. D.; Cavell, K. J.;
Elsevier, C. J. Angew. Chem., Int. Ed. 2005, 44, 2026–2029.
(35) While this work was in progress, precursors to ligands 2135a and
2235b,c and/or metal complexes thereof were prepared independently: (a)
Kuhn, K. M.; Grubbs, R. H. Org. Lett. 2008, 10, 2075–2077. (b) Stylianides,
N.; Danopoulos, A. A.; Pugh, D.; Hancock, F.; Zanotti-Gerosa, A.
Organometallics 2007, 26, 5627–5635. (c) Stewart, I. C.; Ung, T.; Pletnev,
A. A.; Berlin, J. M.; Grubbs, R. H.; Schrodi, Y. Org. Lett. 2007, 9, 1589–
1592.
(36) For a recent report of similar atropisomers caused by bulky N-aryl
substituents in 4,5-dihydroimidazol-2-ylidenes, see: Luan, X.-J.; Mariz, R.;
Gatti, M.; Costabile, C.; Poater, A.; Cavallo, L.; Dorta, R. J. Am. Chem.
Soc. 2008, 130, 6848–6858.

292 Organometallics, Vol. 28, No. 1, 2009
KantcheV and Ying
Scheme 2. Synthesis of Novel 1,3-Bis(o-alkylaryl)-Subs-
Scheme 3. Anion-Exchange Reactions of in Situ Prepared
Palladacyclesa
tituted NHC Ligand Precursorsa
a
Conditions: (a) MeOH-H2O (10:1), room temperature, 1.5-18 h;
(b) THF, 40 °C, 18 h; (c) MeOH-THF (1:1), reflux, 30 min; (d)
concentrated HCl (1.2 equiv), 120 °C, 1 h; (e) H2O, room temperature,
15 min.
a
Conditions: CH3CN, 80°C, 30-45 min.
any formation of the corresponding hydrolysis product. The use
of PdBr2 instead of PdCl2 resulted in the preparation of an
analogous complex containing bromide as the counterion at a
somewhat lower yield (24, 67%) (Table 3, entry 2). Less
sterically hindered analogues (21, 22)35 also formed pallada-
cyclic complexes in excellent yields (90% for 25 and 80% for
26) (Table 3, entries 3 and 4) under these conditions. Surpris-
ingly, compound 25 was obtained as a mixture of two unsepa-
rable isomers in a molar ratio of 2.6:1 (1H NMR spectroscopy).
The X-ray single-crystal structure of complex 25 (Figure 2e)
showed the expected trans configuration between the carbene
carbon and the dimethylamino ligand. Most likely, the two
isomers are atropisomers that only exist in solution and arise
from restricted rotation around one or more bonds in the
saturated NHC ligand carrying N-2-tert-butylphenyl substitu-
ents.36
Table 4. Anion Exchange of IPr-Pd(dmba)Cl (12) with Silver Salts
entry
AgX
solvent
yield, %
1
2
3
4
AgOAc
AgTFAa
AgOTfb
AgPF6
CH2Cl2-acetone (4:1)
CH2Cl2-acetone (4:1)
CH2Cl2-acetone (4:1)
CH2Cl2-CH3CN (5:1)
96 (31)
99 (32)
96 (33)
97 (34)c
a TFA
)
trifluoroacetate. b Tf
)
trifluoromethylsulfonyl. c The
compound was obtained as the Pd-bound acetonitrile adduct (Figure 2).
Table 5. One-Pot, Four-Component, Sequential NHC-Palladacycle
Synthesis and Anion Exchange
The novel carbene precursors used in this work were prepared
from the corresponding N,N-diarylazabutadienes (29 and 30)
(Scheme 2) by either treatment with chloromethyl ethyl ether
to yield the unsaturated imidazolium salts ITbp · HCl (10, 35%)
and IPrp · HCl (11, 62%) or reduction with NaBH4 followed by
cyclization with triethyl orthoformate/HCl to form the corre-
sponding 4,5-dihydro analogues.6,8 For easier purification, the
saturated carbene precursors were isolated as tetrafluoroborates
(SITbp · HBF4, 21, 88%; SIPrp · HBF4, 22, 94%) after simple
precipitation with NH4BF4 from aqueous solution.
Anion Exchange Reactions of NHC-Palladacycle
Chlorides. The nature of the Pd-bound anion could potentially
affect the ease of activation of the palladacyclic precatalysts.
Therefore, we endeavored to develop methods for preparing
NHC-palladacycles carrying anions other than chloride or
bromide. Anion exchange between 12 and various Ag salts
resulted in the formation of the corresponding carboxylate,
sulfonate, or hexafluorophosphate analogues 31-34 (Table 4)
in high yields. As hexafluorophosphate is a noncoordinating
anion, the product was isolated as the acetonitrile adduct (1:1).
This anion-exchange reaction is driven by the formation of
insoluble AgCl. Hence, we reasoned that the limited solubility
of NaCl or KCl in acetonitrile could allow similar reactions to
take place with the less expensive Na or K salts, allowing for
the palladacycle formation, NHC ligation, and anion exchange
to be combined in a single synthetic operation without isolation
of any intermediates. Treatment of amine 5 with PdCl2 in
refluxing acetonitrile using NaOAc instead of K2CO3 resulted
in a clean formation (92% isolated yield) (Scheme 3) of the
known22 acetate-bridged dimeric palladacycle 35. Similarly, the
addition of acetylacetone (acac-H) to the crude palladacycle
entry
X
R1
R2
yield, %
96 (37, X ) I)
61 (12, X ) Cl) + 13 (31, X ) OAc)
46 (12, X ) Cl) + 43 (32, X ) TFA)
94 (33, X ) OTf)
1
2
3
4
I
tBu
iPr
iPr
iPr
H
OAc
TFA
OTf
iPr
iPr
iPr
chloride 6 (using 5 equiv of K2CO3 as the base) led to the
formation of the known37 complex [Pd(dmba)(acac)] (36) in
92% isolated yield. Next, we integrated the in situ NHC-
palladacycle preparation protocol and anion exchange by simply
adding the corresponding Na salt (3 equiv) and heating for an
additional 30 min before workup (Table 5). Whereas the use of
NaI gave complete conversion to the corresponding NHC-ligated
Pd-I palladacycle 37, the use of sodium carboxylates resulted
in incomplete anion exchange. Sodium acetate yielded 61% of
the chloride-ligated complex 12 and its acetate analogue 31
(13%) and sodium trifluoroacetate yielded 46% of 12 and 43%
of the trifluoroacetate analogue 32. In contrast, complete ion
exchange was observed with sodium trifluoromethanesulfonate,
and complex 33 was obtained in 94% yield.
(37) Reichert, B. E.; West, B. O. J. Organomet. Chem. 1974, 71, 291–
297.

N-Heterocyclic Carbene Ligated Palladacycles
Organometallics, Vol. 28, No. 1, 2009 293
Figure 2. ORTEP representations of the X-ray single-crystal structures of NHC-palladacycles: (a) 12; (b) 15; (c) 16; (d) 18; (e) 25; (f) 31
(the unit cell contains two molecules of 31 and one molecule of CH2Cl2; only one molecule of 31 is shown for clarity); (g) 33; (h) 42; (i)
45. The thermal ellipsoids were drawn at the 30% probability level, and the hydrogen atoms were omitted for clarity.
Synthesis of NHC-Palladacycles from 1,3-Dialkylimidazo-
lium and 1,3-Dialkylbenzimidazolium Salts. Our initial experi-
ments with 1,3-dibenzylimidazolium hexafluorophosphate38 (38)
under the conditions of Scheme 1 resulted in only 4% of the
expected product 42 (Table 6). The use of Cs2CO3 at 80 and
100 °C resulted in 22% and 31% of 42, respectively. The
reaction mixture at the end point appeared dark, with a
significant amount of Pd black present. Shortening the reaction
time to 30 min led to an improvement in product yield to 78%.
In an analogous fashion, commercially available 1,3-dimeth-
ylimidazolium chloride (39) yielded 67% and 45% of the
corresponding complex 43 at 30 min and 18 h, respectively.
However, the commercially available isopropyl analogue 40
yielded about the same amount of product 44 irrespective of
the time (65% after 18 h; 62% after 30 min). Presumably, the
bulkier carbene results in a more stable palladacycle complex.
1,3-Diisopropylbenzimidazolium hexafluorophosphate39 (41)
also reacted in good yield, providing the first example of a
benzimidazol-2-ylidene-ligated palladacycle (45).
The actual mechanism of the ligation of palladacycles with
in situ generated NHCs is not clear at this point. Since N
substituents have little effect on the electronic nature and
stability of the unsaturated imidazol-2-ylidenes,1 the relative
difficulty with which alkyl-substituted carbenes react with in
situ formed [Pd(dmba)Cl]2 most likely rules out the involvement
of the free carbene, despite recent reports of NHC generation
under similar conditions.40 The tolerance of the protocol
presented herein to air and moisture also corroborates such a
conclusion. It is plausible that the palladacycle forms a transient
π complex with the imidazolium salts, causing the acidity of
the latter to increase and facilitating the complex formation.
(39) Prepared by anion exchange with NH4PF6 from the known
bromide: Huynh, H. V.; Han, Y.; Ho, J. H. H.; Tan, G. K. Organometallics
2006, 25, 3267–3274.
(40) Bostai, B.; Nova´k, Z.; Be´nyei, A. C.; Kotschy, A. Org. Lett. 2007,
9, 3437–3439.
(38) Prepared by anion exchange with NH4PF6 from the known
chloride: Harlow, K. J.; Hill, A. F.; Welton, T. Synthesis 1996, 697–698.

294 Organometallics, Vol. 28, No. 1, 2009
KantcheV and Ying
Table 6. Synthesis of NHC-Pd(dmba)Cl Complexes from
angle (X ) Cl, O, N), C1-Pd-C8 angle, and N3-Pd-X angle
(X ) Cl, O, N) are 89.37-95.56, 90.86-94.61, and 90.13-
95.37°, respectively. The imidazole and palladacycle rings are
closer to perpendicular than to coplanar: the absolute values
for the N1-C1-Pd-C8 dihedral angle vary between 60.46 and
117.65°. The five-membered palladacycle moiety is puckered
(with the C8-C9-C10-N3 dihedral angle in the range of
25.67-36.20°) due to the presence of sp3 carbon and nitrogen
atoms (the N3-C10-C9 angle ranges from 107.89 to 109.53°,
typical for tetrahedral carbons).
1,3-Dialkylimidazolium and 1,3-Dialkylbenzimidazolium Salts
entry
conditionsa
NHCb
R
X
yield, %
1
2
3
4
5
6
7
8
9
K2CO3, 80 °C, 18 hc
80 °C, 18 h
100 °C, 18 h
100 °C, 30 min
100 °C, 18 h
100 °C, 30 min
100 °C, 18 h
100 °C, 30 min
80 °C, 18 h
100 °C, 30 min
Im
Im
Im
Im
Im
Im
Im
Im
Bn (38)
Bn (38)
Bn (38)
Bn (38)
Me (39)
Me (39)
iPr (40)
iPr (40)
iPr (41)
iPr (41)
PF6
PF6
PF6
PF6
Cl
Cl
Cl
Cl
PF6
PF6
4 (42)
22 (42)
31 (42)
78 (42)
45 (43)
67 (43)
65 (44)
62 (44)
68 (45)d
83 (45)d
Conclusions
We have developed a novel, practical synthesis of NHC-
ligated palladacycles by one-pot, three-component, sequential
reaction of N,N-dimethylbenzylamine, PdCl2 or PdBr2, and a
wide range of imidazolium salts and their 4,5-dihydro- or benzo-
fused counterparts in refluxing acetonitrile in the presence of
mild bases (K2CO3 and Cs2CO3, respectively) in air. Whereas
the complexation of the in situ prepared 1,3-diarylimidazol-2-
ylidenes is facile, the saturated analogues require a stronger base
and the addition of 4 Å molecular sieves to suppress the
competing carbene precursor hydrolysis. 1,3-Dialkylimidazolium
and 1,3-dialkylbenzimidazolium salts generally require more
forcing conditions and strict control of the reaction time for
satisfactory complexation. All complexes show a characteristic
distorted-square-planar Pd(II) center with the carbene and
dimethylamino ligands mutually trans and the planes of the
imidazole ring and the heterocycle close to perpendicular. The
extension of this methodology to other classes of cyclometalated
ligands and the use of the resulting complexes in catalysis are
underway.
BzIm
BzIm
10
a The palladacycle was always prepared at 80 °C. b Im
)
imidazolyl-2-ylidene; BzIm ) benzimidazolyl-2-ylidene. c 5 equiv of
K2CO3 was used. d The reactions were conducted in the presence of 4 Å
molecular sieves.
Perhaps this complexation is more pronounced for 1,3-dia-
rylimidazolium salts: hence, the relative ease of complexation
when such carbene precursors are employed. Alternative
mechanistic explanations also cannot be discounted at this point.
Molecular Structure of NHC-Palladacycles. The NHC-
Pd(dmba)Cl complexes were characterized by X-ray crystal-
lography (Figure 2). Selected bond lengths and angles and
dihedral angles are given in Table 7, and the single-crystal data
and X-ray collection parameters are given in Table 8. In the
crystalline state, the complexes all show the characteristic
slightly distorted square-planar Pd(II) center with the NHC and
dimethylamino ligands mutually trans. The carbene ligand’s
five-membered-ring topology varies slightly for different car-
benes. The NHC ligand retains the bond angles characteristic
for a singlet carbene (∼104° for the imidazol-2-ylidenes and
∼107° for benzimidazol-2-ylidene and 4,5-dihydroimidazol-2-
ylidene). The C1-N1 bond is 1.346-1.362 Å, regardless of
the carbene type, whereas the N1-C4 bond is 1.384-1.394 Å
forimidazol-2-ylideneandbenzimidazol-2-ylideneand1.485-1.490
Å for the backbone saturated analogues. Likewise, the C4-C5
bond length lies in the range of 1.327-1.336 Å for imidazol-
2-ylidene, 1.394 Å for benzimidazol-2-ylidene, and 1.502-1.524
Å for the saturated carbenes, in accordance with changing bond
types from double to single. This is accompanied by puckering
of the carbene ringsthe N1-C4-C5-N2 dihedral angle is close
to 0 (-1.09 to +1.20°) for the unsaturated carbenes (sp2) and
+16.71 to +19.05° for the saturated analogues (sp3). The two
N-aryl rings are in all cases rotated out of conjugation, and the
dihedral angles C1-N1/2-C2/6-C3/7 show large variation
from carbene to carbene (58.83-128.66°). There is no apparent
correlation between compound structure and dihedral angle value
and sign for these low-barrier torsions. In all complexes, the
Pd-C1 (1.966-1.996 Å) and Pd-C8 bonds (1.985-2.004 Å)
are within the expected range for a single Pd-C bond. The
Pd-N3 and Pd-Cl bond lengths are within the ranges of
2.128-2.142 and 2.379-2.424 Å, respectively. The substitution
of the chloride for acetate or acetonitrile leads to shorter C-O
and C-N bonds (2.112 and 2.118 Å, respectively), without
significantly affecting the rest of the structure. The Pd center
exhibits a slight distortion from the perfect square-planar
configuration as a result from its being a part of the five-
membered palladacycle ring. The ranges for the C1-Pd-X
Experimental Section
General Considerations. All reagents and solvents were pur-
chased from commercial sources and were used without further
purification, unless otherwise indicated. Deuterated solvents were
purchased from Sigma-Aldrich. TLC plates were purchased from
VWR. 1H and 13C NMR data were acquired at 25 °C with a Bruker
AV 400 spectrometer. Flash chromatography was performed in a
CombiFlash Sq16 on normal phase silica gel cartridges with
hexane-ethyl acetate gradients.
General Procedure for the Synthesis of NHC-Pd(dmba)Cl
Complexes from 1,3-Diarylimidazolium Salts (Table 1). A pressure
tube was charged with PdCl2 (177 mg, 1.00 mmol) and a stir bar,
and then acetonitrile (5 mL, HPLC grade) and N,N-dimethylben-
zylamine (160 µL, 178 mg, 1.05 mmol) were added. The mixture
was stirred at 80 °C until PdCl2 dissolved completely, forming a
dark orange, clear solution (in ∼20-25 min). Finely powdered
K2CO3 (691 mg, 5.00 mmol) was added, and the mixture was heated
at 80 °C until the palladacycle formation was complete, as indicated
by the change in the color of the mixture to canary yellow (in ∼5
min). Immediately, NHC · HCl (1.10 mmol) was added, and the
mixture was heated at 80 °C with vigorous stirring over 18 h. The
mixture was filtered on a pad of Celite (CH2Cl2), the volatiles were
removed, and the residue was purified by flash chromatography
(ethyl acetate gradient in CH2Cl2: 0% over 5 min, 0-30% over 15
min).
IPr-Pd(dmba)Cl (12). From IPr · HCl (7; 467 mg), 12 (543
1
mg, 82%) was obtained as an off-white solid. H NMR (CDCl3,
400 MHz): δ 7.40 (t, J ) 7.6 Hz, 2H), 7.30 (d, J ) 7.6 Hz, 2H),
7.21 (s, 2H), 6.82-6.70 (m, 3H), 6.53 (d, J ) 7.6 Hz, 1H), 3.46
(s, 2H), 3.37 (m, 2H), 3.15 (m, 2H), 2.39 (s, 6H), 1.49 (d, J ) 6.8
Hz, 6H), 1.18 (d, J ) 6.8 Hz, 6H), 1.02 (d, J ) 6.8 Hz, 6H), 0.81
(d, J ) 6.4 Hz, 6H). 13C NMR (CDCl3, 100 MHz): δ 177.5, 150.5,

N-Heterocyclic Carbene Ligated Palladacycles
Organometallics, Vol. 28, No. 1, 2009 295
Table 7. Selected Bond Lengths, Bond Angles, and Dihedral Angles for NHC-Pd(dmba)X Complexes: X ) Cl for 12, 15, 16, 18, 25, 42, and
45; X ) O for 31, and X ) N for 33
param
C1-N1
12
15
16
1.362
1.402
1.336
1.966
1.997
2.415
2.128
18
25
31
33
1.349
1.384
1.327
1.992
1.985
2.118
2.131
42
45
1.350
1.394
1.392
1.973
1.991
2.393
2.137
Bond Lengths, Å
1.353
1.360
1.395
1.330
1.996
2.004
2.379
2.133
1.362
1.394
1.335
1.988
1.989
2.408
2.133
1.345
1.490
1.524
1.972
1.994
2.424
2.140
1.361
1.387
1.327
1.974
1.985
2.112
2.139
1.346
1.384
1.332
1.978
2.006
2.406
2.138
N1-C4
C4-C5
C1-Pd
C8-Pd
Pd-X
1.485
1.502
1.976
1.999
2.402
2.142
Pd-N3
Bond Angles, deg
N1-C1-N2
C1-Pd-X
C1-Pd-C8
N3-Pd-X
N3-C10-C9
104.94
91.56
94.61
91.40
108.61
103.99
95.88
91.13
95.36
109.53
104.34
90.48
92.15
95.36
109.53
107.76
97.46
91.64
90.13
108.24
107.71
95.53
90.86
91.83
109.39
103.88
93.01
91.47
93.77
108.31
104.57
95.96
90.93
92.05
107.89
104.59
89.37
94.16
94.73
109.04
107.16
89.75
92.74
95.57
108.79
Dihedral Angles, deg
N1-C4-C5-N2
C1-N1-C2-C3
C1-N2-C6--C7
N1-C1-Pd-C8
C8-C9-C10-N3
1.05
120.67
-78.34
-69.37
33.65
0.09
90.51
-73.84
-74.95
28.55
0.32
72.74
-117.70
-96.36
25.67
19.05
128.66
-88.42
117.65
36.20
16.71
-0.81
62.30
86.15
64.63
-34.59
-1.09
58.83
-0.58
1.20
106.71
-113.93
78.46
104.89
-59.44
-68.29
31.44
-100.19
60.46
70.62
-29.81
-36.51
-29.60
147.8, 147.8, 144.7, 136.2, 136.1, 129.7, 125.4, 124.6, 124.0, 123.8,
122.6, 121.5, 72.6, 49.8, 29.0, 28.3, 26.4, 26.2, 23.2, 23.2. Anal.
Calcd for C36H48ClN3Pd (664.66): C, 65.05; H, 7.28; N, 6.32.
Found: C, 65.14; H, 7.41; N, 6.53.
IEt-Pd(dmba)Cl (13). From IEt · HCl (8; 406 mg), 13 (492
mg, 81%) was obtained as a white solid. H NMR (CDCl3, 400
MHz): δ 7.30 (t, J ) 7.6 Hz, 2H), 7.24 (dd, J ) 7.6, 1.2 Hz, 2H),
7.16 (s, 2H), 7.04 (dd, J ) 7.6, 1.2 Hz, 2H), 6.78 (td, J ) 7.6, 1.2
Hz, 1H), 6.71 (d, J ) 7.2, 1.0 Hz, 1H), 6.68 (td, J ) 7.6, 1.2 Hz,
1H), 6.58 (d, J ) 7.2, 1.0 Hz, 1H), 3.44 (s, 2H), 2.94 (m, 2H),
2.87 (m, 2H), 2.70 (m, 2H), 2.61 (m, 2H), 2.40 (s, 6H), 1.19 (t, J
) 7.2 Hz, 6H), 1.18 (d, J ) 7.6 Hz, 6H). 13C NMR (CDCl3, 100
MHz): δ 175.6, 150.0, 147.2, 143.1, 140.0, 137.3, 137.1, 129.1,
126.3, 126.0, 124.1, 123.4, 122.7, 121.1, 72.3, 50.0, 25.9, 25.2,
15.3, 14.7. Anal. Calcd for C32H40ClN3Pd (608.55): C, 63.16; H,
6.63; N, 6.90. Found: C, 63.16; H, 6.69; N, 6.65.
6H). 13C NMR (CDCl3, 100 MHz): δ 176.7, 147.9, 146.1-144.4
(broad), 137.5 (broad), 135.7 (broad), 130.7 (broad), 129.2, 126.5
(broad), 125.4 (broad), 125.4, 123.1, 123.0, 121.6, 71.8, 50.1, 27.7,
25.6 (broad), 24.2 (broad). Anal. Calcd for C32H40ClN3Pd (580.50):
C, 62.07; H, 6.25; N, 7.24. Found: C, 61.93; H, 6.45; N, 6.71.
Reaction of SIPr · HCl (17) and Palladacycle 6 under the
Conditions of Table 2, Entry 1. SIPr · HCl (460 mg, 1.05 mmol)
was used. The crude mixture was purified using flash chromatog-
raphy (solvent A was 9:1 hexane-CH2Cl2, solvent B was ethyl
acetate; gradient of 0-15% B over 30 min). The amide 19 eluted
first (at 8% solvent B), followed by complex 18 (at 12% solvent
B). Compound 19 (113 mg, 26%) was obtained as a white solid.
1H NMR (CDCl3, 400 MHz): δ 8.11 (s, 1H), 7.27-7.22 (m, 2H),
7.09-7.02 (m, 3H), 3.89 (t, J ) 7.1 Hz, 2H), 3.31 (broad s, 1H),
3.21 (heptet, J ) 6.8 Hz, 2H), 3.14 (t, J ) 7.1 Hz, 2H), 3.04 (heptet,
J ) 6.8 Hz, 2H), 1.26 (d, J ) 6.8 Hz, 6H), 1.23-1.13 (m, 18H).
13C NMR (CDCl3, 100 MHz): δ 163.9, 147.6, 142.9, 142.2, 135.4,
129.5, 124.5, 123.8, 123.5, 48.8, 48.7, 28.3, 27.7, 25.4, 24.2, 24.1,
23.5. A small amount (7%) of another carbonyl bond rotamer was
1
IMes-Pd(dmba)Cl (14). From IMes · HCl (9; 375 mg), 14 (555
mg, 96%) was obtained as a white solid. H NMR (CDCl3, 400
1
MHz): δ 7.10 (s, 2H), 6.99 (s, 2H), 6.83-6.76 (m, 4H), 6.70 (td,
J ) 7.6, 1.2 Hz, 1H), 6.58 (d, J ) 7.2, 1.2 Hz, 1H), 3.53 (s, 2H),
2.45 (s, 6H), 2.44 (s, 6H), 2.29 (s, 6H), 2.23 (s, 6H). 13C NMR
(CDCl3, 100 MHz): δ 175.6, 149.3, 147.6, 138.3, 138.3, 137.4,
136.2, 133.9, 129.4, 128.7, 123.9, 123.2, 123.0, 121.2, 72.2, 50.0,
21.1, 20.2, 19.8. Anal. Calcd for C30H36ClN3Pd (580.50): C, 62.07;
H, 6.25; N, 7.24. Found: C, 62.02; H, 6.37; N, 7.40.
1
also observed. Selected resonances: H NMR (CDCl3, 400 MHz)
δ 8.56 (s, 1H), 3.77 (t, J ) 7.1 Hz, 2H).; 13C NMR (CDCl3, 100
MHz) δ 163.0, 52.0, 50.0. Anal. Calcd for C27H40N2O (408.62):
C, 79.36; H, 9.87; N, 6.86. Found: C, 79.04; H, 9.76; N, 6.72.
General Procedure for the Synthesis of NHC-Pd(dmba)Cl
Complexes from 1,3-Diaryl-4,5-dihydroimidazolium Salts (Table
3). A pressure tube was charged with PdCl2 (177 mg, 1.00 mmol),
finely powdered 4 Å molecular sieves (1 g), and a stir bar, and
then acetonitrile (5 mL, HPLC grade kept over 4 Å molecular
sieves) and N,N-dimethylbenzylamine (160 µL, 177 mg, 1.05 mmol)
were added. The mixture was stirred at 80 °C until PdCl2 dissolved
completely, forming a dark orange, clear solution (in ∼20-25 min).
Finely powdered Cs2CO3 (815 mg, 2.50 mmol) was added, and
the mixture was heated at 80 °C until the palladacycle formation
was complete, as indicated by the change in the color of the mixture
to canary yellow (in ∼5 min). Immediately, the required 4,5-
dihydroimidazolium salt (1.05 mmol) was added, and the mixture
was heated at 80 °C with vigorous stirring over 18 h. The mixture
was filtered over a pad of Celite (CH2Cl2), the volatiles were
removed, and the residue was purified by flash chromatography
ITbp-Pd(dmba)Cl (15). From ITbp · HCl (10; 406 mg), 15 (492
mg, 81%) was obtained as an off-white solid. H NMR (CDCl3,
1
400 MHz): δ 7.62 (dd, J ) 8.4, 1.6 Hz, 2H), 7.42 (dd, J ) 8.0, 1.2
Hz, 2H), 7.30-7.22 (m, 2H), 7.29 (s, 2H), 6.93-6.81 (m, 5H),
6.39 (d, J ) 7.2 Hz), 3.51 (s, 2H), 2.42 (s, 6H), 1.51 (s, 18H). 13
C
NMR (CDCl3, 100 MHz): δ 176.7, 151.8, 148.0, 146.7, 137.6,
137.5, 131.8, 129.8, 128.7, 125.1, 125.1, 124.7, 123.0, 121.6, 72.4,
49.6, 36.7, 32.8. Anal. Calcd for C32H40ClN3Pd (608.55): C, 63.16;
H, 6.63; N, 6.90. Found: C, 63.13; H, 6.44; N, 6.56.
IPrp-Pd(dmba)Cl (16). From IPrp · HCl (11; 375 mg), 16 (500
mg, 86%) was obtained as an off-white solid. H NMR (CDCl3,
400 MHz): δ 8.62 (broad s, 2H), 7.38 (broad m, 6H), 7.23 (s, 2H),
6.92-6.15 (broad m, 4H), 3.51 (broad s, 2H), 2.97 (broad s, 2H),
2.50 (broad s, 6H), 1.58-1.18 (broad s, 6H), 1.18-0.76 (broad s,
1

296 Organometallics, Vol. 28, No. 1, 2009
KantcheV and Ying

N-Heterocyclic Carbene Ligated Palladacycles
Organometallics, Vol. 28, No. 1, 2009 297
(solvent A was 9:1 hexane-CH2Cl2, solvent B was ethyl acetate;
gradient of 0-40% B over 20 min).
ITbp · HCl (10). To a solution of the diimine 29 (6.41 g, 10
mmol) in THF (20 mL) were added chloromethyl ethyl ether (1.0
mL, 1.04 g, 11 mmol) and water (0.2 mL) in succession, and the
mixture was stirred at 40 °C over 18 h. The solvent was removed
to dryness, and the residue was partitioned between ethyl acetate
(50 mL) and water (50 mL). The organic layer was further extracted
with water (25 mL × 2). The combined aqueous layers were then
extracted with CH2Cl2 (25 mL × 4). The combined CH2Cl2 layers
were dried (MgSO4), the solvent was removed under reduced
pressure, and the residue was triturated with CH2Cl2/ethyl acetate
(1:5). ITBp · HCl (10; 1.28 g, 35%) was obtained as an off-white
SIPr-Pd(dmba)Cl (18). From SIPr · HCl (17; 460 mg), 18 (640
1
mg, 96%) was obtained as a white solid. H NMR (CDCl3, 400
MHz): δ 7.36-7.28 (m, 4H), 7.22 (m, 1H), 7.10 (dd, J ) 7.2, 1.6
Hz, 2H), 6.85 (m, 3H), 6.78 (m, 1H), 4.16 (m, 2H), 4.10 (m, 2H),
3.59 (m, 4H), 3.40 (s, 2H), 2.32 (s, 6H), 1.60 (d, J ) 6.8 Hz, 6H),
1.26 (d, J ) 6.8 Hz, 6H), 1.21 (d, J ) 6.8 Hz, 6H), 0.74 (d, J )
6.8 Hz, 6H). 13C NMR (CDCl3, 100 MHz): δ 205.4, 150.7, 148.0,
147.9, 146.3, 136.9, 136.0, 128.8, 125.3, 124.4, 124.0, 122.8, 121.7,
72.6, 54.2, 49.5, 29.0, 28.4, 27.0, 26.3, 24.3, 23.6. Anal. Calcd for
C36H50ClN3Pd (666.68): C, 64.86; H, 7.56; N, 6.30. Found: C,
65.19; H, 7.76; N, 6.39.
SIMes-Pd(dmba)Cl (23). From SIMes · HBF4 (20; 414 mg),
23 (570 mg, 98%) was obtained as a white solid. 1H NMR (CDCl3,
400 MHz): δ 7.17 (dd, J ) 7.2, 1.3 Hz, 1H), 6.96 (broad s, 2H),
6.89-6.83 (m, 2H), 6.79 (td, J ) 6.9, 1.9 Hz, 1H), 6.75 (broad s,
2H), 4.12-4.03 (m, 2H), 4.01-3.94 (m, 2H), 3.46 (s, 6H), 2.67
(s, 6H), 2.36 (s, 6H), 2.29 (s, 6H). 13C NMR (CDCl3, 100 MHz):
δ 204.1, 149.0, 147.6, 138.8, 137.8, 137.5, 136.4, 135.3, 129.6,
128.8, 123.9, 123.0, 121.3, 72.1, 51.8, 49.8, 21.0, 20.4, 20.0. Anal.
Calcd for C30H38ClN3Pd (582.52): C, 61.86; H, 6.58; N, 6.93.
Found: C, 61.40; H, 6.68; N, 6.93.
SIMes-Pd(dmba)Br (24). From SIMes · HBF4 (20; 414 mg)
and PdBr2 (266 mg, 1.00 mmol), 24 (417 mg, 67%) was obtained
as a white solid. 1H NMR (CDCl3, 400 MHz): δ 7.18 (dd, J ) 7.2,
1.6 Hz, 1H), 6.96 (broad s, 2H), 6.89-6.85 (m, 2H), 6.79 (m, 1H),
6.74 (broad s, 2H), 4.12-4.05 (m, 2H), 4.01-3.96 (m, 2H), 3.45
(s, 2H), 2.67 (s, 6H), 2.44 (s, 6H), 2.28 (s, 6H), 2.26 (s, 2H). 13C
NMR (CDCl3, 100 MHz): δ 203.8, 150.8, 147.5, 138.6, 137.6,
137.5, 136.5, 135.3, 129.7, 129.6, 128.9, 123.9, 123.1, 121.4, 72.1,
51.9, 50.0, 21.0, 20.9, 20.2. Anal. Calcd for C30H38BrN3Pd (625.13):
C, 57.47; H, 6.11; N, 6.70. Found: C, 57.04; H, 5.81; N, 6.44.
SITBp-Pd(dmba)Cl (25). From SITBp · HBF4 (21; 441 mg),
25 (552 mg, 90%) was obtained as a white solid. The compound
was obtained as a mixture of two inseparable atropisomers: 1H NMR
(CDCl3, 400 MHz): δ 8.68-6.80 (12H), 4.42-3.80 (4H), 3.57-3.27
(4H), 2.40-2.28 (6H), 1.70-1.63 (12H), 1.40-1.38 (6H). 13C NMR
(CDCl3, 100 MHz): δ 208.1, 204.3, 151.8-121.7 (36 aromatic
carbons), 72.3, 72.0, 57.7, 57.0, 56.3, 50.2, 49.4, 49.1, 36.9, 36.5,
33.0, 32.6, 32.3. Anal. Calcd for C32H42ClN3Pd (610.57): C, 62.95;
H, 6.93; N, 6.88. Found: C, 62.90; H, 7.01; N, 6.79.
1
solid. H NMR (CD2Cl2, 400 MHz): δ 11.7-9.7 (broad s, 1H),
8.4-6.7 (broad s, 2H), 7.74 (m, 2H), 7.62 (td, J ) 8.8, 1.6 Hz,
2H), 7.45 (td, J ) 7.6, 1.2 Hz, 2H), 1.36 (s, 18H). 13C NMR
(CD2Cl2, 100 MHz): δ 145.9 (broad), 140.9 (broad), 131.7, 129.1
(broad), 127.6, 125.6 (broad), 36.0, 31.8. Anal. Calcd for
C23H29ClN2 (368.94): C, 74.88; H, 7.92; N, 7.59. Found: C, 75.05;
H, 7.89; N, 7.72.
SITbp · HBF4 (21). In a two-necked 250 mL round-bottomed
flask equipped with a reflux condenser and a stopper, a solution of
diimine 29 (3.20 g, 10 mmol) in THF (50 mL) and methanol (50
mL) was brought to reflux. NaBH4 (3.78 g, 100 mmol) was added
portionwise over 30 min. After the mixture was cooled, the colorless
solution obtained was quenched with water (200 mL), most of the
organic solvent was removed under reduced pressure, and the
residue was extracted with CH2Cl2 (50 mL × 3). The combined
organic layers were dried (MgSO4), and the solvent was removed
under reduced pressure. The residue was dissolved in HC(OEt)3
(25 mL), and concentrated HCl (12.0 M, 1.00 mL, 12 mmol) was
added dropwise. The mixture was heated at 120 °C over 1 h and
cooled. The volatiles were removed under high vacuum to dryness,
the crude chloride salt was dissolved in water (10 mL), and a
solution of NH4BF4 (5.24 g, 50 mmol) in water (20 mL) was added.
The solid was filtered off, rinsed with a minimal amount of water
followed by diethyl ether, and redissolved in CH2Cl2 (50 mL). The
solution was dried (MgSO4) and evaporated. SITBp · HBF4 (21;
3.68 g, 88%) was obtained as an off-white solid after drying under
high vacuum. 1H NMR (CDCl3, 400 MHz): δ 7.88 (broad m, 1H),
7.82-7.72 (broad m, 2H), 7.54 (dd, J ) 8.0, 0.8 Hz, 2H), 7.54 (m,
2H), 7.35 (broad m, 2H), 4.68 (broad s, 4H), 1.48 (s, 18H). 13C
NMR (CDCl3, 100 MHz): δ 159.0, 146.6, 133.6, 131.0, 130.8,
128.7, 128.6, 55.8, 35.9, 32.2. Anal. Calcd for C23H31BF4N2
(422.31): C, 65.41; H, 7.40; N, 6.63. Found: C, 65.40, H, 7.47, N
6.53.
SIPrp-Pd(dmba)Cl (26). From SIPrp · HBF4 (22; 414 mg), 26
1
(469 mg, 80%) was obtained as a white solid. H NMR (CDCl3,
400 MHz): δ 9.0-7.6 (broad s, 2H), 7.39-7.24 (broad m, 6H),
7.05 (broad m, 1H), 6.87-6.79 (broad m, 3H), 4.22 (broad s, 2H),
4.12 (broad s, 2H), 3.45 (s, 2H), 3.16 (broad s, 2H), 2.39 (broad s,
6H), 1.50-1.24 (broad s, 6H), 1.10-0.82 (broad s, 6H). 13C NMR
(CDCl3, 100 MHz): δ 205.7, 147.8, 146.0-144.7 (broad), 138.7,
135.7, 131.7-130.3 (broad), 128.4, 126.5 (broad), 126.2 (broad),
125.2, 123.1, 121.6, 71.6, 54.2, 49.8, 28.2, 25.0 (broad), 23.9
(broad). Anal. Calcd for C30H38ClN3Pd (582.52): C, 61.86; H, 6.58;
N, 6.93. Found: C, 61.64; H, 6.28; N, 7.06.
N,N-(2-tert-Butylphenyl)-1,4-diazabuta-1,4-diene (29). To a
solution of 2-tert-butylaniline (27; 20.4 mL, 19.6 g, 131 mmol) in
a mixture of methanol (50 mL) and water (5 mL) was added glyoxal
solution (40% in water; 7.5 mL, 9.43 g, 65 mmol), and the resulting
mixture was stirred over 1.5 h. The yellow crystalline mass was
filtered off, dried with a stream of air, and vacuum-dried over P2O5.
Diimine 29 was obtained as a yellow solid (19.91 g, 96%) and
used directly for the next step. A small portion was crystallized
(methanol) to provide an analytically pure sample. 1H NMR (CDCl3,
400 MHz): δ 8.28 (s, 2H), 7.44 (m, 2H), 7.27 (m, 4H), 6.94 (m,
2H), 1.47 (s, 18H). 13C NMR (CDCl3, 100 MHz): δ 158.6, 150.2,
143.7, 127.2, 127.1, 126.4, 118.9, 35.8, 30.5. Anal. Calcd for
C22H28N2 (320.47): C, 82.45; H, 8.81; N, 8.74. Found: C, 82.88;
H, 8.65; N, 8.79.
N,N-(2-Isopropylphenyl)-1,4-diazabuta-1,4-diene (30). Freshly
distilled 2-isopropylaniline (28; 5.7 mL, 5.41 g, 40 mmol) and
glyoxal solution (40% in water; 2.3 mL, 2.93 g, 20 mmol) were
added to a degassed (Ar stream over 20 min) mixture of methanol
(20 mL) and water (2 mL), and this mixture was stirred over 18 h.
The dark-colored reaction mixture was diluted with water (50 mL),
and methanol was removed under reduced pressure. The crude
product was extracted in diethyl ether (50 mL × 3), the combined
organic layers were dried (MgSO4), and the volatiles were
evaporated under reduced pressure. Diimine 30 was obtained as a
yellow oil (4.84 g, 71%) after flash chromatography (80 g
Combiflash cartridge, ethyl acetate gradient in hexane: 0% over
10 min, followed by 0-3% over 15 min, and 3% over 10 min). 1H
NMR (CD2Cl2, 400 MHz): δ 8.35 (s, 2H), 7.38 (dd, J ) 7.6, 1.6
Hz, 2H), 7.30 (td, J ) 7.6, 1.6 Hz, 2H), 7.24 (td, J ) 7.6, 1.2 Hz,
2H), 7.00 (dd, J ) 7.6, 1.2 Hz, 2H), 3.59 (heptet, J ) 6.8 Hz, 2H),
1.28 (d, J ) 6.8 Hz, 12H). 13C NMR (CDCl3, 100 MHz): δ 159.7,
148.4, 143.2, 127.7, 126.8, 125.8, 117.4, 27.9, 23.3. Anal. Calcd
for C20H24N2 (292.42): C, 82.15; H, 8.81; N, 8.74. Found: C, 81.86;
H, 9.17; N, 9.00.
IPrp · HCl (11). The procedure for compound 10 was followed,
using diimine 30 (2.92 g, 10 mmol), chloromethyl ethyl ether (1.0
mL, 1.04 g, 12 mmol), and water (0.1 mL) in THF (10 mL).

298 Organometallics, Vol. 28, No. 1, 2009
KantcheV and Ying
IPrp · HCl (11; 2.11 g, 62%) was obtained as an off-white solid.
1H NMR (CDCl3, 400 MHz): δ 11.7-9.7 (broad s, 1H), 8.4-6.7
(broad s, 2H), 7.74 (m, 2H), 7.62 (td, J ) 8.8, 1.6 Hz, 2H), 7.45
(td, J ) 7.6, 1.2 Hz, 2H), 1.36 (s, 9H). 13C NMR (CDCl3, 100
MHz): δ 143.7, 138.2, 132.3, 131.8, 127.8, 127.7, 127.0, 124.7,
28.3, 23.9. Anal. Calcd for C21H25ClN2 (340.89): C, 73.99; H, 7.39;
N, 8.22. Found: C, 74.07; H, 7.24; N, 8.02.
SIPrp · HBF4 (22). The procedure for compound 21 was fol-
lowed, starting with 30 (2.92 g, 10 mmol). SIPrp · HBF4 (22; 3.94 g,
94%) was obtained as an off-white solid after drying under high
vacuum. 1H NMR (CD2Cl2, 400 MHz): δ 7.74-7.70 (m, 3H),
7.48-7.40 (m, 4H), 7.32-7.27 (m, 2H), 4.61 (s, 4H), 3.08 (heptet,
J ) 6.8 Hz), 1.36 (d, J ) 6.8 Hz, 12H). 13C NMR (CDCl3, 100
MHz): δ 156.9, 144.7, 132.5, 131.0, 128.1, 127.7, 127.0, 54.0, 28.4,
24.2. Anal. Calcd for C21H27BF4N2 (394.26): C, 63.97; H, 6.90; N,
7.11. Found: C, 63.57, H, 6.46; N, 6.83.
7.2 Hz, 1H), 3.44 (s, 2H), 3.21 (broad s, 2H), 2.60 (broad s, 2H),
2.36 (s, 6H), 2.05 (s, 3H), 1.59-1.26 (broad s, 6H), 1.26-1.00
(broad m, 12H), 0.92-0.52 (broad s, 6H). 13C NMR (CDCl3, 100
MHz): δ 174.7, 147.7, 145.9 (broad), 145.2 (broad), 135.3, 134.8,
130.5, 126.3, 125.3, 124.9 (broad), 124.5 (broad), 123.9, 122.7,
120.7, 71.8, 50.0, 28.9 (broad), 28.6 (broad), 26.3, 26.3, 22.9
(broad), 22.7 (broad), 21.1. Anal. Calcd for C38H51F3N4PPd
(815.22): C, 55.99; H, 6.31; N, 6.87. Found: C, 55.78; H, 6.65; N,
6.89.
[Pd(dmba)OAc]2 (35). A 100 mL round-bottomed flask was
charged with PdCl2 (885 mg, 5.00 mmol) and a stir bar. Next,
acetonitrile (20 mL, HPLC grade) and N,N-dimethylbenzylamine
(790 µL, 878 mg, 5.25 mmol) were added. The mixture was stirred
at 80 °C until PdCl2 dissolved completely, forming a dark orange,
clear solution (in ∼20-25 min). Finely powdered NaOAc (2.05 g,
25.0 mmol) was added, and the mixture was heated at 80 °C over
20 min. The mixture was filtered over a pad of Celite (CH2Cl2),
the volatiles were removed, and the residue was dissolved in CH2Cl2
(30 mL), washed with water (50 mL × 3), dried (MgSO4), and
evaporated. The dimeric acetate-bridged palladacycle 35 (1.38 g,
92%) was obtained as a yellow solid after trituration with hexanes.
The 1H and 13C NMR spectra were in accordance with the literature.
[Pd(dmba)(acac)]2 (36). A 100 mL round-bottomed flask was
charged with PdCl2 (885 mg, 5.00 mmol) and a stir bar. Next,
acetonitrile (20 mL, HPLC grade) and N,N-dimethylbenzylamine
(790 µL, 878 mg, 5.25 mmol) were added. The mixture was stirred
at 80 °C until PdCl2 dissolved completely, forming a dark orange,
clear solution (in ∼20-25 min). Finely powdered K2CO3 (3.46 g,
25.0 mmol) was added, and the mixture was heated at 80 °C over
10 min. Acetylacetone (540 µL, 527 mg, 5.25 mmol) was added,
and the mixture was stirred for another 15 min. The mixture was
worked up as specified for compound 35. Complex 36 (1.56 g,
92%) was obtained as a yellow solid after trituration with hexanes.
The 1H and 13C NMR spectra were in accordance with the literature.
ITBp-Pd(dmba)I (37, Table 5). A pressure tube was charged
with PdCl2 (177 mg, 1.00 mmol) and a stir bar. Next, acetonitrile
(5 mL, HPLC grade) and N,N-dimethylbenzylamine (160 µL, 178
mg, 1.05 mmol) were added. The mixture was stirred at 80 °C until
PdCl2 dissolved completely, forming a dark orange, clear solution
(in ∼20-25 min). Finely powdered K2CO3 (691 mg, 5.00 mmol)
was added, and the mixture was heated at 80 °C until the
palladacycle formation was complete, as indicated by the change
in the color of the mixture to canary yellow (in ∼5 min).
Immediately, ITbp · HCl (10; 385 mg, 1.05 mmol) was added, and
the mixture was heated at 80 °C with vigorous stirring over 18 h.
Next, NaI (450 mg, 3.00 mmol) was added, and the mixture was
heated at 80 °C for another 30 min. The mixture was filtered over
a pad of Celite (CH2Cl2), the volatiles were removed, and the residue
was purified by flash chromatography (hexane-ethyl acetate:
0-100% over 20 min). Complex 33 (672 mg, 96%) was isolated
as a pale orange solid. 1H NMR (CDCl3, 400 MHz): δ 7.62 (dd, J
) 8.4, 1.6 Hz, 2H), 7.50 (dd, J ) 8.4, 1.6 Hz, 2H), 7.32 (s, 2H),
7.32-7.28 (m, 2H), 6.99 (td, J ) 3.2, 1.6 Hz, 1H), 6.92-6.86 (m,
4H), 6.23 (dd, J ) 4.00, 1.6 Hz, 2H), 3.56 (s, 2H), 2.60 (s, 6H),
1.50 (s, 18H). 13C NMR (CDCl3, 100 MHz): δ 176.1, 156.2, 148.2,
146.3, 137.6, 136.7, 131.7, 130.0, 128.7, 125.6, 125.1, 125.0, 123.5,
122.0, 72.2, 51.7, 36.8, 33.3. Anal. Calcd for C32H40IN3Pd (700.00):
C, 54.91; H, 5.76; N, 6.00. Found: C, 54.68; H, 6.19; N, 6.02.
General Procedure for the Ion Exchange of IPr-Pd(dmba)Cl
Complexes with Ag Salts (Table 4). To a solution of the silver salt
(1 mmol) in acetone (2 mL), a solution of complex 12 (665 mg, 1
mmol) in CH2Cl2 (8 mL) was added. The mixture was stirred over
1 h, filtered, and evaporated to dryness.
IPr-Pd(dmba)OAc (31). From AgOAc (167 mg), 31 (663 mg,
96%) was obtained as a white solid after chromatography with
CH2Cl2-ethyl acetate/methanol (volume ratio 5:1) gradient, 0-100%
1
over 20 min. H NMR (CDCl3, 400 MHz): δ 7.32 (t, J ) 7.6 Hz,
2H), 7.22 (broad d, J ) 7.6 Hz, 2H), 7.12 (s, 2H), 7.10 (broad d,
J ) 7.6 Hz, 2H), 6.68 (t, J ) 7.2 Hz, 2H), 6.61 (m, 3H), 6.38 (d,
J ) 7.6 Hz, 1H), 3.24 (s, 2H), 3.20 (m, 2H), 2.81 (m, 2H), 2.18 (s,
6H), 1.40 (s, 3H), 1.33 (d, J ) 6.4 Hz, 6H), 1.10 (d, J ) 6.4 Hz,
6H), 0.93 (d, J ) 6.8 Hz, 6H), 0.79 (d, J ) 6.4 Hz, 6H). 13C NMR
(CDCl3, 100 MHz): δ 178.3, 176.0, 148.2, 148.0, 147.0, 145.1,
136.0, 135.8, 129.6, 125.4, 124.2, 124.1, 123.8, 122.4, 121.7, 72.3,
49.2, 28.7, 28.5, 26.3, 25.3, 23.0, 22.8. Anal. Calcd for
C38H51N3O2Pd (688.25): C, 66.31; H, 7.47; N, 6.11. Found: C,
66.70; H, 7.63; N, 6.31.
IPr-Pd(dmba)TFA (32). From AgTFA (221 mg), 32 (733 mg,
99%) was obtained as a white solid after chromatography with
1
CH2Cl2-ethyl acetate gradient, 0-100% over 20 min. H NMR
(CDCl3, 400 MHz): δ 7.42 (t, J ) 7.6 Hz, 2H), 7.30 (d, J ) 7.6
Hz, 2H), 7.25 (s, 2H), 7.21 (d, J ) 7.6 Hz, 2H), 6.80 (td, J ) 6.8,
1.2 Hz, 2H), 6.70 (m, 2H), 6.70 (d, J ) 7.6 Hz, 1H), 3.34 (s, 2H),
3.29 (m, 2H), 2.84 (m, 2H), 2.24 (s, 6H), 1.36 (d, J ) 6.8 Hz, 6H),
1.19 (d, J ) 6.8 Hz, 6H), 1.03 (d, J ) 6.8 Hz, 6H), 0.88 (d, J )
6.8 Hz, 6H). 13C NMR (CDCl3, 100 MHz): δ 177.3, 160.8 (q, 2JC-F
) 35 Hz), 147.6, 146.7, 145.9, 145.0, 136.0, 135.8, 135.4, 129.7,
1
125.7, 124.5, 124.3, 123.9, 122.7, 121.0, 116.6 (q, JC-F ) 292
Hz), 72.0, 49.1, 28.6, 28.5, 26.3, 26.3, 23.0, 22.4. Anal. Calcd for
C38H49F3N3O2Pd (742.22): C, 61.49; H, 6.52; N, 5.66. Found: C,
62.06; H, 6.67; N, 5.77.
IPr-Pd(dmba)OTf (33). From AgOTf (257 mg), 33 (745 mg,
96%) was obtained as a white solid after chromatography with
1
CH2Cl2-ethyl acetate gradient, 0-100% over 20 min. H NMR
(CDCl3, 400 MHz): δ 7.45 (t, J ) 4.0 Hz, 2H), 7.31 (d, J ) 4.0
Hz, 4H), 7.30 (s, 2H), 7.21 (td, J ) 7.6, 1.2 Hz, 1H), 6.76-6.71
(m, 2H), 6.36 (dd, J ) 4.2, 0.8 Hz, 1H), 3.45 (s, 2H), 3.03 (broad
s, 4H), 2.38 (s, 6H), 1.35-0.95 (broad, 24H). 13C NMR (CDCl3,
100 MHz): δ 175.5, 160.8, 147.3, 145.7, 141.1, 135.5, 134.6, 130.7,
126.3, 125.1, 124.9, 124.6, 122.6, 121.8, 121.0, 120.3 (q, 1JC-F
)
317 Hz), 71.0, 49.5, 29.0, 26.3, 22.8. Anal. Calcd for
C37H48F3N3O3PdS (778.28): C, 57.10; H, 6.22; N, 5.40. Found:
57.07; H, 6.08; N, 4.94.
IPr-Pd(dmba)PF6 · CH3CN (34). From AgPF6 (257 mg) in
CH2Cl2 (10 mL) and CH3CN (2 mL), 34 (745 mg, 96%) was
General Procedure for the Synthesis of NHC-Pd(dmba)Cl
Complexes from 1,3-Dialkylimidazolium and 1,3-Dialkylbenzimi-
dazolium Salts (Table 1). A pressure tube was charged with PdCl2
(177 mg, 1.00 mmol) and a stir bar. Next, acetonitrile (5 mL, HPLC
grade) and N,N-dimethylbenzylamine (160 µL, 178 mg, 1.05 mmol)
were added. The mixture was stirred at 80 °C until PdCl2 dissolved
completely, forming a dark orange, clear solution (in ∼20-25 min).
Finely powdered Cs2CO3 (814 mg, 2.50 mmol) was added, and
the mixture was heated at 80 °C until the palladacycle formation
obtained as
a
white solid after chromatography with
CH2Cl2-acetonitrile, 0-20% over 20 min. 1H NMR (CDCl3, 400
MHz): δ 7.45 (t, J ) 8.0 Hz, 2H), 7.38 (m, 2H), 7.39-7.17 (broad
m, 4H), 6.88 (t, J ) 6.8 Hz, 2H), 6.79-6.74 (m, 2H), 6.30 (d, J )

N-Heterocyclic Carbene Ligated Palladacycles
Organometallics, Vol. 28, No. 1, 2009 299
was complete, as indicated by the change in the color of the mixture
to canary yellow (in ∼5 min). Immediately, the required imidazo-
lium or benzimidazolium salt (1.10 mmol) was added, the temper-
ature was increased to 100 °C, and the heating was continued for
30 min at that temperature with vigorous stirring. The mixture was
filtered over a pad of Celite (CH2Cl2), the volatiles were removed,
and the residue was purified by flash chromatography (solvent A,
CH2Cl2; solvent B, ethyl acetate-methanol (volume ratio 4:1)
gradient, 0-30% B in A over 15 min.
2H), 3.96 (s, 2H), 2.88 (s, 6H), 1.76 (d, J ) 7.2 Hz, 6H), 1.62 (d,
J ) 7.2 Hz, 6H). 13C NMR (CDCl3, 100 MHz): δ 183.8, 148.3,
136.7, 125.2, 123.8, 122.4, 122.1, 122.1, 112.6, 112.6, 72.2, 54.6,
50.4, 21.1, 20.8. Anal. Calcd for C22H30ClN3Pd (478.32): C, 55.24;
H, 6.32; N, 7.41. Found: C, 55.26; H, 6.38; N, 7.44.
Single-Crystal X-ray Structural Determination. Single crystals
for all complexes were grown by a slow infusion of n-pentane into
concentrated solutions of the NHC-palladacycles in CH2Cl2.
Suitable crystals were mounted on quartz fibers, and X-ray data
were collected on a Bruker AXS APEX diffractometer, equipped
with a CCD detector, using graphite-monochromated Mo KR
radiation (λ ) 0.710 73 Å). The collecting frames of data, indexing
reflection, and determination of lattice parameters and polarization
effects were performed with the SMART suite of programs.41 The
integration of intensity of reflections and scaling was performed
by SAINT.41 The empirical absorption correction was performed
by SADABS.42 The space group determination, structure solution,
and least-squares refinements on |F|2 were carried out with
SHELXTL.43 The structures were solved by direct methods to locate
the heavy atoms, followed by difference maps for the light non-
hydrogen atoms. Anisotropic thermal parameters were refined for
the rest of the non-hydrogen atoms. The hydrogen atoms were
placed in their ideal positions.
Bn2Im-Pd(dmba)Cl (42). From Bn2Im · HPF6 (38; 356 mg),
1
42 (411 mg, 78%) was obtained as an off-white solid. H NMR
(CDCl3, 400 MHz): δ 7.49-7.42 (m, 4H), 7.35-7.29 (m, 6H), 7.02
(dd, J ) 7.6, 1.2 Hz, 1H), 6.98 (td, J ) 7.6, 1.2 Hz, 1H), 6.79 (td,
J ) 7.2, 2.4 Hz, 1H), 6.77 (s, 2H), 6.15 (dd, J ) 7.6, 1.2 Hz, 1H),
5.69 (d, J ) 14.8 Hz, 2H), 5.60 (d, J ) 14.8 Hz, 2H), 3.94 (m,
2H), 2.85 (s, 6H). 13C NMR (CDCl3, 100 MHz): δ 173.5, 149.2,
148.5, 136.1, 135.9, 129.0, 128.8, 128.3, 125.6, 123.8, 122.3, 120.9,
72.2, 55.5, 50.3. Anal. Calcd for C26H28ClN3Pd (524.39): C, 59.55;
H, 5.38; N, 8.01. Found: C, 59.52; H, 5.41; N, 7.90.
Me2Im-Pd(dmba)Cl (43). From Me2Im · HCl (39; 146 mg), 43
(248 mg, 67%) was obtained as an off-white solid. 1H NMR (CDCl3,
400 MHz): δ 7.02 (dd, J ) 1.6, 0.4 Hz, 1H), 6.95 (td, J ) 7.6, 1.2
Hz, 1H), 6.95 (s, 2H), 6.75 (ddd, J ) 7.6, 1.6, 0.4 Hz, 1H), 5.95
(dd, J ) 7.6, 1.2 Hz, 1H), 3.97 (s, 6H), 3.92 (s, 2H), 2.83 (s, 6H).
13C NMR (CDCl3, 100 MHz): δ 173.3, 149.9, 135.3, 124.9, 124.1,
123.4, 123.0, 72.2, 50.1, 38.6. Anal. Calcd for C14H20ClN3Pd
(372.20): C, 45.18; H, 5.42; N, 11.29. Found: C, 45.32; H, 5.47;
N, 11.21.
iPr2Im-Pd(dmba)Cl (44). From iPr2Im · HCl (40; 259 mg), 44
(256 mg, 62%) was obtained as an off-white solid. 1H NMR (CDCl3,
400 MHz): δ 7.03 (s, 2H), 7.00 (dd, J ) 7.2, 0.8 Hz, 1H), 6.94 (td,
J ) 7.6, 1.2 Hz, 1H), 6.72 (td, J ) 7.6, 1.6 Hz, 1H), 5.97 (dd, J )
7.2, 0.8 Hz, 1H), 5.53-5.43 (m, 2H), 3.91 (s, 2H), 2.83 (s, 6H),
1.54 (d, J ) 2.4 Hz, 6H), 1.36 (d, J ) 2.8 Hz, 6H). 13C NMR
(CDCl3, 100 MHz): δ 176.7, 148.5, 147.8, 136.3, 125.2, 123.6,
122.2, 116.9, 116.5, 72.1, 53.1, 50.2, 23.7, 22.8. Anal. Calcd for
C18H28ClN3Pd (428.31): C, 50.48; H, 6.59; N, 9.81. Found: C,
50.12; H, 6.23; N, 9.80.
Acknowledgment. This work was funded by the Institute
of Bioengineering and Nanotechnology (Biomedical Re-
search Council, Agency for Science, Technology and Re-
search, Singapore). We thank Geok Kheng Tan (X-Ray
Crystallography Centre, National University of Singapore)
forthesingle-crystalX-raystructuredeterminationexperiments.
Supporting Information Available: Crystallographic informa-
tion files (CIF) for compounds 12, 15, 16, 18, 25, 31, 33, 42, and
45. This material is available free of charge via the Internet at
http://pubs.acs.org.
OM8008475
iPr2BzIm-Pd(dmba)Cl (45). From iPr2BzIm · HPF6 (41; 383
mg), 45 (399 mg, 83%) was obtained as an off-white solid. At the
beginning of the reaction, finely powdered 4 Å molecular sieves
(1 g) were added. H NMR (CDCl3, 400 MHz): δ 7.63 (m, 2H),
7.28-7.24 (m, 3H); 7.21 (s, 2H), 7.03 (dd, J ) 7.6, 1.2 H), 6.94
(td, J ) 7.6, 1.2 H), 6.67 (td, J ) 7.6, 1.4 Hz, 1H), 6.17-6.06 (m,
(41) SMART (Version 5.631) and SAINT (Version 6.63) Software
Reference Manuals; Bruker AXS: Karlsruhe, Germany, 2000.
(42) Sheldrick, G. M. SADABS: Software for Empirical Absorption
Correction; University of Go¨ttingen, Go¨ttingen, Germany, 2001.
(43) SHELXTL Reference Manual (Version 6.10); Bruker AXS: Karlsru-
he, Germany, 2000.
1